Oxidation and photooxidation of sulfur-containing compounds in the presence of water soluble phthalocyanine complexes

Abstract

Water soluble cobalt(II) 2,9,16,23-tetrasulfophthalocyanine, zinc(II) 2,9,16,23-tetrasulfophthalocyanine, zinc(II) tetracarboxyphthalocyanine, and non-metallic sulfophthalocyanine complexes were studied as catalysts and photocatalysts for oxidation of sulfide, sulfite and thiosulfate ions by dioxygen. The cobalt phthalocyanine complexes exhibit high catalytic activity only in the oxidation of compounds containing sulfur in low oxidation state (sulfides). Oxidation is not complete and the activity of these complexes is not enhanced upon irradiation with visible light. Zinc and non-metallic phthalocyanine complexes demonstrate high catalytic activity only on irradiation with visible light. During interaction between dioxygen and the long-living triplet state of these complexes a chemically more reactive singlet dioxygen is formed which further interacts with compounds containing sulfur of various oxidation state. Upon photooxidation catalyzed by zinc or non-metallic phthalocyanine complexes the alkali sulfides, sulfites and thiosulfates are completely oxidized to sulfates. Aggregation of charged phthalocyanine complexes in aqueous medium decreases their effectiveness as photocatalysts because of lowering of the quantum yield of singlet dioxygen. In the presence of quaternary ammonium salts the equilibrium monomer-dimer is shifted to the monomeric form of the complexes, the photocatalytic activity being enhanced in this case. The mechanism of photocatalytic oxidation of sulfur-containing compounds and the catalyst destruction are briefly discussed.