Oxidation of furan derivatives

Abstract

Process for the production of 2, 5-furandicarboxylic acid comprising (i) contacting a furan derivative with an oxidizing agent and a solvent in the presence of a catalyst system in an oxidation unit to obtain an intermediate product comprising 2, 5-furandicarboxylic acid, (ii) contacting the intermediate product with an oxidizing agent in a post-oxidation unit, adding an oxidizable compound to the post-oxidation unit, the present invention relates to a method for producing 2, 5-furandicarboxylic acid comprising the steps of (iii) separating the raw product into 2, 5-furandicarboxylic acid and a mother liquor comprising a solvent wherein the oxidizable compound is selected from the group consisting of tetrahydrofuran-containing compounds comprising 4 to 10 carbon atoms and alcohols comprising 2 to 8 carbon atoms and mixtures thereof.

Description

Technical Field

[0001] The present invention relates to a method for producing 2,5-furandicarboxylic acid by contacting a furan derivative with an oxidant and a catalyst system, followed by post-oxidation. Background Art

[0002] 2,5-Furandicarboxylic acid (FDCA) is considered in the prior art as a promising component for replacing petroleum-based monomers in the production of high-performance polymers. In recent years, 2,5-Furandicarboxylic acid and the new plant-based polyester polyethylene furanoate (PEF) have attracted a lot of attention. (It is a fully recyclable plastic with excellent properties compared to the currently widely used petroleum-based plastics). These materials can make a significant contribution to reducing dependence on petroleum-based polymers and plastics, while achieving more sustainable management of global resources. Extensive research has been carried out in this field to obtain technologies for producing 2,5-Furandicarboxylic acid and PEF in a commercially viable manner.

[0003] 2,5-Furandicarboxylic acid can be obtained by oxidation of molecules with a furan moiety, which can be, for example, 5-hydroxymethylfurfural (HMF) and its corresponding esters and ethers, such as 5-alkoxymethylfurfural, in particular 5-methoxymethylfurfural (MMF), and similar starting materials, which are usually obtained from plant-based sugars, for example by sugar dehydration. Various oxidation processes are known from the prior art, such as enzyme-catalyzed and metal-catalyzed processes, heterogeneous or homogeneous. WO 2014 / 014981 and WO 2011 / 043661 describe processes for the oxidation of compounds with a furan moiety to 2,5-furandicarboxylic acid using a catalyst system comprising cobalt, manganese and bromine, using oxygen or air as an oxidant.

[0004] The crude 2,5-furandicarboxylic acid product is often not pure enough to be used to make polymers with the desired properties. The product obtained by oxidation may be colored, which is disadvantageous because it indicates the presence of impurities. In order to objectively determine the color, it is necessary to measure the absorbance of the solution at a specific wavelength (such as 400nm). The impurities that cause the color are often difficult to specifically identify and therefore difficult to remove.

[0005] Methods for further purification of the crude oxidation product have been developed. Exemplary purification methods are disclosed in WO 2014 / 014981 and WO 2016 / 195499.

[0006] The application of a post-oxidation step may be advantageous. WO2021 / 123206 describes that a subsequent oxidation reaction is suitable for converting incompletely oxidized products.

[0007] Unfortunately, it was found that the post-oxidation reaction can be unstable, especially in a continuous process where an oscillatory behavior is observed. During this oscillatory behavior, oxygen consumption and the production of carbon monoxide and / or carbon dioxide vary periodically. It was found that the oscillatory behavior of the post-oxidation reaction often results in insufficient post-oxidation or no post-oxidation. It was found that the 2,5-furandicarboxylic acid obtained after insufficient or no post-oxidation has a poor color.

[0008] The article by Xiaobin Zuo et al. (Optimization of Co / Mn / Br-Catalyzed Oxidation of 5-Hydroxymethylfurfural to Enhance 2, 5-Furandicarboxylic Acid Yield and Minimize Substrate Burning", ACS Sustainable Chemistry Engineering 2016, pp. 3659-3668) studied the formation of furandicarboxylic acid by semi-continuous oxidation of 5-hydroxymethylfurfural. The transformation is described as being accompanied by competing side reactions, most notably esterification and over-oxidation to carbon monoxide and carbon dioxide. The yield of furandicarboxylic acid was improved by optimizing operating variables such as catalyst composition, water concentration in the acetic acid solvent, and pressure.

