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Extraction Behaviors of Heavy Rare Earths with Organophosphoric Extractants: The Contribution of Extractant Dimer Dissociation, Acid Ionization and Complexation. A Quantum Chemistry Study Yu Jing, Ji Chen, Li Chen, Wenrou Su, Yu Liu, and Deqian Li J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b01444 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017

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The Journal of Physical Chemistry

Extraction Behaviors of Heavy Rare Earths with Organophosphoric Extractants: The Contribution of Extractant Dimer Dissociation, Acid Ionization and Complexation. A Quantum Chemistry Study Yu Jing, Ji Chen*, Li Chen, Wenrou Su, Yu Liu, Deqian Li

(State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China)

Abstract Heavy rare earths (HREs), namely Ho3+, Er3+, Tm3+, Yb3+ and Lu3+, are more unique and exceptional than light rare earths, due to the stronger extraction capacity for 100,000 times. Therefore, their incomplete stripping and high acidity of stripping become the problems of HRE separation by organophosphoric extractants. However, the theories of extractant structure-performance relationship and molecular design method of novel HRE extractants are still not perfect enough. Beyond the coordination chemistry of the HRE extracted complex, the extractant dimer dissociation, acid ionization and complexation behaviors can be crucial to HRE extraction and reactivity of ionic species for understanding and further improving the extraction performance. To address above issues, 1 ACS Paragon Plus Environment


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three primary fundamental processes, including extractant dimer dissociation, acid ionization and HRE complexation were identified and investigated systematically. The intrinsic extraction performances of HRE cations with four acidic organophosphoric extractants (P507, P204, P227 and Cyanex 272) were studied by using relativistic energy-consistent 4-f core pseudopotentials, combined with density functional theory and a solvation model. Four acidic organophosphoric extractants have been qualified quantitatively from microscopic structures to chemical properties. It has been found that the Gibbs free energy changes of overall extraction process (sequence: P204> P227> P507 > Cyanex 272) and their differences as a function of HREs (sequence: Ho/Er> Er/Tm> Tm/Yb> Yb/Lu) are in good agreement with the experimental maximum extraction capacities and separation factors. These results could provide an important approach to evaluate HRE extractants by the comprehensive consideration of dimer dissociation, acid ionization and complexation processes. This paper also demonstrates the importance of P-O bond, P-C bond, isomer substituent and solvation effects on the structure-performance relationship to guide molecular designs of HRE extraction in future.

1. Introduction Rare earths (REs) are one of the most important strategic mineral resources. Especially heavy rare earths (HREs), including Ho, Er, Tm, Yb and Lu, have a significant

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effect on luminescence, electronics, magnetism, catalysis, metallurgy, and the ceramic industry1, 2. 2-ethylhexyl phosphoric acid mono(2-ethylhexyl) ester (P507) is widely used in industry for HRE extraction. However, its relatively slow kinetics, high stripping acidity and incomplete stripping have limited the high value utilization of HREs3. With the increasing requirements of environmental protection and demands for rare earths, it is necessary to find a more efficient HRE extractants to solve these problems. Up to now, three acidic organophosphoric extractants, including di(2-ethylhexyl) phosphoric acid (P204), 2-ethylhexyl phosphonic acid mono(2-ethylhexyl) ester (P507) and bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272, short for C272) are commercially available for large-scale applications1. Recently, bis(2-ethylhexyl) phosphinic acid (P227)

4, 5

has been selected and synthesized successfully by Shanghai

Institute of Organic Chemistry in Chinese Academy of Sciences to substitute for conventional extractants. The different structures of four acidic organophosphoric extractants are illustrated in Scheme 1. P204 has two oxygen-phosphorus bonds. P507 has one oxygen-phosphorus bond and one carbon-phosphorus bond. P227 has two carbon-phosphorus

bonds,

while

C272,

its

isomer,

2,4,4-trimethylpentyl substituents.


is

substituted

by

two


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Scheme 1. The structures of four acidic organophosphoric extractants When

carbon–phosphorus

bonds

replace

oxygen–phosphorus

bonds,

dialkyl-phosphoric acid demonstrates higher pKa so that the stripping acidity can be decreased significantly. However, the structural change should make a great impact on extraction performances due to the electronic effect, steric hindrance and solvation effect. Comprehensively experimental

evaluation

of

abovementioned

extractants

took

researchers a great amount of time, money and energy due to the minor differences of structural and extraction performance among four extractants. Therefore, it is not easy to make a hasty judgment and conclusion. Moreover, we are completely far from knowing their interaction mechanisms at atomistic level. During the last two decades, various experimental studies6-12 have been carried out to investigate the kinetics, thermodynamics and mechanisms of HRE extraction and separation. The IR, NMR, dynamic interfacial tension, constant interfacial cell with laminar flow and X-ray absorption fine structure (XAFS) measurements have been


