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

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