CO2 Solubility in Ether Functionalized Ionic Liquids on Mole Fraction

Research Article pubs.acs.org/journal/ascecg

CO2 Solubility in Ether Functionalized Ionic Liquids on Mole Fraction and Molarity Scales Mitsuhiro Kanakubo,*,† Takashi Makino,† Takayuki Taniguchi,‡ Toshiki Nokami,‡ and Toshiyuki Itoh‡ †

National Institute of Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University 4-101 Koyama-minami, Tottori 680-8552, Japan

‡

S Supporting Information *

ABSTRACT: The effect of ether functional group(s) in the cation, anion, and both of the ions in ionic liquids on physical absorption of CO2 are revisited in the present work. The solubilities of CO2 in ether functionalized ammonium and pyrrolidinium salts, [N211MEE][Tf2N], [Pyr1MOM][Tf2N], and [Pyr1MOM][FSA] were previously reported together with the corresponding alkyl analogues. In addition to such cation-modified ionic liquids, we investigate a new family of ether functionalized ionic liquids with alkoxy sulfates; [C2mim][C1(OC2)2SO4], [P444ME][C6SO4], [P444ME][C1OC2SO4], [P444ME][C1(OC2)2SO4], and [N221ME][C1(OC2)2SO4]. The CO2 solubility data on the molarity as well as the mole fraction scales are presented in a series of ether functionalized ILs with a brief overview of the previously reported results. It has become apparent that the introduction of an ether functional group in anions more effectively improves the CO2 solubilities on both the mole fraction and molarity scales than that in cations. The effects of the cation, anion, and dual functionalization with ether groups on the physical solubility of CO2 are discussed with the volumetric properties in terms of the molecular structures of ILs. KEYWORDS: Ionic liquids, Ether group, Functionalization, CO2, Solubility, Molarity

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INTRODUCTION Ionic liquids (ILs) are less flammable, nearly involatile, chemically and thermally stable, and exist as liquids over a wide range of temperatures. Because traditional industrial solvents are often volatile organic compounds (VOCs), which possibly leads to atmospheric pollution, nonvolatile ILs are expected to be potential alternatives to such environmentally hazardous solvents. Moreover, ILs show high solubility for acidic gases, whereas they are generally insoluble in gas phase. Accordingly, ILs have received much attention as physical and/ or chemical absorbents to separate acidic gases. Green gas separation technologies as well as a variety of sustainable chemical reaction processes such as alkylation, hydrogenation, hydroformylation, and so on, are expected to be promising achievements. Replacement of organic solvents by ILs will open new opportunities; for examples, high-temperature gases can be directly treated without cooling and possibly without condensers, and IL absorbents can be regenerated at reduced pressures with and without heating. These process develop© XXXX American Chemical Society

ments will be realized by use of ILs. To promote such technologies and processes, precise solubility data of gases in ILs with deep understanding in relation to their molecular structures are of great importance. Since the first report of Blanchard et al.,1 solubilities of gases, particularly solubilities of CO2, in ILs have extensively been investigated by many researchers and reviewed to date.2−9 We have also studied the solubilities of CO2 in a series of ILs at high pressures.10−16 A Brønsted acid−base ionic liquid composed of N,N-dimethylformamide, DMF, and CO2-philic bis(trifluoromethanesulfonyl)amide, HTf2N gave slightly higher solubility of CO2 than DMF.10 High-pressure CO2 solubilities Special Issue: Ionic Liquids at the Interface of Chemistry and Engineering Received: August 29, 2015 Revised: November 25, 2015

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DOI: 10.1021/acssuschemeng.5b00960 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Structures of Ether Functionalized ILs Studied in Our Group

in 1-ethyl-3-methylimidazolium tetracyanoborate, [C2mim][TCB] and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, [C2mim][Tf2N] have been determined; [C2mim][TCB] had a slightly larger solubility for CO2 on the mole fraction scale than [C2mim][Tf2N], whereas [C2mim][TCB] showed much superior solubility on the molarity scale because of the smaller molar volume.16 The effects of ether and ester functional groups in the ammonium cations were investigated with a common anion, [Tf2N]−, revealing that the ether functionalization enhances the molarity scaled CO2 solubility whereas the ester group reduces it.15

There have been many efforts to improve CO2 solubility by designing structures of cation and anion and/or introducing functional groups therein. For example, Brennecke and coworkers2,17 revealed that introduction of fluorine atoms in the cations and anions can effectively enhance CO2 solubility in ILs. They also investigated the effect of CO2-philic functional groups such as ether and ester on CO2 solubility. Noble et al.3,18 synthesized imidazolium salts with oligo(ethylene glycol) substituents, which have similar levels of CO2 solubility but lower solubilities of N2 and CH4 in comparison with the corresponding alkyl analogues. Costa Gomes et al.19,20 also investigated the effect of either ester or ester and ether B


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ACS Sustainable Chemistry & Engineering

functionalized ILs with a brief overview of the results previously reported.

