Single-crystal 2D covalent organic frameworks for high-capacity methane storage

Domain: neuroscience
Published: 2026-02-14
Editorial outcome: minor_revision

Abstract
2D covalent organic frameworks (COFs) usually possess a polycrystalline nature as well as lower porosity and surface area than 3D counterparts, restraining their exploration over gas storage applications. Herein, a substituent strategy has been proposed and employed to generate three robust single-crystal 2D COF isomers with atom-resolution structures determined by 3D electron diffraction. Among three isomers, a precise engineering of their interlayer distance affords the highest Brunauer−Emmett−Teller surface area of ~2100 m2 g−1 and the largest pore volume of 1.40 cm3 g−1 for the desolvated GZU-1. This COF shows the highest total volumetric methane uptake of 240 cm3 (STP) cm−3 at 273 K and 100 bar among 2D COFs, even comparable with those for excellent 3D MOFs. This work not only delivers unique insight into the design of 2D single-crystal COFs by interlayer stacking regulation, but also promotes the application of highly porous 2D COFs in gas storage.

Introduction
Natural gas (NG) with about 95% methane (CH4) component is a clean and widely available energy source with relatively low carbon emission and economic advantage1–5. The low energy density of CH4, 0.04 MJ L−1, retards the application of NG in onboard transportation fuel2–6. Among potential materials and technologies for high methane storage, porous sorbents have shown bright promise, particularly under moderate pressure and ambient temperature, attributed to their extraordinarily high porosities and variable pore structures1,7–10. Accordingly, a number of three-dimensional metal-organic frameworks (3D MOFs) have been reported to exhibit high performance for methane storage3–7. MOFs are generally robust and can sustain thermal/vacuum activation to keep their high porosities3,11. Meanwhile, covalent organic frameworks (COFs) can also exhibit high porosities and stability2,12–14. However, the covalent bonds in COFs are much more directional and less flexible than the coordination bonds in MOFs, meaning that single-crystal COFs are much more difficult to synthesize than MOFs15–31 and remain scarcely explored for methane storage to date2,10,12,32.

Different from their MOFs counterparts dominated by the three-dimensional (3D) extended frameworks, most of the reported COFs are of two-dimensional (2D) layered structures1,33–36. This seems to retard the exploration of their methane storage application due to the still lack of report on high methane storage performance for 2D MOFs associated with the low porosities and instability of the latter materials. Fortunately, the late construction of highly mesoporous 2D COFs with pore volume as high 2.7 cm3 g−1 motivates us to explore 2D COFs for their methane storage functionality35. Additionally, the significant progress achieved on the single-crystal growth of COFs and single-crystal structure determination technologies over the past few years also motivates us to reveal the fundamental science of 2D/3D COFs in a more comprehensive manner16–19. However, it is still challenging to prepare 2D single-crystal COFs due to the uncontrollable nucleation and complicated crystallization processes. Only three examples, including Py-1P37–39, COF-40036, and oDMPA-Py-COF40 with atom-resolution 2D single-crystal structures have been directly obtained from monomer polymerization thus far. In-depth exploration of the growth and structure of 2D COFs single crystals is very difficult, further restraining the structure-originated property and functionality investigation. Previous investigations have revealed that changing the planarity of building blocks through substituents could lead to the preparation of high-quality 3D COFs single crystals16,41.

Herein, along the substituent strategy, we report three highly porous, robust 2D single-crystal COF isomers for their methane storage (Fig. 1), reaching total volumetric methane storage up to 240 cm3 (STP) cm−3 at 273 K and 100 bar, and in particular gravimetric methane delivery working capacity of 213 mg g−1 (5 to 65 bar at 298 K). Detailed crystal structure and molecular modeling studies show that multiple weak intermolecular interactions, such as C–H···π interactions, can collaboratively enforce the COF framework stability and thus maximize their porosities by interlayer packing regulation. This work paves the way towards a systematic development of highly porous 2D COFs for gas storage in the future.

Results
Design and synthesis of single-crystal GZU-1, GZU-2, and GZU-3
In the present case, the methyl or methoxy substituents were introduced onto the positions between the two neighboring phenyl moieties of 1,3,5-tri(phenyl)benzene to afford 1,3,5-trimethoxy-2,4,6-tris(4-aminophenyl)benzene (TOAB), 1,3,5-trimethyl-2,4,6-tris(4-formylphenyl)benzene (TTFB), 1,3,5-trimethyl-2,4,6-tris(4-aminophenyl)benzene (TTAB), and 1,3,5-trimethoxy-2,4,6-tris(4-formylphenyl)benzene (TOFB) (Supplementary Figs. 1–6). Subsequently, [3+3] imine polymerization reaction at 298 K in different solvents with the assistance of aniline and acetic acid afforded three novel 2D single-crystal COFs, GZU-1, GZU-2, and GZU-3 (Fig. 1). GZU-1 was made from TOAB and TTFB in a mixed solution of mesitylene (Mes) and 1,4-dioxane (Diox) for 15 days with a yield of 71%. In contrast, isomeric GZU-2 and GZU-3 were obtained by TTAB and TOFB in trichloromethane (TCM) and mixed Mes/1,4-dioxane, respectively, with the yield of 52 and 63%, respectively. Comparison in FT-IR data of these single-crystal COFs and building blocks indicates the disappearance of N−H stretching bonds (3339 and 3420 cm−1 for TOAB and 3338 and 3410 cm−1 for TTAB) and C=O stretching bonds (1702 cm−1 for TTFB and 1698 cm−1 for TOFB). Moreover, the presence of the C=N stretching bonds (1625 cm−1 for GZU-1, 1628 cm−1 for GZU-2, and 1629 cm−1 for GZU-3) suggests the successful imine condensation (Supplementary Figs. 7–9). This is further confirmed by the observation of the resonance peak due to the carbon from the C=N bonds at 160 ppm for GZU-1 and 157 ppm for GZU-2 and GZU-3 in their solid-state 13C cross-polarization magic-angle-spinning (CP/MAS) NMR spectra (Supplementary Figs. 10–12). Scanning electron microscopy (SEM) images reveal a rod-shaped morphology of GZU-1 with a length size up to 10 μm and hexagonal prism morphologies of GZU-2 and GZU-3 with a length size up to 5 μm (Fig. 2a–c and Supplementary Figs. 13–15). The more than 10 intense, sharp diffraction peaks for these three COFs highlight their high-crystallinity nature (Fig. 2d–f). In particular, GZU-2 and GZU-3, composed of the same building units, display obviously different PXRD patterns, indicating their isomerism nature (Fig. 2e, f and Supplementary Figs. 16–19).

Structural determination of GZU-1, GZU-2, and GZU-3
Single-crystal structural analyses of the three COF isomers were carried out by 3D electron diffraction (ED) at 77 K and resolutions of 0.8~1.0 Å, which show that they have essentially isomeric imine-linked honeycomb layered architectures with the hcb-b topology (Supplementary Figs. 20–22 and Supplementary Table 1–3). They possess similar porous frameworks with the 1D channels (16~17 Å) along the crystallographic c-axis (Fig. 1). GZU-1, GZU-2, and GZU-3 crystallize in distinct space groups, i.e. P3, P-1, P6, respectively. There are different numbers of independent imine-bonded 1,3,5-tris(phenyl)benzene-based building blocks in corresponding asymmetric units of the three COFs.

