JP2016034926A - Production method of 5-hydroxymethylfurfural - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for synthesizing 5-hydroxymethylfurfural by one pot (single stage) reaction by using cellulose and/or a cellulose-containing material as raw materials, and without using a harmful catalyst or organic solvent.SOLUTION: Calcium phosphate is used as a catalyst, and preferably, cellulose and/or a cellulose-containing material which are raw materials are subjected beforehand to crushing/grinding treatment independently or in the mixed state with the catalyst, and then the catalyst and the cellulose or the cellulose-containing material which is the raw material are subjected to a heating treatment in a water solvent, to thereby obtain 5-hydroxymethylfurfural.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing 5-hydroxymethylfurfural, and more particularly to a method for producing 5-hydroxymethylfurfural using cellulose as a raw material.

5-Hydroxymethylfurfural (hereinafter sometimes referred to as “HMF”) is an important chemical intermediate because it is a raw material for various useful chemical substances such as resin materials, fuels, chemical products, surfactants and cosmetics. Is the body. Examples of a method for converting this HMF into a useful chemical substance and a method for effectively using the product obtained by the conversion include the following.
Oxidation can be converted to furandicarboxylic acid, and this product is expected as an alternative to terephthalic acid. Further, according to the hydrogenation reaction, it is converted into dimethylfuran and expected to be used as a fuel additive. Furthermore, levulinic acid is produced by the hydrolysis reaction, and a wide range of uses such as a raw material for γ-valerolactone can be expected.

  This HMF can be produced by hydrolyzing and dehydrating saccharides under various conditions. In recent years, waste biomass such as waste wood has attracted attention as a chemical raw material resource as an alternative to petroleum. Various methods for synthesizing HMF using cellulose as a raw material have been proposed.

  For example, in Patent Document 1, in a method for producing lactic acid and 5-hydroxymethylfurfural and furfural from cellulose, a flow step of flowing water containing cellulose to the path, and the water in a state where a base is added to the water. Is a lactic acid production process in which the temperature is set to a temperature of 150 ° C. or more and less than 400 ° C. and a pressure of 5 MPa or more, and before or after the lactic acid production process, the water is added in a state where an acid is added to the water. It describes that lactic acid and 5-hydroxymethylfurfural and furfural are produced by a 5-hydroxymethylfurfural and furfural production process in which a temperature of 250 ° C. or more and less than 400 ° C. and a pressure of 10 MPa or more is established in the route. However, it is necessary to impose a high pressure condition of a pressure of 5 MPa, and it is expected that an apparatus and a maintenance management for safely performing a high pressure reaction will be costly.

  In addition, in Patent Document 2, in the method for producing 5-hydroxymethylfurfural from cellulose, the cellulose, the acid and the catalyst are allowed to coexist in water having a reaction temperature of 200 ° C. or more and less than 400 ° C. and a pressure of 10 MPa or more and 40 MPa or less. It is described that cellulose is selectively decomposed directly to 5-hydroxymethylfurfural, and scandium triflate, montmorillonite, and platinum-supported alumina are used as the catalyst. However, in this method, it is necessary to keep the water under a high pressure condition of 10 MPa or more and 40 MPa or less, and it is expected that the apparatus cost and the maintenance management cost will be great for a safe reaction. Examples of the acid include inorganic acids such as sulfuric acid, nitric acid and phosphoric acid, and organic acids such as formic acid and acetic acid, and it is necessary to use them as additives in addition to water. In addition, maintenance costs such as equipment corrosion are expected to be expensive.

In Patent Document 3, hexose is brought into contact with subcritical or supercritical water in the presence of a heterogeneous zirconium phosphate catalyst to obtain 5-hydroxymethylfurfural, and cellulose or a cellulose-containing substance is obtained. In the presence of a heterogeneous zirconium phosphate catalyst, it is described that it is decomposed into hexose by contacting with subcritical or supercritical water, and 5-hydroxymethylfurfural is obtained from the obtained hexose. .
However, when the reaction starting material is cellulose or a cellulose-containing material, when it is brought into contact with subcritical or supercritical water, decomposition proceeds and monosaccharides and disaccharides are produced. Even if a zirconium phosphate catalyst is present in the reaction system, there is only a description that it does not interfere with the decomposition reaction of cellulose, and there is no specific description about the decomposition of cellulose. Also, in this method, it is necessary to perform the reaction under subcritical conditions or supercritical conditions, and it is expected that the equipment for maintaining the reaction under high temperature and high pressure conditions and its maintenance will be expensive. .

