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Chemical Engineering Journal 181– 182 (2012) 834– 841

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

jo u r n al hom epage: www.elsev ier .com/ locate /ce j

elective chemical separation of carbondioxide by ether functionalizedmidazolium cation based ionic liquids

ankaj Sharmaa, Soo-Hyun Choia, Sang-Do Parkb, Il-Hyun Baeka,∗, Gil-Sun Leea,∗∗

Greenhouse Gas CenterCarbon Dioxide Reduction & Sequestration R&D Center, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejon 305-343, Republic of Korea

r t i c l e i n f o

rticle history:eceived 10 October 2011eceived in revised form 2 December 2011ccepted 7 December 2011

eywords:LsO2 absorptionther

a b s t r a c t

A series [C2Omim][X] of imidazolium cation-based ILs, with ether functional group on the alkyl side-chain were synthesized, characterized by various techniques such as 1H, 13C NMR, MS-ESI, FTIR and EAand investigated as potential absorbents for CO2 capture. More specifically, the influence of changingthe anion with same cation is carried out. The absorption capacity of CO2 for ILs was investigated at 30and 50 ◦C at ambient pressure (0–1.6 bar). Ether functionalized ILs show significantly high absorptioncapacity for CO2. The CO2 absorption capacity of ILs increased with a rise in pressure and decreased whentemperature was raised. Results showed that absorption capacity reached about 0.9 mol CO2 per mol ofIL at 30 ◦C. The most probable mechanism of interaction of CO2 with ILs was investigated using FTIR and13

midazoleSILs

C NMR, and the result shows that the absorption of CO2 in ether functionalized ILs is a chemical process.The CO2 absorption results and detailed study indicate the predominance of 1:1 mechanism, where theCO2 reacts with one IL to form a carbamic acid. The CO2 absorption capacity of ILs for different anionsfollows the trend: BF4 < DCA ∼ PF6 ∼ TfO < Tf2N. Moreover, the as-synthesized ILs are selective, thermallystable, long life operational and can be recycled at a temperature of 70 ◦C or under vacuum and can beused repeatedly.

© 2011 Elsevier B.V. All rights reserved.

. Introduction

In recent years, carbon dioxide (CO2) capture from flue gasesas become an important issue, in both academia and industry, fornvironmentally benign and sustainable environment. As long asoal, petroleum and natural gas are used as the primary source ofuel, the production of CO2 is inevitable. Therefore, the develop-

ent of economically viable CO2 capture and separation processess a key step in the reduction of CO2.

Extensive research has been conducted by several groups onqueous solutions of alkanolamines, especially monoethanolamineMEA), diethanolamine (DEA) and methyldiethanolamine (MDEA)

or natural gas treating and sweetening [1–14]. Even though thesemine-based methods are highly efficient for CO2 capture, therere several concerns [15–17] which hinder the industrial appli-ation. Therefore, the development of a solvent that could absorb

∗ Corresponding author at: Korea University, Research Institute of Clean Chemicalngineering Systems, Greenhouse Gas Center, Republic of Korea.el.: +82 42 860 3648; fax: +82 42 860 3134.∗∗ Co-corresponding author.

E-mail address: ihbaek@kier.re.kr (I.-H. Baek).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.12.024

CO2 while being non-volatile and non-corrosive and contribute tothe process economy as well as production sustainability by low-ering the solvent inventory and reducing the discharge of volatilechemicals to atmosphere is highly demanded. In this context, ionicliquids (ILs) have been proposed. Their negligible vapor pressure,wide range of polarities, broad liquid range, significant thermaland chemical stability and capability to dissolve CO2 through aphysical interaction make them an attractive candidate for CO2capture.

ILs generally consist of large organic bulky asymmetric cation,such as: quaternary ammonium, imidazolium, pyridinium andphosphonium ions and small sized and more symmetrical shapeeither an inorganic anion such as [Cl]−, [Br]−, [I]−, [BF4]−, [PF6]−,[Tf2N]− or an organic anion such as [RCO2]− [18]. The physical andchemical properties of the ILs are decided by the nature of cationand anion. Therefore, it is possible to achieve specific propertiesby tuning the different combination of a cation and anion [19].The class of imidazolium ILs is used in a wide variety of applica-tions due to their attractive physical and chemical properties, such

as air and moisture stability, low flammability, thermal stability,negligible vapor pressure, being liquid over a wide range of tem-perature, wide electrochemical windows, high conductivities andionic mobilities, easy recycling, tunable miscibility with water and

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P. Sharma et al. / Chemical Engine

rganic solvents, and being a good solvent for a variety of organicnd inorganic compounds [20–22]. More importantly, imidazoliumased ILs have good appetency with CO2 [23].

