Development of Sulfonated Carbon Catalysts from Waste Sugarcane Leaves and Tops for Effective Biodiesel Production from Free Fatty Acids or Waste Cooking Oil

Sulfonated carbon catalysts were prepared by valorization of waste sugarcane leaves and tops (SC), an abundant agricultural residue that is routinely burnt in the field, contributing significantly to PM2.5 pollution in Southeast Asia. Catalyst production was achieved via addition of a chemical activator (ZnCl2, KOH or H3PO4) followed by pyrolysis in N2 at 500 °C for 1 h and subsequent sulfonation with H2SO4. Physically activated samples were prepared by pyrolysis under identical conditions in the presence of N2 (yielding a biochar) or CO2, prior to sulfonation. Resulting catalysts were investigated for fatty acid methyl ester (FAME) production, where sulfonated KOH-activated carbon (Sul-KOH) demonstrated exceptional esterification of palmitic acid with methanol, with a 98 ± 2.65% yield under optimized conditions (65 °C, 1:20 molar ratio, 1% catalyst loading for 2 h). Sul-KOH performance was attributed to the physicochemical properties after sulfonation, which crucially retained significant surface area (270.57 m2 g–1), total pore volume (0.17 cm3 g–1), mesopore volume (0.08 cm3 g–1), high sulfur content (2.45%), and high acid density (6.58 ± 0.21 mmol g–1), ensuring enhanced mass transfer and catalytic efficiency. Crucially, reuse of the Sul-KOH catalyst after five reuse cycles demonstrated a 98.0 ± 0.57% yield, overcoming the deactivation limitations that have frequently compromised biomass-derived sulfonated carbon catalysts. Transesterification of waste cooking oil with methanol was also investigated with Sul-KOH. This achieved an 86.0 ± 1.96% yield of biodiesel, demonstrating its effectiveness for the simultaneous esterification of free fatty acids (FFAs) and transesterification of triglycerides in a single catalytic step. Waste cooking oil (WCO) biodiesel properties were comparable to commercial diesel, with a calorific value of 37.46 MJ/kg, density of 0.85 g/cm3, kinematic viscosity of 6.30 cSt, dynamic viscosity of 5.36 cP, and a flash point of 66.7 oC. The obtaining a calorific value was in typical range reported for biodiesel (ASTM D6751). Such catalysts demonstrate great potential for the conversion of highly degraded oils containing significant proportions of FFA, into sustainable biofuels.