A triazine / heptazine homojunction structure carbon-doped carbon nitride photocatalyst and its preparation method and application

Abstract

The invention provides a preparation method of a triazine / heptazine homojunction structure carbon-doped carbon nitride photocatalyst, which specifically comprises the following steps: (1) placing a mixed powder of melamine and 2,4,6-triaminopyrimidine in air, heating the mixture to 300° C. to 500° C., and calcining the mixture for 2 to 6 hours to form a mixture of carbon-doped melamine and oligomers thereof; (2) placing the mixture obtained in step (1) in water at 60 to 70° C., ultrasonically dispersing the mixture, then slowly cooling the mixture to room temperature and oscillating the mixture for 12 to 36 hours, removing water by rotary evaporation, and further drying the mixture to obtain a solid supramolecular precursor; and (3) fully mixing the solid supramolecular precursor obtained in step (2) with a molten salt, calcining the mixture at 500° C. to 600° C. for 2 to 6 hours, cooling the mixture, and fully washing the mixture with deionized water to remove the mixed salt, separating the solid residue, and drying the mixture to obtain a triazine / heptazine homojunction structure carbon-doped carbon nitride photocatalyst THCN. x The prepared carbon-doped carbon nitride photocatalyst has uniform homojunction distribution, excellent photochemical properties, high efficiency of photogenerated carrier separation, and excellent photocatalytic selective oxidation performance of benzyl alcohol.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalysts, and in particular relates to a preparation method of a triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst, a product thereof, and an application thereof. Background Art

[0002] Aromatic aldehydes are important chemical intermediates widely used in industries such as pesticides, pharmaceuticals, dyes, and food. For example, the industrial production of benzaldehyde, often using toluene as a raw material, involves side-chain chlorination, toluene hydrolysis, or toluene oxidation. These processes present challenges such as demanding reaction conditions, suboptimal yields and selectivity, and the large amounts of acid and base consumed can easily cause secondary pollution, hindering sustainable development. Therefore, finding green and efficient routes for the production of aromatic aldehydes is crucial for the development of modern green chemistry.

[0003] The green synthesis technology of aromatic aldehydes through the oxidation of benzyl alcohol has attracted widespread attention. Compared with traditional industrial methods, it has the advantages of simple process, environmental friendliness, and low cost. Currently, a variety of catalysts have been developed for the liquid-phase oxidation of benzyl alcohol to benzaldehyde, mainly including supported precious metal catalysts (such as Ru, Pd, and Pt-based catalysts) and non-precious metal catalysts (such as Mn, Fe, and Co-based catalysts). However, these processes still require high temperature, high pressure, and organic solvents, and the formation of byproducts often affects efficiency. Therefore, finding greener and more sustainable catalytic systems is crucial.

[0004] Artificial photosynthesis technology can directly utilize solar energy to oxidize benzyl alcohol. By controlling the reaction system under mild conditions, the conversion of target aldehydes or acids can be achieved, making it an effective and green method for the synthesis of aromatic aldehydes. The core of this technology lies in the design and development of high-performance semiconductor photocatalysts. Compared with traditional metal-based semiconductor photocatalysts (such as TiO2, CdS, Ni2P, and InN), graphitic carbon nitride (g-C3N4), as a non-metallic semiconductor photocatalyst, offers advantages such as simple preparation and stable physicochemical properties. Due to its unique network structure, visible light response, and excellent adsorption and activation properties for aromatic compounds, it has become a widely studied photocatalyst for the selective oxidation of benzyl alcohol. However, conventional bulk carbon nitride (g-C3N4), prepared by direct thermal polymerization using nitrogen-containing small molecules (such as melamine, urea, and dicyandiamide) as precursors, often suffers from issues such as small surface area, low visible light utilization, and severe photogenerated electron-hole recombination. Consequently, the photocatalytic oxidation of benzyl alcohol has not yet achieved the desired results.

