Direct Propene Epoxidation over Gold-Titania Catalysts

Transcription

Direct Propene Epoxidation over
Gold–Titania Catalysts: Kinetics and
Mechanism
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de
Technische Universiteit Eindhoven, op gezag van de
rector magnificus, prof.dr.ir. C.J. van Duijn, voor een
commissie aangewezen door het College voor
Promoties in het openbaar te verdedigen
op maandag 18 november 2013 om 14.00 uur
door
Jiaqi Chen
geboren te Rudong, China
Dit proefschrift is goedgekeurd door de promotor:
prof.dr.ir. J.C. Schouten
Copromotor:
dr.ir. T.A. Nijhuis
The Netherlands Organization for Scientific Research (NWO) is kindly acknowledged for
providing an ECHO grant (700.57.044) to make the research described in this thesis
possible.
Chen, Jiaqi
Direct Propene Epoxidation over Gold–Titania Catalysts: Kinetics and Mechanism
Technische Universiteit Eindhoven, 2013
A catalogue record is available from the Eindhoven University of Technology Library
ISBN: 978-90-386-3465-4
Copyright © 2013 by Jiaqi Chen
Cover photo provided by Shutterstock, Inc. under the Single User Standard License
Contents
Summary
vii
1 Introduction
1
1.1 Propene oxide and its production . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.2 Direct epoxidation over gold based catalysts . . . . . . . . . . . . . . . . . . .
3
1.3 Microreactor system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.4 Objectives and outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2 Kinetic study of the direct propylene epoxidation in the explosive regime
19
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.1 Catalyst preparation and characterization . . . . . . . . . . . . . . . . 22
2.2.2 Catalyst testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.3.1 Performance of microreactor and proof of concept . . . . . . . . . . . 24
2.3.2 Catalyst characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.3.3 Product formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.3.4 Propene epoxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3.5 Water formation and link to epoxidation . . . . . . . . . . . . . . . . . 31
2.3.6 Effect of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.3.7 Relationship between selectivity, hydrogen efficiency, catalyst stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.4 Summarizing discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
iv
CONTENTS
Appendix 2.A Deactivation model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Appendix 2.B Activation energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3 Enhancement of catalyst performance: A study into gold–titanium synergy
55
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.1 Preparation of supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.2.2 Deposition of Au . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.2.3 Inversed incorporation of Ti onto Au/SiO2 . . . . . . . . . . . . . . . . 59
3.2.4 Catalytic testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.2.5 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.3.1 Characterization of the supports . . . . . . . . . . . . . . . . . . . . . . 60
3.3.2 Size of Au particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.3.3 PO formation and water formation . . . . . . . . . . . . . . . . . . . . . 64
3.3.4 Performance of the catalyts with inversely-grafted Ti . . . . . . . . . . 70
3.3.5 Effect of supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
3.3.6 Propane formation and its suppression . . . . . . . . . . . . . . . . . . 73
3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
3.4.1 Role of the support in enhanced PO productivity . . . . . . . . . . . . 73
3.4.2 Competition of epoxidation and water formation at the Au–Ti interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.4.3 Origin of the activity in propene hydrogenation . . . . . . . . . . . . . 79
3.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Appendix 3.A Tables of catalyst performance . . . . . . . . . . . . . . . . . . . . . . 80
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4 Switching off propene hydrogenation in the direct epoxidation of propene
85
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.3.1 Activation of hydrogenation activity . . . . . . . . . . . . . . . . . . . . 88
4.3.2 Switching off propene hydrogenation by CO . . . . . . . . . . . . . . . 90
v
CONTENTS
4.3.3 Probing the active site for propene hydrogenation . . . . . . . . . . . 97
4.4 Summarizing discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5 How metallic is gold in the direct epoxidation of propene
115
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5.2.1 Catalyst preparation and testing . . . . . . . . . . . . . . . . . . . . . . 118
5.2.2 Charaterization techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 119
5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
5.3.1 Catalyst performance and characterization . . . . . . . . . . . . . . . . 120
5.3.2 CO adsorption on catalysts after epoxidation and regeneration . . . 121
5.3.3 CO adsorption on Au/Ti-SiO2 treated by O2 and H2 . . . . . . . . . . 130
5.3.4 CO adsorption on Au/TiO2 in the presence of H2 . . . . . . . . . . . . 132
5.3.5 C3 H6 adsorption on Au/TiO2 in the presence of CO . . . . . . . . . . 136
5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
6 Conclusions and outlook
151
6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
6.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
List of Publications
157
Acknowledgements
159
About the Author
163
Summary
Direct Propene Epoxidation over Gold–Titania Catalysts:
Kinetics and Mechanism
Propene oxide (PO) is a major bulk material used in chemical industry. The current
main processes in operation, the chlorohydrin process and the hydroperoxide processes,
are both complex and have a number of disadvantages. As an alternative, the direct
epoxidation of propene to propene oxide using hydrogen, oxygen and propene is highly
selective over a gold–titania based catalyst and has gained considerable attention in the
past 15 years. However, one of the main hurdles for the industry application is the relatively low activity of such Au/Ti-support catalysts. Performing this reaction into the
explosive region of the reactant mixture is supposed to be beneficial for a higher productivity since the formation of the reactive hydroperoxy intermediate from hydrogen
dissociation is rate determining, bringing this process closer towards industrialization.
The usage of a microreactor system for the direct epoxidation of propene over a goldtitania based catalyst system using a mixture of hydrogen, oxygen, and propene allows for
the safe operation of the reaction in the explosive regime. A kinetic study was performed
on the effect of the concentration of hydrogen, oxygen, and propene on the reaction rate
as well as the catalyst deactivation and reactivation. A simple algebraic rate expression
was developed, based on published kinetics, which provided the three reaction rate constants as a function of the feed gas concentrations. The observed rate dependency on the
reactants for the epoxidation and the competing direct water formation is discussed in
relation to the current mechanistic insights in literature. The formation rate of propene
oxide is most dependent on the hydrogen concentration, in which the formation of an
active peroxo species on the gold nanoparticles is the rate determining step. The deactivation is mainly caused by the consecutive oxidation of propene oxide. Oxygen favours
the regeneration of the deactivated catalytic sites. Water formation and propene epoxidation are strongly correlated. Water is formed via two routes: through the active peroxo
viii
SUMMARY
intermediate responsible for epoxidation and from direct water formation without involving this active intermediate. Improving the hydrogen efficiency, defined as the ratio
between PO formation rate and total water formation rate, should distinguish between
these two routes of water formation. The active peroxo intermediate in the epoxidation
is competitively consumed by hydrogenation and epoxidation. The active gold site is
blocked during deactivation, equally inhibiting epoxidation and water formation.
The hydrogen efficiency should distinguish between these two routes of water formation. Based on these understandings, an increase in Au-Ti interface or a better synergy
between Au and Ti should increase the hydrogen efficiency by mitigating direct water
formation. These findings were validated in the catalyst optimization aiming at a catalyst
with a better Au-Ti synergy. An enhanced productivity toward propene oxide in the direct propene epoxidation with hydrogen and oxygen over gold nanoparticles supported
on titanium-grafted silica was achieved by adjusting the gold–titanium synergy. The ob−1
tained PO formation rates in this study (120–130 gPO · kg−1
cat h ) were approximately the
same as the highest rates reported in literature. Highly isolated titanium sites were obtained by lowering the titanium loading grafted on silica. These active catalysts have low
gold loadings of around or below 0.2 wt.%. The tetracoordinated Ti sites were attained
by lowering the Ti loading to 0.2–0.3 wt.% (ca. 0.1 Ti/nm2 ). The tetrahedrally coordinated titanium sites were found to be favorable for attaining small gold nanoparticles
and thus a high dispersion of gold. The improved productivity of propene oxide can be
attributed to the increased amount of the interfacial Au–Ti sites. The active hydroperoxy
intermediate is competitively consumed by epoxidation and hydrogenation at the Au–Ti
interface. The PO/water ratio at the high PO rates obtained in this study ranged between
10 and 20 %. A higher propene concentration is favorable for a lower water formation
rate and a higher formation rate of propene oxide.
Under certain circumstances, propane formation may also happen or even prevail
over the gold-titania catalysts. Propene hydrogenation was encountered during our study
into the site synergy between gold and titanium using Ti-SiO2 as the support. The side
reaction of propene hydrogenation over these gold–titania catalysts was studied in details. The addition of a small amount of carbon monoxide (10–1000 ppmv level) to the
feed gas can completely switch off this propene hydrogenation, while at the same time
also reducing the rate of direct water formation. The formation rate of propene oxide
was not affected by the addition of carbon monoxide. An increase in the formation rate
of carbon dioxide was observed when 1000 ppm carbon monoxide was added and the
SUMMARY
ix
CO conversion was only 10 %. Hydrogenation of carbon monoxide to methane was not
observed. The order of CO on this inhibiting effect is −1. Gold is not necessary for
propane formation. The supports alone showed the same hydrogenation behavior as the
catalysts: 1. enhancement of propene hydrogenation by oxygen; 2. peak activity at ca.
443 K in propene and hydrogen during temperature programmed reaction; 3. switching
off by carbon monoxide with an order of −1. The coordination environment of titanium
and surface hydroxyls may play an important role in propene hydrogenation.
The most important place for PO formation is interfacial Au–Ti sites. Unraveling the
oxidation state of gold is important to the understanding of the direct propene epoxidation on the gold–titania catalysts. Carbon monoxide was used as probe molecule in the
infrared study to investigate the electron density of low-coordinated gold atoms on the
gold–titania catalysts that are active in the direct propene epoxidation. The active gold
sites were fully covered by reaction intermediates and deactivating species after the reaction. These species occupying the gold sites could not desorb even at 573 K. Calcination
in oxygen removed the carbonaceous species on gold. The gold atoms were positively
charged when oxygen was adsorbed on gold or at the interface. Reduction in hydrogen
removed the adsorbed oxygen and the positively charged gold was reduced to its metallic
form. When propene was adsorbed on the catalyst, gold atoms were negatively charged
showing the carbonyl band as low as 2079 cm−1 . Carbon monoxide was replaced by
propene on the catalyst surface and oxidation of carbon monoxide was suppressed by
propene. Hydrogen significantly increased the coverage of carbon monoxide on the titania surface. The results from the infrared study provide a general scheme of electron
transfer via gold on the gold–titania catalysts for the direct propene epoxidation.
Introduction
1
1.1 Propene oxide and its production
O
Propene oxide (PO, CH3 CH CH2 ), also know as propylene oxide, methyloxirane, or
1,2-epoxypropane, is an important basic chemical used in the chemical industry and
consumes over 10 % of all propylene produced [1]. Its major application is in the production of polyether polyols and propylene glycols, which are the starting materials for
polyurethane and polyester production. The total production of propene oxide reached
7.7 million tons in 2012 after recovering from the 2008–2010 recession, and has grown
by 25 % in over a decade when compared to 5.8 million tons in 1999 [1, 2]. The demand
is expected to experience a positive development in general and is on high rise of 8 %
especially in the emerging markets [2]. Contrary to the production of ethylene oxide, a
direct process for the epoxidation is not yet available for propene.
Two main routes are used in industry for PO production, the chlorohydrin process
and the hydroperoxide processes. In the chlorohydrin process propene reacts in aqueous
chlorine solution with hypochlorous acid to produce the propylene chlorohydrins (PCH).
The chlorohydrins thereafter are dechlorinated, using a base (usually calcium hydroxide)
to produce propene oxide. The hydroperoxide processes are a group of processes which
use an alkyl-hydroperoxide as the intermediate oxidant to epoxidize propene producing
propene oxide and an alcohol. In commercial operation, two main hydroperoxide processes, namely, the propene oxide–styrene monomer (SMPO) process and the propene
2
CHAPTER 1. INTRODUCTION
oxide-tert-butyl alcohol (PO/TBA) process are applied. Approximately 60 % of the hydroperoxide plants use the SMPO process. In this process, ethylbenzene is oxidized by
air to ethylbenzene hydroperoxide, which reacts with propene to produce propene oxide
and 2-phenylethanol. After dehydration, 2-phenylethanol is converted to styrene. In the
PO/TBA process, isobutane is oxidized to tert-butyl hydroperoxide (TBHP), which reacts
with propene to produce propene oxide and tert-butyl alcohol (TBA). The third hydroperoxide process is developed by Sumitomo Chemical using cumene hydroperoxide, which
is formed via oxidation of cumene, as the intermediate peroxide. The Sumitomo PO-only
Cumene process produces no co-products since the formed α, α-dimethyl benzy alcohol
is then simultaneously hydrogenated and dehydrated back to cumene for re-use.
Propylene glycol
20%
5%
60–70%
Polyether polyols
Polyurathane (rigid foams)
→ refrigerator, insulator
Polyurathane (flexible foams)
→ car seat, mattress, carpet
Non foams
→ elastomer, adhesive, paint
Unsaturated polyester resins
→ building materials
Industrial use
→ antifreeze, lubricant
Pharmaceuticals
→ toothpaste, cosmetics
Propylene-based glycol ethers
Other propoxylated compounds
Figure 1.1: Uses for propene oxide [3, 4]
The current main processes in operation, the chlorohydrins process and the hydroperoxide processes, are both complex and have a number of disadvantages. The chlorohydrins process produces large quantities of chloride/chlorine containing waste materials.
For this reason no new chlorohydrins based plants have been built over the past 20 years
[5]. The hydroperoxide processes (except Sumitomo PO-only Cumene process) produce
a co-product (usually alcohols) in a quantity 2-3 times larger than that of propene oxide
and, therefore, they are less flexible towards changing market demands.
The recently commercialized process, the hydrogen peroxide process (HPPO) by BASF,
Dow and Solvay as well as Evonik and Uhde, uses hydrogen peroxide to oxidize propene
producing PO and water. Titanium silicalite (TS-1) zeolite is used as the catalyst and the
epoxidation reaction is performed under mild temperature (< 100 o C) and high pressure
3
1.2. DIRECT EPOXIDATION OVER GOLD BASED CATALYSTS
(ca. 25 bar) in liquid phase using methanol as a solvent and co-catalyst [6–9]. This
process is more environmentally friendly and more flexible towards market demands.
However, an on-site H2 O2 production via anthraquinone process is indispensable and a
large amount of methanol needs to be separated and recycled. A high ratio of methanol
is necessary for a stable operation to avoid formation of biphasic liquid [9]. Methanol
in HPPO is generally considered as a co-catalyst[10], while Zwijnenburg et al. suggested
that it may help PO desorption based on their study over a Au/TiO2 /SiO2 catalyst[11]. A
comparison of commercial processes for propene oxide production is given in Table 1.1
1.2 Direct epoxidation over gold based catalysts
The deficiencies of the mentioned chlorohydrin and hydroperoxide processes have spurred
research into the production of PO as a single product derived from the oxidation of
propene. Unlike ethylene epoxidation, the direct oxidation of propene with oxygen over
silver catalyst is insufficiently selective due to a facile combustion via an allylic intermediate [12]. Considerable interest has arisen in the oxidation of propene to propene oxide
in H2 /O2 mixtures after the discovery of gold–titania catalysts by Haruta et al. [13] in
1998. These promising catalysts are the catalysts consisting of gold nanoparticles smaller
than 5 nm or sub-nano clusters on Ti-containing supports, e.g., TiO2 , TiO2 on SiO2 , and
titanosilicates. They are typically used at a temperature of 50 – 200 o C, just above atmo−1
spheric pressure and a GHSV between 5000 and 10000 mL·g−1
cat h . The main advantage
of these gold-based catalytic system is their high selectivity. The following reactions proceed on over the gold–titania catalysts in a mixture of propene, hydrogen and oxygen:
O
C3 H6 + H2 + O2 −→ CH3 CH CH2 + H2 O + 356.9 kJ
(R1.1)
1
H2 + O2 −→ H2 O + 241.8 kJ
2
(R1.2)
C3 H6 + 2 O2 −→ CH3 CHO + CO2 + H2 O + 826.5 kJ
(R1.3)
C3 H6 + H2 + O2 −→ CH3 CH2 CHO + H2 O + 450.9 kJ
(R1.4)
C3 H6 + H2 + O2 −→ CH3 COCH3 + H2 O + 480.7 kJ
(R1.5)
C3 H6 + H2 −→ C3 H8 + 125.1 kJ
(R1.6)
4
Table 1.1: Comparison of commercial processes for PO production [3, 5–7, 9, 14–16]
Process
PCH PO
PO/TBA
PO/SM or SMPO
Sumitomo Cumene
HPPO
(1910– )
(1969– )
(1974, 1980– )
(2003– )
(2008– )
46 %
16 %
30 %
4%
4%
Precursors
Cl2 , H2 O
i-butane
ethylbenzene
cumene
H2 , O2
Intermediate
HOCl, PCH
t-butylhydroperoxide
ethylbenzene hydroperoxide cumene
Catalyst
non-catalytic
molybdenum naphthenate molybdenum naphthenate
Share in capacity a
H2 O2
hydroperoxide
b
Typical conditions
b
steam stripper, 0.1 MPa
o
PO selectivity
(per t PO)
b. only for epoxidation
or silylated titania-on-silica containing Ti
CSTR, 3.5 MPa
CSTR or fixed bed
o
o
fixed bed
fixed bed
o
3–4 MPa, 80–100 C
<5 MPa, ca. 60 C
®3 MPa, <100 o C
87–90 %
¦95 %
∼ 95 %
95 %
>90 %
¾ 4 t t-butanol
¾ 2.2 t styrene
∼ 1.5 t cumyl-alcohol ¾ 0.3 t H2 O
¾ 40 t H2 O
a. based on year 2009
(homogeneous)
TS-1, MeOH
top: 40 C; bottom: 100 C 120 C
Co-products/Recycle ¾ 2 t chloride salts
o
mesoporous silica
5
9
C3 H6 + O2 −→ 3 CO2 + 3 H2 O + 1926.3 kJ
2
(R1.7)
The selectivity towards PO based on propene is generally higher than 90 mol%. The
main side products are acetaldehyde, propionaldehyde, acetone, carbon dioxide and
propane. Combustion of propene can only happen at higher reaction temperature, which
depends on the catalyst, e.g., above 80 o C on Au/TiO2 , or above 200 o C for Au/TS-1.
Propene hydrogenation is generally not encountered if the deposition–precipitation (DP)
method is used for preparing the supported gold catalysts [13, 17, 18]. This side reaction
has only raised concern in the recent two or three years when severe propene hydrogenation was also observed on gold catalysts prepared by DP method [19–21]. To obtain a
PO selectivity above 95 % or even higher is not an issue, but it may sacrifice the activity
which is already low. On the first investigated 1 wt% Au/TiO2 catalyst [13], propene
conversion was below 1 % though with a selectivity of almost 100 %. According to the
study by Haruta et al. [22, 23], the one-pass conversion of propene needs to reach ca. 10
−1
% at PO selectivity of 90 %, or equivalently a space-time-yield above 90 gPO · kg−1
at
cat h
−1
4000 mL·g−1
(10 vol% propene in feed), for an economically viable process in induscat h
try. Besides, the hydrogen efficiency, defined as the amount of PO produced divided by
the amount of water formed, should be increased up to 50 %. In literature, the hydrogen
efficiency is generally below 30 %. Table 1.2 summarizes representative performance of
gold–titania catalysts in the direct epoxidation (or hydro-epoxidation) of propene.
At the beginning phase of catalyst development for the gold–titania system, low conversion of propene (typically <2 %), fast deactivation as well as relatively poor hydrogen
efficiency (defined as the ratio between PO formation rate and total water formation
rate) were the main hurdles towards industrial implementation. Tremendous work has
been done to improve the catalyst performance by means of selecting and optimizing
supports, optimizing the combination between the catalytic Au and Ti sites, or adding
promoters. At present, the activity and stability of the Au/Ti-support catalyst have been
improved to a commercially interesting range while efforts still need be done in enhancing the hydrogen efficiency (considering the production rate of PO and catalyst stability,
47 % in hydrogen efficiency is the best figure). Figure 1.2 gives a concise route map of
the catalyst development. An in-depth review of the full progress in catalyst development
till the year 2008 can be found in [24].
6
Table 1.2: Direct propene epoxidation in hydrogen and oxygen: representative catalytic performance at atmospheric pressure
Catalyst
Temperature GHSVa
o
Conversion
Selectivity Efficiency PO STYa
( C)
−1
(mL·g−1
cat h )
of C3 H6 (%) of PO (%) of H2 (%)
1 wt% Au/TiO2
50
4000
1.1
> 99
35
11
1 wt% Au/TS-1
150
7000
1.1
> 99
5
18
0.1 wt% Au/Ti-
140
3000
8.4–7.8
95
unknown ∼ 35
j
DP method
[13]
organic/inorganic
[26]
hybrid support
1 wt% Au/Ti-SiO2 150
9000
0.83
0.3–0.4 wt% Au/
150
4000
5.0–9.8
87
7.1
18
90–95
27–35
130–150
200
7000
10
76
® 30
134d
200
7000
8.8
81
∼ 30
150
4000
8.5
91
35 f
b
titanosilicate
c
SiO2 , 300 m2 /g
[27]
mesoporous, Ti
[22]
content 2–6%
TS-1 (Si/Ti ∼ 36)
0.05 wt% Au/
Ref.
[25]
trialkoxysilane
0.081 wt% Au/
Notes
stable in 45 h, NH4 NO3 [28]
treatment on TS-1
e
116
steady state
[29]
64–80
silylation, 13–15
[23]
TS-1 (Si/Ti = 36)
titanosilicate
(Si/Ti = 100 : 3)
ppm (CH3 )3 N cofed
Ba(NO3 )2 -Au/
Catalyst
Temperature GHSVa
o
( C)
0.33 wt% Au/TS-1 200
Conversion
Selectivity Efficiency PO STYa Notes
−1
(mL·g−1
cat h )
of C3 H6 (%) of PO (%) of H2 (%)
7000
9.7
87
10 ± 5
132
(Si/Ti = 28)
g
Ref.
carbon pearls in TS-1, [30]
meso-scale defects
0.25 wt% Au/TS-1 200
8000
8.8
82
20
137
TS-1 treated by
[31]
NaOH for 1 h
(Si/Ti = 48)h
0.05 wt% Au/TS-1
6.0
88
47
100
same as above
14.6
∼ 70
∼ 25
164
ionic liquid-enhanced
Table 1.2: (Continued)
(Si/Ti = 48)
h
1.0 wt% Au/TS-1
300
7000
(Si/Ti = 35)
Au/TS-1 (Si/Ti
= 99, 100)
[32]
immobilization of Au
200
14000
∼5
a. generally, H2 /C3 H6 /O2 /balance= 1/1/1/7,
88–94
17 ± 2
158 ± 9 pH∼ 7.3 for DP,
[33]
Au loadings <0.1 wt%
i
−1
gPO · kg−1
cat h ;
b. taken at 0.5 h; c. on a silylated and Ba-promoted catalyst before deactivation; d. steady state;
e. similar to [22], but no direct quantification; f . increased from ∼ 17% by (CH3 )3 N cofeeding; g. steady state in 45 h; h. solid grinding method for gold anchoring;
i. 9 catalysts; j, H2 /C3 H6 /O2 /balance= 75/6/5/14
7
8
Figure 1.2: A brief route map of development of gold–titania catalysts for the direct
propene epoxidation
Mechanism
The mode of operation of the gold–titania catalysts is assumed to be the production of a
peroxide species on the gold nanoparticles, which epoxidize propene over a neighbouring
titanium(IV) site [13, 17, 25, 27, 34]. Titanium(IV) based catalysts are well know for
their capacity to epoxidize propene using organic peroxide, or hydrogen peroxide via the
Ti(η2 -OOR) or Ti(η1 -OOH) intermediate as shown in Figure 1.3.
Figure 1.3: Proposed intermediate for propene epoxidation in SMPO (left) and HPPO
(right) processes [10, 35, 36]
The gold nanoparticles that are active in the direct epoxidation of propene usually
have a size within 2–5 nm and are prepared via the DP method. The fact that the
oxidation reaction of propene to form PO requires the presence of both hydrogen and
oxygen, as well as the fact that propene can be epoxidized very selectively by hydrogen
peroxide over TS-1, creates the assumption that a peroxide species is involved in the re-
9
action mechanism. Theoretical calculations showed that OOH or HOOH can be formed
on small gold particles [37–39]. The hydro-peroxy species (HOOH or OOH) on gold
was spectroscopically confirmed via an inelastic neutron scattering (INS) study by the
group of Goodman [40]. Direct synthesis of hydrogen peroxide over supported gold and
gold–palladium catalysts have been experimentally proved by Ishihara et al. [41] and by
Edwards et al. [42–44], but the selectivity was not high. The DFT study by Ford et al.
[45] explained that the preference of HOOH formation on gold is due to the weak Au−O
bonding and the inefficient scission of O−O bond.
Based on a combination of UV–vis and XANES study, the group of Oyama [34] proposed a mechanism for the propene epoxidation over gold supported on titanosilicates,
in which the formation of HOOH is transferred to Ti(IV) site forming the active Ti−OOH
intermediate as shown in Figure 1.4. However, proof of this ‘sequential’ mechanism is not
flawless. An underlying assumption in their study is that propene does not affect HOOH
formation and decomposition, which may not be true. A ‘simultaneous’ mechanism is
proposed by the group of Delgass [46] based on their DFT study. They proposed that
attack of HOOH to Ti-defect sites is energetically unfavourable and propene is likely adsorbed on Au–Ti interface sites attacking H−Au−OOH species. The most favourable site
for propene adsorption on Au/TiO2 was also found to be Au–Ti interface as evidenced by
the TPD study by Ajo et al. [47].
Figure 1.4: A ‘sequential’ mechanism proposed for propene epoxidation with hydrogen
and oxygen over gold supported on titanosilicates. Reproduced from [34].
Another mechanism proposed by Nijhuis et al. [27] for Au/TiO2 suggests that a bidentate propoxy species formed on titania in adjacent to gold may be a reaction intermediate
10
in the direct epoxidation of propene. Infrared spectroscopic information in their study
indicated that propene is activated by gold nanoparticles and reacts with neighbouring
Ti sites [27]. The formed bidentate propoxy species can be further oxidized to carbonate/carboxylate species leading to catalyst deactivation. This mechanism is depicted in
Figure 1.5. The adsorption of propene on gold in this mechanism was evidenced by insitu XANES study on Au/SiO2 [48]. The participation of lattice oxygen as depicted in
Figure 1.5 was latter investigated by a SSITKA study [49]. However, the role of support
oxygen in forming the bidentate species could not be conclusively answered. Whether or
not this bidentate species is the true reaction intermediate is still in debate. In the study
by Mul et al. [50], the bidentate propoxy species is considered as a spectator formed
from irreversible adsorption of propene oxide. Nevertheless, the effect on short-term deactivation by the carbonate/carboxylate species is generally accepted [27, 50, 51]. The
catalyst activity after deactivation can be restored by calcination in oxygen.
Rate determining
H2 + O2
Au
HOOH
Au
Au
slow
C3H6O
Au
C3H6
+
Au
C3H6
+
Ti
O
Ti
O
fast
+
Ti
O
Ti
O
O
CH2 CH CH3
OH OH
O
OH OH
+
fast
OH OH
O
Au
O
O
O
Ti
Ti
Ti
Ti
O
O
+ H2O
O
Au
O
OH
C
O2
o.a. CO2, CH3CHO
+
CH2CH3
O
O
O Ti
Ti
very slow
OH OH
O
Ti
O
Ti
O
Deactivation
Figure 1.5: The bidentate propoxy mechanism proposed for the direct propene epoxidation over Au/TiO2 by Nijhuis et al. [27]. Reproduced from ref [1].
Kinetic studies
Table 1.3 summarizes the reaction orders in the power-rate-law (PRL) expression for
gold-based propene epoxidaiton in hydrogen and oxygen. The ranges of dependency on
hydrogen, oxygen and propene are 0.5–0.6, 0.2–0.3 and 0.2–0.3, respectively. Taylor
et al. [52] used three Au/TS-1 catalysts to carry out the kinetic analysis. On average,
11
the apparent activation energy in their study is 42 kJ/mol with a range between 35
and 55 kJ/mol for individual catalysts. However, this value is a bit higher than the
apparent activation energy determined on a series of Au/TS-1 catalysts studied earlier in
the same group [29], i.e., 25–36 kJ/mol. The mechanism they proposed to explain the
fractional orders includes the HOOH formation on a single Au site (HOOH-Au) and the
epoxidation of propene on an adjacent Ti site (C3 H6 -Ti). The rate-determining step (RDS)
was proposed to be the epoxidation step between the two surface species, i.e., HOOHAu and C3 H6 -Ti. Lu et al. [53] performed the kinetic study over a barium-promoted
gold catalyst supported on a mesoporous titanosilicate or Ti-TUD. Their explanation for
the fractional dependencies differentiates from what Taylor et al. [52] proposed in two
aspects: 1) the formation of HOOH involves two types of Au sites; 2) HOOH transfers
onto a neighbouring Ti−OH forming Ti(H2 O)−OOH which adsorbs propene leading to
Ti(H2 O)OOH(C3 H6 ). The RDS steps were proposed to be the formation of HOOH on gold
and the transformation from Ti(H2 O)OOH(C3 H6 ) to Ti(H2 O)OH(PO). Based on data
from Lu et al. [53], Bravo-Suárez et al. [54] performed a model validation of Langmuir–
Hinshelwood (L–H) models (the ‘sequential’ or ‘simutaneous’ mechanism; single, two or
three adsorption sites), the PRL model and a PRL/L–H hybrid model. They obtained the
best fit by the hybrid model, rPO = kPO [H2 ] x [O2 ] y [C3 H6 ]/(c + [C3 H6 ]) [54], where c is
a term representing the ratio between HOOH consumed by epoxidation and HOOH that
decomposes [53].
Table 1.3: Dependencies in power-rate-law expressions from kinetic studies of different
gold catalysts
Catalyst
Product
Au/TS-1
PO
Au-Ba/Ti-TUD PO
H2 O
CO2 b
Au/TS-1 c
PO
rProd = kProd [H2 ] x [O2 ] y [C3 H6 ]z a
Ea
Notes
x
y
z
(kJ/mol)
0.60 ± 0.03 0.31 ± 0.04 0.18 ± 0.04
42
0.02–0.06 wt% Au
Si/Ti= 143, 36
170 o C
0.54 ± 0.06 0.24 ± 0.06 0.36 ± 0.06
43
0.11 wt% Au
0.67 ± 0.07 0.16 ± 0.07 0.03 ± 0.07
51
2.4 wt% Ba
0.52 ± 0.14 0.26 ± 0.06 0.07 ± 0.06
80
Si/Ti= 100/3
150 o C
0.53 ± 0.02 0.26 ± 0.02 0.18 ± 0.04
n.d. 0.02 wt% Au
0.46 wt% Ti
180 o C
Ref.
[52]
[53]
[55]
a. with 95 % confidence interval
b. another form is rCO = kCO [H2 ]0.39±0.14 [O2 ]0.21±0.06 [PO]0.22±0.13
2
2
c. performed in an explosive mixture of H2 /O2 up to 40 vol% at atmospheric pressure in a membrane reactor
12
The kinetic and DFT study by Barton and Podkolzin[39] for water formation over
gold catalysts supported on silica, silicate-1 and TS-1 made a foundation for formulating
HOOH formation in the direct propene epoxidation. The power law orders in their study
are 0.7–0.8 on hydrogen, 0.1–0.2 on oxygen. The apparent activation energy was 37–41
kJ/mol. Their rate expression on water formation
rH2 O =
kKOOH KO2 PO2
1+
p
p
KH2 PH2
KH2 PH2 + KO2 PO2 + KOOH KO2 PO2
p
KH2 PH2
PH2
p
1 + KH2 PH2
(1.1)
is based on a two-site model described in the following proposed mechanism:
O2 + ⋆ ←→ O2 ⋆
(R1.8)
H2 + 2 ⋆ ←→ 2 H⋆
(R1.9)
H2 + 2 ←→ 2 H
O2 ⋆ +H ←→ HOO ⋆ +
(R1.10)
HOO ⋆ +H2 + −→ HOOH ⋆ +H (RDS)
(R1.11)
where two types of Au sites are involved, i.e., one for O2 , OOH and HOOH adsorption,
one for dissociative adsorption of H2 . The dissociative adsorption of H2 to form HOOH is
the rate-determining step. The water formation thus follows the cleavage of O−O bond
in HOOH and the hydrogenation of OH. Nijhuis and Weckhuysen [56] also performed a
kinetic study on water formation over gold catalysts supported on TiO2 , SiO2 and silicate1. The apparent activation energy in their study is 42–52 kJ/mol.
1.3 Microreactor system
It has been shown by Oyama et al. [55] that if one increases the hydrogen and oxygen
concentrations in the reactor, one can produce significantly larger quantities of propene
oxide (> 10 % yield) without extensive catalyst deactivation. A problem with increasing
the hydrogen and/or oxygen concentrations, however, is the explosion limits of this gas
combination. The commonly used gas mixture of hydrogen/oxygen/propene of 10 vol%
each is just outside of the explosion limits and increasing either the hydrogen or oxygen
concentration results in a potentially explosive mixture. In the work of Oyama et al. [55],
1.3. MICROREACTOR SYSTEM
13
hydrogen and oxygen reached a 40 vol% composition via a membrane reactor module, in
which hydrogen was gradually fed over the reactor length through inorganic membrane.
The fact that the hydrogen conversion was almost 100 %, meant that although one would
seemingly be operating within an explosive gas composition, but this was not the case
in reality. It is clear, however, that the large increase in hydrogen/oxygen concentration
in this membrane reactor system resulted in a dramatic increase in the propene oxide
productivity, since the same catalyst in a normal flow reactor system with the typical 10
vol% hydrogen/oxygen/propene feed produced less than 2 % in conversion of propene
oxide.
Since microreactor technology first emerged as a scientific discipline in the 1990’s, a
steady increase in the number of chemical reactions and physical changes that have been
successfully performed in such miniature devices could be observed and commercial-scale
applications are now available. Because of their inherently high surface-area-to-volume
ratios, microreactors demonstrate order of magnitude improvements in heat and mass
transfer rates, allowing highly efficient, compact and cost-effective devices to be created
to carry out chemical and thermal reactions more safely, and with greater selectivity and
conversion rates, higher yields, and improved product quality.
In the epoxidation of propene, a significant advantage of the microreactor is its very
small reactant inventory. The reactant gases are mixed just outside the microreactor unit,
after which they immediately proceed to the multi-channel catalytic section. The small
reactant volume makes it possible to work with a gas composition within the explosive
region. For gas phase applications, typical reactor channels are 100–200 µm wide/deep
and a few cm in length, resulting in a total volume of less than 1 µl. The energy liberated
by an explosion in such a channel would be less than 10 mJ, which would not be able
to affect the integrity of the microreactor. Aside from the microreactor being ‘explosionproof’, the excellent heat transfer rates of the reactor make it facile to run the reactor
isothermally and prevent ranaways, which could be an explosion trigger. The potential
of microreactors to operate safely with an explosive hydrogen-oxygen reaction mixtures
has been demonstrated by Jensen et al. [57] in a study in which hydrogen peroxide was
produced directly.
The gold-catalyzed propene epoxidation requires only a relatively short contact time.
Even conventional reactors are typically operated at a GHSV of around 10000 h−1 , which
makes it a very suitable system for a microreactor. The maximum heat production in
the reactor will be 30 W/ml, which can be easily removed from the microreactor system
14
[58]. Thus, risks of a runaway are absent.
1.4 Objectives and outline
The aim is to have an operational microreactor unit for the epoxidation early in the research program, which will be used for studies to determine the optimal conditions at
which the propene epoxidiation will be carried out. For the microreactor, the plan is
to use ‘conventional’ packed-bed reactor as depicted by Losey et al. [59], which then
will limit the arduous development of a special in-situ catalyst preparation in the early
phase. The microreactor will be used for kinetic studies over the entire concentration
range (inside and outside of the explosive regions) as well as for studies to determine
the catalyst stability. Based on the catalyst performance, further optimization of catalysts will be performed. The original final goal is to have a numbered-up microreactor
system combined with an optimized catalyst, which can be connected to a designed micro separation unit, as a demonstration unit for the propene epoxidation with a much
larger operational window. However, when the issue of competitive water formation and
propene hydrogenation arose, which is embedded in the reaction mechnism, developing
such an integrated micro unit became unrealistic. The research then switched to tackle
the problems with side reactions and to gain more mechanistic insight of this catalytic
reaction.
The kinetic and stability studies over a 1 wt% gold catalyst supported on a dispersed
Ti-SiO2 support is presented in Chapter 2. The competitive nature of propene epoxidation
and water formation on the interfacial Au–Ti sites is revealed. A simplified deactivation
model is developed and validated with experimental data. Chapter 3 focuses on catalyst
optimization based on the insights gained in Chapter 2. Chapter 4 describes the general
behaviour of propene hydrogenation over this catalytic system and provides the method
to switch off propene hydrogenation. An infrared study over the gold–titania catalysts
is presented in Chapter 5 trying to give second thoughts on the behaviour of gold in the
direct epoxidation of propene. Final conclusions and outlook come in Chapter 6.
References
[1] T. A. Nijhuis, M. Makkee, J. A. Moulijn and B. M. Weckhuysen, Ind. Eng. Chem. Res., 2006,
45, 3447–3459.
REFERENCES
15
[2] Merchant Research & Consulting. Ltd., Propylene Oxide (PO): 2013 World Market Outlook
and Forecast up to 2018, abstract retrieved from http://mcgroup.co.uk/researches/
propylene-oxide-po, 2013.
[3] J. Tsuji, J. Yamamoto, M. Ishino and N. Oku, SUMITOMO KAGAKU, 2006, 2006-I, 1–8.
[4] Propylene Oxide, http://www.dow.com/propyleneoxide/app/index.htm, 2013.
[5] S. T. Oyama, in Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis, ed.
S. T. Oyama, Elsevier, 2008, ch. 1, pp. 3–99.
[6] G. Thiele (Degussa-Huls AG), US 6372924, 2002.
[7] A. Forlin, G. Paparatto and P. Tegon (Dow Global Technologies Inc.), US 7138534, 2006.
[8] Propylene oxide the Evonik–Uhde HPPO technology: Innovative, profitable, clean, retrieved from
www.thyssenkrupp-uhde.de, 2013.
[9] P. Bassler, H.-G. Göbbel and M. Weidenbach, Chem. Eng. Trnas., 2010, 21, 571–576.
[10] M. G. Clerici and P. Ingallina, J. Catal., 1993, 140, 71–83.
[11] A. Zwijnenburg, M. Makkee and J. A. Moulijn, Appl. Catal., A, 2004, 270, 49–56.
[12] E. A. Carter and W. A. Goddard III, Journal of Catalysis, 1988, 112, 80–92.
[13] T. Hayashi, K. Tanaka and M. Haruta, J. Catal., 1998, 178, 566–575.
[14] P. Blankenstein, M. Crocker, C. J. G. van der Grift and J. J. M. van Vlaanderen (Shell Oil Company), US 7713906, 2010.
[15] T. Seo and J. Tsuji (Sumitomo Chemical Company Ltd.), US 6646139, 2003.
[16] H. A. Wittcoff, B. G. Reuben and J. S. Plotkin, in Chemicals and Polymers from Propylene, John
Wiley & Sons, Inc., 2012, ch. 6, pp. 211–271.
[17] E. E. Stangland, K. B. Stavens, R. P. Andres and W. N. Delgass, J. Catal., 2000, 191, 332–347.
[18] A. Zwijnenburg, A. Goossens, W. G. Sloof, M. W. J. Crajé, A. M. Van der Kraan, L. J. De Jongh,
M. Makkee and J. A. Moulijn, J. Phys. Chem. B, 2002, 106, 9853–9862.
[19] S. T. Oyama, J. Gaudet, W. Zhang, D. S. Su and K. K. Bando, ChemCatChem, 2010, 2, 1582–
1586.
[20] J. Gaudet, K. Bando, Z. Song, T. Fujitani, W. Zhang, D. Su and S. Oyama, J. Catal., 2011,
280, 40–49.
[21] C. Qi, J. Huang, S. Bao, H. Su, T. Akita and M. Haruta, J. Catal., 2011, 281, 12–20.
[22] A. K. Sinha, S. Seelan, S. Tsubota and M. Haruta, Angew. Chem. Int. Ed., 2004, 43, 1546–
1548.
[23] B. Chowdhury, J. J. Bravo-Suárez, M. Daté, S. Tsubota and M. Haruta, Angew. Chem. Int. Ed.,
2006, 45, 412–415.
[24] A. M. Joshi, B. Taylor, L. Cumaranatunge, K. T. Thomson and W. N. Delgass, in Mechanisms
in Homogeneous and Heterogeneous Epoxidation Catalysis, ed. S. T. Oyama, Elsevier, 2008,
ch. 11, pp. 315–338.
[25] T. A. Nijhuis, B. J. Huizinga, M. Makkee and J. A. Moulijn, Ind. Eng. Chem. Res., 1999, 38,
16
884–891.
[26] P. Vogtel, G. Wegener, M. Weisbeck and G. Wiessmeier (Bayer Aktiengesellschaft), WO
2001041921, 2001.
[27] T. A. Nijhuis, T. Visser and B. M. Weckhuysen, J. Phys. Chem. B, 2005, 109, 19309–19319.
[28] L. Cumaranatunge and W. N. Delgass, J. Catal., 2005, 232, 38–42.
[29] B. Taylor, J. Lauterbach and W. N. Delgass, Appl. Catal., A, 2005, 291, 188–198.
[30] B. Taylor, J. Lauterbach and W. N. Delgass, Catal. Today, 2007, 123, 50–58.
[31] J. Huang, T. Takei, T. Akita, H. Ohashi and M. Haruta, Appl. Catal., B, 2010, 95, 430–438.
[32] M. Du, G. Zhan, X. Yang, H. Wang, W. Lin, Y. Zhou, J. Zhu, L. Lin, J. Huang, D. Sun, L. Jia
and Q. Li, J. Catal., 2011, 283, 192–201.
[33] W.-S. Lee, M. C. Akatay, E. A. Stach, F. H. Ribeiro and W. N. Delgass, J. Catal., 2012, 287,
178–189.
[34] J. J. Bravo-Suárez, K. K. Bando, J. Lu, M. Haruta, T. Fujitani and S. T. Oyama, J. Phys. Chem.
C, 2008, 112, 1115–1123.
[35] J. K. F. Buijink, J. J. M. van Vlaanderen, M. Crocker and F. G. M. Niele, Catal. Today, 2004,
93-95, 199–204.
[36] M. G. Clerici, Oil Gas Eur. Mag., 2006, 32, 77–82.
[37] P. P. Olivera, E. M. Patrito and H. Sellers, Surf. Sci., 1994, 313, 25–40.
[38] D. H. Wells Jr., W. N. Delgass and K. T. Thomson, J. Catal., 2004, 225, 69–77.
[39] D. G. Barton and S. G. Podkolzin, J. Phys. Chem. B, 2005, 109, 2262–2274.
[40] C. Sivadinarayana, T. V. Choudhary, L. L. Daemen, J. Eckert and D. W. Goodman, JACS, 2004,
126, 38–39.
[41] T. Ishihara, Y. Ohura, S. Yoshida, Y. Hata, H. Nishiguchi and Y. Takita, Appl. Catal., A, 2005,
291, 215–221.
[42] J. K. Edwards, B. E. Solsona, P. Landon, A. F. Carley, A. Herzing, C. J. Kiely and G. J. Hutchings,
J. Catal., 2005, 236, 69–79.
[43] J. K. Edwards, B. Solsona, P. Landon, A. F. Carley, A. Herzing, M. Watanabe, C. J. Kiely and
G. J. Hutchings, J. Mater. Chem., 2005, 15, 4595–4600.
[44] J. K. Edwards, A. F. Carley, A. A. Herzing, C. Kiely and G. J. Hutchings, Faraday Discuss.,
2008, 138, 225–239.
[45] D. C. Ford, A. U. Nilekar, Y. Xu and M. Mavrikakis, Surf. Sci., 2010, 604, 1565–1575.
[46] A. M. Joshi, W. N. Delgass and K. T. Thomson, J. Phys. Chem. C, 2007, 111, 7841–7844.
[47] H. M. Ajo, V. A. Bondzie and C. T. Campbell, Catal. Lett., 2002, 78, 359–368.
[48] T. A. Nijhuis, E. Sacaliuc, A. M. Beale, A. M. J. van der Eerden, J. C. Schouten and B. M.
Weckhuysen, J. Catal., 2008, 258, 256–264.
[49] T. A. Nijhuis, E. Sacaliuc-Parvulescu, N. S. Govender, J. C. Schouten and B. M. Weckhuysen,
J. Catal., 2009, 265, 161–169.
REFERENCES
17
[50] G. Mul, A. Zwijnenburg, B. Van der Linden, M. Makkee and J. A. Moulijn, J. Catal., 2001,
201, 128–137.
[51] A. Ruiz, B. van der Linden, M. Makkee and G. Mul, J. Catal., 2009, 266, 286–290.
[52] B. Taylor, J. Lauterbach, G. E. Blau and W. N. Delgass, J. Catal., 2006, 242, 142–152.
[53] J. Lu, X. Zhang, J. J. Bravo-Suárez, S. Tsubota, J. Gaudet and S. T. Oyama, Catal. Today,
2007, 123, 189–197.
[54] J. J. Bravo-Suárez, J. Lu, C. G. Dallos, T. Fujitani and S. T. Oyama, J. Phys. Chem. C, 2007,
111, 17427–17436.
[55] S. T. Oyama, X. Zhang, J. Lu, Y. Gu and T. Fujitani, J. Catal., 2008, 257, 1–4.
[56] T. A. Nijhuis and B. M. Weckhuysen, Prepr. Pap.-Am. Chem. Soc., Div. Petr. Chem., 2007, 52,
292–295.
[57] T. Inoue, M. A. Schmidt and K. F. Jensen, Ind. Eng. Chem. Res., 2007, 46, 1153–1160.
[58] E. V. Rebrov, M. H. J. M. De Croon and J. C. Schouten, Catal. Today, 2001, 69, 183–192.
[59] M. W. Losey, M. A. Schmnidt and K. F. Jensen, Ind. Eng. Chem. Res., 2001, 40, 2555–2562.
Kinetic study of the direct
propene epoxidation over
Au/Ti-SiO2 in the explosive
regime
2
This chapter is adapted from:
T. A. Nijhuis, J. Chen, S. M. A. Kriescher, & J. C. Schouten. The direct
epoxidation of propene in the explosive regime in a microreactor – A
study into the reaction kinetics, Ind. Eng. Chem. Res., 2010, 49, 10479–
10485.
J. Chen, S. J. A. Halin, J. C. Schouten, & T. A. Nijhuis. Kinetic study of
propylene epoxidation with H2 and O2 over Au/Ti-SiO 2 in the explosive
regime, Faraday Discuss., 2011, 152, 321–336.
Abstract
A kinetic study of the propene epoxidation with hydrogen and oxygen over a Au/TiSiO2 catalyst has been performed in a wide range of reactant concentrations including
the explosive region in a microreactor. The observed rate dependency on the reactants
for the epoxidation and the competing direct water formation is discussed in relation
to the current mechanistic insights in literature. The formation rate of propene oxide
is most dependent on the hydrogen concentration, in which the formation of an active
peroxo species on the gold nanoparticles is the rate determining step. The deactivation
is mainly caused by consecutive oxidation of propene oxide. Oxygen favours the regeneration of the deactivated catalytic sites. Water formation and propene epoxidation are
strongly correlated. Water is formed via two routes: through the active peroxo intermediate responsible for epoxidation and from direct formation without involving this
active intermediate. Improving the hydrogen efficiency should distinguish between these
two routes of water formation. The active peroxo intermediate in epoxidation is competitively consumed by hydrogenation and epoxidation. The active gold site is blocked
during deactivation.
20
CHAPTER 2. KINETIC STUDY OF THE DIRECT PROPYLENE EPOXIDATION IN THE EXPLOSIVE REGIME
2.1 Introduction
The direct epoxidation of propene to propene oxide (PO) by using the co-reactants hydrogen and oxygen, demonstrated for the first time by Haruta et al. [1], is highly selective
over gold-titania based catalysts and has gained considerable attention in the past decade
[2–10]. Although this catalytic reaction has been investigated extensively, the mechanism
is still not well understood.The most important steps in the reaction mechanism proposed
in literature include the formation of a hydroperoxy species on the gold nanoparticles,
a reactive adsorption of propene on titania sites adjacent to gold, and the consecutive
oxidation of propene by the hydroperoxy species to form propene oxide [11–13]. The
highlights of this catalytic reaction are its exceptionally high selectivity towards propene
oxide and the mild operating conditions. However, the main obstacles to its industrial
application originate from the stability of the catalyst, the relatively low activity, and the
insufficient hydrogen efficiency. Encouraging progress has been made over the past years
concerning the three key factors that determine the catalytic performance, i.e. the size
(or the morphology) of the gold nanoparticles [10, 14, 15], the state of Ti in the support
[3, 8, 16–20], and the preparation method of the catalysts which plays an important role
in the selective dispersion of the gold and its interaction with the Ti sites [6, 15, 21–
23]. Very high productivity of propene oxide has been reported by different groups and
relatively stable performance has been achieved [5, 6, 8, 10, 15, 17]. However, this is
usually at relatively high temperatures (150 – 200 o C), where the hydrogen efficiency is
less. While the hydrogen efficiency of a certain catalyst can be mainly attributed to the
synergy between the gold nanoparticles and the titanium sites in proximity, promoters
[5, 8] and surface modification [24] may additionally depress the hydrogen consumption (or hydroperoxy decomposition) to some extent by adjusting the surface acidity.
Further improvement in hydrogen efficiency seems difficult. To evade the issue of hydrogen efficiency in the direct epoxidation of propene, the Au-clusters–C3 H6 /O2 /H2 O
system has come into view albeit the low activity and the selectivity [25–27]. The reported H2 efficiency of almost 100 % on a Au/TS-1 catalyst with a very low gold loading
[23] reveals the potential in enhancing the H2 efficiency. The highest hydrogen efficiency
(47 %) reported recently over the Au/TS-1 catalyst without any post-treatment, which
still preserves a high conversion of propene, opens the door to a further step in catalyst
development [15].
A kinetic study of the propylene epoxidation using H2 and O2 over a gold–titania
2.1. INTRODUCTION
21
based catalyst is important in revealing the reaction mechanism. However, two aspects are complicating: short-term deactivation due to build-up of carbonate/carboxylate
species on active Ti-sites [11, 28, 29] and possible long-term changes in activity mainly
caused by sintering of the gold nanoparticles [30, 31]. The former difficulty is reversible
by burning the strongly adsorbed species and can be solved in a sense by choosing a relatively stable catalyst where Ti is highly dispersed and by operating at elevated temperatures. The latter situation is irreversible and the effect can be minimized by performing
the catalytic testing in a period as short as possible or preparing catalysts which are stable (low Cl content, surface-stabilized Au nanoparticles). Another factor constraining a
kinetic study is the explosive nature of the reactant mixture. Most reported studies were
performed within the non-explosive region [12, 13]. An important exception is the work
performed by the group of Oyama, who reported propene epoxidation in a packed-bed
catalytic membrane reactor [32]. Feeding (part of) the hydrogen through a membrane
allowed them to work safely with much higher hydrogen and oxygen concentrations (up
to 40% each), which gave them a stable propene conversion of 10% at 80% propene
oxide selectivity at 453 K. Their results showed fractional orders in a power-rate-law expression similar to what had been published previously [12, 13]. Although corresponding
relations between hydrogen consumption and product selectivities as well as the formation rate of propene oxide can be observed among earlier studies [24, 33–35], further
quantitative investigations have not been reported.
In this work, a microreactor system is utilized to perform the propene epoxidation
over a gold on titania–silica catalyst in an extended range of reactant concentrations
including the explosive region. Microreactors have excellent potential for carrying out
this reaction safely using a gas mixture which would be inside of the explosive region
for a number of reasons. First, since microreactors have only a very small volume, the
energy liberated from an explosion in such a channel would be less than 1 J, which
would not be able to affect the integrity of the microreactor. Second, a microreactor
allows for an excellent temperature control, which will dramatically diminish the risk
of a runaway in the reactor. Most importantly, the fact that the characteristic length
(diameter) of the reactor is smaller than the mean free path of the molecules implies
that flame propagation inside the channels is not possible as the molecules transfer their
energy to the wall instead of each other [36].
Different from previously published kinetic studies, the formation rate of propene
oxide in this work is decoupled from the short-term deactivation, which uses the initial
22
activity of the catalyst extrapolated by using a simplified deactivation model based on
the mechanism proposed by Nijhuis et al. [11]. In this study, quantitative description
of the deactivation in a wider range of reactant concentrations will help to gain further
insight into the mechanism of the propene epoxidation in hydrogen and oxygen. The
relationship between the catalyst stability, the water formation during the epoxidation,
and the propene epoxidation itself is thoroughly examined, which may provide mechanistic implication for improving the hydrogen efficiency in the future development of the
catalyst.
2.2 Experimental
2.2.1 Catalyst preparation and characterization
A catalyst consisting of 1 wt% gold on titania dispersed on silica was used, prepared in
a manner similar to the method described by Nijhuis et al. [28]. A total of 15 g of dry
silica support (Davisil 643, Aldrich, 300 m2 /g, pore size 150 Å, pore volume 1.15 cm3 /g)
was dispersed in 250 mL of anhydrous isopropanol (Aldrich, 99.5 %) under a nitrogen
atmosphere in a glove box. The slurry was stirred for 10 min and afterwards 0.70 mL of
tetraethylorthotitanate (TEOT) (Aldrich, 97 %) was added. The amount of TEOT added
was determined by calculating a theoretical titania coverage of 5 % monolayer (Ti/Si
atom ratio) on the silica surface. The slurry was stirred for 30 min. The isopropanol was
slowly evaporated at 318 K and 140 mbar in a rotary evaporator under nitrogen. After
the isopropanol was removed, the powder was dried overnight at 353 K and subsequently
calcined first at 393 K (5 K/min heating) for 2 h and then at 873 K (10 K/min heating)
for 4 h.
Gold was deposited on the catalyst support by a deposition–precipitation method
using aurochloric acid and ammonia. A total of 10 g of support was dispersed in 100 mL
of water. The pH of the slurry was adjusted to 9.5 by dropwise adding ammonia (2.5
wt%). A total of 575 mg of an acidic 30 wt% HAuCl4 solution (Aldrich) was diluted in 20
mL of demineralized water and was added dropwise to the support slurry over a 15 min
period. While HAuCl4 solution was being added, the pH was kept at 9.5 using aqueous
ammonia. After the addition of the gold solution, the slurry was stirred for one hour. The
slurry was filtered and washed 3 times using 200 mL of water. The catalyst was dried
overnight at 353 K and calcined first at 393 K (5 K/min heating) for 2 h and afterwards at
23
2.2. EXPERIMENTAL
673 K (10 K/min heating) for 4 h. Drying and calcination of the support and the catalyst
were performed under atmospheric pressure in stationary air.
Loadings of gold and titanium was determined by inductively coupled plasma atomic
emission spectroscopy (ICP–OES). The average size of gold nanoparticles was determined
by transmission electron microscopy (TEM). The coordination environment of grafted
titanium was analyzed by UV–visible spectroscopy.
2.2.2 Catalyst testing
Catalytic tests were performed using a microreactor system. The microreactor consisted
of a stainless steel capillary tube (0.9 mm inner diameter), which was loaded with 20 mg
of catalyst (sieved, 50 – 60 µm) and was operated at a gas feed rate of 3.33 NmL/min.
The oxygen was mixed with the other two reactants shortly before the catalyst bed. Immediately after the catalyst bed, additional helium was added to the gas stream, diluting
the gases to a composition outside the explosive region. In this manner, the total volume
of a potentially explosive gas mixture was minimized. A normal mili-reactor (quartz, 6
mm inner diameter) was installed together with the microreactor. Both reactors had their
own gas feeding section but were located in the same oven. The mili-reactor shared the
efflux pipeline and GC with the microreactor. Samples can be simultaneously taken from
both reactors via an automated 4-way valve. The mili-reactor was used to confirm the
integrity and performance of the microreactor when necessary.
Scheme 1: Scheme of the microreactor for kinetic study
The catalytic experiments were performed in cycles. Prior to each test the catalyst
was (re)activated by heating it to 573 K (10 K/min) in a 10 % oxygen in helium stream
for 1 h. After cooling to the desired reaction temperature, the gas feed was switched
to the desired composition and a 5 h catalytic test was performed. Experiments were
performed at atmospheric pressure at varying feed concentrations ranging from 2 to 80
vol% for hydrogen and oxygen and 2 to 40 vol% for propene, with the remaining part
24
being helium. The “standard” feed concentration is 10 vol% for each reactant (hydrogen,
oxygen and propene) with helium as balance. In the experiments in which the composition was varied, two reactants were kept at the concentration of 10 vol% with only the
concentration of the third reactant being changed. A kinetic study over the full range of
reactant concentrations was performed at 388 K, 403 K and 418 K. The long-term stability was examined before the kinetic study and was found to be excellent at 388 K. At
the higher temperatures, however, a minor activity loss of 5 – 10 % compared with the
initial activity was observed over a period of 20 days (around 50 cycles). In the kinetic
study, the catalyst was replaced every week in order to minimize the long-term change
in catalyst activity and the activity was monitored by repeating the reaction at the “standard” condition at the end of each series of catalytic cycles. Due to the similarity in trend
of the results at different temperatures, the results at 403 K were mainly reported and
discussed in detail. Product analyses were performed using a fast Interscience Compact
GC system, equipped with a Porabond Q column and a Molsieve 5A column, capable of
analyzing all products in 4 min.
2.3 Results and discussion
2.3.1 Performance of microreactor and proof of concept
In Figure 2.1, the performance of the two reactors at the same reaction conditions is compared. It can be seen that the experiments performed in the two reactors under identical
reaction conditions yielded identical results, and that repeated experiments in the same
reactor after a catalyst reactivation also resulted in identical results. There was a concern
about the ratio between the capillary diameter and the particle size (ca. 15 for the microreactor), which is relatively small and may lead to potential wall channeling and flow
maldistribution [37, 38]. However, the results did not show much difference between
the two reactors. Thus we exclude the possibility of wall channeling. The external and
internal mass transfer limitations were also excluded. The data points in Figure 2.1 show
a small degree of fluctuation, which is due to the valve switching between the two reactors when sampling. The kinetic study in the whole range of reactant concentrations was
thus all performed in the microreactor.
An experiment at a gas composition of 40 vol% hydrogen, 40 vol% oxygen and 20
vol% propene was performed as a first proof of the microreactor concept. The for-
25
2.3. RESULTS AND DISCUSSION
2
Conversion of C3H6 (%)
run1, mili
run1, micro
run2, mili
run2, micro
1.5
1
0.5
0
0
50
100
150
Time (min)
200
250
300
Figure 2.1: Performance of the capilary microreactor and the tubular milli reactor (1 wt
% Au on Ti-SiO2 catalyst, gas feed 10 vol % propene, 10 vol % oxygen, 10 vol % hydrogen
−1
in helium, 393 K, GHSV 10000 mL·g−1
cat h )
mation rate of PO at the elevated reactant concentrations is compared to the “classic”
10/10/10/70 composition as shown in Figure 2.2. It can be seen that performing this reaction under these explosive conditions in a microreactor system greatly improved the reaction rate of propene oxide by a factor of nearly 4 at the beginning. Although the catalyst
experienced a relatively fast deactivation at higher reactant concentrations, the steadystate PO rate in the explosive mixture is still 2.5 times of the rate at the 10/10/10/70
composition.
2.3.2 Catalyst characterization
The gold loading is 0.91 wt%, which is close to the target loading of 1 wt%. The titanium content on the catalyst is 1.29 wt%. TEM analysis showed a narrow particle size
distribution for the gold nanoparticles, centered at 4.5 nm as seen in Figure 2.3. In the
deconvoluted UV-vis spectrum, the bands at 202 and 224 nm are assigned to tetrahedral
tetrapodal Ti and tetrahedral tripodal Ti, respectively, while the bands at 262 nm and 292
nm probably best assigned to penta- and hexacoordinated Ti structures from dinuclear Ti
species. The grafted Ti is highly dispersed mainly in tetrahedral coordination with minor
amount of penta- or hexacoordinatd Ti [39]. No adsorption was found above 300 nm.
26
−7
6
x 10
40% H 40% O 20% C H
PO formation rate (mol⋅g−1
⋅s−1)
cat
2
2
3 6
10% H 10% O 10% C H
2
2
3 6
4.5
3
1.5
0
0
50
100
150
200
Time (min)
250
300
Figure 2.2: Enhanced PO formation rates in the explosive mixture of H2 , O2 and C3 H6
over a 1 wt% Au/Ti-SiO2 catalyst(5 %monolayer of Ti grafted on SiO2 , 393 K, atmo−1
spheric pressure, GHSV 10000 mL·g−1
cat h )
2.3.3 Product formation
In Figure 2.4, the formation rates of propene oxide and water in a single catalytic cycle
at the chosen standard conditions are shown. Both formation rates show a relatively
fast decrease within the first two hours. The rates continue to decrease, much slower,
in the second half of the catalytic cycle. The performance between 150 min and 270
min is used for calculating the averaged H2 efficiency (defined as rPO /rH2 O ) and C3 H6
conversion for a single reaction cycle, which will be used for evaluating the catalyst
performance. Different from the H2 efficiency and C3 H6 conversion, the PO selectivity is
unchanged since the very beginning of a reaction cycle.
The breakthrough curve (green dash) at the beginning of the reaction cycle shown
in Figure 2.4 for the PO formation is due to the strong adsorption of PO on the catalyst
surface [40]. This breakthrough is more obvious when the initial activity of PO formation
is low, but it may be undetectable when the initial activity is high. The concomitant
decrease in the formation rate of water with that of PO along time is commonly observed
in literature for gold–titania catalysts [12, 24, 33–35]. The formation rate of water is the
overall rate unless specified otherwise. The strong correlation between water formation
and propene epoxidation during deactivation indicates that a shared site or a common
reactive intermediate exists between these two reactions. Most likely, the deactivating
species blocks this shared site or a site producing the common intermediate. Based on
the current insights in literature on the mechanisms for both the epoxidation and the
27
80
1.5
(a)
(b)
224
60
1
Abs.
Counts
292
0.5
202
0
200
262
40
20
0
300
400
500
Wavelength (nm)
600
1
2
3
4 5 6 7
Particle size (nm)
8
9
10
Figure 2.3: UV–visible spectrum of the support (a) and size distribution of gold nanoparticles (b)
water formation [9, 12, 13, 41, 42], it is most likely that a peroxo species is this common
intermediate.
2.3.4 Propene epoxidation
A simplified model was developed to describe the time-dependent propene oxide formation in the catalytic cycles on the Au/Ti-SiO2 catalyst. This model is based on the
reaction mechanism proposed previously by Nijhuis et al. [11, 28] as seen in Figure 1.5.
The model is able to accurately describe the propene oxide formation rate in time, using
three parameters:
• rPO,0 : the extrapolated propene oxide formation rate at t = 0, i.e. the activity the
catalyst would have had in the absence of deactivation;
• kdeact : the deactivation rate constant;
• kreact : the reactivation rate constant.
The model is based on the following assumptions:
1. The propene oxide formation rate is proportional to the initial rate rPO,0 (without
deactivation) multiplied with the fraction of active sites available on the catalyst
(i.e., 1−θd , where θd is the fraction of the deactivated sites), according to Equation
2.1.
2. The catalyst deactivation proceeds via a consecutive reaction of propene oxide (or
an intermediate directly correlated to the propene oxide formation rate) adsorbed
−7
6
−6
x 10
rPO
rH2 O
r̂PO
x 10
2.4
3
1.2
1.5
0
0
50
100 150 200
Time (min )
250
2
rH
−1
(×)r PO,0 =3.38×10 −7 mol·g −1
cat ·s
kdeact =1.31×10 3 mol−1 ·g cat
kreact =5.19×10 −5 s−1
O
1.8
rPO (mol⋅g−1
⋅s−1)
cat
4.5
(mol⋅g−1
⋅s−1)
cat
28
0.6
0
300
Figure 2.4: Formation rates of propene oxide and water as a function of time: ◦, propene
, fitted rate of propene oxide by Equation 2.5 (1 wt% Au on Ti-SiO2 ,
oxide; , water;
gas feed 10 vol% hydrogen, 10 vol% oxygen, 10 vol% propene in helium, 403 K, GHSV
−1
10000 mL·g−1
cat h ).
on an active site, forming a deactivated site. The rate of this reaction is according
to Equation 2.2.
3. The catalyst reactivation occurs via the desorption of the deactivating species from
a deactivated site. The rate of this reaction is according to Equation 2.3.
rPO = rPO,0 (1 − θd )
dθd
rdeact =
= kdeact rPO (1 − θd )
d t deactivating
dθd
rreact =
= −kreact θd
d t regenerating
(2.1)
(2.2)
(2.3)
The rate of formation of deactivated sites can be determined from a site balance for the
deactivated sites:
dθd
dt
= kdeact rPO (1 − θd ) − kreact θd
(2.4)
29
By combining equations 1 and 4 and taking the initial conditions of rPO = rPO,0 and θd = 0
at t = 0, the equations can be solved analytically. The analytical solution is provided by
Equation 2.5:



