Weakly-Basic Anion Exchange Resin Scavenges Impurities in Ionic

Aug 24, 2012 - ... groups of the weakly basic anion exchange resin (DOWEX MARATHON WBA) in aqueous solutions, releasing 1-methylimidazole (mim)...
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Weakly-Basic Anion Exchange Resin Scavenges Impurities in Ionic Liquid Synthesized from Trialkyloxonium Salt Koichiro Takao* and Taro Tsubomura Department of Materials and Life Science, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino-shi, 180-8633 Tokyo, Japan S Supporting Information *

ABSTRACT: A new purification method of ionic liquids made from trialkyloxonium salts (Meerwein reagents) has been established. 1H and 19F NMR spectroscopy clarified the presence of 1-methylimidazolium ([Hmim]+) and trifluoro(1methylimidazole)boron ([BF3(mim)]) as impurities occurring in the crude ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim]BF4). The Lewis acidic parts of these impurities, that is, H+ and BF3, were demonstrated to be able to transfer to dimethylamino groups of the weakly basic anion exchange resin (DOWEX MARATHON WBA) in aqueous solutions, releasing 1-methylimidazole (mim). The adsorption isotherms of [Hmim]BF4 and [BF3(mim)] resulted in the equilibrium constants (K) of the reactions with the resin ([Hmim]BF4: log K ≈ 1.7, [BF3(mim)]: log K = −0.17 ± 0.03), indicating that the impurities show a difference in the reactivity with the resin. The volatiles (mim and water) are possible to be removed by evaporation under reduced pressure. Although intense reddish brown color was given to the residual oil of [emim]BF4, further purification and decolorization have been achieved by using silica-gel column chromatography (eluent: acetone) and treatment with activated carbon in water. Finally, the purified [emim]BF4 was successfully obtained as an almost colorless oil. The resin after use can be regenerated simply by washing with 1 M NaOH aq and deionized water.

1. INTRODUCTION Room-temperature ionic liquids, or simply ionic liquids (ILs), consist of only molecular ionic species and possess unique properties, for example, a low melting point near to or below room temperature, negligibly low vapor pressure, moderately high ionic conductivity, and a wide electrochemical potential window. On these characters, ILs are expected to be alternative and attractive materials for various applications such as a green solvent for organic syntheses,1−5 an incombustible liquid electrolyte for batteries,6,7 and a nonvolatile room temperature medium for advanced reprocessing processes of spent nuclear fuels.8−13 The noticeable uniqueness of IL is ascribed mainly to its nonvolatility and low melting point, which are expedient for the green chemistry. However, these characters are kinds of the double-edged sword because purification of ILs through distillation and recrystallization is usually quite impractical. Actually, the physical properties of ILs having the same name are sometimes different in each literature.14,15 Although the performance often tends to be set above the purity of IL especially in the industrial applications,16 an effort to pursue the purity of ILs has to be kept for the fundamental science of this kind of new interesting materials.14,17 A major route to prepare an IL is the following two-step synthesis:6,7 (i) preparation of a halide salt of an organic cation with quaternary N, (ii) anion metathesis from halide to one desired to be introduced (e.g., BF4−, PF6−, (CF3SO2)2N−). © 2012 American Chemical Society

However, this method may result in remarkable contamination with a metal halide, which is a byproduct in the second step, unless the hydrophobicity of the prepared IL is high enough to wash the IL with water. In the former works,18,19 the use of trialkyloxonium salts (Meerwein reagent)20−22 is proposed as an alternative route to metal-halide free ILs (eq 1).

