Method for preparing vanillic acid by depolymerizing lignin under photocatalysis of metal oxide dispersion

Abstract

The invention discloses a method for preparing vanillic acid by photocatalytic lignin depolymerization of a metal oxide dispersion, which comprises the following steps: adding lignin, a reaction solvent and the metal oxide dispersion into a photoreactor according to a certain proportion under different pH values, and carrying out dark treatment and photoreaction until lignin depolymerization is completed; centrifuging, carrying out suction filtration to separate the metal oxide dispersion, and recycling the catalyst; and carrying out rotary evaporation on the filtrate, and re-dissolving to obtain a high-valued product vanillic acid. As the metal oxide dispersion is used as the photocatalyst, the usage amount of the catalyst is greatly reduced, the reaction time is shortened, the lignin depolymerization cost is reduced, the lignin photodepolymerization efficiency is improved, a high-valued product vanillic acid is generated, and the benefits of the method are further improved. Under the optimal process, the photocatalytic lignin depolymerization efficiency can reach 95% or above, the yield of a high-valued product vanillic acid is 15% or above, and the lignin depolymerization rate and the vanillic acid yield are high.

Description

Technical Field

[0001] The present invention belongs to the technical field of effective recycling of waste natural biomass-based macromolecular compounds; more specifically, it relates to a method for preparing vanillic acid by photocatalytic depolymerization of lignin by a metal oxide dispersion. Background Art

[0002] Biomass is a general term for various organisms formed through photosynthesis. It is a medium for storing chemical energy from the sun and an important component of renewable energy. Lignin is the second most abundant natural biomass polymer in the world after cellulose, accounting for 15%-40% of the total biomass. Lignin is a complex, cross-linked and branched aromatic biopolymer that mainly exists in the free space between cellulose and hemicellulose in plant cells and cross-links with them to form a rigid structure. Due to its non-fixed monomer content and complex and diverse bonding methods, it is difficult to depolymerize, which affects the efficient conversion and utilization of lignin.

[0003] At present, the vast majority of lignin is used for combustion in power plants, and only less than 2% of lignin is used to produce value-added products such as adhesives, surfactants, and chelating agents, which not only wastes precious biomass resources, but also easily causes environmental pollution. In order to implement the concept of green and sustainable development, catalytic lignin depolymerization to produce high value-added products has attracted much attention. Vanillic acid (4-hydroxy-3-methoxy-benzaldehyde) is one of the most valuable lignin depolymerization products and is widely used as an intermediate in food additives and pharmaceutical production. For example, after oxidation, it can produce the king of spices - vanillin, which can be further purified and used in food and other industries; it can also be used as a biological precursor for the synthesis of rifamycin antibiotics; it also has significant effects in biological antibacterial, inhibiting tyrosinase activity and promoting biological coagulation.

[0004] According to the different fields of depolymerization, lignin depolymerization methods are divided into physical method, biological method and chemical method. Among them, the physical method is to change the solubility of lignin by heat, microwave and ultrasound. This method is simple and easy, but the mechanical energy consumption is large and the efficiency is low. It is mostly used for lignin pretreatment. The biological method uses biological fungi and biological enzymes to catalyze the breaking of specific chemical bonds of lignin. It is highly targeted, but the process is complicated and the cost is high. There are still certain barriers to industrial application. Compared with the first two methods, the chemical method selectively breaks the chemical bonds such as CC and CO in the molecular structure of lignin through catalytic reactions. The chemical method catalyzes the depolymerization of lignin, which is simple and efficient, and the depolymerization products are rich and diverse. It is easy to apply on a large scale in industry, and the obtained oligomers have high economic utilization benefits. According to the different depolymerization processes, the chemical method includes thermal depolymerization, hydrothermal depolymerization, hydrogenation depolymerization and photocatalytic depolymerization. Among them, photocatalytic depolymerization is the preferred choice for lignin depolymerization because of its advantages such as cleanliness, high efficiency, simple technology and benign ecology.

[0005] In the photocatalytic depolymerization of lignin, the choice of photocatalyst is crucial. Currently, the commonly used photocatalysts are C 3 N 4 、TiO 2 , ZnO powder, etc., but its particle size is large (>100nm), the surface adsorption effect is poor; the band gap is large, the light utilization rate is low; the catalyst addition amount is large (>100mg), the photocatalytic time is long (>2h), the lignin degradation rate is very low and the product is CO 2 and H 2 O, and it is impossible to utilize the product in a high-value manner. For example, Mahyar Mahdavi et al. 3 O 4 Load to C 3 N 4 The photocatalytic degradation of lignin under visible light was achieved, and the optimal depolymerization efficiency reached 90% at 3h and pH = 7 (Mahdavi M, Mirmohammadi M, Baghdadi M, et al. Visible light photocatalytic degradation and pretreatment of lignin using magnetic graphitic carbon nitride for enhancing methane production in anaerobic digestion [J]. Fuel, 2022, 318: 123600.). However, the catalyst particles are large (>200nm), and the catalyst dosage is 2.5g / L, which is 250 times the concentration of lignin substrate. The catalytic time is long and the cost is huge. In recent years, nanomaterials doped with metal ions and constructed with heterojunction structures have been widely used in photocatalytic lignin depolymerization. For example, in the Chinese invention patent document with publication number CN 116371397 A, a Ce-doped lamellar mesoporous Bi / Bi 2 O 3 / Bi 2 O 2.75 Heterojunction catalytic materials. This method has mild conditions and significantly reduces the amount of catalyst (10 mg). However, due to its strong photocarrier efficiency and oxygen vacancies, sodium lignin sulfonate is directly oxidized to CO. 2 and H 2 O, high-value intermediates cannot be prepared.

