Theoretical insights into how the first C–C bond forms in the methanol-to-olefin process catalysed by HSAPO-34

Abstract

We report here a comprehensive understanding of the first C–C coupling during the induction period of the methanol-to-olefin process using density functional theory with the HSE06 hybrid functional. We illustrate the possible routes of formation for the active carbenium ion (CH₃OCH₂⁺), which has been identified to play an important part in triggering the formation of the first C–C bond and the hydrocarbon pool species. CH₃OCH₂⁺ can be generated not only from dimethyl ether and Z(O)–CH₃, but also from the reaction of HCHO and Z(O)–CH₃, which has a lower effective barrier. An understanding of the dominance of CH₃OCH₂⁺ over other carbocations and direct C–C coupling pathways is presented and quantitatively analysed. The charge distribution in the formation of CH₃OCH₂⁺ is revealed and it is confirmed that the carbenium ion is thermodynamically more favoured than the radical. The subsequent reaction after the first C–C coupling was investigated, which uncovered some important active C2 species that possibly led to the formation of the active hydrocarbon pool intermediates and may finally realize the catalytic cycle.