Liquid–liquid extraction to lithium isotope separation based on room-temperature ionic liquids containing 2,2′-binaphthyldiyl-17-crown-5

Abstract

A novel liquid–liquid extraction system was investigated for the selective separation of lithium isotopes using ionic liquids (ILs = C₈mim⁺PF₆⁻, C₈mim⁺BF₄⁻, and C₈mim⁺NTf₂⁻) as extraction solvent and 2,2′-binaphthyldiyl-17-crown-5 (BN-17-5) as extractant. The effects of the concentration of lithium salt, counter anion of lithium salt, initial pH of aqueous phase, extraction temperature, and time on the lithium isotopes separation were discussed. Under optimized conditions, the maximum single-stage separation factor α of ⁶Li/⁷Li obtained in the present study was 1.046 ± 0.002, indicating the lighter isotope ⁶Li was enriched in IL phase while the heavier isotope ⁷Li was concentrated in the solution phase. The formation of 1:1 complex Li(BN-17-5)⁺ in the IL phase was determined on the basis of slope analysis method. The large value of the free energy change (−ΔG° = 92.89 J mol⁻¹) indicated the high separation capability of the Li isotopes by BN-17-5/IL system. Lithium in Li(BN-17-5)⁺ complex was stripped by 1 mol L⁻¹ HCl solution. The extraction system offers high efficiency, simplicity, and green application prospect to lithium isotope separation.

Keywords:

• lithium
• isotope separation
• solvent extraction
• distribution coefficient
• separation factor
• 2,2′-binaphthyldiyl-17-crown-5
• ionic liquids

1. Introduction

Naturally occurring lithium consists of ⁶Li and ⁷Li, two stable isotopes. ⁶Li can be cracked into tritium (T) and helium (He) by neutron bombarding, thus ⁶Li is the important material for nuclear fusion reactor to rapidly absorb neutron and proliferate T. ⁷Li can be used as coolants and ideal heat transfer agent of fusion power reactor and neutral medium of thorium-based fused salt reactor. Therefore, lithium isotopes will play increasingly important role in new energy industry in the future. However, separating lithium isotopes was extremely difficult as ⁶Li and ⁷Li only have tiny mass differences with the same extra nuclear structure.

The only method that was currently utilized to large-scale lithium isotope separation is the amalgam method [1]. It offered a large and compelling single-stage separation factor (α_max = 1.054 ± 0.002), but the use of toxic mercury brought serious biological and environmental problems. In order to avoid these hazards, many new techniques were investigated to separate lithium isotope, including laser [2], electromigration [3], chromatography [4], and membrane separation [5]. Since the solvent extraction has the advantages of high selectivity, high recovery, high capacity, simple equipment, and easy automatic control, liquid–liquid extraction systems of lithium isotopes separation were reported in literatures [6]. However, as Li⁺ has large free energy of hydration, lithium isotope separation effect of the ordinary organic extractant system was very feeble [7]. In addition, high toxicity caused by the use of volatile organic solvents in liquid–liquid extraction process limits its application to large-scale lithium isotope separation. Therefore, there is an urgent need to develop efficient and green liquid–liquid extraction method for lithium isotope separation.

Crown ethers and cryptands, with high affinity and size selectivity for lithium ion, have been investigated with large separation factors of ⁶Li/⁷Li in solvent extraction processes [8,9]. For instance, Li isotopes have been separated successively by liquid–liquid extraction using cryptand(2, 2, 1) [10] and benzo-15-crown-5 [11] with α_max of 1.041 and 1.035, respectively. However, they were not suitable for the isotope separation in the industrial scale because of the large water solubility of cryptand (2, 2, 1) and the relatively small separation factor of benzo-15-crown-5. To overcome the above problems, a new kind of macrocyclic extractant, 2,2′-binaphthyldiyl-17-crown-5 (BN-17-5) (Figure 1(a)) was designed in this paper. Due to the introduction of naphthalene ring, the rigidity of the extractant greatly increased, therefore the water solubility significantly reduced. Besides, the rigid cavity of BN-17-5 matches well with Li ion and achieves elaborate per-organization before coordination with lithium ion. Consequently, BN-17-5 will show strong bonding ability, wonderful selectivity, and relatively large separation factor when coordinated with lithium ion.

Figure 1. Structures of (a) BN-17-5 and (b) ILs.