This method successfully excludes the anion metathesis step. However, the presence of a 1-methylimidazole adduct of BF3 ([BF3(mim)], Chart 1) arising from the unreacted BF3 and Chart 1

Received: May 6, 2012 Accepted: August 17, 2012 Published: August 24, 2012 2497

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Scheme 1. Prospective Conversion of [Hmim]BF4 and [BF3(mim)] on a Weakly Basic Anion Exchange Resin in a Free-Base Form (Sty-DVB-CoP: Stylene-Divinylbenzene Copolymer Framework)

the wet resin was 55.5 ± 0.3 wt %, which was estimated from the weight loss after drying for 15 h at 50 °C under vacuum. The total capacity of the wet resin was 1.81 meq·g−1, which was determined by protonation of all of the resin basic sites with 1 N HCl aq and back-titration of the separated supernatant with 1 N NaOH aq. Batch Tests for Impurity Conversion. Each batch was prepared by loading the crude [emim]BF4 (2 mL, 2.62 g) and water (2 mL) in a glass vial. The wet resins of different weights were added into the batches, followed by shaking for 24 h at 293 K. The volatiles in the filtrate were removed by evaporation at 50 °C then 90 °C under vacuum. The detection and determination of [emim]+, mim, [Hmim]+, [BF3(mim)], and BF4− in the recovered oils and collected volatiles from each batch were performed by using chemical shifts and peak integrals in the 1H and 19F NMR spectra. The data sets in the obtained isotherms should be governed by the equilibria in Scheme 1. Thus, the assumption that an active basic site on the resin can be occupied only by one acidic species is held in the current systems. Therefore, the isotherm obtained should be explained by the Langmuir-type adsorption theory23 with an expansion taking the occurrence of mim in the product part of Scheme 1 into account. For simplicity, the reactions in Scheme 1 are summarized by eq 2 (ionic charge of each species is omitted).

mim added in eq 1 has also been found as an impurity.19 Furthermore, the Meerwein reagent is highly hygroscopic, and tetrafluoroboric acid (HBF4) is readily afforded even under the presence of a trace amount of water. When the acid is formed in the reaction system, it immediately reacts with mim, resulting in [Hmim]BF4 (Chart 1). This salt bears an active proton, which may affect to the solvent property of the desired IL. As a matter of fact, we frequently met significant contamination by these impurities (ca. 10 mol % each) in the desired 1-ethyl-3methylimidazolium tetrafluoroborate ([emim]BF4, Chart 1) prepared from the commercial mim and the Meerwein reagent made from the starting materials without further purification prior to use.20 Our aim in this study is to somehow remove these impurities from [emim]BF4. As mentioned above, the isolation of [emim]BF4 through distillation or recrystallization seems not to be realistic. Taking into account that both [BF3(mim)] and [Hmim]BF4 are obviously produced by Lewis acid−base reaction, where BF3 and H+ are acids and mim is a base, another Lewis base is also able to capture these acids. If the Lewis basic functional group is fixed onto a solid phase, both impurities are decomposed, and the free mim would be released to the liquid phase. We found that this demand is able to perfectly be met by use of weakly basic anion exchange resin in a free-base form, which bears Lewis basic pendants with ternary amino groups (e.g., dimethylamino group) on the polymer framework. The conversion of the impurities is expected to follow Scheme 1. The important point in our method is use of a weakly basic anion exchange resin not to provide cationic sites for the electrostatic attraction of the negatively charged species, but to offer Lewis basic sites to destroy the Lewis acidic impurities in the IL. The feasibility of our purification method has been examined in this study.

where A and S denote an acid (H , BF3) and an adsorption site on the resin, respectively. The equilibrium constant of each reaction in Scheme 1 is defined by K = [AS][mim]/ [Amim][S]. The site occupancy θ is equal to [AS]/([AS] + [S]). Combining these formulas together, the following relationship (eq 3) can be derived.

