Revealing the high activity of WO₃-SBA-15 with isolated tungsten oxide species in cis-cyclooctene epoxidation

Supported tungsten oxide catalysts are promising candidates for olefin epoxidation with hydrogen peroxide (H2O2). Understanding the structure-activity relationship is of great importance for developing more efficient tungsten oxide-based catalysts. Herein, WO3-SBA-15 with an isolated tetrahedron WO4 is synthesized using a direct incorporation strategy, and its TOF is 191 h(-1) in cis-cyclooctene epoxidation with H2O2, which is much higher than that of WO3/SBA-15 (51 h(-1)) prepared using an impregnation method and that of bulk-WO3 (0.4 h(-1)). Its lowest apparent activation energy of 34.6 kJ mol(-1) also corresponded to the remarkable performance of WO3-SBA-15. Kinetic analysis indicated that WO3-SBA-15 bound H2O2 more strongly than other catalysts, resulting in the facilitated activation of H2O2. By correlating the specific activity and the structure characterization, we considered that the high reactivity of WO3-SBA-15 was attributed to (i) more isolated WO4 sites, which were of benefit to the coordination and activation of H2O2 and (ii) a stronger metal-support interaction derived from isolated WO4 species, which favored the dissociation of an O-O bond in the W-OOH intermediate and contributed to the insertion of an oxygen atom into an olefin double bond. Furthermore, DFT calculations indicated that the isolated tungsten oxide was beneficial for lowering the energy barrier of electrophilic oxygen atom transfer, thereby facilitating epoxidation.