_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

12
Mechanisms of industrial
processes clarified by
studies of homogeneous
catalysis by complexes of
transition elements
12.1 lntroduction
In this Chapter a number of organic reactions are discussed which are catalysed
under homogeneous conditions by complexes of the transition elements. A few of
these, notably the carbonylation of methanol to yield acetic acid and the
conversion of alkenes into aldehydes or alcohols with carbon monoxide and
hydrogen (hydroformylation) are used industrially on a large scale. Most
industrial catalysts, however, are heterogeneous, that is, they do not belong to
the same phase as the reactants. Normally a catalyst is a solid, often a highly
dispersed metal or a metal oxide with a high surface area and reactions take place
on adsorption of reactant molecules at its surface.
Table 12.1 represents an attempt to summarize some of the characteristics of
homogeneous and heterogeneous catalysts and their advantages and disadvantages.
In an industrial context the first three factors, activity, selectivity and life are
ofparamount importance. As far as activity is concerned, homogeneous systems
might appear to make the most efficient use of the catalyst material as all metal
atoms are involved in the catalytic process. Only sites on the surface of a
heterogeneous catalyst, including those in pores, are available to the reactants so
that those metal atoms in the interior of the crystallites are not utilized. By using
catalysts of high surface area, however, this wastage can be minimized.
A catalyst cannot alter the position of equilibrium. It merely lowers the
activation energy barrier between reactants and products so that equilibrium is
attained more rapidly than in its absence. Even with the simplest starting
materials. many reaction paths leading to different products are possible. A
catalyst should be able to select o ne of these paths to the exclusion of all others.
High selectivity of this type is desirable in any industrial process. Different
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