Unlocking large-area free-standing MOF-glasses for molecular sieving gas separation membranes

Domain: biology
Published: 2026-03-17
Editorial outcome: minor_revision

Abstract
Membranes from MOF glasses hold significant promise for gas separations due to the absence of grain boundary diffusion, liquid processibility, and tunability. The inherent high viscosity of MOF melts renders them prone to cracking and further handling, and their propensity to densify at high temperatures and long time in molten state severely limit their upscaling potential. A solution to overcome these limitations is demonstrated by selecting suitable materials that fit the thermal and mechanical behaviour to MOF-glass, enabling processing and making of large, crack-free MOF-glass sheets. This is demonstrated on the example of the well-known MOF-glass former ZIF-62. By optimizing each step of the process – from melting to performance testing – we successfully fabricate a crack-free, self-supported ZIF-62 glass membrane. The microstructure is investigated using microscopy as well as SEM-EDX analysis, confirming homogeneous boundary-free MOF-glass, while gas permeation experiments prove the applicability of MOF-glass as gas separation membrane. The membrane exhibits exceptionally sharp methane molecular sieving cut-off with such low permeability that gas chromatography is unable to detect CH4. We conclude this work by giving a brief outlook of the remaining challenges and perspectives for MOF glasses, envisioning transferability of our approach to other glass-forming systems and their scaling perspectives.

Introduction
Membranes are among the key technologies to unlock sustainable, resource and energy-efficient modern industrial separation processes, reducing the energy required for separations by up to 80% compared to conventional separation processes1. A membrane acts as a physical barrier for molecules, with its separation efficiency relying only on the material's parameters, offering an energy-efficient and sustainable technology to disruptively substitute conventional cryogenic distillation for the continuous separation of gases2. For porous membranes, narrow pore openings are favourable, as they offer a specific size selective diffusion limitation via the molecular sieving effect3, offering high selectivity and flux for small molecules, while excluding larger molecules4. Metal-Organic Frameworks (MOFs) – reticular inorganic-organic hybrid materials consisting of metal nodes and organic bridging ligands – are promising candidates for membranes, as they offer well-defined crystalline, porous structures and an almost unlimited number of chemical design possibilities.

Recently, melt-quenched amorphous framework glasses have become a special area of interest for the separation processes with several potential benefits over their crystalline counterparts5. Nowadays, zeolitic imidazolate frameworks (ZIFs) as well as carboxylate-based MOFs6 have been found to be meltable. First, framework glasses can be shaped and processed from powders in their liquid state, sometimes with the help of network formers or modifiers7,8, to form a free-standing layer, and therefore do not require complex thin-film growth techniques on expensive ceramic substrates, commonly used for MOFs and COFs9,10. Second, the universal processability of glasses allows for liquid-state shaping and further polishing, cutting, etc., to achieve the desired size and shape11. Further, most importantly for membranes, the ability of glass to form a monolithic layer, which can be hierarchically structured12 and hinders grain boundary diffusion, and therefore higher selectivity can be achieved9,13. Finally, unique for MOF-derived glasses, their porosity can be tuned by varying the processing parameters to obtain the targeted size of the molecular-sieving windows14.

The separation performance of the MOF glass membranes varies substantially between literature references9,10,13,15–21. Several neat, supported, and composited MOF glass membranes have been reported, showing overall high selectivity for various gas mixtures. Thus, for binary CO2/N2 mixtures, reported values of separation factor α reached α(CO2/N2)=32 for pristine MOF glasses (aZIF-UC-419) and 34.6 for MOF crystal-glass composites (ZIF-8/aZIF-6221). Helium separation has been investigated on supported MOF glass membranes, allowing for its effective separation: pristine, supported aZIF-62 membranes showed selectivity values of 13.9 and 17.4 for He/CH4 and He/N2 mixtures, respectively13. Other groups tackle the challenging CH4/N2 separation (close kinetic diameters of species dkin(N2)=3.64 Å and dkin(CH4)=3.8 Å) for natural gas valorisation with MOF-glasses. A few attempts to employ MOF glass membranes have been made recently, with CH4/N2 selectivity values reaching 6.0 (aZIF-62 foam membrane20) and 6.2 (aZIF-UC-419). Some MOF glass composites with large-pore crystalline MOFs21 and zeolites22 also demonstrated a great promise for hydrocarbon separations, such as propane/propylene (α(propane/propylene) = 18.7) and 1,3-butadiene from different C4 alkanes and olefins, respectively. Comparative data on CO2/N2 separation data has been collected Table S3 and a Robeson-plot23 in Fig. 5S.

Despite the great promise, obtaining neat and free-standing MOF glass membranes of a larger area remains challenging. Practical limitations of melt-quenched MOF glasses are to choose parameters, after which they offer remaining accessible porosity11,14 and, as all glasses in practice, they are prone to cracking24 which complicates the fabrication of the large pieces and commonly necessitates the use of ceramic supports. Several approaches to achieve sufficient sizes of the bubble-free samples were reported, such as hot pressing25 or ball milling with further remelting26. Such glasses show overall good optical quality, but are not suitable for gas separation applications, as any excessive processing leads to significant losses in porosity and inaccessible pore channels. Many groups reported drastic MOF-glass densification caused by the proposed processing techniques27,28. As a result, achieving at least a typical membrane area of the polycrystalline MOF membranes – which is by itself commonly limited to the centimetre scale – while preserving porosity and therefore performance characteristics in MOF glasses remain highly problematic. Moreover, in the absence of such established procedures, further scaling considerations – potentially unlocking industrially-relevant meter-scale membrane areas – remain unexplored.

In this work, we demonstrate a method to achieve larger free-standing MOF glasses from crystalline MOFs (c.f. Fig. 1) on the example of meltable zeolitic imidazolate framework (ZIF) ZIF-62. Specifically, first we were able to produce centimetre-scale, free-standing MOF glass membranes made directly of melt-quenched ZIF-62 (aZIF-62), consisting of zinc metal sites coordinated by imidazolate and benzimidazolate linkers. Mechanical stress, which usually results in glass cracking29, is prevented by two primary changes in the melting procedure. First, we empirically studied the influence of thermo-mechanical parameters and selected, based on the results, suitable substrates for melting and liquid-state processing of aZIF-62 to ensure optimal interfacial interaction. Second, we optimized the glass annealing procedure for aZIF-62, which is performed directly after melting at a temperature below Tg to relieve the internal stress caused by the high viscosity of the melt. This approach did not only provide us with large, crack-free, and utilizable thin layers, but also allowed us to avoid unnecessary processing (such as hot-pressing or remelting) and therefore preserve the most possible accessible porosity in aZIF-62, in accordance with our earlier work14. Further, we designed a procedure to fixate the MOF-glass as a free-standing membrane film, enabling long-lasting MOF-glass membranes for gas permeation testing, with a large active glass area. Our prototype aZIF-62 membrane shows excellent performance in gas-separation experiments, especially in the exclusion of CH4, with an approximate 100% retention. The permeability of CH4 was so low that it became undetectable by our gas chromatography setup in single-gas and mixed-gas experiments, yet it allowed for the highly selective permeation of other gas species, even those with kinetic diameters just slightly higher than that of CH4. This is a step towards perfect homogeneity, crack-free, and defect-free aZIF-62 membranes without any evidence of grain boundary diffusion or leakage. All applied optimizations – from melting and annealing procedures to the prototype membrane fabrication – did not require any energy-intensive modifications and anything but readily available, cheap materials, enhancing the technology transfer potential. As an outlook, we suggest that this or similar – but always systematic, taking both reticular and glassy nature of such systems into account – approaches can be transferred to other glass formers within the MOF family. Further, we provide strategies for the further development of MOF-glass membranes towards real-world application, including a perspective approach for their upscaling to the industrially relevant membrane areas.

The benefits of our advancements in MOF-glass processing demonstrated in this work, are shown in Fig. 1a–h, which starts from melting ZIF-62 crystals into scalable films, annealing them into crack-free, mechanically robust and utilizable aZIF-62 films, which can then be manufactured into stabilized membranes, enabling handling in our technical gas permeation devices.

Results
Overcoming Stress and Strain in Larger aZIF-62 Glasses
Meltable porous ZIF-62 (Fig. 2a), synthesized according to our previously published procedure11,14, was used to fabricate the MOF-glass membrane. Both synthesized material and glass were studied by XRD and the full loss of the crystalline structure after melt-quenching was observed (Fig. 2b).

Free-standing layers of pristine aZIF-62 were prepared through the melt-quenching of slightly hand-ground crystalline ZIF-62 between two identical substrates. Formation of a bulk aZIF-62 sheet without defects and grain boundaries requires the application of some external load due to the very high viscosity of the melt11,30. On the lab scale, simple metal paper clamps simultaneously fix the substrate-powder-substrate system and ensure moderate loading on the material. At the moment the melting temperature (for ZIF-62 in this case, Tm = 450 °C) is reached, the paper clamp (see inset in Fig. 2c) has already irreversibly lost its force (spring force drops significantly beyond 80 °C) and does not have a major impact on the system during the cooling stage. Maintaining only a low load at this point is crucial, as it prevents excessive sample densification through pore collapse, preserving the accessibility of the pore channels for the guest molecules14. Moreover, in this way, large differences in volume change stemming from the elastic mismatch between the soft aZIF-62 (E < 10 GPa31) and the stiffer substrates (E ≥ 70 GPa32) are also avoided.