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adopted, however, most of the results are empirical or semi-empirical experimental conclusions. The extraction performances and extracted complex structures were obtained under some specified conditions and lack of the intrinsic theory from the perspective of atomic and molecular level. Whether previous analysis in the solid state is representative of those in extraction solution, a recurrent question, generally difficult to assess by experiment only, however, can be tackled by quantum chemistry calculation. Luo et al.13 explored the structural and electronic properties of Eu3+ and Am3+ complexes with P507 in nitric acid solution by DFT theory. Boehme et al.14,

15

used quantum mechanical

calculations to gain insight into the ligand-metal bonding in dithiophosphinate complexes of trivalent lanthanides in terms of ligand design for lanthanide or actinide separation. Berny et al.16 studied the interaction of several substituted amides, pyridines, and phosphoryl-containing OPPh3 ligands with La3+, Eu3+, Yb3+, Sr2+ and Na+. Ustynyuk et al.17, 18 adopted the DFT simulation to design a novel RE extractant and it was then proved experimentally to be efficient donor ligands with high and unusual selectivity for the extraction separation of lanthanides. Therefore, quantum chemistry calculation has been a potentially powerful tool in investigating the extraction performance at the atomic level. To sum up, on the experimental side, there is relative scarcity of microscopic structures, particularly those enabling comparison of how different extractants bind to different given HRE ions, because extracted complex single crystal could hardly be obtained so far. It is thus really difficult to investigate the structures of HRE-extracted



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complexes by experiment, so we would make an attempt to evaluate the structures and extraction performances for different extractants from bottom to top by quantum chemistry to predict the experimental results, such as the maximum extraction capacity and separation factor. In our previous publications, we made a comprehensive appraisal of P507, C272, mixture of P507 and C272, P507 adding isooctanol and bifunctional ionic liquid extractant (Bif-ILE) [trialkylmethyl-ammonium] [di-(2-ethylhexyl) orthophosphinate] ([A336] [P507]) based on the coordination equilibrium effect19. Subsequently, the aqueous partition mechanism of traditional extractants, namely P204, P507, P227 and C272, for the extraction of REs was explored20. The results demonstrate that the solubility of extractants decreases with the increase of aqueous acidity, RE loading and electrolyte concentration. Especially, the solubility of P204, P507, P227 and C272 decreases with the increase of RE complex, indicating that aqueous partition of the extractants is accompanied by the RE coordination reaction. Along this line, the structure-performance relationship should be studied in next work to determine intrinsic mechanisms of the preference of an extractant for HRE cations. This possibly also assists in the construction of more efficient extractants. In this paper, we aim to obtain the structural information of four classic acidic organophosphoric extractants and their corresponding ions/complexes, and corresponding Gibbs free energy changes for the extraction in kerosene phase. Firstly, the full extraction


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process was simplified by three main reactions, including dimer dissociation, acid ionizations and HRE complexation, and then they were calculated by quantum chemistry. The results allow us to assess the intrinsic features and contribution of each fundamental reaction. Secondly, the reactivity sequences based on each fundamental reaction of four different extractants were obtained. We could better understand the HRE-ligand binding features and the role of carbon- or oxygen-phosphorous bond and substituents in different extractants. Thirdly, the minor geometry sizes and Gibbs energy changes of different cations were studied by a comparison of given HREs, namely, Ho3+, Er3+, Tm3+, Yb3+ and Lu3+. It will provide insights into the effect of the cation extracted by a given type of extractant on extracted complex. Finally, the overall HRE extraction process consisting of three fundamental reactions will be evaluated as a way to predict the trends in maximum extraction capacities and separation factors of different extractants. 2. Computational Methods 2.1 Quantum chemistry calculation REs have the extremely complex f- electronic structure, which contains numerous low-lying electronic states and exhibits large relativistic effects as well as strong electron correlation contributions, and lead to considerable difficulties to theoretical calculation. There are several popular choices for approximate treatment of relativistic effects of lanthanides21: (1) accurate four-component approaches based on Dirac equation and its alternative relativistic formalisms; (2) two-component quasi-relativistic methods based on



