Preparation method of boron-doped g-C3N4

Abstract

The invention relates to a preparation method of boron-doped g-C3N4, which comprises the following steps: adding K2B4O7. 4H2O or KBH4 as a boron source into g-C3N4, fully grinding and uniformly mixing, calcining in an argon atmosphere, cooling to room temperature, washing and drying to obtain the boron-doped g-C3N4. The boron-doped g-C3N4 prepared by the invention shows higher activity and selectivity in a reaction for preparing benzaldehyde by photocatalytic oxidation of cinnamyl aldehyde, so that the reaction process is more efficient, green and easy to control. By selecting a proper boron precursor, the target of efficient and selective solar driven organic synthesis is achieved. The invention not only provides a new way for generating benzaldehyde through selective oxidation of cinnamyl aldehyde, but also provides a new catalyst and process selection for other organic synthesis reactions.

Description

Technical Field

[0001] The invention belongs to the technical field of synthesis of photocatalytic materials, and in particular relates to boron-doped graphite phase carbon nitride gC 3 N 4 Preparation and application. Background Art

[0002] Benzaldehyde is a spice whose annual consumption is second only to vanillin. It is also an important raw material for medicine. It is mainly obtained by synthesizing petrochemical raw materials or extracting from natural materials. Using cinnamon oil as a raw material to synthesize natural benzaldehyde is one of the three ways to obtain natural benzaldehyde besides extracting from natural materials. Cinnamaldehyde (3-phenyl-2-propenal), the main component of cinnamon oil, is a typical α, β-unsaturated aldehyde. The reaction of preparing benzaldehyde from cinnamaldehyde is actually the process of selectively oxidizing the carbon-carbon double bond in its structure to an aldehyde group. The selective oxidation of cheap and readily available olefins is one of the main ways to obtain high-value-added aldehyde and ketone carbonyl compounds. Using light energy instead of traditional thermal energy or chemical energy to overcome the activation energy required for the reaction can enable the reaction to be completed under milder and greener conditions.

[0003] Graphitic Carbon Nitride gC 3 N 4 As a non-metallic semiconductor photocatalyst without heavy metals, it has been demonstrated to have visible light response and its conduction band (CB) can meet the requirements of O 2 Photoreduction to moderately oxidative oxygen species rather than hydroxyl radicals. However, due to the 3 N 4 The photogenerated carriers recombine seriously, the valence band position is low and the light absorption ability is not ideal, so its photocatalytic activity is poor. In order to improve this problem, non-metallic doping can be used to maintain gC 3 N 4 As a non-metallic semiconductor photocatalyst, it can avoid the problem of heavy metals from the source in the preparation of food and food additives such as spices; it also has the advantages of low cost and a wide range of raw material sources, and its high electronegativity can also 3 N 4 Boron is a non-metallic element, but it exhibits certain metallic properties in the crystalline state. In the crystal structure, it can usually form stable chemical bonds. Boron-doped gC 3 N 4 It can improve its photocatalytic activity and has a higher conversion rate and selectivity in the photocatalytic oxygen oxidation of cinnamaldehyde to prepare benzaldehyde. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a boron-doped graphite phase carbon nitride gC 3 N 4Preparation and application.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A boron-doped gC 3 N 4 The preparation method comprises the following steps:

[0007] K 2 B 4 O 7 ·4H 2 O or KBH 4 Add gC as a boron source 3 N 4 After being fully ground and mixed, it is placed in an inert atmosphere for calcination, cooled to room temperature, washed and dried to obtain boron-doped gC 3 N 4 ,

[0008] The K 2 B 4 O 7 ·4H 2 O and gC 3 N 4 Mix according to the mass ratio of 1 to 5:5;

[0009] The KBH 4 With gC 3 N 4 Mix according to the mass ratio of 1 to 5:5;

[0010] K 2 B 4 O 7 ·4H 2 Boron-doped gC obtained by using O as the boron source 3 N 4 The product code is BCN, and the structural formula is as follows:

[0011]

[0012] KBH 4 Boron-doped gC 3 N 4 The product code is KBCN, and the structural formula is as follows:

[0013]

[0014] The calcination temperature is 350-600° C., and the calcination time is 30-120 minutes.

[0015] The drying is performed by vacuum drying at 50-70° C. for 5-8 hours.

