Page 196 - Introduction to chemical reaction engineering and kinetics

178 Chapter 8: Catalysis and Catalytic Reactions
8.1.2 Types of Catalysis

We may distinguish catalysis of various types, primarily on the basis of the nature of the
species responsible for the catalytic activity:

(1) Molecular catalysis. The term molecular catalysis is used for catalytic systems
where identical molecular species are the catalytic entity, like the molybdenum
complex in Figure 8.1, and also large “molecules” such as enzymes. Many molec-
ular catalysts are used as homogeneous catalysts (see (5) below), but can also be
used in multiphase (heterogeneous) systems, such as those involving attachment
of molecular entities to polymers.
(2) Surface catalysis. As the name implies, surface catalysis takes place on the
surface atoms of an extended solid. This often involves different properties for
the surface atoms and hence different types of sites (unlike molecular catalysis,
in which all the sites are equivalent). Because the catalyst is a solid, surface cata-
lysis is by nature heterogeneous (see (6) below). The extended nature of the
surface enables reaction mechanisms different from those with molecular cata-
lysts.
(3) Enzyme catalysis. Enzymes are proteins, polymers of amino acids, which cat-
alyze reactions in living organisms-biochemical and biological reactions. The
systems involved may be colloidal-that is, between homogeneous and hetero-
geneous. Some enzymes are very specific in catalyzing a particular reaction (e.g.,
the enzyme sucrase catalyzes the inversion of sucrose). Enzyme catalysis is usu-
ally molecular catalysis. Since enzyme catalysis is involved in many biochemical
reactions, we treat it separately in Chapter 10.
(4) Autocatalysis. In some reactions, one of the products acts as a catalyst, and the
rate of reaction is experimentally observed to increase and go through a max-
imum as reactant is used up. ‘Ihis is autocatalysis. Some biochemical reactions
are autocatalytic. The existence of autocatalysis may appear to contradict point
(2) in Section 8.1.1. However, the catalytic activity of the product in question is
a consequence of its formation and not the converse.
A further classification is based on the number of phases in the system: homo-
geneous (1 phase) and heterogeneous (more than 1 phase) catalysis.
(5) Homogeneous catalysis. The reactants and the catalyst are in the same phase.
Examples include the gas-phase decomposition of many substances, including di-
ethyl ether and acetaldehyde, catalyzed by iodine, and liquid-phase esterification
reactions, catalyzed by mineral acids (an example of the general phenomenon of
acid-base catalysis). The molybdenum catalyst in Figure 8.1 and other molecular
catalysts are soluble in various liquids and are used in homogeneous catalysis.
Gas-phase species can also serve as catalysts. Homogeneous catalysis is molec-
ular catalysis, but the converse is not necessarily true. Homogeneous catalysis is
responsible for about 20% of the output of commercial catalytic reactions in the
chemical industry.
(6) Heterogeneous catalysis. The catalyst and the reactants are in different phases.
Examples include the many gas-phase reactions catalyzed by solids (e.g., ox-
idation of SO2 in presence of V,O,). Others involve two liquid phases (e.g.,
emulsion copolymerization of styrene and butadiene, with the hydrocarbons
forming one phase and an aqueous solution of organic peroxides as catalysts
forming the other phase). Heterogeneous, molecular catalysts are made by
attaching molecular catalytic centers like the molybdenum species to solids
or polymers, but heterogeneous catalysts may be surface catalysts. An impor-
tant implication of heterogeneous catalysis is that the observed rate of reaction
may include effects of the rates of transport processes in addition to intrinsic