Modeling the Loading Dependency of Diffusion in Zeolites: The Relevant Site Model

A new model is introduced to describe the loading dependency of diffusion in zeolites. The model is formulated around the idea of segregated adsorption in cage-like zeolites, that is, that molecules are located either in the cage or its window site. Furthermore, only the molecules located at the window site are able to make a successful jump to another cage. This so-called relevant site model (RSM) is based on the Maxwell-Stefan framework for mass transport but includes one extra parameter that describes the adsorption properties of the "relevant site". The RSM describes diffusivity data of N-2 and CO2 in DDR (eight-ring cage-like zeolite) very well up to saturation. The observed diffusivity loading dependency is explained from the relative low window site occupancy that is typically much lower than the total occupancy at lower loadings. The model is successfully extended to nonisothermal diffusivity data of CO2 and N-2 in DDR. Relating intermolecular correlation effects to the relevant site occupancy instead of the total occupancy leads to a quantitative prediction of the observed correlation effects and, consequently, the self-diffusivity. Analysis of the N-2 data suggests positional rearrangements in the DDR cages in a certain loading range. These effects have been incorporated in the model successfully.