Benign by Design: Green and Scalable Synthesis of Zirconium UiO-Metal-Organic Frameworks by Water-Assisted Mechanochemistry

We present a solvent-free, green, and rapid mechanochemical route for the synthesis of a series of zirconium metal-organic frameworks (MOFs) composed of Zr₆ cluster nodes, UiO-66, UiO-66-NH₂, MOF-801, and MOF-804, both on a laboratory scale and by scalable and flow mechanochemical processing. The methodology, based on the use of a nonconventional zirconium dodecanuclear acetate cluster and a minute amount of water as an additive, affords high-quality MOFs in less than 1 h of milling, with minimal requirements for workup processing and eliminating the need for conventional hazardous solvents, such as dimethylformamide. Moreover, the use of a dodecanuclear zirconium acetate precursor circumvents the need for modulators resulting in acetic acid as the only byproduct of the reaction, which does not harm these acid-resistant materials. The porosity, thermal and chemical stability, as well as catalytic activity of mechanochemically prepared Zr-based MOFs are similar to those of solvothermally synthesized counterparts. Finally, the synthesis is readily applicable on a 10 g scale by using a planetary mill, and is also performed by solid-state flow synthesis using twin-screw extrusion (TSE), affording more than 100 g of catalytically active UiO-66-NH₂ material in a continuous process at a rate of 1.4 kg/h.