Tubular C3N4 heterojunction catalyst as well as preparation method and application thereof

Abstract

The invention discloses a tubular C3N4 heterojunction photocatalyst as well as a preparation method and application thereof, the method comprises the following steps: firstly, preparing tubular C3N4 (TCN) from urea and melamine as raw materials, reacting the TCN with H3PO4 to obtain H3PO4 modified TCN (P-TCN), and finally, introducing phosphotungstic acid (PW) to construct the heterojunction photocatalyst (PW / P-TCN). According to the PW / P-TCN catalyst prepared by the invention, O2 adsorption is enhanced, separation and transfer of a photo-generated carrier are improved, exposure of active sites is increased, and meanwhile, efficient reduction and oxidation capabilities of the catalyst are maintained. According to the method, PW / P-TCN is taken as a photocatalyst, acetonitrile is taken as a solvent, a xenon lamp is taken as a light source, oxidative cleavage of C-C bonds of a lignin model compound is realized in a room-temperature oxygen atmosphere, and aromatic monomer products such as benzoic acid and phenylacetaldehyde are obtained.

Description

Technical Field

[0004] The present invention belongs to the technical field of catalyst preparation, and particularly relates to a tubular C 3 N 4 heterojunction photocatalyst and its preparation method and application. Background Art

[0005] Lignin is an important component of lignocellulosic biomass and the largest source of renewable aromatic compounds. As a substitute for fossil fuels, it has great potential in the production of value-added chemicals, fuels, and functional materials. Lignin is interconnected by disordered C-C and C-O bonds, and its complex and robust structure poses significant technical challenges. In recent years, extensive research has been conducted on the cleavage of C-O bonds, and significant progress has been made. However, theoretically, no more than 50% of the theoretical aromatic monomers can be produced by cleaving the C-O bonds in lignin. Therefore, it is of great significance to cleave C-C bonds to obtain valuable aromatic oxides. C-C bonds generally have higher bond dissociation energies than C-O bonds, and their cleavage remains one of the key challenges for lignin valorization. Traditional thermal catalytic processes, including pyrolysis, gasification, and hydrolysis, usually require high temperatures and pressures and produce a large amount of low-functionalized by-products. Therefore, it is of great significance to develop sustainable lignin depolymerization strategies under mild conditions. Summary of the Invention

[0006] The purpose of the present invention is to provide a tubular C 3 N 4 heterojunction photocatalyst and its preparation method and application. The preparation process of this method is simple; when the prepared tubular C 3 N 4 heterojunction photocatalyst PW / P-TCN is applied to the preparation of aromatic monomers by breaking the C-C bonds of lignin model compounds under visible light, it can have high photocatalytic activity.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is: a preparation method of a tubular C 3 N 4 heterojunction photocatalyst, comprising the following steps:

[0008] S1. Mix a certain amount of urea and melamine and add them to deionized water and stir. Pour the obtained solution into a hydrothermal reaction kettle and heat it. After cooling, retain the precipitate and dry it to obtain a precursor;

[0009] S2. Put the precursor obtained in step S2 into a crucible and perform calcination treatment in an inert atmosphere. After natural cooling, obtain tubular C 3 N 4 , named TCN;

[0010] S3. Disperse TCN in a certain amount of H 3 PO 4 solution, and stir to promote the adsorption of H 3 PO 4 on the surface of TCN. Centrifuge to collect the TCN adsorbed with H 3 PO 4 ;

[0011] S4. Place the TCN adsorbed with H 3 PO 4 obtained in step S3 in an oven for drying, and then conduct a calcination treatment under an inert atmosphere, and cool to room temperature to obtain H 3 PO 4 -modified TCN, named P-TCN;

[0012] S5. Prepare the P-TCN obtained in step S4 into a suspension, then add phosphotungstic acid to the P-TCN suspension and stir, and finally dry overnight to obtain tubular C 3 N 4 heterojunction photocatalyst PW / P-TCN.

