Force-Accelerated Autocatalysis in a Knoevenagel Condensation Reaction during Ball Milling.

The kinetics of a Knoevenagel condensation between vanillin and barbituric acid has recently provided insight into the differences between mechanical and traditional solution-based synthetic methods. The solution-based reaction follows first-order reaction kinetics, while mechanochemical reaction kinetics follow a sigmoid pattern, with a rapid acceleration of reactivity following a slow induction period. Previous works theorize the source of the reaction acceleration to be either particle fracture evolution or changes to the reactant's rheological properties. Here, we examined the reaction kinetics in stainless steel, Teflon, zirconia, and aluminum reaction vessels using different milling frequencies to determine the role of reagent mechanics under varying mechanical environments. Reaction vessels with interchangeable midsections and end-caps of different materials were used to discern the role of jar/ball material surface energy and localized shear vs normal loading forces on the reaction kinetics. The kinetics remained sigmoidal regardless of milling jar/ball materials, milling frequency, and observed rheological changes. Based on a kinetic energy model, the reaction is consistent with a force-accelerated autocatalytic process. High mixing in a low-force environment (Teflon) resulted in conversion ∼8x higher than high-force environments (stainless steel), which also showed more ready formation of solid volumes of product that were not free-flowing, reducing yields, suggesting that optimizing interfacial adhesion, impact force, and the relative amounts of shear vs normal forces yields significant rate improvements. Previously reported "cohesive states" likely arise from conditions of shear flow in reaction systems, making the methods introduced here broadly applicable to a host of mechanochemically accelerated chemical systems.