  Patent Document 4 describes that hydroxymethylfurfural is obtained by mixing cellulose in acidic electrolyzed water and stirring the resulting mixture under conditions of a maximum temperature of 210 ° C. and a saturated vapor pressure. Has been. However, since acid electrolyzed water is used as a solvent instead of ordinary water, it is expected to lead to high costs.

In Patent Document 5, a method for converting lignocellulosic materials (cellulose, hemicellulose and lignin) of wood and agricultural waste waste into a value-added material by a one-step hydrolysis method using a solid heterogeneous acid catalyst. Have been proposed, and in the examples of this document, those using only water as a solvent are also described.
However, the analysis of the reaction mixture was performed by HPLC (high performance liquid chromatograph), and all the compounds (xylose, arabinose, glucose, 5-hydroxymethylfurfural and furaldehyde) were calibrated prior to the analysis. Although described, it is only described that a 40% furfural yield was observed for products other than xylose, and there is no specific description for HMF.

Non-Patent Document 1 discloses that a reaction system obtained by dissolving catalytic metal components of CuCl ₂ and CrCl ₂ in 1-ethyl-3-methylimidazolium chloride is effective for cellulose conversion, and the yield of HMF is high. It is described as 55.4 ± 4.0%.
However, chromium, which is a harmful metal, is used for the catalyst, and it is expected that there will be a problem in safety during production, and that it will be costly to treat the catalyst after use and waste liquid treatment.

Furthermore, Non-Patent Document 2 discloses an effective method for obtaining HMF by subjecting cellulose and glucose to microwave treatment in the presence of an ionic liquid, and the catalyst contains chromium trichloride hexahydrate. Are described as effective.
However, with this method, it is expected that the treatment of chromium, which is also a harmful metal, will be very expensive.

JP-A-2005-232116 JP 2007-145736 A JP 2007-196174 A JP 2011-115061 A JP 2013-517792 A

Applied Catalysis A: General, Vol.361, Issues 1-2, 117-122 Tetrahedron Letters Volume 50, (2009) 5403-5405

As described above, in the production of HMF, sugar such as glucose can be produced from cellulose using an acid catalyst or the like, and HMF can be produced from the obtained sugar such as glucose. It is preferable to synthesize HMF by using a substance as a raw material in a one-pot reaction.
However, there are few techniques for producing HMF directly from cellulose in one step, and most of them use harmful chromium catalysts or expensive ionic liquids.
Further, based on the concept of green chemistry, the catalyst is required not to contain harmful metal elements. Furthermore, it is desirable that the solvent is water from the viewpoint of post-treatment and cost.

  The present invention has been made in view of the current situation, and provides a method for synthesizing HMF by a one-pot reaction using cellulose or a cellulose-containing substance as a raw material without using a harmful catalyst or an organic solvent. It is for the purpose.

  As a result of intensive research to achieve the above object, the present inventors have used calcium phosphate as a catalyst, and preferably, the raw material cellulose and / or cellulose-containing substance alone or mixed with the catalyst is used in advance. In addition, by performing pulverization and mixing in a ball mill, an automatic mortar or the like, the crystallinity of the cellulose-containing raw material is reduced or a physically strong contact state is created, and then the catalyst and the raw material cellulose or cellulose-containing substance It was found that the reaction of the target product to HMF proceeds effectively by sealing and heating the mixture together with an aqueous solvent in an autoclave.

The present invention has been completed based on these findings, and according to the present invention, the following inventions are provided.
[1] Production of 5-hydroxymethylfurfural, wherein cellulose and / or a cellulose-containing substance is used as a reaction raw material, and 5-hydroxymethylfurfural is obtained by heat treatment in a catalyst composed of calcium phosphate and an aqueous solvent. Method.
[2] The process for producing 5-hydroxymethylfurfural according to [1], wherein the reaction raw material alone or mixed with the catalyst is subjected to pulverization and rubbing before the heat treatment.
[3] The method for producing 5-hydroxymethylfurfural according to [2], wherein the pretreatment is a ball mill treatment.
[4] The method for producing 5-hydroxymethylfurfural according to any one of [1] to [3], wherein the heat treatment is performed in an inert gas.
[5] The method for producing 5-hydroxymethylfurfural according to any one of [1] to [4], wherein the heat treatment is performed at 180 to 220 ° C.

