Process for producing 2,5-furandicarboxylic acid

Abstract

Process for producing 2,5-furandicarboxylic acid comprising the steps of a) oxidizing substituted furfural with oxidant in the presence of acetic acid solvent and a catalyst system comprising cobalt, manganese and bromine to obtain a crude carboxylic acid composition, b) separating solid 2,5-furandicarboxylic acid from the crude carboxylic acid composition to obtain 2,5-furandicarboxylic acid product and mother liquor, c) adding oxalic acid to mother liquor in an amount of from 0.5 to 1.2 mole of oxalic acid per mole cobalt in the mother liquor and separating a first precipitate containing cobalt to obtain a cobalt depleted stream, and d) adding further oxalic acid to the cobalt depleted stream and separating a second precipitate containing iron from the treated cobalt depleted stream to obtain a purified stream.

Description

[0001] Process for producing 2,5-furandicarboxylic acid

[0002] Technical Field

[0003] The present invention relates to a process for producing 2,5-furandicarboxylic acid by oxidation with oxidant in the presence of acetic acid solvent and a catalyst system comprising cobalt, manganese and bromine.

[0004] Background

[0005] 2,5-Furandicarboxylic acid (FDCA) is known in the art to be a highly promising building block for replacing petroleum-based monomers in the production of high performance polymers. In recent years FDCA and the novel plant-based polyester polyethylenefuranoate (PEF) have attracted a lot of attention. PEF is a recyclable plastic with superior performance properties compared to today' s widely used petroleum-based plastics. These materials could provide a significant contribution to reducing the dependence on petroleum-based polymers and plastics while at the same time allowing for a more sustainable management of global resources. Correspondingly, comprehensive research was conducted in the field to arrive at a technology for producing FDCA and PEF in a commercially viable way.

[0006] FDCA is typically by oxidation of molecules having furan moieties, e.g. 5-methyl furfural, 5-hydroxymethylfurfural as well as the corresponding esters or ethers, e.g. 5- alkoxymethylfurfural, and similar starting materials that are typically obtained from plant-based sugars, e.g. by sugar dehydration. A broad variety of oxidation processes is known from the prior art including enzymatic and metal catalyzed processes.

[0007] One of the most established techniques in the field uses a catalyst system comprising cobalt, manganese and bromine to oxidize compounds having a furan moiety to FDCA using oxygen or air as an oxidizing agent to obtain a crude carboxylic acid composition. Respective processes that are applicable to a wide variety of starting materials are for example disclosed in WO 2014 / 014981 A1 and WO 2011 / 043660 A1 . Solid 2,5-furandicarboxylic acid can be separated from the crude carboxylic acid composition and subsequently be purified further. The remaining liquid, also referred to as mother liquor, comprises the catalyst system and tends to further contain metals which have become entrained during processing such as iron and oftentimes nickel. Iron has been found to be especially detrimental in oxidation for the manufacture of 2,5-furandicarboxylic acid.

[0008] DE 1081445 describes air oxidation of aromatic hydrocarbons containing at least two alkyl groups optionally substituted by halogen atom using as catalyst one or more heavy metal bromides or a mixture of one or more heavy metals and a bromine ion supplying compound.

[0009] US 3840469 describes recovery of cobalt (II) ions from an acetic acid medium to produce a fresh solution of cobalt (II) in acetic acid which can again be used in liquid phase oxidation.

[0010] US 4490297 describes a process for the recovery and recycling of spent cobalt and / or manganese catalysts used in the air oxidation of alkyl aromatics in acetic acid by converting them to insoluble cobalt and / or manganese oxalate dihydrate. The oxalate compounds are converted to a form which is soluble in acetic acid and can be used as an oxidation catalyst by reaction with acetyl bromide, hydrogen bromide or a mixture thereof in acetic acid or acetic anhydride. The still-moist cobalt and / or manganese oxalate can be isolated from an aqueous or acetic acid solution in a bromide-containing acetic acid, and this solution can be used, after adjusting the desired ratios of concentration, as a reaction medium for the oxidation of further aromatics.

