Molecular Characterization of Oxygen-Containing Compounds in Aged Lubricant Base Oils

Characterization of oxidation products at the molecular level enables a better understanding of the lubricant oil oxidation mechanisms. A base oil was subjected to thermal aging for 168 h at 100, 140, and 175 °C. The aged oils were characterized by various analytical techniques, such as elemental composition, group hydrocarbon composition, Fourier transform infrared spectroscopy (FTIR), and high-resolution mass spectrometry (HR MS) combined with electrospray ionization. The oxygen content in the aged oils increased from 0.1 wt % at 100 °C to 2.12% at 175 °C, indicating a sensitive impact on the oxidation temperature and high degree of oxidation in the aging process. Group hydrocarbon composition analysis indicates that a significant proportion of resins (exceeding 10 wt %) are present in the high-temperature aged oils. FTIR spectra provide solid evidence of the presence of carbonyl groups. HR MS reveals the molecular composition of the oxidation products. The O₂-O₄ compound classes with carbon number ranging from C₁₅ to C₄₅ are the major oxygen-containing compounds, in which the O₂ compound class is the most abundant. The nitrogen-containing oxygen compounds show significantly high relative abundance at high-temperature products, indicating that nitrogen compounds may accelerate the oxidation. Combined with the hydrogen-deuterium exchange experiments, the structures of oxygen-containing compounds were discussed.