One-step synthesis method and application of carbon dot composite MOF photocatalyst

Abstract

The application relates to a one-step synthesis method and application of a carbon dot composite MOF photocatalyst, and relates to a preparation method and application of the carbon dot composite MOF photocatalyst, which aims to solve the technical problems of long preparation period, complex steps and low catalytic efficiency of the existing preparation method of carbon dot doped iron-based metal organic framework. The method is as follows: a mixed solution of 2-methyl imidazole, H2O2 and ethylenediamine is put into an ultrasonic cell crusher for crushing and dispersion; the dispersion liquid is mixed with fumaric acid and iron chloride hexahydrate; the mixed solution is transferred into a reaction kettle for hydrothermal reaction; after centrifugation, washing and drying, the carbon dot composite MOF photocatalyst is obtained. When the photocatalyst prepared by using 2-methyl imidazole as raw material and treated by an ultrasonic cell crusher is used for photocatalytic degradation of alkaline lignin, the lignin degradation rate reaches 70% after 4 hours, and the photocatalyst can be used in the field of lignin degradation.

Description

Technical Field

[0001] This invention relates to the preparation method and application of carbon dot composite MOF photocatalysts, which belong to the field of photocatalytic functional materials. Background Technology

[0002] my country boasts abundant vegetation resources, but currently, most lignin is still treated as waste or directly burned as low-grade fuel, leading to a significant waste of lignin resources. Current technologies for lignin depolymerization include thermal depolymerization, ionic liquid depolymerization, and photoelectrocatalytic depolymerization. However, most depolymerization methods suffer from slow efficiency, low selectivity, high cost, difficulty in reagent recovery, high energy consumption, and potential pollution. To address these issues, a novel catalytic lignin depolymerization technology with mild reaction conditions, high efficiency, low cost, and no pollution is needed. Compared to other lignin depolymerization technologies, photocatalysis offers significant advantages such as no pollution and stable controllability, enabling effective utilization of lignin and alleviating resource crises while maintaining sustainable development.

[0003] Metal-organic frameworks (MOFs) possess advantages such as large specific surface area, large pore volume, controllable structure, and high concentration of active metal sites, leading to their widespread application in catalysis and sensing. In the past decade, the iron-based MIL-88A MOF has been applied in water treatment (e.g., heavy metal removal), chemical hydrogen storage (e.g., hydrogen evolution reaction and oxygen evolution reaction), and photochemical and electrochemical energy storage and conversion. However, the potential applications of MIL-88A in photocatalysis and luminescent sensors face significant challenges, including severe charge recombination, low solar energy utilization, low quantum yield, and limited charge transfer between metal ions / clusters and ligands. Recent studies have shown that the rational introduction of carbon dots (CDs) with excellent optical properties, unique quantum confinement, and high electrical conductivity can greatly enhance the functionality of MIL-88A. Existing methods for preparing CDs-MIL-88A composite materials all employ a two-step approach: first, MIL-88A and CDs are prepared separately, and then they are combined. This method is time-consuming and cumbersome, and the resulting material exhibits poor photocatalytic performance when used for photolysis of lignin. This technology mainly solves the above problems by using a one-step hydrothermal method to prepare CDs composite MIL-88A materials, which has a short preparation cycle, simple steps, and high catalytic efficiency. Summary of the Invention

[0004] This invention aims to address the technical problems of existing carbon dot-doped iron-based metal-organic framework preparation methods, which involve long preparation cycles, complex steps, and low catalytic efficiency when used for photolysis of lignin. Instead, it proposes a one-step synthesis method and application of carbon dot composite MOF photocatalysts.

[0005] The present invention discloses a one-step synthesis method for a carbon dot composite MOF photocatalyst, comprising the following steps:

[0006] 1. Dissolve 2-methylimidazole powder in deionized water, then add H2O2 and ethylenediamine (EDA), stir until homogeneous to obtain a mixture; place the mixture in an ultrasonic cell disruptor and ultrasonically disrupt and disperse for 10-30 minutes under ultrasonic frequency of 20-25KHz, power of 700-900W, and ice bath conditions to obtain a dispersion.

[0007] 2. Dissolve fumaric acid in hot water, then add the dispersion and mix well. Then add ferric chloride hexahydrate to obtain a mixed solution. Transfer the mixed solution to a reaction vessel lined with polytetrafluoroethylene and perform a hydrothermal reaction at a constant temperature of 150-160℃ for 4-6 hours. After the reaction is completed, centrifuge and wash the solid phase with water and ethanol in sequence. Dry it in an oven to obtain the carbon dot composite MOF photocatalyst.

