Mechanism insight into the formation of H 2 S from thiophene pyrolysis: A theoretical study

Pyrolysis is an efficient and economical method for the utilization of waste rubber, but the high sulfur content limits its industrial application. Currently, the migration and transformation of the element S during pyrolysis of waste rubber is far from well known. In this work, a density functional theory (DFT) method was employed to explore the possible formation pathways of H2S and its precursors (radicals HS · and S ·) during the pyrolysis of thiophene, which is an important primary pyrolytic product of rubber. In particular, the influence of reactive hydrogen radicals was carefully investigated in the thiophene pyrolysis process. The calculation results indicate that the decomposition of thiophene tends to be initiated by hydrogen transfer between adjacent carbon atoms, which needs to overcome an energy barrier of 312.4 kJ/mol. The optimal pathway to generate H2S in thiophene pyrolysis involves initial H migration and S-C bond cleavage, with an overall energy barrier of 525.8 kJ/mol. In addition, a thiol intermediate that bears unsaturated C-C bonds is essential for thiophene pyrolysis to generate H2S, which exists in multiple critical reaction pathways. Moreover, the presence of hydrogen radicals significantly changes the decomposition patterns and reduces the energy barriers for thiophene decomposition, thus promoting the formation of H2S. The current work on H2S formation from thiophene can provide some theoretical support to explore clean utilization technologies for waste rubber.