Density functional theory calculations and thermodynamic analysis of pyrolysis mechanism of sulfur-containing compounds in shale oil

Sulfur-containing compounds in shale oil pyrolyzed at varying final temperatures are analyzed using a fixed-bed reactor and a gas chromatography-sulfur chemiluminescence detector (GC-SCD). Density functional theory (DFT) calculations are conducted to construct eight sulfur-containing structural models. Focusing on mercaptans, thiophenes, and benzothiophenes, 21 reaction pathways are designed and analyzed. The reaction energy barriers and energies are systematically evaluated to elucidate the pyrolysis mechanisms of these sulfur compounds. The results indicate that 2-pentanethiol undergoes initial homolytic cleavage of the C–S bond, forming a thiyl radical. This radical reacts with an adjacent methyl hydrogen atom, producing hydrogen sulfide and 1-pentene. The energy barrier and reaction energy for this process are 466.1 and 192.2 kJ/mol, respectively. Compared to β-carbon hydrogen, the thiyl group more readily reacts with α-methyl hydrogen to form hydrogen sulfide. In the presence of hydrogen radicals, 2-pentanethiol is more likely to undergo thiol removal, yielding pentane and hydrogen sulfide. For butanethiol, the thiyl group and β-carbon hydrogen cleave with an energy barrier of 456.1 kJ/mol, forming radicals that combine to produce hydrogen sulfide. The reaction energy is 407.3 kJ/mol. The relative concentration of alkyl thiophenes generally decreases with increasing pyrolysis temperature. In butyl-substituted thiophenes, terminal methyl hydrogen cleaves from the thiophene ring, forming hydrogen gas. Radical reactions follow, producing a C8H10S compound, which undergoes two dehydrogenation steps to yield benzothiophene, with a reaction energy of 725.2 kJ/mol. In contrast, the relative content of alkyl benzothiophenes and alkyl dibenzothiophenes increases with pyrolysis temperature. Thiophenes tend to undergo aromatization of alkyl chain substituents, forming benzothiophenes. The butyl substituent on the benzothiophene side chain initially shifts from a linear to a cyclic structure, followed by intra-molecular dehydrogenation aromatization. The energy barrier and reaction energy for this process are 526.9 and 250.8 kJmol, respectively. The type of alkyl substituent significantly influences the energy required for thiophene compound pyrolysis.