Preparation method of a covalent organic framework and its application in photocatalytic hydrogen evolution from seawater

Abstract

The invention discloses a method for preparing a covalent organic framework, comprising the steps of: adding 2,4,6-trihydroxy-1,3,5-benzaldehyde and diamino units to a mixed solution of a mixed organic solvent and an acetic acid aqueous solution, and heating the reaction at high temperature in a microwave reactor; filtering and isolating the solid, washing with ethanol, extracting the crude product by Soxhlet extraction, and drying the crude product under vacuum to obtain a covalent organic framework. The application also provides the application of COFs prepared by the above-mentioned preparation method in photocatalytic seawater hydrogen evolution, compared to the photocatalyst in the prior art, the β-ketoenamine-linked covalent organic framework synthesized using the present application scheme has a promoting effect of photocatalytic decomposition of seawater under visible light irradiation, and shows an extremely high photocatalytic hydrogen production rate in seawater, and has good application prospects in photocatalytic seawater hydrogen evolution.

Description

Technical Field

[0001] The present invention relates to the technical field of material chemistry, and in particular to a method for preparing a covalent organic framework and application of the covalent organic framework in photocatalytic hydrogen evolution from seawater. Background Art

[0002] The development of H2 economy is crucial to alleviate the energy consumption and heavy dependence on fossil fuels and the resulting CO2 emissions or other environmental issues. Photocatalytic water splitting is one of the most attractive methods for sustainable H2 production. In the past few decades, the photocatalytic hydrogen evolution reaction (HER) has developed rapidly, but most of them rely on the decomposition of fresh water. Considering the global shortage of freshwater resources and the abundance of seawater, it is urgent to develop photocatalytic HER systems using seawater as a proton source. However, in the complex seawater environment with high salinity, most of the current photocatalysts have poor hydrogen evolution activity, which is specifically manifested in the low utilization rate of visible light absorption by photocatalysts in seawater, and most photocatalysts have poor stability in seawater, and often suffer from severe salt deactivation. It is still a huge challenge to design efficient and stable photocatalysts that can maintain or even have superior photocatalytic performance when switching from pure water to seawater.

[0003] Covalent organic frameworks (COFs), as a crystalline polymer, are currently being studied for the design of advanced organic photocatalysts due to their precisely controlled periodic skeleton, high porosity, adjustable electronic structure, good light absorption and high carrier mobility. Patent CN114534783A discloses a method for preparing a photocatalyst in which single-atom Pt is embedded in a covalent organic framework and its application. A covalent organic framework photocatalyst with Pt as a cocatalyst is synthesized by photodeposition. A Pt-containing precursor solution is introduced during the preparation of TpPa-1-COF, and a Pt1@TpPa-1-COF photocatalyst is obtained by low-temperature calcination and reduction, which provides favorable conditions for catalyzing hydrogen production from water. However, the crystal structure of COFs prepared by grinding and calcination is not stable and controllable enough. At the same time, the prior art only involves the application of COF in photocatalytic decomposition of fresh water.

[0004] At present, the application of COF in photocatalytic hydrogen evolution from seawater is still rare, and there has been no report on COF materials with high efficiency photocatalytic hydrogen evolution from seawater. At the same time, how to design a new route to synthesize COFs with mild crystallization conditions, short synthesis time, and stable and controllable structure so that it can be applied in the field of photocatalytic seawater decomposition is a problem that needs to be solved at present. Summary of the invention

[0005] In order to solve the problems in the background technology, the present application proposes a method for preparing covalent organic frameworks (COFs), which has mild conditions and short synthesis time, and is achieved by the following scheme:

[0006] A method for preparing a covalent organic framework comprises the following steps:

[0007] 1) adding 2,4,6-trihydroxy-1,3,5-benzaldehyde and diamino units to a mixed solution of a mixed organic solvent and an acetic acid aqueous solution, and heating the mixture at a high temperature in a microwave reactor for reaction;

[0008] 2) filtering and separating the solid, washing with ethanol, extracting the crude product by Soxhlet extraction, and drying the crude product under vacuum conditions to obtain a covalent organic framework.

[0009] Furthermore, the diamino unit is p-phenylenediamine, benzidine or 4,4-diamino-p-terphenyl.