For GZU-1 derived from TOAB and TTFB, its unit cell parameters are a=b=25.600(4) Å, c=27.550(6) Å, α=β=90°, γ=120°, and V=15636(6) Å3 (Fig. 3a and Supplementary Table 1). In the asymmetric unit of GZU-1, there are two crystallographically independent TTFB and two TOAB moieties linked through imine bonds (Fig. 3a and Supplementary Fig. 23). For the TTFB units, the dihedral angles between the central benzene ring and the three peripheral benzene rings are 77.94~82.19°, respectively, disrupting the planarity of this module. This is also true for TOAB units with the dihedral angles of 48.78~56.33° between central and three peripheral benzene rings (Supplementary Fig. 23). The TOAB and TTFB building blocks are interconnected into a 2D covalent honeycomb-like layered structure (Fig. 3b and Supplementary Figs. 24, 25). Moreover, the alternating arrangement of TTAB and TOFB is observed in the neighboring layers. In this compound, there are multiple weak C−H···π interactions between adjacent layers with short H···C distances of 2.71~2.85 Å (Supplementary Fig. 26). Different from the previous 2D COFs with normal AA-, AB-, and ABC-packing structures, GZU-1 shows the unknown ABCDEF 6-fold alternating stacking (AAl6) (Figs. 1, c–e and Supplementary Fig. 27). The neighboring layers crossing the centers of TTFB and TOAB moieties are 4.58 and 4.60 Å (Fig. 3d, e and Supplementary Fig. 27). Density Functional Theory (DFT) calculations explored the relative energy of the different stacking patterns of GZU-1. The results demonstrate that the AAl6 form is the most energetically stable one, being approximately 62, 73, and 122–141 kJ mol−1 per sheet more stable than the slip AAl, the eclipsed AA, and the staggered AB patterns, respectively, as detailed in Supplementary Fig. 28 and Supplementary Table 4. All DFT-optimized geometries and details of the calculations are provided in the Supplementary Information.

For GZU-2 and GZU-3, these two COF isomers are prepared from the same building blocks of TTAB and TOFB, possessing different imine-orientation from that of GZU-1 (Supplementary Figs. 29, 30). The slight difference in the molecular configuration of 1,3,5-tris(phenyl)benzene-based building blocks with the same substituents is displayed for these three COFs (Supplementary Fig. 31). Similar to the dihedral angles between central and three peripheral benzene rings in TTFB for GZU-1, these values for TTAB unit are in a range of 78.11~87.45° for GZU-2 and 82.49~89.19° for GZU-3 (Supplementary Figs. 29 and 30). In contrast, corresponding dihedral angles of TOFB vary in 47.18~57.52° for GZU-2 and 53.34~60.42° for GZU-3. However, the numbers and strength of weak C−H···π interactions among three neighboring layers in GZU-2 and GZU-3 are different from those of GZU-1. In contrast to GZU-1, there are fewer C−H···π weak interactions between adjacent layers in GZU-2 (2.82~2.93 Å) and GZU-3 (2.58~3.03 Å) (Supplementary Figs. 32 and 33). The distances of neighboring layers crossing the centers of TTAB and TOFB units are 4.37/4.47 Å and 4.35/4.44 Å (Supplementary Figs. 34 and 35). It is worth noting that the gradual decrease of interlayered distance from GZU-1, GZU-2 to GZU-3 slightly changes their pore volume from of 1.41, 1.40, to 1.34 cm3 g−1 derived from the single-crystal structures. Furthermore, GZU-2 and GZU-3 exhibit distinctly different packing forms, namely a unidirectional ABCD 4-fold tilted AA stacking (AAt4) structure and AAl6 architecture, respectively (Supplementary Figs. 28, 34, 35). DFT calculations show that these stacking patterns are the most stable among all possible arrangements, with GZU-2 being slightly more stable than GZU-3 by 5 kJ mol−1 per sheet. Notably, the preferential packing in GZU-2 is more stable by 66, 74, and 102–135 kJ mol−1 per sheet than the AAl slip, AA eclipsed, and the staggered AB patterns, respectively, as shown in Supplementary Fig. 28 and Table 4 (see Supplementary Information for more details of the distinct geometries). Interestingly, this energetic stabilization is very similar to that discussed above for GZU-1.

The experimental PXRD patterns of these three COFs align with those derived from the single-crystal structures (Supplementary Figs. 36–38). Following the dynamic vacuum activation, the three activated COFs (GZU-1a, GZU-2a, GZU-3a) exhibit analogous PXRD patterns with parent materials, suggesting a rather rigid behavior of this family of COFs upon desolvation (Supplementary Figs. 36–38). Additionally, the phase purity of all activated GZU-1, GZU-2, and GZU-3 is also demonstrated using whole profile pattern matching by the Le Bail refinement, giving the ultimate R and R values of 4.49 and 6.66% for GZU-1a, 4.54 and 8.25% for GZU-2a, and 6.48 and 8.76% for GZU-3a, respectively (Fig. 2d–f).

To further elucidate the underlying reasons for the synthesizability and high crystallinity of these single-crystal COFs, DFT calculations were further conducted to assess the relative stability of the structures compared to other similar COFs reported in the literature, such as DISTAP-142, TPB-DMTP24, COF-70143, TTI-COF44, and HHTP-DPB45. The data presented in Fig. 4 show that GZU-2 exhibits the most favorable formation energy, at −172 kJ mol−1 per sheet, followed by GZU-3 and GZU-1 with −167 and −165 kJ mol−1 per sheet, respectively. Interestingly, these values are substantially more favorable (i.e., more negative) than those presented by the other COFs, which show values of −113, −106, −103, −71, and −59 kJ mol−1 per sheet for TPB-DMTP, DISTAP-1, COF-701, TTI-COF, and HHTP-DPB, respectively. This more favorable formation energy and consequently predicted greater stability of the resulting crystal structures can directly relate to the higher incidence of C−H⋯π interactions between the sheets of the GZU COFs. These interactions arise from the relative displacement presented by the special stacking pattern of these structures. These C−H,...,π interactions combined, which generally exhibit greater stability than the π⋯π eclipsed interactions, confer to the GZU-1, GZU-2, and GZU-3 a greater overall crystal stability46. This is ultimately associated with the high-quality single crystals obtained for these materials, a task notably difficult for 2D COFs47,48. To further explore the nature of the interaction between the COF layers, we conducted a non-covalent interaction (NCI) analysis based on the DFT electronic density. The result shown in Supplementary Fig. 39 for the GZU-1 indicates that there are two main types of interactions between the adjacent COF layers. The first one is the CH3,...,π interaction between the OMe groups and the aromatic ring from the upper (or lower) layer, with the NCI surface highlighted in green in Supplementary Fig. 39a. The second interaction is the T-shaped benzene-benzene interaction, with the NCI surface highlighted in yellow. Both interactions that can only occur due to the relative shift between the layers are individually stronger than the simple π···π benzene interaction and act as conformational lockers on the crystal structure, playing a decisive role in dictating the relative orientation of the building blocks during crystallization. This indicates the representative structural motif nature of the combination of CH3,...,π and T–shaped benzene interactions within this family of materials.

Permanent porosity of activated single-crystal 2D COFs
The permanent porosities of the activated COFs (GZU-1a, GZU-2a, and GZU-3a) were revealed using N2 adsorption and desorption measurements at 77 K (Fig. 5a and Supplementary Figs. 40 and 41). The N2 adsorption isotherms of these three COFs display a pronounced two-step uptake behavior. A sharp increase in adsorption isotherms is observed at very low relative pressures (P/P0 <0.01), followed by a more gradual uptake in the range of P/P0 ≈ 0.01–0.05. Upon further increasing the relative pressure, the other sharp increase of adsorption appears in the range of P/P0 ≈ 0.05–0.1, achieving a saturated adsorption. For GZU-1a with the largest interlayer distance among the three COFs, it also displays the largest N2 uptake of 917 cm3 g−1 at 1.0 bar with the highest BET area of 2100 m2 g−1, in line with the calculated value from the crystal structure of 2414 m2 g−1 (Supplementary Fig. 42).

