Letter pubs.acs.org/journal/ascecg
Preparation and Properties of CX (X: O, N, S) Based Distillable Ionic Liquids and Their Application for Rare Earth Separation Guofeng Li,† Zhimin Xue,‡ Bobo Cao,§ Chuanyu Yan,† and Tiancheng Mu*,† †
Department of Chemistry, Renmin University of China, No. 59 Zhongguanchun Street, Beijing 100872, China Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, No. 35 Qinghuadong Road, Beijing 100083, China § School of Chemistry and Chemical Engineering, Qufu Normal University, No. 57 Jingxuanxi Road, Qufu 273165, China ‡
S Supporting Information *
ABSTRACT: The negligible vapor pressure of ionic liquids prevents the separation of ionic liquids from other nonvolatile substances by distillation. Most distillable ionic liquids have been reported are protic ionic liquids, and the aprotic ionic liquids are still scarce. In this work, we designed and synthesized a series of unsaturated bond (CX; X is O, N, S) based ionic liquids and some of them could be distilled at a mild condition. Moreover, [MDMF]TfO shows a high efficiency for separation of EuCl3 from the mixture of EuCl3 and NdCl3. KEYWORDS: Distillable ionic liquids, Rare earth separation, Physicochemical properties
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INTRODUCTION
without decomposition, which broke the consensus that ILs are nonvolatile. However, the distillation of AILs requires harsh conditions, such as high vacuum and high temperature, and the distillation rate is very low. For example, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) was distilled at the rate of 0.120 g h−1 at 300 °C and 0.1 mbar.28 Chen et al. developed some novel AILs that could be distilled under a relatively mild condition (at approximate 200 °C and 0.5 mbar).29 However, developing more distillable ILs and gaining comprehensive understanding on the distillable ILs are still urgently demanded. Rare earth elements (REEs) play important roles in industry, such as luminescence, electronics, magnetism, catalysis, metallurgy, and the ceramic industry.30 However, REEs exist together in nature in some minerals, e.g., bastnasite, monazite, xenotime, oil shale, and others, thus it is difficult to separate them from each other due to their similar chemical and physical properties.31,32 Therefore, it is very important to develop efficient methods to separate the mixed REEs. In this study, a series of unsaturated bond (CX; X is O, N, S) based ILs were synthesized and characterized. The physiochemical properties such as density, viscosity, melting point, thermal stability, and conductivity of the ILs were determined. Some of the as-synthesized ILs were proved distillable and the distillation experiments were carried out on a
As the potential solvents of the future, ionic liquids (ILs) have attracted much attention during the past decades because of their unique properties such as negligible vapor pressure, nonflammability, wide electrochemical window, large liquid range and high decomposition temperature (Td).1 These exceptional properties broaden their range of applications, such as reaction solvents, functional materials, supporting electrolytes, and so on.2−5 It is noteworthy that due to their negligible vapor pressure, atmospheric pollution of ILs by evaporation is impossible. So, ILs are expected as the potential substitute for volatile organic compounds (VOCs), whose vapor is harmful to human health and the environment.6−8 However, the low vaporizability of ILs prevents the recycle and further efficient purification of ILs by distillation.9,10 Most ILs would thermally decompose at elevated temperature.11−15 For imidazolium salts, the main thermal decomposition would include the loss of an alkyl chain.16,17 For example, the main thermal decomposition products of 1-butyl3-methylimidazolium dicyanamide ([C4mim][N(CN)2]) are the neutral 1-butylimidazole and methylated dicyanamide via an SN2 reaction.18 Recently, some volatile or distillable ILs have been developed.19−26 Most of distillable ILs are the protic ILs (PILs), [BH]X ([BH]X is formed by combination of B and HX. B is molecular base, and HX is the molecular acid), which can be heated to produce B and HX and then reform [BH]X when condensed.27 Until 2006, Earle et al. discovered that aprotic ILs (AILs) could also be distilled through vaporization © XXXX American Chemical Society
Received: August 19, 2016 Revised: September 19, 2016
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DOI: 10.1021/acssuschemeng.6b01984 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering distillation apparatus.29 The purity of ILs before and after distilling was determined by nuclear magnetic resonance (NMR) spectroscopy. For a better understanding on the structures of synthesized CX based ILs, a density functional theory (DFT) calculation was performed, and Fourier transform infrared (FTIR) spectra and mass specroscopy (MS) of the distillable ILs were provided. These ILs were found very efficient in separating some REEs.
