Molecular dynamics simulations, using a Burkhart-Dreiding force field, was used to determine the self-diffusion coefficients of methane, ethane, propane, n-butane, isobutane, benzene and pyridine in silicalite at 300K. In the case of the alkanes, Einstein diffusion was observed early in a 500ps simulation. Values of the diffusion coefficient for the alkanes ranged from about 9 x 10−4cm2/s for methane to 2 x 10−6cm2/s for isobutane at a loading of 0.5 molecule per unit cell or 2 molecules per simulation box. For the alkanes, there was good agreement between the results of simulations and experimental values of the self-diffusion coefficient obtained using techniques such as pulsed-field gradient nuclear magnetic resonance. In the case of benzene and pyridine, anomalous diffusion was observed for time-scales up to 9ns and apparent diffusion coefficients (10−11 to 10−8cm2/s), calculated from the Einstein relation, increased with increasing loading (up to 3.75 molecules per unit cell) and, as shown for pyridine, with increasing temperature. In the case of benzene, where experimental data were available, the apparent diffusion coefficients calculated at high loadings were within an order of magnitude of experimental values.

Atomistic Simulation of Hydrocarbon Diffusion in Silicalite. Fried, J.R., Weaver, S.: Computational Materials Science, 1998, 11[4], 277-93