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Detrimental effect of glass sample tubes on investigations of BF4−-based room temperature ionic liquid–water mixtures
Journal of Molecular Liquids 219 (2016) 493–496
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Detrimental effect of glass sample tubes on investigations of BF− 4 -based room temperature ionic liquid–water mixtures Koji Saihara a, Yukihiro Yoshimura b,⁎, Hideyuki Fujimoto a, Akio Shimizu a,⁎ a b
Graduate School of Environmental Engineering for Symbiosis, Soka University, 1-236 Tangi-Machi, Hachioji, Tokyo 192-8577, Japan Department of Applied Chemistry, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
a r t i c l e
i n f o
Article history: Received 19 January 2016 Received in revised form 7 March 2016 Accepted 12 March 2016 Available online xxxx Keywords: NMR spectroscopy Hydrolysis Quaternary ammonium ionic liquid Glass sample tube Plastic sample tube
a b s t r a c t Tetrafluoroborate (BF− 4 )-based ILs are susceptible to hydrolysis due to their hygroscopicity, which complicates their applications. The progress of tetrafluoroborate hydrolysis in a 2 mol% N,N-diethyl-N-methyl-N-(2methoxyethyl)ammonium tetrafluoroborate–water mixture was investigated by 19F and 11B NMR in both glass and plastic sample tubes. In the plastic (polypropylene) tube, hydrolysis reached equilibrium after one day, and the intrinsic hydrolysis was estimated to be 4% relative to the initial amount of BF− 4 . In contrast, in the glass tube, hydrolysis continued even 15 days after the initial sample preparation due to reaction between glass and HF produced by the hydrolysis of BF− 4 . Additionally, we found that the dissolution of glass by HF proceeds slowly enough at 4 °C that it can be ignored for approximately 1 day. Therefore, experiments using BF− 4 based ionic liquids can be carried out in glass tube at low temperatures of around 4 °C, provided that they are performed quickly following sample preparation, or in plastic tube. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Room-temperature molten salts, commonly known as ionic liquids (ILs), are composed of only bulky cations and compact anions. In recent years, ILs have received a great deal of attention as new and unique materials due to their valuable properties such as non-volatility, noncombustibility, high thermal stability, and ionic conductivity [1,2]. Their physical properties can be designed/tuned through careful selection of the anion/cation combination. However, despite their advantages, it is also necessary to pay attention to the shelf stability of hydrophilic ILs due to their hygroscopicity. In a preliminary study, Freire et al. [3] reported on the hydrolysis of tetrafluoroborate (BF− 4 ) and hexafluorophosphate in imidazoliumbased ILs, and warned about the production of toxic HF. Subsequently, we reported on the hydrolysis of the quaternary ammonium IL N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (often written [DEME][BF4]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) in water over the entire IL concentration range (0–100 mol%); the hydrolysis of the BF− 4 anion was found to proceed rapidly above a water concentration of 80 mol% [4,5]. NMR spectroscopy has remained a powerful technique for obtaining information regarding substances on the atomic (or molecular) level [6]. For example, the static structures of gases, liquids, and solids can be determined from the chemical shift, area intensity, and through multi-dimensional NMR measurements. Therefore, NMR spectroscopy ⁎ Corresponding authors. E-mail addresses: [email protected] (Y. Yoshimura), [email protected] (A. Shimizu).
http://dx.doi.org/10.1016/j.molliq.2016.03.036 0167-7322/© 2016 Elsevier B.V. All rights reserved.
