Structural chemistry, flexibility, and CO₂ adsorption performance of alkali metal forms of merlinoite with framework Si/Al ratio of 4.2

Small pore zeolites that show framework flexibility, such as merlinoite (topology type MER), possess 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 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 behaviour and expansion from narrow- to wide-pore phases, whilst 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 pelletised form with an alumina binder.