Influence of active-site proximity in zeolites on Brønsted acid-catalyzed reactions at the microscopic and mesoscopic levels

Abstract

Zeolites are the catalytic workhorses of various relevant reactions, primarily because of their acidic properties. However, there have been few studies on unraveling the influence of Al distribution on acid-catalyzed reactions at the mesoscopic and microscopic levels. Taking the methanol-to-aromatics (MTA) process as a paradigm, we compared the catalytic performance of two H-ZSM-5 zeolites with comparable properties, except for Al distribution, i.e., intraparticle distribution and Al proximity at the nanoscale. Our results demonstrate that Al enrichment at positions closer to the external surface results in more paired acid sites that exhibit enhanced activity than the isolated ones. Moreover, such Al organization accelerates progression of cascade reactions to yield aromatics and alleviate their diffusion out. Exploiting this Al-zoning feature, we further fabricated hollow zeolites by desilication. Combining the optimized active sites with the improved diffusion and void-confinement properties of hollow crystals results in the highest capacity to produce aromatics.