[0010] Furthermore, the mixed organic solvent is a mixed solvent of mesitylene and 1,4-dioxane, or a mixed solvent of mesitylene and n-butanol.

[0011] Furthermore, the molar ratio of the 2,4,6-trihydroxy-1,3,5-benzaldehyde to the diamino unit is 1:0.8-3.

[0012] Furthermore, the concentration of the acetic acid aqueous solution is 6 mol / L, and the volume ratio of the acetic acid aqueous solution to the mixed organic solvent is 1:3-5.

[0013] Furthermore, the volume ratio of mesitylene to 1,4-dioxane or n-butanol is 1:0.25-4.

[0014] Furthermore, the reaction temperature of the high temperature heating reaction is 110-140° C., and the reaction time is 40-120 min.

[0015] Furthermore, the extraction time of the crude product by Soxhlet extraction is 12 to 36 hours.

[0016] Furthermore, the drying temperature under vacuum conditions is 50 to 80° C., and the drying time is 12 to 36 hours.

[0017] On the other hand, the present application also provides the use of COFs prepared by the above preparation method in photocatalytic hydrogen evolution from seawater.

[0018] Compared with the photocatalysts in the prior art, the β-ketoenamine covalent organic frameworks (COFs) synthesized by the present invention have a promoting effect on the photocatalytic decomposition of seawater under visible light irradiation. Through the investigation of the building blocks, the constructed COF with high-density β-ketoenamine units showed the highest photocatalytic hydrogen evolution rate in seawater to date (41.3 mmol g -1 h -1), which is about 1.66 times the photocatalytic hydrogen evolution rate in pure water. Therefore, the COFs prepared in this application have good application prospects in photocatalytic hydrogen evolution in seawater.

[0019] In addition, the preparation method provided in the present application uses 1,3,5-triformylphloroglucinol (Tp) as an aldehyde monomer, reacts with a diamino unit, and adopts a microwave-assisted solvothermal method to synthesize the target COFs. Not only are the conditions mild and the synthesis time short, but the process is more stable and controllable than the grinding method in the prior art, and the prepared COFs crystals have higher crystallinity. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The following is a brief description of the contents expressed in the drawings of the specification:

[0021] Figure 1 The schematic diagram is a method for preparing a covalent organic framework according to the present invention and its application in photocatalytic hydrogen evolution from seawater;

[0022] Figure 2 PXRD characterization diagram of COFs prepared in Examples 1-3 of the present invention;

[0023] Figure 3 The PXRD spectrum of the COF prepared in Example 1 of the present invention;

[0024] Figure 4 The PXRD spectrum of the COF prepared in Example 2 of the present invention;

[0025] Figure 5 The PXRD spectrum of the COF prepared in Example 3 of the present invention;

[0026] Figure 6 FI-IR spectrum of the COF prepared in Example 1 of the present invention;

[0027] Figure 7 FI-IR spectrum of the COF prepared in Example 2 of the present invention;

[0028] Figure 8 FI-IR spectrum of the COF prepared in Example 3 of the present invention;

[0029] Fig. 9 The SEM characterization images of COFs prepared in Examples 1-3 of the present invention;

[0030] Fig.10 This is a test result diagram of the COFs prepared in Examples 1-3 of the present invention in the photocatalytic seawater hydrogen evolution performance test;

[0031] Fig.11The graph is a test result of the cyclic stability of COFs prepared in Examples 1-3 of the present invention in the photocatalytic seawater hydrogen evolution performance test;

[0032] Fig.12 The test results of the COFs prepared in Examples 1, 4 and 5 of the present invention in the photocatalytic seawater hydrogen evolution performance test;

[0033] Fig.13 This is a graph showing the test results of the COFs prepared in Examples 1 and 6 of the present invention in the photocatalytic seawater hydrogen evolution performance test. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0035] In the embodiments of the present application, p-phenylenediamine (Pa), benzidine (Bd) or 4,4-diamino-p-terphenyl (TPy) is used as a diamino unit to react with an aldehyde monomer Tp to synthesize COFs, and the photocatalytic seawater decomposition and hydrogenation process is promoted by the in situ polarization effect. The reaction principle is as follows: Figure 1 shown.

