Be all ears to the bead’s tempo: development of a new operando technique to understand what is happening during ball-milling reactions César Leroy, Sébastien Mittelette , Gautier Félix, Nicolas Fabregue, Jessica Špačková, Philippe Gaveau, Thomas-Xavier Métro and Danielle Laurencin ICGM, Univ Montpellier, CNRS, France Mechanochemistry has been used for years as a “green” synthetic method in different domains such as catalysis, crystal engineering or materials science. However, although it makes no doubt that elaborate and valuable molecules and materials can be obtained by this technique, scientists often still lack an understanding of the physical and chemical mechanisms occurring during the mechanical process. In the case of ball-milling, reactions occur in closed jars, mainly opaque, which makes reaction processes difficult to follow. In order to go further, several research groups have developed in-situ and operando analytical methods to monitor reactions and observe evolutions in the structure, crystallinity and/or texture of materials involved in the milling process. Most of these techniques, such as Raman spectroscopy, powder X-Ray diffraction, and solid state NMR, require jars of specific composition and/or geometry to be used. 1-6 Nevertheless, several studies have shown that the size and materials composing the reactor and beads greatly affect the kinetics and the reaction mechanism. 7 In this context, there is an important urge to develop new in-situ/operando analytical techniques that can be used whatever the milling system is. Here, we will present a method based on recording the sound during the milling to help determine what kind of movements the beads can have in the jar, and show if/how this can be related to different physico-chemical changes of the reaction medium. 8 We tested this method on three different reactions: the synthesis of a co-crystal (terephtalic acid + 1,4-diazabicyclo[2.2.2]octane), the hydrolysis of fumed silica, and the hydrolysis of activated- lauric acid. In order to improve our comprehension of these systems, the sound measurements were coupled with other operando techniques such as Raman spectroscopy, and/or temperature measurements. All millings were performed on a vertical mixer mill, and in each case correlations were established between the acoustic variations arising from different movements of the beads to physical and/or chemical changes. Lastly, our attempts to expand this method to other kinds of milling systems will be presented. References 1. D. Gracin et al., Angew. Chem., 2014, 126, 6307– 6311. 2. L. Batzdorf et al., Angew. Chem., Int. Ed., 2015, 54, 1799–1802. 3. H. Kulla et al., Molecules, 2016, 21, 917. 4. S. Lukin et al., Chem. - A Eur. J., 2017, 23, 13941–13949. 5. I. Halasz et al., Nat. Protoc., 2013, 8, 1718–1729. 6. J. G. Schiffmann et al., Solid State Nucl. Magn. Reson., 2020, 109, 101687. 7. L. S. Germann et al., Chem. Sci., 2020, 11, 10092–10100. 8. C. Leroy et al., Chem. Sci., 2022,13, 6328-6334.
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