Molecular-scale co-assembly membranes derived from keplerate cluster: carbonic anhydrase-mimicking nanocapsules for enhanced CO₂/N₂ separation

The development of CO₂ separation membranes with high permeability and high selectivity, as well as ultra-thin selective layers, has always been challenging. Herein, a molecular-scaled co-assembly strategy is employed to fabricate the Pebax-Mo₁₃₂ (Pebax = polyether-block-amide copolymer; Mo₁₃₂ = (NH₄)₄₂[Mo₇₂^VIMo₆₀^VO₃₇₂(CH₃COO)₃₀(H₂O)₇₂]) membranes. The optimal self-standing membrane, Pebax-Mo₁₃₂-5%, shows a CO₂ permeability of ∼384 Barrer and an ultra-high ideal CO₂/N₂ selectivity of ∼244, outperforming most membranes reported in the literature. The CO₂ permeability and ideal CO₂/N₂ selectivity are increased by 70% and 367%, respectively, compared with the pristine Pebax membrane. A thin-film composite membrane prepared by spin-coating technique on a support membrane with gutter layers also exhibits a CO₂ permeance of 838 GPU and a CO₂/N₂ selectivity of 136. Such excellent performance can be attributed to the following reasons: (1) strong hydrogen bonding interactions between {Mo₁₃₂} clusters and Pebax confer excellent interfacial compatibility to the mixed matrix membranes; (2) incorporation of hollow {Mo₁₃₂} clusters into the Pebax molecular chain decreases the crystallinity of Pebax, and thereby accelerates the chain dynamics and increases the free volume of the membrane; (3) in situ diffuse reflectance infrared Fourier-transform spectroscopy demonstrates that the {Mo₁₃₂} clusters can effectively catalyze the hydration reaction of CO₂ and promote the transport of CO₂; (4) furthermore, the 0.35 nm pores of the crown ether-type {Mo₉O₉} allow the accurate size sieving of CO₂ (0.33 nm) and N₂ (0.36 nm) molecules.