Deboronation-assisted construction of defective Ti(OSi)3OH species in MWW-type titanosilicate and their enhanced catalytic performance

Abstract

A thorough investigation of the nature of the Ti species in the boron-containing Ti-MWW zeolite has been carried out. It was found that the inactive extra-framework TiO₆ species can cover up the zeolite Lewis-acid property through the weak atomic binding forces between the “Ti(IV)” atom and “O” atom (TiO₆ species). The nitric acid treatment could remove these TiO₆ species easily from the as-synthesized Ti-MWW(P), followed by exposure of the zeolite Lewis acidity. Importantly, the formation of new Ti(OSi)₃OH species with a strong Lewis-acid strength, is facilitated in the modification process. Thereby, the modified Ti-MWW zeolite has an improved catalytic activity in alkene epoxidation reactions. The removal of framework B(IV) species around the TiO₄ species is an essential step because it can accelerate the transformation of perfect Ti(OSi)₄ species to the defective Ti(OSi)₃OH species in zeolites. Moreover, the strategy to construct Ti(OSi)₃OH species was also applied successfully in boron-containing Ti-MCM-56 zeolites. Considering active species environment can be modulated effectively with the removal of framework tetra-coordinate boron, this strategy could also be beneficial for the preparation of other high-performance titanosilicates.