Iron single-atom composite catalysts with a bilayer carbon dot core-carbon-nitrogen shell structure and their applications

Abstract

This invention relates to iron single-atom composite catalysts with a double-layered carbon dot core-carbon-nitrogen shell structure and their applications. The catalyst preparation method is as follows: carbon source, nitrogen source, and iron metal salt are dispersed in a mixed solution of solvent and coordinating agent and stirred evenly, followed by hydrothermal reaction; after the reaction, the supernatant is filtered and dialyzed, and the dialysate is freeze-dried to obtain Fe. SA / CDs material; add zinc nitrate hexahydrate and iron metal salt to deionized water, add Fe SA / CDs aqueous solution, then add the solution of 2-methylimidazole completely dissolved in deionized water, ultrasonically stir, and after the reaction, obtain Fe. SA / CDs / Fe@ZIF-8 precursor; the precursor was subjected to Joule heat treatment and cooled to room temperature to obtain an iron single-atom composite catalyst with a bilayer carbon dot core-carbon-nitrogen shell structure. This invention effectively improves the catalytic activity and stability of the catalyst in the benzaldehyde oxidative nitrification reaction.

Description

Technical Field

[0001] This invention relates to the field of catalyst technology, specifically to an iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure and its application. Background Technology

[0002] The oxidative nitrification of benzaldehyde is an important organic transformation reaction. Its core lies in using benzaldehyde as a reactant, reacting it with a specific nitrogen-containing reagent in a suitable oxidizing atmosphere to achieve precise construction from an aldehyde group to a cyano group, ultimately yielding benzonitrile. Compared to traditional synthetic routes that rely on highly toxic cyanides, oxidative nitrification typically employs transition metal catalysis systems, using oxygen or air as the oxidant. This improves reaction safety, reduces environmental pollution, and offers high atom economy, fully aligning with the principles of green chemistry and sustainable synthesis. Currently, this type of reaction has become one of the important methods for synthesizing aromatic nitriles and their derivatives.

[0003] In the oxidative nitrification reaction of benzaldehyde, non-precious metal catalysts such as copper, manganese, and iron-based catalysts have significant advantages over precious metal catalysts. Their abundant crustal reserves and low raw material costs significantly reduce production costs, making them more suitable for large-scale industrial production. Furthermore, these catalysts exhibit strong resistance to ammonia poisoning, are less prone to deactivation of active sites in the presence of nitrogen sources, and can efficiently catalyze reactions under mild conditions such as atmospheric pressure and relatively low temperatures. Therefore, they show great potential in the green, economical, and sustainable synthesis of nitrile compounds. Iron-based catalysts have attracted considerable attention due to their low cost, environmental friendliness, and low biotoxicity. Their flexible and tunable coordination environment and easily precise electronic structure control through coordination fields and support effects demonstrate significant advantages in constructing efficient and sustainable catalytic systems. However, these catalysts still have some drawbacks, such as low atom utilization efficiency, difficulty in balancing conversion and selectivity, susceptibility to side reactions, unclear structure-activity relationships and reaction mechanisms, and complex preparation processes with difficulty in controlling uniformity. These problems severely limit their application in the oxidative nitrification reaction of benzaldehyde.

[0004] Based on this, the rational design and construction of an iron-based catalyst that is simple to prepare, low in cost, and has outstanding stability, while also possessing high catalytic conversion rate and excellent selectivity, is of great scientific significance and application value for breaking through the existing technical barriers in the benzaldehyde oxidation-nitrification reaction and promoting its development towards industrialization, greening, and high efficiency. Summary of the Invention

[0005] One objective of this invention is to provide an iron single-atom composite catalyst with a bilayer carbon dot core-carbon-nitrogen shell structure, which solves the problems of difficulty in achieving both conversion and selectivity and complex preparation processes in existing catalysts used for the oxidative nitrification of benzaldehyde; another objective of this invention is to provide applications of the iron single-atom composite catalyst with a bilayer carbon dot core-carbon-nitrogen shell structure.