In contrast, GZU-2a with the second interlayer distance adsorbs smaller N2 volume of 836 cm3 g−1 at 1.0 bar with a slightly lower BET area of 1965 m2 g−1, close to its simulated value of 2396 m2 g−1 (Supplementary Figs. 43 and 44). Its packing isomer, GZU-3a, with the smallest average interlayer distance, accommodates the least N2 amount of 820 cm3 g−1 at 1.0 bar with the smallest BET surface area of 1897 m2 g−1, consistent with the simulated value of 2326 m2 g−1. Interestingly, there exists an overall linear relationship between the average interlayer distance and experimental BET surface area for the three COFs, indicating the capability of the packing mode and imine orientation of 2D COF isomers in finely engineering the interlayer distance and in turn the pore surface area (Supplementary Fig. 45). It is worth noting that the present single-crystal COFs possess much higher BET areas (1897~2100 m2 g−1) than their unsubstituted COF counterpart TAPB-TFPB (~224 m2 g−1)49, further confirming the advantage of substituent strategy in stabilizing the permanent porosity. Following the increased interlayer distance from GZU-3a, GZU-2a, and GZU-1a, their experimental pore volume at P/P0 at 1.0 bar also gets increased in a gradual manner from 1.23, 1.28, to 1.40 cm3 g−1, overall consistent with the calculated values from the simulated structures of 1.34, 1.40, of 1.41 cm3 g−1 (Fig. 5a and Supplementary Figs. 40 and 41). In addition, the pore-size distribution curves of these three single-crystal COFs have been studied by adsorption isotherms employing the density functional theory method, displaying pores centered at 17.3 Å for GZU-1a, 17.0 Å for GZU-2a, and 16.1 Å for GZU-3a, Supplementary Figs. 46–48, aligning with the pore sizes obtained from the single-crystal structures (17.1 Å for GZU-1a, 16.9 Å for GZU-2a, and 16.2 Å for GZU-3a), Supplementary Figs. 49–51. These activated COFs afford moderate CO2 absorption of 20, 25, and 26 cm3 g−1 at room temperature and 1 bar for GZU-1a, GZU-2a, and GZU-3a due to the weak interactions between CO2 molecules and COFs (Supplementary Figs. 52–54). These results further prove their permanent porosity.

High-pressure methane storage of activated COFs
Considering the high permanent porosity and large pore volume of these activated COFs, high-pressure methane adsorption experiments were performed at 273 and 298 K (Fig. 5b and Supplementary Figs. 55, 56). The total gravimetric methane uptake of the activated GZU-1a amounts to 299 mg g−1 (418 cm3 (STP) g−1) at 298 K and 100 bar. While GZU-2a and GZU-3a with different packing mode exhibit total gravimetric methane uptake of 289 mg g−1 (405 cm3 (STP) g−1) and 264 mg g−1 (370 cm3 (STP) g−1), respectively, under the same conditions. The gravimetrically higher methane uptake for GZU-1a than GZU-2a and GZU-3a is associated with its highest gravimetric BET surface area and pore volume among the three COFs. Force field grand Canonical Monte Carlo (GCMC) simulations confirm the attractive methane sorption performance of these three COFs as highlighted in Fig. 5b and Supplementary Figs. 57–59. These simulations reveal that the high methane uptake is associated with the high accessible surface exhibited by the single-crystal COFs, with the periodic and multi-directional layer slipping introducing indentations between the COF layers that favor methane adsorption (Supplementary Fig. 60). Compared with the traditional AA and AB stacking models of 2D COFs, this special layer slipping mode of these three single-crystal COFs can increase the number and cooperative strength of C–H···π interactions between adjacent layers, which help maintain the framework rigidity during activation and high-pressure CH4 adsorption.

On the basis of the crystal densities of 0.523, 0.518, and 0.537 cm g−1 for GZU-1a, GZU-2a, and GZU-3a without guest molecules, the corresponding total volumetric methane uptakes are calculated as 218, 210, and 198 cm3 (STP) cm−3 at 298 K and 100 bar (Supplementary Tables 1–3). In particular for GZU-1a, its total volumetric methane uptake is 240 cm3 (STP) cm−3 at 273 K and 100 bar. After the high-pressure CH4 sorption experiment, three COFs display similar PXRD patterns to the as-prepared species (Supplementary Figs. 61–63), further disclosing the rigid structures of these three COFs with multiple weak C−H···π interactions.

According to the methane isotherms obtained at 273 and 298 K, the adsorption isosteric heat of GZU-1a is calculated as 13.2 kJ mol−1 (Supplementary Figs. 55, 56, 64), in agreement with the simulated value of 11.4 kJ mol−1 and indicating a weak binding interaction between methane and the COF framework4,6. This is also true for GZU-2a and GZU-3a, with the adsorption isosteric heats of 12.3 and 11.8 kJ mol−1 (Supplementary Fig. 64), also showing good consistency with the simulated values of 12.6 and 12.7 kJ mol−1, respectively. This leads to the reduced CH4 capacities at 5 bar and 298 K for GZU-1a (32 mg g−1), GZU-2a (29 mg g−1), GZU-3a (30 mg g−1) (Fig. 5b and Supplementary Fig. 65). This, in combination with the high gravimetric methane uptake of GZU-1a (245 mg g−1), GZU-2a (227 mg g−1), and GZU-3a (215 mg g−1) at 65 bar and 298 K, results in the usable methane gravimetric working capacities of 213, 198, and 185 mg g−1, respectively. It is worth noting that the total gravimetric methane capacity of GZU-1 at 65 bar and 298 K is lower than that for many excellent MOFs absorbents such as UTSA-76a (263 mg g−1)7, NOTT-101a (247 mg g−1)50, MAF-38 (247 mg g−1)11, and NJU-Bai 42 (254 mg g−1)11 (Supplementary Figs. 65–67)9–11. However, its lower adsorption capacity at 5 bar than the above-mentioned MOFs remarkably induces high gravimetric working capacity (Fig. 5c and Supplementary Fig. 67). Noticeably, introduction of substituents onto the COFs frameworks seems to be able to reduce the CH4 affinity at low pressure to enhance the gravimetric working capacity. Indeed, it must be pointed out that thus far, an increasing number of COFs have been synthesized and structurally characterized. However, high-pressure methane storage studies over COFs have been rarely explored due to the lack of single-crystal structural information to evaluate their performances. On the basis of the present result with the help of those reported previously, herein we established the correlation between total gravimetric methane uptake (Ctotal, cm3 (STP) g−1) at 65 bar and 298 K and experimental gravimetric BET area (S, m2 g−1), showing an overall linear relationship with an empirical formula of Ctotal = 0.13383 × S + 5.73085 (R = 0.93), similar to that for MOFs6 (Fig. 5d).

