Deciphering the Link between Zeolite Crystal Size, Brønsted Acid Site Distribution, and Dual-Cycle Selectivity in Methanol-to-Olefins over Zeolite

Abstract

The dual-cycle mechanism, which comprises the aromatic-based cycle and the alkene-based cycle, determines the product distribution and light olefin selectivity in the methanol-to-olefin (MTO) reaction over zeolites. We study how the crystal size of zeolites affects the spatial arrangement and proximity of Brønsted acid sites (BAS) and how the paired BAS modulates the dual-cycle mechanism in the MTO reaction over ZSM-5 zeolites. By using two-dimensional solid-state nuclear magnetic resonance (NMR), ion-exchanged Co²⁺ ion probes and transmission electron microscopy techniques, we demonstrate that the larger-sized ZSM-5 zeolites have a higher concentration of paired BAS, which promotes the aromatic-based cycle in the MTO reaction. The influence of paired BAS on the aromatic-based cycle is isolated from the zeolite morphological effects by comparing ZSM-5 zeolites with the same crystal size and Si/Al ratio but with varying paired BAS content. We reveal the enhanced reactivity of the key hydrocarbon pool species, such as methylbenzenes, cyclopentenyl cations, and pentamethylbenzenium ions, in the MTO reaction using NMR and steady-state isotopic switching experiments. Theoretical calculations corroborate the experimental observation that paired BAS significantly reduces the activation energy barriers for both ethene and propene formation through a paring mechanism, which renders the aromatic-based reaction pathway thermodynamically more favorable.