1 − exp (−At) 


rPO = rPO,0 1 −

1


a − exp (−At)
a
1
kdeact rPO,0
with A = a −
a
and
kreact
kdeact · rPO,0
=a+
1
a
− 2 (a > 1)
(2.5a)
(2.5b)
(2.5c)
This 3-parameter model was fitted to the experimental cycles at different reaction
conditions. An example of such a fit is shown in Figures 2.4 and 2.5. The model describes
the time-dependent experimental results well. The highest error occurs on the fastest
deactivating curve (normally at the highest hydrogen concentration, see discussion later),
but errors from the experimental data are still within 10 % as shown in Figure 2.5.
This model, however, is not yet a reaction mechanism based on the elementary reaction
steps, but rather on a simplified mechanism describing the observed reaction rates well
with parameters which lump a number of elementary steps. For this reason, the two
rate constants are not only dependent on temperature, but also on the concentrations of
the three reactants, viz., hydrogen, oxygen, and propene. The dependency of the three
parameters on the reactant concentrations is shown in Figure 2.6.
30
−7
8
x 10
15
10
80% H2
7
80% H , fitted
0
5% H2
5
5% H2, fitted
Residual (%)
rPO (mol⋅g−1
⋅s−1)
cat
2
6
4
3
2
−10
−15 (b)
10
0
1
−10 (c)
(a)
0
0
60
120
180
Time (min)
240
0
300
60
120
180
Time (min)
240
300
Figure 2.5: Illustration of accuracy of the deactivation model by Equation 2.5: (a) experimental and fitted rates of PO formation at H2 concentrations of 80 vol% and 5 vol%;
residuals of the fitted data for (b) 80 vol% H2 and (c) 5 vol% H2 (1 wt% Au on Ti−SiO2
catalyst, gas feed 10 vol% oxygen, 10 vol% propene in helium, 403 K, GHSV 10000
−1
mL·g−1
cat h , in (b) and (c) residuals are given every third point, green lines are drawn for
eye)
−7
x 10
2400
6
1800
−1
kdeact (mol ⋅ gcat)
⋅s−1)
rPO, 0 (mol⋅g−1
cat
8
4
H2
2
C3H6
(a)
0.2
0.4
0.6
Volume fraction
0.8
H2
600
O2
0
0
1200
O2
CH
(b)
1
0
0
3 6
0.2
0.4
0.6
Volume fraction
0.8
1
−4
1
x 10
k
react
(s−1)
0.8
0.6
0.4
H2
0.2
O2
C3H6
(c)
0
0
0.2
0.4
0.6
Volume fraction
0.8
1
Figure 2.6: Initial activity rPO,0 (a), deactivation rate constant kdeact (b), and reactivation rate constant kreact (c) fitted from the deactivation model (Equation 5) at different
−1
reactant concentrations (1 wt% Au on Ti-SiO2 , 403 K, GHSV 10000 mL·g−1
cat h , the concentration of one reactant is varied while the other two are fixed at 10 vol%).
31
At low concentrations, the propene oxide formation rate is dependent on the concentrations of all three reactants, while at higher concentrations, the rate is only proportional
to the hydrogen concentration (Figure 2.6a). This indicates that at higher reactant concentrations, only hydrogen is involved in the rate determining step. This observation is
in agreement with the most common assumption in literature [2, 11–13] that the formation of peroxo species (the active oxidizing species) on the gold nanoparticles is the rate
determining step. For this reaction a dissociative adsorption of hydrogen is required.
In Figure 2.6b, it can be seen that the deactivation rate constant increases with the
hydrogen concentration, while it decreases with the propene concentration and is only
weakly dependent on the oxygen concentration. This is explained by propene oxide (or
a precursor thereof) being oxidized further by the same peroxo species as is the active
oxidant in the propene epoxidation. If the propene concentration is higher, the concentration of this species on the catalyst is lower, reducing the rate of the unwanted consecutive
oxidation. If the hydrogen concentration is higher, this peroxo species is produced at a
higher rate. The slightly mitigated deactivation at higher oxygen concentration will be
discussed in Section 2.3.7.
In Figure 2.6c, it can be seen that the catalyst reactivation rate is only dependent on
the oxygen concentration. Nijhuis et al. [28] previously determined that the deactivating
species are consecutive oxidation products of propene oxide, which can only leave the
surface once they are completely combusted to produce CO2 . Our current results indicate that this consecutive oxidation to produce carbon dioxide is not by the hydroperoxy
species, but rather by oxygen.
2.3.5 Water formation and link to epoxidation
In Figure 2.4 it can be seen that the propene oxide production and the water formation
show a similar pattern. This can be considered as an indication for a strong correlation
between the water formation and the propene epoxidation. Since the amount of water
produced is far larger than the stoichiometric amount produced from the epoxidation
reaction, this similarity in the rate of formation cannot be explained by the fact that
water and propene oxide would be the co-products of a single reaction. To examine this
correlation further, the water formation rate as a function of the propene oxide formation
rate for a number of catalytic cycles is plotted in Figure 2.7.
It can be seen that the correlation between the water production and the propene
32
−6
−6
x 10
3
x 10
7.5 (a)
(c)
CH
3
2.5
(mol⋅g−1
⋅s−1)
cat
0.8
0.0
2−
2
0.4
1.5
rH
2
O
0.0
2
H
rH
6
e
tim
2−
(mol⋅g−1
⋅s−1)
cat
3
2
4.5
O
6
1.5
1
0.5
0
0
1
2
r
PO
3
4
(mol⋅g−1 ⋅s−1)
5
6
−7
x 10
cat
0
0
1
2
r
PO
3
(mol⋅g−1 ⋅s−1)
cat
4
5
−7
x 10
−6
3
x 10
(b)
2
8
1.5
0.
02
−
1
0
0
O
0.5
1
2
0.
rH
2
O
(mol⋅g−1
⋅s−1)
cat
2.5
2
3
−1 −1
rPO (mol⋅gcat⋅s )
4
5
−7
x 10
Figure 2.7: Formation rate of water as a function of PO formation rate within each catalytic cycle when only the concentration of (a) hydrogen, (b) oxygen, (c) propene is
changed while the other two are fixed at 10 vol% (1 wt% Au on Ti-SiO2 , 403 K, GHSV
−1
10000 mL·g−1
cat h , balance in helium).
oxide production is almost perfectly linear. The intercept of the linear correlation is
not at the origin, which indicates that water is produced via two different routes, one
correlated to the propene oxide formation, and one which is not. The water formation,
which is directly correlated to the propene oxide formation, is tentatively assigned to
the water produced out of a peroxo species (i.e. the species which is also responsible
for the epoxidation of propene). The water formation which is not correlated to the
propene oxide formation, is assigned to the direct water formation from the reaction
between hydrogen and oxygen, not being formed via a peroxo species. These two routes
for water formation are depicted in Scheme 1, which is resembling the scheme presented
previously by Edwards et al. [43] for the direct formation of hydrogen peroxide over
Au–Pd catalysts. It should be mentioned that even though the hydroperoxy species is
33
denoted as HOOH in Scheme 1, this species might also be OOH + H adsorbed in some
other form [44, 45].
H2
H2 + O2
2 H2 O
HOOH
H2 O+
1
2
O2
C3 H6
C3 H6 O+H2 O
Scheme 2: Routes for water formation in propene epoxidation using hydrogen and oxygen.
To more closely examine both the direct water formation and the water formation
from the peroxo species, which is also responsible for the epoxidation, the linear correlation is used to fit each of the data sets according to Equation 2.6:
rH2 O = c · rPO + d
(2.6)
The coefficient c represents the ratio between the change in the overall water formation rate and the loss in the PO formation rate on the active Au–Ti centers. The loss of
activity for both water formation and propene expoxidation can only be attributed to a
blockage on this active Au–Ti center. Occupation of Ti alone by the deactivating species
does not necessarily result in this synchronous decrease in rates in the context of the
sequential mechanism, i.e. HOOH forms first on gold and then spills over to Ti, since
HOOH can decompose to water without Ti. Either gold is occupied in the deactivation,
or the strong metal support interaction [46, 47] is so weakened by the deactivation of Ti
sites that the formation of the active peroxo species is interrupted. The constant d represents the direct formation rate of water. Direct water formation can occur at gold sites
near Ti via a route that does not involve a peroxo species responsible for epoxidation, but
it can also occur at gold sites not in proximity to Ti.
34
x 10
−6
3
10
2.5
H2
1.5
c
d (mol⋅g−1
⋅s−1)
cat
8
2
6
O2
C3H6
1
4
H
2
O2
2
0.5
(a)
0
0
C3H6
(b)
0.2
0.4
0.6
Volume fraction
0.8
1
0
0
0.2
0.4
0.6
Volume fraction
0.8
1
Figure 2.8: Direct formation rate of water not involved in epoxidation (a) and the ratio
between the over all water formation rate and PO formation rate on the active Au–Ti
sites (b) at different reactant concentrations (1 wt% Au on Ti-SiO2 , 403 K, GHSV 10000
−1
mL·g−1
cat h , balance in helium).
In Figure 2.8, the values of c and d are shown as a function of the feed concentrations of hydrogen, oxygen, and propene. In Figure 2.8a, d is mainly a function of the
hydrogen concentration in the feed and not dependent on propene and oxygen. The fact
that parameter d is almost first order dependent on the hydrogen concentration and not
dependent on the oxygen concentration (except for only mildly at very low oxygen concentration) suggests that the direct water formation is rate limited by either the hydrogen
adsorption or the hydrogen dissociation on the gold nanoparticles. Interestingly, at lower
oxygen concentrations the value of d seems to originate from a non-zero value, which
is different for the situation for hydrogen. One possible explanation is that oxygen adsorption on gold is so much stronger than (dissociative) hydrogen adsorption, that only
at much lower oxygen concentrations (beyond our current experimental capabilities) it
would become limiting. The effect by propene on the parameter d is trivial, since the
parameter d represents the direct water formation out of hydrogen and oxygen.
In Figure 2.8b, it can be seen that parameter c is not dependent on the oxygen concentration, but positively dependent on the hydrogen concentration, and inversely dependent on the propene concentration. In Section 2.3.4, it has been discussed that the
propene oxide formation is directly proportional to the formation of a peroxo species.
The parameter c here is not representing the formation of a peroxo species on the catalyst, but the way this peroxo species is reacting further. Scheme 1 is showing the possible
reactions for this peroxo species (for simplicity simply shown as HOOH).
35
In Figure 2.8b, it can be seen that oxygen does not play a role in the reaction or
decomposition of the peroxo species on the gold nanoparticles. This is in line with the
expectation that oxygen is not directly involved in the rate determining step. The observed inverse proportionality of c on the propene concentration is in agreement with
the reaction route of HOOH as shown in Scheme 1. Parameter c is actually inversely
proportional to the efficiency with which the peroxo species is used for the desired epoxidation. A large value of c implies that a lot of water is produced compared to the amount
of propene oxide produced. The inverse proportionality of c to the propene concentration can be easily explained by the fact that as the propene concentration is increasing,
a larger fraction of the peroxo species will react with propene to produce propene oxide,
and less will be lost by decomposition. The positive dependency of c on the hydrogen
concentration can be explained by the fact that excessive hydrogen speeds up the loss of
the peroxo species by hydrogenation to form two water molecules. This is in agreement
with literature, where it is indeed concluded that in the direct hydrogen peroxide formation out of hydrogen and oxygen over gold nanoparticles, the largest loss of peroxide
occurs via the hydrogenation of the peroxide produced [43]. As long as the oxidizing
intermediate is formed, it will be consumed either by decomposition/hydrogenation to
water or by epoxidation to PO and water. This is why even at very low hydrogen concentration the parameter c does not meet at the origin.
2.3.6 Effect of temperature
Important information provided by the formation rates of water and propene oxide at
different temperatures is the activation energy. Water formation and propene oxidation
are two parallel reactions consuming the oxidizing peroxo intermediate originating from
OOH. It might be inaccurate to perform a calculation of the activation energy of propene
oxide formation separately. On the one hand, propene oxide (or consecutively oxidized
species thereof) strongly adsorbs on the surface [3, 28, 48] resulting in an activation
energy influenced by adsorption enthalpies and surface coverages. On the other hand,
the competitive consumption of a reactive peroxo intermediate by epoxidation and water
formation will cause the uneven distribution of this reactive intermediate towards one of
these two reactions when the temperature increases, i.e. in r = Aexp(−Ea /RT ) f (C) the
f (C) may change when T changes. The latter situation can be observed by a saturation
and even a decrease in one of the two reaction rates when the effect of the strong product
36
adsorption is not obvious.
x 10
−6
−7
5
x 10
(a)
473 K
453 K
445 K
407 K
394 K
cat
(mol⋅g−1 ⋅s−1)
3
(b)
2
2
HO
2
4
445 K
435 K
425 K
415 K
407 K
394 K
373 K
r
rPO (mol⋅g−1
⋅s−1)
cat
3
1
1
0
0
60
120
180
Time (min)
240
−12
300
0
0
1
2
−1 −1
3
4
−7
x 10
2
−14
0
−1
ln
−15
r H2 O,θd →1
r PO,0
r C3,0
−16
2.2
2.3
2.4
2.5
−1
1000 ⋅ 1/T (K )
µ
1
¡
¢
−1
ln r/mol · gcat
· s−1
−13
drH2 O
−1
drPO /selectivity
¶
(c)
2.6
−2
2.7
Figure 2.9: Formation rate of propene oxide as a function of time (a) and water formation
rate as a function of PO formation rate (b) at different temperatures and (c) Arrhenius
plots of the direct water formation rate, initial PO formation rate, modified initial rate of
PO, and the ratio between water formation rate and modified PO formation rate at the
active Au–Ti sites (1 wt% Au on Ti−SiO2 , gas feed 10 vol% hydrogen, 10 vol% oxygen,
−1
10 vol% propene in helium, GHSV 10000 mL·g−1
cat h ).
Table 2.1 gives the general performance of the catalyst at different temperatures. The
time-on-stream performance at different temperatures is shown in Figures 2.9(a) and
2.9(b). The main side products are isomers of propene oxide, most probably, due to ring
opening of propene oxide on acidic Ti4+ sites [3, 5, 48]. Propionaldehyde is primary
among these side products, which is consistent with the study on the isomerization of
propene oxide by Namuangruk et al. [49]. Since significant formation of CO2 appears at
temperatures higher than 453 K, we consider these side products as formed by consecutive reaction of propene oxide at temperatures below (and including) 445 K. The overall
formation rate of carbon oxygenates at t = 0 is denoted as rC3,0 . The relation between
37
the activation energy of water formation and that of epoxidation on the Au–Ti site is
given by Equation 2.7, which is explained in more detail in Appendix,
ln

d rH2 O
d rPO /Sel.

