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

Abstract

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.