Calcination

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Calcination
Thermal Treatment of Catalysts
Modern Methods in Heterogeneous Catalysis Research
Friederike C. Jentoft, October 31, 2003
Outline
1. Terminology (calcination)
2. Sample vs. oven set temperature
3. Self-generated atmosphere & self-steaming of zeolites
4. Combustion
5. Glow phenomenon & zirconia catalysts
6. Crystallization
7. Loss of surface area
8. Effect of additives
9. Solid-solid wetting
10.Reductive treatments & SMSI
Steps of Catalyst Preparation
v IUPAC defines 3 steps of catalyst preparation
1. Preparation of primary solid, associating all the useful
compounds
2. Processing of that primary solid to obtain the catalyst
precursor for example by heat treatment
3. Activation of the precursor to give the active catalyst
(reduction to metal, formation of sulfides, deammoniation
of zeolites)
Heat Treatment of Intermediate Solids or Precursors
v drying
v thermal decomposition of salts (nitrates, ammonium salts)
v calcination
v product is a "reasonably inert solid" which can be stored
easily
Annealing
v in a general meaning: a heating of a material over a long
time span; strain and cracks in a crystalline solid can be
removed
Origin of the Term "Calcination"
v latin "calx" = playstone limestone, (greek chálix)
v burning of calcium carbonate (limestone) to calcium
oxide (quicklime)
CaCO3 → CaO + CO2
ΔH(900°C)=3010 kJ mol-1
v used to construct Giza pyramids (ca. 2800 A.C.), burning of
limestone ("Kalkbrennen") mentioned by Cato 184 A.C.
v performed in kilns (ovens) at 900°C
v addition of air to sustain combustion + cool product
Examples for Kilns for Calcination
v Schematic of a vertical shaft kiln.
a) Preheating zone; b) Calcining zone;
c) Cooling zone
v Schematic of a rotary kiln
a) Burner; b) Combustion air;
c) Pre-heater; d) Kiln; e) Cooler
General Definition of "Calcination"
v decomposition of a substance through heating, transformation in
lime-like substance – Duden
v to heat (as inorganic materials) to a high temperature but without
fusing in order to drive off volatile matter or to effect changes (as
oxidation or pulverization) – Websters
v heating (burning) of solids to a certain degree of decomposition,
whereby with e.g. soda, gypsum the crystal water is completely or
partially removed – Römpp's Chemielexikon
v the heating of a solid to a high temperature, below its melting point,
to create a condition of thermal decomposition or phase transition
other than melting or fusing – Hüsing, Synthesis of Inorganic
Materials
Definition of "Calcination" in Catalysis Research
v thermal treatment (of a catalyst) in oxidizing atmosphere. The
calcination temperature is usually slightly higher than that of the
catalyst operating temperature – Ullmann's Encyclopedia of
Industrial Chemistry
v a heat treatment of catalyst precursor in an oxidizing atmosphere for
a couple of hours - Catalysis from A to Z, Eds. Cornil et al.
v heating in air or oxygen; the term is most likely to be applied to a
in the
preparation
a catalyst
- IUPAC
v step
often,
with
respectofto
catalysts,
an Compendium
oxidizing of
Chemical Terminology
treatment is meant
v however, you will find statements such as "calcined
in inert atmosphere"
v sample-generated atmosphere may be oxidative
(nitrate decomposition)
Example from Patent Literature
v extremely vague! unimportant?
v no, there is a secret to it!
v often only temperature + holding time given
E.J. Hollstein, J.T. Wei, C.-Y. Hsu, US Patent 4,918,041
Calcination Procedure: Temperature Program
900
Temperature / K
800
700
600
500
400
300
0
100
200
300
400
500
Time / min
v heating rate, holding time, cooling rate
v cooling usually uncontrolled below certain T, slower
Actual Temperature Program
v oven may not be able to perform selected program (lack
in power): temperature lag of actual temperature behind
set temperature
v poorly tuned controller may give temperature oscillations
v heat needs to be transferred from oven to sample, needs
a gradient: temperature lag of sample temperature vs.
