CN116332743A - Method for preparing levulinic acid by promoting cellulose conversion through formaldehyde - Google Patents
Method for preparing levulinic acid by promoting cellulose conversion through formaldehyde Download PDFInfo
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Abstract
A method for preparing levulinic acid by promoting cellulose conversion by formaldehyde uses microcrystalline cellulose as raw material, organic solvent and water as mixed solvent, alCl 3 ·6H 2 O is used as a catalyst, formaldehyde is used as an accelerator, and levulinic acid is synthesized in a microwave reactor or a high-pressure reaction kettle. The volume ratio of the 2-methyltetrahydrofuran to the water is 1:1-6:1, and the method utilizes the acetalation reaction of formaldehyde and the hydroxyl of a glucose unit in a cellulose structure to promote the hydrolysis and subsequent conversion of cellulose, so that the synthesis efficiency of levulinic acid is high; the generation of humins is less, the boiling point of a solvent is low, and the separation energy consumption is low; formaldehyde is cheap and easy to obtain and is extremely easy to separate.
Description
Technical Field
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
Levulinic acid is an important biomass-based platform chemical that can be used as a chemical feedstock for the production of a variety of high value-added chemicals and liquid fuels. The research on the method for efficiently converting biomass resources into levulinic acid has an important pushing effect on developing new biomass economy and helping to realize the double carbon target in China, and is continuously focused on in academia and industry.
Cellulose is a major component of lignocellulosic biomass, a natural polymer formed by connecting glucose structural units through beta-1, 4-glycosidic bonds, and can be converted into levulinic acid through hydrolysis, dehydration and hydration decomposition reactions, so that the cellulose is an ideal raw material for synthesizing the levulinic acid. In recent years, a great deal of research has been reported on the preparation of levulinic acid from cellulose conversion. However, because of the complex intramolecular and intermolecular hydrogen bond interactions in the cellulose structure, hydrolysis and subsequent conversion thereof are difficult, and in the reported literature, the concentration of cellulose is generally low (< 10.0 wt.%) resulting in low practical synthesis efficiency of levulinic acid, which does not meet the requirements of green chemistry and chemical engineering. Development of a technology for efficiently converting high-concentration cellulose into levulinic acid is extremely important for achieving effective utilization of biomass resources.
Qin et al report a method for preparing levulinic acid from high concentration cellulose conversion in RSC Advance 45 (2016) 39131-39136. Cellulose with the concentration of 12.0wt.% is used as a raw material, water is used as a solvent, phosphoric acid with the concentration of 1.5mol/L is used as a catalyst, sodium chloride with the concentration of 47.0wt.% is used as an auxiliary agent, and the cellulose is reacted for 1h at 170 ℃ under the microwave heating condition, so that the yield of levulinic acid reaches 67.3mol%. The method utilizes high-concentration sodium chloride to destroy the original hydrogen bond structure in the cellulose structure to promote the hydrolysis of cellulose, and oligosaccharide and glucose obtained by hydrolysis are further converted into levulinic acid under the catalysis of phosphoric acid. Since the carbon balance of the reaction is 68.9mol%, 31.1mol% of biochar atoms are converted into humins, and the formation of by-products not only loses biochar raw materials, but also is extremely prone to problems of equipment blockage and difficult separation of products, and the use of high concentrations of sodium chloride and acid phosphorus increases the risk of equipment corrosion.
Wang et al ACS Sustainable Chemistry&10.0wt.% cellulose is used as raw material in Engineering 11 (2018) 15092-15099, sulfolane and water are used as mixed solvent (V Sulfolane (TMP) :V Water and its preparation method =9:1), sulfonated humins are acid catalysts, cellulose is reacted at 180 ℃ for 2h, levulinic acid yield is at most 66.0mol%, while 36.0wt.% humins and a small amount of furfural (13.5 mol%) are produced. The method not only avoids the use of corrosive sodium chloride and phosphoric acid, but also utilizes the by-product humins generated by the reaction as the precursor of the catalystThe utilization value of the by-product is improved by the precursor. But the method uses a large amount of sulfolane with high boiling point, and reduces the actual concentration of the product due to the addition of a large amount of water in the separation process of the humins, and the two greatly increase the energy consumption for separating/purifying the product.
Zhang et al in Renewable Energy 141 (2019) 802-813 report a process for the preparation of levulinic acid by converting cellulose in a biphasic solvent consisting of methyl isobutyl ketone and water. Taking choline cation modified phosphotungstic-titanium heteropolyacid as catalyst, when V Methyl isobutyl ketone :V Water and its preparation method At 10:1, the concentration of cellulose in the water phase is 20.0wt.%, and after 8 hours of reaction at 130 ℃, the yield of levulinic acid is as high as 76.1mol%; when V is Methyl isobutyl ketone :V Water and its preparation method When the reaction rate was reduced to 5:1 and 1:1, the levulinic acid yield was reduced to 67.2mol% and 61.5mol%, respectively. Despite the high V Methyl isobutyl ketone :V Water and its preparation method The method is favorable for improving the yield of levulinic acid and inhibiting the generation of humins, but still has the problem of low concentration of products in an organic phase, and the energy consumption for separating/purifying the products is still higher.
