Heterogeneous Selective Oxidation of Sulfides with H2O2 Catalyzed by Ionic Liquid-Based Polyoxometalate Salts

A group of ionic liquid (IL)-based polyoxometalate (POM) salts were prepared via anion-exchange of imidazolium IL precursors tethered by different carbon chain alkyls with various Keggin POMs. The resultant imidazolium POM salts were tested as the catalysts for oxidation of sulfides with aqueous H2O2. For understanding the POM salt-catalyzed oxidation of sulfides, catalyst compositions, substrates, and reaction conditions were changed. The results show that the imidazolium POM salts are very active and selective heterogeneous catalysts for oxidations of sulfides, adding the advantages of convenient recovery, steady reuse, simple preparation, and flexible composition. Characterizations by Fourier transform infrared (FT-IR), ultraviolet-visible (UV-vis), X-ray diffraction (XRD), and elemental analysis further reveal that the heterogeneous nature of the oxidation processes associates with the good crystallinities of the butyl- and hexyl-functionalized imidazolium POM salts and the hydrogen bonding networks among cations and anions. The POM salts' excellent performances arise from the promoted redox property of the POM-anions by the intramolecular charge transfer from the IL-cations.