Imaging defects and their evolution in a metal–organic framework at sub-unit-cell resolution

Defect engineering of metal–organic frameworks (MOFs) offers promising opportunities for tailoring their properties to specific functions and applications. However, determining the structures of defects in MOFs—either point defects or extended ones—has proved challenging owing to the difficulty of directly probing local structures in these typically fragile crystals. Here we report the real-space observation, with sub-unit-cell resolution, of structural defects in the catalytic MOF UiO-66 using a combination of low-dose transmission electron microscopy and electron crystallography. Ordered ‘missing linker’ and ‘missing cluster’ defects were found to coexist. The missing-linker defects, reconstructed three-dimensionally with high precision, were attributed to terminating formate groups. The crystallization of the MOF was found to undergo an Ostwald ripening process, during which the defects also evolve: on prolonged crystallization, only the missing-linker defects remained. These observations were rationalized through density functional theory calculations. Finally, the missing-cluster defects were shown to be more catalytically active than their missing-linker counterparts for the isomerization of glucose to fructose. Structural defects are known to exist in metal–organic frameworks (MOFs), and to affect the materials’ properties, but their exact structures have remained difficult to determine. Now, missing-linker and missing-cluster defects have been observed in a MOF using low-dose transmission electron microscopy, enabling their distributions, evolutions during crystallization and effects on the material’s catalytic activity to be explored.