UC Riverside

As a powerful oxidizer, perchlorate (ClO4−) has found widespread use in many energetic materials. Over the past two decades, it has increasingly drawn public attention as a pervasive and persistent water pollutant due to the improper disposal of manufacturing waste. Because human exposure to ClO4− can cause thyroid malfunction, many states in the U.S. have set the limits for ClO4− at 0.8−18 μg L−1 in drinking water. The interest in ClO4− treatment has been further fueled by the recent discovery of ClO4− on Mars, Moon, and meteorites, which collectively imply its wide distribution throughout the Solar System. Thus, ClO4− removal technologies are of great interest for water purification, disposal of hazardous materials, and human extraterrestrial exploration.Currently, the commonly used strategy for ClO4− removal from drinking water is ion exchange. While selective resins are effective in physically removing ClO4−, challenges persist, particularly with the disposal of enriched waste brine and spent reins. Catalytic reduction offers a clean and complete reduction of ClO4− to Cl−. However, the real-world application of the best ClO4− reduction catalyst (Re(hoz)2−Pd/C) is hampered by using a rare-earth metal and its short lifespan in the oxidative environment. Therefore, it is imperative to develop an effective, robust, practical, and economic heterogeneous catalyst for aqueous ClO4− reduction. In this work, we have studied the structure-stability relationship of rhenium complexes and discovered that introducing a methyl group on the oxazoline moiety could significantly enhance the overall stability of the complex. Next, we screened a series of molybdenum (Mo) precursors to identify the active Mo species for oxygen atom transfer reaction. Sodium molybdate was found to increase the catalytic activity of virgin Pd/C by 55-fold. This doctoral study has culminated in discovering a simple and straightforward way to construct Mo catalyst for aqueous ClO4− reduction. The initial turnover frequency of (L)MoOx−Pd/C (L = (NH2)2bpy) reached 165 h−1, which is the highest among all reported abiotic ClO4− reduction catalysts. Lastly, we evaluated the performance of (L)MoOx−Pd/C catalyst in synthetic waste brines mimicking ion-exchange resin regeneration. The catalyst has shown limited inhibition in concentrate salt solution and excellent stability under oxidative stress. At the same time, the challenges of deactivation by nitrate and ligand hydrogenation have been identified, and viable solutions were proposed. This doctoral research showcased the power of coordination chemistry in environmental technology innovation. It will guide the ongoing efforts to design catalysts for a wide range of oxygen atom transfer reactions.