Design of Task-Specific Ionic Liquids for Capturing CO2: A Molecular

Ind. Eng. Chem. Res. 2006, 45, 2875-2880

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Design of Task-Specific Ionic Liquids for Capturing CO2: A Molecular Orbital Study Guangren Yu,†,‡ Suojiang Zhang,*,† Xiaoqian Yao,† Jianmin Zhang,† Kun Dong,† Wenbin Dai,§ and Ryohei Mori§ Research Laboratory of Green Chemistry and Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100080, P. R. China, Graduate School of the Chinese Academy of Sciences, Beijing, 100039, P. R. China, and SCF Solution Group, Business Incubation Department, Mitsubishi Materials Corporation, 1002-14, Mukohyama, Naka-shi, Ibaraki-ken, 311-0102, Japan

To design task-specific ionic liquids (TSILs) for capturing CO2, the frontier molecular orbital interactions between CO2 and -NH2 in TSILs, such as 1,1,3,3-tetramethylguanidinium lactate and 1-n-propylamine-3butylimidazolium tetrafluoroborate, were systematically studied by employing B3LYP/6-31G** calculations. A detailed frontier molecular orbital study showed that the inherent intramolecular hydrogen bonds in TSILs and the electron-donating groups attaching to -NH2 could raise the frontier occupied molecular orbital energy on -NH2 and thus enhance the interactions between -NH2 and CO2. Following the underlying interaction mechanism between CO2 and -NH2 in TSILs, two new TSILs, [P(C4)4][Ala] and [P(C4)4][Gly], were designed and the interactions between CO2 and -NH2 in such ionic liquids were investigated. This work showed a good prospect to design TSILs for selectively capturing CO2 from gas streams by applying the molecular orbital calculations. Introduction The increasing accumulation of CO2 in the atmosphere has resulted in global warming effects and serious environmental problems; therefore, the development of efficient methods for capturing CO2 from gas streams is critically important. One of the most widely applied technologies for capturing CO2 industrially is chemical absorption by using organic solvents with -NH2 such as a variety of alkanolamines, which are usually used by mixing with a substantial amount of water.1 Although these aqueous alkanolamine solutions are industrially effective for capturing CO2, there are several serious drawbacks inherently connected to them; for example, the concurrent loss of the volatile amines and the uptake of water into the gas stream cause intensive energy consumption, cost increases, and corrosion problems. In this regard, it is absolutely necessary to seek a new kind of sequestering agent with characteristics such as a negligible vapor pressure and high stability, without additional water. To this end, ionic liquids (ILs) are such sequestering agents. Ionic liquids (ILs) are organic salts that are liquids at ambient temperature or, more generally, below 100 °C. Due to their unique properties, such as negligible vapor pressure, high thermal stability, nonflammability, excellent solvent power, and the ability to be easily modified structurally to the desired physical properties,2-4 ILs have been recognized as a versatile alternative to conventional organic solvents in reactions, separations, material engineering, and other fields. Among a variety of these applications, ILs are particularly applicable in the absorption of CO2 while effectively avoiding the loss of the sequestering agents. The solubility and phase behavior of CO2 in common ILs, such as imidazolium-based,5-13 phosphonium-based,14 and py* To whom correspondence should be addressed. Tel./Fax: +8610-8262-7080. E-mail address: [email protected]. † Institute of Process Engineering, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences. § Mitsubishi Materials Corporation.

