Node Distortions as a Means of Defect Engineering in Zr-Based MOFs.

Defect engineering in Zr-based metal-organic frameworks (Zr-MOFs) has focused primarily on missing-linker defects. However, recent studies suggest that node dehydroxylation-which creates distortions and coordinatively unsaturated Zr sites (Zrcus)-may have a more significant impact on properties. The present work uses pair distribution function (PDF) and thermogravimetric analysis coupled with systematic defect manipulation to study the effect of node dehydroxylation and missing-linker defects in UiO-66. By employing rapid heat treatment (RHT) under humid flow, we tracked the transition from high-symmetry [Zr6O4(OH)4]12+ to distorted [Zr6O6]12+ nodes. This structural evolution significantly improves As(V) uptake, whereas increasing the number of missing linkers-via chemical treatment or RHT of mixed-ligand frameworks-fails to enhance performance. Crucially, our detection of distorted nodes in as-synthesized UiO-66 also raises the possibility that these defects were silently present in many earlier studies that span various applications, where their role in governing performance may have been inadvertently overlooked. The present study challenges the prevailing "missing-linker" paradigm and establishes cluster dehydroxylation as a defect-engineering strategy to enhance Lewis-acidic performance in Zr-MOFs.