Multi-component behaviors of hydrogen isotopes adsorbed on synthetic zeolites 4A and 5A at 77.4K and 87.3K

Competitive Adsorptive Mechanism of H2/N2 in LTA/FAU Zeolites by Molecular Simulations and Experiments

For industrial tail gas to be converted into high-purity hydrogen, the H2-N2 mixture needs to be separated efficiently. This work examined the adsorption characteristics and competitive mechanisms of H2 and N2 on LTA- and FAU-type zeolites, at 77 K, 298 K, and 0.1-10 bar by thoroughly analyzing results of adsorption capacity experiments and molecular simulations. In the Grand Canonical Monte Carlo (GCMC) simulations, the force field causing a molecular dipole of H2 and the polarization force field of N2 are first applied. The accuracy of the force field was experimentally verified. The findings indicate that N2 and H2 loading on Ca-FAU (Ca-LTA) are higher than Na-FAU (Na-LTA). On NaX at 77 K, the highest adsorption selectivity (N2/H2) is observed; on NaA at 298 K, it is the opposite. The GCMC data findings demonstrate that H2 and N2 have remarkably similar adsorption sites, with framework oxygen atoms and non-framework cations serving as the main adsorption sites for adsorbate molecules. Furthermore, the rate at which H2 diffuses is higher than that of N2. The study of redistribution charge before and after adsorption demonstrated that N2 has a greater affinity for the framework oxygen atoms than H2. This study provides a molecular theoretical foundation for the adsorption behavior of H2-N2 mixture in zeolites.

High Efficiency of Na- and Ca-Exchanged Chabazites in D2/H2 Separation by Quantum Sieving

Quantum sieving is a promising approach for separation of hydrogen isotopes using porous solids as sorbents at cryogenic temperatures (<77 K). In the present work, we characterized the properties of two aluminum-rich chabazites: Na-CHA and Ca-CHA (Si/Al = 2.1). The single-gas D₂ and H₂ adsorption isotherms were measured, and the thermodynamic selectivities were determined through coadsorption experiments in the temperature range 38-77 K. We found that at 38 K, Na-CHA shows a selectivity of 25.8 at a loading of 10.6 mmol·g^-1. At the same temperature, Ca-CHA has slightly lower selectivity (18.3), but its uptake (12.9 mmol·g^-1) is higher than that for Na-CHA. Comparison with the literature shows that the obtained values of selectivity are among the highest reported so far. This property combined with robustness and availability on the industrial scale of Al-rich chabazites makes them very promising materials for separation of hydrogen isotopes by quantum sieving.

Adsorption of water in Na-LTA zeolites: an ab initio molecular dynamics investigation

The very wide range of applications of LTA zeolites, including the storage of tritiated water, implies that a detailed and accurate atomic-scale description of the adsorption processes taking place in their structure is crucial. To unravel with an unprecedented accuracy the mechanisms behind the water filling in NaA, we have conducted a systematic ab initio molecular dynamics investigation. Two LTA structural models, the conventional Z4A and the reduced one ZK4, have been used for static and dynamic ab initio calculations, respectively. After assessing this reduced model with comparative static DFT calculations, we start the filling of the α and β cages by water, molecule by molecule. This allowed us to thoroughly study the interaction of water molecules with the zeolite structure and between water molecules, progressively forming H-bond chains and ring patterns as the cage is being filled. The adsorption energies could then be calculated with an unprecedented accuracy, which showed that the interaction of the molecules with the zeolite weakens as their number increases. By these methods, we have been able to highlight the primary role of Na⁺ cations in the interaction of water with zeolite, and inversely, the role of water in the displacement of cations when it is sufficiently solvated, allowing the passage between the α and β cages. This phenomenon is possible thanks to the inhomogeneous distribution of water molecules on the cationic sites, as shown by our AIMD simulations, which allows the formation of water clusters. These results are important because they help in understanding how the coverage of cationic sites by water will affect the adsorption of other molecules inside the Na-LTA zeolite.

Enhanced quantum sieving of hydrogen isotopes via molecular rearrangement of the adsorbed phase in chabazite

Coadsorption experiments reveal an unexpected increase of the D2/H2 selectivity with loading in pure silica chabazite at 47 K. This effect is correlated with the appearance of a step in the adsorption isotherms of H2 and D2. Grand canonical Monte Carlo simulations show that this phenomenon is related to a molecular rearrangement of the adsorbed phase induced by its strong confinement. In the case of a H2 and D2 mixture, this rearrangement favors the adsorption of D2 having a smaller size due to quantum effects.