Green chemistry

The chemical industry today is one of the most important manufacturing industries in the world. The ability of chemists to produce a wide range of different molecules, both simple and staggeringly complex, is very well developed, and nowadays almost anything can be prepared, albeit maybe only on a small scale. On an industrial scale, a great variety of products are synthesised, using chemistry which varies from simple to complex. These products go into almost all the consumer goods we take for granted -colours and fibres for clothes, sports equipment, polymers which go into plastics for e.g. computer and television casings, furnishings, and photographic materials, cleaner fuels, soaps, shampoos, perfumes, and, very importantly, pharmaceuticals. Unfortunately, many of these processes generate a great deal of waste - often more waste is produced than product.

One of the major challenges for chemistry in the opening years of the new millennium is therefore the development of new methods for the clean production of these chemicals. Traditional, so-called end-of-pipe solutions - i.e. treating the waste generated from reactions to render it less harmful - are of limited value in the long term. In the last few years a new, intrinsically more powerful approach has been pioneered. Green chemistry, as it has been called, involves the redesign of chemistry, such that the desired products from a reaction are obtained without generating waste. This massive undertaking involves a wide range of approaches, from the invention of new reactions to developing new catalysts (chemicals which are themselves not used up in the reaction, but which allow the reaction partners to be transformed more rapidly, using less energy, and often more selectively, generating fewer byproducts) which allow more selective reaction to take place, to biotransformations and novel engineering concepts, all of which can also be used to minimise waste. Catalysts can sometimes be developed which allow inherently clean reactions to be invented.

A very important part of such an undertaking is to be clear about what stages of a chemical process generate the most waste. Often this is found to be the separation stage, after the transformation of reactants to products, where all the various components of the final mixture are separated and purified. Approaches to chemical reactions which help to simplify this step are particularly powerful. Such an approach is exemplified by heterogeneous catalysis. This is an area of chemistry where the catalysts used are typically solids, and the reactants are all in the liquid or gas phase. The catalyst can speed up the reaction, increase the selectivity of the reaction, and then be easily recovered by filtration from the liquid, and reused.

One of the newest areas in the realm of catalysis is that of tailored mesoporous materials, which are finding many uses as highly selective catalysts in a range of applications. A mesoporous material is one which has cavities and channels (pores) in the range of 2-5nm (a nanometre is 10~9m) - for comparison, a typical chemical bond is of the order of 0.1 nm, and a small organic molecule is around 0.50nm across. Such mesoporous materials can be thought of as being analogous to the zeolites, which came to prominence in the 1960s. Zeolites are highly structured microporous inorganic solids (pores <2nm), which contain pores of very well defined sizes, in which catalytic groups are situated. A wide range of zeolites is known, each having different pore sizes and channel dimensions. Many are used in large-scale industrial applications. For example, many of the components of petrol are prepared using zeolites, as are precursors for terephthalic acid, used for the manufacture of PET bottles, processes in which millions of tonnes of material is produced annually.

Zeolites are prepared by the linking of basic structural units around a template molecule. The structural units are typically based on oxides of silicon and aluminium, and the templates are usually individual small molecules. Under the right conditions, the silicon and aluminium oxide precursors will link up around the template to form a crystalline three-dimensional matrix containing the template molecules. The template

Figure 4.1. Representation of the pore structure of HZSM5, one of the most important zeolites industrially. The vertical cylinders represent one pore network, and the other cylinders an interconnecting network. The narrow pores, and their almost complete uniformity, means that only some molecules can enter. Others are excluded, and cannot react at the active sites, which are found within the structure. Thus, the reactivity of a molecule is determined by its shape and size, rather than by its electronic properties. Such a situation is almost unique, with the only exception being enzymes, where molecules must fit into the enzyme active site in order to react.

Figure 4.1. Representation of the pore structure of HZSM5, one of the most important zeolites industrially. The vertical cylinders represent one pore network, and the other cylinders an interconnecting network. The narrow pores, and their almost complete uniformity, means that only some molecules can enter. Others are excluded, and cannot react at the active sites, which are found within the structure. Thus, the reactivity of a molecule is determined by its shape and size, rather than by its electronic properties. Such a situation is almost unique, with the only exception being enzymes, where molecules must fit into the enzyme active site in order to react.

molecules can be removed by calcination - i.e. by treatment at high temperatures in air, where the template is effectively burnt out of the structure. This leaves a highly regular structure which has holes where the template molecules used to be. These holes are connected to form pores and cages. It is in these pores and cages, also of very regular size and shape, that the catalytically active groups can be found (Figures 4.1 and 4.2). As we will see, it is this exceptional degree of regularity which is the key to the success of these materials.

