Selective chemical separation of carbondioxide by ether functionalized imidazolium cation based ionic liquids

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

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Selective chemical separation of carbondioxide by ether functionalized imidazolium cation based ionic liquids Pankaj Sharma a , Soo-Hyun Choi a , Sang-Do Park b , Il-Hyun Baek a,∗ , Gil-Sun Lee a,∗∗ a b

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

a r t i c l e

i n f o

Article history: Received 10 October 2011 Received in revised form 2 December 2011 Accepted 7 December 2011 Keywords: ILs CO2 absorption Ether Imidazole TSILs

a b s t r a c t A series [C2 Omim][X] of imidazolium cation-based ILs, with ether functional group on the alkyl sidechain were synthesized, characterized by various techniques such as 1 H, 13 C NMR, MS-ESI, FTIR and EA and investigated as potential absorbents for CO2 capture. More specifically, the influence of changing the anion with same cation is carried out. The absorption capacity of CO2 for ILs was investigated at 30 and 50 ◦ C at ambient pressure (0–1.6 bar). Ether functionalized ILs show significantly high absorption capacity for CO2 . The CO2 absorption capacity of ILs increased with a rise in pressure and decreased when temperature was raised. Results showed that absorption capacity reached about 0.9 mol CO2 per mol of IL at 30 ◦ C. The most probable mechanism of interaction of CO2 with ILs was investigated using FTIR and 13 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 the CO2 reacts with one IL to form a carbamic acid. The CO2 absorption capacity of ILs for different anions follows the trend: BF4 < DCA ∼ PF6 ∼ TfO < Tf2 N. Moreover, the as-synthesized ILs are selective, thermally stable, long life operational and can be recycled at a temperature of 70 ◦ C or under vacuum and can be used repeatedly. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In recent years, carbon dioxide (CO2 ) capture from flue gases has become an important issue, in both academia and industry, for environmentally benign and sustainable environment. As long as coal, petroleum and natural gas are used as the primary source of fuel, the production of CO2 is inevitable. Therefore, the development of economically viable CO2 capture and separation processes is a key step in the reduction of CO2 . Extensive research has been conducted by several groups on aqueous solutions of alkanolamines, especially monoethanolamine (MEA), diethanolamine (DEA) and methyldiethanolamine (MDEA) for natural gas treating and sweetening [1–14]. Even though these amine-based methods are highly efficient for CO2 capture, there are several concerns [15–17] which hinder the industrial application. Therefore, the development of a solvent that could absorb

∗ Corresponding author at: Korea University, Research Institute of Clean Chemical Engineering Systems, Greenhouse Gas Center, Republic of Korea. Tel.: +82 42 860 3648; fax: +82 42 860 3134. ∗∗ Co-corresponding author. E-mail address: [email protected] (I.-H. Baek). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.12.024

CO2 while being non-volatile and non-corrosive and contribute to the process economy as well as production sustainability by lowering the solvent inventory and reducing the discharge of volatile chemicals to atmosphere is highly demanded. In this context, ionic liquids (ILs) have been proposed. Their negligible vapor pressure, wide range of polarities, broad liquid range, significant thermal and chemical stability and capability to dissolve CO2 through a physical interaction make them an attractive candidate for CO2 capture. ILs generally consist of large organic bulky asymmetric cation, such as: quaternary ammonium, imidazolium, pyridinium and phosphonium ions and small sized and more symmetrical shape either an inorganic anion such as [Cl]− , [Br]− , [I]− , [BF4 ]− , [PF6 ]− , [Tf2 N]− or an organic anion such as [RCO2 ]− [18]. The physical and chemical properties of the ILs are decided by the nature of cation and anion. Therefore, it is possible to achieve specific properties by tuning the different combination of a cation and anion [19]. The class of imidazolium ILs is used in a wide variety of applications 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 temperature, wide electrochemical windows, high conductivities and ionic mobilities, easy recycling, tunable miscibility with water and

