Method for preparing non-ionic COF nanosheet based on secondary amine regulator and application of non-ionic COF nanosheet

Abstract

The invention discloses a preparation method for preparing a nonionic COF nanosheet based on a secondary amine regulator, which comprises the following steps: dissolving secondary amine molecules serving as a regulator in an amino monomer aqueous solution to prepare a water-phase monomer solution; dissolving trialdehyde phloroglucinol serving as an aldehyde monomer into the mesitylene solution to prepare an oil-phase monomer solution; and adding the water-phase monomer solution and the oil-phase monomer solution into a container, and carrying out polymerization reaction on an amino monomer and an aldehyde monomer in the water-phase monomer solution under the adjustment of secondary amine molecules to obtain the series of beta-keto-enamine bond connected nonionic COF nanosheets. Secondary amine molecules form an enol imine intermediate through nucleophilic attack of aldehyde groups or imine bonds, ordered expansion in a two-dimensional plane is cooperatively controlled and growth in the vertical direction is inhibited through an activation and inhibition dual regulation mechanism, and efficient and mild preparation of the high-crystallinity nanosheet is realized. The non-ionic COF nanosheets are assembled into the COF membrane through a vacuum auxiliary method, and the COF membrane has high permeability and high selectivity when being applied to organic solvent nanofiltration.

Description

Technical Field

[0001] The present invention relates to the technical field of organic solvent nanofiltration separation, and in particular to a preparation method and application of a non-ionic COF nanosheet. Background Art

[0002] Near-zero discharge and resource recovery of waste organic solvents are major demands for high-quality, sustainable development in green petrochemical, fine chemical and other industries. Low-energy organic solvent nanofiltration (OSN) technology has become a key means to replace traditional distillation. The core of achieving efficient OSN lies in high-performance membrane materials. COF materials, due to their highly ordered and adjustable pore structure and excellent chemical stability, have become ideal OSN membrane materials for achieving precise separation of organic solvents at the molecular scale. COF nanosheets, as the building blocks of COF membranes, can be solution-processed and have excellent film-forming properties. Compared with ionic COF nanosheets, non-ionic COF nanosheets (nCOFNs) have greater flexibility in terms of organic monomer connection, dynamic bonding and reaction network connection.

[0003] Interfacial synthesis is one of the most widely used methods for obtaining nCOFNs. However, due to the highly confined reaction space, this method suffers from the disadvantages of difficult interfacial exfoliation of nanosheets and low volumetric yield. In recent years, the liquid-phase synthesis of nCOFNs has attracted increasing attention due to its excellent solution processability and high yield. Unlike ionic COFs, non-ionic COFs generally form nanoparticles rather than discrete nanosheets during liquid-phase synthesis due to the lack of electrostatic repulsion to offset the π-π interlayer binding forces. Current strategies to inhibit their out-of-plane assembly mainly focus on incorporating non-planar units into the COF backbone or using additives to introduce steric hindrance. However, these strategies often have difficulty in achieving orderly control of in-plane assembly, and the synthesized nCOFNs have low crystallinity or require harsh synthesis conditions such as high temperature and degassing. How to synthesize highly crystalline, large-sized nCOFNs under mild conditions remains a key and urgent challenge. Summary of the Invention

[0004] In response to the above-mentioned prior art, the present invention provides a method for preparing non-ionic COF nanosheets based on secondary amine regulators. This preparation method uses secondary amine molecules as regulators, and expects to obtain a series of high-crystallinity, high-aspect-ratio, and high-yield β-ketoenamine-linked nCOFNs at ambient temperature and pressure through a synergistic activation-inhibition strategy.

