Electrocatalytic reduction of toxic organohalides in surfactant microstructures

Abstract

In a bicontinuous conductive media of 23 :34:43 didodecyldimethylammonium bromide DDAB/n-dodecane/water microemulsion (% by wt.), controlled potential electrolysis was performed at the two diffusion controlled reduction waves of copper phthalocyanine tetrasulphonic acid, tetrasodium (CuPcTS) In absence and in presence of added 1,2- dibromobutane as substrate. From the data obtained, it is possible to establish the mechanism for electro catalytic reduction of vicinal dihalides in surfactant media. The results suggest that vicinal dihalides are reduced to the olefin and the mediator is reoxidized from the active CuPcTS6- state to the CuPcTS5- or CuPcTS4.- state. In the presence of catalyst the reduction of the organohalides occurs at potentials lower than that of direct reduction. Linear sweep voltammetric study indicate that p-chlorophenol is better reduced in isotropic water/acetonitrile media than in the DDAB microemulsion media by copper phthalocyanine mediated electro catalytic dehalogenation. A tentative reason is that in microemulsion reaction occurs via adsorbed film of the surfactant containing both the catalyst and the substrate. In acetonitrile/water media such an environment is not present. A new approach to studies involving destruction of toxic recalcitrant organohalide pollutants has been suggested. This involves fl correlation between molecular mechanics and quantum mechanics calculated parameters with observed voltammetric half-wave potentials. Such an approach has been applied for a few model organohalides. A linear relationship has been-obtained between voltammetric half-wave potentials and theoretically calculated parameters such as heats of formation and lowest unoccupied molecular orbitals (LUMOs). A forward step suggested for the future involves understanding the lowering of the overpotential phenomena by mediators (catalysts) for reductive dehalogenation of organohalides. UV/VIS spectroscopic studies for copper phthalocyanine tetrasulphonic acid, tetrasodium in both aqueous and microemulsion media reflected a high degree of aggregation for this complex in water as compared to microemulsion media. This suggests possible compartmentalization of CuPcTS catalyst in microemulsion. A higher extent of aggregation was found for CuPcTS catalyst in aqueous media, in presence of added tetraethylammonium bromide salt. In buffered aqueous media, voltammetric reduction of copper phthalocyanine tetrasulphonic acid tetrascdiurn was found to be pH dependent. A positive shift in reduction potentials was found with decrease in pH. Thus, control of pH could give another handle on lowering of the overpotential phenomena for reduction of toxic orghanohalides by copper phthalocyanine tetrasulphonate mediator (catalyst).