Method for preparing vanillin by oxidative depolymerization of lignin promoted by alcohol in-situ protection method

Abstract

The invention discloses a method for preparing vanillin through lignin oxidative depolymerization promoted by an alcohol in-situ protection method, and belongs to the field of chemistry. According to the method, a thermocatalysis method is adopted, transition metal salt is used as a catalyst, oxygen is used as an oxidizing agent, and alcohol and an alkali solution are used as additives, so that lignin is converted into vanillin. According to the method, lignin is directly oxidized and depolymerized into vanillin through an alcohol in-situ protection method, and the product yield is up to 24.3% (the theoretical yield is 26.1%). The raw lignin is used as a substrate, and the vanillin is prepared through wood powder one-pot conversion. The catalytic system is mild in condition, the catalyst dosage is small, and high-efficiency and high-selectivity preparation of vanillin can be realized in a relatively short time.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of chemistry, and particularly relates to a method for preparing vanillin by promoting lignin oxidative depolymerization using an alcohol in-situ protection method. BACKGROUND

[0002] Vanillin is a high value-added chemical product, which is widely used in food, cosmetics, pharmaceutical intermediates and other fields. The traditional vanillin production process mainly uses guaiacol and formaldehyde as raw materials to synthesize vanillin through condensation and oxidation. However, this process has problems such as low product purity, environmental pollution, complex products, and the raw materials are heavily dependent on petroleum resources. Therefore, the production of vanillin from renewable resources such as biomass has become a research hotspot. Biomass, including cellulose, lignin and hemicellulose, is the largest renewable carbon source on earth. Among them, lignin is the only macromolecule containing aromatic ring skeleton in nature, and a large amount of aromatic aldehyde compounds can be produced by oxidative cleavage. Based on this, catalytic oxidative depolymerization of lignin to produce vanillin is a method with great application prospect.

[0003] Patent CN 105237371 B mixes lignin with a solid acid catalyst to react under the condition of oxygen pressure 1-3 MPa to prepare vanillin, and the selectivity of vanillin is 50-70%, and the yield is 0.5-6.4%. There are problems such as harsh conditions, low yield, and complex products. The purpose of the present application is to study a method for preparing vanillin by promoting lignin oxidative depolymerization using an alcohol in-situ protection method. This method uses a thermal catalytic method, uses a transition metal salt catalyst to catalyze the cleavage, and efficiently converts lignin into vanillin. This method overcomes the problems of harsh conditions and low yield of lignin in preparing vanillin, realizes high yield of vanillin, greatly improves the resource utilization rate, and has good application prospect. SUMMARY

[0004] The present application provides a method for preparing vanillin by promoting lignin oxidative depolymerization using an alcohol in-situ protection method. The present application also provides a catalyst for preparing vanillin by lignin oxidative depolymerization.

[0005] The technical solution of the present application is as follows:

[0006] A method for preparing vanillin by promoting lignin oxidative depolymerization using an alcohol in-situ protection method, which uses a thermal catalytic method, uses a transition metal salt as a catalyst, uses oxygen as an oxidant, and uses alcohol and alkali solution as additives to convert lignin into vanillin.

[0007] Under the oxygen atmosphere, the alcohol is oxidized into aldehyde, and the aldol condensation occurs between the 1,3-diol structure in the lignin and the alcohol to form a dioxane stable structure, which reduces the influence of internal polycondensation in the lignin depolymerization process. At the same time, the transition metal salt catalyst catalyzes the cleavage, which efficiently converts lignin into vanillin, thereby improving the yield of aromatic aldehyde.

[0008] The alcohol is methanol, ethanol or isopropanol.

[0009] The transition metal salt catalyst includes sulfate (copper sulfate, zinc sulfate, iron sulfate, cobalt sulfate, nickel sulfate, etc.), chloride (copper chloride, zinc chloride, iron chloride, cobalt chloride, nickel chloride, etc.), nitrate (copper nitrate, cobalt nitrate, zinc nitrate, iron nitrate, nickel nitrate, etc.), acetate (copper acetate, zinc acetate, iron acetate, cobalt acetate, nickel acetate, etc.).