ridium-based ILs,11 have been extensively studied experimentally; the results showed that the solubility of CO2 in a variety of ILs is trivial at room temperature and atmospheric pressure, typically around 0.02 mol CO2/mol IL.15-17 More detailed studies on the solubility of CO2 in imidazolium-based ILs through in situ attenuated total reflection IR (ATR-IR) spectroscopy18 and molecular simulation5,19 found that the anions dominated the interactions with CO2 and the cations played a secondary role, and the interactions between the anions and CO2 were weak Lewis acid-base or electrostatic interactions, which are physical in nature. Comparing the numerous studies on the solubility and phase equilibria of CO2 in common ILs, there is little research on ILs devoted to chemically capturing CO2 from a gas stream.4 Bates et al. reported for the first time a task-specific IL with -NH2, 1-n-propylamine-3-butylimidazolium tetrafluoroborate ([pabim][BF4]), for capturing CO2.4 They found that the -NH2 in [pabim][BF4] can effectively capture CO2 by chemically forming a carbamate structure and the absorption capability of [pabim][BF4] is comparable to that of conventional organic amines at room temperature and atmospheric pressure. Han et al. reported another task-specific IL with -NH2, 1,1,3,3-tetramethylguanidinium lactate ([tmg][L]), and found that the -NH2 in [tmg][L] can effectively capture SO2.20 It might be reasonably expected that the -NH2 in [tmg][L] could be as capable of capturing CO2 as that in [pabim][BF4]. Unexpectedly, the experiments showed that this is not true; the equilibrium concentration is only about 0.01 mol CO2/mol [tmg][L] at room temperature and atmospheric pressure,21 which is on the same order as that of the conventional ILs but much lower than that of [pabim][BF4]. Further experiments revealed that the absorption of CO2 into [tmg][L] is actually dominated by physical interactions rather than chemical ones.21 To clarify why the -NH2 in [pabim][BF4] can effectively capture CO2 but that in [tmg][L] cannot and to find some theoretical basis for the rational design of task-specific ILs (TSILs) for capturing CO2, the frontier molecular orbital (FMO) interactions between CO2 and the -NH2 in [tmg][L] and [pabim]-

10.1021/ie050975y CCC: $33.50 © 2006 American Chemical Society Published on Web 02/24/2006

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Figure 1. Hydrogen bonds in [tmg][L] (a) and [pabim][BF4] (b). Bond lengths are in angstroms, and bond angles are in degrees.

[BF4] were systematically studied, and two new TSILs were finally designed. This work provides valuable knowledge for rationally designing TSILs for capturing CO2. Computational Methods The frontier molecular orbital (FMO) theory22-25 has been widely and successfully applied in the interpretation of donoracceptor interactions26-28 since it was proposed by Fukui in

1952. In the framework of FMO theory, the strength of the donor-acceptor interactions is determined by the FMO overlap, i.e., the overlap between the frontier occupied molecular orbital (FOMO) of the donor molecule (generally, it is the highest occupied molecular orbital, HOMO, on the donor atom) and the frontier unoccupied molecular orbital (FUMO) of the acceptor molecule (generally, it is the lowest unoccupied molecular orbital, LUMO, on the acceptor atom). The more overlap, the stronger the interactions between the donor and acceptor atoms. Two preconditions have to be met for an effective FMO overlap: one is that the orbital phases of the HOMO on the donor atom and the LUMO on the acceptor atom have to match, and the other is having a low energy gap between the HOMO and LUMO (generally around 6 eV or less; the less the FMO energy gap, the more effective the FMO overlap). The first precondition decides the possibility of the overlap, and the second one decides the extent of the overlap. The capture of CO2 by -NH2 in ILs will form a -NH2‚CO2 complex intermediate,29-31 which can be recognized as a donoracceptor interaction (wherein the donor atom is the N atom on -NH2 in the ILs and the acceptor atom is the C atom in CO2);31,32 therefore, the interaction strength between CO2 and -NH2 in ILs depends on the overlap between the HOMO on the -NH2 in the ILs and the LUMO of CO2. All calculations have been performed with the GAUSSIAN 03 programs.33 Orbital drawings were carried out using MOLEKEL 4.3 with a contour value of 0.075.34 On the basis of the famous Hohenberg and Kohn theorems, the density functional theory (DFT) provides a sound basis for the development of computational strategies for obtaining information about the energetics, structure, and electron properties of atoms or molecules at much lower costs than traditional ab initio wave function techniques (for example, Hartree-Fock, MøllerPlesset, configuration interaction, coupled cluster theory, and many others) without accuracy loss.35 Previous investigations have proven that the DFT method is suitable for calculation of the ILs.36-38 Considering the wide existence of hydrogen bonds in ILs,36,37,39,40 the well-established Becke’s three-parameter hybrid

Table 1. Molecular Orbital Coefficients and Energies of the HOMOs on - NH2 for [tmg][L], [tmg]+, [pabim][BF4], and [pabim]+ and the LUMO for CO2a atom orbital orbital coefficient atom orbital orbital coefficient

