Oxidative pretreatment of carbohydrate dehydration products including humins

Abstract

Disclosed is a method for oxidative pre-treatment of carbohydrate dehydration products, which involves the discovery that the functional groups of humins (such as functional groups that generate or lead to color formation (e.g. aldehyde groups and / or groups with conjugated double bonds)) can be more easily converted by oxidation (in some cases "bleached") compared to furanic compounds that form FDCA. This selective oxidation of humins can use milder oxidation conditions and / or different reaction systems compared to corresponding conditions and systems for oxidation of furan ring containing compounds. The oxidative pre-treatment can advantageously facilitate the selective oxidation of humin functional groups over furanic compounds that form FDCA, with the overall result that the humins are modified in a way that can provide a number of improvements. These can be particularly present in the further processing of the obtained pre-treated dehydration products, such as in the overall manufacture of bio-based poly(alkylene furandicarboxylate) copolymers, a notable example of which are poly(ethylene furandicarboxylate) or PEF.

Description

Invention Field

[0001] The present invention relates to a method for the oxidation of furans obtained from the dehydration of hexose carbohydrates (e.g., fructose) to prepare the bio-based monomer 2,5-furandicarboxylic acid (FDCA), and more specifically, to processing the crude dehydrated product prior to the oxidation step to resolve the presence of humins therein. Background of the Invention

[0003] The depletion of fossil fuels has greatly spurred the search for alternative resources of petroleum-based carbon for the synthesis of so-called "platform" molecules that can serve as building blocks for commercially valuable products. Biomass is currently considered a potential alternative from which many high-value chemicals can be derived, but the development of sustainable technologies for producing such chemicals from renewable resources remains a significant challenge.

[0004] Bio-based monomers, 2,5-furandicarboxylic acid (FDCA) and its dimethyl ester derivatives, including dimethyl 2,5-furandicarboxylate (FDME), are considered important starting materials in the production of poly(alkylene furanate) polymers, which can replace their known, mass-produced petroleum-derived analogs, namely poly(alkylene terephthalate) polymers, such as polyethylene terephthalate (PET). Prominent examples of poly(alkylene furanate) polymers are polyethylene terephthalate (PEF), obtained by reacting FDCA or FDME with ethylene glycol. Bio-based polymers (bioplastics) such as PEF exhibit superior properties in many respects compared to their petroleum-derived analogs, particularly in the packaging field. For example, blends of PEF and PET can provide improved barrier properties against CO2 and O2, thus extending shelf life (compared to that obtained using pure PET) and providing acceptable containers for products susceptible to oxidative degradation, such as beer. Other packaging applications for PEF include films used to manufacture bags, wrapping materials, and heat-shrinkable materials with high mechanical strength and recyclability.

[0005] Generally, both FDCA and FDME are useful platform molecules for the production of polyamides, polyurethanes, and polyesters with a wide range of applications as plastics, fibers, coatings, adhesives, personal care products, and lubricants. The commercial significance of these molecules has been proven; for example, in a 2004 study by the U.S. Department of Energy, FDCA was identified as one of twelve priority chemicals for building a future “green” chemistry. Due to its structural similarity to terephthalic acid (TPA), the potential of FDCA as an alternative monomer for synthetic polyesters has been recognized at least as early as 1946, for example in GB 621971A, and numerous groups have devoted considerable effort over the years to achieving commercially viable methods for FDCA preparation. Progress in the synthesis of FDCA from bio-based starting materials is described in US 10,538,499, which describes how feedstocks containing hexose units (e.g., fructose) undergo integrated processing steps, the first step of which is a dehydration step to provide 5-hydroxymethylfurfural (HMF) or certain derivatives, such as its esters or ether derivatives. Then, following a similar mid-century oxidation process to that used in the oxidation of p-xylene to prepare TPA, the dehydration product is oxidized to the desired FDCA using a homogeneous catalyst system containing cobalt, manganese, and bromine components. Despite the extensive development of these processing steps for the preparation of bio-based monomers, commercial production of FDCA has not yet been achieved. Further refinement of the proposed synthetic route for FDCA remains necessary to establish economic viability on a commercial scale. Summary of the Invention

