Common features of heterogeneous catalytic eliminations

In discussing the mechanism of eliminations over solids, the nomenclature which has been developed for homogeneous reactions will be used. Therefore the basic mechanisms of olefin formation have first to be outlined and their meaning in heterogeneous catalysis defined. 

The E2 mechanism is so called because the process is bimolecular and in solution consists of an attack by a base on the 3-hydrogen atom with synchronous splitting of the substituent X in the form of an anion. In heterogeneous catalysis, the most important feature is the timing of the fission of the two bonds Ca—X and C —H in the E2 or E2-like mechanism, these bonds are broken simultaneously. Because this can be achieved only by the action of two different centres, a basic one and an acidic one with both present on the surface, the kinetic distinction of the mechanism loses its original sense under these circumstances. 

The El mechanism has, as the rate-determining step in solution, the ionisation of the reactant forming a carbonium ion which then decomposes rapidly. For heterogeneous catalytic reactions, the important features are the occurrence of the reaction in two steps and the presence on the solid surface of carbonium ions or species resembling them closely. Again, the kinetic characterisation by way of an unimolecular process is of little value. Even the relative rates of the two steps may be reversed on solid catalysts. A cooperation of an acidic and a basic site is also assumed, the reaction being initiated by the action of the acidic site on the group X. 

The ElcB mechanism is a two-step process beginning with the abstraction of a proton from the /3-position by a base to give a carbanion. The second step is the loss of the group X as an anion. In heterogeneous catalysis, the corresponding mechanism consist of the primary action of a basic site assisted later by an acidic site which temporarily accomodates the group X . 

It is evident that the simple model of heterogeneous catalytic eliminations assumes the same adsorption complex for all mechanisms, written schematically as 

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