Chemical Catalysis
•
Heterogeneous
- Easy to separate product from catalyst
- Easier for engineer
•
Homogeneous
- More selective
- Milder conditions
- Easier to study, modify, optimize
•
Electrocatalysis
- Reaction occurs at electrode interface
•
Biological
- Enzymes (catalytic proteins)
- Catalytic RNA (Nobel Prize, 1989)
- Antibodies
Catalysis – The Physical Chemistry
Fundamental concepts:
- ΔG≠uncatalyzed > ΔG≠catalyzed
- ΔG of reaction is unchanged
- Catalysts are not reagents.
They enter into the reaction mechanism
(rate expression) but not into the
overall reaction equation.
Definitions:
TOF: turnover frequency (Mproduct/sec.molecat)
TON: turnover number. Total number of moles product / mole catalyst
Selectivity: Ratio of substrate converted to desired product to total
substrate converted. Addresses unwanted reactions.
Catalytic cycle: Cycle of reactions that comprise the mechanism, always
returning the catalyst to its original form
Some homogeneous catalytic processes
(Adapted from J. Halpern, Inorg. Chim. Acta 1981, 50, 11)
from Collman, Stanford
Mechanism:
H2activation prior to
olefin addition
Mechanism:
Olefins add first to cationic
catalyst
Wilkinson’s Catalyst: Mechanism for Olefin Hydrogenation
With the Rh(I) cationic precursor:
Olefin adds prior to H2 oxidative addition.*
*This mechanistic route followed by asymmetric Hydrogenation process
Halpern, Science, 1982
Halpern
(Science, 1982, p. 401)
Halpern
(Science, 1982, p. 401)
from Collman, Stanford
from Coleman, Stanford
from Coleman, Stanford
The SHOP Process
Shell
Higher
Olefin
Process
from Coleman, Stanford