is a natural candidate for investigating the mechanisms of hydrolysis reaction taking place in ILs. NMR measurements are typically performed with a glass sample tube; however, this is problematic for NMR studies − involving BF− 4 -based ILs, as there is a risk of HF generated by BF4 hydrolysis dissolving the glass tube, which would clearly prevent good quality and straightforward NMR spectra from being obtained. With the hope of finding a solution to this problem, we decided to investigate the progress of BF− 4 hydrolysis in a 2 mol% [DEME][BF4]– water mixture using two different sample tube materials: glass and plastic. By comparing the results, our aim was to isolate the effect of the generated HF on the NMR spectra from that corresponding to the actual BF− 4 hydrolysis. 2. Experimental section N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate: [DEME][BF4] (Fig. 1) was purchased from Kanto Chemical Co., Ltd. and used without further purification, and ultra-pure water was obtained from a Synergy UV water purification system (Millipore Inc.). A 2 mol% [DEME][BF4]–water mixture was prepared after weighing the respective components. 19 F and 11B NMR measurements were performed with a ECA-500 (JEOL) spectrometer under the following conditions: 25.0 °C; 470.6 MHz (19F) or 160.5 MHz (11B) nuclear resonance frequency; 140 ppm (19F) or 245 ppm (11B) spectral width; 4 scans; and 0.0021 ppm = 1.01 Hz (19F) or 0.0038 ppm = 0.612 Hz (11B) digital resolution. NMR spectra were obtained using both glass and plastic (polypropylene; hereafter, referred to as PP) sample tubes. Additionally, a double sample tube method
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K. Saihara et al. / Journal of Molecular Liquids 219 (2016) 493–496
Fig. 1. Molecular structure of N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate ([DEME][BF4]).
was employed to prevent contamination of the samples (Fig. 2). For the measurements carried out using glass, a glass inner tube (SP-001, OD = 3.0 mm, Shigemi) containing D2O lock solvent with a chemical shift standard (19F: 5 v/v% trifluoroacetic acid (TFA) = −76.55 ppm [7] and 11B: 0.777 M boric acid = 19.49 ppm [8]) was inserted into a glass outer tube (PS-005, OD = 4.965 mm, Shigemi) containing the sample. For the measurements carried out using plastic, a PP inner tube (NE362-P-5, OD = 4.1 mm, New Era Enterprises Inc.) containing the sample was inserted into a glass outer tube (PS-005) containing the D2O lock solvent. Note that for the PP sample tube system, we did not use TFA because PP is incompatible with concentrated acetic acid. Instead of using a chemical shift standard, the BF− 4 peak was used as a reference by assuming that the chemical shift of BF− 4 has the same value in both sample tubes. The samples were stored in their respective NMR sample tubes in a refrigerator at 4 °C or in an incubator (SLI-220 Tokyo Rikakikai Co., Ltd.) at 36 °C for up to 15 days. After storing at respective conditions, all NMR spectra were obtained at 25 °C. 3. Results and discussion 3.1. Hydrolysis in the plastic and glass NMR sample tubes Typical 19F and 11B NMR spectra of the 2 mol% [DEME][BF4]–water mixture recorded using each sample tube are shown in Fig. 3. Immediately after the sample preparation, two 19F peaks were observed in the spectra around −150 ppm (data not shown). The ratio of the peak intensities was 1:4, which is in accordance with the natural abundance of the boron isotopes 10B and 11B [9,10], and the chemical shift is in 19 F spectra, in addiagreement with the reported value for BF− 4 . For the peak, a quartet at −144 ppm and a singlet at −158 ppm tion to the BF− 4 were observed in the case of the PP tube, whereas a quartet at − 144 ppm and a singlet at − 130 ppm were observed in the case of the glass tube (10 days after the sample preparation, Fig. 3(A)). The
quartet at −144 ppm in both cases was assigned to BF3OH− based on the chemical shift values and peak shapes [12]. Therefore, we can con− clude that BF− 4 decomposes (hydrolyzes) into BF3OH in both types of tubes. There have been some reports [3,11,12] of HF formation from 19 F peak we observed the hydrolysis of BF− 4 in water. To confirm that the here originates from HF, immediately after sample preparation, we measured the spectra of the [DEME][BF4]–water mixture with added HF and of an HF aqueous solution in glass tube, and found that both spectra exhibited an HF peak at −163 ppm. Additionally, the peak intensity observed at −163 ppm increased with increasing HF concentration. We therefore conclude that this peak originates from HF in the glass tube. Interestingly, although the hydrolysis of BF− 4 is also expected to take place, no signal originating from HF was observed from the sample in the glass tube at 10 days after the sample preparation (Fig. 3(A)). However, signal originating from HF (− 157 ppm) was observed in the PP tube. ðIn PP sample tubeÞ