2. EXPERIMENTAL DETAILS Materials. The crude [emim]BF4 was prepared by the procedure described in our previous publication.19 All of the starting materials including diethyl ether (super dehydrated, Wako Chemical) as a reaction solvent were of reagent grade and used without further purification. The weakly basic anion exchange resin taken in this study was DOWEX MARATHON WBA in a free-base form, which was preliminarily washed with deionized water, followed by removal of the supernatant through decantation. This cycle was repeated until no suspended particles were found in the supernatant. The washed resin was filtered off under suction and brought to the adsorption experiments aiming for purification of the crude [emim]BF4. The water content in

C init −1 Csup (3) K where Csup and Cinit are the concentrations of an impurity in the supernatant after contact with the resin (Csup = [Amim]) and at the initial state (i.e., prior to the reaction with the resin), respectively, that is, Cinit = Csup + [AS] after the equilibrium has reached. Purification of [emim]BF4 through Column Process. A conceptual artwork of this process is shown in Figure 1. Note that one should use the glass frit or glass wool to support column filling. The use of cotton plug afforded undesired dissolution of cellulose in the effluents,24−26 which deposited from the purified [emim]BF4 as water was evaporated in the final stage of this process. A column of the anion exchange resin was prepared in a glass tube (inner diameter: 19 mm) with a

(2)

Amim + S = AS + mim +

θ −1 = (1 − K −1) +

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the volume of deionized water. A column of activated carbon (19 mm diameter × 192 mm height) was flushed by this solution and washed with additional deionized water (50 mL). The effluent was passed through another activated carbon column again, followed by removal of the solvent under vacuum (rotary evaporator, then vacuum line). The purity of [emim]BF4 after this process was checked by the 1H, 19F NMR, and UV−vis absorption spectroscopy. The used DOWEX MARATHON WBA resin was regenerated by passing 1 M NaOH aq through the column, followed by washing with deionized water until the effluent showed neutral pH. The total capacity of the regenerated resin was determined by the same method described above. Spectroscopic Analysis. The NMR experiments were performed by using JEOL ECA-500 NMR spectrometer. To a 5 mm diameter NMR tube was loaded a neat liquid sample directly. As an external lock solvent, a glass capillary filled with D2O solution dissolving 0.2 wt % 3-(trimethylsilyl)propionic2,2,3,3-d4 acid sodium salt (TSP-d4) was inserted in each sample tube. In all of the 1H NMR spectra reported here, a signal from trimethylsilyl group of TSP-d4 in the capillary was used as an external reference for the chemical shift. The 19F NMR spectra were recorded versus a 19F signal of boron-11 trifluoride diethyl etherate (11BF3Et2O) external reference. A UV−vis absorption spectrum of [emim]BF4 finally obtained after the purification through the column process was recorded by Agilent 8453 photodiode array spectrophotometer.

Figure 1. Purification of crude [emim]BF4 through a column process. Columns: weakly basic anion exchange resin (A, DOWEX MARATHON WBA), silica gel (B, 200 mesh), and activated carbon (C, granular, ca. 1 mm). (i) Preliminary filtration, (ii) removal of volatiles by evaporation (water, mim), (iii) dilution with acetone, (iv) elution with acetone, (v) removal of acetone by evaporation, (vi) dilution with deionized water, (vii) elution with deionized water, and (viii) removal of water by evaporation.

PTFE stopcock. The initial height of the resin column was 210 mm, corresponding to 24.4 g of the wet resin. The crude [emim]BF4 (37.5 g) diluted by the deionized water (130 mL) was stored at 2 °C in a refrigerator overnight. After filtering the white crystalline precipitate of [BF3(mim)] off, the filtrate was introduced to the column, followed by washing with additional deionized water (50 mL). Both effluents were collected in a round-bottom flask. The solvent and volatiles in this mixture were removed by evaporation at 50 °C under vacuum with a rotary evaporator. The residual volatiles were further evaporated in a vacuum line at 50 °C. The vapor of this volatile was condensed in a liquid nitrogen trap. The reddish brown oil in the initial flask was diluted with twice volume of acetone and then passed through a silica gel column (29 mm diameter × 203 mm height, eluent: acetone). The first grayish band was collected, followed by removal of the solvent with the rotary evaporator. The residual yellow oil was dissolved in twice

