Ultramicroporous covalent organic framework membranes with fortified hydrogen-bond networks for high-performance desalination

Domain: chemistry
Published: 2026-02-27
Editorial outcome: major_revision

Abstract
The increasing global reliance on alternative water sources underscores the critical need for enhanced desalination efficacy. Covalent organic frameworks (COFs), with their ordered porosity and tunable architectures, hold immense potential for next-generation desalination membranes. However, current COF membranes often fail in efficient seawater desalination due to pore sizes largely exceeding hydrated monovalent ion dimensions. Here we present a structurally stable, ultramicroporous COF membrane for low-pressure reverse osmosis (RO) desalination, engineered through a hydrogen-bond fortification strategy. Rational introduction of phenolic hydroxyl adjacent to aldehyde moieties yielded β-ketoenamine configurations enriched with hydrogen bonds, promoting AB-stacking and enhanced crystallinity in Tp-Bth COF membranes. The resultant COF membranes achieved 99.6% sodium chloride rejection with 1.7 L m-2 h-1 bar-1 water permeability at 15 bar, demonstrating high-performance low-pressure RO desalination. Notably, these membranes exhibited high acid resistance, retaining their initial performance after 30 days in a solution at pH 3. This work demonstrates a hydrogen-bond-mediated strategy to precisely tailor COF pore architecture for high-performance desalination.

Introduction
Access to safe freshwater is paramount for human survival, agriculture, and sustained economic development1. However, the projected 22–34% increase in the global population by mid-century will inevitably escalate freshwater demands, directly conflicting with the diminishing availability of clean water2. This scarcity is further exacerbated by climate disruptions and pervasive anthropogenic contamination. Consequently, water desalination and recycling technologies have become increasingly critical for long-term water sustainability3,4. Reverse osmosis (RO) has emerged as the leading membrane technology for potable water production5. Early advancements focused on polyamide thin-film composite (TFC) membranes, which continue to serve as the industrial standard6,7. However, challenges in achieving ordered nanostructure and uniform pore size distribution in these membranes make it difficult to surpass the established permeability-selectivity trade-off8,9.

Covalent organic frameworks (COFs) are crystalline porous materials synthesized via the directional covalent assembly of organic monomers, intrinsically offering aligned pore channels, uniform pore sizes, and high porosity10-13. Despite these advantages, critical challenges for large-area membrane configuration lie in their intrinsically oversized pores (0.8–5 nm)14 and often-insufficient crystallinity. These structural limitations restrict COF membrane applications predominantly to organic solvent nanofiltration15,16, macromolecular dye separation17, and multivalent ion sieving18,19. Notably, achieving effective desalination with COF membranes remains a grand challenge, as the majority of sub-nanometer monovalent ions (e.g., 1–3 Å for naked ions and 6.5–8 Å for hydrated ions)20 typically bypass their pores, resulting in limited exploration of their potential in low-pressure RO desalination. Recently, Liu et al. introduced light-controlled COF membranes for pressure-driven RO seawater desalination. Despite achieving a notable 91.2% NaCl rejection, this figure falls short of the ≥ 95% industrial benchmark for practical RO systems.

Current strategies for engineering COF pore dimensions primarily involve chemical and physical modification21. Chemical approaches involve either pre- or post-synthetic modification, adjusting pore structure before or after membrane formation. Physical modification, or stacking engineering, manipulates pore aperture by altering the COF stacking mode. However, altering monomeric structure with pendant groups or side chains inevitably perturbs reaction kinetics, hindering initial COF nucleation and often leading to synthesis failure due to increased steric hindrance22,23. Moreover, precise control over synthesis parameters—including temperature, solvent, and interfacial conditions—is paramount, as these critically dictate the microstructure and crystallinity of COF membranes. Notably, such modifications frequently result in low-crystallinity COF membranes, which can diminish water permeability ascribed to reduced porosity and compromise selectivity caused by heterogeneous pore size distributions24,25.

Notably, hydrogen bonding plays a pivotal role in regulating COF architecture and functionality. By introducing hydrogen-bond donors or acceptors into the framework, it is possible to enhance inter- and intra-layer interactions, thereby refining pore geometry and improving structural order. Exploiting hydrogen bonding enables precise control of COF pore apertures for efficient molecule and ion separations. Zhu et al. achieved ultra-micropores in hydrophilic aliphatic linker-based COFs using engineered hydrogen-bonding networks26. Similarly, Ma et al. synthesized hydrazone-linked COFs with reduced pore size by modifying monomer hydroxyl sites27. Current strategies to engineer these networks primarily employ amide, acylhydrazone, urea, or β-ketoenamine linkages28-31. β-ketoenamine configurations are particularly promising, establishing extensive hydrogen-bond connectivity while ensuring structural integrity. This combination optimizes crystalline order and stability, potentially mitigating the stability-crystallinity trade-off32. The high hydrogen bond density in β-ketoenamine-linked COFs is crucial for enhanced performance.

To this end, we introduce a facile hydrogen-bond fortification strategy to fabricate ultra-microporous, high-crystalline COF membranes for high-performance water desalination. This approach significantly advances our fundamental understanding of how optimizing the integration of inter- and intra-layer hydrogen bonds— which constrain molecular rotation—can simultaneously mitigate defect formation and modulate COF stacking patterns. The key to this strategy lies in substituting 1,3,5-triformylbenzene (TFB) with 1,3,5-triformylphloroglucinol (Tp) monomers featuring phenolic hydroxyls reacted with benzene-1,3,5-tricarbohydrazide (Bth). This substitution enables the hydrazone linkages to undergo enol-keto tautomerism, leading to the formation of thermodynamically stable β-ketoenamine configurations. The concerted effects of extensive intra-layer hydrogen bonding networks and inter-layer hydrogen bonding interactions effectively restrict molecular rotation, thereby producing COFs with enhanced crystallinity. This structural transformation induces a crystallographic transition from the hydrogen bond-deficient AA-stacking observed in TFB-Bth COF to a highly desirable AB-stacking in Tp-Bth COF, which features well-aligned ultra-micropores and crystallinity. The resultant Tp-Bth COF membranes demonstrate exceptional desalination performance, achieving a 99.6% NaCl rejection and favorable water permeability, thereby far surpassing the desalting performance of advanced COF membranes. Furthermore, the abundant hydrogen-bonding networks intrinsic to the β-ketoenamine Tp-Bth COF provide enhanced structural robustness and stability in acidic conditions, underscoring its potential for sustainable water purification technologies.

Results
Design and synthesis of ultra-microporous COF nanofilms
Fig. 1a illustrates the chemical structure of TFB and Tp molecules, in which three hydroxy groups were inserted to the adjacent phenolic aldehyde. This facile hydroxyl-insertion strategy enables the formation of extensive hydrogen-bonding networks within the Tp-Bth COF film. The resultant intra- and interlayer hydrogen bonds play a critical role in confining molecular rotation, which in turn markedly improves the long-range crystalline ordering of Tp-Bth COF films. Conversely, acylhydrazone-linked TFB-Bth COF films exhibit diminished crystallinity, largely attributed to topological imperfections stemming from weak hydrogen-bonding interactions. Furthermore, the distinctive adjacent hydroxyl configuration of the Tp monomer facilitates enol-keto tautomerism upon Schiff base reaction with hydrazide, yielding a thermodynamically stable Tp-Bth COF characterized by a β-ketoenamine configuration. This structural evolution endows the Tp-Bth membrane with an AB stacking mode and precisely narrowed, uniform ultra-microporous channels, leading to a substantially improved desalination performance compared to the TFB-Bth COF counterpart.

To overcome the inherent water insolubility of Bth monomers, our strategy for fabricating free-standing COF films incorporates a facile preliminary dissolution step into the conventional interfacial polymerization (IP) process. In specific, a dichloromethane (DCM) solution containing either Tp or TFB monomers was introduced into a glass Petri dish. An acetic aqueous solution containing Bth monomers was then gently layered atop the DCM phase, which initiated the Schiff base condensation at the free interface. The inclusion of the acetic acid solution resolved the solubility issue of Bth monomers while also catalyzing the reaction. Both the resulting yellow Tp-Bth COF and white TFB-Bth COF films exhibited uninterrupted continuity and mechanical resilience, allowing for facile retrieval using a wire loop (Figs. 2a, b). These films were then systematically transferred to various substrates—including copper mesh, porous alumina, and silicon wafers—to comprehensively assess their structural and morphological characteristics.

Transmission electron microscopy (TEM) images revealed both transparent and continuous COF films devoid of identifiable defects or slits. This high integrity was further corroborated by scanning electron microscopy (SEM) images, which showed smooth, ultrathin film conformity on porous supports (Supplementary Figs. 1-2). Elemental mapping via energy-dispersive X-ray spectroscopy (EDS) confirms the homogeneous dispersion of carbon, nitrogen, and oxygen constituents across the film surface. Height profile data via atomic force microscopy (AFM) quantify the nanofilm thicknesses at 120.8 ± 0.3 nm for Tp-Bth COF and 40.4 ± 0.8 nm for TFB-Bth COF, respectively (Supplementary Figs. 3-4). The diminished thickness observed in the TFB-Bth COF nanofilm is attributable to the higher hydrophilicity of the TFB monomer (n-octanol/water partition coefficient: 4.85 for Tp vs. 1.32 for TFB). This characteristic facilitates the facile diffusion of TFB into the aqueous phase, promoting bulk phase-transfer polymerization rather than a strictly interfacial reaction33. Conversely, the more hydrophobic nature of Tp monomers restricts their reaction to the interface, thereby leading to the formation of a comparatively thicker Tp-Bth COF nanofilm. Fourier transform infrared (FTIR) spectroscopy provides crucial insights into the COF linkages (Supplementary Fig. 5). The TFB-Bth COF exhibits a distinct vibrational signature at 1674 cm-1, signifying C=N bond formation and corresponding to the imine-linked framework. In contrast, a prominent peak at 1584 cm-1 in the Tp-Bth COF confirms the formation of C=C bonds, demonstrating the formation of irreversible β-ketoenamine linkage.

To elucidate the crystalline structural disparities between the two COFs, we conducted powder X-ray diffraction (PXRD) analyses on both two nanofilms. The Tp-Bth COF exhibited distinct diffraction peaks at 6.9o and 26.8o, whereas the TFB-Bth COF showed broadened and less intense diffraction peaks, suggesting a weakly ordered structure due to structural defects. Pawley refinement confirmed that the Tp-Bth COF adopts an AB stacking arrangement. The observed diffraction peaks at 6.9o and 26.8o correspond to the (100) and (002) crystal planes, respectively, showing favorable agreement with the simulated pattern for AB stacking (Fig. 2c). However, the limited hydrogen-bonding network in TFB–Bth COF yields inherent low crystallinity. This results in closely approximated goodness-of-fit residuals (Rwp, Rp) for both the AA- and AB-stacking models, preventing an unambiguous distinction based on these metrics alone. Crucially, however, a direct visual comparison between the experimental XRD pattern and the simulated profiles (AA- and AB-stacking; Figs. 2d and Supplementary Fig. 7) exhibited a substantially match with the AA-stacking simulation. Consequently, we assign the AA-stacking mode to TFB–Bth COF as the best-supported structural interpretation available from the current data. Further elucidation of crystalline anisotropy using two-dimensional grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) revealed a marked contrast between the two COFs. The Tp-Bth COF exhibited a highly oriented (100) crystallographic plane with vertically aligned nanochannels, as evidenced by its pronounced scattering intensity at Qx,y = 0.49 Å-1 (Fig. 2e). In comparison, the TFB-Bth COF (Fig. 2f) displayed a relatively random orientation. This obvious distinction is primarily attributed to the extensive interlayer hydrogen-bonding networks within Tp-Bth COF. These networks impose supramolecular confinement effects, establishing long-range crystallographic order that markedly enhances channel regularity and minimizes defect formation.

