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APPLICATION OF CHEMICAL KINETICS IN THE HETEROGENEOUS CATALYSIS STUDIES

L. A. PETROVSABIC Chair in Heterogeneous Catalysis

Chemical and Materials Engineering DepartmentCollege of Engineering, King Abdulaziz University, Jeddah

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Kinetics of heterogeneous catalytic reactions is indispensable part of the complex study of the properties and behavior of any catalytic system i.e. catalyst plus reaction media

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Main tasks of kinetics of heterogeneous catalytic reactions

1. Kinetics and mechanism of important in theoretical aspect catalytic processes.

2. Development of the theory of kinetics.

3. Development of kinetic models of non-stationary catalytic processes.

4. Creation of kinetic models of industrially important reactions.

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5. The kinetics of the processes, occurring during the preparation, deactivation and regeneration of the industrial catalysts.

6. Theory and automation of kinetic experiments.

7. The elaboration of laboratory methods for testing and controlling the catalytic activity and selectivity of the industrial catalysts.

8. Development of large scale tests of the catalytic properties of the obtained catalysts.

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Building of kinetic models

Formulation of the possible reaction mechanisms

1. Data from literature

2. Chemical and physical adsorption measurements

3. Kinetic experimental methods a) Steady state experimentsb) Non-steady state experiments

4. Physical methods for catalyst characterization.

5. Chemical methods for catalyst characterization.

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Deriving corresponding to the proposed reaction mechanisms kinetic steady state models

1. Method of Hougen-Watson2. Method of Temkin for stationary heterogeneous

complex catalytic reactions3. Method of graph theory4. Method of group theory5. Any other method

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Mathematical treatment of kinetic experimental data

1. Design of kinetic experiments2. Calculating rates on the independent reactionroutes3. Estimation of the number of independent parameters for a given kinetic model4. Kinetic parameters estimation5. Statistical assessment of the best reaction models

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The effective industrial catalysts are products, which possess very special complex of different properties

1. High catalytic activity;2. High selectivity;3. Proper pore structure;4. High resistance to deactivation;5. High resistance to catalytic poisons;6. Easy regeneration;7. Long life time;

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8. Low operational and light-off temperature;9. High thermal stability;10. High thermal conductivity;11. High mechanical strength;12. High resistance to attrition;13. Low price.

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Industrial catalysts are “performance chemicals”which should be offered on the market

together with information about:

1. Reaction kinetics and kinetic model; 2. Catalytic activity and selectivity; 3. Catalyst pre-treatment regimes; 4. Catalyst deactivation kinetics with respect to

different catalytic poisons; 5. Catalyst regeneration regimes;

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6. Catalyst lifetime: stability, duration of operation, thermal stability; 7. Physical and mechanical properties: strength, abrasion ability, hardness, surface area; 8. Hydrodynamic characteristics of the catalyst grain and of the catalyst bed in the reactor 9. Safety transition regimes in cases of industrial accident; 10. Economy of the process.

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The steady state regimes

The steady state proceeding of heterogeneous catalytic process is realized only in open systems.

The steady state regime means that all reaction parameters (concentrations of reagents and ISC, temperatures, partial and total pressure, regents flow rate) should have constant values which do not change with time. This however does not mean that the parameters should have the same value at different points of the reaction space.

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F(cj, xk) is the function which allow us to express the concentrations of ISC via the concentrations of the reagents, j - number of reagents in the system, i - number of ISC formed on the catalyst surface.At steady state regime

0=dt

dX i 0),( =∑ Kj XcF

( )Kji XcF

dtdX ,=

For every ISC we can write

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∑⟨⟨i i

i

i

i

rXdt

dXX '

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Conditions of Frank-Kamenetzki

1). The rate of formation of the ith ISC from all possible reactions should be smaller than the rate of its consumption along all possible reactions.

2). The life-time of the ith ISC should nearly equal to the time necessary to reach steady state τo and smaller than the time of reaction τr

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,111rrr σ=− −+

1

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σ

−+ −= rrr

∑=

−+ =−=P

p

PP

sSSSRrrr

1

)()(σ

3). The reaction rates of all consecutive elementary steps should be approximately equal to each other and equal to the slowest one called limiting reaction step. For the each step we can write

s

n

sts

n

rrtt XX ∑∑

−

=

−

==

1

1,

1

1, σσ

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Kinetic models of non-stationary catalytic processesshould account for the following factors:

- The rates of the elementary chemical reactions,

- The rates of changing of reactant composition,

- The rates of changing of the activity of the catalyst

- Diffusion of the reacting species in the catalystpores, and etc.

