Lamellar-Confinement-Induced ZIF-67 Nanosheet Mixed Matrix Membranes for Enhanced CH₄/N₂ Separation

The development of high-performance mixed-matrix membranes (MMMs) for gas separation is frequently constrained by the poor interfacial compatibility between traditional isotropic metal-organic framework (MOF) fillers and polymer matrices. To address this limitation, this study introduces a lamellar-confinement strategy for synthesizing two-dimensional ZIF-67 nanosheets (ZIF-67_ns) using a cetyltrimethylammonium bromide (CTAB) liquid crystal template, which effectively restricts crystal growth along the c-axis and promotes lateral expansion. Notably, water serves as a cosolvent for both the CTAB liquid crystal templating system and ZIF-67: this dual role not only facilitates the self-assembly of CTAB molecules into an ordered liquid crystal phase, but also modulates the coordination of Co²⁺/2-methylimidazole and the nucleation kinetics of ZIF-67, thereby synergistically promoting the oriented growth of 2D nanosheets. The resulting nanosheets exhibit high crystallinity, a well-defined hexagonal morphology with an average thickness of 44.3 nm, and a high specific surface area of 1029.3 m²/g. Upon incorporation into a styrene-ethylene-butylene-styrene (SEBS) copolymer matrix, the ZIF-67_ns significantly enhance the polymer-filler interfacial contact area. The fabricated MMMs demonstrate a dramatic improvement in CH₄ permeability, increasing from 24.3 to 278.4 barrer at a 30 wt % loading─an enhancement of approximately 1050%, while maintaining a CH₄/N₂ selectivity of 5.15. This study suggests the significant potential of morphology-controlled two-dimensional MOFs for constructing highly efficient gas separation membranes.