Coverage dependence of aromatic hydrocarbon diffusion in silicalite: predictions of the three-site lattice model

The three-site lattice gas model (TSLG) was used to explain the experimentally observed changes in the transport diffusion coefficient of benzene adsorbed in silicalite. The influence of the surface coverage on the thermodynamic correction factor was studied and compared with the experimental data for the benzene/silicalite and also for the p-xylene/silicalite system at 303 K. In the case of benzene it was demonstrated that the maximum on the coverage-transport diffusion coefficient curve originates from a rapid rearrangement of the adsorbed molecules which takes place at the loading of ∼4 molecules per unit cell (m/uc). In particular, it was shown that the localization of the molecules in the channel intersections and the zigzag channels of silicalite causes the diffusivity to decrease at the loading exceeding 4 m/uc. This phenomenon, combined with the initial increase of the thermodynamic correction factor with the loading (up to 4 m/uc), was considered responsible for the presence of the maximum on the curve.