  According to the method of the present invention, calcium phosphate which is not harmful to the human body is used as a catalyst, and only water is used as a solvent, so that one-pot (one-stage) can be obtained from cellulose or a cellulose-containing substance without using a harmful catalyst or an organic solvent. HMF can be synthesized by reaction.

The figure which shows the outline | summary of the chemical reaction of this invention

In the method for producing 5-hydroxymethylfurfural of the present invention, cellulose and / or a cellulose-containing substance is used as a reaction raw material, the reaction raw material and a catalyst composed of calcium phosphate are mixed in an aqueous solvent, and then the mixture is heated. It is characterized by that.
FIG. 1 is a diagram showing an outline of a chemical reaction of the present invention. According to the present invention, a conventional two-stage or multi-step process can be performed from cellulose and / or a cellulose-containing substance without using harmful catalysts or organic solvents. The HMF can be synthesized by a one-step reaction.

  In the present invention, cellulose suitable for use as a raw material or a cellulose-containing substance is not only purified cellulose provided as a reagent or food additive, but also waste paper, waste cloth, and thinned wood as waste resources. Natural wood, such as sawdust and sawdust, and processed products thereof can be used. In the case of waste, paper contains additives and inks, in the case of cloth, dyes and synthetic fibers, and natural wood contains impurities other than cellulose, such as lignin and hemicellulose. If cellulose-containing raw materials are used, good reaction results can be obtained.

In the present invention, the catalyst used in the one-pot reaction is calcium phosphate (chemical formula: Ca ₃ (PO ₄ ) ₂ ), and commercially available reagents and those synthesized by various known methods can be used.

  In the present invention, the cellulose or the cellulose-containing substance is preferably subjected to pulverization and rubbing treatment using an automatic mortar, a ball mill apparatus, or the like as mechanical energy, alone or mixed with the catalyst before the heat treatment. Later, the reaction is carried out by mixing in an aqueous solvent. By performing pulverization and rubbing treatment using a mechanical processing device, the crystallinity of the raw material cellulose or cellulose-containing substance is reduced and the reactivity is greatly increased. In order to obtain a rate, it is preferable to perform a pretreatment operation. The pretreatment time is preferably about 1 hour to 7 days, particularly preferably about 24 hours to 72 hours.

In the present invention, the heat treatment is performed at a reaction temperature of 180 to 220 ° C., preferably around 200 ° C., and the reaction is performed for 1 to 10 hours, particularly preferably about 2 hours. Since this reaction is a multi-step sequential reaction, if it is too long, undesirable reactions and phenomena such as decomposition of the target product, polymerization, and deposition on the catalyst surface proceed, leading to a decrease in yield. When the time is too short, the yield decreases because the reaction rate of the raw material is low.
The heat treatment is preferably performed in an inert gas atmosphere such as argon gas to eliminate adverse effects such as oxidation due to the presence of air.
In addition, various known methods can be applied for the heat treatment, but representative examples include a batch method and a flow method. In the case of a batch system, it is preferable to keep the sealed state so that the solvent water is in a liquid state at the reaction temperature. In the case of the flow type, it is preferable that the internal pressure is adjusted by a mechanism such as a pressure regulating valve so that the solvent water can pass through the reaction apparatus while being in a liquid state.

  EXAMPLES Hereinafter, although this invention is demonstrated based on an Example, this invention is not limited to this Example.

Example 1
Merck microcrystalline cellulose as a reaction raw material was ground and rubbed for 48 hours using a ball mill. The ball mill used at this time encapsulated 1 kg of alumina balls having a diameter of 2 cm and cellulose in air using a container having a capacity of 3 L. Thereafter, in order to adjust the moisture content, drying treatment was performed in an air dryer at 80 ° C. for 12 hours or more, and then stored at room temperature in a desiccator kept dry with silica gel.
0.2 g of the stored cellulose, 1.8 g of calcium phosphate (manufactured by Merck) as a catalyst, and 20 mL of distilled water were added to a pressure-resistant glass industry reaction vessel (TPR-1 type, internal volume 100 mL). The reaction vessel was sealed, and the space was replaced with argon to eliminate the influence of oxygen in the air, and the initial pressure was adjusted to 0.4 MPa at room temperature. The mixture was heated to 200 ° C. using a heater for heating, and the reaction was performed while stirring at 300 rpm using a stirrer. After 2 hours, the heater was removed, and the reaction vessel was immersed in room temperature water to quench and stop the reaction.

