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Synthesis of succinimide based ionic liquids and comparison of extraction behavior of Co(II) and Ni(II) with bi-functional ionic liquids synthesized by Aliquat336 and organophosphorus acids
Separation and Purification Technology 238 (2020) 116496
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Synthesis of succinimide based ionic liquids and comparison of extraction behavior of Co(II) and Ni(II) with bi-functional ionic liquids synthesized by Aliquat336 and organophosphorus acids Thanh Tuan Trana, Nizakat Azrab, Mudassir Iqbalb, Man Seung Leea, a b
T
⁎
Department of Advanced Materials Science & Engineering, Institute of Rare Metal, Mokpo National University, Chonnam 534-729, Republic of Korea Department of Chemistry, School of Natural Sciences (SNS), National University of Sciences and Technology (NUST), Islamabad 44000, Pakistan
A R T I C LE I N FO
A B S T R A C T
Keywords: Succinimide ionic liquids Cobalt Nickel Extraction bi-functional ILs
Two new succinimide compounds such as 3-butyl-1-methyl-1H-imidazol-3-ium 4-(dioctylamino)-4-oxand 1-butylpyridin-1-ium 4-(dioctylamino)-4-oxobutanoate obutanoate ([N88SA]−[C4min]+) ([N88SA]−[C4Py]+) were synthesized as task-specific ionic liquids (TSILs) for the extraction of metal ions. The extraction behavior of Co(II) and Ni(II) from chloride solution was compared between these TSILs and Bif-ILs synthesized from Aliquat336 and organophosphorus acids such as Cyanex 272, PC 88A and D2EHPA. The extraction percentage of Co(II) and Ni(II) by TSILs was eight times higher than that by Bif-ILs at the initial pH of 4.0. Moreover, the extraction percentage of Ni(II) by the TSILs was slightly higher than that of Co(II). However, Bif-ILs extracted more hydrogen ions than TSILs. The extraction of Co(II) and Ni(II) by [N88SA]−[C4min]+ was higher than that by [N88SA]−[C4Py]+. The addition of TBP to TSILs reduced the extraction of metals but the loss of TSILs into the aqueous was significantly decreased. Succinimide ionic liquids can be employed for the extraction of metals from weak acidic solutions.
1. Introduction Ionic liquids (ILs) are defined as organic salts with a melting point below 100 °C [1]. Due to their low volatility, non-flammability, recyclability, and easy modification as eco-friendly alternatives to volatile organic media, ILs are employed as solvents, catalysts for organic reactions, plasticizers, and electrolytes in batteries [2–6]. In hydrometallurgy, ILs are used as solvents or extractants in recovery of metals [7–10]. The most popular ILs are cations of nitrogen, phosphor, halogenides (Cl−, Br−, I−), nitrate, tetrafluoroborate, trifuoromethanesulfonate, bis(trifluoromethylsulfonyl)imide, bistriflimide (Tf2N−), hexafluorophosphate (PF6−). ILs often acts as ion-exchangers between the aqueous and organic phase, resulting in the loss of IL components [11–13]. The ILs containing fluoride could be hydrolyzed in the aqueous media to become toxic and corrosive chemicals, such as HF, POF3, H2PO3F, and HPO2F2 [14,15]. Thus, the synthesis of new ILs which overcome the above-mentioned drawbacks is desirable. The properties of ILs such as hydrophilicity, hydrophobicity and miscibility with organic solvents or water can be modified by replacing alkyl substituents in the components of ILs or by changing anionic or
⁎
cationic components. Therefore, many ILs have been designed and synthesized for specific purposes, which can be called task-specific ionic liquids (TSILs) [16,17]. Besides, the solvating extractants like TBP (Tributyl phosphate) can act as a modifier to reduce the loss of IL components during extraction. The synthesis of ILs based on the commercial extractants such as Aliquat336 or Cyphos IL 101 (tetradecyl(trihexyl)phos- phonium chloride) has attracted interest owing to some advantages like simplicity in synthesis. The combination of Aliquat336 or Cyphos IL cations and anions containing functional groups will generate bi-functional ILs (Bif-ILs), which can act as both cationic and anionic extractants. The extraction characteristics of metal ions by Bif-ILs such as [A336][P204], [A336][P507], [A336][C272], [A336][C302], [A366][CA-12], [A336] [CA-100], Cyphos IL 104 and Cyphos IL 167 has been reported [18–20]. In this work, new succinimide ionic liquids as TSILs were synthesized for the extraction of metal ions. The components of these TSILs included imidazolium and pyridinium cations and succimide anions containing the functional groups like amide and carboxylate. The combination of these ions would increase the metal extraction. These ILs were employed for testing the extraction of Co(II) and Ni(II) from weak chloride media. In general, solution pH has a profound effect on
Corresponding author. E-mail address: [email protected] (M.S. Lee).
