Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 15, 2009
Publication Date: April 1, 2009
Citation: Moser, B.R. 2009. Comparative Oxidative Stability of Fatty Acid Alkyl Esters by Accelerated Methods. Journal of the American Oil Chemists' Society. 86:699-706. Interpretive Summary: Biodiesel is composed of a number of closely related chemicals that may or may not contain a structural feature referred to as a "double bond." The presence of these double bonds in a chemical results in accelerated oxidative degradation, which means that the chemical is effectively destroyed. If enough of these chemicals are destroyed in biodiesel, it will no longer satisfy important regulatory requirements (ASTM D6751) that define the fuel as biodiesel. A number of factors may influence the rate at which chemicals undergo oxidative degradation, such as exposure to light, heat, oxygen, metals, and other factors. Since biodiesel is composed, at least in part, of chemicals that contain double bonds, it is important to understand the rates at which these chemicals undergo oxidative degradation. The current study measures and compares the oxidative stabilities of the most common chemicals found in biodiesel by accepted standard methods. It was discovered that chemicals that contain more double bonds undergo oxidative degradation more quickly than others. It was also discovered that the location and orientation of the double bond(s) within the chemical are important factors that govern oxidative degradation. The influence of other structural features on oxidative stability was also investigated. The results of this study are expected to inform biodiesel producers as to which feedstocks are best to avoid unwanted oxidative degradation. Different feedstocks for biodiesel production have different compositions, so by employing these results it is possible to select feedstocks that will exhibit superior oxidative stability. Also, by improving the properties of biodiesel, market penetration into the sizable diesel fuel market may be enhanced, which would further benefit American agriculture.
Technical Abstract: Several fatty acid alkyl esters were subjected to accelerated methods of oxidation, including EN 14112 (Rancimat method) and pressurized differential scanning calorimetry (PDSC). Structural trends elucidated from both methods that improved oxidative stability included decreasing the number of double bonds, introduction of trans as opposed to cis unsaturation, location of unsaturation closer to the ester head group, and elimination of hydroxyl groups. Also noticed with EN 14112 was an improvement in oxidative stability when a larger ester head group was utilized. Methyl esters that contained ten or less carbons in the fatty acid backbone were unacceptable for analysis at 110 deg C (EN 14112) due to excessive sample evaporation. With respect to PDSC, a correlation was noticed in which the oxidation onset temperature (OT) of saturated fatty esters increased with decreasing molecular weight (R2 0.7328). In the case of the mono-unsaturates, a very strong inverse correlation was detected between molecular weight and OT (R2 0.9988), which was in agreement with EN 14112. Lastly, a strong direct correlation (R2 0.8759) was elucidated between OT and oil stability index (OSI, EN 14112, 80 deg C). The correlation was not as strong (R2 0.5852) between OSI obtained at 110 deg C and OT.