Ligand Design For The Separation Of Rare Earth Elements
Rare earth elements (La-Lu, Sc, and Y) exhibit unique magnetic and optical properties that make them irreplaceable components of many technologies. Concerns over the rare earth supply chain have focused efforts toward recycling rare earths from end of life technology. Less than 1% of the rare earth elements are currently recycled. This situation is due to the high cost of implementing solvent extraction methods to purify these elements from mixtures, compared to the costs associated with primary mining and purification operations. This dissertation describes work on the development of novel ligands that form water- and oxygen-stable rare earth complexes, to facilitate the separation of rare earth mixtures into purified elements to enable recycling. Derivatization of a tripodal hydroxylamine ligand demonstrated that changing the steric and electronic properties of rare earth complexes impacted their solubility properties and stability to water. A tripodal hydroxypyridonate ligand system with pH-dependent precipitation of the rare earths was achieved. The precipitation separation performance was quantified as a function of pH and ligand equivalents through the development of a high-throughput experimentation screen. The screening results determined optimal rare earth separations conditions that were applied on lab-scale to afford aqueous rare earth separations in a single chelation and separation step. Development of a tripodal catecholate ligand formed homobimetallic rare earth complexes with rare earth-dependent redox properties, determined using cyclic voltammetry, and informed ligand design for future redox-driven separations processes.
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