Room-temperature ionic liquids (ILs), as a new class of burgeoning green solvents, have attracted considerable interest in many fields of chemistry and industry [12,13]. Their non-inflammability and non-volatility provided advantages for using them as a replacement to volatile organic solvent in solvent extraction process. Imidazolium-based ILs (Figure 1(b)), one of the commonly used ILs, were considered to be a prospective solvent for metal ions separarion [14,15]. In recent years, several imidazolium-based ILs have been employed for lithium isotope separation with remarkably high extraction performance [16,17].

In this study, an efficient and green liquid–liquid extraction system was investigated for selective separation of lithium isotope using imidazolium-based ILs as extraction solvent and BN-17-5 as extractant. We studied the extraction performance from many aspects including the extraction efficiency, separation factor, thermodynamic, and kinetic considerations. The large single-stage separation factor and the environmental friendliness of BN-17-5/IL system show the potential of surpassing the amalgam method for the industrial separation of lithium isotopes.

2. Experimental

2.1. Instrumentation

Infrared (IR) spectra were measured with a FTLA 2000-104 spectrometer (ABB Bomem, Canada) with KBr pellets in the 500–4000 cm⁻¹ region. Nuclear magnetic resonance (NMR) spectrum was recorded with Avance III 400 MHz Digital NMR Spectrometer (Bruker Biospin, Switzerland) in deuteriochloroform, and chemical shifts are reported in parts per milion (ppm) downfield from tetramethylsilane. Element analyses were conducted on elementar corporation vario EL III analyzer (EL III, Germany). The mass spectra of extractant and ILs were recorded on a mass spectrometer (LCZ/2690 XE/996, USA). A SHA-2 fully automatic multifunction thermostatic water bath oscillator (Beijing, China) was performed for extracting lithium isotope separation. A TG16-WS high-speed centrifuge (Hunan, China) was employed for sufficient disengagement of the extracting phases and aqueous phases. A Varian Spectra AA atomic absorption spectrophotometer (AAS, Varian, USA) was employed for the determination of lithium concentration. Lithium isotopic ratio was determined by Thermo Scientific Element 2 inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher Scientific, USA) using sample-standard bracketing method. The radio frequency (RF) generator was operated at 1300 W, and the argon gas flows were as follows: nebulizer gas 0.85 L min⁻¹, cooling gas 13.0 L min⁻¹, auxiliary gas 0.82 L min⁻¹, carrier gas 0.933 L min⁻¹, integration time 0.5 s, sampling time 0.01 s, hits of every peak 100, peak width is 100%, and integral width 10%. One percent HNO₃ solution and Li₂CO₃ were used as blank and lithium isotope standard substance for correcting machinery bias, respectively. The determination was performed in triplicate and results reported as mean ± standard deviation.

2.2. Materials and reagents

Lithium salts, 1-methylimidazole, 1-bromooctane, and l,l′-bi-2-naphthol were purchased from Sigma-Aldrich Ltd, Shanghai, China. All other chemicals used were of analytical grade and used without further purification. Demineralized water (18.2 MΩ cm) purified from a Milli-Q purification system was used throughout the experiment.

BN-17-5 used in the present experiment was synthesized by appropriate modified literature procedures [18,19]. A 250 mL round bottom flask was evacuated, filled with nitrogen and charged with l, l′-bi-2-naphthol (2.87 g, 0.010 mol) followed by 100 mL of dry acetonitrile. To this solution, anhydrous cesium fluoride (7.60 g, 0.050 mol) was added, and the mixture was stirred 25 °C for 1 h. A solution of tetraethylene glycol ditosylate (0.010 mol) in acetonitrile (50 mL) was added, and the mixture was stirred at 65 °C for 48 h. After filtration, the solvent was removed in vacuo, and the residue was dissolved in 100 mL CH₂Cl₂, and washed with water (4 × 100 mL). After drying with MgSO₄ and evaporation of the solvent, the crude product was obtained, which was further purified by column chromatography on alumina with petroleum ether–ethyl acetate (2:1) as eluent to give an 85.1% yield of pure product as a light yellow waxy solid. Elemental analysis (%), Calcd for C₂₈H₂₈O₅: C, 75.32; H, 6.47. Found: C, 75.68; H, 6.31. IR (KBr, cm⁻¹): 2959, 2933, 2860, 1468 (C–H); 1690–1650 (C = C); 1120 (C–O). ¹HNMR (400 MHz, CDCl₃): 3.3–3.7(m, 12H), 3.98–4.26(m, 4H), 7.11–7.48(m, 8H), 7.89 (d, 2H, J = 7.7 Hz), 7.92(d, 2H, J = 8.8 Hz). MS: m/e 445.2 (M⁺; base peak). MP: 103–105 °C.

