A process for the preparation of levulinic acid from cellulose promoted by formaldehyde

Abstract

A method for preparing levulinic acid from cellulose promoted by formaldehyde, which uses microcrystalline cellulose as raw material, organic solvent and water as mixed solvent, AlCl3·6H2O as catalyst, and formaldehyde as promoter to synthesize levulinic acid in a microwave reactor or a high-pressure reactor. The volume ratio of 2-methyltetrahydrofuran to water is 1:1-6:1. The method promotes the hydrolysis and subsequent conversion of cellulose by acetalization reaction between formaldehyde and the hydroxyl groups of glucose units in the structure of cellulose, has high efficiency of levulinic acid synthesis, generates less humin, has low boiling point of solvent, low energy consumption of separation, and is cheap and easy to obtain and separate.

Description

Technical Field

[0001] The invention belongs to the field of chemistry, and particularly relates to a method for preparing levulinic acid by promoting cellulose conversion through formaldehyde. Background Art

[0002] Levulinic acid is an important biomass-based platform chemical, serving as a chemical feedstock for the production of a variety of high-value-added chemicals and liquid fuels. Research into efficient methods for converting biomass resources into levulinic acid plays a crucial role in developing a new biomass economy and contributing to my country's "dual carbon goals." Consequently, this research has garnered significant attention from both academia and industry.

[0003] Cellulose is the main component of lignocellulosic biomass. It is a natural polymer composed of glucose structural units connected by β-1,4-glycosidic bonds. It can be converted into levulinic acid through hydrolysis, dehydration and hydration decomposition reactions. Therefore, it is an ideal raw material for the synthesis of levulinic acid. In recent years, there have been a lot of reports on the research on the preparation of levulinic acid by converting cellulose. However, due to the complex intramolecular and intermolecular hydrogen bond interactions in the cellulose structure, its hydrolysis and subsequent conversion are difficult. In the literature that has been reported, the concentration of cellulose is usually low (<10.0wt.%), resulting in low actual synthesis efficiency of levulinic acid, which does not meet the requirements of green chemistry and chemical industry. The development of technology that can efficiently convert high-concentration cellulose into levulinic acid is extremely important for the effective utilization of biomass resources.

[0004] Qin et al. reported in RSC Advance 45 (2016) 39131-39136 a method for producing levulinic acid from high-concentration cellulose. Using 12.0 wt.% cellulose as the raw material, water as the solvent, 1.5 mol / L phosphoric acid as the catalyst, and 47.0 wt.% sodium chloride as the auxiliary agent, the cellulose was heated at 170°C for 1 hour under microwave conditions, resulting in a 67.3 mol% levulinic acid yield. This method utilizes high sodium chloride concentration to disrupt the inherent hydrogen bonds in the cellulose structure, promoting cellulose hydrolysis. Under the catalytic action of phosphoric acid, the resulting oligosaccharides and glucose are further converted into levulinic acid. Since the carbon balance of the reaction is 68.9 mol%, 31.1 mol% of the biocarbon atoms are converted to humins. The production of this byproduct not only wastes the biocarbon raw material but also easily causes equipment clogging and difficulty in product separation. Furthermore, the use of high sodium chloride and acidic phosphorus concentrations increases the risk of equipment corrosion.

[0005] Wang et al. used 10.0 wt.% cellulose as raw material and sulfolane and water as mixed solvent (V环丁砜 :V 水 =9:1), sulfonated humin as the acid catalyst, and cellulose reacted at 180°C for 2 hours. The maximum yield of levulinic acid reached 66.0 mol%, while 36.0 wt.% of humin and a small amount of furfural (13.5 mol%) were produced. This method not only avoids the use of corrosive sodium chloride and phosphoric acid but also utilizes the reaction byproduct humin as a catalyst precursor, increasing the byproduct's value. However, this method uses a large amount of high-boiling sulfolane, and the addition of large amounts of water during the humin separation process reduces the actual product concentration. Both of these factors significantly increase the energy consumption of product separation and purification.

