MOFs are built from metal nodes connected by organic linkers, a modular construction that gives chemists unprecedented control over both pore structure and pore chemistry. They combine exceptionally high surface areas with tunable pore sizes, adjustable organic functionality, architectural flexibility, and thermal and physical stability. This designability sets them apart from conventional porous materials such as zeolites and activated carbons. MOFs can be deployed not only as pure crystalline films but also as fillers dispersed within polymer matrices to form mixed-matrix membranes, known as MMMs. The review, published on 6 August 2026, focuses on how three design strategies—fine-tuning of pore size, functional group engineering, and hybridization with complementary materials—can be tailored to the transport requirements of specific gas pairs, including hydrogen, carbon dioxide, olefin/paraffin mixtures, and oxygen/nitrogen separation.

Pore size engineering is the first and perhaps most fundamental lever. Because MOF apertures can be adjusted through linker length variation, ligand composition, metal center selection, framework flexibility, defect engineering, and post-synthetic treatment, researchers can dial in molecular sieving behavior with sub-angstrom precision. The review highlights ammonia-induced treatment of ZIF-8, which alters the local framework structure and promotes a more rigid selective pathway, and defect-engineered UiO-66, which simultaneously tunes microporosity and mass transport. Strategies such as the polymer-in-cage approach, in which polymers are introduced directly into MOF pore windows, allow separation performance to be optimized for target gases while preserving the intrinsic framework. The message is clear: MOFs should be viewed not simply as porous fillers but as platforms for designing pore environments tailored to specific gas pairs.

Functional group engineering provides the second lever, and it works on two fronts at once. Polar groups such as amino (-NH2) and carboxyl (-COOH) functionalities enhance interactions with acidic gases like CO2 while simultaneously improving interfacial compatibility with polymer matrices. Amino-functionalized ZIF-8-NH2 and UiO-66-NH2 form hydrogen bonds and pi-pi interactions with polymer chains, suppressing interfacial void formation and enabling defect-free MMMs even at high filler loadings. In TEPA-functionalized MIL-101(Cr)/Pebax membranes, grafted amine groups significantly enhanced CO2/CH4 separation by increasing affinity toward CO2. COOH-functionalized MOFs go further: in UiO-66-COOH-based MMMs, thermal treatment induced a dual-crosslinked network between polymer and MOF, enhancing molecular sieving and substantially improving CO2/N2 ideal selectivity. Functional group engineering thus offers not only enhanced gas affinity but also interfacial stabilization, defect suppression, and control over polymer chain packing.

Hybridization with other materials constitutes the third strategy. Graphene oxide nanosheets have proven particularly effective additives: in GO-modified MOF-74/Pebax membranes, GO suppressed MOF aggregation and extended the gas diffusion pathway, markedly improving CO2/N2 ideal selectivity, while in GO-assisted assembly of CuBDC nanosheet membranes, GO filled the voids between rigid nanosheets to enhance continuity and mechanical stability. Ionic liquids add another dimension. In [EMIM][OAc]/CuBTC/Pebax hybrid membranes, incorporation of the IL/MOF composite improved both CO2 permeability and CO2/N2 ideal selectivity, with some membranes even surpassing the Robeson upper bound, the empirical performance ceiling for polymer membranes. Crosslinking strategies between polymers and MOFs, such as chemical crosslinking in Mg-MOF-74/PIM-1 membranes, generate interconnected microporous networks while suppressing polymer chain mobility and interfacial voids.

Hydrogen separation showcases the power of these strategies in their purest form. Because H2 has an extremely small kinetic diameter, sub-nanometer MOF pores can sieve it efficiently from larger molecules. An ammonia-treated ZIF-8 membrane achieved an H2 permeance of 376 gas permeation units (GPU) with an H2/N2 ideal selectivity of 122.4. A CuBDC-NO2 nanosheet membrane exploited controllable stacking of MOF nanosheets to regulate the effective aperture to 0.372 nanometers, delivering an H2 permeance of 1,326 GPU together with H2/CO2 and H2/CH4 selectivities of 266 and 536, respectively. Hybrid architectures pushed performance further: a GO-assisted CuBDC nanosheet membrane reached 2,867 GPU with an H2/CO2 separation factor of 95.1, a MOF-801@MXene membrane combined MOF porosity with well-defined MXene nanochannels for 2,334 GPU, and a soft-solid Zn2(Bim)4/polyamide composite delivered an exceptional H2/CO2 separation factor of 1084 by pairing straight interlayer galleries for rapid H2 transport with a polyamide layer that healed non-selective defects.