[0036] Example 1

[0037] A mixture of mesitylene (8 ml), 1,4-dioxane (2 ml), 2,4,6-trihydroxy-1,3,5-benzaldehyde (Tp) (63 mg, 0.3 mmol), p-phenylenediamine (Pa) (49 mg, 0.45 mmol) and aqueous acetic acid (6M, 2 ml) was heated at 120°C for 1 hour in a microwave reactor (CEM MARS6). Afterwards, the solid was separated by filtration, washed with ethanol (500 ml), and then extracted with Soxhlet for 24 hours, and the collected powder was dried in vacuum at 60°C for 24 hours to obtain the corresponding COF product TpPa.

[0038] Example 2

[0039] A mixture of mesitylene (4 ml), 1,4-dioxane (4 ml), 2,4,6-trihydroxy-1,3,5-benzaldehyde (Tp) (63 mg, 0.3 mmol), benzidine (Bd) (83 mg, 0.45 mmol) and aqueous acetic acid (6M, 2 ml) was heated at 120°C for 1 hour in a microwave reactor (CEM MARS6). Afterwards, the solid was separated by filtration, washed with ethanol (500 ml), and then extracted with Soxhlet for 24 hours, and the collected powder was dried in vacuum at 60°C for 24 hours to obtain the corresponding COF product TpBd.

[0040] Example 3

[0041] A mixture of mesitylene (4 ml), n-butanol (4 ml), 2,4,6-trihydroxy-1,3,5-benzaldehyde (Tp) (63 mg, 0.3 mmol), 4,4-diaminoterphenyl (TPy) (117 mg, 0.45 mmol) and aqueous acetic acid (6 M, 1 ml) was heated at 120 ° C for 1 hour in a microwave reactor (CEM MARS6). After that, the solid was separated by filtration, washed with ethanol (500 ml), and then extracted with Soxhlet for 24 hours. The collected powder was dried under vacuum at 60 ° C for 24 hours to obtain the corresponding COF product TpTPy.

[0042] The COFs prepared in Examples 1-3 were characterized and analyzed. The crystal structures of TpPa (a), TpBd (b) and TpTPy (c) were analyzed by powder X-ray diffraction (PXRD) characterization combined with theoretical structure simulation. The pore sizes were simulated and calculated, and it was found that they belonged to the AA stacking model, such as Figure 2 As shown. TpPa can be calculated as The pore sizes of TpBd and TpTPy are calculated to be and

[0043] The PXRD spectrum of COFs was obtained by powder XRD diffraction experiment. Figure 3-5 As shown, Figure 3 The PXRD spectrum corresponding to Example 1 is: Figure 4 The PXRD spectrum corresponding to Example 2 is: Figure 5 The PXRD spectra corresponding to Example 3. The first strong diffraction peak representing the (100) facet was observed at 2θ degrees of 4.6°, 3.3°, and 2.6° for TpPa, TpBd, and TpTPy, respectively. In the TpPa, TpBd, and TpTPy sequences, the peak gradually shifted to smaller angles, indicating that the unit cells in the a and b axis directions increased due to the increase in the length of the diamine linker.

[0044] Through Fourier transform infrared (FT-IR) spectroscopy, the characteristic peaks of COFs were observed from the figure, such as Figure 6-8 As shown, Figure 6 This is the FI-IR spectrum corresponding to Example 1, Figure 7 This is the FI-IR spectrum corresponding to Example 2, Figure 8 The FI-IR spectra corresponding to Example 3. The framework and surface organic functional groups of these COFs were evaluated by Fourier transform infrared (FTIR) spectroscopy and compared with the corresponding groups. The diamine monomers (Pa, Bd and TPy) were -1 The stretching vibration of the amino group (NH) and the carbonyl stretching peak (1643 cm -1 ) disappears in COFs, corresponding to the acid-catalyzed Schiff base condensation under microwave irradiation. The C=C groups appear at 1573 and 1450 cm -1 About 1240cm -1 The observations around 1620 cm -1 A new C=O was observed, while the signal of C=N was missing. Figure 6 As shown, it reflects the isomerization of keto-ene to keto form.

[0045] The morphological characteristics of the COFs catalysts were observed by scanning electron microscopy (SEM) at the same scale (scale: 1 μm), and the morphological characteristics of these COFs were characterized by scanning electron microscopy (SEM), such as Fig. 9 The synthesized TpPa is rose-shaped and about 1 μm in size, while TpBd and TpTPy are chrysanthemum-shaped and irregularly aggregated, respectively, and the needle-shaped primary particles are within tens of nanometers in size.