[0010] The base is sodium hydroxide, potassium hydroxide or lithium hydroxide.

[0011] The process of aldol condensation forming stable dioxane structure and further producing vanillin is as follows:

[0012]

[0013] The reaction temperature is 120-160ºC; the reaction time is 0.5-3 h; the oxygen partial pressure is 0.2-1.2 MPa; the reaction is continuously stirred; the concentration of the base solution is 1-4 mol / L. -1 The mass ratio of the transition metal salt catalyst to the original lignocellulose is 0.1-30 wt%, preferably 0.4-2.4%. The mass ratio of the base to the original lignocellulose is 8-32%. The volume ratio of the alcohol to the base solution is 1:1-4:1.

[0014] The present application has the following advantages: the present application directly oxidizes and depolymerizes lignin into vanillin by in-situ alcohol protection method, and the product yield is as high as 24.3% (the theoretical yield is 26.1%). The present application realizes one-pot conversion of wood powder to prepare vanillin by using original lignin as the substrate. The catalytic system of the present application has mild conditions, and the catalyst has less dosage, and can realize high efficiency and high selectivity to prepare vanillin in a short time. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 is a gas chromatogram.

[0016] Figure 2 is a mass spectrum. DETAILED DESCRIPTION

[0017] The present application will be described below by specific examples, but the present application is not limited to these examples:

[0018] Comparative Example 1 and Examples 2-24: 0.5 g pine wood, 10 mg copper nitrate, 1 mL NaOH (aq) and a certain volume of alcohol were added to a 20 mL reaction vessel, the reactor was closed, a certain pressure of oxygen was introduced, and the temperature was raised to 140°C under stirring. After reaction at this temperature for a certain time, the sample was cooled, neutralized, and extracted with ethyl acetate. The obtained sample was qualitatively analyzed by gas chromatography-mass spectrometry (GC-MS) and quantitatively analyzed by gas chromatography (GC). The results are shown in Table 1.

[0019] Table 1: Optimization of reaction conditions

[0020]

[0021] From the results in Table 1, it can be seen that the optimal conditions for preparing vanillin by oxidizing and depolymerizing pine lignin using copper nitrate as a catalyst are: NaOH (aq) is 2 mol L -1 , isopropanol / NaOH (aq) is 1:1, O2 pressure is 1 MPa, reaction temperature is 140°C, reaction time is 2 h, and the yield of vanillin is 24.3%.

[0022] Examples 25-48: 0.5 g pine wood, a certain amount of catalyst, 1 mL (2 mol L -1 ) NaOH (aq) and 1 mL isopropanol were added to a 20 mL reaction vessel, the reactor was closed, 1 MPa O2 was introduced, and the temperature was raised to 140°C under stirring. The reaction time was 2 h, and the sample was cooled, neutralized, centrifuged, and extracted. The obtained sample was qualitatively analyzed by gas chromatography-mass spectrometry (GC-MS) and quantitatively analyzed by gas chromatography (GC). The results are shown in Table 2.

[0023] Table 2: Effect of different catalysts on catalytic reaction

[0024]

[0025] From the results in Table 2, it can be seen that copper-based metal salts all have good catalytic activity, among which copper nitrate has the best catalytic performance.

[0026] Examples 49-59: 0.5 g of different reaction substrates, 10 mg of copper nitrate, 1 mL (2 mol L -1 ) NaOH (aq) and 1 mL isopropanol were added to a 20 mL reaction vessel, the reactor was closed, 1 MPa O2 was introduced, and the temperature was raised to 140°C under stirring. The reaction time was a certain time. The obtained sample was qualitatively analyzed by gas chromatography-mass spectrometry (GC-MS) and quantitatively analyzed by gas chromatography (GC). The results are shown in Table 3.

