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Реферат: Untitled Essay Research Paper OXFORD AND

Название: Untitled Essay Research Paper OXFORD AND
Раздел: Топики по английскому языку
Тип: реферат Добавлен 18:30:31 15 ноября 2010 Похожие работы
Просмотров: 2 Комментариев: 6 Оценило: 1 человек Средний балл: 5 Оценка: неизвестно     Скачать

Untitled Essay, Research Paper

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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