The Role of Binary Mixtures of Ionic Liquids in ZIF-8 for Selective Gas Storage and Separation: A Perspective from Computational Approaches

Carbon dioxide is one of the major causes of global climatic changes. Efficient separation of CO2 from different gaseous mixtures can be helpful in flue gas separation, gas sweetening, natural gas processing, and so forth. Adsorption in porous materials is emerging as one of the best techniques in this regard. Also CO2 is easily soluble in ionic liquids (ILs). Hence, we have designed IL (designer solvent)-based composite materials by impregnating ILs into the pores of zeolitic imidazolate framework (ZIF)-8. In this study, we have computationally explored the properties of such composites. Mixture of different types of ILs can enhance composite material properties and leads to efficient gas separation. We studied both simple and binary mixtures of ILs in the nanopores of ZIF-8 using density functional theory (DFT) and grand canonical Monte Carlo (GCMG) methods. ZIF-8 retained its core structure even after IL incorporation, and also the ILs were well dispersed in the confinement. Gas adsorption studies were performed for gases such as CO2, N-2, and CH4 and their binary mixtures. Both single- component and binary mixture adsorptions were similar at a pressure less than 1 bar. The selectivity calculated from the gas mixture adsorption elicited that our materials show good selective adsorption of CO2 compared with the other two gases. The binary mixture of ILs enhanced the CO2 selectivity by 3.5-fold higher than simple ILs@ZIF-8. Especially, the composite with hydrophobic-hydrophobic binary IL mixture has higher CO2 selectivity than other materials, attributed to the strong interactions between CO2 and IL anions. The like-like hydrophobic ion pair and the fluorine content in the IL enhanced the selective adsorption of CO2. The binary IL mixtures at the confinement can be a novel alternative to the prevailing materials in gas separation applications.