Ultralight, compressible covalent organic framework aerogel, preparation method and application thereof

Abstract

The present invention discloses an ultralight, compressible covalent organic framework aerogel, a preparation method, and an application thereof. The method comprises: (1) electrospinning a DMF solution of a urea-based ligand and polyacrylonitrile to obtain a UL / PAN film, crushing and freeze-drying the UL / PAN fiber film to obtain a UL / PAN fiber; (2) dispersing the UL / PAN fiber in a (Diox / TMB mixed solution containing Tp, using acetic acid as a catalyst, to obtain a Tp-COFs / PAN wet gel; and (3) removing PAN by extraction, and after solvent exchange, freeze-drying or supercritical drying to obtain a Tp-COFs fiber aerogel. The Tp-COFs fiber aerogel prepared by the present invention has a large specific surface area, good crystallinity, ultra-low density, excellent mechanical properties, and ultra-high organic solvent adsorption capacity, and shows good prospects in the molding and practical application of COFs materials.

Description

Technical Field

[0001] The present invention belongs to the field of aerogel materials and relates to an ultralight, compressible covalent organic framework aerogel, a preparation method and applications thereof. Background Art

[0002] Covalent organic frameworks (COFs) have the characteristics of structural diversity, multifunctionality, permanent porosity and high specific surface area, and have broad application prospects in the fields of energy storage, catalysis, separation and environmental remediation. However, the micropores or small mesopores of COFs have a great negative impact on diffusion and mass transfer, which greatly weakens their performance. In addition, COFs are usually formed in the form of nano / micrometer-sized powders. The inherent insolubility and infusibility of COFs make it difficult to process them into macroscopic bulk materials, which greatly hinders their practical application. Therefore, it is of great significance to introduce hierarchical pores and shape COFs into macroscopic objects with strong mechanical properties.

[0003] At present, the methods for manufacturing macroscopic COFs blocks mainly include post-crosslinking method, in-situ growth method and self-forming method. For example, COFs blocks are obtained by crosslinking pre-synthesized COFs powder with gel precursors such as chitosan, cellulose, and polyvinyl alcohol (PVA). Although crystalline COFs are introduced into macroscopic objects, the infiltration of crosslinking monomers into the COFs pores often leads to pore blockage. In addition, in-situ growth of COFs on substrates such as GO / rGO and melamine foam / sponge can also improve this situation, but the limited types of substrates limit its wide application.

[0004] Therefore, it is an ideal method to manufacture pure COFs blocks without any binders or other materials. However, it is a considerable challenge to introduce multiple types of pores (especially macropores) into macroscopic COFs while keeping the microporous framework of COFs from collapsing. Existing literature reports several self-forming methods, including in situ gas phase foaming, sol-gel, three-dimensional printing and hard template methods to prepare pure COFs-based blocks with hierarchical porosity, which show excellent performance in removing pollutants and organic solvents. Literature 1 prepared a series of COFs foam blocks by "melt polymerization method", but their density was relatively large (230-410 mg / cm 3), and the adsorption capacity of different organic solvents is small (about 1.2 to 6 times of its own weight) (Zhang P, Wang Z, Yang Y, et al. Melt polymerization synthesis of a class of robust self-shaped olefin-linked COF foams as high-efficiency separators [J]. Sci. China Chem., 2022, 65 (6): 1173-1184.). Reference 2 used the “sol-gel method” to prepare three different COFs aerogels. Although their density is relatively low (about 17.3 to 20.8 mg / cm 3 ), the adsorption capacity of toluene is about 24.7 to 32.6 times of its own weight, but the mechanical properties of the aerogel are poor, and it cannot recover its original shape when compressed, especially when it is deformed by 90%, it will become plastic and the structure will be completely destroyed (Martín-Illán J Rodríguez-San-Miguel D, Castillo O, et al. Macroscopic Ultralight Aerogel Monoliths of Imine-based Covalent Organic Frameworks [J]. Angew. Chem. Int. Ed., 2021, 133 (25): 14088-14096.). However, many applications require not only high porosity, but also require the structure to have strong and flexible building blocks, as well as reversible deformation under large strain. Unfortunately, to date, there has been no report on the preparation of pure COFs macroscopic blocks with satisfactory mechanical properties. Summary of the Invention

[0005] The present invention aims to provide an ultralight, compressible covalent organic framework aerogel, a preparation method and applications thereof.

