A kind of kraft lignin-heptazine conjugated polymer, preparation method and application thereof

Abstract

The present invention discloses a kraft lignin-heptazine conjugated polymer, a preparation method thereof, and an application thereof. The structure of the kraft lignin-heptazine conjugated polymer comprises kraft lignin and a heptazine ring. The kraft lignin-heptazine conjugated polymer of the present invention is prepared by copolymerizing kraft lignin and a carbon-nitrogen polymer in an alkali metal salt. The copolymerization of the kraft lignin of the present invention effectively reduces the band gap of the heptazine-based polymer, extends its light absorption response range to the long-wavelength visible light region, and improves the separation efficiency of photogenerated carriers; the kraft lignin-heptazine conjugated polymer has significant advantages in the photocatalytic reduction of hydrogen peroxide and is reusable; kraft lignin is the main component of papermaking black liquor, has abundant sources, and is low in acquisition cost. The present invention provides a new direction for the high-value utilization of kraft lignin; the present invention also simplifies the preparation of hydrogen peroxide, which can be carried out under normal pressure air, and no additional oxygen is required during the preparation process.

Description

Technical Field

[0001] The invention relates to a kraft lignin-heptazine conjugated polymer, a preparation method and application thereof, and belongs to the technical field of development of new compounds and photocatalytic synthesis of hydrogen peroxide. Background Art

[0002] Hydrogen peroxide (H2O2) is an important inorganic chemical product with widespread applications in biology, chemistry, pharmaceuticals, and the environment. The primary industrial method for producing hydrogen peroxide is the anthraquinone process, which consumes large amounts of energy and causes severe environmental pollution. Photocatalytic synthesis of hydrogen peroxide, which primarily utilizes solar energy, water, and oxygen, can reduce pollution and lower costs, and has therefore attracted extensive research within the scientific community. Currently, research on photocatalytic hydrogen peroxide synthesis systems is still in its early stages, and the development of novel, efficient, stable, and inexpensive photocatalysts is a key research priority for scientists.

[0003] In recent years, polymer-based photocatalytic materials have attracted global attention for the synthesis of hydrogen peroxide. However, existing polymer photocatalytic materials suffer from narrow spectral response, easy recombination of photogenerated charges, poor electron conductivity, and aggregation and adhesion during synthesis. These issues result in low specific surface areas, severely restricting their widespread application.

[0004] Kraft lignin, a black liquor byproduct produced during the papermaking and pulping process, is of great significance for the recovery and utilization of solid waste. Kraft lignin is primarily composed of three phenylpropane monomers: vanillyl alcohol, syringyl alcohol, and p-hydroxyphenylpropanol, forming a complex polymer via ether and carbon-carbon bonds. Its high carbon content and diverse functional groups make it a multifunctional comonomer (organic ligand, carbon source, sulfur source, and oxygen source). Effectively utilizing kraft lignin not only reduces environmental pollution but also provides new economic benefits for the papermaking industry.

[0005] This patent provides a low-cost photocatalyst and preparation method using sustainably sourced kraft lignin as a precursor and comonomer. Experimental data demonstrates the exceptional efficiency of the kraft lignin-heptazine conjugated polymer in the photocatalytic reduction of hydrogen peroxide, with significant advantages in photocatalytic efficiency, stability, and affordability. This patent not only demonstrates the enormous potential of kraft lignin in the functionalization of heptazine-based polymers but also provides a new direction for the high-value utilization of lignin. By applying lignin to the development of new functional materials, it is possible to achieve efficient resource utilization, promote the development of green chemistry, and achieve both environmental and economic benefits. Summary of the Invention

[0006] The present invention provides a kraft lignin-heptazine conjugated polymer, a preparation method and applications thereof. The kraft lignin-heptazine conjugated polymer of the present invention has significant advantages in the photocatalytic reduction of hydrogen peroxide and is reusable, thereby providing a new direction for the high-value utilization of kraft lignin.

[0007] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0008] A kraft lignin-heptazine conjugated polymer comprises a kraft lignin unit and a heptazine ring unit.

[0009] The heptazine ring group, an aromatic ring structure composed of alternating C-N bonds, is an extremely stable electron-withdrawing group. Embedded as an organic unit within the polymer framework, the heptazine ring group not only inhibits photogenerated charge recombination but also effectively modulates the electronic structure and promotes electron transfer between adjacent layers.

[0010] The aforementioned kraft lignin-heptazine conjugated polymer possesses a unique lignin-heptazine ring conjugated structure, which accelerates inter- and intramolecular charge transfer, reduces exciton binding energy, narrows the polymer's band gap, broadens the visible light response range, and promotes n-π* transitions, thereby significantly improving the efficiency of photocatalytic hydrogen peroxide synthesis under visible light. Furthermore, the introduction of kraft lignin increases the material's surface area and surface active sites, expands the π-conjugated system, reduces carrier recombination during the photocatalytic process, and significantly enhances photocatalytic efficiency and stability.

