Verified syntheses of mesoporous materials

Transcription

Microporous and Mesoporous Materials 125 (2009) 170–223
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials
journal homepage: www.elsevier.com/locate/micromeso
Verified syntheses of mesoporous materials
V. Meynen, P. Cool *, E.F. Vansant
Laboratory of Adsorption and Catalysis, Department of Chemistry, University of Antwerpen, Universiteitsplein 1, 2610 Wilrijk, Belgium
a r t i c l e
i n f o
Article history:
Received 30 March 2009
Accepted 31 March 2009
Available online 10 April 2009
Keywords:
Verified syntheses
Mesoporous
Super-microporous
a b s t r a c t
A very large number of different synthesis approaches for the preparation of mesoporous materials has
been reported in literature since the first development of ordered mesoporous materials in the 1990’s.
Since then, the synthesis of advanced mesoporous materials has undergone an explosive growth. Moreover, this type of materials gains growing success in a wide variety of applications. For these reasons and
with the example of the book of verified microporous zeolite syntheses in mind, it is obvious that there is
a growing need for verified synthesis methods of mesoporous materials. In this work, verified synthesis
methods have been compiled for a large number of selected relevant structured mesoporous silica and
titania materials as well as for some super-microporous materials (defined herein as materials with pore
diameters between 1.5 and 2 nm). In addition, for each material, a basic set of material characteristics
have been reported based on the most commonly applied characterization techniques (nitrogen sorption,
XRD, TEM, SEM, NMR, etc.) for mesoporous materials.
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
Microporous zeolites are among the best known and most widely
applied porous materials. Their uniform pore sizes with molecular
dimensions, good stability, selectivity and activity due to their crystallinity, the possibility of incorporating heteroelements into the
structure and their ion exchange capacities make zeolites unique
materials in several processes (catalysis, sorption, membrane separations, etc.). Thanks to innovative synthesis strategies an evolution
towards structured materials with larger pores could be obtained.
After the first reports, introducing the M41S family of ordered mesoporous silicas at the beginning of the 1990s, the synthesis of advanced mesoporous materials has undergone explosive growth.
The exploration of novel compositions and architectures in view of
specific applications in areas as diverse as catalysis, sorption, separations, sensing, optics, drug delivery, etc. has given rise to several
national and international research programmes.
Within the framework of the EU-FP6 Network of Excellence ‘‘INSItu study and DEvelopment of processes involving nanoPORous
Solids (INSIDE-POReS)” bringing together researchers from 16 leading European research groups, it became obvious that there is a
growing need for verified synthesis methods of mesoporous materials. The need and benefit for this type of reference works can be
found in the previous similar work of the verified zeolite syntheses
book coordinated by the International Zeolite Association, which
are indispensable in many laboratories in the field [434]. Emphasis
will be put on various verified preparation methods for selected rel* Corresponding author. Tel.: +32 3 265 23 55; Fax: +32 3 265 23 74.
E-mail address: [email protected] (P. Cool).
1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2009.03.046
evant structured mesoporous silicas and titanias, followed by the
obtained material characterization results (surface area, pore
dimensions, pore volume, XRD, TEM, SEM, NMR, etc.). Some of the
materials listed are not strictly mesoporous, but super-microporous
(defined herein as pores between 1.5 and 2 nm). As this overview
tries by no means to be complete, containing all existing and important mesoporous materials that have been developed, it should be
seen as the start of the publication and documentation of proven
syntheses of mesoporous materials. It will be extended in the future
with more verified syntheses of mesoporous materials.
The synthesis of nanoporous materials and more precisely mesoporous materials, which are defined by IUPAC as materials with
pore sizes between 2 and 50 nm, is an active field of research. As
long as this continues, new mesoporous siliceous and non-siliceous
or inorganic and inorganic-organic hybrid materials will be discovered and new techniques for preparing existing phases will appear.
2. Practical guide
Synthesis experiments reported in the literature are often cryptic, leaving the scientist who wants to reproduce the experiment
many choices of reagents and procedures. Furthermore, the product characterization is often inadequate for an unambiguous choice
for a new application.
In this paper, an outline, covering the essential points for
reporting a synthesis of nanoporous materials, was selected. The
format for all the recipes in this volume follows this outline. It is
intended to assist the scientist by placing the information in the
same relative positions for all recipes. This form supposes that all
synthesis experiments follow the general sequence:
V. Meynen et al. / Microporous and Mesoporous Materials 125 (2009) 170–223
171
List of acronyms and abbreviations
°C
degree Celsius
lm
micrometer
16-12-16 gemini [C16H33(CH3)2N–C12H24–N(CH3)2C16H33]2Br
18-12-18 gemini [C18H37(CH3)2N–C12H24–N(CH3)2C18H37]2Br
2D
two-dimensional
3D
three-dimensional
3D-TEM three-dimensional transmission electron microscopy
a.u.
arbitrary units
AAS
atomic absorption spectroscopy
Ac
autoclave
ATR
attenuated total reflectance
BEA
specific framework type of a zeolite
BET
Brunauer–Emmett–Teller
BJH
Barret–Joyner–Halenda
CASH
combined assembly by soft and hard chemistries
cubic centimeter
cm3
cmc
critical micelle concentration
CP
cross polarization
CRT
cathode ray tube
CTMABr cetyltrimethylammonium bromide
CVD
chemical vapor deposition
DDMS
decyldimethylsilyl
DLCT
direct liquid crystal templating
DOR
double-oriented rotation
DR
Dubinin–Radushkevich
DRIFT
diffuse reflectance infrared Fourier transform
DTMABr decyltrimethylammonium bromide
ED-XRF energy dispersive-X-ray fluorescence
EISA
evaporation induced self-assembly
ENMIX European Nanoporous Materials Institute of Excellence
EO
ethylene glycol
EPMA
electron probe micro analysis
EtOH
ethanol
EU
European Union
EU-FP6 European Union – Framework Programme 6
F127
poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol)
FTIR
Fourier Transform Infrared
FTIR-PAS Fourier transform infrared–photoacoustic spectroscopy
FT-Raman Fourier transform Raman spectroscopy
g
gram
h
hour
HCL
hollow cathode lamp
HDMS
hexyldimethylsilyl
HMS
hexagonal mesoporous silica
HOM
highly ordered silica monoliths
HRTEM high resolution transmission electron microscopy
H-TTNT proton exchanged trititanate nanotube
ICP/AES inductively coupled plasma atomic emission spectroscopy
ICP-OES inductively coupled plasma-optical emission spectroscopy
INSIDE POReS IN-SItu study and DEvelopment of processes
involving nanoPORous Solids
IUPAC
International Union of Pure and Applied Chemistry
kHz
Kilohertz
LCT
liquid crystal template
M
molar
square meter
m2
–
–
–
–
Batch preparation.
Product recovery.
Calcination conditions.
Characterization.
M41S
group name of mesoporous MCM materials (Mobil Composition of Matter)
MAS NMR magic angle spinning nuclear magnetic resonance
MCF
mesostructured cellular foam
MCl
metal chloride salt
MCM
mobil composition of matter
MDD
molecular designed dispersion method
min
minute
mL
milliliter
MMA
monolithic mesoporous aluminosilicate
MOS
molecular based organized systems
MQ MAS multiple-quantum magic angle spinning nuclear magnetic resonance
MSU
Michigan State University
MW
molecular weight
Avogadro number
NA
Na-TTNT sodium trititanate nanotube
NBB
nanobuilding block
Nm
nanometer
NMR
nuclear magnetic resonance
OMM
ordered mesoporous material
p.a.
pro analysis
relative pressure
P/PO
P123
poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol)
PE
polyethylene
PEO
polyethylene glycol
PHTS
plugged hexagonal templated silica
PO
propylene glycol
POS
polymeric based organized systems
PP
polypropylene
ppb
parts per billion
ppm
parts per million
PPO
polypropylene glycol
rpm
rotations per minute
RT
room temperature
SBA
Santa Barbara acids
specific surface area measured by the BET method
SBET
SEM
scanning electron microscopy
STP
standard temperature and pressure
t
multilayer thickness
T
temperature
TDTMABr tetradecyltrimethylammonium bromide
TEAOH tetraethylammonium hydroxide
TEM
transmission electron microscopy
TEOS
tetraethyl orthosilicate/tetraethoxysilane
Ti(OiPr)4 titanium(IV) isopropoxide
TMOS
tetramethyl orthosilicate/tetramethoxysilane
TTMABr tetradecyltrimethylammonium bromide
TTNT
trititanate nanotubes
volume of adsorbed gas
Vads
liquid volume
Vliq
micropore volume
Vmicro
total pore volume
Vtotal
W
Watt
WD-XRF wavelength dispersive-X-ray fluorescence
XRD
X-ray diffraction
XRF
X-ray fluorescence
Hm
magic angle
(a) Name of porous materials
The product name is the name by which the product is usually referred to in the literature and its normal framework
code (letter code followed by a number, reflecting the struc-
172
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
ture). Furthermore, a short description of the material is
added to help the reader in selecting porous materials.
Contributed by
The names indicate the persons who actually prepared the
mesoporous material.
Verified by
Verifiers are those independent investigators who reproduced the synthesis recipe and obtained a satisfactory product by their own evaluation. Again, only names are given
here; for institutional affiliations see the list of contributors.
The verification reports, both positive and negative, are part
of the record of the recipe and are available on request at the
corresponding author P. Cool ([email protected]).
Type of material
The type of the material refers to the chemical content of the
unit cell as indicated by the elemental analysis after washing
and calcination.
Batch composition
The batch composition refers to the product of the batch
preparation stated in molar ratios of oxides, template molecules, neutralization products, reaction media, etc.
Source of materials
The actual materials used to prepare the batch are given
along with their purities and suppliers.
Batch preparation
The batch preparation refers to actual quantities of materials
plus the preparation steps used to prepare the material.
Product recovery
Product recovery refers to the procedure for separating the
desired product from the by-products.
Post-synthesis treatment
These conditions refer to the experimental conditions and
temperatures which convert the finished batch into the final
product. This includes the removal of the template by
extraction and calcination as well as recrystallization or stabilization of the prepared materials.In order to obtain good
quality materials it is advised never to put too much in a calcination crucible.
Product characterization
Product characterization identifies the obtained (crystalline)
product and compares its properties to those of the known standards. For this publication, the basic characterizations are XRD,
N2 sorption at 196 °C (determination of specific surface area,
pore volume, pore diameter), SEM, TEM (HRTEM), NMR, etc.
References
References indicate the primary literature reports dealing
with the nanoporous material plus selected general references recommended by the author.
3. Contributors
The following researchers and laboratories, which are members
of the Network of Excellence INSIDE-POReS, contributed recipes for
ordered mesoporous materials and/or the verification of recipes.
Prof. P. Cool, Prof. E.F. Vansant, Dr. V. Meynen, F. Quiroz, K. De
Witte, G. Smeulders, E. Beyers, S. Ribbens
University of Antwerpen, Department of Chemistry, Laboratory
of Adsorption and Catalysis, Universiteitsplein 1, B 2610 Wilrijk,
Belgium.
[email protected]; [email protected]
Prof. M. Stöcker, Dr. Arjen van Miltenburg, A.I. Spjelkavik, A.M.
Bouzga
SINTEF Materials and Chemistry, P.O. Box 124 Blindern,
Forskningsveien 1, N 0314 Oslo, Norway.
[email protected]
Prof. J. Weitkamp, Prof. R. Gläser, Dr. S.C. Laha, Dr.
D. Pufky-Heinrich, S.A.S. Rezai
University of Stuttgart, Institute of Chemical Technology,
Pfaffenwaldring 55, D 70569 Stuttgart, Germany.
[email protected]
Prof. J. Caro, Prof. P. Behrens, Prof. M. Wark, I. Bannat, F.
Heinroth, B. Seelandt, I. Krueger, N. Witteck, B. Yler, A. Zukal,
R. Marshall, J. Rathousky
University of Hannover, Callinstrasse 3-3A, D 30167 Hannover,
Germany.
[email protected];[email protected];[email protected]
Prof. F. Kapteijn, Dr. M.-O. Coppens, Dr. P.J. Kooyman, Dr. A.F.P.
Ferreira, Dr. S. Aguado, Dr. J. Gascon, A. Denkova, Dr. A.
Quintanilla
Catalysis Engineering DCT – TUDelft, Julianalaan 136, NL 2628
BL Delft, The Netherlands.
[email protected]
Prof. F. Rodríguez-Reinoso, Prof. A. Sepúlveda-Escribano, Dr. J.
Silvestre-Albero, A. Silvestre-Albero, J. Ruiz-Martı́nez
University of Alicante, Department of Inorganic Chemistry, Laboratory of Advanced Materials, Carretera San Vicente del Raspeig, s/n C.P. 03690, San Vicente del Raspeig (Alicante), Spain.
[email protected]; [email protected]; [email protected]
Dr. D.J. Jones, Prof. J. Rozière, Dr. N. Donzel, Dr. M. TailladesJacquin
CNRS – Agrégats, Interfaces et Matériaux pour l’Energie, Institut
Charles Gerhardt for Molecular Chemistry and Materials, Université Montpellier II, Place Eugène Bataillon, F 34095 Montpellier cedex 5, France.
[email protected];
[email protected]; [email protected], Nicolas.
[email protected]
Prof. M-L. Saboungi, F. Meducin, F. Warmont, S. Serieye
CNRS – CRMD, Université d’ Orleans, 1B rue de la Férollerie,
F 45071 Orléans Cedex 2, France.
[email protected]
Dr. N. Kanellopoulos, Dr. G. Karanikolos
National Center for Scientific Research ‘‘Demokritos”, Patriarxou
Grigorio and Neapoleos, Agia Paraskevi, GR 15310 Athens,
Greece.
[email protected]; [email protected]
Dr. K.S. Triantafyllidis, C. Nitsos, S. Karakoulia, Dr. S.D. Sklari
Center for Research and Technology-Hellas (CERTH), Chemical
Process Engineering Research Institute (CPERI), 6th km. Charilaou-Thermi Road, P.O. Box 60361 Thermi, GR 57001 Thessaloniki, Greece.
[email protected]
4. Overview of the main synthetic approaches to mesoporous
materials
Most of the synthesis approaches to form inorganic mesoporous
materials in general are based on the use of organic template molecules that are used in different assembly processes or textural tem-
173
Fig. 4.1. Schematic overview of the main synthetic approaches to mesostructured materials. (A) Liquid crystal templating, (B) self-assembly and cooperative self-assembly,
(C) nanometric building blocks, and (D) sheet-folding mechanism of (titania) nanotubes. (Adapted from [4] and [10]).
plates, around which the inorganic precursor can condense [1–6]
(Fig. 4.1). However, also template-free synthesis mechanisms like
the nanobuilding block (NBB) mechanism [4] and other approaches
(e.g. folded sheet nanotubes) [7–9] (Fig. 4.1) have been reported.
A large diversity in synthesis approaches is known for the formation of different materials, and similar materials (e.g. MCM41, MCM-48, SBA-15, MSU etc.) can be made by different synthesis
methods and surfactants, each of them allowing other parameters
to be altered and controlled. Next to differences in chemical ratios,
the nature of the chemicals and additives that are applied as well
as synthesis temperatures and times, also alternative synthesis setups and combinations thereof are being used to obtain the necessary synthesis conditions (reflux setups, autoclaves for
hydrothermal treatments and microwaves) [11–17]. For this reason, knowledge of the synthesis methods and parameters that
influence the final material will allow pore size engineering and
control of the morphology and structural properties of the obtained material. Basically, the synthesis of mesoporous materials
and its control can be limited to the altering of the combination
of the chosen surfactant type, the specific synthesis mechanism
and the interaction of the silica source with the template molecules (if present) (see Table 4.1) [4].
For example, M41S materials are made by a S+I direct interaction between an ionic, positively charged MOS (molecular based
organized system) surfactant and a negatively charged silica source
in a basic environment. Three types of mechanisms, liquid crystal
templating, self-assembly and cooperative self-assembly have
been suggested for the synthesis of M41S materials based on the
Table 4.1
Schematic overview of the main synthesis parameters to generate a diversity of
mesoporous materials.
Surfactant
Mechanism
Interaction
MOS
POS
Textural templates
LCT
Self-assembly
Cooperative self-assembly
Nanometric building blocks
Direct
Indirect
Ionic
Non-ionic
Ionic
Non-ionic
MOS: molecular based organized systems, POS: polymeric based organized systems
[18], LCT: liquid crystal template.
applied synthesis conditions [4,12,19]. SBA materials, on the other
hand, have been made by use of POS (polymeric based organized
system) surfactants that interact through an indirect reaction of
the template with the positively charged silica source
((S0H+)(XI+)) in acid medium [20–22]. A neutral interaction between MOS surfactants and an inorganic source results in the formation of e.g. HMS materials (S0I0) [4]. Moreover, also other
parameters such as the pH, the presence of additives or not (e.g.
salts, swelling agents, co-solvents, co-surfactants etc.), concentrations, specific silica source, solvent, temperature etc. [1–3,23,24]
will allow fine-tuning of the final material due to small changes
in the characteristics of the surfactant, the mechanism or the interaction. A detailed description of the different surfactants, mechanisms and interactions has been reported in the review papers
by Corma et al. [19] and Soler-Illia et al. [4].
A general synthesis for the preparation of templated mesoporous materials can be described as the dissolution of the template
174
molecules in the solvent (with attention for pH, temperature, additives, co-solvents etc.) followed by addition of the silica source
(TEOS, metasilicate, fumed silica etc.). After a stirring period at a
certain temperature to allow hydrolysis and precondensation, the
temperature will be increased (sometimes combined with hydrothermal treatment or microwave synthesis, the addition of additives or changes in the pH) in order to direct the condensation
process. In a next step, the products will be recovered, washed
and dried. Finally, the template needs to be removed by calcination
procedures or extraction methods. The latter is environmentally
and economically the preferred procedure since it will allow the
recovery and recycling of the templates. However, extraction processes are often incomplete [25] and cannot be executed for all surfactants and materials. Moreover, in contrast to calcination
procedures at high temperatures, extraction methods do not result
in an additional condensation of the silica framework.
For non-siliceous materials, in addition to these important
parameters that control the formation of the structure, special
attention has to be paid to hydrolysis rates, redox reactions, phase
transformations etc. Indeed, the hydrolysis and condensation rates
of transition metal oxides are generally much faster than they are
for silica. Therefore, various approaches have been applied to reduce the uncontrolled hydrolysis and condensation in the synthesis of transition metal oxides so that phase separations are avoided
and good interactions between the inorganic source and the surfactant is obtained. The main factors to control the precursor reactivity are: (1) pH adjustment so that the solubility of the metal
oxides can be increased, in addition, the hydrolysis and condensation will be inhibited by the acid [26–31], (2) retardation of
hydrolysis through precursor complexation [32–34], (3) use of
non-aqueous solvents and a controlled amount of water to inhibit/slow down the hydrolysis [29,31,35,36]. Some synthesis procedures apply only one of the above-mentioned methods for the
formation of mesoporous transition metal oxides, others combine
two or more of the synthesis approaches to obtain maximum control over the synthesis mechanism and properties of the final material. The best known example for the synthesis of transition metal
oxides in this way is the evaporation induced self-assembly (EISA)
which makes use of ethanol as the solvent, MCl as the metal oxide
precursor and a MOS or POS template [29–31,37,38]. Moreover,
transition metal oxides are known to be less stable towards thermal treatments due to possible redox reactions, phase transitions
and crystallization. Several strategies can be applied to prevent
structural collapse or the formation of unwanted crystal phases
etc. upon calcination or application of these materials. Furthermore,
often extraction of the surfactant is done instead of calcination to
avoid structural collapse. Although this is environmentally and eco-
nomically a good approach since it allows surfactant recycling, it
also has some important disadvantages: the surfactant cannot be
removed completely by extraction and the condensation of the
material is limited resulting in low structural and thermal stability.
