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The effect of ionic liquids as co-extractant with crown ether for the extraction of lithium in dichloromethane-water system
Journal of Molecular Liquids 285 (2019) 75–83
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The effect of ionic liquids as co-extractant with crown ether for the extraction of lithium in dichloromethane-water system Wenbo Zhu a,c, Yongzhong Jia a,b, Quanyou Zhang a,b, Jinhe Sun a,b, Yan Jing a,b,⁎, Jie Li d a
Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China Qinghai Engineering and Technology Research Center of Comprehensive Utilization of Salt Lake Resources, 810008 Xining, China University of Chinese Academy of Sciences, 100049 Beijing, China d Zhejiang Jingquan Water Treatment Equipment Co. Ltd, China b c
a r t i c l e
i n f o
Article history: Received 23 August 2018 Received in revised form 2 March 2019 Accepted 8 April 2019 Available online 09 April 2019 Keywords: Lithium Ionic liquids Extraction Infrared spectroscopy [BMIm][NTf2]
a b s t r a c t With different anionic and cationic structures of ionic liquids influencing the efficiency of Li+ extraction, 16 different kinds of ionic liquids with the crown ether-dichloromethane system were investigated primarily. The experimental results showed that smaller relative molecular weight, hydrophobicity, uniform electron distribution on cation, and weak electron donating on anion of ionic liquids with the ligand crown ether promoted the highest extraction of lithium ions in the dichloromethane-water system. The extraction system with [BMIm][NTf2] had the largest extraction rate and distribution coefficient at 26.35% and 0.358, respectively. Cation exchange behavior between Li+ in the aqueous phase and [BMIm]+ in the organic phase was examined through fitting the line of Li+ concentration in the organic phase and [BMIm]+ concentration in the aqueous phase. The anion of [BMIm] [NTf2] coordinating with the cationic lithium-crown ether complex in the organic phase was also verified by infrared spectroscopy. For the thermodynamic studies in the system using [BMIm][NTf2] and crown ether, the extraction process of Li+ was an exothermic spontaneous reaction while lower temperature was beneficial to lithium extraction. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Room temperature ionic liquids, ILs for short, is a class of compounds made up of cations and anions while staying as liquids at room temperature or near room temperature. Compared to traditional organic solvents, the features of ionic liquids are low steam pressure, low volatilization and high-solubility for the organics and the inorganics, especially the chemical and physical properties of ionic liquids can be tailored by the changing the cation- and anion- combination. Last several years, ionic liquids gained considerable attentions as solvent or coextractant for the separation and analytical technique [1,2]. Dietz et al. researched the system employing crown ether for the separation and extraction mechanism of strontium with imidazolium-based ionic liquids [3–7]. Jia et al. used tributyl phosphate or crown ethers as chelating agents for lithium extraction in the liquid-liquid system doped into imidazolium-based ionic liquids or phosphonium-based ionic liquids [8–10]. In all the researches mentioned, ionic liquids were described as a novel high-efficient green solvents or co-extractant. ⁎ Corresponding author at: Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China. E-mail address: [email protected] (Y. Jing).
https://doi.org/10.1016/j.molliq.2019.04.046 0167-7322/© 2019 Elsevier B.V. All rights reserved.
Lithium metal is one of the most important energy materials, widely used in batteries [11,12], nuclear fusion [13], automotive and other industries. Salt lake lithium resources account for 69% of the global lithium reserves. China also has abundant lithium-containing brines, mainly distributed in the salt lakes of Tibet and Qinghai. Technologies of producing lithium and its compounds from spodumene has high-energy consumption with complicated chemical processes. Solvent extraction of lithium is a simple process with low energy consumption, low cost and easy to adjust the production scale. Solvent extraction has been widely used in the literature to extract lithium. The researches focus on alcohol, ketone or diketone [14,15], organophosphorus [16], quaternary ammonium salt-azo-ion chelationassociation and crown ether [17,18] on extractants. The novel and high-efficiency IL extractant [N4444][EHPMEH] has been reported for the extraction and separation of lithium by Zhao et al. The results showed that 3LiCl·4[N4444][EHPMEH] complex was formed and the reaction mechanism of lithium with [N4444][EHPMEH] was the interaction between Li+ in solution and phosphate ester [19]. Shi et al. did a series of works on lithium extraction from brine and achieved the process of multi-stage extraction [14,15,20]. Using large quantities of ionic liquids as solvents or co-extractant could not harm peoples' health and not cause environmental pollution. Using efficient and environmentfriendly ionic liquids as solvents or co-extractants are an important
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direction for the development of solvent extraction methods in the coming future. In this work, the effect of ionic liquids as co-extractant with crown ether as the main ligand for extracting lithium in the liquid-liquid system was studied. We studied different ionic liquids on the extraction of lithium ions and explored the extraction mechanism of lithium with [BMIm][NTf2] in solvent extraction. Lastly, we investigated the reaction process and thermodynamic behavior in the system containing [BMIm][NTf2] and crown ether. 2. Experimental section 2.1. Reagents All of ionic liquids were purchased from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. Crown ether was purchased from Wu Xi Apptec Co., Ltd., Shanghai. Bis(trifluoromethane) sulfonimide lithium salts (LiNTf2) were purchased from 3A Chemical Technology Co., Ltd. Other reagents were from Aladdin Biochemical Technology Co., Ltd. All of reagents used in experiments were analytical reagent grade or highest quality without further purification. Ultrapure water (18.2 MΩ·CM−1) that was purified from a Milli-Q purification system (UPT-II-207, Chengdu, China) was used in experiments (Table 1). 2.2. Analytical method
spectroscopy change of samples before and after extraction, the existence of interactions among lithium ions, crown ether and ionic liquids were confirmed. 2.3. Experimental processing Organic phase was prepared with the concentration of 0.2 mol·L−1 crown ether, 0.5 mol·L−1 ionic liquid and dichloromethane. Aqueous phase was prepared with the concentration of 1 mol·L−1 LiNTf2. Firstly, 2 mL of organic phase and 2 mL of aqueous phase was put into a 10 mL plastic pipe which oscillated for 1 h at a speed of 270 r·min−1 on the horizontal oscillator (GGC-P, Beijing China) so that the organic phase was fully contacted with the aqueous phase and reached the reaction equilibrium. Secondly, the mixed phase was separated by centrifugation at a speed of 7000 r·min−1 for 5 min on a high-speed centrifuge (TG16WS, Hunan China). Then the upper aqueous phase was removed to a new 10 mL plastic pipe. Finally, samples in organic phase or in aqueous phase were ready for various tests and analyses. According to the mathematical Eqs. (1), (2), the extraction rate and distribution coefficient of lithium ions were calculated by testing Li+ concentration difference before and after extraction. E¼
C 0 −C e 100% C0
ð1Þ
D¼
C 0 −C e 100% Ce
ð2Þ
+