[0006] The technical solutions for achieving the purpose of the present invention are as follows:

[0007] The preparation method of ultralight, compressible covalent organic framework aerogel has the following specific steps:

[0008] Step 1: Add polyacrylonitrile (PAN) to a dispersion of a urea-based linker (UL) in N,N-dimethylformamide (DMF), and stir until the mixture is uniform to obtain an electrospinning precursor solution.

[0009] Step 2, setting the electrospinning parameters: the positive voltage of the spinning voltage is 15-20 kV, the negative voltage is 3-5 kV, the injection rate is 0.08-0.15 mm / min, the receiving distance is 15-20 cm, the humidity is 20-40%, and the temperature is 20-50° C., and the electrospinning precursor solution is electrospun to obtain a UL / PAN fiber membrane;

[0010] Step 3, cutting the UL / PAN fiber membrane into pieces, adding water to homogenize, and freeze-drying to obtain UL / PAN fiber;

[0011] Step 4: Disperse the UL / PAN fibers in a mixed solution of dioxane / mesitylene (Diox / TMB) in which trialdehyde phloroglucinol (Tp) is dissolved. After stirring to uniformly disperse the UL / PAN fibers, add a 12M acetic acid solution, react at 90±5°C for 48-72 hours, then raise the temperature to 160-170°C and react for more than 3 hours to obtain a Tp-COF / PAN wet gel.

[0012] Step 5: The Tp-COF / PAN wet gel was washed with acetone and ethanol in sequence, and then extracted with N-methylpyrrolidone (NMP);

[0013] Step 6, sequentially exchanging the solvents of the Tp-COFs wet gel obtained in step 5 with ethanol and water, and then freeze-drying the gel to obtain a Tp-COFs fiber aerogel;

[0014] Alternatively, the Tp-COFs wet gel obtained in step 5 is subjected to solvent exchange with ethanol, and then subjected to supercritical drying to obtain the Tp-COFs fiber aerogel.

[0015] Preferably, in step 1, the urea-based ligand is 1,4-phenylenediurea (BDU), 1,1'-(3,3'-dimethyl-[1,1'-biphenyl]-4,4'-diyl)diurea (DMBDU), [1,1'-biphenyl]-4,4'-diurea (PBDU) or [1,1'-azobenzene]-4,4'-diurea (ABDU).

[0016] Preferably, in step 1, the concentration of PAN in the electrospinning precursor solution is 0.11-0.15 g / mL, and the mass ratio of UL to PAN is 0.5-2:1.

[0017] Preferably, in step 1, the stirring temperature is 20-30° C. to ensure that the viscosity of the electrospinning precursor solution is appropriate.

[0018] Preferably, in step 3, the concentration of UL / PAN fiber membrane is 2-5 mg / L, the homogenization conditions are a rotation speed of 18000-30000 rpm, a time of 1.5-10 min, a freeze-drying temperature of -52°C, and a freeze-drying time of 24-48 h.

[0019] Preferably, in step 4, the Tp concentration in the mixed solution is 0.01-0.02 mmol / mL, the UL / PAN fiber concentration is 8.8-17.6 mg / mL, the volume ratio of dioxane to mesitylene is 1-4:1, and the volume ratio of acetic acid solution to Diox / TMB mixed solution is 1:10-1:5.

[0020] Preferably, in step 5, the extraction temperature is 80-90° C., the NMP is replaced every 8 hours, and the total extraction time is 24-48 hours.

