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.