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

Diffusion of CO2 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 CO2 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 CO2 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 CO2 and N-2 are the same within ITQ-7 and silicalite but not within ITQ-3. In ITQ-3, CO2 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, CO2 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.