The application of biomass-based sources for energy generation has attracted much interest in recent times due to their environmental benefits. In this study, the potential use of rubber seed shells and eggshells in the synthesis of bi-functional catalysts for biodiesel production has been investigated. The catalyst was produced in a sequence of steps that included carbonization, sulfonation, calcination, and impregnation. The catalyst was characterized using Scanning electron microscopy, X-ray fluorescence spectrophotometry, Fourier transform infrared spectroscopy, X-ray diffraction, and Brunauer-Emmett-Teller analysis. The catalysts’ performance was assessed through the simultaneous esterification and transesterification of waste cooking oil with a free fatty acid content of 2.9 %. Response surface methodology based on central composite design was employed to model and optimize the process. The factors investigated included methanol/oil ratio (4:1 – 20:1), catalyst concentration (1 – 5 wt%), reaction temperature (50 – 70 degrees C), and time (1 – 5 h). The catalyst was found to have a specific surface area of 241.0 m2/g, a pore volume of 0.189 cc/g, and a pore size of 1.132 nm. A methanol/oil molar ratio of 14.2:1, catalyst concentration of 2.0 wt%, temperature of 62.5 degrees C, and reaction time of 2.3 h were the optimum reaction conditions required to achieve an optimum average biodiesel yield of 89.4 % and FFA conversion of 91.8 %. Also, the leaching of the catalysts was insignificant, and the catalysts demonstrated good reusability, producing a biodiesel yield of 81.9 % after the fifth cycle. The physicochemical properties of the produced biodiesel fall within the ranges of the recommended ASTM 6751 and EN14214 international standards. The utilization of rubber seeds and eggshells for catalyst synthesis offers a promising, cost-effective, and eco-friendly way to produce biodiesel.
