Molecular Dynamics Simulations of the Diffusion of Small Chain Hydrocarbons in 8-Ring Zeolites

Molecular dynamics (MD) simulations were performed to study the microscopic motion of methane, ethane, propene, and propane adsorbed in three pure silica zeolites with windows made of 8 SiO4 tetrahedral units: Si-LTA, Si-IHW, and Si-ITE. The zeolite framework and guest structures have been simulated allowing full flexibility, using the well-known BKS model and the potential of Oie et al., respectively. The MD approach followed allows us to calculate the intra-and intercage dynamics of the smaller adsorbates, that is, methane and ethane, in statistically meaningful time and length scales in the temperature range studied, whereas for the larger size guest molecules the analysis of intercage motion is limited to higher temperatures. Calculated self-diffusion coefficients for methane, ethane, and propene show a decreasing trend correlated with increasing guest kinetic diameter sizes and decreasing critical window size, confirming experimental measurements on the molecular sieving properties of 8-ring zeolite frameworks. The microscopic motion of propane diffusing in Si-ITE suggests a somewhat anomalous diffusion process, which can be related to the levitation effect. Thus, guest diffusion translational motion is shown to be highly influenced by the topological features of the framework, with the dimensionality of the diffusion path exerting the most noticeable influence. The microscopic understanding of the host-guest dynamics can be used to highlight the trade-off between propane/propylene selectivity and diffusional differentiation in these materials.