Alkali lignin-assisted graphite carbon nitride material as well as preparation method and application thereof

Abstract

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

Description

Technical Field

[0001] The invention relates to the technical field of functional materials, and in particular to an alkali lignin-assisted graphite carbon nitride material and a preparation method and application thereof. Background Art

[0002] Fossil energy is a non-renewable resource. Even if the current mining volume can guarantee global utilization and development, in the near future, oil fields and coal mines will inevitably be exhausted, and the mining of fossil energy will no longer be so convenient. The mining volume will decrease, leading to a real energy crisis. In this situation, the development of renewable energy or transitional energy is one of the important strategies to maintain sustainable social and economic development.

[0003] Pentahydroxymethylfurfural (5-Hydroxymethylfurfural, HMF) can be converted from abundant cellulose, glucose and fructose. It is one of the most promising platform chemicals in the potential utilization of biomass resources. Its selective oxidation to maleic acid (N-(4-fluorophenyl)maleamic acid, MA) is considered to be one of the most promising processes in future biorefining. MA is important in many industrial applications, especially in biorefining processes. They are versatile bulk raw materials for the synthesis of unsaturated polyester resins, pharmaceuticals, agrochemicals, lubricant additives and polymers. At present, the industrial production method of MA is to oxidize fossil benzene (about 27% of the total production) and n-butene (73%) under high temperature and pressure, which usually produces suboptimal yields and multiple by-products. However, the industrial production method of MA is usually a vanadium-based process, which requires high temperature, high pressure and long reaction time, and the biological toxicity of vanadium will bring environmental and health risks. Therefore, this type of industrial production of MA has great application limitations.

[0004] Nitride graphite carbon (gC 3 N 4 ) is a visible light-responsive metal-free semiconductor that has attracted much attention due to its low toxicity, high physical and chemical stability, and efficient electron transfer properties. 3 N 4 The suitable conduction band (CB) and valence band (VB) values ​​make it particularly promising for photocatalytic biorefining. 3 N 4 Their application is often hindered by their suboptimal visible light absorption, low specific surface area, and high probability of charge carrier recombination.

[0005] Therefore, a method based on gC is needed 3 N 4 As the catalytic material of the matrix to break through its single gC 3N 4 application limitations. Summary of the invention

[0006] The invention provides an alkali lignin-assisted graphite carbon nitride material and a preparation method and application thereof.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

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

[0009] gC modified using biomass-based compounds as carbon precursors 3 N 4 The photocatalytic performance of biochar-PFRs was significantly improved. Lignin, as the most abundant aromatic compound on earth, is rich in aromatic rings and phenolic molecules, which can produce 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 conjugated π-electron systems, which can enhance electron transfer under photoexcitation. Biochar-PFRs have a strong affinity for H 2 O 2 and O 2 It has strong catalytic activity and is conducive to the formation of reactive oxygen species (ROS). Therefore, lignin has become an auxiliary gC 3 N 4 is an attractive carbon source.

[0010] The present invention provides an alkali lignin-assisted graphite carbon nitride material, which has nitrogen defects and biochar persistent free radicals, and has a porous coral-like nanosheet structure. During the pyrolysis process, alkali lignin can generate biochar with persistent free radicals through the cracking and rearrangement of weak bonds, which react with gC 3 N 4The alkali lignin-assisted graphite carbon nitride material is connected to each other, generating a large number of oxygen-containing functional groups on the catalyst surface, while introducing nitrogen defects, improving the visible light absorption performance and hydrophilicity of the alkali lignin-assisted graphite carbon nitride material. Furthermore, the alkali lignin-assisted graphite carbon nitride material is oxidatively etched by mixed acid, so that the alkali lignin-assisted graphite carbon nitride material is peeled off into a two-dimensional porous sheet morphology. The formed acidic oxygen-containing functional groups provide Lewis acid sites, which promote the CC bond between the hydroxymethyl and furan ring in HMF under thermal catalytic conditions, providing more free 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] Further preferably, the mass percentage of the alkali lignin to the melamine is 1 to 1.5 wt %.

[0013] Further preferably, the mass percentage of the alkali lignin to the melamine is 1.5-2.5wt%.

[0014] Further preferably, the mass percentage of the alkali lignin to the melamine is 1-2 wt %.

