Molecular dynamics as a tool to study heterogeneity in zeolites – Effect of Na+ cations on diffusion of CO2 and N2 in Na-ZSM-5
Chemical Engineering Science
Diffusion in Na-ZSM-5 zeolite is strongly affected by Al and Na þ cations. MD simulations quantify the effects on diffusion of quadrupolar guest molecules. CO 2 , with its larger quadrupolar moment, is more strongly affected than N 2 . A coarse-grained site-lattice representation for the pore network is derived. Molecular residence times and probabilities to hop between sites are calculated. a b s t r a c t Zeolites typically contain extra-framework cations to charge-compensate for trivalent Al
... te for trivalent Al atom substitutions in the SiO 2 framework. These cations, such as Na þ , directly interact with quadrupolar guest molecules, such as CO 2 and N 2 , which move through their micropores, causing energetic heterogeneity. To assess the effects of heterogeneity in Na-ZSM-5 on diffusion of CO 2 and N 2 , molecular dynamics (MD) simulations are carried out. In silicalite-1, the pure-silicon form of ZSM-5, the self-diffusivity exhibits a monotonic decrease with molecular loading, while the corrected diffusivity shows a relatively constant value. In contrast, the Na þ cations cause a maximum or a flat profile over molecular loading for the selfand corrected diffusivities of CO 2 at T ¼200 and 300 K, while the cations only have minimal impact on the diffusivity of N 2 . The MD simulations allow us to identify energy basins or sites at which guest molecules spend a relatively long time, and construct a coarse-grained lattice representation for the pore network. Average residence times at these sites are calculated for both species. The trends observed in the residence times correlate to the trends observed in the diffusivity. The residence times for CO 2 at T ¼200 K are long at low loading, but decrease with loading as additional CO 2 molecules compete to stay close to a cation. In contrast, the residence times for N 2 are relatively insensitive to the cations, only mildly increasing near a cation. This difference in behavior can be associated to the quadrupole moments of these molecules.