Insights into Water Adsorption in Potassium-Exchanged X-type Faujasite Zeolite: Molecular Simulation and Experiment

In the present study, we have combined several atomic-scale computational techniques [static lattice optimization, density functional theory (DFT), canonical Monte Carlo (MC), and Gibbs ensemble MC (GEMC)] with experiment in order to describe the adsorption of water in potassium-exchanged X-type faujasite. Indeed, by applying DFT calculations, we have evidenced a strongly heterogeneous adsorbent surface and classified the preferential adsorption sites at zero loading for water molecules. The sodalite cage was identified as the preferential location. Moreover, by applying the GEMC technique, we have successfully simulated the water adsorption isotherm in the K-X zeolite. Finally, through the canonical MC simulation, we have described the microscopic mechanisms of water adsorption within the K-X zeolite at various uptakes, ranging from low loading (8 H2O/unit cell) to saturation (240 H2O/unit cell). We have evidenced that at low loading, the sodalite cages host the major part of the adsorbed molecules, while at increasing hydration ratio, the water molecules locate more in supercages. At saturation, each sodalite cage accommodates more than three water molecules on average, whereas nearly 90% of water molecules are located within supercages. Finally, at any hydration ratio, the water molecules in supercages coordinate preferentially with potassium cations at crystallographic site III (or III') rather than at site II. Cations in site II start interacting with water molecules only at uptakes superior to 80 H2O molecules/unit cell, once all cations in site III (or III') are occupied by at least one water molecule.

Automated summary

In this study, a combination of atomic-scale computational techniques and experiments was used to investigate water adsorption in potassium-exchanged X-type faujasite. The DFT calculations revealed a heterogeneous adsorbent surface and identified preferential adsorption sites for water molecules. GEMC technique was employed to simulate water adsorption isotherm, and canonical MC simulation was used to describe the microscopic mechanisms of water adsorption at different uptake levels in the K-X zeolite. Results showed that the adsorbed water molecules were primarily located in sodalite cages at low loading, shifting to supercages at higher hydration ratios. Additionally, coordination of water molecules with potassium cations at crystallographic site III (or III') was favored over site II at all hydration ratios.