Adsorption of CO2 in FAU zeolites: Effect of zeolite composition

Abstract

Many technological processes, mainly in the energy sector, require separation of carbon dioxide from gas mixtures. For that purpose medium and large pore zeolites can be used, provided that the differential CO2 adsorption energy allows thermodynamic separation under equilibrium conditions. Hence the convenience to have a precise knowledge (at the molecular level) about the factors that control CO2 interaction with zeolites, and faujasite-type zeolites in particular, are most relevant in this context on account of their relatively high adsorption capacity. We report on a detailed spectroscopic, calorimetric and theoretical study on the effect of composition on equilibrium CO2 adsorption in alkali-metal exchanged faujasite-type zeolites, which, by combining experimental results with calculations performed at the DFT/CC level on a periodic model of the zeolite yields fine details on the CO2 adsorption complexes and corresponding gas–solid interaction energy. The results obtained are discussed in the broader context of other literature reports; showing, in particular, how the DFT/CC computational approach gives interaction energy values that are in better agreement with experimental data than those obtained using some other computational methods, which show larger limitations to account properly for dispersion interactions. We found out that: (i) dispersion interactions account for about 50% of the overall adsorption enthalpy of CO2 molecules in FAU zeolites, (ii) a very low (experimentally non-detectable) population of sites III and III′ was found for FAU zeolite with Si/Al ratio 2.55:1 and all CO2 molecules are adsorbed on sites II and are tilted toward the zeolite wall due to the stabilizing effect of dispersion interactions between CO2 and zeolite, and (iii) minor heterogeneity of adsorption sites present in the FAU samples originating from differences in the number and geometry of Al atoms in the 6R of sites II.