Asymmetrical covalent organic framework mixed matrix membranes for highly efficient gas separation

Domain: chemistry
Published: 2026-01-22
Editorial outcome: minor_revision

Abstract
The asymmetrical and seamless Covalent organic framework (COF)-mixed matrix membranes (COF-MMMs) extending over the centimeter scale for highly efficient H2/CO2 separation are fabricated via a non-solvent induced phase separation (NIPS)-triggered in situ interfacial polymerization (IP) strategy. Here, a dense and ultrathin COF membrane (15–30 nm thick) is fabricated on the surface of the Polyethersulfone (PES) skin layer, and isolated COF nanocrystals (4–8 nm) are formed and highly dispersed inside the PES matrix through in situ interfacial polymerization. No interfacial defects are detected between the COF nanocrystals and the PES matrix. The COF-MMMs exhibit H2/CO2 selectivity of 88.8 ± 2.46 at an ambient temperature of 298 K, while maintaining a high H2 permeance of 2738 ± 58.02 GPU. This study proposes a facile strategy to fabricate large-scale high-performance COF-MMMs for gas separation.

Introduction
Covalent organic framework (COF) materials exhibit significant potential in gas separation because of their highly ordered porous structure, designable organic building units, and exceptional chemical and thermal stability1–3. Compared with zeolites and metal‒organic frameworks (MOFs), the fabrication of dense and thin COF membranes on porous substrates remains a significant challenge using conventional in situ and secondary-growth methods owing to the inherently limited dynamic covalent reversibility, weak seed/substrate interactions, stringent crystallization requirements with sluggish kinetics, and large molecular dimensions of COF precursors4–6.

The interfacial polymerization (IP) method is the most popular method for fabricating dense and ultrathin COF membranes with exceptional gas separation performance under mild conditions7,8. However, the mechanical strength of the resulting COF films is still limited owing to their inherent brittleness, making them susceptible to cracking or even breaking apart9. Additionally, transferring the membrane onto substrates is technically challenging and difficult to control, and combined with its weak mechanical stability, these issues severely limit the practical application of the interfacial polymerization approach10. Therefore, developing an innovative COF membrane preparation strategy that combines exceptional separation performance with triggered stability represents a critical technological challenge.

Most recently, COF-mixed matrix membranes (COF-MMMs) have attracted both academic and industry attention. By combining the advantages of the processability of polymers with ordered mass transfer channels and abundant functional groups of COFs, great achievements have been obtained in the field of gas separation via COF-MMMs11,12. Currently, the dominant strategy for preparing COF-MMMs is dispersing pre-synthesized COF crystallites directly into polymer matrices13–15. However, micron-scale COF particles exhibit a limited interfacial contact area with the polymer matrix due to their large particle size, resulting in weak interfacial adhesion and the formation of non-selective defects16. Notably, Jin and coworker17 achieved the preparation of an ultrathin (<100 nm) mixed-matrix membrane (MMM) with high filler loading (80.4 vol%) through a solid-solvent processing strategy. The key to this strategy lies in using the polymer as a solid solvent to first form a metal salt@polymer precursor layer, followed by the controlled in situ conversion of the metal salt into a metal-organic framework (MOF). Throughout this process, the polymer segments tightly combine with the MOF particles, resulting in an intact MOF-polymer interface. The resultant membrane exhibits remarkable H2/CO2 separation performance with a selectivity of 76.1 and a hydrogen permeance of 3640 GPU. Similarly, Tan and coworker18 developed an in situ polymerization approach for fabricating COF-MMMs. Uniformly distributed 2D-COFs were synthesized by mixing COF precursors with polyethylene oxide (PEO) monomers, followed by a microwave-assisted Zincke reaction. The resulting COF-MMMs exhibited a remarkable permeability of 803.9 barrer for CO2 and a selectivity of 15.0 for CO2/H2. In situ polymerization effectively overcomes traditional challenges such as COF aggregation and poor interfacial compatibility. However, the low gas separation selectivity still limits the development and exploration of COF-MMMs as selective membranes for gas separation. Therefore, a highly controllable and reproducible method for fabricating COF-MMMs with highly dispersed COF nanocrystals for gas separation is highly desirable.

Non-solvent induced phase separation (NIPS) is a classic method for preparing polymer membranes19,20. Typically, polymer membranes prepared via NIPS exhibit an asymmetric structure comprising a skin layer with molecular sieve selectivity and finger-like pore channels that enhance gas transportation21. Polyether sulfone (PES) has excellent chemical stability, stable mechanical properties, a high glass transition temperature (~225 °C) and a high thermal decomposition temperature (~300 °C), making it a commonly used matrix material for NIPS22. PES membranes prepared via NIPS demonstrate both robust mechanical strength and temperature-resistant gas selectivity23,24, which fully exemplifies the advantages of NIPS technology for producing high-performance separation membranes.

Results
The COF-MMMs with highly dispersed COF nanocrystals extending over the centimeter scale for highly efficient H2/CO2 separation were fabricated via a facile strategy, denoted induced phase separation (NIPS)-triggered in situ interfacial polymerization (IP). The COF-MMMs achieved an excellent H2/CO2 selectivity of 88.8 ± 2.46 with a high H2 permeance of 2738 ± 58.02 GPU (1 GPU = 3.35 × 10−10 mol m−2 s−1 Pa−1). Furthermore, grand canonical Monte Carlo (GCMC) simulations were systematically employed to investigate the adsorption behaviors of H2 and CO2 in TpPa-1, revealing a synergistic separation mechanism involving Knudsen diffusion in the PES matrix, selective adsorption by the COF domains, and molecular sieving through the interlayer spacing of COFs. This study provides a practicable pathway for fabricating COF-MMMs for gas separation.

Synthesis and characterization of COF-MMMs

The fabrication process of COF-MMMs via the NIPS-triggered in situ IP strategy is illustrated in Fig. 1. The experimental section (Supplementary Fig. 1) provides additional methodological details, with the names of all the fabricated membrane materials comprehensively listed in Supplementary Data 1. Typically, the Tp@PES solution was prepared by dissolving the COF precursor 1,3,5-triformylphloroglucinol (Tp, organic aldehyde monomer) directly into a PES solution with 1-methyl-2-pyrrolidone (NMP) as the solvent. A glass fiber (GF) served as the supporting substrate, where the Tp@PES solution was uniformly deposited onto the GF substrate surface (Tp@PES@GF), followed by immersion into an aqueous phase containing p-phenylenediamine (Pa-1) and p-toluenesulfonic acid monohydrate (PTSA). During the NIPS process, the polymer solution immersed in the non-solvent bath (water) undergoes bidirectional diffusion driven by the solvent/non-solvent concentration gradient. The rapid outflow of solvent and influx of non-solvent disrupt thermodynamic equilibrium, triggering liquid‒liquid phase separation that simultaneously generates a polymer‒rich continuous matrix phase and dispersed domains dominated by COF solvent‒nonsolvent21,25. The rapid diffusion and precipitation of solvent at the surface cause the polymer to undergo instantaneous phase separation and deposition, forming a dense skin layer24,26. The polymer-rich phase solidified into a porous framework through chain entanglement or phase transition, establishing a stable solid‒liquid interface with the non-solvent bath. Concurrently, during the initial liquid-state stage of NIPS, the rapid solvent/non-solvent interdiffusion and the polymerization of Pa-1 in the aqueous phase facilitated the migration of Tp precursors toward the PES surface. These Tp precursors then underwent an in-situ interfacial polymerization (IP) reaction with Pa-1 monomers that had diffused to the PES surface from the aqueous phase. At the same time, a portion of the Tp monomers was carried by the solution to the internal surfaces of the finger-like pores. As the polymer phase gradually solidified, the fluidity of the entire system dropped drastically. The diffusion of Tp monomers trapped within the polymer chain network became severely restricted. However, this did not mean the reaction stopped. The Tp monomers that had reached or were in close proximity to the pore surfaces could still react with Pa-1 monomers that continuously diffused in from the aqueous phase, forming a dense, ultrathin COF membrane. The Tp monomers encapsulated within the polymer network likely formed COF nanocrystallites through local chain relaxation or via very short-range diffusion and rearrangement within microscopic pores. In the fabrication of COF-MMMs, the polymer matrix serves as both a solid solvent and a reactive site for subsequent in situ IP.

The PES membrane prepared directly on the GF (PES@GF) appeared white to the naked eye and exhibited a smooth surface, which was the typical structural morphology of the PES film fabricated by NIPS on the basis of the SEM images (Fig. 2a). The COF-MMMs prepared after 72 h of crystallization (TpPa-1@PES@GF-72) appeared reddish-brown. SEM revealed the formation of a membrane composed of irregular COF nanoparticle aggregates on the surface (Fig. 2b). The appearance of small pores in the COF membrane was attributed to the overly mild reaction conditions as well as the fact that the COF membrane consisted of small and irregular nanoparticles27. With prolonged polymerization time, COF nanoparticles formed on the membrane surface, leading to a denser and porous structure (Supplementary Fig. 2), accompanied by a distinct color change from white to reddish-brown (Supplementary Fig. 3). Atomic force microscopy (AFM) characterization revealed porous features of the PES@GF membrane (Fig. 2c), whereas the AFM image of the TpPa-1@PES@GF-72 membrane confirmed the uniform distribution of COF nanoparticles on the polymer matrix surface (Fig. 2d).

The formation of the COF membrane was confirmed by X-ray diffraction (XRD) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). XRD analysis revealed the gradual emergence of characteristic diffraction peaks at 4.7° and 28° (2θ) upon prolonged reaction, which is consistent with reported TpPa-1 COF crystalline structures28 (Supplementary Fig. 4a). Upon COF membrane formation, the broad diffraction peak of the original GF substrate in the 17–37° range was obscured by characteristic PES peaks, confirming complete coverage of the GF surface by the mixed matrix. ATR-FTIR spectroscopy further provided evidence for the formation of COF membrane. The characteristic C=O stretching vibration peak at 1637 cm−1, initially present in Tp@PES@GF membrane and attributable to Tp monomers, progressively diminished during the reaction, indicating complete monomer conversion28 (Supplementary Fig. 4b). The Si-O-Si peaks of GF at 785 cm−1 and 1016 cm−1 disappeared in the COF-MMMs, whereas the newly observed S=O peak at 1240 cm−1 confirmed successful coverage by the PES matrix.

The commercial GF substrate features a unique internal fiber network structure (Supplementary Fig. 5). Cross-sectional SEM characterization revealed that the prepared COF-MMMs exhibited an asymmetrical multilayered architecture (Fig. 2e), comprising a dense COF membrane on the top, a foam-like porous layer in the media, and a bottom layer with finger-like pore channels, similar to those of a pure PES membrane prepared by NIPS24 (Supplementary Fig. 6). X-ray photoelectron spectroscopy (XPS) provided direct evidence for the formation of a dense COF membrane on the top. The strong characteristic peak observed at 399.8 eV in the N 1s spectrum was unambiguously assigned to nitrogen in the imine bond (-C=N-), directly confirming the successful condensation reaction between Tp and Pa-1. Meanwhile, the S 2p signal of PES at 168.0 eV was significantly attenuated in the TpPa-1@PES@GF membrane, demonstrating the formation of a dense COF covering membrane (Supplementary Fig. 7).

