Coordination Chemistry-Driven Approaches to Rare Earth Element Separations

Abstract

Conspectus Current projections for global mining indicate that unsustainable practices will cause supply problems for many elements, called critical raw materials, in the next 20 years. These include elements necessary for renewable technologies as well as artisanal sources. Energy critical elements (ECEs) comprise a group used for clean, renewable energy applications that are in low abundance in the Earth’s crust or require an economic premium to extract from ores. Sustainable practices of acquiring ECEs is an important problem to address through fundamental research to provide alternative energy technologies such as wind turbines and electric vehicles at cheaper costs for our global energy generation and usage. Some of these green technologies incorporate rare-earth (RE) metals (Sc, Y and the lanthanides), which are challenging to separate from mineral sources because of their similar sizes (i.e., ionic radii) and chemical properties. The current process used to provide REs at requisite purities for these applications is counter-current solvent–solvent extraction, which is scalable and works efficiently for any ore composition. However, this method produces large amounts of caustic waste that is environmentally damaging, especially to areas in China that house major separation facilities. Advancement of the selectivity of this process is challenging since exact molecular speciation that affords separations is still relatively unknown. In this context, we developed a program to investigate new RE separations systems that were aimed at minimizing solvent use, controlled by molecular speciation, and could be targeted at problems in recycling these critical metals. The first ligand system that was developed to impart solubility differences between light and heavy rare-earth ions was [{(2- t BuNO)­C6H4CH2}3N]3– (TriNOx3–) (graphic below). A differential solubility allowed for a separation of Nd and Dy of SFNd:Dy = ∼300 in a single step. In other words, a 50:50 Nd/Dy sample was enriched to give 95% pure Nd and Dy through a simple filtration, which is potentially impactful to recycling magnetic materials found in wind turbines. This separations system compares favorably to other state-of-the-art molecular extractants that are based on energetic differences of the thermodynamic parameter to affect separations for neighboring elements. This straightforward, thermodynamically driven method to separate REs primed our future research for new coordination chemistry approaches to separations. Another separations system was accomplished through the variable rate of a redox event from one arm of the TriNOx3– ligand. It was determined that the rate of this one electron oxidation, which operated through an electrochemical-chemical-electrochemical mechanism, was dependent on the identity of the RE ion. This kinetically driven separation afforded a separation factor (SF) of SFEu:Y = 75. We have also described other transformations such as ligand exchange, substituent dependent, and redox-driven chelation processes with well-define...

Lanthanide Contraction: What is Normal?

“…It has been rationalized traditionally as being due to shrinkage of the electron cloud as the nuclear charge increases. Recent reviews have discussed the importance of lanthanide contraction in their separation, coordination chemistry, and biology …”

Highly Tunable 4-Phosphoryl Pyrazolone Receptors for Selective Rare-Earth Separation

“…Yield: 3.83 g (48%); mp 72−74 °C; Raman (80 mW, in cm −1 ) ν: 3083 (16), 2983 (26), 2937 (52), 2929 (53), 2873 (18), 1598 (100), 1570 (13), 1527 (15), 1494 (20), 1450 (23), 1427 (14), 1415 (13), 1402 (19), 1369 (21), 1344 (21), 1313 (14), 1284 (13), 1178 (13), 1095 (25), 1053 (24), 837 (17), 671 (22), 73 (56); IR (ATR, in cm −1 ) ν: 2980 (vw), 2933 (vw), 2877 (vw), 1597 (vw), 1587 (vw), 1562 (w), 1520 (w), 1493 (m), 1468 (w), 1450 (vw), 1414 (w), 1387 (w), 1375 (w), 1336 (w), 1282 (vw), 1269 (vw), 1242 (vw), 1178 (w), 1153 (m), 1101 (w), 1053 (vw), 978 (vs), 937 (w), 899 (w), 887 (w), 850 (vw), 829 (m), 775 (m), 748 (m), 704 (vw), 669 (m), 646 (w), 629 (vw), 607 (w), 575 (s), 530 (w), 519 (w), 503 (w), 422 (w); 1 H NMR (CDCl 3 , 300 K, in ppm): δ 1.28 (6H, d, 3 J HH = 6.2 Hz, H6a), 1.40 (6H, d, 3 J HH = 6.1 Hz, H6b), 2.27 (3H, d, 4 J HP = 0.7 Hz, H4), 4.62 (2H, d sept, 3 J HP = 7.7 Hz, 3 J HH = 6.3 Hz, H5), 7.39 (2H, md, 3 J HH = 9.0 Hz, H9/H10), 7.77 (2H, md, 3 J HH = 9.0 Hz, H9/ H10); 13 4.8. General Procedure for the Syntheses of Lanthanum(III), Europium(III), and Ytterbium(III) Complexes.…”