[0021] Preferably, in step 6, the supercritical drying cooling temperature is 0-15° C., the heating temperature is 35-50° C., and the number of operations is 50-200 times.

[0022] The present invention provides a covalent organic framework aerogel prepared by the above preparation method.

[0023] Furthermore, the present invention provides use of the covalent organic framework aerogel in adsorbing organic solvents.

[0024] Preferably, the organic solvent is dimethyl sulfoxide, chloroform, toluene, 1,4-dioxane, acetone, dimethylacetamide, methanol or ethanol.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] The preparation method of the present invention is simple and universal, and the covalent organic framework aerogel prepared has a large specific surface area (up to 463m 2 / g), good crystallinity, ultra-low density (14.1~15.5mg / cm 3 ), excellent mechanical properties (94% of the height can be restored under 80% deformation, and only 6% stress loss can be achieved after 20 cycles of compression under 50% deformation) and ultra-high organic solvent adsorption capacity (43-94 times its own weight), showing good application prospects in the molding and practical applications of COFs materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the preparation process of the covalent organic framework aerogel of the present invention.

[0028] Figure 2 Structural diagrams of BDU (a) and TpPa-1 COF (b).

[0029] Figure 3 The physical image of TpPa-1 COF aerogel (a) and the SEM image of its structure (b).

[0030] Figure 4SEM images of Tp-BDU / PAN (a) and TpPa-1 / PAN (b), and SEM (c) and TEM (d) images of TpPa-1 COF fibers.

[0031] Figure 5 Structural diagrams of DMBDU (a) and TpBD-Me2 COF (b).

[0032] Figure 6 (a) The physical image of TpBD-Me2 COF aerogel and (b) the TEM image of TpBD-Me2 COF fiber.

[0033] Figure 7 Structural diagrams of PBDU (a) and TpBD COF (b).

[0034] Figure 8 (a) The physical image of TpBD COF aerogel and (b) the TEM image of TpBD COF fiber.

[0035] Figure 9 Structural diagrams of ABDU (a) and TpAzo COF (b).

[0036] Figure 10 (a) The physical image of TpAzo COF aerogel and (b) the TEM image of TpAzo COF fiber.

[0037] Figure 11 Mechanical properties test of the TpPa-1 COF aerogel of Example 1, including the compression-recovery process diagram of the TpPa-1 COF aerogel (a), stress-strain curves at different compressive strains (b), and cyclic stress-strain curves at 50% compressive strain (c).

[0038] Figure 12 The organic solvent adsorption properties of TpPa-1, TpBD-Me2 and TpBD COF aerogels in Examples 1, 2 and 3 are shown.

[0039] Figure 13 This is a diagram showing the process of TpPa-1 COF aerogel adsorbing and removing toluene from the water surface in Example 1.

[0040] Figure 14 The XRD spectra of the materials prepared with different acetic acid concentrations in Comparative Examples 1-3 are shown. DETAILED DESCRIPTION

[0041] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0042] The preparation process of COFs aerogel in the present invention is as follows Figure 1As shown, urea-based ligands were pre-embedded in polyacrylonitrile fibers via electrospinning. A certain amount of UL / PAN fibers was dispersed in a dioxane / mesitylene mixture containing trialdehyde phloroglucinol. Acetic acid was used as a catalyst, and the mixture was reacted at 90°C and 160°C for 72 hours, respectively, to produce a Tp-COFs / PAN wet gel. PAN was removed by extraction, followed by solvent exchange with ethanol and supercritical drying to obtain a Tp-COFs fiber aerogel; alternatively, solvent exchange with ethanol and then water was performed, followed by freeze-drying to obtain a Tp-COFs fiber aerogel. The COFs aerogel prepared by this method consists of intertwined hollow COFs microfibers, with a large number of COFs nanofibers grown in situ on the microfiber surfaces. This intertwined structure of hollow microfibers and nanofibers gives the COFs aerogel a large specific surface area, high porosity, and ultra-low density, which contributes to its improved organic solvent adsorption performance.