[0006] This invention relates to the discovery of methods for improving or even eliminating the adverse effects associated with the presence of humulants, which are highly colored and are often water-insoluble byproducts of the dehydration of such carbohydrates in the bio-based synthesis of FDCA and its derivatives. Given the intended uses of FDCA and its derivatives prepared from dehydrated products containing these humulants (where colorlessness is desired), for example, in the use of FDCA and / or its derivatives as monomers for polyester alternatives to PET used in the manufacture of carbonated beverage bottles, the highly colored nature of these byproducts is a significant hazard.

[0007] However, humin, contained in the dehydration products of carbohydrates with hexose units (e.g., hexoses and their oligomers and polymers), also tends to deplete or limit the usefulness of homogeneous medieval oxidation catalysts. These catalysts have been most frequently proposed and most thoroughly evaluated for the oxidation of 5-hydroxymethylfurfural (HMF) and / or its derivatives (e.g., esters and / or ether derivatives) formed during upstream dehydration. In this respect, it has been determined that operating parameters can be limited when the byproduct humin is present in the oxidation feed along with HMF and / or its derivatives. In homogeneous catalytic oxidation, this can manifest as (i) the need for increased catalyst concentration to maintain desired conversion and yield profiles, and / or (ii) the need for a narrower “operating window” or set of conditions (temperature, pressure, run time) to achieve favorable performance before the reaction is lost or the oxidation “light-off” (e.g., based on measurements of oxygen consumption during the reaction). Unfortunately, humic acid has also been identified as problematic in heterogeneous catalytic methods proposed for the oxidation of HMF and / or its derivatives to provide FDCA, as it leads to catalyst deactivation due to its deposition on solid catalyst particles. Therefore, the increased catalyst consumption in both homogeneous and heterogeneous catalytic oxidation steps negatively impacts the overall economics of the process due to the costs associated with higher quantities of catalyst feed, recovery, and recycling, which can be attributed to the presence of humic acid.

[0008] To address these and other drawbacks associated with the oxidation of carbohydrate dehydration products containing humic acid as a byproduct, mild oxidative pretreatment to modify these humic acid compounds has been found to be particularly advantageous. Generally, oxidative pretreatment can be carried out under milder reaction conditions than those known for oxidizing the desired FDCA-forming furans from carbohydrate dehydration (e.g., and especially HMF and its ester or ether derivatives) to FDCA. For example, oxidative pretreatment can utilize lower temperatures. Oxidative pretreatment can alternatively or in combination be characterized by the substantial retention of the desired FDCA-forming furans in this step, e.g., with relatively unchanged or at least not significantly reduced yields. For example, the oxidation of FDCA-forming furans such as HMF or its esters or ethers to FDCA by oxidative pretreatment may be minimal, or at least relatively much less, than oxidation occurring in downstream or subsequent oxidation. Alternatively or in combination, oxidative pretreatment may feature the use of an oxidant not used in downstream or subsequent oxidation, and / or a catalyst not used in downstream or subsequent oxidation. Otherwise, the oxidation pretreatment may be characterized by the use of different amounts or concentrations of oxidants and / or catalysts relative to those used in downstream or subsequent oxidation, rather than using different oxidants and / or catalysts. Typically, the oxidation pretreatment can be carried out under oxidation pretreatment conditions that differ in one or more respects (e.g., temperature, residence time, concentration of one or more oxidants and / or catalysts, and / or the use of one or more oxidants and / or catalysts) from the oxidation conditions for converting a relatively larger percentage of the desired, non-oligomeric and non-polymeric furan compounds (such as HMF) that form FDCA to FDCA.

