Triggered gate opening and breathing effects during selective CO2 adsorption by merlinoite zeolite

Abstract

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 P42/nmc symmetry, the difference attributed to the preferred locations of the smaller and larger cations. Their performance in CO2 adsorption has been measured by single component isotherms and by mixed gas (CO2/CH4/He) breakthrough experiments. The differing behavior of the cation forms can be related to structural changes during CO2 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 CO2 adsorption and delivery of pure CH4 in CO2/CH4 separation. However, K-MER shows good uptake of CO2 (3.5 mmol g-1 at 1 bar and 298 K), rapid adsorption and desorption kinetics, and promising CO2/CH4 separation. Furthermore, the narrow-to-wide pore transition occurs rapidly and at very low pCO2, via a "triggered" opening. This has the consequence that whereas no CH4 is adsorbed from a pure stream, addition of low levels of CO2 can result in pore opening and uptake of both CO2 and CH4, although in a continuous stream the CH4 is replaced selectively by CO2. This observed cation size-dependent adsorption behavior derives from a fine energetic balance between different framework configurations in these cation-controlled molecular sieves.