Dual active site polyoxometalate-based metal-organic framework compound photocatalyst and preparation method and application thereof

Abstract

The application provides a kind of dual active site polyacid metal organic framework compound photocatalyst and its preparation method and application, it is related to photocatalyst preparation field.The preparation method includes: Keggin type polyoxometalate, chromium nitrate, terephthalic acid are mixed to obtain mixed solution;The molar ratio of Keggin type polyoxometalate, chromium nitrate, terephthalic acid is (0.01-0.10) : 1: 1;The obtained mixed solution is stirred at room temperature, and then the reaction product is obtained by solvothermal reaction, extraction, to obtain dual active site polyacid metal organic framework compound photocatalyst;Wherein, the molecular formula of the Keggin type polyoxometalate is H4SiW 12‑X Mo X O 40 , X=0-12.The obtained photocatalyst can be used for photocatalytic synthesis of urea with water, nitrogen and carbon dioxide at room temperature and normal pressure, the Keggin type polyoxometalate is H4SiW6Mo6O 40 , and the urea yield can reach 1148 μg·h ‑1 ·g ‑1 cat , and the recycling performance is excellent.

Description

Technical Field

[0001] This invention relates to the field of photocatalyst preparation technology, and particularly to dual-active-site polyacid-based metal-organic framework compound photocatalysts, their preparation methods, and applications. Background Technology

[0002] Polyacid-based MOFs combine polyacids (POMs) and metal-organic frameworks (MOFs). POMs possess characteristics such as low specific surface area, aggregation effect, and excellent water solubility. MOFs, on the other hand, suffer from a lack of reactive sites due to saturated metal ion coordination environments, and their structure is partially destroyed by changes in the valence state of metal ions under light. The combination of these two materials, through synergistic effects, effectively promotes improved catalytic performance and has great application potential.

[0003] To date, urea, as a core nitrogen fertilizer, is the main type of nitrogen fertilizer used in most countries. The primary industrial route for urea production employs the Bosch-Meiser process, which involves the coupling of ammonia and carbon dioxide to produce urea at 150-200°C and 15-25 MPa. This synthesis process is accompanied by significant energy consumption and carbon dioxide emissions. Furthermore, the ammonia feedstock mainly relies on the Haber-Bosch process, which converts nitrogen and hydrogen into ammonia at 150-200°C and 15-25 MPa. Due to the harsh reaction conditions, this process also presents energy consumption issues.

[0004] It is evident that developing urea synthesis methods under mild conditions has significant application value for alleviating the energy crisis and environmental pressures. Summary of the Invention

[0005] This invention provides a dual-active-site polyacid-based metal-organic framework compound photocatalyst, its preparation method, and its application. It enables the photocatalytic synthesis of urea using water, nitrogen, and carbon dioxide as raw materials at room temperature and atmospheric pressure, effectively solving the problems of complex urea synthesis processes, high energy consumption, and large carbon dioxide emissions in existing urea synthesis processes.

[0006] This invention proposes a method for preparing a dual-active-site polyacid-based metal-organic framework photocatalyst, comprising:

[0007] A mixture of Keggin-type polyoxometalate, chromium nitrate, terephthalic acid, and water was prepared; the molar ratio of Keggin-type polyoxometalate, chromium nitrate, and terephthalic acid was (0.01-0.10):1:1.

[0008] The resulting mixture was stirred at room temperature, and the reaction product was obtained by solvothermal reaction. The product was then extracted to obtain a dual-active-site polyacid-based metal-organic framework compound photocatalyst.

[0009] The molecular formula of the Keggin-type polyoxometalate is H4SiW. 12-X Mo X O 40 X = 0 to 12.

[0010] Furthermore, the molecular formula of the Keggin-type polyoxometalate is H4SiW6Mo6O 40 .

[0011] Furthermore, the reaction temperature of the solvothermal reaction is 160-200℃; the reaction time of the solvothermal reaction is 6-72 hours.