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

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

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

[0018] Further preferably, the mixed acid is a mixed solution of sulfuric acid and nitric acid, and its pH value is 0-0.5; 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 volume ratio of the mixture to the mixed acid is (3-6) g / L.

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

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

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

[0024] The invention provides an alkali lignin-assisted graphite carbon nitride material. The alkali lignin-assisted graphite carbon nitride material comprises biochar containing oxygen-containing free radicals and graphite carbon nitride. The oxygen-containing free radicals in the biochar are combined with polar groups in the graphite carbon nitride through hydrogen bonds.

[0025] The biochar material with persistent free radicals generated in the present invention shows C=O of quinone group and COC stretching vibration of aryl alkyl ether in infrared characterization, indicating that the surface of biochar generated by calcining alkali lignin has oxygen-containing functional groups anchored on gC 3 N 4 The oxygen-containing functional groups in the persistent free radicals bond with gC 3 N 4 The mixed acid treatment increases the acidic oxygen-containing functional groups on the surface of the material, improves the hydrophilicity of the material, promotes the adsorption of HMF by the material, and is beneficial to the reaction between HMF and the generated 1 O 2 The reaction occurs and the Lewis acidity is increased, which is beneficial to the breaking of the CC bond in HMF under thermal catalytic conditions and provides more furfural free radicals for the generation of MA.

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

[0027] The present invention provides an alkali lignin-assisted graphite carbon nitride material prepared by the above method or the use of the above alkali lignin-assisted graphite carbon nitride material in the selective oxidation of pentahydroxymethylfurfural to maleic acid by photothermal synergistic catalysis. 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 the generation of singlet oxygen free radicals, thereby achieving selective conversion of maleic acid.

[0028] The present invention provides an alkali lignin-assisted graphite carbon nitride material with selective oxidation 1 O 2 HMF is oxidized to MA, and the Lewis acid in the alkaline lignin-assisted graphitic carbon nitride catalytic material promotes the reaction efficiency of HMF to MA by activating the CC bond between the hydroxymethyl group and the furan ring in HMF.

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

[0030] (1) The present invention uses alkaline lignin as a carbon precursor to achieve the gC 3 N 4 The surface modification of HMF precisely regulated the band gap structure of the catalyst and improved the photocatalytic performance. The surface functional groups and Lewis acid sites of the material were adjusted after mixed acid treatment, which promoted the selective breakage of CC bonds in HMF.

[0031] (2) The method provided by the present invention is to prepare gC doped with biochar having persistent free radicals 3 N 4 Catalysts can improve gC 3 N 4 The surface properties and hydrophilicity of the modified material are improved, thereby increasing the adsorption capacity of the modified material for HMF and achieving the 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 pollution to the environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 These are scanning electron microscopy images, where a corresponds to BCN, b corresponds to 1.5% LCN, and c corresponds to 1.5% PLCN.

[0034] Figure 2 Comparison diagram of structural characterization, where a is XRD and b is FT-IR.

[0035] Figure 3 It is a TGA diagram, in which MAm-AL corresponds to a mixture of melamine and alkali lignin, and MAm corresponds to melamine.

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

[0037] Figure 5 Catalytic performance diagram, where a is H 2 O 2 Yield diagram, b is H 2 O 2 Photodegradation diagram.

[0038] Figure 6 Contact angle and infrared image, where a is the contact angle image and b is the infrared characterization image of pyridine.

[0039] Figure 7 Are the characterization results, where a is the photoelectrochemical characterization diagram, b is the electrochemical impedance spectroscopy diagram, and c is the photoluminescence spectrum diagram.

[0040] Figure 8For photothermal catalytic reaction 1 O 2 Electron paramagnetic resonance images of BCN and 1.5% PLCN 1 O 2 The electron paramagnetic resonance images of a and b are those of 1.5% PLCN at different light-on times. 1 O 2 Electron paramagnetic resonance image.

[0041] Fig. 9 The diagram shows the mechanism of selective oxidation of HMF to MA under photothermal catalytic conditions. DETAILED DESCRIPTION

[0042] The present invention is further described below in conjunction with specific embodiments. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. In addition, the embodiments of the present invention involved in the following description are generally only embodiments of a part of the present invention, rather than all embodiments. Therefore, based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work should fall within the scope of protection of the present invention.