[0005] Carbon nitride is compounded with other semiconductors to construct a heterojunction. The heterojunction interface between the two materials and the presence of a built-in electric field are beneficial for promoting the separation and migration of photogenerated carriers and improving the quantum efficiency of light. However, heterojunctions are usually combined by electrostatic attraction or van der Waals interactions, which are usually weak, resulting in poor stability of the catalytic material performance. In contrast, homojunctions are obtained by coupling different phases of the same material, and the physical and chemical properties of each phase are similar. By constructing a homojunction at the molecular level, close contact between the two phases can be ensured, improving the stability of the material. At the same time, the combination of physically and chemically similar phases can effectively eliminate unnecessary interface defects and provide an unobstructed channel for photogenerated carriers. For graphite-phase carbon nitride, the currently reported homojunction construction methods mainly involve the apparent mixing of triazine and heptazine-based precursors. The heptazine monomers such as melem and the triazine-heptazine mixtures obtained at different temperatures are compact and difficult to disperse. The carbon nitride prepared in molten salt has a homojunction interface only at the contact interface between the two phases and is not uniformly distributed, which has limited performance improvement. Summary of the Invention

[0006] The purpose of the present invention is to address the above shortcomings and provide a method for preparing a triazine / heptazine homojunction structure carbon-doped carbon nitride photocatalyst and its products and applications.

[0007] In one aspect, the present invention provides a method for preparing a triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst, comprising the following steps:

[0008] (1) A mixed powder of melamine and 2,4,6-triaminopyrimidine is placed in air, heated to 300°C to 500°C, and calcined for 2 to 6 hours to form a mixture of carbon-doped melamine and its oligomers.

[0009] This step can adjust the ratio of triazine to heptazine in the mixture of carbon-doped melamine and its oligomers by changing the calcination temperature;

[0010] (2) placing the mixture obtained in step (1) in water at 60-70° C., ultrasonically dispersing the mixture, then slowly cooling it to room temperature and shaking it for 12-36 hours, removing water by rotary evaporation, and further drying the mixture to obtain a solid supramolecular precursor;

[0011] This step uses supramolecular self-assembly to rearrange the precursor molecules, making them more fluffy on a macroscopic level so that they can be mixed more evenly with the molten salt; and to allow the triazine and heptaazine components to be evenly mixed on a microscopic level, which is more conducive to obtaining a uniform triazine / heptazine homojunction structure.

[0012] (3) After the solid supramolecular precursor obtained in step (2) is fully mixed with molten salt, it is calcined at 500°C to 600°C for 2 to 6 hours, cooled and fully washed with deionized water to remove the mixed salt, separate the solid residue, and dry to obtain a triazine / heptazine homojunction structure carbon-doped carbon nitride photocatalyst THCN. x -t, (wherein x is the amount of 2,4,6-triaminopyrimidine added based on 10 g of melamine; t is the temperature for preparing the carbon-doped triazine / heptazine mixture precursor).

[0013] This step uses calcination in molten salt to prepare carbon nitride containing a triazine / heptazine homojunction structure. The molten salt reaction environment can maintain the triazine and heptazine units of the raw materials, so that the prepared carbon nitride contains triazine and heptazine structures.

[0014] By the method provided by the present invention, a homojunction structure is formed, which is obtained by coupling the different phases of the same material, and each phase material has similar physical and chemical properties. By constructing a homojunction from a molecular level, it is possible to ensure that the two phases are in close contact, improve material stability, and simultaneously, the phases with similar physical and chemical properties are combined to effectively eliminate unnecessary interface defects, providing unobstructed channels for photogenerated carriers. Graphitic phase carbon nitride has exactly two configurations, namely, triazine-based carbon nitride with triazine ring as a repeating unit, and heptazine-based carbon nitride with heptazine ring as a repeating unit, the carbon nitride of two configurations has a natural advantage in homogeneous structure building, and the homogeneous structure building method currently reported mainly mixes triazine and heptazine-based precursors in appearance, and heptazine monomers such as melem and the triazine-heptazine mixture blocks obtained at different temperatures are compact, difficult to disperse, and the carbon nitride prepared under molten salt, its homojunction interface is only present at the two-phase contact interface, is not uniformly distributed, and has limited performance improvement. In contrast, the present invention's construction of a carbon nitride photocatalyst with a homojunction structure at the molecular level is rarely reported. Furthermore, the present invention's carbon doping of the carbon nitride skeleton disrupts the planar symmetry of the triazine or tris-s-triazine skeleton structure, stimulating π-π* electron transitions and broadening the photocatalyst's light absorption range. Furthermore, the doped carbon also serves as a capture center for photogenerated electrons or holes, improving the separation efficiency of photogenerated carriers and further enhancing the photocatalytic activity of the photocatalyst.

[0015] Furthermore, in step (1), the heating rate is 5°C·min -1 .