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Douglas–Kroll–Hess

Hamiltonian,

zeroth-order

regular

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approximation,

and

quasi-relativistic effective core potential; (3) scalar quasi-relativistic methods without considering the spin-coupling effects. Among the approaches developed in relativistic quantum chemistry, the method of ab initio pseudopotentials (PPs) is one of the most successful ones22. Because of the core-valence separation, only the chemically relevant valence electron system is treated explicitly and relativistic effects are only implicitly accounted for by a proper adjustment of free parameters in the valence-only Hamiltonian. Whereas the first aspect leads to a reduction of the computational effort, the second allows an inclusion of the scalar-relativistic contributions in a nonrelativistic framework. Dolg and Cao

22-24

have made a great contribution by many extraction and separation

applications for this calculation method, which shown relativistic energy-consistent ab initio 4f-in-core and 4f-in-valence PPs are useful and reliable. The method of relativistic energy-consistent ab initio pseudopotentials (PPs) briefly was described here. The valence-only model Hamiltonian for a system with n valence electrons and N nuclear with charges Q is given as

HV = −

1 n ∑∆ + 2 i i

n

∑r

i P204> C272. It is possible that P507, an asymmetric structure with two side chains, results in a heavier tension effect, while C272, a symmetric structure of two tertiary butyl groups, has σ-π hyperconjugated electron effect. Furthermore, large numbers of isomerized methyl groups of C272 can encapsulate P=O and –OH groups to protect the dimers not to be easily attacked by solvent. The C272, P227, P507 and P204 solvation results are -4.87 kcal/mol, -3.17 kcal/mol, 20 ACS Paragon Plus Environment

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-2.35 kcal/mol and -2.25 kcal/mol obtained by the COSMO method as a result of the different extractant volumes20 (C272= 391.56 Å3, P227= 403.13 Å3, P507= 429.65 Å3 and P204= 421.30 Å3). In addition, it can be intuitively explained by the frontier orbital analysis, as shown in Figure 3 and Figure 4. As the organic phase contains a large number of electron-rich reagents, such as alkanes and aromatic compounds, dimer dissociation occurs in the organic phase after the hydrogen bonds are attacked by electron-rich groups of solvents. The lowest unoccupied molecular orbital (LUMO) analysis of extractant dimers can easily reflect the electrophilic activity. It could be observed that the assailable contact areas in –OH groups of P204 and P507 dimers are very large, while those of C272 dimer are very narrow so that it will produce higher energy to solvate. From Figure 4, the highest occupied molecular orbitals (HOMO) of four kinds of extractant monomers can reveal the nucleophilic activity. The active regions of them are really wide. This indicates the extractant monomers in organic phase can attack the dimers and promote the dissociation process. In summary, the sequence of extractant dimer dissociation combined with solvation effect from easy to difficult is P507> P227> P204> C272.



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Figure 3. The LUMO analysis of extractant dimers Key: Red 0.03 eV; Blue -0.03 eV

Figure 4. The HOMO analysis of extractant monomers Key: Red 0.03 eV; Blue -0.03 eV 3.2 Extractant acid ionization 22 ACS Paragon Plus Environment

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Acid ionization is the second step in the extraction process. The difficulty of acid ionization of the extractant monomers (pKa) has always been considered as a key factor32, 42

in the extraction of REs for the past several years. The structures of extractant anions

were calculated in Figure 5 by quantum chemistry based on the calculated extractant monomers in the previous section.

Figure 5. Optimized structures of the extractant anions by quantum chemistry calculation.

From Figure 5, it can be seen that C272 and P227 extractant anions are very close to their corresponding extractant monomer structures after acid ionization, because carbon atoms in two side chains are directly connected to the phosphorus atom. However, P204 and P507, whose oxygen atoms in side chains directly or partly connected to the phosphorus atom, exhibit different features with the conjugate effect. There is a higher charge density in phosphorus atom direction because of the strong electronegativity of oxygen atoms, so that the hydrogen atom of hydroxyl group is easily dropped off to 23 ACS Paragon Plus Environment


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produce the extractant anions. In addition, P507 is an asymmetry structure, so that the two side chains are extended to different directions. The calculated averaged bond lengths and angles are listed in Table 3. Table 3. Calculated averaged bond lengths and angles for extractant anions R(P-R1)/Å R(P-R2) /Å R(P-O1)/ Å R(P-O2)/ Å ∠O1PO2 ∠R1PR2 [C272] 1.870 1.870 1.523 1.518 121.3 102.4 [P204] 1.678 1.666 1.507 1.497 122.8 97.0 [P507] 1.857 1.703 1.511 1.509 122.2 98.0 [P227] 1.878 1.872 1.522 1.514 121.8 101.7 From Table 3, it can be seen that the values of R(P-R1) and R(P-R1) are similar to their extractant dimers and monomers. When phosphorus atom is directly connected to the alkoxy side chain, the bond length is shorter due to the stronger electronegativity. ∠ O1PO2 values of four extractants are close, while ∠R1PR2 values of P204 and P507 are relatively low and both side chains are distorted to some extent due to the electronegativity of oxygen atoms. Another important result is that the distances of R(P-O1) and R(P-O2) between the two oxygen atoms and phosphorus atom are: P204 P227≈ C272. Details of complexation will be described in the next section. From the perspective of acid ionization, actually, the R(O1-H) of the extractant 24 ACS Paragon Plus Environment

