A solid-supported composite of a metal-organic small molecule and g-C3N4, and its preparation method and application

Abstract

The present invention relates to a supported composite of metal-organic small molecules and g-C3N4, and its preparation method and application. Under microwave hydrothermal conditions, the method realizes the fixation of metal-organic small molecule materials on polymer powder materials by carrying out a condensation reaction between the hydroxyl or carboxyl groups on the metal-organic small molecules and the amino groups on g-C3N4. Compared with the prior art, the method of the present invention has the advantages of high loading amount, good dispersibility, etc. The prepared supported composite catalytic material has a high CO2 conversion rate and selectivity, and has good application prospects in the photocatalytic CO2 reduction reaction.

Description

Technical Field

[0001] The present invention relates to the field of catalytic materials, and particularly to a supported composite of metal-organic small molecules and g-C3N4, a preparation method thereof, and an application thereof. Background Art

[0002] At present, photocatalytic technology has become an important environmental protection technology. However, the catalytic performance of photocatalytic materials is restricted by the physical and chemical properties of the materials. Therefore, the development of efficient photocatalytic materials has always been the goal pursued by people. g-C3N4 is a typical layered polymer semiconductor and is considered an ideal material for visible-light water splitting due to its suitable bandgap, chemical stability, and low toxicity. However, traditional solid photocatalytic materials often have an uneven physical structure, which results in low catalytic efficiency. To overcome this problem, researchers have proposed a method of immobilizing and compounding metal-organic small molecules as homogeneous catalysts. However, traditional immobilization and compounding methods often have problems such as low immobilization amount and poor dispersion, thus affecting the catalytic efficiency.

[0003] Patent CN111644192A discloses a g-C3N4@Cd x Zn 1-x Se composite photocatalyst, a preparation method thereof, and an application thereof. In this patent, g-C3N4 nanosheets are subjected to microwave hydrothermal reaction with cadmium salts, zinc salts, and selenium compounds to obtain a g-C3N4@Cd x Zn 1-x Se composite photocatalyst, which improves the photocatalytic hydrogen production activity. Summary of the Invention

[0004] The purpose of the present invention is to provide a supported composite of metal-organic small molecules and g-C3N4, a preparation method thereof, and an application thereof, in order to overcome the problems of low catalytic reduction efficiency of the existing pure g-C3N4, low immobilization amount, and poor dispersion in the immobilization and compounding method.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] One technical solution of the present invention is to provide a preparation method of a supported composite of metal-organic small molecules and g-C3N4, comprising the following steps:

[0007] S1. Mix metal-organic small molecules, g-C3N4, and deionized water, and then carry out microwave hydrothermal reaction to obtain a pretreated composite;

[0008] S2. Add ethanol and hydrochloric acid to the composite obtained in step S1, stir, centrifuge to collect the precipitate, and wash and dry to obtain the supported composite.

[0009] Further, in step S1, the ratio of the metal-organic small molecule, g-C3N4, to deionized water is 5-15 g: 1 g: 60-70 mL.

[0010] Further, in step S1, the metal center component of the metal-organic small molecule is selected from any one of Fe, Cu, Ti, Zn, Zr, Ni, Pt, Co, Mg, Ce, In, Pd, Ag, Rh;

[0011] The ligand of the metal-organic small molecule is selected from any one of aryl compounds, derivatives of aryl compounds, heterocyclic compounds, derivatives of heterocyclic compounds, carboxylic acid compounds or derivatives of carboxylic acid compounds.

[0012] Furthermore, the aryl compound is selected from any one of phenyl, naphthyl, anthryl, biphenyl or perylenyl; the heterocyclic compound is selected from any one of imidazole, thiazole, pyrazole, imidazoline, porphyrin, quinoline, pyrrole, pyridine, thiophene, bipyridine or piperidine; the carboxylic acid compound is selected from any one of acetylacetone carboxylic acid, styrene carboxylic acid or oxalic acid.

[0013] Furthermore, the ligand of the metal-organic small molecule is preferably quinclorac, 5-quinolinol, pyrroloquinoline quinone, chloroiodoquine, 8-hydroxyquinoline, 3-hydroxyquinoline.

[0014] The metal-organic molecule is preferably sodium copper chlorophyllin, zinc protoporphyrin, copper 8-hydroxyquinoline, nickel mesoporphyrin, magnesium protoporphyrin IX dipotassium salt, heme, protoporphyrin IX stannous dichloride.