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

[0044] [Example]

[0045] Example 1

[0046] According to the mass percentage of alkali lignin and melamine being 1.0wt%, alkali lignin and melamine were weighed and ground and mixed for 30min, and then transferred to an alumina crucible and heated at N 2 (99.99%, 50mL·min -1 ), 5℃·min -1 Heated to 550℃ and kept warm for 4h to obtain alkali lignin-assisted gC 3 N 4 The material was recorded as 1% LCN. Then 100 mg of 1% LCN was dispersed in a mixed solution of 20 mL of sulfuric acid and nitric acid (volume ratio of sulfuric acid and nitric acid was 1:1, pH ≈ 0) and magnetically stirred at room temperature for 5 h. After washing with distilled water, all residual acid was removed by centrifugation at 7000 rpm until the pH of the centrifugal supernatant was 7.0. Finally, freeze drying was performed to obtain the etched alkaline lignin-assisted gC 3 N 4 Material, denoted as 1% PLCN.

[0047] Example 2

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

[0049] Example 3

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

[0051] Example 4

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

[0053] Comparative Example 1

[0054] Prepare the alkali lignin by weighing and grinding for 30 min, then transfer it to an alumina crucible and place it under N 2 (99.99%, 50mL·min -1 ), 5℃·min -1 Heat to 550℃ and keep warm for 4h to obtain gC 3 N 4 Material, denoted as BCN.

[0055] Comparative Example 2

[0056] According to the mass percentage of alkali lignin and melamine being 1.5wt%, alkali lignin and melamine were weighed and ground and mixed for 30min, and then transferred to an alumina crucible and heated at N 2 (99.99%, 50mL·min -1 ), 5℃·min -1 Heated to 550℃ and kept warm for 4h to obtain alkali lignin-assisted gC 3 N 4 Material, noted as 1.5% LCN.

[0057]

Performance test

[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 Observation Figure 1 It can be seen that pure gC 3 N 4The material (BCN) structure is irregular and blocky, while the alkali lignin assists gC 3 N 4 The material (1.5% PLCN) showed a highly stacked two-dimensional nanosheet structure, which indicated that the alkaline lignin promoted the regularity and layered structure of the composite material. Figure 1 b and Figure 1 c It can be found that after the mixed strong acid treatment, the amorphous carbon layer on the surface of 1.5% PLCN was etched away, and a porous coral-like nanosheet structure was presented instead.

[0061] ②XRD, FT-IR

[0062] The materials obtained in Examples 1 to 4 and Comparative Example 1 were characterized by XRD and FT-IR, and the results were recorded in Figure 2 middle. Figure 2 In a, BCN and x%PLCN both have characteristic peaks at 13.1° and 27.3°, corresponding to gC 3 N 4 The characteristic crystal planes (100) and (002) of the x% PLCN series materials are similar to those of gC 3 N 4 The standard card corresponds to the gC 3 N 4 Other peaks outside the characteristic peaks indicate that the addition of lignin-derived carbon does not change gC 3 N 4 However, compared with pure gC 3 N 4 In comparison, x% PLCN exhibits a decrease in the overall diffraction peak intensity, which indicates that the crystal structure of x% PLCN is more disordered, which may be caused by the covalent and hydrogen bond interactions within the x% PLCN material.

[0063] Figure 2 b is FT-IR spectrum, 3000~3378cm -1 The broad band between 2153 cm-1 corresponds to the stretching vibrations of residual amino (NH) groups and adsorbed water (OH). -1 The stretching vibrations near the cyano group (-C≡N) are related to the skeletal stretching vibrations of the CNC bond appear at 810cm -1 and 938cm -1 At the same time, at 808cm -1 There is a clear peak at , which corresponds to the breathing mode of the heptazine unit. This indicates that gC 3 N 4No significant structural changes occurred after the addition of alkali lignin, maintaining the integrity of its core structure. x% PLCN at 1610 cm -1 and 1250cm -1 The nearby bands are the C=O stretching vibrations of the quinone group and the COC stretching vibrations of the aryl alkyl ether, which are mainly attributed to the oxygen-containing functional groups in the biochar generated by calcining the alkali lignin anchored on the gC 3 N 4 surface.

