(397f) Differential Solubility of Ethylene and Acetylene in Room Temperature Ionic Liquids by Computational Analysis

The room temperature ionic liquids (RTILs) have potential in realizing the ethylene (C₂H₄) and acetylene (C₂H₂) separation and avoiding solvent loss and environmental pollution compared with traditional solvents. The interaction mechanisms between gases and RTILs are significative for the exploration of new RTILs for gas separation, thus it was studied by quantum chemical calculation and molecular dynamics simulation in this work.

In the quantum chemical calculation, the optimized geometries were obtained for the complexes of C₂H₄/C₂H₂ with anions (Tf₂N^-, BF₄^- and OAc^-), cation (bmim⁺) and their ion pairs, and the analysis for geometry, interaction energy, natural bond orbital (NBO) and atoms in molecules (AIM) was performed. The quantum chemical calculation results show that the hydrogen bonding interaction between gas molecule and anion is the dominant factor in determining the solubility of C₂H₂ in RTILs. However, the hydrogen bonding interaction, p-π interaction in C₂H₄-anion and the π-π interaction in C₂H₄-cation are weak and comparable, which all affect the solubility of C₂H₄ in RTILs with comparable contribution. The calculated results for the distance of H_gas···X(X= O or F in anions), BSSE-corrected interaction energy, electron density of H_gas···X at the bond critical point (ρ_BCP) and the relative second order perturbation stabilization energy (E(2)) are consistent with the experimental data that C₂H₂ is more soluble than C₂H₄ in the same RTILs and the solubility of C₂H₄ in RTILs has the following order: [bmim][Tf₂N] > [bmim][OAc] > [bmim][BF₄]. The calculated results also agree with the order of C₂H₂ solubility in different RTILs that [bmim][OAc] > [bmim][BF₄] > [bmim][Tf₂N]. Furthermore, the calculation results indicate that there is strong C₂H₂-RTIL interaction, which cannot be negligible compared to RTIL-RTIL interaction, thus the regular solution theory is probably not suitable to correlate C₂H₂solubility in RTILs.

In the molecular dynamics simulation, the diffusion coefficients of the ions have been found to increase after C₂H₄/C₂H₂ joins in. The interaction energy is consistent with the quantum chemical calculation results that C₂H₂ have a stronger electrostatic interaction with anion, and C₂H₄ have comparable electrostatic interaction of C₂H₄-anion and van der Waals interaction of C₂H₄-cation. The radial distribution functions show that C₂H₄ and C₂H₂ both have high distribution on the alkyl chain of imidazolium cation and the difference of C₂H₄ and C₂H₂ in ILs are attributed to the specific interaction with anions.