Describing mixture diffusion in microporous materials under conditions of pore saturation

Abstract

In a variety of practical applications involving microporous materials such as zeolites and metal organic frameworks (MOFs), the operating conditions are such that the concentration of guest molecules within the pore space approaches saturation conditions. This situation arises, for example, in CO₂ capture processes operating at high pressures. Pore saturation is also attained in cases where micorporous materials are in contact with a bulk liquid phase. As the concentration of guest molecules within the pores approaches saturation values, the molecular jumps become increasingly correlated. In the Maxwell-Stefan (M-S) description of mixture diffusion, the increase in the correlations causes the exchange coefficient Ð_ij to reduce in values that are significantly lower than that of the M-S diffusivities of the constituent species, Ð_i. We derive simplified expressions for calculation of the fluxes under this correlations dominant scenario: Ð_i/Ð_ij» 1. These expressions are validated by means of molecular dynamics (MD) simulations for a range of mixtures (CH₄-CO₂, CO₂-H₂,CH₄ -C₂H₆, CH₄-C₃H₈, C₂H₆-C₃H₈, CH₄-N₂, CH₄-Ar, Ne-Ar,CH₄-C₂H₆-C₃ H₈) in a variety of zeolites (MFI, BEA, ISV, FAU, NaX, NaY, LTA) and MOFs (IRMOF-1, CuBTC, MIL-53, MIL-47, Co-FA, Co(bdc)dabco, Zn(bdc)dabco).