Orientation-Controlled MOF Membranes Enabling Switchable H₂/CO₂ Separation

Controlling the crystal orientation of metal-organic framework (MOF) membranes introduces a new dimension to chemical separations, as the alignment of anisotropic pore channels dictates the membrane's separation performance. Here, we demonstrate that the gas separation behavior of the MOF membrane can be reversibly switched by tailoring their crystal orientation. Through a solvent-induced liquid-vapor deposition (LVD) strategy, we realize the direct, seed-free growth of a highly (112)-oriented ultrathin KAUST-7 membrane. For comparison, a (001)-pore-aligned KAUST-7 membrane is prepared via secondary growth. In contrast to conventional physical size-preferred orientations, variation in channel alignment here reconfigures the exposure of fluorine-rich NbOF₅^2- adsorption centers toward CO₂, establishing a chemical-preferred orientation that induces transition between entropy and enthalpy dominant molecular transport. Consequently, gas permeation tests reveal that the (112)-oriented KAUST-7 membrane exhibits an H₂/CO₂ selectivity of 52.7, whereas the (001)-pore-aligned KAUST-7 membrane shows a reversed CO₂/H₂ selectivity of 4.7. This marked contrast underscores that the separation properties of MOF membranes are not solely material-dependent but can be deliberately switched and optimized through crystal orientation engineering, offering a versatile design strategy for adaptive separation membranes.