2N(2px) -0.17 4N(2pz) 0.22

2N(2py) 0.29 4N(3s) 0.12

[tmg][L] (EHOMO-5 ) -8.58 eV) 2N(2pz) 2N(3s) 2N(3px) -0.22 0.12 -0.14 4N(3px) 4N(3pz) 5N(2px) 0.16 0.18 0.2

atom orbital orbital coefficient atom orbital orbital coefficient

1C(2pz) -0.11 5N(2pz) -0.24

2N(2px) -0.15 5N(3s) 0.1

[tmg]+ (EHOMO-1 ) -11.87 eV) 2N(2pz) 2N(3px) 2N(3pz) 0.41 -0.14 0.33 5N(3pz) -0.18

atom orbital orbital coefficient

1C(2px) 0.6

1C(3px) 0.6


18C(3s) -0.15 31N(3s) 0.12

24C(2py) -0.12 31N(3px) 0.42


18C(2px) 0.11 31N(2pz) 0.21

18C(3s) 0.16 31N(3s) -0.3

a

CO2 (ELUMO ) 0.95 eV) 2O(2px) 2O(3px) -0.38 -0.4 [pabim][BF4] (EHOMO ) -5.12 eV) 26H(1s) 26H(2s) 31N(2s) 0.15 0.2 -0.14 31N(3py) 0.2 [pabim]+ (EHOMO ) -8.31 eV) 24C(2px) 26H(1s) 26H(2s) -0.12 -0.14 -0.19 31N(3px) 31N(3pz) 0.39 0.17

The atom orbitals with an absolute value of their coefficients less than 0.1 were not presented.

2N(3py) 0.23 5N(3s) -0.15 4N(2pz) -0.24

2O(2px) -0.38

2N(3pz) -0.18 5N(3px) 0.14

4N(2px) 0.22 8H(2s) 0.11

4N(3s) -0.1

4N(3pz) -0.18

2O(3px) -0.4

31N(2px) -0.26

31N(2py) -0.21

31N(2pz) 0.17

31N(2s) -0.12

31N(2px) 0.48

31N(2py) 0.12

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Figure 2. 3-D contour plots of HOMOs on -NH2 for [tmg][L] (a) and [tmg]+ (b). The same atoms in [tmg][L] and [tmg]+ are labeled with the same numbers.

Figure 3. 3-D contour plot of the LUMO for CO2.

function41 coupled with the correlation function of Lee, Yang, and Parr42 (B3LYP), which is known to be reliable, particularly for calculations of closed-shell stable molecules and hydrogenbonded systems, was chosen in this work. The reasonably large basis sets, 6-31G** and 6-311G(2df,p),43 were used in all the calculations. The results of the FMO calculations in terms of these two basis sets were nearly the same; therefore, only the results in terms of the 6-31G** basis set are presented in the following discussions. All the structures of the ILs were obtained by the Berny analytical gradient optimization routines,44,45 and the convergence criteria were same as the defaults in GAUSSIAN 03. The structure of CO2 was directly taken from the experimental data.46 Results and Discussions Considering that the main interest of this work lies in the FMO study, the detailed geometry optimization procedure for the ILs was not presented and only the most stable structures were shown in the FMO analysis. FMO Interactions between -NH2 in [tmg][L] and CO2. The structure of [tmg][L] is illustrated in Figure 1a. The

Figure 4. 3-D contour plots of the HOMOs on -NH2 for [pabim][BF4] (a) and [pabim]+ (b). The same atoms in [pabim][BF4] and [pabim]+ are labeled with the same numbers.

[tmg]+ cation and lactate anion is connected by a hydrogen bond, 2N-3H‚‚‚27O (2N-3H, 1.079 Å, 3H‚‚‚27O, 1.577 Å, ∠(2N-3H‚‚‚27O), 163.9°), which is similar to the wide existence of hydrogen bonds in many IL systems.36,37,39,40 Considering that the -NH2 lies in the [tmg]+ fragment, the FMO calculation was also performed for the [tmg]+ alone. It is found that the FOMO associated with the N atom on -NH2 is HOMO-1 when the [tmg]+ stays alone and is the HOMO-5 when the [tmg]+ is accompanied by the counterion, lactate anion. The molecular orbital coefficients and energies of the HOMO-5 and HOMO-1 are presented in Table 1; the 3-D contour plots are shown in Figure 2a and b, respectively. By comparing Figure 2a and b, it can be found that both the HOMO-5 and HOMO-1 are antibonding π-type orbitals and the antiphase combinations of the p-orbital of the 2N atom on -NH2 with the p-orbitals of the 4N and 5N atoms on the guanidinium plane, which can also be understood from the orbital composition data presented in Table 1.