2.2.1. Method for determining Li concentration The concentration of Li+ in aqueous phases was detected by a directreading inductively coupled plasma emission spectroscopy (ICP-AES, Icap6500 DUO, USA). Before using ICP-AES to test the concentration of Li+, lithium salt solution was diluted for the detection. 2.2.2. Analysis of ultraviolet spectrum The absorption peaks of ionic liquids aqueous solution were determined by using ultraviolet-visible spectrum. The different concentrations of ionic liquids were demonstrated by the absorbance of ionic liquids at maximum wavelength, which was to plot the standard curve of ionic liquids. Furthermore, the concentrations of ionic liquids in aqueous phase after extraction were tested under the standard curve of ionic liquids aqueous solution using ultraviolet visible light spectrophotometer. 2.2.3. Analysis of infrared spectroscopic FTIR spectroscopy is sensitive to small differences in the complex chemical environment and can be proved to characterize different functional group information of the complex. Based on the FTIR
where C0 and Ce were the initial and equilibrium Li concentrations in aqueous phases. 3. Results and discussion 3.1. Effect of cationic structures of ionic liquids on lithium extraction lithium bis(trifluorosulfonyl)imide solution was extracted by 0.5 mol·L−1 ionic liquids including [P4444][NTf2], [Py14][NTf2], [BPy] [NTf2], [PP14][NTf2], [N4444][NTf2], [BMIm][NTf2] with the system containing 0.2 mol·L−1 crown ether as chelating agents and dichloromethane (CH4Cl2) as solvents at 293.15 K. As seen from Fig. 1, different cationic structures of ionic liquids had different distribution coefficient for extracting lithium. [BMIm][NTf2] had a stronger effect on extracting lithium than other classes of ionic liquids. The extraction rate and distribution coefficient of the extraction system containing [BMIm][NTf2] were 26.35%, 0.358, respectively. The reason was that the distribution coefficient of Li+ during lithium separation was related to the positive charge dispersion degree of cationic structures in ionic liquids. Under
Table 1 Purchased ionic liquids. Ionic liquids
Abbreviation
1-Ethyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide 1-Butyl-3-methylimidazolium bis[(trifluoromethyl) solfonyl] imide 1-Hexyl-3-methylimidazolium bis[(trifluoromethyl) solfonyl] imide 1-Octyl-3-methylimidazolium bis[(trifluoromethyl) solfonyl] imide N-Butyl pyridinium bis[(trifluoromethyl) sulfonyl] imide Tributylmethylammonium bis[(trifluoromethyl) sulfonyl] imide Tetrabutylphosphonium bis[(trifluoromethyl) sulfonyl] imide 1-Butyl-1-methylpiperidinium bis[(trifluoromethyl) sulfonyl] imide N-Butyl-N-methylpyrrolidinium bis[(trifluoromethyl) sulfonyl] imide 1-Vinyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide 1-Allyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide 1-Benzyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide 1-Butyl-3-methylimidazolium tetrafluoroborate 1-Butyl-3-methylimidazolim chloride 1-Butyl-3-methylimidazolim hexafluorophosphate 1-Butyl-3-methylimidazolim trifluoroacetate
[EMIm][NTf2] [BMIm][NTf2] [HMIm][NTf2] [OMIm][NTf2] [Bpy][NTf2] [N1444][NTf2] [N4444][NTf2] [PP14][NTf2] [P14][NTf2] [VMIm][NTf2] [AMIm][NTf2] [PhCH2MIm][NTf2] [BMIm][BF4] [BMIm][Cl] [BMIm][PF6] [BMIm][TA]
Purity 99% 99% 99% 98% 99% 98% 98% 99% 99% 99% 99% 99% 99% 99% 99% 99%
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Fig. 1. The effect of different cationic structures of ionic liquids on the efficiency (a) and distribution coefficient (b) of Li+. C (crown ether) = 0.2 mol·L−1, C(ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
Fig. 2. Cationic structure of classes of ionic liquids.
the same anion, the distribution coefficient of Li+ in lithium separation process was increased as the positive charge dispersion degree of ionic liquid cationic structures increased. The increase on the positive charge dispersion degree weakens the interaction between anions and cations of ionic liquids, which leads to an easier exchange of cations of ionic liquids in the organic phase and Li+ in the aqueous phase. In the above ionic liquids, the electronegativity of N and P atoms is higher than that of C atoms. Electrons are mainly distribution on N and P atoms, and positive charges distributed mainly on C atom. Compared with other ionic liquids, the imidazolium cation contains two N atoms located at both ends of the imidazole ring, which is a conjugated system that can distribute the positive charges evenly over the entire imidazole ring. So, the extraction system containing [BMIm][NTf2] that has the
characteristic of greater positive charge dispersion degree achieves the best extractive effect for lithium ions in the extraction process. The cationic structures of [P4444][NTf2], [Py14][NTf2], [BPy][NTf2], [PP14][NTf2], [N4444][NTf2], [BMIm][NTf2] used in this studies are shown in Fig. 2. 3.2. Effect of anionic structures of ionic liquids on lithium extraction The volume, symmetry, ability to form hydrogen bonds, and charge distribution of ionic liquid anions have a great influence on their coordination ability, acidity, alkalinity, viscosity, and solubility. [BMIm][NTf2], [BMIm][PF6], [BMIm][CF3COO], [BMIm][BF4], [BMIm][Cl] were studied. The results can be seen from Fig. 3. Lithium ions extraction becomes difficult with the increasing solubility of ionic liquids. The phenomenon
Fig. 3. The effect of different anionic structures of imidazolium-based ionic liquids on the extraction rate (a) and distribution coefficient (b) of Li+ in the liquid-liquid extraction system. C (crown ether) = 0.2 mol·L−1, C(ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
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Fig. 4. Different anionic structures of imidazolium-based ionic liquids.
may be due to more ionic liquids entering the aqueous phase and ionic liquids in the organic phase will be largely lost, resulting in a decrease of the exchange between cations of ionic liquids and Li+ and leading to the reduction of the distribution coefficient of Li+. Compared to other anions, NTf− 2 has the characteristics of large volume, high symmetry and weak electron supply capability, which increases its binding ability to Li+-ligand in the organic phase and improves the extractive effect and distribution coefficient of Li+ in lithium extraction process. The anionic structures of ionic liquids used in this study are shown in Fig. 4. 3.3. Effect of lengths of alkyl carbon chain of ionic liquids on lithium extraction The lengths of alkyl carbon chain of ionic liquids effect were examined by preparing dichloromethane and four kinds of ionic liquids such as [EMIm][NTf2], [BMIm][NTf2], [HMIm][NTf2] and [OMIm][NTf2]. It can be seen from Fig. 5 that [BMIm][NTf2] showed advantages than other ionic liquids when comparing the extraction rate and distribution coefficient in the same conditions. Using [BMIm][NTf2] as a part of the organic phase resulted in a highest distribution coefficient and
extraction rate. For all the above ionic liquids, it was noteworthy that the lengths of alkyl carbon chain of ionic liquids had a large variation trend for the distribution coefficient and extraction rate. This would be explained that the steric hindrance of ionic liquids improved correspondingly as the carbon chain length of the imidazolium-based ionic liquid increased, resulting in a decrease on the synergistic effect of ionic liquids. Moreover, the increase on the carbon chain length of the imidazolium-based ionic liquid enhanced the hydrophobicity of the ionic liquid. Therefore, when the lengths of carbon chain ionic liquid reach a certain length, the extraction effect of lithium can be optimized in the system. The different alkyl carbon chain lengths of ionic liquids used in this study is shown in Fig. 6. 3.4. Effect of unsaturated functional groups of ionic liquids on lithium extraction The unsaturated functional group of ionic liquids will affect the electron cloud distribution on the imidazole ring, which may have a great influence on the extraction of lithium. In term of different unsaturated functional groups of imidazolium-based ionic liquids, we investigated
Fig. 5. The effect of different carbon chain lengths of imidazolium-based ionic liquids on the extraction rate (a) and distribution coefficient (b) of Li+ in the liquid-liquid extraction system. C (crown ether) = 0.2 mol·L−1, C (ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
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Fig. 6. Cationic structures of different lengths of alkyl carbon chain of imidazolium-based ionic liquids.