[0015] The metal-organic molecule selected in the present invention has a hydroxyl group or a carboxyl group, enabling it to undergo a condensation reaction with the amino group on g-C3N4, thereby forming a bond. This bonding effect is conducive to the formation of a special "double S" mechanism, that is, an S-type heterojunction with a strong covalent bond interface, improving the catalytic performance of the catalytic material.

[0016] Further, in step S1, the reaction temperature of the microwave hydrothermal reaction is 50-200 °C, the reaction time is 5-180 min, the heating rate is 5 °C / min - 60 °C / min, and the microwave power is 600-1000 W.

[0017] Further, in step S2, the addition amount of ethanol is 20-50 mL, the addition amount of hydrochloric acid is 5 mL, and the concentration of the hydrochloric acid is 0.5 M - 5 M.

[0018] Further, in step S2, the stirring temperature is room temperature, the stirring time is 0.5-12 h; the washing solvent is water and ethanol.

[0019] Further, in step S2, the drying process is carried out in a vacuum, air or inert atmosphere, and the inert atmosphere includes nitrogen and argon.

[0020] Furthermore, the drying process is preferably carried out under vacuum conditions.

[0021] The second technical solution of the present invention is to provide a supported composite of metal-organic small molecules and g-C3N4, based on the preparation method described in the above technical solution one.

[0022] The third technical solution of the present invention is to provide an application of the supported composite of metal-organic small molecules and g-C3N4 as described in the above technical solution two, and the supported composite is used as a photocatalyst for photocatalytic reactions.

[0023] Further, the photocatalytic reaction includes a photocatalytic CO2 reduction reaction.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) In the present invention, the condensation reaction between the hydroxyl or carboxyl group on the metal-organic small molecule and the amino group on g-C3N4 is carried out under microwave hydrothermal conditions, realizing the fixation of the metal-organic small molecule material on the polymer powder material, that is, enabling the metal-organic small molecule to be supported on the surface of g-C3N4 to form a supported composite of a homogeneous catalyst. Compared with the traditional supported composite method, the method of the present invention has the advantages of high loading amount and good dispersibility, etc., so that the prepared supported composite catalytic material has high catalytic efficiency, providing a new idea for improving the reduction efficiency and selectivity in the CO2 photocatalytic reduction technology.

[0026] (2) The supported composite catalytic material prepared in the present invention has a narrow band gap, enhanced visible light absorption, improved CO2 reduction performance, and realizes the spatial separation of photogenerated carriers through the good conductivity of the two-dimensional material. Therefore, it shows good application prospects in the photocatalytic CO2 reduction reaction.

[0027] (3) The preparation method of the present invention is simple, the conditions are mild and clean, and basically no environmental pollution is generated during the preparation process; the chemical reagents used are all common reagents, which are cheap and easily available. Compared with other traditional methods for synthesizing materials, this method has the advantages of low energy consumption, fast controllability and precise coordination synthesis. Description of the Drawings

[0028] Figure 1 It is a comparative diagram of the photocatalytic CO2 reduction performance of the CSN composite material prepared with different mass percentages of SCC and g-C3N4 in Example 1 and the pure CN, SCC and the MX composite material prepared in Comparative Example 3 under the same conditions.

[0029] Figure 2 XRD pattern of the CSN-10 composite material prepared in Example 1.

[0030] Figure 3 Scanning electron micrograph of the CSN composite material prepared in Example 1.

[0031] Figure 4 Photocatalytic CO2 reduction performance comparison chart of the CSN composite materials prepared in Comparative Example 1 and Comparative Example 2 and pure CN and SCC under the same conditions. Detailed implementation mode

[0032] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] In the following examples and comparative examples, unless otherwise specified, the raw materials or treatment techniques are all conventional commercially available raw material products or conventional treatment techniques in the art.

