Facile synthesis of heteroporous covalent organic frameworks with dual linkages: a “three-in-one” strategy

INTRODUCTION

As a burgeoning crystalline material, covalent organic frameworks (COFs) have exhibited remarkable potentials in diverse fields, including catalysis^[1-4], adsorption and separation^[5-8], energy storage^[9-12], optoelectronic devices^[13-15], and sensing^[16-19], due to their highly ordered structure, permanent porosity, and excellent stability^[20-25]. Since the first example of boroxine-linked COFs reported in 2005^[26], COFs with different linkages have been investigated, such as −C=N−^[27-29], −C−N−^[30-32], −B−O−^[33-35], −B−N−^[36], −N=N−^[37], and −C=C−^[38-41]. In this context, the investigation of COFs with dual linkages has garnered significant attention due to their ability to combine the physical and chemical properties from two distinct connectors, thereby endowing COFs with novel functionalities. However, the construction of dual-linkage COFs remains a great challenge due to the presence of multiple functional groups within the framework and possible interference between the formation of two or more covalent bonds, thus complicating the synthesis. Consequently, most reported COFs have been synthesized based on single linkages. Therefore, there is an urgent demand for developing effective and convenient strategies to construct COFs with dual or even multiple linkages.

To achieve the construction of COFs with dual linkages, increasing synthetic efforts have been devoted; however, success has been limited^[42-45]. One relatively effective approach is the copolymerization of multi-component monomers, where different functional groups are introduced onto multiple monomers to enable the construction of COFs with various types of covalent bonds. For example, Liang et al. reported the construction of dual-linkage dual-pore COFs composed of reversible hydrazone bonds and boroxine rings through the co-condensation of two-component monomers^[42]. However, the stoichiometry of different monomers and their distinct solubilities exert a significant impact on the construction of COFs with dual linkages. In most cases, only a part of the monomers were fully dissolved during synthesis. Finding a suitable solvent combination that will dissolve the monomers while maintaining a specific stoichiometry to ensure sufficient 2D or 3D polymerization is a challenge^[46]. An alternative strategy involves sequential modulation of reaction conditions to generate diverse covalent bonds for the construction of dual-linkage COFs. For instance, Hu et al. reported a new dual-linkage COF (COF-FD1) containing both alkene and imine linkages through orthogonal Schiff base reaction and Knovenagel condensation^[44]. However, this approach involves multiple reaction steps, leading to an intricate synthesis of COFs with dual linkages. Considering the above dilemmas, we envisioned that the COFs with dual linkages could be constructed by self-condensation of elaborately designed monomers. In our previous work^[47,48], we have developed a “two-in-one” molecular design strategy for the preparation of 2D COFs under simpler conditions. Actually, based on the preparation conditions of COFs, the “three-in-one” strategy to construct COFs with dual linkages by self-condensation of monomers might be an alternate approach to accessing COFs with dual linkages. This strategy not only simplifies the COF synthesis but also improves the solvent adaptability for the monomers, which has, however, not yet been explored.

EXPERIMENTAL

To verify our hypothesis, we designed and synthesized two new trifunctionalized monomers using simple synthetic routes, which were then used to develop COFs with dual linkages ((4-amino-4"-(2,2-dioxan-1,3-dioxan-5-yl)-[1,1':3',1"-terphenyl]-5'-yl)boronic acid (ADTB)-COF and (4'-amino-5'-(4-(2,2-dioxan-1,3-dioxan-5-yl)phenyl)-[1,1':3',1'-teroxan]-5-yl) boronic acid (ADPB)-COF) via the intermolecular self-condensation [Scheme 1]. The corresponding date analysis confirmed that the C=N bond and B₃O₃ ring can be generated simultaneously under solvothermal conditions, demonstrating the successful construction of COFs with dual linkages. In addition, due to the low symmetry of the monomers, the ADTB-COF and ADPB-COF displayed dual pore structures with periodic heterogeneities. Benefiting from the abundant micropores and high surface area, ADPB-COF showed excellent selective adsorption capability of C₃H₈/CH₄ (174, 298 K/1 bar). The current study introduces a novel approach for constructing COFs with dual linkages, which simplified the synthesis process and provided a novel opportunity for developing functional COFs with multiple different linkages.

