Deep eutectic solvents formed by quaternary ammonium salts and aprotic organic compound succinonitrile

Journal of Molecular Liquids 274 (2019) 414–417

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Deep eutectic solvents formed by quaternary ammonium salts and aprotic organic compound succinonitrile Dezhong Yang a,⁎, Shaoze Zhang b, Xiao-guang Sun c, De-en Jiang d,⁎, Sheng Dai c,e,⁎⁎ a

School of Science, China University of Geosciences, Haidian District, Beijing 100083, China Key Laboratory for Advanced Materials and School of Chemistry & Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States d Department of Chemistry, University of California, Riverside, CA 92521, United States e Department of Chemistry, University of Tennessee, Knoxville, TN 37996, United States b c

a r t i c l e

i n f o

Article history: Received 2 August 2018 Accepted 28 October 2018 Available online 30 October 2018 Keywords: Deep eutectic solvents Aprotic organics Hydrogen bonds Ammonium salts Succinonitrile

a b s t r a c t Deep eutectic solvents (DESs) are conventionally prepared by mixing quaternary ammonium salts and protic organic compounds such as alcohol, amide or carboxylic acid. In this work, we report that the DESs can also be obtained by using an aprotic organic compound such as succinonitrile (SN). The eutectic temperature was found to be 12 °C for the mixture of tetrabutylammonium trifluoromethanesulfonate (N4444Tfo) and SN at a molar ratio of 1:6, but no melting point could be observed for several DESs formed by tetrabutylammonium chloride (N4444Cl) and SN, even down to −90 °C. Fourier transform infrared spectroscopy (FTIR), Nuclear Magnetic Resonance (NMR) and theoretical calculations indicated the formation of hydrogen bonds between the anions of the ammonium salts and the methylene group of the SN, which greatly impact the melting behavior of the DESs. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Deep eutectic solvents (DESs) are composed of two or three components that are capable of associating with each other, mainly through hydrogen bond interaction, to form a eutectic mixture [1]. It is generally recognized that there are four types of DESs [1,2], among which the type III DES formed between hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs), has attracted great attention due to its ability to dissolve a wide range of transition metal species [2]. DESs are considered as promising alternatives to ionic liquids (ILs) because they share some similar physicochemical properties with ionic liquids such as nonvolatility, high thermal stability, and a liquid state below 100 °C, but can be easily synthesized from low cost materials [1,3–5]. Therefore, DESs have been widely investigated in many fields, such as electrochemistry, extraction, gas absorption and material synthesis [1,5,6]. DESs are usually obtained by mixing quaternary ammonium salts with a hydrogen bond donors, in which the latter could form intermolecular hydrogen bonds with the anions of the former, leading to the depression of the melting point [6]. So far, a number of hydrogen bond donors have been investigated, including amides, alcohols and

⁎ Corresponding authors. ⁎⁎ Corresponding author at: Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States. E-mail addresses: [email protected] (D. Yang), [email protected] (D. Jiang), [email protected] (S. Dai).

https://doi.org/10.1016/j.molliq.2018.10.150 0167-7322/© 2018 Elsevier B.V. All rights reserved.

carboxylic acids [2]. However, these hydrogen bond donors are all protic organics. Herein, we report that DESs could be formed by quaternary ammonium salts with an aprotic organic compound of succinonitrile (SN). Both the salts and SN studied in this work are solids at room temperature, but homogeneous liquids could be obtained at ambient temperature after mixing the two components. The results indicated that SN can act as a hydrogen bond donor and there are intermolecular hydrogen bonds formed between the methylene group of the SN and the anions of the quaternary ammonium salts. 2. Experimental section 2.1. Materials and characterization Tetrabutylammonium trifluoromethanesulfonate (N4444Tfo) (N99%) and tetrabutylammonium chloride (N4444Cl ≥ 99%) were obtained from Fluka. Succinonitrile (N99%) was supplied by TCI. All the chemicals were used directly as received. The IR spectra were recorded on a PerkinElmer Frontier FTIR spectrometer with wave numbers from 650 to 4000 cm−1. The NMR spectra were taken on a Bruker spectrometer (400 MHz) by using DMSO d6 as the external reference. Differential scanning calorimetry (DSC) experiments were performed in a TA-Q100 instrument. Samples were sealed into aluminum sample pans and an empty sample pan was used as the reference. Samples were cooled to −90 °C at 10 °C/min and then heated at 10 °C/min. The data obtained during the second heating cycles were used. The

