Zinc Coordination-Polymer-Mediated Self-Assembly of Nanoparticles into "Brick-and-Mortar" Membranes for Hydrogen...

Zeolitic imidazolate framework-8 (ZIF-8) with high stability and porosity is a promising candidate for hydrogen separation membranes. However, most ZIF-8 polycrystalline membranes exhibit low H₂/CO₂ (kinetic diameters of 2.9/3.3 Å) mixed gas selectivity, due to the intercrystalline defects and the unprecise molecular sieving originated from framework flexibility of ZIF-8 structure with a theoretical aperture size of 3.4 Å. Here, inspired by nacre's "brick-and-mortar" structure, we develop mixed matrix type composite membranes in which dominant crystalline ZIF-8 nanoparticles (bricks) are interconnected by ultrathin zinc coordination polymer interlayers (mortar) via self-assembling. Driven by coordination bonds between Zn²⁺ from precursor colloid and branched polyethyleneimine (PEI), a zinc coordination polymer network is formed to connect ZIF-8 nanoparticles through interactions between Zn²⁺ of coordination polymer and surface terminal groups on ZIF-8 nanoparticles, thus eliminating intercrystalline void defects and providing a highly selective H₂ transport pathway. Meanwhile, the micropores and large cavities in ZIF-8 allow fast H₂ transport. Benefitting from both highly selective pathway and fast H₂ transport through porous ZIF-8, the optimized ZIF-8-PEI membrane exhibits a record-high H₂ permeability of ~1.78×10⁵ Barrer with a high mixed gas H₂/CO₂ selectivity of 176, surpassing state-of-the-art performance. This bioinspired multifunctional membrane expands the scope of molecular sieving membrane.