Hydrogenolysis of the Lower Alkanes on Single Metal Catalysts Rates, Kinetics, and Mechanisms

HYDROGENOLYSIS OF THE LOWER ALKANES ON SINGLE METAL CATALYSTS RATES, KINETICS, AND MECHANISMS 

The orderly presentation of the very extensive literature falling under this heading presents considerable difficulties. According to Max Planck, The chief problem in every science is that of endeavouring to arrange and collate the numerous individual observations and details which present themselves, in order that they may become part of one comprehensive picture. Let us see how this problem might be addressed. 

The information available is of two types. (1) First is the dependence of rate on the kind of alkane, on the nature of the catalyst, on reactant concentrations and on temperature. (2) Second is the dependence of product selectivities or other descriptive factor (for molecules having three or more carbon atoms) on these variables. This separation is somewhat arbitrary, but the first type focuses on the rate-determining step, and leads us into a discussion of mechanism that lacks the refinements needed to understand the origin of selectivities and their variation with conditions. Some people have combined these two features by evaluating orders of reaction and activation energies for the formation of each individual product, and assume by implication that each reaction that can be formulated to give these products demands a different sort of site but the numbers that emerge can only be used in a qualitative way, and the differences in site architecture cannot be defined. This approach is therefore somewhat sterile, and the alternative, which is to treat the rate of product removal and product selectivities separately, is to be preferred and is the one adopted here. 

References to relevant reviews are collected in the Further Reading section at the end of the chapter. 

It is convenient to start by considering the results obtained many years ago by John Sinfelt and his associates for the hydrogenolysis of ethane they are models of clarity, and they provide a suitable framework for results on other systems. In terms of activity, metals divide themselves easily into three classes (i) the very active (ruthenium and osmium) (ii) the moderately active (the majority) and (iii) the least active (palladium and platinum). This distinction is clearly drawn when the Arrhenius parameters are depicted as a compensation plot 

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