D. Yang et al. / Journal of Molecular Liquids 274 (2019) 414–417

415

temperatures for the glass transition and melting were reported as the peak-maximum temperatures for the endothermic changes of the samples, either due to the overlap of peaks or irregular shape of many peaks. 2.2. Synthesis of the mixtures The mixtures were prepared by mixing quaternary ammonium salts with SN at desired molar ratio, and then kept stirring at 80 °C until a homogeneous solution was formed. 2.3. Computational methodology The Gaussian 09 suite of programs [7] was used to optimize the geometries of the complexes at the theoretical level of B3LYP/6-311+G** [8,9]. No symmetry or geometry constraint was imposed during the optimizations. All the optimized-geometries were corroborated to be factual minima on the potential energy surface by means of frequency calculation at the same theoretical level. In order to correct for the basis set superposition error (BSSE), the standard counterpoise method of Boys and Bernardi was implied [10]. The interaction energies (ΔE) between an ion pair of an ionic liquid and succinonitrile are defined as ΔE ¼ Esystem −EIL −E2SN where Esystem is the energy of the whole system; EIL and E2SN are the total electronic energies of the ion pair and two succinonitrile molecules, respectively. 3. Results and discussion Fig. 1 shows traces of differential scanning calorimeter (DSC) of pure SN and the mixtures of N4444Tfo-SN with different molar ratios (from 1:2 to 1:8). SN shows two endothermic peaks at −38 °C and 58 °C corresponding to the transition from normal crystal to plastic-crystalline phase (Tpc) and melting point (Tm) of SN, respectively [11,12]. As can be seen in Fig. 1, the melting points of the N4444Tfo-SN mixtures are lower than 30 °C, suggesting liquid can be formed by mixing N4444Tfo and SN near room temperature. Moreover, the Tpc peaks of SN disappeared in N4444Tfo-SN (1:3) and N4444Tfo-SN (1:2) system, indicating the strong interactions between N4444Tfo and SN. Fig. 2 shows the phase diagram of N4444Tfo-SN mixture, which is a typical of a binary system with a eutectic point. The eutectic occurs near 45% mass fraction of N4444Tfo (or a molar ratio of 1:6, N4444Tfo:SN). The eutectic temperature is 12 °C (Table S1), which is much lower than the melting point of either N4444Tfo (Tm = 115 °C) or SN (Tm = 58 °C). The depression of the melting point mainly due to the interactions between N4444Tfo and SN. The interactions between N4444Tfo and SN were investigated by using FTIR spectra. As can be seen in Fig. 3a, the shift of\\CN stretching vibration peaks was not obvious after mixing SN with N4444Tfo. The

Fig. 1. DSC traces of the pure SN and mixtures of N4444Tfo-SN.

Fig. 2. Phase diagram of N4444Tfo-SN mixtures.

\\CN peaks shifted from 2254 (SN) to 2253 cm−1[N4444Tfo-SN (1:2)], only showing 1 cm−1 shift, indicating a weak interaction between \\CN group and N4444Tfo. As shown in Fig. 3b, the methylene group (\\CH2\\) stretching peak of SN shifted to lower wavenumber after mixing with N4444Tfo. For example, the\\CH2\\stretching peak shifted from 2988 (SN) to 2966 cm−1[N4444Tfo-SN (1:2)], exhibiting a 22 cm−1 red shift, suggesting the\\CH2\\group is more likely to form hydrogen bonds with the anion of N4444Tfo. We also studied the system formed by tetrabutylammonium chloride (N4444Cl, Tm = 77 °C) and SN. Homogeneous liquid can also be obtained by mixing N4444Cl and SN at ambient temperature. Fig. 4 shows the DSC traces of pure SN and the N4444Cl-SN mixtures. It can be seen in Fig. 4, the melting points were not found for the DESs of N4444Cl-SN (1:2), N4444Cl-SN (1:3), N4444Cl-SN (1:4) and N4444Cl-SN (1:5), indicating the strong interactions between N4444Cl and SN. The glass temperatures are −69 and −77 °C for DES of N4444Cl-SN (1:2) and N4444Cl-SN (1:3), respectively. For DESs of N4444Cl-SN (1:4), N4444Cl-SN (1:5), they remain liquid all the way down to −90 °C, indicating the

Fig. 3. FTIR spectra of N4444Tfo, SN and N4444Tfo-SN mixtures.

