Method for producing furan-2,5-dicarboxylic acid

Abstract

Provided is 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. [Solution] A method for producing furan-2,5-dicarboxylic acid includes an oxidation step 1 in which 5-methylfurfural and an oxygen-containing gas are continuously supplied under supply condition 1 or 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 thereafter in which the oxygen-containing gas is supplied under conditions in which the oxygen concentration and / or the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is higher than those in the oxidation step 1 to carry out an oxidation reaction. In the 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. In the supply condition 2, the oxygen concentration is 15 to 30% by volume, and the molar ratio is 1.0 to 2.5.

Description

[Technical field]

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

[0002] Furan-2,5-dicarboxylic acid (FDCA) is a very useful compound as a raw material or intermediate for resins such as polyethylene furanoate, paints, fibers, etc. In particular, furan-2,5-dicarboxylic acid can be synthesized from a furfural compound derived from cellulose, a biomass raw material, making it useful as a biomass-derived resin and fiber raw material to replace fossil raw materials. Examples of the furfural compounds include 5-substituted furfurals such as 5-hydroxymethylfurfural and 5-methylfurfural. Among these, 5-methylfurfural is expected to be a biomass material that can reduce the impact on the environment because it can be produced from non-edible materials such as cellulose, unlike 5-hydroxymethylfurfural produced from sugars consisting of edible materials such as glucose and fructose.

[0003] As a method for producing furan-2,5-dicarboxylic acid, a method for oxidizing 5-substituted furfural has been used. Among them, a production method by liquid phase oxidation using an oxygen-containing gas such as air has 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, which is intended to improve the selectivity and yield of FDCA, 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. Furthermore, Patent Document 3 discloses a method for producing 2,5-furandicarboxylic acid, which is aimed at obtaining FDCA in a high yield, in which 5-methylfurfural, an oxidation gas, and further alkoxymethyl-2,5-furfural are supplied in the presence of a catalyst containing cobalt, manganese, and bromine and acetic acid, and oxidation is carried out at 150 to 210°C, and solid 2,5-furandicarboxylic acid is separated from the crude product. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2013-507359 [Patent Document 2] U.S. Patent No. 9,321,744 [Patent Document 3] International Publication No. 2023 / 242152 Summary of the Invention [Problem to be solved by the invention]

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

[0006] As a result of intensive 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] to

[16] . [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 the following supply condition 2 in the presence of a lower aliphatic carboxylic acid, a bromine compound and a metal catalyst, and an oxidation reaction is carried out; 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 greater than that in the oxidation step 1, and an oxidation reaction is carried out. 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. [2] A method for producing furan-2,5-dicarboxylic acid according to the above item [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 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 the above [1] or [2], wherein the supply conditions for 5-methylfurfural and oxygen-containing gas in the 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 any one of the above [1] to [3], wherein the supply conditions for 5-methylfurfural and the oxygen-containing gas in the 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 0.4. [5] The method for producing furan-2,5-dicarboxylic acid according to any one of the above [1] to [4], wherein the oxygen partial pressure during supply of the oxygen-containing gas in the oxidation step 1 is 0.05 to 0.2 MPa. [6] The method for producing furan-2,5-dicarboxylic acid according to any one of the above [1] to [5], wherein the reaction temperature in the oxidation step 1 is 130 to 180°C. [7] The method for producing furan-2,5-dicarboxylic acid according to any one of the above [1] to [6], wherein the reaction temperature and the oxygen partial pressure during supply of the oxygen-containing gas in the oxidation step 1 satisfy the following condition 5 or the following 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 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 oxygen-containing gas is 0.05 MPa or more and less than 0.10 MPa. [8] The method for producing furan-2,5-dicarboxylic acid according to any one of the above [1] to [7], wherein the oxygen partial pressure during supply of the oxygen-containing gas in the oxidation step 2 is 0.05 to 0.2 MPa. [9] The method for producing furan-2,5-dicarboxylic acid according to any one of the above [1] to [8], wherein the reaction temperature in the oxidation step 2 is 130 to 180°C.

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

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

[10] , wherein the reaction time of the 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 [1] to

[11] , wherein the metal catalyst is at least one selected from the group consisting of a cobalt catalyst, a manganese catalyst, a zirconium catalyst, a nickel catalyst, and a cerium catalyst.

