Basir Maleki, Hossein Esmaeili
In this study, NiFe2O4@g-C3N4 nanocatalyst was synthesized using ultrasonication and pyrolysis techniques and subsequently applied as a highly efficient and reusable catalyst for biodiesel production from waste cooking oil (WCO). Comprehensive structural analyses such as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman spectroscopy, carbon dioxide temperature-programmed desorption (CO2-TPD), and vibrating sample magnetometry (VSM) confirmed that the NiFe2O4@g-C3N4 nanocatalyst possesses a high specific surface area coupled with notable magnetic features, facilitating its effectiveness in catalysis and ease of separation post-reaction. Under optimized conditions of 2.85 wt% catalyst dosage, 11.72:1 methanol-to-oil molar ratio, 61.17 °C reaction temperature, and 24 kHz ultrasound frequency (250 W), the NiFe2O4@g-C3N4 nanocatalyst achieved a maximum biodiesel yield of 98.83% within 31.02 min. Additionally, the catalyst demonstrated remarkable stability and reusability, maintaining a biodiesel yield of over 90% even after seven consecutive reuse cycles. The reaction kinetics revealed that the process follows a pseudo-first-order model. Kinetic and thermodynamic evaluations of the transesterification process revealed that the reaction is endothermic, with an enthalpy change (ΔH°) of 98.5 kJ/mol. The activation energy of 101.2 kJ/mol indicated that the NiFe2O4@g-C3N4 nanocatalyst possesses sufficient energy to drive the transesterification reaction efficiently. These results highlight the incorporation of g-C₃N₄ nanoparticles to reinforce NiFe₂O₄, which enhances its surface area, magnetic recoverability, and structural stability attributes that significantly improve catalytic activity and reusability in biodiesel production.