The COF-MMMs prepared by the conventional blending method exhibited only an extremely weak nitrogen signal, providing further evidence that the COF membrane was successfully constructed on top of the PES substrate rather than through simple physical blending. Furthermore, the SEM results proved that the Tp@PES solution penetrated into the interior of the GF substrate (Fig. 2f) and formed a robust mechanical interlocking porous structure with the GF fibers (Fig. 2g). Notably, the walls of the finger-like pores in the polymer matrix were extensively covered with irregular COF structures (Fig. 2h), demonstrating that COF formation occurred not only at the top surface but also throughout the internal surface of the polymer matrix. The Tp@PES solution formed numerous foam-like pores within the GF substrate and filled the fiber pores (Fig. 2g). Energy dispersive X-ray spectroscopy (EDS) elemental mapping provided further evidence for the penetration of the COF mixed polymer matrix into the GF interior. Distinct N signals (representative of COF nanoparticles29) were detected on the cross-section of the TpPa-1@PES@GF-72 membrane (Fig. 2j), whereas negligible N signals were observed on both the surface and cross-section of the Tp@PES@GF membrane (Supplementary Fig. 8). Additionally, trace Si detected on the surface of both the Tp@PES@GF and TpPa-1@PES@GF-72 membranes confirmed the complete coverage of the COF mixed polymer matrix over the GF surface (Supplementary Fig. 8).

High-resolution transmission electron microscopy (HRTEM) characterization further revealed the asymmetrical multilayered architecture detected by SEM. A dense, ultrathin COF membrane with a thickness of 15–30 nm was fabricated on the surface of the PES skin layer (Fig. 2k). This was mainly related to the solvent exchange and aggregation-induced effects promoting the migration of Tp to the polymer surface during the NIPS process, where it underwent in situ polymerization and formed a dense, ultrathin COF membrane at the top (Fig. 2l, Supplementary Fig. 9). Scattered COF nanocrystals were also observed on the internal surface of the foam pores within the polymer matrix (Fig. 2l), which was attributable to concurrent solvent/non-solvent interdiffusion that induced localized Tp aggregation and polymerization along the pore walls. Remarkably, highly dispersed COF nanocrystals (4–8 nm) were identified adjacent to the COF membrane (Fig. 2m and Supplementary Fig. 10). Similarly, the same feature was observed in the TpPa-1@PES@GF-24 (Supplementary Fig. 11). The likely explanation was that the formation of a dense COF membrane on the surface slowed solvent/non-solvent diffusion, leading to an increased Tp concentration in the foam-pore layer region near the membrane surface, thereby triggering in situ polymerization and the formation of an nanocrystal-enriched layer. Notably, the inner surface of the finger-like pore walls also contained uniformly distributed COF nanocrystals (Fig. 2n, Supplementary Fig. 12). The slower solvent exchange rate within the finger-like pores of the polymer matrix led to the formation of dispersed COF nanocrystals, which were distributed along the inner surface of the dense skin layer of the polymer (Fig. 2o). Most importantly, the HRTEM images clearly revealed that the COF nanocrystals were enveloped by worm-like polymer chains, forming a seamless COF‒polymer interface with almost no intergranular defects (Fig. 2p). This can be attributed to the formation of hydrogen bonds between the ether (-O-) and sulfone (-SO2-) groups in the PES matrix and the hydroxyl (-OH) groups in the Tp precursors. This interaction was further corroborated by XPS analysis. The O 1s spectrum revealed a significant intensity enhancement of the C–O characteristic peak near 533.0 eV, attributed to phenolic hydroxyl and ether oxygen groups, indicating the successful incorporation of Tp molecules into the PES matrix. More critically, the binding energy of the O=S=O component in the sulfone group shifted positively from 531.4 eV to 531.7 eV, while the binding energy difference between this component and the ether oxygen peak decreased from 1.93 eV to 1.45 eV. These systematic binding energy shifts and the observed energy gap narrowing can be attributed to the decreased electron density around oxygen atoms induced by hydrogen bonding formation, thereby providing direct evidence for the existence of hydrogen bond interactions between PES and Tp (Supplementary Fig. 7). The precursor Tp was completely soluble in the PES solution (Supplementary Fig. 13). During the NIPS process, Tp migrated outward with the diffusion of the NMP solvent. Since Tp was insoluble in the aqueous phase, it became entrapped and concentrated on the surface of the solidifying PES matrix, where it underwent in situ polymerization with Pa-1. This process occurred uniformly across the entire forming interface, effectively preventing the macroscopic aggregation often observed in physical blending methods18. Subsequent in situ IP creates covalent linkages between COF precursors within the cross-linked polymer network, establishing tight interfacial bonding between the COF and the polymer matrix that eliminates interface defects caused by poor compatibility between fillers and polymers via traditional methods. Additionally, the AFM force-distance curve (F-D) measurements revealed a notably high interfacial adhesion force of 2.863 nN, which can be ascribed to the abundant and effective weak van der Waals interactions between PES and the COF (Supplementary Fig. 14). Therefore, the NIPS-triggered in situ IP strategy first achieves a uniform distribution of COF nanocrystals in the polymer matrix to effectively avoid the particle aggregation issues of traditional methods. Second, a perfect COF‒polymer interface was successfully constructed through multiple synergistic interactions, forming a dense, defect‒free ultrathin COF membrane on the COF-MMMs surface for high separation selectivity.

An asymmetrical multilayered architecture analysis of the PES@GF and TpPa-1@PES@GF-72 membranes was performed on the basis of their pore size distributions. The significant difference between the average pore size (0.30 μm) and the most likely pore size (0.016 μm) of the PES@GF membrane indicated the formation of an asymmetrical multilayer structure consisting of foam-like and finger-like pores via NIPS (Supplementary Fig. 16a). The decrease in the average pore size of the TpPa-1@PES@GF-72 membrane to 0.2287 μm (Supplementary Fig. 16b) was caused by the presence of COF nanocrystals on both the topmost polymer surface of the COF-MMMs and on the PES matrix surface within the GF substrate, as confirmed by SEM and HRTEM observations. N2 adsorption‒desorption isotherms revealed that the specific surface areas of the GF, PES@GF and TpPa-1@PES@GF-72 membranes were 2.2319 m2/g, 7.1130 m2/g, and 9.8237 m2/g, respectively (Supplementary Fig. 17). The gradual increase in surface area provided clear evidence for the formation of COF nanocrystals on the PES surface within the GF substrate.

The COF-MMMs exhibited deformation resistance without crease formation under external stress (Fig. 2q). Remarkably, the structural integrity of the COF-MMMs was retained even after the PES matrix was completely dissolved by immersion in DMF, despite the occurrence of stress-induced increases (Fig. 2r). These results confirmed that the polymer matrix not only served as both a solid solvent and a polymerization platform but also provided physical support that enhanced membrane flexibility. For comparison, the COF-MMMs fabricated without the GF substrate were also treated with DMF to selectively remove PES. A complete and free-standing COF membrane was obtained (Supplementary Fig. 18). A prolonged reaction time enhanced membrane integrity, resulting in characteristics comparable to those of liquid‒liquid interfacial polymerized COF membranes (Supplementary Fig. 19). In addition, after the TpPa-1@PES@GF membrane was subjected to external damage tests, such as adhesive tape tearing (Fig. 2s-t), the structural integrity of the membrane layer remained intact. To quantitatively assess the functional integrity of the membrane layer after mechanical damage, we compared the gas separation performance before and after the adhesive tape tearing test. The H2/CO2 selectivity of the membrane after testing was measured at 74.3 (Supplementary Fig. 20). Although this reflects a reduction from the original value of 85.3, it remains significantly higher than that of the pristine PES@GF membrane (17.4) and the selectivity of the COF-MMMs fabricated via the conventional blending method (33.2). This indicates that the core separation layer remained intact under physical damage, and the minor performance variation likely stems from a slight increase in non-selective pores within the support layer. The above experimental results confirmed that the TpPa-1@PES@GF membrane had high mechanical strength and stability.

Gas-separation performance of COF-MMMs

The gas separation performance of PES@GF and COF-MMMs were evaluated using pure H2, CO2, N2, and CH4 at 298 K and a feed pressure of 1 bar. The PES@GF membrane showed a H2/CO2 selectivity of 17.4 with a H2 permeance of 4111 GPU (Fig. 3a). The gas separation mechanism of the PES membrane followed the Knudsen diffusion mechanism, achieving separation via gas‒wall collisions through nanoscale pore diffusion in the dense skin layer24,30. The PES matrix provided low-resistance transport pathways for rapid gas permeation, ensuring high gas flux. The COF-MMMs prepared by a NIPS-triggered in situ IP strategy exhibited improved gas selectivity and stable gas permeability. A prolonged polymerization time markedly increased the H2/CO2 selectivity of the TpPa-1@PES@GF-X membranes. Moreover, the CO2 permeability decreased sharply, primarily due to the selective adsorption of CO2 by the COF31. The introduction of COF led to the formation of more tortuous gas transport pathways and caused a decrease in permeability. However, the porous structure of PES provided fast gas permeation channels for the membrane, stabilizing the H2 permeance at approximately 2700 GPU. The TpPa-1@PES@GF-72 membrane achieved optimal separation performance after 72 h of reaction, with an H2/CO2 selectivity of 88.8 ± 2.46 and an H2 permeance of 2738 ± 58.02 GPU (1 GPU = 3.35 × 10−10 mol m−2 s−1 Pa−1), exceeding the 2008 Robeson upper bound (Fig. 3a). The effect of the operating temperature on the H2/CO2 separation performance of TpPa-1@PES@GF-72 was investigated (Fig. 3b). The elevated gas permeance and reduced H2/CO2 selectivity as the temperature increased to 423 K could be attributed to the more intense molecular motion and weakened CO2 adsorption by the COF at elevated temperatures. Even at an elevated temperature of 423 K, the TpPa-1@PES@GF-72 membrane maintained a high H2/CO2 selectivity of 40.9 ± 0.92, surpassing the 2008 Robeson upper bound, along with H2 permeability of 4919 ± 180.46 GPU (Fig. 3b). During the continuous heating test, the H2 and CO2 permeances of the TpPa-1@PES@GF-72 membrane increased sharply with increasing temperature, whereas the corresponding H2/CO2 selectivity decreased. The TpPa-1@PES@GF-72 membrane exhibited remarkable thermal stability, as evidenced by its nearly consistent separation performance when the temperature was cycled between 423 K and room temperature (Fig. 3c).

To demonstrate the superiority of the NIPS-triggered in situ IP strategy, we fabricated COF-MMMs via the conventional blending approach by admixing the presynthesized COF nanoparticles into the PES polymerization mixture (Fig. 3d). The TpPa-1/PES/GF membrane prepared by the traditional blending method exhibited a H2/CO2 selectivity of 33.2 and a H2 permeance of 3060 GPU (Supplementary Fig. 21, Supplementary Data 1). This inferior performance is likely attributable to the unavoidable aggregation of pre-synthesized COF nanoparticles and the poor interfacial adhesion, which lead to non-selective defects, as commonly encountered in traditional MMMs. And most COF nanoparticles are located at the cross-section (interior) of the COF-MMMs. Increasing the COF loading causes difficulties during fabrication because the high COF content significantly increases the viscosity of the doped solution, which cannot be cast into a film26. In contrast, NIPS-triggered in situ IP strategy ensures the in situ formation of highly dispersed COF nanocrystals and a seamless COF-polymer interface, which is the key to the 162.05% greater selectivity, with a mere 11.76% permeance reduction (Fig. 3e), exceeding the 2008 Robeson upper bound. Moreover, the TpPa-1@PES@GF-72 membrane exhibited operational stability during 40 h of continuous operation (Fig. 3f), with negligible fluctuations in H2 and CO2 permeances. The long-term stability can be attributed to the high chemical stability of the COF and the good COF‒polymer compatibility of the COF-MMMs, as well as the mechanical interlocking structure. We also explored the scaled-up fabrication of COF-MMMs (Fig. 3g), and the separation performance of each part was stable, demonstrating the potential of the NIPS-triggered in situ IP strategy for large-scale applications (Fig. 3h).

