Method for depolymerization of lignin and method for preparing lignin-based antioxidant

Abstract

The present application relates to the technical field of biological resource utilization, and provides a lignin depolymerization method, which comprises the following steps: placing lignin in an anode electrolyte of a flow fuel cell, and making the lignin depolymerize in a discharging process of the flow fuel cell. The lignin is oxidatively depolymerized into small molecular products in the form of the flow fuel cell, the depolymerization method can be carried out under mild conditions, and electric energy is co-produced, so that the high-temperature and high-pressure conditions required by traditional lignin depolymerization processes are avoided. After the lignin depolymerization reaction, a series of small molecular depolymerization products containing phenolic hydroxyl groups and aromatic aldehydes are obtained, the process not only reduces the molecular weight of the lignin, but also increases the content of the phenolic hydroxyl groups and the aromatic aldehydes, and enhances the antioxidant property.

Description

Technical Field

[0001] This invention relates to the field of bioresource utilization technology, specifically to a method for depolymerizing lignin and a method for preparing lignin-based antioxidants. Background Technology

[0002] Lignin is the most abundant polyphenol polymer in nature and can be used as a natural antioxidant. It boasts advantages such as low cost, easy availability, clean and renewable nature, carbon neutrality, high yield, low toxicity, and biodegradability. Lignin is mainly a phenylpropane polymer formed by the oxidative polymerization of three 4-hydroxycinnamyl alcohols (p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol) with different degrees of methoxylation. It is the second most abundant natural polymer on land after cellulose. The lignin macromolecule contains a large number of phenolic hydroxyl groups, which can scavenge free radicals, block free radical chain reactions, and thus terminate the oxidation process. Therefore, lignin itself has certain antioxidant properties. However, the large molecular weight of lignin leads to poor solubility and low antioxidant activity, hindering its large-scale industrial application. Existing methods for depolymerizing lignin require harsh reaction conditions and cannot yield aromatic aldehyde products.

[0003] Therefore, there is an urgent need to develop a method for depolymerizing lignin with mild reaction conditions to obtain aromatic aldehydes. Summary of the Invention

[0004] This invention aims to at least partially solve one of the technical problems in the related art. Therefore, one object of this invention is to provide a method for the depolymerization of lignin to obtain aromatic aldehydes under mild reaction conditions.

[0005] Therefore, in a first aspect, the present invention provides a method for depolymerizing lignin, comprising:

[0006] Lignin is placed in the anode electrolyte of a flow fuel cell, and during the discharge process of the flow fuel cell, the lignin is depolymerized.

[0007] Lignin is oxidized and depolymerized into smaller molecules using a flow fuel cell. This depolymerization method can be carried out under mild conditions and simultaneously generates electricity, avoiding the high temperature and high pressure conditions required for traditional lignin depolymerization processes. After the depolymerization reaction, a series of small-molecule depolymerized products containing phenolic hydroxyl groups and aromatic aldehydes are obtained. This process not only reduces the molecular weight of lignin but also increases the content of phenolic hydroxyl groups and aromatic aldehydes, enhancing its antioxidant properties.

[0008] In some embodiments of the present invention, the concentration of lignin in the anolyte is 0.001 g / L to 80 g / L, optionally 0.001 g / L to 10 g / L. Within the above-mentioned lignin concentration range, the content of aromatic aldehydes in the depolymerization product can be increased.

[0009] In some embodiments of the present invention, during the discharge process of the flow fuel cell, the temperature of the anolyte is 20°C-100°C, optionally 80°C-100°C. Lignin linkages are more easily broken at these temperatures, thereby increasing the yield of depolymerized monomers.

[0010] In some embodiments of the present invention, the anolyte further includes an inorganic base;

[0011] Optionally, the molar concentration of the inorganic base in the anolyte is 0.01 mol / L to 6 mol / L, and can be selected as 0.001 mol / L to 4 mol / L. The inorganic base, as a supporting electrolyte, can improve the conductivity of the anolyte and provide a reaction environment for lignin oxidation, thereby accelerating the depolymerization rate of lignin.

[0012] In some embodiments of the present invention, the flow fuel cell further includes an anode, the anode comprising an anode conductive substrate and an anode electron carrier disposed on the anode conductive substrate, the anode electron carrier comprising at least one of copper oxide, nickel oxide, manganese dioxide, cobalt oxide, iron oxide, nickel phosphide, cobalt phosphide, nickel sulfide, cobalt sulfide, nickel boride, nickel nitride, cobalt tetroxide, nickel hydroxide, cobalt hydroxide, nickel oxyhydroxide, and cobalt oxyhydroxide;

[0013] Optionally, the anode conductive substrate includes at least one of copper foam, nickel foam, cobalt foam, carbon felt, carbon paper, and carbon cloth;

[0014] Optionally, the loading of the anode electron carrier on the anode is 0.001 mg / cm³. 2 -10mg / cm 2 Therefore, under mild reaction conditions, aromatic aldehydes can be obtained.

[0015] In some embodiments of the present invention, the flow fuel cell further includes a cathode, the cathode comprising a cathode conductive substrate and a cathode electron carrier disposed on the cathode conductive substrate, the cathode electron carrier comprising at least one of platinum, ruthenium, carbon black, phthalocyanine iron, iron-nitrogen-carbon composite, cobalt-nitrogen-carbon composite and graphene;

[0016] Optionally, the cathode conductive substrate includes at least one of copper foam, nickel foam, cobalt foam, carbon felt, carbon paper, and carbon cloth;

[0017] Optionally, the cathode electron carrier loading on the cathode is 0.001 mg / cm³. 2 -10mg / cm 2 Therefore, under mild reaction conditions, aromatic aldehydes can be obtained.