[0009] US 4906772 describes the co-production of acetic acid from ethanol in the preparation of terephthalic acid from p-xylene.

[0010] EP-A-2752446 relates to the recycling of polyethylene terephthalate by ethanolysis to form ethyl esters and ethylene glycol and oxidation of the resulting ethyl esters to form carboxylic acids and acetic acid.

[0011] WO 2021 / 231556 describes a method for preparing 2,5-furandicarboxylic acid and terephthalic acid by co-feeding (i) a furanic compound (such as 5-hydroxymethylfurfural) that forms 2,5-furandicarboxylic acid and (ii) p-xylene. Post-oxidation is applied in the example of "Co-oxidation of p-xylene and FDCA to form furanic compounds using Co, Mn, Br catalysts". The main oxidation reaction is allowed to run for a target time before the feed is stopped, and the reaction is allowed to run for a target post-oxidation time without additional substrate.

[0012] WO 2021 / 123189 applies post-oxidation to reduce the amount of monoalkyl esters of 2,5-furandicarboxylic acid in the mother liquor, which otherwise tend to accumulate under certain conditions. Summary of the invention

[0013] The present invention relates to the production of 2,5-furandicarboxylic acid by oxidation of a furan derivative followed by post-oxidation.

[0014] The aim is to ensure that the post-oxidation process remains active, thereby resulting in a higher quality crude 2,5-furandicarboxylic acid. A higher quality 2,5-furandicarboxylic acid contains fewer impurities or impurities that are less detrimental in the further use of the 2,5-furandicarboxylic acid.

[0015] Another object was to improve the color of the 2,5-furandicarboxylic acid obtained, ie to reduce the absorbance of the crude carboxylic acid obtained at 400 nm.

[0016] Surprisingly, it has now been found that by adding an oxidizable compound to the feed or directly to the reactor, post-oxidation remains active. Without wishing to be bound by any theory, it is believed that the oxidizable compound ensures that the catalyst system remains in a reduced state. Alternatively, the additional oxidizable compound may ensure the presence of free radicals. Regardless of the mechanism, additional and thorough oxidation can reduce the amount of color formers and / or compounds of crude 2,5-furandicarboxylic acid, which further can cause retention of catalytic metals in 2,5-furandicarboxylic acid.

[0017] The present invention relates to a method for producing 2,5-furandicarboxylic acid, which comprises (i) contacting a furan derivative with an oxidant and a solvent in the presence of a catalyst system in an oxidation unit to obtain an intermediate product containing 2,5-furandicarboxylic acid, (ii) contacting the intermediate product with an oxidant in a post-oxidation unit, adding an oxidizable compound to the post-oxidation unit to obtain a crude product, and (iii) separating the crude product into 2,5-furandicarboxylic acid and a solvent containing a mother liquor, wherein the oxidizable compound is selected from tetrahydrofuran compounds containing 4 to 10 carbon atoms and alcohols containing 2 to 8 carbon atoms and mixtures thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The general concept of the process flow of the present invention is shown. DETAILED DESCRIPTION

[0019] The process is intended to produce 2,5-furandicarboxylic acid. The product obtained may also contain other compounds besides 2,5-furandicarboxylic acid, such as by-products of the oxidation of furan derivatives. Well-known by-products of the oxidation of HMF and / or MMF are, for example, 5-formyl-2-furancarboxylic acid (FFCA), the monomethyl ester of 2,5-furandicarboxylic acid and the methyl ester of FFCA. The monomethyl ester of 2,5-furandicarboxylic acid can be used to prepare polyesters by transesterification and is therefore also considered a desirable end product.

[0020] Step (i) comprises oxidizing the furan derivative with an oxidizing agent and a solvent in the presence of a catalyst system.

[0021] The furan derivative may be any furan derivative known to be suitable. More specifically, the furan derivative is a furfural derivative, more specifically a furfural derivative selected from the group consisting of 5-methylfurfural (MF) and a derivative of formula (1)

[0022]

[0023] wherein R represents hydrogen, alkyl or acyl. The feed preferably comprises at least 80 wt. % of MF, HMF and / or MMF, more preferably at least 90 wt. % of MF, HMF and / or MMF. The feed preferably consists of HMF and / or MMF.

[0024] The presence of other compounds to be oxidized, such as phenyl-containing compounds to be converted into terephthalic acid or its derivatives, is generally undesirable. Therefore, the reaction mixture of step (i) is preferably substantially free of phenyl-containing compounds. More preferably, the reaction mixture of step (i) is free of phenyl-containing compounds.