Gas-separation mechanisms of COF-MMMs

The gas separation mechanism was investigated through single gas permeation tests (H2, CO2, N2, and CH4) using the TpPa-1@PES@GF-72 membrane (Fig. 4a). The H2, N2, and CH4 permeances significantly surpassed the CO2 permeance, which was attributed to the flexibility of the framework and CO2 adsorption hindering diffusion through the COF channels. The ideal selectivities of H2/CO2, H2/N2, and H2/CH4 were 85.53, 59.57, and 52.57, respectively, significantly exceeding the Knudsen diffusion values (4.69, 3.73, and 2.83)9. The mixed-gas selectivities of 87.07, 57.63 and 54.83 showed minimal deviation from the ideal selectivity, revealing competitive adsorption among gas molecules within the COF framework. According to literature reports, CO2 adsorption can block the pores of COFs, and competitive adsorption occurs within the reduced pores, hindering the transport of H232. For COF TpPa-1 with a free pore size of approximately 18 Å28, the selective adsorption of CO2 molecules created steric hindrance that slowed subsequent rapid CO2 permeation. For smaller H2 molecules, the weak affinity of the COF for H2 and the effective pore size enabled faster diffusion, thereby improving selectivity. Further research revealed that the CH4/CO2 separation factor (4.0) of the mixed gas exceeded the ideal selectivity (3.77) owing to the stronger adsorption affinity of TpPa-1 for CO2. The synergistic mechanism of selective adsorption and competitive diffusion in H2/CO2 separation was confirmed33. In addition, the porous structure of the PES matrix also plays an important role in the separation process. Larger molecules experience greater transport resistance, whereas smaller molecules permeate more rapidly. The significant increase in the gas diffusion difference and synergistic selective adsorption of COFs jointly achieve a balance between the high permeability and high selectivity of membrane materials to meet the requirements of industrial gas separation (Fig. 4b)34. Moreover, the influence of feed pressure on the separation performance of the TpPa-1@PES@GF-72 membrane was also explored. Notably, the membrane’s ultra-high permeance presented a technical challenge for maintaining stable high feed pressures (>1 bar) due to rapid pressure equalization across the membrane. However, at a moderately elevated feed pressure of 0.02 MPa (0.2 bar), the H2 permeance surged to approximately 8000 GPU, underscoring the immense gas flux capability of the membrane. Concurrently, the H2/CO2 selectivity stabilized at around 15. The observed reduction in selectivity may be attributed to enhanced non-selective viscous flow and subtle structural adjustments under applied pressure. Nevertheless, the selectivity remained significantly above the Knudsen limit, confirming the persistence of selective transport mechanisms. The COF-MMMs achieved an optimal balance between ultra-high permeance and considerable selectivity, highlighting their promising potential for high-flux separation applications (Supplementary Fig. 22). Compared with the most advanced H2/CO2 separation membranes (Fig. 4b, Supplementary Data 1), the TpPa-1@PES@GF membrane exhibited reasonable separation performance. Furthermore, the reproducibility demonstrated by tests on ten independent membranes highlights the advantages of the NIPS-triggered in situ IP strategy in fabricating high-performance, highly stable COF-MMMs (Supplementary Table 1).

The H2/CO2 separation mechanism of TpPa-1 was thoroughly studied through GCMC simulations. The simulated gas adsorption isotherms revealed adsorption energies of −20.71 kcal mol-1 for CO2 and −0.71 kcal mol−1 for H2 (Fig. 5a), demonstrating a significantly stronger CO2 adsorption capability for TpPa-1 than for H2. The adsorption energy of TpPa-1 for H2 essentially remained unchanged after CO2 adsorption equilibrium. The structure of TpPa-1 features topologically planar and ordered networks which undergo stacking along the third dimension, thereby generating 1D void channels35. This anisotropic alignment is beneficial for the effective transport of molecules along the stacked columns and pore channels36. During dynamic CO2 adsorption, the effective pore size of TpPa-1 decreased from an initial 16 Å to 13–16 Å (Fig. 5b) with a concomitant reduction in the interlayer spacing (Fig. 5d), thereby impeding rapid CO2 diffusion. In contrast, weakly adsorbed H2 molecules could still rapidly traverse the spatially constrained pore channels and reduce the interlayer spacing without obstruction, enabling highly efficient H2/CO2 separation. This separation mechanism achieved highly efficient H2/CO2 separation by combining Knudsen diffusion in the PES matrix, selective adsorption by the COF domains, and molecular sieving through the COF interlayer spacing. These three mechanisms are interrelated and work synergistically, collectively overcoming the “trade-off” effects in membrane separation processes, thereby enabling the COF-MMMs to simultaneously possess both high selectivity and high permeance. It should be pointed out that several other types of COFs have been recently synthesized via interfacial polymerization as well. Indeed, further studies on the fabrication of other COF-MMMs for gas separation are currently underway in our laboratory.

Methods
Materials
All the chemicals and reagents that are commercially available are used without purification. Polyethersulfone (PES, 80 mesh) was obtained from Dongguan Shuangfu Plastics Trading Co., Ltd., 1-methyl-2-pyrrolidone (NMP, Aladdin, AR), 1,3,5-triformylphloroglucinol (Tp, CoChemist, 95%), p-phenylenediamine (Pa-1, Macklin, AR), p-toluenesulfonic acid monohydrate (PTSA, Aladdin, AR), water (homemade), and GF supports with an open porosity of 94.5%, an average pore size of 4.68 μm (Supplementary Fig. 15) and a diameter of 25 mm (binder-free) were purchased from Shanghai Xinya Purification Equipment Co., Ltd. All gases with a minimum mole fraction of 99.99% were provided by Jinghua (Yinchuan) Gas Co., Ltd., including H2, CO2, N2, CH4 and Ar.

Synthesis of the PES solution. The PES solution was prepared by dissolving 1.5 g of PES granules in 8.5 g of NMP under stirring at room temperature for at least 24 h until a homogeneous solution formed. After that, the solution was kept at room temperature without stirring overnight to remove all the bubbles in the solution. Sealed for future use.

Synthesis of the PES membrane. The PES membrane was prepared by casting the PES solution on a glass plate using a casting knife, followed by immersion in a water bath at room temperature for phase separation. The resulting membrane was thoroughly washed with water and stored in water before use.

Synthesis of the PES@GF membrane. The PES@GF membrane was prepared via the NIPS method. The GF substrate was secured on the spin coater, and the PES solution was spin-coated at 1000 rpm for 60 s. The coated GF membrane was then immersed in an aqueous solution and left undisturbed for 24 h. The resulting membrane was thoroughly rinsed with deionized water and stored in water prior to use. The prepared membrane was designated PES@GF membrane.

Synthesis of the Tp@PES@GF membrane. Tp@PES@GF membrane was prepared via NIPS method. The Tp@PES solution was prepared by dissolving 0.1 g of Tp in 10 g of PES solution under stirring at room temperature for 24 h until a homogeneous solution formed. The GF substrate was secured on the spin coater, and the Tp@PES solution was spin-coated at 1000 rpm for 60 s. The coated GF membrane was then immersed in an aqueous solution and left undisturbed for 24 h. The resulting membrane was thoroughly rinsed with deionized water and stored in water prior to use. The prepared membrane was designated Tp@PES@GF membrane.

Synthesis of the TpPa-1@PES-X membranes. TpPa-1@PES-X membranes were prepared via NIPS-triggered in situ IP. The Tp@PES solution was subsequently cast on the glass panel via a casting knife, followed by immersion in water containing a Pa-1 (97.4 mg) and PTSA (584.4 mg) bath at room temperature. The prepared membranes were abbreviated as TpPa-1@PES-X, where X denotes the polymerization time (e.g., 24 h, 48 h, 72 h, or 96 h). The resulting membranes were thoroughly washed with water and stored in water before use.

Synthesis of the TpPa-1@PES@GF-X membranes. TpPa-1@PES@GF-X membranes were prepared via NIPS-triggered in situ IP. The GF substrate was secured on the spin coater, and the Tp@PES solution was spin-coated at 1000 rpm for 60 s. The coated GF membrane was then immersed in an aqueous solution containing Pa-1 (97.4 mg) and PTSA (584.4 mg) and left undisturbed for varying durations to allow for polymerization. The resulting membrane was thoroughly rinsed with deionized water and stored in water prior to use. The prepared membranes were abbreviated as TpPa-1@PES@GF-X, where X denotes the polymerization time (e.g., 24 h, 48 h, 72 h, or 96 h).

Gas permeation experiments
Gas permeation tests were conducted using a custom-built evaluation system (Nanjing First Experiment Equipment Co., Ltd.). The system incorporated a Wicke‒Kallenbach permeation cell housed in a temperature-controlled oven (Supplementary Fig. 23). Membrane samples (TpPa-1@PES@GF-X) were sealed using perforated aluminum tape, exposing an effective area of 0.5024 cm2. Gas flows were regulated by mass flow controllers (MFCs, SevenStar, D08-4E). The composition of the permeate stream was analyzed by gas chromatography (Panna A91 plus, Changzhou Panna Instruments Co., Ltd.) and its flow rate was measured with a soap bubble flow meter. A flame ionization detector with a methanizer quantified trace CO2, while H2 was detected via a thermal conductivity detector. The permeate side was at atmospheric pressure, with no applied transmembrane pressure drop. All reported data are the average of at least three measurements.

For the single gas permeation tests, the feed and the argon sweep gas flow rates were both set at 100 mL min−1.

Gas permeance (Pi, 1 GPU = 3.35 × 10−10 mol m−2 s−1 Pa−1) of gas “i” is calculated using Eq. (1).

Pi = Ni / (A * Δpi) (1)

where Ni is the permeate rate of gas “i” (mol s−1), A is the effective membrane area (m2), and Δpi is the partial pressure difference of gas “i” (Pa).

Ideal selectivity (IS) is derived from Eq. (2).

IS = Pi / Pj (2)

where Pi and Pj are the permeances of gas “i” and gas “j”, respectively.

For equimolar (1:1) binary gas mixtures, the total feed flow was 100 mL min−1 with N2 as the sweep gas (100 mL min−1). Permeance was calculated according to Eq. (1), and the separation factor (SFi/j) is determined using Eq. (3).

SFi/j = (Yi / Yj) / (Xi / Xj) (3)

where Xi and Yi denote the molar concentration ratios of component “i” on the feed and permeate sides, respectively.

GCMC simulation
GCMC simulations were conducted with the Cassandra code37 to compute CO2 and H2 absorption under TpPa-1 nanoconfinement at 298 K and 101.325 kPa. The configurational bias Monte Carlo method was utilized to increase the efficiency of gas absorption. For all system sorption calculations, equilibration runs of 1 million steps were performed, followed by production runs of 1 million steps. The LJ and electrostatic interactions for the simulation box were truncated at 15 Å. Bond lengths were constrained to their nominal values. The Opls-aa force field38,39 was applied for TpPa-1. CO2 and H2 models were taken from Son et al.40.