The ILs 3-octyl-1-methylimidazole hexafluorophosphate (C₈mim⁺PF₆⁻), 3-octyl-1-methylimidazole tetrafluoroborate (C₈mim⁺BF₄⁻), and 3-octyl-1-methylimidazole bistrifluoromethylsulfonylimide (C₈mim⁺NTf₂⁻) were synthesized according to published procedure with minor variations [20,21]. The yields of C₈mim⁺PF₆⁻, C₈mim⁺BF₄⁻, and C₈mim⁺NTf₂⁻ were 93.5%, 91.3%, and 89.7%, respectively. C₈mim⁺PF₆⁻: IR (cm⁻¹): 3100 (=C–H); 2950, 1180 (C–H); 1600 (C = C); 1115 (C–N); 837 (P–F). ¹HNMR (400 MHz, CD₃CN): 8.46 (s, 1H), 7.38 (s, 1H), 7.33 (s, 1H), 4.14–4.08 (t, 2H, J = 7.6 Hz), 3.82 (s, 3H), 1.83–1.78 (m, 2H, J = 6.8 Hz), 1.29–1.26 (m, 10H), 0.89–0.86 (t, 3H, J = 6.8 Hz). MS: m/e (cation) 195.33, m/e (ation) 144.96. C₈mim⁺BF₄⁻: IR (cm⁻¹): 3060 ( = C–H); 2960, 1170 (C–H); 1640 (C = C); 1120 (C–N); 1060 (B–F). ¹HNMR (400 MHz, CD₃CN): 8.39 (s, 1H), 7.36 (s, 1H), 7.33 (s, 1H), 4.12–4.08 (t, 2H, J = 7.6 Hz), 3.82 (s, 3H), 1.82–1.79 (m, 2H, J = 6.8 Hz), 1.29–1.26 (m, 10H), 0.88–0.85 (t, 3H, J = 6.8 Hz). MS: m/e (cation) 195.33, m/e (ation) 96.8. C₈mim⁺NTf₂⁻: IR (cm⁻¹): 3080 (= C–H); 2950, 1180 (C–H); 1650 (C = C); 1121 (C–N); 1197 (C–F); 1137(O = S = O); 1056 (S–N). ¹HNMR (400 MHz, CD₃CN): 8.41 (s, 1H), 7.36 (s, 1H), 7.32 (s, 1H), 4.13–4.08 (t, 2H, J = 7.6 Hz), 3.82 (s, 3H), 1.82–1.79 (m, 2H, J = 6.8 Hz), 1.29–1.26 (m, 10H), 0.9–0.88 (t, 3H, J = 6.8 Hz). MS: m/e (cation) 195.33, m/e (ation) 362.14.

2.3. Extraction experiments

Extracting phase was prepared by dissolving appropriate amounts of BN-17-5 in 5 mL IL in a 25-mL conical flask. For comparison with the performance of IL, a conventional organic solvent of chloroform (5 mL) containing BN-17-5 was also prepared by the same procedures. Aqueous phases were prepared by dissolving each of the lithium salt in demineralized water. The concentration of lithium salts was various in different extraction experiment (0.2–3 mol L⁻¹). Equal volumes of the aqueous and organic phases were mixed and gently shaken by a fully automatic multifunction thermostatic water bath oscillator. In order to promote adequate disengagement of the two phases, solution samples were centrifuged for 5 min with 7000 r min⁻¹. After centrifugation, the upper aqueous phase was separated and samples were prepared by removing appropriate supernatant solution and diluting proper multiple. Lithium in the extracting phases was back-extracted using 1-mol L⁻¹ HCl solution of equal volumes. In the process of extraction, no third phase was observed at the two-phase boundary and tiny change of phase volume can be ignored. The mass balance of the extraction system is described by the following equation: (1) $$C_o{V}_{\mathrm{aq}}=C_{\mathrm{org}}{V}_{\mathrm{org}}+C_e{V}_{\mathrm{aq}}$$(1) where C_o and C_e are the initial and final concentrations of lithium ion in aqueous phase, which were determined by flame atomic absorption spectrophotometry. C_org is the concentrations of extracted lithium ion in organic phase. V_aq is the volume of the aqueous phase and V_org is the volume of the organic phase. The extraction efficiency of lithium (E) and separation coefficiency (D) were obtained by the following equations: (2) $$E(\%)=\frac{C_o-C_e}{C_o}\times 100$$(2) (3) $$D=\frac{C_{\mathrm{org}}}{C_e}=\frac{C_o-C_e}{C_e}\times \frac{V_{\mathrm{aq}}}{V_{\mathrm{org}}}$$(3)

