New sterically hindered polyvinylamine-containing membranes for CO2 capture from flue gas

Amine-containing facilitated transport membranes can have both high permeability and selectivity due to the reversible reaction between CO2 and amino groups for effective carbon capture from flue gas. We have developed an improved method for the synthesis of high-molecular-weight sterically hindered polyvinylamine (PVAm) as the new fixed-site carrier in thin-film composite membranes for CO2 capture. Commercially available PVAm was N-monomethylated into poly-N-methyl-N-vinylamine (PVAm-CH3) using the stepwise reductive amination with a highly polar fluorinated alcohol as the solvent to enhance the equilibrium shift to the formation of the imine intermediate. The method prevented over-alkylation, resulting in the increased yield of the target product. PVAm-CH3 exhibited excellent CO2 facilitation with a CO2 permeability of 445.7 Barrer (1 Barrer = 3.349 × 10−16 mol m m−2 s−1 Pa−1) and a CO2/N2 selectivity of 70.3, which are above the Robeson 2008 upper bound. The PVAm-CH3 solution retained a sufficiently high viscosity after incorporating the amino acid salt, 2-(1-piperazinyl)ethylamine sarcosinate (PZEA-Sar), as the mobile carrier for the membrane coating on nanoporous polyethersulfone (PES) substrates without any penetration issues. The resultant thin-film composite PVAm-CH3/PZEA-Sar membrane with a thickness of approximately 170 nm exhibited a superior CO2 performance of 1071 GPU (1 GPU = 3.349 × 10−10 mol m−2 s−1 Pa−1) and a CO2/N2 selectivity of 183 at 57 °C and a feed gas pressure of 111.64 kPa (1.5 psig). This PVAm-CH3/PZEA-Sar membrane surpassed the latest redefined 2019 CO2/N2 upper bound and outperformed other polymer-based membranes. Density functional theory calculations also demonstrated that PVAm-CH3 showed a stronger preference, relative to PVAm, toward the more efficient bicarbonate pathway. Thus, the steric hindrance effect of PVAm-CH3 enhanced the solubility of CO2 in the polymer matrix and resulted in the higher CO2 permeance.