According to the crystal densities of COFs, the methane volumetric working capacities (5 to 65 bar) of GZU-1a, GZU-2a, and GZU-3a at 298 K are calculated as 156, 143, and 139 cm3 (STP) cm−3, respectively, (Fig. 5c). It is noteworthy that both methane volumetric and gravimetric working capacities (5 to 65 bar and 298 K) of these three COFs are significantly superior to those for the thus far reported 2D COFs (Fig. 5c). Moreover, both the methane volumetric and gravimetric working capacities (5 to 65 bar and 298 K) of GZU-1a are higher than those for GZU-2a and GZU-3a due to its larger experimentally revealed BET area and pore volume than the latter two species. In addition, the methane volumetric working capacities (5 to 65 bar and 298 K) of GZU-1a (156 cm3 (STP) cm−3) are even comparable with those for 3D COFs like COF-103 (165 cm3 (STP) cm−3)10 and TAM-TFS-COF (157 cm3 (STP) cm−3)12 (Supplementary Fig. 68). The above phenomena clearly indicate the bright perspective of 2D COFs in the methane storage since the further enlargement of pore size of hcb-b topological COFs enables the enhanced BET area and pore volume.

In summary, a substituent strategy has been proposed to design and prepare three single-crystal 2D hcb-b COF isomers with different imine orientation and stacking modes. The atom-resolution crystal structures of these three COFs have been determined using 3D ED. Among the three compounds, GZU-1 and GZU-3 show an unprecedented ABCDEF 6-fold alternating stacking mode (AAl6). Moreover, GZU-2 and GZU-3, with different imine orientations from GZU-1, exhibit the first example of stacking isomers with atom-resolution structures in single-crystal 2D COFs, and GZU-2 depicts the first unidirectional ABCD 4-fold tilted stacking structure (AAt4). Moreover, fine adjustment of interlayer distance of COFs at atomic precision results in the highest total volumetric methane uptake of 240 cm3 (STP) cm−3 at 273 K and 100 bar for the activated sample. In particular, GZU-1a possesses the highest methane gravimetric working capacity (5 to 65 bar at 298 K) of 213 mg g−1, outperforming many state-of-the-art porous materials. The exceptional methane storage capacity of 2D single-crystal COFs demonstrates that the present structure-dependent property is originated from their precisely engineered interlayer packing. These findings will stimulate an improved understanding and leveraging of 2D covalent porous materials in multi-functional applications.

Methods
Synthesis of GZU-1
A 20 mL vial was loaded with TTFB (112.5 mg, 0.26 mmol), aniline (4.0 mL), and dry mesitylene (10.0 mL). The mixture was subjected to ultrasonication until complete dissolution of the solid components. This was followed by the addition of glacial acetic acid (2.0 mL). Subsequently, a solution of TOAB (114.8 mg, 0.26 mmol) in dry 1,4-dioxane (5.0 mL) was introduced into this reaction system. The resulting mixture was left undisturbed at 25 °C for 15 days, affording a large quantity of GZU-1 single crystals with sizes up to 10 μm. The pristine GZU-1 sample was successively solvent-exchanged with 1,4-dioxane, acetone, and n-hexane for 24 h, with the solvent renewed every 8 h. After this process, the material was collected by filtration and desolvated under dynamic vacuum at 100 °C for 12 h, yielding the activated material (147.5 mg, 71%).

Synthesis of GZU-2
A 20 mL vial was loaded with TOFB (124.9 mg, 0.26 mmol), aniline (1.5 mL), and dry TCM (5.0 mL). The mixture was subjected to ultrasonication until complete dissolution of the solid components, then glacial acetic acid (2.0 mL) was added. Subsequently, a solution of TTAB (102.3 mg, 0.26 mmol) in dry TCM (5.0 mL) was immediately introduced into this reaction system. The resulting mixture was left undisturbed at 25 °C for 10 days, affording a large quantity of GZU-2 single crystals with sizes up to 5 μm. The pristine GZU-2 sample was successively solvent-exchanged with TCM and n-hexane for 24 h, with the solvent renewed every 8 h. The solvent-exchanged sample was then collected by filtration and desolvated under dynamic vacuum at 100 °C for 12 h, yielding the activated material (110.1 mg, 52%).

Synthesis of GZU-3
A 20 mL vial was loaded with TOFB (124.9 mg, 0.26 mmol), aniline (4.0 mL), and dry mesitylene (10.0 mL). The mixture was subjected to ultrasonication until complete dissolution of the solid components. This was followed by the addition of glacial acetic acid (2.0 mL). Subsequently, a solution of TTAB (102.3 mg, 0.26 mmol) in dry 1,4-dioxane (5.0 mL) was immediately introduced into this reaction system. The resulting mixture was left undisturbed at 25 °C for 15 days, yielding a large quantity of GZU-3 single crystals with sizes up to 5 μm. The pristine GZU-3 sample was successively solvent-exchanged with 1,4-dioxane, acetone, and n-hexane for 24 h, with the solvent renewed every 8 h. The solvent-exchanged sample was then collected by filtration and desolvated under dynamic vacuum at 100 °C for 12 h, affording the activated material (132.9 mg, 63%).

Density functional theory calculations
All Density functional theory (DFT) calculations were carried out using the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional51 with DFT-D3(BJ)52,53 dispersion corrections using the Quickstep module implemented in the CP2K package (version 2024.3)54. GTH pseudopotentials55,56 were employed in conjunction with triple-ζ Gaussian basis sets augmented with one set of polarization functions, together with an auxiliary plane-wave basis set using a cutoff energy of 1200 Ry mapped onto a five-level multigrid. The orbital transformation (OT) method was applied for electronic structure optimization of all models57. To assess the relative stability of different stacking isomers of the GZU COFs, various stacking configurations were constructed using the pyCOFBuilder package58, as illustrated in Supplementary Fig. 26, and systematically compared with the experimentally determined structures.

Grand canonical Monte Carlo (GCMC) simulations
All methane adsorption isotherms were simulated using the RASPA2 package59,60. For each pressure point of the isotherm, 50000 Monte Carlo cycles were conducted, and the equilibrium adsorption uptake was determined by averaging over the equilibrated portion of the trajectory, as identified using the pyMSER package61. Methane molecules were described using Lennard–Jones parameters from the TraPPE-UA force field62, while COF framework atoms were modeled with the DREIDING force field63. Cross-interaction parameters were derived using the Lorentz–Berthelot mixing rules, and all Lennard–Jones interactions were truncated and shifted to zero at a cutoff distance of 12.8 Å. All simulations were performed using supercells large enough to ensure that each lattice vector exceeds the short-range interaction cutoff, thereby eliminating interactions between periodic images. Adsorption enthalpy was evaluated based on the fluctuations of energy and particle number in the grand canonical ensemble.

