Molecular dynamics simulations of longer n-alkanes in silicalite: a comparison of framework and hydrocarbon models

The diffusion of n-alkanes ranging from length n-C8 to n-C20 in the zeolite silicalite is studied using classical molecular dynamics simulations. Different simulations were performed using a united-atom hydrocarbon model with a rigid zeolite framework, an all-atom hydrocarbon model with a rigid zeolite framework, and an all-atom hydrocarbon model with a flexible zeolite framework, all at 300 K. The latter two models have never previously been used to simulate longer alkanes in silicalite. Diffusion coefficients measured using a rigid zeolite framework exhibited a periodic dependence on chain length in the [010] direction in line with the previously observed phenomenon of resonant diffusion, regardless of the hydrocarbon model used. Explanations are considered in terms of the location of low energy traps within the silicalite structure, presenting a diffusion barrier. A monotonic dependence on diffusivity with chain length was observed however, on using an all-atom hydrocarbon model and a flexible framework, which was attributed to the occurrence of pore 'breathing' assisting diffusion. It was also noted that the calculated diffusion coefficients were up to an order of magnitude lower, and experimental diffusion coefficients are in much closer agreement when the latter model is used.