Simulations using rigid- and flexible-framework models of silicalite, and a rigid-framework model of the aluminophosphate VPI-5, for various sorbate diameters were reported. The sorbate-host interactions were modelled in terms of simple atom-atom Lennard-Jones interactions. The results suggested that the diffusion coefficient exhibited an anomaly as γ approached unity. The molecular dynamics results confirmed the existence of a linear regime for sorbate diameters significantly smaller than the channel diameter, and an anomalous regime was observed for sorbate diameters that were comparable to the channel diameter. The power spectra obtained by Fourier transformation of the velocity autocorrelation function indicated that there was an increase in the intensity of the low-frequency component for the velocity component parallel to the direction of motion for sorbate diameters in the anomalous regime. The results suggested that the diffusion anomaly occurred irrespective of the geometry and topology of the pore structure and of the nature of the host material.

Diffusion Anomaly in Silicalite and VPI-5 from Molecular Dynamics Simulations. Bandyopadhyay, S., Yasbonath, S.: Journal of Physical Chemistry, 1995, 99[12], 4286-92