A mechanism for methanol conversion over HZSM-5 catalyst

Abstract

Mechanistic stages in the conversion of methanol into hydrocarbons, over the zeolitic catalyst HZSM-5, have been studied by means of infrared spectroscopy. Data are reported for both internal surface and gas-phase species. Evidence is presented on the following aspects: the involvement of Lewis sites in the initial dehydration of methanol; the existence of carboxylate side reactions generating CO and CO₂; a role for intermediate isobutene; and the reformation of methanol. The extent of the CO₂ side reaction appears to correlate with the extent of aromatic product formation. This is interpreted in terms of the involvement of the co-product, hydrogen, in the disproportionation of aromatic products. These observations are embodied in a mechanism invoking both organic species and surface sites for the overall process.