Intermediates in the Methanol-to-hydrocarbons (MTH) Reaction: A Gas Phase Study of the Unimolecular Reactivity of Multiply Methylated Benzenium Cations

Abstract

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 CH 3 . 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 gasphase 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).

Conclusive Evidence for Two Unimolecular Pathways to Zeolite‐Catalyzed De‐alkylation of the Heptamethylbenzenium Cation

“…In line with the paring mechanism (Scheme 1), the eliminated propene contained as ingle 13 C atom that originated from aH eptaMB + + ring-carbon atom. Similarly,e liminated ethenec ontained as ingle 13 Ca tom. Although this isotoped istribution is consistent with ar ing expansion/ contraction mechanism,e thene is not commonly associated with the paringm echanism.…”

“…With the exception of pentene, the fragments observedw eret he same as observedp reviously by Svelle et al [13b] with unlabelled reactants. The number of 13 Ca toms in each eliminated alkene is also showni nF igure 1. In line with the paring mechanism (Scheme 1), the eliminated propene contained as ingle 13 C atom that originated from aH eptaMB + + ring-carbon atom.…”

“…The number of 13 Ca toms in each eliminated alkene is also showni nF igure 1. In line with the paring mechanism (Scheme 1), the eliminated propene contained as ingle 13 C atom that originated from aH eptaMB + + ring-carbon atom. Similarly,e liminated ethenec ontained as ingle 13 Ca tom.…”

“…The reactions of these ions weres tudied by gas-phase collisional activation, at echnique that has previously yielded significant insightsa bout the reactivity of aromatic cations. [13] As only unimolecular reactions can occur in av acuum, these experiments provided unprecedented insight into unimolecular alkenef ormationf rom HeptaMB + + . Thereafter,t he gas-phase data were compared to the reactions of de-protonated HeptaMB + + ,H MMC (4), overa na cidic zeolite catalysta t2 25-300 8C.…”

“…In this work, unimolecular PolyMBd e-alkylation was studied for the first time using isotopically labelledH eptaMB + + ,s ynthesised selectively with 13 C-labelled ring-carbon atoms and 12 Cm ethyl groups. The reactions of these ions weres tudied by gas-phase collisional activation, at echnique that has previously yielded significant insightsa bout the reactivity of aromatic cations.…”