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 <i>Immm</i> symmetry, while K- and Cs-MER display <i>P</i>4₂/<i>nmc</i> 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 <i>p</i>_CO₂ 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.