functional groups in the side chain of 1-alkyl-3-methylimidazolium cation, and concluded that their solubilities are of the same order of magnitude as for alkylimidazolium-based ionic liquids. Other than ether and ester functional groups, solubility of CO2 in hydroxyl functionalized ILs, all on the cations, was examined by several researchers.12,17,21−24 Unfortunately, the effect was generally negative, whereas in poor ILs introduction of hydroxyl group can improve CO2 solubility.22 Some of the results were summarized in the recent review,6 targeted for ether- and alcohol functionalized task-specific ILs. On physical solubility of CO2, to our best knowledge, the effects of functional groups in ILs such as ester, hydroxyl, and nitrile groups are always negative or negligible in comparison with those of the corresponding alkyl analogues except for fluoroalkyl and ether groups. Because fluorination is generally cost-intensive, the ether functionalization is one of possible practical candidates to enhance CO2 solubility in ILs. Incorporation of an ether group in ILs improves biodegradability6,25 as well as efficiently reduces viscosity,26−29 of which the latter can favorably increase kinetics in gas absorption and desorption processes. Molecular dynamics simulations and ultrafast dynamics experiments demonstrated that a viscosity decrease is attributed to a more flexible nature of an ether group.30−34 The flexible ether group results in remarkable decrease in intrinsic free volume in ILs, which could lead to increase in CO2 selectivity against N2 and CH4 as revealed by Bara and co-workers.35,36 Recently, Hu et al.4 have also proposed the cavity formation mechanism that the flexibility of ionic structures in ILs is the predominant factor to dissolve gas molecules in relation to nanosegregated domains. Thus, one readily supposes that it is important to understand the pressure−temperature−volume-composition (pTVx) behavior in binary systems of CO2 and ether functionalized ILs; however, most literature dealt with the pressure−temperature−composition (pTx) relations without the volumetric information. Moreover, it is noted that for more acidic and polar SO2, ether functionalized ILs actually exhibited excellent high solubility even if the mechanism is physical absorption.37−39 In the present work, we revisit the effect of ether functional group(s) in the cation, anion, and both of ions in ILs on physical absorption of CO2. The solubilities of CO2 in ether functionalized ammonium and pyrrolidinium salts, N,Ndimethyl-N-ethyl-N-2-(2-methoxyethoxy)ethylammonium bis(trifluoromethanesulfonyl)amide ([N211MEE][Tf2N]), N-methoxymethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([Pyr1MOM][Tf2N]), and Nmethoxymethyl-N-methylpyrrolidinium bis(fluorosulfonyl)amide ([Pyr1MOM][FSA]) were previously reported together with the corresponding alkyl analogues.13,15 In addition to such cation-modified ILs, we newly investigate a family of ether functionalized ILs with alkoxy sulfates; 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy)ethyl sulfate ([C 2 mim][C1(OC2)2SO4]), tri-n-butyl(2-methoxyethyl)phosphonium nhexyl sulfate ([P444ME][C6SO4]), tri-n-butyl(2-methoxyethyl)phosphonium 2-methoxyethyl sulfate ([P444ME][C1OC2SO4]), tri-n-butyl(2-methoxyethyl)phosphonium 2-(2methoxyethoxy)ethyl sulfate ([P444ME][C1(OC2)2SO4]), and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium 2-(2methoxyethoxy)ethyl sulfate ([N221ME][C1(OC2)2SO4]), of which the chemical structures and abbreviations are given in Table 1 together with the cation modified ILs in our previous papers.13,15 The CO2 solubility data on the molarity as well as the mole fraction scales are presented in a series of ether

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EXPERIMENTAL SECTION

Materials. The five ILs, [P444ME][C6SO4], [P444ME][C1OC2SO4], [P444ME][C1(OC2)2SO4], [N221ME][C1(OC2)2SO4], and [C2mim][C1(OC2)2SO4], were prepared at Tottori University. The typical synthetic procedure was given for [P444ME][C1(OC2)2SO4]. First, a precursor salt of ammonium 2-(2-methoxyethoxy)ethyl sulfate, [NH4][C1(OC2)2SO4] was prepared by mixing sulfamic acid (44.7 g, 0.46 mol) and 2-(2-methoxyethoxy)ethanol (110 g, 0.92 mol) at 110 °C for 70 h under argon and dried under reduced pressure (1.0 Torr) at 60 °C for 3 h to give [NH4][C1(OC2)2SO4] (99.9 g, 0.46 mol) as a white solid. This salt was used to the next reaction without further purification. Second, a mixture of [P444ME]Cl (21.0 g, 71 mol) and [NH4][C1(OC2)2SO4] (21.7 g, 100 mmol) in acetone (71 mL) was stirred at room temperature for 24 h, then evaporated to dryness to give a brown oil, which was washed with toluene (3 times) and ether, and again evaporated to dryness. The resulting oil was dissolved in a mixed solvent of acetone with acetonitrile (1:1) and treated with active charcoal, and the charcoal was then removed by filtration. The filtrate was dried under vacuum at 50 °C for 5 h to afford [P444ME][C1(OC2)2SO4] (25.2 g, 54.7 mmol) in 77% yield as a light brown oil. 1H, 13C, and 31P NMR spectra were recorded on a JEOL JNM MH-500: 1H NMR (500 MHz, CDCl3, δ) 0.976 (t, J = 7.0 Hz, 3H), 1.49−1.53 (m, 12H), 2.24−2.30 (m, 6H), 2.75 (dt, JHP = 12.0 Hz, J = 5.0 Hz, 2H), 3.34 (s, 3H), 3.37 (s, 3H), 3.53 (t, J = 5.0 Hz, 2H), 3.56−3.63 (m, 2H), 3.66−3.68 (m, 2H), 3.74−3.82 (m, 2H), 4.19 (dt, J = 5.5 Hz, 2H); 13C NMR (125 MHz, CDCl3, δ) 13.37, 19.18, 19.56, 23.61 (d, JCP = 4.8 Hz), 23.89 (d, JCP = 15.5 Hz), 58.84, 58.96, 61.67, 66.25, 70.19 (d, JCP = 16.9 Hz), 71.88 (d, JCP = 3.6 Hz), 72.43; 31P NMR (202 MHz, CDCl3, δ) 39.14. IR spectra were obtained on SHIMADZU FT-IR 8000: IR (neat) 2959, 2934, 2834, 1641, 1466, 1409, 1384, 1353, 1225, 1029, 919 cm−1. The other ILs were synthesized in a similar manner, of which the detailed procedures are given in the Supporting Information together with 1H and 13C NMR spectra. The purities were evaluated to be better than 95 mol % by NMR (JEOL JNM MH-500, 1H and 13C resonances). The ILs transferred were again dried under vacuum at about 70 °C for 30 h to remove any excess water just prior to measurements at AIST. The appropriate amount of the dried IL was loaded to the high-pressure cell under Ar atmosphere in a glovebox. The water contents of the IL samples were determined by Karl Fischer titration (Kyoto Electronics Manufacturing Co., Ltd., MKC-510) and summarized in Table 1. Measurements. All the measurements were performed at AIST in Sendai. The pVTx behavior in the IL-CO2 system was determined with the volume expansion and CO2 absorption experiments as detailed previously.12 In the former experiment, the volume expansion ΔVL(T, p) in the IL phase was obtained: ΔV L(T , p) =

