Deep Eutectic Solvents as Catalysts for Hydrogen Sulfide Splitting via the Iodine Thermochemical Cycle: A Theoretical Study of a Potential Route to Hydrogen Production

Hydrogen sulfide (H₂S) splitting via the H₂S─I₂ thermochemical cycle is a promising and potentially scalable approach to simultaneously treat H₂S and produce H₂. Yet, the kinetic hindrance of the H₂S─I₂ thermochemical cycle in the aqueous phase remains a challenge that needs to be tackled to enable a more energy-efficient technology for H₂S splitting applications. Therefore, in this computational work, we explore the use of deep eutectic solvents (DESs) in aqueous solution as an alternative medium and catalysts to overcome the kinetic hindrance of the H₂S─I₂ thermochemical cycle and enable its implementation for H₂S treatment and H₂ production applications. In a systematic approach, our extensive computational work assisted in identifying DESs with certain functional groups with catalytic activity to mitigate the inherent kinetic hindrance of the H₂S─I₂ thermochemical cycle. First, we selected three DESs with the potential to interact with sulfur-containing species, i.e., ChCl:2EG, ChCl:2Urea, and ChCl:pTSA. These DESs feature functional groups with different properties: ─OH, ─NH₂, C═O, ─SO₂OH, and an aromatic ring. Then, the previously characterized saddle points for the initiation and rate-limiting steps of the H₂S─I₂ cycle (ACS Sustainable Chem. Eng. 2025, 13, 13365-13385) were docked into the structure of each DES to screen the conformational space of the supermolecule by means of aqueous phase quantum chemistry calculations. This screening was aimed at characterizing numerous saddle point conformers for each targeted reaction of the H₂S─I₂ cycle, resulting in Gibbs free energy barrier distributions that were used to elucidate the potential catalytic activity of each DES. Our results indicate that the ChCl:2EG DES interacts more favorably with the considered saddle points due to its several ─OH polar groups that can be easily accommodated to establish numerous and stabilizing hydrogen bonds, as opposed to the more rigid structure of the ChCl:2Urea and ChCl:pTSA DESs caused by their double bonds and aromatic ring. Moreover, the ChCl:2EG DES revealed an alternative and more kinetically favored route for the investigated reactions in which the DES participates chemically by inducing, with its ─OH groups, a double hydrogen atom transfer reaction. Our theoretical study suggests potential catalytic activity on the H₂S─I₂ cycle by alcohol-based DESs that lack double bonds or ring functional groups. However, the reported catalytic activity and favorable conditions of the proposed reaction medium need to be experimentally proved before any practical implementation can be considered.