Synthesis of silica/soda lime composite catalyst from rice husk ash for efficient biodiesel production

A novel silica/soda-lime composite catalyst was synthesized from rice husk ash and commercial soda lime through a simple impregnation–calcination method. This study represents the first use of soda lime as a CaO-rich basic component in a heterogeneous catalyst for biodiesel production. Structural analyses confirmed enhanced surface area, improved porosity, and strong interactions between silica and soda-lime components. The catalyst showed excellent performance in oleic acid esterification, achieving a biodiesel yield of 96.7% under optimized conditions (1:31 oleic acid: methanol molar ratio, 10 wt% catalyst loading, 70 °C, 4 h). Kinetic evaluation revealed a low activation energy of 19.51 kJ mol⁻¹, indicating efficient catalytic pathways. The 2% silica/soda-lime composite maintained over 75% of activity after ten reuse cycles, demonstrating strong durability. FTIR and XRD of the spent catalyst showed only minor structural changes after repeated use. Overall, the composite offers a low-cost, waste-derived, and highly basic catalyst suitable for scalable biodiesel synthesis with strong environmental and economic advantages.