MAX-PLANCK-GESELLSCHAFT Thermal Treatment of Catalysts€¦ · Thermal Treatment of Catalysts...
Transcript of MAX-PLANCK-GESELLSCHAFT Thermal Treatment of Catalysts€¦ · Thermal Treatment of Catalysts...
Thermal Treatment of Catalysts
Modern Methods in Heterogeneous Catalysis ResearchTU Berlin * HU Berlin * Fritz Haber Institute of the Max Planck Society
December 1, 2006
Friederike Jentoft
MAX-PLANCK-GESELLSCHAFTMAX-PLANCK-GESELLSCHAFT
© F.C. Jentoft FHI Berlin 2006
Outline
1. Terminology (calcination)2. Sample vs. oven set temperature3. Heat effects
CombustionTemplated zirconia catalyst
Crystallization / Loss of surface area Sulfated zirconia catalyst
4. Heat transfer considerations5. Self-generated atmosphere
Self-steaming of zeolites
6. Reactivity of small particles
© F.C. Jentoft FHI Berlin 2006
Steps of Catalyst Preparation
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, deammoniationof zeolites)
© F.C. Jentoft FHI Berlin 2006
Heat Treatment of Intermediate Solids or Precursors
drying
thermal decomposition of salts (nitrates, ammonium salts)
calcination
product is a "reasonably inert solid" which can be stored easily
© F.C. Jentoft FHI Berlin 2006
Origin of the Term "Calcination"
latin "calx" = playstone limestone, (greek chálix)
burning of calcium carbonate (limestone) to calcium oxide (quicklime)
CaCO3 → CaO + CO2 ΔH(900°C)=3010 kJ mol-1
used to construct Giza pyramids (ca. 2800 A.C.), burning of limestone ("Kalkbrennen") mentioned by Cato 184 A.C.
performed in kilns (ovens) at 900°C
addition of air to sustain burner flame + cool product
© F.C. Jentoft FHI Berlin 2006
Examples for Kilns for Calcination
Schematic of a vertical shaft kiln.a) Preheating zone; b) Calcining zone; c) Cooling zone
Schematic of a rotary kiln a) Burner; b) Combustion air; c) Pre-heater; d) Kiln; e) Cooler
© F.C. Jentoft FHI Berlin 2006
General Definition of "Calcination"
decomposition of a substance through heating, transformation in lime-like substance – Dudento 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) – Webstersheating (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 Chemielexikonthe 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
© F.C. Jentoft FHI Berlin 2006
Definition of "Calcination" in Catalysis Research
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 Chemistrya heat treatment of catalyst precursor in an oxidizing atmosphere for a couple of hours - Catalysis from A to Z, Eds. Cornil et al.heating in air or oxygen; the term is most likely to be applied to a step in the preparation of a catalyst - IUPAC Compendium of Chemical Terminology
© F.C. Jentoft FHI Berlin 2006
Definition of "Calcination" in Catalysis Research
In catalysis research usually an oxidizing treatment is meant
when the atmosphere is not specified one can assume it is air but this must not be the case
however, you will find statements such as "calcined in inert atmosphere“
sample-generated atmosphere may be oxidative (nitrate decomposition)
© F.C. Jentoft FHI Berlin 2006
Annealing
in a general meaning: a heating of a material over a long time span; strain and cracks in a crystalline solid can be removed
© F.C. Jentoft FHI Berlin 2006
Example from Patent Literature
extremely vague! unimportant?
no, there is a secret to it!