[0012] Furthermore, the extraction specifically involves cooling the reaction product, removing the lower precipitate, and then performing ultrasonication, centrifugation, washing, and vacuum drying.

[0013] This invention also proposes a dual-active-site polyacid-based metal-organic framework compound photocatalyst prepared by any of the above-described preparation methods.

[0014] This invention also proposes the application of any of the above-described dual-active-site polyacid-based metal-organic framework compound photocatalysts in urea preparation.

[0015] Further, a dual-active-site polyacid-based metal-organic framework compound photocatalyst and water were added to the jacketed reactor. After ultrasonic dispersion, the light source was turned on, a mixture of nitrogen and carbon dioxide was introduced, sodium bicarbonate was added, and the reaction was carried out to obtain urea.

[0016] Furthermore, the concentration of sodium bicarbonate in the reaction system is 0-500 ppm.

[0017] Furthermore, the volume ratio of nitrogen to carbon dioxide is 1–10:1–10;

[0018] Preferably, the volume ratio of nitrogen to carbon dioxide is 1:1.

[0019] Furthermore, the light source is positioned 10 cm from the liquid surface inside the jacketed reactor; the light source is provided by a xenon lamp with a current of 15 A.

[0020] This invention has the following advantages:

[0021] This invention proposes a method for preparing dual-active-site polyoxometalate metal-organic framework (MOF) photocatalysts. Using Keggin-type polyoxometalates (POMs), chromium nitrate, and terephthalic acid as raw materials, polyoxometalates (MOFs) are synthesized via a solvothermal reaction. This method utilizes readily available and low-cost raw materials and involves simple steps. The resulting photocatalyst uses a Keggin-type polyoxometalate (H4SiW) with adjustable proportions. 12-X Mo X O40 (X = 0~12), where tungsten (W) serves as the active site for nitrogen reduction and molybdenum (Mo) serves as the active site for carbon dioxide reduction. By adjusting the type and ratio of polyoxometalates, photocatalysts with different urea selectivity are obtained to meet the requirements of different gas feedstock ratios. The obtained photocatalysts can be used for the photocatalytic synthesis of urea from water, nitrogen, and carbon dioxide at room temperature and atmospheric pressure. The Keggin-type polyoxometalate is H4SiW6Mo6O. 40 The urea yield can reach 1148 μg·h -1 ·g -1 cat It also has excellent recyclability. Attached Figure Description

[0022] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0023] Figure 1 This is a scanning electron microscope (SEM) image of the photocatalyst obtained in Example 1 of the present invention;

[0024] Figure 2 This is the X-ray diffraction (XRD) pattern of the photocatalyst powder obtained in Example 1 of this invention;

[0025] Figure 3 This is a diagram of the reaction apparatus for the urea synthesis process in an embodiment of the present invention;

[0026] Figure 4 This is a test diagram of nitrogen reduction products of the photocatalyst in different concentrations of sodium bicarbonate in Experiment Example 1 of this invention;

[0027] Figure 5 This is a test graph of the cycle performance of the photocatalyst in Experiment Example 1 of this invention;

[0028] Figure 6 This is a scanning electron microscope (SEM) image of the photocatalyst in Example 2 of the present invention;

[0029] Figure 7 This is the X-ray diffraction (XRD) pattern of the photocatalyst powder in Example 2 of the present invention;

[0030] Figure 8 This is a scanning electron microscope (SEM) image of the photocatalyst in Example 3 of the present invention;

[0031] Figure 9 This is the X-ray diffraction (XRD) pattern of the photocatalyst powder in Example 3 of the present invention. Detailed Implementation

[0032] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0033] The present invention will now be described in detail with reference to the accompanying drawings.

[0034] One embodiment of the present invention provides a method for preparing a photocatalyst based on a dual-active-site polyacid-based metal-organic framework compound, comprising:

[0035] A mixture of Keggin-type polyoxometalate, chromium nitrate, terephthalic acid, and water is prepared; wherein the molar ratio of Keggin-type polyoxometalate, chromium nitrate, and terephthalic acid is (0.01-0.10):1:1.

