Methanol to Olefins Reaction over Cavity-type Zeolite: Cavity Controls the Critical Intermediates and Product Selectivity

Methanol to olefins (MTO) reaction over H-RUB-50 zeolite, an eight-membered ring (8-MR) and cavity-type zeolite, presents higher selectivity for ethene. The host–guest interaction was dissected and used to explain the cavity-controlled reaction route and product selectivity. By the aid of the in situ ¹³C MAS NMR spectroscopy, GC-MS, ¹²C/¹³C-methanol switch experiments, and theoretical calculations, the methylbenzenium cations, methylcyclopentenyl cations (triMB⁺, tetraMB⁺, and triMCP⁺), and their deprotonated forms with less methyl groups substitution were captured over LEV zeolite and confirmed as the critical reaction intermediates. The energetic span model was employed to identify the preferred reaction mechanism and provide the theoretical evidence to understand product selectivity. The side-chain methylation mechanism was theoretically predicated to be the energetically favorable route for olefins generation with the participation of these active intermediates. Paring cycle with trimethlycyclopentadienyl cation as the intermediate makes less contribution to ethene formation due to the relatively large energy span. Based on the overall evaluation of the catalytic cycle, the difference of energy span of the whole reaction pathway for ethene and propene formation can give direct theoretical evidence for product selectivity. Additional study to the steps for generating precursors of ethene and propene offers extra support on the understanding of product selectivity and the dominant generation of ethene. This study captured the critical intermediates and established a rational and energetically feasible route of light olefins generation from MTO reaction over H-RUB-50. More importantly, it is exhibited that cavity controls the product selectivity via the important steric constraint for the formation of critical intermediates and the proceeding of critical reaction steps, based on the understanding of the host–guest interaction of the cavity-type zeolite catalyzed MTO reaction.