Precise molecular sieving in polycrystalline covalent organic framework membranes.

Covalent organic frameworks (COFs) unlock opportunities to unite rapid mass transport with molecular-level sieving through membranes. However, achieving long-range crystallographic order within these membranes remains a central challenge that limits their separation precision. Here, we report a monomer-driven strategy for the ambient-condition fabrication of COF polycrystalline membranes featuring definite lattice order and polyhedral textures, reminiscent of benchmark metal-organic framework counterparts. Using a rigid, π-conjugated pyrene-centered tetraamine, we enable nonepitaxial intergrowth of faceted COF crystals on a porous support, yielding a 350-nanometer-thick membrane with a record-high Brunauer-Emmett-Teller surface area of 2121 square meters per gram among COF separation membranes of comparable pore size. The resulting COF membrane preserves its crystallographic order and polyhedral texture even after 1 month of solvent exposure while uniquely combining increased surface stiffness with high tensile ductility. Notably, we demonstrate its superior liquid-phase molecular selectivity, outperforming COF analogs with considerably smaller pore apertures, and elucidate an unambiguous correlation between crystallographic order and permselectivity. This work positions crystallographic ordering as a guiding principle for COF membranes, opening more possibilities at the intersection of framework materials and separation technologies.