CONVERSION OF METHANOL TO GASOLINE - A NEW MECHANISM FOR FORMATION OF THE 1ST CARBON CARBON BOND

A general mechanism is proposed for the critical first C-C bond forming event in the H-ZSM-5-zeolite-catalyzed and related methanol to gasoline (MTG) conversion reactions. We begin with the assumption that the Pearson reaction, in which methanol is converted to hydrocarbons at 200 °C by polyphosphoric acid (PPA), operates by a mechanism similar to the zeolite-catalyzed processes. Results from the literature are combined with new experimental and computational work to present a simple, thermochemically reasonable picture of the overall transformation cycle. Consistent with established gas-phase thermochemistry and recent solid-state NMR results, carbon monoxide appears to be the operative catalytic species. According to the new scheme, after its slow initial generation (induction period), CO is methylated, producing an acylium ion, which is then deprotonated; subsequent methylation of the resulting ketene represents the key new C-C bond forming step. Loss of ethylene completes the cycle. Ab initio calculations support the kinetic and thermodynamic accessibility of the overall reaction sequence. Solution NMR experiments on the Pearson reaction show no exchange of protons between the acidic HO and the hydridic HCH2OX protons of methanol or its phosphate esters, even under conditions where substantial hydrocarbon is being generated. This result, along with simple thermochemical arguments, renders highly doubtful the widely invoked pathway via methylation of an oxonium methylide. © 1990, American Chemical Society. All rights reserved.