https://doi.org/10.1016/j.seppur.2019.116496 Received 11 September 2019; Received in revised form 24 December 2019; Accepted 29 December 2019 Available online 30 December 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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next steps for the synthesis of TSILs. Synthesis of 2-(2-(di-n-octylamino)-succinic acid (8): the mixture of succinic anhydride (6) (1 g, 0.01 mol) and dioctylamine (7) (2.41 g, 0.01 mol) in dry THF (30 mL) were stirred overnight at 50 °C. A lightyellow oily liquid was obtained with 70% yield after evaporation of solvent. FTIR, cm−1; 2932 (CH2 str), 1713 (C]O of acid), 1607 (C]O of amide), 1463 (CH2 bend). 1H NMR ppm (CDCl3); 0.88–0.92 (m, 3H), 1.28–1.32 (m, 20H), 1.57 (t, 4H), 2.52 (t, 2H), 2.76 (t, 2H). 13C NMR ppm (CDCl3); 14.8, 23.2, 26.4, 29.2, 30.1, 32.0, 48.3, 167.2, 172.4. Synthesis of [N88SA]−[C4mim]+ (10): the mixture of 1.02 g (2.9 mmol) of (8) in dry THF (30 mL) and 0.143 g of sodium hydride (1.02 equi) were stirred overnight under argon atmosphere. Subsequently, 0.655 g (2.9 mmol) of (3) was added into the reaction mixture which were stirred for 24 h. After the reaction, precipitates of sodium bromide were obtained which were filtered off; solvent was evaporated under reduced pressure to yield compound (10) in 95% yield. FTIR, cm−1; 3405.22 (C]N), 1615 (C]O of carboxylate), 2924 (CH2 Str.), 1607 (C]O of amide), 1569 (C]C). 1H NMR ppm (CDCl3); 0.87–0.92 (m, 3H), 1.28–1.32 (m, 24 H), 1.56 (t, 4H), 2.51 (t, 2H), 2.65 (t, 2H), 4.1 (s, 3H), 4.3 (t, 2H), 7.4 (s, 1H), 7.5 (s, 1H), 10.4 (s, 1H). 13C NMR ppm (CDCl3); 14.8, 20.8, 23.2, 25.2, 26.4, 29.2, 30.1, 32.0, 34.3, 44.1, 48.3, 118.6, 119.5, 121.4, 167.2, 172.4. Synthesis of [N88SA]−[C4Py]+ (11): The synthetic procedure for (11) was the same as that of (10), in which compound (3) was replaced by (5). Compound (11) is a liquid with 96% yield. FTIR, cm−1; 3405 (C]N), 2924 (CH2 Str.), 1620 (C]O carboxylate), 1605 (C]O amide), 1571 (C]C). 1H NMR ppm (CDCl3); 0.76 (d, 2H), 0.88–0.91 (m, 3H), 1.15–1.24 (m, 6H), 1.27–1.32 (m, 20H), 1.58 (t, 4H), 2.53 (t, 2H), 2.63 (t, 2H), 8.13 (d, 1H), 8.52 (t, 1H), 9.37 (d, 1H). 13C NMR ppm (CDCl3); 13.9, 14.8, 20.8, 23.2, 26.4, 29.2, 30.1, 31.8, 32.0, 48.3, 70.6, 126.2, 141.8, 142.2, 167.2, 172.4.
the extraction of metal ions from weak acid solution. Therefore, solution pH was varied from 3.0 to 6.0 in this work, while reaction temperature and time were fixed. Besides, the effect of adding TBP to these ILs as a modifier and a synergist was investigated. In addition, the extraction of Co(II) and Ni(II) by these TSILs was compared with that by the Bif-ILs (R4N+A−) which were synthesized by Aliquat336 (NMethyl-N, N, N-trioctylammonium chloride) and organophosphorus acids (HA) such as Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid), PC 88A (2-ethylhexyl 2-ethylhexyphospho- nic acid), and D2EHPA (di-(2-ethylhexyl)phosphoric acid). 2. Experimental 2.1. Reagents and chemicals Chemicals used for the synthesis of TSILs, such as succinic anhydride, dioctyl amine, sodium hydride, and butyl bromide were purchased from Sigma Aldrich, whereas methyl imidazolium and pyridine were obtained from DaeJung Chemicals, Korea. All chemicals were of reagent grade and used without further purification. Solvents were dried according to standard procedures and stored over molecular sieves. The synthetic solutions of Co(II) and Ni (II) were prepared by dissolving appropriate amounts of CoCl2⋅6H2O (Junsei Chemical Co., ≥97%) and NiCl2⋅6H2O (Yakuri Pure Chemical Co., Japan, ≥96%) in distilled water. The acidity of the solution was adjusted by adding HCl solution (Daejung Co., 35%). The commercial extractants such as Aliquat336 (ALiCl, BASF Co., 93%), Cyanex 272 (H-CY, Cytec Inc., 85%), PC 88A (H-PC, Cytec Inc., 95%), and D2EHPA (H-D2, Cytec Inc., 95%), TBP (Cytec Inc., 99%) were used without any purification. Organic phases were prepared by diluting synthesized ILs in kerosene. The structure of ILs are presented in Table 1.
2.3.2. Synthesis of the Bif-ILs based on the ALiquat336 (R4NCl) and organophosphorus acids (HA) The Bif-ILs were prepared by mixing an equimolar concentration of Aliquat336 and organophosphorus acids in kerosene and these mixtures were treated by NaHCO3 solution according to the method reported in the paper [21]. The synthesized ILs are represented as Aliquat336-Cyanex 272 (ALi-CY), Aliquat336-PC 88A (ALi-PC), and Aliquat336D2EHPA (ALi-D2).
2.2. Extraction procedure and analytical methods The extraction experiments were carried out by mixing equal volume of aqueous and organic phase (each 20 mL) in a screwed cap bottle for 30 min using a Burrell wrist action shaker (model 75, USA) at ambient temperature (22 ± 1 °C). The shaken solutions were allowed to stand in a glass separatory funnel for phase separation. The concentration of hydrogen ion in the aqueous phases before and after extraction was determined by Orion Star thermal scientific pH meter (model A221, USA). The structures of synthesized compounds were identified by Nuclear Magnetic Resonance Spectroscopy (NMR) and Fourier Transform Spectroscopy (FT-IR). 1H NMR (300 MHz) and 13C NMR (75 MHz) chemical shift values were reported as δ using the residual solvent signal as an internal standard. FT-IR spectra was recorded using Alpha (BRUKER) spectrophotometer. The concentration of nickel and cobalt ions in aqueous phases before and after extraction was determined by ICP-OES (Inductively coupled plasma-optical emission spectrometry, Spectro Arcos).
