A mechanistic understanding of light olefins selectivity in methanol-to-hydrocarbons conversion on MFI

Abstract

Methanol-to-hydrocarbons (MTH) conversion is the final processing step in converting alternative feedstock such as coal, natural gas, and biomass, to hydrocarbon fuels and petrochemicals. Methanol reacts on acidic zeolite catalysts via an indirect “hydrocarbon-pool” mechanism to form a wide variety of hydrocarbons including light olefins, gasoline-range hydrocarbons, and aromatics. The hydrocarbon-pool mechanism involves two reaction cycles simultaneously operating inside the zeolite pores: an olefins-based reaction cycle and an aromatics-based reaction cycle. The observed product distribution in MTH can be rationalized as an effect of the relative rates of propagation of the aromatics-based and the olefins-based reaction cycles. Quantifying the relative propagation of these two catalytic cycles and understanding how these cycles contribute to the overall product distribution under varying reaction conditions, varying feed composition, and on different zeolite topologies or morphologies, is critical for developing structure-function relationships for MTH catalysts. In this work, the effects of independently varying (i) the feed composition (by co-feeding hydrocarbons or oxygenates), (ii) the concentration of active sites (by changing the chemical composition of the zeolite), and (iii) the diffusion characteristics of the zeolite (by changing the crystallite size or silylating the external surface), on the relative rates of propagation of the aromatics- and olefins-based cycles, and consequentially on the observed MTH product selectivity are presented.