3. Results and discussion

3.1. Composition of complex

Exploring the composition ratio of lithium complex has important significance for investigating the extraction process of lithium isotopes separation. The Li⁺ in the aqueous phase are extracted into the IL phase which dissolves BN-17-5 through the process as shown in the following equations: (4) $${\mathrm{Li}}^{+}{\mathrm{Y}}_{\left(\mathrm{aq}\right)}^{-}+n{\mathrm{L}}_{\left(\mathrm{org}\right)}\leftrightarrows {\left({\mathrm{LiL}}_n\right)}^{+}{\mathrm{Y}}_{\left(\mathrm{org}\right)}^{-}$$(4) (5) $$\begin{array}{ccc}\hfill {\left({\mathrm{LiL}}_n\right)}^{+}{\mathrm{Y}}_{\left(\mathrm{org}\right)}^{-}& +& {\mathrm{C}}_8{\mathrm{mim}}^{+}{\mathrm{X}}_{\left(\mathrm{org}\right)}^{-}\leftrightarrows {\left({\mathrm{LiL}}_n\right)}^{+}{\mathrm{X}}_{\left(\mathrm{org}\right)}^{-}\hfill \\ +& {\mathrm{C}}_8{\mathrm{mim}}^{+}{\mathrm{Y}}_{\left(\mathrm{org}\right)}^{-}\hfill \end{array}$$(5)

The overall reaction of this process is shown as follows: (6) $$\begin{array}{ccc}\hfill {\mathrm{Li}}^{+}{\mathrm{Y}}_{\left(\mathrm{aq}\right)}^{-}+n{\mathrm{L}}_{\left(\mathrm{org}\right)}& +& {\mathrm{C}}_8{\mathrm{mim}}^{+}{\mathrm{X}}_{\left(\mathrm{org}\right)}^{-}\leftrightarrows {\left({\mathrm{LiL}}_n\right)}^{+}{\mathrm{X}}_{\left(\mathrm{org}\right)}^{-}\hfill \\ +& {\mathrm{C}}_8{\mathrm{mim}}^{+}{\mathrm{Y}}_{\left(\mathrm{org}\right)}^{-}\hfill \end{array}$$(6) where Li⁺Y⁻_(aq), L_(org), n, and (LiL_n)⁺X⁻_(org) denoted the lithium salt in the aqueous phase, the BN-17-5 dissolved in IL phase, the number of crown ether molecules composing a Li-crown complex and formed complexes in IL phase, respectively. The counter anions of the extracted Li(BN-17-5)⁺ cations were exchangeable with the anions of ILs. A distribution coefficient D of the extraction is shown in the following equation: (7) $$D=K_e(C_o-A){(L_o-nA)}^n$$(7) where K_e is the apparent equilibrium constant defined as (8) $$K_e=\frac{\left[{\left({\mathrm{LiL}}_n\right)}^{+}{\mathrm{Y}}^{-}\right]\left[{\mathrm{C}}_8{\mathrm{mim}}^{+}{Y}^{-}\right]}{\left[\mathrm{LiY}\right]{\left[\mathrm{L}\right]}^n\left[C_8{\mathrm{mim}}^{+}{\mathrm{X}}^{-}\right]}$$(8)

C_o and L_o are the initial concentrations of the Li⁺ in aqueous phase and the BN-17-5 in the IL phase, and A denotes the lithium salt extracted into the IL phase. The activity coefficients of (LiL_n)⁺_org and L_org do not vary and the amount of the Li⁺ extracted into the IL phase was much smaller than the total Li⁺ (L_o = 0.1 mol L⁻¹, C_o = 1 mol L⁻¹, A = 0.03063 mol L⁻¹) within the concentration range from log L_o = −1 to 0. Thus, it would be reasonable to put C_o ≫ A and L_o ≫ A. Then, Equation (7) is abbreviated to (9) $$lgD=nlg{L}_o+C$$(9) where C is constant. As shown in Figure 2, the slope-ratio of lgD vs. lgL_o curve is 0.9998. It means Li(BN-17-5)⁺ 1:1 complex was formed under the experimental conditions. Therefore, the liquid–liquid extraction of lithium isotope separation was mainly due to the coordination of lithium ions with BN-17-5 in IL phase via the ion–dipole bond. The conceptual diagram of reaction mechanism between Li⁺Y⁻_(aq) and L_(org) is described in Figure 3(a).

