| The application of synchrotron vacuum ultraviolet photoionization/mass spectrometry(SVUVPI-MS) in the analysis of the chemical structures of heavy oils has been developed and perfected. We proposed fristly a new method for the analysis of the heavier components in petroleum residues(aromatics and resins), in which laser-induced acoustic desorption(LIAD) is coupled with synchrotron vacuum ultraviolet single-photon ionization and mass spectrometry(LIAD/SVUVPI-MS technique). This new method solves the problem that the infrared laser desorption/synchrotron vacuum ultraviolet single-photon ionization-mass spectrometry(IRLD/SVUVPI-MS) technique we previously explored can only be applied to the analysis of the lightest fraction(saturates) of residues, but it cannot be applied to study the heaviest fraction of asphaltenes yet.The mass spectra of primary saturates exhibit bimodal distribution with the mass range of 100-600 Da, centered at 250 and 400 Da respectively. The components in light mass region are most likely assigned to the hydrocarbons of chain C Cn H2n+2(14 ≤ n ≤ 24) and hydrocarbons CnH2n-6(15 ≤ n ≤ 25), and sulphur-containing compounds: aliphatic compounds Cn H2n+2S(12 ≤ n ≤ 22), alicyclic or aliphatic heterocyclic compounds CnH2 n S(12 ≤ n ≤ 22), nitrogen-containing compounds CnH2n-7N and CnH2n-5N(27 ≤ n ≤ 35); while the heavy mass region contains amount of saturates compounds CnH2n-6 and CnH2n-8(27 ≤ n ≤ 35), which presents higher IEs(> 9.0 eV). After LHAR undergoing the reaction of thermal, hydro-thermal reaction and catalytic hydrotreatment, the components of chain hydrocarbons and sulphur-containing compounds have been significantly reduced but nitrogen-containing still exist with high intensity.Laser-induced acoustic desorption coupled with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry(LIAD/SVUVPI-MS) is employed to analyze aromatics prepared under different conditions from Lungu atmospheric residue(LGAR), i.e., the primary aromatics separated directly from LGAR, and the secondary aromatics after hydrogenation of LGAR and its resins. The mass spectra of the primary aromatics present a bimodal normal distribution in the range of 200-900 Da, in which the relative intensity of the two peaks changes significantly with the SVUV photon energies(9.0, 11.0, and 14.0 eV), indicating that at least two categories of compounds with different ionization energies(IEs) are included, i.e., polycyclic aromatics(IEs < 10.0 eV) in the mass range of 400-900 Da, and aliphatic and alicyclic compounds(IEs close to 11.0 eV) in 200-400 Da. Also detected in the aromatics are metalloporphyrins. Furthermore, the mass spectra of the secondary aromatics separated from LGAR and its resins at different hydrogenation temperatures(390, 400, 410, and 420°C) are also recorded. The results indicate that the hydrogenation process, especially at higher temperatures, results in removal of alkyl-side and bridge chains in the aromatics, and the secondary aromatics from LGAR resins contain more alkyl side and bridge chains and metal compounds than those from LGAR.Although the excellent resolution for residual oil saturation, aromatics and resins with, the current time-of-flight mass spectrometer is still incapable of clear analysis of the most complex component of heavy oil asphaltenes... |
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