Preparation of an imine-linked covalent organic framework and its application in photocatalytic degradation

Abstract

The present invention provides the preparation of a covalent organic framework connected by imine and its application in photocatalytic degradation. The preparation method of the present invention has short reaction time, high yield, low synthesis cost, and simple and easy synthesis operation, and can be obtained by simple solvent heat (200°C oil bath) and room temperature reaction. The prepared D1‑A1‑A2COF, D1‑A1‑A3COF and D2‑A1‑A2COF can be used as photocatalysts, and can have excellent effects on the degradation of microcystin at room temperature. The catalytic conditions for the visible light catalytic degradation of microcystin are simple, and the catalytic process does not need to add other substances except the addition of visible light and photocatalyst, and the photocatalytic process is more green, environmentally friendly and safe.

Description

Technical Field

[0001] The invention belongs to the field of organic materials, and in particular relates to the preparation of an imine-linked covalent organic framework and its application in photocatalytic degradation. Background Art

[0002] The current intensification of industrial and agricultural production and human activities has led to eutrophication of water bodies and the outbreak of cyanobacteria, causing global concerns about water environment quality and water safety. Excessive cyanobacteria in natural water bodies will not only lead to deterioration of water quality, but also release toxic substances into the water body. The main hazard is the release of various types of microcystins into the water body after the death or rupture of cyanobacterial cells. Among them, microcystins (MCs) are a type of intracellular toxins produced by the outbreak of cyanobacteria such as Microcystis aeruginosa, nitrogen-fixing Anabaena, Oscillatoria, Nostoc, and Pseudocylindrica, which have strong liver, kidney, immune and reproductive toxicity. Microcystin is a monocyclic heptapeptide substance, usually composed of five common amino acids (Adda, Glu, Mdha, Ala, MeAsp) and two variable amino acids (generally represented by X and Y). It has multiple isomers, one of which, MC-LR, is the most frequently detected hepatotoxic microcystin during algal blooms. It has the strongest acute toxicity and can specifically bind to protein phosphatases in cells, causing irreversible organ damage. It has been classified as a Class 2B carcinogen by the International Agency for Cancer. The World Health Organization (WHO) stipulates that the concentration of microcystin in drinking water shall not exceed 1.0μg / L.

[0003] MCs have good water solubility and heat resistance, and can exist stably for months or years in aquatic environments with different pH values, making them very difficult to handle. The photocatalytic technology developed in recent years has attracted much attention for its advantages of low cost, environmental friendliness, high efficiency, non-selective mineralization, and mild reaction conditions. Finding efficient and stable photocatalytic materials is the core of photocatalytic technology.

[0004] In recent years, polymer semiconductors such as carbon nitride, conjugated polymers and covalent organic frameworks have become research hotspots in the field of photocatalysis. Among them, covalent organic frameworks (COFs) are an emerging crystalline porous material formed by light organic molecular units connected by covalent bonds. They have the advantages of adjustable structure, ordered pores, large specific surface area, high chemical stability and thermal stability, and a wide range of light absorption. They show excellent application prospects in catalytic reactions, gas adsorption and separation, drug delivery, environmental pollutant adsorption and removal, supercapacitors, etc. However, as a relatively new class of materials, COFs face a series of challenges: the preparation method is cumbersome and time-consuming; the exciton binding energy E is high; the COFs have a large surface area, high chemical stability, and high thermal stability. b Relatively high. Especially E bThe high value means that COFs can only rely on changing the structure and adjusting the band gap to improve the separation and migration rate of photogenerated electron-hole pairs. The introduction of electron donor (Donor) and electron acceptor (Acceptor) structural units into the original COF skeleton can expand the conjugated system and improve the charge transfer efficiency. The introduction of DA units can lead to the rearrangement of the original HOMO energy level and LUMO energy level, forming new HOMO energy level and LUMO energy level, improving electron delocalization and reducing the E of COF. b value, broadening the light absorption range. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention provides a preparation method which is simple, time-saving, has a wide visible light absorption range, and has an b The invention discloses a method for preparing a covalent organic framework connected by imine, which has a small value and high photocatalytic activity, and an application of the imine-type covalent organic framework prepared thereby in the photocatalytic degradation of MC-LR.

[0006] The technical solution of the present invention is as follows: the imine-linked covalent organic framework material, the structural unit is as follows:

[0007]

[0008] Preferably, the imine-linked covalent organic framework material has a structural unit as shown below:

[0009]

[0010] The preparation method of the imine-linked covalent organic framework material D1-A1-A2 COF comprises the following steps:

[0011] 1) weighing trialdehyde phloroglucinol and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine into a reaction container, adding DMSO, and mixing them uniformly by ultrasonication at room temperature;

[0012] 2) introducing nitrogen to remove dissolved oxygen; heating the reaction;

[0013] 3) adding 1,3,5-tris(4-aminophenyl)benzene and heating for reaction;

[0014] 4) Cool to room temperature, wash with water, dry and grind to obtain D1-A1-A2 COF.

[0015] Preferably, the molar ratio of the trialdehyde phloroglucinol, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 1,3,5-tris(4-aminophenyl)benzene is 4:2:1.

[0016] Preferably, the reaction temperature of step 2) is 200° C. in an oil bath, and the reaction time is 3 hours.

[0017] Preferably, the reaction temperature of step 3) is 200° C. in an oil bath, and the reaction time is 9 hours.