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Table 2. Molecular Orbital Coefficients and Energies of the HOMOs on - NH2 for [P(C4)4][Ala] and [P(C4)4][Gly]a atom orbital orbital coefficient atom orbital orbital coefficient

56C(2px) -0.21 64O(2py) 0.13

56C(3s) -0.18 65O(2px) -0.29

[P(C4)4][Ala] (EHOMO ) -4.52 eV) 60N(2px) 60N(2py) 60N(3s) 0.34 -0.11 -0.12 65O(2py) 65O(2pz) 65O(3px) 0.17 -0.25 -0.22


54O(2px) -0.27 56C(2px) -0.19

54O(2py) 0.14 56C(3s) 0.19

[P(C4)4][Gly] (EHOMO ) -4.56 eV) 54O(2pz) 54O(3px) 54O(3py) 0.30 -0.21 0.11 57O(2py) 57O(2pz) 60N(2px) 0.11 0.11 0.33

a

60N(3px) 0.31 65O(3py) 0.13 54O(3pz) 0.22 60N(2py) -0.13

61C(2px) 0.15 65O(3pz) -0.19 55C(2px) 0.14 60N(2pz) -0.11

61C(3s) -0.13

55C(3s) 0.14 60N(3px) 0.31

The atom orbitals with an absolute value of their coefficients less than 0.1 were not presented.

However, the energies of the HOMO-5 and HOMO-1 are quite different. In [tmg]+, there is a carbocation unit on the guanidinium plane (1C-2N, 1.348 Å; 1C-4N, 1.346 Å; 1C-5N, 1.346 Å; ∠(2N-1C-4N), 119.1°; ∠(2N-1C-5N), 119.1°; ∠(4N-1C-5N), 121.8°; ∠(2N-1C-5N-4N), 180.0°), which attaches to the -NH2 (Figure 2b). The strongly electronwithdrawing carbocation makes [tmg]+ bear a much lower HOMO-1 energy (-11.87 eV, Table 1) and, thus, a potential low donorability, which is consistent with the experimental32 or theoretical31 results. Compared with the HOMO-1, the HOMO-5 energy is much higher and dramatically raised to -8.58 eV, which is attributed to the intramolecular hydrogen bond, 2N-3H‚‚‚27O, in [tmg][L] (Figure 1a). The hydrogen bond attracts the 3H atom with a somewhat proton characteristic away from the 2N atom, which can be accounted for by the change of the bond length of 2N-3H from 1.01 Å in [tmg]+ to 1.079 Å in [tmg][L] (Figure 1a). The partial rupture of the 3H atom from the 2N atom increases the electron density around the 2N atom and thus is favorable for raising the HOMO-5 energy, which can be understood by the change of the charge on the 2N atom from -0.619 (Mulliken charge) in [tmg]+ to -0.649 in [tmg][L]. The above discussion shows that the intramolecular hydrogen bond associated with the H atom on -NH2 in ILs could raise the HOMO energy on the -NH2 and thus potentially results in a stronger donorability. The molecular orbital coefficients and energy of the LUMO for CO2 are presented in Table 1, and the 3-D contour plot is shown in Figure 3. From Figure 3 and Table 1, it can be found that the LUMO of CO2 is an antibonding π-type orbital and the antiphase combination of the p-orbital of the 1C atom with the p-orbitals of the 2O and 3O atoms, with the most prominence on the central 1C atom. This orbital characteristic is in agreement with the report by Iron et al.47 Its LUMO energy is high up to 0.95 eV, which is consistent with the hard acid nature of CO2.31 As far as the orbital-phase match is taken into account, the π-type HOMO-5 of [tmg][L] was able to effectively overlap with the π-type LUMO of CO2 in the head-on σ-interaction according to Figures 2a and 3. However, the energy gap between them is too large (9.53 eV, which is far beyond the upper limit for an effective overlap) to overlap effectively. As a result, although the intramolecular hydrogen bond N2-H33‚‚‚O27 in [tmg][L] improves the HOMO energy on -NH2, the strongly electron-withdrawing carbocation in [tmg]+ substantially dominates the HOMO on -NH2 and finally results in a weak interaction between the -NH2 in [tmg][L] and CO2. This explains why [tmg][L] could not effectively capture CO2.21 FMO Interaction between -NH2 in [pabim][BF4] and CO2. The structure of [pabim][BF4] is illustrated in Figure 1b, there are two intramolecular hydrogen bonds: 31N-

