Tuning the CO2 adsorption by the selection of suitable ionic liquids at ZIF-8 confinement: A DFT study

Development of suitable materials for CO₂ storage and conversion has gained tremendous attraction in the past decade. Metal organic frameworks (MOFs) has emerged as suitable porous materials in this aspect. Ionic liquids (ILs) can be confined at the pores of MOFs and these composites are well known for gas storage and separation applications. Here, 1-butyl 3-methylimidazolium ([BMIM]⁺) based ILs@ZIF-8 composites were studied using density functional theory (DFT) based approach for the identification of suitable composite for CO₂ capture. The adsorption of CO₂ has been studied at isolated ILs at gas phase and the same at confined state (i.e. ZIF-8). It is interesting to note that the adsorption behavior of CO₂ at these two environments differ from each other. In gas phase, hydrophilic ILs shows more CO₂ adsorption whereas in the confined state hydrophobic ILs shows dominant adsorption. This is due to the similar hydrophobicities of ZIF-8 and ILs, which play an important role in tuning CO₂ adsorption at the composite. Fluorine containing ILs ([BMIM]⁺ with [BF₄]⁻ and [PF₆]⁻ anions) are showing better adsorption of CO₂ at higher concentration. Our calculations reveal that the adsorption of CO₂ depends on the nature of ILs present in the confined environment. Also we observed maximum adsorption capacity for the empty ZIF-8 and ILs@ZIF-8 composites. The present findings can be helpful to predict the selection of ILs for the CO₂ selectivity at suitable solid-liquid interface.