Enthalpy vs. friction: heat flow modelling of unexpected temperature profiles in mechanochemistry of metal-organic frameworks

Mechanochemical reactions by ball milling are becoming increasingly popular across a wide range of chemical sciences, but understanding and evaluation of temperature during such processes remains a persistent challenge, especially for organic and metal-organic materials. Here, we describe the first methodology for precise real-time measurement of sample temperature during mechanochemical transformations. Using this technique coupled with real-time in situ reaction monitoring by synchrotron X-ray diffraction and numerical simulations of heat flow, we have shown that the temperature profiles of mechanochemical reactions are dominantly determined by the energy dissipated through friction between the sample and the moving milling assembly, while the reaction enthalpy will usually be comparatively insignificant. With the changes in composition during mechanochemical reactions, frictional properties of the milled material change, leading to either better or worse energy absorption upon collisions in the process of milling. This approach explains unexpected and rapid temperature drops during exothermic transformations of ZIF-8 polymorphs. Since reaction kinetics are highly sensitive to changes in temperature, precise temperature profiles provided here will be mandatory to understand kinetics and its changes during milling, and will aid in developing the comprehensive model of mechanochemical reactivity.