The factors which ultimately decide the solubility of CO2 in ILsre related with anion and cation both. Anion played the biggestole in CO2 solubility, a fact that was further supported by aecent X-ray diffraction study by Kanakubo et al. [24]. Anionshat contain fluoroalkyl groups were found to have some of theighest CO2 solubilities, and as the quantity of fluoroalkyl groups

ncreased, the CO2 solubility also increased [25]. For the cations,here were two factors that influenced the CO2 solubility. Theiggest effect was seen by increasing alkyl chain length on theation [25]. The second factor was to attach a functional groupike ether to cation to create greater free volume or by incorporat-ng a CO2-philic carbonyl functional moiety. Some other functional

oieties such as amine, alcohol, carboxylic and nitrile, have alsoeen reported. Due to this aptness for the fine tuning of theirroperties through endless combination of cations and anions andunctional moieties the ILs can be deemed as “designer solvent”.his promising strategy, leads to the synthesis and development ofask specific ionic liquids (TSILs) for a specific requirement is beingchieved.

So finally we designed ether functionalized imidazolium basedLs with different anions to evaluate their combination effect onolubility of CO2. Among the functionalized ILs, ether functional-zed imidazolium ILs have been paid much attention due to theirttractive physical and chemical properties [20–22], which include:ir and moisture stability, low flammability, thermal stability, aegligible vapor pressure, being liquid over a wide temperatureange, wide electrochemical windows, high conductivities andonic mobilities, easy recycling, tunable miscibility with water andrganic solvents and being a good solvent for a wide variety ofrganic and inorganic compounds. As a result they are widely usedn the field of molecular recognition [26], transition metal catalysis27], biocatalysis [28], carbohydrate/nucleoside chemistry [29,30],ntimicrobial activities [31], metal extraction [32], liquid phaserganic synthesis [33], polymer chemistry [34], organic synthe-is [35], self organization [36] and lubricants [37]. In comparisonith other functionalized ILs, ether functionalized ILs are expected

o possess simultaneously peculiar properties and more attractiveor versatile practical applications. Although, imidazolium cationased some ether functionalized ILs have been synthesized withCl]−, [BF4]−, [PF6]−, [CH3SO3]−, [CF3BF3]− and [C2F5BF3]− anionsnd their properties are studied [38,39]. However, there is still somecope to develop new ether functionalized ILs with suitable proper-ies such as hydrophobic, low-melting, and low viscous ionic liquidsy using anions such as [Tf2N]−, [TfO]−, and [DCA]−.

Here in, we report the synthesis, characterization of a series ofther functionalized ILs [C2Omim][X], based on the imidazoliumation that contains additional functional group, ether, on the alkylroup R and their application in capture of CO2. Till date, no reports available involving ether functionalized imidazolium based ILs inpen literature with study of the solubility of CO2.

. Experimental

.1. Reagents and materials

Chloromethylmethylether (CH3OCH2Cl), 1-methylimidazole,iethyl ether, acetone, sodium tetrafluoroborate (NaBF4), mag-esium sulfate (MgSO4), potassium hexafluorophosphate (KPF6),

ithium bis[(trifluoromethyl)sulfonyl] amide Li(Tf2N), sodium tri-uoromethylsulfonate Na(TfO), sodium dicyanamide Na(DCA)ere supplied from Sigma–Aldrich and used without further purifi-

ation.

ournal 181– 182 (2012) 834– 841 835

2.2. Apparatus and procedures

2.2.1. Synthesis of ionic liquidsA previously reported methodology [39] was used for the syn-

thesis of these ionic liquids. Alkylation of 1-methylimidazole withan alkyl halide is followed by halogen (Cl or Br) exchange with slightexcess (1.1 equiv) of NaBF4, KPF6, Li(Tf2N), Na(TfO) and Na(DCA) inorder to reduce the amount of remaining halogen content.

