Macrocyclic chelators and methods of use for the separation of rare earth elements

Abstract

Water-soluble macrocyclic (metal-chelating) compositions having structure (1) wherein: A1, A2, A3, and A4 are independently selected from O, S, and NRa, wherein Ra is H or R, wherein R is a hydrocarbon group containing 1-12 carbon atoms; R1, R2, R3, and R4 are independently selected from H, R, OH, COOH, OR, COOR, CN, and halogen atoms, wherein R1 and R2 and / or R3 and R4 are optionally interconnected to form a ring; R5 and R6 are independently selected from side chains of amino acids; and R7, R8, R9, R10, R11, and R12 are independently selected from H, R, OH, COOH, OR, COOR, CN, and halogen atoms, wherein two of R7, R8, and R9 (or two of R10, R11, and R12) are optionally interconnected to form a ring. Methods of using the above-described compositions for separation of RE elements are also described.

Description

MACROCYCLIC CHELATORS AND METHODS OF USE FOR THE SEPARATION OF RARE EARTH ELEMENTS CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional Application No. 63 / 469,088, filed May 26, 2023, which is herein incorporated by reference in its entirety. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with government support under Grant No. DE- SC0021662 awarded by the U.S. Department of Energy. The government has certain rights in the invention. FIELD OF THE INVENTION

[0003] The present invention generally relates to compositions and methods for separating rare earth elements, and more particularly, to chelator compositions and methods of using these for separating rare earth elements from each other in solution. BACKGROUND

[0004] Rare earth (RE) elements are used in numerous devices and processes, and are thus in high demand. At the same time, obtaining rare earth elements in pure form has been an ongoing challenge particularly due to the difficulty in separating rare earth elements from each other. Although separation methods are currently widely used (e.g., biphasic extraction and precipitation), they are generally costly, inefficient, and environmentally unfriendly.

[0005] RE separations are performed industrially using countercurrent solvent extraction, where an aqueous mixture of lanthanides is contacted with an immiscible organic solvent containing a lipophilic extractant, such as bis(2-ethylhexyl)phosphoric acid (HDEHP) or TODGA. These extractants complex the lanthanides and transfer them into the organic phase. However, this method requires large volumes of solvents. Moreover, from a standpoint of green chemistry, toxicity of the organic solvents (typically kerosene in industry or o-xylene in the laboratory) and extractants used in these systems havebecome substantial threats to the environment and human health. HDEHP exposure, in particular, has the potential to induce serious liver dysfunction (F. Zhang et al., Front Cell Dev Biol.10, 2022).

[0006] These issues have prompted the search for other, lower-energy, more straight- forward and environmentally friendly separation strategies. An approach currently being considered is selective precipitation, in which REEs can be isolated using a simple filtration step. The commonly used precipitants are sulfate, carbonate, fluoride, phosphate, and oxalate. However, these reagents can only precipitate all REEs in one stream from other impurities owing to the similar chemical properties of the lanthanide series (K. N. Han, Minerals, 10(2), 178, 2020). Thus, their separation from one another into elementally pure streams, which is required for use in their respective applications, remains very challenging.

[0007] Many further efforts have been in the search for new precipitators that could better distinguish these similar elements. Despite successful separation of coexisting REEs, these chelating reagents are limited due to the use of organic solvents to precipitate insoluble complexes (J. A. Bogart et al., Angewandte Chemie.127 (2015) 8340–8343; B. E. Cole et al., Inorg Chem.59 (2020) 172–178). In addition, lower reusability of chelators and difficulties associated with the back-extraction process, such as the release of metal ions from complexes, also result in an increase in the separation cost.

[0008] Thus, considering the significant limitations and drawbacks of conventional separation methods, there would thus be a significant benefit in a method that could separate rare earth elements in a highly effective manner while being cost efficient and environmentally friendly. SUMMARY

[0009] In one aspect, the present disclosure is directed to water-soluble macrocyclic chelators (i.e., chelating compounds) that have an exceptional ability for more strongly chelating heavier (smaller radius) rare earth elements compared to lighter (larger radius) rare earth elements, which thereby results in different solubilities, and hence, precipitation abilities, of the resulting metal complexes in an aqueous solution. The chelator compounds described herein advantageously have the ability to separate rareearth elements in a highly effective manner while being cost efficient and environmentally friendly.

[0010] The macrocyclic chelators have the following structure: (1)Rais H or R, wherein R is a hydrocarbon group containing 1-12 carbon atoms optionally substituted with one or more heteroatoms selected from O, N, and S; R1, R2, R3, and R4are independently selected from H, R, OH, COOH, OR, COOR, CN, and halogen atoms, wherein R is selected from hydrocarbon groups containing 1-12 carbon atoms optionally substituted with one or more heteroatoms selected from O, N, and S, wherein R1and R2are optionally interconnected to form a ring, and wherein R3and R4are optionally interconnected to form a ring; R5and R6are independently selected from side chains of the 20 common amino acids; and R7, R8, R9, R10, R11, and R12are independently selected from H, R, OH, COOH, OR, COOR, CN, and halogen atoms, wherein R is selected from hydrocarbon groups containing 1-12 carbon atoms optionally substituted with one or more heteroatoms selected from O, N, and S, wherein two of R7, R8, and R9areoptionally interconnected to form a ring, and wherein two of R10, R11, and R12are optionally interconnected to form a ring.

[0011] In more specific embodiments of Formula (1), the macrocyclic chelators may have the following structure:are as (1).

[0012] In some embodiments, in any of the above formulas, one or more of the following conditions may hold: (i) R1, R2, R3, and R4are H; (ii) R7, R8, R9, R10, R11, and R12are H; and / or (iii) R5and R6are H. In some embodiments, one or more of the foregoing variables is / are methyl, CN, or halogen atom (e.g., F or Cl).

[0013] In a specific embodiment of Formula (1) or (1a), the macrocyclic chelator has the following structure:.

[0014] In another aspect, the present disclosure is directed to a method of separating rare earth elements. The method includes the following steps: (i) contacting an aqueous solution containing at least two rare earth elements with a water-soluble macrocyclic chelator, which may be any of the macrocyclic chelators described herein including but not limited to specific embodiments or conditions disclosed herein, wherein the at least two rare earth elements are dissolved in the aqueous solution and include a first rare earth element having a higher atomic weight (smaller radius) than a second rare earth element, wherein the water-soluble macrocyclic chelator has a stronger binding affinity to the second rare earth element relative to the first rare earth element; and (ii) adding a precipitating salt which more selectively precipitates the first rare earth element relative to the second rare earth element. In some embodiments, steps (i) and (ii) are performed sequentially, while in other embodiments, steps (i) and (ii) are performed simultaneously. Moreover, the method may be inclusive of additional steps (e.g., removal and / or further processing of the precipitate or filtrate) or may be limited to steps (i) and (ii). In some embodiments, one or more additional steps may be included, except that an extraction step (such as with an organic solvent or solution) is excluded.

[0015] The precipitated rare earth element can be separated from the solution by methods well known in the art (e.g., by filtration, settling, decanting, or centrifugation, or combination of any of these) while the complexed rare earth element remains in the aqueous solution. The addition of an acid to the aqueous solution permits restoration and re-use of the water-soluble macrocyclic chelator. The rare earth elements are typically selected from Sc, Y, and the lanthanides but may or may not include the actinides and / or Zr and / or Hf. For example, in an aqueous solution containing Nd and Dy ions, the water-soluble macrocyclic chelator described herein will more selectively chelate the Nd ions over the Dy ions, thereby leaving the Dy ions more uncomplexed or more weakly complexed, and thus, more susceptible to precipitation when contacted with a precipitant. The aqueous solution preferably has a pH of 6-8, 6-7, or 7-8 before the precipitation step.