[0006] The technical solution adopted by this invention to solve its technical problem is as follows: This iron single-atom composite catalyst with a double-layer carbon dot core-carbon-nitrogen shell structure is prepared by the following method: Step 1. Disperse the carbon source, nitrogen source, and iron metal salt in a mixed solution of solvent and complexing agent. The molar ratio of carbon source, nitrogen source, and iron metal salt is (1~30):(10~200):1. After stirring evenly, transfer the solution to a polytetrafluoroethylene-lined hydrothermal reactor for hydrothermal reaction. After the reaction is completed and cooled to room temperature, centrifuge, collect the supernatant, and filter the supernatant through a 0.22 μm microporous membrane to remove impurities from the system. Dialyze the filtered filtrate using a 1000 Da permeation bag. Freeze-dry the dialysate to obtain Fe. SA / CDs material; Step 2. Add zinc nitrate hexahydrate and iron metal salt to deionized water. The molar ratio of zinc nitrate hexahydrate to iron metal salt is 15:1 to 100:1. Stir continuously at room temperature until completely dissolved. Add Fe. SA / CDs aqueous solution, stirred until homogeneous to form solution A; separately, 2-methylimidazole is completely dissolved in deionized water to form solution B, the molar ratio of 2-methylimidazole to the mixed metal salt is 4:1, the mixed metal salt is composed of iron metal salt and zinc nitrate hexahydrate from this step; under ultrasonic conditions, solution B is poured into solution A, ultrasonicated and stirred, after the reaction is complete, centrifuged, washed and dried to obtain Fe SA / CDs / Fe@ZIF-8 precursor; Step 3. Take the Fe obtained in Step 2... SA The / CDs / Fe@ZIF-8 precursor was subjected to Joule heat treatment and then cooled to room temperature to obtain an iron single-atom composite catalyst Fe with a bilayer carbon dot core-carbon-nitrogen shell structure. SA / CDs@NC materials.

[0007] In the above scheme, Fe SA / CDs have a particle size of 2~10nm.

[0008] In step one of the above scheme, the carbon source is any one of citric acid, glucose, sucrose, ascorbic acid, oxalic acid, chitosan, starch, fructose, salicylic acid, polyvinylpyrrolidone, tartaric acid, lignin, and glycerol. The nitrogen source is any one of urea, hexamethylenetetramine, triethylamine, melamine, dicyandiamide, formamide, acetamide, o-phenylenediamine, m-phenylenediamine, acrylamide, polyethyleneimine, triethylenetetramine, and glycine. The solvent is any one of deionized water, anhydrous ethanol, ethylene glycol, isopropanol, and a water / ethanol mixture. The ligand is any one of ethylenediamine, melamine, dicyandiamide, hexamethylenetetramine, and triethylenetetramine.

[0009] In step one of the above scheme, the volume ratio of the ligand to the solvent is 1:10 to 1:50.

[0010] In step one of the above scheme, the hydrothermal reaction temperature is 120~280ºC, the time is 1~24h, and the dialysis time is 12~240h.

[0011] In the above scheme, the iron metal salt is any one of ferric acetate, ferric acetylacetone, ferric chloride hexahydrate, ferric nitrate nonahydrate, ferric sulfate nonahydrate, ferric phosphate dihydrate, ferric citrate trihydrate, ferric ammonium sulfate dodecahydrate, ferric gluconate pentahydrate, ferric oxalate trihydrate, ferrocene, and ferric stearate.

[0012] In step two of the above scheme, the volume of deionized water is 30-100 mL, Fe SA The concentration of the / CDs aqueous solution is 1~20 mg / mL, and the volume is 0.5~10 mL.

[0013] In step two of the above scheme, the stirring speed is 100~800 rpm, the stirring time is 0.5~48h, the ultrasonic time is 10~60 min, the centrifuge speed is 6000~12000 rpm, and the time is 1~60 min.

[0014] In step three of the above scheme, the Joule heat treatment temperature is 500~3200ºC, the heating rate is 1000 ºC / s, and the holding time is 10~1600 s.

[0015] The aforementioned iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure is used for the oxidative nitrification reaction of benzaldehyde, catalyzing the formation of benzonitrile from benzaldehyde. Beneficial effects

[0016] 1. This invention employs an in-situ self-assembly strategy, using carbon dots anchored to iron single atoms as crystal nuclei to induce the epitaxial growth of iron-doped zeolite imidazole ester framework-8 (ZIF-8) on its surface, followed by Joule heat treatment, to successfully construct an iron-based single-atom composite catalyst with a double-layer carbon dot core-carbon-nitrogen shell structure, effectively improving its catalytic activity and stability in the benzaldehyde oxidation nitrification reaction.

[0017] 2. This invention discloses a method for preparing an iron single-atom composite catalyst with a bilayer carbon dot core-carbon-nitrogen shell structure. First, the abundant functional groups on the carbon dot surface are used to achieve efficient anchoring and uniform dispersion of Fe single atoms. Then, an in-situ self-assembly strategy is employed, using the carbon dot as a nucleus to induce the directional epitaxial growth of Fe@ZIF-8 on its surface, forming a well-structured and tightly bound Fe-doped core-shell precursor. Joule heat treatment is then used to construct a multi-level structure of Fe single-atom dispersed carbon dot core-carbon-nitrogen shell, which not only effectively suppresses the aggregation and loss of iron sites but also allows for precise control of the coordination environment and electronic structure of Fe sites on the bilayer carbon dot core-carbon-nitrogen shell.