[0006] Therefore, the present invention uses metal oxide dispersion for the first time for efficient photocatalytic depolymerization of lignin to produce vanillic acid, improves photocatalytic activity through size control and surface modification technology, greatly reduces the amount of photocatalyst used, and obtains high value-added products. Summary of the invention

[0007] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is to provide a method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion. The method overcomes the problems of excessive catalyst dosage, long catalytic time and failure to obtain high-value products during photocatalytic reaction. The method utilizes the advantages of transparent dispersion and small size of the metal oxide dispersion to improve its surface adsorption characteristics, increase its photocatalytic activity, significantly reduce the catalyst dosage and catalytic time, and greatly reduce the depolymerization cost. At the same time, thanks to the appropriate amount of photogenerated electrons, high value-added products such as vanillic acid can be obtained after depolymerization.

[0008] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows :

[0009] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion comprises the following steps:

[0010] S1, after mixing lignin powder, reaction solvent and metal oxide dispersion, adding acid or alkali solution to adjust pH;

[0011] S2, transfer into a photocatalytic reactor for dark treatment;

[0012] S3, turn on the light source for irradiation, and perform photoreaction until the depolymerization of lignin is completed;

[0013] S4, obtaining a filter cake by centrifugation and suction filtration, washing it, and re-dispersing it in a reaction solvent by ultrasonication to recover the metal oxide dispersion;

[0014] S5. After rotary evaporation of the filtrate, a solvent is added to redissolve it to obtain the depolymerization product, vanillic acid.

[0015] As a further improvement of the technical solution, in step S1, the lignin is selected from one or more mixtures of lignin sulfonate, alkali lignin, sulfate lignin, organic lignin and enzymatic lignin.

[0016] Preferably, in step S1, the reaction solvent is selected from one or more mixtures of purified water, alkali solution, ethanol, acetonitrile and the like.

[0017] Preferably, in step S1, the metal oxide dispersion is derived from a mixture of one or more transparent liquid dispersions of zinc oxide, titanium oxide, or copper oxide. The average particle size of the metal oxide particles is between 1-100 nm; more preferably, the average particle size is between 1-20 nm. The preparation process of the metal oxide dispersion refers to the Chinese invention patent publication number CN109181367 A, the invention name: A method for preparing a transparent zinc oxide liquid dispersion; through this method, the surface properties of the metal oxide particles are regulated, so that they can be uniformly dispersed in the reaction medium to obtain a transparent liquid dispersion.

[0018] Preferably, in step S1, the metering ratio of the lignin powder (mg): reaction solvent (mL): metal oxide dispersion (g) is 1:5-100:0.5-50; preferably, the mass ratio of lignin powder to metal oxide dispersion (calculated as solid) is 1:1-2.

[0019] Preferably, in step S1, the acid is selected from one or more mixtures of dilute hydrochloric acid, dilute sulfuric acid, dilute phosphoric acid, dilute nitric acid, and acetic acid; more preferably, in step S1, the acid is dilute hydrochloric acid and dilute sulfuric acid.

[0020] Preferably, in step S1, the base is selected from one or more mixtures of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and aqueous ammonia; more preferably, in step S1, the base is sodium hydroxide and potassium hydroxide.

[0021] Preferably, in step S1, the pH is 1-13; more preferably, in step S1, the pH is 8-11.

[0022] As a further improvement of the technical solution, in step S2, the temperature of the dark treatment is 10-50°C, and the time of the dark treatment is 1-180min; more preferably, in step S2, the temperature of the dark treatment is 20-25°C, and the time of the dark treatment is 10-60min.

[0023] As a further improvement of the technical solution, in step S2, the dark treatment process is stirred; in step S3, the light reaction process is stirred; the stirring rate is 100-1000r / min; more preferably, the stirring rate is 300-500r / min.

[0024] As a further improvement of the technical solution, in step S3, the light source is one or a mixture of two or more of ultraviolet lamps, LED lamps, sunlight, fluorescent lamps, incandescent lamps, mercury lamps, xenon lamps, etc.

[0025] Preferably, in step S3, the temperature of the photoreaction is 10-50°C, and the time of the photoreaction is 30-360 min; more preferably, in step S3, the temperature of the photoreaction is 20-25°C, and the time of the photoreaction is 30-180 min.

[0026] As a further improvement of the technical solution, in step S4, the centrifugal rotation speed is 5000-12000 r / min; more preferably, in step S4, the centrifugal rotation speed is 8000-10000 r / min.

[0027] Preferably, in step S4, the metal oxide dispersion is directly used in step S1 for recycling.

