A finite lattice of adsorption sites, and Monte Carlo simulation, were used to develop a simple hopping model for small molecules within the alpha cage of zeolite NaA. A two-body attractive energetic interaction was used for occupied pairs of nearest-neighbor sites. A many-body repulsive interaction term accounted for the crowding associated with site saturation. This term became important when the site-site spacing was less than the van der Waals diameter of the adsorbate. The dynamic Monte Carlo method was used to evaluate site-to-site hopping frequencies as a function of loading, based upon this potential energy function. As the sorbate-sorbate attractive interaction was increased (or as the temperature was reduced), mobility minima occurred at certain lattice occupancies. These could be explained by the formation of energetically favorable clusters on the cubohedral lattice. Molecular crowding could thus cause sorbate mobility to suffer minima as the loading was increased. This prediction was in agreement with Xe NMR measurements.

Dynamic Monte Carlo Simulation of Sorbate Mobility in Zeolites: the Effects of Molecular Crowding on Sorbate Mobility. P.R.Van Tassel, I.Tantra, H.T.Davis, A.V.McCormick: Materials Research Society Symposium Proceedings, 1993, 290, 147-52