[0005] To solve the above technical problems, the present invention proposes a method for preparing non-ionic COF nanosheets based on a secondary amine regulator. Secondary amine molecules are used as regulators and dissolved in an aqueous amino monomer solution to prepare an aqueous monomer solution; trialdehyde phloroglucinol is used as an aldehyde monomer and dissolved in a mesitylene solution to prepare an oily monomer solution; the aqueous monomer solution and the oily monomer solution are added to a container. Under the regulation of the secondary amine molecules, the amino monomer and the aldehyde monomer undergo a polymerization reaction in the aqueous monomer solution to obtain a series of non-ionic COF nanosheets connected by β-ketoenamine bonds. The specific steps include:

[0006] Step 1) dissolving an amino monomer and a secondary amine molecule in deionized water, wherein the molar concentration of the amino monomer is 0.075-0.9 mmol / L, the molar concentration of the secondary amine molecule is 0.15-3.6 mmol / L, and the concentration equivalent ratio of the secondary amine molecule to the amino monomer is 2-4:1, and ultrasonicating for 20-40 minutes to prepare an aqueous monomer solution. In the present invention, the amount of the secondary amine molecule added is expressed as an equivalent relative to the amino monomer concentration.

[0007] Step 2) dissolving trialdehyde phloroglucinol (Tp) in mesitylene, wherein the molar concentration of trialdehyde phloroglucinol is 0.05-0.6 mmol / L, and ultrasonicating for 20-40 minutes to prepare an oil phase monomer solution;

[0008] step 3) adding the aqueous monomer solution prepared in step 1) and the oily monomer solution prepared in step 2) dropwise to a container at a molar ratio of amino monomer to aldehyde monomer of 3:2, and separating the oily and water phases after the addition is complete, with the upper layer being the oily phase and the lower layer being the aqueous phase;

[0009] Step 4) After sealing the container with plastic wrap, the reaction system was allowed to stand at room temperature and pressure for 7 days, and the lower aqueous phase was a brown-red transparent solution;

[0010] Step 5) discarding the upper oil phase, filtering the lower aqueous phase and placing it into a dialysis bag for 3 days to obtain a non-ionic COF nanosheet solution.

[0011] Furthermore, the method for preparing non-ionic COF nanosheets of the present invention comprises:

[0012] In step 1), the amino monomer is one of p-phenylenediamine (Pa), benzidine (BD), and hydrazine (Hz); the secondary amine molecule is one of pyrrolidine (Py), dimethylamine (Da), 2-methylpyrrolidine (MP), and diisopropylamine (NPA).

[0013] The amino monomer is preferably p-phenylenediamine (Pa), and the secondary amine molecule is preferably pyrrolidine (Py). In the aqueous monomer solution, the molar concentration of p-phenylenediamine (Pa) is preferably 0.3 mmol / L, and the molar concentration of pyrrolidine (Py) is preferably 0.6 mmol / L.

[0014] In step 2), the molar concentration of the trialdehyde phloroglucinol in the oil phase monomer solution is preferably 0.2 mmol / L.

[0015] In steps 1) and 2), the ultrasonic time is preferably 30 min.

[0016] In step 5), the molecular weight cut-off of the dialysis bag is preferably 30 kDa.

[0017] At the same time, the present invention also proposes to assemble the non-ionic COF nanosheets prepared in the present invention into a COF membrane by a vacuum-assisted method, wherein the amount of the non-ionic COF nanosheet solution is 100-300 μL / 3.14 cm 2 The film thickness was calculated to be 60-180 nm.

[0018] Preferably, the non-ionic COF nanosheets are assembled into a COF membrane by a vacuum-assisted method, wherein the amount of the non-ionic COF nanosheet solution is 200 μL / 3.14 cm 2 The membrane thickness is calculated to be 120 nm and the methanol flux of the membrane is 127 Lm -2 h -1 bar -1 , molecular cut-off is 580 g mol -1 .

[0019] The present invention also proposes the application of the above-mentioned non-ionic COF membrane, which is used for the separation and purification of cannabidiol oil (CBD). The cannabidiol oil / chlorophyll separation factor is 180, the chlorophyll retention rate is 99.6%, and the cannabidiol oil recovery rate is 81%.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) In the preparation method of the present invention, it is proposed to use secondary amine molecules as regulators for the liquid phase synthesis of nCOFNs. Secondary amine molecules can form enolimine intermediates by nucleophilic attack on aldehyde groups or imine bonds, thereby achieving both activation and inhibition regulatory effects. On the one hand, the reversibility of enone-type interconversion is conducive to strengthening the imine exchange process and promoting orderly planar assembly. On the other hand, the non-planar side groups of the secondary amine molecules introduce steric hindrance, inhibiting the interlayer stacking during the growth of nCOFNs. Therefore, a series of β-ketoenamine-linked nCOFNs with high crystallinity, high aspect ratio and high yield can be obtained at ambient temperature and pressure.

