DFT and COSMO-RS studies on dicationic ionic liquids (DILs) as potential candidates for CO2 capture: the effects of alkyl side chain length and symmetry in cations

Abstract

A few studies on CO₂ capture using dicationic ionic liquids (DILs) show that they are more promising absorbents for CO₂ capture than monocationic ILs (MILs). Ion–ion, ion–CO₂ and DIL molecule–CO₂ interactions are important for understanding the performance–structure–property relationships for the rational design of DILs for CO₂ capture applications. However, the role of these interactions in determining CO₂ solubility in DILs is unclear. In this study, we used DFT methods to understand these interactions in three selected DILs)considering the effects of alkyl side chain length and symmetry in cations (by exploring different aspects, such as the electronic and geometrical structures, topological properties and the strength and nature of interactions, charge transfer, etc. The results showed that the most suitable solvent for CO₂ is the symmetric DIL with a longer side chain length, i.e. [Bis(mim)C₅-(C₄)₂][NTf₂]₂. In addition, we used the COSMO-RS calculations to obtain the macroscopic solubility of CO₂ in the studied DILs, which was in good agreement with the DFT results. Gas selectivity results calculated using COSMO-RS theory indicated that the selectivity of CO₂ from H₂, CO and CH₄ gases decreases slightly with increasing the length of side alkyl chains.