Triggered Gate Opening and Breathing Effects during Selective CO₂ Adsorption by Merlinoite Zeolite

Zeolites with flexible structures that adapt to coordinate extraframework cations when dehydrated show a rich variety of gas adsorption behavior and can be tuned to optimize kinetics and selectivity. Merlinoite zeolite (topology type MER) with Si/Al = 3.8 has been prepared in Na, K, and Cs forms and its structural response to dehydration measured: the unit cell volumes decrease by 9.8%, 7.7%, and 7.1% for Na-, K-, and Cs-MER, respectively. Na-MER adopts Immm symmetry, while K- and Cs-MER display P 4 2 / nmc symmetry, the difference attributed to the preferred locations of the smaller and larger cations. Their performance in CO 2 adsorption has been measured by single-component isotherms and by mixed gas (CO 2 /CH 4 /He) breakthrough experiments. The differing behavior of the cation forms can be related to structural changes during CO 2 uptake measured by variable-pressure PXRD. All show a “breathing” transition from narrow to wide pore forms. Na- and Cs-MER show non-Type I isotherms and kinetically-limited CO 2 adsorption and delivery of pure CH 4 in CO 2 /CH 4 separation. However, K-MER shows good uptake of CO 2 (3.5 mmol g –1 at 1 bar and 298 K), rapid adsorption and desorption kinetics, and promising CO 2 /CH 4 separation. Furthermore, the narrow-to-wide pore transition occurs rapidly and at very low p CO 2 via a “triggered” opening. This has the consequence that whereas no CH 4 is adsorbed from a pure stream, addition of low levels of CO 2 can result in pore opening and uptake of both CO 2 and CH 4, although in a continuous stream the CH 4 is replaced selectively by CO 2 . This observed cation size-dependent adsorption behavior derives from a fine energetic balance between different framework configurations in these cation-controlled molecular sieves.