Page 204 - Introduction to chemical reaction engineering and kinetics

186 Chapter 8: Catalysis and Catalytic Reactions

carbocation intermediates:

H+ + R’-CH=CH-R + R’-CH2-CH+-R

which then participate in a variety of reactions such as alkylation, rearrangements,
and cracking.

8.2.3 Other Liquid-Phase Reactions
Apart from acid-base catalysis, homogeneous catalysis occurs for other liquid-phase
reactions. An example is the decomposition of H,O, in aqueous solution catalyzed by
iodide ion (II). The overall reaction is

2H,O,(A) + 2H,O + 0,

and the rate law is

(-f-*) = k*c*+ ; EA = 59 kJ mol-’

A possible mechanism is
H,O, + I- AH,0 + IO- (slow)

IO- + H20,%H,0 + 0, + I- (fw

with I- being used in the first step to form hypoiodite ion, and being regenerated in the
second step. If the first step is rate-determining, the rate law is as above with kA = k,.
For the uncatalyzed reaction, EA = 75 kJ mol-l.
This reaction can be catalyzed in other ways: by the enzyme catalase (see enzyme
catalysis in Chapter lo), in which EA is 50 kJ mol-l, and by colloidal Pt, in which EA is
even lower, at 25 kJ mol-l.
Another example of homogeneous catalysis in aqueous solution is the dimerization
of benzaldehyde catalyzed by cyanide ion, CN- (Wilkinson, 1980, p.28):

2C,H,CHO(A) 4 C,H,CH(OH)COC,H,
(-I-*) = kAc;cCN-
Redox cycles involving metal cations are used in some industrial oxidations.

8.2.4 Organometallic Catalysis
Many homogeneous catalytic chemical processes use organometallic catalysts (Par-
shall and Ittel, 1992). These, like the example in Figure 8.1, consist of a central metal
atom (or, rarely, a cluster) to which is bonded a variety of ligands (and during reac-
tion, reaction intermediates). These catalysts have the advantage of being identifiable,
identical molecular catalysts and the structures of the catalytic sites can be altered
by use of specific ligands to change their activity or selectivity. With the addition of
specific ligands, it is possible to make reactions stereoselective (i.e., only one of a
possible set of enantiomers is produced). This feature has extensive application in
polymerization catalysis, where the polymer properties depend on the stereochemistry,
and in products related to biology and medicine, such as drug manufacture and food
chemistry.