High-resolution TEM (HRTEM) imaging of Tp-Bth COFs clearly revealed long-range ordered lattice fringes with a characteristic spacing of 0.30 nm, which is indexed to the (002) crystallographic plane (Fig. 2g). Complementary selected area electron diffraction (SAED) patterns, exhibiting bright diffraction spots, further corroborated the crystallinity of the Tp-Bth COF film. In sharp contrast, the HRTEM image of the TFB-Bth COF (Fig. 2h) shows a weakly ordered structure, with the corresponding SAED pattern (inset) displaying diffuse rings, characteristic of low crystallinity. Notably, HRTEM images acquired from multiple regions across the Tp-Bth COF (Supplementary Figs. 8-9) consistently illustrated that crystalline domains predominantly occupied the entire prepared COF nanofilms, highlighting their uniform, high-quality microstructure.

It is reported that robust hydrogen-bonding interactions often induce spectral displacements in the IR spectrum of their constituent molecular units28. To gain real-time spectroscopic evidence of these dynamic interactions under thermal perturbation, we performed variable-temperature FT-IR (VT-IR) experiments (Fig. 2i). As predicted, hydrogen bonding was disrupted at elevated temperatures, resulting in blue shifts in the stretching frequencies of the β-ketoenamine carbonyl and N–H bonds. This spectral change directly signifies an increase in the bond orders of these moieties33-35. For each VT-IR experiment, spectra were collected at temperatures ranging from room temperature up to 150 °C. In specific, the β-ketoenamine N–H peak, initially at 3066 cm−1 at room temperature, gradually shifted to 3079 cm−1 at 150 °C, an overall shift of 13 cm−1. This thermal response upon heating directly indicates the disruption of the hydrogen bonding and a resulting enhancement in the N–H bond order arising from the formation of unbound amine groups28. Similarly, the β-ketoenamine carbonyl peak at 1677 cm-1 blue-shifted by 9 cm−1 to 1686 cm−1 when heated to 150 °C. This shift indicates an increased bond order due to the formation of free amide carbonyl moieties at higher temperatures.

To further determine the structural configurations of COF nanofilms, solid-state 13C nuclear magnetic resonance (13C NMR) spectroscopy was conducted (Fig. 2j). The TFB-Bth-COF spectrum showed a clear resonance at 146.59 ppm, corresponding to the imine carbon (C=N) derived from Schiff base reaction between TFB and Bth monomers, thereby confirming its successful framework construction. Furthermore, characteristic chemical shifts at 180.17 ppm and 99.85 ppm are assigned to carbonyl (C=O) and olefinic (C=C) moieties, respectively, validating the formation of β-ketoenamine linkages in Tp-Bth COF. Supplementary Fig. 12 shows the temporal evolution of Tp-Bth COF film surface architectures under controlled synthesis durations. As the reaction time increased, the resulting films exhibited progressively denser microstructures, indicative of enhanced structural consolidation during the growth process. This structural refinement correlates with reaction time-dependent crystallization kinetics, where prolonged monomer assembly enables defect annihilation via hydrogen-bonding modulated self-correction.

Morphologies and structural characteristics of COF membranes
Building upon the synthesis of free-standing COF nanofilms, we engineered two distinct COF variants in situ atop a porous Kevlar support via IP reaction in a custom-built diffusion cell (Supplementary Figs. 13-14). The resultant Tp-Bth COF membrane presented a distinct yellow color, differentiating it from the pale-yellow Kevlar substrate, while the TFB-Bth COF membrane exhibited a grayish-white appearance (Supplementary Fig. 15). SEM images (Figs. 3a-b) showed continuous and compact surface morphologies of both COF membranes in contrast to porous Kevlar support (Supplementary Fig. 16). This validates the formation of defect-free COF layers with good mechanical flexibility.

The impact of monomeric concentration on the surface morphological structure of as-fabricated COF membranes was conducted through characterization by SEM and AFM. Evidently, elevating either Bth or Tp concentration led to a significant enhancement of nucleation densities and film compactness (Supplementary Figs. 17-19). Higher monomeric concentrations contributed to accelerated reaction kinetics that favor heterogeneous nucleation, leading to the formation of more polymeric nodules that coalesce into densely packed surface36. Similarly, prolonged reaction duration facilitated the subsequent growth and crystallization, thereby leading to the formation of well-structured and compact COF selective layers (Supplementary Fig. 20). AFM analyses further revealed a marked decrease from 19.7 nm to 8.9 nm in surface roughness when increasing the reaction time from 6 to 24 h (Supplementary Fig. 21). This decrease primarily correlates with particle growth and coalescence, which led to smoother and thicker COF layers. Likewise, the TFB-Bth membrane exhibited a more densely packed surface as increasing monomeric concentrations (Supplementary Figs. 22-24). The presence of larger polymeric nodules on the surface is likely related to higher hydrophilicity and reactivity of TFB molecules, leading to uncontrolled phase-change polymerization that yielded randomly distributed polymeric particles.

Furthermore, the as-fabricated COF membranes exhibited relatively hydrophobic surfaces, characterized by water contact angles ranging from 80o to 90o (Supplementary Figs. 25-26), showing minor dependence on monomer concentrations. This surface hydrophobicity is primarily attributed to the abundance of non-polar aromatic skeletons and the low-polarity imine linkages integrated within the framework structure. Crucially, however, the extensive hydrogen-bonding networks inherent to the COF structure can function as preferential pathways for water molecules. Overall, the observed surface hydrophobicity is unlikely to significantly impede their RO application in water desalination. Cross-sectional TEM images (Figs. 3c-d) revealed the COF layer thickness of 92.3 nm (Tp-Bth) and 88.3 nm (TFB-Bth), consistent with cross-sectional SEM data (Supplementary Fig. 27). Furthermore, cross-sectional TEM imaging was conducted on Tp-Bth COF membranes synthesized using varying reaction times. As visualized in Supplementary Fig. 28, the membrane thickness exhibits a progressive increase corresponding to the extension of the reaction time from 3 h to 12 h. Furthermore, our engineered Tp-Bth COF membranes exhibited significantly lower surface roughness (Ra = 7.26 nm) compared to Kevlar supports (Ra = 23.5 nm, Supplementary Fig. 16). This distinct contrast in nanoscale smoothness not only provides evidence for the formation of a smooth COF layer but also demonstrates its potentially high fouling resistance.

The elemental composition of the hydrogel support and the two COF membranes was determined by XPS analyses, with results detailed in Fig. 3f and Table S1. Tp-Bth COF membranes exhibited atomic ratios of 64.50% carbon, 5.69% nitrogen, and 29.81% oxygen, which is characteristic of the pre-designed oxygen-enriched framework structure. In contrast, TFB-Bth COF showed a distinct distribution: 69.35% carbon, 17.25% nitrogen, and 13.4% oxygen, reflecting the substantial architectural divergence between the two COF membranes. Furthermore, high-resolution deconvoluted XPS spectra provided additional confirmation for the presence of β-ketoenamine linkages in Tp-Bth and imine bonds in TFB-Bth, aligning with our previous FT-IR and 13C NMR analyses (Supplementary Fig. 29). In specific, N 1s peaks at 400.9 eV (N–H) and 400.1 eV (N–C), along with O 1s peaks at 532.8 eV (O=C) and 532.2 eV (O-C), confirmed the β-ketoenamine formation within Tp-Bth COF membranes (Supplementary Figs. 29d-f). Conversely, the C 1s spectrum of TFB-Bth COF membranes showed a distinct peak at 285.2 eV, indicative of imine bond formation, which is further corroborated by corresponding peaks in the N 1s and O 1s spectra (Supplementary Figs. 29g-i). Intriguingly, Zeta potential measurements revealed that the presence of the Tp-Bth COF layer substantially increased the isoelectric points of the composite membranes compared to both the TFB-Bth membrane and the hydrogel support (Fig. 3g). This charge alteration mainly stems from the protonation of the densely populated amine (-NH) groups within the crystalline Tp-Bth framework, a process facilitated by the presence of acetic acid. The resulting local electropositivity is critical, as it promotes the strong electrostatic repulsion of monovalent cations, thereby enhancing the overall water desalination efficacy.

Given the critical role of COF layer pore size in membrane desalination, their intrinsically specific surface area and pore size distribution were analyzed using Brunauer-Emmett-Teller (BET) characterizations. The nitrogen adsorption and desorption isotherms (Figs. 3h-i) for both Tp-Bth and TFB-Bth nanofilms consistently display a classic Type I isotherm, marked by a pronounced low-pressure adsorption surge (P/P₀ < 0.1). This characteristic profile establishes the prevalence of microporous structures within both COF films. BET data revealed a substantially higher specific surface area for the Tp-Bth film (60.5 m2 g-1) compared to that of the TFB-Bth film (40.5 m2 g-1). Notably, the Tp-Bth COF film exhibited a smaller pore size of 0.64 nm and a distinctly narrower ultramicroporous distribution in contrast to its TFB-Bth counterpart. This distinct difference in pore architecture provides strong evidence for their disparate stacking configurations and indicates structural regularity along with a narrower pore size distribution for the Tp–Bth film. As shown in Supplementary Fig. 30, the intrinsic pore size of TFB–Bth COF is measured to be 12.1 Å, which aligns closely with the pore size determined by BET measurements, further corroborating its AA-stacking mode. Conversely, the notable discrepancy observed between the intrinsic pore size and the experimentally measured pore size in Tp–Bth COF validates its AB-stacking configuration. We attribute this structural to the high crystallinity of the Tp-Bth film, a characteristic that simultaneously facilitates the formation of a long-range ordered pore architecture and yields the narrower, more uniform ultramicropores essential for high-selectivity desalination.

To assess pore characteristics in an aqueous environment, we performed molecular weight cut-off (MWCO) measurements on both COF membranes, using uncharged alcohols as model solutes (Table S2). The determined MWCO values were 70.4 Da for Tp-Bth and 384 Da for TFB-Bth membranes, respectively (Fig. 3j), aligning with the BET characterization results. Furthermore, probability density distribution profiles were derived from these rejection curves, as depicted in the Supplementary Fig. 31. A comparative analysis revealed a distinction between the hydrodynamic diameter calculations via MWCO (0.35 nm) and the BET-derived geometric pore size (0.64 nm) for Tp-Bth COF membranes. We presume this dimensional mismatch arises from the formation of hydrogen-bonded hydration layers, where oxygen/nitrogen moieties within the framework establish hydrogen-bonding interactions with adjacent water molecules, reducing accessible pore dimensions37. The abundance of ultra-microporous pores within high-crystalline Tp-Bth COF films is expected to enable a high desalination efficiency.