[0022] (2) The nCOFNs prepared in this invention have excellent solution processability, which enables them to be easily assembled into ultrathin, defect-free, and mechanically strong COF membranes. When applied to the OSN process, the methanol flux was 127 L m -2 h -1 bar -1 , far exceeding the most advanced organic membranes with similar molecular weight cut-off (MWCO), while also maintaining long-term stability for 1,000 hours of continuous cross-flow filtration. Furthermore, the COF membrane prepared by this invention exhibits excellent purification of high-value pharmaceuticals. In the separation and purification of cannabidiol oil (CBD), the CBD oil / chlorophyll separation factor was 180, the chlorophyll rejection was 99.6%, and the CBD oil recovery rate was 81%. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the preparation and structure of TpPa nCOFNs based on secondary amine regulators of the present invention;

[0024] Figure 2 Powder X-ray diffraction characterization images of TpPa nCOFNs synthesized with different concentrations of pyrrolidine;

[0025] Figure 3 Scanning electron microscopy images of TpPa nCOFNs synthesized with different pyrrolidine concentrations;

[0026] Figure 4 Photos and scanning electron microscope images of TpPa nCOFNs synthesized without the addition of pyrrolidine;

[0027] Figure 5 Powder X-ray diffraction characterization images of TpPa nCOFNs synthesized with different secondary amine modifiers;

[0028] Figure 6 : This is a transmission electron microscope image of a cross section of the TpPa COF film obtained in Example 6;

[0029] Figure 7The results of different dye retention and permeation performance tests on TpPa COF membranes;

[0030] Figure 8 This is the CBD / chlorophyll separation performance test result of TpPa COF membrane. DETAILED DESCRIPTION

[0031] The proposed method for preparing nonionic COF nanosheets using a secondary amine modifier is designed to utilize a secondary amine molecule as the modifier, dissolving it in an aqueous amino monomer solution to produce an aqueous monomer solution. Trialdehyde phloroglucinol, an aldehyde monomer, is then dissolved in a mesitylene solution to produce an oily monomer solution. This allows for the controlled synthesis of nonionic COF nanosheets in a phase transfer polymerization system. The aqueous and oily monomer solutions are added to a single container. Under the regulation of the secondary amine molecule, the amino and aldehyde monomers polymerize in the aqueous monomer solution to produce a series of β-ketoenamine-linked nonionic COF nanosheets (nCOFNs). In phase transfer polymerization, secondary amines nucleophilically attack aldehyde or imine bonds to form enolimine intermediates. This improves the reversibility of enone interconversion and strengthens the imine exchange process. Furthermore, the side groups of the secondary amine molecules introduce steric hindrance, hindering interlayer stacking during COF growth. The dual regulatory mechanism of activation and inhibition by secondary amines can synergistically control ordered expansion within the two-dimensional plane and inhibit vertical growth, enabling the efficient and gentle preparation of highly crystalline, high-aspect-ratio nanosheets. Because the resulting nCOFNs have excellent solution processability, they can be assembled into ultrathin, defect-free, and mechanically strong COF membranes via vacuum-assisted methods. Application of these membranes in organic solvent nanofiltration (OSN) processes can achieve both high permeability and high selectivity.

[0032] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention in any way.

