Effect of the aging temperature on soybean biodiesel oxidation, quantifying the formed gums and sediments by thermogravimetry

Biodiesel has been widely used as an alternative fuel in diesel engines. However, although its physicochemical properties make it a substitute for traditional diesel fuel, biodiesel is more susceptible to oxidation and degradation during storage than diesel. The high content of unsaturated fatty acid methyl (or ethyl) esters in biodiesel causes its fast oxidative degradation when in contact with precursor agents as oxygen. These reaction products are highly unstable, resulting in a mixture of different species, some of them of high molecular weight, producing sediments and undesirable polymeric compounds. In this study, a sample of commercial soybean biodiesel (BS100) was aged at 50 degrees C, 110 degrees C, 150 degrees C and 200 degrees C in a Rancimat apparatus. This was performed bubbling air into the sample at 10 L h(-1)flow rate for 5 h. The original biodiesel and the aged samples were analyzed by thermogravimetry (TG/DTG), in dynamic mode, under nitrogen atmosphere. Data were compared with those obtained from the analyses performed by size-exclusion chromatography (SEC). Fourier transform infrared spectroscopy (FTIR) analyses were also performed. The objective of this work was to evaluate the effect of the aging temperature on the oxidation of biodiesel and on the degraded product composition, as well as to quantify the formed gums and sediments. The thermogravimetric analysis shows that biodiesel samples aged at 150 degrees C and 200 degrees C have a higher content of heavier products (gum), 33.4% and 44.9%, respectively, than the non-aged biodiesel (2.8%). The SEC analysis indicates molecular weight value of 283, 1241 and 1380 g mol(-1)for samples aged at 150 degrees C, 200 degrees C and for the non-aged sample, respectively, which confirms the results of the TG/DTG analysis. The widening of ester carbonyl stretching band (C=O, 1740 cm(-1)) in the FTIR spectra, as the aging temperature rises, confirms the degradation of biodiesel into acids, ketones and other compounds containing this functional group. The decrease in percentage transmittance in the region corresponding to the fingerprint (1500-500 cm(-1)) indicates that heavy products (gums and sediments) are produced. These results indicate that the degradation product content in the aged biodiesel is strongly dependent on the temperature of the aging process.