[0011] As one preferred implementation scheme, the preferred structure of the above-mentioned kraft lignin-heptazine conjugated polymer is:

[0012] This structure is the preferred structure of this application. When the amount of kraft lignin added is higher, the carbon content of the resulting material is higher and the nitrogen content is lower, corresponding to the fact that each heptazine unit in the structure is connected to more kraft lignin. The catalytic effect advantage of this structure is very significant.

[0013] The kraft lignin-heptazine conjugated polymer is formed by copolymerizing kraft lignin and heptazine ring in alkali metal molten salt.

[0014] As one preferred implementation scheme, the kraft lignin-heptazine conjugated polymer is prepared by copolymerizing kraft lignin and a carbon-nitrogen polymer in an alkali metal salt.

[0015] To further ensure catalytic performance, the mass ratio of kraft lignin to carbon-nitrogen polymer solid powder is (0.001-0.2):1, and the mass ratio of alkali metal salt to carbon-nitrogen polymer solid powder is (2-20):1. More preferably, the mass ratio of kraft lignin to carbon-nitrogen polymer solid powder is (0.006-0.1):1, and the mass ratio of alkali metal salt to carbon-nitrogen polymer solid powder is (8-12):1.

[0016] As one of the specific preferred implementation schemes, the preparation method of the above-mentioned kraft lignin-heptazine conjugated polymer comprises the following steps:

[0017] 1) uniformly mixing kraft lignin, a carbon-nitrogen polymer solid powder, and an alkali metal salt, calcining under a nitrogen atmosphere, naturally cooling, and grinding to obtain a mixture (a solid powder mixture of kraft lignin-heptazine conjugated polymer and alkali metal salt), wherein the calcination temperature is 450-600° C. and the calcination time is 1-4 hours;

[0018] 2) The mixture obtained in step 1) is centrifuged and washed to remove the remaining salt, and then dried and ground to obtain a kraft lignin-heptazine conjugated polymer.

[0019] The preparation method is simple, efficient, and suitable for large-scale production. Experimental verification shows that the yield of hydrogen peroxide of the prepared kraft lignin-heptazine conjugated polymer is significantly improved under visible light irradiation.

[0020] In the above step 1), the heating rate during calcination is 1-3°C / min.

[0021] In the above step 1), the preparation method of the carbon-nitrogen polymer solid powder is: calcining the nitrogen-containing precursor under nitrogen atmosphere, then naturally cooling and grinding to obtain the carbon-nitrogen polymer solid powder, wherein the calcination temperature is 400-550°C and the calcination time is 1-3h; the nitrogen-containing precursor is at least one of melamine, melamine, cyanuric acid, 5-aminotetrazole, guanidine thiocyanate, thiourea, and urea.

[0022] In order to improve product performance, the nitrogen-containing precursor is preferably melamine.

[0023] The inventors have found that a single alkali metal salt needs to be polymerized at a higher temperature, which may cause carbonization of the polymer. However, the melting point of mixed alkali metal salts is lowered, and the reaction temperature is lowered, which is conducive to the catalytic reaction. Preferably, the mixed molten salt method is adopted in this application.

[0024] In step 1), the alkali metal salt is a mixture of at least two of lithium chloride, sodium chloride, potassium chloride, zinc chloride, lithium bromide, potassium bromide, sodium bromide, and mixed eutectic salts thereof. More preferably, the alkali metal salt is a mixture of lithium chloride and potassium chloride in a mass ratio of 9:(10-12).

[0025] The kraft lignin-heptazine conjugated polymer can be used for photocatalytic production of hydrogen peroxide. After the reaction is completed, the kraft lignin-heptazine conjugated polymer is filtered out, washed by centrifugation, dried, and then reused.

[0026] After filtering out the kraft lignin-heptazine conjugated polymer, the precipitate was centrifuged and washed three times (at a centrifugal speed of 8,000 to 10,000 rpm, with each centrifugation lasting 3 to 5 minutes). The precipitate was placed in a vacuum oven and dried at 50 to 60° C. for 10 to 12 hours, and then naturally cooled to room temperature. The solid powder was ground in an agate mortar to obtain a particle size of less than 100 μm, which was the recovered kraft lignin-heptazine conjugated polymer material after the reaction.

[0027] Compared with traditional catalysts, the kraft lignin-heptazine conjugated polymer of the present invention has a stable structure, a long service life, and can realize the repeated use of the catalyst.

