CHBE 553 Lecture 27 Continue Mechanisms Of Catalyst Action 1

Mechanisms Of Catalyst Action Catalysts can be designed to help initiate reactions. Catalysts can be designed to stabilize the intermediates of a reaction. Catalysts can be designed to hold the reactants in close proximity. Catalysts can be designed to hold the reactants in the right configuration to react. 2

Mechanisms Continued Catalysts can be designed to block side reactions. Catalysts can be designed to sequentially stretch bonds and otherwise make bonds easier to break. Catalysts can be designed to donate and accept electrons. Catalysts can be designed to act as efficient means for energy transfer. 3

Other Important Concepts One needs a catalytic cycle to get reactions to happen. Mass transfer limitations are more important when a catalyst is present. 4

Main Effect: Stabilizing Intermediates 5 Figure A Sachtler-Frahrenfort and Tanaka-Tamaru plot for the hydrogenation of ethylene.

Stabilizing Intermediates Not Entire Effect Leads to increases in rates – -need other effects to get to Does not lead to selectivity 6

Last Time Started Mentioned The Use Of Catalyst To Affect Selectivity Catalysts can be designed to hold the reactants in close proximity. Catalysts can be designed to hold the reactants in the right configuration to react. Catalysts can be designed to block side reactions. Catalysts can be designed to sequentially stretch bonds and otherwise make bonds easier to break. 7

Holding Reactants In The Right Configuration 8 Figure A cartoon of the reaction of ethanol and NAD + on the active site of liver alcohol dehydrogenase. Adapted from Oppenheimer and Handlon (1992) (In the Enzyme, vol 20 (1992) 453. (12.92)

Catalysts Can Be Designed To Hold The Reactants In The Correct Configuration To React, Make Bonds Easier To Break 9 Figure The active site for reaction (12.91) on a palladium catalyst. (12.91) Figure A Picture of Lysozyme 161L. This figure was generated using a program called RASMOL, using data in the protein data base from an x-ray diffraction spectrum generated by Weaver and Matthews[1987]

Catalysts Make Bonds Easier To Break 10 Figure A Picture of Lysozyme 161L. This figure was generated using a program called RASMOL, using data in the protein data base from an x-ray diffraction spectrum generated by Weaver and Matthews[1987]

Catalytic Antibodies 11 (12.94)

Transition Metals – Weaken Bond By Attaching To Antibonding Orbitals 12 Figure A diagram of the key interactions during the dissociation of hydrogen on platinum.

Acid Catalysts: Charges Simplify Reactions 13

Possible Mechanism: Ion 14 Ion Mechanism In both cases isomerization is rate determining step Radical Mechanism

Orbitals For Radical Case 15 Filled Empty Figure A rough diagram of the key MO's during reactions (12.101). Note: Only 1 filled with ions.

Catalysts Can Block Side Reactions Consider syncs of isotatic polypropylene 16 (12.104) (12.105)

Possible Reaction Step In PE Production 17 Figure A rough diagram of one step during the production of isotatic polypropylene.

Catalyst For PE Production: Block Side Reactions 18 Figure A diagram of propylene polymerization in a Ziegler-Natta catalyst.

Catalysts Can Be Designed To Donate And Accept Electrons 19 (12.106) (12.107)

A Diagram Of A Polymer Fuel Cell 20 (12.25)

Summary Catalysts can be designed to help initiate reactions. Catalysts can be designed to stabilize the intermediates of a reaction. Catalysts can be designed to hold the reactants in close proximity. Catalysts can be designed to hold the reactants in the right configuration to react. 21

Summary Continued Catalysts can be designed to block side reactions. Catalysts can be designed to sequentially stretch bonds and otherwise make bonds easier to break. Catalysts can be designed to donate and accept electrons. Catalysts can be designed to act as efficient means for energy transfer. 22