2.2.1.1. Synthesis of [C2Omim][Cl]. Chloromethylmethylether(70 mL, 0.93 mol) and 1-methylimidazole (50 mL, 0.63 mol) wereadded to a round-bottomed flask fitted with a reflux condenserfor 24 h at 80 ◦C with stirring. The product was washed withdiethyl ether (4× 25 mL), heated at 80 ◦C and stirred under vacuum(0.5 mmHg) for 2 d. The product was obtained as a slightly yellowliquid which solidified on cooling (82% yield) with water content of3.46 �g (H2O) mL−1 (RTIL). 1H NMR (700 MHz), [D6] acetone, 25 ◦C:ı = 3.29 (s, 3H), 3.86 (s, 2H), 3.81 (s, 3H), 7.34 (s, 1H), 7.40 (s, 1H),8.64 (s, 1H); 13C NMR (700 MHz), [D6] acetone, 25 ◦C: ı = 36.3, 49.1,69.7, 122.5, 123.3, 137.4; MS-ESI: m/z (%): 126 (100) [C2Omim]+,35 (100) [Cl]−; FTIR (neat): 3414, 3150, 3096, 2970, 2880, 1638,1574, 1462, 1173, 1083, 1011, 970.3, 835.5, 761.2, 651.2 cm−1;elemental analysis calcd (%) for C6H10N2OCl (161): C 44.7, H 6.2, N17.3; found: C 44.4, H 6.1, N 17.1

2.2.1.2. Synthesis of [C2Omim][BF4]. [C2Omim][Cl] (25.00 g,0.15 mol) was transferred to a plastic Erlenmeyer flask (250 mL).Acetone (150 mL) was added followed by NaBF4 (19.00 g, 0.17 mol).This mixture was stirred at room temperature for 24 h. The result-ing waxy solid precipitate was collected by filtration and washedwith acetone (2× 100 mL). The organic layer was collected, dried(MgSO4), filtered and the solvent removed in vacuum to givethe product (89% yield) as a light brown liquid; water content of2.66 �g (H2O) mL−1 (RTIL). 1H NMR (700 MHz), [D6] acetone, 25 ◦C:ı = 3.28 (s, 3H), 3.86 (s, 2H), 3.80 (s, 3H), 7.33 (s, 1H), 7.42 (s, 1H),8.66 (s, 1H); 13C NMR (700 MHz), [D6] acetone, 25 ◦C: ı = 36.4, 49.2,69.4, 123.2, 124.1, 136.6; MS-ESI: m/z (%): 126 (100) [C2Omim]+,87 (100) [BF4]−; FTIR (neat): 3400, 3140, 3090, 2969, 2885, 1642,1570, 1469, 1169, 1079, 1059, 1010, 970, 835.5, 758.2 cm−1;elemental analysis calcd (%) for C6H10N2OBF4 (213): C 33.8, H 4.69,N 13.1; found: C 33.2, H 4.62, N 12.9

2.2.1.3. Synthesis of [C2Omim][PF6]. [C2Omim][Cl] (25.00 g,0.15 mol) was transferred to a plastic Erlenmeyer flask (250 mL).Acetone (150 mL) was added followed by KPF6 (31.00 g, 0.168 mol).This mixture was stirred at room temperature for 24 h. The result-ing waxy solid precipitate was collected by filtration and washedwith acetone (2× 100 mL). The organic layer was collected, dried(MgSO4), filtered and the solvent removed in vacuum to givethe product (90% yield) as a dark brown liquid; water contentof 1.84 �g (H2O) mL−1 (RTIL). 1H NMR (700 MHz), [D6] acetone,25 ◦C): ı = 3.28 (s, 3H), 3.80 (s, 2H), 3.83 (s, 3H), 7.31 (s, 1H), 7.39(s, 1H), 8.61 (s, 1H); 13C NMR (700 MHz), [D6] acetone, 25 ◦C:ı = 35.3, 49.4, 69.6, 122.2, 123.55, 137.44; MS-ESI: m/z (%): 126(100) [C2Omim]+, 145 (100) [PF6]−; FTIR (neat): 3410, 3148, 3101,2976, 2881, 1641, 1575, 1460, 1170, 1086, 1017, 967, 836, 833, 761,558 cm−1; elemental analysis calcd (%) for C6H10N2OPF6 (271): C33.8, H 4.69, N 13.14; found: C 33.2, H 4.59, N 13.1