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),,,( TxcfArc

αrr=

),,,(2

Txcfdtda αrr=

),,,(1

TxcfAdtdx

xαrr=

Catalytic processes under non-steady state regime

║A║c - is stoichiometric matrix for reagents;

║A║x is stoichiometric matrix for intermediate surface compounds (ISC)

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The reasons for the deviation from steady state regimes are numerous: presence of catalyst redox cycle, presence in the reaction mechanism a non-linear elementary steps, changes of the reaction mechanism due to changes of the degree of conversion, catalyst re-crystallisation, poisoning, deactivation,etc.

Most of these factors acts spontaneously and are part of the properties of the system catalyst-reaction media and as a result different phenomena are observed like multiple steady states, oscillating reactions, chaotic behaviour, heat explosion, etc

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Use of Kinetics Models (in %)

115275525Engineering

10471413164Catalyst producer

2283327107Oil

8222729148Chemical

Mechanism studies

Catalyst develop-

ment

Process optimi-sation

Process develop-

ment

Trouble-shouting

Number of companies

Type of company

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Design of catalysts pellets

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Influence of diffusion on the catalysts performance

External diffusion regime

oeffo

c

o

c

ced CkC

kk

Ckk

kkr .111.

=+

=+

=

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Internal diffusion regime

MT

SDD

pgmm

KeffK ρτ

θτθ

..319400. 2

, ==pgg

g

e SSV

rρθ.

22==

effeffKeff DDD ,12,

111+=

D12,eff = D12.θ/τ

)31

31(1

φφφη −=

tgh eff

n

s

DCkR

1. −

=φeffs DC

rH.

.γ=

n

SCkr ..η=

232.503.20Ar - H24333.242.23Ar - H23535.261.13Ar - H2293BNTKred

4.504.71H2 - Ar4336.843.13H2 - Ar35312.801.41H2 - Ar293BNTKred

5.420.55O2 - Ar4338.120.48O2 - Ar35313.950.42O2 - Ar293BNTKoxid

1.573.12Ar - H24331.982.16Ar - H23533.671.25Ar - H2293M-19red

4.703.72H2 - Ar4335.172.91H2 - Ar3537.021.85H2 - Ar293M-19red

5.230.49O2 - Ar4336.000.41O2 - Ar3539.290.36O2 - Ar293M-19oxid

Tortuosity factor, τDeff.x102 cm2/sGas pairT, KCatalyst

Effective diffusion coefficients Deff and tortuosity factor τ obtained from the dynamic method of Wicke-Kallenbach for oxide and reduced forms of the M-19 and WGSR catalysts.

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Influence of the reduction on the values of Deff (in cm2/s) obtained from chromatographic and Wicke-Kallenbachmethods for M-19 and BNTK catalysts

7183.950.555762.890.504335452.670.494672.010.433533001.260.422761.050.38293BNTK6983.420.495302.440.464336172.530.414661.820.393534301.550.362770.860.31293M-19

Change %

Change %

MWKCMDTemperature K

Catalystoxy

effD oxy

effDred

effD red

effD

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The tortuosity factor τ and equivalent pore radius reobtained from the chromatographic method for oxide and reduced forms of the M-19 and WGSR catalysts.

3.283.053.222.402.252.31Equivalent pore radius re.10-7 cm

4.236.558.743.895.217.04Tortuosity factor, τ433353293433353293Temperature, K

BNTKM 19CatalystsParameters

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Catalyst deactivation

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Diffusion effects in processes accompanied by catalyst deactivation

1.Changes in the reaction rate caused by diffusion restrictions;

2.Diffusion modified deactivation caused by diffusion restrictions;

3.Diffusion modified deactivation influence on the main reaction

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Types of Deactivation1. Sintering2. Poisoning3. Changes of catalytic activity due to interactions

with reagents:a. Strong and irreversible adsorption of some

reagents;b. Interaction of reactants with catalytic centers;c. Induced diffusion of lattice components towards

catalyst surface;d. Formation of coke precursors blocking the

catalyst surface.

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Classification of mechanisms of deactivation.Parallel deactivation

slow stepA + Z = [AZ] ======== [BZ] ↔ B + Z

↓[PZ]―――→coke

Consecutive deactivationslow step

A + Z = [AZ] ======== [BZ] ↔ B + Z↓

[PZ]―――→coke

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Kinetic description of deactivationInseparable deactivationCoke formation is inseparable part of the reaction mechanism. This process is described by special term in the kinetic equation for reaction of paraffins dehydrogenation.

( )0*

211

1

cckPkPkPr

−++= σ

P1 – paraffins; P2 – reaction product; σ – reaction reversibility; c – coke concentration, co – threshold coke concentration; k – rate constant; k1 – olefins adsorption constant, k* - rate constant of coke formation.

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Separable deactivation Coke formation is described by rate model whichis uncoupled from rate equation describing main reaction.

