Cyclic thiophene oligomer-based three-dimensional covalent organic framework material and preparation method thereof

Abstract

The invention relates to the technical field of preparation of covalent organic framework materials, in particular to a cyclic thiophene oligomer-based three-dimensional covalent organic framework material and a preparation method thereof. The cyclic thiophene oligomer-based three-dimensional covalent organic framework material is selected from any one of compounds shown in the following structural formula: # imgabs0 #, and R represents H or C1-C3 alkyl. The cyclic thiophene oligomer based three-dimensional covalent organic framework material has high crystallinity, permanent porosity and excellent stability.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of covalent organic framework materials, and more particularly to a three-dimensional covalent organic framework material based on cyclic thiophene oligomers and a preparation method thereof. Background Art

[0002] Covalent organic framework materials (COFs) are a class of novel crystalline porous framework materials assembled by covalent bonding of organic units. They have the characteristics of adjustable structure design, good chemical and thermal stability, easy modification and functionalization, and high porosity, and are widely used in the fields of adsorption and separation, ion transport, catalysis, energy storage, and sensing. According to the spatial dimension, COFs are mainly divided into two-dimensional (2D) structures stacked layer by layer through π-π interactions and three-dimensional (3D) network structures with interconnected pores. Since the concept of COFs was first proposed in 2005, most of the research on COFs has focused on the 2D level, and the structures and properties of 2D COFs have been designed and explored. In contrast, due to the limited design and selection of 3D organic building units, the research progress of 3D COFs is much slower. Currently, the construction of 3D COFs is more based on 3D organic units with a tetrahedral configuration, such as tetraphenylmethane, tetraphenylsilane, spirobifluorene, adamantane, etc. The design and synthesis of novel multi-connected three-dimensional organic units directly limit the development of 3D COFs, making the design and construction of novel 3D COFs still a challenge.

[0003] In view of this, the present invention is specifically proposed. Summary of the Invention

[0004] The purpose of the present invention is to provide a three-dimensional covalent organic framework material based on cyclic thiophene oligomers and a preparation method thereof. The three-dimensional covalent organic framework material based on cyclic thiophene oligomers provided by the embodiments of the present invention has high crystallinity, permanent porosity, and excellent stability.

[0005] The present invention is implemented as follows:

[0006] In a first aspect, the present invention provides a three-dimensional covalent organic framework material based on cyclic thiophene oligomers, which is any one of the compounds shown in the following structural formulas:

[0007]

[0008] Wherein, R represents H or a C1-C3 alkyl group.

[0009] In an alternative embodiment, R is H or methyl.

[0010] Second aspect, the present invention provides a method for preparing the cyclic thiophene oligomer-based three-dimensional covalent organic framework material described in the foregoing embodiments, including: mixing a cyclic thiophene oligomer compound containing six aldehyde groups and a diamine monomer to carry out a Schiff base reaction.

[0011] In an alternative embodiment, it includes: mixing a cyclic thiophene oligomer compound containing six aldehyde groups, a diamine monomer, an organic solvent, and a catalyst, then cyclically performing freezing-thawing-degassing, then heating to room temperature, and then reacting at 120-140 °C for 5-7 days.

[0012] In an alternative embodiment, the structural formula of the cyclic thiophene oligomer compound containing six aldehyde groups is as follows:

[0013]

[0014] The structural formula of the diamine monomer is as follows:

[0015] Wherein, R is hydrogen and C1-C3 alkyl; preferably, R is hydrogen and methyl.

[0016] In an alternative embodiment, the molar ratio of the cyclic thiophene oligomer compound containing six aldehyde groups to the diamine monomer is 1:(3.0-3.5).

[0017] In an alternative embodiment, the organic solvent is selected from at least one of alcohol solvents and halogen-substituted phenyls.

[0018] In an alternative embodiment, the organic solvent is at least one of o-dichlorobenzene and n-butanol. Preferably, the volume ratio of o-dichlorobenzene to n-butanol is 1:(6-9).

[0019] In an alternative embodiment, the catalyst is acetic acid, preferably an aqueous acetic acid solution, and the concentration of the aqueous acetic acid solution is 6-9M.

