Deciphering the\nLink between Zeolite Crystal Size,\nBrønsted Acid Site Distribution, and Dual-Cycle Selectivity in\nMethanol-to-Olefins over Zeolite

The dual-cycle mechanism, which comprises the aromatic-based\ncycle\nand the alkene-based cycle, determines the product distribution and\nlight olefin selectivity in the methanol-to-olefin (MTO) reaction\nover zeolites. We study how the crystal size of zeolites affects the\nspatial arrangement and proximity of Brønsted acid sites (BAS)\nand how the paired BAS modulates the dual-cycle mechanism in the MTO\nreaction over ZSM-5 zeolites. By using two-dimensional solid-state\nnuclear magnetic resonance (NMR), ion-exchanged Co<sup>2+</sup> ion\nprobes and transmission electron microscopy techniques, we demonstrate\nthat the larger-sized ZSM-5 zeolites have a higher concentration of\npaired BAS, which promotes the aromatic-based cycle in the MTO reaction.\nThe influence of paired BAS on the aromatic-based cycle is isolated\nfrom the zeolite morphological effects by comparing ZSM-5 zeolites\nwith the same crystal size and Si/Al ratio but with varying paired\nBAS content. We reveal the enhanced reactivity of the key hydrocarbon\npool species, such as methylbenzenes, cyclopentenyl cations, and pentamethylbenzenium\nions, in the MTO reaction using NMR and steady-state isotopic switching\nexperiments. Theoretical calculations corroborate the experimental\nobservation that paired BAS significantly reduces the activation energy\nbarriers for both ethene and propene formation through a paring mechanism,\nwhich renders the aromatic-based reaction pathway thermodynamically\nmore favorable.