Carbon dioxide separation is the most extensively investigated application, and the review documents how defect engineering, functionalization, crosslinking, and hybridization all contribute. Defect-engineered UiO-66/PEGDA MMMs introduced additional microporosity, yielding a CO2 permeability of 470 Barrer with a CO2/N2 ideal selectivity of 41, roughly a four-fold permeability increase over the pristine polymer. A polymer-in-cage strategy using high-aspect-ratio NZIF-8 nanoplates created tortuous diffusion pathways, achieving a CO2 permeance of 333 GPU with a CO2/N2 ideal selectivity of 80. Amino-functionalized ZIF-8-NH2 in a carboxyl-containing polyimide enabled defect-free fabrication at 50 weight percent filler loading with a CO2/CH4 ideal selectivity of 89.4. Ionic-liquid-modified MOF-177/Pebax membranes achieved a remarkably high CO2/N2 ideal selectivity of 622. Perhaps most strikingly, MOF-derived glass membranes, which eliminate crystalline grain boundaries while maintaining continuous transport pathways, achieved a CO2 permeability of 19,157 Barrer in a MIL-101/coordination-polymer-glass composite and a CO2 permeance of 37,000 GPU in a topology-engineered ZIF-derived glass.

Beyond hydrogen and carbon dioxide, MOF membranes are tackling separations once considered nearly impossible for polymers. Olefin/paraffin separation is industrially vital but difficult because the gas pairs share very similar molecular sizes and physicochemical properties. Defect-engineered AZIF-8 nanoparticles enhanced framework rigidity and molecular sieving precision while improving polymer compatibility, allowing 6FDA-DAM/AZIF-8 MMMs to reach a propylene permeability of 79.4 Barrer with a propylene/propane ideal selectivity of 39.8. Layered MOF membranes modified with an ionic liquid/AgBF4 composite exploited silver-mediated facilitated transport, transforming an intrinsically non-selective membrane into an olefin-selective one with mixed-gas separation factors of 28.8 for propylene/propane and 12.0 for ethylene/ethane. Gallate-based MOFs also emerged as promising fillers: a Co-gallate/PIM-1 MMM with a three-dimensionally interconnected porous framework achieved a propylene permeability of 3,552 Barrer with a selectivity of 8.8, improvements of 106.5 and 131.6 percent over pristine PIM-1. For air separation, an o-carborane-modified ZIF-8/PIM-1 membrane combined molecular sieving with preferential nitrogen adsorption inhibition, achieving an O2 permeability of 796.5 Barrer and an O2/N2 separation factor of 8.9, surpassing the 2015 Robeson upper bound.

The review is careful to note that reported performance values from different studies were obtained under different temperatures, pressures, feed compositions, and membrane thicknesses, and should not be interpreted as direct quantitative comparisons. It also acknowledges that practical deployment still faces significant hurdles: scale-up fabrication, substrate adhesion, defect formation during growth or activation, and stability under humidity, pressure, and gas impurities. Pure MOF films suffer from grain-boundary defects and mechanical fragility, while MMMs are limited by filler agglomeration, interfacial voids, plasticization, and physical aging, especially under mixed-gas and high-pressure conditions. Future progress, the authors argue, will depend on more robust interface-engineering strategies, more scalable fabrication routes, and more realistic testing protocols that reflect industrial operating environments.

Even with these caveats, the trajectory is unmistakable. The modular combination of metal nodes and organic linkers gives MOFs a degree of structural diversity and chemical tunability that no other membrane material class currently matches. By linking pore architecture, pore chemistry, and interfacial design to the transport behavior of target gas systems, MOF-based membranes are converging on solutions to the permeability-selectivity trade-off that has constrained polymer membranes for decades. As materials design and process integration mature, these crystalline frameworks are expected to play an increasingly central role in energy-efficient separations, from hydrogen purification for the emerging hydrogen economy to carbon capture at industrial scale, positioning MOFs as a foundational platform for next-generation membrane technology.

Subject of Research: Metal-organic framework membranes for gas separation

Article Title: Designable features of metal-organic frameworks for diverse gas separation membranes

Article References: Son, H. J., & Lee, C. S. (2026). Designable features of metal-organic frameworks for diverse gas separation membranes. Frontiers in Membrane Science and Technology, 5, Article 1909558. https://doi.org/10.3389/frmst.2026.1909558

Image Credits: AI Generated

DOI: 10.3389/frmst.2026.1909558

Keywords: metal-organic frameworks, gas separation, membranes, hydrogen purification, carbon dioxide capture, mixed-matrix membranes, pore size engineering, functional group engineering, graphene oxide, ionic liquids, olefin paraffin separation, Robeson upper bound