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

Abstract

The process for producing a carboxylic acid composition including 2,5-furandicarboxylic acid comprises the steps of: a) oxidizing an oxidizable compound including 5-alkoxymethylfurfural at a temperature in the range of 160-210° C. in an oxidation reactor in the presence of a saturated organic acid solvent having 2-6 carbon atoms and a catalyst system including cobalt, manganese and bromine using an oxidizing gas to obtain a crude carboxylic acid composition containing a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid, and b) separating the 2,5-furandicarboxylic acid from the crude carboxylic acid composition in a solid-liquid separation zone. The invention relates to an oxidation reactor comprising: a) separating at least a portion of the solid 2,5-furandicarboxylic acid from the oxidizing reactor to produce a solid filter cake and a mother liquor, b) determining the amount of manganese and / or cobalt in the filter cake, and c) increasing the amount of one or more control acids in the oxidation reactor if the measured amount of manganese and / or cobalt in the filter cake exceeds a predetermined threshold, wherein the control acid is selected from the group consisting of hydrobromic acid and a mono- or di-carboxylic acid having 2-5 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 wt % to 7 wt % relative to the weight of the mother liquor.

Description

Technical Field

[0001] The present invention relates to a process for producing a carboxylic acid composition including 2,5-furandicarboxylic acid, and in particular, to a process for producing a carboxylic acid composition including 2,5-furandicarboxylic acid using 5-alkoxymethylfurfural as a starting material. Background Art

[0002] 2,5-Furandicarboxylic acid (FDCA) is known in the art as a very promising base material for replacing petroleum-based monomers in the production of high-performance polymers. In recent years, FDCA and the new plant-based polyesters - polyethylene furanoate (PEF) have attracted a lot of attention as fully recyclable plastics with improved properties compared to the petroleum-based plastics widely used today. These materials can make a significant contribution to reducing the dependence on petroleum-based polymers and plastics, while allowing a more sustainable management of global resources. Accordingly, comprehensive research is being conducted in this field to obtain technologies for the production of FDCA and PEF in a commercially viable manner, allowing these promising materials to successfully enter the market.

[0003] FDCA is usually obtained as a crude carboxylic acid composition by oxidation of molecules with furanic moieties, such as 5-hydroxymethylfurfural (5-HMF) and the corresponding 5-HMF esters or 5-HMF ethers, such as 5-alkoxymethylfurfural, and similar starting materials, which are usually obtained from plant sugars, such as by dehydration of sugars. Various oxidation processes are known from the prior art, including, for example, enzyme-catalyzed or metal-catalyzed processes.

[0004] One of the most mature technologies in the art is to oxidize compounds having a furan moiety to FDCA using a catalyst system comprising cobalt, manganese and bromine, using oxygen or air as an oxidant. Corresponding processes suitable for various 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 insufficient to reach the purity required for the polymerization of FDCA into PEF or other high-performance polymers. Therefore, in order to produce a purified carboxylic acid composition, a purification process for further purifying the crude carboxylic acid composition has been developed. These processes include, for example, a hydrogenation step, a post-oxidation step, a distillation step, a recrystallization step or a similar method, often combined with a comprehensive purification scheme with several steps of washing and separating 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, it has been found that one of the most promising methods for obtaining FDCA in an economically viable manner is to use 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 processes contains not only the free diacid, i.e. FDCA, but also significant amounts of monoalkyl esters of FDCA, of which those processes currently appear to be the most mature, which use 5-methoxymethylfurfural as the starting material for oxidation and produce significant amounts of the monomethyl ester of FDCA (FDCA-Me).

[0007] Although some prior art documents are eager to report the high yields and good purity of their claimed oxidation processes, little attention is paid to the fact that in most cases the underlying reactions are very difficult to carry out in practice and / or very sensitive to external influences. This is especially true for batch experiments with long residence times or (semi-)continuous processes that require long-term operation (preferably in steady state). In particular, these difficulties are very serious if several subsequent process steps need to be linked together to obtain the desired product, because small deviations in one process step may have multiple negative effects on its downstream reactions.

[0008] Furthermore, in most prior art documents, only laboratory-scale experiments are disclosed. However, producing new compounds in a commercially viable manner requires large-scale reactors, which makes it more challenging to keep the reaction running. In a real industrial-scale plant, gradients in process parameters, such as temperature and concentration, changes in the mass flow rate of compounds, or other effects may cause the process to stop completely or produce undesirable products. For example, the use of recycle streams to improve process efficiency or economy can lead to a buildup of materials, whether desired or undesirable.

[0009] Unfortunately, although oxidation processes starting with 5-alkoxymethylfurfural as a starting material have several advantages, such as efficient dehydration of sugars and product recovery to produce 5-alkoxymethylfurfural, high yield and good product purity compared to prior art processes that do not produce monoalkyl esters of FDCA, these processes sometimes prove to be particularly difficult to control. When the technology is established, it is found difficult to keep the process running for a long time (technicians usually refer to such processes as "active" or "alive"). For several sets of process parameters, several of which are reported to be preferred in the prior art based on laboratory scale experiments, the process tends to stop after a period of time, (technicians usually refer to such processes as "dead" or "terminated"), and sometimes the process cannot even be started at the beginning. Although the coloring oxidation process for the production of FDCA can be manifested in several ways, it is often observed that the color of the obtained product (obtained in the form of a solid filter cake in most cases) changes from white to yellow and further to brown, and the yield of FDCA decreases significantly as more and more undesirable by-products are produced. Therefore, the color of the filter cake on the white to brown scale is a good qualitative indicator if the process has deviated from the desired state and is operating in an undesirable state, or even in the process of being completely "stopped." In addition, the cessation of the reaction can usually be evidenced by a rapid increase in the oxygen content of the reactor outlet gas stream and a decrease in the production of CO2 and CO.

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

[0011] Incorporation of catalyst metal into the product filter cake appears to be particularly problematic for the oxidation of furan-containing moieties to form 2,5-furandicarboxylic acid. Although this problem has not been previously reported, to the inventors' knowledge, it is particularly evident when manganese is incorporated into the filter cake. In this case, the product filter cake is particularly rich in manganese relative to the cobalt in the filter cake compared to the catalyst feed. The inventors believe that this effect is different from the sometimes observed over-oxidation of manganese (II) to manganese (IV), which is sometimes reported in the Co / Mn / Br literature for the production of organic acids by oxidation, where manganese is oxidized to form Mn(IV)O2, which subsequently precipitates out of solution as black specks in the product filter cake. The observations and reports made herein show the appearance of a pink color in the filter cake, which is associated with excess manganese in the filter cake.

[0012] While not wishing to be bound by any theory, we believe that a relatively insoluble complex is formed between 2,5-furandicarboxylic acid and manganese, possibly involving doubly ionized forms of 2,5-FDCA and Mn(II).

[0013] The two effects described above, the challenge of keeping the process going and the unfavorable tendency to incorporate metals from the catalyst system into the filter cake, appear to be two separate effects observed in the oxidation process starting with 5-alkoxymethylfurfural. For example, incorporation of metals from the catalyst system is also observed for the "live" process, and the problem itself is qualitatively indicated by the filter cake having a pink color. However, the two effects may potentially have similar or at least related origins. In any case, it is believed that the removal of catalyst from the process into the filter cake (i.e., precipitation) will likely at least increase the difficulty of keeping the process running, since a metal catalyst is required to run the process. Summary of the invention

[0014] In view of the above described problems, the main purpose is to overcome the shortcomings of the prior art oxidation process using 5-alkoxymethylfurfural as a starting material while maintaining the overall advantages of the basic process. In particular, there is a need for a process for producing 2,5-furandicarboxylic acid from 5-alkoxymethylfurfural, which can be reliably initiated and operated for a long time without deviating from an acceptable scheme, or at least reduce the possibility of deviation, even if it is carried out on an industrial scale. In addition, there is a need for a process for producing 2,5-furandicarboxylic acid from 5-alkoxymethylfurfural, which reduces the problem of metal incorporation into the product filter cake. Specifically, one purpose is to provide a process for producing 2,5-furandicarboxylic acid, which allows the operating process to be adjusted when an increase in metal incorporation is observed in the filter cake, wherein it is expected that only minor adjustments are required, preferably adjustments to parameters that can be precisely controlled and quickly adjusted. It would be particularly desirable if the process for producing 2,5-furandicarboxylic acid could achieve the various benefits through complex process control without the need for additional substances or devices.