[0036] The above mixture was stirred at room temperature and then subjected to a solvothermal reaction to obtain the reaction product. The product was extracted to obtain a dual-active-site polyacid-based metal-organic framework compound photocatalyst.

[0037] The molecular formula of the Keggin-type polyoxometalate is H4SiW. 12-X Mo X O 40 X = 0 to 12.

[0038] The method for preparing dual-active-site polyoxometalate (POM) metal-organic framework (MOF) photocatalysts proposed in this invention uses Keggin-type polyoxometalates (POMs), chromium nitrate, and terephthalic acid as raw materials. The polyoxometalates are prepared via a solvothermal reaction. This method uses readily available and low-cost raw materials, and the preparation process is simple, requiring only sealed heating. 12-X Mo X O 40 (X = 0–12), where tungsten (W) serves as the active site for nitrogen reduction and molybdenum (Mo) serves as the active site for carbon dioxide reduction. By adjusting the type and ratio of polyacids, photocatalysts with different urea selectivity can be obtained to meet the requirements of different gas feedstock ratios. The obtained photocatalyst can be used for the photocatalytic synthesis of urea from water, nitrogen, and carbon dioxide at room temperature and atmospheric pressure, with a urea yield of up to 1148 μg·h⁻¹. -1 ·g -1 cat It also has excellent recyclability.

[0039] Specifically, the molecular formula of the Keggin-type polyoxometalate is H4SiW 12-X Mo X O 40Where X = 0 to 12. It should be noted that W represents tungsten, the active site for nitrogen reduction; Mo represents molybdenum, the active site for carbon dioxide reduction; X can be selected from 0 to 12, and different proportions of POMs can be adapted to different proportions of gas components. Preferably, when X is 6, the molecular formula of the Keggin-type polyoxometalate is H4SiW6Mo6O. 40 .

[0040] In one embodiment of the present invention, the reaction temperature of the solvothermal reaction is 160-200°C. Preferably, the reaction temperature of the solvothermal reaction is 170-190°C. More preferably, the reaction temperature of the solvothermal reaction is 180°C.

[0041] In one embodiment of the present invention, the reaction time of the solvothermal reaction is 6-72 hours. Preferably, the reaction time of the solvothermal reaction is 12-48 hours. More preferably, the reaction time of the solvothermal reaction is 24 hours.

[0042] In one embodiment of the present invention, the molar ratio of Keggin-type polyoxometalate, chromium nitrate, and terephthalic acid is (0.01-0.10):1:1. Preferably, the molar ratio of Keggin-type polyoxometalate, chromium nitrate, and terephthalic acid is 0.03:1:1. In this embodiment of the present invention, the loading of POMs will lead to the filling of the pores of the metal-organic framework compound (MOF), thereby weakening the adsorption of the raw material gas by the photocatalyst and thus reducing the reaction activity of the photocatalyst. As active sites for catalytic reactions, insufficient loading of POMs will also have a negative impact on the reaction activity of the photocatalyst. At this ratio, the loading of POMs and the activity of the catalyst are optimal, and the resulting material has a stable crystal structure.

[0043] The mixture described in this embodiment of the invention is a suspension formed by Keggin-type polyoxometalate, chromium nitrate, terephthalic acid, and water. The ratio of chromium nitrate, terephthalic acid, and water is 1 mol: 1 mol: 10–50 mL.

[0044] In one embodiment of the present invention, the extraction involves extracting a dual-active-site polyacid-based metal-organic framework photocatalyst from the reaction product. Specifically, the reaction product obtained from the solvothermal reaction is cooled to room temperature, the lower precipitate is removed, and the product is then subjected to ultrasonication, centrifugation, washing, and vacuum drying.

