Effect of stressing conditions on mechanochemical Knoevenagel synthesis

The study examines the interaction of process conditions of a mechanochemical process in a planetary ball mill. The investigated Knoevenagel condensation of vanillin and barbituric acid exhibits sigmoidal feedback kinetics under solvent free conditions. The temperature development in the non-cooled chamber increases the reaction velocity along the process. In contrast to mere grinding processes that are determined by the stressing intensity, the mechanochemical process is thus mainly dependent on the power input which is connected to the heating rate. Consequently, significantly different temperatures are obtained by the time of full conversion depending on the process parameters. Finally, lowest energy demands are reached at shortest reaction times for the Knoevenagel condensation in a planetary ball mill. However, a thermal treatment of educt powders under steady mixing but without particle refinement do not result in a sufficient yield. The feedback kinetics are attributed to the improved crystallization induced by the cumulating reaction product and to the significant temperature increase as function of heating rate and process time.