often only temperature + holding time given
E.J. Hollstein, J.T. Wei, C.-Y. Hsu, US Patent 4,918,041
© F.C. Jentoft FHI Berlin 2006
Calcination Procedure: Temperature Program
heating rate, holding time, cooling rate
0 100 200 300 400 500300
400
500
600
700
800
900
Tem
pera
ture
/ K
Time / min
cooling usually uncontrolled below certain T, slower
© F.C. Jentoft FHI Berlin 2006
Actual Temperature Program
oven may not be able to perform selected program (lack in power): temperature lag of actual temperature behind set temperature
poorly tuned controller may give temperature oscillations
heat needs to be transferred from oven to sample, needs a gradient: temperature lag of sample temperature vs. oven temperature
© F.C. Jentoft FHI Berlin 2006
Role of Sample
strongly endo- or exothermic events may interfere with the heating program
endothermic events: solvent evaporation
exothermic events: combustion of organics, crystallization, loss of surface area
© F.C. Jentoft FHI Berlin 2006
Evaporation of Water: Thermal Effects
example: a 10 g sample containing 18 % water (0.1 mol)
ΔHevap(H2O, 373 K) = 41 kJ mol-1
to evaporate in 1 minute: ≈ 70 Watt
© F.C. Jentoft FHI Berlin 2006
Example: Calcination of Zirconium Hydroxide
with a 10 g sample, deviations can occur already at moderate heating rates
0.0 0.5 1.0 1.5 2.0 2.5 3.0
300
400
500
600
70010 K min-1
1 K min-1
3 K min-1
Sam
ple
bed
tem
pera
ture
/ K
Time / h0.0 0.5 1.0 1.5 2.0 2.5 3.0
300
400
500
600
70010 K min-1
1 K min-1
3 K min-1
Sam
ple
bed
tem
pera
ture
/ K
Time / h
© F.C. Jentoft FHI Berlin 2006
Exothermic Reactions: Combustion
organic matter may be present, e.g. from sol-gel process, surfactant-assisted synthesis
will combust upon thermal treatment in oxygen-containing environment
look at thermochemical dataCRC Handbook of Thermophysical and Thermochemical DataEds. David R. Lide, Henry V. Kehiaian, CRC Press Boca Raton New York 1994 FHI library 50 E 55D'Ans Lax, Taschenbuch für Chemiker und PhysikerEd. C. Synowietz, Springer Verlag 1983, FHI library 50 E 54
© F.C. Jentoft FHI Berlin 2006
Example: Surfactant-Assisted Synthesis Mesoporous Zirconia
surfactants (hexadecyl-trimethyl-ammonium bromide) form micelles
inorganic matter forms around micelles
© F.C. Jentoft FHI Berlin 2006
Example: TG/DTA Analysis of ZrO2-Precursor
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
200 400 600 800 1000 1200
40
50
60
70
80
endo
exo
TG (m
g)
Temperature (K)
-10
0
10
20
30
40
50
60
DTA
(mV
)
200 400 600 800 1000 1200
40
50
60
70
80
endo
exo
TG (m
g)
Temperature (K)
-10
0
10
20
30
40
50
60
DTA
(mV
)
H2O loss
SO42- decomposition
combustion of organics
© F.C. Jentoft FHI Berlin 2006
Example: Pentane Combustion
C5H12 (g) + 8 O2 (g) → 5 CO2 (g) + 6 H2O (g)
∑=
Θ °Δ=ΔL
iifir HH
1ν
12522165 HCfOHfCOfr HHHH °Δ−°Δ+°Δ=Δ Θ
)44.146(1)82.241(6)51.393(5 111 −−−Θ −−−+−=Δ kJmolkJmolkJmolHr
10.3272 −Θ −=Δ kJmolHr
combustion is strongly exothermic!
© F.C. Jentoft FHI Berlin 2006
Calculated Heat from Surfactant Combustion
Assume a 10 g sample with 10 % organic matter
Heat per g pentane combusted: 45 kJ
Molar heat of capacity of precursor / intermediate assumed similar to that of ZrO2, which is 0.744 J g-1 K-1
© F.C. Jentoft FHI Berlin 2006
Other Exothermic Reactions
Example: calcination of X-ray amorphous zirconium hydroxide
"ZrO2 * 2.5 H2O"
© F.C. Jentoft FHI Berlin 2006
640 660 680 700 720 740 760550
600
650
700
750
800
850
900
950
Sam
ple
bed
tem
pera
ture
/ K
Oven temperature / K
115 120 125 130 135 140 145 150 155 Heating time / min
Heating of Zirconium Hydroxide
640 660 680 700 720 740 760550
600
650
700
750
800
850
900
950
2.2 ml boat
Sam
ple
bed
tem
pera
ture
/ K
Oven temperature / K
115 120 125 130 135 140 145 150 155 Heating time / min
2.2 ml
8.4 ml
17.1 ml
strong influence of batch size / heat transfer
rapid overheating (up to 40-50 K/s)
overshoot of up to 300 K
640 660 680 700 720 740 760550
600
650
700
750
800
850
900
950
2.2 ml boat
8.4 ml boat
17.1 ml boat
Sam
ple
bed
tem
pera
ture
/ K
Oven temperature / K
115 120 125 130 135 140 145 150 155 Heating time / min
© F.C. Jentoft FHI Berlin 2006
History of Glow
Overheating is so violent, it is accompanied by emission of
visible light ("glow")Berzelius 1812 (antimonates, antimonites)
© F.C. Jentoft FHI Berlin 2006
The Glow Phenomenon
© F.C. Jentoft FHI Berlin 2006
Oxides Showing a Glow
© F.C. Jentoft FHI Berlin 2006
Origin of Heat?
