Page 21 - Principles of Catalyst Development
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CATALYTIC FUNCTIONS 7
Catalysis occurs through complexing and rearrangement between
molecules and ligands of the catalyst. Reactions can be very specific, with
high yields of desired products. Since the mechanisms involve readily
identified species, these reactions are easily studied in the laboratory with
the techniques of organometallic chemistry. They are, however, difficult to
operate commercially. Liquid phase operation places restrictions on tem-
perature and pressure, so equipment is complicated. The catalyst must be
separated from the products, imposing additional difficulties.
For these reasons, homogeneous catalysis is found only in limited
industrial use, appearing usually in the manufacture of specialty chemicals,
drugs, and food. Exceptions are acetic acid production, olefin alkylation,
and hydroformylation.
1.2.2. Heterogeneous Catalysis
In these systems the reactants and catalyst exist in different phases.
Most commonly, solid catalysts are used with gaseous or liquid reactants,
sometimes both. Other permutations are possible but less often encountered.
Introducing a separate catalyst phase immediately complicates the
mechanism. Interfacial phenomena now become important. Diffusion,
absorption, and adsorption all play critical roles in establishing the rate.
These additional steps become difficult to separate from surface chemistry.
Accordingly, heterogeneous systems are difficult to study in the laboratory.
Disappearance of reactants and appearance of products are easily followed,
but important features such as the rates and energetics of adsorption,
structure of the active surface, and nature of reactive intermediates, require
separate experimentation using a constantly changing arsenal of techniques.
Often, the sequence of surface steps can only be deduced from accumulated
evidence, with many uncertainties. In every important application of
heterogeneous catalysis there is much controversy on the exact details of
chemistry. Ammonia synthesis, for example, was the first important catalytic
process, emerging over 70 years ago.(13) Yet the nature of the catalytic
surface is still debated.(14) Another case is hydrodesulfurization. Process
and catalyst development dates from the early 1950s(15) but a large amount
of scientific research is now appearing in the current literature.(J6)
Nevertheless heterogeneous catalysts are convenient to use commer-
cially. Easily prepared solid catalyst pellets, packed in tubes through which
reactants flow, satisfy process requirements for simple construction and
dependable operation. Control is good, product quality high. It is not
surprising that the vast majority of industrial catalytic processes adopt this
approach.