Modern Ideas About the Mechanisms of Homogeneous and Heterogeneous Acid Catalysis: Similarities and Differences

The modern ideas about the nature of the protonated states in aqueous solutions in non-polar solvents and on the surfaces of heterogeneous oxide catalysts of the acid type are examined. It is concluded that in heterogeneous systems mechanisms analogous to the mechanism of homogeneous acid catalysis in aqueous solutions operate only in catalytic reactions of polar molecules. Owing to the absence of solvation effects, the catalytic reactions of hydrocarbons take place, contrary to the widely held view, via intermediate covalent and not ionic structures. On the other hand, the properties of carbenium ions are characteristic of transition states, which in many instances also possess features resembling those of adsorbed carbenes of surface ylides. The role of synchronous or concerted mechanisms in heterogeneous acid catalysis is discussed.
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