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Synthesis of ionic liquid using a four-compartment configuration electrodialyzer
Journal of Membrane Science 318 (2008) 1–4
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Rapid communication
Synthesis of ionic liquid using a four-compartment configuration electrodialyzer Hong Meng, Hui Li, Chunxi Li ∗ , Liangshi Li College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 18 October 2007 Received in revised form 13 February 2008 Accepted 15 February 2008 Available online 10 March 2008 Keywords: Ionic liquids Electrodialysis 1-Butyl-3-methylimidazolium tetrafluoroborate
a b s t r a c t In this paper, a novel method was proposed to synthesize ionic liquids by using a specially designed four-compartment configuration electrodialyzer. 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4 ), used as a model ionic liquid, was successfully synthesized and concentrated. The products of [BMIM]BF4 could be easily obtained by using the permselectivity of ion-exchange membranes and the migration of ions under dc electric field. The structure of the products was identified with Fourier transform spectra (FTIR) and 1 H NMR. It was noted that 92% yield ratio could be achieved while the purity was over 95% after operating 1 h under the voltage of 10 V. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) are one of the most important organic salts, which have been widely used in a number of fields. The study of the ILs has attracted much interest in recent years. However, synthesis of ILs with high purity is still a major research goal in this area. The IL is composed exclusively of ions, with the forces overwhelmingly Coulombic. The cations and anions of ILs can be varied virtually at will to change their chemical and physical properties. Generally, the cation is a bulk organic structure with low symmetry, which so far is based on ammonium, sulfonium, phosphonium, imidazolium, pyridinium, picolinium, pyrrolidinium, thiazolium, oxazolium and pyrazolium cations. 1-Butyl-3-methyl and 1-ethyl3-methylimidazolium cations are probably the most investigated structures of this class [1–3]. A two-step method was commonly used to prepare ionic liquids. First, the cation halide salts were prepared by the quarternerization of the appropriate amine [4]. Subsequently, the metathesis of a halide salt with a silver or ammonium salt was followed to generate ionic liquid and silver sediments or ammonia gas. For instance, the synthesis of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4 ) by the metathesis of a halide salt with silver salt was listed as follows: AgBF4 + [BMIM]Cl = [BMIM]BF4 + AgCl ↓
∗ Corresponding author. Tel.: +86 10 64444911; fax: +86 10 64410308. E-mail address: [email protected] (C. Li). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.02.020
In this process, the synthesis cost was relatively high due to the addition of expensive silver salts. Furthermore, the excessive silver salts were usually still remained in the system. Since the purity is essential for many solvent applications, it is necessary to remove the impurity ions before the use of ILs. However, because the distillation is not suitable to purify ionic liquids due to their nonvolatile character, the relatively high purity possible should be attained during the synthesis process. Electrodialysis (ED) is a membrane separation process based on the selective migration of aqueous ions through ion-exchange membranes as a result of an electrical driving force [5]. This technique represents one of the most important methods for desalting solutions. In this study, it was thought that the ion liquids could possibly be synthesized by using a specially designed fourcompartment electrodialyzer as the cations and anions of ILs have their own charges. It was also expected that the impurity ions could be avoided during the synthetical process [6]. For that purpose, [BMIM]BF4 was used as a model ionic liquid. A schematic diagram, which simply describes the principles of ILs synthesis using a fourcompartment electrodialyzer, is shown in Fig. 1. As shown in Fig. 1, the feed solutions of [BMIM]Cl and NaBF4 were added to compartments II and IV, respectively. Two electrodes are positioned, one on either side parallel to the membranes. Applying a voltage to the electrodes generates an electric field. It is well known that the anion-exchange membrane (AEM) permits the passage of anions only while the cation-exchange membrane (CEM) permits the passage of cations only. Therefore, BF4 − migrated through the anion-exchange membranes while [BMIM]+ ions passed through the cation-exchange membranes under the
2
H. Meng et al. / Journal of Membrane Science 318 (2008) 1–4
Fig. 1. Principles of ILs synthesis using a four-compartment electrodialyzer: A, anion-exchange membrane; C, cation-exchange membrane.
