Acid Treatment for Regulating Hf Sites of the Hierarchical Hf-B-BEA Zeolite, Thereby Boosting Its MPV Reaction Performance

Abstract

Catalyst engineering plays a key role in bridging gaps between the structure of the Lewis acid center and its corresponding catalytic performance. In this regard, work that has been done in tuning active sites of Hf-containing zeolites, which hold promising catalytic activity for a variety of hydrogen-shift reactions, is far from complete. Herein, a Hf-B-BEA catalyst was rapidly prepared (within hours) by a hydrothermal semi-reconstruction strategy, involving partial framework etching of parent B-BEA and Hf grafting during the reassembling process. Then, an acid treatment was performed for expelling the boron atoms from the framework; more importantly, Hf migration into the silanol nests was triggered. The acid treatment worked powerfully as the driving force for transforming the semi-framework Hf into well-defined tetrahedrally coordinated framework Hf, thus endowing Lewis acidity of the obtained Hf-BEA. Consequently, this operation greatly boosted the Meerwein–Ponndorf–Verley reduction performance of Hf-BEA, which was almost inactive over the pristine Hf-B-BEA. The rational regulation of Hf sites provides new concepts in designing Hf-containing zeolites and understanding the structural–activity relationship.