Finding the Compromise between Brønsted Acidity and Mesoporosity in Hierarchical ZSM-5 Zeolites

Abstract

Mesoporous ZSM-5 catalysts were prepared from a commercial ZSM-5 (Si/Al=15), using a 2-step micelle-templating procedure. By varying the NaOH/Si synthesis ratio in the range of 0.3-0.7, ZSM-5 catalysts with different textural and acidic properties were obtained. Increasing the NaOH/Si ratio in the synthesis gel leads to zeolites that feature reduced number of Brønsted acid sites, but higher mesopore volume, which allows for reducing the diffusion path length. In order to assess the best compromise between Brønsted acidity and mesopore volume two test reactions were run. The alkylation of phenol with tert-butyl alcohol allowed to evidence the accessibility of bulky molecules to active sites. The esterification of benzyl alcohol with hexanoic acid allowed to probe the specificity of Brønsted acid sites in ZSM-5. The best compromise was found for micro-/mesoporous ZSM-5 catalysts prepared with the ratio 0.38 <0.5.