Biosynthesis of SnO2 nanoparticles using roselle (Hibiscus sabdariffa L.) flower extract as photocatalyst for biodiesel production from waste cooking oil

The rising demand for renewable energy and the need to manage waste cooking oil drive the development of efficient, eco-friendly photocatalysts for biodiesel production. This study reports the green synthesis of SnO 2 nanoparticles using roselle ( Hibiscus sabdariffa L.) flower extract as a natural bioreducing and stabilizing agent. Rich in anthocyanins and polyphenols, roselle enables nanoparticle formation without toxic chemicals. SnO 2 was selected due to its high electron mobility, chemical stability, strong UV absorption, and tunable bandgap, making it suitable for photocatalytic transesterification. Nanoparticles were synthesized via hydrothermal and sol–gel methods and applied in biodiesel production from waste cooking oil under UV irradiation. XRD confirmed a tetragonal rutile structure with crystallite size of 12 ± 3 nm. FE-SEM showed hydrothermal SnO 2 formed uniform cubic particles with smaller sizes, while sol–gel produced larger, agglomerated ones. DRS UV–Vis analysis revealed band gaps of 3.66 eV (hydrothermal) and 3.67 eV (sol–gel), supporting efficient UV absorption. GSA indicated hydrothermal SnO 2 had a larger surface area (73.5 m 2 /g) than sol–gel (51.2 m 2 /g) with an average pore size of 10.7 nm. These properties enhanced catalytic activity, with hydrothermal SnO 2 achieving the highest methyl ester yield of 99.38 %, confirmed by GC–MS. Major biodiesel components included methyl oleate (55.79 %), methyl palmitate (20.46 %), methyl stearate (14.87 %), and methyl myristate (8.26 %). FTIR further verified biodiesel formation through the ester C=O peak at 1748 cm −1 . This work highlights a sustainable, low-cost strategy for fabricating high-performance photocatalysts while valorizing waste cooking oil into biodiesel. • SnO 2 nanoparticles were synthesized using roselle ( Hibiscus sabdariffa L.) flower extract as a green bioreducing agent. • Hydrothermal and sol–gel methods were compared for SnO 2 synthesis under identical conditions. • Hydrothermally synthesized SnO 2 exhibited a higher surface area and a larger and uniform size than sol–gel SnO 2 . • UV-irradiated hydrothermally synthesized SnO₂ achieved 98.53% biodiesel yield from waste cooking oil.