Electrolytic Reduction of Low Molecular Weight Chlorinated Aliphatic Compounds:  Structural and Thermodynamic Effects on Process Kinetics

Abstract

A series of chlorinated low molecular weight alkanes and alkenes was transformed electrolytically using a porous nickel cathode at potentials from −0.3 to −1.4 V (versus standard hydrogen electrode). Kinetics were first-order with respect to the concentration of the halogenated targets. The dependence of the first-order rate constants on cathode potential followed the Butler−Volmer equation, modified to account for mass transfer resistance to reaction. The mass-transfer-limited rate constant for reaction of all species was about 1.55 L m-2 min-1. Log-transformed reaction rate constants for reduction of chlorinated alkanes, derived via experiments at the same cathode potential (E c = −1.0 or −1.2 V vs SHE), were linearly related to carbon−halogen bond enthalpies, as expected based on a physical model that was developed from transition state theory. The chlorinated ethenes reacted much faster than predicted from bond enthalpy calculations and the alkane-based correlation, suggesting that alkenes are not transformed via the same mechanism as the chlorinated alkanes. Dihalo-elimination was the predominant pathway for reduction of vicinal polychlorinated alkanes. For chlorinated alkenes and geminal chlorinated alkanes, sequential hydrogenolysis was the major reaction pathway.

Electrocatalysis and electron transfer mechanisms in the reduction of organic halides at Ag

“…The great interest for this process is due to the important roles it plays in synthetic [2,4] and environmental applications, [5][6][7] especially the abatement of volatile polychlorinated organic compounds, but also to mechanistic aspects.…”

Abiotic Dehalogenation by Metals

“…This mechanism satisfactorily explains the formation of hydrocarbon products without the formation of lesser chlorinated ethane intermediates. The products observed by Gregory et al (55) for the reduction of 1,1,1-TCA in a column containing steel wool are consistent with such a mechanism, as are the products of electrolysis of 1,1,1-TCA and 1,1-DCA on a porous nickel cathode reported by Liu et al (82). In contrast, in the presence of granular aluminum 1,1,1,-TCA was reported to undergo dehydrohalogenation to yield significant amounts of 1,1-DCE (5).…”

“…When granular iron serves as the reductant, the pathway dominates, accounting for 85-99% of products for all of the vicinal polychloroethenes (8). Liu et al (82) noted that chlorinated ethenes react via similar pathways during electrolysis on a porous nickel cathode. This elimination pathway is advantageous in that each elimination step allows the loss of two halogens leading to full dehalogenation while circumventing the production of generally toxic intermediates.…”

“…The latter product resulted in the production of CO and formate. Liu et al (82) reported that electrolytic reduction of CT produces similar products (CF, DCM, and methane, but no chloromethane) regardless of the cathode material (Ag, Al, Au, Cu, Fe, Ni, Pd, or Zn). With the nickel cathode, ethane, ethene, and propane were detected as products.…”

Spectroelectrochemical study of perchloroethylene reduction at copper electrodes in neutral aqueous medium

“…On the other hand, electrochemical approaches are also being investigated. Special attention merits the work done by Liu et al [25]. For perchloroethylene (PCE), trichloroethylene (TCE) was the major intermediate and transdichloroethylene (DCE) and acetylene were intermediates at the trace level.…”