Discriminative separation of gases by a "molecular trapdoor" mechanism in chabazite zeolites

Separation of molecules based on molecular size in zeolites with appropriate pore aperture dimensions has given rise to the definition of "molecular sieves" and has been the basis for a variety of separation applications. We show here that for a class of chabazite zeolites, what appears to be "molecular sieving" based on dimension is actually separation based on a difference in ability of a guest molecule to induce temporary and reversible cation deviation from the center of pore apertures, allowing for exclusive admission of certain molecules. This new mechanism of discrimination permits "size-inverse" separation: we illustrate the case of admission of a larger molecule (CO) in preference to a smaller molecule (N₂). Through a combination of experimental and computational approaches, we have uncovered the underlying mechanism and show that it is similar to a "molecular trapdoor". Our materials show the highest selectivity of CO₂ over CH₄ reported to date with important application to natural gas purification. © 2012 American Chemical Society.