Synergizing tetra-coordinated Ti sites anchored onto micropore-free silica with Au sites to boost propylene hydro-oxidation

Abstract

Constructing tetra-coordinated Ti sites on the micropore-free oxide surface as a substitute for the Ti-containing zeolites to synergize with Au sites for propylene epoxidation with H2 and O2 to propylene oxide (HOPO) can solve the problems resulting from micropores but remains a great challenge. Here, we report a facile strategy to anchor tetra-coordinated Ti sites onto micropore-free silica by precisely cleaving the surface hydroxyl and siloxane bridges of silica to generate under-coordinated O sites to bind Ti sites. Multiple spectroscopic characterizations, including ultraviolet Raman, UV–vis and Fourier transformation infrared and 29Si solid nuclear magnetic resonance spectroscopy, demonstrate the anchored Ti sites on the surface of silica are mainly the target tetra-coordinated ones. It is further unveiled that the removal of surface hydroxyl also improves the hydrophobicity of synthesized Ti-SiO2, which enhances the desorption of PO. Benefitting from the weak acidic properties, high hydrophobicity and micropore-free structures, the tetra-coordinated Ti sites on silica synergize well with Au sites in propylene epoxidation, demonstrating high activity, selectivity and stability, which are superior to those achieved on the most extensively researched Au/TS-1 catalysts.