Alkaline lignin assisted graphite carbon nitride material and preparation method and application thereof

Abstract

The application relates to the technical field of functional materials, in particular to an alkali lignin assisted graphite carbon nitride material and a preparation method and application thereof. The alkali lignin is used as a carbon precursor, biomass-based macromolecules realize surface modification of g-C3N4, the band gap structure of the catalyst is accurately controlled, and the photocatalytic performance is improved; after mixed acid treatment, the surface functional groups and Lewis acid sites of the material are adjusted, and the selective breaking of C-C bonds in HMF is promoted. Finally, the alkali lignin assisted graphite carbon nitride material can realize selective oxidation of HMF into MA.

Description

Technical Field

[0001] This invention relates to the field of functional materials technology, specifically to an alkali lignin-assisted graphite carbon nitride material and its preparation method and application. Background Technology

[0002] Fossil fuels are non-renewable resources. Even if current extraction levels can guarantee global utilization and development, oil fields and coal mines will inevitably be depleted in the near future. Extraction of fossil fuels will become less convenient, leading to a decline in extraction volumes and a genuine energy crisis. Under these circumstances, developing renewable or transitional energy sources is one of the important strategies for maintaining sustainable social and economic development.

[0003] 5-Hydroxymethylfurfural (HMF), synthesized from abundant cellulose, glucose, and fructose, is one of the most promising platform chemicals for the potential utilization of biomass resources. Its selective oxidation to maleic acid (N-(4-fluorophenyl)maleamic acid, MA) is considered one of the most promising processes for future biorefining. MA is important in many industrial applications, particularly in biorefining. It is a versatile bulk feedstock for synthesizing unsaturated polyester resins, pharmaceuticals, agrochemicals, lubricant additives, and polymers. Currently, industrial-scale MA production involves the oxidation of fossil benzene (approximately 27% of total production) and n-butene (73%) under high temperature and pressure, typically yielding suboptimal yields and various byproducts. However, industrial-scale MA production is often based on vanadium processes, which require high temperatures, high pressures, and long reaction times. Furthermore, the biotoxicity of vanadium poses environmental and health risks, thus limiting the application of such industrial-scale MA production methods.

[0004] Nitrogen-based graphite carbon (g-C3N4) is a visible-light-responsive metal-free semiconductor that has attracted considerable attention due to its low toxicity, high physicochemical stability, and efficient electron transfer properties. The suitable conduction band (CB) and valence band (VB) values ​​of g-C3N4 make it particularly promising for photocatalytic biorefining. However, its applications are often hindered by the unsatisfactory visible-light absorption, low specific surface area, and high recombination probability of charge carriers in pristine g-C3N4.

[0005] Therefore, a catalytic material based on g-C3N4 is needed to overcome the application limitations of g-C3N4 alone. Summary of the Invention

[0006] This invention provides an alkali lignin-assisted graphite carbon nitride material, its preparation method, and its application.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides a method for preparing alkali lignin-assisted graphite carbon nitride material, comprising: grinding and mixing melamine and alkali lignin to obtain a mixture; after calcining the mixture, immersing it in a mixed acid reaction; and after removing residual acid from the surface, obtaining a biochar-doped graphite carbon nitride material with persistent free radicals, namely alkali lignin-assisted graphite carbon nitride material; wherein, the persistent free radicals include oxygen-containing functional groups.

[0009] The photocatalytic performance of g-C3N4 modified with biomass-based compounds as carbon precursors was significantly improved. Lignin, as the most abundant aromatic compound on Earth, is rich in aromatic rings and phenolic molecules. These molecules can generate persistent free radicals (PFRs) through the cleavage and rearrangement of weak bonds during pyrolysis. Biochar-PFRs extracted from lignin contain abundant oxygen-containing functional groups, graphitized and semiquinone structures, and a conjugated π-electron system, which can enhance electron transfer under photoexcitation. Biochar-PFRs exhibit strong catalytic activity towards H2O2 and O2, which is conducive to the formation of reactive oxygen species (ROS). Therefore, lignin is an attractive carbon source for the production of biochar-assisted g-C3N4.

