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

Abstract

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 Xtype 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 a verage, 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.

Cs-RHO Goes from Worst to Best as Water Enhances Equilibrium CO2 Adsorption via Phase Change

“…Our results motivate the need to investigate CO 2 adsorption in materials under wet conditions, since these are characteristic of flue gas, including at different hydration levels. 29 We are currently in the process of proving this hypothesized mechanism by conducting a Rietveld refinement of in situ PXRD data in parallel with advanced ab initio metadynamics simulations.…”

Cation-Induced Disruption of the Local Structure of Water in Faujasite

“…All density functional theory (DFT) calculations and ab initio molecule dynamics (AIMD) simulations were performed with the CP2K , package utilizing the general gradient approximation and the PBE functional . The PBE functional has been used to model water in zeolites ,,,− to simulate the relatively large periodic systems with DFT-MD. The core electrons were modeled with norm-conserving GTH pseudopotentials, while the valence electrons were modeled with a polarizable double-ζ basis set .…”

“…34 Monte Carlo simulations were carried out to study water adsorption in faujasite and how it varies with the Si/Al ratio and sodium content, 31 as well as to understand water adsorption into potassium-exchanged faujasite. 35 pores varied with water loading and the total amount of sodium exchanged. 33 In hydrogen-exchanged zeolites, density functional theory-based MD or MC simulations and experiments have been done to study proton dissociation and how it affects the structure of water for varying water loadings.…”

“…The structure of confined water in zeolites has been quantified through X-ray diffraction measurements, , infrared spectroscopy, and molecular dynamics (MD) or Monte Carlo (MC) simulations. − Results from molecular simulations have shown that zeolitic confinement changes the structure of water; for example, changes in the water dipole and infrared spectra of water in zeolite A were observed with Car–Parrinello MD . Infrared spectroscopy and classical MD simulations showed that changes in the structure of water depend on the pore size, and they affect the catalytic reaction rates .…”

“…Also, classical MC simulations have shown that water in confinement differs from bulk water in various zeolitic frames . Monte Carlo simulations were carried out to study water adsorption in faujasite and how it varies with the Si/Al ratio and sodium content, as well as to understand water adsorption into potassium-exchanged faujasite . Water and cations will affect each other; classical MD and MC simulations have shown that sodium positions in faujasite pores varied with water loading and the total amount of sodium exchanged .…”