(71e) Molecular Simulation of CO2 Absorption in the Ionic Liquid [P2228][2-Cnpyr] Using Reaction Ensemble Monte Carlo

CO2 capture using monoethanolamine (MEA) suffers from amine degradation, solvent emissions, and large heat requirements to regenerate the solvent. Ionic liquids (ILs) are an attractive alternative to MEA due to their thermal stability, minimal vapor pressure, and tunable properties. Gurkan et al. [1] designed a pyrrolide-based anion that binds CO2 more weakly than MEA, but still absorbs CO2 in near equimolar amounts at standard conditions. Further IL optimization is complicated by the exceptionally large number of possible cation-anion pairs. Molecular simulation could be a valuable tool to screen proposed ILs for CO2 absorption, if it were possible to simulate both the condensed phase IL environment and the C-N bond formation as CO2 binds to the pyrrolide anion. Quantum mechanical methods can accurately model bond formation and breaking, but are limited to small system sizes representative of the gas phase environment. Classical simulations, on the other hand, can be used to model the bulk liquid at a molecular level, but cannot describe covalent bond formation. Weâ??ve developed a methodology for predicting CO2 absorption in ILs using reaction ensemble Monte Carlo (RxMC) that utilizes both quantum and classical simulations. Liquid phase configurations are sampled using traditional translation, rotation and regrowth Monte Carlo moves. The equilibrium composition is sampled using reaction moves that delete reactant molecules and insert product molecules. The acceptance probability of reaction moves depends on the liquid-phase change in potential energy due to molecular insertions and deletions and the gas-phase change in free energy due to the reaction. The latter is determined by computing the potential of mean force using ab initio molecular dynamics. We investigate the absorption of CO2 in the ionic liquid triethyloctylphosphonium 2-cyanopyrrolide, [P2228][2-CNpyr], and present CO2 absorption isotherms at a range of temperatures.

[1] Gurkan, B., Goodrich, B.F., Mindrup, E.M., Ficke, L.E., Massel, M., Seo, S., Senftle, T.P., Wu, H., Glaser, M.F., Shah, J.K., Maginn, E.J., Brennecke J.F. and Schneider W.F., "Molecular design of high capacity, low viscosity, chemically tunable ionic liquids for CO2 capture." The Journal of Physical Chemistry Letters 1.24 (2010): 3494-3499.