Preparation method of magnesium-modified porous material catalyst and application of magnesium-modified porous material catalyst in catalytic production of hydrogen peroxide

Abstract

The invention provides a preparation method of a magnesium-modified porous material catalyst and application of the magnesium-modified porous material catalyst in catalytic production of hydrogen peroxide. The preparation method comprises the following steps: successively dispersing a porous covalent triazine framework containing a 2, 2 '-dipyridyl unit and soluble magnesium salt in a water polar solvent, then adding a certain amount of acid and a chelating agent, carrying out a chelation reaction at a certain temperature, and finally filtering, washing and drying to obtain a target product of chelating magnesium salt in inner holes of the covalent triazine framework. The magnesium modified porous material is simple in preparation process and small in environmental pollution, and has excellent performance of catalyzing reduction of oxygen into hydrogen peroxide. And 85-88% of two-electron selectivity (the rest is reduced into water, and no pollution is caused) and relatively high reaction activity are shown.

Description

Technical Field

[0001] The invention relates to the preparation of an electrocatalyst, and is applied to fields such as green industrial synthesis represented by electrocatalytic production of hydrogen peroxide. Background Art

[0002] Hydrogen peroxide (H2O2) is an important chemical product with the advantages of high active oxygen content, strong oxidizing property and environmental protection. It is widely used in many fields such as chemical synthesis, paper bleaching, wastewater treatment, disinfection and sterilization, and fuel cells. At present, the anthraquinone method is generally used in the industrial preparation of hydrogen peroxide. This method has the disadvantages of high energy consumption and high pollution, and requires the use of hydrogen as a hydrogen source, which cannot meet the requirements of green and sustainable development. In contrast, the electrocatalytic two-electron oxygen reduction reaction (2e - ORR) can use protons in water as the source of hydrogen and realize hydrogen peroxide synthesis under the drive of electrical energy, which is energy-saving and environmentally friendly and has great development potential. - ORR electrosynthesis of hydrogen peroxide has attracted extensive attention. - In the ORR reaction process, catalysts play a very critical role. Efficient catalysts can effectively reduce the overpotential of the reaction and improve the activity and selectivity of the reaction. Covalent triazine frameworks (CTFs) are nitrogen-containing aromatic polymer frameworks with triazine rings. Their highly porous properties give them a sufficiently large specific surface area, which provides a large number of adsorption sites for gallium salts. Their excellent stability gives CTFs great potential as ORR electrocatalyst substrate materials. It is reported that Mg-N x Magnesium single-atom materials with magnesium as the central unit can be used as highly active catalysts for oxygen reduction reactions (Nature Communications, DOI:10.1038 / s41467-020-14565-w), but the report uses a pyrolysis strategy to prepare the catalyst, the preparation process used is relatively complex, the reaction conditions are relatively harsh; and the reaction selectivity of oxygen reaction to generate hydrogen peroxide is low. In contrast, the present invention uses a solvothermal method, which not only has a simple preparation process, mild preparation conditions, and can obtain the target product in one step; and the designed catalyst has good selectivity for two-electron reduction of oxygen to hydrogen peroxide. Summary of the invention

[0003] In view of the above technical problems, the present invention provides a method for preparing magnesium-modified porous materials by a non-pyrolysis method and using the materials for catalytic production of hydrogen peroxide, comprising the following steps: The first step: uniformly dispersing the covalent triazine framework CTF and the soluble magnesium salt in deionized water to obtain a dispersion.

[0004] The mass ratio of CTF to soluble magnesium salt is 1:0.5~1:5, and the dispersion of CTF in deionized water is 1~3 g L -1.

[0005] The CTF is prepared by mixing [2,2'-bipyridyl]-5,5'-dimethylmethanol and benzene monomer biphenyl-4,4'-dicarboxamidine or terephthalamidine, adding cesium carbonate, and washing and freeze-drying the obtained solid product after precipitation.

[0006] The soluble magnesium salts include magnesium acetate, magnesium nitrate hexahydrate and magnesium sulfate.

[0007] Step 2: Add pH adjuster and chelating agent to the above dispersion in batches for multiple times, and after ultrasonic homogenization, heat and stir to react.

[0008] The pH value of the dispersion is 0-6, and the pH regulator used includes nitric acid, acetic acid, etc. The chelating agents added in the present invention are some stable ligands that can provide oxygen atom coordination. CTF contains a large number of bidentate nitrogen binding sites, and the entire environment is negatively charged; the chelating agent contains oxygen coordination sites, and some of them are weakly acidic (negatively charged after ionization). Therefore, the positively charged magnesium ions are well complexed with CTF and the chelating agent, and the prepared complex has a stable structure.

