Diffusional behaviour of simple sorbates in zeolites: effect of anisotropic frameworks and geometrical correlations

Molecular dynamics (MD) studies of helium and argon diffusion in all-silica analogues of ZSM-5, ZSM-11 and ferrierite are used to study diffusional anisotropy. All three zeolites have channels running parallel to only two of the three crystallographic axes. ZSM-5 and ZSM-11, however, allow for three-dimensional diffusional motion due to the presence of geometrical correlations. Motion in the correlated direction, which has no direct displacement channels, is distinctly subdiffusional on time scales of the order of 1 ns. The ballistic to diffusional crossover times are much greater for displacements in the direction showing correlated, as opposed to, direct displacements.