Enhanced Biodiesel Production from Waste Cooking Oil via Molten Salt-Synthesized M/g-CN (M = Fe, Cu, and Fe-Cu) Catalysts

Biodiesel production through base-catalyzed homogeneous transesterification requires high-purity raw materials, such as triglycerides and anhydrous alcohol, and oils with a low free fatty acid (FFA) content to prevent undesirable saponification reactions that consume the catalyst and reduce process yield. In this way, the development of new and efficient heterogeneous catalysts is extremely important for producing biodiesel from various raw materials. The present study reports a new heterogeneous catalyst of type M/g-C₃N₄ (named as M/g-CN, where M = Fe, Cu, and Fe-Cu) synthesized using the molten salt method (MSM) and characterized structurally and morphologically. The synthesized catalysts were evaluated for reducing the acid value (AV) and producing fatty acid ethyl esters (FAEE) from high-acid-content waste cooking oil (WCO). The influence of temperature, time, and catalyst amount was investigated and optimized to achieve mild reaction conditions. The results showed that adding metals to g-C₃N₄ improved catalyst efficiency. Under optimal conditions, the Fe-Cu/g-CN catalyst showed the best performance, achieving an AV reduction efficiency of approximately 90%, yielding a product with a free fatty acid content of 0.6%. The catalyst was easily separated from the reaction mixture, maintaining an average AV reduction after five reuse cycles. The catalytic activity was also evaluated in the model esterification reaction between oleic acid and ethanol, achieving 95% reduction in oleic acid and 95% ethyl oleate yield with the Fe-Cu/g-CN catalyst. These findings demonstrate that the proposed system operates under significantly less stringent conditions than those reported in the literature, establishing it as a simple and more sustainable route for waste oil valorization.