[0018] In some embodiments of the present invention, the flow fuel cell further includes a cathode electrolyte, which includes an electrolyte comprising at least one of vanadium oxysulfate, ferric chloride, ferric nitrate, ferric citrate, phosphomolybdic acid, phosphomolybdic vanadate, copper chloride, and potassium ferricyanide.

[0019] Optionally, the molar concentration of the electrolyte in the cathode electrolyte is 0.001 mol / L to 4 mol / L;

[0020] Optionally, the cathode electrolyte further includes a supporting electrolyte, which includes an inorganic acid or an inorganic base;

[0021] Optionally, the cathode electrolyte further includes a supporting electrolyte with a molar concentration of 0.01 mol / L to 6 mol / L. Thus, the reaction conditions are mild, and aromatic aldehydes can be obtained.

[0022] A second aspect of the present invention provides a method for preparing a lignin-based antioxidant, comprising:

[0023] The depolymerization products of lignin are polycondensed with compounds containing carbonyl groups to obtain lignin-based antioxidants.

[0024] The depolymerization product of lignin is obtained by the method described in the first aspect of the present invention.

[0025] The method for preparing lignin-based antioxidants provided by this invention produces lignin-based antioxidants with advantages such as good oil solubility, strong antioxidant capacity, and low toxicity. These antioxidants can be used as antioxidants dissolved in oily solvents in oil products, for example, to improve the oxidative stability of biodiesel to meet national standards.

[0026] In some embodiments of the present invention, the carbonyl-containing compound includes at least one of acetaldehyde, acetone, methyl ethyl ketone, pentanone, and hexanone;

[0027] Optionally, the mass ratio of the carbonyl-containing compound to the depolymerization product of the lignin is (50-300):1. This results in a lignin-based antioxidant with good antioxidant properties.

[0028] In some embodiments of the present invention, the depolymerization product of lignin is reacted with a carbonyl-containing compound via a polycondensation reaction to obtain a lignin-based antioxidant, comprising:

[0029] The depolymerization products of lignin, carbonyl compounds, and solvents are mixed, and an alkaline catalyst is added to carry out the reaction to obtain a lignin-based antioxidant.

[0030] Optionally, the solvent includes at least one of methanol, ethanol, propanol, tert-butanol, ethyl acetate, tetrahydrofuran, diethyl ether, and 1,4-dioxane;

[0031] Optionally, the alkaline catalyst includes at least one of sodium hydroxide, potassium hydroxide, calcium oxide, calcium hydroxide, magnesium hydroxide, and magnesium oxide;

[0032] Optionally, the reaction temperature is 10°C-70°C;

[0033] Optionally, the reaction time is 2 h to 180 h. As a result, the lignin-based antioxidant prepared exhibits good antioxidant properties.

[0034] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0035] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0036] Figure 1 A schematic diagram of the structure of a flow fuel cell according to an embodiment of the present invention is shown;

[0037] Figure 2 A schematic diagram of the preparation process of a lignin-based antioxidant according to an embodiment of the present invention is shown;

[0038] Figure 3 The infrared spectrum of the product involved in the process of Embodiment 1 of the present invention is shown;

[0039] Figure 4 A comparative graph showing the antioxidant properties of the products involved in the process of Embodiment 1 of the present invention is displayed;

[0040] Figure 5 The gel permeation chromatogram of the product involved in the process of Embodiment 1 of the present invention is shown;

[0041] Figure 6 The graph shows the effect of the amount of product added in the process of Example 1 of the present invention on the oxidative stability of palm oil biodiesel.

[0042] Reference numerals: 1: Anode storage tank; 2: Anode discharge chamber; 3: Anode; 4: Ion exchange membrane; 5: Cathode; 6: Cathode discharge chamber; 7: External load; 8: Cathode storage tank;

[0043] a: Depolymerization reaction; b: Acidification; c: Centrifugation; d: Extraction; e: Drying; f: Condensation reaction; g: Vacuum distillation; h: Acidification; i: Extraction; j: Drying. Detailed Implementation

[0044] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0045] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0046] The large molecular weight of lignin results in poor solubility and low antioxidant activity, hindering its large-scale industrial application. Existing methods for depolymerizing lignin require harsh reaction conditions and cannot yield aromatic aldehyde products.

[0047] Therefore, the first aspect of the present invention proposes a method for depolymerizing lignin, which aims to provide a method for depolymerizing lignin with mild reaction conditions that can yield aromatic aldehydes.

[0048] The method for depolymerizing lignin proposed in this invention includes:

[0049] S10: Place lignin in the anode electrolyte of a flow fuel cell and depolymerize the lignin during the discharge process of the flow fuel cell.

[0050] Figure 1 This is a schematic diagram of the structure of a flow fuel cell according to an embodiment of the present invention. Please refer to [link / reference]. Figure 1 In a flow fuel cell, an anode 3, a cathode 5, an ion exchange membrane 4, and a graphite bipolar plate are assembled to form a flow fuel cell device with an anode discharge chamber 2 and a cathode discharge chamber 6. The anode discharge chamber 2 and cathode discharge chamber 6 are connected to an anode storage tank 1 and a cathode storage tank 8, respectively. The anode storage tank 1 contains an anolyte containing lignin, and the cathode storage tank 8 contains a redox reversible electrolyte containing a cathode electrolyte. The anolyte and cathode electrolyte are continuously pumped into the anode discharge chamber 2 and cathode discharge chamber 6 of the flow fuel cell device, respectively, and then circulated back to the anode storage tank 1 and cathode storage tank 8, respectively. During discharge, the cathode 5 and anode 3 of the flow fuel cell are connected to an external load 7 to form a circuit, generating electrical energy while oxidizing the lignin in the anode electrolyte. Finally, air is introduced into the cathode discharge chamber 6 and cathode storage tank 8 to regenerate the cathode electron carrier and the redox electrolyte.

