Rational design of imine‐linked three‐dimensional mesoporous covalent organic frameworks with bor topology

Three‐dimensional (3D) covalent organic frameworks (COFs) possess great potential applications in various fields. Constructing 3D COFs with large pore sizes is extremely challenging due to the interpenetration and collapse. Herein, we report a series of crystalline imine‐linked 3D COFs (3D‐bor‐COF‐1, 3D‐bor‐COF‐2, 3D‐bor‐COF‐3) with mesoporous channels through rationally designing the topology configuration. These 3D‐bor‐COFs display permanent porosity and Brunauer–Emmett–Teller (BET) surfaces of 3205.5, 1752.7, and 2077.3 m2 g−1 (SLangmuir = 4277.7, 2480.3, and 2698.0 m2 g−1), respectively. The pore sizes of 3D‐bor‐COFs were confirmed by the lattice fringes from high‐resolution transmission electron microscopy, as well as structural simulation and nitrogen adsorption isotherm analysis. 3D‐bor‐COFs display large pore sizes (3.8 nm for 3D‐bor‐COF‐3), which is among the highest record of 3D COFs. Owing to the unstacked‐aromatic pore environment and high specific surface area, 3D‐bor‐COFs display excellent adsorption capacity for benzene vapor (1203.9 mg g−1 for 3D‐bor‐COF‐3) under 298 K, which is three times higher than that of the best‐reported 2D COF. This work not only provides inspiration for designing 3D mesoporous imine‐COFs, but also demonstrates a strategy for constructing aromatics adsorption materials.


INTRODUCTION
larger guests or anchoring functional groups, thus limiting mass-transport related applications, such as molecular capture/separation, 17 catalysis, 18 and drug delivery. 19,20 Therefore, developing COFs with large pores is very essential and urgent.
Nevertheless, synthesizing mesoporous crystalline materials is difficult since larger pore tends to arise interpenetration and collapse 21 ; and the construction of mesoporous imine-linked 3D COFs is more challenging for the following reasons. First, the formation reaction of imine is less reversible than the early developed boroxine and boronate-ester linked COFs, since it hinders the error correction mechanism of COFs. 22,23 Second, iminelinkage is a chain structure with three bonds, which not only leads to more spatial conformations compared with the ring structures of boroxine/boronate, but also creates challenges in getting precise reticular network, especially for 3D-imine-COFs. 24 The theoretical specific surface area of two-dimensional (2D) COFs is usually 2000-3000 m 2 g −1 , the theoretical limitation of 3D COFs could be as high as 7000-10 000 m 2 g −1 . 25 However, the usually obtained surface area of 3D COFs was significantly lower than theoretical value, resulting from the widely observed crystallinity issue and interpenetration phenomena. [26][27][28][29][30][31] Currently, methods to reduce the interpenetration are focused on: (1) shortening the length of the linkers, 32 (2) increasing the steric hindrance, 33 (3) controlling reaction conditions, 27,31 and (4) increasing the repulsive effect of the linkers. 34 Herein, we selected the bor topology that rarely occurs interpenetration phenomenon to prepare mesoporous imine-linked 3D COFs. [35][36][37] In 2007, Yaghi's group reported two boronate-esterlinked COFs (COF-105 and COF-108) with ctn and bor topologies that have 1.8 and 2.96 nm pore sizes, respectively; notably, it was also the first report of 3D COFs. 36 After that, McGrier's group reported the second example of bor COFs, in which the boronate-ester-linked DBA-3D-COF 1 has a pore size of 2.8 nm. 37 It is worth mentioning that these two bor topology boronate-esterlinked COFs also represent the high surface area in 3D COFs. Unfortunately, except for two cases, iminelinked COF with bor topology has not been reported so far.
Considering the high surface area, mesoporous feature, and the exposed aromatic pore environment of our 3D COFs, we explored their gas adsorption application. Unlike 2D COFs, the strong π-π stacking of planar precursors on 2D COFs is forced to be separated in 3D COFs, providing more efficient adsorption active sites. 45 And surprisingly, we found the 3D-bor-COFs could serve as excellent capture materials for benzene vapor, with an adsorption capacity of 983.3 mg g −1 for 3D-bor-COF-1, 953.5 mg g −1 for 3D-bor-COF-2, and 1203.9 mg g −1 for 3D-bor-COF-3 F I G U R E 2 N 2 sorption isotherms collected at 77 K and corresponding pore size distributions based on the nonlocal density functional theory (NLDFT) method of 3D-bor-COF-1(A and B), 3D-bor-COF-2 (E and F), and 3D-bor-COF-3 (I and J). High-resolution transmission electron microscopy (HRTEM) images and lattice fringe distance measurements in red regions of 3D-bor-COF-1 (C and D), 3D-bor-COF-2 (G and H), and 3D-bor-COF-3 (K and L) F I G U R E 3 Benzene vapor adsorption-desorption isotherms at 298 K (Table S3) under 298 K (Figure 3), respectively, which are among the highest record of porous materials, also the first attempt in the 3D COFs field. And 3D-bor-COFs demonstrates high hydrogen adsorption capacities of 172.05 cm 3 g −1 for 3D-bor-COF-1, 128.67 cm 3 g −1 for 3D-bor-COF-2, and 134.31 cm 3 g −1 for 3D-bor-COF-3 under condition of 77 K and 1 atm ( Figures S32-S37), respectively.

