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 tricyan degrees methane ([S-221][CCN3]), triethylsulfonium acetate ([S-222][Ace]), and 1-2(-hydroxyethyl )-3-methylimidazolium bisKtrifluoromethyl)sulfonyllimide ([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 (rho(BCP)) and Laplacian values (del(2)rho(BCP)). Symmetry adapted perturbation theory revealed that electrostatic energies are the main contributors to the total attractive energies between ILs and gases ([OHemim][NTf2]-NH3, [S-221][CCN3]-CO2, and [S-222][Ace]-C2H2). Therefore, based on the above results, we can design the ILs with specific functional groups, which are easy to form Hbonds with the gases to be separated. (C) 2018 Elsevier Ltd. All rights reserved.