A method for preparing 2-methylfuran from biomass raw materials

Abstract

The present invention discloses a route for preparing 2-methylfuran from hemicellulose. Under the system of high-concentration brine / organic extractant, xylose can be directly converted into 2-methylfuran in one step, and the highest yield can reach 82%. This process ingeniously utilizes the hydrodechlorination reaction, and by regulating the brine concentration, the selectivity of the final product is successfully regulated. There is no acid or alkali in the whole reaction process, and there is little corrosion to the reaction equipment, which is conducive to large-scale production. It has great significance for the sustainable development of China's energy, environmental protection, and the rapid realization of carbon neutrality and carbon peak.

Description

Technical Field

[0001] The present invention relates to a new method for catalytically converting biomass raw materials to prepare 2-methylfuran. Specifically, it can convert hemicellulose into 2-methylfuran in one step under a high-concentration brine / organic extractant system. The highest yield can reach 82%. Background Art

[0002] 2-Methylfuran is an important organic synthesis intermediate with broad application prospects. It can be used to prepare various drugs, such as antibiotics, analgesics, anti-tumor drugs, etc. In addition, 2-methylfuran can also be used as an additive in the fields of food, spices, and dyes. Therefore, the research on its efficient, economical, and green synthesis method is of great significance. The traditional method for synthesizing 2-methylfuran uses sulfuric acid as a catalyst, with 2-methyl-2-butanol and formaldehyde as raw materials. The esterification reaction is carried out under heating conditions, and then dehydration and cyclization are carried out to obtain 2-methylfuran. However, this method has problems such as complex operation, harsh reaction conditions, and difficulty in purifying the product. Therefore, researchers seek to develop a simple, efficient, and environmentally friendly method for synthesizing 2-methylfuran to solve the problems existing in the existing methods.

[0003] Biomass energy is widely distributed and inexpensive. Compared with biomass energy, the impact of the exploitation and utilization of fossil energy on the environment has become increasingly prominent in recent years. The research on using biomass energy to prepare 2-methylfuran has become a hot topic in the field of biomass energy research today. So far, the preparation of 2-methylfuran is mostly obtained by selective hydrogenation of furfural. In recent years, the price of furfural has been increasing, and furfural itself is not easy to preserve and is very easy to form furoic acid. Therefore, it is very necessary to optimize the raw materials for preparing 2-methylfuran and study new production processes. Hemicellulose is one of the three major components of lignocellulose, mainly composed of C5 sugars linked by glycosidic bonds. Through recent research, the authors in the scientific research found that hemicellulose is very easy to extract from lignocellulose and can exist in the extract in the form of xylose and is easy to preserve. Compared with furfural, the extraction process of xylose is more environmentally friendly. According to literature reports, xylose can be extracted from lignocellulose under the reaction conditions of 120 °C using a γ-valerolactone / water / eutectic solvent system. Therefore, it is necessary to study a process for preparing 2-methylfuran using xylose as a raw material. Summary of the Invention

[0004] To solve the above problems, the present invention provides a new route for preparing 2-methylfuran from xylose. Under the system of high-concentration brine / organic extractant, xylose can be directly converted into 2-methylfuran in one step. The highest yield can reach 82%. This process ingeniously utilizes the hydrodechlorination reaction. By regulating the brine concentration in this application, the selectivity of the final product is successfully regulated. The whole reaction process is acid-free and alkali-free, and there is little corrosion to the reaction equipment, which is beneficial to large-scale production. It is of great significance for the sustainable development of China's energy, environmental protection, rapid realization of carbon neutrality and carbon peak.

[0005] The present invention relates to a new method for catalytic conversion of hemicellulose to prepare 2-methylfuran, which is achieved by the following means.

[0006] Using a high-concentration halogenated brine / organic solvent two-phase system and a common hydrodechlorination catalyst, under a hydrogen atmosphere and a specific reaction temperature, the reaction raw materials are directly converted into 2-methylfuran in one step through isomerization, dehydration, and hydrodechlorination reactions.