[0016] In some embodiments, the precipitating salt is a bicarbonate salt. In further or separate embodiments, the at least two rare earth elements in the aqueous solution are selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Lu, Sc, and Y, or the at least two rare earth elements are at least first and second lanthanide elements selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, and Lu. In more specific embodiments, the first and second rare earth elements in the aqueous solution are Dy and Nd, respectively, and Dy is more selectively precipitated in step (ii) relative to Nd. One or more additional rare earth elements (i.e., in addition to Dy and Nd) may or may not be present in the aqueous solution. In further or separate embodiments, the first and second rare earth elements are separated in step (ii) by a separation factor of at least 50, 100, 200, 300, 350, or 400. In further or separate embodiments, the molar ratio (L / M) of the chelator (L) to total amount of rare earth element (M) is at least 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, or 15. BRIEF DESCRIPTION OF THE FIGURES

[0017] FIG.1 is a chart comparing the lanthanide (Ln) stability constants (logKML) of metal complexes with Gmacropa, macropa, and bpa 18C6 (structures shown in FIG.5).

[0018] FIG.2 is a schematic depiction of the separation of biphasic Nd / Dy mixtures using a Gmacropa / HCO3- bi-extractant system and subsequent stripping of the metals using HCl.

[0019] FIGS.3a-3d plot data of the Gmacropa(macropa) / HCO3- bi-extractant precipitation system using an equivalent mixture of Nd and Dy at various L / M ratios. FIGS.3a and 3c are graphs plotting the yield of Nd in the filtrate and Dy in the solid using Gmacropa and macropa as extractant, respectively, relative to the initial amount at different L / M molar ratios (where L is chelator and M is metal). FIGS.3b and 3d are graphs plotting REE distribution percentage in filtrate and solid using Gmacropa and macropa as extractant, respectively, relative to the total metal in each phase.

[0020] FIG.4 is a graph plotting separation factors (SFs) for Nd / Dy pairs from L / HCO3- precipitation carried out at different L / M ratios. L represents Gmacropa and macropa.

[0021] FIG.5 shows chemical structures of several types of chelators. DETAILED DESCRIPTION

[0022] As used herein, the term "hydrocarbon group" (also denoted by the group R) is, in a first embodiment, composed solely of carbon and hydrogen. The hydrocarbon group may contain, for example, one, two, three, four, five, six, seven, eight, nine, ten, eleven, or twelve carbon atoms, or a number of carbon atoms within a range therein (e.g., 1-12, 2-12, 3-12, 1-6, 2-6, 3-6, 1-4, 2-4, or 1-3 carbon atoms). The hydrocarbon group composed solely of carbon and hydrogen can be, for example, a linear or branched alkyl, linear or branched alkenyl, cycloalkyl, cycloalkenyl (aliphatic), or aromatic group, as further discussed below.

[0023] Some examples of straight-chained (linear) alkyl groups include methyl, ethyl, n- propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, and n- dodecyl groups. Some examples of branched alkyl groups include isopropyl (2-propyl), isobutyl (2-methylprop-1-yl), sec-butyl (2-butyl), t-butyl, 2-pentyl, 3-pentyl, 2- methylbut-1-yl, isopentyl (3-methylbut-1-yl), 1,2-dimethylprop-1-yl, 1,1-dimethylprop- 1-yl, neopentyl (2,2-dimethylprop-1-yl), 2-hexyl, 3-hexyl, 2-methylpent-1-yl, 3- methylpent-1-yl, and isohexyl (4-methylpent-1-yl), wherein the “1-yl” suffix represents the point of attachment of the group.

[0024] Some examples of straight-chained olefinic (alkenyl) groups include vinyl, propen-1-yl (allyl), 3-buten-1-yl (CH2=CH-CH2-CH2-), 2-buten-1-yl (CH2-CH=CH-CH2- ), butadienyl, and 4-penten-1-yl groups. Some examples of branched olefinic groupsinclude propen-2-yl, 3-buten-2-yl (CH2=CH-CH.-CH3), 3-buten-3-yl (CH2=C.-CH2- CH3), 4-penten-2-yl, 4-penten-3-yl, 3-penten-2-yl, 3-penten-3-yl, and 2,4-pentadien-3-yl, wherein the dot in the foregoing exemplary formulas represents a radical (i.e., the point of attachment of the group).

[0025] Some examples of cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl groups. The cycloalkyl group can also be a polycyclic (e.g., bicyclic) group by either possessing a bond between two ring groups (e.g., dicyclohexyl) or a shared (i.e., fused) side (e.g., decalin and norbornane). Some examples of cycloalkenyl (aliphatic) groups include cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclopentadienyl, cyclohexenyl, cyclohexadienyl, cycloheptenyl, cycloheptadienyl, cyclooctenyl, cyclooctadienyl, and cyclooctatetraenyl groups. Some examples of aromatic groups include phenyl and benzyl. The aromatic hydrocarbon group can also be a polycyclic group (such as a bicyclic or tricyclic polyaromatic group) by either possessing a bond between two of the ring groups (e.g., biphenyl) or a shared (i.e., fused) side, as in naphthalene, anthracene, phenanthrene, phenalene, or indene.

[0026] In another embodiment, the term "hydrocarbon group" (R) contains at least one heteroatom (i.e., non-carbon and non-hydrogen atom), such as one or more heteroatoms selected from oxygen (O), nitrogen (NRa), sulfur (S), and halide atoms (e.g., F, Cl, or Br), or groups containing one or more of these heteroatoms (i.e., heteroatom-containing groups), wherein Rais H or a hydrocarbon group (R). One or more of the heteroatoms (e.g., oxygen, nitrogen, and / or sulfur atoms) can be inserted between carbon atoms (e.g., as -O-, -NRa-, or -S-) in any of the hydrocarbon groups described above. Alternatively, or in addition, one or more heteroatom-containing groups, such as any of these described below (e.g., OH, OR, CN, or C(O)OR), can replace one or more hydrogen atoms on the hydrocarbon group. In some embodiments, the hydrocarbon group contains at least one halogen atom (such as in -CF3 or -OCF3).

[0027] Some examples of oxygen-containing groups include hydroxy (OH), alkoxy (OR), carbonyl-containing (e.g., carboxylic acid, ketone, aldehyde, carboxylic ester, amide, and urea functionalities), nitro (NO2), carbon-oxygen-carbon (ether), sulfonyl, and sulfinyl (i.e., sulfoxide) groups. Some particular examples of alkoxy groups (–OR) include methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, isobutoxy, t-butoxy, phenoxy, benzyloxy, 2-hydroxyethoxy, 2-methoxyethoxy, 2-ethoxyethoxy, vinyloxy,and allyloxy groups. In the case of an ether group, the ether group can also be a polyalkyleneoxide (polyalkyleneglycol) group, such as a polyethyleneoxide group. Some examples of nitrogen-containing groups include primary amine, secondary amine, tertiary amine (i.e., -NR’2, wherein R’ is independently selected from H and hydrocarbon groups set forth above), nitrile (CN), amide (i.e., -C(O)NR’2or -NRC(O)R’, wherein R’ is independently selected from hydrogen atom and hydrocarbon groups set forth above), imine (e.g., -CR’=NR’, wherein R’ is independently H or a hydrocarbon group), urea (- NR’-C(O)-NR’2, wherein R’ is independently H or a hydrocarbon group), and carbamate groups (-NR’-C(O)-OR’, wherein R’ is independently H or a hydrocarbon group). Some examples of sulfur-containing groups include mercapto (i.e., -SH), thioether (i.e., sulfide, e.g., -SR), disulfide (-R-S-S-R), sulfoxide (-S(O)R), sulfone (-SO2R), sulfonate (- S(=O)2OR”, wherein R” is H, a hydrocarbon group, or a cationic group), and sulfate groups (-OS(=O)2OR”, wherein R” is H, a hydrocarbon group, or a cationic group). Some examples of halide atoms include fluorine, chlorine, bromine, and iodine. In some embodiments, any one or more of the above heteroatoms and / or heteroatom-containing groups may be excluded from Formula (1) or (1a).

