The Effect of Electron-Donating Groups and Hydrogen Bonding on H2S Capture over Polyethylene Glycol/Amine Sites

The reversible adsorption of H2S on tetraethylenepentamine (TEPA) was investigated using attenuated total reflection infrared (ATR-IR) spectroscopy, mass spectrometry, and density functional theory (DFT). The in situ infrared method revealed H2S ability to selectively poison the TEPA amine active site by the formation of a strongly adsorbed species in the form of (HS–) NH3+-TEPA. In addition, the H2S–amine interaction, in the presence of oxygen, resulted in the oxidative degradation of the amine, indicated by the formation of an −NO2 species. The addition of polyethylene glycol (PEG) affects the H2S-amine interactions and prevents the amine site from oxidative degradation. Both experimental IR and DFT calculations revealed that PEG affected the electronic and geometric environment around the amine binding site through hydrogen bonding and electron-donating effects. The addition of PEG decreased the (+) charge of the ammonium ion site, effectively decreasing electrostatic anion–cation interaction of the strongly bound HS– species at the amine active site. This decrease in anion–cation interaction brought about by PEG resulted in lowering the binding energy of H2S on the amine site and decreasing the extent of oxidative degradation of the amine site. The addition of PEG also increased the adsorption capacity of the TEPA sorbent from 0.71 μmol H2S to 1.10 μmol H2S/g sorbent at 9.1 wt % PEG in the TEPA thin film, increasing the H2S adsorption capacity by 1.5 times. The qualitative aspect of this study suggests a way to tailor the amine active site using PEG to improve the adsorption capacity and regenerability of amine-based sorbents.