Spectroscopic Study of Biodiesel Degradation Pathways

Oxidative degradation of biodiesel under accelerated conditions has been examined by Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR), and gravimetric measurement of deposit formation. The formation of gums and deposits caused by oxidation in storage or in an engine fuel system is a significant issue because of the potential for fuel pump and injector fouling. The results of this study indicate several important pathways for degradation and two pathways leading to formation of oligomers and, ultimately, deposits. Peroxides formed in the initial stage of oxidation can decompose to form aldehydes, ketones, and acids. These can react further in aldol condensation to form oligomers. Additionally, peroxides can react with fatty acid chains to form dimers and higher oligomers. Deposits form when the polarity and molecular weight of these oligomers is high enough. An antagonistic effect on deposit formation is observed for ultra-low sulfur diesel-(ULSD) biodiesel blends because of the lower polarity of ULSD relative to higher sulfur grades of diesel. Additionally, methyl esters can decompose by hydrolysis, reverse transesterification through reaction with glycerin and glycerides, and other related mechanisms. The use of antioxidants at the point of manufacture to prevent formation of hydroperoxides is recommended.