Antioxidant potential of sulfenimides in waste walnut oil biodiesel: kinetic stability, phosphomolybdenum activity, and reducing power capacity

This study investigated the influence of synthetic sulfenimides on biodiesel-diesel fuel blends. Three previously synthesized sulfenimide derivatives were structurally characterized using spectroscopic techniques. We evaluated their antioxidant properties through phosphomolybdenum activity and reducing power capacity assays. The tested fuel samples were designated as D100, B10D90, B10D90_AA (ascorbic acid), B10D90_<b>3</b>, B10D90_<b>2</b>, and B10D90_<b>1</b>. The effect of adding 2000 ppm of the additive was measured using an Oxifast instrument following the ASTM D7545 standard, and the results were compared to those of the chemical antioxidant ascorbic acid. Inhibition time values were derived from the oxidation stability results. The starting temperatures for crystallization (Tc) of the mixtures were found using differential scanning calorimetry (DSC), and the measured temperatures were -7.24, -7.64, -7.89, -8.06, -8.39, and -8.52 °C. FT-IR spectra exhibited characteristic absorption bands associated with antioxidant functional groups, which means the sulfenimide compounds were successfully added. The addition of sulfenimides improved the oxidative stability of biodiesel blends. Furthermore, we conducted thermogravimetric analysis (TGA) at multiple heating rates to investigate the thermal decomposition kinetics of the sulfenimides. The activation energies for compounds <b>1</b>, <b>2</b>, and <b>3</b> were calculated using the Kissinger-Akahira-Sunose (KAS) method and were found to be 125.28, 111.34, and 88.11 kJ mol^-1, respectively.