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

organic solvents, and being a good solvent for a variety of organic and inorganic compounds [20–22]. More importantly, imidazolium based ILs have good appetency with CO2 [23]. The factors which ultimately decide the solubility of CO2 in ILs are related with anion and cation both. Anion played the biggest role in CO2 solubility, a fact that was further supported by a recent X-ray diffraction study by Kanakubo et al. [24]. Anions that contain fluoroalkyl groups were found to have some of the highest CO2 solubilities, and as the quantity of fluoroalkyl groups increased, the CO2 solubility also increased [25]. For the cations, there were two factors that influenced the CO2 solubility. The biggest effect was seen by increasing alkyl chain length on the cation [25]. The second factor was to attach a functional group like ether to cation to create greater free volume or by incorporating a CO2 -philic carbonyl functional moiety. Some other functional moieties such as amine, alcohol, carboxylic and nitrile, have also been reported. Due to this aptness for the fine tuning of their properties through endless combination of cations and anions and functional moieties the ILs can be deemed as “designer solvent”. This promising strategy, leads to the synthesis and development of task specific ionic liquids (TSILs) for a specific requirement is being achieved. So finally we designed ether functionalized imidazolium based ILs with different anions to evaluate their combination effect on solubility of CO2 . Among the functionalized ILs, ether functionalized imidazolium ILs have been paid much attention due to their attractive physical and chemical properties [20–22], which include: air and moisture stability, low flammability, thermal stability, a negligible vapor pressure, being liquid over a wide temperature range, wide electrochemical windows, high conductivities and ionic mobilities, easy recycling, tunable miscibility with water and organic solvents and being a good solvent for a wide variety of organic and inorganic compounds. As a result they are widely used in the field of molecular recognition [26], transition metal catalysis [27], biocatalysis [28], carbohydrate/nucleoside chemistry [29,30], antimicrobial activities [31], metal extraction [32], liquid phase organic synthesis [33], polymer chemistry [34], organic synthesis [35], self organization [36] and lubricants [37]. In comparison with other functionalized ILs, ether functionalized ILs are expected to possess simultaneously peculiar properties and more attractive for versatile practical applications. Although, imidazolium cation based some ether functionalized ILs have been synthesized with [Cl]− , [BF4 ]− , [PF6 ]− , [CH3 SO3 ]− , [CF3 BF3 ]− and [C2 F5 BF3 ]− anions and their properties are studied [38,39]. However, there is still some scope to develop new ether functionalized ILs with suitable properties such as hydrophobic, low-melting, and low viscous ionic liquids by using anions such as [Tf2 N]− , [TfO]− , and [DCA]− . Here in, we report the synthesis, characterization of a series of ether functionalized ILs [C2 Omim][X], based on the imidazolium cation that contains additional functional group, ether, on the alkyl group R and their application in capture of CO2 . Till date, no report is available involving ether functionalized imidazolium based ILs in open literature with study of the solubility of CO2 .

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2.2. Apparatus and procedures 2.2.1. Synthesis of ionic liquids A previously reported methodology [39] was used for the synthesis of these ionic liquids. Alkylation of 1-methylimidazole with an alkyl halide is followed by halogen (Cl or Br) exchange with slight excess (1.1 equiv) of NaBF4 , KPF6 , Li(Tf2 N), Na(TfO) and Na(DCA) in order to reduce the amount of remaining halogen content. of [C2 Omim][Cl]. Chloromethylmethylether 2.2.1.1. Synthesis (70 mL, 0.93 mol) and 1-methylimidazole (50 mL, 0.63 mol) were added to a round-bottomed flask fitted with a reflux condenser for 24 h at 80 ◦ C with stirring. The product was washed with diethyl 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 yellow liquid which solidified on cooling (82% yield) with water content of 3.46 ␮g (H2 O) mL−1 (RTIL). 1 H 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); 13 C 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) [C2 Omim]+ , 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 C6 H10 N2 OCl (161): C 44.7, H 6.2, N 17.3; found: C 44.4, H 6.1, N 17.1 2.2.1.2. Synthesis of [C2 Omim][BF4 ]. [C2 Omim][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 resulting waxy solid precipitate was collected by filtration and washed with acetone (2× 100 mL). The organic layer was collected, dried (MgSO4 ), filtered and the solvent removed in vacuum to give the product (89% yield) as a light brown liquid; water content of 2.66 ␮g (H2 O) mL−1 (RTIL). 1 H 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); 13 C 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) [C2 Omim]+ , 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 C6 H10 N2 OBF4 (213): C 33.8, H 4.69, N 13.1; found: C 33.2, H 4.62, N 12.9

