Gas separation by ionic liquids: A theoretical study

Ionic liquids (ILs) have great potential for separating gases as well as avoiding solvent loss and environmental pollution. An in-depth understanding of the interaction mechanism between ILs and gases is extremely important for exploring and developing high-performing ILs for gas separation. Quantum chemistry is a powerful approach for gaining insight into separation mechanisms. Herein, with the aid of this method, the interaction mechanisms of three representative ILs, i.e., diethylmethylsulfonium tricyanomethane ([S221][CCN3]), triethylsulfonium acetate ([S222][Ace]), and 1-2(-hydroxyethyl)-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([OHemim][NTf2]), with CO2/CH4, C2H2/C2H4, and NH3/CH4 mixtures are studied. The results show that hydrogen bonds (H-bonds), which are mainly electrostatic dominant (and sometimes even covalent dominant when the H-bond is sufficiently strong), play a crucial role in gas separation. Furthermore, there is a linear relationship between the distance of the H-bonds and both electron density (ρBCP) and Laplacian values (∇2ρBCP). Symmetry adapted perturbation theory revealed that electrostatic energies are the main contributors to the total attractive energies between ILs and gases ([OHemim][NTf2]-NH3, [S221][CCN3]-CO2, and [S222][Ace]-C2H2). Therefore, based on the above results, we can design the ILs with specific functional groups, which are easy to form H-bonds with the gases to be separated.