The Role Catalysts In Chemical Reactions, Their Importance In Industry, Problems
and New Developments


OXFORD AND CAMBRIDGE SCHOOLS EXAMINATION BOARD. General Certificate Examination
- Advanced Level Chemistry (Salters') - Paper 3 mock.

ROBERT TAYLOR U6JW.

A Catalyst is a substance that alters the rate of a reaction. The catalyst
remains unchanged at the end of the reaction. The process is called catalysis.
In this report I aim going to explain the role of catalysts in chemical
reactions and their importance in industry. I will also outline the problems
associated with the use of some catalysts and discuss, using appropriate
examples, new developments in this area which will help reduce damage to the
environment.

The process of catalysis is essential to the modern day manufacturing industry.
Ninety per cent, over a trillion dollars' worth, of manufactured items are
produced with the help of catalysts every year. It is therefore logical that
scientists are constantly searching for new improved catalysts which will
improve efficiency or produce a greater yield.
An acidic catalyst works due its acid nature. Catalysts are strong
acids and readily give up hydrogen ions, or protons: H+. Protons can be released
from hydrated ions, for example H3O+, but more commonly they are released from
ionisable hydroxyl groups (R-OH) where the O-H bond is broken to produce R-O-
and H+. When the reactant receives protons from an acid it undergoes a
conformational change, (change in shape and configuration), and becomes a
reactive intermediate. The intermediate can then either become an isomer by
returning a proton to the catalyst, or it may undergo a further reaction and
form a completely new molecule.

Up until the mid - 1960's silica-alumina gels were used to catalyse the cracking
of hydrocarbons. This form of cracking is where the large molecules in oil are
converted into small, highly volatile molecules. However because the size of the
pores of silica-alumina gels was so variable, (ranging from 0.1nm to 50nm), and
the fact that their shape was so variable, they were hardly ideal catalysts. Due
to the large size of their cavities, large carbonaceous products were able to
form in the cavities thus lowering the reactivity if the catalyst. Catalysis
with alumina silica-gels was also difficult to control precisely because of
their indefinite structure, and therefore uneven distribution of protons.

By the mid-1960's it was obvious that silica-alumina gels were inefficient as
catalysts and they were replaced by zeolites. Zeolites are highly porous
crystals with minute channels ranging from 0.3nm to 0.8nm in diameter. Due to
their definite crystalline structure and the fact that their pores are too small
to contain carbonaceous build-up, zeolites do not share the problems of silica-
alumina gels.

Zeolites are able to exhibit shape-selective crystals i.e.. their active sites
are specific to only a few product molecules (the ones that will fit into the
tiny pores).

An example of this is when the zeolite ZSM-5 is used to catalyse the synthesis
of 1,4-dimethylbenzine. When molecules of methylbenzene combine with methanol in
the ZSM-5 catalyst, only rod-shaped molecules 1,4-dimethylbenzene are released,
(these are the commercially desirable ones). The boomerang shaped molecules are
unable to pass through the catalysts pores and are therefore not released.

Until relatively recently, one of the large drawbacks with catalysts was the
highly toxic by-products which they became after use. This was because the
catalysts were often corrosive acids with a high toxicity level in liquid form.
Examples include hydrogen fluoride. Once these catalysts had been used this
promoted great problems in terms of disposal as these acids corrode disposal
containers and are highly dangerous to transport and handle.

These problems have been solved by a new type of catalyst. Solid acid catalysts,
such as silica-alumina gels and zeolites, hold their acidity internally and are
therefore much safer to work with and to dispose of.

More recently, pressure from environmentalists has led to a search for more
environmentally friendly forms of catalysis. There is now a need to replace both
the Friedel - Crafts process which involves the unwanted production of hydrated
aluminium chloride and the Oxidation process which forms by-products containing
nitric acid, chromate (VI) and manganate (VII). The leading contender for an
environmentally acceptable alternative to the Friedel - Crafts and Oxidation
processes is the process of using Supported reagents. These are materials where
a reagent such as ZnCl2 or FeCl3 has been absorbed on to an insoluble inorganic
or organic solid (e.g. silica, alumina, clay or charcoal). When a reagent has
been well dispersed on the surface of the support material, the effective
surface area of the reagent can be increased by up to one hundred times. This
improves reagent activity and selectivity, along with the fact