Carbenium ions in zeolites: Two important carbenium ions have been observed for the first time under working conditions of the methanol‐to‐olefins (MTO) reaction over chabazite zeolites using 13C NMR spectroscopy. Their crucial roles in the MTO reaction cycles have been demonstrated by combining experiments and theoretical calculations.

Organic reaction intermediates confined in zeolite cavities are very important for product formation during methanol-to-olefins (MTO) conversion; however, direct evidence is still required to understand the particular function of these intermediates. Herein, for the first time, by careful selection of SAPO molecular sieves with different cavity size but identical 8MR pore openings, the reactivity and role of these intermediates played in olefin generation are verified by isotopic tracing method and theoretical calculations based on the observation of two types of carbenium intermediates, polyMB+ and polyMCP+, under the real MTO reaction conditions. It demonstrates that cavity size controls the molecular size and reactivity of these confined species, which results in different MTO activity and product selectivity.