Investigating the relative influences of molecular dimensions and binding energies on diffusivities of guest species inside nanoporous crystalline materials

The primary objective of this article is to investigate the relative influences of molecular dimensions and adsorption binding energies on unary diffusivities of guest species inside nanoporous crystalline materials such as zeolites and metal-organic frameworks (MOFs). The investigations are based on molecular dynamics (MD) simulations of unary diffusivities, along with configurational-bias Monte Carlo (CBMC) simulations of the isosteric heats of adsorption (−Qst) of a wide variety of guest molecules (CO2, H2, N2, He, Ne, Ar, Kr, CH4, C2H4, C2H6, C3H6, C3H8, and nC4H10) in 24 different host materials spanning a wide range of pore sizes, topologies, and connectivities. For cage-type materials with narrow windows, in the 3.2-4.2 Å size range, separating adjacent cages (e.g., LTA, CHA, DDR, and ZIF-8),the diffusivities are primarily dictated by the molecular dimensions, bond lengths, and bond angles. However, for channel structures (e.g., AFI, MFI, MgMOF-74, NiMOF-74, MIL-47, MIL-53, and BTP-COF) and "open" frameworks with large windows separating adjacent cavities (NaY, NaX, CuBTC, IRMOF-1, MOF-177, and MIL-101), the diffusivities of guest species in any given host material are strongly dependent on the binding energies of the guest species that can be quantified by −Qst. The stronger the binding energy, the higher the "sticking tendency", and the lower the corresponding diffusivity. The insights gained from our study are used to rationalize published experimental data on diffusivities and trans-membrane permeances. The results of our study will be valuable in choosing the right material with the desired diffusion characteristics for a given separation application