−1
=−
obs
obs
Ea,H
O − Ea,PO
2
RT
+ const.
(2.7)
where Sel is the selectivity towards propene oxide.
The Arrhenius plots of rH2 O,θd →1 (the parameter d in Equation 2.6), rPO,0 , and rC3,0
are presented in Figure 2.9c. The difference in overall activation energy between water
formation and epoxidation at the Au–Ti site given by Figure 2.9c is 22±3 kJ/mol, while
the overall activation energy of the direct water formation is 51±5 kJ/mol. The activation
energy estimated by rPO,0 at the three lowest temperatures is 24 kJ/mol. These numbers
are in agreement, i.e., the sum of 22 and 24 is close to 51. This leads to the conclusion
that the water formation through Au–Ti is not much different from the water formation
through the other Au sites. So for a high hydrogen efficiency, a low reaction temperature
is desired.
Equation 2.7 should satisfy the precondition that within a short period of time the
amount of deactivating species accumulated on the active Au–Ti sites is far less than the
amount of propene oxide formed. This should be checked by comparing the turn over
frequency of deactivating species formation (TOFd ) and that of propene oxide formation
(TOFPO ) based on the total number of Au–Ti sites. TOFd can be easily estimated from
Equation 2.4 by kdeact rPO . At the standard condition at 403 K, kdeact rPO is estimated to
be at the level of 5 × 10−4 s−1 . The total amount of gold on the catalyst used in this
study is 4.6 × 10−5 mol · g−1
cat . The fraction of active gold atoms at corners and edges is
roughly estimated to be 0.15 based on the mean particle size 4.5 nm [50, 51]. Assuming
that each titanium alkoxide molecule forms three Ti–O–Si bonds on the silica surface,
the chance that a gold atom on the edge contacts a titanium atom is therefore 0.15.
The estimated TOFPO is at the level of 0.3 s−1 , which means that the deactivation is
indeed 3 orders of magnitude lower. Actually, this precondition is expected to be satisfied,
otherwise a complete deactivation within few minutes should be observed. Furthermore,
since in our deactivation model, the deactivation and subsequent reactivation should
produce complete oxidation products, thus we could also predict that the deactivation is
much slower than the epoxidation based on the product composition. Stable performance
is achieved starting from the temperature 453 K and the CO2 formation rate increases
38
significantly above that temperature (Figure 2.9a and Table 2.1).
Table 2.1: General performance of 1 wt% Au on Ti–SiO2 in direct propene epoxidation
at different temperatures and standard reactant concentrations
T
(K)
373
394
407
415
425
435
445
453
473
C3 H6 conversion
(%)
0.56
0.73
0.91
1.06
1.26
1.45
1.62
2.05
3.13
PO
>99
99
94
91
84
77
69
48
17
Selectivity (%)
EA
PA
AC
0
0
0
0.7 0.1
0
1.8 3.8 0.3
2.0 6.2 1.1
2.5 9.8 3.9
3.2
13
5.7
4.2
17
7.7
4.9
28
12
8.4
42
17
CO2
0
0
0
0
0
1.8
1.2
6.9
16
H2 efficiency
(%)
20
12
9.1
8.2
6.6
5.2
4.0
2.1
0.6
PO, propene oxide; EA, acetaldehyde; PA, propionaldehyde; AC, acetone
Table 2.2: Power-law fitting of kinetic parameters fitted form Equation 2.5 at three temperatures
T (K)
388
418
403
f
rPO,0
kdeact
rPO,0
kdeact
rPO,0
kdeact
rH O,0
2
d rH2 O /d rPO − 1
rH2 O,θd →1
f = k[H2 ] x [O2 ] y [C3 H6 ]z
x
y
z
0.54
0.35
0.19
0.04 −0.10
−0.24
0.43
0.31
0.34
0.25 −0.11
−0.32
0.46
0.30
0.29
0.20 −0.10
−0.29
0.72
0.25
−0.06
0.26
0.85
0.02
0.05
−0.42
−0.10
R2
0.9835
0.9543
0.9516
0.9543
0.9781
0.9662
0.9922
95% confidence
∆x
∆y
∆z
0.03 0.03 0.05
0.02 0.02 0.03
0.05 0.05 0.07
0.03 0.03 0.05
0.03 0.03 0.05
0.02 0.02 0.03
0.03 0.03 0.04
0.9541
0.9919
0.03
0.03
0.03
0.03
0.05
0.05
2.3.7 Relationship between selectivity, hydrogen efficiency, catalyst
stability
Figure 2.10 gives the averaged H2 efficiencies and PO selectivities at different reactant
concentrations. It can be seen that increasing the H2 concentration causes the H2 efficiency to decrease monotonously, while changing the C3 H6 concentration shows an inverse trend. This phenomenon makes it clear again that H2 and C3 H6 consume the same
oxidizing peroxo intermediate competitively as we have discussed in Section 2.3.5. It is
39
interesting that O2 has a slightly positive effect on the H2 efficiency as it does on reducing
the deactivation constant kdeact (see Figure 2.6b), since it is not directly involved in the
rate determining step of water formation or epoxidation. This slight effect by oxygen on
the H2 efficiency can also be observed from the work by Lu et al. [13]. Probably, oxygen may play a role as the stabilizer of excessive hydrogen in the form of OOH and may
therefore slightly inhibit the redundant formation of the oxidizing peroxo species, which
will subsequently either oxidize PO causing catalyst deactivation or directly decompose
to water.
100
20
H2
O2
80
CH
PO selectivity (%)
H2 efficiency (%)
15
3 6
10
5
60
O2
20
C3H6
(b)
(a)
0
0
H2
40
0.2
0.4
0.6
Volume fraction
0.8
1
0
0
0.2
0.4
0.6
Volume fraction
0.8
1
Figure 2.10: Hydrogen efficiency (a) and selectivity to propene oxide (b) at different
−1
reactant concentrations (1 wt% Au on Ti−SiO2 , 403 K, GHSV 10000 mL·g−1
cat h , the
concentration of one reactant is varied while the other two are fixed at 10 vol%).
It can be seen that the PO selecitivity possesses the same trend as the H2 efficiency
for different reactants when Figures 2.10b and 2.10a are compared. The PO selectivity is
plotted against the H2 efficiency in Figure 2.11a. Interestingly, the trend shown in Figure
2.11a, in which the PO selectivity and hydrogen efficiency increase together, can also
be obtained from the study by Huang et al. [15](see Figure 2.12) on their gold catalysts
supported on the same TS-1(48), despite the fact that these catalysts have either different
pre-treatment methods on the support or different gold loadings. The catalyst in this
study has a titanium density of 2.7 × 10−4 mol · g−1
cat , which is far more than the amount of
Ti that functions in epoxidation.† Propene oxide can easily react with these Lewis acidic
sites and form mainly its isomers. A higher formation rate of water may increase the
extent of hydrolysis of Ti–O–Si bonds, which yields a stronger Brønsted acidity on the
catalyst surface favouring the ring opening of propene oxide.
In Figure 2.11b, the deactivation rate constant kdeact is plotted against the H2 effi-
40
ciency at different reactant concentrations. It can be clearly seen that a higher H2 efficiency corresponds to a lower deactivation rate indicating that the deactivation process is
closely linked to water formation, or more precisely, to the way how the (excessive) oxidizing peroxo species is used. A catalyst with a higher hydrogen efficiency should have
a more stable performance and a better PO selectivity. A very important conclusion that
can be drawn from Figure 2.11 is that a stable, selective and hydrogen efficient catalyst
should be possible.
PO selectivity (%)
100
95
H2
90
O2
CH
3 6
(a)
85
0
5
10
15
H2 efficiency (%)
20
2400
H
kdeact (mol−1⋅ gcat)
2
1800
O
2
C3H6
1200
600
(b)
0
0
5
10
15
H efficiency (%)
20
2
Figure 2.11: Selectivity towards propene oxide (a) and deactivation rate constant (b) as
a function of hydrogen efficiency at different reactant concentrations (1 wt% Au on Ti−1
SiO2 , 403 K, GHSV 10000 mL·g−1
cat h , the concentration of one reactant is varied while
the other two are fixed at 10 vol%).
2.4 Summarizing discussion
Deactivation on the catalyst in this study is mainly caused by consecutive oxidation of
propene oxide as proposed by Ruiz et al. [29], while the water productivity reflects
41
2.4. SUMMARIZING DISCUSSION
100
Selectivity (%)
80
60
PO
CO2
40
20
0
0
10
20
30
40
Hydrogen efficiency (%)
50
Figure 2.12: Relationship between product selectivities and the hydrogen efficiency on
Au/TS-1 catalysts (adapted from data in the paper by Huang et al. [15]).
the formation rate of the oxidizing peroxo species. The deactivating species in propene
epoxidation was found to be carbonate/carboxylate adsorbed on active Ti sites [28].
Building up of carbonates on gold is also the cause of deactivation in CO oxidation on
gold catalysts [52–54]. The concurrent decrease in water formation and epoxidation
observed in this sutdy indicates that deactivating species block the active Au–Ti sites.
The competing roles of propene and hydrogen in consuming the active peroxo intermediate indicate that a moderate hydrogen concentration is preferred for an acceptable
propene conversion without much loss in hydrogen efficiency. Including the decomposition of the active peroxo intermediate into the rate expression based on a real mechanism
[13] can well explain the saturation of PO formation at higher propene concentrations
and the fractional order on propene in the power-rate-law expression, which is close to
zero and normally within the range of 0.18–0.35 [12, 13, 32]. Most likely, hydrogen
speeds up this decomposition by increasing surface coverage of dissociated hydrogen.
This results in a rate expression for PO formation in the following form,
rPO = kHOOH θOOH
PH2
p
1 + KH2 PH2
kPO PC3 H6
p
KH2 PH2
kPO PC3 H6 + kH2 O
p
1 + KH2 PH2
(2.8)
which has no essential difference from the rate expression proposed by Lu et al. [13],
but may explain the generally observed lower order (0.55–0.60) on hydrogen in propene
epoxidation than that in hydrogen oxidation (0.7–0.8) on gold–titania catalysts [2, 12,
13, 32, 41].
42
Due to the parallel consumption of the active peroxo species by hydrogen and propene,
a higher propene concentration will increase the utilization efficiency of this active intermediate towards propene oxide and suppresses the unwanted water formation, which
accordingly enhances the catalyst stability. Higher oxygen concentrations favour the regeneration of the deactivated sites and might stabilize the excessive hydrogen alleviating
the deactivation process. The elementary reaction steps happening at the Au–Ti interface
can be described by the following reactions:
O2 + ⋆ ←→ O2 ⋆
(R2.1)
H2 + 2 △ ←→ 2 H△
(R2.2)
O2 ⋆ +H△ ←→ HOO ⋆ +△
(R2.3)
C3 H6 + HOO ⋆ +H2 + △ −→ PO + H2 O ⋆ +H△ (RDS)
(R2.4)
−−
* PO +
PO −)
(R2.5)
−−
* C3 H6
C3 H6 + −)
(R2.6)
PO + HOO ⋆ +H2 + 2 △ −→ SÎ + H2 O ⋆ +H△
(R2.7)
where ‘△’ denotes Au sites for hydrogen dissociation, ‘⋆’ denotes a second Au sites for
O2 /OOH adsorption as proposed by Barton and Podkolzin [41], ‘’ denotes Ti in proximity to Au and ‘S’ is the deactivating species. For simplicity, adsorption of C3 H6 on gold
and competing formation of water are not listed here.
Further improvement in hydrogen efficiency will place a premium on the theoretical
investigation into the pathway of water formation on gold. Barton and Podkolzin [41]
proposed the HOOH pathway through which the O–O bond cleaves and two hydroxyls
form. The study by Ford et al. [45] reinforced this perspective and suggested another
energetically competitive pathway on Au(111) facets, in which OOHH is formed and
O–O bond scission occurs leaving an oxygen atom on gold. Similar hydrogen-induced
OOH dissociation on gold surface is also proposed in hydrogen-promoted CO oxidation
[44] and in direct H2 O2 synthesis [55]. Recent DFT study by Li et al. [56] on propene
epoxidation in oxygen and water on gold clusters suggested a pathway in which the scission of the oxygen bond in OOH on gold surface is preferred and the oxygen atom left
epooxidizes propene. Considering our low activation energy of propene epoxidation and
2.5. CONCLUSIONS
43
the proposal by Joshi et al. [57] that there is an extra energy barrier for HOOH attacking Ti–OH, the reaction route on our catalyst might not be the sequential mechanism
involving H2 O2 transfer on Au/titanosilicate catalysts [9]. In general, a lower reaction
temperature is preferred in our system for a higher hydrogen efficiency, but this makes
catalyst regeneration more difficult. An efficient activation of hydrogen on gold nanoparitles/clusters [58–61] as well as the synergy between Au and Ti is the key issue for a
desirable performance.
2.5 Conclusions
A kinetic study of propene epoxidation with hydrogen and oxygen over the Au/Ti–SiO2
catalyst has been performed over a wide range of reactant concentrations including the
explosive region by utilizing a micro reactor system. Analysis of the dynamic deactivation
process at different reactant concentrations showed that the formation rate of propene
oxide is most dependent on the hydrogen concentration and that the formation of an
active peroxo species on the gold nanoparticles is the rate determining step. Deactivation
is mainly caused by the consecutive oxidation of propene oxide (or a precursor thereof).
Higher hydrogen concentrations speed up the deactivation by increasingly forming the
oxidizing peroxo species. When the propene concentration is higher, the concentration
of this oxidizing species is lower by epoxidizing propene to form the desired propene
oxide and therefore the deactivation is mitigated. Oxygen favours the regeneration of
the deactivated sites.
Water formation and epoxidation are strongly correlated. It can be concluded from
our results that there are two routes for water formation, i.e. the water formation on the
Au–Ti center through the active peroxo intermediate which is also responsible for epoxidation, and the direct water formation not related to epoxidation. Water formation and
propene epoxidation on the active Au–Ti sites are two parallel reactions competitively
consuming the same active peroxo intermediate. When the hydrogen concentration is
higher, the hydrogenation of the peroxo intermediate to form water is more dominant
instead of its consumption by epoxidation. Higher propene concentrations are preferred
for the efficiency of utilizing this peroxo intermediate to form propene oxide. Oxygen
has no influence on the direct water formation and does not affect the ratio of the water
productivity and epoxidation rate on Au–Ti centers indicating that OOH is the true in-
44
termediate and that its reaction with hydrogen forming the active peroxo species is the
rate determining step. Catalyst deactivation is caused by the blockage of the active Au–Ti
center.
Saturation of propene oxide formation is observed, which can be attributed to the nature of the parallel water formation and propene epoxidation consuming a common intermediate. The activation energy of propene epoxidation is 22 kJ/mol lower than the water
formation on the Au-Ti center suggesting a low reaction temperature for propene epoxidation is favoured. A moderate hydrogen concentration combined with high propene
and oxygen concentrations is preferred for a desirable performance of the catalyst, i.e. a
higher hydrogen efficiency and consequently better stability and selectivity.
2.A Deactivation model
Combining Equations 2.1 and 2.4 results in
dθd
dt
= kdeact rPO,0 (1 − θd )2 − kreact θd
(2.A.1)
Equation 2.A.1 can be rewritten as
dθd
θd2
−

2+
kreact
kdeact rPO,0

= kdeact rPO,0 d t
(2.A.2)
θd + 1

We denote a(a > 1) as the root of θd2 − 2 + kreact / kdeact rPO,0 θd + 1 = 0; then we
have
a+
1
a
=2+
kreact
kdeact rPO,0
(2.A.3)
Analytically solving Equation 2.A.2 results in
1
1 − exp − a −
kdeact rPO,0 t
a
θd =
1
1
kdeact rPO,0 t
a − exp − a −
a
a
(2.A.4)
Combining Equations 2.A.4 and 2.1 results in the solution Equation 2.5.
Initial guesses for the least-squares fitting are based on the following physical meanings of parameters.
45
2.A. DEACTIVATION MODEL
1. The value of 1/a represents the coverage of deactivating species at steady-state.
rPO, t=5h
The initial guess for 1/a is 1 −
;
rPO, t=0
2. Multiplying rPO,0 on both sides of Equation 2.A.1 results in
d rPO,0 (1 − θd )
dt

= −rPO,0 · kdeact rPO,0 (1 − θd )2 − kreact θd ,
which can be rewritten as
d rPO
dt
= −kdeact rPO,0 · rPO (1 − θd ) + rPO,0 · kreact θd
(2.A.5)
Integrating Equation 2.A.5 leads to
Z
rPO
d r = −kdeact rPO,0
rPO,0
Z
t
rPO (1 − θd )d t +
0
rPO = rPO,0 − kdeact rPO,0
Z
Z
t
rPO,0 kreact θd d t
0
t
rPO (1 − θd )d t +
0
Z
t
rPO,0 kreact θd d t.
(2.A.6)
0
When t → 0, θd → 0; equation 2.A.6 can be simplified to
rPO = rPO,0 − kdeact rPO,0
Then we plot rPO against
Rt
0
Z
t
rPO d t.
0
rPO d t (the accumulative amount of PO produced by the time
t), the slope at t = 0 should be very close to (−kdeact rPO,0 ).
3. The initial guess of rPO,0 is by fitting the data using r = a1 e−b1 t + a2 e−b2 t + c, where
a1,2 , b1,2 and c are constants. This is explained in the following section.
H2 + O2 + C3 H6
Au
OOH⋆ + △ + H△
C3 H6
PO
r
s
b
i
Figure 2.13: Schematic model of occupied sites on the Au–Ti center
Ti
46
1.
−−
* s
s + −)
C3 H6 adsorption, desorption
2.
s −→ i
PO formation
3.
i −→ r
PO oxidation, site deactivation
4.
r −→ r +
site regeneration
5.
−−
*i+
i −)
PO desorption, re-adsorption
The above reactions represent the transform of adsorbed species on the Ti sites in
proximity to Au. The site balance gives:
blank site
C3 H6 site
PO site
deactivated site
db
dt
ds
dt
di
dt
dr
dt
= −(k1 + k−5 )b +
k−1 s
=
k1 b
− (k−1 + k2 )s
=
k−5 b
+
k2 s
+
k5 i
+ k4 r
− (k3 + k5 )i
=
k3 i
− k4 r
′
where k−5 = k−5
PPO , k1 = k1′ PC3 H6 , k2 and k3 are most relevant to, strictly speaking, the
instantaneous concentration of H2 .
If the Au–Ti sites are fully occupied by C3 H6 , PO and the deactivating species under reaction conditions, and if the re-adsorption of PO on the active sites is neglectable
compared with the adsorption of C3 H6 to the active sites, the following equations can be
obtained:
di
= k2 s − (k3 + k5 )i


dt


dr
= k3 i − k4 r


dt


s = 1 − i − r.

Therefore,

di
dt
 dr

dt
= −(k2 + k3 + k5 )i − k2 r + k2
= k3 i − k4 r.
The above two equations are re-written as

 x′
1
 x′
2
=
−(k2 + k3 + k5 )x 1 − k2 x 2 + k2
=
k3 x 1 − k4 x 2
(2.A.7)
(2.A.8)
47
2.A. DEACTIVATION MODEL
with the initial conditions x 1 (0) = 0 and x 2 (0) = 0.
If k2 and k3 are simply treated as a constant, after the Laplace transform one can
obtain

 s x̃ − x (0)
1
1

s x̃ 2 − x 2 (0)
=
−(k2 + k3 + k5 ) x̃ 1 − k2 x̃ 2 +
=
k3 x̃ 1 − k4 x̃ 2
k2
s
and subsequently

 (s + k + k + k ) x̃ + k x̃
2
3
5
1
2 2

k3 x̃ 1 − (s + k4 ) x̃ 2
=
=
k2
+ x 1 (0)
s
−x 2 (0).
Solving the above equations yields
(s + k2 + k3 + k5 )(s + k4 ) + k2 k3 x̃ 1 =
k2
s
+ x 1 (0) (s + k4 ) − k2 x 2 (0).
Substitution of the initial conditions yields
x̃ 1 =
k2 (s + k4 )
s(s2 + αs + β )
where α = k2 + k3 + k4 + k5 , β = k4 (k2 + k3 + k5 ) + k2 k3 .
The roots of s(s2 + αs + β ) = 0 are
s1,2 =
1
2
−α ±
p
α2 − 4β , s3 = 0.
According to the residue theorem, one can obtain
x1
=
3
P
Res( x̃ 1 est , s j )
j=1
=
k2 (s1 + k4 )
s1 (s1 − s2 )
e s1 t +
k2 (s2 + k4 )
s2 (s2 − s1 )
e s2 t +
k2 k4
s1 s2
(s − s j ) · k2 (s + k4 )
.
(s − s1 )(s − s2 )(s − s3 )
Due to the time resolution of GC analysis, it is hard to get information of the very
where Res( x̃ 1 est , s j ) = lim
s→s j
beginning period when the occupation of the clean Au–Ti sites by the formed PO occurs
from the observed rates of PO formation. When fitting the time-on-stream rates of PO
48
formation, for simplicity one can fit the data only after the peak appears leaving the
adsorption of PO on the Ti–Ti sites out of account. This treatment in data fitting requires
a modification to the initial conditions, i.e., using x 1 (0) = f0 and x 2 (0) = 0 instead of
x 1 (0) = 0 and x 2 (0) = 0, since the breakthrough information is lost/discarded. This
is a reasonable assumption when the establishment of adsorption equilibrium of PO on
Au–Ti sites is fast and this time is far shorter when compared with the duration of a
single catalytic cycle. In another word, one can simply consider that the fitting using
the modified initial conditions is valid from t 0 , for example, t 0 = 5 s. Therefore, the
time-dependent coverage of PO on the Au–Ti sites is
x1 =
( f0 s1 + k2 )(s1 + k4 )
s1 (s1 − s2 )
e s1 t +
( f0 s2 + k2 )(s2 + k4 )
s2 (s2 − s1 )
e s2 t +
k2 k4
s1 s2
(2.A.9)
Since the re-adsorption of PO on the active Au–Ti sites is neglected, the observed rate of
PO formation (the descending part) is
rPO = k5 x 1 · NAu−Ti
(2.A.10)
where NAu−Ti is the total number of Au–Ti sites.
Combining equations 2.A.9 and 2.A.10 yields a general form for fitting the observed
PO rates,
rPO = a1 e−b1 t + a2 e−b2 t + c.
(2.A.11)
Equation 2.A.11 fits each individual set of experimental data very well. But due to the
complexity of the coefficients, it is not easy to get the kinetic information directly.
2.B Activation energy
The formation rate of water due to the decomposition or hydrogenation of the active peroxo intermediate, which is also responsible for epoxidation on the active Au–Ti centers,
can be given by Equation 2.B.1,
Au
rH2 O,d = k1 θOOH
PH2 θoAu · θHAu · Nactive
(2.B.1)
where Nactive = NAu−Ti (1 − θd ), NAu−Ti is the total number of Au–Ti sites, and θd is the
fraction of deactivated sites. The rH2 O,d here is excluded from the water formed from the
49
2.B. ACTIVATION ENERGY
epoxidation, i.e., from H2 + O2 + C3 H6 −→ PO + H2 O.
The observed formation rate of propene oxide is given by Equation 2.B.2 with the
assumption that one molecule of propene oxide may cause one deactivated Au–Ti center:
Au
rPO = k2 θOOH
PH2 θoAu · θCTi H · Nactive − NAu−Ti ·
3
6
dθd
dt
(2.B.2)
deactivating
Au
The θOOH
, θoAu , θHAu , and θCTi H in Equatons 2.B.1 and 2.B.2 are the coverage based on the
3
6
sites that remain active but not on the total Au–Ti sites.
When the amount of deactivating species accumulated on the active Au–Ti sites is far
less than the amount of propene oxide formed within a short period of time, the second
term on the RHS of Equation 2.B.2 can be omitted:
Au
PH2 θoAu θCTi H · Nactive
rPO = k2 θOOH
3
(2.B.3)
6
Since Eaobs = RT 2 (∂ ln r/∂ T )P , the difference between the overall activation energy of
water formation and PO formation can be expressed by
obs
obs
Ea,H
− Ea,PO
O
2
RT 2

rH2 O,d
∂
ln
r

PO 
=

∂T

(2.B.4)
P
By integrating Equation 2.B.4, Equation 2.7 in Section 2.3.6 can be obtained.
From Equations 2.B.1 and 2.B.3, we have
rH2 O,d
rPO
ln
rH2 O,d
rPO
= ln
A1
A2
−
=
k1
·
θHAu
(2.B.5)
k2 θCTi H
3
Ea,H2 O − Ea,PO
RT
6
+ ln θHAu − ln θCTi H
3
(2.B.6)
6
When the quasi equilibrium is reached on the active sites, θHAu and θCTi H can be given
3
6
by
θHAu
and
θCTi H =
3
6
=
p
KH2 PH2
p
1 + KH2 PH2
KC3 H6 PC3 H6
1 + KC3 H6 PC3 H6 + KPO PPO
(2.B.7)
(2.B.8)
50
Thus we have

∂ ln θHAu
∂T

=
P
=
and


∂ ln θCTi H
3
∂T
6

1
2
1
2
 = (1 − θ Ti )
C H
3
6
P
(1 − θHAu )
(1 − θHAu )

