Method for producing furan-2,5-dicarboxylic acid

Abstract

Provided is a method for producing furan-2,5-dicarboxylic acid, the method comprising: an oxidation step 1 for performing an oxidation reaction by continuously supplying 5-methylfurfural and an oxygen-containing gas under a supply condition 1 or 2 in the presence of a lower aliphatic carboxylic acid, a bromine compound, and a metal catalyst; and an oxidation step 2 for performing an oxidation reaction under a supply condition in which the oxygen concentration of the oxygen-containing gas and / or the molar ratio of oxygen in the oxygen-containing gas to the 5-methylfurfural is greater than those in the oxidation step 1. In the supply condition 1, the oxygen concentration of the oxygen-containing gas is 10 vol% or more and less than 15 vol%, and the molar ratio of oxygen in the oxygen-containing gas to the 5-methylfurfural is 2.5-4.2. In the supply condition 2, the oxygen concentration is 15-30 vol% and the molar ratio is 1.0 or more and less than 2.5.

Description

Method for producing furan-2,5-dicarboxylic acid

[0001] The present invention relates to a method for producing furan-2,5-dicarboxylic acid.

[0002] Furan-2,5-dicarboxylic acid (FDCA) is a highly useful compound as a raw material or intermediate for resins such as polyethylene furanoate, paints, fibers, and the like. Furan-2,5-dicarboxylic acid can be synthesized from furfural compounds derived from cellulose, a biomass raw material, making it particularly useful as a biomass-derived resin and fiber raw material that can replace fossil fuels. Examples of such furfural compounds include 5-substituted furfurals such as 5-hydroxymethylfurfural and 5-methylfurfural. Unlike 5-hydroxymethylfurfural, which is produced from edible sugars such as glucose and fructose, 5-methylfurfural can be produced from non-edible materials such as cellulose, making it a promising biomass raw material with reduced environmental impact.

[0003] Methods for producing furan-2,5-dicarboxylic acid include oxidation of 5-substituted furfural. Among these, liquid-phase oxidation methods using oxygen-containing gases such as air have been developed. For example, Patent Document 1 discloses a method for oxidizing 5-hydroxymethylfurfural and its esters to furan-2,5-dicarboxylic acid in the presence of a cobalt- and manganese-based oxidation catalyst containing bromine at 140 to 200°C and an oxygen partial pressure of 1 to 10 bar. Patent Document 2 discloses a method for producing furan-2,5-dicarboxylic acid by oxidizing a furan composition containing a first compound such as 5-(acetoxymethyl)-2-furoic acid and a second compound such as 5-substituted furfural in the presence of a catalyst, with the aim of improving the selectivity and yield of FDCA. Furthermore, Patent Document 3 discloses a method for producing 2,5-furandicarboxylic acid, which is intended to obtain FDCA in high yield, by supplying 5-methylfurfural, an oxidation gas, and further alkoxymethyl-2,5-furfural in the presence of a catalyst containing cobalt, manganese, and bromine and acetic acid, oxidizing the mixture at 150 to 210°C, and separating solid 2,5-furandicarboxylic acid from the crude product.

[0004] Patent Publication No. 2013-507359 U.S. Patent No. 9,321,744 International Publication No. 2023 / 242152

[0005] Patent Documents 1 and 2 describe methods for producing FDCA using 5-methylfurfural as a raw material. However, the FDCA yield remains low, ranging from 39.94% to 46.25%. Patent Document 3 describes a method for producing FDCA using a mixture of alkoxymethylfurfural and 5-methylfurfural as a raw material, and Comparative Examples 2 to 4 describe a method for producing FDCA using only 5-methylfurfural as a raw material and air diluted to an 8% oxygen concentration as the oxidizing agent (oxidizing gas). However, using a gas with an 8% oxygen concentration as the main oxidizing gas poses many industrial challenges for industrial production, such as the cost of gas recovery and the nitrogen used to reduce the oxygen concentration. Furthermore, there is no description regarding the amount of air relative to the substrate furfural compound. Therefore, when the reaction is carried out using only air, which is industrially advantageous, under the conditions described in Patent Document 3, an increase in the off-gas oxygen concentration cannot be suppressed. Furthermore, even if the air supply rate is reduced to increase the oxygen conversion rate and suppress the increase in the off-gas oxygen concentration, there is the problem of a deterioration in the reaction yield. Therefore, there has been a demand for a method for producing furan-2,5-dicarboxylic acid in a high yield that is safe from an industrial standpoint. Therefore, an object of the present invention is to provide a method for producing furan-2,5-dicarboxylic acid in a high yield, with an excellent conversion rate, and with high industrial safety.

[0006] As a result of extensive research, the present inventors have found that the above-mentioned problems can be solved by a method having two oxidation steps in which 5-methylfurfural and an oxygen-containing gas are supplied under specific conditions. That is, the present invention relates to the following: [1] A method for producing furan-2,5-dicarboxylic acid, comprising: an oxidation step 1 in which 5-methylfurfural and an oxygen-containing gas are continuously supplied to a reaction vessel in the presence of a lower aliphatic carboxylic acid, a bromine compound, and a metal catalyst under the following supply condition 1 or the following supply condition 2 to carry out an oxidation reaction; and an oxidation step 2 in which, after the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are continuously supplied to the reaction vessel under supply conditions in which at least one of the oxygen concentration of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is higher than that of the oxidation step 1. Supply condition 1: The oxygen concentration of the oxygen-containing gas is 10% by volume or more and less than 15% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.2. Supply condition 2: The oxygen concentration of the oxygen-containing gas is 15 to 30% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 1.0 to 2.5. [1a] A method for producing furan-2,5-dicarboxylic acid, comprising: an oxidation step 1 in which 5-methylfurfural and an oxygen-containing gas are continuously supplied to a reaction vessel under the following supply condition 1 or the following supply condition 2 in the presence of a lower aliphatic carboxylic acid, a bromine compound, and a metal catalyst to carry out an oxidation reaction; and an oxidation step 2 in which, after the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are continuously supplied to the reaction vessel to carry out an oxidation reaction under supply conditions such that at least one of the oxygen concentration of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is higher than that in the oxidation step 1, wherein the bromine compound is at least one selected from the group consisting of hydrogen bromide and bromide salts, and the metal catalyst is at least one selected from the group consisting of cobalt catalysts and manganese catalysts.Supply condition 1: the oxygen concentration of the oxygen-containing gas is 10% by volume or more and less than 15% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.2; Supply condition 2: the oxygen concentration of the oxygen-containing gas is 15 to 30% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 1.0 to less than 2.5. [2] A method for producing furan-2,5-dicarboxylic acid according to [1] or [1a] above, wherein the supply conditions for oxidation step 1 are supply condition 1 and the oxygen concentration of the oxygen-containing gas in oxidation step 2 is 5% by volume or more higher than the oxygen concentration of the oxygen-containing gas in oxidation step 1, or the supply conditions for oxidation step 1 are supply condition 2 and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in oxidation step 2 is 1.5 or more higher than the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in oxidation step 1. [3] The method for producing furan-2,5-dicarboxylic acid according to [1], [1a] or [2] above, wherein the supply conditions for 5-methylfurfural and the oxygen-containing gas in oxidation step 2 are supply condition 3 below. Supply condition 3: The oxygen concentration of the oxygen-containing gas is 15 to 30% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.2. [4] The method for producing furan-2,5-dicarboxylic acid according to any one of [1], [1a], [2] to [3] above, wherein the supply conditions for 5-methylfurfural and the oxygen-containing gas in oxidation step 1 are supply condition 4 below. [5] A method for producing furan-2,5-dicarboxylic acid according to any one of [1], [1a], and [2] to [4] above, wherein the oxygen partial pressure during supply of the oxygen-containing gas in oxidation step 1 is 0.05 to 0.2 MPa. [6] A method for producing furan-2,5-dicarboxylic acid according to any one of [1], [1a], and [2] to [5] above, wherein the reaction temperature during oxidation step 1 is 130 to 180°C. [7] A method for producing furan-2,5-dicarboxylic acid according to any one of [1], [1a], and [2] to [6] above, wherein the reaction temperature during oxidation step 1 and the oxygen partial pressure during supply of the oxygen-containing gas satisfy the following condition 5 or condition 6:Condition 5: the reaction temperature is 130°C or higher and lower than 160°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.10 to 0.2 MPa. Condition 6: the reaction temperature is 160 to 180°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.05 MPa or higher and lower than 0.10 MPa. [8] A method for producing furan-2,5-dicarboxylic acid according to any one of [1], [1a], and [2] to [7] above, wherein the oxygen partial pressure during supply of the oxygen-containing gas in oxidation step 2 is 0.05 to 0.2 MPa. [9] A method for producing furan-2,5-dicarboxylic acid according to any one of [1], [1a], and [2] to [8] above, wherein the reaction temperature in oxidation step 2 is 130 to 180°C.

