Atomistic Simulations of CO2 During "Trapdoor" Adsorption onto Na-Rho Zeolite

Behavior of CO2 within Na-Rho was studied using atomistic simulations. This zeolite is known to experience a phenomenon called "cation gating" which allows carbon dioxide but not other sorbents to permeate the zeolite, giving rise to very high adsorption selectivities for CO2. Our goal is to provide further insight into the reasons behind this intriguing phenomenon. We show that CO2's favorable electrostatic interactions with the zeolite framework result in preferential binding in the opening of the channels between cages. This leads us to suggest a novel mechanism to explain carbon dioxide's unique "gate opening behavior" in which this preference for binding inside the "gate" allows CO2 to "squeeze" by the gatekeeping cation as it moves around slightly due to thermal fluctuations. This proposed mechanism is distinct from a previously proposed mechanism in which carbon dioxide mediates the displacement of gatekeeping cations via electrostatic interactions and may be in better agreement with experimental evidence.