Process for producing 2,5-furandicarboxylic acid from ether of 5-hydroxymethylfurfural

Abstract

A method for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid, comprising the steps of: a) oxidizing an oxidizable compound comprising 5-alkoxymethylfurfural in an oxidation reactor using an oxidizing gas at a temperature ranging from 160 to 210°C in the presence of a saturated organic acid solvent having 2 to 6 carbon atoms and a catalyst system comprising cobalt, manganese, and bromine to obtain a crude carboxylic acid composition comprising a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid; and b) separating at least a portion of the solid 2,5-furandicarboxylic acid from a crude carboxylic acid composition in a solid-liquid separation zone. a) isolating a manganese and / or cobalt content from an oxidation reactor to produce a solid cake and a mother liquor; b) determining the amount of manganese and / or cobalt in the cake; and c) increasing the amount of one or more control acids in the oxidation reactor if the determined amount of manganese and / or cobalt in the cake exceeds a predetermined threshold, wherein the one or more control acids are selected from the group consisting of hydrobromic acid and mono- or dicarboxylic acids having 2 to 5 carbon atoms and a pKa of less than 3.2, and the mother liquor comprises a monoalkyl ester of 2,5-furandicarboxylic acid in the range of 0.5 to 7 wt.%, based on the weight of the mother liquor.

Description

[Technical field]

[0001] The present invention relates to a process for producing a carboxylic acid composition containing 2,5-furandicarboxylic acid, and more particularly to a process for producing a carboxylic acid composition containing 2,5-furandicarboxylic acid using 5-alkoxymethylfurfural as a starting material. [Background technology]

[0002] 2,5-Furandicarboxylic acid (FDCA) is known in the art as a very promising building block to replace petroleum-based monomers in the production of high performance polymers. In recent years, a novel plant-based polyester, polyethylene furanoate (PEF), a fully recyclable plastic with superior properties compared to FDCA and the currently widely used petroleum-based plastics, has attracted great interest. These materials could contribute significantly to reducing the dependency on petroleum-based polymers and plastics, while at the same time allowing a more sustainable management of the earth's resources. In response, comprehensive research has been carried out in the field to establish commercially viable production technologies for these promising materials, with the aim of successfully bringing FDCA and PEF to the market.

[0003] FDCA is typically obtained as a crude carboxylic acid composition by oxidation of molecules containing a furan moiety, such as 5-hydroxymethylfurfural (5-HMF), which is typically obtained from plant-based sugars, e.g., by dehydration of sugars, and the corresponding 5-HMF esters or 5-HMF ethers, e.g., 5-alkoxymethylfurfural, and similar starting materials. A wide variety of oxidation processes are known in the prior art, including, e.g., enzymatic and metal-catalyzed processes.

[0004] One of the most established techniques in the field is the use of a catalytic system containing cobalt, manganese, and bromine to oxidize compounds containing a furan moiety to FDCA using oxygen or air as an oxidizing agent. Corresponding processes, which are applicable to a wide variety of starting materials, are disclosed, for example, in WO 2014 / 014981 A1 or WO 2011 / 043660 A1.

[0005] The purity of the crude carboxylic acid composition obtained in the above-mentioned process is often not sufficient to achieve the required purity required for the polymerization of FDCA to PEF and other high-performance polymers, so purification processes have been developed to further purify the crude carboxylic acid composition to produce a purified carboxylic acid composition. These processes include, for example, hydrogenation steps, post-oxidation steps, distillation steps, recrystallization steps, or similar methods, and are often combined with a comprehensive purification scheme with several steps of washing and isolating the obtained carboxylic acid composition. Exemplary purification processes are disclosed, for example, in WO 2014 / 014981 A1 or WO 2016 / 195499 A1.

[0006] In recent years, one of the most promising approaches to obtain FDCA in an economically viable manner has been found to be the use of large amounts of ethers of 5-HMF, such as 5-alkoxymethylfurfural, as starting materials for oxidation. As a result, the crude carboxylic acid composition obtained in such a process not only contains the free diacid, i.e., FDCA, but also contains a significant amount of monoalkyl esters of FDCA, and currently the process using 5-methoxymethylfurfural as starting material for oxidation, which results in a significant amount of monomethyl ester of FDCA (FDCA-Me), is considered to be the most established.

[0007] While some prior art documents enthusiastically report high yields and good purity for their claimed oxidation processes, little attention is often paid to the fact that the underlying reactions are in most cases very difficult to carry out in practice and / or are quite sensitive to external influences. This is especially true for batch experiments with long residence times and for (semi-)continuous processes that need to be operated (preferably at steady state) for long periods of time. These problems are especially acute when several subsequent process steps need to be chained together to obtain the desired product, since a small deviation in one process step can have a multifold negative impact on downstream reactions.

[0008] Moreover, most prior art documents disclose only laboratory-scale experiments. However, to produce new compounds in a commercially viable manner, large-scale reactors are required, and it is even more difficult to keep the reaction going. In a real industrial-scale plant, gradients in process parameters, such as temperature and concentration, fluctuations in the mass flow rate of compounds, or other effects can cause the process to stop completely or produce undesirable products. For example, the use of recycle streams to improve the efficiency and economy of the process can lead to the accumulation of materials, both desirable and undesirable.

[0009] Unfortunately, although the oxidation process starting from 5-alkoxymethylfurfural has several advantages over comparable prior art processes that do not produce monoalkyl esters of FDCA, such as efficient sugar dehydration, product recovery to produce 5-alkoxymethylfurfural, high yields, and good product purity, such processes have sometimes proven to be particularly difficult to control. In establishing this technology, it was found that it can be difficult to keep the process running for long periods of time (those skilled in the art often refer to such processes as "alive" or "living"). For some sets of process parameters, sometimes reported as favorable in the prior art based on laboratory-scale experiments, the process tends to stop after a certain time (those skilled in the art often refer to such processes as "dead" or "dying"), and in some cases, it may even be impossible to start the process in the first place. The oxidation process for producing FDCA can show signs of dying in several ways, but it is often observed that the FDCA yield drops significantly as more unwanted by-products are produced and the color of the resulting product, often a solid cake, changes from white to yellow to brown. As a result, the white to brown scale of cake color is a good qualitative indicator of whether the process is running undesirably off-target or even "dying" completely. Furthermore, reaction termination is usually evidenced by a sudden increase in oxygen content in the reactor outlet gas stream and a decrease in CO2 and CO production.

[0010] In addition to the above problems of oxidation processes starting from 5-alkoxymethylfurfural, it has also been unfortunately observed that these processes are more likely to suffer from catalytic metal contamination of the product cake than other prior art processes, which not only contaminates the product but also removes valuable catalyst from the system that could otherwise be reused or recycled.

[0011] Contamination of the catalyst metal into the product cake appears to be particularly problematic with respect to oxidation of furan-containing moieties to form 2,5-furandicarboxylic acid. This problem has not been reported before, but to our knowledge, it appears particularly strong with respect to contamination of the cake with manganese. In such cases, the product cake is particularly enriched in manganese relative to cobalt in the cake compared to the catalyst feed. We believe this effect is different and distinct from the occasionally observed over-oxidation of manganese(II) to manganese(IV) (manganese is oxidized to form Mn(IV)O2 and then precipitates from solution into the product cake as black specks) occasionally reported in the Co / Mn / Br literature for organic acid production via oxidation. We report here the appearance of a pink color in the cake, which is associated with excess manganese in the cake.

[0012] Without intending to be bound by any theory, it is believed that a relatively insoluble complex is formed between 2,5-furandicarboxylic acid and manganese, possibly involving a doubly ionized form of 2,5-FDCA and Mn(II).