[10] A method for producing furan-2,5-dicarboxylic acid according to any one of [1], [1a], and [2] to [9] above, wherein the reaction temperature and the oxygen partial pressure during supply of the oxygen-containing gas in oxidation step 2 satisfy the following condition 7 or the following condition 8. Condition 7: the reaction temperature is 130°C or higher and lower than 160°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.15 to 0.2 MPa. Condition 8: the reaction temperature is 160 to 180°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.11 MPa or higher and lower than 0.15 MPa.

[11] The method for producing furan-2,5-dicarboxylic acid according to any one of the above items [1], [1a], and [2] to

[10] , wherein the reaction time of oxidation step 1 is 3 to 200 minutes.

[12] The method for producing furan-2,5-dicarboxylic acid according to any one of the above items [1], [1a], and [2] to

[11] , wherein the metal catalyst is at least one selected from the group consisting of aliphatic carboxylates of cobalt and aliphatic carboxylates of manganese.

[13] The method for producing furan-2,5-dicarboxylic acid according to any one of the above items [1], [1a], and [2] to

[12] , wherein the lower aliphatic carboxylic acid is acetic acid.

[14] The method for producing furan-2,5-dicarboxylic acid according to any one of [1], [1a], and [2] to

[13] above, wherein the bromine compound is at least one selected from the group consisting of hydrogen bromide, sodium bromide, potassium bromide, and ammonium bromide.

[15] The method for producing furan-2,5-dicarboxylic acid according to any one of [1], [1a], and [2] to

[14] above, wherein the gas other than oxygen contained in the oxygen-containing gas in oxidation step 1 contains 95% by volume or more of nitrogen.

[16] The method for producing furan-2,5-dicarboxylic acid according to any one of [1], [1a], and [2] to

[15] above, wherein the gas other than oxygen contained in the oxygen-containing gas in oxidation step 2 contains nitrogen in an amount of 95% by volume or more.

[0007] According to the present invention, it is possible to provide a method for producing furan-2,5-dicarboxylic acid, which can produce furan-2,5-dicarboxylic acid in high yield, has an excellent conversion rate, and is industrially safe.

[0008] The method for producing furan-2,5-dicarboxylic acid of the present invention includes an oxidation step 1 in which 5-methylfurfural and an oxygen-containing gas are continuously supplied to a reaction vessel in the presence of a lower aliphatic carboxylic acid, a bromine compound, and a metal catalyst under the following supply condition 1 or supply condition 2 to carry out an oxidation reaction, and an oxidation step 2 in which, after the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are continuously supplied to the reaction vessel under supply conditions in which at least one of the oxygen concentration of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is higher than that of the oxidation step 1. Supply condition 1: the oxygen concentration of the oxygen-containing gas is 10% by volume or more but less than 15% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.2. Supply condition 2: the oxygen concentration of the oxygen-containing gas is 15 to 30% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 1.0 or more but less than 2.5.

[0009] The method for producing furan-2,5-dicarboxylic acid of the present invention preferably includes an oxidation step 1 in which 5-methylfurfural and an oxygen-containing gas are continuously supplied to a reaction vessel under the following supply condition 1 or supply condition 2 in the presence of a lower aliphatic carboxylic acid, a bromine compound, and a metal catalyst to carry out an oxidation reaction, and an oxidation step 2 in which, after the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are continuously supplied to the reaction vessel under supply conditions such that at least one of the oxygen concentration of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is higher than that in the oxidation step 1, and the bromine compound is at least one selected from the group consisting of hydrogen bromide and bromide salts, and the metal catalyst is at least one selected from the group consisting of cobalt catalysts and manganese catalysts. Supply condition 1: The oxygen concentration of the oxygen-containing gas is 10% by volume or more and less than 15% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.2. Supply condition 2: The oxygen concentration of the oxygen-containing gas is 15 to 30% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 1.0 or more and less than 2.5. The production method of the present invention will be described in detail below.

[0010] [Oxidation Step 1] In the method for producing furan-2,5-dicarboxylic acid of the present invention, first, oxidation step 1 is carried out by continuously supplying 5-methylfurfural and an oxygen-containing gas to a reaction vessel in the presence of a lower aliphatic carboxylic acid, a bromine compound, and a metal catalyst under the following supply condition 1 or the following supply condition 2, and carrying out an oxidation reaction. Supply condition 1: The oxygen concentration of the oxygen-containing gas is 10% by volume or more and less than 15% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.2. Supply condition 2: The oxygen concentration of the oxygen-containing gas is 15 to 30% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 1.0 to less than 2.5.

