Kinetic study and confirmation of epoxide-dependent oligomerization of methyl oleate

The aging of biodiesel proceeds via multiple pathways, with oligomerization playing a central role. In this study, we investigated an epoxide-dependent oligomerization pathway, which had previously only been postulated. Using methyl oleate (C18:1) as a model monounsaturated fatty acid and acetic acid as a representative, reactive nucleophile and known biodiesel aging product with a suitable boiling point, oligomeric products were identified by size-exclusion chromatography (SEC) coupled with electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-QTOF-MS). The concentration of 20 wt% was chosen to ensure a measurable kinetic effect while maintaining stable reaction conditions. Furthermore, the role of methyl oleate during biodiesel aging was addressed. Time-resolved analysis confirmed the proposed sequential order of reactions. It was pointed out that elimination reactions may occur. The data support the formation of epoxides, despite the isobaric overlap with ketones, and show that hydroxyl intermediates undergo esterification and etherification. Moreover, experiments with pure C18:1 demonstrated that acetic acid–derived oligomers are generated. Under Rancimat conditions, addition of 20 wt% acetic acid resulted in an approximately 1.1 – 2.6 increase in product yield. Kinetic analysis revealed structure-dependent formation and decay behavior of the aging products, with slightly faster epoxidation and shifted product distributions toward higher oligomeric species in the presence of acetic acid. Reactive intermediates were consumed more rapidly than oligomeric species and all decay processes followed apparent second-order kinetics. These findings provide direct experimental evidence for the involvement of epoxide-dependent pathways in biodiesel aging.