33H‚‚‚34F associated with the 33H atom on -NH2 (31N-33H, 1.022 Å; 33H‚‚‚34F, 2.011 Å; ∠(31N-33H‚‚‚34F), 155.0°), and 2C-6H‚‚‚34F associated with the 6H atom on the imidazolium ring (2C-6H, 1.082 Å; 6H‚‚‚34F, 1.941 Å; ∠(2C-6H‚‚‚34F), 153.3°). Similar to the above study on [tmg]+, [pabim]+ was also calculated alone. The FOMOs associated with the N atom on -NH2 in both [pabim][BF4] and [pabim]+ are HOMOs. The molecular orbital coefficients and energies of the HOMOs are presented in Table 1, and the 3-D contour plots are shown in Figure 4a and b, respectively. Both the HOMOs are π-type orbitals and mainly composed of the p-orbital of the 31N atom with a little antiphase contribution from the s-orbital of the 26H atom (Figure 4 and Table 1). When [pabim]+ lies alone, there are two competing factors that influence the HOMO energy: the three electron-donating methylenes attaching to the -NH2 and the electron-withdrawing imidazolium ring with a positive charge. The first one raises the HOMO energy, and the latter one does not; these two opposite factors result in a compromise HOMO energy of [pabim]+ (-8.31 eV, Table 1). However, when the [pabim]+ is accompanied by [BF4]-, the HOMO energy of [pabim][BF4] dramatically rises to -5.12 eV. This is attributed to the following two hydrogen bonds: 31N-33H‚‚‚34F and 2C-6H‚‚‚34F. The effect of the first hydrogen bond on the HOMO is similar to that of 2N-3H‚‚‚27O on the HOMO-5 of [tmg][L]. The second hydrogen bond partially counteracts the positive charge of the imidazolium ring and thus weakens its electron-withdrawing effect on the HOMO, so it is favorable for raising the HOMO energy. From Figures 3 and 4a, it can be found that the π-type HOMO of [pabim][BF4] is capable of effectively overlapping with the π-type LUMO of CO2 in a head-on σ-interaction. Caused by the two intramolecular hydrogen bonds, the HOMO energy of [pabim][BF4] rises to a higher value (-5.12 eV), and the energy gap between the HOMO of [pabim][BF4] and the LUMO of CO2 becomes smaller (6.07 eV); therefore, the -NH2 in [pabim][BF4] can interact better with CO2 comparing to the case of [tmg][L]. This explains why [pabim][BF4] can effectively capture CO2.4 Class of New TSILs with -NH2 for Capturing CO2. On the basis of the above understandings and systematic calculations, two new TSILs, tetrabutylphosphonium alanine ([P(C4)4][Ala]) and tetrabutylphosphonium glycine ([P(C4)4][Gly]) were finally designed. The molecular orbital coefficients and energies of the HOMOs for [P(C4)4][Ala] and [P(C4)4][Gly] are presented in Table 2, and the 3-D contour plots are shown in Figure 5. From Figure 5 and Table 2, it can be found that both the HOMOs are π-type orbitals and are mainly composed of a p-orbital of the N atom on -NH2 and p-orbitals of the O atoms on carboxylate groups. Similar to the HOMO of [pabim][BF4], the HOMOs of [P(C4)4][Ala] and [P(C4)4][Gly] are able to

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Figure 5. 3-D contour plots of HOMOs on -NH2 for [P(C4)4][Ala] (a) and [P(C4)4][Gly] (b). Some H atoms are not labeled for clarity.

