Influence of intracrystalline diffusion constraints enhanced by channel fluctuation on zeolite catalyst performance in isomerization and aromatization

Abstract

In the realm of zeolite catalysis, understanding the role of intracrystalline diffusion constraints is of paramount significance, especially in the context of isomerization and aromatization reactions. This study delves deep into the structure-performance relationships, shedding light on the profound impact of channel fluctuation on isomerization and aromatization performance. Through meticulous analysis, we scrutinized the intracrystalline diffusion constraints of olefin intermediates integral to these reactions. ZSM-5, ZSM-22, Beta, and ZSM-12 zeolites, while possessing similar crystallinity, Si/Al ratio, acidity, and diffusion path length, display distinct channel fluctuations. Insightful in situ infrared desorption experiments coupled with molecular dynamic simulations underscore that these channel fluctuations intensify the intracrystalline diffusion constraints of olefin intermediates via an entropic trapping effect. Consequently, the isomers selectivity in n-octane isomerization adopts the sequence of Pt/Al2O3-ZSM-5 < Pt/Al2O3-ZSM-22 < Pt/Al2O3-Beta < Pt/Al2O3-ZSM-12, inversely correlating with channel fluctuation. This arises from the acceleration of undesired β-scission of olefin intermediates by intracrystalline diffusion constraints accentuated by channel fluctuation, leading to cracking by-products. Contrarily, the aromatic selectivity in 1-octene aromatization mirrors the opposite trend: ZSM-5 > ZSM-22 > Beta > ZSM-12, directly associating with channel fluctuation. This owes to the augmented intracrystalline diffusion constraints fostering the cyclization and hydrogen transfer of olefin intermediates, culminating in aromatic products. This study presents an in-depth investigation into how the diffusion constraints related to channel fluctuation influence product selectivity across different zeolite catalysts, offering valuable insights for future research and application in heterogeneous catalysis.