Breaking new ground in direct mechanocatalysis: Knoevenagel condensation via supported organo-catalysts on zirconia

Abstract

Mechanocatalysis combines mechanical energy and chemical reactivity to perform solvent-free catalytic transformations. Direct mechanocatalysis involves using reactor-supported catalysts to boost reaction performances while avoiding solvents and energy-consuming post-reaction purification steps. This process aims at increasing the reaction yield and decreasing drastically the E-factor. Herein, we report the first examples of direct mechano-organocatalysis with a piperazine-based organocatalyst, covalently grafted onto amine-functionalized zirconia milling balls. This unprecedented milling system catalyzed Knoevenagel condensations under solvent-free conditions, operating faster (with a thousand-fold less catalyst used than that in traditional methods), achieving full conversion within only3 hours, and remained active after multiple reaction cycles. The turnover frequency (TOF) reached 5700 h⁻¹, far exceeding that of homogeneous analogues (40 h⁻¹), due to the low catalyst loading and efficient energy transfer. Comprehensive surface characterization of the milling balls (XPS and original PM-IRRAS), before and after grafting, and after catalysis, elucidated the structure–activity relationship. This work establishes the first demonstration that a robust organocatalyst can be efficiently used in supported mechanocatalysis, highlighting the promise of surface-engineered zirconia systems for green chemistry.