Method for preparing levulinic acid through synchronous separation and conversion of lignocellulose biomass

Abstract

The invention discloses a method for preparing levulinic acid through synchronous separation and conversion of lignocellulose biomass, which is characterized in that levulinic acid is synthesized in a microwave reactor or a high-pressure reaction kettle by taking lignocellulose biomass as a raw material, water or a water-containing organic solvent as a solvent, formaldehyde as an accelerant and AlCl3. 6H2O as a catalyst. The lignocellulose biomass is selected from straw, pine, moso bamboo, willow, corn straw, corncob, wheat straw and sorghum straw. And the organic solvent is selected from tetrahydrofuran, 1, 4-dioxane and gamma-valerolactone. According to the method, chemical bonds between cellulose / hemicellulose and lignin are destroyed through acetalation reaction between formaldehyde and cellulose / hemicellulose in lignocellulose biomass, separation of cellulose / hemicellulose components and lignin components is achieved, synchronous depolymerization and directional conversion of the cellulose / hemicellulose components and the lignin components are promoted, the reaction steps are few, energy consumption is low, and the method is suitable for industrial production. No solid humin is generated, the utilization rate of biological carbon atoms is high, and sugar components in the raw materials are utilized to the maximum extent.

Description

Technical Field

[0001] The invention belongs to the field of chemistry, and in particular relates to a method for preparing levulinic acid by synchronous separation and conversion of lignocellulose biomass. Background Art

[0002] Levulinic acid is an important biomass-based platform chemical that can be used to synthesize a variety of high-value-added fine chemicals and liquid fuels, the latter of which is expected to replace existing petroleum-based chemicals and fuels, thereby alleviating the dual pressures from resources and the environment currently faced by human society. In recent years, the development of methods and technologies for synthesizing levulinic acid from biomass resources has received increasing attention.

[0003] Lignocellulosic biomass, such as agricultural and forestry waste, is the most abundant and cheap biomass resource on earth. Its main components are 30-50wt.% cellulose, 20-40wt.% hemicellulose and 10-20wt.% lignin. In theory, both cellulose and hemicellulose can be converted into levulinic acid: cellulose is hydrolyzed to obtain glucose, glucose is dehydrated to 5-hydroxymethylfurfural, and the latter can be hydrated and decomposed to obtain levulinic acid; hemicellulose is hydrolyzed to obtain xylose, xylose is dehydrated to furfural, furfural is hydrogenated to furfuryl alcohol, and the latter can be hydrated and decomposed to obtain levulinic acid. In addition, furfural can also be hydroxymethylated to generate 5-hydroxymethylfurfural and then hydrated and decomposed to obtain levulinic acid. From the current research, high levulinic acid yields can be obtained directly using cellulose as raw material, while the hemicellulose route has received little attention. However, both the cellulose route and the hemicellulose route require the separation of cellulose and hemicellulose components from lignocellulosic biomass first. However, since the lignin component is wrapped outside the cellulose and hemicellulose structures through phenyl ether bonds, phenyl glycosidic bonds, and acetal bonds, the difficulty of separating cellulose and hemicellulose is increased. Not only does it require a large amount of mixed reagents containing alkali, sulfate, and sulfide to separate cellulose and hemicellulose, but it also produces a large amount of lignin waste liquid, which causes great environmental pollution, high separation energy consumption, and serious carbon loss of biomass raw materials. Therefore, the development of a method for preparing levulinic acid directly from lignocellulosic biomass by synchronous separation and conversion is particularly necessary for reducing environmental pollution, reducing production energy consumption, and improving the carbon utilization rate of biomass raw materials. It is in line with the production concept of green chemistry and chemical industry, but it is also very challenging due to the complexity of lignocellulose structure.

