A method for preparing a highly selective electrosynthetic covalent organic framework for hydrogen peroxide

Abstract

This invention relates to the field of covalent organic framework materials technology, and discloses a method for preparing a highly selective covalent organic framework for the electrosynthesis of hydrogen peroxide. The covalent organic framework material of this invention is synthesized from N,N,N',N'-tetra(4-aminophenyl)-1,4-phenylenediamine (TPDA) and thiophene dialdehyde monomers via a solvothermal method. The thiophene dialdehyde monomers are selected from 2,5-thiophene dicarboxaldehyde (TDC), 2,2'-bithiophene-5,5'-dicarboxaldehyde (bTDC), or 2,2':5',2''-terthiophene-5,5''-dicarboxaldehyde (tTDC). This invention uses a mixed solvent of o-dichlorobenzene and n-butanol, reacting at 120°C for 3 days under sealed conditions. After washing, Soxhlet extraction, and drying, a covalent organic framework material with good crystallinity and high specific surface area is obtained. This material exhibits excellent catalytic activity and selectivity in the electrocatalytic reduction of oxygen to produce hydrogen peroxide, and can be applied in fuel cells, metal-air batteries, and other fields. The method of this invention is simple to operate, has a high yield, and a controllable structure, and has good prospects for industrial application.

Description

Technical Field

[0001] This invention relates to the field of electrocatalysis technology, specifically to a method for preparing a highly selective electrosynthetic covalent organic framework for hydrogen peroxide. Background Technology

[0002] Covalent organic frameworks (COFs) are a class of porous crystalline organic polymer materials formed by covalent bonds connecting lightweight elements (C, H, O, N, etc.). COFs possess unique advantages such as high specific surface area, regular and ordered pore structure, good thermal and chemical stability, and designable framework structure, showing broad application prospects in fields such as gas adsorption and separation, catalysis, sensing, energy storage and conversion.

[0003] Thiophene compounds have attracted widespread attention in the construction of functional COFs due to their excellent electronic conductivity and structural modifiability. Thiophene-containing COFs can enhance the charge transport properties of materials through the coordination of the lone pair electrons of sulfur atoms with metal ions, or through the π-π stacking effect of the thiophene ring, thus possessing potential application value in the field of electrocatalysis.

[0004] However, current methods for synthesizing COFs based on thiophene dialdehyde monomers still suffer from problems such as harsh reaction conditions, poor crystallinity, and low yields, limiting their further development and application. In existing technologies, imine-linked COFs typically require several days of reaction under high temperature and vacuum conditions, and the crystallinity and specific surface area of ​​the product are affected by various factors such as monomer structure, solvent system, and catalyst, resulting in poor batch stability. Therefore, developing a simple, mild, high-yield, and structurally controllable method for synthesizing thiophene-based COFs, and exploring its application in the electrocatalytic oxygen reduction reaction (ORR), is of significant research importance and practical value. Summary of the Invention

[0005] The present invention aims to provide a method for preparing covalent organic frameworks for the highly selective electrosynthesis of hydrogen peroxide, in order to solve the technical problems of poor crystallinity and low yield in existing thiophene-based COF synthesis methods.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for preparing a highly selective electrosynthetic covalent organic framework for hydrogen peroxide, comprising the following steps: Step 1: Add N,N,N',N'-tetratetra(4-aminophenyl)-1,4-phenylenediamine (TPDA) and thiophene dialdehyde monomers to the reaction vessel at a molar ratio of 1:2. Step 2: Add a mixed solvent of o-dichlorobenzene and n-butanol to the reaction vessel and disperse it evenly by ultrasonication; Step 3: Perform freeze-thaw cycle degassing on the reaction vessel, then evacuate and seal it; Step 4: Heat the sealed reaction vessel to 120°C and react for 3 days; Step 5: After the reaction is complete, separate the precipitate, wash and dry it to obtain the TDC-based covalent organic framework material.

