Controlled alkali etching of MOFs with secondary building units for low-concentration CO(2) capture

Low-concentration CO(2) capture is particularly challenging because it requires highly selective adsorbents that can effectively capture CO(2) 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 CO(2) capture experiments demonstrated improved CO(2) capture performance of the MOF-808-X series compared to the pristine MOF-808 under dry conditions (400 ppm CO(2)). Among them, MOF-808-0.07 with abundant Zr–OH sites showed the highest CO(2) 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 CO(2) 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 (13)C CP-MAS ssNMR characterizations revealed that the enhanced low-concentration CO(2) 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.