[0025] The mixture in step (i) comprises, in addition to the furan derivative, an oxidant, a solvent and a catalyst system.

[0026] Preferably, the solvent is an organic acid, water or a mixture thereof. Preferably, the solvent comprises water and / or acetic acid. The solvent may be a recycle stream or may be freshly added. A particularly suitable stream is a mother liquor obtained from the separation and / or purification of a 2,5-furandicarboxylic acid slurry. More preferably, the solvent is selected from the group consisting of water, acetic acid and a mixture thereof. After the reaction is started, the amount of water present in the reaction mixture of step (i) tends to increase due to the oxidation of the furan derivatives producing more water.

[0027] The catalyst system comprises cobalt, manganese and bromine as elements or derivatives thereof. The catalyst system also tends to contain a limited amount of water. The catalyst system preferably has a weight ratio of cobalt to manganese in the catalyst system of 5 or more, preferably 10 or more, preferably 15 or more, and / or a weight ratio of bromine to the total weight of cobalt and manganese in the catalyst system of 1 or more, preferably 1.5 or more, most preferably 2 or more, wherein the value is preferably less than 4.0, more preferably less than 3.5, more preferably less than 3, more preferably less than 2. If the catalyst system comprises other metals other than cobalt and manganese in an amount of 5% by weight or more, it is preferred that the above ratio is a weight ratio of bromine to the total weight of all metals in the catalyst system. The metal is preferably added in the form of a salt soluble in the reaction mixture. Typically, the amount of cobalt is selected in the range of 500 to 6000 ppm by weight based on the weight of the reaction mixture including the furan derivative, the solvent and the catalyst system in the oxidation unit. The amount of manganese is typically in the range of 20 to 6000 ppm by weight based on the weight of the reaction mixture including the furan derivative, the solvent and the catalyst system in the oxidation unit. Typically, the bromine content will be 30 to 8000, preferably 50 to 4500 ppm by weight, based on the weight of the reaction mixture comprising furan derivative, solvent and catalyst system in the oxidation unit. Alternatively, the bromine content is 3000 to 8000 ppm by weight.

[0028] The oxidant used in step (i) and step (ii) may be any suitable gas known to those skilled in the art. Preferably, the oxidant comprises molecular oxygen. Most preferably, the source of the oxidant is air.

[0029] Obviously, the amount of oxidant required in step (i) depends on the furan derivative to be oxidized. The amount of oxidant in step (i) is generally 0.5 to 10 moles of oxygen per mole of furan derivative, preferably 1 to 8 moles, more preferably 1 to 5 moles of oxygen per mole of furan derivative.

[0030] The present process can be carried out in batch or semi-batch or continuous mode. The process is particularly suitable for continuous operation, which can be continued uninterruptedly due to sufficient post-oxidation.

[0031] The process is considered to be carried out continuously if the furan derivative is added to the oxidation reactor essentially continuously. Preferably, the intermediate product is additionally added continuously to the post-oxidation unit. More preferably, the intermediate product is additionally removed continuously from the oxidation unit and the crude product is removed continuously from the post-oxidation unit. Preferably, if the intermediate product is added continuously, the oxidizable compound may be added to the post-oxidation unit intermittently or continuously. Most preferably, the oxidizable compound is added continuously. Operation is considered to be continuous if the flow continues for most of the time, preferably for at least 70% of the time. Operation is considered to be continuous even if the fluid flow is frequently stopped for short periods of time due to periodic closing and reopening of valves.

[0032] Step (i) is carried out in a temperature range of 150° C. to 210° C., preferably in a temperature range of 160° C. to 190° C., more preferably in a temperature range of 165° C. to 180° C. Preferably, the pressure in step (i) is in the range of 6 to 20 barg. These parameters have been found to be preferred for obtaining intermediate products of good purity in good yield.

[0033] In addition to 2,5-furandicarboxylic acid, the intermediate product tends to contain a monoalkyl ester of 2,5-furandicarboxylic acid. As mentioned above, an alkyl ester of 2,5-furandicarboxylic acid is also considered a desirable product.