[0015] Another object is to provide a process for producing 2,5-furandicarboxylic acid from 5-alkoxymethylfurfural using an optimized catalyst system which increases the stability of the corresponding process and reduces the tendency of metals to be incorporated into the filter cake while maintaining the beneficial properties reported for these processes.

[0016] Still another object is to provide a process for producing 2,5-furandicarboxylic acid using acetic acid or acetic acid containing a small amount of water as a primary washing liquid for the crude filter cake.

[0017] Without wishing to be bound by any theory, the presence of a monoalkyl ester of 2,5-furandicarboxylic acid in the oxidation reactor appears to produce some of the beneficial effects generally associated with the corresponding processes of the prior art. Therefore, it was found that a minimum amount of a monoalkyl ester of 2,5-furandicarboxylic acid in the oxidation reactor is desirable. In particular, a certain amount of a monoalkyl ester of 2,5-furandicarboxylic acid in the oxidation reactor feed appears to reduce the tendency of manganese to appear in the product filter cake.

[0018] However, it has also been found that monoalkyl esters of 2,5-furandicarboxylic acid appear to be the cause of some of the problems associated with the corresponding processes described above. In particular, if the concentration of monoalkyl esters of 2,5-furandicarboxylic acid exceeds a maximum value, it appears that the stability of the process and the ability to wash metals out of the filter cake will be adversely affected. Monoalkyl esters of 2,5-furandicarboxylic acid (e.g., monomethyl 2,5-furandicarboxylate) are much more soluble in acetic acid-rich systems than 2,5-furandicarboxylic acid. While FDCA will crystallize in large quantities with only a small residue remaining in solution, FDCA-Me will tend to remain in solution with only a portion co-crystallized into the product filter cake. Therefore, FDCA-Me will remain in the "mother liquor" and tend to accumulate in the system. If this level accumulates high enough, it will exceed the solubility limit at the temperature used for product separation, and a second phase, which is mainly FDCA-Me, will crystallize out of solution. We found this precipitate to be particularly difficult to filter, consisting of fluffy, waxy particles, which increased filtration time, made filter cake washing difficult, and resulted in increased metal incorporation rates (similar to the rates of the catalyst metal feed).

[0019] Without wishing to be bound by any theory, it is believed that the amount of monoalkyl ester of 2,5-furandicarboxylic acid needs to be kept within a certain range, wherein it was found that the most suitable reference system for concentration is the mother liquor, i.e. the liquid obtained from the reaction mixture and the crude carboxylic acid composition after separation of FDCA in the solid-liquid separation zone, since the mother liquor allows to collect information about the reaction medium in the oxidation reactor.

[0020] Accordingly, in the process for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid of the present process, the mother liquor comprises the monoalkyl ester of 2,5-furandicarboxylic acid in the range of 0.5 to 7% by weight relative to the weight of the mother liquor.

[0021] However, we have found that this limitation alone is not sufficient to completely eliminate the problem of metal incorporation from the catalyst into the filter cake. Fortunately, the inventors have found a solution to control the process in a way that metal incorporation into the filter cake can be prevented at an early stage without having to stop the process.

[0022] In this process, the amount of manganese and / or cobalt in the filter cake is determined, and only when the measured amount exceeds a predetermined threshold value, additional process steps are performed to adjust the process. It is particularly evident that manganese is incorporated into the filter cake in a ratio greater than cobalt. A useful measure of this trend is the ratio of manganese to cobalt in the filter cake divided by the ratio of manganese to cobalt in the catalyst feed. When the ratio is about a value of 1.0, the metal in the dry filter cake accurately reflects the metal in the catalyst system. When the ratio is significantly higher than 1, such as 2 or higher, manganese is preferentially divided into the filter cake, and remedial control measures as described herein should be taken.

[0023] Surprisingly, we have found that the incorporation of metals into the filter cake can be suppressed if the amount of one or more control acids in the oxidation reactor is increased, wherein comprehensive experiments have shown that for proper operation of the process the one or more control acids need to be selected from the group consisting of hydrobromic acid and a mono- or dicarboxylic acid having 2-5 carbon atoms and a pKa of less than 3.2.

[0024] Furthermore, we have found that for an oxidation process which will produce monoalkyl esters of 2,5-furandicarboxylic acid, specific temperatures are required in order to allow for reasonable operation, wherein a specific catalyst system has been identified which has proven to be particularly stable and which inherently reduces the tendency for metals to be incorporated into the filter cake, even in the presence of relatively large amounts of monoalkyl esters of 2,5-furandicarboxylic acid.

[0025] The present invention relates to a process for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid, comprising the following steps:

[0026] a) oxidizing an oxidizable compound including 5-alkoxymethylfurfural in an oxidation reactor using an oxidizing gas at a temperature in the range of 160° C. to 210° C. in the presence of a saturated organic acid solvent having 2 to 6 carbon atoms and a catalyst system including cobalt, manganese and bromine to obtain a crude carboxylic acid composition including a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid,

[0027] b) separating at least a portion of solid 2,5-furandicarboxylic acid from the crude carboxylic acid composition in a solid-liquid separation zone to produce a solid filter cake and a mother liquor,

[0028] c) determining the amount of manganese and / or cobalt in the filter cake, and

[0029] d) increasing the amount of one or more control acids in the oxidation reactor if the measured amount of manganese and / or cobalt in the filter cake exceeds a predetermined threshold,

[0030] wherein the one or more controlling acids are selected from the group consisting of hydrobromic acid and mono- or di-carboxylic acids having 2 to 5 carbon atoms and a pKa of less than 3.2, and

[0031] Wherein the mother liquor comprises a monoalkyl ester of 2,5-furandicarboxylic acid in a range of 0.5% to 7% by weight relative to the weight of the mother liquor.

[0032] Adopt the process of the present invention, can overcome the shortcomings of prior art oxidation process, use ether of 5-HMF as starting material simultaneously, and keep the overall benefit relevant to this technology, such as high yield, good product purity and the availability of cheap starting material.The process of the present invention can be started reliably and operated for a long time, has acceptable product quality system, even when carried out as industrial scale.Use the process of the present invention, can solve the problem that metal is incorporated into the product filter cake.Once the metal amount in the filter cake exceeds the predetermined threshold value, just define suitable countermeasure, directly allow to adjust the process to produce the product filter cake that the metal amount in the filter cake reduces.In the process according to the present invention, process control only needs quite little intervention, wherein the addition of control acid can be accurately controlled, and if necessary, can quickly adjust the intensity of intervention. DETAILED DESCRIPTION

[0033] Step a) of the present process corresponds to a typical oxidation reaction for obtaining FDCA, wherein the temperature is defined within a range found to be particularly advantageous for producing FDCA from the starting material and is particularly suitable for process control that allows the use of controlled acid. Likewise, it was found that this temperature ensures the formation of sufficient monoalkyl esters of 2,5-furandicarboxylic acid. Furthermore, the oxidizable compound oxidized as a starting material in step a) is defined as 5-alkoxymethylfurfural, i.e., an ether of 5-hydroxymethylfurfural.

