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Cluster-Based Supramolecular Ionic Frameworks: From Construction to Separation Functions
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DOI：10.1021/acs.accounts.5c00866.png)
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Abstract
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ConspectusSynthetic molecular/nanoporous structures have shown wide utilization in multidisciplinary aspects of catalysis, adsorption/loading, surface property control, precise separations, and so forth. In contrast to conventional disordered pores, chemically synthesized molecular frameworks through covalent/coordination bonds exhibit more controllable ordered channel architectures for various functional purposes. The definite dimensions and topologies provide tunable pathways for selective molecular recognition and nanorecognition, delivery, and interface activation. However, the structure of these frameworks is relatively rigid, making it difficult for them to be processed and fine-tuned, and they are not easy to repair or recycle. Moreover, when combined with polymers, they are prone to phase separation, which limits their application in membrane sustainability. Therefore, developing distinctive strategies to overcome these limitations remains a critical scientific and technological priority. Given that the main cause of these problems lies in the stiff chemical bonds and crystalline synthesis, the changes in binding form and building units become decisive factors. One of the effective strategies is to use multiple intermolecular interactions as the driving forces to construct supramolecular ionic frameworks (SIFs). Although the assembly mode reduces the rigidity and orderliness of the framework structure, such a route simultaneously brings about significant structural flexibility, convenient modifications, excellent membrane-forming performance, and processability through a simple preparation method. To further enhance the functions of SIFs, the selection of building units that can balance both driving forces and diversified topological configurations becomes a prerequisite for rational design. As a class of stable nanoscale particles, polyoxometalates (POMs) offer more possibilities for the arrangement of the framework structures and possess rich functionalities. Additionally, POMs provide multiple driving forces, including ionic interactions, host–guest interactions, and hydrogen bonds, thereby endowing the framework with both structural stability and flexibility. Relying on the induced structural characteristic, such cluster-based SIFs (CSIFs) directly lead to multiple potentials upon size exclusion, the hydrophobic/hydrophilic effect, and electrostatic adsorption stemming from inherent charges to POMs, demonstrating a landscape of advanced applications for precise separations at nano/molecular scales.To outline the fundamental strategy and typical results, this Account focuses on the construction of CSIFs for various topological architectures, synergistic principles of building units and driving forces, structural analysis of frameworks, membrane separations, and the discussion of separation mechanisms. While exploring the advances of CSIFs, we analyze the limitations and possible breakthroughs of assembly structures. The optimized routes for the precise isolation of a series of substrates via CSIF-based membranes are discussed. Besides primary size screening, the composition adjustment brings about sieving nanoparticles/molecules with a certain charge and chiral selectivity. The tunable hydrophilicity and hydrophobicity in the pores hold the liquid-switchable ability for the separations of incompatible liquids. Finally, the local surface electrical potential difference enables selective adsorption with high capacities for gases based on molecular/bond polarity. The systematic evaluation highlights the distinct advantages of CSIFs in high structural flexibility for processability, recyclability, and the feasibility of engineering diverse morphologies. Moreover, CSIFs show potential in the fields of selective ion separation, anode protection of batteries, and catalysis-possessing functions in the future.
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