Facile Synthesis of Hierarchical Sn-Beta Zeolite by Tuning Chemical Equilibrium of Fluoride Etching

Abstract

The Sn-Beta zeolite is commonly known as a unique Lewis acid catalyst, however, usually suffers from the drawback of diffusion limitation within zeolitic micropores. To address these issues, the present work reports a facile fluoride etching approach for the synthesis of hierarchical Sn-Beta zeolites by tuning chemical equilibrium of HF and NH₄F mixed solutions. It was found that the etching of HF solution alone only extracts Sn species in the Sn-Beta framework while adding NH₄F into HF solution shifts the chemical equilibria to produce more reactive HF₂ ⁻ for Si, thereby achieving unbiases selective extraction of Si and Sn from *BEA framework. The obtained hierarchical Sn-Beta zeolite possessed excellent catalytic activity in Baeyer-Villiger oxidation of bulky 2-adamantanone with aqueous hydrogen peroxide. Structure-performance relationship revealed that the Lewis acid site and the pore structure are two crucial factors determining the catalytic activity. The former was identified as a catalytic active center while the latter increase the accessibility to active sites in microporous Sn-Beta zeolites by forming hierarchical porosity. Therefore, the sample obtained after optimizing the conditions of NH₄F- HF mixed solutions presented a remarkably enhanced catalytic oxidation activity for bulky molecules.