Article pubs.acs.org/IECR
Ether-Functionalized Trialkylimidazolium Ionic Liquids: Synthesis, Characterization, and Properties Yide Jin,† Shaohua Fang,*,† Ming Chai,† Li Yang,*,† and Shin-ichi Hirano‡ †
School of Chemistry and Chemical Technology, and ‡Hirano Institute for Materials Innovation, Shanghai Jiaotong University, Shanghai 200240, China S Supporting Information *
ABSTRACT: A family of new ether-functionalized ILs based on trialkylimidazolium cations with one or two ether groups and TFSA− anion was synthesized and characterized. Their properties including melting point, thermal stability, viscosity, conductivity, and electrochemical windows were determined and compared to those of the trialkylimidazolium ILs without ether group. The relationship between the cations structure and IL physicochemical properties was systematically studied. Most of these ether-functionalized ILs were liquids at room temperature, and the melting points of 21 ILs were lower than −60 °C. At room temperature, 26 ILs owned the viscosities lower than 100 mPa s, and the viscosities of IM(2o1)12TFSA and IM(2o2)12TFSA were 57.4 and 54.4 mPa s.
1. INTRODUCTION Ionic liquids (ILs) are molten salts with melting points at or below room temperature, which are composed of organic cations and various anions. During the past decade, ILs have attracted great interest for application as new media in organic reactions,1 catalysis processes,2 and separation technologies,3−6 and more often as potential electrolytes in various electrochemical devices, including lithium ion batteries,7−9 electrochemical capacitors,10−12 fuel cells,13,14 and dye-sensitized solar cells.15−17 This is mainly attributable to their unique properties, such as very low vapor pressure, nonflammability, good thermal stability, great chemical and electrochemical stability, and high conductivity.1,18,19 Currently, most of the research on ILs is associated with the 1,3-dialkylimidazolium ILs, for this kind of ILs not only is prepared easily but also has low viscosities and high conductivities, which are of benefit to IL applications.10,20−26 Nevertheless, the electrochemical applications of the 1,3dialkylimidazolium ILs have been restricted, because of the low cathodic stability caused by the acidic proton at the C-2 position of the imidazolium ring (i.e., the reduction potential is about 1 V versus Li/Li+).20−22 1,2,3-Trialkylimidazolium ILs are acquired after substituting the acidic proton at the C-2 position in 1,3-dialkylimidazolium cations by alkyl group, and the electrochemical stabilities can be improved obviously, which makes these ILs become very attractive electrolytes in some electrochemical devices (i.e., lithium ion batteries) requiring wide electrochemical windows, although their viscosities increase slightly with increasing cation size.25,30,31 Nowadays, functionalized IL is a very noticeable topic in the field of IL research. By introducing different functional groups into IL cations, the physicochemical and electrochemical properties of ILs can be greatly tuned, providing more choices for the applications.23,24 When electron-withdrawing groups, such as the nitrile group and ester group, were introduced into the 1,3-dialkylimidazolium, quaternary ammonium, pyrrolidinium, and piperidinium cations, the thermal stability and © 2012 American Chemical Society
electrochemical stability were improved, but the high viscosity restricted their applications.26,27,34,35 Contrastingly, incorporating ether group with electron-donating ability into cations can help to reduce viscosities and molting points, and not cause the obvious degradation of electrochemical stability.22,36−38 One ether group has already been introduced into different cations, such as 1,3-dialkylimidazolium, quaternary ammonium, pyrrolidinium, piperidinium, morpholinium, oxazolidinium, guanidinium, sulfonium, and quaternary phosphonium ILs.9,25−33 Meanwhile, researchers have paid attention to the ILs with two or more ether groups. A series of symmetrical 1,3dialkoxymethyl-substituted imidazolium ILs have been reported, but the viscosities of these ILs are higher than 80 mPa s at 25 °C due to long chains of ether groups.34 Several quaternary ammonium ILs based on cations with two identical ether groups (2-ethoxyethyl or 4-methoxybenzyl group) are synthesized, and the thermal properties of these ILs have been investigated.35 Recently, some quaternary ammonium ILs comprised of multi identical ether groups (2-methoxyethyl group) in cation are prepared, and their physicochemical properties have been studied.26 Our group also has prepared guanidinium, pyrrolidinium, and piperidinium ILs with two ether groups and quaternary ammonium ILs with three or four different ether groups, and their possibility as electrolytes for lithium ion battery is also explored.7,8,36 Thus far, research involving the ether-functionalized 1,2,3trialkylimidazolium ILs is quite rare. Two salts based on the 1(2-methoxyethyl)-2,3-dimethylimidazolium cation (IM(2o1)11+) and Cl− or PF6− anions have been investigated for their thermal behaviors.27 Monteiro et al. have studied the transport properties of lithium ion in the 1-(2-ethoxyethyl)-2,3dimethylimidazolium bis(trifluoromethylsulfonyl) imide (IMReceived: Revised: Accepted: Published: 11011
March 31, 2012 August 2, 2012 August 7, 2012 August 7, 2012 dx.doi.org/10.1021/ie300849u | Ind. Eng. Chem. Res. 2012, 51, 11011−11020
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Article
Figure 1. Structures of ether-functionalized trialkylimidazolium ILs with one or two ether groups and the trialkylimidazolium ILs without ether group.
