(750a) Facilitated Transport Membranes with Tunable Amine-CO2 Chemistry for Highly Selective CO2/H2 Separation

CO₂-selective, amine-containing facilitated transport membranes are of great interest for syngas purification since high-pressure H₂ can be retained upon CO₂ removal. A variety of amine-containing polymers have shown decent chemical and thermal stability at aggressive conditions, but their CO₂/H₂ separation properties are largely limited by the hydrostatic compaction and severe carrier saturation associated with the high syngas pressure. Herein, we report a new approach to enhance the CO₂ permeability by manipulating the steric hindrance of the amine carriers. A series of α-aminoacids with different alkyl or hydroxyethyl substituents are deprotonated by 2-(1-piperazinyl)ethylamine, leading to nonvolatile amine carriers with different degrees of steric hindrance. For hosting the low MW amine carriers, a highly crosslinked poly(vinyl acetal) is synthesized as a water-swellable polymer network. In order to avoid membrane compaction, perforated graphene oxide mono-sheets are dispersed as reinforcement fillers. In the presence of moisture, a bulkier alkyl substituent to the amino site (increasing steric hindrance) destabilizes the carbamate adduct and thus drastically increases the chemisorption of CO₂, while the incorporation of ethylene oxide groups provides additional physisorption of CO₂. The enhanced CO₂ solubility significantly mitigates the carrier saturation behavior, and an unprecedented CO₂/H₂ selectivity greater than 100 is demonstrated at 107 °C and 12.5 atm of CO₂ partial pressure. As the CO₂ partial pressure reduces to 0.4 atm, a less hindered amine yields a higher reactive diffusivity of CO₂, resulting in a CO₂ permeance of 435 GPU with a selectivity greater than 500. These reaction-mediated polymeric membranes are well above the theoretical upper bound, and they are of great interest for designing a highly-selective membrane process for syngas purification.