[0007] Further, in the above technical solution, the reaction raw materials are hemicellulose components and their monomers in biomass, including xylose, arabinose, or a mixture of two or more of them.

[0008] Further, in the above technical solution, the high-concentration halogenated brine, that is, the salt concentration needs to exceed 50 wt.% under the reaction conditions, so as to provide sufficient halogen ions to promote the hydrodehalogenation reaction under the reaction conditions.

[0009] Further, in the above technical solution, the halogenated salts with a solubility exceeding 50 wt.% under the reaction conditions include one or a mixture of more of KCl, KBr, and NaBr.

[0010] Further, in the above technical solution, the organic solvent in the halogenated brine / organic solvent two-phase system is a variety of organic solvents with extraction function that are immiscible with brine, including toluene, methyltetrahydrofuran, tetrahydrofuran, and methyl isobutyl ketone.

[0011] Further, in the above technical solution, the common hydrodechlorination catalysts include: Pd / C, NiAu, Pt / C, PdCu, RuCu.

[0012] Further, in the above technical solution, under a hydrogen atmosphere, the hydrogen pressure is 2-8 MPa, and the reaction temperature is 140-240 °C.

[0013] Further, in the above technical solution, the volume ratio of halogenated brine to organic solvent is 1:10 - 1:1. Specific embodiments

[0014] Examples 1 - 3

[0015] The conversion experiments of xylan, xylose, and arabinose were carried out using a Pd / C catalyst. 0.3 g of the reaction substrate was loaded into the reaction kettle, and 20 mL of 70 wt.% NaBr solution and 20 mL of toluene were added. Hydrogen gas was charged into the reaction kettle at 3 Mpa, and it was reacted under the reaction conditions of 160 °C and 500 rpm for 4 hours. The reaction results are shown as follows.

[0016] Table 1. Catalytic conversion of different reaction substrates of xylose to prepare 2 - methylfuran

[0017]

[0018]

[0019] Reaction conditions: React at 160 °C and 500 rpm for 4 hours

[0020] As described in Table 1, under these reaction conditions, xylan, xylose, and arabinose can all produce methylfuran. Among them, cyclopentanone is the main by - product. As shown in Reaction Route 1, it is the reaction route for the catalytic conversion of xylose to prepare 2 - methylfuran. The reaction steps are as follows: 1) Xylose undergoes an isomerization reaction to form xylulose; 2) Xylulose undergoes a dehydration reaction to form furfural; Furfural forms furfuryl alcohol under the action of a hydrogenation catalyst. After xylose forms furfuryl alcohol, the reaction route is further divided into the following three: 1) Furfural alcohol is directly hydrogenated to form tetrahydrofurfuryl alcohol; 2) Furfural alcohol is hydrogenated and dechlorinated to form 2 - methylfuran (the target product); 3) Furfural alcohol undergoes isomerization, dehydration, and then hydrogenation to form cyclopentanone.

[0021] In this reaction system, since the hydrogenation catalyst Pd / C is poisoned by Br in the high - concentration NaBr solution, almost no over - hydrogenation reaction occurs to form tetrahydrofurfuryl alcohol. However, the isomerization, dehydration, and hydrogenation of furfural alcohol and the dechlorination still constitute a competitive relationship, so cyclopentanone is the main by - product.

[0022]

[0023] Reaction Route 1. Reaction route for catalytic conversion of xylose to prepare dimethylfuran

[0024] Examples 4 - 9

[0025] The conversion experiment of xylose was carried out using a Pd / C catalyst. 0.3 g of the reaction substrate was loaded into the reaction kettle, and 20 mL of NaBr solution and 20 mL of toluene were added. Hydrogen gas was charged into the reaction kettle at 3 Mpa, and it was reacted under the reaction conditions of 160 °C and 500 rpm for 4 hours. The reaction results are shown as follows.