[0064] ③TGA

[0065] Pyrolysis tests were performed on mixtures of melamine and alkali lignin, and on melamine, and the results were recorded in Figure 3 It was found that before 370℃, the pyrolysis behaviors of the two samples were similar, indicating that before 370℃, melamine (MAm) in the mixture of alkali lignin and melamine (MAm-AL) preferentially decomposed to form gC 3 N 4 Before 700 °C, the reduced weight loss in MAm-AL compared with pure melamine (MAm) is attributed to gC 3 N 4 Strong hydrogen bonds were formed between the biochar-PFRs derived from alkali lignin and gC. These bonds enhanced the thermal stability of the alkali lignin-melamine mixture, slowing down the decomposition rate and reducing the total weight loss during pyrolysis. 3 N 4 The degree of carbonization increases.

[0066] ④EPR

[0067] Electron paramagnetic resonance (EPR) spectroscopy was used to analyze the generation of PFRs in nitrogen defects and biochar. Both nitrogen defects and biochar-PFRs effectively promoted the generation of unpaired electrons, thereby increasing the EPR signal intensity. Figure 4 It can be seen that all samples show a Lorentzian signal centered at a g value of 2.003; and compared with BCN, the EPR spectrum of x%PLCN shows significantly stronger and more symmetrical signals, indicating the presence of nitrogen defects in the x%PLCN structure and the formation of biochar-PFR.

[0068] The concentration of PFRs increased with the increase of alkali lignin doping, and 1.5% PLCN showed the strongest signal intensity. However, when the alkali lignin content exceeded 1.5%, the concentration of PFRs began to decrease, which may be due to the formation of free radical-free radical complexes, thereby reducing the concentration of PFRs.

[0069] 2. Application performance

[0070] ① Catalytic performance

[0071] Test method: 30 mg of the catalyst was dispersed in 20 mL of a mixture of γ-valerolactone (GVL) and formic acid (FAc) (1:1, v / v). The dark reaction was carried out at room temperature (25°C) under oxygenated conditions for 30 minutes. After reaching the equilibrium of adsorption and desorption, the photocatalytic reaction was carried out under visible light irradiation to produce H. 2 O 2 At regular intervals, the suspension was extracted from the system and filtered through a 0.45 μm microporous membrane. The filtrate was analyzed by iodine measurement for H 2 O 2 In the photodegradation of H 2 O 2 30 mg of the catalyst was dispersed in 20 mL of 0.2 mmol H 2 O 2 The mixture of GVL and FAc (1:1, v / v) was treated in the dark, and then samples were taken every 30 minutes under light conditions. After filtration, H 2 O 2 concentration.

[0072] The catalytic materials obtained in Examples 1 to 4 and Comparative Example 1 were tested according to the above test method. 2 O 2 The generation and H 2 O 2 The degradation results are recorded in Figure 5 In. From Figure 5 a It can be seen that after 2 hours of illumination, the H 2 O 2 The yields were 92.57 μmol·L -1 ·h -1 , 133.14 μmol·L -1 ·h -1 , 156.68μmol·L -1 ·h -1 , 107.71μmol·L -1 ·h -1 and 99.62 μmol·L -1 ·h -1 . Figure 5 b in H 2 O 2 The photodegradation efficiencies were 75.32%, 80.08%, 92.55%, 88.17% and 86.38%, respectively. 2 O 2 The yield and photodegradation efficiency are the lowest; among the x% PLCN series materials, 1.5% PLCN has the best H yield2 O 2 Based on the performance of the x% PLCN series materials, it is speculated that they can better convert H into 2 O 2 Decompose into substances with stronger oxidizing ability to promote the selective oxidation of HMF.

[0073] ②Selective oxidation of HMF to produce MA

[0074] Catalytic oxidation test method: 20mM HMF was dissolved in 10mL GVL and FAc solvent mixture (1:1, v / v), and then 15mg catalyst was added. The reaction time was 4h at 90℃, magnetic stirring under light, and 0.6Mpa oxygen pressure. After the reaction was completed, the mixture was cooled to room temperature, the catalyst was separated by filtration, and the reaction was carried out using an XDB-C18 reverse phase column and a UV detector (mobile phase 5:95 acetonitrile:0.01% phosphoric acid, flow rate 0.6mL·min -1 The product was quantified by HPLC (HPLC, 240 nm).

[0075] The catalytic materials obtained in Examples 1 to 4 and Comparative Examples 1 to 2 were tested according to the above test method, and a table of the selectivity of different catalysts for MA was drawn up. The results are shown in Table 1.

