The role of the extra-framework cations in the adsorption of CO₂ on faujasite y

A purely electrostatic picture predicts that small, more polarizing cations adsorb more strongly than the large ones. In the case of the adsorption of CO₂ on faujasite Y, however, the inverse order is found at low pressure: CsY and KY adsorb stronger than NaY and LiY. This trend cannot be explained by a simple monopole-dipole or monopole-quadrupole interaction of CO₂ with a single cation. Therefore, we have conducted a combined adsorption, IR and DFT study in order to shed light on this phenomenon. Our results show that a simultaneous interaction of CO₂ with the cations (located in the hexagonal window between supercage and sodalite cage) and framework oxygen atoms (in the 12-ring connecting two supercages) can explain the strong adsorption of CO₂ on CsY and KY. We also discuss the effect of van der Waals interactions. Although there is a redistribution of the charge of three framework oxygen atoms in the 12-ring towards the carbon atom of CO₂, the geometry of CO₂ remains almost linear. The adsorption mode can, therefore, be interpreted as a carbonate precursor. In zeolite Y, this acid-base like interaction is more important than polarization of CO₂ by the cation only. Furthermore, evidence for an adsorption mode where CO₂ binds simultaneously to two cations in the supercage was detected by DFT, as already reported for Na-ferrierite. This adsorption mode is, however, strongly dependant on the distance between two cations in the supercage and is only favorable in the case of KY.