[0004] The "Biofine" process, which was industrialized in 2005, is a typical example of preparing levulinic acid using lignocellulosic biomass as raw materials. This process uses willow, poplar, Miscanthus, switchgrass and other biomass as raw materials, water as solvent, sulfuric acid as catalyst, and reacts in the first reactor at a temperature of 210-220°C for 12 seconds. The cellulose and hemicellulose components in the biomass are hydrolyzed into glucose and xylose respectively; the reaction mixture then enters the second reactor at a temperature of 190-200°C for 20 minutes, and glucose is further converted into levulinic acid and formic acid, and xylose is dehydrated into furfural. The yields of levulinic acid and furfural are 70.0 mol% and 50.0 mol% respectively, accompanied by a large amount of solid humin and undissolved lignin.

[0005] In order to improve the synthesis efficiency of levulinic acid, Alonso et al. reported a method for converting rice straw into levulinic acid in Energy & Environmental Science 6 (2013) 76-80. The rice straw was reacted at 170°C for 1 hour using γ-valerolactone and water as a mixed solvent and sulfuric acid as a catalyst. The yield of levulinic acid was 61.0 mol% and the yield of furfural was 56.0 mol%. In this method, the mixed solvent of γ-valerolactone and water is conducive to the dissolution of lignin, thereby promoting the contact of the catalyst and solvent with the cellulose and hemicellulose components, reducing the reaction temperature, and the residual lignin component can be precipitated and separated by adding water.

[0006] Qing et al. reported a method for converting corn cobs, corn stalks, rice straw, bagasse and bamboo shoots into levulinic acid in Bioresource Technology 260 (2018) 150-156. The biomass was reacted at 193 ° C for 17 minutes with water as solvent and SnCl4 as catalyst. The yield of levulinic acid was 58.9-76.6 mol%, and the yield of furfural was 8.1-26.7 mol%. This method can efficiently convert the cellulose component in lignocellulosic biomass into levulinic acid in a short time, but the hemicellulose component is mainly converted into solid humin.

[0007] According to existing research reports, the existing methods for preparing levulinic acid by separating and converting lignocellulosic biomass are basically centered around converting the cellulose component in the raw material into levulinic acid and the hemicellulose component into furfural. However, there are usually the following problems: (1) The yield of cellulose to levulinic acid is high, but the yield of hemicellulose to furfural is low. The further conversion of furfural into levulinic acid requires additional reaction steps, which increases production costs; (2) Humin is generated seriously and the utilization rate of raw material carbon atoms is low; (3) Solid humin is difficult to separate from lignin components, which hinders the subsequent value-added utilization of lignin components. Considering the theoretical feasibility of converting hemicellulose components into levulinic acid, if a method is developed to separate the cellulose / hemicellulose components from the lignin components in the biomass raw material in one step and simultaneously convert cellulose / hemicellulose into levulinic acid, it can further improve the utilization rate of biomass raw materials, reduce production energy consumption, and reduce environmental pollution, which is of great significance for achieving efficient utilization of all components of lignocellulosic biomass. Summary of the invention

[0008] The present invention aims to provide a method for preparing levulinic acid by synchronous separation and conversion of lignocellulosic biomass.

[0009] Summary of the invention: A method for preparing levulinic acid by synchronous separation and conversion of lignocellulosic biomass, characterized in that lignocellulosic biomass is used as raw material, water or water-containing organic solvent is used as solvent, formaldehyde is used as promoter, AlCl3·6H2O is used as catalyst, and levulinic acid is synthesized in a microwave reactor or a high-pressure reactor, the lignocellulosic biomass is rice straw, pine, bamboo, willow, corn stalk, corn cob, wheat straw, sorghum straw, the concentration of lignocellulosic biomass is 0.5-10.0wt.%, the organic solvent is tetrahydrofuran, 1,4-dioxane, γ-valerolactone, the volume ratio of organic solvent to water is 1:1, the mass ratio of formaldehyde to lignocellulose is 0.2:1-1.7:1, the concentration of AlCl3·6H2O is 50.0-200.0mmol / L, the reaction temperature is 165-195°C, and the reaction time is 5-120 minutes.