action of dc electric field. Both [BMIM]+ and BF4 − ions entered into compartment III and in turn to form new products of [BMIM]BF4 . At the same time, the by-products, NaCl, was formed in compartment I as the Na+ and Cl− was migrated from compartments IV and II under the action of electric field, respectively. The same more matrixes could be settled in an electrodialysis apparatus according to the above mentioned arrangement principle. The purpose of this study is to investigate the feasibility of synthesis of the ionic liquid by using a four-compartment electrodialysis system. De-ionized water, [BMIM]Cl solution, de-ionized water and NaBF4 solution were in series initially fed into the four compartments in the electrodialyzer, respectively. The [BMIM]BF4 is expected to be obtained by compulsorily migrate [BMIM]+ and BF4 − into a product compartment (compartment III). The structure of the products was identified with Fourier transform spectra (FTIR) and 1 H NMR. The yield ratio and purity of the products were determined by HPLC. Additionally, the variations of conductivities in each compartment over the operating time were investigated.
2. Experimental
Table 1 Properties of JCM-1 and JAM-l membranes Homogeneous ion-exchange membranes
Degree of cross-linking Thickness (mm) Water content (%) Exchange capacity mequiv./g (dry) Transport number (%) Bursting strength (MPa)
JCM-1
JAM-1
10 0.11–0.13 ≥24 ≥1.8 ≥95 ≥0.15
10 0.11–0.12 ≥20 ≥1.4 ≥88 ≥0.15
In this study, Na2 SO4 solution with a conductivity of around 5000 S/cm was used as electrode solution. Note Na2 SO4 solution was supplied to both cathode compartment and anode compartment and recycled with the same pump. [BMIM]Cl solution and NaBF4 solution (0.46 mol/l) were added into compartments II and IV, respectively. The solutions in compartments I–IV were circulated at the flow rate of 40 l/h by using four peristaltic pumps, respectively. After operation for 1 h, the electrodialyzer was turned off. The respective concentration of the [BMIM]+ , BF4 − , and Na+ , Cl− in compartments I, II, III and IV were measured. The components of the remained feed solution were also analyzed.
2.1. ED setup and materials 2.2. Analytical methods The experimental ED setup is comprised of a circulation system, a dc power supply unit, ion-exchange membranes and spacer. The dimensional sizes of the self-made electrodialyzer are 300 mm × 120 mm. The electrodialyzer has eight stacks. The thickness of each compartment is 2.7 mm. The electrode materials are titanium plated with ruthenium for the anode and stainless steel for the cathode, respectively. Four solution tanks are used for holding feed, dilute solution, concentrated solution and electrode solution, five pumps are applied to circulate these solutions; each pump has a maximum capacity of 100 l/h. Commercial cation- and anionexchange membranes, JCM-1 and JAM-l, were supplied by Beijing Huan Yu Li Da Equipment Co. Ltd. and were arranged alternately in the electrodialyzer in this study. Table 1 shows the properties of JCM-1 and JAM-l membranes. In our laboratory, [BMIM]Cl was synthesized by the reaction of 1-methylimidazole and 1-chlorobutane under argon atmosphere for 4 days at 70 ◦ C with magnetic agitation. After the reaction was completed, the product and unreacted starting materials were separated in two liquid phases. The products were washed with ethylacetate for several times. Vacuum was used for the complete removal of unreacted materials and washing liquid.
The concentrations of [BMIM]Cl and [BMIM]BF4 were detected by using high-performance liquid chromatography (SCL-10Avp, Shimadzu). Agilent Zorbax 300-SCX (4.6 mm × 150 mm, 5 m) was
Fig. 2. FTIR analysis of the products sampled from compartment II (voltage: 10 V; operating time: 1 h).