What must be considered is the interaction between the glass melt and a substrate during cooling, as incompatibility at this interface can lead to the build-up of additional internal stress in the resulting glass. The same problem is very likely to occur while melting the grown crystalline MOF layer on alumina to form a supported glass membrane19,33. To tackle this, various inert substrates have been initially selected for the melting of ZIF-62 to form aZIF-62 based on their coefficient of thermal expansion (CTE) and either brittle or ductile nature: soda-lime silica glass (SL; low CTE, brittle), aluminium (high CTE, ductile), and platinum (low CTE, ductile) (Fig. 2c). Among the employed substrates, our empirical finding is that only aluminium yielded bulk and crack-free layers (Fig. 2d), whilst contact with soda-lime silica glass and platinum resulted in significant cracking. This can be explained by both aZIF-62 and aluminium exhibiting relatively large CTE (32.1×10-6 K-1 34 and 24.0×10-6 K-1 35, respectively). This stands in contrast with platinum and soda-lime silica glass, which have low CTEs (9.0×10-6 K-1 35, and 9.2×10-6 K-1 36, respectively): while tensile stresses are induced in aZIF-62 during cooling, compressive stresses develop in the surrounding materials. Additional context and quantitative estimations to support this explanation are provided in Table S1, Supplementary Note 1 and Table S2.

As ductile platinum showed a result similar to the soda-lime silica glass, it can be concluded that the ability of the substrate to deform plastically cannot compensate for the difference in their thermal behaviour (c.f. Fig. 2c). As ductility by itself is not a limiting parameter, it is expected that the last untested combination – brittle material with high CTE – could also potentially yield a crack-free aZIF-62; however, designing such an experiment is nearly impossible due to the absence of inert and temperature-stable materials with this set of characteristics (for instance, matching for the role, phosphate glasses with high CTE would normally undergo a glass transition Tg way below the melting point of ZIF-62)32.

Testing other inert substrates such as borosilicate glass BOROFLOAT® 33, referred to as BF33 (brittle, very low CTE of 3.25×10-6 K-1) and gold (ductile, relatively low CTE of 14.2×10-6 K-1) also resulted in significant cracking (Figs. S1 and S2). Worth noting is that the visual “turbidity” of the glass layer (Fig. 2b, c) is only caused by the transferred surface pattern of the aluminium foil used for melting (Fig. S3), and the material remains transparent in its volume.

Although the selection of a compatible substrate allows us to make larger layers, the fabrication procedure is not comprehensive without a sufficiently designed annealing step. If melt-quenched and cooled to room temperature directly after the melting, aZIF-62 films can only keep their structural integrity for hours, or at most up to a few days, as at some point the residual stress causes spontaneous cracking (Fig. 2d, e). To avoid that, MOF glass was annealed slightly below the Tg of aZIF-62 at the temperature of 300 °C (Tg(ZIF-62)=322 °C) for 2 h before cooling down to room temperature. This temperature choice appears close to the optimum, as it allows for the balance between not densifying the glass in its liquid state and keeping the matrix mobile enough at the local scale to allow atomic rearrangements, releasing the residual inner stress.

Membrane fabrication
As membrane performance testing and utilization require proper sealing, normally involving the application of a certain load to O-rings and the membrane, the obtained thin brittle glass layer must be somehow stabilized to avoid cracking and damage. We demonstrate that a pristine free-standing MOF glass membrane can be fabricated by stabilizing with soda-lime glass and sealing with epoxy resin, enabling unproblematic handling. The aZIF-62 thin film was fabricated as described above – by melting the crystals between aluminium foils, which is carried on soda lime glass sheets, fixed by paper clamps (Fig. 3a), and followed by an annealing step prior to cooling to room temperature to release residual stress of the material. After removal of the aluminium foil, the aZIF-62 thin film was manually glued between two identical ring-shaped soda-lime silica glasses with epoxy resin (Fig. 3b, c), allowing for proper contact and stabilizing the active area. Excess aZIF-62 was then removed by breaking off the edges and polishing the outer ring (Fig. 3d, e) to maintain the required circular shape.

As a binding component, epoxy resin is readily available and easy to handle, while providing sufficient thermal and mechanical stability, as well as being gas-tight, thereby preventing leakages. The resulting aZIF-62 membrane remains undamaged and stable upon handling and is easily transported, sealed, and can thereby run as a prototypical membrane in the gas permeation setup (Fig. 3f). It requires harsh handling to break the obtained membranes after they have been fixed.

Performance
Gas permeation of single gases and binary gas mixtures was evaluated using a Wicke-Kallenbach setup and a gas chromatography system (GC). The gas separation performance of the fabricated aZIF-62 membrane with a thickness of 330 µm was tested under atmospheric conditions at 1 atm and room temperature (RT). A total of 100 ml/min of the feed gases (He, H2, CO2, N2, and CH4) were used, while sweep gas (N2, or CH4) flow rate was kept at 1 ml/min. The single and mixed gases were measured twice, with different carrier gases (He and Ar) for maximum intensity and accuracy of the individual peaks and their area, with a minimum of 99 cycles per experiment. The mounting of the aZIF-62 membrane is demonstrated in the photograph in Fig. 4a, featuring the large, transparent, and free-standing thin film in the middle with ⌀ 18 mm in diameter of which ⌀ 10 mm – and thus about 78.54 mm² – can be considered active membrane area. Screwing the O-rings gas-tight requires high pressure to seal the cell. The proposed design allows MOF glass to withstand this stress due to the epoxy resin and soda-lime glass rings fixing it from both sides, ensuring the mechanical stability of the whole cell. The measured single gas permeability shows a decreasing trend with an increase of the kinetic diameters, dkin, of the tested gas species: He (dkin=2.6 Å), H2 (dkin=2.89 Å), CO2 (dkin=3.3 Å), N2 (dkin=3.64 Å), and CH4 (dkin=3.8 Å) (Fig. 4b). Notably, the methane permeability P(CH4) tends to zero with no molecules being detected over the course of 16.5 h (99 cycles) of each measurement (carrier gas He, Ar) – single gas CH4 and mixed gas He/CH4, H2/CH4, CO2/CH4, N2/CH4 are thus leading to an estimated separation factor α(Pi/PMethane)=∞ in all cases. The minimum concentration of a gas in a sample that can be detected by the gas chromatography system with a thermal conductivity detector is 0.00009%, while it is safe to quantify concentrations at 0.001% (>10 ppm) or above. We do not observe a peak in CH4, even after switching carrier gases from He to Ar across all measurements, indicating that aZIF-62 is completely impermeable to CH4. The monolithic nature of glass, not allowing grain boundary diffusion, kinetically excludes larger molecules, starting at the kinetic diameter of methane. This trend can be observed in single-gas permeabilities, as high permeabilities for smaller species, such as He (P(He)=5500 barrer) and H2 (P(H2)=5400 barrer), are measured. Further, significant loss of permeability occurs for CO2 with P(CO2)=500 Barrer, then very low permeability of P(N2)=17 Barrer for N2 is determined, indicating the drastic molecular sieving cut-off. To the best of our knowledge, such a sharp molecular-sieving cut-off has not been shown for MOF glasses or any other MOF-membranes, yet. This result is supported by our previous preliminary work, where an absolute exclusion of ethane (dkin(C2H6)=4.4 Å) diffusion was achieved after liquid-state processing of aZIF-62, suggesting that this is likely possible, also for CH4 and other relatively large gas molecules. In particular, the pore channel size distribution in such, but millimetre-scale glasses prepared by following the same initial synthetic and melting protocols have been investigated in detail via HR-TEM: pore channel sizes varying from approximately 1.7 Å to 4 Å with an average pore-limiting diameter of 2.7±0.5 Å have been observed in the normal distribution for the analogous to the current work samples14. The above-demonstrated permeation results are indeed in agreement with this data: a disordered glassy nature results in a relatively broad pore channel size distribution, but the ≥3.8 Å (potentially allowing for CH4 (dkin=3.8 Å) permeation) fraction is almost negligible compared to the smaller sizes. At the same time, the 3.64 Å-3.8 Å fraction, enabling selective N2 permeation, is more present and therefore a notable N2 signal is observed. Aside from H2/CH4, He/CH4, and CO2/CH4 gas pairs, where pristine supported9,10,13 and composite37 MOF-glass membranes have already demonstrated great promise, a highly challenging N2/CH4 separation with relatively low reported selectivities19,20 can now be addressed more effectively.

