Mechanochemical syntheses, e.g., by milling or grinding, can be used to reduce the need for solvents and to allow reactions that are challenging in solution. These processes usually need milling or grinding media or rotating screws to exert the required forces on the reactants. However, ultrasonic or acoustic waves could be used instead—a process called resonant acoustic mixing, or RAM. This could make mechanochemical syntheses simpler and easier to scale up.

Tomislav Friščić, McGill University, Montreal, Canada, and colleagues have developed a mechanochemical method for the synthesis of metal–organic frameworks (MOFs) that uses liquid-assisted resonant acoustic mixing (LA-RAM). The team made common MOFs such as ZIF-8 or HKUST-1, as well as rarer MOFs such as the metastable ZIF-L, without bulk solvent or any grinding media. They combined the respective reactants with small amounts of MeOH and NH₄NO₃ and agitated the mixture using a RAM system at a frequency of 60 Hz.

The team found that the liquid additive improves the efficiency of RAM for MOF synthesis and allows the preparation of both stable MOFs and ones that were previously inaccessible by other mechanochemical methods, such as ZIF-L. The researchers attribute this to LA-RAM being a milder method. The process can be scaled up to at least tens of grams of product.

• Simple, scalable mechanosynthesis of metal organic frameworks using liquid-assisted resonant acoustic mixing (LA-RAM),
  Hatem M. Titi, Jean-Louis Do, Ashlee J. Howarth, Karthik Nagapudi, Tomislav Friščić,
  Chem. Sci. 2020.
  https://doi.org/10.1039/d0sc00333f