[0010] This invention provides an alkali lignin-assisted graphitic carbon nitride material with nitrogen defects and persistent biochar radicals, exhibiting a porous, coral-like nanosheet structure. During pyrolysis, alkali lignin generates biochar with persistent radicals through the cleavage and rearrangement of weak bonds. This biochar is then linked to g-C3N4 via hydrogen bonding and π-π covalent bonds, generating numerous oxygen-containing functional groups on the catalyst surface. Simultaneously, nitrogen defects are introduced, enhancing the visible light absorption and hydrophilicity of the alkali lignin-assisted graphitic carbon nitride material. Furthermore, oxidative etching of the alkali lignin-assisted graphitic carbon nitride material with a mixed acid exfoliates it into a two-dimensional porous sheet-like morphology. The resulting acidic oxygen-containing functional groups provide Lewis acidic sites, promoting the C / C bond between the hydroxymethyl group and the furan ring in HMF under thermocatalytic conditions, thus providing more radical intermediates for the selective production of MA.

[0011] Preferably, the mass percentage of the alkali lignin to the melamine is 1 to 2.5 wt%.

[0012] More preferably, the alkali lignin to melamine has a mass percentage of 1 to 1.5 wt%.

[0013] More preferably, the alkali lignin to melamine has a mass percentage of 1.5 to 2.5 wt%.

[0014] More preferably, the alkali lignin to melamine has a mass percentage of 1-2 wt%.

[0015] Preferably, the calcination atmosphere is a nitrogen atmosphere, the calcination temperature is 500-700℃, the calcination heating rate is 1-5℃ / min, and the calcination time is 3-8h.

[0016] More preferably, the calcination temperature is 550°C, the calcination heating rate is 5°C / min, and the calcination time is 4 hours.

[0017] Preferably, the mixed acid is a mixed solution of sulfuric acid and nitric acid with a pH of 0 to 2; wherein the volume ratio of sulfuric acid to nitric acid is 1:(0.8 to 1.5).

[0018] More preferably, the mixed acid is a mixed solution of sulfuric acid and nitric acid with a pH of 0 to 0.5; wherein the volume ratio of sulfuric acid to nitric acid is 1:1.

[0019] The pH of the mixed acid can be infinitely close to 0.

[0020] Preferably, the mass-to-volume ratio of the mixture to the mixed acid is (3-6) g / L.

[0021] More preferably, the mass-to-volume ratio of the mixture to the mixed acid is 4 g / L.

[0022] Preferably, the immersion time in the mixed acid reaction is 3 to 6 hours.

[0023] Preferably, the method for removing residual acid from the surface is as follows: after the reaction is completed, centrifuge at 5000-10000 rpm until the pH of the supernatant after centrifugation is 7.0.

[0024] This invention provides an alkali lignin-assisted graphitic carbon nitride material, which includes biochar containing oxygen-containing free radicals and graphitic carbon nitride. The oxygen-containing free radicals in the biochar are bonded to the polar groups in the graphitic carbon nitride through hydrogen bonds.

[0025] The biochar material with persistent free radicals generated in this invention exhibits C=O of quinone groups and COC stretching vibrations of aryl alkyl ethers in infrared characterization, indicating that oxygen-containing functional groups are anchored on the surface of g-C3N4 biochar generated from calcined alkali lignin. The oxygen-containing functional groups in the persistent free radicals bind to the polar groups in g-C3N4 via hydrogen bonds, and are uniformly distributed in the alkali lignin-assisted graphitic carbon nitride material. Mixed acid treatment increases the acidic oxygen-containing functional groups on the material surface, improves the hydrophilicity of the material, promotes the adsorption of HMF, and facilitates the interaction between HMF and the generated biochar. 1The reaction with O2 increases Lewis acidity, which, under thermocatalytic conditions, facilitates the breaking of the C-C bonds in HMF, providing more furfural radicals for the formation of MA.

