Fundamental of CO2 Adsorption and Diffusion in Subnanoporous Materials: Application to CALF-20

Abstract

We propose an approach for approximating the thermodynamics and kinetics of guest molecules in nanoporous materials. This statistical mechanics-based method requires a minimal set of parameters as input and has been applied to CO₂ molecules in the recently highlighted CALF-20 metal–organic framework for adsorption at different temperatures. The physical parameters of the adsorption model are extracted from one CO₂ isotherm at a given temperature and analyzed by the adsorption energy distribution method. The model is then used to approximate isotherms at different temperatures, Henry’s constant, saturation density, as well as enthalpies of adsorption at infinite dilution. The approach was further applied in MFI zeolite with CH₄ guests to ensure the transferability of the method. We then express molecular kinetics through the transition state theory, allowing one to approximate molecular diffusion, in part from thermodynamics, and further compare self-diffusion coefficients with molecular dynamics used as a numerical experiment. The approach proposed allows to express the molecular adsorption and self-diffusion in CALF-20 based on a formalism fed by physical parameters. The model proposed may be used to choose an appropriate isotherm model or, alternatively, can serve to help in setting an initial guess in a standard fitting procedure.