3. RESULTS AND DISCUSSION Identification of Impurities Occurring in [emim]BF4. The impurities in the crude [emim]BF4 were detected and identified by means of 1H NMR spectroscopy. The obtained spectrum is shown in Figure 2. The 1H NMR signals of [emim]+ were observed at 1.20 (t, 3H, N−CH2CH3), 3.67 (s, 3H, N−CH3), 3.97 (quartet, 2H, N−CH2CH3), 7.23 and 7.30 (t, 2 × 1H, 4-/5-H), and 8.36 ppm (s, 1H, 2-H). The signals at 3.76 ppm, 7.02 ppm, 7.07 ppm, and 8.05 ppm showing the 3:1:1:1 peak integral ratio are indicative of the presence of the impurity [BF3(mim)] in the prepared IL. All of the abovementioned assignments for the 1H signals are consistent with our former publication.19 Furthermore, additional signals at 3.68, 7.28, 8.40, and 11.39 were found in this sample. The

Figure 2. 1H NMR spectrum of crude [emim]BF4. Chemical shift range: 0.9 ppm to 4.2 ppm (a), 6.5 ppm to 12.0 ppm (b). The dashed line is identical to the solid one but magnified to clarify the presence of impurities. *Spinning sideband of [emim]+. 2499

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Figure 3. 19F NMR spectrum of crude [emim]BF4. The dashed line is identical to the solid one but magnified to clarify the presence of impurities.

remarkable downfield shift of the peak at 11.39 ppm suggests that the electron density around this proton is considerably low. The most reliable species attributable to these signals should be [Hmim]+, which arises from contamination of moisture into the reaction between the Meerwein reagent and mim (eq 1) as described in the Introduction. This is supported by the appearance of other 1H signals at 3.68 (N−CH3), 7.28 (4- or 5H), and 8.40 ppm (2-H), similar to [emim]+. The shoulders at 3.68 ppm and 7.28 ppm superpose on the much larger signals of N−CH3 and 4-/5-H of [emim]+ at 3.67 ppm and 7.30 ppm, respectively, disturbing the calculation of their individual peak integrals. The remaining peaks at 8.40 ppm and 11.39 ppm exhibit the almost identical peak areas. There are five nonequivalent protons in [Hmim]+, while one of 4-H or 5-H is missing. Taking the similarity with [emim]+ in the molecular structure and charge distribution into account, the undetected 1 H signal of 4- or 5-H would be hidden in the much more intense peak of [emim]+ in this region. The difference in the peak integrals of the 4-/5-H signals of [emim]+ is equal to the sum of 4-/5-H of [Hmim]+, implying the superposition of these signals. In accordance with the 2-H peak integrals, mole fractions of the 1H NMR detectable species in the crude [emim]BF4 of Figure 2 were evaluated as [emim]+:[Hmim]+: [BF3(mim)] = 0.879:0.094:0.027 (these values depend on the synthesizing lot). Tetrafluoroborate and related species in the crude [emim]BF4 were identified by using 19F NMR spectroscopy. The recorded spectrum is displayed in Figure 3. The signals of 11 BF4− and 10BF4− appear at 2.64 ppm and 2.69 ppm (isotopic shift: 0.05 ppm).27 The peak integral ratio between these signals is in line with the natural abundance ratio of boron isotopes (10B:11B = 0.199:0.801). The 19F multiplet signal around 4.9 ppm is attributable to [BF3(mim)] because of agreement in the peak integral ratio with the 1H NMR data. Another set of 19F signals was found at 4.46 ppm and 4.52 ppm, which seems to exhibit the peak integral ratio similar to 11BF4− and 10BF4− described above. Although no reliable assignments for them are available for the moment, this unknown species eventually disappeared through the purification process discussed later. The mole fractions of the 19F NMR detectable species in the crude [emim]BF4 were estimated as BF4−: [BF3(mim)]:(unknown) = 0.949:0.021:0.030. Batch Tests for Impurity Conversion. In Figure 4 the mole fractions of [Hmim]BF4 and [BF3(mim)] were plotted as a function of weight ratio of the wet resin versus crude [emim]BF4. From this figure, it is confirmed that both impurities are able to be adsorbed onto the resin. Regarding selectivity, the adsorption of [Hmim]+ was clarified to be more preferred than [BF3(mim)]. This is reasonable in the viewpoint of steric demand on the ternary nitrogen atom offering its lone pair to the acids; that is, the abstraction of BF3 affords much more steric hindrance on the ternary nitrogen atom compared