Figure 2. lg D vs. lg L_o (the initial concentrations of LiTFA in the aqueous phase is 1.0 mol L⁻¹, L_o is the concentrations of BN-17-5 in C₈mim⁺NTf₂⁻). The inset graph show plots of A vs. L_o.

Figure 3. Illustration of the mechanism of (a) the reaction between LiY_(aq) and L_(org) and (b) the isotope exchange reaction between ⁶Li⁺_(aq) and ⁷Li⁺_(aq).

3.2. Separation factor

The separating capabilities of BN-17-5 for the Li⁺ isotopes can be compared by the apparent equilibrium constants (K) of the following chemical exchange equilibrium : (10) $$\begin{array}{c}\hfill {}^6{\mathrm{LiY}}_{\left(\mathrm{aq}\right)}+{{(}^7\mathrm{LiL})}^{+}{\mathrm{X}}_{\left(\mathrm{org}\right)}^{-}\leftrightarrows {\left({}^6\mathrm{LiL}\right)}^{+}{\mathrm{X}}_{\left(\mathrm{org}\right)}^{-}+{}^7{\mathrm{LiY}}_{\left(\mathrm{aq}\right)}\end{array}$$(10)

where the subscripts (aq) and (org), and X⁻ represent the ions found in the aqueous and in the IL phase, the counter ion of IL, respectively. The conceptual diagram of the above process is depicted in Figure 3(b).

The K is the same with the single stage separation factor α, as can be seen from the definition of α: (11) $$K=\frac{\left[{\left({}^6\mathrm{LiL}\right)}^{+}{\mathrm{X}}_{\left(\mathrm{org}\right)}^{-}\right]{[}^7{\mathrm{LiY}}_{\left(\mathrm{aq}\right)}]}{{[}^6{\mathrm{LiY}}_{\left(\mathrm{aq}\right)}]\left[{\left({}^7\mathrm{LiL}\right)}^{+}{\mathrm{X}}_{\left(\mathrm{org}\right)}^{-}\right]}$$(11) (12) $$\alpha =\frac{{({[}^6{\mathrm{Li}}^{+}]/{[}^7{\mathrm{Li}}^{+}])}_{\left(\mathrm{org}\right)}}{{({[}^6{\mathrm{Li}}^{+}]/{[}^7{\mathrm{Li}}^{+}])}_{\left(\mathrm{aq}\right)}}$$(12)

where ([⁶Li⁺]/[⁷Li⁺]) represents the isotopic ratio, which was determined by using an inductively coupled plasma mass spectrometry. The maximum single-stage separation factor α of ⁶Li/⁷Li of BN-17-5/C₈mim⁺NTf₂⁻ system was 1.046 ± 0.002 with TFA⁻ as counter ion. This value is approximately equal to that of cryptand(2_B, 2, 1) polymer (α_max = 1.047 ± 0.002) as extractant [22]. Under the same conditions, α_max was 1.040 ± 0.001 with chloroform as solvent. This improvement in the separation factor was attributed to two reasons. On one hand, BN-17-5 achieves elaborate per-organization before coordination, and the forming rigid cavity matches well with Li ion. On the other hand, imidazolium-based ILs used in this experiment were acted as ionic associated agent and extraction solvent. The results show that BN-17-5 dissolved in C₈mim⁺NTf₂⁻ was able to extract lithium ions more effectively than that in conventional organic solvent of chloroform.

3.3. Effect of the lithium salts concentration

The initial concentration of lithium salts in aqueous phase was one of the significant characteristics in the separation process. With fixed concentration of BN-17-5 (0.186 mol L⁻¹) and fixed volume of aqueous (5 mL) and organic phase (5 mL), a series of concentrations of trifluoroacetic acid lithium (LiTFA) from 0.2 to 3 mol L⁻¹ were tested for Li⁺ extraction (Figure 4). The data showed that with the increase of LiTFA concentration, the extraction efficiency changed greatly. The maximum extraction efficiency of Li⁺ removal was 15.1% in 1 mol L⁻¹ LiTFA with C₈mim⁺NTf₂⁻ as solvent, 14.2% in 1 mol L⁻¹ LiTFA with C₈mim⁺PF₆⁻ as solvent, 12.5% in 1.4 mol L⁻¹ LiTFA with C₈mim⁺BF₄⁻ as solvent, and 9.6% in 1.8 mol L⁻¹ LiTFA with CHCl₃ as solvent, respectively. The effect of LiTFA concentration on distribution coefficient also was studied for better comparison. The curve of log D vs. log C_LiTFA did not present linear correlation because the concentration of the extracted lithium ion was non-ignorable compared with the concentration of BN-17-5. When the concentration of LiTFA was higher than the corresponding optimum value, the extraction efficiency and distribution coefficient decreased sharply.