[0011] US4786752 describes a method for recovery and recycle of metal catalyst components from liquid phase oxidation of pseudocumene to trimellitic acid in the presence of an oxidation catalyst comprising cobalt, manganese and bromine components and optionally zirconium wherein said catalyst components are recovered by oxalate precipitation from aqueous residue. The precipitated metal oxalates are separated such as by filtration while the filtrate which contains the bulk of the bromine and soluble metals such as sodium is sent to disposal. The metal oxalates can be directly recycled or subjected to application of heat to convert said salts to more catalytically active forms of said salts.

[0012] US 5183933 describes the preparation of 2,6-naphthalenedicarboxylic acid by oxidation of 2,6-dimethylnaphthalene with the help of a catalyst comprising cobalt, manganese and bromine wherein the atom ratio of manganese to cobalt is about 5:1 to about 0.3:1 . Valuable catalyst metals can be precipitated with oxalic acid or its salt from mother liquor. Example 12 found that a molar ratio of oxalic acid to cobalt and manganese in the range of 0.6 to 2.1 adequately recovered the cobalt and manganese while leaving most of the undesirable trimellitic acid in the mother liquor. Iron was mentioned as a typical corrosion metal found in the mother liquor. Cobalt and iron were recovered to the same extent when applying a molar ratio of 0.85 and 2.1 of oxalic acid.

[0013] EP0373578 is described to be especially useful for the preparation of terephthalic acid by the oxidation of para-xylene or the preparation of isophthalic acid by the oxidation of metaxylene. It aims to recover and regenerate cobalt and manganese catalyst from mother liquor further containing acetic acid and corrosion metal compounds of iron and nickel comprising (a) precipitating cobalt and manganese by the addition of (1) oxalic acid and alkali metal hydroxide, the amount of alkali metal hydroxide being at least about equal on a molar basis to the amount of oxalic acid, or (2) mono alkali metal oxalate, (b) isolating the precipitate, (c) dispersing the precipitate in a solution containing aqueous acetic acid, and (d) forming cobalt acetate and manganese acetate by oxidizing the oxalic acid moiety to carbon oxides. The mother liquor from which the precipitated cobalt and manganese oxalate have been removed, may be evaporated to recover the acetic acid portion while the residue containing the iron and nickel corrosion metal besides undesirable organic reaction products may be burned or disposed of in some other manner. The addition of alkali metal hydroxide to the oxalic acid is described to improve the purge of iron and nickel in other words results in less iron and nickel precipitating with the cobalt and manganese oxalates. Mono alkali metal oxalate is described to achieve the same result.

[0014] It is advantageous for commercial operation to be able to recycle cobalt catalyst. Furthermore, it is advantageous to be able to recycle mother liquor which remains after recovering FDCA. Unfortunately, the presence of corrosion metals can cause problems in oxidation of substituted furfural. An amount of only several parts per million by weight (ppmw) of iron on total amount of oxidation reaction mixture can be detrimental.

[0015] The present aim was to recover cobalt catalyst such that it again can be used in the oxidation of substituted furfural. A further aim was to be able to again use mother liquor in the oxidation step.

[0016] Disclosure of the invention

[0017] The present invention concerns a process for producing 2,5-furandicarboxylic acid by oxidation of a substituted furfural in the presence of acetic acid solvent and a catalyst system comprising cobalt, manganese and bromine. It now has surprisingly been found that staged addition of oxalic acid to mother liquor from substituted furfural oxidation allows to remove catalytic metal, especially cobalt, separate from catalyst poison metals such as iron. This absence of iron from both the recovered cobalt and mother liquor would allow to recycle each of these to the oxidation reaction.

[0018] The present invention relates to a process for producing 2,5-furandicarboxylic acid, comprising the steps of a) oxidizing substituted furfural with oxidant in the presence of acetic acid solvent and a catalyst system comprising cobalt, manganese and bromine to obtain a crude carboxylic acid composition comprising solid 2,5-furandicarboxylic acid, b) separating at least part of the solid 2,5-furandicarboxylic acid from the crude carboxylic acid composition to obtain 2,5-furandicarboxylic acid product and mother liquor, c) adding oxalic acid to at least part of the mother liquor and separating a first precipitate containing cobalt to obtain a cobalt depleted stream wherein the amount of oxalic acid added is of from 0.5 to 1 .2 mole of oxalic acid per mole of cobalt present in the mother liquor which is treated, and d) adding further oxalic acid to the cobalt depleted stream and separating a second precipitate containing iron to obtain a purified stream.