[0013] The two effects mentioned above, namely the challenge of keeping the process alive and the undesirable tendency of metals from the catalytic system to be introduced into the cake, appear to be two separate effects observed for oxidation processes starting from 5-alkoxymethylfurfural. For example, metals from the catalytic system are also observed in the "live" process and are qualitatively manifested by a pink color in the cake. However, these two effects may have a similar or at least related origin. In any case, since the process requires a metal catalyst to run, it is believed that the removal of the catalyst from the process in the cake, i.e., in the precipitate, may at least contribute to the difficulties of keeping the process alive. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] WO 2014 / 014981 A1 [Patent Document 2] WO 2011 / 043660 A1 [Patent Document 3] WO 2016 / 195499 A1 Summary of the Invention [Problem to be solved by the invention]

[0015] In view of the above problems, the main objective was to overcome the shortcomings of the prior art oxidation process using 5-alkoxymethylfurfural as starting material, while maintaining the overall advantages of the basic process. In particular, there was a need for a process for producing 2,5-furandicarboxylic acid from 5-alkoxymethylfurfural, which can be reliably started and operated for a long time, even when carried out on an industrial scale, without leaving the tolerance range, or at least with a reduced possibility of doing so. Furthermore, there was a need for a process for producing 2,5-furandicarboxylic acid from 5-alkoxymethylfurfural, which reduces the problem of metal contamination in the product cake. In particular, one objective was to provide a process for producing 2,5-furandicarboxylic acid, which can affect the running process when an increase in the amount of metal contamination in the cake is observed, and which requires only minor adjustments, preferably to parameters that can be accurately controlled and rapidly adjusted. It would be particularly desirable if the process for producing 2,5-furandicarboxylic acid could achieve the respective advantages by means of advanced process control, without the need for additional substances or devices.

[0016] Another object was to provide a process for the production of 2,5-furandicarboxylic acid from 5-alkoxymethylfurfural using an optimized catalyst system that increases the robustness of the respective process and reduces the tendency of metals to be incorporated into the cake, while also maintaining the beneficial properties reported for such processes.

[0017] Yet another object was to provide a process for producing 2,5-furandicarboxylic acid, which uses acetic acid or acetic acid with traces of water as a primary wash for the crude cake.

[0018] Without intending to be bound by any theory, it is believed that the presence of the monoalkyl ester of 2,5-furandicarboxylic acid in the oxidation reactor is responsible for some of the beneficial effects typically associated with this technology in the prior art. Therefore, it has been found that it is desirable to have a minimum amount of the monoalkyl ester of 2,5-furandicarboxylic acid in the oxidation reactor. In particular, the presence of a certain amount of the monoalkyl ester of 2,5-furandicarboxylic acid in the feed to the oxidation reactor appears to reduce the tendency of manganese to appear in the product cake.

[0019] However, it was also found that monoalkyl esters of 2,5-furandicarboxylic acid appear to be the reason for some of the above problems associated with the process. In particular, when the concentration of monoalkyl esters of 2,5-furandicarboxylic acid exceeds a maximum value, it appears to have a detrimental effect on both the robustness of the process and the ability to wash metals out of the cake. The solubility of monoalkyl esters of 2,5-furandicarboxylic acid, such as monomethyl-2,5-furandicarboxylate, in acetic acid-rich systems is much higher than that of 2,5-furandicarboxylic acid. FDCA-Me tends to remain in solution and only partially co-crystallize in the product cake, whereas FDCA crystallizes to a significant extent and only a small amount remains in solution. As a result, FDCA-Me tends to be retained in the "mother liquor" and accumulate in the system. If the level rises sufficiently, the solubility limit at the temperature used for product isolation is exceeded and a second phase consisting mainly of FDCA-Me crystallizes out of solution. This precipitate, consisting of fluffy waxy particles, proved to be particularly difficult to filter, resulting in long filtration times, difficult cake washing, and increased metal loading (in proportions similar to the catalyst metal feed).

[0020] Without intending to be bound by any theory, it is believed that the amount of monoalkyl ester of 2,5-furandicarboxylic acid must be kept within a certain range, and it has been found that the most convenient reference system for its concentration is the mother liquor, i.e. the liquid obtained from the reaction mixture and the crude carboxylic acid composition after the FDCA has been separated in the solid-liquid separation zone, since it allows to gather information about the reaction medium in the oxidation reactor.

[0021] Accordingly, in the present process for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid, the mother liquor comprises a monoalkyl ester of 2,5-furandicarboxylic acid in the range of 0.5 to 7% by weight, based on the weight of the mother liquor.

[0022] However, it has been found that this limitation alone does not completely eliminate the problem of metal contamination from the catalyst into the cake. Fortunately, the inventors have found a solution to control the process in such a way that metal contamination into the cake can be addressed early without having to shut down the running process.

[0023] In this process, the amount of manganese and / or cobalt in the cake is determined, and only if the amount exceeds a predetermined threshold value is an additional process step implemented to adjust the process. Manganese has a particular tendency to be contaminated in the cake in greater proportion than cobalt. A useful indicator of this tendency is the ratio of manganese to cobalt in the cake divided by the ratio of manganese to cobalt in the catalyst feed. When this ratio has a value of about 1.0, the metals in the dried cake accurately reflect the metals in the catalyst system. When this ratio is significantly higher than 1, for example 2 or higher, manganese is preferentially trapped in the cake, and remedial control measures should be taken as described herein.

[0024] Surprisingly, it has been found that increasing the amount of control acid(s) in the oxidation reactor can address metal cake contamination. Extensive experimentation has revealed that the control acid(s) must be selected from the group consisting of hydrobromic acid and mono- or dicarboxylic acids having 2 to 5 carbon atoms and a pKa less than 3.2 for the process to operate properly.

[0025] Furthermore, it has been discovered that the oxidation process resulting in the production of mono-alkyl esters of 2,5-furandicarboxylic acid requires specific temperatures to enable reasonable operation, and specific catalyst systems have been identified which have proven to be particularly robust, and which themselves reduce the tendency of metals to become incorporated into the cake, even in the presence of greater amounts of mono-alkyl esters of 2,5-furandicarboxylic acid. [Means for solving the problem]

[0026] The present invention provides a process for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid, comprising: a) oxidizing an oxidizable compound comprising 5-alkoxymethylfurfural in an oxidation reactor using an oxidizing gas at a temperature ranging from 160 to 210° C. in the presence of a saturated organic acid solvent having 2 to 6 carbon atoms and a catalyst system comprising cobalt, manganese, and bromine to obtain a crude carboxylic acid composition comprising a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid; b) isolating at least a portion of the solid 2,5-furandicarboxylic acid from the crude carboxylic acid composition in a solid-liquid separation zone to produce a solid cake and a mother liquor; c) determining the amount of manganese and / or cobalt in the cake; d) increasing the amount of one or more control acids in the oxidation reactor if the determined amount of manganese and / or cobalt in the cake exceeds a predetermined threshold. Including, the one or more control acids are selected from the group consisting of hydrobromic acid and mono- or dicarboxylic acids having from 2 to 5 carbon atoms and a pKa of less than 3.2; The mother liquor comprises a monoalkyl ester of 2,5-furandicarboxylic acid in the range of 0.5 to 7% by weight, based on the weight of the mother liquor. Regarding the process.

[0027] The process overcomes the drawbacks of the oxidation processes of the prior art, while also making it possible to use ethers of 5-HMF as starting material and to maintain the overall advantages associated with this technology, such as high yields, good purity of the product, and availability of cheap starting materials. The process of the invention, even when carried out on an industrial scale, can be reliably started and run for long periods of time without leaving the acceptable range of product quality. The process of the invention makes it possible to address the problem of metal contamination in the product cake. As soon as the amount of metal in the cake exceeds a certain threshold, suitable countermeasures are defined that directly allow adjusting the process to yield a product cake with a reduced amount of metal in the cake. In the process of the invention, relatively little intervention is required for process control, the addition of control acid can be precisely controlled, and the intensity of the intervention can be rapidly adjusted as required. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Step a) of the process corresponds to a typical oxidation reaction to obtain FDCA, where the temperature is defined to be in a range that has been found to be particularly advantageous for producing FDCA from starting materials and particularly suitable for allowing process control using a control acid. This temperature has also been found to ensure sufficient formation of monoalkyl esters of 2,5-furandicarboxylic acid. Furthermore, the oxidizable compound oxidized as starting material in step a) is defined to be 5-alkoxymethylfurfural, i.e., ethers of 5-hydroxymethylfurfural.

