SELF-DIFFUSION OF RARE-GASES IN SILICALITE STUDIED BY MOLECULAR-DYNAMICS

The motion of rare gases adsorbed in silicalite has been simulated by using simple Lennard-Jones atom-atom potentials. The silicalite framework has been assumed to be rigid and was represented by the oxygen atoms. The MD simulations have been carried out in the microcanonical (NVE) ensemble. Four loadings have been considered: infinite dilution and 2, 5, and 9 atoms per unit cell (u.c.). The diffusion coefficients have been computed from the Einstein relation at different temperatures in the range 200-500 K. At 300 K and for 2 atoms/u.c., values of 24, 12, and 2 x 10(-9) m2.s-1 have been obtained for neon, argon, and xenon, respectively. Values of 2 and 3 kJ.mol-1 for the activation energies have been derived for neon and argon; they arc nearly independent of the loading. For xenon, the activation energy varies from 6 kJ.mol-1 at 0 atoms/u.c. to 3 kJ.mol-1 at 9 atoms/u.c. The distribution of the adsorbed atoms along the channels has been investigated in relation with the potential energy values inside the void space. The concept of supermobility has been revisited.