[0006] Zhang et al. reported a method for preparing levulinic acid by converting cellulose in a biphasic solvent consisting of methyl isobutyl ketone and water in Renewable Energy 141 (2019) 802-813. Using choline cation-modified phosphotungstic titanic heteropoly acid as a catalyst, when V 甲基异丁基酮 :V 水 When the ratio is 10:1, the cellulose concentration in the aqueous phase is 20.0 wt.%, and after reaction at 130 °C for 8 h, the yield of levulinic acid is as high as 76.1 mol%. 甲基异丁基酮 :V 水 When the ratio was reduced to 5:1 and 1:1, the yield of levulinic acid dropped to 67.2 mol% and 61.5 mol%, respectively. 甲基异丁基酮 :V 水 It is beneficial to improve the yield of levulinic acid and inhibit the formation of humin, but there is still the problem of low product concentration in the organic phase, and the energy consumption of product separation / purification is still high.

[0007] Zhu Liangfang et al. reported in Chinese invention patent ZL 202210142280.4 a method for preparing levulinic acid by synergistically promoting cellulose conversion with alkyl ammonium halide and sodium halide. This method uses a biphasic solvent consisting of 2-methyltetrahydrofuran and water as the reaction solvent, benzenesulfonic acid as the catalyst, and uses low concentrations of sodium chloride (1.6wt.%) and alkyl ammonium halide (91.3mmol / L) to synergistically promote cellulose hydrolysis and stabilize levulinic acid, thereby greatly inhibiting the formation of humin. When V 2-甲基四氢呋喃 :V 水 =2:1, 15.0 wt.% wheat straw cellulose was reacted at 180°C for 2 h, and the yield of levulinic acid reached 81.0 mol%. This method uses a low amount of organic phase and reduces equipment corrosion. However, the alkyl ammonium halide, which has surfactant properties, can partially dissolve in the organic phase, reducing the recyclability of the reaction system.

[0008] At present, the preparation of levulinic acid by converting high-concentration cellulose still faces the following problems: (1) The efficiency of levulinic acid synthesis is not ideal, and a large amount of humin is still produced, resulting in high energy consumption for product separation / purification; (2) The use of a large amount of high-boiling-point organic solvents further increases the energy consumption for product separation / purification; (3) The recyclability of the reaction system needs to be improved. Summary of the Invention

[0009] The present invention aims to provide a method for preparing levulinic acid by promoting cellulose conversion through formaldehyde.

[0010] Summary of the invention: A method for preparing levulinic acid by promoting the conversion of cellulose by formaldehyde, characterized in that microcrystalline cellulose is used as a raw material, an organic solvent and water are used as a mixed solvent, AlCl3·6H2O is used as a catalyst, and formaldehyde is used as a promoter. Levulinic acid is synthesized in a microwave reactor or a high-pressure reactor, the concentration of cellulose in water is 1.0-30.0 wt.%, the organic solvent is 2-methyltetrahydrofuran, tetrahydrofuran, 1,4-dioxane, γ-valerolactone, or methyl isobutyl ketone, the volume ratio of the organic solvent to water is 1:2-10:1, the molar ratio of formaldehyde to the glucose structural unit in the cellulose is 0.7:1-7:1, the concentration of AlCl3·6H2O is 150.0-700.0 mmol / L, the reaction temperature is 160-195°C, and the reaction time is 5-120 min.

[0011] The concentration of the cellulose in water is 2.5 to 10.0 wt.%.

[0012] The volume ratio of the organic solvent to water is 1:1 to 6:1.

[0013] The molar ratio of the formaldehyde to the glucose structural unit in the cellulose is 2.9:1 to 5.0:1.

[0014] The concentration of AlCl3·6H2O is 400-550 mmol / L.

[0015] The reaction temperature is 175-180°C.

[0016] The reaction time is 30 to 60 minutes.

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

[0018] (1) Formaldehyde reacts with the hydroxyl groups of the glucose structural units in cellulose to promote cellulose hydrolysis and subsequent conversion, resulting in high efficiency in the synthesis of levulinic acid;

[0019] (2) Humin is generated in small amounts, the solvent has a low boiling point, and the separation energy consumption is low;

[0020] (3) Formaldehyde is cheap, readily available and extremely easy to separate.

