CHBE 452 Lecture 27 Catalysis I 1

Importance Of Catalysis 90% of all chemical processes use catalysts Changes in catalysts have a giant influence on rates and selectivity’s of reactions. More than anything else Most real reactor design associated with optimizing performance of catalyst 2

Catalysis Definition Ostwald defined a catalyst as a substance which changed the rate of reaction without itself being consumed in the process Not being consumed  catalyst does change 3

Catalytic Reaction Occurs Via A Catalytic Cycle: reactants + catalyst  complex complex  products + catalyst 4

Example: Rhodium Catalyzed CH 3 OH+CO  CH 3 COOH 5 Printing press analogy

The Rate Enhancement Of A Number Of Reactions In The Presence Of A Catalyst

Examples Of Effects Of Solvent Table 13.2 The rate of the S N 2 reaction NaCl + CH 3 I  NaI + CH 3 Cl at 350 K ______________________________________________________________ Rate Constant, Rate Constant, Solventliter/(mol-second)Solvent liter/(mol-second) ______________________________________________________________ Gas phase ~ ______ _______ Water 3.5 x Methylcyanide0.13 Methanol 3.1 x Dimethylformamide 2.5 ______________________________________________________________ 7

Types Of Catalysts Homogeneous catalysts: Soluble compounds that go in solutions Heterogeneous catalysts: Solids that sit in reactors 8

Common Examples Of Catalysts Homogeneous Enzymes (detergents, digestion, tears) Heterogeneous Catalytic converter 9

Rate Laws Different Than With Gas Phase Reactions Rate proportional to the surface area not the volume. 10

Effects Of Surface Area Consider a platinum catalyzed reaction. You can run the reaction 1) Run the reaction on the wire 2) Take the wire and smash it with a hammer and then run the reaction. The rate will be higher on the wire you smashed with a hammer! 11

Why Does Smashing A Wire Change The Rate? When you squashed the platinum you created more surface area. You also changed the shape of the surface which can affect the rate. 12

Turnover Numbers Rates of catalytic reactions often expressed as turnover number 13 R A = Rate per unit area (molecules/cm 2 -sec) N S = Number of exposed metal atoms / unit area (Atoms/cm 2 )

Turnover Numbers For Some Typical Reactions 14

Typical Catalytic Kinetics 15 Figure 2.15 The influence of the CO pressure on the rate of CO oxidation on Rh(111). Data of Schwartz, Schmidt, and Fisher

Typical Catalyst Kinetics 16 Called a Langmuir-Hinshelwood rate law. Also called Monod rate law.

Temperature Dependence 17 Figure 2.18 The rate of the reaction CO + 2 O 2  CO 2 on Rh(111). Data of Schwartz, Schmidt and Fisher[1986].

Catalysts Do Not Work Over A Broad Temp Range 18

Types Of Catalysts: Homogeneous Catalysts Heterogeneous Catalysts 19

Homogeneous Catalysts: Acids or Bases Metal salts Enzymes Radical initiators 20

Table 12.2-Some Reactions Commonly Catalyzed By Acids And Bases 21 ReactionExampleTypical Application Isomerization (Rearranging the structure of a molecule) CH 2 =CHCH 2 CH 3  CH 3 CH=CHCH 3 Octane Enhancement Monomer Production Paraxylene Production Alkylation (Making too little molecules into a bigger one) CH 3 CH=CHCH 3 + CH 3 CH 2 CH 2 CH 3  (CH 3 CH 2 )CH(CH 3 )(C 4 H 9 ) Pharmaceutical Production Monomer Production Fine Chemicals Butane + olefin  octane Cracking (Taking a big molecule and making it into two littler ones). C 12 H 24  C 7 H 14 + C 5 H 10 Crude Oil Conversion Digestion

Acids And Bases As Catalysts 22

Acids As Catalysts Continued 23

Solid Acids And Bases As Catalysts 24

Very Complex Pore Structure 25 Figure 12.4 A diagram of the pore structure in Faugasite.

Enzymes As Catalysts Oxidoreductases (promote oxidation reduction reactions) Transferases (promote transfer of functional groups) NADH peroxidase (Oxidizes NADH with peroxides NADH + H 2 O 2   NAD(+)+2 H 2 O. Dimethylallylcis- transferase (Transfer dimethylallyl groups) Dimethylallyl diphosphate + isopentenyl disphosphate  diphosphate + dimethylallylcis- isopentenyldiphosphate Ferroxidase (oxidizes Iron) 4 Fe H + + O 2  4 Fe H 2 O Glycoaldehyde transferase (Transfer’s Glucoaldeydes) Also called Transketolase Sedoheptulose 7-phosphate + D- glyceraldehyde 3-phosphate  D- ribose 5-phosphate + D-xylulose 5-phosphate Glucose oxidase (oxidizes Glucose)  -D-Glucose + O 2  D- glucono-1,5- lactone + H 2 O 2 Alanine aminotransferase (Transfer amino groups from alanine) L-Alanine + 2-oxoglutarate  pyruvate + L-glutamate 26

Solvents: As Catalysts 27

Next: Heterogeneous Catalysis Examples of heterogeneous catalysts include: Supported Metals Transition Metal Oxides and Sulfides Solid Acids and Bases Immobilized Enzymes and Other Polymer Bound Species 28

Supported Metal Catalysts 29 Figure.12.3 A picture of a supported metal catalyst. Use support because platinum very expensive and only the surface is active. Spread platinum out on cheap support. Support also provides strength

Advantage Of Heterogeneous Catalysts Compared To Homogeneous: Cheaper separation More selective Generally cheaper Disadvantage Not quite as active or a per metal atom basis 30

Typical Mechanism Of Heterogeneous Catalysis (H 2 +C 2 H 4  C 2 H 6 ) 31

Summary Catalysts make a tremendous difference to rates enhancement average possible Kinetics change – rarely linear Optimum conditions often at max rate Zero order kinetics Two types of catalysts Homogeneous Heterogeneous Homogeneous more active Heterogeneous less expensive to use/control 32

Query What did you learn new in this lecture? 33