[0034] There are also many cases of using lower oxidation temperatures in the prior art. However, the current process is preferred because higher temperatures allow the oxidation reactor to operate at elevated pressures while still allowing a large amount of heat generated by the oxidation reaction to be removed by evaporation. This is referred to as "adiabatic" operation by those skilled in the art, a way in which the heat of a reaction is not removed by an external source (such as a cooler, heat dissipation through a wall, etc.). In general, higher temperatures require higher pressures for "adiabatic" operation. Higher pressures allow higher oxygen partial pressures in the reactor (at a specified oxygen volume %) and reduce the risk of hypoxia. For safety reasons, the oxygen volume percentage in the exhaust gas is generally limited to below the lower explosion limit, for example at a level of 10 volume %, or more preferably below about 6 volume %, to allow for a safety margin. This results in the formation of a crude carboxylic acid composition comprising 2,5-furandicarboxylic acid and a monoalkyl ester of 2,5-furandicarboxylic acid. In the process according to the present invention, the catalyst system for oxidation comprises cobalt, manganese and bromine, wherein these compounds are preferably provided in the form of cobalt acetate, manganese acetate and hydrobromic acid, wherein hydrobromic acid is particularly preferably used.

[0035] The oxidation reactor may be any typical oxidation reactor known in the art. The saturated organic acid solvent used in the reaction has 2 to 6 carbon atoms, with acetic acid being particularly preferred.

[0036] In step b), at least a portion of the solid 2,5-furandicarboxylic acid is separated, which means separated from the crude carboxylic acid composition, wherein the separation is carried out in a solid-liquid separation zone.

[0037] 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, most preferably at least 80% by weight relative to the weight of the crude carboxylic acid composition.

[0038] In the solid-liquid separation zone, solid filter cake and mother liquor are generated.

[0039] In step c) of the process of the invention, the amount of manganese and / or cobalt in the filter cake is determined. Preferably, the amount of manganese is determined. A person skilled in the art understands that the filter cake containing solid FDCA may be wet due to the presence of residual mother liquor. However, a skilled person who intends to determine the amount of compound in the filter cake will either dry the respective filter cake sufficiently to be sure of the measurement result, or will adjust or correct the measurement and its result, respectively, for the residual amount of mother liquor.

[0040] Preferably, the filter cake in step c) comprises greater than 90% solids by weight, more preferably greater than 95% solids by weight, most preferably greater than 99% solids by weight relative to the weight of the filter cake, wherein the filter cake is also referred to as a "dried filter cake". The "moisture" content of the filter cake can be determined using any of several techniques known in the art, for example, by weight loss under controlled heating conditions, and the results of the metal assay are reported on a "moisture-free" basis.

[0041] The amount of manganese and / or cobalt in the filter cake can be determined using any suitable measurement technique, various of which are well known to those skilled in the art, such as appropriately calibrated X-ray fluorescence (XRF) or inductively coupled plasma (ICP). In addition to chemical analysis, spectroscopic and optical measurement methods are particularly preferred. In the most basic case, the amount of manganese in the filter cake is determined by the process operator through optical inspection of the filter cake, wherein the color of the filter cake obtained is evaluated based on the intensity of the pink color, which is now known to be generally related to manganese in the filter cake when the furfural-related compounds are oxidized to form FDCA.

[0042] If the measured amount of manganese and / or cobalt (preferably manganese) exceeds a predetermined threshold, for example because the filter cake is found to be too pink, the amount of one or more control acids in the oxidation reactor tends to be increased.

[0043] The term controlling acid is arbitrarily chosen to clearly denote a particular group of acids found to be suitable for use in the present process.

[0044] Suitable threshold values ​​are defined by the person skilled in the art depending on the individual process characteristics and the amount of metal considered acceptable in the filter cake for subsequent processing steps and / or further use.

[0045] We have found that in addition to the total concentration of catalyst metals in the filter cake (which may also be affected by the retention of catalyst-rich mother liquor), the enrichment of manganese in the filter cake relative to cobalt relative to the feed catalyst is also a suitable indicator. This ratio is particularly useful because applicants have found that manganese is the most sensitive indicator of problems when oxidizing furfural-related compounds to form FDCA with a catalyst system comprising cobalt, manganese and bromine, and appears at abnormally high levels in the filter cake. A manganese enrichment factor has been developed, which is defined according to the following equation:

[0046] (Mn / Co) 滤饼 / (Mn / Co) 催化剂

[0047] The advantage of this equation is that it is not affected by the absolute content of catalyst or the presence of, for example, washing liquid, while reflecting the unwanted enrichment of manganese in the filter cake. A value less than or equal to 1 indicates that there is no preferential enrichment of manganese in the filter cake. For practical reasons, when the absolute manganese content is low, for example, less than about 10ppm in the filter cake, the value may fluctuate up to about 1.5. Values ​​above this level, or, for example, above 2.5, indicate that manganese is not undesirably incorporated into the filter cake and corrective measures need to be taken. Accordingly, a preferred process is one in which the ratio of the weight ratio of manganese to cobalt in the filter cake to the weight ratio of manganese to cobalt in the catalyst system is less than 2.5, preferably less than 2, and more preferably less than 1.5.

[0048] 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 and is the first control acid identified by the inventors because it is usually available in the factory as part of the catalyst system commonly used in step a). However, other organic acids were tested and the inventors found that only those acids could act as control acids with a certain acidity. It was found that the pKa of a suitable control acid should be less than 3.2, wherein the pKa is measured in water.

[0049] In addition, it was confirmed in the screening experiment that only monocarboxylic acids or dicarboxylic acids can be used as control acids, because high polycarboxylic acids may potentially cause activity loss due to complex formation. For example, trimellitic acid and pyromellitic acid are relatively strong aromatic polycarboxylic acids, and their first pKa values ​​are 2.52 and 1.92 respectively. However, these acids can cause activity loss in some oxidations and are therefore not suitable as control acids. In this study, it was found that FDCA, a relatively strong aromatic dicarboxylic acid, can also cause activity loss or system startup failure. As far as we know, this effect has not been observed in the process of p-xylene oxidation to produce terephthalic acid. Although it is not desired to be limited by theory, the applicant speculates that the higher solubility of FDCA and its higher acidity are at least part of the reason for this effect. Due to the significant increase in solubility at higher temperatures and the resulting compounding of diacids and catalyst components, the oxidation of higher temperatures exacerbates this problem.

[0050] When added as a control acid, the monomethyl ester of FDCA, a monocarboxylic acid with the desired pKa but 7 carbon atoms, can provide the desired effect. However, the monoester of FDCA tends to accumulate in recycle operations and has been found to have an adverse effect on oxidation at high levels, resulting in slower reaction rates and more intermediates, as well as brown or even reaction cessation. In addition, the monoester of FDCA can cause difficulties in filtration and difficulty in removing mother liquor from the filter cake, resulting in a higher total metal content. However, surprisingly, it was found that the monocarboxylic acid FCA, i.e. 2-furancarboxylic acid, is a control acid very suitable for the process of the present invention. However, when FCA is used, if FDCA is desired to be used for polymerization, the purification system used after oxidation must be able to remove it from the FDCA composition. Accordingly, the inventors infer that the control acid can be defined as hydrobromic acid and a monocarboxylic acid or dicarboxylic acid having 2-5 carbon atoms and a pKa less than 3.2. In the framework of the present invention, the monoester of FDCA, which functions similarly to the control acid, is considered separately due to the special problems associated with its accumulation and is therefore limited to a certain range of use.