[0051] Lignin is oxidized and depolymerized into smaller molecules using a flow fuel cell. This depolymerization method can be carried out under mild conditions and simultaneously generates electricity, avoiding the high temperature and high pressure conditions required for traditional lignin depolymerization processes. After the lignin depolymerization reaction, a series of small-molecule depolymerized products containing phenolic hydroxyl groups and aromatic aldehydes are obtained. This process not only reduces the molecular weight of lignin but also increases the content of phenolic hydroxyl groups and aromatic aldehydes, enhancing the antioxidant properties of the depolymerized products.

[0052] According to the method of the present invention, by using substances with redox capabilities as cathode and anode electron carriers, lignin is oxidized to generate the corresponding target products, while electrons are transferred to oxygen molecules, thereby achieving directional electron transfer to generate electrical energy. In the anode discharge chamber of the flow fuel cell, the high-valence anode electron carrier oxidizes lignin to generate monomers such as aromatic aldehydes. It is then reduced and rapidly transfers electrons to the oxidized cathode electron carrier in the cathode discharge chamber via an external circuit, thereby generating electrical energy. At this time, the anode electron carrier recovers to the high-valence state and continues to participate in the catalytic cycle. The cathode electron carrier reduced in the cathode discharge chamber is oxidized by the redox electrolyte or the introduced air, and the electrolyte can also be further oxidized and regenerated by contact with the introduced air in the cathode reservoir. Therefore, oxygen is the final electron acceptor, and the overall reaction is that oxygen oxidizes lignin and depolymerizes it into monomers such as aromatic aldehydes. The depolymerization products of lignin contain benzaldehyde-like substances with para-phenolic hydroxyl groups (such as para-hydroxybenzaldehyde, vanillin, syringaldehyde, etc.).

[0053] Lignin undergoes oxidative depolymerization via discharge to yield a series of small monomers. Most of these monomers contain phenolic hydroxyl groups, exhibiting certain antioxidant properties. Benzaldehyde compounds containing phenolic hydroxyl groups at the para-position constitute a significant proportion of the depolymerization products, primarily including p-hydroxybenzaldehyde, vanillin, and syringaldehyde. These products serve as the main substrates for subsequent condensation reactions. Additionally, the depolymerization products also contain p-hydroxyacetophenone, acetylvanillin, and acetylsyringone. Notably, the depolymerization mixture contains p-coumaric acid, ferulic acid, and sinapic acid. These compounds possess unsaturated carbon-carbon double bonds in their side chains, exhibiting a conjugated structure of carbon-carbon double bonds and benzene rings. Their phenolic hydroxyl groups demonstrate strong hydrogen-donating capabilities and good antioxidant properties.

[0054] In some embodiments of the present invention, the flow fuel cell includes an anode, which comprises an anode conductive substrate and an anode electron carrier disposed on the anode conductive substrate. The anode electron carrier is selected from at least one of copper oxide, nickel oxide, manganese dioxide, cobalt oxide, iron oxide, nickel phosphide, cobalt phosphide, nickel sulfide, cobalt sulfide boronide, nickel nitride, cobalt tetroxide, nickel hydroxide, cobalt hydroxide, nickel oxyhydroxide, and cobalt oxyhydroxide. These metal oxides have certain redox potentials, which can oxidize and break the bonds between phenylpropane constituent units (e.g., β-O-4 bonds, 4-O-5 bonds, etc.), and are poorly soluble in water, avoiding the problem of product separation from the catalyst. Selecting the above-mentioned anode electron carrier can accelerate the electron transfer kinetics from lignin to the electron carrier and improve the product yield of monomers such as aromatic aldehydes.

[0055] In some embodiments of the present invention, the anode conductive substrate is selected from one of the following: copper foam, nickel foam, cobalt foam, carbon felt, carbon paper, and carbon cloth. These substrate materials have excellent conductivity and porosity, providing abundant reaction sites for catalyst loading and reaction. The selection of the above-mentioned anode conductive substrate materials also takes into account the acidity or alkalinity of the electrolyte, which can avoid the dissolution and corrosion of the substrate material.

[0056] In some embodiments of the present invention, the loading of the anode electron carrier on the anode is 0.001 mg / cm³. 2 -10mg / cm 2 For example, the loading of the anodic electron carrier can be 0.001 mg / cm³. 2 1mg / cm 2 2mg / cm 2 3mg / cm 2 5mg / cm 2 7mg / cm 2 9mg / cm 2 10mg / cm 2 If the load is too small, the electron transfer rate will be too slow and the current will be too low, which is not conducive to the oxidation of lignin and the production of electricity; if the load is too large, it will waste raw materials and there will be obvious shedding after long-term operation.

[0057] In some embodiments of the present invention, the flow fuel cell includes a cathode, which comprises a cathode conductive substrate and a cathode electron carrier disposed on the cathode conductive substrate. The cathode electron carrier is selected from platinum, ruthenium, carbon black, phthalocyanine iron, iron-nitrogen-carbon composite, cobalt-nitrogen-carbon composite, and graphene. These electron carriers can promote electron transport kinetics through valence state changes of active centers, thereby catalyzing the four-electron reduction of oxygen to water.

[0058] In some embodiments of the present invention, the cathode conductive substrate material is selected from one of the following: copper foam, nickel foam, cobalt foam, carbon felt, carbon paper, and carbon cloth. These substrate materials have excellent conductivity and porosity, providing abundant reaction sites for catalyst loading and reaction. The selection of the above-mentioned cathode conductive substrate material also takes into account the acidity or alkalinity of the electrolyte, which can avoid the dissolution and corrosion of the substrate material.