Desalination performance of ultra-microporous COF membranes
The desalination performance of as-fabricated COF membranes was assessed using a 2000 ppm NaCl solution as the feed, operating at 15 bar and room temperature. As plotted in Fig. 4a, elevating the Bth concentration from 0.5 to 1.25 mM (Tp = 1.75 mM) led to a consistent increase in NaCl rejection from 99.2% to 99.6%, with a minor trade-off in water permeability (1.7 L m-2 h-1 bar-1). Moreover, the as-synthesized membranes demonstrated good reproducibility, with the performance error bars remaining within an acceptable range. This observed trend is primarily ascribed to the formation of a more densely packed membrane surface at higher Bth concentrations, supported by SEM and AFM analyses. However, at Bth concentrations exceeding 1.5 mM, a double volume of acetic acid was required to ensure a homogeneous aqueous phase. The presence of this excessive acid negatively impacted film formation and compromised the intrinsic hydrogen-bond density, leading to a noticeable decline in desalination performance. This hypothesis is further corroborated by the lower desalination performance observed in Supplementary Fig. 32 across varying Tp concentrations (0.5 to 2 mM) at a fixed 1.5 mM Bth concentration. On the other hand, low monomeric concentrations (Tp ≤ 1.25 mM, Bth ≤ 1 mM) led to relatively loose crosslinked networks in the COF layer, resulting in salt rejection below 99.0% (Supplementary Fig. 33).

Fig. 4b illustrates a non-monotonic relationship between NaCl rejection and reaction time: initial increases are followed by subsequent declines. This trend arises because extended reaction duration facilitates epitaxial growth and crosslinking, elevating water/salt selectivity, but then causes over-aggregation and structural defects that compromise performance38. Following reaction parameter optimization, our engineered Tp-Bth COF membrane exhibited an exceptionally high NaCl rejection (99.6%) and good water permeability of 1.7 L m-2 h-1 bar-1. We attribute this performance primarily to its narrow ultra-microporous pore size distribution (centered at 0.64 nm) resulting from H-bond mediated high crystallinity, as well as an elevated isoelectric point that promotes effective sodium ion rejection. Using the identical methodology, the optimal desalination performance of TFB-Bth membranes was also explored (Supplementary Figs. 35-37). In contrast, TFB-Bth membranes showed a substantially lower NaCl rejection of 92.7%. This diminished performance correlates with their larger average pore size and higher MWCO, likely due to a comparatively low crystallinity. Furthermore, we investigated the effect of salt concentration on the rejection performance. Notably, the membrane exhibited minimal loss in rejection rate when the NaCl concentration was raised from 2000 ppm to 10,000 ppm (Supplementary Fig. 38). This demonstrates its exceptional stability under high osmotic pressure and confirms its strong potential for use in seawater desalination.

The substantial difference in NaCl rejection (92.7% vs. 99.6%) between the two COF membranes is attributed to the following key factors: (i) Hydrogen-bond-stabilized β-ketoenamine structure: The presence of adjacent phenolic hydroxyl groups on the Tp monomer enables enol-keto tautomerism, forming a stable β-ketoenamine linkage. This oxygen-rich structure promotes the formation of dense hydrogen-bond networks contributing to enhanced salt rejection; (ii) Enhanced crystallinity via molecular restriction: Extensive hydrogen-bonding networks restrict molecular rotation and suppress disordered stacking, promoting higher crystallinity and long-range ordered pore channels. Moreover, these networks largely strengthen interlayer binding energy and reduce interlayer spacing; (iii) Precise stacking transition mediated by hydrogen bonds: Rich intralayer hydrogen-bonds minimize stacking energy discrepancies during COF growth, enabling a controlled transition from the AA-stacking in TFB-Bth COF to the AB-stacking in Tp-Bth COF; (iv) Defect suppression and ultramicropore formation: This crystallinity consequently inhibits defect formation. The AB-stacking mode provides ultra-microporous channels (~0.64 nm), smaller than the hydrated sodium ion diameter (~0.72 nm), enabling highly efficient size exclusion. In contrast, the hydrogen-bond-deficient TFB-Bth COF membrane exhibits disordered stacking and a relatively large, non-uniform pore size distribution, leading to a lower desalination efficiency.

Given the exceptional desalination performance of Tp-Bth COF membranes, we further investigated their efficiency in boron removal from seawater. Boron, a prevalent natural element, predominantly exists as boric acid in aquatic environments, with typical seawater concentrations ranging from 4 to 6 ppm39. Boron removal poses a significant challenge, generally requiring multi-stage RO to meet drinking water standards40. Notably, in a single pass, Tp-Bth membranes effectively diminished boron concentration from 5 ppm to 1.14–1.25 ppm (75.9 ± 1.1% rejection, Fig. 4c), thus meeting stringent drinking water standards (1.0–2.4 ppm)41,42. Furthermore, a comparative analysis was conducted between Tp-Bth COF membranes and a state-of-the-art commercial brackish water RO membrane (BW30, DuPont). As illustrated in Fig. 4c, the Tp-Bth membrane presented distinctly higher NaCl and boron rejection than the BW30. In specific, the Tp-Bth membrane demonstrated a water permeability of 1.7 ± 0.09 L m-2 h-1 bar-1, with 99.6 ± 0.05% NaCl rejection and 75.9 ± 0.11% boron rejection, compared with 3.24 ± 0.16 L m−2 h−1 bar−1, 96.6 ± 0.6% and 32.3 ± 0.18% for BW3039,43. The demonstrated improvement of the Tp-Bth membrane in both salt and boron rejection establishes it as a highly promising candidate, directly enabled by our hydrogen-bond fortification strategy. To further evaluate the practical utility of our fabricated Tp-Bth membranes for water desalination, we subsequently conducted systematic investigations into their antifouling performance, acid stability, and operational stability.

To address the inherent acid sensitivity of conventional imine-linked COFs, we engineered β-ketoenamine-linked COF membranes to overcome this persistent limitation. Following a 30-day immersion in a pH 3 acidic solution, SEM confirmed the membranes retained their structural integrity (Figs. 4d-e), and their water permeability and salt rejection remained consistently stable (Fig. 4f). ATR-FTIR analysis (Supplementary Fig. 39) showed unchanged spectra of the Tp-Bth COF membrane after prolonged acid exposure, confirming robust acid stability, mainly due to their stable β-ketoenamine linkages. The non-reversible nature of the enol-to-keto tautomerization equilibrium and the robust hydrogen-bonding interactions between adjacent layers play a dual role in conferring this enhanced stability in acidic environments44. SEM and AFM images (Supplementary Figs. 40-41) further supported this, revealing negligible structural morphological changes after 60 days of acid immersion, which correlated with only a slight decrease in desalination performance (Table S3).

To assess the anti-fouling properties of Tp-Bth COF membranes, bovine serum albumin (BSA) and sodium alginate (SA) were utilized as model organic foulants. Normalized water flux trends in response to 200 ppm foulant exposure and subsequent water rinsing were monitored (Fig. 4g). All membranes showed a reduction in flux when exposed to BSA or SA, followed by flux restoration after water flushing. Our comprehensive anti-fouling assessment utilized four key performance indicators: flux recovery ratio (FRR), flux decline ratio (FDR), reversible flux decline rate (DRr), and irreversible flux decline rate (DRir). The fouling resistance of the Tp-Bth membrane against both BSA and SA model foulants is evident in Fig. 4h, with FRR/FDR/DRr/DRir values of 95.0/11.5/6.5/5.0% and 98.3/18.6/2.0/16.6%, respectively. The absence of a consolidated cake layer on the fouled Tp-Bth COF membrane in SEM images (Supplementary Fig. 42) further supports its antifouling capability. We attribute this low fouling propensity to the low surface roughness of the Tp-Bth COF selective layer, coupled with the introduced H-bond networks. These networks may facilitate the effective immobilization of a significant number of water molecules, creating a stable hydrated layer on the membrane surface that serves as a highly effective antifouling barrier.

Long-term stability is of paramount significance for assessing the industrial viability of RO membranes. As displayed in Fig. 4j, the Tp-Bth membrane exhibits significant durability, sustaining continuous operation at 15 bar pressure for 80 hours with NaCl rejection consistently at 99.5%. Despite a slight flux decrease, the membrane retained a high desalination performance, underscoring its potential for practical use. Concurrently, this provides additional evidence for the unparalleled stability inherent to the β-ketoenamine architecture. As demonstrated in Fig. 4k, the desalination performance of our Tp-Bth COF membrane sets a benchmark for COF membranes and outperforms many conventional alternatives, establishing its competitiveness in water purification technologies.

Mechanistic insights into hydrogen-bond fortification
To understand the critical role of hydrogen-bonding interactions in COF membrane crystallization, we conducted systematic density functional theory (DFT) simulations. These calculations examined charge changes during hydrogen-bond formation and the energy relationships of different conformational states. The comparative analysis reveals that Tp-Bth COF, featuring its extensive hydrogen-bonding network, offers improved interlayer interactions, higher structural stability, and boosted solvent affinity relative to the TFB-Bth COF.

Differential charge densities in Fig. 5a-b reveal charge redistribution localized predominantly within individual layers of the TFB-Bth COF. The consistent electron loss and gain patterns across adjacent layers demonstrate electrostatic repulsion, thus confirming the van der Waals (vdW) interactions as the primary mode of interlayer bonding for TFB-Bth COF45. In contrast, Fig. 5b shows a pronounced charge density decrease at the amine-associated hydrogen atoms (light blue shade), coupled with electron density accumulation at oxygen and nitrogen centers within adjacent molecular layers (yellow shade), indicative of hydrogen-bonding interactions.

A direct comparative analysis of interlayer binding energies between the two COF systems (Fig. 5c) reveals substantially augmented interfacial cohesion in the hydrogen-bond-rich Tp-Bth COF compared to its TFB-Bth counterpart. Furthermore, structural characterization shows a decreased interlayer distance of 3.2 Å for Tp-Bth COF, aligning with our previous TEM measurements and distinctly lower than that of TFB-Bth COF (3.7 Å). The intralayer hydrogen-bonding geometries for both COF systems were further quantified. As depicted in Supplementary Fig. 43, Tp-Bth COF exhibits significantly shorter hydrogen-bond distances compared to TFB-Bth COF (1.7 vs. 2.3 Å), demonstrating stronger interaction energies for Tp-Bth COF according to Jeffrey's classification criteria46. The combined charge density distribution analysis, bond length measurements, and binding energy calculations reveal that N–H···O hydrogen bonding interactions are crucial for substantially enhancing interlayer cohesion within COF structures. These collective findings provide multiscale evidence for the indispensable role of robust hydrogen-bonding networks in fine-tuning interlayer interactions and stabilizing the framework architecture.

Using the central N-N dihedral angle within COF monolayers as a structural probe, we comparatively analyzed bond rotation energetics in TFB-Bth and Tp-Bth systems (Fig. 5d). In the weakly hydrogen-bonded TFB-Bth COF, the dihedral angle exhibits unrestricted rotation across a broad angular range, primarily constrained by the steric interactions between adjacent functional groups. This considerable rotational freedom drives the formation of a three-dimensional hyperbranched architecture, while simultaneously leading to out-of-plane side-chain deviations that disrupt periodic stacking alignment along the z-axis. In contrast, Tp-Bth COF exhibits rotational constraints enforced by hydrogen bonds, wherein energy barriers sharply increase with angular deviation from equilibrium. This inherent molecular locking effectively suppresses stacking defects and spatial propagation errors during framework assembly through precise bond orientation control29. The fundamental difference in rotational dynamics—molecular confinement regulated by hydrogen bonds versus free rotation primarily limited by steric hindrance effects—dictates their distinct structural stabilities and functional performance.