Permeances of gases in a binary mixture of H2/CO2, CO2/N2, He/CO2 and H2/CO2 are shown in Fig. 4c, with corresponding real selectivity α in Fig. 4d. Straightforward comparison of this data with other MOF-glass membranes is complicated by the fact that – as demonstrated in the introduction – currently reported membranes vary significantly in their type (pristine or composited with crystalline MOFs or polymers), support nature (free-standing or supported), and the initial glass-former composition. All of these variations are employed to design the membrane exhibiting the molecular-sieving characteristics that specifically target one or another separation, while our membrane in particular works best for the challenging methane separations. In Fig. S5, we present a qualitative overview of the various reported MOF-glass membranes tested for different gas separations. Thus, our experimental value of the real selectivity αreal(CO2/N2)=21,8 from a mixed-gas CO2/N2 measurement is in good agreement with the ideal selectivity, published for supported membranes of aZIF-62 (αideal(CO2/N2)=23)9 and also a TIF-4 (αideal(CO2/N2)=25)10 with the latter MOF glass appearing to be very similar to aZIF-62, structurally and property-wise. In our present work, however, the ideal selectivity we measure reaches a value of αideal(N2/CO2)=29. Smaller species, such as H2 and He, were also effectively separated from CO2 with our aZIF-62 membrane, calculating an ideal selectivity of αideal(H2/CO2)=10 and αideal(He/CO2)=11 and real selectivity of αreal(H2/CO2)=20,2 and αreal(He/CO2)=23, respectively. A mixed gas separation factor much larger than an ideal separation factor is not often seen; however, it can be explained by the nature of the aZIF-62 having mainly pore channels, and not much of pore space for diffusion, in which the gases block each other, and the faster permeating species are highly favored to overcome the surface barrier38,39. Although the mixed-gas performance of MOF glass-based membranes was not reported for H2/CO2 and He/CO2 pairs, our estimations of ideal separation factors from other works13,19 give notably lower values, proving the defect-free nature of the present membrane. Table 3S and Fig. 5S puts our data in comparison with data on different MOF-glass membranes.

Upon completion of performance testing, the measurement cell was opened and the membrane was cut along the stacking axis to observe the cross-section, which was further polished (Fig. 5a). That the membrane architecture survives mechanical stress is shown by the technical drawings on how the membrane is mounted to become gas tight in Fig. 5b, c. From micrographs and SEM image it is observable that all layers – aZIF-62, epoxy resin binder, and soda-lime silica glass – remained macroscopically (Fig. 5a) and microscopically undamaged after cutting and polishing with no signs of crumbling or detaching (Fig. 5d). In Fig. 5d the SEM image and corresponding energy dispersive X-ray (EDX) mapping of the aZIF-62 membrane’s cross-section are presented. The EDX mapping proves the homogeneous structure of all layers with no signs of voids or detachments – thus, a great binding between epoxy resin and MOF/soda-lime silica glasses – which is perfectly visible through characteristic elemental signals: silicon (green) and oxygen (light blue) for soda-lime substrate, carbon (red) for epoxy resin, zinc (blue), nitrogen (yellow) and carbon, all well-distributed for aZIF-62. Single element maps can be found in the Supplementary Information (Fig. S4).

The MOF glass herein investigated does not possess any defects like bubbles, cracks, or densified areas throughout the volume, which is a result of sufficient cleaning of the MOF particles and high-purity chemicals in their synthesis. The SEM analysis once again confirms the absence of grain boundaries and explains the observed exceptional selectivity for CH4 and smaller gas species. Second, no gaps or defects are observed at the interfaces, demonstrating that this material combination is suitable for stabilizing the active layer while eliminating potential leaks.

Methods
Materials
Zinc nitrate hexahydrate (≥99%) was purchased from ABCR. Benzimidazole (≥99%) was supplied by Alfa Aesar, and Imidazole (≥99.5%) was purchased from Sigma-Aldrich. N,N-Dimethylformamide (≥99.9%) was supplied by VWR, dichloromethane (≥99%, stabilized with ethanol) – by Acros Organics. Epoxy resin DIPOXY-2K-4000 Geode-ART by Dipoxy-Germany. Au, Pt, Al foil had a purity of 99.99% as provided by the vendors. BF33 was provided by Schott AG (technical data sheet: https://www.schott.com/en-gb/products/borofloat-p1000314/downloads).

Synthesis of ZIF-62(Zn)
The synthetic procedure for ZIF-62(Zn) was fully reproduced from our previous work11. A total of 12.79 g of benzimidazole and 38.02 g of imidazole were sequentially dissolved in 480 ml of DMF, with stirring for 5 min. Following this, 19.93 g of zinc nitrate hexahydrate was added to the mixture, and stirring was continued until complete dissolution was achieved. The solution was then transferred to a 500 ml glass jar and maintained at 130 °C for 60 hours. The white sediment formed was separated from the mother liquor by centrifugation (10 minutes, 8.528 × g) and washed thoroughly twice with both DMF and DCM. Finally, the ZIF-62 powder was activated in a vacuum furnace (25 mbar, 150 °C) for 72 h.

Melting of aZIF-62
The melting of ZIF-62(Zn) was performed following the procedure outlined in our previous report with some changes. The crystals were carefully ground into a fine powder without applying to much mechanical pressure. The powder was then placed between two silica glass slides (microscope slides). Depending on the experiment, melting was performed directly on soda-lime silica or borosilicate glass, or on aluminium, gold, or platinum foil. The foils were placed between powder and silica glass substrates. The assembly was secured with two metal paper clamps and then heated to 450 °C in a nitrogen atmosphere. The clamps are from an office supply, thus they soften above 80 °C and should apply only a very minimal load to the sample, as they lose their spring force. Heating from room temperature to 450 °C was done using a heating rate of 10 K/min. After maintaining 450 °C for 5 min, the setup was allowed to cool to room temperature with a cooling rate of 2 K/min. In case of adding an annealing step, the system was cooled down from 450 °C to 300 °C first with 2 K/min cooling rate and kept there for 120 min before cooling down to room temperature with 2 K/min cooling rate. The resulting aZIF-62 was carefully removed from the substrate.

Optical microscopy
A digital microscope (VHX-6000, Keyence) equipped with a universal zoom lens (VH-Z100UR and VHX-S650) and a free-angle XYZ observation stage was employed to capture digital images of aZIF-62 membrane and its cross-section.

Powder X-ray diffraction (PXRD)
PXRD data for crystalline and amorphous samples were collected using a Rigaku MiniFlex diffractometer configured with Bragg-Brentano geometry. The instrument was equipped with a 600 W X-ray generator producing CuKα radiation at a wavelength of 1.54059 Å. Diffraction patterns were recorded over a 2θ range of 5–50°, with a step size of 0.02°.

Gas Permeation Measurements and Calculations
Gas permeation was measured using a Wicke-Kallenbach setup using sweep gas as a transport medium to the gas chromatography system (GC). Bronkhorst EL-Flow mass flow controllers were used to transport gases upstream and downstream. The gas composition was analyzed using an online Shimadzu Nexis GC-2030, equipped with a TCD detector and switchable carrier gas option (He, Ar, N2, and H2). The GC has a two-column setup, where permanent gases are fractioned on a 13x molecular sieve column, while CH4 and CO2 are fractioned using an SH-Q-Bond (100% divinylbenzene) plot column. Feed gas was fed to the upstream at 100 ml/min, while sweep gas was flown to the downstream at 1 ml/min. No heating was applied, and the feed and sweep sides were unpressurized so that the measurement was performed at room temperature (T=22 °C) and 1 atm (atmospheric pressure). Binary gas mixtures were fed with partial pressures of 0.5 atm each. Permeability of gas i (P) is the thickness and chemical potential normalized flux through the membrane, calculated from Eq. (1)

P_i = (n_i * d) / (A * Δp) (1)

where n is the amount of gas molecules, d is the thickness of the membrane, A is the effective area of the membrane, and Δp is the transmembrane pressure (chemical potential gradient, driving force). Dividing the molar ratios of the gases i, j in the permeate by the molar ratio of the components i, j in the retentate gives the separation factor α in Eq. (2). The feed gas is pumped to the membrane in a large excess and thus, the retentate quotient becomes 1. This simplifies the quotient to contain only the components in the permeate.

α(i=j) = (n_i,permeate / n_j,permeate) / (n_i,retentate / n_j,retentate) = n_i,permeate / n_j,permeate (2)

The permeability is given for better comparison to literature data in Barrer, a non SI-unit, which is equal to 1 Gas Permeability Unit (GPU). In Eq. 1, the transformation into SI units is given:

1 Barrer = 1 GPU = 3.346 * 10^-16 mol * m / (m^2 * s * Pa) (3)

Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy
First, the membrane stacks were carefully cut using a diamond saw to expose the cross section, manually ground using SIC abrasive paper with progressively increasing grit from P1200 to P4000 and polished using a water-based 1 µm diamond polishing suspension (Struers DiaPro). Further, a piece of the membrane was fixed to a carbon adhesive disc, and a small amount of conductive silver paint was added to ensure good conductivity. Finally, the samples were coated with Pt (8 nm) in a high-vacuum sputter coater (Safematic CCU-O10 HV) to avoid charging effects.