[0016] A method for preparing benzaldehyde by photocatalytic oxygen oxidation of cinnamaldehyde comprises the following steps:

[0017] (1) In a reactor, add a photocatalyst gC to cinnamaldehyde, or dissolve cinnamaldehyde in a solvent. 3 N 4 , BCN or KBCN;

[0018] (2) introducing oxygen into the reactor, turning on the light source to illuminate the reaction system, and reacting under vigorous stirring;

[0019] (3) Filter out the catalyst, extract the reaction product and separate the benzaldehyde.

[0020] Photocatalyst gC 3 N 4 The added amount of BCN or KBCN is 1 to 30% of the mass of cinnamaldehyde.

[0021] The solvent is acetonitrile, alcohol, ether, water or petroleum ether.

[0022] The light source is a xenon lamp with a light power density of 200 to 500 mw / cm 2

[0023] During the reaction, the temperature of the reaction system was maintained at room temperature by circulating cooling water.

[0024] Photocatalyst gC 3 N 4 , BCN or KBCN is used in the photocatalytic oxygen oxidation of cinnamaldehyde to prepare benzaldehyde.

[0025] A boron-doped gC 3 N 4 The photocatalyst BCN has the following structural formula:

[0026]

[0027] A boron-doped gC 3 N 4 The photocatalyst KBCN has the following structural formula:

[0028]

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The boron-doped gC prepared by the present invention 3 N 4The photocatalytic oxidation of cinnamaldehyde to benzaldehyde showed higher activity and selectivity, making the reaction process more efficient, green and easy to control. By selecting a suitable boron precursor, the goal of efficient and selective solar-driven organic synthesis was achieved. This not only provides a new way for the selective oxidation of cinnamaldehyde to benzaldehyde, but also provides new catalysts and process options for other organic synthesis reactions. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 gC 3 N 4 , BCN and KBCN X-ray diffraction patterns.

[0032] Figure 2 gC 3 N 4 SEM photograph of.

[0033] Figure 3 This is the SEM picture of BCN.

[0034] Figure 4 This is the SEM photo of KBCN.

[0035] Figure 5 This is the energy dispersive X-ray spectrometer (EDS) image of BCN.

[0036] Figure 6 This is the energy dispersive X-ray spectrometer (EDS) image of KBCN.

[0037] Figure 7 is the measurement result of X-ray photoelectron spectroscopy (XPS) spectrum. Intensity is the intensity and Bindingenergy is the binding energy.

[0038] Figure 8 gC 3 N 4 , BCN and KBCN photocatalytic cinnamaldehyde oxidation performance. Conversion is the conversion rate and Selectivity is the selectivity. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is further described below through embodiments.

[0040] Example 1

[0041] 1. Photocatalyst gC 3 N 4 Preparation of BCN and KBCN

[0042] 10 g of urea was placed in a ceramic crucible and then placed in a muffle furnace and calcined at 550 °C for 4 h in air at a heating rate of 5 °C / min to obtain gC 3 N 4 .

[0043] 0.30 g of gC 3 N 4 and 0.17 g of K 2 B 4 O 7 ·4H 2 After being fully ground and mixed at room temperature, the mixture was placed in a covered porcelain boat, which was placed in a tube furnace and calcined at 450°C for 90 min in an argon atmosphere with a program temperature increase of 2.3°C / min. After the sample was cooled to room temperature, it was washed three times with water and three times with alcohol, and dried in vacuum at 60°C to obtain boron-doped gC 3 N 4 Product, codenamed BCN.

[0044] 0.30 g of gC 3 N 4 and 0.12 g of KBH 4 After being fully ground and mixed at room temperature, the mixture was placed in a covered porcelain boat, which was placed in a tube furnace and calcined at 450°C for 30 min under an argon atmosphere with a program temperature increase of 2.3°C / min. After the sample was cooled to room temperature, it was washed three times with water and three times with alcohol, and dried in vacuum at 60°C to obtain boron-doped gC 3 N 4 Product, codenamed KBCN.

[0045] 2. Photocatalyst gC 3 N 4 X-ray diffraction analysis of BCN and KBCN

[0046] The crystal structure of the photocatalyst was analyzed by X-ray diffraction (SmartLab3KW, Rigaku Corporation, Japan) using a working voltage of 40 kV and a working current of 30 mA. 3 N 4 , BCN and KBCN X-ray diffraction (XRD) patterns, such as Figure 1 As shown, two characteristic peaks appear at 13.4° and 27.3°, which are consistent with gC 3 N 4 The signal is consistent with the standard card (PDF Card No.87-1525).

