Aqueous Reductive Dechlorination of Chlorinated Ethylenes with Tetrakis(4-carboxyphenyl)porphyrin Cobalt

The catalytic dechlorination of chlorinated ethylenes by 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin cobalt ((TCPP)Co), a cobalt complex structurally similar to vitamin B12, was studied. It was found to have superior aqueous-phase
dechlorination activity on chlorinated ethylenes (CEs) relative to vitamin B12. Bimolecular rate constants for the
degradation of CEs by (TCPP)Co of 250, 24, 0.24, and 1.5 M-1 s-1 were found for perchloroethylene (PCE),
trichloroethylene (TCE), cis-dichloroethylene (cDCE), and trans-dichloroethylene (tDCE), respectively. Through kinetic
analysis, the rate laws for PCE and TCE were determined to be first order in substrate and catalyst, and PCE
degradation was shown to be sensitive to the concentration of the titanium citrate bulk reductant and pH. The
importance of the CoI oxidation state on dehalogenation was studied with UV−vis absorbance spectroscopy, a
variety of reducing agents, and cyclic voltammetry. Evidence of chlorovinyl complexes as potential catalytic cycle
intermediates was obtained through the preparation of (TPP)Co(trans-C2H2Cl) and the observation of (TPP)Co(C2HCl2) and (TCPP)Co(C2HCl2) by mass spectrometry. The X-ray crystal structure of (TPP)Co(trans-C2H2Cl) is
reported.