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

[13] , wherein the bromine compound is at least one selected from the group consisting of hydrogen bromide, bromide salts, and organic bromine compounds.

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

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

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

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

[0007] According to the production method of the present invention, it is possible to provide a method for producing furan-2,5-dicarboxylic acid in high yield, with an excellent conversion rate, and with high industrial safety. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 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 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 to carry out the 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 or more and less than 2.5. The production method of the present invention will be described in detail below.

[0009] [Oxidation step 1] In the method for producing furan-2,5-dicarboxylic acid of the present invention, first, oxidation step 1 is carried out 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. 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.

[0010] In addition, it is preferable to place the lower aliphatic carboxylic acid, the bromine compound and the metal catalyst in the reaction vessel before supplying 5-methylfurfural and the oxygen-containing gas, and it is preferable to continuously supply 5-methylfurfural and the oxygen-containing gas under supply condition 1 or supply condition 2 to the reaction vessel containing the lower aliphatic carboxylic acid, the bromine compound and the metal catalyst. The amounts of lower aliphatic carboxylic acid, bromine compound and metal catalyst preliminarily placed in the reaction vessel may be appropriately adjusted depending on the shape, volume, etc. of the reaction vessel. However, it is preferable to use amounts that can be sufficiently mixed with 5-methylfurfural and oxygen-containing gas and can adjust the initial reaction temperature to the target temperature so that the oxidation reaction in this step can be smoothly started.

[0011] 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. 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 the oxidation step 2 carried out after the oxidation step 1, it is preferable to also mix a lower aliphatic carboxylic acid, a bromine compound and a metal catalyst with the 5-methylfurfural that is continuously supplied. In other words, the oxidation step 2 is preferably a step in which, after the oxidation step 1, 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.

[0012] 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 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 continuously supplied 5-methylfurfural.

[0013] <Lower aliphatic carboxylic acids> 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. As a specific lower aliphatic carboxylic acid, preferably, 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 only acetic acid may be used. From the viewpoint of facilitating dissolution of the bromine compound and the metal catalyst, it is preferable to use hydrous acetic acid, which is a mixed solution of water and acetic acid. The use of the above lower aliphatic carboxylic acids is preferred because it can enhance the activity of the catalyst.

[0014] 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 the lower aliphatic carboxylic acid within the above range, it is possible to adjust the viscosity to an appropriate level in this step and the subsequent oxidation step 2, making the reaction easier to handle. It is also possible to control the reaction heat. In addition, "total 5-methylfurfural" refers to "all 5-methylfurfural introduced into the reaction vessel from oxidation step 1 to the end of oxidation step 2 and used in the oxidation reaction." The same applies below. The lower aliphatic carboxylic acids may be used alone or in combination of two or more.

[0015] <Bromine compounds> 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, ammonium bromide, and the like.

[0016] The amount of the bromine compound used in the present production method is, in terms of bromine, 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, relative to 100 parts by mass of total 5-methylfurfural. By setting the amount of the bromine compound within the above range, corrosion of the reaction vessel and the like can be suppressed, while the reaction rate in this step and the subsequent oxidation step 2 can be improved, and the yield can be improved, which is preferable. The bromine compound may be used alone or in combination of two or more kinds.

[0017] <Metal catalyst> The metal catalyst used in the present production method is preferably at least one selected from the group consisting of transition metal catalysts and rare earth metal catalysts, and more preferably a transition metal catalyst. A specific transition metal catalyst is preferably at least one selected from the group consisting of a cobalt catalyst, a manganese catalyst, a zirconium catalyst, a nickel catalyst, and a cerium catalyst, more preferably at least one selected from the group consisting of a cobalt catalyst and a manganese catalyst. It is further 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 a cobalt catalyst, a manganese catalyst, a zirconium catalyst, a nickel catalyst, and a cerium catalyst, more preferably at least one selected from the group consisting of a cobalt catalyst and a manganese catalyst. It is further preferable to use both a cobalt catalyst and a manganese catalyst. The metal catalyst can be used in the form of a salt, a simple metal, an oxide, a hydroxide, etc., but 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, and even more preferably at least one selected from the group consisting of cobalt acetate and manganese acetate. The use of the above metal catalyst is preferred because it allows the production of furan-2,5-dicarboxylic acid in high yield.