[0028] In a first aspect, the present disclosure is directed to a water-soluble macrocyclic chelator composition (i.e., “compound” or “chelator”) having the following structure:(1)

[0000] e var a es , , , an n ormu a ( ) are n epen ent y se ected from O, S, and NRa, wherein Rais H or a hydrocarbon group (R) containing 1-12 carbon atoms optionally substituted with (contain) one or more heteroatoms selected from O, N, and S, as described above. In a first set of embodiments, one, two, three, or all of A1, A2, A3, and A4is / are O atoms. In a second set of embodiments, one, two, three, or all of A1, A2, A3, and A4is / are NRalinkers. In a third set of embodiments, one, two, three, or all of A1, A2, A3, and A4is / are S atoms. Any of the first, second, and third embodiments provided above may also be combined. For example, in some embodiments, one, two, or three of A1, A2, A3, and A4is / are O atoms while the remaining A1, A2, A3, and A4is / are selected from NRalinkers and / or S atoms. Rais independently selected from H and R groups for each instance where an NRalinker is present, and R may or may not contain one or more heteroatoms. In some embodiments, R may contain one or more F atoms, while in other embodiments, R does not contain F atoms or other halogen atoms. In some embodiments, Rais independently selected from H and methyl, or Rais H in all instances where an NRalinker is present.

[0030] The variables R1, R2, R3, and R4in Formula (1) are independently selected from the following groups: (i) hydrogen atom, (ii) hydrocarbon groups (R) containing 1-12 carbon atoms optionally substituted with one or more heteroatoms, as described above; (iii) halogen atoms (e.g., F, Cl, Br, or I); (iv) -C(=O)OH (i.e., COOH) groups; (v) -C(=O)OR (i.e., COOR) groups; (vi) -OH groups; (vii) -OR groups; and (viii) CN groups. In a first set of embodiments, one, two, three, or all of R1, R2, R3, and R4is / are H atoms. In a second set of embodiments, one, two, three, or all of R1, R2, R3, and R4is / are independently R groups. In a third set of embodiments, one, two, three, or all of R1, R2, R3, and R4is / are halogen atoms. In a fourth set of embodiments, one, two, three, or all of R1, R2, R3, and R4is / are COOH groups. In a fifth set of embodiments, one, two, three, or all of R1, R2, R3, and R4is / are independently COOR groups. In a sixth set of embodiments, one, two, three, or all of R1, R2, R3, and R4is / are OH groups. In a seventh set of embodiments, one, two, three, or all of R1, R2, R3, and R4is / are independently OR groups. In an eighth set of embodiments, one, two, three, or all of R1, R2, R3, and R4is / are CN groups. Any of the first through eighth embodiments provided above may also be combined. For example, in some embodiments, one, two, or three of R1, R2, R3, and R4is / are H atoms while the remaining R1, R2, R3, and R4is / are selected from hydrocarbon groups (R), such as methyl or ethyl, and / or halogen atoms. In some embodiments, R1and R2are optionally interconnected to form a ring, and / or R3and R4are optionally interconnected to form a ring. In other embodiments, R1and R2are not permitted to form a ring and / or R3and R4are not permitted to form a ring. Moreover, any one of the above described possibilities for R1, R2, R3, and R4may be combined with any one of the above described possibilities for A1, A2, A3, and A4.

[0031] The variables R5and R6in Formula (1) are independently selected from side chains of amino acids, or more particularly, the 20 common amino acids, all of which are herein understood to be alpha-amino acids. In some embodiments, one or both of R5and R6is / are H atoms, wherein H is a glycine side chain. In other embodiments, one or both of R5and R6is / are hydrocarbon groups, typically containing 1-12, 1-6, 1-4, or 1-3 carbon atoms (as in R described earlier above), more typically linear or branched alkyl groups, such as methyl (as found in alanine), isopropyl (as found in valine), isobutyl (as found in leucine), and sec-butyl (as found in isoleucine). Alternatively, R5and / or R6may contain an aromatic or heteroaromatic ring, such as benzyl (as found inphenylalanine), indole (as found in tryptophan), p-phenol (as in tyrosine), and imidazole (as found in histidine). Alternatively, R5and / or R6may be or include a highly hydrophilic side chain, such as carboxymethyl (i.e., -CH2COOH, as found in aspartic acid), 2-carboxylethyl (i.e., -CH2CH2COOH, as found in glutamic acid), guanidinium (as found in arginine), 4-aminobutyl (as found in lysine), hydroxymethyl (as found serine), 1-hydroxyethyl (as found in threonine), mercaptomethyl (as found in cysteine), methyl ethyl sulfide (as found in methionine), amidomethyl (as found in asparagine), and 2- amidoethyl (as found in glutamine). R5and / or R6may also interconnect with the adjacent NH to form a ring, as in proline. R5and / or R6may also be selected from amino acids slightly differing in structure from any of the 20 common amino acids, such as by one carbon in the side chain or presence of a different hydrophilic group, e.g., α- aminobutyric acid (homoalanine), homolysine, phenylglycine, or taurine. Moreover, any one of the above described possibilities for R5and R6may be combined with any of the above described possibilities for R1, R2, R3, and R4and further combined with any one of the above described possibilities for A1, A2, A3, and A4.

[0032] The variables R7, R8, R9, R10, R11, and R12are independently selected from the following groups: (i) H, (ii) R, (iii) OH, (iv) COOH, (v) OR, (vi) COOR, (vii) CN, and (viii) halogen atoms (e.g., F, Cl, Br, or I), wherein R is selected from hydrocarbon groups containing 1-12 carbon atoms optionally substituted with one or more heteroatoms selected from O, N, and S, as described in detail earlier above. In a first set of embodiments, one, two, three, four, five, or all of R7, R8, R9, R10, R11, and R12is / are H atoms. In a second set of embodiments, one, two, three, four, five, or all of R7, R8, R9, R10, R11, and R12is / are independently R groups. In a third set of embodiments, one, two, three, four, five, or all of R7, R8, R9, R10, R11, and R12is / are OH groups. In a fourth set of embodiments, one, two, three, four, five, or all of R7, R8, R9, R10, R11, and R12is / are COOH groups. In a fifth set of embodiments, one, two, three, four, five, or all of R7, R8, R9, R10, R11, and R12is / are independently OR groups. In a sixth set of embodiments, one, two, three, four, five, or all of R7, R8, R9, R10, R11, and R12is / are independently COOR groups. In a seventh set of embodiments, one, two, three, four, five, or all of R7, R8, R9, R10, R11, and R12is / are CN groups. In an eighth set of embodiments, one, two, three, four, five, or all of R7, R8, R9, R10, R11, and R12is / are halogen atoms. Any of the first through eighth embodiments provided above may also be combined. For example,in some embodiments, one, two, three, four, or five of R7, R8, R9, R10, R11, and R12is / are H atoms while the remaining R7, R8, R9, R10, R11, and R12is / are selected from hydrocarbon groups (R), such as methyl or ethyl, and / or halogen atoms. In some embodiments, two of R7, R8, and R9(e.g., R7and R8, or R8and R9) are optionally interconnected to form a ring. In separate or further embodiments, two of R10, R11, and R12(e.g., R10and R11, or R11and R12) are optionally interconnected to form a ring. Moreover, any one of the above described possibilities for R7, R8, R9, R10, R11, and R12may be combined with any of the above described possibilities for R5and R6, which may be further combined with any of the above described possibilities for R1, R2, R3, and R4and further combined with any one of the above described possibilities for A1, A2, A3, and A4.