oven temperature
Role of Sample
v strongly endo- or exothermic events may interfere with
the heating program
v endothermic events: solvent evaporation
v exothermic events: combustion of organics,
crystallization
Evaporation of Water: Thermal Effects
v example: a 10 g sample containing 18 % water (0.1 mol)
v ΔHevap(H2O, 373 K) = 41 kJ mol-1
v to evaporate in 1 minute: ≈ 70 Watt
Example: Calcination of Zirconium Hydroxide
Sample bed temperature / K
700
-1
10 K min
-1
3 K min
600
500
400
-1
1 K min
300
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Time / h
v with a 10 g sample, deviations can occur already at
moderate heating rates
Evaporation of Water: Gas Formation
v example: a 10 g sample containing 18 % water (0.1 mol)
v at 400 K: corresponds to ≈ 3.3 l of water vapor
v depending on the
- form of the bed
- the type of furnace (tubular / muffle)
- static / dynamic atmosphere (no flow / flow)
the sample will be exposed to vapor for minutes!
Zeolite Y as a Cracking Catalyst
v zeolite Y is used as a cracking catalyst (FCC)
Faujasite structure
v first synthesize NaY, then exchange Na+ by NH4+ (liquid
phase)
v obtain active HY through thermal decomposition of NH4Y
v regular HY not very stable
Ultrastable Y zeolite
v McDaniel and Maher 1967 report „new ultra-stable form
of faujasite“
v worked with 100 g of zeolite, first exchange then heat
treatment
900
Temperature / K
800
700
600
500
400
300
0
100
200
300
400
500
Time / min
v „keeping the elapsed time between the exchange step
and the heating step at 815°C to a minimum is quite
critical“
C.V. McDaniel, P.K. Maher in Molecular Sieves, Soc. Chem. Ind. London, 1968, p. 186
Ultrastable Y Zeolite
v Kerr 1967: treatment of HY at 700-800°C in inert static
atmosphere
v „any technique keeping this water in the system during
the heating process will result in a stable product“
v published comparison of heating in „deep bed“ or
„shallow bed“: deep bed produces stable product
v ascribes success of McDaniel & Maher to the large
amount that they used
G.T. Kerr, J. Phys. Chem. 1967, 71, 4155 and J. Catal. 1969, 15, 200.
Influence of Packing
MS analysis
m/e = 18 (H2O)
v Packing of a solid
influences evolution
of gas (water vapor)
Evaporation – Autogeneous Pressure
5.8 mg H2O, sealed
explosion
2.09 mg H2O, 50 µm hole in lid
onset
6.2 mg H2O
no lid
v boiling point of
water is
determined
properly only in
crucible with lid
+ hole in lid!
Stabilization Through Dealumination
v water vapor removes aluminum from zeolite framework
(extra-framework aluminum)
v leads to stabilization
v today, ultrastable Y or USY is obtained through
„steaming“, treatment of NH4-Y 600-800°C in rotary kilns
v USY is used in „fluid catalytic cracking“ and
„hydrocracking“ and „hydroprocessing“
v „steaming“ is a general method for dealumination of
zeolites
Exothermic Reactions: Combustion
v organic matter may be present, e.g. from sol-gel
process, surfactant-assisted synthesis
v will combust upon thermal treatment in oxygencontaining environment
v look at thermochemical data
CRC Handbook of Thermophysical and Thermochemical Data
Eds. David R. Lide, Henry V. Kehiaian, CRC Press Boca Raton New York
1994 FHI library 50 E 55
D'Ans Lax, Taschenbuch für Chemiker und Physiker
Ed. C. Synowietz, Springer Verlag 1983, FHI library 50 E 54
Example: Pentane Combustion
C5H12 (g) + 8 O2 (g) → 5 CO2 (g) + 6 H2O (g)
∆r H
Θ
L
= ∑ νi ∆ f Hi °
i =1
∆ r H Θ = 5 ∆ f H ° CO2 + 6 ∆ f H ° H 2O − 1 ∆ f H ° C5 H12
∆ r H Θ = 5 ( −393.51 kJmol −1 ) + 6 ( −241.82 kJmol −1 ) − 1 (−146.44 kJmol −1 )
∆ r H Θ = 3272.0 kJmol −1
v combustion is strongly exothermic!