2.1. Reagents and materials

2.2.1.3. Synthesis of [C2 Omim][PF6 ]. [C2 Omim][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 resulting waxy solid precipitate was collected by filtration and washed with acetone (2× 100 mL). The organic layer was collected, dried (MgSO4 ), filtered and the solvent removed in vacuum to give the product (90% yield) as a dark brown liquid; water content of 1.84 ␮g (H2 O) mL−1 (RTIL). 1 H 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); 13 C 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) [C2 Omim]+ , 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 C6 H10 N2 OPF6 (271): C 33.8, H 4.69, N 13.14; found: C 33.2, H 4.59, N 13.1

Chloromethylmethylether (CH3 OCH2 Cl), 1-methylimidazole, diethyl ether, acetone, sodium tetrafluoroborate (NaBF4 ), magnesium sulfate (MgSO4 ), potassium hexafluorophosphate (KPF6 ), lithium bis[(trifluoromethyl)sulfonyl] amide Li(Tf2 N), sodium trifluoromethylsulfonate Na(TfO), sodium dicyanamide Na(DCA) were supplied from Sigma–Aldrich and used without further purification.

2.2.1.4. Synthesis of [C2 Omim][Tf2 N]. [C2 Omim][Cl] (25.00 g, 0.15 mol) was transferred to a plastic Erlenmeyer flask (250 mL). Acetone (150 mL) was added followed by Li(Tf2 N) (49.00 g, 0.170 mol). This mixture was stirred at room temperature for 24 h. The resulting waxy solid precipitate was collected by filtration and washed with acetone (2× 100 mL). The organic layer was collected, dried (MgSO4 ), filtered and the solvent removed in