Characterization of 3D-bor-COFs
To demonstrate the generality of this bor COF construction strategy, we extended the triangular structure unit to two benzene rings and prepared other two new COFs (3D-bor-COF-2 and 3D-bor-COF-3). Fouriertransform infrared spectroscopy (FT-IR) displayed the   -S19) indicates that 3D-bor-COF-1, 3D-bor-COF-2, and 3D-bor-COF-3 start to decom-pose at around 500 • C, where a weight loss was ∼5% under nitrogen atmosphere. Moreover, to test the chemical stability of 3D-bor-COFs, 20 mg samples were exposed to common organic solvents (including acetone, tetrahydrofuran, hexane, acetonitrile, dichloromethane, chloroform, DMF, DMAc, and DMSO) and water (pH = 1, 7, 14) for 24 h at room temperature. 3D-bor-COF-1, 3D-bor-COF-2, and 3D-bor-COF-3 maintain their crystallinity well after treatment under above conditions, which were demonstrated by the PXRD patterns ( Figures S20-S22). The stability of COFS depends largely on the bond type, vinylene-linked and sp2 carbonconjugated COFs is more stable than the all imine-linked COFs reported because of its low reversibility. However, for the above two examples, their crystallinity and BET surface area are worse than our 3D-bor-COFs. In fact, it is not easy for imine-linked COFs to have high crystallinity, high specific surface area, and high stability at the same time.
As shown in Figure 4(A), 3D-bor-COF-1 had peaks at 2.13 • , and 3.14 • , and 6.14 • corresponding to (1 0 0), (2 0 0), and (2 2 0) 46 we compared it with other two possible topologies, ctn and ofp ( Figure 5 and Tables S4-S12). However, the calculated patterns did not match with the experimental PXRD patterns. Therefore, we identify 3D-bor-COF-1, 3D-bor-COF-2, and 3D-bor-COF-3 as bor network. It is worth noting that the peak at 3.88 • disappeared in the XRD obtained from the experiment, which may be due to F I G U R E 4 Powder XRD patterns of (A) 3D-bor-COF-1, (C) 3D-bor-COF-2, and 3D-bor-COF-3 (E). The observed XRD patterns are shown in black, the patterns calculated on the basis of the bor network in crimson, and the difference between the observed and refined profiles in blue. Calculated PXRD patterns and experimental XRD patterns (black curves) of 3D-bor-COF-1 (B), 3D-bor-COF-2 (D), and 3D-bor-COF-3 (F) based on bor (pink curves) net, ctn (red curves) net, and ofp (blue curves) the influence of crystal defects or trapped molecules in the materials.

Benzene vapor adsorption tests
Aromatic molecules are commonly used solvents and important synthetic reactants in the laboratory and industry. However, most of them are volatile molecules, and thus cause serious environmental problems, especially threat to the health of humans and various organisms. In recent years, significant efforts have been put into developing effective methods to reduce aromatics emissions. In the past decade, a lot of exploration have been made on the adsorption of benzene by metalorganic frameworks. 49 Considering 3D-bor-COFs possess unstacked-aromatic pore environment, high specific surface area, and mesoporous properties, we believed that the material has excellent adsorption properties for large volatile molecules such as aromatic hydrocarbon. 3Dbor-COFs showed high benzene capacities under 298 K (983.3 mg g −1 for 3D-bor-COF-1, 953.5 mg g −1 for 3Dbor-COF-2, and 1203.9 mg g −1 for 3D-bor-COF-3). 3D-bor-COF-3 is around the three times of the reported 2D COF materials, TTPE-COF (452.4 mg g −1 ). 50 And it is also comparable with the best results of the porous frameworks (Table S3) PAF-1 (1306.0 mg g −1 , S BET = 5600 m 2 g −1 ), 51 MIL-101 (1302.6 mg g −1 , S BET = 3900 m 2 g −1 ). 52 Notably, we found that 3D-bor-COF-2 has an adsorption capacity of up to 711.28 mg g −1 in the low-pressure zone (P/P 0 = 0.1), which suggests it can effectively adsorb benzene vapor under ultra-low pressure (Figure 3). We think that high specific surface area, central nitrogen atom, and large pore size are conducive to aromatic hydrocarbon adsorption. In order to better understand the trend of benzene vapor adsorption, we calculated the distribution density of benzene in 3D-bor-COFs using Metropolis Monte Carlo method. As shown in Figures S38-S40, exposed π-plane and central nitrogen atoms provide a great influence on benzene vapor adsorption. To further study the enhanced benzene vapor adsorption capacities of 3D-bor-COFs, we investigated the intermolecular interactions between the building units and benzene vapor using density-functional theory calculation. We built a model (Figures S41-S44) for TAPB, TABPA, TABPB, and TFBM. As shown in Figure  S41, exposed π-plane of central phenyl-, phenyl-, and imine bond of the unstacked TAPB showed adsorption energy of −27.47, −24.04, and −25.07 kJ mol −1 , respectively. Further, as for the aromatic ring-based adsorption sites, the position of central nitrogen for TABPA showed adsorption energy of −29.48 kJ mol −1 ; this result suggests that nitrogen atom provides different charge environments resulting in higher adsorption heat than exposed π-plane of phenyl-(−28.24 kJ mol −1 ) and imine group (−26.26 kJ mol −1 ; Figure S42). TABPB central phenyl-, exposed π-plane of phenyl-, and imine bond showed adsorption energy of −27.54, −24.37, and −25.07 kJ mol −1 , respectively ( Figure  S43); the biphenyl-position of TFBM unit showed much stronger interaction with benzene vapor, with an adsorption energy of −35.36 and −27.82 kJ mol −1 , imine bond showed −26.85 kJ mol −1 ( Figure S44). This result suggests the exposed aromatic rings in 3D-COFs would have a stronger interaction with benzene vapor, and the unique structure of TFBM make it a preferred adsorption site for benzene vapor.