[0009] The heating temperature is 30~100 ℃ and the time is 6~24 h.

[0010] Step 3: Obtain the catalyst through filtering, washing and drying.

[0011] In some embodiments, one or more non-aqueous polar solvents such as methanol, tetrahydrofuran, NN-dimethylformamide (DMF) are selected and washed repeatedly for at least three times, and after filtering, the sample is placed in a vacuum drying oven and dried for 6 to 24 hours to obtain a catalyst. Taking phenanthrenequinone as an example, the preparation process and corresponding structural formula of the prepared magnesium-modified porous material are shown as follows:

[0012] The above preparation process is simple and has little pollution to the environment. The prepared magnesium-modified porous material has excellent electrocatalytic performance of oxygen reduction to hydrogen peroxide, showing 85-89% two-electron selectivity and high reaction activity. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 , a is the ORR diagram of the sample in Example 1, b is the hydrogen peroxide selectivity of the sample in Example 1, and c is the number of transferred electrons of the sample in Example 1.

[0014] Figure 2 , a is the ORR diagram of the sample in Example 2, b is the hydrogen peroxide selectivity of the sample in Example 2, and c is the number of transferred electrons of the sample in Example 2.

[0015] Figure 3 , a is the ORR diagram of the sample in Example 3, b is the hydrogen peroxide selectivity of the sample in Example 3, and c is the number of transferred electrons of the sample in Example 3.

[0016] Figure 4 , a is the ORR diagram of the sample in Example 4, b is the hydrogen peroxide selectivity of the sample in Example 4, and c is the number of transferred electrons of the sample in Example 4.

[0017] Figure 5 , a is the ORR diagram of the sample in Example 5, b is the hydrogen peroxide selectivity of the sample in Example 5, and c is the number of transferred electrons of the sample in Example 5.

[0018] Figure 6 , are the XRD diagrams of Examples 1 and 2. DETAILED DESCRIPTION

[0019] Characterization conditions: The ORR test method in the embodiment of the present invention is as follows: 980 μL of water, 980 μL of ethanol and 40 μL of Nafion are taken and mixed evenly, and then 1 mg of sample is taken and added to the solution, and ultrasonicated for 2 h to make it evenly dispersed. The obtained slurry is then dropped on the rotating ring disk electrode with a loading of 0.02 mg cm -2 , and then a three-electrode system using a reference electrode (mercuric oxide), a counter electrode (carbon rod), and a working electrode was tested in an oxygen-saturated 0.1 M KOH solution.

[0020] Preparation of covalent triazine framework (CTF): In a round-bottom flask, add [2,2'-bipyridyl]-5,5'-dimethylmethanol, benzene monomer biphenyl-4,4'-dicarboxamidine or terephthalamidine and cesium carbonate to the dimethyl sulfoxide solution. The precipitate in the yellow suspension is washed thoroughly with dilute hydrochloric acid to remove the residual cesium carbonate, and then washed several times with water, ethanol and tetrahydrofuran. After filtration, the solid is freeze-dried to obtain yellow CTF.

[0021] Embodiment 1: In a round-bottom flask (25 mL), add 1.2 ml of Mg(CH3COO)2 solution (120.32 mg Mg(CH3COO)2 dissolved in 100 ml water), 5 mg CTF, and 2 ml of deionized water, and ensure that the dispersion of Mg(CH3COO)2 and CTF in up water is 0.36 g L -1 , 1.25 g L -1 ; Then add 0.5 ml of nitric acid to adjust the pH value of the solution to 0-3. The dispersion of nitric acid in the system is 0.02 g L -1, and then the mixture was ultrasonically homogenized, heated and stirred at 65°C for 2 hours, and then the ligand o-naphthol was added, and 100 ul o-naphthol solution (2.03 mg o-naphthol was dissolved in 900 ul water) was added every 30 minutes, and the cycle was repeated three times, and the heating and stirring at 65°C was continued for 6 hours. After the reaction was completed, the precipitate was filtered to obtain the precipitate, and the precipitate was washed several times with methanol and ethanol to remove the residual Mg(CH3COO)2 in the precipitate, and finally placed in a vacuum drying oven for 12 hours to obtain a covalent triazine framework / magnesium salt complex.

[0022] Figure 1 a is the ORR diagram of the sample in Example 1, with an onset potential of 0.74 V vs RHE and a limiting current density of 1.04 mA cm -2 , the half-wave potential is 0.54 V vs RHE, Figure 1 b is the hydrogen peroxide selectivity of the sample in Example 1, and it can be concluded that the hydrogen peroxide selectivity is 88.42%. Figure 1 c is the number of transferred electrons of the sample in Example 1, and it can be concluded that the number of transferred electrons is 2.23.