2.2.1.4. Synthesis of [C2Omim][Tf2N]. [C2Omim][Cl] (25.00 g,0.15 mol) was transferred to a plastic Erlenmeyer flask (250 mL).Acetone (150 mL) was added followed by Li(Tf2N) (49.00 g,

0.170 mol). This mixture was stirred at room temperature for 24 h.The resulting waxy solid precipitate was collected by filtrationand washed with acetone (2× 100 mL). The organic layer wascollected, dried (MgSO4), filtered and the solvent removed in

8 ering Journal 181– 182 (2012) 834– 841

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36 P. Sharma et al. / Chemical Engine

acuum to give the product (89% yield) as a light brown liquid;ater content of 2.21 �g (H2O) mL−1 (RTIL). 1H NMR (700 MHz),

D6] acetone, 25 ◦C): ı = 3.27 (s, 3H), 3.81 (s, 2H), 3.82 (s, 3H), 7.31s, 1H), 7.42 (s, 1H), 8.66 (s, 1H); 13C NMR (700 MHz), [D6] acetone,5 ◦C: ı = 35.9, 49.2, 68.7, 122.1, 123.2, 137.1; MS-ESI: m/z (%):26 (100) [C2Omim]+, 280 (100) [Tf2N]−; FTIR (neat): 3414, 3150,096, 2979, 2970, 2880, 2876, 1638, 1574, 1462, 1348, 1336, 1181,173, 1135, 1083, 1055, 1013, 968.3, 833.5, 789, 760.2, 739 cm−1;lemental analysis calcd (%) for C8H10N3O5F6S2 (406): C 23.6, H.46, N 10.34, S 15.76; found: C 23.1, H 2.43, N 10.1, S 15.69

.2.1.5. Synthesis of [C2Omim][TfO]. [C2Omim][Cl] (25.00 g,

.15 mol) was transferred to a plastic Erlenmeyer flask (250 mL).cetone (150 mL) was added followed by Na(TfO) (29.50 g,.171 mol). This mixture was stirred at room temperature for 24 h.he resulting waxy solid precipitate was collected by filtration andashed with acetone (2× 100 mL). The organic layer was collected,ried (MgSO4), filtered and the solvent removed in vacuum toive the product (90% yield) as a light brown liquid; water contentf 2.25 �g (H2O) mL−1 (RTIL). 1H NMR (700 MHz), [D6] acetone,5 ◦C: ı = 3.28 (s, 3H), 3.84 (s, 2H), 3.81 (s, 3H), 7.31 (s, 1H), 7.42s, 1H), 8.68 (s, 1H); 13C NMR (700 MHz), [D6] acetone, 25 ◦C):

= 35.1, 49.2, 69.4, 122.2, 123.6, 137.8; MS-ESI: m/z (%): 126 (100)C2Omim]+, 149 (100) [TfO]−; FTIR (neat): 3410, 3155, 3100, 2976,879, 1642, 1569, 1458, 1422, 1363, 1253, 1223, 1170, 1089, 1021,060, 980, 965.3, 919, 844, 775, 763, 739, 717, 655, 636, 573,29 cm−1; elemental analysis calcd (%) for C7H10N2O4F3S (275): C0.5, H 5.2, N 10.18, S 11.6; found: C 30.1, H 4.98, N 10.1, S 11.1.

.2.1.6. Synthesis of [C2Omim][DCA]. [C2Omim][Cl] (25.00 g,

.15 mol) was transferred to a plastic Erlenmeyer flask (250 mL).cetone (150 mL) was added followed by Na(DCA) (15.00 g,.168 mol). This mixture was stirred at room temperature for 24 h.he resulting waxy solid precipitate was collected by filtration andashed with acetone (2× 100 mL). The organic layer was collected,ried (MgSO4), filtered and the solvent removed in vacuum toive the product (88% yield) as a light yellow liquid; water contentf 2.52 �g (H2O) mL−1 (RTIL). 1H NMR (700 MHz), [D6] acetone,5 ◦C: ı = 3.29 (s, 3H), 3.86 (s, 2H), 3.81 (s, 3H), 7.34 (s, 1H), 7.40s, 1H), 8.64 (s, 1H); 13C NMR (700 MHz), [D6] acetone, 25 ◦C:

= 35.3, 49.3, 69.1, 121.2, 122.1, 136.9; MS-ESI: m/z (%): 126 (100)C2Omim]+, 66 (100) [DCA]−; FTIR (neat): 3400, 3138, 3089, 2971,876, 1643, 1575, 1459, 1168, 1077, 1011, 965, 830, 761 cm−1;lemental analysis calcd (%) for C8H10N5O (192): C 50.0, H 5.2, N6.45; found: C 49.4, H 5.1, N 35.9.