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RESULTS AND DISCUSSION The synthesized ILs (Figure 1) were combined between methyl trifluoromethanesulfonate (MeTfO) and CO, CN, and
Figure 2. Geometry parameters of ILs (methyl of MeTfO combined with amide X) calculated at B3LYP/6-311++G(d,p) level.
TfO have been fully optimized at B3LYP/6-311++G(d,p) level (Figures 2 and S1). In the case that the methyl of MeTfO is combined with amide X (Figure 2), the calculated electronic energy of [MDMF]TfO, [MDMSF]TfO, [MDBU]TfO, and [MtMG]TfO is −1250.1235, −1573.0953, −1463.4782, and −1364.2861 au, respectively. For the methyl of MeTfO combined with amide N (Figure S1), the electronic energy of the above four ILs is −1250.0992, −1573.0500, −1464.3162, and −1364.2185 au, respectively. The electronic energy of ILs in Figure S1 is higher than that in Figure 2, which indicates that the bonding type in Figure 2 is more stable than that in Figure S1. The energy variations of the reactions can be estimated by eq 1, and the results are given in Table 1. Although the interaction energy of [MDBU]TfO and [MtMG]TfO in Figure 2 is larger than that in Figure S1, the methyl of MeTfO combined with amide X (Figure 2) should also be considered as a reasonable mechanism because all the values for ΔGA and ΔHA are negative in Figure 2, which suggests that the interaction between the cations and anions is thermodynamically favorable and the formation processes of structures in Figure 2 are spontaneous. NMR, FTIR and MS further verify the structure of the ILs. The NMR and MS of [MDMF]TfO, and IR spectra of DMF, TFO and [MDMF]TfO are given in Figure S2. The absorption bands near 1665 cm−1 can be attributed to CO stretching in DMF. The absorption in 1708 cm−1 corresponds to the CN in the [MDMF]TfO. Figures S3−S14 show the NMR and IR spectra of other as-synthesized ILs, which indicate that these ILs are formed. The as-synthesized ionic liquids are hydrophobic and water stable. As shown in Figure S15, the synthesized ILs show a broad temperature range of stability. CN based ILs show the highest thermal stability, where the decomposition temperature is about 300−450 °C; CS based ILs show the Tonset within 250−350 °C and the CO based ILs show the lowest Tonset, which is lower than 200 °C. We can also find that the cation has a great influence on the thermal stability of as-synthesized ILs, which is consistent with other types of ILs.14
Figure 1. Synthesis of the distilled ILs and their structures, and abbreviations of raw materials forming ILs. The abbreviate and full name of the cations and anion of ILs are given below. [MDMF], N(methoxy)methylidene-N,N-dimethylammonium; [MDMA], N(methoxy)ethylidene-N,N-dimethylammonium; [MMHA], N(methoxy)ethylidene-N-methylammonium; [MDMP], N-(methoxy)propylidene-N,N-dimethylammonium; [MU], 2-methyl-pseudourea; [MtMG], N,N,N′,N′,N″-pentamethyl-guanidinium; [MDBN], 1-methyl-1,5-diazabicyclo[4.3.0]non-5-ene; [MDBU], 1-methyl-1,8diazabicyclo[5.4.0]undec-7-ene; [MDMSF], N-(methylthio)methylidene-N,N-dimethylammonium; [MMSU], N-(1-amino-1methylthio)methylidene-N-methylammonium; [MMMSU], N-(1methylamino-1-methylthio)methylidene-N-methylammonium; [MtriMSU], N-(1-methylamino-1-methylthio)methylidene-N,N-dimethylammonium; [MtMSU], N-(1-dimethylamino-1-methylthio)methylidene-N,N-dimethylammonium; [TfO], trifluoromethanesulfonate.
