Investigation into the mechanism of polyoxotungstates-catalyzed cyclooctene epoxidation by ESI-MS

Abstract

The reaction process of cyclooctene epoxidation was real-time monitored by ESI-MS using H₃PW₁₂O₄₀ and H₂WO₄ as catalysts. By observing the species changes of the catalysts in the reaction solutions and the combination of catalysts with the substrate, a catalytic reaction mechanism of polyoxometalate clusters was proposed from the aspect of aggregation and dissociation. When H₃PW₁₂O₄₀ was used as the catalyst, it was degraded into minor peroxo-species [PW₄O_16−x(O₂)_x]³⁻ (x = 0, 2, 4, 6, 8) which performed as the active center of the epoxidation of cyclooctene. When H₂WO₄ was used as the catalyst, it reacted with H₂O₂ and generated mononuclear tungsten peroxo-species [HWO₂(O₂)₂]⁻ and [HWO₂(O₂)₃]⁻. After the addition of cyclooctene, the peroxo-species catalyzed the epoxidation of cyclooctene, generating 1,2-epoxycyclooctane and combining with 1,2-epoxycyclooctane to form the intermediate [HW₂(O₂)₂O₅(C₈H₁₄O)₂]⁻.