[0020] In an alternative embodiment, the volume ratio of the catalyst to the organic solvent is 1:(5-10).

[0021] The present invention has the following beneficial effects: The embodiments of the present invention provide a polyhedral three-dimensional organic unit hexaaldehyde cyclic thiophene oligomer, and the single crystal structure of the organic unit is obtained. Based on this organic unit, a cyclic thiophene oligomer-based three-dimensional covalent organic framework material is synthesized. The three-dimensional covalent organic framework of the present invention is a fully conjugated structure and contains electron-rich thiophene units, and has good application prospects in catalytic fields such as photocatalysis and electrocatalysis. Description of the Drawings

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other relevant drawings can also be obtained based on these drawings.

[0023] Figure 1 1H NMR spectrum of CDHt-6CHO provided by the embodiment of the present invention;

[0024] Figure 2 Crystal photograph of CDHt-6CHO provided by the embodiment of the present invention;

[0025] Figure 3 Single crystal structure diagram of CDHt-6CHO provided by the embodiment of the present invention;

[0026] Figure 4 Powder X-ray diffraction pattern of COF-NUST-68 and its organic monomer provided by the embodiment of the present invention;

[0027] Figure 5 Powder X-ray diffraction pattern of COF-NUST-68 provided by the embodiment of the present invention;

[0028] Figure 6 Fourier transform infrared spectrum of COF-NUST-68 and its organic monomer provided by the embodiment of the present invention;

[0029] Figure 7 N2 adsorption-desorption and pore size distribution diagram of COF-NUST-68 provided by the embodiment of the present invention;

[0030] Figure 8 Scanning electron microscope image of COF-NUST-68 provided by the embodiment of the present invention;

[0031] Figure 9 Transmission electron microscope image of COF-NUST-68 provided by the embodiment of the present invention. Specific embodiments

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For the reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchase.

[0033] In the first aspect, the present invention provides a cyclic thiophene oligomer-based three-dimensional covalent organic framework material, which is selected from any one of the compounds shown in the following structural formulas:

[0034]

[0035] Among them, R represents H or C1-C3 alkyl. For example, R is H or C1-C3 alkyl such as hydrogen, methyl, ethyl, and propyl.

[0036] In a second aspect, the present invention provides a method for preparing a cyclic thiophene oligomer-based three-dimensional covalent organic framework material as described in the foregoing embodiment, including: mixing a cyclic thiophene oligomer compound containing six aldehyde groups and a diamine monomer for a Schiff base reaction.

[0037] Among them, the preparation steps of the cyclic thiophene oligomer compound (CDHt-6CHO) containing six aldehyde groups include:

[0038] Step 1: Add the cyclic thiophene oligomer compound (CDHt-6TMS) containing six TMS groups into a single-necked flask, add chloroform solution, dropwise add trifluoroacetic acid, react under light protection, after the reaction is completed, wash with water, saturated sodium bicarbonate solution, and water, add anhydrous sodium sulfate to dry the organic phase, and remove the solvent under reduced pressure. Use dichloromethane and methanol for rapid precipitation to obtain the cyclic thiophene oligomer compound.

[0039] Step 2: Dissolve the cyclic thiophene oligomer compound in N,N-dimethylformamide, add N-bromosuccinimide in batches under stirring, react at room temperature, after the reaction is completed, remove the solvent under reduced pressure, add methanol, filter to obtain a filter cake, and wash it repeatedly with ethanol to obtain a cyclic thiophene oligomer compound containing six Br atoms.

[0040] Step 3: Add the cyclic thiophene oligomer compound containing six Br atoms, anhydrous potassium carbonate, tetrakis(triphenylphosphine)palladium, and 4-formylphenylboronic acid into a mixed solution of tetrahydrofuran and water, stir the mixture evenly and then reflux and stir the reaction in a nitrogen atmosphere. After the reaction is complete, cool to room temperature, remove the organic solvent under reduced pressure, then extract with chloroform, dry with anhydrous sodium sulfate, remove the solvent to obtain a crude product, and purify the crude product by silica gel column chromatography. The eluent is ethyl acetate and dichloromethane to obtain CDHt-6CHO.

