Preparation and application of a three-dimensional covalent organic polymer with electrocatalytic hydrogen peroxide production properties

Abstract

The preparation and application of a three-dimensional covalent organic polymer (COP) with electrocatalytic hydrogen peroxide production performance belong to the environmental fields such as wastewater treatment. Imidazole-linked fully conjugated three-dimensional COPs are obtained in a one-step process by carrying out a multi-component Debus-Radziszewski condensation reaction of a cyclooctatetraene tetrathiophene derivative ligand with a three-dimensional saddle-shaped structure and fully π-conjugated characteristics with pyrene-4,5,9,10-tetraone and amine acetate. The imidazole-linked three-dimensional COPs of this invention exhibit good chemical stability, porosity, and intrinsic semiconductor properties. This material possesses excellent electrocatalytic performance for the synthesis of hydrogen peroxide (H2O2), and the H2O2 generated by its electrocatalysis can effectively degrade methylene blue dye.

Description

Technical Field

[0001] This invention belongs to the field of porous organic polymer materials, specifically relating to the preparation of a novel imidazole-linked three-dimensional fully conjugated covalent organic polymer material. This covalent organic polymer can be used for the electrocatalytic synthesis of hydrogen peroxide via 2-electron oxygen reduction and can effectively degrade methylene blue dye. Background Technology

[0002] Covalent organic polymers (COPs) are a class of multidimensional, multifunctional porous organic materials assembled by covalent bonds using organic ligands of different geometries and lengths. Due to their excellent stability, high specific surface area, large pore volume, controllable pore structure, and ease of pore modification, COPs have attracted widespread attention in recent years and show promising application prospects in fields including heterogeneous catalysis, gas adsorption and storage, drug sustained release, and water treatment (Chem. Mater. 2019, 31, 3313-3323).

[0003] Furthermore, COPs are composed of organic structural units linked by covalent bonds, and one of the most important scientific questions in this field is how to construct stable connections (Acc. Chem. Res. 2015, 48, 3053). Currently, COPs based on heterocycles such as cyclotriazine, phenazine, benzothiazole, quinoline, and benzimidazole as linking units have been ingeniously constructed. The introduction of these heterocycles not only increases the stability of the material but also introduces heteroatoms, which is beneficial for improving and enhancing the semiconductor properties of the material, such as electrocatalysis. Based on this, in 2019, Wang et al. reported a covalent organic framework (COF) with imidazole linkages synthesized through a multi-component reaction (J. Am. Chem. Soc. 2019, 141, 18004-18008). Due to the formation of imidazole linkages, this type of material exhibits good crystallinity and ultra-high stability.

[0004] However, compared to the widely reported two-dimensional covalent organic polymers, constructing new three-dimensional topologies and developing novel three-dimensional covalent organic polymer materials is of great significance for promoting their practical applications in fields such as wastewater treatment and environmental protection. To this end, we used imidazole rings as connecting units and utilized the three-dimensional conjugated saddle structure characteristics of cyclooctatetraene tetrathiophene derivatives to construct a novel imidazole-linked fully conjugated three-dimensional COP material. This material exhibits excellent two-electron oxygen reduction electrocatalytic performance, enabling the high-selectivity and high-yield synthesis of hydrogen peroxide (H2O2). Furthermore, the synthesized H2O2 can be effectively used to degrade methylene blue dye. This invention provides new ideas for the development of COP materials in environmental and other fields.

[0005] Currently, there are no research reports or patent publications at home and abroad regarding the preparation method of three-dimensional covalent organic polymer materials with imidazole units as links, the generation of H2O2 by oxygen reduction, and the effective degradation of methylene blue dye by the Fenton reaction. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing a highly stable imidazole-linked fully conjugated three-dimensional covalent organic polymer material, which has the ability to electrocatalyze the production of H2O2 and can be used to degrade methylene blue dye.

[0007] To achieve the above objectives, the technical solution adopted by the present invention specifically includes the following:

[0008] The first aspect of this invention provides a method for preparing an imidazole-linked three-dimensional covalent organic polymer material, comprising the following steps:

[0009] (1) The aromatic heterocyclic substituted saddle-shaped cyclooctatetraene tetrathiophene derivative ligand COThP-CHO, pyrene-4,5,9,10-tetraone (PyT), and ammonium acetate (NH4OAc), which serves as both a reaction component and a catalyst, are dispersed and dissolved in a solvent; wherein COThP-CHO is a conjugated saddle-shaped cyclooctatetraene tetrathiophene derivative ligand, and the substituted aromatic ring is selected from benzene ring, furan, thiophene, selenophene, and other aromatic heterocycles.

