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.