NMR Study on the Reactivity of Hydrogen Sulfide in Aqueous Amine Solutions

Removing hydrogen sulfide (H₂S) from gas streams is a critical task in many industrial processes, e.g. in natural gas cleaning where it is generally carried out together with the removal of carbon dioxide (CO₂) by reactive absorption. The most common solvents for the reactive CO₂ absorption are aqueous amine solutions, sometimes with alcohols as cosolvents. The sour gases H₂S and CO₂ dissociate in these basic solutions, leading to high solubilities, which are further enhanced by chemical reactions of CO₂ with many amines. It is usually assumed that H₂S only dissociates in these solvents but does not react chemically as CO₂ does; however, no experimental proof of this hypothesis has been given in the literature so far. Therefore, we have carried out an NMR study of the speciation of aqueous amine solutions containing N-methyldiethanolamine (MDEA), monoethanolamine (MEA), piperazine (PZ), and ethanol (EOH), which were loaded with H₂S at pressures between 2.7 and 4.8 bar at 323.15 K. To identify possible chemical reactions of H₂S with the solvents, the resulting NMR spectra were compared with the NMR spectra of the unloaded aqueous amine solutions. The dissociation of H₂S in the basic amine solutions leads to protonation of the amines and strong NMR peak shifts, but no reaction products of H₂S with MDEA, MEA, PZ, and EOH were found. We expect that this is also true for other amines and alcohols and confirms the hypothesis that the solubility of H₂S in the studied solvent class is not affected by chemical reactions with the solvents.