[0047] 3. Photocatalyst gC 3 N 4 Scanning electron microscopy (SEM) analysis of BCN and KBCN

[0048] C 3 N 4 Scanning electron microscope images of BCN and KBCN are shown in Figure 2. Figure 2 , Figure 3 and Figure 4 As shown, they all show thick and dense layered stacking structures. Figure 4 The surface of KBCN is rougher. After magnification, the situation on the catalyst surface is more obvious, with more pores and larger pore diameters.

[0049] 4. Energy dispersive spectroscopy (EDS) analysis of photocatalysts BCN and KBCN

[0050] like Figure 5 As shown, the energy dispersion image of BCN shows that B is successfully doped into gC 3 N 4 structure and evenly distributed.

[0051] like Figure 6 As shown, the energy dispersion spectrum image of KBCN shows that B is successfully doped into gC 3 N 4 structure and evenly distributed.

[0052] 5. X-ray photoelectron spectroscopy (XPS) spectroscopy (K-Alpha, Thermo Scientific, USA) was used to characterize the photocatalyst gC 3 N 4 Chemical composition determination of BCN and KBCN, with a reference C1s binding energy of 284.8 eV

[0053] Figure 7 (a) is gC 3 N 4 The C1s spectra of BCN and KBCN show that the three peaks at 288.2, 286.3 and 284.8 eV are respectively assigned to N–C=N, C–NH x and C–C.

[0054] Figure 7 The three peaks in the N1s spectrum in (b) are 401.3, 400.4 and 398.7 eV, which are respectively assigned to C–N–H, N–(C) 3 At the same time, the C1s and N1s signals of doped BCN and KBCN shift slightly to higher binding energies, indicating that boron enters gC smoothly. 3 N 4 The structure of the molecule changes the chemical environment around the original C and N atoms.

[0055] Figure 7The B1s binding energy values ​​of (c) show that K 2 B 4 O 7 ·4H 2 O and KBH 4 Effect of two different boron sources on gC 3 N 4 During doping, boron atoms enter gC 3 N 4 The sites are not completely consistent. However, the binding energy of the B1s peaks of BCN and KBCN is between 193.0 eV of boron oxides and hydroxides and 190.0 eV of hexagonal boron nitride, which is the binding energy when B and N atoms are connected. This shows that the doped boron atoms are introduced and replace the gC 3 N 4 The B1s binding energy of BCN is 192.2 eV, which is due to the boron atom using three sp 2 The hybrid orbitals are sp 2 Hybrid orbitals form σ bonds, while the empty p orbital of boron forms a π bond with the lone pair of electrons on the p orbital of one of the nitrogen atoms that is not involved in the hybridization. Therefore, in >B=N<, boron carries a positive charge because its electronegativity is less than that of nitrogen, while the nitrogen atom carries a negative charge. The B1s binding energy of KBCN is 191.4eV, which is produced by two types of boron atoms, that is, the boron doped in KBCN has the same type as that in BCN, and there is another type that is only connected to two nitrogen atoms. In this case, the three sp 2 Only two of the hybrid orbitals form σ bonds with the nitrogen atom, so there is a single electron in its outer shell.

[0056] According to the above measurement results and analysis, K 2 B 4 O 7 ·4H 2 O is the boron source to obtain boron-doped gC 3 N 4 The structural formula of the product BCN is as follows:

[0057]

[0058] According to the above test results and analysis, KBH 4 Boron source to obtain boron-doped gC 3 N 4 The structural formula of the product KBCN is as follows:

[0059]

[0060] 6. Photocatalyst gC 3 N 4Activity test of BCN and KBCN for photocatalytic oxygen oxidation of cinnamaldehyde to prepare benzaldehyde

[0061] (1) Method for preparing benzaldehyde by photocatalytic oxygen oxidation of cinnamaldehyde

[0062] In the reactor, a mixture of 30 mg of photocatalyst, 1 mmol (mass 132.159 mg) of cinnamaldehyde and 10 mL of acetonitrile was added. The reactor was evacuated to a vacuum state, high-purity oxygen gas entered the reactor to reach ambient pressure, and then a magnetic stirrer was used to vigorously stir in the dark for 30 minutes. Then a 300W xenon lamp was turned on as a light source to irradiate from the top. In addition, during the photocatalytic oxidation of cinnamaldehyde, the solution temperature was maintained at 25 ° C by circulating cooling water. After 14 hours, the reaction product was extracted and the catalyst was filtered out.