CONCLUSION
We here developed the first example of imine-linked threedimensional COFs with bor topology. The mesoporous 3D-bor-COF-3 possesses a pore size of 3.8 nm, while 3Dbor-COF-1 owns a BET surface area of 3205 m 2 g −1 and a Langmuir surface area of 4277.7 m 2 g −1 , which are among the best results of 3D imine COFs. We demonstrate a new method to measure the pore size by HRTEM. Moreover, 3D-bor-COFs with the plane of exposed aromatic rings, high specific surface area, and large pore size displayed excellent adsorption capacity for benzene vapor. This work not only enriches the family of 3D COFs, but also provides inspiration for the design of mesoporous 3D COFs. Meanwhile, it also opens up a possibility for the application of 3D COFs in the capture of aromatic hydrocarbon.

Materials
All starting materials and solvents, unless otherwise noted, were obtained from J&K scientific LTD. and used without purification. All products were isolated and handled under nitrogen using either glovebox or Schlenk line techniques. The reaction mixture was heated at 120 • C for 4 days to obtain a light-yellow precipitate, which was isolated by filtration over a medium glass frit and washed with anhydrous acetone (40.0 ml). The product was immersed in anhydrous acetone (40.0 ml) for 24 h. During this period, the activation solvent was decanted and freshly replenished several times. The 3D-bor-COF-1 was obtained by removing solvent in vacuum at 80 • C, which was a light yellow crystalline solid. Under the same conditions, replacing TAPB with TABPA (20.72 mg and 0.4 mmol) and TABPB (23.20 mg and 0.4 mmol) respectively will obtain dark red and dark brown precipitates named 3D-bor-COF-2 and 3Dbor-COF-3.

Characterizations
1 H NMR spectra were recorded on a JEOL JNM-ECA600 NMR spectrometer. 13 C NMR spectra were recorded on a JEOL JNM-ECZ600R 600 MHz solid-state NMR spectrometer. The FT-IR spectra was obtained by using a Bruker VERTEX 70 Fourier transform infrared spectrophotometer. TGA was carried out under nitrogen on a METTLER TOLEDO 1600LF thermal analyzer, measured over the temperature from 50 to 750 • C with a heating rate of 10 • C min −1 and a N 2 flow rate of 30 ml min −1 . Samples for TGA were dried under vacuum at RT for 4 h, and then 80 • C for 6 h. EAs were performed by using a Thermo Scientific FlashSmart elemental analyzer. PXRD data were col-lected on a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å) over the range of 2θ = 1.0−40.0 • with a step size of 0.02 • and 2 s per step. The sorption isotherm for N 2 was measured by using a BEL sorp-max II analyzer with ultra-high-purity gas (99.999% purity). To estimate pore size distributions for 3D-bor-COF-1, 3D-bor-COF-2, and 3D-bor-COF-3, NLDFT was applied to analyze the N 2 isotherm on the basis of the model of N 2 at 77 K on carbon with slit pores and the method of non-negative regularization. The sorption isotherm for benzene was measured by using a BSD-VVS Vacuum Vapor/Gas Sorption Analyzer. The SEM images were obtained by Zeiss Gemini scanning electron microscope. The TEM images were obtained by JEOL JEM-2100PLUS Transmission electron microscope.

A C K N O W L E D G M E N T S
The authors acknowledge the financial support from the National Natural Science Foundation of China (52073161) and the Tsinghua University Initiative Scientific Research Program (no. 2021Z11GHX010). The authors also thank Tsinghua University-Zhangjiagang Joint Institute for Hydrogen Energy and Lithium Ion Battery Technology.

C O N F L I C T O F I N T E R E S T
The authors declare no conflict of interest.