[0033] In some embodiments, A1, A2, A3, and A4in Formula (1) are all O, in which case Formula (1) has the following structure:, and described above under Formula (1), and any of the definitions, specific examples,and combinations of these variables, as provided earlier above, can be selected in Formula (1a). In specific embodiments of Formula (1a), one, two, three, or all of R1, R2, R3, and R4is / are H atoms. In separate or further specific embodiments of Formula (1a), one, two, three, four, five, or all of R7, R8, R9, R10, R11, and R12is / are H atoms. In separate or further specific embodiments of Formula (1a), one or both of R5and R6is / are H atoms, wherein H is a glycine side chain, or one or both of R5and R6is / are CH3 groups, wherein CH3is an alanine side chain. In embodiments where one or more variables are not H atoms, the non-H variables are independently selected from any of the other groups described above, such as hydrocarbon groups (R), such as methyl or ethyl, and / or halogen atoms.

[0035] In a particularly specific embodiment of Formula (1a), the water-soluble macrocyclic chelator has the following structure, which is also herein referred to as Gmacropa or Gly-macropa:by adapting methods well known in the art. For example, Gmacropa, as shown in Formula (1b), can be prepared by reacting H2macropa (a free dicarboxylic acid precursor) with an ester- protected amino acid under conditions conducive for amidation between the alpha-aminogroup of the amino acid and carboxylic acid groups of H2macropa, followed by deprotection of the ester group followed by acidification to afford the final product. FIG. 2 shows a possible reaction scheme for producing Gmacropa, as an example. Although linkage of glycine is depicted in FIG.2, the synthetic method can be used to link any other type of amino acid, such as any of those described earlier above. The choice of amino acid being attached to the H2macropa precursor will determine the nature of the R5and R6groups in Formula (1) or (1a), wherein the R5and R6groups correspond to side chains of the amino acid being attached. If the amino acid being attached contains a carboxylic-reactive group in its side chain, the reactive group will likely need to be protected before the amidation reaction by using methods well known in the art.

[0037] In another aspect, the present disclosure is directed to methods for separating rare earth (RE) elements by use of any of the above described water-soluble macrocyclic chelators (i.e., “chelators”). In an initial step of the method, an aqueous solution containing at least two RE elements is contacted with any of the above described chelators, or specifically the chelator of Formula (1b), to result in binding of the chelator to the RE elements. The initial step may be referred to as step (i). The at least two RE elements are dissolved in the aqueous solution. In some embodiments, the aqueous solution contains precisely two RE elements, while in other embodiments, the aqueous solution contains precisely or at least three, four, five, six, or more different RE elements. The term “contacted” or “contacting,” as used herein, typically involves mixing, or more typically, dissolving, the chelator into the aqueous solution containing the at least two RE elements. The chelator can be mixed or dissolved into the aqueous solution by, for example, directly adding the chelator in solid (typically, powder) form to the aqueous solution followed by agitation, or by first mixing or dissolving the chelator into a separate aqueous solution and adding the separate aqueous solution into the aqueous solution containing the RE elements. Alternatively, the aqueous solution containing the RE elements may be added to a solid form or solution form of the chelator. The chelator can be dissolved into the aqueous solution by any of the intimate mixing methods known in the art, e.g., stirring, agitating, shaking, etc., which is typically done at room temperature (typically about 25°C) but may employ a slightly elevated temperature of up to 50 or 100°C if needed to ensure full dissolution. The at least two RE elements in the aqueous solution include at least a first rare earth element having ahigher (heavier) atomic weight (smaller radius) than a second rare earth element. The pH of the aqueous solution containing the RE elements before contact with the chelator is generally near neutral, e.g., 5.5-8.5, or 6-8 or around 7. In some embodiments, the aqueous solution contains only water as a solvent, while in other embodiments, the aqueous solution contains water admixed with a water-soluble organic solvent, such as an alcohol, acetonitrile, DMF, DMSO, or the like.

[0038] By virtue of the unique structure of the chelators described herein, the chelator has a stronger binding affinity to the second (lower atomic weight, larger radius) RE element relative to the first (higher atomic weight, smaller radius) RE element in the aqueous solution. The RE element that has a stronger binding affinity to the chelator will be more resistant to precipitation in the presence of a precipitating salt compared to the RE element that has a weaker binding affinity to the chelator. Thus, the RE element that has a stronger binding affinity to the chelator (i.e., lower atomic weight, larger radius RE) will be more inclined to remain in solution in the presence of a precipitating salt while the RE that has a weaker binding affinity to the chelator (i.e., higher atomic weight, smaller radius) will be more inclined to precipitate from solution.

[0039] The term “rare earth element” (i.e., “RE element” or “rare earth metal” or “RE metal”), as used herein, includes the lanthanide elements, which are elements having an atomic number of 57-71, and may or may not also include scandium (Sc) and / or yttrium (Y) and / or zirconium (Zr) and / or hafnium (Hf). The RE elements may or may not be or include the actinide elements, which are elements having an atomic number of 90-103 (e.g., uranium (U) and / or thorium (Th)). In some embodiments, the at least two RE elements in the aqueous solution may be exclusively at least two, three, four, or more lanthanide elements, or may be exclusively non-lanthanide RE elements, or may be a mixture of lanthanide and non-lanthanide RE elements.

[0040] The lanthanide elements present in the aqueous solution may include one or more of the following elements: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Any one, two, three, four, or more of the above lanthanide elements may be present in the aqueous solution, which may or may not be in the presence of one, two, or more non-lanthanide RE elements, provided that at least two, three, or four REelements altogether (i.e., as the sum of lanthanide and non-lanthanide RE elements) are present in the aqueous solution. In some embodiments, the aqueous solution contains precisely or at least two, three, four, or more lanthanide elements, either in the presence or absence of one, two, or more non-lanthanide elements (e.g., Sc, Y, and actinide elements), wherein the two or more lanthanide elements may be selected from any two or more lanthanide elements provided above. In more specific embodiments, the aqueous solution may contain Nd along with one, two, or more lanthanide or non-lanthanide RE elements (e.g., at least Dy and / or Sm), or the aqueous solution may contain Dy along with one, two, or more lanthanide or non-lanthanide RE elements (e.g., at least Nd and / or Sm), or the aqueous solution may contain La or Ce along with one, two, or more lanthanide or non-lanthanide RE elements (e.g., at least Nd, Dy, and / or Sm). Any two or more of the foregoing lanthanide elements may be at least partially separated from each other using the method described herein.

[0041] In the aqueous solution, the RE elements are generally present in salt form, i.e., as a complex between an ionic RE element (e.g., Nd+3or Dy+3) and a negatively charged counterion (i.e., anion). The RE salt should be fully dissolved in the aqueous solution before the aqueous solution is contacted with the chelator. The anion is typically a halide, such as chloride or bromide, but may alternatively or in addition be nitrate, oxide, or hydroxide, provided that the anion maintains the RE elements dissolved in the aqueous solution prior to being contacted with the chelator.

[0042] The molar ratio of the chelator (L) to total amount of rare earth elements (M) denoted herein as the ratio L / M. The ratio L / M is typically at least 1. In different embodiments, the ratio L / M is precisely, at least, or greater than 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or within a range bounded by any two of the foregoing values, e.g., 1-10, 1-8, 1-6, 1- 5, 1-4, 1-3, 2-10, 2-8, 3-10, 3-8, 4-10, or 4-8. Notably, any of the above described chelators with any selection of variables described above may be used in any of the molar ratios or ranges thereof as described above. Moreover, any of the above described chelators and molar ratios may be used for separating any combination of RE elements described above.

