MnO_2催化KMnO_4氧化降解酚类化合物

研究了MnO2强化KMnO4氧化降解酚类化合物的效能与机制.在假一级动力学实验条件下(KMnO4初始浓度是目标有机物初始浓度的10倍),考察了KMnO4对酚类化合物(2-氯酚和4-氯酚)的氧化降解规律.发现在KMnO4氧化降解酚类化合物过程中存在着明显的自催化现象,即原位产生的胶体MnO2可以促进KMnO4对有机物的氧化降解.实验进一步考察了MnO2浓度、粒径大小和溶液pH对MnO2催化KMnO4氧化降解酚类化合物的影响.结果表明,外加胶体MnO2和颗粒MnO2都可以催化KMnO4氧化降解酚类化合物,而且假一级动力学常数(K)随着MnO2浓度(30～180μmol·L-1)的增加呈线性增加;与胶体MnO2相比,颗粒MnO2的催化能力较弱;随着溶液pH的增加,MnO2催化能力逐渐减弱.实验中还发现外加MnO2能够催化KMnO4氧化降解2-硝基酚(单独MnO2和KMnO4均不能将其氧化),但对于二甲基亚砜(其不具有与金属离子络合配位的能力)则没有催化作用.由此推测MnO2催化KMnO4氧化降解有机物的作用机制可能为表面吸附络合催化,即吸附在MnO2表面形成的络合物比存在于溶液中的有机物本身更易被KMnO4氧化.

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.