Figure 4. Mole fractions of [Hmim]BF4 (solid) and [BF3(mim)] (open) as a function of weight ratio between wet DOWEX MARATHON WBA resin and crude [emim]BF4.

to H+. Furthermore, presence of mim in the volatiles collected from the liquid phase in each batch was also confirmed by 1H NMR, indicating that the conversion of the impurities depicted in Scheme 1 certainly proceeds as expected. In the left panel of Figure 5, the reciprocal fraction of the active resin sites

Figure 5. Adsorption isotherms of impurities (left: [BF3(mim)], right: [Hmim]BF4) captured onto wet DOWEX MARATHON WBA resin. Points in which an impurity was not detected in the supernatant of Figure 4 were omitted. Cinit: 0.280 M for [BF3(mim)], 0.332 M for [Hmim]BF4.

occupied by BF3, θ−1, is plotted against the reciprocal concentrations of [BF3(mim)] in the liquid phase, Csup−1. The least-squares fit of eq 3 to the plots in Figure 5 afforded log K = −0.17 ± 0.03. The similar analysis has been performed for [Hmim]BF4 (right panel of Figure 5), where points in which this species was not found in the supernatant were omitted. Although the plot for [Hmim]BF4 involves significant 2500

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NMR spectra. However, its peak intensity is as low as spinning side bands of the signals arising from [emim]BF4. This oil diluted by deionized water was passed through the activated carbon column, followed by evaporation of all of the volatiles in the effluent. Finally, the purified [emim]BF4 was obtained as an almost colorless oil (16.0 g, 47 % yield). The 1H NMR spectrum shows characteristic signals of [emim]BF4 at 1.22 (t, 3H, N−CH2CH3), 3.68 (s, 3H, N−CH3), 3.99 (quartet, 2H, NCH2CH3), 7.26 and 7.33 (t, 2 × 1H, 4-, 5-H), 8.38 (s, 1H, 2H). The contamination by [BF3(mim)] was still suggested by a signal at 8.07 ppm (2-H), of which the peak integral was, however, less than the spinning sideband of the 2-H signal arising from [emim]BF4. It is also noteworthy that none was indicative of the presence of [Hmim]+. The 19F NMR result is in line with 1H. Thus, 11BF4− and 10BF4− provide the signals at 2.43 ppm and 2.48 ppm, respectively. The slight upfield shifts of these signals by 0.21 ppm from the crude state may arise from elimination of the impurities. Furthermore, the 19F signal of [BF3(mim)] was also detected around 4.9 ppm. At a viewpoint of the peak integrals, both 1H and 19F NMR data are consistent with each other. From the 1H NMR peak integrals, the purity of [emim]BF4 and fraction of the contaminant [BF3(mim)] were determined as 99.8 mol % and 0.2 mol %, respectively. Figure S1 in the Supporting Information is a UV− vis absorption spectrum of the purified [emim]BF4, which exhibits no significant absorption at the wavelengths longer than 300 nm. Regarding regeneration of the weakly basic anion exchange resin after the purification of [emim]BF4, the conventional method could be utilized. The H+-adsorbing sites would be readily deprotonated by a basic regenerant. In contrast, the removal of BF3 from its adsorbing sites might be more difficult because of the coordination bond between B and N. However, the hydrolysis of BF3, which tends to be more promoted in a basic solution, may facilitate the regeneration of these sites. The DOWEX MARATHON WBA resin column after the abovementioned process was washed by a coflowing 1 M NaOH aq regenerant and deionized water. The column height decreased with elapse of the regeneration and finally returned to the initial one (210 mm). This implies that the regeneration is successfully done. As a matter of fact, the total capacity of the wet resin after this regeneration has fallen into 1.84 meq·g−1 (cf. 1.81 meq·g−1 at the initial state), which reveals entire recovery of the basic sites on the resin.

