Influence of natural antioxidants on the thermo-oxidative stability of soybean ethyl and bovine tallow methyl biodiesels

• The synthetic antioxidant BHT exhibited the highest antioxidant effect, followed by soursop, pitanga, mango, and acerola leaves. • The results demonstrated the potential of natural antioxidants as sustainable and environmentally friendly alternatives to synthetic antioxidants. • It can contribute to developing more eco-friendly and cost-effective strategies. • It provides a comparative evaluation of natural and synthetic antioxidants on the thermooxidative stability of SEB and BTMB. Biodiesel is susceptible to oxidation, which can degrade fuel quality and performance. This study investigates the effect of natural plant-derived antioxidants on the thermo-oxidative behaviour of soybean ethyl and bovine tallow methyl biodiesels. Biodiesels were synthesised, and additives were prepared from the leaves and extracts of the selected plants. Pure and additive biodiesels were stored under two conditions (ambient and 60 °C) and monitored over time (0, 168, 504, 1176, and 1848 h). Physicochemical characterisation, including acid value, kinematic viscosity, density, thermogravimetry (TGA), Fourier-transform infrared spectroscopy (FTIR), and inductively coupled plasma optical emission spectrometry (ICP-OES), was performed. The addition of natural antioxidants improved the thermooxidative stability of biodiesels, with varying degrees of effectiveness. BHT (Butylated Hydroxytoluene) showed the highest antioxidant effect, followed by soursop, pitanga, mango, and acerola leaves. TGA and FTIR analyses confirmed the oxidative degradation of biodiesels over time and the protective effect of the additives. The findings demonstrate that natural antioxidants offer an environmentally friendly and effective alternative to synthetic stabilisers, thereby improving fuel durability and sustainability. These results advance the understanding of biodiesel stabilisation mechanisms and support the adoption of renewable antioxidant additives in biofuel formulations.