[0026] Table 2. Catalytic conversion of xylose with different NaBr concentrations to prepare 2 - methylfuran[a]

[0027]

[0028]

[0029] Reaction conditions: React at 160 °C and 500 rpm for 4 hours

[0030] As shown in Table 2, when the brine concentration is 20 wt.%, the product selectivity is not good, and the main products are 2-methyltetrahydrofuran, 2-methylfuran, cyclopentanone, cyclopentanol, tetrahydrofurfuryl alcohol, and xylitol.

[0031] In this reaction, the functions of Br in the high-concentration NaBr solution are mainly as follows: 1) Promote the isomerization of compounds, such as the isomerization of xylose to xylulose and the isomerization of furfuryl alcohol to hydroxycyclopentenone; 2) Poison the catalyst appropriately to prevent over-hydrogenation to products such as 2-methyltetrahydrofuran, xylitol, and tetrahydrofurfuryl alcohol. As shown in Reaction Route 1, when the concentration of NaBr is too low, Pd / C will over-hydrogenate the substrate to xylitol, tetrahydrofurfuryl alcohol, and cyclopentanol. At the same time, part of the furfuryl alcohol that has not been over-hydrogenated acts as an intermediate and is converted into cyclopentanone and 2-methylfuran. In the reaction system, the isomerization dehydration and hydrogenation of furfuryl alcohol and the hydrodechlorination reaction form a competitive relationship. When the halogen ion content is low, as shown in the results of Example 5 and Example 6. The isomerization dehydration and hydrogenation of furfuryl alcohol to form cyclopentanone play a dominant role. As the concentration of NaBr gradually increases, the hydrodechlorination reaction becomes the dominant role. This is exactly the reason why high-concentration brine is required for the preparation of 2-methylfuran in this patent.

[0032] Examples 10 - 12

[0033] The solubilities of NaBr, KCl, and KBr in water at 20 °C are 90.5 g / 100 mL, 31.5 g / 100 mL, and 75.0 g / 100 mL respectively. The solubility of KCl in water at 100 °C can reach 56.7 g / 100 mL. According to the above properties, the examples are as follows. Conduct an experiment on the conversion of xylose using a Pd / C catalyst. Load 0.3 g of the reaction substrate into the reaction kettle, add 20 mL of distilled water and appropriate amounts of NaBr, KCl, and KBr, with the requirement that the brine concentration is 50 wt.%, and then add 20 mL of toluene. Charge 3 Mpa of hydrogen into the reaction kettle and react it at 160 °C and 500 rpm for 4 hours. The reaction results are as follows.

[0034] Table 3. Preparation of 2-methylfuran by catalytic conversion of xylose with different salts

[0035]

[0036]

[0037] Reaction conditions: React at 160 °C and 500 rpm for 4 hours.

[0038] As shown in Table 3, NaBr, KCl, and KBr can all catalyze the conversion of xylose to prepare 2-methylfuran. There is no obvious difference in the types of halogen ions in the reaction system. A sufficiently high concentration of halogen ions seems to be an important indicator to promote the hydrodechlorination reaction and at the same time inhibit the conversion of xylose to cyclopentanone.

[0039] Examples 13 - 16

[0040] Perform an experiment on the conversion of xylose using a Pd / C catalyst. Charge 0.3 g of the reaction substrate into a reaction kettle, add 20 mL of a 70 wt.% NaBr solution and 20 mL of an organic solvent. Charge 3 Mpa of hydrogen into the reaction kettle and react it at 160 °C and 500 rpm for 4 hours. The reaction results are as shown below.

[0041] Table 4. Catalytic conversion of different reaction substrates of xylose to prepare 2-methylfuran

[0042]

[0043] Reaction conditions: React at 160 °C and 500 rpm for 4 hours

[0044] As shown in the above table, different solvents have little effect on the catalytic conversion of xylose to prepare 2-methylfuran. Under the above four solvent conditions, xylose can be catalytically converted to prepare 2-methylfuran. The highest yield of 82% can be obtained in toluene solvent.