Textural properties assessment
Pore size distribution, pore volume, and gravimetric surface area were calculated using the Zeo++ software (v0.3)64,65. A probe molecule with a radius of 1.86 Å was employed, and all calculations were performed in the High Accuracy mode with 100000 sampling cycles.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript describes synthesis of three single-crystal 2D COFs using a substituent-assisted strategy. Their single crystal structures are determined by 3D ED. Combining gas sorption, DFT, and GCMC, the authors reveal very high surface areas and methane uptakes/working capacities that are competitive with leading porous adsorbents. The topic and results should be interesting for the readers of Nat. Commun. However, there are several scientific flaws need further study and clarification. In addition, there are some overstatements which require modification. Therefore, I suggest a revision based on the current state of the manuscript. The detailed comments and questions are listed as following.\n1. In the abstract, the authors mentioned “…, significant 2D COF structure-originated superiority in methane storage.” The authors need to elaborate the mechanism about why the 6-fold stacking has the advantage over e.g., AB stacking, ABC stacking, etc. In addition, the structure-originated “superiority” is overstated, as there are reported 3D COFs and MOFs shows better methane storage performance (Science, 2024, 386, 693; J. Am. Chem. Soc. 2014, 17, 6207, etc.).\n2. The authors mentioned ““The N₂ sorption isotherms of these three COFs possess the Type IV form, …”. According to IUPAC, a Type IV isotherm is characteristic of mesopores with capillary condensation and often hysteresis. None of those presents in the isotherm of the COFs. The authors need to revise this part and the following claims.\n3. The authors report a total volumetric CH4 uptake of 240 cm3 (STP) cm-3 at 273 K and 100 bar for GZU-1a, and claim “very close to the DOE target”. However, the DOE target of 263 cm3 (STP) cm-3 is explicitly defined for 65 bar and ~298 K (room temperature). Therefore, the statement from the authors is misleading and need to be revised.\n4. The authors concluded “…, GZU-1 exhibit the first example of stacking isomers in single-crystal 2D COFs …”. However, COF isomers with different stacking modes have been reported (ACS Appl. Mater. Interfaces 2021, 13, 29471; J. Mater. Chem. A, 2025, 13, 27661). The authors need to reconsider their claim about the first.\n5. In the Supplementary Information, the authors calculated “formation energies” per COF sheet for GZU and other COFs. The authors further claim that the more negative values “highlight the high thermodynamic stability of the GZU series compared to other structurally related frameworks, facilitating the growth of high-quality single crystals.” In solvothermal COF synthesis, kinetics, solubility, reversibility of bond formation, etc., are all critical. Claiming that more exergonic formation energies “facilitate the growth of high-quality single crystals” is oversimplified.\n6. While the authors have done it partly, they need to address all A and B alerts in the checkcif files.", "reviewer_2": "Baoqiu Yu et al. report the synthesis of three new imine-linked COFs based on methyl- and methoxy-substituted triphenylbenzene monomers. The COFs were obtained as small single crystals via a modulated synthesis using aniline. Single crystal structures were obtained via 3D electron diffraction (3DED). They reveal rather complex stacking patterns of slightly shifted COF layers. The COFs are highly porous with BET surface areas well above 2000 m2 g-1 and methane storage capacities of up to 213 mg g-1 at 298 K and 5 – 65 bar.\n2D COF single crystals have remained scarce due to their challenging synthesis. Structure analysis of honeycomb lattice 2D COFs with their complex layer stacking is an important finding. There has been indirect evidence that these materials are generally more complex than the simple AA stacking pictures that are commonly used to describe most 2D COFs, but the direct evidence here from single crystal data will certainly increase the understanding of these COF geometries.\nHowever, I was wondering if the authors could quantify the density of stacking faults – I assume they should be quite common for ABCDEF layer sequences?\nThe materials are highly porous with very well-defined N2 sorption isotherms and high BET surface areas. However, I could not find the pore size distributions. These should be added.\nRegarding the BET surface areas, the ranges where the BET model was applied might be incorrect. BET plots are up to 0.05 which is in the region of the mesopore filling step. However, the BET model applies only to the multilayer formation without capillary condensation effects. Typically one will find two linear regions in the BET plots of mesoporous COFs and it would be the one at lower relative pressure that should be used.\nGZU 2 seems to change slightly upon vacuum activation. But the single crystal structure is also recorded under high vacuum, and its simulation seems closer to the PXRD of the activated COF. I think the patterns used for the PXRD refinements in Figure 2 should be the ones of the activated (solvent-free) COFs.\nMinor points:\nLine 80 should read methoxy (not methoxyl) substituents\nSuppl figure 41 – please plot separately or shifted to be able to see individual isotherms\nIn summary, this is an important and convincing study. A couple of technical aspects should be updated/corrected as pointed out above. After these changes I recommend the publication in Nature Communications.", "reviewer_3": "I enjoyed reading this interesting article titled \"Single-crystal 2D Covalent Organic Frameworks for High-capacity Methane Storage\" by Yu et al. The authors propose and demonstrate a substituent strategy to prepare three single-crystal 2D hcb-b COFs isomers with different imine orientation and stacking modes. The high degree of crystallinity evidenced by the e-diffraction is noteworthy and as explained by the authors, this is quite rare in 2D COFs. The structural modeling is well done and brings important design aspects in layer stacking. The importance of the functional side groups in fine-tuning the stacking to achieve high methane storage is valuable. The authors have identified through computational modeling the superior formation energy arising from the efficient molecular interactions and also comparison with other related COF structure has been used to elucidate this. I think the figures from the SI need to be moved into the main text for easy readability and this part is important to the work. Currently, the DFT studies identify the higher crystallinity of these phases via formation energy which is attributable more to the periodic lattice, however, an alternate modeling at molecular level could help identify the impact of the side groups such as -OCH3 in influencing the packing and thereby the order. This can be included to garner further insights. Overall, this is a neat well-investigated work on a high-quality COF material contributing further to this interesting class of 2D COFs. I recommend its publication in Nature Communications."}, "author_response": "Response to reviewers' reports:\nReviewer 1:\nComment: This manuscript describes synthesis of three single-crystal 2D COFs using\na substituent-assisted strategy. Their single crystal structures are determined by 3D\nED. Combining gas sorption, DFT, and GCMC, the authors reveal very high surface\nareas and methane uptakes/working capacities that are competitive with leading\nporous adsorbents. The topic and results should be interesting for the readers of Nat.\nCommun. However, there are several scientific flaws need further study and\nclarification. In addition, there are some overstatements which require modification.\nTherefore, I suggest a revision based on the current state of the manuscript. The\ndetailed comments and questions are listed as following.\nResponse: Thanks a lot for the time and effort of this reviewer in reviewing our paper.\nWe are also very grateful for his/her encouragements and professional comments\ntowards further improving the quality of this paper.