Page 80 - Visions of the Future Chemistry and Life Science

Chemistry on the inside 69

Figure 4.6. General scheme for the synthesis of linear alkyl benzenes, precursors
to surfactants. Control over pore size of the catalyst can suppress the second
alkylation almost completely. Given the ease with which the pore size can be
chosen, one can design an effective catalyst for any particular reaction, and allow
the selective and clean production of the desired mono-alkyl product, thus
eliminating much of the waste associated with the process.

As can be seen, the reaction will proceed to the monoalkylated
product, but does not stop there. The alkylated product is more reactive
than the starting material, and will alkylate again, giving products which
are useless. Control over this aspect of the reaction can only be achieved
with difficulty in traditional systems, and very high dilutions are used to
control the product distribution. The use of the new mesoporous materi-
als allows a more concentrated (and thus more efficient) process to be
developed. This is because the dialkylated product is bigger than the mono-
alkylated product. Careful choice of the pore size of the material will mean
that the space inside the pore is too small for the dialkylated product to
form, but is big enough for the desired monoalkylated product to form
readily. Thus, the reaction can run selectively at high concentrations,
solving the selectivity problem and using a catalyst which can be easily
recovered. Waste is thus reduced dramatically.
While most work has been concentrated on aluminium-containing
zeolites, the discovery of titanium-containing zeolites by an Italian
company, Enichem, in the 1980s represented another major breakthrough
in zeolites. They showed that these titanium-containing zeolites are
excellent catalyst for the selective oxidation of a variety of simple, small
molecules. Such oxidations are amongst the most important reactions in
organic chemistry, as they allow the introduction of a huge range of
important functions into the basic hydrocarbon feedstocks derived from
oil. Larger pore size versions of the material would allow a much wider
range of organic molecules to be functionalised. This type of reaction is
of enormous importance in large molecule chemistry too, with some