.2.2. Characterization1H and 13C NMR spectra were obtained using a Bruker Avance-

00 FT-NMR Spectrophotometer and [D6] acetone was used as

olvent. The chemical shift values are reported in ppm with respecto TMS internal reference. The mass of the samples was measuredn a Hewlett Packard 1100 Series, Mass Spectrophotometer, Agilent200 Series. The FT-IR spectra were recorded on Thermo, Model:

able 1he various components of ILs, density and viscosity of ILs of the series [C2Omim][X].

S. No. Ionic liquid Anion Density

X− (g L−1) at 25 ◦C

1. [C2Omim][Cl] Cl− 1.14

2. [C2Omim][BF4] BF4− 1.24

3. [C2Omim][PF6] PF6− 1.38

4. [C2Omim][Tf2N] NTf2− 1.46

5. [C2Omim][TfO] TfO− 1.31

6. [C2Omim][DCA] DCA− 1.05

a Viscosity of CO2 saturated ILs.

Fig. 1. Experimental apparatus: (1) CO2 reservoir; (2) control valve; (3) pressuregauge; (4) trifurcate valve; (5) control valve; (6) control valve; (7) isochoric cell; (8)thermostat; (9) magnetic stirrer; (10) vacuum pump.

Nicolet 6700. The elemental analysis (C, H, and N) was performedat the Thermo Finnigan Flash EA-2000 Elemental Analyzer (EA).The water content in the ILs was detected by Karl–Fischer titration(Mitsubishi Chem., Model CA-07). All the salts were tested aftervacuum drying at 70–100 ◦C for 24 h. The TGA was performed ona thermal analysis system (Mettler Toledo, Model: TGA/SDTA 851e). An average sample weight of 5 mg was placed in a platinumpan and heated at 10 ◦C/min from ca. 30 to 100 ◦C under a flow ofN2. The density of the liquid salts was approximately determinedby measuring the weight of 1.0 mL of the salt three times at 25 ◦C.The viscosity was measured with a viscometer (Brookfield, ModelDV-III+) using 0.6 mL sample at 10, 20 and 30 ◦C.

2.2.3. CO2 absorption isotherm measurementThe experimental method used for the gas solubility mea-

surement was based on an isochoric saturation technique. Theexperimental apparatus used in this study is schematically repre-sented in Fig. 1. The main composition of the apparatus includesa gas reservoir, a thermostat, a pressure gauge, an isochoric cell,a vacuum pump, and a magnetic stirrer. The temperature wasdetermined with two calibrated platinum resistance thermometersplaced in the heating jacket of the cell with an uncertainty below±0.1 ◦C. The uncertainty of the pressure gauge is approximately±0.001 bar in the experimental pressure range. The carbon dioxideabsorption isotherm measurement was performed at 30 and 50 ◦C.In a typical experiment, about 2 mL ILs was loaded into the isochoriccell, the air in the system was eliminated by vacuum pump, thenCO2 was charged into the cell from the gas reservoir and the ionicliquid was stirred. The system was considered to have reached equi-librium if the pressure of the system had been unchanged within2 h. Then the pressure of the system was recorded and the weightof the cell was determined with an electronic balance (SartoriusBS224S) with an uncertainty ±0.0001 g. The solubility of carbon

dioxide in the ionic liquid was determined by the shifted quality ofthe isochoric cell. The quantity of absorbed CO2 in ILs was measuredgravimetrically by weighing the solution successively (with closedvalves) until the mass remains constant at the particular pressure

Viscosity (cP)

10 ◦C 20 ◦C 30 ◦C 30 ◦Ca

592.2 401.1 201.2 252.4370.1 252.7 130.1 178.2598.3 281.1 141.2 193.3

70.1 44.2 29.5 45.4117.7 100.2 61.2 78.8

40.4 31.6 22.9 36.5

P. Sharma et al. / Chemical Engineering J

N N

CH3 (CH2)O

CH3

X

dwmr

2

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3

spa

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f

T −

e[iTb

−

TS

X= [BF4], [PF6], [Tf2N], [TfO], [DCA ]

Fig. 2. Chemical structures of the ILs [C2Omim][X].

uring the experiment. To verify the validation of the apparatus,e determined the solubility of CO2 in a choline chloride + ureaixture with this apparatus, and the result was consistent with

eference [40].

