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

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.