Molecular dynamics simulations of alkanes in the zeolite silicalite: Evidence for resonant diffusion effects

Molecular dynamics simulations have been conducted to study n-alkane dynamics in the zeolite silicalite. Chains ranging in length from n-C-4 to n-C-20 have been examined at various loadings and temperatures. An interesting chain-length dependence for the individual components of the self-diffusivity tensor was observed. While the self-diffusivity of chains in the [100] and [001] directions exhibits a monotonic decrease as a function of chain length, the self-diffusivity along the [010] axis is a periodic function of chain length. Local maxima in the self-diffusivity along this axis occur for n-C-8 and n-C-16, while local minima are observed for n-C-6 and n-C-14. The apparent activation energy for diffusion is also periodic with chain length. Periodicity in the diffusivity and activation energies are most pronounced at low temperature. A physical explanation for this behavior is given in terms of a resonant diffusion mechanism. The essential features of previous theoretical treatments describing how resonant diffusion effects might occur in zeolites are confirmed by these simulations. Sorbate conformational structure and time constants for motion between channel systems are also computed. These calculations indicate that, at low temperature, long chains become localized in one channel system. Interchange between channels for these chains is very slow. Under these conditions, silicalite essentially acts as a one-dimensional zeolite. The fact that the time constant associated with molecular rearrangement may be greater than the time constant for diffusion along a given channel suggests a possible explanation for some of the differences often seen between zeolite self-diffusivities measured using transient and equilibrium techniques.