[0034] In some specific embodiments, a preparation method of a solid-supported composite of a metal-organic small molecule and g-C3N4 includes the following steps:

[0035] Disperse x wt% of a metal-organic small molecule (x wt% is the mass percentage of the metal-organic small molecule and g-C3N4, x = 5, 7, 10, 13, 15) in 20 mL of deionized water to obtain solution A, disperse 0.71 g of g-C3N4 in 30 mL of deionized water to obtain solution B, put the mixture of solution A and solution B into a microwave reactor, in the temperature range of 90 °C to 180 °C, in the microwave power range of 600 - 1000 W, carry out microwave hydrothermal reaction for 30 min to 90 min, carry out condensation reaction between the hydroxyl or carboxyl group on the metal-organic small molecule and the amino group on g-C3N4 to form a pretreated composite, mix the composite in 50 mL of ethanol and 2 M 5 mL of hydrochloric acid and stir and mix at room temperature for 2 h, then centrifuge to collect the precipitate, wash it with water and ethanol, and dry it under vacuum conditions to finally obtain a solid-supported composite of a metal-organic small molecule and g-C3N4.

[0036] The above implementation mode will be described in more detail below with reference to specific embodiments.

[0037] Example 1:

[0038] Using sodium copper chlorophyllin (SCC) as a metal-organic small molecule and g-C3N4 (CN) as a substrate material, x wt% (x wt% is the mass percentage of SCC to CN, x = 5, 7, 10, 13, 15) of sodium copper chlorophyllin was respectively mixed with 20 mL of deionized water to obtain solution A. 0.71 g of g-C3N4 was mixed with 30 mL of deionized water to obtain solution B. The mixture of solution A and solution B was placed in a microwave reactor for a microwave hydrothermal reaction with a microwave power of 1000 W, the temperature was raised to 150 °C within 15 min and kept at this temperature for 30 min. The carboxyl group generated by the hydrolysis of sodium copper chlorophyllin was condensed with the amino group on g-C3N4. The reaction product was mixed in 50 mL of ethanol and 5 mL of 2M hydrochloric acid and stirred at room temperature for 2 h. Then the precipitate was collected by centrifugation, washed with water and ethanol, and dried under vacuum to form a solid-supported composite of copper chlorophyll and g-C3N4 (CSN).

[0039] Example 2:

[0040] Using zinc protoporphyrin as a metal-organic small molecule and g-C3N4 as a substrate material, x wt% (x wt% is the mass percentage of zinc protoporphyrin to CN, x = 5, 7, 10, 13, 15) of zinc protoporphyrin was respectively mixed with 20 mL of deionized water to obtain solution A. 0.71 g of g-C3N4 was mixed with 30 mL of deionized water to obtain solution B. The mixture of solution A and solution B was placed in a microwave reactor for a microwave hydrothermal reaction with a microwave power of 1000 W, the temperature was raised to 150 °C within 15 min and kept at this temperature for 30 min. The carboxyl group generated by the hydrolysis of zinc protoporphyrin was condensed with the amino group on g-C3N4. The reaction product was mixed in 50 mL of ethanol and 5 mL of 2M hydrochloric acid and stirred at room temperature for 2 h. Then the precipitate was collected by centrifugation, washed with water and ethanol, and dried under vacuum to form a solid-supported composite system of zinc protoporphyrin and g-C3N4.

[0041] Example 3:

[0042] Using copper 8-hydroxyquinoline as the metal-organic small molecule and g-C3N4 as the substrate material, x wt% (x wt% is the mass percentage of copper 8-hydroxyquinoline to CN, x = 5, 7, 10, 13, 15) of copper 8-hydroxyquinoline was respectively mixed with 20 mL of deionized water to obtain solution A. 0.71 g of g-C3N4 was mixed with 30 mL of deionized water to obtain solution B. The mixture of solution A and solution B was placed in a microwave reactor for a microwave hydrothermal reaction with a microwave power of 1000 W, the temperature was raised to 150 °C within 15 min and kept at this temperature for 30 min. The hydroxyl groups generated by the hydrolysis of copper 8-hydroxyquinoline underwent a condensation reaction with the amino groups on g-C3N4. The reaction product was mixed in 50 mL of ethanol and 5 mL of 2 M hydrochloric acid and stirred at room temperature for 2 h. Then, the precipitate was collected by centrifugation, washed with water and ethanol, and dried under vacuum conditions to form a supported composite system of copper 8-hydroxyquinoline and g-C3N4.

