Diffusion in zeolites: Extension of the relevant site model to light gases and mixtures thereof in zeolites DDR, CHA, MFI and FAU

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 diffusion in zeolite DDR is successfully extended to a variety of light gases (CH4, CO2, Ar and Ne) and binary mixtures thereof in other zeolite topologies, DDR, CHA, MFI and FAU, utilizing the extensive diffusivity dataset published by Krishna and van Baten for this variety of zeolite-guest systems (e.g. Chem. Eng. Sci., 63 (2008), 3120 (supplementary material)).The RSM is formulated around the central idea of segregated adsorption in structures consisting of cages connected by windows, distinguishing cage and window adsorption sites. 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 and composition instead of to the overall loading, which can be very different.From the RSM approach a measure for the level of adsorption segregation is derived: the ratio of the RS and total occupancy. The predicted level of adsorption segregation correlates well with the level of confinement of a molecule at the RS: the molecule diameter to zeolite pore diameter. The predicted degree of adsorption segregation of the studied light gases in DDR is in good agreement with molecular simulations results, indicating the physical meaningfulness of the estimated RS adsorption parameters.The binary mixture diffusivity modelling points out that in case of the small-pore zeolites (DDR and CHA) the data is described best with equal RS saturation loadings for both components. For the large pore zeolite FAU the ratio of the RS saturation loadings equals that of the bulk saturation loadings. The geometry of the RS strongly influences the RS saturation loading: in case of the small-pore zeolites the RS (= window site) is restricted to only one molecule but when the RS becomes larger its saturation loading becomes similar to that of the bulk.