[0033] Example 1

[0034] Using secondary amine molecules as regulators, nonionic COF nanosheets TpPa nCOFNs were synthesized under mild conditions using phase transfer polymerization. Figure 1 As shown, the preparation steps of the nanosheet are as follows:

[0035] Step 1) Weigh 1.6 mg (0.015 mmol) of p-phenylenediamine monomer, add 50 mL of deionized water, pipette 2.48 μL (0.03 mmol, i.e., 2 eq) of pyrrolidine and dissolve it, and sonicate for 30 min;

[0036] Step 2) Weigh 2.1 mg (0.01 mmol) of trialdehyde phloroglucinol (Tp) monomer, add 50 mL of mesitylene, and sonicate for 30 min;

[0037] Step 3) The reaction was carried out in a 200 mL beaker. The oil phase monomer solution containing Tp was slowly added dropwise along the wall of the beaker to the top of the aqueous phase using a dropper. After the addition was complete, the system volume was 100 mL. The oil and water phases separated, with the upper layer being the oil phase and the lower layer being the aqueous phase.

[0038] Step 4) After sealing with plastic wrap, the reaction system was allowed to stand at room temperature and pressure for 7 days. After 7 days of reaction, the lower aqueous phase gradually turned into a brown-red transparent solution;

[0039] Step 5) The upper oil phase was discarded, and the lower aqueous solution was filtered and placed into a dialysis bag with a molecular weight cutoff of 30 kDa for 3 days for further purification to obtain non-ionic COF nanosheets, which were recorded as TpPa-nCOFNs-Py-2.

[0040] Figure 3 It can be seen that the COF nanosheets synthesized under 2eq conditions have obvious lamellar structure.

[0041] Example 2

[0042] TpPa nCOFNs were synthesized under mild conditions using phase transfer polymerization. The preparation process of Example 2 was essentially the same as that of Example 1, except that in step 1, the amount of pyrrolidine added was changed from 2.48 μL (2 eq) to 4.96 μL (4 eq). The resulting nanosheets were designated TpPa-nCOFN-Py-4. Figure 3 It can be seen that the COF nanosheets synthesized under 4eq conditions also have obvious lamellar structure.

[0043] Comparative Example 1

[0044] TpPa nCOFNs were synthesized under mild conditions using phase transfer polymerization. The preparation process of Comparative Example 1 was basically the same as that of Example 1, except that in step 1), the amount of pyrrolidine added was changed from 2.48 μL (2 eq) to 1.24 μL (1 eq). The resulting nanosheets were designated TpPa-nCOFNs-Py-1. Figure 3 It can be seen that the COFs synthesized under 1 eq conditions exhibit a block structure.

[0045] Comparative Example 2

[0046] TpPa nCOFNs were synthesized under mild conditions using phase transfer polymerization. The preparation process of Comparative Example 2 was basically the same as that of Example 1, except that in step 1), the amount of pyrrolidine added was changed from 2.48 μL (2 eq) to 9.92 μL (8 eq). The resulting nanosheets were designated TpPa-nCOFNs-Py-8. Figure 3 It can be seen that the COFs synthesized under 8eq conditions present a granular structure.

[0047] Comparative Example 3

[0048] TpPa nCOFNs were synthesized under mild conditions using phase transfer polymerization. The preparation process of Comparative Example 3 was basically the same as that of Example 1, except that in step 1), the amount of pyrrolidine added was changed from 2.48 μL (2 eq) to no pyrrolidine. The resulting nanosheets were designated TpPa-nCOFNs-Py-0. Figure 4 This is a photograph of TpPa-nCOFNs-Py-0 synthesized without adding a pyrrolidine regulator.

[0049] Example 3

[0050] TpPa nCOFNs were synthesized under mild conditions using phase transfer polymerization. The preparation process of Example 5 was essentially the same as that of Example 1, except that in step 1), the added regulator was changed from 2.48 μL (2 eq) pyrrolidine to 2.07 μL (2 eq) dimethylamine (Da). The resulting nanosheets were designated TpPa-nCOFNs-Da-2.

[0051] Example 4

[0052] TpPa nCOFNs were synthesized under mild conditions using phase transfer polymerization. The preparation process of Example 4 was essentially the same as that of Example 1, except that in step 1), the added regulator was changed from 2.48 μL (2 eq) pyrrolidine to 3.06 μL (2 eq) 2-methylpyrrolidine (MP). The resulting nanosheets were designated TpPa-nCOFNs-MP-2.