[0063] (2) Analysis of product components after photocatalytic reaction

[0064] (2.1) Analysis methods

[0065] Using o-dichlorobenzene as an internal standard, the volume was fixed with anhydrous ethanol and the cinnamaldehyde component after the reaction was analyzed by gas chromatography (GC-2018, Shimadzu, Japan) internal standard method (CN 116496153A).

[0066] (2.2) Test reagents

[0067] Potassium borohydride (AR grade) was purchased from Nanning Blue Sky Experimental Equipment Co., Ltd.

[0068] Urea (AR grade), cinnamaldehyde (AR grade) and o-dichlorobenzene (GR grade) were purchased from Aladdin Biochemical Reagent Research Co., Ltd.

[0069] Potassium tetraborate tetrahydrate (AR grade) was purchased from Shanghai MacLean Biochemical Technology Co., Ltd.

[0070] Anhydrous ethanol (AR grade) and acetonitrile (AR grade) were purchased from Xilong Technology Co., Ltd.

[0071] Ultra-high purity argon (99.999%), ultra-high purity nitrogen (99.999%), ultra-high purity air, and oxygen (99.9%) were purchased from Guangxi Ruida Chemical Technology Co., Ltd.

[0072] (2.3) GC-2018 analysis conditions

[0073] The gas chromatographic column was a 30m×0.32mm×0.25μm AE.SE-54 capillary column, the column temperature was 160°C, the vaporization chamber temperature was 260°C, and the hydrogen flame ionization detector (FID) temperature was 260°C.

[0074] (2.4) GC-MS analysis results of product components after photocatalytic reaction

[0075] Table 1 GC-MS analysis results of cinnamaldehyde after KBCN catalytic reaction

[0076]

[0077] (2.5) Photocatalyst gC 3 N 4 , BCN and KBCN selectivity and conversion rate test results

[0078] like Figure 8 As shown, with gC 3 N 4 In comparison, the conversion rate of cinnamaldehyde in the KBCN sample reached 73.8% and the selectivity of benzaldehyde reached 74.1% after 14 h of photoreaction. 4 The strong reducing property of the gC changes with the incorporation of boron 3 N 4 Select the same alkali metal element but without reducing K 2 B 4 O 7 ·4H 2 O as a boron source for boron doping gC 3 N 4 Experiment. After the photoreaction under the same conditions, the conversion rate of the BCN sample reached 65.2%, the selectivity reached 61.3%, and the activity increase effect was weaker than that of KBH 4 gC as a boron source 3 N 4 .

[0079] Example 2

[0080] A boron-doped gC 3 N 4 The preparation method comprises the following steps: 2 B 4 O 7 ·4H 2 O as a boron source added to gC 3 N 4 After being fully ground and mixed, it is placed in an inert atmosphere for calcination, cooled to room temperature, washed and dried to obtain boron-doped gC 3 N 4 , codenamed BCN. 2 B 4 O 7 ·4H 2 O and gC 3 N 4The mixture is mixed at a mass ratio of 1:5. The calcination temperature is 350° C. and the calcination time is 30 min. The drying is vacuum drying at 50° C. for 5 h.

[0081] Example 3

[0082] A boron-doped gC 3 N 4 The preparation method comprises the following steps: 2 B 4 O 7 ·4H 2 O as a boron source added to gC 3 N 4 After being fully ground and mixed, it is placed in an inert atmosphere for calcination, cooled to room temperature, washed and dried to obtain boron-doped gC 3 N 4 , codenamed BCN. 2 B 4 O 7 ·4H 2 O and gC 3 N 4 The mixture is mixed at a mass ratio of 5:5. The calcination temperature is 600° C. and the calcination time is 120 min. The drying is performed at 70° C. in vacuum for 8 h.

[0083] Example 4

[0084] A boron-doped gC 3 N 4 The preparation method comprises the following steps: 2 B 4 O 7 ·4H 2 O as a boron source added to gC 3 N 4 After being fully ground and mixed, it is placed in an inert atmosphere for calcination, cooled to room temperature, washed and dried to obtain boron-doped gC 3 N 4 , codenamed BCN. 2 B 4 O 7 ·4H 2 O and gC 3 N 4 The mixture is mixed at a mass ratio of 5:5. The calcination temperature is 400° C. and the calcination time is 90 min. The drying is performed at 60° C. in vacuum for 6.5 h.