CS based amide. However, it is unclear whether the electrophilic reagents (methyl of MeTfO) combined with the amide nitrogen atom or X (O, N, S) because the amide N and X are both nucleophilic. To investigate the structures of the ILs, we consider two possible bonding types, i.e., the methyl of MeTfO is combined with amide X, which is shown in Figure 2, and the methyl of MeTfO is combined with amide N, which is shown in Figure S1. The structures of four typical ILs [MDMF]TfO, [MDMSF]TfO, [MDBU]TfO, and [MtMG]B
DOI: 10.1021/acssuschemeng.6b01984 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering Table 1. Energy Variations of the Reactions Calculated at B3LYP/6-311++G(d,p) Level (in au) species
methyl of MeTfO combined with amide X
[MDMF]TfO [MDMSF]TfO [MDBU]TfO [MtMG]TfO
ΔGA
ΔHA
ΔEB
ΔGB
ΔHB
−0.8996 −0.0189 −0.0487 −0.0541
−0.9833 −0.0013 −0.0284 −0.0344
−1.0012 −0.0183 −0.0485 −0.0539
0.0884 0.0283 −0.5609 −0.9871
0.0402 0.0476 0.4584 0.0317
0.0218 0.0278 0.5607 0.9872
was set up including an oil pump but no condensation of liquid N2 (−196 °C), which is of simple operation, low cost and more close to the normal distillation conditions compared with the references.28,29 Then, 6.0 g of [MDMF]TfO was added. When [MDMF]TfO was gradually heated in the oil bath, the solution bubbled and the mist was produced. After the mist was condensated in a receiving flask, the products were verified by NMR measurement and we found that 95.2% [MDMF]TfO was recovered and the other is DMF (Figure S2). It is consistent to the results obtained by Lee et al.29 The presence of DMF in the received product proved that the O-alkylation reaction is reversible when distilling. After adding an equivalent of CF3SO3CH3 to the residual DMF, [MDMF]TfO could be obtained. So, the distillable ILs could be recycled and reused while most other ILs could not be recycled by distilling. Besides [MDMF]TfO, the recovery rates of other distillable ILs were also studied (Table 3).
For further understanding of these ILs, important physicochemical properties, such as thermal decomposition temperature (Td) and melting point (Tm), and density (d), viscosity (η) and conductivity (κ) at room temperature were determined (Table 2). The density and viscosity of the asTable 2. Physiochemical Properties of TfO Based ILsa ILs [MDMF]TfO [MDMA]TfO [MMHA]TfO [MDMP]TfO [MU]TfO [MtMG]TfO [MDBN]TfO [MDBU]TfO [MDMSF]TfO [MMSU]TfO [MMMSU] TfO [MtriMSU] TfO [MtMSU]TfO
methyl of MeTfO combined with amide N
ΔEA
Td (°C)
Tm (°C)
169 182 198 172 201 338 427 444 328 278 268
13.9 67.6 liq. at 38.4 liq. at liq. at 71.9 liq. at 23.6 74.8 134
257 329
d (g cm−3)
η (cPa s) κ (μS cm−1)
1.4065
29.60
6727
RT
1.4054
76.31
2473
RT RT
1.5413
340.35
RT
1.3395 1.4321
48.19
278 5730
21.7
1.4063
162.49
1386
liq. at RT
1.3759
204.52
1043
834.7 936
Table 3. Recycling of the TfO Based ILs by Vacuum Distillation
a
The density, viscosity, and conductivity measurements were carried out at 25 °C. The density and viscosity at different temperatures (35, 45, 55 °C) are listed in Table S1.
