Mechano-chemical stability and water effect on gas selectivity in mixed-metal zeolitic imidazolate frameworks: a systematic investigation from van der Waals corrected density functional theory

Abstract

A series of Zn/Cu Zeolitic Imidazolate Frameworks (ZIFs) ZIF-202, -203, and -204 are systematically investigated by Density Functional Theory (DFT) with and without van der Waals (vdW) corrections. The elastic constants for non-solvent structures indicate that ZIF-202 and -204 are mechanically stable while ZIF-203 is unstable, which arises from the stiffness along the x-axis under a uniaxial strain in the PBE-D3 method. By considering the presence of solvents in ZIF-203, a structural phase transformation from a monoclinic to a triclinic structure is found which could be explained by the Jahn–Teller distortion. From the chemical bonding point of view, it is found that vdW interactions and hybridization between d-orbitals (copper) and p-orbitals (imidazolate) are the main-driving forces in stabilizing ZIF-202 and -204, respectively. The electronic structure calculations predict the presence of two optical transitions in the visible region in agreement with the experimental observation for ZIF-204 both without and with water. The DFT simulations reveal that CO₂ molecules prefer to locate near imidazolate and water in dry and hydrated ZIF-204, respectively. The analysis of Canonical Monte Carlo (GCMC) simulations reveals that Coulomb interaction between CO₂ and H₂O molecules is mainly responsible for the enhanced CO₂ uptake and selectivity under humid conditions compared to dry ones.