Organic solvents critically control COF crystallization, molecular alignment, and pore structure via their physicochemical properties (viscosity, polarity)47-50. Computational analyses reveal distinct solvent-framework interactions: TFB-Bth COF shows a large intermolecular energy difference (0.44 eV) between water and dichloromethane, leading to random molecular orientation and heterogeneous pores51. Conversely, Tp-Bth COF exhibits a much smaller energy difference (0.07 eV) due to balanced interfacial forces. This near-equilibrium affinity promotes directional alignment during assembly, enhancing crystallization and yielding uniform pores. Overall, DFT calculations reveal that the synergy between hydrogen bonding interactions and reduced solvent affinity differences yields a more ordered crystal structure, resulting in more uniform pores.

Methods
Fabrication of Kevlar hydrogel support membrane
Kevlar hydrogel support membranes were fabricated via wet phase inversion52-54. A clarified solution was first prepared by sonicating deionized water (1.2 mL) and KOH (1.2 g), followed by adding Kevlar fiber (1.2 g) and DMSO (60 mL) with 48 h of magnetic stirring. The resulting dark red solution was centrifuged at 6578 x g to remove bubbles and precipitates. The dope solution was manually cast (250 μm thickness) onto PP/PE non-woven fabric fixed to a glass plate. Phase inversion was achieved by immersion in 25°C water, where DMSO/water exchange formed a transparent 3D porous hydrogel. After 10 minutes, the Kevlar hydrogel was transferred to a freshwater bath for over 6 hours. The resulting supports exhibited water permeation flux of 500-600 LMH bar-1.

Fabrication of COF free-standing nanofilm and membrane
Free-standing COF nanofilms were synthesized at ambient conditions via interfacial polymerization. The organic phase contained Tp (1.75 mM, 22.05 mg) in DCM (15 mL), while the aqueous phase dissolved Bth (1.25 mM, 4.7 mg) and acetic acid (0.1 M) in water (15 mL). After layering the aqueous solution over the DCM phase in a Petri dish to form a static interface, the sealed system was reacted undisturbed for 12 h.