[0029] Also, lower oxidation temperatures are often used in the prior art. However, current processes are preferred because higher temperatures allow the oxidation reactor to operate under high pressure while still allowing the large heat generated by the oxidation reaction to be removed by vaporization. This is known to those skilled in the art as "adiabatic" operation, meaning that the heat of reaction is not removed by external sources such as coolers, losses through the walls, etc. In general, "adiabatic" operation requires operation at higher pressures at higher temperatures. Higher pressures allow for higher oxygen partial pressures in the reactor (prohibited oxygen volume percentages) and reduce the risk of oxygen starvation. The oxygen volume percentage in the off-gas is generally limited for safety reasons to a level below the lower explosion limit, e.g., 10% by volume, and more preferably, less than about 6% by volume to provide a safety margin. This results in the formation of a crude carboxylic acid composition comprising monoalkyl esters of 2,5-furandicarboxylic acid and 2,5-furandicarboxylic acid. In the process according to the invention, the catalytic system for the oxidation comprises cobalt, manganese and bromine, these compounds being preferably provided as cobalt acetate, manganese acetate and hydrobromic acid, with the use of hydrobromic acid being particularly preferred.

[0030] The oxidation reactor can be any typical oxidation reactor known in the art. The saturated organic acid solvent used in the reaction has from 2 to 6 carbon atoms, with acetic acid being particularly preferred.

[0031] In step b), at least a portion of the solid 2,5-furandicarboxylic acid is isolated, which means separated from the crude carboxylic acid composition, wherein the isolation is carried out in a solid-liquid separation zone.

[0032] Within the framework of the present invention, the term at least a portion preferably means at least 10% by weight, more preferably at least 50% by weight and most preferably at least 80% by weight, based on the weight of the crude carboxylic acid composition.

[0033] In the solid-liquid separation zone, a solid cake and a mother liquor are produced.

[0034] In step c) of the process of the present invention, the amount of manganese and / or cobalt in the cake is determined. Preferably, the amount of manganese is determined. A person skilled in the art will understand that the cake containing solid FDCA may be wet due to the presence of residual mother liquor. However, a person skilled in the art who wants to determine the amount of the compound in the cake will either dry the cake sufficiently or adjust or correct the measurement and its result, respectively, for the residual amount of mother liquor in order to be confident about the measurement result.

[0035] Preferably, the cake in step c) comprises more than 90% solids by weight, more preferably more than 95% solids by weight, most preferably more than 99% solids by weight, the latter also referred to as "dry cake". The "moisture" content of the cake can be determined using any of several techniques known in the art, for example by mass loss under controlled heating conditions, and metal results are reported on a "moisture free" basis.

[0036] The amount of manganese and / or cobalt in the cake can be determined using any suitable measurement technique, where each technique, such as appropriately calibrated x-ray fluorescence (XRF) or inductively coupled plasma (ICP), is well known to those skilled in the art. Besides chemical analysis, spectroscopic and optical measurement methods are particularly preferred. In the most basic case, the amount of manganese in the cake is determined by optical inspection of the cake by the process operator, where the color of the resulting cake is evaluated in terms of the intensity of the pink color, which is currently known to be typically associated with manganese in the cake when it oxidizes furfural-related compounds to form FDCA.

[0037] If the determined amount of manganese and / or cobalt, preferably manganese, exceeds a certain threshold, e.g. because the cake is found to be too pink, the amount of one or more control acids in the oxidation reactor tends to be increased.

[0038] The term control acid has been arbitrarily chosen to clearly denote a particular group of acids which have been found to be suitable in the present process.

[0039] A suitable threshold value will be defined by one skilled in the art based on the individual process characteristics and the amount of metal that is considered acceptable in the cake for subsequent processing steps and / or further applications.

[0040] The total catalytic metal concentration in the cake may also be affected by the retention of catalyst-rich mother liquor, but other than that, it has been found that the enrichment of manganese in the cake relative to the feed catalyst is a better indicator than cobalt. This ratio is particularly useful because applicants have found that it is manganese that is the most sensitive indicator of problems and is found at unusually high levels in the cake when oxidizing furfural-related compounds to form FDCA using catalytic systems containing cobalt, manganese, and bromine. A manganese enrichment factor has been developed and is defined according to the following formula: (Mn / Co) ケーキ / (Mn / Co) 触媒

[0041] This formula has the advantage that it reflects the unwanted enrichment of manganese in the cake while being unaffected by the absolute level of catalyst or the presence of, for example, wash liquid. A value of 1 or less indicates that no preferential enrichment of manganese in the cake is occurring. For practical reasons, this value can vary as high as about 1.5 when the absolute manganese content is low, for example less than about 10 ppm in the cake. Values ​​above this level, or for example above 2.5, indicate unwanted contamination of the cake with manganese and require corrective action. Accordingly, preferred are processes in which the ratio of the mass ratio of manganese to cobalt in the cake to the mass ratio of manganese to cobalt in the catalyst system is less than 2.5, preferably less than 2, more preferably less than 1.5.

[0042] As defined above, the one or more control acids are selected from the group consisting of hydrobromic acid and mono- or dicarboxylic acids having 2 to 5 carbon atoms and a pKa of less than 3.2. Hydrobromic acid is particularly preferred, as it is often available in plant as part of the catalytic system typically used in step a), and is one of the first control acids identified by the inventors. However, when further organic acids were tested, the inventors found that only acids with a certain acidity function as control acids. It was found that the pKa of a suitable control acid should be less than 3.2, where pKa is the index in water.

[0043] Furthermore, screening experiments confirmed that only mono- or dicarboxylic acids can apparently function as control acids. Higher polycarboxylic acids can cause activity loss, possibly due to complex formation. For example, trimellitic acid and pyromellitic acid are relatively strong aromatic polycarboxylic acids with primary pKa values ​​of 2.52 and 1.92, respectively. However, these acids are not suitable as control acids due to the possibility of activity loss in certain oxidations. This study found that FDCA, a relatively strong aromatic dicarboxylic acid, can also cause activity loss and failure to start the system. To the best of the inventors' knowledge, this effect has not been observed during the oxidation of p-xylene to produce terephthalic acid. Without intending to be bound by theory, applicants speculate that the high solubility of FDCA and its high acidity are at least partially responsible for this effect. Oxidation at higher temperatures magnifies this problem due to the significantly increased solubility at higher temperatures and the resulting tendency of diacids to form complexes with the catalyst components.

[0044] The monomethyl ester of FDCA, which has the required pKa but is a monocarboxylic acid with seven carbon atoms, can have the desired effect when added as a control acid. However, this monoester of FDCA has a tendency to accumulate in recycle operations and has been found to have a detrimental effect on the oxidation at high levels, resulting in slower reaction rates, more intermediates, browning, and even reaction death. In addition, the monoester of FDCA can lead to difficulties in filtration, reduced removal of mother liquor from the cake, and higher overall metal levels. However, it has surprisingly been found that a monocarboxylic FCA, namely 2-furan carboxylic acid, is a very suitable control acid for the process of the present invention. However, when using FCA, the purification system used after oxidation must be capable of removing FCA from the FDCA composition if it is desired to use the FDCA in polymerization. This has led the inventors to reason that the control acid can be defined as hydrobromic acid and mono- or dicarboxylic acids having 2 to 5 carbon atoms and a pKa of less than 3.2. Within the framework of the present invention, the monoesters of FDCA also function like control acids, but are considered differently due to the special problems associated with their accumulation, which limits their scope of use.

