RSM optimization of biodiesel production from Spirogyra as environmentally friendly and the fastest growing algae using phycosynthesized CuZnO green catalyst

This study introduces an integrated and environmentally friendly approach for biodiesel production from Spirogyra algae oil. The oil was extracted using supercritical CO2 (SC-CO2) enhanced with ethanol as a co-solvent. The extracted lipids were subsequently converted to biodiesel through transesterification catalyzed by a heterogeneous CuZnO nanocatalyst synthesized from the same algal extract. Response Surface Methodology (RSM) was employed to optimize the key reaction parameters (methanol-to-oil molar ratio, catalyst concentration, and reaction temperature) resulting in a maximum biodiesel yield of 91.05% under optimal conditions (methanol-to-oil ratio: 10.23, catalyst concentration: 1.86 wt%, temperature: 79 °C). Experimental validation showed excellent agreement between the predicted and observed yields. FTIR analysis of the CuZnO nanoparticles confirmed the formation of dual-metal oxides with characteristic Zn–O and Zn–O–Zn vibrational bands (429–660 cm−1 and 800–1045 cm−1). Residual bio-ligands from the algal extract were also observed, suggesting their contribution to nanoparticle stabilization and surface functionalization. Furthermore, FTIR spectra of the extracted oil and produced biodiesel confirmed successful transesterification, showing distinct methyl ester peaks at 1455 cm−1 and strong C–OCH3 stretching vibrations between 1150 and 1280 cm−1. Gas chromatography analysis of fatty acid methyl esters (FAMEs) indicated that saturated fatty acids (SFAs), particularly palmitic acid (C16:0, 41.22%), were predominant, along with notable amounts of unsaturated fatty acids such as oleic (C18:1 cis, 14.08%) and linoleic acid (C18:2 cis, 8.76%). Overall, the findings highlight Spirogyra algae as a promising and sustainable feedstock for biodiesel production. The combination of green extraction techniques and bio-derived nanocatalysts enhances conversion efficiency. Moreover, the dynamic viscosity of Spirogyra biodiesel was lower than that of conventional palm oil biodiesel, which could improve fuel atomization during combustion in diesel engines.