(Product analysis methods and results)
After completion of the reaction in the reaction vessel, the catalyst and unreacted raw material were separated using a membrane filter (manufactured by Advantech) to obtain a yellow transparent aqueous solution that passed through the filter. The aqueous solution was diluted 10 times with distilled water and analyzed with a high performance liquid chromatograph (hereinafter referred to as HPLC) (manufactured by Shimadzu Corporation). A calibration curve was prepared using a commercially available reagent as a standard substance, and the concentration in the aqueous solution was calculated from the calibration curve. The unit structure of cellulose is C ₆ H ₁₀ O ₅ , expressed as (C ₆ H ₁₀ O ₅ ) _n, and the case where it was all hydrolyzed to glucose (C ₆ H ₁₂ O ₆ ) was defined as a glucose yield of 100%. For HMF, the concentration of HMF obtained from 100% glucose in 100% yield was calculated and taken as 100% yield.
As a result of the reaction, the yield of glucose was 9.0%, and the yield of HMF was 35.1%. Other by-products showed minute amounts of fructose close to the detection limit, but other minute peaks could not be identified by comparison with the reagents. These are presumed to be dimers other than glucose and HMF monomers and oligomers that are polymerized.

The above-mentioned experiment was performed by changing the reaction temperature condition to 205 ° C. and without changing the other conditions. As a result, the yield of HMF was 25.0%, and glucose was a minute amount close to the lower limit of detection.
Next, the above-mentioned experiment was performed by changing the reaction temperature condition to 195 ° C. without changing the other conditions. As a result, the yield of HMF was 25.9% and the yield of glucose was 12.2%.
Furthermore, when the reaction time was extended to 3 hours under the condition of a temperature of 195 ° C., the HMF yield was 26.5% and the glucose yield was 8.2%.

(Example 2)
In this example, in order to confirm the versatility of the reaction raw material, instead of Merck microcrystalline cellulose, SERVA microcrystalline cellulose was used as a raw material, and grinding and rubbing were performed for 48 hours using a ball mill. The treatment was carried out in the same way as in the previous examples. Thereafter, the same reaction operation as described above was performed, and HMF was synthesized at a reaction temperature of 200 ° C. and a reaction time of 2 hours.
As a result, the yield of HMF was 27.0% and the yield of glucose was 9.4%.
As a result, it was clarified that HFF could be synthesized, although some variation was observed depending on the manufacturer of the raw material cellulose.

(Example 3)
Using the same Merck microcrystalline cellulose as in Example 1, the Merck microcrystalline cellulose was subjected to grinding and rubbing treatment in air for 10 hours using an automatic mortar at room temperature. Thereafter, in order to adjust the moisture content, drying treatment was performed in an air dryer at 80 ° C. for 12 hours or more, and then stored at room temperature in a desiccator kept dry with silica gel.
0.2 g of the stored cellulose, 1.8 g of calcium phosphate (manufactured by Merck) as a catalyst, and 30 mL of distilled water were added to a pressure-resistant glass industry reaction vessel (TPR-1 type). The reaction vessel was sealed, and the space was replaced with argon to eliminate the influence of oxygen in the air, and the initial pressure was adjusted to 0.4 MPa at room temperature. The reaction was conducted while heating to 200 ° C. using a heater and performing a stirring operation at 300 rpm using a stirrer. After 2 hours, the heater was removed, and the reaction vessel was immersed in room temperature water to quench and stop the reaction. HPLC analysis was performed in the same manner as in Example 1.
As a result of the analysis of the reaction, the yield of glucose was 8.7% and the yield of HMF was 25.1%. Other by-products showed minute amounts of fructose close to the detection limit, but other minute peaks could not be identified. It is presumed to be a dimer other than glucose or HMF monomer or a plurality of polymerized oligomers.
As a result, as a pretreatment method, it can be said that the ball mill 48-hour treatment is superior to the automatic mortar treatment 10 hours.