[0018] The preparation method of the imine-linked covalent organic framework material D1-A1-A3 COF comprises the following steps:

[0019] 1) weighing trialdehyde phloroglucinol and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine into a reaction container, adding DMSO, and mixing them uniformly by ultrasonication at room temperature;

[0020] 2) introducing nitrogen to remove dissolved oxygen; heating the reaction;

[0021] 3) adding melamine and heating for reaction;

[0022] 4) Cool to room temperature, wash with water, dry and grind to obtain D1-A1-A3 COF.

[0023] Preferably, the molar ratio of the trialdehyde phloroglucinol, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and melamine is 4:2:1.

[0024] Preferably, the reaction temperature of step 2) is 200° C. in an oil bath, and the reaction time is 3 hours.

[0025] Preferably, the reaction temperature of step 3) is 200° C. in an oil bath, and the reaction time is 9 hours.

[0026] The preparation method of the imine-linked covalent organic framework material D2-A1-A2 COF comprises the following steps:

[0027] 1) Weigh 2,5-dimethoxybenzene-1,4-dicarbaldehyde and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine into a reaction container, add acetonitrile, and mix them evenly by ultrasonication at room temperature;

[0028] 2) Add glacial acetic acid, seal the tube, and stir to react at room temperature;

[0029] 3) adding 1,3,5-tris(4-aminophenyl)benzene to close the reaction;

[0030] 4) Wash with ethanol, collect the precipitate, dry it, and grind it to obtain D2-A1-A2 COF.

[0031] Preferably, the molar ratio of the 2,5-dimethoxybenzene-1,4-dicarbaldehyde, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 1,3,5-tris(4-aminophenyl)benzene is 4:2:1.

[0032] Preferably, the reaction time of step 2) is 3 hours.

[0033] Preferably, the reaction time of step 3) is 9 hours.

[0034] The imine-linked covalent organic framework material acts as a photocatalyst to catalytically degrade secondary pollutants produced by cyanobacteria or water blooms under visible light wavelengths.

[0035] Preferably, the secondary pollutant produced by the cyanobacteria or water bloom is microcystin MC-LR.

[0036] Preferably, the wavelength of the visible light is greater than 420 nm.

[0037] Beneficial effects of the present invention: The present invention provides an imine-linked covalent organic framework D1-A1-A2 COF, D1-A1-A3COF and D2-A1-A2 COF. The method for preparing D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF of the present invention has short reaction time, high yield, low synthesis cost, and simple and easy synthesis operation, and can be obtained by simple solvent heat (200°C oil bath). The prepared D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF photocatalysts have excellent effects on the degradation of microcystins at room temperature, and the catalytic conditions for the visible light catalytic degradation of microcystins are simple. In addition to the addition of visible light and photocatalysts, the catalytic process no longer requires the addition of other substances, and the photocatalytic process is more green, environmentally friendly and safe. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Flow chart of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF visible light photocatalysts prepared for the invention;

[0039] Figure 2 SEM images (a), (b), (c) of D1-A1-A2 COF prepared in the present invention; SEM images (d), (e), (f) of D1-A1-A3 COF and SEM images (g), (h), (i) of D2-A1-A2 COF;

[0040] Figure 3XRD patterns of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF prepared in the present invention;

[0041] Figure 4 The full spectrum of D1-A1-A2 COF prepared by the present invention (a); the full spectrum of D1-A1-A3 COF (b) and the full spectrum of D2-A1-A2COF (c); the high-resolution C spectrum of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF from top to bottom (d); the high-resolution N spectrum of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF from top to bottom (e); the high-resolution O spectrum of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF from top to bottom (f);

[0042] Figure 5 D1-A1-A2 COF (a), D1-A1-A3 COF (b) and D2-A1-A2 COF (c) prepared in the present invention 13 C-NMR spectrum;

[0043] Figure 6 FT-IR graphs of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF prepared in the present invention;

[0044] Figure 7 Specific surface area diagram (a) of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF prepared in the present invention; pore size diagram (b) of D1-A1-A2 COF, D1-A1-A3 COF and pore size diagram (c) of D2-A1-A2 COF;

[0045] Figure 8 UV-visible diffuse reflectance spectra of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF prepared in the present invention;

[0046] Fig. 9 Flat band potential diagrams of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF prepared in the present invention;

[0047] Fig.10 The energy band diagrams of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF relative to the standard hydrogen electrode;

[0048] Fig.11Impedance graphs of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF prepared in the present invention;

[0049] Fig.12 The instantaneous photocurrent diagram of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF prepared in the present invention;

[0050] Fig.13 Cyclic voltammograms of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF prepared in the present invention;

[0051] Fig.14 Degradation rate curves of MC-LR photocatalytic degradation by D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF prepared in the present invention, 3mg-500ppb MC-LR (a); 3mg-2ppm MC-LR (b);

[0052] Fig.15 The effects of coexisting ions (a), humic acid (b), initial solution pH (c) and adsorption at different pH values ​​(d) on the photocatalytic degradation of MC-LR by the D1-A1-A3 COF prepared in the present invention;

[0053] Fig.16 The repeatability of the photocatalytic degradation of MC-LR by the D1-A1-A3 COF prepared in the present invention;

[0054] Fig.17 The photocatalytic degradation rate of MC-LR by D1-A1-A3 COF prepared by the present invention for adding different active capture agents;

[0055] Fig.18 The mechanism diagram of photocatalytic degradation of MC-LR by D1-A1-A3 COF prepared by the present invention; DETAILED DESCRIPTION

[0056] The following embodiments can enable those skilled in the art to more fully understand the present invention, but the present invention is not limited to the scope of the embodiments.

