L. E. Shmukler*,
M. S. Gruzdev,
N. O. Kudryakova,
Yu. A. Fadeeva,
A. M. Kolker and
L. P. Safonova
G. A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, 153045, Akademicheskaya St., 1, Ivanovo, Russia. E-mail: les@isc-ras.ru; Fax: +7 4932 336237; Tel: +7 4932 336259
First published on 3rd November 2016
Protic ionic liquids (PILs) composed of the triethylammonium cation with dihydrogen phosphite, tosylate, and trifluoroacetate anions were synthesized. All samples were salts with melting points below 100 °C and were characterized via NMR spectroscopy, attenuated total reflection (ATR) spectroscopy, differential scanning calorimetry (DSC), and thermogravimetry (TG). The electrochemical characteristics of each protic ionic liquid were obtained using a combination of impedance spectroscopy and cyclic voltammetry. Moreover, the influence of water on the thermal behavior and conductivity of triethylammonium tosylate is studied. A linear correlation between the temperatures of melting and crystallization is established for a number of PILs, including both derivatives of triethylamine with different acids reported in the literature and the newly synthesized PILs. The optimal combination of thermal characteristics and electroconductivity was observed for triethylammonium trifluoroacetate and tosylate.
There are two main families of ILs: aprotic ionic liquids (AILs) and protic ionic liquids (PILs). Aprotic ionic liquids have no active protons in their chemical structure. Typical AILs, such as imidazolium salts, are generally synthesized by the quaternization (alkylation) of an amine, followed by anion exchange. PILs are prepared by the proton transfer reactions from Brønsted acids to Brønsted bases and have exchangeable (active) protons that are able to form H-bonds. PILs have been less studied in comparison to AILs. Herein, our attention is focused on the PILs due to their remarkable properties, which make them potentially usable in many industrial and technical applications. Moreover, the wide variety of substances that are the potential components of PILs allows the synthesis of PILs with desired properties.
The incorporation of PILs into the confined geometry of polymers1 makes these systems applicable as electrolytes in non-humidified fuel cell systems.1–6 Polymer electrolytes based on ILs surpass the conventional electrolytes in some characteristics, viz. they are safer and highly thermostable, as well as have a wide range of operating temperatures. There are many studies reporting the synthesis of protic ionic liquids and ionic melts with high thermal stability in combination with high ionic conductivity under anhydrous conditions. For example, it was shown3 that anhydrous diethylmethylammonium trifluoromethanesulfonate ([dema]/[TfO] = 1/1) exhibited high thermal stability (360 °C), low melting point (−6 °C), and high ionic conductivity (4.3 × 10−2 S cm−1 at 120 °C). H. Nakamoto et al.5 succeeded in synthesizing a neutral salt composed of bis(trifluoromethanesulfonyl)imide and benzimidazole, which was hydrophobic and electrochemically stable. The decomposition temperature of this salt was higher than 350 °C and its proton conductivity was equal to 8.3 × 10−3 S cm−1 at 140 °C. In7 two series of liquid PILs, which were composed of secondary and/or tertiary ammonium cations and trifluoroacetate, methanesulfonate, trifluoromethanesulfonate, and tosylate anions, were synthesized. These salts had high conductivity values (1.4–4.9 mS cm−1 at 70 °C) and were thermostable up to 200–300 °C.
Herein, we report the first results of our investigation of the anion nature effect on the thermal behavior and conductivity of protic ionic liquids composed of triethylammonium cation and anions of different acids.