Tp-Bth/TFB-Bth thin films were fabricated in situ on Kevlar hydrogel supports via interfacial Schiff-base reactions. A 4.5 cm diameter Kevlar membrane was vertically mounted in a U-shaped diffusion cell (dual 18 mL chambers). The organic and aqueous phases were loaded into opposing chambers, followed by sealed reaction at 29°C/30% RH for 12 h. Membranes were rinsed with water, soaked in ultrapure water, and characterized. TFB-Bth membranes were synthesized identically.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a novel COF membrane designed for high-performance RO desalination. The membrane was\nsynthesized via extensive hydrogen-bonding networks through β-ketoenamine linkages, concurrently enhancing crystallinity\nand introducing ultramicroporosity. The resulting Tp-Bth COF membrane demonstrates outstanding desalination\nperformance, achieving 99.6% NaCl rejection and a water permeability of 1.7 L m-2 h-1 bar-1 under a low operating\npressure of 15 bar. Furthermore, the membrane exhibits excellent acid resistance, retaining its structural integrity and\nseparation performance even after 30 days of exposure to pH 3 conditions, in addition to superior antifouling properties and\nlong-term operational stability. Given these notable achievements, I endorse its publication in Nature Communications after\nminor revisions.\n1. The introduction could be made more concise by removing redundant details. Moreover, the discussion on the critical role\nof hydrogen bonding networks in COF materials is relatively limited and could be appropriately expanded.\n2. The scale bars in Figures S1 and S2 are inconsistent. The authors should ensure uniform scaling for accurate\ncomparison.\n3. Apart from promoting the dissolution of BTH in the aqueous phase, does acetic acid serve additional functions in\ninterfacial polymerization? How do different acetic acid concentrations affect the reaction?\n4. The description of Fig. 2. in the manuscript should strictly align with the corresponding images and their discussed\ncontent. Some parts of the text lack direct correlation with the figures and should be clearly labeled. Additionally, the authors\nshould supplement the discussion regarding Figs. 2f and 2h, preferably with a comparative analysis to highlight the\nadvantages of Tp-Bth COF.\n5. Why do the thicknesses of the free-standing Tp-Bth COF and TFB-Bth COF films differ significantly from those of their\ncomposite membranes? Additionally, what role does Kevlar substrate play in the membrane formation process for each\nsystem?\n6. The trends of performance variation with concentration and time in Figure 4 are not sufficiently pronounced. The authors\nshould provide additional data to more clearly demonstrate the direct correlation between performance evolution and\nparameter modulation.", "reviewer_2": "Overall, the work is interesting and the experiments are carefully done. I think this is a solid use of the COF platform, and I\nrecommend it for publication after the authors address the following comments:\n1) On pg. 7 lines 169-187, the authors use different names for the COFs when describing the thin-films (i.e., Tp-COF instead\nof Tp-Bth COF). Are the authors talking about the same materials? If so, I would recommend using the same names to avoid\nconfusion (i.e., Tp-Bth COF nanofilm etc.). This can be done for the other sections as well.\n2) When the films are grown using the IP method, is it possible to control the thickness for each COF film? If not, do the\nvarious sizes of the films affect the performance and reproducibility of the data?\n3) On pg 12 Lines 316-318, the authors state \" We attribute this structural superiority tot the high crystallinity of the Tp-Bth\nfilm, which facilitates the formation of a long-range ordered architecture.\" Is the performance due to the narrow pores or the\nhigher crystallinity? Both samples do not look very crystalline, but the data shows that Tp-Bth COF does have smaller pores.\n4) The experimental PXRD of TFB-Bth COF looks like it also exhibits AB stacking based on the simulated data. Are the\nauthors sure that TFB-Bth COF exhibits AA stacking? Is there additional data to support the AA stacking?\n5) Is TFB-Bth COF nonporous? The surface area is not provided in the narrative. If so, would this also explain why the\nperformance is not as good as the Tp-Bth COF?"}, "author_response": "Response to Reviewers: NCOMMS-25-57236-T\nReviewer #1:\nWe would like to preface our responses by summarizing the key revisions made to the\nmanuscript in response to the reviewers’ comments. The main modifications primarily\ninclude the following aspects:\n1. Clarified the role of hydrogen-bonding networks and streamlined the Introduction\nby expanding relevant discussion and removing redundant details to improve focus.\n2. Added further discussion on the thickness variation between Tp-Bth and TFB-Bth\nCOF membranes and the influence of interfacial polymerization on film thickness\n(new Fig. S28).\n3. Provided further XRD refinement data confirming the high crystallinity and AB\nstacking of Tp-Bth COF versus the weaker AA stacking of TFB-Bth COF (Figs. S7,\nS30).\n4. Revised ambiguous statements and polished the language throughout the\nmanuscript to enhance overall clarity and precision.\nThis manuscript presents a novel COF membrane designed for high-performance RO\ndesalination. The membrane was synthesized via extensive hydrogen-bonding networks\nthrough β-ketoenamine linkages, concurrently enhancing crystallinity and introducing\nultramicroporosity. The resulting Tp-Bth COF membrane demonstrates outstanding\ndesalination performance, achieving 99.6% NaCl rejection and a water permeability of 1.7\nL m-2 h-1 bar-1 under a low operating pressure of 15 bar. Furthermore, the membrane\nexhibits excellent acid resistance, retaining its structural integrity and separation\nperformance even after 30 days of exposure to pH 3 conditions, in addition to superior\nantifouling properties and long-term operational stability. Given these notable\nachievements, I endorse its publication in Nature Communications after minor revisions.\nResponse: We sincerely appreciate the professional and constructive comments provided\nduring the review of our manuscript. We have addressed each of your suggestions diligently\nand have revised the paper accordingly to enhance its clarity and rigor.\n1. The introduction could be made more concise by removing redundant details. Moreover,\nthe discussion on the critical role of hydrogen bonding networks in COF materials is\nrelatively limited and could be appropriately expanded.\nResponse: We sincerely thank the reviewer for the careful reading and for pointing out this\nissue. In response, we have streamlined the content of the “Introduction” section and\nappropriately expanded the discussion on the critical role of hydrogen-bonding networks\nin covalent organic framework (COF) materials. The corresponding revisions have been\nmade accordingly in the revised manuscript.\nIn Manuscript Page 2-3 Line 48-73\nAccess to safe freshwater is paramount for human survival, agriculture, and sustained\neconomic development1. However, the projected 22–34% increase in the global population\nby mid-century will inevitably escalate freshwater demands, directly conflicting with the\ndiminishing availability of clean water2. This scarcity is further exacerbated by climate\ndisruptions and pervasive anthropogenic contamination. Consequently, water desalination\nand recycling technologies have become increasingly critical for long-term water\nsustainability3,4. Reverse osmosis (RO) has emerged as the leading membrane technology\nfor potable water production5. Early advancements focused on polyamide thin-film\ncomposite (TFC) membranes, which continue to serve as the industrial standard6,7.\nHowever, challenges in achieving ordered nanostructure and uniform pore size distribution\nin these membranes make it difficult to surpass the established permeability-selectivity\ntrade-off8,9.\nCovalent organic frameworks (COFs) are crystalline porous materials synthesized via\nthe directional covalent assembly of organic monomers, intrinsically offering aligned pore\nchannels, uniform pore sizes, and high porosity10-13. Despite these advantages, critical\nchallenges for large-area membrane configuration lie in their intrinsically oversized pores\n(0.8–5 nm)14 and often-insufficient crystallinity. These structural limitations restrict COF\nmembrane applications predominantly to organic solvent nanofiltration15,16,\nmacromolecular dye separation17, and multivalent ion sieving18,19. Notably, achieving\neffective desalination with COF membranes remains a grand challenge, as the majority of\nsub-nanometer monovalent ions (e.g., 1–3 Å for naked ions and 6.5–8 Å for hydrated\nions)20 typically bypass their pores, resulting in limited exploration of their potential in\nlow-pressure RO desalination. Recently, Liu et al. introduced light-controlled COF\nmembranes for pressure-driven RO seawater desalination. Despite achieving a notable 91.2%\nNaCl rejection, this figure falls short of the ≥ 95% industrial benchmark for practical RO\nsystems.\n2. The scale bars in Figures S1 and S2 are inconsistent. The authors should ensure\nuniform scaling for accurate comparison.\nResponse: We are grateful for the reviewer’s careful attention to detail. The scale bars in\nFig. S1 and S2 have been corrected and unified in the revised manuscript to ensure\nconsistent scaling and improve the accuracy of visual comparisons.\nIn Supplementary Information Page 9-10\nFig. S1. Characterization of Tp-Bth free-standing nanofilms. (a) SEM image of the\ntransfer free-standing film on the surface of anodic aluminum oxide (AAO); (b) TEM\nimage of the surface transfer free-standing film of copper mesh; (c) TEM/EDS mapping of\nelemental distributions of free-standing nanofilms.\nFig. S2. Characterization of TFB-Bth free-standing nanofilms. (a) SEM image of the\ntransfer free-standing film on the surface of anodic aluminum oxide (AAO); (b) TEM\nimage of the surface transfer free-standing film of copper mesh; (c) TEM/EDS mapping of\nelemental distributions of free-standing nanofilms.\n3. Apart from promoting the dissolution of BTH in the aqueous phase, does acetic acid\nserve additional functions in interfacial polymerization? How do different acetic acid\nconcentrations affect the reaction?\nResponse: We appreciate the reviewer's careful and insightful question concerning the role\nof acetic acid (AcOH) in the interfacial polymerization (IP) process. We confirm that a\nprimary function of AcOH is indeed the solubilization of the Bth monomer in the aqueous\nphase. However, our study reveals that AcOH fulfills several other critical roles.\nSpecifically, its concentration is a key variable that dictates the reaction kinetics, the degree\nof membrane crystallinity, and the final desalination performance.\nAdditional Functions of Acetic Acid:\nThe addition of acetic acid protonates the amino groups in the Bth monomer, resulting in a\nTp-Bth membrane with a higher isoelectric point than the Kevlar support. This positive\nsurface charge enhances the rejection of monovalent cations through electrostatic repulsion,\nthereby contributing to the membrane’s exceptional desalination performance. [Nature\nWater, 3(1), 110-121 (2024); Nature Communications, 13(1): 7123 (2022)]\nFurthermore, in the Tp-Bth system, the addition of acetic acid is crucial for catalyzing the\nsubsequent enol-to-keto tautomerization. This irreversible reaction converts the initially\nformed hydrazone linkage into the thermodynamically stable β-ketoenamine configuration,\nwhich serves as the foundation for constructing an extensive hydrogen-bonding network.\nAs discussed in detail, this hydrogen-bonding network is responsible for the enhanced\ncrystallinity, the formation of AB-stacking, and the superior stability observed in the Tp-\nBth COF membrane. [Nano letters, 24(46), 14797-14805 (2024); Nature Communications,\n16(1), 1223. (2025); Nature Communications, 14(1), 5926. (2023)]\nImpact of Acetic Acid Concentration:\nThe concentration of acetic acid is a critical parameter that requires precise optimization,\nand this optimization is interdependent on the concentration of the Bth monomer. As\ndetailed in the Results and Discussion section (Fig. 4a and associated text under\nPerformance and application), we observed that when the Bth concentration exceeded 1.5\nmM, a double volume of acetic acid (increasing its final concentration in the aqueous phase\nfrom 0.1 M to 0.2 M) was necessary to maintain a homogeneous solution.\nOptimal Concentration (0.1 M): At this concentration, acetic acid effectively balances its\ndual roles as a solubilizing agent and a catalyst, leading to the formation of a dense, highly\ncrystalline Tp-Bth membrane. This results in an exceptional NaCl rejection rate of 99.6%.\nExcessive Concentration (0.2 M): Although the use of excess acid is necessary for\nsolubility at high monomer loadings, it adversely affects film formation and compromises\nthe intrinsic hydrogen-bond density, ultimately leading to increased defects and reduced\ncrystallinity. We therefore attribute the subsequent decline in desalination performance (as\nshown in Fig. S29) to this impaired structural integrity.\nIn conclusion, acetic acid functions not merely as a solubilizing agent but as a crucial\nmodulator that catalyzes the reaction and governs the final microstructure of the COF\nmembrane. As demonstrated in our work, its concentration must be carefully fine-tuned in\nconjunction with the monomer concentrations to achieve the targeted high-performance\nmembrane. In response to the reviewer’s comment, we have enhanced our discussion in\nthe main text to clarify the multifaceted role of acetic acid addition and the influence of its\nconcentration. The primary modifications are as follows:\nIn Manuscript Page 7 Line 155-162:\nTo overcome the inherent water insolubility of Bth monomers, our strategy for\nfabricating free-standing COF films incorporates a facile preliminary dissolution step into\nthe conventional interfacial polymerization (IP) process. In specific, a dichloromethane\n(DCM) solution containing either Tp or TFB monomers was first introduced into a glass\nPetri dish. An acetic aqueous solution containing Bth monomers was then gently layered\natop the DCM phase, which initiated the Schiff base condensation at the free interface. The\ninclusion of the acetic acid solution resolved the solubility issue of Bth monomers while\nalso catalyzing the reaction.\nIn Manuscript Page 12 Line 313-321:\nIntriguingly, Zeta potential measurements revealed that the presence of the Tp-Bth\nCOF layer substantially increased the isoelectric points of the composite membranes\ncompared to both the TFB-Bth membrane and the hydrogel support (Fig. 3g). This charge\nalteration mainly stems from the protonation of the densely populated amine (-NH) groups\nwithin the crystalline Tp-Bth framework, a process facilitated by the presence of acetic\nacid. The resulting local electropositivity is critical, as it promotes the strong electrostatic\nrepulsion of monovalent cations, thereby enhancing the overall water desalination efficacy.