[0018] 3. The iron single-atom composite catalyst prepared by this invention has a unique structure with a bilayer carbon dot core-carbon-nitrogen shell structure, forming a multi-level structure of a bilayer carbon dot core-carbon-nitrogen shell supported by Fe single atoms. Fe single atoms are distributed on both the core and shell, which can significantly improve the density and space utilization of active sites. Furthermore, the porous structure of the nitrogen-doped carbon support effectively prevents the aggregation of Fe single atoms. While enhancing structural stability, it can also regulate the electronic structure of active sites and accelerate electron transport efficiency, synergistically improving the catalytic activity and stability of the catalyst.

[0019] 4. The catalyst prepared by this invention has excellent catalytic performance and low cost, with extremely high Fe atom utilization. When applied to the benzaldehyde oxidation nitrification reaction, it can achieve high conversion rate and high product selectivity. Moreover, the catalyst is easy to separate from the reaction system and the recovery process is simple, which is suitable for the needs of industrial green production. Attached Figure Description

[0020] Figure 1 This is a scanning electron microscope image of the iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure obtained in Example 1.

[0021] Figure 2 The images show the iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure obtained in Example 1 and the X-ray diffraction patterns of each comparative example.

[0022] Figure 3 The images show the iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure obtained in Example 1 and the Fourier transform infrared spectra of each comparative example.

[0023] Figure 4 This is a time-yield graph of the iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure obtained in Example 1 in the benzaldehyde oxidative nitrification reaction.

[0024] Figure 5This is a comparison chart of the conversion frequencies of the iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure obtained in Example 1 and other catalysts. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings: Example 1 This iron single-atom composite catalyst with a bilayered carbon dot core-carbon-nitrogen shell structure was prepared by the following method: Step 1. Weigh 6.0g urea, 3.0g citric acid, and 0.4848g ferric nitrate nonahydrate and dissolve them in 25mL deionized water and 1mL ethylenediamine solution. Stir until the system is homogeneous, then transfer the mixture to a 50mL PTFE-lined hydrothermal reactor. React the mixture at 200ºC for 8 hours. After cooling to room temperature, centrifuge and collect the supernatant. Filter the supernatant using a 0.22μm microporous membrane. Place the filtrate in a 1000Da permeation bag and dialyze at room temperature for 3 days, changing the deionized water every 6 hours. Freeze-dry the dialysate to obtain Fe. SA / CDs material.

[0026] Step 2. Weigh 3.1289 g of zinc nitrate hexahydrate and 0.1057 g of ferric acetate, and dissolve them in 75 mL of deionized water. Stir continuously at room temperature until both metal salts are completely dissolved. Add 1 mL of Fe3+ solution with a concentration of 5 mg / mL. SA / CDs aqueous solution, stirred until the system is homogeneous to form solution A. Weigh 3.6360 g of 2-methylimidazole, dissolve it in 50 mL of deionized water, and stir until completely dissolved to form solution B. Under ultrasonic conditions, pour solution B into solution A, sonicate for 30 min, and stir for 4 h. After the reaction is complete, centrifuge, wash, and dry to obtain Fe. SA / CDs / Fe@ZIF-8 precursor.

[0027] Step 3. Take the Fe obtained in Step 2 SA The / CDs / Fe@ZIF-8 precursor was subjected to Joule heat treatment at a heating rate of 1000 ºC / s, a Joule heat treatment temperature of 1200ºC, and a treatment time of 30 s. After cooling to room temperature, Fe was obtained. SA / CDs@NC materials.

[0028] The iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure obtained in Example 1 above was characterized.

[0029] like Figure 1 The image shows the Fe obtained in Example 1. SA Scanning electron microscope image from / CDs@NC. Figure 1 Fe can be seen from SAIn / CDs@NC, single-atom carbon dots of iron act as crystal nuclei, encapsulated by nitrogen-doped carbon material, exhibiting a polygonal morphology.

[0030] like Figure 2 The image shows the Fe obtained in Example 1. SA / CDs@NC and comparative X-ray diffraction patterns. From Figure 2 Fe can be seen from SA The / CDs@NC catalyst exhibits a broad diffraction peak at a diffraction angle of 24º, indicating a low degree of graphitization. Furthermore, the absence of metallic characteristic peaks demonstrates that the iron species within the catalyst are highly dispersed, with no aggregation of iron metal atoms.

