Methanol conversion to hydrocarbons over zeolite catalysts: comments on the reaction mechanism for the formation of the first carbon-carbon bond

The reaction mechanism for the conversion of methanol to hydrocarbons using zeolite catalysts is discussed. In particular, the mechanism of the format ion of the initial carbon-carbon bond is considered in terms of the reactio n of a surface ylide intermediate with adsorbed methanol. It is suggested t hat the formation of the initial carbon-carbon bond involves the interactio n of the surface-bound ylide intermediate and its associated Bronsted acid site with a methanol molecule. This leads to the formation of a surface eth oxy group from which ethene can be formed by beta-elmination. alpha-Elimina tion will lead to a higher carbon nucleophile for further reaction with met hanol or dimethyl ether. To investigate the suggested scheme, density funct ional theory calculations are used to discern the nature of the surface yli de species and its interaction with adsorbed methanol. We find that the sur face ylide formation step actually results in the insertion of a CH2 group into the Al-O bond of the cluster representing the zeolite surface. Simple analysis of the wavefunction in terms of atomic charges and bond orders ind icates that this CH2 group has the required nucleophilic character to react with a methanol carbon atom. On the formation of a surface ethoxy group, w e find that the Al-O bond is reformed and so the reaction appears to procee d by the formation of transient defects in the zeolite framework. We also s uggest that this C-C bond formation will be more facile for clusters of met hanol molecules at the active site. (C) 1999 Elsevier Science B.V. All righ ts reserved.