Heterogeneous photoassisted catalysis: Conversions of perchloroethylene, dichloroethane, chloroacetic acids, and chlorobenzenes

Abstract

The chlorinated hydrocarbons perchloroethylene, dichloroethane, monochloroacetic and dichloroacetic acids, in dilute aqueous solutions, are completely mineralized to HCl and CO 2 by photoassisted heterogeneous catalysis with an aqueous slurry of near-uv illuminated TiO 2. Trichloroacetic acid was dehalogenated at a negligible rate. Rate parameters from the present and our earlier studies indicate that the relative chlorocarbon mineralization rates at the 1 to 50-ppb levels of relevance to water supply contamination appear to be dichloroacetaldehyde ⪢ trichloroethylene > perchloroethylene > dichloroacetic acid ∼ dichloromethane > trichloromethane ∼ (1,2)-dichloroethane ∼ monochloroacetic acid > tetrachloromethane > trichloroacetic acid. The heterogeneously photocatalyzed conversion of monochlorobenzene in dilute (70–400 ppm) aqueous solutions yields (1) ortho- and para-chlorophenol, which may subsequently be dechlorinated to yield aromatic oxygenates ( ortho- and para-benzoquinone, and the corresponding hydroquinones), or (2) condensation products such as 4,4′-dichloro-1,1′-biphenyl. No evidence of ring opening was noted. Similar but more complex behavior was noted with a dichlorobenzene.