RT
∂T

P
∆Hads,H2
∆Hads,C3 H6
2
∂ ln KH2
(2.B.9)
RT 2
Ti
− θPO
∆Hads,PO
RT 2
(2.B.10)
The difference in apparent activation energy can be described by
1
obs
obs
Ea,H
− Ea,PO
= Ea,H2 O − Ea,PO + (1 − θHAu )∆Hads,H2 −
O
2
2
Ti
(1 − θCTi H )∆Hads,C3 H6 + θPO
∆Hads,PO
3
6
(2.B.11)
REFERENCES
51
References
201, 128–137.
2006, 45, 412–415.
[7] T. A. Nijhuis, T. Visser and B. M. Weckhuysen, Angew. Chem. Int. Ed., 2005, 44, 1115–1118.
1548.
C, 2008, 112, 1115–1123.
[11] T. A. Nijhuis and B. M. Weckhuysen, Catal. Today, 2006, 117, 84–89.
2007, 123, 189–197.
[14] J. Lu, X. Zhang, J. J. Bravo-Suárez, K. K. Bando, T. Fujitani and S. T. Oyama, J. Catal., 2007,
250, 350–359.
884–891.
[18] T. Liu, P. Hacarlioglu, S. T. Oyama, M. Luo, X. Pan and J. Lu, J. Catal., 2009, 267, 202–206.
[19] H. Yang, D. Tang, X. Lu and Y. Yuan, J. Phys. Chem. C, 2009, 113, 8186–8193.
[20] Y. Liu, H. Yu, X. Zhang and J. Suo, Acta. Phys. Chim. Sin., 2010, 26, 1585–1592.
[21] E. E. Stangland, B. Taylor, R. P. Andres and W. N. Delgass, J. Phys. Chem. B, 2005, 109,
2321–2330.
[22] E. Sacaliuc-Parvulescu, H. Friedrich, R. Palkovits, B. M. Weckhuysen and T. A. Nijhuis, J.
Catal., 2008, 259, 43–53.
[23] J. Lu, X. Zhang, J. J. Bravo-Suárez, T. Fujitani and S. T. Oyama, Catal. Today, 2009, 147,
186–195.
[24] B. S. Uphade, T. Akita, T. Nakamura and M. Haruta, J. Catal., 2002, 209, 331–340.
52
[25] M. Ojeda and E. Iglesia, Chem. Commun., 2009, 45, 352–354.
[26] J. Huang, T. Akita, J. Faye, T. Fujitani, T. Takei and M. Haruta, Angew. Chem. Int. Ed., 2009,
48, 7862–7866.
[27] S. Lee, L. M. Molina, M. J. López, J. A. Alonso, B. Hammer, B. Lee, S. Seifert, R. E. Winans,
J. W. Elam, M. J. Pellin and S. Vajda, Angew. Chem. Int. Ed., 2009, 48, 1467–1471.
[30] N. Yap, R. P. Andres and W. N. Delgass, J. Catal., 2004, 226, 156–170.
[31] S. C. Parker and C. T. Campbell, Top. Catal., 2007, 44, 3–13.
[32] S. T. Oyama, X. Zhang, J. Lu, Y. Gu and T. Fujitani, J. Catal., 2008, 257, 1–4.
[33] C. Qi, M. Okumura, T. Akita and M. Haruta, Appl. Catal., A, 2004, 263, 19–26.
[34] A. K. Sinha, S. Seelan, M. Okumura, T. Akita, S. Tsubota and M. Haruta, J. Phys. Chem. B,
2005, 109, 3956–3965.
[36] E. V. Rebrov, M. H. J. M. de Croon and J. C. Schouten, Catal. Today, 2001, 69, 183–192.
[37] D. Mehta and M. C. Hawley, Ind. Eng. Chem. Proc. Des. Dev., 1969, 8, 280–282.
[38] M. Winterberg and E. Tsotsas, AlChE J., 2000, 46, 1084–1088.
[39] J. M. Fraile, J. I. García, J. A. Mayoral and E. Vispe, J. Catal., 2005, 233, 90–99.
[40] T. A. Nijhuis, T. Q. Gardner and B. M. Weckhuysen, J. Catal., 2005, 236, 153–163.
[41] D. G. Barton and S. G. Podkolzin, J. Phys. Chem. B, 2005, 109, 2262–2274.
[42] T. A. Nijhuis, M. Makkee, J. A. Moulijn and B. M. Weckhuysen, Ind. Eng. Chem. Res., 2006,
45, 3447–3459.
[43] J. K. Edwards, A. F. Carley, A. A. Herzing, C. J. Kiely and G. J. Hutchings, Faraday Discuss.,
2008, 138, 225–239.
[44] E. Quinet, L. Piccolo, F. Morfin, P. Avenier, F. Diehl, V. Caps and J.-L. Rousset, J. Catal., 2009,
268, 384–389.
[45] D. C. Ford, A. U. Nilekar, Y. Xu and M. Mavrikakis, Surf. Sci., 2010, 604, 1565–1575.
[46] D. W. Goodman, Catal. Lett., 2005, 99, 1–4.
[47] M. Boronat and A. Corma, Dalton Trans., 2010, 39, 8538–8546.
[48] F. Sun and S. Zhong, J. Nat. Gas Chem., 2006, 15, 45–51.
[49] S. Namuangruk, P. Khongpracha, P. Pantu and J. Limtrakul, J. Phys. Chem. B, 2006, 110,
25950–25957.
[50] A. Carlsson, A. Puig-Molina and T. V. W. Janssens, J. Phys. Chem. B, 2006, 110, 5286–5293.
[51] C. J. Weststrate, A. Resta, R. Westerström, E. Lundgren, A. Mikkelsen and J. N. Andersen, J.
Phys. Chem. C, 2008, 112, 6900–6906.
[52] M. M. Schubert, A. Venugopal, M. J. Kahlich, V. Plzak and R. J. Behm, J. Catal., 2004, 222,
REFERENCES
32–40.
[53] Y. Denkwitz, B. Schumacher, G. Kuc̆erová and R. J. Behm, J. Catal., 2009, 267, 78–88.
[54] F. Gao, T. E. Wood and D. W. Goodman, Catal. Lett., 2010, 134, 9–12.
[55] R. Todorovic and R. J. Meyer, Catal. Today, 2011, 160, 242 – 248.
[56] C. Chang, Y. Wang and J. Li, Nano Res., 2011, 4, 131–142.
[57] A. M. Joshi, W. N. Delgass and K. T. Thomson, J. Phys. Chem. C, 2007, 111, 7841–7844.
[58] A. G. Sault, R. J. Madix and C. T. Campbell, Surf. Sci., 1986, 169, 347–356.
[59] T. V. Choudhary and D. W. Goodman, Appl. Catal., A, 2005, 291, 32–36.
[60] A. Corma, M. Boronat, S. González and F. Illas, Chem. Commun., 2007, 43, 3371–3373.
[61] M. Boronat, F. Illas and A. Corma, J. Phys. Chem. A, 2009, 113, 3750–3757.
53
Enhancement of catalyst
performance in the direct
propene epoxidation: A study
into gold–titanium synergy
3
This chapter has been published as:
J. Chen, S. J. A. Halin, E. A. Pidko, M. W. G. M. Verhoeven, D. M.
Perez Ferrandez, E. J. M. Hensen, J. C. Schouten, & T. A. Nijhuis, Enhancement of catalyst performance in the direct propene epoxidation:
A study into gold–titanium synergy, ChemCatChem, 2013, 5, 467–478.
Abstract
Enhanced productivity toward propene oxide in the direct propene epoxidation with hydrogen and oxygen over gold nanoparticles supported on titanium-grafted silica was
achieved by adjusting the gold–titanium synergy. Highly isolated titanium sites were
obtained by lowering the titanium loading grafted on silica. The tetrahedrally coordinated titanium sites were found to be favorable for attaining small gold nanoparticles
and thus a high dispersion of gold. The improved productivity of propene oxide can be
attributed to the increased amount of the interfacial Au–Ti sites. The active hydroperoxy
interface. A higher propene concentration is favorable for a lower water formation rate
and a higher formation rate of propene oxide. Propene hydrogenation, if occurring, can
be switched off by a small amount of carbon monoxide.
56
CHAPTER 3. ENHANCEMENT OF CATALYST PERFORMANCE: A STUDY INTO GOLD–TITANIUM SYNERGY
3.1 Introduction
The direct epoxidation of propene to propene oxide (PO) using the co-reactants hydrogen
and oxygen, demonstrated for the first time by Haruta et al. [1], is highly selective over
gold–titania based catalysts and has gained considerable attention in the past fifteen
years. This catalytic system requires highly dispersed gold nanoparticles or clusters and
tetrahedral-coordinated Ti4+ sites in the support for a high PO productivity [2–6]. The
most important steps in the reaction mechanism proposed in the literature include the
formation of a hydroperoxy species on the gold nanoparticles, a reactive adsorption of
propene on titania sites adjacent to gold, and the consecutive oxidation of propene by
the hydroperoxy species to form propene oxide [7, 8]. The rate-determining step in this
reaction is the formation of the hydroperoxy species on gold [9–11], which involves the
dissociative adsorption of hydrogen, most likely, at low-coordinated gold atoms close to
the Au–Ti interface [12–14]. The selectivity to PO from propene is generally high (>
90%), which makes this process attractive to the industry [15, 16].
One important issue hindering this process toward a commercial level is the unwanted
hydrogen combustion. Besides the stoichiometric water formed in the epoxidation reaction, a large amount of hydrogen is directly converted into water resulting in a relatively
low hydrogen efficiency. Though studies have addressed this problem, for example, by
silylating the hydrophilic surface of the support or by introducing promotors and additives [17–20], the answer to an intrinsically higher hydrogen efficiency remains vague in
most cases when a high PO yield is the main aim.
Our previous study on the kinetics of this catalytic system has shown that the active
hydroperoxy species is competitively consumed by epoxidation and hydrogenation at the
Au–Ti interface [11]. Water is formed via two routes: through the active hydroperoxy intermediate responsible for expoxidation and from the direct formation not involving this
active intermediate. Gold sites not neighbouring to Ti sites are inactive in the epoxidation
and only produce water. Thus, in principle, the hydrogen efficiency may be improved by
enhancing the synergy between gold and titanium to decrease the proportion of gold
sites not adjacent to titanium. On the other hand, a catalyst with the same gold loading
but having smaller gold nanoparticles shows a higher rate in hydrogen dissociation [12],
which is preferred to speed up the rate-determining step, i.e., the formation of the active
hydroperoxy species. Therefore, developing a catalyst with higher PO productivity and
hydrogen efficiency should focus on achieving small gold nanoparticles and sufficient
3.2. EXPERIMENTAL
57
Au–Ti interface.
In this study, the effect of the synergy between gold and titanium by adjusting metal
loadings was investigated in terms of PO productivity and hydrogen efficiency. Titaniumgrafted silica (Ti-SiO2 ) was chosen as the support. Though titanium silicalite-1 (TS-1) is
often preferred due to its hydrophobicity leading to less hydrogen loss, the zeolite needs
special treatment to anchor gold efficiently [21], and a reproducible preparation method
for the catalysts can be an issue [22]. One beneficial point using Ti-SiO2 as the support
is that almost all the titanium sites are accessible to gold when the silica is mesoporous.
Furthermore, the relatively simple preparation allows for an sufficient amount of support
that can be prepared in one batch; thus the effect of gold loading can be easily and fairly
compared. In this study, the relation between water formation and PO formation was
systematically examined over a wide range of metal loadings and reactant compositions.
Propene hydrogenation was encountered over catalyts using supports with short grafting
time of titanium. The performance of these catalysts is compared and discussed.
3.2 Experimental
3.2.1 Preparation of supports
The Ti-SiO2 supports were prepared by grafting Ti on silica. In a typical synthesis, 15
g of dry as-received silica (Davisil 643, 300 m2 /g, pore size 150 Å, pore volume 1.15
cm3 /g) was dispersed in 250 mL of anhydrous 2-propanol (Aldrich, 99.5 %) under a nitrogen atmosphere in a glove box. The slurry was stirred for 10 minutes and afterwards
tetraethylorthotitanate (TEOT, Aldrich, 97 %) was added. The amount of TEOT added
was determined by calculating the desired theoretical titania coverage of 1 – 10 % monolayer (Ti/Si atom ratio based on ca. 8 atom Si/nm2 ) on the silica surface. A coverage
of 5 % monolayer on 15 g of silica corresponds to 0.7 mL TEOT in total, or 1 wt.% of Ti
with respect to the amount of silica. The slurry was stirred for 30 min. The 2-propanol
was evaporated at 333 K under vacuum in a rotary evaporator. The solvent was removed
within 45 min. The powder was dried overnight at 353 K and subsequently calcined first
at 393 K (5 K/min heating) for 2 hours and then at 873 K (10 K/min heating) for 4 hours.
The prepared supports are denoted as Ti-SiO2 -x, where x stands for the nominal coverage of x % monolayer of Ti on silica surface. A blank support (Ti-SiO2 -0) was prepared
following the above-mentioned procedure but without adding TEOT.
58
An additional Ti-SiO2 support with 1 % monolayer coverage of Ti was prepared on
the as-received silica (Davisil 643) according to the above described procedure. The only
difference in preparation is that the grafting time of titanium alkoxide was prolonged by
controlling the evaporation temperature and pressure of 2-propanol (ca. 313 K and 140
mbar, respectively) [11]. The solvent was evaporated mildly in 3 hours. This support is
denoted as Ti-SiO2 -1L, where ‘L’ stands for ‘prolonged’.
The TS-1 zeolite was prepared following a method similar to the one described by
Chen et al. [23]. An aliquot of 0.76 g of titanium butoxide (TBOT, Aldrich, 97 %) was
mixed with 19.2 g of tetraehtylorthosilicate (TEOS, Aldrich, 98 %) at 273 K in an ice–
water bath and stirred for half an hour. Afterwards, 15 mL of tetrapropylammonium
hydroxide (TPAOH, Merck, 40 wt.% aqueous solution), previously mixed with 15 mL
of de-ionized water, was added dropwise. The mixture was stirred for another half an
hour at 273 K. The solution was then heated in an oil bath to 333 K and was stirred
at this temperature for 2 hours. The crystallization was carried out in a PEEK-lined
stainless steel autoclave at 448 K for 48 hours. The resulting white powder was separated
by centrifugation, washed three times with plenty of de-ionized water, dried at 393 K
overnight and calcined at 813 K for 6 hours.
3.2.2 Deposition of Au
Gold was deposited on the support by a deposition–precipitation (DP) method using
aurochloric acid and ammonia. In a typical synthesis, 2 g of support was dispersed in
100 mL of water. The pH of the slurry was adjusted to 9.5 by dropwise adding ammonia
solution(2.5 wt%). The calculated amount of HAuCl4 solution (Aldrich, 30 wt.% aqueous
solution) diluted in water (approximately 20 mL in total) was added dropwise within
15 min. The amount of gold solution added to the support was exactly the amount
corresponding to the target loading of gold on the catalyst. The catalyst slurry was stirred
for 1 hour after adding gold. During the preparation procedure, the pH of the slurry was
kept between 9.4 and 9.5 by dropwise adding ammonia from time to time, while the
slurry was vigorously stirred. The solid was then filtered (centrifuged for the Au/TS-1
catalyst) and washed 3 times with de-ionized water. The catalyst was dried overnight at
353 K and calcined first at 393 K (5 K/min heating) for 2 hours and afterwards at 673 K
(10 K/min heating) for 4 hours. The prepared gold catalysts on the Ti-SiO2 -x supports
are denoted as m-Au/Ti-SiO 2 -x, where m represents a nominal content of m wt.% of gold
3.2. EXPERIMENTAL
59
on this catalyst. After preparation, catalysts were stored in sealed amber bottles in an
2o C refrigerator.
3.2.3 Inversed incorporation of Ti onto Au/SiO2
A batch of 10 g Au/SiO2 was prepared following the DP method and calcination procedure described in section 3.2.2. The as-received silica (Davisil 643) was used as the
support. The support was dispersed in 250 mL water instead of above-mentioned 100
mL in this batch. The gold loading was nominally 1 wt.%. 4 g of the obtained Au/SiO2
catalyst was dispersed in 250 mL anhydrous 2-propanol (Aldrich, 99.5 %) under nitrogen
atmosphere. Thereafter TEOT was added to obtain 5 % or 10 % monolayer coverage of
Ti on silica surface. The slurry was stirred in a glove box for 30 minutes. The solvent was
evaporated in the rotating evaporator under vacuum at 333 K. The obtained catalysts
were dried overnight in an oven at 353 K. No calcination was performed. These two
catalysts with inversely-incorporated Ti are denoted as 1.0-Au/SiO2 -Ti-x (x = 5, 10).
3.2.4 Catalytic testing
Catalytic tests were performed in a flow setup equipped with a fast Interscience Compact
GC system (3 min analysis time) containing a Porabond Q column and a Molsieve 5A
column in two separate channels, each with a thermal conductivity detector. 300 mg of
catalyst was mounted into the tubular quartz reactor (6 mm inner diameter, 1.5 mm wall
thickness). The catalytic performance was tested under the standard reaction condition
in a 5-hour reaction cycle, typically with a gas feed rate of 50 mL min−1 in total (GHSV
−1
10000 mL·g−1
cat h ), consisting of 10 vol% (each) of hydrogen, oxygen and propene (he-
lium balance). Between each 5-hour reaction cycle, the catalyst was regenerated in 10
vol.% oxygen in helium at 573 K for 1 hour. The temperature of the catalyst bed was
monitored by a K-type thermocouple tightly attached to the outer wall of the reactor.
Carbon monoxide could be introduced with concentrations ranging from 70 ppm to 1
vol.% into the gas feed through an extra mass flow controller. The experiments with
ammonia gas in the feed were carried out by passing the gas feed through an evaporator
containing diluted ammonia solution at 273.6 K.
Besides the 6-mm tube reactor, a microreactor system [11] was used for testing the
0.05-Au/Ti-SiO 2 -1 catalyst at high hydrogen and propene concentrations. The microreactor consisted of a stainless steel capillary tube (0.9 mm inner diameter), which was
60
loaded with 20 mg of catalyst and was operated at a gas feed rate of 3.33 NmL/min. The
volumetric concentration of hydrogen or propene ranged from 2 vol.% to 80 vol.%, while
the oxygen fraction was kept constant at 10 vol.%. The catalysts are generally prepared
and tested within one month after the supports are prepared.
3.2.5 Characterization
Diffuse reflectance UV–visible (DR UV–vis) spectra were recorded on a Shimadzu UV2401PC spectrometer using BaSO4 as a reference. Transmission electron microscope
(TEM) images were recorded with a FEI Tecnai G2 Sphera transmission electron microscope at an acceleration voltage of 200 kV. The catalyst samples for TEM analyses were
finely ground and suspended in drops of ethanol. The suspension was then deposited
onto the TEM grid. The size distribution of gold nanoparticles was determined for each
catalyst by counting all visible gold particles from around 20–40 images (at least 150
gold particles in total) at the magnification of 100 k. Loadings of gold and titanium were
determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using a Spectra CirosCCD system. In ICP analyses, gold was dissolved with aqua regia and
grafted titanium was etched by 5 mol/L H2 SO4 solution. The H2 SO4 solution containing
dissolved titanium was then diluted to 2.9 mol H2 SO4 /L for analyses. The TS-1 zeolite
was dissolved in an equal-volume mixture of HNO3 , HCl and HF. The thermogravimetric
(TG–DTG) profiles were measured on METTLER TOLEDO TGA/DSC STARe system with
20 mL/min nitrogen or helium flow as protective gas and 40 mL/min oxygen flow as
reactive gas. The effluent was monitored by ThermoStarTM mass spectrometry system.
3.3 Results
3.3.1 Characterization of the supports
Figure 3.1 shows the DRUV–vis spectra of different supports directly after calcination.
The TS-1(Si/Ti ratio of 78, 1.0 wt.% of Ti) sample shows a main adsorption band at
210 nm indicating the tetrahedral coordination environment of Ti in the framework.
The sample Ti-SiO2 -1L(see Experimental section) with a Ti loading of 0.22 wt.% has a
very similar coordination environment of Ti to the TS-1 sample. The main part of the
UV–vis band of Ti on this sample can be assigned to tetrahedral tripodal Ti species [24–
61
3.3. RESULTS
27]. A weak adsorption of Ti-SiO2 -1L at 250 nm may be assigned to the tripodal Ti
site coordinated with an extra hydroxyl or water molecule coordinated. The supports
Ti-SiO2 -x (x = 1, 2, 5, 10, x% monolayer in terms of Ti/Si on the surface) were prepared
with a much shorter grafting time of Ti (see Experimental section). The intention of
shortening the grafting time was to obtain more Ti-defects, which may be preferable for
stabilizing gold nanoparticles [21, 28]. Compared to Ti-SiO2 -1L, the Ti-SiO2 -1 sample
shows a broad adsorption at around 275 nm, which can be assigned to penta-coordinated
Ti structures from TiO x moieties [24–27]. As seen from the Ti-SiO2 -x samples, increasing
the loading of grafted Ti broadens the adsorption band to longer wavelengths in the
region of 250 – 300 nm indicative of higher fraction of penta- and hexacooridnated Ti
structures formed.
(a)
F
E: Ti−SiO2−5
D: Ti−SiO2−2
C: Ti−SiO2−1
E
Delta abs.
B: Ti−SiO2−1L
D
Abs.
F−E
E−D
D−C
C−B
B−A
(b)
F: Ti−SiO2−10
A: TS−1
C
B
A
F−E
E−D
0.2
D−C
C−B
0.2
200
B−A
300
400
500
Wavelength (nm)
600
200
250
300
350
Wavelength (nm)
400
Figure 3.1: DRUV–vis spectra of the freshly calcined supports (a) and subtracted spectra
from each two supports (b). The Ti contents (wt.%) of samples A–F are 1.00, 0.22, 0.27,
0.44, 1.07, and 1.78 respectively, determined by ICP analysis.
Figure 3.2 shows the thermogravimetric (TG) and differential thermogravimetric
(DTG) profiles of the fresh and the aged (stored in sealed bottle at RT for certain time)
Ti-SiO2 supports. The as-received SiO2 is not fully hydroxylated. Estimated from the
weight loss between 120 and 800 o C, the surface hydroxyl density of the as-received
SiO2 is about 1.8 OH/nm2 , which is close to the literature data for this type of silica
[29]. The hydroxylated SiO2 shows a main dehydroxylation peak (DTG, dashed line)
above 500 o C as shown in Figure 3.2. Grafting of Ti was performed on the as-received
SiO2 in our study.
62
x 10
0
100.5
−0.2
100.5
−0.2
100
−0.4
100
−0.4
99.5
−0.6
99.5
−0.6
99
98.5
−1.2
800
98
−0.8
SiO2
hydroxylated
50 120 120 200
400
600
o
Temperature ( C)
−3
101
x 10
0
100.5
−0.2
100.5
−0.2
100
−0.4
100
−0.4
99.5
−0.6
99.5
−0.6
99
98
DTG (%/s)
x 10
0
98.5
−0.8
Ti−SiO2−1
fresh prepared
50 120 120 200
400
600
Temperature (oC)
−1
99
98.5
−1.2
800
98
−0.8
Ti−SiO2−1
aged 8 months
50 120 120 200
400
600
Temperature (oC)
−1
−1.2
800
−3
x 10
0
101
x 10
0
100.5
−0.2
100.5
−0.2
100
−0.4
100
−0.4
99.5
−0.6
99.5
−0.6
99
98.5
98
DTG (%/s)
101
−0.8
Ti−SiO2−5
fresh prepared
50 120 120 200
400
600
o
Temperature ( C)
Normalized weight (%)
−3
−1
99
98.5
−1.2
800
98
−0.8
Ti−SiO2−5
aged 7 months
50 120 120 200
400
600
o
Temperature ( C)
−3
−1
−1.2
800
−3
x 10
0
101
x 10
0
100.5
−0.2
100.5
−0.2
100
−0.4
100
−0.4
99.5
−0.6
99.5
−0.6
99
−0.8
Ti−SiO −10
98.5
98
2
fresh prepared
50 120 120 200
400
600
Temperature (oC)
−1
−1.2
800
DTG (%/s)
101
−1.2
800
101
−3
−1
DTG (%/s)
50 120 120 200
400
600
o
Temperature ( C)
−1
DTG (%/s)
98
−0.8
SiO2 as received
99
DTG (%/s)
99
98.5
DTG (%/s)
101
DTG (%/s)
−3
x 10
0
−3
101
−0.8
Ti−SiO −10
98.5
98
2
aged 8 months
50 120 120 200
400
600
Temperature (oC)
−1
−1.2
800
Figure 3.2: Thermogravimetric (TG, blue solid line) and differential thermogravimetric
(DTG, red dashed line) profiles of dehydration and dehydroxylation of SiO2 and Ti-SiO2 x supports ( heated at 10 K/min from 323 K to 1073 K, temperature stabilized at 393 K
for 10 min; the hydroxylated SiO2 was obtained by hydrolyzing the as-received SiO2 in
373 K water for 48 h in an autoclave.)
The Ti-SiO2 supports after calcination were slowly hydroxylated when stored sealed
under ambient conditions. In general, the peak from the DTG curve at around 280 o C
63
3.3. RESULTS
may be attributed to the hydrolyzed Si–O–Si bond in the aged supports, while the shoulder or peak at around 170 o C for the aged Ti-SiO2 -5 and Ti-SiO2 -10 is best attributed to
the hydrolyzed Ti–O–Ti or Ti–O–Si bond. The support Ti-SiO2 -10 shows a much higher
affinity to moisture, very likely due to an higher amount of Lewis acidic sites as well as
defective Ti–OH on the surface. The re-hydroxylation of supports increases the coordination number of Ti even after re-calcination at 300 o C as indicated by the UV-vis spectra
(not shown).
3.3.2 Size of Au particles
The titanium loading affects both the coordination state of Ti as seen from Figure 3.1
and the size distribution of gold nanoparticles. Figure 3.3 summarizes the averaged gold
particle sizes of catalysts with different gold and titanium loadings. From this summary
it can be seen that the measured particle sizes are between 1 and 4 nm and that lower
gold and titanium loadings generally lead to smaller gold nanoparticles. Detailed information on the size distribution and metal loadings determined by ICP are provided in the
Appendix.
bel
ow
d
lim etect
ion
it
Figure 3.3: A visual summary of the averaged sizes of gold nanoparticles on Ti-SiO2 -x
(x = 1, 2, 5, medians are used for x = 1 due to skewness)
Figure 3.4(e) shows the average sizes of gold nanoparticles with gold loading of 1
wt.% over the Ti-SiO2 -x (x = 1, 2, 5, 10) supports, which are 2.5 ± 0.9 nm, 2.9 ± 1.1 nm,
3.5±1.1 nm, and 6.0±1.4 nm respectively. As seen from these four samples, the average
size of gold nanoparticles decreases when the titanium loading is lower. Figure 3.4(a–d)
shows representative TEM images of these four samples.
64
Au 1 wt.%
Ti 10ML.%
Au 1 wt.%
Ti 5ML.%
30
(e)
Au 1wt.%
Ti 10ML.%
20
10
0
40
Au 1wt.%
Ti 5ML.%
20 nm
(a)
Au 1 wt.%
Ti 2ML.%
20 nm
(b)
Au 1 wt.%
Ti 1ML.%
Counts
20
0
60
Au 1wt.%
Ti 2ML.%
40
20
0
80
Au 1wt.%
Ti 1ML.%
60
40
20
20 nm
(c)
20 nm
(d)
0
0 1 2 3 4 5 6 7 8 9 10
Particle size (nm)
Figure 3.4: TEM images of catalysts with the gold loading of nominal 1 wt.% on the
Ti-SiO2 -x supports and their size distributions of gold nanoparticles (In (e), the numberaveraged particle size and its ‘standard deviation’ are shown by a circle with an error bar;
the median size is illustrated by the stem.)
Figure 3.5(a–d) shows representative TEM pictures of catalysts with lower gold loadings. Figure 3.5(e) compares the size distribution of gold nanoparticles on the Ti-SiO2 -1
support. As seen from Figure 3.5(e), lower gold loadings lead to smaller gold particles.
The particle sizes of catalysts with gold loadings lower than 0.2 wt.% were difficult to
measure, because very few particles can be observed and our TEM is unable to detect
subnanometer particles. Figure 3.6 shows TEM images of the 0.05-Au/TS-1 sample. The
average crystal size is ca. 120 nm. Gold nanoparticles can rarely be observed on TS-1
and the size of those observed gold particles is around 3 nm.
3.3.3 PO formation and water formation
The time-on-stream productivity of PO over the catalysts is generally stable at 433 K
as shown in Figure 3.7(a). Compared to 0.5-Au/Ti-SiO 2 -5, the 0.5-Au/Ti-SiO 2 -1 and
0.5-Au/Ti-SiO 2 -2 catalysts showed a much higher activity. In general, the catalysts on
supports with lower Ti loadings, which have a higher fraction of isolated Ti sites as
65
3.3. RESULTS
Au 0.2wt.%
Ti 5ML.%
Au 0.2wt.%
Ti 2ML.%
100
(e)
Au 1wt.%
Ti 1ML.%
50
(a)
Au 0.5wt.%
Ti 1ML.%
20 nm
(b)
Au 0.2wt.%
Ti 1ML.%
0
120
Counts
20 nm
Au 0.5wt.%
Ti 1ML.%
80
40
0
90
Au 0.2wt.%
Ti 1ML.%
60
30
20 nm
(c)
20 nm
(d)
0
0
1
2
3 4 5 6 7 8
Particle size (nm)
9 10
Figure 3.5: (a–d)TEM images of catalysts with lower gold loadings and (e) size distributions of gold nanoparticles on the Ti-SiO2 -1 support (the number-averaged particle size
and its ‘standard deviation’ are shown by a circle with an error bar; the median size is
illustrated by the stem)
indicated by UV–vis spectra, gave a higher PO productivity as summarized by Figure
3.7(b). The 1.0-Au/Ti-SiO 2 -5 catalyst (ca. 1 wt.% of Au and Ti) gave its highest PO
−1
rate at around 30 gPO · kg−1
at 433 K. In the series of catalysts over the Ti-SiO2 -5
cat h
support, the PO formation rate decreased as the gold loading was lowered, while the
PO rate reaches the maximum over the 0.5-Au/Ti-SiO 2 -Ti-2 catalyst on the Ti-SiO2 -2
support. Detailed performance and elemental analyses of all the catalysts at different
temperatures are provided in the supplemental information.
−1
PO formation rates higher than 100 gPO · kg−1
were achieved over the catalysts
cat h
using the Ti-SiO2 -1 support. Maximum PO formation rates obtained on the five catalysts
over Ti-SiO2 -1 are plotted in Figure 3.8(a). What should be mentioned here is that these
best rates for each catalyst were obtained at different tempratures ranging from 150 o C
to 210 o C (Table 3.1). The highest PO rates are from the catalysts with gold loadings not
higher than 0.2 wt.%. Suprisingly, the 0.05-Au/Ti-SiO 2 -1 catalyst with a gold loading as
low as 0.05 wt.% also showed a very high activity, though no visible gold particles could
be observed by TEM. The corresponding PO productivity per gram of Au is plotted in
66
20 nm
Figure 3.6: TEM images of the 0.05-Au/TS-1 sample
−1
Figure 3.8(b) for these five catalysts. The activity per unit weight of gold (gPO · g−1
Au h )
increases significantly towards the low Au loadings.
Table 3.1: Maximum PO formation rates obtained on the Au/Ti−SiO2 catalysts with different gold loadings over the 1% monolayera Ti−SiO2 and the corresponding selectivities
Loading
(Au wt%)
0.05
0.09
0.20
0.48
0.84
T
(K)
483
465
457
437
424
rPO,max c
h−1 )
(g · kg−1
cat
113 (226)
122 (135)
131 (67)
85 (18)
60 (10)
Selectivity(%)
PO
C3 H8
26
69
23
73
38
53
45
40
67
7
H2 efficiency b
(%)
7.8
8.1
5.4
4.0
2.2
a. 0.27 wt% Ti as determined by induced coupled plasma (ICP) analysis
b. determined as rPO /(rH
2
O
+ rC
H
3 8
)
−1
c. numbers in parentheses are in terms of gPO · g−1
Au h
The formation rates of water and PO are correlated in the catalytic reaction. In general, they decrease concurrently when the active Au–Ti sites are blocked during the catalyst deactivation [11]. Since the amount of water produced per amount of PO is a
determining factor for the viability of a process based on these catalysts, plotting the
water formation rate against the PO rate for each single catalytic test is an informative
67
3.3. RESULTS
−7
x 10
0.5−Au/Ti−SiO −1
(a)
2
(b)
0.5−Au/Ti−SiO2−2
5
−1 −1
2
4
3
2
1
0
0
60
120
180
Time (min)
240
300
Figure 3.7: (a) Time-on-stream formation rate of PO during a 5-hour catalytic test for
three Au/Ti-SiO 2 catalysts: 0.5 wt.% Au, different Ti loadings; (b) summary of PO productivity for different Au/Ti-SiO2 catalysts. (H2 :O2 :C3 H6 :He= 1 : 1 : 1 : 7, 433 K, GHSV
−1
−7
−1 −1
−1
10000 mL·g−1
= 20.9 gPO · kg−1
cat h ; under this condition, 1 × 10 mol PO · gcat s
cat h ;
productivity averaged from 150 min to 270 min)
method to compare the performance of catalysts. Figure 3.9(a) shows the trajectories of
reaction rates for several catalysts in a single 5-hour cycle. The length of the trajectory
reflects the short-term stability of a catalyst. The trajectory starts at the highest reaction
rates of a catalyst and ends at the lowest rates after 5-hour deactivation. The hydrogen efficiency in terms of rPO /rH2 O can be easily calculated for each point. The slope
of a trajectory represents the ratio of hydrogen consumed by water formation and by
epoxidation at the Au–Ti interface when the blockage of the active site is the main cause
of short-term activity change [11]. The experiments in this figure were performed at 3
different temperatures. On the left part of the diagram as shown in Figure 3.9(a), the
activity of the catalysts with 1 wt.% gold loading is compared at 403 K, a relatively low
temperature at which the deactivation is obvious. The 1.0-Au/Ti-SiO 2 -1 and 1.0-Au/TiSiO2 -2 catalysts show much higher rH2 O /rPO ratios when compared to the catalysts with
higher Ti loadings. There are two possible reasons: 1) the smaller gold particles on the
1.0-Au/Ti-SiO 2 -1 and 1.0-Au/Ti-SiO 2 -2 catalysts have higher hydrogen dissociation rates
while the epoxidation rate is relatively limited at the Au–Ti interface; 2) less likely, the
gold sites not adjacent to Ti but producing water may also be blocked. The comparison of
0.2-Au/Ti-SiO 2 -2 and 0.5-Au/Ti-SiO 2 -2 at 433 K shows that a higher gold loading result
in unwanted water formation since these two catalysts have the same support and similar
gold sizes. By adjusting the Au and Ti loadings, the synergy between the two sites can
be tuned. As seen from Figure 3.9(a), a much higher PO productivity can be achieved on
68
150
250
Au efficiency (g ⋅g ⋅h )
−1 −1
PO Au
120
−1
−1
rPO, max (gPO⋅kgcat⋅h )
m−Au/Ti−SiO2−1
90
60
30
0.1 0.2
2
200
150
100
50
(a)
0
0
(b)
0.5
Au loading (wt.%)
0.8
1
0
0
0.1 0.2
0.5
Au loading (wt.%)
0.8
1
Figure 3.8: (a)Maximum PO formation rates obtained on the Au/Ti-SiO2 -1 catalysts and
(b) corresponding gold efficiency. (data see Table 3.1; 1.0-Au/Ti-SiO 2 -5 used as a refer−1
ence has a maximum PO rate of 31 gPO · kg−1
at 433 K and its actual gold loading is
cat h
0.80wt.%; H2 :O2 :C3 H6 :He= 1 : 1 : 1 : 7, GHSV 10000 mL·g−1
h−1 ; performance avercat
aged from 150 min to 270 min in each 5-hour catalytic test)
the 0.05-Au/Ti-SiO 2 -1 catalyst even at 483 K without increasing the rH2 O /rPO ratio much
if compared with the performance of 1.0-Au/Ti-SiO 2 -5 at 403 K.
In Chapter 2, it has been found that the active hydroperoxy species is competitively
consumed by hydrogenation and epoxidaion at the Au–Ti interface. Figure 3.9(b) confirms this finding on the 0.05-Au/Ti-SiO 2 -1 catalyst, which showed no visible gold particles but having very high epoxidation activity. When the hydrogen concentration was
raised, both formation rates of water and PO increased. However, hydrogenation of the
hydroperoxy species is more dominant at higher hydrogen concentrations leading to further loss in the hydrogen efficiency. On the other hand, higher propene concentrations
are favourable for suppressing water formation as demonstrated in Figure 3.9(b). The
activity of the 0.05-Au/Ti-SiO 2 -1 catalyst was stable in each 5-hour test resulting in a
compact cluster of points in the water–PO plot. The activity at the same condition (10
vol% for each reactant) is not overlapping in the two series of H2 and C3 H6 experiments
as shown in Figure 3.9(b), which means that a slight long term change of the catalyst
had occurred because of the gold sintering and/or the support hydroxylation. The results
presented in Figure 3.9 indicate that the composition of catalysts (Au/Ti contents) and
reactant concentrations should be optimized together for a better catalytic performance.
69
3.3. RESULTS
x 10
8
6
(a)
433 K
403 K
483 K
(b)
2%
20%
3%
1
(mol⋅g ⋅s )
5%
6
Pr
1
cat
0.5-Au/Ti-SiO2-2
op
en
1.0-Au/Ti-SiO2-1
8%
e(+
10%
)
8%
0.05-Au/Ti-SiO2-1
4
r
2
HO
5%
1.0-Au/Ti-SiO2-2
3%
0.2-Au/Ti-SiO2-2
2
2%
Hy
1.0-Au/Ti-SiO2-5
dr
e
og
n(
)
20% 30%
40%
50% 80%
1.0-Au/Ti-SiO2-10
0
0
1
2
r
PO
3
4
1 1
(mol⋅g ⋅s )
5
cat
6
7
x 10
Figure 3.9: (a) Water vs. PO formation rates for different catalysts at selected tempera−1
tures in a 5-hour catalytic test (H2 :O2 :C3 H6 :He= 1 : 1 : 1 : 7, GHSV 10000 mL·g−1
cat h );
and (b) competitive effect of hydrogen and propene on water and PO formation over
0.05-Au/Ti-SiO 2 -1 at 483 K (varied hydrogen or propene concentrations, 10 vol% oxy−1
gen fixed, helium balance, GHSV 10000 mL·g−1
cat h , tested in the microreactor system,
5-hour test for each condition; the series of experiments changing C3 H6 concentrations
were performed after the series of H2 ).
1.0−Au/SiO
2
60
40
Counts
20
0
1.0−Au/SiO −Ti−5
40
2
30
20
10
*
0
0
2
4
6
8 10 12
Particle size (nm)
14
Figure 3.10: Gold particle size distribution of the 1 wt.% Au/SiO2 and the reverselygrafted 1 wt.% Au/SiO2 -Ti-5 catalyst (∗: particles bigger than 14 nm are grouped in the
column of 14 nm)
70
3.3.4 Performance of the catalyts with inversely-grafted Ti
In Figure 3.9(a), the activity of the catalysts with 1 wt.% of gold was compared. These
catalysts have different gold particle sizes (Figure 3.4) in addition to different Ti loadings.
In order to exclude the influence from the gold particle size, the method with which Ti
was grafted onto Au/SiO2 was applied (see Experimental section). The resulting catalysts
are denoted as 1.0-Au/SiO 2 -Ti-x. The 1 wt.% Au/SiO2 has an average gold particle size
of 3.1 ± 0.8 nm. However, the inverse-grafting method was not successful to obtain
an identical size distribution of gold particles. The resulting 1.0-Au/SiO 2 -Ti-5 catalyst
has an average gold size of 4.8 ± 2.7 nm as shown in Figure 3.10. A large number
of big gold particles was formed concluding that during Ti depostion gold particle size
changed. Additionally, the obtained 1.0-Au/SiO 2 -Ti-x (x = 5, 10) catalysts both showed
very high activity in propene hydrogenation (7% – 10% in propane yield), while the 1
wt.% Au/SiO2 had no activity in propene hydrogenation and negligible conversion (less
than 0.02%) to PO at the same conditions given in Table 3.2.
3.3.5 Effect of supports
The catalyst made on a support with grafted Ti can easily achieve high PO productivity
−1
(greater than 100 gPO · kg−1
cat h ) at 473 K as listed in Table 3.3. The rPO /rH2 O ratio on
these catalysts at 473 K ranges between 10% and 20% at the standard reaction conditions
with 10% of each reactant. The gold loadings of these highly active catalysts are generally
as low as 0.2 wt.%. The Ti loadings of these supports are lower than 0.3 wt.%. If the
specific surface area of the silica chosen in this study is taken into account (300 m2 /g),
the surface density of Ti is around 0.1 Ti/nm2 . Although the TS-1 zeolite used in this
study has a much higher Ti loading than the Ti-SiO2 -1 and Ti-SiO2 -1L supports, the
activity of 0.05-Au/TS-1 is very limited. The TEM images of the Au/TS-1 catalyst are
provided in the supplemental information.
Despite the difference in the UV–vis spectra, the most interesting difference between
Ti-SiO2 -1 and Ti-SiO2 -1L is that a substantive amount of propane was formed over the
catalysts on the Ti-SiO2 -1 support as shown in Table 3.3. The Ti-SiO2 -1L support was
prepared with longer grafting time for Ti (3 hours instead of less than 45 min for TiSiO2 -x). The activity in propene hydrogenation over the Au/Ti-SiO2 -1 catalysts is more
than two magnitudes higher than the Au/Ti-SiO 2 -1L catalysts. The 0.05-Au/TS-1 catalyst
also has a low activity in propene hydrogenation.
3.3. RESULTS
Table 3.2: Performance of the catalysts with inversely-grafted Ti at 433 K and the comparison with the catalyts prepared in the normal
sequence a
Sample ID b
Ti loading
(wt.%)
Au loading
(wt.%)
Au particle size c
(nm)
1.02
1.85
1.07
1.78
0.95
0.95
0.80
0.83
4.8 ± 2.7 (4.0)
-d
3.5 ± 1.1 (3.5)
6.0 ± 1.4 (5.9)
1.0-Au/SiO2 -Ti-5
1.0-Au/SiO2 -Ti-10
1.0-Au/Ti-SiO2 -5
1.0-Au/Ti-SiO2 -10
Formation rate
(×10−7 mol · g−1
s−1 )
cat
PO
C 3 H8
H2 O
0.38 11.89 19.74
0.28
9.05
20.88
1.47
0.45
28.95
0.73
8.44
14.40
PO yield
(%)
0.31
0.23
1.18
0.59
Selectivity
(%)
PO
C3 H8
3.1
95.2
2.9
93.0
64.1
19.5
7.7
87.9
H2 efficiency e
(%)
1.2
0.9
5.0
4.9
−1
a. performance averaged from 150 min to 270 min in each 5-hour catalytic test; H2 : O2 : C3 H6 : He= 1 : 1 : 1 : 7, GHSV 10000 mL·g−1
cat h
b. the catalysts with inversely-grafted Ti are denoted as 1.0-Au/SiO2 -Ti-x (x = 5, 10)
c. median in brackets
d. not determined
e. determined as rPO /(rH
2
O
+ rC
H
3 8
)
71
72
Sample ID
Ti loading
(wt.%)
Au loading
(wt.%)
0.22
0.22d
0.22d
0.27
0.27e
0.27e
1.00
0.20
0.11
0.06
0.20
0.09
0.05
0.04
0.2-Au/Ti-SiO2 -1L
0.1-Au/Ti-SiO2 -1L
0.05-Au/Ti-SiO2 -1L
0.2-Au/Ti-SiO2 -1
0.1-Au/Ti-SiO2 -1
0.05-Au/Ti-SiO2 -1
0.05-Au/TS-1
Formation rate
(×10−7 mol · g−1
s−1 )
cat
PO
C3 H8
H2 O
5.86
0.14
61.20
5.68
0.19
43.76
3.03
0.14
14.56
5.44
4.59
113.1 b
5.47 15.23
60.46
5.36 17.21
47.14
0.72
0.25
8.49
PO yield
(%)
4.73
4.58
2.44
4.66
4.41
4.32
0.58
Selectivity
(%)
PO
C3 H8
82.8
1.9
84.8
2.9
89.4
4.1
40.2
33.9
24.7
68.8
23.1
74.1
59.6
20.8
H2 efficiency c
(%)
CO2
4.9
0.0
0.0
9.4
1.9
0.2
0.0
9.6
12.9
20.6
7.2
8.3
8.2
−1
a. performance averaged from 150 min to 270 min in each 5-hour catalytic test; H2 :O2 :C3 H6 :He= 1 : 1 : 1 : 7, GHSV 10000 mL·g−1
cat h ,
−1
−1
under this condition, 1 × 10−7 mol PO · g−1
= 20.9 gPO · kg−1
cat s
cat h
b. Hydrogen was fully combusted.
c. determined as rPO /(rH
2
O
+ rC
H
3 8
)
d. same support as 0.2-Au/Ti-SiO 2 -1L, Ti content assumed to be the same
e. same support as 0.2-Au/Ti-SiO2 -1, Ti content assumed to be the same
Table 3.3: Performance of catalyts over Ti-SiO2 -1L, Ti-SiO2 -1 and TS-1 at 473 K a
3.4. DISCUSSION
73
3.3.6 Propane formation and its suppression
Significant propane formation was encountered sepecially for the low-gold-loaded catalysts on the Ti-SiO2 -x supports in this study. Figure 3.11(a) summarizes the rC3 H8 /rPO
ratio at 433 K over these catalysts, which ranges from 0.1 to 10. The catalysts with lower
gold loadings show a higher selectivity towards propane. In this study, the first catalytic
test for each catalyst produced less propane than the consecutive cycles. Therefore, the
first catalytic cyle was not used when comparing the performance. The first 2-hour testing for the fresh 0.2-Au/Ti-SiO 2 -2 catalyst as shown in Figure 3.11(b) demonstrates such
instability of propane formation. Such ascending trend in propene hydrogenation is general for the fresh catalysts in our study and will be described in detail in Chapter 4. A
small amount of carbon monoxide in the feed can completely shut off propene hydrogenation while not affecting PO formation. In Figure 3.11(b), the inhibiting effect on
propene hydrogenation by carbon monoxide is shown. After removing carbon monoxide,
the activity in propene hydrogenation was restored. The propane formation can also be
suppressed by ammonia. However, all the three reactions were inhibited in presence of
ammonia as seen from Figure 3.11(b). After removing ammonia from the feed gas, only
a portion of its original activity was restored. Detailed experimental results on propene
hydrogenation will be given in Chapter 4.
3.4 Discussion
3.4.1 Role of the support in enhanced PO productivity
The highest PO formation rates reported in literature using gold catalysts in the direct
epoxidation of propene using hydrogen and oxygen are generally at the level of 120
−1
– 150 gPO · kg−1
[4–6, 18, 21, 22, 30], despite some differences in space velocity
cat h
under the testing conditions. These rates almost meet the industrial requirement for PO
productivity [18]. Most of these records were obtained on catalysts using TS-1 as the
support. The isolated tetrahedral-coordinated Ti4+ cations abundant in TS-1 make this
material an ideal support for the hydro-epxoidation of propene. The TS-1 zeolite active in
selective oxidation normally preserves a small crystal size (< 400 nm) facilitating to the
use of the exterior surface [25, 31], especially in the case of hydro-epoxidation of propene
when the gold nanoparticles are bigger than the micropores. The chemical environment
74
(a)
−6
−7
x 10
2.5
PO
C3H8
4
H2O
2
remove NH3
3
1.5
2
1
rH
2
H
rPO or rC
(mol⋅g−1
⋅s−1)
cat
1000 ppm NH3
3 8
(mol⋅g−1
⋅s−1)
cat
200 ppm CO
500 ppm CO
O
10
5
1
0.5
remove CO
(b)
0
0
2
4
Time (h)
6
0
8
Figure 3.11: (a) Ratio between the C3 H8 formation rate and the PO formation rate at 433
K of catalysts over the Ti-SiO2 -x (x = 1, 2, 5) supports and (b) effect of CO and NH3 on
inhibiting C3 H8 formation on 0.2-Au/Ti-SiO 2 -2 at 423 K (H2 :O2 :C3 H6 :He= 1 : 1 : 1 : 7,
−1
GHSV 10000 mL·g−1
cat h ; in (a), performance averaged from 150 min to 270 min in each
5-hour test)
of titanium in TS-1, to a great extend, is in the closed Ti(OSi)4 structure. The smooth
surface of TS-1 crystals with little defective hydroxyls is also believed to be beneficial to
