Corn Stover-Derived Sulfonated Carbon as a Reusable Solid Acid Catalyst for the Green Multicomponent Synthesis of Tetrahydrodipyrazolopyridines.

The open-field burning of agricultural residues such as corn stover contributes significantly to air pollution and greenhouse gas emissions, highlighting the need for sustainable waste-valorization strategies. In this study, corn stover was converted into a sulfonated carbon catalyst through KOH activation, pyrolysis, and controlled sulfonation, providing a biomass-derived carbon material for heterogeneous catalytic applications. The resulting catalyst (KCS-100-3) exhibited a high surface area (620.22 m2 g-1) and a high density of Brønsted acidic sites (1.55 mmol g-1), enabling efficient catalytic performance in the aqueous-phase multicomponent synthesis of tetrahydrodipyrazolopyridine derivatives. Under optimized conditions (75 °C, 40 min), the desired products were obtained in excellent yields (94-99%) across a range of aromatic aldehydes and acetoacetate esters. Acidity characterization, control experiments, and TON/TOF analysis indicated that catalytic activity is primarily associated with accessible sulfonic acid functionalities. Mechanistic studies using ESI-MS identified pyrazolone and benzylidene pyrazolone intermediates, while hot-filtration experiments supported the heterogeneous nature of the catalytic process. The catalyst exhibited good recyclability, maintaining yields above 90% for seven consecutive cycles, although gradual deactivation was observed due to the loss of sulfur-containing acidic groups during extended reuse. Furthermore, gram-scale synthesis afforded up to 4.80 g of product in 90% isolated yield, demonstrating the practical applicability of the method. This work demonstrates a practical approach for transforming agricultural waste into value-added carbon-based catalysts and highlights the potential of biomass-derived materials for heterogeneous catalytic applications.