[0018] The amount of metal catalyst used in this production method, in terms of metal element, is 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 per 100 parts by mass of total 5-methylfurfural used as a raw material in the oxidation reaction. By setting the amount of the metal catalyst within the above range, side reactions are suppressed while the reaction rate in this step and the subsequent oxidation step 2 is increased, and the yield is improved, which is preferable. When the catalyst concentration is equal to or higher than the lower limit, the reaction rate and the yield are improved, whereas when the catalyst concentration is equal to or lower than the upper limit, the catalyst cost is low and no adverse effect is caused on the reaction. The metal catalyst may be used alone or in combination of two or more. The reason why the production method including the oxidation step 1 can produce furan-2,5-dicarboxylic acid in high yield with excellent conversion using 5-methylfurfural as the raw material and can also produce the product industrially with high safety is unclear, but is thought to be as follows. It is believed that by oxidizing 5-methylfurfural under specific mild conditions in this oxidation step 1, a portion of the 5-methylfurfural becomes an active species, which allows the oxidation reaction in the subsequent oxidation step 2 to proceed smoothly. As a result, oxygen does not remain and the oxygen concentration in the exhausted gas (off-gas) can be kept low, enabling production with high safety. Furthermore, since the raw materials are consumed evenly during the reaction, side reactions are suppressed, and it is believed that the conversion rate is excellent and the yield is high.

[0019] <Water> In the production method of the present invention, water may be used, but it is preferable to use water since the bromine compound is easily soluble in water. The amount of water used in this production method 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 even 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, and therefore the yield can be improved.

[0020] <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 the oxidation step 1 preferably contains 95% by volume or more of nitrogen, more preferably 98% by volume or more of nitrogen. As the gas containing nitrogen and oxygen, air is preferred. In order to adjust the oxygen concentration, oxygen gas or an oxygen-containing gas having a high oxygen concentration may be mixed with an inert gas such as nitrogen. For example, a gas having an oxygen concentration of 10% by volume or more and less than 15% by volume under supply condition 1 in this step can be obtained by mixing nitrogen with air having an oxygen concentration of 20.9% by volume. Also, air having an oxygen concentration of 20.9% by volume can be used as it is as a gas having an oxygen concentration of 15 to 30% by volume under supply condition 2 in this step. As described above, in this step, it is preferable to obtain an oxygen-containing gas by adjusting the oxygen concentration with air and nitrogen. It is preferable to use air or a mixed gas of air and nitrogen as the oxygen-containing gas from the viewpoints of safety and economy.

[0021] <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 to carry out the oxidation reaction (oxidation step 1).

[0022] Here, "continuously supplying 5-methylfurfural and oxygen-containing gas" means that the oxidation reaction of 5-methylfurfural in the reaction vessel and the supply of 5-methylfurfural and oxygen-containing gas are carried out in parallel, and 5-methylfurfural and oxygen-containing gas are supplied for at least 50% of the reaction time during 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 the reaction time of the 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 the reaction time of the oxidation step 1, more preferably 80% or more, even more preferably 90% or more, and may be carried out for 100%.

[0023] The above supply condition 1 or the above supply condition 2 is suitable as a condition for causing an oxidation reaction under a relatively mild condition. By carrying out the oxidation reaction under such conditions, it is believed that a part of 5-methylfurfural can be made into an active species, and the oxidation reaction in the subsequent oxidation step 2 can proceed smoothly.

[0024] 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 further preferably 11 to 13% by volume. When the oxygen concentration of the oxygen-containing gas is within the above range, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion.

[0025] When 5-methylfurfural and an oxygen-containing gas are supplied under the 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, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion rate. It is believed that under supply condition 1, by setting the oxygen concentration at a low level, active species for the subsequent oxidation reaction can be prepared industrially safely.

[0026] 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 the 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 further preferably 20 to 22% by volume. When the oxygen concentration of the oxygen-containing gas is within the above range, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion.

[0027] When 5-methylfurfural and an oxygen-containing gas are supplied under the supply condition 2, the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is 1.0 or more and 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, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion rate. It is believed that under the supply condition 2, the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural is set to a low value, making it possible to prepare active species for the subsequent oxidation reaction in an industrially safe manner.

