Applications

The initial applications of MTSs were, perhaps not surprisingly, simply attempts to reproduce zeolite chemistry on larger molecules. This chemistry is based on the fact that the aluminium centres in zeolites cause a negative charge to exist on the framework of the solid; this charge must be balanced by a cation. When the cation is a hydrogen ion (proton), the material is an acid, and indeed some zeolites are very strong acids indeed. However, the acidity of the corresponding MTSs is much lower, and initially this limited their applicability somewhat. Nevertheless, the MTSs are often found to be very effective as mild acid catalysts. Much work has therefore been aimed at the production of other materials using the same concept, but with either different templating systems, or with combinations of elements other than Si and Al in the framework.

However, many industrial processes are based on the use of very strong acids, and there is great pressure to find replacements for the liquid acids currently used in industrial processes. One method which has been successfully applied to increase the acidity of these systems is the immobilisation of aluminium chloride onto the pore walls. Aluminium chloride is itself a very strong acid, and is one of the commonest in industrial chemistry. It is used in a wide range of transformations, but cannot be recovered intact from reactions. Its destruction leads to large quantities of waste being generated. Aluminium chloride has been successfully attached to the walls of HMS materials, without any reduction in activity - i.e. the resultant material has the same activity as unsupported aluminium chloride. A major advantage over free aluminium chloride is the ease of removal of the solid catalyst from reaction mixtures, simplifying the process and reducing waste dramatically. The catalyst can then be easily recovered from the raction mixture, and reused. A second important advantage is the ability to control product distributions by tailoring the pore size of the material. This is best illustrated by the preparation of linear alkyl benzenes (LABs) which are precursors to detergents, and are produced on a massive scale using either aluminium chloride or hydrogen fluoride, both of which have many problems associated with their use. The general scheme is shown in Figure 4.6.

alkylating desired product alkyl alkylating alkyl undesired product alkyl agent agent desired product alkyl undesired product alkylating agent is

Figure 4.6. General scheme for the synthesis of linear alkyl benzenes, precursors to surfactants. Control over pore size of the catalyst can suppress the second alkylation almost completely. Given the ease with which the pore size can be chosen, one can design an effective catalyst for any particular reaction, and allow the selective and clean production of the desired mono-alkyl product, thus eliminating much of the waste associated with the process.

for example:

Figure 4.6. General scheme for the synthesis of linear alkyl benzenes, precursors to surfactants. Control over pore size of the catalyst can suppress the second alkylation almost completely. Given the ease with which the pore size can be chosen, one can design an effective catalyst for any particular reaction, and allow the selective and clean production of the desired mono-alkyl product, thus eliminating much of the waste associated with the process.

As can be seen, the reaction will proceed to the monoalkylated product, but does not stop there. The alkylated product is more reactive than the starting material, and will alkylate again, giving products which are useless. Control over this aspect of the reaction can only be achieved with difficulty in traditional systems, and very high dilutions are used to control the product distribution. The use of the new mesoporous materials allows a more concentrated (and thus more efficient) process to be developed. This is because the dialkylated product is bigger than the mono-alkylated product. Careful choice of the pore size of the material will mean that the space inside the pore is too small for the dialkylated product to form, but is big enough for the desired monoalkylated product to form readily. Thus, the reaction can run selectively at high concentrations, solving the selectivity problem and using a catalyst which can be easily recovered. Waste is thus reduced dramatically.

While most work has been concentrated on aluminium-containing zeolites, the discovery of titanium-containing zeolites by an Italian company, Enichem, in the 1980s represented another major breakthrough in zeolites. They showed that these titanium-containing zeolites are excellent catalyst for the selective oxidation of a variety of simple, small molecules. Such oxidations are amongst the most important reactions in organic chemistry, as they allow the introduction of a huge range of important functions into the basic hydrocarbon feedstocks derived from oil. Larger pore size versions of the material would allow a much wider range of organic molecules to be functionalised. This type of reaction is of enormous importance in large molecule chemistry too, with some existing processes being far from 'green'. Thus researchers have been active in preparing analogous MTS structures containing titanium. Results with these MTS materials have shown that these materials are indeed capable of carrying out many of the desired reactions, but without the limitations of size which hamper the zeolites. For example, one of the important applications of titanium-containing zeolites is the hydroxyla-tion of benzene to phenol (which is used as a precursor to antioxidants for food and cosmetic use), and then further to hydroquinone, used in the photography industry as a developer. Ti containing MTSs are known and can carry out the same type of transformations as the corresponding zeolite. Larger molecules such as naphthalenes, which cannot enter the pores of the zeolites, can access the pores of the MTSs, and react in the expected manner. One important target is Vitamin K3, a derivative of naphthalene, formed by hydroxylation. Current practice still involves the use of acidic chromium reagents which are used in large quantities, and are highly toxic. Significant success has been reported with the use of Ti containing mesoporous materials of this reaction type, and further progress is expected (see Figure 4.7).

The organically modified versions of these materials have also been investigated as catalysts. These materials have great potential, as the incorporation of organic groups will allow a much wider variety of materials to be prepared, and thus a much wider range of applications can be investigated. Simple amine (an example of a basic group, which will remove a proton from a molecule, thus making it able to react in many ways) con-

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