3. Results and discussion 3.1. FT-IR spectra of Bif-ILs In order to elucidate the structure of these Bif-ILs, FT-IR spectra of organophosphorus acids, Aliquat336 and Bif-ILs were compared. The analysis was mainly based on the change of characteristic bands in three main types of functional groups, including P]O, PeOH, and a dimeric peak of hydrogen bond. These changes occur during the interaction between ions after the synthesis of Bif-ILs. The frequencies of characteristic vibrational bands of analysts are presented in Fig. 2 and Table 2. The peaks in range from 1105 to 1292 cm−1 are ascribed to the stretching vibration of P]O bonds [22,23]. Comparison of the peaks of Bif-ILs with those of organophosphorus acids shows that the increase in intensity of P]O stretching vibration leads to the shifting of the corresponding peak to the lower wavenumbers. On the contrary, the peaks of PeO or PeOH groups of organophosphorus acids in range from 899 to 1105 cm−1 are shifted to the higher wavenumbers compared with the corresponding Bif-ILs. Moreover, the peaks approximately 2325 cm−1 in the spectra of organophosphorus acids, which are also ascribed to PeOH vibration were significantly decreased in intensity of Bif-ILs [22,23]. All of these changes in P]O, PeO and PeOH bands may be attributed to the interaction between organophosphate anions and the cations of Aliquat336.
2.3. Synthesis of the ionic liquids 2.3.1. Synthesis of the TSILs The synthetic process is presented in Fig. 1. The reaction process was observed by thin layer chromatography (TLC, silica gel 60 F-254 on aluminum plate, Merck). 1-Methyl-3-butylimidazolium bromide (3) and N-Butyl pyridinium bromide (5) were synthesized by treating 1-bromobutane (2) with corresponding 1-methyl-1H-imidazolium (1) and pyridine (4) in acetonitrile (molar ratio 1:1) under reflux. After the reactions were completed, obtained products were a light yellow color oily liquid for (3) and light brown oily liquid for (5). These products will be used for the 2
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Table 1 The structure of Ionic Liquids. ILs
Anions −
Cations
+
[N88SA] [C4mim]
3-butyl-1-methyl-1H-imidazol-3-ium, [C4min]+
[N88SA]−[C4Py]+ 4-(dioctylamino)-4-oxobutanoate, [N88SA]−
1-butylpyridin-1-ium, [C4Py]+ ALi-CY
bis(2,4,4trimethylpentyl)phosphinate ALi-PC
N-methyl-N,N-dioctyloctan-1-aminium
2-ethylhexyl(2-ethylhexyl)phosphonate ALi-D2
bis(2-ethylhexyl)phosphate
3.2. Extraction of Co(II) and Ni(II) by TSILs
The bands of dimeric peaks of hydrogen bond between molecules of organophosphorus acids were also observed at 1698.2, 1671.2, and 1686.9 cm−1 corresponding to Cyanex 272, PC 88A, and D2EHPA [22,23]. These bands were not observed in the Bif-ILs. Based on these FT-IRs, the synthesis of Bif-ILs was verified.
3.2.1. Effect of pH on metal extraction The extraction of Co(II) and Ni(II) by TSILs was investigated in the initial pH range from 3.0 to 6.0. The concentration of metal ions in the feed phase was fixed at 100 ppm. The concentration of ILs in the organic phase was controlled to 0.01 M in kerosene. Figs. 3 and 4 show that both cobalt and nickel are extracted above 65% by [N88SA]−[C4mim]+ and [N88SA]−[C4Py]+. The extraction percentage 3
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Fig. 1. Synthesis of succinimide based Task specific ionic liquids.
of metal ions increased with increasing pH values. Co(II) and Ni(II) exist as an octahedral configuration in weak hydrochloric acid solution (e.g [Co(H2O)6]2+, [Co(H2O)5Cl]+, Co(H2O)4Cl2 and [Ni(H2O)6]2+) [20]. Therefore, the extractability of the metal ions by the TSILs could be attributed to the interaction between anions of TSILs and metal cations. Apart from the extractability of metal ions, the extraction of hydrogen ions by TSILs was observed (see Table 3.). The increase in the equilibrium pH of the aqueous indicates that not only metal ions but also hydrogen ion was extracted into TSILs. According to literatures reported [24–26], the solubility of TSILs cations in aqueous phase were confirmed. Hence, the solvent extraction reaction of Co(II) and Ni(II) by these TSILs can be proposed as: + Me2aq. + 2A−B+org. = MeA2org. + 2B+aq. 2+
(1) −
−
where Me represents metal ions. A represents [N88SA] , while B+ represent [C4mim]+ and [C4Py]+, respectively. The results showed that TSILs are efficient for the extraction of Ni (II) and Co (II) from the chloride solution. However, decreasing the limpidity of aqueous phase after the extraction was observed in both extractants as pH values increased. The solubility of TSILs in water was ascribed to this phenomenon. The loss of TSILs components will be disadvantageous in solvent extraction process. Therefore, the effect of adding TBP as a modifier to the ILs will be investigated. Fig. 2. FT-IR spectra of organophosphorus acids, Aliquat336, and Bif-ILs in kerosene.
3.2.2. Effect of TBP on the extraction of metals In order to investigate the effect of the addition of TBP on the extraction of metals by TSILs, the extraction experiments were done at the same experimental conditions in the previous sections. When the 4
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Table 2 Frequencies of characteristic vibrational bands of Aliquate336, organophosphorus acids and Bif-ILs in kerosene. Extractants
P]O(cm−1)
PeOeC and PeOH (cm−1)
CeH (cm−1)
Cyanex272 D2EHPA PC88A Aliquat336 ALi-CY ALi-D2 ALi-PC
1170.9 1227.6 1197.2 – 1133.7 1216.8 1164.9
957.5 1025.2 1034.7 and 980.4 – 1023.9 1053.1, 1036.0, and 972.74 1039.1 and 972.4
2954.5, 2957.2, 2957.2, 2955.8, 2954.1, 2956.7, 2965.5,
Fig. 3. The extraction percentage of Co(II) and Ni(II) in chloride solution at various pH by 0.01 M [N88SA]−[C4mim]+ in kerosene.