[0010] (2) After the reaction system is briefly dissolved and dispersed by ultrasound, it is then frozen-vacuum-thawed in a cycle, and then heated under sealed conditions until the reaction is complete;

[0011] (3) The product was filtered, washed, and then purified by extraction to obtain a three-dimensional covalent organic polymer with an imidazole-linked fully conjugated structure (named BUCT-COP-7).

[0012] The structural formula of COThP-CHO is as follows: where This refers to aromatic rings, such as those with O / S / Se hybridization, where the aromatic ring is benzene, naphthalene, etc. For example... for wait.

[0013]

[0014] Furthermore, in step (1), the molar ratio of COThP-CHO, pyrene-4,5,9,10-tetraone and NH4OAc is 1:2:2 to 20 (the molar ratio of NH4OAc relative to COThP-CHO is preferably 12).

[0015] Furthermore, in step (1), the solvent is a mixture of N,N-dimethylacetamide and mesitylene.

[0016] Furthermore, in step (1), the solvent is a mixed solvent of N,N-dimethylacetamide and mesitylene in a volume ratio of 1 to 9:9 to 1 (preferably 2:8).

[0017] Furthermore, in step (2), the ultrasonic time is 1 to 30 minutes, and the freezing-vacuum-thawing cycle method is as follows: the reactants are frozen with liquid nitrogen, vacuumed, and thawed with nitrogen gas.

[0018] Furthermore, in step (2), the temperature of the heating reaction is between 50 and 200°C, and the reaction time is between 1 and 10 days.

[0019] Furthermore, in step (3), the polymer is washed with dichloromethane and ethanol, extracted with organic solvents such as tetrahydrofuran using Soxhlet extraction, and finally vacuum dried to obtain the imidazole-linked fully conjugated three-dimensional covalent organic polymer.

[0020] This invention utilizes the Debus-Radziszewski condensation reaction to synthesize imidazole-linked, fully conjugated three-dimensional COP materials in one step from COThP-CHO, pyrene-4,5,9,10-tetraone, and NH4OAc. The synthetic route is as follows:

[0021]

[0022] The substituted aromatic ring can be a benzene ring, furan, thiophene, selenophene, or other aromatic or heterocyclic rings. The substituent R on PyT can be a hydrogen atom, a C1-20 alkyl group, or other substituent groups.

[0023] The second aspect of the present invention provides an electrochemical test of the product obtained by the preparation method of the above-mentioned three-dimensional covalent organic polymer material in the 2-electron oxygen reduction to generate H2O2, comprising the following steps: the covalent organic polymer prepared by the present invention and single-walled carbon nanotubes are mixed in a certain mass ratio (preferably 1:1) to form an ink droplet and dropped onto a rotating ring-disk electrode (RRDE), which is used as the cathode, a carbon rod as the anode, and a calomel electrode as the reference electrode to form a three-electrode system. After oxygen saturation for a certain period of time, its electrocatalytic performance is tested.

[0024] Furthermore, the mass loading of the ink on the RRDE is 0.1–0.3 mg / cm³. 2 .

[0025] Furthermore, the material was tested in 0.1M KOH solution to obtain electrochemical LSV curves and hydrogen peroxide selectivity curves.

[0026] The third aspect of this invention provides a method for applying the 3D COP described in the second aspect, comprising the following steps: The covalent organic polymer material and a conductive agent (which may be single-walled or multi-walled carbon nanotubes, acetylene black, Ketjen black, etc.) are mixed at a certain mass ratio (preferably 1:1) to form an ink droplet, which is then dropped onto a gas diffusion electrode as a cathode catalyst. A carbon rod is used as the anode, and the two electrodes are separated by Nafion 115 to assemble a gas diffusion battery. Electrolysis is performed using O2 from the air. After a certain time, an appropriate amount of cathode electrolyte containing H2O2 is taken, and a certain amount of methylene blue solution is titrated using the Fenton reaction to observe the dye degradation.

[0027] Furthermore, the mass loading of the ink on the gas diffusion electrode is 0.2-1 mg / cm³. 2 .

[0028] Furthermore, the volume of electrolyte used was 3 mL, and the volume of methylene blue solution was 6 mL.