[0076] In Table 1, the MA and FAc yields of all alkali lignin-assisted catalysts showed significant improvements compared with BCN. This is because the PFRs in the alkali lignin-derived biochar mediated the electron transfer to O under light irradiation. 2 On, induced H 2 O 2 The generation of H 2 O 2 It can be further converted into 1 O 2 At the same time, the quinone-like groups in biochar can also interact with O through energy transfer. 2 Reaction formation 1 O 2 On the other hand, the presence of alkali lignin can effectively adjust the band gap structure of the prepared catalyst, thereby improving its light absorption and photoelectron-hole separation efficiency and promoting H 2 O 2 and the generation of oxygen-containing free radicals.

[0077] After the different catalysts were pressed into pellets, contact angle tests and pyridine infrared characterization were performed. The results are as follows: Figure 6 Observation Figure 6It can be seen from a that 1.5% PLCN exhibits greater hydrophilicity compared to BCN. At the same time, the contact angle of the x% PLCN series of materials decreases with the increase of the amount of alkali lignin doping. 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 bond in HMF gives its molecules strong polarity, making them easily adsorbed to the surface of the catalytic material through hydrogen bonds.

[0078] Figure 6 b is the pyridine infrared characterization result, which further explains the degradation process of 1.5% PLCN. Figure 5 b, at 1540cm -1 、1490cm -1 and 1450cm -1 The signals observed at Acidic sites (BA), and Lewis acid sites (LA+BA) and Lewis acid sites (LA). 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 may be due to gC 3 N 4 The amine functional groups on the surface were generated, and 1.5% PLCN showed the strongest Lewis acidity, indicating that the etching process helped to form acidic oxidative functional groups and increase 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 play an important role in enhancing the H 2 O 2 Adsorption and subsequent conversion to 1 O 2 In addition, higher reaction temperatures reduce the apparent activation energy required for photocatalysis, thereby increasing the efficiency of photogenerated charge carrier migration and promoting 1 O 2 The Lewis acidic sites on the surface of x% PLCN promoted the cleavage of the CC bond between the hydroxymethyl group and the furan ring in HMF, providing more free radical intermediates for the selective generation of MA.

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

[0081]

[0082] Note: HMF is pentahydroxymethylfurfural, MA is maleic acid, MAN is maleic anhydride (HMF intermediate), and FAc is formic acid (a by-product in the process of HMF oxidation to MA).

[0083] 3. Mechanism

[0084] ① Photocurrent

[0085] In order to explore the specific mechanism of HMF oxidation of MA in “2. Application Performance”, photocurrent, electrochemical impedance and photoluminescence tests were performed on Examples 1 to 4 and Comparative Example 1.

[0086] Figure 7 a) Periodic photocurrents of the catalysts before and after alkali lignin modification. 1.5% PLCN exhibits significantly higher photocurrent density than other catalysts, indicating that 1.5% PLCN achieves higher photogenerated electron-hole separation efficiency under repeated switching of lamp illumination. Figure 7 (b) The electrochemical properties of the prepared samples were analyzed using electrochemical impedance spectroscopy. Compared with the other materials, 1.5% PLCN showed a much smaller semicircle diameter, which reflects its higher conductivity and lower photoelectron transfer resistance. Figure 7 The photoluminescence spectrum of c shows that the photoluminescence intensity of the catalyst modified with alkali lignin is reduced, mainly due to the delayed recombination of photoexcited electrons and holes in non-radiative charge transfer or other long-lived processes. Among all the catalytic materials, the fluorescence intensity of 1.5% PLCN is the weakest, indicating that its separation efficiency of photogenerated electron-hole pairs is the highest.

[0087] ② Free radicals

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

[0089] In order to verify that the free radicals that play a major role in the catalytic oxidation process are 1 O 2 , for 1.5% PLCN 1 O 2 Electron paramagnetic resonance test. Figure 8 It can be seen that after TEMP was added to the reaction system, singlet oxygen ( 1 O 2 ) detectable signal peaks. These 1 O 2 The signal increases with the reaction time, indicating that hydroxyl radicals (·OH) and superoxide radicals (·O 2 - ) is continuously reorganized and converted into 1 O2 In contrast, BCN showed almost no detectable 1 O 2 peak, indicating that the biochar-PFRs generated by calcining alkali lignin 1 O 2 Plays a major role in.

