(642c) Selective H2s/CO2 Separation Using Sterically Hindered Amine Membranes

H₂S, an extremely toxic and corrosive gas, is a common contaminant in fuel gas mixtures like syngas and natural gas. The selective removal of H₂S from such CO₂-containing fuel gases is important for enabling their industrial processing. This work describes the development of amine-based facilitated transport membranes (FTMs) with high H₂S/CO₂ selectivity at elevated temperature (> 100 °C) and explores the underlying mechanisms of acid gas transport in FTMs.

A series of primary, secondary, and tertiary amine carriers with different alkyl substituents was used to synthesize FTMs. Their H₂S/CO₂ transport performances were evaluated at 107 °C and 7 bar. Sterically hindered amines showed enhanced H₂S/CO₂ separation performance, with the H₂S/CO₂ selectivity rising from 4 to nearly 20 as the amine hindrance was increased. The best performance was obtained using a severely hindered di-tert-butylamine carrier, which showed a high H₂S/CO₂ selectivity of 19.6 and an H₂S permeance of 560 GPU. These results are supported by Density Functional Theory (DFT) calculations. The DFT results indicated that the presence of sterically bulky groups on the N site of the amine can restrict its reactivity to CO₂ through the carbamate pathway. Conversely, the amine-H₂S reaction was found to be relatively insensitive to steric hindrance. Finally, the carrier saturation behavior of the di-tert-butylamine carrier was investigated at H₂S concentrations of 0.5—30%. The carrier was found to be nearly inert to CO₂, achieving H₂S/CO₂ selectivities of 25.4 — 9 and H₂S permeances of 760—220 GPU. Overall, the newly synthesized membranes show good potential for syngas desulfurization, and the learnings offer some insight into the design of carriers for acid gas purification.