All PILs were prepared by the neutralization reaction between equimolar amounts of triethylamine (Brønsted base) and different Brønsted acids according to the following (Scheme 1):
The reaction was carried out under argon. A certain amount of TEA (approximately 0.07 mol) was placed in a 100 mL round-bottom flask equipped with a Teflon coated stir bar. Since the reaction is highly exothermic, the flask was immersed in an ice bath and each acid was added dropwise to TEA, separately. Then, the mixture was stirred for 8 h at 90 °C and was dried under vacuum. Triethylammonium tosylate contains a lot of water because p-toluenesulfonic acid monohydrate was used for its synthesis. Therefore, lyophilization of the salt was carried out to reduce its water content.8
Thus, the following resulting products were obtained: white transparent solid, yellow liquid, and white crystals in the case of triethylammonium dihydrogen phosphite, triethylammonium trifluoroacetate, and triethylammonium tosylate, respectively. The structural formulae of the PILs synthesized are shown below:
The water content in the synthesized ionic liquids was measured using a Karl Fischer titrator (V30, Mettler Toledo). The synthesized PILs were characterized via the 1H NMR, 13C NMR and ATR spectroscopic techniques.
13C NMR (CDCl3, TMS, δ, ppm): 7.61 (CH3–), 44.61 (–CH2–).
13C NMR (CDCl3, TMS, δ, ppm): 7.56 (CH3–); 20.3 (–CH3); 45.30 (–CH2–); 124.87 (C–H); 127.76 (C–H); 138.87 (C–H); 141.68 (C–H).
13C NMR (CDCl3, TMS, δ, ppm): 9.74 (CH3–), 46.98 (–CH2–), 119.12–116.79 (–CF3–), 163.35 (CO).
The spectra of all the synthesized PILs, and pure TFA and TEA were obtained using the attenuated total reflection (ATR) technique with a MVP 2 Series™ (Harrick) instrument with a diamond crystal. The spectra of solid samples (H2PHO3 and PTSA) were measured under vacuum using the KBr pellet. Every sample was ground with spectral grade KBr to the state of a homogeneous powder. The mixtures were then pressed into disc-pellets and analyzed.
FT-IR measurements were conducted to characterize the products with respect to the interactions between the Brønsted acids and TEA. The ATR spectra of the PILs together with the spectra of their source compounds over the whole frequency range are shown in the ESI.† In general, the spectrum of each PIL can be considered as a superposition of the spectra of its pure components with some band displacements, observed as expected. However, the appearance of some new modes within the frequency range of 2500–2700 cm−1 (Fig. 1) is an evidence for the formation of proton transferred salts (or the protonated amine (CH3–CH2)3NH+).9,10
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Fig. 1 ATR spectra of the synthesized PILs and the spectra of their source substances in the range of 2000–3750 cm−1. (a) Triethylammonium dihydrogen phosphite, (b) triethylammonium tosylate, and (c) triethylammonium trifluoroacetate. The spectrum of pure TFA was obtained from ref. 11. |
TGA of the PILs was performed using a NETZSCH TG 209 F1 analyzer in a flow of argon (20 mL·min−1) at a heating rate of 10 °C min−1.
Theoretically, the freezing temperature is equal to the melting point. However, for many ILs these two temperatures are not the same. Therefore, substantial supercooling is observed when PILs are cooled from relatively high temperatures and the supercooled melt devitrifies (crystallizes) at the temperature of so-called “cold crystallization” (Tc). This behavior is typical for all the salts synthesized. As an example, the DSC scans for [TEA]/[TFA] are shown in Fig. 2.
The thermochemical characteristics of the synthesized PILs are shown in Table 1. It should be noted that no glass transition was observed for the PILs under the experimental conditions. Also, one can see that the melting point (Tm) of triethylammonium tosylate increases but the conductivity decreases after salt lyophilization.
