| Nov 13, 2023 |
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(Nanowerk News) Membrane-based separation technologies offer major advantages over conventional processes like distillation in terms of energy efficiency and environmental impact. However, current membrane materials still face limitations in their ability to achieve highly selective separations at industrially viable flux rates. Metal-organic frameworks (MOFs) have emerged over the past two decades as an exciting new class of ultra-porous materials with potential to overcome these challenges. But translating their exceptional molecular sieving properties into high-performance membranes has proven difficult.
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Now researchers in China have demonstrated a new “modular customization” strategy that finally unleashes the full separation potential of MOFs in defect-free membrane form. They report their findings in Angewandte Chemie (“Modular Customization and Regulation of Metal–Organic Frameworks for Efficient Membrane Separations”).
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| Graphical abstract of the work. (© Angewandte Chemie)
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Membrane separations rely on selective permeability – the ability of a membrane to preferentially transport certain molecules over others. This makes pore size and chemistry crucially important. But commercial polymeric membranes lack the uniform porosity and tunability needed for more discriminating separations. Inorganic materials like zeolites exhibit molecular sieving at small pore sizes, but become challenging to process at larger scales. MOFs can combine high porosity with both tunability and processability. Their metal-organic hybrid framework structures contain regular, customizable pores ideal for separation. Yet after intense study, few of the many thousands of known MOFs have successfully transitioned into membranes able to realize their intrinsic separation capacity.
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The main roadblock has been manufacturability. Conventional MOF membrane fabrication techniques often create polycrystalline films riddled with defects between grains. These microscopic pinholes and cracks allow unselective transport, negating the MOF’s molecular sieving. Obtaining flawless polycrystalline growth across an entire membrane remains remarkably difficult for most MOFs. This renders many candidate materials unusable despite looking promising as powders. Even the rare MOFs that can be coerced into functioning membranes require bespoke processing tricks, limiting broader applications.
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The new research demonstrates a modular approach that bypasses these limitations. Instead of trying to grow continuous MOF films, the method creates a composite by separately synthesizing MOF microparticles then integrating them with an ultrathin polymer coating. The micron-scale MOF crystals serve as discrete molecular sieving modules, providing selective pathways for transport through exposed pores. The polymer eliminates defects by sealing gaps between crystals. This modular configuration sidesteps intercrystalline defects while maintaining access to intrinsic MOF separation capacity.
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The researchers illustrate the concept by fabricating four different modular membranes using MOFs that normally resist polycrystalline growth. Scanning electron microscopy confirms a consistent morphology with no cracks or pinholes. Tests show excellent molecular sieving, with selectivity directly linked to MOF pore sizes. For the challenging hydrogen separation from carbon dioxide, selectivities reached as high as 1656 – surpassing previous benchmarks. The modular membranes also demonstrate high permeance, stability for thousands of hours, and tolerance to impurities.
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According to the researchers, modular MOF integration represents a versatile strategy to rapidly customize separation membranes. The scalable fabrication method works reliably across diverse MOF types, pore sizes, and crystal morphologies. Modules synthesized separately under ideal conditions provide reproducibility and quality control. The polymeric coating then repairs inherent MOF limitations like brittleness and poor machinability.
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Crucially, modularity also enables selective membranes to be optimized for specific separations through post-synthetic adjustments. The study demonstrates tuning molecular sieving after membrane fabrication via ligand substitution – something difficult with conventional MOF films. This “retrofitting” can improve performance without remaking the membrane from scratch.
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The simplicity and flexibility of the modular approach opens doors to expand MOF applications. By drawing from the vast library of MOF materials containing over 70,000 structures and counting, researchers can now mix-and-match appropriate molecular sieves with tailored pore sizes and chemistries for a given separation. They demonstrated this by using a hydrophilic large-pore MOF to effectively filter dye molecules from water via nanofiltration. Significantly easier processing also lowers barriers for translating designs into practice.
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Overall the work illuminates a path forward to finally convert longstanding MOF potential into tangible membrane advances. Modular MOF composites could provide a versatile platform to enhance separations ranging from gas purification to wastewater treatment. Companies are already beginning to commercialize specialized membranes containing MOF fillers. Now with an expanding menu of modular MOF possibilities, researchers can systematically target separations that offer the greatest efficiency gains and environmental benefits. The study provides a powerful exemplar of rational design and scalable manufacturing principles converging to maximize real-world impact.
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