Interdependence between porosity, acidity, and catalytic performance in hierarchical ZSM-5 zeolites prepared by post-synthetic modification

Abstract

Adopting a systematic demetallation strategy to prepare hierarchical ZSM-5 zeolites with a wide range of concomitant micro- and mesoporosity, we precisely examine the relation between variations in the porous and acidic properties. A comparative assessment of the type, concentration, and strength of acid sites is attained through the infrared study of adsorbed probe molecules (pyridine, 2,4,6-trimethylpyridine, 2,6-di-tert-butylpyridine, and d3-acetonitrile), the temperature-programmed surface reaction (TPSR) of n-propylamine and the cracking of n-hexane. The impact of these core intrinsic properties on the hierarchical zeolite performance is quantified in liquid-phase reactions with distinct acidity demands and diffusion constraints, including the alkylation of toluene with isopropanol or benzyl alcohol and the esterification of hexanoic acid with benzyl alcohol. Optimal post-synthetic modification greatly improves the initial turnover rates per Brønsted acid site. The relative performance is strongly influenced by both the mesopore surface area and by the associated concentration and strength of acid sites. The need for improved characterization of the mesopore surface acidity is highlighted.