Exploring cluster formation in Zr-MOF synthesis in situ using X-ray absorption spectroscopy

Metal-organic frameworks (MOFs) with a Zr-oxo cluster [(Zr₆O₄(OH)₄)]₁₂^- are exceptionally stable and offer vast potential for a wide range of applications. Synthesis parameters strongly affect the quality, stability, morphology, etc., of the MOFs. In this study, we present an experimental set-up that allows following in situ element-specific chemical transformations during synthesis and the effect of various reaction parameters on the reaction products. Zr-fumarate formation was monitored using X-ray absorption spectroscopy (XAS) in ZrCl₄-DMF-based solutions. We have studied (i) the local (∼2-5 Å) environment around Zr⁴⁺ ions in the early stages of the Zr-oxo cluster formation and MOF-synthesis reaction, (ii) kinetics of the synthesis reaction and its dependence on water and modulator concentrations, and (iii) the effect of the reaction parameters on MOF product quality. XAS data have provided direct evidence that the increased amounts of water and modulator accelerate the reaction. It has also confirmed that water is essential for MOF formation in DMF. Zr-cluster formation and the synthesis reaction were not observed in the absence of water in ZrCl₄-DMF and ZrCl₄-DMF-linker-modulator solutions. According to the EXAFS data, Zr⁴⁺ ions are octahedrally coordinated by chlorine atoms in anhydrous solutions with and without the linker. In contrast, the Zr-oxo clusters and MOFs were formed only in the presence of water. The results of the in situ experiments were correlated with post-synthetic characterization of the resultant MOF products.