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

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)C-5-(C-4)(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 H-2, CO and CH4 gases decreases slightly with increasing the length of side alkyl chains.