Alkali and alkaline-earth exchanged faujasites: Strength of Lewis base and acid centres and cation site occupancy in Na- and BaY and Na- and BaX zeolites

Sodium and barium forms of X (Si:Al = 1.3) and Y (Si:Al = 3.5) zeolites were studied by IR spectroscopy of adsorbed CO (at low temperature) and CO₂ (at room temperature). In BaY, Ba²⁺ ions substitute almost completely the Na⁺ ones at sites SII, and new, stronger Lewis acid sites are formed. In the supercages of X zeolite in sodium form, the Na⁺ ions are present at both SII and SIII sites. After exchange with Ba²⁺ almost all alkali ions in the supercages are substituted by alkaline-earth ions, which occupy sites SII only. Also, in this case, Ba²⁺ ions behave as stronger Lewis acid centres than Na⁺ ions at such sites. However, their Lewis acidity is weaker than for Ba²⁺ species at sites SII in Y zeolite, because of the lower Si:Al ratio. IR spectra of adsorbed CO₂ revealed that the Lewis basicity of the framework oxygen atoms of NaX, put in evidence by the formation of carbonate-like species, is strongly decreased by exchanging Na⁺ with Ba²⁺, as in BaX CO₂ is linearly polarised on the cations only. Calculation of the partial negative charge on the framework oxygens (carried out on the basis of the Sanderson's electronegativity equalisation method) confirmed this effect for X systems, and indicated that the exchange of Na⁺ with Ba²⁺ resulted in a decrease in basicity in the case of Y zeolite also, (which even in the sodium form was unable to give rise to carbonate-like species by interaction with CO₂).