Zeolites based on silicon and aluminium are acidic catalysts and are extremely thermally stable. This makes them ideal for use in the petrochemical industry, where some of the largest scale and most high energy transformations are carried out. These transformations are carried out in

Figure 4.2. The structure of Faujasite, a more open, larger pore zeolite. Larger molecules can enter this structure, which is more open, and slightly less regular than HZSM5 (Figure 4.1). Nevertheless, there are still many important molecules which cannot enter the pores of this zeolite, one of the most accessible of the class.

the gas phase at high temperatures and involve small molecules such as dimethyl benzenes and small alkanes - these are the materials which are used in petrol and PET, as mentioned above. Since the catalytic groups of the zeolite are found within the structure, the molecules must be able to diffuse into the structure before they can react. The size of the pores and channels of the zeolites are designed to be very close to the dimensions of the molecules to be reacted. This means that small changes in size and shape can dramatically alter the ability of the molecule to reach the active site. Under 'normal' chemical conditions, molecules react according to their electronic properties - i.e. since the electrons in the molecule must be rearranged during a reaction, their exact positioning and energy within the molecule usually determines both the rate and the nature of the reaction in a given situation. Harsh conditions usually allow many different reactions to take place, and are thus to be avoided if, as is almost always the case, a selective reaction is required. However, in the case of zeolites, the only molecules which can react are those which can fit into the pore structure and get to the active site. Similarly, the only products which can be formed are those which are of the right shape and size to escape from the catalytic sites, migrate through the pores, and out of the catalyst. This phenomenon is known as shape selectivity, although size selectivity might be a more accurate description.

Isomer relative diffusion rate

Figure 4.3. Relative diffusion rates in HZSM5. The shaded areas are the pore walls, the unshaded parts the vertical pore system from Figure 4.1. As can be seen, the rate of diffusion varies enormously with only very small changes in molecular size and shape. This allows the zeolite to discriminate almost completely between the three molecules shown, a situation which is unprecedented in traditional, homogeneous chemistry.

Isomer relative diffusion rate

Figure 4.3. Relative diffusion rates in HZSM5. The shaded areas are the pore walls, the unshaded parts the vertical pore system from Figure 4.1. As can be seen, the rate of diffusion varies enormously with only very small changes in molecular size and shape. This allows the zeolite to discriminate almost completely between the three molecules shown, a situation which is unprecedented in traditional, homogeneous chemistry.

An example of this is the commercial process for preparing para--xylene, the precursor to terephthalic acid, which is polymerised to give polyethylene terephthalate) (PET). In this case, the mixture of xylenes obtained from crude oil is reacted in a zeolite (known as HZSM5). The relative rates of diffusion in and out of the pores are sufficiently different (by a factor of about ten thousand) to allow the extremely efficient and selective conversion of all the isomers to the desired para isomer, which is the narrowest and can thus move through the structure most rapidly (Figure 4.3).

This type of selectivity is extremely valuable, as it gives chemists the opportunity to direct reactions in different ways to those available using conventional, electronically controlled, systems. With this in mind, chemists have searched for many years for materials with the same degree of uniformity displayed by the zeolites, but with larger pores. This would allow the concept of shape selectivity to be extended to larger molecules such as pharmaceutical intermediates, and other highly functional compounds. Other forms of selectivity will also benefit from a very regular structure.

The pore size of most zeolites is <1.5nm. This microporosity limits their utility in most areas of chemistry, where the molecules used are much larger, and for which mesoporous materials would be necessary. Unfortunately, attempts to use larger template molecules in the zeolite synthesis, an approach which should in theory lead to larger pore size zeolites, have met with very little success. Indeed, some zeolitic materials have been prepared which have mesopores - none of these has ever displayed any real stability and most collapse on attempts to use them. A new methodology was thus required.

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