[Oxidation of phenolic compounds with permanganate catalyzed by manganese dioxide]

The effectiveness and mechanism of manganese dioxide (MnO2) enhancing permanganate (KMnO4) oxidation of phenolic compounds were investigated. Under the pseudo-first-order kinetic conditions (i. e., the initial concentration of KMnO4 was ten times higher than that of phenolic compounds), the oxidation kinetics of 2-chlorophenol and 4-chlorophenol by KMnO4 were examined. The reactions displayed autocatalysis, suggesting a catalytic role of in situ formed final products MnO2. Further experiments were conducted to evaluate the effects of the initial concentration and particle size of MnO2 additives as well as solution pH on KMnO4 oxidation. It was found that both ex situ-preformed colloidal and particulate MnO2 additives could significantly enhance KMnO4 oxidation of these phenolic compounds. Also, the pseudo-first-order rate constants K for the degradation of these phenolic compounds were observed to increase linearly with the increase of MnO2 concentration (in the range of 30 - 180 micromol x L(-1)). For the same concentration, colloidal MnO2 had much stronger ability than particulate MnO2 to promote KMnO4 oxidation. The increase of solution pH decreased the catalytic ability of MnO2. Moreover, it was found that MnO2 could catalyze KMnO4 oxidation of 2-nitrophenol which otherwise exhibited negligible reactivity toward MnO2 and KMnO4, respectively, but had no effect on KMnO4 oxidation of dimethyl sulfoxide (DMSO) which did not contain the moieties capable of forming complexes with the surface bound metals of MnO2. On the basis of these results, a catalytic mechanism was proposed; that was, the surface adsorption (i. e., the formation of surface complexes) was necessary and the surface bound phenolic compounds were more susceptible toward KMnO4 oxidation than their solution counterparts.