Zhou Liangfang et al report in Chinese patent application ZL 202210142280.4 a process for the preparation of levulinic acid from the synergistic promotion of cellulose conversion by alkyl ammonium halides and sodium halides. The method takes a biphasic solvent consisting of 2-methyltetrahydrofuran and water as a reaction solvent, takes benzenesulfonic acid as a catalyst, and utilizes low-concentration sodium chloride (1.6 wt.%) and alkylammonium halide (91.3 mmol/L) to synergistically promote cellulose hydrolysis and stabilize levulinic acid, thereby inhibiting the generation of humins to a great extent. When V is 2-methyltetrahydrofuran :V Water and its preparation method When=2:1, 15.0wt.% of wheat straw cellulose was reacted at 180 ℃ for 2h, with levulinic acid yields as high as 81.0mol%. The method has low organic phase consumption and low equipment corrosion, but the alkyl ammonium halide with surfactant property can be partially dissolved in the organic phase, so that the recycling property of the reaction system is reduced.
At present, the preparation of levulinic acid from high-concentration cellulose conversion still faces the following problems: (1) The levulinic acid has poor synthesis efficiency, a large amount of humins are still generated, and the energy consumption for separating/purifying the product is high; (2) The use of large amounts of high boiling organic solvents further increases the energy consumption for product separation/purification; (3) the recycling property of the reaction system is required to be improved.
Disclosure of Invention
The present invention aims to provide a method for preparing levulinic acid by promoting cellulose conversion by formaldehyde.
The key points of the invention are as follows: a method for preparing levulinic acid by promoting cellulose conversion by formaldehyde is characterized in that microcrystalline cellulose is used as a raw material, an organic solvent and water are used as mixed solvents, and AlCl is used as a solvent 3 ·6H 2 O is a catalyst, formaldehyde is an accelerator, levulinic acid is synthesized in a microwave reactor or a high-pressure reaction kettle, the concentration of cellulose in water is 1.0-30.0 wt%, an organic solvent is 2-methyltetrahydrofuran, tetrahydrofuran, 1, 4-dioxane, gamma-valerolactone and methyl isobutyl ketone, the volume ratio of the organic solvent to water is 1:2-10:1, the molar ratio of formaldehyde to glucose structural units in the cellulose is 0.7:1-7:1, and AlCl is prepared by the steps of 3 ·6H 2 The concentration of O is 150.0-700.0 mmol/L, the reaction temperature is 160-195 ℃ and the reaction time is 5-120 min.
The concentration of cellulose in water is 2.5 to 10.0wt.%.
The volume ratio of the organic solvent to the water is 1:1-6:1.
The molar ratio of formaldehyde to glucose structural units in cellulose is 2.9:1-5.0:1.
The AlCl 3 ·6H 2 The O concentration is 400-550 mmol/L.
The reaction temperature is 175-180 ℃.
The reaction time is 30-60 min.
Compared with the prior art, the invention has the following characteristics and advantages:
(1) The hydroxyl of glucose structural units in formaldehyde and cellulose is subjected to acetalation reaction to promote cellulose hydrolysis and subsequent conversion, so that the levulinic acid synthesis efficiency is high;
(2) The generation of humins is less, the boiling point of a solvent is low, and the separation energy consumption is low;
(3) Formaldehyde is cheap and easy to obtain and is extremely easy to separate.
Formaldehyde is a high-reactivity reagent, and can destroy the original hydrogen bond structure in the cellulose structure by acetalation reaction with the hydroxyl of glucose structural units in the cellulose, thereby playing a role in promoting cellulose hydrolysis and stabilizing hydrolysis product glucose, and effectively inhibiting the generation of humins by matching with the use of a low-boiling point solvent and a low-corrosivity acid catalyst, improving the synthesis efficiency of levulinic acid, reducing the energy consumption for separation/purification, and realizing the efficient conversion of high-concentration cellulose to prepare levulinic acid.