416

D. Yang et al. / Journal of Molecular Liquids 274 (2019) 414–417

Fig. 4. DSC traces of pure SN and N4444Cl-SN mixtures.

completely amorphous samples [13]. However, the DESs formed by N4444Cl and ethylene glycol (EG) showed melting points of −30.13, −30.88 and −17.02 °C for N4444Cl-EG (1:2), N4444Cl-EG (1:3) and N4444Cl-EG (1:4), respectively [14]. As the concentration of SN was increased, the melting points were found at −13 °C and 4 °C for N4444Cl-SN (1:6) and N4444Cl-SN (1:8), respectively (Table S2). In contrast, the melting points for N4444Tfo-SN (1:6) and N4444Tfo-SN (1:8) were found at 12 °C and 15 °C, respectively (Table S1). The different melting behavior of the N4444Cl-SN DESs may arise from the strong interaction between SN and the anion of Cl−, which will be discussed in the following section. The FTIR spectra of SN and the N4444Cl-SN mixtures are shown in Fig. 5. As can be seen from Fig. 5a the stretching vibration peak of \\CN shifted from 2254 cm−1 for SN to 2250 cm−1 for N4444Cl-SN (1:2), probably due to the interaction between \\CN and the Cl− anion. Moreover, the wavenumber of \\CN in N4444Cl-SN (1:2) was lower than that in N4444Tfo-SN (1:2) (2253 cm−1), suggesting the interaction between Cl− and\\CN was stronger than that between Tfo− and.

Fig. 5. FTIR spectra of N4444Cl, SN and N4444Cl-SN mixtures.

\\CN. As can be seen, the stretching peak of the \\CH2\\ of SN shifted from 2988 (SN) to 2963 [N4444Cl-SN (1:2)] cm−1, showing a 25 cm−1 red shift (Fig. 5b), suggesting the strong interaction between the \\CH2\\ of SN and Cl−. The wavenumber of the \\CH2 of SN in N4444Cl-SN (1:2) was also lower than that in N4444Tfo-SN (1:2) (2966 cm−1), indicating the hydrogen bond between Cl− and CH2 was stronger than that between Tfo− and CH2 [15]. Moreover, the interactions between SN with the two quaternary ammonium salts were investigated by NMR and the results are shown in Fig. 6. The signals of the methylene group of \\CH2-CN shifted downfield from 2.77 ppm in N4444Tfo-SN (1:2) to 3.09 ppm in N4444Cl-SN (1:2) (Fig. 6a), indicating that the hydrogen bond in Cl−⋯⋯H2C-CN was stronger than that of Tfo−⋯⋯H2C-CN. This is mainly due to that the hydrogen bonding ability of Cl− is stronger than that of Tfo− [16]. Fig. 6b shows the 13C signals of the\\CN group. The chemical shifts of the carbon of\\CN were 118.5 and 119.2 ppm for N4444Tfo-SN (1:2) and N4444Cl-SN (1:2) system, respectively. This result indicated that the Cl−⋯CN interaction was stronger than that of Tfo−⋯CN. The NMR results are consistent with the FTIR analyses. The interactions between the quaternary ammonium salts and SN mainly impact the melting behaviors of DESs. As the interactions between the anion and SN enhanced, it will be more difficult to crystallize, resulting in lower melting points. To further understand the interactions between salts and SN, quantum chemical calculations at the level of B3LYP/6-311+G** were performed to probe the local interaction geometries and energetics based a gas-phase cluster model. As can be seen from Fig. 7a, the O atoms in the Tfo− anion interact with the methylene H atoms and \\CN groups of the SN in N4444Tfo-SN (1:2) system. Fig. 7b shows that the hydrogen bonds are formed between Cl− and the methylene H atoms of SN in the N4444Cl-SN (1:2) system. Moreover, the interaction between N4444Cl

Fig. 6. 1H NMR (a) and 13C NMR (b) spectra of N4444Tfo-SN (1:2) and N4444Cl-SN (1:2).