[N88SA]−[C4Py]+
ALi-Cy
ALi-PC
ALi-D2
pH
7.0
6.8
8.0
7.3
7.1
2856.2 2857.3 2856.9 2854.63 2856.9 2856.8 856.6
1698.2 1686.9 1671.2 – – – –
2296.5 2326.9 2325.4 – – – –
Fig. 6. The extraction percentage of Co(II), Ni(II) in chloride solution at various pH by the mixture of 15% (v/v) TBP and 0.01 M [N88SA]−[C4Py] in kerosene.
concentration of TBP in the organic phase was controlled to 15 vol%, the aqueous phase has become clear after the extraction. Figs. 5 and 6 indicate that the addition of TBP to the TSILs led to a decrease in the extraction of Co(II) and Ni(II) compared to the TSILs without TBP. Compared to the TSILs without TBP, the addition of TBP to the TSISs increased the difference in the extraction percentage between Ni(II) and Co(II). The interaction between TBP and the ions of TSILs is responsible for this decrease in the extraction percentage of metal ions. Thus, the
Table 3 Change in the pH of aqueous phase after extraction by ILs. (The initial pH of the aqueous before extraction was fixed at 4.0). [N88SA]−[C4mim]+
and and and and and and and
PeO (cm−1)
Fig. 5. The extraction percentage of Co(II), Ni(II) in chloride solution at various pH by the mixture of 15% (v/v) TBP and 0.01 M [N88SA]−[C4mim]+ in kerosene.
Fig. 4. The extraction percentage of Co(II) and Ni(II) in chloride solution at various pH by 0.01 M [N88SA]−[C4Py]+ in kerosene.
ILs
2924.5, 2924.2, 2924.0, 2923.2, 2925.1, 2925.0, 2924.8,
POeH (cm−1)
5
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as TSILs for the extraction of metals were synthesized. The spectroscopy methods such as 1H NMR, 13C NMR, and FT-IR were used to identify their structure. The extraction behavior of Co(II) and Ni(II) and the change in equilibrium pH was compared between TSILs and Bif-ILs. The extraction of metals by TSILs occurs following the ion exchange mechanism. The results showed that the extraction by TSILs with [C4min]+ cations was better than with [C4Py]+ . Although the addition of TBP into TSILs did not lead to synergism in the extraction of Co(II) and Ni(II), the decrease in the loss of ILs components into the aqueous phase was observed. The extraction of Ni(II) was slightly higher than that of Co(II) by TSILs and the difference in the extraction percentage of Ni(II) and Co(II) became larger in the presence of TBP. It is noticeable that the extraction percentage of Co(II) and Ni(II) by TSILs was eight times higher than that of Bif-ILs. However, the extractability of hydrogen ions by Bif-ILs is better than TSILs. The extraction of hydrogen by Bif-ILs increased with the increase in pKa of organophosphorus acids. Taking into consideration that Co(II) is selectively extracted over Ni (II) from weak acid solution by most extractants, the selectivity of the TSILs for Ni(II) could be employed for the separation of Ni(II) and Co (II). Moreover, the extraction of hydrogen ions could circumvent the necessity of the saponification of organophosphorus extractants for the extraction of metal ions from weak acid solution.
Fig. 7. The extraction percentage of Co(II) and Ni(II) in chloride solution at pH 4.0 by TSILs and Bif-ILs in kerosene. The concentration of extractants were fixed 0.01 M.
CRediT authorship contribution statement
addition of TBP to the TSILs would have a favorable effect on the separation of Co(II) and Ni(II).
Thanh Tuan Tran: Data curation, Writing - original draft. Nizakat Azra: . Mudassir Iqbal: Conceptualization. Man Seung Lee: Supervision, Writing - review & editing.
3.3. Comparison of the extraction of Co(II) and Ni(II) between TSILs and Bif-ILs
Declaration of Competing Interest
In order to compare the extractability of metal ions between TSILs and Bif-ILs, the concentration of ILs was fixed at 0.01 M in kerosene. The concentration of Ni(II) and Co(II) ions in the feed phase were 100 ppm and initial pH was controlled to 4.0. Fig. 7 showed that the extraction percentage of Co(II) and Ni(II) by TSILs was higher than 80%, whereas that by Bif-ILs was below 10%. This indicates that TSILs are more efficient than Bif-ILs in extracting Co(II) and Ni(II). The solvent extraction reaction of metal ions by R4NA can be represented as [19,20,27]:
MeCl2aq + 2R 4NA org = 2R 4NCl org + MeA2org
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the article. Acknowledgements This work was supported by a grant from National Research Foundation of Korea (2018R1D1A1BO7044951). We gratefully thank the Gwangju branch of the Korea Basic Science (KBSI) for ICP data.
(2) References
where Me2+ are Co(II) and Ni(II) ions. The solvent extraction reactions indicate that the extractability of metal ions by Bif-ILs depends on the structure of their anions, which could form complexes with metal ions. The formation of stable complexes between metal ions and [N88SA]− ions is responsible for the efficient extraction of metals by TSILs in comparison with Bif-ILs. It is noticeable that the selective extraction of cobalt by ALi-Cy is higher than that by other Bif-ILs. The formation of stable complexes of Co(II) with Cyanex272 anions could be attributed to this selective extraction. The change in pH values before and after extraction is listed in Table 3. For TSILs, the pH values in raffinate solution extracted by [N88SA]−[C4mim]+ is higher than [N88SA]−[C4Py]+. This may be attributed to the difference in ion exchange between hydrogen ions and the cations of TSILs, which is responsible for the extraction of hydrogen ions. In the extraction with Bif-ILs, the order in the increase of pH values was ALi-Cy > ALi-PC > ALi-D2, corresponding to the increase of pKa values of the acids such as Cyanex272 (pKa = 6.37), PC88A (pKa = 4.51), and D2EHPA (pKa = 3.24). This indicates that the extraction of hydrogen ions by Bif-ILs depends on the base strength of organophosphate ions. Thus, the application of ILs as extractants of hydrogen ions might be considerable.