( ) ( )( )τ

φ TPkTPkrriddikin,,.,,=

rkin(k, Pi, T) – kinetic model of main reactionat constant catalyst activity;

( )TPk idd ,,φ kinetic model of the reaction of catalyst deactivation.

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⎠

⎞⎜⎝

⎛=

)()(

)()(

0

0

tata

rr

trtr

K

d

k

d

k

d if t → 0 then ad and ak → 1

0)()(

→

⎟⎠

⎞⎜⎝

⎛≠

tk

d

k

d

aa

taTa

Diffusion modified deactivation caused by diffusion restrictions

( ) ( )trtr kd ⟩

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xPQxkP

r k0

0

1)1(

α++−

=

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

−++

++

+++

=Φ )exp(.11 0

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0

320 QQ

xPQxkk

xPQxkk

Pk µαα

3/2CQ =

basic reaction rate under kinetic control

Deactivation function

C – coke deposited per gram-catalyst.

Dehydrogenation of i-pentenes

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Decomposition of 4,4-dimetildioxan-1,3

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Reaction mechanism

1) C6H5NO2 + [K] = [C6H5NO2K] 2) [C6H5NO2K] + H2 → [C6H5NOK] + H2O3) [C6H5NOK] + H2 → [C6H5NHOHK]4) [C6H5NHOHK] + H2 → [C6H5NH2K] + H2O5) [C6H5NH2K] = C6H5NH2 + [K] 6) [C6H5NO2K] + [C6H5NHOHK] → [RK] + nH2

7) n[RK] → coke

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Kinetic model of the reaction in the absenceof diffusion limitations and deactivation

Nb

HNb

kin PkPPk

r2

1

12

+=

k1= 2,4.103, E1 = 43300 J/molk2= 6,8.10-4, E2 = 34500 J/mol.

The mean deviation between experimental calculated rates was 20,3%.

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Kinetic model of nitrobenzene hydrogenation to aniline considering the catalyst deactivation in

the absence of diffusion limitations

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟⎟

⎠

⎞⎜⎜⎝

⎛−

+=

0

*1

2

1)(

222 11

)(Nb

H

Nb

H

Nb

HNbkindeac P

PPP

kPkPPk

trτ

k1* = 10,34

Mean deviation between experimental and calculated rates is 13,7%.

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Kinetic model of nitrobenzene hydrogenation to aniline in presence of diffusion limitations

( )5.0

2

2

1 )1ln(..2

)( 2 ⎟⎠

⎞⎜⎝

⎛+−=

NbNb

Heff

eff

difPkP

kPD

Rktr

Mean deviation between experimental and calculated rates is 11,8%.

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Kinetic model of the reaction of nitrobenzene hydrogenation to aniline with diffusion limitations

and catalyst deactivation

[ ] ZPkPk

PDRktr

NbNb

Heff

eff

deacdif.)1ln(

.).(2)( 5.0

)(2)(

5.0

2

1

.

2

ττ

τ +−⎟⎠

⎞⎜⎝

⎛=

⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

0)(

)(* 11Nb

Nb

PP

KZ τ

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Prognosis of catalyst life time

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1.Process of gas phase hydrogenation of nitroben-zene to aniline on copper catalysts M-19 produced by Neftochim AD. Reactors used: 1.1. Laboratory glass reactor containing 1 g catalyst

with grain size 0.1 mm, 1.2. Hungarian made pilot system OL-105/01

containing 5 g catalyst with grain size 1 mm, 1.3. Pilot plant reactor containing 2 kg industrial

pellets with size 6x6, 1.4. Pilot plant reactor containing 20 kg industrial

pellets with size 6x6, 1.5. Industrial unit for aniline production with four

packed bed reactors arranged in series. Each reactor contains 4000 kg catalyst with pellet size 6x6.

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2. Process of gas phase crotonealdehyde hydroge-nation to butanol on kieselguhr-supported copper catalyst produced by Neftochim AD. Reactors used:

2.1. Laboratory glass reactor containing 1 g catalyst with grain size 0.1 mm,

2.2. Hungarian made pilot system OL-105/01 containing 5g catalyst with grain size 1 mm,

2.3. Industrial unit for butanol production with four tubular reactors arranged in parallel. Each reactor contains 4000 kg catalyst with pellet size 6x6.

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3. Liquid phase hydrogenation of crotonealdehyde to butiraldehyde on alumina supported nickel catalyst produced by Neftochim AD. Reactors used:

(i) Laboratory liquid phase glass reactor containing 1g catalyst with grain size 0.01 mm,

(ii) Industrial unit for butyraldehyde production with four liquid phase reactors arranged in parallel. Each reactor contains 150 kg powder catalyst with particle size 0.01mm.