PILs | Tc (°C) | ΔH (J g−1) | Tm (°C) | ΔH (J g−1) | Tdec (°C) | κ (mS cm−1) | Water (wt%) |
---|---|---|---|---|---|---|---|
a Tc – crystallization temperature; Tm – melting temperature; Td – decomposition temperature; κ – ionic conductivity. | |||||||
[TEA]/[H2PHO3] | −2.7 | −29.1 | 42.8 | 23.4 | 106.1 | 3.92 (100 °C) | 2 |
[TEA]/[PTSA] | 12.0 | −33.4 | 67.5 | 41.8 | 324.6 | 7.47 (105 °C) | 3.4 |
[TEA]/[PTSA] after lyophilization | 18.5 | −55.6 | 80.2 | 65.1 | 311.3 | 5.28 (105 °C) | <1 |
[TEA]/[TFA] | −41.4 | −51.7 | 8.6 | 59.5 | 186.3 | 10.22 (105 °C) | <1 |
The TG curves of the PILs under investigation are presented in Fig. 3, which show their thermal stabilities; PILs decomposition temperatures, which are stated as the temperatures corresponding to 3% loss of the sample weight, are listed in Table 1.
The temperature corresponding to the beginning of weight loss of the PIL based on H2PHO3 was low due to the condensation reaction of phosphites.3
The thermal properties of the synthesized PILs along with the reported data for triethylammonium salts are listed in Table 2.
Salts | H2O | Tg, °C | Tm, °C | Tc, °C | Td, °C | κ, mS cm−1 (25 °C) | Ref. |
---|---|---|---|---|---|---|---|
a Ac: acetic acid; TFSA: bis(tetrafluoromethylsulfonyl)imide; TFA: trifluoroacetic acid; TFSI: bis(trifluoromethanesulfonyl)amide; MsOH: methanesulfonic acid; BF: heptafluorobutyrate; PFBSu: perfluorobenzenesulfonic acid; TfOH: trifluoromethanesulfonic acid; H2PHO3: phosphonic acid; FBSu: fluorobenzenesulfonic acid; TFBSu: trifluorobenzenesulfonic acid; PFBu: perfluorobutanesulfonic acid; BA: benzoic acid; PTSA: p-toluenesulfonic acid; Sacc: saccharin; H2SO4: sulfuric acid; PFOc: perfluoroctane sulfonic acid; BF4: tetrafluoroborate; BSu: benzenesulfonic acid; N: nitric acid; F: formic acid; H3PO4: phosphoric acid; H2PO3F: fluorophosphoric acid; and (BuO)2POOH: di-n-butyl-phosphate. | |||||||
TEA Ac | 0.42% | −93 | −18 | 48 | 20 | ||
TEA TFSA | 600 ppm | −0.8 | −29 | 5.64 | 21 | ||
TEA TFA | 50 ppm | −83 | 2 | −42 | 190 | 22 | |
0.42% | 8.6 | −41.4 | 186.3 | 2.45 | This work | ||
TEA TFSI | <1 ppm | 3.5 | 350 | 32.3 (130 °C) | 4 | ||
10 ppm | 3 | 4.4 | 23 | ||||
TEA ms | 100 ppm | −78.9 | 24.3 | −31.6 | 1.91 | 24 | |
50 ppm | 33 | −24 | 305 | 22 | |||
2 mol% | 25 | −31 | 218 | 2.03 | 16 | ||
−62.1 | 17.4 | 225 | 16.3 (120 °C) | 3 | |||
<1 ppm | −96.5 | 21.6 | 269.7 | 25 | |||
TEA BF | 0.3 | 25 | −62 | 0.679 | 26 | ||
TEA PFBSu | 50 ppm | −58 | 29 | −5 | 370 | 27 | |
TEA TfOH | 50 ppm | −58 | 32 | −20 | 376 | 22 | |
36 | |||||||
2 mol% | 26 | −13 | 322 | 4.79 | 16 | ||
34.3 | 358 | 2.76 | 3 | ||||
<1 ppm | 312.5 | 25 | |||||
TEA H2PHO3 | −78.4 | 135 | 3 | ||||
2% | 42.8 | −2.7 | 106.1 | 3.92 (100 °C) | This work | ||
TEA FBSu | 50 ppm | −70 | 59 | 2 | 320 | 27 | |
TEA TFBSu | 50 ppm | 60 | 3 | 340 | 27 | ||
TEA PFBu | 50 ppm | 61 | 50 | 383 | 27 | ||
TEA BA | 50 ppm | 62 | 30 | 202 | 22 | ||
TEA PTSA | 3.4% | 67.5 | 12.0 | 324.6 | 0.46 | This work | |
TEA Sacc | −27.4 | 72 | 36 | 24 | |||
59.4 | |||||||
TEA H2SO4 | 74.7 | 270 | 13.1 (120 °C) | 3 | |||
<1 ppm | −100.1 | 84.2 | 262.8 | 25 | |||
TEA PFOc | 50 ppm | −48 | 82 | 71 | 390 | 27 | |
TEA BF4 | <1 ppm | 104.3 | 296.7 | 25 | |||