[0007] Preferably, as an improvement, the thiophene dialdehyde monomer is any one of 2,5-thiophene dicarboxaldehyde (TDC), 2,2'-bithiophene-5,5'-dicarboxaldehyde (bTDC), or 2,2':5',2''-terthiophene-5,5''-dicarboxaldehyde (tTDC).

[0008] Preferably, as an improvement, in step two, the volume ratio of o-dichlorobenzene to n-butanol is 1:1.

[0009] Preferably, as an improvement, in step two, the amount of the mixed solvent used is 8 to 12 mL of mixed solvent per 0.3 mmol TPDA.

[0010] Preferably, as an improvement, in step three, the freeze-thaw cycle treatment is performed in three freeze-thaw cycles; the vacuum is drawn until the internal pressure is 0.005 mmHg.

[0011] Preferably, as an improvement, in step five, the washing is performed sequentially with N,N-dimethylformamide (DMF) and acetone; the drying is performed under reduced pressure at 80°C for 24 h, or further with Soxhlet extraction with dichloromethane followed by drying.

[0012] Preferably, as an improvement, the yield of the product obtained in step five is 85% to 90%.

[0013] Preferably, as an improvement, the present invention also provides a TDC-based covalent organic framework material prepared by the above method.

[0014] Preferably, the material is an orange-red fluffy powder with a typical crystal structure of a covalent organic framework.

[0015] Preferably, as an improvement, the present invention also provides the application of the above-mentioned TDC-based covalent organic framework material in electrocatalytic oxygen reduction reaction.

[0016] 1. This scheme selects TPDA as the tetraamine monomer, which has a rigid benzene ring structure and four amino groups, and can form a two-dimensional or three-dimensional network structure with imine bonds through Schiff base reaction with dialdehyde monomers. The symmetry and rigidity of TPDA contribute to the formation of a highly ordered crystalline COF structure.

[0017] 2. This scheme uses a mixed solvent system of o-dichlorobenzene and n-butanol. o-Dichlorobenzene is a good aromatic solvent, which can effectively dissolve aromatic monomers and promote π-π stacking; n-Butanol, as a polar solvent, can adjust the polarity of the reaction system, promote the reversible formation and repair of imine bonds, thereby improving the crystallinity of COF. The 1:1 volume ratio of the two solvents showed the best solvent effect in the experiment.

[0018] 3. This scheme employs a freeze-thaw cycle degassing method to remove oxygen from the reaction system and prevent oxidation side reactions; after evacuating to 0.005 mmHg, the system is sealed to ensure that the reaction proceeds under oxygen-free and constant-pressure conditions. The reaction temperature of 120℃ provides sufficient heat energy to drive the Schiff base reaction while avoiding thermal decomposition of the skeleton caused by excessively high temperatures.

[0019] 4. The reaction time of this scheme is 3 days, which is sufficient to ensure the full conversion of monomers and the self-repair of the crystal lattice, thereby obtaining COF materials with high crystallinity. Post-treatment uses DMF and acetone washing to remove unreacted monomers and oligomers, and further purification by Soxhlet extraction can obtain high-quality pure-phase COF.

[0020] 5. By altering the conjugation length of thiophene dialdehyde monomers (from monothiophene TDC to bithiophene bTDC and then to trithiophene tTDC), the electronic structure and pore size of the COF framework can be tuned, thereby optimizing its performance in electrocatalytic ORR. Longer conjugated structures are beneficial for charge transport but may reduce the specific surface area of ​​the material; the bithiophene-based COF prepared in this scheme exhibits excellent H2O2 selectivity and yield.

[0021] 6. Electrochemical tests show that the bTDC-based COF obtained in this invention exhibits excellent oxygen reduction catalytic activity in 0.1 M KOH solution, with an electron transfer number close to a 2-electron pathway, high hydrogen peroxide yield, and good stability. Therefore, this invention provides a new approach for developing non-noble metal electrocatalysts, which can be applied to cathode catalytic materials for fuel cells and metal-air batteries. Attached Figure Description

[0022] Figure 1 This is the X-ray diffraction (XRD) pattern of the TDC-COF obtained in Example 1 of the present invention.

