Residual Hydrogen Sulfide in Recovered Liquid Sulfur: A Computational Investigation.

The nature, structure, stability and reaction of sulfur clusters (Sn) with H2S leading to the formation of hydrogen polysulfanes (HSn+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 Sn (n ≤ 4) possess open chain ground state structures while S5 and S8 have cyclic ground state structures and that as n 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 HSn+1H are only formed on the singlet surface of the Sn + H2S reaction where Sn are open chain structures. The unbranched HSn+1H are formed in exergonic processes while branched HSn+1H are either formed in exergonic or endergonic processes depending on n. 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.