Precursors of tetragonal tungsten bronzes as catalysts in selective reactions: Liquid phase oxidation of diphenyl sulfide and gas phase oxidation of hydrogen sulfide

Abstract

Mixed metal oxides, of the type (KNaM)xP0.11WV0.2NbOx and (NH4)x/(Na)xP0.11WV0.2NbOx, were prepared by hydrothermal synthesis from lacunary heteropolytungstates, [M4(H2O)2(PW9O34)2]10− (MPW9, with M(II) = Co, Cu or Mn) and their precursor Na8H(PW9O34) (NaPW9), chemically modified by Nb(V) and V(IV). For comparison mixed metal oxides were also synthesized by using a Keggin heteropolytungstates [PW12O40]3− as tungsten precursor. The pristine samples were finally heat-treated at 500 °C in flowing N2 for 2 h. The resulting mixed metal oxides were characterized by several physico-chemical techniques and tested in both the liquid phase catalytic oxidation of diphenyl sulfide (DPS), in friendly environmental conditions (using tert-butyl hydroperoxide as oxidant), and in the gas phase partial oxidation of hydrogen sulfide to sulfur (using air at oxidant). Although the catalytic behavior of these catalysts strongly depends on the composition of catalysts, high activity and selectivity have been achieved over mixed metal oxides in both reactions. The good catalytic performance of species containing Cu(II) was evidenced in both reactions studied and the reactivity of this element could be explained by its structural and vibrational spectroscopic properties.