[0051] In step d), the amount of one or more control acids is increased by intentionally and purposefully adding one or more control acids to the oxidation reactor. Any in situ formation of control acids that may occur in the oxidation reactor is not considered to be an intentional and purposeful addition of one or more control acids and does not correspond to an increase in the amount of one or more control acids in the oxidation reactor. Because hydrobromic acid is often used to provide bromide ions to the catalyst of the oxidation reaction, it is advantageous to discuss what is considered to be an increase in hydrobromic acid within the framework of the present invention. Most (semi-)continuous processes add additional catalyst, typically including additional hydrobromic acid, to compensate for the loss of bromine during the reaction, i.e., for example, in the overhead distillate or mother liquor, thereby maintaining the desired catalyst concentration. Adding hydrobromic acid to the oxidation reactor to maintain the desired hydrobromic acid concentration in the oxidation reactor does not constitute an increase in the amount of hydrobromic acid in the sense of the present invention. In other words, step d) requires changing the catalyst composition in the reactor by increasing the ratio of bromine to metal. Accordingly, step d) of the process of the present invention can be:

[0052] d) if the measured amount of manganese and / or cobalt in the filter cake exceeds a predetermined 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 weight ratio greater than 2, even more preferably to a weight ratio greater than 2.5, wherein the one or more control acids are selected from the group consisting of mono- or di-carboxylic acids having 2 to 5 carbon atoms and a pKa less than 3.2.

[0053] In other words, if the filter cake using the existing catalyst system is found to have a high metal content, particularly manganese, and the catalyst composition is then changed to increase the bromine content by adding HBr, this change will constitute an increase in the amount of controlling acid within the meaning of the present invention. In sharp contrast, the addition of bromine by, for example, NaBr or NH4Br will not constitute and increase bromine, as these are not controlling acids within the meaning of the present invention.

[0054] Those skilled in the art know several suitable methods to adjust the concentration of the monoalkyl ester of 2,5-furandicarboxylic acid in the oxidation reactor and adjust the concentration in the mother liquor to the above-mentioned level accordingly. Among the options known in the art, those skilled in the art select the appropriate option based on their general knowledge. For example, those skilled in the art can increase the amount of solvent or other starting materials to dilute the solution in the oxidation reactor, or, if mother liquor recycling is used, those skilled in the art can intentionally 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 the fresh mother liquor. In order to reduce the monoester and / or control the concentration of the acid, a portion of the mother liquor can also be removed or purged from the system. If a portion of the mother liquor is purged, acetic acid can still be recovered, for example by distillation, and the residue is either discarded or processed to recover the catalyst for recovery and reuse.

[0055] However, we have found other convenient options for reducing the amount of monoalkyl esters of 2,5-furandicarboxylic acid in the mother liquor, which otherwise tends to increase under certain conditions. It has been found that a convenient way to reduce the amount of the corresponding substance can be obtained 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. It has been found that the post-oxidation step is particularly effective when used at high temperatures.

[0056] In view of this observation, a preferred embodiment of the process of the present invention 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 reducing 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), wherein the post-oxidation is carried out in the post-oxidation reactor under the conditions described in step a).

[0057] The most preferred process is one in which the temperature in step a) is 170°C or more and in which a post-oxidation step a1) is applied after step a), wherein the post-oxidation is carried out in a post-oxidation reactor under the conditions as described for step a). This process is particularly preferred because it was found that in this case the amount of monoalkyl ester of FDCA tends to level off, meaning that it only rises to a certain level, wherein the level was found to be well within the desired range defined above.

[0058] In fact, this observation has proven to be very useful for operating several processes starting from ethers of 5-HMF, independent of the problem of metal incorporation. Accordingly, disclosed herein is a process for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid, comprising the following steps:

[0059] a) oxidizing an oxidizable compound including 5-alkoxymethylfurfural in an oxidation reactor using an oxidizing gas at a temperature in the range of 170° C. to 210° C. in the presence of a saturated organic acid solvent having 2 to 6 carbon atoms and a catalyst system including cobalt, manganese and bromine to obtain a crude carboxylic acid composition including a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid,

[0060] a1) oxidizing the crude carboxylic acid composition of step a) in a post-oxidation reactor in the presence of a saturated organic acid solvent having 2 to 6 carbon atoms and a catalyst system comprising cobalt, manganese and bromine using an oxidizing gas at a temperature in the range of 170° C. to 210° C. to obtain a crude carboxylic acid composition comprising a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid,

[0061] b) separating at least a portion of solid 2,5-furandicarboxylic acid from the crude carboxylic acid composition in a solid-liquid separation zone to produce a solid filter cake and a mother liquor,

[0062] c) determining the amount of manganese and / or cobalt in the filter cake, and

[0063] d) increasing the amount of one or more control acids in the oxidation reactor if the measured amount of manganese and / or cobalt in the filter cake exceeds a predetermined threshold,

[0064] The mother liquor comprises a monoalkyl ester of 2,5-furandicarboxylic acid in an amount ranging from 0.5% to 7% by weight relative to the weight of the mother liquor.

[0065] Preferred processes are those wherein 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.

[0066] The above process is preferred because it was found that the corresponding control acid provides particularly good and significant effects, while being either relatively cheap and easy to handle, or being available as waste and / or by-product of the process according to step a), and being obtainable 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 that frequent replenishment is not required.

[0067] Preferred is a process according to the present invention, wherein the process for producing the 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 conveyed from the solid-liquid separation zone to the oxidation reactor as a recycle mother liquor stream, wherein the portion of the mother liquor not conveyed to the oxidation reactor as a recycle mother liquor stream is preferably treated in an evaporation step to recover the organic acid solvent as a condensed vapor stream, and / or wherein one or more bases are preferably added to the mother liquor treated in the evaporation, preferably in an amount of base equal to or greater than the amount of free bromide ions in the mother liquor on a molar basis.

[0068] The process of the present invention provides acceptable results for batch processes, wherein for example a sample of the crude carboxylic acid composition containing solid precipitation is taken out from a batch reactor and treated in a solid-liquid separation zone according to step b). If desired, a control acid can be added to the oxidation reactor of the ongoing batch process. Similarly, a first batch of processes can be completed to analyze the resulting product filter cake and, in the event that the metal content in the first batch of process filter cake exceeds a predetermined threshold, additional control acid can be provided to a second batch of processes.

[0069] However, the process defined above is obviously preferred because the process of the present invention shows its full potential in a continuous or semi-continuous process, because these processes require a suitable control mechanism that allows the least intrusive adjustment of the operating system, which is suitable for solving the problem of metal incorporation into the filter cake. This process generally comprises the continuous or intermittent addition of an oxidizable compound and the removal of a carboxylic acid composition comprising 2,5-furandicarboxylic acid. Advantageously, in order to increase the amount of control acid in the oxidation reactor, the mother liquor obtained in step b) can be reused in subsequent batch experimental processes. However, designing the process of the present invention as a continuous or semi-continuous process allows the mother liquor from the solid-liquid separation zone to be transported back to the oxidation reactor as a recycled mother liquor stream. If the mother liquor stream contains a control acid, this allows a person skilled in the art to increase the amount of control acid in the oxidation reactor.

[0070] Preferred is a process according to the invention wherein the oxidizable compound comprises 5-methoxymethylfurfural and wherein the crude carboxylic acid composition comprises the monomethyl ester of 2,5-furandicarboxylic acid.

[0071] It is believed that the present process can be used for 5-alkoxymethylfurfurals regardless of the length of the alkoxy chain, and in particular for 5-alkoxymethylfurfurals in which the alkoxy group contains 1 to 6 carbon atoms. It was found that the best results were obtained when 5-methoxymethylfurfural was used as the oxidizable compound. This is particularly beneficial because 5-methoxymethylfurfural has been shown to be one of the most economically viable starting materials for the production of FDCA.