Detailed Description
Example 1:
to a 30mL microwave reaction tube was added sequentially a magnetic stirrer, 750.0mg (15.0 wt.%) microcrystalline cellulose, 3.5mL water, 10mL 2-methyltetrahydrofuran, 450.0mmol/L AlCl 3 ·6H 2 O, 1.5mL of formaldehyde aqueous solution (concentration: 37.0 wt.%) was used to seal the reaction tube, and the reaction tube was heated to 185℃with microwaves for 45min. After the reaction is finished, the reaction tube is purged by compressed air, after the reaction tube is cooled to room temperature, solid residues are centrifugally separated, ethanol is added into the reaction liquid for mixing and dissolving, the volume is fixed, a certain volume of reaction liquid is taken for dilution, the product is quantitatively analyzed by high performance liquid chromatography, and the yield of levulinic acid is 87.3mol%.
Examples 2 to 8:
the reaction was carried out using microcrystalline cellulose at various concentrations according to the method of example 1, and the reaction conditions and results are shown in Table 1.
TABLE 1
| Example sequence number | Cellulose/water mass ratio (wt.%) | Yield of levulinic acid (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 |
Examples 9 to 12:
the reaction was carried out according to the method of example 1 using different reaction solvents (volume ratio of organic solvent to water is 2:1), and the reaction conditions and results are shown in Table 2.
TABLE 2
Examples 13 to 18:
the reaction was carried out according to the method of example 1 using different volume ratios of 2-methyltetrahydrofuran/water mixed solvent, and the reaction conditions and results are shown in Table 3.
TABLE 3 Table 3
Examples 19 to 26:
the reaction was carried out according to the method of example 1 using different molar ratios of glucose structural units in formaldehyde/cellulose, and the reaction conditions and results are shown in Table 4.
TABLE 4 Table 4
Examples 27 to 38:
according to the method of example 1, different concentrations of AlCl are used 3 ·6H 2 O was reacted under the reaction conditions and the results shown in Table 5.
TABLE 5
Examples 39 to 45:
the reaction was carried out according to the method of example 1 using different reaction temperatures, and the reaction conditions and results are shown in Table 6.
TABLE 6
| Example sequence number | Reaction temperature (. Degree. C.) | Yield of levulinic acid (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 |
Examples 46 to 52:
the reaction was carried out according to the method of example 1 using different reaction times, and the reaction conditions and results are shown in Table 7.
TABLE 7
| Example sequence number | Reaction time (min) | Yield of levulinic acid (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 |
Example 53:
to a 100mL autoclave were added in order a magnetic stirrer, 1500.0mg (15.0 wt.%) microcrystalline cellulose, 7.0mL water, 20mL 2-methyltetrahydrofuran, 450.0mmol/L AlCl 3 ·6H 2 O, 3.0mL of formaldehyde aqueous solution (concentration: 37.0 wt.%) was purged with nitrogen, pressurized to 1.5MPa, heated to 185℃and reacted for 30min. After the reaction is finished, purging the kettle body by using air, centrifuging to separate solid residues after the reaction kettle is cooled to room temperature, adding ethanol into the reaction liquid to mix and fix the volume, taking a certain volume of the reaction liquid to dilute, and quantitatively analyzing the product by using high performance liquid chromatography, wherein the yield of levulinic acid is 61.3mol%.
Claims (7)
1. A method for preparing levulinic acid by promoting cellulose conversion by formaldehyde is characterized in that microcrystalline cellulose is used as a raw material, an organic solvent and water are used as a mixed solvent, and AlCl is used as a solvent 3 ·6H 2 O is a catalyst, formaldehyde is an accelerator, levulinic acid is synthesized in a microwave reactor or a high-pressure reaction kettle, the concentration of cellulose in water is 1.0-30.0 wt%, an organic solvent is 2-methyltetrahydrofuran, tetrahydrofuran, 1, 4-dioxane, gamma-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 glucose structural units in the cellulose is 0.7:1-7:1, and AlCl is prepared by the steps of 3 ·6H 2 The concentration of O is 150.0-750.0 mmol/L, the reaction temperature is 160-195 ℃ and the reaction time is 5-120 min.
2. The method according to claim 1, characterized in that the concentration of cellulose in water is 2.5-10.0 wt.%.
3. The method for preparing levulinic acid from formaldehyde-promoted cellulose according to claim 1, wherein the volume ratio of the organic solvent to water is 1:1 to 6:1.
4. The method of claim 1, wherein the molar ratio of formaldehyde to glucose structural units in cellulose is from 2.9:1 to 5.0:1.
5. The method of preparing levulinic acid from formaldehyde-promoted cellulosic conversion according to claim 1, wherein said AlCl 3 ·6H 2 The O concentration is 400-550 mmol/L.
6. The method for preparing levulinic acid from formaldehyde-facilitated cellulose conversion according to claim 1, wherein said reaction temperature is from 175 ℃ to 180 ℃.
7. The method for preparing levulinic acid from formaldehyde-promoted cellulose conversion according to claim 1, wherein said reaction time is 30 to 60 minutes.
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