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monomers calculated from the quantum chemistry in the previous section can accurately predict the ease or complexity of acid ionization, but very tiny differences of R(O1-H) exist in four extractant monomers. We could better compare them by thermodynamic properties. The changes in the Gibbs free energy of acid ionization are listed in Table 4. Table 4. Calculated changes in the Gibbs free energy contribution in gas phase, solvation effect and both total in extractant acid ionization ∆Gg2/kcal ∆∆Gsol2/kcal ∆G2/kcal C272 319.65 -278.21 41.44 P204 317.95 -279.93 38.02 P507 322.41 -280.53 41.88 P227 327.34 -280.52 46.82 Combining the Gibbs free energy change in gas phase with the solvation effect, we can find that the final sequence of acid ionization from easy to difficult is P204> C272≈ P507> P227. The main reason is that the electron and substituent effects of different side chains in extractants influence the distance between oxygen atom and hydrogen atom of the hydroxyl group. As described in the previous section, P204 and P507, whose side chains are connected to oxygen atoms suffer a higher charge attraction for oxygen atoms of P=O and –OH groups, so they have slightly longer bond lengths of R(O-H). Meanwhile, the electron-donating effect of alkoxy side chain leads to a stronger acidity of P204 and P507. On the contrary, the side chains of C272 and P227 have little effect of induction and conjugation effect, so their acidities are less than P204 and P507. Particularly, the core P=O and –OH groups of C272 has been wrapped by a large number of isomerized methyl groups, so C272 anion is more stable in the solvent than P227, 25 ACS Paragon Plus Environment


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which indicates the acid ionization capacity of P227 is the weakest among the four extractants. Therefore, the sequence of acid ionization from strong to weak is P204> P507> C272≈ P227. This calculated result is in good accordance with the experiment42, i.e., pKa(P204) = 2.79, pKa(P507) = 3.30, pKa(C272) = 3.73 and pKa(P227) = 4.98. It is worth noting that extractant acid ionization is not a key step by quantum chemistry method, if the initial pH in aqueous solution is appropriate. In actual system, the initial pH condition is usually constant and not fit for all elements of RE extraction, especially for different light, medium and heavy REs. Although the mistaken idea that the acid ionization plays a vital role in RE extraction has been circulated last decade, a large number of extraction and stripping experiments have recently proved the pKa only controls the acidity of stripping instead of extraction capacity and separation factor4, 5. Fortunately, this present work has firstly reconfirmed this conclusion by theory. When one neutral extractant monomer rips off one hydrogen cation and dissociates into one anion, the distances of the P atom and the side chains (whether alkyl chain or alkoxy chain) become longer, because the positive and negative charges reassign to two moieties after acid ionization. The value of R(P=O) decreases, while that of R(P-OH) slightly increases and keeps very close to each other. This demonstrates that the core binding site, i.e., two oxygen atoms and phosphorus atom, has produced a three-center and four-electron conjugate effect. In addition, P507, an asymmetry structure, is different from the completely symmetry structure of P204, P227 and C272, two side chains of its


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monomer (See Figure 2) and acid anion (See Figure 5) extend in the opposite directions. In summary, the final sequence of acid ionization for extractant monomers from strong to weak is P204> C272≈ P507> P227.

3.3 HRE complexation In order to tentatively evaluate the effect of different structures of extractants on HRE complexation, the HOMOs of four extractant anions, which reflect the ability of binding to metal cations, are shown in Figure 6. The main active sites are located in the region of acid radical of organophosphorus extractants. We can find that the HOMOs of C272 and P227 anions are different from P204 and P507. The positive and negative regions of the former have their respective certain directions from up to bottom (See Figure 6), while the active binding sites of P204 and P507 have a wide range. This shows that P204 and P507 have the more opportunities and wider ranges for attacking metal cations than C272 and P227. This interesting result can supply and explain the kinetic extraction experiment that the HRE extraction equilibrium time of C272 and P22719, 42, 43 (30-60 min) is much longer than P204 and P50719, 42, 44 (10-20 min).