combustion of organic contaminants
heat of crystallization
loss of surface energy through sintering
© F.C. Jentoft FHI Berlin 2006
Effect of Combustion?
atmosphere little influence on overheating effect
heat not caused by combustion of organic contaminants
780 790 800 810 820 830 840
760
780
800
820
840
860
880
900Oxygen
AirArgon
Sam
ple
bed
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 Heating time /min
© F.C. Jentoft FHI Berlin 2006
Origin of Heat?
combustion of organic contaminants
heat of crystallization
loss of surface energy through formation of larger particles
© F.C. Jentoft FHI Berlin 2006
Estimation of OverheatingHeat of crystallization of t-ZrO2 (kJ·mol-1 )
29.3 to 33.4 Livage, 1968
30.1 ± 0.8 Haberko et al., 1975
23.2 Srinivasan et al., 1988
19.6 Mercera et al., 1990
12.9 Tatsumi et al., 1996
4.3 to 22.5 Chuah et al., 1998
53 Molodetsky, 2000
© F.C. Jentoft FHI Berlin 2006
Estimation of Temperature Rise through Crystallization
assume a medium heat Q (=-ΔH) of 25 kJ mol-1
process assumed quasi-adiabatic (δQ = 0)
molar heat of precursor / intermediate assumed similar to that of ZrO2, which is 77.66 J mol-1 K-1 at 700 K
KcQT
p
320≈=Δ
corresponds approximately to observation
© F.C. Jentoft FHI Berlin 2006
Surface Energy
surface energy of t-ZrO2(101) ≈ 1.1 J m-2
surface area shrinks from 250 to 150 m2 g-1 during calcination → heat of 110 J g-1 or 13.5 kJ mol-1 (ZrO2)
is not negligible in comparison to crystallization
loss of surface area through formation of larger crystals
© F.C. Jentoft FHI Berlin 2006
Causes of Glow? Example Zirconium Hydroxide
100 110 120 130 140 150 160 170 180
600
700
800
900 Oven set T
Tem
pera
ture
/ K
Time / min
"before"
"after"
20 30 40 50 60 70 80 900.00.20.40.60.81.0
MM
T
T
T
M
Nor
mal
ized
inte
nsity
2 Theta (°)
M
T
0.20.40.60.81.0
T
"after"
"before"
Crystallization starts before T overshoot and continues throughoutBET surface area “before”: 244 m2/g
“after”: 122 m2/g
© F.C. Jentoft FHI Berlin 2006
Estimation of Overheating
"there is no zirconium hydroxide"Riedel, Anorganische Chemie, deGruyter 2002, p. 776
an undefined initial and an undefined final state
Heat of crystallization of t-ZrO2 (kJ·mol-1 )
29.3 to 33.4 Livage, 1968
30.1 ± 0.8 Haberko et al., 1975
23.2 Srinivasan et al., 1988
19.6 Mercera et al., 1990
12.9 Tatsumi et al., 1996
4.3 to 22.5 Chuah et al., 1998
53 Molodetsky, 2000
precursor
cryst. oxide
ΔHcryst
Reaction coordinate
E
© F.C. Jentoft FHI Berlin 2006
Origin of Heat
combustion of organic contaminants
heat of crystallization - yes!
