Method for the preparation of covalently grafted phenol formaldehyde resins with cobalt porphyrins and their use in the electrocatalytic reduction of oxygen

Abstract

This invention discloses a method for preparing cobalt porphyrin covalently grafted onto phenolic resin and its application in electrocatalytic oxygen reduction. First, a cobalt porphyrin with a flexible alkyl chain is prepared. Then, the phenolic hydroxyl groups on the phenolic resin nucleophilically replace the bromine at the end of the flexible alkyl chain of the cobalt porphyrin, thereby covalently grafting the cobalt porphyrin onto the phenolic resin. Compared to the simple physical mixing of 5,10,15,20-tetra(pentafluorophenyl)cobalt porphyrin and phenolic resin, and the electrocatalytic oxygen reduction performance of unmodified phenolic resin, the cobalt porphyrin of this invention, due to its flexible alkyl chain, provides a higher local proton concentration for the covalently linked cobalt porphyrin with phenolic hydroxyl groups on the phenolic resin. Furthermore, during the electrocatalytic oxygen reduction process, the protons from the phenolic hydroxyl groups can act on the porphyrin metal center, accelerating the proton transfer rate and thus improving the performance of the electrocatalytic oxygen reduction reaction.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalytic oxygen reduction technology. Specifically, it involves first modifying a cobalt porphyrin with a flexible alkyl chain ending in bromine, then using the nucleophilic phenolic hydroxyl group on a phenolic resin to replace the bromine at the end of the flexible alkyl chain of the cobalt porphyrin, and covalently grafting the cobalt porphyrin onto the phenolic resin. The resulting cobalt porphyrin covalently grafted phenolic resin is used for electrocatalytic oxygen reduction. Background Technology

[0002] The ever-increasing demand for fossil fuels has led to energy depletion and environmental pollution from their combustion, problems that cannot be ignored. Developing clean, efficient, and renewable energy sources is urgently needed. Zinc-air batteries, due to their environmental friendliness, high energy density, and low cost, have great potential for energy storage and conversion. The performance of zinc-air batteries primarily depends on the oxygen reduction reaction (ORR) that occurs at the air cathode during discharge. Although noble metal catalysts such as Pt / C, Ir / C, or RuO2 exhibit excellent ORR performance, their scarcity and high cost limit their large-scale industrial application. Therefore, developing inexpensive and efficient ORR catalysts is crucial for the future development of zinc-air batteries.

[0003] Porphyrins are considered ideal electrocatalytic materials due to their rich redox properties, modifiable functional groups, and excellent ORR activity demonstrated in numerous studies. Introducing phenolic hydroxyl groups around porphyrins to increase the local proton concentration can accelerate the proton transfer rate during electrocatalytic oxygen reduction, thereby improving the performance of electrocatalytic ORR. Phenolic resins are polymers obtained by reacting phenol or substituted phenols with formaldehyde, and these polymer chains contain a large number of phenolic hydroxyl groups. Currently reported metalloporphyrin-phenolic resins are formed by the dehydration condensation of m-tetra(p-hydroxyphenyl)metalloporphyrin and phenol as hydroxyl donors with formaldehyde. In compounds synthesized by this method, the porphyrin molecule is located in a rigid environment, and the phenolic hydroxyl groups on the phenolic resin are far from the porphyrin metal center. During electrocatalytic oxygen reduction, it is difficult for protons to act on the metal center. Therefore, redox activity can only be acquired by calcination. Summary of the Invention

[0004] To address the above problems, this invention proposes a method for preparing cobalt porphyrin covalently grafted phenolic resin and its application in electrocatalytic oxygen reduction.

[0005] The synthetic route and specific preparation method of the cobalt porphyrin covalently grafted phenolic resin used in this invention are as follows:

[0006] In the formula, n takes the value of an integer from 4 to 12, and R represents... The values ​​of m and p are integers from 0 to 8, and m + p = n - 4.

[0007] Step 1: Add p-hydroxybenzaldehyde, pentafluorobenzaldehyde and pyrrole to dichloromethane, stir until homogeneous, add boron trifluoride diethyl ether (BF3Et2O) and stir for 3-6 hours, then add 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and stir for 1-3 hours. Then purify by column chromatography to obtain compound a, which is chemically named 5-(4-hydroxyphenyl)-10,15,20-tris(pentafluorophenyl)porphyrin;

[0008] Step 2: Compound a, 1,6-dibromohexane and potassium carbonate were added to N,N-dimethylformamide and stirred at room temperature for 8-10 hours. The mixture was then purified by column chromatography to obtain compound b, which was chemically named 5-[4-[(6-bromohexyl)oxy]phenyl]-10,15,20-tris(pentafluorophenyl)porphyrin.

