O2-Mediated Reactivity of Confined Carbenium Ions in Zeolites.

Carbenium ions are critical intermediates in zeolite-catalyzed hydrocarbon transformations; yet, their fundamental reactivity and mechanistic pathways remain incompletely understood, particularly under conditions relevant to practical catalysis. Here, we demonstrate that molecular oxygen─often present in catalytic systems but rarely mechanistically considered─profoundly alters carbenium ion chemistry. Through a combination of solid-state 13C NMR, operando UV-vis, and online mass spectrometry, we show that O2 selectively promotes the oxidative transformation of cyclopentenyl cations via cyclopentenone-type intermediates on ZSM-5 zeolite. This oxygen-mediated pathway enhances the formation of methylated aromatics, modulates reaction selectivity, and accelerates the evolution of polyaromatic species, ultimately impacting the catalyst stability. These effects are consistently observed in both methanol-to-hydrocarbon and ethene conversion over zeolites.