[0013] Preferably, in step S1, the mass ratio between urea and melamine is 1.6:1; pour the obtained solution into a hydrothermal reaction kettle and heat it to 100 - 180 °C for 20 h.

[0014] Preferably, in step S2, calcine in an N 2 atmosphere at a rate of 2 °C / min from room temperature to 550 °C for 4 h.

[0015] Preferably, in step S3, the concentration of the H 3 PO 4 solution is 0.3 mol / L; the ratio between the mass of TCN and the volume of the H 3 PO 4 solution is 1 mg:1 ml.

[0016] Preferably, in step S4, under an N 2 atmosphere, heat to 300 °C at a rate of 5 °C / min and calcine for 90 min.

[0017] Preferably, in step S5, add P-TCN to ethanol and stir to form a suspension of 5 mg / ml; add phosphotungstic acid to the P-TCN suspension at 50 °C and stir for 12 h, and then dry at 80 °C, and the mass fraction of phosphotungstic acid is 5% - 30%.

[0018] To achieve the above object, the present invention also provides the tubular C 3 N 4 heterojunction photocatalyst prepared by the above preparation method.

[0019] To achieve the above object, the present invention also provides the tubular C prepared by the above preparation method 3 N 4 The application of the heterojunction photocatalyst in the preparation of aromatic monomers by breaking the C-C bond of lignin model compounds under visible light

[0020] Furthermore, the specific application process is as follows: taking the tubular C 3 N 4 The heterojunction photocatalyst PW / P-TCN as a catalyst, acetonitrile as a solvent, and a xenon lamp as a light source, oxidatively break the C-C bond of the lignin model compound to obtain aromatic monomers under an oxygen atmosphere at room temperature

[0021] Preferably, the model compound is one of 1,2-diphenylethanol, 1,2-diphenylethanone, 1,2-diphenylethane, 2-phenoxy-1-phenylethanol, and the aromatic monomer is one or more of benzoic acid, phenylacetaldehyde, phenyl formate

[0022] Compared with the prior art, the present invention has the following beneficial effects

[0023] (1) By preparing the tubular C 3 N 4 The specific surface area is increased and the surface transfer distance is shortened, so that the catalyst has higher photoinduced charge separation and transfer ability, and a heterojunction catalyst (PW / P-TCN) is constructed by introducing phosphotungstic acid (PW), and the doped PW acts as a high-level electron acceptor to inhibit the recombination of photoelectrons and holes

[0024] (2) The preparation process of the method of the present invention is simple; the prepared PW / P-TCN photocatalyst exhibits high oxidation activity and reusability, and can be used in a green photocatalytic reaction system to selectively and effectively cleave the C-C bond in lignin, replacing the traditional high-temperature and high-pressure hydrogen conditions to achieve the depolymerization of lignin, with the advantages of green, safe, mild conditions, low equipment requirements and simple operation; the present invention prepares high-value chemicals from pure biomass-based raw materials, greatly increasing the sustainability of the reaction process, and providing an effective and sustainable strategy for producing high-value aromatic chemicals from abundant renewable biomass resources under mild conditions Description of the Drawings

[0025] Figure 1 SEM image of PW / P-TCN prepared in Example 1 of the present invention

[0026] Figure 2 Photocatalytic conversion rate of the photocatalyst prepared in Example 1 of the present invention for different lignin model compounds and yield of products Detailed Embodiments

[0027] The following further describes in detail the specific embodiments of the present invention in conjunction with the accompanying drawings.

[0028] All raw material reagents in the embodiments of this application are purchased on the market or prepared by conventional methods well-known to those skilled in the art.