[0009] Step 3: Add compound b, cobalt acetate tetrahydrate and sodium acetate to anhydrous and oxygen-free N,N-dimethylformamide, bubble with argon for 20-30 minutes, reflux at 100°C for 2-4 hours, then extract, dry the organic phase with anhydrous sodium sulfate, and purify by column chromatography to obtain compound Co-1064, chemically named 5-[4-[(6-bromohexyl)oxy]phenyl]-10,15,20-tris(pentafluorophenyl)cobalt porphyrin;

[0010] Step 4: Dissolve compound Co-1064, phenolic resin (PR), and potassium carbonate in N,N-dimethylformamide (DMF) and stir at room temperature for 8-10 hours. After the reaction is complete, filter the solution and concentrate the filtrate by rotary evaporation to remove N,N-dimethylformamide. Wash the concentrated solid product with water, ethanol, and dichloromethane in sequence and then dry it under vacuum to obtain compound Co-1064-PR, i.e., cobalt porphyrin covalently grafted phenolic resin.

[0011] In step 1 above, the preferred molar ratio of p-hydroxybenzaldehyde, pentafluorobenzaldehyde, pyrrole, boron trifluoride ether, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone is 1:3:4:0.5 to 1:1 to 2.

[0012] In step 2 above, the preferred molar ratio of compound a, 1,6-dibromohexane, and potassium carbonate is 1:1 to 5:1 to 10.

[0013] In step 3 above, the preferred molar ratio of compound b, cobalt acetate tetrahydrate, and sodium acetate is 1:1 to 10:1 to 10.

[0014] In step 4 above, the preferred molar ratio of compound Co-1064, PR, and potassium carbonate is 1:20 to 100:1 to 10. This invention also protects the application of cobalt porphyrin covalently grafted onto phenolic resin for electrocatalytic oxygen reduction.

[0015] The Co-1064-PR prepared by the method of the present invention can be used for electrocatalytic oxygen reduction. The specific method is as follows: Co-1064-PR and carbon nanotubes are mixed evenly at a mass ratio of 1:1, and then dispersed together with Nafion in acetonitrile to obtain a slurry. The slurry is then coated on the electrode surface and dried before electrocatalytic oxygen reduction is performed.

[0016] The beneficial effects of this invention are as follows:

[0017] 1. In this invention, cobalt porphyrin with flexible alkyl chains is grafted onto phenolic resin. The phenolic hydroxyl groups on the phenolic resin provide a higher local proton concentration for the covalently linked cobalt porphyrin, thereby accelerating the proton transfer rate during the electrocatalytic oxygen reduction process. As a result, the performance of Co-1064-PR electrocatalytic ORR is significantly better than that of the comparative sample PR and Co-TPFPP+PR.

[0018] 2. Compared with commercial catalysts containing precious metals such as Pt / C, Ir / C, and RuO2, the ORR catalyst synthesized by covalently grafting cobalt porphyrin onto phenolic resin in this invention has a lower cost and is more suitable for large-scale industrial applications. It also provides a new approach for further developing the fusion of polymer molecules with porphyrin molecules as a high-efficiency catalyst. Attached Figure Description

[0019] Figure 1 These are the UV-vis spectra of Co-1064-PR, Co-1064, and PR obtained in Example 1.

[0020] Figure 2 These are the IR spectra of Co-1064-PR and Co-1064 obtained in Example 1.

[0021] Figure 3 The ORR RRDE spectra of the Co-1064-PR, Co-TPFPP+PR and PR slurries prepared in Example 1 were tested on a rotating ring disk electrode. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to these embodiments.

[0023] Example 1

[0024] The synthetic route and specific preparation method of cobalt porphyrin covalently grafted phenolic resin in this embodiment are as follows:

[0025]

[0026]

[0027] In the formula, n takes the value of an integer from 4 to 12, and R represents... The values ​​of m and p are integers from 0 to 8, and m + p = n - 4.

[0028] Step 1: Add 300 mL of dichloromethane to a 500 mL round-bottom flask equipped with a stir bar, then add p-hydroxybenzaldehyde (367 mg, 3 mmol). After it is completely dissolved, add pentafluorobenzaldehyde (1.1 mL, 9 mmol) and pyrrole (0.83 mL, 12 mmol) sequentially. Stir for 10 min, then slowly add boron trifluoride diethyl ether (0.38 mL, 3 mmol) dropwise to the mixed solution. After stirring for 3 h, the solution changes from pale yellow to dark brown. Then, add 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (681 mg, 3 mmol) to the solution and stir for 1 h. The solution fluoresces red under ultraviolet light. Remove dichloromethane from the reaction solution using a rotary evaporator. The crude product is purified by column chromatography (eluent ratio of petroleum ether:dichloromethane = 4:1) to obtain purple crystalline compound a (594 mg, yield 22%).

