Charge transformation of framework titanium (Ⅳ) into titanium (Ⅲ) in vanadium-titanium doped MFI zeolite for enhanced alkene epoxidation

Abstract

Simultaneous insertion of double heteroatom in the MFI framework generally gives rise to synergistic catalysis effect, however the specific origins for synergistic catalysis is still nebulous. In this study, vanadium-titanium doped MFI zeolite was synthesized via one-pot hydrothermal synthesis with the addition of (NH4)2CO3 and applied as heterogeneous catalyst for the synergistic catalysis of 1-hexene and cyclohexene epoxidation. Simultaneous incorporation of vanadium and titanium induced the partial charge transformation of titanium (Ⅳ) into titanium (Ⅲ), which was verified by EPR and XPS characterizations. UV-Raman and UV–Vis spectra disclosed that partial charge transformation mainly occurred for framework titanium (Ⅳ). In addition, simultaneous insertion of vanadium and titanium gave rise to more Lewis acid sites as well as larger external surface area for VTS(0.15) zeolite in comparison to VS(0.15) and TS(0.15) zeolite. The prepared VTS(0.15) zeolite presented a TOF value of 118.8 h−1 for 1-hexene epoxidation, which is 7.8 folds and 1.3 folds of VS(0.15) and TS(0.15) respectively. Partial charge transformation of framework titanium (Ⅳ) into titanium (Ⅲ), robust Lewis acid sites as large external surface area may induce the synergistic catalysis effect for VTS(0.15) zeolite in the epoxidation reactions. Moreover, the obtained VTS(0.15) zeolite particles showed good reusability for more than five cycles.