In H2 O
BF4− → BF3 OH− þ F− ðHFÞ
ð1Þ
In H2 O
ðIn glass sample tubeÞ BF4− → B F3 OH− þ F− ðHFÞ ↓ 6HF þ SiO2 →H2 SiF6 þ 2H2 O
ð2Þ HF is known to dissolve glass. Therefore, the most likely reactions (outlined below) that were expected to take place in each of the glass NMR sample tubes were the dissolution of SiO2, the main constituent of glass, by HF, followed by the formation of H2SiF6 and H2O via the reduction of HF. Note that HF should have been generated in the glass sample tube, but no HF peak was observed in the 19F NMR spectrum, most likely due to the high rate of partial dissolution of the glass. The transparency of glass tube did not changed visually even 15 days later after the sample preparation, though three–four months later the wall of the glass tube became slightly opaque by reacting with HF. The chemical shift [8]. The peak observed at − 130 ppm was therefore assigned to SiF2− 6 originating from HF in the PP tube (− 157 ppm) was observed at a slightly lower magnetic field than that observed in the glass tube (− 163 ppm). This may be due to the difference in the interaction between HF and the two types of NMR sample tubes. In this study, two 11B NMR peaks were observed. In the early stages of the reaction, a single peak was observed both for the glass and plastic sample tubes; however, after 11 days an additional peak was observed (Fig. 3(B)). On the other hand, four 19F NMR peaks were observed, − (− 143 ppm), which were assigned to BF− 4 (− 150 ppm), BF3OH 2− − SiF6 (−130 ppm), and F (−157 ppm). Among these, only BF− 4 and BF3OH− can be assigned to the two observed 11B peaks. In fact, the chemical shifts and shapes of the two observed peaks are in agreement − with the 11B NMR peaks reported for BF− 4 and BF3OH [9,10]. The key result that we would like to stress is that the intrinsic hydrolysis reaction for BF− 4 can be precisely traced when using PP tube for the NMR measurements, while if glass tube are employed, the dissolution of glass by HF complicates the overall reaction scheme. 3.2. Time course analysis of BF− 4 hydrolysis in plastic and glass sample tubes
Fig. 2. The two types of NMR sample tubes used in this study: plastic tube (left) and glass tube (right).
Next, we calculated the amounts of the various decomposition prod2− − ucts from the respective 19F NMR peak intensities (BF− 4 , BF3OH , SiF6 , − F ) over the course of 15 days in both types of sample tubes. For the spectra acquired using the glass tube, the F− peak did not grow in considerably with time at either 36 °C (Fig. 4(B)) and 4 °C (Fig. 4(C)), even peaks were clearly obafter 15 days. However, both BF3OH− and SiF2− 6 served even at 4 °C, which indicates that the hydrolysis of BF− 4 proceeds at this lower temperature. Based on the results, it appears that the reaction in the 2 mol% [DEME][BF4]–water mixture at 36 °C reached
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495
Fig. 3. Typical 19F (A: day 10) and 11B (B: day 11) NMR spectra of 2 mol% [DEME][BF4]–water mixture stored at 36 °C using glass (upper) and plastic (lower) NMR sample tubes. NMR spectra were obtained at 25 °C. Note that only the region of the spectra relevant to the discussion is shown.
equilibrium after one day in the PP tube, and the amount of decomposition product BF3OH− was 4% relative to the initial amount of BF− 4 (Fig. 4(A)). On the other hand, the amount of BF3OH− measured in the glass tube went up drastically after one day, and continued to increase gradually over the next two weeks (Fig. 4(B)). Comparing the results of Fig. 4(A) and Fig. 4(B) at 36 °C, i.e., PP vs. glass, it is clear that the hydrolysis of BF− 4 proceeds accompanied by the consumption of HF through dissolution of the glass sample tube. In contrast, at 4 °C, although the hydrolysis did not proceed during the first 2–3 days of measurement (Fig. 4(C)), it then proceeded rapidly, and equilibrium conditions were not observed even by the final (15th) day of measurement. In Fig. 4(A), a filled circle (11B NMR result) deviates slightly (about 1%) from the open ones (19F NMR results), although they match in Fig. 4(B) and (C). This may be ascribed to the error of calculated peak area of BF3OH−, because the S/N ratio of 11B NMR spectra in the plastic cell was lower than that in the glass cell and the BF3OH− peak intensity was very small as compared to BF− 4 peak intensity.