An Oxford EDX system in combination with a Sigma VP Field Emission Scanning Electron Microscope (Carl-Zeiss AG, Germany) was used to analyse the sample. The SEM micrographs were obtained with an acceleration voltage of 6 kV an emission current of 271 pA and using secondary electrons and in-lens detectors. EDX investigations were performed with a 50 mm2 XMax detector with a resolution of 127 eV. Mapping was performed within 20-30 minutes with an acceleration voltage of 20 kV.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a highly significant and meticulously executed study on the fabrication of large-area, free-standing\nMOF-glass membranes for gas separation. The work successfully addresses a major bottleneck in the field—the inherent\nbrittleness and cracking of MOF-glasses—through a clever combination of empirical substrate selection (matching CTE),\nand an optimized annealing protocol. The results are outstanding: the demonstrated perfect molecular sieving of CH₄, with\nundetectable permeability, is a remarkable achievement that underscores the defect-free, grain-boundary-free nature of the\nprepared agZIF-62 glass films. The methodology is well-described, the data (XRD, SEM-EDX, permeation tests) are\nconvincing and fully support the claims, and the discussion is thoughtful. This work represents a substantial step forward\ntowards the practical application of MOF-glasses in high-precision separation membranes. It is recommended for publication\nafter minor revisions addressing the points below.\n1. As shown in Fig. 2b, the crystalline structure completely disappears after the transformation of ZIF-62 crystalline seeds\ninto a ZIF-62 glassy membrane. What is the underlying reason for this phenomenon?\n2. How does the pore size distribution change after the conversion of ZIF-62 crystalline seeds into a ZIF-62 glassy\nmembrane? The pore size distribution of MOFs can be further determined.\n3. Membrane thickness is a critical parameter for gas separation applications. Can the thickness of the MOF-glass\nmembranes be controlled using your fabrication method? What is the relevant influence?", "reviewer_2": "In this manuscript, Knebel et.al., reported the design and engineering of a MOF glass membrane for gas separation. By\ncarefully optimizing each step of the process – from the initial substrate choice to the materials melting and final performance\nvalidation, a dense and free-standing ZIF-62 glass membrane was successfully fabricated. The membrane microstructure\nwas investigated in details. Benefiting from the absence of grain boundaries, an exceptional selectivity for CH4 and smaller\ngas specie was achieved. The concept of fabricating free-standing gas separation membranes from MOF glass presents a\npromising approach, with the potential to inspire new strategies for creating high-performance molecular sieving\nmembranes. I would like to recommend its publication in Nature Communication.\nThere are some minor comments to be considered as following:\n1. MOF glass based molecular sieving membranes have become a research hotspot in the past decade, what are the\nprincipal advantages of MOF glass membranes over polycrystalline MOF membranes? A detailed comparison is suggested\nto be provided.\n2. This manuscript describes the fabrication of a “large-area” MOF glass membrane with a diameter of 18 mm. It would be\nhelpful if the authors could clarify the benchmark for this description, as this scale is common for polycrystalline MOF\nmembranes. What is “large-area” relative to?\n3. In line 243, Page 9, the authors claimed that “our experimental value of the real selectivity αreal(N2/CO2) = 22…..”. We\nobserved that the CO2 permeability of the resulting glass membrane was higher than that of N2 (Fig. 4b), thus the\nαreal(N2/CO2) should be αreal(CO2/N2). Please carefully check it.\n4. The mechanical stability of membrane materials is important for real-world application, what about the stability of ZIF-62\nglass membrane?\n5. What is the pore channels size of the resulting ZIF-62 glass membrane?", "reviewer_3": "This paper presents a significant advancement in the field of MOF-based membranes for gas separation by addressing key\nlimitations in the production and application of MOF glasses, specifically the issues of cracking during processing and the\ndifficulty in creating large, free-standing membranes. I recommend acceptance of this article for publication, as the research\naddresses a significant challenge in the field and the results demonstrate a promising solution for producing high-\nperformance MOF-glass membranes for gas separation. The clarity of presentation and thoroughness of the characterization\nfurther support the value of this work. However, I urge the authors to temper their enthusiasm and acknowledge the\nlimitations of their findings. The study convincingly demonstrates the feasibility of their approach for ZIF-62 glass\nmembranes, but also not more as other ZIFs, MOFs, or related materials are not included. The authors should frame the\nstudy as a proof-of-concept and carefully revise the manuscript to remove any overly generalized statements regarding the\nbroader applicability of their study.", "reviewer_4": "The authors present an innovative study on free-standing glassy MOF membranes for molecular sieving based on ZIF-62.\nThey were able to produce a crack-free and bubble-free membrane with accessible pore structure of approximately 330\nmicrons in thickness. The authors circumvented many of the problems associated with melt-quenched approaches by\ndirectly annealing the solidified melt. In addition, the authors placed their glassy MOF membrane within an appropriate ring\nstructure, creating a robust, free-standing membrane with a large surface area that is suitable for permeation tests in a\nWicke-Kallenbach setup with online gas chromatography coupling. Their approach greatly simplifies processing and has\nhuge potential for technology transfer. Gas permeation was measured for single gases (He, H2, CO2, N2 and CH4) and\nbinary gas mixtures (H2/CO2, CO2/N2, He/CO2, H2/CH4) with their kinetic diameters in the range of 2.5 to 4.0 Angstrom.\nAlthough, the overall gas permeability is low, due to the relatively thick membrane, the selectivities are comparable to\nliterature, and in the case of H2/CH4 separation they exceed literature values clearly in an impressive a manner. The\nH2/CH4 selectivity is infinity. If methane (CH₄) is permeating at all, it is below the detection limit of the authors’ state-of-the-\nart setupAdditionally, the authors present ideas for the design of membrane modules, including flexible forms. In summary,\nthe authors set a new benchmark for the H2/CH4 selectivity with glassy MOF membranes, bringing technological\napplications of this membrane class closer. The results hold considerable potential significance, promising to advance\nunderstanding in the field and contribute meaningfully to its progression. The conclusions are well supported. The\nmanuscript would, however, benefit from some minor revisions, including the following:\n1. The manuscript would benefit greatly from a more explicit comparison of the authors' results with those of the current state\nof the art, which has developed since Wang et al.'s early paper on the subject, A MOF Glass Membrane for Gas Separation,\nAngew. Chem. Int. Ed. 59 (2020), 4365-4369, https://doi.org/10.1002/anie.201915807. These authors used melt-quenched\nZIF-62 and achieved a similar CO2/N2 selectivity at one order of magnitude higher permeability at an overall thickness of\nthe membrane of 20 to 30 microns compared to a thickness of 330 microns in the authors’ manuscript.\n2. “Large area” should be quantified and compared to the current state of the art.\n3 .Permeation and selectivities should be quantified and compared to the current state of the art.\n3. Is there a rational behind the selection and order of permeability test displayed in Figure 4c,d?\n4. Why the possible binary gas mixtures where restricted to four (i.e. H2/CO2, CO2/N2, He/CO2, H2/CH4)? Including\nHe/CH4 and CO2/CH4 separation could enhance the impact of the manuscript.\n5. Line 310ff: In the context of the mentioned “perovskite-based membranes”, it would be helpful to direct the reader by citing\ntwo relevant reviews.\n6. Supporting material, section 1.7: Avoid duplicating content in the second paragraph and use a consistent acronym for X-\nray spectroscopy (e.g. EDS or EDX).", "reviewer_5": "This work reports a type of ZIF-62 glass membranes, fabricated through thermal expansion coefficient matching between the\nMOF glass and the processing devices. The authors observed impressive selectivity particularly for bulkier gas molecules,\nand also discussed the possible pathway forward towards the practical applications. Overall, this contribution address some of the\ncritical challenges associated with the MOF glass, particularly ZIF-62, and the fabricated membrane could offer some\nsolutions to challenging separations. The paper can be considered for publication in Nature Communications, after some\nfurther improvement.\n1. The authors claims the preservation of mechanically robust, cracking free membrane is originated from the CTE matching\nbetween MOF glass and Al foil. This could be the reason but more investigation will be required to fully support this claim.\nFor example, does the ZIF-62 attach strongly with the foil after sintering? Or the mechanism could be just a low energy\ninterface that does not provide additional stress to MOFs? More types of foils/substrates with different CTE would be\nwelcomed.\n2. It is impressive to see the high selectivity of this membrane, which address one of the critical challenges in separation.\nThere are some recent argument on the use of W-K setup for membrane permeation test - for example, the sweep gas of He\ncan back diffuse to the feed side, and this will lead to a higher measured membrane permeability than the actual value.\nCould the author provide further discussion on this?\n3. It would be interesting to see if the membrane performance variance with different thickness, and/or different\nbenzimidazolate ratio.\n4. There are multiple typos in the manuscript, better to revise them."}, "author_response": "We would like to thank all five referees, for the time they dedicated to reviewing our manuscript and for\ntheir general evaluation of our work. The referees’ comments – to which our point-by-point responses are pro-\nvided below – helped us strengthen the manuscript. The referees’ comments are formatted in italic, our responses\nin green, changes in the manuscript in yellow, and parts removed from the main text – in strikethrough red.\nThe key revisions include:\n- Figure 1c was transformed from a scheme into a graph.\n- We have tested two additional inert substrates for ZIF-62 melting – borosilicate float glass BF33 and pure\ngold. As expected from the CTE mismatch, both substrates resulted in significant glass cracking, confirm-\ning that the initially suggested aluminum remains the optimal choice. The results and corresponding dis-\ncussion have been incorporated into the manuscript.