Microwave-Assisted Transesterification of Soybean Oil Using Eggshell-Derived SrFe/CaO Catalysts

Abstract

This work proposes the use of a novel non-magnetic SrFe/CaO catalyst derived from waste materials, which have not been reported for biodiesel production under microwave conditions. Also, it evaluates the interaction of multiple reaction factors through statistical modeling. While CaO has been widely used in biodiesel production, its leaching and limited stability have motivated the search for improved materials. Hence, the Sr/CaO, Fe/CaO, and SrFe/CaO catalysts were synthesized by incipient impregnation of the calcined eggshells with Sr and Fe salts, followed by calcination at 800 °C. N₂-physisorption, XRD, and Hammett indicators were used as characterization techniques of the catalysts, and the reaction was performed in a batch reactor under microwave irradiation at 60 °C. The SrFe/CaO catalyst exhibited the highest basicity (60 mmol g⁻¹), possibly due to the formation of the α-Fe₂O₃ phase, and achieved a biodiesel yield of 92%. Further optimization using the Box-Behnken response surface methodology revealed that reaction time and the methanol-to-oil ratio significantly affected the yield, while microwave power had a minor influence. The optimal conditions for this system were 56 min, 150 W, and a methanol-to-oil ratio of 10:1, which resulted in a maximum biodiesel yield of 98.87%. Despite the promising activity, catalyst stability decreased over reuse cycles, likely due to CaO leaching.