The search began with nearly 97,000 Scopus records matching terms associated with membranes and gas separation. Adding terms for permeability, permeance, and transport reduced the pool to approximately 38,500 documents. Excluding adjacent subjects such as membrane distillation, desalination, pervaporation, reverse osmosis, water purification, and filtration removed about 6,000 records.

Requiring terms such as biopolymer, biodegradable, or biobased then produced 914 documents, comparable to the 751 records retrieved for polymers of intrinsic microporosity, or PIMs. The query was not exhaustive: a 2001 paper on gas permeation in poly(lactic acid), for example, did not use “biopolymer” terminology. Adding terms for renewable resources and feedstocks captured that paper and 57 others.

Biopolymers are also studied as gas-barrier materials for food packaging, where the same transport measurements may be reported without being framed as membrane-separation research. Expanding the search to include packaging and “gas barrier,” and adding barrier to the transport terms, increased the final corpus to 2,909 documents, more than three times the original biopolymer-specific result. The author also notes that the database search is only an approximation of the field, so the figures describe the literature captured by the structured search rather than a complete census of every investigation involving a biopolymer membrane.

The volume of identified work has grown sharply since 1990. The cumulative count reached 1,193 documents by 2020, compared with 193 in 2010, and the corpus continued to expand through 2025. At that point, 70% of the documents were articles, 16% were reviews, and the remainder were book chapters and conference papers.

Only 34% of the full collection was open access, although the proportion was 45% for 2023, 36% for 2024, and 42% for 2025. The perspective argues that greater open access could reduce data gatekeeping and make transport measurements more available for comparison and synthesis. Polysaccharides were the most frequently mentioned material class, with 1,054 mentions among the articles classified for the analysis.

These included cellulose, chitosan, and starch, as well as lignin, lignocellulose, and algae-based materials. Cellulose, including nano- and microfibrillated cellulose, nanofibers, and nanocrystals, occupied the leading position within the class. Their biobased origin and general biodegradability are major attractions, but chemical modifications made to improve processability can substantially reduce biodegradability.

Cellulose acetate, a material commonly used in gas separations, illustrates that compromise. The figures are counts of article mentions rather than counts of distinct materials, because some articles were assigned to more than one category. Aliphatic polyesters generated 768 mentions and included poly(lactic acid), poly(butylene succinate), polyhydroxyalkanoates, and polycaprolactone.

All four are biodegradable, although poly(butylene succinate) and polycaprolactone) are generally not biobased. PLA is usually produced by biomass fermentation followed by chemical conversion from lactic acid to lactide and then polylactide, a route that limits its use as a single-use material in the European Union. Naturally occurring polyhydroxyalkanoates are generally produced by microbial fermentation, with their final chemical structure largely determined by the feedstock.

Other polymers fell into a group that could not be classified as polysaccharides or aliphatic polyesters. These included poly(vinyl alcohol), terephthalate- and furan-based aliphatic-aromatic copolyesters such as PBAT and PEF, polyethylene glycol, polynorbornenes, and polyurethane. Many have limited biodegradability but are biobased and are being investigated as potentially more sustainable alternatives to synthetic commodity polymers.

Biobased PEF, for example, is under investigation as an alternative to PET, while biodegradable PBAT is being studied as an alternative to LDPE. A final category comprised natural biomolecules such as gelatine, beeswax, silk, acacia gum, citrus pectin, and spirulina. These are generally used as additives with antioxidant, antibacterial, or other functions, and their effects on transport are often secondary to the original purpose of the investigation.

The distribution of measured penetrants reveals a pronounced mismatch with many separation applications. Water and oxygen dominate the literature because they are central to food packaging; both may also occur in flue gas, where water can act as a contaminant. Among the 2,031 articles examined, only 14% reported carbon dioxide permeability, 3% reported nitrogen permeability, and 2% reported methane permeability.

Helium, hydrogen, and hydrogen sulfide were also reported only in small numbers of papers. In the material-specific analysis, 71 articles reported nitrogen transport properties and 42 reported methane transport properties, making it difficult to evaluate many biopolymers for separations involving those gases. Some studies also addressed air, ethylene, ammonia, aroma barriers using limonene, organic vapors, and one propene–propane separation based on cellulose acetate.

Composite structures were prominent in the research record. More than 62% of the articles examined reported structural modification through the inclusion of particles or nanofibers, layering, or blending. Graphene was the most common filler, appearing in 97 articles, followed by ionic liquids in 35, metal-organic frameworks in 21, and zeolites in 17.

Combining materials can substantially change transport properties, but the perspective emphasizes that measurements for the pristine polymer should accompany results for each modification, including results that do not improve performance. The evidence base also contains limited modelling. Only 206 articles included modelling or simulation; 45 used molecular dynamics or density functional theory, and 20 examined process-level applicability.

More complete datasets would support structure–property relationships, material screening, and assessment under process conditions, although comparable operational data can be limited even for established membrane materials. Metadata analysis showed a global but fragmented research network. The 2,909 documents contained affiliations from 88 countries.

China led with 750 publications, followed by India with 400 and the United States with 279. Publications ranged from single-author papers to collaborations with as many as 19 authors from 10 countries, while almost half had three, four, or five authors. Only 221 documents, less than 8%, included at least one commercial industrial partner, with such partnerships recorded across 40 countries.

China accounted for 98 of the identified industrial collaborations. The count excluded engagement through public or private research and technology organizations, so it does not represent the full extent of participation by organizations that support translation and scale-up. The findings point to a possible research focus without establishing industrial readiness.

Many biopolymers are semicrystalline, which can restrict transport, and disrupting crystallinity or using reliable simulations may be necessary to characterize the behaviour of their amorphous phases. The author proposes that crystallinity could eventually be treated as a parameter adjusted for a particular application. Many biopolymers are also rubbery at or close to room temperature, and gas and vapor transport in rubbery polymers is driven mainly by solubility.

This creates a proposed opportunity for selective transport of highly condensable molecules, including volatile organic compounds, but the perspective stresses that the relevant transport properties still need to be measured. Consistent reporting of permeability, selectivity, unmodified controls, operating conditions, modelling, and process-level evaluation would help researchers identify which material–gas combinations are worth advancing and clarify where further work is required.

Subject of Research: Bibliometric analysis of biopolymer membranes for gas and vapor separation

Article Title: Biopolymers as membranes for gas separation: it is all about data

Article References: Papchenko, K. (2026). Biopolymers as membranes for gas separation: it is all about data. Frontiers in Membrane Science and Technology, 5, Article 2009069. https://doi.org/10.3389/frmst.2026.2009069

Image Credits: AI Generated

DOI: 10.3389/frmst.2026.2009069

Keywords: biopolymers, gas separation, membranes, gas permeability, literature analysis, industrial collaboration, composite membranes, modelling, research networks, sustainable materials, separation, data

The study, published in the Journal of Materials Science, describes a composite membrane built in three layers, each with a distinct job. At the base sits a hollow fiber of alpha-alumina, a hard ceramic chosen for its mechanical strength and tolerance of harsh chemical and thermal conditions. On top of that ceramic skeleton, the researchers deposited an ultrathin film of polydopamine, a synthetic mimic of the adhesive proteins that mussels use to cling to rocks in the surf. Finally, over the polydopamine primer, they laid down a selective layer of graphene oxide nanosheets just 245 nanometers thick, roughly a thousand times thinner than a human hair.