V L(T , p) − V L(T , p0 ) V L(T , p0 )

(1)

L

where V (T, p) is the volume in the IL phase at a temperature T and pressure p, and VL(T, p0) is the volume in the IL phase at the same T and atmospheric pressure p0. Several measurements repeated at different pressures provided a polynomial of ΔVL(p) as a function of p at each T. In the latter experiment, the molar amount of CO2 nL2 (T, p) dissolved in the known amount n1 of IL was obtained by reading the pressure drop in the high-pressure system:

n2L(T , p) = n2i −

Vcell(T , p) − V L(T , p) V mG(T , p)

(2)

ni2

is the molar amount of loaded CO2, Vcell(T, p) is the total inner volume of the apparatus that has a slight positive temperature dependence but negligible small pressure dependence in the present experimental condition (Vcell(T, p) ≈ Vcell(T)), VGm(T, p) is the molar volume of CO2 in the gas phase calculated by NIST REFPROP Ver. 9.0,40 and VL(T, p) is given by eq 1. We assume that dissolution and/ C


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ACS Sustainable Chemistry & Engineering or vapor of ILs in the CO2 phase is negligibly small. Actually, this is the case for not only aprotic but also protic ILs composed of strong acids and bases at moderate temperatures.14 The mole fraction x2 of CO2 in the IL phase, the molarity c2 of CO2 in IL, and the molar volume VmL in the liquid phase were derived from n1, nL2 , and VL. The uncertainties in the temperature and pressure are within ±0.02 K and ±0.002 MPa, respectively. The uncertainties are estimated as ±0.002 for x2, ± 0.8% for c2, and ±0.7% for VLm. The atmospheric pressure densities and viscosities were measured using a vibrating tube densimeter (Anton Paar, DMA 5000M) and rotating-cylinder viscometer (Anton Paar, Stabinger SVM 3000).41,42 The built-in viscosity correction for the densimeter was employed. The uncertainty in the temperature is ±0.01 K for both measurements. The estimated expanded uncertainties are less than ±0.05 kg m−3 and ±2% for the densimeter and viscometer, respectively. The van der Waals volumes, VvdW, of cations and anions were evaluated by ab initio calculations. Geometrical optimizations in the gas phase for the isolated ions were performed at the DFT/6-311G+ level using Gaussian 09 program suites.43 In the calculations, alkyl and alkoxy chains in ILs were optimized as a fully extended configuration.

compared to the corresponding alkyl analogues between ([N7211][Tf2N] and [N211MEE][Tf2N]) and ([Pyr1MOM][Tf2N] and [Pyr31][Tf2N]). However, it is interesting that both the functionalized and nonfunctionalized ILs have almost the same solubilities in view of the fact that the free volumes in ether functionalized ILs, which can accommodate CO2 molecules, are still smaller than the corresponding alkyl analogues. This point is again discussed in the later section. Figure 1 also involves the literature values of CO2 solubilities in ether functionalized ILs at the same temperature under high pressures; Ecoeng 500 ([N13,1,2O2OH,2O2O2OH][C1SO4] in our notation),17 [(C2O)2im][Tf2N],44 and [N221ME][Tf2N].45 The earlier work of Brennecke et al. demonstrated that Ecoeng 500 having three ether groups showed similar levels of CO2 solubility, even though it is composed of the poor anion (methyl sulfate) and the cation containing the negative hydroxyl groups. A molecular weight, i.e., volume of Econeng 500, is exceptionally much larger than those in other ILs, keeping the high solubility of CO2, as is seen from the poor molarity scaled solubility in the following section. The results of the other two ether functionalized ILs are in harmony with our observation. A slight increase was observed for the CO2 solubility in [(C2O)2im][Tf2N] in comparison with [C4mim][Tf2N].44 Nonthanasin et al.45 has found that the solubility of CO2 in [N221ME][Tf2N] was comparable to that in [N4111][Tf2N] and higher than those in the hydroxyl functionalized ammonium ILs. It is noted that they claimed that [N221ME][Tf2N] has the highest CO2 solubility in the ammonium-base ILs, but the ammonium and pyrrolidinium ILs studied in our group show higher solubilities. On the basis of the solubility data, the Henry constants, k∞ H, for CO2 were calculated as

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RESULTS In this study, we have newly determined the pTVx behaviors in binary systems of CO2 and five ether functionalized ILs, [C 2 mim][C 1 (OC 2 ) 2 SO 4 ], [P 444ME ][C 6 SO 4 ], [P 444ME ] [C 1 OC 2 SO 4 ], [P 444ME ][C 1 (OC 2 ) 2 SO 4 ], and [N 221ME ] [C1(OC2)2SO4] at 298.15 K, which are summarized in Tables S1−S5, respectively. In the following sections, we discuss the effects of the cation functionalization, and the anion and dual functionalization with ether groups on the physical solubility of CO2 with a brief overview of the previously reported data. In the successive section, the volumetric properties in ether functionalized IL and CO2 systems are also considered. Effect of Cation Functionalization. The mole fraction scaled solubilities, x2, of CO2 in ILs composed of ether functionalized cations are plotted against pressure, p, at 333 K in Figure 1, together with those in [C2mim][Tf2N] as a reference. As is frequently observed in ILs, the CO2 solubility is less significantly dependent on the cation species, in contrast to the anion, though a slight difference is detected. The effect of the ether functionalization on the cations’ side chains are negligibly small on the mole fraction scaled solubilities

