Impact of Coke Distribution on the Diffusion of Alkanes in the MFI Zeolite

Abstract

The diffusion behavior of molecules within the intricate channels of zeolite catalysts is a key factor of catalytic performance and product selectivity. In this study, we systematically investigated the influence of coke distribution within the zigzag and straight channels of the MFI zeolite on the diffusion behavior of alkanes. Our findings reveal that short-chain alkanes (e.g., CH₄) demonstrate dynamic interconversion between zigzag and straight channels of the MFI zeolite in coke-free systems. When one set of channels is obstructed by coke, they preferentially diffuse through the alternative set of channels. However, long-chain alkanes (e.g. n–C₁₂H₂₆) primarily diffuse through straight channels in the coke-free MFI zeolite, with minimal migration to zigzag channels. Therefore, this unidirectional transport behavior becomes severely compromised when coke deposits accumulate in the straight channels, where molecules are effectively trapped between the coke layers. To overcome these diffusion limitations, effective approaches are proposed: (1) elevating the temperature to activate the zigzag channel and (2) introducing mesopores to mitigate coke deposition and diffusion limitations. This study provides valuable insights into the diffusion pathway preferences of molecules within coke-containing zeolite systems and offers a practical reference for optimizing and selecting zeolite catalytic reaction conditions.