UiO-66: structure, synthesis and properties

UiO-66 is built from two parts. The node is a Zr₆O₄(OH)₄ cluster: six zirconium atoms at the corners of an octahedron, with four oxide and four hydroxide ions capping its eight faces. Twelve carboxylate groups bridge its twelve edges, so the cluster is a 12-connected building unit (Valenzano et al. 2011).

The linker is terephthalate (BDC²⁻), the same linker as in MOF-5. Each linker joins two clusters, and each cluster takes twelve linkers. The clusters sit like the atoms of a face-centred cubic metal, giving the 12-connected fcu net (Wu et al. 2013). The cell is cubic (Fm-3m, a ≈ 20.7 Å) and holds four clusters.

Like a face-centred cubic metal, the framework has two kinds of cavity: larger octahedral cages and smaller tetrahedral cages. When UiO-66 is heated, the cluster loses its μ₃-OH groups as water and becomes Zr₆O₆; the change is reversible and does not disturb the carboxylates, which is part of why the framework is so stable (Cavka et al. 2008; Valenzano et al. 2011).

1. Solvothermal synthesis. Zirconium tetrachloride (ZrCl₄) and terephthalic acid are dissolved in N,N-dimethylformamide (DMF), sealed and heated at 120 °C for 24 h (Cavka et al. 2008). Small octahedral crystals form.
2. Modulation. A monocarboxylic acid such as acetic or benzoic acid is often added. It competes with terephthalate for the Zr₆ clusters: with more modulator the product changes from intergrown to separate crystals of tunable size, and water is needed for well-ordered products (Schaate et al. 2011).
3. Washing and activation. The solid is washed, the DMF in the pores is exchanged for a volatile solvent, and the pores are emptied by heating under vacuum.
4. Other routes. UiO-66 has been made at the half-kilogram scale under reflux (Ragon et al. 2014) and by ball milling with little solvent (Užarević et al. 2016).

Most early MOFs were fragile: heat, water or mechanical stress broke them down. UiO-66 introduced the Zr₆ cluster, a building unit whose strong Zr–O bonds give frameworks with a decomposition temperature above 500 °C, resistance to most chemicals, and crystallinity after 10 tons/cm² of external pressure (Cavka et al. 2008). The same cluster with longer linkers gives UiO-67 and UiO-68 (Cavka et al. 2008; Schaate et al. 2011).

UiO-66 is also the model system for defects in MOFs. Real samples have linkers missing: neutron diffraction found about 10% missing in a typical sample, and varying the acetic acid modulator and the synthesis time tunes the vacancies, raising the BET surface area from 1,000 to 1,600 m² g⁻¹ (Wu et al. 2013). In modulated samples, missing clusters were found to be the predominant defect, their number set by the concentration and acidity of the modulator (Shearer et al. 2016). Thermogravimetry counts the linkers: for ideal Zr₆O₆(BDC)₆ the mass before the linkers burn is 2.20 times the ZrO₂ residue (calculated from the formulas), and it is lower when linkers are missing (Shearer et al. 2016).

• Catalysis. Missing-linker sites expose zirconium ions that act as Lewis acids, and modulated synthesis raises the catalytic activity of UiO-66 (Vermoortele et al. 2013).
• Gas adsorption. Missing linkers change how UiO-66 takes up gases, CO₂ in particular, and the hydroxylated form adsorbs better than the dehydroxylated one (Wu et al. 2013).
• Functional fabrics. The amino version, UiO-66-NH₂, grown on polymer fibres catalyses the hydrolysis of chemical warfare agent simulants (Lee et al. 2017).
• Defect engineering. Modulators, water and time set the crystal size and the number of defects, so UiO-66 is the standard system for studying how defects change porosity and reactivity (Schaate et al. 2011; Shearer et al. 2016).

For a paper, slide or poster, show the Zr₆O₄(OH)₄ cluster (often as a green octahedron) with its twelve carboxylates, one terephthalate linker, and the fcu net, with a defect marked if your work is about missing linkers. The two figures above, the defect-engineering figure and the ZIF-8/UiO-66 synthesis figure, open as editable LabCanvas figures. The crystal structure SVG is drawn from the UiO-66 crystal file by the LabCanvas figure editor, where you can switch between atoms and polyhedra and change the number of cells; the 3D model on this page opens in the LabCanvas 3D Studio, where you can turn it and export a high-resolution image.