Figure 4. Effect of LiTFA concentration on the extraction efficiency (A) and distribution coefficient (B): (a) C₈mim⁺NTf₂⁻, (b) C₈mim⁺PF₆⁻, (c) C₈mim⁺BF₄⁻, and (d) CHCl₃.

3.4. Effect of aqueous phase initial pH

In order to search the optimal pH value of the extraction process, the solutions of LiTFA with optimum concentration were extracted by 0.186 mol L⁻¹ BN-17-5 in different solvents with pH ranging from 1 to 6 for 8 h. The pH of the aqueous phase was adjusted by adding the appropriate hydrochloric acid solution. The extraction behavior of Li⁺ as a function of pH in the aqueous phase is shown in Figure 5. The results confirmed that there was an increase in the extraction efficiencies and distribution coefficient with increasing pH from 1 to 6 for each system but not correlations linear. The extraction efficiencies attained maximum at optimum pH 6 for C₈mim⁺NTf₂⁻, C₈mim⁺PF₆⁻, C₈mim⁺BF₄⁻, and CHCl₃ systems were 15.1%, 14.2%, 12.5%, and 9.6%, respectively. The extraction efficiencies and distribution coefficient were very tiny in pH below 3. The unpaired electrons of oxygen atoms on BN-17-5 point to inside of the ring forming a rich electronic cavity. Thus, at high concentration of H⁺, BN-17-5 will be protonated and the formation of lithium complex will be inhibited. Moreover, the higher acidity will cause a gradual loss of [C₈mim]⁺ to aqueous phase. Therefore, hydrochloric acid was prospective for acting as the stripping reagent in back-extraction experiments.

Figure 5. Effect of aqueous phase initial pH on the extraction efficiency (A) and distribution coefficient (B): (a) C₈mim⁺NTf₂⁻, (b) C₈mim⁺PF₆⁻, (c) C₈mim⁺BF₄⁻, and (d) CHCl₃.

3.5. Effects of counter anion

The complex cation formed was hydrophobic because of the greasy exterior of BN-17-5 used in the present study. In order to preserve electrical neutrality, the complex cation must accompany the anion to be extracted into the IL phase. Therefore, the distribution coefficients were influenced by the affinity of the anion to the ILs. As shown in Table 1, the order of the distribution coefficients was I⁻ > Br⁻ > C1⁻ for lithium halides. The order was in agreement with the previous extraction work [23]. For a series of acetic derivate, the order was trifluoroacetate (TFA⁻) > difluoroacetate (DFA⁻) > acetate (CH₃COO⁻). The results can be understood by an idea of the hard and soft of acid and base (HSAB) [24,25]. In general, the softer anion has the greater affinity to the organic solvent.

Table 1. Effect of counter anion on the separation of Li⁺ with BN-17-5.

Solvents		LiTFA	LiSCN	LiI	LiBr	LiCl	LiDFA	CH3COOLi	LiNO3
C8mim^+ NTf2^−	E (%)	15.1	14.36	14.2	13.91	12.68	12.34	11.75	12.41
	D	0.0554	0.0431	0.0344	0.0183	0.0162	0.0352	0.00288	0.0134
	α	1.046 ± 0.002	1.042 ± 0.002	1.039 ± 0.003	1.026 ± 0.002	1.019 ± 0.002	1.024 ± 0.002	1.001 ± 0.001	1.022 ± 0.003
C8mim^+
PF6^−	E (%)	14.2	14.02	13.87	13.16	12.23	11.01	10.89	12.22
	D	0.0504	0.0410	0.0621	0.0405	0.0338	0.0306	0.00194	0.0141
	α	1.042 ± 0.002	1.041 ± 0.002	1.040 ± 0.002	1.024 ± 0.002	1.015 ± 0.002	1.025 ± 0.002	1.0015 ± 0.001	1.019 ± 0.003
C8mim^+ BF4^−	E (%)	12.5	12.39	12.06	11.97	10.38	11.06	9.48	11.13
	D	0.0497	0.0397	0.0401	0.0262	0.0102	0.0286	0.00184	0.0238
	α	1.041 ± 0.002	1.039 ± 0.001	1.039 ± 0.001	1.026 ± 0.002	1.011 ± 0.003	1.018 ± 0.002	1.0002 ± 0.002	1.025 ± 0.002
CHCl3	E (%)	9.6	9.23	9.03	8.2	7.89	9.12	5.41	8.13
	D	0.048	0.0399	0.0412	0.0231	0.0101	0.0155	0.00112	0.0091
	α	1.040 ± 0.001	1.035 ± 0.002	1.034 ± 0.001	1.018 ± 0.003	1.002 ± 0.003	1.021 ± 0.002	1.0003 ± 0.002	1.015 ± 0.002

