Understanding the Adsorption and Diffusion of Carbon Dioxide in Zeolitic Imidazolate Frameworks: A Molecular Simulation Study

In this work, atomic partial charges in the framework atoms of two typical zeolitic imidazolate frameworks (ZIFs), ZIF-68 and ZIF-69, were calculated using density functional theory, and a suitable force field for describing CO, adsorption in ZIFs was identified. On the basis of this, a combined grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation study was per-formed to investigate the adsorption and diffusion behaviors of CO2 in ZIFs. The results show that the small pores formed by the nIM linkers in ZIF-68 and ZIF-69 are the preferential adsorption sites for CO2 molecules, with the corners formed by the phenyl rings in the large pores being less preferential adsorption sites. This work demonstrates that the chlorine atoms in cbIM linkers in ZIF-69 lead to enhanced binding energy but reduced diffusivity for CO2, the electrostatic interactions produced by the frameworks are important that cannot be ignored, and, down to 180 K, no steps are found in isotherms. In addition, this work demonstrates that the diffusion of CO2 in ZIFs is likely to be much slower than that in other MOFs.