[0029] Example 1

[0030] A method for preparing a tubular C 3 N 4 heterojunction photocatalyst, comprising the following steps:

[0031] S1. Mix 8 g of urea and 5 g of melamine and add them to deionized water and stir. Pour the obtained solution into a hydrothermal reaction kettle, heat it to 100 - 180 °C, keep it for 20 h, cool it, and retain the precipitate and dry it to obtain a precursor;

[0032] S2. Put the precursor obtained in step S2 into a crucible and heat it from room temperature to 550 °C at a rate of 2 °C / min in N 2 for 4 h of calcination, and naturally cool it to obtain tubular C 3 N 4 , named TCN;

[0033] S3. Disperse 100 mg of TCN in 100 ml of 0.3 mol / L H 3 PO 4 solution, stir for 5 h to promote the adsorption of H 3 PO 4 on the surface of TCN, and centrifuge and collect the TCN after adsorbing H 3 PO 4 ;

[0034] S4. Place the TCN after adsorbing H 3 PO 4 obtained in step S3 in an oven to dry it, and then heat it to 300 °C at a rate of 5 °C / min in an N 2 atmosphere for 90 min of calcination, and cool it to room temperature to obtain H 3 PO 4 -modified TCN, named P-TCN;

[0035] S5. Add 100 mg of P-TCN obtained in step S4 to 20 ml of ethanol, stir to form a suspension of 5 mg / ml, then add phosphotungstic acid to the P-TCN suspension at 50 °C and stir for 12 h. The mass fraction of phosphotungstic acid is 20%, and finally dry it overnight at 80 °C to obtain a tubular C 3 N 4 heterojunction photocatalyst PW / P-TCN.

[0036] It can be seen from Figure 1 that the sample prepared in this embodiment generally presents a tubular structure. The surface of the sample tube wall is smooth and the texture is light. The existence of the tubular structure greatly increases the specific surface area of the catalyst, providing more active sites for chemical reactions such as adsorption and separation.

[0037] In order to further test the photocatalytic activity of the product prepared in the embodiment, the application experiment process is as follows. The oxidation cleavage of the C-C bond of different lignin model compounds to obtain aromatic monomer products is shown in Table 1, and the photocatalytic conversion rate of the lignin model compound and the yield diagram of the aromatic monomer are as shown in Figure 2 shown.

[0038] Under room temperature and xenon lamp illumination conditions, using the PW / P-TCN prepared in this embodiment as the photocatalyst, 1,2-diphenylethanol (DPol) as the lignin model compound, and acetonitrile as the solvent, in an O 2 atmosphere, the oxidation cleavage of the C-C bond of 1,2-diphenylethanol is achieved. The conversion rate of DPol is 99.9%, and the yield of the aromatic monomer benzaldehyde is 67.5% and the yield of benzoic acid is 33.6%.

[0039] Example 2

[0040] Under room temperature and xenon lamp illumination conditions, using the PW / P-TCN prepared in Example 1 as the photocatalyst, 1,2-diphenylethanone (DPone) as the lignin model compound, and acetonitrile as the solvent, in an O 2 atmosphere, the oxidation cleavage of the C-C bond of 1,2-diphenylethanone is achieved. The conversion rate of DPone is 74.3%, the yield of the aromatic monomer benzoic acid is 25.5%, and the yield of benzoic acid is 43.3%.

[0041] Example 3

[0042] Under room temperature and xenon lamp illumination conditions, using the PW / P-TCN prepared in Example 1 as the photocatalyst, 1,2-diphenylethane (DPe) as the lignin model compound, and acetonitrile as the solvent, in an O 2 atmosphere, the oxidation cleavage of the C-C bond of 1,2-diphenylethane is achieved. The conversion rate of DPe is 97.4%, and the yield of the aromatic monomer benzoic acid is 55.2% and the yield of benzoic acid is 31.4%.

[0043] Example 4

[0044] Under room temperature and xenon lamp illumination conditions, using the PW / P-TCN prepared in Example 1 as the photocatalyst, 2-phenoxy-1-phenylethanol (PPol) as the lignin model compound, and acetonitrile as the solvent, in an O 2Under this atmosphere, the oxidative cleavage of the C-C bond of 2-phenoxy-1-phenylethanol was achieved. The conversion rate of PPol was 99.4%, the yield of the aromatic monomer benzoic acid was 34.2%, the yield of benzoic acid was 15.3%, and the yield of phenyl formate was 40.8%.