[0023] Figure 2 The image shows the XRD pattern of b-TDC COF obtained in Example 2 of this invention.

[0024] Figure 3 The image shows the XRD pattern of t-TDC COF obtained in Example 3 of this invention.

[0025] Figure 4 This is a transmission electron microscope (TEM) image of the TDC-COF obtained in Example 1 of the present invention.

[0026] Figure 5 This is a high-magnification TEM image of TDC-COF obtained in Example 1 of the present invention.

[0027] Figure 6 This is an elemental distribution diagram of the TDC-COF obtained in Example 1 of the present invention.

[0028] Figure 7 This is a TEM image of b-TDC COF obtained in Example 2 of the present invention.

[0029] Figure 8 This is a high-magnification TEM image of b-TDC COF obtained in Example 2 of the present invention.

[0030] Figure 9 This is an elemental distribution diagram of b-TDC COF obtained in Example 2 of the present invention.

[0031] Figure 10 This is a TEM image of t-TDC COF obtained in Example 3 of the present invention.

[0032] Figure 11 This is a high-magnification TEM image of t-TDC COF obtained in Example 3 of the present invention.

[0033] Figure 12 This is an elemental distribution diagram of t-TDC COF obtained in Example 3 of the present invention.

[0034] Figure 13 This is a graph showing the electron transfer number and H2O2 selectivity of the three COF catalysts of this invention.

[0035] Figure 14 The graph shows the linear sweep voltammetry (LSV) curves of oxygen reduction for the three COF catalysts of this invention.

[0036] Figure 15 The diagram shows the Faradaic efficiency (FE) of the three COF catalysts of this invention at different potentials for H2O2 production.

[0037] Figure 16 This is a durability test diagram of bTDC-COF obtained in Example 1 of the present invention. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto. Unless otherwise specified, the technical means used in the following embodiments and experimental examples are conventional means well known to those skilled in the art, and the materials and reagents used can all be obtained commercially. Example

[0039] A method for preparing a highly selective electrosynthetic covalent organic framework for hydrogen peroxide includes the following steps: Step 1: Add N,N,N',N'-tetratetra(4-aminophenyl)-1,4-phenylenediamine (TPDA) (141.78 mg, 0.3 mmol, 1.0 eq.) and 2,5-thiophene dicarboxaldehyde (TDC) (84.096 mg, 0.6 mmol, 2 eq.) to a dry Pyrex glass tube (10 mL). Step 2: Add a mixed solvent of o-dichlorobenzene (5 mL) and n-butanol (5 mL), and sonicate in an ultrasonic bath for 30 min to disperse it evenly. Step 3: Degas the glass tube by performing three freeze-thaw cycles, and then seal it after evacuating it to an internal pressure of 0.005 mmHg. Step 4: Place the sealed glass tube on the heating table and heat it at 120°C for 3 days. Step 5: After the reaction is complete, a dark red precipitate is formed at the bottom of the separation tube. It is washed successively with N,N-dimethylformamide (DMF) and acetone, and then dried. It is then further extracted with dichloromethane (DCM) for 24 h. Finally, it is dried under reduced pressure at 80℃ for 24 h to obtain orange-red fluffy powder TDC-COF with a yield of 85.4%. Example

[0040] This embodiment is basically the same as Example 1, except that the dialdehyde monomer used in step one is 2,2'-bithiophene-5,5'-dicarboxaldehyde (bTDC) (133.368 mg, 0.6 mmol, 2 eq.). The remaining steps are the same as in Example 1, yielding an orange-red, fluffy powder of bTDC-COF with a yield of 86.7%. Example

[0041] This embodiment is basically the same as Example 1, except that the dialdehyde monomer used in step one is 2,2':5',2''-terthiophene-5,5''-dicarboxaldehyde (tTDC) (182.646 mg, 0.6 mmol, 2 eq.). The remaining steps are the same as in Example 1, yielding an orange-red, fluffy powder tTDC-COF with a yield of 89.7%.