[0028] The above-mentioned photocatalytic method for producing hydrogen peroxide comprises mixing a catalyst, water, and an alcohol sacrificial agent, uniformly dispersing the mixture, and then conducting a hydrogen peroxide production reaction under conditions of visible light irradiation with a wavelength of 420-600 nm and a temperature of 5-40°C in a reaction atmosphere of atmospheric pressure air. The catalyst is the aforementioned kraft lignin-heptazine conjugated polymer. This method does not require additional oxygen flow and is simple and highly efficient.

[0029] The above reaction time is preferably 1 to 4 hours.

[0030] During the photocatalytic synthesis of hydrogen peroxide, alcohol sacrificial agents can donate electrons, preventing the recombination of photogenerated charge carriers (electron-hole pairs) on the photocatalyst surface, thereby improving photocatalytic efficiency. Photogenerated electrons effectively reduce oxygen to hydrogen peroxide, while photogenerated holes are captured by the alcohol sacrificial agent. Furthermore, the alcohol sacrificial agent traps photogenerated holes, reducing the occurrence of other unwanted side reactions.

[0031] The alcohol sacrificial agent is at least one of isopropyl alcohol, methanol, ethanol, ethylene glycol, glycerol or benzyl alcohol; the volume ratio of the alcohol sacrificial agent to water is 1:(2-20).

[0032] The concentration of the above-mentioned kraft lignin-heptazine conjugated polymer material is 50-5000 mg / L.

[0033] The technologies not mentioned in this invention are all referred to the prior art.

[0034] This application includes at least the following beneficial technical effects:

[0035] 1. By using kraft lignin copolymerization and a molten salt method, aromatic carbon ring structures and heteroatoms were doped into heptazine-based polymers to form nanorod-like structures, enhancing the surface area and active sites. The expansion of the π-conjugated system and the reduction of the band gap improve the separation efficiency of photogenerated carriers and significantly enhance the photocatalytic efficiency.

[0036] 2. The copolymerization of kraft lignin effectively reduces the band gap of the heptazine-based polymer, extending its light absorption response range to the long-wavelength visible light region and improving the separation efficiency of photogenerated carriers. Under visible light irradiation, the material exhibits excellent photocatalytic ability for the synthesis of hydrogen peroxide, significantly improving light capture and utilization efficiency, and has great potential for solar photocatalytic applications.

[0037] 3. By using kraft lignin as a comonomer, this new approach to high-value utilization of kraft lignin is being explored. This material can be used not only for photocatalytic synthesis of hydrogen peroxide but also in water treatment, air purification, and batteries, demonstrating broad application prospects.

[0038] 4. Kraft lignin, a major component of papermaking black liquor, is abundant and inexpensive to obtain. Compared to traditional photocatalytic materials, the present invention utilizes kraft lignin as a comonomer, significantly reducing material costs. Furthermore, the molten salt process employed is simple and efficient, eliminating the need for expensive equipment and complex process steps, further reducing production costs. This makes this material significantly economically advantageous and practically feasible for large-scale industrial production.

[0039] 5. The material formed by the copolymerization of kraft lignin and heptazine ring has a stable structure, long service life, can realize the reuse of catalysts, and is acid and alkali resistant. It complies with the principles of green chemistry and sustainable development, reduces the impact on the environment, and has great potential and broad application prospects in the field of photocatalysis.

[0040] 6. The preparation of hydrogen peroxide is simplified and can be carried out under normal pressure air. No additional oxygen is required during the preparation process. At the same time, the yield of hydrogen peroxide is significantly improved, which is simple and efficient.

[0041] 7. The irradiation wavelength band of visible light during the preparation of hydrogen peroxide is broadened. Existing materials have almost no way to generate hydrogen peroxide under irradiation of 600 nanometers, while the present application still has a high yield under irradiation of 600 nanometers. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1This is a scanning electron microscope image of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention, showing that the material exhibits a dense nanorod-like morphology;

[0043] Figure 2 This is a transmission electron microscopy image of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention, in which nanorod-like structures and nanosheet morphologies are observed;

[0044] Figure 3 (a) Nitrogen adsorption and desorption and (b) pore size distribution of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention and Comparative Example 1, showing the mesoporous structure characteristics of the material;

[0045] Figure 4 X-ray powder diffraction patterns of the kraft lignin-heptazine conjugated polymer materials prepared in Examples 1-4 of the present invention and Comparative Example 1, showing the crystal structures of the materials;

[0046] Figure 5 Fourier transform infrared spectra of the kraft lignin-heptazine conjugated polymer materials prepared in Examples 1-4 of the present invention and Comparative Example 1;

[0047] Figure 6 Solid-state nuclear magnetic resonance spectra of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention and Comparative Example 1;

[0048] Figure 7 X-ray photoelectron spectra of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention and Comparative Example 1: (a) overall spectrum, (b) carbon 1s spectrum, (c) nitrogen 1s spectrum, and (d) potassium 2p spectrum, indicating the elemental composition and chemical structure of the materials;

