Intermediates in the methanol-to-hydrocarbons (MTH) reaction: a gas phase study of the unimolecular reactivity of multiply methylated benzenium cations

In order to reach a deeper insight into the reaction mechanism of the zeolite catalyzed methanol to hydrocarbons reaction (MTH), the proposed reaction intermediates, i.e., a series of multiply methyl-substituted benzenium ions has been generated in the gas phase by chemical ionization. The fragmentations of the corresponding long-lived (metastable) ions have been investigated. While expulsion of H-2 dominates for the lower homologues, elimination of methane dominates for the higher homologues, accompanied by increasing amounts CH3 center dot. Loss of larger fragments relevant to the MTH-reaction, in particular ethene, propene and even butene, is also observed in minor amounts. This latter finding is consistent with a proposed reaction cycle in the MTH reaction known as the paring mechanism, and the feasibility of this mechanism has thus been demonstrated. The metastable gas-phase ions studied here are considerably more energetic than those residing in a zeolite catalyst, but they were found to decompose with markedly higher selectivity towards alkenes as compared to those activated by collision-induced dissociation (CID).