Evaluation of the influence of chromium(III) ions and protoporphyrin IX on the oxidative stability of biodiesel

Abstract

This study aimed to investigate the impact of porphyrin complexes with Cr³⁺ on the oxidative stability of biodiesel. Specifically, it focused on assessing the induction period as well as the fluorescence and FTIR spectroscopy, kinetic and thermodynamic parameters of oxidation under varying temperature conditions. The concentration of the metal added in the biodiesel samples, with and without protoporphyrin IX (PPIX), was established based on previous literature. Oxidative stability tests were carried out at 105, 110, 115, and 120°C. The Cr³⁺ transition metal ion exhibited low catalytic activity in biodiesel oxidation reactions, and the tests without PPIX showed lower induction period values for all temperatures. PPIX exhibited antioxidant action, delaying both the initiation and propagation stages of chain reactions responsible for the formation of free radicals, thereby enhancing the stability of the biofuel even in the presence of Cr³⁺, when compared to the same test without the addition of the compound. The fluorescence intensity of PPIX decreased as a function of the contact time with the metal ion, and the FTIR analysis of the biodiesel with PPIX presented the most significant variations in the spectra. The tests containing PPIX at all temperatures presented lower values of reaction rate than the control samples, while the test without PPIX with Cr³⁺ ion resulted in higher k in comparison to control. The activation energy values ranged from 43.36 to 106.37 kJ mol⁻¹. The results of thermodynamic parameters indicated greater stability for biodiesel containing PPIX, with enthalpy activation (ΔH^‡) and entropy activation (ΔS^‡) values of 103.16 kJ mol⁻¹ and -52.61 J.K⁻¹.mol⁻¹, respectively.