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Figure 6. The HOMO analysis of extractant anions Key: Red 0.03 eV; Blue -0.03 eV

HRE (Ho3+, Er3+, Tm3+, Yb3+ and Lu3+) extracted complexes with four different extractants were obtained by quantum chemistry calculation. The structures of complexes with different HREs are extremely similar. The representative structures of Lu extracted complexes with different extractants are shown in Figure 7. It can be seen from Figure 7 that the HRE cations of the extracted complexes are not completely in the middle of molecules but slightly slanted towards hydrogen atoms of one side chain. The HRE cation of C272 extracted complex tends to be attracted at an angle owing to a large number of isomerized methyl groups. Likewise, that of P507 extracted complex inclines upward to the alkoxy chains due to its asymmetry structure. The HRE cations of P204 and P227 extracted complexes are firmly “seized” by two side chains to locate in the center of molecules as a result of symmetry structures.


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Figure 7. Optimized structures of the representative Lu extracted complexes by quantum chemistry calculation.

Moreover, two alkoxy side chains (6C+1O atoms) of P204 extracted complex are much more flexible and can wrap the HRE cation much stronger because of the longer chains, while two short alkyl side chains (4C atoms) of P227 extracted complex attract the HRE cation caused by the insufficient length of side chains without two oxygen atoms. This structural effect also demonstrates the reason why the extraction performances of P204 and P227 vary so much in spite of the slight difference of two oxygen atoms. The geometry data of HRE extracted complexes are very close due to the high similarity in physical and chemical properties. For the sake of brevity, the detailed bond lengths and angles for representative extracted complexes of Ho3+ and Lu3+ with single ligand are listed in Table 5. The full data of HRE extracted complexes are removed to 29 ACS Paragon Plus Environment


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Supporting Information. Table 5. Calculated averaged bond lengths and angles for the representative extracted complexes R(O1-M)/Å R(O2-M)/Å R(P-M)/Å ∠O1PO2 ∠R1PR2 ∠O1MO2 C272-Ho 2.080 2.107 2.731 97.7 116.6 70.8 C272-Lu 2.037 2.065 2.684 97.3 117.2 72.3 P204-Ho 2.140 2.130 2.755 100.7 108.3 69.4 P204-Lu 2.098 2.090 2.711 100.3 108.4 70.8 P507-Ho 2.118 2.078 2.740 98.2 110.6 70.5 P507-Lu 2.077 2.037 2.693 97.8 110.9 72.0 P227-Ho 2.078 2.076 2.718 99.2 116.6 72.4 P227-Lu 2.035 2.032 2.670 98.9 117.9 74.1 According to the lanthanide contraction principle32 (The radius of Ho3+, Er3+, Tm3+, Yb3+ and Lu3+ are 0.894 Å, 0.881 Å, 0.869 Å,0.858 Å and 0.848 Å, respectively), 4f electron of valence electron contribution for ligands is more intensity, so the ability of HRE cations binding to ligands is improved with the increase of HRE element number. From the results of quantum chemical calculations, the distances between the HRE cations and the two oxygen atoms connected to the phosphorus atom become gradually smaller with the increase of HRE element number, indicating that the HRE extraction capacity is gradually enhanced. To sum up, the calculated results follow the lanthanide contraction theory precisely. Furthermore, ∠ O1PO2 values gradually decrease with the increase of the complexation ability and HRE element number, while ∠R1PR2 values gradually increase. That means that the two oxygen atoms of the acid radical are pulled more seriously by HRE cations and the lone pair electrons of ligands are donated more strongly. The final 30 ACS Paragon Plus Environment

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HRE extracted complex structure is stretched longer when complexation reaction occurs. In addition, in order to analyze the regularity of the extracted complexes with different extractants, the distances between HRE cation and the central phosphorus atom are shown in Figure 8. From the calculation results, we can see that the sequence of the bond lengths from long to short is P204> P507> C272> P227, because the alkoxy chains of P204 and P507 directly or partly connected to the central phosphorus atom gradually weaken the contribution of local conjugation effect for two binding oxygen atoms. A large number of isomerized methyl groups of C272 also weaken the local charge density for two binding oxygen atoms due to the σ-π hyperconjugation effect. Therefore, HRE cations and the phosphorus atom of P227 prefers a closer distance. However, this result cannot provide the stability of extracted complexes owing to the intricate HRE extraction system. The stability of the extracted complexes is not only related to the electronic effect and extracted complex structure, but also to the steric hindrance effect, solvation effect and actual operation condition (i.e., the acidity of initial aqueous phase, oil water ratio, interface feature, mixing behavior and dispersing phenomenon). Therefore, the extraction properties need to be inferred from the final thermodynamic Gibbs free energy.