[0016] Furthermore, in the step (1), the mass ratio of melamine to 2,4,6-triaminopyrimidine is 10:0.02-0.5.

[0017] Furthermore, in step (1), the mass ratio of melamine to 2,4,6-triaminopyrimidine is 10:0.05.

[0018] Furthermore, in step (2), the mixture is ground into fine powder before being ultrasonically dispersed in water, and the concentration of the mixture in water is 25 g / L.

[0019] Furthermore, in step (3), the molten salt is a composition of KCl and LiBr, and the mass ratio of the solid supramolecular precursor, KCl and LiBr is 1:2.75:2.25.

[0020] The second aspect of the present invention further provides a triazine / heptazine homojunction structure carbon-doped carbon nitride photocatalyst prepared by the above preparation method.

[0021] The third aspect of the present invention further provides an application of the above triazine / heptazine homojunction structure carbon-doped carbon nitride photocatalyst in photocatalytic selective oxidation.

[0022] Furthermore, the application is to use benzyl alcohol as a reaction substrate, acetonitrile as a reaction medium, carbon-doped carbon nitride as a photocatalyst, a xenon lamp as a light source, and an oxidant to form a reaction system to prepare benzaldehyde and / or benzoic acid.

[0023] Furthermore, the reaction conditions are as follows: the reaction temperature is controlled at 60°C, the reaction time is 1 to 12 hours, and the concentration of benzyl alcohol in the reaction system is 50 mmol·L -1 , the carbon nitride photocatalyst concentration is 4 g·L -1 , the oxidant is oxygen.

[0024] Beneficial effects of the present invention:

[0025] (1) The triazine / heptazine homojunction structure carbon-doped carbon nitride photocatalyst does not use any organic solvents during the preparation process, and only uses water as the solvent, which is safe, environmentally friendly and simple to prepare.

[0026] (2) The ratio of triazine molecules to heptazine molecules in the precursor can be adjusted by controlling the preparation temperature of the mixture. This method is simple and effective.

[0027] (3) Prepolymerization of 2,4,6-triaminopyrimidine and melamine to form a carbon-doped triazine / heptazine precursor for the preparation of carbon nitride can effectively prevent the rapid sublimation of 2,4,6-triaminopyrimidine and improve the structural integrity of the carbon nitride skeleton to facilitate the rapid transmission of photogenerated carriers.

[0028] (4) The molten salt method is conducive to the directional regulation of the repeating units of carbon nitride photocatalysts. Using a mixture of triazine and heptazine as a precursor, a carbon nitride catalyst with a triazine / heptazine homojunction structure can be obtained.

[0029] (5) The operation of first crushing the precursor and then self-assembling it in water can promote the rearrangement of triazine and heptazine molecules, improve their dispersibility in molten salt, and is conducive to obtaining a carbon nitride photocatalyst with uniform distribution at the triazine / heptazine homojunction interface. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 THCN prepared in Example 3 0.05 -SEM image of 400 photocatalyst;

[0031] Figure 2 XRD patterns of the photocatalysts prepared in Examples 1 to 6 and Comparative Example 1;

[0032] Figure 3 UV-visible diffuse reflectance spectra of the photocatalysts prepared in Example 3 and Comparative Example 1;

[0033] Figure 4 Tauc plots of the photocatalysts prepared in Example 3 and Comparative Example 1;

[0034] Figure 5 THCN prepared in Example 3 0.05 -400 photocatalyst photocatalytic benzyl alcohol oxidation cycle performance diagram (each cycle 4h);

[0035] Figure 6 THCN prepared in Example 3 0.05 -400 photocatalyst photocatalytic benzyl alcohol oxidation cycle performance diagram (each cycle is 12h). DETAILED DESCRIPTION

[0036] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the examples in the specification. Unless otherwise specified, the methods described are all conventional methods, and the original photocatalysts described can be obtained from public commercial channels unless otherwise specified.

[0037] As used herein, "one embodiment" or "embodiment" refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0038] The preparation method of the triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst provided by the present invention comprises the following specific steps:

[0039] (1) placing a mixed powder of melamine and 2,4,6-triaminopyrimidine in air, heating it to 300° C. to 500° C., and calcining it for 2 to 6 hours to form a mixture of carbon-doped melamine and its oligomers;

[0040] (2) placing the mixture obtained in step (1) in water at 60-70° C., ultrasonically dispersing the mixture, then slowly cooling it to room temperature and shaking it for 12-36 hours, removing water by rotary evaporation, and further drying the mixture to obtain a solid supramolecular precursor;

[0041] (3) After the solid supramolecular precursor obtained in step (2) is fully mixed with molten salt, it is calcined at 500°C to 600°C for 2 to 6 hours, cooled and fully washed with deionized water to remove the mixed salt, separate the solid residue, and dry to obtain a triazine / heptazine homojunction structure carbon-doped carbon nitride photocatalyst THCN. x -t.