(2o2)11TFSA) electrolyte.37 Furthermore, IM(2o1)11TFSA and IM(2o2)11TFSA have also been applied as new electrolytes for rechargeable magnesium battery.38 In the present work, we reported the synthesis and characterization of 30 ether-functionalized trialkylimidazolium ILs with one or two ether groups. The structures of these ILs are shown in Figure 1, and 28 ILs were reported for the first time except IM(2o1)11TFSA and IM(2o2)11TFSA. The melting point, thermal stability, viscosity, conductivity, and electrochemical windows of these ILs were systematically investigated, and compared to the four trialkylimidazolium ILs based on small cation without ether group.
with deionized water until no residual halide anions in the deionized water used to rinse the IL could be detected by AgNO3. The dichloromethane was removed by rotating evaporation. The product IL was dried under high vacuum for more than 24 h at 105 °C. The detailed synthesis procedure for each IL and its NMR characterization data of 1H and 13C could be seen in the Supporting Information. 2.2. Preparation of Trialkylimidazolium ILs without Ether Group. The four trialkylimidazolium ILs without ether group (IM112TFSA, IM113TFSA, IM114TFSA, IM115TFSA) were synthesized according to reported procedures and dried before use.10,39,40 2.3. Measurement. The structure of synthesized IL was confirmed by 1H NMR and 13C NMR (Avance III 400), and chloroform-d or acetone-d6 was used as solvent. The water content of the dried IL was detected by a moisture titrator (Metrohm 73KF coulometer) based on the Karl Fischer method, and the value was less than 50 ppm. Calorimetric measurement of IL was performed by using a differential scanning calorimeter (DSC, Perkin-Elmer Pyris 1) in the temperature range −60 °C to a predetermined temperature. Each sample with an average weight of 4−6 mg was sealed in an aluminum pan in a dry chamber, and then heated and cooled at a scan rate of 10 °C min−1. The thermal data were collected during heating in the second heating− cooling scan. The thermal stability was measured with TGA (Perkin-Elmer, 7 series thermal analysis system). Each sample with an average weight of 4−6 mg was placed in the platinum pan and heated at 10 °C min−1 from room temperature to 600 °C under nitrogen. The viscosity value was measured by using a viscometer (DV-III ULTRA, Brookfield Engineering Laboratories, Inc.). The density was determined by measuring the weight of prepared IL (1.0 mL) in a dry chamber at 25 °C. The ionic conductivity was measured by using DDS-309+ conductivity
2. EXPERIMENTAL SECTION 2.1. Preparation of Ether-Functionalized Trialkylimidazolium ILs. In a typical experiment, the synthesize procedure of IM(2o1)12TFSA was regarded as an example. The imidazole with one ether group IM(2o1)1 was prepared by the following steps: 2-methyl-imidazole reacted with a slightly excess amount of chloroethyl methyl ether at 140 °C for 72 h in a 100 mL autoclave. The brown liquid product was washed with 100 mL of an ethanol and ether (1/4 v/v) mixed solution three times and then reacted with triethylamine (20.2 g, 200 mmol) at 130 °C for 24 h in a 100 mL autoclave again. The solid salt formed was filtered off, and the residue was distilled under reduced pressure using a 20 cm vigreux-columne. The product was collected at 