DFT Study on the Dibenzothiophene Pyrolysis Mechanism in Petroleum

Abstract

Dibenzothiophene sulfur (DBTs), the most stable sulfur species, is present in remarkable concentration in petroleum. Removal of DBTs is with profound significance in environmental protection. In this work, a density functional theory method was adopted to investigate the pyrolysis mechanism of DBT. It was found that the pyrolysis of DBT is possibly started by H-migration or S–C bond rupture. Three main reaction pathways were found. Two dominating pyrolysis pathways are through thiol intermediate pyrolysis and another is through DBT carbene direct dissociation. The dominating products are sulfur-free atoms, 2-ethynyl-benzothiophene, 3-ethynyl-benzothiophene, ethyne, biphenylene, and 1,8-dihydrocyclopentaindene, with a certain amount of benzothiophene and SH radicals. For DBT, BT, and thiophene, the most difficult step is the initial step. The highest energy barrier of DBT is about 17 kcal/mol higher than BT and thiophene, indicating that pyrolysis of DBT needs more intense reaction conditions. Thiophenes can be stable in extreme conditions, which is possibly caused by the high energy barrier of the initial reaction step. After the initial reaction steps, BT and DBT can be desulfurized more easily through thiol intermediates. Searching the catalyst that could lower the energy barrier of the initial step and the reagent that could convert thiophenes directly into thiols may be a potential desulfurization approach for thiophenes.

Unimolecular Pyrolysis Mechanism of Thiophene and Furan: An Ab Initio Comparative Study

“…C 2,3 -carbene/F-C 2,3 -carbene, also known as α-carbene, is the IM formed through the 2,3-H transfer. In the preceding study, − we have found that the H transfer along the carbon skeleton generally has a low energy barrier. Prior to the illustration of α-carbene cracking, the conversion of α-carbene to other carbenes needs to be manifested.…”

Theoretical Study on the Unimolecular Pyrolysis of Thiophene and Modeling

“…The major mechanism of thiophene unimolecular pyrolysis is concluded based on our preceding studies. − All possible initiations, including H shifts, ring bond ruptures, isomerization (non-H-migration), and C–H fission, have been considered in this work.…”

A Probable Origin of Dibenzothiophenes in Coals and Oils

“…In addition, the radical (I) can self-combine to a bithiophenolate (III), whose hydroarylation produces species (IV) with MW = 109 + 32 m +76 k (m = 1; k = 0, 1, 2, 3). With -SNa in an ortho-position of the benzene ring, (IV) cyclize to DBT or NaS-substituted DBT or those with MW = 215 + 76 n (n = 0, 1, 2) in Figure 7 as shown in Reaction (9). On the other hand, the sulfur radical couples with the radical (I) to form a benzenedithiolate and subsequently benzenetrithiolate, accounting for species with MW = 109 + 32 m (m = 0, 1, 2).…”

“…Dibenzothiophene and its derivatives are major polycyclic aromatic sulfur heterocyclics (PASHs) in coals, crude oils and sedimentary organic matter [1][2][3][4]. Due to the excellent micro-biological degradation resistance and a high thermal stability of DBTs [5][6][7][8][9], their geochemical parameters such as distribution and concentration have been widely applied to evaluate the maturity of coals and suggested as the depositional environment indicator of sedimentary organic matter as well as the tracer of hydrocarbon migration pathways [10][11][12][13][14][15][16]. In view of their valuable applications as geochemical markers, great efforts have been devoted to elucidating the origin of DBTs [17][18][19][20].…”