three kinds of unsaturated functional groups ionic liquids in separating lithium. As shown in Fig. 7, the unsaturated functional groups of ionic liquids had an effect in a small variation trend for the distribution coefficient of Li+. The distribution coefficient of lithium ions in the extraction system decreased with the increase of relative molecular weight of ionic liquids, indicating that the extraction system with low relative molecular weight of ionic liquids had high extractive effect to lithium. This may be due to the fact that the Van der Waals forces between molecules and solvent molecules grew with increasing relative molecular weight of ionic liquids, resulting in a weaker extraction capability of the extraction system for lithium. In addition, the functional group of [VMIm][NTf2] cation formed a conjugated system with the imidazole ring, which allowed the positive charge to be uniformly dispersed throughout the imidazole ring and its branches, which weakened the interaction between the anion and cation of ionic liquid. Thereby, Li+ in the aqueous phase were more easily exchanged with the cation of ionic liquid in the organic phase. The N-atom of the imidazole ring of
[AMIm][NTf2], [BzMIm][NTf2], which had the highest electronegativity, was easy to form an inducing effect with the unsaturated group on the side chain of the imidazole ring, and induced its electron cloud distribution through electrostatic field induction. This increased the interaction between the anion and cation of ionic liquid, which resulted in ion exchange difficulty between Li+ and cations of ionic liquids. The functional groups of ionic liquids used in this study are shown in Fig. 8. 4. Extraction mechanism For an in-depth understanding of lithium separation, we investigated the concentration of hydrophobic [BMIm]+ in the aqueous phase. First, 1-butyl-3-methylimidazolium bis(trifluorosulfonyl)imide salt aqueous solution was tested by Beijing Puxi TU-1810 UV–Vis spectrophotometer to obtain its ultraviolet spectrum. The absorbance of [BMIm]+ at maximum wavelength was 2.302 Abs, and its maximum wavelength was 211 cm−1 (Fig. 9). Next, the standard curve of the
Fig. 7. The effect of unsaturated functional groups of imidazolium-based ionic liquids on the extraction rate (a) and distribution coefficient (b) of Li+ in the liquid-liquid extraction system. C (crown ether) = 0.2 mol·L−1, C (ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
Fig. 8. Cationic structure of different unsaturated functional groups of imidazolium-based ionic liquids.
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Fig. 9. Ultraviolet absorption curves of [BMIm][NTf2] aqueous solution.
Fig. 11. Fitted curve of [BMIm]+ concentration in water and Li+ concentration in organic phase after extraction. C(crown ether) = 0.2 mol·L−1, C([BMIm]+) = 0.5 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
the concentration of [BMIm]+ in the aqueous phase and the concentration of Li+ in the organic phase. It can be seen in Fig. 11 that the slope of the fitted curve is 1.05824, closed to 1. It indicated that Li+ and [BMIm]+ were 1:1 exchanged in lithium extraction process, which fully agreed with the principle of charge balance of aqueous and organic phases. 5. FTIR analysis of organic phases
Fig. 10. Standard curve of different concentrations [BMIm][NTf2] aqueous solution at 293.15 K.
absorbance at maximum wavelength 211 cm−1 vs [BMIm]+ concentrations in aqueous solution was plotted in Fig. 10. 100 mL [BMIm][NTf2] water solution was prepared by weighing 0.0235 g [BMIm][NTf2] in 100 ml volumetric flask and further diluted to achieved different concentrations. Extraction experiments were carried out respectively in solutions of 0 mol·L−1, 0.25 mol·L−1, 0.5 mol·L−1, 1.0 mol·L−1, 1.5 mol·L−1 and 2.0 mol·L−1 LiNTf2 in the same experimental conditions. The results showed that the concentration of [BMIm]+ in the aqueous phase increased during the lithium extraction as the concentration of Li+ in the organic phase increased (Table 2). [BMIm]+ were involved in the extraction process and formed an exchange with Li+ in the aqueous phase. To further verify Li+ and [BMIm]+ exchange process in the above experiment, we fitted the linear relationship between
To further understand the role of ionic liquids on the extraction of lithium, we studied the infrared spectrum of organic phase before and after extraction. FT-IR spectra of [BMIm][NTf2] were shown in Fig. 12. For [BMIm][NTf2], the peaks at 3158 cm−1 and 3153 cm−1 were regarded as the stretching vibration of unsaturated C\\H bond on imidazole ring. The stretching vibration of saturated C\\H bond were at 2968 cm−1, 2940 cm−1, 2880 cm−1. The vibration of imidazole ring skeleton was at 1573 cm−1, 1468 cm−1. The peaks at 1058 cm−1 corresponded to the stretching vibration of S-N-S bond. As shown in Fig. 13 that the IR spectra was used to detect changes in the structure of [BMIm][NTf2] in the organic phase. The most obvious change of [BMIm][NTf2] was that the vibration peak associated with the cation [BMIm]+ did not change. Moreover, the stretching vibration of S-N-S bond located at 1058 cm−1 transformed into band at 1053 cm−1. The results showed that [BMIm]+ was not involved in the chelating reaction of Li+ and crown ether in the organic phase. In addi+ tion, NTf− 2 anions might formed a weak coordinate bond with the Li complex, resulting in the changes of electronic cloud distribution on NTf− 2 , which reduced the force constant and the vibration frequency of S-N-S bond. 6. Analysis of thermodynamics It is important to obtain the thermodynamic parameters of the extraction of lithium from the aqueous phase to the organic phase in combination with the crown ether. The distribution coefficients from 278.15 K to 303.15 K were shown in Fig. 14. Obviously, this system
Table 2 The changes of Li+ and [BMIm]+ concentration in the aqueous phase or organic phase before and after lithium extraction at 293.15 K. The concentration of Li+ in the aqueous phase before extraction (mol·L−1) The concentration of Li+ in the aqueous phase after extraction (mol·L−1) The concentration of Li+ in the organic phase after extraction (mol·L−1) The concentration of [BMIm]+ in the aqueous phase after extraction (mol·L−1)
0.0 0.0 0.0 0.006
0.25 0.174 0.076 0.088
0.5 0.357 0.143 0.159
1.0 0.736 0.264 0.290
1.5 1.222 0.278 0.307
2.0 1.715 0.285 0.305
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Fig. 12. Infrared spectrum of [BMIm][NTf2]. Fig. 14. The effect of temperature on the distribution coefficient of Li+ in the liquid-liquid extraction system from 278.15 K to 303.15 K. C (crown ether) = 0.2 mol·L−1, C(ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
was more favorable for extracting lithium at low temperature. In the extraction process, Li+ ions entered the organic phase from the aqueous phase exchanged with [BMIm]+ in the organic phase. Coordination reactions between lithium ions and crown ethers formed the Li+-ligand complex. As shown in the following Eqs. (3), (4): þ − þ − þ Liþ Li A aq þ ½BM Imþ org ⇌ ½BM Im ½A org aq Liþ org
þ − þ B− org þ nLorg ⇌ ½Li nL ½B org
ð3Þ ð4Þ
The complete chemical reaction of this extraction system was shown in Eq. (5): þ − Li A aq þ ½BM Imþ ½B− org þ nLorg ⇌ ½Li nLþ ½B− org þ ½BM Imþ ½A− aq
ð5Þ
where the subscripts ‘aq’ and ‘org’ meant the corresponding reactants in organic phase and aqueous phase; A−, B−, L, n was donated by the counter anion of lithium salt in the aqueous phase, the anion of ionic liquids in the organic phase, crown ether, and the number of crown ether taking part in the reaction respectively. [Li-ligand] + and the anion of
Fig. 13. Infrared spectrum measured by equal volume organic phase before (a) and after (b) lithium extraction.