[0059] In some embodiments of the present invention, the cathode electron carrier loading on the cathode is 0.001 mg / cm³. 2 -10mg / cm 2 For example, the loading of the cathode electron carrier can be 0.001 mg / cm³. 2 1mg / cm 2 2mg / cm 2 3mg / cm 2 5mg / cm 2 7mg / cm 2 9mg / cm 2 10mg / cm 2 If the load is too small, the electron transfer rate will be too slow and the current will be too low, which is not conducive to the oxidation of lignin and the production of electricity; if the load is too large, it will waste raw materials and there will be obvious shedding after long-term operation.

[0060] In some embodiments of the present invention, the flow fuel cell includes an ion exchange membrane selected from anion exchange membranes and cation exchange membranes. The type and ion permeability of the ion exchange membrane significantly affect the internal resistance of the cell, and thus the electrode reaction rate. According to embodiments provided by the present invention, preferred ion exchange membranes include cation exchange membranes, such as perfluorosulfonic acid membranes, and anion exchange membranes, such as hydroxide ion exchange membranes. The selection of the ion exchange membrane is also related to the cathode electrolyte and anolyte used.

[0061] In some embodiments of the present invention, during the discharge process of the flow fuel cell, the temperature of the anolyte is 20°C-100°C, optionally 80°C-100°C. For example, the temperature of the anolyte can be 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, etc. Specifically, the storage tank controls the anolyte temperature at 20°C-100°C using a heating device to adjust the reaction rate. Similar to chemical reaction rates, electrode reactions are significantly affected by temperature. Increasing the temperature is beneficial to increasing the reaction rate on the electrode surface, the diffusion rate of active materials, and thus increasing the electron transfer rate. Furthermore, lignin linkages are more easily broken down at higher temperatures, thereby increasing the yield of depolymerized monomers. In a preferred embodiment of the present invention, during the discharge process of the flow fuel cell, the temperature of the anolyte is 80°C-100°C.

[0062] In some embodiments of the present invention, the concentration of lignin in the anolyte is 0.001 g / L to 80 g / L. For example, the concentration of lignin in the anolyte can be 0.001 g / L, 1 g / L, 10 g / L, 20 g / L, 40 g / L, 50 g / L, 70 g / L, 80 g / L, etc. Increasing the lignin concentration is beneficial for obtaining a higher product concentration, but lignin may not react sufficiently on the electrode surface. Therefore, controlling the concentration of lignin in the anolyte within the above-mentioned range can obtain a higher yield of depolymerization products. In a preferred embodiment of the present invention, the concentration of lignin in the anolyte is 0.001 g / L to 10 g / L.

[0063] In some embodiments of the present invention, the anolyte further includes an inorganic base as a supporting electrolyte. This can improve the conductivity of the solution and provide a reaction environment for the oxidation of lignin, thereby accelerating the reaction rate.

[0064] In some embodiments of the present invention, the concentration of the inorganic base supporting electrolyte in the anolyte is 0.01 mol / L-6 mol / L. For example, the molar concentration of the inorganic base in the anolyte is 0.01 mol / L, 0.5 mol / L, 1 mol / L, 2 mol / L, 3 mol / L, 5 mol / L, 6 mol / L, etc. Too low a base concentration results in a low charge conduction rate in the electrolyte, leading to a high internal resistance of the battery; while too high a base concentration leads to violent side reactions, reducing the yield of the target product. Stronger bases, compared to weaker bases, can better promote the deprotonation and hydrolysis oxidation processes of lignin in the depolymerization reaction, thereby obtaining more low molecular weight depolymerization products. However, excessively high base concentrations can also increase electrode corrosion and aging of the ion exchange membrane, reducing the lifespan of the electrode and the mill. In a preferred embodiment of the present invention, the concentration of the inorganic base supporting electrolyte is 0.01 mol / L-6 mol / L.

[0065] In some embodiments of the present invention, the cathode electrolyte includes an electrolyte containing at least one redox reversible electrolyte selected from vanadium oxysulfate, ferric chloride, ferric nitrate, ferric citrate, phosphomolybdic acid, phosphomolybdic vanadate, copper chloride, and potassium ferricyanide. These electrolytes can increase the conductivity of the cathode electrolyte on the one hand, and on the other hand, they can also act as redox electron carriers to promote further electron transfer to oxygen.

[0066] In some embodiments of the present invention, the molar concentration of the electrolyte in the cathode electrolyte is 0.001 mol / L to 4 mol / L. For example, the molar concentration of the electrolyte in the cathode electrolyte can be 0.001 mol / L, 0.01 mol / L, 0.1 mol / L, 0.5 mol / L, 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, etc. Within the above concentration range, the conductivity of the cathode electrolyte can be increased, which can promote the discharge of the liquid flow fuel cell and thus cause lignin depolymerization.

[0067] In some embodiments of the present invention, the cathode electrolyte further includes a supporting electrolyte, which comprises an inorganic acid or an inorganic base. Using an inorganic acid or inorganic base as a supporting electrolyte can improve the conductivity of the cathode electrolyte and provide an acidic or alkaline environment for the electrode reaction, thereby increasing the reaction rate.

[0068] In some embodiments of the present invention, the cathode electrolyte further includes a supporting electrolyte, the molar concentration of which is 0.01 mol / L to 6 mol / L. For example, the molar concentration of the supporting electrolyte in the cathode electrolyte can be 0.01 mol / L, 0.1 mol / L, 0.5 mol / L, 1 mol / L, 2 mol / L, 5 mol / L, 6 mol / L, etc. Within the above molar concentration range, the conductivity of the cathode electrolyte can be improved, and an acidic or alkaline environment can be provided for the electrode reaction, thereby increasing the reaction rate.

