Atomistic Simulations of CO2 and N2 within Cage-Type Silica Zeolites

The behavior of CO2 and N-2, both as single components and as binary mixtures, in two cage-type silica zeolites was studied using atomistic simulations. The zeolites considered, ITQ-3 and paradigm cage-type zeolite ZK4 (the all-silica analog of LTA), were chosen so that the principles illustrated can be generalized to other adsorbent/adsorbate systems with similar topology and types of interactions. N-2 was chosen both because of the potential uses of N-2/CO2 separations and because it differs from CO2 most significantly in the magnitude of its Coulombic interactions with zeolites. Despite similarities between N-2 and CO2 diffusion in other materials, we show here that the diffusion of CO2 within cage-type zeolites is dominated by an energy barrier to diffusion located at the entrance to the narrow channels connecting larger cages. This barrier originates in Coulombic interactions between zeolites and CO2's quadrupole and results in well-defined orientations for the diffusing molecules. Furthermore, CO2's favorable electrostatic interactions with the zeolite framework result in preferential binding in the windows between cages. N-2's behavior, in contrast, is more consistent with that of molecules previously studied. Our analysis suggests that CO2's behavior might be common for adsorbates with quadrupoles that interact strongly with a material that has narrow windows between cages.