160−162 °C (boiling point) when the pressure was about 10 Pa. Next, the imidazole with one ether group was mixed with an excess amount of bromoethane at 60 °C for 48 h in a 250 mL flask with acetonitrile as solvent. The produced bromide was acquired after washing with ether. The bromide was recrystallized twice from acetone and THF, and dried under high vacuum at 60 °C. The bromide and LiTFSA were dissolved in deionized water and mixed for 24 h at ambient temperature. The crude IL was dissolved with dichloromethane and washed 11012
dx.doi.org/10.1021/ie300849u | Ind. Eng. Chem. Res. 2012, 51, 11011−11020
Industrial & Engineering Chemistry Research
Article
meter in a dry chamber. The electrochemical stability was investigated by linear sweep voltammogram (LSV) measurement, which was performed by using CHI 660D electrochemical working station in an argon-filled UNILAB glovebox ([O2] < 5 ppm, [H2O] < 1 ppm). Glassy carbon (3 mm diameter) was used as the working electrode. Platinum wire (2 mm diameter) was used as the counter electrode. Silver wire (2 mm diameter) was used as the quasi-reference electrode as in many researchers’ similar works.41−46 The glassy carbon electrode was polished with alumina paste (d = 0.1 μm), and the polished electrode was washed with deionized water and dried under vacuum. In the LSV test, the freshly polished glassy carbon electrode was used in two scans (positive and negative, respectively) for one IL, and the fresh IL was used in each scan.
Table 1. Physical and Thermal Properties of the Ionic Liquids ionic liquids IM(2o1)11TFSA IM(2o1)12TFSA IM(2o1)13TFSA IM(2o1)14TFSA IM(2o1)15TFSA IM(2o1)1(2o1)TFSA IM(2o1)1(2o2)TFSA IM(2o1)1(3o1)TFSA IM(2o1)21TFSA IM(2o1)22TFSA IM(2o1)23TFSA IM(2o1)24TFSA IM(2o1)25TFSA IM(2o1)2(2o1)TFSA IM(2o1)2(2o2)TFSA IM(2o1)2(3o1)TFSA IM(2o2)11TFSA IM(2o2)12TFSA IM(2o2)13TFSA IM(2o2)14TFSA IM(2o2)15TFSA IM(2o2)1(2o2)TFSA IM(2o2)1(3o1)TFSA IM(2o2)21TFSA IM(2o2)22TFSA IM(2o2)23TFSA IM(2o2)24TFSA IM(2o2)25TFSA IM(2o2)2(2o2)TFSA IM(2o2)2(3o1)TFSA IM112-TFSA IM113-TFSA IM114-TFSA IM115-TFSA
3. RESULTS AND DISCUSSION 3.1. Thermal Properties. The physical and thermal properties of 30 ether-functionalized trialkylimidazolium ILs with one or two ether group and four trialkylimidazolium ILs without ether group, including melting point, density, viscosity, conductivity, and thermal decomposition temperature, are indicated in Table 1. The phase transitions of these ether-functionalized trialkylimidazolium ILs were investigated by differential scanning calorimetry (DSC), and the DSC traces of five ILs are shown in Figure 2 as examples. IM(2o1)15TFSA (Figure 2c), IM(2o1)24TFSA (Figure 2d), and IM(2o1)1(2o2)TFSA owned a crystallization transition (Tc) before melting transition (Tm), the IM(2o1)13TFSA (Figure 2b) and IM (2o1)2(2o1)TFSA showed a solid−solid transition (Ts‑s) before melting, and IM(2o1)22TFSA (Figure 2e), IM(2o1)23TFSA, IM(2o1)25TFSA, and IM(2o1)1(2o1)TFSA exhibited only a melting transition. Like IM(2o1)11TFSA (Figure 2a), the other 20 ether-functionalized ILs did not show any phase transition behaviors until −60 °C during the heat−cooling scan processes, which is the inferior temperature limit of our DSC measurement. It was possible that some ILs may form supercooling liquid, and we denoted their melting points by “
