Confined Growth of MOFs into Angstrom-Scale Defect-Free Membranes for Precise Molecular Separation

Metal-organic frameworks (MOFs), featuring ordered nanochannels and versatile pore chemistry, have emerged as promising materials for developing next-generation separation membranes. MOF-based membranes are typically synthesized either by the intergrowth of crystals to form polycrystalline membranes or by incorporating crystals into polymers to form mixed-matrix membranes (MMMs). During the membrane formation processes, multiscale defects─ranging from microscale intercrystalline cracks and nanoscale interfacial gaps to angstrom-scale lattice defects─are prone to arise. These undesirable defects compromise the membrane structural integrity and molecular transport properties, thus impairing the separation efficiency and impeding the implementation of MOF-based membranes for precise molecular separation. In this Account, we present our group's efforts to construct defect-free molecular sieving membranes based on the "confined growth of MOFs". We first establish MOF design and selection principles by correlating molecular transport mechanisms with framework characteristics. We then demonstrate how confined growth enables the elimination of multiscale defects in both polycrystalline membranes and MMMs. For polycrystalline membranes, the elimination of angstrom-scale lattice defects has been a long-standing challenge. To address this, we proposed a high-probability theoretical coordination strategy that creates sufficient chemical potential to overcome the steric hindrance that occurs when connecting ligands to metal clusters. By further regulating the confinement of coordination reactants (e.g., ligands, metal cluster precursor), coordination environment (e.g., solvent, modulator), and coordination space (e.g., pore size, topology), we inhibited the linker-missing and metal cluster-missing defects and restored intrinsic lattice integrity in MOF membranes. This leads to a family of high-connectivity MOF membranes exhibiting outstanding molecular separation performance, not only for gases but also for water/ions and organic mixtures. By employing atomic-level characterizations (e.g., real-space high-resolution transmission electron microscopy) and molecular simulations, we revealed the underlying thermodynamics and kinetic mechanism of defect suppression and confined molecular transport through the lattice aperture. For MMMs, despite the structural integrity achieved in the as-synthesized crystals, incorporating these crystals into polymers remains another grand challenge because this process often introduces multiscale defects resulting from filler aggregation and interfacial incompatibility. Recognizing that the space between MOF fillers and the polymer matrix represents a typical confined environment, we pursued molecular-confined strategies to solve the dispersion and compatibility issues in MMMs. Recently, we proposed a solid-solvent processing strategy to fabricate an ultrathin and highly loaded defect-free MMM for molecular sieving. The polymer serves as a solid solvent, confining MOF nucleation and in situ growth on the polymer chains to avoid crystal aggregation and interfacial defects. This yields a new-generation nanocomposite membrane with MOF crystals as the continuous phase, enabling fast molecular transport through interconnected MOF nanochannels. Notably, this general polymer-confined growth allows for the scalable fabrication of MMMs toward diverse molecular separation applications including hydrogen separation, CO2 capture, and organic azeotropic separations. By integrating machine learning and high-throughput calculations, our group also explored the AI-accelerated design and customized synthesis of MOF-based membranes. We conclude this Account by discussing the remaining limitations, challenges, and opportunities of further exploring angstrom-scale defect-free MOF membranes. We hope that this Account can provide insights in developing next-generation molecularly selective separation membranes toward broad applications and stimulate or deepen interest in multiple disciplines crossing chemistry, materials science, and chemical engineering.