[0026] Preferably, the alkali lignin-assisted graphitic carbon nitride material has a porous coral-like nanosheet structure, and the particle size of the alkali lignin-assisted graphitic carbon nitride material is 1-3 μm; the content of graphitic carbon nitride is 97.5-99 wt% based on the weight of the alkali lignin-assisted graphitic carbon nitride material.

[0027] This invention provides the application of the alkali lignin-assisted graphite carbon nitride material prepared by the above method, or the above alkali lignin-assisted graphite carbon nitride material, in the photothermal synergistic catalysis of selective oxidation of pentahydroxymethylfurfural to maleic acid. In the photothermal synergistic catalysis process, the alkali lignin-assisted graphite carbon nitride material reduces the mineralization of pentahydroxymethylfurfural and its derived free radical intermediates by selectively oxidizing singlet oxygen free radicals, thereby achieving selective conversion of maleic acid.

[0028] The present invention provides a selective oxidation process for the production of alkali lignin-assisted graphite carbon nitride materials. 1 O2 oxidizes HMF to MA, while alkali lignin assists the Lewis acid in the graphite carbon nitride catalyst to promote the conversion efficiency of HMF to MA by activating the C-C bond between the hydroxymethyl group and the furan ring in HMF.

[0029] Therefore, the present invention has the following beneficial effects:

[0030] (1) This invention uses alkali lignin as a carbon precursor to achieve surface modification of g-C3N4 by biomass-based macromolecules, precisely control the band gap structure of the catalyst, and improve the photocatalytic performance; after mixed acid treatment, the surface functional groups and Lewis acid sites of the material are adjusted, which promotes the selective breaking of CC bonds in HMF.

[0031] (2) The method provided by the present invention can improve the surface properties and hydrophilicity of g-C3N4 by preparing biochar-doped g-C3N4 with persistent free radicals, thereby improving the adsorption capacity of the modified material for HMF and enabling selective oxidation of HMF to MA.

[0032] (3) The preparation process of the present invention is simple, easy to control, short in time, low in cost, and has little environmental pollution. Attached Figure Description

[0033] Figure 1 The images are scanning electron microscope (SEM) images, where a corresponds to BCN, b corresponds to 1.5% LCN, and c corresponds to 1.5% PLCN.

[0034] Figure 2The images show a comparison of structural characterizations, where a represents XRD and b represents FT-IR.

[0035] Figure 3 The image is a TGA diagram, where MAm-AL corresponds to a mixture of melamine and alkali lignin, and MAm corresponds to melamine.

[0036] Figure 4 This is an electron paramagnetic resonance image.

[0037] Figure 5 Catalytic performance graphs, where a is the H2O2 yield graph and b is the H2O2 photodegradation graph.

[0038] Figure 6 Here are the contact angle and infrared diagrams, where a is the contact angle diagram and b is the pyridine infrared characterization diagram.

[0039] Figure 7 The figures show the characterization results, where a is the photoelectrochemical characterization figure, b is the electrochemical impedance spectroscopy figure, and c is the photoluminescence spectrum figure.

[0040] Figure 8 In photothermal catalytic reactions 1 Electron paramagnetic resonance (EPR) plot of O2, where a represents BCN and 1.5% PLCN. 1 Electron paramagnetic resonance (EPR) images of O2, b represents the electron paramagnetic resonance (EPR) of 1.5% PLCN at different lamp-on times. 1 Electron paramagnetic resonance image of O2.

[0041] Figure 9 This is a schematic diagram of the mechanism for the selective oxidation of HMF to MA under photothermal catalysis. Detailed Implementation

[0042] The present invention will be further described below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0043] The alkali lignin in this section is from Shandong Longli Biotechnology Co., Ltd., and its main components include lignin; melamine is from Sinopharm Chemical Reagent Co., Ltd.; sulfuric acid is from Shanghai Maclean Biochemical Technology Co., Ltd., with a concentration of 98%; and nitric acid is from Laiyang Economic and Technological Development Zone Fine Chemical Plant, with a concentration of 68%.