[0043] Example 1

[0044] Step 1: 0.8 g of BDU was added to 6 mL of DMF and completely dispersed by ultrasound. 0.8 g of PAN was added and stirred at room temperature until the mixture was uniformly mixed to obtain an electrospinning precursor solution.

[0045] Step 2: Pour the electrospinning precursor solution into a 5 mL syringe, and use electrospinning technology to set the positive pressure to 15 kV, the negative pressure to 5 kV, the injection rate to 0.1 mm / min, the receiving distance to 15 cm, the humidity to 30%, and the temperature to 25°C to obtain BDU / PAN fibers.

[0046] Step 3: Take 0.8 g of the BDU / PAN fiber membrane obtained in step 2, cut it into pieces, add it into a homogenizer with 100 mL of deionized water, crush it at a speed of 30,000 rpm for 1.5 minutes, then quick-freeze it with liquid nitrogen, and freeze-dry it to obtain BDU / PAN fiber.

[0047] Step 4: Dissolve 42 mg of Tp in 10 mL of a Diox / TMB (v / v = 4 / 1) mixture. Then, add 116.4 mg of BDU / PAN fibers to the mixture, stir to disperse evenly, and then add 2 mL of 12 M acetic acid. Place the reactor in an oven at 90°C for 72 hours, then increase the temperature to 160°C for another 72 hours to obtain a TpPa-1 / PAN wet gel.

[0048] Step 5: The TpPa-1 / PAN wet gel obtained in step 4 was washed with acetone and ethanol in sequence, placed in NMP, and extracted at 90° C. for 48 hours, during which the NMP was replaced every 8 hours.

[0049] Step 6: Solvent exchange was performed on the TpPa-1 wet gel obtained in step 5 with ethanol and water in sequence, and then the gel was frozen in a refrigerator overnight, and then freeze-dried for 24 hours to obtain the TpPa-1 aerogel.

[0050] Figure 2 The structural formulas of BDU and TpPa-1 can be seen in the figure. At 90°C, BDU reacts with excess Tp in the solution to form a layer of Tp-BDU polymer. When the temperature is further raised to 160°C, the insoluble BDU in the Tp-BDU / PAN fiber undergoes hydrolysis to form soluble 1,4-phenylenediamine (Pa), which further reacts with Tp in the solution to form TpPa-1 nanofibers in situ on the microfiber surface. Simultaneously, the Tp-BDU polymer undergoes reconstruction, transforming into TpPa-1 COF. Figure 3 This is a physical picture of TpPa-1 aerogel, which is composed of a large number of fibers entangled with each other. Figure 4 The changes in fiber morphology during the preparation of TpPa-1 aerogel can be seen in the figure, and the TEM image clearly shows the structure of TpPa-1COF fibers.

[0051] Example 2

[0052] Step 1: 0.8 g of DMBDU was added to 6 mL of DMF and completely dispersed by ultrasound. 0.8 g of PAN was added and stirred at room temperature until the mixture was uniformly mixed to obtain an electrospinning precursor solution.

[0053] Step 2: Pour the electrospinning precursor solution obtained in step 1 into a 5 mL syringe, and use electrospinning technology with a positive pressure of 15 kV, a negative pressure of 5 kV, an injection rate of 0.1 mm / min, a receiving distance of 15 cm, a humidity of 30%, and a temperature of 25°C to obtain DMBDU / PAN fibers.

[0054] Step 3: Take 0.8 g of the DMBDU / PAN fiber membrane obtained in step 2, cut it into pieces, add it into a homogenizer with 100 mL of deionized water, crush it at a speed of 30,000 rpm for 1.5 minutes, then quick-freeze it with liquid nitrogen, and freeze-dry it to obtain DMBDU / PAN fiber.