⎛f ⎞ kH∞ = lim ⎜ 2 ⎟ p → 0⎝ x 2 ⎠

(3)

where f 2 is the fugacity of CO2 obtained by NIST REFPROP Ver. 9.0.40 The results of k∞ H are listed in Table 2 with the physical properties. Although the high-pressure phase equilibria data were unavailable, Table 2 contains the reported values of k∞ H for several 1-ether-functionalized-3-methylimidazolium salts (entries 13−18), which were obtained by an isochoric saturation technique at pressures close to atmospheres. Noble and co-workers 1 8 have determined k H∞ values for [C1(OC2)nmim][Tf2N] with n = 1−3 (entries 13−15). The k∞ H values for [C1(OC2)nmim][Tf2N] were slightly larger than those for the corresponding alkyl analogues, [C1(C3)nmim][Tf2N], for n = 1 and 2, and equivalent for n = 3. Costa Gomes et al.19 revealed that incorporation of ether group in pentoxycarbonylmethyl C5OCOC1− side chain efficiently decreased k∞ H , and one more ether group brought about a similar effect, though the derived thermodynamic properties, enthalpy and entropy, for CO2 dissolution showed similar trends both for the functionalized and nonfunctionalized ILs. Sharma el al.46−48 reported unbelievably high solubilities of CO2 in [C1OC1mim]+ and [C1OC2mim]+ salts with different anions, and proposed a chemisorption mechanism to form a covalent bond between CO2 and ether oxygen. Because all the present ILs consisting of ether functionalized cations, anions, and both ions clearly showed a simple reversible physical absorption behavior, their data are omitted in this paper. Actually, Seo et al.49 recently pointed out that the results

Figure 1. Solubility x2 of CO2 in cation functionalized ILs with ethers against p at 333 K. Symbols: red ●, [N211MEE][Tf2N];15 red ▲, [N221ME][Tf2N];45 red ◆, [N13,1,2O2OH,2O2O2OH][C1SO4] (=Ecoeng 500);17 red ○, [N7211][Tf2N];15 blue ●, [Pyr1MOM][Tf2N];13 blue ▲, [Pyr1MOM][FSA];13 blue ○, [Pyr31][Tf2N];13 gold ●, [(C2O)2im][Tf2N];44 gold ○, [C2mim][Tf2N].14 The errors in x2 are within the symbols in our measurements. D


a

E

3.12 3.38

4.30 3.66 6.95

8.71

3.45 2.96 2.90 3.35 3.29 4.05 4.82

298 K

4.18 4.28 4.35 3.73 4.04 3.63 3.19 4.03 4.42

4.54 3.82 3.83 4.38 4.26 5.36

313 K

0.993 0.961

0.663 0.703 0.488

0.452

1.09 0.980 1.07 1.01 1.08 1.08 0.512

(∂c/∂p)T (mol dm−3 MPa−1)

424.39 421.34 465.39 509.44 490.42 492.39 564.50 424.43 440.39

416.56 460.61 343.44

310.37

391.32 452.48 456.42 408.39 410.36 310.34 442.64

M (g mol−1)

The values of (∂c/∂p)T, ρ, Vm, and η are measured at 298 K. bPresent work. cRef 71.

12 13 14 15 16 17 18 19 20

9 10 11

8

1 2 3 4 5 6 7

cation functionalized [C2mim][Tf2N] [N7211][Tf2N] [N211MEE][Tf2N] [Pyr31][Tf2N] [Pyr1MOM][Tf2N] [Pyr1MOM][FSA] [P444ME][C6SO4] anion functionalized [C2mim][C1(OC2)2SO4] Dual functionalized [P444ME][C1OC2SO4] [P444ME][C1(OC2)2SO4] [N221ME][C1(OC2)2SO4] literature and others [N221ME][Tf2N] [C1OC2mim][Tf2N] [C1(OC2)2mim][Tf2N] [C1(OC2)3mim][Tf2N] [C5OCOC1mim][Tf2N] [C2OC2OCOC1mim][Tf2N] [C4(OC2)2OCOC1mim][Tf2N] [N5211][Tf2N] [N211MEE][Tf2N]

IL

k∞ H /MPa

1408 1500 1510 1430 1414 1483 1387 1341 1459

1078 1078 1193

1268

1519 1291 1391 1428 1483 1405 1075

ρ (kg m−3)

a Table 2. Henry Constants k∞ H of CO2 and Physical Properties for Ether Functionalized ILs

301.4 281 308 356 347 332 407 316.4 301.9

386.4 427.1 287.9

244.7

246.0 350.5 328.1 286.0 276.8 220.8 411.6

Vm (cm3 mol−1)

251 231 269 301 284 287 350 257 259

325 357 262

229

218 293 276 228 235 201 373

VvdW (cm3 mol−1)

50.6 49.5 38.9 55.3 63.0 45.5 56.8 58.9 42.7

61.2 70.2 25.9

15.2

27.7 57.4 52.4 58.0 41.5 20.0 38.9

Vm − VvdW (cm3 mol−1)

0.168 0.176 0.126 0.155 0.182 0.137 0.140 0.186 0.141

0.158 0.164 0.090

0.062

0.113 0.164 0.160 0.203 0.150 0.091 0.095

(Vm − VvdW) /Vm

45 18 18 18 19 19 19 15, 70 15, 70

64.0c

128 208

p.w.b p.w.b p.w.b

p.w.b

14 15, 70 15, 70 13 13 13 p.w.b

ref

491 359 714

325

33.0 168 58.6 61.5 40.6 28.5 1628

η (mPa s)

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ACS Sustainable Chemistry & Engineering presented by Sharma et al.46 are in errors in a Letter to the Editor. Effect of Anion and Dual Functionalization. The mole fraction scaled solubilities, x2, of CO2 in anion and dual functionalized ILs are plotted against pressure, p, at 298 K in Figure 2. There have been the solubility data reported for

dual functionalized ILs at 298 K are plotted as a function of pressure, p, in Figures 3 and 4, respectively. Over the pressure

Figure 3. Molarity c2 of CO2 in cation functionalized ILs with ethers against p at 333 K. Symbols: red ●, [N211MEE][Tf2N];15 red ◆, [N13,1,2O2OH,2O2O2OH][C1SO4] (=Ecoeng 500);17 red ○, [N7211][Tf2N];15 blue ●, [Pyr1MOM][Tf2N];13 blue ▲, [Pyr1MOM][FSA];13 blue ○, [Pyr31][Tf2N];13 red ○, [C2mim][Tf2N].14 The errors in c2 are within the symbols in our measurements.