uncertainty because of the small number of the data points, the estimated log K for this impurity is ca. 1.7. The difference between these values is also indicative of the stronger adsorption of [Hmim]BF4 than [BF3(mim)]. The removal of [BF3(mim)] (mole fraction at initial state: 0.082) with the DOWEX MARATHON WBA resin was not completely achieved in Figure 4. In accordance with the rough extrapolation from this figure, approximately twice weight of the wet resin would be necessary to convert all this impurity. This is not practical in the actual experiment. On the other hand, we have found that significant amount of [BF3(mim)] deposits from the mixture of the crude [emim]BF4 and excess volume of water, i.e., [BF3(mim)] is insoluble in water. After filtration and removal of volatiles in the supernatant, the amount of [BF3(mim)] in [emim]BF4 significantly decreased. Therefore, the [BF3(mim)] contamination in the crude [emim]BF4 can be preliminarily reduced by addition of excess water and filtration, and then its residual can be further scavenged by the reaction with the weakly basic anion exchange resin. Refrigeration of the mixture lowers the solubility of [BF3(mim)], which more facilitates the separation of this contaminant (see Experimental Details and vide infra). The residual oil obtained after removal of the volatiles in the supernatant of each batch was considerably colored in reddish brown. The source of the color is still uncertain, because no signals except for [emim]BF4 and the known impurities were detected in the NMR spectra. Such colorization is frequently observed in the preparation of ILs and could arise from mim resulting from Scheme 1. Thin-layer chromatography tests clarified that most of the colored substances is held onto the silica-gel surface (Rf < 0.1), when acetone is taken as a mobile phase. In contrast, [emim]BF4 efficiently follows the eluent flow (Rf = 0.5 to 0.6). Therefore, the separation of [emim]BF4 from the colored substances (but not all) is facilitated by silicagel column chromatography successive to the treatment with the anion exchange resin. The residual colored impurities may be further removed upon contact with activated carbon.19 Purification of [emim]BF4 through Column Process. By combining the above-mentioned findings and conventional methods for purification of substances, we established a methodology for purification of crude [emim]BF4 prepared by the Meerwein reagent (Figure 1). The crude [emim]BF4 (37.5 g) was loaded to this process. The preliminary removal of 3.3 g of [BF3(mim)] as crystalline precipitate from the crude [emim]BF4 via filtration accounted for 82 % of this contaminant’s initial presence. The composition of the crude [emim]BF4 at this stage was 87.9 mol % [emim]BF4 (89.7 wt %), 9.4 mol % [Hmim]BF4 (8.2 wt %), and 2.7 mol % [BF3(mim)] (2.1 wt %), which were determined by 1H NMR peak integrals. After the first column of the DOWEX MARATHON WBA resin, mim was detected in the volatiles of the effluent by 1H NMR, indicating the conversion of the impurities to mim through the column as shown in Scheme 1. As the crude [emim]BF4 solution passed through the resin, the column height increased by 18 mm (8.6 %), implying the desired adsorption of the impurities. The residual oil was significantly colored as observed in the batch experiments. The second column, silica-gel/acetone, retained the colored materials at several centimeters from the top and successfully released a grayish band of [emim]BF4. Although the residual oil after evaporation of volatiles was still yellow, no [Hmim]BF4 appeared in its 1H NMR spectrum. The presence of [BF3(mim)] in this oil was still detected in both 1H and 19F

