N-doped sulfonated biochar as a solid acid catalyst for biodiesel production via esterification: preparation, response surface optimization, kinetics, and mechanism

The synergistic valorization of waste molasses and chitin is pivotal for establishing a sustainable and economically feasible route to produce biodiesel. Herein, a novel N-doped sulfonated biochar catalyst (110-6-SO₃H-KOH-700NC₂) was synthesized via KOH activation, nitrogen doping, and subsequent sulfonation, and systematically evaluated for the esterification of oleic acid (OA) with methanol. The 110-6-SO₃H-KOH-700NC₂ catalyst exhibited an ultra-high specific surface area of 2970.18 m²/g, which, combined with abundant carbon defects (ID/IG = 1.35), facilitated efficient grafting of -SO₃H groups, and achieved a total acidity of 2.97 mmol/g. Comprehensive characterizations confirmed the successful incorporation of -SO₃H onto the N-doped carbon framework, while the material retained its hierarchical porous architecture, ensuring excellent mass transfer. Single-factor experiments and Box-Behnken design (RSM-BBD) were employed to optimize the reaction parameters. The maximum OA conversion reached 95.68% under optimal conditions, closely matching the model prediction (95.79%). Kinetic studies revealed that the esterification follows pseudo-first-order kinetics with a low apparent activation energy (E_a) of 45.76 kJ/mol, significantly outperforming many conventional solid acid catalysts. Notably, the catalyst exhibits excellent stability over ten successive reaction cycles with no notable decline in activity. A plausible acid-base synergistic catalysis mechanism was proposed, wherein the protonated carbonyl oxygen and subsequent nucleophilic attack by methanol govern the reaction pathway. This work provides a sustainable and scalable strategy for converting biomass-derived porous carbon into high-performance solid acid catalysts for efficient biodiesel production.