[0011] It is preferable to previously place the lower aliphatic carboxylic acid, bromine compound, and metal catalyst in the reaction vessel before supplying 5-methylfurfural and oxygen-containing gas, and it is preferable to continuously supply 5-methylfurfural and oxygen-containing gas to the reaction vessel containing the lower aliphatic carboxylic acid, bromine compound, and metal catalyst under supply condition 1 or supply condition 2. The amounts of the lower aliphatic carboxylic acid, bromine compound, and metal catalyst previously placed in the reaction vessel may be adjusted appropriately depending on the shape, volume, etc. of the reaction vessel, but it is preferable to use amounts that can be sufficiently mixed with 5-methylfurfural and oxygen-containing gas and that can adjust the initial reaction temperature to the target temperature so that the oxidation reaction in this step can be smoothly initiated.

[0012] In addition, it is preferable to mix a lower aliphatic carboxylic acid, a bromine compound, and a metal catalyst with the 5-methylfurfural supplied in this step and then supply the resulting mixture. That is, it is preferable to continuously supply 5-methylfurfural, a lower aliphatic carboxylic acid, a bromine compound, a metal catalyst, and an oxygen-containing gas under supply condition 1 or supply condition 2. Furthermore, in oxidation step 2, which is performed after oxidation step 1, it is preferable to mix a lower aliphatic carboxylic acid, a bromine compound, and a metal catalyst with the 5-methylfurfural that is continuously supplied. That is, it is preferable that oxidation step 2 is a step, which is performed after oxidation step 1, in which 5-methylfurfural, a lower aliphatic carboxylic acid, a bromine compound, a metal catalyst, and an oxygen-containing gas are continuously supplied to the reaction vessel to carry out an oxidation reaction.

[0013] The amount of lower aliphatic carboxylic acid, the amount of bromine compound, and the amount of metal catalyst are preferably determined relative to the total 5-methylfurfural used in this production method, and when oxidation step 2 is carried out continuously for a long period of time, it is preferable to mix them with the 5-methylfurfural and supply them to the reaction vessel in a ratio (amount) similar to the preferred ratio (amount) of each component shown below relative to the 5-methylfurfural continuously supplied.

[0014] <Lower aliphatic carboxylic acid> The lower aliphatic carboxylic acid used in the present production method is preferably an aliphatic carboxylic acid having 1 to 4 carbon atoms, more preferably an aliphatic carboxylic acid having 2 to 3 carbon atoms, and even more preferably an aliphatic carboxylic acid having 2 carbon atoms. Specific examples of the lower aliphatic carboxylic acid include at least one selected from the group consisting of formic acid, acetic acid, propionic acid, and butyric acid, more preferably at least one selected from the group consisting of acetic acid and propionic acid, and even more preferably acetic acid. When using acetic acid, a mixed solution may be prepared in advance by mixing water and acetic acid, as described below, or acetic acid alone may be used. From the viewpoint of facilitating the dissolution of the bromine compound and the metal catalyst, it is preferable to use aqueous acetic acid, which is a mixed solution of water and acetic acid. The use of the above lower aliphatic carboxylic acid is preferable because it can enhance the activity of the catalyst.

[0015] The amount of lower aliphatic carboxylic acid used in this production method is preferably 100 to 1500 parts by mass, more preferably 200 to 800 parts by mass, even more preferably 300 to 700 parts by mass, and even more preferably 400 to 600 parts by mass, relative to 100 parts by mass of total 5-methylfurfural. By setting the amount of lower aliphatic carboxylic acid within the above range, the viscosity of the reaction system can be appropriately adjusted in this step and the subsequent oxidation step 2, making it easier to handle. It also makes it possible to control the reaction heat. Note that "total 5-methylfurfural" refers to "all 5-methylfurfural introduced into the reaction vessel from oxidation step 1 through the end of oxidation step 2 and used in the oxidation reaction." The same applies hereinafter. One type of lower aliphatic carboxylic acid may be used, or two or more types may be used.

[0016] <Bromine Compound> The bromine compound used in the present production method is preferably hydrogen bromide, a bromide salt, or an organic bromine compound, more preferably at least one selected from the group consisting of hydrogen bromide and a bromide salt, and even more preferably hydrogen bromide. Hydrogen bromide is preferably used as an aqueous solution. Specific bromide salts include sodium bromide, potassium bromide, and ammonium bromide.

[0017] The amount of the bromine compound used in this production method is preferably 1.0 to 6.0 parts by mass, more preferably 1.5 to 5.5 parts by mass, even more preferably 2.0 to 4.5 parts by mass, and even more preferably 3.0 to 3.5 parts by mass, calculated as bromine, relative to 100 parts by mass of total 5-methylfurfural. Setting the amount of the bromine compound within the above range is preferable because it inhibits corrosion of the reaction vessel and the like, improves the reaction rate in this step and the subsequent oxidation step 2, and improves the yield. One type of bromine compound may be used, or two or more types may be used.

[0018] <Metal Catalyst> The metal catalyst used in this production method is preferably at least one selected from the group consisting of transition metal catalysts and rare earth metal catalysts, more preferably a transition metal catalyst. A specific transition metal catalyst is preferably at least one selected from the group consisting of cobalt catalysts, manganese catalysts, zirconium catalysts, nickel catalysts, and cerium catalysts, more preferably at least one selected from the group consisting of cobalt catalysts and manganese catalysts. It is even more preferable to use both a cobalt catalyst and a manganese catalyst. As described above, the metal catalyst used in this step is preferably at least one selected from the group consisting of cobalt catalysts, manganese catalysts, zirconium catalysts, nickel catalysts, and cerium catalysts, more preferably at least one selected from the group consisting of cobalt catalysts and manganese catalysts. It is even more preferable to use both a cobalt catalyst and a manganese catalyst. The metal catalyst can be used in the form of a salt, elemental metal, oxide, hydroxide, or the like. The metal catalyst used in this step is preferably a salt, more preferably an aliphatic carboxylate, even more preferably a lower aliphatic carboxylate, and even more preferably an acetate. Of these, at least one selected from the group consisting of cobalt acetate and manganese acetate is even more preferred. Use of the above metal catalyst is preferred because furan-2,5-dicarboxylic acid can be obtained in high yield.

[0019] The amount of metal catalyst used in this production method is, in terms of metal element, preferably 0.05 to 4.0 parts by mass, more preferably 0.1 to 3.5 parts by mass, even more preferably 0.15 to 3.0 parts by mass, and even more preferably 0.2 to 2.5 parts by mass, relative to 100 parts by mass of the total 5-methylfurfural used as a raw material in the oxidation reaction. Setting the amount of metal catalyst within the above range is preferable because it improves the reaction rate and yield in this step and the subsequent oxidation step 2 while suppressing side reactions. When the catalyst concentration is equal to or greater than the lower limit, the reaction rate and yield are improved. When the catalyst concentration is equal to or less than the upper limit, the catalyst cost is reduced and there is no adverse effect on the reaction. One type of metal catalyst may be used, or two or more types may be used. The reasons why this production method including oxidation step 1 can produce furan-2,5-dicarboxylic acid in high yield with excellent conversion using 5-methylfurfural as a raw material and also enables industrially safe production are unclear, but are thought to be as follows. By oxidizing 5-methylfurfural under specific mild conditions in this oxidation step 1, a portion of the 5-methylfurfural becomes active species, which is thought to enable the oxidation reaction in the subsequent oxidation step 2 to proceed smoothly. As a result, oxygen does not accumulate, and the oxygen concentration in the discharged gas (off-gas) can be kept low, which is thought to enable highly safe production. Furthermore, because the raw materials are consumed evenly during the reaction, side reactions are suppressed, and the conversion rate is excellent and the yield is thought to be high.