H. Meng et al. / Journal of Membrane Science 318 (2008) 1–4
3
Table 2 Operation data of four-compartment electrodialysis under the voltage of 10 V Time
V (V)
I (A)
Compartment I conductivity (S/cm)
Compartment II conductivity (S/cm)
Compartment III conductivity (S/cm)
Compartment IV conductivity (S/cm)
Electrode compartment conductivity (S/cm)
0 5 10 15 20 25 30 35 40 45 50 55 60
10 10 10 10 10 10 10 10 10 10 10 10 10
0 0.7 1.2 1.9 2.3 3.2 3.7 3.7 3.6 3.1 2.8 1.5 0.7
3.5 1020 3000 7000 9000 18,900 21,100 22,300 23,100 23,700 24,500 25,100 25,400
18,680 18,010 17,300 16,500 15,400 14,500 13,360 11,700 9,800 7,800 5,300 3,000 176
3.5 800 1100 5500 11,800 12,900 14,730 15,900 16,700 17,100 17,500 17,900 18,100
23,000 21,100 18,300 17,100 15,900 14,300 12,660 10,500 8,100 4,010 1,200 550 149
5000 5003 5011 5013 5018 5020 5024 5027 5032 5036 5040 5044 5051
used. The mobile phase was the mixture of methanol and KH2 PO4 at the ratio of 3:7. The mobile phase flow was 1.0 ml/min. The detection wavelength was 212 nm. Injection volume was 60 l and column temperature was room temperature. Conductivity was measured by SevenGo proTM conductivity meter, Metler Toledo. FTIR were obtained using a Tensor-27 spectrophotometer (Bruker, Germany). Nuclear magnetic resonance spectroscopy was performed (AVANCE+400, Bruker) to further confirm the structure of the products. 2.3. Calculation The recovery ratio (R) of ILs is defined by the following equation:
R (%) =
1 V 1 − C0 V 0 Cpl p pl p
Cfl0 Vf0
× 100
1 and C 0 are the final where R is the yield ratio of ionic liquids, Cpl pl
and initial concentrations of [BMIM]+ in product solution (compart-
Fig. 3.
1
ment III), respectively, Vp1 and Vp0 are the final and initial volume of product solution, Cfl0 and Vf0 are the initial concentration and volume of [BMIM]+ in feed solution (compartment II). 3. Results and discussion A four-compartment configuration electrodialyzer was operated to synthesize the IL of [BMIM]BF4 in this study. The variations of conductivities of each compartment over the operating time (under 10 V) are shown in Table 2. It was noted from Table 2 that the conductivities of compartment I and III increased while conductivities of compartment II and IV decreased with the proceeding of ED operation. This is because the cations and anions migrated through the cation- and anion-exchange membranes under the action of electric field, respectively. These caused the formation and concentration of NaCl and [BMIM]BF4 in compartments I and III, respectively. To verify the feasibility of synthesis process, the structure of the products sampled from compartment III was identified by FTIR
H NMR spectra (voltage: 10 V; operating time: 1 h).
4
H. Meng et al. / Journal of Membrane Science 318 (2008) 1–4
and 1 H NMR. Fig. 2 shows an example of FTIR spectra of the products after operating 1 h under the voltage of 10 V. The C H bond stretch was clearly observed in the region of 4000–2000 cm−1 . Among them, >3100 cm−1 adsorption was attributed to the typical aromatic C H bands while the peak range 3000–2700 cm−1 was due to the C H stretch of aliphatic saturated hydrocarbon. The FTIR spectra also contain a significant number of imidazole rings in the region of 1584–1465 cm−1 . A significant absorbance band BF4 − at around 1061 cm−1 was also found. Based on the FTIR analyses, it could be primarily concluded that [BMIM]BF4 was synthesized by using four-compartment ED method. In order to obtain further information about the structure of imidazolium salts, 1 H NMR analyses were carried out and the records are shown in Fig. 3. As we expected, no anomaly peak occurred in the 1 H NMR spectra. These signals further confirmed that the [BMIM]BF4 with high purity was successfully synthesized. After operating 1 h, the canary yellow ionic liquid was observed. The concentration of [BMIM]BF4 was determined as 0.40 mol/l by HPLC. The yield ratio could reach 92%. The water contained in ILs was then vaporized by vacuum distillation. After drying at 70–80 ◦ C for 8–24 h, the transparent [BMIM]BF4 was finally obtained. The purity of IL was over 95% based on the HPLC results. These findings suggest that the new ED method offers some advantages over the traditional two-step method, such as much shorter synthesis period, higher purity products, lower material costs and potentially industrial applications. For example, in the case of metathesis with NaBF4 in the second step [7], it took more than 24 h to obtain [BMIM][BF4 ]. As a comparison, in this case, the second step was completed within only 1 h. In short, the specially designed fourcompartment configuration electrodialyzer could also potentially be used in other syntheses process of aqueous ILs prepared by the two-step method.
