Amino-Functionalized ZIF-7 Nanocrystals: Improved Intrinsic Separation Ability and Interfacial Compatibility in Mixed-Matrix Membranes for CO₂/CH₄ Separation

Highly permeable and selective, as well as plasticization-resistant membranes are desired as promising alternatives for cost- and energy-effective CO₂ separation. Here, robust mixed-matrix membranes based on an amino-functionalized zeolitic imidazolate framework ZIF-7 (ZIF-7-NH₂) and crosslinked poly(ethylene oxide) rubbery polymer are successfully fabricated with filler loadings up to 36 wt%. The ZIF-7-NH₂ materials synthesized from in situ substitution of 2-aminobenzimidazole into the ZIF-7 structure exhibit enlarged aperture size compared with monoligand ZIF-7. The intrinsic separation ability for CO₂/CH₄ on ZIF-7-NH₂ is remarkably enhanced as a result of improved CO₂ uptake capacity and diffusion selectivity. The incorporation of ZIF-7-NH₂ fillers simultaneously makes the neat polymer more permeable and more selective, surpassing the state-of-the-art 2008 Robeson upper bound. The chelating effect between metal (zinc) nodes of fillers and ester groups of a polymer provides good bonding, enhancing the mechanical strength and plasticization resistance of the neat polymer membrane. The developed novel ZIF-7 structure with amino-function and the resulting nanocomposite membranes are very attractive for applications like natural-gas sweetening or biogas purification.