§10.5 Catalytic reactions

Catalyzed reactions

Out-class extensive reading:

Levine, p.577

17.16 Catalysis

17.17 Enzyme catalysis

10.5 Catalytic reactions

5.1 Catalysts and catalysis

Catalyst

A substance of small amount that can

change the rate of a chemical reaction

without themselves undergoing any

chemical change.

Catalysis

The phenomenon of acceleration or

retardation of the rate of a chemical

reaction by addition of small amount of

foreign substances to the reaction system.

10.5 Catalytic reactions

Catalyst: Pt-Rh alloy

Catalyzed reaction:

4NH3 + 5O2 = 4NO + 6H2O

5.2 type of catalysis

Types Definition Examples

1) Homogeneous

catalysis

The catalyst is present

in the same phase as

the reactant.

Hydrolysis of sucrose with inorganic

acid.

NO2 catalyzes the oxidation of SO2

2) Heterogeneous

catalysis

The catalyst constitutes

a separate phase from

the reaction system

Haber’s process for ammonia synthesis;

contact oxidation of sulphur dioxide;

Hydrogenation of alkene, aldehyde, etc.

3) Biological /

enzyme catalysis

Reaction catalyzed

with biological

catalysts: enzyme

Hydrolysis of starch in stomach with

the aid of diastase

10.5 Catalytic reactions

5.3 General characteristics of catalyzed reactions

1) Catalyst takes part in the reaction.

(CH3)3COH (CH3)2C=CH2 + H2O

without catalyst: k = 4.8 1014 exp(-32700/T) s-1

with HBr (g) as catalyst: kc = 9.2 1012 exp(-15200/T) dm3mol-1s-1

23

14

12

101.432700

exp108.4

15200exp102.9

T

T

k

kc

10.5 Catalytic reactions

with HBr serving as catalyst:

1 2

1

[A][B][A][B]

k kr k

k

1,2,1,, aaaappa EEEE

By altering reaction path, catalyst can lower

activation energy of the overall reaction

significantly and change the reaction rate

dramatically.

1

1

2

A C A C

A C + B A B + C

k

k

k

10.5 Catalytic reactions

2) No impact on the thermodynamic features of the reaction

(1) Catalyst cannot start or initiate a thermodynamically non-spontaneous reaction;

(2) Catalyst can change the rate constant of forward reaction and backward reaction

with the same amplitude and does not alter the final equilibrium position.

Catalyst can only shorten the time for

reaching equilibrium.

e

e

ln ( )( )

xk k t kt

x x

(3) Catalyst is effective both for forward reaction and backward reaction.

Study on the catalyst for formation of ammonia can be done with easy by making use

of the decomposition of ammonia.

cat.

3 2 22NH N 3H

10.5 Catalytic reactions

3) Selectivity of catalysts

(1) The action of catalyst is specific. Different reaction calls for different catalyst.

Hydrogenation? Isomerization through carbonium?

(2) The same reactants can produce different products over different catalysts.

CH2

Ag

200~300 oCCH2 CH2

O

+2

1O2CH2

CH2200~300 oC

+2

1O2CH2

PdCl2 CuCl2CH3 C

O

H

10.5 Catalytic reactions

Activity and retention：

activity: mol cm-2; mol g-1

Turn-over frequency (TOF): h-1

catalyzed reaction per mole of catalyst in

unit time. initial rate, activity-stability;

4) Catalytic activity:

10.5 Catalytic reactions

(1) The chemical composition of catalyst

remains unchanged at the end of the

reaction;

(2) Only a small amount of catalyst is

required;

(3) Catalyst has optimum temperature;

(4) Catalyst can be poisoned by the

presence of small amount of poisons; anti-

poisoning, recovering/renewing.

5) Other characteristics:

T

k

Type II

(5) The activity of a catalyst can be

enhanced by promoter;

10.5 Catalytic reactions

(7) Catalyst usually loaded on support with high specific area, such as activated carbon,

silica. free standing catalyst vs. supported catalyst

10.5 Catalytic reactions

Effects of catalyst support:

better distribution;

low loading;

better catalytic activity

better stability;

Electron effect

bi-functional effect for

recovery of active sites.

5) Other characteristics:

Porous carbon

Molecular sieve

10.5 Catalytic reactions

5) Other characteristics:

5.4 kinetics of homogeneous catalysis

1 2

1 2

[S][C][S][C] '[S]

k kr k k

k k

For homogeneous reaction, the reactant is usually named as substrate.

S C M P Ck k

k 1 2

1

' [C]k k

C12H22O11 + H2O C6H12O6 + C6H12O66

12 22 11 2[C H O ][H O] [H ]r k

When C is acid, rate constant is

proportional to dissociation constant (Ka) as

pointed out by Brønsted et al. in the 1920s:Dehydration of acetaldehyde catalyzed by different acids.

0 2 4 6 8 10

-2

-1

0

1

2

3

log k

a

- lgKa

lgKa

lgk a

10.5 Catalytic reactions

Where Ga and is experimental constants.

ranges between 0 ~ 1.

aaa KGk lglglg aaa KGk

In aqueous solution, the acid may be H+ or H3O+ but in general it may be any

species HA capable of being a proton donor (Brønsted acid) or a electron acceptor

(Lewis acid).

For base-catalyzed reaction there also exists:

bbb KGk

5.4 kinetics of homogeneous catalysis

10.5 Catalytic reactions

Ziegler-Natta catalysts are typically based

on titanium compounds and organometallic

aluminum compounds, such as the

undefined methylaluminoxane or well

defined triethylaluminum, (C2H5)3Al.

5.5 Typical coordination catalysis

10.5 Catalytic reactions

Ziegler-Natta Catalysis，Advances in Organometallic

Chemistry，Volume 18, 1980, Pages 99-149

5.5 Typical coordination catalysis

Oxidation of ethylene to produce acetaldehyde with the presence of PdCl2 and CuCl in

aqueous solution.

2 2 2 2 3

2

2 2 2

(a) C H + PdCl + H O CH CHO + Pd + 2HCl

(b) 2CuCl + Pd 2CuCl + PdCl

(c) 2CuCl + 2HCl + O 2CuCl + H O

2 1 2

4 2 2 3 [PdCl ][C H ][H O] [Cl ]r k

2 2PdCl CuCl

2 2 2 3

1 C H + O CH CHO

2

10.5 Catalytic reactions

1

2

3

4

2

4 2 4 2 4 3

2

2 4 3 2 2 4 2 2

2

2 4 2 2 2 2 4 2 3

2 4 2

(1) [PdCl ] + C H [C H PdCl ] + Cl

(2) [C H PdCl ] + H O [C H Pd(H O)Cl ]+ Cl

(3) [C H Pd(H O)Cl ] + H O [C H Pd(HO)Cl ] + H O

(4) [C H Pd(HO)Cl ] Cl P

K

K

K

k

5

2 2

2 2 3

d CH CH OH + Cl

(5) Cl Pd CH CH OH HCl + Pd + CH CHOk

5.5 Typical coordination catalysis

10.5 Catalytic reactions