The above-mentioned experiment was performed by changing the reaction temperature condition to 205 ° C. and without changing the other conditions. As a result, the HMF yield was 21.2% and the glucose yield was 5.4%.
Moreover, the above-mentioned experiment was performed by changing the reaction temperature condition to 195 ° C. and without changing the other conditions. As a result, the HMF yield was 18.1% and the glucose yield was 8.5%.

Example 4
MERVA microcrystalline cellulose was used instead of Merck cellulose for 10 hours in an automatic mortar and used as a raw material. 1.8 g of calcium phosphate catalyst was added and dispersed in 30 mL of water, placed in the same reaction vessel as in Example 1, filled with argon, and reacted at 205 ° C. for 2 hours with stirring at 300 rpm. As a result, glucose was obtained at a yield of 5.1% and HMF was obtained at a yield of 15.1%. In addition, some minor by-product peaks that could not be identified and quantified were detected.
The result of this example shows that the reaction result is not a phenomenon specific to Merck cellulose but a general reaction result.

(Example 5)
In this example, in order to confirm the versatility of the raw material, a reaction was performed using a used cloth (a shirt, 100% cotton, manufactured by UNIQLO) as the cellulose-containing raw material.
The used cloth was cut into a size of 1 cm × 5 cm and the thread used for sewing was removed, followed by pulverization with a pulverizer to obtain cotton-like cellulose. When mixed with the catalyst and reacted in water as in Example 1, 10.4% of HMF was produced. As other by-products, many minute peaks that were difficult to identify were observed, and brown deposits were also observed on the catalyst surface. These are presumed to be dimers other than glucose and HMF monomers and oligomers that are polymerized. The quantitative calculation assumes that the raw material is all cellulose.

Subsequently, the cotton-like cellulose raw material obtained by pulverizing the above-mentioned old cloth with a pulverizer was further treated with a ball mill for 48 hours. That is, it is a raw material that has been subjected to both pulverizer processing and ball mill processing. As a result of the ball mill treatment, most of the cotton-like raw material changed from cotton-like to fine powder. To 0.2 g of old cloth (cellulose-containing raw material) pulverized into a fine powder, 1.8 g of calcium phosphate catalyst was added and dispersed in 30 mL of water, placed in a reaction vessel similar to that in Example 1, and sealed with argon. Then, the reaction was carried out with stirring at 300 rpm for 2 hours.
As a result, glucose was obtained at a yield of 6.8% and HMF was obtained at a yield of 30.6%. It can be said that the effect of the ball mill treatment increased the reactivity of the raw materials, leading to a high yield. In addition, some minor by-product peaks that could not be identified and quantified were detected. These by-products are presumed to be dimers other than glucose and HMF monomers and oligomers that are polymerized in plural. The quantitative calculation assumes that the raw material is all cellulose.
As a result, although the yield was slightly reduced as compared with microcrystalline cellulose as a reagent, it became clear that HMF can be produced from old cloth, which is one of cellulose-containing wastes, and the versatility of the raw material could be demonstrated.

(Example 6)
In this example, in order to confirm the versatility of the raw material, the cellulose raw material was reacted using copy paper.
As a pretreatment, the printed copy paper was processed in a pulverizer. Using 0.2 g of a cotton-like pulverized material as a raw material, 1.8 g of a catalyst was added, dispersed in 30 mL of water, and reacted at 200 ° C. for 2 hours. The quantification was calculated assuming that all 0.2 g of raw material was cellulose. As a result, an HMF yield of 8.3% was obtained, but it was not significantly high.
The HMF yield improved to 21.7% when using raw materials that had been ball milled for 48 hours to demonstrate the need to add mechanical energy such as ball milling. The glucose yield at that time was close to the detection limit, and an accurate quantitative analysis was not possible. Since various additives are used for improving the quality of copy paper, the cellulose content is lower than 100%. Therefore, the HMF yield based on cellulose is considered to be higher than the above value.

