Insight into the formation of framework titanium species during acid treatment of MWW-type titanosilicate and the effect of framework titanium state on olefin epoxidation

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Acid treatment before calcination is indispensable for MWW-type titanosilicate to achieve excellent catalytic performance. Our previous study indicated that not only about half of the absolutely dominant low active non-framework TiO 6 species in as-synthesized B–Ti-MWW zeolite was removed but also the other part was transformed into high active framework TiO 4 species by acid treatment. However, it is still not very clear about the transformation mechanism of titanium species and the state of formed framework TiO 4 species during acid treatment, which significantly affects the catalytic performance. Herein, based on the elaborate experimental design/implementation and structural characterizations including UV Raman spectroscopy excited at 244 nm, 11 B MAS NMR, 29 Si MAS NMR and FT-IR spectroscopy, etc., the underlying transformation mechanism of low active non-framework TiO 6 species into high active framework TiO 4 species during acid treatment of as-synthesized B–Ti-MWW zeolite has been studied in depth and revealed as deboration-based titanium incorporation into MWW framework. Specifically, titanium incorporation was almost simultaneous with deboronation, which was unexpectedly nearly accomplished at the very early stage of acid treatment. Moreover, the newly formed framework TiO 4 species was almost inactive in epoxidation of 1-hexene. Encouragingly, with the gradual increase of acid-treated time, the state of framework TiO 4 species gradually became favorable and the catalytic performance in 1-hexene epoxidation was improved significantly. This study further deepens the understanding of B–Ti-MWW zeolite and would be beneficial for developing titanosilicate zeolites with high catalytic performance. • The evolution of titanium species during acid treatment of as-synthesized B–Ti-MWW zeolite has been studied in detail. • The underlying transformation mechanism of titanium species has been revealed as deboration-based titanium incorporation. • Low active framework TiO 4 species was formed at the very early stage of acid treatment. • The state of formed framework TiO 4 species gradually became favorable, resulting in attractive catalytic performance.