Energy Fuels 2010, 24, 6689–6692 Published on Web 11/08/2010
: DOI:10.1021/ef101143t
Alkyl-fluoroalkylimidazolium-Based Ionic Liquids as Efficient CO2 Absorbents Hyunjoo Lee,*,† Min Ho Cho,† Bang Sook Lee,† Jelliarko Palgunadi,‡ Honggon Kim,† and Hoon Sik Kim*,‡ †
Energy Division, Korea Institute of Science and Technology, Hawolgok-dong, Sungbuk-gu, Seoul 136-791, Korea, and ‡ Department of Chemistry, Kyung Hee University, 1 Hoegi-dong, Dondaemoon-gu, Seoul 130-701, Korea Received August 25, 2010. Revised Manuscript Received October 27, 2010
Scheme 1. One-Pot Preparation Method for 1-Alkyl-3-fluoroalkylimidazolium-Based ILs16
CO2 capture from the exhaust flue gases of fossil-fuel power plants has become one of the hot issues for a sustainable society because of the strong impact of CO2 on global warming. Industrially, CO2 is being removed from flue gases by a variety of methods, including the chemical absorption by aqueous solutions of alkanol amines and the adsorption with K2CO3. Of these, alkanol-amine-based aqueous solutions are more preferred because of their rapid reactions with CO2, forming ammonium carbamate. However, the absorption process using alkanol amines has several drawbacks, such as the decomposition of amines by small amounts of S-containing compounds present in flue gases, loss of absorbents during regeneration because of their high volatilities, and the requirement of enormous energy to beak the strong C-N bonds of carbamates in the regeneration of alkanol amines.1,2 To avoid the problems encountered in the chemical absorption processes, physical CO2 absorbents, such as dimethyl ethers of polyethylene glycol, are being used at the expense of absorption capacity, but they also suffer from the drawbacks of low thermal stability, high volatility, and flammability.3 Recently, ionic liquids (ILs) have received considerable interest as potential candidates to replace current amineand organic-solvent-based CO2 absorbents, owing to their high thermal stability, negligible volatility, and tunable acidity and basicity for the interaction with acidic gases, such as CO2 and SO2. Accordingly, a large number of reports have been published on the application of ILs as CO2-capture agents. Although the achievements have been significant, much needs to be improved in terms of CO2 absorption capacity and price of ILs.4-9 Introducing carbonyl groups and long alkyl chains with a branching or ether linkages in ILs was suggested as a means of improving CO2 solubility by increasing the CO2-philic property and also by creating free volume, where CO2 molecules
could be captured.10 ILs with basic anions, such as acetates, phosphates, or amino acid moieties, were also proven to have high CO2 absorption capacities.11-14 The increase in the number of fluorine atoms in ILs is another way of enhancing the CO2 dissolution ability. Muldoon et al.11 reported that, for an IL bearing a fluorinated anion, the capability of dissolving CO2 increased with an increasing number of fluorine atoms on the anion. They found that CO2 solubility in 1-butyl-3-methylimidazolium-based ILs is in the following order: tris(pentafluoroethyl)trifluorophosphate [eFAP] > bis(trifluoromethyl sulfonyl)imide [NTf2] > hexafluorophosphate [PF6] > tetrafluoroborate [BF4]. However, as far as we know, 1-methyl-3-(nonafluorohexyl)imidazolium bis(trifluorosulfonyl)imide ([C6H4F9MIm][NTf2]) and 1-methyl-3-(tridecafluorooctyl)imidazolium bis(trifluorosulfonyl)imide ([C8H4F13MIm][NTf2]) are the only ILs bearing a fluorine group on the cations, which were tested as CO2 absorbents. These ILs were shown to have higher CO2 solubility compared to alkyl-substituted ILs having the same anion. In general, introduction of a fluorinated functional group on the imidazolium cation has been conducted by a two-step reaction comprising the reaction of alkyl imidazole with fluoroalkyl iodide and the following metathesis with an alkali metal salt. However, this methodology has some drawbacks, such as the use of costly fluoroalkyl iodide, low product selectivity, and the involvement of a tedious and problematic anion-exchange reaction.15 Recently, we have shown that 1-alkyl-3-fluoroalkylimidazolium-based ILs having CF3CO2-, Cl-, NO3-, BF4-, and NTf2- anions can be prepared almost quantitatively from the reaction of an alkylimidazole with a corresponding acid in the presence of a fluorinate olefin, such as hexafluoropropylene (CF3CFdCF2) and tetrafluoroethylene (CF2dCF2).16
