Page 79 - Visions of the Future Chemistry and Life Science

68 D. J. MACQUARRIE

surfaces. All these shapes could one day find application, not only in catal-
ysis, but in adsorption of e.g. pollutants from water, molecular wires, and
a host of other devices.

4.3 Applications

The initial applications of MTSs were, perhaps not surprisingly, simply
attempts to reproduce zeolite chemistry on larger molecules. This chem-
istry is based on the fact that the aluminium centres in zeolites cause a
negative charge to exist on the framework of the solid; this charge must be
balanced by a cation. When the cation is a hydrogen ion (proton), the
material is an acid, and indeed some zeolites are very strong acids indeed.
However, the acidity of the corresponding MTSs is much lower, and ini-
tially this limited their applicability somewhat. Nevertheless, the MTSs
are often found to be very effective as mild acid catalysts. Much work has
therefore been aimed at the production of other materials using the same
concept, but with either different templating systems, or with combina-
tions of elements other than Si and Al in the framework.
However, many industrial processes are based on the use of very strong
acids, and there is great pressure to find replacements for the liquid acids cur-
rently used in industrial processes. One method which has been successfully
applied to increase the acidity of these systems is the immobilisation of alu-
minium chloride onto the pore walls. Aluminium chloride is itself a very
strong acid, and is one of the commonest in industrial chemistry. It is used
in a wide range of transformations, but cannot be recovered intact from reac-
tions. Its destruction leads to large quantities of waste being generated.
Aluminium chloride has been successfully attached to the walls of HMS
materials, without any reduction in activity – i.e. the resultant material has
the same activity as unsupported aluminium chloride. A major advantage
over free aluminium chloride is the ease of removal of the solid catalyst from
reaction mixtures, simplifying the process and reducing waste dramatically.
The catalyst can then be easily recovered from the raction mixture, and
reused. A second important advantage is the ability to control product dis-
tributions by tailoring the pore size of the material. This is best illustrated
by the preparation of linear alkyl benzenes (LABs) which are precursors to
detergents, and are produced on a massive scale using either aluminium
chloride or hydrogen fluoride, both of which have many problems associated
with their use. The general scheme is shown in Figure 4.6.