[0008] Furthermore, the concentration of 2-methylimidazole in the mixture described in step one is 15–16 mg / mL.

[0009] Furthermore, in the mixture described in step one, the mass ratio of 2-methylimidazole to the volume of H2O2 is 1 g: (13-15) mL; the volume ratio of H2O2 to EDA is (9-10): 1.

[0010] Furthermore, the volume ratio of the dispersion to the molar amount of fumaric acid in step two is 10 mL: (3-4) mmol.

[0011] Furthermore, in step two, the molar ratio of the organic ligand fumaric acid to the metal ion ferric chloride hexahydrate is (1-1.3):1.

[0012] Furthermore, the temperature of the hot water in step two, 55–60°C, is conducive to the complete dissolution of fumaric acid.

[0013] Furthermore, in step two, the centrifugation speed is 10000 r / min, and the centrifugation time is 5 to 10 min.

[0014] The application of the carbon dot composite MOF photocatalyst prepared by the above method is to use it for the photocatalytic degradation of basic lignin. Specifically:

[0015] The carbon dot composite MOF photocatalyst was dispersed in an alkaline lignin aqueous solution and stirred under light conditions for 0.5–4 h to complete the degradation of alkaline lignin.

[0016] Furthermore, the mass of carbon dot composite MOF photocatalyst added is 10% to 50% of the mass of alkaline lignin.

[0017] The beneficial effects of this invention are:

[0018] 1. This invention utilizes a one-step synthesis method to prepare structurally stable carbon dot composite MOF photocatalysts, which simplifies the preparation process, improves the preparation efficiency, and allows for controllable preparation, high product purity, and easy large-scale synthesis, making it possible to rapidly prepare carbon dot composite MOF materials.

[0019] 2. Carbon dots were prepared using 2-methylimidazole as a carbon source. Through a one-step hydrothermal synthesis method, the carbon dots were distributed on the surface and embedded in the internal structure of MIL-88A, generating more active sites. This resulted in better conductivity and effectively accelerated the separation of photogenerated charges, inhibiting the recombination of photogenerated electrons and holes. This enabled the photocatalytic degradation of alkaline lignin. After 4 hours of photocatalytic degradation, the degradation rate of lignin reached 70%, shortening the degradation time. It is a reliable heterogeneous catalytic material with outstanding technical advantages and can be used in the field of lignin degradation. Attached Figure Description

[0020] Figure 1 A photograph of the carbon dot composite MOF photocatalyst prepared in Example 1;

[0021] Figure 2 SEM image of the carbon dot composite MOF photocatalyst prepared in Example 1;

[0022] Figure 3 XRD pattern of pure MIL-88A prepared for Comparative Example 1;

[0023] Figure 4 The XRD pattern of the carbon dot composite MOF photocatalyst prepared in Example 1;

[0024] Figure 5 TEM image of the carbon dot composite MOF photocatalyst prepared in Example 1;

[0025] Figure 6 The XRD pattern of the product obtained in Comparative Example 5 is shown below.

[0026] Figure 7 This is a comparison chart showing the results of photocatalytic degradation of alkaline lignin using the photocatalysts prepared in Example 1 and Comparative Example 5.

[0027] Figure 8 The images show alkaline lignin before and after degradation of the carbon dot composite MOF photocatalyst prepared in Example 1. Detailed Implementation

[0028] The beneficial effects of the present invention are verified using the following examples:

[0029] Example 1: The one-step synthesis method of the carbon dot composite MOF photocatalyst in this example is carried out according to the following steps:

[0030] 1. Dissolve 0.75g of 2-methylimidazole powder in 50mL of deionized water, then add 10mL of H2O2 and 1mLEDA, stir well to obtain a mixture; place the mixture in an ultrasonic cell disruptor and ultrasonically disrupt and disperse for 30min under ultrasonic frequency of 25KHz, power of 720W, and ice bath conditions to obtain a dispersion.

[0031] 2. Dissolve 0.58g of fumaric acid in 50mL of hot water at 55℃, then add 10mL of the dispersion prepared in step 1, mix well, and then add 1.35g of ferric chloride hexahydrate to introduce metal ions, mix well to obtain a mixed solution; transfer the mixed solution to a reaction vessel lined with polytetrafluoroethylene, and perform a hydrothermal reaction at 150℃ for 4h. After the reaction is completed, centrifuge at 10000r / min for 5min, wash the solid phase three times each with water and ethanol, and then dry it in an oven at 60℃ for 12h to obtain the carbon dot composite MOF photocatalyst, denoted as CDs / MIL.