Moreover, the obtained material consists for a high percentage of an
amorphous phase. Therefore, when crystalline phases are crucial
for their application (e.g. anatase in case of titania photocatalysts),
heat treatments cannot be left out. For that reason, various in-situ
and post-treatment methods have been developed to stabilize mesoporous transition metal oxides. The most important stabilization
methods are summarized in Table 4.2.
Knowing the main mechanisms to form structured materials, a
short description of the verified mesoporous materials included in
this publication will be given in order to assist the reader in the
selection of the optimal porous material for a given application.
4.1. M41S materials
The first ordered mesoporous materials (IUPAC: 2 nm <
dp < 50 nm) that were reported are known as the M41S-type of silica mesophases. They were first reported in 1992 by Mobil [71–77].
The Mobil researchers introduced self-assembling surfactants as
structure directing agents to direct the formation of the SiO2 mesostructured materials. Also, Chiola et al. [78], Di Renzo et al. [79] and
Yanagisawa et al. [80] already reported on the formation of mesoporous materials by making use of self-assembling molecules.
However, due to an only limited description of the synthesis and
characteristics of the materials, their synthesis did not lead to the
great breakthrough as the publications and patents of Mobil.
M41S is the generic term for the various types of MCM (Mobil
Composition of Matter) materials in the mesoporous range. All
M41S materials have well-defined uniform pores that are ordered
in the long range. However, the walls of the pores consist of amorphous silica that can contain various heteroelements (e.g. Al [81–
90], Ti [91,92], Co [93], Zr [94], Cu [88], Fe [88,95], Zn [88] etc.).
By changing the synthesis conditions, it is possible to alter the
ordering of the material and therefore create new types of structures belonging to the M41S family. The various types of M41S
materials can be distinguished by the number after the acronym.
The three most important are: MCM-41 which is hexagonal
[12,19,96–101], cubic MCM-48 [19,81,101–103] and lamellar
MCM-50 [104–106]. In general, most M41S materials are made
in basic environment with quaternary ammonium salts
(CnH2n+1(CmH2m+1)3NX with n = 6–22, m = 1–4 and X = OH/Cl, OH,
Cl, Br or HSO4) or gemini surfactants ([CmH2m+1(CH3)2N–CsH2s–
N(CH3)2CnH2n+1]2Br with m and n = 16–18 and s = 2–12). However,
also sulphates, sulphonates, phosphates and carboxylic acids with
Table 4.2
Summary of various in-situ and post-treatment methods to increase the thermal stability of mesoporous transition metal oxides.
Stabilization method
In-situ/Posttreatment
References
Pre-crystallized precursor particles
Hydrothermal (re)crystallization
In-situ
In-situ or Posttreatment
In-situ
In-situ
[39–43]
[44–47]
Post-treatment
Post-treatment
Post-treatment
Post-treatment
In-situ or Posttreatment
Post-treatment
[52]
[53]
[54–59]
[60]
[4,61,62]
POS surfactants and synthesis conditions for thicker walls
Thermally stable surfactants/templates such as porous or non-porous carbon, CASH (combined assembly by soft and hard
chemistries), hydrocarbon additives, porous silica etc.
CVD (chemical vapour deposition) of metal tetraalkoxy groups with subsequent hydration to produce thicker pore walls
Doping with sodium oxide (stability increase with increasing concentration of doped element)
Ammonia treatment in gas or liquid phase (formation of small crystal domains before template removal)
Base treatment (formation of small crystal domains before template removal)
Addition of mineral anions or cations (postponing crystallization to higher temperatures)
Phosphate modification (postponing crystallization to higher temperatures) (high amounts will have negative influence on
catalysis!)
[29,48]
[49–51]
[63–70]
175
Fig. 4.2. Schematic representation of the values of the packing parameters.
Fig. 4.3. TEM image of the honeycomb structure of MCM-41 and a schematic
representation of the hexagonal shaped one-dimensional pores.
100
110
Intensity / a.u.
4.1.1. MCM-41
MCM-41 is the most widely studied M41S material. It is often
used as a model to compare with other materials or to study fundamental aspects in sorption, catalysis [117] etc. This is due to the
simplicity and ease in its preparation with negligible pore-networking and pore-blocking effects [12]. It consists of an amorphous (alumino, metallo)-silicate framework forming hexagonal
pores. MCM-41 has high surface areas of up to 1200 m2/g and large
pore volumes. The pores are very uniform causing narrow pore size
distributions [118]. The pores are unidirectional and arranged in a
honeycomb structure over micrometer length scales (Fig. 4.3).
A typical X-ray diffraction pattern of MCM-41 shows the hexagonal symmetry of the pore ordering (space group: p6m) (Fig. 4.4).
It typically contains four main reflection lines (d100, d110, d200 and
d210) or more at low angles (2h = 10°). Since MCM-41 consists of
amorphous silica, it has no crystallinity at the atomic level. Therefore, no reflections can be observed at higher degrees 2h.
For classical MCM-41, the pores can be tailored to diameters between dp = 1.5 and 20 nm. The largest pores can only be obtained
with the addition of swelling agents. The pore walls are quite thin
with a thickness between 1 and 1.5 nm. The presence of these thin
pore walls leads to low chemical and hydrothermal stabilities
[12,119]. In order to improve the stability of these materials,
Intensity / a.u.
long alkyl chains are applied for the synthesis of MCM-41 and
MCM-50 [107]. The key parameters for the M41S synthesis are
the hydrogel composition, the type and length of the surfactant,
the alkalinity, the temperature and the synthesis time. The type
of mesophase that will be obtained after a specific M41S synthesis
with quaternary ammonium salts can be predicted by the packing
factor (g-factor), which is a measure for the local effective surfactant packing [96,108–110]. It includes the hydrophobic–hydrophilic balance and therefore describes the tendency of the alkyl
chains to minimize their water contact and maximize their interorganic interactions [110]. Also the Coulomb interactions between
charged head groups are included. The solvating energies that also
determine the packing or shape of the surfactants in aqueous medium are not included in this g-packing factor. It can be expressed as
g ¼ aV0 l, where V represents the total volume of the surfactant chains
plus any co-solvent or organic additive between the chains, a0 the
effective head group area at the ‘‘micelles” surface and l the kinetic
surfactant tail length or curvature elastic energy (Fig. 4.2). The value of g increases as V increases or l or a0 decreases.
In classical micelle chemistry, as the value of g increases above a
critical value, mesophase transitions occur. The expected mesophases as a function of the g packing factor can be summarized
as follows (Table 4.3) [1,96]:
These transitions reflect a decrease of the surface curvature
from cubic through vesicular and lamellar. When the polar head
group has a large surface area, spherical structures are obtained.
On the other hand, lamellar or rod packing occurs when the head
groups are packed tightly with large aggregation numbers. By
changes in the synthesis conditions, the g packing factor and therefore also the ordering of the materials can be altered.
Additional information concerning the synthesis and characterization of M41S materials as well as the modification and applications can be found in various reviews [5,12,19,96,111–114]. The
M41S materials can be synthesized as powder materials, thin films
on various supports or as monolithic materials. The monolithic
materials have, in addition to the ordered uniform micro- and mesopores, also controlled macropores and macroscopic morphologies
[115,116].
200
210
3
4
5
6
7
8
2θ /º
Table 4.3
Overview of the value of g and the predicted mesophase [1,96].
110
Packing factor (g)
Mesophase
1/3
1/2
1/2–2/3
1
Cubic (Pm3n)
Hexagonal (p6m)
Cubic (Ia3d)
Lamellar
0
1
2
3
4
2θ /º
200
5
6
7
8
Fig. 4.4. Typical XRD pattern of MCM-41 with indices of the diffraction planes.
Inset: enlargement of the XRD pattern. The corresponding d-spacings:
(1 0 0) = 3.90 nm; (1 1 0) = 2.26 nm; (2 0 0) = 1.95 nm and (2 1 0) = 1.48 nm.
176
various techniques have been applied [12]. Some of these methods
include in-situ techniques like the addition of various salts
[120,121]. Other methods are post-modification methods such as
ion exchange [122], treatment in acid [123,124], grafting of organosilane functional groups to produce hydrophobic organic chains
on the surface [5,12,125,126] etc. On the other hand, attempts have
been made to increase the condensation and crystallization degree
in the pore walls of MCM-41 by hydrothermal treatments, introduction of zeolite functionality by recrystallization in the presence
of zeolite templates, the formation of M41S materials with the insitu addition of molecular templates of zeolites or using zeolite
precursor particles as the inorganic source [127–132].
4.1.2. MCM-48
Due to the smaller synthesis regime for MCM-48 when applying quaternary ammonium salts, the MCM-48 structure has been
far less studied than MCM-41 [133]. MCM-48 could be only
obtained with surfactant to silica ratios higher than 1
[104,108,133,134]. However, gemini surfactants have the intrinsic
ability to favour a cubic symmetry over a wide variety of conditions [102]. MCM-48 is cubic and has BET surface areas, pore sizes
and volumes similar to MCM-41. The wall thickness of the pores
is thin for MCM-48 as for MCM-41 causing only limited chemical
and hydrothermal stabilities. The structure of MCM-48 is of particular interest since the pores are three-dimensional. The XRD
pattern consists of several lines in the region between 2h = 0
and 10° (Fig. 4.5). Due to the amorphous nature of the pore walls,
again no diffraction patterns can be observed at high angles. Only
a broad band between 2h = 20 and 35° is present, indicating the
amorphous nature.
MCM-48 has a Ia3d symmetry that was confirmed by Monnier
et al. as being a 3D bicontinuous pore system [135]. The cubic pore
structure is mathematically ordered according to the ‘‘minimal
surface” which was described for the first time by Schoen [136].
The minimal surface defining the MCM-48 structure can be identified as a gyroid G or G-surface. Further mathematical aspects can
be found in the literature, see e.g. Ravikovitch and Neiman [137]
or Anderson [138]. The gyroid surface divides the cube into two
identical but separate compartments, creating two independent
but intertwinning enantiomeric 3D pore systems. Fig. 4.6 shows
a schematic representation of the unit cell with two micelle systems (red and blue rods) following the pore system. The two independent pore systems are interlocked and will run along the (111)
and (100) directions, but will never cross or join each other. The
pore systems are represented by micelle rods that progress in spirals around each other towards the (100) direction.
332
211
Intensity / a.u.
Intensity / a.u.
420
321400
3
422
431
4
5
6
7
8
2θ /º
Fig. 4.6. Cubic unit cell of MCM-48 with two independent micelle systems (red and
blue rods) separated by the pore wall (upper right). Mathematical representation of
a G gyroid minimal surface (upper left). Representation of 2 4 cubic unit cells
without the pore walls. The rods represent two independent micelle systems (red
and blue) moving towards the (100) direction (bottom). (For interpretation of the
references in colour in this figure legend, the reader is referred to the web version of
this article.)
The interest in three-dimensional structured materials has increased over the last years. This can be attributed to the expectation that the 3D pore network could have some important
advantages in catalysis and separation technology compared with
one-dimensional systems. There is more agitation in the system
due to an increased curvature in the pores. Moreover, the 3D network reduces the chance of restrictions in diffusion, which is not
limited to one dimension [111].
4.2. SBA materials
In 1998, a new family of highly ordered mesoporous silica
materials has been synthesized in an acid medium by the use of
commercially available non-ionic triblock copolymers (EOnPOmEOn) with large polyethyleneoxide (EO)n and polypropyleneoxide
(PO)m blocks [18,139,140]. Different materials with a diversity of
periodic arrangements have been prepared and denoted as SBA
materials (the acronym for Santa Barbara acids). A wide variety
of SBA materials has been reported in the literature, such as
SBA-1 (cubic) [141,142], SBA-11 (cubic) [139,143], SBA-12 (3D
hexagonal network) [139,143], SBA-14 (lamellar) [139], SBA-15
(2D hexagonal) [139,140,143] and SBA-16 (cubic cage-structured)
[139,143–145]. SBA-15 immediately attracted a lot of attention
because of its desirable features and is now the most intensely
studied SBA structure.
220
0
1
2
3
4
5
6
7
8
2θ /º
Fig. 4.5. Typical XRD pattern of MCM-48 (Ia3d space group) with indices of the
diffraction planes. Inset: enlargement of the XRD pattern. The corresponding dspacings: (2 1 1) = 3.80 nm; (2 2 0) = 3.29 nm; (3 2 1) = 2.50 nm; (4 0 0) = 2.35 nm;
(4 2 0) = 2.10 nm; (3 3 2) = 2.00 nm; (4 2 2) = 1.93 nm and (4 3 1) = 1.85 nm
4.2.1. SBA-15
SBA-15 is a combined micro- and mesoporous material with
hexagonally ordered tuneable uniform mesopores (4–14 nm)
[21,146]. The size of the micropores was found to depend on the
synthesis conditions and can vary between 0.5 and 3 nm in size
[147–153]. It consists of thick microporous silica pore walls (3–
6 nm) responsible for the high hydrothermal stability of SBA-15
compared to other mesoporous materials with thin pore walls like
MCM-41, MCM-48 and HMS [119,139]. X-ray diffraction patterns
of the SBA-15 materials reveal the 2D hexagonally structured pores
(p6mm space group) at low angles, whereas no diffraction pattern
can be observed at high angles due to the amorphous nature of the
pore walls (Fig. 4.7).
TEM investigation of the SBA-15 materials revealed the curved
nature of the pores [154,155] (Fig. 4.8).
However, it has been reported that SBA-15 materials with short
or straight channels can be synthesized as well by, respectively,
decreasing the stirring time or adding salts during the synthesis
(Fig. 4.9) [156,157]. Moreover, these short-channel SBA-15 materials also give rise to smaller, less aggregated particles.
The shape and curvature of the pores was claimed to be important for the diffusion of molecules through the structure [154,158]
and the ultimate adsorption capacity [158]. The micropores in the
walls of the SBA-15 mesopores originate from the polyethyleneoxide blocks (PEO) in the triblockcopolymers that are directed to the
Intensity / a.u.
100
110 200
0
1
2
3
4
2θ /º
5
6
7
8
Fig. 4.7. Typical XRD pattern for SBA-15 materials (p6mm space group) with
indices of the diffraction planes. The corresponding d-spacings: (1 0 0) = 9.80 nm;
(1 1 0) = 5.45 nm and (2 0 0) = 4.74 nm.
177
aqueous solution [22,146,153,159,160], whereas the polypropyleneoxide blocks (PPO) are more hydrophobic and give rise to the
internal structure of the mesopore [18,22,159–161]. A schematic
representation of the structure-directing assembly of the PEO
and PPO blocks in SBA-15 can be seen in Fig. 4.10.
By changing the length of the polyethyleneoxide blocks, different amounts of micropores and changes in the pore wall thickness
could be obtained [21,160,161]. Moreover, the ratio of the number
of polyethyleneoxide units to the number of polypropyleneoxide
units directs the mesophase (lamellar, hexagonal, cubic etc.) of
the structure [5,160,162]. On the other hand, altering the length
of the polypropyleneoxide blocks will result in different mesopore
diameters [160]. Furthermore, synthesis parameters like temperature [5,146,160,163–165], pH [166] and the addition of additives
such as co-surfactants, swelling agents, electrolytes, salts etc.
[5,167–170] will allow pore size engineering and tuning of the general properties and morphologies of SBA-15 to a large extent. A
wide diversity of morphologies [167–175] has been reported for
SBA-15 such as rods, fibers, gyroids, discoid-like, doughnut-like,
spheres (micrometer and millimeters sized), rope-like, etc.
(Fig. 4.11).
In addition, SBA-15 can be synthesized using low-cost silicon
sources [176–180] and fast synthesis procedures [14–17]. Due to
these desirable features, SBA-15 has attracted a lot of attention.
Since the development of SBA-15 a lot of research has been
done on the development and modification of materials with a
combined micro- and mesoporosity. It can be produced both as
bulk powder in large and small batches as well as in the form of
thin films [181–184] or as monoliths [185–189]. In case of monoliths, the materials often are denoted differently, e.g. HOM-2
[188,189]. SBA-15 has been modified with a wide diversity of transition metal oxides (V [190–193], Ti [157,190,194–197], Al [198–
203], Zr [17], Ru [202], Rh [202], Fe [202] etc.) and organic functional groups [204–209] by post-synthesis and in-situ processes.
This gives the active SBA-15 materials the possibility to be used
in catalysis [190,192–196,198–200,202,210–212], controlled release of drugs or antioxidants [213,214], removal of heavy metals
[215], photoluminescence [216,217], lithium batteries [218],
immobilization of enzymes [158,219], proton conductivity [209]
etc. Moreover, one of the interesting applications of SBA-15 is its
use as a template for the synthesis of (inverse) carbon replicas
[173,220–224] and nanowires of various metals [225–232].
4.2.2. SBA-16
When block copolymers with larger EO chains (e.g. EO106PO70EO106 = F127) are used as templates under acidic conditions,
SBA-16 can be formed at room temperature [20]. The large EO
chains will favour the formation of globular aggregated structures
[20,233]. SBA-16 can be obtained only in a narrow range of diluted
Fig. 4.8. TEM images and diffraction pattern showing the hexagonal ordering of
SBA-15 and the curved pores.
Calcination
PPO
PEO
mesopore
(4-14 nm)
micropore
Fig. 4.9. SEM image of SBA-15 (a) (inset: enlarged particle) and SBA-15 with short
channels (b).
Fig. 4.10. Schematic representation of SBA-15 before and after calcination.
178
Fig. 4.11. SEM images showing examples of a few different morphologies of SBA-15 (spheres, fibers and rods).
110
Intensity / a.u.
200
Intensity / a.u.
surfactant concentration (3–5%) [20]. Similar to SBA-15, the template can be removed by extraction in ethanol at low temperatures
or by calcination at elevated temperatures. After template removal,
a combined micro- and mesoporous material is obtained due to the
presence of PEO and PPO chains responsible for the formation of
the micropores and mesopores, respectively [146,249]. The narrow
pore size distributions, mesopore sizes around 6 nm, high surface
areas, large total pore volumes and thick pore walls (4–6 nm) of
SBA-16 resemble that of SBA-15. The thick pore walls result in high
chemical, thermal and hydrothermal stabilities of SBA-16
[119,234]. As in the case of SBA-15, the total pore volume, pore size
and relative fraction of micro- and mesopores can be controlled by
changes in the synthesis conditions (time, temperature, Si/surfactant ratio, pH, type of surfactant, additives, cosurfactants etc.)
[235–244,249,253]. Moreover, a wide variety of morphologies such
as spheres, cubes, rods etc. can be formed by careful control of the
synthesis method [240–247]. Apart from various similarities with
SBA-15, the pore geometry and ordering of SBA-16 is different.
SBA-16 shows a broad hysteresis loop in nitrogen adsorption–
desorption isotherms which closes around P/P0 = 0.45, indicating
the presence of inkbottle pores (Fig. 4.12) [248–250].
X-ray diffraction patterns of SBA-16 reveal its three-dimensional cubic cage structure (Im3m space group) (Fig. 4.13)
[20,249]. A clear model of the structure based on TEM was published by Sakamoto et al. [251]. They observed that the cubic phase
consists of two non-interpenetrating 3D channel systems with
spherical cavities at the intersection of the channels. This 3D structure is expected to provide favourable mass transfer kinetics in
comparison to the 2D network of SBA-15.
211
220
1
1.5
2
2.5
3
3.5
4
4.5
5
2θ /°
200
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
2θ /°
Fig. 4.13. XRD pattern of SBA-16. Inset: enlargement of the diffraction pattern with
indices of the diffraction planes. The corresponding d-spacings: (1 1 0) = 11.31 nm;
(2 0 0) = 8.02 nm; (2 1 1) = 6.39 nm and (2 2 0) = 5.88 nm.
SBA-16 can be prepared in economically and environmentally
more friendly ways under fast [252] and mild conditions [253] or
via microwave synthesis [243,245]. SBA-16 materials can be made
in bulk (powder) or deposited as films on various substrates
[254,255]. In accordance with all other mesoporous materials,
SBA-16 is often functionalized with various hetereoelements
[256,257] or organic functional groups [209,258,259] to improve
their performance in various processes such as catalysis, sorption,
separations etc. The functionalization can be achieved by in-situ
methods during synthesis or by post-modification processes.