ionic liquid formed a complex without charge. Thus, the distribution coefficient was described by the following Eq. (6): D¼
½Liorg ½Liaq
¼
½Li nLþ ½B− org þ − Li A aq
ð6Þ
Throughout the complete reaction for Li+ extraction in this system, the chemical equilibrium constant Ke was defined as an Eq. (7): h i h i ½BM Imþ ½A− aq ½Li nLþ ½B− org K e ¼ h i h i n Liþ A− aq ½BM Imþ ½B− org Lorg
ð7Þ
In order to better explain the relationship between the distribution coefficient (D) and the chemical equilibrium constant (Ke), the new
Fig. 15. Plot of log D versus 103/T on lithium extraction in the crown ether-[BMIm][NTf2] system. The aqueous phase: C (LiNTf2) = 1 mol·L−1. C (crown ether) = 0.2 mol·L−1, C (ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1. Temperature changes from 278.15 K to 303.15 K.
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Table 3 Thermodynamic parameters of the crown ether-dichloromethane extraction system containing [BMIm][NTf2] at 293.15 K. Ionic liquids [BMIm][NTf2]
Temperature (K)
n = 1 [21,24–25] ΔG°1 1.213
293.15
n = 2 [22–23] ΔH°1 −21.190
ΔS°1 −76.423
ΔG°2 −0.489
ΔH°2 −21.190
ΔS°2 −70.615
The units of ΔG°, ΔH° and ΔS° are kJ·mol−1, kJ·mol−1, J·mol−1·K−1, respectively.
expression of Eq. (7) would be modified by taking base-e logarithm on both sides of Eqs. (6) and (7). h i ln D ¼ ln K e þ ln ½BMImþ ½A− org þ n h i ln Lorg − ln ½BM Imþ ½A− aq ΔG ° ¼ −RT ln K e ln K e ¼ −
ð8Þ ð9Þ
ΔH ° ΔS ° þ RT R
ð10Þ
As shown in Fig. 15. Based on Van't Hoff Eq. (10), Eq. (8) could be described as Eq. (11). The enthalpy change ΔH° for Li+ extraction in this system could be obtained by the slope of fitting the linear equations of log D and 103/T using Eq. (13). At 293.15 K, Concentrations of [BMIm]+ mentioned in the above experiments in the aqueous phase and in the organic phase were 0.290 mol·L−1 and 0.210 mol·L−1, respectively. Nishizawa [21] and Xiao [22,23] obtained n value of 1 and 2, which was the number of crown ethers coordinated to Li+. As shown in Eqs. (13), (14), (15), (16), the entropy change ΔS° at 293.15 K could be calculated from the intercept of the fitted line. Then, the Gibbs free energy ΔG° was obtained in accordance with the thermochemical Eq. (17). h i ΔH ° ΔS ° þ þ ln ½BM Imþ ½B− org þ n RT h R i ln Lorg − ln ½BM Imþ ½A− aq
ln D ¼ ‐
ð11Þ
The Eq. (12) would be modified by taking base-10 logarithm on both sides of Eq. (11). logD ¼
h i 1 ΔH ° ΔS ° − þ þ log ½BM Imþ ½B− org 2:303 RT R h i þn log Lorg −log ½BM Imþ ½A− aq
ð12Þ
parameters of the liquid-liquid extraction system containing [BMIm] [NTf2] and crown ether at 293.15 K were listed in Table 3. When n = 1, the value of ΔG° was greater than 0. This indicated that the extraction reaction could not be carried out under this condition. When n = 2, negative values of ΔH° and ΔG° on Li+ extraction in the crown etherdichloromethane system containing [BMIm][NTf2] indicated that this system was an exothermic spontaneous reaction in extracting lithium process, which is consistent with the experimental phenomenon. So the complexation ratio n of crown ether to lithium ion is 2 in the extraction system. 7. Conclusion In this work, we reported a series of ionic liquids as synergists or solvents incorporated into the organic phase of the extraction system containing crown ether for extracting lithium. A series of ionic liquids to the extractive separation lithium ions were studied under same conditions. The factors such as cationic structures of ionic liquids, lengths of alkyl carbon chain of ionic liquids, anionic structures of ionic liquids, unsaturated functional groups of ionic liquids were investigated. The hydrophobicity, weakly anion coordination, uniform cation charge distribution or conjugation, smaller relative molecular weight of ionic liquids were the keys to effective separation of lithium. A study on Li+ and ionic liquids exchange behavior explained that the exchange of cationic structures of ionic liquids in the organic phase with Li+ in the aqueous phase promoted lithium ions entering into the organic phase and forming the Li+-ligand complex. Li+ and the cations of ionic liquids exchange conformed to the principle of charge balance in organic and aqueous phases by fitting the concentrations of Li+ entering into organic phases and cations swapping into aqueous phases. The anion of [BMIm][NTf2] combining with the Li+-crown ether complex in the organic phase was also verified by infrared spectroscopy. The system containing crown ether and [BMIm][NTf2] was an exothermic spontaneous reaction in extracting lithium process and the lower temperature was of benefit to lithium extraction. Acknowledgments
ΔH ° þC logD ¼ − 2:303RT
ð13Þ
f
ð14Þ
h i ΔS ° þ log ½BM Imþ ½B− org R h i þn log Lorg −log ½BM Imþ ½A− aq
C¼
1 2:303
g
( ) 1 ΔS ° þ logð0:290Þ þ n logð0:200Þ−logð0:210Þ C¼ 2:303 R
C¼
ΔS ° þ 0:060868−0:303504n 2:303R
ΔG ° ¼ ΔH ° −TΔS °
This work was supported by the National Science Foundation of China (U1407117) and the Natural Scientific Foundation of Qinghai Province (2011-Z-931Q). References
ð15Þ
ð16Þ ð17Þ
where R was the gas constant value of 8.314 J·mol−1·K−1, C was a constant that can be the intercept of log D versus 103/T. Thermodynamic
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The effect of ionic liquids as co-extractant with crown ether for the extraction of lithium in dichloromethane-water system Wenbo Zhu a,c, Yongzhong Jia a,b, Quanyou Zhang a,b, Jinhe Sun a,b, Yan Jing a,b,⁎, Jie Li d a
Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China Qinghai Engineering and Technology Research Center of Comprehensive Utilization of Salt Lake Resources, 810008 Xining, China University of Chinese Academy of Sciences, 100049 Beijing, China d Zhejiang Jingquan Water Treatment Equipment Co. Ltd, China b c
a r t i c l e
i n f o
Article history: Received 23 August 2018 Received in revised form 2 March 2019 Accepted 8 April 2019 Available online 09 April 2019 Keywords: Lithium Ionic liquids Extraction Infrared spectroscopy [BMIm][NTf2]
a b s t r a c t With different anionic and cationic structures of ionic liquids influencing the efficiency of Li+ extraction, 16 different kinds of ionic liquids with the crown ether-dichloromethane system were investigated primarily. The experimental results showed that smaller relative molecular weight, hydrophobicity, uniform electron distribution on cation, and weak electron donating on anion of ionic liquids with the ligand crown ether promoted the highest extraction of lithium ions in the dichloromethane-water system. The extraction system with [BMIm][NTf2] had the largest extraction rate and distribution coefficient at 26.35% and 0.358, respectively. Cation exchange behavior between Li+ in the aqueous phase and [BMIm]+ in the organic phase was examined through fitting the line of Li+ concentration in the organic phase and [BMIm]+ concentration in the aqueous phase. The anion of [BMIm] [NTf2] coordinating with the cationic lithium-crown ether complex in the organic phase was also verified by infrared spectroscopy. For the thermodynamic studies in the system using [BMIm][NTf2] and crown ether, the extraction process of Li+ was an exothermic spontaneous reaction while lower temperature was beneficial to lithium extraction. