Electrocatalytic reduction of organic halides in microemulsions and surfactant dispersions

Sharp peaks in capacitance current vs. potential curves were observed at mercury and silver electrodes in aqueous micellar systems containing hexadecyltrimethylammonium bromide (CTAB) and straight chain alcohols of two to five carbons. These peaks are attributed to rapid structural reorganization of a mixed alcohol/CTAB layer on the electrode surface. Longer chain alcohols give peaks at more positive potentials and at lower concentrations. When type and concentration of alcohol were adjusted so that capacitance peak potentials were matched with reduction potentials of 1,2-dicyanobenzene, perylene, and tetracene, cyclic voltammograms in alcohol/CTAB/water suggested their reversible reductions in a CTAB/alcohol layer adsorbed on the electrode surface. Organic anion radical products of these reductions were much more stable in the CTAB/alcohol layer than in aqueous CTAB micelles. This is consistent with a decrease in polarity of the microenvironment for uncharged solutes by incorporation of alcohol in the surfactant layer on the electrode. This decrease in polarity compared to CTAB micelles may reflect exclusion of water from solubilization sites on the electrode surface. Anion radicals stabilized in the CTAB/alcohol layer on a mercury electrode were used for catalytic reduction of 2,3,5-trichlorophenol and 2,5,2{dollar}spprime{dollar},5{dollar}spprime{dollar}-PCB.; Aqueous suspensions of didodecyldimethylammonium bromide (DDAB) facilitate strong coadsorption of the catalyst, zinc phthalocyanine (ZnPC) and PCBs on negative carbon felt cathodes minimizing catalyst decomposition. Electrolyses results obtained in DDAB dispersions were superior to those obtained in CTAB micellar and microemulsion systems. The product of these reactions in DDAB was not only biphenyl as in the case of CTAB, but also reduced biphenyls (cyclohexylbenzene, 2-cyclohexenylbenzene, and a cyclohexadienylbenzene). The reduction mechanism of biphenyl in DDAB is believed to be similar to that in organic solvents such as DMF. Optimum electrolyses conditions were 0.08 M DDAB/0.05 M TEAB in a pH 3.5 acetate buffer. Addition of ultrasound increased the amounts of biphenyl and reduced biphenyls two-fold. Ultrasound improved yields, presumably by increasing mass transport of PCBs to the DDAB electrode film and biphenyl products out of the film. Mercury cathodes yielded the fastest rate of dehalogenation followed by carbon felt and lead. Nearly quantitative yields of biphenyl and reduced biphenyls were obtained after 18 h electrolysis of 0.100 g of the PCB mixture, Aroclor 1016 using a mercury cathode.