Solvent‐Controlled Pathways Enable Structure‐Programmable Metal‐Organic Framework Membranes for Isomer Separation

The separation of structurally similar aliphatic and aromatic isomers remains a critical industrial challenge, as their nearly identical sizes render conventional distillation highly energy-intensive. Molecular-sieving metal-organic framework (MOF) membranes offer an energy-efficient alternative; however, reliably programming their growth pathways to achieve targeted pore architectures is difficult. Here, we report a solvent-triggered pathway control strategy that directs distinct growth routes from a single vertically aligned Zn─Al layered double hydroxide (LDH) nanoarray template. In N,N-dimethylformamide (DMF), selective activation of LDH metal sites stabilizes the framework and guides a surface-induced interstitial growth mechanism, yielding a dense Zn-BODC membrane with ∼0.5 nm apertures for highly selective n-hexane/2,3-dimethylbutane (Hex/23DMB) separation. In water, rapid LDH dissolution triggers a complete template-conversion pathway that replicates the honeycomb morphology, producing an Al-BODC membrane with ∼0.7 nm pores capable of efficient para-/ortho-xylene (PX/OX) discrimination. These solvent-defined pathways enable programmable microstructures and complementary separation performances. This work establishes a versatile platform for the rational design of ultramicroporous MOF membranes with tailored sieving properties for demanding isomer separations.