[0019] Figure

[0020] Fig. 1 shows a general concept of a process line-up of the present invention.

[0021] Fig. 2 shows the same general concept combined with 2,5-furandicarboxylic acid purifici cation.

[0022] Detailed description

[0023] The current process is aimed at producing 2,5-furandicarboxylic acid. The product obtained can contain further compounds besides 2,5-furandicarboxylic acid such as sideproducts of the oxidation of the substituted furfural compound. Well known side-products of the oxidation of HMF and / or MMF are for example 5-formyl-2-furancarboxylic acid (FFCA), monomethyl ester of 2,5-furandicarboxylic acid, methyl ester of FFCA and bis-carbonyl-furoic acid (also referred to as 5,5’-carbonyl-bis-furan-2-carboxylic acid furoic acid, BCFCA). Monomethyl ester of 2,5-furandicarboxylic acid can be used in the preparation of polyesters by transesterification and therefore is considered a desirable end-product as well.

[0024] The substituted furfural compound can be any substituted furfurfal known to be suitable. More specifically, the furfural compound is according to the formula (1) wherein R is an organic substituent containing of from 1 to 3 carbon atoms, and preferably is selected from the group consisting of methyl, methoxymethyl, hydroxymethyl and mixtures thereof, more preferably the group consisting of methoxymethyl, hydroxymethyl and mixtures thereof. The feed preferably comprises at least 80 % by weight of 5-methylfurfural (MF), hydroxymethyl furfural (HMF) and / or methoxymethyl furfural (MMF), more preferably at least 90 % by weight of MF, HMF and / or MMF. The feed preferably consists of HMF and / or MMF.

[0025] Besides the substituted furfural, the mixture present in step a) comprises acetic acid solvent, a catalyst system and oxidant.

[0026] The acetic acid solvent can be a recycle stream or can be freshly added. Preferably, a mixture will be applied. A recycle stream which is especially suitable is mother liquor which is obtained by separating solid 2,5-furandicarboxylic acid from reaction mixture. The solvent generally will be an aqueous solution of acetic acid more specifically an aqueous solution of acetic acid containing at least 50, preferably at least 70, % by weight of acetic acid.

[0027] The catalyst system comprises cobalt, manganese and bromine. Each of cobalt and manganese can be present as such or as a derivative thereof. The amount of bromine is the amount of ionic bromine and excludes bromine present as part of a derivative such as bromoacetic acid. The catalyst system preferably has a weight ratio of cobalt to manganese of 5 or higher, preferably 10 or higher, preferably 15 or higher. The weight ratio of bromine to the combined weight of cobalt and manganese in the catalyst system preferably is 1 or higher, preferably 1.5 or higher, most preferably 2 or higher. The weight ratio of bromine to the combined weight of cobalt and manganese in the catalyst system is preferably less than 4.0, more preferably less than 3.5, more preferably less than 3, more preferably less than 2.5. If the catalyst system comprises other metals besides cobalt and manganese in an amount of 5 % by weight or more, it is preferred that the above ratios are achieved for the weight ratio of bromine to the combined weight of all metals in the catalyst system. The metals preferably are added as salts which are soluble in the reaction mixture. The weight ratios and amounts are to be calculated based on the elements per se. 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 during oxidation. The amount of manganese typically is in the range from 20 to 6000 ppm by weight, based on the weight of the reaction mixture during oxidation.

[0028] Typically, the bromine concentration would be from 30 to 8000, preferably 50 to 4500 ppm by weight of bromine, based on weight of the reaction mixture during oxidation. Alternatively, the bromine content is from 3000 to 8000 ppm by weight.

[0029] The oxidant can be any gas known to be suitable by the person skilled in the art. Preferably, the oxidant comprises molecular oxygen. Most preferably, the source of the oxidant is air.