Figure 2. Solubility x2 of CO2 in anion and dual functionalized ILs with ethers against p at 298 K. Symbols: black ■, [P444ME][C1(OC2)2SO4]; black ▲, [P444ME][C1OC2SO4]; black ○, [P444ME][C6SO4]; red ■, [N221ME][C1(OC2)2SO4]; gold ■, [C2mim][C1(OC2)2SO4]; gold □ and ⊞, [C2mim][C1(OC2)2SO4] (with and without buoyancy correction, respectively, at 303 K);50 gold ○, [C2mim][Tf2N].14 The errors in x2 are within the symbols in our measurements.

[C2mim][C1(OC2)2SO4].50 The present data at 298 K are systematically a little smaller than those measured by Soriano et al.50 using a thermogravimetric microbalance with a buoyancy correction at slightly higher temperature of 303 K. If the buoyancy correction was not made, their results are coming closer to ours. The solubilities in [C4mim][C1(OC2)2SO4] (=Ecoeng 41 M in their notation) of Muldoon et al.,17 which are only available at temperatures higher than 313 K, significantly drop downward (no data point). The variation in the solubility is observed more noticeably for the anion functionalization in Figure 2 than for the cation functionalization in Figure 1. The introduction of ether group(s) in sulfate anions with a common [P444ME]+ cation considerably increases the solubility of CO2 in the order of [C6SO4]− < [C1OC2SO4]− < [C1(OC2)2SO4]−. Although sulfates are not good but relatively poor anions for CO2 dissolution in view of the solubility results in a series of [C2mim]X by Mejiá et al.,51 the ether functionalization can successively improve the CO2 solubility at the same level of [C2mim][Tf2N]. On the other hand, with a common [C1(OC2)2SO4]− anion, the solubilities of CO2 are also strongly dependent on the cations, which follow the order of [C2mim]+ < [N221ME]+ < [P444ME]+. This suggests that the ether functionalization in the cations can make appreciably positive contribution to CO2 solubility in dual functionalized ILs and/or in relatively poor ILs, as it was not very significantly observed in [Tf2N]− salts as mentioned in the former section. In [P444ME][C1(OC2)2SO4], the larger volume of bulky cation, compared to [C2mim]+ and [N221ME]+, also plays an important role as pointed out by several researchers.51−53 There have been no other solubility data, as far as we know, in anion and dual functionalized ILs with ether groups. Volumetric Properties. The molarity scaled solubilities, c2, of CO2 in cation functionalized ILs at 333 K, and in anion and

Figure 4. Molarity c2 of CO2 in anion and dual functionalized ILs with ethers against p at 298 K. Symbols: black ■, [P444ME][C1(OC2)2SO4]; black ▲, [P444ME][C1OC2SO4]; black ○, [P444ME][C6SO4]; red ■, [N221ME][C1(OC2)2SO4]; gold ■, [C2mim][C1(OC2)2SO4]; gold ○, [C2mim][Tf2N].14 The errors in c2 are within the symbols in our measurements.

range studied, the molarity scaled solubility increases almost linearly with p in every IL. The data points in cation functionalized ILs with [Tf2N]− and [FSA]− anions appear to fall on a single straight line except in Ecoeng 500 with [C1SO4]−, of which the data points significantly deviate downward due to their own large molar volume. A careful examination provides that the slopes, (∂c/∂p)T, for the ether functionalized ILs obtained in Table 2 are larger than those in the corresponding alkyl analogues, whereas the Henry constants based on the mole fraction scale give similar values. This mainly arises from the smaller molar volumes of ether functionalized ILs. As is seen from Figure 4, on the other hand, the CO2 solubilities in anion and dual functionalized ILs are quite different and cannot be normalized even on the molarity scale. One of the reasons is that the volume change in the anion and dual functionalized ILs is rather widespread compared to that in the cation functionalized ILs. However, the effect of ether incorporation in the sulfate anions on c2 is still markedly observed with a common [P444ME]+ cation; [C6SO4]− ≪ F


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ACS Sustainable Chemistry & Engineering [C1OC2SO4]− < [C1(OC2)2SO4]−. On the effect in the cations with a common [C1(OC2)2SO4]− anion, the molarities in [P444ME][C1(OC2)2SO4] are much larger than those in [C2mim][C1(OC2)2SO4] and [N221ME][C1(OC2)2SO4], but the difference between the latter two ILs is not so significant. It is noted that the molality scaled solubilities, m2, of CO2 in ether functionalized ILs show almost the similar behaviors with those observed for the molarity scaled solubilities c2 (see Figure S1 in the Supporting Information). In the molality scale, the data points for [P 4 44 M E ][C 1 OC 2 SO 4 ] and [P 4 4 4 M E ] [C1(OC2)2SO4] come closer to those for [C2mim][Tf2N] and the other cation functionalized ILs with [Tf2N]− and [FSA]− anions. Aki et al.54 showed that the volume expansion in the liquid phase accompanied by CO2 dissolution is relatively small in ILs in comparison with molecular solvents. They also revealed that such volume expansion is normalized based on the molar volume scale. The definitions of the volume expansion, ΔVL, and the change in molar volume, ΔVLm, are rewritten as ΔV L(p) =

Figure 6. Expansion of molar volume ΔVLm in the IL phase as a function of x2 at 298 K. Symbols: red ●, [N211MEE][Tf2N];15 red ■, [N221ME][C1(OC2)2SO4]; red ○, [N7211][Tf2N];15 blue ●, [Pyr1MOM][Tf2N];13 blue ▲, [Pyr1MOM][FSA];13 blue ○, [Pyr31][Tf2N];13 black ■, [P444ME][C1(OC2)2SO4]; black ▲, [P444ME][C1OC2SO4]; black ○, [P 444ME ][C 6 SO4 ]; gold ■ , [C2 mim][C1 (OC 2) 2 SO4 ]; gold ○ , [C2mim][Tf2N].14