[0021] Formaldehyde is a highly reactive reagent that can destroy the original hydrogen bond structure in the cellulose structure by undergoing acetalization reaction with the hydroxyl groups of the glucose structural units in cellulose, thereby promoting cellulose hydrolysis and stabilizing the hydrolysis product glucose. Combined with the use of low-boiling point solvents and low-corrosive acid catalysts, it effectively inhibits the formation of humin, improves the synthesis efficiency of levulinic acid, reduces the energy consumption of separation / purification, and realizes the efficient conversion of high-concentration cellulose to produce levulinic acid. DETAILED DESCRIPTION

[0022] Example 1:

[0023] A magnetic stirrer, 750.0 mg (15.0 wt.%) microcrystalline cellulose, 3.5 mL of water, 10 mL of 2-methyltetrahydrofuran, 450.0 mmol / L AlCl3·6H2O, and 1.5 mL of formaldehyde solution (37.0 wt.%) were added to a 30 mL microwave reaction tube. The tube was sealed with a reaction cap and microwave-heated to 185°C for 45 min. After the reaction, the tube was purged with compressed air. After cooling to room temperature, the solid residue was separated by centrifugation. Ethanol was added to the reaction solution and the volume was fixed. 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 87.3 mol%.

[0024] Example 2-8:

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

[0026] Table 1

[0027] Example No. Cellulose / water mass ratio (wt.%) Levulinic acid yield (mol%) 2 1.0 80.2 3 2.5 88.4 4 5.0 88.4 5 10.0 88.8 6 20.0 67.6 7 25.0 45.7 8 30.0 42.4

[0028] Examples 9-12:

[0029] The reaction was carried out according to the method of Example 1 using different reaction solvents (the volume ratio of organic solvent to water was 2:1). The reaction conditions and results are shown in Table 2.

[0030] Table 2

[0031]

[0032]

[0033] Examples 13-18:

[0034] According to the method of Example 1, the reaction was carried out using 2-methyltetrahydrofuran / water mixed solvents in different volume ratios. The reaction conditions and results are shown in Table 3.

[0035] Table 3

[0036]

[0037] Examples 19-26:

[0038] According to the method of Example 1, the reaction was carried out using different molar ratios of formaldehyde / glucose structural units in cellulose. The reaction conditions and results are shown in Table 4.

[0039] Table 4

[0040]

[0041] Examples 27-38:

[0042] According to the method of Example 1, the reaction was carried out using different concentrations of AlCl3·6H2O. The reaction conditions and results are shown in Table 5.

[0043] Table 5

[0044]

[0045]

[0046] Examples 39-45:

[0047] The reaction was carried out according to the method of Example 1 at different reaction temperatures. The reaction conditions and results are shown in Table 6.

[0048] Table 6

[0049] Example No. Reaction temperature (℃) Levulinic acid yield (mol%) 39 160 17.1 40 165 33.2 41 170 48.5 42 175 75.6 43 180 85.4 44 190 64.4 45 195 51.5

[0050] Examples 46-52:

[0051] The reaction was carried out according to the method of Example 1 with different reaction times. The reaction conditions and results are shown in Table 7.

[0052] Table 7

[0053] Example No. Reaction time (min) Levulinic acid yield (mol%) 46 5 36.1 47 10 48.9 48 15 56.1 49 30 67.2 50 60 73.9 51 90 65.9 52 120 58.2

[0054] Example 53:

[0055] A magnetic stirrer, 1500.0 mg (15.0 wt.%) of microcrystalline cellulose, 7.0 mL of water, 20 mL of 2-methyltetrahydrofuran, 450.0 mmol / L AlCl3·6H2O, and 3.0 mL of aqueous formaldehyde (37.0 wt.%) were sequentially added to a 100 mL autoclave. The mixture was purged with nitrogen and pressurized to 1.5 MPa. The mixture was then heated to 185°C and allowed to react for 30 min. After the reaction, the autoclave was purged with air. After the autoclave was cooled to room temperature, the solid residue was separated by centrifugation. Ethanol was added to the reaction solution, which was mixed and then fixed to volume. 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 61.3 mol%.

Claims

1. A method for preparing levulinic acid by promoting cellulose conversion by formaldehyde, characterized in that: Levulinic acid is synthesized in a microwave reactor or a high-pressure reactor using microcrystalline cellulose as a raw material, an organic solvent and water as a mixed solvent, AlCl3·6H2O as a catalyst, and formaldehyde as a promoter. The concentration of cellulose in water is 2.5-10.0 wt.%, the organic solvent is 2-methyltetrahydrofuran, 1,4-dioxane, γ-valerolactone or methyl isobutyl ketone, the volume ratio of the organic solvent to water is 1:1-6:1, the molar ratio of formaldehyde to the glucose structural unit in the cellulose is 1.5:1-6:1, the concentration of AlCl3·6H2O is 250-750 mmol / L, the reaction temperature is 175-190 DEG C, and the reaction time is 30-90 min.

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