[0090] ③Degradation pathway

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

[0092] Fig. 9 The mechanism process is summarized as follows: when light is irradiated on the surface of 1.5% PLCN, the generation of photogenerated electrons and holes is stimulated, and the PFRs and quinone-like groups in the biochar are activated. The photogenerated electrons react with molecular oxygen to generate ·O 2 - and H 2 O 2 PFRs transfer electrons to O 2 Induced H 2 O 2 The generation of H 2 O 2 After the generation of ·OH, the surface-bound ·OH mediates the reaction to generate 1 O 2 After the quinone-like group is irradiated by light to form an excited state, it reacts with O 2 Energy transfer induced generation 1 O 2 .exist 1 O 2 At the same time, holes are generated by activating C β -H key, release C β Free radical (intermediate 1). Intermediate 1 and 1 O 2 The reaction forms C β Peroxyl radical (intermediate 2), intermediate 2 contacts H + After that, intermediate 3 is formed. Intermediate 3 selectively breaks the OO bond through electron transfer rearrangement to generate alkoxy radical (4). Alkoxy radical (4) further undergoes β-cleavage to produce furfural (5) and release a molecule of formic acid (detected by HPLC). In the photothermal catalytic reaction, Lewis acid can activate the CC bond between the hydroxymethyl group and the furan ring in HMF, causing it to break and directly generate furfural (5). Furfural (5) 1 O 2and H+ attack to generate 5-hydroxyfuran-2-acetaldehyde (6), which then undergoes 1,4 rearrangement to generate 5-oxo-2H-furan-2-carboxaldehyde (7). 5-oxo-2H-furan-2-carboxaldehyde (7) is 1 O 2 After the hydrogen atom is extracted from the aldehyde group, acyl group (8) is generated, and acyl group (8) is decarbonized to generate intermediate 9. Intermediate 9 is 1 O 2 and H + , forming 5-hydroxy-2(5H)-furanone (10), which is further oxidized to form maleic anhydride (11), and 11 is hydrolyzed to produce MA.

Claims

1. A method for preparing an alkali lignin-assisted graphite carbon nitride material, characterized in that: include: Melamine and alkali lignin are ground and mixed to obtain a mixture, which is then immersed in a mixed acid reaction after calcination, and after the surface residual acid is removed, a biochar-doped graphite carbon nitride material with persistent free radicals is obtained, namely, an alkali lignin-assisted graphite carbon nitride material; wherein the persistent free radicals include oxygen-containing functional groups.

2. The preparation method according to claim 1, characterized in that The mass percentage of the alkali lignin to the melamine is 1 to 2.5 wt %.

3. The preparation method according to claim 1, characterized in that: The calcination atmosphere is a nitrogen atmosphere, the calcination temperature is 500-700° C., the calcination heating rate is 1-5° C. / min, and the calcination time is 3-8 hours.

4. The preparation method according to claim 1, characterized in that: The mixed acid is a mixed solution of sulfuric acid and nitric acid, and its pH value is 0-2; wherein the volume ratio of sulfuric acid to nitric acid is 1:(0.8-1.5).

5. The preparation method according to claim 1 or 4, characterized in that: The mass volume ratio of the mixture to the mixed acid is (3-6) g / L.

6. The preparation method according to claim 1 or 4, characterized in that: The immersion time for mixed acid reaction is 3 to 6 hours.

7. The preparation method according to claim 6, characterized in that: The method for removing the residual acid on the surface is: after the reaction is completed, centrifugation is performed at 5000-10000 rpm until the pH of the supernatant after centrifugation is 7.

0.

8. The alkali lignin-assisted graphite carbon nitride material obtained by the preparation method according to any one of claims 1 to 7, characterized in that: The alkali lignin-assisted graphite carbon nitride material comprises biochar containing oxygen-containing free radicals and graphite carbon nitride, and the oxygen-containing free radicals in the biochar are combined with polar groups in the graphite carbon nitride through hydrogen bonds.

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

10. Use of the alkali lignin-assisted graphite carbon nitride material prepared by the preparation method according to any one of claims 1 to 7 or the alkali lignin-assisted graphite carbon nitride material according to claim 8 or 9 in the photothermal synergistic catalytic selective oxidation of pentahydroxymethylfurfural to maleic acid, characterized in that: In the photothermal synergistic catalytic process, the alkaline lignin assisted graphite carbon nitride material reduces the mineralization of pentahydroxymethylfurfural and its derived free radical intermediates by selectively oxidizing the generation of singlet oxygen free radicals, thereby achieving selective conversion of maleic acid.

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