[0009] Regardless of how oxidative pretreatment can be distinguished or classified from the specific pathways by which such FDCA-forming furans are oxidized to FDCA, the examples described herein involve the important finding that, under certain oxidative pretreatment conditions, particularly compared to HMF, HMF esters, or HMF ethers, the functional groups of humulins (such as those that produce or cause color formation (e.g., aldehydes and / or groups with conjugated double bonds)) can be more readily converted (in some cases “bleached”) by oxidation. This selective oxidation of humulins can be achieved using milder oxidative conditions and / or different reaction systems compared to those of desired, non-oligomeric and non-polymeric furan precursors for FDCA oxidation, a focus of previous research efforts. Oxidative pretreatment can advantageously facilitate the selective oxidation of humulin functional groups, with the overall result being the modification of humulins in ways that can provide numerous improvements, particularly in terms of the pretreated dehydrated products obtained from further processing.

[0010] In other words, the oxidative pretreatment step described herein can be considered as resulting in the “selective” oxidation of humulin while substantially preserving intact the desired, non-oligomeric and non-polymeric furan compounds (such as HMF, their esters, or ethers) that form FDCA for subsequent oxidation, thereby converting them to FDCA in the presence of the humulin that has been advantageously modified in the oxidative pretreatment step. Modification of humulin in the pretreated dehydration product that has undergone the oxidative pretreatment step can lead to a number of improved properties. These may include superior color due to the “bleaching” of humulin, which can be measured by a reduction in the absorption of UV or visible light radiation by the pretreated dehydration product. Other improved properties of the pretreated dehydration product may include, or extend to, color reduction and / or improved color stability in downstream processing steps (e.g., oxidation, esterification and / or transesterification, and / or copolymerization steps). Alternatively or in combination, further improved properties may include: (i) a reduction in the average molecular weight of humin in the pretreated dehydrated product, (ii) an increase in the amount of precipitated humin in the product (promoting the separation of humin from the product), and / or (iii) improved “processability,” “oxidative robustness,” or performance of the product in downstream oxidation steps in terms of providing more stable operation.

[0011] The amount (e.g., weight percentage) of dissolved and / or solid humic acid in a given dehydration product can be used as a basis for adjusting the operating conditions of the oxidative pretreatment, such as increasing the stringency of the pretreatment when the humic acid content increases. For example, the amount of oxidant added or fed into the oxidative pretreatment, relative to the amount of dehydration product added or fed into the pretreatment (or the amount of furan compounds in the product that form FDCA), can be increased when the amount of dissolved and / or solid humic acid increases (e.g., in the case of continuous pretreatment). Otherwise, the amount of oxidant present in the oxidative pretreatment (e.g., in an oxidative pretreatment vessel or reactor), relative to the amount of dehydration product present in the pretreatment (or the amount of furan compounds in the product that form FDCA), can be increased when the amount of dissolved and / or solid humic acid increases (e.g., in the case of batch pretreatment). In other embodiments, the dry solids content of the dehydration feed (corresponding to the weight percentage of hexose carbohydrates used in the feed) can be used in the same manner as the content of dissolved and / or solid humin as a basis for adjusting the operating conditions for the oxidative pretreatment. This dry solids content, apart from the average molecular weight of these humin, is generally related to the content of dissolved and / or solid humin in the resulting dehydrated product. That is, an increase in dry solids content leads to an increase in both the humin content and the average molecular weight of the humin.

[0012] These and other aspects, embodiments, and related advantages will become apparent from the following detailed description. Detailed Implementation

[0013] As used herein, the terms “wt-%” and “wt-ppm” are used to specify percentage by weight and parts per million by weight, respectively. The term “mol-%” is used to specify mole percentage.