loss of surface energy through formation of larger particles - yes
© F.C. Jentoft FHI Berlin 2006
Effect of Additives
additives shift glow to higher temperatures and reduce overshoot
650 700 750 800 850600
650
700
750
800
850
900
950
1000
Rampe 3°/min17,1 ml-Schiffchen
ZH
Prob
ente
mpe
ratu
r / K
Ofentemperatur / K
120 130 140 150 160 170 180 190Zeit / min
650 700 750 800 850600
650
700
750
800
850
900
950
1000
Rampe 3°/min17,1 ml-Schiffchen
SZH
ZH
Prob
ente
mpe
ratu
r / K
Ofentemperatur / K
120 130 140 150 160 170 180 190Zeit / min
650 700 750 800 850600
650
700
750
800
850
900
950
1000
Rampe 3°/min17,1 ml-Schiffchen
FeSZH
SZH
ZH
Prob
ente
mpe
ratu
r / K
Ofentemperatur / K
120 130 140 150 160 170 180 190Zeit / min
650 700 750 800 850600
650
700
750
800
850
900
950
1000
Rampe 3°/min17,1 ml-Schiffchen
MnSZHFeSZH
SZH
ZH
Prob
ente
mpe
ratu
r / K
Ofentemperatur / K
120 130 140 150 160 170 180 190Zeit / minTime / min
Oven set temperature / K
Sam
ple
bed
tem
pera
ture
/ K
© F.C. Jentoft FHI Berlin 2006
Effect of the Calcined Amount: MnSZ and FeSZ
780 800 820 840 860
750
800
850
900
950
1000
Sam
ple
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 185 190Heating time / min
780 800 820 840 860
750
800
850
900
950
1000
2%MnSZ
Sam
ple
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 185 190Heating time / min
780 800 820 840 860
750
800
850
900
950
1000
2%MnSZ
25 g
12 g
3 g
Sam
ple
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 185 190Heating time / min
780 800 820 840 860
750
800
850
900
950
1000
2%FeSZ
2%MnSZ
25 g
12 g
3 g3 g12 g
25 g
Sam
ple
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 185 190Heating time / min
2.2 ml2.2 ml
8.4 ml
17.1 ml
Promoters: influence calcination chemistry, Fe different than Mn
Strong effect of batch size
Planned Tmax may be exceeded
© F.C. Jentoft FHI Berlin 2006
Effect of the Calcined Amount: MnSZ and FeSZ
780 800 820 840 860
750
800
850
900
950
1000
Sam
ple
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 185 190Heating time / min
780 800 820 840 860
750
800
850
900
950
1000
2%MnSZ
Sam
ple
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 185 190Heating time / min
780 800 820 840 860
750
800
850
900
950
1000
2%MnSZ
25 g
12 g
3 g
Sam
ple
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 185 190Heating time / min
780 800 820 840 860
750
800
850
900
950
1000
2%FeSZ
2%MnSZ
25 g
12 g
3 g3 g12 g
25 g
Sam
ple
tem
pera
ture
/ K
Oven temperature / K
165 170 175 180 185 190Heating time / min
0 100 200 300 400 500300
400
500
600
700
800
900
1000
Tem
pera
ture
/ K
Time / min
Promoters: influence calcination chemistry, Fe different than Mn
Strong effect of batch size
Planned Tmax may be exceeded
© F.C. Jentoft FHI Berlin 2006
samples calcined in larger batches are more active (1 vol% n-butane at 338 K)
Influence on Catalytic Activity?!
780 800 820 840 860
750
800
850
900
950
1000 25 g
12 g
3 g3 g12 g
25 g
Prob
ente
mpe
ratu
r / K
Ofentemperatur / K
165 170 175 180 185 190Zeit / min
0 120 240 360 480
02468
10121416 17.1 ml boat
8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min
2%FeSZ
0 120 240 360 480
0
2
4
6
8
10
12
14
17.1 ml boat 8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min
2%MnSZ
characterize catalysts
© F.C. Jentoft FHI Berlin 2006
Surface Area & Calcination Batch Size
surface area increases with calcination batch size
differences in activity exceed differences in surface area
0 2 4 6 8 10 12 14 16 18 2080
85
90
95
100
105
110
115
120
FeSZ MnSZ
Surf
ace
area
/ m
2 g-1
Boat size / ml
0 120 240 360 480
02468
10121416 17.1 ml boat
8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min0 120 240 360 480
0
2
4
6
8
10
12
14
17.1 ml boat 8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min
2%FeSZ 2%MnSZ
© F.C. Jentoft FHI Berlin 2006
Porosity of Iron-Promoted Sulfated Zirconia
large batch calcination: formation of mesopores, 1-4 nm
0.0 0.2 0.4 0.6 0.8 1.010
20
30
40
50
60
70
p/p0
Ads
orbe
d vo
lum
e / c
m3 *g
-1
2.2 ml boat 8.4 ml boat 17.1 ml boat
© F.C. Jentoft FHI Berlin 2006
0 120 240 360 480
02468
10121416 17.1 ml boat
8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min0 120 240 360 480
0
2
4
6
8
10
12
14
17.1 ml boat 8.4 ml boat 2.2 ml boat
Yiel
d i-b
utan
e (%
)
Time on stream / min
Bulk Structure
lattice parameters of tetragonal ZrO2 change
0 5 10 15 201.438
1.439
1.440
1.441
1.442
1.443
1.444
1.445
c/a
Boat size / ml0 5 10 15 20
1.438
1.439
1.440
1.441
1.442
1.443
1.444
1.445
c/a
Boat size / ml
2%MnSZ 2%FeSZ
© F.C. Jentoft FHI Berlin 2006
Effect of Calcination Batch Size
samples from the same raw material (precursor) are converted into different products by variation of the batch size during calcination
© F.C. Jentoft FHI Berlin 2006
Why Does Overheating Occur?