[0043] Example 4:

[0044] Using nickel protoporphyrin as the metal-organic small molecule and g-C3N4 as the substrate material, x wt% (x wt% is the mass percentage of nickel protoporphyrin to CN, x = 5, 7, 10, 13, 15) of nickel protoporphyrin was respectively mixed with 20 mL of deionized water to obtain solution A. 0.71 g of g-C3N4 was mixed with 30 mL of deionized water to obtain solution B. The mixture of solution A and solution B was placed in a microwave reactor for a microwave hydrothermal reaction with a microwave power of 1000 W, the temperature was raised to 150 °C within 15 min and kept at this temperature for 30 min. The hydroxyl groups generated by the hydrolysis of copper 8-hydroxyquinoline underwent a condensation reaction with the amino groups on g-C3N4. The reaction product was mixed in 50 mL of ethanol and 5 mL of 2 M hydrochloric acid and stirred at room temperature for 2 h. Then, the precipitate was collected by centrifugation, washed with water and ethanol, and dried under vacuum conditions to form a supported composite of nickel protoporphyrin and g-C3N4.

[0045] Comparative Example 1:

[0046] Compared with Example 1, most of them are the same, except that the microwave hydrothermal reaction was replaced with a hydrothermal reaction. Specifically:

[0047] Using sodium copper chlorophyllin as a metal-organic small molecule and g-C3N4 as a substrate material, x wt% (x wt% is the mass percentage of SCC and CN, x = 5, 7, 10, 13, 15) of sodium copper chlorophyllin was respectively mixed with 20 mL of deionized water to obtain solution A, 0.71 g of g-C3N4 was mixed with 30 mL of deionized water to obtain solution B, and the mixture of solution A and solution B was subjected to a hydrothermal reaction for 12 h at a temperature of 200 °C. The reaction product was mixed in 50 mL of ethanol and 5 mL of 2 M hydrochloric acid and stirred at room temperature for 2 h, then the precipitate was collected by centrifugation, washed with water and ethanol, and dried under vacuum to form a solid-supported complex (CSN) of copper chlorophyll and g-C3N4.

[0048] Comparative Example 2:

[0049] Compared with Example 1, most of them are the same, except that the microwave hydrothermal reaction is replaced by an ultrasonic mixing reaction, specifically:

[0050] Using sodium copper chlorophyllin as a metal-organic small molecule and g-C3N4 as a substrate material, x wt% (x wt% is the mass percentage of SCC and CN, x = 5, 7, 10, 13, 15) of sodium copper chlorophyllin was respectively mixed with 20 mL of deionized water to obtain solution A, 0.71 g of g-C3N4 was mixed with 30 mL of deionized water to obtain solution B, and the mixture of solution A and solution B was ultrasonically mixed under the condition of 40 kHz. After the compounding was completed, it was cooled to room temperature. A certain amount of the sample was placed in a reactor and subjected to a solid-phase microwave reaction in vacuo for 15 min at a microwave power of 400 W, and then cooled to room temperature to form a solid-supported complex (CSN) of copper chlorophyll and g-C3N4.

[0051] Comparative Example 3:

[0052] Compared with Example 1, most of them are the same, except that the microwave hydrothermal reaction is replaced by mechanical mixing, specifically:

[0053] Using sodium copper chlorophyllin as a metal-organic small molecule and g-C3N4 as a substrate material, sodium copper chlorophyllin and g-C3N4 were mechanically mixed according to the mass ratio of g-C3N4 to SCC of 10:1 to form MX samples.

[0054] Comparative Example 4:

[0055] Compared with Example 2, most of them are the same, except that the microwave hydrothermal reaction is replaced by a hydrothermal reaction, specifically:

[0056] Using zinc protoporphyrin as a metal-organic small molecule and g-C3N4 as a substrate material, x wt% (x wt% is the mass percentage of zinc protoporphyrin to CN, x = 5, 7, 10, 13, 15) of zinc protoporphyrin was respectively mixed with 20 mL of deionized water to obtain solution A, 0.71 g of g-C3N4 was mixed with 30 mL of deionized water to obtain solution B, the mixture of solution A and solution B was subjected to a hydrothermal reaction for 12 h at a temperature of 200 °C, the reaction product was mixed in 50 mL of ethanol and 5 mL of 2 M hydrochloric acid and stirred at room temperature for 2 h, then the precipitate was collected by centrifugation, washed with water and ethanol, and dried under vacuum conditions to form a supported composite system of zinc protoporphyrin and g-C3N4.

