(525a) Molecular and Multi-Scale Simulations of CO2 and N2 Transport in ZSM-5 Zeolite Membranes with Framework Substitutions

Zeolites are crystalline aluminosilicates with an ordered network of micropores. Their uses include membrane separations, catalysis, and sequestration. The zeolite ZSM-5 contains a regular array of straight channels in the y-direction, and zigzag channels in the x-z plane. The pores have a 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 membrane separation of CO₂ from flue gases. The molecules CO₂ and N₂ have different kinetic diameters and a different affinity for the zeolite surface. Therefore, they adsorb into and diffuse through the zeolite pores at different rates, which allow them to be separated in a membrane operation. We considered the cases of 0 and 1 Al substitution of Si in the ZSM-5 unit cell to study the role of heterogeneity. The type and 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 ZSM-5. Both N₂ and CO₂, but especially CO₂, have a strong quadrupolar interaction with the Na⁺ cation. We have used molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulations of CO₂ and N₂ diffusing in ZSM-5, with some of the Si atoms substituted by Al- and Na⁺ counterions. The MD simulations were run for 20-26 ns at T = 200, 300 and 400 K. Diffusivities and residence times around preferred sites were calculated from the simulated trajectories. The diffusivity of CO₂ is of the order of 10^-6 - 10^-5 cm² / s, while the diffusivity of N₂ is of the order of 10^-5 - 10^-4 cm² / s. The residence times for CO₂ are in the typical range of 30 - 100 ps when no strong sites are present, but dramatically increase to 1 - 2 ns with Na⁺ present. The Na⁺ causes an increase in the diffusivity at low loading, until crowding effects cause a decrease. The activation barrier for CO₂ strongly increases due to the presence of Na⁺, with the diffusivity of CO₂ showing a maximum. GCMC simulations were used to determine the adsorption isotherms, showing a substantial increase of CO₂ loading with Na⁺. content. Though the N₂ molecules are more mobile, they adsorb less strongly than CO₂. To cope with the broad range of time and length scales of membrane transport, we use dynamic Monte-Carlo simulations and mean-field theory of diffusion on a site lattice model representing the pore network topology. These coarse-grained simulations make use of the residence times on the sites and the adsorption isotherms calculated using the just described atomistic models. This multi-scale approach allows a calculation of the selectivity of CO₂ over N₂ and the throughput of a membrane. Current results show a strong dependence of the permeances with the level of heterogeneity. Increasing the Na⁺ content typically decrease the permeance at a same, desired level of selectivity, since the CO₂ and N₂ molecules diffuse at a slower rate. The trends of the permeances agree well with experimental measurements, though the quantitative predictions are higher in this defect-free membrane model. Experimental permeance measurements are suspect to intercrystalline barriers, which cause a decrease in the observed permeances. The selectivity of the ZSM-5 membranes is always predicted to be in favor of CO₂, again in agreement with prior experimental observations. Heterogeneity plays an important role in activated diffusion. The residence times and diffusivities indicate that Na⁺ cations can dramatically affect the behavior of CO₂ and N₂ molecules, allowing for a more rational basis for membrane design and operation.