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.