4.3. Plugged hexagonal templated silica (PHTS): an analogue to SBA-15
Volume adsorbed STP / cm³ g
-1
600
500
400
300
200
100
0
0.0
0.1
0.2
0.3
0.4
0.5
P/P 0
0.6
0.7
0.8
0.9
1.0
Fig. 4.12. Nitrogen sorption isotherm at 196 °C for SBA-16 showing clearly a
broad hysteresis loop.
By increasing the silica over surfactant ratio in the synthesis of
SBA-15, Plugged Hexagonal Templated Silica (PHTS) is formed
[154,224,260–263].
PHTS has the same basic characteristics as SBA-15. It consists of
hexagonally ordered mesopores with diameters that are similar to
those of SBA-15. Moreover, it has thick pore walls (3–6 nm) perforated with micropores making PHTS a combined micro- and mesoporous material [264]. In addition, PHTS possesses microporous
amorphous nanoparticles (plugs) in the uniform mesoporous channels resulting in higher micropore volumes. The pillaring effect of
the nanoparticles gives PHTS a higher mechanical stability compared to the pure SBA-15 [260,265]. In addition, PHTS possesses
a high hydrothermal stability, which improves when applying high
synthesis temperatures and longer synthesis times [260,266].
Therefore, PHTS is put forward as a good candidate for industrial
applications. Indeed, stability is one of the major factors that hinder prospective catalytic applications of mesoporous materials
179
[266]. Furthermore, PHTS materials have a characteristic nitrogen
sorption isotherm due to the presence of a tuneable amount of
both open and plugged sections induced by the dispersion of nanoparticles in the mesopores (see Fig. 4.14 schematic representation)
[224,260,262,267].
Nitrogen sorption isotherms at 196 °C for both SBA-15 and
PHTS are of type IV according to the IUPAC classification
(Fig. 4.14). The N2 sorption isotherms of PHTS show a one-step capillary condensation in the adsorption branch, indicating the filling
of the uniform mesopores. However, in contrast to SBA-15, PHTS
exhibits a two-step desorption. The first desorption step is similar
to desorption from pure SBA-15 and can be assigned to desorption
of nitrogen from the open mesopores according to the normal Kelvin model. Desorption occurs at equilibrium conditions via a receding meniscus. The second desorption step can be attributed to the
nanoparticles (plugs) within the mesopores, plugging/narrowing
part of the mesoporous channels and creating inkbottle-like sections. Therefore, nitrogen that is present between two nanoparticles can only be desorbed through the restricted pore entrance.
For these plugged pores, desorption is delayed and results in a second desorption step at lower relative pressure compared to the
open mesopores (first desorption step).
Interpretations of the size of the nanoparticles and therefore
also the diameter of the pore at the plugs should be done with care.
Indeed, the relative pressure at P/P0 = 0.42 depends weakly on the
pore size and pore geometry in nitrogen sorption measurements
due to the lower closure point of the hysteresis (P/P0 = 0.42) for
nitrogen [224,268–270]. When this lower closure limit is reached,
capillary evaporation can no longer be delayed in the plugged sections. Kruk et al. proved that using argon adsorption–desorption
isotherms (lower closure point limit at a relative pressure of ca.
0.3), all constrictions in the porous structure of PHTS, plugging
the mesopores, are likely to exhibit diameters above ca. 4–5 nm
[224]. By modification with hexyldimethylsilyl (HDMS) and decyldimethylsilyl (DDMS), it was estimated that the largest diameter of
the constrictions were larger than 2.4 nm but smaller than 3.4 nm
[266]. Therefore, for materials with hysteresis loops closing at
P/P0 = 0.42, argon adsorption–desorption measurements or surface
modification techniques should be carried out if information concerning the real size of the plugs and pore entrances are needed
[191,224,266,269]. The ratio of open to plugged pores can be tuned
from 100% open pores to fully plugged pores (inkbottle pores with
nanoparticles at the pore mouth) by simply adjusting the synthesis
parameters [127,224,260]. In addition, altering the synthesis conditions will allow controlling the size and stability of the plugs
[266]. It was found that the minimum time required to obtain good
structural materials was 4 hours [262]. Moreover, increasing synthesis temperatures will result in increasing pore diameters and
enlarged particle sizes. Changes in synthesis temperature result
in similar phenomena observed in SBA-15 materials and PHTS.
However, in contrast to SBA-15, the micropore volumes in PHTS increase when the synthesis temperature is raised, which was explained by the increase in the microporosity of the plugs [262].
The synthesis mechanism of PHTS is described in the literature
[127]. By careful control of the stirring temperature and the
amount of TEOS (tetraethylorthosilicate) used for the synthesis of
PHTS, different morphologies could be formed (Fig. 4.15).
At low temperatures and low TEOS concentrations smooth rods
are formed, whereas at high temperatures spherical morphologies
will be obtained. The differences in morphologies were based on
the cloud point of the surfactant and the balance between the rate
of polymerization of the silica source and the rate of the mesostructure formation [263].
Catalytically active elements can be introduced in PHTS-type
materials in different ways. On the one hand, metal acetylacetonate complexes can be deposited on the surface of the PHTS materials by use of the liquid-phase molecular designed dispersion
method (MDD) [271]. This way, a dispersed layer of metal oxides
is formed on the surface of PHTS after calcination. Depending on
the size of the molecule and the temperature applied, molecules
can be deposited in the entire pore or can be excluded from the
plugged sections. Also other post-synthesis impregnation methods
have been applied to introduce heteroelements on the surface of
PHTS materials [272,273]. On the other hand, PHTS materials can
be activated by introducing metal oxide or zeolitic nanoparticles
into SBA-15 [274–279]. This way the catalytically active elements
are introduced into the pores of SBA-15 at the same time as the
transformation of the material into a PHTS-type material. By an
incipient wetness impregnation of a preformed nanoparticles suspension in SBA-15, PHTS materials and their properties can be tailor-made in a controlled way. Both the plugging as well as the
heteroelement are introduced by post-synthesis modifications. Recently, also in-situ introduction of heteroelements in PHTS has
been achieved [280,281]. By carefully choosing the right method
600
-1
500
-1
350
Open
mesopore
400
plugged
mesopore
300
200
PHTS
100
SBA-15
0
300
250
200
150
100
50
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
0.1
P /P 0
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P /P 0
plug
SBA-15
open
mesopore
PHTS
plugged
mesopore
SBA-15 with
inkbottle pores
Fig. 4.14. Nitrogen sorption isotherms at 196 °C for SBA-15, PHTS and SBA-15 with inkbottle pores. Schematic representation of the different pore structures.
180
Fig. 4.15. Different morphologies (smooth fibers, rough fibers, spherical) of PHTS synthesized by changes in the stirring temperature [263].
to introduce the heteroelements, it is possible to control the location of the active elements: (1) on the surface in the entire pore
system, (2) on the surface only in the open pores, (3) only in the
plugs, (4) incorporated in the silica walls, and combinations of
the previous.
4.4. MCF
Mesostructured cellular foam (MCF) is made by addition of a
swelling agent to the synthesis of SBA-15 [282–284]. Often, mesitylene (1,3,5-trimethylbenzene) is chosen, but other swelling
agents are applied as well. The swelling agent causes an
enlargement of the micelle resulting in a sponge-like foam with
three-dimensional structure with large uniform spherical cells
(15–50 nm), accessible via large windows (5–20 nm) (Fig. 4.16).
Therefore, MCF is a very open structure with large uniform pore
diameters and large pore volumes (Fig. 4.17). It has thick pore
walls resulting in a high hydrothermal stability. The addition of
ammonium fluoride can selectively enlarge the windows by 50–
80% [282,284].
The nitrogen sorption isotherms of MCF show inkbottle pores
due to the presence of the large windows that provide accessibility
to the spherical cells. The structural properties of MCF materials
can be controlled by adjusting the synthesis parameters (time,
temperature, pH, additives, swelling agent etc.) [157,282–287].
Moreover, the morphology and particle size of the MCF materials can be altered by simple adjustments during the synthesis
[157,288]. Various heteroelements [289–292,295] and functional
groups [293,296–302] have been introduced into MCFs via postsynthesis modification or in-situ techniques in order to increase
their performance in (bio)catalysis, sorption, controlled release,
separations etc. MCFs have been produced as bulk powders or
monoliths [303–305].
MCF materials are often desirable due to their fast mass transfer
kinetics, good accessibility for large molecules (e.g. polymers, enzymes, etc.) and their large pore volumes that are beneficial in various processes [157,290,294,298].
4.5. MSU
Fig. 4.16. TEM image of an MCF structure.
1800
1500
d v (r )
-1
2100
1200
900
600
0
5
10
15
20
25
Radius / nm
30
35
40
300
MCF
SBA-15
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P /P 0
Fig. 4.17. Nitrogen adsorption-desorption isotherm at 196 °C of SBA-15 and MCF.
Inset: pore size distribution deduced from the adsorption branches.
The acronym MSU stands for Michigan State University where
the synthesis of MSU was developed. MSU materials are prepared
with non-ionic polyethylene glycol polymers (not necessarily triblock copolymers) in neutral environment in contrast to SBA-15,
PHTS and MCF which are made in acidic media [306]. The various
types of MSU materials are often called the MSU-X family of
materials where X is a number or a letter. The numbers and letters are used to distinguish among the different materials. For
example, MSU-1 is made with alkyl-PEO alcohols like tergitol,
MSU-2 with alkyl-aryl-PEO surfactants like triton-X or igepal,
MSU-3 with block copolymers like pluronic, MSU-4 with ethoxylated derivatives of the fatty esters of sorbitan like tween
[306,307]. Most of the MSU materials present only local structural
ordering and are called wormhole framework structures
[306,308]. However, also highly ordered mesoporous MSU materials have been developed that were denoted MSU-H (hexagonal)
[307,309] as well as foam structures made with the addition of
swelling agents (MSU-F) [308]. Other types of letters that can
be placed after the acronym are MSU-S used for aluminosilicate
MSU materials that are synthesized using zeolite seeds and exhibit improved hydrothermal stability compared to conventional
MCM-41 aluminosilicate materials. This often coincides with the
addition of HBEA or WBEA to the nomenclature meaning that the
structure was made by using zeolite beta (BEA) seeds and H
and W indicate the hexagonal or wormhole-like ordering of the
materials, respectively [310,311]. When vesicular particle architectures are obtained and a lamellar pore arrangement, the material is denoted as MSU-V [312,313]. Also MSU-G exhibits a vesicle
like hierarchical structure [314].
Since the template is not necessarily a triblock copolymer, not
all MSU materials will possess micropores next to the controlled
and uniform mesopores. Due to the fact that the synthesis occurs
in neutral environment, the interaction between surfactant and
the inorganic source is of the type S0I0 instead of the bridged interaction with the counterion of the acid ((S0H+)(XI +)) in case of SBA15, PHTS and MCF. This neutral interaction allows easy extraction
of the surfactant after synthesis. Therefore a more economical synthesis is attained because of higher recovery yields of the surfactant after extraction. As in case of the preparation of all
mesoporous templated materials, the structural and morphological
properties of MSU can be designed by careful control of the synthesis parameters (time, temperature, type of template, additives, etc.)
[315–327]. Addition of fluoride causes an in-depth mineralization
improving the structural quality of the final material without
changing the pH of the solution [306,307,328]. For the synthesis
with tween surfactants, the addition of fluoride ions is even a prerequisite to obtain good structures [307]. Moreover, cheap silicon
sources can be used to make the synthesis more cost-effective
[308,329,330].
To achieve high activity and performance in a variety of applications, MSUs can be modified with several heteroelements
[310,314,331–335]. The MSU materials have been prepared as bulk
materials as well as in films [336] and monoliths [185,304,337–
339]. The monolitic materials are designed to feature also a controlled macroporosity and macroscopic morphologies next to the
adjustable micro- and mesoporosity. Their basic structure resembles that of MSUs but some special attention is needed in the synthesis to allow control over the macroscopoic properties. In case of
monoliths, the materials often have alternative terminology.
4.6. Direct liquid crystal templating (DLCT)
An alternative synthesis route to mesoporous materials uses direct templating by preformed lyotropic liquid crystal phases that
are prepared under high surfactant concentrations, generally >20
wt.% [340–345]. Both non-ionic and ionic surfactant types may
be used. The pore size can be controlled in the super-microporous
to mesoporous range. Particular aspects of the synthesis are that:
(1) It uses a molecular source of silicon (tetramethoxy- or tetraethoxysilane, TMOS, TEOS). (2) The synthesis is essentially solventfree, TMOS or TEOS are being used as the solvent for heteroatom
salts (generally nitrates or metal organic compounds). (3) The synthesis takes place under conditions of controlled alkoxide hydrolysis and condensation, coupled with removal of the produced
alcohol in order to prevent disruption of the liquid crystal phase
structure. The high concentration of surfactant required for direct
liquid crystal templating is maintained throughout the whole process and the phase structure is preserved by the sol–gel formation
of silica in the hydrophilic domains. This results in a solid replica, a
nanocast of the liquid crystal phase. Precipitation of a powder form
product is avoided and macroscopic (monolithic) porous objects
are obtained. The shape and size of the monoliths can be controlled. (4) The synthesis gel contains no cations other than protons, avoiding any need for an ion-exchange step.
The DLCT methodology has been used for the preparation of
super-microporous and mesoporous silica and aluminosilicates
using a range of non-ionic surfactants of the Brij, Tergitol and
Montanox types [344]. Alkylene oxide segments of the surfactants
can form crown ether-type complexes with inorganic ions through
weak coordination bonds, allowing direct preparation of heteroatom-containing mesoporous silicas [344,346]. The interaction between metal ions and alkylene oxide segments ensures the
dispersion of heteroatoms throughout the nascent mesophase,
while the strength of interaction can impact the extent to which
a given metal ion is incorporated in the silica framework. In this
181
way not only aluminium but noble and base metal ions have been
dispersed throughout mesoporous aluminosilicates and silica.
They can be either reduced to the metallic form (Rh, Pt, Ir, Ru)
[346] or oxidized (Co, Ni, Cu, Ag) [347–349].
Through 27Al MAS NMR, the environment of aluminium in DLCT
aluminosilicates was shown to depend on the Si/Al ratio. The aluminium was predominantly tetra-coordinated in materials of Si/Al
ratio up to 15, tetra- and hexa-coordinate in samples of Si/Al of 10.
Penta-coordination was observed in materials of low Si/Al ratio
[347]. The strength of the acid sites has been investigated in detail
using a combination of calorimetric, gas phase adsorption and
spectroscopic techniques [350]. For all Si/Al ratios, the acid
strength is higher than that of aluminium substituted MCM-41
types [351], which confers activity of interest in catalysis where
medium acid strength is required. Bimetallic Pd/Pt and Pd/Rh particles supported on DLCT aluminosilicates catalyse the hydrogenation and ring opening of polyaromatics components of light cycle
oil to molecules of high cetane number [352–354]. Copper and silver oxide functionalized DLCT aluminosilicates show activity in the
separation of propene from propane/propene mixtures [347,348],
while functionalization with supported cobalt, copper and iron oxides confers de-NOx activity [349]. The direct liquid crystal templating approach has been extended from the use of oligomeric
non-ionic surfactants to block copolymer templates [355–357]
which enables preparation of large transparent aluminosilicate
monolithic mesophases and pore sizes in corresponding calcined
materials in the range 3–7 nm [358].
4.7. Mesoporous titania
Templated mesoporous transition metal oxides are made in a
similar way as their silica counterparts. However, special attention
needs to be given to the higher reactivity of the transition metal
precursors in comparison to the silica sources. Only control over
the hydrolysis and condensation reactions and other aspects such
as phase transformations etc. will allow the formation of defined
porous structures. Therefore, various methods to temper the reactivity are applied in combination with a wide variety of in-situ or
post-synthesis approaches to prevent the occurrence of uncontrolled redox reactions, phase transformations etc. These are described in the first part of this chapter containing the general
synthesis mechanisms and in several reviews [4,8]. Titania is one
of these metal oxides that suffer fast hydrolysis and condensation
reactions resulting in poorly structured and even non-porous
materials. This is regarded as the first fundamental issue towards
the synthesis of mesoporous titania. Various methods for the synthesis of mesoporous titania exist [4,8,65]. However, the best
known synthesis approach to obtain the controlled formation of
mesoporous titania is the evaporation induced self assembly
(EISA). This method combines different ways to temper reactivity
such as synthesis in non-aqueous solvents (often ethanol) and
acidic inhibition to temper condensation. The EISA method was
first described by the group of Stucky for the formation of a wide
variety of transition metal oxides (TMOs) using ethanol as the solvent, MCln as the TMO source and a polymeric template [29,31]. Later, the EISA method was further optimized by Soler-Illia et al. via
complexation of the titania precursor with alkoxides and the addition of a controlled amount of water resulting in further retardation of the hydrolysis and condensation kinetics [30]. This
approach was done in combination with the use of MOS templates
such as CTABr (cetyltrimethylammonium bromide), influencing
the kinetics of hydrolysis and condensation even more strongly
due to ligand assisted templating. Although the mesoporous titania
can be formed in a controlled way, it often possesses only local
ordering. Nevertheless, materials with high degree of ordering
can be obtained by mixing two types of titania precursors. It
182
consists of combining acid–base pairs which are titania precursors
such as a titanium salt (TiCl4) (inducing acidity) in combination
with a titaniumalkoxide (inducing alkalinity). They can easily react
to form low polymerized homogeneous precursors and self-control
of the inorganic polymerization is obtained [359–361]. With the
EISA method, a critical micelle concentration (cmc) will be obtained upon evaporation of the solvent. The synthesis should be
controlled in such a way that hydrolysis and condensation occur
upon micelle formation while a good interaction between the inorganic titania source and the template is ensured. After formation of
the network, the material is subjected to a mild thermal treatment
to allow further condensation and strengthening of the network.
Afterwards, the template needs to be removed by extraction or calcination procedures. During calcination and any other heat treatments, care must be taken to prevent structural collapse or
uncontrolled phase transformation. A wide variety of methods is
known. They are described in Table 4.2 in the first part of this
chapter.
Since the discovery of carbon nanotubes in 1991 by Iijima [362],
much research has been devoted to the development of nanoscale
tubular materials. Especially, nanostructured titania has lately received much attention due to its potential in a broad range of
applications like photocatalysis [9] (e.g. water purification), solar
cells [363], electronics [364,365] (e.g. lithium ion batteries), sensors [366] (e.g. hydrogen sensing), membranes [367,368], medicine
[369,370] (e.g. bone tissue regeneration), adsorbers [371] (e.g.
treatment of radioactive liquid waste) and catalyst supports
[372] (e.g. CO oxidation). In 1998, Kasuga et al. synthesized TiO2
nanotubes for the first time by a quick, cheap, simple and template-free hydrothermal synthesis procedure [373]. The formation
mechanism of the Ti-based nanotubes is, however, still under discussion. According to Kasuga et al. [373] and other researchers
[374,375], the acid washing process of the precipitate after hydrothermal treatment is essential for formation of trititanate nanotubes. However, several other authors [376,377] found evidence
that trititanate nanotubes are formed during hydrothermal treatment. Nevertheless, there is a consensus that after chemical bond
breaking of the bulk titania in NaOH, two-dimensional nanosheets
are formed which can be converted into nanotubes by a sheet-folding mechanism [378–381] (Fig. 4.1D). Also, the crystalline phase of
the synthesized nanotubes is still a matter of controversy. Indeed,
various different crystalline phases were claimed to be obtained
after hydrothermal treatment of a chosen TiO2 source in a NaOH
solution, such as TiO2 (anatase) [373], monoclinic sodium trititanate (Na2Ti3O7)[382] or Na2Ti2O4(OH)2 crystal phases [383,384].
