Molecular understanding of carbon dioxide interactions with ionic liquids

Abstract

The effectiveness of CO2 absorption in ionic liquids (ILs) depends on the physical properties of cations/anions of the IL and their influence on ion-ion and ion-CO2 interactions. A molecular understanding of these cation-anion and ion-CO2 interactions is vital to identify promising ILs for CO2 capture. Koopmans' theorem based on quantum mechanics was used to compute several interaction descriptors, such as ionization potential (IP), electron affinity (EA), chemical potential, chemical hardness, softness, and electrophilicity index for cations/anions/ILs. Density Functional Theory (DFT) studies were performed to obtain interaction energies (IE) for ion-ion and ion-CO2 interactions. Several cations (imidazolium-based and pyridinium-based), inorganic (F¯, BF4¯, etc.) and organic anions ([TF2N]¯, [bFAP]¯, etc.) and cation-anion combinations were considered in the present study. The analysis of different interaction descriptors provided a better understanding of the relationships between the EA of cation, the IP of anion, and cation-anion IE values with the experimental CO2 solubility. Several inferences can be made based on the results obtained: a) the cation-anion interaction energy and IP of the anion have a strong correlation with the CO2 solubility, b) the IE trends of ion-CO2 and CO2-IL alone do not correlate well with the experimental CO2 solubility trends, (c) free molecular volume or interstitial spaces resulting from entropic effects of cation-anion interactions due to the nature/structure of cation and anion, play a dominant role for an effective CO2 absorption, (d) for a particular anion, changing the cation does not significantly influence cation-anion IE and ion-CO2 IE values, while for a particular cation, the nature of anion dramatically influences cation-anion IE and ion-CO2 IE values – consistent with the experimental CO2 solubility trends. Of the anions and cations studied, the combination of anions containing fluoroalkyl groups, such as [TF2N]¯ and [bFAP]¯, with the cations, such as [hmim]+ and [hmpy]+ constitute promising solvents for efficient CO2 capture. The findings of this study emphasize the importance of assessing cation-anion and ion-CO2 interactions in conjunction with other interaction descriptors to design efficient solvents for CO2 capture.