Analysis of oxidized biodiesel by 1 H‐NMR and effect of contact area with air

Abstract

Biodiesel is continuously gaining attention and significance as an alternative diesel fuel. An important issue facing biodiesel is fuel stability upon exposure to air due to its content of unsaturated fatty acids. Numerous factors influence the oxidative stability of biodiesel, and several methods for its assessment have been developed. In the present work, a defined amount of biodiesel (methyl soyate) was heated in open beakers, with the only difference being the size of the beaker, i.e. the surface area of the biodiesel exposed to air. Biodiesel oxidized in this fashion was analyzed by 1 H-NMR, kinematic viscosity and acid value. Acid values and kinematic viscosity increased with time and surface area. A previously developed 1 H-NMR procedure was used to evaluate the unsaturation and “residual” fatty acid composition. The amounts of saturated fatty acids determined by this method increased, with monounsaturated and diunsaturated species increasing and then decreasing with time. After “flash” (3 h, 165 7C) oxidation, NMR shows the greatest effect on saturates and compounds with two double bonds, the former increasing and the latter decreasing. The double bond originally located at D15 in 18:3 is largely retained, showing that other double bond positions in 18:3 are initially affected by oxidation. The methyl ester signal decreases, coinciding with the increase in acid value. An increasingly strong absorption was observed in the UV-VIS spectra. Increasing surface area accelerated oxidation and affected fatty acid composition.