Abstract
We report an investigation of the “missing-linker phenomenon” in the Zr-based metal–organic framework UiO-66 using atomistic force field and quantum chemical methods. For a vacant benzene dicarboxylate ligand, the lowest energy charge-capping mechanism involves acetic acid or Cl–/H2O. The calculated defect free energy of formation is remarkably low, consistent with the high defect concentrations reported experimentally. A dynamic structural instability is identified for certain higher defect concentrations. In addition to the changes in material properties upon defect formation, we assess the formation of molecular aggregates, which provide an additional driving force for ligand loss. These results are expected to be of relevance to a wide range of metal–organic frameworks.
Highlights
Metal−organic frameworks (MOFs) are materials formed via the coordination of metal centers and organic linkers in three dimensions
Acetic acid (CH3COOH) and/or HCl are commonly used as acidic modulators to promote linker removal from the structure
Our findings suggest that a small concentration of water during synthesis may increase the number of linker vacancies within the material
Summary
Metal−organic frameworks (MOFs) are materials formed via the coordination of metal centers and organic linkers in three dimensions. The varied chemical compositions and structural topologies of MOFs make them suitable for a broad range of applications including gas storage and separation, solar energy conversion, and heterogeneous catalysis.[1−6]. One MOF that has attracted particular attention is UiO-66 (Figure 1), which was first synthesized by Cavka et al.[7] This material features a high coordination of 12 benzene-1,4dicarboxylate (BDC) ligands around each ZrIV node and is thermally stable up to 813 K.8. Each octahedral cage is edge-sharing with eight tetrahedral cages and face-sharing with eight octahedral cages.[9] The inner-sphere coordination of Zr in UiO-66 is 6, but additional face-sharing oxide and hydroxide ligands lead to an outer-sphere coordination of 12
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