2.78

Ho

Er

Tm

Yb

Lu

69

70

71

2.76 2.74 R (M-P) / Å

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2.72 2.70 C272 P204 P507 P227

2.68 2.66 2.64 67

68

RE Number

Figure 8. The distances between the HRE cations and the central phosphorus atom of different extracted complexes

The changes in the Gibbs free energy of different extracted complexes are listed in Table 6. The absolute values of Gibbs free energy change in gas phase (negative value) increase significantly with the increase of HRE element number. Meanwhile, the absolute values of Gibbs free energy change of the solvation effect (positive value) increase gradually. The former embodies that the complexation capacity is improved from Ho3+ to Lu3+ caused by the lanthanide contraction principle. In contrast to this, the later reflects that the solvation energy gradually increases with the increase of HRE element number in order to overcome the greater energies from gas phase into kerosene solvent. It could be observed that the change of Gibbs free energy in gas phase gives much more contribution than that in solvent phase, so the absolute value of total Gibbs free


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energy change (negative value) increases with the increase of HRE element number in the actual solvent system, indicating that the HRE complexation ability is improved remarkably.

Table 6. Calculated changes in the Gibbs free energy contribution in gas phase, solvation effect and both total in HRE complexation ∆Gg3/kcal ∆∆Gorg3/kcal ∆G3/kcal C272-Ho -610.45 144.25 -466.21 C272-Er -615.36 144.96 -470.40 C272-Tm -619.79 146.01 -473.78 C272-Yb -624.49 147.68 -476.81 C272-Lu -628.74 149.87 -478.87 P204-Ho -625.35 149.56 -475.78 P204-Er -630.83 150.34 -480.49 P204-Tm -635.84 151.45 -484.39 P204-Yb -640.98 153.18 -487.80 P204-Lu -645.67 154.91 -490.76 P507-Ho -610.19 147.19 -463.00 P507-Er -615.26 147.93 -467.34 P507-Tm -619.85 149.00 -470.85 P507-Yb -624.66 150.70 -473.96 P507-Lu -629.01 152.75 -476.25 P227-Ho -619.14 146.92 -472.21 P227-Er -624.65 147.65 -477.00 P227-Tm -629.29 148.73 -480.57 P227-Yb -634.16 150.42 -483.74 P227-Lu -638.47 152.52 -485.95 For the same HRE cation, the final sequence of complexation ability from strong to weak is P204> P227> C272≈ P507. It may seem contradictory that the distance between HRE and phosphorus atom from long to short is P204> P507> C272> P227 for geometry structure in Fig 8. In fact, the structure property of HRE extracted complex is 33 ACS Paragon Plus Environment


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only one ingredient in complexation ability. It is also difficult, even incorrect to apparently evaluate the extractant performance just from a single factor of the HRE extracted complex structure or complexation reaction, because the HRE extraction is a comprehensive process involving in the extractant dimer dissociation, extractant monomer acid ionization, HRE complexation and etc. Considering the combined effect of geometry structure, steric hindrance, electron effect, solvation effect and HRE cation size, we obtain the final result of HRE complexation ability is P204> P227> C272≈ P507. Firstly, it is shown that the oxygen atoms (i.e., P204 or P507) inserted into the bond between the alkyl side chain and the central phosphorus atom play an important role in the HRE complexation. Secondly, it is interesting to note that the complexation capacities of P227 and C272, whose side chains are alkyl groups without the oxygen atom, are slightly stronger than that of P507. This interesting phenomenon may be explained by the notion that an sp2 alkoxy oxygen in P507 hybridization system is less capable of π bonding to phosphorus atom. Therefore, the negative charge on the phosphorus atom, the hydroxyl oxygen and phosphoryl oxygen are decreased45, 46. We can observe visually in Figure 7 that the methyl groups at both branched side chains of P227 can affect the HRE cation, but P507 provides only one alkoxy side chain to seize the HRE cation. The complexation ability of C272, of which both side chains have shorter chains and a large number of isomerized methyl groups, is lower than P227 due to the weaker contribution for HRE cations.


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In summary, the final sequence of HRE complexation ability for different extractants from strong to weak is P204> P227> C272≈ P507. It's worth noting that these values of the relationship are really close to each other. Although the slight differences exist in each process, the accumulations of them gather here can make a great difference to extraction performance. Hence, the sequence of HRE extraction capacity will be slightly changed if extractant dimer dissociation and monomer acid ionization are taken into account. Therefore, we will finally investigate the overall extraction process, considering the above three main contributions in next section. It is another prospective exploration of the stability of HRE extracted complexes with single ligand. In Figure 9, the LUMOs show that HRE single-coordinated complexes still have a wide range of electrophilic active sites, which prefer to be attacked by electron-rich substituents. As a result, the single-coordinated extracted complexes can be further coordinated one by one to form the final HRE six-ligand extracted complexes.