[0053] Example 5

[0054] TpPa nCOFNs were synthesized under mild conditions using phase transfer polymerization. The preparation process of Example 5 was essentially the same as that of Example 1, except that in step 1), the added regulator was changed from 2.48 μL (2 eq) pyrrolidine to 4.23 μL (2 eq) diisopropylamine (NPA). The resulting nanosheets were designated TpPa-nCOFNs-NPA-2.

[0055] The formulas of the aqueous monomer solutions and oily monomer solutions of Examples 1-5 and Comparative Examples 1-3 are shown in Table 1.

[0056] Table 1 Formulation data of Examples and Comparative Examples

[0057]

[0058] Example 6

[0059] TpPa nCOFNs were synthesized under mild conditions using phase transfer polymerization, the process being the same as in Example 1, and then TpPa-nCOFNs-Py-2 was filtered onto a polyacrylonitrile (PAN) base membrane by vacuum filtration to obtain the corresponding TpPaCOF membrane, which was recorded as TpPa-nCOFNs-Py-2 membrane. In the present invention, the base membrane can be one of polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), polypropylene (PP) and polyethersulfone resin (PES). In Example 6, the base membrane is a circular PAN base membrane with a diameter of 1 cm, the amount of TpPa-nCOFNs-Py-2 solution used is 200 μL, and the thickness of the membrane is 120 nm, as shown in FIG. Figure 6 As shown; the methanol flux of this membrane is 127Lm -2 h -1 bar -1 , molecular cut-off is 580 g mol -1 ,like Figure 7 As shown in (a) and (b) in .

[0060] The organic solvent nanofiltration (OSN) separation performance of the TpPa-nCOFNs-Py-2 membrane prepared in Example 6 was evaluated using a cross-flow apparatus, with dye rejection and organic solvent flux as the main evaluation indicators:

[0061] Retention rate calculation formula: R = (1-C p / C f )×100%, where R represents the retention rate, C p and C f are the dye concentrations (ppm) in the permeate and feed solutions, respectively.

[0062] Organic solvent flux (L m-2h -1 bar -1 ) is defined as: under certain operating pressure conditions, the volume of organic solvent that passes through a unit effective membrane area per unit pressure and unit time.

[0063] The flux of various organic solvents of the membrane obtained in Example 6 is shown in Table 2, and the retention performance of dyes with different molecular weights is shown in Table 3.

[0064] Table 2 Flux data of various organic solvents

[0065]

[0066] Table 3 Various dye retention performance data

[0067] organic solvents Dye name Molecular weight (Da) Retention rate (%) 1 Methanol Methyl orange 327 61.20 2 Methanol Chrome Black T 461 82.93 3 Methanol Acid fuchsin 586 93.36 4 Methanol Congo Red 697 97.83 5 Methanol Methylene blue 800 97.80 6 Methanol Evans Blue 961 99.57

[0068] Example 7

[0069] The CBD purification performance of the TpPa-nCOFNs-Py-2 membrane prepared in Example 6 was evaluated using dead-end filtration to explore the practicality of TpPa COF membranes for the separation of high-value pharmaceuticals. The organic solvent flux and retention during the CBD / chlorophyll purification process are shown in Table 4.

[0070] Table 4 Organic solvent flux and retention data of CBD and chlorophyll

[0071]

[0072] The structural characterization of the TpPa nCOFNs prepared in Examples 1-5 and Comparative Examples 1-3 is analyzed below, and the performance test of the TpPa COF membranes prepared in Examples 6-7 is evaluated.

[0073] The TpPa-nCOFNs-Py-x prepared in Example 1-2 and Comparative Example 1-2 were characterized by powder X-ray diffraction (PXRD). Figure 2 shown.

[0074] The crystallinity of the nanosheets was compared based on the intensity of the diffraction peak at the (100) plane. The crystallinity of TpPa-nCOFNs-Py-x initially increased and then decreased with increasing Py concentration. Highly crystalline TpPa nCOFNs were generated when the Py concentration was 2 eq (relative to Pa).