[0028] In the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are supplied under the above-mentioned supply condition 1 or the above-mentioned supply condition 2. However, regardless of the supply condition, it is preferable that the supply condition for 5-methylfurfural and an oxygen-containing gas in the oxidation step 1 is the supply condition 4 described below. 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.

[0029] 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 even 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, the conversion rate is high, and furan-2,5-dicarboxylic acid can be obtained in high yield.

[0030] In the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are supplied under the above-mentioned supply condition 1 or the above-mentioned supply condition 2. Regardless of the supply condition, the oxygen partial pressure during supply of the oxygen-containing gas in the oxidation step 1 is preferably 0.05 to 0.2 MPa. 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 in the oxidation step 1 is preferably 130 to 180°C. There are preferable combinations of oxygen partial pressure and reaction temperature, as shown below.

[0031] In the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are supplied under the above-mentioned supply condition 1 or the above-mentioned supply condition 2. However, regardless of the supply condition, it is preferable that the reaction temperature in the oxidation step 1 and the oxygen partial pressure when the oxygen-containing gas is supplied are the following condition 5 or the following 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 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 oxygen-containing gas is 0.05 MPa or more and less than 0.10 MPa.

[0032] 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 further 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 high conversion.

[0033] 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 further preferably 0.15 to 0.2 MPa. By keeping the oxygen partial pressure within the above range, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion. Condition 5 is considered to be an industrially safe method for preparing active species for the subsequent oxidation reaction by setting the oxygen partial pressure at a low level.

[0034] 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 further preferably 170 to 175° 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 high conversion.

[0035] 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, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion. Condition 6 is considered to be an industrially safe method for preparing active species for the subsequent oxidation reaction by setting the reaction temperature at a low temperature.

[0036] The reaction time of the 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, still more preferably 3 to 20 minutes, and still more preferably 3 to 10 minutes. By keeping the reaction time within the above range, it is considered that active species for the subsequent oxidation reaction can be prepared industrially safely.

[0037] [Oxidation process 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 those in the oxidation step 1. The oxidation step 2 can provide furan-2,5-dicarboxylic acid in high yield.

[0038] 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. In other words, the oxidation step 2 is preferably a step of continuously supplying 5-methylfurfural, a lower aliphatic carboxylic acid, a bromine compound, a metal catalyst, and an oxygen-containing gas to the reaction vessel after the oxidation step 1 to carry out an oxidation reaction.

[0039] The amount of lower aliphatic carboxylic acid, the amount of bromine compound and the amount of metal catalyst are preferably determined based on 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 in a ratio similar to the preferred ratio relative to the 5-methylfurfural continuously supplied and supply them to the reaction vessel.

[0040] In the oxidation step 1, it is believed that 5-methylfurfural is oxidized under relatively mild conditions, so that a portion of 5-methylfurfural can be made into an active species, and in the subsequent step, the main oxidation reaction is carried out to obtain furan-2,5-dicarboxylic acid. Therefore, in the oxidation step 2, it is believed that the oxidation reaction can be smoothly carried out by increasing the oxygen concentration and / or the molar ratio of oxygen to the raw material, compared to the oxidation step 1. Therefore, it is believed that oxygen does not remain, and a highly safe production can be achieved. Furthermore, since the raw material is consumed evenly during the reaction, side reactions are suppressed, the conversion rate is excellent, and the yield is increased.

[0041] <Oxygen-containing gas> The oxygen-containing gas used in this step is the same as the oxygen-containing gas described in the oxidation step 1, but its oxygen concentration is the same as or greater than that of the oxygen-containing gas used in the 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 the oxidation step 1 preferably contains 95% by volume or more of nitrogen, more preferably contains 98% by volume or more of nitrogen. As the gas containing nitrogen and oxygen, air is preferred. In order to adjust the oxygen concentration, oxygen gas or an oxygen-containing gas having a high oxygen concentration may be mixed with an inert gas such as nitrogen. For example, air having an oxygen concentration of 20.9% by volume can be used as is. In this step, it is preferable to adjust the oxygen concentration with air and nitrogen to obtain an oxygen-containing gas. It is preferable to use air or a mixed gas of air and nitrogen as the oxygen-containing gas from the viewpoints of safety and economy.

[0042] <Conditions for oxidation step 2> This process is a process in which 5-methylfurfural and an oxygen-containing gas are continuously supplied to the reaction vessel and an oxidation reaction is carried out 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 those in the oxidation process 1.