[0072] Preferred is a process according to the invention, wherein the mother liquor comprises a monoalkyl ester of 2,5-furandicarboxylic acid, preferably a monomethyl ester of 2,5-furandicarboxylic acid, in the range of 1.0 to 4% by weight relative to the weight of the mother liquor.

[0073] The above process is preferred because we have found that the above range of the amount of alkyl 2,5-furandicarboxylate in the mother liquor ensures a strong enough beneficial effect of this compound to be felt, while establishing a sufficient buffer to approach the upper limit of the monoalkyl ester of FDCA determined, so that the process has a high flexibility for changes and peaks in the concentration of alkyl ester of 2,5-furandicarboxylic acid.

[0074] Preferred processes are those wherein the mother liquor comprises preferably 0.5% by weight or more bromoacetic acid, relative to the weight of the mother liquor, and / or the mother liquor comprises preferably 0.1% by weight or more dibromoacetic acid, relative to the weight of the mother liquor, and / or the mother liquor comprises preferably 0.02% by weight or more 5-bromo-2-furoic acid, relative to the weight of the mother liquor.

[0075] Depending on the parameters chosen, the mother liquor was found to contain bromoacetic acid and / or dibromoacetic acid and / or 5-bromo-2-furoic acid. The formation of these compounds may advantageously play a role in controlling the acid, has not been reported before in other processes for the production of FDCA, such as processes starting from 5-HMF, and may be a feature of processes using 5-alkoxymethylfurfural as the oxidizable compound, at least if the specific reaction conditions are set as defined in step a) above.

[0076] A preferred process is one in which the predetermined threshold value of cobalt in the filter 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 in which the predetermined threshold value of manganese in the filter 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 the corresponding threshold values ​​ensure that a solid filter cake is obtained that is sufficiently free of metals to allow efficient further processing. Furthermore, a process according to the invention is preferred, in which the predetermined threshold value also comprises the ratio of the weight ratio of manganese to cobalt in the filter cake to the weight ratio of manganese to cobalt in the catalyst system.

[0077] A preferred process is one 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 delivered to the oxidation reactor as a recycle mother liquor stream.

[0078] This process is preferred because no additional control acid needs to be handled and / or stored on the production side, thereby reducing costs and eliminating the need for additional equipment. In particular for continuous or semi-continuous processes, the amount of control acid in the oxidation reactor can be increased by conveying the mother liquor to the oxidation reactor as a recirculating mother liquor stream. 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 arrangement allows for complex process control, wherein the amount of control acid in the oxidation reactor can be increased by increasing the portion of the mother liquor conveyed to the oxidation reactor as a recirculating mother liquor stream.

[0079] Preferred is a process according to the invention wherein the weight ratio of cobalt to manganese in the catalyst system is 10 or more, preferably 15 or more, and / or wherein 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, most preferably 2 or more, wherein this value is preferably less than 4.0, more preferably less than 3.5. If the catalyst system comprises 5% or more by weight of other metals besides cobalt and manganese, preferably the weight ratio of bromine to the combined weight of all metals in the catalyst system reaches the above ratios. This process is particularly preferred because we have found that the above catalyst system is significantly superior to other catalyst systems under the conditions defined in step a) above. In particular, the inventors surprisingly found that the corresponding catalyst system also reduces the tendency of manganese and cobalt in the catalyst system to be incorporated into the product filter cake even in the presence of increased amounts of monoalkyl esters of 2,5-furandicarboxylic acid and reduced enrichment of manganese compared to cobalt in the filter cake. Likewise, the use of the above-described catalyst system opens up a much wider range of process parameters, such as pressure or residence time, whereby the process according to the invention can be operated in a very reliable manner, significantly reducing the probability of undesired process stops and / or reducing the formation of undesired by-products.

[0080] Preferred is a process according to the present invention, wherein separating 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 washing 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 metal in the filter cake can be further reduced if a washing step is used in the solid-liquid separation zone. It is completely surprising here that a washing solution mainly comprising a saturated organic acid solvent (preferably acetic acid) has achieved considerable success in removing metals from the filter cake. In the prior art, it is generally believed that it is necessary to use larger amounts of water to ensure sufficient removal of metals from the filter cake. However, using the process of the present invention, it was found that washing with an organic acid is sufficient to obtain a filter cake that is substantially free of metals. This is considered to be particularly beneficial because for any system that uses mother liquor recycle into the oxidation reactor, large amounts of water that may be introduced into the system with the washing solution are undesirable because it is found that the oxidation reaction is generally sensitive to higher water concentrations.

[0081] A preferred process is one in which the weight ratio of manganese to cobalt in the filter cake divided by the weight 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 well-defined criterion for the process operator to judge whether his process is operating under the desired conditions, thereby allowing a very easy and reliable way to identify errors in the system.

[0082] The process of the present invention requires the addition of potentially large amounts of control acids, several of which contain ionic bromine or brominated organic compounds. These strong acids are corrosive and may form highly oxidizing gaseous compounds. Accordingly, the mother liquor stream contains highly acetic acid and / or corrosive compounds and / or additional metal ions, such as iron, nickel or chromium, from the reactor equipment. In addition, oxidizing gaseous substances may damage the top equipment of the oxidation reactor. Therefore, another object of the present invention is to provide countermeasures for protecting equipment, particularly protecting the top equipment and pipelines in contact with the mother liquor stream, and / or removing unwanted metals from the mother liquor. The inventors have found that the following processes that address these challenges are preferred.

[0083] Preferred is a process according to the present invention, wherein the process further comprises the following steps:

[0084] 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 increase the pH to greater than 7, wherein one or more metal hydroxides or carbonates are precipitated from the mother liquor, wherein the metal is selected from the group consisting of cobalt, manganese, iron, nickel or chromium.

[0085] A preferred process is one in which the mother liquor contains greater than 2000 ppm by weight of cobalt and greater than 130 ppm by weight of manganese relative to the weight of the mother liquor. This process is preferred because it allows efficient recycling of the mother liquor, wherein sufficiently high amounts of catalyst metals can be retained in the oxidation reactor based on the cobalt and manganese provided with the recycled mother liquor stream.

[0086] A preferred process is one in which the solid 2,5-furandicarboxylic acid separated in step b) is further washed with a second washing solution comprising more than 95% by weight (preferably more than 99% by weight) of water relative to the weight of the washing solution. This process is beneficial because it can provide an FDCA filter cake containing minimal amounts of manganese and cobalt. However, as mentioned above, it is often found that the corresponding process is more suitable for a batch process without mother liquor recycling. If the second washing solution is mixed with the mother liquor in the solid-liquid separation zone, the reuse of the mother liquor stream becomes more difficult because the presence of large amounts of water in the oxidation reaction defined in step a) is generally undesirable.

[0087] The preferred process is one in which the organic acid solvent is acetic acid. This process is preferred because acetic acid has been repeatedly shown to be the most suitable solvent used in most prior art processes. Acetic acid is cheap, readily available and relatively acceptable if environmental factors are taken into account.

[0088] A preferred process is one in which the oxidizing gas comprises molecular oxygen, preferably air. This process is preferred because the use of air is in most cases the most economically viable method for oxidizing 5-alkoxymethylfurfural to FDCA.

[0089] A preferred process is one in which the temperature in step a) is in the range of 170°C to 190°C. This process is preferred because the inventors have found that within this particular temperature range, the amount of monoalkyl ester of 2,5-furandicarboxylic acid tends to strive to reach a plateau value just within the above-mentioned desired range. In addition, the corresponding temperatures were found to produce high yields and high purity FDCA, as qualitatively indicated by the white filter cake found in several experiments using the corresponding temperatures.

