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Designer Catalysts for Clean Chemistry

Dr. Dewi W. Lewis from the Department of Chemistry, University College London describes how designing catalysts allows us to find cleaner routes to key chemicals.

One of the major challenges facing chemists and the modern chemical industry is the efficient production of a wide range of key chemicals, such as such as petrochemicals, polymers, flavours and fine chemicals for the production of pharmaceuticals. Significantly, in recent years, we have seen a shift to more environmentally friendly chemistry, increasing the need for cleaner processes. The use of catalysts is widespread and should allow for more efficient reactions and lessen the production of by-products. However, there is a continued need for improved catalysts, particularly those that reduce reliance upon costly separation processes, that reduce waste and can be efficiently recycled.

Catalysts can be either homogenous — that is, in the same phase as the reagents — or heterogeneous, when they are in a different phase. Homogeneous catalysts are typically transition metals (the active site) that are coordinated to organic ligands and can be made to be highly selective; in other words, they do not produce any unwanted by-products. Selectivity is achieved by carefully choosing the organic ligands to restrict the geometry of the reactive molecules so that they favour only the formation of the desired product. Chemists can now effectively tune such organo-metallic compounds so that they catalyse very specific reactions and are highly efficient. For example, a kilogram of catalyst produces many tonnes of polypropylene. Therefore, even though such catalysts are expensive, it is not economical to remove the catalysts from the product for re-use. The separation process is expensive and is one of the major disadvantages of homogeneous catalysts: the need to recover the catalyst from the product.

Heterogeneous catalysts, on the other hand, are in a different phase to the reactants and products. There is, therefore, considerable advantage in the use of solid heterogeneous catalysts: separation of the catalysts is easily achieved and handling solids is significantly easier (and hence less expensive) than handling often corrosive and poisonous liquid catalysts. However, most solids have the disadvantage that very few (~1%) of their atoms are at the surface — hence, most of the solid is not used. Furthermore, most solid catalyst surfaces are relatively flat, allowing reactants to approach in essentially any orientation, which leads to poor product selectivity (i.e. we get unwanted by-products).

One class of solids, however, avoids such problems: microporous solids. These crystalline materials have cavities, tunnels and cages within their structure, which are both regular and also of molecular dimensions (a few millionths of a meter). They therefore resemble a sponge, but with smaller, regular holes. With such pores, the whole material can be accessed by small molecules, essentially making the inside of the solid a surface as well (98% of the atoms in such a material are now accessible for reaction). Zeolites are the most common microporous solid and are used on a massive industrial scale. For example, almost all oil refineries use zeolite Y as the catalyst for cracking crude oil into gasoline. Zeolites also find uses as ion-exchange materials: look for zeolite A on the side of a packet of washing powder, where it softens the water.

Zeolites have the catalytic sites inside the pores and so impose size and shape selectivity. Only molecules of a particular size can enter the pores to react. Similarly, the pores control which products are formed: only those that are smaller than the pore — and can diffuse out of the pore — can form. A good example is the acid catalysed addition of a methyl group to methylbenzene to give di-methylbenzene. If we carry out the reaction in solution, we get three products (1,2-dimethylbenzene, 1,3-dimethylbenzene and 1,4-dimethylbenzene) in roughly similar amounts. However, when carried out in the zeolite ZSM-5 the product is predominantly 1,4-dimethylbenzene since the formation of the other two isomers (which have a larger diameter) is restricted by the size of the zeolite channel.

However, choosing which catalyst is suitable for a given reaction is often a case of trial and error. One approach is simply to try as many catalysts as possible for the reaction to find which is best: a method that is time-consuming and expensive. An even more difficult problem is how make a new material with the kind of catalytic properties that we desire.

Researchers at the Department of Chemistry and at the Royal Institution of Great Britain are pioneering combined theoretical and experimental approaches. The aim is to first determine — using computer modelling — which material would be best before having to conduct expensive experiments. They are also trying to design new catalysts from scratch, using computers, and also work out how to make them.

For example, the ZSM-5 catalyst (mentioned above) will also convert methanol to hydrocarbons such as octane, providing a way to make petrol. However, the catalyst is not particularly selective, making lots of other alkanes as well as alkenes: the reaction is the addition of the carbon from the methanol onto the end of other molecules. This is fine for petrol, which is basically a mixture. But if, for instance, we want to make only propene (for making polypropylene, for example), this catalyst is not suitable. We want the same reaction to happen, but once it reaches propene we want it to stop. The solution was to change to a microporous solid which had small cages about the same length as propene; now no larger molecules can be made because they won’t fit.

Through the combined use of computer modelling and experiments, we are able to bring an element of design into the process that will not only reduce the time required to identify potential catalysts, but also ensure that we are finding the best catalyst.

Chemists are at the forefront of such initiatives in finding routes to cleaner and more efficient ways of making the materials we use in our modern world — from fuels to CDs, from perfumes to new drugs. A degree in chemistry will provide you with the knowledge and skills required to contribute to such efforts.

Author:
Dr. Dewi W. Lewis, Department of Chemistry, University College London
 
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