Metal–organic frameworks (MOFs) are crystalline, porous materials composed of metal centers and organic linkers. They have applications, e.g., in catalysis or gas separation. Zirconium-based MOFs are commonly used in catalysis. To slow the rate of growth in Zr-MOFs and allow larger crystals to form, growth-modulator ligands such as acetate, trifluoroacetate, or benzoate can be used. These ligands can be removed during activation of the MOF. However, decomposition of the solvent molecules during this process can replace the modulator ligands with Zr-node-blocking formate ligands, and thus, hamper catalytic performance.

Zhiyong Lu, Hohai University, Nanjing, China, and Northwestern University, Evanston, IL, USA, Joseph T. Hupp, Northwestern University, and colleagues have developed an alternative activation protocol that effectively removes modulator ligands and avoids the formation of formate ligands. Instead, the sites not occupied by linkers are occupied by easily displaceable OH/H₂O pairs. The team converted NU-1000, a MOF with 1,3,5,8-(p-benzoate)pyrene (TBAPy^4–) linkers, into formate-free NU-1000 (called NU-1000-FF) via a chloride-containing intermediate (called NU-1000-FF-Cl).

First, the researchers synthesized NU-1000 from ZrOCl₂·8H₂O, benzoic acid, H₄TBAPy, and trifluoroacetic acid (TFA). The as-synthesized MOF was then solvent-exchanged with dimethyl sulfoxide (DMSO) and dispersed in DMSO and aqueous HCl. The resulting NU-1000-FF-Cl was dried and then thermally activated. The intermediate NU-1000-FF-Cl was solvent-exchanged with ethanol and washed with ethanol and triethylamine to remove the chlorine. The resulting free NU-1000-FF was dried and activated at 120 °C.

The team found that the intermediate NU-1000-FF-Cl has eight aqua ligands and four non-ligated chlorides. In the free NU-1000-FF, the non-linker ligands are fully replaced by OH/H₂O pairs. The aqua ligands are easier to remove and free Zr sites can be accessed at lower temperatures. This makes NU-1000-FF much more catalytically active than the analogous formate-blocked MOF.

• Node-Accessible Zirconium MOFs,
  Zhiyong Lu, Jian Liu, Xuan Zhang, Yijun Liao, Rui Wang, Kun Zhang, Jiafei Lyu, Omar K. Farha, Joseph T. Hupp,
  J. Am. Chem. Soc. 2020.
  https://doi.org/10.1021/jacs.0c09782