RSM-optimized biodiesel production from Ethiopian Podocarpus falcatus seed oil using a CaO-CeO2 heterogeneous composite catalyst and oxidative stability assessment

This study investigates the conversion of Podocarpus falcatus seed oil into biodiesel using a calcium-oxide-cerium-oxide (CaO-CeO₂) heterogeneous composite catalyst. The work addresses the critical need for sustainable non-edible feedstocks and robust catalysts that overcome the drawbacks of conventional homogeneous systems, such as soap formation, poor recyclability, and high wastewater generation. The CaO-CeO₂ composite catalyst was synthesized through ethanol-assisted impregnation and calcination at 550 °C, yielding a nanocrystalline structure with high surface reactivity. Transesterification parameters including temperature, reaction time, catalyst loading, and methanol to-oil molar ratio were optimized using response surface methodology based on a central composite design. The optimum conditions (63.2 °C, 1.52 h, 1.59 wt% catalyst, and 11.82:1 molar ratio) produced a maximum biodiesel yield of 98.32%, with strong model correlation (R² = 0.9916) validated by analysis of variance. Physicochemical characterization confirmed compliance with international fuel standards, while gas chromatography-mass spectrometry and Fourier-transform infrared spectroscopy verified complete conversion of triglycerides to methyl esters. The biodiesel exhibited acceptable oxidative stability (induction period = 26.5 h), high cetane number, and favorable cold-flow properties, indicating suitability for compression-ignition engines. The catalyst maintained over 78% activity after eight reuse cycles, confirming its durability and economic viability. This work introduces Podocarpus falcatus as a novel indigenous biodiesel feedstock and establishes CaO-CeO₂ composites as an efficient, reusable, and environmentally benign catalyst system for sustainable fuel production.