Determination of Composition Range for "Molecular Trapdoor" Effect in Chabazite Zeolite

Highly selective separation of small molecules, such as CO2, N-2, and CH4, is difficult to achieve if all of the molecules can access the internal surface so that the selectivity depends only on differences in interaction of these molecules with the surface. Recently, we reported on a "molecular trapdoor" mechanism (Shang, J.; et al. J. Am. Chem. Soc. 2012, 134, 19246-19253), which provides a record high selectivity through a guest-induced cation deviation process where the adsorbent exclusively admits "strong" molecules (e.g., CO2 and CO) but excludes "weak" ones (e.g., N2 and CHO. In this study, we have investigated the range of zeolite compositions (varying Si/Al and cation type) for which a trapdoor effect is present and summarize this composition range with a simple "rule of thumb". Cation density and cation type are the controlling factors in achieving the molecular trapdoor effect on chabazites. Specifically, the "rule" requires every pore aperture connecting the supercages to accommodate one door-keeping cation of an appropriate type. This "rule" will help guide the synthesis of "trapdoor" chabazite adsorbents for the deployment of carbon capture as well as help the development of molecular trapdoor adsorbents/membranes for other small-pore zeolites, such as RHO, LTA, and other porous materials.