4. Conclusions This study presents a novel approach to synthesize ionic liquid by using a four-compartment configuration electrodialyzer. It was found that the ionic liquid could be easily formed by specially designing the ED configuration. In the synthesis process of 1-butyl-3-methylimidazolium tetrafluoroborate, the FTIR and 1 H NMR spectra validated the formation of [BMIM]BF4 . Under the dc voltage of 10 V, 92% yield ratio could be achieved after operating 1 h. Moreover, the purity was over 95%. These results indicated that the synthesis of ionic liquid with ED is a feasible and promising method in the future study. Acknowledgements The project was supported by National Natural Science Foundation of China (20606001 and 20776015) and the Beijing Nova Programme (No. 2007A020). References [1] P. Wasserscheid, W. Keim, Ionic liquids—new “solutions” for transition metal catalysis, Angew. Chem. Int. Ed. 39 (21) (2000) 3772–3774. [2] R.D. Rogers, K.R. Sedden, Ionic liquids—solvents of the future, Science 302 (2003) 792–793. [3] C. Chiappe, D. Pieraccini, Ionic liquids: solvent properties and organic reactivity, J. Phys. Org. Chem. 18 (2005) 276–277. [4] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (8) (1999) 2071–2083. [5] H. Meng, C.S. Peng, S.X. Song, D.Y. Deng, Electro-regeneration mechanism of ion-exchange resins in electrodeionization, Surf. Rev. Lett. 11 (6) (2004) 599– 606. [6] L. Madzingaidzo, H. Danner, R. Braun, Process development and optimisation of lactic acid purification using electrodialysis, J. Biotechnol. 96 (2002) 223–239. [7] A. Paul, P.K. Mandal, A. Samanta, On the optical properties of the imidazolium ionic liquids, J. Phys. Chem. B 109 (18) (2005) 9148–9153.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Rapid communication
Synthesis of ionic liquid using a four-compartment configuration electrodialyzer Hong Meng, Hui Li, Chunxi Li ∗ , Liangshi Li College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 18 October 2007 Received in revised form 13 February 2008 Accepted 15 February 2008 Available online 10 March 2008 Keywords: Ionic liquids Electrodialysis 1-Butyl-3-methylimidazolium tetrafluoroborate
a b s t r a c t In this paper, a novel method was proposed to synthesize ionic liquids by using a specially designed four-compartment configuration electrodialyzer. 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4 ), used as a model ionic liquid, was successfully synthesized and concentrated. The products of [BMIM]BF4 could be easily obtained by using the permselectivity of ion-exchange membranes and the migration of ions under dc electric field. The structure of the products was identified with Fourier transform spectra (FTIR) and 1 H NMR. It was noted that 92% yield ratio could be achieved while the purity was over 95% after operating 1 h under the voltage of 10 V. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) are one of the most important organic salts, which have been widely used in a number of fields. The study of the ILs has attracted much interest in recent years. However, synthesis of ILs with high purity is still a major research goal in this area. The IL is composed exclusively of ions, with the forces overwhelmingly Coulombic. The cations and anions of ILs can be varied virtually at will to change their chemical and physical properties. Generally, the cation is a bulk organic structure with low symmetry, which so far is based on ammonium, sulfonium, phosphonium, imidazolium, pyridinium, picolinium, pyrrolidinium, thiazolium, oxazolium and pyrazolium cations. 1-Butyl-3-methyl and 1-ethyl3-methylimidazolium cations are probably the most investigated structures of this class [1–3]. A two-step method was commonly used to prepare ionic liquids. First, the cation halide salts were prepared by the quarternerization of the appropriate amine [4]. Subsequently, the metathesis of a halide salt with a silver or ammonium salt was followed to generate ionic liquid and silver sediments or ammonia gas. For instance, the synthesis of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4 ) by the metathesis of a halide salt with silver salt was listed as follows: AgBF4 + [BMIM]Cl = [BMIM]BF4 + AgCl ↓