[0020] <Water> Water may be used in the production method of the present invention. The use of water is preferred because it makes the bromine compound more soluble. The amount of water used in the production method of the present invention is preferably 10 to 200 parts by mass, more preferably 10 to 100 parts by mass, even more preferably 10 to 50 parts by mass, and still more preferably 10 to 40 parts by mass, relative to 100 parts by mass of the total 5-methylfurfural used as a raw material in the oxidation reaction. When the water concentration is within the above range, the bromine compound can be dissolved while preventing a decrease in catalytic activity, thereby improving the yield.

[0021] <Oxygen-Containing Gas> The oxygen-containing gas used in this step is a mixed gas of oxygen and a gas other than oxygen. The gas other than oxygen mixed with oxygen is preferably an inert gas, more preferably nitrogen. The gas other than oxygen contained in the oxygen-containing gas in oxidation step 1 preferably contains 95% or more by volume of nitrogen, and more preferably 98% or more by volume of nitrogen. Air is preferred as the gas containing nitrogen and oxygen. Furthermore, to adjust the oxygen concentration, oxygen gas or an oxygen-containing gas with a high oxygen concentration may be mixed with an inert gas such as nitrogen. For example, a gas having an oxygen concentration of 10% or more by volume but less than 15% by volume under supply condition 1 of this step can be obtained by mixing nitrogen with air having an oxygen concentration of 20.9% by volume. Furthermore, air having an oxygen concentration of 20.9% by volume can be used as a gas having an oxygen concentration of 15 to 30% by volume under supply condition 2 of this step. As described above, in this step, it is preferable to adjust the oxygen concentration using air and nitrogen to obtain an oxygen-containing gas. As the oxygen-containing gas, it is preferable to use air or a mixed gas of air and nitrogen from the viewpoints of safety and economy.

[0022] <Conditions for Oxidation Step 1> In this step, 5-methylfurfural and an oxygen-containing gas are continuously supplied to the reaction vessel under the above-mentioned supply condition 1 or the above-mentioned supply condition 2, and oxidation reaction is carried out in the oxidation step 1.

[0023] Here, "continuously supplying 5-methylfurfural and an oxygen-containing gas" means that the oxidation reaction of 5-methylfurfural in the reaction vessel and the supply of 5-methylfurfural and an oxygen-containing gas are carried out in parallel, and that the supply of 5-methylfurfural and an oxygen-containing gas is carried out for at least 50% of the reaction time from the start of the oxidation reaction to the end of the oxidation reaction. The supply of 5-methylfurfural is preferably carried out for 60% or more of the reaction time of oxidation step 1, more preferably 80% or more, even more preferably 90% or more, and may be carried out for 100%. The supply of oxygen-containing gas is preferably carried out for 60% or more of the reaction time of oxidation step 1, more preferably 80% or more, even more preferably 90% or more, and may be carried out for 100%.

[0024] The above supply condition 1 or supply condition 2 is suitable as a condition for causing an oxidation reaction under relatively mild conditions. By carrying out the oxidation reaction under such conditions, it is believed that a portion of the 5-methylfurfural can be converted into an active species, which allows the oxidation reaction in the subsequent oxidation step 2 to proceed smoothly.

[0025] Supply condition 1 is a condition in which the oxygen concentration of the oxygen-containing gas is 10% by volume or more and less than 15% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.2. When 5-methylfurfural and an oxygen-containing gas are supplied under supply condition 1, the oxygen concentration of the oxygen-containing gas used is 10% by volume or more and less than 15% by volume, preferably 10 to 14% by volume, more preferably 10 to 13% by volume, and even more preferably 11 to 13% by volume. When the oxygen concentration of the oxygen-containing gas is within the above range, industrial safety is high, conversion is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0026] When 5-methylfurfural and an oxygen-containing gas are supplied under supply condition 1, the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.2, preferably 3.0 to 4.0, more preferably 3.1 to 3.8, and even more preferably 3.2 to 3.5. When the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield. Supply condition 1 is thought to be able to prepare active species for the subsequent oxidation reaction industrially safely by setting the oxygen concentration low.

[0027] Supply condition 2 is a condition in which the oxygen concentration of the oxygen-containing gas is 15 to 30% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 1.0 or more and less than 2.5. When 5-methylfurfural and an oxygen-containing gas are supplied under supply condition 2, the oxygen concentration of the oxygen-containing gas used is 15 to 30% by volume, preferably 18 to 28% by volume, more preferably 20 to 25% by volume, and even more preferably 20 to 22% by volume. When the oxygen concentration of the oxygen-containing gas is within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0028] When 5-methylfurfural and an oxygen-containing gas are supplied under supply condition 2, the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 1.0 or more but less than 2.5, preferably 1.0 to 2.3, more preferably 1.3 to 2.1, and even more preferably 1.5 to 2.0. When the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield. Supply condition 2 is thought to be able to prepare active species for the subsequent oxidation reaction industrially safely by setting the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural low.

[0029] In the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are supplied under the above supply condition 1 or the above supply condition 2. Regardless of the supply condition, the supply condition for 5-methylfurfural and an oxygen-containing gas in the oxidation step 1 is preferably the following supply condition 4. Supply condition 4: The product of the oxygen concentration (volume) of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 0.3 to 0.5.

[0030] The product of the oxygen concentration (volume) of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is preferably 0.3 to 0.5, more preferably 0.35 to 0.45, even more preferably 0.37 to 0.45, and still more preferably 0.40 to 0.44. When the product of the oxygen concentration (volume) of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is within the above range, industrial safety is high, and furan-2,5-dicarboxylic acid can be obtained in a high yield with a high conversion rate.

[0031] In oxidation step 1, 5-methylfurfural and an oxygen-containing gas are supplied under the above supply condition 1 or the above supply condition 2. Regardless of the supply condition, the oxygen partial pressure during supply of the oxygen-containing gas in oxidation step 1 is preferably 0.05 to 0.2 MPa. In oxidation step 1, 5-methylfurfural and an oxygen-containing gas are supplied under the above supply condition 1 or the above supply condition 2. Regardless of the supply condition, the reaction temperature in oxidation step 1 is preferably 130 to 180°C. There are preferred combinations of oxygen partial pressure and reaction temperature. Specific examples are shown below.

