Why does CaX zeolite have such a high CO2 capture capacity and how is it affected by water?

Abstract

CO2 capture is important in solving one of the major current environmental problems, namely global warming. Among the different approaches, adsorption is considered the most promising and CaX zeolite is one of the materials with the highest capacity. However, the knowledge on the mechanism of its interaction with CO2 is still not complete which hinders further optimization. In this work we report the results of a combined study of CO2 adsorption on a CaX zeolite by in-situ FTIR spectroscopy, adsorption measurements and DFT calculations. It was found that at ambient temperature CO2 initially forms linear Ca2+−OCO species which, with increasing equilibrium pressure, are first converted to Ca2+(CO2)2 and then to Ca2+(CO2)3 species. This is the reason for the very high CO2 adsorption capacity of the material. Because of the practical importance, the effect of H2O on the processes was also studied. Water suppress CO2 adsorption but the effect is softened by the formation of mixed ligand Ca2+(CO2)x(H2O)y complexes (x + y = 2 or 3). In agreement with the spectroscopic results it was found that the adsorption capacity of the material increases with the rise of activation temperature up to 673 K due to dehydration. Its value, 22.2 wt% at 273 K and 490 mbar, is among the highest values reported in the literature at similar conditions.