[0049] Figure 8 UV-visible diffuse reflectance spectra of the kraft lignin-heptazine conjugated polymer materials prepared in Examples 1-4 of the present invention and Comparative Example 1;

[0050] Figure 9 This is a performance graph of the photocatalytic preparation of hydrogen peroxide by the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention under varying incident light wavelengths, showing the performance changes of the material under different light wavelengths;

[0051] Figure 10 This is a performance diagram of the photocatalytic preparation of hydrogen peroxide by the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention at different reaction times, showing the effect of different reaction times on the amount of product generated;

[0052] Figure 11This is a cyclic experimental diagram of the photocatalytic production of hydrogen peroxide by the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention, showing the stability of the material. DETAILED DESCRIPTION

[0053] In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with the examples, but the content of the present invention is not limited to the following examples.

[0054] Example 1

[0055] (1) 3 g of melamine was weighed and added to a porcelain boat, which was then placed in a tube furnace. Under nitrogen atmosphere, the temperature was raised to 425°C at a rate of 2°C / min and calcined at 425°C for 2 hours. The mixture was then naturally cooled to room temperature and ground in an agate mortar to a particle size of less than 100 μm to obtain a carbon-nitrogen polymer solid powder.

[0056] (2) Weigh 0.6 g of the carbon-nitrogen polymer solid powder obtained in step (1), mix it with 5 mg of kraft lignin, 3.3 g of potassium chloride and 2.7 g of lithium chloride, and grind it in a mortar to obtain a uniform mixture; wherein, kraft lignin (Kraftlignin) is provided by the Institute of Forest Products Chemical Industry, Chinese Academy of Forestry; or kraft lignin is prepared according to the following literature: Yan Zhenyu, Wei Lulu, Deng Yongjun, Tian Qingwen, Fang Guigan. Fractionation and performance comparison of kraft lignin (for sample L3). Applied Chemical Industry, 2020, 49(7): 4-7.

[0057] (3) The mixture obtained in step (2) was placed in a porcelain boat and placed in a tube furnace. Under nitrogen atmosphere, the temperature was increased to 550°C at a heating rate of 2°C / min, and the mixture was calcined at 550°C for 4 hours. The mixture was then naturally cooled to room temperature and ground in an agate mortar to a particle size of less than 100 μm to obtain a solid powder of the mixture.

[0058] (4) The solid powder of the mixture obtained in step (3) was placed in a beaker, 300 mL of water was added, and the mixture was heated and stirred at 80° C. for 10 minutes to disperse it evenly. The mixture was then centrifuged and washed (at a centrifugal speed of 10,000 rpm and a centrifugal time of 3 minutes each time). The above steps were repeated 8 times to obtain a precipitate.

[0059] (5) The precipitate obtained in step (4) was placed in a vacuum oven, dried at 60° C. for 12 hours, naturally cooled to room temperature, and ground with an agate mortar to a particle size of less than 100 μm, thereby obtaining a sulfate lignin-heptazine conjugated polymer material (S-CN5).

[0060] Taking the kraft lignin-heptazine conjugated polymer described in Example 1 as an example, the left side of the dotted line is the heptazine conjugated polymer, and the right side of the dotted line is the kraft lignin. The reaction equation is shown in Formula 1:

[0061]

[0062] Formula 1, wherein the first horizontal arrow has “nitrogen, heating” below it; the second horizontal arrow has “addition of sulfate lignin and alkali metal salt” above it and “nitrogen, heating” below it.

[0063] Example 2

[0064] The difference between this embodiment and embodiment 1 is that the amount of kraft lignin added in step (2) is 10 mg. The other steps and parameters are the same as those in embodiment 1. The kraft lignin-heptazine conjugated polymer material (S-CN 10 ).

[0065] Example 3

[0066] The difference between this embodiment and embodiment 1 is that the amount of kraft lignin added in step (2) is 15 mg. The other steps and parameters are the same as those in embodiment 1. The kraft lignin-heptazine conjugated polymer material (S-CN 15 ).

[0067] Example 4

[0068] The difference between this example and Example 1 is that the amount of kraft lignin added in step (2) is 1 mg. The other steps and parameters are the same as those in Example 1. A kraft lignin-heptazine conjugated polymer material (S-CN1) is obtained.

[0069] Comparative Example 1

[0070] This embodiment differs from embodiment 1 in that no sulfate lignin is added in step (2). The other steps and parameters are the same as those in embodiment 1. A heptazine conjugated polymer (KPHI) is obtained.

[0071] Comparative Example 2

[0072] This example differs from Example 1 in that in step (2), kraft lignin is replaced with alkaline lignin (Aladdin, L330880-25g, CAS No. 8068-05-1). All other steps and parameters are the same as in Example 1. An alkaline lignin-heptazine conjugated polymer is obtained.