[0028] As a further improvement of the technical solution, in step S5, the solvent is one or more of pure water, ethanol, and acetonitrile.

[0029] Any range described in the present invention includes the end value and any numerical value between the end values ​​and any sub-range formed by the end value or any numerical value between the end values.

[0030] Unless otherwise specified, all raw materials in the present invention can be purchased from the market, and the equipment used in the present invention can adopt conventional equipment in the relevant field or refer to the existing technology in the relevant field.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. The present invention utilizes metal oxide dispersion as a photocatalyst to depolymerize lignin. At room temperature (20-25°C), pH between 8-11, dark treatment for 10-60 minutes, and light reaction for 30-180 minutes, the lignin depolymerization efficiency reaches more than 95%, and the yield of the high-value product vanillic acid reaches more than 15%.

[0033] 2. The invention uses metal oxide dispersion as a photocatalyst, which has low preparation cost, simple and operable process, and can be redispersed in the medium after being recovered after the reaction. The catalytic reaction has high repeatability, which reduces the catalyst cost.

[0034] 3. In the present invention, when the mass ratio of lignin powder to metal oxide dispersion (calculated as solid) is 1:1-2, the amount of metal oxide dispersion (photocatalyst) used is significantly reduced, and the process cost is significantly reduced; at the same time, the operation is simple and the conditions are mild, and it can be popularized in the recycling and utilization of various lignin-based materials, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0036] Figure 1 This is the molecular formula diagram of sodium lignin sulfonate used in Example 1. From the diagram, it can be seen that the lignin structure is a phenylpropane unit;

[0037] Figure 2 This is a transmission electron microscopy image of the zinc oxide dispersion used in Example 1, from which it can be seen that the average particle size is about 4 nm;

[0038] Figure 3 This is the XRD pattern of the zinc oxide dispersion used in Example 1;

[0039] Figure 4 This is a physical picture of the reaction solution being centrifuged to separate zinc oxide in Example 1. From the picture, it can be seen that zinc oxide is precipitated at the bottom of the container;

[0040] Figure 5 This is a graph showing the change in lignin concentration after the reaction detected by a UV-visible spectrophotometer in Example 1. From the graph, it can be seen that as the light reaction time progresses, the lignin structure is destroyed and the content continues to decrease;

[0041] Figure 6 This is a liquid chromatogram of the high-value product vanillic acid obtained in Example 1. From the figure, it can be seen that a high-value product is generated. DETAILED DESCRIPTION

[0042] In order to explain the present invention more clearly, the present invention is further described below in conjunction with embodiments. It should be understood by those skilled in the art that the content specifically described below is illustrative rather than restrictive, and should not be used to limit the scope of protection of the present invention.

[0043] As one aspect of the present invention, a method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion comprises the following steps:

[0044] S1, after mixing lignin powder, reaction solvent and metal oxide dispersion, adding acid or alkali solution to adjust pH;

[0045] S2, transfer into a photocatalytic reactor for dark treatment;

[0046] S3, turn on the light source for irradiation, and perform photoreaction until the depolymerization of lignin is completed;

[0047] S4, obtaining a filter cake by centrifugation and suction filtration, washing it, and re-dispersing it in a reaction solvent by ultrasonication to recover the metal oxide dispersion;

[0048] S5. After rotary evaporation of the filtrate, a solvent is added to redissolve it to obtain the depolymerization product, vanillic acid.

[0049] In certain embodiments of the present invention, in step S1, the lignin is selected from one or more mixtures of lignin sulfonate, alkali lignin, kraft lignin, organic lignin and enzymatically hydrolyzed lignin.

[0050] In certain embodiments of the present invention, in step S1, the reaction solvent is selected from one or more mixtures of purified water, alkali solution, ethanol, acetonitrile, etc.

[0051] In certain embodiments of the present invention, in step S1, the metal oxide dispersion is derived from a mixture of one or more transparent liquid dispersions of zinc oxide, titanium oxide, or copper oxide. The average particle size of the metal oxide particles is between 1-100 nm; more preferably, the average particle size is between 1-20 nm. The preparation process of the metal oxide dispersion refers to the Chinese invention patent publication number CN 109181367 A, the invention name: A method for preparing a transparent zinc oxide liquid dispersion; through this method, the surface properties of the metal oxide particles are regulated, so that they can be uniformly dispersed in the reaction medium to obtain a transparent liquid dispersion; in the present invention, the metal oxide dispersion is used as a photocatalyst, which can greatly reduce the amount of photocatalyst used and increase the yield of the high-value product vanillic acid. For TiO 2 Catalyst, under the condition that the amount of powder catalyst is 50-100 times the mass of lignin, the photocatalytic efficiency of lignin depolymerization is 88-99% for 4-6 hours. The amount of catalyst is large and the reaction time is long, and the overall efficiency is low (Sharma S, Kumar S, Arumugam SM, et al. Promising photocatalytic degradation of lignin over carbon quantum dots decorated TiO 2 nanocomposite in aqueous condition [J]. Applied Catalysis A: General, 2020, 602: 117730.). Also as a ZnO catalyst, under the condition that the powder catalyst dosage is 10 times the mass of lignin, the photocatalytic efficiency of lignin depolymerization is 45% after 1 hour of photocatalysis; the photocatalytic efficiency is 93% after 2 hours of photocatalysis, and no high-value products are generated (Lenka S, Badamali SK. Nanostructured ZnO as an efficient heterogeneous photocatalyst towards degradation of lignin under visible light irradiation [J]. Molecular Catalysis, 2023, 536: 112918.). The present invention uses a ZnO dispersion catalyst, and under the condition that the dosage is 1 times the mass of lignin, the photocatalytic efficiency of lignin depolymerization is 88% after 1 hour of photocatalysis; the photocatalytic efficiency is 95% after 2 hours of photocatalysis, and the yield of high-value product vanillic acid is more than 15%. The mass of photocatalyst required for lignin depolymerization is less, the depolymerization efficiency is higher, and the yield of the high-value product vanillic acid is more prominent. This is related to the fact that the dispersed metal oxides expose more active sites, the photocatalytic intensity is moderate, and the intermediate reactions can be effectively cut off.