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4. Conclusion Two new succinimide ionic liquids with non-fluorine which can act 6
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[21]
[22]
[23]
[24]
[25]
[26]
[27]
7
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Synthesis of succinimide based ionic liquids and comparison of extraction behavior of Co(II) and Ni(II) with bi-functional ionic liquids synthesized by Aliquat336 and organophosphorus acids Thanh Tuan Trana, Nizakat Azrab, Mudassir Iqbalb, Man Seung Leea, a b
T
⁎
Department of Advanced Materials Science & Engineering, Institute of Rare Metal, Mokpo National University, Chonnam 534-729, Republic of Korea Department of Chemistry, School of Natural Sciences (SNS), National University of Sciences and Technology (NUST), Islamabad 44000, Pakistan
A R T I C LE I N FO
A B S T R A C T
Keywords: Succinimide ionic liquids Cobalt Nickel Extraction bi-functional ILs
Two new succinimide compounds such as 3-butyl-1-methyl-1H-imidazol-3-ium 4-(dioctylamino)-4-oxand 1-butylpyridin-1-ium 4-(dioctylamino)-4-oxobutanoate obutanoate ([N88SA]−[C4min]+) ([N88SA]−[C4Py]+) were synthesized as task-specific ionic liquids (TSILs) for the extraction of metal ions. The extraction behavior of Co(II) and Ni(II) from chloride solution was compared between these TSILs and Bif-ILs synthesized from Aliquat336 and organophosphorus acids such as Cyanex 272, PC 88A and D2EHPA. The extraction percentage of Co(II) and Ni(II) by TSILs was eight times higher than that by Bif-ILs at the initial pH of 4.0. Moreover, the extraction percentage of Ni(II) by the TSILs was slightly higher than that of Co(II). However, Bif-ILs extracted more hydrogen ions than TSILs. The extraction of Co(II) and Ni(II) by [N88SA]−[C4min]+ was higher than that by [N88SA]−[C4Py]+. The addition of TBP to TSILs reduced the extraction of metals but the loss of TSILs into the aqueous was significantly decreased. Succinimide ionic liquids can be employed for the extraction of metals from weak acidic solutions.
1. Introduction Ionic liquids (ILs) are defined as organic salts with a melting point below 100 °C [1]. Due to their low volatility, non-flammability, recyclability, and easy modification as eco-friendly alternatives to volatile organic media, ILs are employed as solvents, catalysts for organic reactions, plasticizers, and electrolytes in batteries [2–6]. In hydrometallurgy, ILs are used as solvents or extractants in recovery of metals [7–10]. The most popular ILs are cations of nitrogen, phosphor, halogenides (Cl−, Br−, I−), nitrate, tetrafluoroborate, trifuoromethanesulfonate, bis(trifluoromethylsulfonyl)imide, bistriflimide (Tf2N−), hexafluorophosphate (PF6−). ILs often acts as ion-exchangers between the aqueous and organic phase, resulting in the loss of IL components [11–13]. The ILs containing fluoride could be hydrolyzed in the aqueous media to become toxic and corrosive chemicals, such as HF, POF3, H2PO3F, and HPO2F2 [14,15]. Thus, the synthesis of new ILs which overcome the above-mentioned drawbacks is desirable. The properties of ILs such as hydrophilicity, hydrophobicity and miscibility with organic solvents or water can be modified by replacing alkyl substituents in the components of ILs or by changing anionic or
⁎
cationic components. Therefore, many ILs have been designed and synthesized for specific purposes, which can be called task-specific ionic liquids (TSILs) [16,17]. Besides, the solvating extractants like TBP (Tributyl phosphate) can act as a modifier to reduce the loss of IL components during extraction. The synthesis of ILs based on the commercial extractants such as Aliquat336 or Cyphos IL 101 (tetradecyl(trihexyl)phos- phonium chloride) has attracted interest owing to some advantages like simplicity in synthesis. The combination of Aliquat336 or Cyphos IL cations and anions containing functional groups will generate bi-functional ILs (Bif-ILs), which can act as both cationic and anionic extractants. The extraction characteristics of metal ions by Bif-ILs such as [A336][P204], [A336][P507], [A336][C272], [A336][C302], [A366][CA-12], [A336] [CA-100], Cyphos IL 104 and Cyphos IL 167 has been reported [18–20]. In this work, new succinimide ionic liquids as TSILs were synthesized for the extraction of metal ions. The components of these TSILs included imidazolium and pyridinium cations and succimide anions containing the functional groups like amide and carboxylate. The combination of these ions would increase the metal extraction. These ILs were employed for testing the extraction of Co(II) and Ni(II) from weak chloride media. In general, solution pH has a profound effect on
Corresponding author. E-mail address: [email protected] (M.S. Lee).