[0031] like Figure 3 The image shows the Fe obtained in Example 1. SA / CDs@NC and Fourier transform infrared spectra of each comparative example. From Figure 3 It can be seen from 3433 cm -1 The presence of a distinct O−H stretching vibration peak nearby indicates the presence of adsorbed hydroxyl species in the catalyst. (2925 cm⁻¹) -1 The presence of a C-H stretching vibration peak nearby confirms that the catalyst possesses a C-H structure. (1618 cm⁻¹) -1 The presence of a distinct C=C(N) stretching vibration characteristic absorption peak nearby indicates that the carbon support of this catalyst material contains a large number of nitrogen-doped structures.

[0032] The application of the above-mentioned iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure in the catalytic reaction of benzaldehyde oxidation and nitrification is specifically carried out as follows: 2.5 mg of iron single-atom composite catalyst, 0.5 mmol of benzaldehyde, 200 μL of ammonia, and 3 mL of deionized water were added to a Schlenk reaction tube and reacted at 80 ºC in air for 2.5 h. After centrifugation, the supernatant was collected, and the product was analyzed by gas chromatography. The results were calculated using the area normalization method.

[0033] Example 2 The difference from Example 1 is that the iron metal salt in step 2 is 0.0522 g of ferric acetate.

[0034] Example 3 Compared with Example 1, the difference is that the iron metal salt in step 2 is 0.2236 g of ferric nitrate nonahydrate, and the Joule heat treatment temperature in step 3 is 1000 ºC and the treatment time is 60 s.

[0035] Comparative Example 1 Step 1. Weigh 3.1289 g of zinc nitrate hexahydrate and 0.1057 g of ferric acetate, and dissolve them in 75 mL of deionized water. Stir until completely dissolved to form solution A. Weigh 3.6360 g of 2-methylimidazole, dissolve it in 50 mL of deionized water, and stir until completely dissolved to form solution B. Under ultrasonic conditions, add solution B to solution A, sonicate for 30 min, and stir for 4 h. After the reaction is complete, centrifuge, wash, and dry to obtain the Fe@ZIF-8 precursor.

[0036] Step 2. The Fe@ZIF-8 precursor obtained in Step 1 was heated to 950 ºC at a heating rate of 5 ºC / min and calcined for 2 h under Ar atmosphere protection to obtain Fe SA / NC.

[0037] Comparative Example 2 Compared with Example 1, the difference is that 0.4848 g of ferric nitrate nonahydrate is not added in step 1, 3.2935 g of zinc nitrate hexahydrate is added in step 2, and 0.1057 g of ferric acetate is not added, thus obtaining CDs@NC.

[0038] Comparative Example 3 Commercially available Fe(NO3)3 was used as comparative example 3.

[0039] Comparative Example 4 Commercially available FeSO4 was used as Comparative Example 4.

[0040] Comparative Example 5 Commercially available Fe(C5H5)2 was used as Comparative Example 5.

[0041] Comparative verification: In this invention, an iron single-atom composite catalyst with a bilayer carbon dot core-carbon-nitrogen shell structure catalyzes the oxidative nitrification reaction of benzaldehyde. 2.5 mg of the iron single-atom composite catalyst, 0.5 mmol of benzaldehyde, 200 μL of ammonia, and 3 mL of deionized water are added to a Schlenk reaction tube, and the reaction is carried out at 80 ºC in air for 2.5 h. After centrifugation, the supernatant is collected, and the product is analyzed using gas chromatography. The results are calculated using the area normalization method.

[0042] like Figure 4 The image shows the Fe obtained in Example 1. SA Time-yield graph of / CDs@NC in the oxidative nitrification of benzaldehyde. Benzaldehyde was efficiently converted to benzonitrile after 2.5 h of reaction (selectivity > 99%, conversion > 99%, yield > 99%). No byproducts were formed when the reaction time was extended to 24 h, indicating that the catalyst has excellent catalytic activity and selectivity.

[0043] like Figure 5The image shows the Fe obtained in Example 1. SA Conversion frequency plot of / CDs@NC with other catalysts. Fe SA / CDs@NC catalytic oxidation of benzaldehyde to benzonitrile has a conversion frequency as high as 1054 h⁻¹ -1 It is far superior to the comparative example, and far superior to Fe in Comparative Example 1. SA / NC (188 h) -1 ), Comparative Example 2 CDs@NC (0 h -1 ), and also significantly exceeded Fe(NO3)3 (11 h) in Comparative Example 3. -1 FeSO4 in Comparative Example 4 (4 h) -1 ) and Fe(C5H5)2 (2 h) in Comparative Example 5 -1 It exhibits excellent intrinsic catalytic activity.

