Correlating mesoporosity/acidity with catalytic performances for surfactant-templated mesoporous FAU zeolites

Hierarchical zeolite catalysts containing additional meso- or macropores have attracted significant interest due to their beneficial performance in various reactions involving bulky molecules. A surfactant-templated post-synthesis of zeolites in basic medium (NaOH) advances from the uniform mesopore size distribution and easy regulation of micro-to-mesopore volume ratio in thus prepared hierarchical zeolites, although their synthesis-acidity-performance relationships remain elusive. In this study, we tackled this problem by designing a series of surfactant-templated hierarchical FAU catalysts with systematically varied textural and acidic characteristics, while assessing their catalytic performance in two model reactions: (1) acid-site-strength insensitive O-alkylation of bulky alcohols (1-octadecanol and 1-adamatylmethanol) and (2) acid-site-strength sensitive Friedel-Crafts benzoylation of p-xylene. A systematic increase in the NaOH/Si molar ratio in the synthesis from 0.075 to 0.25 decreased the number of strong Bronsted acid sites in hierarchical FAU zeolites, while it increased the concentration of Bronsted acid sites accessible for bulky molecules (up to 67% in hierarchical zeolites vs. 21% in the parent FAU zeolite). The development of transport mesopores was shown to positively affect the outcome of the studied reactions independently of the required acid site strength. Although, the optimal interplay between the concentration of strong Bronsted acid sites and their 'accessibility' (i.e., fraction of acid sites accessible to molecules with a kinetic diameter larger than the micropore opening in zeolites) was found to differ for O-alkylation of bulky alcohols or Friedel-Crafts benzoylation of p-xylene, the possibility of balancing these characteristics in the hierarchical surfactant-templated zeolites was shown by adjusting the NaOH/Si ratio used for their preparation. The results of this study contribute to the understanding of the synthesis-property-performance relationships of surfactant-templated hierarchical zeolite catalysts and provide a guide to tailor the best catalyst for a given reaction. A surfactant-templated synthesis results in hierarchical zeolites with controllable micropore-to-mesopore volume ratio. However, the relationships between the synthesis conditions, acidity and catalytic performance of thus prepared zeolites are not fully understood and require further investigation.