[0052] In certain embodiments of the present invention, in step S1, the stoichiometric ratio of lignin powder (mg): reaction solvent (mL): metal oxide dispersion (g) is 1:5-100:0.5-50; preferably, the mass ratio of lignin powder to metal oxide dispersion (calculated as solid) is 1:1-2.

[0053] In certain embodiments of the present invention, in step S1, the acid is selected from one or more mixtures of dilute hydrochloric acid, dilute sulfuric acid, dilute phosphoric acid, dilute nitric acid, and acetic acid; more preferably, in step S1, the acid is dilute hydrochloric acid and dilute sulfuric acid.

[0054] In certain embodiments of the present invention, in step S1, the base is selected from one or more mixtures of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and aqueous ammonia; more preferably, in step S1, the base is sodium hydroxide and potassium hydroxide.

[0055] In certain embodiments of the present invention, in step S1, the pH is 1-13; more preferably, in step S1, the pH is 8-11.

[0056] In some embodiments of the present invention, in step S2, the temperature of the dark treatment is 10-50°C, and the time of the dark treatment is 1-180 min; more preferably, in step S2, the temperature of the dark treatment is 20-25°C, and the time of the dark treatment is 10-60 min.

[0057] In some embodiments of the present invention, in step S2, the dark treatment process is stirred; in step S3, the light reaction process is stirred; the stirring rate is 100-1000r / min; more preferably, the stirring rate is 300-500r / min.

[0058] In certain embodiments of the present invention, in step S3, the light source is one or a mixture of two or more of ultraviolet lamps, LED lamps, sunlight, fluorescent lamps, incandescent lamps, mercury lamps, xenon lamps, and the like.

[0059] In some embodiments of the present invention, in step S3, the temperature of the photoreaction is 10-50°C, and the time of the photoreaction is 30-360 min; more preferably, in step S3, the temperature of the photoreaction is 20-25°C, and the time of the photoreaction is 30-180 min.

[0060] In certain embodiments of the present invention, in step S4, the centrifugal rotation speed is 5000-12000 r / min; more preferably, in step S4, the centrifugal rotation speed is 8000-10000 r / min.

[0061] In certain embodiments of the present invention, in step S4, the metal oxide dispersion is directly used in step S1 for recycling.

[0062] In certain embodiments of the present invention, in step S5, the solvent is one or more of purified water, ethanol, and acetonitrile.

[0063] Example 1

[0064] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion comprises the following steps:

[0065] 1) Mix 5 mL of 1 g / L sodium lignin sulfonate aqueous solution and 0.25 g of zinc oxide aqueous dispersion (2 wt%) (the actual amount of zinc oxide used is 5 mg), add water to make the volume 50 mL, add 0.1 mL of 0.1 M NaOH solution dropwise, adjust the pH to pH = 8, and the reaction solution is light yellow;

[0066] 2) The reaction solution was transferred into a photocatalytic reactor, the reaction vessel was sealed, and the reaction mixture was kept away from light and dark for 60 min under 500 rpm magnetic stirring and 25° C. circulating water protection;

[0067] 3) Turn on the UV light source and perform photoreaction for 2 hours until the lignin reaction solution is completely decolorized;

[0068] 4) The concentration of lignin after the reaction can be detected by UV-visible spectrophotometer;

[0069] 5) separating zinc oxide from the reaction solution at a centrifugal speed of 10000 r / min, further filtering the filtrate to obtain a filter cake, combining the two precipitates, and ultrasonically dispersing them in pure water to obtain a zinc oxide dispersion;

[0070] 6) The filtrate obtained in step 5) is subjected to rotary evaporation and then redissolved in pure water to obtain the depolymerized product vanillic acid. The concentration of the high-value product vanillic acid can be obtained by liquid chromatography.

[0071] After the reaction, the depolymerization efficiency of lignin was measured to be 95%, and the yield of high-value product vanillic acid was 15%.

[0072] Figure 1 This is the molecular formula diagram of sodium lignin sulfonate used in Example 1. From the diagram, it can be seen that the lignin structure is a phenylpropane unit.

[0073] Figure 2 This is a transmission electron microscopy image of the zinc oxide dispersion used in Example 1. It can be seen from the image that the average particle size is about 4 nm.

