An Operationally Simple Method for Separating the Rare‐Earth Elements Neodymium and Dysprosium

Abstract

Rare-earth metals are critical components of electronic materials and permanent magnets. Recycling of consumer materials is a promising new source of rare earths. To incentivize recycling there is a clear need for simple methods for targeted separations of mixtures of rare-earth metal salts. Metal complexes of a tripodal nitroxide ligand [{(2-(t) BuNO)C6 H4 CH2 }3 N](3-) (TriNOx(3-) ), feature a size-sensitive aperture formed of its three η(2) -(N,O) ligand arms. Exposure of metal cations in the aperture induces a self-associative equilibrium comprising [M(TriNOx)thf]/ [M(TriNOx)]2 (M=rare-earth metal). Differences in the equilibrium constants (Keq ) for early and late metals enables simple Nd/Dy separations through leaching with a separation ratio SNd/Dy =359.

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

“…The η 2 -(N,O) bonding mode of the three hydroxylaminato arms was retained, and each complex contained a single molecule of THF coordinated to the central metal cation. The η 2 -(N,O) bonding mode had been observed previously by us and others in the RE(TriNOx)THF (RE = La, Ce, Nd, Dy, and Y) (28,40) (44,45). As shown in Table 1, there was a steady decrease in the metalligand bond lengths across the series, consistent with the decrease in ionic radii of the central metal cations.…”

“…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).…”

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

“…[13] In contrast to the thermodynamic control of conventional metal separations,k inetic control is essential for high selectivity in biological systems. [12] "Naked" [Yb Thes hort NÀOb ond in the k 1 -O bonding arm (average 1.286 ,w hich is in the typical NÀOb ond length range for nitroxide radicals (1.23-1.30 )a nd very comparable to that of the 2,2,6,6-tetramethyl-1-piperidinyloxy radical (1.296-(3) ) [ In analogy to going from dimeric [RE(TriNOx)] 2 (RE = La to Sm, except Pm) to monomeric [RE(TriNOx)thf], [12] the monomeric cation [Eu(TriNOx)Cthf] + (Figure 2) was obtained by dissolving [Eu(TriNOx)C] 2 [BAr F ] 2 in thf (see the Supporting Information). [16] Inspired by work on redox-active ligands,which have been widely used in organometallic chemistry, [17] we initiated an investigation of the redox chemistry of [RE(TriNOx)]/[RE-(TriNOx)thf] (thf = tetrahydrofuran) compounds (Scheme 1).…”

“…[3] Because of the growing global demand for REs (an annual growth rate of 5% through 2020 is projected [4] )a nd their indispensable roles in diverse applications,t hey are categorized as critical raw materials by the U.S. Department of Energy and the European Commission. [12] Notably, the fundamental theories underlying both the well-established and the new separation approaches are based on thermodynamic properties,s uch as the equilibria for the coordination of ligands to RE 3+ ions,ordistribution equilibria in the aqueous/organic interfacial complex fluid. [7] Research on simpler and more environmentally friendly RE separation methods with higher single-step efficiencies has attracted attention in the past.…”

“…[7] Research on simpler and more environmentally friendly RE separation methods with higher single-step efficiencies has attracted attention in the past. [12] Notably, the fundamental theories underlying both the well-established and the new separation approaches are based on thermodynamic properties,s uch as the equilibria for the coordination of ligands to RE 3+ ions,ordistribution equilibria in the aqueous/organic interfacial complex fluid. Recently,o ur group successfully separated early REs from late ones by leaching with high single-step efficiency using the tripodal hydroxylamine ligand [{2-(tBuN-(O))C 6 H 4 CH 2 } 3 N] 3À (TriNOx 3À ), which induced as elf-association (dimerization) equilibrium between monomeric [RE-(TriNOx)thf] and dimeric [RE(TriNOx)] 2 species.…”