(351x) Facilitated Transport Membranes with Tunable Amine–CO2 Chemistry for Hydrogen Purification

CO₂-selective, amine-containing facilitated transport membranes (FTMs) are of great interest for syngas purification since high-pressure H₂ can be retained upon CO₂ removal. Various FTMs have shown decent chemical and thermal stability at aggressive conditions, but their CO₂/H₂ separation properties are largely limited by the severe carrier saturation at high syngas pressure. Herein, we report a new approach to enhance the CO₂ permeability by manipulating the steric hindrance of the amine carrier. A series of α-aminoacids with different alkyl or hydroxyethyl substituents were deprotonated by 2-(1-piperazinyl)ethylamine, resulting in nonvolatile amine carriers with different degrees of steric hindrance. For hosting the low MW amine carriers, a water-swellable polymer network was synthesized from poly(vinyl alcohol) crosslinked by a bidentate tertiary aminosilane. In the presence of moisture, a bulkier alkyl substituent increased the steric hindrance and hence destabilized the carbamate adduct to afford bicarbonate through hydrolysis. Thus, this drastically increased the chemisorption of CO₂. Further, density functional theory (DFT) calculations were conducted to study the function of the hydroxyethyl substituent, which indicated that the hydroxyl group stabilized the bicarbonate through strong hydrogen bonding, thus further improving the CO₂ sorption. The enhanced CO₂ solubility significantly mitigated the carrier saturation, and an unprecedented CO₂/H₂ selectivity greater than 130 was demonstrated at 107°C and 12.5 atm of CO₂ partial pressure. As the CO₂ partial pressure reduced to 0.4 atm, a less hindered amine yielded a higher reactive diffusivity of CO₂, resulting in a CO₂ permeance of 435 GPU with a selectivity greater than 500. The CO₂/H₂ separation performance of these reaction-mediated polymeric membranes is well above the theoretical upper bound, and they open up a new avenue for designing a highly selective membrane process for syngas purification.