[0057] Comparative Example 5:

[0058] Compared with Example 3, most of them are the same, except that the microwave hydrothermal reaction is replaced with a hydrothermal reaction, specifically:

[0059] Using copper 8-hydroxyquinoline as a metal-organic small molecule and g-C3N4 as a substrate material, x wt% (x wt% is the mass percentage of copper 8-hydroxyquinoline to CN, x = 5, 7, 10, 13, 15) of copper 8-hydroxyquinoline was respectively mixed with 20 mL of deionized water to obtain solution A, 0.71 g of g-C3N4 was mixed with 30 mL of deionized water to obtain solution B, the mixture of solution A and solution B was subjected to a hydrothermal reaction for 12 h at a temperature of 200 °C, the reaction product was mixed in 50 mL of ethanol and 5 mL of 2 M hydrochloric acid and stirred at room temperature for 2 h, then the precipitate was collected by centrifugation, washed with water and ethanol, and dried under vacuum conditions to form a supported composite system of copper 8-hydroxyquinoline and g-C3N4.

[0060] Comparative Example 6:

[0061] Compared with Example 4, most of them are the same, except that the microwave hydrothermal reaction is replaced with a hydrothermal reaction, specifically:

[0062] Using nickel protoporphyrin as a metal-organic small molecule and g-C3N4 as a substrate material, x wt% (x wt% is the mass percentage of nickel protoporphyrin to CN, x = 5, 7, 10, 13, 15) of nickel protoporphyrin was respectively mixed with 20 mL of deionized water to obtain solution A, 0.71 g of g-C3N4 was mixed with 30 mL of deionized water to obtain solution B, the mixture of solution A and solution B was subjected to a hydrothermal reaction for 12 h at a temperature of 200 °C, the reaction product was mixed in 50 mL of ethanol and 5 mL of 2 M hydrochloric acid and stirred at room temperature for 2 h, then the precipitate was collected by centrifugation, washed with water and ethanol, and dried under vacuum conditions to form a supported complex of nickel protoporphyrin and g-C3N4.

[0063] The supported composites prepared in the above examples and comparative examples were structurally characterized by the following methods:

[0064] X-ray diffraction measured on a Bruker D8 ADVANCE X-ray diffractometer in Germany was used for the structural analysis of the samples;

[0065] Scanning electron microscope photos obtained on a FEI Quanta FEG 450 scanning electron microscope in the United States were used to analyze the morphological structure of the samples.

[0066] The performance test method of the catalytic material in the photocatalytic CO2 reduction reaction includes the following steps:

[0067] Gas-solid phase photocatalytic CO2 reduction occurs in a closed quartz glass reactor at room temperature without adding any sacrificial agent or cocatalyst.

[0068] (1) Add 20 mg of photocatalyst and 20 mL of deionized water to the bottom of the quartz reactor and perform ultrasonic dispersion.

[0069] (2) Transfer the reactor to a vacuum oven and dry it at 60 °C. After the water has completely evaporated, the catalyst powder remains at the bottom of the reactor, forming a uniform photocatalytic layer.

[0070] (3) Take 0.084 g of sodium bicarbonate (NaHCO3) powder and add it to the reactor tank. The reactor is purged with N2 for 30 min.

[0071] (4) Inject 30 μL of sulfuric acid (H2SO4) (2 M) into the reaction tank to obtain the CO2 and H2O vapor required for photocatalytic reduction of CO2 through the reaction of H2SO4 and NaHCO3.

[0072] (5) Place the quartz reactor under a 300 W Xe lamp source (MC-X301, Beijing Meiruichen Technology Co., Ltd.). Use a filter (AM1.5 G), and the distance between the catalyst and the light source is 10 cm. After 4 hours of illumination, assemble the reactor on a gas chromatograph to analyze its composition and content.

[0073] Figure 1 It is a comparative diagram of the photocatalytic CO2 reduction performance of the CSN composite materials prepared with different mass percentages of SCC and g-C3N4 in Example 1 and pure CN, SCC, and MX prepared in Comparative Example 3 under the same conditions. CSN-5, CSN-7, CSN-10, CSN-13, and CSN-15 represent the mass percentages of SCC and CN as 5, 7, 10, 13, and 15 respectively. It can be seen from the figure that the CO production rate of the CSN composite material is significantly higher than that of CN, SCC, and MX. The CO production rate of CSN-10 is the highest, which is 31.42 μmol g-1 h -1 。

[0074] Figure 2 XRD pattern of CSN-10 prepared in Example 1 (i.e., the solid-supported cobalt and penta prepared from 10% by mass of SCC and CN), where the abscissa is the angle and the ordinate is the intensity. The strong intensity of the (002) diffraction peak of CSN-10 in the figure indicates its high crystallinity. In addition, the diffraction peak of the (002) plane of CSN-10 shifts 0.1° to the left, indicating that the incorporated SCC increases the interlayer distance of CN.