[0041] Specifically, it includes: mixing the cyclic thiophene oligomer compound containing six aldehyde groups, the diamine monomer, an organic solvent, and a catalyst, then freezing, evacuating, that is, cyclically performing freezing-thawing-degassing, then heating to room temperature, and then reacting at 120-140°C for 5-7 days.

[0042] Among them, the structural formula of the cyclic thiophene oligomer compound (abbreviated as CDHt-6CHO) containing six aldehyde groups is shown as follows:

[0043]

[0044] The structural formula of the diamine monomer is as follows:

[0045] Among them, R is hydrogen and C1-C3 alkyl; preferably, R is hydrogen and methyl. For example,

[0046] Furthermore, the molar ratio of the cyclic thiophene oligomer compound containing six aldehyde groups to the diamine monomer is 1:(3.0-3.5). The organic solvent is selected from at least one of alcohol solvents and halogen-substituted phenyls. For example, the organic solvent is at least one of o-dichlorobenzene and n-butanol. Preferably, the volume ratio of o-dichlorobenzene to n-butanol is 1:(6-9).

[0047] The catalyst is acetic acid, preferably an aqueous acetic acid solution, and the concentration of the aqueous acetic acid solution is 6-9M. The volume ratio of the catalyst to the organic solvent is 1:(5-10).

[0048] The features and properties of the present invention will be further described in detail below in conjunction with examples.

[0049] Example 1

[0050] The embodiment of the present invention provides a cyclic thiophene oligomer-based three-dimensional covalent organic framework material (abbreviated as COF-NUST-68), and its structural formula is as follows:

[0051]

[0052] The embodiment of the present invention provides a preparation method of the above-mentioned cyclic thiophene oligomer-based three-dimensional covalent organic framework material, including:

[0053] S1. Synthesize a cyclic thiophene oligomer compound containing six aldehyde groups (CDHt-6CHO)

[0054] Synthesize with reference to the following synthetic route:

[0055]

[0056] , it can be understood that the above Br can be changed to halogens such as chlorine and iodine.

[0057] Specific steps are as follows:

[0058] Synthesis of Compound 1: Add 3-thiopheneboronic acid (1.27 g, 10 mmol), anhydrous potassium carbonate (4.14 g, 30 mmol) and tetrakis(triphenylphosphine)palladium (231 mg) into a 250 mL dry Schlenk flask. Evacuate the reaction system for 30 min and refill with Ar three times during this period. Under Ar protection, add 50 mL of anaerobic tetrahydrofuran, 15 mL of anaerobic water and 3-bromothiophene (1.55 g, 9.5 mmol). Then transfer the reaction system to an oil bath at 100 °C and stir and reflux the reaction in the dark for 24 h. After the reaction is completed, let it cool to room temperature naturally, remove the solvent under reduced pressure, add 100 mL of dichloromethane for extraction, wash the organic phase with water, dry it with anhydrous sodium sulfate, and remove the solvent under reduced pressure to obtain the crude product. Using petroleum ether as the eluent, separate and purify by silica gel column chromatography to obtain white flaky solid Compound 1 with a yield of 1.46 g and a yield of 93%.

[0059] Synthesis of Compound 2: Weigh Compound 1 (3.32 g, 20 mmol) and add it into a 250 mL dry Schlenk flask. Under Ar protection, add 80 mL of chloroform, N-bromosuccinimide (NBS, 7.15 g, 40.1 mmol) and 20 mL of glacial acetic acid (HOAc). Stir the reaction system in the dark at room temperature for 8 h. After the reaction is completed, pour the reaction solution into 100 mL of water, separate the organic phase, and wash the organic phase with water and saturated sodium bicarbonate solution in sequence. Add anhydrous sodium sulfate to dry the organic phase, remove the solvent under reduced pressure to obtain the crude product. Then use petroleum ether as the eluent, separate and purify by silica gel column chromatography to obtain white powdery Compound 2 with a yield of 5.7 g and a yield of 88%.