[0042] The following is a detailed description of the raw material feeding amounts and specific reaction conditions in the examples.

[0043] Triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst labeled THCN x -t, wherein x is the amount of 2,4,6-triaminopyrimidine added based on 10 g of melamine; and t is the preparation temperature of the carbon-doped triazine / heptazine mixture precursor.

[0044] Example 1

[0045] THCN 0.05 Preparation of -300 photocatalyst:

[0046] (1) 10 g of melamine and 0.05 g of 2,4,6-triaminopyrimidine powder were placed in a mortar and mixed thoroughly. The mixture was then transferred to a crucible and calcined at 300 °C for 4 h (the heating rate was 5 °C min -1 ), to obtain a carbon-doped triazine / heptazine mixture.

[0047] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0048] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, it was fully washed with deionized water to remove the mixed salt, separate the solid residue, and dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN. 0.05 -300.

[0049] Example 2

[0050] THCN 0.05 Preparation of -350 photocatalyst:

[0051] (1) 10 g of melamine and 0.05 g of 2,4,6-triaminopyrimidine powder were placed in a mortar and mixed thoroughly. The mixture was then transferred to a crucible and calcined at 350 °C for 4 h (the heating rate was 5 °C min -1 ), to obtain a carbon-doped triazine / heptazine mixture.

[0052] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0053] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, it was fully washed with deionized water to remove the mixed salt, separate the solid residue, and dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN. 0.05 -350.

[0054] Example 3

[0055] THCN 0.05 Preparation of -400 photocatalyst:

[0056] (1) 10 g of melamine and 0.05 g of 2,4,6-triaminopyrimidine powder were placed in a mortar and mixed thoroughly. The mixture was then transferred to a crucible and calcined at 400 °C for 4 h (the heating rate was 5 °C min -1 ), to obtain a carbon-doped triazine / heptazine mixture.

[0057] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0058] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1), after natural cooling, it was fully washed with deionized water to remove the mixed salt, separate the solid residue, and dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN. 0.05 -400.

[0059] Figure 1 THCN prepared in Example 3 0.05 -400 catalyst, which exhibits an amorphous structure and a loose and porous surface.

[0060] Example 4

[0061] THCN 0.05 -425 Photocatalyst Preparation:

[0062] (1) 10 g of melamine and 0.05 g of 2,4,6-triaminopyrimidine powder were placed in a mortar and mixed thoroughly. The mixture was then transferred to a crucible and calcined at 425 °C for 4 h (the heating rate was 5 °C min -1 ), to obtain a carbon-doped triazine / heptazine mixture.

[0063] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0064] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, it was fully washed with deionized water to remove the mixed salt, separate the solid residue, and dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN. 0.05 -425.

[0065] Example 5

[0066] THCN 0.05 Preparation of -450 photocatalyst:

[0067] (1) 10 g of melamine and 0.05 g of 2,4,6-triaminopyrimidine powder were placed in a mortar and mixed thoroughly. The mixture was then transferred to a crucible and calcined at 450 °C for 4 h (the heating rate was 5 °C min -1 ), to obtain a carbon-doped triazine / heptazine mixture.

[0068] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0069] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, it was fully washed with deionized water to remove the mixed salt, separate the solid residue, and dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN. 0.05 -450.

[0070] Example 6

[0071] THCN 0.05 Preparation of -500 photocatalyst:

[0072] (1) 10 g of melamine and 0.05 g of 2,4,6-triaminopyrimidine powder were placed in a mortar and mixed thoroughly. The mixture was then transferred to a crucible and calcined at 500 °C for 4 h (the heating rate was 5 °C min -1 ), to obtain a carbon-doped triazine / heptazine mixture.

[0073] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0074] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, it was fully washed with deionized water to remove the mixed salt, separate the solid residue, and dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN. 0.05 -500.