∗ Corresponding author. Tel.: +86 10 64444911; fax: +86 10 64410308. E-mail address: [email protected] (C. Li). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.02.020
In this process, the synthesis cost was relatively high due to the addition of expensive silver salts. Furthermore, the excessive silver salts were usually still remained in the system. Since the purity is essential for many solvent applications, it is necessary to remove the impurity ions before the use of ILs. However, because the distillation is not suitable to purify ionic liquids due to their nonvolatile character, the relatively high purity possible should be attained during the synthesis process. Electrodialysis (ED) is a membrane separation process based on the selective migration of aqueous ions through ion-exchange membranes as a result of an electrical driving force [5]. This technique represents one of the most important methods for desalting solutions. In this study, it was thought that the ion liquids could possibly be synthesized by using a specially designed fourcompartment electrodialyzer as the cations and anions of ILs have their own charges. It was also expected that the impurity ions could be avoided during the synthetical process [6]. For that purpose, [BMIM]BF4 was used as a model ionic liquid. A schematic diagram, which simply describes the principles of ILs synthesis using a fourcompartment electrodialyzer, is shown in Fig. 1. As shown in Fig. 1, the feed solutions of [BMIM]Cl and NaBF4 were added to compartments II and IV, respectively. Two electrodes are positioned, one on either side parallel to the membranes. Applying a voltage to the electrodes generates an electric field. It is well known that the anion-exchange membrane (AEM) permits the passage of anions only while the cation-exchange membrane (CEM) permits the passage of cations only. Therefore, BF4 − migrated through the anion-exchange membranes while [BMIM]+ ions passed through the cation-exchange membranes under the
2
H. Meng et al. / Journal of Membrane Science 318 (2008) 1–4
Fig. 1. Principles of ILs synthesis using a four-compartment electrodialyzer: A, anion-exchange membrane; C, cation-exchange membrane.
action of dc electric field. Both [BMIM]+ and BF4 − ions entered into compartment III and in turn to form new products of [BMIM]BF4 . At the same time, the by-products, NaCl, was formed in compartment I as the Na+ and Cl− was migrated from compartments IV and II under the action of electric field, respectively. The same more matrixes could be settled in an electrodialysis apparatus according to the above mentioned arrangement principle. The purpose of this study is to investigate the feasibility of synthesis of the ionic liquid by using a four-compartment electrodialysis system. De-ionized water, [BMIM]Cl solution, de-ionized water and NaBF4 solution were in series initially fed into the four compartments in the electrodialyzer, respectively. The [BMIM]BF4 is expected to be obtained by compulsorily migrate [BMIM]+ and BF4 − into a product compartment (compartment III). The structure of the products was identified with Fourier transform spectra (FTIR) and 1 H NMR. The yield ratio and purity of the products were determined by HPLC. Additionally, the variations of conductivities in each compartment over the operating time were investigated.
2. Experimental
Table 1 Properties of JCM-1 and JAM-l membranes Homogeneous ion-exchange membranes
Degree of cross-linking Thickness (mm) Water content (%) Exchange capacity mequiv./g (dry) Transport number (%) Bursting strength (MPa)
JCM-1
JAM-1
10 0.11–0.13 ≥24 ≥1.8 ≥95 ≥0.15
10 0.11–0.12 ≥20 ≥1.4 ≥88 ≥0.15
In this study, Na2 SO4 solution with a conductivity of around 5000 S/cm was used as electrode solution. Note Na2 SO4 solution was supplied to both cathode compartment and anode compartment and recycled with the same pump. [BMIM]Cl solution and NaBF4 solution (0.46 mol/l) were added into compartments II and IV, respectively. The solutions in compartments I–IV were circulated at the flow rate of 40 l/h by using four peristaltic pumps, respectively. After operation for 1 h, the electrodialyzer was turned off. The respective concentration of the [BMIM]+ , BF4 − , and Na+ , Cl− in compartments I, II, III and IV were measured. The components of the remained feed solution were also analyzed.