[0043] Following the initial step of contacting the aqueous solution containing the RE elements with the chelator, a precipitating salt is added to the aqueous solution that contains the RE elements in contact with the chelator. The precipitating step may also bereferred to as step (ii). The precipitating salt should possess an anionic portion known to form a poorly aqueous-soluble (or substantially aqueous-insoluble) complex with the two or more RE elements. Depending on the RE elements contained in the aqueous solution, the precipitating salt may be select from, for example, bicarbonate, carbonate, sulfate, carboxylate (e.g., oxalate formate, or acetate), or phosphate salts.

[0044] According to the invention, one or more RE elements having a stronger binding affinity to the chelator will have a greater tendency to remain in solution in the presence of the precipitating salt while the RE element that has a weaker binding affinity to the chelator will have a greater tendency to precipitate from solution. Thus, by virtue of the difference in binding affinity, the addition of the precipitating salt more selectively precipitates the first RE element (higher atomic weight, smaller radius) relative to the second RE element (lower atomic weight, larger radius), thereby achieving at least a partial separation of the first and second RE elements, wherein additional RE elements may or may not be present and may also simultaneously or subsequently be at least partially separated by the same precipitation method. The degree of separation is typically quantified as the separation factor (SF), as further discussed below. As a particular example, the aqueous solution may contain first and second RE elements as Dy and Nd, respectively, in which case Dy (the heavier RE element) will be more selectively precipitated out of the aqueous solution in step (ii) relative to Nd (the lighter RE element). Conversely, Nd will be more selectively retained in the aqueous solution (i.e., in the filtrate after precipitation), and thus, Nd will be more highly enriched in the aqueous solution (i.e., filtrate) after precipitation.

[0045] Typically, following the precipitation step, the precipitate is separated (e.g., by filtration, decanting, and / or centrifuging) from the aqueous solution. Thus, the second (lower atomic weight, larger radius) RE element will be more enriched in the filtrate after precipitation and separation of the first (higher atomic weight, smaller radius) RE element. The term “more enriched,” as used herein, is relative to the molar ratios of RE elements in the starting aqueous solution. Thus, in the case of the starting aqueous solution containing Nd and Dy elements, the resulting filtrate after precipitation will contain a greater Nd:Dy molar ratio relative to the starting aqueous solution.

[0046] In some embodiments, the RE element and / or chelator present in the aqueous solution, after precipitation and removal of the precipitated product, is / are recovered.Any suitable method of recovery may be employed. By a preferred method, the RE element and / or chelator are recovered by acidifying the aqueous solution after precipitation and removal of the precipitated product. The acid is typically an inorganic (mineral) acid. The inorganic acid may be, for example, a hydrohalide (i.e., HX, wherein X is typically Cl, Br, or I), sulfuric acid (H2SO4), nitric acid (HNO3), or phosphoric acid (H3PO4). Upon acidification, the chelator becomes protonated and releases the RE element. The protonated chelator will typically precipitate out of the aqueous solution and can thus be separated and removed by filtration, settling, decanting, and / or centrifugation while leaving the decomplexed RE element in solution. The decomplexed RE element can be isolated from the aqueous solution by adding well known RE precipitants, which are counter anions that form insoluble salts with RE elements. The precipitants might be oxalate salts, carbonate salts or bicarbonate salts. The chelator and RE element present in the solid precipitate may also be recovered.

[0047] In some embodiments, the method further includes a first step of producing the initial aqueous solution containing the RE elements, i.e., prior to the above-described initial step of contacting the aqueous solution containing the RE elements with the chelator. In some embodiments, the aqueous solution results directly or indirectly from a recycling process of one or more materials that contain the at least two RE elements. The materials being recycled may be, for example, magnetic materials, such as any of the neodymium-containing, samarium-containing, or dysprosium-containing permanent magnets known in the art. The materials being recycled may alternatively be, for example, catalysts used in the petroleum industry; components in lighting; components in television and computer screens; phosphors; lighter flints; and materials used in superconductors and lasers. In other embodiments, the aqueous solution containing the RE elements results directly or indirectly from a manufacturing process. In other embodiments, the aqueous solution containing the RE elements results directly or indirectly from an RE element mining or extraction process.

[0048] The extent of the separation is typically indicated by the selectivity of the process. The separation factor (SF) is calculated using Equation 1, as follows:wherein nRE1.filtrateis the number of moles of a first RE element (RE1) remaining in the filtrate, nRE2.filtrate is the number of moles of a second RE element (RE2) remaining in the filtrate, nRE1.solidis the number of moles of the first RE element (RE1) in the solid precipitate, and nRE2.solid is the number of moles of the second RE element (RE2) in the solid precipitate.

[0049] RE1 and RE2 refer to the two RE elements for which the separation factor is being calculated. The method described herein has the ability to separate first and second rare earth elements (i.e., at completion of step ii) by a separation factor of at least 25. In different embodiments, the first and second rare earth elements are separated by a separation factor of at least or above, for example, 25, 50, 75, 100, 150, 200, 250, 300, 350, or 400.

[0050] Examples have been set forth below for the purpose of illustration and to describe the best mode of the invention at the present time. However, the scope of this invention is not to be in any way limited by the examples set forth herein. EXAMPLES

[0051] Overview

[0052] In this work, a new macropa chelator referred to as Gmacropa is reported. Gmacropa contains a glycine introduced via a coupling reaction between carboxyl in the picolinate arm and the amine group in glycine. Gmacropa can be used as an aqueous stabilizer for light Ln3+in precipitation partition, resulting in a separation factor of SNd / Dy=372.2 and a precipitate of ACS grade Dy2(CO3)3. Overall, via amplifying the subtle coordination differences of Ln3+, the present method provides an REE separation strategy that is highly efficient, environmentally friendly, and low cost.

[0053] A bi-extractant system can amplify the negligible difference across the series, resulting in a more remarkable selectivity than where only one extractant is employed. The selectivity can be further enhanced when these two extractants possess the opposite lanthanide binding preference. Following this separation principle, a two-extractant precipitation process was established using Gmacropa as stabilizer for light Ln3+and NaHCO3was used to precipitate heavy Ln3+in a binary Nd / Dy separation. The concept is schematically shown in FIG.2. NaHCO3 was picked for a few reasons. 1. Nd and DylogK for Gmacropa and carbonate follows a sequence of [Nd(Gmacropa)]+> Nd2(CO3)3> Dy2(CO3)3 > [Dy(Gmacropa)]+. This binding preference order facilitates the separation. 2. The solubility product constant (pKsp) of Dy2(CO3)3is down to 31.5±0.9, suggesting its extremely high insolubility. 3. NaHCO3 also works as a buffer solution to keep the acidity of the aqueous system stable at pH 8.4, where Gmacropa shows the highest binding affinity for Nd3+.

[0054] Synthesis and Characterization of the Glycine-macropa (“Gmacropa”) Chelator

[0055] The Gmacropa chelator was synthesized using a two-step procedure. An exemplary process for producing Gmacropa starting from H2macropa is shown in the following scheme:

[0056] The glycine ethyl ester was conjugated to H2macropa via the coupling reaction of carboxyl acid groups in macropa with the amino group of the glycine ethyl ester molecule, followed by the deprotection of the ester group under basic conditions. Acidicwork-up of the crude reaction mixture by adding HCl (~6 M) affords G-macropa, which is isolated by filtration.

[0057] Synthesis of Gmacropa ethyl ester. The H2macropa starting material was obtained following a literature procedure. To a solution of macropa (2H2O 2HCl adduct, 500 mg, 1eq., 0.78 mmol) in DMF (8 mL) were added HBTU (4 eq., 1.186 g) and DIPEA (10 eq., 0.98 mL) at room temperature. Glycine ethyl ester was added after a homogenous solution obtained. The reaction mixture was heated at 45 ºC overnight, during which a significant quantity of precipitate formed, then the reaction mixture was stored at 4 ºC overnight and the precipitate was collected via filtration. The collected precipitate was washed with cold DMF (10 mL) and DCM (10 mL) successively, and dried under vacuum to yield the product as a white solid (410 mg, 73% yield).