[0090] Preferred is a process according to the invention, wherein the pressure in step a) is in the range of 700 kPa to 2000 kPa, and / or wherein the oxidation reactor comprises one or more continuously stirred tank reactors, preferably two or more continuously stirred tank reactors connected in series. Among the possible parameters and equipment sets tested by the inventors, the above parameters were found to be ideal for obtaining high purity FDCA in good yields while minimizing the energy costs required for pressurizing the reactor.

[0091] Preferred is a process according to the invention, wherein the solid-liquid separation zone comprises a filter or a centrifuge, preferably a filter, more preferably a rotary pressure filter. The above process is beneficial because filters and centrifuges have been found to be particularly suitable means for separating solid FDCA from mother liquors comprising monoalkyl esters of 2,5-furandicarboxylic acid, even though these compounds are generally difficult to separate from solid FDCA.

[0092] Preference is given to a process according to the invention, wherein the filter cake comprises more than 95% by weight (preferably more than 98% by weight) of 2,5-furandicarboxylic acid relative to the weight of the dry filter cake, and the filter cake preferably comprises a monoalkyl ester of 2,5-furandicarboxylic acid in an amount in the range of 0.1% to 3% by weight (preferably in the range of 0.15% to 2.3% by weight) relative to the weight of the dry filter cake, and / or wherein the filter cake comprises a combined amount of cobalt and manganese of less than 300 ppm by weight (preferably less than 75 ppm by weight) relative to the weight of 2,5-furandicarboxylic acid in the filter cake.

[0093] It is believed that the amount of monoalkyl ester of FDCA should be lower than 3% by weight relative to the weight of the dry cake in order to prevent the effect of this compound from being impaired in the subsequent purification process.

[0094] In view of the above disclosure of the present invention, it will be appreciated by those skilled in the art 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 utilizes all of the above information to provide a process that can be operated reliably and provides low initial metal incorporation into the filter cake, thereby requiring less process control effort. Accordingly, the relevant aspects of the present process may be:

[0095] A process for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid, comprising the following steps:

[0096] a1) oxidizing an oxidizable compound comprising 5-alkoxymethylfurfural in an oxidation reactor using an oxidizing gas at a temperature in the range of 160°C 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 relative to the weight of the liquid phase, wherein the bromine is provided in the form of hydrobromic acid, wherein the weight ratio of cobalt to manganese in the catalyst system is 10 or more, preferably 15 or more, and wherein 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, most preferably 2 or more.

[0097] It is clear to the person skilled in the art that preferred embodiments of the process correspond to the preferred embodiments of the process disclosed above, for example with regard to the ranges of solvent, starting materials, catalyst, temperature and alkyl ester of 2,5-furandicarboxylic acid.

[0098] Hereinafter, the present invention is described in more detail using experiments.

[0099] Example

[0100] Unless otherwise specified, the oxidation reactor is a 600 ml stirred pressure vessel with two impellers. The reactor is pre-charged with a solvent comprising acetic acid and water in a weight ratio of 95 / 5, and a catalyst component to form a specific composition of a catalyst system comprising cobalt, manganese and bromine. The catalyst component is provided in the form of cobalt (II) acetate tetrahydrate, manganese (II) acetate tetrahydrate and 48% by weight HBr in water. A typical amount of "pre-charge" is 310 grams.

[0101] 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 5-methoxymethylfurfural (MMF) or a mixture of MMF with 5-hydroxymethylfurfural and a small amount of levulinic acid. The process starts with a typical feed rate of 8.3 mmol / min and lasts for 60 minutes (total feed 500 mmol). The flow rate of lean air (8% oxygen) starts with a typical flow rate of 10 standard liters / min. The reaction usually starts within 3 minutes, with a sharp decrease in oxygen at the outlet and an increase in CO and CO2. Heat is generated during the reaction, and a vapor stream is taken from the top of the tower and condensed. The vapor stream mainly contains acetic acid and water. The amount of solvent captured in the overhead distillate is continuously monitored and supplemented in the oxidation reactor with a fresh solvent stream flowing to the reactor.

[0102] Typical operating pressures are 12 to 14 bar at 160°C and 17.5 bar at 175°C.

[0103] At the end of the desired feed time, the feed of the oxidizable compound is stopped and the contents of the oxidation reaction are "quenched" to room temperature (or desired filtration temperature) or subjected to an extended post-oxidation time at the same reaction temperature and oxygen flow rate as above.

[0104] Example A—Co / Mn Ratio

[0105] The experiment in Example A used a 160°C MMF feed, which was post-oxidized for 1 hour and 15 minutes. The filter cake was separated by filtration and washed with 1 part solvent (95 parts acetic acid to 5 parts water, by weight) to 1 part estimated dry cake weight. The results are shown in Table 1.

[0106] Table 1: Co / Mn ratio

[0107]

[0108] Table 1 shows that different catalyst systems can be used in this process. A high cobalt to manganese ratio appears to reduce the overall tendency of the system to incorporate metals into the filter cake.

[0109] Example B - Ratio of Metals in Filter Cake Relative to Ratio of Metals in Catalyst

[0110] The experiment of Example B used 160 ℃ of MMF feed for 1 hour and 15 minutes of post-oxidation. For all experiments, the catalyst was 3300ppm Co, 185ppm Mn and 7000ppm Br from aqueous HBr. After each test, the reaction slurry was cooled to 80 ℃ and filtered. The remaining mother liquor was replaced from the filter cake using a minimum wash of acetic acid / water (95 / 5 weight ratio). The combined mother liquor and wash liquor were analyzed and corrected for cobalt, manganese, ionic bromine and water (5%). This material was then used as a precharge for the next test to simulate a recycle operation. Therefore, subsequent experiments were performed in the oxidation reactor with an increased amount of control acid. The cobalt recovery rate in each case was 90-95%, allowing a high recycle content. In each case, after adjustment to bring the catalyst to the desired level, the mother liquor from the previous test was used as a precharge for recycle.

[0111] A total of 8 trials were performed, with a total of 7 additional recoveries. Table 2 shows the filter cake mass and mother liquor composition. The yield is the total yield of FDCA recovered in the filter cake and the monoester of FDCA. The results are summarized in Table 2.

[0112] Table 2 - Mn / Co ratios in filter cake relative to Mn / Co ratios in catalyst

[0113]

[0114] The results show that in Run B1, the fresh feed run (no recycle), the Mn / Co ratio of the filter cake divided by the same ratio in the catalyst is very high, indicating that excess Mn precipitated into the filter cake (usually associated with a pink filter cake). In addition, relatively high levels of Co and Mn were detected in the filter cake. The remaining runs, all of which included some FDCA-Me and control acids (bromoacetic acid, dibromoacetic acid, acetoxyacetic acid, 5-bromo-2-furoic acid, and fumaric acid) due to recycle, maintained constant yields and did not show excess Mn / Co precipitation.

[0115] It is found that FDCA-Me accumulates continuously with each recovery. The experimental setting does not allow a further significant increase in the control acid, because only the mother liquor is used in each case, and the accumulation of the acid at a certain point is controlled by the stable state reached between the new generation and removal. It can be seen that in experiments B2 to B4, as the concentration of FDCA-Me and the control acid initially increases, the amount of metal in the filter cake is reduced to a very ideal value. In addition, the ratio of Mn / Co in the filter cake to the Mn / Co in the catalyst feed is always close to 1, indicating that excessive metal (Mn) is not incorporated into the filter cake. However, when the amount of the control acid reaches a stable level, the accumulation of FDCA-Me continues, and its harmful effects on the metal incorporation obviously begin to dominate the system. When the FDCA-Me level is very high, we observe that the filtration difficulty increases. In this case, the metal keeps the same ratio as in the catalyst feed, but it is quite high, indicating that there is a problem in obtaining a good washing and the corresponding removal of the mother liquor.