\n- We have deepened the discussion on substrate-a ZIF-62 interactions and supported the attribution of the\ng\nobserved cracking behavior primarily to residual thermal stresses through quantitative estimations.\n- More context and detail have been added to the introduction, including a thorough discussion of the po-\ntential benefits of glassy MOF membranes over their crystalline counterparts. The remainder of the intro-\nduction has been slightly restructured and shortened to improve flow and clarity and to avoid repetition.\n- Permeation data for He/CH was added and small but significant typos (for example N /CO instead of\n4 2 2\nCO /N ) were corrected.\n2 2\n- Data for He/CH was added to the new Figure 4c and 4d with the expected results, no CH was passing\n4 4\nthrough the membrane.\n- The manuscript now includes a detailed qualitative summary of the reported MOF-glass membranes from\nliterature and their tested results for various gas separations, which have been collected in a table, plotted\nand discussed accordingly.\n- The material and methods moved from the SI to the main manuscript to fit the style of Nat. Commun.\n- Other minor revisions are covered in our point-by-point responses.\nWe believe the revised manuscript has improved a lot and we thank everyone involved again for their con-\nstructive feedback.\nOn behalf of all authors,\nDr. Alexander Knebel\nReviewer #1:\nThis manuscript presents a highly significant and meticulously executed study on the fabrication of large-area,\nfree-standing MOF-glass membranes for gas separation. The work successfully addresses a major bottleneck in\nthe field—the inherent brittleness and cracking of MOF-glasses—through a clever combination of empirical sub-\nstrate selection (matching CTE), and an optimized annealing protocol. The results are outstanding: the demon-\nstrated perfect molecular sieving of CH₄, with undetectable permeability, is a remarkable achievement that un-\nderscores the defect-free, grain-boundary-free nature of the prepared agZIF-62 glass films. The methodology is\nwell-described, the data (XRD, SEM-EDX, permeation tests) are convincing and fully support the claims, and the\ndiscussion is thoughtful. This work represents a substantial step forward towards the practical application of\nMOF-glasses in high-precision separation membranes. It is recommended for publication after minor revisions\naddressing the points below.\nWe are sincerely grateful to the referee for their positive and highly encouraging evaluation of our manuscript.\nBelow, we provide a point-by-point response to all comments.\n1. As shown in Fig. 2b, the crystalline structure completely disappears after the transformation of ZIF-62 crys-\ntalline seeds into a ZIF-62 glassy membrane. What is the underlying reason for this phenomenon?\nX-ray diffraction (XRD) relies on constructive interference from regularly spaced lattice planes. However, when\na crystalline solid – or a mixture of solids thereof – is melted into a glass, the long‐range periodic atomic order is\nlost. Indeed, the quenching of a glass results in the kinetically driven freezing of a long-range disordered melt\nstructure, thereby preventing the atoms from re-ordering into thermodynamically more stable crystalline struc-\ntures. Thus, the absence of periodicity eliminates sharp Bragg reflections, leaving only broad halos in place of\ndistinct XRD peaks. This behavior is typical for all glasses and particularly for melt-quenched ZIF-62, in agree-\nment with all relevant previous reports (e.g., 10.1126/sciadv.aao6827, 10.1038/s41563-023-01738-3,\n10.1063/5.0031941, 10.1016/j.jnoncrysol.2019.119806, 10.1038/s41467-024-49428-1).\n2. How does the pore size distribution change after the conversion of ZIF-62 crystalline seeds into a ZIF-62\nglassy membrane? The pore size distribution of MOFs can be further determined.\nWe thank the referee for this comment. Since ZIF-62 is a well-studied glass-former among MOFs, its pore size\ndistribution and its evolution have been investigated in detail in previous works, e.g. physisorption experiments\nby Frentzel-Beyme et al. in 10.1038/s41467-022-35372-5 or PALS data reported by Wang et. al.\n/10.1002/anie.201915807. Nevertheless, we would like to respectfully note that while the pore size is an im-\nportant characteristic of a porous material, parameters like channel size or pore window size provide more com-\nprehensive information for the diffusive gas separation, as performed in our membrane study, since, for instance,\na large pore’s guest accommodating ability might be severely limited by a narrow pore window. It is equally\napplicable to the disordered glasses, where preservation of the accessible channels plays critical role – even more\nimportant than preserving the large pores – since they determine the molecular pathways and therefore the per-\nmeation ability.\nThus, the pore channel size distribution in ZIF-62 glasses prepared by following the same initial synthetic and\nmelting protocols have been investigated in detail via HR-TEM in our previous study in Nat. Mater.\n(10.1038/s41563-023-01738-3) and demonstrated that our melting methodology allows for preserving the chan-\nnels accessible for the guest molecules upon glass formation. Overall, due to the non-ordered glassy nature, pore\nchannel sizes varying from approximately 1,7 Å to 3,4 Å with an average pore-limiting diameter of 2,7±0,5 Å for\na melt-pressed sample (as we used here) have been observed in the normal distribution for the analogous to the\ncurrent study samples. This distribution explains the gradual loss of permeability in the row He (d = 2.6 Å), H\nkin 2\n(d = 2.89 Å), CO (d = 3.3 Å), N (d = 3.64 Å), and CH (d = 3.8 Å). We have now included this data\nkin 2 kin 2 kin 4 kin\nand discussed it accordingly:\n[…] This result is supported by our previous preliminary work, where an absolute exclusion of ethane (d (C H )\nkin 2 6\n= 4.4 Å) diffusion was achieved after liquid-state processing of a ZIF-62, suggesting that this is likely possible,\ng\nalso for CH and other relatively large gas molecules. In particular, the pore channel size distribution in such, but\n4\nmillimeter-scale glasses prepared by following the same initial synthetic and melting protocols have been inves-\ntigated in detail via HR-TEM: pore channel sizes varying from approximately 1.7 Å to 4 Å with an average pore-\nlimiting diameter of 2.7 Å have been observed in the normal distribution for the analogous to the current work\nsamples.[11] The above-demonstrated permeation results are indeed in agreement with this data: a disordered\nglassy nature results in a relatively broad pore channel size distribution but the ≥3.8 Å (potentially allowing for\nCH (d = 3.8 Å) permeation) fraction is almost negligible compared to the smaller sizes. At the same time, the\n4 kin\n3.64 Å - 3.8 Å fraction, enabling selective N permeation, is more present and therefore notable N signal is\n2 2\nobserved. […]\n3. Membrane thickness is a critical parameter for gas separation applications. Can the thickness of the MOF-\nglass membranes be controlled using your fabrication method? What is the relevant influence?\nWe thank the referee for this comment. Our method indeed allows for the thickness control of the resulting mem-\nbranes via two ways – direct and indirect. The direct way involves initial control of the amount of the crystalline\npowder used for the glass formation per substrate area – the more powder is used, the thicker membrane it will\nresult in. The indirect way, on the other hand, involves post-processing techniques such as polishing to reduce the\nthickness – as has been shown before in (10.1038/s41467-024-49428-1) ZIF-62 glass is prone to the polishing\ntechniques standard for glass handling. Those methods are straightforward and do not rely on the complicated\nmembrane thickness control techniques demonstrated for the supported MOF glass membranes, e.g., precise but\ntime- and energy-demanding cathodic deposition-assisted method in 10.1002/anie.202401817. We have updated\nthe relevant discussion as follows:\nAs we demonstrated before, the material easily withstands polishing and other types of conventional glass treat-\nment, such as liquid-state shaping, so thinner layers can be indeed fabricated by such processing techniques.\nMoreover, the initial membrane thickness can be modified within a certain range by controlling the amount of the\ncrystalline powder per substrate area before melting.11-14\nThen, as demonstrated in other studies on MOF glass gas separation membranes (e.g., 10.1038/s41467-025-\n56295-x), reducing the thickness leads to some increase in permeance while sacrificing selectivity. This trade off\nis intuitive and is anticipated for our membrane as well; yet, the need to maintain the mechanical integrity of the\nfree-standing glass membrane would not allow for the significant decrease in the membrane’s thickness (e.g., like\nup to 10 times in the abovementioned 10.1002/anie.202401817). While significant changes would indeed be ex-\npected when decreasing the membrane thickness down to at least the tens-of-micrometers range, such thicknesses\ncannot yet be achieved for neat free-standing MOF glass membranes by either our or other existing methods and\nare therefore outside the scope of this study.\nReviewer #2:\nIn this manuscript, Knebel et.al., reported the design and engineering of a MOF glass membrane for gas separa-\ntion. By carefully optimizing each step of the process – from the initial substrate choice to the materials melting\nand final performance validation, a dense and free-standing ZIF-62 glass membrane was successfully fabricated.\nThe membrane microstructure was investigated in details. Benefiting from the absence of grain boundaries, an\nexceptional selectivity for CH4 and smaller gas specie was achieved. The concept of fabricating free-standing\ngas separation membranes from MOF glass presents a promising approach, with the potential to inspire new\nstrategies for creating high-performance molecular sieving membranes. I would like to recommend its publication\nin Nature Communication.\nThere are some minor comments to be considered as following:\nThe authors thank the referee for the positive evaluation of our work. Below, we provide a point-by-point response\nto all comments.\n1. MOF glass based molecular sieving membranes have become a research hotspot in the past decade, what are\nthe principal advantages of MOF glass membranes over polycrystalline MOF membranes? A detailed compari-\nson is suggested to be provided.