[0023] Figure 6 This is the XRD diagram of the sample in Example 1. Combined with the standard card, it can be concluded that magnesium does not form compounds such as oxide salts, and magnesium salts and magnesium acetate exist, indicating that magnesium is chelated with CTF and small molecule chelating agents in the form of single atoms, thereby roughly maintaining the crystal structure of CTF, and also indicating the successful preparation of the covalent triazine framework / magnesium salt complex.

[0024] Embodiment 2: In a round-bottom flask (25 mL), add 1.2 ml of Mg(CH3COO)2 solution (120.32 mgMg(CH3COO)2 dissolved in 100 ml water), 5 mg CTF, and 2 ml of deionized water; then add 0.5 ml of nitric acid to adjust the pH value of the solution to 0-3. The dispersion of nitric acid in the system is 0.02 g L -1 , and then the mixture was ultrasonically homogenized, heated and stirred at 65°C for 2 hours, and then the ligand phenanthrenequinone was added, and 100 ul phenanthrenequinone solution (7.92 mg dissolved in 900 ul water) was added every 30 minutes, and the cycle was repeated three times, and the heating and stirring at 65°C were continued for 6 hours. After the reaction was completed, the precipitate was filtered to obtain the precipitate, and the precipitate was washed several times with methanol and ethanol to remove the residual Mg(CH3COO)2 in the precipitate, and finally placed in a vacuum drying oven for 12 hours to obtain a covalent triazine framework / magnesium salt complex.

[0025] Figure 2 a is the ORR diagram of the sample in Example 2, with an initial potential of 0.73 V vs RHE and a limiting current density of 1.11 mA cm -2, the half-wave potential is 0.56 V vs RHE, Figure 2 b is the hydrogen peroxide selectivity of the sample in Example 2, and it can be concluded that the hydrogen peroxide selectivity is 87.82%. Figure 2 c is the number of transferred electrons of the sample in Example 2, and it can be concluded that the number of transferred electrons is 2.24.

[0026] Figure 6 This is the XRD diagram of the sample in Example 1. Combined with the standard card, it can be concluded that magnesium does not form compounds such as oxide salts, indicating that magnesium is chelated with CTF and small molecule chelating agents in the form of single atoms, thereby substantially maintaining the crystal structure of CTF, and also indicating the successful preparation of the covalent triazine framework / magnesium salt complex.

[0027] Embodiment 3: In a round-bottom flask (25 mL), add 1.2 ml of Mg(CH3COO)2 solution (120.32 mgMg(CH3COO)2 dissolved in 100 ml water), 5 mg CTF, and 2 ml of deionized water; then add 0.5 ml of nitric acid to adjust the pH value of the solution to 0-3. The dispersion of nitric acid in the system is 0.02 g L -1 , and then the mixture was ultrasonically homogenized, heated and stirred at 65°C for 2 hours, and then the ligand salicylic acid was added, and 100 ul of salicylic acid solution (5.25 mg dissolved in 900 ul of water) was added every 30 minutes, and the cycle was repeated three times, and the heating and stirring at 65°C was continued for 6 hours. After the reaction was completed, the precipitate was filtered to obtain the precipitate, and the precipitate was washed several times with methanol and ethanol to remove the residual Mg(CH3COO)2 in the precipitate, and finally placed in a vacuum drying oven for 12 hours to obtain a covalent triazine framework / magnesium salt complex.

[0028] Figure 3 a is the ORR diagram of the sample in Example 3, with an onset potential of 0.78 V vs RHE and a limiting current density of 1.71 mA cm -2 , the half-wave potential is 0.78 V vs RHE, Figure 3 b is the hydrogen peroxide selectivity of the sample in Example 3, and it can be concluded that the hydrogen peroxide selectivity is 65.7%. Figure 3 c is the number of transferred electrons of the sample in Example 3, and it can be concluded that the number of transferred electrons is 2.67. FIG. 9 is the XRD diagram of the sample in Example 3. Combined with the standard card, it can be concluded that there are magnesium salts and magnesium acetate, proving the successful preparation of CTF / magnesium acetate.