[0042] Comparative Example 1 This comparative example is basically the same as Example 1, except that ultrasonic treatment is not performed in step two; instead, the monomer and solvent are directly mixed and then degassed and sealed. As a result, the product yield decreased to 57%, and XRD showed a significant reduction in crystallinity.

[0043] Comparative Example 2 This comparative example is basically the same as Example 1, except that in step two, only o-dichlorobenzene (10 mL) was used as a single solvent, without the addition of n-butanol. The resulting product yield was 68%, but the particles showed severe agglomeration and a decrease in specific surface area.

[0044] Comparative Example 3 This comparative example is basically the same as Example 1, except that the reaction temperature in step four is 100°C. The monomer conversion was incomplete, with a yield of only 35%.

[0045] Comparative Example 4 This comparative example is basically the same as Example 1, except that the reaction temperature in step four is 150°C. As a result, the product underwent thermal decomposition, yielding a black charred substance, with a yield of less than 25%.

[0046] Comparative Example 5 This comparative example is basically the same as Example 1, except that in step three, freeze-thaw degassing is not performed; instead, the tube is directly sealed in air. As a result, a black gel-like substance is formed after the reaction, and powdered COF cannot be obtained.

[0047] X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and elemental distribution analysis were performed on the covalent organic framework materials prepared in Examples 1, 2, and 3. The results are as follows: Figures 1-12 As shown.

[0048] Figure 1 The XRD pattern of TDC-COF obtained in Example 1 of this invention is shown. The results show that four distinct diffraction peaks appear at 2θ of approximately 2.4°, 13.2°, 18.4° and 21.8°, which are attributed to the (100), (420), (620) and (550) crystal planes, respectively, indicating that the material has high crystallinity and forms a regular covalent organic framework structure.

[0049] Figure 2 The XRD pattern of bTDC-COF obtained in Example 2 of this invention is shown. The results show that three distinct diffraction peaks appear at 2θ of approximately 3.4°, 6.7° and 10.1°, which are attributed to the (100), (220) and (330) crystal planes, respectively, indicating that the material has high crystallinity and forms a regular covalent organic framework structure.

[0050] Figure 3 The image shows the XRD pattern of tTDC-COF obtained in Example 3 of this invention. Two distinct diffraction peaks appear at approximately 1.8° and 5.1° 2θ, belonging to the (100) and (300) crystal planes respectively, indicating that the material has high crystallinity and forms a regular covalent organic framework structure. This indicates that as the thiophene chain grows, the pore size and cell parameters of the material further increase.

[0051] Figure 4This is a TEM image of the TDC-COF obtained in Example 1 of the present invention. The results show that the material exhibits a distinct spherical nanoparticle morphology with a size of approximately 500 nm. The surface is rough and porous, which is beneficial for electrolyte penetration and exposure of active sites.

[0052] Figure 5 This is a high-magnification transmission electron microscope (HRTEM) image of the TDC-COF obtained in Example 1 of this invention. The results show that clear lattice fringes can be observed, corresponding to the (100) crystal plane of COF, with a crystal plane spacing of approximately 0.348 nm, confirming that the material has a typical crystal structure.

[0053] Figure 6 This is an elemental distribution diagram of the TDC-COF obtained in Example 1 of the present invention. The results show that carbon, nitrogen, and sulfur are uniformly distributed in the material, with no elemental segregation, indicating that the composition of the covalent organic framework is uniform.

[0054] Figure 7 This is a TEM image of the bTDC-COF obtained in Example 2 of the present invention. The results show that the material also exhibits a smooth nanosphere morphology with a size of approximately 400 nm and a uniform structure.

[0055] Figure 8 The image shows the HRTEM image of bTDC-COF obtained in Example 2 of this invention. The results show that the material has a distinct lamellar structure, clear lattice fringes, an interplanar spacing of approximately 0.358 nm, and good crystallinity.

[0056] Figure 9 This is an elemental distribution diagram of the bTDC-COF obtained in Example 2 of the present invention. The results show that carbon, nitrogen, and sulfur elements are uniformly distributed, proving that the bithiophene-based COF structure is homogeneous.