[0073] Comparative Example 3

[0074] This embodiment differs from embodiment 1 in that potassium sulfate is used instead of kraft lignin in step (2). The other steps and parameters are the same as those in embodiment 1. A potassium sulfate-modified heptazine conjugated polymer is obtained.

[0075] Comparative Example 4

[0076] 3g of melamine was weighed and placed in a corundum crucible. The crucible was then placed in a tube furnace and heated to 550°C at a rate of 2°C / min under a nitrogen atmosphere. The mixture was then calcined at 550°C for 4 hours. The mixture was cooled to room temperature and ground to obtain a solid powder. This was a bulk carbon nitride polymer (without the addition of molten salt or sulfate lignin).

[0077] Comparative Example 5

[0078] This embodiment differs from embodiment 1 in that potassium chloride and lithium chloride are not added in step (2). The other steps and parameters are the same as those in embodiment 1. A kraft lignin copolymer-phase carbon nitride polymer (without molten salt) is obtained.

[0079] Comparative Example 6

[0080] 3g of kraft lignin was weighed and placed in a corundum crucible. The crucible was then placed in a tube furnace. Under a nitrogen atmosphere, the temperature was increased at a rate of 2°C / min to 550°C. The crucible was then calcined at 550°C for 4 hours. The mixture was cooled to room temperature and then ground to obtain a solid powder. This resulted in a kraft lignin carbon material.

[0081] Figure 1 This is a scanning electron microscope image of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention. It can be observed from the image that the sample presents a dense nanorod-like structure.

[0082] Figure 2 This is a transmission electron micrograph of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention. The image shows densely packed nanorod-like structures and nanosheets. It is inferred that the unique morphology of the kraft lignin-heptazine conjugated polymer is due to the ability of kraft lignin to generate abundant microporous and mesoporous carbon nanotubes during heat treatment in a nitrogen atmosphere using a molten salt method. These carbon nanotubes accumulate with the heptazine units in the molten salt environment, and the low adhesion between lithium chloride and potassium chloride and the heptazine rings leads to the formation of nanorods within the salt space.

[0083] Figure 3Figures 1 and 2 show (a) nitrogen adsorption and desorption and (b) pore size distribution of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention and Comparative Example 1. According to the IUPAC classification system, the nitrogen adsorption isotherm of the kraft lignin-heptazine conjugated polymer sample exhibits a typical type IV curve with an H1 hysteresis loop, indicating that the sample has mesoporous structure characteristics. The specific surface area of ​​the KPHI sample before modification is 47 m 2 ·g -1 , while the specific surface area of ​​S-CN5 sample increased to 71m 2 ·g -1 This shows that the addition of sulfate lignin can increase the specific surface area and active sites of the modified material, thereby improving the adsorption efficiency of oxygen and further enhancing the performance of photocatalytic production of hydrogen peroxide. The reason for the increase in specific surface area and pore size may be that sulfate lignin can be used to prepare porous carbon materials using the molten salt method in a nitrogen atmosphere, and the addition of sulfate lignin is beneficial to the S-CN X Increase in specific surface area and pore size.

[0084] Figure 4 The X-ray powder diffraction patterns of the sulfate lignin-heptazine conjugated polymer materials prepared in Examples 1-4 of the present invention and Comparative Example 1. As can be seen from the figure, there is a characteristic peak at about 8°, which is due to the rearrangement of the heptazine unit and the charging effect of potassium. The diffraction peaks of 13.0° and 27.4° correspond to the (100) and (002) crystal planes of the graphite material, respectively. The main characteristic peaks of S-CN are similar to those of the KPHI sample, and no new peaks are observed. This means that with the copolymerization of sulfate lignin, its basic graphite-like structure is not destroyed, no new crystal structure is introduced, and it is not affected by thermal polymerization. When the sulfate lignin content increases, the overall diffraction peak intensity of the sulfate lignin-heptazine conjugated polymer is relatively weak. This result is because the sulfate lignin copolymerization partially destroys the crystal structure of the heptazine-based polymer, making it more disordered.

[0085] Figure 5 The Fourier transform infrared spectra of the kraft lignin-heptazine conjugated polymer materials prepared in Examples 1-4 of the present invention and Comparative Example 1. It was found that all samples had a 0.1% IR spectrum at 812 cm -1 There is a sharp peak at 1240-1637 cm -1 The peaks in the range of 3000–3600 cm-1 can be attributed to the characteristic stretching vibration modes of aromatic nitrogen heterocycles. -1 The broad peaks between the two groups can be attributed to NH stretching vibrations, further confirming the presence of NH or NH2 groups on the surface. This indicates that there are some incompletely polymerized amino functional groups and adsorbed water molecules on the surface of the kraft lignin-heptazine conjugated polymer.

