Article pubs.acs.org/IECR
Synthesis, Characterization, and Properties of Ether-Functionalized 1,3-Dialkylimidazolium Ionic Liquids Jianhao Zhang,† Shaohua Fang,*,† Long Qu,† Yide Jin,† Li Yang,*,†,‡ and Shin-ichi Hirano‡ †
School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, No. 800, Dongchuan Road, MinHang District, Shanghai 200240, China ‡ Hirano Institute for Materials Innovation, Shanghai Jiaotong University, Shanghai 200240, China S Supporting Information *
ABSTRACT: A family of new ether-functionalized ionic liquids based on 1,3-dialkylimidazolium cation with alkoxymethyl or alkoxyethyl group and TFSA anion were synthesized and characterized. The properties of these ionic liquids, including melting point, thermal stability, density, viscosity, conductivity, and electrochemical window, were determined and compared with those of 3 typical 1,3-dialkylimidazolium ionic liquids without ether group. The effect of ether group on the properties was systematically investigated. All these ionic liquids were liquid at room temperature, and their melting points were lower than −60 °C. It was demonstrated that alkoxyethyl group was favorable to decreasing viscosity of 1,3-dialkylimidazolium ionic liquids, and alkoxymethyl group was not helpful for decreasing viscosity. At room temperature, 6 new ionic liquids had the viscosities lower than 45 cP, and the viscosity of IM2o2-2-TFSA was only 31.3 cP. leading to evident degradation of electrochemical stability.41−43 One ether group has already been introduced to imidazolium, tetraalkylammonium, tetraalkylphosphonium, pyrrolidinium, piperidinium, pyrazolium, guanidinium, oxazolidinium, and morpholinium cations.25,44−48 Besides, ILs with two or more ether groups also attract researchers’ interests. Han et al. have reported a series of tertaalkylammonium ILs with two, three, or four 2-methoxyethyl groups.49 Chen et al. have synthesized a series of tetraalkylammonium ILs with two identical or different ether groups, and these ILs are used as solvents of microcrystalline cellulose.50 Recently, our group has prepared pyrrolidinium, piperidinium, guanidinium, tetraalkylammonium, and 1,2,3-trialkylimidazolium ILs with two or more ether groups, and their performances as electrolytes in lithiumion batteries are also discussed.51−54 For ether-functionalized ILs, the structure of an ether group can influence their physicochemical properties, especially for thermal stability and viscosity. Usually, alkoxymethyl (1oR) group and alkoxyethyl (2oR) group are chosen and connected to nitrogen or phosphorus atom in the IL cation. Ether group can reduce thermal stability of ILs, and the 1oR group is more negative for thermal stability than the 2oR group.45,46,50,55 For instance, the thermal decomposition temperature of Nmethoxymethyl-N,N,N-methylammonium bis(trifluoromethyldulfonyl)imide (N222(1o1)-TFSA, 287 °C) is lower than that of N-ethoxymethyl-N,N,N-methylammonium TFSA (N222(2o1)-TFSA, 384 °C).45 The relationship between the structure of the ether group and the viscosity of ILs is intricate. For tetraalkylammonium, tetraalkylphosphonium, and pyrrolidinium ILs, the 1oR group is more effective in
1. INTRODUCTION Molten salts with melting points at or below room temperature are called as ionic liquids (ILs).1−3 Owing to their desirable properties including low volatility and melting point, good thermal and electrochemical stability, and high ion conductivity, ILs have been applied as new media in organic reactions,4−7 separation technologies,8−11 catalytic processes,12−17 and also safe electrolytes for electrochemical devices.18−21 Usually ILs are based on organic cations with nitrogen, phosphorus, or sulfur as central atoms, such as imidazolium, tetraalkylammonium, pyrrolidinium, piperidinium, pyrazolium, guadinium, tetraalkylphosphonium, and sulfonium cations.22,23 1,3-Dialkylimidazolium ILs have been investigated more widely in various kinds of ILs, due to their outstanding advantages including convenient preparation, low viscosity, and high conductivity.24−27 1-Ethyl-3-methylimidazolium (EMI) ILs are representatives of 1,3-dialkylimidazolium ILs.25,28 In 1982, the report of EMI chloroaluminate (EMI-AlCl4) with the viscosity of 18 cP at 25 °C29,30 is regarded as a milestone in the research area of ILs, but the moisture sensitivity of AlCl4 anion restricts its application. Afterward, a series of relatively more fluid ILs composed of EMI cation and different anions have been reported, such as EMI fluorohydrogenate (EMI-[F(HF)2.3], 4.8 cP at 25 °C),31 EMI dicyanamide (EMI[N(CN)2], 21 cP at 20 °C),32 EMI bis(fluorosulfonyl)imide (EMI-FSI, 18 cP at 25 °C),33 and EMI 2,2,2-trifluoro-N(triluoromethylsulfonyl)acetamide (EMI-TSAC, 25 cP at 25 °C).34 An attractive feature of ILs is that their physicochemical and electrochemical properties can be tuned remarkably by introducing different functional groups into cation.35,36 The ester group and nitrile group with electron-withdrawing effect can result in inevitable increasing of viscosity.37−40 Nevertheless, incorporating an ether group with electron-donating effect can decrease melting point and viscosity of ILs without © XXXX American Chemical Society
Received: July 8, 2014 Revised: October 1, 2014 Accepted: October 5, 2014
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dx.doi.org/10.1021/ie502716p | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. Structures of these 1,3-dialkylimidazolium ILs.
