Tuning selectivity in hydrocarbon conversion catalysis

Abstract

An argument is made here for the importance of the kinetics of surface reactions in determining selectivity in catalytic processes. Particular emphasis is given to hydrocarbon conversion. Three examples from our research are used to illustrate how such selectivity can be tuned by changing the nature of the catalyst, by altering specific molecular features of the reactants, and/or by tuning the reaction conditions. Our discussion starts with the role of regiospecificity in dehydrogenation steps from adsorbed hydrocarbon intermediates in reforming, by showing how β-hydride elimination can explain hydrogenation, double bond migration and cis– trans isomerization in olefins. Next, the role of similar dehydrogenation steps in partial oxidations is illustrated by an example where atomic substitutions within the reactants can tip the balance between β- and γ-hydride elimination rates, and with that the selectivity between dehydrogenation and dehydration products from alcohols. Finally, the use of cinchona chiral modifiers to impart enantioselectivity to platinum hydrogenation catalysts is explained, and the role that reaction conditions such as concentrations, dissolved gases, and the nature of the solvent play in defining the performance of those systems is interpreted by using a molecular picture of the adsorption of the modifier.