Triggered Gate Opening and Breathing Effects during Selective CO 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 symmetry, while K- and Cs-MER display 4/ symmetry, the difference attributed to the preferred locations of the smaller and larger cations. Their performance in CO adsorption has been measured by single-component isotherms and by mixed gas (CO/CH/He) breakthrough experiments. The differing behavior of the cation forms can be related to structural changes during CO 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 adsorption and delivery of pure CH in CO/CH separation. However, K-MER shows good uptake of CO (3.5 mmol g at 1 bar and 298 K), rapid adsorption and desorption kinetics, and promising CO/CH separation. Furthermore, the narrow-to-wide pore transition occurs rapidly and at very low via a "triggered" opening. This has the consequence that whereas no CH is adsorbed from a pure stream, addition of low levels of CO can result in pore opening and uptake of both CO and CH, although in a continuous stream the CH is replaced selectively by CO. This observed cation size-dependent adsorption behavior derives from a fine energetic balance between different framework configurations in these cation-controlled molecular sieves.