Effect of Humidity on the Sorption of H₂S from Multicomponent Acid Gas Streams on Silica-Supported Sterically Hindered and Unhindered Amines

The sorption of H₂S on a silica-supported sterically hindered secondary amine and a supported unhindered tertiary amine is investigated using fixed bed breakthrough experiments. In situ Fourier transform infrared spectroscopy and computational analysis are used to elucidate the structure of the surface species formed during humid H₂S sorption. Additionally, the cyclic stability of the amine-functionalized silica sorbent is assessed using NMR spectroscopy, N₂ physisorption, and elemental analysis. For all sorbents under dry conditions, the physisorption of H₂S on the amines led to low H₂S sorption capacities, but under humid conditions, a strongly chemisorbed species is formed, which leads to an increase in the H₂S sorption capacity. These findings are supported by density functional theory (DFT) calculations, which support the formation of a chemisorbed and stable HS^– species in the presence of H₂O. As a result of the strongly bound species, the sorbents require humid temperature swing sorption–desorption cycles for efficient sorbent regeneration under the conditions studied, with temperature swings using dry gas failing to effectively desorb the H₂S. For competitive H₂S/CO₂ sorption under mixed gas conditions (CH₄/CO₂/H₂S/H₂O), the selectivity for H₂S sorption over CO₂ is demonstrated to result from kinetic factors, with reaction barriers playing a major role under humid conditions. Overall, the structure of the silica framework and organic moieties are maintained for both sorbents after extended humid H₂S exposure. The results in this work confirm the positive effect of humidity on the H₂S sorption capacity of silica-supported sterically hindered and unhindered amines.