The isotope separation factors decreased in the order: TFA⁻ > SCN⁻ > I⁻ > DFA⁻ > Br⁻ > Cl⁻ > CH₃COO⁻ > CH₃COHCOO⁻. Obviously, the counter anions of lithium salts had great influence on lithium isotope separation. The order of α can also be explained by applying the HSAB theory with the anions of softness orders SCN⁻ > I⁻ > Br⁻ > Cl⁻ for halides, and TFA⁻ > DFA⁻ > acetate for acetate series. However, there was an exception for SCN⁻ and TFA⁻. SCN⁻ has more softness order than TFA⁻, while the value of α(LiSCN) was smaller than α(LiTFA). The SCN⁻ gave the second-large separation factor following TFA⁻ in the present experiment. Therefore, LiTFA was most suitable for the industrial separation among a variety of lithium salts. In order to achieve maximum extraction performance, the extraction efficiency, distribution coefficient, and the single-stage separation factor for the lithium isotopes extraction were investigated under the optimum conditions and listed in Table 1.

3.6. Kinetic consideration

The kinetics of Li⁺ extraction is indispensable for designing and selecting the operating conditions for metal removal process. Under the optimum operating conditions at room temperature, the extraction was carried out by shaking mechanically the organic phase of 0.186 mol L⁻¹ BN-17-5 and the lithium ion aqueous solution for 2–10 hours (Figure 6). The results showed that there was a similar trend for ILs or CHCl₃ as solvent. It is evident that the rate of removal was very fast initially and became slower with the lapse of time, then attained gradual stabilization and the extraction equilibrium of Li⁺ by BN-17-5 was achieved within 6 h. However, the lithium isotope exchange equilibrium between organic phase and aqueous phase quickly reached within 1 min [16]. The extraction equilibrium was a relative long-term process in this experiment. The configuration of pre-organized BN-17-5 is not easy to be adjusted when forming active intermediates, thus the formation of Li(BN-17-5)⁺ complexes required slow kinetics of bonding.

Figure 6. Effect of extration time (a) C₈mim⁺NTf₂⁻, (b) C₈mim⁺PF₆⁻, (c) C₈mim⁺BF₄⁻, and (d) CHCl₃.

3.7. Thermochemical consideration

To further understand the extraction performance of the extraction system with BN-17-5, various thermodynamic parameters were obtained by studying the effect of different temperatures on the lithium separation factor [26,27]. According to the Bigeleisen–Mayer theory: (1) ln α is reciprocally proportional to T² for high temperatures (above 100 °C), (2) ln α is proportional to 1/T for low temperatures (below 50 °C), and (3) the relationship between ln α and T is complicated for intermediate region. Considering a separation factor can be regarded as an equilibrium constant for an isotope exchange reaction, the manner in which the separation factor depends on the temperature can be derived by thermodynamics [1]. Assuming that ΔH° is independent of the temperature in the temperature range investigated, the following van't Hoff equation (13) holds for the separation factor: (13) $$\frac{\mathrm{d}ln\alpha }{\mathrm{d}(1/T)}=-\Delta H^o/R$$(13) where R is the gas constant. Accordingly, ΔH° (⁶Li/⁷Li) can be obtained from the slope of the line determined by plotting log α vs. 1/T (Figure 7). The free energy change ΔG° (⁶Li/⁷Li) and entropy change ΔS° (⁶Li/⁷Li) could be calculated by applying the well-known thermochemical equations (14) and (15): (14) $$\Delta G^o=-RTlnK=-2.303RTlogK$$(14) (15) $$\Delta G^o=\Delta H^o-T\Delta S^o$$(15)

Figure 7. The influence of temperatures on separation factor.

The change of thermodynamic parameters of isotopic exchange is listed in Table 2. The data indicated that the extraction process was a spontaneous exothermic process. The separation factor was decreased with the increase of operating temperature. Thus, 25 °C of operating temperature was used for lithium isotope separation to obtain a relatively high separation factor.

Table 2. Thermodynamic parameters of lithium isotope extraction process.

