A Dynamic Monte Carlo Simulation of Sorbate Mobility in Zeolites: The Effects of Molecular Crowding on Sorbate Mobility
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A DYNAMIC MONTE CARLO SIMULATION OF SORBATE MOBILITY IN ZEOLIfTES: THE EFFECTS OF MOLECULAR CROWDING ON SORBATE MOBRILITY PAUL R. VAN TASSEL, IWAN TANTRA, H. TED DAVIS, AND ALON V. MCCORMICK University of Minnesota, Department of Chemical Engineering and Materials Science, 421 Washington Avenue SE, Minneapolis, MN 55455
ABSTRACT A finite lattice of adsorption sites, as shown by Monte Carlo simulation, is used to develop a simple hopping model of small molecules within the alpha cage of zeolite NaA. A two body attractive energetic interaction is employed for occupied pairs of nearest neighbor sites. A many body repulsive interaction term accounts for the crowding associated with site saturation. This term becomes important when the site-site spacing is less than the van der Waals diameter of the adsorbate. The dynamic Monte Carlo method is used to evaluate site to site hopping frequencies as a function of loading based on this potential energy function. As the sorbate-sorbate attractive interaction is increased (or, equivalently, as the temperature is reduced), mobility minima occur at certain lattice occupancies which may be explained by the formation of energetically favorable clusters on the cubocathedral lattice. In other words, molecular crowding can cause sorbate mobility to suffer minima as loading is increased. This prediction is in agreement with recent Xe NMR measurements.
INTRODUCTION Although processes involving adsorption and diffusion in zeolite micropores is of great interest to the chemical and materials industries, simple models describing such phenomena are not yet available. In the absence of an acceptable theory, recent efforts have focused on using molecular simulation to explain and expand upon experimental findings. Equilibrium Monte Carlo simulations have yielded accurate adsorption isotherms and thermodynamic properties [I6] and molecular dynamic simulations have provided diffusion coefficients and other kinetic data [7-17]. Although, simulation is capable of providing valuable molecular level information, it is often at the expense of considerable computational effort. With this in mind, we have directed recent efforts toward using the molecular data available from simulation studies to develop simple zeolite adsorption and diffusion models. Recently, we presented results of Monte Carlo simulations of Xe atoms adsorbed in the interior of a NaA alpha cage (Fig. 1) [ 18]. The sodium ion content of the alpha cage was varied to investigate the influence of framework cations on the adsorbed phase. We found that when sodium ions occupy both the 8- and 6-membered rings (a cation-rich cage), adsorption occurs at sites in front of the 4-membered rings. This simple arrangement of sites is cubocahedral, i.e. is in the form of a twelve vertex polyhedron with each site connected to four nearest neighbors (Fig. 2). Further, it was observed that the position of these sites remains constant with loading, thus making this system amenable to a simple modelinig technique involving a lattice of adsorption si
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