heat is generated faster than transferred away
look at heat transfer
© F.C. Jentoft FHI Berlin 2006
Heat Transfer Modes
all of them play role during calcination
estimations can be made!
© F.C. Jentoft FHI Berlin 2006
Heat Transfer by Convection
free vs. forced convection makes a considerable difference
© F.C. Jentoft FHI Berlin 2006
Thermal Conductivity
Kingery 1955
ZrO2
data are for solids, conduction worse in loose powders
exact material during calcination unknown
© F.C. Jentoft FHI Berlin 2006
Radiation
conduction and convection are proportional to difference between the temperatures (T gradient , ΔT) of the body of interest and the surrounding
radiation is proportional to the difference between the temperatures to the forth power
© F.C. Jentoft FHI Berlin 2006
Self-Generated Atmosphere
example: a 10 g sample containing 18 % water (0.1 mol)
at 400 K: corresponds to ≈ 3.3 l of water vapor
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!
© F.C. Jentoft FHI Berlin 2006
Zeolite Y as a Cracking Catalyst
zeolite Y is used as a cracking catalyst (FCC)
first synthesize NaY, then exchange Na+ by NH4+ (liquid
phase)
obtain active HY through thermal decomposition of NH4Y
regular HY not very stable
Faujasite structure
© F.C. Jentoft FHI Berlin 2006
Ultrastable Y zeolite
McDaniel and Maher 1967 report „new ultra-stable form of faujasite“
worked with 100 g of zeolite, first exchange then heat treatment
„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
0 100 200 300 400 500300
400
500
600
700
800
900
Tem
pera
ture
/ K
Time / min
© F.C. Jentoft FHI Berlin 2006
Ultrastable Y Zeolite
Kerr 1967: treatment of HY at 700-800°C in inert static atmosphere
„any technique keeping this water in the system during the heating process will result in a stable product“
published comparison of heating in „deep bed“ or „shallow bed“: deep bed produces stable product
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.
© F.C. Jentoft FHI Berlin 2006
Influence of Packing
Packing of a solid influences evolution of gas (water vapor)
MS analysism/e = 18 (H2O)
© F.C. Jentoft FHI Berlin 2006
Evaporation – Autogeneous Pressure
boiling point of water is determined properly only in crucible with lid + hole in lid!
6.2 mg H2Ono lid
2.09 mg H2O, 50 µm hole in lidexplosion
onset
5.8 mg H2O, sealed
© F.C. Jentoft FHI Berlin 2006
Stabilization Through Dealumination
water vapor removes aluminum from zeolite framework (extra-framework aluminum)
leads to stabilization
today, ultrastable Y or USY is obtained through „steaming“, treatment of NH4-Y 600-800°C in rotary kilns
USY is used in „fluid catalytic cracking“ and „hydrocracking“ and „hydroprocessing“
„steaming“ is a general method for dealumination of zeolites
© F.C. Jentoft FHI Berlin 2006
IUPAC Recommendations on Calcination
all particles of catalyst should be subjected (..) to exactly the same (..) conditionsonly possible in moving beds (fluid beds, rotating furnaces, spray drying)
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
© F.C. Jentoft FHI Berlin 2006
Tammann and Hüttig Temperature
Tammann temperaturetemperature necessary for lattice (bulk) recrystallizationfor metal oxides TTammann ≈ 0.52 TF
Hüttig temperaturetemperature necessary for surface recrystallizationfor metal oxides THüttig ≈ 0.26 TF
with TF the absolute melting temperature
© F.C. Jentoft FHI Berlin 2006
Influence of Particle Size
smaller particles react at lower temperature
50-80 µm
1 µm
0.2 µm
decomposition of Al(OH)3,
thermogravimetric analysis
© F.C. Jentoft FHI Berlin 2006
Influence of Particle Size on Melting Point of Au
Melting point can decrease drastically with decreasing particle size!
Ph. Buffat, J. P. Borel, Phys. Rev. A 13 (1975) 2287-2298