[0044]

Example

[0045] Example 1

[0046] Prepare a mixture of alkali lignin and melamine at a mass percentage of 1.0 wt%. Grind and mix the alkali lignin and melamine for 30 min, then transfer the mixture to an alumina crucible and heat under N2 (99.99%, 50 mL / min). -1 At 5℃·min -1 Heating to 550℃ and holding for 4 hours yielded alkali lignin-assisted g-C3N4 material, denoted as 1% LCN. Subsequently, 100 mg of 1% LCN was dispersed in 20 mL of a mixed solution of sulfuric acid and nitric acid (sulfuric acid to nitric acid volume ratio 1:1, pH≈0), and magnetically stirred at room temperature for 5 hours. After washing with distilled water, the solution was centrifuged at 7000 rpm to remove all residual acid until the pH of the supernatant reached 7.0. Finally, freeze-drying yielded the etched alkali lignin-assisted g-C3N4 material, denoted as 1% PLCN.

[0047] Example 2

[0048] This embodiment is basically the same as Example 1, except that the mass percentage of alkali lignin and melamine is 1.5 wt%, and the final product is denoted as 1.5% PLCN.

[0049] Example 3

[0050] This embodiment is basically the same as Example 1, except that the mass percentage of alkali lignin and melamine is 2.0 wt%, and the final product is denoted as 2.0% PLCN.

[0051] Example 4

[0052] This embodiment is basically the same as Example 1, except that the mass percentage of alkali lignin and melamine is 2.5 wt%, and the final product is denoted as 2.5% PLCN.

[0053] Comparative Example 1

[0054] Prepare by weighing alkali lignin and grinding it for 30 min, then transferring it to an alumina crucible and heating it under N2 (99.99%, 50 mL / min). -1 At 5℃·min -1 Heating to 550℃ and holding for 4 hours yields g-C3N4 material, denoted as BCN.

[0055] Comparative Example 2

[0056] Prepare a mixture of alkali lignin and melamine at a mass percentage of 1.5 wt%. Weigh out the alkali lignin and melamine, grind and mix for 30 min, then transfer to an alumina crucible and heat under N2 (99.99%, 50 mL / min). -1 At 5℃·min -1Heating to 550℃ and holding for 4 hours yielded alkali lignin-assisted g-C3N4 material, denoted as 1.5% LCN.

[0057] [Performance Testing]

[0058] 1. Morphology and Structure

[0059] ①SEM

[0060] The materials obtained in Example 2 and Comparative Examples 1-2 were characterized by SEM, and the results are as follows: Figure 1 As shown. Observation Figure 1 It can be seen that the pure g-C3N4 material (BCN) exhibits an irregular blocky structure, while the alkali lignin-assisted g-C3N4 material (1.5% PLCN) shows a highly stacked two-dimensional nanosheet structure. This indicates that alkali lignin promotes the regularity and layered structure of the composite material. Furthermore, observation... Figure 1 b and Figure 1 It can be observed that after treatment with mixed strong acid, the amorphous carbon layer on the surface of 1.5% PLCN is etched away, and it instead exhibits a porous coral-like nanosheet structure.

[0061] ②XRD, FT-IR

[0062] The materials obtained in Examples 1-4 and Comparative Example 1 were characterized by XRD and FT-IR, and the results were recorded in Figure 2 middle. Figure 2 Both BCN and x%PLCN exhibit characteristic peaks at 13.1° and 27.3°, respectively, corresponding to the characteristic crystal planes (100) and (002) of g-C3N4, representing the interlayer stacking of aromatic rings and the layered graphite structure. The x%PLCN series materials correspond to the standard card of g-C3N4 and do not produce any other impurity peaks besides the characteristic peaks of g-C3N4, indicating that the addition of lignin-derived carbon does not change the basic structure of g-C3N4. However, compared with pure g-C3N4, x%PLCN shows a decrease in overall diffraction peak intensity, indicating that the crystal structure of x%PLCN is more disordered, which may be due to the covalent and hydrogen bond interactions within the x%PLCN material.

