Reductive Dechlorination of Tetrachloroethylene and Trichloroethylene Catalyzed by Vitamin B₁₂ in Homogeneous and Heterogeneous Systems

TLDR

The study proposes that reductive β‑elimination is a viable pathway for reducing vicinal polyhaloethenes in other systems. Vitamin B12, with Ti(III) citrate as reductant, catalyzes PCE and TCE reduction in both homogeneous and agarose‑bound heterogeneous systems, and a competing reductive β‑elimination pathway is proposed to explain acetylene formation. Catalytic activity of surface‑bound vitamin B12 matches that of homogeneous B12, retains activity upon reuse, achieves 81–84 % PCE and 89 % TCE carbon recoveries, and demonstrates potential for engineered aqueous degradation of chlorinated ethenes.

Abstract

The reduction of tetrachloroethylene (PCE) and trichloroethylene (TCE) catalyzed by vitamin B12 was examined in homogeneous and heterogeneous (B12 bound to agarose) batch systems using titanium(III) citrate as the bulk reductant. The solution and surface-mediated reaction rates at similar B12 loadings were comparable, indicating that binding vitamin B12 to a surface did not lower catalytic activity. No loss in PCE reducing activity was observed with repeated usage of surface-bound vitamin B12. Carbon mass recoveries were 81−84% for PCE reduction and 89% for TCE reduction, relative to controls. In addition to sequential hydrogenolysis, a second competing reaction mechanism for the reduction of PCE and TCE by B12, reductive β-elimination, is proposed to account for the observation of acetylene as a significant reaction intermediate. Reductive β-elimination should be considered as a potential pathway in other reactive systems involving the reduction of vicinal polyhaloethenes. Surface-bound catalysts such as vitamin B12 may have utility in the engineered degradation of aqueous phase chlorinated ethenes.