Unveiling the mechanisms behind high CO2 adsorption by the selection of suitable ionic liquids incorporated into a ZIF-8 metal organic framework: A computational approach

Abstract

There is growing focus on the crucial task of effectively capturing carbon dioxide (CO₂) from the atmosphere to mitigate environmental consequences. Metal-organic frameworks (MOFs) have been used to replace many conventional materials in gas separation, and the incorporation of ionic liquids (ILs) into porous MOFs has shown promise as a new technique for improving CO₂ capture and separation. However, the driving force underlying the electronic modulation of MOF nanostructures and the mechanisms behind their high CO₂ adsorption remain unclear. This study reports the effect of encapsulating different imidazolium ILs in porous ZIF-8, to clarify the adsorption mechanism of CO₂ using density functional theory (DFT)-based approaches. For this purpose, a range of anions, including bis(trifluoromethylsulfonyl)imide [NTf₂], methanesulfonate [MeSO₃], and acetate [AC], were combined with the 1-ethyl-3-methylimidazolium [EMIM]⁺ cation. [EMIM]⁺-based ILs@ZIF-8 composites were computationally investigated to identify suitable materials for CO₂ capture. First, the intermolecular and intramolecular interactions between [EMIM]⁺ and different anions were examined in detail, and their effects on CO₂ adsorption were explored. Subsequently, the integration of these ILs into the ZIF-8 solid structure was studied to reveal how their interactions influenced the CO₂ adsorption behavior. Our results demonstrate that the incorporation of ILs strongly affects the adsorption capability of CO₂, which is highly dependent on the nature of the ILs inside the ZIF-8 framework. DFT simulations further confirmed that the incorporation of ILs into ZIF-8 led to superior CO₂ capture compared to isolated ILs and pristine ZIF-8. This improvement was attributed to the mutual interactions between the ILs and ZIF-8, which effectively fine-tuned CO₂ adsorption within the composite structure. This understanding may act as a general guide for gaining more insight into the interfacial interactions between ILs and ZIFs structures and how these molecular-level interactions can help predict the selection of ILs for CO₂ adsorption and separation, thereby addressing environmental challenges with greater precision and effectiveness.

Keywords: CO(2) adsorption; Inter-/intra-molecular interaction; Interfacial mechanism; Ionic liquids; Molecular simulations; ZIF-8.