[0027] Table 3: Catalytic reaction effect of different reaction substrates and their amounts

[0028]

[0029] From the results in Table 3, it can be seen that the 11 reaction substrates tested all have high yields.

[0030] The product was analyzed by gas chromatography-mass spectrometry (GC-MS) and the retention time was compared with that of a standard sample; the quantitative analysis was performed by internal standard gas chromatography (GC).

[0031] Theoretical yield = (mass of vanillin generated by oxidation of nitrobenzene / mass of lignin in raw lignin) x 100%

[0032] Actual yield = (mass of vanillin generated by reaction / mass of raw lignin) x 100%.

Claims

1. A method for preparing vanillin by oxidative depolymerization of lignin promoted by an alcohol in-situ protection method, characterized in that: The method adopts a thermal catalytic method, uses a transition metal salt as a catalyst, oxygen as an oxidant, and alcohol and alkaline solutions as additives to convert lignin into vanillin.

2. The method for preparing vanillin by oxidative depolymerization of lignin promoted by an alcohol in-situ protection method according to claim 1, characterized in that: In an oxygen atmosphere, alcohol is oxidized to aldehyde and undergoes aldol condensation with the 1,3-diol structure in lignin to form a stable dioxane structure, reducing the impact of internal condensation during lignin depolymerization. At the same time, the transition metal salt catalyst catalyzes cracking, efficiently converting lignin into vanillin, thereby increasing the yield of aromatic aldehydes.

3. The method for preparing vanillin by oxidative depolymerization of lignin promoted by an alcohol in-situ protection method according to claim 1, characterized in that: The alcohol is methanol, ethanol or isopropanol.

4. The method for preparing vanillin by oxidative depolymerization of lignin promoted by an alcohol in-situ protection method according to claim 1, characterized in that: The transition metal salt catalyst includes sulfates, chlorides, nitrates or acetates.

5. The method for preparing vanillin by oxidative depolymerization of lignin promoted by an alcohol in-situ protection method according to claim 4, characterized in that: Sulfates include copper sulfate, zinc sulfate, iron sulfate, cobalt sulfate, and nickel sulfate; chlorides include copper chloride, zinc chloride, iron chloride, cobalt chloride, and nickel chloride; nitrates include copper nitrate, cobalt nitrate, zinc nitrate, iron nitrate, and nickel nitrate; acetates include copper acetate, zinc acetate, iron acetate, cobalt acetate, and nickel acetate.

6. The method for preparing vanillin by oxidative depolymerization of lignin promoted by an alcohol in-situ protection method according to claim 1, characterized in that: The alkali is sodium hydroxide, potassium hydroxide or lithium hydroxide.

7. The method for preparing vanillin by oxidative depolymerization of lignin promoted by an alcohol in-situ protection method according to claim 1, characterized in that: The process of aldol condensation to form a stable dioxane structure and further produce vanillin is as follows: 。 8. The method for preparing vanillin by oxidative depolymerization of lignin promoted by an alcohol in-situ protection method according to claim 1, characterized in that: The reaction temperature is 120-160°C; the reaction time is 0.5-3 h; the oxygen partial pressure is 0.2-1.2 MPa; and stirring is continued during the reaction.

9. The method for preparing vanillin by oxidative depolymerization of lignin promoted by in-situ alcohol protection according to claim 1, characterized in that: The concentration of the alkali solution is 1-4 molL-1; the mass ratio of the alkali to the native wood cellulose is 8-32%; and the volume ratio of the alcohol to the alkali solution is 1:1-4:

1.

10. The method for preparing vanillin by oxidative depolymerization of lignin promoted by in-situ alcohol protection according to claim 1, characterized in that: The mass ratio of the used transition metal salt catalyst to the native wood cellulose is: 0.1-30wt%.

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