[0058] Synthesis of Gmacropa. To a solution of the Gmacropa ethyl ester (white solid) obtained above (400 mg) in a mixed solvent of MeOH (28 mL) and THF (12 mL) was added 0.68 mL NaOH solution (5M). The resulting solution was heated at 80oC overnight. After cooling to room temperature, the volatiles were removed under vacuum at 40oC to give a colorless residue. HCl (6 M, 10 mL) was added to the solution to yield a white solid, which was collected by filtration to yield Gmacropa•3H2O•2HCl (340 m, pD~10g, 0.44 mmol, 77%).1H NMR (500 MHz, D2O, pD~10) δ 2.79 (t, J = 5.8 Hz, 8H, CH2N), 3.54 (s, 8H, NCH2O), 3.60 (t, J = 5.8 Hz, 8H, CH2O), 3.82 (s, 4H, CH2NH), 3.90 (d, J = 2.4 Hz, 4H, CH2O), 7.53 (dd, J = 2.1, 6.7 Hz, 2H, py-CH), 7.86 (s, 4H, py-CH).13C NMR (126 MHz, D2O, pD 10) δ 43.42, 53.12, 59.54, 66.63, 68.27, 69.56, 121.04, 127.54, 138.79, 148.60, 157.28, 166.89, 176.56. ESI-HRMS: m / z=324.15 ([M+2H]2+, Calcd; 324.15); m / z=647.30 ([M+H]+, Cacld; 647.30); m / z=669.28 ([M+Na]+, Cacld; 669.28). Elemental analysis: found %: C 54.72, H 5.90, N 7.52; calcd % for C30H42N6O10∙2HCl∙3H2O: C 46.64, H 6.55, N 10.65. Analytical HPLC (method quick- MeCN): tR = 12.95 min.

[0059] The ligands were fully characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and high-performance liquid chromatography (HPLC). The solid- state crystal structure of Gmacropa was determined by X-ray crystallography of a single crystal grown from an acidic solution of concentrated Gmacropa solution. Under this condition, Gmacropa crystallizes in the P1 ̄ space group. The macrocycle stands on a crystallographic inversion center. The pendant arms are positioned above and below theplane of the macrocycle, and pyridyl glycine outstretches away from the core of macrocycle.

[0060] Solution and Solid-State Characterization of La3+Complexes of Gmacropa

[0061] To gain insight on the REEs coordination chemistry of Gmacropa in solution, NMR spectroscopy and UV-Vis were employed to analyze the structures of its complexes for largest La3+and smallest Lu3+in D2O at pD7. The1H,13C, and 2D (COSY and HMBC) NMR spectra and ESI-MS data of La(Gmacropa)+complex indicate the successful formation of one symmetric complex species in solution. However, identical spectra were observed in the presence and absence of Lu3+. This observation was further confirmed by UV-Vis spectroscopy analysis, which indicates an unsuccessful synthesis of the Lu-Gmacropa complex. By contrast, macropa is able to form complexes with both of these Ln3+under identical condition. Paramagnetic1H and13C NMR analyses of the Ce and Nd complexes for Gmacropa were also conducted. Similar to La, both of these REEs can form on-fold complexes, which indicates that the lack of success in producing the Lu complex is a result of a decreasing binding affinity across the lanthanide series and not a shortcoming of the synthetic method.

[0062] Thermodynamic Solution Studies

[0063] Metal complex stability constants provide a quantitative measure of the thermodynamic affinity of ligands for metal ions. The magnitudes of these stability constants for different ligands are used to assess their values in different metal chelation applications. In this context, the stability constants of Gmacropa with a series of rare earth metal ions via potentiometric titrations were evaluated. The results are shown in FIG.1. The stability constant is defined as KLnL = [LnL] / [Ln3+][L] at chemical equilibrium.

[0064] The protonation constants (log Ka), as well as the stability constants (KLnL) of its complexes with light lanthanide (Ln) elements were determined by potentiometric titration. The corresponding protonation stability constants of macropa, bpa 18C6, and macrophosphi and the stability constants of their Ln complexes were collected for comparison. The structures of these chelators are shown in FIG.5.

[0065] The calculation equations of Kaand KLnLare expressed as follows, respectively:(2)(3)

[0066] For Gmacropa, four protonation constants were determined from the potentiometric titration in the absence of metal ions over a pH range of 2.5-11.3. The first two protonation constants are assigned to the successive protonation of the pivotal nitrogen atoms of the macrocycle. The third and fourth protonation steps occur on the carboxylic acid groups. Despite up to eight basic sites on this chelator, the fifth to eighth protonation constants (four total N atoms in the pyridine and amide groups) could not be accurately determined by potentiometric titrations in the pH range employed. The sum of the first two protonation constants of the glycine conjugate (14.41) is nearly identical to macropa (14.26), which suggests that the loading of glycine does not change the total basicity of the macrocycle.

[0067] Next, in order to evaluate the thermodynamic affinity of this new ligand for Ln ions, stability constants (log KML) were determined by conducting pH-independent potentiometric titration in the presence of early Ln(III)(La-Tb) in a ligand-to-metal ratio of 1:0.95. However, the stability constants of late Ln (Dy-Lu) ions were too low (<5) to be determined precisely even in a situation where 3 equivs of L is employed to suppress the formation of insoluble Ln hydroxide species at higher pH. Similar to macropa, Gmacropa maintains the reversed-size selectivity across the Ln series, reflected by its higher binding affinity for the large Ln ions over the small ones. This observation is in agreement with the above NMR and UV-vis data, which demonstrates that the modification on the methyl picolinate-carboxylate arm with glycine does not cause the transformation of the selectivity pattern. Remarkably, Gmacropa shows a 2-6 log unit lower in thermodynamic affinity for all Ln ions determined compared to macropa and macrophosphi.

[0068] Another noteworthy consequence is that infrequent deprotonated complex species (LnLH-1) were observed from the refinement of La, Ce, and Pr titration data. This is most likely due to the deprotonation of the coordinated water molecule in basic solution, which does not happen to macropa and macrophosphi (macropa derivative containingmethylphosphonate groups attached to the pyridine rings). The premature deprotonation may be due to the weakened affinity of Gmacropa for these three Ln3+ions. For Nd-Tb, the typical Ln hydroxide formed at the late stage of titration, which indicates that Gmacropa in the Ln complex was replaced by hydroxide ions. This typically occurs to Ln-L complexes of low thermodynamic stability. Generally, the sum of all log Kavalues reflects the overall basicity of a ligand, which is what the metal-ion affinity scale is based on.

[0069] It appears that larger Σ log Kavalue of ligand indicates higher thermodynamic affinity to metal ions. In the present case, although proximate Σ logKa of 20.4 and 19.4 are observed in Gmacropa and macropa, respectively, macropa exhibits 4-7 log units higher in logK value for La-Tb than that of Gmacropa. It can be reasonably deduced that the coordination structure of Gmacropa for Ln3+is in the absence of the carboxyl group. Alternatively, the carbonyl in the amide, with weaker basicity, participates in the coordination as a result of a favorable stereochemistry. This situation is quite similar to the Ln3+coordination configuration for bpa 18C6, whose entire logKa value is only 13.48 (M. Regueiro-Figueroa, et al., Inorg Chem 2015, 54, 4940–4952). Therefore, the effective Σ logKa of Gmacropa is 14.41 (sum of first and second logKa).