.2.4. CO2 absorption mechanism verificationThe mechanism of CO2 absorption in this report is inves-

igated by FTIR and 13C NMR of CO2 saturated ILs. The CO2aturated sample of ILs was collected after experiment and charac-erized for FTIR and 13C NMR. The same instruments and conditionsere used which were used previously for characterization ofaterials.

. Results and discussion

The chemical structures of the ILs [C2Omim][X] synthesized andtudied in the present work are presented in Fig. 2. The various com-onents of ILs, density and viscosity of ILs of the series [C2Omim][X]re reported in Table 1.

In general, with a constant cation, the density increased withncreasing the bulkiness of the anion [39,41]. These trends areonsistent with the reported observations in other ILs with imi-azolium cations. The density data for ILs are reported in Table 1.

The trend observed for the density of the [C2Omim][X] is asollows for different anions:

f2N− > C2F5BF3 > CF3BF3 > PF6− > TfO− > BF4

− > Cl− > DCA

Therefore, the density values of ILs are in good agreement asxpected. The ILs containing [Tf2N]− anion have higher density and

DCA]− containing anion have lower density. But [C2F5BF3] contain-ng ionic liquids are almost equally dense to [Tf2N]− ionic liquids.hese results indicate that the densities of these ionic liquids cane fine-tuned with slight structural changes in cation and anion.

able 2olubility data of CO2 (mol CO2/mol IL) in different ILs at different temperatures.

Pressure (bar) Solubility of CO2 in different ILs (mol CO2/mol IL)

[C2Omim][BF4] [C2Omim][PF6] [C2Omim][Tf2

0 0 0 0

0.1 0.8 0.86 0.9

0.2 0.84 0.9 0.95

0.4 0.86 0.93 0.97

0.6 0.89 0.95 0.975

0.8 0.9 0.95 0.976

1.0 0.91 0.955 0.9775

1.2 0.915 0.956 0.978

1.4 0.92 0.957 0.979

1.6 0.925 0.958 0.979

0 0 0 0

0.1 0.62 0.69 0.7

0.2 0.69 0.72 0.75

0.4 0.73 0.785 0.81

0.6 0.75 0.8 0.84

0.8 0.76 0.82 0.86

1.0 0.77 0.825 0.865

1.2 0.78 0.828 0.868

1.4 0.782 0.833 0.87

1.6 0.785 0.83 0.871

ournal 181– 182 (2012) 834– 841 837

The viscosity of an ionic liquid is essentially determined bytheir tendency to form hydrogen bonds and by the strength of vander Waals interactions (dispersion and repulsion), being stronglydependent on the anion type [42–50]. An anion combined a goodcharge distribution and a flat shape e.g., [F(HF)2.3]− [51], [N(CN)2]−

[45,46] and [C(CN)3]− [46,47] or an irregular shape (e.g., [Al2Cl7]−

[44], [(CF3SO2)(CF3CO)N]− [48], and [(CF3SO2)2N]−) [43,49] tendsto form low-viscous ILs, while that with high symmetry e.g., BF4

−

[42,49,50,52], PF6− [42,50,53], AsF6

− [54], SbF6− [54], TaF6

− [54]usually produces high-viscous and/or high melting salts in spite ofits weak coordinating ability. The viscosity data for ILs are reportedin Table 1.

The trend observed for the viscosity of the ILs is as follows fordifferent anions:

Cl− > PF6− > BF4

− > TfO− > CF3BF3 > C2F5BF3 > Tf2N− > DCA

The trend obtained for viscosity of ILs is in good agreement withthe earlier reported results and it shows that viscosity of ILs ismainly decided by anion. We observe that ILs containing [DCA] −

anion are least viscous and [PF6]− containing ILs are most viscous.The viscosity of ether functionalized ionic liquids having [CF3BF3]anion is higher than ionic liquids having [C2F5BF3] anion. It alsostates that the viscosity of these ionic liquids is less than [TfO]−

anions. But there are enough studies carried out by many researchgroups and it is concluded that the structure of the cation also influ-ences the viscosity of ILs [39,41]. All this results suggest that, insearch of low-viscous ILs, we should focus not only on weak coor-dinating ability of the anion, but also the shape (symmetry) andsize of the anion should also be taken into consideration.