[0032] In the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are supplied under the above supply condition 1 or the above supply condition 2. Regardless of the supply condition, the reaction temperature and the oxygen partial pressure during supply of the oxygen-containing gas in the oxidation step 1 preferably satisfy the following condition 5 or 6. Condition 5: Reaction temperature is 130°C or higher and lower than 160°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.10 to 0.2 MPa. Condition 6: Reaction temperature is 160 to 180°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.05 MPa or higher and lower than 0.10 MPa.

[0033] Condition 5 is a condition in which the reaction temperature is 130°C or higher and lower than 160°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.10 to 0.2 MPa. When 5-methylfurfural and an oxygen-containing gas are supplied under Condition 5, the reaction temperature is preferably 130°C or higher and lower than 160°C, more preferably 135 to 155°C, and even more preferably 140 to 150°C. By keeping the reaction temperature within the above range, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and a high conversion rate.

[0034] When 5-methylfurfural and an oxygen-containing gas are supplied under condition 5, the oxygen partial pressure during supply of the oxygen-containing gas is preferably 0.10 to 0.2 MPa, more preferably 0.12 to 0.2 MPa, and even more preferably 0.15 to 0.2 MPa. By keeping the oxygen partial pressure within the above range, industrial safety is high, and furan-2,5-dicarboxylic acid can be obtained with a high conversion rate and high yield. Condition 5 is thought to be able to prepare active species for the subsequent oxidation reaction industrially safely by setting the reaction temperature low.

[0035] Condition 6 is a condition in which the reaction temperature is 160 to 180°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.05 MPa or more and less than 0.10 MPa. When 5-methylfurfural and an oxygen-containing gas are supplied under Condition 6, the reaction temperature is preferably 160 to 180°C, more preferably 165 to 180°C, and even more preferably 170 to 175°C. By keeping the reaction temperature within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0036] When 5-methylfurfural and an oxygen-containing gas are supplied under condition 6, the oxygen partial pressure during supply of the oxygen-containing gas is preferably 0.05 MPa or more and less than 0.10 MPa, more preferably 0.05 to 0.09 MPa, and even more preferably 0.05 to 0.08 MPa. By keeping the oxygen partial pressure within the above range, industrial safety is high, and furan-2,5-dicarboxylic acid can be obtained with a high conversion rate and high yield. Condition 6 is thought to be able to prepare active species for the subsequent oxidation reaction industrially safely by setting the oxygen partial pressure low.

[0037] The reaction time for oxidation step 1 is not particularly limited, but is preferably 3 to 200 minutes, more preferably 3 to 100 minutes, even more preferably 3 to 60 minutes, still more preferably 3 to 30 minutes, even more preferably 3 to 20 minutes, and even more preferably 3 to 10 minutes. By keeping the reaction time within the above range, it is believed that active species for the subsequent oxidation reaction can be prepared industrially safely.

[0038] [Oxidation Step 2] The method for producing furan-2,5-dicarboxylic acid of the present invention includes, after the oxidation step 1, an oxidation step 2 in which 5-methylfurfural and an oxygen-containing gas are continuously supplied to the reaction vessel to carry out an oxidation reaction under supply conditions in which at least one of the oxygen concentration of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is higher than that in the oxidation step 1. By the oxidation step 2, furan-2,5-dicarboxylic acid can be obtained in high yield.

[0039] In the oxidation step 2, it is preferable to mix a lower aliphatic carboxylic acid, a bromine compound, and a metal catalyst with the continuously supplied 5-methylfurfural. That is, the oxidation step 2 is preferably a step that follows the oxidation step 1, in which 5-methylfurfural, a lower aliphatic carboxylic acid, a bromine compound, a metal catalyst, and an oxygen-containing gas are continuously supplied to the reaction vessel to carry out the oxidation reaction.

[0040] The amount of lower aliphatic carboxylic acid, the amount of bromine compound, and the amount of metal catalyst are preferably determined relative to the total amount of 5-methylfurfural used in this production method, and when oxidation step 2 is carried out continuously for a long period of time, it is preferable to mix them in a ratio similar to the preferred ratio relative to the 5-methylfurfural continuously supplied and supply them to the reaction vessel.

[0041] In oxidation step 1, 5-methylfurfural is oxidized under relatively mild conditions, which is thought to convert a portion of the 5-methylfurfural into an active species. The main oxidation reaction is then carried out in the subsequent step to obtain furan-2,5-dicarboxylic acid. Therefore, in oxidation step 2, the oxidation reaction is thought to proceed smoothly by increasing the oxygen concentration and / or the molar ratio of oxygen to the raw material compared to oxidation step 1. This is thought to prevent oxygen retention and enable highly safe production. Furthermore, because the raw materials are consumed evenly during the reaction, side reactions are suppressed, resulting in a high conversion rate and a high yield.

[0042] <Oxygen-Containing Gas> The oxygen-containing gas used in this step is the same as the oxygen-containing gas described in Oxidation Step 1, but its oxygen concentration is the same as or higher than that of the oxygen-containing gas used in Oxidation Step 1. Specifically, the oxygen-containing gas used in this step is a mixed gas of oxygen and a gas other than oxygen. The gas other than oxygen mixed with oxygen is preferably an inert gas, more preferably nitrogen. The gas other than oxygen contained in the oxygen-containing gas in Oxidation Step 2 preferably contains 95% by volume or more of nitrogen, and preferably 98% by volume or more of nitrogen. Air is preferred as the gas containing nitrogen and oxygen. Furthermore, in order to adjust the oxygen concentration, oxygen gas or an oxygen-containing gas with a high oxygen concentration may be mixed with an inert gas such as nitrogen. For example, air with an oxygen concentration of 20.9% by volume can be used as is. In this step, it is preferable to adjust the oxygen concentration using air and nitrogen to obtain the oxygen-containing gas. From the standpoints of safety and economy, it is preferable to use air or a mixed gas of air and nitrogen as the oxygen-containing gas.

[0043] <Conditions for Oxidation Step 2> This step is a step in which 5-methylfurfural and an oxygen-containing gas are continuously supplied to the reaction vessel to carry out an oxidation reaction under supply conditions in which at least one of the oxygen concentration of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is greater than that in Oxidation Step 1.

[0044] Here, "continuously supplying 5-methylfurfural and an oxygen-containing gas" means that the oxidation reaction of 5-methylfurfural in the reaction vessel and the supply of 5-methylfurfural and an oxygen-containing gas are carried out in parallel, and that the supply of 5-methylfurfural and an oxygen-containing gas is carried out for at least 50% of the reaction time from the start of the oxidation reaction to the end of the oxidation reaction. The supply of 5-methylfurfural is preferably carried out for at least 60% of the reaction time of oxidation step 2, more preferably at least 80%, even more preferably at least 90%, and may be carried out for 100%. The supply of oxygen-containing gas is preferably carried out for at least 60% of the reaction time of oxidation step 2, more preferably at least 80%, even more preferably at least 90%, and may be carried out for 100%.

