Corn stalk-derived magnetic humic acid nanocatalyst for green synthesis of tetrahydrodipyrazolopyridines in water

A sustainable magnetic nanocatalyst (Fe 3 O 4 /HA) was prepared by coating humic acid (HA) derived from corn stalk agricultural waste onto Fe 3 O 4 nanoparticles via a hydrothermal-co-precipitation method. The catalyst combines the Brønsted acidity of biomass-derived humic acid with the magnetic recoverability of Fe 3 O 4 , enabling efficient and recyclable heterogeneous catalysis. Structural and magnetic analyses confirmed the successful formation of the Fe 3 O 4 /HA composite with preserved magnetic properties. The catalytic activity was evaluated in the one-pot multicomponent synthesis of tetrahydrodipyrazolopyridines (THDPPs) in water. Under optimized conditions, Fe 3 O 4 /0.5HA afforded excellent yields of up to 99% and exhibited broad substrate scope, including aromatic, heteroaromatic, and aliphatic aldehydes. The catalyst was readily recovered using an external magnet and reused for ten consecutive cycles with only a slight loss of activity. Green chemistry assessment demonstrated the environmental benefits of the protocol. This work highlights the valorization of agricultural waste into a functional magnetic catalyst and provides an efficient and environmentally benign approach for the synthesis of bioactive heterocycles.