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4. Liquid phase gasoline sweetening process. Used catalyst sulfophtalocyanine produced in Neftochim AD. Reactors used:

4.1 Laboratory trickle-bed liquid phase metal reactor containing 1 g catalyst with grain size 0.6-1 mm,

4.2. Pilot plant trickle-bed reactor containing 5 kg catalyst with particle size 0.5-2 mm and 8 atm. working pressure

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Reaction in presence of selectivity promotor

Ethylene oxidation

1) 2C2H4 + O2 = 2C2H4O2) C2H4 + 3O2 = 2CO2 + 2H2O

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EthyleneO

EthyleneO

PkPkPPk

R32

1

2

2

1)1(

++=

EthyleneO

EthyleneO

PkPkPPk

R32

4

2

2

1)2(

++=

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1

)2()1()1(

kkk

RRRS

m +=

+=

100..2.5

.6

2OEO

EO

CCCS

∆+∆∆

=

2

).17.0(83.0

1

1

4

O

Ethylene

FF

kk

S++

=

Selectivity on ethylene oxide production

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bCaSDHE+= .

bS =at CDCE = 0

Ethylene inhibited oxidation

a – sensitivity of selectivity to DCE concentration;b – selectivity without presence of DCE

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Sensitivity of selectivity of ethylene oxidation on DCE concentration

1.322.000.942:1

1.230.900.501:1

0.870.580.341:2

292264240

Catalyst bed temperature, oCFeed ratio O2 : C2H4

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EthyleneO

DCEEthyleneOEthyleneO

PkPkPPPkPPk

R65

19.0

21

2

22

1)1(

++−

=

EthyleneO

DCEEthyleneOEthyleneO

PkPkPPPkPPk

R65

07.0

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2

22

1)2(

++−

=

)()( 07.0

43

19.0

21

19.0

21

DCEDCE

DCE

m PkkPkkPkkS

−+−−

=

))(6)(5(

)(607.0

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19.0

21

19.0

21

22

DCE

O

Ethylene

DCE

O

Ethylene

DCE

m

PkkF

FPkk

FF

PkkS−+−+

−=

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Catalyst deactivation

unpromoted

promoted

RR

A)1()1(

1=

unpromoted

promoted

RR

A)2()2(

2=

19.0

1

2

11

DCEP

kkA −= 07.0

3

4

21

DCEP

kkA −=

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DCE critical concentrations and partial pressures

300224192Complete oxidation of ethylene oxide

413.93.10-5656.18.10-5999.44.10-5Complete oxidation of ethylene

1201.15.10-41601.53.10-420822Selective oxidation of ethylene

CDCE, ppm

PDCE, atm

CDCE, ppm

PDCE, atm

CDCE, ppm

PDCE, atm

292292264264240240Temperature°C

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Ethylene oxidation. Influence of water vapor

A concentration of 0.2-16 % of water in the feed inhibits the epoxidation of ethylene to ethylene oxide and promotes the process of complete oxidations

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Ethylene oxidation. Influence of carbon dioxide

22

222

765

21

1)1(

COEthyleneO

COEthyleneOEthyleneO

PkPkPkPPPkPPk

R+++

−=

22

222

765

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1)2(

COEthyleneO

COEthyleneOEthyleneO

PkPkPkPPPkPPk

R+++

−=

A concentration of 12-33 % of carbon dioxide in the feed inhibits both the epoxidation of еthyleneto ethylene oxide and complete oxidation, but the second effect is much stronger, which means that it promotes the selectivity to ethylene oxide.

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Ethylene oxide oxidation

( )25.0

54

5.0

32

1

222

2

1)1(

−++++=

OEOEOOO

EOO

PPkPkPkPkPPk

R

( )25.0

54

5.0

32

6.0

21

222

22

1)2(

−++++−

=OEOEOOO

DCEEOOEOO

PPkPkPkPkPPPkPPk

R

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CONCLUSIONS

1. Detailed study of the kinetics is expensive and time consuming process and should be done on the catalysts that have already passed other chemical, physical, mechanical and physico-chemical tests.

2. Reliable and precise data can be obtained only if the laboratory catalytic reactors are properly selected and analytical methods used for are fast, precise, and reliable.

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3. Interpretation of the results from kinetic studies should be done always taking into account the results obtained from the application of other characterization techniques - chemical, physical, morphological and mechanical. Very important in this sense is the opportunity to combine catalytic activity measurements with application “in situ” of some physical methods by which the additional information about the catalyst changes during the catalytic runs can be obtained.

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4. Heat and mass transfer processes always accompany heterogeneous catalytic reactions. It is very important to properly evaluate the heat and mass transfer effect on a catalytic reaction.

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It is essential that the analytical methods and equipment used to analyse reaction mixture compositions at the reactor inlet and outlet are precise, fast, and reliable.

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The progress of chemical industry depends on its attitude toward chemical science and interest

in research work, while the good shape of science is determined by the fact how far it turns

its face to the demand and prospects for developments in industry.

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