CO2 capture using dicationic ionic liquids (DILs): Molecular dynamics and DFT-IR studies on the role of cations

Abstract

Dicationic ionic liquids (DILs) have been shown to be useful as an effective solvent for the absorption of CO2. However, compared to monocationic ionic liquids (MILs), they have been less investigated for this application. Previous studies on MIL–CO2 systems have shown that anions play the main role in tuning CO2 capture, but the partial negative charge on the oxygens of CO2 may interact with cation centers and, especially, for DILs with two charge centers, the role of cations can be significant. Therefore, the current work focuses on how cation symmetry and the length of side chains affect interactions and also the dynamical and structural properties of DIL–CO2 systems using molecular dynamics simulation. In addition, the effect of CO2 on the infrared vibrational spectra of isolated ions and ion triplet (DIL molecules) was studied using density functional theory calculations and the observed red and blue shifts have been interpreted. The results indicated that symmetric cation with longer side chains tend to interact more strongly with CO2 molecules. It seems that increasing the length of the side chains causes more bending of the middle chain, and in addition to increasing the free fraction volume, it weakens the interaction between cations and anions, and as a result more interaction between gas and cation. The results of this work may contribute to the rational molecular design of DILs for CO2 capture, DIL-based gas sensors, etc.