Polymer supported metal complexes as catalysts for oxidation of thiosalts by molecular oxygen. III. Comparative kinetic studies with poly(4-vinyl pyridine)-Cu(II) complex

Abstract

Abstract A water-insoluble copper(II) complex, prepared by cross-linking poly(4-vinyl pyridine) with copper sulfate, has been used as a catalyst for oxidation of thiosalts by molecular oxygen at ambient temperatures. This polychelate has been found to catalyze oxidation of sulfite (SO2−3), thiosulfate (S2O2−3), dithionite (S2O2−4), disulfite (S2O2−5), trithionate (S3O2−6) and tetrathionate (S4O2−6) to sulfate (SO2−4). The kinetics of oxidation of S2O2−3, S3O2−6 and S4O2−6 has been studied in the presence of the catalyst. In each case the initial rate behavior is well explained by a Langmuir—Hinshelwood type rate law. The differences in the rate of oxidation of these thiospecies appear to arise mainly from the differences in their adsorption behavior on the catalyst. Two mechanistic pathways are proposed for the oxidation of S2O2−3, one involving a direct electron transfer from S2O2−3 to Cu(II) and another involving electron transfer from S2O2−3 to O2 through the central Cu(II) atom in a Cu(Py)n(S2O2−3)(O2) complex. The initial concentration of S2O2−3 influences the reaction mechanism and the product distribution pattern, a lower concentration favoring its oxidation to SO2−4 over that to S4O26. The rate of oxidation is mildly augmented by the acidity of the reaction medium down to a pH of 2.0, but it falls abruptly thereafter due to the decomposition of the complex. The presence of acetate ions in the reaction medium produces a marked promotional effect on the catalytic oxidation of S2O2−3 to SO2−4, but it greatly inhibits the oxidation of S3O2−6.