2.1. ED setup and materials 2.2. Analytical methods The experimental ED setup is comprised of a circulation system, a dc power supply unit, ion-exchange membranes and spacer. The dimensional sizes of the self-made electrodialyzer are 300 mm × 120 mm. The electrodialyzer has eight stacks. The thickness of each compartment is 2.7 mm. The electrode materials are titanium plated with ruthenium for the anode and stainless steel for the cathode, respectively. Four solution tanks are used for holding feed, dilute solution, concentrated solution and electrode solution, five pumps are applied to circulate these solutions; each pump has a maximum capacity of 100 l/h. Commercial cation- and anionexchange membranes, JCM-1 and JAM-l, were supplied by Beijing Huan Yu Li Da Equipment Co. Ltd. and were arranged alternately in the electrodialyzer in this study. Table 1 shows the properties of JCM-1 and JAM-l membranes. In our laboratory, [BMIM]Cl was synthesized by the reaction of 1-methylimidazole and 1-chlorobutane under argon atmosphere for 4 days at 70 ◦ C with magnetic agitation. After the reaction was completed, the product and unreacted starting materials were separated in two liquid phases. The products were washed with ethylacetate for several times. Vacuum was used for the complete removal of unreacted materials and washing liquid.
The concentrations of [BMIM]Cl and [BMIM]BF4 were detected by using high-performance liquid chromatography (SCL-10Avp, Shimadzu). Agilent Zorbax 300-SCX (4.6 mm × 150 mm, 5 m) was
Fig. 2. FTIR analysis of the products sampled from compartment II (voltage: 10 V; operating time: 1 h).
H. Meng et al. / Journal of Membrane Science 318 (2008) 1–4
3
Table 2 Operation data of four-compartment electrodialysis under the voltage of 10 V Time
V (V)
I (A)
Compartment I conductivity (S/cm)
Compartment II conductivity (S/cm)
Compartment III conductivity (S/cm)
Compartment IV conductivity (S/cm)
Electrode compartment conductivity (S/cm)
0 5 10 15 20 25 30 35 40 45 50 55 60
10 10 10 10 10 10 10 10 10 10 10 10 10
0 0.7 1.2 1.9 2.3 3.2 3.7 3.7 3.6 3.1 2.8 1.5 0.7
3.5 1020 3000 7000 9000 18,900 21,100 22,300 23,100 23,700 24,500 25,100 25,400
18,680 18,010 17,300 16,500 15,400 14,500 13,360 11,700 9,800 7,800 5,300 3,000 176
3.5 800 1100 5500 11,800 12,900 14,730 15,900 16,700 17,100 17,500 17,900 18,100
23,000 21,100 18,300 17,100 15,900 14,300 12,660 10,500 8,100 4,010 1,200 550 149
5000 5003 5011 5013 5018 5020 5024 5027 5032 5036 5040 5044 5051
used. The mobile phase was the mixture of methanol and KH2 PO4 at the ratio of 3:7. The mobile phase flow was 1.0 ml/min. The detection wavelength was 212 nm. Injection volume was 60 l and column temperature was room temperature. Conductivity was measured by SevenGo proTM conductivity meter, Metler Toledo. FTIR were obtained using a Tensor-27 spectrophotometer (Bruker, Germany). Nuclear magnetic resonance spectroscopy was performed (AVANCE+400, Bruker) to further confirm the structure of the products. 2.3. Calculation The recovery ratio (R) of ILs is defined by the following equation:
R (%) =
1 V 1 − C0 V 0 Cpl p pl p
Cfl0 Vf0
× 100
1 and C 0 are the final where R is the yield ratio of ionic liquids, Cpl pl
and initial concentrations of [BMIM]+ in product solution (compart-
Fig. 3.
1
ment III), respectively, Vp1 and Vp0 are the final and initial volume of product solution, Cfl0 and Vf0 are the initial concentration and volume of [BMIM]+ in feed solution (compartment II). 3. Results and discussion A four-compartment configuration electrodialyzer was operated to synthesize the IL of [BMIM]BF4 in this study. The variations of conductivities of each compartment over the operating time (under 10 V) are shown in Table 2. It was noted from Table 2 that the conductivities of compartment I and III increased while conductivities of compartment II and IV decreased with the proceeding of ED operation. This is because the cations and anions migrated through the cation- and anion-exchange membranes under the action of electric field, respectively. These caused the formation and concentration of NaCl and [BMIM]BF4 in compartments I and III, respectively. To verify the feasibility of synthesis process, the structure of the products sampled from compartment III was identified by FTIR
H NMR spectra (voltage: 10 V; operating time: 1 h).
