MOLECULAR-DYNAMICS STUDY OF METHANE AND XENON IN SILICALITE

The dynamic and equilibrium properties of two different sorbate molecules, xenon and methane, in the pentasil zeolite silicalite were studied with molecular dynamics simulations. A rigid polyatomic model was employed for methane, while xenon was represented as a Lennard-Jones sphere. The zeolite lattice was assumed rigid. The effect of intracrystalline occupancy and temperature on the structure and dynamics of the sorbed phase was investigated. Predicted self-diffusivities are in good agreement with NMR measurements. Except at the lowest temperature studied, 200 K, the sorbate self-diffusivities were found to decrease monotonically with occupancy. At 200 K, the self-diffusivities exhibit a weak maximum before decreasing at high loadings. This maximum is attributed to the trapping of sorbate molecules in small potential wells formed within the pore network of the zeolite at low occupancy. The single particle density distribution function, ρ1(r), shows that at low loadings sorbate molecules are localized preferentially in the sinusoidal channels and avoid the energetically less favorable channel intersections and straight channels. At higher loadings, partial order is observed in the intracrystalline fluid. © 1990 American Chemical Society.