[0030] It will be clear that the amount of oxidant required depends on the substituted furfural compound to be oxidized and the reaction conditions applied. The amount of oxidant generally is 0.5 to 10 mol of oxygen per mole of substituted furfural compound, preferably of from 1 to 8, more preferably of from 1 to 5 mole of oxygen per mole of substituted furfural.

[0031] The current process can be carried out in batch or semi-batch, or continuously. The process is especially advantageous for continuous operation which will allow to make best use of the recycle streams.

[0032] A process is considered to be carried out continuously if the substituted furfural is added to an oxidation reactor substantially continuously. Preferably, the crude carboxylic acid is removed from the oxidation reactor continuously. Preferably, the oxidant is added intermittently or continuously. Most preferably, the oxidant is added continuously. An operation is considered to be continuous if the oxidant flows continuously for the majority of the time, preferably during at least 70 % of the time. Operation is considered to be continuous even if the flow of fluid is frequently stopped for a short time for example due to a valve regularly closing and opening again.

[0033] Step a) preferably is carried out at a temperature in the range of 150 to 210 °C, preferably a temperature of 160 to 190 °C, more preferably a temperature in the range of from 165 to 180 °C. Preferably, the pressure in step a) is in the range of 6 to 20 barg. These parameters were found to be preferred for producing 2,5-furandicarboxylic acid and its monomethyl ester in good yield.

[0034] The above temperature range allows the oxidation reactor to run at elevated pressure while still allowing a large amount of heat generated by the exothermic oxidation reaction to be removed by vaporization. This is known to one skilled in the art as “adiabatic” operation wherein heat of reaction is not being removed by sources such as coolers, loss through the walls, and the like but in majority by evaporation of the solvent and recycling of cold condensate. In general, the higher temperature requires a higher pressure for “adiabatic” operation. A higher pressure, in turn, allows for a higher oxygen partial pressure in the reactor and reduces the risk of oxygen starvation. The latter means that the reaction is limited by the oxygen available.

[0035] The oxidation can be carried out in any typical oxidation reactor that is known in the art. The oxidation of step a) preferably is carried out in a continuously stirred tank reactor.

[0036] Part or all of the product obtained in step a) can be subjected to post oxidation before being subjected to step b). It is preferred to submit to post oxidation substantially all of the reaction mixture including solvent and catalyst. Such post oxidation preferably is carried out at a temperature in the range of 150 to 210 °C, more specifically of 160 to 210 °C. Preferably, the pressure during post oxidation is in the range of 6 to 20 barg. The reaction product generally is allowed to cool.

[0037] Subsequently, at least part of the solid 2,5-furandicarboxylic acid can be separated from the crude product in a solid-liquid separation zone to obtain a solid and mother liquor. The separation of step b) can be carried out in any way known to the person skilled in the art. Preferred is a process wherein the solid-liquid separation zone comprises a filter or centrifuge, preferably a filter, more preferably a rotary pressure filter. The separation preferably is carried out by filtration. At higher temperatures, more dissolved 2,5- furandicarboxylic acid remains in the mother liquor. Therefore, the filtration preferably is carried out at a temperature in the range of 40 to 90 °C. Not all of the 2,5-furandicarboxylic acid generally will be removed from the crude carboxylic acid composition while generally not all of the solid cake which is separated will be 2,5-furandicarboxylic acid. Furthermore, the solid cake tends to contain some mother liquor.

[0038] The 2,5-furandicarboxylic acid product obtained in step b) can be treated or washed with solvent selected from the group consisting of water and mono- and / or dicarboxylic acids. The acid preferably is a mono-carboxylic acid containing of from 1 to 3 carbon atoms.

[0039] In a preferred embodiment, the process further comprises (i) contacting at least part of the 2,5-furandicarboxylic acid product with polar solvent to obtain a 2,5-furandicarboxylic acid solution; (ii) contacting the 2,5-furandicarboxylic acid solution with hydrogen in the presence of a hydrogenation catalyst at hydrogenation conditions yielding a hydrogenated solution; and (iii) separating purified 2,5-furandicarboxylic acid from the hydrogenated solution. Preferred process conditions comprise contacting with hydrogen at a temperature in the range of 150 to 200 °C and a contact time with the hydrogenation catalyst in the range of 5 seconds to 15 min. Preferably, the polar solvent is selected from the group consisting of water, acetic acid and mixtures thereof. Preferred embodiments are described in WO2016 / 195490.