[0029] Furthermore, based on the Fenton reaction principle, the methylene blue used was at pH=1 and a concentration of 200 ppm, containing 0.1 mmol of Fe. 2+ The solution.

[0030] The advantages of this invention are:

[0031] 1) This invention synthesizes a novel fully conjugated three-dimensional covalent organic polymer material with high stability by means of the Debus-Radziszewski reaction in one step.

[0032] 2) The three-dimensional covalent organic polymer prepared by the present invention can be used for the electrocatalytic production of H2O2 by 2-electron oxygen reduction, with an average electron transfer number n of 2.49 and an H2O2 selectivity of 82.1%.

[0033] 3) The three-dimensional covalent organic polymer prepared in this invention is electrolyzed using a gas diffusion electrolysis cell device with O2 in the air at a relatively low current density of 10 mA / cm². 2 After 3 hours of discharge, a relatively high yield of H2O2 of 1090 mmol / g can be produced, with an average yield of 363.3 mmol / g / h and a Faraday efficiency of about 80%.

[0034] 4) The three-dimensional covalent organic polymer prepared in this invention is subjected to a gas diffusion electrolysis cell at a current of 10 mA / cm². 2 After electrolysis for 3 hours, 3 mL of the cathode electrolyte was added to 6 mL of a solution with pH = 1 containing 0.1 mmol of Fe. 2+ In a methylene blue solution, after a brief shaking, the color fading can be clearly observed, indicating that the dye has been effectively degraded. Attached Figure Description

[0035] Figure 1 The image shows the X-ray photoelectron spectroscopy (XPS) spectrum of the covalent organic polymer material (BUCT-COP-7) prepared in this invention.

[0036] Figure 2 The Fourier Transform Infrared (FTIR) spectrum of the covalent organic polymer material (BUCT-COP-7) prepared in this invention;

[0037] Figure 3 The solid-state NMR spectrum of the covalent organic polymer material (BUCT-COP-7) prepared in this invention;

[0038] Figure 4 The LSV curve of the covalent organic polymer material prepared for this invention under a rotating ring-disk electrode (RRDE) in 0.1M KOH solution.

[0039] Figure 5 Hydrogen peroxide selectivity curves of the covalent organic polymer material prepared in this invention under a rotating ring-disk electrode (RRDE) in 0.1M KOH solution.

[0040] Figure 6 This is a photograph of the actual assembly used in testing the gas diffusion battery of this invention.

[0041] Figure 7 This invention relates to the H2O2 yield and corresponding Faraday efficiency obtained during gas diffusion battery testing at different current densities.

[0042] Figure 8 In this invention, during the gas diffusion battery test, the current density was 10 mA / cm². 2 Stability of discharge for 3 hours and H2O2 production at different times.

[0043] Figure 9 In this invention, when conducting gas diffusion cell tests, a value of 10 mA / cm was used. 2 The concentration of the degraded cathode electrolyte was tested after adding an appropriate amount of the cathode electrolyte to a 200 ppm methylene blue solution 3 hours after discharge.

[0044] Figure 10 This is a color comparison of the present invention before and after degradation of a 200 ppm methylene blue solution. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0046] Example 1

[0047] The three-dimensional COP product synthesized using the above-mentioned synthetic route of covalent organic polymer materials is labeled as BUCT-COP-7.

[0048] The steps are as follows:

[0049] A phenyl-substituted saddle-shaped ligand COThP-CHO (0.02 mmol), pyrene-4,5,9,10-tetraone (0.04 mmol), and NH4OAc (0.24 mmol), which serves as both a reactant and a catalyst, were placed in a Pyrex tube. N,N-dimethylacetamide / trimethylbenzene (v / v = 2:8, total volume 1 mL) was added, and the mixture was sonicated for 10 minutes. The Pyrex tube was then rapidly frozen in a liquid nitrogen bath, followed by evacuation to 0 mbar. After three freeze-vacuum-thaw cycles, the tube was vacuum-sealed, heated to room temperature, and then placed in an oven at 150–180 °C for 3–7 days. Following the reaction, the solution was filtered through filter paper, washed with dichloromethane and ethanol, and finally extracted with tetrahydrofuran using a Soxhlet extractor for 24–48 hours to remove the reaction solvent and small oligomers.