[0045] In step d), the amount of one or more control acids is increased by deliberately and intentionally adding one or more control acids in the oxidation reactor. Any in situ formation of control acids that may occur in the oxidation reactor is not considered as deliberately and intentionally adding one or more control acids and does not amount to increasing the amount of one or more control acids in the oxidation reactor. Since hydrobromic acid is often used to provide bromide ions to the catalyst of the oxidation reaction, it is convenient to discuss what is considered as an increase in hydrobromic acid within the framework of the present invention. In most (semi-)continuous processes, additional catalyst is added to maintain the desired catalyst concentration, which often includes additional hydrobromic acid, to compensate for the loss of bromine during the reaction, e.g., lost in the overhead or mother liquor. Adding hydrobromic acid to the oxidation reactor to maintain the desired hydrobromic acid concentration in the oxidation reactor does not amount to increasing the amount of hydrobromic acid within the meaning of the present invention. In other words, step d) requires changing the catalyst composition in the reactor by increasing the bromine to metal ratio. Accordingly, step d) of the process of the invention may be: d) if the determined amount of manganese and / or cobalt in the cake exceeds a pre-determined threshold, increasing the amount of one or more control acids in the oxidation reactor or increasing the bromine to metal ratio in the catalyst system, preferably to a mass ratio of greater than 2, even more preferably to a mass ratio of greater than 2.5, said one or more control acids being selected from the group consisting of mono- or dicarboxylic acids having from 2 to 5 carbon atoms and having a pKa of less than 3.2.

[0046] In other words, if a cake using an existing catalyst system is found to be high in metals, particularly manganese, and the catalyst composition is subsequently modified to increase the bromine level via the addition of HBr, such a modification would increase the amount of control acid within the meaning of the present invention In sharp contrast, increasing the bromine, for example via NaBr or NHBr, would not increase the amount of control acid, as these are not control acids within the meaning of the present invention.

[0047] The skilled person knows several suitable methods for adjusting the concentration of the monoalkyl ester of 2,5-furandicarboxylic acid in the oxidation reactor and its concentration in the corresponding mother liquor to the above-defined level. Among the options known in the art, the skilled person can choose the appropriate option based on general knowledge. For example, the skilled person can increase the amount of solvent or other starting materials to dilute the solution in the oxidation reactor, or, if a mother liquor recycle is used, the skilled person can systematically remove the monoalkyl ester of 2,5-furandicarboxylic acid from the mother liquor stream to change the concentration of the ester in the oxidation reactor and in the fresh mother liquor. It is also possible to remove or purge part of the mother liquor from the system to reduce the concentration of the monoester and / or the control acid. Even if part of the mother liquor is purged, acetic acid can still be recovered, for example by distillation, and the residue can be discarded or subjected to a recovery process to recover and reuse the catalyst.

[0048] However, it has been found that there are other convenient options for reducing the amount of monoalkyl esters of 2,5-furandicarboxylic acid in the mother liquor, which tend to accumulate under certain conditions. It has been found that a convenient way of reducing the amount of said substances is available by increasing the temperature in the oxidation reactor of step a) and / or by increasing the residence time of the crude carboxylic acid in the oxidation reactor of step a) and / or by applying a post-oxidation step a1) after step a) and / or by reducing the temperature in the solid-liquid separation zone. A post-oxidation step has been found to be particularly effective when used at high temperatures.

[0049] In view of this observation, a preferred embodiment of the present process comprises controlling, preferably reducing, the amount of monoalkyl ester of 2,5-furandicarboxylic acid in the mother liquor by increasing the temperature in the oxidation reactor of step a) and / or by decreasing the temperature in the solid-liquid separation zone and / or by increasing the residence time of the crude carboxylic acid in the oxidation reactor of step a) and / or by applying a post-oxidation step a1) after step a), where the post-oxidation is carried out in the post-oxidation reactor under the conditions as described for step a).

[0050] Most preferred is a process in which the temperature in step a) is 170° C. or higher, and a post-oxidation step a1) is applied after step a), the post-oxidation being carried out in a post-oxidation reactor under the conditions as described for step a), this process being particularly preferred since it has been found that the amount of monoalkyl ester of FDCA in this case tends to plateau, i.e. increases only up to a certain level, and this level is well within the desired range defined above.

[0051] In fact, this observation has proven very useful in operating some processes starting from ethers of 5-HMF without metal contamination problems. a) oxidizing an oxidizable compound comprising 5-alkoxymethylfurfural in an oxidation reactor using an oxidizing gas at a temperature ranging from 170 to 210° C. in the presence of a saturated organic acid solvent having 2 to 6 carbon atoms and a catalyst system comprising cobalt, manganese, and bromine to obtain a crude carboxylic acid composition comprising a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid; a1) oxidizing the crude carboxylic acid composition of step a) in a post-oxidation reactor using an oxidizing gas at a temperature ranging from 170 to 210° C. in the presence of a saturated organic acid solvent having 2 to 6 carbon atoms and a catalyst system comprising cobalt, manganese, and bromine to obtain a raw carboxylic acid composition comprising a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid; b) isolating at least a portion of the solid 2,5-furandicarboxylic acid from the raw carboxylic acid composition in a solid-liquid separation zone to produce a solid cake and a mother liquor; c) determining the amount of manganese and / or cobalt in the cake; d) increasing the amount of one or more control acids in the oxidation reactor if the determined amount of manganese and / or cobalt in the cake exceeds a predetermined threshold. A process for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid, comprising: The mother liquor comprises a monoalkyl ester of 2,5-furandicarboxylic acid in the range of 0.5 to 7% by weight, based on the weight of the mother liquor. It is a process.

[0052] Preferred is a process in which the one or more control acids are selected from the group consisting of hydrobromic acid, bromoacetic acid, dibromoacetic acid, 5-bromo-2-furoic acid, fumaric acid, acetoxyacetic acid, maleic acid, and furoic acid. More preferably, the control acid is selected from the group consisting of hydrobromic acid, bromoacetic acid, dibromoacetic acid, acetoxyacetic acid, and 5-bromo-2-furoic acid. More preferably, the control acid is selected from the group consisting of bromoacetic acid, dibromoacetic acid, acetoxyacetic acid, and 5-bromo-2-furoic acid.

[0053] The above process is preferred because it has been found that the respective control acid provides particularly good and significant effects while being relatively cheap and easy to handle or available as waste and / or by-products of the process according to step a) and obtained in the mother liquor of step b). Most preferably, a mixture of control acids comprising bromoacetic acid, dibromoacetic acid, acetoxyacetic acid and 5-bromo-2-furoic acid is added to the oxidation reactor. It is also preferred that the control acid is relatively stable, i.e. resistant to chemical decomposition, so as not to require frequent replenishment.

[0054] Preferred is the process according to the invention, wherein the process for producing a carboxylic acid composition is a continuous or semi-continuous process, preferably a continuous process, wherein at least a portion, preferably at least 60% by weight, more preferably at least 80% by weight, of the mother liquor is directed from the solid-liquid separation zone to the oxidation reactor as a recycle mother liquor stream, and preferably the part of the mother liquor which is not directed to the oxidation reactor as a recycle mother liquor stream is treated in an evaporation step in order to recover the organic acid solvent as a condensed vapour stream, and / or preferably one or more bases are added to the mother liquor which is treated in the evaporation, preferably in an amount equal to or greater than the amount of free bromide ions in the mother liquor, in molar terms.

[0055] The process of the present invention provides acceptable results for a batch process, for example, where a sample of the crude carboxylic acid composition, including the solid precipitate, is taken from the batch reactor and treated in a solid-liquid separation zone according to step b). If necessary, control acid can be added to the oxidation reactor of the batch process during operation. Similarly, it is also possible to complete a first batch process, analyze the resulting product cake, and provide additional control acid to a second batch run if the amount of metals in the cake of the first run exceeds a predetermined threshold.

