Synthesis of waste-derived Fe3O4-CaO nanomagnetic catalysts for efficient biodiesel production under mild operating conditions

The development of high-performance catalysts from waste-derived, circular economy precursors is crucial for the advancement of renewable energy systems. This study reports a novel, energy-efficient magnetic Fe₃O₄/CaO nanocatalyst synthesized entirely from waste and natural resources: iron sand and eggshell waste. Unlike traditional methods, a synchronous impregnation strategy was utilized to introduce CaO during the formation of the Fe₃O₄ phase. This approach successfully modified the Fe₃O₄ crystalline structure, embedding strong basic catalytic sites while maintaining high magnetic susceptibility for rapid recovery. Structural characterization confirmed that this synergistic integration prevents CaO leaching and enhances stability. Catalytic performance was evaluated for the transesterification of soybean oil using a Taguchi L16 design to minimize energy expenditure. The system achieved a 91.8% biodiesel conversion under significantly milder conditions than existing literature (60 °C, 1 wt% total catalyst loading, and 60 min reaction time) using the optimal 3 g CaO-loaded magnetic nano-catalyst (FeCa3). By reducing operating severity, this catalyst addresses the limitation of high energy consumption in biodiesel production. The Fe₃O₄/CaO system facilitates rapid magnetic separation, which simplifies catalyst recovery and downstream processing. This work presents a scalable, waste-to-catalyst platform that bridges the gap between material sustainability and process intensification, offering a viable pathway for cost-effective, low-carbon biofuel manufacturing. • Magnetic Fe₃O₄/CaO was synthesized from iron sand and eggshell waste. • FeCa3 catalyst achieved 91.8% biodiesel conversion at only 60 °C. • Synchronous impregnation enhanced basic site density and magnetic recovery. • Taguchi L16 design optimized performance under low-severity conditions. • Waste-derived catalysts offer a circular path for sustainable biofuel.