[0050] Figure 1 XPS C1s spectra were used to verify that the two monomers formed a good bond in the solvent system, which is a covalent organic polymer. Figure 2 The infrared spectrum of BUCT-COP-7 is given. Figure 3 This is the solid-state NMR spectrum of BUCT-COP-7, and the formation of the imidazole unit was confirmed by FT-IR and solid-state NMR spectroscopy. The FT-IR spectrum shows the imidazole group at approximately 1639 cm⁻¹. -1 and 3412cm -1 The typical bands at that location are similar to those in the imidazole compound model.

[0051] The ORR activity of COP was determined by testing the LSV curve of a rotating ring-disc electrode (RRDE) in 0.1M KOH alkaline solution (see attached diagram). Figure 4 This indicates that some H2O2 is produced. Calculations show that the H2O2 yield can reach 82.1% between 0.6-0.7V (vs. RHE). (See attached diagram) Figure 5 ).

[0052] Example 2: The application of electrocatalytic synthesis of H2O2 from the three-dimensional fully conjugated covalent organic polymer BUCT-COP-7 and its effective degradation of methylene blue dye is carried out according to the following steps.

[0053] An ink was prepared by mixing BUCT-COP-7 covalent organic polymer and single-walled carbon nanotubes at a certain mass ratio (e.g., 1:1) with anhydrous ethanol:water:Nafion (solution concentration 5wt%) = 10:10:1 (volume ratio). This ink was then dropped onto a gas diffusion electrode with a mass loading of 0.5 mg cm⁻¹. -2 Using it as the cathode catalyst, a carbon rod as the anode, and Nafion 115 as the electrode separator, a 0.1M KOH electrolyte was used to assemble a gas diffusion cell (see attached image). Figure 6 ).

[0054] Electrolysis was performed using O2 in the air. Electrolysis was carried out for 10 minutes at different current densities, and curves showing the relationship between H2O2 yield and current density were obtained (see attached figure). Figure 7 ), of which at 50mA cm -2 Under electrolysis, the yield of H2O2 can reach 1680.39 mmol g. -1 h -1 At a relatively low current density of 10 mA / cm² 2 After 3 hours of discharge, a relatively high yield of H2O2 of 1090 mmol / g was achieved, with an average yield of 363.3 mmol / g / h and a Faraday efficiency of approximately 80%. (See attached image) Figure 8 Take 10 mA / cm 2 After 3 hours of discharge, 3 ml of cathode electrolyte containing H2O2 was added dropwise to 6 mL of solution using the Fenton reaction. The solution had a pH of 1 and contained 0.1 mmol of Fe. 2+ In a 200 ppm methylene blue solution, liquid ultraviolet testing showed that the methylene blue content was almost zero after the reaction (see attached image). Figure 9 Meanwhile, it was observed that the deep blue dye turned colorless before and after degradation (see attached image). Figure 10 Experiments have shown that imidazole-linked three-dimensional COPs have good electrocatalytic oxygen reduction capabilities to produce H2O2. The generated H2O2 can also be effectively applied to the degradation of organic dye wastewater.

[0055] The above embodiments are merely examples to clearly illustrate the present invention and should not be construed as limiting the claims. The present invention is not limited to the above embodiments, and variations in its specific structure are permitted. All variations made within the scope of the independent claims of this invention are also within the scope of protection of this invention.

[0056] Appendix Figure 1 The peaks at 284.2 eV and 285.4 eV show the characteristic C=C and C=N peaks of the imidazole unit, proving the successful synthesis of 3D-COP.

[0057] Appendix Figure 2 This indicates that the imidazole group in BUCT-COP-7 is located at approximately 1648 cm⁻¹.-1 The presence of typical bands, similar to those in the imidazole model compound, confirms the synthesis of the imidazole unit.

[0058] Appendix Figure 3 The presence of characteristic peaks of imidazole units in the solid-state NMR spectrum further confirms the synthesis of COP.

[0059] Appendix Figure 4 The disk current and ring current during the rotating ring-disk electrode (RRDE) test of BUCT-COP-7 show that the material undergoes a 2-electron oxygen reduction reaction, producing a certain amount of H2O2.

[0060] Appendix Figure 5 The yield of H2O2 produced under different voltages is approximately 80% between 0.6-0.7V (vs. RHE).

[0061] Appendix Figure 6 This is a picture of the actual assembly used for testing a gas diffusion battery.

