On the mechanism of adsorption and separation of CO2 on LTA zeolites: An IR investigation

The adsorption of CO2 on the zeolites 3A (K-LTA), 4A (Na-LTA) and 5A (Ca,Na-LTA), all with Si/Al a.r. = 1, has been investigated by IR spectroscopy. CO2 adsorption on 3A zeolite is mostly limited at the external surface, both in the form of linear molecular species and of carbonate-like species. In the case of 4A and 5A zeolites, which are applied in the industry for CO2 separation, the adsorption of both linear molecular species and carbonate species occurs mostly in the cavities. The adsorption in the form of carbonates is definitely stronger than that of linear molecules. Much more carbonate-like species are formed on 4A than on 5A zeolite. This is explained with the partial poisoning of the cations of 5A zeolite in the form of calcium hydroxyl and carbonate species, which are already present in the sample after activation. The relevance of the participation of framework oxygen species in adsorption in metal exchanged zeolites is shown.