Therefore, by either using plastic sample tube or maintaining a low sample temperature, NMR measurements of BF− 4 -based ILs can be carried out without being obscured by hydrolysis. 3.3. Elimination rate of HF in glass sample tube As explained in the previous section, we believe that no HF peak was observed in the 19F NMR spectrum due to the high rate of partial dissolution of the glass. Therefore, we investigated time course of the reaction between HF and a glass sample tube, by adding HF to the [DEME][BF4]–water mixture. The decay of HF over time, as calculated based on the 19F NMR peak (−163 ppm) intensity in the glass sample tube at 36 °C is shown in Fig. 5 for two mixtures: 5 vol.% HF in H2O (○) and 0.7 vol.% HF added to a 95% [DEME][BF4]–water mixture (△). In both cases, the peak intensity at − 163 ppm decreased over time, and that at −143 ppm increased. The elimination rate of HF was calculated by assuming that the dissolution of glass by HF is a pseudo first-
− 11 Fig. 4. Time dependence of rates of decomposition products calculated from 19F NMR peak intensities ( : BF3OH−, △: SiF2− B NMR peak intensities (●: BF3OH−) of 6 , □: F (HF) and 2 mol% [DEME][BF4]–water obtained after storing under the following conditions: (A) plastic sample tube at 36 °C, (B) glass sample tube at 36 °C, and (C) glass sample tube at 4 °C.
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K. Saihara et al. / Journal of Molecular Liquids 219 (2016) 493–496
we conclude that the elimination rate of HF produced by hydrolysis of BF− 4 is fast in the glass tube, and the HF peak was not observed one day after the sample preparation in the 19F NMR spectrum. In summary, we showed that the intrinsic hydrolysis is estimated to be 4% relative to the initial amount of BF− 4 in 2 mol% [DEME][BF4]– water. On the other hand, HF produced by the hydrolysis dissolved the glass tube, and hydrolysis continued even 15 days after the initial sample preparation. In addition, we found that the dissolution of glass by HF proceeds slow enough at 4 °C that it can be ignored for approximately 1 day. Therefore, experiments using BF− 4 -based ionic liquids should be carried out in glass tube at low temperatures of around 4 °C, provided that they are performed quickly following sample preparation, or in PP tube. We hope that the results presented here will be useful to other groups performing research involving BF− 4 -based ILs.
References
Fig. 5. Decay of HF concentration over time calculated from 19F NMR peak intensity after storing under the following conditions: ( ) glass sample tube at 36 °C; 4.7 vol.% HF added to H2O, and (△) 0.67 vol.% HF in 95.5% aqueous [DEME][BF4].
order process. The elimination rate constants for the former and latter mixtures are 3.4 × 10−2 vol.%/min and 2.4 × 10−2 vol.%/min, respectively. These results indicated that the elimination rate of HF is not influenced remarkably by the HF concentration (up to 5 vol.%) and in the presence of [DEME][BF4] (2 mol%). Therefore, the half-lives for these two systems are about 30 min, and they are reduced to less than 1% of their initial amounts in 24 h. In the 2 mol% [DEME][BF4]–water mixture, it is expected that the HF produced by hydrolysis of BF− 4 is small compared to the added HF in the above experiments. Based on these results,
[1] T. Welton, Chem. Rev. 99 (1999) 2071–2084. [2] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH Verlag GmbH and Co, KGaA, Germany, 2002. [3] M.G. Freire, C.M.S.S. Neves, I.M. Marrucho, J.A.P. Coutinho, A.M. Fernandes, J. Phys. Chem. A 114 (2010) 3744–3749. [4] S. Ohta, A. Shimizu, Y. Imai, H. Abe, N. Hatano, Y. Yoshimura, Open J. Phys. Chem. 1 (2011) 70–76. [5] K. Saihara, A. Shimizu, H. Abe, Y. Yoshimura, J. Jpn. Inst. Energy 94 (2014) 329–333. [6] I. Furo, J. Mol. Liq. 117 (2014) 117–137. [7] Compilation of Reported F19 NMR Chemical Shifts, 1951 to Mid-1967 by C. H. Dungan, J. R. Van Wazer, John Wiley & Sons Inc., 1970 [8] Chemical shift standards and standard materials, https://staff.aist.go.jp/hayashi.s/ table1e.pdf [9] R.D. Falcone, B. Baruah, E. Gaidamauskas, C.D. Rithner, N.M. Correa, J.J. Silber, D.C. Crans, N.E. Levinger, Chem. Eur. J. 17 (2011) 6837–6846. [10] H. Maki, Y. Okumura, H. Ikuta, M. Mizuhata, J. Phys. Chem. C 118 (2014) 11964–11974. [11] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D. Rogers, Green Chem. 3 (2001) 156–164. [12] M. Tseng, Y. Liang, Y. Chu, Tetrahedron Lett. 46 (2005) 6131–6136.