V L(p) − V L(p0 ) V L(p0 )

(4)

Figure 6, the molar volume scaling can normalize the volume expansion to a large extent. This is because the change in VLm with x2 is enlarged (the molar volume of CO2 in the IL phase is reduced) as the molar volume of IL is increased (see Figure S2). However, there could be a minor difference among the cations in Figure 6. The slope for the plot of ΔVLm vs x2 tends to increase in magnitude, the imidazolium (orange) ∼ pyrrolidinium (blue) < ammonium (red) < phosphonium (black). This could be attributed to the tendency of the free volume in the ILs (see Vm − VvdW in Table 2), though the uncertainties hinder further discussion.

and ΔVmL(p) = L

VmL(p) − VmL(p0 ) Vm L(p0 )

(5)

VLm

where V and are the absolute volume and the molar volume in the liquid phase, and p0 is atmospheric pressure. The percent volume expansions of ΔVL and ΔVLm for the present ether functionalized ILs at 298 K are given as a function of x2 in Figures 5 and 6, respectively. ΔVL increases with increasing x2

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DISCUSSION In this section, we discuss the present solubility results in ether functionalized ILs in terms of the molecular structures on the basis of some theoretical frameworks developed so far. Regular Solution Theory and COSMO Approach. Camper et al.55−57 attempted to predict the solubilities of gases in ILs by the simple regular solution theory. Later, as shown by Shi and Maggin,58 the regular solution theory underpredicts the CO2 solubilities based on the experimentally measured solubility parameter, but could yield accurate predictions only if the IL solubility parameter was fit to experimental data. Coutinho et al.59 also pointed out the complexity (expressed as “chameleonic behavior”) of the solubility parameters for ILs. Thus, the regular solution theory is not a strictly quantitative model; nevertheless, it is useful to qualitatively estimate the trend of gas solubilities, for example, the relationship between the solubility and the molar volume of IL. Actually, Sumon and Henni60 derived that the Henry constants of CO2 decrease in general with increase in molar volumes by conductor-like screening model for real solvents (COSMO-RS) calculations for huge numbers of ILs. The present results partially pursue such relation of the mole fraction scaled Henry constant with the molar volume; viz., k∞ H decreases with increasing molar volume, more exactly, with extension of side chains, in fixed cation−anion pairs. The molar volume effect will intrinsically involve both the enthalpic and entropic contributions. The enthalpic part of contributions was pointed out by Palomar et al.61 that the van der Waals interactions associated with the solute CO2 in ILs primarily

Figure 5. Volume expansion ΔVL in the IL phase as a function of x2 at 298 K. Symbols: red ●, [N211MEE][Tf2N];15 red ■, [N221ME][C1(OC2)2SO4]; red ○, [N7211][Tf2N];15 blue ●, [Pyr1MOM][Tf2N];13 blue ▲, [Pyr1MOM][FSA];13 blue ○, [Pyr31][Tf2N];13 black ■, [P444ME][C1(OC2)2SO4]; black ▲, [P444ME][C1OC2SO4]; black ○, [P 444ME][C6 SO4 ]; gold ■, [C 2mim][C 1(OC 2 )2 SO4 ]; gold ○, [C2mim][Tf2N].14

up to ∼25% at x2 ≈ 0.6, which is very similar to the previous results for common ILs.54 Roughly, larger the molar volume is, smaller ΔVL is. [P444ME][C6SO4] gives extraordinary smaller values of ΔVL, even though the molar volumes are comparable between [P444ME][C6SO4] and [P444ME][C1(OC2)2SO4]. The data points for the ether functionalized and nonfunctionalized alkyl analogue fall on a single curve, leading to high volumetric capacities of CO2 in ether functionalized ILs. As shown in G


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such as [BF4]− and [PF6]−, the larger cavities are primarily created by small angular rearrangements of the anions,66 whereas in the ILs based on the “flexible” ions such as [Tf2N]− and [methide]−, the larger cavity formations in ionic domains can be achieved by the conformational conversions of the anions.67 The authors also explained that the greater solubility of CO2 in [Pyr41][Tf2N] than in [Cnmim][Tf2N] (n = 2, 4) is promoted by the cavities in the ionic domains, which are generated by the conformational changes mainly occurred at the pyrrolidinium ring. Surely, the Henry constants of CO2 in [Pyr31][Tf2N] and [Pyr1MOM][Tf2N] (entries 4 and 5 in Table 2) are smaller than that in [C2mim][Tf2N]. The cavity formation model can be straightforwardly extended to the ether functionalized ILs. The flexible ether groups can produce the cavities, which can accommodate CO2 molecules, at lower energy levels of conformational changes. Interestingly, as contrasted to the incorporation of ether group, the ester functionalization in the cations generally reduce the CO2 solubilities (compare entries 19 and 20 in Table 2).15,20 This is probably because the ester group is less flexible in accordance with the cavity formation mechanism. The increase in the Henry constants in [Pyr1MOM]+ salts from [Tf2N]− to [FSA]− (entries 5 and 6) will be explained in a similar manner. Hu et al.4 also claimed that for CO2 dissolution, it is crucially important to introduce flexible moieties in ionic domains comprising the charged portions of cations and anions. The modifications of nonpolar domains such as elongation of alkyl chain, fluorination, incorporation of cyano group have less influence on the CO2 solubilities. The present results have proven that the ether functionalization in the anions is much more effective in improving the CO2 solubilities than that in the cations. This might be similarly interpreted in terms of intrinsic nanodomain segregations in ILs as mentioned above. The ether functionalization in the anions can expand the polar ionic domains that accommodate CO2 molecules, whereas that in the cations cannot. It was revealed that the polyether side chains in imidazolium cations strongly interact with the imidazolium ring via mainly intramolecular interactions between the ether oxygen atoms and imidazolium ring hydrogen atoms, in particular, more acidic 2-hydrogen atoms.30,33,68 Canongia Lopes et al.68 express such intramolecular complex as “scorpion-like.” They also demonstrated that the nanodomain segregations are suppressed in the ether functionalized ILs by X-ray diffraction as well as MD simulation studies. Luo and coworkers69 found that the imidazolium and pyridinium cations can form 1:1 complexes with poly(ethylene glycol) and poly(propylene glycol). These evidence indicate that the ether groups in the cations cannot aggregate each other and cannot form polar regions in ILs. As mentioned in the previous section, incorporation of ether groups in ester-linking side chain in imidazolium salts decreased k∞ H successively (entries 16−18 in Table 2). Therefore, the scorpion-like interactions in [C1(OC2)nmim][Tf2N] will be weakened by the ester insertion between the imidazolium ring and ether groups. For the ILs consisting of ether functionalized sulfates, it is expected that the ether groups could interact with the cations, in particular, imidazolium cations with acidic ring hydrogens, which is competitive to Coulombic interactions between the cation and sulfate (−SO3− group). In fact, the solubility of CO2 in [C2mim][C1(OC2)2SO4] is rather smaller than those in the other sulfate ILs. The ammonium and phosphonium cations are relatively bulky and have no acidic hydrogen like the ring hydrogens in imidazolium cations. Hence, the interactions

