Structural Chemistry, Flexibility, and CO₂ Adsorption Performance of Alkali Metal Forms of Merlinoite with a Framework Si/Al Ratio of 4.2

Small pore zeolites that show framework flexibility, such as merlinoite (topology type MER), possess a high potential for the selective adsorption of small gas molecules including CO₂. The CO₂ adsorption properties of Na-, K-, and Cs-exchanged forms of a merlinoite zeolite with Si/Al = 4.2 have been measured at 298 K, and in situ PXRD was used to follow their structural response to dehydration and CO₂ uptake. The Na- and Cs-forms convert from a wide-pore to a narrow-pore form upon dehydration, while the K-form remains in the wide-pore form. The Na- and Cs-forms exhibit stepped CO₂ adsorption isotherms, consistent with breathing behavior and expansion from narrow- to wide-pore phases, while K_6.2-MER remains in the wide-pore structure throughout. Synchrotron PXRD of the K- and Cs-forms reveals the effects of CO₂ adsorption on the cation site distributions and the framework configuration. All cation forms of MER (4.2) show enhanced adsorption kinetics for Ar compared to those with lower Si/Al, and the wide-pore structure of K_6.2-MER (4.2) shows particularly rapid sorption for both Ar and CO₂. Breakthrough curves over K_6.2-MER (4.2) demonstrate good separation of CO₂ from CH₄ in flowing CO₂/CH₄ mixtures, even in pelletized form with an alumina binder.