SELECTIVITY IN HETEROGENEOUS CATALYTIC OXIDATION

Abstract

Selectivity of the catalytic oxidation of hydrocarbons is discussed in terms of the type of intermediate complex formed. Two different intermediate complexes initiate two different routes of the reaction. Activation of oxygen leads to total oxidation through electrophilic attack on the double bond and formation of the complex of the peroxide or epoxide type, whereas partial oxidation requires activation of the hydrocarbon molecule resulting in the formation of the allyl species bonded as ligand in the intermediate complex. It proceeds then in a series of consecutive oxidative elementary steps, each of them requiring different active centers to be present at the catalyst surface. The type of product obtained and its selectivity depend thus on the type and mutual proportions of these centers. Oxidation is performed by the cationic active centers, whereas oxygen is inserted by a nucleophilic attack which may also be considered as an acid-base process. Mechanisms of these elementary steps is are discussed on the basis of the quantum chemical description of intermediate complexes.