Selective and Reversible Cation-Gating Adsorption Behavior in Gmelinite Zeolites for Efficient CO₂ Separation

Zeolites exhibiting stepwise adsorption behavior, i.e., a two-step increase in carbon dioxide (CO₂) uptake, have attracted attention in the field of zeolite research due to the potential to recover CO₂ using a small amount of energy. In this study, a Na⁺-type gmelinite (GME) zeolite exhibited stepwise adsorption behavior due to the migration of Na⁺ ions in the GME framework. Gas adsorption measurements, in situ powder X-ray diffraction (PXRD) analysis, and magic-angle spinning (MAS) nuclear magnetic resonance (NMR) analysis revealed that Na⁺ serves as a gate-opening cation that induces the migration of CO₂ from straight channels to the grain-like cages, resulting in notable stepwise adsorption. Remarkably, repetitive CO₂ adsorption measurements clarified that this stepwise adsorption performance was reversibly induced. Other cation-type GME zeolites such as Li⁺- and K⁺-GME zeolites exhibited type-I CO₂ adsorption isotherms, indicating only Na⁺ ions were capable of inducing the cation-gating CO₂ adsorption performance. Furthermore, time-resolved PXRD analysis revealed that the migration rate of Na⁺ was high under the CO₂ adsorption and desorption processes. In addition, stepwise adsorption behavior was observed even when the GME zeolite powder was pelletized. These findings pave the way for the development of highly efficient CO₂ separation processes that rely on zeolites with stepwise adsorption properties.