Degradation of coordinated β-diketonates as iron chelate catalysts during the oxidation of H2S to S8 by molecular oxygen

The iron–acetylacetonate (acac−) and iron–benzoylacetonate (bzac−) complexes were investigated as catalysts in the oxidation of H2S in aqueous, in 75% ethanol–25% water, and in various concentrations of N-methyl-2-pyrrolidone (v/v) mixed solvent systems. From the formation constants, the calculated reduction potentials indicated favorable oxidation of hydrogen sulfide to elemental sulfur and favorable reduction of dioxygen to water. The iron–acac chelate complex catalyzes the formation of elemental sulfur and water as the Fe(III) chelates of acac− are reduced to the Fe(II) chelates. However, the degradation of acac, producing acetate and acetone, was observed in the reoxidation of the Fe(II) complex to the Fe(III) complex. A mechanism for the degradation of acac− is proposed, involving attack of the ligand by a hydroxyl radical. A decrease in the dielectric constant of the mixed solvent systems lowers the rate of degradation of the chelating agent. The reaction mechanism and solvent effects are discussed.