4. CONCLUSION In this study, we established a new purification method of ionic liquids made from the Meerwein reagent. The preliminary NMR experiments revealed that the major impurities present in the crude [emim]BF4 were 1-methylimidazole (mim) adducts of H+ and BF3 as expected from the reaction scheme to prepare this ionic liquid. The capability of a weakly basic anion exchange resin (DOWEX MARATHON WBA) on the conversion of the impurities was examined in a batch method. As a result, both Hmim+ and BF3(mim) could be converted to mim on the resin surface, although a difference in reactivity of the impurities was also found. On the basis of these findings, the practical column process was proposed, and its efficiency was demonstrated. Through this process, the purified [emim]BF4 was successfully obtained as an almost colorless oil. The feasibility of regeneration of the used resin was also confirmed. 2501

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(14) Poole, C. F. Chromatographic and spectroscopic methods for the determination of solvent properties of room temperature ionic liquids. J. Chromatogr., A 2004, 1037, 49−82. (15) Seddon, K. R.; Stark, A.; Torres, M.-J. Influence of chloride, water, and organic solvents on the physical properties of ionic liquids. Pure Appl. Chem. 2000, 72, 2275−2287. (16) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; WileyVCH: Darmstadt, 2008. (17) Ren, S.; Hou, Y.; Wu, W.; Liu, W. Purification of Ionic Liquids: Sweeping Solvents by Nitrogen. J. Chem. Eng. Data 2010, 55, 5074− 5077. (18) Egashira, M.; Yamamoto, Y.; Fukutake, T.; Yoshimoto, N.; Morita, M. A Novel Method for Preparation of Imidazolium Tetrafluoroborate Ionic Liquids. J. Fluorine Chem. 2006, 127, 1261− 1264. (19) Takao, K.; Ikeda, Y. Alternative Route to Metal Halide Free Ionic Liquids. Chem. Lett. 2008, 37, 682−683. (20) Meerwein, H. Triethyloxonium Fluoborate. Org. Synth. 1973, Coll. Vol. 5, 1080−1082. (21) Meerwein, H. Trimethyloxonium Fluoborate. Org. Synth. 1973, Coll. Vol. 5, 1096−1097. (22) Curphey, T. J. Trimethyloxonium Tetrafluoroborate. Org. Synth. 1988, Coll. Vol. 6, 1019−1023. (23) Sing, K. S. W. Reporting Physisorption Data for Gas/Solid Systems with Special Reference to the Determination of Surface Area and Porosity. Pure Appl. Chem. 1985, 57, 603−619. (24) Ohno, H.; Fukaya, Y. Task Specific Ionic Liquids for Cellulose Technology. Chem. Lett. 2009, 38, 2−7. (25) Rao, C. J.; Venkatesan, K. A.; Nagarajan, K.; Srinivasan, T. G.; Rao, P. R. V. Treatment of Tissue Paper Containing Radioactive Waste and Electrochemical Recovery of Valuables Using Ionic Liquids. Electrochim. Acta 2007, 53, 1911−1919. (26) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. Dissolution of Cellose with Ionic Liquids. J. Am. Chem. Soc. 2002, 124, 4974−4975. (27) Harris, R. K.; Mann, B. E. NMR and the Periodic Table; Academic Press, Inc.: London, U.K., 1978.

ASSOCIATED CONTENT

S Supporting Information *

UV−vis absorption spectrum of purified [emim]BF4. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was supported by JSPS KAKENHI Grant No. 24750074, Grant-in-Aid for Young Scientists (B). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Miss Yurina Tone, Mr. Shota Sawamura, Mr. Masataka Inoue, and Mr. Shohei Inoue for their technical assistance.

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REFERENCES

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dx.doi.org/10.1021/je3005003 | J. Chem. Eng. Data 2012, 57, 2497−2502