Carbon-supported metal-modified lacunary tungstosilicic polyoxometallates used as catalysts in the selective oxidation of sulfides

Abstract

Lacunary tungstosilicic polyoxometallates modified with transition metal ions [SiW 11 O 39 M(H 2 O)] 6-, where M = Mn 2+, Fe 2+, Co 2+ or Cu 2+, were synthesized and supported on activated carbon to obtain the SiW 11 MC catalysts. The samples were characterized by FT-IR, XRD, N 2 adsorption-desorption measurements, and the acidic properties were determined using the isopropanol dehydration test reaction. The activity and selectivity of the catalysts were evaluated in the selective oxidation of a series of sulfides to sulfoxides or sulfone. The reaction was carried out in acetonitrile as solvent using H 2 O 2 35% p/V as a clean oxidant. The conversion values decreased in the following order: SiW 11 MnC > SiW 11 FeC > SiW 11 CuC > SiW 11 CoC. The catalysts were reused without appreciable loss of their catalytic activity. It was found that the activity of the catalysts decreases in parallel with the increment in the reduction temperature values. The most easily reducible catalyst displayed the highest conversion values. We found a convenient and selective procedure for oxidizing sulfides to sulfoxides or sulfones using aqueous hydrogen peroxide and a catalytic amount of lacunary tungstosilicic polyoxometallates supported on carbon at low temperatures (20-50 °C) in a reasonably short reaction time