Formation of Sulfur Species from Thiophene Decomposition and the Impact of Alkali Metal Ions: A Density Functional Theory Study

Pyrolysis is an efficient utilization method for S-containing organic solid wastes to produce value-added chemicals and fuels but is accompanied by gaseous pollutants (e.g., H2S and COS). The selective transformation of sulfur to H2S in the pyrolysis gas will be beneficial for the quality improvement of liquid and solid products, as well as the further management of sulfur. However, the S-containing gas formation mechanism, especially the influence mechanism of widely presented alkali metals, is still unclear. Herein, the formation pathways of S-containing radicals/gases (S, SH, CS radicals, and H2S) was investigated by combining density functional theory (DFT), kinetic calculations, and wave function analysis with thiophene as the model compound. The impact of K+ and Na+ on S-containing species formation was carefully studied to elucidate the enrichment method for H2S. Thiophene decomposition is preferentially initialized by the 1,2-hydrogen transfer reaction. The formation competitiveness of S-containing species is in the order SH radical ≈ H2S > CS radical ≈ S radical, indicating the dominance of H2S compared with those of COS and CS2 formed through CS radicals. Alkali metal ions can enhance the decomposition of thiophene and are beneficial to the direct formation of H2S and S radicals through enhanced hydrogen transfer reactions. An inferior generation path of SH radicals is significantly enhanced, and the CS radical formation is inhibited in the presence of K+ and Na+. Consequently, alkali metal ions are conducive to direct and indirect generation and enrichment of H2S in the gas phase. The elucidation of the enhanced H2S formation method can lay a theoretical basis for the waste-to-value utilization of H2S in the pyrolysis gas.