Propane/Propylene Diffusion in Zeolites: Framework Dynamics

Pure silica zeolites with narrow eight-membered (8-MR) oxygen rings are good candidates for separations of olefin/paraffin mixtures based on molecular sieving effects. We have used a molecular dynamics technique to study the structural features of two pure silica 8-MR frameworks: Si-CHA and Si-SAS. CHA and SAS are framework types with 3- and 1-dimensional porous systems, respectively, with cages connected through 8-MR windows. Window size temperature fluctuations were studied from equilibrium molecular dynamics simulations by allowing full flexibility of the framework. The thermal influence on diffusional features of propane/propylene molecules adsorbed in Si-CHA and Si-SAS zeolites is studied using classical molecular dynamics simulations. Potential energy surface effects are taken into account by comparing the results from the well-known van Beest, Kramer, and van Santen potential [Phys. Rev. Lett. 1990, 64, 1995] and the Pedone, Malavasi, Menziani, Cormack, and Segre potential [J. Phys. Chem. B 2006, 24, 11780].