[0034] The above temperature range allows the oxidation reactor to be operated at elevated pressures while still allowing a large amount of heat generated by the exothermic oxidation reaction to be removed by evaporation. This is referred to as "adiabatic" operation by those skilled in the art, where the heat of reaction is not removed by a cooler, by a source of loss through a wall, etc., but is primarily removed by evaporation of the solvent and recycling of the condensation. Typically, higher temperatures require higher pressures to operate "adiabatic". Higher pressures in turn allow for higher oxygen partial pressures in the reactor and reduce the risk of oxygen deficiency. The latter means that the reaction is limited by available oxygen.

[0035] The unit for carrying out the oxidation may be any typical oxidation reactor known in the art. The oxidation unit of step (i) is preferably a continuous stirred tank reactor.

[0036] Part or all of the intermediate product obtained in step (i) is post-oxidized in step (ii). At least a portion of the gaseous product of step (i) can be removed as waste gas. This gas stream can be separated and subsequently cooled. Compounds (such as acetic acid) can be separated from the intermediate product and sent back to step (i) or other parts of the process.

[0037] In a preferred embodiment, the gaseous intermediate product and / or the gaseous crude product are separated, cooled and recycled to step (i). For safety reasons, the oxygen volume percentage in the waste gas is generally limited to a level below the lower explosion limit, for example below 10 volume %, or more preferably below about 6 volume % to allow for a safety margin. The oxygen-containing gas can be recycled to step (i) or used in step (ii), or recycled or used in other parts of the method. Due to the low oxygen content, it can be used as a conveying gas or for operating a dryer and / or a filter.

[0038] Preferably, all reaction slurries produced in step (i) are submitted to step (ii), more particularly all catalyst systems present in step (i). The slurry consists of solids and liquids of the intermediate product. Preferably, all reaction slurries are submitted to step (ii) without separating liquids or solids from the intermediate product. Submitting all reaction slurries of step (i) to step (ii) ensures that the catalyst system of step (i) is also useful in step (ii).

[0039] The post-oxidation of step (ii) is preferably carried out at a temperature in the range of 150° C. to 210° C., more specifically 160° C. to 210° C. Preferably, the pressure in step (ii) is in the range of 6 to 20 barg.

[0040] Surprisingly, it has now been found that post-oxidation proceeds more efficiently if an oxidizable compound is added to the feed or directly to the reactor during the post-oxidation. The oxidizable compound is a compound other than any intermediate product obtained by oxidation of the furan derivative and other than any other compound already present during the first oxidation.

[0041] The oxidizable compound is selected from tetrahydrofuran-containing compounds containing 4 to 10 carbon atoms and alcohols containing 2 to 8 carbon atoms and mixtures thereof. The tetrahydrofuran-containing compound most preferably contains 4 carbon atoms. Preferably, the oxidizable compound is selected from the group consisting of ethanol and tetrahydrofuran, more preferably ethanol. The addition of ethanol has the advantage that its oxidation product, acetic acid, can serve as a solvent for the process. Ethanol can be added as such or in the form of an ethyl ester of ethanol formed under the reaction conditions of the post-oxidation unit. Most preferably, ethanol is added as such.

[0042] The amount of oxidizable compound present in step (ii) is preferably at least 0.01 wt.%, more preferably at least 0.05 wt.%, more preferably at least 0.1 wt.%, based on the total amount of liquid intermediates. The amount of oxidizable compound is preferably at most 7 wt.%, more preferably at most 5 wt.%, more preferably at most 3 wt.%, more preferably at most 2 wt.%, based on the total amount of liquid intermediates in the post-oxidation unit. The amount of intermediates is the total amount of liquid and / or solid reaction mixture including the oxidized furan derivative and the solvent.

[0043] Preferably, the oxidant (preferably oxygen) added to step (ii) is at least 0.001 mole of oxidant per mole of 2,5-furandicarboxylic acid and esters thereof. Preferably, the ratio is at least 0.05. Preferably, the ratio is at most 5, more preferably at most 3, more preferably at most 2.

[0044] Preferably, at least a portion of the gaseous crude product is removed. This gaseous crude product may be separated and cooled before separating out compounds (such as acetic acid) which may be returned to step (i), step (ii) or other parts of the process.

[0045] The crude product obtained in step (ii) tends to be a slurry which comprises solid 2,5-furandicarboxylic acid and a liquid comprising the solvent, water and dissolved catalyst system and possibly other organic impurities.

