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...