[0010] The lignocellulosic biomass is rice straw, bamboo, willow, or sorghum straw.

[0011] The concentration of the lignocellulosic biomass is 0.5-6.0 wt.%.

[0012] The solvent is water.

[0013] The mass ratio of the formaldehyde to the lignocellulosic biomass is 0.7:1 to 1.7:1.

[0014] The AlCl3·6H2O concentration is 95.0-175.0 mmol / L.

[0015] The reaction temperature is 180-190°C.

[0016] The reaction time is 45 to 90 minutes.

[0017] Compared with the prior art, the present invention has the following characteristics and advantages:

[0018] (1) The acetalization reaction between formaldehyde and the hydroxyl groups in the cellulose and hemicellulose structures in lignocellulosic biomass is used to destroy the complex chemical bonds between lignin-cellulose and lignin-hemicellulose, thereby achieving the separation of cellulose and hemicellulose components from lignin components;

[0019] (2) Utilizing the interaction between formaldehyde and cellulose and hemicellulose, the synchronous depolymerization and directional conversion of cellulose and hemicellulose are promoted, so that the lignin component can be retained in a solid form, which is beneficial for subsequent value-added utilization;

[0020] (3) The hydroxymethylation reaction between formaldehyde and furfural opens up a route for synthesizing levulinic acid from hemicellulose via hydrolysis, dehydration, hydroxymethylation and hydration decomposition, thereby achieving the simultaneous conversion of cellulose and hemicellulose in lignocellulosic biomass into levulinic acid, reducing the number of reaction steps and lowering production energy consumption;

[0021] (4) No solid humin is produced, the yield of levulinic acid is high, and the sugar components in the biomass raw materials are maximized. DETAILED DESCRIPTION

[0022] Embodiment 1:

[0023] A magnetic stirrer, 750.0 mg (5.0 wt.%) straw, 13.5 mL water, 362.1 mg AlCl3·6H2O (100.0 mmol / L), and 1.5 mL formaldehyde solution (concentration of 37.0 wt.%, 600.0 mg formaldehyde) were added to a 30 mL microwave reaction tube in sequence, and the reaction tube was sealed with a reaction cover, and microwave heating was performed at 185° C. for 75 minutes. After the reaction was completed, the reaction tube was purged with compressed air, and after the reaction tube was cooled to room temperature, the solid residue was separated by centrifugation, and water was added to the reaction solution and the volume was fixed. After a certain volume of the reaction solution was diluted, the product was quantitatively analyzed by high performance liquid chromatography, and the yield of levulinic acid was 90.9 mol% based on the molar number of the structural units of cellulose and hemicellulose in the lignocellulose.

[0024] Embodiment 2-7:

[0025] According to Example 1, the reaction was carried out using rice straw of different concentrations. The reaction conditions and results are shown in Table 1.

[0026] Table 1

[0027] Example No. Rice straw concentration (wt.%) Levulinic acid yield (mol%) 2 0.5 88.9 3 2.0 89.1 4 4.0 89.0 5 6.0 89.8 6 7.5 58.7 7 10.0 39.4

[0028] Embodiment 8-15:

[0029] According to Example 1, formaldehyde and straw in different mass ratios were used for reaction. The reaction conditions and results are shown in Table 2.

[0030] Table 2

[0031]

[0032]

[0033] Embodiment 16-24:

[0034] According to Example 1, different concentrations of AlCl3·6H2O were used for the reaction. The reaction conditions and results are shown in Table 3.

[0035] Table 3

[0036] Example No. <![CDATA[Concentration of AlCl3·6H2O (mmol / L)]]> Levulinic acid yield (mol%) 16 50.0 36.5 17 75.0 46.1 18 80.0 60.4 19 90.0 66.4 20 95.0 79.7 21 125.0 93.9 22 150.0 97.1 23 175.0 97.6 24 200.0 64.3

[0037] Embodiment 25-27:

[0038] According to Example 1, different solvents were used for the reaction. The reaction conditions and results are shown in Table 4.