[0046] In step (iii), 2,5-furandicarboxylic acid can be separated from the crude product in a solid-liquid separation zone to obtain a solid filter cake and a mother liquor. The separation of step (iii) can be carried out in any manner known to those skilled in the art. A preferred method is that the solid-liquid separation zone comprises a filter or a centrifuge, preferably a filter, more preferably a rotary pressure filter. The separation is preferably carried out by filtration. Typically, not all 2,5-furandicarboxylic acid is removed from the crude carboxylic acid composition, and typically not all separated solid filter cakes are 2,5-furandicarboxylic acid.

[0047] The 2,5-furandicarboxylic acid obtained in step (iii) may be treated or washed with a solvent selected from the group consisting of water, monocarboxylic acids and / or dicarboxylic acids. The acid is preferably a monocarboxylic acid containing 1 to 3 carbon atoms.

[0048] In a preferred embodiment, the method further comprises (iv) contacting 2,5-furandicarboxylic acid with a polar solvent to obtain a solution; (v) contacting the solution with hydrogen in the presence of a hydrogenation catalyst under hydrogenation conditions to obtain a hydrogenated solution; and (vi) separating purified 2,5-furandicarboxylic acid from the hydrogenated solution. Suitable process conditions are described, for example, in WO2016 / 195490. Preferred process conditions include contacting with hydrogen for a time in the range of 5 seconds to 15 minutes at a temperature in the range of 150°C to 200°C. Preferably, the polar solvent is selected from the group consisting of water, acetic acid, and mixtures thereof.

[0049] It will be clear to the skilled person that preferably all of the solution obtained in step (v) is subjected to step (vi), although only part of the solution may be used.

[0050] The 2,5-furandicarboxylic acid obtained in step (iii) or step (vi) preferably contains at least 50% by weight, more preferably at least 70% by weight, more preferably at least 80% by weight, more preferably at least 90% by weight, and most preferably at least 95% by weight relative to the weight of the dry filter cake of 2,5-furandicarboxylic acid. The wet filter cake may contain residual mother liquor. The dry filter cake may contain derivatives of 2,5-furandicarboxylic acid, such as esters of 2,5-furandicarboxylic acid, 2-carboxy-5-(formyl)furan (FFCA) and dicarbonylfurancarboxylic acid (BCFCA).

[0051] The crude 2,5-furandicarboxylic acid obtained in step (iii) is preferably purified by the method specifically described in WO2021 / 123206 or WO2021 / 123203.

[0052] Figure 1 The general concept of the arrangement of the method of the present invention is shown.

[0053] exist Figure 1 In the process, a feed comprising hydroxymethylfurfural and / or methoxymethylfurfural and a solvent and a catalyst system is fed to the oxidation unit 1 via line 10. Separately, the oxidant is added to the unit 1 via line 20. Offgases can be removed from the top via line 30, while liquid and solid intermediate products, and optionally part of the gaseous intermediate products, can be sent to the post-oxidation unit 2 via line 40. Air is added to the post-oxidation unit 2 via line 50. The oxidizable compound, pure or mixed with a solvent, can be added to line 40 (via an addition point not shown) to be mixed with the feed into the post-oxidation unit 2, or directly added to the post-oxidation unit 2 via line 60. Gaseous crude product can be removed via line 70. The offgases of the oxidation unit can be sent to the unit 3 via line 30 and / or the offgases of the post-oxidation unit via line 70, where these gaseous effluents can be cooled and optionally further treated to obtain a liquid solvent stream, which can be recycled to the oxidation unit 1 or other parts of the process (not shown) via line 80. The waste gas of unit 3 can be removed through pipeline 90. The liquid and solid crude products can be sent to separation unit 4 through pipeline 100 to obtain 2,5-furandicarboxylic acid filter cake and mother liquor. It is preferred to cool the liquid and solid crude products between the post-oxidation step and the separation step. The mother liquor can be removed through pipeline 120. The 2,5-furandicarboxylic acid filter cake can be sent to unit 5 through pipeline 110 for further purification. The purified 2,5-furandicarboxylic acid can be removed through pipeline 130.

[0054] The invention is described in more detail below by way of non-limiting examples.

[0055] Example

[0056] The present experiments were performed as part of a continuous operation.