[0085] Example 5

[0086] A boron-doped gC 3 N 4The preparation method comprises the following steps: 2 B 4 O 7 ·4H 2 O as a boron source added to gC 3 N 4 After being fully ground and mixed, it is placed in an inert atmosphere for calcination, cooled to room temperature, washed and dried to obtain boron-doped gC 3 N 4 , codenamed BCN. 2 B 4 O 7 ·4H 2 O and gC 3 N 4 The mixture is mixed in a mass ratio of 1:5, 2:5, 3:5, 4:5 or 5:5. The calcination temperature is 350, 400, 450, 500, 550 or 600° C., and the calcination time is 30, 40, 50, 60, 70, 80, 90, 100, 110 or 120 min. The drying is vacuum drying at 50, 55, 60, 65 or 70° C. for 5, 6, 7 or 8 h.

[0087] Example 6

[0088] A boron-doped gC 3 N 4 The preparation method comprises the following steps: KBH 4 Add gC as a boron source 3 N 4 After being fully ground and mixed, it is placed in an inert atmosphere for calcination, cooled to room temperature, washed and dried to obtain boron-doped gC 3 N 4 , codenamed KBCN, the KBH 4 With gC 3 N 4 The mixture is mixed in a mass ratio of 1:5, 2:5, 3:5, 4:5 or 5:5. The calcination temperature is 350, 400, 450, 500, 550 or 600° C., and the calcination time is 30, 40, 50, 60, 70, 80, 90, 100, 110 or 120 min. The drying is vacuum drying at 50, 55, 60, 65 or 70° C. for 5, 6, 7 or 8 h.

[0089] Example 7

[0090] A method for preparing benzaldehyde by photocatalytic oxygen oxidation of cinnamaldehyde comprises the following steps:

[0091] (1) In a reactor, cinnamaldehyde is dissolved in a solvent and then a photocatalyst gC is added. 3 N4 ;

[0092] (2) introducing oxygen into the reactor, turning on the light source to illuminate the reaction system, and reacting under vigorous stirring;

[0093] (3) Filter out the catalyst, extract the reaction product and separate the benzaldehyde.

[0094] Photocatalyst gC 3 N 4 The added amount is 1%, 3%, 6%, 9%, 12%, 15%, 18%, 21%, 24%, 27% or 30% of the mass of cinnamaldehyde.

[0095] The solvent is acetonitrile, alcohol, ether, water or petroleum ether. The concentration of cinnamaldehyde in the solvent can be varied, and in this embodiment, it is 500-1000 mg / L.

[0096] During the reaction, the temperature of the reaction system was maintained at room temperature by circulating cooling water.

[0097] Example 8

[0098] A method for preparing benzaldehyde by photocatalytic oxygen oxidation of cinnamaldehyde comprises the following steps:

[0099] (1) In a reactor, cinnamaldehyde is dissolved in a solvent, and then a photocatalyst BCN is added;

[0100] (2) introducing oxygen into the reactor, turning on the light source to illuminate the reaction system, and reacting under vigorous stirring;

[0101] (3) Filter out the catalyst, extract the reaction product and separate the benzaldehyde.

[0102] The added amount of the photocatalyst BCN is 1%, 3%, 6%, 9%, 12%, 15%, 18%, 21%, 24%, 27% or 30% of the mass of cinnamaldehyde.

[0103] The solvent is acetonitrile, alcohol, ether, water or petroleum ether. The concentration of cinnamaldehyde in the solvent can be varied, and in this embodiment, it is 500-1000 mg / L.

[0104] During the reaction, the temperature of the reaction system was maintained at room temperature by circulating cooling water.

[0105] Example 9

[0106] A method for preparing benzaldehyde by photocatalytic oxygen oxidation of cinnamaldehyde comprises the following steps:

[0107] (1) In a reactor, cinnamaldehyde is dissolved in a solvent, and then a photocatalyst KBCN is added;

[0108] (2) introducing oxygen into the reactor, turning on the light source to illuminate the reaction system, and reacting under vigorous stirring;

[0109] (3) Filter out the catalyst, extract the reaction product and separate the benzaldehyde.

[0110] The added amount of the photocatalyst KBCN is 1%, 5%, 10%, 15%, 20%, 25% or 30% of the mass of cinnamaldehyde.

