Mechanochemical synthesis of Knoevenagel condensation products from biorenewable furaldehydes using crustacean waste-derived chitosan as a sustainable organocatalyst

Abstract

The biorefinery processes employing renewable feedstock can benefit from sustainable synthetic practices, such as mechanochemistry, organocatalysis, and renewable catalysts. This work reports using crustacean waste-derived chitosan (CS) as an eco-friendly and recyclable heterogeneous organocatalyst for the Knoevenagel condensation reaction between biorenewable 5-substituted-2-furaldehydes and malononitrile. The reaction was performed under solvent-free, mechanochemical conditions in a mortar and pestle. The reaction kinetics were faster, and the product selectivity was higher under mechanochemical conditions than in solvent-mediated synthesis. The CS catalyst was conveniently recovered and recycled. Moreover, the Knoevenagel condensation reaction was extended to substituted benzaldehydes to demonstrate the broad substrate scope of the process. In all cases, the Knoevenagel condensation products were isolated in excellent yields (>85%) in <30 min at RT. The CS (fresh and recycled) catalysts were characterized by UV-Vis, FTIR, PXRD, SEM-EDX, DSC, TGA, and elemental analysis techniques.