[0045] Specifically, the solvent used for washing is at least one of N,N-dimethylformamide, ethanol, and water, and the washing process can be repeated multiple times by centrifugation. The vacuum drying temperature is 50-120℃, and the vacuum drying time is 10-48 hours.

[0046] It should be noted that the room temperature can be 5 to 30°C.

[0047] An embodiment of the present invention also proposes a dual-active-site polyacid-based metal-organic framework compound photocatalyst prepared by any of the above-described preparation methods.

[0048] In the dual-active-site polyoxometalate-based metal-organic framework (MOF) photocatalyst obtained in this invention, Keggin-type polyoxometalate and chromium nitrate are uniformly supported in the pores (including macropores and micropores) of the obtained MOFs. Both maintain their complete chemical and molecular structures, and the material is free of other impurities and exhibits good crystallinity. The obtained dual-active-site polyoxometalate-based MOF photocatalyst has a large specific surface area, reaching 1500-2500 m². 2 ·g -1 The total pore volume is 0.8-1.2 cm³. 3 ·g -1 It can effectively adsorb gaseous raw materials, carry out catalytic reactions, and increase catalytic efficiency.

[0049] The dual-active-site polyoxometalate metal-organic framework photocatalyst obtained in the embodiments of the present invention, with the addition of the polyoxometalate H4SiW 12-X Mo X O 40 The ratio of tungsten to molybdenum can be adjusted, ultimately resulting in a composite structure with adjustable polyacid content, which can meet the usage requirements of different gas feedstock ratios.

[0050] An embodiment of the present invention also proposes the application of any of the above-mentioned dual-active-site polyacid-based metal-organic framework compound photocatalysts in urea preparation.

[0051] Specifically, the application includes the following steps:

[0052] A dual-active-site polyacid-based metal-organic framework compound photocatalyst and water were added to a jacketed reactor. After ultrasonic dispersion, the light source was turned on, a mixture of nitrogen and carbon dioxide was introduced, sodium bicarbonate was added, and the reaction was carried out to obtain urea.

[0053] In one embodiment of the present invention, the volume ratio of nitrogen to carbon dioxide in the reaction raw materials is 1-10:1-10. More preferably, the volume ratio of nitrogen to carbon dioxide is 1:1. In this embodiment of the present invention, tungsten is used to reduce nitrogen and molybdenum is used to reduce carbon dioxide. By adjusting the elemental ratio in the polyoxometalate, dual-active-site polyoxometalate-based metal-organic framework photocatalysts with different elemental ratios can be obtained, thereby meeting the requirements for different gas raw material ratios. When the molecular formula of the Keggin-type polyoxometalate is H4SiW6Mo6O 40 At this time, the ratio of tungsten to molybdenum in polyacid is 1:1, which is more suitable when the volume ratio of nitrogen to carbon dioxide is 1:1, and the conversion rate of urea is optimal.

[0054] In one embodiment of the present invention, the concentration of sodium bicarbonate in the reaction system is 0-500 ppm. In the process conditions described in this embodiment, since the reduction products of nitrogen gas include not only urea but also ammonia, sodium bicarbonate is used to selectively control the urea content. Sodium bicarbonate can promote the conversion of nitrogen reduction products from ammonia to urea, but excessive sodium bicarbonate will inhibit urea synthesis. Preferably, the concentration of sodium bicarbonate in the reaction system is 100 ppm.

[0055] Preferably, the ratio of the dual-active-site polyacid-based metal-organic framework photocatalyst to water is 10–50 mg: 100 mL. More preferably, the ratio of the dual-active-site polyacid-based metal-organic framework photocatalyst to water is 30 mg: 100 mL.

[0056] Preferably, the reaction temperature is 25°C. Specifically, the thermostatic bath is started, and 25°C condensate is introduced into the jacket of the jacketed reactor to control the reaction temperature.

[0057] Preferably, a stirrer is installed inside the reactor for stirring during the reaction. More preferably, the stirrer rotates at a speed of 600 rpm·min. -1 .

