Sustainable biodiesel production from waste cooking oil using biosynthesized ZnO catalysts derived from Hibiscus sabdariffa flower extract

Biodiesel production from waste lipids using environmentally benign catalysts represents a sustainable strategy for renewable fuel generation. In this study, flower extract of Hibiscus sabdariffa (roselle), an industrial crop by-product, was utilized as a green reducing and stabilizing agent for the sol–gel synthesis of zinc oxide nanoparticles (ZnO NPs). The effect of extract concentrations (1, 4, and 8 % w/v) on the physicochemical properties of ZnO and its photocatalytic performance in the transesterification of waste cooking oil was investigated. X-ray diffraction confirmed the formation of crystalline ZnO NPs, while FTIR analysis verified Zn–O functional groups. FE-SEM and elemental analysis indicated uniform particle morphology with Zn (25–60 %), O (4–15 %), and residual carbon (13–49 %). Gas sorption analysis revealed a mesoporous structure with pore sizes of 2–50 nm and a maximum specific surface area of 19.84 m2 g−1 for the 4 % extract-derived sample. The band gap energies is slightly decreased from 3.18 to 3.14 eV with increasing roselle extract concentration. The optimum photocatalytic transesterification of waste cooking oil was conducted using 0.2 g of ZnO-roselle catalyst, a methanol-to-oil ratio of 12:1, at 60 °C for 2 h under the UV irradiation. The ZnO NPs synthesized with 1 % roselle extract exhibited the highest catalytic activity and achieving a maximum yield of 93.22 %. Finally, the results demonstrate that roselle-mediated ZnO synthesis provides a sustainable ZnO NPs catalyst production and the potential of plant-derived resources for industrial biodiesel applications and biomass valorization in Indonesia.