4
H. Meng et al. / Journal of Membrane Science 318 (2008) 1–4
and 1 H NMR. Fig. 2 shows an example of FTIR spectra of the products after operating 1 h under the voltage of 10 V. The C H bond stretch was clearly observed in the region of 4000–2000 cm−1 . Among them, >3100 cm−1 adsorption was attributed to the typical aromatic C H bands while the peak range 3000–2700 cm−1 was due to the C H stretch of aliphatic saturated hydrocarbon. The FTIR spectra also contain a significant number of imidazole rings in the region of 1584–1465 cm−1 . A significant absorbance band BF4 − at around 1061 cm−1 was also found. Based on the FTIR analyses, it could be primarily concluded that [BMIM]BF4 was synthesized by using four-compartment ED method. In order to obtain further information about the structure of imidazolium salts, 1 H NMR analyses were carried out and the records are shown in Fig. 3. As we expected, no anomaly peak occurred in the 1 H NMR spectra. These signals further confirmed that the [BMIM]BF4 with high purity was successfully synthesized. After operating 1 h, the canary yellow ionic liquid was observed. The concentration of [BMIM]BF4 was determined as 0.40 mol/l by HPLC. The yield ratio could reach 92%. The water contained in ILs was then vaporized by vacuum distillation. After drying at 70–80 ◦ C for 8–24 h, the transparent [BMIM]BF4 was finally obtained. The purity of IL was over 95% based on the HPLC results. These findings suggest that the new ED method offers some advantages over the traditional two-step method, such as much shorter synthesis period, higher purity products, lower material costs and potentially industrial applications. For example, in the case of metathesis with NaBF4 in the second step [7], it took more than 24 h to obtain [BMIM][BF4 ]. As a comparison, in this case, the second step was completed within only 1 h. In short, the specially designed fourcompartment configuration electrodialyzer could also potentially be used in other syntheses process of aqueous ILs prepared by the two-step method.
4. Conclusions This study presents a novel approach to synthesize ionic liquid by using a four-compartment configuration electrodialyzer. It was found that the ionic liquid could be easily formed by specially designing the ED configuration. In the synthesis process of 1-butyl-3-methylimidazolium tetrafluoroborate, the FTIR and 1 H NMR spectra validated the formation of [BMIM]BF4 . Under the dc voltage of 10 V, 92% yield ratio could be achieved after operating 1 h. Moreover, the purity was over 95%. These results indicated that the synthesis of ionic liquid with ED is a feasible and promising method in the future study. Acknowledgements The project was supported by National Natural Science Foundation of China (20606001 and 20776015) and the Beijing Nova Programme (No. 2007A020). References [1] P. Wasserscheid, W. Keim, Ionic liquids—new “solutions” for transition metal catalysis, Angew. Chem. Int. Ed. 39 (21) (2000) 3772–3774. [2] R.D. Rogers, K.R. Sedden, Ionic liquids—solvents of the future, Science 302 (2003) 792–793. [3] C. Chiappe, D. Pieraccini, Ionic liquids: solvent properties and organic reactivity, J. Phys. Org. Chem. 18 (2005) 276–277. [4] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (8) (1999) 2071–2083. [5] H. Meng, C.S. Peng, S.X. Song, D.Y. Deng, Electro-regeneration mechanism of ion-exchange resins in electrodeionization, Surf. Rev. Lett. 11 (6) (2004) 599– 606. [6] L. Madzingaidzo, H. Danner, R. Braun, Process development and optimisation of lactic acid purification using electrodialysis, J. Biotechnol. 96 (2002) 223–239. [7] A. Paul, P.K. Mandal, A. Samanta, On the optical properties of the imidazolium ionic liquids, J. Phys. Chem. B 109 (18) (2005) 9148–9153.