\nIn Manuscript Page 15 Line 383-391:\nHowever, at Bth concentrations exceeding 1.5 mM, a double volume of acetic acid\nwas required to ensure a homogeneous aqueous phase. The presence of this excessive acid\nnegatively impacted film formation and compromised the intrinsic hydrogen-bond density,\nleading to a noticeable decline in desalination performance. This hypothesis is further\ncorroborated by the inferior desalination performance observed in Fig. S32 across varying\nTp concentrations (0.5 to 2 mM) at a fixed 1.5 mM Bth concentration.\n4. The description of Fig. 2 in the manuscript should strictly align with the corresponding\nimages and their discussed content. Some parts of the text lack direct correlation with the\nfigures and should be clearly labeled. Additionally, the authors should supplement the\ndiscussion regarding the Figs. 2f and 2h, preferably with a comparative analysis to\nhighlight the advantages of Tp-Bth COF.\nResponse: We are thankful for the reviewer’s constructive comments for highlighting the\nneed for improved alignment between the description of Fig. 2 and the corresponding\nimages, as well as for the suggestion to elaborate on the discussion of Figs. 2f and 2h. In\nresponse, we have thoroughly revised the relevant text in the Results and discussion section\nto ensure precise and explicit referencing of each figure panel. Additionally, we have\nenriched the analysis of Figs. 2f and 2h by incorporating a direct comparative evaluation\nto better emphasize the structural advantages of the Tp-Bth COF.\nIn Manuscript Page 8 Line 192-207:\nPawley refinement confirmed that the Tp-Bth COF adopts an AB stacking\narrangement. The observed diffraction peaks at 6.9o and 26.8o correspond to the (100) and\n(002) crystal planes, respectively, showing excellent agreement with the simulated pattern\nfor AB stacking (Fig. 2c). In contrast, the low hydrogen-bond density in TFB–Bth COF\nresults in a weak crystallinity. This leads to relatively close goodness-of-fit residuals (Rwp,\nRp) for both the AA- and AB-stacking models, which hindered definitive discrimination\nbetween the two curves. Nevertheless, a direct comparison between the experimental XRD\npattern and the simulated profiles (Figs. 2d and S7) demonstrates superior alignment with\nthe AA-stacking simulation. Therefore, we assign the AA-stacking mode to TFB–Bth COF\nas the most consistent interpretation. Further elucidation of crystalline anisotropy using\ntwo-dimensional grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) revealed\na marked contrast between the two COFs. The Tp-Bth COF exhibited a highly oriented\n(100) crystallographic plane with vertically aligned nanochannels, as evidenced by its\npronounced scattering intensity at Qx,y = 0.49 Å-1 (Fig. 2e). In comparison, the TFB-Bth\nCOF (Fig. 2f) displayed a relatively random orientation.\nIn Manuscript Page 8-9 Line 211-221:\nHigh-resolution TEM (HRTEM) imaging of Tp-Bth COFs clearly revealed long-\nrange ordered lattice fringes with a characteristic spacing of 0.30 nm, which is indexed to\nthe (002) crystallographic plane (Fig. 2g). Complementary selected area electron\ndiffraction (SAED) patterns, exhibiting bright diffraction spots, further corroborated the\nsuperior crystallinity of the Tp-Bth COF film. In sharp contrast, the HRTEM image of the\nTFB-Bth COF (Fig. 2h) shows a weakly ordered structure, with the corresponding SAED\npattern (inset) displaying diffuse rings, characteristic of low crystallinity. Notably,\nHRTEM images acquired from multiple regions across the Tp-Bth COF (Figs. S8-S9)\nconsistently illustrated that crystalline domains predominantly occupied the entire prepared\nCOF nanofilms, highlighting their uniform, high-quality microstructure.\n5. Why do the thicknesses of the free-standing Tp-Bth COF and TFB-Bth COF films differ\nsignificantly from those of their composite membranes? Additionally, what role does Kevlar\nsubstrate play in the membrane formation process for each system?\nResponse: We acknowledge the reviewer’s observation regarding raising these perceptive\nquestions concerning the distinct thicknesses of the freestanding COF films versus their\ncomposite counterparts and the specific function of the Kevlar substrate. These inquiries\ntouch upon fundamental aspects of our fabrication strategy, and addressing them allows us\nto elucidate the critical roles played by synthesis configuration and substrate-matrix\ninteractions in determining the final membrane's architecture and properties. Our response\nbelow integrates experimental observations with a discussion of the underlying\nphysicochemical mechanisms to provide a comprehensive explanation.\nFor the free-standing nanofilms, the inherent hydrophobicity of the Tp monomer confines\nthe reaction tightly to the interface, leading to the formation of a comparatively thicker Tp-\nBth nanofilm (120.8 ± 0.3 nm). In contrast, the greater hydrophilicity of the TFB monomer\npromotes its diffusion into the aqueous phase (Angewandte Chemie, 133(32), 17779-17787\n(2021)), resulting in a phase-transfer polymerization and a much thinner TFB-Bth nanofilm\n(40.4 ± 0.8 nm). For the composite membranes, the reaction occurs in situ on the confined\nsurface of the Kevlar hydrogel support (Advanced Functional Materials, 2504997 (2025)).\nIn the Tp-Bth system, the confinement restricts the growth area, leading to a denser and\nslightly thinner selective layer (92.3 nm) compared to its free-standing counterpart. More\nnotably, for the TFB-Bth system, the Kevlar substrate confinement forces the\npolymerization to occur predominantly at the interface, thereby promoting the formation\nof a more substantial and continuous selective layer (88.3 nm) than what is possible in an\nunconfined, free-interface synthesis.\nThe Kevlar hydrogel provides a robust and stable platform for interfacial polymerization.\nIts nanoscale pore structure effectively confines the reaction zone, directing the nucleation\nand growth of the covalent organic framework (COF) layer. This spatial confinement\nfacilitates the formation of a continuous, defect-free selective layer with tunable thickness.\nAdditionally, the hydrogel’s surface chemistry and porous architecture modulate the local\nconcentration and diffusion kinetics of the aqueous-phase monomer (Bth), enabling a more\ncontrolled and uniform polymerization compared to reactions at a free interface. This\nregulation significantly reduces the formation of structural defects. Furthermore, the\nflexible yet mechanically strong Kevlar support imparts essential durability to the ultrathin\nCOF layer, allowing for practical handling and stable operation under applied pressure\nduring desalination processes—capabilities that would be unattainable with a fragile, free-\nstanding film.\nIn Manuscript Page 7 Line 176-182:\nThe diminished thickness observed in the TFB-Bth COF nanofilm is attributable to\nthe higher hydrophilicity of the TFB monomer (n-octanol/water partition coefficient: 4.85\nfor Tp vs. 1.32 for TFB). This characteristic facilitates the facile diffusion of TFB into the\naqueous phase, promoting bulk phase-transfer polymerization rather than a strictly\ninterfacial reaction33. Conversely, the more hydrophobic nature of Tp monomers restricts\ntheir reaction to the interface, thereby leading to the formation of a comparatively thicker\nTp-Bth COF nanofilm.\n6. The trends of performance variation with concentration and time in Figure 4 are not\nsufficiently pronounced. The authors should provide additional data to more clearly\ndemonstrate the direct correlation between performance evolution and parameter\nmodulation.\nResponse: We sincerely thank the reviewer for this insightful comment regarding the\nclarity of performance trends in Fig. 4. We agree that a more pronounced demonstration of\nthe correlation is crucial. In response, we have supplemented our dataset and revised the\ncorresponding text in the Results and discussion section to more explicitly highlight the\nperformance evolution with parameter modulation.\nIn Manuscript Page 15 Line 377-379:\nAs plotted in Fig. 4a, elevating the Bth concentration from 0.5 to 1.25 mM (Tp = 1.75\nmM) led to a consistent increase in NaCl rejection from 99.2% to 99.6%, with a minor\ntrade-off in water permeability (1.7 L m-2 h-1 bar-1).\nReviewer #2:\nWe would like to preface our responses by summarizing the key revisions made to the\nmanuscript in response to the reviewers’ comments. The main modifications primarily\ninclude the following aspects:\n1. Clarified the role of hydrogen-bonding networks and streamlined the Introduction\nby expanding relevant discussion and removing redundant details to improve focus.\n2. Added further discussion on the thickness variation between Tp-Bth and TFB-Bth\nCOF membranes and the influence of interfacial polymerization on film thickness\n(new Fig. S28).\n3. Provided further XRD refinement data confirming the high crystallinity and AB\nstacking of Tp-Bth COF versus the weaker AA stacking of TFB-Bth COF (Figs. S7,\nS30).\n4. Revised ambiguous statements and polished the language throughout to enhance\nclarity and precision.\nOverall, the work is interesting and the experiments are carefully done. I think this is a\nsolid use of the COF platform, and I recommend it for publication after the authors address\nthe following comments:\nResponse: We sincerely thank the reviewer for the positive assessment of our work and\nthe valuable feedback provided. Your insightful comments have been thoroughly\nconsidered, and we have implemented corresponding revisions to further strengthen the\nmanuscript.\n1. On pg. 7 lines 169-187, the authors use different names for the COFs when describing\nthe thin-films (i.e., Tp-COF instead of Tp-Bth COF). Are the authors talking about the\nsame materials? If so, I would recommend using the same names to avoid confusion (i.e.,\nTp-Bth COF nanofilm etc.). This can be done for the other sections as well.\nResponse: We feel great thanks for this careful observation and for pointing out the\ninconsistency in nomenclature. The reviewer is absolutely correct. In the mentioned section\nand throughout the manuscript, “Tp-COF” and “TFB-COF” were used as abbreviated\nforms referring to the “Tp-Bth COF” and “TFB-Bth COF” materials, respectively. We\nacknowledge that this inconsistent usage could indeed cause confusion for readers.\nIn response to this valuable comment, we have systematically revised the entire manuscript\nto ensure uniform nomenclature. We have replaced all instances of the abbreviated forms\n(e.g., Tp-COF, TFB-COF) with the full, descriptive names (Tp-Bth COF and TFB-Bth COF)\nfor both the nanofilms and the composite membranes. This includes corrections in the main\ntext, figure captions, and the supporting information. We believe this revision has\nsignificantly improved the clarity and consistency of the manuscript, and we are grateful\nfor the reviewer’s suggestion.\nIn Manuscript Page 4-5 Line 113-122:\nThis structural transformation induces a crystallographic transition from the hydrogen\nbond-deficient AA-stacking observed in TFB-Bth COF to a highly desirable AB-stacking\nin Tp-Bth COF, which features well-aligned ultra-micropores and superior crystallinity.\nThe resultant Tp-Bth COF membranes demonstrate exceptional desalination performance,\nachieving a 99.6% NaCl rejection and favorable water permeability, thereby far surpassing\nthe desalting performance of advanced COF membranes. Furthermore, the abundant\nhydrogen-bonding networks intrinsic to the β-ketoenamine Tp-Bth COF provide enhanced\nstructural robustness and superior stability in acidic conditions, underscoring its potential\nfor sustainable water purification technologies.\n2. When the films are grown using the IP method, is it possible to control the thickness for\neach COF film? If not, do the various sizes of the films affect the performance and\nreproducibility of the data?\nResponse: We thank the reviewer for raising this important point regarding thickness\ncontrol and its implications for performance and reproducibility. Below, we provide a\ndetailed response based on our experimental system and results.\nInterfacial polymerization occurs when reactive monomers—or monomers paired with\ninitiators/catalysts—dissolved in two immiscible phases come into contact and react at the\ninterface. This method offers distinct advantages for polymer synthesis, including high\nmolecular weight products, rapid reaction kinetics, and mild operating conditions, owing\nto the unique physicochemical environment of the liquid–liquid interface. The resulting\nfilms are inherently uniform and defect-free, as monomer diffusion preferentially continues\nin regions where the polymer layer is underdeveloped, thereby promoting self-repair and\ncontinuous growth. Furthermore, film morphology can be tailored by modulating monomer\nconcentration and solvent selection. The process evolves through two kinetic stages: an\ninitial rapid polymerization phase, governed by direct monomer contact, forms an incipient\nthin film, followed by a diffusion-limited stage wherein monomer permeation through the\ngrowing film becomes rate-determining. Eventually, polymerization ceases due to\nsignificant diffusion resistance. This leads to a self-limiting growth profile, characterized\nby an initially fast thickness increase that gradually decelerates and approaches a maximum\nvalue. (Angewandte Chemie International Edition, 59(49), 21840-21856. (2020)).\nIn our current IP setup, precise and independent control over the thickness of each COF\nfilm is challenging. The final thickness is an emergent property resulting from the interplay\nof several key synthesis parameters, including: Monomer concentrations (Journal of\nMembrane Science, 93(2), 175-192. (1994)), reaction time ( Journal of applied polymer\nscience, 70(11), 2169-2172. (1998)), and monomer reactivity (Macromolecules, 33(2),\n624-633. (2000)). The presence and nature of the substrate (as discussed in our response to\na previous comment) While we cannot independently \"dial-in\" an arbitrary thickness, our\nwork demonstrates that the thickness can be influenced in a predictable and reproducible\nmanner by systematically varying these parameters. For instance, as shown in Fig. S28,\nincreasing the reaction time generally leads to a thicker and more consolidated selective\nlayer. Therefore, while not independently controllable in isolation, thickness is a tunable\nparameter within the constrained optimization of the synthesis conditions.\nWe have revised the manuscript to provide a clearer explanation of our control over film\nthickness and the reproducibility of the performance data, as detailed in the modifications\nbelow.