Trititanate nanotubes (TTNT) are prepared via a hydrothermal
synthesis method [9,385]. Hydrothermal treatment of a TiO2/NaOH
mixture yields a well formed tubularly shaped nanomaterial. These
sodium-containing multiwall tubular structures consist of a number of shells with interlayer distances of about 0.74 nm and an inner pore diameter of 4–4.2 nm (Fig. 4.18). The fact that the
nanotubes are not symmetric confirms the nanosheets fold up
according to a rolling up mechanism during hydrothermal treatment. Furthermore, the obtained nanotubes are open at both ends,
which makes the inner pore easily accessible. This is in contrast to
most carbon nanotubes that have caps closing off their ends.
The titania and trititanate nanotubes consist of small nanocrystalline domains due to the nanometer size dimensions of the tubes.
The trititanate crystals are built up by the interconnection of three
TiO6 octahedra which share edges. These chains of octahedra join
at the corners to form a stepped, zigzag ribbon layered structure.
layers, sodium cations
Between these negatively charged Ti3 O2
7
are located [386]. Results of ab initio calculations by Zhang et al.
Intensity / a.u.
4.8. Titania nanotubes
Fig. 4.18. HRTEM image of: (a) Na-TTNT, and (b) H-TTNT.
b
a
0
10
20
30
40
50
2θ /º
60
70
80
90
100
Fig. 4.19. X-ray diffraction patterns of: (a) Na2Ti3O7, and (b) acid washed H2Ti3O7
nanotubes.
[387] based on the density functional theory were consistent with
the assumption that the intercalated sodium ions can be replaced,
even though the Na2Ti3O7 structure is very stable. This is possible
since the sodium ions are only weakly bonded to the negatively
charged Ti3 O2
7 layers. Sodium can easily be exchanged for hydrogen by acid washing, producing H-TTNT materials (Fig. 4.19). However, also transition metals like Co2+ or Cu2+ can be intercalated
[388]. This is interesting in view of photocatalysis under visible
light.
Nitrogen sorption results reveal that there is a significant difference in the specific surface areas of the different trititanate nanotubes (205 m2/g for Na-TTNT and 333 m2/g for H-TTNT) [9]. This
can be explained by taking into consideration that the large sodium cations with large hydration spheres are exchanged by small,
poorly hydrated protons during the acid treatment. The ion exchange process results in a smaller interlayer distance and a decrease in strain energy. The relaxation of these tensed bond
lengths and angles, originating from the curvature of the layers,
leads to small differences in the nanotube dimensions which are
responsible for the increase in surface area.
Next to ion exchange, anatase crystal domains can be obtained
within the nanotubes by means of simple calcination processes or
hydrothermal treatments [9,389,390]. This will result in a significant increase in photocatalytic activity.
Besides the classical synthesis of titania and trititanate nanotubes by hydrothermal treatment in conventional ovens, also
microwave assisted syntheses have been reported [9,391–394].
5. Characterization techniques
This chapter involves characterization techniques briefly described, that have been used in order to characterize the synthesized materials. Only a limited number of techniques are
described that are most commonly applied for the characterization
of ordered mesoporous materials. It is clear that in addition other
more advanced techniques can be applied as well.
Detailed descriptions of the techniques are given, together with
some recommendations for good practice.
183
(once all these requirements are fulfilled the systems jumps
to the next point in the adsorption isotherm). The operator
should define the stringency of the pressure readings criteria
depending on the nature of the sample. In this sense, care must
be taken when using samples where the presence of narrow
microporosity is expected.
5.1. Adsorption
5.1.1. N2 adsorption isotherms
The correct characterization of the textural properties of nanoporous materials is very helpful to verify the success of the synthesis process [395]. Additionally, the exact knowledge of the porous
network (micropores, mesopores and macropores) will be useful to
understand the behavior of the new material in a future application. Commonly, the textural characterization of porous solids is
performed using adsorption of probe molecules (Ar, N2, CO2,
etc.). Among them, N2 adsorption at low temperature (196 °C)
is the most widely used [396]. N2 adsorption covers the relative
pressure (P/P0) range from 106 to 1 and provides information
about the whole microporosity (up to 2 nm) and the mesoporosity.
Although the use of N2 adsorption at 196 °C for the characterization of porous materials is widely spread, the reported values
not always reflect the real porous structure of the material. Usually, there are many inherent errors associated with the sample,
the equipment, the operator, etc., which must considered carefully
in order to obtain reliable measurements. Although most of these
problems are not so important in mesoporous materials, they become very important when there is microporosity present [397–
399]:
– Concerning the sample, there are critical parameters to be considered such as the amount of sample used in each measurement (commonly 0.1 g) and the shape of the sample (powder,
grain, pellet, monolith, etc.). It is noteworthy to mention that
the textural properties of a certain sample should differ for the
powder or grain to the pellet or the monolith (e.g. breaking a
monolith to fit the sample holder could provide an erroneous
characterization). Additionally, when using heavy pellets or
grains, care must be taken with the amount of sample since
the use of a single pellet or grain could differ considerable from
other individual pellets or grains. Thus, the use of several pellets
or grains in order to obtain an average value is highly encouraged. In these cases an increase of the amount of sample used
will drastically reduce the error.
– Concerning the automated equipment, the errors usually come
from the lack of precision at low relative pressures. Automated
equipments are frequently supplied with only a pressure-transducer (1.33 105 Pa) which fails when trying to perform high-precision isotherms (below P/P0 103–104). The incorporation of
a second pressure-transducer with a higher precision (133.3 or
1333 Pa) avoids the aforementioned drawbacks and allows
achieving high-precision adsorption measurement starting at
relatives pressures below 106.
– Concerning the operator, the errors usually come from the lack
of experience in the manipulation of the equipment, together
with the ignorance about the way the adsorption equipment
operates. First of all, the automated equipment must be frequently calibrated (pressure reading, fixed volumes, etc.) to
avoid uncertainty in the adsorption data. Secondly, the operator should ensure that the adsorption data are obtained under
true equilibrium conditions. Usually, the software of the
adsorption equipment allows to define for each point in the
adsorption isotherm: (i) the time left between consecutive
pressure readings to ensure that the equilibrium has been
reached, (ii) the number of pressure readings to be considered,
and (iii) the deviation allowed within these pressure readings
A conclusive proof to asses the presence of true equilibrium
conditions would be to check the effect on the adsorption isotherm
of either: (i) an increase in the stringency of the pressure readings
criteria, e.g. an increase in the ‘‘equilibrium time”, (ii) the effect of
an increase in the adsorption temperature and/or (iii) to check the
uptake in the desorption branch. In the absence of kinetic restrictions: (i) the uptake must be independent of the pressure reading
criteria used over the whole relative pressure range, (ii) at a higher
adsorption temperature the adsorption capacity must decrease
over the whole relative pressure range, as it corresponds to an exothermic process, and (iii) the desorption branch of the isotherm
must always close with the adsorption branch once the hysteresis
loop is closed (it must never remain above or below).
5.1.2. Analysis of the N2 adsorption isotherms
As stated above adsorption–desorption of N2 at 196 °C may be
used for the characterization of the porous texture on newly developed materials. Prior to the measurement, the sample must be outgassed overnight under high vacuum at around 200 °C. The N2
isotherm can provide information about the surface area, the
porosity and the pore volume [395,400].
The adsorption isotherms for mesoporous materials are type IV
according to IUPAC classification [401], with capillary condensation at medium relative pressure being indicative of mesoporosity
[118]. In some cases the desorption branch is different to the
adsorption path, the isotherm then exhibiting a hysteresis loop.
The shape of the hysteresis loop provides information about the
shape and the size of the mesopores.
The ‘‘apparent” or equivalent surface area is usually calculated
by application of the BET equation to the N2 adsorption isotherm.
The model calculates the volume of the monolayer of adsorbed
molecules on the surface and the surface area is directly obtained
by application of the following equation:
SBET ¼ nm A N A 1018 m2 =g
with nm is the number of molecules adsorbed at the monolayer coverage; A the mean cross-sectional area of the gas molecule (e.g. N2:
0.162 nm2); NA the Avogadro number.
In most mesoporous materials the range of linearity of the BET
equation, after plotting [(P/P0)/(n (1 P/P0))] vs. P/P0, is in the relative pressure range (P/P0) from 0.05 to 0.30. However, if microporosity is also present this range will be leading to false values
because the linearity of the BET plot will be greatly reduced to
about the 0.05–0.15 range. It is recommended to do the analysis
of the BET equation after plotting the experimental data and checking the range of linearity, not using the information directly provided by the automatic equipment. However, it is important to
remember that the intercept must be always positive independently of the porous nature of the material.
The micropore volume (Vmicro) can be estimated following three
approximations: applying the Dubinin–Radushkevich (DR) equation to the N2 adsorption data or using two different comparative
methods, the as-method and the t-method. In the DR method,
the logarithm of the amount adsorbed (log Vads) is plotted against
the square logarithm of the inverse relative pressure (log2 (P0/P)).
Only the central region of the plot, which corresponds to the
micropores range (log2(P0/P) between 2 and 15), must be fitted
to obtain a straight line. The extrapolation of this line provides
the volume of micropores (log Vmicro).
184
The as and the t methods, both are based in the comparison of
the adsorption isotherm with that of a non-porous reference sample [400]. In the as-method the amount of N2 adsorbed is plotted
against the as value, which corresponds to the ratio Va/V0.4, obtained from the reference isotherm at each relative pressure (Va/
V0.4 represents the ratio between the amount adsorbed at each relative pressure and that adsorbed at P/P0 = 0.4, once the monolayer
is already formed and the micropores are already filled, on the reference sample). The extrapolation of the initial linear part of the
as-plot intercepts with the Y-axis and provides the volume of
micropores. In the t-method the amount of N2 adsorbed is plotted
against t, which corresponds to the multilayer thickness for the
adsorption of N2 on the non-porous reference solid. The t-values
are calculated from the reference isotherm as follows:
t ¼ n=nm d
with n is the amount of gas adsorbed at each relative pressure; nm
the monolayer capacity calculated from the linear BET-equation; d
the mean thickness of the N2 monolayer (0.354 nm).
In the same way as the as-method, the extrapolation of the linear part of the t plot intercepts with the Y-axis and provides the
micropore volume of the material. Due to their similar principles,
the micropore volume obtained from both methods must be quite
similar.
The total pore volume (Vtotal) is obtained from the amount of
nitrogen adsorbed at a relative pressure P/P0 0.95–0.99. The reported value can be expressed both as a volume of adsorbed gas
or, most commonly, as the volume of adsorbed liquid using the following equation:
V liq ¼ 1:54 103 V ads
The pore size distribution in the mesopore range can be obtained using the BJH (Barret–Joyner–Halenda) method. The method is based on the Kelvin equation, predicting the formation of
liquid N2, at the capillary condensation step, in the larger pores
of the material. The equation expresses the relation between the
condensation of N2 in the mesopores of a certain size. In this
way, based on the model the relation between P/P0 and the pore radius can be deduced. This allows the formation of the pore size distribution of the material.
X-ray diffraction can also be carried out on powders, and this is
the way it is typically applied to mesoporous materials. Fig. 5.1
shows powder X-ray diffraction patterns of different substances,
the basic composition of which is that of silica, SiO2 (disregarding
the presence of different amounts of surface silanol groups).
Quartz, of course, is a crystalline material, and the regular periodic
arrangement of the atoms gives rise to sharp signals (Fig. 5.1a). On
the other hand, silica gel is amorphous and does not show sharp
signals (Fig. 5.1c); only with magnification, a broad hump becomes
visible between 17° and 30° 2h. As in the gel phase, the silica in
mesoporous materials is amorphous. Correspondingly, in the mesoporous sample (Fig. 5.1b), no sharp peaks are seen in the 2h region
where crystalline quartz displays its Bragg reflections. However,
mesoporous silicas do exhibit reflection at low diffraction angles,
typically in the region between 0.8° and 5° 2h. In Fig. 5.1, this region is displayed on an enlarged 2h-scale in the inset. The presence
of these peaks is not due to a regular periodic arrangement of
atoms, but to a regular array of pores with diameters in the small
nanometer range. The peaks can be indexed on a cubic lattice and
from their arrangement, it can be deduced that the sample has the
MCM-48 pore topology.
More precisely, as it is the electrons that scatter the X-ray
beams, it is the difference in electron density between the pore
walls and the empty pore space which gives rise to these reflections. When there is some material inside the pores (for example
the templating surfactant molecules in the as-synthesized state),
this electron density contrast is smaller and, correspondingly, the
intensity of these low-angle diffraction peaks is diminished. In extreme cases, the intensity of these peaks can even vanish, in spite
of the fact that a perfectly ordered pore system is present.
The MCM-48 material, the diffraction pattern of which is displayed in Fig. 5.1b, can be considered as an ordered mesoporous
material (OMM). Often, products of surfactant-templated syntheses only give a single, broadened diffraction peak at low angles.
Then, it can only be deduced that within the sample, the electron
density varies with a certain periodicity. This periodicity corresponds to the d value which can be calculated from the peak position 2h via the Bragg equation. The broadening indicates variations
of the periodicity around the calculated value. Whereas such
samples can be highly mesoporous (as deduced for example from
5.2. X-ray diffraction (XRD)
X-ray diffraction is one of the most important and most powerful
methods for the investigation of materials. This method is based on
the scattering of X-rays by the electrons of atoms. The wavelengths
of X-rays are similar to interatomic distances, and so the X-rays scattered by different atoms will interfere destructively or constructively, in the latter case giving rise to diffracted beams. In the case
of crystalline samples, sharp diffraction phenomena result. The
geometry of the corresponding diffraction events can be described
by Bragg’s law, which combines a measure of the lattice of the crystal structure, namely the distance d between lattice planes, the
wavelength k of the X-ray radiation and the diffraction angle h:
2d sin h ¼ k:
The Bragg equation treats diffraction as the reflection of X-rays
at the lattice planes; correspondingly, a diffraction event is usually
called a reflection.
By analyzing the geometry of the diffracted beams, information
can be gained on the geometry of the lattice of the structure under
investigation. By further analyzing the intensity distribution of the
reflections, information on the positions of the atoms can be obtained. This is usually carried out by measuring X-ray reflections
on a single-crystal and forms the basis of X-ray single-crystal
structural analysis.
Fig. 5.1. Powder X-ray diffraction patterns of silica samples: (a) crystalline aquartz, (b) mesoporous MCM-48, and (c) amorphous silica gel.
sorption measurements) they are not OMMs, as the characteristic
order of the pores is missing.
5.3. Scanning electron microscopy (SEM)
Scanning electron microscopy characterization is used primarily for the study of surface topography and morphology of solid
materials on a scale down to about 10 nm. Topographical features,
void content, particle agglomeration as well as compositional and
structural differences within the material can be revealed.
The technique works on the principle that an electron beam is
passing through an evacuated column and focused by electromagnetic lenses onto the material [402]. The beam is scanned over the
specimen surface in synchronism with the beam of a cathode ray
tube (CRT) display screen. Inelastically scattered secondary electrons are emitted from the sample surface and collected by a scintillator, the signal from which is used to modulate the brightness of
the cathode ray tube. In this way the secondary electron emission
from the sample is used to form an image on the CRT display
screen. Differences in secondary emission result from changes in
surface topography. If (elastically) backscattered electrons are collected to form the image, contrast results from compositional and
structural differences and often diffraction patterns can be obtained for crystalline materials.
Materials can only be studied properly when they are electrically conducting, as the electrons otherwise give rise to charging
phenomena resulting in blurred images. Non-conducting materials
(amongst which silica-based materials) need to be sputtered with a
thin layer of conducting material before being inserted into the
SEM, and connected to a conducting sample holder. In general,
Au or C is used for sputtering in a plasma sputter-coater.
5.4. High resolution transmission electron microscopy
5.4.1. Transmission electron microscopy (TEM)
The ultimate technique to obtain direct structural information
at nanometer scale resolution for porous materials is transmission
electron microscopy. Whereas in SEM the detectors are mounted
on the same side of the sample as the impinging beam in order
to detect the scattered secondary electrons, for TEM the detectors
are mounted behind the sample to detect the electrons transmitted
through a thin section (preferably less than 100 nm) of the material [403]. The image in TEM is the result of diffraction contrast.
The sample is oriented so that some of the beam is transmitted
and some is diffracted out. Any local structural variation in the
sample causes a different fraction of the incident beam intensity
to be ‘diffracted out’, leading to a variation in image darkness on
a viewing screen at the base of the microscope. Magnification is
achieved by using lenses underneath the sample to project the image formed by the diffracted electrons onto a recording device. The
magnification is determined by the optical system and the resolution by the aberrations in the lens performance.
Atomic resolution can be obtained for crystalline materials.
Powders can be crushed lightly, to separate primary particles,
and supported on special, electrically conducting and partially
electron transparent sample holders (TEM grids). Large particles
of non-conducting material will show charging phenomena leading to blurred images.
5.4.2. Three-dimensional TEM (3D TEM) or electron tomography
Study of the shape and arrangement of individual pores at the
nanometer scale in three-dimensions can be done by electron
tomography [404]. In contrast to TEM, which gives three-dimensional information projected into a 2D image, tomography reveals
real structural information in the third dimension. With electron
tomography (3D-TEM) a 3D-reconstruction is calculated from a
185
series of TEM images taken at a tilt angle range of +70° to 70°. Because the beam direction is fixed in electron tomography, the sample is rotated around a single axis to obtain the images at different
tilt angles. The reconstruction can be visualized with contour surfaces that give information about the surface of the sample, as well
as with slices though the reconstruction that give detailed information on the interior porous structure of the sample. The resolution of a 3D reconstruction is approximately given by the relation:
Q
Resolution = * thickness of the sample/number of images. In the
late 1960’s, 3D TEM has already been pioneered in the field of biology. Nowadays, with the power of the current generation of computers for the calculation of the reconstruction, advanced
tomography has evolved into a breakthrough technique for the
characterization of nano-structured solid materials and catalysts.
For any microscopy method, care has to be taken that the few
particles actually imaged are representative for the bulk of the
material under investigation. Many particles (or sample areas)
need to be inspected by the operator first before deciding on which
particles or areas will be recorded. Especially, mesoporous materials can be very inhomogeneous, consisting of a mixture of ordered
and disordered mesoporous material with dense amorphous material. Additionally, specks of dust or of other samples can be caught
on the microscopy sample holder during sample preparation or
during transport to the microscope, giving rise to contaminated
data.
5.5. Nuclear magnetic resonance (NMR)
A number of atoms in the framework of solid materials possess
isotopes with nuclear spin, which makes these isotopes observable
by NMR (nuclear magnetic resonance) spectroscopy. The information obtained by solid-state NMR spectroscopy is complementary
to that of diffraction techniques, such as X-ray and neutron diffraction, since the latter are long-range methods, while solid-state
NMR spectroscopy allows the study of the local structure
[405,406]. 27Al and 29Si isotopes with the natural abundance of
100% and 4.7%, respectively, are important nuclei for investigating
the local structure of mesoporous materials on the basis of silicates
and aluminosilicates. In contrast to liquids with high mobility of
molecular compounds, the anisotropic nuclear interactions in solids are not averaged by mobility. Therefore, the NMR spectra of solids are too broad for a direct evaluation. The nuclear interactions
being responsible for this line broadening in the NMR spectra of
solids are dipolar interactions of the resonating nuclei with neighbouring nuclear spins, the anisotropic chemical shift due to the
anisotropic shielding effect of electrons, and the quadrupolar interaction of the nuclear electric quadrupole moment with the electric
field gradient at the site of the nuclei. For nuclei with spin I = 1/2,
only the first two interactions are dominating broadening mechanisms. For nuclei with spin I > 1/2, which exhibit an electric quadrupole moment, also the quadrupolar interaction affects the shape
of the solid-state NMR signal.
29
Si nuclei have a nuclear spin of I = 1/2 and sufficient line narrowing is reached by the conventional magic angle spinning (MAS)
NMR technique. On the other hand, 27Al nuclei are characterized by
a nuclear spin of I > 1/2 and, therefore, by an electric quadrupole
moment responsible for the quadrupolar interaction. The averaging of this interaction may require the application of more sophisticated solid-state NMR techniques, such as double-oriented
rotation (DOR) or multiple-quantum MAS NMR (MQMAS).