the reduction of unnecessary decomposition of the hydroperoxy intermediate during the
catalytic reaction. However, the limited amount of Ti–OH on the surface of TS-1 makes an
efficient deposition of gold close to Ti very difficult, since the surface defects seems crucial
for the interaction between gold and the support in the deposition–precipitation method
[32, 33]. This dilemma encouraged researchers to develop synthesis methods roughening
the surface of TS-1 and greatly improved PO productivity was achieved [21, 28, 30].
Using Ti-grafted silica as an alternative may overcome the problem in anchoring gold
3.4. DISCUSSION
75
in a cheaper way, since the grafted Ti should be in the form of an open structure on
the surface. However, productivities obtained on gold catalysts supported on Ti-grafted
silica in earlier studies were generally inferior when compared to that on TS-1, which
may be attributed to the existence of higher number (penta or hexa-) coordianted Ti
[34, 35]. In our study, we were aiming at a support mainly with highly isolated Ti4+
sites, which would be reflected in a narrow band in UV–vis spectra at around 200–210
nm as Ti(OSi)4 or at 220–230 nm as Ti(OH) x (OSi)3−x [24–27]. The principle of grafting
Ti onto the silica surface is through the reaction between titanium alkoxide with surface
silanols. The surface silanol density of the silica used in this study was 1.8 OH/nm2 as
estimated from the weight loss during dehydroxylation. Though not fully hydroxylated, it
is known for this type of silica (Davisil) that most of the silanol sites are paired across the
entire silica surface [36]. For simplicity, if we assume that a titanium alkoxide molecule
randomly reacts with one silanol first and that the silanol sites are evenly distributed
in a small area, the distribution of Ti on the silica surface may look like how Figure
3.12 shows. Sites occupied by Ti in vicinity may end up in Ti–O–Ti connectivities after
calcination and accordingly penta- or hexa-coordination environment. The Ti density
on the silica surface calculated in our study is around 0.1, 0.2, 0.5 and 0.8 Ti/nm2 for
the Ti-SiO2 -x (x = 1, 2, 5, 10) supports based on the actual Ti loadings and the specific
area of the silica. The Ti-SiO2 supports with a Ti loading of 0.2 – 0.3 wt.% showed the
most similar adsorption band to TS-1 in UV-vis spectra as seen in Figure 3.1. Indeed,
−1
the highest PO rates obtained in this study (120 – 130 gPO · kg−1
cat h ) were from the Ti-
SiO2 -1 and Ti-SiO2 -1L supports. Increasing the Ti loading may result in a full coverage
of Ti on the silica surface as illustrated in Figure 3.12. Such a higher coverage of Ti can
also be evidenced by the shoulder or peak at around 170 o C in the DTG curve during
dehydroxylation as shown in Figure 3.2 for Ti-SiO2 -5 and Ti-SiO2 -10, which is known as
the dehydroxylation temperature of Ti–OH [37].
It seems that highly isolated Ti4+ sites are also favourable for attaining smaller gold
nanoparticles as indicated by Figure 3.4. In our preparation, the gold precursor was
added into the slurry at a relatively high pH (9.4–9.5), which may lead to a fast formation
of a hydroxo-gold complex interacting with Ti–OH [32, 33]. The decreasing trend in gold
particle size on the Ti-SiO2 -x (x = 10, 5, 2, 1) supports but with the same gold loading
may be considered as an indirect evidence of the interaction between gold precursors
and defective Ti–OH sites. The TS-1 used in this study has a relatively high Ti loading
(1 wt.%, or Si/Ti= 78). The 0.05-Au/TS-1 catalyst showed very poor activity when
76
compared to the catalysts with the same loading of gold on Ti-SiO2 as listed in Table 3.3.
It is, very likely, due to the limited number of Ti-OH sites on the surface of TS-1 crystals
so that efficient gold deposition to Ti could not occur.
(a)
(b)
(c)
(d)
Figure 3.12: Schematic of Ti distribution on silica surface for different Ti-SiO2 -x supports
(a–d corresponding to x = 1, 2, 5, 10; filled square: Ti, isolated or connected; empty
circle: unoccupied Si–OH)
3.4.2 Competition of epoxidation and water formation at the Au–Ti
interface
In this study, the most active catalysts on Ti-SiO2 -1 and Ti-SiO2 -1L have gold loadings
at around and lower than 0.2 wt.%. The gold particle sizes on these catalysts should be
very small (not greater than 2 nm) inferred from the trend shown in Figure 3.5 and the
fact that few nanoparticles were visible. The range of gold loadings of the highly active
catalysts in this study is in accordance with those in literature reporting the highest PO
formation rates [4–6, 21]. It has been shown that the turnover frequencies of hydrogen
3.4. DISCUSSION
77
dissociation are relatively constant over Au/TiO2 and independent of gold particle sizes
[12]. The high activity in propene epoxidation over the low-loaded catalysts may be
attributed to the increased number of interfacial Au–Ti sites from the high dispersion of
gold. A good synergy between gold and titanium may, to a great extend, limit the direct
water formation which occurs on the gold sites not adjacent to titanium as indicated by
Figure 3.9(a). However, hydrogen, as a scavenger forming the active hydroperoxy species
at the Au–Ti interface, also hydrogenates this active intermediate to water [11, 38]. Recent studies on hydrogen dissociation over Au/TiO2 suggests that hydrogen dissociation
occurs at low-coordinated gold atoms near the metal–support interface [13, 14]. On the
other hand, it has been shown in our earlier study that the inhibiting effect of propene
on water formation over gold catalysts may be due to the propene adsorption and activation on gold [39]. The Au–Ti interface is also a site for propene adsorption. [40].
As demonstrated in Figure 3.9(b) and in Chapter 2, higher propene concentrations suppress water formation while increasing the PO formation rates. The active hydroperoxy
intermediate at the Au–Ti interface is competitively consumed by epoxydation and hydrogenation, while higher propene concentrations do not inhibit the rate-determining
step in the hydro-epoxidation of propene. This may explain why in an industrial example
a higher propene concentration was used [15]. In conclusion, our findings suggest that
a high propene concentration is preferred in real operation besides a moderate concentration for hydrogen.
The hydrogen efficiency of catalysts with highest PO productivity in this study is not
high enough. It is within the range of 10 – 20 % if propane formation is not counted,
which is lower than the level at 20 – 30 % of Au/TS-1 catalysts with their best performance [21]. There may be still room for our catalysts to improve the hydrogen efficiency
due to the nature of hydrophilicity in Ti-SiO2 since no sylilation had been performed. It
is known for hydrophilic support that silylation can help reduce hydrogen consumption.
An example can be seen from the work by Uphade et al. [17] as given in Figure 3.13.
However, their study shows that the elimination of surface hydroxyls can decrease the
direct water formation significantly, while the ratio of rH2 O /rPO at the Au–Ti interface
was not much changed.
In industry, a typical world scale PO plant can have capacity of 300, 000 t/a. PO pro−1
ductivites of state-of-the-art gold catalysts are generally above 150 gPO · kg−1
cat h , which
is equal to the activity of our most active catalyst shown in Figure 7b. Simple calculations based on these two figures suggest that a new plant based on the hydro-epoxidation
78
60
H2 conversion (%)
50
40
30
20
Au/Ti−MCM−48
10
Au/Ti−MCM−48(silylated)
0
0
1
2
3
C3H6 conversion (%)
4
5
Figure 3.13: An example of improvement in hydrogen efficiency by hydrophobic treatment on the catalyst support. Data adapted from the paper by Uphade et al. [17]. (Fig.
12 in ref [17]. The catalyst was 0.09 wt% Au/Ti-MCM-48 (Si/Ti= 50). Testing was
performed at 423 K in a 10-mm tube reactor. H2 :O2 :C3 H6 :He= 1 : 1 : 1 : 7, GHSV
−1
4000 mL·g−1
cat h . The time-on-stream performance in the original study is plotted in the
H2 O-vs-PO fashion here.)
technology would require 228 tons of catalyst with gold as much as 456 kg (assuming
a gold loading of 0.2 wt.%), which is still rather large. By further catalyst development
and process optimization, the catalyst cost can be reduced further. Especially preparing
catalysts in which the gold–titanium interaction is optimized can be a significant step
forward. Even though we did not observe significant deactivation over a period of up
to one month, the long-term stability of catalysts on a longer time scale was not examined. However, to an industrial process this longer term stability will be essential for
the process economics. On the other hand, an intrinsically high efficiency in hydrogen
utilization is indispensable to make this a viable process. Although PO productivity has
been improved in this study, the PO/water ratio is still lower than 20%, which is inferior to the 30% level achieved on Au/TS-1 [21]. The hydrophilic nature of our support
may be the reason. Previously in Chapter 2 and once again illustrated in Figure 3.9b,
we demonstrated that operating the process with low hydrogen concentrations is also an
efficient manner to increase the hydrogen efficiency, although this is at the expense of
the catalyst productivity.
3.4. DISCUSSION
79
3.4.3 Origin of the activity in propene hydrogenation
In this study, severe propene hydrogenation was observed on most catalysts. Propene hydrogenation was only supposed to happen on gold nanoparticles smaller than 2 nm in the
hydro-epoxidation of propene over the gold–titania system [1]. This side reaction was
rarely reported in earlier studies [41]. In the recent two years, however, severe propene
hydrogenation has been reported by different groups over this catalytic system [42–44].
It was suggested by Qi et al. [42] that there is an strict boundary for the gold particle
size (2 – 5 nm) to avoid propene hydrogenation and that contamination of alkali metals
like sodium in TS-1 is the reason why propene hydrogenation hardly occurs on Au/TS-1.
In the study by Oyama et al. [43, 44], the Au/TS-1 catalyst with an average gold particle size of 3 nm showed around 40% selectivity towards propane. After they treated
this Au/TS-1 catalyst with NaCN solutions for leaching, Au+ appeared and an 100% selectivity to propane arose. In our study, the activity of propene hydrogenation does not
seem to be dependent on the gold particle size. The catalysts with activity in propene
hydrogenation in this study show gold particle sizes ranging from sub-nanometer up to
6 nm. On the catalyst using TS-1 as the support (0.05-Au/TS-1), propene hydrogenation was also observed. Discrepancies in our study and literature stimulated us to try
to find the possible reasons for the propene hydrogenation. Blank experiments on the
as-recieved SiO2 , the hydroxylated SiO2 and the Ti-SiO2 -0 (see Experimental section)
showed no activity in any reaction, which excludes the influence of contaminants in the
silica (0.005 wt.% Fe2 O3 and 0.07 wt.% Na2 O as analyzed by the distributor) and the possibility of strained siloxane [45] formed during support preparation and calcination. The
gold oxidation state was analyzed for several catalysts in situ and gold was found in its
metallic state (detailed in Chapter 4). However, there are common features for our catalysts in propene hydrogenation: 1) propene hydrogenation can be switched off by carbon
monoxide at very low concentrations; 2) programmed reaction in propene and hydrogen
shows an peak activity at around 170 o C (see Chapter 4). The anomalous behaviour
in propene hydrogenation of the catalysts with inversely grafted Ti, when compared to
Au/SiO2 , implies the role of Ti–OH in this side reaction, which has been addressed by
Sykes et al. [46, 47]. On the other hand, the distinct activity in propene hydrogenation
between catalysts supported on Ti-SiO2 -1L and Ti-SiO2 -1 suggests that the grafting time
of Ti onto the silica plays an important role on the dispersion of Ti as mentioned by Gao
et al. [48]. It appears more to us that certain type of TiO x on the support contributes to
80
the activity in propene hydrogenation over our catalysts. However, this cannot explain
the propene hydrogenation activity of Au/MCM-41(Au< 0.01 wt.%) in the study by Qi
et al. [42]. Our results strongly indicate that propane formation occurs via the catalyst
support, but we cannot yet provide conclusive information on the reactive site. Further
investigation is provided in Chapter 4.
3.5 Conclusions
By adjusting the site synergy between Au and Ti, highly active catalysts in the direct
epoxidation of propene were achieved by using Ti-grafted silica as the support in this
study. These active catalysts have low gold loadings of around 0.2 wt.%. The tetracoordinated Ti sites were attained by lowering the Ti loading to 0.2 – 0.3 wt.% (ca. 0.1
Ti/nm2 ) on the silica. High dispersion of gold close to the tetra-coordinated Ti provides
sufficient Au–Ti interface leading to the high activity of these catalysts. The obtained
−1
PO formation rates in this study (120 – 130 gPO · kg−1
cat h ) are at the same level of the
highest rates reported on Au/TS-1. The PO/water ratio at these high PO rates ranges
between 10 % and 20 %. The active hydroperoxy species are competitively consumed
by epoxidation and hydrogenation at the Au–Ti interface. High propene concentrations
are favourable for a lower water formation rate and a higher PO formation rate. Propene
hydrogenation, if occurring, can be switched off by introducing a small amount of carbon
monoxide.
3.A Tables of catalyst performance
Sample ID
Loading
(wt%)
1.0-Au/Ti-SiO2 -1
Ti
Au
0.27
0.84
Au particle size
(nm)
a
2.5 ± 0.9 (2.4)
Temperature
Formation rate b
PO yield
Selectivity
(K)
(×10−7 mol · g−1
s−1 )
cat
(%)
(%)
PO
C3 H8
H2 O
403
1.43
1.31
43.62
433
2.69
0.21
108.3
1.09
7.76
18.43
H2 efficiency
(%)
PO
C3 H8
CO2
1.16
49.6
45.4
0.0
2.17
46.9
3.8
21.7
2.0
0.88
12.2
86.9
0.0
4.2
3.2
0.5-Au/Ti-SiO2 -1
0.48
2.4 ± 1.5 (1.9)
403
433
4.06
3.62
98.40
3.27
43.6
38.9
5.5
4.0
0.2-Au/Ti-SiO2 -1
0.20
1.7 ± 0.6 (1.6)
403
1.10
14.56
10.66
0.89
7.0
92.6
0.0
4.4
433
2.84
15.62
26.43
2.29
15.2
83.6
0.0
6.7
473c
5.44
4.59
113.1
4.66
40.2
33.9
9.4
–
403
1.72
31.33
18.32
1.38
5.2
94.2
0.0
3.5
433
3.84
30.83
35.43
3.09
10.9
87.7
0.0
5.8
473
5.47
15.23
60.46
4.41
24.7
68.8
1.9
7.2
403
433
1.25
2.78
28.51
29.96
13.45
23.36
1.01
2.23
4.2
8.4
95.3
90.7
0.0
0.0
3.0
5.2
473
5.36
17.21
47.14
4.32
23.1
74.1
0.2
8.3
0.1-Au/Ti-SiO2 -1
0.05-Au/Ti-SiO2 -1
0.09
0.05
–
–
3.A. TABLES OF CATALYST PERFORMANCE
Table 3.A.1: Metal loadings, gold particle sizes and catalytic performance of catalysts on the Ti-SiO2 -1 support
a. averaged size ± ‘standard deviation’, the ‘standard deviation’ is calculated by assuming a normal distrubution; the median is given in parentheses.
−1
−7
−1 −1
−1
= 20.9 gPO · kg−1
b. H2 :O2 :C3 H6 :He= 1 : 1 : 1 : 7, GHSV 10000 mL·g−1
cat h ; under this condition, 1 × 10 mol PO · gcat s
cat h .
c. hydrogen was fully combusted; propylene glycol was formed at this temperature.
81
82
Table 3.A.2: Metal loadings, gold particle sizes and catalytic performance of catalysts on the Ti-SiO2 -x (x = 2, 5, 10) supports
Sample ID
Au particle size
(nm)
Temperature
(K)
403
433
403
433
403
433
403
433
403
433
403
433
403
433
403
433
403
433
403
433
1.0-Au/Ti-SiO2 -2
0.44a
0.90
2.9 ± 1.1 (2.8)
0.5-Au/Ti-SiO2 -2
0.42
0.47
2.9 ± 1.1 (2.8)
0.2-Au/Ti-SiO2 -2
0.20
2.4 ± 1.0 (2.3)
0.1-Au/Ti-SiO2 -2
0.10
–
0.05-Au/Ti-SiO2 -2
0.05
–
0.80
3.5 ± 1.1 (3.5)
0.5-Au/Ti-SiO2 -5
0.43
3.1 ± 1.2 (3.0)
0.2-Au/Ti-SiO2 -5
0.19
3.1 ± 1.1 (3.2)b
0.1-Au/Ti-SiO2 -5
0.12
–
0.83
6.0 ± 1.4 (5.9)
1.0-Au/Ti-SiO2 -5
1.0-Au/Ti-SiO2 -10
1.07
1.78
Formation rate
(×10−7 mol · g−1
s−1 )
cat
PO
C3 H8
H2 O
1.28
2.94
1.57
3.72
1.39
3.32
0.62
1.35
0.34
0.69
0.85
1.47
0.96
1.24
0.59
1.01
0.33
0.62
0.56
0.73
1.09
0.70
1.60
1.40
2.30
2.67
1.72
1.80
1.75
2.83
0.37
0.45
0.29
0.17
0.32
0.41
0.56
0.80
5.61
8.44
26.84
90.07
20.25
63.48
9.59
26.75
2.34
6.60
2.10
5.06
9.93
28.95
7.55
12.79
2.84
6.71
1.52
3.82
5.71
14.40
PO yield
(%)
1.03
2.37
1.26
3.00
1.12
2.68
0.50
1.09
0.28
0.56
0.68
1.18
0.78
1.00
0.48
0.82
0.26
0.50
0.45
0.59
a. a different batch of support was used for the 1.0-Au/Ti-SiO2 -2 catalyst; Ti loading of this support was determined by XPS.
b. only 97 particles were counted from 40 TEM images.
Selectivity
(%)
PO
C3 H8 CO2
51.9
59.1
48.1
62.0
37.4
52.7
35.7
42.6
16.3
19.6
66.2
64.1
74.1
78.8
64.6
64.9
36.8
42.1
8.9
7.7
44.1
14.0
49.0
23.3
62.0
42.4
64.3
56.8
83.7
80.2
29.2
19.5
22.3
10.9
34.7
26.3
63.2
54.7
89.3
87.9
0.0
6.3
0.0
3.6
0.0
0.6
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
H2 efficiency
(%)
4.6
3.2
7.2
5.7
11.7
11.3
17.9
16.1
8.9
8.8
8.2
5.0
12.3
9.6
18.7
14.2
15.7
13.4
4.9
3.2
Loading
(wt%)
Ti
Au
REFERENCES
83
References
201, 128–137.
[3] C. Qi, T. Akita, M. Okumura and M. Haruta, Appl. Catal., A, 2001, 218, 81–89.
186–195.
[7] T. A. Nijhuis and B. M. Weckhuysen, Catal. Today, 2006, 117, 84–89.
C, 2008, 112, 1115–1123.
2007, 123, 189–197.
[11] J. Chen, S. J. A. Halin, J. C. Schouten and T. A. Nijhuis, Faraday Discuss., 2011, 152, 321–336.
[12] T. Fujitani, I. Nakamura, T. Akita, M. Okumura and M. Haruta, Angew. Chem. Int. Ed., 2009,
48, 9515–9518.
[14] I. X. Green, W. Tang, M. Neurock and J. T. Yates Jr., Angew. Chem. Int. Ed., 2011, 50, 10186–
10189.
[15] S. S. Dhingra (Dow Chemical), US 2010/0076208, 2010.
[16] R. G. Bowman (Dow Chemical), WO 2009/061623, 2009.
[17] B. S. Uphade, T. Akita, T. Nakamura and M. Haruta, J. Catal., 2002, 209, 331–340.
1548.
2006, 45, 412–415.
[20] B. Chowdhury, K. K. Bando, J. J. Bravo-Suárez, S. Tsubota and M. Haruta, J. Mol. Catal. A:
Chem., 2012, 359, 21–27.
[22] W.-S. Lee, M. C. Akatay, E. A. Stach, F. H. Ribeiro and W. N. Delgass, J. Catal., 2012, 287,
178–189.
[23] L. Y. Chen, G. K. Chuah and S. Jaenicke, J. Mol. Catal. A: Chem., 1998, 132, 281–292.
[24] B. Notari, Adv. Catal., 1996, 41, 253–334.
[25] P. Ratnasamy, D. Srinivas and H. Knözinger, Adv. Catal., 2004, 48, 1–169.
84
[27] O. A. Kholdeeva, I. D. Ivanchikova, M. Guidotti, C. Pirovano, N. Ravasio, M. V. Barmatova
and Y. A. Chesalov, Adv. Synth. Catal., 2009, 351, 1877–1889.
[29] D. Harrison, I. M. Coulter, S. Wang, S. Nistala, B. A. Kuntz, M. Pigeon, J. Tian and S. Collins,
J. Mol. Catal. A: Chem., 1998, 128, 65–77.
[30] M. Du, G. Zhan, X. Yang, H. Wang, W. Lin, Y. Zhou, J. Zhu, L. Lin, J. Huang, D. Sun, L. Jia
and Q. Li, J. Catal., 2011, 283, 192–201.
[31] A. J. H. P. van der Pol, A. J. Verduyn and J. H. C. van Hooff, Appl. Catal., A, 1992, 92, 113–130.
[32] R. Zanella, L. Delannoy and C. Louis, Appl. Catal., A, 2005, 291, 62–72.
[33] E. Sacaliuc-Parvulescu, H. Friedrich, R. Palkovits, B. M. Weckhuysen and T. A. Nijhuis, J.
Catal., 2008, 259, 43–53.
[34] E. Sacaliuc, A. M. Beale, B. M. Weckhuysen and T. A. Nijhuis, J. Catal., 2007, 248, 235–248.
[35] C.-H. Liu, Y. Guan, E. J. M. Hensen, J.-F. Lee and C.-M. Yang, J. Catal., 2011, 282, 94–102.
[36] K. G. Proctor, S. J. Markway, M. Garcia, C. A. Armstrong and C. P. Gonzales, in Colloidal Silica:
Fundamentals and Applications, ed. H. E. Bergna and W. O. Roberts, CRC Press, 2006, ch. 32,
pp. 385–396.
[37] C. Morterra, J. Chem. Soc., Faraday Trans. I, 1988, 84, 1617–1637.
[38] J. K. Edwards, A. F. Carley, A. A. Herzing, C. J. Kiely and G. J. Hutchings, Faraday Discuss.,
2008, 138, 225–239.
1586.
[44] J. Gaudet, K. K. Bando, Z. Song, T. Fujitani, W. Zhang, D. S. Su and S. T. Oyama, J. Catal.,
2011, 280, 40–49.
[45] V. K. Rajagopal, R. D. Guthrie, T. Fields and B. H. Davis, Catal. Today, 1996, 31, 57–63.
[46] E. C. H. Sykes, M. S. Tikhov and R. M. Lambert, Catal. Lett., 2002, 78, 7–11.
[47] E. C. H. Sykes, M. S. Tikhov and R. M. Lambert, J. Phys. Chem. B, 2002, 106, 7290–7294.
[48] X. Gao and I. E. Wachs, Catal. Today, 1999, 51, 233–254.
Switching off propene
hydrogenation in the direct
epoxidation of propene over
gold–titania catalysts
4
Part of this chapter has been published as:
J. Chen, S. J. A. Halin, D. M. Perez Ferrandez, J. C. Schouten and T. A.
Nijhuis, 2012, J. Catal. 285, 324–327.
Abstract
The study into site synergy of gold and titanium yielded catalysts with high performance
in propene epoxidation as well as catalysts where propene hydrogenation prevails. The
side reaction of propene hydrogenation over these gold–titania catalysts were studied
in details. The addition of a small amount of carbon monoxide to the feed gas can
completely switch off this propene hydrogenation, while at the same time also reducing
the rate of direct water formation. The formation rate of propene oxide was not affected
by addition of CO. The order of CO on this inhibiting effect is −1. Gold is not necessary
for this side reaction. The supports alone showed the same hydrogenation behavior as
the catalysts: 1) enhancement of propene hydrogenation by O2 ; 2) peak activity at ca.
443 K in propene and hydrogen during temperature programmed reaction; 3) switching
off by CO with an order of −1. The coordination environment of titanium and surface
hydroxyls may play an important role in propene hydrogenation.
86
CHAPTER 4. SWITCHING OFF PROPENE HYDROGENATION IN THE DIRECT EPOXIDATION OF PROPENE
4.1 Introduction
Gold nanoparticles or clusters supported on titanium-containing supports are capable of
epoxidizing propene to propene oxide (PO) by using hydrogen and oxygen at a high
selectivity [1–4]. However, propene hydrogenation may occur or even prevail under certain circumstances as reported in literature: the particle size of gold supported on TiO2
is smaller than 2 nm [1]; or, Ti-based oxides are used as supports while the gold size
is out of the range of 2–5 nm [5]; and recently, supported gold (+1) cyanide particles
were reported to show a high selectivity towards propene hydrogenation [6]. Significant
propane formation is rarely reported when titanium silicalite-1 (TS-1) is used as the support. Qi et al. [5] proposed that the contaminant of sodium on the catalysts is responsible
for inhibiting propene hydrogenation on Ti-based gold catalysts.
In Chapter 2, we have examined the effect of reactant concentrations on the epoxidation reaction over a Au/Ti-SiO2 catalyst. Based on insights provided in Chapter 2, we
performed a study concerning the synergy between gold and titanium sites by simply adjusting the metal loadings. The results on the catalyst optimization are given in Chapter
3. In literature, the trend for the gold–titania system is to go to low gold loadings with
highly isolated Ti4+ sites [7–10]. These low-loaded catalysts generally have a higher PO
productivity, although a higher reaction temperature (generally above 423 K) is needed.
The gold particle size of these catalysts is smaller. As a result, the relative amount of Au–
Ti interface may be higher, which is often seen as the active site [11–15]. The smaller
particle size, however, makes these catalysts more susceptible to propane formation [1].
Sacaliuc et al. [16] showed that the epoxidation activity of Au/Ti–SBA-15 can be related
to the differences in the amount of grafted Ti. The work by Delgass’s group [17] has
shown that Ti-defects in TS-1 can boost the activity of Au/TS-1 in direct propene epoxidation. It is also known that Ti-defects on a silica surface can stabilize the supported gold
nanoparticles or clusters [18, 19]. In our study, titanium was grafted onto a commercial
silica using titanium alkoxide as precursor by the surface sol–gel method. Gold was then
deposited to the calcined support by the deposition–precipitation method resulting in the
Au/Ti-SiO2 catalysts.
The aim of the study on the site synergy was to obtain a catalyst with a higher epoxidation activity and higher hydrogen efficiency. Although catalysts with higher epoxidation activities were obtained, significant propane formation was observed over almost all
the catalysts we prepared with a short grafting time for Ti, which consequently reduced
4.2. EXPERIMENTAL
87
the hydrogen efficiency to a large extent. On the other hand, we found that introducing
small amount of carbon monoxide in the reactant feed suppressed propene hydrogenation over these catalysts without affecting propene epoxidation. Here we provide the
evidence for the inhibiting effect of carbon monoxide on propene hydrogenation in the
direct epoxidation of propene over Au/Ti-SiO2 catalysts. The root cause of this side reaction, i.e. propene hydrogenation, is also investigated and discussed in this chapter.
4.2 Experimental
The catalysts synthesized in Chapter 3, which are active in propene hydrogenation, are
further investigated in this chapter. Thus the names of the catalysts follow the same
nomenclature given in the previous chapter. Since the preparation method used for
our catalysts does not introduce sodium, an amount of 300 mg of the 0.05 wt% Au/TiSiO2 (1% ML) catalyst (i.e. 0.05-Au/Ti-SiO 2 -1) was impregnated with sodium sulfate
solution and was dried in vacuum at room temperature overnight. The resulting catalyst
has a sodium loading of 0.5 wt%. The 0.05-Au/Ti-SiO 2 catalyst impregnated with 0.5
wt% sodium was used to validate the effect of sodium on suppressing propene hydrogenation in the direct propene epoxidation over gold–titania catalysts as proposed by Qi
et al. [5]. The reason for using Na2 SO4 is based on the proposal by Mul et al. that highly
acidic sites can be neutralized by a neutral salt while the active Ti site remains active [3].
Catalytic tests were performed in a flow setup equipped with a fast Interscience Compact
gas chromatography system (3 min analysis time) consisting a Porabond Q column and a
Molsieve 5A column in two separate channels, each with a thermal conductivity detector.
The quartz reactor is located in an oven and has an inner diameter of 6 mm. When ’reaction cycle’ is mentioned, catalyst regeneration in 10 vol.% oxygen diluted in helium at
573 K for 1 hour was performed between each cycle. Each reaction cycle normally lasted
for 5 hours.
The X-ray photoelectron spectroscopy (XPS) measurements were carried out with
a Kratos AXIS Ultra spectrometer, equipped with a monochromatic X-ray source and a
delay-line detector (DLD). Spectra were obtained using the aluminium anode (Al Kα =
1486.6 eV) operating at 150 W. A U-shape Pyrex tubular reactor (6 mm inner diameter)
with detachable valves on both ends was used for post-reaction catalysts. The fresh catalysts were activated for 4–5 hours at 423 K in flowing hydrogen, oxygen and propene
88
(diluted in helium) in the U-shape reactor. The temperature was maintained using an
oil bath. The post-reaction catalysts were then flushed in helium and afterwards sealed
in the U-shape reactor. The reactor was transferred into a nitrogen glove box free from
oxygen (< 2 ppm) and moisture (< 0.5 ppm). The post-reaction samples for XPS measurements were prepared in the glove box. Transport of the samples from the glove box
to the spectrometer was performed in an inert atmosphere by using a small nitrogenpurged chamber equipped with a magnetic arm. In this way, the post-reaction catalysts
were kept ‘in-situ’. The XPS spectra of these post-reaction catalysts were used to compare
with their counterparts prior to reaction.
Infrared spectra were recorded with a Bruker Vertex 70v spectrometer at a 2 cm−1
optical resolution. The samples were pressed in self-supporting discs (diameter: 12.7
mm, ca. 7.9 mg cm−2 ). Progressive adsorption of carbon monoxide at 90 K was carried
out by dosing carbon monoxide through a 50 µL gas-sample loop until a pressure of 1
mbar in the IR cell was reached. The IR cell was cooled in flowing liquid nitrogen.
4.3 Results and discussion
4.3.1 Activation of hydrogenation activity
The fresh calcined catalysts showed a gradual activation in their activity of propene hydrogenation during the first catalytic cycle. Figure 4.1 shows such a phenomenon of the
catalysts over the Ti-SiO2 -1 support. In the first cycle, the formation rate of PO started
at a high level from the very beginning, while the activity in the propene hydrogenation
experienced an activating period, whose duration mainly depended on the reaction temperature. In general, a higher reaction temperature led to a shorter activating period
for propene hydrogenation. For the catalysts supported on Ti-SiO2 -1 and Ti-SiO2 -2, this
activating period can only be seen in the first reaction cycle. Their activity in propene hydrogenation remained relatively stable in the sequential catalytic testings, between which
the regeneration in 10 vol% O2 at 573 K was performed, despite that sometimes a very
small increase (less than 10%) in propene hydrogenation could be observed along time.
The gold particle sizes of the catalysts shown in Figure 4.1 are small with an average size
less than 2 nm.
This take-off phenomenon in propene hydrogenation is much more obvious on the
catalysts over the supports with higher Ti loadings, e.g., Ti-SiO2 -5 and Ti-SiO2 -10. Figure
89
−7
−6
x 10
3
1.5
2
1.5
O
or rC
3
4.5
H
H2O
4.5
rH
rPO (mol⋅g−1
⋅s−1)
cat
PO
C3H8
(mol⋅g−1
⋅s−1)
cat
x 10
6
3 8
6
(a)
0
0
1
2
3
0
5
4
Time (h)
−7
−6
x 10
4.5
H2O
3
1.5
2
1.5
O
or rC
3
H
PO
C3H8
3 8
4.5
(mol⋅g−1
⋅s−1)
cat
x 10
6
rH
rPO (mol⋅g−1
⋅s−1)
cat
6
(b)
0
0
1
2
3
0
5
4
Time (h)
−6
−5
x 10
x 10
1
H2O
1.2
0.6
H
2
0.4
rH
rPO or rC
0.8
(mol⋅g−1
⋅s−1)
cat
0.8
PO
C3H8
O
1.6
3 8
(mol⋅g−1
⋅s−1)
cat
2
0.4
0.2
(c)
0
0
1
2
3
Time (h)
4
0
5
Figure 4.1: Time-on-stream performance in the reaction cycle for catalyst activation: (a)
0.05-Au/Ti-SiO 2 -1 at 423 K; (b) 0.05-Au/Ti-SiO 2 -1 at 483 K; (c) 0.2-Au/Ti-SiO 2 -1 at 473
K (gas feed 10 vol% hydrogen, 10 vol% oxygen, 10 vol% propene in helium, GHSV 10000
−1
mL·g−1
cat h ).
90
4.2 shows the activity in propene hydrogenation over the 1.0-Au/Ti-SiO 2 -10 catalyst, of
which the average size of gold nanoparticles is 6.0 ± 1.4 nm (see Chapter 3 for details),
in the first two reaction cycles. By the end of the second reaction cycle, the activity in
propene hydrogenation had increased by one magnitude when compared to the activity
at the very beginning of the first cycle. However, the formation rates of PO and water
showed very little change in the first two cycles. The irrelevance between PO formation and propane formation over these catalysts implies that the two catalytic reactions
may proceed over different sites. And this take-off phenomenon for propene hydrogenation indicates that this side reaction may be suppressed if the catalyst would remain its
original status at the very beginning of the first reaction cycle.
−6
1.2
x 10
0.8
0.6
rC
H
3 8
(mol⋅g−1
⋅s−1)
cat
1
0.4
0.2
cycle 1
cycle 2
0
0
1
2
3
4
5
Time (h)
Figure 4.2: Increasing activity in propene hydrogenation over the 1.0-Au/Ti-SiO 2 -10 catalyst during the first two catalytic cycles at 423 K (gas feed 10 vol% hydrogen, 10 vol%
−1
oxygen, 10 vol% propene in helium, GHSV 10000 mL·g−1
cat h ).
4.3.2 Switching off propene hydrogenation by CO
Carbon monoxide was found to be able to suppress propene hydrogenation while not
affecting the epoxidation over the catalysts investigated in Chapter 3. In this section, the
0.05-Au/Ti-SiO 2 -1 catalyst is used as an example to illustrate such an effect of CO. Figure
4.3 gives the general performance of 0.05-Au/Ti-SiO 2 -1 at different temperatures. The
conversion level of propene on this catalyst is very similar to the conversions reported by
Qi et al. over their Au/Ti-TUD-1 (Ti/Si= 2/100 ) catalysts with very low gold loadings
[5]. The conversion of propene on 0.05-Au/Ti-SiO 2 -1 decreased at higher temperatures,
which was mainly due to the decreased activity in propene hydrogenation. At ca. 433 K,
91
30
Conversion
Yield
Conversion, yield (%)
25
20
15
10
5
0
423
433
443
453
463
473
Temperature (K)
483
493
100
rest
PO
C3H8
Selectivity (%)
80
60
40
20
0
432
443
454
463
473
Temperature (K)
483
Figure 4.3: Catalyst performance of the 0.05-Au/Ti-SiO 2 -1 catalyst at different temperatures: (a) conversion of C3 H6 and yield to PO; (b) product selectivity (tested in 5-hour
reaction cycles, gas feed 10 vol% H2 , 10 vol% O2 and 10 vol% C3 H6 in helium, GHSV
−1
10000 mL·g−1
cat h , performance averaged between 150 min and 270 min in each cycle,
1-hour regeneration in 10 vol% O2 at 573 K between each cycle).
the selectivity towards propane was as high as 90%. The activity in the propene epoxidation monotonously increased when the reaction temperature was raised. The selectivity
towards oxygenates also increased at higher temperatures, which is in accordance with
the behaviour of a normal catalyst which is only active in propene epoxidation as discussed in Chapter 2.
In Figure 4.4, the effect of carbon monoxide addition on the formation rates of PO,
water and propane is shown for the 0.05-Au/Ti-SiO 2 -1 catalyst. When the experiment
started, the catalyst bed temperature increased sharply from ca. 463 K for over 10 K;
due to the exothermic reactions. The temperature stabilized at around 473 K after about
1 h. At t = 2.5 h, the catalyst bed temperature was raised to 483 K to determine the
−6
−6
x 10
5
4
0.6
3
0.4
2
1
2
O
H
3 8
0.8
or rC
x 10
rH
rPO (mol⋅g−1
⋅s−1)
cat
1
(mol⋅g−1
⋅s−1)
cat
92
PO
C3H8
0.2
H2O
0
0
5
Temperature (K)
493
10
Time (h)
15
0
483
473
463
453
Catalyst bed
Oven setting
0
5
10
Time (h)
15
Figure 4.4: Time-on-stream performance of the 0.05-Au/Ti-SiO 2 -1 catalyst (top) and the
catalyst bed temperature (bottom) with and without CO addition (gas feed 10 vol% H2 ,
10 vol% O2 and 10 vol% C3 H6 in helium, GHSV 10000 mL·g−1
h−1 , grayed area: 1 vol%
cat
CO introduced between 5 – 7.5 h, 10 – 12.5 h).
temperature influence on the formation rate of propane. From t = 2.5 h till the end of the
experiment, the oven temperature was kept constant. Carbon monoxide was introduced
into the gas feed at a low concentration of 1 vol% during the time intervals between
5 h – 7.5 h, 10 h – 12.5 h. The formation rate of propane dropped immediately by
two orders of magnitude without affecting the epoxidation reaction as carbon monoxide
was added. Meanwhile, water formation was also suppressed. Accordingly, the catalyst
bed temperature dropped by about 5 K. During the co-feeding of carbon monoxide, the
hydrogen efficiency increased from 8 % to 17 %. The conversion of carbon monoxide was
low at the level of 10 %. An increase in the formation rate of carbon dioxide was observed
when carbon monoxide was added, but this was only the result of carbon monoxide
conversion while not of propene combustion. Hydrogenation of carbon monoxide to
methane was not observed. Removing carbon monoxide restored the catalyst activity in
propene hydrogenation (7.5 h – 10 h, 12.5 h – 17.5 h). The result in the first 5 h showed
that propane formed at a higher rate when the reaction temperature was lower on this
catalyst. Therefore, the suppression of propane formation between 5 h – 7.5 h and 10 h
93
– 12.5 h can be completely attributed to carbon monoxide addition.
−6
−6
x 10
x 10
5
PO
C3H8
4
3
0.4
2
0.2
1
2
O
H
3 8
0.6
or rC
H2O
rH
rPO (mol⋅g−1
⋅s−1)
cat
0.8
(mol⋅g−1
⋅s−1)
cat
1
0
0
5
Temperature (K)
493
10
Time (h)
15
0
483
473
463
453
Catalyst bed
Oven setting
0
5
10
Time (h)
15
Figure 4.5: Time-on-stream performance of the 0.05-Au/Ti-SiO 2 -1 catalyst impregnated
with 0.5 wt% sodium (top) and the catalyst bed temperature (bottom) with and without
CO addition (gas feed 10 vol% H2 , 10 vol% O2 and 10 vol% C3 H6 in helium, GHSV 10000
−1
mL·g−1
cat h , grayed area: 1 vol% CO introduced between 7.5 – 10 h).
The effect of adding sodium to the 0.05-Au/Ti-SiO 2 -1 catalyst was also investigated
to compare the effect of the carbon monoxide addition on suppressing the hydrogenation reaction to the recently published approach of adding sodium [5]. The results are
shown in Figure 4.5. When the reaction started, the catalyst bed temperature increased
instantly from ca. 463 K to 468 K. The catalyst bed temperature was raised to around
480 K at t = 2.5 h by increasing the oven temperature. Carbon monoxide (1 vol%) was
introduced into the gas feed from t = 7.5 h till the end (t = 10 h). As seen from Figure
4.5, the existence of sodium has little effect on propene hydrogenation over our catalyst.
However, the formation of PO and water was suppressed after sodium sulfate impregnation comparing to the original activity shown in Figure 4.5. It should be noted here
that both sodium and sulfate might have great potential in modifying the catalyst acidity [20]. Figure 4.5 clearly shows that addition of carbon monoxide inhibited propene
hydrogenation without affecting propene epoxidation over the sodium-impregnated catalyst while sodium had little effect on hydrogenation. The work by Haruta’s group [5]
94
proposed that propane formation could be reduced by sodium addition to the catalysts.
Our results show that such a modification to the catalyst is apparently not as simple as
it seems. Carbon monoxide addition, however, is a very simple and highly effective way
to block propane formation (for the case where propene hydrogenation is unavoidable
in the propene epoxidation under an atmosphere of hydrogen and oxygen), which was
determined to be effective for all the catalysts in this study.
−5
10
0.05% 0.1%
CO
CO
0.2%
CO
0.3%
CO
0.5%
CO
0%
CO
−6
10
−7
10
PO
C3H8
(a)
H2O
−8
10
0
2.5
5
7.5
Time (h)
10
12.5
−6
−6
x 10
x 10
5
4
0.6
3
0.4
2
1
2
O
H
3 8
0.8
or rC
(b)
rH
rPO (mol⋅g−1
⋅s−1)
cat
1
(mol⋅g−1
⋅s−1)
cat
r (mol⋅g−1
⋅s−1)
cat
0%
CO
0.2
PO
CH
3 8
H2O
0
0
2.5
5
Temperature (K)
493
7.5
Time (h)
10
12.5
0
(c)
483
473
463
453
Catalyst bed
Oven setting
0
2.5
5
7.5
Time (h)
10
12.5
Figure 4.6: Effect of CO concentrations (volumetric) on the formation rates of propene
oxide, propane and water over the 0.05-Au/Ti-SiO 2 -1 catalyst: (a) formation rates in
logarithmic scale; (b) formation rates in decimal scale; (c) catalyst bed temperature (gas
feed 10 vol% H2 , 10 vol% O2 and 10 vol% C3 H6 in helium, CO introduced when required,
−1
cat h ).
95
Figure 4.6 shows the effect of CO concentrations on propene hydrogenation over the
0.05-Au/Ti-SiO 2 -1 catalyst. Over this catalyst while the addition of 0.05 vol% of carbon
monoxide can already reduce 80% of propane formation on our catalysts, a concentration
of 1 vol% for carbon monoxide may be needed to keep the ratio of propane and PO
formation rates below 0.05.
Such an effect of CO on propene hydrogenation is not only valid for the catalysts
supported on Ti-SiO2 , but also true for the 0.05-Au/TS-1 catalyst studied in Chapter 3.
Figure 4.7 shows how CO suppressed the propane formation during the direct epoxidation of propene over 0.05-Au/TS-1. The overal activity of this 0.05-Au/TS-1 catalyst
was very low. The hydrogen efficiency was around 10 %. When no CO was introduced,
the conversion of propene was 0.75 % at 473 K. The main side products were propane
and propanal. The selectivity towards propane and propanal was 20 % and 17 %, respectively. When CO was introduced, the propane formation was completely suppressed,
while the rates for PO and propanal were not affected. The concurrent decrease in water
and propanal formation in the first 2 hours as shown in Figure 4.7 indicates that some
strong acidic Ti−OH sites were passivated probably due to strong adsorption of reaction products. These acidic Ti−OH sites close to gold normally cause additional water
formation and PO isomerization.
−7
−6
0.8
0.8
0.6
0.6
PO
C3H8
0.4
2
rH
H
rPO, rC
0.4
H2O
(mol⋅g−1
⋅s−1)
cat
x 10
1
O
x 10
3 8
or rpropanal (mol⋅g−1
⋅s−1)
cat
1
propanal
0.2
0
0
0.2
1
2
3
4
Time (h)
5
6
7
0
Figure 4.7: Inhibiting effect of CO on propene hydrogenation over the 0.05-Au/TS-1
catalyst (at 473 K, gas feed 10 vol% H2 , 10 vol% O2 and 10 vol% C3 H6 in helium, 1 vol%
−1
CO introduced between 2.5 h – 5 h, GHSV 10000 mL·g−1
cat h )
96
−6
1.0−Au/SiO2−Ti−5
−7
10
rC
H
3 8
(mol⋅g−1
⋅s−1)
cat
10
−8
10
1
10
2
3
10
10
4
10
PCO (ppm)
Figure 4.8: Inhibition effect of CO on propene hydrogenation over different catalysts (gas
feed 10 vol% H2 , 10 vol% O2 and 10 vol% C3 H6 in helium, CO concentration varied,
−1
o
o
cat h ; 200 C for 0.05-Au/Ti-SiO 2 -1, 130 C for 1.0-Au/Ti-SiO 2 -2,
o
150 C for 1.0-Au/SiO 2 -Ti-5 and 1.0-Au/Ti-SiO 2 -10).
There is no clear correlation between the activity in propene hydrogenation and the
amount of gold or titanium loaded. On the other hand, the activity in propene hydrogenation can change during reactions or simply after storage for a long time and seems
irrelevant to PO formation. The catalytic performance of the catalysts studied in Chapter
3 does not support the hypothesis that there is a strict boundary for the gold particle
size as proposed in literature [5]. Adding sodium also had no influence on propene hydrogenation as shown in Figure 4.5. Nevertheless, CO can effectively suppress propene
hydrogenation as long as it happens over the gold–titania catalysts. Meanwhile, it does
not affect the reaction rate of PO formation. The order of CO on this inhibiting effect
is determined to be −1.06 ± 0.13 over different catalysts as shown in Figure 4.8. The
reaction rates of propene hydrogenation under conditions given in Figure 4.8 were very
stable. Due to the exothermicity of the three reactions, i.e., water formation, PO formation and propene hydrogenation, the formation rates of propane under different CO
concentrations were those at a constant temperature within an error of ±1 o C. In other
words, they were those at relatively high CO concentrations (still ppm level) where the
change in temperature due to the change in reaction rates became small. The conversion
of CO to CO2 was not tracked for the experiments with CO concentrations (introduced)
97
below 103 ppm. However, the combustion of CO to CO2 was limited since CO could still
be observed by GC in all cases.
4.3.3 Probing the active site for propene hydrogenation
Oxidation state of gold and titanium
There are concerns about the oxidation states of gold and titanium which may contribute
to propene hydrogenation over the gold–titania catalysts: 1. oxidized gold, i.e., Au+ as
proposed by Oyama et al. [6]; 2. reduced titanium, e.g., Ti3+ suggested by Sykes et al.