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Room temperature ionic liquids, ILs for short, is a class of compounds made up of cations and anions while staying as liquids at room temperature or near room temperature. Compared to traditional organic solvents, the features of ionic liquids are low steam pressure, low volatilization and high-solubility for the organics and the inorganics, especially the chemical and physical properties of ionic liquids can be tailored by the changing the cation- and anion- combination. Last several years, ionic liquids gained considerable attentions as solvent or coextractant for the separation and analytical technique [1,2]. Dietz et al. researched the system employing crown ether for the separation and extraction mechanism of strontium with imidazolium-based ionic liquids [3–7]. Jia et al. used tributyl phosphate or crown ethers as chelating agents for lithium extraction in the liquid-liquid system doped into imidazolium-based ionic liquids or phosphonium-based ionic liquids [8–10]. In all the researches mentioned, ionic liquids were described as a novel high-efficient green solvents or co-extractant. ⁎ Corresponding author at: Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China. E-mail address: [email protected] (Y. Jing).
https://doi.org/10.1016/j.molliq.2019.04.046 0167-7322/© 2019 Elsevier B.V. All rights reserved.
Lithium metal is one of the most important energy materials, widely used in batteries [11,12], nuclear fusion [13], automotive and other industries. Salt lake lithium resources account for 69% of the global lithium reserves. China also has abundant lithium-containing brines, mainly distributed in the salt lakes of Tibet and Qinghai. Technologies of producing lithium and its compounds from spodumene has high-energy consumption with complicated chemical processes. Solvent extraction of lithium is a simple process with low energy consumption, low cost and easy to adjust the production scale. Solvent extraction has been widely used in the literature to extract lithium. The researches focus on alcohol, ketone or diketone [14,15], organophosphorus [16], quaternary ammonium salt-azo-ion chelationassociation and crown ether [17,18] on extractants. The novel and high-efficiency IL extractant [N4444][EHPMEH] has been reported for the extraction and separation of lithium by Zhao et al. The results showed that 3LiCl·4[N4444][EHPMEH] complex was formed and the reaction mechanism of lithium with [N4444][EHPMEH] was the interaction between Li+ in solution and phosphate ester [19]. Shi et al. did a series of works on lithium extraction from brine and achieved the process of multi-stage extraction [14,15,20]. Using large quantities of ionic liquids as solvents or co-extractant could not harm peoples' health and not cause environmental pollution. Using efficient and environmentfriendly ionic liquids as solvents or co-extractants are an important
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direction for the development of solvent extraction methods in the coming future. In this work, the effect of ionic liquids as co-extractant with crown ether as the main ligand for extracting lithium in the liquid-liquid system was studied. We studied different ionic liquids on the extraction of lithium ions and explored the extraction mechanism of lithium with [BMIm][NTf2] in solvent extraction. Lastly, we investigated the reaction process and thermodynamic behavior in the system containing [BMIm][NTf2] and crown ether. 2. Experimental section 2.1. Reagents All of ionic liquids were purchased from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. Crown ether was purchased from Wu Xi Apptec Co., Ltd., Shanghai. Bis(trifluoromethane) sulfonimide lithium salts (LiNTf2) were purchased from 3A Chemical Technology Co., Ltd. Other reagents were from Aladdin Biochemical Technology Co., Ltd. All of reagents used in experiments were analytical reagent grade or highest quality without further purification. Ultrapure water (18.2 MΩ·CM−1) that was purified from a Milli-Q purification system (UPT-II-207, Chengdu, China) was used in experiments (Table 1). 2.2. Analytical method
spectroscopy change of samples before and after extraction, the existence of interactions among lithium ions, crown ether and ionic liquids were confirmed. 2.3. Experimental processing Organic phase was prepared with the concentration of 0.2 mol·L−1 crown ether, 0.5 mol·L−1 ionic liquid and dichloromethane. Aqueous phase was prepared with the concentration of 1 mol·L−1 LiNTf2. Firstly, 2 mL of organic phase and 2 mL of aqueous phase was put into a 10 mL plastic pipe which oscillated for 1 h at a speed of 270 r·min−1 on the horizontal oscillator (GGC-P, Beijing China) so that the organic phase was fully contacted with the aqueous phase and reached the reaction equilibrium. Secondly, the mixed phase was separated by centrifugation at a speed of 7000 r·min−1 for 5 min on a high-speed centrifuge (TG16WS, Hunan China). Then the upper aqueous phase was removed to a new 10 mL plastic pipe. Finally, samples in organic phase or in aqueous phase were ready for various tests and analyses. According to the mathematical Eqs. (1), (2), the extraction rate and distribution coefficient of lithium ions were calculated by testing Li+ concentration difference before and after extraction. E¼
C 0 −C e 100% C0
ð1Þ
D¼
C 0 −C e 100% Ce
ð2Þ
+
2.2.1. Method for determining Li concentration The concentration of Li+ in aqueous phases was detected by a directreading inductively coupled plasma emission spectroscopy (ICP-AES, Icap6500 DUO, USA). Before using ICP-AES to test the concentration of Li+, lithium salt solution was diluted for the detection. 2.2.2. Analysis of ultraviolet spectrum The absorption peaks of ionic liquids aqueous solution were determined by using ultraviolet-visible spectrum. The different concentrations of ionic liquids were demonstrated by the absorbance of ionic liquids at maximum wavelength, which was to plot the standard curve of ionic liquids. Furthermore, the concentrations of ionic liquids in aqueous phase after extraction were tested under the standard curve of ionic liquids aqueous solution using ultraviolet visible light spectrophotometer. 2.2.3. Analysis of infrared spectroscopic FTIR spectroscopy is sensitive to small differences in the complex chemical environment and can be proved to characterize different functional group information of the complex. Based on the FTIR