The polydopamine interlayer is the quiet hero of the fabrication strategy. Ceramic supports are porous by design, but their rough, grainy surfaces make it difficult to spread a continuous, defect-free coating of two-dimensional nanosheets. Using atomic force microscopy, the team measured the surface roughness of the pristine alumina fibers at about 120 nanometers. After a 24-hour immersion in a dilute dopamine solution at pH 8.5, a conformal polydopamine film roughly 154 nanometers thick formed on the fiber, smoothing the surface down to about 102 nanometers. The film’s catechol hydroxyl and amine functional groups then acted as anchoring sites for the graphene oxide, which bonds to the interlayer through hydrogen bonding and, the authors propose, covalent coupling reactions such as Schiff-base and Michael-addition pathways.

The graphene oxide itself was synthesized by a modified Hummers method, the workhorse oxidation route that rips apart graphite crystals and decorates the resulting sheets with oxygen-containing groups. Characterization confirmed the transformation: infrared spectroscopy revealed hydroxyl, epoxy, and carboxylic signatures; Raman spectroscopy showed the disorder ratio jump from 0.150 in pristine graphite to 0.998 in the oxidized product; and X-ray diffraction pinned the interlayer spacing between stacked sheets at 7.86 angstroms, of which about 4.52 angstroms remains as open gallery height after accounting for the electron clouds of the carbon planes themselves. That spacing is the critical dimension. It exceeds the kinetic diameters of both hydrogen, at 2.89 angstroms, and carbon dioxide, at 3.30 angstroms, so neither molecule is strictly excluded by size alone, yet the confined nanochannels still impose a strong transport bias toward the smaller, faster hydrogen.

What sets this study apart from earlier graphene oxide membrane work is not just the membrane but the test protocol. Most reports evaluate separation performance under a single feed composition, typically an equimolar mixture. The Brazilian team instead swept the hydrogen content of their hydrogen-carbon dioxide feed from 25 to 75 mole percent, closer to the variable streams that real industrial reformers and gasifiers actually deliver. The results were strikingly composition-dependent. At the leanest feed, 25 percent hydrogen, the membrane still enriched the permeate to just over 52 percent hydrogen. At an even 50-50 split, the permeate climbed to nearly 93 percent hydrogen with a separation factor of about 11. And at the richest feed of 75 percent hydrogen, the membrane delivered a permeate stream of 99.04 percent hydrogen purity, with a separation factor of 27.44.

The physics behind that trend is a story of competing molecules fighting for the same nanochannels. Carbon dioxide has a quadrupole moment that makes it stick preferentially to the oxygen-containing functional groups lining the graphene oxide galleries. When carbon dioxide is abundant in the feed, it adsorbs inside the channels and travels by an adsorption-assisted mechanism, crowding out hydrogen and degrading selectivity. As the hydrogen fraction rises and the carbon dioxide partial pressure falls, that competitive adsorption weakens, freeing the channels for hydrogen and simultaneously boosting hydrogen permeance and suppressing carbon dioxide leakage. The authors conclude that transport through the membrane is a hybrid of Knudsen diffusion, molecular sieving, and preferential carbon dioxide adsorption, a combination that explains why performance tracks feed composition so closely.

The raw numbers are competitive by any standard. The composite membrane achieved a hydrogen permeance of about 2811 gas permeation units, a measure of flux normalized by pressure and membrane area, alongside that peak separation factor of 27.44. For comparison, a graphene oxide membrane on a zeolitic imidazolate framework support reported by other researchers managed roughly 433 GPU and a separation factor of 14.9 at 250 degrees Celsius, while an externally assembled graphene oxide membrane on alumina reached 876 GPU with a separation factor of 17.2. Even an ethylenediamine-crosslinked variant, which pushed selectivity to about 22.9, paid for it with a permeance drop to around 73 GPU. The Brazilian membrane’s combination of high flux and respectable selectivity, achieved at room temperature, suggests that the thin, well-adhered selective layer is doing exactly what the polydopamine primer was designed to enable.

Durability matters as much as performance, and here too the results are encouraging. Over 12 hours of continuous pure-gas permeation at a transmembrane pressure of 1 bar, the hydrogen permeance held steady at about 4285 GPU with a net drift of only 0.2 percent between the first and last measurements, while carbon dioxide permeance averaged about 619 GPU with similarly tight variation. The absence of any systematic flux decline over that window indicates that the polydopamine anchor is keeping the graphene oxide layer intact under sustained gas flow, addressing one of the chronic failure modes of nanosheet membranes, which tend to delaminate or reorganize over time.

The broader significance lies in what the composition sweep reveals about how such membranes should be evaluated and deployed. The authors argue that assessing performance across a wide range of feed compositions is essential to represent real-world process conditions, where hydrogen content in reformer off-gas or syngas fluctuates rather than sitting at a convenient laboratory benchmark. Their findings align with established behavior in palladium-silver and ceramic dual-phase membranes, where higher hydrogen partial pressure increases the driving force for permeation. For graphene oxide, the added wrinkle is the adsorption interplay, meaning that process designers cannot simply extrapolate from a single-point measurement.

Challenges remain before dopamine-engineered graphene oxide membranes reach industrial scale. The 12-hour stability test, while reassuring, is short compared with the months of continuous operation a plant would demand, and the tests were run at room temperature rather than at the elevated temperatures where reforming actually occurs. Still, the demonstration that a cheap, bio-inspired polymer primer can turn a rough ceramic fiber into a platform for an ultrathin, high-flux, composition-tolerant hydrogen sieve is a meaningful step. As decarbonization pressures mount across ammonia production, refining, and steelmaking, membranes that can wring high-purity hydrogen from mixed streams without the energy penalty of pressure swing adsorption may prove to be one of the quiet enabling technologies of the energy transition.

Subject of Research: Graphene oxide composite membranes on polydopamine-modified alumina hollow fibers for hydrogen and carbon dioxide gas separation

Article Title: High-purity hydrogen for decarbonization via dopamine-engineered α-Al2O3/graphene oxide membranes: the impact of gas mixture feed composition on H2/CO2 separation

Article References: Ascendino, G. G., Dias, W. A., Reis, M. H. M., & Hori, C. E. (2026). High-purity hydrogen for decarbonization via dopamine-engineered α-Al2O3/graphene oxide membranes: the impact of gas mixture feed composition on H2/CO2 separation. Journal of Materials Science, 61(42), 32709-32726. https://doi.org/10.1007/s10853-026-13661-4

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13661-4

Keywords: hydrogen purification, graphene oxide membrane, polydopamine, gas separation, carbon dioxide capture, hollow fiber membrane, decarbonization, steam methane reforming, molecular sieving, ceramic membrane, hydrogen economy, membrane technology

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

The solution–diffusion model was born in the 1960s, when researchers studying cellulose acetate reverse osmosis membranes had no way to see inside a polymer film at the molecular scale. Because no instrument could resolve the subnanometer structure, scientists assumed the membrane was a homogeneous, pore-free medium. The model describes transport in three steps: molecules partition from the feed into the polymer according to their solubility, diffuse independently through the matrix, and desorb into the permeate. Crucially, it assumes that free volume inside the polymer exists only as isolated, transient cavities created by thermal fluctuations of the polymer chains, never as continuous channels. This simplicity became the model’s greatest strength, allowing generations of researchers to compare membranes using just two numbers: solubility and diffusivity.

