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 structuresssilicalite, ITQ-3, and ITQ-7swas 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.

Efficient separation of N2 and he at low temperature using MFI membranes

“…For He, we used the constants reported for H 2 previously of 1.0 and 0.35, respectively, see Krishna et al for details . The activation energies of diffusion ( E A,diff ) were 3.1 kJ/mol and 1.0 kJ/mol for N 2 and He, respectively . The diffusivities and parameters for describing the dependence of diffusivities with loadings have been reported for N 2 previously .…”

Sorption, Structure and Dynamics of CO2 and Ethane in Silicalite at High Pressure: A Combined Monte Carlo and Molecular Dynamics Simulation Study

“…The sorption and mobility of gases such as CO 2 and ethane inside ZSM-5 has been studied by several authors [11,12,13,14,15]. More recently, mixtures containing CO 2 in silicalite have also been studied [16,17,18]. Moisture-driven CO 2 sorption has also been studied.…”

Improved Hill–Sauer Force Field for Accurate Description of Pores in 8-Ring Zeolites

“…The zeolites LTA-5A and ITQ-29 have smaller 8-ring windows unobstructed by cations but showed diffusivity lower by an order of magnitude compared to the cationic structures CaA and partially exchanged NaCaA. The efficiency of hydrocarbon separation processes in 8-ring zeolites relies on a combination of adsorption and diffusion selectivity of the molecules in the target mixture 2,10 .…”