where C0 and Ce were the initial and equilibrium Li concentrations in aqueous phases. 3. Results and discussion 3.1. Effect of cationic structures of ionic liquids on lithium extraction lithium bis(trifluorosulfonyl)imide solution was extracted by 0.5 mol·L−1 ionic liquids including [P4444][NTf2], [Py14][NTf2], [BPy] [NTf2], [PP14][NTf2], [N4444][NTf2], [BMIm][NTf2] with the system containing 0.2 mol·L−1 crown ether as chelating agents and dichloromethane (CH4Cl2) as solvents at 293.15 K. As seen from Fig. 1, different cationic structures of ionic liquids had different distribution coefficient for extracting lithium. [BMIm][NTf2] had a stronger effect on extracting lithium than other classes of ionic liquids. The extraction rate and distribution coefficient of the extraction system containing [BMIm][NTf2] were 26.35%, 0.358, respectively. The reason was that the distribution coefficient of Li+ during lithium separation was related to the positive charge dispersion degree of cationic structures in ionic liquids. Under
Table 1 Purchased ionic liquids. Ionic liquids
Abbreviation
1-Ethyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide 1-Butyl-3-methylimidazolium bis[(trifluoromethyl) solfonyl] imide 1-Hexyl-3-methylimidazolium bis[(trifluoromethyl) solfonyl] imide 1-Octyl-3-methylimidazolium bis[(trifluoromethyl) solfonyl] imide N-Butyl pyridinium bis[(trifluoromethyl) sulfonyl] imide Tributylmethylammonium bis[(trifluoromethyl) sulfonyl] imide Tetrabutylphosphonium bis[(trifluoromethyl) sulfonyl] imide 1-Butyl-1-methylpiperidinium bis[(trifluoromethyl) sulfonyl] imide N-Butyl-N-methylpyrrolidinium bis[(trifluoromethyl) sulfonyl] imide 1-Vinyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide 1-Allyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide 1-Benzyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide 1-Butyl-3-methylimidazolium tetrafluoroborate 1-Butyl-3-methylimidazolim chloride 1-Butyl-3-methylimidazolim hexafluorophosphate 1-Butyl-3-methylimidazolim trifluoroacetate
[EMIm][NTf2] [BMIm][NTf2] [HMIm][NTf2] [OMIm][NTf2] [Bpy][NTf2] [N1444][NTf2] [N4444][NTf2] [PP14][NTf2] [P14][NTf2] [VMIm][NTf2] [AMIm][NTf2] [PhCH2MIm][NTf2] [BMIm][BF4] [BMIm][Cl] [BMIm][PF6] [BMIm][TA]
Purity 99% 99% 99% 98% 99% 98% 98% 99% 99% 99% 99% 99% 99% 99% 99% 99%
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Fig. 1. The effect of different cationic structures of ionic liquids on the efficiency (a) and distribution coefficient (b) of Li+. C (crown ether) = 0.2 mol·L−1, C(ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
Fig. 2. Cationic structure of classes of ionic liquids.
the same anion, the distribution coefficient of Li+ in lithium separation process was increased as the positive charge dispersion degree of ionic liquid cationic structures increased. The increase on the positive charge dispersion degree weakens the interaction between anions and cations of ionic liquids, which leads to an easier exchange of cations of ionic liquids in the organic phase and Li+ in the aqueous phase. In the above ionic liquids, the electronegativity of N and P atoms is higher than that of C atoms. Electrons are mainly distribution on N and P atoms, and positive charges distributed mainly on C atom. Compared with other ionic liquids, the imidazolium cation contains two N atoms located at both ends of the imidazole ring, which is a conjugated system that can distribute the positive charges evenly over the entire imidazole ring. So, the extraction system containing [BMIm][NTf2] that has the
characteristic of greater positive charge dispersion degree achieves the best extractive effect for lithium ions in the extraction process. The cationic structures of [P4444][NTf2], [Py14][NTf2], [BPy][NTf2], [PP14][NTf2], [N4444][NTf2], [BMIm][NTf2] used in this studies are shown in Fig. 2. 3.2. Effect of anionic structures of ionic liquids on lithium extraction The volume, symmetry, ability to form hydrogen bonds, and charge distribution of ionic liquid anions have a great influence on their coordination ability, acidity, alkalinity, viscosity, and solubility. [BMIm][NTf2], [BMIm][PF6], [BMIm][CF3COO], [BMIm][BF4], [BMIm][Cl] were studied. The results can be seen from Fig. 3. Lithium ions extraction becomes difficult with the increasing solubility of ionic liquids. The phenomenon
Fig. 3. The effect of different anionic structures of imidazolium-based ionic liquids on the extraction rate (a) and distribution coefficient (b) of Li+ in the liquid-liquid extraction system. C (crown ether) = 0.2 mol·L−1, C(ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
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Fig. 4. Different anionic structures of imidazolium-based ionic liquids.
may be due to more ionic liquids entering the aqueous phase and ionic liquids in the organic phase will be largely lost, resulting in a decrease of the exchange between cations of ionic liquids and Li+ and leading to the reduction of the distribution coefficient of Li+. Compared to other anions, NTf− 2 has the characteristics of large volume, high symmetry and weak electron supply capability, which increases its binding ability to Li+-ligand in the organic phase and improves the extractive effect and distribution coefficient of Li+ in lithium extraction process. The anionic structures of ionic liquids used in this study are shown in Fig. 4. 3.3. Effect of lengths of alkyl carbon chain of ionic liquids on lithium extraction The lengths of alkyl carbon chain of ionic liquids effect were examined by preparing dichloromethane and four kinds of ionic liquids such as [EMIm][NTf2], [BMIm][NTf2], [HMIm][NTf2] and [OMIm][NTf2]. It can be seen from Fig. 5 that [BMIm][NTf2] showed advantages than other ionic liquids when comparing the extraction rate and distribution coefficient in the same conditions. Using [BMIm][NTf2] as a part of the organic phase resulted in a highest distribution coefficient and
extraction rate. For all the above ionic liquids, it was noteworthy that the lengths of alkyl carbon chain of ionic liquids had a large variation trend for the distribution coefficient and extraction rate. This would be explained that the steric hindrance of ionic liquids improved correspondingly as the carbon chain length of the imidazolium-based ionic liquid increased, resulting in a decrease on the synergistic effect of ionic liquids. Moreover, the increase on the carbon chain length of the imidazolium-based ionic liquid enhanced the hydrophobicity of the ionic liquid. Therefore, when the lengths of carbon chain ionic liquid reach a certain length, the extraction effect of lithium can be optimized in the system. The different alkyl carbon chain lengths of ionic liquids used in this study is shown in Fig. 6. 3.4. Effect of unsaturated functional groups of ionic liquids on lithium extraction The unsaturated functional group of ionic liquids will affect the electron cloud distribution on the imidazole ring, which may have a great influence on the extraction of lithium. In term of different unsaturated functional groups of imidazolium-based ionic liquids, we investigated
Fig. 5. The effect of different carbon chain lengths of imidazolium-based ionic liquids on the extraction rate (a) and distribution coefficient (b) of Li+ in the liquid-liquid extraction system. C (crown ether) = 0.2 mol·L−1, C (ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
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Fig. 6. Cationic structures of different lengths of alkyl carbon chain of imidazolium-based ionic liquids.
