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+.