2. Experimental

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

vacuum to give the product (89% yield) as a light brown liquid; water content of 2.21 ␮g (H2 O) mL−1 (RTIL). 1 H NMR (700 MHz), [D6 ] acetone, 25 ◦ C): ı = 3.27 (s, 3H), 3.81 (s, 2H), 3.82 (s, 3H), 7.31 (s, 1H), 7.42 (s, 1H), 8.66 (s, 1H); 13 C NMR (700 MHz), [D6 ] acetone, 25 ◦ C: ı = 35.9, 49.2, 68.7, 122.1, 123.2, 137.1; MS-ESI: m/z (%): 126 (100) [C2 Omim]+ , 280 (100) [Tf2 N]− ; FTIR (neat): 3414, 3150, 3096, 2979, 2970, 2880, 2876, 1638, 1574, 1462, 1348, 1336, 1181, 1173, 1135, 1083, 1055, 1013, 968.3, 833.5, 789, 760.2, 739 cm−1 ; elemental analysis calcd (%) for C8 H10 N3 O5 F6 S2 (406): C 23.6, H 2.46, N 10.34, S 15.76; found: C 23.1, H 2.43, N 10.1, S 15.69 2.2.1.5. Synthesis of [C2 Omim][TfO]. [C2 Omim][Cl] (25.00 g, 0.15 mol) was transferred to a plastic Erlenmeyer flask (250 mL). Acetone (150 mL) was added followed by Na(TfO) (29.50 g, 0.171 mol). This mixture was stirred at room temperature for 24 h. The resulting waxy solid precipitate was collected by filtration and washed with acetone (2× 100 mL). The organic layer was collected, dried (MgSO4 ), filtered and the solvent removed in vacuum to give the product (90% yield) as a light brown liquid; water content of 2.25 ␮g (H2 O) mL−1 (RTIL). 1 H NMR (700 MHz), [D6 ] acetone, 25 ◦ C: ı = 3.28 (s, 3H), 3.84 (s, 2H), 3.81 (s, 3H), 7.31 (s, 1H), 7.42 (s, 1H), 8.68 (s, 1H); 13 C NMR (700 MHz), [D6 ] acetone, 25 ◦ C): ı = 35.1, 49.2, 69.4, 122.2, 123.6, 137.8; MS-ESI: m/z (%): 126 (100) [C2 Omim]+ , 149 (100) [TfO]− ; FTIR (neat): 3410, 3155, 3100, 2976, 2879, 1642, 1569, 1458, 1422, 1363, 1253, 1223, 1170, 1089, 1021, 1060, 980, 965.3, 919, 844, 775, 763, 739, 717, 655, 636, 573, 529 cm−1 ; elemental analysis calcd (%) for C7 H10 N2 O4 F3 S (275): C 30.5, H 5.2, N 10.18, S 11.6; found: C 30.1, H 4.98, N 10.1, S 11.1. 2.2.1.6. Synthesis of [C2 Omim][DCA]. [C2 Omim][Cl] (25.00 g, 0.15 mol) was transferred to a plastic Erlenmeyer flask (250 mL). Acetone (150 mL) was added followed by Na(DCA) (15.00 g, 0.168 mol). This mixture was stirred at room temperature for 24 h. The resulting waxy solid precipitate was collected by filtration and washed with acetone (2× 100 mL). The organic layer was collected, dried (MgSO4 ), filtered and the solvent removed in vacuum to give the product (88% yield) as a light yellow liquid; water content of 2.52 ␮g (H2 O) mL−1 (RTIL). 1 H 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); 13 C NMR (700 MHz), [D6 ] acetone, 25 ◦ C: ı = 35.3, 49.3, 69.1, 121.2, 122.1, 136.9; MS-ESI: m/z (%): 126 (100) [C2 Omim]+ , 66 (100) [DCA]− ; FTIR (neat): 3400, 3138, 3089, 2971, 2876, 1643, 1575, 1459, 1168, 1077, 1011, 965, 830, 761 cm−1 ; elemental analysis calcd (%) for C8 H10 N5 O (192): C 50.0, H 5.2, N 36.45; found: C 49.4, H 5.1, N 35.9. 2.2.2. Characterization 1 H and 13 C NMR spectra were obtained using a Bruker Avance700 FT-NMR Spectrophotometer and [D6 ] acetone was used as solvent. The chemical shift values are reported in ppm with respect to TMS internal reference. The mass of the samples was measured on a Hewlett Packard 1100 Series, Mass Spectrophotometer, Agilent 1200 Series. The FT-IR spectra were recorded on Thermo, Model:

3

2

6

5

P

4 8

7 9

1 10 Fig. 1. Experimental apparatus: (1) CO2 reservoir; (2) control valve; (3) pressure gauge; (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 performed at 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 after vacuum drying at 70–100 ◦ C for 24 h. The TGA was performed on a thermal analysis system (Mettler Toledo, Model: TGA/SDTA 85 1e). An average sample weight of 5 mg was placed in a platinum pan and heated at 10 ◦ C/min from ca. 30 to 100 ◦ C under a flow of N2 . The density of the liquid salts was approximately determined by measuring the weight of 1.0 mL of the salt three times at 25 ◦ C. The viscosity was measured with a viscometer (Brookfield, Model DV-III+) using 0.6 mL sample at 10, 20 and 30 ◦ C. 2.2.3. CO2 absorption isotherm measurement The experimental method used for the gas solubility measurement was based on an isochoric saturation technique. The experimental apparatus used in this study is schematically represented in Fig. 1. The main composition of the apparatus includes a gas reservoir, a thermostat, a pressure gauge, an isochoric cell, a vacuum pump, and a magnetic stirrer. The temperature was determined with two calibrated platinum resistance thermometers placed 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 dioxide absorption isotherm measurement was performed at 30 and 50 ◦ C. In a typical experiment, about 2 mL ILs was loaded into the isochoric cell, the air in the system was eliminated by vacuum pump, then CO2 was charged into the cell from the gas reservoir and the ionic liquid was stirred. The system was considered to have reached equilibrium if the pressure of the system had been unchanged within 2 h. Then the pressure of the system was recorded and the weight of the cell was determined with an electronic balance (Sartorius BS224S) with an uncertainty ±0.0001 g. The solubility of carbon dioxide in the ionic liquid was determined by the shifted quality of the isochoric cell. The quantity of absorbed CO2 in ILs was measured gravimetrically by weighing the solution successively (with closed valves) until the mass remains constant at the particular pressure

Table 1 The various components of ILs, density and viscosity of ILs of the series [C2 Omim][X]. S. No.

