275006 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 Hgas···X (X= O or F in anions), BSSE-corrected interaction energy, electron density of Hgas···X at the bond critical point (ρBCP) 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.