Product Selectivity in Alkane Conversion within Cavities of Enzymes and Zeolites: Influence of Cavity Volume on Product Selectivity

Abstract

Alkane monooxygenases and proton-exchanged zeolites are catalysts that have cavities within their molecular structure and a catalytic site for n-alkane conversion in the cavity. The effect of the spatial volume of the cavity in the catalysts and the molecular volume of the reacting molecules on the product selectivity in catalytic reactions was evaluated by analysis of the product selectivity in the oxidation of C4-C8 n-alkanes to alcohols by alkane monooxygenases, and in the cracking of n-pentane by proton-exchanged zeolites. The selectivity for production of the 2-hydroxyalkane enantiomer from n-alkanes depended on the cavity volume of the substrate binding site in the alkane monooxygenases. Further, the selectivity for ethane and propene in n-pentane cracking by proton-exchanged zeolite also depended on the cavity volume of the zeolite. From these dependencies, we propose that product selectivity is expressed if the molecular volume of the reacting molecules or reaction intermediates is almost identical to the volume of the cavity in which the catalytic reactions proceed.