\n(1) In the abstract, the authors mentioned “…, significant 2D COF\nstructure-originated superiority in methane storage.” The authors need to elaborate\nthe mechanism about why the 6-fold stacking has the advantage over e.g., AB stacking,\nABC stacking, etc. In addition, the structure-originated “superiority” is overstated, as\nthere are reported 3D COFs and MOFs shows better methane storage performance\n(Science, 2024, 386, 693; J. Am. Chem. Soc. 2014, 17, 6207, etc.).\nResponse: Thanks a lot for this professional comment. According to Density\nFunctional Theory (DFT) calculations, the ABCDEF 6-fold stacking structure of\nGZU-1 is the most energetically stable one, being approximately 62, 73, and 122-141\nkJ mol−1 per sheet more stable than the slip AAl, the eclipsed AA, and the staggered\nAB patterns, respectively, Supplementary Fig. 26 and Supplementary Table 4. In\naddition, the 6-fold stacking structure of GZU-1 introduces a larger degree of\ninterlayer slip and generates periodic interlayer indentations that expose additional\nadsorption-accessible surface. Moreover, this stacking mode can increase the number\nand cooperative strength of C–H···π interactions between adjacent layers, which help\nmaintain the framework rigidity during activation and high-pressure CH adsorption.\nhigh-pressure methane storage and have been widely studied. It is worth noting that\ndifferent from their MOFs counterparts which are dominated by the three-dimensional\n(3D) extended frameworks, most of the reported COFs are of two-dimensional (2D)\nlayered structures. This seems to retard the exploration over their methane storage\napplication due to the still lack of report on high methane storage performance for 2D\nMOFs associated with their low porosities and instability. Fortunately, the late\nconstruction of highly mesoporous 2D COFs with pore volume as high as 2.7 cm3 g−1\nmotivates us to explore 2D COFs for their methane storage functionality (Adv. Mater.\n2020, 32, 1905776). In addition, following the instruction of this reviewer, we have\nremoved the expression “structure-originated superiority,” which might be interpreted\nas overstated. The corresponding result has been corrected in the revised version of\nmanuscript.\n(2) The authors mentioned ““The N₂ sorption isotherms of these three COFs possess\nthe Type IV form, …”. According to IUPAC, a Type IV isotherm is characteristic of\nmesopores with capillary condensation and often hysteresis. None of those presents in\nthe isotherm of the COFs. The authors need to revise this part and the following\nclaims.\nResponse: Thank this reviewer very much for this important clarification. We fully\nagree with this reviewer in the point that the N₂ sorption isotherms of these three\nCOFs at 77 K do not exhibit the capillary condensation or hysteresis feature\ncharacteristic of IUPAC Type IV isotherm. As a consequence, we have revised the\nmanuscript to remove the Type IV classification. Instead we re-describe in this way\n“The N₂ adsorption isotherms of these three COFs at 77 K display a pronounced\ntwo-step uptake behavior. A sharp increase in adsorption isotherms is observed at\nvery low relative pressures (P/P₀ < 0.01), followed by a more gradual uptake in the\nrange of P/P₀ ≈ 0.01–0.05. Upon further increasing the relative pressure, the other\nsharp increase of adsorption appears in the range of P/P₀ ≈ 0.05–0.1, achieving a\nsaturated adsorption.” This sorption behavior is similar to the mesoporous COFs with\npore size close to 2 nm. The above behavior allows these three COFs to be classified\nas quasi-mesoporous materials. All subsequent statements that rely on the previous\nType IV assignment in the present revised version of manuscript have also been\ncorrected to ensure consistency with the IUPAC classification. These revisions do not\naffect the conclusion regarding the permanent porosity of the materials. In addition,\nthe pore-size distribution curves of these three single-crystal COFs have been studied\nby adsorption isotherms employing the density functional theory method, displaying\npores centered at 17.3 Å for GZU-1a, 17.0 Å for GZU-2a, and 16.1 Å for GZU-3a,\nSupplementary Figs. 44−46, aligning with the pore sizes obtained from the\nsingle-crystal structures (17.1 Å for GZU-1a, 16.9 Å for GZU-2a, and 16.2 Å for\nGZU-3a), Supplementary Figs. 47−49. The above results further confirm that these\nthree COFs are quasi-mesoporous materials. Corresponding result has been added into\nthe revised version of manuscript and Supporting Information.\n(3) The authors report a total volumetric CH uptake of 240 cm3 (STP) cm-3 at 273 K\n4\nand 100 bar for GZU-1a, and claim “very close to the DOE target”. However, the\nDOE target of 263 cm3 (STP) cm-3 is explicitly defined for 65 bar and ~298 K\n(room temperature). Therefore, the statement from the authors is misleading and need to be\nrevised.\nResponse: We are indeed very sorry for having made such kind of mistakes. The\nincorrect comparisons have been removed in the revised manuscript. In addition, the\nwhole manuscript has been carefully checked to avoid such kind of errors.\n(4) The authors concluded “…, GZU-1 exhibit the first example of stacking isomers in\nsingle-crystal 2D COFs …”. However, COF isomers with different stacking modes\nhave been reported (ACS Appl. Mater. Interfaces 2021, 13, 29471; J. Mater. Chem. A,\n2025, 13, 27661). The authors need to reconsider their claim about the first.\nResponse: Good point. Just as pointed out by this reviewer, COF isomers with\ndifferent stacking modes have been reported. The precise control over the interlayer\nstacking mode for 2D COFs is a challenging pursuit due to the complicated and\nuncontrollable interactions of 2D sheets. To the best of our knowledge, about 10 cases\nof work focusing on the stacking mode control of 2D COFs have been reported thus\nfar (refs: Nat. Commun. 2024, 15, 194; J. Am. Chem. Soc. 2023, 145, 21798-21806; J.\nAm. Chem. Soc. 2022, 144, 20363-20371; ACS Appl. Mater. Interfaces 2021, 13,\n29471; Adv. Mater. 2025, 37, 2500468; Angew. Chem. Int. Ed. 2025, 64, e202512603;\nand J. Mater. Chem. A, 2025, 13, 27661, etc.). Notably, these aforementioned COFs\nstructures were identified through PXRD studies with the help of theoretical\nsimulations, thus bringing somehow structural uncertainties due to their\npolycrystalline nature. As a result, constructing single-crystal 2D COFs with\natom-resolution single-crystal structures and modulating their interlayer stacking\npatterns are important in this field. These 2D COF isomers (GZU-1, GZU-2, and\nGZU-3) feature atom-resolution structures determined by 3D electron diffraction, a\nprecise level of structural elucidation not achieved in previous studies. To avoid any\npotential ambiguity, we have added further clarification in the revised version of\nmanuscript.\n(5) In the Supplementary Information, the authors calculated “formation energies”\nper COF sheet for GZU and other COFs. The authors further claim that the more\nnegative values “highlight the high thermodynamic stability of the GZU series\ncompared to other structurally related frameworks, facilitating the growth of\nhigh-quality single crystals.” In solvothermal COF synthesis, kinetics, solubility,\nreversibility of bond formation, etc., are all critical. Claiming that more exergonic\nformation energies “facilitate the growth of high-quality single crystals” is\noversimplified.\nResponse: Thanks a lot for this important and insightful comment. We fully agree\nwith this reviewer in the point that COF crystallization is governed by a complex\ninterplay of factors including reaction kinetics, monomer solubility, bond reversibility,\nand nucleation–growth dynamics, which cannot be determined solely by\nthermodynamic formation energies. Our previous wording suggesting that more\nexergonic formation energies directly “facilitate the growth of high-quality single\ncrystals” is indeed an oversimplification. In the revised version of Supplementary\nInformation, we have removed this causal implication and now restrict the discussion\nof formation energies to their role as indicators of relative thermodynamic stability\namong different stacking arrangements and frameworks, which might help rationalize\nthe observed preference for certain packing modes.\n(6) While the authors have done it partly, they need to address all A and B alerts in\nthe checkcif files.