Electrocatalytic dechlorination of volatile organic compounds at a copper cathode. Part I: Polychloromethanes

The electrochemical reductive dehalogenation of CCl4, CHCl3, CH2Cl2 and CH3Cl was investigated at Cu in dimethylformamide (DMF). The principal aim of the study was to check whether Cu has good electrocatalytic properties and is stable to fouling in preparative-scale electrolyses. All polychloromethanes (PCMs) have been investigated by cyclic voltammetry in DMF + 0.1 M Pr4NBF4 in order to evaluate the reduction mechanism and the catalytic activity of Cu. The electrochemical reduction of CCl4 and CHCl3 has also been studied by controlled-potential electrolysis in DMF + 0.1 M Pr4NBF4 both in the absence and presence of H2O or CH3CO2H, using chromatographic techniques to determine the intermediates and final products of the process. Cu exhibits a good electrocatalytic activity, in some cases the positive shifts of the reduction potentials with respect to GC being comparable with those at Ag, which is considered to be one of the best candidates for environmental applications in dehalogenation reactions. The results point out that the reduction mechanism at Cu is similar to what was already observed for Ag on which reduction of PCMs takes place through two competing reaction pathways: hydrodehalogenation and hydrogenolysis of carbenes. The proton availability of the medium affects drastically both the catalytic effect of Cu and the distribution of reduction products. The presence of an added proton donor promotes the hydrodehalogenation pathway, enhancing the concentration of intermediate PCMs and the final yield of methane, which is the main product of the exhaustive electrolysis.