[0014] This document describes a method for preparing 2,5-furandicarboxylic acid (FDCA) using an oxidative pretreatment step and an oxidation step. It will be understood from this disclosure that (i) the oxidative pretreatment and (ii) the oxidation leading to a greater conversion of the furan compounds that form FDCA to FDCA can be carried out in separate containers or reactors, as in the case where (i) the oxidation pretreatment container or reactor is carried out and (ii) the oxidation container or reactor is carried out in a separate oxidation container or reactor, wherein the operation of the oxidation pretreatment container or reactor utilizes the oxidation pretreatment conditions as described herein. For example, all or part of the reaction product obtained from the oxidation pretreatment container or reactor can be conveyed to a downstream oxidation container or reactor, as in continuous process operation. Alternatively, (i) and (ii) can be carried out in separate areas within a single container or reactor, as in the case where (i) the oxidation pretreatment area and (ii) the oxidation area are carried out within the same container or reactor, wherein the operation of the oxidation pretreatment area utilizes the oxidation pretreatment conditions as described herein. For example, the reaction product obtained from the oxidation pretreatment area can be conveyed to a downstream oxidation area, as in continuous process operation. Otherwise, (i) and (ii) can be carried out in the same container or reactor, such as in the case where (i) the pretreated dehydrated product is subjected to oxidative pretreatment conditions as described herein to provide a first initial period of time for sustained effectiveness, and (ii) the pretreated dehydrated product is subjected to different oxidative conditions as described herein to provide a second subsequent period of time for sustained effectiveness, providing a composition containing FDCA (e.g., because the degree of conversion of furans forming FDCA under oxidative conditions is greater than that occurring under oxidative pretreatment conditions). Carrying (i) and (ii) in the same container or reactor may be characteristic of batch processes.

[0015] The methods described herein may include any of a number of further processing steps while still benefiting from certain advantages associated with the oxidative pretreatment steps as described herein. Such further processing steps may include a step of “esterification” to form one or more ester derivatives of FDCA, by which one or preferably both of the carboxylic acid groups of the dicarboxylic acid are converted to ester groups, such as alkyl ester groups (in the case of mono- or dialkyl ester derivatives) or aryl ester groups (in the case of mono- or diaryl ester derivatives), wherein methyl ester groups, ethyl ester groups, or phenyl ester groups are specific examples. In the case of methyl ester groups, a preferred ester derivative of FDCA is dimethyl 2,5-furandicarboxylate (FDME), which can be formed by reacting FDCA with sufficient methanol.

[0016] Those skilled in the art who understand this disclosure will appreciate that the methods described herein, including the oxidative pretreatment step, can also be applied to the production of a wide range of other FDCA derivatives, or practically to the oxidation of carbohydrate dehydration products to produce other oxidized products (other than FDCA) and / or to the production of desired derivatives of these other oxidized products, including, but not limited to, ester derivatives; and hydroxy (alcohol) derivatives, including hydroxyalkyl derivatives; ether derivatives, including alkoxy derivatives; amino derivatives; acyl derivatives, including acyl halide derivatives such as acyl chloride derivatives; isocyanate derivatives; aldehyde derivatives; and acetal derivatives, wherein unmodified humic acid will present similar difficulties as the oxidative preparation of FDCA. As an example of such other oxidized products in which the oxidative pretreatment step may be relevant, dialdehyde dicarboxyfuran (which can be prepared by the oxidation of substances produced during fructose dehydration) can be converted to diamino derivatives more easily and effortlessly than FDCA. In the case of such derivatives, and more specifically for illustrative purposes, in the case of FDCA, one or both of the two carboxyl-substituted furan ring members may instead be substituted with a carboxylic acid group other than the carboxyl group and / or with other groups to provide, for example, the corresponding diol, dialkylol (e.g., diethanol), diamino, diacyl (e.g., diacyl chloride), diester, diisocyanate, ether acid, ether ester, ester acid, ester aldehyde, ether aldehyde, ether acetal, ester acetal, acetal acid, hydroxy acid, hydroxyalkyl acid, hydroxy ester, hydroxyalkyl ester, hydroxy acetal, hydroxyalkyl-hydroxyalkyl, diacetal and / or aldehyde-acetal derivatives, wherein the "acid" substituent refers to a carboxyl group formed from a carboxylic acid other than acetic acid or a group such as carboxymethyl.

[0017] This article also describes further processing steps beyond the formation of esters and other derivatives of FDCA, extending to the production of bio-based copolymers (e.g., copolyesters).