[0074] Figure 3 This is the XRD pattern of the zinc oxide dispersion used in Example 1.

[0075] Figure 4 This is a physical picture of zinc oxide being separated from the reaction solution by centrifugation in Example 1. From the picture, it can be seen that zinc oxide is precipitated at the bottom of the container.

[0076] Figure 5 This is a graph showing changes in lignin concentration after the reaction detected by a UV-visible spectrophotometer in Example 1. From the graph, it can be seen that as the light reaction time progresses, the lignin structure is destroyed and the content continues to decrease.

[0077] Figure 6 This is a liquid chromatogram of the high-value product vanillic acid obtained in Example 1. From the figure, it can be seen that a high-value product is generated.

[0078] Example 2

[0079] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion comprises the following steps:

[0080] 1) 5 mL of 1 g / L sodium lignin sulfonate aqueous solution and different amounts of zinc oxide aqueous dispersion (2 wt%) were mixed, water was added to make the volume 50 mL, 0.1 mL of 0.1 M NaOH solution was added dropwise, and the pH was adjusted to pH = 8. At this time, the reaction solution was light yellow;

[0081] 2) The reaction solution was transferred into a photocatalytic reactor, the reaction vessel was sealed, and the reaction mixture was kept away from light and dark for 60 min under 500 rpm magnetic stirring and 25° C. circulating water protection;

[0082] 3) Turn on the UV light source and perform photoreaction for 2 hours until the lignin reaction solution is completely decolorized;

[0083] 4) The concentration of lignin after the reaction can be detected by UV-visible spectrophotometer;

[0084] 5) separating zinc oxide from the reaction solution at a centrifugal speed of 10000 r / min, further filtering the filtrate to obtain a filter cake, combining the two precipitates, and ultrasonically dispersing them in pure water to obtain a zinc oxide dispersion;

[0085] 6) The filtrate obtained in step 5) is subjected to rotary evaporation and then redissolved in pure water to obtain the depolymerized product vanillic acid. The concentration of the high-value product vanillic acid can be obtained by liquid chromatography.

[0086] The lignin depolymerization efficiency and vanillic acid yield are shown in Table 1:

[0087] Table 1: Lignin depolymerization efficiency and vanillic acid yield

[0088]

[0089] It can be seen that as the amount of zinc oxide dispersion increases, the lignin depolymerization efficiency gradually increases to reach a threshold value, but when the amount of zinc oxide dispersion exceeds 0.25 g (at this time, the amount of lignin: the amount of catalyst = 1:1), the photoreaction will be violent, and the amount of high-value product vanillic acid will continue to decrease. Therefore, considering the depolymerization efficiency and the yield of high-value product vanillic acid, the amount of zinc oxide dispersion is preferably 0.25-0.50 g (at this time, the amount of lignin: the amount of catalyst = 1:1-2).

[0090] Example 3

[0091] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion comprises the following steps:

[0092] 1) Mix 5 mL of 1 g / L sodium lignin sulfonate aqueous solution and 0.25 g of zinc oxide aqueous dispersion (2 wt%) (the actual amount of zinc oxide is 5 mg), add water to make the volume 50 mL, add 0.1-1 mL of 0.1 M NaOH solution or dilute hydrochloric acid dropwise, adjust the pH to 6-12, and the reaction solution is light yellow;

[0093] 2) The reaction solution was transferred into a photocatalytic reactor, the reaction vessel was sealed, and the reaction mixture was kept away from light and dark for 60 min under 500 rpm magnetic stirring and 25° C. circulating water protection;

[0094] 3) Turn on the UV light source and perform photoreaction for 2 hours until the lignin reaction solution is completely decolorized;

[0095] 4) The concentration of lignin after the reaction can be detected by UV-visible spectrophotometer;

[0096] 5) separating zinc oxide from the reaction solution at a centrifugal speed of 10000 r / min, further filtering the filtrate to obtain a filter cake, combining the two precipitates, and ultrasonically dispersing them in pure water to obtain a zinc oxide dispersion;

[0097] 6) The filtrate obtained in step 5) is subjected to rotary evaporation and then redissolved in pure water to obtain the depolymerized product vanillic acid. The concentration of the high-value product vanillic acid can be obtained by liquid chromatography.

[0098] The lignin depolymerization efficiency and vanillic acid yield are shown in Table 2:

[0099] Table 2: Lignin depolymerization efficiency and vanillic acid yield

[0100] pH Lignin depolymerization efficiency / % Vanillic acid yield / % pH = 6 86.6 5.0 pH = 7 88.8 7.4 pH=8 95.2 15.0 pH=9 95.8 3.8 pH = 10 96.1 2.1 pH=11 99.2 1.5 pH = 12 99.2 0.0

[0101] It can be seen that with the increase of pH, the lignin depolymerization efficiency shows an increasing trend. However, since the high-value product vanillic acid is not alkali-resistant and excessive acid will inhibit its formation, the depolymerization efficiency and the yield of the high-value product vanillic acid are considered, and the preferred pH is 8-11.

