Single-component structural correlation and self-diffusion of N2 and O2 through nanopores of Li-LSX zeolite: The role of temperature, loading, and Li-III cations

Abstract

Diffusion-selective N2/O2 separation and competitive adsorption by nanoporous Li-LSX zeolite, as the best commercially available adsorbent, is a scientifically interesting phenomenon with important technological applications. In a recent paper (J. Phys. Chem. C 121, 2017, 1770–1780), we reported the first molecular dynamics (MD) study of the adsorption of the binary N2/O2 mixtures within this zeolite. In the present paper, the microscopic structure and intracrystalline self-diffusion coefficient of N2 and O2 as single-component guest species in Li-LSX zeolite are determined at different temperatures and practical loadings. The simulation results are found to be in very good complementary agreement with experimental adsorption findings as well as with the recent predictions based on the binary air mixture simulations. The influences of the internal surface dynamics of extra-framework Li+ cation in site III (Li-III) on the guest static and dynamic processes within the zeolite are determined. O2 has no significant association with Li-III, while all structural analyses prove remarkable adsorption and hence localization of N2 around the Li-III in the supercages. The degree of localization and the residence time of N2 on Li-III in particular increases with decreasing temperature and also with fixing of Li-III cations during the simulation. As a guide for future design of N2/O2 separation by zeolite frameworks at ambient temperature, an excellent adsorption selectivity ratio can be achieved if it is practically possible to reduce the dynamics of accessible Li-III sorption sites up to extremely fixed situations. The calculated Arrhenius activation energy for N2 diffusion process through Li-LSX zeolite is several times higher than that of O2. These results are due to the larger quadrupole moment of N2 and stronger Coulombic attraction between N2 and Li-III that causes the translational motion of N2 to be significantly hindered. Overall, the intracrystalline self-diffusion coefficients of both N2 and O2 slightly decrease with loading. Current simulations also correctly approve that the Li+ cations in sites I′ and II are inaccessible to interact with guest molecules.