	Concentration of polyether
Complex	in organic phase(mol·L^−1)	ΔH (J mol^−1)	ΔS (J mol^−1 K^−1)	ΔG (J mol^−1)	T (°C)
Li^+-crown^a-I^−	3.98 × 10^−2	−861.1	−2.8	−27.29	1–25 [11]
Li^+-crown-I^−	3.98 × 10^−2	−354.00	−1.08	−30.01	25–40 [11]
Li^+-crown-I^−	0.186	−466.8	−1.36	−61.52	1–40 [11]
Li^+-CR^b-SCN^−	0.78	−201.00	−0.46	−63.95	13–35 [11]
Li^+-L^c-TFA^−	0.186	−170.41	−0.26	−92.89	0–40
Li^+-cryp^d-I^−	−	−146.3	−0.17	−95.3	0–40 [22]

^aCrown: benzo-15-crown-5

^bCR: 4-tertiarybutyl-benzo-15-crown-5

^cL: BN-17-5

^dcryp: cryptand (2_B,2,1) polymer.

The −ΔG° of the system of Li⁺-L-TFA⁻ is bigger than that of the reported system of Li⁺-crown-I⁻ and Li⁺-CR-I⁻, but slightly smaller than the system of Li⁺-cryp-I⁻. The larger −ΔG° is beneficial to the formation of stabile complexes and the extraction separation of the lithium isotope. There are two main factors contributing to the remarkably high extraction performance of Li⁺-L-TFA⁻ system. Frist, BN-17-5 has the rigid cavity and elaborate per-organization before coordination with lithium, which greatly improved the compatibility and bonding ability between extractant and lithium ion. Namely, this extraction system has low energy of rearrangement and high lithium ion bonding ability. Second, the high polarity of ILs solvent influences the extraction performance and presents the large separation factor. Consequently, the extraction system developed a wonderful extraction performance with green, high efficiency, large separation factor.

3.8. Back-extraction of lithium ion

The recovery of extracted metal ions into a receiving phase is also important for the separation and concentration of a target metal ion. There has, however, been little progress in striking a balance between improvements in extraction and recovery by back extraction in IL-based extraction systems [28,29]. According to the above discussion, the cation-binding capabilities of BN-17-5/IL system are extremely pH sensitive. At low pH, the binding constant of BN-17-5 for metal cations will be severely reduced due to the protonation and charge repulsion. The binding affinity can accordingly be switched by varying the pH, with metal ion binding taking place at neutral or high pH, and metal ion release taking place at low pH. This switchable binding property forms the main rationale for the development of IL-based separation processes using BN-17-5.

Thus, relatively high concentration of hydrochloric acid (1 mol L⁻¹) was employed for stripping lithium ion in organic phase with o/a of 1. The single-stage back-extraction efficiency of C₈mim⁺NTf₂⁻, C₈mim⁺PF₆⁻, C₈mim⁺BF₄⁻, and CHCl₃ systems was up to 34.5%, 21.4%, 15.3%, and 13.2%, respectively. After repeated stripping process for four times, lithium ion in C₈mim⁺NTf₂⁻ phase (0.143 mol L⁻¹) was completely transferred into the acid solution. No trace amount of Li ions was detected in the acid of the fifth back-extraction. The experimental results demonstrated the extraction performance of BN-17-5/C₈mim⁺NTf₂⁻ system is superior to others. The IL phase containing BN-17-5 was reused for solvent extractions after being processed with ultra-pure water. Compared with the first lithium isotope separation, no significant change of extraction efficiency was observed. The stripping process based on BN-17-5/C₈mim⁺NTf₂⁻ system is feasible and BN-17-5 and C₈mim⁺NTf₂⁻ are stable under these experimental conditions. These results highlight the great potential of BN-17-5 as extractants for lithium isotopes separation in IL systems.

4. Conclusions

In summary, an efficient and green liquid–liquid extraction system was developed in this paper for the selective separation of lithium isotopes with BN-17-5 as an extractant and ILs as an extraction solvent. The bonding ratio of extractant and lithium ion, the optimal concentration of LiTFA, the best extraction time, the maximum single-stage separation factor, relevant thermodynamic parameters, and back-extraction experiments were investigated. The experiment results demonstrated that the BN-17-5/IL extraction system offers high efficiency, green nature, easier recycling strategy, and good application prospect to lithium isotopes separation.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [grant number 21101078], [grant number 21276105], [grant number 21176101], the Program for New Century Excellent Talents in University of China [grant number NCET-515 11-0657], and the Natural Science Foundation of Jiangsu Province [grant number BK2011143].