Simultaneously Probing Gaseous Diffusion and Adsorption Properties in Porous Materials Using Molecular Dynamics Simulations

An extended two-phase thermodynamics (2PT) model, denoted as ext-2PT, is developed to simultaneously extract both the adsorption and diffusion properties of adsorbates in porous materials based on molecular dynamics (MD) simulations. The model describes the chemical potential as a linear combination of the gas-like and solid-like contribution with a newly introduced g fraction, aside from the fluidicity used in the original 2PT method. The ext-2PT method can be applied to compute the chemical potential of an adsorbed system as a function of adsorption uptake. An interpolation scheme can then be employed to determine the equilibrium loading at any given gas-phase chemical potential values, thus yielding adsorption isotherms. The diffusion coefficients of adsorbates can, meanwhile, be directly computed from the density of state (DOS) at zero frequency by the same set of MD simulations. The applicability of the ext-2PT model is demonstrated by its accurate predictions on the adsorption and diffusion of methane in two metal-organic frameworks (MOFs), IRMOF-1 and Cu-BTC, as well as in zeolite FAU with inaccessible volume. Overall, this ext-2PT model represents an effective and efficient approach capable of simultaneously computing adsorption isotherms and diffusion coefficients by a single set of MD simulations.