[21, 22], hydride complexes of titanium proposed by Yermakov et al. [23]. In section
4.3.1, the activation of catalysts for propene hydrogenation has been described. If the
activation period for propene hydrogenation is due to the change in oxidation states of
gold and/or titanium, this change may be observed by comparing the fresh catalysts and
their counterparts under reactions for a couple of hours. Pseudo in-situ XPS experiments
were thus performed over three representative catalysts: 1.0-Au/SiO2 -Ti-5, 1.0-Au/TiSiO2 -10, and 0.2-Au/Ti-SiO 2 -1. Their catalytic performances are listed in the appendix
of Chapter 3. The 1.0-Au/SiO 2 -Ti-5 catalyst was prepared by grafting Ti onto a 1 wt.%
Au/SiO2 catalyst and gained activity in propene hydrogenation after grafting Ti.
8
7.5
PO
C H
3
8
Yield (%)
6
4
2
1.06
0
0.25
0
1.0 wt% Au/Ti-SiO
2
1.0-Au/SiO -Ti-5
2
Figure 4.9: Comparison of catalytic performance between the 1 wt.% Au/Ti-SiO 2 catalyst
investigated in Chapter 2 for kinetic studies and the 1.0-Au/SiO 2 -Ti-5 catalyst by inverse
Ti grafting in Chapter 3 (423 K, gas feed 10 vol% H2 , 10 vol% O2 and 10 vol% C3 H6 in
−1
helium, GHSV 10000 mL·g−1
cat h , performance averaged between 150 min and 270 min
in a 5-hour reaction cycle)
98
Au
4f7/2= 83.3 eV
as prepared
Ti
as prepared
(1.35)
2p3/2= 459.0 eV
(2.85)
∆ = 3.68 eV
∆ = 5.70 eV
4f7/2= 83.3 eV
activated
2p3/2= 458.9 eV
activated
(1.49)
(2.81)
∆ = 3.64 eV
4f7/2= 83.4 eV
as prepared
(1.48)
∆ = 3.71 eV
4f7/2= 83.4 eV
activated
(1.47)
∆ = 3.71 eV
x4
as prepared
4f7/2= 83.4 eV
(1.90)
Intensity (arb. units)
Intensity (arb. units)
∆ = 5.68 eV
2p3/2= 459.0 eV
as prepared
(2.71)
∆ = 5.65 eV
2p3/2= 459.1 eV
activated
(2.76)
∆ = 5.65 eV
x4
as prepared
2p3/2= 459.5 eV
(2.70)
∆ = 3.70 eV
activated
x4
4f7/2= 83.4 eV
(1.61)
∆ = 5.51 eV
activated
x4
2p3/2= 459.5 eV
∆ = 3.70 eV
92
90
88 86 84 82
Binding energy (eV)
80
(2.99)
∆ = 5.50 eV
470
465
460
455
Binding energy (eV)
450
Figure 4.10: XPS spectra of Au 4f and Ti 2p for catalysts before and after reaction (‘as
prepared’: fresh and flushed in helium at 423 K for 1 hour; ‘activated’: after 5-hour
reaction in 2 % H2 , 2 % O2 and 10 % C3 H6 at 423 K and then flushed in helium at 423 K
for 1 hour; spectra referenced to Si 2p line at 103.3 eV).
Figure 4.10 shows the XPS spectra of Au 4f and Ti 2p lines for the selected catalysts
before and after reaction. Trivial difference can be told from the comparison of Au 4f lines
or Ti 2p lines. The gold atoms are determined to be Au0 and Ti is determined to be Ti4+ .
The 1.0-Au/SiO 2 -Ti-5 catalyst showed an significant increase in intensity of Au 4f line
after reaction, which is most likely due to the observed desorption of acetone (converted
by gold from 2-propanol used for Ti grafting since no calcination was performed, see
Experimental in Chapter 3 for details) and 2-propanol during heating up to the reaction
temperature. The existence of oxidized gold is doubtful under the reaction atmosphere
containing H2 and C3 H6 . Infrared spectroscopy using CO as probe molecule was also
implemented to check if oxidized gold can be formed by calcination in O2 .
99
0.04
(a) 1.0
O2
5
act vate ex s t
s.
0.03
CO OH
0.02
CO ph s sor e
0.01
CO press re
0
2200
2150
aven m er (cm
2100
2050
1
0.04
( ) 1.0
s.
0.03
O2
5 CO OH
a ter calc nat on n O2
0.02
CO
4+
CO ph s sor e
0.01
CO press re
0
CO
2200
d+
CO
2150
aven m er (cm
0
2100
2050
1
Figure 4.11: Difference infrared spectra of progressive CO adsorption on the 1.0Au/SiO2 -Ti-5 catalyst at 90 K (the spectrum before CO dosing in each experiment was
taken as the background for substraction; CO pressure: from 0.07 to 1.0 mbar, ca. 0.07
mbar interval; in ‘a’, the sample was activated ex-situ in a mixture of H2 /O2 /C3 H6 at 423
K and then dehydrated at 573 K in-situ; ‘b’ was performed after ‘a’, the same pellet was
used, calcination was performed in-situ at 573 K and then evacuated at 573 K for 10
min.)
Figure 4.11 shows the progressive CO adsorption on 1.0-Au/SiO 2 -Ti-5. For the activated catalyst, it seems that both Au and Ti sites were occupied by organic components
and no adsorption was observed for CO. After calcination in O2 , the carbonaceous surface was cleaned. The band at 2179 cm−1 is assigned to CO adsorbed on Ti4+ . The broad
band between 2100 and 2140 cm−1 is composed of contributions from CO on Auδ+ and
Au0 . Progressive CO adsorption on 1.0-Au/Ti-SiO 2 -10 showed similar IR spectra to Figure 4.11. In Figure 4.2, the 1.0-Au/Ti-SiO2-10 catalyst showed continuous increase in
hydrogenation activity in the first 2 reaction cycles, between which calcination in O2 was
100
performed. It is very unlikely that such a phenomenon can be attributed to oxidized gold.
In-situ electron paramagnetic resonance spectroscopy (EPR, 9.4 GHz, 2.0 mW, RT or 110
K) was also performed to detect Ti3+ if there is any. However, no signal for paramagnetic species was observed for the activated catalysts. In summary, no obvious change in
oxidation states of gold and titanium was observed.
Effect of hydroxyls and O2
Although the root cause of propene hydrogenation as a side reaction remains obscure,
there is a very important feature for the catalysts active for propene hydrogenation. Without oxygen, the time-on-stream performance in propane formation simply declines. Another consideration hinted by the work of Sykes et al. [21, 22] is that for their reduced
TiO2 certain degree of hydroxylation is needed for the hydrogenation activity. If hydroxyls play a role in propene hydrogenation, temperature programmed reaction in C3 H6 and
H2 only may be used to validate this hypothesis. The 0.05-Au/Ti-SiO 2 -1 catalyst is again
used as the first example for illustration here.
Figure 4.12 shows the results of an experiment designed to illustrate the effect of
hydroxyls and O2 . In the first hour, the temperature of catalyst bed was raised from
303 K to 473 K with a ramping rate of 3 K/min. The activity in propene hydrogenation
reached its peak at ca. 443 K and then started to decrease. CO was introduced in period
III and propene hydrogenation was consequently switched off. In period IV, water vapor
was introduced with an attempt to restore the activity in propene hydrogenation since the
hydroxylation of Ti−O−Ti or Ti−O−Si was deemed important for this reaction. However,
water vapor had no effect on restoring the declined hydrogenation activity. Thus, in
period V, the reaction feed was replaced by 2 vol.% H2 and 2 vol.% O2 . Water was formed
and the bed temperature increased significantly in period V. After the treatment in H2 and
O2 , the hydrogenation activity of this catalyst was checked in the last hour (after t = 17.5
h). The hydrogenation activity was restored and showed the same pattern as the activity
during t = 1–3 h. Attempts to restore the hydrogenation activity by treating with pure H2
or O2 were also performed. However, these attempts failed to restore the catalyst activity
in propene hydrogenation.
Figure 4.13 shows the temperature programmed reaction in H2 and C3 H6 but without O2 over different catalysts. No matter how active these catalysts are in propene
hydrogenation, all of them showed a peak activity at 443 ± 5 K, a temperature where
101
−6
2.5
x 10
II
III
CO
0
2
4
6
8
10 12
Time (h)
14
16
18
20
0
2
4
6
8
10 12
Time (h)
14
16
18
20
I
III
CO
II
II
III
CO
IV V II
1.5
1
rC
H
3 8
(mol⋅g−1
⋅s−1)
cat
2
0.5
0
225
200
Temperature (oC)
175
150
125
100
75
50
25
Figure 4.12: Propane formation rate (top) and catalyst bed temperature (bottom) over
the 0.05-Au/Ti-SiO 2 -1 catalyst in 10 vol% C3 H6 and 10 vol% H2 (Prior to the experiment,
the catalyst was hydroxylated in 2 vol% H2 + 2 vol% O2 at 423 K for 1 hour and was
then flushed in helium at 423 K for 1 hour, after which the catalyst was cooled down to
303 K. I: temperature programmed reaction, 3 K/min from 303 K to 478 K; II: no CO
introduced; III: 1 vol% CO introduced; IV: no CO, 0.5 vol% water vapor introduced; V:
no CO, re-hydroxylation in 2 vol% H2 + 2 vol% O2 instead of the reaction atmosphere
−1
of 10 vol% H2 + 10 vol% C3 H6 . GHSV 10000 mL·g−1
cat h , helium balance. II, III, IV, V,
constant oven setting temperature).
either dehydroxylation or removal of water with a stronger adsroption occurred. Even
for the 0.05-Au/TS-1 catalyst, whose activity in propene hydrogenation is very low, such
a temperature-related feature can be observed.
There are also some other interesting phenomena during propene hydrogenation
without O2 when CO was introduced or in a case where the reaction started at a different temperature but after an identical pretreatment. Figures 4.14 and 4.15 show such
phenomena using 1.0-Au/SiO 2 -Ti-5 as an example. In Figure 4.14, the activity in propene
102
2.5
x 10
−6
x 10
1
o
−8
−7
1
x 10
x 10
1
−7
200 ( C)
o
0.5
2
150
0.6
0.6
0.05−Au/TS−1
0.4
0.4
2
100
0.2
0.2
50
50
60
0
180
120
0
0
60
Time (min)
x 10
1
−6
1
o
C3H8
1
0.2
(mol⋅g−1
⋅s−1)
cat
H
0.4
100
2
2
3 8
2
rC
0.6
O
150
rH
Temperature
0.8
(mol⋅g−1
⋅s−1)
cat
0.8
H2O
3
0
200 ( C)
C3H8
0.8
H2O
Temperature
0.6
150
0.6
2
0.4
0.4
100
0.2
50
0
x 10
1
o
200 ( C)
rC
H
3 8
(mol⋅g−1
⋅s−1)
cat
4
−7
x 10
2
x 10
−7
rH
5
0
180
120
Time (min)
−7
(mol⋅g−1
⋅s−1)
cat
0
O
0
0.8
3 8
Temperature
(mol⋅g−1
⋅s−1)
cat
(mol⋅g−1
⋅s−1)
cat
H
0.2
CH
HO
3 8
rC
2
H
0.4
rC
100
1
0.6
(mol⋅g−1
⋅s−1)
cat
O
1.5
200 ( C)
0.8
rH
(mol⋅g−1
⋅s−1)
cat
Temperature
150
3 8
0.8
2
rH
3 8
HO
O
CH
2
0.2
50
60
120
Time (min)
0
180
0
0
60
120
0
180
Time (min)
Figure 4.13: Temperature programmed reaction in hydrogen and propene for different
catalysts (pretreatment: the catalyst was hydroxylated in 2 vol.% H2 + 2 vol.% O2 at
423 K for 1 hour and then flushed in helium at 423 K for 1 hour; reaction conditions:
−1
10 vol.% H2 + 10 vol.% C3 H6 , helium balance, GHSV 10000 mL·g−1
cat h , ramping rate 3
K/min)
hydrogenation showed the identical pattern to Figure 4.13 in the first 150 min, that is, a
peak activity at ca. 443 K and then a continuous decay. When a very small amount of CO
was introduced, the hydrogenation activity was suppressed to a low level but remained
constant. There were some water formed in the period when 200 ppm and 500 ppm CO
was introduced. The origin of this water is unclear since there is no O2 in the gas feed
and the GC used in this study is also very sensitive to O2 at ppm level. Probably there
was a trace amount of contaminant from air remained in the gas pipelines. In the other
temperature programmed experiments where CO was co-fed from the very beginning,
propene hydrogenation was suppressed at a very low level all the time (not shown).
Figure 4.15 shows the activity evolution in another temperature programmed experiment over 1.0-Au/SiO 2 -Ti-5. After pretreatment in H2 and O2 , the reaction started at 403
K (130 o C) instead of 303 K. The reaction temperature was kept at 403 K (130 o C) for 30
min and then temperature ramping started. At 403 K, the activity also kept decreasing.
Once the temperature started to increase, the reaction rate increased accordingly and
103
reached its maximum at ca. 443 K, after which decay started again.
−7
−7
3.5
x 10
x 10
1
o
C3H8
C3H6 an 10
H2
1.4
0.6
500 ppm CO 10
10
10 H2 0 H2
100
CO
0.7
0.2
0 ppm
200 ppm 500 ppm
50
0
C3H6 0.4
H2
0
60
120
180
240
Time
me (min)
(m n)
300
O
10
2.1 150
2
emperat re
(mol⋅g−1
⋅s−1)
cat
0.8
H2O
rC
H
3 8
(mol⋅g−1
⋅s−1)
cat
2.8
rH
( C)
200
0
360
Figure 4.14: Effect of CO on propene hydrogenation without O2 (1.0-Au/SiO 2 -Ti-5, pretreatment: the catalyst was hydroxylated in 2 vol.% H2 + 2 vol.% O2 at 423 K for 1
hour and then flushed in helium at 423 K for 1 hour. reaction conditions given on graph,
−1
cat h )
−7
6
−7
x 10
x 10
1
o
0.8
H2O
Temperature
0.6
0.4
100
2
2.4
150
rH
3.6
(mol⋅g−1
⋅s−1)
cat
C3H8
O
200 ( C)
rC
H
3 8
(mol⋅g−1
⋅s−1)
cat
4.8
1.2
0.2
50
0
0
60
120
0
180
Time (min)
Figure 4.15: Temperature programmed reaction in hydrogen and propene over 1.0Au/SiO2 -Ti-5. (pretreatment: the catalyst was hydroxylated in 2 vol.% H2 + 2 vol.%
O2 at 423 K for 1 hour and then flushed in helium at 423 K for 1 hour; reaction condi−1
tions: 10 vol.% H2 + 10 vol.% C3 H6 , helium balance, GHSV 10000 mL·g−1
cat h , ramping
rate 3 K/min)
Both the experiments in Figure 4.14 and Figure 4.15 are highly reproducible in terms
of the quantified reaction rate. But when one compares the reaction rates at 403 K(130
o
C) or 443 K(170 o C) (where the activity reached its maximum) for the same catalyst
in Figure 4.14 and Figure 4.15, they are quite different. It seems that propene hydro-
104
genation over these catalysts are very sensitive to the amount of hydroxyls present on
the surface. The history of catalyst treatment heavily influences the activity in propene
hydrogenation. Later in this chapter, the high reproducibility after identical pretreatment
will also be shown.
In order to validate the importance of dehydroxylation or dehydration at ca. 443 K to
propene hydrogenation, another experiment was performed over the 1.0-Au/SiO 2 -Ti-5
catalyst for illustration. The results are given in Figure 4.16. The fresh catalyst showed
no activity in propene hydrogenation at 423 K without hydroxylation or hydration in H2
and O2 . Pretreatment in 2 vol% water vapor only (diluted in helium) at 423 K showed
no help either. Thus, the catalyst was activated in 2 vol% H2 and 2 vol% O2 at 423 K for
3 hours. The water formation rate increased along time and gradually stabilized. The
change in water formation rate may be attributed to the gradual removal of adsorbed
propoxy species due to 2-propanol used in grafting Ti on this catalyst (see Experimental
in Chapter 3 for details). After drying in helium at 423 K, the activated catalyst still
showed a relatively high activity in propene hydrogenation (ca. 5 % conversion of C3 H6
to C3 H8 at t = 210 min) as shown in Figure 4.16(a). The activity in propene hydrogenation continued to decrease in only C3 H6 and H2 . After O2 was co-fed, the formation rate
of propane was increased as shown in Figure 4.16(a). However, after drying in helium
at 573 K, the activated catalyst showed almost no activity in propene hydrogenation as
shown in Figure 4.16(b). Co-feeding of O2 boosted propene hydrogenation. It is clear
that hydrogen dissociation was enhanced by the presence of O2 and that the removal of
hydroxyls or water had a negative impact on propene hydrogenation. However, still, the
observation of simultaneous dehydroxylation or dehydration and decrease in hydrogenation activity can only be considered as an indirect evidence.
Propene hydrogenation over the supports
Since the dehydroxylation or dehydration is only relevant to the support, if gold nanoparticles are not the only source for hydrogen dissociation, propene hydrogenation should be
observed on the supports only. This is true. Parallel to the experiment performed on the
0.05-Au/Ti-SiO 2 -1 catalyst as shown in Figure 4.12, the same experiment was performed
over the Ti-SiO2 -1 support. The result is given in Figure 4.17. The Ti-SiO2 -1 support
showed exactly the same catalytic features as the 0.05-Au/Ti-SiO 2 -1 catalyst in propene
hydrogenation as shown in Figure 4.12. The support itself showed an even higher hy-
105
-6
3.0x10
10% H +10% C H +
/s)
(a)
2
6
2
o
He, 150 C
cat
Formation rate (mol/g
3
o
10% O , 150 C, 2 hours
-6
2% H
2.0x10
2
0.5 hours
+ 2% O
H O
2
2
o
150 C, 3 hours
10% H
2
+ 10% C H
3
6
o
150 C, 2 hours
C H
-6
1.0x10
3
8
PO x 5
0.0
0
60
120
180
240
300
360
420
t (min)
-6
3.0x10
/s)
(b)
o
Formation rate (mol/g
cat
He, 300 C
H O
2
0.5 hours then
2% H
-6
2
2.0x10
+ 2% O
2
o
10% H +10% C H +
cool down
2
150 C, 3 hours
3
6
o
10% O , 150 C, 2 hours
2
10% H
2
+ 10% C H
3
6
o
150 C, 2 hours
C H
3
-6
1.0x10
8
PO x 5
0.0
0
60
120
180
360
420
480
540
t (min)
Figure 4.16: Effect of hydroxyls and O2 on propene hydrogenation (1.0-Au/SiO 2 -Ti-5,
−1
cat h )
drogenation activity at ca. 443 K or after restoring the activity by H2 /O2 treatment.
Further check of the hydrogenation activity over different supports by the temperature
programmed reaction in H2 and C3 H6 also confirmed the catalytic features shown in Figure 4.13 over the gold-containing catalysts. That is, a peak activity at ca. 443 K and
activity decay when O2 is not present. It is clear that in the presence of O2 , H2 can
dissociate on the support. Gold is not necessary for propene hydrogenation over the catalysts investigated. This explains why the propene epoxidation was not affected when
propene hydrogenation was switched off. It seems that propene hydrogenation is merely
catalyzed over the support, which is consistent with the findings by Sykes et al. [21, 22].
106
−6
2.5
x 10
I
III
CO
II
II
III
CO
II
IV
II
III
CO
II
1.5
1
rC
H
3 8
(mol⋅g−1
⋅s−1)
cat
2
0.5
0
0
2
4
6
8 10 12 14 16 18 20 22 24
Time (h)
0
2
4
6
8 10 12 14 16 18 20 22 24
Time (h)
225
200
Temperature (oC)
175
150
125
100
75
50
25
Figure 4.17: Propane formation rate (top) and catalyst bed temperature (bottom) over
the Ti-SiO2 -1 support in 10 vol% C3 H6 and 10 vol% H2 (Prior to the experiment, the
support was hydroxylated in 2 vol% water vapor in helium at 423 K for 2 hours and then
in 2 vol% H2 + 2 vol% O2 at 423 K for 1 hour, after which the support was flushed in
helium at 423 K for 1 hour and then cooled down to 303 K. I: temperature programmed
reaction, 3 K/min from 303 K to 478 K; II: no CO introduced; III: 1 vol% CO introduced;
IV: no CO, re-hydroxylation in 2 vol% H2 + 2 vol% O2 instead of the reaction atmosphere
−1
of 10 vol% H2 + 10 vol% C3 H6 . GHSV 10000 mL·g−1
cat h , helium balance. II, III, IV,
constant oven setting temperature).
The order of CO on inhibiting propene hydrogenation was also checked over the
Ti-SiO2 -10 support. The results are summarized in Table 4.1 and Figure 4.18. Over
the Ti-SiO2 -10 support, the order of CO on propene hydrogenation gives −0.91, which
is almost the same as the order on the gold-containing catalysts. The CO order was
calculated based on low formation rates of propane to exclude the influence from the
temperature. What should be mentioned here is that the highest rates shown in Figure
4.18 for each catalyst or support were already less than 20 % of the formation rates of
107
propane when there was no CO present.
Table 4.1: CO order of the inhibiting effect on propene hydrogenation
Sample ID
0.05-Au/Ti-SiO2 -1
0.05-Au/Ti-SiO2 -1
T (K)
Reaction conditionsa
473
10/10/10 H2 /C3 H6 /O2
1.4 × 10
−6
−6
−1.26
rC
H
3 8
(mol·g−1
h−1 )
cat
b
CO order
−1.10
473
10/10/10 H2 /C3 H6 /O2
2.9 × 10
1.0-Au/Ti-SiO2 -2
403
10/10/10 H2 /C3 H6 /O2
1.4 × 10−7
−0.99
1.0-Au/SiO2 -Ti-5
423
10/10/10 H2 /C3 H6 /O2
1.1 × 10−6
−1.01
1.0-Au/Ti-SiO2 -10
423
Ti-SiO2 -10
473
c
10/10/10 H2 /C3 H6 /O2
1.2 × 10
−6
−0.94
10/10/0.2 H2 /C3 H6 /O2
3.3 × 10−6
−0.91
−1
a. GHSV 10000 mL·g−1
cat h .
b. formation rate when CO was not introduced
c. catalyst re-prepared after the support was aged for 9 months
−6
Ti−SiO2−10
−7
10
rC
H
3 8
(mol⋅g−1
⋅s−1)
cat
10
−8
10
1
10
2
3
10
10
4
10
PCO (ppm)
Figure 4.18: Inhibition effect of CO on propene hydrogenation over the Ti-SiO2 -10 support (473 K, gas feed 10 vol% H2 , 10 vol% C3 H6 and 0.2 vol% O2 in helium, CO concen−1
tration varied, GHSV 10000 mL·g−1
cat h ; for the testing conditions for the gold catalysts,
see Figure 4.8 or Table 4.1).
What caused deactivation in propene hydrogenation
If one looks at period II in Figure 4.17 or Figure 4.12 after restoring the activity by H2 /O2
treatment, a question may arise. That is: even though the formation rate of propane may
be sensitive to the amount of hydroxyls or water on the surface, what causes a fast de-
108
activation at a constant temperature where the population of surface hydroxyl or water
molecules should be relatively constant. In Figure 4.15, the deactivation can also be observed at a constant temperature below 443 K and is slower than the deactivation rate
above 443 K. In order to answer this question, experiments were performed as shown in
Figure 4.19 over the Ti-SiO2 -10 support. The original thought behind was that propene
may form propoxy species on the surface and thus deactivates the hydrogenation activity as proposed for the propene hydrogenation over H-ZSM-5 [24, 25]; or that H2 may
further reduce the active species causing deactivation; or that deactivation is only due
to dehydroxylation or dehydration. The time-on-stream performance of cycles 2, 4 and
6 (interval flush with C3 H6 ) in Figure 4.19(b) confirmed that propene caused the deactivation in propene hydrogenation. It can be seen that flush in propene lowered the
hydrogenation activity significantly. Another interesting phenomenon not shown here
is that when CO and C3 H6 were co-fed during the interval flushing, the hydrogenation
activity could not be lowered. Flush in pure helium or 10 vol% H2 as shown in Figure
4.19(a) did not lead to an obvious change in activity. This is in accordance with our
experience that storage under ambient conditions or flush in inert atmosphere does not
lower the hydrogenation performance of the activated catalysts or supports. Since CO
can effectively suppress propene hydrogenation in a reversible way, it is very likely that
CO adsorbs on or interacts with the active sites for propene hydrogenation much stronger
than C3 H6 . But this contradicts the hypothesis that propene may form propoxy species
causing deactivation in hydrogenation activity since this form of deactivation should be
irreversible and should prevail over the reversible effect of CO. That is to say, the formation of propoxy species cannot explain at the same time these phenomena: the reversible
effect of CO, a constant formation rate of propane when CO is fed as illustrated by Figure
4.14, and the deactivating effect of C3 H6 .
Further investigation was performed by means of IR spectroscopy. Results are given
in Figure 4.20. The Ti-SiO2 -10 support was first activated at 423 K to obtain a high
activity in propene hydrogenation. The sample was then investigated in-situ by IR after prolonged dehydroxylation and contact with C3 H6 at 473 K, which mimicked the
conditions in Figure 4.19(b). The extent of dehydroxylation at 473 K in vacuum for a
prolonged period was very limited as seen from spectra a–c in Figure 4.20(A). However,
after contacting with C3 H6 , dehydroxylation was greatly facilitated (spectra d, e, f) as
evidenced by the decrease of intensity at 3480 cm−1 . Further evacuation in vacumm
made no difference on the IR spectra as seen by the overlapped spectra f–h. On the
109
1.6
x 10
6
1.2
0.8
rC
H
3 8
(mol⋅gcat1⋅s 1)
(a)
0.4
0
0
1.6
30
x 10
60
0
120
me (m n)
150
180
210
60
0
120
me (m n)
150
180
210
6
1.2
0.8
rC
H
3 8
(mol⋅gcat1⋅s 1)
(b)
0.4
0
0
30
Figure 4.19: Effect of flush in He, H2 and C3 H6 on the deactivation of propene hydrogenation over the Ti-SiO2 -10 support. Before each cycle, the support was calcined in 10
vol% O2 at 573 K for 1 hour, then was treated in 2 vol% H2 + 2 vol% O2 at 423 K for 1
hour, and then was kept at 423 K for 0.5 hour; afterwards, the temperature was raised
to 473 K with a ramping rate of 3 K/min in He. During propene hydrogenation at 473
K, the feed gas was 10 vol% H2 + 10 vol% C3 H6 balanced by He. Flushing in He, or 10
vol% H2 in He, or 10 vol% C3 H6 in He was performed between t = 30 − 90 min and
between t = 120 − 180 min. Cycle 5 in (a) was a cycle without interval flushing and was
−1
used as a reference. GHSV was 10000 mL·g−1
cat h . All cycles were performed within a
short period of time in a sequential order as indicated by the number.
other hand, no adsorption in the CH stretching or bending region was observed, which
excludes the formation of propoxy species on the support. It appears that the only effect
of C3 H6 on the deactivation of hydrogenation activity is the accelerated dehydroxylation.
This in turn confirms the importance of surface hydroxyls in propene hydrogenation over
Ti-containing oxides as discussed in the previous section.
110
Adsorbance (a.u.)
0.05
0.05
e
3200 2800 2400 2000 1600
d
a-c
f,g,h
(A)
3800
3600
3400
3200
3000
2800
-1
Wavenumber (cm )
0.005
Adsorbance (a.u.)
b
c
d
e
(B)
f
3800 3600 3400 3200 3000 2800
1800
1600
1400
-1
Wavenumber (cm )
Figure 4.20: IR spectra of Ti-SiO2 -10 at 473 K. The support was pretreated ex-situ in 2
vol% H2 + 2 vol% O2 at 423 K for 1 hour and then transferred into IR cell. Spectra a:
5 min in vacuum; b: 30 min in vacuum; c: 60 min in vacuum; d: after c, in 17.5 mbar
C3 H6 for 30 min and after subsequent evacuation; e: after d, in 17.5 mbar C3 H6 for 30
min and after subsequent evacuation; f: after e, in 17.5 mbar C3 H6 for 30 min and after
subsequent evacuation; g: after f, 30 min in vacuum; h: after f, 60 min in vacuum. In
section A, spectra f, g, and h overlapped. Section B gives the difference spectra of b–f
subtracted from spectrum a.
4.4 Summarizing discussion
It was observed in this study that the formation rate of propane kept increasing from
zero until it leveled off during the activation of the catalysts while PO formation rate was
stable at a high level from the very beginning as shown in section 4.3.1. The results in
4.4. SUMMARIZING DISCUSSION
111
this chapter imply that propene hydrogenation and epoxidation may happen on two different sites. The size effect of gold nanoparticles is undoubtedly important in hydrogen
dissociation during the direct propene epoxidation [1, 5, 26], since Au–Ti4+ interface
is indispensable for the direct propene epoxidation. Carbon monoxide might be competitively adsorbed on gold where hydrogen adatoms locate and thus abate the propene
hydrogenation. It could also be the situation that Ti−OH induces hydrogen dissociation
(on gold or with Ti) while carbon monoxide may poison the Ti−OH group probably by
forming surface formate species, or the Ti site if the stability of possible titanium species
at reduced oxidation states is an issue [27]. It has been observed by Sykes et al. [21, 22]
that hydroxylated titania surface under partial reduction is capable of hydrogenating a
terminal alkene. It has also been proposed that transition metals anchored on SiO2 can
hydrogenate alkene probably by forming hydride complexes [23]. The catalytic performance of our catalyst, and one step further, of the hydroxylated support only, in hydrogen
and propene but without oxygen clearly shows that the surface hydroxyl, most likely Ti–
OH, plays an important role in propene hydrogenation and that gold is not of necessity
for propane formation (section 4.3.3). The enhancement of propane formation by oxygen
observed by Qi et al. [5] may be partly attributed to water formation in presence of oxygen and accordingly the hydration or hydroxylation of catalyst surface. Oxygen vacancy
must be present on the supports in the study performed in this chapter, which facilitates the dissociation of H2 , since propene hydrogenation can be greatly enhanced by the
presence of O2 and the activation process needs O2 . On the other hand, it is generally accepted that tripodal Ti4+ sites are favoured in liquid-phase epoxidation when concerning
the coordination environment of titanium in silica-supported titanium catalysts [28–30].
Hydrothermal instability [31] of such tripodal Ti sites leads to the formation of bipodal
and monopodal Ti sites on silica surface, which, combined with vicinal silanols, might
contribute to propene hydrogenation in the complex system of propene, hydrogen and
oxygen over gold–titania catalysts. Our experimental observations suggest that on the
supports where Ti−O−Si or Ti−O−Ti linkage is not that resistant to hydrolysis, the role
of support in propene hydrogenation should be taken into account.
On the other hand, the temperature of ca. 443 K, at which the hydrogenation activity
of the catalysts and supports investigated in this study reach the peak during TPR in only
H2 and C3 H6 , is very close to the temperature for dehydroxylation of Ti−OH over TiO2
supported on SiO2 [32]. This temperature has also been observed as the dehyroxylation
or dehydration temperature of the Ti-SiO2 supports in this study as shown in Chapter 3.
112
Primet et al. [33] indicated that for incompletely coordinated titanium atoms in TiO2 (the
structure below), coordination-bonded water is removed at 423 K. Depending on the
distance between the two ajacent Ti atoms, C3 H6 may facilitate the removal of the two
hydroxyls as found in experiments shown in Figures 4.19 and 4.20. This dehydroxylation
may be via the C−
−C double bond forming a π complex with one OH and an interaction
between the allylic hydrogen and the other –OH. However, checking with C2 H4 has not
been performed. In this sense, the role of CO in switching off propene hydrogenation can
also be the competitive adsorption on the Ti−OH or defective TiO x sites against C3 H6 .
Regarding to the low hydrogenation activity on the 0.05-Au/TS-1 catalyst, there may
be also some incompletely coordinated Ti atoms since the TS-1 support used here has
a very small crystal size of 120 nm on average (see Figure 3.6). This structure shown
above is also similar to a structure named strained siloxane in thermally activated SiO2 ,
which can also hydrogenate alkenes as found by Rajagopal et al. [34] at 473–623 K.
In his study, it was found that silica gained hydrogenation activity after heating in Ar
between 603 and 703 K for hours. However, blank experiments on pure silica (either
hydroxylated or calcined) used in this study did not show any activity. Propene can also
be hydrogenated by TiO2 P25 when TiO2 is reduced above 573 K. The hydrogenation
activity on reduced TiO2 can also be suppressed when CO is present. But the appearance
of peak activity at ca. 443 K on the Ti-SiO2 supports was not found on reduced TiO2 . It
seems that defective TiO x and hydroxyls are necessary for propene hydrogenation over
the Ti-SiO2 supports. Gold may play as an extra source for H2 dissociation but is not
necessary for propene hydrogenation.
4.5 Conclusions
Propene hydrogenation over the gold–titania catalysts can also proceed over the supports and can be switched off by CO without affecting the formation of propene oxide.
The presence of O2 enhances propene hydrogenation over the catalysts and supports.
Temperature programmed reaction in H2 and C3 H6 shows a peak activity at ca. 443 K,
which corresponds to dehydroxylation or dehydration of the support. Activity decrease
REFERENCES
113
in propene hydrogenation is due to accelerated dehydroxylation in the presence of C3 H6 .
References
201, 128–137.
884–891.
[6] J. Gaudet, K. Bando, Z. Song, T. Fujitani, W. Zhang, D. Su and S. Oyama, J. Catal., 2011,
280, 40–49.
186–195.
[12] B. Chowdhury, J. J. Bravo-Suárez, N. Mimura, J. Lu, K. K. Bando, S. Tsubota and M. Haruta,
J. Phys. Chem. B, 2006, 110, 22995–22999.
2007, 123, 189–197.
[16] E. Sacaliuc, A. M. Beale, B. M. Weckhuysen and T. A. Nijhuis, J. Catal., 2007, 248, 235–248.
[18] W. Yan, B. Chen, S. M. Mahurin, E. W. Hagaman, S. Dai and S. H. Overbury, J. Phys. Chem. B,
2004, 108, 2793–2796.
[19] W. T. Wallace, B. K. Min and D. W. Goodman, J. Mol. Catal. A: Chem., 2005, 228, 3–10.
[20] X. Gao and I. E. Wachs, Catal. Today, 1999, 51, 233–254.
[21] E. C. H. Sykes, M. S. Tikhov and R. M. Lambert, Catal. Lett., 2002, 78, 7–11.
[22] E. C. H. Sykes, M. S. Tikhov and R. M. Lambert, J. Phys. Chem. B, 2002, 106, 7290–7294.
[23] Yu. I. Yermakov, Y. A. Ryndin, O. S. Alekseev, D. I. Kochubey, V. A. Shmachkov and N. I.
Gergert, J. Mol. Catal., 1989, 49, 121–132.
[24] G. Spoto, S. Bordiga, G. Ricchiardi, D. Scarano, A. Zecchina and E. Borello, J. Chem. Soc.,
114
Faraday Trans., 1994, 90, 2827–2835.
[25] A. Zheng, S.-B. Liu and F. Deng, Microporous Mesoporous Mater., 2009, 121, 158–165.
48, 9515–9518.
[27] Y. Li, B. Xu, Y. Fan, N. Feng, A. Qiu, J. He, J. Miao, H. Yang and Y. Chen, J. Mol. Catal. A:
Chem., 2004, 216, 107–114.
[28] M. Crocker, R. H. M. Herold, A. G. Orpen and M. T. A. Overgaag, Dalton Trans., 1999, 3791–
3804.
[29] J. K. F. Buijink, J. J. M. van Vlaanderen, M. Crocker and F. G. M. Niele, Catal. Today, 2004,
93-95, 199–204.
[31] J. Zhou, Z. Hua, X. Cui, Z. Ye, F. Cui and J. Shi, Chem. Commun., 2010, 46, 4994–4996.
[32] S. Haukka, E. L. Lakomaa and A. Root, J. Phys. Chem., 1993, 97, 5085–5094.
[33] M. Primet, P. Pichat and M. V. Mathieu, J. Phys. Chem., 1971, 75, 1216–1220.
[34] V. K. Rajagopal, R. D. Guthrie, T. Fields and B. H. Davis, Catal. Today, 1996, 31, 57–63.
How metallic is gold in the
direct epoxidation of propene :
An FTIR study
5
This chapter has been published as:
J. Chen, E. A. Pidko, V. Ordomsky, M. W. G. M. Verhoeven, E. J. M.
Hensen, J. C. Schouten, & T. A. Nijhuis, How metallic is gold in the
direct epoxidation of propene: An FTIR study, Catal. Sci. Technol.,
2013, DOI: 10.1039/C3CY00358B.
Abstract
Unraveling the oxidation state of gold is important to understand the role of gold in
the direct propene epxoidation on the gold–titania catalysts. Fourier transform infrared
study of low-temperature carbon monoxide adsorption was performed over Au/TiO2 and
Au/Ti-SiO2 under atmospheres of the reaction mixture, oxygen, hydrogen, and propene,
respectively. Data reveals that the active gold sites treated by the reaction mixture are
fully covered by reaction intermediates and deactivating species. Oxidation at 573 K removes these carbonaceous species on gold. Oxygen adsorption at reaction temperatures
leads to positively charged gold, which can be reduced to metallic gold in the presence
of hydrogen. Propene plays as an electron donor to gold atoms resulting in negatively
charged gold with the carbonyl band at 2079 cm−1 . The results in this study may provide
a general scheme of electron transfer via gold on the gold–titania catalysts for the direct
propene epoxidation.
116
CHAPTER 5. HOW METALLIC IS GOLD IN THE DIRECT EPOXIDATION OF PROPENE
5.1 Introduction
Gold nanoparticles in the direct propene epoxidation over gold–titania catalysts are considered to have two main functions. The first is the formation of the active oxidizing
species, the hydro-peroxy species (HOOH, OOH), which has been spectroscopically confirmed via an inelastic neutron scattering (INS) study [1]. From a combination of in-situ
UV–Vis and XANES study [2], it was suggested that the hydro-epoxidation of propene
over gold–titania catalysts involves the formation of HOOH on gold and a sequential
transfer of HOOH to Ti4+ forming the real active intermediate Ti−OOH. This route resembles the chemistry of propene epoxidation by H2 O2 over TS-1 in the HPPO process
[3]. The second function of gold in the epoxidation of propene, which is less prominent,
is that propene may be activated on gold nanoparticles [4], or in another sense, that the
adsorption of propene on gold strongly affects the activity of gold in the formation of
hydro-peroxy species, since the inhibiting effect of the alkene in the hydrogen oxidation
on gold catalysts is generally observed [5].
The rate-determining step in the hydro-epoxidation of propene on gold–titania catalysts is considered to be the dissociative adsorption of hydrogen [6–8]. The study by
Boronat et al.[9] suggests that the hydrogen dissociation over Au/TiO2 occurs only on
low coordinated and neutral gold atoms at corner or edge sites of gold nanoparticles but
not directly bonded to the support. The H2 –D2 exchange reaction studied by Fujitani
et al.[10] on model Au/TiO2 (110) catalysts with gold nanoparticles of controlled sizes
has shown a constant turnover frequency of HD formation when based on the perimeter length of gold nanoparticles, indicating that hydrogen dissociation on gold–titania
catalysts may very likely be an interfacial phenomenon. Green et al.[11] studied lowtemperature hydrogen oxidation over Au/TiO2 by infrared (IR) spectroscopy and density
functional theory (DFT) calculations. Their results suggest a pathway of hydrogen dissociation at the perimeter sites at the interface between Au and TiO2 , in which oxygen
molecules adsorb on Ti5c perimeter sites and hydrogen dissociates into Au–H and Ti–
OOH in the first step. Yang et al.[12] performed DFT study of hydrogen dissociation and
diffusion at the perimeter sites of Au/TiO2 (110) and depicted two ways of hydrogen dissociation. In their study, the heterolytic dissociation of hydrogen with one H atom on gold
and another on the bridge oxygen atom in the support is energetically more favourable,
while the homolytic hydrogen dissociation mainly occurs after all Au–O–Ti sites are passivated into Au–O(H)–Ti. Using coadsorbed CO in an IR study of hydrogen dissociation
117
5.1. INTRODUCTION
over Au/TiO2 at room temperature, Panayotov et al.[13] revealed that the most active
sites for hydrogen dissociation are defect Au0 sites away from Au–O–Ti interface. Passivation of Au–O–Ti to Au–O(H)–Ti was also observed. The rate of hydrogen atom spillover
1/2
to the support was determined to be proportional to PH . The coadsorption (or com2
petitive adsorption) of CO does not change the chemistry of hydrogen dissociation and
spillover, but it can suppress the initial rate of hydrogen dissociation by a factor of 2.6
as demonstrated by Panayotov et al. [13]. In summary, hydrogen dissociation occurs on
low-coordinated Au0 atoms at edges and corners and the role of the support cannot be
neglected over gold–titania catalysts.
Another interesting phenomenon in the hydro-epoxidation of propene over gold–
titania catalysts is that gold can catalyze the propene hydrogenation [14–17]. Although
in our previous study [18], it has been demonstrated that the support itself can contribute
significantly to the propene hydrogenation and that the presence of a small amount of
CO (10 ppm to 1000 ppm) can switch off the propene hydrogenation, we cannot exclude
contribution from gold. Gold-catalyzed alkene hydrogenation has long been known [19].
Specifically over the gold–titania system, the explanation to this side reaction can be that
propene hydrogenation is sensitive to the structures of gold nanoparticles or clusters
[14, 16, 20]. It can also be that Au+ exists and functions as proposed by Oyama et al.
[17]. The Au+ in their study is in the form of Au(CN)–1
2 resulting from NaCN leaching of
Au/TS-1 and has no activity in propene epoxidation, but is active in hydrogenation. Bond
and Thompson[21] suggested that gold atoms with a low coordination number are more
prone to be oxidized than those with a high coordination number. The findings by Oyama
and coworkers[17] thus provide another mechanistic view towards the hydrogenation of
propene, in which small gold clusters have probably oxidized interphase atoms. Nevertheless, Au+ should be neither easy to be produced nor stable in the reaction conditions
for the direct propene hydrogenation where dihydrogen and propene are also present.
Dekkers et al.[22] observed Au+ by pre-oxidizing Au/TiO2 in pure dioxygen at 573 K
for one hour and found that Au+ was gradually reduced to Au0 in a CO–O2 mixture at
room temperature. Venkov et al.[23] could not obtain Au+ by treating Au/TiO2 under
oxygen at 573–773 K and only observed Au+ after oxidation in NO+O2 at 773 K. Thus it
is doubtful that Au+ would exist under a H2 /O2 /C3 H6 mixture at 323–473 K.
The most informative technique used in gold catalysis to determine gold oxidation
state is using CO as the probe molecule to observe the frequency of ν(CO) bands by the
IR spectrum. The sites on gold nanoparticles, where CO adsorbs, are also those gold
118
atoms with low coordination numbers at edges and corners, where all the chemistry is
supposed to happen. Spectral regions of surface carbonyls on gold with different status
are well summarized in reviews by Freund et al.[24] and Mihaylov et al.[25]. Though
some of the bands overlap with CO interacting with Ti4+ and hydroxyls, they can be
distinguished by progressive adsorption/desorption.
The aim of this work is to have a general image of the oxidation state of gold in
Au/Ti-SiO2 and Au/TiO2 which are active in the direct propene epoxidation. The catalysts were investigated by low-temperature CO adsorption after different pretreatment
or in different atmospheres. We also have the two following questions left from previous
studies [5, 18]. The first is that what may be the reason for the suppressing effect of
propene adsorption on water formation over gold. The second is that to what extent can
we observe the competitive adsorption of CO and H2 if the inhibiting effect of CO on
propene hydrogenation is due to CO occupying gold. Co-adsorption of CO and H2 /C3 H6
was thus investigated and discussed in this study.
5.2 Experimental
5.2.1 Catalyst preparation and testing
The catalyst consisting of 1 wt% gold on titania dispersed on silica, which has been fully
investigated in our earlier study [8], was used in this study and denoted as Au/Ti-SiO2
(SiO2 , Davisil 643, Aldrich, 300 m2 /g, pore size 150 Å, pore volume 1.15 cm3 /g). The
preparation of 1 wt% Au/SiO2 (Davisil 643, Aldrich) and 1 wt.% Au/TiO2 (P25, Degussa,
70% anatase, 30% rutile, 45 m2 /g) followed the deposition–precipitation method using
ammonia described in our earlier study [4]. Safety concerns and suggestions on the possible formation of explosive fulminating gold when using ammonia have been addressed
earlier [4]. Due to the limited amount of gold and ammonia, the risks are very minor
in this study. A total of 2 g of the support was dispersed in 100 mL of water. The pH of
the slurry was adjusted to 9.5 by dropwise adding ammonia (2.5 wt%). A total of 115
mg of an acidic 30 wt% HAuCl4 solution (Aldrich, 99.99% trace metal basis) was diluted
in 20 mL of demineralized water and was added dropwise to the support slurry over a
15 min period. While HAuCl4 solution was being added, the pH was kept at 9.5 using
aqueous ammonia. After the addition of the gold solution, the slurry was stirred for one
hour. The slurry was filtered and washed 3 times using 200 mL of water. The catalyst
5.2. EXPERIMENTAL
119
was dried overnight at 353 K and calcined first at 393 K (5 K/min heating) for 2 h and
afterwards at 673 K (10 K/min heating) for 4 h. Drying and calcination of the support
and the catalyst were performed under atmospheric pressure in stationary air.