[0102] Example 3

[0103] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion comprises the following steps:

[0104] 1) Mix 5 mL of 1 g / L sodium lignin sulfonate aqueous solution and 0.25 g of zinc oxide aqueous dispersion (2 wt%) (the actual amount of zinc oxide used is 5 mg), add water to make the volume 50 mL, add 0.1 mL of 0.1 M NaOH solution dropwise, adjust the pH to pH = 8, and the reaction solution is light yellow;

[0105] 2) The reaction solution was transferred into a photocatalytic reactor, the reaction vessel was sealed, and the reaction mixture was treated in dark and light-proof conditions for different time periods under 500 rpm magnetic stirring and 25° C. circulating water protection;

[0106] 3) Turn on the UV light source and perform photoreaction for 2 hours until the lignin reaction solution is completely decolorized;

[0107] 4) The concentration of lignin after the reaction can be detected by UV-visible spectrophotometer;

[0108] 5) separating zinc oxide from the reaction solution at a centrifugal speed of 10000 r / min, further filtering the filtrate to obtain a filter cake, combining the two precipitates, and ultrasonically dispersing them in pure water to obtain a zinc oxide dispersion;

[0109] 6) The filtrate obtained in step 5) is subjected to rotary evaporation and then redissolved in pure water to obtain the depolymerized product vanillic acid. The concentration of the high-value product vanillic acid can be obtained by liquid chromatography.

[0110] The lignin depolymerization efficiency and vanillic acid yield are shown in Table 3:

[0111] Table 3: Lignin depolymerization efficiency and vanillic acid yield

[0112] Time / min Lignin depolymerization efficiency / % Vanillic acid yield / % 0 80.6 13.9 10 91.2 13.9 20 90.0 14.2 30 92.1 15.1 60 95.2 15.0 90 95.2 15.0 150 95.1 14.8 300 95.2 14.9

[0113] It can be seen that without dark treatment, the efficiency of lignin depolymerization is reduced, because there is no stable adsorption relationship between the surface of zinc oxide dispersion and lignin, which is not conducive to the subsequent light reaction. However, if the dark treatment time is too long, the lignin depolymerization efficiency and vanillic acid yield will remain unchanged, resulting in a waste of time cost. Therefore, considering the depolymerization efficiency and the high-value product vanillic acid yield, the dark treatment time is preferably 10-60 minutes.

[0114] Example 5

[0115] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion comprises the following steps:

[0116] 1) Mix 5 mL of 1 g / L sodium lignin sulfonate aqueous solution and 0.25 g of zinc oxide aqueous dispersion (2 wt%) (the actual amount of zinc oxide used is 5 mg), add water to make the volume 50 mL, dropwise add 0.1 mL of 0.1 M NaOH solution or dilute hydrochloric acid, adjust the pH to pH = 7, and the reaction solution is light yellow;

[0117] 2) The reaction solution was transferred into a photocatalytic reactor, the reaction vessel was sealed, and the reaction mixture was kept away from light and dark for 60 min under 500 rpm magnetic stirring and 25° C. circulating water protection;

[0118] 3) Turn on the ultraviolet light source and perform light reaction for different times until the lignin reaction solution is completely decolorized;

[0119] 4) The concentration of lignin after the reaction can be detected by UV-visible spectrophotometer;

[0120] 5) separating zinc oxide from the reaction solution at a centrifugal speed of 10000 r / min, further filtering the filtrate to obtain a filter cake, combining the two precipitates, and ultrasonically dispersing them in pure water to obtain a zinc oxide dispersion;

[0121] 6) The filtrate obtained in step 5) is subjected to rotary evaporation and then redissolved in pure water to obtain the depolymerized product vanillic acid. The concentration of the high-value product vanillic acid can be obtained by liquid chromatography.

[0122] The lignin depolymerization efficiency and vanillic acid yield are shown in Table 4:

[0123] Table 4: Lignin depolymerization efficiency and vanillic acid yield

[0124] Time / min Lignin depolymerization efficiency / % Vanillic acid yield / % 10 30.12 0.0 30 65.4 5.0 60 88 7.1 90 90.2 8.6 120 95.2 15.0 150 95.6 2.3 300 99.9 0.0 420 99.9 0.0

[0125] It can be seen that the extension of the light reaction time (30-420min) is beneficial to the depolymerization of lignin, but too long a light reaction time (greater than 120min) will cause the decomposition of the high-value product vanillic acid, which is not conducive to the formation of high-value products. Therefore, considering the depolymerization efficiency and the yield of the high-value product vanillic acid, the preferred light reaction time is 30-180min.

[0126] Example 6

[0127] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion comprises the following steps:

[0128] 1) Mix 5 mL of 1 g / L sodium lignin sulfonate aqueous solution and 0.25 g of zinc oxide aqueous dispersion (2 wt%) (the actual amount of zinc oxide used is 5 mg), add water to make the volume 50 mL, add 0.1 mL of 0.1 M NaOH solution dropwise, adjust the pH to pH = 8, and the reaction solution is light yellow;

[0129] 2) The reaction solution was transferred into a photocatalytic reactor, the reaction vessel was sealed, and the reaction mixture was kept away from light and dark for 60 min under 500 rpm magnetic stirring and 25° C. circulating water protection;

[0130] 3) Turn on the UV light source and perform photoreaction for 2 hours until the lignin reaction solution is completely decolorized;

[0131] 4) The concentration of lignin after the reaction can be detected by UV-visible spectrophotometer;

[0132] 5) separating zinc oxide from the reaction solution at a centrifugal speed of 10000 r / min, further filtering the filtrate to obtain a filter cake, combining the two precipitates, and ultrasonically dispersing them in pure water to obtain a zinc oxide dispersion;

[0133] 6) The zinc oxide dispersion separated in step 5) is used for the next photocatalytic lignin depolymerization reaction under the same conditions, and the process is repeated 5 times.