[0039] Table 4

[0040] Example No. Solvents Levulinic acid yield (mol%) 25 Tetrahydrofuran-water 45.7 26 1,4-Dioxane-Water 61.6 27 γ-Valerolactone-water 75.2

[0041] Embodiment 28-33:

[0042] According to Example 1, the reactions were carried out at different reaction temperatures. The reaction conditions and results are shown in Table 5.

[0043] Table 5

[0044] Example No. Reaction temperature (℃) Levulinic acid yield (mol%) 28 165 56.8 29 170 56.7 30 175 65.2 31 180 81.7 32 190 75.6 33 195 66.8

[0045] Examples 34-41:

[0046] According to Example 1, the reaction was carried out using different reaction times. The reaction conditions and results are shown in Table 6.

[0047] Table 6

[0048]

[0049]

[0050] Embodiment 42-48:

[0051] According to Example 1, different lignocellulosic biomasses were used as raw materials for the reaction. The reaction conditions and results are shown in Table 7.

[0052] Table 7

[0053] Example No. Lignocellulosic biomass Levulinic acid yield (mol%) 42 Pine 59.1 43 bamboo 80.7 44 Willow 86.2 45 Corn stalks 64.0 46 Corn cob 60.1 47 Wheat straw 66.9 48 Sorghum straw 75.7

[0054] Embodiment 49:

[0055] A magnetic stirrer, 1500.0 mg (5.0 wt.%) straw, 27.0 mL water, 724.2 mg AlCl3·6H2O (100.0 mmol / L), and 3.0 mL formaldehyde solution (concentration of 37.0 wt.%) were added to a 50 mL autoclave in sequence, and the mixture was heated to 185° C. and reacted for 60 minutes. After the reaction was completed, the autoclave was purged with air, and after the autoclave was cooled to room temperature, solid residues were separated by centrifugation, and water was added to the reaction solution to make the volume constant. A certain volume of the reaction solution was diluted and the product was quantitatively analyzed by high performance liquid chromatography. The yield of levulinic acid was 78.8 mol% based on the molar number of the structural units of cellulose and hemicellulose in the lignocellulose.

Claims

1. A method for preparing levulinic acid by synchronous separation and conversion of lignocellulosic biomass, characterized in that: The method uses lignocellulosic biomass as a raw material, water or an organic solvent containing water as a solvent, formaldehyde as a promoter, and AlCl3·6H2O as a catalyst to synthesize levulinic acid in a microwave reactor or a high-pressure reactor. The lignocellulosic biomass is rice straw, pine, bamboo, willow, corn stalk, corn cob, wheat stalk, and sorghum stalk. The concentration of the lignocellulosic biomass is 0.5-10.0wt.%, the organic solvent is tetrahydrofuran, 1,4-dioxane, and gamma-valerolactone, the volume ratio of the organic solvent to water is 1:1, the mass ratio of formaldehyde to lignocellulose is 0.2:1-1.7:1, the concentration of AlCl3·6H2O is 50.0-200.0mmol / L, the reaction temperature is 165-195°C, and the reaction time is 5-120 minutes.

2. The method according to claim 1, characterized in that: The lignocellulosic biomass is rice straw, bamboo, willow and sorghum straw.

3. The method according to claim 1, characterized in that: The concentration of the lignocellulosic biomass is 0.5-6.0wt.%.

4. The method according to claim 1, characterized in that: The solvent is water.

5. The method according to claim 1, characterized in that: The mass ratio of formaldehyde to lignocellulosic biomass is 0.7:1 to 1.7:

1.

6. The method according to claim 1, characterized in that: The concentration of AlCl3·6H2O is 95.0-175.0 mmol / L.

7. The method according to claim 1, characterized in that: The reaction temperature is 180-190°C.

8. The method according to claim 1, characterized in that: The reaction time is 45 to 90 minutes.

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