Page 71 - Visions of the Future Chemistry and Life Science

60 D. J. MACQUARRIE

invention of new reactions to developing new catalysts (chemicals which
are themselves not used up in the reaction, but which allow the reaction
partners to be transformed more rapidly, using less energy, and often more
selectively, generating fewer byproducts) which allow more selective reac-
tion to take place, to biotransformations and novel engineering concepts,
all of which can also be used to minimise waste. Catalysts can sometimes
be developed which allow inherently clean reactions to be invented.
A very important part of such an undertaking is to be clear about what
stages of a chemical process generate the most waste. Often this is found
to be the separation stage, after the transformation of reactants to products,
where all the various components of the final mixture are separated and
purified. Approaches to chemical reactions which help to simplify this step
are particularly powerful. Such an approach is exemplified by heterogene-
ous catalysis. This is an area of chemistry where the catalysts used are typ-
ically solids, and the reactants are all in the liquid or gas phase. The
catalyst can speed up the reaction, increase the selectivity of the reaction,
and then be easily recovered by filtration from the liquid, and reused.
One of the newest areas in the realm of catalysis is that of tailored
mesoporous materials, which are finding many uses as highly selective cat-
alysts in a range of applications. A mesoporous material is one which has
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cavities and channels (pores) in the range of 2–5nm (a nanometre is 10 m)
– for comparison, a typical chemical bond is of the order of 0.1nm, and a
small organic molecule is around 0.50nm across. Such mesoporous mate-
rials can be thought of as being analogous to the zeolites, which came to
prominence in the 1960s. Zeolites are highly structured microporous inor-
ganic solids (pores
2nm), which contain pores of very well defined sizes,
in which catalytic groups are situated. A wide range of zeolites is known,
each having different pore sizes and channel dimensions. Many are used in
large-scale industrial applications. For example, many of the components
of petrol are prepared using zeolites, as are precursors for terephthalic acid,
used for the manufacture of PET bottles, processes in which millions of
tonnes of material is produced annually.
Zeolites are prepared by the linking of basic structural units around a
template molecule. The structural units are typically based on oxides of
silicon and aluminium, and the templates are usually individual small
molecules. Under the right conditions, the silicon and aluminium oxide
precursors will link up around the template to form a crystalline three-
dimensional matrix containing the template molecules. The template