Unraveling Molecular Diffusion Behaviors on the Acidic Sites of H‐ZSM‐5 Zeolite using Time‐Resolved in‐situ FT‐IR Technique

Abstract

Zeolites, renowned for their abundant crystalline structures and moderate acidities, have garnered significant attention in industrial chemical processes. Among them, the diffusion behaviors of various hydrocarbons within zeolite play a pivotal role due to their profound impact on product selectivity and separation efficiency. While acid sites are essential in determining the catalytic performance of zeolites, their effect on intra‐crystalline diffusivities has often been neglected in catalyst design. Herein, we employ a homemade time‐resolved in‐situ Fourier Transform Infrared (TR in‐situ FT‐IR) spectroscopy to investigate the intricate interplay between Brønsted acid sites and various probe molecules. Our study reveals that an augmentation in the density of Brønsted acid sites within H‐ZSM‐5 zeolites remarkably enhances the diffusivity of 1‐butene, in stark contrast to the behavior observed for iso‐butane. This contrasting effect in diffusivity is attributed to the distinct nature of interactions between alkenes and alkanes with Brønsted acid sites. Specifically, the π‐H interactions between alkenes and acid sites act as a driving force, propelling the alkene molecules forward through the zeolite pores. These findings offer valuable insights into designing tailored zeolites with specific acid site properties, controlling the transport behaviors of various probe molecules, and promising new avenues for catalysis and separation.