[0057] Figure 10 This is a TEM image of tTDC-COF obtained in Example 3 of the present invention. The results show that the material is a multi-level layered nanoflower structure with a relatively large diameter of approximately 700 nm.

[0058] Figure 11 The image shown is an HRTEM image of the tTDC-COF obtained in Example 3 of this invention. The results show that the lattice fringes are clear, the interplanar spacing matches the unit cell structure, and the crystallinity is high.

[0059] Figure 12 This is an elemental distribution diagram of tTDC-COF obtained in Example 3 of the present invention. The results show that the elements are uniformly distributed, indicating that the terthiophene-based COF also has a uniform composition.

[0060] The electrochemical oxygen reduction performance of the three COF catalysts prepared in Examples 1, 2, and 3 was tested. The tests were conducted in a 0.1 M KOH aqueous solution using a standard three-electrode system, with a rotating ring disk electrode as the working electrode, a mercury oxide electrode as the reference electrode, and a graphite rod as the counter electrode. The catalyst ink was prepared as follows: 4 mg of catalyst powder and 2 mg of carbon black were added to 1400 μL of ethanol and 15 μL of 5 wt% Nafion solution, and ultrasonically dispersed for 1 h. 13 μL of the dissolved oxygen was then dropped onto a glassy carbon disk electrode (0.2475 cm²). 2 Before testing, pre-activation was performed using cyclic voltammetry (60 scans, scan rate 50 mV / s). ORR polarization curves were acquired using linear scan voltammetry in O2-saturated 0.1 M KOH solution, with a potential range of 1.1–0 V (relative to the reversible hydrogen electrode), a scan rate of 10 mV / s, and a rotation speed of 1600 rpm. Hydrogen peroxide selectivity and electron transfer number were calculated using disk current and ring current. The collection efficiency of the rotating ring-disk electrode was 0.37, as shown in the figure. Figures 13-16 As shown.

[0061] Figure 13 The diagram shows the electron transfer number and H2O2 selectivity of the three COF catalysts of this invention. The results show that all three COF catalysts (TDC-COF, bTDC-COF, and tTDC-COF) exhibit stable electrocatalytic oxygen reduction reaction characteristics within the potential range of 0–0.6 V. Among them, bTDC-COF has an electron transfer number of approximately 2.1–2.2 and an H2O2 selectivity exceeding 90%, closest to the 2-electron transfer pathway, making it the catalyst with the highest H2O2 selectivity among the three. TDC-COF also predominates on the 2-electron pathway but with slightly lower selectivity. tTDC-COF has an electron transfer number of approximately 2.7–2.8 and an H2O2 selectivity of approximately 60%, favoring a mixed 2-electron and 4-electron pathway for oxygen reduction. Based on the above electrochemical performance tests, bTDC-COF exhibits the best selectivity for oxygen reduction to hydrogen peroxide.

[0062] Figure 14 The following are oxygen reduction LSV curves for the three COF catalysts of this invention. The results show that the onset potential of the TDC-COF in Example 1 is approximately 0.76 V (relative to the reversible hydrogen electrode), and the limiting diffusion current density is approximately -2.5 mA / cm². 2 The onset potential of the bTDC-COF in Example 2 is approximately 0.77 V, and the limiting current density is approximately -2.55 mA / cm². 2 The onset potential of the tTDC-COF in Example 3 was approximately 0.62 V, but the limiting current density was significantly lower than that in Examples 1 and 2, at approximately -2.18 mA / cm². 2Among them, bTDC-COF in Example 2 exhibited the best oxygen reduction activity.

[0063] Figure 15 The figure shows the Faradaic efficiency (FE) of the three COF catalysts of this invention at different potentials for the production of H2O2. The results show that the tTDCC-COF of Example 3 has the lowest FE, followed by the TDC-COF of Example 1. The bTDC-COF catalyst of Example 2 has the highest hydrogen peroxide Faradaic efficiency at different potentials, indicating that the bithiophene structure facilitates the reduction of oxygen to produce hydrogen peroxide.