[0045] Examples 17 - 19

[0046] Take 1 g of Al2O3 and add it to a beaker; then add 0.017 g of AuCl3 and 0.493 g of Ni(NO3) 26 H2O in another beaker. Add a certain amount of water to mix the two salts into a homogeneous solution, and then pour them into the beaker containing Al2O3 for equal impregnation. Let the sample stand in a cool and windless place for 24 h, and then put it into an oven to dry. After the sample is dried, calcine it at 400 °C in an air atmosphere for 4 h, then put it into a tubular furnace and reduce it at 500 °C in a hydrogen and nitrogen mixed gas for 4 h. After the reduction is completed and the tubular furnace cools to room temperature, passivate the sample in a nitrogen and air mixed gas for 24 h. Name the sample NiAu / Al2O3. And prepare PtCu / Al2O3 and RuCu / Al2O3 catalysts in the same way, that is, use Al2O3 as the carrier and prepare them by the above co-impregnation method.

[0047] The conversion experiment of xylose was carried out using Pd / C, NiAu / Al2O3, and Pt / C catalysts. 0.3 g of the reaction substrate was loaded into the reaction kettle, and 20 mL of 70 wt.% NaBr solution and 20 mL of toluene were added. Hydrogen gas was charged into the reaction kettle at 3 Mpa, and the reaction was carried out at 160 °C and 500 rpm for 4 hours. The reaction results are shown as follows.

[0048] Table 5. Catalytic conversion of different reaction substrates of xylose to prepare 2-methylfuran

[0049]

[0050] Reaction conditions: reaction at 160 °C and 500 rpm for 4 hours

[0051] As shown in the table, the three catalysts in Examples 17-19 are all metal catalysts with good hydrodechlorination effects. Among them, it can be clearly seen that the hydrodechlorination activity of Pd / C is superior to that of the other two catalysts.

[0052] Examples 22-28

[0053] The conversion experiment of xylose was carried out using Pd / C catalyst. 0.3 g of the reaction substrate was loaded into the reaction kettle, and 20 mL of 70 wt.% NaBr solution and 20 mL of toluene were added. Hydrogen gas was charged into the reaction kettle, and the reaction was carried out at 160 °C and 500 rpm for 4 hours. The reaction results are shown as follows.

[0054] Table 6. Catalytic conversion of xylose to prepare 2-methylfuran under different hydrogen pressures

[0055]

[0056]

[0057] Reaction conditions: reaction at 160 °C and 500 rpm for 4 hours

[0058] As can be seen from the above table, when the hydrogen pressure is low, xylose will be converted into furfural. As shown in reaction route 1, this reaction route requires furfuryl alcohol as an intermediate, so sufficient hydrogen pressure is needed to promote the selective hydrogenation of furfural to furfuryl alcohol. Furfural is then hydrodechlorinated to prepare 2-methylfuran. From the results of the above table, the optimal pressure is 3 MPa.

Claims

1. A method for preparing 2-methylfuran from biomass raw materials, characterized in that: A high-concentration brine / organic solvent two-phase system and a common hydrodechlorination catalyst are used. Under a hydrogen atmosphere and a specific reaction temperature, under a hydrogen atmosphere, the hydrogen pressure is 2-8 MPa, and the reaction temperature is 140-240 °C. The reaction raw materials are directly converted into 2-methylfuran in one step through isomerization, dehydration, and hydrodechlorination reactions; The reaction raw materials are the hemicellulose components and their monomers in biomass, including one or a mixture of two or more of xylose and arabinose; The high-concentration brine has a brine salt concentration of at least 50 wt.%; The halide salt is selected from one or a mixture of more than one of KCl, KBr, and NaBr; In the brine / organic solvent two-phase system, the organic solvent has an extraction function and is immiscible with the brine.

2. The method according to claim 1, wherein: The organic solvents include toluene, methyltetrahydrofuran, tetrahydrofuran, and methyl isobutyl ketone.

3. The method according to claim 1, characterized in that: The common hydrodechlorination catalysts include: Pd / C, NiAu catalyst, Pt / C, PdCu catalyst, and RuCu catalyst.

4. The method according to claim 1, wherein: The volume ratio of brine to organic solvent is 1:10 to 1:1.

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