\nResponse: Thanks a lot for pointing out this matter. According to this instruction, we\nhave carefully revisited all A and B alerts in the CheckCIF reports and provided\ndetailed explanations in the revised crystallographic data. Many of these alerts\noriginate from the intrinsic limitations of 3D electron diffraction (3D ED) and the\nweakly scattering, beam-sensitive nature of 2D single-crystal COFs. Unlike X-ray\ndiffraction, 3D ED data collection is typically constrained by limited tilting ranges,\nreduced high-angle completeness, and unavoidable dynamical scattering effects, all of\nwhich can trigger alerts related to completeness, data redundancy, or refinement\nrestraints.\nWe have also refined the structures and reprocessed the data multiple times, making\nevery reasonable effort to reduce or eliminate these alerts (such as the anisotropic\nrefinement). However, despite extensive attempts and considerable effort, some A and\nB alerts could not be completely removed due to the less data collected by 3D ED and\npoor single-crystal quality of 2D COFs. For example, (1) RINTA01_ALERT_3_A,\nthe value of Rint is greater than 0.25; (2) PLAT020_ALERT_3_A, the value of Rint is\ngreater than 0.12. As can be easily expected, for single-crystal COFs characterized by\n3D ED, the presence of such alerts is widely recognized as common and often\nunavoidable (refs: Nat. Chem. 2025 17, 571–581; Nat. Chem. 2025, 17, 226–232; J.\nAm. Chem. Soc. 2023, 145, 13537−13541; Chem 2024, 10, 2170−2179; J. Am. Chem.\nSoc. 2023, 145, 25332−25340; and .etc), reflecting the experimental constraints rather\nthan structural inaccuracies. To ensure transparency, we have now provided explicit\njustifications for each alert in the revised CIF files. Importantly, the remaining alerts\ndo not compromise the validity of the structural models, which are strongly supported\nby consistent PXRD patterns, pore characteristics, and DFT calculations.\nReviewer 2:\nComment: Baoqiu Yu et al. report the synthesis of three new imine-linked COFs\nbased on methyl- and methoxy-substituted triphenylbenzene monomers. The COFs\nwere obtained as small single crystals via a modulated synthesis using aniline. Single\ncrystal structures were obtained via 3D electron diffraction (3D ED). They reveal\nrather complex stacking patterns of slightly shifted COF layers. The COFs are highly\nporous with BET surface areas well above 2000 m2 g-1 and methane storage\ncapacities of up to 213 mg g-1 at 298 K and 5–65 bar.\n2D COF single crystals have remained scarce due to their challenging synthesis.\nStructure analysis of honeycomb lattice 2D COFs with their complex layer stacking is\nan important finding. There has been indirect evidence that these materials are\ngenerally more complex than the simple AA stacking pictures that are commonly used\nto describe most 2D COFs, but the direct evidence here from single crystal data will\ncertainly increase the understanding of these COF geometries.\nResponse: Many thanks for the time and effort of this reviewer paid in reviewing this\npaper. As can be found in the revised version of manuscript and Supporting\nInformation, all the comments have been thoroughly addressed accordingly.\n(1) However, I was wondering if the authors could quantify the density of stacking\nfaults – I assume they should be quite common for ABCDEF layer sequences?\nResponse: Quite interesting question, thanks a lot. Actually it is hard to quantify the\ndensity of stacking faults for the 2D COFs reported thus far due to the lack of their\neither single crystal X ray diffraction analysis- or 3D ED analysis-determined\nstructures. On the other hand, without either single crystal X ray diffraction analysis-\nor 3D ED analysis-determined structures, at this stage we cannot say that those 2D\nCOFs with structures determined on the basis of powder X ray diffraction analysis\nwith the help of theoretical modeling are truly correct or accurate. At the end of this\nparagraph, we would like to emphasize that for most previously reported 2D COFs,\ntheir structural models were derived from powder X-ray diffraction combined with\ntheoretical simulations, which inherently provide averaged structural information (ref:\nChem. Sci., 2020, 11, 12647–12654).\n(2) The materials are highly porous with very well-defined N sorption isotherms and\n2\nhigh BET surface areas. However, I could not find the pore size distributions. These\nshould be added.\nResponse: Thanks a lot for this very nice advice! According to the instruction, the\npore-size distribution curves of these three single-crystal COFs have been studied by\nadsorption isotherms employing the density functional theory method, displaying\npores centered at 17.3 Å for GZU-1a, 17.0 Å for GZU-2a, and 16.1 Å for GZU-3a,\nSupplementary Figs. 44−46, aligning with the pore sizes obtained from the\nsingle-crystal structures (17.1 Å for GZU-1a, 16.9 Å for GZU-2a, and 16.2 Å for\nGZU-3a), Supplementary Figs. 47−49. It is worth noting again that The N₂ adsorption\nisotherms of these three COFs at 77 K display a pronounced two-step uptake behavior.\nA sharp increase in adsorption isotherms is observed at very low relative pressures\n(P/P₀ < 0.01), followed by a more gradual uptake in the range of P/P₀ ≈ 0.01–0.05.\nUpon further increasing the relative pressure, the other sharp increase of adsorption\nappears in the range of P/P₀ ≈ 0.05–0.1, achieving a saturated adsorption, Fig. 5a and\nSupplementary Figs. 38 and 39. This sorption behavior is similar to the mesoporous\nCOFs with pore size close to 2 nm. Corresponding result has been added into the\nrevised version of manuscript and Supporting Information.\n(3) Regarding the BET surface areas, the ranges where the BET model was applied\nmight be incorrect. BET plots are up to 0.05 which is in the region of the mesopore\nfilling step. However, the BET model applies only to the multilayer formation without\ncapillary condensation effects. Typically one will find two linear regions in the BET\nplots of mesoporous COFs and it would be the one at lower relative pressure that\nshould be used.\nResponse: Thanks again for this good suggestion towards further improving the\nquality of this work. On the basis of this instruction, the BET plots and reassessed\nfitting ranges of these three COFs have been re-examined to ensure the utilization of a\nsingle-layer N filling range. To avoid fitting errors in BET calculations, the\n2\nfilling is up to 0.046 for GZU-1a, 0.048 for GZU-2a, and 0.048 for GZU-3a,\nrespectively. As shown is Supplementary Figs. 40−42 (Figs. R1−R3 also given\nbelow), the linear regions at lower relative pressure are used to calculated the BET\nsurface areas of activated GZU-1, GZU-2, and GZU-3, which are 2100, 1965, and\n1897 m² g⁻¹, respectively. Corresponding figures and data have been updated in the\nrevised version of manuscript and Supporting Information. These corrections do not\nalter the overall trends or the structure–property relationships discussed in the work.\nFigure R1. The BET surface area calculation for GZU-1a based on the N adsorption isotherm at\n2\ntaken from P/P = 0.0015 – 0.046. [C: the BET constant; V : the monolayer capacity]\n0 m\nThe following values obtained for the BET Surface Area, Slope, Y-Intercept, C, V , R2, 1/(√C + 1),\nm\nand P/P at V Using the Fittings.\n0 m\nBET surface area 2099.8700 ± 39.3132 m2 g–1\nSlope 0.002066 ± 0.000039 g cm–3\nY-intercept 0.000007 ± 0.000001 g cm–3\nC 317.785372\nV 482.4428 cm3 g–1\nm\nR2 0.998\n1/(√C + 1) 0.0531\nP/P at V 0.0432\n0 m\nFigure R2. The BET surface area calculation for GZU-1a based on the N adsorption isotherm at\n2\ntaken from P/P = 0.0002 – 0.048.\nm\nand P/P at V Using the Fittings. [C: the BET constant; V : the monolayer capacity]\n0 m m\nBET surface area 1965.0422 ± 36.1635 m2 g–1\nSlope 0.002209 ± 0.000041 g cm–3\nY-intercept 0.000006 ± 0.000001 g cm–3\nC 400.628167\nV 482.4428 cm3 g–1\nm\nR2 0.998\n1/(√C + 1) 0.0476\nP/P at nm 0.0411\n2\ntaken from P/P = 0.0003 – 0.048.\nm\nand P/P at V Using the Fittings. [C: the BET constant; V : the monolayer capacity]\n0 m m\nBET surface area 1897.1534 ± 30.8696 m2 g–1\nSlope 0.002289 ± 0.000037 g cm–3\nY-intercept 0.000005 ± 0.000001 g cm–3\nC 450.471415\nV 435.8689 cm3 g–1\nm\nR2 0.998\n1/(√C + 1) 0.0450\nP/P at nm 0.0397\n0\n(4) GZU 2 seems to change slightly upon vacuum activation. But the single crystal\nstructure is also recorded under high vacuum, and its simulation seems closer to the\nPXRD of the activated COF. I think the patterns used for the PXRD refinements in\nFigure 2 should be the ones of the activated (solvent-free) COFs.