Understanding the dynamics, self-diffusion, and microscopic structure of hydrogen inside the nanoporous Li-LSX zeolite

Abstract

The dynamical and structural properties of hydrogen (H2) guest gas inside nanoporous Li-LSX zeolite were studied by molecular dynamics (MD) simulation for different loadings (8, 12, 16, and 20) of H2 per unit cell at temperatures of 200, 298, 400, and 500 K. Three equal mean-square displacement (MSD) components (in the x, y, and z-directions) for the center of mass of H2 guest molecules show that the translational motion of H2 in this zeolite medium is isotropic due to the high symmetry of the zeolite framework. At these conditions, H2 guest molecules freely move without blocking each other's path into the supercages and channels of the Li-LSX zeolite. The order of calculated self-diffusion coefficient of H2 guest molecules at different temperatures, in the range of 10−9 up to 10−7 m2 s−1, and corresponding activation energy, ∼2 kcal mol−1, followed using the Arrhenius equation is in good agreement with the pores size of Li-LSX zeolite (7.4–12 Å) and compatible with inter-region of well-known Knudsen and Configurational diffusion. The H2 self-diffusion coefficients increase with temperature, while showing no quantifiable changes with loading within this loading range. A further study with a broader guest loading range would be appropriate to fully understand the loading effect on the self-diffusion of H2 guest molecules in the Li-LSX zeolite as the H2 storage candidate. In addition to determining the temperature and loading effects on the H2 guest behavior, current simulations also show that the Li-III cations are specific H2 sorption sites and the structural correlation, dynamics, self-diffusion coefficient, and adsorption of H2 molecules are strongly dependent on the mobility or immobility of the key extraframework Li-III cationic sites of Li-LSX zeolite. The results of the simulation help in the choice of favorable structure, best design, and operative manufacture of zeolites or other microporous materials for similar applications.