Correlating the Adsorption Preference and Mass Transfer of Xenon in RHO-Type Molecular Sieves

Abstract

The adsorption and diffusion of adsorbates or reactants are the most essential steps in zeolite-based processes for gas separation and catalysis. An explicit description of the relationship between adsorption and mass transport in confined environments of microporous materials is a prerequisite to catalytic reaction and gas separation. The dimensions, shapes, and types of pores are the most essential properties that govern the adsorption and diffusion behaviors of guest molecules. Here, a xenon atom was used as a sensitive probe to provide information about the dynamic adsorption process between the lta cavity and double eight-membered rings (D8R) of DNL-6 molecular sieves with RHO topology by 129Xe NMR and pulsed field gradient (PFG) NMR. Loading-dependent 129Xe NMR presents the two types adsorption environments with different probabilities of xenon distribution, and D8R is the preferential adsorption site. Furthermore, 129Xe PFG NMR exhibits the mass-transport limitations of the xenon atom in DNL-6 as the loading increases. On the basis of molecular dynamics (MD) and Monte Carlo (MC) simulations, the interaction energies between RHO framework and xenon were predicted and the preferred adsorption character of D8R was displayed visually, which further contribute to the understanding of adsorption and diffusion behavior, especially for the loading dependence of intracrystalline diffusion. The diffusion limitation caused by the preferential adsorption of D8R can depress the mass transport in RHO-type molecular sieves. A direct relationship between the adsorption preference and diffusion was established at the molecular level.