[0057] The methods described in the embodiments of the present invention are conventional methods unless otherwise specified. The materials and reagents used can be obtained from biological or chemical reagent companies unless otherwise specified. The water used in this experiment is ultrapure water with a resistivity of 18.25 MΩ·cm.

[0058] Example 1 Preparation of D1-A1-A2 COF

[0059] In this embodiment, trialdehyde phloroglucinol, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 1,3,5-tris(4-aminophenyl)benzene are used to prepare D1-A1-A2 type covalent organic framework D1-A1-A2 COF, and the reaction formula is as follows: Figure 1 As shown, the preparation method comprises the following steps:

[0060] (1) Weigh 0.084 g of trialdehyde phloroglucinol (0.4 mmol) and 0.0709 g of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (0.2 mmol) into a 50 mL three-necked flask, add 25 mL of DMSO, and mix them evenly by ultrasonication at room temperature;

[0061] (2) nitrogen was introduced for 10 min to remove dissolved oxygen;

[0062] (3) Place the three-necked flask with condensed water in a 200°C oil bath and react for 3 hours;

[0063] (4) 0.0436 g of 1,3,5-tris(4-aminophenyl)benzene (0.1 mmol) was added from one side of the three-necked flask and reacted at 200° C. for 9 h;

[0064] (5) After cooling to room temperature, the product was washed several times with UP water, dried in a 40°C forced air drying oven, and ground to obtain a reddish brown powder, namely D1-A1-A2 COF.

[0065] Example 2 Preparation of D1-A1-A3 COF

[0066] In this embodiment, trialdehyde phloroglucinol, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and melamine are used to prepare D1-A1-A3 type covalent organic framework D1-A1-A3 COF, and the reaction formula is as follows: Figure 1 As shown, the preparation method comprises the following steps:

[0067] (1) Weigh 0.084 g of trialdehyde phloroglucinol (0.4 mmol) and 0.0709 g of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (0.2 mmol) into a 50 mL three-necked flask, add 25 mL of DMSO, and mix them evenly by ultrasonication at room temperature;

[0068] (2) nitrogen was introduced for 10 min to remove dissolved oxygen;

[0069] (3) Place the three-necked flask with condensed water in a 200°C oil bath and react for 3 hours;

[0070] (4) 0.0126 g of melamine (0.1 mmol) was added from one side of the three-necked flask and reacted at 200 °C for 9 h;

[0071] (5) After cooling to room temperature, the product was washed several times with UP water, dried in a 40°C forced air drying oven, and ground to obtain a reddish brown powder, namely D1-A1-A3 COF.

[0072] Example 3 Preparation of D2-A1-A2 COF

[0073] In this embodiment, 2,5-dimethoxybenzene-1,4-dicarbaldehyde, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 1,3,5-tris(4-aminophenyl)benzene are used to prepare D2-A1-A2 type covalent organic framework D2-A1-A2 COF. The reaction formula is as follows: Figure 1 As shown, the preparation method comprises the following steps:

[0074] (1) Weigh 0.084 g of 2,5-dimethoxybenzene-1,4-dicarbaldehyde (0.4 mmol) and 0.0709 g of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (0.2 mmol) into a 100 mL beaker, add 30 mL of acetonitrile, and mix by ultrasonication at room temperature for 30 s;

[0075] (2) Inject 1.6 mL of glacial acetic acid (17.5 M), seal the beaker with a parafilm, and stir vigorously at room temperature for 3 h;

[0076] (3) Add 0.0436 g of 1,3,5-tris(4-aminophenyl)benzene (0.1 mmol), continue the blocking reaction for 9 h, wash with ethanol several times, collect the precipitate, dry it in a forced air drying oven at 40 °C, and grind it to obtain the yellow product D2-A1-A2 COF;

[0077] Comparative Example 1

[0078] Preparation of D1-A1 COF: 0.13 mmol of trialdehyde pyrogallol + 0.07 mmol of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine were mixed in 5 mL of DMSO, and then the mixture was stirred in an air atmosphere at 120°C for 42 h. After cooling, the precipitant was thoroughly washed with methanol and then dried in a vacuum oven at 70°C for 5 h.

[0079] Comparative Example 2

[0080] Preparation of D1-A1 COF: 0.084g of trialdehyde pyrogallol + 0.0709g of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine were mixed in 25mL DMSO and reacted at 200°C for 3h under nitrogen protection to form a conjugated organic framework. Then 0.0436g of Melem (cas number: 1502-47-2) was added and the reaction was continued at 200°C for 36h. The product was washed with ultrapure water, dried with air at 40°C, and ground.

[0081] In summary, the preparation method of Examples 1-3 only takes 12 hours to complete the reaction, which is shorter than Comparative Example 1 (42 hours) and Comparative Example 2 (39 hours), more energy-saving and environmentally friendly, and the experiment does not need to be conducted overnight. From a safety perspective, the preparation method we designed is safer.

[0082] Test Example 1 Characterization of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF

[0083] The morphology of the samples was analyzed by scanning electron microscopy (SEM, ZEISS Sigma 300, Germany). The crystal structure of the samples was tested by X-ray powder diffractometer (XRD, Bruker D 8, Germany) with Cu Ka radiation source, tube voltage of 40KV, current of 30mA, and scanning range of 5-90°; the chemical composition of the samples was analyzed by X-ray photoelectron spectroscopy (ThermoScientific K-Alpha+, USA) with Al Ka ​​radiation as the excitation source; the functional groups of the samples were determined by FT-IR spectrometer from Thermo Fisher Scientific (Thermo Scientific Nicolet iS20, USA); the light absorption properties of the samples were determined by ultraviolet visible spectrophotometer (Shimaduz UV-12600, Japan), and BaSO 4 As the test background, the scanning range is 200-800nm; the N 2 The adsorption-desorption isotherms were measured by BSD-specific surface area and pore size analyzer (BSD-660M A3M), and the specific surface area and pore size distribution of the samples were calculated by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively; the structural information of the samples was analyzed by wide-cavity solid-state nuclear magnetic resonance (BrukerAvance Neo 400WB, Germany).