[0086] Figure 6 The solid-state nuclear magnetic resonance spectrum of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention; 13 Analysis of C NMR data revealed that KPHI had peaks at 163.1 ppm and 156.5 ppm, and S-CN5 had similar peaks at 163.0 ppm and 156.2 ppm. The peak near 163.0 ppm corresponded to [CN2(NH X )], while the peak near 156.2ppm is attributed to the C(e) atom in (CN3), which indicates the presence of a heptaazine chemical structure. In addition, new peaks appeared at 166.3ppm, 165.0ppm, 160.3ppm, 158.7ppm and 157.1ppm in S-CN5, which are attributed to the syringyl signal peak of sulfate lignin and the carbon signal peak of guaiacyl. Although KPHI and S-CN5 have similar carbon nitride skeleton structures, multiple new characteristic peaks appeared in the S-CN5 sample, verifying the successful introduction of sulfate lignin and the change in the chemical structure of the heptaazine-based polymer material. The appearance of these new peaks further proves the existence of the sulfate lignin-heptazine conjugated structure.

[0087] Figure 7 X-ray photoelectron spectra of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention and Comparative Example 1: (a) Overall spectrum, (b) Carbon 1s spectrum, (c) Nitrogen 1s spectrum, and (d) Potassium 2p spectrum. The KPHI and S-CN5 samples contain only C, N, and K elements, with a small amount of oxygen detected due to water and carbon dioxide adsorbed on the sample surface. The C1s spectra of both samples show three characteristic peaks corresponding to the heptazine-based polymer framework: N=CN, C-NH x and CC (284.8eV). The binding energy of NC=N of S-CN5 shifts to a higher direction compared with KPHI, indicating that the copolymerization of kraft lignin leads to changes in the electronic structure. The N 1s spectrum shows four characteristic peaks: bidentate nitrogen atom (C=NC), tertiary amine group (N-(C)3), amino functional group (CNH) and π excitation. In the XPS spectrum, the K 2p3 / 2 and K 2p1 / 2 peaks are located at 293eV and 296eV, respectively, proving the presence of potassium ions in the kraft lignin-heptazine conjugated polymer material. The kraft lignin-heptazine conjugated polymer material has a high negative charge density (due to the lone pair electrons of the nitrogen atom). Potassium ions, as a source of positive charge, can neutralize these negative charges, play a role in charge compensation, and achieve charge balance, thereby improving the structural stability, photocatalytic performance and conductive properties of the material.

[0088] Table 1 shows the X-ray photoelectron spectroscopy analysis results of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention and Comparative Example 1. Compared with KPHI, S-CN5 shows significantly increased area ratios at the N-(C)3 peak at 400.30 eV and the N=CN peak at 288.33 eV. The increased C / N ratio also indicates that the kraft lignin was successfully incorporated into the structure of the heptazine-based polymer.

[0089] Table 1

[0090]

[0091] From the above characterization, it can be seen that Example 1 successfully obtained the product structure shown in Formula 1.

[0092] Figure 8 The UV-visible diffuse reflectance spectra of the sulfate lignin-heptazine conjugated polymer materials prepared in Examples 1-4 of the present invention and Comparative Example 1. As the sulfate lignin content increases, the optical edge of the sulfate lignin-heptazine-based conjugated polymer sample shows a red shift phenomenon. The sulfate lignin-heptazine-based conjugated polymer sample moves toward the long wavelength direction at the optical edge, resulting in a significant enhancement of light absorption in the wavelength range of 450 to 800 nanometers. This change means that the sulfate lignin-heptazine-based conjugated polymer can more effectively absorb and utilize the energy of more incident photons, inducing the generation of n→π* transitions, thereby significantly improving the photocatalytic performance. The introduction of sulfate lignin effectively solves the challenges of the narrow response and insufficient intensity of the heptazine-based conjugated polymer in the visible light range, and further improves the absorption and utilization efficiency of the heptazine-based conjugated polymer for sunlight.

[0093] Photocatalytic hydrogen peroxide production performance test:

[0094] The samples prepared by Example 1-4 and Comparative Example 1-6 are respectively used as catalysts, and catalyst (30mg), deionized water (25mL) and isopropanol (5mL) are put into a cylindrical beaker reactor with an open top of 100mL, and the reaction atmosphere is the air of normal pressure. After ultrasonic treatment for 5 minutes, 420 nanometers, 450 nanometers, 500 nanometers, 550 nanometers and 600 nanometers LED lamps are used as light sources, and the LED lamp holder is directly facing the top of the beaker to irradiate the reaction solution. After 1 hour of reaction, the experiment ends. Take a 50mL centrifuge tube, add 7.795mL pure water, 200 μL potassium iodide solution with a concentration of 0.1mol / L and 5 μL ammonium molybdate solution, as indicators. The liquid after the reaction for 1 hour is poured into another centrifuge tube, and it is 10000r / min to set the centrifugal speed, and the centrifugal time is 5min. Afterwards, 2mL supernatant is quickly taken out and added to the indicator tube. Finally, the absorbance was measured under a UV-visible spectrophotometer, and the hydrogen peroxide concentration was calculated based on the corresponding absorbance at 352 nm and the standard curve.

