A one-step rapid synthesis of TS-1 zeolites with highly catalytically active mononuclear TiO6 species

Abstract

The modulation and determination of the coordination environments of Ti active sites in titanosilicate zeolites are key challenges in the rational design of high-performance heterogeneous catalysts. Up to now, the highly catalytic active Ti species, while not yet unambiguously determined, is mainly constructed by the complex post-treatment method, in which it is difficult to precisely control the distribution of the created Ti species. Herein, we showcase a facile strategy for the one-step rapid synthesis of TS-1 (MFI framework type) zeolites with highly catalytically active Ti species via active seed-assisted microwave irradiation. The coordination environment and local structure of the Ti species in TS-1 zeolites were investigated via UV-vis, UV-Raman, and X-ray absorption spectroscopies. Significantly, according to the elaborate characterizations based on the extended X-ray absorption fine structure (EXAFS), these highly catalytically active Ti species were clearly identified as novel mononuclear TiO₆ species. Experimental studies revealed that the active seeds can provide plenty of high-coordinated Ti precursors, which play a critical role in creating mononuclear TiO₆ in the zeolite framework coupled with microwave irradiation. Moreover, such mononuclear TiO₆ species remain stable upon calcination. The obtained catalyst affords a high turnover number value (272) in the 1-hexene epoxidation reaction, which is almost 70% higher than that of the conventional TS-1 zeolite (161). This study may open new perspectives for the designed synthesis and applications of titanosilicate zeolite catalysts in various important selective oxidation reactions.