[0075] (1) The TpPa-nCOFNs-Py-x prepared in Example 1-2 and Comparative Example 1-2 were characterized by scanning electron microscopy (SEM). The results are as follows: Figure 3 shown.

[0076] In Examples 1 and 2, TpPa nCOFNs synthesized with 2 and 4 eq of pyrrolidine both exhibited distinct lamellar structures, with the 2 eq nanosheets exhibiting a more pronounced lamellar structure than the 4 eq nanosheets. In Comparative Example 1, COFs synthesized with 1 eq of pyrrolidine exhibited a blocky structure, while those synthesized with 8 eq in Comparative Example 2 exhibited a granular structure. Therefore, in practical applications, a 2 eq pyrrolidine addition is preferred.

[0077] (2) Scanning electron microscopy (SEM) characterization test was performed on the TpPa-nCOFNs-Py-0 prepared in Comparative Example 3. The results are as follows Figure 4 shown.

[0078] When pyrrolidine was not added, significant flocculation was observed in the lower aqueous solution. This is due to the aggregation and precipitation of COFs due to π-π stacking during growth. Freeze-dried precipitates revealed a blocky structure composed of stacked sheets under a scanning electron microscope. Comparison with SEM images of TpPa nCOFNs regulated by pyrrolidine confirmed that the addition of pyrrolidine effectively modulated the synthesis of TpPa nCOFNs.

[0079] (III) The TpPa nCOFNs prepared in Example 1 and Examples 3-5 were characterized by powder X-ray diffraction (PXRD). Figure 5 shown.

[0080] In Example 3, the nCOFNs synthesized using Da exhibited high crystallinity, but their (001) diffraction peak exhibited a broad bulge, due to nanoscale disorder during the stacking process. In Examples 4 and 5, the nCOFNs synthesized using Mp and NPA, respectively, exhibited relatively weak (100) diffraction peaks and low crystallinity, likely due to the adverse effects of steric hindrance on imine exchange. Therefore, in practical applications, Py is preferred as a modulator for the synthesis of nCOFNs.

[0081] (IV) The TpPa COF membrane prepared in Example 6 was subjected to different dye retention and permeability tests. The results are as follows: Figure 7 As shown, (a) is a graph showing the test results of different dye retention, and (b) is a graph showing the test results of different organic solvent penetration performance.

[0082] For molecular weights between 327 and 960 g mol -1 Membrane selectivity in methanol was evaluated for a range of dye solutes, such as Figure 7 As shown in (a), the rejection rate increases with the increase of dye molecular weight, indicating that the selectivity is size-dependent. The molecular weight cutoff of the TpPa-COF-Py-2 membrane is 586 g mol -1 .

[0083] The permeability of nonpolar solvents (n-hexane, mesitylene), polar protic solvents (methanol, ethanol, propanol, butanol) and polar aprotic solvents (acetone, ethyl acetate) were evaluated. Figure 7 As shown in (b), the membrane exhibits high permeability to both polar and nonpolar solvents. Solvent transport is primarily controlled by viscosity, a viscous flow behavior attributed to its rigid pore structure. The TpPa COF membrane exhibits superior selectivity and permeability compared to commercial membranes and recently investigated organic OSN membranes.

[0084] (V) In Example 7, the TpPa COF membrane prepared in Example 6 was tested for CBD purification performance. The results were as follows: Figure 8 shown.

[0085] The TpPa COF membrane achieved 99.6% chlorophyll rejection while allowing CBD to pass through with low rejection (<28%), resulting in a CBD / chlorophyll separation factor of 180. After 7 hours of operation, methanol permeability and solute rejection remained stable. CBD purity in the permeate remained above 98%, while chlorophyll purity in the retentate increased from 57% to 88%. CBD recovery reached 81%.

[0086] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments. The above-mentioned specific embodiments are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can make many improvements and changes without departing from the purpose of the present invention, which are all protected by the present invention.