Claims

1. An iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure, characterized in that... Prepared by the following method: Step 1. Disperse the carbon source, nitrogen source, and iron metal salt in a mixed solution of solvent and complexing agent. The molar ratio of carbon source, nitrogen source, and iron metal salt is (1~30):(10~200):

1. After stirring evenly, transfer the solution to a polytetrafluoroethylene-lined hydrothermal reactor for hydrothermal reaction. After the reaction is completed and cooled to room temperature, centrifuge, collect the supernatant, and filter the supernatant through a 0.22 μm microporous membrane to remove impurities from the system. Dialyze the filtered filtrate using a 1000 Da permeation bag. Freeze-dry the dialysate to obtain Fe. SA / CDs material; Step 2. Add zinc nitrate hexahydrate and iron metal salt to deionized water. The molar ratio of zinc nitrate hexahydrate to iron metal salt is 15:1 to 100:

1. Stir continuously at room temperature until completely dissolved. Add Fe. SA / CDs aqueous solution, stirred until homogeneous to form solution A; separately, 2-methylimidazole is completely dissolved in deionized water to form solution B, the molar ratio of 2-methylimidazole to the mixed metal salt is 4:1, the mixed metal salt is composed of iron metal salt and zinc nitrate hexahydrate from this step; under ultrasonic conditions, solution B is poured into solution A, ultrasonicated and stirred, after the reaction is complete, centrifuged, washed and dried to obtain Fe SA / CDs / Fe@ZIF-8 precursor; Step 3. Take the Fe obtained in Step 2... SA The / CDs / Fe@ZIF-8 precursor was subjected to Joule heat treatment and then cooled to room temperature to obtain an iron single-atom composite catalyst Fe with a bilayer carbon dot core-carbon-nitrogen shell structure. SA / CDs@NC materials.

2. The iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure according to claim 1, characterized in that: In step one, Fe SA / CDs have a particle size of 2~10nm.

3. The iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure according to claim 2, characterized in that: In step one, the carbon source is any one of citric acid, glucose, sucrose, ascorbic acid, oxalic acid, chitosan, starch, fructose, salicylic acid, polyvinylpyrrolidone, tartaric acid, lignin, and glycerol. The nitrogen source is any one of urea, hexamethylenetetramine, triethylamine, melamine, dicyandiamide, formamide, acetamide, o-phenylenediamine, m-phenylenediamine, acrylamide, polyethyleneimine, triethylenetetramine, and glycine. The solvent is any one of deionized water, anhydrous ethanol, ethylene glycol, isopropanol, and a water / ethanol mixture. The ligand is any one of ethylenediamine, melamine, dicyandiamide, hexamethylenetetramine, and triethylenetetramine.

4. The iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure according to claim 3, characterized in that: In step one, the volume ratio of the ligand to the solvent is 1:10 to 1:

50.

5. The iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure according to claim 4, characterized in that: In step one, the hydrothermal reaction temperature is 120~280ºC, the time is 1~24h, and the dialysis time is 12~240h.

6. The iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure according to claim 5, characterized in that: The iron metal salt is any one of ferric acetate, ferric acetylacetone, ferric chloride hexahydrate, ferric nitrate nonahydrate, ferric sulfate nonahydrate, ferric phosphate dihydrate, ferric citrate trihydrate, ferric ammonium sulfate dodecahydrate, ferric gluconate pentahydrate, ferric oxalate trihydrate, ferrocene, and ferric stearate.

7. The iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure according to claim 6, characterized in that: In step two, the volume of deionized water is 30-100 mL, Fe SA The concentration of the / CDs aqueous solution is 1~20 mg / mL, and the volume is 0.5~10 mL.

8. The iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure according to claim 7, characterized in that: In step two, the stirring speed is 100~800 rpm, the stirring time is 0.5~48 h, the ultrasonic time is 10~60 min, the centrifuge speed is 6000~12000 rpm, and the time is 1~60 min.

9. The iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure according to claim 8, characterized in that: In step three, the Joule heat treatment temperature is 500~3200ºC, the heating rate is 1000 ºC / s, and the holding time is 10~1600 s.

10. The application of the iron single-atom composite catalyst with a bilayer carbon dot core-carbon-nitrogen shell structure as described in claim 9, characterized in that: The iron single-atom composite catalyst with a double-layered carbon dot core-carbon-nitrogen shell structure is used for the oxidative nitrification reaction of benzaldehyde, catalyzing the formation of benzonitrile from benzaldehyde.

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