[0056] However, the process of the present invention realizes its full potential in a continuous or semi-continuous process, and since these processes require suitable control mechanisms that allow for minimal invasive adjustments of the running system, suitable for addressing the problem of metal contamination in the cake, the above defined process is clearly preferred. Such processes generally involve continuous or intermittent addition of oxidizable compounds and withdrawal of carboxylic acid compositions comprising 2,5-furandicarboxylic acid. Advantageously, the mother liquor obtained in step b) can be reused in subsequent batch experimental runs to increase the amount of control acid in the oxidation reactor. However, designing the process of the present invention as a continuous or semi-continuous process allows the mother liquor to be returned from the solid-liquid separation zone to the oxidation reactor as a recycled mother liquor stream. This allows the skilled person to increase the amount of control acid in the oxidation reactor when the mother liquor stream contains control acid.

[0057] Preferred is the process according to the invention, wherein the oxidizable compound comprises 5-methoxymethylfurfural and the crude carboxylic acid composition comprises the monomethyl ester of 2,5-furandicarboxylic acid.

[0058] It is believed that the present process can be used for 5-alkoxymethylfurfural regardless of the length of the alkoxy chain, especially for 5-alkoxymethylfurfural in which the alkoxy group contains from 1 to 6 carbon atoms. It has been found that the best results are obtained when 5-methoxymethylfurfural is used as the oxidizable compound. This is particularly advantageous since 5-methoxymethylfurfural has proven to be one of the most economically viable starting materials for the production of FDCA.

[0059] Preferred is the process according to the invention, wherein the mother liquor comprises in the range of 1.0 to 4% by weight of monoalkyl ester of 2,5-furandicarboxylic acid, preferably monomethyl ester of 2,5-furandicarboxylic acid, relative to the weight of the mother liquor.

[0060] The above process is preferred because it has been found that the amount of alkyl ester of 2,5-furandicarboxylic acid in the mother liquor in the above range ensures that the beneficial effects of the compound are fully exerted, while at the same time establishing sufficient buffering against the upper limit of the identified mono alkyl ester of FDCA, giving the process high flexibility with respect to fluctuations and peaks in the concentration of alkyl ester of 2,5-furandicarboxylic acid.

[0061] Preferred is a process, wherein the mother liquor comprises bromoacetic acid in an amount preferably of 0.5% by weight or more, relative to the weight of the mother liquor, and / or dibromoacetic acid in an amount preferably of 0.1% by weight or more, relative to the weight of the mother liquor, and / or 5-bromo-2-furoic acid in an amount preferably of 0.02% by weight or more, relative to the weight of the mother liquor.

[0062] Depending on the parameters selected, the mother liquor was found to contain bromoacetic acid, and / or dibromoacetic acid, and / or 5-bromo-2-furoic acid. These compounds can advantageously act as control acids, but their formation has not been reported previously in other processes for producing FDCA, such as processes starting from 5-HMF, and may be a characteristic feature of the process using 5-alkoxymethylfurfural as the oxidizable compound, at least when certain reaction conditions, as defined above in step a), are established.

[0063] Preferred is a process in which the predetermined threshold for cobalt in the cake is 200 ppm by weight, preferably 50 ppm by weight, most preferably 30 ppm by weight, relative to the weight of 2,5-furandicarboxylic acid, and / or the predetermined threshold for manganese in the cake is 100 ppm by weight, preferably 25 ppm by weight, most preferably 15 ppm by weight, relative to the weight of 2,5-furandicarboxylic acid. This process is preferred because each threshold ensures that a solid cake is obtained with a sufficiently low content of metals that it can be efficiently further processed. Further preferred is a process according to the invention, in which the predetermined threshold also includes the ratio of the mass ratio of manganese to cobalt in the cake to the mass ratio of manganese to cobalt in the catalyst system.

[0064] Preferred is a process in which the amount of one or more control acids in the oxidation reactor is increased by adding one or more control acids to the oxidation reactor by increasing the portion of the mother liquor that is directed to the oxidation reactor as a recycle mother liquor stream.

[0065] This process is preferred because it eliminates the need for handling and / or storage of additional control acids at the production side, thereby reducing costs and eliminating the need for additional equipment. The amount of control acid in the oxidation reactor can be increased by directing the mother liquor to the oxidation reactor as a recycled mother liquor stream, especially in the case of a continuous or semi-continuous process. If the mother liquor stream contains one or more control acids and replaces fresh solvent in the oxidation reactor, the concentration of the one or more control acids contained in the mother liquor will increase in the oxidation reactor. This setup allows for a high degree of process control, allowing the amount of control acid in the oxidation reactor to be increased by increasing the portion of the mother liquor that is directed to the oxidation reactor as a recycled mother liquor stream.

[0066] Preferred is a process according to the invention, in which the weight ratio of cobalt to manganese in the catalytic system is 10 or more, preferably 15 or more, and / or the weight ratio of bromine to the combined weight of cobalt and manganese in the catalytic system is 1 or more, preferably 1.5 or more, most preferably 2 or more, preferably less than 4.0, more preferably less than 3.5. When the catalytic system contains metals other than cobalt and manganese in an amount of 5% by weight or more, the above ratios are preferably achieved for the weight ratio of bromine to the combined weight of all metals in the catalytic system. This process is particularly preferred, since it has been found that said catalytic system significantly outperforms other catalytic systems under the conditions defined above for step a). In particular, the inventors have surprisingly found that said catalytic system reduces the tendency of manganese and cobalt of the catalytic system to contaminate the product cake, reducing the enrichment of manganese in the cake relative to cobalt, even when the amount of monoalkyl ester of 2,5-furandicarboxylic acid is increased. Likewise, the use of the catalyst system allows the process of the present invention to be carried out in a very reliable manner, as the range of process parameters, such as pressure and residence time, is significantly increased, significantly reducing the possibility of undesired process stoppages and / or reducing the formation of unwanted by-products.

[0067] Preferred is the process according to the invention, in which the step of isolating at least a portion of the solid 2,5-furandicarboxylic acid in the solid-liquid separation zone comprises washing the solid 2,5-furandicarboxylic acid with a wash solution comprising a saturated organic acid solvent having 2 to 6 carbon atoms, preferably acetic acid, and less than 15% by weight, preferably less than 10% by weight, of water. The above process is preferred because the amount of metals in the cake can be further reduced if a wash step is used in the solid-liquid separation zone. It was quite surprising here that a wash solution mainly comprising a saturated organic acid solvent, preferably acetic acid, has reasonable success in removing metals from the cake. In the prior art, it was often believed that a larger amount of water would be necessary to ensure sufficient metal removal from the cake. However, with the process of the invention, it has been found that washing with organic acid is sufficient to obtain a cake with a sufficiently low content of metals. This is considered to be particularly beneficial, since the oxidation reaction is often found to be sensitive to high concentrations of water, and therefore a large amount of water that may be introduced into the system with the wash solution is undesirable for any system using mother liquor recycle to the oxidation reactor.

[0068] Preferred is a process in which the mass ratio of manganese to cobalt in the cake divided by the mass ratio of manganese to cobalt in the catalyst system is less than 1.5, preferably less than 1.3. This process is preferred because it introduces a clear criterion for the process operator to determine whether his process is operating in a desired state, thereby allowing him to identify errors in the system in a very simple and reliable way.

[0069] The process of the present invention may require the addition of large amounts of control acids, some of which contain ionic bromine or bromine-based organic compounds. These strong acids are corrosive and may form highly oxidizing gaseous compounds. As a result, the mother liquor stream contains highly acetic and / or corrosive compounds, and / or additional metal ions, such as iron, nickel, or chromium, originating from the reaction equipment. Furthermore, the oxidizing gaseous materials may damage the overhead equipment of the oxidation reactor. Therefore, it was a further object of the present invention to provide measures to protect the equipment, especially the overhead equipment and tubing in contact with the mother liquor stream, and / or to remove unwanted metals from the mother liquor. The inventors have found that the following process, which can address these challenges, is preferred.