The MAS technique is based on a rapid rotation of the sample
with spinning frequencies between mrot = 3 and 30 kHz. Most of
the above-mentioned nuclear interactions depend on the geometric term (3cos2H 1), where H denotes the angle between the
direction of the external magnetic field and the sample spinning
axis. The maximum averaging of solid-state interactions, i.e., best
186
line narrowing, is achieved for the magic angle of Hm = 54.74°, i.e.,
when (3cos2Hm 1) becomes zero. An important reason for the
residual line width of MAS NMR signals is the distribution of the
isotropic chemical shift. Often, this effect causes the line broadening in the spectra of mesoporous materials because of the distribution of structural parameters in the amorphous walls of these
materials.
The DOR technique removes the quadrupolar line broadening of
the central transition of nuclei by the simultaneous sample spinning around two axes. By this way, an averaging of the two geometric terms in the equation describing the shape of the central
transition of quadrupole nuclei is reached. The DOR device consists
of a large outer rotor reaching a spinning frequency of up to
1.5 kHz and a small inner rotor with a spinning frequency of up
to 7 kHz. The angle H1 between the external magnetic field and
the rotational axis of the outer rotor corresponds to the magic angle Hm. The angle H2 between the rotational axes of the inner and
the outer rotor amounts to 30.5°.
The MQMAS technique combines an excitation of non-observable multiple-quantum transitions {+m, m} with the experimentally observed single-quantum transition {+1/2, 1/2}. At a
specific time of the pulse-sequence, the anisotropic part of the
quadrupolar interaction, responsible for the line broadening, is
refocused. In a simple form of the experiment, the multiple-quantum transitions are excited by a single high-power radio frequency
pulse. Subsequently, the multiple-quantum coherence is allowed
to evolve for the time t1. After the evolution period t1, a second
pulse is applied, which converts the multiple-quantum coherences
into an observable single-quantum coherence, which is recorded
during the echo in the time period t2. Finally, the two-dimensional
Fourier transformation of the decays in the domain t2 for different
pulse delays t1 leads to a two-dimensional MQMAS spectrum with
narrow isotropic and featured signals along the frequency axes of
m1 and m2, respectively.
A suitable way to enhance the intensities of NMR signals of nuclei with a small magnetogyric ratio or low concentration (rare
spins S, such as 29Si nuclei), which interact with abundant spins I
(such as 1H nuclei), is the polarization transfer from the spins I
to the spin S ensemble via a cross polarization (CP). An additional
advantage of this technique is the selective enhancement of the
NMR signals of rare spins S in the vicinity of abundant spins I.
The CP experiment starts with a p/2 pulse applied to the abundant
spins I. Spin polarization is transferred from the spins I to the spins
S during the contact pulse, if the condition cI B1I = cS B1S is fulfilled. In this case, B1I and B1S denote the amplitudes of the magnetic fields of the contact pulses applied to the spins I and S,
while cI and cS are the magnetogyric ratios of the spins I and S,
respectively.
The basic structural units of mesoporous materials on the basis
of silicates and aluminosilicates are TO4 tetrahedra with silicon
atoms at the central T-positions. In the second coordination sphere
of these T-atoms, aluminum can be incorporated into the framework. Depending on the amount of aluminum atoms, which are
incorporated, the tetrahedrally coordinated silicon atoms (Q4) in
aluminosilicates may be characterized by up to five different environments denoted as Si(nAl) with n = 0, 1, 2, 3, and 4. Each type of
Si(nAl) species has a characteristic chemical shift. Typically, the
29
Si MAS NMR signal of Si(0Al) species occurs at ca. 110 ppm (referenced to tetramethylsilane). The addition of one tetrahedrally
coordinated aluminum atom in the local structure of Si(nAl) species leads to a shift of the corresponding 29Si MAS NMR signal by
ca. 5 ppm to positive values. For mesoporous materials, however,
the line broadening due to chemical shift distribution is so large
that the different signals of the various Si(nAl) species can not be
resolved. Another important species influencing the 29Si MAS
NMR spectra of mesoporous materials are hydroxyl groups bound
Q
4
3
Q
Q2
-60.0 -70.0 -80.0 -90.0 -100.0 -110.0 -120.0 -130.0 -140.0 -150.0 -160.0 -170.0
δ 29Si / ppm
Fig. 5.2. 29Si MAS NMR spectrum of SBA-15 consisting of signals due to Q4, Q3, and
Q2 silicon species.
to silicon atoms at the outer surface or at internal framework defects. Generally, silicon atoms bound to one (Q3: Si(3Si, 1OH)) or
two (Q2: Si(2Si, 2OH)) hydroxyl groups can be distinguished by
their signals at chemical shifts of ca. 103 ppm and 90 ppm,
respectively. It is important to note, that the signals of Si(1Al) species (d29Si = 95 to 105 ppm) occur at the similar resonance positions to those of Si(3Si, 1OH) species. In this case, application of the
cross polarization experiment has the advantage that this technique causes a selective enhancement of the signals of silicon
atoms with hydroxyl protons in their vicinity. This behavior supports the correct assignment of the nature of neighbouring species.
As an example, Fig. 5.2 shows the 29Si MAS NMR spectrum of a
mesoporous SBA-15 material consisting of signals due to Q4, Q3,
and Q2 silicon atoms at chemical shifts of 110, 102, and 91
ppm, respectively.
According to Loewenstein’s rule, the formation of Al–O–Al
bonds in aluminosilicates is forbidden, and only Al(4Si) species
can exist in the corresponding frameworks. Therefore, 27Al MAS
NMR spectra of hydrated aluminosilicates consist, in general, of
only one signal of tetrahedrally coordinated framework aluminum
(Altet) at chemical shifts of ca. 50–60 ppm (referenced to a 0.1 M
aqueous solution of Al(NO3)3 in D2O). In hydrated aluminosilicates,
only small deviations from the ideal tetrahedral symmetry of the
AlO4 units may occur, which lead to weak quadrupolar interactions
and weak second-order quadrupolar line broadenings.
Octahedrally coordinated aluminum species (Aloct) in hydrated
aluminosilicates, which can be due to extra-framework aluminum
compounds, induce 27Al MAS NMR signals at ca. 0 ppm. If these
AlO6 species exists as polymeric aluminum oxides or oxide hydrates, a strong quadrupolar line broadening may occur owing to
distortions of the octahedral symmetry. In some cases, an additional broad 27Al MAS NMR signal appears at 30–50 ppm indicating
the presence of aluminum atoms in a disturbed tetrahedral coordination or a fivefold coordinated state.
As an example, Fig. 5.3 shows the 27Al MAS NMR spectrum of an
aluminum-containing hydrated MCM-41 consisting of narrow signals at 53 ppm due to tetrahedrally coordinated framework aluminum and at ca. 0 ppm due to octahedrally coordinated aluminum
species. The broad background signal at ca. 0 ppm indicates the
presence of polymeric aluminum oxides or oxide hydrates. An improved resolution, e.g., of the different signals at ca. 0 ppm would
require the application of the DOR or MQMAS technique.
5.6. Elemental analysis
5.6.1. Atomic absorption spectroscopy (AAS)
Atomic absorption spectroscopy is a widely used method for the
quantitative determination of single elements incorporated in a
187
tet
Al
oct
Al
180
160
140
120
100
80
60
40
20
0
-20
-40
-60
-80
-100
-120 -140
δ 27Al / ppm
Fig. 5.3. 27Al MAS NMR spectrum of an aluminum-containing hydrated MCM-41 with signals at 53 and 0 ppm due to tetrahedrally and octahedrally coordinated aluminum
species, respectively.
material [407]. When photons are sent in on an atom, an electron
of the outermost electron shell can be excited towards an elevated
energy level. Since the different energy levels in an atom are quantisized, this excitation will only occur if the wavelength of the photon is equal to an allowed transition state of the atom. The allowed
transitions are specific for each element. The amount of absorbed
photons depends on the concentration of the element under determination. This results in the quantitative character of the
technique.
Before the actual measurement can be executed, it is necessary
to atomize the investigated element. Different techniques can be
applied, whereby flame and electrothermal (using a graphite furnace) atomizers are the most common. Flame atomization appears
to be superior to the electrothermal method in terms of reproducibility. On the other hand, the sensitivity of the electrothermal
atomizing technique is markedly better since in this case the entire
sample is atomized in a short period, while for flame-AAS it is necessary to first destruct, dilute and vaporize the sample. Furthermore, the average residence time of the atoms in the optical path
is longer for the electrothermal method. The radiation with a
monochromatic source occurs after the atomization. The most
common source for this type of measurements is a hollow cathode
lamp (HCL). Since a HCL is an element specific source, the emitted
light possesses the proper wavelength to excite the atoms. The extent of absorption reflects the element concentration of the
sample.
5.6.2. Electron probe microanalysis (EPMA)
Electron probe microanalysis is a qualitative and quantitative
technique, which is commonly used for the determination of the
elemental composition and distribution within a micro volume
of solid material [407]. The investigated materials are bombarded
with a high energy electron beam (compare with electron microscopy), exciting the electrons from the lower K or L shells towards
elevated energy levels. Those electrons (primary electrons) will
emit characteristic X-rays while returning to their ground state.
The element can be determined using ‘‘Moseley’s Law”: k = K/
(Z r), whereby K and r are constants, Z the atomic number
and k the wavelength of the emitted radiation. The intensity of
the radiation can be correlated to the concentration of the
element.
The returning of excited primary electrons to lower energy levels, can give rise to the removal of secondary electrons out of more
innermost shells, so called Auger electrons. Since Auger electron
emission predominates with atoms of low atomic numbers, this
technique is more suitable for the determination of heavier elements (from boron to uranium).
5.6.3. X-ray fluorescence (XRF)
X-ray fluorescence can be used for the qualitative and quantitative determination of all elements in the periodic table with an
atomic number greater than that of oxygen. The measurement of
lighter elements is less convenient, since difficulties in detection
become progressively worse as atomic numbers become smaller
than 23 (vanadium) due to a competing process, namely Auger
emission [407].
The samples are exposed to X-ray radiation, which causes ionization of inner shell electrons, creating vacancies in the inner shells
(K, L, . . .). The transition of outer shell electrons into these vacancies can create the emission of characteristic X-ray fluorescent
radiation. The measurement of the wavelength or the energy and
intensity of the characteristic photons emitted from the sample
are the basis of the XRF principle. This enables the identification
of the elements present in the sample and the determination of
their mass or concentration.
Different types of XRF instruments exist, whereby wavelength
dispersive (WD) and energy dispersive (ED) instruments are the
most common. The primary difference is the way the fluorescent
X-rays are detected and analyzed. A wavelength dispersive XRF
has a more complex set-up, which results in a lowered efficiency
compared to energy dispersive XRF. However, in comparison to
ED-XRF, WD-XRF has a better resolution.
5.6.4. Inductively coupled plasma-optical emission spectroscopy/
atomic emission spectroscopy (ICP-OES/AES)
Inductively coupled plasma-atomic emission spectroscopy (ICPAES), also referred to as inductively coupled plasma-optical emission spectrometry (ICP-OES), is an analytical technique used to
determine concentrations of a wide range of elements in solution.
ICP-AES/OES is a fast multi-element technique with a dynamic linear range and moderate-low detection limits (0.2–100 ppb). ICPAES/OES makes use of the fact that the atoms of elements can take
up energy from an inductively coupled plasma, are thereby excited,
and fall back into their ground state again emitting electromagnetic radiation at wavelengths characteristic of a particular element. The identification of this radiation permits the qualitative
analysis of a sample. A quantitative determination takes place on
the basis of the proportionality of radiation intensity and element
concentration in calibration and analysis samples. Calibration can
be performed with multi-element solutions mixed from standard
solutions.
To generate plasma, argon gas is supplied to a torch coil, and
high frequency electric current is applied to the work coil at the
tip of the torch tube. Using the electromagnetic field created in
the torch tube by the high frequency current, argon gas is ionized
188
and plasma is generated. This plasma has high electron density
and temperature (6727–9727 °C) and this energy is used in the
excitation of the sample. Solution samples are introduced into
the plasma in an atomized state through the narrow tube in the
center of the torch tube. A peristaltic pump delivers the aqueous
or organic liquid sample into a nebulizer where it is atomized and
introduced directly inside the plasma flame. The sample immediately collides with the electrons and other charged ions in the
plasma and is broken down into charged ions. The various molecules break up into their respective atoms which then lose electrons and recombine repeatedly in the plasma, giving off the
characteristic wavelengths of the elements involved. The emitted
light is collected by a spectrometer and passes through a diffraction grating that serves to resolve the light into a spectrum of its
constituent wavelengths. Within the spectrometer, this diffracted
light is then collected by wavelength and amplified to yield an
intensity measurement that can be converted to an elemental
concentration by comparison with the calibration standards.
Digestion methods such as microwave, high-pressure, fusion,
and acid digestion can be employed for the liquid sample preparation of solid sample material.
Some specifics of the ICP-AES/OES methods are:
–
–
–
–
Simultaneous, sequential analysis of multiple elements.
Wide linear region of analytical curve.
Few chemical interference or ionization interference.
High sensitivity (low limit of detection for majority of elements
is 10 ppb or lower).
– High number of measurable elements–elements that are difficult to analyze in atomic absorption spectrometry such as Zr,
Ta, rare earth, P and B can be easily analyzed.
– Stable.
The majority of the above features are derived from the structure and characteristics of the light source plasma.
5.7. FT–Raman spectroscopy
Raman spectroscopy is used to determine molecular structures
and compositions of materials. It is based on the principle of the
interaction of monochromatic laser light with solid material
[402]. The light can be scattered in all directions with the frequency the same as that of the original light, this effect is known
as the Rayleigh scattering. Another type of scattering that can occur is known as the Raman effect. It occurs at frequencies both
higher and lower than the original frequency and with diminished intensities. The differences Dv between the incident and
scattered frequencies are equal to the actual vibrational frequencies of the material. Therefore, Raman provides characteristic frequencies of various functional groups. In a typical Raman
spectrum the intensity is plotted against the Raman shift
(cm1), the difference in frequency between the incident and
scattered beam. All molecules vibrating giving rise to a change
in polarizability can be measured with Raman. Raman and infrared spectroscopy provide complementary information to structure determination.
However, the primary limitation of the Raman technique resides in the intrinsic weakness of the Raman effect. As the total
intensity of scattered radiation is only 0.1% of that of the source,
it is essential that a very sensitive detector and efficient optical
systems are being used in the apparatus. In addition, if components
with fluorescence are present in the samples, this gives interference when excited by visible laser radiation. Also heating effects,
especially for colored materials, can influence the spectra in FT-Raman spectroscopy.
5.8. FT-infrared spectroscopy (FT-IR)
Fourier transform infrared spectroscopy (FT-IR) is an economic
and multidisciplinary analytical tool, which yields information
concerning the structural details of a siliceous inorganic material
[402,406]. In addition, it can be used to confirm surface characteristics (such as acidity) and isomorphous substitution by other elements in the material. The technique allows to relate different
materials by their common structural features, such as a classification of zeolite structures. Analysis by FT-IR is based on the fact
that molecules have specific frequencies of internal vibrations.
These frequencies occur in the infrared region of the electromagnetic spectrum. When a sample is placed in a beam of infrared
radiation, the sample will absorb radiation at frequencies corresponding to molecular vibrational frequencies, and this is being
measured in the infrared spectrometer. The result is an infrared
spectrum which represents a plot of absorbed energy vs. frequency. The vibration frequency of a bond is related to the masses
of the vibrating atoms (m1 and m2 of atom 1 and atom 2, respectively) and the force constant (f in g s2) of the vibrating bond,
according to:
1
t¼
2pc
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðm1 þ m2 Þ
f
m1 m2
Not all matter is capable of producing an IR-spectrum. In order
to interact with IR-radiation, the molecule must have a permanent
dipole moment and must vibrate with a variation in the bond
length (stretching vibration) or the bond angle (bending vibration).
In general, for a non-linear n-atomic molecule, there are 3n 6
fundamental vibrational modes. Standard FT-IR analysis requires
sample preparation such as first measuring the background spectrum of a KBr pressed pellet, followed by measuring the sample,
pressed and diluted (2%) in KBr.
The mid-infrared region of the spectrum (4000 to 400 cm1) is
the most interesting part of the spectrum when dealing with siliceous materials as it contains the fundamental framework vibrations of the Si(Al)O4 groupings. Identification of materials in this
region is possible because different materials will absorb at different frequencies. In addition to the characteristic nature of the
absorptions, the magnitude of the absorption in the spectrum
due to a given species can be related to the concentration of that
species (quantitative analysis). Although any quantitative data derived from the technique must be treated with caution. In fact,
there are some special methods that can be applied for quantitative interpretation of the spectrum, such as the use of calibration
graphs, analysis using absorbance ratio methods, standard addition
techniques, and transformation of the FT-IR spectrum according to
the Kubelka–Munk theory.
Besides the standard applications of identification and quantitative analysis, there are a number of special FT-IR techniques. Some
of these are briefly listed as follows:
FT-IR photoacoustic spectroscopy (FT-IR-PAS) involves infrared
absorption in the sample, followed by conversion of the absorbed
energy into heat [408]. The subsequent heat-induced thermal
expansion in the sample and adjacent media produces a photoacoustic signal when the incident beam intensity is modulated at
a frequency in the acoustic range. Microphonic detection of this
signal, processed by the normal detector amplification electronics
of the FT-IR spectrometer yields the spectrum. FT-IR-PAS technique
allows the measurement to take place under absolute dry conditions (He gas atmosphere) and can be successfully applied for the
study of the surface hydroxyl groups of a siliceous material.
Special infrared reflectance techniques involve attenuated total
reflectance (ATR) and diffuse reflectance infrared spectroscopy
(DRIFT).
189
6. Syntheses recipes
Intensity / a.u.
ATR makes use of a crystal (zinc selenide or germanium) as the
reflecting medium, and which is in close contact with the sample.
The ATR technique is very useful for obtaining spectra of very thin
samples, e.g. films, coatings.
DRIFT is based on the principle of detection of diffusely scattered radiation at a sample powder surface. The sample absorbs
some frequencies of the incident radiation so the scattered radiation will be devoid of energy in some of these frequencies. When
scanning the scattered radiation a spectrum is obtained. The most
significant advantage of DRIFT is that it allows spectra to be obtained from solid powder samples, with virtually no sample preparation other than scattering a small amount of the powdered
sample on a bed of powder KBr in a small cup.
1
2
3
4
5
6
7
8
2θ / º
Fig. 6.1. X-ray diffraction pattern of MCM-41 made from aerosil. The corresponding
d-spacings:
(1 0 0) = 4.28 nm;
(1 1 0) = 2.43 nm;
(2 0 0) = 2.10 nm
and
(2 1 0) = 1.59 nm.
6.1. MCM-41 (from fumed silica)
Type of material: Silica
Batch composition: 1 SiO2:0.25 CTMABr:39.36 H2O:0.20 TEAOH.
Source of materials:
Cetyltrimethylammonium bromide 99+% (CTMABr) (Acros
Organics).
Tetraethylammoniumhydroxide 20% (TEAOH 20%) (Sigma–
Aldrich).
Fumed silica (aerosil 380, Degussa).
Batch preparation:
1. Add 6.2 g CTMABr to 40.4 g H2O, stir at room temperature until
dissolved.
2. Add 10 g of TEAOH 20% solution.
3. Add 4.1 g fumed silica and stir at 70 °C for 2 h (stirring is crucial.
If stirring is stopped due to a high viscosity, it is possible to add
between 1 and 5 ml of water to keep it stirring).
4. Stir for a duration of 24 h at room temperature.
5. Transfer the viscous solution into an autoclave and heat to 130–
150 °C for 48 h.
6. After the heat treatment, quench the autoclave and filter the
solution.
