Accomplishing simple, solubility-based separations of rare earth elements with complexes bearing size-sensitive molecular apertures

Abstract

Rare earth (RE) metals are critical components of electronic materials and permanent magnets. Recycling of consumer materials is a promising new source of rare REs. To incentivize recycling, there is a clear need for the development of simple methods for targeted separations of mixtures of RE metal salts. Metal complexes of a tripodal hydroxylaminato ligand, TriNOx 3-, featured a size-sensitive aperture formed of its three η 2 -(N,O) ligand arms. Exposure of cations in the aperture induced a self-associative equilibrium comprising RE(TriNOx)THF and [RE(TriNOx)] 2 species. Differences in the equilibrium constants K dimer for early and late metals enabled simple separations through leaching. Separations were performed on RE1/RE2 mixtures, where RE1 = La-Sm and RE2 = Gd-Lu, with emphasis on Eu/Y separations for potential applications in the recycling of phosphor waste from compact fluorescent light bulbs. Using the leaching method, separations factors approaching 2,000 were obtained for early-late RE combinations. Following solvent optimization, >95% pure samples of Eu were obtained with a 67% recovery for the technologically relevant Eu/Y separation. (11)(12)(13)(14). Limitations associated with their beneficiation and separations, especially their solvent-, waste-, and energy intensities, have contributed to the concentration of suppliers in the People's Republic of China. Supply risks for these elements have emerged, particularly in the face of current and growing demand in the next 20 y (15, 16). Because the global marketplace for these elements is dominated by a single source (17), prices for primary rare earth (RE) materials are volatile (18). As a result, the US Department of Energy has classified many of these elements as "critical" (19). There is a clear need to find potential new supplies of these elements.Recent life cycle assessments have indicated that recycling of consumer materials is a promising alternative to conventional production processes (20). Despite this assertion, as recently as 2011, less than 1% of RE-containing materials were being recycled (21). These low recycling rates stem from a combination of sporadic collection procedures and lack of efficient separations and preprocessing steps (22)(23)(24)(25)(26)(27).To contribute to incentivizing the "urban mining" of REcontaining materials, we recently initiated efforts toward new, simplified methods in RE separations (28). Our initial work focused on the separation of neodymium (Nd) and dysprosium (Dy), two key components of neomagnets (Nd 2 Fe 14 B). We disclosed the development of the tripodal nitroxide ligand, [((2-t BuNO)C 6 H 4 CH 2 ) 3 N] 3− (TriNOx 3-), which induced a selfassociation equilibrium between monomeric Nd(TriNOx)THF/ dimeric [Nd(TriNOx)] 2 species. The position of this equilibrium was found to be strongly dependent on the size of the RE cation. We showed proof of concept that differences in the self-association equilibrium constants between Nd and Dy could be exploited to achieve 95% pure materials through a simple leachi...

Electro‐kinetic Separation of Rare Earth Elements Using a Redox‐Active Ligand

“…The separation factors for a single iteration could thus be calculated based on the ICP‐OES results as S Y/Eu =74.8, S Y/Yb =55.6, and S Y/Lu =261. Notably, the separation factor for the Eu/Y mixture (S Y/Eu ), was higher than the factors obtained in the thermodynamic separations of RE‐TriNOx previously reported by our group …”

Synthesis and Characterization of a Tripodal Tris(nitroxide) Aluminum Complex and Its Catalytic Activity toward Carbonyl Hydroboration

“…The aluminum ion is pentacoordinate with a distorted-trigonal-bipyramidal geometry (τ 5 = 0.88). 8 The average N−O distance in 2-py is 1.44 Å, which compares well with analogous parameters for the series of rare-earth (TriNOx 3− )RE complexes prepared by the Schelter group (N−O ave = 1.44 Å) 6,9,10 and suggests a fully reduced TriNOx 3− ligand.…”

Magnetic Field Directed Rare‐Earth Separations

“…Initial studies of temperature and concentration effects were also performed. A separation factor of SF Dy:Y =4.14±0.46 was determined for neighboring elements in [RE(TriNOx)] complexes, demonstrating the potential of magnetic separations of RE elements even without the established solubility differences of the [RE(TriNOx)] system …”