But the model’s experimental foundation, according to the new analysis, is shakier than textbooks suggest. The classic evidence came from stacked-membrane experiments, in which researchers measured water content layer by layer after permeation and found a gradient decreasing from the feed side to the permeate side. That gradient was interpreted as the concentration difference driving diffusion. The Rice-led team, drawing on their own molecular dynamics simulations and continuum poromechanical modeling, offers a different explanation: the gradient is a mechanical artifact of membrane compaction. Under hydraulic pressure, the pressure inside the membrane falls continuously from feed to permeate, so the polymer skeleton is compressed progressively harder toward the permeate side, squeezing out porosity along the way.

To settle the question experimentally, the researchers repeated stacked-membrane experiments with Nafion 117 ion-exchange membranes, the same material used in recent work by Freeman and co-workers. If the gradient were truly diffusion-driven, releasing the pressure should erase it within minutes as water redistributes. Instead, after more than 24 hours of permeation, depressurization, and a 10-minute relaxation, a pronounced porosity gradient remained. Soaking the individual layers in deionized water for three hours did not remove it either. Thickness measurements confirmed progressively greater irreversible compaction toward the permeate side, and short 15-minute permeation runs produced essentially no gradient at all. Flux–pressure cycling from 30 to 120 bar and back caused roughly a 20 percent irreversible loss in water permeance, independent evidence of permanent structural deformation. The gradient, the authors conclude, is not the driving force for transport but the structural fingerprint of pressure-driven flow through a compressible, interconnected pore network.

The structural evidence goes far beyond compaction experiments. Positron annihilation lifetime spectroscopy can measure the size and distribution of free-volume elements; small-angle neutron and X-ray scattering reveal nanoscale heterogeneity and solvent-rich domains; wide-angle X-ray scattering probes chain packing; and synchrotron-based X-ray absorption and emission spectroscopy show that the hydrogen-bond network of water confined in polyamide membranes is distorted relative to bulk water. Neutron-scattering studies have even resolved distinct populations of bound and mobile water coexisting within the active layer of reverse osmosis membranes. Molecular dynamics simulations complement these measurements by mapping the three-dimensional, solvent-accessible free volume and testing whether pathways actually connect across the membrane. Together, these techniques consistently indicate that membranes long classified as dense are riddled with connected, dynamic, subnanometer porosity.

Importantly, the authors stress that

The pervaporation case deserves particular attention because it forces a reconsideration of what happens inside a membrane when liquid and vapor coexist on either side. In the classical picture, a liquid feed dissolves into the polymer, diffuses across, and evaporates only upon reaching the downstream surface, where vacuum or sweep gas maintains near-zero partial pressure. The Perspective argues instead that spectroscopic and simulation evidence points to liquid-like and vapor-like domains coexisting within the membrane interior, meaning the liquid–vapor transition occurs inside the material rather than exclusively at its downstream boundary. This internal phase transition reframes pervaporation as a process whose selectivity is set partly by where and how that phase change unfolds within the pore network, a question the solution–diffusion framework was never designed to address.

Gas separation in glassy polymers offers a complementary line of evidence. Glassy polymers are frozen far below their relaxation timescales, so their free-volume elements are comparatively rigid and long-lived, making them ideal systems for probing whether transport proceeds through connected channels or isolated cavities. Recent molecular simulations described in the article show that permeability and selectivity in these materials emerge from pore connectivity, molecular sieving at constrictions, and the strength of gas–wall interactions, with adsorption-mediated hopping between sites replacing the notion of a homogeneous diffusion matrix. This mechanistic picture naturally explains long-standing empirical observations, such as the strong dependence of gas permeability on polymer chain rigidity and the distinctive sieving behavior of highly rigid ladder-type polymers, without requiring ad hoc corrections to diffusivity coefficients.

The practical stakes of this reframing are considerable. If transport proceeds through connected subnanometer pores, then the levers that matter most for membrane design are geometric and chemical rather than phenomenological: pore size distribution, network connectivity, tortuosity of the pathways, and the chemistry of the pore walls that governs solvent–polymer and gas–polymer interactions. This shifts the design paradigm from fitting solubility and diffusivity parameters after the fact toward deliberately engineering the architecture of the free-volume network itself. For polyamide reverse osmosis membranes, for example, it suggests that controlling the nanovoid structure formed during interfacial polymerization, and how that structure responds to sustained pressure, may matter as much as the polymer chemistry that determines water affinity.

The framework also carries implications for how membrane aging and compaction are understood. The irreversible permeance losses observed under cyclic pressurization, on the order of twenty percent in the experiments highlighted, become interpretable as progressive collapse of pore connectivity rather than as a mysterious drift in an effective diffusivity. Similarly, fouling and solvent-induced swelling can be reexamined as phenomena that alter pore geometry and wall chemistry in real time, which helps explain why membrane performance often depends strongly on operating history in ways that equilibrium sorption and diffusion coefficients cannot capture.

Methodologically, the convergence across PALS, SANS, SAXS, WAXS, synchrotron X-ray spectroscopy, and molecular dynamics is itself significant. Each technique probes a different length and timescale, from picosecond chain fluctuations to static nanoscale morphology, and their agreement on the existence of connected free-volume networks reduces the likelihood that any single measurement is an artifact. Continued progress will likely depend on coupling these experimental probes with simulations that can resolve how pore networks reconfigure under realistic operating pressures and solvent compositions, and on developing characterization methods capable of mapping connectivity, not merely cavity size, in operating membranes.

The authors are careful to note that the two conceptual frameworks need not be mutually exclusive at every scale. The solution–diffusion model remains a useful correlation tool, and continuum-level descriptions built on it have guided decades of engineering practice. The argument is instead about mechanism: the molecular picture underlying those correlations is better represented by viscous flow and hopping through connected nanoscale pathways than by diffusion through a truly continuous dense matrix. Recognizing this distinction matters because models that encode the wrong mechanism can mislead extrapolation, particularly as new membrane materials push into regimes of tighter sieving, higher pressures, and more aggressive solvents where the difference between the two pictures becomes experimentally consequential.

What emerges from the synthesis is a unifying thread across processes that have traditionally been treated separately. Reverse osmosis, organic solvent nanofiltration, pervaporation, and glassy polymer gas separation have each accumulated their own empirical models and design heuristics. Framing all of them around interconnected subnanometer pore networks provides a common physical substrate: pressure-driven viscous flow in liquid separations, internal phase coexistence in pervaporation, and sieving with adsorption-mediated transport in gas separation are all manifestations of the same underlying structural reality. Whether this perspective ultimately reshapes how next-generation membranes are designed will depend on translating the structural picture into quantitative, predictive tools, but the conceptual realignment it proposes is already substantial, touching everything from how compaction is interpreted to how the internal state of a pervaporation membrane is defined.