[0116] Example C - Different feeds

[0117] The experiment in Example C was carried out in the same manner as Example B, but using a feed containing a mixture of 5-HMF (6.4% by weight), MMF (86.4% by weight) and a small amount of levulinate (2.3% by weight) and small amounts of other compounds. The re-recovery rate for each run from C2 to C4 was 90+%. The results are summarized in Table 3.

[0118] Table 3 - Different feed materials

[0119]

[0120] As in Example B, the first test, i.e., the test without addition of a highly acidic component, resulted in a high manganese content in the filter cake. In tests C2 to C4, where a control acid was included in the oxidation reactor, the yields were good and the total amount of metal in the filter cake was low. These experiments illustrate the process of the invention for different feed compositions. In particular, a mixture of 5-HMF and MMF gave good results.

[0121] Example D - Temperature

[0122] The experiments in Example D were conducted using the same feed as Experiment C, but at a temperature of 175°C (except that the first two experiments were conducted at 160°C). The re-recovery was set at 80% of the mother liquor based on cobalt as a standard. A 1 hour post-oxidation was used, also at 175°C. The pressure was increased to 17.5 bar to accommodate the higher temperature and solvent vapor pressure. The first two experiments were conducted at 160°C in order to construct a "mother liquor" composition for the high temperature experiments. The results are summarized in Table 4.

[0123] Table 4 - Temperature

[0124]

[0125] Similar to Examples B and C, the first run without added control acid produced very high levels of manganese in the filter cake. Likewise, the first run with increased amounts of control acid in the oxidation reactor (still at 160°C) provided high metal contents in the filter cake, even though the amount of control acid was quite low. The third run, the first run conducted at 175°C, showed an increase in manganese content in the filter cake, which can be attributed to the new process steady state established at the elevated temperature relative to the mother liquor composition. In other words, D3 would have likely shown even more metal in the filter cake if conducted without control acid. In fact, when the run was attempted directly at 170°C without added control acid, the process was expected to stall. The first two runs were conducted at 160°C in order to establish a system that was continuously reacting. Subsequent runs all showed expected low levels of metal in the filter cake and provided excellent yields and surprisingly low metal contents in the filter cake, even for the last run. It can be seen that when post oxidation and 80% recycle were performed at 175°C, the monoester content stabilized at a lower level compared to full recycle without post oxidation at 160°C. Together with the other experiments, this shows that if the monoester content is controlled, excellent results can be obtained in several trials. Furthermore, Example D again demonstrates the beneficial effect of controlling the acid.

[0126] Example E - Effect of Bromine Content

[0127] The experiment of Example E was carried out using the same setup as described above. The feed used was purified 5-methoxymethylfurfural (MMF), with a total of 500 mmol of MMF used, fed at a steady rate over a total of 1 hour. The reaction temperature was 160°C and the pressure was 12 bar. No post-oxidation was used. The reactor was "pre-loaded" with 310 grams of catalyst and the alkyl monoester of FDCA (FDCA-Me) added, as described below. The reported total yield is the sum of FDCA and FDCA-Me, minus the initial FDCA-Me, based on the number of moles relative to the MMF feed. In addition to the precise measurement, the filter cake color is used as a qualitative indicator of the manganese concentration in the filter cake (scaled from white to pink) and the concentration of undesirable by-products and colored bodies (scaled from white to yellow to brown). These qualitative analyses are fast and provide a very reliable impression of the quality of the filter cake, since even small amounts of impurities can cause a noticeable discoloration of the filter cake. The results are summarized in Table 5.

[0128] Table 5 – Effect of bromine content

[0129]

[0130] Comparison of E1 and E2, or E6 and E7-E9, shows that FDCA-Me seems to reduce the problem of manganese enrichment in the filter cake, as evidenced by the decrease in absolute values ​​and the decrease in the Mn / Co filter cake / catalyst ratio, from numbers well above 1 when no FDCA-Me is added to numbers close to or below 1 when FDCA-Me is added. However, the studies of E1 to E4 show that excess FDCA-Me has an adverse effect on the process, with the process of E4 giving a brown product and being considered a failure. This example also shows that good washing of the filter cake is more difficult in the presence of excess FDCA-Me, as indicated by the following: although the Mn / Co filter cake / catalyst ratio is close to 1, the levels of both metals are increased. Comparison of E5 and E6 demonstrates that the addition of additional Br in the form of hydrobromic acid significantly reduces the amount of manganese in the filter cake. In addition, E5 ends prematurely with only 362 mmol feed (planned 500 mmol) when the reaction stops abruptly, while E6 runs without problems. Based on these results, it can be inferred that HBr can act as a controlling acid in the sense of the present invention. In fact, it can be seen that the catalyst system used in E6 to E9 is the best catalyst system for the oxidation of 5-alkoxymethylfurfural. While E3 gave a yellow-brown filter cake at 6 wt% FDCA-Me, the color of the filter cake did not reach the brown level in E7 to E9. On the contrary, in E9 even a "white" filter cake could be obtained. Finally, the yield of E6 was significantly increased compared to E1, or the yield of E7-E9 was also significantly increased compared to E3.

[0131] Example F - Controlled Acid

[0132] The experiment of Example F was carried out using the same apparatus as described above. The feed used was purified 5-methoxymethylfurfural (MMF), with a total of 500 mmol of MMF used, fed at a steady rate over a total of 1 hour. The reaction temperature was 160° C. and the pressure was 12 bar. No post-oxidation was used. The reactor was "pre-loaded" with 310 grams of catalyst and added components, as described below. The reported total yield is the sum of FDCA and FDCA-ME, minus any initial FDCA-ME, based on the number of moles relative to the MMF feed. In all cases, the catalyst was 3300 ppm cobalt, 185 ppm manganese and 7000 ppm bromine. The results are summarized in Table 6, where a pink filter cake was observed for experiments F4 and F7.

[0133] Table 6 – Controlled Acids

[0134]

[0135] Not all acids added in the experiments resulted in the desired white filter cake, which also contained small amounts of precipitated metals and no manganese enrichment, as evidenced by the Mn / Co filter cake / catalyst ratio. The reference case (no control acid added) used the strong catalyst system identified previously and had a relatively white filter cake, but still showed the undesirable manganese enrichment in the filter cake. The addition of formic acid and 2-carboxy-5-(formyl)furan (FFCA) did not result in the desired white filter cake or the desired low Mn / Co filter cake / catalyst values. Considering that in some cases a white filter cake can be obtained with this catalyst setup even without the addition of a control acid, it can be concluded that the various acids may even have a detrimental effect. Each of the other acids in the table had a positive effect on reducing the Mn / Co filter cake / catalyst ratio, demonstrating their suitability for reducing the problem of excess manganese in the filter cake. In most cases, the total metal content was also good, indicating that the filter cake also had good washability. Taking all the experimental evidence into account, it is concluded that a suitable controlling acid is 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.

[0136] Example G - Addition of HBr or Bromoacetic Acid

[0137] The experiment of Example G was carried out using the same set-up as described above. The feed was a mixture of 5-HMF (6.4% by weight), MMF (86.4% by weight) and a small amount of levulinate (2.3% by weight) and small amounts of other compounds. In each case, the reactor was "pre-loaded" with 310 grams of 95 / 5 by weight acetic acid / water and the catalyst composition described. The reactor temperature was 170°C. The catalyst in all cases was 3300 / 185 / 7000 ppm by weight Co / Mn / Br, with aqueous HBr solution used to provide the bromine. It was observed that the catalyst composition that ran well at 160°C, allowing a total of 500 mmol for a full 1 hour feed, would not run for a full hour at a temperature of 170°C and a pressure of 17-18 bar. At a certain point during the experiment, it was observed that the reaction suddenly stopped, which was evidenced by a rapid increase in the oxygen content in the outlet gas stream and a decrease in the production of CO2 and CO.