[0084] Test Example 2 Electrochemical Characterization of D1-A1-A2-COF, D1-A1-A3 COF and D2-A1-A2 COF

[0085] 1. Flat belt potential test

[0086] The flat band potential of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF was tested using a standard three-electrode system. The three electrodes included indium tin oxide (ITO) conductive glass coated with the sample as the working electrode, saturated calomel electrode (SCE) as the reference electrode and platinum wire electrode as the counter electrode. Conductive glass with a size of 1 cm × 2 cm was ultrasonically cleaned with distilled water, acetone, ethanol and distilled water for 30 min in turn, and dried for later use. 20 mg of the sample was dissolved in a mixed solution of 2.5 mL of ethanol and 2.5 mL of ethylene glycol. After magnetic stirring for 12 h, a viscous dispersion containing the catalyst was obtained. 20 μL of the dispersion was applied to a working area of ​​1 cm 2 The working electrode was obtained by drying the sample with an infrared lamp after 30 minutes of standing. The “open circuit potential-time” technique was selected to study the stable voltage of the sample. The “impedance-potential” technique was selected and the scanning range was set to -1.2-0.4V. 2 SO 4 The Mott-Schottky curve of the sample is tested in solution at a frequency of 1500-3000Hz (1500, 2000, 2500, 3000Hz). The flat band potential of the sample can be calculated based on the Mott-Schottky curve, and the type of semiconductor can be determined by the positive and negative values ​​of the tangent slope.

[0087] 2. Photoelectrochemical properties test

[0088] The photocatalyst response to light and the carrier separation efficiency were detected by photocurrent test. The transient photocurrent response of the working electrode was measured by CHI660E electrochemical workstation (Chenhua Instrument, Shanghai, China), abbreviated as IT, and the stable voltage of the open circuit potential was set as the initial voltage.

[0089] Electrochemical impedance spectroscopy was used to detect the charge transfer rate. The electrolyte solution contained 5 mM K 3 [Fe(CN) 6 ] / K 4 [Fe(CN) 6 ](1:1) solution. The electrochemical impedance spectroscopy model used in this work is CHI660E.

[0090] Application Example 1

[0091] This application example illustrates the application of D1-A1-A2 COF in visible light catalytic degradation system

[0092] The D1-A1-A2 COF prepared in Example 1 was subjected to a photocatalytic degradation activity test of MC-LR. 3 mg of D1-A1-A2 COF was added to a quartz beaker containing 50 mL of MC-LR (2 ppm); after ultrasonic mixing, it was transferred to a photocatalytic reaction device and adsorbed in the dark for 60 min to reach adsorption-desorption equilibrium. Subsequently, a 300 W xenon lamp (CEL-HXF300-T3, Beijing Zhongjiao Jinyuan Technology Co., Ltd.) was used for illumination with a constant current of 14 A. A cutoff filter with a λ ≥ 420 nm was used to transmit visible light. The reaction solution was irradiated for 2 h for a photocatalytic reaction.

[0093] The concentration change of MC-LR during the degradation process was analyzed using ultra-high performance liquid chromatography / triple quadrupole mass spectrometry (Agilent, USA, 1290infinty22 / 6470B). The performance of D1-A1-A2 COF under visible light was obtained as follows: Fig.14 As shown in (b), the degradation rate of MC-LR by 3 mg of D1-A1-A2 COF after 2 h of illumination reached 97%, which has very excellent photocatalytic performance.

[0094] Application Example 2

[0095] This application example illustrates the application of D1-A1-A3 COF in visible light catalytic degradation system

[0096] The D1-A1-A3 COF prepared in Example 2 was subjected to a photocatalytic degradation activity test of MC-LR. 3 mg of D1-A1-A3 COF was added to a quartz beaker containing 50 mL of MC-LR (500 ppb / 2 ppm); after ultrasonic mixing, the mixture was transferred to a photocatalytic reaction device and adsorbed in the dark for 60 min to reach adsorption-desorption equilibrium. Subsequently, a 300 W xenon lamp (CEL-HXF300-T3, Beijing Zhongjiao Jinyuan Technology Co., Ltd.) was used for illumination with a constant current of 14 A. A cutoff filter with a λ ≥ 420 nm was used to transmit visible light. The reaction solution was irradiated for 2 h for a photocatalytic reaction.

[0097] The concentration change of MC-LR during the degradation process was analyzed using ultra-high performance liquid chromatography / triple quadrupole mass spectrometry (Agilent, USA, 1290infinty22 / 6470B). The performance of D1-A1-A3 COF under visible light was obtained as follows: Fig.14 As shown in (a), the degradation rate of MC-LR (500 ppb) by 3 mg of D1-A1-A3 COF after 40 min of illumination reached 86%. Considering its high efficiency in degradation of 500 ppb of MC-LR and for comparison with D1-A1-A2 COF, we used 3 mg of D1-A1-A3 COF to degrade 2 ppm of MC-LR, as shown in Fig.14 As shown in (b), the degradation rate of MC-LR (2ppm) reached 95% after 80 minutes of illumination, which has very excellent photocatalytic performance.

