ChE 553 Lecture 22 Introduction To Catalysis 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

Catalysts Do Not Work Over A Broad Temp Range 7

Types Of Catalysts: Homogeneous Catalysts Heterogeneous Catalysts 8

Homogeneous Catalysts: Acids or Bases Metal salts Enzymes Radical initiators 9

Table 12.2-Some Reactions Commonly Catalyzed By Acids And Bases 10 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

Some Reactions Commonly Catalyzed By Acids And Bases Continued 11

Acids And Bases As Catalysts 12

Acids As Catalysts Continued 13

Metal Atoms: As Catalysts 14

Examples Of Reactions Catalyzed By Homogeneous Transition Metal Catalysts 15

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 16

Enzymes Continued Hydrolases (Promote hydrolysis/cleavage reactions) Lyases (promote addition of CO 2, H 2 O and NH 3 to double bonds or formations of double bonds via elimination of CO 2, H 2 O or NH 3 ) Carboxylesterase (Promotes hydrolysis of ester linkages) A carboxylic ester + H 2 O  an alcohol + a carboxylic anion Carbonate dehydratase (Dehydrates carbonates) H 2 CO 3  CO 2 + H 2 O 1,4-ALPHA-D- Glucan glucanohydrolase (also called ALPHA- Amylase) Hydrolysis of 1,4- ALPHA-glucosidic linkages in oligosaccharides and polyasaccharides. Citrate dehydratase Citrate  CIS- aconitate + H 2 O Interleukin 1-beta converting enzyme Release of interleukin 1-beta by specific hydrolysis at 116-ASP- |-ALA-117 and 27- ASP-|-GLY-28 bonds Pyruvate decarboxylase A 2-OXO acid  an aldehyde + CO 2 17

Enzymes Continued 18

Radical Initiators As Catalysts Example: 19

Free Radical Polymerization Catalysts: 20

Free Radicals Catalysts For Ozone Destruction 21

Some Examples Of Reactions Initiated Or Catalyzed By Free Radicals And Similar Species 22

Solvents: As Catalysts 23

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 24

Supported Metal Catalysts 25 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

Pictures Of Some Heterogenous Catalysts 26

Pores In Heterogeneous Catalysts 27 Figure 14.3 A cross sectional diagram of a typical catalyst support.

Advantage Of Heterogeneous Catalysts Compared To Homogeneous: Cheaper separation More selective Generally cheaper Disadvantage Not quite as active or a per metal atom basis 28

A Selection Of The Reactions Catalyzed By Supported Metals 29

A Selection Of The Reactions Catalyzed By Supported Metals 30

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

Transition Metal Oxides, Nitrides, Sulfides: Bond transition Metal to O, N, S to reduce activity and to increase selectivity 32

A Selection Of The Reactions Catalyzed By Transition Metal Oxides, Nitrides, And Sulfides 33

A Selection Of The Reactions Catalyzed By Transition Metal Oxides, Nitrides, And Sulfides 34

Solid Acids And Bases As Catalysts 35

Very Complex Pore Structure 36 Figure 12.4 A diagram of the pore structure in Faugasite.

Leads To Shape Selective Catalysis 37 Figure An interconnecting pore structure which is selective for the formation of paraxylene.

Summary Two types of catalysts –Homogeneous –Heterogeneous Homogeneous more active Heterogeneous less expensive, easier to use/control 38