[0070] Preferred is a process according to the invention, comprising h) contacting at least a portion of the mother liquor with a composition comprising a base selected from the group consisting of Na2CO3 and NaOH to raise the pH to greater than 7, whereby one or more metal hydroxides or carbonates precipitate from the mother liquor, the metal being selected from the group consisting of cobalt, manganese, iron, nickel, or chromium. The process further comprises:

[0071] Preferred is a process in which the mother liquor contains cobalt in an amount of more than 2000 ppm by weight and manganese in an amount of more than 130 ppm by weight, based on the weight of the mother liquor, which is preferred because it allows efficient mother liquor recycling and allows for the preservation of a sufficiently large amount of catalytic metals, based on cobalt and manganese, in the oxidation reactor to which the recycled mother liquor stream is fed.

[0072] Preferred is the process in which the solid 2,5-furan dicarboxylic acid isolated in step b) is further washed with a second washing solution containing water in an amount of more than 95% by weight, preferably more than 99% by weight, based on the weight of the washing solution. This process is beneficial because it can provide an FDCA cake containing minimal amounts of manganese and cobalt. However, as shown above, the process is often found to be more suitable for batch processes in which no mother liquor recycle is used. If the second washing solution is mixed with the mother liquor in the solid-liquid separation zone, it becomes more difficult to reuse the mother liquor stream, since the presence of a large amount of water is often undesirable in the oxidation reaction as defined in step a).

[0073] Preferred is the process where the organic acid solvent is acetic acid, as this has proven time and again to be the most suitable solvent used in the majority of prior art processes, which is cheap, readily available and relatively acceptable when considering environmental aspects.

[0074] Preferred is the process where the oxidizing gas comprises molecular oxygen, preferably air, as in most cases using air is the most economically viable way to oxidize 5-alkoxymethylfurfural to FDCA.

[0075] Preferred is the process in which the temperature in step a) is in the range of 170 to 190° C. This process is preferred because the inventors have found that in this particular temperature range, the amount of monoalkyl ester of 2,5-furandicarboxylic acid tends to progress toward a plateau value that is well within the desired range defined above. Moreover, the temperatures have been found to produce FDCA in high yields and good purity, as qualitatively indicated by the white cake observed in some of the experiments using each temperature.

[0076] Preferred is the process according to the invention, in which the pressure in step a) is in the range of 700 to 2000 kPa and / or the oxidation reactor comprises one or more continuous stirred tank reactors, preferably two or more continuous stirred tank reactors connected in series. Among the possible sets of parameters and equipment tested by the inventors, the above parameters were found to be ideal for obtaining high purity FDCA in good yields while at the same time minimizing the energy costs required for pressurizing the reactor.

[0077] Preferred is a process according to the invention, in which the solid-liquid separation zone comprises a filter or a centrifuge, preferably a filter, more preferably a rotary pressure filter. The above process is advantageous because filters and centrifuges have been found to be particularly suitable means for isolating solid FDCA from mother liquors containing monoalkyl esters of 2,5-furandicarboxylic acid, despite the fact that these compounds are often difficult to separate from the solid FDCA.

[0078] Preferred is a process according to the invention, wherein the cake comprises 2,5-furandicarboxylic acid in an amount of more than 95%, preferably more than 98%, by weight, based on the weight of the dried cake, and monoalkyl ester of 2,5-furandicarboxylic acid, preferably in an amount ranging from 0.1 to 3%, preferably from 0.15 to 2.3%, by weight, based on the weight of the dried cake, and / or wherein the cake comprises a combined amount of cobalt and manganese of less than 300 ppm by weight, preferably less than 75 ppm by weight, based on the weight of 2,5-furandicarboxylic acid in the cake.

[0079] It is believed that the amount of monoalkyl ester of FDCA should be below 3% by weight, based on the weight of the dry cake, to prevent compromising the effectiveness of this compound in subsequent purification processes.

[0080] In view of the above disclosure of the present invention, a person skilled in the art will understand that the results obtained by the inventors of the present invention also allow the definition of an optimized oxidation process for the production of FDCA. Such a process will utilize all the information discussed above to provide a process that can be reliably operated and requires less effort in process control due to lower initial metal contamination in the cake. Accordingly, the relevant aspects of the process can be as follows: 1. A process for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid, comprising: a1) oxidizing an oxidizable compound comprising 5-alkoxymethylfurfural in an oxidation reactor using an oxidizing gas at a temperature ranging from 160 to 210° C. in the presence of a saturated organic acid solvent having 2 to 6 carbon atoms and a catalyst system comprising cobalt, manganese, and bromine to obtain a crude carboxylic acid composition comprising a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid, wherein the liquid phase in the reactor comprises a monoalkyl ester of 2,5-furandicarboxylic acid in the range of 0.5 to 7% by weight based on the weight of the liquid phase, bromine is provided as hydrobromic acid, the weight ratio of cobalt to manganese in the catalyst system is 10 or more, preferably 15 or more, and the weight ratio of bromine to the combined weight of cobalt and manganese in the catalyst system is 1 or more, preferably 1.5 or more, and most preferably 2 or more. The process includes:

[0081] It will be apparent to one skilled in the art that the preferred embodiments of this process correspond to the preferred embodiments of the process disclosed above, for example with respect to the ranges of solvent, starting material, catalyst, temperature, and alkyl ester of 2,5-furandicarboxylic acid.

[0082] The present invention will now be described in more detail using experiments. EXAMPLES

[0083] Unless otherwise noted, the oxidation reactor is a 600 ml stirred pressure vessel equipped with two impellers. The reactor is pre-charged with a solvent comprising acetic acid and water in a 95 / 5 weight ratio, and catalyst components for a specific catalyst system comprising cobalt, manganese, and bromine. The catalyst components are provided as a 48% by weight aqueous solution of cobalt(II) acetate tetrahydrate, manganese(II) acetate tetrahydrate, and HBr. A typical amount of the "pre-charge" is 310 grams.

[0084] The oxidation reactor is purged, pressurized, and heated to the desired operating temperature with stirring at 2000 rpm. The oxidizable compound provided as feed is either 5-methoxymethylfurfural (MMF) or a mixture of MMF with 5-hydroxy-methylfurfural and a small amount of levulinic acid. The process is started with a typical feed rate of 8.3 mmol / min and is continued for 60 minutes (total feed of 500 mmol). The flow rate of lean air (8% oxygen) is started with a typical flow rate of 10 normal L / min. The reaction typically starts within 3 minutes and is recognized by a rapid decrease in oxygen at the outlet and an increase in CO and CO2. During the reaction, heat is generated and a vapor stream is entrapped overhead and condensed. This vapor stream contains mainly acetic acid and water. The amount of solvent captured overhead is continuously monitored and is compensated in the oxidation reactor by a fresh solvent stream to the reactor.

[0085] Typical operating pressures are 12 to 14 barg at 160°C and 17.5 barg at 175°C.

[0086] After the desired feed period has ended, the feed of oxidizable compound is stopped and the contents of the oxidation reaction are either "quenched" to room temperature (or to a desired filtration temperature) or subjected to post-oxidation for an extended period of time at the same reaction temperature and oxygen flow rate as indicated above.

[0087] Example A Co / Mn ratio The experiment in Example A used a feed of MMF and continued the feed at 160° C. for 1 hour with 15 minutes of post-oxidation. The cake was isolated by filtration and washed with 1 part solvent (95 parts acetic acid to 5 parts water, by weight) per part estimated dry cake weight. The results are shown in Table 1.

[0088] [Table 1]

[0089] Table 1 shows that different catalyst systems can be used in this process. A higher ratio of cobalt to manganese appears to reduce the overall tendency of the system to introduce metals into the cake.