7. Wash the solid with 150 mL H2O.
8. Transfer the solid into an autoclave and add some fresh water
(until the solid is just covered) and heat it a second time to
130–150 °C for 72 h.
Product recovery: Filter, wash with 3 25 mL water and dry.
Post-synthesis treatment: Calcine the product at 550 °C during
6 h with a heating rate of 1 °C/min in ambient atmosphere. Cooling
down occurs slowly.
Product characterization:
XRD: (See Fig. 6.1).
N2-sorption: (See Figs. 6.2 and 6.3).
Range of data derived from the isotherms:
SBET = 1000–1100 m2/g.
Vtotal = 1–1.2 cm3/g.
Pore diameter (BJH, adsorption branch) = 2.5–4.0 nm.
SEM: (See Fig. 6.4).
-1
700
3
600
500
400
300
200
100
0
0.0
0.1
0.2
0.3
0.4
0.5
P /P 0
0.6
0.7
0.8
0.9
1.0
Fig. 6.2. N2-sorption isotherm at 196 °C of MCM-41 made from aerosil.
ads
des
d v (r )
K.S. Triantafyllidis, C. Nitsos, S. Karakoulia, S.D. Sklari
A. Silvestre-Albero, J. Silvestre-Albero, A. Sepúlveda-Escribano,
F. Rodríguez-Reinoso
800
Volume adsorbed STP / cm g
Short description of material: Hexagonally ordered mesoporous
material with small mesopores.
Contributed by: P. Cool, E.F. Vansant, V. Meynen
Verified by:
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Radius / nm
Fig. 6.3. Pore size distribution of MCM-41 made from aerosil. It was obtained with
the BJH method.
Remarks:
MCM-41 can also be prepared with an alternative surfactant,
gemini 16-8-16, following a similar synthesis procedure as for
MCM-48 [101] (recipe 6).
6.2. MCM-41 (from metasilicate)
190
Tetradecyltrimethylammonium bromide P98% (TTMABr)
(Fluka).
Decyltrimethylammonium bromide P98% (DTMABr) (Fluka).
Ethyl alcohol P99% (EtOH) (Merck).
H2SO4 98% (Merck).
Batch preparation:
MCM-41 materials are synthesized using different templates.
1. Add 8.13 g sodium metasilicate to 120 g of demineralized water
and stir for 30 min in a PP-beaker (400 mL).
2. Add 4.84 g cetyltrimethylammonium bromide or 4.8 g tetradecyltrimethylammonium bromide or 3.73 g decyltrimethylammonium bromide to 30 g demineralized water and 10 g EtOH,
mix and stir for 30 min.
3. Add (2) to (1) and stir for 30 min to obtain a clear gel.
4. Add 15 g of 2 M H2SO4 slowly to solution (3) within 7 min under
vigorous stirring – then stir the mixture at room temperature
for 30 min. Cover the beaker with parafilm during the synthesis
to prevent evaporation.
5. Add 15 g 2 M H2SO4 slowly to solution (4) within 7 min while
stirring vigorously – then stir the mixture at room temperature
for 1 h. Cover with parafilm to prevent evaporation.
6. Transfer the gel into a Teflon-lined steel autoclave (Teflon insert
200 mL) (only 2/3 of the gel can be transferred, the rest is
thrown away). Temperature: 150 °C in air oven (preheated)
during 24 h, without agitation.
7. Quench the autoclave with cold water.
Product recovery:
1. Add 25 mL ethanol to the precipitate, mix and filter.
2. Wash the precipitate four times with 10 mL ethanol and three
times with 200 mL demineralized water.
3. Dry in air at 80 °C overnight prior to calcination.
Post-synthesis treatment:
Calcine the solid at 540 °C for 1 h in nitrogen with a heating rate
of 2 °C/min. Substitute the nitrogen by air and continue the calcination for 8 h at 540 °C (gas flow 30 cm3/min).
SBET = 770–1030 m2/g.
Vtotal = 0.6–0.95 cm3/g.
Fig. 6.4. SEM images of MCM-41 made from aerosil.
F. Quiroz, V. Meynen, P. Cool, E.F. Vansant
A. Ferreira, S. Aguado, J. Gascon, F. Kapteijn
Batch composition: 1 SiO2:1 Na2O:0.2 CTMABr (or TTMAB or
DTMABr):143 H2O:0.9 H2SO4.
Intensity / a.u.
Contributed by: S.C. Laha, R. Gläser, D. Pufky-Heinrich, J.
Weitkamp
Verified by:
0
Sodium metasilicate 99% (Na2SiO3, Fluka).
Cetyltrimethylammonium bromide 95% (CTMABr) (Sigma–
Aldrich).
1
2
3
4
2θ /º
5
6
7
8
Fig. 6.5. X-ray diffraction pattern of MCM-41 from metasilicate. The corresponding
d-spacings: (1 0 0) = 3.58 nm; (1 1 0) = 2.05 nm; and (2 0 0) = 1.80 nm.
191
-1
600
500
400
300
200
100
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P /P 0
Fig. 6.6. N2-sorption isotherm at 196 °C of MCM-41 made from metasilicate.
Based on material prepared using the C14 template (tetradecyltrimethylammonium
bromide).
d v (r )
ads
des
0
1
2
3
4
5
6
Radius / nm
7
8
9
10
Fig. 6.7. Pore size distribution of MCM-41(BJH) made from metasilicate. Based on
material prepared using the C14 template (tetradecyltrimethylammonium
bromide).
TEM: (See Fig. 6.9).
6.3. MCM-41
Contributed by: J. Rathousky, A. Zukal, R. Marschall, I. Bannat, J.
Caro, M. Wark
Fig. 6.9. TEM image of MCM-41 made from metasilicate.
Verified by:
D. Pufky-Heinrich, J. Weitkamp
Batch
composition:
CH3COOC2H5:1018 H2O.
1
Na2SiO3:0.329
CTMABr:1.88
Sodium metasilicate (Na2SiO3, Sigma–Aldrich).
Cetyltrimethylammonium bromide 95% (CTMABr) (Sigma–
Aldrich).
Ethyl acetate p.a. >99% (Fluka).
Batch preparation:
Fig. 6.8. SEM image of MCM-41made from metasilicate.
1. Dissolve 2.61 g cetyltrimethylammonium bromide in 400 mL
water at 30 °C.
2. Add 2.67 g sodium metasilicate and stir until completely
dissolved.
3. Add 4 mL ethyl acetate under vigorous stirring and stop stirring
after 15 s.
192
4. Leave the mixture in a closed PE bottle for 24 h at room
temperature.
5. Treat the obtained mixture hydrothermally at 100 °C for 48 h.
Product recovery:
Filter the hot precipitate, wash with ethanol and water and dry
at RT.
Calcine the product at 600 °C for 20 h, with heating/cooling rate
of 1 °C/min in ambient atmosphere.
Intensity / a.u.
Fig. 6.13. SEM images of MCM-41.
0
1
2
3
4
5
6
7
8
2θ /º
Fig. 6.10. X-ray diffraction pattern of MCM-41. The corresponding d-spacings:
(1 0 0) = 3.59 nm; (1 1 0) = 2.07 nm and (2 0 0) = 1.77 nm.
SBET = 900–1200 m2/g.
Vtotal = 0.8–1.2 cm3/g.
-1
700
600
Fig. 6.14. TEM images of MCM-41.
500
400
300
6.4. MCM-41 (spherical)
200
100
0
0
0.1
0.2
0.3
0.4
0.5
P /P 0
0.6
0.7
0.8
0.9
1
material with small mesopores and a spherical morphology.
Contributed by: F. Rodríguez-Reinoso, A. Sepúlveda-Escribano, J.
Silvestre-Albero
Verified by:
R. Marschall, M. Wark, J. Caro
Fig. 6.11. N2-sorption isotherm at 196 °C of MCM-41.
Batch composition: 1
EtOH:37.3 NH3.
ads
des
SiO2:0.30
CTMABr:122.6
H2O:57.7
d v (r)
Cetyltrimethylammonium bromide 99+% (CTMABr) (Sigma–
Aldrich).
Ammonia 30% (Panreac or ROTH or Acros Organics).
Ethanol absolute (Panreac).
Tetraethyl ortosilicate 98% (TEOS) (GC Sigma–Aldrich).
Methanol 99.8+% (Sigma–Aldrich).
0
1
2
3
4
5
6
Radius / nm
7
8
Fig. 6.12. Pore size distribution (BJH) of MCM-41.
9
10
Batch preparation:
1. Add 7.5 g CTMABr to 150 g H2O and stir at room temperature
until complete dissolution.
193
dv (r )
Intensity / a.u.
ads
des
1
2
3
4
5
6
7
8
9
10
2θ /º
0.0
Fig. 6.15. X-ray diffraction pattern of spherical MCM-41. The corresponding
d-spacings: (1 0 0) = 3.66 nm; (1 1 0) = 2.08 nm and (2 0 0) = 1.83 nm.
-1
2.0
3.0
Radius / nm
4.0
5.0
Fig. 6.17. Pore size distribution (BJH) of spherical MCM-41.
Calcine the product at 550 °C during 5 h with a heating rate of
1 °C/min in ambient atmosphere.
800
700
600
500
400
300
200
100
0
0.0
0.1
0.2
0.3
0.4
0.5
P/P 0
0.6
0.7
0.8
0.9
1.0
Fig. 6.16. N2-sorption isotherm at 196 °C of spherical MCM-41.
2.
3.
4.
5.
1.0
Add 42.9 g NH3 and 180 g ethanol.
Stir the solution for 30 min at 500 rpm.
Add 14.1 g TEOS.
Stir the solution for 3 h at 500 rpm and then for 12 h at 300 rpm
at 25 °C.
Product recovery:
Filtrate the solution and subsequently wash the solid with
300 mL distilled water and thereafter with 300 mL methanol. Dry
the solid at 90 °C for 20 h.
SBET = 1100–1500 m2/g.
Vtotal = 0.7–1.0 cm3/g.
Remarks:
Dissolving CTMABr at room temperature appears to be slow.
Increasing the temperature to 30 °C results in faster dissolution.
When ethanol is applied, a homogeneous solution is obtained
resulting in the spherical morphology [413].
6.5. Al-MCM-41
material with small mesopores and Si/Al ratios of 20, 40 and 60.
Fig. 6.18. SEM images of spherical MCM-41.
Intensity / a.u.
194
0
1
2
3
4
5
6
7
8
2θ /º
Tetradecyltrimethylammonium bromide ca. 99% (TDTMABr,
C14H29(CH3)3NBr) (Sigma–Aldrich).
Sodium aluminate (54% Al2O3, 41% Na2O) (Riedel-de Haën).
Sodium silicate 5 hydrate (8.9% Na2O + 28% SiO2) (VWR BDH).
Sulfuric acid (H2SO4, 10%, 50% and 95%) (Merck p.a., 95–97%).
Batch preparation:
1. Dissolve 15 g template (C14H29(CH3)3NBr) in 95 g water and stir.
2. Add sodium aluminate (0.43, 0.21 or 0.14 g, depending on the
Si/Al ratio) and continue stirring for 2 h.
3. Add 18.7 g sodium silicate under stirring, immediately followed
by 5.6 g 10% sulphuric acid.
4. Add 15 g water and stir the entire solution for 30 min.
5. Adjust the pH to 10 with 50% sulphuric acid.
6. Transfer the final solution to teflon-lined autoclaves and heat
for 144 h at 100 °C or 150 °C. MCM-41 is formed at both
temperatures.
Product recovery:
Filter the Al-MCM-41 samples, wash with water and dry at
room temperature until constant weight (three days).
Calcine the product at 540 °C with a heating rate of 5 °C/min,
during 1 h in flowing nitrogen, followed by 6 h in flowing air with
flow rates of 100 mL/min.
SBET = 850–950 m2/g.
Vtotal = 0.7–1.3 cm3/g.
Sample
EPMA
XRF
AAS
ICP-AES
Al-MCM-41 (20)
Al-MCM-41 (40)
Al-MCM-41 (60)
28.5
34.5
62.7
20.4
34.2
51.3
21.3
19.8
63.1
700
-1
Type of material: Aluminosilicate
Batch composition: 1 Si:0.06/0.02/0.01 Al:0.4 C14H29(CH3)3NBr:
68 H2O.
Si/Al ratio:Chemical composition: Si/Al ratios of the (calcined)
parent mesoporous Al-MCM-41 materials (desired Si/Al ratios
indicated).
600
Contributed by: M. Stöcker
Verified by:
K.S. Triantafyllidis, S. Karakoulia, C. Nitsos, S.D. Sklari
Fig. 6.20. X-ray diffraction pattern of Al-MCM-41 (Si/Al 20). The corresponding dspacings: (1 0 0) = 3.59 nm; (1 1 0) = 2.09 nm and (2 0 0) = 1.80 nm.
500
400
300
200
100
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P /P 0
Fig. 6.21. N2-sorption isotherm at 196 °C of Al-MCM-41 (Si/Al 20).
ads
des
d v (r )
Fig. 6.19. TEM image of spherical MCM-41.
0
1
2
3
4
5
6
7
8
9
10
Radius / nm
Fig. 6.22. Pore size distribution of Al-MCM-41(Si/Al 20) by the BJH method.
195
Verified by:
A. Ferreira, S. Aguado, J. Gascon, A. Quintanilla, F. Kapteijn
K.S. Triantafylidis, S. Karakoulia, C. Nitsos, S.D. Sklari
D. Pufky-Heinrich, J. Weitkamp
Batch composition: 1 SiO2:100 H2O:0.26 NaOH:0.1 surfactant.
Gemini surfactant:
1,12-Dibromododecane (Acros Organics).
N,N-dimethylhexadecylamine (Fluka)/N,N-dimethyloctadecylamine (Acros Organics).
Acetone p.a. (Acros Organics).
MCM-48:
Fumed silica (aerosil 380, Degussa).
Sodium hydroxide pellets p.a. (Acros Organics).
Gemini 16-12-16 or gemini 18-12-18 (home-made; see below).
Fig. 6.23. SEM image of Al-MCM-41 (Si/Al 20).
Batch preparation:
Intensity / a.u.
Fig. 6.24. TEM image of Al-MCM-41 (Si/Al 20).
calcined sample
(Si/Al 60)
calcined sample
(Si/Al 20)
150
Fig. 6.25.
and 60.
100
27
0
50
Chemical shift/ ppm
-50
-100
Al MAS NMR spectra of the Al-MCM-41 sample with Si/Al ratio of 20
NMR: (See Fig. 6.25).
Remarks:
Characterization of samples with other Si/Al ratios gives similar
results for XRD, N2-sorption, SEM and TEM.
6.6. MCM-48
Short description of material: Cubic structured mesoporous
(A) Synthesis gemini surfactant:
1. Add 12 g 1.12-dibromododecane and 25.5 mL N,N-dimethylhexadecylamine to 50 mL acetone.
2. Increase temperature until refluxing starts (around the
boiling point of acetone), reflux during 24 h.
3. Cool down (the gemini will start to crystallize).
4. Filter and wash with acetone p.a.
5. Recrystallize the gemini in a beaker (with a watch glass
on top) with acetone p.a.
6. Filter and wash with acetone p.a.
The gemini crystals are recovered by filtration on a buchner filter or with a rotavap. The gemini surfactant is dried
at room temperature in ambient conditions.
(B) Synthesis MCM-48
1. Add 60 mL H2O and 0.3461 g NaOH to 2.8867 g gemini
16-12-16 or to 3.0735 g gemini 18-12-18.
2. Stir in the teflon part of an autoclave until gemini is
dissolved.
3. Add 2 g aerosol and stir for 30 min.
4. Age the autoclave in an oven at 130 °C during 3 days.
5. After heat treatment, quench the autoclave to room
temperature.
6. Filter and wash with 30 mL water.
7. Transfer the residue back in an autoclave and add 30 mL
of fresh water.
8. Heat the autoclave in an oven at 130 °C for 1 day.
(Repeat 1 (total synthesis = 3 days base/1 day water/1
day water).
Product recovery:
Filter, wash with 3 25 mL water and dry at room temperature.
2 °C/min in ambient atmosphere. Cooling down occurs slowly.
SBET = 1200–1700 m2/g.
Vtotal = 1.0–1.4 cm3/g.
Intensity / a.u.
196
1
2
3
4
5
6
7
8
2θ / º
Fig. 6.26. The X-ray diffraction pattern of MCM-48. The corresponding d-spacings:
(2 1 1) = 3.40 nm and (2 2 0) = 2.98 nm.
Fig. 6.29. SEM image of MCM-48.
800
3
-1
700
600
500
400
300
200
100
0
0.0
0.1
0.2
0.3
0.4
0.5
P /P 0
0.6
0.7
0.8
0.9
1.0
Fig. 6.27. N2-sorption isotherm at 196 °C of MCM-48.
ads
des
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Intensity / a.u.
d v (r )
Fig. 6.30. TEM image of MCM-48.
Radius / nm
Fig. 6.28. Pore size distribution of MCM-48 calculated with the BJH method.
Remarks:
16-12-16 gemini surfactant: The pattern should look like the
one shown in Fig. 6.31, otherwise another recrystallization is
necessary.
Detailed information about the influence and optimization of
the synthesis conditions on the properties of the materials can
be found in the literature [102,414].
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
2θ /º
Fig. 6.31. X-ray diffraction pattern of the 16-12-16 gemini surfactant.
6.7. SBA-15
Short description of material: Large pore hexagonal mesoporous
material with micropores in the walls.
Contributed by: P. Cool, E. F. Vansant, V. Meynen
197
Verified by:
A. Denkova, M.O. Coppens, F. Kapteijn
SBET = 650–950 m2/g.
Vtotal = 0.65–1.0 cm3/g.
Vmicro = 0.10–0.3 cm3/g.
Batch composition: 1 TEOS:5.87 HCl:194 H2O:0.017 P123.
Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly
(ethylene glycol) (MW 5800) (EO20PO70EO20, P123) (Sigma–
Aldrich).
HCl 37% (Acros Organics).
Tetraethylorthosilicate 98% (TEOS) (Acros Organics).
Remarks:
The morphology can be changed in a wide range [24,173].
It is also possible to use pluronic P123 from BASF (MW 5800)
(same product as from Sigma–Aldrich).
Batch preparation:
6.8. SBA-15
Add 4 g P123 to 130 mL H2O and 20 mL HCl.
Stir until complete dissolution.
Add 9.14 mL TEOS and stir during 7.5 h at 45 °C.
Ageing for 15.5 h at 80 °C (without stirring).
Cool down.
1.
2.
3.
4.
5.
Product recovery:
Filter, wash with 3 25 mL water and dry.
Intensity / a.u.
d v (r )
ads
des
0
1
2
3
4
5
6
Radius / nm
7
8
9
10
Fig. 6.34. Pore size distribution of SBA-15 obtained by the BJH method.
0
0.5
1
1.5
2
2θ /º
2.5
3
3.5
4
Fig. 6.32. X-ray diffraction pattern of SBA-15. The corresponding d-spacings:
(1 0 0) = 9.70 nm; (1 1 0) = 5.58 nm and (2 0 0) = 4.82 nm.
-1
700
600
500
400
300
200
100
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P /P 0
Fig. 6.33. N2-sorption isotherm at 196 °C of SBA-15.
Fig. 6.35. SEM images of SBA-15.
Intensity / a.u.
198
0
1
2
3
4
2θ /º
5
6
7
8
Fig. 6.36. TEM images of SBA-15.
Verified by:
(1 0 0) = 10.76 nm; (1 1 0) = 6.04 nm; (2 0 0) = 5.32 nm and (2 1 0) = 3.97 nm.
1000
Batch composition: 1 Si:0.018 EO20PO70EO20:2.08 HCl:112 H2O.
Aldrich).
Tetraethylorthosilicate (TEOS) (Sigma–Aldrich, reagent grade
98%).
Hydrochloric acid 1 M (HCl) (Merck).
-1
900
800
700
600
500
400
300
200
100
0
0.0
Product recovery:
Centrifugate and wash with warm distilled water until pH of 4–
5 is observed.
