The dynamic and equilibrium properties of xenon and methane in the pentasil zeolite silicalite were studied using molecular dynamics simulations. A rigid polyatomic model was used for methane, while xenon was represented by a Lennard-Jones sphere. The zeolite lattice was assumed to be rigid. The effect of intracrystalline occupancy and temperature upon the structure and dynamics of the sorbed phase was investigated. The predicted self-diffusivities were in good agreement with NMR measurements. The self-diffusivity of methane at 300K was predicted to be 1.6 x 10-4cm2/s, with an activation energy of 5.6kJ/mol. Except at the lowest temperature (200K) studied, the sorbate self-diffusivities were found to decrease monotonically with occupancy. At 200K, the self-diffusivities exhibited a weak maximum before decreasing at high loadings. This maximum was 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 showed that, at low loadings, sorbate molecules were localized preferentially in the sinusoidal channels and avoided the energetically less favorable channel intersections and straight channels. At higher loadings, partial order was observed in the intracrystalline fluid.

Molecular Dynamics Study of Methane and Xenon in Silicalite. R.L.June, A.T.Bell, D.N.Theodorou: Journal of Physical Chemistry, 1990, 94[21], 8232–40

Figure 14

Diffusivity of Alkanes in Silicalite as a Function of Carbon Number

(a: 70C, b: 50C, c: 30C)