\nWe thank the referee for their suggestion. The principal advantages of MOF glass membranes over polycrystalline\nMOF membranes are now summarized in the introduction as follows:\nRecently, melt-quenched amorphous framework glasses have become a special area of interest for the\nseparation processes with several potential benefits over their crystalline counterparts. First, framework glasses\ncan be shaped and processed from powders in their liquid state to form a free-standing layer, and therefore do not\nrequire complicated thin film growing techniques on expensive ceramic substrates, commonly used for MOFs\nand COFs. Second, the universal processability of glasses allows for liquid-state shaping and further polishing,\ncutting, etc., to achieve the desired size and shape. Further, most importantly for membranes, the ability of glass\nto form a monolithic layer hinders grain boundary diffusion and therefore higher selectivity can be achieved.2,5\nFinally, unique for MOF-derived glasses, their porosity can be tuned by varying the processing parameters to\nobtain the targeted size of the molecular-sieving windows.\nThe rest of the introduction has been slightly restructured and shortened accordingly for the flow and clarity and\nalso to avoid repetitions.\n2. This manuscript describes the fabrication of a “large-area” MOF glass membrane with a diameter of 18 mm.\nIt would be helpful if the authors could clarify the benchmark for this description, as this scale is common for\npolycrystalline MOF membranes. What is “large-area” relative to?\nWe thank the referee for this comment and agree that this description needs clarification. Indeed, more mature\npolycrystalline MOF membranes can be commonly fabricated within this and sometimes larger size range. In this\nwork, the “large-area” remark refers, first of all, to the free-standing neat MOF glass layers which have been\ncomplicated to fabricate even at the demonstrated here centimeter-scale. Second, this remark further refers to the\nscaling possibilities that we envision for this type of membranes particularly – e.g., modular designs in Figure 6.\nWe have now clarified it throughout the text, for instance in the following sections:\nDespite the great promise, obtaining neat and free-standing MOF glass membranes of a larger area remains\nchallenging. […] As a result, achieving at least typical membrane area of the polycrystalline MOF membranes –\nwhich is by itself commonly limited to the centimeter scale – while preserving porosity and therefore performance\ncharacteristics in MOF glasses remains highly problematic. Moreover, in the absence of such established proce-\ndures further scaling considerations – potentially unlocking industrially-relevant meter-scale membrane areas –\nremain unexplored.\n[…]\nSpecifically, first we were able to produce large area centimetre-scale, free-standing MOF glass membrane\n[…] As an outlook we will provide strategies for the further development of MOF-glass membranes towards real-\nworld application, including a perspective approach for their upscaling to the industrially relevant membrane\nareas.\n3. In line 243, Page 9, the authors claimed that “our experimental value of the real selectivity αreal(N2/CO2) =\n22…..”. We observed that the CO2 permeability of the resulting glass membrane was higher than that of N2 (Fig.\n4b), thus the αreal(N2/CO2) should be αreal(CO2/N2). Please carefully check it.\nWe thank the reviewer for pointing this out. We did indeed refer to a CO /N separation throughout the manu-\n2 2\nscript. Writing N /CO was a typo, but also a significant error. We corrected it everywhere.\n2 2\n4. The mechanical stability of membrane materials is important for real-world application, what about the sta-\nbility of ZIF-62 glass membrane?\nThe mechanical stability of the membrane material is indeed of crucial importance for concrete applications. The\nmechanical properties of hybrid glasses compare less favorably with respect to conventional inorganic glass fam-\nilies, for which elastic moduli, hardness, and fracture toughness are about an order of magnitude larger (compare\n10.1038/s41467-020-16382-7 and 10.1111/j.1551-2916.2007.01945.x ). Nevertheless, they prove sufficient for\nthe needs of gas separation applications; this does not prevent their processing (polishing, handling; see our work\n10.1038/s41467-024-49428-1) nor their shaping and loading resistance to achieve larger-scale membranes, as\npresented in this study and highlighted throughout the text. Moreover, this task is notably made easier thanks to\nthe sufficiently large thicknesses obtained (330 µm), which associate high gas separation selectivity with pre-\nserved material integrity during use.\nHowever, further scaling of such thin layers as is would inevitably and undoubtedly lead to breakage due to the\ntheir non-flexible nature. For that reason, we would like to again stress out the point that further scaling consid-\nerations must always acknowledge this fact and suggest solutions relevant to this particular system – as proposed,\nfor instance, with the modular design in Figure 6, which could simultaneously (1) ensure mechanical robustness\nof the overall membrane, (2) make it easily transportable if made with flexible (e.g. resin) connections, and (3)\nallow to change the modules independently if needed and therefore prolonging the life cycle of the whole system.\n5. What is the pore channels size of the resulting ZIF-62 glass membrane?\nWe thank the referee for this comment – indeed, this important information was missing in the report. We thank\nthe referee for this comment. The pore channel size distribution in ZIF-62 glasses prepared by following the same\ninitial synthetic and melting protocols have been investigated in detail via HR-TEM in our previous study in Nat.\nMater. (10.1038/s41563-023-01738-3) and demonstrated that our melting methodology allows for preserving the\nchannels accessible for the guest molecules upon glass formation. Overall, due to the non-ordered glassy nature,\npore channel sizes varying from approximately 1,7 Å to 3,4 Å with an average pore-limiting diameter of 2,7±0,5\nÅ for a melt-pressed sample (as we used here) have been observed in the normal distribution for the analogous to\nthe current study samples. This distribution explains the gradual loss of permeability in the row He (d = 2.6 Å),\nkin\nH (d = 2.89 Å), CO (d = 3.3 Å), N (d = 3.64 Å), and CH (d = 3.8 Å). We have now included this data\n2 kin 2 kin 2 kin 4 kin\nand discussed it accordingly:\n[…] This result is supported by our previous preliminary work, where an absolute exclusion of ethane (d (C H )\nkin 2 6\n= 4.4 Å) diffusion was achieved after liquid-state processing of a ZIF-62, suggesting that this is likely possible,\ng\nalso for CH and other relatively large gas molecules. In particular, the pore channel size distribution in such, but\n4\nmillimeter-scale glasses prepared by following the same initial synthetic and melting protocols have been inves-\ntigated in detail via HR-TEM: pore channel sizes varying from approximately 1.7 Å to 4 Å with an average pore-\nlimiting diameter of 2.7 Å have been observed in the normal distribution for the analogous to the current work\nsamples.[11] The above-demonstrated permeation results are indeed in agreement with this data: a disordered\nglassy nature results in a relatively broad pore channel size distribution but the ≥3.8 Å (potentially allowing for\nCH (d = 3.8 Å) permeation) fraction is almost negligible compared to the smaller sizes. At the same time, the\n4 kin\n3.64 Å - 3.8 Å fraction, enabling selective N permeation, is more present and therefore notable N signal is\n2 2\nobserved. […]\nReviewer #3:\nThis paper presents a significant advancement in the field of MOF-based membranes for gas separation by ad-\ndressing key limitations in the production and application of MOF glasses, specifically the issues of cracking\nduring processing and the difficulty in creating large, free-standing membranes. I recommend acceptance of this\narticle for publication, as the research addresses a significant challenge in the field and the results demonstrate\na promising solution for producing high-performance MOF-glass membranes for gas separation. The clarity of\npresentation and thoroughness of the characterization further support the value of this work.\nWe would like to thank the referee for the positive evaluation of our work.\nHowever, I urge the authors to temper their enthusiasm and acknowledge the limitations of their findings. The\nstudy convincingly demonstrates the feasibility of their approach for ZIF-62 glass membranes, but also not more\nas other ZIFs, MOFs, or related materials are not included. The authors should frame the study as a proof-of-\nconcept and carefully revise the manuscript to remove any overly generalized statements regarding the broader\napplicability of their study.\nWe thank the referee for this comment and agree that we might have included some overstatements throughout\nthe text. We have carefully revised the text accordingly and pasted some examples of our corrections below. Yet\nwe would like to note that although this study is indeed limited to ZIF-62 as the most straightforward example –\nwhich is now stated more clearly – we remain hopeful that this work will inspire the researchers from the field to\nemploy this or other systematic fabrication approaches, carefully considering both reticular and glassy sides of\nsuch systems, when also looking into other glass-forming MOF systems. On the other hand, we fully understand\nthat MOF-glass membranes (including ZIF-62) remain at the premature stage due to the overall field’s infancy,\nfrequently conflicting published data, and complications related to the system itself. To this end, we transparently\noutline the remaining challenges and uncertainties within the outlook section and – to the best of our experience\n– suggest the ways of how those can be approached.\nRelevant revisions are exemplified in – but not limited to – the following parts:\nAbstract:\n[…] Here, we present suggest a solution to overcome these limitations and demonstrate that - by selecting suitable\nmaterials particularly fitting thermal and mechanical behavior to those of MOF-glass – it is possible to process\nlarge, crack-free MOF-glass sheets, as demonstrated on the example of the well-known MOF-glass former ZIF-\n62. […] We conclude this work by giving a brief outlook of the remaining challenges and perspectives for MOF\nglasses in a view of gas separation membranes, envisioning transferability of our approach to other glass-forming\nsystems and their scaling perspectives.\nIntroduction:\n[…] In this work, we demonstrate a method to achieve larger free-standing MOF glasses from crystalline MOFs\n(c.f. Figure 1) on the example of meltable zeolitic imidazolate framework ZIF-62. […]\n[…] As an outlook we suggest that this or similar – but always systematic, taking both reticular and glassy nature\nof such systems into account – approaches can be transferred to other glass formers within MOF family. […]\nConclusions and outlook:\n[…] As demonstrated above, MOF glass membranes possess tremendous potential for high precision separation\ntechnologies as they exhibit very sharp molecular-sieving cut-offs. However, handling these materials is still a\nbig challenge, and the herein presented approach offers a solution to this challenge. The high selectivity is achiev-\nable because we the system combines the well-defined porosity in MOFs with the processability of glasses, al-\nlowing for the fabrication of monolithic defect-free layers without grain boundaries. […] We envision that sys-\ntematic fabrication and processing considerations within this work can be applied to other MOF-glass-formers\naside from ZIF-62 – in such cases, intrinsic properties and behaviors of these specific systems must be always\ntaken into account. However, such complex materials at the early stages inevitably raise some challenges. […]\nReviewer #4:\nThe authors present an innovative study on free-standing glassy MOF membranes for molecular sieving based\non ZIF-62. They were able to produce a crack-free and bubble-free membrane with accessible pore structure of\napproximately 330 microns in thickness. The authors circumvented many of the problems associated with melt-\nquenched approaches by directly annealing the solidified melt. In addition, the authors placed their glassy MOF\nmembrane within an appropriate ring structure, creating a robust, free-standing membrane with a large surface\narea that is suitable for permeation tests in a Wicke-Kallenbach setup with online gas chromatography coupling.