[0134] 7) The filtrate obtained in step 5) is subjected to rotary evaporation and then redissolved in pure water to obtain the depolymerized product vanillic acid. The concentration of the high-value product vanillic acid can be obtained by liquid chromatography.

[0135] The lignin depolymerization efficiency and vanillic acid yield are shown in Table 5:

[0136] Table 5: Lignin depolymerization efficiency and vanillic acid yield

[0137] Cycle times Lignin depolymerization efficiency / % Vanillic acid yield / % 1 95.2 15.0 2 90.4 14.7 3 88.7 13.9 4 85.3 12.0 5 80.9 9.7

[0138] It can be seen that after 5 cycles of experiments, the zinc oxide dispersion can still maintain a depolymerization efficiency of 80.9% and a vanillic acid yield of 9.7%, and has a certain light stability.

[0139] Example 7

[0140] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion is repeated, except that: in step 1), sodium lignin sulfonate is replaced by alkali lignin, and the reaction solvent is replaced by alkali solution. After the reaction, the lignin depolymerization efficiency is 90.1%, and the vanillic acid yield is 7.1%.

[0141] It can be seen that in the presence of alkali solution, zinc oxide dispersion still has an excellent effect on photocatalytic alkali lignin depolymerization, but due to the presence of alkali solution, the yield of high-value product vanillic acid decreases to a certain extent, but still remains at a high level.

[0142] Example 8

[0143] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion is repeated in Example 1, except that in step 1), the reaction solvent is replaced with acetonitrile, and after the reaction, the lignin depolymerization efficiency is 92.7%, and the vanillic acid yield is 14.2%.

[0144] It can be seen that in acetonitrile solvent, zinc oxide dispersion still has an excellent effect on photocatalytic lignin depolymerization, and 14.2% of the high-value product vanillic acid is generated.

[0145] Example 9

[0146] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion is repeated, except that in step 1), sodium lignin sulfonate is replaced by organic lignin, and the reaction solvent is replaced by ethanol. After the reaction, the lignin depolymerization efficiency is 92.7%, and the vanillic acid yield is 14.2%.

[0147] It can be seen that in the ethanol solvent, the zinc oxide dispersion still has an excellent effect on the photocatalytic depolymerization of organic lignin, and 14.2% of the high-value product vanillic acid is generated.

[0148] Example 10

[0149] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion is repeated in Example 1, except that in step 1), sodium lignin sulfonate is replaced with lignin sulfate, and after the reaction, the lignin depolymerization efficiency is 88.7% and the vanillic acid yield is 6.2%.

[0150] This shows that zinc oxide dispersion still has excellent effect on photocatalytic kraft lignin depolymerization.

[0151] Embodiment 11

[0152] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion is repeated in Example 1, except that in step 1), the zinc oxide dispersion is replaced with a titanium oxide dispersion, and after the reaction, the lignin depolymerization efficiency is 95.0%, and the vanillic acid yield is 3.2%.

[0153] It can be seen from this that titanium oxide dispersion has the same effect on photocatalytic lignin depolymerization as zinc oxide dispersion, but because titanium oxide dispersion has a stronger ability to utilize light and the photoreaction is more thorough, the yield of the high-value product vanillic acid is lower.

[0154] Example 12

[0155] A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion is described. Example 1 is repeated, except that in step 1), the zinc oxide dispersion is replaced with a copper oxide dispersion. After the reaction, the lignin depolymerization efficiency is 81.3%, and the vanillic acid yield is 4.1%.

[0156] It can be seen that copper oxide dispersion has the same effect on photocatalytic lignin depolymerization as zinc oxide dispersion, but because copper oxide dispersion has a weaker ability to utilize light, the rate of photoreaction is slower, the efficiency of lignin depolymerization is lower, and the yield of high-value product vanillic acid is lower.

[0157] Comparative Example 1

[0158] A method for preparing vanillic acid by photocatalytic depolymerization of lignin by metal oxide dispersion is repeated in Example 1, except that: in step 2), subsequent operations are performed without stirring, and after the reaction, the lignin depolymerization efficiency is 56.1%, and the vanillic acid yield is 0%.

[0159] It can be seen that the stirring operation can enhance mixing, help the zinc oxide dispersion to have a greater contact with lignin, and increase the possibility of light reaction.

[0160] Comparative Example 2

[0161] A method for preparing vanillic acid by photocatalytic depolymerization of lignin with a metal oxide dispersion is repeated in Example 6, except that: in step 5), the filtrate is not filtered, and only the precipitate after centrifugation is taken. After 5 cycles of reaction, the lignin depolymerization efficiency is 30.1%, and the vanillic acid yield is 2.4%.

