Full-field X-ray absorption tomography reveals the chemical structure of defects in metal-organic frameworks

A better understanding of the relationship between defect-engineered MOF synthesis conditions and the structure of the resulting material is essential. Through the use of full-field X-ray absorption near-edge structure microscopy and tomography, we were able to visualize the chemical heterogeneity in defect-engineered HKUST-1 MOF single crystals. Tomographic analysis revealed a spatially non-uniform incorporation of the defective linker, leading to cluster formation of a secondary coordination polymer throughout the crystals. These clusters differ from the surrounding HKUST-1 framework in metal concentration and copper coordination. Although the frequency of clusters is positively correlated with the defective linker concentration in the crystallisation environment, clusters can equally be found in HKUST-1 crystals synthesized in absence of a defective linker. This suggests that secondary coordination polymer formation is associated with a post-synthesis collapse of the MOF lattice driven by an increased density of crystallographic defects and in turn enhanced by the zonation of defective linkers.

The visualisation of heterogeneities, including the detection of secondary phases of reduced copper coordination, within entire defect-engineered MOF crystals, allowed us to provide an unprecedented point of view towards explaining the structure-property relationship of defect-engineered MOFs and introduce full-field X-ray absorption near-edge structure microscopy and tomography as a powerful tool to investigate the chemistry of such important materials and characterize MOFs in the 3D space.