*To whom correspondence should be addressed. Telephone: þ822-958-5868. Fax: þ82-2-958-5809. E-mail:
[email protected] (H.L.); khs2004@ khu.ac.kr (H.S.K.). (1) Baltus, R. E.; Counce, R. M.; Culbertson, B. H.; Luo, H.; DePaoli, D. W.; Dai, S.; Duckworth, D. C. Sep. Sci. Technol. 2005, 40, 525–541. (2) Alejandre, J.; Rivera, J. L.; Mora, M. A.; Garza, V. D. L. J. Phys. Chem. B 2000, 104, 1332–1337. (3) Kohl, L.; Nielsen, R. B. Gas Purification, 5th ed.; Gulf Publishing Co.: Houston, TX, 1997; p 1395. (4) Blanchard, L. A.; Hancu, D.; Beckmanm, E. J.; Brennecke, J. F. Nature 1999, 399, 28–29. (5) Zhang, W.; Li, Z.; Han, B.; Ha, S.; Song, J.; Xie, Y.; Zhou, X. Green Chem. 2009, 10, 1142–1145. (6) Cadena, C.; Anthony, J. L.; Shah, J. K.; Morrow, T. I.; Brennecke, J. F.; Maginn, E. J. J. Am. Chem. Soc. 2004, 126, 5300–5308. (7) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H., Jr. J. Am. Chem. Soc. 2002, 124, 926–927. (8) Gutowski, K. E.; Maginn, E. J. J. Am. Chem. Soc. 2008, 130, 14690–14704. (9) Lee, H.; Kim, D. S.; Kim, H.; Kim, H. S. KIC News 2009, 12, 1–8. r 2010 American Chemical Society
(10) Aki, S. N. V. K.; Mellein, B. R.; Saurer, E. M.; Brennecke, J. F. J. Phys. Chem. B 2004, 108, 20355–20365. (11) Muldoon, M. J.; Aki, S. N. V. K.; Anderson, J. L.; Dixon, J. K.; Brennecke, J. F. J. Phys. Chem. B 2007, 111, 9001–9009. (12) Shariati, A.; Peters, C. J. J. Supercrit. Fluids 2004, 29, 43–48. (13) Zhang, J. M.; Zhang, S. J.; Dong, K.; Zhang, Y. Q.; Shen, Y. Q.; Lv, X. M. Chem.;Eur. J. 2006, 12, 4021–4026. (14) Jiang, Y. Y.; Wang, G. N.; Zhou, Z.; Wu, Y. T.; Geng, J.; Zhang, Z. B. Chem. Commun. 2008, 505–507. (15) Singh, R. P.; Manandhar, S.; Shreeve, J. M. Tetrahedron Lett. 2002, 43, 9497–9499.
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Energy Fuels 2010, 24, 6689–6692
: DOI:10.1021/ef101143t
Figure 1. Structures of ILs used in the current study. Table 1. Some Physical Properties of ILs Used in the Current Study ILs
Tg (°C)
viscosity (mPa s)
density (g/cm3)
molar volume (mL/mol)
water content (ppm)
H2.1 (MPa)
1 2 3 4 5
-66.8 -58.3 -63.3 -85.8 -69.4
525 566 575 52 (52)b 74 (73)b
1.38 1.41 1.37 1.43 1.20
402.43 264.10 233.74 293.26 210.19
453 480 457 538 674
3.08 3.38 3.88 3.36 (3.33)c 6.64
a
a Viscosities were measured at 30 °C, and the error was estimated at (2.5%. b The number in parentheses is the reference value.17 c The number in parentheses is the reference value.11
The reactions could be processed in one pot without isolating intermediates, quaternary alkylimidazolium salts (Scheme 1). In this paper, we report the CO2 absorption capacities of several ILs bearing a fluorinated alkyl group on the cations. The viscosities of all of the fluorinated ILs were measured at 30 °C using a rotational viscometer (TV-22, Toki Sangyo Co., Ltd., Japan), which was precalibrated with a standard liquid for the selection of the ILs suitable for the CO2 absorption tests. For this reason, 3-butyl-1-(1,1,2,3,3,3-hexafluoropropyl)imidazolium bis(trifluoromethanesulfonyl)imide ([B(C3HF6)Im][NTf2], 1), 3-butyl-1-(1,1,2,3,3,3-hexafluoropropyl)imidazolium trifluoroacetate ([B(C3HF6)Im][CF3CO2], 2), and 3-butyl-1-(1,1,2,2,-tetrafluoroethyl)imidazolium trifluoroacetate ([B(C2HF4)Im][CF3CO2], 3) (see Figure 1 and Table 1) with viscosities lower than 600 mPa s were used for CO2 absorption. For comparison, dialkylimdiazolium-based ILs with a fluorinated anion, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIm][NTf2], 4) and 1-butyl3-methylimidazolium trifluoroacetate ([BMIm][CF3CO2], 5), were prepared and measured for their CO2 dissolution capacities. The measurements of CO2 solubilities in ILs were performed at 298 ( 1 K and at pressures ranging from 2 to 23 bar using an apparatus described elsewhere (see also the Supporting Information).18 The amount of dissolved CO2 in an IL was calculated on the basis of a pressure-decay observation by employing Soave modification of the Redlich-Kwong (SRK) equation of state (EOS) (see the Supporting Information).19-22 The accuracy of the CO2 absorption was estimated at around (2.5%. A comparison of Henry’s law constants for the CO2 absorption in [BMIm][NTf2] reveals that Henry’s law
Figure 2. CO2 solubilities in fluorinated ILs with pressure: 1, [B(C3HF6)Im][NTf2]; 2, [B(C3HF6)Im][CF3CO2]; 3, [B(C2HF4)Im][CF3CO2]; 4, [BMIm][NTf2]; and 5, [BMIm][CF3CO2].