[0032] Comparative Example 1: This comparative example is for the preparation of pure MIL-88A. The specific steps are as follows: 0.58g of fumaric acid was dissolved in 40mL of hot water at 55℃, and then 1.35g of ferric chloride hexahydrate was added and mixed evenly. The mixed solution was transferred to a reaction vessel lined with polytetrafluoroethylene and subjected to a constant temperature hydrothermal reaction at 150℃ for 4h. After the reaction was completed, the mixture was centrifuged at 10000r / min for 5min. The solid phase was washed three times each with water and ethanol, and then dried in an oven at 60℃ for 12h to obtain pure MIL-88A.

[0033] A photograph of the carbon dot composite MOF photocatalyst prepared in Example 1 is shown below. Figure 1 As shown, it is a dark brick-red viscous powder with a fine texture.

[0034] Scanning electron microscope (SEM) images of the carbon dot composite MOF photocatalyst prepared in Example 1 are shown below. Figure 2 As shown, from Figure 2 It can be seen that the carbon dot composite MOF photocatalyst prepared in Example 1 has the traditional structure of MIL-88A, and the doping of carbon dots has not changed the special structure of MIL-88A.

[0035] Figure 3 The image shows the XRD pattern of pure MIL-88A prepared in Comparative Example 1. Figure 4 The XRD pattern of the carbon dot composite MOF photocatalyst prepared in Example 1 is shown below. Figure 3 and Figure 4It can be seen that the carbon dot composite MOF photocatalyst prepared in Example 1 shows the characteristic peak of carbon dots at 2θ = 21℃, indicating that MIL-88A contains the characteristic peak of carbon dots, and that carbon dots have been composited into MIL-88A.

[0036] Figure 5 This is a transmission electron microscope (TEM) image of the carbon dot composite MOF photocatalyst prepared in Example 1. Figure 5 As can be seen in B, the carbon dots were successfully composited onto MIL-88A and are uniformly distributed; through Figure 5 As can be seen from C, carbon dots are not only distributed on the surface of MIL-88A, but also embedded in the structure of MIL-88A. This composite structure not only improves the adsorption performance of the material, but also greatly enhances its photocatalytic performance.

[0037] Comparative Example 2: This comparative example differs from Example 1 in that a conventional stirrer is used instead of an ultrasonic cell disruptor in step one. The specific steps are as follows:

[0038] 1. Dissolve 0.75g of 2-methylimidazole powder in 50mL of deionized water, then add 10mL of H2O2 and 1mLEDA, and stir until homogeneous to obtain a mixture; place the mixture on a magnetic stirrer and stir for 30min at a stirring speed of 120 rpm to obtain a dispersion.

[0039] 2. Dissolve 0.58g of fumaric acid in 30mL of hot water at 55℃, then add 10mL of the dispersion prepared in step 1, mix well, and then add 1.35g of ferric chloride hexahydrate, mix well to obtain a mixed solution; transfer the mixed solution to a reaction vessel lined with polytetrafluoroethylene, and perform a hydrothermal reaction at a constant temperature of 150℃ for 4h. After the reaction is completed, centrifuge at a high speed of 10000r / min for 5min, wash the solid phase three times each with water and ethanol, and then dry it in an oven at 60℃ for 12h to obtain the product.

[0040] Comparative Example 2 yielded a light green solution with a small amount of black particles after the isothermal hydrothermal reaction in step two. After centrifugation and drying, the product was obtained and subjected to lignin photocatalytic degradation experiments. It was found that the product had no photocatalytic effect at all and its appearance color was completely different from that of normal MIL-88A. The stirring conditions in step one had a significant impact on the product. The carbon dot precursor was broken down using an ultrasonic cell disruptor, and the precursor was removed from the MOF construction. Only then could the carbon dot composite MOF photocatalyst be synthesized in one step in the subsequent hydrothermal reaction.

[0041] Comparative Example 3: This comparative example differs from Example 1 in that a conventional ultrasonic cleaner is used instead of an ultrasonic cell disruptor in step one. The specific steps are as follows:

[0042] 1. Dissolve 0.75g of 2-methylimidazole powder in 50mL of deionized water, then add 10mL of H2O2 and 1mLEDA, stir well to obtain a mixture; put the mixture into an ultrasonic cleaner and ultrasonically treat it for 30min at a power of 600w and a frequency of 40KHz to obtain a dispersion.