[0069] In some embodiments of the present invention, the resistance of the external load of the flow fuel cell is 0-2000 ohms. For example, the resistance of the external load can be 0, 100 ohms, 500, 1000 ohms, 1200 ohms, 1500 ohms, 2000 ohms, etc. Connecting an external load is a prerequisite for forming a circuit and realizing the transfer of electrons from lignin to air; otherwise, the oxidative depolymerization of lignin cannot proceed. The magnitude of the resistance of the external load directly determines the rate of electron transfer. When the resistance of the external load is 0 ohms, that is, the battery discharge occurs in a short-circuit manner, and the electron transfer rate is the fastest. When the resistance of the external load increases, the ohmic resistance of electron transfer increases, and the rate of oxidative depolymerization of lignin at the anode decreases. When the external load is infinitely large, that is, in an open-circuit state, no current is generated, electrons cannot be transferred to oxygen, and the oxidative depolymerization reaction of lignin cannot proceed. Therefore, when the resistance of the external load is within the above range, good power generation and lignin oxidative depolymerization effects can be obtained.

[0070] In some embodiments of the present invention, the depolymerization products need to be extracted, the extraction including acidifying the discharged anolyte, centrifuging, extracting and drying.

[0071] In some embodiments of the present invention, the acidification involves adjusting the pH of the system to 1-3. For example, the pH of the system may be adjusted to 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, etc. According to the method of the embodiments of the present invention, the depolymerization product is present in an alkaline aqueous solution. Acidifying the depolymerization product converts it from a phenolate ionic form to a phenolic form, allowing for better extraction from the reaction solution.

[0072] In some embodiments of the present invention, the extractant used for extraction includes at least one of ethyl acetate, dichloromethane, chloroform, and diethyl ether. According to the method of the embodiments of the present invention, the above-mentioned extractant is immiscible with water, the organic phase and the aqueous phase can be well separated into layers, and it has a low boiling point, facilitating vacuum distillation recovery.

[0073] In some embodiments of the present invention, the drying includes at least one of freeze drying, vacuum drying, and rotary evaporation drying.

[0074] A second aspect of this invention provides a method for preparing a lignin-based antioxidant, comprising:

[0075] S100: The depolymerization product of lignin is reacted with a compound containing a carbonyl group to produce a lignin-based antioxidant.

[0076] The depolymerization product of lignin is obtained by the method described in the first aspect of the present invention.

[0077] The method for preparing lignin-based antioxidants provided by this invention produces lignin-based antioxidants with advantages such as good oil solubility, strong antioxidant capacity, and low toxicity. These antioxidants can be used as antioxidants dissolved in oily solvents in oil products, for example, to improve the oxidative stability of biodiesel to meet national standards.

[0078] In some embodiments of the present invention, the carbonyl-containing compound includes aldehydes and ketones containing α-H, specifically, the carbonyl-containing compound includes at least one selected from acetaldehyde, acetone, methyl ethyl ketone, pentanone, and hexanone. According to the method of the embodiments of the present invention, methyl ethyl ketone is superior to other ketones because the α-H reactivity at both ends of the ketone group of methyl ethyl ketone differs. The steric hindrance at the ethyl end makes the condensation reaction more likely to occur at the α-H at the methyl end, resulting in better selectivity during preparation and facilitating control of product production. Furthermore, the condensation product formed by methyl ethyl ketone has a short side chain and good antioxidant properties.

[0079] In some embodiments of the present invention, the mass ratio of the carbonyl-containing compound to the depolymerization product of the lignin is (50-300):1. For example, the mass ratio of the carbonyl-containing compound to the depolymerization product of the lignin can be 50:1, 100:1, 150:1, 200:1, 250:1, 300:1, etc. The carbonyl-containing compound and the depolymerization product of lignin undergo a condensation reaction according to the above ratio. The amount of the carbonyl-containing compound is far in excess relative to the depolymerization product, which allows the depolymerization product to react fully and achieve maximum conversion. Unreacted methyl ethyl ketone can be recovered and recycled in subsequent steps through vacuum distillation, avoiding waste of raw materials.

[0080] In some embodiments of the present invention, S100 includes:

[0081] S101: The depolymerization product of lignin, a carbonyl compound and a solvent are mixed and reacted with an alkaline catalyst to obtain a lignin-based antioxidant.

[0082] In some embodiments of the present invention, the solvent includes at least one selected from methanol, ethanol, propanol, tert-butanol, ethyl acetate, tetrahydrofuran, diethyl ether, and 1,4-dioxane. The inventors have found that methanol has the best solubility for the depolymerization products, effectively dispersing them to facilitate sufficient reaction with ketone compounds.

[0083] In some embodiments of the present invention, the reaction occurs under alkaline catalysis, i.e., aldol condensation. According to the method of the embodiments of the present invention, a carbonyl compound loses its α-H atom under alkaline catalysis to form a carbanion. The carbanion acts as a nucleophile, attacking the carbonyl carbon atom of another aldehyde molecule, resulting in a nucleophilic addition reaction to generate a β-hydroxy aldehyde (ketone). The β-hydroxy aldehyde (ketone) readily dehydrates upon heating to generate an α,β-unsaturated aldehyde (ketone). According to the method of the embodiments of the present invention, a benzaldehyde compound containing a para-phenolic hydroxyl group in the depolymerization product reacts with methyl ethyl ketone. Since benzaldehyde compounds do not possess α-H, they can only react with the α-H of the ketone group as C+ ions. The steric hindrance at the ethyl-terminal α-H of methyl ethyl ketone makes the condensation reaction more likely to occur at the methyl-terminal α-H; therefore, this condensation reaction has high selectivity.

