Zeolites are inorganic aluminosilicates with an ordered atomistic-scale porous topology that have found usage in membrane separations, catalysis, and sequestration. The zeolite ZSM-5 has a well-defined topology with straight channels in the y-direction, zigzag channels in the x-z plane, and with a typical pore diameter of ~5.5 Å. The zeolite lattice has a nominal composition of SiO₂, but the Si atoms can be replaced by Al atoms, along with extra framework cations, such as Na⁺, to maintain charge balance.

One possible application of ZSM-5 is the separation of CO₂ from flue gases. The molecules CO₂ and N₂ have different kinetic diameters and therefore will diffuse through the zeolite pores at different rates, hence separating molecules in a membrane operation.

We have used molecular dynamics (MD) simulations of CO₂ and N₂ diffusing in ZSM-5 with Na⁺ and Al present. We performed the simulations with the DL_POLY software package, using the Nose-Hoover thermostat, the Ewald summation for Coulombic forces, a pretabulation of the zeolite potential energy landscape, and for runs over time lengths of 20-26 ns. In the MD runs we collect the trajectories to determine the mean squared displacement over time curve. From this information we calculated the self- and corrected-diffusivities of CO₂ and N₂ at T=200, 300, and 400 K. The diffusivities provide insight into the mobility of molecules as a function of temperature and concentration.

We consider the cases of 0 and 1 Al substitution of Si in the ZSM-5 unit cell to study the role of heterogeneity. In practice the concentration of the cation (that accompanies the Al) is a property that can be controlled via ion exchange to tune the adsorptive and diffusive properties of molecules in the ZSM-5. The CO₂ and N₂ molecules have a strong electrostatic interaction to the Na⁺ cation that is tethered near the Al atom. We find that the diffusivities of CO₂ at T=200 and 300 K show a maximum as a function of loading at 1 Al per ZSM-5 unit cell, due to the Na⁺ strongly attracting the molecules. The Na⁺ ions also cause pore blockage by occupying space in the pore normally available for transport. The activation barrier for CO₂ strongly increases due to the presence of Na⁺.