[0098] Application Example 3

[0099] This application example illustrates the application of D2-A1-A2 COF in visible light catalytic degradation system

[0100] The D2-A1-A2 COF prepared in Example 3 was subjected to a photocatalytic degradation activity test of MC-LR. 3 mg of D1-A1-A2 COF was added to a quartz beaker containing 50 mL of MC-LR (500 ppb); after ultrasonic mixing, it was transferred to a photocatalytic reaction device and darkly adsorbed for 60 min to reach adsorption-desorption equilibrium. Subsequently, a 300 W xenon lamp (CEL-HXF300-T3, Beijing Zhongjiao Jinyuan Technology Co., Ltd.) was used for illumination with a constant current of 14 A. A cutoff filter with a λ ≥ 420 nm was used to transmit visible light. The reaction solution was irradiated for 2 h for a photocatalytic reaction.

[0101] The concentration change of MC-LR during the degradation process was analyzed using an ultra-high performance liquid chromatography / triple quadrupole mass spectrometer (Agilent, USA, 1290infinty22 / 6470B). The performance of D2-A1-A2 COF under visible light was obtained as follows: Fig.14 As shown in (a), the degradation rate of MC-LR by 3 mg of D2-A1-A2 COF after 2 h of illumination reached 80%, which has very excellent photocatalytic performance.

[0102] Application Example 4

[0103] By comparing application examples 1, 2 and 3, it is shown that the performance of D1-A1-A3 COF is better than that of D1-A1-A2 COF and better than that of D2-A1-A2 COF, and the ranking is: D1-A1-A3 COF>D1-A1-A2 COF>D2-A1-A2 COF. In addition to characterization, subsequent experiments will be conducted with D1-A1-A3 COF.

[0104] Experimental Results and Discussion

[0105] 1. Scanning electron microscope (SEM) analysis

[0106] Figure 2(a), (b), (c); (d), (e), (f); (g), (h), (i) are the morphologies of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF observed by scanning electron microscopy. For D1-A1-A2 COF and D1-A1-A3COF, their morphologies are fibrous structures similar to those of sea corals; for D2-A1-A2 COF, its morphology is similar to that of the succulent "jade pendant".

[0107] 2. X-ray powder diffraction (XRD) analysis

[0108] Figure 3 The XRD spectra of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF samples are shown, confirming the crystallinity of the covalent organic framework material.

[0109] 3. X-ray Photoelectron Spectroscopy (XPS) Analysis

[0110] XPS was used to analyze the chemical composition and surface chemical state of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF. Figure 4 (a), (b) and (c) are the full spectra of the three samples prepared, respectively, which confirm that the elements on the surface of the three photocatalysts include C, N and O. Figure 4 (d), (e) and (f) are the detailed spectrum analysis results of C, N and O elements in samples D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF from top to bottom and from left to right. It can be seen from the analysis that the carbon atoms in the three samples are in the form of SP 2 The peaks with binding energies of 398.6 eV, 398.7 eV, and 398.7 eV are attributed to the SP of C=NC in the triazine unit. 2 Nitrogen, the peak with a binding energy of 400.1 eV is attributed to the SP of CN 3 Hybridized nitrogen atoms; the O element in D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF samples mainly exists in the form of CO and C=O. The peaks at binding energies of 536.0eV, 536.0eV and 535.7eV in the three samples can be attributed to the binding energy of hydroxyl groups of water adsorbed on the sample surface.

[0111] 4. Solid-state NMR analysis

[0112] like Figure 5As shown in (a) and (b), the solid-state NMR carbon spectra show that the clear peaks of D1-A1-A2 COF and D1-A1-A3 COF at 182 / 184.8, 167 / 169.2 / 165.3, and 145 / 147.3 ppm can be attributed to the carbonyl carbon, triazine carbon, and enamine carbon atoms, respectively; Figure 5 As shown in (c), the D2-A1-A2 COF sample 13 The signal at 170.1 ppm in the C-CPMAS-NMR spectrum was assigned to the triazine carbon, and the signal at 160 ppm was assigned to the imine carbon atom. The framework structure of the imine-linked covalent organic framework of the present invention was confirmed.

[0113] 5. Fourier transform infrared spectroscopy (FT-IR) analysis

[0114] Figure 6 The FT-IR spectra of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF are shown at 810 / 810 / 814 cm -1 、1620-1625cm -1 The absorption peaks that appear correspond to the stretching vibration of the triazine unit in the TAPT monomer and the Melamine monomer and the stretching vibration of C=O, respectively, proving that the -OH group in the TFP monomer exists in the form of -C=O; 1503-1510cm -1 and 1174-1179cm -1 The characteristic stretching bands at 1676 cm-1 can be assigned to -C=N and -CN respectively; D2-A1-A2COF -1 The appearance of a characteristic C=N stretching band at , proved that DHTP, TAPT and TAPB monomers were successfully introduced into the covalent organic framework.

[0115] 6. Specific surface area analysis

[0116] The specific surface area and pore size of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF were analyzed by nitrogen adsorption-desorption technology. Figure 7 The specific surface area diagram (a) and pore size diagrams (b) and (c) of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF prepared in the present invention are shown in FIG. Figure 7 As shown in (a), the specific surface areas of D1-A1-A2 COF, D1-A1-A3 COF, and D2-A1-A2 COF are 921.9, 1001.8, and 404.3 m 2 / g; such as Figure 7As shown in (b) and 7(c), the pore sizes of D1-A1-A2 COF, D1-A1-A3 COF, and D2-A1-A2 COF are 15.0 nm, 11.4 nm, and 11.0 nm, respectively; and the pore volumes are 1.3 cm 3 / g, 1.2cm 3 / g and 3.5cm 3 / g.