Subject of Research: Transport mechanisms and pore-network structure of dense polymer membranes used in water, solvent, and gas separations

Article Title: Interconnected pore networks as a unifying framework for transport in dense polymer membranes

Article References: Wang, R., Fan, H., Qian, J., Wang, L., Li, Y., & Elimelech, M. (2026). Interconnected pore networks as a unifying framework for transport in dense polymer membranes. Frontiers in Membrane Science and Technology, 5, Article 1955546. https://doi.org/10.3389/frmst.2026.1955546

Image Credits: AI Generated

DOI: 10.3389/frmst.2026.1955546

Keywords: polymer membranes, pore networks, solution-diffusion model, reverse osmosis, organic solvent nanofiltration, pervaporation, gas separation, molecular dynamics, free volume, membrane compaction, molecular sieving, desalination

The research, led by a team working with carbon molecular sieve (CMS) hollow fiber membranes, tackles a challenge that has dogged the field for decades: how to manufacture ultra-selective carbon membranes in a form that can be produced at industrial scale without sacrificing performance. CMS membranes work by pyrolyzing polymer precursors at high temperatures, in this case 900 degrees Celsius, to create carbon structures with ultramicropores so precisely sized that they can discriminate between molecules differing by fractions of an angstrom. Flat-sheet versions of these membranes have posted extraordinary selectivities before, including a cellulose-derived membrane with a hydrogen-to-methane selectivity of 3,498 and a hollow fiber variant reaching 5,506 at 130 degrees Celsius. But translating those laboratory achievements into hollow fibers, the geometry favored for industrial deployment because of its high surface-area-to-volume packing density, has proven stubbornly difficult.

The breakthrough hinges on a chemical pretreatment of the polymer precursor before carbonization. The team started with Matrimid, a commercially available and relatively inexpensive polyimide, spun into asymmetric hollow fibers with a thin selective layer of roughly 150 nanometers atop a porous support. They then treated these fibers with a dilute solution of diethyltoluenediamine (DETDA) followed by trimesoyl chloride (TMC), both dissolved in hexane. This sequence cross-links the entire fiber matrix, including both the selective skin and the porous substructure, transforming the polymer into a network precursor with fundamentally different pyrolysis behavior. Spectroscopic analysis confirmed the chemistry: Fourier transform infrared spectra showed the ratio of amide to imide peaks rising from 0.52 to 0.71 after treatment, while x-ray photoelectron spectroscopy revealed nitrogen concentrations climbing from 5.9 to 8.0 atomic percent and oxygen from 14.9 to 18.5 atomic percent, clear fingerprints of the grafted diamine and acyl chloride.

Crucially, the researchers paired this cross-linking with a second stabilization step using vinyltrimethoxysilane (VTMS) sol-gel chemistry. Without such protection, the porous support layer of the hollow fiber tends to collapse completely during the intense heat of pyrolysis, densifying the entire wall and choking off gas transport. In previous work by the same group, fibers lacking VTMS treatment suffered exactly this fate. With the dual DETDA/TMC and VTMS treatment, the final carbon membrane retains a hierarchical architecture: a dense carbon separation layer only about one micrometer thick, supported by an open, porous substrate. This combination resolves what the authors describe as a long-standing collapse-stability trade-off, and it does so using cheap commodity chemicals rather than the expensive ionic-liquid solvents or fluorinated polyimides that earlier approaches required.

The separation results are striking. In mixed-gas tests using a feed of 10 percent hydrogen and 90 percent methane at 35 degrees Celsius and 100 psi, the dual-treated membranes delivered hydrogen at 99.56 percent purity one day after pyrolysis, with a permeance of 19.9 gas permeation units. After 24 days of aggressive vacuum aging, the same membranes actually improved, reaching a mixed-gas hydrogen-to-methane selectivity of 2,639.8 with 15.6 GPU permeance and a downstream hydrogen purity of 99.8 percent. For comparison, the celebrated cellulose-derived CMS membrane achieved a pure-gas selectivity of 5,506 only at an elevated 130 degrees Celsius, and its performance at practical ambient temperatures would be substantially lower. Both of the new membranes also surpass the 2015 upper bound for polymer membranes in hydrogen-methane separation, a benchmark that has defined the frontier of polymeric materials for a decade.

The same platform proved equally adept at a second, very different separation problem: recovering helium from natural gas. Global helium demand is growing by roughly two million cubic meters per year, driven by applications ranging from MRI magnets to semiconductor manufacturing, yet most helium-bearing reservoirs contain only between 0.2 and 4 percent helium. Conventional cryogenic distillation is energy- and capital-intensive, and ordinary polymeric membranes suffer from an intrinsic trade-off between permeability and selectivity that limits their efficiency. Testing their membranes against a feed of 5 percent helium and 95 percent methane, the researchers found that the dual-treated CMS achieved 98.8 percent helium purity, rising to 98.96 percent after aging as the selectivity climbed from 1,617.8 to 2,000.8. The VTMS-only variant delivered 99.3 percent purity initially. These purities, achieved in a single membrane stage from such a dilute feed, represent a proof of concept that could reshape how helium is harvested.

Perhaps the most intriguing finding is that aging, usually the enemy of membrane performance, actually improved the dual-treated membranes. Over 24 days of vacuum aging at 35 degrees Celsius, the helium permeance of the DETDA/TMC-plus-VTMS membrane fell by only 5.9 percent, compared with a 29 percent drop for the VTMS-only version, while methane permeance fell sharply by 38 percent. The researchers attribute this to the tightening of imperfectly packed carbon structures: cross-linking introduces small defects into the selective Langmuir domains during carbonization, and controlled aging closes those defects into narrower slits that still admit tiny, spherical helium atoms but increasingly exclude the bulkier methane molecule. Probe-gas experiments with molecules of varying kinetic diameters confirmed a molecular sieving mechanism, with a sharp permeance cutoff between small gases like hydrogen and helium and larger ones like carbon dioxide, oxygen, and nitrogen.

The underlying physics of why DETDA/TMC treatment changes the carbon structure so profoundly remains partly hypothetical, and the authors are candid about this. They propose that the higher density of cross-linked sites promotes stress-induced chain scission during pyrolysis, yielding shorter carbon strands that assemble into smaller carbon plates which stack more tightly. This tightening reduces permeance for medium-sized gases and enhances oxygen-nitrogen selectivity, while partially retained cross-linked branches create a small population of larger bypass pores that raise methane permeance. The data cannot yet establish whether the chemical modification distributes uniformly through the dense selective layer, and the team notes that multistage process design and techno-economic analysis will be needed to quantify real-world benefits. Still, the empirical performance under realistic mixed-gas, high-pressure, aged conditions speaks for itself.

