J. Phys. Chem. B 2009, 113, 7591–7598
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Absorption of CO2 in the Ionic Liquid 1-n-Hexyl-3-methylimidazolium Tris(pentafluoroethyl)trifluorophosphate ([hmim][FEP]): A Molecular View by Computer Simulations Xiaochun Zhang,† Feng Huo,† Zhiping Liu,*,† Wenchuan Wang,† Wei Shi,‡ and Edward J. Maginn*,‡ DiVision of Molecular and Materials Simulation, Key Lab for Nanomaterials, Ministry of Education, Beijing UniVersity of Chemical Technology, Beijing 100029, China, and Department of Chemical and Biomolecular Engineering, UniVersity of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, Indiana 46556-5637 ReceiVed: January 14, 2009; ReVised Manuscript ReceiVed: March 14, 2009
Using a computational screening methodology, we predicted (AIChE J. 2008, 54, 2717) that the anion tris(pentafluoroethyl)trifluorophosphate ([FEP]) should increase the solubility of CO2 in ionic liquids (ILs) relative to a wide range of conventional anions. This prediction was confirmed experimentally. In this work, we develop a united-atom force field for the [FEP] anion and use the continuous fractional component Monte Carlo (CFC MC) method to predict CO2 absorption isotherms in 1-n-hexyl-3-methylimidazolium ([hmim]) [FEP] at 298.2 and 323.2 K and pressures up to 20.0 bar. The simulated isotherms overestimate the solubility of CO2 by about 20% but capture the experimental trends quite well. Additional Monte Carlo (MC) and molecular dynamics (MD) simulations are performed to study the mechanisms of CO2 absorption in [hmim][FEP] and [hmim][PF6]. The site-site radial distribution functions (RDFs) show that CO2 is highly organized around the [PF6] anion due to its symmetry and smaller size, while less ordered distributions were found around [FEP] and [hmim]. However, more CO2 can be found in the first coordination shell of [FEP] compared with [PF6]. The structures of ILs, illustrated by P-P radial distribution functions, change very little upon the addition of as much as 50 mol % CO2. An energetic analysis shows that the van der Waals interactions between CO2 and ILs are generally larger than electrostatic interactions. 1. Introduction Ionic liquids (ILs) are salts which remain liquid near ambient temperatures. Due to their negligible vapor pressure, these liquids have been suggested as alternatives to conventional volatile organic compounds (VOCs) in many industrial processes.1 More importantly, the properties of ILs can be changed to meet the needs of specific tasks by selecting from among thousands of combinations of cations and anions. Since one cannot synthesize all of these ILs and measure their properties experimentally, prediction methods based on ab initio calculations, atomistic simulations, and thermodynamic models are of critical importance in the molecular design of ILs. Recently, we proposed a computational screening methodology to hunt the candidate ILs for use in the capture of carbon dioxide (CO2).2 Specifically, we sought the cation-anion pairs that yielded the highest physical solubility of CO2 possible. We predicted that the solubilities of CO2 in ILs with the tris(pentafluoroethyl)trifluorophosphate ([FEP]) anion should be among the highest, and this was confirmed experimentally. For example, with the same cation 1-n-hexyl-3-methylimidazolium ([hmim]), [hmim][FEP] can absorb more than 70% more CO2 compared with [hmim][PF6] at 8.0 bar and 298 K. The screening method is efficient and useful, but it provides no detailed mechanistic explanation for the observed solubility differences. Here, we report a series of atomistic simulation results that seek to provide * To whom correspondence should be addressed. E-mail: liuzhp@ mail.buct.edu.cn (Z.L.);
[email protected] (E.M.). † Beijing University of Chemical Technology. ‡ University of Notre Dame.
a clearer picture as to why the [FEP] anion enhances the solubility of CO2. Atomistic simulations of CO2-IL mixtures have been reported by several groups,3-11 but most have focused on the hexafluorophosphate ([PF6]) anion paired with the 1-n-butyl3-methylimidazolium ([bmim]) cation.3-7,12 A few simulation studies have been reported on other ILs, including 1,1,3,3tetramethylguanidium lactate ([TMG][L])9 and 1-n-hexyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]).10 We have shown3 that the substitution of the “acidic” hydrogen in the imidazolium ring (see H5 in Figure 1a) by a methyl group has limited influence on the solubility of CO2. Strong organization of CO2 around the [PF6] anion was observed. In contrast, there is a significantly weaker interaction between CO2 and [bmim]. The finding is consistent with in situ attenuated total reflectance (ATR)-IR studies13 and recent ab initio molecular dynamics (AIMD) results.12 Huang et al. carried out a Voronoi analysis and found5 that the small cavities in pure ILs would assemble into bigger ones to accommodate CO2 molecules by angular rearrangements, which cause little change in radial distribution functions (RDFs). Balasubramanian and co-workers studied14 the complex of CO2 and various anions by ab initio calculations and observed, interestingly, an inverse correlation between the binding energy of the complex and the experimental solubility. The objectives of the present work are as follows: (1) Develop a united-atom (UA) force field for the [FEP] anion, consistent with our previous UA force fields15 for imidazolium cations.
10.1021/jp900403q CCC: $40.75 2009 American Chemical Society Published on Web 05/01/2009
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Zhang et al. have been used extensively for other systems,32 and we have shown6,15,33,34 that they can reduce the computational cost to one-fourth or less, without loss of accuracy, compared with AA force fields. The UA force field of the [FEP] anion was developed using methods similar to those previously used.15,22 The force field for the [hmim] cation was taken from previous work15 The functional form for the total energy U is given by the standard AMBER35 model:
U)
∑
bonds
∑
dihedrals
Kr(r - r0)2 +
∑
Kθ(θ - θ0)2 +
angles
Kx[1 + cos(nχ - δ)] +
∑ 4εij[(σij/rij)12 i