[0117] 7. Solid UV diffuse reflectance (DRS) analysis

[0118] like Figure 8 As shown in (a), the UV-visible diffuse reflectance spectra confirm that D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2COF absorb light in the range of 200-800nm, with the maximum absorption concentrated in 250-500nm, and the light response of D1-A1-A3 COF is stronger, which indicates that D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF can capture visible light and generate photogenerated electrons, which is beneficial to the photocatalytic reaction. Figure 8 As shown in (b), the band gaps of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF were calculated by the Kubelka-Munk formula, and their values ​​were 1.58 eV, 1.37 eV and 2.14 eV, respectively. Therefore, the stronger light absorption and lower band gap of D1-A1-A3 COF may be the reasons for its higher photocatalytic efficiency.

[0119] 8. Mott-Schottky analysis

[0120] Through the electrochemical workstation, Mott-Schottky curve tests at different frequencies can be performed to obtain more information about the energy band structure of the catalyst. Fig. 9 As shown in (a), (b) and (c), the slopes of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2COF are all positive, which is consistent with the characteristics of a classical n-type semiconductor. The CBMs of D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF are calculated to be -0.52 V, -0.40 V and -0.57 V respectively. Combined with the band gap values ​​of the three samples calculated previously, the valence bands of the three photocatalysts can be calculated to be 1.06 V, 0.97 V and 1.57 V respectively. The corresponding band structure diagrams of D1-A1-A2COF, D1-A1-A3 COF and D2-A1-A2 COF samples are plotted as follows: Fig.10 shown.

[0121] 9. Chemical impedance spectroscopy (EIS) analysis

[0122] In order to gain a deeper understanding of the charge transfer capabilities of D1-A1-A2 COF, D1-A1-A3 COF, and D2-A1-A2 COF, EIS Nyquist curves were tested. Fig.11 As shown, the arc radius of D1-A1-A3 COF is smaller than that of D1-A1-A2COF and smaller than that of D2-A1-A2 COF, indicating that the electrons on the interface of D1-A1-A3 COF have lower resistance, resulting in faster charge transfer and thus exhibiting better photocatalytic activity.

[0123] 10. Cyclic voltammetry (CV) analysis

[0124] The electronic efficiency was investigated by cyclic voltammetry, such as Fig.12 As shown, D1-A1-A3 COF exhibits the highest oxidation peak current and the lowest reduction peak current compared to D1-A1-A2COF and D2-A1-A2 COF, indicating that it has the highest redox ability.

[0125] 10. Transient photocurrent response analysis (IT)

[0126] like Fig.13 As shown in the figure, the photocurrent intensity of D1-A1-A3 COF is significantly stronger than that of D1-A1-A2 COF and D2-A1-A2COF, indicating that there is a rapid separation of electrons and holes in the D1-A1-A3 COF material. As the light-on time increases, the photocurrent response of D1-A1-A2COF, D1-A1-A3 COF and D2-A1-A2 COF materials will gradually decrease, which may be because the electrons are partially lost in the closed circuit when the current is excited.

[0127] 11. Performance comparison experiment

[0128] The photocatalytic performance of the synthesized D1-A1-A2 COF, D1-A1-A3 COF, and D2-A1-A2 COF was evaluated by the degradation efficiency of MC-LR under visible light. Before light irradiation, in a dark environment, the adsorption-desorption equilibrium between microcystin and the photocatalyst was reached in 60 min. Fig.14As shown in (a), under visible light irradiation, 3 mg of D1-A1-A3 COF and D2-A1-A2 COF were irradiated with visible light for 2 hours, and the degradation rates of microcystin (500 ppb) were 89% and 80%, respectively. However, it is interesting that D1-A1-A3 COF had already degraded 86% of MC-LR in 40 minutes, while D2-A1-A2 COF had only degraded 70% of MC-LR in 40 minutes; and when exploring the use of 3 mg of D1-A1-A2 COF for the degradation of 500 ppb MC-LR, it was found that in the adsorption-desorption equilibrium stage, 3 mg of D1-A1-A2 COF had already adsorbed 61% of 500 ppb MC-LR in just 40 minutes, and could not reach the adsorption-desorption equilibrium ( Fig.14 (a)). Therefore, the concentration of MC-LR was increased from 500ppb to 2ppm, and the dark adsorption behavior of 3mg D1-A1-A2 COF on 2ppm MC-LR was studied. It was found that 3mg D1-A1-A2 COF could reach dark adsorption equilibrium for 2ppm MC-LR. Therefore, 3mg D1-A1-A2 COF and D1-A1-A3 COF were used to degrade 2ppm MC-LR under visible light to compare the performance difference between the two.

[0129] like Fig.14 As shown in (b), for 3 mg of D1-A1-A2 COF, the removal rate of 2 ppm MC-LR reached 91% after 100 min of illumination, and 97% after 120 min of illumination; for 3 mg of D1-A1-A3 COF, 95% of the MC-LR was degraded after 80 min of illumination, and 98% of the MC-LR was removed after 2 h of illumination. Therefore, the photocatalytic performance of D1-A1-A3 COF is much better than that of D1-A1-A2 COF. Based on this, 3 mg of D1-A1-A3 COF will be selected to degrade 500 ppb MC-LR in subsequent experiments. In addition, we also compared the photocatalytic activity of the synthesized D1-A1-A2 COF, D1-A1-A3COF and D2-A1-A2 COF materials with the reported work. As shown in Table 3, the photocatalyst we synthesized showed very superior performance, so this will be a more potential method for MC-LR degradation.