TEA BSu | 50 ppm | 119 | 106 | 330 | 27 | ||
TEA N | 0.84% | 115 | 28 | ||||
113–114 | 29 | ||||||
TEA F | 0.19% | −119 | 13.05 | 28 | |||
0.51% | 54 | 20 | |||||
TEA H3PO4 | −27.8 | 175 | 3 | ||||
<1 ppm | −34.4 | 349.2 | 25 | ||||
−32.2 | |||||||
TEA H2PO3F | <1 ppm | −59.7 | 25 | ||||
TEA(BuO)2POOH | 100 ppm | −91 | 0.23 | 24 |
The thermal properties of the PILs synthesized on the basis of phosphorous and trifluoroacetic acids agree well with the reported data. The discrepancy observed can be explained, first of all, by the experimental conditions. The parameters presented in Table 2 for the salts based on methanesulfonic and trifluoromethanesulfonic acids show that the changes of thermal characteristics do not correlate directly with the different water contents in the salts.
However, some regularity in Tm variation with the nature of acids is observed. Thus, the melting temperatures of the PILs obtained on the basis of carbon acids are, in general, lower as compared to those of the PILs based on sulfonic acids. The substitution of one hydroxyl-group in sulfuric acid by a methyl-group, specifically going from sulfuric to methanesulfonic acid, results in a decrease in Tm. At the same time, a significant increase in Tm was observed when the aliphatic methyl-group in methanesulfonic acid (Tm = 17.4 to 33 °C) was replaced by an aromatic group (Tm = 119 °C for BSu). According to ref. 27, this phenomenon can be explained by the structure, rigidity, and high packing ability of the aromatic ring. There is a considerable increase in Tm when CH3COO− (−18 °C) is replaced by CF3COO− (8.6 °C). The melting point values also depend on the length of the perfluorinated alkyl anionic chain. Thus, the Tm of the salts increases from 61 to 82 °C when going from perfluorobutanesulfonic acid to perfluoroctane sulfonic acid.
Most ionic liquids based on the triethylammonium cation are thermostable up to 200–350 °C. Triethylammonium formate and acetate are the least thermostable salts. In ref. 30 it was also noted that the acetate based PILs are less stable compared to the protic ionic liquids based on other anions.
The onset of thermal decomposition for the ionic liquids based on the alkylimidazolium cation showed that the nature of the anion substantially determines this property, whereas the effect of the cation is much weaker.31,32
In some studies7,33–35 a correlation between Tm, Td, and Δpκ values was established. For TEA-based salts (Table 2), no similar correlation is observed. The only correlation established is the correlation between the temperatures of crystallization and melting (Fig. 4).
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Fig. 4 The correlation between the melting and crystallization temperatures of the triethylamine-based PILs. |
The conductivity values of triethylamine-based ionic liquids (Table 2) are within 10−4 to 10−3 S cm−1 at 25 °C. The largest conductivity value was obtained for the salt containing formic acid.
The ionic conductivity of the synthesized PILs was evaluated from their impedance spectra. As an example, the spectra of [TEA]/[TFA] at different temperatures are presented in Fig. 5. The spectra of other PILs were similar.
