Catalytic Dehalogenatin of Perchloroethylene in a Redox Environment

Abstract

The catalytic dehalogenation of tetrachloroethylene (PCE) occurs via oxidation or reductive hydrodechlorination. Catalytic oxidation uses oxygen to dehalogenate PCE into CO₂ and Cl₂. This process requires higher temperatures >350°C then reductive hydrodechlorination and can produce undesirable toxic products, such as dioxins and furans. Hydrodechlorination uses a reductant to reduce PCE to ethane, and intermediate products such as less chlorinated hydrocarbons. Catalyst deactivation and associated loss of activity are commonly observed. Here, we examined a redox environment for the destruction of PCE on commercially available and laboratory made precious metal loaded catalysts. When a mixture of PCE, oxygen and hydrogen are passed over the catalyst, the PCE is converted to ethane, CO₂, water, and HCl as a function of temperature (ambient to 450°C) and hydrogen to oxygen ratio in the feed (0 to 5). In the laboratory experiments, high conversion of PCE was observed for relatively high H₂/O₂ ratios (84% conversion with H₂/O₂ = 2.15, 63% with H₂/O₂ = 1.18 at 350°C, for commercial catalyst) for retention time of ~ 1 s. The conversion of PCE generally increased with increasing temperature for all H₂/O₂ ratios. In the strictly oxidation environment (H₂/O₂ = 0), PCE conversion was lower than with hydrogen at any given temperature (<30% at 464°C). At lower temperature (<350°C) the dominant carbon-containing product was ethane, under redox conditions. At high temperature (>380°C) CO₂ eluted from the reactor, suggesting that oxidation of reduction products or PCE occurs. Experiments were conducted by using a laboratory made catalyst. A mixture of three types of precious metals (Pt, Pd, and Rh) was impregnated onto a monolithic alumina support. These studies show no apparent performance difference between the two catalysts at high temperatures (>280°C). However, at low temperatures the laboratory catalyst outperforms the commercial catalyst. It was speculated that this difference due to high metal loading of the laboratory catalyst (38.61 mg versus 1.27 mg). A field scale study of the commercial catalyst was undertaken at the Superfund Park-Euclid site in Tucson, Arizona, where the soil is contaminated with PCE and other volatile hydrocarbons. Gases from a soil-vapor extraction unit were fed to the reactor, Even though the soil vapor contained high oxygen (>17%), high PCE conversion with and without hydrogen was observed. Due to the relatively high cost associated with the use of hydrogen, propane, methane, and diesel were investigated as replacement reductants. The results indicate that propane and diesel are promising replacements for hydrogen that deserve further investigation.