Acidic and basic sites in NaX and NaY faujasites investigated by NH₃, CO₂ and CO molecular probes

The acid-base properties of samples of NaY and NaX faujasites have been investigated by adsorbing different probe molecules and measuring IR spectra. In the case of the NaY sample, only Na⁺ ions were found to be involved in the adsorption of CO₂ and CO, confirming the overwhelming Lewis acid character of this material. In contrast, carbonate-like species were formed by adsorbing carbon dioxide on the NaX sample, due to the reaction of basic framework oxygen atoms with CO₂ molecules polarised on neighbour Na⁺ ions. The spectroscopic analysis of NaX also showed evidence of Brønsted acid hydroxyls and OH groups bonded to extra-framework Al atoms. Ammonia adsorption revealed that the amount of Brønsted acid hydroxyls is significantly lower than the Lewis acid Na⁺ countercations. Moreover, small oxide particles, carrying carbonate-like species on their surface, are present in the zeolitic cavities. These particles could be responsible for the basic reactivity towards CO observed after outgassing the NaX sample at high temperature.