Data availability
All data are available in the main text or the Supplementary Information. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript dealt with asymmetrical PES membranes with a thin COF top layer and embedded COF nanocrystals. The membrane showed high H2 permeation and H2/CO2 selectivity. The experiments were carefully conducted based on well-established hypothesis. My comments are shown below.\n1. The authors report an asymmetrical PES membrane with a COF top layer and embedded COF nanocrystals. I am not sure if we should refer to this type of membrane as a mixed matrix membrane.\n2. From Figure S1, it seems the author prepared the TpPa 1@PES on glass fibre support by using the spin coating method, however, the experimental detail in the Supporting material mentions the polymer solution was cast on the glass panel via a casting knife. So it confuses me.\n3. Line 130 and 133, Fig. 1a and Fig. 1b appear to be mislabelled. The same issue appears from Line 168-215.\n4. Line 217, authors state “This ensured the uniform distribution of Tp precursors in the polymer matrix and avoided COF aggregation caused by physical mixing in traditional methods.” However, this conclusion is based only on localised TEM observations. More macroscopic or statistical characterisation techniques should be provided to demonstrate uniform distribution and avoid COF aggregation throughout the membrane. Also, the interaction between PES matrix and COFs should be characterised.\n5. Several figures in the supporting information have never been mentioned in the manuscript, such as Fig S8-S11.\n6. Line 237, “The average pore size of the TpPa-1@PES@GF membrane decreased to 0.2287 μm (Supplementary Fig. 12b), confirming that the COF nanocrystals were present both on the topmost polymer surface of the COF-MMMs and on the PES matrix surface within the GF substrate.” I do not quite understand how the decreased average pore size could lead to the position of COF.\n7. Line 258, authors state “In addition, after the TpPa-1@PES@GF membrane was subjected to external damage tests, such as adhesive tape tearing (Fig. 2s-t), the structural integrity of the membrane layer remained intact”, gas permeation performance of the membranes after adhesive tape tearing needs to be evaluated to confirm the structural integrity of the membrane layer.", "reviewer_2": "This study proposed a new \"non-solvent induced phase separation (NIPS)-triggered in situ interfacial polymerization (IP)\" strategy, fabricating large-scale asymmetric COF-MMMs for gas separation. The synergistic formation of the COF and the PES membrane could overcome the defects by conventional physical blending technique. It is recommended that the manuscript be accepted after revisions upon addressing the following issues.\n1.There is a spelling mistake in Fig.1 “aqueous solution”.\n2.Fig. 1a-1d was wrongly labelled in the discussion. They should be Fig. 2a to 2d. The same problem with the Fig. 1k, 1l, etc. Please check carefully throughout the manuscript.\n3.The sequence of the discussion of Figure 2j and 2h should be reversed.\n4.The article proposes a synergistic mechanism consisting of \"Knudsen diffusion, selective adsorption, and interlayer molecular sieving\". Please further elaborate on which mechanism plays the key role in achieving high H2/CO2 selectivity?\n5.Is there a significant chronological order between the occurrence of \"solvent exchange\" and \"interfacial polymerization\"? Please explain in detail the migration of Tp monomers before and after the solidification of the polymer matrix: does it instantaneously \"fix\" on the inner surface of the finger-like pores during phase separation, or continue to diffuse from the interior of the matrix to the interface during subsequent immersion?\n6.Compared to the membrane prepared by the traditional blending method (H2/CO2 selectivity of 33.2), the selectivity of the new method increased by 162%. Please provide an in-depth analysis of the main reasons for this significant improvement.\n7.GCMC simulations show that TpPa-1 has a strong adsorption affinity for CO2. However, high adsorption capacity typically reduces diffusion rates, leading to decreased permeance. Please combine the simulation results to explain why the H2 permeance remains at a very high level despite the presence of strong competitive CO2 adsorption.\n8.The article demonstrates the TpPa-1 system. Have the authors considered whether this method is applicable to other COF systems?\n9.The authors compare the membrane performance of this work with membranes prepared by the \"traditional blending method\" and conclude a 162% performance improvement. However, the COF nanoparticles used in the blending method are \"pre-synthesized\".Could the difference in COF morphology affect the performance comparison?", "reviewer_3": "The authors propose a pioneering non-solvent induced phase separation (NIPS)-triggered in situ interfacial polymerization (IP) strategy for fabricating centimeter-scale asymmetrical COF-mixed matrix membranes (COF-MMMs). This approach uses the polymer matrix as both a solid solvent and an IP platform, effectively addressing the long-standing issues of filler aggregation and poor interfacial compatibility in traditional MMM preparation. However, the manuscript has several weaknesses as follows:\n1. This manuscript lacks innovation. The textual description is rather ordinary, and no obvious innovative aspects are identified.\n2. While the authors cite relevant studies on COF membranes, MMMs, and NIPS/IP methods, there are notable gaps in the literature coverage. For instance, recent (post-2022) breakthroughs in COF-based MMMs for gas separation, such as novel in situ polymerization strategies or advanced interfacial modification techniques, are not adequately discussed. Additionally, the comparison with other emerging membrane materials (e.g., metal-organic framework (MOF)-based MMMs with similar or superior performance) is insufficient. This incomplete literature review limits the authors' ability to fully contextualize their work within the latest research landscape and highlight the unique advantages of their strategy over all competing approaches.\n3. Although the authors report the average values and standard deviations of separation performance (e.g., 88.8 ± 2.46 for H₂/CO₂ selectivity), they provide limited information on the reproducibility of membrane fabrication. There is no detailed description of the number of independent membrane preparations (e.g., n = 3 or more) or the variation in performance across different batches. In high-impact journals like Nature Communications, rigorous reproducibility data is essential to confirm that the reported performance is not an outlier and that the fabrication method is reliable for widespread adoption.\n4. While the authors study the effect of temperature on membrane performance, other critical operating parameters that are relevant to industrial applications are not thoroughly investigated. For example, the influence of feed gas pressure (beyond 1 bar) on separation performance is not reported, even though pressure is a key factor in industrial gas separation processes (e.g., natural gas processing, hydrogen purification).\n5. The authors mention that hydrogen bonding between the -OH groups of Tp precursors and the -O-/-SO₂- groups of PES, along with weak van der Waals interactions, contributes to good interfacial adhesion. However, they provide limited direct evidence to support these interactions. Advanced characterization techniques, such as X-ray photoelectron spectroscopy (XPS) for chemical state analysis at the interface or atomic force microscopy-based force spectroscopy (AFM-FS) for quantifying interfacial adhesion forces, are not employed. This lack of direct evidence weakens the argument for the formation of a seamless COF-polymer interface and limits the understanding of the interfacial interaction mechanism."}, "author_response": "Point-by-point response to the reviewer comment\nThank you very much for giving us an opportunity to revise our manuscript. We appreciate the editor and reviewers very much for their constructive comments and suggestions on our manuscript originally entitled “Facile fabrication of asymmetrical COF-mixed matrix membranes for highly efficient H /CO separation”.\n2 2\nAll comments and suggestions from reviewers are valuable and helpful for revising and improving our manuscript. We respond carefully to each of the issues raised by the referees, outlining all corrections and improvements made as follows. All changes or the corrections have been marked in red ink in the revised manuscript. In addition, another version of the manuscript, without any marked text, is also provided.\nIn the text below, comments by the referee are underlined; our response is in normal text. Text that has been added or edited was marked in red ink. We hope that these amendments are sufficient and that the revised manuscript can be accepted for publication.\nIn addition, during the revision process, we conducted a thorough re-examination of all our data and identified an error in the original submission. The images previously used in Figures 3f and 3g were inadvertently misapplied. We have now corrected these figures with the accurate images and corresponding data analysis in the revised manuscript. We wish to emphasize that this correction does not affect any of the scientific interpretations or conclusions discussed in the main text. To ensure complete transparency, we have also provided the original, underlying source data for these figures as part of the “Supplementary Information”. We sincerely apologize for this oversight and thank you for the opportunity to correct it.\nFig. 2 SEM images of the membrane surface (a) PES@GF (inset: photographic image of PES@GF), (b) TpPa-1@PES@GF-72, (inset: photographic image of TpPa-1@PES@GF), (c) PES@GF, (d) TpPa-1@PES@GF-72, and cross-sectional SEM images of the TpPa-1@PES@GF-72 membrane in different regions (e-h), EDS elemental mapping of the cross-section of the TpPa-1@PES@GF-72 membrane (i-j), cross-sectional HRTEM images of the TpPa-1@PES@GF-72 membrane at different magnifications (k-p), and photographic images of the TpPa-1@PES@GF-72 membrane before (q) and after (r) treatment in a DMF solution; photographic images of the TpPa-1@PES@GF-72 membrane before (s) and after (t) detachment with tape.\nBest regards.\nDr. Zheng Wang\nCollege of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, People’s Republic of China.\nHelanshan Rd. 539, Yinchuan 750021, China\nTel: +86 951 2062008, Fax: +86 951 2062323\nE-mail: wzheng@nxu.edu.cn\nResponses to the reviewers\nWe appreciate all the valuable suggestions. The revised portion are marked in red in the revised manuscript and our responses to the comments are as follows:\nReviewer #1 (Remarks to the Author):\nComments: This manuscript dealt with asymmetrical PES membranes with a thin COF top layer and embedded COF nanocrystals. The membrane showed high H permeation and H /CO\n2 2 2\nselectivity. The experiments were carefully conducted based on well-established hypothesis. My comments are shown below.\nQuestion 1: The authors report an asymmetrical PES membrane with a COF top layer and embedded COF nanocrystals. I am not sure if we should refer to this type of membrane as a mixed matrix membrane.\nResponse: Your prudent consideration is highly valuable.\nMixed-matrix membranes (MMMs) are composite membranes in which a dispersed phase (e.g., nanofillers such as zeolites, MOFs, or COFs) is uniformly distributed within a continuous polymer matrix, thereby combining the advantages of both components (Chem. Soc. Rev., 2019, 48, 2665–2681; Science, 2023, 381(6664): 1350–1356). The concept of MMMs has evolved significantly in recent years. Specifically, Jin et al. fabricated an MOF-MMM using a solid-solvent strategy for the first time (Science, 2023, 381(6664): 1350–1356). This MOF-MMM was composed of a thin MOF layer (thickness less than 100 nanometers) with high MOF loading (up to 80%) within the polymer matrix. When the MOF loading exceeds approximately 50 vol%, MOF particles will no longer be isolated but instead form interconnected MOF nano-channels within the polymer matrix, thereby dominating molecular permeation. This demonstrates that even when the filler phase is highly continuous, the structure still belongs to the category of MMMs as long as a polymer matrix constitutes the continuous phase and forms a favorable interface.\nIn our study, the membranes prepared using a NIPS-triggered in situ interfacial polymerization (IP) strategy featured a thin COF top layer (15–30 nm thick) with well dispersed COF nanocrystals. The continuous COF membrane on the surface can be regarded as a special case of extreme enrichment and interconnection of the filler phase in the surface region. In brief, the structure of membrane we developed is quite similar to that prepared by Jin et al. Based on these similarities, we believe it is both accurate and consistent with current developments in the field to refer to the membrane in this study as a “mixed-matrix membrane.” The strategy developed here produces an asymmetric COF-MMM that combines the high selectivity of pure COF films with the excellent mechanical stability typical of conventional MMMs.