A survey of selective oxidation

The selective oxidation of organic molecules is one of the most important processes used in the chemical industry. Its reactions fall into two broad categories (i) gas-phase oxidation of hydrocarbons (alkanes, alkenes) to oxygenated products and (ii) further oxidation of molecules containing one or more oxygen atoms (mainly in the liquid phase). 

The first set of reactions is the mainstay of the petrochemical industry 1 outstanding examples are the oxidation of propene to propenal (acrolein) catalysed by bismuth molybdate, and of ethene to oxirane (ethylene oxide) catalysed by silver. In general these processes work at high but not perfect selectivity, the catalysts having been fine-tuned by inclusion of promoters to secure optimum performance. An especially important reaction is the oxidation of ethene in the presence of acetic (ethanoic) acid to form vinyl acetate (ethenyl ethanoate) catalysed by supported palladium-gold catalysts this is treated in Section 8.4. Oxidation reactions are very exothermic, and special precautions have to be taken to avoid the catalyst over-heating. 

The second set of reactions is more related to the fine chemicals and pharmaceutical industries, although some of them are carried out industrially on a very significant scale. Temperature-control in three-phase systems is easier, and is rarely a problem, but adequate mixing of the phases is essential to avoid mass-transport limitation. Selectivity here is more directed towards securing the desired product, which may be one of several closely related ones. 

Turning to current work employing gold catalysts, the reaction that has attracted most attention is the oxidation of propene to methyloxirane (propene oxide or 1,2-epoxypropane).2 While silver catalysts work very well for epoxidising ethene, they fail utterly with propene, because the selective intermediate is more reactive than the reactant, and carbon dioxide alone 

The most dramatic results obtained so far with gold catalysts have been with the liquid phase processes. They are conducted with oxygen or air, often using water as solvent, and are therefore felt to be environmentally benign. Particular success has been obtained with reducing sugars (Section 8.3.2) and other aldehydes (Section 8.3.3), and with alcohols and other hydroxy-compounds (Sections 8.3.4-8.3.7). Reactions that use soluble gold complexes to catalyse selective oxidation are reported in Chapter 12. 

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