What makes this work resonate beyond the laboratory is the paradigm it suggests. Rather than designing an entirely new polymer for every separation, the method takes ordinary, defect-free asymmetric hollow fibers already spun from cheap commercial materials and chemically retunes them into stabilized network precursors with controllable pyrolysis behavior. This unlocks a vast pool of commercially available polyimides that were previously unusable for CMS membranes because highly cross-linked polymers cannot be spun directly into fibers. The researchers envision extending the approach to different diamines and acid chlorides, and to gas pairs far beyond hydrogen-methane and helium-methane. If the vision holds, the natural gas pipeline network itself could become a distributed hydrogen delivery system, with membrane modules sited at points of use quietly sifting ultrapure hydrogen from the flowing blend, and low-concentration natural gas wells becoming viable helium sources. A simple chemical treatment, applied before the fire, may have redrawn the map of what molecular sieves can do.

Subject of Research: Scalable carbon molecular sieve hollow fiber membranes for hydrogen and helium separation from natural gas

Article Title: Transforming Gas Separations with Scalable Carbon Molecular Sieve Membranes Enabled by Chemically Tuned Hollow‐Fiber Precursors

Article References: Cao, Y., Liu, Z., Shomali, Z., Kim, J., Schlosser, S., Lively, R. P., & Koros, W. J. (2026). Transforming Gas Separations with Scalable Carbon Molecular Sieve Membranes Enabled by Chemically Tuned Hollow‐Fiber Precursors. Advanced Science, Article e78223. https://doi.org/10.1002/advs.78223

Image Credits: AI Generated

DOI: 10.1002/advs.78223

Keywords: carbon molecular sieve membranes, gas separation, hydrogen purification, helium recovery, hollow fiber membranes, pyrolysis, Matrimid polyimide, cross-linking, mixed gas separation, natural gas pipelines, membrane aging, molecular sieving

The central obstacle the team set out to solve is a familiar one in membrane science. Mixed-matrix membranes combine the easy processing of polymers with the precisely engineered pores of crystalline materials, but the recipe has a built-in ceiling. When the fraction of porous filler climbs beyond a modest threshold, the particles aggregate, defects appear at the polymer–filler interface, and the membrane’s performance collapses rather than improves. Most strategies have therefore kept filler loadings low, which means the polymer dominates transport and the carefully designed pores of the filler contribute little. The intrinsic transport properties of the crystalline material are effectively wasted.

The Tianjin-led group overturned the conventional design philosophy. Rather than maximizing strong interactions between polymer and filler to glue the two phases together, they pursued what they call a weak-interaction inlaying strategy. By balancing the interaction energies between polymer and filler on one hand and between filler particles themselves on the other, they found that the single-crystal COFs could be dispersed uniformly at loadings reaching 75.5 percent by volume, an extraordinary fraction for a mixed-matrix membrane. At such loadings the filler no longer acts as a minor additive; it becomes the dominant, continuous transport network through which gas molecules travel.

The choice of filler matters as much as the loading. Covalent organic frameworks are a class of porous crystalline polymers built from light elements such as carbon, nitrogen, oxygen and hydrogen, linked by strong covalent bonds into periodic lattices with regularly sized pores. Unlike metal–organic frameworks, they contain no metal nodes, making them fully organic and compatible with an organic polymer matrix. The team used single-crystal COFs, whose well-defined, defect-minimized lattices preserve the exact pore geometry designed into the material. When these crystals are embedded intact, the membrane inherits their intrinsic transport channels rather than a degraded approximation of them.

The resulting membranes create what the authors describe as filler-dominated pathways for fast carbon dioxide transport. Gas molecules moving through the membrane encounter a continuous network of COF pores whose size and chemistry favor carbon dioxide over nitrogen, while the surrounding polymer simply holds the architecture together. Because the interfaces remain clean and compatible even at very high filler content, there are no leaky voids that would undermine selectivity and no dense polymer regions that would choke off flow. The combination of high permeability and maintained selectivity pushes the materials well beyond the conventional upper-bound trade-off line that has long constrained polymer membranes for carbon dioxide–nitrogen separations.

The numbers are striking in context. A permeability of 74,800 Barrer is orders of magnitude above typical commercial polymer membranes for post-combustion capture, which often sit in the tens to low hundreds of Barrer. High permeability translates directly into smaller membrane areas and lower driving pressures for a given capture duty, which in turn reduces the capital cost and energy consumption of a capture plant. Previous life-cycle assessments have suggested that membrane separation can compete with the amine-based chemical absorption processes used today, but only if membrane materials reach the permeability–selectivity combination that makes module sizes practical. This work moves that target substantially closer.

Performance on day one means little if a membrane degrades, and polymer membranes are notorious for physical ageing, the gradual collapse of free volume that throttles transport over months and years. The team addressed this concern directly with long-term tests. Membranes based on polyimide retained approximately 85 percent of their initial permeability after six months of ageing at room temperature, a result that suggests the rigid crystalline COF network helps stabilize the membrane structure against the densification that normally erodes polymer performance. For a material operating at such extreme filler loadings, this stability is itself a notable finding.

The work also carries a broader methodological lesson for materials chemistry. The weak-interaction inlaying strategy reframes the interface problem: instead of fighting aggregation with ever-stronger polymer–filler bonding, the researchers tuned the relative strengths of competing interactions so that dispersion remains thermodynamically comfortable. That principle, balancing polymer–filler against filler–filler interaction energies, could plausibly extend beyond COFs to other crystalline fillers and beyond carbon capture to other molecular separations, from hydrogen purification to hydrocarbon cracking. The study’s authors, including co-first authors Hanze Ma and Shilin Guo, with Guangwei He and Zhongyi Jiang as corresponding authors, frame the approach as a general route to exploiting the intrinsic properties of porous crystals inside processable membranes.

Scalability remains the decisive question for any laboratory membrane advance. The team emphasizes solution processability as a design requirement, and the fact that the membranes are all-organic, formulated from polymer and COF in solution, aligns with existing membrane fabrication methods rather than requiring exotic deposition techniques. Still, translating a laboratory coupon into square meters of defect-free membrane module, and validating performance on real flue gas with its humidity, sulfur impurities and temperature swings, will demand further engineering. The six-month ageing data and the robust interfacial design are encouraging early signals, but industrial pilots will be the true test.

Even with those caveats, the study represents one of the most compelling demonstrations yet that the long-standing permeability–selectivity trade-off can be beaten by design rather than merely nudged. By making the crystalline filler the membrane rather than a garnish on it, the researchers have effectively built a processable, polymer-locked single-crystal separation device. If the approach survives scale-up, membranes of this kind could shrink the footprint and energy bill of post-combustion carbon capture, a technology the Intergovernmental Panel on Climate Change and the International Energy Agency both identify as essential to limiting warming. In a field where incremental gains are the norm, a seventy-thousand-Barrer membrane with intact selectivity is the kind of result that redraws the map.

