Extra-framework zirconium clusters in metal organic framework DUT-67 controlled by the choice of the metal precursor

DUT-67 zirconium-thiophenedicarboxylate (TDC) MOFs have been synthesized in the presence of formic acid from different zirconium precursors. The materials formed have been characterized by X-ray diffraction, nitrogen sorption, scanning electron microscopy, and thermal gravimetry. Rietveld structure refinement has permitted to localize inside the microporosity zirconium and oxygen atoms of extraframework clusters (EFCs), whose conformation depended on the nature of the zirconium precursor. Dissolution-condensation processes of ZrCl4 and ZrOCl2 during the synthesis gave rise to Zr6 EFCs in one of every six cuboctahedral pores. Less numerous zirconia-like Zr12 oxo-clusters were formed from Zr isopropylate, whereas characteristic Zr8 nitro-clusters were formed from Zr oxonitrate. The distances between oxygen atoms of EFCs and framework were compatible with the presence of additional TDC linkers connected to the EFCs. The results have been interpreted in terms of competition between Zr species during the condensation of the MOF network.