Unraveling the Role of Open Metal Sites and Their Capping Ligands in MOFs Stability and Liquid-Phase Separation

The stability of metal–organic frameworks (MOFs) is crucial for their practical applications. Open metal sites (OMS), essential active sites in various applications, also significantly impact stability, yet their role remains insufficiently understood. This study investigates MOF-808, a zirconium-based MOF with six OMS per cluster, revealing its higher stability in liquid water compared to gaseous water. Coordination of formic acid at OMS notably enhances water stability, while recrystallization experiments elucidate the mechanisms of instability in gaseous water. Thermal stability is determined by the decomposition temperature of capping ligands on OMS. In liquid-phase adsorption of high-value bio-oil compound creosol, uptake results demonstrate that capping ligand polarity modulates pore hydrophilicity/hydrophobicity, affecting adsorption selectivity. Remarkably, MOF-808 capped with pentafluorobenzoic acid achieves 100% selective pore occupancy during adsorption. This study highlights OMS as critical to MOF stability and emphasizes the role of capping ligands in improving stability and tuning adsorption properties. It underscores nonstructural ligand modifications as a powerful strategy to enhance MOF performance, broadening their application potential in challenging environments.