Flexibility-frustrated porosity for enhanced selective CO2 adsorption in an ultramicroporous metal-organic framework

Selective CO₂ capture from industry is crucial for reducing emissions from fossil fuel combustion. Flexible metal-organic frameworks (MOFs) have shown promise for CO₂ adsorption via differential binding and size-exclusion mechanisms. However, achieving precise pore-size control to selectively capture CO₂, particularly in the presence of N₂ and water, remains a challenge. Here, we demonstrate a strategy for frustrating framework flexibility in a MOF to create an optimal, confined pore environment that enhances selective CO₂ recognition while maintaining high working capacity. We designed a flexible MOF, Cambridge University (CU)-4, by using a bulky cubane-derived ligand and In³⁺ ions that undergo dynamic breathing with a 2 Å contraction upon solvent exchange and removal. In situ synchrotron X-ray diffraction and molecular simulations reveal that the stable narrow-pore configuration creates a hydrogen-rich cavity that selectively binds CO₂ via multiple hydrogen bonds. This physisorption-based CO₂ recognition remains effective even at 80% humidity, making CU-4 promising for post-combustion carbon capture.