Residual Hydrogen Sulfide in Recovered Liquid Sulfur: A Computational Investigation

The nature, structure, stability and reaction of sulfur clusters (S_<i>n</i>) with H₂S leading to the formation of hydrogen polysulfanes (HS_<i>n</i>+1H) in a critical equilibrium in the modified thermal Claus process are investigated using density functional theory (DFT) and coupled cluster (CC) theory. The computations reveal that S_<i>n</i> (<i>n</i> ≤ 4) possess open chain ground state structures while S₅ and S₈ have cyclic ground state structures and that as <i>n</i> increases for open chain structures, a switch in stability of spin states occurs at S₃ and S₅. Triplet state open chains of S₅ and S₈ are computed to be more stable than their singlet state analogues, even though their global minimum cyclic structures are singlet state species. This provides a plausible explanation for the paramagnetic behavior of liquid sulfur. Furthermore, the results suggest that branched and unbranched HS_<i>n</i>+1H are only formed on the singlet surface of the S_<i>n</i> + H₂S reaction where S_<i>n</i> are open chain structures. The unbranched HS_<i>n</i>+1H are formed in exergonic processes while branched HS_<i>n</i>+1H are either formed in exergonic or endergonic processes depending on <i>n</i>. The reaction product on the triplet surface in all cases may best be described as weakly attracted doublet species that are always formed in endergonic processes. This suggests that the exergonicity of the reverse of the triplet surfaces for these reactions may be responsible for the residual H₂S found in recovered liquid sulfur.