Dry the solid at 90 °C over night.
Calcine the product at 500 °C during 6 h in flowing air (flow rate
of 100 mL/min). Heating rate: 1 °C/min. Cooling down occurs slowly.
SBET = 550–900 m2/g.
Vtotal = 1.0–1.4 cm3/g.
Vmicro = 0.02–0.06 cm3/g.
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P/P 0
Batch preparation:
ads
des
d v (r )
1. Add 20 g EO20PO70EO20 to 400 mL 1 M HCl and heat to 50 °C.
2. Stir the mixture over night at about 30 °C to obtain a homogeneous mixture.
3. Heat the mixture to 40 °C, add 40 g TEOS under stirring. A white
suspension (precipitation) is formed after 1 h.
4. Keep the mixture at 40 °C under stirring for an additional 24 h.
5. Transfer the mixture into Teflon-lined autoclaves and keep it at
100 °C for 72 h.
0.1
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Radius / nm
7.0
8.0
9.0
10.0
Fig. 6.39. Pore size distribution of SBA-15 determined by the BJH method.
6.9. SBA-15 (from sodium silicate)
Contributed by: S.C. Laha, R. Gläser, J. Weitkamp
Verified by:
B. Seelandt, M. Wark, J. Caro
Remarks:
Synthesis can be done in an autoclave or in PP bottles at temperatures of 100 °C.
Batch composition: 1 SiO2:0.33 Na2O:0.017 P123:1.4 HCl:95
H2O.
199
Calcine the solid at 540 °C for 1 h in nitrogen with a heating rate
of 2 °C/min. Substitute the nitrogen by air and continue the calcination for 8 h at 540 °C (gas flow 30 cm3/min).
N2-sorption: (See Fig. 6.43 and 6.44).
SBET = 500–700 m2/g.
Vtotal = 0.7–1.0 cm3/g.
Vmicro = 0.06–0.1 cm3/g.
Intensity / a.u.
Fig. 6.40. SEM image of SBA-15.
Aldrich).
Sodium silicate solution extra-pure, (25.5–28.5% SiO2 and 7.5–
8.5% Na2O) (Merck).
HCl 37 wt.% (Merck).
0
1
2
3
4
5
6
7
8
2θ /º
Product recovery:
500
450
-1
1. Add 43.2 g of dimineralized water to 8.5 g of concentrated HCl,
mix.
2. Add 6.92 g P123 triblock copolymer to (1), mix and stir for 1 h
at 35 °C.
3. Add 69.5 g dimineralized water to 0.27 g NaOH, mix until
dissolved.
4. Add (3) to 15.9 g sodium silicate.
5. Add (4) to (2) and stir at 35 °C for 24 h.
6. Hydrothermal treatment in a 300 mL Teflon-lined steel autoclave at 100 °C in a preheated oven, during 24 h, without any
agitation.
(1 0 0) = 9.80 nm; (1 1 0) = 5.45 nm and (2 0 0) = 4.74 nm.
Batch preparation:
400
350
300
250
200
150
100
50
1. Add 25 mL ethyl alcohol to the precipitate, mix and filter.
2. Wash the precipitate three times with 10 mL ethyl alcohol and
three times with 200 mL demineralized water.
3. Dry in air at 80 °C.
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
P /P 0
Fig. 6.43. N2-sorption isotherm of SBA-15.
0.8
0.9
1
200
d v (r )
ads
des
0
1
2
3
4
5
6
Radius / nm
7
8
9
10
Fig. 6.46. TEM image of SBA-15.
6.10. SBA-15
Contributed by: F. Heinroth, P. Behrens
Verified by:
Batch composition: 1 TEOS:6.2 4 M HCl:128.6 H2O:0.017 P123.
Aldrich).
HCl 37% (Sigma–Aldrich).
Tetraethylorthosilicate >98% (TEOS) (Fluka).
Batch preparation:
4. After 24 h at 35 °C, transfer the solution into a PP-bottle and
place it in an oven at 80 °C for 24 h.
Product recovery:
Filter the reaction solution; wash the generated white powder
with water and dry at 50 °C.
Calcine the product at 500 °C during 24 h in ambient atmosphere with a heating rate of 1 °C/min. Cooling down occurs slowly.
SBET = 600–900 m2/g.
Vtotal = 0.8–1.1 cm3/g.
Vmicro = 0.09–0.17 cm3/g.
Remarks:
1. Add 34.2 g P123 to 810 mL H2O and 540 mL 4 M HCl and stir.
2. Heat the solution to 35 °C.
3. After 17.5 h, add 77 mL TEOS and stir.
Calcination at 500 °C for 24 h without applying a heating ramp is
possible.
201
Contributed by: I. Bannat, J. Caro, M. Wark
Verified by:
Intensity / a.u.
G. Smeulders, V. Meynen, P. Cool, E.F. Vansant
0
1
2
3
4
2θ /º
5
6
7
(ethylene glycol) (MW 5800) (EO20PO70EO20, P123) (BASF).
Tetraethylorthosilicate >98% p.s (TEOS) (Merck).
Concentrated hydrochloric acid >37% p.a. (Fluka).
8
(1 0 0) = 9.00 nm; (1 1 0) = 5.32 nm and (2 0 0) = 4.55 nm.
1. Dissolve 1 g P 123 in 25.63 mL of water.
2. Add 2.13 g of TEOS and add 8.13 g HCl quickly under vigorous
stirring.
3. Stir the mixture at 40 °C for 4 h and transfer then into a teflonbased reaction vessel.
4. Carry out microwave treatment in a microwave system model
ETHOS 1 (MLS Company, Germany) for 2 h at 100 °C. Use continuous microwave radiation with a maximum power of 400 W.
700
-1
600
Batch preparation:
500
400
Product recovery:
Filter the solution and wash the white product with water.
Afterwards, dry the products at ambient temperature.
Calcine at 550 °C for 6 h in ambient atmosphere and with a
heating rate of 1 °C/min. Cooling down occurs slowly.
300
200
100
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
SBET = 700–900 m2/g.
Vtotal = 0.8–1.0 cm3/g.
Vmicro = 0.10–0.20 cm3/g.
P /P 0
d v (r )
ads
des
Remarks:
The template can also be removed by extraction instead of calcination [418].
6.12. Short-channel SBA-15
0
1
2
3
4
5
6
7
8
9
10
Radius / nm
This synthesis was made at a larger scale in the order of 20 g (3–
4 times more compared to the other syntheses of SBA-15).
Reduced reaction temperature circumvents hydrothermal treatment and allows to carry out the reaction in polypropylene (PP)
bottles.
material with micropores in the walls. In addition, the pore lengths
are reduced in size allowing improved diffusion rate.
Verified by:
6.11. SBA-15 (microwave synthesis)
Aldrich).
202
HCl (37% Acros Organics).
Tetraethylorthosilicate TEOS (98% Acros Organics).
Batch preparation:
1. Add 4 g P123 to 127 mL H2O and 20 mL HCl, stir until complete
dissolution (this takes about 1 h).
2. Heat till 40 °C.
3. Add 9.14 mL TEOS and stir for 8 min at 40 °C.
4. Stop stirring and allow the mixture to age for 24 h at 40 °C.
5. Hydrothermal treatment in an autoclave at 100 °C for 24 h.
6. Quench the synthesis mixture.
Product recovery:
Filter, wash with 3 20 mL water and dry in ambient
atmosphere.
Calcine the product at 550 °C during 6 h, with a heating rate of
1 °C/min. Cooling down occurs slowly.
SBET = 500–800 m2/g.
Vtotal = 0.9–1.2 cm3/g.
Vmicro = 0.04–0.09 cm3/g.
Remarks:
The ‘‘short-channels” can be identified both from the TEM
images and the SEM images. Next to the shorter pores, also
the particles morphology consists of shorter rods that are not
agglomerated in longer secondary particles as those of normal
SBA-15.
Addition of KCl during the synthesis (before TEOS is added) gives
rise to SBA-15 with short straight channels [419].
Addition of a swelling agent, mesitylene, during the synthesis
(before TEOS is added) enlarges the pores of the short-channel
SBA-15 [172].
Due to the short length of the mesopores, higher loadings can be
obtained upon impregnation [421].
6.13. Plugged hexagonal templated silica (PHTS)
material with micropores in the walls and amorphous microporous
silica plugs in the channels.
Verified by:
Literature verification by E.B. Celer et al. [266].
203
Intensity / a.u.
-1
600
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
500
400
300
200
100
0
0.0
0.1
0.2
Fig. 6.52. X-ray diffraction pattern of SBA-15 made by microwave assisted
synthesis. The corresponding d-spacings: (1 0 0) = 8.57 nm; (1 1 0) = 4.93 nm and
(2 0 0) = 4.22 nm.
Aldrich).
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P/P 0
2θ /º
Fig. 6.53. N2-sorption isotherm at 196 °C of SBA-15 made by microwave assisted
synthesis.
Batch preparation:
1. Add 4 g P123 to 130 mL H2O and 20 mL HCl; stir until complete
dissolution.
2. Add 15 g (about 16 mL) TEOS and stir for 7.5 h at 60 °C.
3. Stop stirring and age for 15.5 h at 80 °C.
4. Cool down to room temperature.
204
dv (r )
Intensity / a.u.
ads
des
0
1
2
3
4
5
6
7
8
2θ / º
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Radius / nm
Fig. 6.57. X-ray diffraction pattern of short channel SBA-15. The corresponding dspacings: (1 0 0) = 10.10 nm; (1 1 0) = 5.78 nm; (2 0 0) = 5.02 and (2 1 0) = 3.32 nm.
Fig. 6.54. Pore size distribution of SBA-15 made by microwave assisted synthesis
via the BJH method.
800
3
-1
700
600
500
400
300
200
100
0
0.00
Fig. 6.55. SEM image of microwave assisted synthesized SBA-15.
0.10
0.20
0.30
0.40
0.50
P /P 0
0.60
0.70
0.80
0.90
1.00
Fig. 6.58. N2-sorption isotherm at 196 °C of SBA-15 with short channels.
d v (r )
ads
des
0.0
1.0
Fig. 6.56. TEM image of microwave assisted synthesized SBA-15.
Product recovery:
Filter, wash with 3 25 mL water and dry at ambient
atmosphere.
SBET = 700–900 m2/g.
Vtotal = 0.65–0.8 cm3/g.
Vmicro = 0.15–0.3 cm3/g.
2.0
3.0
4.0
5.0
6.0
Radius / nm
7.0
8.0
9.0
10.0
Fig. 6.59. Mesopore size distribution of SBA-15 with short channels determined
with the BJH method.
Pore diameter (BJH, adsorption branch) = 5.0–7.0 nm for the
open pores. The plugged pores are smaller than 4 nm.
Remarks:
Stability of the plugs is depending on the synthesis conditions
[266].
For more information about the influence of the TEOS amount,
the stirring and ageing temperature [262].
205
Fig. 6.60. SEM images of short-channel SBA-15.
Fig. 6.61. TEM images of short-channel SBA-15.
Intensity / a.u.
-1
600
0
1
2
3
4
5
6
7
8
9
2θ /º
Fig. 6.62. X-ray diffraction pattern of PHTS. The corresponding d-spacings:
(1 0 0) = 9.80 nm; (1 1 0) = 5.70 nm and (2 0 0) = 4.96 nm.
PHTS can also be formed by post-synthesis depositions of nanoparticles on SBA-15 [127].
6.14. Al-SBA-15
material with micropores in the walls and a Si/Al ratio of 32.
Verified by:
500
400
300
200
100
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P /P 0
Fig. 6.63. N2-sorption isotherm at 196 °C of PHTS.
Batch composition: 1 Si:0.06 Al:0.018 EO20PO70EO20:0.21
HCl:115 H2O.
Aldrich).
206
d v (r )
ads
des
0
1
2
3
4
5
6
Radius / nm
7
8
9
10
Fig. 6.64. Mesopore size distribution of PHTS determined with the BJH method.
Tetraethylorthosilicate (TEOS) (Sigma–Aldrich, reagent grade
98%).
Sodiumaluminate (54% Al2O3, 41% Na2O) (Riedel-de Haën).
HCl (0.1 M) (Merck).
Fig. 6.66. TEM image of PHTS.
1. Heat 20 g template (EO20PO70EO20) at 50 °C and dissolve in
400 mL hydrochloric acid 0.1 M.
2. Stir the solution over night at 25–30 °C.
3. Add 40 g silicate source (TEOS) under stirring.
4. Stir the solution for 1 h.
5. Add 0.92 g sodiumaluminate.
6. Stir over night at 25 °C.
7. Measure the pH in the solution; it should be around 2.
8. Change the temperature to 40 °C, after about 30 min a gel is
formed, after 2–3 h a white precipitate is observed.
9. Stir the white precipitate solution for 4 h.
10. Change the pH of the solution to 2.5 at 40 °C by addition of
4 M NaOH.
11. Stir the solution for 1 h.
12. Transfer the mixture to Teflon bottles and keep it at 100 °C
for 72 h.
Product recovery:
Wash the formed white products with distilled water and centrifuge until pH 5. Dry the samples at 90 °C for 3 days followed by
drying at 100 °C overnight.
Intensity / a.u.
Batch preparation:
0
1
2
3
4
5
6
7
8
2θ / º
Fig. 6.67. X-ray diffraction pattern of Al-SBA-15 (Si/Al = 32). The corresponding dspacing: (1 0 0) = 8.02 nm.
Calcine the product at 500 °C for 6 h in flowing air with flow
rates of 100 mL/min and with a heating rate of 1 °C/min.
Fig. 6.65. SEM images of PHTS.
207
1200
6.15. Mesoporous silica films
1000
Short description of material: Mesoporous silica film with pores
in the nanometer range.
Contributed by: I. Krueger, N. Witteck, P. Behrens
Verified by:
3
-1
800
600
V. Meynen, P. Cool, E.F. Vansant
400
200
0
0.00
0.20
0.40
0.60
0.80
1.00
P /P 0
Fig. 6.68. N2-sorption isotherm at 196 °C of Al-SBA-15 (Si/Al = 32).
Batch composition: 1 TEOS:48.9 CH3CH2OH:0.06 2 N HCl:26.9
H2O:0.0135 EO20PO70EO20.
Aldrich).
Ethanol abs. for synthesis (Merck).
Millipore water.
Hydrochloric acid purum p.a. (Fluka).
Tetraethoxysilane purum (TEOS) (Fluka).
Standard glass slides (Karl Hecht KG, Elka Objektträger).
d v (r )
ads
des
Batch preparation:
The nanostructured film is prepared on standard glass slides in
a dip-coating procedure. The dip-coating solution:
0
2
4
6
8
10
12
14
16
18
20
Radius / nm
Fig. 6.69. Pore size distribution of Al-SBA-15 (Si/Al = 32) obtained with the BJH
method.
SBET = 800–1100 m2/g.
Vtotal = 1.0–1.6 cm3/g.
Vmicro = 0.05–0.2 cm3/g.
Chemical composition:
The Si/Al ratio of the different verified Al-SBA-15 samples were
determined as 32 by XRF, 32 by EPMA and 42 by ICP/AES.
1. Add 0.0135 mol of P123 to 48.9 mol of ethanol, 0.06 mol of
hydrochloric acid and 26.9 mol of millipore water until all the
block copolymer is dissolved.
2. Add 1 mol of TEOS to the mixture and stir for 10 min.
3. Transfer the dip-coating solution to a modified desiccator (see
Fig. 6.73) wherein the air humidity is adjusted with a glucose
solution (50 wt.%).
4. Position the glass slide for 5 min above the dip-coating solution
in the closed desiccator.
5. Dip the glass slide into the solution for 30 s, pull out very slowly
(approximately 2 mm/s) and keep in the closed desiccator for
5 min.
Product recovery: Not applicable.
The glass slides are put in a drying oven at 60 °C in ambient
atmosphere overnight and are then calcined at 415 °C for 4 h in
ambient atmosphere without a heating rate.
Remarks:
Intensity / a.u.
Successful deposition of the film can also be observed by a
change in contact angle. For example: original glass slide about
28° and after deposition of the silica film about 14°.
calcined sample
(Si/Al 32)
150
Fig. 6.70.
100
27
0
50
Chemical shift/ ppm
-50
6.16. SBA-16
-100
Al MAS NMR spectra of the Al-SBA-15 sample with a Si/Al ratio of 32.
Short description of material: Cubic mesoporous material with
micropores in the walls.
Contributed by: W.J.J. Stevens, P. Cool, E.F. Vansant, V. Meynen
Verified by:
208
Intensity / a.u.
Fig. 6.71. SEM images of calcined sample Al-SBA-15 (Si/Al = 32).
0
1
Fig. 6.72. TEM images of calcined sample Al-SBA-15 (Si/Al = 32).
2
3
2θ /º
4
5
6
Fig. 6.74. X-ray diffraction pattern of the SBA-15 film. The corresponding dspacing: (1 0 0) = 5.81 nm.
1-Butanol.
Batch preparation:
1.
2.
3.
4.
5.
Fig. 6.73. Schematic representation of the modified desiccator for dip-coating.
D. Pufky-Heinrich, R. Gläser, J. Weitkamp
Batch composition: 1 TEOS:0.88 HCl:111 H2O:0.0032 F127:1.70
BuOH.
(ethylene glycol) (MW 12600) (EO106PO70EO106, F127) (Sigma–
Aldrich).
Add 7 mL HCl to 190 mL H2O in a reflux setup.
Add 14.8 mL BuOH.
Add 4 g F127 and stir until complete dissolution.
Add 21 mL TEOS and stir during 24 h at 45 °C.
Stop stirring and age at 100 °C during 24 h.
Product recovery:
atmosphere.
Calcine the product at 550 °C during 6 h in ambient atmosphere
and with a heating rate of 1 °C/min. Cooling down occurs slowly.
SBET = 700–800 m2/g.
Vtotal = 0.55–0.65 cm3/g.
Vmicro = 0.15–0.3 cm3/g.
209
-1
600
500
400
300
200
100
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P/P 0
Fig. 6.77. N2-sorption isotherm of cubic SBA-16 at 196 °C.
d v (r )
ads
des
0
1
2
3
4
5
6
7
8
9
10
Radius / nm
Fig. 6.75. SEM images: surface structure of two different silica films.
6.17. SBA-16
Intensity / a.u.
Short description of material: Cubic mesoporous material with
micropores in the walls.
Contributed by: F. Heinroth, P. Behrens
Verified by:
D. Pufky-Heinrich, S.A.S. Rezai, J. Weitkamp
Batch composition: 1 TEOS:116 H2O:4 HCl:0.004 F-127.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
2θ /º
Fig. 6.76. X-ray diffraction pattern of cubic SBA-16. The corresponding d-spacings:
(1 1 0) = 11.31 nm; (2 0 0) = 8.02 nm; (2 1 1) = 6.39 and (2 2 0) = 5.88 nm.
HCl (Sigma–Aldrich).
(ethylene glycol) (MW 12600) (EO106PO70EO106, F-127)
(Sigma–Aldrich).
TEOS >98% (Fluka).
Batch preparation:
Remarks:
Addition of more or less butanol during the synthesis (before
TEOS is added) gives rise to differently ordered materials [426].
Changing the amount of TEOS added changes the observed morphology of the SBA-16 material as well as its porous characteristics [246].
1. Add 134 mL concentrated HCl to 836 mL H2O.
2. Add 20 g Pluronic F-127 and stir.
3. Heat the solution to 35 °C and add after 4.5 h 89.1 mL TEOS
under stirring.
4. Transfer the solution after 20 h into a PP-bottle and treat it for
24 h in an oven at 80 °C.
210
Intensity / a.u.
Fig. 6.79. SEM images of cubic SBA-16.
Fig. 6.80. TEM images of cubic SBA-16.
0
Product recovery:
Filter the reaction solution and wash the generated white powder with water, dry at 50 °C.
Calcine the product at 500 °C during 24 h in ambient atmosphere with a heating ramp of 1 °C/min.
SBET = 550–800 m2/g.