[0111] The solvent is acetonitrile, alcohol, ether, water or petroleum ether. Regardless of the concentration of cinnamaldehyde in the solvent, the photocatalytic reaction can occur, which is 500-1000 mg / L in this embodiment.

[0112] During the reaction, the temperature of the reaction system was maintained at room temperature by circulating cooling water.

[0113] Example 10

[0114] A method for preparing benzaldehyde by photocatalytic oxygen oxidation of cinnamaldehyde comprises the following steps:

[0115] (1) Adding photocatalyst BCN to cinnamaldehyde in a reactor;

[0116] (2) introducing oxygen into the reactor, turning on the light source to illuminate the reaction system, and reacting under vigorous stirring;

[0117] (3) Filter out the catalyst, extract the reaction product and separate the benzaldehyde.

[0118] The added amount of the photocatalyst BCN is 1%, 3%, 6%, 9%, 12%, 15%, 18%, 21%, 24%, 27% or 30% of the mass of cinnamaldehyde.

[0119] During the reaction, the temperature of the reaction system was maintained at room temperature by circulating cooling water.

[0120] Embodiment 11

[0121] A method for preparing benzaldehyde by photocatalytic oxygen oxidation of cinnamaldehyde comprises the following steps:

[0122] (1) Adding photocatalyst KBCN to cinnamaldehyde in a reactor;

[0123] (2) introducing oxygen into the reactor, turning on the light source to illuminate the reaction system, and reacting under vigorous stirring;

[0124] (3) Filter out the catalyst, extract the reaction product and separate the benzaldehyde.

[0125] The added amount of the photocatalyst KBCN is 1%, 5%, 10%, 15%, 20%, 25% or 30% of the mass of cinnamaldehyde.

[0126] During the reaction, the temperature of the reaction system was maintained at room temperature by circulating cooling water.

Claims

1. A method for preparing boron-doped g-C3N4, characterized in that: The following steps are involved: K2B4O7·4H2O or KBH4 is added to g-C3N4 as a boron source, and after being fully ground and mixed, it is calcined in an inert atmosphere, cooled to room temperature, washed and dried to obtain boron-doped g-C3N4. The K2B4O7·4H2O and g-C3N4 are mixed in a mass ratio of 1 to 5:5; The KBH4 and g-C3N4 are mixed in a mass ratio of 1 to 5:5; The boron-doped g-C3N4 product obtained using K2B4O7·4H2O as the boron source is coded as BCN, and its structural formula is as follows: The boron-doped g-C3N4 product obtained using KBH4 as the boron source is coded as KBCN, and its structural formula is as follows:

2. The preparation method according to claim 1, characterized in that: The calcination temperature is 350-600° C., and the calcination time is 30-120 minutes.

3. The preparation method according to claim 1, characterized in that: The drying is performed by vacuum drying at 50-70° C. for 5-8 hours.

4. A method for preparing benzaldehyde by photocatalytic oxygen oxidation of cinnamaldehyde, characterized in that: The following steps are involved: (1) In a reactor, add the photocatalyst g-C3N4, BCN or KBCN to cinnamaldehyde, or dissolve cinnamaldehyde in a solvent; (2) introducing oxygen into the reactor, turning on the light source to illuminate the reaction system, and reacting under vigorous stirring; (3) Filter out the catalyst, extract the reaction product and separate the benzaldehyde.

5. The method for preparing benzaldehyde by photocatalytic oxidation of cinnamaldehyde according to claim 4, characterized in that: The added amount of the photocatalyst g-C3N4, BCN or KBCN is 1 to 30% of the mass of cinnamaldehyde.

6. The method for preparing benzaldehyde by photocatalytic oxidation of cinnamaldehyde according to claim 4, characterized in that: The solvent is acetonitrile, alcohol, ether, water or petroleum ether.

7. The method for preparing benzaldehyde by photocatalytic oxidation of cinnamaldehyde according to claim 4, characterized in that: During the reaction, the temperature of the reaction system was maintained at room temperature by circulating cooling water.

8. Application of photocatalyst g-C3N4, BCN or KBCN in the photocatalytic oxygen oxidation of cinnamaldehyde to prepare benzaldehyde.

9. A boron-doped g-C3N4 photocatalyst BCN, characterized in that: Its structural formula is as follows:

10. A boron-doped g-C3N4 photocatalyst KBCN, characterized in that: Its structural formula is as follows:

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