Selection of a suitable ZIF-8/ionic liquid (IL) based composite for selective CO2 capture: the role of anions at the interface

Abstract

The effective capture of CO₂ from the atmosphere is much needed to reduce its environmental impact. The design and development of CO₂ capturing materials is getting much attention. A zeolitic imidazolate framework (ZIF) can replace many of the conventional materials in gas separation due to its stability and high performance. Here, we analyzed the effect of encapsulation of ionic liquids (ILs) into the pores of ZIF-8 for selective CO₂ capture and separation. The [BMIM]⁺ cation with a series of anions was selected to study suitable carbon capture materials using density functional theory (DFT) approaches. Our calculations suggest that the nitrogen containing anions are not well adsorbed on the ZIF-8 surface but their gas separation performance is not affected by these interfacial interactions. This is confirmed from the CO₂/N₂ and CO₂/CH₄ selectivity of these composites, calculated using grand canonical Monte Carlo (GCMC) simulations. A suitable force field for the composites was identified by comparing the available force fields with the experiments. The IL@ZIF-8 composite shows better CO₂ selectivity compared to pristine ZIF-8. Fluorinated hydrophobic anions (such as [BF₄]⁻, [PF₆]⁻ and [Tf₂N]⁻) in the composites show better CO₂ adsorption and significant CO₂ selectivity than pristine ZIF-8, especially at low pressure. The nature of the anion plays an important role in CO₂ separation, rather than its stability at the pores of ZIF-8. Close scrutiny of the results reveal that the CO₂ selectivity of these composite materials depends on the anion of the IL and thus through the selection of a suitable anion we can significantly enhance the CO₂ selectivity for different flue gas mixtures. Our molecular level design shows that the selection of suitable anions in IL based composites is very important in identifying potential carbon capture materials for industrial applications.