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OXFORD AND CAMBRIDGE SCHOOLS EXAMINATION BOARD.General Certificate Examination – Advanced LevelChemistry (Salters’) – Paper 3 mock.ROBERT TAYLOR U6JW.THE ROLE CATALYSTS IN CHEMICAL REACTIONS, THEIR IMPORTANCE IN INDUSTRY,
PROBLEMS AND NEW DEVELOPMENTS. 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
that supported reagents are easier to handle as they invariably low-toxic, non-corrosive
free flowing powders. Also the reagents can be filtered from the mixture after use and
therefore be subsequently re-used. Supported reagents have good thermal and mechanical
stability’s and their reactions are more often than not carried out in non-polar solvents.
This is due to the fact that the reaction takes place on the surface of the solid
therefore the solvent only acts as a form of heat transfer and a working fluid.In summary I see Supported reagents as the best possible solution to the problems
associated
with catalysis due to their easy use and their ability to be recovered and re-used.
They have a high level of activity and improved selectivity in reactions. This is
accompanied by their highly catalytic activity which leads to the best possible level of
performance in commercial uses. This has already been proven by the use of active reagents
in Friedel – Crafts reactions. These reactions originally had the drawbacks of firstly
the hydrolysed aluminium chloride containing aqueous effluent which is produced, and
secondly the by-products such as polymeric tars and di- and polysubstituted by-products
which are produced which unless they can be successfully removed make the product impure.
By using a supported reagent catalyst, in most cases the desired level of activity can be
achieved but the catalyst can be removed easily from the reaction mixture and re-used. I
personally therefore feel that the future of environmentally friendly catalysis lies with
supported reagent catalysts.WORD COUNT = 998
NB: Athough this essay is headed as being a mock exam it was never assessed by an
examining
board, only my chemistry teacher so there is no chance of an examining board also having a
copy. Rob Taylor 25/11/1996. ~:o)
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