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

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.