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Detrimental effect of glass sample tubes on investigations of BF− 4 -based room temperature ionic liquid–water mixtures Koji Saihara a, Yukihiro Yoshimura b,⁎, Hideyuki Fujimoto a, Akio Shimizu a,⁎ a b
Graduate School of Environmental Engineering for Symbiosis, Soka University, 1-236 Tangi-Machi, Hachioji, Tokyo 192-8577, Japan Department of Applied Chemistry, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
a r t i c l e
i n f o
Article history: Received 19 January 2016 Received in revised form 7 March 2016 Accepted 12 March 2016 Available online xxxx Keywords: NMR spectroscopy Hydrolysis Quaternary ammonium ionic liquid Glass sample tube Plastic sample tube
a b s t r a c t Tetrafluoroborate (BF− 4 )-based ILs are susceptible to hydrolysis due to their hygroscopicity, which complicates their applications. The progress of tetrafluoroborate hydrolysis in a 2 mol% N,N-diethyl-N-methyl-N-(2methoxyethyl)ammonium tetrafluoroborate–water mixture was investigated by 19F and 11B NMR in both glass and plastic sample tubes. In the plastic (polypropylene) tube, hydrolysis reached equilibrium after one day, and the intrinsic hydrolysis was estimated to be 4% relative to the initial amount of BF− 4 . In contrast, in the glass tube, hydrolysis continued even 15 days after the initial sample preparation due to reaction between glass and HF produced by the hydrolysis of BF− 4 . Additionally, we found that the dissolution of glass by HF proceeds slowly enough at 4 °C that it can be ignored for approximately 1 day. Therefore, experiments using BF− 4 based ionic liquids can be carried out in glass tube at low temperatures of around 4 °C, provided that they are performed quickly following sample preparation, or in plastic tube. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Room-temperature molten salts, commonly known as ionic liquids (ILs), are composed of only bulky cations and compact anions. In recent years, ILs have received a great deal of attention as new and unique materials due to their valuable properties such as non-volatility, noncombustibility, high thermal stability, and ionic conductivity [1,2]. Their physical properties can be designed/tuned through careful selection of the anion/cation combination. However, despite their advantages, it is also necessary to pay attention to the shelf stability of hydrophilic ILs due to their hygroscopicity. In a preliminary study, Freire et al. [3] reported on the hydrolysis of tetrafluoroborate (BF− 4 ) and hexafluorophosphate in imidazoliumbased ILs, and warned about the production of toxic HF. Subsequently, we reported on the hydrolysis of the quaternary ammonium IL N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (often written [DEME][BF4]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) in water over the entire IL concentration range (0–100 mol%); the hydrolysis of the BF− 4 anion was found to proceed rapidly above a water concentration of 80 mol% [4,5]. NMR spectroscopy has remained a powerful technique for obtaining information regarding substances on the atomic (or molecular) level [6]. For example, the static structures of gases, liquids, and solids can be determined from the chemical shift, area intensity, and through multi-dimensional NMR measurements. Therefore, NMR spectroscopy ⁎ Corresponding authors. E-mail addresses: [email protected] (Y. Yoshimura), [email protected] (A. Shimizu).
http://dx.doi.org/10.1016/j.molliq.2016.03.036 0167-7322/© 2016 Elsevier B.V. All rights reserved.