determine the absorption capacity of CO2 in ILs, again, with a COSMO-RS method. The comparative examination of the mole fraction and molarity scaled solubilities as presented in this paper can provide meaningful insight about this type of contribution. As is seen from the molarity scaled solubilities in Figure 4, the ether functionalization in anions efficiently increases the molarities of CO2 in the ILs, presumably due to solute−solvent (CO2−ether) interactions, like CO2−fluorine interactions in anion-fluorinated ILs. Note that fluorination of alkyl side chain in imidazolium cations is less effective than that in anions in fluorinated ILs.17,62 For the ether functionalized cations in Figure 3, the differences in the molarity scaled solubilities are considerably minimized, in particular for a common anion, [Tf2N]− in this case. This is emphasized by Carvalho and Coutinho63 that the molality scaled CO2 solubilities are dominated by entropic effects and expressed by a single universal correlation as a function of p and T, independent of ILs and low volatile solvents. Unfortunately, their model is critically oversimplified.5,64,65 Free Volume Model. As mentioned in the Introduction, Bara et al.35 pointed out that the flexible ether group decreases the intrinsic free volume in ILs, which could lead to increase in CO2 selectivity against N2 and CH4. They also obtained the relationship between the molar volume of IL and the fractional free volume, defined as (Vm − VvdW)/Vm, of which the van der Waals volume was replaced by the COSMO volume, VCOSMO in their original paper.36 In the imidazolium-based ILs, the fractional free volumes for the ether functionalized cations are smaller, and increased more moderately with increasing the molar volume than those for the nonfunctionalized alkyl cations. In the ammonium and pyrrolidinium ILs studied in this work, the ether functionalization in the cations brings about the similar decrement in the fractional free volume, though the absolute values of free volume change dependently on the molar volumes and kind of cations, of which the latter trend is roughly expressed as imidazolium < ammonium < pyrrolidinium. Although the effect of the ether functionalization in the phosphonium cation on the free volume cannot be exactly evaluated for lack of the corresponding alkyl counterpart, the similar decrease in the free volume to the other cations may be expected. The effect of the ether functionalization in the sulfate anions is ambiguous. With a common cation, [P444ME]+, the replacement of [C6SO4]− by [C1OC2SO4]− increases the free volume, and the further replacement by [C1(OC2)2SO4]− again raises the free volume slightly. In the recent review, Vlugt and co-workers5 showed that the molarity scaled Henry constant approximately decreases with increasing the molar volume of IL as well as the free volume in IL, where the data points scatter over ± ∼ 20%. Such correlations are inapplicable to the pairs of ILs, composed of the ether and nonfunctionalized cations in this study. The molarity scaled solubilities, (∂c/∂p)T, which are the inverses of the molarity scaled Henry constants, increase in the ether-containing cations with the smaller molar volumes and free volumes. In the present ILs with the sulfate anions, it seems that (∂c/∂p)T increases with the molar volume and free volume. Cavity Formation Mechanism and Domain Structure. Another related approach is the cavity formation mechanism proposed by Hu et al.4 that attempt to describe the gas dissolution phenomena in ILs in relation to the flexibility of ionic structures at molecular level. Their model originates from a question, “how to produce the required cavities?” They considered that in the ILs comprising relatively “rigid” ions H


