Can two CO2 molecules be simultaneously bound to one Na+ site in NaY zeolite? A detailed FTIR investigation

Efficient CO2 capture is necessary to solve the current environmental problems of global warming. Porous materials are promising for this purpose and adsorption capacity is a critical issue. Large cations in zeolites can often attach two small molecules, but it is not yet well established whether this applies to CO2. Here we present a detailed FTIR spectroscopic study of CO2 adsorption on NaY zeolite. At ambient temperature and pressures up to 50 mbar, Na+⋯O–C–O species are formed and the CO2 molecules are tilted, which allows interaction of the C-atom with framework oxygen. At higher pressure or at lower temperature reorientation of the CO2 molecules occurs, leading to a significant decrease in the extinction of the ν1 and 2ν2 bands and to a gradual red shift of the ν3(13CO2), (ν1+ν3) and (2ν2+ν3) modes with 3 cm−1. At lower temperature, two additional conversions occur: (i) formation of (Na+⋯O–C–O)2 dimers, causing an additional 2-3 cm−1 red shift of the combination bands, and (ii) attachment of a second CO2 molecule to the same Na+ site, resulting in a discrete red shift of ν3(13CO2), (ν1+ν3) and (2ν2+ν3) with 10 cm−1. Addition of N2 or CO to pre-adsorbed CO2 leads to compression of the formal CO2 adsorption layer. The obtained results demonstrate the possibility of simultaneous binding of two CO2 molecules to one cationic site in zeolites which opens up new horizons in the design of high-capacity CO2 adsorbents.