Scheme 1. Schematic diagram of dual-pore 2D COFs with dual linkages constructed by a “three-in-one” strategy. COFs: Covalent organic frameworks; ADTB: (4-amino-4"-(2,2-dioxan-1,3-dioxan-5-yl)-[1,1':3',1"-terphenyl]-5'-yl) boronic acid; ADPB: (4'-amino-5'-(4-(2,2-dioxan-1,3-dioxan-5-yl)phenyl)-[1,1':3',1'-teroxan]-5-yl) boronic acid.

RESULTS AND DISCUSSION

The new trifunctionalized monomers, ADTB and ADPB [Supplementary Scheme 1], were thoroughly characterized by various methods, including nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry [Supplementary Figures 1-12]. The ADTB-COF and ADPB-COF were synthesized via the self-polycondensation of ADTB and ADPB, respectively, under solvothermal conditions [Supplementary Schemes 2 and 3,Supplementary Figures 13 and 14]. These trifunctionalized modules can be conveniently used to construct dual-linkage COFs in a variety of solvents, unlike most reported COFs, which involved the co-condensation of two or more types of monomers with different functional groups separately. Notably, this strategy could largely avoid the deviation of the stoichiometry between the reaction functional groups, thus highlighting the benefits of this three-in-one strategy.

The as-synthesized ADTB-COF and ADPB-COF were systematically characterized by various techniques. The successful self-condensation of two trifunctionalized monomers has been confirmed by means of Fourier transform infrared spectroscopy [Supplementary Figure 15]. The distinctive vibrations of boric acid groups (≈ 3,578 cm^-1), amino groups (≈ 3,372 and ≈ 3,376 cm^-1), and aliphatic C−H bonds (≈ 2,845 cm^-1 and ≈ 2,948 cm^-1) in ADTB and ADPB monomers disappeared in the resulting COFs. Furthermore, the distinctive bands of the C=N unit and the B₃O₃ ring were identified at ≈ 1,620 cm^-1 and ≈ 730 cm^-1, respectively, indicating high polymerization in the obtained dual-linkage COFs. In addition, as shown in Supplementary Figure 16, the characteristic signals that appeared at ca. 159 and 127 ppm could be attributed to the carbon of imine bands and the carbon atoms connected with boroxine rings in the solid state ¹³C CP/MAS NMR spectra^[43], which further confirmed the presence of imine units and boroxine rings in both COFs with dual linkages. On the other hand, Elemental analysis was used to determine the C, H, and N contents of two dual-linkage COFs, which were found to be in agreement with theoretical values. The above results demonstrate the successful construction of the two heteroporous COFs with dual linkages and further validate the effectiveness of the “three-in-one” strategy.

The crystalline structures of ADTB-COF and ADPB-COF have been characterized by the powder X-ray diffraction (PXRD) and high-resolution transmission electron microscopy (HRTEM). The PXRD pattern of ADTB-COF [Figure 1A] shows a distinct diffraction peak at 3.01°, corresponding to the (100) lattice plane, with some weaker diffraction peaks at 5.33°, 8.20°, 11.07°, and 26.34°, which could be assigned to (110), (210), (130), and (001) planes, respectively. The simulated PXRD patterns of ADTB-COF with eclipsed (AA) stacking and periodic heterogeneous pore features using the Materials Studio software match better with the experimental patterns than those with the staggered (AB) stacking or the ABC stacking models [Supplementary Figure 17 and Supplementary Table 1]^[49]. The Pawley refinement based on experimental PXRD patterns revealed the unite cell parameters of a = b = 34.18 Å and c = 3.39 Å with the P6/M space group (R_p = 2.00%, R_wp = 2.83%) [Figure 1B and C]. HRTEM images showed distinct lattice fringes of the (100) planes with d₁₀₀ = 29.0 Å, in good agreement with the structure having the AA stacking heteroporous topology. [Figure 2A and B]. The heteroporous topology of ADPB-COF was analogous to that of ADTB-COF and displayed strong PXRD peaks at 2.45°, 4.67°, 8.10°, and 9.31°, which could be indexed to (100), (200), (220), and (400) planes, respectively. The crystal structure of ADPB-COF was also in good agreement with the AA stacking manner (space group: P6/M, unit cell parameters: a = b = 42.69 Å, c = 3.45 Å,Figure 1D-F, Supplementary Figure 18 and Supplementary Table 2), and the difference suggests that experimental result matches well with the refined PXRD pattern in the Pawley refinement (R_p = 2.93% and R_wp = 3.83%). Furthermore, the HRTEM images of ADPB-COF also revealed well-defined lattice fringes of the (100) planes with d₁₀₀ = 36.2 Å, consistent with the structure having the AA stacking heteroporous topology [Figure 2C and D]. Scanning electron microscope (SEM) images of two COFs with dual linkages revealed resembling spherical morphology [Figure 2E and F]. Thermogravimetric analysis (TGA) was used to evaluate the thermal stability of the two COFs with dual linkages. The findings showed that both ADTB and ADPB retained 95% of their original weight up to 500 °C in an atmosphere of N₂, demonstrating excellent thermostability [Supplementary Figure 19]. Furthermore, the stability of two COFs was evaluated under a 97% relative humidity (RH) environment. As shown in Supplementary Figure 20, the results demonstrated the efficacy of the dual-linkage strategy in enhancing the water resistance of boroxine-based COFs.

