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.

Fig. 1. Structures of [Bis(mim)C₅]²⁺, [N₁₁₁-C₅-mim]²⁺, [Bis(mim)C₅-(C₄)₂]²⁺ cations and [NTf₂]⁻ anion along with their atomic labeling schemes.

Fig. 2. Calculated ESP maps (isovalue = 0.0004) of (a) isolated ions and (b) the studied DILs at M06-2X/ccpVDZ level of theory.

Fig. 3. Optimized structures of (a) [Bis(mim)C₅][NTf₂]₂, (b) [N₁₁₁-C₅-mim][NTf₂]₂ and (c) [Bis(mim)C₅-(C₄)₂][NTf₂]₂ at M06-2x/cc-pVDZ level of theory. Broken lines show hydrogen bonding interactions (distances are in Å).

Fig. 4. Calculated spatial distribution of HOMO and LUMO for DILs at M06-2X/ccpVDZ level.

Fig. 5. (a) Kinetic energy density, G(r), as a function of interaction energies and (b) E_HB as a function of G(r).

Fig. 6. Most stable structures of isolated ion–CO₂ complexes at M06-2X/ccpVDZ level. Broken lines show the most important intermolecular interactions (distances are in Å).

Fig. 7. Most stable structures of DIL–CO₂ complexes at M06-2X/ccpVDZ level. Broken lines show the most important intermolecular interactions (distances are in Å).

Fig. 8. Color-filled RDG isosurfaces and scatter graphs of RDG for (a) [Bis(mim)C₅]²⁺–CO₂, (b) [N₁₁₁-C₅-mim]²⁺–CO₂, (c) [Bis(mim)C₅-(C₄)₂]²⁺–CO₂ and (d) [NTf₂]⁻–CO₂. Green and red colors denote vdW interactions and steric contributions, respectively.

Fig. 9. (a) CO₂ solubility in the studied DILs at 1 bar and at different temperatures. (b) CO₂ solubility in DILs at 303.15 K and at different pressures calculated by COSMO-RS theory.

Fig. 10. (a) Henry's constant versus temperature for the studied DILs. (b) Selectivity of CO₂ from CO, H₂ and CH₄ in the studied DILs at 303.15 K calculated by COSMO-RS theory.