The temperature dependencies of the ionic conductivity of the synthesized PILs are presented in Fig. 6. The conductivity values increase according to the following order: [TEA]/[H2PHO3] < [TEA]/[PTSA] < [TEA]/[TFA]. At temperatures below 70 °C the protic IL synthesized on the basis of p-toluenesulfonic acid was in the supercooled state due to its high viscosity. Therefore, the temperature dependence of its specific conductivity is shown as a dotted line within the temperature range from 25 to 70 °C.
From the Arrhenius plots (Fig. 6), it can be seen that the temperature dependencies of the ionic conductivity are not strictly linear. Therefore, these dependencies can be fitted with the nonlinear Vogel–Tamman–Fulcher (VTF) equation:36,37
The VFT parameters calculated from the temperature dependencies of the ionic conductivities of the [TEA]/[TFA] and [TEA]/[PTSA] salts are summarized in Table 3.
PILs | κo (S cm−1) | B (K) | To (K) | R2 |
---|---|---|---|---|
[TEA]/[TFA] | 0.07 ± 0.02 | 360 ± 104 | 193 ± 21 | 0.997 |
[TEA]/[PTSA] | 0.06 ± 0.01 | 285 ± 37 | 246 ± 6 | 0.998 |
The electrochemical window is an important parameter for the electrochemical stability of PILs. There are many factors that affect the ECW of ionic liquids. Among other things, the electrode material has a strong effect on the value of ECW. Since the synthesized PILs are expected for their application in electrochemical cells, in which Pt-electrodes are the most widely used, therefore, Pt was used as the electrode material for ECW measurements. Fig. 7 displays the cyclic voltammograms obtained using a platinum electrode at 50 °C for the three determined PILs.
The electrochemical window (ECW) value of TEA-based PILs considerably depends on the nature of the anion and temperature. It should be mentioned that the largest ECW values were obtained at 50 °C for all the PILs measured. The other important observation is that triethylammonium cation reduces at different potentials, subject to the counterion nature. Thus the cathodic limits for [TEA]/[TFA], [TEA]/[PTSA] and [TEA]/[H2PHO3] are equal to −0.75, −0.85, and −0.45 V, respectively, at 50 °C. A similar dependence of the cathodic potential on the nature of anion was observed for ILs based on metals halides38 and for pyrrolidinium-based PILs.39 This is attributed to changes in the medium acidity. In addition, water impurities, which are detected in all the PILs synthesized, probably contribute to the value of the cathodic response.38,40 The influence of temperature on the electrochemical stability of the TEA-based PILs is shown in Fig. 8.
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Fig. 8 Cyclic voltammograms of the synthesized PILs at different temperatures: (a) [TEA]/[PTSA]; (b) [TEA]/[TFA]; and (c) [TEA]/[H2PHO3]. |
For all the synthesized PILs, ECW narrowing with an increase in temperature is typical. Such behavior was also observed earlier for RTILs.41,42 The ECW decreases both in the cathodic and anodic areas of potential that is probably connected to the influence of temperature on the kinetics of redox processes.34 Here, it should be noted that the magnitude of ECW narrowing is not the same in the cathodic and anodic potential areas. Moreover, the ECW for all synthesized the PILs decreases by less than 300 mV in the cathodic area, whereas in the anodic area the decrease is about 100 mV for [TEA]/[TFA] and 400 mV for the other two salts. Thus, it is obvious that the [TEA]/[H2PHO3] salt is the most sensitive to an increase in temperature, and the potential range of its electrochemical stability decreases by about 700 mV upon heating from 50 to 120 °C.
Footnote |
† Electronic supplementary information (ESI) available: 1H and 13C NMR graphs of triethylammonium trifluoroacetate, dihydrogen phosphite, and tosylate; ATR spectra of both PILs synthesized and initial compounds. See DOI: 10.1039/c6ra21360j |
This journal is © The Royal Society of Chemistry 2016 |