[0162] It can be seen from this that the suction filtration operation can extract the zinc oxide dispersion remaining in the filtrate, utilize the zinc oxide dispersion to a greater extent, and improve the recycling rate of the catalyst.

[0163] Comparative Example 3

[0164] A method for preparing vanillic acid by photocatalytic depolymerization of lignin by a metal oxide dispersion is repeated in Example 6, except that: in step 5), the reaction solution is centrifuged at 5000 r / min, and after 5 cycles of reaction, the lignin depolymerization efficiency is 35.5%, and the vanillic acid yield is 4.8%.

[0165] It can be seen that the centrifugal operation helps to precipitate the zinc oxide dispersion after the photoreaction, utilize the zinc oxide dispersion to a greater extent, and improve the recycling rate of the catalyst.

[0166] Comparative Example 4

[0167] A method for preparing vanillic acid by photocatalytic depolymerization of lignin with a metal oxide dispersion, repeating Example 6, except that: in step 7), the filtrate is not subjected to rotary evaporation, and the vanillic acid is directly stored in the filtrate. The vanillic acid solution thus obtained has a short shelf life, and after being placed for 1-3 days and then retested, the vanillic acid content is reduced by 80%.

[0168] It can be seen that the rotary evaporation operation is helpful to purify the reaction solution, making the vanillic acid solution clearer and facilitating the long-term preservation of vanillic acid.

[0169] Obviously, the above embodiments and comparative examples of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the embodiments here. Any obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion. It is characterized in that The following steps are involved: S1, after mixing lignin powder, reaction solvent and metal oxide dispersion, adding acid or alkali solution to adjust pH; S2, transfer into a photocatalytic reactor for dark treatment; S3, turn on the light source for irradiation, and perform photoreaction until the depolymerization of lignin is completed; S4, obtaining a filter cake by centrifugation and suction filtration, washing it, and re-dispersing it in a reaction solvent by ultrasonication to recover the metal oxide dispersion; S5. After rotary evaporation of the filtrate, a solvent is added to redissolve it to obtain the depolymerization product, vanillic acid.

2. The method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion according to claim 1, Features: In step S1, the lignin is selected from one or more mixtures of lignin sulfonate, alkali lignin, kraft lignin, organic lignin and enzymatic lignin.

3. The method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion according to claim 1, Features: In step S1, the reaction solvent is selected from one or more mixtures of purified water, alkali solution, ethanol, acetonitrile, etc.

4. The method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion according to claim 1, Features: In step S1, the metal oxide dispersion is derived from a mixture of one or more transparent liquid dispersions of zinc oxide, titanium oxide, or copper oxide; the average particle size of the metal oxide particles is between 1-100 nm; more preferably, the average particle size is between 1-20 nm.

5. The method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion according to claim 1, Features: In step S1, the stoichiometric ratio of lignin powder (mg): reaction solvent (mL): metal oxide dispersion (g) is 1:5-100:0.5-50; preferably, the mass ratio of lignin powder to metal oxide dispersion (calculated as solid) is 1:1-2.

6. The method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion according to claim 1, Features: In step S1, the acid is selected from one or more mixtures of dilute hydrochloric acid, dilute sulfuric acid, dilute phosphoric acid, dilute nitric acid, and acetic acid; more preferably, in step S1, the acid is dilute hydrochloric acid and dilute sulfuric acid; Preferably, in step S1, the base is selected from one or more mixtures of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, and aqueous ammonia; more preferably, in step S1, the base is sodium hydroxide and potassium hydroxide; Preferably, in step S1, the pH is 1-13; more preferably, in step S1, the pH is 8-11.

7. The method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion according to claim 1, Features: In step S2, the temperature of the dark treatment is 10-50°C, and the time of the dark treatment is 1-180 min; more preferably, in step S2, the temperature of the dark treatment is 20-25°C, and the time of the dark treatment is 10-60 min.

8. The method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion according to claim 1, Features: In step S2, the dark treatment process is stirred; in step S3, the light reaction process is stirred; the stirring rate is 100-1000r / min; more preferably, the stirring rate is 300-500r / min.

9. The method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion according to claim 1, Features: In step S3, the light source is one or a mixture of two or more of ultraviolet lamps, LED lamps, sunlight, fluorescent lamps, incandescent lamps, mercury lamps, xenon lamps, etc. Preferably, in step S3, the temperature of the photoreaction is 10-50°C, and the time of the photoreaction is 30-360 min; more preferably, in step S3, the temperature of the photoreaction is 20-25°C, and the time of the photoreaction is 30-180 min.

10. The method for preparing vanillic acid by photocatalytic depolymerization of lignin using a metal oxide dispersion according to claim 1, Features: In step S4, the centrifugal speed is 5000-12000 r / min; more preferably, in step S4, the centrifugal speed is 8000-10000 r / min; Preferably, in step S4, the metal oxide dispersion is directly used in step S1 for recycling; Preferably, in step S5, the solvent is one or more of purified water, ethanol, and acetonitrile.

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