Résumé
Mechanocatalysis combines mechanical energy and chemical reactivity to perform chemical transformations without solvents. Zirconia and especially yttria-stabilized zirconia (3Y-TZP) is commonly used as balls and jar material for mechanochemical reactors due to its excellent mechanical and chemical resistance. The challenge is to develop zirconia balls with supported organic catalysts resistants to shocks and friction generated in mechanochemical conditions. This new feature would open up unexplored routes for chemical synthesis. First examples of direct organo-mechanocatalysis with a piperazine-based organocatalyst, covalently grafted onto amine-functionalized zirconia milling balls are reported. The amine surface coverage was optimized up to 27 NH2/nm² with a multistep strategy : initial grafting of phosphonic acid on zirconia, amide bonding via a linker, and subsequent reaction of the remaining surface anhydride groups of the linker with the primary amines of a fourth generation PAMAM dendrimer, which contains 64 terminal primary amines per molecule. This unprecedented system catalyzed Knoevenagel condensations under solvent-free conditions, operating with thousand-fold less catalyst than traditional methods, achieving full conversion within 3 hours only. Its robustness was demonstrated with a constant catalytic activity after several cycles (more than 10 runs). The turnover frequency (TOF) reached 5700 h⁻¹, far exceeding that of homogeneous analogues (40 h⁻¹) due to catalyst low loading and to efficient energy transfer. Several aldehydes featuring a wide range of electronic substituents (electron-donating groups such as -OMe and -OH, and electron-withdrawing groups such as -Cl and -NO2) and melting points (from -26 to 116 °C) were successfully converted with 100% conversion and very high isolated yields (from 95% to 99%). 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 prepared and used in supported mechanocatalysis, highlighting the promise of surface-engineered zirconia systems.