Correlation effects in the diffusion of CH4 and CF4 in MFI zeolite were investigated with the help of molecular dynamics simulations and the Maxwell-Stefan formulation. For single-component diffusion, the correlations were reflected by the self-exchange coefficient, Diiorr. This coefficient was usually assumed to be equal to the single-component Maxwell-Stefan diffusivity, Di. A detailed analysis of single-component diffusivity data from molecular dynamics studies, together with published kinetic Monte Carlo simulations, revealed that Diiorr/Di was a decreasing function of the molecular loading and depended upon the guest-host combination. It was affected by intermolecular repulsion (attraction) forces. A comparison of published kinetic Monte Carlo simulations for diffusion of various molecules in MFI, with those for primitive square and cubic lattices, showed that the self-exchange coefficient increased with increasing connectivity. Correlations in CH4/CF4 binary mixtures were described by the binary exchange coefficient, D12orr. This exchange coefficient was examined using Onsager transport coefficients deduced from molecular dynamics simulations. Analysis of the molecular dynamics data led to the development of a logarithmic interpolation formula which related D12orr to the self-exchange coefficient, Diiorr, of the constituents. A suggested procedure for the estimation of D12orr was confirmed by comparison with molecular dynamics simulations of the Onsager and Fick transport coefficients for a variety of loadings and compositions. The studies showed that a combination of the Maxwell-Stefan formulation, and ideal adsorbed solution theory, permitted reliable predictions of binary mixture transport on the basis of pure-component diffusion and sorption data.

Correlation Effects in Diffusion of CH4/CF4 Mixtures in MFI Zeolite. a Study Linking MD Simulations with the Maxwell-Stefan Formulation. A.I.Skoulidas, D.S.Sholl, R.Krishna: Langmuir, 2003, 19[19], 7977-88