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


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)


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


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


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


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


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


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)


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


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


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


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


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


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


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


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


Example: Surfactant-Assisted Synthesis Mesoporous Zirconia

surfactants (hexadecyl-trimethyl-ammonium bromide) form micelles

inorganic matter forms around micelles


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


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!


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


Other Exothermic Reactions

Example: calcination of X-ray amorphous zirconium hydroxide

"ZrO2 * 2.5 H2O"


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


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


115 120 125 130 135 140 145 150 155 Heating time / min


History of Glow

Overheating 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 Heat?

combustion of organic contaminants

heat of crystallization

loss of surface energy through sintering


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


165 170 175 180 Heating time /min


Origin of Heat?


heat of crystallization

loss of surface energy through formation of larger particles


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


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


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


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


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


Origin of Heat


heat of crystallization - yes!

loss of surface energy through formation of larger particles - yes


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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 (%

)


2%MnSZ

characterize catalysts


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


Yiel

d i-b

utan

e (%

)

Time on stream / min0 120 240 360 480

0

2

4

6

8

10

12

14


Yiel

d i-b

utan

e (%

)


2%FeSZ 2%MnSZ


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



0 120 240 360 480

02468

10121416 17.1 ml boat


Yiel

d i-b

utan

e (%

)

Time on stream / min0 120 240 360 480

0

2

4

6

8

10

12

14


Yiel

d i-b

utan

e (%

)


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


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


Why Does Overheating Occur?

heat is generated faster than transferred away

look at heat transfer


Heat Transfer Modes

all of them play role during calcination

estimations can be made!


Heat Transfer by Convection

free vs. forced convection makes a considerable difference


Thermal Conductivity

Kingery 1955

ZrO2

data are for solids, conduction worse in loose powders

exact material during calcination unknown


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


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!


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


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


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.


Influence of Packing

Packing of a solid influences evolution of gas (water vapor)

MS analysism/e = 18 (H2O)


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


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


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


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


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


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

Date post:	09-Sep-2020
Category:	Documents
Upload:	others
View:	1 times
Download:	0 times

MAX-PLANCK-GESELLSCHAFT Thermal Treatment of Catalysts€¦ · Thermal Treatment of Catalysts...

Documents