is a natural candidate for investigating the mechanisms of hydrolysis reaction taking place in ILs. NMR measurements are typically performed with a glass sample tube; however, this is problematic for NMR studies − involving BF− 4 -based ILs, as there is a risk of HF generated by BF4 hydrolysis dissolving the glass tube, which would clearly prevent good quality and straightforward NMR spectra from being obtained. With the hope of finding a solution to this problem, we decided to investigate the progress of BF− 4 hydrolysis in a 2 mol% [DEME][BF4]– water mixture using two different sample tube materials: glass and plastic. By comparing the results, our aim was to isolate the effect of the generated HF on the NMR spectra from that corresponding to the actual BF− 4 hydrolysis. 2. Experimental section N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate: [DEME][BF4] (Fig. 1) was purchased from Kanto Chemical Co., Ltd. and used without further purification, and ultra-pure water was obtained from a Synergy UV water purification system (Millipore Inc.). A 2 mol% [DEME][BF4]–water mixture was prepared after weighing the respective components. 19 F and 11B NMR measurements were performed with a ECA-500 (JEOL) spectrometer under the following conditions: 25.0 °C; 470.6 MHz (19F) or 160.5 MHz (11B) nuclear resonance frequency; 140 ppm (19F) or 245 ppm (11B) spectral width; 4 scans; and 0.0021 ppm = 1.01 Hz (19F) or 0.0038 ppm = 0.612 Hz (11B) digital resolution. NMR spectra were obtained using both glass and plastic (polypropylene; hereafter, referred to as PP) sample tubes. Additionally, a double sample tube method
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K. Saihara et al. / Journal of Molecular Liquids 219 (2016) 493–496
Fig. 1. Molecular structure of N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate ([DEME][BF4]).
was employed to prevent contamination of the samples (Fig. 2). For the measurements carried out using glass, a glass inner tube (SP-001, OD = 3.0 mm, Shigemi) containing D2O lock solvent with a chemical shift standard (19F: 5 v/v% trifluoroacetic acid (TFA) = −76.55 ppm [7] and 11B: 0.777 M boric acid = 19.49 ppm [8]) was inserted into a glass outer tube (PS-005, OD = 4.965 mm, Shigemi) containing the sample. For the measurements carried out using plastic, a PP inner tube (NE362-P-5, OD = 4.1 mm, New Era Enterprises Inc.) containing the sample was inserted into a glass outer tube (PS-005) containing the D2O lock solvent. Note that for the PP sample tube system, we did not use TFA because PP is incompatible with concentrated acetic acid. Instead of using a chemical shift standard, the BF− 4 peak was used as a reference by assuming that the chemical shift of BF− 4 has the same value in both sample tubes. The samples were stored in their respective NMR sample tubes in a refrigerator at 4 °C or in an incubator (SLI-220 Tokyo Rikakikai Co., Ltd.) at 36 °C for up to 15 days. After storing at respective conditions, all NMR spectra were obtained at 25 °C. 3. Results and discussion 3.1. Hydrolysis in the plastic and glass NMR sample tubes Typical 19F and 11B NMR spectra of the 2 mol% [DEME][BF4]–water mixture recorded using each sample tube are shown in Fig. 3. Immediately after the sample preparation, two 19F peaks were observed in the spectra around −150 ppm (data not shown). The ratio of the peak intensities was 1:4, which is in accordance with the natural abundance of the boron isotopes 10B and 11B [9,10], and the chemical shift is in 19 F spectra, in addiagreement with the reported value for BF− 4 . For the peak, a quartet at −144 ppm and a singlet at −158 ppm tion to the BF− 4 were observed in the case of the PP tube, whereas a quartet at − 144 ppm and a singlet at − 130 ppm were observed in the case of the glass tube (10 days after the sample preparation, Fig. 3(A)). The
