Molecular Evolution, Heteroatom Transformation, and Asphaltenes Formation during Controlled Autoxidation of Vacuum Residue: Insights from Complementary Atmospheric Pressure Photoionization and Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Controlled autoxidation induces substantial molecular evolution in vacuum residue (VR) through oxygen incorporation and molecular restructuring, yet the associated heteroatom transformations remain insufficiently understood. In this study, complementary atmospheric pressure photoionization (APPI(+)) and electrospray ionization (ESI(±)) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were employed to characterize the structural evolution of vacuum residue and its derived asphaltenes during controlled autoxidation, with and without tetralin. APPI-MS revealed an increase in the relative abundance of oxygen–sulfur (OSx) molecular classes from 6.10% in vacuum residue-derived asphaltenes (VR-Asph) to 14.76% in asphaltenes derived from oxidized vacuum residue (VR-ox-Asph), consistent with oxygen-containing transformations involving sulfur-bearing molecular populations. Complementary ESI-MS analyses showed substantial transformation of nitrogen-containing molecular populations, with nitrogen-only species decreasing while nitrogen–oxygen species increased in both positive- and negative-ion modes. Together, the APPI and ESI results reveal oxygen-containing molecular transformations together with changes in molecular unsaturation and composition that are consistent with molecular-restructuring pathways. Oxygen incorporation is reflected by increased relative abundances of oxygenated sulfur- and nitrogen-containing species, whereas higher double bond equivalent (DBE) and lower H/C indicate changes in molecular unsaturation and aromatic character that are consistent with molecular restructuring. Compared with VR-ox-Asph, the lower-temperature, tetralin-containing asphaltenes (VR-ox-T-Asph) exhibited lower relative APPI OSx and ESI(+) NO signals and higher APPI-derived DBE with lower H/C. These differences reflect the relative molecular-class distributions within the isolated asphaltenes and do not establish lower overall oxygen incorporation into the bulk treated vacuum residue. However, the individual effects of treatment temperature and tetralin addition cannot be separated because these variables were varied simultaneously.