[0058] Preferably, the light source is provided by a xenon lamp with a current of 15A. The light source is positioned 1-10 cm above the liquid surface in the jacketed reactor; preferably, the light source is positioned 10 cm above the liquid surface. Under these conditions, the photocatalyst performance can be optimized.

[0059] In this embodiment of the invention, the photocatalyst prepared by the aforementioned method is used for the photocatalytic synthesis of urea with water, nitrogen, and carbon dioxide at room temperature and atmospheric pressure. When the volume ratio of nitrogen to carbon dioxide is 1:1 and the concentration of sodium bicarbonate is 100 ppm, the urea yield reaches 1148 μg·h⁻¹. -1 ·g -1 cat No inactivation occurred in five cycles of experiments, demonstrating excellent recyclability. This effectively solves the problem of high energy consumption and large carbon dioxide emissions associated with existing urea synthesis methods that use ammonia and carbon dioxide as raw materials under high temperature and pressure conditions.

[0060] The present invention will now be described in detail with reference to the embodiments.

[0061] Example 1 Methods for preparing dual-active-site polyacid-based metal-organic framework photocatalysts include:

[0062] Weigh out 2 mmol of terephthalic acid, 2 mmol of chromium nitrate, and 0.06 mmol of Keggin-type H4SiW6Mo6O. 40Place the mixture in a beaker and add 30 mL of deionized water. Stir at room temperature for 1 hour to obtain a mixture.

[0063] The above mixture was placed in a reaction vessel for a solvothermal reaction at 180°C for 24 hours, and then allowed to cool naturally to room temperature. The product obtained in the above steps was placed in a centrifuge tube and centrifuged at 8000 rpm. -1 The precipitate was ultrasonically cleaned at least three times with N,N-dimethylformamide and ethanol at a rotation speed of 1000 rpm. The precipitate was then transferred to a vacuum drying oven and kept at 80°C for 24 hours to obtain the product SiW6Mo6@MOFs, denoted as SiW6Mo6@MIL-101(Cr).

[0064] Its scanning electron microscope (SEM) image is as follows Figure 1 As shown; X-ray diffraction (XRD) pattern as follows Figure 2 As shown.

[0065] Example 2 Methods for preparing dual-active-site polyacid-based metal-organic framework photocatalysts include:

[0066] Same as Example 1, except that Keggin type H4SiW9Mo3O is weighed. 40 Replace H4SiW6Mo6O 40 All other conditions remained unchanged. The resulting product was denoted as SiW9Mo3@MIL-101(Cr).

[0067] Its SEM image is as follows Figure 6 As shown; its XRD pattern is as follows. Figure 7 As shown.

[0068] Example 3 Methods for preparing dual-active-site polyacid-based metal-organic framework photocatalysts include:

[0069] Same as Example 1, except that Keggin type H4SiW3Mo9O 40 Replace H4SiW6Mo6O 40 All other conditions remained unchanged. The resulting product was denoted as SiW3Mo9@MIL-101(Cr).

[0070] Its SEM image is as follows Figure 8 As shown; its XRD pattern is as follows. Figure 9 As shown.

[0071] Experimental Example 1 Photocatalyst performance testing

[0072] Using the photocatalyst SiW6Mo6@MOFs prepared in Example 1 of this invention, urea was synthesized photocatalystically at room temperature and atmospheric pressure using water, nitrogen, and carbon dioxide as raw materials. Specifically:

[0073] Add 100 mL of deionized water and 30 mg of photocatalyst to the jacketed reactor and sonicate for 10 min to uniformly disperse the material. Turn on the thermostat and introduce 25°C condensate into the reactor. Set the stirrer to 600 rpm. -1 Stirring was performed, the light source was adjusted to 10 cm above the surface of the stirred liquid, the xenon lamp current was adjusted to 15 A, and a nitrogen and carbon dioxide mixture with a volume ratio of 1:1 was introduced. Sodium bicarbonate was added to the reaction system to adjust the selectivity of urea. Catalytic activity was tested at different sodium bicarbonate concentrations (100 ppm, 20 ppm, 50 ppm, 100 ppm, 200 ppm, 500 ppm), using the following reaction apparatus: Figure 3 As shown, its performance test is as follows: Figure 4 As shown.

