Mixed matrix membranes of PIM-1 incorporating MOF-808 functionalized with amino acids for enhanced CO2/CH4 separation

Abstract

This study investigates the enhancement of CO₂/CH₄ separation performance in mixed matrix membranes (MMMs) by incorporating amino acid-functionalized MOF-808 (MOF-808@AA; AA = Lys, Arg and Cys) into a polymer of intrinsic microporosity (PIM-1) matrix. MMMs containing 10 wt% MOF-808@AA were fabricated via solution casting. The structural and physicochemical properties of PIM-1, MOF-808@AA, and the resulting MMMs were characterized using ¹H-NMR, FTIR, TGA, SEM, XRD, SAXS, N₂/77 K and CO₂/15 °C isotherms, and high-pressure CO₂ adsorption. Collectively, the characterization results confirm the successful incorporation of MOF-808@AA into the PIM-1 membranes, with no detectable alteration of the nanostructure of the PIM-1 matrix. Gas separation measurements revealed a notable increase in both CO₂ permeability and CO₂/CH₄ selectivity upon incorporation of MOF-808@AA fillers. Among the tested membranes, the PIM-1/MOF-808@Lys MMM functionalized with lysine exhibited the best performance, achieving a CO₂ permeability of 14 354 ± 201 Barrer and a CO₂/CH₄ selectivity of 26.4 ± 1.8. Aging effects were also studied, showing a reduction in CO₂ permeability accompanied by a slight increase in selectivity over time. In addition, the single and mixed gas experiments at elevated pressures (up to 6 bar) showed that CO₂ permeability under mixed gas conditions was slightly higher than in single gas experiments, supporting the potential of these MMMs for application in realistic CO₂/CH₄ separation processes under practical operating conditions. Inelastic neutron scattering (INS) and density functional theory (DFT) calculations confirmed that CO₂ molecules preferentially interact with the amino groups introduced by AA functionalization. DFT simulations further revealed that while CO₂ and CH₄ access similar adsorption sites, the interaction energy of CO₂ with amino groups is approximately three times higher than that of CH₄. These findings demonstrate that MOF-808@AA fillers can significantly enhance the CO₂-philicity of PIM-1-based MMMs, offering a promising approach for efficient CO₂/CH₄ separation compared to pristine PIM-1 or unmodified PIM-1/MOF-808 composites.