quartet at −144 ppm in both cases was assigned to BF3OH− based on the chemical shift values and peak shapes [12]. Therefore, we can con− clude that BF− 4 decomposes (hydrolyzes) into BF3OH in both types of tubes. There have been some reports [3,11,12] of HF formation from 19 F peak we observed the hydrolysis of BF− 4 in water. To confirm that the here originates from HF, immediately after sample preparation, we measured the spectra of the [DEME][BF4]–water mixture with added HF and of an HF aqueous solution in glass tube, and found that both spectra exhibited an HF peak at −163 ppm. Additionally, the peak intensity observed at −163 ppm increased with increasing HF concentration. We therefore conclude that this peak originates from HF in the glass tube. Interestingly, although the hydrolysis of BF− 4 is also expected to take place, no signal originating from HF was observed from the sample in the glass tube at 10 days after the sample preparation (Fig. 3(A)). However, signal originating from HF (− 157 ppm) was observed in the PP tube. ðIn PP sample tubeÞ
In H2 O
BF4− → BF3 OH− þ F− ðHFÞ
ð1Þ
In H2 O
ðIn glass sample tubeÞ BF4− → B F3 OH− þ F− ðHFÞ ↓ 6HF þ SiO2 →H2 SiF6 þ 2H2 O
ð2Þ HF is known to dissolve glass. Therefore, the most likely reactions (outlined below) that were expected to take place in each of the glass NMR sample tubes were the dissolution of SiO2, the main constituent of glass, by HF, followed by the formation of H2SiF6 and H2O via the reduction of HF. Note that HF should have been generated in the glass sample tube, but no HF peak was observed in the 19F NMR spectrum, most likely due to the high rate of partial dissolution of the glass. The transparency of glass tube did not changed visually even 15 days later after the sample preparation, though three–four months later the wall of the glass tube became slightly opaque by reacting with HF. The chemical shift [8]. The peak observed at − 130 ppm was therefore assigned to SiF2− 6 originating from HF in the PP tube (− 157 ppm) was observed at a slightly lower magnetic field than that observed in the glass tube (− 163 ppm). This may be due to the difference in the interaction between HF and the two types of NMR sample tubes. In this study, two 11B NMR peaks were observed. In the early stages of the reaction, a single peak was observed both for the glass and plastic sample tubes; however, after 11 days an additional peak was observed (Fig. 3(B)). On the other hand, four 19F NMR peaks were observed, − (− 143 ppm), which were assigned to BF− 4 (− 150 ppm), BF3OH 2− − SiF6 (−130 ppm), and F (−157 ppm). Among these, only BF− 4 and BF3OH− can be assigned to the two observed 11B peaks. In fact, the chemical shifts and shapes of the two observed peaks are in agreement − with the 11B NMR peaks reported for BF− 4 and BF3OH [9,10]. The key result that we would like to stress is that the intrinsic hydrolysis reaction for BF− 4 can be precisely traced when using PP tube for the NMR measurements, while if glass tube are employed, the dissolution of glass by HF complicates the overall reaction scheme. 3.2. Time course analysis of BF− 4 hydrolysis in plastic and glass sample tubes
Fig. 2. The two types of NMR sample tubes used in this study: plastic tube (left) and glass tube (right).
Next, we calculated the amounts of the various decomposition prod2− − ucts from the respective 19F NMR peak intensities (BF− 4 , BF3OH , SiF6 , − F ) over the course of 15 days in both types of sample tubes. For the spectra acquired using the glass tube, the F− peak did not grow in considerably with time at either 36 °C (Fig. 4(B)) and 4 °C (Fig. 4(C)), even peaks were clearly obafter 15 days. However, both BF3OH− and SiF2− 6 served even at 4 °C, which indicates that the hydrolysis of BF− 4 proceeds at this lower temperature. Based on the results, it appears that the reaction in the 2 mol% [DEME][BF4]–water mixture at 36 °C reached
K. Saihara et al. / Journal of Molecular Liquids 219 (2016) 493–496
495
Fig. 3. Typical 19F (A: day 10) and 11B (B: day 11) NMR spectra of 2 mol% [DEME][BF4]–water mixture stored at 36 °C using glass (upper) and plastic (lower) NMR sample tubes. NMR spectra were obtained at 25 °C. Note that only the region of the spectra relevant to the discussion is shown.