[0063] Figure 2 b is the FT-IR spectrum, 3000–3378 cm⁻¹ -1 The wide band between them corresponds to the stretching vibrations of the residual amino (NH) group and adsorbed water (OH). 2153 cm⁻¹ -1 The stretching vibrations in the vicinity are related to the cyano group (-C≡N). The skeletal stretching vibration of the CNC bond occurs at 810 cm⁻¹. -1 and 938cm -1 Between. Meanwhile, at 808cm -1A distinct peak appears at [a specific location], corresponding to the respiration pattern of the heptamethrin unit. This indicates that g-C3N4 did not undergo significant structural changes after the addition of alkali lignin, maintaining the integrity of its core structure. x%PLCN at 1610 cm⁻¹ -1 and 1250cm -1 The nearby bands are the C=O of quinone groups and the COC extension vibrations of aryl alkyl ethers, which are mainly attributed to the anchoring of oxygen-containing functional groups in the biochar produced by calcination of alkali lignin to the g-C3N4 surface.

[0064] ③TGA

[0065] A pyrolysis test was performed on a mixture of melamine and alkali lignin, and the results were recorded in [the relevant documentation]. Figure 3 The study found that the pyrolysis behavior of the two samples was similar before 370℃, indicating that before 370℃, melamine (MAm) in the alkali lignin-melamine mixture (MAm-AL) preferentially decomposes to form g-C3N4. Before 700℃, the reduced weight loss in MAm-AL compared to pure melamine (MAm) is attributed to the formation of strong hydrogen bonds between g-C3N4 and alkali lignin-derived biochar-PFRs. These bonds enhance the thermal stability of the alkali lignin-melamine mixture, thereby slowing down the decomposition rate and reducing the total weight loss during pyrolysis. After 700℃, the degree of carbonization of alkali lignin and g-C3N4 increased.

[0066] ④EPR

[0067] Electron paramagnetic resonance (EPR) spectroscopy was used to analyze nitrogen vacancies and the generation of PFRs in biochar. Both nitrogen vacancies and biochar-PFRs effectively promote the generation of unpaired electrons, thereby increasing the EPR signal intensity. Therefore, observation... Figure 4 It can be seen that all samples showed a Lorentz signal centered at a g value of 2.003; and compared with BCN, the EPR spectrum of x%PLCN showed a significantly stronger and more symmetrical signal, indicating that there are nitrogen defects in the x%PLCN structure, and biochar-PFR is formed.

[0068] With increasing alkali lignin doping, the concentration of PFRs increased, with 1.5% PLCN exhibiting the strongest signal intensity. However, when the alkali lignin content exceeded 1.5%, the PFR concentration began to decrease, possibly due to the formation of free radical-free radical complexes, which reduced the PFR concentration.

[0069] 2. Application performance

[0070] ① Catalytic performance

[0071] Test method: 30 mg of catalyst was dispersed in 20 mL of a 1:1, v / v mixture of γ-valerol (GVL) and formic acid (FAc). The mixture was subjected to a dark reaction at 25°C under oxygen-filled conditions for 30 minutes until adsorption and desorption equilibrium was reached. Then, a photocatalytic reaction to produce H2O2 was carried out under visible light irradiation. The suspension was extracted from the system at regular intervals and filtered through a 0.45 μm microporous membrane. The H2O2 content of the filtrate was analyzed using the iodometric method. For the photodegradation of H2O2, 30 mg of catalyst was dispersed in 20 mL of a 1:1, v / v mixture of GVL and FAc containing 0.2 mmol H2O2. After dark treatment, samples were taken every 30 minutes under light irradiation. The samples were filtered and the H2O2 concentration was determined using the iodometric method.

