Cu-exchanged phosphomolybdic acid induced a solvent effect to enhance alkene epoxidation with H2O2

Abstract

Solvent-mediated alkene epoxidation using H₂O₂ oxidant is significantly enhanced by a Cu-substituted phosphomolybdic acid anchored on triphenylamine-functionalized porous organic polymers (CuPMA/POPs). The immobilization of the CuPMA cluster on the POP supports suppresses the stronger coordinative interactions between the Cu ion and the PMA cluster, resulting in an obvious solvent effect. In 1,4-dioxane, CuPMA/POPs achieves 90% conversion of cyclooctene and markedly improves epoxide selectivity in styrene oxidation (65.5% vs. 36.9% for PMA/POPs). Various characterizations indicate that phosphomolybdic acid did not form the active species [PO₄MoO(O₂)]³⁻, which typically arises from the interactions of PMA-H₂O₂. Instead, the incorporation of Cu modulates the electronic structure of PMA, facilitating the generation of active peroxo intermediates such as Cu-OOH and Mo-OOH. These species promote oxygen-atom transfer to the CC bond, thus leading to high epoxide selectivity. Additionally, the hydrogen-bonding network resulting from the solvent effect between 1,4-dioxane and H₂O₂ further enhances the reaction efficiency. The catalyst also exhibits excellent recyclability due to strong host–guest interactions, underscoring the synergy between Cu substitution and solvent effects in promoting efficient and stable epoxidation catalysis.