[0130] Table 3 Performance comparison of the photocatalysts synthesized in this work and those reported in the literature for the degradation of MC-LR

[0131]

[0132] 12. Effects of coexisting ions, humic acid and initial solution pH on the degradation of MC-LR by D1-A1-A3 COF

[0133] (1) Coexisting ions

[0134] To evaluate the effects of inorganic ions present in natural water environments, SO 4 2- 、NO 3 - , CO 3 2- , HCO 3 - , Cl - , Ca 2+ and Mg 2+ Therefore, by adding the same amount of 10 mM sodium sulfate, sodium nitrate, sodium carbonate, sodium bicarbonate, hydrochloric acid, magnesium chloride and calcium chloride into the photocatalytic reaction solution, their effects on the photocatalytic degradation of MC-LR by D1-A1-A3COF were studied. Fig.15 As shown in (a), compared with the control group, the addition of nitrate and sulfate had little effect on the degradation performance of D1-A1-A3COF for MC-LR. + 、NO 3 - and SO 4 2- The effect of is negligible; for bicarbonate and carbonate, the degradation efficiency of MC-LR is severely inhibited. Bicarbonate is a powerful hydroxyl radical scavenger, and the reaction occurs as follows:

[0135] HCO 3 - + ·OH→CO 3 2- +H 2 O

[0136] ·OH participates in the photocatalytic degradation of MC-LR by D1-A1-A3 COF. As the reaction proceeds, ·OH is captured, which affects the degradation of MC-LR. Carbonates have a scavenging effect on photogenerated holes, resulting in a reduction in active sites and a decrease in activity. Interestingly, Cl - The presence of Cl significantly promoted the degradation of MC-LR. - The presence of promotes the production of non-free radicals such as hypochlorous acid, which greatly enhances the selective removal and mineralization of MC-LR; calcium and magnesium ions may be due to adsorption on the surface of catalyst D1-A1-A3 COF to form hydroxide complex precipitation, occupying the active sites, resulting in a decrease in photocatalytic efficiency.

[0137] (2) Impact of humic acid

[0138] There are a large number of humic substances commonly found in natural water bodies. They react with free radicals and interfere with the removal of pollutants. Therefore, it is necessary to evaluate the effect of humic substances on the efficiency of MC-LR degradation by D1-A1-A3 COF. Humic substances are macromolecular organic matter formed by complex biochemical processes of bacteria, algae and other microorganisms in the environment of plants and animals, and are commonly found in natural ecosystems. Natural humus is mainly divided into three main components: humic acid (HA), fulvic acid (FA) and humins. In natural water and sediment environments, HA accounts for a large proportion of dissolved organic matter (DOM), and has a complex structure and contains a variety of active functional groups. In the coexistence reaction system, HA inevitably interacts with coexisting substances, thereby making the reaction system complicated. Due to its unique chemical activity, it has attracted much attention in the field of environmental governance. Therefore, we selected HA as a typical organic matter in the system and explored the effect of different concentrations of HA on the degradation of MC-LR by D1-A1-A3 COF. Fig.15 As shown in (b), when the HA concentration increased from 1 mg / L to 40 mg / L, the effect of HA on the degradation of MC-LR was obviously negligible, which indicates that the constructed D1-A1-A3 COF material system for degrading MC-LR has good anti-interference ability.

[0139] (3) Effect of initial solution pH on the degradation of MC-LR by D1-A1-A3 COF

[0140] The initial solution pH plays a crucial role in the photocatalytic degradation of MC-LR. The effect of pH is indispensable for further evaluating the degradation performance of the catalyst for MC-LR removal in practical applications. Therefore, we studied the effect of different pH values ​​from 3.08 to 8.77 on the photocatalytic degradation of MC-LR by D1-A1-A3 COF during the degradation process. Fig.15 As shown in (c), D1-A1-A3 COF showed the best effect of removing MC-LR under the strong acidic condition of pH = 3.08; under the weak acidic and neutral conditions of pH = 5.03 and 7.05, the degradation rate of MC-LR by D1-A1-A3 COF under visible light irradiation for 2h can still reach 91%; and when the pH value reaches 9, the efficiency of D1-A1-A3 COF in removing MC-LR is only 10%. We can explain that: Fig.15(d) In the solution system with different pH conditions, D1-A1-A3COF showed a huge difference in adsorption of MC-LR. In the acidic environment of pH = 3.08, the adsorption amount of D1-A1-A3 COF on MC-LR was as high as 91%. As the pH of the solution continued to increase, the adsorption amount gradually decreased. This adsorption behavior with pH changes can be explained by the ionization process of the adsorbent and the adsorbate. The net charge of the adsorbent and the adsorbate will change with the change of solution pH. In an alkaline environment, MC-LR is deprotonated, and the higher the pH, the more negative charge it carries. The measured Zeta potential of D1-A1-A3COF is negative. The electrostatic repulsion leads to low adsorption capacity in the alkaline environment; in a low pH environment, the electrostatic repulsion is reduced, and intermolecular hydrogen bonds are formed, which promotes adsorption.