[0075] Figure 3 SEM image of CSN-10 (i.e., the catalytic material prepared from 10% by mass of SCC and g-C3N4) prepared in Example 1, as Figure 3 shown in a and b, the morphology of the wrinkled CN nanosheets is well maintained in CSN-10. The uniform distribution of C, N, O, and Cu elements in the energy-dispersive spectroscopy (EDS) elemental mapping ( Figure 3 c-f) confirms the coexistence of SCC and CN in CSN-10. The fluffy hollow structure of CSN-10 can be clearly observed in the TEM image ( Figure 3 g and h). No obvious lattice fringes are found in the corresponding HRTEM image ( Figure 3 i). In addition, we speculate that the black dots marked by the white arrows in the red magnified area ( Figure 3 j) are SCC molecules, indicating the successful synthesis of CSN-10.

[0076] Figure 4 Photocatalytic CO2 reduction performance comparison chart of CSN composites prepared in Comparative Example 1 and Comparative Example 2 with pure CN and SCC under the same conditions, where CSN-10-H represents that the mass percentage of SCC and CN is 10, and the preparation condition is hydrothermal preparation; CSN-10-U represents that the mass percentage of SCC and CN is 10, and the preparation condition is ultrasonic preparation. Compared with Figure 1 CSN-10 in, the CO yields of CSN-10-H and CSN-10-U are significantly lower than that of CSN-10.

[0077] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A preparation method of a solid-supported composite of a metal-organic small molecule and g-C3N4, characterized in that, It includes the following steps: S1. Mix a metal-organic small molecule, g-C3N4, and deionized water, and then carry out microwave hydrothermal reaction to obtain a pretreated composite; S2. Add ethanol and hydrochloric acid to the composite obtained in step S1, stir, centrifuge to collect the precipitate, and wash and dry to obtain a supported composite; Among them, the metal center component of the metal-organic small molecule is selected from any one of Fe, Cu, Ti, Zn, Zr, Ni, Pt, Co, Mg, Ce, In, Pd, Ag, Rh; the ligand of the metal-organic small molecule is selected from any one of aryl compounds, derivatives of aryl compounds, heterocyclic compounds, derivatives of heterocyclic compounds, carboxylic acid compounds or derivatives of carboxylic acid compounds; the aryl compound is selected from any one of phenyl, naphthyl, anthryl, biphenyl or perylenyl; the heterocyclic compound is selected from any one of imidazole, thiazole, pyrazole, imidazoline, porphyrin, quinoline, pyrrole, pyridine, thiophene, bipyridine or piperidine; the carboxylic acid compound is selected from any one of acetylacetone carboxylic acid, styrene carboxylic acid or oxalic acid; In step S1, the reaction temperature of the microwave hydrothermal reaction is 50-200 °C, the reaction time is 5-180 min, the heating rate is 5 °C / min - 60 °C / min, and the microwave power is 600-1000 W.

2. The preparation method of the supported composite of the metal-organic small molecule and g-C3N4 according to claim 1, characterized in that, In step S1, the ratio of the metal-organic small molecule: g-C3N4: deionized water is 5-15 g: 1 g: 60-70 mL.

3. The preparation method of the supported complex of the metal-organic small molecule and g-C3N4 according to claim 1, wherein, In step S2, the addition amount of ethanol is 20-50 mL, the addition amount of hydrochloric acid is 5 mL, and the concentration of the hydrochloric acid is 0.5 M - 5 M.

4. The preparation method of the immobilized complex of the metal-organic small molecule and g-C3N4 according to claim 1, characterized in that, In step S2, the stirring temperature is room temperature, the stirring time is 0.5-12 h; the washing solvent is water and ethanol.

5. A solid-supported composite of a metal-organic small molecule and g-C3N4, characterized in that, Prepared based on the preparation method according to any one of claims 1-4.

6. Use of the immobilized complex of the metal-organic small molecule and g-C3N4 as described in claim 5, characterized in that, The supported composite is used as a photocatalyst for photocatalytic reaction.

7. Use of the supported composite of the metal-organic small molecule and g-C3N4 according to claim 6, characterized in that, The photocatalytic reaction is a photocatalytic CO2 reduction reaction.

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