Hydrodehalogenation of 1- to 3-Carbon Halogenated Organic Compounds in Water Using a Palladium Catalyst and Hydrogen Gas

Abstract

Supported palladium (Pd) metal catalysts along with H{sub 2} gas show significant potential as a technology which can provide rapid, on-site destruction of halogenated groundwater contaminants. Pd catalyzes the rapid hydrodehalogenation of nine 1- to 3-carbon HOCs, resulting in little or no production of halogenated intermediates. Initial transformation rates were compared for 12 HOCs using 1 w/w% Pd-on-Al{sub 2}O{sub 3} (Pd/Al) and metallic Pd catalysts in clean, aqueous batch systems at ambient pressure and temperature. Half-lives of 4--6 min were observed for 5--100 {micro}M aqueous concentrations of PCE, TCE, cis-, trans-, 1,1-DCE, carbon tetrachloride, and 1,2-dibromo-3-chloropropane at ambient temperature and pressure with 0.22 g/L of catalyst. Using Pd/Al, TCE transformed quantitatively to ethane without formation of any detectable chlorinated intermediate compounds. This implies a direct conversion of TCE to ethane at the Pd surface. Carbon tetrachloride transformed primarily to methane and ethane and minor amounts of ethylene, propane, and propylene. Chloroform is a reactive intermediate. Formation of C2 and C3 products implies a free radical mechanism. Methylene chloride, 1,1-dichloroethane, and 1,2-dichloroethane were nonreactive. Reaction mechanisms and kinetic models are postulated for TCE, carbon tetrachloride, and chloroform transformation.