Vtotal = 0.25–0.4 cm3/g.
Vmicro = 0.1–0.2 cm3/g.
1
2
3
4
2θ /º
5
6
7
8
(1 1 0) = 10.14 nm; (2 0 0) = 6.35 nm and (2 1 1) = 2.15 nm.
Remarks:
Calcination at 500 °C for 24 h without applying a heating ramp is
possible.
This synthesis was made at a larger scale in the order of 20 g
(3–4 times more compared to the other syntheses of
SBA-16).
211
-1
250
200
150
100
50
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P/P 0
ads
des
dv (r )
Fig. 6.85. TEM image of SBA-16.
6.18. MCF
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Radius / nm
Fig. 6.83. Pore size distribution of SBA-16 calculated by the BJH method.
Short description of material: Mesoporous cellular foam; porous
material with very large mesopores. Three-dimensional pore network with cellular pore morphology.
Verified by:
M. Stöcker, A.M. Bouzga
212
Batch composition: 1 TEOS:5.87 HCl:194 H2O:0.017 EO20PO70EO20:0.031 NH4F:0.815 mesitylene.
d v (r )
Aldrich).
NH4F p.a. (Merck).
Mesitylene (=trimethylbenzene) 99% (Acros Organics).
ads
des
0
5
10
15
20
25
30
35
40
45
50
Radius / nm
Batch preparation:
Fig. 6.87. Pore size distribution of MCF determined with the BJH method.
1. Add 4 g P123 and 20 mL HCl to 130 mL H2O; stir until the surfactant is dissolved.
2. Add 0.0467 g NH4F and 4.6 mL mesitylene.
3. Stir for 1 h at 35–40 °C.
4. Add 9.14 mL of TEOS; stir for 20 h at 35–40 °C.
5. Transfer the mixture to an autoclave and keep it at 100 °C for
24 h.
6. After heat treatment, quench the autoclave to room
temperature.
Product recovery:
atmosphere.
Calcine the product at 550 °C during 6 h in ambient atmosphere
with a heating rate of 1 °C/min. Cooling down occurs slowly.
Fig. 6.88. SEM images of MCF.
XRD:The pores are too large for X-ray diffraction. The first order
peak should appear below 0.2° 2h.
SBET = 550–700 m2/g.
Vtotal = 2.1–2.6 cm3/g.
Vmicro = 0.07–0.1 cm3/g.
Pore diameter (BJH, adsorption branch) = 20.0–30.0 nm
(desorption about 10.0–15.0 nm).
Fig. 6.89. TEM image of MCF.
Remarks:
2000
1800
Volume adsorbed STP / cm³g
-1
The synthesis can also be executed without the addition of
NH4F. However, without the presence of F ions in the synthesis
mixture, the hysteresis loop will be much broader [282,284].
1600
1400
1200
1000
6.19. MMA (monolithic mesoporous aluminosilicates)
800
600
Short description of material:
Highlights :
400
200
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
P/P 0
Fig. 6.86. N2-sorption isotherm at 196 °C of MCF.
0.9
1
– Tunable porosity from super-micropores (1 nm < dp < 2 nm) to
small mesopores.
– Tunable concentration of surface acid sites (moderate acid
strength).
213
– Monolithic solid with controlled morphology.
– Thermostability–hydrostability.
– One-step synthesis.
(7) Store at 40 °C for 2 h.
(8) Store at 60 °C until solid phase condensation occurs (usually
2 or 3-days period).
Applications:
– Heterogeneous catalysis.
– Adsorption, gas separation. . .
Contributed by: D. J. Jones, J. Rozière, N. Donzel, M. TailladesJacquin
Verified by:
Product recovery:
The material contracts during the drying and calcination steps,
facilitating their removal form the moulds.
Calcine at 560 °C during 6 h in ambient atmosphere with a temperature ramp of 0.5 °C/min.
N2-sorption: (See Figs. 6.91–6.96).
Pore diameter of the various MMA materials as determined by
the as-plot method
M. Stöcker, A. van Miltenburg
Industrial verification by UOP-LLC (Des Plaines) [427].
Batch composition: 1 TEOS:3.88 H2O:0.007 HNO3:0.19 Brij 30:
Al (NO3)3.9H20.
Tetraethoxysilane, purity 98%, Si(OC2H5)4 (Sigma–Aldrich).
65% Nitric acid solution (Fluka).
Aluminiumnitrate nonahydrate, purity 99+%, Al(NO3)3.9H2O
(Acros Organics).
Polyoxyethylene (4) lauryl ether (MW 362) (C12H25(OCH2CH2)4OH, Brij 30) (Sigma–Aldrich).
MMA material
dp/ nm
Si/Al = 1
Si/Al = 40
Si/Al = 20
Si/Al = 15
Si/Al = 10
Si/Al = 5
2.0
2.2
2.6
2.5
2.5
3.4
SBET = 650–1000 m2/g.
Vtotal = 0.4–0.65 cm3/g.
Batch preparation:
Remarks:
The dilute 0.1 M HNO3 aqueous solution serves as both solvent
and pH regulator.
-1
300
3
(1) Add 4.5 g Brij 30 to 4.5 g 0.1 M HNO3 and 13.5 g TEOS while
stirring the solution until complete dissolution (clear and
transparent solution). The reaction vessel is kept at a constant temperature (usually 25 °C) by means of a water bath.
(2) In case that Al is introduced in the system, the necessary
amount of Al(NO3)3.9.H2O is added to the reaction mixture
while stirring. (e.g. 0.61 g in case of Si/Al = 40; 1.62 g for
Si/Al = 15).
(3) Remove the ethanol produced by the reaction by vacuum
treatment (water pump or reduced pressure of 20–35 mbar)
at ambient temperature for 1 h.
(4) Store the solution in a drier at 60 °C until an enough viscous
solution is obtained (usually between 1 and 2 h).
(5) Transfer the solution in a mould (glass tube) by means of a
syringe.
(6) Store at ambient temperature for 2 h.
250
200
150
100
50
0
0
0.1
0.2
0.3
0.4
Si/Al = 10
Si/Al = 20
0.6
0.7
0.8
0.9
1
-1
Fig. 6.91. Adsorption isotherms of nitrogen at 196 °C of MMA (Si/Al = 1).
300
3
Si/Al = 40
Intensity / a.u.
0.5
P/P 0
Si/Al = ∞
0
1
2
3
4
5
2θ /º
6
7
8
9
10
250
200
150
100
50
0
0
Fig. 6.90. X-ray diffraction pattern of MMA. The corresponding d-spacings: (Si/
Al = 10) = 4.67 nm;
(Si/Al = 20) = 4.39 nm;
(Si/Al = 40) = 3.63
and
(Si/
Al = 1) = 3.38 nm.
0.1
0.2
0.3
0.4
0.5
P/P0
0.6
0.7
0.8
0.9
1
214
3
-1
300
250
Intensity / a.u.
200
150
100
50
calcined sample
Si/Al = 15
calcined sample
Si/Al = 40
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P/P 0
150
100
50
0
-50
-100
Chemical shift / ppm
Fig. 6.97.
27
Al MAS NMR spectra of various MMA materials.
3
-1
350
300
250
200
150
100
50
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P/P 0
Fig. 6.98. SEM image of MMA.
3
-1
300
250
200
150
100
50
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P/P 0
300
3
-1
250
200
150
100
50
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P/P 0
Fig. 6.99. TEM image of MMA.
6.20. Monolithic material
Adaptations of the above procedure were performed through the
introduction of aluminium nitrate to the diluted HNO3 prior to
addition of surfactant and TEOS, using Si/Al ratio in the range
5 < Si/Al < 40.
The materials can be successfully modified with Pt and Pd [428].
Instead of applying moulds, the materials can also be made via
extrusion with or without the addition of binders.
Short description of material: Monolithic material with supermicropores (1 nm < dp < 2 nm).
Contributed by: B. Ufer, P. Behrens
Verified by:
K. De Witte, V. Meynen, P. Cool, E.F. Vansant
F. Meducin, F. Warmont, S. Serieye, M.-L. Saboungi
215
Batch composition: 1 TMOS:0.196 tetraethylene glycol monododecyl ether:0.00075 HCl:4.17 H2O.
d v (r )
Tetraethylene glycol monododecyl ether >98% (Fluka).
Hydrochloric acid 0.01 mol/L (Riedel-de Haën) FixanalÒ.
Tetramethylorthosilicate >98% (Fluka).
ads
des
Batch preparation:
1. Mix 200 lL of tetraethylene glycol monododecyl ether with
200 lL 0.01 mol/L hydrochloric acid and 400 lL tetramethylorthosilicate in a safe lock tube (Eppendorf).
2. Shake the used safe lock tube (Eppendorf) for 1 min.
3. Transfer the synthesis gel to a container providing the wanted
geometry.
4. Leave the synthesis gel in the oven for one week at 40 °C to
evaporate the resulting methanol and allow condensation to
take place.
0
1
2
3
4
5
6
7
8
9
10
Radius / nm
Fig. 6.102. Pore size distribution of the powdered monolithic material determined
with the BJH method.
Product recovery:
Recover the resulting monolithic material without further steps.
Calcine at 500 °C during 24 h in ambient atmosphere with a
heating ramp that is as low as possible.
Intensity / a.u.
Fig. 6.103. SEM images of monolithic material.
SBET = 1060–1170 m2/g.
Vtotal = 0.40–0.50 cm3/g.
0
1
2
3
4
5
6
7
8
Remarks:
2θ /º
Fig. 6.100. XRD of the powdered monolithic material. The corresponding dspacings: (1 0 0) = 3.22 nm.
350
Volume adsorbed / cm³ g
-1
300
250
200
150
100
The container to form the monoliths does not need to be of any
special type of material. Examples of materials that have been
used: cell-culture dishes, plastic drinking straws, etc.
To obtain crack free monoliths, they should stay in an oven at
40 °C for at least a week and evaporation or heating should
always be very slow to retain the monolithic structure.
After calcination the monolith will show some crack formation
and becomes super-microporous (1 nm < dp < 2 nm). Nevertheless, it possesses short range order of the pores and high surface
areas.
Due to the super-microporous nature of the materials, it must be
noted that the BJH method to determine the pore size needs to
be applied with care and is only indicative.
50
6.21. Stabilized mesoporous titania
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P /P 0
Fig. 6.101. N2-sorption isotherm at 196 °C of the powdered monolithic material.
Short description of material: mesoporous titania (partially anatase). Stabilization of the structure by post-synthesis modification
with NH4OH or NaOH.
Intensity / a.u.
216
20
25
30
35
40
45
50
55
60
2θ /º
Intensity / a.u.
Fig. 6.105. X-ray diffraction pattern of mesoporous titania stabilized with a base
treatment of NH4OH.
1000
900
800
700
600
500
400
300
200
100
0
-1
Raman shift / cm
Fig. 6.106. Raman spectrum of mesoporous titania after base treatment with
NH4OH and calcination.
Contributed by: E. Beyers, P. Cool, E.F. Vansant, V. Meynen
Verified by:
M. Stöcker, A.I. Spjelkavik
J. Ruiz-Martínez, J. Silvestre-Albero, A. Sepúlveda-Escribano,
Fig. 6.104. TEM images of monolithic material.
-1
250
200
150
100
50
0
Type of material: Titania
Batch composition: 1 Ti(OiPr)4:0.16 CTMABr:1.4 HCl:17 H2O:20
EtOH.
Ethanol p.a. (EtOH) (Merck, absolute GR for analysis).
Titanium (IV) isopropoxide Ti(OiPr)4 97% (Sigma–Aldrich).
Cetyltrimethylammonium bromide CTMABr (Acros Organics).
NH4OH 28–30 wt.% solution of NH3 in water p.a. (Acros
Organics).
Batch preparation:
(A)
1. Add 0.59 g CTMABr to 6 mL EtOH.
2. Add 3 mL Ti(OiPr)4to 5.7 mL EtOH and 1.18 mL HCl.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P /P 0
Fig. 6.107. N2-sorption at 196 °C of mesoporous titania stabilized with NH4OH
treatment.
3. Add the CTMABr solution (1) to the Ti(OiPr)4 solution (2).
4. Add dropwise 2.06 mL H2O and stir during 15 min.
5. Transfer the solution into an open Petri Dish and put this
in an oven at 60 °C during 7 days.
(B)
Base treatment of the solid with NH4OH for stabilization:
1. Add 1 g of solid to 50 mL H2O (pH 9–10, by addition of
NH4OH).
2. Reflux for 48 h, keep the pH constant between pH 9–10
(NH4OH).
217
Product recovery:
conditions.
2 °C/min under air flow. Alternatively, it is possible to calcine the
material during 2 h at 450 °C with a heating rate of 2 °C/min without air flow.
Raman: (See Fig. 6.106).
SBET = 250–500 m2/g.
Vtotal = 0.25–0.8 cm3/g.
Remarks:
The stabilization step with NH4OH is also possible with NaOH
and leads to products with a higher surface area and total pore
volume [60]. Fifty milliliter of 0.112 M NaOH solution was used
for each gram of product [60].
Fig. 6.110. TEM image (B and C) and diffraction pattern (A) of mesoporous titania
stabilized by the post-synthesis NH4OH treatment.
The mechanism for stabilization can be found in the literature
[56,60,431].
ads
des
d v (r )
6.22. Titanate nanotubes
0
1
2
3
4
5
6
7
8
9
10
Short description of material: tubular-shaped material with inner
diameter of approximately 5 nm and outer diameter of approximately 10 nm. Titanate crystalline structure.
Contributed by: S. Ribbens, E. Beyers, E.F. Vansant, P. Cool, V.
Meynen
Verified by:
radius / nm
Fig. 6.108. Pore size distribution (BJH) of mesoporous titania stabilized with a
NH4OH treatment.
I. Bannat, M. Wark, J. Caro
A. Silvestre-Albero, J. Ruiz-Martínez, J. Silvestre-Albero, A. Sepúlveda-Escribano, F. Rodríguez-Reinoso
N. Kanellopoulos, G. Karanikolos
Type of material: Sodium trititanate and after ion exchange
hydrogen trititanate.
Batch composition:
TiO2-powder (Riedel-de Haën).
NaOH pellets (Acros Organics).
Batch preparation [432]:
Fig. 6.109. SEM image of mesoporous titania stabilized with the NH4OH method.
1. Disperse 4.5 g TiO2 into 80 mL 10 M NaOH solution under
vigorous stirring.
2. Stir the mixture for an hour.
3. Transfer the mixture in an autoclave (AC).
4. Heat the AC to 150 °C for 48 h.
5. Quench the AC to room temperature.
6. Recover the solid by centrifugation and wash three times
with deionized water.
7. Disperse the wet cake into 240 mL 0.1 M HCl and stir for
30 min.
8. Recover the solid by centrifugation.
9. Wash with 0.1 M HCl to pH 5–6 to obtain the Na-titanate
tube.
10. Further washing up to pH 1–1.5 to convert the Na-titanate
tube into an H-titanate tube.
Product recovery:
Centrifugation, wash with water (3 times), ethanol (2 times)
and dry at 100 °C in ambient atmosphere.
No further steps required
N2-sorption: (See Fig. 6.112 and 6.113).
SBET = 200–300 m2/g.
Vtotal = 0.5–0.9 cm3/g.
Pore diameter (BJH, adsorption branch) = 3.0–4.3 nm (inner
diameter).
H-tube
Na-tube
d v (r )
218
0
5
10
15
20
25
30
35
40 45 50
Radius / nm
55
60
65
70
75
80
Fig. 6.113. Pore size distribution (BJH, adsorption branch) of a sodium and
hydrogen trititanate nanotube.
Intensity / a.u.
Na2Ti 3O7
H2Ti 3O7
0
10
20
30
40
50
60
70
80
90
100
2θ /º
Fig. 6.111. X-ray diffraction pattern of Na2Ti3O7 and acid washed H2Ti3O7
nanotubes.
Remarks:
1200
H-Tube
Na-tube
The titanate nanotubes can be converted to anatase nanotubes
by calcining under Ar atmosphere at 500 °C [432] or in ambient
atmosphere [9].
The titanate nanotubes can also be prepared with microwave
assisted synthesis [9].
-1
1000
Fig. 6.114. TEM and HRTEM of protonated titanate nanotubes (a–c), sodium
titanate nanotubes (d). The nanotubes have outer diameters of 10–15 nm and
lengths of 200–400 nm.
800
600
7. Conclusions
400
200
0
0
0.2
0.4
0.6
0.8
1
P /P 0
Fig. 6.112. N2-sorption isotherm at 196 °C of a sodium and hydrogen trititanate
nanotube.
This publication covers the verified syntheses of the selected
relevant structured super-microporous and mesoporous siliceous
and titania materials. The number of listed materials is by no
means complete, containing all existing and important mesoporous materials that have been developed in recent years. Moreover,
the number of verified syntheses is much smaller than the verified
syntheses of zeolites collected in the well-known zeolite syntheses
book [434]. However, this achievement should be seen as the start
of the publication and documentation for proven syntheses of mesoporous materials, with the possibility to be further explored in the
future.
The Network of Excellence (INSIDE POReS) was indispensable
in accomplishing this publication on verified mesoporous materials. It was the ideal network of researchers from different fields
(synthesis, characterization, catalysis and membranes) that synthesize and/or apply these mesoporous materials for different
application purposes. The publication was a joint work by experts in the field working together to realize a large amount of
verification work in synthesizing and characterizing these materials. This work will be carried on in the future in the frame of
the European Nanoporous Materials Institute of Excellence (ENMIX), a collaboration within the Network of Excellence (NoE) INSIDE POReS.
As the number of mesoporous materials to be developed is still
growing, it is planned to come out with a second edition in the future containing new and more verified syntheses of mesoporous
materials. The field is open for further additions as new materials
will be added. Readers are invited to report their experience, success and ideas concerning the verified syntheses to the corresponding author. The authors will be pleased to include useful comments
from the readership in the second edition of this publication.
Acknowledgments
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
This work was funded by the EU and executed in the frame of
the European FP6 Network of Excellence INSIDE-POReS, coordinated by Dr. N. Kanellopoulos. All contributors express their great
appreciation for the support and encouragements of Dr. S. Bøwadt
(European Commission).
V. Meynen is grateful to the FWO-Flanders for financial support.
The authors would like to thank Prof. G. Van Tendeloo (EMAT,
University of Antwerpen), S. Brouwer (TUDelft), V. ButselaarOrthelieb (TUDelft) and U. Lafont (TUDelft) for their aid in some
of the SEM/TEM analyses, and M. Mertens from VITO (Flemish
Institute for Technological Research) for measuring part of the
XRD patterns. Prof. Dr. M. Hunger from the Institute of Chemical
Technology, University of Stuttgart, is gratefully acknowledged
for the 27Al MAS NMR measurements. Linn Sommer from the
Department of Chemistry, University of Oslo (Norway) is recognized for recording some of the nitrogen adsorption–desorption
isotherms.
The contributors, K.S. Triantafyllidis and C. Nitsos would like to
acknowledge the aid of the Department of Chemistry of the Aristotle University of Thessaloniki for providing means and infrastructure for performing the synthesis of the mesoporous materials in
cooperation with CERTH/CPERI. They would also like to thank the
Laboratory for Analysis and Characterization of Solids at CPERI/
CERTH and SINTEF (M. Stöcker) for performing part of the characterization of the materials.
Chapter 4, ‘‘The overview of the main synthetic approaches to
mesoporous materials” was composed by V. Meynen, E. Beyers, S.
Ribbens, D.J. Jones, P. Cool and E.F. Vansant.
Chapter 5, ‘‘Characterization Techniques” was composed by P.
Cool, C. Van Oers, F. Rodríguez-Reinoso, K.S. Triantafyllidis, M.
Hunger, P.J. Kooyman, P. Behrens and E.F. Vansant.
Chapter 6 has been established by all researchers mentioned in
the contributors list (Chapter 3).
The authors gratefully acknowledge the Elsevier Inc. for their
help in connection with the production of the paper.
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Verified syntheses of mesoporous materials

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