[0043] 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 5-methylfurfural and an oxygen-containing gas are supplied 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 the reaction time of the oxidation step 2, more preferably 80% or more, even more preferably 90% or more, and may be carried out for 100%. The supply of the oxygen-containing gas is preferably carried out for 60% or more of the reaction time of the reaction time of the oxidation step 2, more preferably 80% or more, even more preferably 90% or more, and may be carried out for 100%.

[0044] When the supply conditions for the oxidation step 1 are the above supply conditions 1, the oxygen concentration is set low, so it is preferable to increase the oxygen concentration in this step. Also, when the supply conditions for the oxidation step 1 are the above supply conditions 2, the molar ratio of oxygen to the raw material is 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 conditions 1 and the oxygen concentration of the oxygen-containing gas in oxidation step 2 is 5 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 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.

[0045] When the supply condition for oxidation step 1 is supply condition 1, the oxygen concentration of the oxygen-containing gas in oxidation step 2 is preferably 1 volume % or more higher, preferably 3 volume % or more higher, preferably 5 volume % or more higher, preferably 7 volume % or more higher, preferably 8 volume % or more higher than the oxygen concentration of the oxygen-containing gas in oxidation step 1, and the upper limit is preferably 20 volume % or less higher, preferably 15 volume % or less higher, preferably 10 volume % or less higher than the oxygen concentration of the oxygen-containing gas in oxidation step 1. By having the oxygen concentration of the oxygen-containing gas in the above range, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion.

[0046] When the supply condition for the oxidation step 1 is the supply condition 2, the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in the oxidation step 2 is preferably 0.5 or more larger than the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in the oxidation step 1, preferably 1.0 or more larger, preferably 1.5 or more larger, preferably 1.6 or more larger, preferably 1.8 or more larger, and the upper limit is preferably 3.0 or less larger, preferably 2.5 or less larger, preferably 2.0 or less larger than the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in the oxidation step 1. By setting the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in the above range, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion rate.

[0047] In the oxidation step 1, 5-methylfurfural and an oxygen-containing gas are supplied under the above-mentioned supply condition 1 or the above-mentioned 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 supply condition 3 described 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.

[0048] 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 further preferably 20 to 22% by volume. When the oxygen concentration of the oxygen-containing gas is within the above range, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion.

[0049] 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, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion.

[0050] The oxygen partial pressure when the oxygen-containing gas is supplied 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 preferable combinations of oxygen partial pressure and reaction temperature, as shown below.

[0051] 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 condition 8. Condition 7: Reaction temperature is 130°C or higher and lower than 160°C, and the oxygen partial pressure during supply of 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 oxygen-containing gas is 0.11 MPa or more and less than 0.15 MPa. Condition 7 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 7, the reaction temperature is preferably 130° C. or higher and lower than 160° C., more preferably 135 to 155° C., and further 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 high conversion.

[0052] 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 0.10 to 0.2 MPa, more preferably 0.12 to 0.2 MPa, and further preferably 0.15 to 0.2 MPa. By keeping the oxygen partial pressure within the above range, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion.

[0053] 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 0.10 to 0.2 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 further preferably 170 to 175° 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 high conversion.

[0054] 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, it is possible to obtain furan-2,5-dicarboxylic acid in high yield with high industrial safety and high conversion.

[0055] There is no particular limitation on the reaction time of the oxidation step 2, and the reaction may be carried out until a 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 for a long period of time, which is 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 for a long period of time. In addition, it is more preferable to extract the resulting furan-2,5-dicarboxylic acid appropriately, remove it from the reaction system, and recover it as the 5-methylfurfural and the lower aliphatic carboxylic acid are continuously supplied for a long period of time. By extracting the furan-2,5-dicarboxylic acid, the reaction can be carried out for a longer period of time, which is more preferable industrially. It is even more preferable to continuously extract the resulting furan-2,5-dicarboxylic acid, remove it from the reaction system, and recover it. It is even more preferable to continuously extract the resulting furan-2,5-dicarboxylic acid, remove it from the reaction system, and recover it at the same time as continuously supplying the 5-methylfurfural and the lower aliphatic carboxylic acid.

