Molecular Insight into CO₂ “Trapdoor” Adsorption in Zeolite Na-RHO

Abstract

Among other zeolites, sodium-substituted zeolite Na-RHO exhibits a phenomenon known as “trapdoor” adsorption, with extra-framework cations acting as gates to small windows in the zeolite structures. While carbon dioxide can diffuse through these gates, methane and other guests cannot, leading to strong potential for gas separation. This effect has been attributed in the literature to specific cation–guest interactions that would allow CO 2 to pull open the trapdoor. We investigate here the gating phenomenon using ab initio molecular dynamics simulations combined with free energy methods. Our findings invalidate this previously proposed mechanism, showing that the presence of CO 2 does not significantly affect the motion of the gating cation. We put forward and demonstrate an alternative mechanism, showing that the amplitude of the thermal motion of the cations is large and that CO 2 is able to squeeze while the gate is open, while nonpolar guests such as methane cannot. This leads to an image of the mechanism that is closer to swinging doors than trapdoors.