Tuning the CHA framework composition by isomorphous substitution for CO₂/CH₄ separation

Chemically and structurally flexible zeolites that adapt to coordinate extra-framework cations show a rich variety of gas adsorption behavior. Zeolite framework composition can be tuned to optimize pore geometry and host-guest interaction to improve selectivity. Herein, we report the study on the influence of zeolite framework nature on the separation of CO₂/CH₄, for CHA system containing variable isomorphously substituted heteroelements (none, B, Al, Ga, and Ti). Their performance has been evaluated by single-component isotherms and the overall separation ability was found to be related to both the geometry of 8-ring pore apertures in CHA and interaction between zeolite host and guest molecules, as revealed by Rietveld refinement of PXRD after dehydration and variable-pressure FTIR spectroscopy. Nearly isotropic geometry of the 8-ring pore opening in Al-CHA (3.84×3.89 Å) was obtained, while Ga-CHA exhibits the largest distortion (3.76×3.93 Å). Despite the smallest ionic radius of B, both the pore size of 8-ring (3.76×3.86 Å) as well as the unit cell volume are the smallest amongst the studied materials. Due to the combination of both the textural and the chemical factors, the maximum CO₂ capacity and separation selectivity over CH₄ follows the order Al-CHA > Ga-CHA > B-CHA ≈ Si-CHA > Ti-CHA. These observations suggest that tuning the framework composition of zeolite can strongly influence the separation of small molecules such as CO₂ and CH₄.