D. Yang et al. / Journal of Molecular Liquids 274 (2019) 414–417

417

Fig. 7. The optimized structures and interaction energies of quaternary ammonium salts and succinonitrile based on a gas-phase cluster model: (a) N4444Tfo-SN (1:2); (b) N4444Cl-SN (1:2). C, gray; H, white; N, blue; O, red; S, yellow; F, cyan; Cl, green.

and SN (−107.2 kJ/mol) is stronger than that between N4444Tfo and SN (−83.0 kJ/mol). 4. Conclusion In summary, DESs can also be formed by quaternary ammonium salts with aprotic organic compound. Hydrogen bonds were formed between the anions of the ammonium salts and the methylene group of the SN. Moreover, the anions of the quaternary salts have great effects on the melting behavior of the DESs. This work enlarges the scope of DESs and provides new opportunities for the development of DESs. Notes The authors declare no competing financial interests. Acknowledgment This work was supported financially by the National Natural Science Foundation of China (No. 21503196). X.S was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. S.D. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2018.10.150. References [1] Q.H. Zhang, K.D. Vigier, S. Royer, F. Jerome, Deep eutectic solvents: syntheses, properties and applications, Chem. Soc. Rev. 41 (2012) 7108–7146.

[2] E.L. Smith, A.P. Abbott, K.S. Ryder, Deep eutectic solvents (DESs) and their applications, Chem. Rev. 114 (2014) 11060–11082. [3] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Novel solvent properties of choline chloride/urea mixtures, Chem. Commun. (2003) 70–71. [4] A.P. Abbott, D. Boothby, G. Capper, D.L. Davies, R.K. Rasheed, Deep eutectic solvents formed between choline chloride and carboxylic acids: versatile alternatives to ionic liquids, J. Am. Chem. Soc. 126 (2004) 9142–9147. [5] R. Germani, M. Orlandini, M. Tiecco, T. Del Giacco, Novel low viscous, green and amphiphilic N-oxides/phenylacetic acid based deep eutectic solvents, J. Mol. Liq. 240 (2017) 233–239. [6] G. Garcia, S. Aparicio, R. Ullah, M. Atilhan, Deep eutectic solvents: physicochemical properties and gas separation applications, Energy Fuel 29 (2015) 2616–2644. [7] M.T. Frisch, H.B. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. Petersson, Gaussian 09, Revision D. 01, Gaussian, Inc., Wallingford CT, 2009. [8] C. Lee, W. Yang, R.G. Parr, Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density, Phys. Rev. B 37 (1988) 785–789. [9] W.J. Hehre, R. Ditchfield, J.A. Pople, Self—consistent molecular orbital methods. XII. Further extensions of Gaussian—type basis sets for use in molecular orbital studies of organic molecules, J. Chem. Phys. 56 (1972) 2257–2261. [10] S.F. Boys, F. Bernardi, The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors, Mol. Phys. 19 (1970) 553–566. [11] P.-J. Alarco, Y. Abu-Lebdeh, A. Abouimrane, M. Armand, The plastic-crystalline phase of succinonitrile as a universal matrix for solid-state ionic conductors, Nat. Mater. 3 (2004) 476. [12] L.-Z. Fan, X.-L. Wang, F. Long, All-solid-state polymer electrolyte with plastic crystal materials for rechargeable lithium-ion battery, J. Power Sources 189 (2009) 775–778. [13] G. Annat, M. Forsyth, D.R. MacFarlane, Ionic liquid mixtures—variations in physical properties and their origins in molecular structure, J. Phys. Chem. B 116 (2012) 8251–8258. [14] F.S. Mjalli, J. Naser, B. Jibril, V. Alizadeh, Z. Gano, Tetrabutylammonium chloride based ionic liquid analogues and their physical properties, J. Chem. Eng. Data 59 (2014) 2242–2251. [15] V.H. Paschoal, L.F.O. Faria, M.C.C. Ribeiro, Vibrational spectroscopy of ionic liquids, Chem. Rev. 117 (2017) 7053–7112. [16] P.A. Hunt, C.R. Ashworth, R.P. Matthews, Hydrogen bonding in ionic liquids, Chem. Soc. Rev. 44 (2015) 1257–1288.