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Black plate (251,1) hydroperoxides, RO2H, e.g. tert-butyl hydroperoxide or cumyl hydroperoxide, may be used successfully to promote oxidations in both the gas and liquid phases. Heterogeneous catalysts are often further advantageous in that they are readily recovered after the reaction is completed, and so may be recycled, and are easily used in continuous processes. This is beneficial in the design of ‘‘green chemistry’’ in order to diminish the environmental impact of the chemical industry. When a metal centre in placed inside an inorganic matrix, a catalyst can be obtained which allows specific activities and specificities to be retained by preventing the oligo- merisation of active monomeric oxometal (M5O) or related species to m-oxo complexes. The pronounced propensity for oxometal complexes to undergo such transformations is one main reason for the low activities often found in solution. Titanium(IV) catalysts provide an apposite example of this and are most effective as isolated in polymer matrices which prevent them forming oligo- mers, or a redox-active metal centre may be incorporated at the surface of an oxide support material, e.g. the heterogeneous Ti(IV)ySiO2 catalysts used by Shell to promote the formation of propylene oxide (1,2-epoxypropane) by reacting propylene (propene) with ethylbenzene hydroperoxide79. The redox centre may be incorporated in the aluminosilicate framework or within the cavities of zeolites and other molecular sieves. Such ‘‘redox molecular sieves’’ have a regular arrange- ment of micropores with homogeneous internal cavities and channels of molecular size80 – 82. It is possible to tailor-make catalysts where the reaction dimension is tightly controlled by the pore size of the zeolite, and also by the degree of hydro- phobicity which in combination act to sieve the kind of molecules for access to the active centre on the basis of their size and relative hydrophobicyhydrophilic character. There is a clear analogy between such materials and enzymes, and thus they have been termed ‘‘zeozymes’’83. For many decades, the strategy relied almost entirely on the introduction of redox-active metal ions into zeolites by cation-exchange, but here the metal cation is susceptible to leaching and escapes, often quite rapidly, into the prevailing liquid medium. In 1983, researchers at Enichem found84 that the framework-substituted titanium silicalite (TS-1) was stable for the selective oxidation of a range of organic substrates with a 30% aqueous solution of H2O2. This resulted in a surge of activity to find alternative titanium(IV) molecular sieves and indeed those containing other metal ions for use in organic synthesis85–88. www.scienceprogress.co.uk Properties and applications of zeolites 251PDF Image | Properties and applications of zeolites
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