COMPUTER-SIMULATION OF THE STRUCTURE, ENERGETICS, AND DIFFUSION PROPERTIES OF P-XYLENE IN ZEOLITE NA-Y

The properties of p-xylene adsorbed in zeolite Na-Y are studied by mol ecular simulations. Constrained reaction coordinate minimizations are used to calculate the minimum-energy path for the diffusion. The predi cted adsorption site inside the supercage is found to be in good agree ment with neutron diffraction results. Extensive molecular dynamics si mulations are performed in order to study the behavior of p-xylene as a function of loading at temperatures above 500 K. As important inform ation for the interpretation of experimental data; the distribution of the molecules over the supercages is obtained. Due to the very long s imulation runs the diffusion coefficients of p-xylene in the model sys tem are determined with high accuracy. The diffusion coefficients obta ined for low sorbate concentration are dose to the largest experimenta l values, which scatter over more than 2 orders of magnitude. It is fo und that the diffusivity decreases drastically with increased sorbate loading. Furthermore, the microscopic details of the diffusion process are analyzed by the investigation of site and cage residence times an d by orientation correlation functions. The simulations reveal that si te-to-site migration inside a supercage is often associated with inver sion of the orientation of the aromatic plane.