[0056] 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 for a long 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 for a long period of time and extract the resulting furan-2,5-dicarboxylic acid. And 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 for a long period of time and extract the resulting furan-2,5-dicarboxylic acid continuously at the same time. The amount of lower aliphatic carboxylic acid, the amount of bromine compound and the amount of metal catalyst supplied here 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 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 continuously supplied 5-methylfurfural.

[0057] 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. As the components used in this step, it is convenient and preferable to use the components used in the oxidation step 1 as they are.

[0058] [Other processes] 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. Optional steps included in the method for producing furan-2,5-dicarboxylic acid of the present invention include a solvent removal step, a purification step, and the like.

[0059] The solvent removal step is a step of removing lower aliphatic carboxylic acids, which are solvents with low boiling points, and water from the product containing furan-2,5-dicarboxylic acid, which is the target of this production method. By removing these solvents before removing the by-products generated in the oxidation reaction and the raw material 5-methylfurfural in the next 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 a purification step described below.

[0060] The purification step may be carried out by any method as long as it is possible to separate and recover the target furan-2,5-dicarboxylic acid in high purity, and for example, reprecipitation, recrystallization, etc. may be carried out. Furthermore, hydrogenation may be carried out in order to remove coloring components produced as by-products in the oxidation reaction. EXAMPLES

[0061] The present invention will be described in detail below based on the following examples, but the present invention is not limited to these examples.

[0062] [evaluation] <Outlet oxygen concentration> The outlet oxygen concentration was determined as follows. Equipment: Shimadzu Corporation Portable oxygen meter: POT-101 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 viewpoint of safety during industrial production, it is preferable that the outlet oxygen concentration is 10% by volume or less, which is the explosion limit of acetic acid, the solvent, and the lower the outlet oxygen concentration, the greater the degree of freedom in reaction conditions, which is preferable from the viewpoint of safety. In the examples and comparative examples, the concentration was evaluated based on 8% by volume as the standard concentration at which the explosion limit is not reached even with a sudden change in concentration. For the comparative examples in which the outlet oxygen concentration exceeded 8% by volume, Table 1 shows the time (minutes) from the start of the oxidation reaction as the time when the outlet oxygen concentration exceeded 8% by volume. (Evaluation Criteria) ○: The outlet oxygen concentration was always 8% by volume or less during the oxidation reaction. ×: The outlet oxygen concentration exceeded 8% by volume during the oxidation reaction.

[0063] <5-Methylfurfural conversion rate> The amount (moles) of 5-methylfurfural contained in the oxidation reaction product after oxidation step 2 was calculated by the internal standard method using gas chromatography (internal standard: triphenylmethane), and subtracted from the amount (moles) of 5-methylfurfural in the raw material to determine the amount (moles) of 5-methylfurfural consumed. The conversion rate was calculated from the amount of 5-methylfurfural consumed and the amount of 5-methylfurfural in the raw material by the following formula. The conversion rate is the raw material conversion rate. 5-Methylfurfural conversion rate (%)=(amount of 5-methylfurfural consumed (moles)) / (amount of 5-methylfurfural in the raw material (moles))×100 A higher 5-methylfurfural conversion rate is preferable since it allows the raw material to be converted more efficiently into the product furan-2,5-dicarboxylic acid.

[0064] <Yield of furan-2,5-dicarboxylic acid> The amount of furan-2,5-dicarboxylic acid contained in the oxidation reaction product after the oxidation step 2 (the amount of furan-2,5-dicarboxylic acid produced) (mol) was calculated by the 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) according to the following formula. Furan-2,5-dicarboxylic acid selectivity (%)=(amount (mol) of furan-2,5-dicarboxylic acid produced) / (amount (mol) of 5-methylfurfural consumed)×100 The yield of furan-2,5-dicarboxylic acid is a value calculated by multiplying the conversion rate by the selectivity of furan-2,5-dicarboxylic acid. The higher the selectivity of furan-2,5-dicarboxylic acid and the yield of furan-2,5-dicarboxylic acid, the more efficiently a high-purity furan-2,5-dicarboxylic acid can be obtained, which is preferable.

