LATTICE MODEL AND SIMULATION OF DYNAMICS OF ADSORBATE MOTION IN ZEOLITES

Previous Monte Carlo simulations of the distribution of adsorbates in zeolites have suggested that adsorbates are confined to a lattice of s ites in the micropore. In this work, we examine the effect of the stru cture and energetics of the adsorption site lattice on the mobility of small molecules in cage-like micropores using Monte Carlo lattice dyn amics (MCLD) simulations. In MCLD, motion is modeled as a series of ac tivated site-to-site hops over energetic and entropic barriers whose m agnitudes can be functions of zeolite structure, sorbate chemistry, an d loading. The topology of the lattice is such that adsorbate hops are of two types: (i) between sites in a given cage, and (ii) between sit es in neighboring cages. For comparison, an analytical model of adsorb ate self-diffusivity is constructed by applying a random walk theory t o this lattice. This model is exact at low loading and approximates th e dynamics well even for a crowded lattice. We also compare MCLD to mo lecular dynamics (MD) simulations of methane adsorbed in zeolite A and observe qualitative agreement between the two approaches. However, th e computational cost of MCLD is an order of magnitude lower than MD.