(29c) Promotion of Gas-Phase Catalytic Perchloroethylene Dechlorination Using Low Molecular Weight Alkanes Under Redox Conditions

Chlorinated hydrocarbons, such as perchloroethylene (PCE) and trichloroethylene (TCE), are persistent environmental hazards affecting groundwater sources located near a variety of industrial processes, due to their improper disposal. As these compounds are suspected carcinogens, there is great interest in developing inexpensive and environmentally sound technologies for the remediation of contaminated sites. Current efforts focus on the use of soil-vapor extraction (SVE) to pass gas phase contaminants through a granular activated carbon bed (GAC), which creates solid toxic waste, and possibly more harmful by-products during GAC regeneration. Our research has focused on the use of short-chain alkanes, in combination with oxygen, to promote the conversion of PCE over a Pt/Rh three-way catalyst. The use of both of a hydrocarbon and oxygen creates mixed reducing-oxidizing (redox) conditions. Although both catalytic oxidation and catalytic reductive hydrodechlorination are well-known processes, they have important drawbacks, such as catalyst deactivation (reductive conditions), and the possibility of generating toxic by-products such as dioxins, especially in complex matrices (oxidative conditions). These drawbacks raise concerns on the use of catalytic processes for long-term remediation efforts. Our results show that redox conditions prevent these shortcomings while completely removing the target compound and producing primarily CO₂, H₂O and HCl. The process has proven to be most effective near stoichiometric conditions with respect to the reducing and oxidizing agents (2:1 for H₂:O₂ and 1:5 for propane:O₂). Hydrogen has been used as the primary reducing agent in the laboratory, while alkanes were studied as hydrogen sources both in the laboratory and in a pilot scale study involving a site contaminated with PCE, TCE and other volatile organics. Residence times in the reactor are typically on the order of 0.5 seconds and catalyst surface temperatures range from 200 °C to 550 °C, with PCE conversion greater than 99% starting at 450 °C under slightly oxidizing conditions. Laboratory results suggest that the catalytic mechanism is a multiple step surface reaction involving the three reactants (H₂/C₃H₈, O₂ and PCE). A mechanism based on Langmuir-Hinshelwood kinetics has been developed in an attempt to model the process. The model satisfactorily captures the trends of the laboratory data. The effectiveness of C1-C4 alkanes on PCE conversion was studied, with PCE conversion increasing as the hydrocarbon chain length increases. Butane proved to be the most effective, but propane was chosen for further study due to cost considerations involved with environmental remediation. Results of the field test show complete destruction of PCE and TCE in the presence of propane over 240 days of operation with no noticeable catalyst deactivation.