[0060] Synthesis of Compound 3: Under Ar protection, 15 mL of diethyl ether was added to a 100 mL dry Schlenk flask, and anhydrous and anaerobic diisopropylamine (3.3 mL, 23 mmol) was added dropwise thereto. The reaction flask was placed in an ice-water bath to keep the system temperature at about 0 °C. Under Ar protection, n-BuLi (9.2 mL, 22 mmol) was slowly added dropwise, and the reaction was stirred in the ice-water bath for 30 min to prepare a lithium diisopropylamide (LDA) solution required for the reaction. Compound 2 (3.24 g, 10 mmol) was weighed into a 250 mL dry Schlenk flask. Under Ar protection, 80 mL of diethyl ether was added, and after cooling to 0 °C, the prepared LDA was slowly added dropwise, and the reaction was stirred at 0 °C for 2 h. Then, the reaction system was transferred to a cryostat, cooled to -78 °C, and anhydrous and anaerobic trimethylchlorosilane solution (5.43 g, 50 mmol) was slowly added dropwise under Ar protection, and the reaction was stirred at -78 °C for 2 h. It was naturally warmed to room temperature and stirred overnight. After the reaction was completed, water was slowly added dropwise at 0 °C to quench the reaction. The solvent was removed under reduced pressure, and the concentrated solution was diluted with dichloromethane. The aqueous phase and the organic phase were separated using a separatory funnel. The aqueous phase was extracted with dichloromethane, and the organic phases were combined. The organic phase was washed successively with water and saturated sodium bicarbonate solution. The organic phase was dried over anhydrous sodium sulfate, filtered, and the solvent was removed under reduced pressure to obtain a crude product. Using petroleum ether as an eluent, the white solid product Compound 3 was obtained by silica gel column chromatography separation and purification. The yield was 3.93 g, and the yield rate was 84%.

[0061] Synthesis of Compound CDHt-6TMS: Compound 3 (5.62 g, 12 mmol) was weighed and added to a 250 mL dry Schelenk flask. The reaction system was evacuated for 30 min, and Ar was introduced three times during this period. Under Ar protection, 120 mL of anhydrous and anaerobic diethyl ether was added and stirred until dissolved. The reaction system was cooled to -78 °C, and n-BuLi (10.5 mL, 25.2 mmol) was added dropwise under Ar protection, and the reaction was stirred at -78 °C for 2 h. Subsequently, anhydrous copper chloride (4.84 g, 30 mmol) was added under Ar, and the reaction was continued to be stirred at -78 °C for 2 h. Then, the reaction system was naturally raised to room temperature and stirred overnight. After the reaction was completed, water was slowly added dropwise at low temperature to quench the reaction. The solvent was removed under reduced pressure, and dichloromethane was added for extraction. The organic phase was separated, and the aqueous phase was extracted with dichloromethane. The organic phases were combined and washed with water and saturated ammonium chloride solution. The organic phase was dried over anhydrous sodium sulfate, and the solvent was removed under reduced pressure to obtain a crude product. Using petroleum ether as an eluent, the pale yellow oily product CDHt-6TMS was obtained by silica gel column chromatography separation and purification. The yield was 1.26 g, and the yield rate was 34%.

[0062] Synthesis of compound CDHt: Weigh 2.0 g (2.16 mmol) of compound CDHt-6TMS into a 100 mL round-bottom flask, add 60 mL of chloroform to dissolve the sample. While stirring, add trifluoroacetic acid (3.0 mL). Stir the reaction mixture in the dark at room temperature for 30 min, and monitor the reaction progress using a TLC plate. After the reaction is completed, add 30 mL of water to quench the reaction. Separate the aqueous phase and the organic phase. Extract the aqueous phase with dichloromethane, combine the organic phases, and wash them successively with water and saturated sodium bicarbonate solution. Dry the organic phase with anhydrous sodium sulfate and remove the solvent under reduced pressure. Use dichloromethane and methanol for rapid precipitation to obtain a grayish-white powder product CDHt, with a yield of 0.99 g and a yield rate of 93%.

[0063] Synthesis of compound CDHt-6Br: Weigh 985 mg (2.0 mmol) of compound CDHt into a 100 mL round-bottom flask, add 50 mL of N,N-dimethylformamide. While stirring, add N-bromosuccinimide (3.58 g, 20 mmol) in batches. Then stir the reaction mixture in the dark at room temperature overnight until the reaction system becomes a milky yellow emulsion, and end the reaction. Remove the solvent under reduced pressure, add 100 mL of methanol, ultrasonically disperse for 5 min, filter under vacuum, collect the filter cake, and wash the solid product repeatedly with ethanol. The obtained solid is dried under vacuum at 60 °C to obtain an off-white powder product CDHt-6Br, with a yield of 1.65 g and a yield rate of 85%.