Claims

1. A method for preparing nonionic COF nanosheets based on a secondary amine modifier, characterized in that: A secondary amine molecule is used as a regulator and dissolved in an amino monomer aqueous solution to prepare an aqueous monomer solution; trialdehyde phloroglucinol is used as an aldehyde monomer and dissolved in a mesitylene solution to prepare an oily monomer solution; the aqueous monomer solution and the oily monomer solution are added to a container, and under the regulation of the secondary amine molecule, the amino monomer and the aldehyde monomer undergo a polymerization reaction in the aqueous monomer solution to obtain a series of non-ionic COF nanosheets connected by β-ketoenamine bonds.

2. The method for preparing nonionic COF nanosheets according to claim 1, characterized in that: The following steps are involved: Step 1) dissolving an amino monomer and a secondary amine molecule in deionized water, wherein the molar concentration of the amino monomer is 0.075-0.9 mmol / L, the molar concentration of the secondary amine molecule is 0.15-3.6 mmol / L, and the concentration equivalent ratio of the secondary amine molecule to the amino monomer is 2-4:1, and ultrasonicating for 20-40 minutes to prepare an aqueous monomer solution; Step 2) dissolving trialdehyde phloroglucinol in mesitylene, wherein the molar concentration of trialdehyde phloroglucinol is 0.05-0.6 mmol / L, and ultrasonicating for 20-40 minutes to prepare an oil phase monomer solution; step 3) adding the aqueous monomer solution prepared in step 1) and the oily monomer solution prepared in step 2) dropwise to a container at a molar ratio of amino monomer to aldehyde monomer of 3:2, and separating the oily and water phases after the addition is complete, with the upper layer being the oily phase and the lower layer being the aqueous phase; Step 4) After sealing the container with plastic wrap, the reaction system was allowed to stand at room temperature and pressure for 7 days, and the lower aqueous phase was a brown-red transparent solution; Step 5) discarding the upper oil phase, filtering the lower aqueous phase and placing it into a dialysis bag for 3 days to obtain a non-ionic COF nanosheet solution.

3. The method for preparing nonionic COF nanosheets according to claim 2, wherein: In step 1), the amino monomer is one of p-phenylenediamine (Pa), benzidine (BD), and hydrazine (Hz); the secondary amine molecule is one of pyrrolidine (Py), dimethylamine (Da), 2-methylpyrrolidine (MP), and diisopropylamine (NPA).

4. The method for preparing nonionic COF nanosheets according to claim 2 or 3, characterized in that: In step 1), the amino monomer is p-phenylenediamine (Pa), the secondary amine molecule is pyrrolidine (Py), and the molar concentration of p-phenylenediamine (Pa) in the aqueous monomer solution is 0.3 mmol / L, and the molar concentration of pyrrolidine (Py) is 0.6 mmol / L; in step 2), the molar concentration of trialdehyde phloroglucinol in the oil phase monomer solution is 0.2 mmol / L.

5. The method for preparing nonionic COF nanosheets according to claim 2, wherein: In steps 1) and 2), the ultrasonic time is 30 minutes.

6. The method for preparing nonionic COF nanosheets according to claim 2, wherein: In step 5), the molecular weight cut-off of the dialysis bag is 30 kDa.

7. A non-ionic COF membrane, characterized in that The nonionic COF nanosheets obtained by any of the preparation methods of claims 1 to 7 are assembled into a COF membrane by a vacuum-assisted method, wherein the amount of the nonionic COF nanosheet solution used is 100-300 μL / 3.14 cm 2 The film thickness was calculated to be 60-180 nm.

8. The non-ionic COF membrane according to claim 7, characterized in that: The nonionic COF nanosheets were assembled into a COF membrane by a vacuum-assisted method, wherein the amount of the nonionic COF nanosheet solution was 200 μL / 3.14 cm 2 The membrane thickness is calculated to be 120 nm and the methanol flux of the membrane is 127 L m -2 h -1 bar -1 , molecular cut-off is 580 g mol -1 .

9. An application of a non-ionic COF membrane, characterized in that: The non-ionic COF membrane described in claim 7 or 8 is applied to the separation and purification of cannabidiol oil (CBD), with a cannabidiol oil / chlorophyll separation factor of 180, a chlorophyll retention rate of 99.6%, and a cannabidiol oil recovery rate of 81%.

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