constant obtained in this study (3.36) is very close to the literature value (3.33), supporting the accuracy of our measurements. Figure 2 shows the CO2 dissolution capacities of the prepared fluoroalkyl-substituted ILs. Among the ILs tested, [B(C3HF6)Im][NTf2] (1) showed the highest CO2 solubility of 0.45 in mole fraction at 25 °C and 20 bar. It is worth noticing that compound 1 exhibited a similar CO2 absorption capacity to compound 4 at 2 bar and showed a slightly higher absorption capacity than compound 4 at higher pressures. The effect of fluoroalkyl substitution on the imidazolium cation on the CO2 dissolution ability was more pronounced for compound 5 bearing a CF3CO2 anion. At the CO2 pressure of 2 bar, the mole fractions of dissolved CO2 in compounds 2 and 3 were measured at 0.067 and 0.050, respectively, which are considerably higher than 0.030 in compound 5. Compounds 2 and 3 also showed relatively high CO2 absorption capacities of 0.43 and 0.40 in mole fraction at 20 bar, which are 43 and 33% higher than the value of 0.3 in compound 5. It has been reported that CO2 solubility in ILs primarily depends upon the strength of the interaction of CO2 with the anion. Two types of interactions are known: one is the acid-base
(16) Kim, D. S.; Natalia, D.; Nguyen, D. Q.; Suh, D. J.; Kim, H.; Kim, H. S.; Lee, H. Synlett 2009, 13, 2101–2104. (17) Bonh^ ote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.; Gr€ atzel, M. Inorg. Chem. 1996, 35, 1168–1178. (18) Palgunadi, J.; Kang, J. E.; Cheong, M.; Kim, H.; Lee, H.; Kim, H. S. Bull. Korea Chem. Soc. 2009, 30, 1749–1754. (19) Soave, G. Chem. Eng. Sci. 1972, 27, 1196–1203. (20) Shiflett, M. B.; Yokozeki, A. J. Phys. Chem. 2007, 111, 2070– 2074. (21) Li, H.; Yan, J. Appl. Energy 2009, 86, 2760–2770. (22) Redlich, O.; Kwong, J. N. S. Chem. Rev. 1949, 44, 233–244.
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: DOI:10.1021/ef101143t
interaction between CO2 and the anion in ILs, and the other is between CO2 and fluorine in the anion. Another major factor that affects the CO2 dissolution capacity is the molar volume of the solvent. In general, the larger the molecular size, the larger the free volume in which CO2 can occupy.23-25 In this context, the higher CO2 absorption capacity of the IL with a larger sized NTf2- anion than those with a CF3CO2- anion can be rationalized. The higher CO2 absorption in ILs with a NTf2- anion was also confirmed in our studies. As revealed in Figure 2, compounds 1 and 4 show higher CO2 absorption capacities compared to those bearing the CF3CO2- anion (compounds 2, 3, and 5), irrespective of the presence of a fluorinated alkyl group on the imidazolium cation. The lower CO2 absorption capacities of the ILs with a CF3CO2- anion than those bearing a NTf2- anion suggest that, at least for this case, the molar volume of the IL exerted a more pronounce effect than the anion basicity (pKa of HNTf2, -4; pKa of CF3CO2H, 0),26 which is one of the key factors in determining the CO2 solubility in ILs. It was hoped that the CO2 absorption capacity of compound 4 bearing a NTf2- anion could be improved significantly by the introduction of a fluorinated alkyl group on the imidazolium cation. However, the CO2 absorption capacity of compound 4 increased only slightly by the substitution of a methyl group on the imidazolium ring with a hexafluoropropyl group, although the molar volume of compound 4 increased by 37.2% by the substitution. This result may suggest that the ILs with a NTf2- anion already possess sufficient free volume for the incorporation of CO2, and thus, a further increase of the free volume does not seem to affect the solubility of CO2. The positive effect exerted by the fluorinated alkyl group is counterbalanced by the viscosity increase with the substitution. On the contrary, the effect of fluoroalkyl substitution was much more pronounced for compound 5 having a CF3CO2- anion. The CO2 absorption capacity of compound 5 increased almost twice at 2 bar when the methyl group on the imidazolium ring was replaced by a hexafluoropropyl group. This is a rather surprising result because the molar volume increases only by 25.6% by the substitution of the methyl group with a hexafluoropropyl group (Table 1). A similar phenomenon was also observed when the methyl group was replaced by a tetrafluoroethyl group. Smith et al. and Russina et al. suggest that there exist extensive nanostructures in the fluorinated alkyl-substituted imidazolium trifluoroacetates.27,28 It is expected that the CO2 molecules can reside in not only the ionic regions but also the nonpolar domains that are composed of the fluorinated alkyl groups of the ILs because of the highly CO2-philic nature of fluorinated alkyl groups and the presence of the stable nonpolar domains containing the fluorinated alkyl groups. In this context, the considerably higher CO2 solubility in compounds
Figure 3. Repeated CO2 absorption-desorption cycles using compound 1 ([B(C3HF6)Im][NTf2]).
