Using the Maxwell-Stefan theory for diffusion, a simple formula was used to correlate the tracer (self) diffusivity, D*, and Maxwell-Stefan or jump diffusivity, D. The presence of the interchange coefficient, Dij, in the Maxwell-Stefan formulation caused the self-diffusivity to be lower than the jump diffusivity. Assuming that the interchange coefficient was given by D/F, D* = D1 + Fθ was derived, where F was a factor which took account of topological effects within the zeolite matrix. The validity of the Maxwell-Stefan formulation was established by performing kinetic Monte Carlo simulations of the diffusion of methane, perfluoromethane, 2-methylhexane and iso-butane in silicalite. It was shown that the exchange coefficient, Dij, was a quantification of correlation effects during the hopping of molecules. For iso-butane, the isotherm inflection led to a sharp inflection in the diffusion behaviour. The influence of molecular repulsive forces on the loading dependence of the jump and self-diffusivities was reviewed with the aid of published molecular dynamics simulations for methane.

Verification of the Maxwell-Stefan Theory for Tracer Diffusion in Zeolites. R.Krishna, D.Paschek: Chemical Engineering Journal, 2002, 85[1], 7-15