[0040] More specific embodiments for treating the 2,5-furandicarboxylic acid product obtained in step b) are described in WO2021 / 123206 or WO2021 / 123203.

[0041] In many cases, it will be possible to recycle to step a) a substantial part of the mother liquor obtained in step b) without being treated in step c). At least part of the mother liquor obtained in step b) will be treated in step c) in order to be able to separately remove iron contaminant in step d). The amount of mother liquor which is obtained in step b) and is sent to step c) can be of from 5 to 60 %, preferably of from 5 to 40 %, by weight of the mother liquor. The amount of mother liquor obtained in step b) which can be sent to step a) can be of from 40 to 95 %, preferably of from 60 to 95 %, by weight of mother liquor obtained in step the b).

[0042] In step c), oxalic acid is added to at least part of the mother liquor obtained in step b). The amount of oxalic acid added is of from 0.5 to 1.2 mole based on molar amount of cobalt present in the part of the mother liquor which is treated in this step c). This amount allows to recover a substantial amount of the cobalt catalyst without precipitating a substantial amount of iron.

[0043] The expression oxalic acid also can indicate partly neutralized oxalic acid such as the monoanion of oxalic acid containing both a carboxylic acid and a carboxylate group. For the present process, the oxalic acid preferably is ethanedioic acid.

[0044] The exact amount of oxalic acid generally will depend on the amounts of the various metals, especially the amount of cobalt present in the mother liquor which is actually treated in step b). The amount of cobalt tends to be high while cobalt tends to readily precipitate. This makes that mainly cobalt can be recovered in step c). While only a limited amount of iron tends to be present, its absence from the first precipitate is highly desirable. The amount of oxalic acid added in step c) preferably is at least 0.5 mole of oxalic acid based on total molar amount of cobalt present, more preferably at least 0.7, more preferably at least 0.8, more preferably at least 0.9. In order to prevent iron to precipitate, the amount of oxalic acid preferably is at most 1.2 of oxalic acid based on total molar amount of cobalt present, more preferably at most 1.1, most preferably at most 1.0.

[0045] The first precipitate tends to consist of fine particles. Such particles are preferably removed by centrifugation.

[0046] A cobalt depleted stream is obtained after the cobalt containing first precipitate has been removed. At least part of the first precipitate can be sent back to the oxidation step a). Preferably, the first precipitate is added to either a fresh stream or a recycle stream to ensure that the cobalt catalyst is dissolved before being used in the oxidation reaction.

[0047] The cobalt depleted stream tends to still contain a substantial amount of manganese and bromine and metal corrosion products besides solvent and organic by-products. It was found that especially iron becomes easily entrained besides being a serious catalyst poison in the current oxidation process.

[0048] An option is to remove acetic acid from the cobalt depleted stream, for example by distillation, for recycle to step a). The remainder of the cobalt depleted stream can be removed from the process. This has the disadvantage that valuable compounds are removed as well.

[0049] It now was surprisingly found that the iron corrosion product can be precipitated with oxalic acid after cobalt has been substantially removed. The amount of oxalic acid to be added depends on the further compounds present. The person skilled in the art will appreciate that the amount of oxalic acid will have to be higher if the cobalt depleted stream sill contains a substantial amount of cobalt. The presence of a substantial amount of manganese can also require a higher amount of oxalic acid to be added. The amount of oxalic acid to be added in step d) generally is at least 5 mole of oxalic acid per mole of iron present in the cobalt depleted stream treated in step d). An upper limit will depend on further metals present. The amount of oxalic acid added in step d) generally will be at most 1 mole, preferably at most 0.6 mole, of oxalic acid per mole of metals present in the cobalt depleted stream treated in step d). These amounts of oxalic acid are based on the amount of metal present in the cobalt depleted stream actually treated in step d).

