Optimisation of Biodiesel Production from Microalgae Using Sorghum Stalk Ash as Catalyst

Microalgal oils and agricultural residue catalysts offer potential resources for renewable biodiesel production. This study investigated the production and characterisation of biodiesel from Nannochloropsis oculata oil using calcined sorghum stalk ash (SSA) as a heterogeneous base catalyst. Process optimisation employed response surface methodology with a central composite design in Design-Expert® software, examining methanol-to-oil ratio (9:1–17:1), catalyst loading (0.5–5.0 wt%), reaction time (30–120 min), and agitation speed (200–1000 rpm). X-ray fluorescence analysis identified K₂O as the predominant oxide constituent of the catalyst. The extracted oil exhibited a viscosity of 34.87 mm²/s, acid value of 7.60 mg KOH/g, iodine value of 59.76 g I₂/100 g, and saponification value of 216.10 mg KOH/g. The maximum reported biodiesel yield was 91.55%, obtained at an agitation speed of 1000 rpm, reaction time of 120 min, and catalyst loading of 5.0 wt%. The reduced quadratic model achieved an adjusted R² of 0.8713. Gas chromatography–mass spectrometry analysis indicated a total fatty acid methyl ester content of 75.00%, dominated by C18 and C16 methyl esters. Reported biodiesel properties included a kinematic viscosity of 4.46 mm²/s, flash point of 125.33°C, cetane number of 50.01, and higher heating value of 67.67 MJ/kg. Fourier-transform infrared spectroscopy detected characteristic ester carbonyl absorption bands at 1740–1750 cm⁻¹. These findings support the potential of SSA as a low-cost heterogeneous catalyst and N. oculata oil as a renewable feedstock for biodiesel production, contributing to the development of processes that combine microalgal resources with agricultural residue utilisation.