Mixed Matrix Membranes of PIM-1 Incorporating MOF-808 Functionalized with Amino Acids for Enhanced CO2/CH4 Separation.

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.% of 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, N2/77K and CO2/15ºC 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 (14354 ± 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. Keywords: MOF-808@amino acid, PIM-1 polymer, Mixed matrix membrane (MMM), CO2/CH4 separation, DFT calculation.