\nResponse: Thanks a lot for this careful and technically insightful observation and\ngood suggestion. As pointed out by the reviewer, the single-crystal structure of these\nthree COFs was collected under high-vacuum conditions, and the simulated PXRD\npatterns derived from the solvent-free structures show better agreement with the\nPXRD of the activated COFs. In the revised manuscript, we have clarified that the\nPXRD refinements in Figure 2 are based on the solvent-free/activated structures.\nThese revisions improve consistency between the structural models and the\nexperimental PXRD data.\n(5) Line 80 should read methoxy (not methoxyl) substituents\nResponse: We are indeed very sorry for having made such kind of mistakes, which\nhave been corrected in the revised version of manuscript. In addition, the whole\nmanuscript has been carefully checked to avoid such kind of errors.\n(6) Suppl figure 41 – please plot separately or shifted to be able to see individual\nisotherms\nResponse: Thanks again for this good suggestion. As can be seen in Fig. 5a and\nSupplementary Figs. 38 and 39, the N sorption isotherms at 77 K of these three\n2\nactivated single-crystal COFs have been presented separately for clarity in the revised\nversion of Supporting Information.\n(7) In summary, this is an important and convincing study. A couple of technical\naspects should be updated/corrected as pointed out above. After these changes I\nrecommend the publication in Nature Communications.\nResponse: Thanks a lot for the time and effort of this reviewer in reviewing our paper.\nWe are also very grateful for his/her encouragements and professional comments\ntowards further improving the quality of this paper.\nReviewer 3:\nComment: I enjoyed reading this interesting article titled \"Single-crystal 2D Covalent\nOrganic Frameworks for High-capacity Methane Storage\" by Yu et al. The authors\npropose and demonstrate a substituent strategy to prepare three single-crystal 2D\nhcb-b COFs isomers with different imine orientation and stacking modes. The high\ndegree of crystallinity evidenced by the e-diffraction is noteworthy and as explained\nby the authors, this is quite rare in 2D COFs. The structural modeling is well done\nand brings important design aspects in layer stacking. The importance of the\nfunctional side groups in fine-tuning the stacking to achieve high methane storage is\nvaluable. The authors have identified through computational modeling the superior\nformation energy arising from the efficient molecular interactions and also\ncomparison with other related COF structure has been used to elucidate this. I think\nthe figures from the SI need to be moved into the main text for easy readability and\nthis part is important to the work. Currently, the DFT studies identify the higher\ncrystallinity of these phases via formation energy which is attributable more to the\nperiodic lattice. However, an alternate modeling at molecular level could help\nidentify the impact of the side groups such as -OCH in influencing the packing and\n3\nthereby the order. This can be included to garner further insights. Overall, this is a\nneat well-investigated work on a high-quality COF material contributing further to\nthis interesting class of 2D COFs. I recommend its publication in Nature\nCommunications.\nResponse: Many thanks for the time paid in reviewing this paper as well as the\nprofessional comments and encouragements of this reviewer. According to the\ninstruction, the figure comparing the formation energy between the GZU COFs with\nsimilar structures reported previously has been added into the revised version of\nmanuscript from the Supporting Information.\na b\nT-shape benzene\ninteraction\nCH-π\n3\ninteraction c\nFigure R4. Non-covalent interactions (NCI) surface plots of the between GZU-1\nCOF layers. (a) The main interactions between the layers arises from the CH ···π\n3\nfrom the OMe groups and benzene from adjacent layers, represented in green, and the\nT-shaped benzene-benzene interactions, represented in yellow. Other small lateral\ninteractions between the adjacent layers are represented in blue. Perpendicular (b) and\nin-plane (c) views with all the interactions between the layers. gray: carbon, white:\nhydrogen, red: oxygen, and blue: nitrogen.\nIn addition, based on the suggestion of this reviewer, we perform additional\nsimulations to elucidate the impact of the OMe side groups on the intermolecular\ninteractions between the COF layers, and thereby its impact on the overall\ncrystallinity. Specifically, we performed non-covalent interaction (NCI) plot analysis\nbased on the QTAIM theory (ref: Comput. Phys. Commun., 2014, 185, 1007–1018.)\nusing the electronic density for both the crystal structure and molecular models.\nThis new result helps confirm that the high crystallinity of the GZU COFs series is\ndriven by specific interlayer interactions that function as \"conformational lockers\".\nNCI analysis reveals that the layers are stabilized by CH ···π interactions between\n3\nmethoxy groups and aromatic rings, alongside T-shaped benzene-benzene interactions,\nSupplementary Fig. 37 (Fig. R4 also given above). These forces, which are stronger\nthan standard π···π stacking, dictate the precise orientation of the building blocks\nduring the crystallization process. This structural motif of GZU-1 provides a\nrepresentative explanation for the superior structural order found throughout this\nfamily of materials.\nWe are very much grateful for the reviewer's overall positive evaluation and\nrecommendation for publication. The insightful comments have helped us improve\nthe clarity, presentation, and mechanistic discussion of the manuscript, and we believe\nthe revised version more clearly conveys the significance of this work to the broader\nCOF field."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all my comments. I am happy with the improvements being made and recommend the manuscript for publication.", "reviewer_2": "Baoqiu Yu et al. report the synthesis of three new imine-linked COFs based on methyl- and methoxy-substituted triphenylbenzene monomers. The COFs were obtained as small single crystals via a modulated synthesis using aniline. Single crystal structures were obtained via 3D electron diffraction (3DED). They reveal rather complex stacking patterns of slightly shifted COF layers. The COFs are highly porous with BET surface areas well above 2000 m2 g-1 and methane storage capacities of up to 213 mg g-1 at 298 K and 5–65 bar.\nBET analysis and pore size distributions have been updated/added.\nThe authors have fully addressed my comments. I suggest the publication without further changes.", "reviewer_3": "The revisions are satisfactory and has certainly improved the manuscript's thoroughness. Authors have responded to all the technical queries with rational explanations. I recommend its publication in this revised form."}, "author_response": "Response to reviewers' reports:\nReviewer 1:\nComment: The authors have addressed all my comments. I am happy with the\nimprovements being made and recommend the manuscript for publication.\nResponse: Many thanks for the time and efforts paid by this reviewer in reviewing\nthis paper.\nReviewer 2:\nComment: Baoqiu Yu et al. report the synthesis of three new imine-linked COFs\nbased on methyl- and methoxy-substituted triphenylbenzene monomers. The COFs\nwere obtained as small single crystals via a modulated synthesis using aniline. Single\ncrystal structures were obtained via 3D electron diffraction (3DED). They reveal\nrather complex stacking patterns of slightly shifted COF layers. The COFs are highly\nporous with BET surface areas well above 2000 m2 g-1 and methane storage\ncapacities of up to 213 mg g-1 at 298 K and 5–65 bar.\nBET analysis and pore size distributions have been updated/added.\nThe authors have fully addressed my comments. I suggest the publication without\nfurther changes.\nResponse: Thanks a lot for the time and efforts of this reviewer in reviewing our\npaper again.\nReviewer 3:\nComment: The revisions are satisfactory and has certainly improved the manuscript's\nthoroughness. Authors have responded to all the technical queries with rational\nexplanations. I recommend its publication in this revised form.\nResponse: Many thanks for the time paid in reviewing this paper as well as the\nencouragements of this reviewer."}]

Decision letter