Programming Local Microenvironments in Reticular Frameworks for Enhanced CO2 Capture: A Demonstration of Spatial Active-Site Engineering
The capture of carbon dioxide (CO2) from dilute streams, such as ambient air or flue gas, is a critical step toward mitigating anthropogenic emissions. While metal-organic frameworks (MOFs) featuring zinc-hydroxide (Zn–OH) sites have shown promise for CO2 binding through bicarbonate formation, their performance is often limited by the spatial arrangement of these active sites. In this work, we demonstrate a reticular chemistry strategy to program the spatial relationship among Zn–OH sites within a shared cavity, moving beyond simple surface area or site density optimization. By designing two isoreticular MOFs, NU-6000 and NU-6001, with distinct pore environments, we achieve differential CO2 adsorption behaviors. Notably, NU-6000-OH, which features a confined cage structure, exhibits significantly enhanced CO2 uptake at low pressures (0.4 mbar) compared to NU-6001-OH, with a site efficiency that surpasses representative MOFs. Structural characterization, including single-crystal X-ray diffraction, reveals the formation of Zn-bound bicarbonate species, confirming the cooperative binding mechanism. This work highlights the importance of the second coordination sphere in governing molecular recognition and suggests that programmed microenvironments could be extended to catalytic applications, such as CO2 reduction, where intermediate stabilization and proton transfer are crucial. Our findings establish reticular chemistry as a powerful tool for engineering local chemical environments, offering a pathway to design advanced sorbents and catalysts with tailored functionalities.