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

Abstract

Pyrolysis is an efficient utilization method for Scontaining organic solid wastes to produce value-added chemicals and fuels but is accompanied by gaseous pollutants (e.g., H 2 S and COS). The selective transformation of sulfur to H 2 S 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 Scontaining radicals/gases (S, SH, CS radicals, and H 2 S) 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 Scontaining species formation was carefully studied to elucidate the enrichment method for H 2 S. 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 ≈ H 2 S > CS radical ≈ S radical, indicating the dominance of H 2 S compared with those of COS and CS 2 formed through CS radicals. Alkali metal ions can enhance the decomposition of thiophene and are beneficial to the direct formation of H 2 S 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 H 2 S in the gas phase. The elucidation of the enhanced H 2 S formation method can lay a theoretical basis for the waste-to-value utilization of H 2 S in the pyrolysis gas.