“…Yield: 4.82 g (66%); Raman (80 mW, in cm −1 ) ν: 3080 (31), 2983 (40), 2937 (79), 2927 (81), 2873 (29), 2738 (11), 2730 (12), 1600 (100), 1568 (18), 1554 (17), 1525 (24), 1502 (27), 1490 (21), 1471 (17), 1452 (34), 1409 (27), 1386 (20), 1369 (30), 1350 (31), 1319 (23), 1298 (20), 1276 (19), 1178 (17), 1159 (20), 1107 (15), 1057 (18), 1028 (29), 999 (68), 835 (23), 752 (15), 702 (18), 644 (26), 611 (17), 580 (14), 424 (13), 252 (16), 247 (17), 225 (15), 218 (16), 156 (17), 75 (89); IR (ATR, in cm V1 ) ν: 2980 (w), 2931 (vw), 2877 (vw), 1593 (w), 1556 (w), 1531 (m), 1500 (w), 1456 (w), 1406 (w), 1387 (w), 1375 (w), 1350 (vw), 1273 (vw), 1180 (w), 1153 (m), 1103 (w), 1068 (vw), 976 (vs), 937 (w), 908 (w), 887 (m), 833 (vw), 775 (m), 756 (s), 702 (m), 690 644 (m), 609 (w), 577 (s), 565 (s), 548 (m), 528 (w), 505 (w), 494 (w), 447 (w), 424 (w); 1 H NMR (CDCl 3 , 300 K, in ppm): δ 1.25 (6H, d, 3 J HH = 6.3 Hz, H6a), 1.36 (6H, d, 3 J HH = 6.2 Hz, H6b), 2.25 (3H, s, H4), 4.59 (2H, d sept, 3 J HP = 7.7 Hz, 3 J HH = 6.3 Hz, H5), 7.22 (1H, t, 3 J HH = 7.4 Hz, H11), 7.39 (2H, mt, 3 J HH = 8.0 Hz, H10), 7.76 (2H, md, 3 J HH = 7.7 Hz, H9); 13 C{ 1 H} NMR (CDCl 3 , 300 K, in ppm): δ 13.9 (1C, s, C4), 23.7 (2C, d, 3…”

“…Over the past few decades, various techniques have been employed for the separation of REEs, which include liquid–liquid extraction (LLE), − solid–liquid extraction (SLE), , electrochemical processes, ,, and selective crystallization and precipitation, ,− as well as the use of biogenic material-based adsorbent (e.g., protein). ,, Among these, LLE is the most commonly used technique due to both its high processing capacity and separation efficiency along with its rather simple operation. , Various extractants have been investigated for the separation of REEs in LLE processes. Organophosphorus compounds such as di-(2-ethylhexyl)­phosphoric acid (D2EHPA, P204), ,, 2-ethyl­(hexyl)­phosphonic acid mono-2-ethylhexyl ester (HEH­[EHP], P507), ,, bis­(2,4,4-trimethylpentyl)­phosphinic acid (Cyanex 272), tributyl phosphate (TBP), trioctylphosphine oxide (TOPO), β-diketones such as benzoyl-1,1,1-trifluoroacetone and 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione, primary amines such as tri-alkyl methylamine, and quaternary amines such as tri-octyl methylammonium nitrate (Aliquat 336) have all been widely investigated .…”

Solution Structures of Europium Terpyridyl Complexes with Nitrate and Triflate Counterions in Acetonitrile

“…Lanthanide (Ln) ions bound with organic ligands serve as components in a wide array of applications, such as luminescent optical probes for imaging and sensing, − magnets for magnetic refrigeration, − contrast agents for magnetic resonance imaging, , and single-molecule magnets for quantum information science. ,− Rare-earth complexes may also function as homogeneous catalysts for a variety of industrially relevant processes including polymerization, dehalogenation, and redox reactions. − Due to these application areas, lanthanides have been designated as critical materials, and there is great interest in their separation and recovery from complex mixtures. This is complicated by the similar sizes of the ions and the common oxidation state of +3 in solution, thus making separation and recovery highly active areas of research. − The ability to resolve the solution structures of Ln–ligand complexes will provide insight and guidance toward not only improving Ln extraction methods but will also facilitate enhancing the magnetic, optical, and catalytic functionalities of Ln-containing materials. However, determining Ln solution structures is particularly challenging both experimentally and computationally; Ln ions have large coordination spheres in which they primarily bind to ligands through ionic interactions, resulting in a multitude of different possible configurations having different numbers of coordinated ligands, counterions, and solvent molecules.…”