[0072] The catalytic materials obtained in Examples 1-4 and Comparative Example 1 were tested according to the above-described test methods. The results of H2O2 generation and degradation under light irradiation were recorded. Figure 5 From. Figure 5 As can be seen, after 2 hours of illumination, the H2O2 yields of the BCN and x%PLCN samples were 92.57 μmol·L⁻¹. -1 ·h -1 133.14 μmol·L -1 ·h -1 156.68 μmol·L -1 ·h -1 107.71 μmol·L -1 ·h -1 and 99.62 μmol·L -1 ·h -1 . Figure 5 The H2O2 photodegradation efficiencies in b were 75.32%, 80.08%, 92.55%, 88.17%, and 86.38%, respectively. BCN exhibited the lowest H2O2 yield and photodegradation efficiency; among the x%PLCN series materials, 1.5%PLCN showed the best photocatalytic performance in H2O2 production. Based on this performance of the x%PLCN series materials, it is speculated that during the catalytic oxidation of HMF, it can better decompose H2O2 into substances with stronger oxidizing capabilities, thereby promoting the selective oxidation of HMF.

[0073] ② Selective oxidation of HMF to generate MA

[0074] Catalytic oxidation assay: 20 mM HMF was dissolved in a solvent mixture of 10 mL GVL and FAc (1:1, v / v), and then 15 mg of catalyst was added. The mixture was magnetically stirred under light at 90 °C and an oxygen pressure of 0.6 MPa for 4 h. After the reaction was complete, the mixture was cooled to room temperature, the catalyst was separated by filtration, and the reaction was performed using an XDB-C18 reversed-phase column and a UV detector (mobile phase 5:95 acetonitrile: 0.01% phosphoric acid, flow rate 0.6 mL / min). -1 HPLC quantitative product (240 nm).

[0075] The catalytic materials obtained in Examples 1-4 and Comparative Examples 1-2 were tested according to the above-described test methods, and the selective production yields of MA by different catalysts were plotted. The results are shown in Table 1.

[0076] In Table 1, compared with BCN, all alkali lignin-assisted catalysts showed significantly improved MA and FAc yields. This is because under light irradiation, PFRs in alkali lignin-derived biochar mediate the transfer of electrons to O2, inducing the production of H2O2. H2O2 can be further converted into PFRs in the presence of biochar and PFRs through Fenton-like and ·OH recombination reactions. 1 O2. Meanwhile, the quinone groups in biochar can also react with O2 through energy transfer to form... 1 O2. On the other hand, the presence of alkali lignin can effectively regulate the band gap structure of the prepared catalyst, thereby improving its light absorption and photoelectron-hole separation efficiency, and promoting the generation of H2O2 and oxygen-containing free radicals.

[0077] Contact angle tests and pyridine infrared characterization were performed on different catalysts after tableting. The results are as follows: Figure 6 As shown. Observation Figure 6 As can be seen from this, 1.5% PLCN exhibits greater hydrophilicity compared to BCN. Meanwhile, the contact angle of the x% PLCN series materials decreases with increasing alkali lignin doping concentration. This is mainly due to the introduction of alkali lignin, which increases the content of oxygen-containing functional groups (-OH, COC, -COOH) in the catalytic material. The C=O bonds in HMF endow their molecules with strong polarity, making them readily adsorbed onto the surface of the catalytic material via hydrogen bonding.

[0078] Figure 6 b represents the infrared characterization results of pyridine, which further elucidates the degradation process of 1.5% PLCN. Figure 5 In b, at 1540cm -1 1490cm -1 and 1450cm -1 The signals observed at these locations correspond to Brønsted and Steady. Acidic site (BA), Lewis acid sites (LA+BA) and Lewis acid sites (LA) are also shown. The Lewis acid sites in BCN, 1.5% LCN, and 1.5% PLCN (based on integrated peak area) are the main pyridine adsorption sites. The Lewis acidity is likely due to the generation of amine functional groups on the g-C3N4 surface. 1.5% PLCN exhibits the strongest Lewis acidity, indicating that the etching process facilitates the formation of acidic oxide functional groups and increases the density of Lewis acid sites.

