Absorption of CO2 in the Ionic Liquid 1-n-Hexyl-3-methylimidazolium Tris(pentafluoroethyl)trifluorophosphate ([hmim][FEP]): A Molecular View by Computer Simulations

Abstract

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.