[0084] In some embodiments of the present invention, the alkaline catalyst includes at least one of sodium hydroxide, potassium hydroxide, calcium oxide, calcium hydroxide, magnesium hydroxide, and magnesium oxide.

[0085] In some embodiments of the present invention, the mass ratio of the alkali catalyst to the depolymerization product of the lignin is (1-10):10. For example, the mass ratio of the alkali catalyst to the depolymerization product of the lignin can be 1:10, 3:10, 5:10, 6:10, 8:10, 10:10, etc. There is a significant interaction between the type and amount of alkali catalyst used; that is, when the alkali catalyst used is more alkaline, its amount is less, and when the alkali catalyst used is less alkaline, its amount is more. The amount of alkali used needs to be optimized and controlled to avoid the generation of heavy polycondensation byproducts.

[0086] In some embodiments of the present invention, the temperature of the condensation reaction is 10°C-70°C. For example, the reaction temperature can be 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, etc. According to the method of the embodiments of the present invention, in order to ensure the effective conversion of depolymerization products and reduce repolymerization side reactions, the most suitable condensation reaction temperature is 40°C-60°C.

[0087] In some embodiments of the present invention, the condensation reaction time is 2h-180h. For example, the reaction time can be 2h, 10h, 20h, 50h, 80h, 100h, 130h, 150h, 180h, etc. According to the method of the embodiments of the present invention, in order to ensure the effective conversion of depolymerization products and reduce repolymerization side reactions, the most suitable condensation reaction time is 24-48 hours.

[0088] In some embodiments of the present invention, the reaction is further followed by extraction and purification of the condensation reaction product. According to the method of the embodiments of the present invention, extraction and purification of the condensation reaction product facilitates the acquisition of a high-purity lignin-based antioxidant, which is convenient for subsequent use.

[0089] In some embodiments of the present invention, the extraction and purification include vacuum distillation, acidification, extraction, and drying.

[0090] In some embodiments of the present invention, the temperature of vacuum distillation is 30°C-70°C. For example, the temperature of vacuum distillation can be 30°C, 40°C, 50°C, 60°C, 70°C, etc.

[0091] In some embodiments of the present invention, the pressure of vacuum distillation is 1 kPa-10 kPa. For example, the pressure of vacuum distillation can be 1 kPa, 3 kPa, 5 kPa, 7 kPa, 9 kPa, 10 kPa, etc.

[0092] According to the method of the present invention, vacuum distillation can remove solvent and unreacted carbonyl-containing compounds, resulting in higher purity of the product.

[0093] In some embodiments of the present invention, the acidification involves adjusting the pH of the system to 1-4. In some embodiments of the present invention, the pH of the system can be adjusted to 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, or 4.0. According to the method of the embodiments of the present invention, acidifying the target product to convert it from an ionic form to a phenolic form allows for better extraction from the reaction solution.

[0094] In some embodiments of the present invention, the extractant in the extraction includes at least one of dichloromethane, trichloromethane, carbon tetrachloride, and diethyl ether. According to the method of the embodiments of the present invention, the above-mentioned extractant has a low boiling point so that it can be easily recovered by vacuum distillation.

[0095] In some embodiments of the present invention, the drying includes at least one of freeze drying, vacuum drying, and rotary evaporation drying.

[0096] The depolymerization products of lignin contain benzaldehyde compounds with para-phenolic hydroxyl groups (p-hydroxybenzaldehyde, vanillin, syringaldehyde). These compounds can undergo condensation reactions with carbonyl-containing compounds to obtain an enone structure. This enone structure can form a conjugated large π-bond system with the benzene ring, which is beneficial for enhancing the antioxidant properties of the phenolic hydroxyl group. It is important to note that, in addition to benzaldehyde compounds, acetophenone compounds can also be used as reaction substrates in the method of this invention, such as p-hydroxyacetophenone, acetylvanillin, and acetylsyringone. When benzaldehyde compounds undergo condensation reactions with ketone compounds, because benzaldehyde compounds do not possess α-H, they can only generate C+ ions that react with the α-H of the ketone group. Furthermore, the cyclic structure of benzaldehyde compounds produces a significant steric hindrance effect, which is beneficial for improving product selectivity. It is noteworthy that acetophenone compounds possess α-H; therefore, both acetophenone compounds and ketone compounds can form carbanions, which act as nucleophiles to attack the carbonyl carbon atom of another molecule, thus initiating a condensation reaction. However, when two ketone molecules condense, due to electronic and steric effects, only a small amount of condensation product is obtained under the same conditions. Unless a special apparatus is used to continuously remove the product from the equilibrium system, most of the ketone can be converted to a β-hydroxyketone. This system does not possess such conditions, therefore the conversion rate of this reaction pathway is extremely low.

[0097] It is important to clarify that the types and contents of monomers in the depolymerization products vary depending on the type of lignin raw material and the processing method. Through repeated experiments, the inventors discovered that the content of benzaldehyde-like substances in the depolymerization products is significantly higher than that of acetophenone-like substances with corresponding structures, and the reactivity of benzaldehyde-like substances in condensation reactions is also much greater than that of the corresponding acetophenone-like substances. Therefore, the condensation reaction in this system mainly refers to the condensation of benzaldehyde-like substances (p-hydroxybenzaldehyde, vanillin, syringaldehyde) containing para-phenolic hydroxyl groups in the depolymerization mixture with ketone substances to form enone compounds. The benzene ring, carbon-carbon double bonds, and carbon-oxygen double bonds on the side chains of this enone compound form a conjugated structure, which enhances the hydrogen-donating capacity of the phenolic hydroxyl groups, thereby improving antioxidant properties.