[0064] Figure 16 This is a durability test diagram of the TDC-COF obtained in Example 1 of the present invention. The results show that in the 50-hour long-term stability test of the rotating ring-disk electrode (RRDE), the disk electrode current density is approximately -2.3 to -2.0 mA / cm². 2 The current remained stable with no significant attenuation; the ring electrode current consistently remained at 0.7 mA / cm². 2 The calculated H2O2 selectivity remained above 90%, indicating that the catalyst exhibited excellent catalytic stability during the long-term oxygen reduction reaction. Furthermore, the reaction pathway was mainly based on efficient 2-electron transfer to generate hydrogen peroxide, with negligible side reactions, demonstrating good potential for practical application.

[0065] Therefore, the method for preparing a highly selective electrosynthetic covalent organic framework for hydrogen peroxide has the following advantages: (1) The preparation method is simple and feasible, the reaction conditions are mild, the yield is high (85% to 90%), and no precious metals or complex catalysts are required.

[0066] (2) The resulting material has high crystallinity and large specific surface area, which is conducive to electrolyte mass transfer and full exposure of active sites.

[0067] (3) The material exhibits excellent catalytic activity, high two-electron selectivity and good stability in electrocatalytic oxygen reduction reaction, and can replace platinum-based catalysts in fuel cells and metal-air batteries.

[0068] In summary, this invention successfully prepared a series of covalent organic framework materials via a solvothermal method, which have broad application prospects in the field of energy electrocatalysis.

[0069] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing a highly selective electrosynthetic covalent organic framework for hydrogen peroxide, characterized in that, Includes the following steps: Step 1: Add N,N,N',N'-tetratetra(4-aminophenyl)-1,4-phenylenediamine (TPDA) and thiophene dialdehyde monomers to the reaction vessel at a molar ratio of 1:

2. Step 2: Add a mixed solvent of o-dichlorobenzene and n-butanol to the reaction vessel and disperse it evenly by ultrasonication; Step 3: Perform freeze-thaw cycle degassing on the reaction vessel, then evacuate and seal it; Step 4: Heat the sealed reaction vessel to 120°C and react for 3 days; Step 5: After the reaction is complete, separate the precipitate, wash and dry it to obtain the covalent organic framework material.

2. The preparation method according to claim 1, characterized in that: The thiophene dialdehyde monomer is any one of 2,5-thiophene dicarboxaldehyde (TDC), 2,2'-bithiophene-5,5'-dicarboxaldehyde (bTDC), or 2,2':5',2''-terthiophene-5,5''-dicarboxaldehyde (tTDC).

3. The preparation method according to claim 1, characterized in that: In step two, the volume ratio of o-dichlorobenzene to n-butanol is 1:

1.

4. The preparation method according to claim 1, characterized in that: In step two, the amount of the mixed solvent used is 8 to 12 mL per 0.3 mmol TPDA.

5. The preparation method according to claim 1, characterized in that: In step three, the freeze-thaw cycle treatment consists of three freeze-thaw cycles; the vacuum is evacuated to an internal pressure of 0.005 mmHg.

6. The preparation method according to claim 1, characterized in that: In step five, the washing is performed sequentially with N,N-dimethylformamide (DMF) and acetone; the drying is performed under reduced pressure at 80°C for 24 h, or further with Soxhlet extraction with dichloromethane followed by drying.

7. The preparation method according to claim 1, characterized in that: The yield of the product obtained in step five is 85% to 90%.

8. A method for preparing a highly selective electrosynthetic covalent organic framework for hydrogen peroxide, characterized in that: Prepared by the method described in any one of claims 1 to 7.

9. The covalent organic framework material according to claim 8, characterized in that: The material is an orange-red fluffy powder with a typical crystalline structure of a covalent organic framework.

10. The application of the covalent organic framework material according to claim 8 or 9 in the electrocatalytic oxygen reduction reaction.

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