[0064] Synthesis of compound CDHt-6CHO: Add compound CDHt-6Br (1.93 g, 2.0 mmol), 4-formylphenylboronic acid (2.40 g, 16.0 mmol), tetrakis(triphenylphosphine)palladium (416 mg, 0.36 mmol), and anhydrous potassium carbonate (4.97 g, 36 mmol) into a 250 mL dry Schlenk flask. Evacuate the reaction system for 30 min, and replace the gas with Ar three times during this period. Under Ar protection, add 100 mL of anaerobic tetrahydrofuran and 20 mL of anaerobic water, and stir until dissolved. Transfer the reaction system to an oil bath and stir the reaction mixture in the dark at 100 °C for 48 h. After the reaction is completed, cool it to room temperature naturally. Remove the solvent under reduced pressure, add chloroform for extraction, separate the aqueous phase and the organic phase. Extract the aqueous phase with chloroform, combine the organic phases, and wash with water. Dry the organic phase with anhydrous sodium sulfate and remove the solvent under reduced pressure. Use ethyl acetate and dichloromethane as eluents for separation and purification by silica gel column chromatography to obtain a bright yellow solid product CDHt-6CHO, with a yield of 1.46 g and a yield rate of 66%.

[0065] S2. Synthesis of COF-NUST-68;

[0066] In an ampoule glass bottle, CDHt-6CHO (22.3 mg, 0.02 mmol) and p-phenylenediamine DAB (6.5 mg, 0.06 mmol) were added successively. Then o-dichlorobenzene (3.6 mL) and n-butanol (0.4 mL) were added, and ultrasonic treatment was carried out for 5 min to make the mixture uniform. 0.4 mL of 9M HOAc solution was added as a catalyst. The mixture in the ampoule bottle was quickly frozen using liquid nitrogen, and vacuum was pumped for 3 min. A total of three freeze-thaw degassing operations were carried out, and then the tube mouth was sealed with a flame gun. After naturally rising to room temperature, the ampoule bottle was placed in an oven at 120 °C for reaction for 6 days. The reaction bottle was taken out and naturally cooled to room temperature. The solid was separated by filtration. The powder sample was successively soaked and filtered with tetrahydrofuran, chloroform, methanol, and acetone, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain a pale yellow solid powder COF-NUST-68 with a yield of 24 mg and a yield rate of 90%.

[0067] Example 2

[0068] An embodiment of the present invention provides a cyclic thiophene oligomer-based three-dimensional covalent organic framework material (abbreviated as COF-NUST-69), and its structural formula is shown as follows:

[0069]

[0070] An embodiment of the present invention provides a preparation method of COF-NUST-69, including:

[0071] In an ampoule glass bottle, CDHt-6CHO (22.3 mg, 0.02 mmol) and 2,5-dimethyl-1,4-phenylenediamine DAB-Me (8.2 mg, 0.06 mmol) were added successively. Then o-dichlorobenzene (3.6 mL) and n-butanol (0.4 mL) were added, and ultrasonic treatment was carried out for 5 min to make the mixture uniform. 0.4 mL of 9M HOAc solution was added as a catalyst. The mixture in the ampoule bottle was quickly frozen using liquid nitrogen, and vacuum was pumped for 3 min. A total of three freeze-thaw degassing operations were carried out, and then the tube mouth was sealed with a flame gun. After naturally rising to room temperature, the ampoule bottle was placed in an oven at 120 °C for reaction for 6 days. The reaction bottle was taken out and naturally cooled to room temperature. The solid was separated by filtration. The powder sample was successively soaked and filtered with tetrahydrofuran, chloroform, methanol, and acetone, and then dried in a vacuum drying oven at 80 °C for 24 h to obtain a pale yellow solid powder COF-NUST-68 with a yield of 24 mg and a yield rate of 90%.

