Atomistic Simulations of CO₂ and N₂ Diffusion in Silica Zeolites: The Impact of Pore Size and Shape

Abstract

Diffusion of CO 2 and N 2, both as single components and as binary mixtures, in three zeolites with identical chemical composition but differing pore structures—silicalite, ITQ-3, and ITQ-7—was studied using atomistic simulations. In all materials CO 2 diffuses slower than N 2, but otherwise the behavior of these gases within ITQ-7 and silicalite is quite different than within ITQ-3. In ITQ-7 and silicalite, the loading dependence of diffusion is very similar for CO 2 and N 2; the apparent activation energies for diffusion of each adsorbate are similar in the two materials, and the diffusion properties of adsorbed mixtures can easily be understood. In contrast, none of these are true within ITQ-3. Free energy and potential energy profiles are used to uncover the roots of these differences. The preferential sites of adsorption for CO 2 and N 2 are the same within ITQ-7 and silicalite but not within ITQ-3. In ITQ-3, CO 2 molecules preferentially adsorb in the windows that separate the material’s cages, whereas for N 2 this is the site of the largest barrier to diffusion. As a consequence, CO 2 hinders N 2 diffusion very effectively. Our analysis suggests that this behavior might be common for adsorbates that interact strongly with a material that has narrow windows between cages.