\nIn Manuscript Page 11 Line 291-294:\nFurthermore, cross-sectional TEM imaging was conducted on Tp-Bth COF\nmembranes synthesized using varying reaction times. As visualized in Fig. S28, the\nmembrane thickness exhibits a progressive increase corresponding to the extension of the\nreaction time from 3 h to 12 h.\nIn Manuscript Page 15 Line 375-391:\nThe desalination performance of as-fabricated COF membranes was assessed using a\n2000 ppm NaCl solution as the feed, operating at 15 bar and room temperature. As plotted\nin Fig. 4a, elevating the Bth concentration from 0.5 to 1.25 mM (Tp = 1.75 mM) led to a\nconsistent increase in NaCl rejection from 99.2% to 99.6%, with a minor trade-off in water\npermeability (1.7 L m-2 h-1 bar-1). Moreover, the as-synthesized membranes demonstrated\ngood reproducibility, with the performance error bars remaining within an acceptable range.\nThis observed trend is primarily ascribed to the formation of a more densely packed\nmembrane surface at higher Bth concentrations, supported by SEM and AFM analyses.\nHowever, at Bth concentrations exceeding 1.5 mM, a double volume of acetic acid was\nrequired to ensure a homogeneous aqueous phase. The presence of this excessive acid\nnegatively impacted film formation and compromised the intrinsic hydrogen-bond density,\nleading to a noticeable decline in desalination performance. This hypothesis is further\ncorroborated by the inferior desalination performance observed in Fig. S32 across varying\nTp concentrations (0.5 to 2 mM) at a fixed 1.5 mM Bth concentration. On the other hand,\nlow monomeric concentrations (Tp ≤ 1.25 mM, Bth ≤ 1 mM) led to relatively loose\ncrosslinked networks in the COF layer, resulting in salt rejection below 99.0% (Fig. S33).\n3. On page 12 Lines 316-318, the authors state \" We attribute this structural superiority\nto the high crystallinity of the Tp-Bth film, which facilitates the formation of a long-range\nordered architecture.\" Is the performance due to the narrow pores or the higher\ncrystallinity? Both samples do not look very crystalline, but the data shows that Tp-Bth\nCOF does have smaller pores.\nResponse: We sincerely thank the reviewer for this insightful comment and the opportunity\nto clarify the relationship between crystallinity, pore size, and desalination performance.\nThe reviewer is correct in noting that both factors are crucial, and our performance results\nare indeed due to the synergistic effect of both the higher crystallinity and the resulting\nnarrower, more uniform pore size distribution of the Tp-Bth COF. We apologize for any\nlack of clarity in our original statement and have revised the manuscript to reflect this more\nprecisely.\nThe reviewer’s observation that the samples may not appear “very crystalline” is noted.\nWe wish to clarify that within the context of thin-film COF membranes, the relative\ncrystallinity and, more importantly, the long-range structural order of Tp-Bth are\nsignificantly superior to those of TFB-Bth. This is unambiguously demonstrated by:\nPXRD (Fig. 2c): the sharper and more intense peaks for Tp-Bth indicate better crystalline order.\nGIWAXS (Figs. 2e,f): The highly oriented diffraction pattern of Tp-Bth confirms a long-\nrange ordered structure with vertically aligned channels, in stark contrast to the relatively\nrandom orientation in TFB-Bth.\nHRTEM & SAED (Figs. 2g,h): The clear lattice fringes and sharp diffraction spots for Tp-Bth directly visualize its high crystallinity, which is\nabsent in the TFB-Bth film. This enhanced crystallinity is critical for suppressing defects\nand ensuring the structural integrity of the membrane. A highly crystalline, ordered\nstructure minimizes non-selective pathways (gaps and voids) that would otherwise allow\nsalt ions to bypass the molecular sieving pores, thereby directly contributing to the high\nsalt rejection.\nThe desalination performance is intrinsically linked to the crystallinity of the thin films. As\nclearly demonstrated by Brunauer–Emmett–Teller (BET) surface area analysis and\nmolecular weight cut-off (MWCO) measurements (Figs. 3h–j), the Tp–Bth COF exhibits\na smaller and more uniform pore size (~0.64 nm) compared to TFB–Bth. This is the\nprimary factor enabling effective size-exclusion-based sieving of hydrated monovalent salt\nions (~0.65–0.8 nm). The smaller pore size in Tp–Bth is a direct consequence of its higher\ncrystallinity and specific AB-stacking mode, both of which are regulated by the reinforced\nhydrogen-bonding network. Hydrogen bonding restricts molecular rotation and locks the\nCOF layers into a more compact and regular AB-stacking arrangement, in contrast to the\nless-ordered AA-stacking observed in TFB–Bth. Therefore, the hydrogen-bonding network\nserves as the fundamental design feature that simultaneously enhances crystallinity\n(thereby reducing defects) and refines the pore structure (by narrowing the pores and\nimproving selectivity).\nTo better elucidate the connection between crystallinity, pore size, and desalination\nperformance, and to specifically address the query regarding crystallinity of the reviewer,\nwe have implemented the following revisions in the main text.\nIn Manuscript Page 13 Line 339-342:\nWe attribute this structural superiority to the high crystallinity of the Tp-Bth film, a\ncharacteristic that simultaneously facilitates the formation of a long-range ordered pore\narchitecture and yields the narrower, more uniform ultramicropores essential for high-\nselectivity desalination.\nIn Manuscript Page 8 Line 188-200:\nTo elucidate the crystalline structural disparities between the two COFs, we conducted\npowder X-ray diffraction (PXRD) analyses on both two nanofilms. The Tp-Bth COF\nexhibited distinct diffraction peaks at 6.9o and 26.8o, whereas the TFB-Bth COF showed\nbroadened and less intense diffraction peaks, suggesting a weakly ordered structure due to\nstructural defects. Pawley refinement confirmed that the Tp-Bth COF adopts an AB\nstacking arrangement. The observed diffraction peaks at 6.9o and 26.8o correspond to the\n(100) and (002) crystal planes, respectively, showing excellent agreement with the\nsimulated pattern for AB stacking (Fig. 2c). In contrast, the low hydrogen-bond density in\nTFB–Bth COF results in a weak crystallinity. This leads to relatively close goodness-of-fit\nresiduals (Rwp, Rp) for both the AA- and AB-stacking models, which hindered definitive\ndiscrimination between the two curves. Nevertheless, a direct comparison between the\nexperimental XRD pattern and the simulated profiles (Figs. 2d and S7) demonstrates\nsuperior alignment with the AA-stacking simulation.\n4. The experimental PXRD of TFB-Bth COF looks like it also exhibits AB stacking based\non the simulated data. Are the authors sure that TFB-Bth COF exhibits AA stacking?\nIs there additional data to support the AA stacking?\nResponse: We sincerely thank the reviewer for raising this insightful question regarding\nthe stacking mode of TFB–Bth COF, which has allowed us to clarify this important\nstructural aspect. We also apologize for any misunderstanding that may have arisen from\nour initial description. The reviewer makes a valid observation that the diffraction peak\npositions of TFB–Bth COF may appear visually similar to the simulated AB-stacking\npattern. However, based on a comprehensive analysis of PXRD Pawley refinement, BET\npore size distribution, and supplementary structural characterizations, we are confident in\nassigning the AA-stacking mode to TFB–Bth COF.\nFirstly, we performed additional Pawley refinement on the XRD data of TFB–Bth COF.\nHowever, due to its low hydrogen-bond density and the resulting poor crystallinity, the\ngoodness-of-fit residuals (Rwp, Rp) for both the AA- and AB-stacking models were\nrelatively close, posing challenges for definitive discrimination between the two modes.\nNevertheless, by comparing the experimental XRD pattern with the simulated AA- and\nAB-stacking profiles (as shown in Fig. 2d and Fig. S7), it is evident that the experimental\ndata align more closely with the simulated AA-stacking pattern. Therefore, we have\npreliminarily assigned the AA-stacking mode to TFB–Bth COF. Secondly, the\nexperimental pore size obtained from BET and molecular weight cut-off (MWCO)\nmeasurements is in good agreement with the theoretical pore size of TFB–Bth COF derived\nfrom Materials Studio (MS) simulations (Fig. S30), further corroborating the AA-stacking\nassignment. To more clearly elucidate the stacking mode of TFB–Bth COF, we have\nrevised the manuscript as follows.\nIn Manuscript Page 8 Line 195-202:\nIn contrast, the low hydrogen-bond density in TFB–Bth COF results in a weak\ncrystallinity. This leads to relatively close goodness-of-fit residuals (Rwp, Rp) for both the\nAA- and AB-stacking models, which hindered definitive discrimination between the two\ncurves. Nevertheless, a direct comparison between the experimental XRD pattern and the\nsimulated profiles (Figs. 2d and S7) demonstrates superior alignment with the AA-stacking\nsimulation. Therefore, we assign the AA-stacking mode to TFB–Bth COF as the most\nconsistent interpretation.\nIn Manuscript Page 13 Line 332-339:\nThis distinct difference in pore architecture provides strong evidence for their\ndisparate stacking configurations and indicates superior structural regularity along with a\nnarrower pore size distribution for the Tp–Bth film. As shown in Fig. S30, the intrinsic\npore size of TFB–Bth COF is measured to be 12.1 Å, which aligns closely with the pore\nsize determined by BET measurements, further corroborating its AA-stacking mode.\nConversely, the notable discrepancy observed between the intrinsic pore size and the\nexperimentally measured pore size in Tp–Bth COF validates its AB-stacking configuration.\n5. Is TFB-Bth COF nonporous? The surface area is not provided in the narrative. If so,\nwould this also explain why the performance is not as good as the Tp-Bth COF?\nResponse: We sincerely appreciate the reviewer for raising these pertinent questions\nregarding the porosity of TFB–Bth COF and its impact on performance, and we regret any\nmisunderstanding our original description may have caused. The TFB–Bth COF is not\nnonporous; however, it exhibits a lower specific surface area and a larger, less uniform pore\nsize distribution compared to the Tp–Bth COF. The BET surface area of the TFB–Bth\nnanofilm was measured to be 40.5 m2 g-1, which is considerably lower than the 60.5 m2 g-\n1 of the Tp–Bth nanofilm. In accordance with the reviewer’s suggestion to improve\nreadability, we have revised the relevant text as shown below.\nIn Manuscript Page 12-13 Line 328-332:\nBET data revealed a substantially higher specific surface area for the Tp-Bth film\n(60.5 m2 g-1) compared to that of the TFB-Bth film (40.5 m2 g-1). Notably, the Tp-Bth COF\nfilm exhibited a smaller pore size of 0.64 nm and a distinctly narrower ultramicroporous\ndistribution in contrast to its TFB-Bth counterpart."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The revised manuscript reports an ultra-microporous covalent organic framework (COF) membrane with enhanced\ncrystallinity through a hydrogen-bond strengthening strategy. The membrane exhibits excellent RO desalination\nperformance as well as remarkable acid resistance and antifouling stability. The authors have provided comprehensive\nexperimental characterization and simulation analyses to support their conclusions.\nIn response to the reviewers’ previous comments, the authors have made thorough and careful revisions. Overall, the\nmanuscript has been substantially improved and can be considered for publication after minor revisions.\n1、Please provide the simulated data for both AA and AB stacking models of Tp-Bth and TFB-Bth COF. It would be clearer\nand more informative if the results were plotted in the same figure for direct comparison.\n2、Both COF membranes exhibit relatively high water contact angles (80–90°), which raises concerns for water treatment\napplications. Have the authors considered how this hydrophobicity might influence the energy barrier for water penetration\ninto the pores?\n3、Some performance tests are shown only for the Tp-Bth membrane. Please clarify why TFB-Bth was not included in those\ncomparisons.\n4、The choice of 2000 ppm NaCl as the feed concentration should be further justified. Considering the relevance to\nseawater desalination (≈35,000 ppm), performance under higher salinity conditions should be discussed or at least\ncommented on.\n5、The concentration units “2000 mg L⁻1” (line 360–361) and “2000 ppm” should be unified for consistency.", "reviewer_2": "My comments have been addressed and I'm satisfied with the revised manuscript."}, "author_response": "Response to Reviewers: NCOMMS-25-57236-T\nReviewer #1:\nWe would like to commence our response by outlining the principal revisions enacted in\nthe manuscript to address the reviewers’ comments. These revisions include the\nfollowing key points:\n1. Revised Fig. S7 for direct comparison of the simulated PXRD patterns for AA and\nAB stacking modes of TFB-Bth COF, clarifying structural assignment.\n2. Expanded discussion on hydrophobicity and internal hydrogen-bonding networks\nfor water molecule transport.\n3. Clarified rationale for focusing performance tests on the Tp-Bth membrane as the\nkey innovation.\n4. Included performance data at enhanced salinity (6000 and 10,000 ppm NaCl),\ndemonstrating robust performance.\nThe revised manuscript reports an ultra-microporous covalent organic framework (COF) membrane with enhanced\ncrystallinity through a hydrogen-bond strengthening strategy. The membrane exhibits excellent RO desalination\nperformance as well as remarkable acid resistance and antifouling stability. The authors have provided comprehensive\nexperimental characterization and simulation analyses to support their conclusions.\nIn response to the reviewers’ previous comments, the authors have made thorough and\ncareful revisions. Overall, the manuscript has been substantially improved and can be\nconsidered for publication after minor revisions.\nResponse: We are deeply grateful for the insightful feedback and valuable suggestions\noffered by the reviewers. Each comment has been carefully considered and incorporated\ninto the revised version of our manuscript, resulting in significant improvements in both\nclarity and scientific rigor.\n1. Please provide the simulated data for both AA and AB stacking models of Tp-Bth and\nTFB-Bth COF. It would be clearer and more informative if the results were plotted in the\nsame figure for direct comparison.\nResponse: We sincerely thank the reviewer for this valuable feedback and for suggesting\na clearer and more direct comparison of the data. We fully appreciate the intent behind this\nsuggestion and have revised the manuscript accordingly to address it.