three kinds of unsaturated functional groups ionic liquids in separating lithium. As shown in Fig. 7, the unsaturated functional groups of ionic liquids had an effect in a small variation trend for the distribution coefficient of Li+. The distribution coefficient of lithium ions in the extraction system decreased with the increase of relative molecular weight of ionic liquids, indicating that the extraction system with low relative molecular weight of ionic liquids had high extractive effect to lithium. This may be due to the fact that the Van der Waals forces between molecules and solvent molecules grew with increasing relative molecular weight of ionic liquids, resulting in a weaker extraction capability of the extraction system for lithium. In addition, the functional group of [VMIm][NTf2] cation formed a conjugated system with the imidazole ring, which allowed the positive charge to be uniformly dispersed throughout the imidazole ring and its branches, which weakened the interaction between the anion and cation of ionic liquid. Thereby, Li+ in the aqueous phase were more easily exchanged with the cation of ionic liquid in the organic phase. The N-atom of the imidazole ring of
[AMIm][NTf2], [BzMIm][NTf2], which had the highest electronegativity, was easy to form an inducing effect with the unsaturated group on the side chain of the imidazole ring, and induced its electron cloud distribution through electrostatic field induction. This increased the interaction between the anion and cation of ionic liquid, which resulted in ion exchange difficulty between Li+ and cations of ionic liquids. The functional groups of ionic liquids used in this study are shown in Fig. 8. 4. Extraction mechanism For an in-depth understanding of lithium separation, we investigated the concentration of hydrophobic [BMIm]+ in the aqueous phase. First, 1-butyl-3-methylimidazolium bis(trifluorosulfonyl)imide salt aqueous solution was tested by Beijing Puxi TU-1810 UV–Vis spectrophotometer to obtain its ultraviolet spectrum. The absorbance of [BMIm]+ at maximum wavelength was 2.302 Abs, and its maximum wavelength was 211 cm−1 (Fig. 9). Next, the standard curve of the
Fig. 7. The effect of unsaturated functional groups of imidazolium-based ionic liquids on the extraction rate (a) and distribution coefficient (b) of Li+ in the liquid-liquid extraction system. C (crown ether) = 0.2 mol·L−1, C (ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
Fig. 8. Cationic structure of different unsaturated functional groups of imidazolium-based ionic liquids.
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Fig. 9. Ultraviolet absorption curves of [BMIm][NTf2] aqueous solution.
Fig. 11. Fitted curve of [BMIm]+ concentration in water and Li+ concentration in organic phase after extraction. C(crown ether) = 0.2 mol·L−1, C([BMIm]+) = 0.5 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
the concentration of [BMIm]+ in the aqueous phase and the concentration of Li+ in the organic phase. It can be seen in Fig. 11 that the slope of the fitted curve is 1.05824, closed to 1. It indicated that Li+ and [BMIm]+ were 1:1 exchanged in lithium extraction process, which fully agreed with the principle of charge balance of aqueous and organic phases. 5. FTIR analysis of organic phases
Fig. 10. Standard curve of different concentrations [BMIm][NTf2] aqueous solution at 293.15 K.
absorbance at maximum wavelength 211 cm−1 vs [BMIm]+ concentrations in aqueous solution was plotted in Fig. 10. 100 mL [BMIm][NTf2] water solution was prepared by weighing 0.0235 g [BMIm][NTf2] in 100 ml volumetric flask and further diluted to achieved different concentrations. Extraction experiments were carried out respectively in solutions of 0 mol·L−1, 0.25 mol·L−1, 0.5 mol·L−1, 1.0 mol·L−1, 1.5 mol·L−1 and 2.0 mol·L−1 LiNTf2 in the same experimental conditions. The results showed that the concentration of [BMIm]+ in the aqueous phase increased during the lithium extraction as the concentration of Li+ in the organic phase increased (Table 2). [BMIm]+ were involved in the extraction process and formed an exchange with Li+ in the aqueous phase. To further verify Li+ and [BMIm]+ exchange process in the above experiment, we fitted the linear relationship between
To further understand the role of ionic liquids on the extraction of lithium, we studied the infrared spectrum of organic phase before and after extraction. FT-IR spectra of [BMIm][NTf2] were shown in Fig. 12. For [BMIm][NTf2], the peaks at 3158 cm−1 and 3153 cm−1 were regarded as the stretching vibration of unsaturated C\\H bond on imidazole ring. The stretching vibration of saturated C\\H bond were at 2968 cm−1, 2940 cm−1, 2880 cm−1. The vibration of imidazole ring skeleton was at 1573 cm−1, 1468 cm−1. The peaks at 1058 cm−1 corresponded to the stretching vibration of S-N-S bond. As shown in Fig. 13 that the IR spectra was used to detect changes in the structure of [BMIm][NTf2] in the organic phase. The most obvious change of [BMIm][NTf2] was that the vibration peak associated with the cation [BMIm]+ did not change. Moreover, the stretching vibration of S-N-S bond located at 1058 cm−1 transformed into band at 1053 cm−1. The results showed that [BMIm]+ was not involved in the chelating reaction of Li+ and crown ether in the organic phase. In addi+ tion, NTf− 2 anions might formed a weak coordinate bond with the Li complex, resulting in the changes of electronic cloud distribution on NTf− 2 , which reduced the force constant and the vibration frequency of S-N-S bond. 6. Analysis of thermodynamics It is important to obtain the thermodynamic parameters of the extraction of lithium from the aqueous phase to the organic phase in combination with the crown ether. The distribution coefficients from 278.15 K to 303.15 K were shown in Fig. 14. Obviously, this system
Table 2 The changes of Li+ and [BMIm]+ concentration in the aqueous phase or organic phase before and after lithium extraction at 293.15 K. The concentration of Li+ in the aqueous phase before extraction (mol·L−1) The concentration of Li+ in the aqueous phase after extraction (mol·L−1) The concentration of Li+ in the organic phase after extraction (mol·L−1) The concentration of [BMIm]+ in the aqueous phase after extraction (mol·L−1)
0.0 0.0 0.0 0.006
0.25 0.174 0.076 0.088
0.5 0.357 0.143 0.159
1.0 0.736 0.264 0.290
1.5 1.222 0.278 0.307
2.0 1.715 0.285 0.305
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Fig. 12. Infrared spectrum of [BMIm][NTf2]. Fig. 14. The effect of temperature on the distribution coefficient of Li+ in the liquid-liquid extraction system from 278.15 K to 303.15 K. C (crown ether) = 0.2 mol·L−1, C(ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1.
