CO2 Adsorption/Desorption in FAU Zeolite Nanocrystals: In Situ Synchrotron X-ray Powder Diffraction and in Situ Fourier Transform Infrared Spectroscopic Study

The host-guest and guest-guest interactions governing the CO2 adsorption/desorption in two nanosized zeolite samples with FAU framework type and different Si/Al ratios (Na-X Si/Al = 1.24 and Na-Y Si/Al = 2.54) and cation distribution were investigated by in situ synchrotron high-resolution X-ray powder diffraction (XRPD) and in situ Fourier transform infrared (FTIR) spectroscopy. The two complementary techniques allow probing the CO2 adsorption/desorption in the FAU zeolites at different levels, that is, average structure by XRPD versus local structure by FTIR spectroscopy. The presence of physisorbed CO2 molecules in both zeolites was detected by XRPD, whereas only a high amount of chemisorbed CO2 in the Na X zeolite was found. The presence of unshielded Na cations and H2O molecules in the supercage of the Na X sample induces the formation of stable bidentate bicarbonate groups. Evacuating CO2-loaded samples resulted in the efficient removal of physisorbed CO2 from both nanosized zeolites; on the contrary, high temperature is required to remove the chemisorbed species from the nanosized Na X zeolite. Understanding the CO2 sorption behavior and capacity of nanosized zeolites is of great importance in broadening their use in environmental, clinical, and biomedical applications.