[0090] Example B Metal ratio in cake to catalyst The run in Example B used a feed of MMF and lasted for 1 hour at 160°C with a 15 minute post-oxidation. In all runs, the catalyst was 3300 ppm Co, 185 ppm Mn, and 7000 ppm Br from aqueous HBr. After each run, the reaction slurry was cooled to 80°C and filtered. Residual mother liquor was removed from the cake with minimal washing with acetic acid / water (95 / 5 by weight). The combined mother liquor and washes were analyzed and corrected for cobalt, manganese, ionic bromine, and water (5%). This material was then used as a precharge for the next run to simulate a recycle operation. As a result, subsequent runs were run with an increased amount of control acid in the oxidation reactor. In all cases, cobalt recovery was 90-95%, and high recycle rates were achieved. In all cases, mother liquor from the previous run was used as a precharge for the recycle after adjustments were made to bring the catalyst to the desired level.

[0091] A total of eight runs were performed with a total of seven recycles. Table 2 shows the cake quality and mother liquor composition. The yield is the combined yield of FDCA and monoester of FDCA recovered in the cake. The results are summarized in Table 2.

[0092] [Table 2]

[0093] The results show that for run B1, a run with fresh feed (no recycle), the cake Mn / Co ratio divided by the same ratio in the catalyst was very high, indicating excessive precipitation of Mn into the cake (often associated with a pink cake color). Furthermore, relatively large amounts of Co and Mn were detected in the cake. The remaining runs all contained some FDCA-Me and control acids (bromoacetic acid, dibromoacetic acid, acetoxyacetic acid, 5-bromo-2-furoic acid, and fumaric acid) due to recycle, but the yields were consistent and no excess precipitation of manganese over cobalt was observed.

[0094] FDCA-Me was found to accumulate consistently with each recycle. This experimental setup did not allow for any significant further increase in the control acid, since in each case only the mother liquor was used and the acid accumulation at any given time was governed by the steady state achieved between new production and removal. In experiments B2 to B4, as the concentrations of both FDCA-Me and control acid were initially increased, it was seen that the amount of metals in the cake decreased to a very desirable value. Furthermore, the ratio of Mn / Co in the cake to Mn / Co in the catalyst feed was consistently close to 1, indicating that no excess metal (Mn) was being incorporated into the cake. However, while the amount of control acid plateaued, FDCA-Me continued to accumulate and its detrimental effect on metal contamination clearly began to dominate the system. It was observed that filtration became more difficult when the level of FDCA-Me became very high. In this case, the metals remained in the same ratio as in the catalyst feed, but at relatively high values, indicating problems in achieving good washing and corresponding removal of the mother liquor.

[0095] Example C Different Feeds The experiment in Example C was carried out similarly to Example B, but using a feed containing a mixture of 5-HMF (6.4 wt%), MMF (86.4 wt%), and small amounts of levulinate (2.3 wt%), along with traces of other compounds. Recycle rates were 90+% for each of runs C2 through C4. The results are summarized in Table 3.

[0096] [Table 3]

[0097] Similar to Example B, the first run, i.e., the run without the addition of a strong acidic component, had high manganese levels in the cake. Runs C2 to C4, which included a control acid in the oxidation reactor, had better yields and lower total metals in the cake. These experiments illustrate the process of the present invention for different feed compositions. In particular, a mixture of 5-HMF and MMF gave good results.

[0098] Example D temperature The experiment in Example D was carried out using the same feed as experiment C, but at a temperature of 175°C (except for the first two runs at 160°C). The recycle rate was set at 80% of the mother liquor based on cobalt. A 1 hour post-oxidation was also carried out at 175°C. The pressure was increased to 17.5 barg to accommodate the higher temperature and solvent vapor pressure. The first two runs were carried out at 160°C to establish the "mother liquor" composition for the high temperature experiments. The results are summarized in Table 4.

[0099] [Table 4]

[0100] Similar to Examples B and C, the first run without the addition of control acid resulted in very high levels of manganese in the cake. Again, the first run with increased amounts of control acid in the oxidation reactor (still at 160°C) results in excellent metal content in the cake, even with a relatively low amount of control acid. The third run, the first run at 175°C, shows a slightly elevated level of manganese in the cake, which may be due to a new steady state of the process with respect to the mother liquor composition established at the elevated temperature. In other words, it is speculated that D3 would show much more metals in the cake if it were run without control acid. In fact, if a run at 170°C directly without the addition of control acid were attempted, the process would be expected to die. The first two runs were run at 160°C to establish a living system. All subsequent runs show the desired low levels of metals in the cake, and even the last run results in excellent yields and surprisingly low amounts of metals in the cake. It can be seen that the monoester content levels off at a lower level when post-oxidation is used and run at 175°C with 80% recycle compared to full recycle at 160°C without post-oxidation. Combined with other experiments, this shows that controlling the monoester content can provide superior results over several runs. Furthermore, the beneficial effect of the control acid is again confirmed by Example D.

[0101] Example E Effect of Bromine Content The experiment in Example E was carried out using the same setup as above. The feed used was purified 5-methoxymethylfurfural (MMF), with a total of 500 mmol of MMF used, fed at a constant rate for a total of 1 hour. The reaction temperature was 160° C. and the pressure was 12 barg. No post-oxidation was performed. The reactor “pre-charge” was 310 grams, with catalyst and added alkyl monoester of FDCA (FDCA-Me) as noted below. The total yield reported is the sum of FDCA and FDCA-Me, on a molar basis relative to the MMF feed, after subtracting the initial FDCA-Me. In addition to the precise measurement, the color of the cake was used as a qualitative indicator of the concentration of manganese in the cake (on a scale from white to pink), and the concentration of undesirable by-products and color bodies (on a scale from white to yellow to brown). These qualitative analyses are rapid and provide a fairly reliable impression of the quality of the cake, as even trace amounts of impurities can cause noticeable coloring of the cake. The results are summarized in table 5.

[0102] [Table 5]

[0103] Comparing E1 with E2, or E6 with E7-E9, FDCA-Me appears to reduce the problem of manganese concentration in the cake. This is supported by both the absolute reduction and the Mn / Co cake / catalyst ratio, which is a number well above 1 without FDCA-Me, dropping to near or below 1 with the addition of FDCA-Me. However, E1-E4 show that excess FDCA-Me has a detrimental effect on the process, and the process in E4, which results in a brown product, can be considered a failure. This example also suggests that the presence of excess FDCA-Me makes good washing of the cake more difficult, as shown by the elevated levels of both metals, despite the Mn / Co cake / catalyst ratio being near 1. Comparing E5 with E6, it is evidenced that the amount of manganese in the cake is significantly reduced by adding additional Br as hydrobromic acid. Furthermore, the reaction died suddenly, with E5 dying early with only 362 mmol of feed (out of the planned 500 mmol), while E6 ran without problems. From these results, it is deduced that HBr can function as a control acid in the sense of the present invention. Indeed, it can be seen that the catalytic systems used in E6 to E9 are optimized catalytic systems for the oxidation of 5-alkoxymethylfurfural. Whereas E3 gives a yellow-brown cake at 6 wt.% FDCA-Me, the color of the cakes in E7 to E9 does not reach the brown level. On the contrary, even a "white" cake can be obtained in E9. Finally, the yield in E6 compared to E1, or in E7 to E9 compared to E3, is also significantly increased.

[0104] Example F Control Acid The experiment in Example F was run using the same setup as above. The feed used was purified 5-methoxymethylfurfural (MMF), with a total of 500 mmol of MMF used, fed at a steady rate for a total of 1 hour. The reaction temperature was 160° C. and the pressure was 12 barg. No post-oxidation was performed. The reactor “pre-charge” was 310 grams, with the catalyst and added ingredients as noted below. The total yield reported is the sum of FDCA and FDCA-ME on a molar basis relative to the MMF feed, after subtracting any initial FDCA-ME. In all cases, the catalyst was 3300 ppm cobalt, 185 ppm manganese, and 7000 ppm bromine. The results are summarized in Table 6, and a pink cake was observed in experiments F4 and F7.

