Submitted to: European Journal of Science and Lipid Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 21, 2006
Publication Date: April 21, 2006
Citation: Knothe, G.H. 2006. Analysis of oxidized biodiesel by 1H-NMR and effect of contact area with air. European Journal of Science and Lipid Technology. 108:493-500. Interpretive Summary: Biodiesel is an alternative diesel fuel derived from vegetable oils such as soybean oil or other sources such as animal fats and waste frying oils. It is largely a mixture of compounds called fatty acid methyl esters. Some of the fatty acid methyl esters contained in biodiesel easily react with air in a process called oxidation. Ultimately, this oxidation process causes the quality of the biodiesel fuel to deteriorate. It is important to know how the oxidation process affects the composition of fatty acid methyl esters in the biodiesel fuel. Gaining such knowledge will affect methods for stabilizing biodiesel towards oxidation. This work describes how an analytical method often used in organic chemistry and known as nuclear magnetic resonance (NMR) can be applied to biodiesel. The results show how the composition of the fatty acid methyl esters in biodiesel changes upon oxidation. Also studied was the effect of the surface area of biodiesel with air. The greater the surface area, the more rapid the oxidation process. The NMR results coincide with this observation.
Technical Abstract: Biodiesel, the mono-alkyl esters of vegetable oils and animal fats, is continuously gaining attention and significance as an alternative diesel fuel. One of the most important technical isues facing biodiesel is fuel stability upon exposure to air due to its content of unsaturated fatty acids. Numerous factors influencing the oxidative stability of biodiesel have been determined and several methods for its assessment 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 being the only variable. The biodiesel oxidized in this fashion was analyzed by 1H-NMR, kinematic viscosity, acid value, and UV-Vis spectroscopy. Acid values and kinematic viscosity incresed with time and increasing surface area. The increase in viscosity can be traced to various causes including free fatty acids the formation of some higher molecular weight species, which was confirmed by GPC. A previously developed 1H-NMR procedure was used to assess the presence and nature of remaining double bonds and a "residual" fatty acid composition. The reactions were conducted at 80 deg C for 168 h and at 165 deg C for 3 h ("flash" oxidation). The amounts of saturated fatty acids increased considerably while the amounts of monounsaturated and diunsaturated species increase and then decrease with time. When conducting the reaction at 80 deg C, after 48 h no double bond influencing the signal of terminal methyl groups is detectable in case of the largest surface area while it was still significant in case of the smallest area. After 168 h at 48 h, virtually no double bond influencing the terminal signal is observed. After flash oxidation, NMR shows the greatest effect on saturates and compounds with two double bonds, the former increasing the latter decreasing. The double bond at delta-15 in C18:3 is largely retained under "flash" oxidation conditions, showing that other double bond positions in C18:3 are initially affected by oxidation. The methyl ester signal also decreases, indicating the formation of free fatty acids during oxidation and coinciding with the increase in acid value. An increasingly strong absorption was observed in the UV-Vis spectra. Increasing surface area considerably accelerated oxidation and the fatty acid composition.