[0057] The liquid feed of oxidation reactor is prepared by sugar dehydration. The liquid feed comprises 18 wt % of 5-methoxymethylfurfural and 1.4 wt % of 5-hydroxymethylfurfural as furan derivatives, and a mixture of a large amount of acetic acid and a small amount of water as solvent. The liquid feed also comprises a small amount of by-product of sugar dehydration, including levulinic acid and methyl levulinate. In addition, the liquid feed comprises an oxidation catalyst system consisting of cobalt acetate hydrate, manganese acetate hydrate and hydrobromic acid. The oxidant fed to the reactor is air. The molar ratio of oxygen per hour to the total molar amount of 5-methoxymethylfurfural and 5-hydroxymethylfurfural per hour is about 4.

[0058] The liquid feed and oxidant were continuously fed into a continuously stirred tank reactor where the liquid feed was contacted with oxygen at 175°C and 11 bar with a residence time of approximately 1.2 hours.

[0059] The liquid slurry produced in the oxidation reactor was continuously fed to the post-oxidation reactor and contacted with air at 165° C., 10 bar pressure and at a molar ratio of 0.4 of oxygen per hour to the total molar amount of furandicarboxylic acid and monomethyl furandicarboxylic acid ester per hour. The residence time was 1.2 hours. In addition, ethanol was fed to the post-oxidation reactor at about 0.2 mol of ethanol per hour to the total molar amount of furandicarboxylic acid and monomethyl furandicarboxylic acid ester per hour.

[0060] The vapors from the oxidation and post-oxidation reactors are removed overhead and condensed. The vapors are partially recycled into the oxidation reactor.

[0061] The activity of the post-oxidation reactor was monitored by oxygen consumption and the production of carbon dioxide and carbon monoxide.

[0062] After 231 hours of continuous operation, the ethanol flow to the post-oxidation reactor was stopped. Subsequently, pulses of ethanol were added every two hours to check activity. If there was a spike in oxygen consumption after the ethanol pulse, the reaction was considered to be still active. If there was no response, it was assumed that there was no more post-oxidation activity. After the start of the pulsed addition of ethanol, oxygen consumption and the production of carbon dioxide and carbon monoxide began to fluctuate.

[0063] After 250 hours of operation (i.e. 19 hours after cessation of ethanol addition), the ethanol pulse no longer caused an increase in oxygen consumption. It was concluded that the activity in the post-oxidation reactor had ceased.

[0064] The post-oxidation reactor can then be restarted (ie oxygen consumption and carbon dioxide and carbon monoxide production begin again) by a pulse of methoxymethylfurfural.

[0065] The crude product of the post-oxidation reactor is fed into a filter to obtain a 2,5-furandicarboxylic acid filter cake. It was found that when the ethanol pulse no longer causes an increase in oxygen consumption, i.e., when the post-oxidation is inactive, the color of the filter cake increases significantly. The color is determined by the absorbance of the filter cake solution at a wavelength of 400 nm.

Claims

1. A method for producing 2,5-furandicarboxylic acid, the method comprising (i) contacting a furan derivative with an oxidizing agent and a solvent in the presence of a catalyst system in an oxidation unit to obtain an intermediate product comprising 2,5-furandicarboxylic acid, (ii) contacting the intermediate product with an oxidant in a post-oxidation unit to which an oxidizable compound is added to obtain a crude product, and (iii) separating the crude product into 2,5-furandicarboxylic acid and a mother liquor containing a solvent, The oxidizable compound is selected from the group consisting of tetrahydrofuran-containing compounds containing 4 to 10 carbon atoms, alcohols containing 2 to 8 carbon atoms, and mixtures thereof.

2. The method according to claim 1, wherein the oxidizable compound is selected from the group consisting of ethanol and tetrahydrofuran, more preferably ethanol.

3. The process according to claim 1 or 2, wherein the amount of the oxidizable compound is 0.01 wt% to at most 7 wt% based on the total amount of the liquid intermediate product.

4. The method according to any one of claims 1 to 3, wherein the furan derivative has the chemical formula (1): wherein R represents hydrogen, alkyl or acyl.

5. The method according to any one of claims 1 to 4, wherein the method further comprises (iv) contacting the 2,5-furandicarboxylic acid with a polar solvent to obtain a solution; (v) contacting the solution with hydrogen in the presence of a hydrogenation catalyst under hydrogenation conditions to obtain a hydrogenated solution; and (vi) isolating purified 2,5-furandicarboxylic acid from the hydrogenated solution.

6. The process according to any one of claims 1 to 5, wherein the process is continuous and the furan derivative is added continuously to the oxidation unit.

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