\nTheir approach greatly simplifies processing and has huge potential for technology transfer. Gas permeation\nwas measured for single gases (He, H2, CO2, N2 and CH4) and binary gas mixtures (H2/CO2, CO2/N2, He/CO2,\nH2/CH4) with their kinetic diameters in the range of 2.5 to 4.0 Angstrom. Although, the overall gas permeability\nis low, due to the relatively thick membrane, the selectivities are comparable to literature, and in the case of\nH2/CH4 separation they exceed literature values clearly in an impressive a manner. The H2/CH4 selectivity is\ninfinity. If methane (CH₄) is permeating at all, it is below the detection limit of the authors’ state-of-the-art\nsetupAdditionally, the authors present ideas for the design of membrane modules, including flexible forms. In\nsummary, the authors set a new benchmark for the H2/CH4 selectivity with glassy MOF membranes, bringing\ntechnological applications of this membrane class closer. The results hold considerable potential significance,\npromising to advance understanding in the field and contribute meaningfully to its progression. The conclusions\nare well supported. The manuscript would, however, benefit from some minor revisions, including the following:\nWe thank the referee for their positive and encouraging evaluation of our manuscript. Below, we provide a point-\nby-point response to all comments.\n1. The manuscript would benefit greatly from a more explicit comparison of the authors' results with those of the\ncurrent state of the art, which has developed since Wang et al.'s early paper on the subject, A MOF Glass Mem-\nbrane for Gas Separation, Angew. Chem. Int. Ed. 59 (2020), 4365-\n4369, https://doi.org/10.1002/anie.201915807. These authors used melt-quenched ZIF-62 and achieved a similar\nCO2/N2 selectivity at one order of magnitude higher permeability at an overall thickness of the membrane of 20\nto 30 microns compared to a thickness of 330 microns in the authors’ manuscript.\nWe thank the referee for this comment. Our manuscript now includes a detailed summary of the state-of-the-art\nMOF-glass gas separation membranes (Table S3 and Figure S5). Although quantitatively the “infinite” methane\nselectivity demonstrated in our work is outstanding within the field, we believe that, at the current stage of devel-\nopment, qualitative overview and not the quantitative benchmarking is more informative for the MOF-glass-\nmembranes field. Our reasoning behind such considerations is now included in the main text as follows:\n[…] Straightforward comparison of this data with other MOF-glass membranes is complicated by the fact that –\nas demonstrated in the introduction – currently reported membranes vary significantly in their type (pristine or\ncomposited with crystalline MOFs or polymers), support nature (free-standing or supported), and the initial glass-\nformer composition. All of these variations are employed to design the membrane exhibiting the molecular-siev-\ning characteristics that specifically target one or another separation – while our membrane in particular works\nbest for the challenging methane separations. In Figure S 5, we present a qualitative overview of the various\nreported MOF-glass membranes tested for different gas separations. […]\n2. “Large area” should be quantified and compared to the current state of the art.\nWe thank the referee for this comment and agree that this description needs clarification. Indeed, more mature\npolycrystalline MOF membranes can be commonly fabricated within this and sometimes larger size range. In this\nwork, the “large-area” remark refers, first of all, to the free-standing neat MOF glass layers which have been\ncomplicated to fabricate even at the demonstrated here centimeter-scale. Second, this remark further refers to the\nscaling possibilities that we envision for this type of membranes particularly – e.g., modular designs in Figure 6.\nWe have now clarified it throughout the text, for instance in the following sections:\nDespite the great promise, obtaining neat and free-standing MOF glass membranes of a larger area remains\nchallenging. […] As a result, achieving at least typical membrane area of the polycrystalline MOF membranes –\nwhich is by itself commonly limited to the centimeter scale – while preserving porosity and therefore performance\ncharacteristics in MOF glasses remains highly problematic. Moreover, in the absence of such established proce-\ndures further scaling considerations – potentially unlocking industrially-relevant meter-scale membrane areas –\nremain unexplored.\n[…]\nSpecifically, first we were able to produce large area centimeter-scale, free-standing MOF glass membrane\n[…] As an outlook we will provide strategies for the further development of MOF-glass membranes towards real-\nworld application, including a perspective approach for their upscaling to the industrially relevant membrane\nareas.\n3 .Permeation and selectivities should be quantified and compared to the current state of the art.\nThis data is now provided in Table S3 and Figure S5 and discussed accordingly in the main text, as described in\ndetail in our response to the Comment 1 of this referee.\n3. Is there a rational behind the selection and order of permeability test displayed in Figure 4c,d?\nWell, the permeation tests were done mainly for applications in carbon capture, H purification and He upgrading.\n4. Why the possible binary gas mixtures where restricted to four (i.e. H2/CO2, CO2/N2, He/CO2, H2/CH4)?\nIncluding He/CH4 and CO2/CH4 separation could enhance the impact of the manuscript.\nWe thank the referee and add our permeation tests for He/CH additionally. That was a measurement we tested\n4\nlong and rigorously. We stopped measuring gases against CH , as there was almost no difference to the single gas\n4\ndata. CH is simply not permeating through the MOF-glass membranes undetectable, and the measurement is not\n4\ngiving us any more information as this that we get from single gas permeation, which we thought is not rendering\nthe mixed gas quantification very significant.\n5. Line 310ff: In the context of the mentioned “perovskite-based membranes”, it would be helpful to direct the\nreader by citing two relevant reviews.\nWe thank the referee for noting this – indeed, the relevant reference has been accidentally removed when revising\nthe manuscript prior to submission. We added one reference that we thought is significant for the advancement\nof the field of perovskite membranes from recent times.\n6. Supporting material, section 1.7: Avoid duplicating content in the second paragraph and use a consistent\nacronym for X-ray spectroscopy (e.g. EDS or EDX).\nWe thank the referee for this comment – the acronyms are now consistent within the document.\nReviewer #5:\nThis work reports a type of ZIF-62 glass membranes, fabricated through thermal expansion coefficient matching\nbetween the MOF glass and the processing devices. The authors observed impressive selectivity particularly for\nbulkier gas molecules, and also discussed the possible pathway forward towards the practical applications. Over-\nall, this contribution address some of the critical challenges associated with the MOF glass, particularly ZIF-62,\nand the fabricated membrane could offer some solutions to challenging separations. The paper can be considered\nfor publication in Nature Communications, after some further improvement.\nThe authors thank the referee for their time and positive evaluation of our work. Below, we provide a point-by-\npoint response to all comments.\n1. The authors claims the preservation of mechanically robust, cracking free membrane is originated from the\nCTE matching between MOF glass and Al foil. This could be the reason but more investigation will be required\nto fully support this claim. For example, does the ZIF-62 attach strongly with the foil after sintering? Or the\nmechanism could be just a low energy interface that does not provide additional stress to MOFs? More types of\nfoils/substrates with different CTE would be welcomed.\nWe thank the referee for this valuable comment. First, we have now performed experiments with two additional\nsubstrates – gold and borosilicate glass (BOROFLOAT®, Schott AG). Both brittle borosilicate glass with very\nlow CTE (3.25×10-6 K-1) and ductile gold with relatively low CTE (14.2×10-6 K-1) yielded similar to soda-lime\nglass and platinum results – ZIF-62 glasses cracked significantly because of the CTE mismatch. We would like\nto highlight again that although brittle material with high CTE could also potentially yield a crack-free a ZIF-62\ng\nand such experiment would indeed be interesting, there is no readily available materials with such characteristics\nthat are at the same time inert and temperature-stable enough to withstand the experiment.\nFigure S1 and S2 has been now added:\nFigure S 1 Results of ZIF-62 melting while pressed between borosilicate glass BOROFLOAT®.\nFigure S 2 Results of ZIF-62 melting while pressed between pure gold foil (0.2 mm thickness).\nAs well as the corresponding text:\nTesting other inert substrates such as borosilicate glass BOROFLOAT® (brittle, very low CTE of 3.25×10-6 K-1)\nand gold (ductile, relatively low CTE of 14.2×10-6 K-1) also resulted in significant cracking (Figure S 1 and S2).\nThis discussion is now included in the Supplementary Note 1 and referred to in the main text.