synthesized ILs at different temperatures (35, 45, and 55 °C) were also measured and given in Table S1. Table 2 indicates that about half of the investigated ILs are liquid at room temperature, and generally the remaining ILs have a relatively low melting point. The densities of the ILs are similar and fall in the range from 1.3395 to 1.5413 g mL−1. Because of the restrictions of the instrument, [MDBU]TfO was too viscous to measure. It was found from Table 2 and Table S1 that the viscosities of the diamide based ILs were higher than those of single-amide ionic liquids. For example, the viscosity of [MDMF]TfO and [MDMSF]TfO is 29.60 and 48.19 cP s at 25 °C, whereas those of [MtriMSU]TfO and [MtMSU]TfO are 162.49 and 204.52 cP s at 25 °C, respectively. It may be ascribed to the relatively higher symmetry of diamide based ILs. Generally speaking, ILs with lower viscosity have the higher conductivity. Table S1 shows that the density decreases slightly with temperature, while the viscosity decreases dramatically with temperature. Figure S16 indicates that the viscosity has a great influence on the conductivity. The conductivity decreased rapidly with the increase of the viscosity in the low viscosity range while it decreased much slower when the viscosity was higher. The amide O-alkylation reaction is reversible with the increase of temperature, and the ILs can be reverted to volatile molecular precursors (amide and alkyl triflate).29 In our experiment, a simple short-path vacuum distillation apparatus
IL
recovery rate [wt %]
purity [mol %]
[MDMF]TfO [MDMA]TfO [MMHA]TfO [MDMP]TfO [MMSU]TfO [MtMSU]TfO
94.3 73.2 56.1 93.1 30.1 28.7
95.2 85.5 29.6 71.4 57.1 53.7
The distilled properties of these ILs would attract more attention and expand the application of ILs in new fields, for example, in separating the rare earth elements (REEs).33 Because of the wide applications of REEs in many hi-tech fields, such as lamp phosphors, rechargeable batteries, permanent magnets, catalysis and the ceramic industry, separation technologies are more and more important because REEs share similar properties, thus tending to exist together naturally.34 Although acidic organophosphorous,35 functional ILs,36 and amine extractants have been used for REEs separation,37 they were of high cost and difficult to be recycled. The distillable ILs are a good choice to solve these problems. Let us take the separation of the mixture of Eu3+ and Nd3+ as an example. Our experiments indicate that the solubility of pure EuCl3 in [MDMF]TfO is 5.3 g/100 g at 25 °C, whereas the NdCl3 is insoluble in [MDMF]TfO. 1 g of [MDMF]TfO was added into the mixture of EuCl3 and NdCl3 with the same quality of 50 mg in a beaker. After stirring at room temperature overnight, the phases were separated by centrifugation for 10 min and the upper liquid phase was separated by decantation. Then, [MDMF]TfO was distilled and EuCl3 was obtained. The solid phase after centrifugation is shown to be NdCl3. The mixture of EuCl3 and NdCl3 was successfully separated in this way. Because YCl3 is insoluble in [MDMF]TfO, [MDMF]TfO could also be used to separate EuCl3 and YCl3 or extract EuCl3 C
DOI: 10.1021/acssuschemeng.6b01984 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
placed at room temperature and after stirring overnight, a colorless liquid [MDMF]TfO formed (Figure 1). The characterization of the asprepared ionic liquids by NMR is given in ESI (Figures S3−S15). Computational Methods. The geometries of all structures were fully optimized using the B3LYP density functional at 6-311++G(d,p) level. The B3LYP/6-311++G(d,p) basis set, containing polarizable and diffuse functions, is considered to describe well the weak interactions in the binary systems, especially for intermolecular H-bonded interactions. The interaction energy (ΔE) was calculated at the same level, in which the counterpoise (CP) method was employed for the Gaussian basis sets’ superposition (BSSE). For the studied system, ΔE can be evaluated as the following equation:
from the mixture of EuCl3, NdCl3 and YCl3. To demonstrate this separation process of different REEs, pictures of pure [MDMF]TfO, EuCl3 dissolved in [MDMF]TfO, NdCl3 in [MDMF]TfO, and YCl3 in [MDMF]TfO are given in Figure S17. The primilary application of the distillable ILs on separation REEs seems promising. Therefore, we plan to conduct a further comprehensive investigation on the dissolution of other REEs in the distillable ILs and explore the mechanism of dissolution. These efforts could broaden the application of ILs and are important for sustainable chemistry and engineering.
