13C-Methylation and 13C NMR Reveal Sulfur-Containing Structures in Residual Oil and Its Hydrogenated Product

To support the optimization and screening of catalysts for the residue hydrodesulfurization (HDS) process, it is essential to conduct in-depth characterization of sulfur-containing compounds (SCCs) in the residue. In this study, a combination of 13 C-methylation derivatization and 13 C nuclear magnetic resonance (NMR) spectroscopy was employed to investigate the structural details of sulfur-containing compounds in residual oil on the basis of diverse model compounds. The results showed that the chemical shift of the 13 C-methyl group attached to the sulfur atom in the internal-type thiophenic aromatic compounds (such as dibenzothiophene) is greater than 35 ppm, while the chemical shift of the 13 C-methyl group attached to the sulfur atom in the thiophenes, terminal-type thiophenic aromatic compounds (such as naphthothiophene), and those with alkyl substituents is 29–35 ppm. When the 13 C-methyl is substituted on the carbon atom in the terminal thiophene ring, its chemical shift is within the range of 10–15 ppm. In addition, the chemical shift range of 15–29 ppm is observed and attributed to the 13 C-methyl and alkyl groups substituted on the aromatic ring in both the internal-type and terminal-type thiophenic aromatic compounds. These rules were further applied to the characterization of the residue oil and its hydrodesulfurization product. The analytical results indicate that the molecular structure of refractory sulfur compounds in the deep hydrotreated residue is dominated by the sterically hindered internal-type thiophenes with long alkyl side chains. The established method can be used for the characterization of isomers of sulfur-containing compounds in the residue and other petroleum products, thereby providing reference information for subsequent hydrodesulfurization and hydrogenation research.