Mechanochemistry is widely used for the synthesis of new organic, metal-organic, and inorganic compounds. The success of mechanochemistry is based on fast and quantitative reactions together with a simple implementation. Furthermore, the reduction of solvents makes this method more environmentally friendly as compared to solution chemistry. We present an in situ triple coupling of synchrotron X-ray diffraction with Raman spectroscopy, and thermography to study milling reactions in real time. This combination of methods allows a correlation of the structural evolution with temperature information. The temperature information is crucial for understanding both thermodynamics and reaction kinetics. Exemplarily, the reaction mechanism of three prototypical mechanochemical syntheses, a cocrystal formation, a C-C bond formation, and the formation of metal-organic compound, a manganese-phosphonate, were elucidated. General trends regarding the temperature development during milling could be identified. The heat of reaction and latent heat of crystallization of the product contribute to the overall temperature increase. A decrease in temperature occurs via release of e.g. water as a by-product. The three independent methods complement each other thus maximizing the gain of information and enabling a detection of solid and liquid intermediates. As a generally applicable combination of analytical methods, information can be collected simultaneously on the crystalline, molecular, and temperature state of the material during grinding. Therefore, a correlation of the chemical composition with even slight changes in temperature is possible. In the beginning of the reaction, the impact of the balls induces mixing and particle size reduction of the reactants. Both phenomena generally lead to a rapid increase in temperature. An additional increase in temperature is provided by the chemical energy from the heat of reaction and the latent heat of crystallization of the product. Although general trends are observed, the chosen reactions clearly reflect a diversity of mechanochemical reaction pathways. Temperature effects in ball milling syntheses have just begun to be considered and will become even more important for the optimization of reaction conditions in the future of mechanochemistry.

Real-time <i>in situ</i> investigations of mechanochemical reactions

256th National Meeting (2018)

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Real-time in situ investigations of mechanochemical reactions