Catalytic tests were performed in a flow setup equipped with a fast Interscience Compact GC system (3 min analysis time) containing a Porabond Q column and a Molsieve
5A column in two separate channels, each with a thermal conductivity detector. 300 mg
of catalyst was loaded into the tubular quartz reactor (6 mm inner diameter) and tested
with a gas feed rate of 50 mL min−1 in total consisting of 10 vol.% each for hydrogen,
−1
oxygen, and propene with helium as the balance (GHSV 10000 mL·g−1
cat h ). The term
‘spent’ or ‘after reaction’ hereinafter means that the catalyst had been tested in the reaction mixture for epoxidation for at least 2 hours. The term ‘regenerated’ hereinafter
means that the catalyst or sample had been calcined in 5 vol.% or 10 vol.% oxygen diluted in helium at 573 K for at least 30 min. When mentioned in an IR experiment, the
regeneration was always conducted in-situ in 5 vol.% oxygen at 573 K for 30 min.
5.2.2 Charaterization techniques
Transmission electron microscope (TEM) images were recorded with a FEI Tecnai G2
Sphera transmission electron microscope at an acceleration voltage of 200 kV. Loadings
of gold and titanium were determined by inductively coupled plasma optical emission
spectrometry (ICP-OES) using a Spectra CirosCCD system. In ICP analyses, gold was dissolved with aqua regia and grafted titanium was etched by 5 mol/L H2 SO4 solution. The
H2 SO4 solution containing dissolved titanium was then diluted to 2.9 mol H2 SO4 /L for
analyses. The X-ray photoelectron spectroscopy (XPS) measurements were carried out
with a Kratos AXIS Ultra spectrometer, equipped with a monochromatic X-ray source and
a delay-line detector (DLD). Spectra were obtained using the aluminium anode (Al Kα =
1486.6 eV) operating at 150W. A detachable U-shape Pyrex tubular reactor (6 mm inner
diameter) with valves on both ends was used for post-reaction catalysts. The U-shape
reactor can be heated using an aluminum heating jacket up to 573 K. The post-reaction
catalysts were then flushed in helium and afterwards sealed in the U-shape reactor. The
reactor was transferred into a nitrogen glove box free from oxygen (< 2 ppm) and moisture (< 0.5 ppm). The post-reaction samples for XPS measurements were prepared in
the glove box. Transport of the samples from the glove box to the spectrometer was performed in an inert atmosphere by using a small nitrogen-purged chamber equipped with
120
a magnetic arm. In this way, the post-reaction catalysts were kept ‘in-situ’. XPS spectra
were referenced to the C 1s line at 284.9 eV.
Infrared spectra were recorded with a Bruker IFS 113v spectrometer at a 2 cm−1
optical resolution and accumulation of 128 scans (ca. 2 min of acquisition time per
spectrum). The samples were pressed in self-supporting discs at 0.5 MPa (diameter:
12.7 mm, ca. 7.9 mg cm−2 ). The sample wafer was placed in a homemade doublewalled IR cell [26]. The high-vacuum (below 10−5 mbar) transmission IR cell was inhouse made and can be cooled by liquid nitrogen and heated by electric wire. The IR
cell is connected to a gas dosing system, through which the progressive adsorption of
adsorbates can be performed via a 5-µL or 50-µL sample loop on a Valco six-port valve.
In a typical CO dosing experiment, the CO pulse was automatically carried out upon
the acquisition of each spectrum (128 scans) finished. The stabilization of one spectrum
was fast and normally within a few scans. An extra pair of inlet and outlet is located
on top of the IR cell, through which in-situ calcination or pretreatment under different
atmospheres can be performed. The gas for sample pretreatment can be dried by a liquid
nitrogen trap when necessary. The catalysts were tested in the tube reactor confirming the
activity before IR experiments. The catalyst discs were generally used for 3-4 sequential
IR measurements. In-situ calcination in 5 vol.% oxygen diluted by helium at 573 K for
30 min followed by evacuation at 573 K for 30 min was carried out between each IR
experiment.
5.3 Results
5.3.1 Catalyst performance and characterization
The 1 wt.% Au/TiO2 and Au/Ti-SiO 2 catalysts were tested for propene epoxidation in hydrogen and oxygen. Their catalytic performance was listed in Table 5.1. The conversion
of propene and the selectivity to PO were in the typical range for these two catalysts at
these conditions [4]. The side products were mainly propionaldehyde, acetone, acetaldehyde and carbon dioxide. No propane formation was found. The size distribution of gold
nanoparticles was narrow for both catalysts (spent), centered at 4 nm for Au/TiO2 and
4.5 nm for Au/Ti-SiO 2 . The catalyst stability was investigated for these two catalysts by
repeatedly performing catalytic testing followed by 1-hour regeneration at 573 K in 10
vol.% oxygen. The results are shown in Figure 5.1. The time-on-stream performance
121
5.3. RESULTS
Table 5.1: Charaterization and general performance of three gold catalysts
Sample ID
Au/Ti-SiO2
Au/TiO2
Au/SiO2
Au loading
(wt.%)
Ti loading
(wt.%)
Au size
(nm)
0.91
0.93
0.95
1.29
–
–
4.5±1.1
4.0±1.2
3.1±0.8
Temp.
(K)
423
333
Catalytic performancea
C3 H6 conv. PO sel. H2 eff. b
(%)
(%)
(%)
1.32
85.5
7.1
0.20
98.0
15.9
–
a. no activity in propene hydrogenation observed, activity taken at 2 h, H2 :O2 :C3 H6 :He= 1 : 1 : 1 : 7,
−1
cat h .
b. determined as rPO /rH O.
2
of both catalysts was highly reproducible. The reason for these stability tests was to exclude a possible catalyst change in the IR experiments since each disc was re-used for 3–4
times. The effect of high vacuum at 573 K after calcination in oxygen in the IR cell on
possible gold sintering was examined by repeatedly treating one Au/TiO2 sample under
such condition before cooling down to 323 K in vacuum for 9 times. The average size
of gold particles after such treatment increased slightly by less than 0.5 nm, but it was
still well below 5 nm as shown in Figure 5.2 and the size distribution remained narrow.
Thus sintering of gold nanoparticles in IR experiments for each pellet was not supposed
to occur or had very limited effect on CO adsorption.
5.3.2 CO adsorption on catalysts after epoxidation and regeneration
Figure 5.3 shows the changes in IR spectra during progressive CO adsorption on a clean
Au/SiO2 sample at 90 K. The spectrum of the dehydrated sample before CO dosing is
quite simple and similar to what can be observed for SiO2 . In the OH stretching region,
the band at 3743 cm−1 is assigned to unassociated Si−OH. The band at 3715 cm−1 is assigned to the terminal silanol in a pair or chain of hydrogen-bonded hydroxyls (hydrogen
perturbed OH), while the broad band centered at 3552 cm−1 is assigned to those within
the hydrogen-bonded hydroxyl groups (oxygen perturbed OH) [27]. The adsorption at
3653 cm−1 is from inaccessible Si−OH. The adsorption at 1980 cm−1 and 1875 cm−1 is
attributed to the skeleton vibrations of silica [28]. The 1640 cm−1 band is most likely to
be assigned to the bending mode of some remaining adsorbed water. Upon adsorption,
the CO molecules interact with terminal OH groups leading to a rise of the perturbed
band at ca. 3550 cm−1 , which gradually shifted to lower wavelengths when CO coverage increased. CO also interacted with unassociated OH groups giving the decreased
122
x 10
−7
−6
4
(a)
cycle 1
cycle 2
cycle 3
cycle 4
cycle 5
Au/Ti−SiO , PO
2
2
3
2
2
1
0
2
1
0
60
x 10
120
180
Time (min)
240
0
300
0
−7
60
120
180
Time (min)
240
300
−6
2
(c)
x 10
cycle 1
cycle 2
cycle 3
Au/TiO2, PO
(d)
(mol⋅g−1
⋅s−1)
cat
1.5
1.5
1
2
O
1
cycle 1
cycle 2
cycle 3
Au/TiO2, H2O
rH
rPO (mol⋅g−1
⋅s−1)
cat
2
O
2
cycle 1
cycle 2
cycle 3
cycle 4
cycle 5
Au/Ti−SiO , H O
rH
rPO (mol⋅g−1
⋅s−1)
cat
3
x 10
(b)
(mol⋅g−1
⋅s−1)
cat
4
0.5
0
0.5
0
30
60
Time (min)
90
120
0
0
30
60
Time (min)
90
120
Figure 5.1: Tests of activity stability for the 1 wt.% Au/Ti-SiO2 (top, a and b) and
Au/TiO2 (bottom, c and d) catalysts in 10/10/10/70 H2 /O2 /C3 H6 /He mixture. GHSV
−1
10000 mL·g−1
cat h , 423 K for Au/Ti-SiO 2 , 333 K for Au/TiO2 . A few hours of continuous
testing in the reaction mixture followed by 1 hour calcination in 10 vol.% oxygen at 573
K constitutes one ‘cycle’.
band at ca. 3746 cm−1 (shifted from 3743 cm−1 before CO was introduced) and lowered
transparency at ca. 3610 cm−1 . The isosbestic point located at ca. 3680 cm−1 . Three
carbonyl bands were observed after introducing CO. The band at 2158 cm−1 is known for
CO interacting with OH groups. The band at 2136 cm−1 can be assigned to physisorbed
CO [29, 30]. The 2136 cm−1 band is unlikely to be assigned to P-branch adsorption
of gas phase CO. The contribution from gas phase CO at 1 mbar and 90 K was found
to be very minor in our study. Besides, the R-branch component at higher frequencies
is lacking as seen for Au/SiO2 . The band at 2099 cm−1 is assigned to Au0 −CO, which
shifted from 2105 cm−1 at low CO coverage. A red shift of ν(CO/Au) with increased
coverage of CO was generally observed and can be explained by the balancing between
123
5.3. RESULTS
50
(a)
Counts
40
30
20
10
0
1
60
2
3
4 5 6 7
Particle size (nm)
8
9
10
2
3
4 5 6 7
Particle size (nm)
8
9
10
(b)
Counts
50
40
30
20
10
0
1
Figure 5.2: Change in the size distribution of gold nanoparticles on the 1 wt.% Au/TiO2
catalyst after repeating 1-hour calcination in vacuum at 573 K in the IR cell and cooling
down to 323K for 9 times: (a) before treatment; (b) after treatment.
dipole–dipole coupling, which leads to a blue shift, and the chemical shift, which leads
to a wavenumber decrease since the donation from the weakly antibonding 5σ orbital
dominates in Au–CO interaction whereas the backbonding from gold atoms to 2π* CO
orbital is lacking [31–33].
Progressive adsorption of CO on the spent and regenerated Au/Ti-SiO 2 sample is
shown in Figure 5.4. Comparing to CO adsorption on Au/SiO2 , a new band above 2180
cm−1 appeared as seen from Figure 5.4, which can be assigned to CO adsorbed on Ti4+
ions [29]. The spent catalyst showed very weak adsorption of CO on both Ti4+ and Au.
The weak band of linear carbonyl on Au0 at 2095 cm−1 was blue shifted by 12 cm−1
to 2107 cm−1 on the regenerated sample, on which propoxy species adsorbed at Au–Ti
interface are supposed to be fully removed. After regeneration, the sample showed a
much stronger adsorption of CO on gold indicating that during reaction the gold sites
124
m
(A)
a
s.
0
0.1
3000
3610
(C)
CO-OH
0.03
2099
3552
CO-Au0
m
0.02
2136
3653
3 43
1500
0.04
a
m
0
n vacuum
0.01
a
0
.05
3800
2000
1
(B)
3 15
s.
2500
avenum er (cm
0.05
2158
3500
3700
3600
3500
avenum er (cm−1)
3400
2200
2150
2100
2050
Figure 5.3: Development of IR spectra during progressive CO adsorption on the 1 wt.%
Au/SiO2 sample at 90 K: (A) wide-frequency range; (B) range of ν(OH); (C) difference
spectra in the range of ν(CO), substracted from the spectrum ‘0’ before CO dosing. The
following CO pressures after each dose were used (in mbar, spectra a–m): 0.06, 0.14,
0.23, 0.30, 0.36, 0.43, 0.50, 0.58, 0.66, 0.73, 0.81, 0.90, 0.99. Prior to low-temperature
CO adsorption, the catalyst was calcined in 10 vol.% O2 at 573 K and then tested in
2 vol.% H2 and 2 vol.% O2 at 423 K in the flowing reactor. Afterwards the pellet was
prepared ex-situ and transferred into IR cell for evacuation at 473 K for 1 hour.
active for epoxidation were fully covered by strongly adsorbed species though the water
formation rate remained high as shown in Figure 5.1. Upon dosing CO on the regenerated
sample as shown in Figure 5.4(B), Auδ+ can be detected by the shoulder of ν(CO) at 2125
cm−1 [23, 34, 35].
The spectra of the spent and regenerated sample before CO dosing are given and
compared in Figure 5.5. The difference spectrum given in Figure 5.5 confirmed the
presence of strongly adsorbed propoxy on the catalyst surface. In Figure 5.5(A), it is
clear that alkoxy adsorbed on the support cannot be fully removed by oxidation at 573 K
125
5.3. RESULTS
2158
(A)
2158
0.04
0.04
(B)
CO-OH
CO-OH
0.03
0.03
0
0
2200
2150
2100
Wavenumber (cm 1
2050
2107
2125
0.01
n'
a'
2185
CO-Ti
a
CO-Ti4+
CO-Au0
2095
0.01
4+
2136
n
2136
0.02
0.02
2181
Abs.
CO-Au0
CO-Auδ+
2200
2150
2100
2050
Figure 5.4: Progressive CO adsorption on the (A) spent and (B) regenerated 1 wt.%
Au/Ti-SiO2 catalyst. The spectra were taken at 90 K and the following CO pressures after
each dose were used (in mbar): 0.06, 0.12, 0.19, 0.26, 0.33, 0.40, 0.46, 0.53, 0.61, 0.67,
0.74, 0.82, 0.89, 0.96 for a – n; 0.06, 0.12, 0.18, 0.25, 0.32, 0.39, 0.45, 0.53, 0.62, 0.68,
0.76, 0.83, 0.90, 0.97 for a′ – n′ . The sample in vacuum was used as the background.
as indicated by the remaining bands in the CH stretching region after regeneration. The
difference spectrum between the spent and regenerated sample given in Figure 5.5(B)
did show increased transparency by regeneration in the the CH stretching and bending
regions, which was due to the removed propoxy species similar to what was observed
by Mul et al. [36]. In the CH stretching region, the bands at 2989 and 2979 cm−1
are assigned to νas (CH3 ), while the bands at 2938, 2906, 2881, and 2860 cm−1 can
be assigned to νas (CH2 ), ν(CH), νs (CH3 ), and νs (CH2 ), respectively [4, 36, 37]. A
weak band at 2825 cm−1 was observed, which may be assigned to ν(CH) of surface
formate [38]. In the CH bending region, the bands at 1459/1452, 1382, and 1347
cm−1 can be assigned to δas (CH3 )/δ(CH2 ), δs (CH3 ), and δ(CH), respectively. The bands
at 1719 and 1697 cm−1 are assigned to C−
−O stretching vibration from surface species
containing carbonyl group. Since the sample of the spent catalyst was already evacuated
at 573 K, the carbonyl group is mostly likely from dehydrogenation of intermediates such
as Ti−O−CH2 CH(OH)CH3 or Ti−O−CH(CH3 )CH2 OH (preferred, however no obvious
adsorption of ν(CH) at 2720–2750 cm−1 for aldehydes) by the ring opening of propene
oxide on the surface [36, 39, 40]. Less intensive bands at 1682, 1626, 1591, and 1423
cm−1 may be attributed to surface carbonates and carboxylate [41].
The spectrum given in Figure 5.6A provides information on species adsorbed on the
spent Au/TiO2 catalyst. Figure 5.6B–C shows CO adsorption on the spent and regen-
126
2884
2979
2990
regenerated
2938
Abs.
1461
spent
1453
1725
1380
(A)
1800 1700 1600 1500 1400 1300
0.1
3500
3000
2000
1500
-1
1297
1347
1452
1423
1459
1591
1382
1682
1626
1697
1719
2825
2881
2938
2860
Abs.
2906
2989
(B)
2979
Wavenumber (cm )
.005
3000
2800
1800
-1
1600
1400
Wavenumber (cm )
Figure 5.5: IR spectra of the 1 wt.% Au/Ti-SiO2 sample after reaction and regeneration:
(A) wide-frequency range; (B) difference spectrum of the spent subtracted from the regenerated. Both spectra were taken at 90 K in vacuum. The sample of the spent catalyst
−1
(2 hours in 10/10/10/70 H2 /O2 /C3 H6 /He mixture at 423 K, GHSV 10000 mL·g−1
cat h ,
ex-situ) was evacuated in vacuum at 573 K in the IR cell for 1 hour and then cooled
down to 90 K. After CO adsorption was performed, the same sample was then calcined
in flowing 5 vol.% O2 at 573 K in-situ for 0.5 hour to clean the carbonaceous surface,
evacuated at 573 K for 0.5 hour and then cooled down to 90 K.
erated Au/TiO2 catalyst. The left inset in Figure 5.6A compares the IR spectra of the
spent Au/TiO2 sample after evacuation at 573 K (spectrum b) and the same sample after regeneration (spectrum c). In the left inset, the regenerated sample clearly shows
OH stretching bands above 3500 cm−1 . However, there are hardly OH stretching bands
observed on the spent catalyst but evacuated at 573 K. These OH groups on a clean or
regenerated catalyst sample can hardly be completely removed by evacuation at even
673 K. As for the spent catalyst, evacuation at 573 K eliminated all the surface hydroxyl
127
5.3. RESULTS
d
1100
1050
1000
a
1147
1227
1331
1303
1375
1723
1674
2716
1544
1448
1150
1576
2869
2970
2905
2982
Abs.
2932
1200
1123
a-d
3500 3000 2500 2000 1500 1000
1020
a
c
1053
(A)
1090
b
0.02
3200
2800
1600
1200
-1
108
185
0
2150
2100
Wavenumber (cm 1
CO Au0
107
1 7 1 CO β-Ti4+
a'
0
2200
'
2050
1
a
3
1 8
0.02
09
0.02
9 CO
CO Au0
14(
190
Abs.
0.04
Abs.
0.04
179
(C)
CO β-Ti
0.06
CO β-Ti4+
4+
(B)
-Ti4+
0.06
178
Wavenumber (cm )
2200
2150
2100
Wavenumber (cm 1
2050
Figure 5.6: Difference spectrum of the spent and regenerated sample of Au/TiO2 (section
A) and progressive CO adsorption on the spent (section B, pre-evacuated at 573 K) and
the regenerated (section C) sample. The left inset in section A compares the IR spectra of
the spent and regenerated sample. The spent catalyst was under the reaction conditions
ex-situ at 333 K for 1 hour after a 5-hour catalytic testing and regeneration. The sample
was then evacuated at 323 K and 573 K in the IR cell. All spectra were taken at 90 K.
The following CO pressures after each dose were used (in mbar): 0.01, 0.03, 0.05, 0.07,
0.09, 0.11, 0.13, 0.16, 0.18, 0.20 for a – j in section B; 0.01, 0.03, 0.05, 0.06, 0.08,
0.10, 0.13, 0.16, 0.18, 0.21 for a′ – j′ in section C. In section A, spectrum a is difference
spectrum between the sample evacuated at 573 K (spectrum b) and the regenerated
sample (spectrum c); spectrum d is difference spectrum between the sample evacuated
at 323 K (not shown) and the regenerated sample (spectrum c). The green spectrum
‘a−d’ in the right inset is the subtraction of ‘a’ by ‘d’.
128
groups indicating that probably some adsorbed monodentate propoxy species reacted
with remaining Ti−OH during heating. The right inset in Figure 5.6A shows the change
of the spent sample in the Ti–O–C/C–C stretching region during heating. After heating
the spent sample in vacuum at 573 K, the intensity decrease of two bands at 1135 and
1090 cm−1 was observed. As seen from Figure 5.6A (spectrum a), the spent catalyst was
fully covered by bidentate propoxy species, carboxylates as well as aldehyde adsorbed on
the surface. The presence of a bidentate propoxy species on the spent catalyst is indicated
by the pattern of bands in the CH stretching region at 2970 [νas (CH3 )], 2932 [νas (CH2 )],
2905 [ν(CH)], and 2869 cm−1 [νs (CH3 )] together with two sharp bands in the Ti−O−C
and C−C stretching region at 1123 and 1090 cm−1 [4, 37]. The band at 1123 cm−1 can
be assigned to Ti−O−C stretching, while for PO or 1,2-propanediol adsorbed on Au/TiO2
frequency of this band is ca. 20 cm−1 higher [36, 37]. The assignment for the 1090 cm−1
band can be ν(Ti−O−C) [4], or ν(C−C) [36, 42]. Aldehyde remaining on the surface
is evidenced by the peak at 1723 cm−1 for C−
−O vibration and the weak peak at 2716
−1
cm−1 for H−C−
−O, while the peak at ca. 2820 cm is superimposed by bands of νs (CH3 )
and νs (CH2 ). Originally, the band at 1723 cm−1 was superimposed by a much stronger
band at 1685 cm−1 of the spent sample evacuated at 323 K (not shown). However, the
band at 1685 cm−1 disappeared after evacuation at 573 K, while the band at 1723 cm−1
remained intact. The strong and broad bands at 1435 cm−1 (overlapped with the sharp
band at 1448 cm−1 for δas (CH3 )/δ(CH2 )) and 1544 cm−1 are assigned to νs (COO) and
νas (COO) of surface acetate [43–45]. Additional bands at 1576 cm−1 and 1331 cm−1
are tentatively assigned to surface formate [45, 46]. The surface acetate and formate
species are mainly from the oxidation of the strongly adsorbed bidentate propoxy species
as proposed in literature [4, 37]. IR adsorption by acetate/formate is less pronounced
on the spent catalyst evacuated at 323 K. After heating the sample to 573 K in vacuum,
the increase of bands at 1435 cm−1 and 1544 cm−1 corresponded to the decrease of
bands in the Ti−O−C/C−C stretching region between 1050 and 1200 cm−1 (the right
inset in Figure 5.6A). It is more likely to assign the bands at 1140 and 1090 cm−1 to
ν(Ti−O−C) since these two bands turned into O−
−C−O vibrations and have the same
width and reduced intensity. Probably lattice oxygen took part in the breakage of C−C
bonds and consequent oxidation while heating up to 573 K in vacuum. After regeneration, a small amount of carbonate or carboxylates remained on the surface as indicated
by strong adsorption at ca. 1540 and 1440 cm−1 (Figure 5.6 spectrum c), which were
merely spectators on the surface since the catalyst activity was fully restored after regen-
129
5.3. RESULTS
eration.
As shown in Figure 5.6, on the spent Au/TiO2 catalyst there was limited adsorption
of CO on both gold and Ti4+ similar to what was observed for the spent Au/Ti-SiO2
catalyst. The position of weak CO band located at 2096 cm−1 (shifted from 2108 cm−1
at low coverage) indicates that the accessible gold atoms are typical Au0 , although 2096
cm−1 is at the lower side for CO adsorption on Au0 [24, 25]. On the regenerated sample,
CO adsorption on metallic gold is clearly featured by the sharp band at 2107 cm−1 ,
which was originally at 2123 cm−1 at low CO coverage. The red shift of ν(CO/Au0 ) is
very typical for supported gold catalysts due to the fact that the backbonding from gold
atoms to 2π* CO orbital is lacking [31–33]. A weak peak located at 2138 cm−1 upon
CO adsorption can also be identified and it gradually evolved to lower frequencies at
higher CO overages, seemingly saturated as the small band at 2127 cm−1 . The small
band at 2127 cm−1 can be assigned to vibration of
13
CO arising from natural abundance,
4+
which interacts with 5-fold coordinated Ti atoms (Ti4+
5c , or β-Ti ) [23, 29, 47]. This
assignment can also be supported by Figure 5.13, which shows CO adsorption on bare
TiO2 (P25). However, it is not unambiguous to distinguish the contribution from CO
adsorbed on Auδ+ , which also gives a peak located between 2140 and 2125 cm−1 . The
−1
−1
band of ν(CO) adsorbed on Ti4+
at
5c located at 2179 cm , which shifted from 2190 cm
low CO coverage on the clean and dehydrated Au/TiO2 , while on the spent catalyst this
band was at a bit lower frequency (2185 cm−1 ) at low CO coverage. On the regenerated
Au/TiO2 the peak at 2214 cm−1 (shifted from 2229 cm−1 ) is assigned to ν(CO) on the
4+
stronger Lewis acid site Ti4+
[23, 29, 47].
4c , or α-Ti
The XPS survey spectrum given in Figure 5.7 also indicates the carbonaceous surface
after the reaction. The O 1s, Ti 2p, and Au 4f XPS peaks of the spent catalyst are weaker
than their counterparts after regeneration, while the C 1s peak is much stronger. Since
the samples were kept in-situ, the surface carbon was most likely not from contamination.
Peak fitting of the O 1s XPS spectra shows five components on the catalyst surface, which
are located at BE 529.9, 531.2, 532.0, 532.8, and 533.6 eV, respectively. The components
at 529.9 and 531.2 eV are assigned to oxygen from Ti−O−Ti and Ti−OH, respectively
[48]. Combining the IR data by Figure 5.6 and their intensity change after regeneration,
the components of the O 1s line at 532.0, 532.8, and 533.6 eV can be assigned to CO2–
3
or O−
−O in ketone/aldehyde, and C−O−Ti species, respectively
−C−O in carboxylates, C−
[49, 50]. The Au 4f7/2 line is located at BE 83.4 eV for both the spent and regenerated
catalyst and the spin doublet separation is 3.6 eV for the Au 4f line. The surface gold is
130
O 1s
(A)
Intensity (a.u.)
Ti 2p
Regenerated
C 1s
Spent
Au 4f
Ti 3s
Ti 3p
800
700
600
500
400
300
200
100
0
Binding energy (eV)
(B)
Au 4f
(C)
7/2
Regenerated
Spent
92
O 1s
5/2
Intensity (a.u.)
Intensity (a.u.)
Au 4f
Regenerated
Spent
88
84
80
Binding energy (eV)
536
532
528
524
Binding energy (eV)
Figure 5.7: XP survey spectrum (A), Au 4f photoelectron lines (B), and O 1s photoelectron lines (C) of the spent and regenerated Au/TiO2 catalyst. The spent catalyst was
dehydrated in flowing helium at 473 K. Spectra were referenced to the C 1s line at 284.9
eV.
simply in its metallic form [15, 51]. The difference in the intensity of the Au 4f lines from
the spent and regenerated catalyst is much less profound than the difference observed
from CO adsorption by IR. It can be concluded that XPS is not sensitive for detecting the
active gold sites in our reaction since this method counts all the gold atoms at the surface
rather than those specifically at edges and corners.
5.3.3 CO adsorption on Au/Ti-SiO2 treated by O2 and H2
The effect of O2 /H2 pretreatment on the gold oxidation state is shown in Figure 5.8. On
the regenerated Au/Ti-SiO2 sample (Figure 5.8A), two bands can be distinguished: the
sharp one at 2108 cm−1 and a shoulder at 2128 cm−1 . They are assigned to CO adsorbed
131
5.3. RESULTS
0.025
δ+
1 8 CO Au
CO Au0
1 1
c1
0
2150
2100
Wavenumber (cm 1
111
c10
185
0.005
0
2200
δ+
1 8 CO Au
158
0.01
a10
a1
1 1
0.005
Abs.
0.01
(B)
0.02
0.015
158
185 CO Ti4+
Abs.
0.015
CO OH
0.02
108
(A)
CO Au0
0.025
2200
2050
2150
2100
Wavenumber (cm 1
2050
0.025
0.015
a2-a1
2128
0.02
Abs.
0.002
0.01
b1
118
185
0.005
b2-b1
b10
158
Abs.
(D)
CO Au0 10
(C)
c2-c1
0
2200
2150
2100
Wavenumber (cm 1
2050
2180
2160
2140
2120
2100
-1
Wavenumber (cm
2080
2060
)
Figure 5.8: Progressive CO adsorption on the Au/Ti-SiO2 sample after different pretreatment: (A) after regeneration at 573 K followed by 0.5 hour in vacuum at 573 K and
quenching to 90 K; (B) after regeneration at 573 K, 0.5 hour in vacuum at 573 K, 0.5
hour in 5 vol% O2 at 423 K, 0.5 hour in vacuum at 423 K and quenching to 90 K; (C)
after regeneration at 573 K, 0.5 hour in vacuum at 573 K, 0.5 hour in 5 vol% O2 at 423
K, 0.5 hour in vacuum at 423 K, 0.5 hour in 5 vol% H2 at 423 K, 0.5 hour in vacuum
at 423 K and quenching to 90 K. Section (D) gives incremental adsorption between the
second and first dose. The treated sample in vacuum at 90 K was used as background.
CO pressure increased from 0.01 mbar to 0.20 mbar after 10 doses with increment of ca.
0.02 mbar per dose.
on Au0 and Auδ+ , respectively. After pre-oxidation at 423 K (Figure 5.8B), the ν(CO/Au0 )
band became much weaker, while the ν(CO/Auδ+ ) almost remained the same. By further
treating the pre-oxidized sample in H2 (Figure 5.8C), the ν(CO/Auδ+ ) band could not
be clearly observed. The incremental adsorption of CO between the second and first
dose (Figure 5.8D) indicates that on the reduced Au/Ti-SiO2 there were mainly Au0 sites
and that the remaining small amount of Auδ+ may come from the interfacial Au−O−Ti.
132
As for the regenerated sample, there may be more Au−O−O−Ti sites at the perimeter
as proposed in literature [35]. And on the pre-oxidized sample, it seems that Au0 atoms
were covered by molecular oxygen even after evacuation at 423 K, which is in accordance
with the high desorption temperature of O2 on small gold nanoparticles [52]. However,
when Au/TiO2 was used, the disturbance of ν( 13CO) on Ti4+ to ν(CO) on Auδ+ obscured
the band assignment and thus not discussed. On the other hand, treating Au/TiO2 in the
same way as Figure 5.8 but at 323 K (the typical epoxidation temperature for Au/TiO2 )
showed no significant change in the adsorption intensity of CO on Auδ+ and Au0 when
comparing to literature [23, 35], probably due to the low pre-oxidation temperature and
low gold loading.
5.3.4 CO adsorption on Au/TiO2 in the presence of H2
Figure 5.9 shows the progressive CO adsorption on Au/TiO2 in the presence of H2 at 90
K. In the OH stretching region before CO dosing, three sharp bands at 3734, 3675, and
3420 cm−1 together with several weak shoulders at 3715, 3690, 3658, and 3648 cm−1
can be observed. During the H2 treatment at 323 K, trace amount of H2 O was formed
as evidenced by the weak adsorption of δ(H2 O) at 1617 cm−1 . Upon CO dosing, a sharp
band in the OH stretching region at 3648 cm−1 appeared together with a band for CO2 at
ca. 2350 cm−1 . Surprisingly, the pressure in the IR cell continued to decrease till the 3rd
or 4th dose. CO adsorbed on gold from the first 3 doses (via the 5 µL loop) was almost
fully and instantly converted to CO2 . The CO2 vibration band reached it peak intensity
after the 3rd dose and gradually decreased after subsequent CO dosage very likely due
to the replacement of CO on the surface. The change in the OH stretching region after
CO dosing was more difficult to explain. The band at 3734 cm−1 remained at its original
position, which is generally assigned to impurities such as Si−OH [36, 47]. The weak
shoulder at 3715 cm−1 shifted to 3706 cm−1 upon CO dosing and can be assigned to
isolated Ti−OH of the anatase phase [53, 54]. The major band centered at 3679 cm−1
decreased its intensity as the CO coverage increased and shifted to the broad band at
3547 cm−1 due to the interaction with CO. This major band can be assigned to Ti−OH of
anatase [47, 55]. The assignment of the 3425 cm−1 band was proposed to be from water
molecules or hydroxy groups on TiO2 (rutile) [36, 55, 56]. The sharp band at 3648 cm−1
developed into the band at ca. 3500 cm−1 with a shift of 150 cm−1 as the CO coverage
increased. A clear assignment of the 3648 cm−1 band is difficult in our case. This band
133
5.3. RESULTS
(mbar)
0.04
(A)
Pressure in cell
4.80
4.75
0
2
4
6
8
10
Abs.
dose of CO
11
2
1
dose 0
90 K in 4.82 mbar H
3800
3600
3400
3200
2250
2000
1750
1500
2
1250
1000
-1
(C)
2103
2179
(B)
501
547
648
0.1
0.08
se 11
0.06
2127
0.04
0.02
se 1
2213
3492
Abs.
Abs.
425
7 4
0.01
679
se 0
2165
2157
Wavenumber (cm )
dose 11
dose 1
0
3800
3700
3600 3500 3400
Wavenumber (cm 1
3300
3200
2200
2150
2100
Wavenumber (cm 1
2050
Figure 5.9: Progressive CO adsorption on Au/TiO2 in the presence of H2 : (A) wide
frequency range, the inset shows the pressures in the IR cell after each dose of CO;
(B) spectra in the ν(OH) region; (C) difference spectra in the ν(CO) region using the
spectrum of dose 0 as the background. The regenerated Au/TiO2 sample was kept in 50
mbar H2 (pre-dried with liquid nitrogen trap) at 323 K for 10 min followed by quenching
down to 90 K and adjusting the H2 pressure by evacuation to 4.82 mbar before CO dosing.
All spectra were taken at 90 K.
was very weak before CO dosing. Upon CO dosing in the presence of H2 , this band
became very strong and sharp. Considering the decrease of cell pressure after CO dosing
and the CO2 formation, it is clear that CO reacted with lattice oxygen in the presence of
H2 forming CO2 and hydroxy groups and/or water. Water formation can be evidenced
by the weak band at 3492 cm−1 upon CO dosing (red spectrum, dose 1 in Figure 5.9B),
which can be assigned by ν(OH) bonded to water. On the other hand, from analysis
of the difference spectra in the OH stretching region(not shown), the perturbation of a
component at 3658 cm−1 upon CO dosing also contributed to the band increase at 3648
134
cm−1 but to a much lesser extent. The contribution of pre-adsorbed water to the bands at
3648 cm−1 , either by the reaction with CO or by the dissociation of water on coordination
unsaturated Ti4+ sites, was excluded by the experiment shown in Figure 5.10.
3435
f
3500
a
2107
Abs.
f
1616
a
0.02
-1
Wavenumber (cm )
Figure 5.10: Progressive CO adsorption on Au/TiO2 pre-covered by trace amount of H2 O:
(a) in vacuum; (b–e) dose 1 to 4, in 0.01, 0.03, 0.05, 0.07 mbar CO respectively; (f) dose
10, in 0.21 mbar CO. The sample was pre-treated by partially dried H2 at 323 K and
evacuated at 323 K for 1 hour. All spectra were taken at 90 K.
In Figure 5.10, the coverage of water before CO adsorption was evidenced by the
relatively stronger band of δ(H2 O) at 1616 cm−1 and a band of ν(OH) perturbed by H2 O
at 3500 cm−1 . After CO adsorption, no significant change occurred to the 3648 cm−1
band. With the increased coverage of CO, the band for δ(H2 O) gradually shifted to 1645
cm−1 . The band at 3500 cm−1 gradually shifted to 3435 cm−1 . The band for ν(CO) on
Au0 shifted from 2119 cm−1 at the low CO coverage and saturated at 2107 cm−1 . It seems
that water is not necessary for the band at 3648 cm−1 although its decrease in intensity
was observed concomitantly with outgassing of water molecules on TiO2 surface in other
studies [56].
Figure 5.9 shows that the bands of ν(CO) at 2213(α-Ti4+ ), 2179(β-Ti4+ ), 2165(γTi4+ ), and 2103(Au0 ) cm−1 did not shift with the CO coverage in the presence of H2 . The
band at 2157 cm−1 is assigned to ν(CO) interacting with OH groups. The small band at
2127 cm−1 is assigned to vibration of
4+
with β-Ti
13
CO (arising from natural abundance) interacting
[23]. The bands at 2165 and 2157 cm−1 would only appear at very high CO
pressures if there were no H2 in presence. After the experiment performed in Figure 5.9,
the IR cell was immediately evacuated at 90 K and the progressive CO adsorption on this
135
5.3. RESULTS
sample was repeated but without H2 in presence. The results are given in Figure 5.11.
The pressure in the IR cell increased monotonously by each dose of CO with an average
increment of ca. 0.02 mbar, which is significantly different from what was observed
in Figure 5.9A, where CO and H2 reacted leading to a pressure decrease. The band at
3648 cm−1 remained. The bands at 2165 and 2157 cm−1 were not observed. It can be
concluded that the presence of H2 significantly enhanced the adsorption of CO on TiO2
at low temperature.
3680
3648
2106
0.06
0
3700
3600 3500 3400
Wavenumber (cm−1)
3300
3200
2127
dose 11
2117
2213
2188
Abs.
0.04
0.02
3800
(B)
0.08
3425
Abs.
3735
0.01
2179
0.1
(A)
2200
2150
2100
Wavenumber (cm 1
dose 1
2050
Figure 5.11: CO adsorption on Au/TiO2 after removing hydrogen (to be compared with
Figure 5.9, see text). The following CO pressures after each dose were used (in mbar):
0.00, 0.01, 0.03, 0.05, 0.07, 0.09, 0.11, 0.13, 0.16, 0.18, 0.20 for dose 1 to dose 11 (red
to blue). All spectra were taken at 90 K.
The same experiments were performed on the bare TiO2 (P25) sample for reference.
The results are shown in Figures 5.12 and 5.13. The pressure in the IR cell increased
monotonously after each CO dose when H2 was present as shown in Figure 5.12, which
in turn confirmed that CO and H2 reacted on Au/TiO2 with lattice oxygen as indicated
in Figure 5.9. The results shown in this study are consistent with what Widmann and
Behm [57] revealed. They showed that only for Au on TiO2 can TiO2 be reduced by CO.
The presence of H2 simply improved the CO coverage at low CO pressures featured by
the stronger interaction of CO with hydroxy groups (3400 – 3600 cm−1 , 2159 cm−1 ) and
the weak acidic γ-Ti4+ (2165 cm−1 ) as shown in Figure 5.12B/C. After the experiment in
Figure 5.12 was done, the cell was fully evacuated and CO adsorption on TiO2 without
H2 present was performed at the same temperature (Figure 5.13). The only and obvious
difference in ν(CO) region as compared to Figure 5.11 is that no CO on gold was observed. The remaining peaks were exactly the same, which also confirms the assignment
136
(A)
Pressure in cell
(mbar)
0.04
5.30
5.20
5.10
0
2
4
6
8
10
Abs.
dose of CO
9
2
1
dose 0
90 K in 5.12 mbar H
3800
3600
2
3400
3200
2250
2000
1750
1500
1250
-1
0.08
3432
(C)
0.06
Abs.
Abs.
2165
2159
2179
(B)
3637
0.01
3729
0.1
3547
3520
3681
Wavenumber (cm )
dose 1 to 9
0.04
2213
0.02
2127
dose 0
dose 9
dose 1
0
3800
3700
3600 3500 3400
Wavenumber (cm−1)
3300
3200
2200
2150
2100
Wavenumber (cm 1
2050
Figure 5.12: Progressive CO adsorption on TiO2 (P25) in the presence of H2 : (A) wide
frequency range, the inset shows the pressures in the IR cell after each dose of CO;
(B) spectra in the ν(OH) region; (C) difference spectra in the ν(CO) region using the
spectrum of dose 0 as the background. The TiO2 sample was calcined in O2 at 573 K and
treated in 50 mbar H2 (pre-dried with liquid nitrogen trap) at 323 K for 20 min followed
by quenching down to 90 K and adjusting the H2 pressure by evacuation to 5.12 mbar
before CO dosing. All spectra were taken at 90 K.
of the 2127 cm−1 band to
13
CO. In the OH stretching region showed no obvious sharp
bands at 3648 and ca. 3420 cm−1 .
5.3.5 C3 H6 adsorption on Au/TiO2 in the presence of CO
Figure 5.14 shows the IR spectra of C3 H6 adsorption on Au/TiO2 in the presence of
CO at 230 K (above the boiling point of C3 H6 226 K at atmospheric pressure). The main
adsorption band of ν(CO) on gold in 10 mbar CO located at 2108 cm−1 . Two weak bands
137
5.3. RESULTS
2179
3675
0.1
0.01
3644
2188
0.04
0.02
2213(2229)
Abs.
3433
Abs.
0.06
2127(2139)
3727
0.08
0
3800
3700
3600 3500 3400
Wavenumber (cm 1
3300
3200
2200
2150
2100
Wavenumber (cm−1)
2050
Figure 5.13: CO adsorption on TiO2 after removing hydrogen. The following CO pressures after each dose were used (in mbar): 0.02, 0.04, 0.07, 0.11, 0.14, 0.18, 0.22, 0.26,
0.30 for dose 1 to dose 9 (red to blue). All spectra were taken at 90 K.
at 2132 and 2069 cm−1 were also observed. The band at 2132 cm−1 can be assigned
to CO on Auδ+ . The broad band at 2060 – 2070 cm−1 was also observed in several
other studies when at relatively high CO pressures (e.g. 20 mbar) and temperatures
(e.g., at room temperature) [58, 59]. In our experiment, this band was observed after a
few minutes after the sample contacted with CO. It stabilized without further evolution.
As proposed in literature, this weak band is more likely due to morphology change of
gold nanoparticles (lowered coordination number of gold atoms) in CO but to a small
extent in our case. The progressively dosed C3 H6 gradually replaced CO on the surface
interacting with both Ti4+ and Au as indicated by the weakened CO bands in Figure
5.14A. It is known that C3 H6 forms a stronger π-complex with Ti4+ than CO [60], thus
all CO adsorbed on Ti4+ was almost removed. It seems that C3 H6 has also a stronger
interaction with gold than CO does. When there was 2.5 mbar C3 H6 present, the band
of CO on gold red-shifted by 30 cm−1 to 2079 cm−1 . Interaction with C3 H6 does not
lead to reconstruction of gold nanoparticles. This was confirmed by low temperature CO
adsorption on the Au/TiO2 sample which had contacted with 50 mbar C3 H6 at 323 K
for 30 min followed by evacuation. Thus, the red shift of ν(CO/Au) was more likely an
effect of change in the electron density of gold atoms. The band at 2079 cm−1 may be
assigned to ν(CO/Auδ− ) [61].
The presence of C3 H6 on the catalyst surface was confirmed by the IR spectra in the
CH stretching and bending regions as given in Figure 5.14B. The details and assignment
138
2183
2108
0.01
4+
CO-Ti
0.05
2132
2178
g
2069
a
b
2200
2150
2100
2050
c
d
e
f
g
2079
Absorbance (a.u.)
b
CO-Au
(A)
2150
2100
2050
2000
1950
-1
a - g
0.02
b - g
2894
2923
2856
0.01
1300
2950
1400
2977
a - g
1500
3009
1600
3081
1700
3059
3736
Absorbance (a.u.)
1373
1453
1632
3421
3648
3676
Wavenumber (cm )
(B)
3800
3600
3400
3200
3000
2800
-1
Wavenumber (cm )
Figure 5.14: Progressive C3 H6 adsorption on Au/TiO2 in the presence of CO at 230 K:
(A) the CO stretching region; the spectra were shifted for clarity; spectra in the inset
were not shifted; (B) the OH and CH stretching region; the inset shows C−
−C stretching
and CH bending region. Spectra (gas phase compensated): (a) in vacuum before CO and
C3 H6 adsorption; (b) in 10.3 mbar CO after 20 min; (c) in 10.3 mbar CO + 0.5 mbar
C3 H6 ; (d) in 10.3 mbar CO + 1.0 mbar C3 H6 ; (e) in 10.3 mbar CO + 1.5 mbar C3 H6 ; (f)
in 10.3 mbar CO + 2.0 mbar C3 H6 ; (g) in 10.3 mbar CO + 2.5 mbar C3 H6 .
139
5.3. RESULTS
g
.005
Abs.
Abs.
.005
f
e
g
c
d
c
3600
3400
3200
3000
Wavenumber (cm−1)
2800
2600
1800
1600
1400
1200
Wavenumber (cm−1)
1000
Figure 5.15: Difference spectra of progressive C3 H6 adsorption on Au/TiO2 in the presence of CO at 230 K (see Figure 5.14, spectrum b as the background).
of bands from C3 H6 adsorbed on the Au/TiO2 sample are given in Figure 5.15 and Table
5.3. The decrease of the main bands at 3676 and 3648 cm−1 and increase of the broad
band centred at 3410 cm−1 is attributed to the formation of hydrogen bond between the
allylic hydrogen of C3 H6 and oxygen in Ti−OH [62]. The sharp band of ν(C−
−C)at 1632
cm−1 is slightly lower than the gas-phase C3 H6 , indicating a weak π-bonding to Ti4+ and
Au. Three bands above 3000 cm−1 were observed. The bands at 3081 and 3009 cm−1 can
be assigned to νas (CH2 ) and ν(CH), respectively. The origin of the 3059 cm−1 is unclear.
This band was not observed for C3 H6 adsorbed on TiO2 (anatase) [62]. We tentatively
assign this 3059 cm−1 band as νas (CH2 ) of C3 H6 adsorbed at the Au–Ti interface or on
gold. However, further investigation may be needed. After evacuation, no bands in the
CH stretching region and for C−
−C could be observed. The weak bands at 1576 and 1252
cm−1 (Figure 5.15) can be assigned to carbonates due to the presence of CO [41].
Since the co-adsorption of CO and C3 H6 was performed at a low temperature of 230
K, it is interesting to know how C3 H6 would compete with CO at temperatures where
the epoxidation occurs. In Chapter 4, we demonstrated that CO can switch off propene
hydrogenation over the gold–titania catalysts. However, it is not clear if the role of CO
is on gold or the support or both. If the effect of CO is on gold, propene should not
have much effect on CO oxidation if we assume all the chemistry happens on the low
coordinated gold atoms close to the Au–Ti interface.
140
Table 5.2: Summary of frequency observed (cm−1 ) for CO adsorption for different catalysts in this study a
Au/SiO2
Au/Ti-SiO2
S
R
2181 2185
S
Au/TiO2
R
2158
2136
2158
2136
2158
2136
2138
2127
2105
2099
2095
2125, 2128
2118, 2121
2107
a: S, spent; R, regenerated
2108
2096
2123
2107
2079
Ref.
on Ti4+ of Ti-SiO2
on α-Ti4+ of TiO2 , at low CO coverage
on α-Ti4+ of TiO2 , at high CO coverage
on β-Ti4+ of TiO2 , at low CO coverage
on β-Ti4+ of TiO2 , at high CO coverage
on γ-Ti4+ of TiO2
with isolated OH groups on TiO2
with OH groups on SiO2 and/or Ti-SiO2
physisorbed CO
not fully resolved, in Figure 5.6C, tentatively 13 CO on β-Ti4+ at low CO coverage
13
CO on β-Ti4+
on Auδ+
on Au0 , low CO coverage
on Au0 , high CO coverage
on Auδ− , C3 H6 present
[29]
[23, 29, 47]
[23, 29, 47]
[23]
[23]
[29, 30]
[23, 29, 47]
[23, 34, 35]
[24, 25]
[61]
2185
2178
2229
2214
2190
2179
2165
2157
Assignment
141
5.3. RESULTS
The effect of C3 H6 on CO oxidation over Au/TiO2 is shown in Figure 5.16. It can be
seen that CO oxidation was suppressed by C3 H6 at either 303 K or 373 K. No significant
deactivation was observed in the first 8 hours. However, after flushing in C3 H6 (period
III) and further flushing in helium till C3 H6 concentration lower than 10 ppm, the activity