[0045] When the supply conditions for oxidation step 1 are the above-mentioned supply condition 1, the oxygen concentration was set low, so it is preferable to increase the oxygen concentration in this step. Furthermore, when the supply conditions for oxidation step 1 are the above-mentioned supply condition 2, the molar ratio of oxygen to the raw material was set low, so it is preferable to increase the molar ratio of oxygen to the raw material in this step. For example, it is preferable that the supply conditions for oxidation step 1 are supply condition 1 and the oxygen concentration of the oxygen-containing gas in oxidation step 2 is 5% by volume or more higher than the oxygen concentration of the oxygen-containing gas in oxidation step 1, or that the supply conditions for oxidation step 1 are supply condition 2 and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in oxidation step 2 is 1.5 or more higher than the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in oxidation step 1.

[0046] When the supply conditions for oxidation step 1 are supply conditions 1, the oxygen concentration of the oxygen-containing gas in oxidation step 2 is preferably at least 1 vol % higher, preferably at least 3 vol %, preferably at least 5 vol %, preferably at least 7 vol %, preferably at least 8 vol %, and the upper limit is preferably at most 20 vol %, preferably at most 15 vol %, preferably at most 10 vol % higher than the oxygen concentration of the oxygen-containing gas in oxidation step 1. When the oxygen concentration of the oxygen-containing gas is within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0047] When the supply conditions for oxidation step 1 are supply conditions 2, the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in oxidation step 2 is preferably at least 0.5 higher, preferably at least 1.0 higher, preferably at least 1.5 higher, preferably at least 1.6 higher, preferably at least 1.8 higher than the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in oxidation step 1, and the upper limit is preferably at most 3.0 higher, preferably at most 2.5 higher, preferably at most 2.0 higher than the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in oxidation step 1. When the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is within the above range, industrial safety is high, conversion is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0048] In the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are supplied under the above supply condition 1 or the above supply condition 2. Regardless of the supply condition, the supply condition for 5-methylfurfural and an oxygen-containing gas in the oxidation step 2 is preferably the following supply condition 3. Supply condition 3: The oxygen concentration of the oxygen-containing gas is 15 to 30% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.2.

[0049] When 5-methylfurfural and an oxygen-containing gas are supplied under supply condition 3, the oxygen concentration of the oxygen-containing gas used is 15 to 30% by volume, preferably 18 to 28% by volume, more preferably 20 to 25% by volume, and even more preferably 20 to 22% by volume. When the oxygen concentration of the oxygen-containing gas is within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0050] When 5-methylfurfural and an oxygen-containing gas are supplied under supply condition 3, the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.2, preferably 3.0 to 4.0, more preferably 3.1 to 3.8, and even more preferably 3.2 to 3.5. When the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0051] The oxygen partial pressure when supplying the oxygen-containing gas in the oxidation step 2 is preferably 0.05 to 0.2 MPa. The reaction temperature in the oxidation step 2 is preferably 130 to 180°C. There are preferred combinations of oxygen partial pressure and reaction temperature. Specific examples are shown below.

[0052] The reaction temperature and the oxygen partial pressure during supply of the oxygen-containing gas in the oxidation step 2 preferably satisfy the following condition 7 or 8. Condition 7: Reaction temperature is 130°C or higher but lower than 160°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.15 to 0.2 MPa. Condition 8: Reaction temperature is 160 to 180°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.11 MPa or higher but lower than 0.15 MPa. Condition 7 is a condition in which the reaction temperature is 130°C or higher but lower than 160°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.15 to 0.2 MPa. When 5-methylfurfural and an oxygen-containing gas are supplied under condition 7, the reaction temperature is preferably 130°C or higher but lower than 160°C, more preferably 135 to 155°C, and even more preferably 140 to 150°C. By keeping the reaction temperature within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0053] When 5-methylfurfural and an oxygen-containing gas are supplied under Condition 7, the oxygen partial pressure during supply of the oxygen-containing gas is preferably 0.15 to 0.2 MPa. By keeping the oxygen partial pressure within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0054] Condition 8 is a condition in which the reaction temperature is 160 to 180°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.11 MPa or more and less than 0.15 MPa. When 5-methylfurfural and an oxygen-containing gas are supplied under condition 8, the reaction temperature is preferably 160 to 180°C, more preferably 165 to 180°C, and even more preferably 170 to 175°C. By keeping the reaction temperature within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0055] When 5-methylfurfural and an oxygen-containing gas are supplied under Condition 8, the oxygen partial pressure during supply of the oxygen-containing gas is preferably 0.11 MPa or more and less than 0.15 MPa, more preferably 0.11 to 0.14 MPa, and even more preferably 0.11 to 0.13 MPa. By keeping the oxygen partial pressure within the above range, industrial safety is high, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0056] The reaction time for the oxidation step 2 is not particularly limited, and the reaction may be carried out until the desired amount (production amount) of furan-2,5-dicarboxylic acid is obtained. In particular, if the oxidation reaction proceeds smoothly, a large amount of furan-2,5-dicarboxylic acid can be efficiently obtained by continuously supplying 5-methylfurfural and a lower aliphatic carboxylic acid over an extended period of time. This is therefore industrially preferable. It is more preferable to continuously supply 5-methylfurfural, a lower aliphatic carboxylic acid, a bromine compound, a metal catalyst, and an oxygen-containing gas over an extended period of time. Furthermore, it is more preferable to appropriately withdraw the resulting furan-2,5-dicarboxylic acid, remove it from the reaction system, and recover it, in conjunction with the continuous supply of 5-methylfurfural and a lower aliphatic carboxylic acid over an extended period of time. By withdrawing furan-2,5-dicarboxylic acid, the reaction can be carried out for a longer period of time, which is industrially preferable. Furthermore, it is even more preferable to continuously withdraw the resulting furan-2,5-dicarboxylic acid, remove it from the reaction system, and recover it. It is even more preferable to continuously supply 5-methylfurfural and a lower aliphatic carboxylic acid and simultaneously continuously withdraw the resulting furan-2,5-dicarboxylic acid, remove it from the reaction system, and recover it.

[0057] It is even more preferable to continuously supply 5-methylfurfural, a lower aliphatic carboxylic acid, a bromine compound, a metal catalyst, and an oxygen-containing gas over an extended period of time and extract the resulting furan-2,5-dicarboxylic acid. It is even more preferable to continuously supply 5-methylfurfural, a lower aliphatic carboxylic acid, a bromine compound, a metal catalyst, and an oxygen-containing gas over an extended period of time and extract the resulting furan-2,5-dicarboxylic acid. It is even more preferable to continuously supply 5-methylfurfural, a lower aliphatic carboxylic acid, a bromine compound, a metal catalyst, and an oxygen-containing gas over an extended period of time and extract the resulting furan-2,5-dicarboxylic acid continuously. The amounts of the lower aliphatic carboxylic acid, the bromine compound, and the metal catalyst supplied here are preferably determined relative to the total 5-methylfurfural used in this production method. When oxidation step 2 is carried out continuously over an extended period of time, it is preferable to mix the components with the 5-methylfurfural and supply them to the reaction vessel in the same ratios (amounts) as the preferred ratios (amounts) of each component shown below relative to the continuously supplied 5-methylfurfural.