reducing the viscosity than the 2oR group.45,47,50,55 For instance, the viscosity of N-methyl-N-ethoxymethylpyrrolidinium TFSA (Py1(1o2)-TFSA, 38.7 cP) is lower than that of Nmethyl-N-methoxyethylpyrrolidinium TFSA (Py1(2o1) -TFSA), 54.8 cP) at 25 °C.56 For 1,3-dialkylimidazolium ILs, the available data seem to be contradictory. Fei et al. have reported that 1,3-dialkylimidazolium ILs with 1oR group had lower viscosities than ILs with 2oR group. For example, the viscosity of 1-methyl-3-methoxymethylimidazolium TFSA (IM1o1-1-TFSA, 49.9 cP) is lower than that of 1-methyl-3methoxyethylimidazolium TFSA (IM2o1-1-TFSA, 58.5 cP) at 21 °C.57 Then, Zhou et al. have illustrated that the viscosities of 1,3-dialkylimidazolium ILs with 2oR group were lower than those of ILs with 1oR group. For example, the viscosity of 1methyl-3-methoxyethylimidazolium perfluoroethyltrifluoroborate (IM2o1-1-C2F5BF3, 38 cP) is lower than that of 1methyl-3-methoxymethylimidazolium perfluoroethyltrifluoroborate (IM1o1-1-C2F5BF3, 47 cP) at 25 °C.46 Hence, it is meaningful to further confirm the relationship between the structure of the ether group and the viscosity of 1,3dialkylimidazolium ILs. Until now, all the reported 1,3-dialkylimidazolium ILs with one ether group are derivatives of N-methylimidazole.25,27,42,46,56−64 Moreover, the research about 1,3-dialkylimidazolium ILs with two ether groups is very rare. Pernak et al. have synthesized a series of 1,3-dalkylimidazolium ILs with two identical ether groups,65 but long ether group and high
symmetry result in a high viscosity and melting point of these ILs. In order to discover more ILs with low viscosity and melting point, we prepared 23 ILs based on 1,3-dialkylimidaozlium cations with one or two short ether groups in this work. The structures of ILs are shown in Figure 1. The alkyl groups are represented by Arabic numbers. 1o1, 1o2, 2o1, and 2o2 correspond to methoxyethyl, ethoxymethyl, methoxyethyl, and ethoxyethyl groups, respectively. Nineteen ILs were reported for the first time apart from IM1o1-1-TFSA, IM1o21-TFSA, IM2o1-1-TFSA, and IM2o2-1-TFSA. The physicochemical properties, including melting point, thermal decomposition temperature, density, viscosity, conductivity, and electrochemical window, were systematically studied and compared with those of 3 typical 1,3-dialkylimidazolium ILs without ether group. It was proven that the 2oR group was favorable to decreasing viscosity of 1,3-dialkylimidazolium ILs, and 1oR group was not helpful for decreasing viscosity.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Ether-Functionalized 1,3-Dialkylimidazolium ILs. At first, one ether group was introduced into the imidazole ring to substitute the hydrogen atom at the N-1 position. Then, another alkyl group or ether group was added into the imidazole ring to acquire a 1,3-dialkylimidazolium cation. IM2o1-2-TFSA was selected as an example to illustrate the process of synthesis. Imidazole (10.2 g, 0.15 mol) was mixed B
dx.doi.org/10.1021/ie502716p | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Table 1. Properties of These 1,3-Dialkylimidazolium ILs ionic liquids IM1o1-R Series IM1o1-1-TFSA IM1o1-2-TFSA IM1o1-3-TFSA IM1o1-4-TFSA IM1o1-2o1-TFSA IM1o1-2o2-TFSA IM1o2-R Series IM1o2-1-TFSA IM1o2-2-TFSA IM1o2-3-TFSA IM1o2-4-TFSA IM1o2-2o1-TFSA IM1o2-2o2-TFSA IM2o1-R Series IM2o1-1-TFSA IM2o1-2-TFSA IM2o1-3-TFSA IM2o1-4-TFSA IM2o1-2o1-TFSA IM2o2-R Series IM2o2-1-TFSA IM2o2-2-TFSA IM2o2-3-TFSA IM2o2-4-TFSA IM2o2-2o1-TFSA IM2o2-2o2-TFSA IMm-R Series EMI-TFSA PMI-TFSA BMI-TFSA
MWa/g mol−1
Tmb/°C
dc/g cm−3 ± 5%
ηd/mPa s ± 5%
σe/mS cm−1 ± 5%
Tdf/°C
407.4 421.4 435.4 449.4 451.4 465.4
