It was recalled that molecular diffusion in zeolites was often likened to a random walk between specific adsorption sites within the channels and cavities of the materials. Several types of correlation between the steps of the walk supported this assumption: kinetic correlations due to incomplete relaxation of the molecule in its final site, vacancy correlations which arose at high loading because the molecules blocked each other and geometrical correlations because the zeolite channels and cages featured non-symmetrical sites. The first and last correlation effects could be observed at infinite dilution. A method for calculating the exact diffusion coefficient at infinite dilution, as a function of the microscopic rate constants, was presented here. Account was taken of both geometrical and kinetic correlation effects. This was achieved by separating the molecular motion into uncorrelated sequences of jumps; where all jumps of one sequence were correlated with each other. The method was used to study the geometrical correlations of benzene in NaY, and the kinetic and geometrical correlations of ethane in silicalite.

Correlation Effects in Molecular Diffusion in Zeolites at Infinite Dilution. F.Jousse, S.M.Auerbach, D.P.Vercauteren: Journal of Chemical Physics, 2000, 112[3], 1531-40