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

Thiophene sulfur is the most stable and abundant organic sulfur species in petroleum. In this work, the unimolecular pyrolysis mechanism of thiophene was investigated using high-level ab initio methods. As a contrast, similar reactions of furan were computed using the same methods. It was found that the most likely initiation reactions for thiophene unimolecular pyrolysis are 2,3-H and 3,2-H transfers. The 3,2-H shift forms buta-2,3-dienethial, which subsequently decomposes to CS + CH3C2H and HC2CH2 radical (*) + HCS*, whereas the 2,3-H migration corresponds to alpha-carbene and ultimately leads to C2H2 + SCCH2, CS + CH3C2H, HC2CH2* + HCS*, and S + C2CHC2H. H2S is supposed to be yielded through secondary reactions. Combined with thermodynamics, the favored products are concluded to be C2H2 + SCCH2. Thiophene is kinetically and thermodynamically more stable due to the higher energy barrier of the reaction pathway and the lower thermal stability of the products.