[0043] 2. Dissolve 0.58g of fumaric acid in 30mL of hot water at 55℃, then add 10mL of the dispersion prepared in step 1, mix well, and then add 1.35g of ferric chloride hexahydrate, mix well to obtain a mixed solution; transfer the mixed solution to a reaction vessel lined with polytetrafluoroethylene, and perform a hydrothermal reaction at a constant temperature of 150℃ for 4h. After the reaction is completed, centrifuge at a high speed of 10000r / min for 5min, wash the solid phase three times each with water and ethanol, and then dry it in an oven at 60℃ for 12h to obtain the product.

[0044] Comparative Example 3 yielded a light green solution after the isothermal hydrothermal reaction in step two. After centrifugation and drying, the resulting product was subjected to a lignin photocatalytic degradation experiment, which revealed that the product had no photocatalytic effect whatsoever. This indicates that the ordinary ultrasonic cleaner used in step one, due to its low power and the obstruction caused by the cleaner and container walls, could not directly act and thus failed to meet the conditions for synthesizing carbon dot composite MOF photocatalysts.

[0045] Comparative Example 4: This comparative example differs from Example 1 in that the 2-methylimidazole in step one of Example 1 is replaced with glucose, citric acid, amino acid and sodium citrate, respectively. The other steps and parameters are the same as in Example 1.

[0046] Comparative Example 4 yielded colored solutions after the isothermal hydrothermal reaction in step two, which showed no photocatalytic effect on lignin degradation. This indicates that 2-methylimidazole is a necessary material for the synthesis of carbon dot precursors, and carbon dot composite MOF photocatalysts can only be synthesized by selecting 2-methylimidazole as a raw material.

[0047] Comparative Example 5: This comparative example demonstrates the two-step preparation of carbon dot composite MOF photocatalysts. The specific steps are as follows:

[0048] I. Preparation of carbon dots: Dissolve 0.75g of 2-methylimidazole powder in 50mL of deionized water, then add 1mLEDA and stir until homogeneous to obtain a mixture. Place the mixture in an ultrasonic cell disruptor and ultrasonically disrupt and disperse for 30min under conditions of 25KHz, 720W, and ice bath. Transfer the solution to a reaction vessel lined with polytetrafluoroethylene and hydrothermally react at a constant temperature of 150℃ for 4h. Filter the solution three times through a 0.22μm filter membrane, and then dialyze it through a 1000Da dialysis membrane with deionized water. Change the water every 12 hours until the deionized water shows no color change to obtain a carbon dot dispersion.

[0049] 2. Dissolve 0.58g of fumaric acid in 30mL of hot water at 55℃, then add 10mL of the carbon dot dispersion prepared in step 1, mix well, and then add 1.35g of ferric chloride hexahydrate, mix well to obtain a mixed solution; transfer the mixed solution to a reaction vessel lined with polytetrafluoroethylene, and perform a hydrothermal reaction at 150℃ for 4h. After the reaction is completed, centrifuge at 10000r / min for 5min, wash the solid phase three times each with water and ethanol, and then dry it in an oven at 60℃ for 12h to obtain the product, denoted as two-step CDs / MIL.

[0050] The product obtained in Comparative Example 5 was a darker brick-red powder, and its XRD pattern is shown below. Figure 6 As shown, it also exhibited a special peak for carbon points at 2θ = 21℃, indicating successful doping. However, compared to... Figure 4 The XRD patterns show that, under the same conditions, the one-step method has an advantage over the two-step method in terms of the amount of carbon incorporated, and the one-step method is more effective. It can be seen that the one-step method allows the carbon dots to be distributed more evenly with MIL-88A. It is speculated that because the precursors in the one-step method not only have a small molecular weight, but are also even smaller after being broken down by a cell sonicator, they are easier to be evenly distributed and doped with MOFs inside or on the surface during the construction process, thus resulting in better performance.

[0051] The carbon dot composite MOF photocatalysts prepared in Example 1 and Comparative Example 5 were used for photocatalytic degradation of alkaline lignin.