[0058] In this step, 5-methylfurfural and an oxygen-containing gas are continuously supplied to the reaction mixture that has been subjected to the oxidation step 1. Preferably, 5-methylfurfural, a lower aliphatic carboxylic acid, a bromine compound, a metal catalyst, and an oxygen-containing gas are continuously supplied to the reaction mixture that has been subjected to the oxidation step 1. For convenience, it is preferable to use the components used in the oxidation step 1 as they are in this step.

[0059] [Other Steps] The method for producing furan-2,5-dicarboxylic acid of the present invention may include any step other than the oxidation step 1 and the oxidation step 2. Examples of the optional step included in the method for producing furan-2,5-dicarboxylic acid of the present invention include a solvent removal step and a purification step.

[0060] The solvent removal step is a step of removing a lower aliphatic carboxylic acid, which is a solvent with a low boiling point, and water from a product containing furan-2,5-dicarboxylic acid, which is the target of this production method. By removing these solvents before removing the by-products produced in the oxidation reaction and the raw material 5-methylfurfural in the subsequent purification step, the subsequent purification step can be carried out efficiently. In the solvent removal step, in order to efficiently remove the solvent, the solvent may be removed by heating and distillation under reduced pressure, or the solvent may be removed by heating and distillation under normal pressure. Alternatively, the solvent may be removed in the purification step described below.

[0061] The purification step may be carried out by any method as long as it can separate and recover the target furan-2,5-dicarboxylic acid with high purity, and may involve, for example, reprecipitation, recrystallization, etc. Furthermore, hydrogenation may be carried out in order to remove coloring components produced as by-products in the oxidation reaction.

[0062] The present invention will be specifically described based on the following examples, but the present invention is not limited to these examples.

[0063] [Evaluation] <Outlet Oxygen Concentration> The outlet oxygen concentration was determined as follows. Apparatus: Portable oxygen meter: POT-101 manufactured by Shimadzu Corporation. Measurement method: The gas exhaust pipe of the autoclave was connected to the oxygen meter, and the outlet oxygen concentration was measured in real time and evaluated according to the following criteria. From the perspective of safety during industrial production, it is preferable that the outlet oxygen concentration be 10% by volume or less, which is the explosion limit of acetic acid, the solvent. A lower outlet oxygen concentration is preferable because it increases the degree of freedom in reaction conditions from a safety perspective. In the examples and comparative examples, evaluation was performed using 8% by volume as the standard concentration at which the explosion limit is not reached even with a sudden concentration fluctuation. For comparative examples in which the outlet oxygen concentration exceeded 8% by volume, the time (minutes) from the start of the oxidation reaction to the point at which the outlet oxygen concentration exceeded 8% by volume is shown in Table 1. (Evaluation Criteria) A: The outlet oxygen concentration was always 8% by volume or less during the oxidation reaction. B: The outlet oxygen concentration exceeded 8% by volume during the oxidation reaction.

[0064] <5-methylfurfural Conversion Rate> The amount (moles) of 5-methylfurfural contained in the oxidation reaction product after oxidation step 2 was calculated by an internal standard method using gas chromatography (internal standard: triphenylmethane), and subtracted from the amount (moles) of 5-methylfurfural as the raw material to determine the amount (moles) of 5-methylfurfural consumed. The conversion rate was calculated using the amount of 5-methylfurfural consumed and the amount of 5-methylfurfural in the raw material using the following formula. The conversion rate refers to the raw material conversion rate: 5-methylfurfural conversion rate (%) = (amount (moles) of 5-methylfurfural consumed) / (amount (moles) of 5-methylfurfural in the raw material) × 100. A higher 5-methylfurfural conversion rate is preferable because it allows the raw material to be converted more efficiently into the product furan-2,5-dicarboxylic acid.

[0065] <Yield of Furan-2,5-dicarboxylic Acid> The amount of furan-2,5-dicarboxylic acid (amount of furan-2,5-dicarboxylic acid produced) (moles) contained in the oxidation reaction product after oxidation step 2 was calculated by an internal standard method using gas chromatography (internal standard: triphenylmethane). The selectivity of furan-2,5-dicarboxylic acid was calculated from the amount of furan-2,5-dicarboxylic acid produced and the amount of 5-methylfurfural in the raw material (amount of 5-methylfurfural consumed) using the following formula: Furan-2,5-dicarboxylic acid selectivity (%) = (amount of furan-2,5-dicarboxylic acid produced (moles)) / (amount of 5-methylfurfural consumed (moles)) × 100. The yield of furan-2,5-dicarboxylic acid is a value calculated as the product of the conversion rate and the furan-2,5-dicarboxylic acid selectivity. Higher furan-2,5-dicarboxylic acid selectivity and yield of furan-2,5-dicarboxylic acid are preferable because they enable more efficient production of highly pure furan-2,5-dicarboxylic acid.

[0066] <Amount of High Molecular Weight Substances> The gas exhaust pipe of the autoclave was connected to a portable gas concentration measuring device (CGT-7100, manufactured by Shimadzu Corporation), and the amount of exhausted carbon-containing gas (carbon monoxide and carbon dioxide) was measured. The carbon amount of the carbon-containing gas was calculated from the measured amount of the carbon-containing gas. In addition, the carbon amount of the raw material 5-methylfurfural and the carbon amount of furan-2,5-dicarboxylic acid in the product were calculated. The ratio obtained by the following formula was taken as the amount of high molecular weight substances in the product. Note that all of the carbon amounts below are in molar terms. The smaller the amount of high molecular weight substances, the higher the purity of the resulting furan-2,5-dicarboxylic acid, and therefore is preferred. Amount of high molecular weight substances (mol %) = (carbon amount of raw material 5-methylfurfural - carbon amount of the carbon-containing gas - carbon amount of furan-2,5-dicarboxylic acid in the product) / (carbon amount of raw material 5-methylfurfural) x 100

[0067] Example 1 (Production of furan-2,5-dicarboxylic acid) (1. Oxidation Step 1) Cobalt acetate tetrahydrate, manganese acetate tetrahydrate, a 48% by mass aqueous hydrogen bromide solution, glacial acetic acid, and water were mixed to obtain a catalyst solution such that the cobalt metal atom concentration, manganese metal atom concentration, bromide ion concentration, acetic acid concentration, and water concentration were the values ​​shown in Table 1. 120 g of the catalyst solution was charged into a 500 mL titanium autoclave equipped with a gas outlet pipe with a reflux condenser, a gas inlet pipe, a continuous raw material feed pump, and a stirrer, and the temperature and pressure were increased to 140°C and 1.0 MPa under a nitrogen atmosphere. Next, the catalyst solution and 5-methylfurfural were mixed in a mass ratio of catalyst solution / 5-methylfurfural = 13 / 5 (72 / 28) to obtain a raw material solution. The supply of the raw material liquid into the reactor and the supply of air diluted with nitrogen (oxygen concentration 12% by volume) into the reactor were started simultaneously, and the raw material liquid and air diluted with nitrogen were continuously supplied for 42 minutes to perform oxidation step 1. The conditions for oxidation step 1 are shown in Table 1. The molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is a value calculated from the amount of 5-methylfurfural contained in the raw material liquid and the amount of oxygen in the oxygen-containing gas supplied simultaneously with the raw material liquid. The oxygen partial pressure is a value calculated from the oxygen concentration of the oxygen-containing gas and the vapor pressure of acetic acid.

