Green Synthesis of Iron Oxide-Based Superacid Catalysts Modified with Graphene Oxide for Efficient Esterification Reactions

Iron oxide-based superacid catalysts were successfully prepared using impregnation method. Graphene oxide was prepared from sugarcane bagasse, the biowaste of sugar factories, modified Hummer’s method. Fe₂O₃ prepared by precipitation method followed by impregnation with ammonium persulfate and doped with graphene oxide. These catalysts were applied to the esterification of oleic acid with methanol at various conditions such as temperature, reaction time, and catalyst dosage. The highest yield of 96% was achieved using 0.2 g of S₂/Fe₂O₃ at 100 °C for 3 h, indicating a high density of Brønsted and Lewis acid sites on its surface. Catalyst characterization (XRD, SEM, FTIR, N₂ adsorption-desorption analysis, and TGA) confirmed good nanoparticle crystallinity, effective intercalation of metal oxides with persulfate and graphene oxide, and a predominantly mesoporous structure. Sulfation with ammonium persulfate significantly enhanced the acidity of iron oxide, while the addition of graphene oxide provided a moderate acidity increase. This increase in acidity positively impacted the conversion rate of oleic acid with methanol. All three catalysts (GF, SF, GSF) demonstrated strong reusability for oleic acid esterification, maintaining conversion efficiency above 50% after six cycles, indicating economic feasibility with minimal efficiency loss. Overall, this solid superacid catalyst presents a promising green pathway for ester synthesis and biodiesel production.