Characteristics of Products of Thermal Decomposition of Heavy Oil Asphaltenes under Supercritical Conditions

This paper presents the results of a comparative study of the composition of asphaltenes of two heavy oils and liquid products of their conversion in a supercritical hexane (SCH) stream. Using elemental analysis, cryoscopy in benzene, infrared and 1H nuclear magnetic resonance spectroscopy, structural group analysis and chromatography, and mass spectrometry, the secondary asphaltenes, the resulted resins and oils, and concentrations of high-molecular-weight nitrogen bases isolated from maltene fractions of thermolysates are characterized. It is shown that the molecular mass of secondary asphaltenes increases as a result of SCH conversion, while the molecular mass of resins decreases, compared to the initial samples. This is due to an increase in the number of structural blocks in mean molecules of secondary asphaltenes and a decrease in their number in mean resin molecules. In both cases, the structural blocks become more compact, because of the decrease in the total number of rings, mainly naphthenic. The structural fragments of the molecules of asphaltenes of both oils are normal alkanes, alkenes, alkyl-substituted monocyclic, tetracyclic, and pentacyclic naphthenes, which are similar in the molecular mass distribution, including unsaturated, monocyclic, bicyclic, tricyclic, tetracyclic, pentacyclic, and hexacyclic aromatic hydrocarbons, benzothiophenes, dibenzothiophenes, naphthobenzothiophenes, and carbazoles. Some of these compounds can be bound in asphaltene molecules via methylene, ether, and sulfide bridges. Among the nitrogen bases of asphaltenes, C11–C15 alkylquinolines, C15–C17 benzoquinolines, C18 dibenzoquinolines, and C16H33NO, C18H35NO, C18H37NO aliphatic amides are identified. Their presence in the composition of asphaltene structures is most likely associated with the participation of high-molecular-weight nitrogen bases in intermolecular interactions, which are due to the structural similarity of their molecules with asphaltene molecules.