UC Riverside

Heterogeneous transformations of halogenated and oxygenated chemicals have been studied for decades; however, there has been very limited progress in developing metal-ligand coordination systems for environmental applications. Transition metals coordinated with organic ligands can substantially enhance the reactivities of heterogeneous reaction systems. Similar to the evolution of metal centers in various enzymes, organic ligands used in synthetic catalytic systems can be rationally modified with diverse functionalities. In this dissertation, we systematically designed and tested a series of organic ligands bearing various functional groups at multiple positions. Two primary and highly recalcitrant water pollutants, per- and polyfluoroalkyl substances (PFAS) and perchlorate, are used as substrates to investigate the novel relationship between ligand structure and catalyst activity/stability.The second chapter of this dissertation studied ligand-enabled reductive defluorination of PFAS by metallic zinc. The addition of neutral N,N-bidentate ligands, such as substituted 2,2'-bipyridine (R2bpy), to nano-sized zinc powder (nZn0) enabled rapid and significant defluorination from linear long-chain perfluorocarboxylates (CnF2n+1–COO−), which did not degrade by the biogenic cobalt-catalyzed systems (e.g., vitamin B12). The third and fourth chapters of this dissertation developed a highly stable heterogeneous bimetallic catalyst, which incorporated an oxorhenium complex Re(O)(R-hoz)2 onto Pd/C. Modification of the original 2-(2’-hydroxyphenyl)-2-oxazoline (hoz, a bioinspired anionic N,O-bidentate ligand) resulted in substantially improved catalyst stability. A comprehensive collection of Re(O)(R-hoz)2Cl complexes bearing various functional groups with electron-donating/withdrawing and steric effects were synthesized. Upon immobilization onto the water-Pd/C interface, selected complexes exhibited unprecedented high stability during the rapid reduction of concentrated perchlorate. A novel set of ligand design rationale was obtained from this catalytic system. Both studies demonstrate the critical roles of developing organic ligands to enhance both the reactivity and stability of heterogeneous catalytic systems for the reductive degradation of recalcitrant water pollutants. The experimental results and mechanistic insights highlight the long-neglected significance of coordination chemistry for environmental engineering.