[0105] [Table 6]

[0106] Not all acids added in the experiments resulted in the desired white cake with low precipitated metals and no manganese enrichment, as supported by the Mn / Co cake / catalyst ratio. The reference case (no control acid added) used the robust catalyst system identified above and produced a relatively white cake, but still had unwanted manganese enrichment in the cake. The addition of formic acid and 2-carboxy-5-(formyl)furan (FFCA) did not produce the desired white cake or the desired low Mn / Co cake / catalyst values. Given that white cakes were obtained in some cases with this catalyst set-up without the addition of control acid, it was concluded that each acid may even have a detrimental effect. Each of the other acids in the table had a positive effect on reducing the Mn / Co cake / catalyst ratio, supporting the suitability of reducing the problem of excess manganese in the cake. In most cases, good overall metal content was also shown, as was good cake washability. Considering all the experimental evidence, it was deduced that a suitable control acid would be selected from the group consisting of hydrobromic acid and mono- or dicarboxylic acids having 2 to 5 carbon atoms and a pKa less than 3.2.

[0107] Example G Addition of HBr to bromoacetic acid The experiment in Example G was run using the same setup as above. The feed was a mixture of 5-HMF (6.4 wt%), MMF (86.4 wt%), and a small amount of levulinate (2.3 wt%), along with traces of other compounds. In each case, the reactor was "pre-charged" with 310 grams of acetic acid / water in a 95 / 5 wt ratio and the catalyst composition as described. The reaction temperature was 170°C. In all cases, the catalyst was 3300 / 185 / 7000 wt ppm Co / Mn / Br, and bromine was fed using an aqueous solution of HBr. It was observed that the catalyst composition, which flowed well at 160°C and allowed a total of 500 mmol of feed to be passed through for 1 hour, did not flow through the 1 hour period at a temperature of 170°C and a pressure of 17-18 barg. At some point during the run, the reaction was observed to suddenly stop. This is evidenced by a sharp increase in the oxygen content of the exit gas stream and a reduction in the production of CO2 and CO.

[0108] [Table 7]

[0109] In run G1 (no acid added), the process died without any meaningful analysis of yield and cake color. In run G4 (no FDCA added), the process could not be started. In G2, mother liquor from a previous run was added, which increased the amount of FDCA-Me and control acid in the oxidation reactor (by a relatively small amount). G2 achieved a viable process with acceptable yield, although manganese contamination in the cake was significant. In runs G3 (bromoacetic acid added) and G5 (HBr added), the viable process gave a desirable white cake and showed low manganese amounts.

[0110] Example H Addition of HBr This example was carried out using a single continuous stirred tank oxidation reactor (CSTR) where the cobalt and manganese levels were kept constant throughout and feeds were run with different levels of HBr to observe the effect of added HBr on the system.

[0111] The reactor is fitted with a reflux condenser and pump to send the reflux back to the reactor while allowing the removal of heat by evaporation of the solvent. The reactor was pre-loaded with approximately 100 grams of the specific catalyst package in acetic acid. The reactor was heated to 160° C. under nitrogen pressure. After reaching temperature, the gas was switched to a mixture of air and nitrogen with 8% oxygen, with a flow rate of 3.3 Nl / min. A feed of 20% by weight of "RMF" (a mixture of 5-HMF, MMF and levulinic acid as used previously) in acetic acid, and the desired cobalt and manganese (3000 ppm and 300 ppm, respectively). The feed contains approximately 1% by weight water in total, which establishes a steady concentration in the reactor of about 6% due to water formed during oxidation. A valve at the bottom of the reactor is opened about every 30 seconds to remove a small amount of material to keep the level constant and establish "CSTR" conditions. The temperature was maintained at 160°C, the pressure was 13 barg and the residence time was 60 minutes. After at least 3 hours on-stream time the reactor was considered to be at steady state and sampling began. At the end of each run the feed was shut off and post-oxidation was carried out. After post-oxidation the reactor was cooled and the contents were filtered, washed with acetic acid / water and dried before analysis. The metals content of the cake is given in the table below.

[0112] [Table 8]

[0113] Runs H1 and H2 both have high levels of metals in the cake overall, as well as high Mn / Co cake / catalyst ratios. The remaining runs all have low metal contamination overall, which decreases with increasing HBr, and the Mn / Co cake / catalyst ratios are all near 1, indicating very good results.

Claims

1. 1. A method for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid, comprising: a) oxidizing an oxidizable compound comprising 5-alkoxymethylfurfural in an oxidation reactor using an oxidizing gas at a temperature ranging from 160 to 210° C. in the presence of a saturated organic acid solvent having 2 to 6 carbon atoms and a catalyst system comprising cobalt, manganese, and bromine to obtain a crude carboxylic acid composition comprising a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid; b) isolating at least a portion of the solid 2,5-furandicarboxylic acid from the crude carboxylic acid composition in a solid-liquid separation zone to produce a solid cake and a mother liquor; c) determining the amount of manganese and / or cobalt in the cake; d) increasing the amount of one or more control acids in the oxidation reactor if the determined amount of manganese and / or cobalt in the cake exceeds a predetermined threshold. Including, the one or more control acids are selected from the group consisting of hydrobromic acid, bromoacetic acid, dibromoacetic acid, 5-bromo-2-furoic acid, fumaric acid, acetoxyacetic acid, maleic acid, and furoic acid; The mother liquor comprises a monoalkyl ester of 2,5-furandicarboxylic acid in the range of 0.5 to 7% by weight, based on the weight of the mother liquor. method.

2. 10. The process of claim 1 which is a continuous process wherein at least 60 weight percent of the mother liquor is directed from the solid-liquid separation zone to the oxidation reactor as a recycle mother liquor stream.

3. 3. The method of claim 1 or 2, wherein the oxidizable compound comprises 5-methoxymethylfurfural and the crude carboxylic acid composition comprises the monomethyl ester of 2,5-furandicarboxylic acid.

4. 4. The method according to claim 1, wherein the mother liquor comprises the monomethyl ester of 2,5-furandicarboxylic acid in the range of 1.0 to 4% by weight, based on the weight of the mother liquor.

5. 5. The process according to claim 1 , wherein the mother liquor comprises bromoacetic acid, and / or dibromoacetic acid, and / or 5-bromo-2-furoic acid.

6. 6. The method according to claim 1, wherein the predetermined threshold value for cobalt in the cake is 200 ppm by weight, relative to the weight of 2,5-furandicarboxylic acid, and / or the predetermined threshold value for manganese in the cake is 100 ppm by weight, relative to the weight of 2,5-furandicarboxylic acid.

7. 7. The process of any one of claims 3 to 6, wherein the amount of one or more control acids in the oxidation reactor is increased by adding one or more control acids to the oxidation reactor by increasing the portion of the mother liquor that is directed to the oxidation reactor as a recycle mother liquor stream.

8. 8. The process according to claim 1 , wherein the mass ratio of cobalt to manganese in the catalytic system is 10 or more and / or the mass ratio of bromine to the combined mass of cobalt and manganese in the catalytic system is 1 or more.

9. 9. The method of claim 1, wherein the step of isolating at least a portion of the solid 2,5-furandicarboxylic acid in a solid-liquid separation zone comprises washing the solid 2,5-furandicarboxylic acid with a wash solution comprising acetic acid and less than 15% by weight water.

10. 10. The method according to claim 1, wherein the solid 2,5-furandicarboxylic acid isolated in step b) is further washed with a second wash solution comprising water in an amount greater than 95% by weight, based on the weight of the wash solution.

11. 11. The process according to any one of claims 1 to 10, wherein the temperature in step a) is in the range of from 170 to 190°C.

12. 12. The process according to any one of claims 1 to 11, wherein the pressure in step a) is in the range of 700 to 2000 kPa and / or the oxidation reactor comprises one or more continuous stirred tank reactors.

13. 13. The process of any one of claims 1 to 12, wherein the ratio of manganese to cobalt in the cake to the ratio of manganese to cobalt in the catalyst system is less than 2.

5.

14. 14. The method according to claim 1, wherein the cake comprises 2,5-furandicarboxylic acid in an amount of more than 95% by weight, based on the weight of the dry cake.

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