[0046] Example 4

[0047] A mixture of mesitylene (8 ml), 1,4-dioxane (2 ml), 2,4,6-trihydroxy-1,3,5-benzaldehyde (Tp) (63 mg, 0.3 mmol), p-phenylenediamine (Pa) (49 mg, 0.45 mmol) and aqueous acetic acid (6M, 2 ml) was heated at 120°C for 2 hours in a microwave reactor (CEM MARS6). Afterwards, the solid was separated by filtration, washed with ethanol (500 ml), and then extracted with Soxhlet for 24 hours, and the collected powder was dried in vacuum at 60°C for 24 hours to obtain the corresponding COF product TpPa.

[0048] Example 5

[0049] A mixture of mesitylene (8 ml), 1,4-dioxane (2 ml), 2,4,6-trihydroxy-1,3,5-benzaldehyde (Tp) (63 mg, 0.3 mmol), p-phenylenediamine (Pa) (49 mg, 0.45 mmol) and aqueous acetic acid (6M, 2 ml) was heated at 120°C for 3 hours in a microwave reactor (CEM MARS6). Afterwards, the solid was separated by filtration, washed with ethanol (500 ml), and then extracted with Soxhlet for 24 hours, and the collected powder was dried in vacuum at 60°C for 24 hours to obtain the corresponding COF product TpPa.

[0050] The comparison of reaction parameters and related experimental results of Examples 4-5 and Example 1 is shown in Table 1.

[0051] Table 1 Results of adjusting reaction time to prepare TpPa

[0052]

[0053] Example 6

[0054] A mixture of mesitylene (8 ml), 1,4-dioxane (2 ml), 2,4,6-trihydroxy-1,3,5-benzaldehyde (Tp) (63 mg, 0.3 mmol), p-phenylenediamine (Pa) (49 mg, 0.45 mmol) and aqueous acetic acid (6M, 3 ml) was heated at 120°C for 1 hour in a microwave reactor (CEM MARS6). Afterwards, the solid was separated by filtration, washed with ethanol (500 ml), and then extracted with Soxhlet for 24 hours, and the collected powder was dried in vacuum at 60°C for 24 hours to obtain the corresponding COF product TpPa.

[0055] The comparison of reaction parameters and related experimental results of Example 6 and Example 1 is shown in Table 2.

[0056] Table 2 Results of adjusting the amount of acetic acid to prepare TpPa

[0057]

[0058] Photocatalytic hydrogen evolution performance test of seawater

[0059] The photocatalytic experiments were carried out in a homemade quartz photoreactor (80 ml) with a circulating cooling water system to keep the reaction solution at ambient temperature. In a typical reaction of photocatalytic decomposition of pure water, the COF photocatalyst (5 mg) was dispersed in an aqueous solution of sacrificial agent ascorbic acid (0.1 M, 20 mL) and ultrasonicated for 10 minutes. The above reaction solution was bubbled with argon (Ar) for 5 minutes to remove impurity gases. Then a certain volume of hexachloroplatinum aqueous solution (H2PtCl6, 0.5wt%) was added to in situ photodeposit Pt species on COF before the photocatalytic reaction. A 300WXe lamp equipped with a filter (420≤λ≤780nm) was used as the light source, and the product hydrogen was quantitatively analyzed by gas chromatography (GC-9860-5CNJ, China) containing a thermal conductivity detector (TCD) and a 5A molecular sieve packing column (2 meters). The same procedure was used to evaluate the performance of the catalyst in photocatalytic hydrogen evolution from seawater, using artificial simulated seawater (26.518 g / L NaCl, 3.305 g / L MgSO4, 2.447 g / L MgCl2, 1.141 g / L CaCl2, 0.725 g / L KCl, 0.202 g / L NaHCO3 and 0.083 g / L NaBr) instead of pure water in the reaction.

