Metal ion-catalyzed oxidative degradation of Orange II by H2O2. High catalytic activity of simple manganese salts

In an effort to develop new routes for the clean oxidation of non-biodegradable organic dyes, a detailed study of some environmentally friendly Mn(II) salts that form very efficient in situ catalysts for the activation of H2O2 in the oxidation of substrates such as Orange II under mild reaction conditions, was performed. The studied systems have advantages from the viewpoint of green chemistry in that simple metal salts can be used as very efficient catalyst precursors and H2O2 is used as a green oxygen donor reagent. Oxidations were carried out in a glass reactor over a wide pH range in aqueous solution at room temperature. Under optimized conditions it was possible to degrade Orange II in a carbonate buffer solution in less then 100 s using 0.01 M H2O2 in the presence of only 2 x 10(-5) M Mn(II) salt. To gain insight into the manganese catalyzed oxidation mechanism, the formation of the active catalyst was followed spectrophotometrically and appears to be the initiating step in the oxidative degradation of the dye. High valent manganese oxo species are instable in the absence of a stabilizing coordinating ligand and lead to a rapid formation of catalytically inactive MnO2. In this context, the role of the organic dye and HCO3- as potential stabilizing ligands was studied in detail. In situ UV-Vis spectrophotometric measurements were performed to study the effect of pH and carbonate concentration of the buffer solution on the formation of the catalytically active species. Electrochemical measurements and DFT (B3LYP/LANL2DZp) calculations were used to study the in situ formation of the catalytic species. The catalytic cycle could be repeated several times and demonstrated an excellent stability of the catalytic species during the oxidation process. A mechanism that accounts for the experimental observations is proposed for the overall catalytic cycle.