[0075] Example 7

[0076] THCN 0.02 Preparation of -400 photocatalyst:

[0077] (1) 10 g of melamine and 0.02 g of 2,4,6-triaminopyrimidine powder were placed in a mortar and mixed thoroughly. The mixture was then transferred to a crucible and calcined at 400 °C for 4 h (the heating rate was 5 °C min -1 ), to obtain a carbon-doped triazine / heptazine mixture.

[0078] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0079] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, it was fully washed with deionized water to remove the mixed salt, separate the solid residue, and dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN. 0.02 -400.

[0080] Example 8

[0081] THCN 0.1 Preparation of -400 photocatalyst:

[0082] (1) 10 g of melamine and 0.1 g of 2,4,6-triaminopyrimidine powder were placed in a mortar and mixed thoroughly. The mixture was then transferred to a crucible and calcined at 400 °C for 4 h (the heating rate was 5 °C min -1 ), to obtain a carbon-doped triazine / heptazine mixture.

[0083] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0084] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, it was fully washed with deionized water to remove the mixed salt, separate the solid residue, and dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN. 0.1 -400.

[0085] Example 9

[0086] THCN 0.5 Preparation of -400 photocatalyst:

[0087] (1) 10 g of melamine and 0.5 g of 2,4,6-triaminopyrimidine powder were placed in a mortar and mixed thoroughly. The mixture was then transferred to a crucible and calcined at 400 °C for 4 h (the heating rate was 5 °C min -1 ), to obtain a carbon-doped triazine / heptazine mixture.

[0088] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0089] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, it was fully washed with deionized water to remove the mixed salt, separate the solid residue, and dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN. 0.5 -400.

[0090] Comparative Example 1

[0091] Preparation of Bulk Carbon Nitride (Bulk-CN) Photocatalyst:

[0092] 1g of melamine powder was placed directly in a crucible with a lid and calcined at 550℃ for 4h (heating rate was 5℃·min -1 ), and after natural cooling, fully grind to obtain bulk carbon nitride Bulk-CN.

[0093] Figure 2The XRD patterns of the catalysts prepared in Examples 1 to 6 and Comparative Example 1 are shown. As the pretreatment temperature increases, the peaks at 2θ=8.1° and 28.0° gradually increase, corresponding to the (100) and (002) crystal planes of the heptazine structure carbon nitride. The XRD diffraction peak positions of Bulk-CN with a heptazine structure deviate slightly due to the different preparation methods. The Bulk-CN obtained by direct calcination of melamine has severe stacking of flakes and a high diffraction intensity of the (002) crystal plane. The diffraction peaks at 2θ=12.0, 21.0, 24.3, 26.6, 29.3 and 32.2° weaken and disappear. This group of diffraction peaks corresponds to triazine structure carbon nitride. The diffraction peaks disappear as the pretreatment temperature increases, indicating that the triazine structure in the precursor gradually decreases. After the pretreatment temperature is above 450°C, almost only the heptazine structure exists in the precursor, and the prepared carbon nitride is a heptazine structure carbon nitride. At a pretreatment temperature of 400°C, XRD diffraction peaks were observed to represent the coexistence of heptazine- and triazine-structured carbon nitride, confirming that the carbon nitride prepared at this pretreatment temperature contained a triazine / heptazine homojunction. When the pretreatment temperature was below 350°C, the triazine structure dominated the carbon nitride prepared.

[0094] Figure 3 The UV-visible diffuse reflectance spectra of Example 3 and Comparative Example 1 are shown. Compared with Bulk-CN, THCN in Example 3 0.05 -400 absorption edge band is red-shifted, with a wider light absorption range.

[0095] Figure 4 The Tauc plots of Example 3 and Comparative Example 1 are shown. 0.05 The band gap of -400 is 2.75eV, which is smaller than that of Bulk-CN (2.82eV), which is more conducive to light absorption and thus obtains better catalytic activity.

[0096] Comparative Example 2

[0097] Preparation of THCN-300 photocatalyst:

[0098] (1) 10 g of melamine powder was calcined at 300 °C for 4 h (heating rate was 5 °C min -1 ), to obtain a triazine / heptazine mixture.

[0099] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0100] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, the mixture was fully washed with deionized water to remove the mixed salt, the solid residue was separated, and the mixture was dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN-300.

[0101] Comparative Example 3

[0102] Preparation of THCN-400 photocatalyst:

[0103] (1) 10 g of melamine powder was calcined at 400 °C for 4 h (heating rate was 5 °C min -1 ), to obtain a triazine / heptazine mixture.

