Formation and Evolution of Methylcyclohexene in the Initial Period of Methanol to Olefins over H-ZSM-5

Identification of the reaction intermediates and probing into their evolution in the conversion of methanol to light olefins (MTO) are rather challenging but meaningful in unraveling the hydrocarbon pool (HCP) mechanism as well as in exploring more efficient catalysts in MTO. Hence, we would like to report a finding made in this regard that methylcyclohexene (MCH) is a crucial reaction intermediate in the initial period of MTO over H-ZSM-5 through various measures, including transient pulse experiment, 13C cross-polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR), gas chromatography–mass spectrometry (GC–MS), and density-functional theory (DFT) calculation. The early alkenes (mostly ethene and propene) produced in the induction stage can rapidly construct the long-chain alkenes through oligomerization and create dienes through the Prins reaction with formaldehyde from methanol dehydrogenation. After that, MCH is facilely formed via the Diels–Alder (D–A) reaction between dienes and monoenes. Compared with the oligomerization and cyclization of alkenes, the D–A reaction shows higher activity with a lower energy barrier in building larger molecule products. MCH is highly reactive and can be quickly transformed either into methylcyclopentene (MCP) via ring contraction or into methylbenzene (MB) via hydride transfer and deprotonations; the formation of MCP may take the priority over that of MB at a low temperature, as the former reaction needs a lower energy barrier. Accordingly, MCH acts as a bridge connecting MCP and MB and plays a vital role in the establishment of initial HCP in MTO. These findings should be helpful for an in-depth understanding of the MTO reaction mechanism and then benefit further research into MTO.