[0055] Step 4: Dissolve 42 mg of Tp in 10 mL of a Diox / TMB (v / v = 4 / 1) mixture. Then, add 179.2 mg of the DMBDU / PAN fibers obtained in Step 3 to the mixture, stir to evenly disperse them, and then add 2 mL of a 12 M acetic acid solution. Place the reactor in an oven at 90°C for 72 hours, then raise the temperature to 160°C for another 72 hours to obtain a TpBD-Me2 / PAN wet gel.

[0056] Step 5: The TpBD-Me2 / PAN wet gel obtained in step 4 was washed with acetone and ethanol in sequence, placed in NMP, and extracted at 90° C. for 48 hours, during which the NMP solution was replaced every 8 hours.

[0057] Step 6: Solvent exchange the TpBD-Me2 wet gel obtained in step 5 with ethanol and water in sequence, then freeze it in a refrigerator overnight, and then freeze-dry it for 24 hours to obtain TpBD-Me2 aerogel.

[0058] Example 3

[0059] Step 1: 0.8 g of PBDU was added to 6 mL of DMF and completely dispersed by ultrasound. 0.8 g of PAN was added and stirred at room temperature until the mixture was uniform to obtain an electrospinning precursor solution.

[0060] Step 2: Pour the electrospinning precursor solution obtained in step 1 into a 5 mL syringe, and use electrospinning technology with a positive pressure of 15 kV, a negative pressure of 5 kV, an injection rate of 0.1 mm / min, a receiving distance of 15 cm, a humidity of 30%, and a temperature of 25°C to obtain PBDU / PAN fibers.

[0061] Step 3: Take 0.8 g of the PBDU / PAN fiber membrane obtained in step 2, cut it into pieces, add it into a homogenizer with 100 mL of deionized water, crush it at a speed of 30,000 rpm for 1.5 minutes, then quick-freeze it with liquid nitrogen, and freeze-dry it to obtain PBDU / PAN fiber.

[0062] Step 4: Dissolve 42 mg of Tp in 10 mL of a mixture of dioxane / mesitylene / TMB (v / v = 4 / 1). Then, add 162.0 mg of the PBDU / PAN fibers obtained in Step 3 to the mixture, stir to evenly disperse them, and then add 2 mL of 12 M acetic acid. Place the reactor in an oven at 90°C for 72 hours, then raise the temperature to 160°C for another 72 hours to produce a TpBD / PAN wet gel.

[0063] Step 5: The TpBD / PAN wet gel obtained in step 4 was washed with acetone and ethanol in sequence, placed in NMP, and extracted at 90° C. for 48 hours, during which the NMP was replaced every 8 hours.

[0064] Step 6: Solvent exchange was performed on the TpBD wet gel obtained in step 5 with ethanol and water in sequence, and then the gel was frozen in a refrigerator overnight, and then freeze-dried for 24 hours to obtain the TpBD aerogel.

[0065] Example 4

[0066] Step 1: 0.8 g of ABDU was added to 6 mL of DMF and completely dispersed by ultrasonication. 0.8 g of PAN was added and stirred at room temperature until the mixture was uniform to obtain an electrospinning precursor solution.

[0067] Step 2: Pour the electrospinning precursor solution obtained in step 1 into a 5 mL syringe, and use electrospinning technology with a positive pressure of 15 kV, a negative pressure of 5 kV, and an injection rate of 0.1 mm / min to obtain ABDU / PAN fibers.

[0068] Step 3: Take 0.8 g of the ABDU / PAN fiber membrane obtained in step 2, cut it into pieces, add it to a homogenizer with 100 mL of deionized water, and crush it at a speed of 30,000 rpm for 1.5 minutes. Then, quick-freeze it with liquid nitrogen and freeze-dry it to obtain ABDU / PAN fiber;

[0069] In step 4, 42 mg of Tp was dissolved in 10 mL of a Diox / TMB (v / v = 4 / 1) mixture. Then, 178.8 mg of the ABDU / PAN fibers obtained in step 3 were added to the mixture and stirred to uniformly disperse. Then, 2 mL of a 12 M acetic acid solution was added. The reaction vessel was placed in an oven and reacted at 90°C for 72 hours. The temperature was then raised to 160°C and reacted for another 72 hours to obtain a TpAzo / PAN wet gel.