v oxygenates have higher enthalpies of formation, i.e.
enthalpy of combustion becomes smaller
Example: Surfactant-Assisted Synthesis
Mesoporous Zirconia
v surfactants (hexadecyl-trimethyl-ammonium bromide)
form micelles
v inorganic matter forms around micelles
Example: TG/DTA Analysis of ZrO2-Precursor
80
H2O loss
60
exo
50
70
combustion
of organics
60
30
endo
20
50
DTA (mV)
TG (mg)
40
10
SO42- decomposition
0
40
200
400
600
800
1000
1200
-10
Temperature (K)
v ZrO2/CTAB composite synthesized with Zr(O-nPr)4 in the
presence of sulfate ions at Zr:S:CTAB = 2:2:1, measured
with 10 K/min in an air stream
Other Exothermic Reactions
v Example: calcination of X-ray amorphous zirconium
hydroxide
v "ZrO2 * 2.5 H2O"
Heating of Zirconium Hydroxide
Sample bed temperature / K
Heating time / min
115 120 125 130 135 140 145 150 155
950
17.1 ml boat
900
2.2 ml
8.4 ml boat
850
800
8.4 ml
2.2 ml boat
750
17.1 ml
700
650
600
550
640
660
680
700
720
Oven temperature / K
740
760
v strong influence of batch size / heat transfer
v rapid overheating (up to 40-50 K/s)
v overshoot of up to 300 K
History of Glow
vOverheating is so violent, it is accompanied by emission of
visible light ("glow")
Berzelius 1812 (antimonates, antimonites)
The Glow Phenomenon
Oxides Showing a Glow
Origin of Glow
v combustion of organic contaminants
v heat of crystallization
v loss of surface energy through sintering
Sample bed temperature / K
Effect of Combustion?
900
Heating time /min
165
170
175
180
Oxygen
880
Argon
Air
860
840
820
800
780
760
780
790
800
810
820
Oven temperature / K
830
840
v atmosphere little influence on overheating effect
v heat not caused by combustion of organic contaminants
Origin of Glow
v combustion of organic contaminants
v heat of crystallization
v loss of surface energy through formation of larger
particles
Heat of Crystallization of ZrO2 (kJ mol-1)
?-ZrO2
any ZrO2
t- or m-ZrO2
t- or m-ZrO2
t-ZrO2
t-ZrO2
t-ZrO2
t-ZrO2
t-ZrO2
m-ZrO2
–28.7
–23.2
–4.3 to –22.5
–12.9
–29.3 to –33.4
–19.6
d
n
:
a
2
–30.1
rO ± 0.8
.3
Z
4
t
n
e
of –53
e
n
o
tw
i
t
e
–13
a
b
iz
l
l
re –58.6 ± 3.3
a
u
t
t
rys
era
c
lit
f
o
o
t
t in surface
a
g energy upon a-ZrO2 → t-ZrO2
e
Change
n
i
h
rd
o
2
-1
c
(assumed
ac ABET = 100 m •g ): +14.6
Transition t-ZrO2 to m-ZrO2:
–5
–6
–6
Keshavaraja
Srinivasan
Chuah
Tatsumiol-1
m
J
Livage
k
.6Mercera
3
5
Haberko
Molodetsky
Xie
Molodetsky
Molodetsky
Molodetsky
Xie
Coughlin
Estimation of Temperature Rise through
Crystallization
v assume a medium heat of 25 kJ mol-1
v process assumed quasi-adiabatic (δQ = 0)
v molar heat of precursor / intermediate assumed similar
to that of ZrO2: 82.3 J mol-1 K-1
∆H
∆T =
≈ 300 K
cp
v corresponds approximately to observation
Does Crystallization Happen During Glow?