\nQuestion 2: From Figure S1, it seems the author prepared the TpPa 1@PES on glass fibre support by using the spin coating method, however, the experimental detail in the Supporting material mentions the polymer solution was cast on the glass panel via a casting knife. So it confuses me.\nResponse: We apologize for any confusion caused by the original version.\nThe membranes discussed in this work can be categorized into two types: those with a glass fiber as the substrate and those without. Membranes with a glass fiber substrate are prepared via spin-coating, which ensures uniform distribution of the polymer within the glass fibers. In contrast, membranes without a glass fiber substrate are directly prepared by blade-coating on a glass plate. To clearly differentiate between the two types, we have included the preparation process for the blade-coated membranes and labeled the membrane types in Figure 1 of the Supporting Information (Supplementary Information, Page 1, lines 11–12). Additionally, we have replaced ambiguous terminology in the text: “casting solution” is now specified as “Tp@PES solution”, and “PES casting solution” is replaced with “PES solution”.\nSupplementary Fig. 1 Schematic diagram of the TpPa-1@PES@GF membrane and TpPa-1@PES membrane preparation process.\nQuestion 3: Line 130 and 133, Fig. 1a and Fig. 1b appear to be mislabelled. The same issue appears from Line 168-215.\nResponse: We were really sorry for our careless mistakes. Thank you for your reminder.\nCorrection has been made and marked in red in the revised manuscript:\n“Fig. 1a” changed to “Fig. 2a”. “Fig. 1b” changed to “Fig. 2b”. “Fig. 1c” changed to “Fig. 2c”. “Fig. 1d” changed to “Fig. 2d”. “Fig. 1h” changed to “Fig. 2h”. “Fig. 1g” changed to “Fig. 2g”. “Fig. 1k” changed to “Fig. 2k”. “Fig. 1l” changed to “Fig. 2l”. “Fig. 1m” changed to “Fig. 2m”. “Fig. 1n” changed to “Fig. 2n”. “Fig. 1o” changed to “Fig. 2o”. “Fig. 1p” changed to “Fig. 2p”.\nQuestion 4: Line 217, authors state “This ensured the uniform distribution of Tp precursors in the polymer matrix and avoided COF aggregation caused by physical mixing in traditional methods.” However, this conclusion is based only on localised TEM observations. More macroscopic or statistical characterisation techniques should be provided to demonstrate uniform distribution and avoid COF aggregation throughout the membrane. Also, the interaction between PES matrix and COFs should be characterised.\nResponse: We sincerely thank the reviewer for these insightful comments, which have helped us strengthen the evidence for our claims.\nWe agree with the reviewer that macroscopic and statistical evidence is valuable to corroborate the TEM observations. In addition to TEM observations in localized areas, EDS elemental mapping revealed distinct and widespread nitrogen signals across the entire cross-section of the TpPa-1@PES@GF-72 membrane. This combined with the observed reduction in average pore size (Supplementary Fig. 16), clearly confirms that the COF is not only confined to the surface but is also distributed throughout the internal porous structure.\nFurthermore, we have included TEM images from multiple regions (Supplementary Fig. 10), which clearly demonstrate the highly dispersed distribution of COF particles across these areas. In response to your suggestions, the discussion has been revised and highlighted in red to improve the manuscript:\n“This ensured the uniform distribution of Tp precursors in the polymer matrix and avoided COF aggregation caused by physical mixing in traditional methods” have been changed into “The precursor Tp was completely soluble in the PES solution (Supplementary Fig. 13). During the NIPS process, Tp migrated outward with the diffusion of the NMP solvent. Since Tp was insoluble in the aqueous phase, it became entrapped and concentrated on the surface of the solidifying PES matrix, where it underwent in situ polymerization with Pa-1. This process occurred uniformly across the entire forming interface, effectively preventing the macroscopic aggregation often observed in physical blending methods.” (Page 14, lines 261–268).\nSupplementary Fig. 10 Cross-sectional HRTEM images of the TpPa-1@PES@GF-72 membrane.\nAdditionally, the interaction between the PES matrix and COFs was characterized using Atomic Force Microscopy (AFM). The force–distance (F–D) curves provided direct physical evidence of the adhesion forces between the PES matrix and COF nanocrystals, supporting the presence of van der Waals interactions as proposed (Supplementary Fig. 14). Furthermore, the seamless interface observed in HRTEM (Fig. 2p) with no visible gaps, offers strong morphological evidence of excellent compatibility and robust interfacial adhesion. The mechanical stability tests (Fig. 2q–t), in which the membrane withstands deformation and tape-peeling without delamination, provide macroscopic validation of this strong interfacial interaction.\nWe have revised the discussion and marked in red in the main text: “weak van der Waals interactions between PES and the COF further enhanced the interfacial adhesion” have been changed into “The AFM force-distance curve (F-D) measurements revealed a notably high interfacial adhesion force of 2.863 nN, which can be ascribed to the abundant and effective weak van der Waals interactions between PES and the COF (Supplementary Fig. 14).” (Page 14, lines 272-275).\nSupplementary Fig. 14 AFM Force-Distance Curve of the (a) PES@GF membrane and (b) TpPa-1@PES@GF-72 membrane.\nIn the revised text, we have more clearly stated that the F-D characterization and XPS characterization collectively demonstrate the strong interfacial adhesion, arising from both van der Waals forces and potential hydrogen bonding as proposed.\nQuestion 5: Several figures in the supporting information have never been mentioned in the manuscript, such as Fig S8-S11.\nResponse: We sincerely apologize for our oversight and thank the reviewer for pointing it out.\nSupplementary Figs. 9–23 have now been referenced in the manuscript and highlighted in red.\nQuestion 6: Line 237, “The average pore size of the TpPa-1@PES@GF membrane decreased to 0.2287 μm (Supplementary Fig. 12b), confirming that the COF nanocrystals were present both on the topmost polymer surface of the COF-MMMs and on the PES matrix surface within the GF substrate.” I do not quite understand how the decreased average pore size could lead to the position of COF.\nResponse: We sincerely thank the reviewer for careful reading.\nThis conclusion is a reasonable inference drawn from the integration of multiple characterization techniques, including porometry analysis, SEM, and TEM. In the NIPS-triggered in situ IP strategy, the COF (formed by the polymerization of Tp and Pa-1 monomers) not only forms at the outermost surface of the membrane (air/polymer interface) but also polymerizes and grows on the inner surfaces of various pores (such as finger-like and spongy pores) formed within the PES matrix during phase separation. The observed reduction in average pore size is directly attributed to the in situ formation and growth of COF nanocrystals on the inner pore surfaces, effectively creating a coating layer on the original pore walls and thereby reducing the pore diameter. As a result, extensive COF formation within the internal pores of the membrane leads to a decrease in the overall average pore size.\nWe have provided multiple direct experimental evidences. SEM images (Figure 2h) show irregular COF structures covering the walls of finger-like pores in the polymer matrix, visually confirming the formation of COF within the membrane. HRTEM images (Figures 2l, n, o) further demonstrate the presence of uniformly distributed COF nanocrystals not only on the inner surfaces of spongy pores (Figure 2l) but also on the inner surfaces of finger-like pore walls (Figure 2n) and in regions beneath the dense skin layer (Figure 2o).\nTherefore, the reduction in average pore size is attributed to the widespread distribution of COF both on the surface and within the interior of the membrane. According to your suggestions, the discussion has been revised and marked in red in order to improve the manuscript: “The average pore size of the TpPa-1@PES@GF membrane decreased to 0.2287 μm (Supplementary Fig. 13b), confirming that the COF nanocrystals were present both on the topmost polymer surface of the COF-MMMs and on the PES matrix surface within the GF substrate.” have been changed into “The decrease in the average pore size of the TpPa-1@PES@GF-72 membrane to 0.2287 μm (Supplementary Fig. 16b) was caused by the presence of COF nanocrystals on both the topmost polymer surface of the COF-MMMs and on the PES matrix surface within the GF substrate, as confirmed by SEM and HRTEM observations.” (Page 15, lines 287-292).\nWe hope that the above clarifications and the corresponding revisions in the manuscript have fully addressed your concerns. Your insightful feedback has significantly enhanced the scientific rigor of our paper.\nQuestion 7: Line 258, authors state “In addition, after the TpPa-1@PES@GF membrane was subjected to external damage tests, such as adhesive tape tearing (Fig. 2s-t), the structural integrity of the membrane layer remained intact”, gas permeation performance of the membranes after adhesive tape tearing needs to be evaluated to confirm the structural integrity of the membrane layer.\nResponse: Thank you for your valuable feedback. We have supplemented the experimental data with gas permeation tests to verify the structural integrity of the membrane after the tape peeling test.\nGas separation performance tests were performed on the TpPa-1@PES@GF-72 membrane after tape peeling. The original membrane exhibited an H /CO selectivity of 85.3 and an H permeance\n2 2 2\nof 2741 GPU. After peeling, the H /CO selectivity measured 74.3, while the H permeance\n2 2 2\nincreased to 3034 GPU.\nThe slight increase in H permeance and the moderate decrease in H /CO selectivity after peeling\n2 2 2\ncan be attributed to the introduction of non-selective defects at the micron or submicron scale within the polymer matrix or support layer due to external physical damage. Alternatively, some previously closed finger-like pores may have reopened, creating additional transport pathways not controlled by the COF layer. Moreover, the peeling process may have subtly affected the arrangement or porosity of the top-layer COF nanocrystals, sufficient to cause changes in permeation rates. Most critically, even after undergoing such rigorous physical damage, the membrane retains a high H /CO selectivity of 74.3, which is substantially higher than that of the\n2 2\npristine PES membrane (17.4) and markedly surpasses conventional blended COF-MMMs (33.2), maintaining its position within the performance range of high-quality gas separation membranes. This strongly demonstrates that the ultrathin COF separation layer and the seamless interface between the well-dispersed COF nanocrystals and the PES matrix retained structural integrity and functional stability during the tape-peeling test. The introduced minor damage primarily occurred in the non-selective support layer without compromising the separation structure. The supplementary data have been included as Supplementary Fig. 20 in the revised manuscript and referenced in the main text: “To quantitatively assess the functional integrity of the membrane layer after mechanical damage, we compared the gas separation performance before and after the adhesive tape tearing test. The H /CO selectivity of the membrane after testing was measured at\n2 2\n74.3 (Supplementary Fig. 20). Although this reflects a reduction from the original value of 85.3, it remains significantly higher than that of the pristine PES@GF membrane (17.4) and the selectivity of COF-MMMs fabricated via the conventional blending method (33.2). This indicates that the core separation layer remained intact under physical damage, and the minor performance variation likely stems from a slight increase in non-selective pores within the support layer.” (Page 16, lines 312–322).\nSupplementary Fig. 20 Permeance and H /CO separation selectivity of TpPa-1@PES@GF-72\n2 2\nmembrane before and after detachment with tape.\nWe believe this experiment further strengthens the rigor and persuasiveness of the conclusion regarding the membrane’s excellent mechanical stability.\nReviewer #2 (Remarks to the Author):\nComments: This study proposed a new \"non-solvent induced phase separation (NIPS)-triggered in situ interfacial polymerization (IP)\" strategy, fabricating large-scale asymmetric COF-MMMs for gas separation. The synergistic formation of the COF and the PES membrane could overcome the defects by conventional physical blending technique. It is recommended that the manuscript be accepted after revisions upon addressing the following issues.\nQuestion 1: There is a spelling mistake in Fig.1 “aqueous solution”.\nResponse: We sincerely apologize for our oversight and thank the reviewer for pointing it out. In the Fig.1, the word “Aaqueous” has been replaced with “Aqueous”.\nFig. 1 Fabrication of COF-MMMs via the NIPS-triggered in situ IP strategy.\nQuestion 2: Fig. 1a-1d was wrongly labelled in the discussion. They should be Fig. 2a to 2d. The same problem with the Fig. 1k, 1l, etc. Please check carefully throughout the manuscript.