Figure 1. (A) Experimental PXRD patterns (red), refined patterns (black dot), simulated profiles for the eclipsed (AA) stacking model (blue), and the deviation (pink) between experimental and refined patterns of ADTB-COF; (B) Top views and (C) side views of the eclipsed (AA) stacking model for ADTB-COF; (D) Experimental PXRD patterns (red), refined patterns (black dot), simulated profiles for the eclipsed (AA) model (blue), and the deviation (pink) between experimental and refined patterns of ADPB-COF; (E) Top views and (F) side views of the eclipsed (AA) stacking model for ADPB-COF. PXRD: Powder X-ray diffraction; COF: covalent organic framework; ADTB: (4-amino-4"-(2,2-dioxan-1,3-dioxan-5-yl)-[1,1':3',1"-terphenyl]-5'-yl) boronic acid; ADPB: (4'-amino-5'-(4-(2,2-dioxan-1,3-dioxan-5-yl)phenyl)-[1,1':3',1'-teroxan]-5-yl) boronic acid.

Figure 2. TEM images for (A and B) ADTB-COF and (C and D) ADPB-COF; SEM images for (E) ADTB-COF and (F) ADPB-COF. TEM: Transmission electron microscopy; ADTB: (4-amino-4"-(2,2-dioxan-1,3-dioxan-5-yl)-[1,1':3',1"-terphenyl]-5'-yl) boronic acid; COF: covalent organic framework; ADPB: (4'-amino-5'-(4-(2,2-dioxan-1,3-dioxan-5-yl)phenyl)-[1,1':3',1'-teroxan]-5-yl) boronic acid; SEM: scanning electron microscopy.

The inherent porosity of the two COFs with dual linkages was evaluated by N₂ sorption measurement at 77 K. A Brunauer-Emmett-Teller (BET) surface area of 247 m²·g^-1 for ADTB-COF and 893 m²·g^-1 for ADPB-COF were calculated by analyzing the low-pressure region (P/P₀ < 0.20), as shown in Figure 3A and B[Supplementary Figures 21 and 22]. ADTB-COF showed a lower BET surface area, which may be ascribed to the formation of oligomers during the polymerization process that could obstruct ultra-micropores pores^[42,43]. The pore size distributions of ADTB-COF and ADPB-COF were estimated to be 2.23 and 2.06 nm, respectively [Figure 3C and D], using the nonlocal density functional theory (NLDFT) method. These calculated values matched with the pore size predicted from the simulated structures (2.29 nm).

Figure 3. Nitrogen adsorption and desorption isotherms at 77 K of (A) ADTB-COF and (B) ADPB-COF; Pore size distribution by NLDFT of ADTB-COF (C) and (D) ADPB-COF. ADTB: (4-amino-4"-(2,2-dioxan-1,3-dioxan-5-yl)-[1,1':3',1"-terphenyl]-5'-yl) boronic acid; COF: covalent organic framework; ADPB: (4'-amino-5'-(4-(2,2-dioxan-1,3-dioxan-5-yl)phenyl)-[1,1':3',1'-teroxan]-5-yl) boronic acid; NLDFT: nonlocal density functional theory.

