UV resonance Raman spectroscopic insight into titanium species and structure-performance relationship in boron-free Ti-MWW zeolite

•Ti species and structure-performance relationship in Ti-MWW were studied by UVRRS.•Acid treatment resulted in the transformation of TiOm (m = 5, 6) into TiO4 species.•Only acid-treated and subsequently calcined Ti-MWW showed catalytic activity.•Framework Ti(OSi)4 species is much active than framework Ti(OSi)3OH species.

The titanium species and structure-performance relationship in boron-free Ti-MWW zeolite have been thoroughly studied by 244 nm excited UV resonance Raman spectroscopy for the first time. It was surprisingly found out that acid treatment not only removed a part of the dominant non-framework TiOm (m = 5, 6) species but also made most of them transformed into TiO4 species, which has never been reported before. Interestingly, acid treatment of as-synthesized and directly calcined Ti-MWW resulted in the formation of active framework TiO4 species and inactive TiO4 species in amorphous SiO2 matrix, respectively. Only the samples obtained from acid treatment of as-synthesized Ti-MWW and subsequent calcination showed catalytic activity in 1-hexene epoxidation. Importantly, despite higher Si/Ti ratio, framework Ti(OSi)4 species-dominant Ti-MWW showed better catalytic performance than the relatively hydrophilic framework Ti(OSi)3OH species-dominant one. These understandings would be beneficial for further study of Ti-MWW and development of highly efficient titanosilicate zeolites.

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