3.1. CO2 absorption isotherm measurements

The absorption isotherm of CO2 in different ILs was determinedat 30 and 50 ◦C at ambient pressure (Figs. 3 and 4). It was noted thatether functionalized ILs show significantly high absorption capacityfor CO2. The CO2 absorption capacity of ILs increased with a risein pressure and decreased when temperature was raised. Resultsshowed that absorption capacity reached about 0.9 mol CO2 per

mol of ILs at 30 ◦C. The CO2 absorption capacity of ILs for differentanions follows the trend: BF4 < DCA ∼ PF6 ∼ TfO < Tf2N. It is alreadyreported that anions that contain fluoroalkyl groups were foundto have highest CO2 solubilities, and as the quantity of fluoroalkyl

Temperature (◦C)

N] [C2Omim][TfO] [C2Omim][DCA]

0 0 300.88 0.80.92 0.850.94 0.880.96 0.90.965 0.910.967 0.920.968 0.930.969 0.9350.97 0.935

0 0 500.696 0.650.74 0.70.8 0.740.82 0.760.84 0.770.845 0.780.848 0.790.855 0.7920.85 0.795

838 P. Sharma et al. / Chemical Engineering Journal 181– 182 (2012) 834– 841

1.61.41.21.00.80.60.40.20.0

0.0

0.2

0.4

0.6

0.8

1.0

mol

CO

2/mol

IL

Pressure (bar )

[C2Omim][Tf2N]

[C2Omim][TfO]

[C2Omim][ PF6]

[C2Omim][DCA]

[C2Omim][BF4]

Fig. 3. Mole fraction of CO2 (�) in different ILs as a function of pressure at 30 ◦C.

1.51.00.50.0-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

mol

CO

2/mol

IL

[C2Omim][Tf2N]

[C2Omim][TfO]

[C2Omim][ PF6]

[C2Omim][DCA]

[C2Omim][BF4]

F

gt

3

Fa

−1

TC

Press ure (b ar)

ig. 4. Mole fraction of CO2 (�) in different ILs as a function of pressure at 50 ◦C.

roups increased, the CO2 solubility also increased. So the result andrend is in good agreement with previous reports (Tables 2 and 3).

.2. Mechanism of CO2 absorption

The interaction of CO2 with different ILs was investigated usingTIR and result shows that the absorption of CO2 in ether function-lized ILs is a chemical process. The FTIR spectra of CO2 absorbed

able 3omparison of the solubility of CO2 in TSIL from previous literature and this work.

S. No. Name of TSIL

1. Trihexyl(tetradecyl)phosphonium amino acid; [P66614][AA]

2. Tetrabutylphosphonium amino acid; [P(C4)4] [AA]

3. (3-Aminopropyl) tributylphosphonium amino acid; [aP4443] [AA]

4. 3-Aminoethyl-2-methyl-1-methylimidazolium taurine; [aemmim] [Tau]

5. 1-Butyl-3aminopropyl imidazolium tetrafluoroborate

6. [C2Omim][Tf2N]

Fig. 5. (a) FTIR spectra of different ILs and (b) FTIR spectra of CO2 absorbed differentILs.

ILs (Fig. 5(b)) show two new peaks appeared around 1700 cmand 1405 cm−1 for ( C Ostr) and ( O H in planebend) of the car-boxylic acid. The 13C NMR spectra of carbondioxide absorbed etherfunctionalized ILs are shown in Fig. 6(a)–(e). The NMR spectra of all

Test conditions Solubility of CO2 molof CO2/mol IL

Reference

0–1 bar; 22 ◦C 0.9 [55]1 atm; 22 ◦C; water (1%, mass fraction) 0.12 [57]Supported on porous silica 0.51 atm; 22 ◦C 0.2 [58]Supported on porous silica 1.10–1.2 bar; 30 ◦C 0.7 [59]0–1.2 bar; 50 ◦C 0.91 atm; 22 ◦C 0.5 [64]0–1.6 bar; 30 ◦C 0.9 This work

P. Sharma et al. / Chemical Engineering Journal 181– 182 (2012) 834– 841 839

F[

tpg

cie

NNCH2

O

CH3

+

C OO

NNCH2

O

CH3

C

O O

(1)