[0052] 20 mg of photocatalyst was dispersed in 50 mL of 1000 mg / L alkaline lignin solution by ultrasonication for 3 min, and magnetically stirred in the dark for 30 min to reach adsorption-desorption equilibrium. Then, a xenon lamp was turned on, and 2 mL of the suspension was taken at 0 min, 10 min, 30 min, 60 min, 90 min, 150 min, and 240 min, respectively. The catalyst was removed by passing the suspension through a 0.22 μm filter membrane. The absorbance of the samples was then measured using a UV spectrophotometer. A standard curve was plotted for the absorbance of lignin solutions of different concentrations at an incident wavelength of 280 nm, yielding the corresponding regression equation: y = 0.298x + 0.0276. The lignin concentration was calculated using the measured absorbance of the samples according to the regression equation. The degradation efficiency was calculated based on the change in lignin concentration. The degradation efficiency is calculated using the following formula:

[0053] η = (1 - Ct / C0) × 100%

[0054] In the formula:

[0055] η — Degradation efficiency, %

[0056] C0 – Initial concentration of the lignin solution, mg / mL

[0057] Ct — Concentration of the lignin solution at time t, mg / mL

[0058] The degradation results of alkaline lignin by the carbon dot composite MOF photocatalysts prepared in Example 1 and Comparative Example 5 are as follows: Figure 7 As shown, from Figure 7 It can be seen that the carbon dot composite MOF photocatalyst prepared in Example 1 has better photocatalytic performance and good degradation efficiency in alkaline lignin. After 4 hours of degradation, the degradation efficiency is as high as 70%, while the two-step photocatalyst in Comparative Example 5 has a degradation efficiency of only 30% after 4 hours of degradation.

[0059] Figure 8 These are photographs of alkaline lignin before degradation and after 4 hours of photocatalytic degradation using the carbon dot composite MOF photocatalyst prepared in Example 1. Figure 8 It can be seen that alkaline lignin is a yellow slurry before degradation and a white slurry after degradation.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A one-step synthesis method of carbon dot composite MOF photocatalyst, characterized in that, The method is carried out according to the following steps: I. Dissolve 2-methylimidazole powder in deionized water, then add H2O2 and ethylenediamine, stir uniformly, and obtain a mixed solution; put the mixed solution into an ultrasonic cell disrupter, and ultrasonically disrupt and disperse for 10-30 min under the conditions of ultrasonic frequency of 20-25 KHz, power of 700-900 W, and ice bath; and obtain a dispersion liquid; II. Dissolve fumaric acid in hot water, then add the dispersion liquid and mix uniformly, and then add ferric chloride hexahydrate, and obtain a mixed solution; transfer the mixed solution into a reaction kettle lined with polytetrafluoroethylene, and hydrothermally react for 4-6 h under the condition of temperature of 150-160 DEG C; after the reaction is completed, centrifugally separate, wash the solid phase with water and ethanol in sequence, and dry in an oven, and obtain a carbon dot composite MOF photocatalyst.

2. A one-step synthesis method of carbon dot composite MOF photocatalyst according to claim 1, wherein, The concentration of 2-methylimidazole in the mixed solution in step I is 15-16 mg / mL.

3. The one-step synthesis of carbon dot composite MOF photocatalyst according to claim 1 or 2, characterized in that, The mass ratio of 2-methylimidazole to the volume of H2O2 in the mixed solution in step I is 1 g:(13-15) mL; and the volume ratio of H2O2 to EDA is (9-10):

1.

4. The one-step synthesis of carbon dot composite MOF photocatalyst according to claim 1 or 2, characterized in that, The volume ratio of the dispersion liquid to the amount of substance of fumaric acid in step II is 10 mL:(3-4) mmol.

5. The one-step synthesis of carbon dot composite MOF photocatalyst according to claim 1 or 2, characterized in that, The molar ratio of fumaric acid to ferric chloride hexahydrate in step II is (1-1.3):

1.

6. The one-step synthesis of carbon dot composite MOF photocatalyst according to claim 1 or 2, characterized in that, The temperature of the hot water in step II is 55-60 DEG C.

7. The one-step synthesis method of carbon dot composite MOF photocatalyst according to claim 1 or 2, characterized in that, The rotation speed during centrifugal separation in step II is 10000 r / min, and the centrifugal time is 5-10 min.

8. The use of a carbon dot composite MOF photocatalyst prepared by the method of claim 1, characterized in that The application is to use the carbon dot composite MOF photocatalyst for photocatalytic degradation of alkaline lignin.

9. The use of a carbon dot composite MOF photocatalyst according to claim 8, characterized in that The method for photocatalytic degradation of alkaline lignin by using the carbon dot composite MOF photocatalyst is: disperse the carbon dot composite MOF photocatalyst into an alkaline lignin aqueous solution, and stir for 0.5-4 h under light irradiation, and complete the degradation of alkaline lignin.

10. The use of a carbon dot composite MOF photocatalyst according to claim 9, characterized in that, The added mass of the carbon dot composite MOF photocatalyst is 10%-50% of the mass of alkaline lignin.

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