Controlling the Mechanism of Nucleation and Growth Enables Synthesis of UiO-66 Metal-Organic Framework with Desired Macroscopic Properties

By combining in situ X-ray diffraction, Zr K-edge X-ray absorption spectroscopy and ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, we show that the properties of the final MOF are influenced by H₂O and HCl via affecting the nucleation and crystal growth at the molecular level. The nucleation implies hydrolysis of monomeric zirconium chloride complexes into zirconium-oxo species, and this process is promoted by H₂O and inhibited by HCl, allowing to control crystal size by adjusting H₂O/Zr and HCl/Zr ratios. The rate-determining step of crystal growth is represented by the condensation of monomeric and oligomeric zirconium-oxo species into clusters, or nodes, with the structure identical to that in secondary building units (SBU) of UiO-66 framework. The rapid crystallization in the absence of HCl leads to formation of defective secondary building units with missing zirconium atoms, providing a pathway to control the number of defects in UiO-66 crystals. Remarkably, we have shown that assembling of the metal nodes and linkers into the UiO-66 structure is not the rate-limiting step, and the degree of deprotonation of the linker has no direct effect on the crystallization kinetics or crystal size of product.