[0104] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0105] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, the mixture was fully washed with deionized water to remove the mixed salt, the solid residue was separated, and the mixture was dried to obtain a carbon-doped carbon nitride photocatalyst THCN-400 with homogeneous junction uniform distribution.

[0106] Comparative Example 4

[0107] Preparation of THCN-500 photocatalyst:

[0108] (1) 10 g of melamine powder was calcined at 500 °C for 4 h (heating rate was 5 °C min -1 ), to obtain a triazine / heptazine mixture.

[0109] (2) The mixture obtained in step (1) was ultrasonically dispersed in water at 60°C, then slowly cooled to room temperature and shaken for 24 hours, and the water was removed by rotary evaporation (the heating temperature was controlled at 60°C), and then placed in a vacuum oven at 40°C for further drying to obtain a solid supramolecular precursor.

[0110] (3) The supramolecular precursor (1 g) obtained in step (2) was uniformly mixed with KCl (2.75 g) and LiBr (2.25 g) and transferred to a covered crucible and calcined at 550 °C for 4 h (heating rate of 5 °C min-1) in air atmosphere. -1 ), after natural cooling, the mixture was fully washed with deionized water to remove the mixed salt, the solid residue was separated, and the mixture was dried to obtain a homogeneously distributed carbon-doped carbon nitride photocatalyst THCN-500.

[0111] Test Case

[0112] The carbon nitride photocatalysts obtained in the above examples and comparative examples were used as catalysts to carry out photocatalytic benzyl alcohol oxidation reaction. The specific reaction method is as follows:

[0113] 15 mg of each carbon nitride photocatalyst powder obtained in the above examples and control examples was ultrasonically dispersed in 8 mL of acetonitrile. 0.2 mmol of benzyl alcohol was added, and the reaction mixture was transferred to a quartz reaction tube. After displacing the remaining air in the tube with oxygen, the tube was connected to an oxygen balloon for reaction. A Xe lamp was used as the light source, the reaction temperature was controlled at 60°C, and the reaction time was 4 hours. The product was detected by gas chromatography, and the conversion and selectivity were calculated using the normalization method.

[0114] Table 1 Comparison of oxidative performance of various photocatalysts

[0115]

[0116]

[0117] (Conv. 苯甲醇 (%) indicates conversion rate, Sel. 苯甲醛 (%) is the proportion of benzaldehyde in the product, Sel. 苯甲酸 (%) is the proportion of benzoic acid in the product)

[0118] Table 1 compares the performance of the above-mentioned photocatalysts for the photocatalytic oxidation of benzyl alcohol under the conditions described in the test examples. Bulk carbon nitride (Comparative Example 1) has the lowest conversion rate and the worst oxidation performance. This is because carbon nitride photocatalysts prepared by direct thermal polymerization of melamine typically have severe flake stacking, a small comparative area, and severe photogenerated carrier recombination. For homojunction carbon-doped carbon nitride photocatalysts (Examples 1-6), the photocatalytic performance first increases and then decreases with increasing pretreatment temperature. Example 3 exhibits the best catalytic activity, with the product primarily being benzaldehyde at a 4-hour reaction time. In comparison, the performance of catalysts without carbon doping is less than adequate (Comparative Examples 2-4). Catalytic performance can also be reduced with either too little or too much carbon doping (Examples 7-9). This is because carbon doping promotes the separation of photogenerated electron-hole pairs. Low doping levels result in fewer separation centers and insufficient carrier separation. Excessive doping, however, destroys the carbonized carbon skeleton structure, causing the photocatalyst to transition from a semiconductor to a conductor, which is detrimental to light absorption and photogenerated carrier separation. In summary, thanks to the molecular rearrangement under supramolecular self-assembly, the precursors can be uniformly mixed at the microscale, which can make the triazine / heptazine homojunction structure evenly distributed. Combined with carbon doping, the photocatalytic oxidation performance of benzyl alcohol by carbon nitride photocatalyst can be effectively improved.