Figure 9. The LUMO analysis of the representative Lu extracted complexes Key: Red 0.03 eV; Blue -0.03 eV 35 ACS Paragon Plus Environment


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3.4 Overall HRE extraction Table 7. Changes in the Gibbs free energy contribution in dimer dissociation, acid ionization, HRE complexation and their total ∆Gtotal/kcal ∆∆G ∆G1/kcal ∆G2/kcal ∆G3/kcal C272-Ho 19.95 41.44 -466.21 -2613.06 0.00 C272-Er 19.95 41.44 -470.40 -2638.24 -25.18 C272-Tm 19.95 41.44 -473.78 -2658.53 -20.29 C272-Yb 19.95 41.44 -476.81 -2676.68 -18.15 C272-Lu 19.95 41.44 -478.87 -2689.02 -12.34 P204-Ho 13.66 38.02 -475.78 -2699.68 0.00 P204-Er 13.66 38.02 -480.49 -2727.90 -28.22 P204-Tm 13.66 38.02 -484.39 -2751.31 -23.41 P204-Yb 13.66 38.02 -487.80 -2771.75 -20.44 P204-Lu 13.66 38.02 -490.76 -2789.53 -17.78 P507-Ho 3.06 41.88 -463.00 -2643.16 0.00 P507-Er 3.06 41.88 -467.34 -2669.19 -26.03 P507-Tm 3.06 41.88 -470.85 -2690.28 -21.09 P507-Yb 3.06 41.88 -473.96 -2708.93 -18.65 P507-Lu 3.06 41.88 -476.25 -2722.70 -13.77 P227-Ho 4.76 46.82 -472.21 -2678.55 0.00 P227-Er 4.76 46.82 -477.00 -2707.27 -28.72 P227-Tm 4.76 46.82 -480.57 -2728.67 -21.40 P227-Yb 4.76 46.82 -483.74 -2747.68 -19.02 P227-Lu 4.76 46.82 -485.95 -2760.99 -13.31

The overall Gibbs free energy changes of HRE extraction process are summarized in Table 7, including the dissociation of extractant dimers, the acid ionization of extractant monomers and the HRE complexation. At the same time, the differences of Gibbs free energy change between adjacent HREs (∆∆G) were calculated with the reference to ∆∆G (Ho3+) =0 kcal, in order to inspect the prediction method of HRE separation factors. Here, we are not able to consider the three hydrogen bonds of the HRE six-ligand extracted 36 ACS Paragon Plus Environment

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complex due to the limit of computational source at the present stage, where the asymmetrical geometries and electronic structures are extremely complicated. Based on the comprehensive consideration of the extractant dimer dissociation, the acidic ionization, the HRE complexation and their corresponding solvation effects, the sequence of the absolute values of total Gibbs free energy changes (negative value) from high to low is P204> P227> P507> C272. The total Gibbs free energy change reflects the extraction capacity and the extreme of chemical reactivity. Therefore, this calculated sequence is in good agreement with the experimental maximum extraction capacity19, i.e., 0.0352 mol/L for P204, 0.0332 mol/L for P227, 0.0328 mol/L for P507 and 0.0209 mol/L for C272. (Extraction operation conditions: [Extractant](o) = 0.16 mol/L, [Lu3+](a) = 5.8 × 10−3 mol/L, pH0 = 2.0, O/A = 1 and T= 298 K) For the same extractant, the sequence of maximum extraction capacity is: Lu3+> Yb3+> Tm3+> Er3+> Ho3+ with the increase of HRE element number, which is also in agreement with the lanthanide contraction principle. These results demonstrate that the quantum chemistry method in present work can nicely reproduce the experimental extraction performance and evaluate the different HRE extractants in future. Our study points out the importance of substituent effects on the extractants. As shown in dialkylphosphoric acids (P204), mono-alkylphosphonates (P507) and dialkylphosphinic acids (P227) with the same alkyl substituent, the presence or absence of oxygen atoms makes a great difference on HRE extraction. When carbon–phosphorus