equilibrium after one day in the PP tube, and the amount of decomposition product BF3OH− was 4% relative to the initial amount of BF− 4 (Fig. 4(A)). On the other hand, the amount of BF3OH− measured in the glass tube went up drastically after one day, and continued to increase gradually over the next two weeks (Fig. 4(B)). Comparing the results of Fig. 4(A) and Fig. 4(B) at 36 °C, i.e., PP vs. glass, it is clear that the hydrolysis of BF− 4 proceeds accompanied by the consumption of HF through dissolution of the glass sample tube. In contrast, at 4 °C, although the hydrolysis did not proceed during the first 2–3 days of measurement (Fig. 4(C)), it then proceeded rapidly, and equilibrium conditions were not observed even by the final (15th) day of measurement. In Fig. 4(A), a filled circle (11B NMR result) deviates slightly (about 1%) from the open ones (19F NMR results), although they match in Fig. 4(B) and (C). This may be ascribed to the error of calculated peak area of BF3OH−, because the S/N ratio of 11B NMR spectra in the plastic cell was lower than that in the glass cell and the BF3OH− peak intensity was very small as compared to BF− 4 peak intensity.
Therefore, by either using plastic sample tube or maintaining a low sample temperature, NMR measurements of BF− 4 -based ILs can be carried out without being obscured by hydrolysis. 3.3. Elimination rate of HF in glass sample tube As explained in the previous section, we believe that no HF peak was observed in the 19F NMR spectrum due to the high rate of partial dissolution of the glass. Therefore, we investigated time course of the reaction between HF and a glass sample tube, by adding HF to the [DEME][BF4]–water mixture. The decay of HF over time, as calculated based on the 19F NMR peak (−163 ppm) intensity in the glass sample tube at 36 °C is shown in Fig. 5 for two mixtures: 5 vol.% HF in H2O (○) and 0.7 vol.% HF added to a 95% [DEME][BF4]–water mixture (△). In both cases, the peak intensity at − 163 ppm decreased over time, and that at −143 ppm increased. The elimination rate of HF was calculated by assuming that the dissolution of glass by HF is a pseudo first-
− 11 Fig. 4. Time dependence of rates of decomposition products calculated from 19F NMR peak intensities ( : BF3OH−, △: SiF2− B NMR peak intensities (●: BF3OH−) of 6 , □: F (HF) and 2 mol% [DEME][BF4]–water obtained after storing under the following conditions: (A) plastic sample tube at 36 °C, (B) glass sample tube at 36 °C, and (C) glass sample tube at 4 °C.
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we conclude that the elimination rate of HF produced by hydrolysis of BF− 4 is fast in the glass tube, and the HF peak was not observed one day after the sample preparation in the 19F NMR spectrum. In summary, we showed that the intrinsic hydrolysis is estimated to be 4% relative to the initial amount of BF− 4 in 2 mol% [DEME][BF4]– water. On the other hand, HF produced by the hydrolysis dissolved the glass tube, and hydrolysis continued even 15 days after the initial sample preparation. In addition, we found that the dissolution of glass by HF proceeds slow enough at 4 °C that it can be ignored for approximately 1 day. Therefore, experiments using BF− 4 -based ionic liquids should be carried out in glass tube at low temperatures of around 4 °C, provided that they are performed quickly following sample preparation, or in PP tube. We hope that the results presented here will be useful to other groups performing research involving BF− 4 -based ILs.
References
Fig. 5. Decay of HF concentration over time calculated from 19F NMR peak intensity after storing under the following conditions: ( ) glass sample tube at 36 °C; 4.7 vol.% HF added to H2O, and (△) 0.67 vol.% HF in 95.5% aqueous [DEME][BF4].
order process. The elimination rate constants for the former and latter mixtures are 3.4 × 10−2 vol.%/min and 2.4 × 10−2 vol.%/min, respectively. These results indicated that the elimination rate of HF is not influenced remarkably by the HF concentration (up to 5 vol.%) and in the presence of [DEME][BF4] (2 mol%). Therefore, the half-lives for these two systems are about 30 min, and they are reduced to less than 1% of their initial amounts in 24 h. In the 2 mol% [DEME][BF4]–water mixture, it is expected that the HF produced by hydrolysis of BF− 4 is small compared to the added HF in the above experiments. Based on these results,
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