[0141] 13. Cyclic stability study of MC-LR degradation by D1-A1-A3 COF

[0142] The recycling test of MC-LR degradation by D1-A1-A3 COF was used to evaluate the recyclability of the material. Fig.16 As shown, the degradation efficiency of MC-LR can still be maintained at 90% after 3 consecutive cycles, indicating that the D1-A1-A3 COF system has good reusability.

[0143] 14. Active species capture experiment

[0144] The main active species involved in the degradation of MC-LR by D1-A1-A3 COF were studied by free radical quenching experiments. p-Benzoquinone (p-BQ), EDTA-2Na, potassium dichromate, furfuryl alcohol (FFA), and isopropyl alcohol (IPA) were used to capture ·O 2 - 、h + 、e - , 1 O 2 , and OH. Fig.17 As shown, p-BQ and K 2 Cr 2 O 7 The inhibitory effect of ·O was the most significant, followed by FFA. On the other hand, the inhibitory effects of IPA and EDTA-2Na were negligible. 2 - 、e - and 1 O 2 is the main reactive oxygen species in MC-LR degradation, followed by h + and OH.

[0145] 15. Analysis of the mechanism of MC-LR degradation by D1-A1-A3 COF

[0146] Based on the above free radical capture experimental results, the mechanism of photocatalytic degradation of MC-LR by D1-A1-A3 COF was proposed. Fig.18 As shown in the figure, under visible light irradiation, the electrons on the HUMO level of the prepared D1-A1-A3 COF material transition to the LUMO level to form electron-hole pairs. The potential on the LUMO level (-0.40 eV) is more negative than the generation potential of superoxide anions (-0.33 eV), so it can react with the O on the surface of D1-A1-A3 COF. 2 The reaction generates superoxide radicals O 2 - Finally, when the electron returns from any energy level of the first excited state to the vibrational energy level of the first excited state, due to S1 * The lowest vibration energy level is related to TI * The highest vibrational energy levels of the photocatalysts overlap, and a crossover occurs between systems. The direction of the electron spin changes, thereby achieving a crossover from the singlet excited state to the triplet excited state, thereby interacting with the O 2 The molecules react to generate singlet oxygen. In the subsequent photocatalytic degradation process, superoxide radicals and singlet oxygen act together on MC-LR, undergoing corresponding redox reactions with it, destroying its structure and mineralizing it into CO 2 and H 2 O and other inorganic small molecules to reduce their toxicity.

[0147] in conclusion

[0148] The present invention successfully applies the synthesized D1-A1-A2 COF, D1-A1-A3 COF and D2-A1-A2 COF to the photocatalytic degradation of MC-LR. By comparing the photocatalytic performance, the D1-A1-A3 COF with the best photocatalytic performance is found. In subsequent experiments, the effects of factors such as pH, humic acid, and coexisting ions on the photocatalytic degradation of MC-LR system are explored. The experimental results show that the pH of the system seriously affects the adsorption of MC-LR by the photocatalyst D1-A1-A3 COF, thereby further affecting the photocatalytic performance; organic humic acid has little effect on the system, indicating that the constructed D1-A1-A3 COF photocatalytic degradation of MC-LR system has a strong ability to resist the interference of humic acid; in addition, the effects of coexisting ions in the solution on the system are also explored. The experimental results show that compared with the control group, the addition of nitrate and sulfate has almost no effect on the performance of D1-A1-A3 COF in degrading MC-LR, so Na + 、NO 3 - and SO 4 2-The effect of is negligible; for bicarbonate and carbonate, the degradation efficiency of MC-LR is severely inhibited. Bicarbonate is a powerful scavenger of hydroxyl radicals, which reacts as follows: HCO 3 -+·OH→CO 3 2- +H 2 O, while ·OH participates in the photocatalytic degradation of MC-LR by D1-A1-A3 COF. As the reaction proceeds, ·OH is captured, thus affecting the degradation of MC-LR. Carbonates have a scavenging effect on photogenerated holes, resulting in a reduction in active sites and a decrease in activity. Interestingly, the presence of Cl- in the solution significantly promotes the degradation of MC-LR, because the presence of Cl- promotes the production of non-free radicals such as hypochlorous acid, which greatly enhances the selective removal and mineralization of MC-LR. Calcium and magnesium ions may be due to the adsorption on the surface of the catalyst D1-A1-A3 COF to form hydroxide complex precipitation, occupying the active sites, resulting in a decrease in photocatalytic efficiency. Finally, through the active species capture experiment, the mechanism of D1-A1-A3 COF photocatalytic degradation of MC-LR was proposed.

Claims

1. A covalent organic framework material is used as a photocatalyst to catalyze the degradation of secondary pollutants produced by cyanobacteria under visible light wavelength; the secondary pollutant produced by cyanobacteria is microcystin MC-LR; the wavelength of the visible light is greater than 420nm; the structural unit of the covalent organic framework material is as follows, 2. The use according to claim 1, characterized in that: The preparation method of the covalent organic framework material D1-A1-A3 COF comprises the following steps: 1) weighing trialdehyde phloroglucinol and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine into a reaction container, adding DMSO, and mixing them uniformly by ultrasonication at room temperature; 2) introducing nitrogen to remove dissolved oxygen; heating the reaction; 3) adding melamine and heating for reaction; 4) Cool to room temperature, wash with water, dry and grind to obtain D1-A1-A3 COF.

3. The use according to claim 2, characterized in that: The molar ratio of the trialdehyde phloroglucinol, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and melamine is 4:2:1; the reaction temperature of step 2) is 200°C and the reaction time is 3 hours; the reaction temperature of step 3) is 200°C and the reaction time is 9 hours.

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