[0095] Table 2 shows the photocatalytic hydrogen peroxide synthesis activities of Examples 1-4 and Comparative Examples 1-6 under incident light wavelengths of 420 nm and 600 nm. The effect of different contents of kraft lignin copolymerization on catalytic performance was investigated. Under 420 nm incident light wavelength, the original KPHI catalyst (Comparative Example 1) had a low activity in producing hydrogen peroxide (290 μmol·g -1 ·Hour- 1 However, after adding a certain mass range (1-15 mg) of kraft lignin, all synthesized S-CN X The activity of samples (Examples 1-4) in producing hydrogen peroxide was significantly improved. Among them, the catalytic performance of sample S-CN5 was the best, with a yield of 1169 μmol·g -1 ·Hour- 1 , which is about 4 times that of KPHI. The improvement in the catalytic efficiency of S-CN5 is mainly attributed to the copolymerization of sulfate lignin, which not only promotes the transfer of interfacial electrons to oxygen and inhibits the degradation of hydrogen peroxide, but also prolongs the existence time of transient substances by forming a charge separation state. However, the excessive addition of sulfate lignin (>10mg) will cause the catalytic activity to begin to decline, because the high-temperature pyrolysis of sulfate lignin usually leads to a large amount of carbon deposition, which destroys the π-conjugated structure and crystallinity of the polymer. Therefore, appropriate sulfate lignin copolymerization is crucial for optimizing the electronic structure of heptazine conjugated polymers and significantly improving the photocatalytic activity of hydrogen peroxide.

[0096] At the same time, in order to exclude the influence of other factors, the photocatalytic preparation of hydrogen peroxide activity of heptazine conjugated polymer materials synthesized by other comonomers or other methods was tested. Under the same added mass, the sulfate lignin copolymerization (Example 1) showed better photocatalytic preparation of hydrogen peroxide performance than the alkaline lignin (Comparative Example 2). When the comonomer was replaced with an inorganic sulfate (Comparative Example 3), no hydrogen peroxide was detected. This may be because the lack of aromatic groups makes it impossible to synthesize carbon-nitrogen polymer materials with melamine during the thermal polymerization process. The activity of sulfate lignin-heptazine conjugated polymer is significantly higher than that of bulk carbon nitride polymer prepared by typical melamine thermal polymerization (Comparative Example 4). The activity of the carbon nitride polymer sample obtained by sulfate lignin copolymerization without the molten salt method is not significantly improved compared with Comparative Example 4 (Comparative Example 5). The sulfate lignin carbon material (Comparative Example 6) prepared under molten salt conditions also has basically no photocatalytic activity.

[0097] Table 2

[0098]

[0099]

[0100] Table 2 shows the data obtained after 1 h of reaction.

[0101] Figure 9 Figure 1 shows the performance of the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention for photocatalytic production of hydrogen peroxide under varying incident light wavelengths. To investigate the effect of incident light wavelength on the photocatalytic reduction performance of S-CN5, the hydrogen peroxide yield of the S-CN5 photocatalyst gradually decreased with increasing wavelength, consistent with the trend in the UV-visible absorption spectrum. These results indicate that the photocatalytic hydrogen peroxide yield of S-CN5 is strongly correlated with incident light wavelength, with visible light being the primary driving force.

[0102] Bulk carbon nitride polymer materials and heptazine conjugated polymers are almost unable to generate hydrogen peroxide under 600 nm red light irradiation, mainly because their band gaps are wide, the photon energy is insufficient to stimulate electron transitions, and the electron-hole separation efficiency is still low. It is worth noting that the modified S-CN5 has photocatalytic activity under 600 nm red light irradiation (hydrogen peroxide yield is 151 μmol·g -1 ·Hour- 1 ). Kraft lignin-heptazine conjugated polymer material has n-π * The reason for this is that its structure contains abundant lone pairs of electrons (from oxygen, nitrogen, and sulfur atoms) and a wide range of π *orbital (derived from the conjugated system of the heptazine ring). This transition enhances the light absorption range, especially under 600-nanometer red light, improving the separation efficiency of electron-hole pairs, thereby enhancing the photocatalytic activity for hydrogen peroxide synthesis. As a photocatalyst that can respond to 600-nanometer red light, S-CN5 demonstrates its unique band structure and electronic properties, enabling more efficient use of sunlight, thereby improving overall light energy utilization efficiency, representing an innovation and breakthrough in photocatalytic materials.

