Correlation Effects in Diffusion of CH₄/CF₄ Mixtures in MFI Zeolite. A Study Linking MD Simulations with the Maxwell−Stefan Formulation

Abstract

Correlation effects in diffusion of CH4 and CF4 in MFI zeolite have been investigated with the help of molecular dynamics (MD) simulations and the Maxwell−Stefan (M−S) formulation. For single-component diffusion, the correlations are captured by the self-exchange coefficient ; in the published literature this coefficient has been assumed to be equal to the single-component M−S diffusivity, Đi. A detailed analysis of single-component diffusivity data from MD, along with published kinetic Monte Carlo (KMC) simulations, reveals that /Đi is a decreasing function of the molecular loading, depends on the guest−host combination, and is affected by intermolecular repulsion (attraction) forces. A comparison of published KMC simulations for diffusion of various molecules in MFI, with those of primitive square and cubic lattices, shows that the self-exchange coefficient increases with increasing connectivity. Correlations in CH4/CF4 binary mixtures are described by the binary exchange coefficient ; this exchange coefficient has been examined using Onsager transport coefficients computed from MD simulations. Analysis of the MD data leads to the development of a logarithmic interpolation formula to relate with the self-exchange coefficient of the constituents. The suggested procedure for estimation of is validated by comparison with MD simulations of the Onsager and Fick transport coefficients for a variety of loadings and compositions. Our studies show that a combination of the M−S formulation and the ideal adsorbed solution theory allows good predictions of binary mixture transport on the basis of only pure component diffusion and sorption data.