Heterogeneous Catalysis of Liquid Phase Oxidations

Many results obtained from homogenous processes also appear to apply to heterogeneous catalysts. A more direct relationship has begun to emerge in the study of new types of systems. These combine the best features of homogeneous and heterogeneous systems in which known metal complexes are chemically bonded to specific groups on a catalyst support. 

The principle of hard and soft catalysts developed for homogeneous catalysts may also be extended to heterogeneous catalysts. Soft surfaces are typified by transition metals and their alloys as well as transition metal oxides having metal-type conduction bands. Hard surfaces are typified by most transition metal oxides. In the first case, chemisorption can best be compared to 

Typical autoxidation catalysts, such as Co, Mn, Cu and Fe, afforded mainly cyclohexen-l-ol and cyclohexen-l-one. Other epoxidation catalysts, such as Mo or V, afforded mainly cyclohexene oxide and cyclohexen-l-ol (see Section III.B.2). It was concluded that the atomic number and the oxidation state of the transition metal are more important than the detailed catalyst structure in determining the course of the reaction. 

The gas phase oxidation of ethylene to ethylene oxide over silver catalysts has been studied extensively.49 la-c It has been suggested that epoxide formation involves transfer of oxygen from a silver-oxygen complex to the olefin on the catalyst surface.4913 Silver-on-silica also catalyzes the liquid phase oxidation of cumene to cumene hydroperoxide. A mechanism that involved insertion of coordinated oxygen into a C—H bond was proposed630  

However, this interpretation has been questioned by Vreugdenhil.631 He showed in a careful kinetic study that the catalytic activity of Ag-on-Si02 is mainly, if not entirely, due to its capacity to decompose hydroperoxides into chain-initiating radicals. (Approximately one-third of the ethylene is burned to carbon dioxide during the silver-catalyzed epoxidation.) 

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