[0029] Step 2: Weigh compound a (450 mg, 0.5 mmol) and dissolve it in 20 mL of N,N-dimethylformamide. Then add 1,6-dibromohexane (0.38 mL, 2.5 mmol) and potassium carbonate (690 mg, 5 mmol). Stir at room temperature (25 °C) for 10 h. Remove N,N-dimethylformamide by vacuum distillation and purify by column chromatography (eluent ratio of petroleum ether: dichloromethane = 3:1) to obtain purple crystalline compound b (357 mg, yield 67%).

[0030] Step 3: Compound b (106 mg, 0.1 mmol), Co(OAc)₂·4H₂O (249 mg, 1 mmol), and NaOAc (82 mg, 1 mmol) were dissolved in dry N,N-dimethylformamide and degassed by bubbling with argon for 30 min. The resulting reaction solution was refluxed at 100 °C for 3 h. After cooling to room temperature, the mixture was extracted with water and dichloromethane. The crude product obtained from the extraction was dried over anhydrous sodium sulfate and purified by column chromatography (eluent ratio of petroleum ether:dichloromethane = 4:1). The product was recrystallized from tetrahydrofuran and n-hexane to give compound Co-1064 (97 mg, yield 87%).

[0031] Step 4: Compound Co-1064 (5 mg, 0.0045 mmol), phenolic resin (28 mg, 0.23 mmol), and potassium carbonate (6 mg, 0.043 mmol) were dissolved in 7 mL of N,N-dimethylformamide and stirred at room temperature for 10 h. After the reaction was completed, the mixture was filtered, and the N,N-dimethylformamide was removed by rotary evaporation. The resulting solid product was washed three times with water, ethanol, and dichloromethane, and dried in a vacuum drying oven at 70 °C for 24 h to obtain a reddish-brown solid compound Co-1064-PR (20 mg, yield 62%).

[0032] The Co-1064-PR synthesized in this embodiment was characterized and tested. For example... Figure 1 As shown, in the UV-vis absorption spectrum, Co-1064-PR exhibits characteristic absorption peaks of porphyrin compounds at 410 nm and 534 nm (Soret and Q bands), and shows a similar characteristic absorption peak to PR at 300 nm. Co-1064 exhibits characteristic absorption peaks of porphyrin compounds at 409 nm and 525 nm (Soret and Q bands), and Co-1064-PR shows a redshift relative to Co-1064. Figure 2 As shown, in the IR spectrum, Co-1064-PR at 1208 cm⁻¹ -1 The absorption peaks observed are attributed to the stretching vibrations of the CO bonds on the phenolic resin chain in the long phenolic resin chain; Co-1064 shows a peak at 803 cm⁻¹. -1 The absorption peaks that appeared were attributed to the stretching vibration of the C-Br bond on the cobalt porphyrin alkyl bromide. The presence of phenolic hydroxyl groups and the disappearance of bromine at the end of the alkyl chain in Co-1064-PR compared to Co-1064 prove that Co porphyrin was successfully covalently grafted onto the phenolic resin.

[0033] Example 2

[0034] The synthetic route of cobalt porphyrin covalently grafted phenolic resin in this embodiment is the same as that in Example 1, and the specific preparation method is as follows:

[0035] Step 1: Add 300 mL of dichloromethane to a 500 mL round-bottom flask equipped with a stir bar, then add p-hydroxybenzaldehyde (367 mg, 3 mmol). After it is completely dissolved, add pentafluorobenzaldehyde (1.1 mL, 9 mmol) and pyrrole (0.83 mL, 12 mmol) sequentially. Stir for 10 min, then slowly add boron trifluoride diethyl ether (0.19 mL, 1.5 mmol) dropwise to the mixed solution. After stirring for 6 h, the solution changes from pale yellow to dark brown. Then, add 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (1022 mg, 4.5 mmol) to the solution and stir for 3 h. The solution fluoresces red under ultraviolet light. Remove dichloromethane from the reaction solution using a rotary evaporator. The crude product is purified by column chromatography (eluent ratio of petroleum ether:dichloromethane = 4:1) to obtain purple crystalline compound a (648 mg, yield 24%).