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bonds replace oxygen–phosphorus bonds, dialkylphosphinic acid demonstrates higher pKa values than mono phosphonic acid (P507) and phosphoric acid (P204). The sequence of acid ionization from strong to weak is P204> P507> P227, so the stripping acidity of HRE could be decreased. On the other hand, the asymmetry structure of mono phosphonic acid (P507), of which one side chain is an alkyl chain and the other is an alkoxy chain, decreases π bonding to phosphorus atom and the negative charge on the hydroxyl oxygen atom. This leads mono phosphonic acid (P507) to an easier dimer dissociation (P507> P227> P204) but a weaker complexation capacity (P204> P227> P507). As shown in the same phosphinic acids of C272 with bis(2,4,4-trimethylpentyl) substituents and its isomer P227 with bis(2-ethylhexyl) substituents, these two show entirely different performances for HRE extraction. The replacement of the bis(2-ethylhexyl) groups by bis(2,4,4-trimethylphentyl) reduces the dimer dissociation capacity (P227> C272) and interaction with lanthanide cations (P227> C272), because C272 has a higher steric hindrance and smaller molecular volume than P227. Furthermore, the solvation effect of C272 is relatively weaker and its anion is more stable due to the σ-π hyper conjugation effect than P227. This may be an important theoretical foundation for the structural design of binding sites and substituents for extractant molecules to sufficiently capture HRE cations. Another important thing should be noticed is that three processes of extractant dimer dissociation, monomer acid ionization and HRE complexation are not isolated. Taking a


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simple example, the extractant monomer affects not only the extractant dimer, but also the acid ionization. In the same way, the extractant anion affects not only the acid ionization, but also the HRE complexation. Therefore, as soon as one given extractant is selected, three abovementioned processes of HRE extraction are determined. We should sum up in one comprehensive evaluation method by a series of consecutive reactions in HRE extraction system, depending one upon another. One interesting difficulty of HRE extraction and separation that actual industry faces is how to obtain and enhance the separation factor, because it is very sensitive to the experimental conditions (the concentration of HRE, initial pH in aqueous phase, oil/water ratio and etc.) as well as a great manual operation error. Now the separation factors of HRE have not united accurate data, but only a wide range (1.10 – 2.40) is approved. Quantum chemistry is a good way to calculate the separation factor by providing a certain and repeatable reference value. The separation factors of different extractants acquired by Eq. (19) are shown in Figure 10. The differences of Gibbs free energy changes of Ho/Er, Er/Tm, Tm/Yb and Yb/Lu decrease gradually for the same extractant, which means the separation factors between adjacent HREs decrease gradually with the increase of HRE element number. Here, the values of separation factors, i.e. 1.02-1.05, are lower than the experimental ones, i.e. 1.10-2.40. because the actual HRE solvent system is a more complex process, which depends on the HRE cation: extractant ratio (chemical stoichiometry = 1:6, actual operating consumption P227> P204> C272, P204> C272≈ P507> P227 and P204> P227> C272≈ P507 by thermodynamic analysis, respectively. The essential mechanisms and each contribution in HRE extraction process were summarized for the first time. HRE complexation plays a prominent role in the three processes, accounting for 93-95%. Based on the comprehensive consideration of three above processes, the calculated sequence of extraction capacity and extreme of chemical reactivity, i.e. P204> P227> P507> C272, is in good agreement with the experimental maximum extraction capacity. In conclusion, we emphasize the importance of quantum chemistry approaches to compare the intrinsic binding features of various classes of extractants used in the complexation and liquid-liquid extraction of HRE cations. Our study provides a reasonable way to simplify the complicated HRE extraction process to three reactions, which displays experimental similar results of the maximum extraction capacity and


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separation factor. The comparison of P204, P507, P227 and C272 also demonstrates the importance of P-O bond, P-C bond and isomer substituent effects on the complexation strengths. Such computational method should contribute to a better understanding of the structure-performance relationship of the complexes and the basis of efficient extraction and separation of HREs.

Corresponding Author *Ji Chen. E-mail: [email protected]. Tel/Fax: +86 0431-85262646. Acknowledgment This project was supported by the National Basic Research Program of China (Grant 2012CBA01202), Selected Postdoctoral Science Foundation Funded Project of Jilin Province and Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase). We greatly acknowledge National Supercomputing Center in Shenzhen for providing the computational resources and Gaussian09 D 0.1. Supporting Information Available Tables of data for the quantum chemistry calculations. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Xie, F.; Zhang, T. A.; Dreisinger, D.; Doyle, F., A Critical Review On Solvent Extraction of Rare Earths From Aqueous Solutions. Miner. Eng. 2014, 56, 10-28. (2) Binnemans, K.; Jones, P. T.; Blanpain, B.; Van Gerven, T.; Yang, Y.; Walton, A.; Buchert, M., Recycling of Rare Earths: A Critical Review. J. Cleaner Prod. 2013, 51, 1-11. (3) Chen, J., Application of Ionic Liquids on Rare Earth Green Separation and Utilization; Springer Verlag Press: Berlin, Germany, Springer: 2016. 45 ACS Paragon Plus Environment


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