Molecular dynamics simulations were used to study n-alkane dynamics in silicalite. Chains ranging in length from n-C4 to n-C20 were examined for various loadings and temperatures. A chain-length dependence of individual components of the self-diffusivity tensor was observed. The self-diffusivity of chains in the [100] and [001] directions exhibited a monotonic decrease as a function of chain length, but the self-diffusivity along the [010] axis was a periodic function of chain length. Local maxima in the self-diffusivity along this axis occurred for n-C8 and n-C16, while local minima were observed for n-C6 and n-C14. The apparent activation energy for diffusion was also periodic as a function of chain length. The periodicity of the diffusivity and activation energies was most pronounced at low temperatures. An explanation for this behavior was given in terms of resonant diffusion. Sorbate conformational structure and time constants for motion between channel systems were also computed. These calculations indicated that, at low temperatures, long chains became localized in one channel system. Interchange between channels for these chains was very slow. Under these conditions, silicate acted essentially as a one-dimensional zeolite. The fact that the time constant associated with molecular rearrangement could be greater than the time constant for diffusion along a given channel suggested a possible explanation for the differences seen between the zeolite self-diffusivities measured using transient and equilibrium techniques.

Molecular Dynamics Simulations of Alkanes in the Zeolite Silicalite: Evidence for Resonant Diffusion Effects. Runnebaum, R.C., Maginn, E.J.: Journal of Physical Chemistry B, 1997, 101[33], 6394-408