(696b) Steric Hindrance Effect Demonstrated in Polyvinylamine Facilitated Transport Membranes for CO2 Separation

CO₂ capture from flue gas (main component: N₂) is essential for eliminating greenhouse gas emission, and CO₂ separation from synthesis gas is effective for hydrogen purification. Due to the inherent advantages of membrane separation, membrane technology is receiving more and more attention for CO₂ separation. For most conventional polymer membranes, CO₂/N₂ separation follows the solution-diffusion mechanism. The key challenge of solution-diffusion membranes is the trade-off between permeability and selectivity, i.e., the Robesonâ??s Upper Bound. However, for facilitated transport membranes, the non-reactive gas component, e.g., N₂, diffuses through the membrane via the solution-diffusion mechanism, whereas CO₂ follows the facilitated transport mechanism via its reversible chemical reaction with carriers (often amines). Therefore, CO₂ transport is facilitated, and both high CO₂ permeability and high CO₂/N₂ selectivity can be achieved.

Amine carriers are the key for the CO₂ separation performance of a facilitated transport membrane. Amine carriers exhibiting a high CO₂ loading capacity and reaction rate are always desirable. It has been reported that the CO₂ loading capacity of sterically hindered amine can be twice of unhindered amine in aqueous solution. In this work, high-molecular-weight polyvinylamine (PVAm) was successfully synthesized and modified into sterically hindered polyamines with different degrees of steric hindrance via alkylation reactions. The membranes were synthesized by coating the sterically hindered polyamine solutions with suitable viscosity onto a nanoporous polyethersulfone substrate. Under the typical conditions for CO₂ capture from flue gas (57 °C, 1 atm, 100% relative humidity, 20% CO₂ in the feed gas balanced with N₂ on a dry basis), polyvinylamine membranes showed an average CO₂ permeability of 214 Barrers and CO₂/N₂ selectivity of 48.5. Although the content of amino groups reduced by incorporation of alkyl groups, CO₂ permeability was improved by 24% and CO₂/N₂ selectivity was increased by 14% by modifying polyvinylamine into poly(N-methyl-N-vinylamine). These results have demonstrated the effect of steric hindrance with the incorporation of the methyl group for enhancing CO₂ separation performance in the solid membrane phase. This is one of very few pieces of work demonstrating the steric hindrance in the solid membrane phase. Poly(N-isopropyl-N-vinylamine) and poly(N-tert-butyl-N-vinylamine) membranes showed 15% and 11% improvements on CO₂ permeability, respectively, with no significant changes in CO₂/N₂ selectivity. Comparing the CO₂ separation performances of the above polyamine membranes and their corresponding steric hindrance degrees, we found that the CO₂ permeability increased first significantly with increasing the steric hindrance degree and then less pronouncedly with further increasing the steric hindrance degree. This could be explained by the reduced amino group content in the membranes and the lower reaction rate due to the incorporation of bulky groups onto amino groups. Based on the gas transport results, poly(N-methyl-N-vinylamine) has demonstrated the largest improvement on membrane performance, making it a promising candidate as the next-generation fixed-site carrier for CO₂ facilitated transport membranes.