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

PAN Shuo; ZHANG Yu; WANG Qing; BAI Jingru; XU Xiangming; WANG Qi; LIU Bin

doi:10.13225/j.cnki.jccs.2024.0918

Abstract：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 systemat-ically 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 re-moval,yielding pentane and hydrogen sulfide.For butanethiol,the thiyl group and β-carbon hydrogen cleave with an en-ergy 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 dibenzo-thiophenes 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 pro-cess are 526.9 and 250.8 kJmol,respectively.The type of alkyl substituent significantly influences the energy required for thiophene compound pyrolysis.