(691h) On the Dual Role of the Reactant during Aqueous Phase Hydrodechlorination of Trichloroethylene Using Pd Supported on Swellable Organically Modified Silica

Trichloroethylene (TCE) is a major groundwater contaminant which can adversely affect human health upon exposure to even trace quantities. Hence, the allowable concentration limit of TCE in groundwater has been restricted to 5 ppb by U.S.E.P.A. Pd-catalyzed hydrodechlorination (HDC) of TCE has been found to be effective for degradation of TCE. However, deactivation of conventional catalysts such as Pd/Al₂O₃ due to HCl, an unavoidable HDC product, has prompted researchers to develop highly deactivation resistant HDC catalysts. Recently, our research group has demonstrated the superior deactivation resistance of Pd supported on swellable organically modified silica (SOMS), a novel material which possesses hydrophobicity and ability to swell 3-4 times its original volume on exposure to organics. In our previous studies, ethanol-induced swelling of Pd/SOMS was found to result in faster removal of HCl formed inside the pores, thereby protecting Pd from deactivation. In this study, the role of TCE, as a reactant and as a swelling agent for Pd/SOMS, is investigated. Soaking Pd/SOMS with TCE prior to the batch reaction led to 45% higher conversion than dry Pd/SOMS. Results of ethanol-swollen Pd/SOMS have been compared with the TCE-swollen counterparts. To that end, similar steady-state activities obtained in flow experiments, with and without ethanol, indicated that TCE can act as a reactant and as a swelling agent. Additionally, it was found that Pd(0) is not a pre-requisite for HDC reaction as the H₂ present in the system can itself reduce supported Pd precursor to active Pd particles during the reaction. In-situ characterization using time-resolved X-ray absorption spectroscopy (XAS) demonstrated rapid reduction of Pd(2+) to Pd(0) under reaction conditions. Overall, the findings from this study demonstrated the economic viability of the process by eliminating the need for an external swelling agent (such as ethanol) and a pre-reduced catalyst.