Synthesis of two new Hf-MOFs with UiO-66 and CAU-22 structure employing 2,5-pyrazinedicarboxylic acid as linker molecule.

Abstract

Two different metal-organic frameworks containing 2,5-pyrazinedicarboxylate (PzDC2−) as linker molecule and hexanuclear {Hf6(μ3-O)4(μ3-OH)4} clusters were obtained under solvothermal reaction conditions at 120 °C. Formic acid as the solvent leads to the formation of Hf-UiO-66-PzDC ([Hf6(μ3-O)4(μ3-OH)4(OH)4(H2O)4(PzDC)4]), while the reaction in a water/formic acid mixture resulted in the crystallisation of Hf-CAU-22-PzDC ([Hf6(μ3-O)4(μ3-OH)4(μ-OH)2(OH)4(H2O)4(PzDC)3]). The UiO-66 compound is a new member of the large class of UiO-66-type MOFs. Hf-CAU-22 contains a chain of condensed {Hf6O4(OH)4(μ-OH)4} clusters as the IBU, which is new to Hf-MOF chemistry. For both MOFs, a detailed characterization, i. e. structure refinement from powder X-ray diffraction data, 1H-NMR-spectroscopy, elemental and thermogravimetric analyses and N2 sorption measurements, was carried out. Hf-UiO-66-PzDC undergoes a dehydration around 215 °C and framework decomposition starts at 315 °C. From N2 sorption measurements, a maximum BET surface area of 460 m2/g was determined. The dehydration of Hf-CAU-22-PzDC starts at 200 °C and the framework decomposition at 325 °C. The BET surface area was determined to 170 m2/g.