was more favorable for extracting lithium at low temperature. In the extraction process, Li+ ions entered the organic phase from the aqueous phase exchanged with [BMIm]+ in the organic phase. Coordination reactions between lithium ions and crown ethers formed the Li+-ligand complex. As shown in the following Eqs. (3), (4): þ − þ − þ Liþ Li A aq þ ½BM Imþ org ⇌ ½BM Im ½A org aq Liþ org
þ − þ B− org þ nLorg ⇌ ½Li nL ½B org
ð3Þ ð4Þ
The complete chemical reaction of this extraction system was shown in Eq. (5): þ − Li A aq þ ½BM Imþ ½B− org þ nLorg ⇌ ½Li nLþ ½B− org þ ½BM Imþ ½A− aq
ð5Þ
where the subscripts ‘aq’ and ‘org’ meant the corresponding reactants in organic phase and aqueous phase; A−, B−, L, n was donated by the counter anion of lithium salt in the aqueous phase, the anion of ionic liquids in the organic phase, crown ether, and the number of crown ether taking part in the reaction respectively. [Li-ligand] + and the anion of
Fig. 13. Infrared spectrum measured by equal volume organic phase before (a) and after (b) lithium extraction.
ionic liquid formed a complex without charge. Thus, the distribution coefficient was described by the following Eq. (6): D¼
½Liorg ½Liaq
¼
½Li nLþ ½B− org þ − Li A aq
ð6Þ
Throughout the complete reaction for Li+ extraction in this system, the chemical equilibrium constant Ke was defined as an Eq. (7): h i h i ½BM Imþ ½A− aq ½Li nLþ ½B− org K e ¼ h i h i n Liþ A− aq ½BM Imþ ½B− org Lorg
ð7Þ
In order to better explain the relationship between the distribution coefficient (D) and the chemical equilibrium constant (Ke), the new
Fig. 15. Plot of log D versus 103/T on lithium extraction in the crown ether-[BMIm][NTf2] system. The aqueous phase: C (LiNTf2) = 1 mol·L−1. C (crown ether) = 0.2 mol·L−1, C (ILs) = 0.5 mol·L−1, C(LiNTf2) = 1.0 mol·L−1, volume ratio: Vorg: Vaq = 1:1. Temperature changes from 278.15 K to 303.15 K.
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Table 3 Thermodynamic parameters of the crown ether-dichloromethane extraction system containing [BMIm][NTf2] at 293.15 K. Ionic liquids [BMIm][NTf2]
Temperature (K)
n = 1 [21,24–25] ΔG°1 1.213
293.15
n = 2 [22–23] ΔH°1 −21.190
ΔS°1 −76.423
ΔG°2 −0.489
ΔH°2 −21.190
ΔS°2 −70.615
The units of ΔG°, ΔH° and ΔS° are kJ·mol−1, kJ·mol−1, J·mol−1·K−1, respectively.
expression of Eq. (7) would be modified by taking base-e logarithm on both sides of Eqs. (6) and (7). h i ln D ¼ ln K e þ ln ½BMImþ ½A− org þ n h i ln Lorg − ln ½BM Imþ ½A− aq ΔG ° ¼ −RT ln K e ln K e ¼ −
ð8Þ ð9Þ
ΔH ° ΔS ° þ RT R
ð10Þ
As shown in Fig. 15. Based on Van't Hoff Eq. (10), Eq. (8) could be described as Eq. (11). The enthalpy change ΔH° for Li+ extraction in this system could be obtained by the slope of fitting the linear equations of log D and 103/T using Eq. (13). At 293.15 K, Concentrations of [BMIm]+ mentioned in the above experiments in the aqueous phase and in the organic phase were 0.290 mol·L−1 and 0.210 mol·L−1, respectively. Nishizawa [21] and Xiao [22,23] obtained n value of 1 and 2, which was the number of crown ethers coordinated to Li+. As shown in Eqs. (13), (14), (15), (16), the entropy change ΔS° at 293.15 K could be calculated from the intercept of the fitted line. Then, the Gibbs free energy ΔG° was obtained in accordance with the thermochemical Eq. (17). h i ΔH ° ΔS ° þ þ ln ½BM Imþ ½B− org þ n RT h R i ln Lorg − ln ½BM Imþ ½A− aq
ln D ¼ ‐
ð11Þ
The Eq. (12) would be modified by taking base-10 logarithm on both sides of Eq. (11). logD ¼
h i 1 ΔH ° ΔS ° − þ þ log ½BM Imþ ½B− org 2:303 RT R h i þn log Lorg −log ½BM Imþ ½A− aq
ð12Þ
parameters of the liquid-liquid extraction system containing [BMIm] [NTf2] and crown ether at 293.15 K were listed in Table 3. When n = 1, the value of ΔG° was greater than 0. This indicated that the extraction reaction could not be carried out under this condition. When n = 2, negative values of ΔH° and ΔG° on Li+ extraction in the crown etherdichloromethane system containing [BMIm][NTf2] indicated that this system was an exothermic spontaneous reaction in extracting lithium process, which is consistent with the experimental phenomenon. So the complexation ratio n of crown ether to lithium ion is 2 in the extraction system. 7. Conclusion In this work, we reported a series of ionic liquids as synergists or solvents incorporated into the organic phase of the extraction system containing crown ether for extracting lithium. A series of ionic liquids to the extractive separation lithium ions were studied under same conditions. The factors such as cationic structures of ionic liquids, lengths of alkyl carbon chain of ionic liquids, anionic structures of ionic liquids, unsaturated functional groups of ionic liquids were investigated. The hydrophobicity, weakly anion coordination, uniform cation charge distribution or conjugation, smaller relative molecular weight of ionic liquids were the keys to effective separation of lithium. A study on Li+ and ionic liquids exchange behavior explained that the exchange of cationic structures of ionic liquids in the organic phase with Li+ in the aqueous phase promoted lithium ions entering into the organic phase and forming the Li+-ligand complex. Li+ and the cations of ionic liquids exchange conformed to the principle of charge balance in organic and aqueous phases by fitting the concentrations of Li+ entering into organic phases and cations swapping into aqueous phases. The anion of [BMIm][NTf2] combining with the Li+-crown ether complex in the organic phase was also verified by infrared spectroscopy. The system containing crown ether and [BMIm][NTf2] was an exothermic spontaneous reaction in extracting lithium process and the lower temperature was of benefit to lithium extraction. Acknowledgments
ΔH ° þC logD ¼ − 2:303RT
ð13Þ
f
ð14Þ
h i ΔS ° þ log ½BM Imþ ½B− org R h i þn log Lorg −log ½BM Imþ ½A− aq
C¼
1 2:303
g
( ) 1 ΔS ° þ logð0:290Þ þ n logð0:200Þ−logð0:210Þ C¼ 2:303 R
C¼
ΔS ° þ 0:060868−0:303504n 2:303R
ΔG ° ¼ ΔH ° −TΔS °
This work was supported by the National Science Foundation of China (U1407117) and the Natural Scientific Foundation of Qinghai Province (2011-Z-931Q). References
ð15Þ
ð16Þ ð17Þ
where R was the gas constant value of 8.314 J·mol−1·K−1, C was a constant that can be the intercept of log D versus 103/T. Thermodynamic
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