Subject of Research: All-organic mixed-matrix membranes with single-crystal covalent organic frameworks for energy-efficient carbon capture

Article Title: All-organic mixed-matrix membranes with single-crystal covalent organic frameworks for carbon capture

Article References: Ma, H., Guo, S., Su, P., Zhao, Q., Wang, S., Ren, Y., Fu, J., Zhang, H., Zhang, T., Yuan, S., Pan, Q., Villalobos, L. F., Li, Y., Ma, X., Agrawal, K. V., He, G., & Jiang, Z. (2026). All-organic mixed-matrix membranes with single-crystal covalent organic frameworks for carbon capture. Nature Energy. https://doi.org/10.1038/s41560-026-02149-9

Image Credits: AI Generated

DOI: 10.1038/s41560-026-02149-9

Keywords: carbon capture, covalent organic frameworks, mixed-matrix membranes, gas separation, CO2 permeability, polyimide, membrane technology, porous materials, post-combustion capture, polymer membranes, Nature Energy, materials science

Pressure swing adsorption, or PSA, works by exploiting the fact that different gas molecules bind to porous solids with different strengths and at different speeds. A gas mixture is pushed through a bed of adsorbent material under pressure; components that bind strongly are retained while others pass through. Depressurizing the bed then releases the captured gas, regenerating the material for the next cycle. Compared with cryogenic distillation, which demands enormous energy input to reach low temperatures, or membrane separation, whose thin films foul easily and remain largely confined to the laboratory, PSA offers low energy consumption, simple operation, and proven industrial scalability. The catch is that the entire process hinges on the adsorbent itself. And for methane and nitrogen, the two dominant components of low-concentration coalbed methane, that is a formidable challenge, because the two molecules are nearly identical in size and chemical behavior.

Methane has a kinetic diameter of 3.8 angstroms, nitrogen 3.64 angstroms, a difference of barely two-tenths of an angstrom. Both are nonpolar gases. Methane’s higher polarizability tends to make porous solids preferentially adsorb it through thermodynamic equilibrium effects, while nitrogen’s smaller size and quadrupole moment can give it an edge in kinetic adsorption rates. Any successful adsorbent must therefore navigate a subtle competition between two separation mechanisms, and the review argues that the future lies in materials engineered to exploit both simultaneously. The authors organize the field into four material families: carbon-based adsorbents, zeolite molecular sieves, metal-organic frameworks, and composite hybrids, tracing how each has evolved from empirical trial and error toward rational, knowledge-driven design.

Carbon-based materials, chiefly activated carbon and carbon molecular sieves, remain the workhorses of the field. They are cheap, abundant, and easy to manufacture from wood, coconut shells, coal, or even agricultural waste such as coffee grounds, camellia seed husks, and mung bean powder. Activated carbon typically relies on equilibrium separation: methane, with its stronger affinity for the carbon surface, is retained in the bed while nitrogen breaks through. Recent work has pushed selectivity upward through clever chemistry. Nitrogen-doped porous carbon spheres made from glucose achieved a separation selectivity of 3.76, while palm-sheath-derived porous carbon dominated by ultramicropores smaller than 7 angstroms reached 7.6. Coffee-ground-derived columnar carbon exhibited a selectivity of 10.3 along with water resistance and mechanical strength. A systematic study of 38 coal-based activated carbons established a quantitative design rule: pores between 4 and 7 angstroms, matching the kinetic diameters of the two gases, are critical for effective separation.

Carbon molecular sieves take a different tack. With uniform micropores of 3 to 5 angstroms, they exploit kinetic separation: nitrogen diffuses through the narrow pores far faster than methane, so nitrogen is preferentially adsorbed and methane is left enriched in the gas phase. This approach preserves residual pressure in the bed, which facilitates downstream methane liquefaction. Chemical vapor deposition with methane or toluene can precisely tune pore sizes; one material achieved an equilibrium separation factor of 4.74, exceeding the industrial threshold of 3. Iron-ion-modified sieves raised the separation ratio from 2.01 to 6.03, and in-situ ion activation strategies cut corrosive KOH consumption by 93 percent while maintaining selectivity of 5.7. One phenolic-resin-derived sieve concentrated a 75 percent methane feed to roughly 90 percent purity with recovery above 80 percent in actual PSA cycling. The persistent weakness, the review notes, is moisture: water molecules compete for adsorption sites and can block the very micropores that make these materials work.

Zeolites, crystalline aluminosilicates with perfectly ordered pore networks, offer superior thermal and chemical stability, and recent years have seen a leap in their performance. Conventional zeolites such as 4A, 5A, and 13X show only modest selectivity, typically between 4 and 7. But targeted modification has changed the calculus dramatically. Nanocrystalline Ag-ZK-5, in which silver cations create strong electric field gradients that favor quadrupolar methane molecules, achieved an IAST selectivity of 11.8, the highest among zeolite-based materials. Ammonium-exchanged mordenite delivered selectivity 2.4 times that of its sodium form. Perhaps most strikingly, researchers demonstrated that applying an external electric field to the trap-door zeolite ZSM-25 induced lattice expansion and enhanced cation oscillation, boosting methane-nitrogen selectivity by 60 percent, a proof of concept that molecular discrimination can be actively controlled rather than passively fixed at synthesis.

Metal-organic frameworks, or MOFs, represent the most designable class of adsorbents. Built from metal ions or clusters linked by organic ligands, they combine ultrahigh surface areas with pores that can be tuned almost atom by atom. Early MOF studies achieved selectivities of 3 to 6 by decorating ligands with polarizable groups such as bromine and nitro, strengthening van der Waals attraction to methane. The current generation goes further, deploying multiple mechanisms at once. The zirconium-based MIP-203-F overcame the classical trade-off between capacity and selectivity through dual binding sites, formate bridges and hydroxyl clusters, that cooperatively polarize methane molecules. The titanium framework ZSTU-1 features engineered nano-traps that bind methane with record selectivity. The hydrophobic framework TUTQ-1Ni maintained a selectivity of 11.0 even at 100 percent relative humidity, the highest reported for any MOF under fully saturated conditions, directly addressing the moisture problem that plagues the field.

Machine learning is now accelerating the search. One model trained on molecular simulation data accurately predicted gas uptake across 4,612 experimentally reported MOF frameworks with correlation coefficients between 0.93 and 0.97, enabling rapid virtual screening before any synthesis is attempted. Meanwhile, engineers are tackling the practical problem of shaping: MOF powders must be formed into robust beads or extrudates for industrial fixed beds. Using sustainable binders such as sodium alginate and carboxymethyl cellulose, researchers have produced shaped MOFs that retain more than 90 percent of their original porosity and outperform commercial zeolite 13X, with one aluminum-based material achieving 99.5 percent methane recovery and 97.3 percent purity in simulated vacuum PSA operation.

Composite materials aim to have it all, combining the high pore volume of carbons, the stability of zeolites, and the tunable active sites of MOFs. A graphene-oxide-promoted carbon aerogel reached a surface area of 3140 square meters per gram and a methane capacity of 6.87 millimoles per gram, with breakthrough time extended 200 percent over the pristine material. Growing aluminum-derived carbon nanosheets inside macroporous polyacrylate cut water uptake by 67 percent and boosted methane adsorption 1.73-fold. A hybrid of UiO-66-Br2 with mesoporous SBA-15 silica achieved a selectivity parameter of 20.06, far exceeding either component alone, while MOF-5 dispersed on water-absorbing clinoptilolite gained dramatic moisture resistance because the hydrophilic scaffold sacrificially captures water before it can degrade the MOF.