(Example 7)
In this example, in order to confirm the versatility of the raw material, the reaction was performed using natural wood as a raw material.
The reaction was carried out using Akita cedar powdered with a pulverizer as raw material. As a result of measuring the content of the sugar component constituting the raw wood, it has been found that glucose derived from cellulose and hemicellulose is 43.9% (weight ratio). The other sugar components are 9.9% xylose, 7.5% galactose and 12.3% mannose, and the components other than the sugar component are lignin.
The amount of the raw material used was calculated based on the sugar content in the wood flour, and the weight was adjusted so that the amount of glucose was 0.2 g, and 0.456 g was used. As a result of the reaction, HMF was obtained in a yield of 14.3%. Glucose was close to the detection limit and could not be accurately quantified. The calculation method of the yield when using natural wood as a raw material is as follows. Based on this analysis value, the HMF yield was calculated based on the amount of glucose converted into HMF. Therefore, based on the total weight of wood, the above reaction result is 6.3% of HMF yield (based on total weight).
The wood flour pulverized by a pulverizer was further subjected to ball mill treatment for 48 hours and used as a reaction raw material. As a result, an HMF yield of 35.5% and a glucose yield of 9.3% were obtained. The reference value used for the calculation of this yield is also based on glucose (content 43.9%).
As a result of this example, it became clear that wood raw materials can also be efficiently converted to HMF, and the versatility of the raw materials could be demonstrated.

(Comparative Example 1)
The same Merck microcrystalline cellulose used in Example 1 was ball milled for 48 hours, and 0.2 g was used as a reaction raw material. The catalyst was dispersed in 30 mL of water without adding any catalyst, placed in a reaction vessel similar to the example, filled with argon, and reacted at 200 ° C. for 2 hours with stirring at 300 rpm. As a result, HMF was obtained in a yield of 16.7% and glucose was obtained in a yield of 14.8%. In addition, some minor by-product peaks that could not be identified and quantified were detected.
This reaction result showed that the reaction was slow and the HMF yield was small unless a catalyst was used, and the effect of adding the catalyst into the reaction system could be verified.

(Example 8)
In this example, Merck microcrystalline cellulose was used as a raw material without any ball mill treatment. As in Example 1, 1.8 g of the catalyst was added and dispersed in 30 mL of water, placed in a reaction vessel similar to that in Example 1, sealed with argon, heated at 200 ° C. for 2 hours with stirring at 300 rpm, and allowed to react. It was.
As a result, HMF was obtained in a yield of 12.8% and glucose was obtained in a yield of 5.8%.

Example 9
In Example 1, the inside of the reaction vessel was replaced with argon to eliminate the influence of oxygen in the air. However, in this example, in order to clarify the adverse effect of residual air, air was not replaced without performing argon replacement. The reaction was carried out while remaining. Other conditions were the same as in Example 1.
As a result, the HMF yield decreased to 23.1% and the glucose yield also decreased to 4.9%.
This result shows the adverse effect of air, and the effect of replacing the inside of the reaction vessel with an inert gas such as argon became clear.

(Comparative Example 2)
With respect to Example 7 using natural wood, an experiment was conducted to examine the effect of the presence or absence of a catalyst.
As the raw material obtained by subjecting the powder of Akita cedar to ball mill treatment for 48 hours, which has obtained excellent reaction results in Example 7, 0.456 g of wood so that the amount of glucose becomes 0.2 g was used. Using this raw material, the reaction was conducted under the same conditions as in Example 7 without adding any catalyst. As a result, the HMF yield was 24.2% and the glucose yield was 11.0%.
As a result, when the catalyst was not used, the HMF yield was reduced, and the effect of using the catalyst became clear.

Claims (5)

  A method for producing 5-hydroxymethylfurfural, wherein cellulose and / or a cellulose-containing substance is used as a reaction raw material and heat-treated in a catalyst composed of calcium phosphate and an aqueous solvent to obtain 5-hydroxymethylfurfural.   2. The method for producing 5-hydroxymethylfurfural according to claim 1, wherein before the heat treatment, the reaction raw material alone or mixed with the catalyst is subjected to pulverization and rubbing treatment.   The method for producing 5-hydroxymethylfurfural according to claim 2, wherein the pretreatment is a ball mill treatment.   The method for producing 5-hydroxymethylfurfural according to any one of claims 1 to 3, wherein the heat treatment is performed in an inert gas.   The method for producing 5-hydroxymethylfurfural according to any one of claims 1 to 4, wherein the heat treatment is performed at 180 to 220 ° C.

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