v use method that allows structure-determination and good
time resolution
v X-ray absorption spectroscopy at Zr K edge at ESRF
(1 spectrum per s; 10 K min-1 heating rate)
v allows observation of local environment around Zr4+ ions
In situ X-ray Absorption Spectroscopy
v use large pellet to create
a sufficiently large glow
effect
v put small pellet inside that
is transparent for X-rays
5 mm
5mm
Norm. absorption
XAS Spectra
1.0
0.75
0.5
d
on
200
0.25
100
50
0.0
e
,s
150
18
18.25
18.5
18.75
Photon energy [keV]
19
m
Ti
e
c
s
Sample Temperature vs. Time
600
Temperature, °C
550
Heating rate
≈ 50 K s-1
500
450
400
350
110 112 114 116 118 120 122 124 126 128 130
Time, seconds
Structural Evolution
FT(χ(k)*k2)
6.0
4.0
547 °C
499 °C
447 °C
439 °C
436 °C
2.0
0
2
4
R [Å]
6
8
Origin of Glow
v combustion of organic contaminants
v heat of crystallization - yes!
v loss of surface energy through formation of larger
particles
Sintering
v a heat treatment at 2/3 to 3/4 of the melting point to
solidify shaped bodies from pressed metal powders,
occurs in 3 steps
v 1. increase of particle contacts through "sinter bridges"
2. formation of a contiguous backbone, original particles
lose their identity, shrinkage, formation of new grain
boundaries
3. rounding and elimination of pores, further shrinkage,
closed pores
Tammann and Hüttig Temperature
v Tammann temperature
temperature necessary for lattice (bulk) recrystallization
for metal oxides TTammann ≈ 0.52 TF
v Hüttig temperature
temperature necessary for surface recrystallization
for metal oxides THüttig ≈ 0.26 TF
with TF the absolute melting temperature
Influence of Particle Size
v decomposition of Al(OH)3,
thermogravimetric analysis
v smaller particles react at
lower temperature
1 µm
50-80 µm
0.2 µm
Influence of Particle Size on Melting Point of Au
v Melting point
can decrease
drastically with
decreasing
particle size!
Ph. Buffat, J. P. Borel, Phys.
Rev. A 13 (1975) 2287-2298
Surface Energy
v loss of surface area
through formation of
larger crystals
v surface energy of t-ZrO2(101) ≈ 1.1 J m-2
v surface area shrinks from 250 to 150 m2 g-1 during calcination
→ 110 J g-1 or 13.5 kJ mol-1 (ZrO2)
v is not negligible in comparison to crystallization
Formation of a Crystalline Solid from
Amorphous Precursor
ΔH???
v nature of amorphous precursor usually not well-known
v ΔH depends on nature of precursor & product
v if, through variation of the treatment conditions, for the
same precursor different ΔHs are obtained, the products
will be different
Effect of Additives
Probentemperatur
/K /K
Sample
bed temperature
Time
min
Zeit // min
120
1000
130
140
150
160
170
FeSZH
180
190
MnSZH
950
900
850
SZH
ZH
800
750
700
Rampe 3°/min
17,1 ml-Schiffchen
650
600
650
700
750
800
850
/K/K
OvenOfentemperatur
set temperature
v additives shift glow to higher temperatures and reduce
overshoot
Glow Phenomenon: MnSZ and FeSZ
185
190
1000
1000
25 g
25 g
950
900
Temperature / K
Sample temperature / K
165
Heating time / min
170
175
180
12 g
900
12 g
3g
3g
850
800
800
820
840
Oven temperature / K
600
500
400
0
2%FeSZH
780
700
300
2%MnSZH
2%MnSZ
750
800
100
200
300
400
500
Time / min
860
v max. calcination T may be exceeded
v promoters influence calcination chemistry (systemic), Fe and Mn different
v strong batch size dependence
Influence on Catalytic Activity?!