[0098] In some embodiments of the present invention, the resulting lignin-based antioxidant is a mixture comprising monomers, dimers, and oligomers. These molecules all possess varying degrees of antioxidant activity. Among them, the monomers with stronger antioxidant activity exhibit higher degrees of conjugation between the unsaturated bonds and benzene rings on their side chains, primarily forming enone-type compounds through condensation reactions. These enone-type compounds can undergo further self-condensation or condensation with benzaldehyde-like substances in the depolymerization products to generate dimers with even higher degrees of conjugation, resulting in superior antioxidant activity.

[0099] In some embodiments of the present invention, the lignin-based antioxidant can be applied in fields such as pharmaceuticals, plastics and rubber, and biofuels. In some embodiments of the present invention, the raw material for the lignin-based antioxidant is derived from lignin in biomass, which is green, renewable, inexpensive, and readily available. The lignin-based antioxidant prepared according to embodiments of the present invention has good oil solubility and can be used for antioxidant purposes in oil-based products (such as biodiesel), effectively improving the oxidation stability of biodiesel and ensuring that the oxidation stability performance of biodiesel meets national standards.

[0100] The present disclosure will be explained below with reference to embodiments. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the disclosure. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0101] Example 1

[0102] Please refer to the preparation process of lignin-based antioxidants. Figure 2 , Figure 2 In the diagram, a: depolymerization reaction; b: acidification; c: centrifugation; d: extraction; e: drying; f: condensation reaction; g: vacuum distillation; h: acidification; i: extraction; j: drying.

[0103] Depolymerization reaction: Cobalt sulfide-supported nickel foam anodes were prepared using electrodeposition. The anolyte contained 3 mol / L potassium hydroxide as the supporting electrolyte and 2 g / L lignin. Carbon felt was used as the cathode, with 0.37 mol / L VO₂(SO₄)₂ as the redox electrolyte and 2 mol / L sulfuric acid as the supporting electrolyte. A Nafion 115 membrane was used as the ion exchange membrane. After assembling the battery device, a circulating pump continuously pumped the anolyte and cathode electrolyte into the anolyte and cathode discharge chambers, and then circulated them back to the anolyte and cathode storage tanks. After 2 hours of discharge, the anolyte contained lignin oxidative depolymerization monomers such as aromatic aldehydes.

[0104] Depolymerization product collection: The resulting reaction mixture was acidified to pH 2 with 12 mol / L hydrochloric acid, then centrifuged to obtain a supernatant and unreacted lignin precipitate. The supernatant and lignin precipitate were extracted with ethyl acetate, respectively, and centrifuged to obtain an upper organic phase and a solid precipitate. The organic phase was collected. The organic phases were mixed and the solvent was evaporated to dryness using a vacuum rotary evaporator to obtain a residue. The residue was dissolved in methanol to obtain the lignin depolymerization product (depolymerization product), which was used as a raw material for the condensation reaction.

[0105] Condensation reaction and purification extraction: Methyl ethyl ketone and NaOH aqueous solution were added to the methanol solution after lignin depolymerization, and the mixture was placed in a shaker at 55℃ for 36 h. The reaction solution was then removed by vacuum rotary evaporation at 40℃ to remove most of the solvents such as methyl ethyl ketone and methanol. The residue was then dissolved in water and acidified with hydrochloric acid to pH=2. The solid-liquid mixture was then extracted with dichloromethane. The lower dichloromethane extract phase was separated by centrifugation. The extraction was repeated 3 times to obtain the dichloromethane phase, which was washed 3 times with deionized water and the solvent was removed by vacuum evaporation at 35℃ to obtain the lignin-based antioxidant. The solid part was regarded as a byproduct of the condensation reaction and was freeze-dried.

[0106] Except for the parameters, the preparation steps and methods of Examples 2-17 are the same as those of Example 1 (see Table 1).

[0107] Comparative Example 1

[0108] Conventional method: Lignin is decomposed by ethanol in a heated reactor, and the depolymerization product is used directly as an antioxidant. 2 g / L lignin and 25 mL of ethanol are placed in a 50 mL reactor. The reactor is sealed and heated to a reaction temperature of 160°C and maintained for 60 minutes. The reactor is then rapidly cooled to room temperature in cold water. The reactor is opened, and the reaction solution (containing the depolymerization product) is recovered. The inside of the reactor is rinsed with 60 mL of ethanol and mixed with the recovered product. The insoluble lignin residue is further separated by centrifugation (3500 rpm, 20 minutes). The soluble fraction is dried at 40°C for 48 hours to obtain the lignin-based antioxidant.

[0109] Table 1

[0110]

[0111] The infrared spectra of the products involved in the process of Embodiment 1 of the present invention are attached. Figure 3 As shown, it can be seen that at 1691cm -1 At this point, the peak intensity of the lignin-based antioxidant increased relative to both the lignin raw material and the depolymerization product, indicating an increase in the content of conjugated ketone carbonyl groups, which suggests the effectiveness of the subsequent condensation reaction.

[0112] After obtaining the lignin-based antioxidant according to the method in Example 1, its DPPH free radical scavenging ability was compared and analyzed with that of the reaction product. The results are as follows: Figure 4 As shown in the figure. The results indicate that all three substances possess certain antioxidant properties. The IC50 of the depolymerization product... 50 The value was 0.0348 mg / mL, and the IC50 value of the condensation reaction byproducts was 0.0348 mg / mL. 50 The value was 0.0354 mg / mL, and the antioxidant properties of the two were not significantly different. After aldol condensation, the lignin-based antioxidant IC50 of the lignin depolymerization products... 50 The concentration was 0.0294 mg / mL, indicating a significant improvement in antioxidant activity. Therefore, the product obtained from the depolymerization and condensation of lignin is an excellent antioxidant.

[0113] IC of lignin-based antioxidants of Examples 1-17 and Comparative Example 1 of the present invention 50 The values ​​are shown in Table 2.

