Study of the addition of Gabiroba leaves extract in the biodiesel oxidation reaction in the presence of metal ions

Abstract Biodiesel is considered a renewable energy source, making it possible to reduce the use of fossil fuels and the emissions of gasses derived from sulfur oxides. It is one of the alternatives to diesel due to its similar physicochemical properties. However, this biofuel contains unsaturated methyl esters in its composition, which makes it susceptible to oxidation. Among the factors that alter its chemical stability is its contamination by transition metal ions. This study aimed to evaluate the influence of Cu2+, Fe2+ and Fe3+ ions in the oxidative stability of biodiesel using Gabiroba leaves extract as a natural antioxidant. Metal ions were added to biodiesel in the presence and absence of the extract to evaluate their effect in the induction period and the kinetics and thermodynamic parameters of the oxidation reaction. The Gabiroba leaves extract inhibited the catalytic action of Fe2+ and Cu2+ cations in biodiesel, but this effect was not observed with Fe3+. The thermodynamic parameters of the reactions evaluated by the Eyring equation, based on the activated complex theory, indicated that in all cases the process was non-spontaneous and endothermic: ΔG‡ > 0, ΔH‡ > 0, and negative ΔS‡ values.