Modeling the Loading Dependency of Diffusion in Zeolites: the Relevant Site Model Extended to Mixtures in DDR-Type Zeolite

The recently introduced relevant site model (RSM) (Van den Bergh, et al, J. Phys. Chem. C, 113, 2009, 17840) to describe the loading dependency of single-component diffusion in zeolites is extended to mixtures. The model is formulated around the central idea of segregated adsorption in structures consisting of cages connected by windows, distinguishing cage and window adsorption sites, and only the molecules located at the window site (i.e., the relevant site RS) are able to make a Successful jump to the next cage. The RSM is based on the Maxwell-Stefan framework for mass transport but includes only one extra parameter that describes the adsorption properties of the 'relevant site'. Key feature of the RSM as applied to mixtures is that competitive adsorption effects and 'speeding up and slowing down' (exchange) effects between guest molecules are related to the relevant site loading instead of the overall loading, which can be very different. Analysis of all extensive set of diffusivity data of N-2/CO2 and Ne/Ar mixtures in zeolite DDR, directly computed using molecular dynamics, shows that the RSM provides excellent mixture diffusivity predictions from single component data. The results arc comparable to the 'Reed-Ehrlich' approach as put forward by Krishna and co-workers (e.g., Sep. Purif. Technol. 61, 2008, 414). Although the model predictions are comparable, the two approaches are fundamentally different since in the Reed-Ehrlich approach the loading dependency of diffusion is described by intermolecular repulsions. A clear improvement by the RSM approach is found in the case of the N-2 diffusivity in N-2/CO2 mixtures, attributed to the specific window blocking effect by CO2 and inherently incorporated in the RSM by relating adsorption to the relevant (window) site.