effectively overlap with the LUMO of CO2 in a head-on σ-interaction. Because of the effect of the strongly electrondonating carboxylate groups, coupled with the methylene groups, the HOMO energies of [P(C4)4][Ala] and [P(C4)4][Gly] rise to -4.52 eV and -4.56 eV, respectively, which are near the -5.12 eV of [pabim][BF4]. The higher HOMO energies make the FMO energy gaps between their HOMOs and the LUMO of CO2 become smaller, 5.47 and 5.51 eV for [P(C4)4][Ala] and [P(C4)4][Gly], respectively. As a result, the -NH2 in [P(C4)4][Ala] and [P(C4)4][Gly] should be capable of effectively capturing CO2. Such predictions were proved by the experiments;48 the equilibrium concentrations reach a value of about 1 mol CO2/2 mol [P(C4)4][Ala] or [P(C4)4][Gly] at room temperature and atmospheric pressure, which is comparable to [pabim][BF4] on a molar basis.4 Conclusion In this work, the FMO interactions between CO2 and -NH2 in [tmg][L] and [pabim][BF4] were systematically studied. The detailed FMO studies showed that the electron-donating groups attaching to -NH2 coupled with the intramolecular hydrogen bonds associated with the H atom on -NH2 in ILs could raise the FOMO energy on -NH2 and thus enhance the interactions between -NH2 and CO2. On the basis of such theoretical understandings and systematic calculations, two new TSILs, [P(C4)4][Ala] and [P(C4)4][Gly], were designed, and such designed ILs were finally proved to be effective for capturing CO2 by the experiments. This work provides valuable knowledge for designing TSILs for capturing CO2 by applying molecular orbital calculations.

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (20436050) and the National HighTech Project of China (863 Project; 2004AA649030). Literature Cited (1) White, C. M.; Strazisar, B. R.; Granite, E. J.; Hoffman, J. S. 2003 Critical Review: Separation and Capture of CO2 from Large Stationary Sources and Sequestration in Geological FormationssCoalbeds and Deep Saline Aquifers. J. Air Waste Manage. Assoc. 2003, 53, 645. (2) Rogers, R. D., Seddon, K. R., Eds. Ionic Liquids: Industrial Applications to Green Chemistry; American Chemical Society (distributed by Oxford University Press): Washington, D.C., 2002. (3) Rogers, R. D., Seddon, K. R., Eds. Ionic Liquids as Green SolVents: Progress and Prospects; American Chemical Society (distributed by Oxford University Press): Washington, D.C., 2003. (4) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H., Jr. CO2 Capture by a Task-Specific Ionic Liquid. J. Am. Chem. Soc. 2002, 124, 926. (5) Cadena, C.; Anthony, J. L.; Shah, J. K.; Morrow, T. I.; Brennecke, J. F.; Maginn, E. J. Why is CO2 so Soluble in Imidazolium-Based Ionic Liquids? J. Am. Chem. Soc. 2004, 126, 5300. (6) Blanchard, L. A.; Brennecke, J. F. Recovery of Organic Products from Ionic Liquids Using Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 2001, 40, 287. (7) Kamps, A. P. S.; Tuma, D.; Xia, J. Z.; Maurer, G. Solubility of CO2 in the Ionic Liquid [bmim][PF6]. J. Chem. Eng. Data 2003, 48, 746. (8) Zhang, Z. F.; Wu, W. Z.; Gao, H. X.; Han, B. X.; Wang, B.; Huang, Y. Tri-phase Behavior of Ionic Liquid-water-CO2 System at Elevated Pressures. Phys. Chem. Chem. Phys. 2004, 6, 5051. (9) Aki, S. N. V. K.; Mellein, B. R.; Saurer, E. M.; Brennecke, J. F. High-Pressure Phase Behavior of Carbon Dioxide with Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2004, 108, 20355. (10) Shiflett, M. B.; Yokozeki, A. Solubilities and Diffusivities of Carbon Dioxide in Ionic Liquids: [bmim][PF6] and [bmim][BF4]. Ind. Eng. Chem. Res. 2005, 44, 4453.

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ReceiVed for reView August 27, 2005 ReVised manuscript receiVed December 17, 2005 Accepted January 31, 2006 IE050975Y

Design of Task-Specific Ionic Liquids for Capturing CO2: A Molecular

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