https://doi.org/10.1016/j.seppur.2019.116496 Received 11 September 2019; Received in revised form 24 December 2019; Accepted 29 December 2019 Available online 30 December 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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next steps for the synthesis of TSILs. Synthesis of 2-(2-(di-n-octylamino)-succinic acid (8): the mixture of succinic anhydride (6) (1 g, 0.01 mol) and dioctylamine (7) (2.41 g, 0.01 mol) in dry THF (30 mL) were stirred overnight at 50 °C. A lightyellow oily liquid was obtained with 70% yield after evaporation of solvent. FTIR, cm−1; 2932 (CH2 str), 1713 (C]O of acid), 1607 (C]O of amide), 1463 (CH2 bend). 1H NMR ppm (CDCl3); 0.88–0.92 (m, 3H), 1.28–1.32 (m, 20H), 1.57 (t, 4H), 2.52 (t, 2H), 2.76 (t, 2H). 13C NMR ppm (CDCl3); 14.8, 23.2, 26.4, 29.2, 30.1, 32.0, 48.3, 167.2, 172.4. Synthesis of [N88SA]−[C4mim]+ (10): the mixture of 1.02 g (2.9 mmol) of (8) in dry THF (30 mL) and 0.143 g of sodium hydride (1.02 equi) were stirred overnight under argon atmosphere. Subsequently, 0.655 g (2.9 mmol) of (3) was added into the reaction mixture which were stirred for 24 h. After the reaction, precipitates of sodium bromide were obtained which were filtered off; solvent was evaporated under reduced pressure to yield compound (10) in 95% yield. FTIR, cm−1; 3405.22 (C]N), 1615 (C]O of carboxylate), 2924 (CH2 Str.), 1607 (C]O of amide), 1569 (C]C). 1H NMR ppm (CDCl3); 0.87–0.92 (m, 3H), 1.28–1.32 (m, 24 H), 1.56 (t, 4H), 2.51 (t, 2H), 2.65 (t, 2H), 4.1 (s, 3H), 4.3 (t, 2H), 7.4 (s, 1H), 7.5 (s, 1H), 10.4 (s, 1H). 13C NMR ppm (CDCl3); 14.8, 20.8, 23.2, 25.2, 26.4, 29.2, 30.1, 32.0, 34.3, 44.1, 48.3, 118.6, 119.5, 121.4, 167.2, 172.4. Synthesis of [N88SA]−[C4Py]+ (11): The synthetic procedure for (11) was the same as that of (10), in which compound (3) was replaced by (5). Compound (11) is a liquid with 96% yield. FTIR, cm−1; 3405 (C]N), 2924 (CH2 Str.), 1620 (C]O carboxylate), 1605 (C]O amide), 1571 (C]C). 1H NMR ppm (CDCl3); 0.76 (d, 2H), 0.88–0.91 (m, 3H), 1.15–1.24 (m, 6H), 1.27–1.32 (m, 20H), 1.58 (t, 4H), 2.53 (t, 2H), 2.63 (t, 2H), 8.13 (d, 1H), 8.52 (t, 1H), 9.37 (d, 1H). 13C NMR ppm (CDCl3); 13.9, 14.8, 20.8, 23.2, 26.4, 29.2, 30.1, 31.8, 32.0, 48.3, 70.6, 126.2, 141.8, 142.2, 167.2, 172.4.
the extraction of metal ions from weak acid solution. Therefore, solution pH was varied from 3.0 to 6.0 in this work, while reaction temperature and time were fixed. Besides, the effect of adding TBP to these ILs as a modifier and a synergist was investigated. In addition, the extraction of Co(II) and Ni(II) by these TSILs was compared with that by the Bif-ILs (R4N+A−) which were synthesized by Aliquat336 (NMethyl-N, N, N-trioctylammonium chloride) and organophosphorus acids (HA) such as Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid), PC 88A (2-ethylhexyl 2-ethylhexyphospho- nic acid), and D2EHPA (di-(2-ethylhexyl)phosphoric acid). 2. Experimental 2.1. Reagents and chemicals Chemicals used for the synthesis of TSILs, such as succinic anhydride, dioctyl amine, sodium hydride, and butyl bromide were purchased from Sigma Aldrich, whereas methyl imidazolium and pyridine were obtained from DaeJung Chemicals, Korea. All chemicals were of reagent grade and used without further purification. Solvents were dried according to standard procedures and stored over molecular sieves. The synthetic solutions of Co(II) and Ni (II) were prepared by dissolving appropriate amounts of CoCl2⋅6H2O (Junsei Chemical Co., ≥97%) and NiCl2⋅6H2O (Yakuri Pure Chemical Co., Japan, ≥96%) in distilled water. The acidity of the solution was adjusted by adding HCl solution (Daejung Co., 35%). The commercial extractants such as Aliquat336 (ALiCl, BASF Co., 93%), Cyanex 272 (H-CY, Cytec Inc., 85%), PC 88A (H-PC, Cytec Inc., 95%), and D2EHPA (H-D2, Cytec Inc., 95%), TBP (Cytec Inc., 99%) were used without any purification. Organic phases were prepared by diluting synthesized ILs in kerosene. The structure of ILs are presented in Table 1.
2.3.2. Synthesis of the Bif-ILs based on the ALiquat336 (R4NCl) and organophosphorus acids (HA) The Bif-ILs were prepared by mixing an equimolar concentration of Aliquat336 and organophosphorus acids in kerosene and these mixtures were treated by NaHCO3 solution according to the method reported in the paper [21]. The synthesized ILs are represented as Aliquat336-Cyanex 272 (ALi-CY), Aliquat336-PC 88A (ALi-PC), and Aliquat336D2EHPA (ALi-D2).
2.2. Extraction procedure and analytical methods The extraction experiments were carried out by mixing equal volume of aqueous and organic phase (each 20 mL) in a screwed cap bottle for 30 min using a Burrell wrist action shaker (model 75, USA) at ambient temperature (22 ± 1 °C). The shaken solutions were allowed to stand in a glass separatory funnel for phase separation. The concentration of hydrogen ion in the aqueous phases before and after extraction was determined by Orion Star thermal scientific pH meter (model A221, USA). The structures of synthesized compounds were identified by Nuclear Magnetic Resonance Spectroscopy (NMR) and Fourier Transform Spectroscopy (FT-IR). 1H NMR (300 MHz) and 13C NMR (75 MHz) chemical shift values were reported as δ using the residual solvent signal as an internal standard. FT-IR spectra was recorded using Alpha (BRUKER) spectrophotometer. The concentration of nickel and cobalt ions in aqueous phases before and after extraction was determined by ICP-OES (Inductively coupled plasma-optical emission spectrometry, Spectro Arcos).