\nResponse: We sincerely apologize for the oversight and thank you for your helpful reminder. The necessary corrections have been made and are marked in red in the revised manuscript.\n“Fig. 1a” changed to “Fig. 2a”. “Fig. 1b” changed to “Fig. 2b”. “Fig. 1c” changed to “Fig. 2c”. “Fig. 1d” changed to “Fig. 2d”. “Fig. 1h” changed to “Fig. 2h”. “Fig. 1g” changed to “Fig. 2g”. “Fig. 1k” changed to “Fig. 2k”. “Fig. 1l” changed to “Fig. 2l”. “Fig. 1m” changed to “Fig. 2m”. “Fig. 1n” changed to “Fig. 2n”. “Fig. 1o” changed to “Fig. 2o”. “Fig. 1p” changed to “Fig. 2p”.\nQuestion 3: The sequence of the discussion of Figure 2j and 2h should be reversed.\nResponse: We sincerely thank the Reviewer for careful reading. The sequence of the discussion of Fig. 2j and 2h have been reversed and marked in red in the revised manuscript (Page 11, lines 209-216).\nQuestion 4: The article proposes a synergistic mechanism consisting of \"Knudsen diffusion, selective adsorption, and interlayer molecular sieving\". Please further elaborate on which mechanism plays the key role in achieving high H /CO selectivity?\n2 2\nResponse: We thank the reviewer for this insightful question regarding the key mechanism behind the high H /CO selectivity.\n2 2\nThrough gas separation experiments and simulation calculations, the proposed synergistic separation mechanism in this study comprises three components: Knudsen diffusion within the PES matrix, selective adsorption in COF regions, and molecular sieving through COF interlayer spacing. These three elements work collectively to achieve efficient H /CO separation.\n2 2\nWe contend that these mechanisms form an inseparable synergistic system where their individual contributions cannot be strictly prioritized; rather, they function complementarily to collectively enable the remarkable enhancement in membrane performance.\nSpecifically, PES@GF membrane that rely solely on Knudsen diffusion demonstrate high H\n2\npermeance (4111 GPU), yet their selectivity remains quite low (H /CO of 17.4), failing to meet\n2 2\nefficient separation requirements. Should separation depend exclusively on the selective adsorption of COFs, the absence of both the rapid transport pathways provided by the PES matrix and the dynamic sieving effect of COF interlayer spacing would likely lead to a substantial reduction in overall gas permeance, making it difficult to balance high selectivity with high permeability.\nGCMC simulations and structural analyses presented in this work indicate that variations in COF interlayer spacing induced by CO adsorption (the molecular sieving effect) can partially hinder\n2 2\nwith selective adsorption, ultimately creating a synergistically enhanced separation performance.\nAccording to your suggestions, the discussion has been revised and marked in red in order to improve the manuscript: “This separation mechanism achieved synergistic separation by combining Knudsen diffusion in the PES matrix, selective adsorption by the COF domains, and molecular sieving through the COF interlayer spacing to overcome “trade-off” effects” have been changed into “This separation mechanism achieved highly efficient H /CO separation by\n2 2\ncombining Knudsen diffusion in the PES matrix, selective adsorption by the COF domains, and molecular sieving through the COF interlayer spacing. These three mechanisms are interrelated and work synergistically, collectively overcoming the “trade-off” effects in membrane separation processes, thereby enabling the COF-MMMs to simultaneously possess both high selectivity and high permeance” (Page 24, lines 467-473).\nQuestion 5: Is there a significant chronological order between the occurrence of \"solvent exchange\" and \"interfacial polymerization\"? Please explain in detail the migration of Tp monomers before and after the solidification of the polymer matrix: does it instantaneously \"fix\" on the inner surface of the finger-like pores during phase separation, or continue to diffuse from the interior of the matrix to the interface during subsequent immersion?\nResponse: Thank you for raising this crucial question, which helps us articulate the mechanism of the \"NIPS-triggered in situ interfacial polymerization\" dynamic process more precisely.\nIn the NIPS process, solvent exchange serves as the initial triggering event. The moment the casting solution is immersed in the aqueous phase (non-solvent bath), rapid bidirectional diffusion between the solvent (NMP) and non-solvent (water) begins. This process simultaneously brings Pa-1 monomers from the aqueous phase to the reaction interface and provides the driving force for the migration of Tp monomers toward the interface, thereby triggering interfacial polymerization. Thus, at the onset, both processes occur simultaneously. However, the duration of these two processes differs. Liquid-liquid phase separation and polymer solidification are relatively rapid, whereas the interfacial polymerization and crystallization of COFs proceed as a slower kinetic process. Therefore, interfacial polymerization continues for an extended period even after the polymer matrix network has largely solidified.\nThe migration of Tp monomers is a “phased and constrained” process, rather than a single, instantaneous fixation or free diffusion. We propose that the migration behavior of Tp monomers changes significantly before and after phase separation solidification, which can be specifically divided into two stages:\nStage 1: During the initial polymer solidification, the system remains in a liquid state. At this point, molecular diffusion driven by the solvent/non-solvent concentration gradient is intense and rapid. In this liquid environment, Tp monomers can migrate relatively freely. A portion of the Tp monomers are carried by the rapid solvent outflow to the outermost surface, where they react swiftly with Pa-1 to form a dense top-layer COF membrane. Simultaneously, another portion of Tp monomers enters the inner surfaces of finger-like pore channels along with the solution. During this process, Tp monomers do indeed become instantaneously \"fixed\" on the forming inner surfaces of structures such as finger-like pores, providing sites for subsequent reactions.\nStage 2: After the polymer matrix solidifies, the fluidity of the entire system drops drastically. At this stage, the diffusion of Tp monomers trapped within the polymer chain network becomes extremely difficult. However, this does not mean the reaction stops. Tp monomers that have already reached or are very close to the inner surfaces of the pore channels in Stage 1 can continue to react with Pa-1 monomers that persistently diffuse in through the aqueous phase, allowing the COF nanocrystals to gradually grow and mature. Meanwhile, Tp monomers encapsulated within the polymer network may undergo very short-range diffusion and rearrangement through local chain relaxation or within tiny pores, contributing to the formation of COF nanocrystalline grains.\nAccording to your suggestions, the discussion has been revised and marked in red in order to improve the manuscript: “Concurrently, the Tp precursor was induced to migrate toward the PES surface by diffusion of the polymer solution and polymerization of Pa-1. As the polymer phase solidified, Tp became uniformly embedded within the polymer surface and underwent in situ IP with the Pa-1 monomer in the aqueous phase to form a dense and ultrathin COF film. Notably, during solvent exchange, Tp precursors were also distributed along the internal surface of the finger-like pores within the polymer matrix and polymerized to form COF nanocrystals.” have been changed into “Concurrently, during the initial liquid-state stage of NIPS, the rapid solvent/non-solvent interdiffusion and the polymerization of Pa-1 in the aqueous phase facilitated the migration of Tp precursors toward the PES surface. These Tp precursors then underwent an in-situ interfacial polymerization (IP) reaction with Pa-1 monomers that had diffused to the PES surface from the aqueous phase. At the same time, a portion of the Tp monomers was carried by the solution to the internal surfaces of the finger-like pores. As the polymer phase gradually solidified, the fluidity of the entire system dropped drastically. The diffusion of Tp monomers trapped within the polymer chain network became severely restricted. However, this did not mean the reaction stopped. The Tp monomers that had reached or were in close proximity to the pore surfaces could still react with Pa-1 monomers that continuously diffused in from the aqueous phase, forming a dense, ultrathin COF film. The Tp monomers encapsulated within the polymer network likely formed COF nanocrystallites through local chain relaxation or via very short-range diffusion and rearrangement within microscopic pores.” (Page 6, lines 125-140).\nQuestion 6: Compared to the membrane prepared by the traditional blending method (H /CO\n2 2\nselectivity of 33.2), the selectivity of the new method increased by 162%. Please provide an in-depth analysis of the main reasons for this significant improvement.\nResponse: We thank the reviewer for this critical question, which allows us to highlight the fundamental advantages of our NIPS-triggered in situ IP strategy.\nThe remarkable performance enhancement can be attributed to the combined effects of \"the formation of a defect-free ultrathin COF separation layer at the top surface\" and \"the highly dispersed COF nanocrystals with excellent interfacial compatibility within the matrix.\" The defect-free ultrathin COF layer serves as the primary selective separation layer, effectively transforming the non-selective polymer interfaces commonly found in conventional MMMs into functional COF layers with molecular sieving capabilities. The existence of this continuous layer is clearly demonstrated by SEM and HRTEM images (Fig. 2b, k in the original text). Its defect-free nature directly contributes to the significant improvement in H₂/CO₂ selectivity. Meanwhile, the well-dispersed COF nanocrystals within the matrix provide additional selective adsorption sites, further substantially enhancing the separation selectivity.\nA clarification was added in the revised manuscript and marked in red in order to improve the manuscript: “In contrast, the COF-MMMs achieved 162.05% greater selectivity” have been changed into “In contrast, NIPS-triggered in situ IP strategy ensures the in situ formation of highly dispersed COF nanocrystals and a seamless COF-polymer interface, which is the key to the 162.05% greater selectivity, with a mere 11.76% permeance reduction (Fig. 3e), exceeding the 2008 Robeson upper bound.” (Page 20, lines 383-387).\nQuestion 7: GCMC simulations show that TpPa-1 has a strong adsorption affinity for CO .\n2\nvery high level despite the presence of strong competitive CO adsorption.\nThe fact that H permeance remains high despite strong CO adsorption is not a contradiction but\n2 2\nrather the key to the high selectivity. This phenomenon can be explained by the distinct transport behaviors of H and CO , as revealed by combining our GCMC simulations with the membrane's\n2 2\nasymmetrical structure. While strong CO adsorption does reduce its diffusion rate through COF\n2\nchannels, the overall gas permeance of the membrane is not determined solely by the COF layer, but rather by the integrated system comprising the PES porous matrix and the COF layer.\nThe PES matrix, with its macroporous and finger-like pore structure, offers extremely low resistance to gas transport, providing a high baseline gas permeance (H permeance > 4000 GPU\n2\nfor pure PES membrane). After constructing an ultrathin (approximately 15-30 nm), defect-free COF layer on the surface, the additional transport resistance introduced is limited. This ultimately manifests as only an 11% decrease in permeance compared to the PES baseline, while achieving a 162% enhancement in selectivity.\nIn essence, the GCMC simulations explain how the COF layer provides high selectivity, while the asymmetric structure fabricated via NIPS ensures high gas permeance.\nA clarification was added in the revised manuscript and marked in red in order to improve the manuscript: “The PES matrix provided low-resistance transport pathways for rapid gas permeation, ensuring high gas flux (Page 17, lines 331-332).”\nQuestion 8: The article demonstrates the TpPa-1 system. Have the authors considered whether this method is applicable to other COF systems?\nResponse: We thank the reviewer for raising this pertinent question regarding the applicability of our method to other COF systems. We fully agree that the generality of a new fabrication strategy is a key indicator of its significance.\nIn fact, we have systematically evaluated the feasibility of extending this methodology to other COF systems and have formally incorporated it into our research agenda. Preliminary investigations have been conducted on COF TpPa-1 within alternative polymer matrices, including PVDF and polyimide. Furthermore, we have successfully synthesized other COF variants, such as COF TpHz, COF-LZU-1, and COF TpTAPB, through interfacial polymerization, and are currently assessing their compatibility with the NIPS-triggered in situ interfacial polymerization strategy.