[0070] Despite the identical configuration of Gmacropa and bpa 18C6 in the coordination inner sphere, Gmacropa displays 1.86 and 0.96 log units higher in stability constants for La3+and Eu3+than bpa 18C6, respectively. The presence of the glycine residue in Gmacropa thus influences the Ln3+-binding affinity via outer-sphere interactions, wherein the term “outer-sphere” refers to effects that do not immediately arise from direct metal ion coordination. These effects may be a result of several factors. First, the unbonded carboxyl group can form hydrogen bonding with the solvent molecule (water). Second, the charged pendant arms of Gmacropa tune the thermodynamic affinity for Ln3+by outer-sphere electric fields. The influence of the latter may be dominant. An apparent configuration contraction can be observed before and after the chelator bonds to the metals, and this configuration transformation results in the two negatively charged pendant arms getting closer to the metal center. This gives rise to an additional binding force compared to bpa 18C6.

[0071] Upon further analysis of the log KML plot versus ionic radius, another significant difference in the selectivity patterns of Gmacropa, bpa 18C6, and macropa was found,with the results shown in FIG.1. Gmacropa appears in a span of 4.44 log K units for early Ln3+(La-Eu), while this value decreases to 3.46 logK units for bpa 18C6 and becomes relatively more static for macropa (Δlog K=1.98). For the related Nd / Dy REEs pair in secondary RE resource, a Δlog K of 2.64 was observed for macropa, while a value higher than 3.21 for Gmacropa was observed. Here, 3.21 is the stability constant difference of La(III) and Tb(III) complexes. As mentioned above, the exact logK of Dy- Gmacropa complex is too low to determine using potentiometric titration. This significant amplification in the binding affinity difference observed in early Ln3+elements indicates Gmacropa as a particularly promising extractant for separations of these REEs.

[0072] DFT calculations

[0073] DFT calculations can provide valuable insights into the bonding nature of complexes, including bond lengths and bond energies. This information can help to obtain a better understanding of the Ln3+affinity preference trend, as well as the origin of the thermodynamic discrimination of Ln-Gmacropa complexes compared to those of macropa. Specifically, the structures of largest La3+and smallest Lu3+complexes of two ligands were optimized using hybrid functional TPSSh / LC and the Stuttgart large-core effective core potential and basis set for the lanthanides and / 6-31G(d,p) basis set for the other atoms, and their standard free energies were calculated. Solvation influence in an aqueous environment was taken into account using the PCM solvation model. Δ(δλδ)(δλδ) and Δ(λδλ)(λδλ) have been demonstrated as the most energetically stable conformations for La3+and Lu3+macropa complexes, respectively. Therefore, these two conformations were employed in the structure optimization. With suitably optimized structures in hand, the ΔG° values for the chelation reaction between L2-and Ln3+were calculated. La(macropa)- + Gmacropa2-→ [La(Gmacropa)]++ macropa2-

[0074] Nd / Dy Separation Experiments

[0075] The separation of Nd / Dy in precipitation schemes was evaluated. The precipitation experiments were conducted in two steps: complexation (coordination) step and precipitation step. In the first step, Gmacropa and the RECl3 (RE= Nd and Dy) were combined to form their complexes over 4 hours. NaHCO3solution was added in thesecond step to precipitate Dy3+selectively. The effect of the ratio of chelator to total Ln3+(L / M) was performed in order to determine the most favorable separation conditions. As shown in FIGS.3a-3d, this REE pair can be well separated using a L / M of 8 with a separation factor of more than 360. Pure Nd3+aqueous solutions can be obtained in all separation experiments except for the no L control. These results demonstrate the use of the Gmacropa / HCO3- process for environmentally friendly RE separation and purification.

[0076] Following the procedure shown in FIG.2, ICP-MS data were calculated and summarized in FIGS.3a-3d. Both Nd and Dy were precipitated out by HCO3- in the absence of Gmacropa. A monotonic increase of the Nd yield (40.6%-96.62%) was observed in the filtrate as the addition of Gmacropa at L / M ratio of 1-8 (results shown in FIG.3a). Remarkably, negligible entrainment (<3%) happens to Dy in all filtrates due to its higher binding ability of HCO3- than Gmacropa. As a result, isolated Nd with high purities of 97.20%-96.37% was obtained in all L-related precipitation experiments (results shown in FIG.3b). The rise of L / M ratio to 8 leads to filtrate highly enriched to 96.37% Nd and the solid enriched to 94.95% Dy. However, the precipitation of this REE pair using macropa as Nd aqueous stabilizer showed very different results. Separation at higher L / M ratio of 2 and 4 neither yield any precipitate due to the REE ultrahigh affinity of macropa (results shown in FIG.3c), where all Dy were captured by HCO3- in Gmacropa scheme. Although there was some generation of solid products (42.66% Dy and 9.4% Nd), macropa disorders the REE distribution in both filtrate (39.17 Nd / 60.83 8Dy) and solid (81.77 Nd / 18.23 Dy) and gave rise to an inefficient separation. Further decreasing L / M ratio to 0.5, 70.29 % Dy was enriched in the precipitate, while a co- precipitation of 24.03% Nd results in a mixture of two REEs in each phase. Collectively, Gmacropa exhibits an overwhelming selectivity and precipitate efficiency of larger over smaller Ln3+compared to macropa in our L / HCO3- bi-extractant system. The great disparity in binding affinity for REEs between macropa and HCO3-makes it difficult to control the separation of Nd / Dy REEs pair.

[0077] The separation factor (SF) is used for determining the selectivity in the precipitation of one Ln3+ion over another, which is derived from distribution coefficients (D) and reflects the partitioning of metal ions between aqueous phase and non-aqueous phase (see equations 4 and 5 below). SFNd / Dy values of separation experiments usingGmacropa substantially augment in magnitude as the increase of L / M ratio, from 1.5 for no L to 372.2 for L / M=8, exhibiting 1-2 magnitudes higher than the control precipitations employing macropa (SFNd / Dy1.1-7.5). This great separation with a SF of 372.2 reaches a similar state-of-the-art as TriNOx3-precipitants (SFNd / Dy 359), where TriNOx3-is a chelator that can be used for Nd / Dy precipitations, and 10-fold exceeds those seen in solvent extraction based on HDEHP and hundreds of times improvement compared to separation employing TOGDA. (4)

[0078] of Gmacropa to Nd3+and Dy3+in solution before and after the addition of NaHCO3, an analysis was conducted on the products (L / M=1) in aqueous phase at these two stages by1H NMR spectroscopy in D2O at pD 7. The1H NMR spectrum only showed free Gmacropa and its Nd complex character peaks in a 1:1 molar ratio without HCO3-, which indicates that 100% Nd3+ions are coordinated by Gmacropa while Dy3+ions do not bind. After adding HCO3-, NMR spectroscopy revealed the presence of both Gmacropa and Nd(Gmacropa)+but in a ratio of 1.67:1. This observed loss of Nd(Gmacropa)+in the aqueous phase can be attributed to a trans- chelation reaction between partial Nd(Gmacropa)+and Nd2(CO3)3. The calculated precipitation efficiency (37.4%) of Nd is coherent with ICP-MS data (40.6%, FIG.3a). Here, the aqueous phase at L / M=1 was used for collecting apparent Nd complex peaks. In all, these NMR data elucidated the chelation and precipitation processes in the bi- extractant system. The precipitate was isolated by centrifugation followed by water washing and lyophilization. X-ray photoelectronic spectrum (XPS) analysis of the precipitates shows excellent correlation with the ACS grade Dy2(CO3)3. A high intensity of the Dy feature signals and negligible Nd were observed in both spectra. The direct application of precipitate product in the RE market make the present L / HCO3- precipitation scheme highly promising compared to the known separation strategies in which follow-up treatment procedures are needed, such as in reverse extraction.

