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

We propose a theoretical approach for predicting thermodynamics and kinetics of guest molecules in nanoporous materials. This statistical mechanical-based method requires a minimal set of physical parameters that may originate from experiments or numerical simulations. We applied it to CO2 molecules in the recently highlighted CALF-20 metal-organic framework for adsorption and molecular self-diffusion at different temperatures. All the physical parameters of the model are extracted from one CO$_2$ isotherm 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. We then express molecular kinetics through the transition state theory allowing to predict molecular diffusion in part from the prior knowledge of thermodynamics, and further compared self-diffusion coefficients to one from molecular dynamics used as a numerical experiment. The approach proposed allows to express molecular adsorption and diffusion based on a fitting procedure allowing to get physical parameters with a view on thermodynamics and kinetics mechanisms at play in the system.