[0119] Table 2 THCN 0.05 -400 catalyst selectivity test results under different conditions

[0120]

[0121]

[0122] (“+” in the table means optional; “-” means not optional)

[0123] Table 2 THCN 0.05 -400 catalyst photocatalytic oxidation results of benzyl alcohol under different conditions. Comparing No. 1 and No. 2, it can be seen that the reaction must be carried out under light conditions. Comparing No. 3 and No. 4, due to superoxide radicals, photogenerated holes are the key factors affecting the reaction. Under anaerobic conditions, photogenerated holes can only oxidize benzyl alcohol to benzaldehyde, and due to the existence of some photogenerated electron-hole pair recombination phenomena, the conversion rate is not high; under aerobic conditions, superoxide radicals are generated in the system, and if the reaction time is long enough, benzaldehyde can be further completely oxidized to benzoic acid. Lowering the reaction temperature slightly reduces the catalyst effect (No. 5). Comparing No. 6 and No. 7, when K2S2O8 is added as an electron capture agent, the complete conversion of benzaldehyde or benzoic acid can be achieved by controlling the reaction atmosphere.

[0124] Figure 5 THCN prepared in Example 3 0.05Performance diagram of the photocatalytic oxidation of benzyl alcohol by the 4-hour cycle of the 400-400 catalyst. After five cycles, the catalytic effect decreased slightly, and the main product was benzaldehyde.

[0125] Figure 6 THCN prepared in Example 3 0.05 -400 catalyst photocatalytic benzyl alcohol oxidation cycle performance diagram, each cycle time is 12h. After 5 cycles, the benzyl alcohol conversion rate did not decrease significantly, and the product was almost entirely benzoic acid. However, during the last two cycles, a small amount of benzaldehyde appeared in the product. The test results under both conditions showed that THCN 0.05 -400 catalyst has excellent cyclic stability.

Claims

1. A method for preparing a triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst, characterized in that: The specific steps are: (1) placing a mixed powder of melamine and 2,4,6-triaminopyrimidine in air, heating it to 300° C. to 500° C., and calcining it for 2 to 6 hours to form a mixture of carbon-doped melamine and its oligomers; (2) placing the mixture obtained in step (1) in water at 60-70° C., ultrasonically dispersing the mixture, then slowly cooling it to room temperature and shaking it for 12-36 hours, removing water by rotary evaporation, and further drying the mixture to obtain a solid supramolecular precursor; (3) After the solid supramolecular precursor obtained in step (2) is fully mixed with molten salt, it is calcined at 500°C to 600°C for 2 to 6 hours, cooled and fully washed with deionized water to remove the mixed salt, separate the solid residue, and dry to obtain a triazine / heptazine homojunction structure carbon-doped carbon nitride photocatalyst THCN. x -t, Wherein, x is the amount of 2,4,6-triaminopyrimidine added based on 10 g of melamine; and t is the temperature for preparing the carbon-doped triazine / heptazine mixture precursor.

2. A method for preparing the triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst according to claim 1, characterized in that: In the step (1), the heating rate is 5°C·min -1 .

3. The method for preparing the triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst according to claim 1, wherein: In the step (1), the mass ratio of melamine to 2,4,6-triaminopyrimidine is 10:0.02-0.

5.

4. The method for preparing the triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst according to claim 3, wherein: In the step (1), the mass ratio of melamine to 2,4,6-triaminopyrimidine is 10:0.

05.

5. The method for preparing the triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst according to claim 1, wherein: In the step (2), the mixture is ground into fine powder before being ultrasonically dispersed in water, and the concentration of the mixture in water is 25 g / L.

6. The method for preparing the triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst according to claim 2, wherein: In the step (3), the molten salt is a composition of KCl and LiBr, and the mass ratio of the solid supramolecular precursor, KCl and LiBr is 1:2.75:2.

25.

7. A triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst, characterized in that: The invention is prepared by the method according to any one of claims 1 to 6.

8. Use of the triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst according to claim 7 in photocatalytic selective oxidation.

9. Use of the triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst in photocatalytic selective oxidation according to claim 8, characterized in that: Benzyl alcohol is used as a reaction substrate, acetonitrile is used as a reaction medium, a triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst is used as a photocatalyst, a xenon lamp is used as a light source, an oxidant is added, and a reaction system is formed to prepare benzaldehyde and / or benzoic acid.

10. Use of the triazine / heptazine homojunction carbon-doped carbon nitride photocatalyst in photocatalytic selective oxidation according to claim 9, characterized in that: The reaction conditions are as follows: the reaction temperature is controlled at 60°C, the reaction time is 1 to 12 hours, and the concentration of benzyl alcohol in the reaction system is 50 mmol·L -1 , the carbon nitride photocatalyst concentration is 4 g·L -1 , the oxidant is oxygen.

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