Thin Films Based on Polyimide/Metal–Organic Framework Nanoparticle Composite Membranes with Substantially Improved Stability for CO₂/CH₄ Separation

As one of the gas separation membranes, the thin-film nanocomposite (TFN) membrane can effectively reduce gas transport resistance and improve gas permeance. However, due to the high mobility of the chain in the thin film, it is still a great challenge to realize the TFN membrane with stable separation performance. In this work, we report a less destructive and efficient cross-linking strategy based on metal-ion coordination to improve the stability of the TFN membrane for CO2/CH4 separation. The selective layer is made of carboxylated polyimide as a matrix and UiO-66 nanoparticles (diameters of ∼50 nm) as fillers. By simply immersing TFN membranes in Cu(NO3)2 solution, coordination bonds are successfully constructed between Cu2+ and carboxyl groups (−COOH). The cross-linked TFN membranes exhibit high CO2/CH4 selectivity of up to 29–35 with CO2 permeance of 110–350 GPU, outperforming most previously reported membranes. Moreover, the plasticization pressure of the cross-linked membranes improves from 0.3–0.9 to 0.9–1.5 MPa, higher than most reported asymmetric and hollow fiber membranes. In the CO2/CH4 (50:50 v/v) mixed-gas permeation tests, the cross-linked membranes retain constant CO2/CH4 selectivity at 23.9–25.2 with increasing mixed-gas feed pressure. The cross-linked membranes also demonstrate stable CO2/CH4 selectivity in the range of 27.2–31.2 within 100 h of testing time under a 1.0 MPa CO2 partial pressure.