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ΔE = E IL − (E(MeTfO) + E(amide))
CONCLUSIONS In conclusion, a series of unsaturated bond (CX; X is O, N, S) based ILs have been synthesized and characterized by NMR and FTIR. Then, some basic physicochemical properties including melting point, thermal stability, density, viscosity, and conductivity were investigated. Moreover, the stable structure of these ILs was supported by the density functional theory calculation. Most importantly, the distillation experiments showed that some of the ILs could be distilled at a milder condition. Therefore, they could be used in the separation and purification processes. For example, [MDMF]TfO can be used to separate EuCl3 from the mixture of EuCl3 and NdCl3. We believe that the distillation properties of AILs would contribute to the research on AILs in the future, and we will also have a further investigation on them.
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(1)
The reaction Gibbs energy (ΔGr) and the reaction enthalpy (ΔHr) were also calculated at B3LYP/6-311++G(d,p) level, which can be used to judge the feasibility of the reaction.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01984. Geometry parameters calculated at B3LYP/6-311++G(d,p) level, TGA curves and related 1H NMR, FTIR, MS, and the density and viscosity at different temperatures, of the synthesized distilled ILs (PDF)
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EXPERIMENTAL SECTION
AUTHOR INFORMATION
Corresponding Author
Materials. N-Methylacetamide (MHA) (99%) and N,N-dimethylpropionamide (DMP) (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Urea (U) (99%) was obtained from J&K Chemical. And other materials were purchased from TCI Shanghai. EuCl3 and NdCl3 were purchased from China Minmetals (Beijing) Research Institute of RC Co., Ltd. All of the materials were used without further purification. Instruments. 1H NMR spectra of the distillable ionic liquids were recorded on a Bruker AM 400 MHz spectrometers with CDCl3 as solvent. Infrared spectra of the ILs were recorded with an FTIR spectrometer (IRPrestige-21, Shimadzu Corporation, Japan) with a Pike ATR accessory (ZnSe crystal, Pike Technologies, USA). Mass spectrometry (MS) analysis experiments were carried out using an electrospray-ionization mass spectrometry (ESI-MS, LCMS-2010, Shimadzu, Japan). The melting point was measured by differential scanning calorimetry measurement (DSC 8000, PerkinElmer, UK) with the samples sealed in aluminum pans under a nitrogen atmosphere, at a flow rate of 20 mL min−1 and heating rate 10 °C min−1. The thermal stability was studied by thermal gravimetric analysis (TGA) instrument (Q 50, TA Instrument company, America) with the samples put in platinum pan under a nitrogen atmosphere, at a flow rate of 40 mL min−1 and heating rate 10 °C min−1. The density (d) was measured at 25, 35, 45, and 55 °C using an Anton Paar DMA 4500 densimeter with a precision of ±0.05 kg m−3. The viscosity (η) was obtained at 25, 35, 45, and 55 °C using an Anton Paar AMVn falling ball automated viscometer with a uncertainty lower than 0.2 mPa·s. The conductivity was measured by using a conductivity meter (FE30, Mettler-Toledo Instruments Co., Ltd., China) with a LE703 conductivity electrode, which was calibrated by 0.1 M KCl solution. The deviation of the conductivity measurement was less than ±0.5%. Each sample was measured three times and the average value was reported. Preparation of Distillable Ionic Liquids. The distillable ionic liquids were synthesized in a similar way. Here, we take [MDMF]TfO as an example to show the process. At first, 2.83 mL of CF3SO3CH3 (25 mmol) was slowly dropped into 1.93 mL of DMF (25 mmol) in a round-bottomed flask in an ice bath. Then, the reaction flask was
*. Email:
[email protected],
[email protected]. Phone: +86-10-62514925. Fax: +86-10-62516444. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21173267, 21473252). REFERENCES
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