Fluoride etching opens access for bulky molecules to active sites in microporous Ti-beta zeolite

Abstract

Fluoride etching is a facile and efficient approach for hierarchical porosity engineering of microporous zeolites to overcome the inherent diffusion limitations. Hierarchical Ti-beta zeolites with abundant secondary meso-/macro-pores have been first prepared by chemical fluoride etching with various HF and NH₄F solutions. The physicochemical properties of the prepared Ti-beta (*BEA) zeolites are characterized extensively by complementary methods. Diluted HF solution preferentially extracts titanium from a Ti-beta zeolite framework, while the addition of NH₄F into HF solution shifts the chemical equilibria to produce more reactive HF₂⁻ for silicon, thereby achieving non-chemically selective extraction of Si and Ti from the *BEA framework. The hierarchical characters of fluoride-etched Ti-beta zeolites can be improved by regulation of the HF and NH₄F concentrations in the etching solutions. Compared with the parent microporous Ti-beta zeolite, the hierarchical materials prepared by etching in HF and NH₄F mixed solutions not only retain the chemical composition but also increase the accessibility to the active sites in microporous Ti-beta zeolites by forming hierarchical porosity, thus promising a remarkably enhanced catalytic oxidation activity for bulky molecules.