\nIn the initial submission, we had indeed presented the simulated AA and AB stacking\npatterns for both Tp-Bth and TFB-Bth COFs together in a single figure. However, during\nthe first round of review, another reviewer rightly pointed out that the experimental PXRD\npattern of the TFB-Bth COF, due to its lower crystallinity and broader diffraction peaks,\nexhibited visual similarities in certain regions with the simulated pattern for AB stacking.\nThis raised a valid concern that, without detailed explanation, this visual overlap could\npotentially lead to misinterpretation regarding its actual stacking mode.In response to that\nvalid concern, and to ensure absolute rigor in our conclusions and prevent any potential\nmisunderstanding for readers, we decided in the revised manuscript to present the PXRD\nanalyses for the two COFs in separate panels (i.e., Fig. 2c and 2d in the main text). This\nformat allowed us sufficient space to provide a more thorough discussion and justification\nfor the stacking mode assignment of each COF individually.\nTo actively address your current request for a \"clearer and more direct\" comparison, we\nhave implemented the following measures: We have added a new dedicated comparison\nfigure (see revised Fig. S7 in the Supporting Information). This figure places the\nexperimental PXRD data for TFB-Bth COF side-by-side with the simulated patterns for\nboth its AA and AB stacking models, accompanied by the corresponding Pawley\nrefinement.\nWe believe that this approach—using separate main-text figures for detailed discussion\nwhile providing direct side-by-side simulations in the Supporting Information for reader\nverification—effectively addresses the scholarly concerns of both reviewers. It avoids\npotential misinterpretation arising from the visual similarity of broadened peaks while\nsimultaneously providing you and all readers with the complete data necessary for a direct\nand clear comparison.\nWe are grateful again for your insightful comment, which has prompted us to present the\ndata in a more comprehensive manner, thereby significantly enhancing the clarity and\nscientific rigor of the manuscript. As suggested by the reviewer, we have revised the main\ntext to include a more detailed discussion on the precise stacking arrangement of TFB-Bth\nCOF and Tp-Bth COF. The principal amendments are enumerated as follows:\nIn Manuscript Page 8 Line 195-202:\nHowever, the limited hydrogen-bonding network in TFB–Bth COF yields inherent\nlow crystallinity. This results in closely approximated goodness-of-fit residuals (Rwp, Rp)\nfor both the AA- and AB-stacking models, preventing an unambiguous distinction based\non these metrics alone. Crucially, however, a direct visual comparison between the\nexperimental XRD pattern and the simulated profiles (AA- and AB-stacking; Figs. 2d and\nS7) exhibited a substantially superior match with the AA-stacking simulation.\nConsequently, we assign the AA-stacking mode to TFB–Bth COF as the best-supported\nstructural interpretation available from the current data.\nIn Supplementary Information Page 15:\nFig. S7. Experimental, Pawley refined (AB stacking), and simulated PXRD profiles of the\nTFB-Bth COF.\n2. Both COF membranes exhibit relatively high water contact angles (80–90°), which\nraises concerns for water treatment applications. Have the authors considered how this\nhydrophobicity might influence the energy barrier for water penetration into the pores?\nResponse: We are grateful to the reviewer for their pertinent observation concerning the\nenergy barrier for water penetration that may arise from the hydrophobic nature of the\nmembranes. We agree that this is a critical consideration for water treatment membranes.\nPlease allow us to address this concern with the following explanations based on our\nexperimental observations and the specific structure of our COF membranes. We\nacknowledge that the measured water contact angles (80–90o) indicate a relatively\nhydrophobic surface character for our COF membranes. This is primarily attributed to the\nabundance of non-polar aromatic skeletons in their framework. However, we would like to\nemphasize that despite this intrinsic hydrophobicity, the optimized Tp-Bth COF membrane\ndemonstrated a promising water permeability of 1.7 L m-2 h-1 bar-1 alongside an exceptional\nNaCl rejection of 99.6% under the tested conditions (15 bar, 2000 ppm NaCl). This robust\ndesalination performance itself suggests that any energy barrier for water entry is\neffectively overcome under operational pressures, and does not preclude high permeability.\nWhile the overall membrane surface appears hydrophobic, the internal pore channels of the\nTp-Bth COF are lined with polar functional groups from the β-ketoenamine linkages (C=O,\nN-H). The extensive intra-framework hydrogen-bonding networks can act as preferential\npathways for water molecules. Water molecules can form hydrogen bonds with these polar\nsites, effectively reducing the activation energy for pore entry and facilitating a \"relay\"\nmechanism for water diffusion through the one-dimensional channels. This phenomenon,\nwhere localized hydrophilic sites within an otherwise hydrophobic framework enhance\nwater transport, has been reported in other molecular-sieving membranes. (Angewandte\nChemie, 137(20), e202502204. (2025))\nWe thank the reviewer again for raising this important point, which allows us to provide a\ndeeper discussion on the structure-property relationship in crystalline porous membranes.\nAccordingly, the manuscript has been revised, incorporating a more detailed discussion on\nthe design of the membrane structure and the issue of hydrophobicity.\nIn Manuscript Page 11-12 Line 286-294:\nFurthermore, the as-fabricated COF membranes exhibited relatively hydrophobic\nsurfaces, characterized by water contact angles ranging from 80o to 90o (Figs. S25-26),\nshowing minor dependence on monomer concentrations. This surface hydrophobicity is\nprimarily attributed to the abundance of non-polar aromatic skeletons and the low-polarity\nimine linkages integrated within the framework structure. Crucially, however, the\nextensive hydrogen-bonding networks inherent to the COF structure can function as\npreferential pathways for water molecules. Overall, the observed surface hydrophobicity is\nunlikely to significantly impede their RO application in water desalination.\n3. Some performance tests are shown only for the Tp-Bth membrane. Please clarify why\nTFB-Bth was not included in those comparisons.\nResponse: We thank the reviewer for this pertinent observation regarding the performance\ntests. The reviewer is correct that some advanced performance evaluations, such as the\nlong-term acid stability, antifouling tests, and boron removal, were primarily focused on\nthe Tp-Bth membrane. This focused approach was based on a strategic and scientific\nrationale, which we clarify below.\nThe central theme of our work is to introduce and validate a hydrogen-bond fortification\nstrategy for creating ultra-microporous, high-performance COF desalination membranes.\nThe Tp-Bth COF membrane is the key innovation and the main subject of this study, as it\nembodies this strategy and demonstrates its success. In contrast, the TFB-Bth membrane\nserves as a critical control sample to highlight the fundamental structural and performance\ndifferences arising from the absence of the fortified hydrogen-bond network.Our initial\ncomprehensive characterization (Figs. 2-3) and basic desalination tests (Fig. 4a-c)\nestablished a clear and significant performance gap between the two membranes. The TFB-\nBth membrane exhibited a substantially lower NaCl rejection (~92.7%) compared to the\nTp-Bth membrane (~99.6%). This baseline performance clearly indicated that the TFB-Bth\nmembrane, with its larger pore size and lower crystallinity, did not meet the high bar\nrequired for effective reverse osmosis desalination. Consequently, we directed our efforts\ntowards a more in-depth investigation of the Tp-Bth membrane's potential for practical\napplication, which naturally included assessing its stability (acid, long-term) and\nantifouling properties. We would like to reassure the reviewer that the TFB-Bth membrane\nwas thoroughly included in all essential structural and functional comparisons necessary to\nestablish the fundamental structure-property relationship central to our study. Specifically,\nit was comprehensively characterized alongside the Tp-Bth membrane across all core\nanalyses, including structural elucidation via PXRD, FTIR, and solid-state NMR,\nmorphological examination by SEM and AFM, pore structure analysis through BET\nmeasurements, and the critical evaluation of basic desalination performance, such as water\npermeability and NaCl rejection, as summarized in Figure 4c. These comparative data\ncollectively and unequivocally demonstrate the profound impact of the hydrogen-bond\nfortification strategy, thereby providing a solid foundation for our subsequent focus on the\nmore promising Tp-Bth membrane for advanced application-oriented testing. In summary,\nthe selective application of advanced performance tests was a reasoned decision. It was\nbased on the Tp-Bth membrane's clearly demonstrated superiority in the initial screening,\nmaking it the more scientifically relevant and promising candidate for a detailed\ninvestigation of application-oriented properties.\nWe have now clarified this rationale in the revised manuscript in the Results and discussion\nsection to prevent any potential misunderstanding for the reader. The specific changes are\nsummarized below:\nIn Manuscript Page 17 Line 448-453:\nThe demonstrated superiority of the Tp-Bth membrane in both salt and boron rejection\nestablishes it as a highly promising candidate, directly enabled by our hydrogen-bond\nfortification strategy. To further evaluate the practical utility of our fabricated Tp-Bth\nmembranes for water desalination, we subsequently conducted systematic investigations\ninto the their antifouling performance, acid stability, and operational stability.\n4. The choice of 2000 ppm NaCl as the feed concentration should be further justified.\nConsidering the relevance to seawater desalination (≈35,000 ppm), performance under\nhigher salinity conditions should be discussed or at least commented on.\nResponse: We feel great thanks to the reviewer for raising this important point regarding\nthe feed concentration. We would like to clarify that the primary focus of this study is the\ndevelopment of membranes for the desalination of brackish water, not full-strength\nseawater. This scope directly informs and justifies our choice of a 2000 ppm NaCl feed\nsolution for performance evaluation.\nThe use of a 2000 ppm NaCl solution is a long-established and conventional standard for\nevaluating and benchmarking the performance of membranes specifically designed for\nbrackish water desalination. This is evident in both the scientific literature and industry\npractices, allowing for a direct and meaningful comparison of our novel COF membrane\nwith existing commercial BWRO membranes (such as the BW30 membrane included in\nour Fig. 4c for comparison) and other emerging materials reported in prior studies. From a\nscientific perspective, testing at 2000 ppm provides a sufficiently challenging environment\nto accurately probe the intrinsic separation properties (water permeability and salt rejection)\nof our material, which was the central goal of this foundational study. The excellent\nperformance of our Tp-Bth membrane under these brackish water-simulating conditions\n(99.6% rejection, 1.7 L m-2 h-1 bar-1) strongly demonstrates its potential for this specific\napplication. In direct response to this insightful suggestion, we have conducted additional\nexperiments at higher salinities (6000 ppm and 10,000 ppm NaCl) to further probe the\nperformance limits of our Tp-Bth membrane. The results confirm that the membrane\nmaintains rejections of 97.67% and 95.62% at 6000 ppm and 10,000 ppm, respectively,\ndemonstrating its robustness in high-salinity brackish water scenarios and indicating its\npotential for even more challenging feeds. These new findings have been added to the\nrevised manuscript. They reinforce that, while the core application scope of this work\nremains brackish water desalination, the membrane also shows promising performance\nunder elevated salinity, effectively addressing the reviewer’s comment and providing a\nmore complete picture of its capabilities. We are grateful for this suggestion, which has\nstrengthened the discussion of our membrane's potential.\nIn Manuscript Page 16 Line 410-414:\nFurthermore, we investigated the effect of salt concentration on the rejection\nperformance. Notably, the membrane exhibited minimal loss in rejection rate when the\nNaCl concentration was raised from 2000 ppm to 10,000 ppm (Fig. S38). This\ndemonstrates its exceptional stability under high osmotic pressure and confirms its strong\npotential for use in seawater desalination.\nIn Supplementary Information Page 46:\nFig. S38. The salt rejection changes of the Tp-Bth membranes with the increase of salt\nconcentration.\n5. The concentration units “2000 mg L-1” (line 360–361) and “2000 ppm” should be\nunified for consistency.\nResponse: We thank the reviewer for pointing out this inconsistency. The reviewer is\nabsolutely correct. We have now unified the concentration unit throughout the entire\nmanuscript (including the main text and Supplementary Information) to \"2000 ppm\" for\nconsistency. We apologize for this oversight.\nIn Manuscript Page 14 Line 365-366:\nFig. 4. Performance and application of Tp-Bth and TFB-Bth COF membranes. (a-b) Water\npermeance and NaCl rejection of Tp-Bth COF membranes under different Bth content and reaction time;\n(c) Separation performance of the TFB-Bth, Tp-Bth, and SW30 membranes toward a 2000 ppm NaCl\nand 5 ppm boron feed solution; (d-e) Digital photographs, SEM images (scale bar 1 μm) of Tp-Bth\nmembrane acid immersion process; (f) Performance of the long-term pH stability (pH = 3) of the Tp-\nBth membrane at a feed pressure of 15 bar and room temperature (2000 ppm NaCl); (g) Evaluation of\nantifouling performance: Normalized water flux versus operation time for Tp-Bth COF membrane using\nBSA and SA as model foulants. The fouling tests comprise three stages: (1) collecting baseline of water\nflux using a feed solution of 2,000 ppm NaCl, (2) recording flux when fouled by 200 ppm model foulants\nin a feed solution of 2,000 ppm NaCl and (3) evaluating flux recovery using a feed solution of 2,000 ppm\nNaCl after cleaning the membrane via deionized water flushing; (h) flux recovery ratio (FRR), flux\ndecline Ratio (FDR), reversible flux decline rate (DRr) and irreversible flux decline rate (DRir) for Tp-\nBth COF membrane; (i) Radar plot comparing the physicochemical properties of Tp-Bth and TFB-Bth\nCOF membranes; (j) Long-term operational stability performance of Tp-Bth membrane; (k) Membranes\nperformance in this work compared to other membranes in the literature."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The manuscript can be accepted now."}, "author_response": ""}]

Decision letter