An investigation was made of how sorbate loading controlled the diffusion of spherically symmetrical Lennard-Jones molecules in a cation-free zeolite. This was studied by taking account of the mechanical flexibility of the framework, which allowed for energy relaxation of the sorbed molecules together with fluctuations of the window diameters. The number density of the adsorbed molecules was increased from 1 up to 15 methane molecules per cavity. That is, from 8 to 120 particles adsorbed in one unit cell. It was shown that the sorbate loading strongly altered the diffusion mechanism by inducing a transition, at 7 molecules per cavity, from an increasing to a decreasing mobility regime.

Sorbate-Loading Dependence of Diffusion Mechanism in a Cubic Symmetry Zeolite of Type ZK4. a Molecular Dynamics Study. Demontis, P., Suffritti, G.B.: Journal of Physical Chemistry B, 1997, 101[30], 5789-93