Reaction Pathways and Rate-Determining Steps in Reactions of Alkanes on H-ZSM5 and Zn/H-ZSM5 Catalysts

Abstract

Reactions of ethane and propane on H-ZSM5 lead primarily to undesired products of β-scission reactions. Turnover rates, aromatic site-time yields, and the rate of hydrogen disposal during alkane conversion increase as the density of exchanged Zn cations increases in H-ZSM5. Zn cations decrease β-scission rates and the residence time of alkenes within oligomerization/β-scission cycles, leading to higher rates and selectivity for the synthesis of C6-C8aromatic products. Reactions of propene/2-13C propane mixtures show that initial dehydrogenation steps become increasingly irreversible as Zn cations catalyze the recombinative desorption of H-atoms and H2. This process depletes the surface hydrogen pool available for the reverse hydrogenation reactions and it increases the overall rate of propene conversion to aromatics. Zn cations promote not only alkane dehydrogenation, but also the subsequent removal of hydrogen as H2, leading to unsaturated intermediates required in cyclization steps. C3H8/C3D8isotopic exchange rates are much higher than propane conversion rates. Exchange rates are similar on H-ZSM5 and Zn/H-ZSM5, but negligible on Na-ZSM5, consistent with the involvement of Bronsted acid sites in isotopic scrambling within alkanes and with a predominant role of Zn in the recombinative desorption of H-atoms. This proposal is consistent with the effects of Zn cations on the rate of H2/D2isotopic equilibration, with the role of added H2on alkane reaction rates, and with the rate of incorporation of D-atoms into reaction products formed from C3H8/D2mixtures.