[0050] The second precipitate tends to consist of fine particles. These particles are preferably removed by centrifugation.

[0051] A purified stream is obtained after the iron precipitate has been removed from the cobalt depleted stream. At least part of the purified stream obtained in step d) can be recycled to the oxidation reaction of step a). In order to prevent build-up of organic contaminants, it is preferred that a limited amount of purified stream is removed from the process as a bleed stream.

[0052] The purified stream obtained in step d) preferably contains less than 5 parts per million by weight (ppmw) of iron, preferably at most 3 ppmw.

[0053] Fig. 1 shows a general concept of a process line-up of the present invention.

[0054] In the process of Fig. 1, a composition comprising substituted furfural such as methoxymethyl furfural and / or hydroxymethyl furfural, acetic acid solvent and a catalyst system comprising cobalt, manganese and bromine is sent via line 21 to oxidation unit 1. Cobalt catalyst can have been added to line 21 via recycle stream 25 and solvent can have been added via recycle stream 35. Oxidant such as air is added via line 22. The oxidation tends to be carried out at elevated temperature such as of from 170 to 190 °C and a pressure in step in the range of 700 to 2000 kPa. The oxidation unit can be a continuously stirred tank reactor.

[0055] The product obtained is a crude carboxylic acid composition which is cooled and sent via line 23 to separation unit 2 where solid 2,5-furandicarboxylic acid product can be separated off for example by filtration. The 2,5-furandicarboxylic acid product can be removed from the process as product.

[0056] After separation of at least part of the 2,5-furandicarboxylic acid product, the remaining crude carboxylic acid composition is mother liquor which is removed via line 24. A substantial portion of the mother liquor can be recycled to the oxidation unit 1 via lines 25 and 21 while another substantial portion will be sent to unit 3. Oxalic acid is added to unit 3 via line 26. Mother liquor is treated with oxalic acid in unit 3 and sent via line 27 to unit 4. In unit 4, a first precipitate containing cobalt is separated off by centrifuging and removed via line 28. The first precipitate is sent via lines 28 and 21 to oxidation unit 1. At least part of the cobalt depleted stream obtained in unit 4 is sent via line 29 to unit 5 where further oxalic acid is added via line 31. Part of the cobalt depleted stream can be removed from the process via line 30 to allow to remove waste including organic compounds to prevent having to treat these further with oxalic acid and a subsequent separation. The treated cobalt depleted stream obtained in unit 5 is sent via line 32 to unit 6 where a second precipitate containing iron is separated and removed via line 33 to obtain a purified stream 34. At least a portion of the purified stream 34 can be recycled to oxidation unit 1 via lines 35 and 21. A minor portion of the purified stream can be removed as a bleed stream via line 36 to prevent building up undesired metal or organic compounds.

[0057] Figure 2 shows the general concept of Figure 1 in combination with 2,5- furandicarboxylic acid purification. The line-up described for Figure 1 is the same in Figure 2. In Figure 2, the 2,5-furandicarboxylic acid product is sent via line 38 to unit 7. In unit 7, this product is contacted with polar solvent such as an aqueous composition containing acetic acid added via line 37 to obtain a 2,5-furandicarboxylic acid solution. This solution is sent via line 39 to hydrogenation unit 8. Hydrogen is added to unit 8 via line 40. Unit 8 contains heterogeneous hydrogenation catalyst. In unit 8, the 2,5-furandicarboxylic acid solution is treated with hydrogen in the presence of the hydrogenation catalyst at hydrogenation conditions yielding a hydrogenated solution. Such hydrogenated solution is cooled and sent via line 41 to unit 9 where purified 2,5-furandicarboxylic acid is separated from the hydrogenated solution by filtration and removed via line 42. The remainder of the solution is removed via line 43. Part of this solution can be recycled via line 44 and be added again via line 37 to unit 7. The remainder of the solution can be removed from the process via line 45.

