Density Functional Theory Studies on Zeolitic Imidazolate Framework-8 and Ionic Liquid-Based Composite Materials

The identification of suitable density functional methods for predicting the properties of nanoporous composite materials is highly significant in the field of chemical and material sciences. The stability of the composite materials depends on the nature of bonding and dispersive interaction at the interface. Thus, we have studied the effect of dispersion correction in the incorporation of hydrophobic and hydrophilic ionic liquids (ILs) into zeolitic imidazolate framework-8 (ZIF-8) nanostructures using the density functional theory (DFT)-based approaches. These structures were analyzed employing selected methods (Becke-Lee-Yang-Parr and Perdew-Burke-Ernzerhof) with dispersion correction (D2 or D3) and different basis sets (such as double-zeta valence polarized (DZVP), triple-zeta valence polarized (TZVP), and triple-zeta valence doubly polarized (TZV2P)) for the understanding of microscopic features of IL@ZIF-8 nanopores. It is found that the result obtained from DFT-D2/TZVP is more reliable for the prediction of the experimental crystal structure as well as stability and spectral information of the complexes. Furthermore, the microscopic analysis of geometries reveals that ILs are highly dispersed and stabilized at the nanopores of ZIF-8, particularly the ZIF-8 structure is highly preferable for the hydrophobic group in ILs. It is found that fluorine-containing anions are highly dispersed on the ZIF-8 surface compared to the nonfluorinated anion (i.e., [BMIM]⁺[Cl]^-). This is confirmed from the adsorption energies (E _ads), charge transfer, electron density analyses, and IR spectral analysis. These findings can provide more insights into the stability of composite materials, which are suitable for applications of catalytic conversion at the confined state, gas storage, and separation techniques.