Gate Opening and Breathing Effects during Selective CO 2 Adsorption by Merlinoite Zeolite

Zeolites with flexible structures that adapt to coordinate extra-framework cations when dehydrated show a rich variety of gas adsorption behaviour, and can be tuned to optimise kinetics and selectivity. Merlinoite zeolite (topology type MER) with Si/Al = 3.8 has been prepared in Na-, Kand Cs-forms and its structural response to dehydration measured: the unit cell volumes decrease by 9.7, 7.7 and 7.1% for Na-, Kand Cs-MER, respectively. Na-MER adopts Immm symmetry, while Kand CsMER 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 behaviour 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. Naand Cs-MER show non-Type I isotherms and kinetically-limited CO2 adsorption and delivery of pure CH4 in CO2/CH4