3. Results and discussion 3.1. FT-IR spectra of Bif-ILs In order to elucidate the structure of these Bif-ILs, FT-IR spectra of organophosphorus acids, Aliquat336 and Bif-ILs were compared. The analysis was mainly based on the change of characteristic bands in three main types of functional groups, including P]O, PeOH, and a dimeric peak of hydrogen bond. These changes occur during the interaction between ions after the synthesis of Bif-ILs. The frequencies of characteristic vibrational bands of analysts are presented in Fig. 2 and Table 2. The peaks in range from 1105 to 1292 cm−1 are ascribed to the stretching vibration of P]O bonds [22,23]. Comparison of the peaks of Bif-ILs with those of organophosphorus acids shows that the increase in intensity of P]O stretching vibration leads to the shifting of the corresponding peak to the lower wavenumbers. On the contrary, the peaks of PeO or PeOH groups of organophosphorus acids in range from 899 to 1105 cm−1 are shifted to the higher wavenumbers compared with the corresponding Bif-ILs. Moreover, the peaks approximately 2325 cm−1 in the spectra of organophosphorus acids, which are also ascribed to PeOH vibration were significantly decreased in intensity of Bif-ILs [22,23]. All of these changes in P]O, PeO and PeOH bands may be attributed to the interaction between organophosphate anions and the cations of Aliquat336.
2.3. Synthesis of the ionic liquids 2.3.1. Synthesis of the TSILs The synthetic process is presented in Fig. 1. The reaction process was observed by thin layer chromatography (TLC, silica gel 60 F-254 on aluminum plate, Merck). 1-Methyl-3-butylimidazolium bromide (3) and N-Butyl pyridinium bromide (5) were synthesized by treating 1-bromobutane (2) with corresponding 1-methyl-1H-imidazolium (1) and pyridine (4) in acetonitrile (molar ratio 1:1) under reflux. After the reactions were completed, obtained products were a light yellow color oily liquid for (3) and light brown oily liquid for (5). These products will be used for the 2
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Table 1 The structure of Ionic Liquids. ILs
Anions −
Cations
+
[N88SA] [C4mim]
3-butyl-1-methyl-1H-imidazol-3-ium, [C4min]+
[N88SA]−[C4Py]+ 4-(dioctylamino)-4-oxobutanoate, [N88SA]−
1-butylpyridin-1-ium, [C4Py]+ ALi-CY
bis(2,4,4trimethylpentyl)phosphinate ALi-PC
N-methyl-N,N-dioctyloctan-1-aminium
2-ethylhexyl(2-ethylhexyl)phosphonate ALi-D2
bis(2-ethylhexyl)phosphate
3.2. Extraction of Co(II) and Ni(II) by TSILs
The bands of dimeric peaks of hydrogen bond between molecules of organophosphorus acids were also observed at 1698.2, 1671.2, and 1686.9 cm−1 corresponding to Cyanex 272, PC 88A, and D2EHPA [22,23]. These bands were not observed in the Bif-ILs. Based on these FT-IRs, the synthesis of Bif-ILs was verified.
3.2.1. Effect of pH on metal extraction The extraction of Co(II) and Ni(II) by TSILs was investigated in the initial pH range from 3.0 to 6.0. The concentration of metal ions in the feed phase was fixed at 100 ppm. The concentration of ILs in the organic phase was controlled to 0.01 M in kerosene. Figs. 3 and 4 show that both cobalt and nickel are extracted above 65% by [N88SA]−[C4mim]+ and [N88SA]−[C4Py]+. The extraction percentage 3
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Fig. 1. Synthesis of succinimide based Task specific ionic liquids.
of metal ions increased with increasing pH values. Co(II) and Ni(II) exist as an octahedral configuration in weak hydrochloric acid solution (e.g [Co(H2O)6]2+, [Co(H2O)5Cl]+, Co(H2O)4Cl2 and [Ni(H2O)6]2+) [20]. Therefore, the extractability of the metal ions by the TSILs could be attributed to the interaction between anions of TSILs and metal cations. Apart from the extractability of metal ions, the extraction of hydrogen ions by TSILs was observed (see Table 3.). The increase in the equilibrium pH of the aqueous indicates that not only metal ions but also hydrogen ion was extracted into TSILs. According to literatures reported [24–26], the solubility of TSILs cations in aqueous phase were confirmed. Hence, the solvent extraction reaction of Co(II) and Ni(II) by these TSILs can be proposed as: + Me2aq. + 2A−B+org. = MeA2org. + 2B+aq. 2+
(1) −
−
where Me represents metal ions. A represents [N88SA] , while B+ represent [C4mim]+ and [C4Py]+, respectively. The results showed that TSILs are efficient for the extraction of Ni (II) and Co (II) from the chloride solution. However, decreasing the limpidity of aqueous phase after the extraction was observed in both extractants as pH values increased. The solubility of TSILs in water was ascribed to this phenomenon. The loss of TSILs components will be disadvantageous in solvent extraction process. Therefore, the effect of adding TBP as a modifier to the ILs will be investigated. Fig. 2. FT-IR spectra of organophosphorus acids, Aliquat336, and Bif-ILs in kerosene.
3.2.2. Effect of TBP on the extraction of metals In order to investigate the effect of the addition of TBP on the extraction of metals by TSILs, the extraction experiments were done at the same experimental conditions in the previous sections. When the 4
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Table 2 Frequencies of characteristic vibrational bands of Aliquate336, organophosphorus acids and Bif-ILs in kerosene. Extractants
P]O(cm−1)
PeOeC and PeOH (cm−1)
CeH (cm−1)
Cyanex272 D2EHPA PC88A Aliquat336 ALi-CY ALi-D2 ALi-PC
1170.9 1227.6 1197.2 – 1133.7 1216.8 1164.9
957.5 1025.2 1034.7 and 980.4 – 1023.9 1053.1, 1036.0, and 972.74 1039.1 and 972.4
2954.5, 2957.2, 2957.2, 2955.8, 2954.1, 2956.7, 2965.5,
Fig. 3. The extraction percentage of Co(II) and Ni(II) in chloride solution at various pH by 0.01 M [N88SA]−[C4mim]+ in kerosene.