[0045] Table 1 Evaluation results of the photocatalytic cleavage of the C-C bond of lignin model compounds

[0046]

[0047] In summary, the present invention uses the cleavage of the lignin C-C bond under light instead of the traditional high-temperature and high-pressure hydrogen conditions to achieve the depolymerization of lignin, which has the advantages of being green, safe, mild conditions, low equipment requirements, and simple operation. The present invention prepares high-value chemicals from pure biomass-based raw materials, greatly increasing the sustainability of the reaction process.

Claims

1. A method for preparing a tubular C3N4 heterojunction photocatalyst, characterized in that: The steps include: S1, mixing a certain amount of urea and melamine and adding them into deionized water and stirring, pouring the obtained solution into a hydrothermal reactor for heating, and retaining the precipitate after cooling and drying to obtain a precursor; S2, placing the precursor obtained in step S2 into a crucible, calcining it in an inert atmosphere, and obtaining tubular C3N4 after natural cooling, which is named TCN; S3, dispersing TCN in a certain amount of H3PO4 solution, stirring to promote the adsorption of H3PO4 on the surface of TCN, and collecting the TCN after adsorbing H3PO4 by centrifugation; S4, placing the TCN after adsorbing H3PO4 obtained in step S3 in an oven to dry, then calcining it under an inert atmosphere, and cooling it to room temperature to obtain H3PO4-modified TCN, named P-TCN; S5. The P-TCN obtained in step S4 is prepared into a suspension, and then phosphotungstic acid is added to the P-TCN suspension and stirred, and finally dried overnight to obtain a tubular C3N4 heterojunction photocatalyst PW / P-TCN.

2. The method for preparing a tubular C3N4 heterojunction photocatalyst according to claim 1, characterized in that: In step S1, the mass ratio of urea to melamine is 1.6:1; the obtained solution is poured into a hydrothermal reactor and heated to 100-180° C. for 20 hours.

3. The method for preparing a tubular C3N4 heterojunction photocatalyst according to claim 1 or 2, characterized in that: In step S2, the mixture is heated from room temperature to 550°C at a rate of 2°C / min in a N2 inert atmosphere and calcined for 4 hours.

4. The method for preparing a tubular C3N4 heterojunction photocatalyst according to claim 1 or 2, characterized in that: In step S3, the concentration of the H3PO4 solution is 0.3 mol / L; the ratio between the mass of TCN and the volume of the H3PO4 solution is 1 mg:1 ml.

5. The method for preparing a tubular C3N4 heterojunction photocatalyst according to claim 1 or 2, characterized in that: In step S4, the mixture is heated to 300° C. and calcined at a rate of 5° C. / min for 90 min under a N 2 atmosphere.

6. The method for preparing a tubular C3N4 heterojunction photocatalyst according to claim 1 or 2, characterized in that: In step S5, P-TCN is added to ethanol and stirred to form a 5 mg / ml suspension; phosphotungstic acid is added to the P-TCN suspension at 50° C. and stirred for 12 hours, and then dried at 80° C., and the mass fraction of the phosphotungstic acid is 5%-30%.

7. A tubular C3N4 heterojunction photocatalyst prepared according to the preparation method according to any one of claims 1 to 6.

8. Use of the tubular C3N4 heterojunction photocatalyst according to claim 7 in preparing aromatic monomers by breaking CC bonds of lignin model compounds under visible light.

9. The use according to claim 8, characterized in that: The specific application process is: using the tubular C3N4 heterojunction photocatalyst PW / P-TCN as a catalyst, acetonitrile as a solvent, and a xenon lamp as a light source, the CC bond of the lignin model compound is oxidatively broken under room temperature oxygen atmosphere to obtain an aromatic monomer.

10. The use according to claim 9, characterized in that: The model compound is one of 1,2-diphenylethanol, 1,2-diphenylacetophenone, 1,2-diphenylethane, and 2-phenoxy-1-phenylethanol, and the aromatic monomer is one or more of benzoic acid, phenylacetaldehyde, and phenyl formate.

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