\nThis evaluation involves multiple critical considerations. Theoretically, this approach can be applied to any COF system suitable for interfacial polymerization, provided that one monomer is soluble in the polymer casting solution (organic phase) while the other remains soluble in the non-solvent bath (aqueous phase). A representative example includes imine-linked COFs, where aldehyde monomers dissolve in NMP and amine monomers are water-soluble, making them particularly suitable candidates for this strategy. We are actively advancing the application of this methodology to additional COF systems in our ongoing research.\nA clarification was added in the revised manuscript and marked in red in order to improve the manuscript: “It should be pointed out that several other types of COFs have been recently synthesized via interfacial polymerization as well. Indeed, further studies on the fabrication of other COF-MMMs for gas separation are currently underway in our laboratory” (Page 24, lines 473-476).\nQuestion 9: The authors compare the membrane performance of this work with membranes prepared by the \"traditional blending method\" and conclude a 162% performance improvement. However, the COF nanoparticles used in the blending method are \"pre-synthesized\".Could the difference in COF morphology affect the performance comparison?\nResponse: We sincerely thank the reviewer for this critical and insightful question. The reviewer is absolutely correct to highlight that the difference in COF morphology (highly dispersed nanocrystals in situ formed versus pre-synthesized particles) is a key factor in the performance comparison. In fact, we argue that this morphological difference is not a confounding variable but is the direct and intentional consequence of our novel fabrication strategy, and it constitutes the very reason for the performance improvement.\nThe COFs prepared by the two methods indeed exhibit inherent differences in morphology and size, which fundamentally result from their distinct preparation routes.\nThe fundamental limitation of the conventional blending method lies in its difficulty in incorporating ultrasmall nanocrystals (e.g., 4-8 nm). Pre-synthesized nanocrystals are prone to irreversible aggregation during drying, storage, and membrane casting processes, leading to the incorporation of larger aggregates rather than highly dispersed individual nanocrystals into the membrane.\nIn contrast, the NIPS-triggered in situ IP strategy effectively overcomes the issues of dispersion and interfacial compatibility inherent in traditional methods, thereby enabling enhanced separation performance. This comparison reasonably reflects the morphological improvements achieved through the optimization of the preparation methodology.\nA clarification was added in the revised manuscript and marked in red in order to improve the manuscript: “This inferior performance is likely attributable to the unavoidable aggregation of pre-synthesized COF nanoparticles and the poor interfacial adhesion, which lead to non-selective defects, as commonly encountered in traditional MMMs” (Page 19, lines 376-379).\n“In contrast, the COF-MMMs achieved 162.05% greater selectivity” have been changed into “ In contrast, NIPS-triggered in situ IP strategy ensures the in-situ formation of highly dispersed COF nanocrystals and a seamless COF-polymer interface, which is the key to the 162.05% greater selectivity, with a mere 11.76% permeance reduction (Fig. 3e), exceeding the 2008 Robeson upper bound.” (Page 20, lines 383-387).\nReviewer #3 (Remarks to the Author):\nComments: The authors propose a pioneering non-solvent induced phase separation\n(NIPS)-triggered in situ interfacial polymerization (IP) strategy for fabricating centimeter-scale\nasymmetrical COF-mixed matrix membranes (COF-MMMs). This approach uses the polymer\nmatrix as both a solid solvent and an IP platform, effectively addressing the long-standing issues\nof filler aggregation and poor interfacial compatibility in traditional MMM preparation. However,\nthe manuscript has several weaknesses as follows:\nQuestion 1: This manuscript lacks innovation. The textual description is rather ordinary, and no obvious innovative aspects are identified.\nResponse: We sincerely thank the reviewer for this critical feedback. We have carefully considered this point and have made substantial efforts in the revised manuscript to strengthen and clarify the core innovative aspects of our research. We acknowledge that our initial submission may not have highlighted our contributions clearly enough.\nIndeed, the novelty of this work is primarily demonstrated in the following aspects:\n1. For the first time, the asymmetrical and seamless COF-MMMs extending over the centimeter scale for highly efficient H /CO separation were fabricated via a non-solvent induced phase\n2 2\nseparation (NIPS)-triggered in situ interfacial polymerization (IP) strategy.\n2. A dense and ultrathin COF membrane with a thickness of 15‒30 nm was fabricated on the surface of the PES skin layer, and isolated COF nanocrystals with a particle size of 4‒8 nm were formed and highly dispersed inside the PES matrix with excellent interfacial compatibility.\n3. The COF-MMMs exhibited an excellent H /CO selectivity of 88.8 ± 2.46, while maintaining a\n2 2\nhigh H permeance of 2738 ± 58.02 GPU.\n2\n4. The COF-MMMs exhibited remarkable thermal stability even increasing temperature to 423 K and long-term operational durability for 40 h.\n5. The grand canonical Monte Carlo (GCMC) simulation revealed that a synergistic separation mechanism of COF-MMMs comprising Knudsen diffusion in the PES matrix, selective adsorption by the COF domains, and molecular sieving through the COF interlayer spacing.\nFurthermore, we have sincerely incorporated the constructive suggestions from the other two reviewers and have carefully revised and supplemented the manuscript accordingly. We hope that the clarifications provided above, along with the corresponding revisions, have fully addressed your concerns. Your insightful feedback has significantly enhanced the scientific rigor of our paper.\nThe primary modifications and enhancements are marked in red within the text. We sincerely hope that the revised version meets your expectations and kindly request your re-evaluation.\nQuestion 2: While the authors cite relevant studies on COF membranes, MMMs, and NIPS/IP methods, there are notable gaps in the literature coverage. For instance, recent (post-2022) breakthroughs in COF-based MMMs for gas separation, such as novel in situ polymerization strategies or advanced interfacial modification techniques, are not adequately discussed. Additionally, the comparison with other emerging membrane materials (e.g., metal-organic framework (MOF)-based MMMs with similar or superior performance) is insufficient. This incomplete literature review limits the authors' ability to fully contextualize their work within the latest research landscape and highlight the unique advantages of their strategy over all competing approaches.\nResponse: We sincerely thank the reviewer for this invaluable feedback regarding the literature coverage. We agree that situating our work within the context of the most recent advancements is crucial for accurately highlighting its significance. We apologize for the oversight and have undertaken a comprehensive revision of the introduction and discussion sections to address this point.\nWe have now incorporated a discussion on the latest advancements in MOF-based MMMs. This includes citing and discussing recent studies on novel in situ polymerization strategies (Science, 2023, 381, 1350-1356) that have been published since 2022. “Jin et al. proposed an innovative “solid-solvent processing” strategy to fabricate an ultrathin (< 100 nm), highly loaded (80.4 vol%) mixed-matrix membrane (MMM). In this strategy, the polymer acts as a solid solvent to dissolve metal salts, forming an ultrathin precursor layer that immobilizes the metal salt and regulates its conversion to a metal-organic framework (MOF), thereby providing adhesion to the MOF in the matrix. The resultant membrane exhibits remarkable H /CO separation performance, with a\n2 2\nhydrogen permeance of 3640 GPU and a H /CO selectivity of 76.1”. (Page 3, lines 58-67). These\n2 2\nnew citations help to establish the current state-of-the-art and demonstrate the rapid evolution of this field.\nIn response to your advice, we have supplemented the revised manuscript by adding a direct comparison with high-performance MOF-based membranes and citing key recent works on MOF-MMMs. This comparison allows us to more clearly articulate the rationale behind our selection of COF fillers and, within the context of competitive material platforms, better highlight the unique advantages of our specific membrane strategy (Fig. 4b, Supplementary Table S1).\nFig. 4 (a) Kinetic diameter-dependent single-gas permeance through the TpPa-1@PES@GF-72 membrane and corresponding gas pair selectivity and (b) comparison of the separation performance of previously reported results with those of the TpPa-1@PES@GF membrane (detailed in Supplementary Table 1). The solid black line represents the 2008 upper bound for polymeric membranes for H /CO separation, with permeability converted to permeance assuming\n2 2\na 0.1 μm thick selective membrane.\nSupplementary Table S1 Representative examples of COF-based membranes and MOF-based membranes for H\n2\n/CO\n2\nseparation.\nSubstrat PH2 a) SH2\nMembrane Methods OPc) Ref.\ne (GPU) /CO2 b)\n298K,\nM\nZJU-74a In situ growth α-Al2O3 206 100 33\n2bar\nO\nF\nZr-MOF Confined-coordin α-Al2O3 1200 67 298K, 34\nation induced 1bar\n303K,\nPEI/PI@ZIF-67-3 In situ growth PINM 9.62 9.3 35\n1bar\nZIF-8 2653.7 17.1\nSFSC - RTd) 36\nZn(EtIm)2 6268.7 22.1\nZIF-L 2597 31.1\n298K,\nIn situ growth PSf 37\n1bar\nZn2-ZIF-L 2202 33.6\n298K,\nCo-Zn-ZIF-L IP PSf 2156 31.6 38\n1bar\nIn situ interfacial 298K,\nZn2(bim)4 α-Al2O3 6300 86.5 39\nassembly 1bar\nSpin-coating/blad 298K,\nUiO-66/gel/PEI PAN 845 16.8 40\ne casting 1bar\n313K,\nPAA/(PVP+MOF) LbL assembly Anodisc 1.2 20.3 41\n1bar\n298K,\nZIF-8-5 Contra-diffusion mPSF 5225 26.6 42\n1bar\nCu(SiF6)(pyz)3@PEG 298K,\n3640 76.1\nMMM 1.5 bar\nCu(SiF6)(pyz)3@PVA\nSSP PAN 3000 30 393K, - 43\nMMM\nNi(NbOF5)(pyz)3@PVA 393K,\n115 32.7\nMMM 1 bar\nZIF-L@PVA MMMs SSP PAN 622.5 21.1 298K, 44\nZIF-8@PMMA MMMs Nylon 444 18.3 1.5 bar\nMIS-ZIF-L ISIA α-Al2O3 4033 321 45\n-\nZIF-8/GO In-situ conversion α-Al2O3 406 25 46\n-\nsoc-MOF Seeded Growth Ceramic 120 48 47\n-\nCo-gallate MOF Electrodeposition AAO 124 67.8 298K, - 48\nZIF-8 Electrodeposition AAO 439 69.1 298K, - 49\nMOF-801 Counter diffusion PVDF 119 66.3 298K, - 34\nZIF-90-NH2/6FDA-Durene 308K,\nBlending - 487 35.8 50\nMMM 5 bar\nThese additions and revisions place our work within a broader and more forward-looking research context, while also more clearly articulating its advantages and uniqueness compared to existing and emerging competing approaches. Once again, we sincerely appreciate your guidance, which has significantly improved the quality and impact of our paper.\nQuestion 3: Although the authors report the average values and standard deviations of separation performance (e.g., 88.8 ± 2.46 for H /CO selectivity), they provide limited information on the\n2 2\nreproducibility of membrane fabrication. There is no detailed description of the number of independent membrane preparations (e.g., n = 3 or more) or the variation in performance across different batches. In high-impact journals like Nature Communications, rigorous reproducibility data is essential to confirm that the reported performance is not an outlier and that the fabrication method is reliable for widespread adoption.\nResponse: We sincerely appreciate you for raising this critical question. More detailed membrane reproducibility data are entirely reasonable and necessary for validating the reliability and scientific soundness of our approach.\nFollowing your suggestion, we independently prepared ten TpPa-1@PES@GF-72 membranes (n\n= 10) under identical conditions and evaluated their gas separation performance. The average\nH /CO selectivity across all membranes was 84.18 with a standard deviation of ±11, while the\n2 2\naverage H flux was 2699 with a standard deviation of ±222.72. The originally reported values fall\n2\nwithin these ranges.\nWe have incorporated these detailed reproducibility data into the revised manuscript as a supplementary table (Supplementary Table S2). Additionally, the following statement was added in the text: “Furthermore, the excellent reproducibility demonstrated by tests on ten independent membranes highlights the advantages of the NIPS-triggered in situ IP strategy in fabricating high-performance, highly stable COF-MMMs (Supplementary Table S2).” (Page 22, lines 441-444).\nSupplementary Table S2 H\n2\n/CO\n2\nselectivity of the TpPa-1@PES@GF-72 membrane.\nMembrane\nPH2 (GPU) PCO2 (GPU) SH2/CO2\n1 2681 31.28 86.5\n2 2765 28.96 95.8\n3 2908 37.35 79.3\n4 2576 31.38 81.3\nTpPa-1@PES@GF-72\n5 2680 37.56 73.1\nmembrane\n6 2796 37.06 76.9\n7 2938 44.72 67.9\n8 2262 22.91 98.7\n9 2942 34.84 85.3\n10 2442 25.22 97\nAverage 2699 ± 222.72 33.34 ± 6.87 84.18 ± 11\nWe believe that these supplementary reproducibility data strongly demonstrate that the NIPS-triggered in situ IP strategy is a reliable, stable, and repeatable fabrication method. The high performance achieved is not incidental, thereby providing solid data support for the robustness of our conclusions. Once again, we thank you for helping us enhance the scientific rigor of this paper.