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(3) Bara, J. E.; Carlisle, T. K.; Gabriel, C. J.; Camper, D.; Finotello, A.; Gin, D. L.; Noble, R. D. Guide to CO2 Separations in ImidazoliumBased Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2009, 48, 2739−2751. (4) Hu, Y.-F.; Liu, Z.-C.; Xu, C.-M.; Zhang, X.-M. The Molecular Characteristics Dominating the Solubility of Gases in Ionic Liquids. Chem. Soc. Rev. 2011, 40, 3802−3823. (5) Ramdin, M.; de Loos, T. W.; Vlugt, T. J. H. State-of-the-Art of CO2 Capture with Ionic Liquids. Ind. Eng. Chem. Res. 2012, 51, 8149− 8177. (6) Tang, S.; Baker, G. A.; Zhao, H. Ether- and AlcoholFunctionalized Task-Specific Ionic Liquids: Attractive Properties and Applications. Chem. Soc. Rev. 2012, 41, 4030−4066. (7) Makino, T.; Sakurai, M.; Kanakubo, M. Solubilities of Gases in Ionic Liquids and its Applications to Separation Processes. J. Vac. Soc. Jpn. 2013, 56, 88−96. (8) Lei, Z.; Dai, C.; Chen, B. Gas Solubility in Ionic Liquids. Chem. Rev. 2014, 114, 1289−1326. (9) Chiappe, C.; Pomelli, C. S. Point-Functionalization of Ionic Liquids: An Overview of Synthesis and Applications. Eur. J. Org. Chem. 2014, 28, 6120−6139. (10) Kodama, D.; Kanakubo, M.; Kokubo, M.; Ono, T.; Kawanami, H.; Yokoyama, T.; Nanjo, H.; Kato, M. CO2 Absorption Properties of Brønsted Acid−Base Ionic Liquid Composed of N, N-Dimethylformamide and Bis(trifluoromethanesulfonyl)amide. J. Supercrit. Fluids 2010, 52, 189−192. (11) Kodama, D.; Kanakubo, M.; Kokubo, M.; Hashimoto, S.; Nanjo, H.; Kato, M. Density, Viscosity, and Solubility of Carbon Dioxide in Glymes. Fluid Phase Equilib. 2011, 302, 103−108. (12) Makino, T.; Kanakubo, M.; Umecky, T.; Suzuki, A. CO2 Solubility and Physical Properties of N-(2-Hydroxyethyl)pyridinium Bis(trifluoromethanesulfonyl)amide. Fluid Phase Equilib. 2013, 357, 64−70. (13) Makino, T.; Kanakubo, M.; Umecky, T.; Suzuki, A.; Nishida, T.; Takano, J. Pressure−Volume−Temperature−Composition Relations for Carbon Dioxide + Pyrrolidinium-Based Ionic Liquid Binary Systems. Fluid Phase Equilib. 2013, 360, 253−259. (14) Makino, T.; Kanakubo, M.; Masuda, Y.; Umecky, T.; Suzuki, A. CO2 Absorption Properties, Densities, Viscosities, and Electrical Conductivities of Ethylimidazolium and 1-Ethyl-3-methylimidazolium Ionic Liquids. Fluid Phase Equilib. 2014, 362, 300−306. (15) Makino, T.; Kanakubo, M.; Umecky, T. CO2 Solubilities in Ammonium Bis(trifluoromethanesulfonyl)amide Ionic Liquids: Effects of Ester and Ether Groups. J. Chem. Eng. Data 2014, 59, 1435−1440. (16) Makino, T.; Kanakubo, M.; Masuda, Y.; Mukaiyama, H. Physical and CO2-Absorption Properties of Imidazolium Ionic Liquids with Tetracyanoborate and Bis(trifluoromethanesulfonyl)amide Anions. J. Solution Chem. 2014, 43, 1601−1613. (17) Muldoon, M. J.; Aki, S. N. V. K.; Anderson, J. L.; Dixon, J. K.; Brennecke, J. F. Improving Carbon Dioxide Solubility in Ionic Liquids. J. Phys. Chem. B 2007, 111, 9001−9009. (18) Bara, J. E.; Gabriel, C. J.; Lessmann, S.; Carlisle, T. K.; Finotello, A.; Gin, D. L.; Noble, R. D. Enhanced CO2 Separation Selectivity in Oligo(ethylene glycol) Functionalized Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2007, 46, 5380−5386. (19) Deng, Y.; Morrissey, S.; Gathergood, N.; Delort, A.-M.; Husson, P.; Costa Gomes, M. F. The Presence of Functional Groups Key for Biodegradation in Ionic Liquids: Effect on Gas Solubility. ChemSusChem 2010, 3, 377−385. (20) Deng, Y.; Husson, P.; Delort, A.-M.; Besse-Hoggan, P.; Sancelme, M.; Costa Gomes, M. F. Influence of an Oxygen Functionalization on the Physicochemical Properties of Ionic Liquids: Density, Viscosity, and Carbon Dioxide Solubility as a Function of Temperature. J. Chem. Eng. Data 2011, 56, 4194−4202. (21) Yuan, X.; Zhang, S.; Liu, J.; Lu, X. Solubilities of CO2 in Hydroxyl Ammonium Ionic Liquids at Elevated Pressures. Fluid Phase Equilib. 2007, 257, 195−200. (22) Jalili, A. H.; Mehdizadeh, A.; Shokouhi, M.; Sakhaeinia, H.; Taghikhani, V. Solubility of CO2 in 1-(2-Hydroxyethyl)-3-methyl-

between the cations and ether groups of sulfate anions may not be significantly stabilized in the ammonium and phosphonium ILs. The ether functionalization in the ammonium and phosphonium cations in the ether functionalized sulfate ILs, i.e., the dual functionalization, would efficiently weaken such interactions, which makes a positive contribution to the CO2 solubility. These remarks are qualitatively derived from the present solubility data in the ether functionalized ILs. Further discussion will be made with help from MD simulations and diffraction experiments.

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CONCLUSIONS The CO2 solubility data in a series of ILs, of which the cations, anions, and both are functionalized with ether groups, have been summarized with newly and previously reported results. It has become apparent, like fluorination, that the introduction of ether group(s) in anions more effectively enhances the CO2 solubilities on both the mole fraction and molarity scales than that in cations. The ether functionalization is less cost-intensive than fluorination. Moreover, incorporation of ether groups in ILs improves biodegradability as well as efficiently reduces viscosity. Hence, the ether functionalization is one of promising and practical approaches to increase the CO2 solubility in ILs. The effects of ether functionalization on physical absorption of CO2 are discussed in detail in terms of the molecular structures of ILs. In conclusion, it is of great importance to design the structures of anions and cations in ionic liquids for individual purposes by taking account of inter- and intramolecular interactions, in other words, direct and indirect contributions, in the vicinity of solvation sphere as well as the intrinsic nanodomain segregations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00960. Syntheses and identifications of ILs, CO2 solubilities in ILs studied in this work, and plots of molality of CO2 in ether functionalized ILs against p and of molar volume in the IL phase as a function of x2 (PDF).

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AUTHOR INFORMATION

Corresponding Author

*M. Kanakubo. E-mail: [email protected]. Fax: +81-22232-7002. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS It is a pleasure to thank Mr. Atsuhiro Oguni, Ms. Kaori Takeshita, and Ms. Eriko Niitsuma at AIST for their careful assistance with the measurements.

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REFERENCES

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CO2 Solubility in Ether Functionalized Ionic Liquids on Mole Fraction

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