[0036] Step 2: Weigh compound a (450 mg, 0.5 mmol) and dissolve it in 20 mL of N,N-dimethylformamide. Then add 1,6-dibromohexane (0.15 mL, 1 mmol) and potassium carbonate (345 mg, 2.5 mmol). Stir at room temperature (25 °C) for 10 h. Remove N,N-dimethylformamide by vacuum distillation and purify by column chromatography (eluent ratio of petroleum ether: dichloromethane = 3:1) to obtain purple crystalline compound b (229 mg, yield 43%).

[0037] Step 3: Compound b (106 mg, 0.1 mmol), Co(OAc)₂·4H₂O (125 mg, 0.5 mmol), and NaOAc (41 mg, 0.5 mmol) were dissolved in dry N,N-dimethylformamide and degassed by argon bubbling for 30 min. The resulting reaction solution was refluxed at 100 °C for 3 h. After cooling to room temperature, the solution was extracted with water and dichloromethane. The crude product obtained from the extraction was dried over anhydrous sodium sulfate and purified by column chromatography (eluent ratio of petroleum ether:dichloromethane = 4:1). The product was recrystallized from tetrahydrofuran and n-hexane to give compound Co-1064 (89 mg, yield 80%).

[0038] Step 4: Compound Co-1064 (5 mg, 0.0045 mmol), phenolic resin (55 mg, 0.45 mmol), and potassium carbonate (6 mg, 0.043 mmol) were dissolved in 7 mL of N,N-dimethylformamide and stirred at room temperature for 10 h. After the reaction was completed, the mixture was filtered, and the N,N-dimethylformamide was removed by rotary evaporation. The resulting solid product was washed three times with water, ethanol, and dichloromethane, and dried in a vacuum drying oven at 70 °C for 24 h to obtain a reddish-brown solid compound Co-1064-PR (47 mg, yield 79%).

[0039] Example 3

[0040] The synthetic route of cobalt porphyrin covalently grafted phenolic resin in this embodiment is the same as that in Example 1, and the specific preparation method is as follows:

[0041] Step 1: Add 300 mL of dichloromethane to a 500 mL round-bottom flask equipped with a stir bar, then add p-hydroxybenzaldehyde (367 mg, 3 mmol). After it is completely dissolved, add pentafluorobenzaldehyde (1.1 mL, 9 mmol) and pyrrole (0.83 mL, 12 mmol) sequentially. Stir for 10 min, then slowly add boron trifluoride diethyl ether (0.57 mL, 4.5 mmol) dropwise to the mixed solution. After stirring for 3 h, the solution changes from pale yellow to dark brown. Then, add 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (1022 mg, 4.5 mmol) to the solution and stir for 1 h. The solution fluoresces red under ultraviolet light. Remove dichloromethane from the reaction solution using a rotary evaporator. The crude product is purified by column chromatography (eluent ratio of petroleum ether:dichloromethane = 4:1) to obtain purple crystalline compound a (1093 mg, yield 27%).

[0042] Step 2: Weigh compound a (450 mg, 0.5 mmol) and dissolve it in 20 mL of N,N-dimethylformamide. Then add 1,6-dibromohexane (0.38 mL, 2.5 mmol) and potassium carbonate (690 mg, 5 mmol). Stir at room temperature (25 °C) for 10 h. Remove N,N-dimethylformamide by vacuum distillation and purify by column chromatography (eluent ratio of petroleum ether: dichloromethane = 3:1) to obtain purple crystalline compound b (357 mg, yield 67%).

[0043] Step 3: Compound b (106 mg, 0.1 mmol), Co(OAc)₂·4H₂O (249 mg, 1 mmol), and NaOAc (41 mg, 0.5 mmol) were dissolved in dry N,N-dimethylformamide and degassed by argon bubbling for 30 min. The resulting reaction solution was refluxed at 100 °C for 3 h. After cooling to room temperature, the solution was extracted with water and dichloromethane. The crude product obtained from the extraction was dried over anhydrous sodium sulfate and purified by column chromatography (eluent ratio of petroleum ether:dichloromethane = 4:1). The product was recrystallized from tetrahydrofuran and n-hexane to give compound Co-1064 (93 mg, yield 83%).

[0044] Step 4: Compound Co-1064 (5 mg, 0.0045 mmol), phenolic resin (12 mg, 0.1 mmol), and potassium carbonate (3 mg, 0.022 mmol) were dissolved in 7 mL of N,N-dimethylformamide and stirred at room temperature for 10 h. After the reaction was completed, the mixture was filtered, and the N,N-dimethylformamide was removed by rotary evaporation. The resulting solid product was washed three times with water, ethanol, and dichloromethane, and dried in a vacuum drying oven at 70 °C for 24 h to obtain a reddish-brown solid compound Co-1064-PR (11 mg, yield 67%).