[0029] Embodiment 4: In a round-bottom flask (25 mL), add 1.2 ml of Mg(CH3COO)2 solution (120.32 mgMg(CH3COO)2 dissolved in 100 ml water), 5 mg CTF, and 2 ml of deionized water; then add 0.5 ml of nitric acid to adjust the pH value of the solution to 0-3. The dispersion of nitric acid in the system is 0.02 g L -1 , and then the mixture was ultrasonically homogenized, heated and stirred at 65°C for 2 hours, and then the ligand benzoic acid was added, and 100 ul of benzoic acid solution (4.8 mg dissolved in 900 ul of water) was added every 30 minutes, and the cycle was repeated three times, and the heating and stirring at 65°C were continued for 6 hours. After the reaction was completed, the precipitate was filtered to obtain the precipitate, and the precipitate was washed several times with methanol and ethanol to remove the residual Mg(CH3COO)2 in the precipitate, and finally placed in a vacuum drying oven for 12 hours to obtain a covalent triazine framework / magnesium salt complex.

[0030] Figure 4 a is the ORR diagram of the sample in Example 4, with an initial potential of 0.74 V vs RHE and a limiting current density of 1.14 mAcm -2 , the half-wave potential is 0.51 V vs RHE, Figure 4 b is the hydrogen peroxide selectivity of the sample in Example 4, and it can be concluded that the hydrogen peroxide selectivity is 78.16%, Figure 4 c is the number of transferred electrons of the sample in Example 4, and it can be concluded that the number of transferred electrons is 2.44.

[0031] Embodiment 5: In a round-bottom flask (25 mL), add 1.2 ml of Mg(CH3COO)2 solution (120.32 mgMg(CH3COO)2 dissolved in 100 ml water), 5 mg CTF, and 2 ml of deionized water; then add 0.5 ml of nitric acid to adjust the pH value of the solution to 0-3. The dispersion of nitric acid in the system is 0.02 g L -1 , and then the mixture was ultrasonically homogenized, heated and stirred at 65°C for 2 hours, and then the ligand salicyl alcohol was added, and 100 ul of salicyl alcohol solution (4.72 mg dissolved in 900 ul water) was added every 30 minutes, and the cycle was repeated three times, and the heating and stirring at 65°C was continued for 6 hours. After the reaction was completed, the precipitate was filtered to obtain the precipitate, and the precipitate was washed several times with methanol and ethanol to remove the residual Mg(CH3COO)2 in the precipitate, and finally placed in a vacuum drying oven for 12 hours to obtain a covalent triazine framework / magnesium salt complex.

[0032] Figure 5 a is the ORR diagram of the sample in Example 5, with an initial potential of 0.7 V vs RHE and a limiting current density of 0.99 mAcm -2, the half-wave potential is 0.57 V vs RHE, Figure 5 b is the hydrogen peroxide selectivity of the sample in Example 5, and it can be concluded that the hydrogen peroxide selectivity is 72.5%. Figure 5 c is the number of transferred electrons of the sample in Example 5, and it can be concluded that the number of transferred electrons is 2.55.

Claims

1. A method for preparing a magnesium-modified porous material catalyst, characterized in that: The method comprises the following preparation steps: S1, uniformly dispersing the covalent triazine framework CTF and the soluble magnesium salt in deionized water to obtain a dispersion; S2, adding a pH adjuster to the dispersion in S1, and then adding a chelating agent, after ultrasonic homogenization, heating and stirring to react; S3, filtering, washing and drying to obtain a catalyst.

2. The preparation method according to claim 1, characterized in that The mass ratio of CTF to soluble magnesium salt described in S1 is 1:0.5~1:5, and the dispersion of CTF in deionized water is 1~3 g L -1 .

3. The preparation method according to claim 2, characterized in that: The CTF is prepared by mixing [2,2'-bipyridyl]-5,5'-dimethylmethanol and benzene monomer biphenyl-4,4'-dicarboxamidine or terephthalamidine, adding cesium carbonate, and washing and freeze-drying the obtained solid product after precipitation.

4. The preparation method according to claim 1, characterized in that: The soluble magnesium salt includes any one of magnesium acetate, magnesium nitrate and magnesium sulfate.

5. The preparation method according to claim 1, characterized in that: The chelating agent comprises one or more of benzoic acid, phenanthrenequinone, o-naphthol, salicylic acid and salicyl alcohol; the molar ratio of the amount thereof to the atomic weight of magnesium metal in the soluble magnesium salt is 1.0-2.0:

1.

6. The preparation method according to claim 1, characterized in that The heating temperature in S2 is 30~100 ℃ and the time is 6~24 h.

7. The covalent triazine framework / magnesium salt complex prepared according to the preparation method according to any one of claims 1 to 6 is used as a catalyst for producing hydrogen peroxide.

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