Considering the abundant micropores and high surface area of ADPB-COF, the adsorption of light hydrocarbons was assessed^[50]. As methane (CH₄), carbon dioxide (CO₂), ethane (C₂H₆), and propane (C₃H₈) constitute the primary constituents of natural gas, the adsorption isotherms of these four gases on ADPB-COF were measured at 298 and 273 K. There are considerable differences in adsorption isotherms for all four gases [Figure 4A and B]. Specifically, the uptakes of CH₄, CO₂, C₂H₆, and C₃H₈ for ADPB-COF were 3.7, 16.2, 25.3, and 44.1 cm³·g^-1 at 298 K and 1.0 bar, respectively. Notably, the C₃H₈ uptake shows a steep initial rise and then flattens with increasing pressure, belonging to typical type I sorption, indicating a high level of interaction between C₃H₈ molecules and ADPB-COF pores. The higher affinity for ADPB-COFs and the increased uptake is due to the greater polarizability of C₃H₈ compared to the other three gases in this study [62.9 × 10²⁵ cm³ (C₃H₈) > 44.3 × 10²⁵ cm³ (C₂H₆) > 27.6 × 10²⁵ cm³ (CO₂) > 25.9 × 10²⁵ cm³(CH₄)]^[51]. Consequently, the C₃H₈ is more susceptible to polarization by the framework, resulting in the formation of an induced dipole and an increase in adsorption capacity. Notably, among these gases, C₃H₈ exhibits the highest critical temperature [369.8 K (C₃H₈) > 305.4 K (C₂H₆) > 304.2 K (CO₂) > 190.6 K (CH₄)]^[52], making it easily condensable and adsorbable on the porous surfaces.

Figure 4. Sorption isotherms for CH₄, CO₂, C₂H₆, and C₃H₈ of ADPB-COF at (A) 298 K and (B) 273 K; (C) Calculated isosteric heats of CH₄, CO₂, C₂H₆, and C₃H₈ on ADPB-COF and (D) IAST calculations for adsorption selectivity of equimolar CO₂/CH₄, C₂H₆/CH₄, and C₃H₈/CH₄ gas mixtures on ADPB-COF at 298 K. ADPB: (4'-amino-5'-(4-(2,2-dioxan-1,3-dioxan-5-yl)phenyl)-[1,1':3',1'-teroxan]-5-yl) boronic acid; COF: covalent organic framework; IAST: ideal adsorption solution theory.

Furthermore, as shown in Figure 4C, the isosteric heat was used to evaluate the adsorption affinity between CH₄/CO₂/C₂H₆/C₃H₈ and ADPB-COF. The isosteric heats of ADPB-COF for C₃H₈, C₂H₆, CO₂, and CH₄ are 30.8, 27.3, 25.8, and 23.6 kJ/mol, respectively. Therefore, the steepness of the initial gas uptake and the adsorption capacity of the four gases follow a descending trend in the order (C₃H₈ > C₂H₆ > CO₂ > CH₄), which can also be attributed to their distinct physicochemical properties. The significant difference in isosteric heat among CH₄, CO₂, C₂H₆, and C₃H₈ on ADPB-COF also indicates the high potential for separating light hydrocarbons.

Furthermore, the selectivity of binary components with an equal molar ratio was assessed using the ideal adsorption solution theory (IAST). The models were constructed with high precision (R² > 0.999) to accurately fit the experimental single-component isotherms at 298 K and 1.0 bar through the single site Langmuir-Freundlich equation [Supplementary Figures 23-26 and Supplementary Table 3]. The gas mixtures of C₃H₈/CH₄, C₂H₆/CH₄, and CO₂/CH₄ were set at a ratio of 1:1, and the selectivities obtained were plotted as a function of the mixed gas pressure, ranging from 0 to 1.0 bar. As shown in Figure 4D, ADPB-COF showed excellent selectivity for the mixtures of C₃H₈ and CH₄ at 298 K and 1.0 bar, with a selectivity as high as 174 (4 for CO₂/CH₄, 20 for C₂H₆/CH₄). Compared with other representative porous materials based on the separation of C₃H₈/CH₄ [Supplementary Table 4], ADPB-COF featured a favorable selectivity. These results indicate that the dual-linkage COF materials may exhibit superior performance in methane separation from natural gas, thereby offering a new approach for light hydrocarbon purification.

CONCLUSIONS

In conclusion, we propose a new “three-in-one” strategy for synthesizing heteroporous 2D COFs with dual linkages (ADTB-COF and ADPB-COF) through self-polycondensation of two trifunctionalized simplex monomers, respectively. Benefitting from the abundant micropores and high surface area, ADPB-COF showed excellent selectivity of C₃H₈ over CH₄ (174, 298 K/1 bar). The current study introduced a novel approach for constructing COFs with dual linkages, which greatly simplified the synthetic process and provided a new opportunity for developing functional COFs with multiple different linkages.