Controlled alkali etching of MOFs with secondary building units for low-concentration CO₂ capture

Low-concentration CO₂ capture is particularly challenging because it requires highly selective adsorbents that can effectively capture CO₂ from gas mixtures containing other components such as nitrogen and water vapor. In this study, we have successfully developed a series of controlled alkali-etched MOF-808-X (where X ranges from 0.04 to 0.10), the FT-IR and XPS characterizations revealed the presence of hydroxyl groups (-OH) on the zirconium clusters. Low-concentration CO₂ capture experiments demonstrated improved CO₂ capture performance of the MOF-808-X series compared to the pristine MOF-808 under dry conditions (400 ppm CO₂). Among them, MOF-808-0.07 with abundant Zr-OH sites showed the highest CO₂ capture capacity of 0.21 mmol g^-1 under dry conditions, which is 70 times higher than that of pristine MOF-808. Additionally, MOF-808-0.07 exhibited fast adsorption kinetics, stable CO₂ capture under humid air conditions (with a relative humidity of 30%), and stable regeneration even after 50 cycles of adsorption and desorption. In situ DRIFTS and ¹³C CP-MAS ssNMR characterizations revealed that the enhanced low-concentration CO₂ capture is attributed to the formation of a stable six-membered ring structure through the interaction of intramolecular hydrogen bonds between neighboring Zr-OH sites via a chemisorption mechanism.