Design of ligands for the complexation of Fe(II)/Fe(III) in the catalytic oxidation of H2S to sulfur

Abstract

Examination of the stability constants of Fe(II) and Fe(III) chelates of a wide variety of ligands that contain only acetate and amino groups shows a linear correlation. A separate linear correlation displaced toward the ferric ion was obtained for those ligands that contain more basic phenolate donor groups. The ligands of the latter group generally are unsuited to the oxidation of H2S to sulfur, while ligands in the first correlation are suitable for that purpose. Ligands that do not contain α-methylene groups are relatively resistant to oxidative degradation. Molecular mechanics is employed to compare the Fe(III) complexes of three such ligands, dipicolinic acid (DIPIC), pyridine-2-phosphonic-6-carboxylic acid (2PP6C), and 2-carboxy-8-hydroxyquinoline (CHOX). The calculations show that there is considerable strain in the formation of the Fe(II) and Fe(III) complexes of these ligands, both with respect to the O-O distance and the O-Fe-O angle. The relatively high stability of the CHOX complex compared to that of DIPIC, therefore, is due mainly to the higher basicity of the phenolate oxygen compared to the carboxylate groups of DIPIC. The higher stability of the Fe(III) complex of 2PP6C relative to that of DIPIC is due to the higher basicity and charge of the phosphonate group compared to a carboxylate group of DIPIC that it replaces. In spite of the lack of α-methylene groups in CHOX, the ligand undergoes rapid oxidative degradation when its iron chelate is used as a catalyst for the oxidation of H2S to sulfur by air. However, the oxidation products were found to also be effective catalysts for this process.