X

X= [BF 4], [PF 6], [Tf 2N], [TfO], [DCA]

X

NNCH2

O

CH3

C

O O

X

NNCH2

O

CH2

C

X

ig. 6. 13C spectra of carbondioxide absorbed ILs. (a) [C2Omim][BF4]; (b)C2Omim][PF6]; (c) [C2Omim][Tf2N]; (d) [C2Omim][TfO]; (e) [C2Omim][DCA].

he ILs show a new peak is appeared around ı = 200–210 ppm. Thiseak is attributed to carbonyl carbon atom of the formed carboxylicroup after absorption of CO2.

Based on this analysis, it can be concluded that absorption pro-ess of CO2 by ether functionalized ILs is a chemical process. Thisndicates that the lone pair of electrons on oxygen atom of thether group attack as a nucleophile on the carbon atom of CO2 and

O OH

Fig. 7. Proposed mechanism of CO2 absorption by ether functionalized ILs.

lead to the formation of carboxylic acid. The detailed mechanism ispresented in Fig. 7. This indicates the predominance of 1:1 mech-anism, where the CO2 reacts with one IL to form a carbamic acid,over further reaction with another IL to make a carbamate (the 1:2mechanism). This type of mechanism was also reported by previousresearchers in case of amine tethered to anion of the ILs [55,56].

Here, it is very interesting to note that the maximum absorptionof CO2 is 0.9 mol carbondioxide per mol of ILs at 30 ◦C at 1.6 barpressure. The reason why the absorption capacity cannot reach themaximum may be the high viscosity of ILs. It is reported that viscos-

ity of the ILs decreases dramatically as the temperature increases.Another important phenomenon observed in the absorption pro-cess is that viscosity of the liquid phase increases rapidly due toformation of a hydrogen bonding network. The viscosity of CO2

840 P. Sharma et al. / Chemical Engineering

1098765432100.80

0.85

0.90

0.95

1.00

mol

CO

2/mol

IL

Recycle number

[C2Omim][Tf2N]

[C2Omim][TfO ]

[C2Omim][ PF6]

[C2Omim][D CA]

[C2Omim][BF4]

F3

snfsgsa

3

brlc7fn

4

saridtnphHl

A

CoS

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

ig. 8. Mole fraction of CO2 (�) in different recycled ILs as a function of pressure at0 ◦C.

aturated ILs is reported in Table 1. Previous researchers alsooticed the non-ideal absorptions of CO2 in TSILs due to viscosity

actor [57–63]. This viscosity factor in absorption process can beignificantly controlled either by decreasing the number of hydro-ens on the anion available for hydrogen bonding or by addingome organic solvents or water. This will help to achieve the idealbsorption results in ILs.

.3. Thermal stability and recycle of ILs

Thermal stability of the ether functionalized ILs has been studiedy TGA in nitrogen environment (flow rate 20 mL/min), heatingate 10 ◦C/min. Results shows that there is no considerable weightoss up to 100 ◦C that means all ILs are stable under experimentalonditions (30 and 50 ◦C). The CO2 saturated ILs were heated at0 ◦C or under 20 Pa vacuum to desorbed CO2 and repeatedly usedor 10 cycles continuously. The result shows (Fig. 8) that there waso significant change in the CO2 capture capability of ILs.

. Conclusions

A series [C2Omim][X] of hydrophobic, chemically thermallytable ILs have been synthesized and characterized. The CO2bsorption capacity of ILs was investigated. The absorption capacityeaches 0.9 mol CO2 per mol of ILs at ambient pressure. The max-mum CO2 absorption is shown by ILs having [Tf2N] anion. This isue to the reason that the extent of fluoroalkylation is maximum inhis anion as compared to the other anions. The absorption mecha-ism investigated by FTIR proved that the absorption is a chemicalrocess. The absorbed CO2 can be easily desorbed by heating atigher temperature or under vacuum and can be repeatedly used.ence, these ether functionalized ILs are selective, thermally stable,

ong life operational, a promising candidate for CO2 capture.

cknowledgement

This research was supported by a grant (code CE3-101) from thearbon Dioxide Reduction & Sequestration Research Center, onef the 21st Century Frontier Programs funded by the Ministry ofcience and Technology of the Korean government.

[

Journal 181– 182 (2012) 834– 841

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