[0068] (2. Oxidation Step 2) After completion of oxidation step 1, air (oxygen concentration 20.9% by volume) was used instead of air diluted with nitrogen (oxygen concentration 12% by volume), and oxidation step 2 was carried out under the conditions shown in Table 1 to obtain a product containing furan-2,5-dicarboxylic acid. The raw material liquid and air were continuously supplied for 129 minutes. Note that the outlet oxygen concentration never exceeded 8% by volume during oxidation steps 1 and 2. The results of the 5-methylfurfural conversion rate, furan-2,5-dicarboxylic acid yield, and amount of high molecular weight substances in the obtained product are shown in Table 1. Note that the amount of 5-methylfurfural finally supplied is shown in Table 1 as the "total amount of 5-methylfurfural."

[0069] Examples 2 and 3 and Comparative Examples 1 to 7 (Production of furan-2,5-dicarboxylic acid) Products containing furan-2,5-dicarboxylic acid were obtained in the same manner as in Example 1, except that the conditions for the oxidation step were changed to those shown in Table 1. In all cases, the raw material liquid and air or air diluted with nitrogen were continuously supplied. In Example 3, the oxygen-containing gas was supplied so that the molar ratio of oxygen to 5-methylfurfural was 3.4 for the first 35 minutes of oxidation step 2, and then the air supply rate was increased, and the oxygen-containing gas was supplied so that the molar ratio of oxygen to 5-methylfurfural was 4.0 for the last 122 minutes. In Comparative Examples 1 to 4 and 6, the oxidation step was carried out under the same conditions from start to finish. The results for the 5-methylfurfural conversion rate, furan-2,5-dicarboxylic acid yield, and amount of high molecular weight substances for the obtained products are shown in Table 1.

[0070]

[0071] As shown in Table 1, according to the production method of the example, furan-2,5-dicarboxylic acid is obtained in high yield, and the conversion rate from the raw material 5-methylfurfural is also excellent. Furthermore, it is understood that the oxidation reaction can be carried out while suppressing an increase in the outlet oxygen concentration (oxygen concentration in the off-gas). Therefore, according to the production method of furan-2,5-dicarboxylic acid of the present invention, furan-2,5-dicarboxylic acid is obtained in high yield, and the conversion rate is also excellent. Furthermore, it is understood that the production method of furan-2,5-dicarboxylic acid of the present invention is highly safe from an industrial standpoint.

Claims

1. A method for producing furan-2,5-dicarboxylic acid, comprising: an oxidation step 1 in which 5-methylfurfural and an oxygen-containing gas are continuously supplied to a reaction vessel under the following supply condition 1 or supply condition 2 in the presence of a lower aliphatic carboxylic acid, a bromine compound, and a metal catalyst to carry out an oxidation reaction; and an oxidation step 2 in which, after the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are continuously supplied to the reaction vessel to carry out an oxidation reaction under supply conditions such that at least one of the oxygen concentration of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is higher than that in the oxidation step 1, wherein the bromine compound is at least one selected from the group consisting of hydrogen bromide and bromide salts, and the metal catalyst is at least one selected from the group consisting of cobalt catalysts and manganese catalysts. Supply condition 1: The oxygen concentration of the oxygen-containing gas is 10% by volume or more and less than 15% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.

2. Supply condition 2: The oxygen concentration of the oxygen-containing gas is 15 to 30% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 1.0 to 2.

5.

2. A method for producing furan-2,5-dicarboxylic acid according to claim 1, wherein the supply conditions for oxidation step 1 are supply conditions 1 and the oxygen concentration of the oxygen-containing gas in oxidation step 2 is 5% by volume or more higher than the oxygen concentration of the oxygen-containing gas in oxidation step 1, or the supply conditions for oxidation step 1 are supply conditions 2 and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in oxidation step 2 is 1.5 or more higher than the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in oxidation step 1.

3. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the supply conditions for 5-methylfurfural and oxygen-containing gas in oxidation step 2 are the following supply condition 3. Supply condition 3: the oxygen concentration of the oxygen-containing gas is 15 to 30% by volume, and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 2.5 to 4.

2.

4. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the supply conditions for 5-methylfurfural and oxygen-containing gas in oxidation step 1 are the following supply condition 4: Supply condition 4: The product of the oxygen concentration (volume) of the oxygen-containing gas and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 0.3 to 0.

5.

5. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the oxygen partial pressure when supplying the oxygen-containing gas in oxidation step 1 is 0.05 to 0.2 MPa.

6. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the reaction temperature in oxidation step 1 is 130 to 180°C.

7. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the reaction temperature and the oxygen partial pressure during supply of the oxygen-containing gas in oxidation step 1 satisfy the following condition 5 or 6. Condition 5: The reaction temperature is 130°C or higher but lower than 160°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.10 to 0.2 MPa. Condition 6: The reaction temperature is 160 to 180°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.05 MPa or higher but lower than 0.10 MPa.

8. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the oxygen partial pressure when supplying the oxygen-containing gas in oxidation step 2 is 0.05 to 0.2 MPa.

9. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the reaction temperature in oxidation step 2 is 130 to 180°C.

10. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the reaction temperature and the oxygen partial pressure during supply of the oxygen-containing gas in oxidation step 2 satisfy the following condition 7 or 8. Condition 7: The reaction temperature is 130°C or higher but lower than 160°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.15 to 0.2 MPa. Condition 8: The reaction temperature is 160 to 180°C, and the oxygen partial pressure during supply of the oxygen-containing gas is 0.11 MPa or higher but lower than 0.15 MPa.

11. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the reaction time in oxidation step 1 is 3 to 200 minutes.

12. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the metal catalyst is at least one selected from the group consisting of aliphatic carboxylates of cobalt and aliphatic carboxylates of manganese.

13. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the lower aliphatic carboxylic acid is acetic acid.

14. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the bromine compound is at least one selected from the group consisting of hydrogen bromide, sodium bromide, potassium bromide, and ammonium bromide.

15. A method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the gas other than oxygen contained in the oxygen-containing gas in oxidation step 1 contains 95% by volume or more of nitrogen.

16. A method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the gas other than oxygen contained in the oxygen-containing gas in oxidation step 2 contains 95% by volume or more of nitrogen.

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