\nSupplementary Note 1\nEstimation of substrate-dependent residual thermal stresses in ZIF-62 glass\nA rough estimate of these resulting thermal stresses can be derived on the basis of the thin film’s elastic properties\n(elastic modulus E, and Poisson’s ratio ν), and the mismatch in thermal expansion coefficients between the sub-\nstrate and the film (α and α, respectively), and by the temperature difference ΔT (e.g., from the synthesis tem-\ns f\nperature down to room temperature), following:\nE (1)\nσ = α -α ΔT\nth s f\n1-ν\n( )\nSuch an approach can be applied to the present case of free-standing MOF glass processing to achieve semi-\nquantitative insights into their post-synthesis cracking behavior. First of all, it is worthy of mention that the elastic\nproperties of ZIF-62 glasses can vary by over 25% (e.g., 6.58 ± 0.02 GPa and 5.2 ± 0.9 GPa)5,6 from one report\nto another (stemming from differences in sample purity or resulting porosity after melting)7. Nevertheless, such\npreliminary values are sufficient to estimate the order of magnitude of the residual thermal stresses; hence, elastic\nproperties of E = 5.2 GPa and ν = 0.343 will be employed from ref. 1.6 Second, the temperature difference ΔT is\nthe difference between the glass transition temperature T of 322 °C (at which the glass ceases to flow over the\ng\nlaboratory time scale) and room temperature. As a means of evaluation, the strength of ZIF-62 glasses – measured\nby three-point bending tests using parallelepipedic specimens, and reported to be 8 MPa by To et al.6 – is em-\nployed. Note that, to the authors’ knowledge, this is the only experimentally obtained strength reported for ZIF-\n62 glasses and that, like their processing-dependent optical and mechanical properties, this value can be greatly\naffected by the employed synthesis and characterization procedures (and, notably, by the surface state following\npolishing). From this, approximations of the residual thermal stresses can be estimated for each of the initially\nconsidered substrates, namely soda-lime silica glass, aluminum, and platinum and newly added borosilicate glass\nand gold:\nTable S 1 Substrate-dependent residual thermal stresses.\nSubstrate Coefficient of thermal α -α , 10-6 °C-1 σ\ns f th\nexpansion α, 10-6 °C-1\nSoda-lime silica glass 9.2 -22.9 54.7 MPa\nAluminum 24.0 -8.1 19.4 MPa\nPlatinum 9.0 -23.1 55.2 MPa\nBOROFLOAT® 3.25 -28.8 68.8 MPa\nGold 14.2 -17.9 42.8 MPa\nThe obtained values of residual thermal stresses thus range in the order of a few tens of MPa, with that obtained\nusing aluminum foil being substantially smaller than those obtained using other substrates. These values therefore\ncompare quantitatively to the previously reported ZIF-62 glass strength, and thus support ascribing the observed\ncracking behavior primarily to residual thermal stresses; while the σ when using aluminum foil is close to the\nth\nlatter, the σ when using other substrates greatly exceeds it, resulting in the cracking of the specimens.\nth\n2. It is impressive to see the high selectivity of this membrane, which address one of the critical challenges in\nseparation. There are some recent argument on the use of W-K setup for membrane permeation test - for example,\nthe sweep gas of He can back diffuse to the feed side, and this will lead to a higher measured membrane perme-\nability than the actual value. Could the author provide further discussion on this?\nThe referee is completely right about that. The Wicke-Kallenbach tech was originally developed for larger hy-\ndrocarbons and their diffusion on active coal, where no back diffusion occurs. In our case we are working with a\nporous system, in which backdiffusion can occure. We use the sweep gas, because it is very easy to bring the\nanalyte gas (the permeate in our case) to the GC system. There are other ways in membrane science, such as\npressure decay measurements, which also come with their own problems, for instance long waiting times and\nlarge sample area requirements, or the need for a mass spectrometer. In the research context, we always making\nassumptions. Since we are measuring with 1 ml/min sweep gas and 100 ml/min feed gas (in mixed gas 50 mln\ngas 1, 50 mln gas 2), the back diffusion probability of gas is very low due to a low chemical potential gradient.\nThe sweep gas is not detectable in the retentate at these rates and we assume that back diffusion is very low and\ncan be presumable be ignored.\n3. It would be interesting to see if the membrane performance variance with different thickness, and/or different\nbenzimidazolate ratio.\nAs demonstrated in other studies on MOF glass gas separation membranes (e.g., 10.1038/s41467-025-56295-x),\nreducing the thickness leads in the first approach to an increase in flux, but often sacrifices selectivity. This\ntradeoff is intuitive and is anticipated for our membrane as well; yet, the need to maintain the mechanical integrity\nof the free-standing glass membrane would at this stage not allow for the significant decrease in the membrane’s\nthickness (compare 10.1002/anie.202401817). While significant changes would indeed be expected when de-\ncreasing the membrane thickness down to at least the tens-of-micrometers range, such thicknesses cannot yet be\nachieved for neat free-standing MOF glass membranes by either our or other existing methods and are therefore\noutside the scope of this study. On a similar note, we agree with the referee that investigating influence of different\nlinker ratios on the separation performance would be interesting and could represent another tool to adjust the\nperformance for specific targeted separations. Yet we respectfully note that investigating this influence in a com-\nprehensive manner would deserve a separate study.\n4. There are multiple typos in the manuscript, better to revise them.\nWe have now carefully proof-read the manuscript and corrected the typos."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revision and the response from the authors have addressed the questions from the referee. It can be recommended for\npublication in this current version.", "reviewer_2": "The authors have well addressed the reviewers' comments. I think the revised manuscript can be accepted by Nature\nCommunn.", "reviewer_3": "This is an excellent paper that provides a practical and scientifically rigorous solution to a long-standing problem in MOF-\nglass membrane fabrication. The authors successfully demonstrate the production of large-area, crack-free, self-supported\nMOF-glass membranes with outstanding molecular sieving capabilities. The methodology is well-detailed, and the results\nare compelling. Especially I like the detailed analysis and clear presentation in this paper. I have only the two comments for\nimproving this strong paper:\nThe authors states that methane permeability was so low that it was \"undetectable\" by their gas chromatography setup.\nTherefore, please include a quantitative upper bound for the methane permeability, rather than simply stating it's\n\"undetectable\".\nThe gas permeation experiments were conducted at room temperature (22°C) and 1 atm. A brief discussion or\nideally, some data on how the separation performance (permeability and selectivity) changes with variations in temperature and pressure\nwould provide a more complete (impactful) picture of the membrane's versatility and applicability in different industrial\nscenarios.", "reviewer_4": "The authors addressed my concerns in a reasonable manner. The manuscript is recommended for publication.", "reviewer_5": "The authors have addressed the comments raised by the last round of review - now the paper can be accepted"}, "author_response": "Rebuttal Letter\nWe thank all five excellent reviewers for the time dedicated to reviewing our manuscript and for their\ngeneral evaluation of our work. We further want to thank the editor for his work and guidance.\nThe reviewers’ comments are answered point-by-point below. The reviewers’ comments are formatted in\nitalic, our responses in green, changes in the manuscript in yellow, and parts removed from the main text – in\nstrikethrough red.\nThe main revision are:\n- We did everything requested from the editorial office, which was mainly a shortening of the abstract and\nfollowing the guidelines provided throughout the AIP list, source data etc.\n- We added an author contribution list.\n- We have asked for a quantitative device limit from the device manufacturer and added the values to our\npaper in response to the comments of reviewer 3.\nWe are looking forward to publication of our manuscript and thank everyone involved again for their con-\nstructive feedback.\nOn behalf of all authors,\nDr. Alexander Knebel\nReviewer #1:\nThe revision and the response from the authors have addressed the questions from the referee. It can be recom-\nmended for publication in this current version.\nWe thank the referee for the time and effort spent on the improving our manuscript.\nReviewer #2:\nThe authors have well addressed the reviewers' comments. I think the revised manuscript can be accepted by\nNature Communn.\nWe thank the reviewer for increasing the quality of our manuscript through his valuable comments, and the time\ncommitted.\nReviewer #3:\nThis is an excellent paper that provides a practical and scientifically rigorous solution to a long-standing problem\nin MOF-glass membrane fabrication. The authors successfully demonstrate the production of large-area, crack-\nfree, self-supported MOF-glass membranes with outstanding molecular sieving capabilities. The methodology is\nwell-detailed, and the results are compelling.Especially I like the detailed analysis and clear presentation in this\npaper. I have only the two comments for improving this strong paper:\nWe thank the reviewer for his positive evaluation of our paper.\nThe authors states that methane permeability was so low that it was \"undetectable\" by their gas chromatography\nsetup. Therefore, please include a quantitative upper bound for the methane permeability, rather than simply\nstating it's \"undetectable\".\nThis section was already added to the paper during the last round of revisions; however, the detection limits and\nquantification limits have been asked from Shimadzu (producer of the GC device) and they stated that a quanti-\nfication is usually possible until 10 ppm, while the detection limit is much lower. We have corrected our values\nto even lower numbers than in our previous version:\nThe minimum concentration of a gas in a permeate sample that can be detected by the gas chromatography system\nwith thermal conductivity detector is 0,00009 %, while it is safe to quantify concentrations at 0.001 % (>10 ppm)\nor above.\nThe gas permeation experiments were conducted at room temperature (22°C) and 1 atm. A brief discussion or\nideally, some data on how the separation performance (permeability and selectivity) changes with variations in\ntemperature and pressure would provide a more complete (impactful) picture of the membrane's versatility and\napplicability in different industrial scenarios.\nThe referee has a good point here. We will certainly do that in our upcoming studies. The data we generated until\nnow on p- and T-variation is too preliminary to be published with this work. The proof-of-concept for a working,\nlarge and free-standing MOF-glass membrane has been demonstrated in this paper.\nReviewer #4:\nThe authors addressed my concerns in a reasonable manner. The manuscript is recommended for publication.\nWe thank the reviewer for his valuable time and effort and that he increased the quality of our paper.\nReviewer #5:\nThe authors have addressed the comments raised by the last round of review - now the paper can be accepted\nWe thank the reviewer for the time spent on the evaluation of our manuscript and acknowledge the value added."}]

Decision letter