Testing the consistency of the Maxwell-Stefan formulation when predicting self-diffusion in zeolites with strong adsorption sites

The Maxwell-Stefan (MS) formulation, as applied to zeolites that contain both weak and strong adsorption sites, such as ZSM-5, is compared to dynamic Monte Carlo simulations, for the limiting case of single-component self-diffusion. This study is intended as a consistency check, and as a step towards an analytical or semi-analytical theory for self-diffusion in zeolites with multiple types of sites. In its original form, when it is assumed that, the ratio of the self-exchange coefficient to the corrected diffusivity, is equal to 1, the MS formulation performs well for silicalite, the all-Si version of ZSM-5. However, when there are lattice heterogeneities or the topology of the pore network differs from that of silicalite, it is necessary to assume zeta not equal 1. Because zeta is generally occupancy dependent, the theory is unsuited as a fully predictive theory for self-diffusion in heterogeneous microporous solids, unless a theory for is derived. However, since several Studies have demonstrated that the MS formulation is able to predict multi-component diffusivities from single-component diffusivities for zeolites with one type of site, an extension to zeolites with multiple types of sites would be very valuable.