165
170
Zeit / min
175
180
Probentemperatur / K
1000
2%FeSZ
185
190
25 g
25 g
950
12 g
900
12 g
2%MnSZ
3g
3g
850
800
750
Yield i-butane (%)
16
17.1 ml boat
8.4 ml boat
2.2 ml boat
14
12
10
8
6
4
2
800
820
840
Ofentemperatur / K
860
14
Yield i-butane (%)
780
17.1 ml boat
8.4 ml boat
2.2 ml boat
12
10
8
6
4
2
0
0
0
120
240
360
480
Time on stream / min
0
120
240
360
480
Time on stream / min
samples calcined in larger batches are more active (1 vol% n-butane at 338 K)
characterize catalysts
Surface Area & Calcination Batch Size
14
12
Yield i-butane (%)
17.1 ml boat
8.4 ml boat
2.2 ml boat
14
10
8
6
4
120
2
0
0
120
240
360
115
2%FeSZ
2 -1
Time on stream / min
17.1 ml boat
8.4 ml boat
2.2 ml boat
12
10
8
6
4
2
0
FeSZ
MnSZ
480
Surface area / mg
Yield i-butane (%)
16
0
120
240
360
480
Time on stream / min
110
2%MnSZ
105
100
95
90
85
80
0
2
4
6
8
10 12 14 16 18 20
Boat size / ml
v surface area increases with calcination batch size
v differences in activity exceed differences in surface area
Activity and Bulk Structure
17.1 ml boat
8.4 ml boat
2.2 ml boat
12
10
8
6
4
2
16
Yield i-butane (%)
2%MnSZ
Yield i-butane (%)
14
17.1 ml boat
8.4 ml boat
2.2 ml boat
14
12
10
2%FeSZ
8
6
4
2
0
0
0
120
240
360
480
0
240
1.445
1.445
1.444
1.444
1.443
1.443
1.442
1.442
c/a
c/a
120
1.441
1.440
1.439
1.439
0
5
10
Boat size / ml
15
480
1.441
1.440
1.438
360
Time on stream / min
Time on stream / min
20
1.438
0
5
10
Boat size / ml
v lattice parameters of tetragonal ZrO2 change
15
20
3
Adsorbed volume / cm*g
-1
Porosity of Iron-Promoted Sulfated Zirconia
70
2.2 ml boat
8.4 ml boat
17.1 ml boat
60
50
40
30
20
10
0.0
0.2
0.4
0.6
0.8
1.0
p/p0
v large batch calcination: formation of mesopores, 1-4 nm
Effect of Calcination Batch Size
v samples from the same raw material (precursor) are
converted into different products by variation of the batch
size during calcination
Why Does Overheating Occur?
v heat is generated faster than transferred away
v look at heat transfer
Heat Transfer Modes
v all of them play role during calcination
v estimations can be made!
Heat Transfer by Convection
v free vs. forced convection makes a considerable
difference
Thermal Conductivity
v data are for solids,
conduction worse in
loose powders
v exact material during
calcination unknown
ZrO2
Kingery 1955
Radiation
v conduction and convection are proportional to difference
between the temperatures (T gradient , ΔT) of the body
of interest and the surrounding
v radiation is proportional to the difference between the
temperatures to the forth power
IUPAC Recommendations on Calcination
v all particles of catalyst should be subjected (..) to exactly
the same (..) conditions
only possible in moving beds (fluid beds, rotating
furnaces, spray drying)
v supply a sufficient quantity of gas or liquid to the reactor
to ensure complete reaction (..); special consideration
should be given to mass and heat transfer
J. Haber, J.H. Block, B. Delmon, Pure & Appl. Chem. 67 (1995) 1257-1306
Preparation of Supported Catalysts
v goal: disperse an active phase on an inexpensive and
inert (?) support
increase surface area
support
Interaction Between Active Phase and Support
Spreading Mechanisms
v transport via gas phase also possible
Preparation of SCR Catalyst by Solid-Solid-Wetting
v MoO3/TiO2: typical catalyst for selective catalytic reduction
(reaction of NOx with NH3 to give N2 and H2O)
720 K heating in O2,
saturated with H2O
v start out with physical mixture: small particles, intimate
mixture
v evoke spreading through thermal treatment
v analysis of surface composition!
Ion Scattering Spectroscopy
v topmost layer of surface is probed with ion beam (He+)
He+
He+
Ekin (~kV)
E'kin
v also: LEIS = low energy ion scattering
v kinetic energy after interaction depends on mass of scattering
atom
v highly surface sensitive, destructive
Spreading
v Spreading of MoO3
on TiO2 can be
achieved by
thermal treatment
in O2/H2O
Supported Metal Catalysts:
Metal-Support Interaction
Depth Profiling of Supported Noble Metal Catalyst
v Rh/TiO2: a system with strong metal support interaction
(SMSI)
Conclusion
vMany things can happen during a thermal
treatment!

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