Rates of Oxidative Coupling of Humic Phenolic Monomers Catalyzed by a Biomimetic Iron-Porphyrin

A synthetic water-soluble meso-tetra(2,6-dichloro-3- sulfonatophenyl)porphyrinate of iron(III) chloride (FeP) was used as biomimetic catalyst in the oxidative coupling of three monomeric phenols (catechol, caffeic, and p-coumaric acids), which are common constituents of natural humic substances. The extent of oxidation induced by the FeP catalyst in solutions of phenolic monomers was followed in the presence of an oxygen donor such as hydrogen peroxide or dissolved oxygen under daylight radiation. Both UV- and fluorescence-detected liquid chromatograms indicated that primary oxidation products had a larger electronic conjugation and molecular mass than the original phenols, thereby suggesting that the biomimetic oxidative catalysis produced covalently linked phenylene and oxyphenylene oligomers. However, the polyphenolic products were further oxidized in the progress of the catalytic reaction to possible undetectable aliphatic acids or even to complete mineralization. Rate constants describing the initial reaction period were larger for the catalyzed oxidation with hydrogen peroxide than those for the noncatalyzed control solutions under autoxidation or hydrogen peroxide treatment. However, the rate constants measured for the phenol solutions treated with just the FeP catalyst showed that the presence of dissolved oxygen and the action of the daylight radiation were sufficient to significantly increase the reaction rate in respect to control solutions. These results confirmed previous findings, showing that humic materials may undergo oxidative coupling catalyzed by metal-porphyrins in the presence of either an oxygen donor or, simply, dissolved molecular oxygen under daylight. The increase of molecular mass of natural humic and polyphenolic substances by this biomimetic technology may have useful applications in environmental chemistry.