1. 2. 3. 4. 5. 6. a

Ionic liquid

[C2 Omim][Cl] [C2 Omim][BF4 ] [C2 Omim][PF6 ] [C2 Omim][Tf2 N] [C2 Omim][TfO] [C2 Omim][DCA] Viscosity of CO2 saturated ILs.

Anion

Density

Viscosity (cP)

X−

(g L−1 ) at 25 ◦ C

10 ◦ C

20 ◦ C

30 ◦ C

30 ◦ Ca

Cl− BF4 − PF6 − NTf2 − TfO− DCA−

1.14 1.24 1.38 1.46 1.31 1.05

592.2 370.1 598.3 70.1 117.7 40.4

401.1 252.7 281.1 44.2 100.2 31.6

201.2 130.1 141.2 29.5 61.2 22.9

252.4 178.2 193.3 45.4 78.8 36.5

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

N

N

(CH2)

CH3

X

O

CH3

X= [BF4], [PF6], [Tf2N], [TfO], [DCA] Fig. 2. Chemical structures of the ILs [C2 Omim][X].

during the experiment. To verify the validation of the apparatus, we determined the solubility of CO2 in a choline chloride + urea mixture with this apparatus, and the result was consistent with reference [40]. 2.2.4. CO2 absorption mechanism verification The mechanism of CO2 absorption in this report is investigated by FTIR and 13 C NMR of CO2 saturated ILs. The CO2 saturated sample of ILs was collected after experiment and characterized for FTIR and 13 C NMR. The same instruments and conditions were used which were used previously for characterization of materials. 3. Results and discussion The chemical structures of the ILs [C2 Omim][X] synthesized and studied in the present work are presented in Fig. 2. The various components of ILs, density and viscosity of ILs of the series [C2 Omim][X] are reported in Table 1. In general, with a constant cation, the density increased with increasing the bulkiness of the anion [39,41]. These trends are consistent with the reported observations in other ILs with imidazolium cations. The density data for ILs are reported in Table 1. The trend observed for the density of the [C2 Omim][X] is as follows for different anions: Tf 2 N− > C2 F5 BF3 > CF3 BF3 > PF6 − > TfO− > BF4 − > Cl− > DCA− Therefore, the density values of ILs are in good agreement as expected. The ILs containing [Tf2 N]− anion have higher density and [DCA]− containing anion have lower density. But [C2 F5 BF3 ] containing ionic liquids are almost equally dense to [Tf2 N]− ionic liquids. These results indicate that the densities of these ionic liquids can be fine-tuned with slight structural changes in cation and anion.

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The viscosity of an ionic liquid is essentially determined by their tendency to form hydrogen bonds and by the strength of van der Waals interactions (dispersion and repulsion), being strongly dependent on the anion type [42–50]. An anion combined a good charge 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., [Al2 Cl7 ]− [44], [(CF3 SO2 )(CF3 CO)N]− [48], and [(CF3 SO2 )2 N]− ) [43,49] tends to 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 of its weak coordinating ability. The viscosity data for ILs are reported in Table 1. The trend observed for the viscosity of the ILs is as follows for different anions: Cl− > PF6 − > BF4 − > TfO− > CF3 BF3 > C2 F5 BF3 > Tf 2 N− > DCA− The trend obtained for viscosity of ILs is in good agreement with the earlier reported results and it shows that viscosity of ILs is mainly 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 [CF3 BF3 ] anion is higher than ionic liquids having [C2 F5 BF3 ] anion. It also states that the viscosity of these ionic liquids is less than [TfO]− anions. But there are enough studies carried out by many research groups and it is concluded that the structure of the cation also influences the viscosity of ILs [39,41]. All this results suggest that, in search of low-viscous ILs, we should focus not only on weak coordinating ability of the anion, but also the shape (symmetry) and size of the anion should also be taken into consideration. 3.1. CO2 absorption isotherm measurements The absorption isotherm of CO2 in different ILs was determined at 30 and 50 ◦ C at ambient pressure (Figs. 3 and 4). It was noted that ether functionalized ILs show significantly high absorption capacity for CO2 . The CO2 absorption capacity of ILs increased with a rise in pressure and decreased when temperature was raised. Results showed that absorption capacity reached about 0.9 mol CO2 per mol of ILs at 30 ◦ C. The CO2 absorption capacity of ILs for different anions follows the trend: BF4 < DCA ∼ PF6 ∼ TfO < Tf2 N. It is already reported that anions that contain fluoroalkyl groups were found to have highest CO2 solubilities, and as the quantity of fluoroalkyl