Figure 4. Comparison of CO2 solubility in various ILs: 1, [(C3HF6)Im][NTf 2 ]; 2, [B(C 3 HF 6 )Im][CF 3 CO 2 ]; [C 8 H 4 F 13 MIm][NTf 2 ]; 11 [C6H4F9MIm][NTf2];11 and [HMIm][eFAP].11
2 and 3 than that in compound 5 can be rationalized. However, in the case of compounds 1 and 4, such a drastic enhancement in CO2 solubility was not observed by the substitution of CH3 with CF2CHCF3 on the imidazolium ring. It is likely that the solubility enhancement by the presence of highly CO2philic fluorinated alkyl groups and the nonpolar domains is counterbalanced by the increase in viscosity. The hydrogen bond interactions between C(2)-H and the oxygen atom of CO2 seems to have little influence on the CO2 solubility in these ILs. A comparison of the CO2 absorption capacity of compound 4 with that of compound 5 shows that the effect of the free volume in ionic regions overrides that of the interactions between CO2 and the anion because CF3CO2- is more basic than NTf2-. Recyclability of the IL bearing a fluorinated alkyl group on the imidazolium ring was also evaluated with compound 1. The equilibrium cell containing compound 1 was pressured to 11 bar of CO2 at 25 °C, and the pressure drop was monitored until equilibrium. The desorption of dissolved CO2 was carried out at 25 °C for 1 h by flowing N2 at a rate of 20 mL/min. As depicted in Figure 3, the initial CO2 absorption capacity of compound 1 was maintained up to five cycles. 1H nuclear magnetic resonance (NMR) studies were conducted with compounds 1 and 5 to ensure that there were no changes in
(23) Seki, T.; Grunwaldt, J.-D.; Baiker, A. J. Phys. Chem. B 2009, 113, 114–122. (24) Kazarian, S. G.; Briscoe, B. J.; Welton, T. Chem. Commun. 2000, 2047–2048. (25) Anthony, J. L.; Anderson, J. L.; Marginn, E. J.; Brennecke, J. F. J. Phys. Chem. B 2005, 109, 6366–6374. (26) MacFarlane, D. R.; Pringle, J. M.; Johansson, K. M.; Forsyth, S. A.; Forsyth, M. Chem. Commun. 2006, 1905–1917. (27) Smith, G. D.; Borodin, O.; Magda, J. J.; Boyd, R. H.; Wang, Y.; Bara, J. E.; Miller, S.; Gin, D. L.; Noble, R. D. Phys. Chem. Chem. Phys. 2010, 12, 7064–7076. (28) Russina, O.; Triolo, A.; Gontrani, L.; Caminiti, R.; Xiao, D.; Hines, L. G., Jr.; Bartsch, R. A.; Quitevis, E. L.; Plechkova, N.; Seddon, K. R. J. Phys.: Condens. Matter 2009, 21, 424121–424129.
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: DOI:10.1021/ef101143t
the ILs during the CO2 absorption or desorption (see the Supporting Information). It should be mentioned here that the CO2 dissolution capacities of compounds 1 and 2 are comparable to those of [C6H4F9mim][NTf2], [C8H4F13MIm][NTf2], and [HMIm][eFAP], which are regarded as the best CO2-philic ILs reported thus far (see Figure 4). In conclusion, compounds 1 and 2 could also be candidates to potential CO2 absorbents in consideration of their high CO2 absorption capacity and easy preparation.
Acknowledgment. This work was financially supported by the National Energy Resources Technology Development R&D Program for Greenhouse Gas Mitigation under the Korea Ministry of Knowledge Economy and the Program of the Korea Institute of Science and Technology. Supporting Information Available: Experimental section, schematic diagram of the apparatus used to measure CO2 solubility (Figure S1), SRK EOS, and 1H and 19F NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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