[0114] Table 2

[0115]

[0116]

[0117] As can be seen from Table 2, the IC50 of the lignin-based antioxidants prepared in Examples 1-17 of this invention... 50 The values ​​are small and the antioxidant properties are high, while the antioxidant prepared by the conventional method in Comparative Example 1 needs to be carried out under high temperature and high pressure, which makes the preparation process complicated.

[0118] The gel permeation chromatogram of the product involved in the process of Example 1 of this invention is as follows: Figure 5As shown in Table 3, the average molecular weights of the products involved in the process are as follows. It can be seen that after the depolymerization reaction of lignin, the weight-average molecular weight decreases from 5550 g / mol to 1000 g / mol, a significant reduction that is beneficial for improving its solubility. After the depolymerization products undergo a condensation reaction, two parts are produced: a lignin-based antioxidant and a condensation reaction byproduct, with molecular weights of 520 g / mol and 1810 g / mol, respectively. The average molecular weight of the depolymerization products falls between that of the lignin-based antioxidant and the condensation reaction byproduct, indicating that the subsequent purification steps after the condensation reaction are beneficial for further enriching the low molecular weight products as lignin-based antioxidants. The other part, with higher molecular weight byproducts, may also be due to further condensation reactions.

[0119] Table 3

[0120] Depolymerization products Lignin-based antioxidants Condensation reaction byproducts Lignin <![CDATA[M w (g / mol)]]> 1000 520 1810 5550 <![CDATA[M n (g / mol)]]> 450 280 820 1310 PDI 2.22 1.86 2.21 4.24

[0121] Oxidative stability tests were conducted using a Rancimat 873 oxidation stability tester according to EN14112 and Chinese standard NB / SH / T 0873-2013. 0–3000 ppm of the lignin-based antioxidant prepared in Example 1 was added to palm oil biodiesel. The induction period of the biodiesel was determined by measuring the time it took for the conductivity to increase rapidly. A longer induction period indicates a stronger antioxidant activity against the oil. Results are as follows: Figure 6 As shown in the figure. According to the oxidation stability test results, the addition of all components has a certain effect on prolonging the induction time of biodiesel, but there are significant differences between different components. When 2000 ppm is added, the lignin-based antioxidant prolongs the induction period by 4.79 h compared to the depolymerization product, which is greater than the national standard of 6 hours. Meanwhile, the condensation reaction byproducts hardly improve the oxidation stability of biodiesel. This indicates that the lignin-based antioxidant prepared according to the method of this invention has good antioxidant properties.

[0122] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," "some implementations," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0123] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a lignin-based antioxidant, characterized in that, include: The depolymerization products of lignin are polycondensed with compounds containing carbonyl groups to obtain lignin-based antioxidants. The method for preparing the lignin depolymerization product includes: Lignin is placed in the anode electrolyte of a flow fuel cell, and during the discharge process of the flow fuel cell, the lignin is depolymerized. The flow fuel cell further includes an anode, which includes an anode conductive substrate and an anode electron carrier disposed on the anode conductive substrate, wherein the anode electron carrier is cobalt sulfide. The anode conductive substrate includes at least one of copper foam, nickel foam, cobalt foam, carbon felt, carbon paper, and carbon cloth; On the anode, the loading of the anode electron carrier is 5 mg / cm³. 2 -10mg / cm 2 ; The temperature of the anolyte is 80℃-100℃; The anolyte further includes an inorganic base, wherein the molar concentration of the inorganic base in the anolyte is 3 mol / L; The flow fuel cell further includes a cathode, which includes a cathode conductive substrate and a cathode electron carrier disposed on the cathode conductive substrate. The cathode electron carrier includes at least one of platinum, ruthenium, carbon black, iron phthalocyanine, iron-nitrogen-carbon composite, cobalt-nitrogen-carbon composite and graphene. The cathode electron carrier loading is 0.001 mg / cm³. 2 -10mg / cm 2 ; The concentration of the lignin is 2 g / L; The carbonyl-containing compound includes at least one of acetone and methyl ethyl ketone; The mass ratio of the carbonyl-containing compound to the depolymerization product of the lignin is (100-300):

1.

2. The method according to claim 1, characterized in that, The cathode conductive substrate includes at least one of copper foam, nickel foam, cobalt foam, carbon felt, carbon paper, and carbon cloth.

3. The method according to claim 1, characterized in that, The flow fuel cell further includes a cathode electrolyte, which includes an electrolyte, and the electrolyte includes at least one of the following: vanadium oxysulfate, ferric chloride, ferric nitrate, ferric citrate, phosphomolybdic acid, phosphomolybdic vanadate, copper chloride, and potassium ferricyanide. The concentration of the electrolyte in the cathode electrolyte is 0.001 mol / L to 4 mol / L. The cathode electrolyte also includes a supporting electrolyte, which includes an inorganic acid or an inorganic base; The cathode electrolyte also includes a supporting electrolyte, the molar concentration of which is 0.01 mol / L to 6 mol / L.

4. The method according to claim 1, characterized in that, The depolymerization products of lignin are polycondensed with compounds containing carbonyl groups to obtain lignin-based antioxidants, including: The depolymerization products of lignin, carbonyl compounds, and solvents are mixed, and an alkaline catalyst is added to carry out the reaction to obtain a lignin-based antioxidant. The solvent includes at least one of methanol, ethanol, propanol, tert-butanol, ethyl acetate, tetrahydrofuran, diethyl ether, and 1,4-dioxane; The alkaline catalyst includes at least one of sodium hydroxide, potassium hydroxide, calcium oxide, calcium hydroxide, magnesium hydroxide, and magnesium oxide. The reaction temperature is 10℃-70℃; The reaction time is 2h-180h.

Images