[N88SA]−[C4Py]+
ALi-Cy
ALi-PC
ALi-D2
pH
7.0
6.8
8.0
7.3
7.1
2856.2 2857.3 2856.9 2854.63 2856.9 2856.8 856.6
1698.2 1686.9 1671.2 – – – –
2296.5 2326.9 2325.4 – – – –
Fig. 6. The extraction percentage of Co(II), Ni(II) in chloride solution at various pH by the mixture of 15% (v/v) TBP and 0.01 M [N88SA]−[C4Py] in kerosene.
concentration of TBP in the organic phase was controlled to 15 vol%, the aqueous phase has become clear after the extraction. Figs. 5 and 6 indicate that the addition of TBP to the TSILs led to a decrease in the extraction of Co(II) and Ni(II) compared to the TSILs without TBP. Compared to the TSILs without TBP, the addition of TBP to the TSISs increased the difference in the extraction percentage between Ni(II) and Co(II). The interaction between TBP and the ions of TSILs is responsible for this decrease in the extraction percentage of metal ions. Thus, the
Table 3 Change in the pH of aqueous phase after extraction by ILs. (The initial pH of the aqueous before extraction was fixed at 4.0). [N88SA]−[C4mim]+
and and and and and and and
PeO (cm−1)
Fig. 5. The extraction percentage of Co(II), Ni(II) in chloride solution at various pH by the mixture of 15% (v/v) TBP and 0.01 M [N88SA]−[C4mim]+ in kerosene.
Fig. 4. The extraction percentage of Co(II) and Ni(II) in chloride solution at various pH by 0.01 M [N88SA]−[C4Py]+ in kerosene.
ILs
2924.5, 2924.2, 2924.0, 2923.2, 2925.1, 2925.0, 2924.8,
POeH (cm−1)
5
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as TSILs for the extraction of metals were synthesized. The spectroscopy methods such as 1H NMR, 13C NMR, and FT-IR were used to identify their structure. The extraction behavior of Co(II) and Ni(II) and the change in equilibrium pH was compared between TSILs and Bif-ILs. The extraction of metals by TSILs occurs following the ion exchange mechanism. The results showed that the extraction by TSILs with [C4min]+ cations was better than with [C4Py]+ . Although the addition of TBP into TSILs did not lead to synergism in the extraction of Co(II) and Ni(II), the decrease in the loss of ILs components into the aqueous phase was observed. The extraction of Ni(II) was slightly higher than that of Co(II) by TSILs and the difference in the extraction percentage of Ni(II) and Co(II) became larger in the presence of TBP. It is noticeable that the extraction percentage of Co(II) and Ni(II) by TSILs was eight times higher than that of Bif-ILs. However, the extractability of hydrogen ions by Bif-ILs is better than TSILs. The extraction of hydrogen by Bif-ILs increased with the increase in pKa of organophosphorus acids. Taking into consideration that Co(II) is selectively extracted over Ni (II) from weak acid solution by most extractants, the selectivity of the TSILs for Ni(II) could be employed for the separation of Ni(II) and Co (II). Moreover, the extraction of hydrogen ions could circumvent the necessity of the saponification of organophosphorus extractants for the extraction of metal ions from weak acid solution.
Fig. 7. The extraction percentage of Co(II) and Ni(II) in chloride solution at pH 4.0 by TSILs and Bif-ILs in kerosene. The concentration of extractants were fixed 0.01 M.
CRediT authorship contribution statement
addition of TBP to the TSILs would have a favorable effect on the separation of Co(II) and Ni(II).
Thanh Tuan Tran: Data curation, Writing - original draft. Nizakat Azra: . Mudassir Iqbal: Conceptualization. Man Seung Lee: Supervision, Writing - review & editing.
3.3. Comparison of the extraction of Co(II) and Ni(II) between TSILs and Bif-ILs
Declaration of Competing Interest
In order to compare the extractability of metal ions between TSILs and Bif-ILs, the concentration of ILs was fixed at 0.01 M in kerosene. The concentration of Ni(II) and Co(II) ions in the feed phase were 100 ppm and initial pH was controlled to 4.0. Fig. 7 showed that the extraction percentage of Co(II) and Ni(II) by TSILs was higher than 80%, whereas that by Bif-ILs was below 10%. This indicates that TSILs are more efficient than Bif-ILs in extracting Co(II) and Ni(II). The solvent extraction reaction of metal ions by R4NA can be represented as [19,20,27]:
MeCl2aq + 2R 4NA org = 2R 4NCl org + MeA2org
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the article. Acknowledgements This work was supported by a grant from National Research Foundation of Korea (2018R1D1A1BO7044951). We gratefully thank the Gwangju branch of the Korea Basic Science (KBSI) for ICP data.
(2) References
where Me2+ are Co(II) and Ni(II) ions. The solvent extraction reactions indicate that the extractability of metal ions by Bif-ILs depends on the structure of their anions, which could form complexes with metal ions. The formation of stable complexes between metal ions and [N88SA]− ions is responsible for the efficient extraction of metals by TSILs in comparison with Bif-ILs. It is noticeable that the selective extraction of cobalt by ALi-Cy is higher than that by other Bif-ILs. The formation of stable complexes of Co(II) with Cyanex272 anions could be attributed to this selective extraction. The change in pH values before and after extraction is listed in Table 3. For TSILs, the pH values in raffinate solution extracted by [N88SA]−[C4mim]+ is higher than [N88SA]−[C4Py]+. This may be attributed to the difference in ion exchange between hydrogen ions and the cations of TSILs, which is responsible for the extraction of hydrogen ions. In the extraction with Bif-ILs, the order in the increase of pH values was ALi-Cy > ALi-PC > ALi-D2, corresponding to the increase of pKa values of the acids such as Cyanex272 (pKa = 6.37), PC88A (pKa = 4.51), and D2EHPA (pKa = 3.24). This indicates that the extraction of hydrogen ions by Bif-ILs depends on the base strength of organophosphate ions. Thus, the application of ILs as extractants of hydrogen ions might be considerable.
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4. Conclusion Two new succinimide ionic liquids with non-fluorine which can act 6
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