[0103] Figure 10 This graph shows the photocatalytic production of hydrogen peroxide using the kraft lignin-heptazine conjugated polymer material prepared in Example 1 of the present invention at different reaction times. Under the influence of a 420-nanometer LED light source and the photocatalyst, the amount of hydrogen peroxide produced gradually increases with reaction time, eventually reaching a plateau. This observation indicates that the photocatalyst achieves optimal catalytic activity within 2-3 hours.

[0104] Figure 11 The cyclic experimental diagram of the photocatalytic preparation of hydrogen peroxide performance of the kraft lignin-heptazine conjugated polymer material obtained in Example 1 of the present invention. The circulation method is: after the preparation of hydrogen peroxide is completed, the kraft lignin-heptazine conjugated polymer is filtered out, centrifuged and washed (centrifugal speed 10000 rpm, each centrifugation time is 3min) 3 times, dried (dried at 60°C for 12 hours, naturally cooled to room temperature), and ground with an agate mortar to obtain a solid powder with a particle size of less than 100 microns, that is, the recovered kraft lignin-heptazine conjugated polymer material after reaction, which is then circulated for the catalytic preparation of hydrogen peroxide. The above process is repeated 5 times (5 cycles), the incident light wavelength is 420 nanometers, and the reaction time is 1h. The fluctuation between each cycle is very small. After 5 cycles, the output of hydrogen peroxide does not decrease significantly, indicating that the S-CN5 photocatalyst has excellent stability.

Claims

1. A kraft lignin-heptazine conjugated polymer, characterized in that: Its structure includes a kraft lignin unit and a heptazine ring unit, and its structural formula is:

2. A method for preparing the kraft lignin-heptazine conjugated polymer according to claim 1, characterized in that: The invention is prepared by copolymerizing sulfate lignin and carbon-nitrogen polymer in alkali metal salt, wherein the mass ratio of sulfate lignin to carbon-nitrogen polymer solid powder is (0.001-0.2):1, and the mass ratio of alkali metal salt to carbon-nitrogen polymer solid powder is (2-20):

1.

3. The preparation method according to claim 2, wherein: The steps include: 1) uniformly mixing kraft lignin, carbon-nitrogen polymer solid powder and alkali metal salt, roasting under nitrogen atmosphere, naturally cooling and grinding to obtain a mixed material, wherein the roasting temperature is 450-600° C. and the roasting time is 1-4 hours; 2) The mixture obtained in step 1) is centrifuged and washed to remove the remaining salt, and then dried and ground to obtain a kraft lignin-heptazine conjugated polymer.

4. The preparation method according to claim 3, wherein: In step 1), the preparation method of the carbon-nitrogen polymer solid powder is: calcining the nitrogen-containing precursor under nitrogen atmosphere, then naturally cooling and grinding to obtain the carbon-nitrogen polymer solid powder, wherein the calcination temperature is 400-550°C and the calcination time is 1-3h; the nitrogen-containing precursor is at least one of melamine, melamine, cyanuric acid, 5-aminotetrazole, guanidine thiocyanate, thiourea, and urea.

5. The preparation method according to any one of claims 2 to 4, characterized in that: The alkali metal salt is a mixture of at least two of lithium chloride, sodium chloride, potassium chloride, zinc chloride, lithium bromide, potassium bromide, sodium bromide, and mixed eutectic salts thereof.

6. The preparation method according to claim 5, wherein: The alkali metal salt is a mixture of lithium chloride and potassium chloride in a mass ratio of 9:(10-12).

7. Use of the kraft lignin-heptazine conjugated polymer according to claim 1, characterized in that: The method is used for photocatalytic preparation of hydrogen peroxide; after the preparation of hydrogen peroxide is completed, the kraft lignin-heptazine conjugated polymer is filtered out, centrifugally washed, dried, and then reused.

8. The use according to claim 7, characterized in that: The method for photocatalytically preparing hydrogen peroxide comprises: mixing a catalyst, water, and an alcohol sacrificial agent, uniformly dispersing the mixture, and then conducting a hydrogen peroxide production reaction in a reaction atmosphere of normal pressure air, irradiated with visible light having a wavelength of 420-600 nanometers, and at a temperature of 5-40°C; wherein the catalyst is the kraft lignin-heptazine conjugated polymer described in claim 1.

9. The use according to claim 8, characterized in that: The alcohol sacrificial agent is at least one of isopropanol, methanol, ethanol, ethylene glycol, glycerol or benzyl alcohol; the volume ratio of the alcohol sacrificial agent to water is 1:(2-20); and the concentration of the kraft lignin-heptazine conjugated polymer material is 50-5000 mg / L.

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