Non-crystalline heterogeneous catalysts

The final section in this chapter will discuss several areas of current research, which have attempted to use non-crystalline materials such as metallosilicates or supported oxidation catalysts on high surface area solids. The definition of non-crystalline employed here refers to the support system and not necessarily the active catalytic centre. 

The previous section discussed the use of crystalline metal silicalites as catalysts for the liquid-phase oxidation of organic substrates with peroxygens. The unfortunate feature of many of the crystalline catalysts is their relatively difficult synthesis, requiring specialized expertise, equipment and the use of proprietary knowledge. Also the small pores of the zeolite limit access to the catalytic sites by large organic substrates, which are often more prevalent in the 

Whilst the metal containing xerogels show reasonable catalytic activity with hydrogen peroxide, they are still inferior to the redox zeolites mentioned earlier. 

One problem with them is their sensitivity to the medium they are employed in, as regards metal oxide leaching. For example, in f-butanol, the catalysts are relatively stable, but in co-ordinating solvents such as acetonitrile, considerable leaching is detected. It is possible that if the leaching problem can be overcome, the MOx-Si02 xerogel catalysts could have a future on an industrial scale. 

The penicillin-G-potassium salt has been oxidized using supported polyoxo-metallates.62 Specifically, the oxidation of the sulfide group to give the sulfoxide was achieved, the product being an intermediate in the production of Cephalosporin antibiotics.63 The selective oxidation of benzenethiol to benzenesulfonic acid with hydrogen peroxide was effectively catalysed by phosphotungstic acid on y-alumina.62 This was in contrast to the homogeneous oxidation where the disulfide was produced in substantial yields. 

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