[0065] <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) to measure the amount of exhausted carbon-containing gas (carbon monoxide and carbon dioxide). 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 regarded as the amount of high molecular weight substances in the product. Note that the carbon amounts below are all calculated on a molar basis. The smaller the amount of high molecular weight substances, the higher the purity of the resulting furan-2,5-dicarboxylic acid, which is preferable. 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)×100

[0066] Example 1 (Production of furan-2,5-dicarboxylic acid) (1. Oxidation step 1) Cobalt acetate tetrahydrate, manganese acetate tetrahydrate, a 48 mass% aqueous hydrogen bromide solution, glacial acetic acid, and water were mixed so that the cobalt metal atom concentration, manganese metal atom concentration, bromide ion concentration, acetic acid concentration, and water concentration would be the values ​​shown in Table 1, to obtain a catalyst solution. 120 g of the catalyst liquid was charged into a 500 mL titanium autoclave equipped with a gas exhaust pipe with a reflux condenser, a gas inlet pipe, a raw material continuous liquid delivery 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 at 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 the air diluted with nitrogen were continuously supplied for 42 minutes to carry out the oxidation step 1. The conditions in the 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.

[0067] (2. Oxidation step 2) After the completion of the oxidation step 1, air (oxygen concentration 20.9% by volume) was used instead of the air diluted with nitrogen (oxygen concentration 12% by volume), and the 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. During the oxidation steps 1 and 2, the outlet oxygen concentration never exceeded 8% by volume. 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. The amount of 5-methylfurfural finally supplied is shown in Table 1 as the "total amount of 5-methylfurfural".

[0068] Examples 2 to 3 and Comparative Examples 1 to 7 (Production of furan-2,5-dicarboxylic acid) A product containing furan-2,5-dicarboxylic acid was 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 each case, the raw material liquid and air or air diluted with nitrogen were continuously supplied. In Example 3, oxygen was supplied in the oxygen-containing gas so that the molar ratio of oxygen to 5-methylfurfural was 3.4 for the first 35 minutes of the oxidation step 2, and then the amount of air supplied was increased, and the oxygen-containing gas was supplied in the last 122 minutes so that the molar ratio of oxygen to 5-methylfurfural was 4.0. Comparative Examples 1 to 4 and 6 were carried out under the same conditions from the beginning to the end of the oxidation step. The results of the 5-methylfurfural conversion rate, furan-2,5-dicarboxylic acid yield, and amount of high molecular weight substances of the obtained products are shown in Table 1.

[0069] [Table 1]

[0070] As shown in Table 1, according to the production method of the embodiment, 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 found that the oxidation reaction can be carried out while suppressing the outlet oxygen concentration (oxygen concentration in the off-gas). Therefore, it is found that the method for producing furan-2,5-dicarboxylic acid of the present invention can produce furan-2,5-dicarboxylic acid in high yield and has an excellent conversion rate, and is also highly safe from an industrial standpoint.

Claims

1. The method for producing furan-2,5-dicarboxylic acid 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 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 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, 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 or more and less than 2.

5.

2. The method for producing furan-2,5-dicarboxylic acid according to claim 1, wherein the supply conditions for the oxidation step 1 are supply conditions 1 and the oxygen concentration of the oxygen-containing gas in the oxidation step 2 is 5 volume % or more higher than the oxygen concentration of the oxygen-containing gas in the oxidation step 1, or the supply conditions for the oxidation step 1 are supply conditions 2 and the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in the oxidation step 2 is 1.5 or more higher than the molar ratio of oxygen in the oxygen-containing gas to 5-methylfurfural in the 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 the oxidation step 2 are the following supply conditions 3. Supply condition 3: 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 the oxidation step 1 are the following supply conditions 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. 3. The method for producing furan-2,5-dicarboxylic acid according to claim 1, wherein the oxygen partial pressure when the oxygen-containing gas is supplied in the 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 the 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 the oxidation step 1 satisfy the following condition 5 or the following condition 6. Condition 5: Reaction temperature is 130° C. or higher and lower than 160° C., and oxygen partial pressure during supply of oxygen-containing gas is 0.10 to 0.2 MPa. Condition 6: Reaction temperature is 160 to 180° C., and oxygen partial pressure during supply of oxygen-containing gas is 0.05 MPa or more and less than 0.10 MPa.

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

11. The method for producing furan-2,5-dicarboxylic acid according to claim 1 or 2, wherein the reaction time of the 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. 3. The method for producing furan-2,5-dicarboxylic acid according to claim 1, wherein the gas other than oxygen contained in the oxygen-containing gas in the oxidation step 1 contains 95% by volume or more of nitrogen.

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

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