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

Abstract

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 K6.2-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 K6.2-MER (4.2) shows particularly rapid sorption for both Ar and CO2. Breakthrough curves over K6.2-MER (4.2) demonstrate good separation of CO2 from CH4 in flowing CO2/CH4 mixtures, even in pelletized form with an alumina binder.