Heterogeneous catalytic oxidation and fine chemicals Sheldon

HETEROGENEOUS CATALYTIC OXIDATION AND FINE CHEMICALS R.A. Sheldon 

Catalytic oxidation is the most important technology for the conversion of hydrocarbon feedstocks (olefins, aromatics and alkanes) to a variety of bulk industrial chemicals.1 In general, two types of processes are used heterogeneous, gas phase oxidation and homogeneous liquid phase oxidation. The former tend to involve supported metal or metal oxide catalysts e.g. in tne manufacture of ethylene oxide, acrylonitrile and maleic anhydride whilst the latter generally employ dissolved metal salts, e.g. in the production of terephthalic acid, benzoic acid, acetic acid, phenol and propylene oxide. 

Both homogeneous and heterogeneous catalytic oxidations can be divided into the same three categories based on the type of mechanism involved (a) autoxidation, (b) direct oxidation of (coordinated) substrates and (c) catalytic oxygen transfer. 

In contrast to catalytic hydrogenation, where no reaction takes place in the absence of a catalyst, catalytic oxidations with molecular oxygen are complicated by the fact that oxygen reacts with organic substrates even in the absence of a catalyst. This involves the so-called free radical autoxidation mechanism with the following as key steps  

A major problem associated with such autoxidations is that they are largely indiscriminate, i.e. they exhibit poor chemo- and regio- selectivities. They are synthetically useful only with relatively simple substrates containing one reactive position, e.g. the oxidation of toluene to benzoic acid or p-xylene to terephthalic acid. Any catalytic oxidation has to complete with this non-catalytic pathway. Moreover, the situation is further complicated by the fact that transition metal ions also catalyze autoxidations by mediating the decomposition of trace amounts of hydroperoxides into chain-initiating radicals, via the so-called Haber-Weiss mechanism  

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