in CO oxidation was severely reduced. Co-feeding of C3 H6 completely suppressed CO
oxidation. At 373 K after C3 H6 flushing (11.5 – 13.5 h), the activity in CO oxidation
gradually restored very likely due to desorption of adsorbed propene. However, after
period IV when C3 H6 was removed from the CO/O2 stream (the last 1 hour), the activity
could not be restored. It seems that propene formed a strongly adsorbed species at
the Au–Ti interface when CO oxidation was proceeding (O adatoms available) and thus
deactivated the catalytic site. The Au/TiO2 catalyst, after being tested at 303 K as shown
in Figure 5.14a, was then heated to 333 K and tested for the direct propene epoxidation.
The epoxidation activity at 333 K is given in Figure 5.17. There was only ca. 30% loss
in the maximum PO formation rate. The trajectory of rH2 O against rPO was still on par
with the original performance of the catalyst as indicated by Figure 5.17c. It can be
concluded that the strongly adsorbed species formed from C3 H6 during CO oxidation can
be removed by hydro-epoxidation.
Table 5.3: IR frequencies (in cm−1 ) of C3 H6 and their assignment
vibrations
gas phase[63]
in polyethylene[64]
Ir4 /Al2 O3 [65]
on TiO2 [62]
this work
νas (CH2 )
3090
3075
3077
3090
3081, 3059
ν(CH)
3013
3008
3014
3010
3009
νs (CH2 )
2992
2977
2979
2990
2977
νas (CH3 )
2954, 2933
2958, 2933
2968
2960
2950, 2923
νs (CH3 )
ν(C−C)
2870
2912
2859
2930, 2870
2894, 2856
1652
1645
1640
1635
1632
δas (CH3 )
1474, 1443
1449, 1443
1458, 1442
1460, 1440
1453, 1435
δ(CH)
1419, 990
1411, 988
1296
δs (CH3 )
1378
1370
1376
δ(CH2 )
1298
1293
1415
ρ(CH2 )
1229
1169
1170
1171
ρ(CH3 )
1171, 1045
1040
1049, 1007
1047
1415, 1010
1380
1373
1295
142
as c ncen ra
n ( m)
5000
II
I
I
II
4000
II
III
I
IV
I
3000
×0.5
6
2000
1000
(a)
0
0
2
4
6
8
T me (h)
10
12
14
16
as c ncen ra
n ( m)
5000
4000
3000
×0.5
6
2000
1000
(b)
2
0
0
2
4
6
8
T me (h)
10
12
14
16
Figure 5.16: Effect of C3 H6 on CO oxidation over Au/TiO2 : (a) at 303 K; (b) at 373 K . I,
1500/4750 ppm of CO/O2 , helium balance; II, helium flush; III, 10000 ppm C3 H6 flush;
IV, 1500/4750 ppm of CO/O2 , co-feeding 10000 ppm C3 H6 . 150 mg catalyst, GHSV
−1
20000 mL·g−1
cat h .
5.4 Discussion
The oxidation state of gold in the direct propene epoxidation can be summarized in the
scheme given in Figure 5.18. Our data of propene adsorption on the Au/TiO2 catalyst
suggests that the low-coordinated gold atoms become negatively charged in the presence
of propene, which leads to a broad carbonyl band at a low frequency of 2079 cm−1 . The
negatively charged gold has been also recently well described by Chakarova et al. on
Au/SiO2 system by CO reduction [61]. Combining our previous work of propene adsorption on Au/SiO2 [5], we would not expect much difference for Au/Ti-SiO2 in terms of
charge transfer over gold in different atmospheres. In our case for Au/TiO2 , contributions from propene adsorbed on gold and the support near the interface may both count
143
5.4. DISCUSSION
2
x 10
−7
−6
2
(a)
x 10
(b)
Au/TiO , PO
Au/TiO , H O
2
2
(mol⋅g−1
⋅s−1)
cat
rPO (mol⋅g−1
⋅s−1)
cat
1.5
1.5
1
rH
2
O
1
2
0.5
0
0.5
0
30
60
Time (min)
90
120
0
0
30
60
Time (min)
90
120
−6
1
x 10
(c)
0.6
0.4
rH
2
O
(mol⋅g−1
⋅s−1)
cat
0.8
0.2
0
0
0.5
1
1.5
rPO (mol⋅g−1
⋅s−1)
cat
2
−7
x 10
Figure 5.17: The activity of Au/TiO2 in the direct propene epoxidation at 333 K after
testing the effect of C3 H6 on CO oxidation as shown in Figure 5.16a. GHSV 10000
−1
mL·g−1
cat h , 10/10/10/70 H2 /O2 /C3 H6 /He mixture. (a) PO formation rate; (b) water
formation rate; (c)relation between the PO and water formation rates. Grey dots are
data of three reaction cycles from Figure 5.1 for reference.
to increase the electron density on gold. But for Au/Ti-SiO2 , the contribution of charge
transfer from the support and the role of the support oxygen may be less pronounced
[66]. Propene adsorption on gold in this study was performed at a relatively low temperature (230 K) and in an atmosphere where oxygen was absent. However, under the real
reaction conditions, the species adsorbed on gold are much more complicated. Oxygen
molecules are activated on gold by charge transfer from reduced gold forming gold–
oxygen complex [67, 68]. When hydrogen is present, OOH is formed [1]. The study
by Bravo-Suárez et al. on propane oxidation in hydrogen and oxygen over gold catalysts suggests that even propane can be oxidized by Ti–OOH forming isopropoxy species
144
[69]. The real role of propene modifying the reactivity of gold may be complicated. On
one hand, propene can pi-bonding to gold and reduce the water formation as shown
by Nijhuis et al. in their XANES study [5]. The spectra shown in Figure 5.14 provide
another evidence of the electron-donating ability from propene to gold, which seems to
be even stronger than CO. On the other hand, oxidants adsorbed on gold may also react
with propene forming strongly adsorbed species as proposed by Nijhuis et al.[5] and as
suggested by the CO oxidation experiment cofed with propene in this study.
Although propene adsorption on either gold single crystal surfaces or a model Au/TiO2
catalyst is found to be weak [70, 71], it may be not the case for a real catalyst. On a model
Au/TiO2 (110) catalyst in the work by Ajo et al. [71], the most stable sites for propene
adsorption were those at the edge of gold islands with a TPD peak at ca. 240 K, while for
2D Au islands the temperature was higher till 310 K. For a real Au/TiO2 catalyst active
for the direct propene epoxidation, the TPD peak of propene desorption on gold and/or
at the perimeter was found to be at 323 K [14]. TPD data over Au/Ti-SiO2 by BravoSuárez et al. suggests that the presence of gold and titania can significantly enhance
the adsorption of C3 H6 and a desorption temperature higher than 450 K was reported
by them [72]. However, it would be more interesting to know if propene and oxygen
can cooperatively adsorb on gold just as CO and oxygen on gold clusters [73, 74]. In
the coadsorption of CO and oxygen on gold clusters, it is proposed that CO acts as the
electron donor and oxygen as the acceptor, both not competing for adsorption sites but
cooperating [73, 74]. Our finding on the stronger electron-donating ability of propene
than CO on gold might provide a starting point to this question.
The desorption temperature for O2 on gold is generally high as reported in literature.
In the early work by Hayashi et al. [14], the TPD curve of Au/TiO2 which had been
treated at 573 K in O2 showed that O2 desorbed at above ca. 500 K. The desorption
of recombined O adatoms was reported to be 470 K on Au(100) and above 500 K on
Au(111) [70], and even higher for thin gold nanoparticles supported on TiO2 [52]. The
results in Figure 5.8 of Au/Ti-SiO2 treated by O2 and H2 also indicate that the removal of
O2 needs a relatively high temperature. Evacuation at 573 K seems not enough for us to
fully remove oxygen on gold for Au/Ti-SiO2 . The presence of O2 on gold, or preferably
at the interface [75], makes the gold atoms electron deficient. The Auδ+ sites can be
reduced by hydrogen to Au0 as shown in Figure 5.8.
The CO adsorption on spent catalysts, as shown in Figures 5.4 and 5.6, indicates
that the gold sites during reaction are fully covered by deactivating species and reac-
5.5. CONCLUSIONS
145
Figure 5.18: Schematic of gold oxidation state under conditions for propene epoxidation
tion intermediates. Very limited gold sites were accessible for the probe molecule CO,
even if the Au/Ti-SiO2 catalyst still had half its activity in PO formation after deactivation. CO adsorbed on these remaining gold sites shows the typical band location for Au0 .
However, these CO vibration bands show a long tailing shape towards lower wavenumbers indicating certain degree of reduction of the supports due to the adsorbed organic
species and thus less electron transfer from gold to the support. IR using CO as the probe
molecule may be not the preferred method to detect gold oxidation state under the real
reaction conditions due to the fully covered surface. When using other bulk methods like
XANES or XAFS one may also need to pay attention to the small contribution from the
low-coordinated gold atoms if the gold particle size is not small enough.
A very interesting phenomenon observed in this study is that the presence of H2
greatly increased the amount of CO adsorbed on the TiO2 support at low temperature,
as shown in Figures 5.9 and 5.12. This may provide one explanation for the preferential
oxidation (PROX) of CO over Au/TiO2 with aid of H2 . On Au/TiO2 , the metal–support
interface is considered to be the active site for O2 dissociation [75, 76]. It is also found
that the diffusion of CO adsorbed on TiO2 to the Au–Ti interface contributes more to CO
oxidation over Au/TiO2 at low temperature [76]. Thus, besides a facile dissociation of
O2 in the presence of H2 , H2 may also promote the adsorption of CO on the support and
consequently increase the reaction rate.
5.5 Conclusions
On both Au/TiO2 and Au/Ti-SiO 2 catalysts investigated after use in the direct propene
epoxidation, the active gold sites can hardly be detected by CO molecules. The low-
146
coordinated gold atoms are most likely occupied by propoxy species and carboxylates,
which are most abundant on the catalyst surface after the reaction. Those gold atoms
which can still be detected by CO show a very weak band at 2095 cm−1 indicating their
metallic state. XPS spectra show that the oxidation state of surface gold atoms is metallic
before and after reaction. However, XPS is not a sensitive tool to pinpoint those lowcoordinated atoms at the perimeter of gold nanoparticles.
Calcination in oxygen at 573 K restores the catalyst activity by removing the deactivating species and carbonaceous spectators. Oxygen can strongly adsorb on gold or at
the perimeter of gold nanoparticles over the Au/Ti-SiO 2 catalyst at the reaction temperature for propene epoxidation (e.g., 423 K), leading to positively charged gold atoms
as evidenced by the carbonyl band on gold at ca. 2125 cm−1 . Reduction in hydrogen
at the reaction temperature removes adsorbed oxygen on the catalyst and restores the
low-coordinated gold atoms to electron-neutral state, which give carbonyl bands at 2100
- 2110 cm−1 . Over the Au/TiO2 catalyst, it has been observed that the presence of hydrogen at low temperatures can significantly enhance CO adsorption on the support, which
may help explain the role of hydrogen in the preferential oxidation of CO.
The low-coordinated gold atoms become negatively charged in the presence of propene.
Propene shows a stronger chemisorption than CO on both gold and Ti4+ . Progressive
propene adsorption leads to the weakening and broadening of CO band on gold with a
red shift of ca. 30 cm−1 towards 2079 cm−1 . This implies a strong interaction between
propene and gold through electron donation, which may explain the suppressed water
formation over the gold catalysts when propene is present.
References
[1] C. Sivadinarayana, T. V. Choudhary, L. L. Daemen, J. Eckert and D. W. Goodman, JACS, 2004,
126, 38–39.
C, 2008, 112, 1115–1123.
[3] M. G. Clerici and P. Ingallina, J. Catal., 1993, 140, 71–83.
REFERENCES
147
2007, 123, 189–197.
48, 9515–9518.
[11] I. X. Green, W. Tang, M. Neurock and J. T. Yates Jr., Angew. Chem. Int. Ed., 2011, 50, 10186–
10189.
[12] B. Yang, X.-M. Cao, X.-Q. Gong and P. Hu, Phys. Chem. Chem. Phys., 2012, 14, 3741–3745.
[13] D. A. Panayotov, S. P. Burrows, J. T. Yates and J. R. Morris, J. Phys. Chem. C, 2011, 115,
22400–22408.
2011, 280, 40–49.
[18] J. Chen, S. J. A. Halin, D. M. Perez Ferrandez, J. C. Schouten and T. A. Nijhuis, J. Catal.,
2012, 285, 324–327.
[19] A. S. K. Hashmi and G. J. Hutchings, Angew. Chem. Int. Ed., 2006, 45, 7896–7936.
[20] J. Chou, N. R. Franklin, S.-H. Baeck, T. F. Jaramillo and E. W. McFarland, Catal. Lett., 2004,
95, 107–111.
[21] G. C. Bond and D. T. Thompson, Gold Bull., 2000, 33, 41–50.
[22] M. A. P. Dekkers, M. J. Lippits and B. E. Nieuwenhuys, Catal. Lett., 1998, 56, 195–197.
[23] Tz. Venkov, K. Fajerwerg, L. Delannoy, Hr. Klimev, K. Hadjiivanov and C. Louis, Appl. Catal.,
A, 2006, 301, 106–114.
[24] R. Meyer, C. Lemire, Sh. K. Shaikhutdinov and H.-J. Freund, Gold Bull., 2004, 37, 72–124.
[25] M. Mihaylov, H. Knözinger, K. Hadjiivanov and B. C. Gates, Chem. Ing. Tech., 2007, 79, 795–
806.
[26] E. J. M. Hensen, D. G. Poduval, D. A. J. M. Ligthart, J. A. R. van Veen and M. S. Rigutto, J.
Phys. Chem. C, 2010, 114, 8363–8374.
[27] Characterization and Chemical Modification of the Silica Surface, ed. E. F. Vansant, P. Van
Der Voort and K. C. Vrancken, Elsevier, 1995, pp. 59–77.
[28] A. Burneau and J. P. Gallas, in The Surface Properties of Silicas, ed. A. P. Legrand, John Wiley
and Sons, 1998, ch. 3. Hydroxyl Groups on Silica Surfaces, pp. 147–234.
[29] K. Hadjiivanov, B. M. Reddy and H. Knözinger, Appl. Catal., A, 1999, 188, 355–360.
[30] D. A. Boyd, F. M. Hess and G. B. Hess, Surf. Sci., 2002, 519, 125–138.
148
[31] J. France and P. Hollins, J. Electron. Spectrosc. Relat. Phenom., 1993, 64–65, 251–258.
[32] F. Boccuzzi, S. Tsubota and M. Haruta, J. Electron. Spectrosc. Relat. Phenom., 1993, 64–65,
241–250.
[33] C. Ruggiero and P. Hollins, J. Chem. Soc., Faraday Trans., 1996, 92, 4829–4834.
[34] F. Boccuzzi, A. Chiorino and M. Manzoli, Mater. Sci. Eng., C, 2001, 15, 215–217.
[35] I. X. Green, W. Tang, M. McEntee, M. Neurock and J. T. Yates, JACS, 2012, 134, 12717–12723.
201, 128–137.
[38] F. Meunier, D. Reid, A. Goguet, S. Shekhtman, C. Hardacre, R. Burch, W. Deng and
M. Flytzani-Stephanopoulos, J. Catal., 2007, 247, 277–287.
[39] J. Gong, D. W. Flaherty, T. Yan and C. B. Mullins, ChemPhysChem, 2008, 9, 2461–2466.
[40] S. Y. Liu and S. M. Yang, Applied Catalysis A: General, 2008, 334, 92–99.
[41] M. A. Bollinger and M. A. Vannice, Appl. Catal., B, 1996, 8, 417–443.
[42] W.-C. Wu, C.-C. Chuang and J.-L. Lin, J. Phys. Chem. B, 2000, 104, 8719–8724.
[43] G. A. M. Hussein, N. Sheppard, M. I. Zaki and R. B. Fahim, J. Chem. Soc., Faraday Trans. 1,
1989, 85, 1723–1741.
[44] V. Sanchez Escribano, G. Busca and V. Lorenzelli, J. Phys. Chem., 1990, 94, 8939–8945.
[45] F. P. Rotzinger, J. M. Kesselman-Truttmann, S. J. Hug, V. Shklover and M. Grätzel, J. Phys.
Chem. B, 2004, 108, 5004–5017.
[46] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet and B. Delmon, J. Catal.,
1993, 144, 175–192.
[47] G. Busca, H. Saussey, O. Saur, J. C. Lavalley and V. Lorenzelli, Appl. Catal., 1985, 14, 245–260.
[48] B. Erdem, R. A. Hunsicker, G. W. Simmons, E. D. Sudol, V. L. Dimonie and M. S. El-Aasser,
Langmuir, 2001, 17, 2664–2669.
[49] G. P. López, D. G. Castner and B. D. Ratner, Surf. Interface Anal., 1991, 17, 267–272.
[50] J. Stoch and J. Gablankowska-Kukucz, Surf. Interface Anal., 1991, 17, 165–167.
[51] M. P. Casaletto, A. Longo, A. Martorana, A. Prestianni and A. M. Venezia, Surf. Interface Anal.,
2006, 38, 215–218.
[52] V. A. Bondzie, S. C. Parker and C. T. Campbell, Catal. Lett., 1999, 63, 143–151.
[53] M. Primet, P. Pichat and M. V. Mathieu, J. Phys. Chem., 1971, 75, 1216–1220.
[54] H. Lin, J. Long, Q. Gu, W. Zhang, R. Ruan, Z. Li and X. Wang, Phys. Chem. Chem. Phys., 2012,
14, 9468–9474.
[55] P. Du, A. Bueno-López, M. Verbaas, A. R. Almeida, M. Makkee, J. A. Moulijn and G. Mul, J.
Catal., 2008, 260, 75–80.
[56] C. Deiana, E. Fois, S. Coluccia and G. Martra, J. Phys. Chem. C, 2010, 114, 21531–21538.
[57] D. Widmann and R. J. Behm, Angew. Chem. Int. Ed., 2011, 50, 10241–10245.
REFERENCES
149
[58] T. Diemant, Z. Zhao, H. Rauscher, J. Bansmann and R. J. Behm, Top. Catal., 2007, 44, 83–93.
[59] M. Boronat, P. Concepción and A. Corma, J. Phys. Chem. C, 2009, 113, 16772–16784.
[60] A. A. Efremov, Yu. D. Pankratiev, A. A. Davydov and G. K. Boreskov, React. Kinet. Catal. Lett.,
1982, 20, 87–91.
[61] K. Chakarova, M. Mihaylov, S. Ivanova, M. A. Centeno and K. Hadjiivanov, J. Phys. Chem. C,
2011, 115, 21273–21282.
[62] A. A. Efremov and A. A. Davydov, React. Kinet. Catal. Lett., 1980, 15, 327–331.
[63] R. C. Lord and P. Venkateswarlu, J. Opt. Soc. Am., 1953, 43, 1079–1085.
[64] J. G. Radziszewski, J. W. Downing, M. S. Gudipati, V. Balaji, E. W. Thulstrup and J. Michl,
JACS, 1996, 118, 10275–10284.
[65] A. M. Argo, J. F. Goellner, B. L. Phillips, G. A. Panjabi and B. C. Gates, JACS, 2001, 123,
2275–2283.
[66] T. A. Nijhuis, E. Sacaliuc-Parvulescu, N. S. Govender, J. C. Schouten and B. M. Weckhuysen,
J. Catal., 2009, 265, 161–169.
[67] J. A. van Bokhoven, C. Louis, J. T. Miller, M. Tromp, O. V. Safonova and P. Glatzel, Angew.
Chem. Int. Ed., 2006, 45, 4651–4654.
[68] N. Weiher, A. M. Beesley, N. Tsapatsaris, L. Delannoy, C. Louis, J. A. van Bokhoven and S. L. M.
Schroeder, JACS, 2007, 129, 2240–2241.
[69] J. J. Bravo-Suárez, K. K. Bando, J. Lu, T. Fujitani and S. T. Oyama, J. Catal., 2008, 255,
114–126.
[70] K. A. Davis and D. W. Goodman, J. Phys. Chem. B, 2000, 104, 8557–8562.
[72] J. J. Bravo-Suárez, J. Lu, C. G. Dallos, T. Fujitani and S. T. Oyama, J. Phys. Chem. C, 2007,
111, 17427–17436.
[73] W. T. Wallace and R. L. Whetten, JACS, 2002, 124, 7499–7505.
[74] A. Prestianni, A. Martorana, F. Labat, I. Ciofini and C. Adamo, J. Phys. Chem. B, 2006, 110,
12240–12248.
[75] M. Boronat and A. Corma, Dalton Trans., 2010, 39, 8538–8546.
[76] I. X. Green, W. Tang, M. Neurock and J. T. Yates, Science, 2011, 333, 736–739.
Conclusions and outlook
6
6.1 Conclusions
The usage of a microreactor system for the direct epoxidation of propene over a gold–
titania-based catalyst system using a mixture of hydrogen, oxygen, and propene allows
for the safe operation of the reaction in the explosive regime. The kinetic study performed over a very wide range of reactant concentrations in the capillary reactor as discussed in Chapter 2 provides the mechanistic insights for both catalyst development and
optimization of the operation. The formation rate of propene oxide is most dependent
on the hydrogen concentration, confirming that the dissociative adsorption of hydrogen
is the rate determining step in the direct epoxidation of propene. The formation of an active hydroperoxo species on the gold nanoparticles, which is responsible for the propene
epoxidation, is thus influenced by this rate determining step. The dependencies of PO
formation rate over the 1 wt% Au/Ti-SiO 2 catalyst investigated are ca. 0.5, 0.3 and 0.3
for hydrogen, oxygen and propene, respectively. Although performing the direct epoxidation of propene at higher reactant concentrations can increase the formation rate of
PO further, a higher hydrogen concentration also plays an adverse role in the utilization
efficiency of hydrogen, which leads to much more water formation than the extra PO that
can be gained. The active hydroperoxo species responsible for PO formation is competitively consumed by hydrogenation and epoxidation at the Au–Ti interface. The apparent
activation energy for the water formation at the Au–Ti interface is determined to be 22
kJ/mol higher than the PO formation, which implies that a lower reaction temperature
152
CHAPTER 6. CONCLUSIONS AND OUTLOOK
Table 6.1: Qualitative guideline in reactant concentrations for a better catalyst performance
H2 ↑
O2 ↑
C3 H6 ↑
PO productivity
++
+
+
Hydrogen efficiency
−−
+/0
+
Catalyst stability
−
0
+
is preferred for a better hydrogen efficiency. Besides the water formation via the hydrogenation of the active hydroperoxo species responsible for PO formation, there is an
extra route for hydrogen oxidation, i.e., so called ‘direct water formation’. The direct water formation happens very likely on gold not adjacent to a Ti(IV) site. A higher propene
concentration is beneficial to reducing both the hydrogenation of the active hydroperoxo
species at the Au–Ti interface and the direct water formation. In regard to the catalyst
deactivation, it is found that the deactivating species blocks the Au–Ti interfacial sites
and diminishes both water formation and PO formation. The rate of deactivation and the
hydrogen efficiency can be correlated. A low hydrogen efficiency usually corresponds to
a higher deactivation rate. A lower hydrogen concentration or a higher propene concentration has a positive effect on improving the hydrogen efficiency.
An enhanced productivity of propene oxide was achieved by adjusting the gold–
titanium synergy over catalysts supported on titanium-grafted silica. The original idea
was to reduce the amount of gold sites that are not in proximity to a Ti(IV) site to mitigate the unwanted direct water formation. Highly isolated titanium sites were obtained
by lowering the amount of titanium grafted on silica. The tetrahedrally coordinated titanium sites were found to be favourable for attaining small gold nanoparticles and thus
−1
a high dispersion of gold. A PO productivity as high as 100gPO · kg−1
can be easily
cat h
achieved on gold catalysts supported on the Ti-SiO2 with mostly tetrahedrally coordinated Ti(IV) sites. The improved productivity of propene oxide can be attributed to the
increased amount of the interfacial Au–Ti sites and less deactivation. There is no need
for a high gold loading to obtain a high PO productivity. A gold loading smaller than
0.2 wt% is sufficient for high PO formation rates as long as enough Au–Ti interfacial
sites can be created by a dedicated method of gold deposition. The active hydroperoxy
interface, which has been confirmed again on the catalysts with invisible gold clusters. A
6.1. CONCLUSIONS
153
higher propene concentration is favourable for a lower water formation rate and a higher
formation rate of propene oxide. The hydrogen efficiency achieved on the catalysts with
high PO productivity was with the range of 10–20 %.
Under certain circumstances, propane formation may also happen or even prevail
over the gold–titania catalysts under the conditions for the direct propene epoxidation.
Propene hydrogenation was encountered during our study into the site synergy between
gold and titanium using Ti-SiO2 and TS-1 as the supports. It is found that propene hydrogenation can be completely suppressed by adding a small amount of carbon monoxide
while the propene epoxidation was not affected. Further investigation showed that the
support itself plays an important role in propene hydrogenation, since the gold particle
size varied between 1–6 nm on different catalysts and gold is not necessary for this side
reaction. Catalysts with similar gold/titanium loadings can give different performance in
propene hydrogenation. Gold on the catalysts producing propane only can hardly be detected by CO adsorption while an in-situ XPS study showed that the gold is in its metallic
form. The supports alone showed the same hydrogenation behavior as the gold–titania
catalysts: 1) enhancement of propene hydrogenation by dioxygen; 2) peak activity at ca.
443 K accompanying the process of dehydroxylation during temperature programmed
reaction in only propene and hydrogen ; 3) switching off by CO with an order of −1. The
activity in propene hydrogenation deactivates by flushing in propene and can be restored
by a treatment in a mixture of hydrogen and oxygen. The hydroxylated catalyst can
hydrogenate propene in the absence of oxygen but the dehydroxylated one cannot. An
investigation by infrared spectroscopy confirmed that the effect of propene flush on the
loss of activity in propene hydrogenation is merely related to a loss of surface hydroxyls.
Although a few papers argue that oxidized gold may be responsible for propene hydrogenation [1, 2], unraveling the general oxidation state of gold is important to the
understanding of the direct propene epoxidation on the gold–titania catalysts. Carbon
monoxide was used as probe molecule in the infrared study to investigate the electron
density of low-coordinated gold atoms on the gold–titania catalysts that are active in the
direct propene epoxidation. The active gold sites were fully covered by reaction intermediates and deactivating species after the reaction. These species occupying the gold sites
could not desorb even at 573 K. Calcination in oxygen removed the carbonaceous species
on gold. The gold atoms were positively charged when oxygen was adsorbed on gold
154
Table 6.2: Typical operating conditions of an industrial EO (ethylene oxide) reactor and
state-of-the-art performance for a PO process based on gold catalysts
EO [3]
PO
0.15–0.40
0.10
O2
0.05–0.09
0.10
H2
–
0.10
Molar fraction at inlet
C2 H4 /C3 H6
One-pass conversion
7–15 %
10 %
Selectivity
80 %
90 %
Space velocity
4500–7500 h−1
4000 mL·g−1
h−1
cat
3
Space-time yield
0.13–0.26 g/h/cm
0.10 g/h/gcat
Temperature
500–550 K
423–473 K
Pressure
1.0–2.2 MPa
0.1 MPa
or at the interface. Reduction in hydrogen removed the adsorbed oxygen and the positively charged gold was reduced to its metallic form. When propene was adsorbed on the
catalyst, gold atoms were negatively charged showing the carbonyl band as low as 2079
cm−1 . Carbon monoxide was replaced by propene on the catalyst surface and oxidation
of carbon monoxide was suppressed by propene. Hydrogen significantly increased the
coverage of carbon monoxide on the titania surface at low temperatures.
6.2 Outlook
After more than a decade of research, the direct propene epoxidation over gold–titania
catalysts has progressed significantly in terms of PO productivity and short-term stability
over different supports as summarized in Table 1.2. The research goal for this catalytic
system set by the group of Haruta [4] is to have one-pass conversion of 10 %, selectivity
of 90 % and a hydrogen efficiency of 50 %. However, an economic feasibility study needs
to be solidified based on comparison with PO/TBA, SMPO and HPPO processes just as
one performed by Ghanta et al. [5] for a new PO process. From the engineering point of
view, the direct PO process based on the gold catalyst will be no much difference from the
existing EO production based on the silver catalyst, nor different from other gas-phase
oxidation processes based on fixed-bed reactor technology. A high propene concentration
will be implemented to overcome the issue of hydrogen efficiency and safety when using
the hydrogen/oxygen/propene mixture. The product stream would contain a significant
6.2. OUTLOOK
155
amount of unreacted gas, including propene, hydrogen, and oxygen, which needs to be
recycled. Water and PO would be very likely condensed at a relatively low temperature
in the first step of gas separation since the boiling point (at atmospheric pressure) of
PO is only 307 K. To overcome the energy consumption by refrigeration, the reactor and
the first separator would be preferably operated at a few bars. The following separation
step would be water–PO separation, likely, by fractional distillation. Nevertheless, the
bottleneck of the whole process still remains at the catalyst development phase to address
more efficient utilization of hydrogen and long-term stability. It is also very important
for the industry to experimentally determine the operation window outside the explosive
region.
In our earliest kinetic study, which is not included in this thesis, the catalyst activity
for water formation doubled after storage of a few months, but there was no deactivation
in PO formation observed. Possible cause of this problem may be attributed to hydroxylation of the support, which may be supported by Figure 3.2. Considering the relatively
large amount of water co-produced in the direct epoxidation of propene, one may pay
additional attention to improving the hydrogen efficiency to avoid a long-term change of
catalyst in a humid environment at relatively high temperatures. Hydrophobic supports
are preferred. The highest records in the hydrogen efficiency are exclusively from the
TS-1 support as seen in Table 1.2. Thus developing a type of TS-1 crystal with a specific
flat shape may be helpful to enhance gold dispersion. Another option for a hydrophobic
support is hybrid organic/inorganic Ti-containing materials [6, 7], in which the silanols
are replaced or passivated by alkoxy groups. This kind of hybrid supports, when used
in the direct propene epoxidation, showed a high PO yield approaching 10 % with very
limited deactivation for as long as 10 days [6] due to the absence of surface hydroxyls.
Although not much data about hydrogen efficiency has been reported on gold catalysts
supported on this type of supports, it may be interesting to expand work into this area.
The competitive hydrogenation and epoxidation at the Au–Ti interface consuming
the active hydroperoxo species are very similar to what is happening in the direct synthesis of hydrogen peroxide over Au–Pd catalysts [8]. It was found that hydrogenation
of hydrogen peroxide over the Au–Pd catalysts can be switched off [8]. This may also
provide alternatives to improve the hydrogen efficiency since hydrogen peroxide is often supposed to be the intermediate in the direct propene epoxidation over gold–titania
catalysts.
156
References
1586.
2011, 280, 40–49.
[3] H. Kestenbaum, A. Lange de Oliveira, W. Schmidt, F. Schüth, W. Ehrfeld, K. Gebauer, H. Löwe,
T. Richter, D. Lebiedz, I. Untiedt and H. Züchner, Ind. Eng. Chem. Res., 2002, 41, 710–719.
[4] A. K. Sinha, S. Seelan, S. Tsubota and M. Haruta, Angew. Chem. Int. Ed., 2004, 43, 1546–1548.
[5] M. Ghanta, D. R. Fahey, D. H. Busch and B. Subramaniam, ACS Sustainable Chem. Eng., 2013,
1, 268–277.
[6] P. Vogtel, G. Wegener, M. Weisbeck and G. Wiessmeier (Bayer Aktiengesellschaft), WO
2001041921, 2001.
[8] J. K. Edwards, B. Solsona, E. Ntainjua N, A. F. Carley, A. A. Herzing, C. J. Kiely and G. J.
Hutchings, Science, 2009, 323, 1037–1041.
List of Publications
Journal publications
• Chen, J., Pidko, E.A., Ordomsky, V., Verhoeven, M.W.G.M., Hensen, E.J.M., Schouten, J.C.,
Nijhuis, T.A. (2013). How metallic is gold in the direct epoxidation of propene : An FTIR
study. Catalysis Science & Technology, DOI: 10.1039/c3cy00358b.
• Chen, J., Halin, S.J.A., Pidko, E.A., Verhoeven, M.W.G.M., Perez Ferrandez, D.M., Hensen,
E.J.M., Schouten, J.C., Nijhuis, T.A. (2013). Enhancement of catalyst performance in the
direct propene epoxidation : A study into gold-titanium synergy. ChemCatChem, 5, 467–
478.
• Chen, J., Halin, S.J.A., Perez Ferrandez, D.M., Schouten, J.C., Nijhuis, T.A. (2012). Switching off propene hydrogenation in the direct epoxidation of propene over gold–titania catalysts. Journal of Catalysis, 285, 324–327.
• Chen, J., Halin, S.J.A., Schouten, J.C., Nijhuis, T.A. (2011). Kinetic study of propylene
epoxidation with H2 and O2 over Au/Ti-SiO2 in the explosive regime. Faraday Discussions,
152, 321–336.
• Nijhuis, T.A., Chen, J., Kriescher, S.M.A., Schouten, J.C. (2010). The direct epoxidation
of propene in the explosive regime in a microreactor – A study into the reaction kinetics.
Industrial & Engineering Chemistry Research, 49, 10479–10485.
The author also contributed to the following publications outside the scope of this thesis:
• Wu, C., Chen, J., Cheng, Y. (2010). Thermodynamic analysis of coal pyrolysis to acetylene
in hydrogen plasma reactor. Fuel Processing Technology, 91, 823–830.
• Chen, J., Cheng, Y. (2009). Process development and reactor analysis of coal pyrolysis to
acetylene in hydrogen plasma reactor. Journal of Chemical Engineering of Japan, 42, 103–
110.
158
PUBLICATIONS
• Chen, J., Cheng, Y., Xiong, X., Wu, C., Jin, Y. (2009). Research progress of coal pyrolysis to
acetylene in thermal plasma reactor. Chemical Industry and Engineering Progress (China),
28, 361–367.
• Cheng, Y., Chen, J.Q., Ding, Y.L., Xiong, X.Y., Jin, Y. (2008). Inlet effect on the coal pyrolysis to acetylene in a hydrogen plasma downer reactor. Canadian Journal of Chemical
Engineering, 86, 413–420.
• Cheng, Y., Chen, J., Ding, S. (2007). Controlled synthesis of nano-sized TiO2 powders using high-temperature vapor phase process. Journal of Chemical Industry and Engineering
(China), 58, 2103–2109.
Conference presentations
• Chen, J., Nijhuis, T.A., Schouten, J.C. (2012). The role of support in propene hydrogenation
over gold-titania catalysts. The 6th International Conference on Gold Science Technology and
its Applications (GOLD 2012), 5–8 September, 2012, Tokyo, Japan. [Poster]
• Chen, J., Ordomskiy, V., Schouten, J.C., Nijhuis, T.A. (2012). Probing the active site of
propene hydrogenation in the direct propene epoxidation over gold–titania catalysts. The
15th International Congress on Catalysis (ICC 2012), 1–6 July, 2012, Munich, Germany.
[Oral]
• Chen, J., Nijhuis, T.A., Schouten, J.C. (2012). The inhibition effect of carbon monoxide on propene hydrogenation over gold-titania catalysts. The 13th Netherlands’ Catalysis
and Chemistry Conference (NCCC XIII), 5–7 March 2012, Noordwijkerhout, the Netherlands.
[Poster]
• Chen, J., Nijhuis, T.A., Schouten, J.C. (2011). Kinetic study of direct propylene epoxidation
over Au/Ti-SiO2 in the explosive regime. The 12th Netherlands’ Catalysis and Chemistry
Conference (NCCC XII), 28 February – 2 March, 2011, Noordwijkerhout, the Netherlands.
[Oral]
• Chen, J., Halin, S.J.A., Nijhuis, T.A., Schouten, J.C. (2011). Kinetic study of the epoxidation
of propene over gold–titania catalysts into the explosive regime. The 22nd North American
Catalysis Society Meeting (NAM22), 5–10 June, 2011, Detroit, USA. [Poster]
• Chen, J., Nijhuis, T.A., Schouten, J.C. (2010). Direct epoxidation of propylene to propene
oxide in a microreactor. The 11th Netherlands’ Catalysis and Chemistry Conference (NCCC XI),
1–3 March, 2010, Noordwijkerhout, the Netherlands. [Poster]
• Chen, J., Nijhuis, T.A., Schouten, J.C. (2010). Direct epoxidation of propylene to propene
oxide in a micro reactor. Netherlands Process Technology Symposium 2010 (NPS-10), 25–27
October, 2010, Veldhoven, the Netherlands. [Oral]
Acknowledgements
Finally the research project came to an end where I am so grateful to all the people who
have helped me, encouraged me and inspired me through the whole journey. I enjoyed
the great time here in Eindhoven and learned a lot from all of you.
I would like to thank my promoter Prof.dr.ir. J.C. Schouten. Jaap, thanks for providing
me such a great opportunity to pursue my doctor’s degree in the group SCR. You were
always helpful and open-eared in our progress meetings. I appreciate your trust and
always constructive suggestions which lead me on the way towards the end of the project.
My special thanks are for my daily supervisor Dr.ir. T.A. Nijhuis. Xander, I benefited a
lot from the fruitful discussions with you, from your expertise, enthusiasm and optimism.
You showed me into the wonderful world of catalysis since the beginning of the project.
You are so peaceful in mind, patient, always approachable and never put pressure on
your students. I really appreciate your saying of being prepared to be an independent
researcher. Xander, many thanks for your guidance all the way. Besides, I would also like
to thank my former daily co-supervisor, Prof.dr. Evgeny Rebrov. Evgeny, although you’ve
been promoted and left Eindhoven to Belfast, thank you very much for your scientific
input through discussion and progress meetings in the early phase of my project.
I’m very grateful to the skillful craftsmen in the workshop. Without the timely completion of small or big changes on my setup with high quality by you guys, I would not
be able to implement my ideas and would not have enough and reliable data to complete
my thesis. Erik, you are so professional, efficient and always willing to help. Thank you
so much for all the efforts you made. I would also like to thank Dolf, Theo, Chris and
Madan for your help whenever I bothered you guys in the workshop. In addition, I want
to thank Anton Bombeeck, our technical coordinator. Anton, I bothered you also quite
a lot and thanks for your help with making orders and hunting treasures in the storage
room.
160
ACKNOWLEDGEMENTS
My heartfelt thanks are given to Peter, Carlo and Marlies. Peter, thank you so much
for all the help that you had offered me. Carlo, thanks a lot for the automation work
done by you and I really appreciate your professionalism on safety. Marlies, you always
helped me solve the GC problems in the first place, many thanks.
I would like to thank people who helped me with different spectrometers and analytical tools. I am very grateful to Prof.dr.ir. Emiel Hensen for allowing me to use the IR
spectrometer in your group. I want to express my sincere gratitude to Dr. Evgeny Pidko
for the time and energy that you spent in my IR experiments and for your tolerance to my
mistakes. I am also much obliged to Tiny Verhoeven for performing all the XPS analyses
and for your helpful discussion. And again, Carlo Buijs, thank you for taking countless
TEM and SEM pictures for me. As well as my lab mate, Serdar, thank you very much for
your time on my TEM samples. In addition, I would also like to thank Prof.dr.ir. René
Janssen for EPR measurements.
I’d like to thank my two master students, Stefanie Kriescher and Sander Halin. Thank
you for your contribution to the experimental work of this thesis. Both of you are diligent
not only in academic research, but also in sports, where you impressed me by persistence
and self-discipline, as well as your achievements.
And Denise, it is very kind of you to help me arrange all the administrative documents.
Whenever I needed help, you were always there and gave me directly the answer. I want
to express my deep gratitude to you for all the things you’ve done for me all the years.
I’d also like to thank John, Mart and Martin. Thank you all for making SCR a splendid
place for research and you helped make it possible for me to meet so many excellent and
young researchers from all over the world.
I spent a lot of enjoyable time with my colleagues in SCR. Ma’moun, Marco and
Christine, my dear office mates in STW 1.38, thank you for the memorable time we spent
together and also thanks for all the help you provided me in either research or daily life.
Dulce and Serdar, my dear lab mates, I really enjoyed our discussion and chat in the
lab. As well as Jack, thanks a lot for your patient explanation about your delicate setup
even when you were busy in the last phase of your PhD. Vitaly, I really appreciate your
insightful input in IR experiments and your exemplary role of hard working. The same
gratitude is given to my lovely colleagues for the pleasant time in SCR: Bianca, Bruno,
Carlos, Chattarbir, Emila, Faysal, Fernanda, Frans, Gregory, Halabi, Ivana, Joost, Jovan,
Kevin, Lara, Lidia, Maria, Michiel, Minjing, Narendra, Nopi, Oki, Paola, Patrick, Roman,
Slavisa, Stijn, Shohreh, Tom, Violeta. I’d also like to thank the people from the third
ACKNOWLEDGEMENTS
161
floor: Chaochao, Leilei, Sami, Guanna, Michel. Thank you all for the assistance when
using the equipment in your group.
There are many people who have enriched my life here in the Netherlands. The name
list is not short: Gao Yang, Harrison, Donglin, Jinbao, Wang Qi and Chuanbo, Yue Jun
couple, Qingling and Maarten, Pengzhao, Sijun and Jijing, Yongjian couple, Xixi, Danqing, Gu Xi, Xiaoran, Changwen couple, Xueqing couple, Junlin and Wang Juan, Piming
couple, Delei, Lianghui, Wu Jing, Xu Wei couple, Jogesh, Dries, Elham, Lou Xianwen, Yu
Fangli, Zhang Yanmei, Gao Lu, Ma Zhe, Wenhao. It is destiny to meet you guys in the
boundless huge crowd here in the Netherlands. I really cherish this experience. Special
thanks are given to Shih-chin and Te-hui. You showed me endless love and care and how
life can always be positive and definite. I also want to thank David and Prisca, Sandy and
other saints met in Eindhoven for sharing and encouragement.
Furthermore, I am very grateful to my former supervisor Prof. Yi Cheng. Without
your encouragement, I would not take the first step to embrace the challenging but also
wonderful experience in the Netherlands. As well as sister Gong Weiping, thank you for
all the support during my stay in Beijing and your hospitality in Brussels. I also want to
thank my college mates, Baoxiang, Chen Zhen, Chen Bo and Hangzhou for the fellowship
in the past years.
Huihui, my beloved wife, it is lucky for me to have you, your continuous loving and
caring. You are magic and can always make my troubles disappear. I don’t know how to
express my gratitude to you in words. You and I will grow together and I am forever in
your debt, that’s for sure. My dear parents, thank you so much for your understanding
and support, your continuous teaching and sharing. I also want to express my deep
gratitude to my parents in law. Thank you for your trust and support. I appreciate all the
support, understanding and encouragement from my family members and friends. 我深
深地感谢我的爸爸妈妈，谢谢你们这些年来的关心、支持和分享。我同样感谢我的
岳父和岳母，谢谢你们的信任和支持。最后，我真诚地感谢一直以来家人和朋友们
对我的鼓励和支持。
About the Author
Jiaqi Chen (陈家琦) was born on January 27, 1984, in Rudong, Jiangsu Province, China.
After finishing the secondary school in 2001, he started his study in the department of
Chemical Engineering at Tsinghua University, Beijing. In 2005, he worked on the topic
‘Controlled synthesis of titanium dioxide nanoparticles in a flame reactor’ for his graduation project under the supervision of Prof. Yi Cheng and obtained his bachelor’s degree in
July. Afterwards, he continued with the master programme in the group of reaction engineering in the same department. In 2008, he obtained his master’s degree after working
on a joint project ‘Coal pyrolysis to acetylene in a hydrogen plasma reactor’ with Xinjiang
Tianye Group. After having spent 7 years in Beijing, he started off to Eindhoven, the
Netherlands. From 2008 to 2012, he worked as a PhD student on ‘propylene epoxidation over gold catalysts’ in the Laboratory of Chemical Reactor Engineering in Eindhoven
University of Technology under the supervision of Prof.dr.ir. J.C. Schouten and Dr.ir.
T.A. Nijhuis. His PhD project involved mainly the kinetic study, catalyst development
and mechanistic investigations into the bi-functional gold–titania system. Since October
2012, he has worked as a researcher in Shell Global Solutions in Amsterdam.