Non-selective Defect Minimization towards Highly Efficient Metal-Organic Framework Membranes for Gas Separation

The persistence of defects in polycrystalline membranes poses a substantial obstacle to reaching the theoretical molecular sieving separation and scaling up production. The low membrane selectivity in most reported literature is largely due to the unavoidable non-selective defects during synthesis, leading to a mismatch between the well-defined pore structure of polycrystalline molecular sieve materials. This paper presents a novel approach for minimizing non-selective defects in metal-organic framework (MOF) membranes by a constricted crystal growth strategy in a confined environment. The in situ ZIF formation using the densely packed seeding array between the substrate and the pre-grown top ZIF layer yields a confined membrane interlayer, which is highly uniform with a tightly packed crystalline structure. Unlike uncontrolled crystal growth, we purposely regulate the interlayer membrane growth in the direction parallel to the substrate. A notable 99 % decrease in defects in the confined interlayer was achieved compared to the random-grown top layer, leading to a ~353 % increment in H₂/N₂ selectivity over the non-confined reference MOF membrane. The performance of this new membrane sits in the optimal range above the Robeson upper bound. The membrane boasts a balanced high H₂ permeability (>5000 Barrer) and selectivity (>50), significantly surpassing peer ZIF membranes.