[0058] The invention will be further illustrated by means of the following example. Example

[0059] Liquid feed for oxidation was prepared by sugar dehydration. The liquid feed comprised 5-methoxymethylfurfural and 5-hydroxymethyl furfural and as solvent a mixture consisting of a major amount of acetic acid and a minor amount of water. The liquid feed also comprised minor amounts of by-products of sugar dehydration including levulinic acid and methyl levulinate. Further, the liquid feed comprised an oxidation catalyst system composed of cobalt acetate hydrate, manganese acetate hydrate and hydrobromic acid. The oxidant fed into the reactor was air. The liquid feed was oxidized in an oxidation reactor and subsequently in a post-oxidation reactor. The product obtained was filtered to recover solid 2,5-furandicarboxylic acid.

[0060] The filtrate was mother liquor containing 1500 ppm cobalt and 120 ppm manganese. To this mother liquor were added extra metals to obtain a liquid additionally containing a total of 5 ppm iron, 10 ppm of chromium and 20 ppm nickel. Subsequently, oxalic acid was added gradually and precipitate was formed. The amount of metals in the remaining liquid was determined. When the amount of oxalic acid added was 95 molar % based on original total amount of metals, the mixture was centrifuged to remove the precipitate. Subsequently, the addition of oxalic acid to the remaining liquid was continued until 165 % of oxalic acid was added based on original total amount of cobalt.

[0061] It was surprisingly found that staged addition of oxalic acid to a substituted furfural oxidation product allows to substantially recover cobalt catalyst metal and some manganese while the majority of the iron contaminant remains in solution. Subsequently, the iron contaminant can be precipitated and removed to recover a mother liquor which is suitable for recycling. The mother liquor contains solvent, bromine and some manganese and can further contain dissolved 2,5-furandicarboxylic acid and its ester.

Claims

CLAIMS1. Process for producing 2,5-furandicarboxylic acid which process comprises the steps of: a) oxidizing substituted furfural with oxidant in the presence of acetic acid solvent and a catalyst system comprising cobalt, manganese and bromine to obtain a crude carboxylic acid composition comprising solid 2,5-furandicarboxylic acid, b) separating at least part of the solid 2,5-furandicarboxylic acid from the crude carboxylic acid composition to obtain 2,5-furandicarboxylic acid product and mother liquor, c) adding oxalic acid to at least part of the mother liquor and separating a first precipitate containing cobalt to obtain a cobalt depleted stream wherein the amount of oxalic acid added is of from 0.5 to 1.2 mole of oxalic acid per mole cobalt in the mother liquor, and d) adding further oxalic acid to the cobalt depleted stream and separating a second precipitate containing iron to obtain a purified stream.

2. Process according to claim 1 wherein at least part of the first precipitate containing cobalt obtained in step c) is recycled to step a).

3. Process according to claim 1 or 2 wherein at least part of the purified stream obtained in step d) is recycled to step a).

4. Process according to any one of claims 1 to 3 wherein in step d) at least 5 mole of oxalic acid is added per mole of iron in the cobalt depleted stream.

5. Process according to any one of claims 1 to 4 wherein the substituted furfural is according to the formula (1)wherein R is an organic substituent containing of from 1 to 3 carbon atoms, and preferably is selected from the group consisting of methyl, methoxymethyl, hydroxymethyl and mixtures thereof, more preferably the group consisting of methoxymethyl, hydroxymethyl and mixtures thereof.

6. Process according to any one of claims 1 to 5 wherein of from 5 to 60 %, preferably of from 5 to 40 %, by weight of the mother liquor obtained in step b) is sent to step c).

7. Process according to any one of claims 1 to 6 wherein the purified stream obtained in step d) contains less than 5 parts per million by weight (ppmw) of iron, preferably at most 3 ppmw.

8. Process according to any one of claims 1 to 7 which process further comprises(i) contacting at least part of the 2,5-furandicarboxylic acid product obtained in step b) with polar solvent to obtain a 2,5-furandicarboxylic acid solution;(ii) contacting the 2,5-furandicarboxylic acid solution with hydrogen in the presence of a hydrogenation catalyst at hydrogenation conditions yielding a hydrogenated solution; and(iii) separating purified 2,5-furandicarboxylic acid from the hydrogenated solution.

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