Gas-Specific and Tunable Hydrogen-Selective ZIF Membrane through Combined Physical Confinement and Sealing Techniques

Grain boundaries in polycrystalline metal-organic framework (MOF) membranes impede high-level gas separation performance. Herein, a new sealing method is introduced by combining a fluorinated polymer and reactive ion plasma treatment to physically tune the permeation of the polycrystalline zeolitic imidazolate framework-8 (ZIF-8) membranes. In particular, the sealing technique is applied to the ZIF-8 membrane hybridized with graphene nanoribbon with an intrinsic aperture size of 3.4 Å, and the sealing method can be easily tuned depending on the target gas pairs. First, the ultrathin perfluoropolyether (PFPE) layer, formed via dip-coating, effectively blocks the permeation of large molecules through the non-selective grain boundaries, resulting in the H₂ permeance of 1.3 × 10^-6 mol m^-2·Pa·s and 209 of the ideal H₂/N₂ selectivity. Second, methane permeation can be further hindered by additional reactive ion plasma treatment, which enhances the fluorination of the sealing layer, resulting in 1218 of the ideal H₂/CH₄ selectivity. Due to the physical confinement with graphene nanoribbon and thin sealing layer, the membrane is highly selective and permeable for hydrogen, far surpassing the performance of previous MOF-type membranes. Molecular dynamics simulations reveal MOF membranes' critical grain boundary gap degrading selectivity.