Heterogeneous catalytic kinetics solvent effects

The zeolite is rigid and ordered, and lacks conformational adaptability, in contrast to an enzyme, which can coil, uncoil, and twist around. Yet the zeolite can incorporate transition metal functions—these are of prime importance in enzyme catalysis—and it can effect redox reactions reactions over zeolites can be inhibited by competitive adsorption of reactants, products, solvents, or poisons—a phenomenon observed in biological and some other inorganic heterogeneous catalytic systems Rideal kinetics have been identified in some zeolite-catalyzed alkylations, a pattern which has its parallels in the enzyme field a few cases of stereospecificity (such as orfho-alkylation effects, unusual olefin isomer ratios), where a transition state not otherwise attainable intervenes, may exist. What better group of catalysts than zeolites might there have been to activate the evolutionary process in the dark, fermenting Pre-Cambrian seas some 1,000,000,000 years ago ... 

The procedure most frequently employed in heterogeneous catalysis consists in comparing the activity of catalysts determined by a standard testing reaction with some property of these catalysts. A less frequent procedure is based on the determination of the reactivity of several compounds on a single catalyst, which is subsequently related to some property of the reacting molecules (76). The third type of correlation of kinetic data involves the determination of the effect of solvents on catalytic reactions in the liquid state by means of a standard testing reaction on the same catalyst. 

A higher form of interpretation of the effect of solvents on the rate of heterogeneously catalyzed reactions was represented by the Langmuir-Hinshelwood kinetics (7), in the form published by Hougen and Watson (2), where the effect of the solvent on the reaction course was characterized by the adsorption term in the kinetic equation. In catalytic hydrogenations in the liquid state kinetic equations of the Hougen-Watson type very frequently degrade to equations of pseudo-zero order with respect to the concentration of the substrate (the catalyst surface is saturated with the substrate), so that such an interpretation is not possible. At the same time, of course, also in these cases the solvent may considerably affect the reaction. As is shown below, this influence is very adequately described by relations of the LFER type. 

In spite of the large number of suggested relations describing the effect of solvents on reaction kinetics, only partial success has been observed in their application, mostly always in similar reaction systems relations failed on passing to another system. The situation becomes much more complicated in heterogeneous systems. Their considerable complexity greatly impedes an adequate description of the effect of solvent on the course of the catalytic reaction using a one-parameter relation. 

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