Understanding CO2 capture kinetics and energetics by ionic liquids with molecular dynamics simulation

Room temperature ionic liquids (ILs) are recognized to be potential media for CO2 capture, but the interaction nature is less documented which hinders the future development of ILs with high CO2 solvation capability. Here, through all atom molecular dynamics (MD) simulations, the solvation process of CO2 with four representative ILs, [EMIM][BF4], [BMIM][BF4], [EMIM]CL and [BMIM]CL was systematically studied. Our data clearly reflect the fact that hydrophobic components from both cations and anions dominate CO2 solvation because IL-CO2 attraction is mainly driven by the van der Waals attractions. Consequently, cations with longer alkyl chain (for instance, [BMIM](+) than [EMIM](+)) and anions with higher hydrophobicity (for instance, [BF4](-) than CL-) effectively enhance CO2 solvation. For all the ILs under study, addition of water into ILs abates CO2 solvation through regulating the anion-CO2 interactions. Moreover, it is worth mentioning that ILs with a hydrophobic anion ([BF4](-) in this study) are more resistant to the existence of water to capture CO2 than ILs with a hydrophilic anion (Cl- in this study). Free energy decomposition analyses were conducted which support the above findings consistently. Generally, it is predicted that cations with long alkyl chain, anions with high hydrophobicity, and ILs with smaller surface tension are potentially effective CO2 capturing media. Our present study helps the deep understanding of the CO2 capturing mechanism by ILs and is expected to facilitate the future design and fabrication of a novel IL medium for gas capture and storage.