(762a) Aqueous Phase Hydrodechlorination of Trichloroethylene Using Pd Supported on Swellable Organically Modified Silica (SOMS): Deactivation Due to Chloride and Sulfur Species

Trichloroethylene (TCE) is a major groundwater contaminant which is strictly regulated by U.S. E.P.A. The maximum allowable concentration of TCE in water is 5 ppb, which warrants complete removal of TCE from groundwater[1]. Catalytic hydrodechlorination (HDC) appears to be the most promising technique to date, among several physical, biological and chemical ones that are employed for degradation of TCE[2, 3]. However, the challenges faced during HDC of TCE involve slow kinetics due to lower miscibility and hence, low concentration of TCE available in aqueous phase, inhibition due to HCl formed as the reaction by-product and deactivation due to several anionic species such as chlorides, sulfides, sulfates etc. that are usually found in groundwater.

The main focus of this study lies in the implementation of SOMS as a catalyst support for HDC of TCE. SOMS belongs to the class of bridged polysilsesquioxanes, which are hybrid organic-inorganic materials, synthesized by sol-gel technique. Post gel formation, the surface of SOMS is made hydrophobic by derivatizing the silanol groups with hexamethyldisilazane. SOMS swells to almost 3 times it’s dry volume, in presence of organic solvents. Apart from this, properties such as, mesoporosity, high surface area and high absorption capacity towards organics, also make SOMS a strong candidate for a catalyst support[4]. This study aims at investigating the effects of chloride and sulfur species on the performance of Pd supported on SOMS for HDC of TCE.

Chloride and sulfur treated catalysts were characterized by extended X-ray absorption fine structure (EXAFS), inductively coupled plasma optical emission spectrometry (ICP-OES), transmission electron microcopy (TEM) in order to investigate the resistance exhibited by SOMS matrix towards these species[5, 6]. Additionally, phenomena such as reduction of Pd by H₂ dissolved in aqueous phase under ambient conditions and swelling of SOMS by solvents such as ethanol, were investigated by in-situ X-ray absorption near edge structure (XANES) and near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) respectively. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and laser Raman spectroscopy were also used to explore different characteristics of Pd/SOMS and their role in HDC of TCE[7]. Commercially obtained Pd/Al₂O₃ was used as the basis of comparison for performance and deactivation resistance of Pd/SOMS. The results elucidate the effects of different chloride and sulfur species, eg. perchlorates, sulfides and sulfates, with varied pH environments on the activity of Pd/SOMS and the role of SOMS in preventing deactivation. Finally, they show that SOMS has great potential to serve as a catalyst support for HDC of TCE.

[1] National Primary Drinking Water Regulations: EPA 816-F-09-004, in, United States Environmental Protection Agency, 2009.

[2] C.G. Schreier, M. Reinhard, Catalytic hydrodehalogenation of chlorinated ethylenes using palladium and hydrogen for the treatment of contaminated water, Chemosphere, 31 (1995) 13.

[3] H.H. Russell, J.E. Matthews, W.S. Guy, TCE removal from contaminated soil and groundwater, EPA Environmental Engineering Sourcebook, (1992).

[4] C.M. Burkett, L.A. Underwood, R.S. Volzer, J.A. Baughman, P.L. Edmiston, Organic–Inorganic Hybrid Materials that Rapidly Swell in Non-Polar Liquids: Nanoscale Morphology and Swelling Mechanism, Chemistry of Materials, 20 (2008) 10.

[5] G. Celik, S.A. Ailawar, S. Gunduz, J.T. Miller, P.L. Edmiston, U.S. Ozkan, Aqueous-phase hydrodechlorination of trichloroethylene over Pd-based swellable organically-modified silica (SOMS): Catalyst deactivation due to chloride anions, Applied Catalysis B: Environmental, 239 (2018) 654-664.

[6] G. Celik, S.A. Ailawar, S. Gunduz, J.T. Miller, P.L. Edmiston, U.S. Ozkan, Aqueous-Phase Hydrodechlorination of Trichloroethylene over Pd-Based Swellable Organically Modified Silica: Catalyst Deactivation Due to Sulfur Species, Industrial & Engineering Chemistry Research, 58 (2019) 4054-4064.

[7] G. Celik, S. Ailawar, H. Sohn, Y. Tang, F. Tao, J.T. Miller, P.L. Edmiston, U.S. Ozkan, Swellable Organically-modified Silica (SOMS) as a Catalyst Scaffold for Catalytic Treatment of Water Contaminated with Trichloroethylene, ACS Catalysis, (2018).