Table 2 Solubility data of CO2 (mol CO2 /mol IL) in different ILs at different temperatures. Pressure (bar)

Temperature (◦ C)

Solubility of CO2 in different ILs (mol CO2 /mol IL) [C2 Omim][BF4 ]

[C2 Omim][PF6 ]

[C2 Omim][Tf2 N]

[C2 Omim][TfO]

[C2 Omim][DCA]

0 0.1 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0 0.8 0.84 0.86 0.89 0.9 0.91 0.915 0.92 0.925

0 0.86 0.9 0.93 0.95 0.95 0.955 0.956 0.957 0.958

0 0.9 0.95 0.97 0.975 0.976 0.9775 0.978 0.979 0.979

0 0.88 0.92 0.94 0.96 0.965 0.967 0.968 0.969 0.97

0 0.8 0.85 0.88 0.9 0.91 0.92 0.93 0.935 0.935

30

0 0.1 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0 0.62 0.69 0.73 0.75 0.76 0.77 0.78 0.782 0.785

0 0.69 0.72 0.785 0.8 0.82 0.825 0.828 0.833 0.83

0 0.7 0.75 0.81 0.84 0.86 0.865 0.868 0.87 0.871

0 0.696 0.74 0.8 0.82 0.84 0.845 0.848 0.855 0.85

0 0.65 0.7 0.74 0.76 0.77 0.78 0.79 0.792 0.795

50

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

1.0

mol CO2/mol IL

0.8

0.6

0.4 [C2Omim][Tf2N] [C2Omim][TfO]

0.2

[C2Omim][ PF6] [C2Omim][DCA]

0.0

[C2Omim][BF4]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

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

0.9 0.8

mol CO2/mol IL

0.7 0.6 0.5 0.4 0.3

[C2Omim][Tf2N]

0.2

[C2Omim][TfO]

0.1

[C2Omim][ PF6]

0.0

[C2Omim][DCA] [C2Omim][BF4]

-0.1 0.0

0.5

Pressure (bar)

1.0

1.5

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

groups increased, the CO2 solubility also increased. So the result and trend is in good agreement with previous reports (Tables 2 and 3). 3.2. Mechanism of CO2 absorption The interaction of CO2 with different ILs was investigated using FTIR and result shows that the absorption of CO2 in ether functionalized ILs is a chemical process. The FTIR spectra of CO2 absorbed

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

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

Table 3 Comparison of the solubility of CO2 in TSIL from previous literature and this work. S. No.

Name of TSIL

Test conditions

Solubility of CO2 mol of CO2 /mol IL

Reference

1. 2.

Trihexyl(tetradecyl)phosphonium amino acid; [P66614 ][AA] Tetrabutylphosphonium amino acid; [P(C4 )4 ] [AA] (3-Aminopropyl) tributylphosphonium amino acid; [aP4443 ] [AA]

4.

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

5. 6.

1-Butyl-3aminopropyl imidazolium tetrafluoroborate [C2 Omim][Tf2 N]

0.9 0.12 0.5 0.2 1.1 0.7 0.9 0.5 0.9

[55] [57]

3.