[0070] Step 5: The TpAzo / PAN wet gel obtained in step 4 was washed with acetone and ethanol in sequence, placed in NMP, and extracted at 90° C. for 48 hours, during which the NMP was replaced every 8 hours.

[0071] Step 6: Solvent exchange was performed on the TpAzo wet gel obtained in step 5 with ethanol and water in sequence, and then the gel was frozen in a refrigerator overnight, and then freeze-dried for 24 hours to obtain the TpAzo aerogel. Figure 10 (a) is a physical picture of TpAzo COF aerogel, which is composed of a large number of fibers entangled with each other, and (b) is a TEM image of TpAzo COF fiber (b), which can also recover its original shape after being compressed.

[0072] Performance test case

[0073] 1. Mechanical properties

[0074] The TpPa-1 aerogel in Example 1 was used to test mechanical properties:

[0075] Cylindrical TpPa-1 aerogels with a diameter of 25.8 mm and a height of 10.6 mm were used as samples for testing. Compressive stress-strain curves and 20-cycle fatigue tests were performed at a strain rate of 2 mm / min on a WDT-5 universal machine equipped with a 100 N load cell.

[0076] like Figure 11 As shown in (a), TpPa-1 aerogel is able to recover after compression and maintain its aerogel structure. The compression stress-strain curve shows that TpPa-1 aerogel can fully recover at 50% strain and can recover 94% of its height at 80% strain. The results of cyclic compression experiments show that TpPa-1 aerogel only loses 7% of its stress and less than 5% of its height after being compressed 20 times at 50% strain. These results demonstrate that TpPa-1 aerogel has excellent mechanical properties.

[0077] 2. Organic solvent adsorption performance

[0078] The TpPa-1, TpBD-Me2 and TpBD COF aerogels in Examples 1, 2 and 3 were used to test the organic solvent adsorption performance:

[0079] At room temperature, the samples were immersed in different organic solvents for 5 min. before ) and after absorption (W after The absorption capacity (Q) of the sample for organic solvents is calculated based on the mass of the sample. The formula is as follows:

[0080] Q=(W after -W before ) / W before ×100%

[0081] The process for removing organic solvents from water surfaces involves dissolving Oil Red O dye in toluene and dropping it onto the water surface. Tweezers are then used to grasp a piece of TpPa-1 COF aerogel and remove the dyed toluene from the water surface.

[0082] Depend on Figure 12 As shown in Table 1, TpPa-1, TpBD-Me2 and TpBD COF aerogels can adsorb 43-90 times, 43-94 times and 44-83 times of their own weight of organic solvents, respectively, indicating that the aerogels prepared by this method have ultra-high organic solvent adsorption capacity. Figure 13 It shows that the aerogel can be used to remove organic solvent pollution on water surfaces.

[0083] Table 1 Organic solvent adsorption properties of TpPa-1, TpBD-Me2, and TpBD COF aerogels and pure COF bulk materials reported in the literature

[0084]

[0085]

[0086] Comparative Example 1

[0087] This comparative example is similar to Example 1, except that the acetic acid concentration used is 3M. ​​In this case, aerogel cannot be formed. Figure 14 From the XRD pattern, it can be seen that no COF is formed.

[0088] Comparative Example 2

[0089] This comparative example is similar to Example 1, except that the acetic acid concentration used is 6 M. In this case, aerogel cannot be formed. Figure 14 From the XRD pattern, it can be seen that no COF is formed.

[0090] Comparative Example 3

[0091] This comparative example is similar to Example 1, except that the acetic acid concentration used is 17.5 M. In this case, aerogel cannot be formed. Figure 14 From the XRD pattern, it can be seen that no COF is formed.

