Constructing titanosilicate enriched with octahedral titanium sites containing amine ligand for superior alkene epoxidation

Titanosilicates are important microporous catalysts for the selective liquid-phase oxidations using H₂O₂. While the conventional paradigm held isolated tetrahedral Ti sites as the exclusive active centers for peroxo intermediate formation, recent evidence demonstrates that modulating the Ti coordination environment via organic amine ligand (e.g., piperidine, PI) significantly enhances the catalytic performance through electron donation from amine to Ti sites. However, achieving a high concentration of such organic-inorganic hybrid active sites has remained a key synthetic challenge, with prior work achieving ligation in only approximately 25% of octahedral Ti species and often accompanying by the formation of anatase impurity. In this study, we overcome this synthetic challenge by constructing the MWW-type titanosilicate containing an unprecedented >85% PI-ligated octahedral Ti species via post-treatment crystallization. The resulting Re-Ti-MWW(C₇)-PI catalyst delivers a state-of-the-art catalytic activity in epoxidation of small alkenes. In particular, it exhibits a 7.2-fold increase in propylene turnover frequency (TOF) compared to traditional Ti-MWW with only tetrahedral Ti sites. This enhancement could extend robustly across C₅-C₈ linear olefins, maintaining a consistent performance superiority. This work not only achieves a recorded catalytic efficiency but also fundamentally validates the ligand engineering as a powerful strategy for tailoring the active site for selective oxidations.