[0079] Combination Figure 4 , Figure 6 As shown in Table 1, the nitrogen-deficient sites generated on the catalyst surface and the increased Lewis acidity enhance H2O2 adsorption and subsequent conversion. 1 O2 plays a crucial role. Furthermore, the higher reaction temperature lowers the apparent activation energy required for photocatalysis, thereby improving the efficiency of photogenerated carrier migration and promoting [the process / catalysis]. 1 The generation of O2 and the Lewis acidic sites on the x%PLCN surface promote the breaking of the C-C bond between the hydroxymethyl group and the furan ring in HMF, providing more radical intermediates for the selective generation of MA.

[0080] Table 1. Selective oxidation of HMF to MA under photothermal synergistic catalysis

[0081]

[0082] Note: HMF is pentahydroxymethylfurfural, MA is maleic acid, MAN is maleic anhydride (an intermediate of HMF), and FAc is formic acid (a byproduct of the oxidation of HMF to MA).

[0083] 3. Mechanism

[0084] ①Photocurrent

[0085] To investigate the specific mechanism of HMF oxidation of MA in "2. Application Performance", photocurrent, electrochemical impedance and photoluminescence tests were performed on Examples 1-4 and Comparative Example 1.

[0086] Figure 7 a. Periodic photocurrent of the catalyst before and after alkali lignin modification. 1.5% PLCN showed a significantly higher photocurrent density than other catalysts, indicating that 1.5% PLCN achieved higher photogenerated electron-hole separation efficiency under repeated switching of lamp illumination. Figure 7 Electrochemical impedance spectroscopy was used to analyze the electrochemical properties of the prepared samples. Compared with other materials, 1.5% PLCN showed a much smaller semicircle diameter, which reflects its higher conductivity and lower photoelectron transfer resistance. Figure 7The photoluminescence spectrum of the catalyst modified with alkali lignin showed a decrease in photoluminescence intensity, mainly due to delayed recombination of photoexcited electrons and holes during nonradiative charge transfer or other long-lived processes. Among all catalytic materials, 1.5% PLCN exhibited the weakest fluorescence intensity, indicating its highest efficiency in separating photogenerated electron-hole pairs.

[0087] ② Free radicals

[0088] Detection method: TEMP concentration 10 mmol·L -1 The catalyst was 15 mg and the solvent was 10 mL. The reaction was carried out under light irradiation.

[0089] To verify that the free radicals that play a major role in the catalytic oxidation process are... 1 O2, for 1.5% PLCN 1 Electron paramagnetic resonance (EPR) testing of O2. (By...) Figure 8 It can be seen that after adding TEMP to the reaction system, 1.5% PLCN singlet oxygen was observed under light irradiation. 1 The detectable signal peaks of O2). 1 The O2 signal increases with increasing reaction time, indicating the presence of hydroxyl radicals (·OH) and superoxide radicals (·O2). - Throughout the reaction process, it continuously recombines and transforms into... 1 O2. In contrast, BCN showed almost no detectable O2. 1 The O2 peak indicates that the biochar-PFRs generated by calcination of alkali lignin are produced... 1 It plays a major role in O2.

[0090] ③ Degradation pathway

[0091] Suspensions generated during the catalytic oxidation test of 1.5% PLCN at different time periods were collected and subjected to HPLC-MS analysis to refine the degradation mechanism of HMF catalytic oxidation by 1.5% PLCN.

