Pressure as the driving force for mechanochemical reactions on the example of ion metathesis of alkali halides upon ball milling

Abstract

We report an in situ X-ray diffraction study of the mechanochemical ion metathesis between sodium iodide (NaI) and potassium chloride (KCl) to form sodium chloride (NaCl) and potassium iodide (KI) upon ball milling in a shaker mill. The data permit insights into the fundamental processes occurring during mechanochemistry. The reaction proceeds in incremental steps upon ball impact and consequently follows pseudo-zero order kinetics after an induction period needed for mixing and reduction of the sizes of the salt crystals. The total energy input required for full conversion is a constant value irrespective of the shaking frequency. Different shaking frequencies imply different average kinetic energies of the milling balls and thus different energy transfer per impact. The time for the total energy transfer to the powder thus varies as a function of the kinetic energy of the balls and number of impacts. At lower shaking frequency, i.e., at lower kinetic energy of the balls and a lower impact rate, the time required for full conversion is simply longer. The data reported provide strong evidence that pressure generated by the impact of milling balls is the driving force for the metathesis reaction rather than a temperature increase. The observed pseudo-zero order kinetics complies well with periodic pressure pulses driving the salt metathesis reaction.