The review’s authors conclude that no single material family has yet solved every requirement: high capacity, high selectivity, moisture resistance, cyclic durability, fast kinetics, and low cost all at once. Carbon materials need better oxidation resistance and humidity tolerance; zeolites must overcome their hydrophilic sites; MOFs face scale-up and mechanical fragility; composites must perfect interfacial bonding so components do not delaminate over thousands of cycles. But the trajectory is unmistakable. The field is shifting from experience-driven modification to atomic-level rational design, from single-mechanism separation to multi-mechanism synergy, and from laboratory curiosities to engineering validation under real industrial conditions. If those threads continue to converge, the methane now vented from the world’s coal mines, a resource once written off as unusable, could become a meaningful pillar of low-carbon energy supply, turning one of the energy sector’s most stubborn waste streams into a genuine asset.

Subject of Research: Adsorbent materials for methane/nitrogen separation in pressure swing adsorption purification of low-concentration coalbed methane

Article Title: Recent advances in adsorbent materials for the efficient purification of low-concentration coalbed methane through pressure swing adsorption

Article References: Qiao, Y., Deng, B., Kong, Y., Han, Y., Chen, P., & Zhao, R. (2026). Recent advances in adsorbent materials for the efficient purification of low-concentration coalbed methane through pressure swing adsorption. Journal of Saudi Chemical Society, 30(3), Article 31. https://doi.org/10.1007/s44442-026-00081-x

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00081-x

Keywords: coalbed methane, pressure swing adsorption, gas separation, activated carbon, carbon molecular sieves, zeolites, metal-organic frameworks, composite adsorbents, methane purification, greenhouse gas, porous materials, adsorption selectivity

The research, led by Tiantian Qiao and Sainan Zhou of Liaoning University together with Xiaoqing Lu and Yongqing Li of the China University of Petroleum, focuses on C12N8, a porous single-layer carbon nitride framework. Using density functional theory calculations paired with grand canonical Monte Carlo simulations, the team systematically explored what happens when scandium atoms are incorporated into the lattice and when the sheet is stretched under tensile strain. Their central finding is that the two modification strategies play fundamentally different roles: doping is the dominant lever for performance, while strain acts as a secondary, double-edged adjustment.

The chemistry behind the improvement is electrostatic. When a scandium atom substitutes into the C12N8 framework, it behaves as a strong cationic center, withdrawing electron density from its surroundings and dramatically increasing the polarity of the nanosheet. Carbon dioxide, with its linear structure and pronounced quadrupole moment, is highly sensitive to such electric field gradients. The doped scandium sites become primary adsorption anchors, gripping CO2 molecules far more tightly than the pristine carbon nitride ever could. Nitrogen and methane, which lack the same charge distribution, feel a much weaker pull, and that asymmetry is what produces the material’s remarkable selectivity.

The numbers are striking. Under dry gas conditions at ambient temperature, unstrained Sc-doped C12N8 achieves a CO2/N2 selectivity of 369 and a CO2/CH4 selectivity of 468 — figures that place it among the best-performing adsorbents modeled to date for these industrially critical separations. Flue gas from power plants is dominated by nitrogen, and natural gas upgrading hinges on stripping CO2 from methane, so a material that discriminates this strongly against both contaminants addresses two of the largest separation problems in the energy economy simultaneously.

Capacity tells a complementary story. When the Sc-doped nanosheet is stretched by five percent, the simulated CO2 uptake reaches 8.19 millimoles per gram at 298 kelvin and 1 bar, conditions close to those of real post-combustion capture. Strain accomplishes this by reshaping the material’s geometry rather than its chemistry. Tensile deformation flattens the lattice topology and enlarges the accessible pore volume, giving CO2 molecules more room to pack inside the sheet. In effect, the stretched nanosheet becomes a roomier molecular warehouse, and the electrostatic anchors installed by scandium doping ensure that the additional guests are overwhelmingly carbon dioxide.

Yet the study’s most instructive result is what strain does not do — and what it costs. Applied to pristine C12N8, tensile strain alone delivers no improvement in CO2 uptake whatsoever; without the cationic anchors, extra pore volume is simply not enough. More importantly, in the doped system, strain boosts capacity at the expense of selectivity. The same flattening and pore expansion that lets more CO2 squeeze in also relaxes the geometric and energetic discrimination that made the unstrained material such a precise sieve. The highest selectivities belong to the unstrained doped sheet; the highest capacity belongs to the five-percent-stretched one. Engineers designing a real capture process would have to decide which figure matters more for their application, or find an intermediate strain that balances the two.

This capacity-selectivity trade-off is a recurring theme in adsorbent design, and the study frames it with unusual molecular-level clarity. Strong electrostatic interactions between open metal sites and CO2 drive both uptake and selectivity upward together, which is why scandium doping enhances both metrics. Strain, by contrast, is purely structural: it promotes CO2 packing but dilutes the selectivity advantage. The authors describe Sc-C12N8 as a promising two-dimensional molecular sieve, and the phrase is apt — the material’s performance depends on pore architecture and pore chemistry working in concert, and the simulations show precisely how each contribution can be dialed independently.

The choice of scandium is not arbitrary. The element sits among the transition metals known to form strong bonds with nitrogen, and prior work on scandium nitride films and Sc-embedded graphyne has highlighted its affinity for nitrogen-rich lattices. Embedding scandium in a carbon nitride framework therefore yields stable cationic sites rather than loosely bound decoration atoms that might cluster or leach. The study also builds on a body of earlier computational work by the same groups showing that alkali, alkaline earth, and first-row transition metal dopants can transform graphyne and covalent organic frameworks into high-performance CO2 sorbents, with the dopant’s charge and coordination environment governing how CO2 molecules arrange themselves in the pores.

Methodologically, the work illustrates the modern pipeline of adsorbent discovery. Density functional theory, with dispersion corrections to capture the weak van der Waals forces that dominate gas adsorption, provides the electronic structure and binding energetics; charge partitioning analysis quantifies the polarity that scandium introduces; and grand canonical Monte Carlo simulations using established force fields then translate those microscopic interactions into macroscopic adsorption isotherms and mixture selectivities at realistic temperatures and pressures. Because every step is computational, the results are predictions rather than measurements, but they define a concrete, testable target for synthetic chemists working on two-dimensional carbon nitrides, several of which have already been realized in the laboratory.

The broader significance lies in the design principle. Rather than searching blindly for a material that happens to capture CO2 well, the study demonstrates a modular strategy: choose a stable, porous two-dimensional framework, install cationic adsorption sites by doping, and then use lattice strain as a fine-tuning knob to trade capacity against selectivity as the application demands. As atmospheric CO2 concentrations continue to climb and carbon capture and storage moves from demonstration to deployment, materials that can be engineered at this level of precision — a single atomic layer, a single dopant species, a few percent of strain — may prove essential to making separation technology both effective and energy-efficient. The scandium-doped C12N8 nanosheet is a simulation today, but it is a simulation with a blueprint attached.