0–1 bar; 22 ◦ C 1 atm; 22 ◦ C; water (1%, mass fraction) Supported on porous silica 1 atm; 22 ◦ C Supported on porous silica 0–1.2 bar; 30 ◦ C 0–1.2 bar; 50 ◦ C 1 atm; 22 ◦ C 0–1.6 bar; 30 ◦ C

[58] [59] [64] This work

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

N

839

CH 3

CH 2

N

O

X

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

O

O

(1)

N

CH 3

CH 2

N

X

O

C O

N

N

O

CH 3

CH 2

X

O

C O

N

N

O

CH 2

CH 2

X

O

C O

OH

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

Fig. 6. 13 C spectra of carbondioxide absorbed ILs. (a) [C2 Omim][BF4 ]; (b) [C2 Omim][PF6 ]; (c) [C2 Omim][Tf2 N]; (d) [C2 Omim][TfO]; (e) [C2 Omim][DCA].

the ILs show a new peak is appeared around ı = 200–210 ppm. This peak is attributed to carbonyl carbon atom of the formed carboxylic group after absorption of CO2 . Based on this analysis, it can be concluded that absorption process of CO2 by ether functionalized ILs is a chemical process. This indicates that the lone pair of electrons on oxygen atom of the ether group attack as a nucleophile on the carbon atom of CO2 and

lead to the formation of carboxylic acid. The detailed mechanism is presented in Fig. 7. This indicates the predominance of 1:1 mechanism, where the CO2 reacts with one IL to form a carbamic acid, over further reaction with another IL to make a carbamate (the 1:2 mechanism). This type of mechanism was also reported by previous researchers in case of amine tethered to anion of the ILs [55,56]. Here, it is very interesting to note that the maximum absorption of CO2 is 0.9 mol carbondioxide per mol of ILs at 30 ◦ C at 1.6 bar pressure. The reason why the absorption capacity cannot reach the maximum may be the high viscosity of ILs. It is reported that viscosity of the ILs decreases dramatically as the temperature increases. Another important phenomenon observed in the absorption process is that viscosity of the liquid phase increases rapidly due to formation of a hydrogen bonding network. The viscosity of CO2

840

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

1.00

References

mol CO2/mol IL

0.95

0.90

[C2Omim][Tf2N] [C2Omim][TfO]

0.85

[C2Omim][ PF6] [C2Omim][DCA] [C2Omim][BF4]

0.80 0

1

2

3

4

5

6

7

8

9

10

Recycle number Fig. 8. Mole fraction of CO2 () in different recycled ILs as a function of pressure at 30 ◦ C.

saturated ILs is reported in Table 1. Previous researchers also noticed the non-ideal absorptions of CO2 in TSILs due to viscosity factor [57–63]. This viscosity factor in absorption process can be significantly controlled either by decreasing the number of hydrogens on the anion available for hydrogen bonding or by adding some organic solvents or water. This will help to achieve the ideal absorption results in ILs.

3.3. Thermal stability and recycle of ILs Thermal stability of the ether functionalized ILs has been studied by TGA in nitrogen environment (flow rate 20 mL/min), heating rate 10 ◦ C/min. Results shows that there is no considerable weight loss up to 100 ◦ C that means all ILs are stable under experimental conditions (30 and 50 ◦ C). The CO2 saturated ILs were heated at 70 ◦ C or under 20 Pa vacuum to desorbed CO2 and repeatedly used for 10 cycles continuously. The result shows (Fig. 8) that there was no significant change in the CO2 capture capability of ILs.

4. Conclusions A series [C2 Omim][X] of hydrophobic, chemically thermally stable ILs have been synthesized and characterized. The CO2 absorption capacity of ILs was investigated. The absorption capacity reaches 0.9 mol CO2 per mol of ILs at ambient pressure. The maximum CO2 absorption is shown by ILs having [Tf2 N] anion. This is due to the reason that the extent of fluoroalkylation is maximum in this anion as compared to the other anions. The absorption mechanism investigated by FTIR proved that the absorption is a chemical process. The absorbed CO2 can be easily desorbed by heating at higher temperature or under vacuum and can be repeatedly used. Hence, these ether functionalized ILs are selective, thermally stable, long life operational, a promising candidate for CO2 capture.

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

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