Claims

1. A method for preparing an ultralight, compressible covalent organic framework aerogel, characterized in that: The specific steps are as follows: Step 1, adding polyacrylonitrile to the DMF dispersion of the urea-based ligand, stirring until the mixture is uniformly mixed, to obtain an electrospinning precursor solution; Step 2, setting the electrospinning parameters: the positive voltage of the spinning voltage is 15-20 KV, the negative voltage is 3-5 KV, the injection rate is 0.08-0.15 mm / min, the receiving distance is 15-20 cm, the humidity is 20-40%, and the temperature is 20-50°C, and the electrospinning precursor solution is electrospun to obtain a UL / PAN fiber membrane; Step 3, cutting the UL / PAN fiber membrane into pieces, adding water to homogenize, and freeze-drying to obtain UL / PAN fiber; Step 4: Disperse the UL / PAN fibers in a dioxane / mesitylene mixed solution containing trialdehyde phloroglucinol and stir until the UL / PAN fibers are evenly dispersed. Then, add a 12 M acetic acid solution and react at 90 ± 5 °C for 48 to 72 hours. Then, raise the temperature to 160 to 170 °C and react for more than 3 hours to obtain a Tp-COF / PAN wet gel. Step 5, the Tp-COF / PAN wet gel was washed with acetone and ethanol in sequence, and then extracted with NMP; Step 6, sequentially exchanging the solvents of the Tp-COFs wet gel obtained in step 5 with ethanol and water, and then freeze-drying the gel to obtain a Tp-COFs fiber aerogel; Alternatively, the Tp-COFs wet gel obtained in step 5 is subjected to solvent exchange with ethanol, and then subjected to supercritical drying to obtain the Tp-COFs fiber aerogel.

2. The preparation method according to claim 1, characterized in that In step 1, the urea-based ligand is 1,4-phenylenediurea, 1,1'-(3,3'-dimethyl-[1,1'-biphenyl]-4,4'-diyl)diurea, [1,1'-biphenyl]-4,4'-diurea or [1,1'-azobenzene]-4,4'-diurea.

3. The preparation method according to claim 1, wherein In step 1, the concentration of PAN in the electrospinning precursor solution is 0.11~0.15 g / mL, the mass ratio of urea-based ligand to polyacrylonitrile is 0.5~2:1, and the stirring temperature is 20~30°C.

4. The preparation method according to claim 1, wherein In step 3, the concentration of UL / PAN fiber membrane is 2~5 mg / L, the homogenization conditions are a rotation speed of 18000~30000 rpm, a time of 1.5~10 min, a freeze-drying temperature of -52°C, and a freeze-drying time of 24~48 h.

5. The preparation method according to claim 1, wherein In step 4, the concentration of trialdehyde phloroglucinol in the mixed solution is 0.01-0.02 mmol / mL, the concentration of UL / PAN fiber is 8.8-17.6 mg / mL, the volume ratio of dioxane to mesitylene is 1-4:1, and the volume ratio of acetic acid solution to dioxane / mesitylene mixed solution is 1:10-1:

5.

6. The preparation method according to claim 1, wherein In step 5, the extraction temperature is 80-90°C, NMP is replaced every 8 hours, and the total extraction time is 24-48 hours.

7. The preparation method according to claim 1, wherein In step 6, the supercritical drying cooling temperature is 0-15°C, the heating temperature is 35-50°C, and the number of operations is 50-200 times.

8. The covalent organic framework aerogel prepared according to the preparation method according to any one of claims 1 to 7.

9. Use of the covalent organic framework aerogel according to claim 8 in adsorption of organic solvents.

10. The use according to claim 9, characterized in that The organic solvent is dimethyl sulfoxide, chloroform, toluene, 1,4-dioxane, acetone, dimethylacetamide, methanol or ethanol.

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