Selective dechlorination of chlorinated phenoxy herbicides in aqueous medium by electrocatalytic reduction over palladium-loaded carbon felt

The highly selective dechlorinations of phenoxy herbicides (2,4-D and 2,4,5-T) and other chlorinated phenoxyacetic acids in aqueous solution with MeOH, trifluoroacetic acid and tetraalkylammonium salt were achieved under very mild experimental conditions by electrocatalytic reduction with the solution permeable cathodes made of Pd-loaded carbon felt. The chlorinated phenoxyacetic acids tested in this work were wholly dechlorinated to phenoxyacetic acid with 80-93% yields within 4 h of galvanostatic electrolysis. The chlorine atoms attached to 3- and 4-positions of the aromatic ring were removed faster than the chlorine atom at 2-position. For the polychlorinated phenoxyacetic acids, the stepwise dechlorination and the new reaction route, i.e., simultaneous cleavage of more than one chlorine atom in one step, were suggested. (C) 1999 Elsevier Science B.V. All rights reserved.