10.5 Catalytic reactions Catalyzed reactions. Out-class extensive reading: Levine, p Catalysis Enzyme catalysis

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1 10.5 Catalytic reactions Catalyzed reactions Out-class extensive reading: Levine, p Catalysis Enzyme catalysis

2 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.

3 5.2 type of catalysis Types Definition Examples 1) Homogeneous catalysis 2) Heterogeneous catalysis 3) Biological catalysis / enzyme catalysis The catalyst is present in the same phase as the reactant. The catalyst constitutes a separate phase from the reaction system Reaction catalyzed with biological catalysts: enzyme Hydrolysis of sucrose with inorganic acid. NO 2 catalyzes the oxidation of SO 2 Haber s process for ammonia synthesis; contact oxidation of sulphur dioxide; Hydrogenation of alkene, aldehyde, etc. Hydrolysis of starch in stomach Heterogeneous catalysis is going to be discussed in Surface Chemistry.

4 5.3 General characteristics of catalyzed reactions 1) Catalyst takes part in the reaction. (CH 3 ) 3 COH (CH 3 ) 2 C=CH 2 + H 2 O without catalyst: k = exp(-32700/t) s -1 with HBr (g) as catalyst: k c = exp(-15200/t) dm 3 mol -1 s -1 k c k exp T exp T

5 with HBr serving as catalyst: 10.5 Catalytic reactions k 1 A C A C k1 k2 A C + B A B + C r kk 1 2 [A][B] k[a][b] k 1 E E E E a, app a,1 a,2 a, 1 By altering reaction path, catalyst can lower activation energy of the overall reaction significantly and change the reaction rate dramatically.

6 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. x e ln ( ) ( xe x) k k t kt Catalyst can only shorten the time for reaching equilibrium. (3) Catalyst is effective both for forward reaction and backward reaction. cat NH N 3H Study on the catalyst for formation of ammonia can be done with easy by making use of the decomposition of ammonia.

7 3) Selectivity of catalysts 10.5 Catalytic reactions (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. CH 2 CH O 2 Ag 200~300 o C CH 2 CH 2 1 PdCl 2 CuCl 2 CH 2 CH 2 + O 2 2 CH 3 C 200~300 o C O O H

8 4) Other characteristics: 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; antipoisoning, recovering/renewing. k Type II T (5) The activity of a catalyst can be enhanced by promoter; (6) Catalyst usually loaded on support with high specific area, such as activated carbon, silica.

9 4) Other characteristics: (7) Effects of catalyst support: free standing catalyst/supported catalyst better distribution; low loading; better catalytic activity, and better stability; Electron effect, bi-functional effect for recovery of active sites.

10 4) Other characteristics: 10.5 Catalytic reactions Porous carbon Molecular sieve

11 4) Other characteristics: Turn-over frequency (TOF): h -1 Activity and retention: catalyzed reaction per catalyst in unit time. activity: mol cm -2 ; mol g -1 initial rate, activity-stability;

12 Catalysts Reaction systems Reaction time/min Temperature/ o C TOF/h -1 references Pd/MSC-30 HCOOH-HCOONa < [1] Pd/AC HCOOH-HCOONa < [2] Pd/AC HCOOH-HCOONa < [2] Pd/C HCOOH-HCOONa [3] Pd/C HCOOH-HCOONa [4] Pd/C HCOOH-HCOONa [5] Pd/resin3 HCOOH-HCOONa [6] Pd/PDA-GO HCOOH-HCOONa < [7] Pd-S-SiO 2 HCOOH-HCOONa [8] Pd/C HCOOH-HCOONH this study Pd/C HCOOH-HCOONH this study

13 5.4 kinetics of homogeneous catalysis 6 C 12 H 22 O 11 + H 2 O C 6 H 12 O 6 + C 6 H 12 O 6 r k[c H O ][H O] [H ] For homogeneous reaction, the reactant is usually named as substrate. log k a lgk a lgk a - lgk a Dehydration of acetaldehyde catalyzed by different acids. 1 k2 S C M P C kk k k k 1 [S][C] k 1 2 r k[s][c] k k' 1 2 k[c] '[S] When C is acid, rate constant is proportional to dissociation constant (K a ) as pointed out by Brønsted et al. in the 1920s:

14 5.4 kinetics of homogeneous catalysis 10.5 Catalytic reactions lg k a lg G a lg K a k G a a K a Where G a and is experimental constants. ranges between 0 ~ 1. In aqueous solution, the acid may be H + or H 3 O + 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: k G b b K b

15 5.5 Typical coordination catalysis Ziegler-Natta catalysts are typically based on titanium compounds and organometallic aluminium compounds, such as the undefined methylaluminoxane or well defined triethylaluminium, (C 2 H 5 ) 3 Al.

16 5.5 Typical coordination catalysis Oxidation of ethylene to produce acetaldehyde with the presence of PdCl 2 and CuCl in aqueous solution. 1 2 PdCl2CuCl2 C H + O CH CHO r k[pdcl ][C H ][H O] [Cl ] (a) C H + PdCl + H O (b) 2CuCl + Pd 2CuCl + PdCl CH CHO + Pd + 2HCl (c) 2CuCl + 2HCl + O 2CuCl + H O 2

17 5.5 Typical coordination catalysis 10.5 Catalytic reactions K (1) [PdCl ] + C H [C H PdCl ] + Cl 2 2 (2) [C H PdCl ] + H O [C H Pd(H O)Cl ]+ Cl K (3) [C H Pd(H O)Cl ] + H O [C H Pd(HO)Cl ] + H O 4 (4) [C H Pd(HO)Cl ] k Cl P (5) Cl Pd CH CH OH k HCl + Pd + CH CHO K d CH CH OH + Cl

Chemical Kinetics. Chapter 13. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chemical Kinetics. Chapter 13. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chemical Kinetics Chapter 13 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chemical Kinetics Thermodynamics does a reaction take place? Kinetics how fast does