[0060] The COFs prepared in Examples 1-3 of this application were subjected to photocatalytic freshwater hydrogen evolution performance tests and photocatalytic seawater hydrogen evolution performance tests according to the above process and compared. Under visible light irradiation, photocatalytic HER experiments were carried out in the presence of ascorbic acid (AA) and hexachloroplatinic acid (H2PtCl6), respectively as hole sacrificial agents and Pt precursors, and Pt species were loaded by in-situ photodeposition process. The hydrogen evolution rate of TpPa, TpBd and TpTPy photocatalytic decomposition of pure water was evaluated in a 0.1M ascorbic acid aqueous solution, and the theoretical loading amount of Pt species relative to the mass of COF was 0.5wt.%. Fig.10 As shown in Figure 2, a continuous hydrogen evolution process was observed, with hydrogen evolution rates of 24.8, 14.9, and 14.5 mmol h for TpPa, TpBd, and TpTPy, respectively. -1 g -1 The photocatalytic hydrogen evolution activity of these COFs in seawater was evaluated by replacing pure water with artificial simulated seawater. The amount of hydrogen produced by all three COFs materials during the photocatalytic hydrogen evolution process in seawater increased linearly with time. The hydrogen evolution rates of TpPa, TpBd, and TpTPy were 41.3, 21.7, and 14.9 mmol h, respectively. -1 g -1, which are 1.66, 1.46 and 1.03 times the photocatalytic hydrogen evolution rate of pure water under the same conditions, respectively. The recyclability of TpPa was evaluated in the process of photocatalytic hydrogen evolution from seawater. In the cycle experiment, after each round, the powder catalyst was filtered through a cellulose membrane (pore size = 200nm) and washed with deionized water. The recovered catalyst was placed in a fresh 0.1M ascorbic acid solution for the next round of reaction. The results show that in the long-period photocatalytic reaction of 25 hours, the photocatalytic hydrogen evolution rate of seawater by TpPa remained basically unchanged after each cycle, such as Fig.11 shown.

[0061] It can be seen that compared with pure water, β-ketoenamine-linked COFs in seawater promote the rate of visible light-driven water hydrogen evolution. This promotion is caused by the in-situ polarization effect, that is, the metal ions in seawater are adsorbed on the skeleton of COFs, thereby increasing the dielectric constant of the COF material and reducing the exciton dissociation energy of the organic semiconductor, thereby improving the charge separation and transfer ability, and ultimately enhancing the hydrogen evolution activity.

[0062] The photocatalytic hydrogen evolution performance of TpPa prepared under different reaction time conditions was tested in fresh water and seawater and compared. The test results are as follows: Fig.12 shown.

[0063] The photocatalytic hydrogen evolution performance of TpPa prepared under different acetic acid dosage conditions was tested in fresh water and seawater and compared. The test results are as follows: Fig.13 shown.

[0064] from Fig.12 and Fig.13 It can be seen that within the parameter range claimed in the present application, COFs have good performance for photocatalytic hydrogen evolution from seawater.

[0065] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. Application of a covalent organic framework in photocatalytic hydrogen evolution from seawater, characterized in that: The preparation method of the covalent organic framework comprises the following steps: 1) adding 2,4,6-trihydroxy-1,3,5-benzaldehyde and diamino units to a mixed solution of a mixed organic solvent and an acetic acid aqueous solution, and heating the mixture at a high temperature in a microwave reactor for reaction; 2) filtering and separating the solid, washing with ethanol, extracting the crude product by Soxhlet extraction, and drying the crude product under vacuum conditions to obtain a covalent organic framework; Wherein, the diamino unit is p-phenylenediamine, benzidine or 4,4-diamino-p-terphenyl; the mixed organic solvent is a mixed solvent of mesitylene and 1,4-dioxane, or a mixed solvent of mesitylene and n-butanol; the molar ratio of 2,4,6-trihydroxy-1,3,5-benzaldehyde to the diamino unit is 1:0.8-3; the concentration of the acetic acid aqueous solution is 6 mol / L, and the volume ratio of the acetic acid aqueous solution to the mixed organic solvent is 1:3-5; the volume ratio of mesitylene to 1,4-dioxane or n-butanol is 1:0.25-4; The reaction temperature of the high temperature heating reaction in step 1) is 110-140°C, and the reaction time is 40-120min; The extraction time of the crude product by Soxhlet extraction in step 2) is 12 to 36 hours; the drying temperature under vacuum conditions is 50 to 80° C. and the drying time is 12 to 36 hours.

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