[0074] As shown in the figure, the photocatalyst SiW6Mo6@MOFs exhibits the highest urea catalytic activity of 1148 μg·h⁻¹ at a sodium bicarbonate concentration of 100 ppm. -1 ·g -1 cat .

[0075] The results above show that, under a certain light source intensity, tungsten serves as the active site for nitrogen reduction and molybdenum as the active site for carbon dioxide reduction in the photocatalyst SiW6Mo6@MOFs. The tungsten-to-molybdenum ratio in the SiW6Mo6@MOFs photocatalyst is 1:1, and the volume ratio of nitrogen to carbon dioxide in the raw materials is also 1:1. This allows tungsten to effectively reduce nitrogen and molybdenum to effectively reduce carbon dioxide. Combined with the effect of 100 ppm sodium bicarbonate, the urea catalytic activity reaches a maximum of 1148 μg·h⁻¹. -1 ·g -1 cat .

[0076] To further verify the stability of the catalyst, the photocatalyst SiW6Mo6@MOFs was subjected to catalytic activity cycling tests at a sodium bicarbonate concentration of 100 ppm in the aforementioned urea synthesis process. The performance tests are as follows: Figure 5 As shown, the photocatalyst SiW6Mo6@MOFs did not show significant changes in catalytic activity during 5 cycles, demonstrating excellent cycling stability.

[0077] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. 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. The application of a dual-active-site polyacid-based metal-organic framework compound photocatalyst in urea preparation, characterized in that, The preparation method of the dual-active-site polyacid-based metal-organic framework compound photocatalyst includes: A mixture of Keggin-type polyoxometalate, chromium nitrate, terephthalic acid, and water was prepared; the molar ratio of Keggin-type polyoxometalate, chromium nitrate, and terephthalic acid was (0.01-0.10):1:

1. The resulting mixture was stirred at room temperature, and the reaction product was obtained by solvothermal reaction. The product was then extracted to obtain a dual-active-site polyacid-based metal-organic framework compound photocatalyst. The molecular formula of the Keggin-type polyoxometalate is H4SiW. 12-X Mo X O 40 X = 3~9.

2. The application according to claim 1, characterized in that, The molecular formula of the Keggin-type polyoxometalate is H4SiW6Mo6O 40 .

3. The application according to claim 1, characterized in that, The reaction temperature of the solvothermal reaction is 160-200 °C; the reaction time of the solvothermal reaction is 6-72 hours.

4. The application according to claim 1, characterized in that, The extraction process specifically involves cooling the reaction product, removing the lower precipitate, followed by ultrasonication, centrifugation, washing, and vacuum drying.

5. The application according to claim 1, characterized in that, A dual-active-site polyacid-based metal-organic framework compound photocatalyst and water were added to a jacketed reactor. After ultrasonic dispersion, the light source was turned on, a mixture of nitrogen and carbon dioxide was introduced, sodium bicarbonate was added, and the reaction was carried out to obtain urea.

6. The application according to claim 5, characterized in that, The concentration of sodium bicarbonate in the reaction system is 0-500 ppm.

7. The application according to claim 6, characterized in that, The concentration of sodium bicarbonate in the reaction system is 100 ppm.

8. The application according to claim 5, characterized in that, The volume ratio of nitrogen to carbon dioxide is 1~10:1~10.

9. The application according to claim 8, characterized in that, The volume ratio of nitrogen to carbon dioxide is 1:

1.

10. The application according to claim 5, characterized in that, The light source is located 10 cm away from the liquid surface inside the jacketed reactor; the light source is provided by a xenon lamp with a current of 15A.

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