[0072] Characterization 1

[0073] The CDHt-6CHO prepared in Example 1 was characterized, and the results are shown in Figures 1 to 3 . According to Figure 1It can be seen that single crystals of CDHt-6CHO were obtained by the solvent evaporation method. The N,N-dimethylformamide solution of CDHt-6CHO was placed in a constant-temperature air blast oven at 50 °C to slowly evaporate N,N-dimethylformamide, and regular light yellow transparent polyhedral crystals were obtained. The photo of the crystals is as shown in Figure 2 shown. The crystal structure of the sample was determined by an X-ray single crystal diffractometer. The single crystal structure of CDHt-6CHO is as shown in Figure 3 shown. It can be seen that the cyclododeca-hexathiophene core of CDHt-CHO has a 3D polyhedral configuration, and six benzaldehydes extend outward along the six vertices of the polyhedron, showing a 3D 6-connected structure.

[0074] Characterization 2

[0075] The COF-NUST-68 prepared in Example 1 was characterized, and the results are shown in Figures 4 to 9 .

[0076] According to Figure 4 it can be seen that the PXRD curve of COF-NUST-68 shows diffraction peaks different from those of its organic monomers, indicating that a new type of three-dimensional covalent organic framework material based on cyclic thiophene oligomers was successfully synthesized by the examples of the present invention.

[0077] According to Figure 5 it can be seen that COF-NUST-68 has an acs-a topological structure, and the unit cell parameters are α = β = 90°, γ = 75°, and there are three obvious peaks at 3.25°, 3.79° and 7.65°, corresponding to the (011), (110) and (222) Bragg crystal planes respectively.

[0078] According to Figure 6 it can be seen that COF-NUST-68 shows a stretching vibration peak of C=N near 1621 cm -1 , indicating that the organic monomers reacted successfully to form C=N.

[0079] According to Figure 7 it can be seen that the specific surface area of COF-NUST-68 reaches 1229 m 2 g -1 , and the pore size is microporous, distributed around 1.88 nm.

[0080] According to Figure 8 and Figure 9 it can be seen that COF-NUST-68 has a rough spherical morphology.

[0081] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A cyclic thiophene oligomer-based three-dimensional covalent organic framework material, characterized in that: It is selected from any one of the compounds represented by the following structural formulas: Wherein, R represents H or C1-C3 alkyl.

2. The cyclic thiophene oligomer-based three-dimensional covalent organic framework material according to claim 1, characterized in that: R is H or methyl.

3. A method for preparing the cyclic thiophene oligomer-based three-dimensional covalent organic framework material according to claim 1, characterized in that: include: A cyclic thiophene oligomer compound containing six aldehyde groups and a diamine monomer are mixed to carry out a schiff base reaction.

4. The preparation method according to claim 3, characterized in that: include: A cyclic thiophene oligomer compound containing six aldehyde groups, a diamine monomer, an organic solvent and a catalyst are mixed and then cycled through freezing-thawing-degassing, then heated to room temperature, and then reacted at 120-140° C. for 5-7 days.

5. The preparation method according to claim 3, characterized in that: The structural formula of the cyclic thiophene oligomer compound containing six aldehyde groups is as follows: The structural formula of the diamine monomer is shown below: Wherein, R is hydrogen and C1-C3 alkyl; preferably R is hydrogen and methyl.

6. The preparation method according to any one of claims 3 to 5, characterized in that: The molar ratio of the cyclic thiophene oligomer compound containing six aldehyde groups to the diamine monomer is 1:(3.0-3.5).

7. The preparation method according to claim 4, characterized in that: The organic solvent is selected from at least one of an alcohol solvent and a halogen-substituted phenyl.

8. The preparation method according to claim 7, characterized in that: The organic solvent is at least one of o-dichlorobenzene and n-butanol. Preferably, the volume ratio of o-dichlorobenzene to n-butanol is 1:(6-9).

9. The preparation method according to claim 4, characterized in that: The catalyst is acetic acid, preferably an acetic acid aqueous solution, and the concentration of the acetic acid aqueous solution is 6-9M.

10. The preparation method according to claim 4, characterized in that: The volume ratio of the catalyst to the organic solvent is 1:(5-10).

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