[0079] To further explore the Gmacropa / HCO3- system for use in the Nd / Dy RE pair, precipitations on 75:25 and 95:5 mole ratio mixtures were also conducted using 8 equivalents of Gmacropa and 20 equivalents of HCO3- relative to total metal ions. Ndand Dy are utilized in Nd2F14B high-performance permanent magnets and electronic wastes. The precipitation percentage of Dy remained essentially static even in the presence of a 3-fold and 19-fold excess Nd. The filtrate was enriched to 96.36% Nd for the 75:25 mole ratio system. This purity was nearly identical to the result obtained from the 50:50 mixtures (96.37%). Notably, a pure Nd (99.28%) filtrate was obtained from the precipitation experiment starting with an initial ratio 95Nd:5Dy, which suggests that selectivity was enhanced at lower absolute Dy concentrations. This result can be explained by a reduced level of Nd entrainment during the precipitation of Dy.

[0080] Finally, the easy dissociation of Ln3+from their Gmacropa complexes and the low solubility of Gmacropa at acidic solution permit the recovery of ligand from filtrate in a straight-forward manner. The filtrate was isolated by filtration and treated with HCl, and white solid was observed in 5 min.1H NMR confirmed that this white solid was pure Gmacropa ligand. A recovery yield of 74.2% was achieved in a 0.07 mmol scale Gmacropa / HCO3- precipitation, comparable to that in a larger scale synthesis (0.6 mmol) of Gmacropa (yield: 77%). The ease of the release of RE ions under acidic conditions and the high reproductivity of Gmacropa reflected the cost-efficiency of the above described bi-extractant precipitation system.

[0081] Conclusions

[0082] Herein is demonstrated an efficient, entirely aqueous REE precipitation method using a glycine macrocyclic conjugate that functions as a large REE stabilizer and with HCO3- as a precipitant. In the above described separation strategy, the chelator (Gmacropa) and precipitant (HCO3-) have different selectivities for lanthanide ions, which improves the overall separation of REEs. By this method, a newly formed Ln2(CO3)3solid phase replaces the organic solvent phase in conventional separations. This along with the use of an all-aqueous processing medium advantageously eliminates a major source of environmental concern. The presence of the glycine in the amide portion of the Gmacropa chelator leads to a successive decrease in binding affinity for Gmacropa across the Ln series from lower to higher weight Ln elements, which facilitates the retention of light RE elements and precipitation of heavy RE elements. By this method, Nd and Dy, which are among the most critical REEs in spent permanent magnet sand electronic waste, can be efficiently separated in both yield (>92.4%-98.4%)and purity (94.95%-98.45%). Moreover, the method described herein provides facile stripping of the RE from the chelator and recovery of the chelator for re-use.

[0083] While there have been shown and described what are at present considered the preferred embodiments of the invention, those skilled in the art may make various changes and modifications which remain within the scope of the invention defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A water-soluble macrocyclic chelator composition having the following structure: (1)A1, A2, A3, and A4are independently selected from O, S, and NRa, wherein Rais H or R, wherein R is a hydrocarbon group containing 1-12 carbon atoms optionally substituted with one or more heteroatoms selected from O, N, and S; R1, R2, R3, and R4are independently selected from H, R, OH, COOH, OR, COOR, CN, and halogen atoms, wherein R is selected from hydrocarbon groups containing 1-12 carbon atoms optionally substituted with one or more heteroatoms selected from O, N, and S, wherein R1and R2are optionally interconnected to form a ring, and wherein R3and R4are optionally interconnected to form a ring;R5and R6are independently selected from side chains of the 20 common amino acids; and R7, R8, R9, R10, R11, and R12are independently selected from H, R, OH, COOH, OR, COOR, CN, and halogen atoms, wherein R is selected from hydrocarbon groups containing 1-12 carbon atoms optionally substituted with one or more heteroatoms selected from O, N, and S, wherein two of R7, R8, and R9are optionally interconnected to form a ring, and wherein two of R10, R11, and R12are optionally interconnected to form a ring.

2. The composition of claim 1, wherein the water-soluble macrocyclic chelator has the following structure:wherein R1, R2, R3, R4, R5, R6R7, R8, R9, R10, R11, and R12are as defined in claim 1.

3. The composition of any one of claims 1-2, wherein R1, R2, R3, and R4are H.

4. The composition of any one of claims 1-3, wherein R7, R8, R9, R10, R11, and R12are H.

5. The composition of any one of claims 1-4, wherein R5and R6are H.

6. The composition of claim 1, wherein the water-soluble macrocyclic chelator has the following structure: .

7. A method of separating rare earth elements, the method comprising: (i) contacting an aqueous solution comprising at least two rare earth elements with a water-soluble macrocyclic chelator, wherein the at least two rare earth elements are dissolved in the aqueous solution and comprise a first rare earth element having a higher atomic weight (smaller radius) than a second rare earth element, wherein the water- soluble macrocyclic chelator has a stronger binding affinity to the second rare earth element relative to the first rare earth element; and (ii) adding a precipitating salt which more selectively precipitates the first rare earth element relative to the second rare earth element;wherein the water-soluble macrocyclic chelator has the following structure: (1)A1, A2, A3, and A4are independently selected from O, S, and NRa, wherein Rais H or R, wherein R is a hydrocarbon group containing 1-12 carbon atoms optionally substituted with one or more heteroatoms selected from O, N, and S; R1, R2, R3, and R4are independently selected from H, R, OH, COOH, OR, COOR, CN, and halogen atoms, wherein R is selected from hydrocarbon groups containing 1-12 carbon atoms optionally substituted with one or more heteroatoms selected from O, N, and S, wherein R1and R2are optionally interconnected to form a ring, and wherein R3and R4are optionally interconnected to form a ring; R5and R6are independently selected from side chains of the 20 common amino acids; andR7, R8, R9, R10, R11, and R12are independently selected from H, R, OH, COOH, OR, COOR, CN, and halogen atoms, wherein R is selected from hydrocarbon groups containing 1-12 carbon atoms optionally substituted with one or more heteroatoms selected from O, N, and S, wherein two of R7, R8, and R9are optionally interconnected to form a ring, and wherein two of R10, R11, and R12are optionally interconnected to form a ring.

8. The method of claim 7, wherein the water-soluble macrocyclic chelator has the following structure:are as 9. The method of any one of claims 7-8, wherein R1, R2, R3, and R4are H.

10. The method of any one of claims 7-9, wherein R7, R8, R9, R10, R11, and R12are H.

11. The method of any one of claims 7-10, wherein R5and R6are H.

12. The method of claim 7, wherein the water-soluble macrocyclic chelator has the following structure: .any one salt is a bicarbonate salt.

14. The method of any one of claims 7-13, wherein the at least two rare earth elements are at least two lanthanide elements comprising first and second lanthanide elements.

15. The method of any one of claims 7-14, wherein the first and second rare earth elements are first and second lanthanide elements selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Lu, Sc, and Y.

16. The method of claim 15, wherein the first and second rare earth elements are Dy and Nd, respectively, and Dy is more selectively precipitated in step (ii) relative to Nd.

17. The method of any one of claims 7-16, wherein the first and second rare earth elements are separated in step (ii) by a separation factor of at least 50.

18. The method of any one of claims 7-16, wherein the first and second rare earth elements are separated in step (ii) by a separation factor of at least 100.

19. The method of any one of claims 7-16, wherein the first and second rare earth elements are separated in step (ii) by a separation factor of at least 200.

20. The method of any one of claims 7-16, wherein the first and second rare earth elements are separated in step (ii) by a separation factor of at least 300.

21. The method of any one of claims 7-20, wherein the ratio (L / M) of the chelator (L) to total amount of rare earth (M) is at least 1.

22. The method of any one of claims 7-20, wherein the ratio (L / M) of the chelator (L) to total amount of rare earth (M) is at least 2.

23. The method of any one of claims 7-20, wherein the ratio (L / M) of the chelator (L) to total amount of rare earth (M) is at least 4.

24. The method of any one of claims 7-20, wherein the ratio (L / M) of the chelator (L) to total amount of rare earth (M) is at least 8.

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