Ab Initio Screening of Divalent Cations for CH₄, CO₂, H₂, and N₂ Separations in Chabazite Zeolite

The efficient separation and adsorption of critical gases are, more than ever, a major focus point in important energy processes, such as CH₄ enrichment of biogas or natural gas, CO₂ separation and capture, and H₂ purification and storage. Thanks to its physicochemical properties, cation-exchanged chabazite is a potent zeolite for such applications. Previous computational screening investigations have mostly examined chabazites exchanged with monovalent cations. Therefore, in this contribution, periodic density functional theory (DFT) calculations in combination with dispersion corrections have been used for a systematic screening of divalent cation exchanged chabazite zeolites. The work focuses on cheap and readily available divalent cations, Ca(II), Mg(II), and Zn(II), Fe(II), Sn(II), and Cu(II) and investigates the effect of the cation nature and location within the framework on the adsorption selectivity of chabazite for specific gas separations, namely, CO₂/CH₄, N₂/CH₄, and N₂/H₂. All the cationic adsorption sites were explored to describe the diversity of sites in a typical experimental chabazite with a Si/Al ratio close to 2 or 3. The results revealed that Mg-CHA is the most promising cation for the selective adsorption of CO₂. These predictions were further supported by ab initio molecular dynamics simulations performed at 300 K, which demonstrated that the presence of CH₄ has a negligible impact on the adsorption of CO₂ on Mg-CHA. Ca(II) was found to be the most favorable cation for the selective adsorption of H₂ and CO₂. Finally, none of the investigated cations were suitable for the preferential capture of N₂ and H₂ in the purification of CH₄ rich mixtures. These findings provide valuable insights into the factors influencing the adsorption behavior of N₂, H₂, CH₄, and CO₂ and highlight the crucial role played by theoretical calculations and simulations for the optimal design of efficient adsorbents.