Subject of Research: Computational design of scandium-doped two-dimensional C12N8 nanosheets for CO2 capture and separation

Article Title: Balancing CO2 capture and selectivity through scandium doping and strain engineering in two-dimensional C12N8 nanosheets

Article References: Qiao, T., Zhou, S., Lu, X., & Li, Y. (2026). Balancing CO2 capture and selectivity through scandium doping and strain engineering in two-dimensional C12N8 nanosheets. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13796-4

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13796-4

Keywords: carbon capture, CO2 adsorption, two-dimensional materials, C12N8 nanosheets, scandium doping, strain engineering, density functional theory, grand canonical Monte Carlo, gas separation, molecular sieve, carbon nitride, selectivity

The concept of a secondary building unit is familiar to anyone who has followed the evolution of framework chemistry. In metal–organic frameworks, clusters of metal ions bridged by organic linkers act as rigid, geometrically predictable joints. Because each node presents a fixed number of connection points arranged in a defined geometry, chemists can link them with linear or angular ligands and predict the topology of the resulting network. Connectivity, in this sense, is the key design variable: the more connection points a node offers, the more complex and topologically rich the network it can support. Achieving high connectivity with purely organic, non-metal nodes has long been difficult, because molecular clusters held together only by weak interactions tend to fall apart or rearrange during crystallization.

The new study tackles that challenge by turning to ionic clusters. These are aggregates in which charged organic building blocks associate through strong electrostatic attractions, often reinforced by networks of hydrogen bonds, to form discrete, well-defined assemblies that behave like single structural units. Rather than relying on a metal center to fix the geometry, the cluster’s charge distribution and directional ionic pairing lock its shape. When such a cluster carries a high number of available charged sites around its perimeter, it can bind many counter-charged organic partners simultaneously, functioning exactly like the multi-connected nodes of traditional framework chemistry while remaining entirely metal-free.

What emerges from this strategy is a crystalline porous organic salt: a material in which positively and negatively charged organic components assemble into an ordered lattice whose architecture is governed by electrostatic complementarity. The authors show that using ionic clusters as high-connectivity nodes produces frameworks with far greater structural complexity than could be achieved from simple ion pairs. The high connectivity translates directly into richer topologies and, crucially, into structures whose pores survive the removal of guest molecules from the crystal. That permanence is the defining requirement for useful porous materials, since a framework that collapses when emptied cannot capture, separate, or store anything.

The physics behind the stability is worth unpacking. In ordinary organic salts, each ion typically partners with just one or a few counterions, and the resulting crystals can be fragile, prone to phase transitions, and sensitive to humidity or solvent exchange. In an ionic cluster built from multiple charged units, the electrostatic charges are effectively distributed and multiply bridged. Each ionic contact reinforces the others, and the cluster presents a cohesive, internally satisfied structure to the outside world. At the same time, the outward-facing charged groups remain available for assembly. The result is a node that is simultaneously stable on the inside and reactive on the outside, precisely the combination needed for reliable crystallization into open frameworks.

Porous organic salts as a class carry distinct advantages over their covalent and coordination-based cousins. Their synthesis is often a matter of simple mixing in solution, with no need for solvothermal conditions, metal precursors, or elaborate catalysts. The components are frequently derived from abundant, inexpensive organic acids and bases. Because no metal is involved, the materials avoid issues of metal cost, toxicity, and scarcity that can complicate applications in water treatment or biomedicine. And because ionic frameworks are inherently charged, they offer strong, tunable electrostatic fields inside their pores, an attribute of particular interest for capturing carbon dioxide, whose quadrupolar charge distribution responds strongly to polar pore environments, and for separating similarly sized molecules that differ in polarity.

The introduction of high-connectivity ionic clusters addresses what has arguably been the field’s most serious limitation: topological simplicity. When the building blocks of an organic salt connect at only two or three points, the achievable network types are limited, and the resulting pores tend to be small, one-dimensional, or easily blocked by interpenetration. High-connectivity nodes change the calculus dramatically. With four, six, or more bonding directions available from a single node, designers can target three-dimensional pore networks with large cavities, intersecting channels, and controlled pore apertures. The study’s demonstration that such connectivity can be achieved through purely ionic assembly suggests that the design principles matured for metal–organic frameworks can now be ported into the metal-free world.

The implications extend across several application domains. In carbon capture, charged pore surfaces can be tuned to bind carbon dioxide preferentially over nitrogen, and a crystalline salt whose pore chemistry is dictated by interchangeable counterions offers a natural knob for optimization. In gas separation, the combination of size-selective apertures and strong electrostatic fields could discriminate between molecules that conventional adsorbents struggle to distinguish. In proton conduction, hydrated ionic frameworks are natural candidates for electrolyte membranes in fuel cells, since the same ionic pathways that define the structure can also transport charge. And in sensing and drug delivery, the ability to exchange guests within a stable, water-tolerant crystalline host makes these salts attractive platforms for uptake and release studies.

There are also fundamental scientific payoffs. Framework chemistry has long been organized around a handful of canonical secondary building units whose geometries dictate network topology. Adding ionic clusters to that catalog expands the mathematical space of achievable structures and raises new questions about how charge delocalization, hydrogen bonding, and counterion identity shape crystallization outcomes. The work also highlights a conceptual bridge between solution-phase supramolecular chemistry and solid-state materials design: the same electrostatic forces that assemble ions into discrete clusters in solution can be harnessed, under the right conditions, to assemble those clusters into extended, permanently porous crystals. Understanding and controlling that hierarchy, from ion to cluster to framework, is a central theme of modern supramolecular materials science.

Challenges remain before such materials reach practical deployment. Water sensitivity, long a weakness of ionic solids, must be quantified and, where necessary, engineered away through hydrophobic pore linings or reinforced ionic networks. Scalability of synthesis, reproducibility of crystal quality, and long-term stability under cycling conditions all require systematic study. Yet the demonstration that ionic clusters can serve as reliable high-connectivity secondary building units represents a genuine conceptual advance, giving chemists a metal-free, solution-processable route to complex porous architectures. As the toolkit of ionic nodes grows, crystalline porous organic salts may move from a laboratory curiosity to a serious competitor in the crowded field of functional porous materials, proving that the humble electrostatic bond, when organized with sufficient geometric ambition, can build structures as sophisticated as any forged from metal or covalent links.

Subject of Research: Use of ionic clusters as high-connectivity secondary building units for constructing crystalline porous organic salts.

Article Title: Ionic clusters as high-connectivity secondary building units for crystalline porous organic salts

Article References: Lu, Y.-L., Chen, Y., Qu, H., Zhang, L., O’Shaughnessy, M., Fellowes, T., & Cooper, A. I. (2026). Ionic clusters as high-connectivity secondary building units for crystalline porous organic salts. Nature Chemistry. https://doi.org/10.1038/s41557-026-02248-w

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02248-w

Keywords: ionic clusters, porous organic salts, secondary building units, crystalline frameworks, metal-free materials, supramolecular chemistry, porosity, carbon capture, gas separation, framework chemistry, electrostatic assembly, hydrogen bonding