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Direct Observation of Cyclic Carbenium Ions and Their Role in the Catalytic Cycle of the Methanol‐to‐Olefin Reaction over Chabazite Zeolites

“…The appearance of the resonance peaks at 240-255, 155, and 56 ppm in the 13 C MAS NMR spectra implies the formation of five-membered ring cations (polymethylcyclopentenyl cations) over the two catalysts, [17,19] while for H-SSZ-13, apart from these signals from polymethylcyclopentenyl cations, the appearance of the peaks at 203, 190, and 144 ppm also confirms the formation of six-membered ring cations. According to the previous studies carried out over HBeta [18] and DNL-6 [20] catalysts, the benzenium cation observed here is ascribed to heptamethylbenzenium (heptaMB + ), which will be demonstrated later by theoretical calculations of the 13 C chemical shift (Supporting Information, Table S1). Therefore, for the first time, in the catalysts with chabazite topologies, the most important reaction intermediates, polymethylcyclopentenyl cations and polymethylbenzenium cations, were successfully captured and directly observed under real MTO reaction conditions.…”

“…Even though some carbenium ions in the MTO reaction, such as cations 1-4 (see Scheme 1) have been identified using in situ solid-state NMR, the formation of these carbenium ions in the catalyst was verified mostly indirectly or by the aid of the reaction of co-feeding methanol and benzene or methylbenzene. [16][17][18][19] Owing to the difficulty in the direct observation of these HCP species, understanding of their formation and significance in methanol conversion over zeolites or SAPO catalysts under real working conditions is still a huge challenge. Until now, only cation 2 on H-ZSM-5 and cation 3 on DNL-6 with framework type RHO were observed directly under real working conditions using methanol as the only reactant.…”

Direct Observation of Cyclic Carbenium Ions and Their Role in the Catalytic Cycle of the Methanol‐to‐Olefin Reaction over Chabazite Zeolites

“…The appearance of the resonance peaks at 240-255, 155, and 56 ppm in the 13 C MAS NMR spectra implies the formation of five-membered ring cations (polymethylcyclopentenyl cations) over the two catalysts, [17,19] while for H-SSZ-13, apart from these signals from polymethylcyclopentenyl cations, the appearance of the peaks at 203, 190, and 144 ppm also confirms the formation of six-membered ring cations. According to the previous studies carried out over HBeta [18] and DNL-6 [20] catalysts, the benzenium cation observed here is ascribed to heptamethylbenzenium (heptaMB + ), which will be demonstrated later by theoretical calculations of the 13 C chemical shift (Supporting Information, Table S1). Therefore, for the first time, in the catalysts with chabazite topologies, the most important reaction intermediates, polymethylcyclopentenyl cations and polymethylbenzenium cations, were successfully captured and directly observed under real MTO reaction conditions.…”

Zeolite Shape‐Selectivity in the gem‐Methylation of Aromatic Hydrocarbons

“…[11] Additionally, there is strong experimental evidence for cyclic resonancestabilized cations as persistent species in the pores, such as cyclopentenyl and pentamethylbenzenium cations in HZSM-5 [12,13] and hexamethylbenzenium and heptamethylbenzenium (7MB + ) cations in HBEA. [14,15] Geminal methylbenzenium ions form the main starting point from which commonly proposed HP routes (such as the "paring" and "side-chain" mechanisms) originate. [16] The heptamethylbenzenium cation, for example, is formed from hexamethylbenzene (HMB) through one-step geminal methylation by methanol as shown in Scheme 1.…”