\nQuestion 4: While the authors study the effect of temperature on membrane performance, other critical operating parameters that are relevant to industrial applications are not thoroughly investigated. For example, the influence of feed gas pressure (beyond 1 bar) on separation performance is not reported, even though pressure is a key factor in industrial gas separation processes (e.g., natural gas processing, hydrogen purification).\nResponse: We sincerely thank the reviewer for raising this insightful and valuable point. You correctly pointed out that the influence of feed pressure is a key parameter for assessing the practical application potential of membrane materials in industrial gas separation processes, such as natural gas treatment and hydrogen purification.\nFollowing your suggestion, we conducted supplementary studies on the effect of feed pressure on the separation performance of the TpPa-1@PES@GF-72 membrane. However, due to the inherently ultra-high gas flux of this membrane, achieving and maintaining a high, stable feed pressure (e.g., > 1 bar) in our experimental system presented technical difficulties, as the high gas permeation rate made it challenging to sustain a significant pressure gradient.\nDespite this challenge, we successfully performed separation performance tests at a slightly elevated feed pressure of 0.02 MPa (0.2 bar). The experimental results indicate that even at this low pressure: The H permeance significantly increased to approximately 8000 GPU. Meanwhile,\n2\nthe H /CO selectivity remained around 15.\n2 2\nSupplementary Fig. 22 Permeance and separation selectivity of the TpPa-1@PES@GF-72\nmembranes at 0.02Mpa.\nWe fully acknowledge that a more comprehensive pressure-dependence study would be ideal. However, in industrial gas separation, the primary purpose of applying feed pressure is typically to enhance the transmembrane mass transfer driving force, thereby increasing gas flux. Our membrane inherently possesses an ultra-high flux even at near-atmospheric pressure, which is a desirable characteristic as it can significantly reduce the membrane area and capital cost required for a specific separation task.\nMore importantly, the membrane's ability to maintain a H /CO selectivity of 15 under such high\n2 2\nflux conditions (8000 GPU) is, in our view, remarkable. In many high-flux membranes, a significant trade-off between permeability and selectivity is often observed, particularly when the driving force is increased. The fact that our TpPa-1@PES@GF-72 membrane can deliver an ultra-high permeance while maintaining considerable selectivity underscores its potential for applications requiring high throughput.\nIn summary, while we agree that broader pressure stability is an important aspect for future investigation, the results obtained at 0.02 MPa convincingly demonstrate the exceptional performance of our membrane, characterized by an excellent combination of ultra-high flux and satisfactory selectivity under practical operating driving forces. We have incorporated these results and the corresponding discussion into the revised manuscript: “Moreover, the influence of feed pressure on the separation performance of the TpPa-1@PES@GF-72 membrane was also explored. Notably, the membrane's ultra-high permeance presented a technical challenge for maintaining stable high feed pressures (>1 bar) due to rapid pressure equalization across the membrane. However, at a moderately elevated feed pressure of 0.02 MPa (0.2 bar), the H permeance surged\n2\nto approximately 8000 GPU, underscoring the immense gas flux capability of the membrane. Concurrently, the H /CO selectivity stabilized at around 15. The observed reduction in selectivity\n2 2\nmay be attributed to enhanced non-selective viscous flow and subtle structural adjustments under applied pressure. Nevertheless, the selectivity remained significantly above the Knudsen limit, confirming the persistence of selective transport mechanisms. The COF-MMMs achieved an optimal balance between ultra-high permeance and considerable selectivity, highlighting their promising potential for high-flux separation applications (Supplementary Fig. 22).” (Page 21,\nLines 424-438).\nWe sincerely believe that this supplementary study not only addresses your question but also deepens our understanding of the developed membrane material and more precisely defines its most competitive application areas. We thank the reviewer again for raising this important point, which prompted us to undertake this enlightening exploration.\nQuestion 5: The authors mention that hydrogen bonding between the -OH groups of Tp precursors and the -O-/-SO - groups of PES, along with weak van der Waals interactions, contributes to good\n2\ninterfacial adhesion. However, they provide limited direct evidence to support these interactions. Advanced characterization techniques, such as X-ray photoelectron spectroscopy (XPS) for chemical state analysis at the interface or atomic force microscopy-based force spectroscopy (AFM-FS) for quantifying interfacial adhesion forces, are not employed. This lack of direct evidence weakens the argument for the formation of a seamless COF-polymer interface and limits the understanding of the interfacial interaction mechanism.\nResponse: We sincerely thank the reviewer for this excellent suggestion, which has guided us to obtain direct and compelling evidence to solidify our proposed interfacial interaction mechanism. We fully agree that advanced characterization is crucial for understanding the interface. In direct response to the reviewer's comment, we have now employed both X-ray photoelectron spectroscopy (XPS) and atomic force microscopy-based force distance spectroscopy (F-D) to probe the interfacial interactions.\nWe performed high-resolution XPS analysis on PES@GF, Tp@PES@GF, TpPa-1@PES@GF-72, and TpPa-1/PES/GF membranes prepared via the conventional blending method (Supplementary Fig. 7). We have revised the discussion and marked in red in the main text: “X-ray photoelectron spectroscopy (XPS) provided direct evidence for the formation of a dense COF membrane on the top. The strong characteristic peak observed at 399.8 eV in the N 1s spectrum was unambiguously assigned to nitrogen in the imine bond (-C=N-), directly confirming the successful condensation reaction between Tp and Pa-1. Meanwhile, the S 2p signal of PES at 168.0 eV was significantly attenuated in the TpPa-1@PES@GF membrane, demonstrating the formation of a dense COF covering membrane (Supplementary Fig. 7). The COF-MMMs prepared by the conventional blending method exhibited only an extremely weak nitrogen signal, providing further evidence that the COF membrane was successfully constructed on top of the PES substrate rather than through simple physical blending.” (Page 10, Lines 192-203).\n“This interaction was further corroborated by X-ray photoelectron spectroscopy (XPS) analysis. The O 1s spectrum revealed a significant intensity enhancement of the C–O characteristic peak near 533.0 eV, attributed to phenolic hydroxyl and ether oxygen groups, indicating the successful incorporation of Tp molecules into the PES matrix. More critically, the binding energy of the O=S=O component in the sulfone group shifted positively from 531.4 eV to 531.7 eV, while the binding energy difference between this component and the ether oxygen peak decreased from 1.93 eV to 1.45 eV. These systematic binding energy shifts and the observed energy gap narrowing can be attributed to the decreased electron density around oxygen atoms induced by hydrogen bonding formation, thereby providing direct evidence for the existence of hydrogen bond interactions between PES and Tp (Supplementary Fig. 7).” (Page 13, Lines 249-261).\nSupplementary Fig. 7 XPS Spectra of the PES@GF, Tp@PES@GF, TpPa-1@PES@GF, and TpPa-1/PES/GF membranes.\nAFM force-distance spectroscopy (Supplementary Fig. 14) characterized the adhesion force on the COF membrane surface, offering direct evidence for the non-covalent interfacial adhesion that promotes the formation of a seamless interface. We have revised the discussion and marked in red in the main text: “weak van der Waals interactions between PES and the COF further enhanced the interfacial adhesion” have been changed into “The AFM force-distance curve (F-D) measurements revealed a notably high interfacial adhesion force of 2.863 nN, which can be ascribed to the abundant and effective weak van der Waals interactions between PES and the COF (Supplementary Fig. 14).” (Page 14, lines 272-275).”\nSupplementary Fig. 14 AFM Force-Distance Curve of the (a) PES@GF membrane and (b) TpPa-1@PES@GF-72 membrane.\nWe are sincerely grateful for your feedback, which prompted this valuable analysis. All new data, corresponding figures, and discussions have been added to the revised manuscript (marked in red). We are confident that this new evidence greatly strengthens the robustness and persuasiveness of our findings concerning the “interfacial interactions”.\nReviewer #1 (Remarks to the Author):\nThe revision has addressed my comments appropriately so I support acceptance of this paper.\nResponse: Thank the reviewer for the highly positive remarks and support of publication of this work.\nReviewer #2 (Remarks to the Author):\nThe authors have well responded the comments and carefully revised the manuscript. I think that all issues have been addressed. I support its publication.\nResponse: Thank the reviewer for the highly positive remarks and support of publication of this work.\nReviewer #3 (Remarks to the Author):\nThe innovativeness of this article needs to be further refined, and performance data does not fall into the category of the article's innovation.\nResponse: Thank you for your thorough review and valuable comments on our manuscript. We have carefully read and fully understood your point regarding the need to further refine the article's innovation and that performance data itself does not constitute innovation. We completely agree with your view. Performance data are not the innovation, but rather the results delivered by an innovative method. Accordingly, we have systematically refined and restated the “innovativeness” section, focusing on the originality of the method design, breakthroughs in mechanistic understanding, and synergistic strategies in interface engineering, rather than merely emphasizing performance metrics.\nWe have now clearly summarized the innovative points into the following three aspects and highlighted them at the end of the Introduction and in the Conclusion section:\n1. Methodological Innovation\nProposed a “NIPS-triggered in situ IP” strategy, which organically combines non-solvent induced phase separation with in situ interfacial polymerization to achieve controllable and uniform growth of a COF membrane on the polymer matrix surface.\nDual functionalization of the polymer matrix: It acts not only as mechanical support but also as a “solid solvent” and “reaction platform” addressing issues of filler agglomeration and interfacial defects common in traditional methods.\n2. Synergistic Innovation in Interface Engineering\nConstructed a seamless COF-polymer interface: Through in situ polymerization coupled with physical entanglement of polymer chains and hydrogen-bonding interactions (e.g., between PES and Tp), a defect-free interfacial structure was formed.\nAchieved multi-scale structural control: A dense, ultrathin COF layer (15–30 nm) was formed on the membrane surface, while simultaneously, uniformly dispersed COF nanocrystals (4–8 nm) were anchored on the internal pore walls, resulting in an asymmetrical multilayered architecture.\n3. In-depth Elucidation of the Separation Mechanism\nProposed a “three-mechanism synergy” separation model: This model integrates Knudsen diffusion (in the PES matrix), selective adsorption (by COF domains), and molecular sieving (through COF interlayer spacing) to explain the gas separation mechanism.\nThrough GCMC simulations and experimental validation, clarified the enhancing effect of pore size dynamic contraction induced by CO adsorption on H /CO separation performance.\n2 2 2\nWe believe these revisions have more clearly articulated the scientific novelty and methodological contribution of our work. The performance data are now appropriately framed as outcomes of the innovative fabrication strategy.\nThank you again for your valuable feedback, which has significantly improved the clarity and impact of our manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revision has addressed my comments appropriately so I support acceptance of this paper.", "reviewer_2": "The authors have well responded the comments and carefully revised the manuscript. I think that all issues have been addressed. I support its publication.", "reviewer_3": "The innovativeness of this article needs to be further refined, and performance data does not fall into the category of the article's innovation."}, "author_response": ""}]

Decision letter