[0092] Figure 9 The mechanism was summarized as follows: When light irradiates the surface of 1.5% PLCN, it excites the generation of photogenerated electrons and holes, activating PFRs and quinone-like groups in the biochar. The photogenerated electrons react with molecular oxygen to generate ·O2. - And H2O2. PFRs transfer electrons to O2 to induce the generation of H2O2, and after activating H2O2 to generate ·OH, it generates H2O2 through a surface-bound ·OH-mediated reaction. 1 When O2 is irradiated with light, the quinone-like group is excited to a state and then induced to generate O2 through energy transfer. 1 O2. In 1 While O2 is generated, holes activate C in the HMF.β -H key, release C β Free radical (intermediate 1). Intermediate 1 and... 1 O2 reacts to form C β Peroxy radical (intermediate 2), intermediate 2 contacts H + Intermediate 3 is then formed. Intermediate 3 selectively breaks the OO bond through electron transfer rearrangement to generate an alkoxy radical (4). The alkoxy radical (4) further undergoes β-cleavage to produce a furfural group (5) and releases one molecule of formic acid (detected by HPLC). In the photothermal catalytic reaction, Lewis acid can activate the C / C bond between the hydroxymethyl group and the furan ring in HMF, causing it to break and directly generate a furfural group (5). The furfural group (5) in 1 Under the attack of O2 and H+, 5-hydroxyfuran-2-acetaldehyde (6) is generated, which then undergoes a 1,4 rearrangement to generate 5-oxo-2H-furan-2-carboxaldehyde (7). 5-oxo-2H-furan-2-carboxaldehyde (7) is then... 1 After O2 abstracts a hydrogen atom from its aldehyde group, an acyl group (8) is generated. After the acyl group (8) undergoes decarbonylation, intermediate 9 is generated. Intermediate 9 then undergoes... 1 O2 and H + The attack forms 5-hydroxy-2(5H)-furanone (10), which is further oxidized to produce maleic anhydride (11), and 11 is hydrolyzed to produce MA.

Claims

1. The application of alkali lignin-assisted graphite carbon nitride materials in the photothermal synergistic catalytic selective oxidation of 5-hydroxymethylfurfural to maleic acid, characterized in that, The preparation method of the alkali lignin-assisted graphite carbon nitride material includes: grinding and mixing melamine and alkali lignin to obtain a mixture; after the mixture is calcined, it is immersed in a mixed acid reaction; after removing the residual acid on the surface, a biochar-doped graphite carbon nitride material with persistent free radicals is obtained, namely, alkali lignin-assisted graphite carbon nitride material; wherein, the persistent free radicals include oxygen-containing functional groups.

2. The application as described in claim 1, characterized in that, The mass percentage of the alkali lignin to the melamine is 1~2.5 wt%.

3. The application as described in claim 1, characterized in that, The calcination atmosphere is a nitrogen atmosphere, the calcination temperature is 500~700℃, the calcination heating rate is 1~5℃ / min, and the calcination time is 3~8 h.

4. The application as described in claim 1, characterized in that, The mixed acid is a mixed solution of sulfuric acid and nitric acid with a pH of 0 to 2; wherein the volume ratio of sulfuric acid to nitric acid is 1:(0.8 to 1.5).

5. The application as described in claim 1 or 4, characterized in that, The mass-to-volume ratio of the mixture to the mixed acid is (3~6) g / L.

6. The application as described in claim 1 or 4, characterized in that, The immersion time in the mixed acid reaction is 3-6 hours.

7. The application as described in claim 6, characterized in that, The method for removing residual acid from the surface is as follows: after the reaction is completed, centrifuge at 5000~10000 rpm until the pH of the supernatant after centrifugation is 7.

0.

8. The application as described in any one of claims 1 to 4, characterized in that, The alkali lignin-assisted graphitic carbon nitride material includes biochar containing oxygen-containing free radicals and graphitic carbon nitride, wherein the oxygen-containing free radicals in the biochar are bonded to the polar groups in the graphitic carbon nitride through hydrogen bonds.

9. The application as described in claim 8, characterized in that, The alkali lignin-assisted graphitic carbon nitride material has a porous coral-like nanosheet structure, and the particle size of the alkali lignin-assisted graphitic carbon nitride material is 1~3 μm; the content of graphitic carbon nitride is 97.5~99 wt% based on the weight of the alkali lignin-assisted graphitic carbon nitride material.

10. The application as described in any one of claims 1 to 4, characterized in that, In the photothermal synergistic catalysis process, the alkali lignin-assisted graphitic carbon nitride material selectively oxidizes the generation of singlet oxygen free radicals, reducing the mineralization of 5-hydroxymethylfurfural and its derived free radical intermediates, thereby achieving selective conversion of maleic acid.

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