[0062] Appendix Figure 7 The curves show the H2O2 yield and Faraday efficiency at different current densities during the gas diffusion battery test. It can be seen that at a current density of 30 mA / cm², the yield of H2O2 and the Faraday efficiency are [data missing]. -2 At the above levels, the yield of H2O2 can reach 1000 mmol g. cat -1 h -1 Meanwhile, Faraday efficiency remains at around 80%.

[0063] Appendix Figure 8 During the gas diffusion cell test, at a current density of 10 mA / cm² 2 The stability and H2O2 production after 3 hours of discharge were measured. It can be seen that at a relatively low current density of 10 mA / cm², [the following parameters were observed]. 2 After 3 hours of discharge, a relatively high yield of H2O2 of 1090 mmol / g can be produced.

[0064] Appendix Figure 9 After the stability test of the gas diffusion battery, 3 mL of cathode electrolyte containing H2O2 was taken and added dropwise to 6 mL of electrolyte with pH = 1 containing 0.1 mmol Fe2O3 using the Fenton reaction. 2+ In a 200 ppm methylene blue solution, liquid ultraviolet testing showed that the methylene blue content was almost zero after the reaction.

[0065] Appendix Figure 10 The color change of methylene blue dye before and after degradation was investigated, and it can be seen that the dark blue dye was significantly degraded.

Claims

1. Use of imidazole-linked three-dimensional covalent organic polymer material for 2-electron oxygen reduction to produce H2O2 and degrade methylene blue dye. A method for preparing imidazole-linked three-dimensional covalent organic polymer material, comprising the following steps: (1) dispersing uniformly in a solvent a three-component reagent of an aromatic ring-substituted saddle-shaped cyclooctatetraene tetra-thiophene derivative ligand COThP-CHO, a pyrene-4,5,9,10-tetraone (PyT) derivative and ammonium acetate (NH4OAc) as both a reaction component and a solid catalyst; (2) after the reaction system is dispersed by short-time ultrasonic, it is subjected to a freeze-vacuum-thaw cycle, and then is heated to complete the reaction under a sealed condition; (3) the product is filtered, washed and subjected to Soxhlet purification to obtain the imidazole-linked three-dimensional covalent organic polymer.

2. Use according to claim 1, characterized in that: The covalent organic polymer material and a conductive agent are mixed in a mass ratio of 1:1 to form an ink, which is dropped on a gas diffusion electrode to serve as a cathode catalyst, a carbon rod is used as an anode, and a Nafion film is used to separate the two electrodes to assemble a gas diffusion battery. The conductive agent is selected from single-walled or multi-walled carbon nanotubes, acetylene black and ketjen black.

3. The use according to claim 2, wherein O2 in air is used for electrolysis, after a certain time, a proper amount of cathode electrolyte containing H2O2 is taken, a methylene blue solution is titrated by using a Fenton reaction to determine the degradation of the dye.

4. Use according to claim 1, characterized in that: The COThP-CHO is a saddle-shaped conjugated molecular building block, and the structural formula of the COThP-CHO is as follows: representing an aromatic ring, O / S / Se-hybridized aromatic ring, the aromatic ring is benzene, naphthalene; ； , the substituent R on the PyT is a hydrogen atom, an alkyl group.

5. The use according to claim 1, characterized in that: The molar ratio of COThP-CHO, pyrene-4,5,9,10-tetraone and NH4OAc is 1:2:2-20.

6. Use according to claim 1, characterized in that: The molar ratio of COThP-CHO, pyrene-4,5,9,10-tetraone and NH4OAc is 1:2:

12.

7. Use according to claim 1, characterized in that: In the step (1), the solvent is a mixed solvent of N,N-dimethylacetamide and mesitylene, and the volume ratio of N,N-dimethylacetamide to mesitylene is 1-9:9-1.

8. The use according to claim 1, characterized in that: In the step (2), the ultrasonic time is 1-30 min, the freeze-vacuum-thaw cycle method is: freezing the reactants with liquid nitrogen, vacuumizing and thawing by nitrogen, and the sealing reaction is carried out under a nitrogen atmosphere or vacuum condition. In the step (2), the heating reaction temperature is 50-200℃, and the reaction time is 1-10 days.

9. The use according to claim 1, characterized in that: In the step (3), the product is washed with dichloromethane and ethanol, and is subjected to Soxhlet extraction with tetrahydrofuran for 1-5 days, and then is vacuum dried to obtain the imidazole-linked three-dimensional covalent organic polymer.

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