[0045] Example 4

[0046] Application of Co-1064-PR electrocatalytic oxygen reduction in Example 1

[0047] 1 mg of Co-1064-PR and 1 mg of carbon nanotubes were mixed evenly and then dispersed together with 20 μL of Nafion in 1 mL of acetonitrile to obtain Co-1064-PR slurry. The slurry was then coated on the electrode surface and dried for electrocatalytic oxygen reduction.

[0048] Control experiment 1: 24 μg of 5,10,15,20-tetra(pentafluorophenyl)cobalt porphyrin, 1 mg of PR and 1 mg of carbon nanotubes were mixed evenly and then dispersed together with 20 μL of nafion in 1 mL of acetonitrile to obtain Co-TPFPP+PR slurry. The slurry was then coated on the electrode surface and dried for electrocatalytic oxygen reduction.

[0049] Control experiment 2: 1 mg PR and 1 mg carbon nanotubes were mixed evenly and then dispersed together with 20 μL nafion in 1 mL acetonitrile to obtain PR slurry. The slurry was then coated on the electrode surface and dried for electrocatalytic oxygen reduction.

[0050] like Figure 3 As shown, in the RRDE test, the half-wave potentials of Co-1064-PR, Co-TPFPP+PR, and PR measured by linear sweep voltammogram (LSV) were 784 mV, 715 mV, and 701 mV, respectively. Compared with Co-TPFPP+PR and PR, the half-wave potential of Co-1064-PR shifted more than 60 mV towards the anode, indicating that it has better ORR catalytic activity. This suggests that the protons of the phenolic hydroxyl groups on the phenolic resin can be rapidly transferred to the covalently grafted Co porphyrin center, thereby promoting the ORR process.

Claims

1. A method for the preparation of a covalently grafted phenol-formaldehyde resin with cobalt porphyrin characterized in that This method consists of the following steps: Step 1: Add p-hydroxybenzaldehyde, pentafluorobenzaldehyde and pyrrole to dichloromethane, stir until homogeneous, add boron trifluoride diethyl ether and stir for 3-6 hours, then add 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and stir for 1-3 hours, then purify by column chromatography to obtain compound a; Step 2: Add compound a, 1,6-dibromohexane and potassium carbonate to N,N-dimethylformamide, stir at room temperature for 8-10 hours, and purify by column chromatography to obtain compound b; Step 3: Add compound b, cobalt acetate tetrahydrate and sodium acetate to anhydrous and oxygen-free N,N-dimethylformamide, bubble with argon for 20-30 minutes, reflux at 100°C for 2-4 hours, then extract, dry the organic phase with anhydrous sodium sulfate, and purify by column chromatography to obtain compound Co-1064; Step 4: Dissolve compound Co-1064, phenolic resin and potassium carbonate in N,N-dimethylformamide, stir at room temperature for 8-10 hours, filter after the reaction is complete, concentrate the filtrate by rotary evaporation to remove N,N-dimethylformamide, wash the concentrated solid product with water, ethanol and dichloromethane in sequence and then dry under vacuum to obtain compound Co-1064-PR, i.e. cobalt porphyrin covalently grafted phenolic resin; In the formula, n takes the value of an integer from 4 to 12, and R represents... The values ​​of m and p are integers from 0 to 8, and m + p = n - 4.

2. The method for preparing cobalt porphyrin covalently grafted phenolic resin according to claim 1, characterized in that: In step 1, the molar ratio of p-hydroxybenzaldehyde, pentafluorobenzaldehyde, pyrrole, boron trifluoride ether, and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone is 1:3:4:0.5~2:1~3.

3. The method for preparing cobalt porphyrin covalently grafted phenolic resin according to claim 1, characterized in that: In step 2, the molar ratio of compound a, 1,6-dibromohexane, and potassium carbonate is 1:1 to 5:1 to 10.

4. The method for preparing cobalt porphyrin covalently grafted phenolic resin according to claim 1, characterized in that: In step 3, the molar ratio of compound b, cobalt acetate tetrahydrate, and sodium acetate is 1:1 to 10:1 to 10.

5. The method for preparing cobalt porphyrin covalently grafted phenolic resin according to claim 1, characterized in that: In step 4, the molar ratio of the compound Co-1064, phenolic resin, and potassium carbonate is 1:20 to 100:1 to 10.

6. The application of the cobalt porphyrin covalently grafted phenolic resin prepared by the method of claim 1 for electrocatalytic oxygen reduction.

Images