[0138] Table 7 – Addition of HBr or bromoacetic acid

[0139]

[0140] In experiment G1 (no acid added), the process was terminated and no meaningful analysis of the yield and filter cake color was allowed. In experiment G4 (no FDCA added), the process could not be started. In G2, the mother liquor of the previous test was added to increase the amount of FDCA-Me and control the acid (at a relatively low amount) in the oxidation reactor. Although G2 showed significant manganese incorporation in the filter cake, it obtained an active process with acceptable yield. In experiment G3 (adding bromoacetic acid) and experiment G5 (adding HBr), the active process obtained an ideal white filter cake, indicating that the manganese content was low.

[0141] Example H—Addition of HBr

[0142] This example was conducted using a single continuous stirred tank oxidation reactor (CSTR). The levels of cobalt and manganese were kept constant throughout and the process had different levels of HBr feed in order to observe the effect of added HBr on the system.

[0143] The reactor was equipped with a reflux condenser and a pump so that heat was removed by evaporation of the solvent while the reflux was pumped back into the reactor. The reactor was pre-loaded with about 100 grams of a 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, including 8% oxygen, with a flow rate of 3.3 Nl / min. 20 wt% of "RMF" (a mixture of 5-HMF, MMF and levulinic acid as used before) in acetic acid and the required cobalt and manganese (3000 ppm and 300 ppm, respectively) were fed. The feed contained about 1 wt% of water in total, which established a steady-state concentration of about 6% in the reactor due to the formation of water during the oxidation process. The valve at the bottom of the reactor was opened approximately every 30 seconds to remove a small amount of material in order to maintain a constant liquid level and establish "CSTR" conditions. The temperature was maintained at 160°C, the pressure was 13 bar, and the residence time was 60 minutes. After running for at least 3 hours, the reactor was considered to be in a stable state and sampling could be started. At the end of each test, the feed was stopped and post-oxidation was performed. After post-oxidation, the reactor was cooled, the contents were filtered, washed with acetic acid / water, and dried before analysis. The following table shows the metal content of the filter cake.

[0144] Table 8 – Addition of HBr

[0145]

[0146] Runs H1 and H2 show high overall metal content in the filter cake and high ratios of Mn / Co filter cake / catalyst. The remaining runs all have low total metal incorporation, becoming even lower with increasing HBr, and all show very good ratios, close to 1, for Mn / Co filter cake / catalyst.

Claims

1. A process for producing a carboxylic acid composition comprising 2,5-furandicarboxylic acid, comprising the following steps: a) oxidizing an oxidizable compound including 5-alkoxymethylfurfural in an oxidation reactor in the presence of acetic acid and a catalyst system including cobalt, manganese and bromine using an oxidizing gas at a temperature in the range of 160° C. to 210° C. to obtain a crude carboxylic acid composition including a monoalkyl ester of 2,5-furandicarboxylic acid and solid 2,5-furandicarboxylic acid, b) separating 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 filter cake and a mother liquor, c) determining the amount of manganese and cobalt in the filter cake, and d) increasing the amount of one or more control acids in the oxidation reactor if the ratio of the weight ratio of the measured amount of manganese to the measured amount of cobalt in the filter cake to the weight ratio of manganese to cobalt in the catalyst system exceeds 2.5, wherein 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, and wherein the mother liquor comprises a monoalkyl ester of 2,5-furandicarboxylic acid in a range of 0.5% to 7% by weight relative to the weight of the mother liquor.

2. A process according to claim 1, wherein the process for producing a carboxylic acid composition is a continuous process, wherein at least 60% by weight of the mother liquor is conveyed from the solid-liquid separation zone to the oxidation reactor as a recycled mother liquor stream.

3. A process according to claim 2, wherein the process for producing a carboxylic acid composition is a continuous process, wherein at least 80% by weight of the mother liquor is conveyed from the solid-liquid separation zone to the oxidation reactor as a recycled mother liquor stream.

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

5. The process according to claim 1, wherein the mother liquor comprises a monoalkyl ester of 2,5-furandicarboxylic acid in the range of 1.0% to 4% by weight relative to the weight of the mother liquor.

6. The process according to claim 5, wherein the mother liquor comprises the monomethyl ester of 2,5-furandicarboxylic acid in the range of 1.0% to 4% by weight relative to the weight of the mother liquor.

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

8. The process according to claim 7, wherein the mother liquor comprises bromoacetic acid in an amount of 0.5% by weight or more relative to the weight of the mother liquor and / or dibromoacetic acid in an amount 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 of 0.02% by weight or more relative to the weight of the mother liquor.

9. A process according to claim 2, wherein the one or more control acids are added to the oxidation reactor by increasing the portion of the mother liquor that is transported to the oxidation reactor as a recycled mother liquor stream, thereby increasing the amount of one or more control acids in the oxidation reactor.

10. The process of claim 1, wherein the weight ratio of cobalt to manganese in the catalyst system is 10 or more, and / or wherein the weight ratio of bromine to the combined weight of cobalt and manganese in the catalyst system is 1 or more.

11. The process of claim 10, wherein the weight ratio of cobalt to manganese in the catalyst system is 15 or more, and / or wherein the weight ratio of bromine to the combined weight of cobalt and manganese in the catalyst system is 1.5 or more.

12. The process of claim 11 wherein the weight ratio of bromine to the combined weight of cobalt and manganese in the catalyst system is 2 or more.

13. The process of claim 1, wherein separating 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.

14. The process of claim 13, wherein the scrubbing solution comprises acetic acid and less than 10% by weight water.

15. The process according to claim 1, wherein the solid 2,5-furandicarboxylic acid separated in step b) is further washed with a second washing solution comprising water, the second washing solution comprising water in an amount greater than 95% by weight relative to the weight of the washing solution.

16. The process according to claim 15, wherein the solid 2,5-furandicarboxylic acid separated in step b) is further washed with a second washing solution comprising water, the second washing solution comprising more than 99% by weight of water relative to the weight of the washing solution.

17. The process according to claim 16, wherein the temperature in step a) is in the range of 170°C to 190°C.

18. The process of claim 1, wherein step a) is carried out at a pressure in the range of 700 kPa to 2000 kPa, and / or wherein the oxidation reactor comprises one or more continuous stirred tank reactors.

19. The process of claim 18, wherein the oxidation reactor comprises two or more continuously stirred tank reactors connected in series.

20. The process of claim 1 wherein the ratio of the weight ratio of manganese to cobalt in the filter cake to the weight ratio of manganese to cobalt in the catalyst system is less than 2.

5.

21. The process of claim 20, wherein the ratio of the weight ratio of manganese to cobalt in the filter cake to the weight ratio of manganese to cobalt in the catalyst system is less than 2.

22. The process of claim 21 wherein the ratio of the weight ratio of manganese to cobalt in the filter cake to the weight ratio of manganese to cobalt in the catalyst system is less than 1.

5.

23. The process of claim 1, wherein the filter cake comprises 2,5-furandicarboxylic acid in an amount greater than 95% by weight relative to the weight of the dry filter cake.

24. The process according to claim 23, wherein the filter cake comprises 2,5-furandicarboxylic acid in an amount greater than 98% by weight relative to the weight of the dry filter cake.

25. The process according to claim 23, wherein the filter cake comprises the monoalkyl ester of 2,5-furandicarboxylic acid in an amount ranging from 0.1% to 3% by weight relative to the weight of the dry filter cake.

26. The process according to claim 25, wherein the filter cake comprises the monoalkyl ester of 2,5-furandicarboxylic acid in an amount ranging from 0.15% to 2.3% by weight relative to the weight of the dry filter cake.

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