Catalytic Water Treatment
Groundwater contamination by organic compounds is a cause for concern. These compounds pose serious risk to human health and the environment due to their toxicity. Physical methods of removal of these contaminants from water merely transfer them to another phase (adsorbent). Biological techniques have proven to be inefficient as these molecules are toxic and are difficult to be metabolized by microorganisms. We are employing catalytic hydrogenation/hydrogenolysis to degrade the toxic molecules to benign products.
Among several classes of contaminants, our research is focused on degradation of chlorinated ethylenes and aromatic compounds. Trichloroethylene and toluene are the representative compounds chosen from these classes of compounds, respectively. Our research focuses on studying the influence of unique properties of swellable organically modified silica (SOMS) on the catalytic performance and the deactivation resistance towards extraneous poisons while conducting hydrodechlorination and hydrogenation reactions.
SOMS is a novel hybrid organic-inorganic material which is hydrophobic, mesoporous and swellable in nature. It is synthesized by sol-gel method using bis(trimethoxysilyethyl) benzene. Pd is chosen as the active site for TCE degradation while Ru is chosen as an active site for toluene degradation. Catalysts are synthesized by incipient wetness impregnation followed by chemical reduction. We perform catalytic reactions in batch as well as flow mode to investigate the kinetics and evaluate the catalytic performance. Several in-situ and ex-situ techniques such as X-ray photoele ctron spectroscopy (ultra-high vacuum and near-ambient pressure) diffuse reflectance fourier transform infrared spectroscopy, X-ray absorption spectroscopy (near-edge and extended fine structure), temperature programmed reduction/oxidation using mass spectrometry, scanning and transmission emission microscopy are employed to study the catalytic material under different conditions.
Heterogeneous catalysis researchers have often dealt with non-animated or static materials. However, in this project we are studying a novel material, SOMS, which is animated and stimulus driven. This material swells to almost 3-4 times its original volume when contacted with organic compounds.
Hydrodechlorination (HDC) of trichloroethylene (TCE)
HDC of TCE is a metal catalyzed reaction in which chlorine atoms are removed from the TCE molecule to form partially chlorinated ethenes, which are further dechlorinated to produce ethane. The chlorine removed, as a result of HDC reaction, forms HCl, which in turn is found to deactivate the metal. Studies have employed chloride scavenging agents such as NaOH and KOH and bimetallic catalysts to tackle this problem. It is important to understand this deactivation behavior to develop catalysts with enhanced deactivation resistance. In this research, we are using SOMS as the catalyst support due to its hydrophobicity and swellability.
Anionic species such as Cl- , NO3-, HS-, SO42-, HCO3- present in groundwater are known to poison supported metal catalysts. Our research is focused on exploring the deactivation resistance of SOMS catalyst and understanding the role of hydrophobicity and swellability of SOMS in hydrodechlorination.
We study the effect of in-situ and ex-situ poisoning with different anionic species on catalytic performance of Pd/SOMS and Pd/Al2O3. X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) is used to observe and quantify poisoning/leaching of metal sites on the catalyst. Ethanol is used as an external swelling agent and the role of ethanol in kinetics of HDC is explored. Techniques such as cryogenic scanning electron microscopy (Cryo-SEM) and near-ambient pressure XPS are used to observe swelling of SOMS and its effect on the availability of active Pd sites for the reaction. We perform in-situ X-ray absorption experiments in flow mode at Argonne National Lab to understand the sorption of organics in the catalyst.
Swellability of SOMS provides a large surface area for the reaction. Also, high affinity of SOMS to organics favors kinetics of the reaction by increasing the local concentration of the organic reactant in the pores as compared to the bulk aqueous reaction mixture. SOMS can protect active metal sites due to its hydrophobicity.
Hydrogenation (HYD) of Toluene
Hydrogenation of aromatics is widely studied in the literature owing to its applications in the petroleum industry. HYD of toluene produces partially and completely hydrogenated products such as methylcyclohexenes and methylcyclohexane respectively. Results obtained for HDC with SOMS catalyst motivated our research on HYD of toluene. Our research is focused on synthesizing an active Ru/SOMS catalyst and fine-tuning the synthesis procedure to make it more industrially relevant.
We are also running batch reactor experiments to assess the activity of these catalysts for HYD of toluene, optimizing the metal loading and reaction temperature, conducting the reactions in altered feed environments, investigating mass transfer limitations and kinetics of the reaction, characterizing the active metal sites by techniques such as X-ray absorption near-edge spectroscopy (XANES), temperature programmed reduction (TPR) and in-situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS).
We are exploring the effect of polarity and structure of the organic molecule on affinity of SOMS to that organic molecule. This will help in developing a catalyst with enhanced performance.