[0018] Dehydration products and humic acid present in these products

[0019] Specific embodiments relate to methods for pretreating dehydrated products of one or more carbohydrates (e.g., fructose) having a six-carbon sugar unit, and in specific embodiments, to dehydrated products that have not been refined to remove humin (“crude” dehydrated products). That is, the dehydrated products are obtained from the dehydration of such one or more carbohydrates. Carbohydrates having a six-carbon sugar unit mean hexoses, oligomers of hexoses, or polymers of hexoses. Such carbohydrates include starch, amylose, galactose, cellulose, hemicellulose, inulin, fructan, glucose, fructose, sucrose, maltose, cellobiose, lactose, and sugar oligomers. These carbohydrates may also be referred to as hexose carbohydrates and can be obtained from one or a combination of products, byproducts, or intermediate products of wet or dry particle milling or cellulose / hemicellulose hydrolysis processes, wherein such products include one or more of fructose syrup, crystalline fructose, high-fructose corn syrup, crude fructose, purified fructose, or molasses. The preferred carbohydrate is fructose, which can be provided to the dehydration step in pure or purified form (e.g., with a purity greater than 90% or 95%).

[0020] As described above, the dehydration step typically results in the formation of humin as a byproduct in the dehydration product containing furan compounds that form FDCA. Huminin refers to highly colored, typically brown to black, amorphous or non-crystalline polymers produced by the dehydration of sugars. The concentration of humin in the dehydration product will depend at least in part on the conditions used in the dehydration step, and particularly on the vigor of the reaction, such that this concentration can be correlated with single-pass conversion. The concentration of humin in a given sample and their relative molecular weight can be determined by analytical methods, such as gel permeation chromatography (GPC-RID) using a refractive index detector. Huminin is generally considered detrimental to the downstream oxidation of furan compounds that form FDCA and is also undesirable in terms of its ability to cause coloration in the final bio-based polymer product, but it must also be considered as representing some intrinsic potential of hexose sugars for the production of FDCA and its derivatives or other value-added products that, ideally, would not be lost as waste.

[0021] Therefore, representative methods may further include a step of separating at least a portion (e.g., an insoluble or precipitated portion) of humin from the dehydration product prior to oxidative pretreatment (e.g., providing an oxidative pretreatment feed having a reduced humin concentration relative to the dehydration product). Alternatively or in combination, representative methods may further include a step of separating at least a portion (e.g., an insoluble or precipitated portion, such as a portion that becomes insoluble or precipitated by oxidative pretreatment) of humin from the pretreated dehydration product after oxidative pretreatment, to the extent that humin may become insoluble by oxidative pretreatment. If at least a portion of humin needs to be removed before oxidizing a dehydration product containing furan compounds that form FDCA, the fact that at least a portion of humin may be insoluble in an aqueous medium can be utilized by filtering the dehydration product or the pretreated dehydration product, such as using an ultrafiltration (UF) membrane. Other methods for removing humin from the dehydration product or the pretreated dehydration product, or from other intermediate products in the overall synthesis of bio-based polymers, include distillation and sublimation. Specific methods for removing humectants are described, for example, in US 10,457,657; US 10,017,486; and US 9,611,241, which disclose these methods by reference incorporation.

[0022] However, certain aspects of this disclosure relate to the advantageous effects achieved herein by modifying humulin through oxidative pretreatment, such as the reaction stabilizing effect of using the pretreated dehydrated product in subsequent oxidation to obtain monomer compositions containing FDCA. These effects eliminate the need for an additional step to remove humulin (assuming no alternative of higher value to upgrade or directly use a portion of the humulin, which provides an independent reason for humulin removal), and therefore, according to some embodiments, the dehydrated product is not processed (e.g., filtered) to remove humulin, or the pretreated dehydrated product is not processed (e.g., filtered) to remove humulin, or neither of these products is processed to remove humulin.

[0023] Oxidative pretreatment of dehydration products

[0024] A representative method involves contacting a dehydration product with an oxidant in an oxidative pretreatment step to provide a pretreated dehydration product having improved properties (e.g., relative to the dehydration product) due to modification of the humicin present in the dehydration product. This modification can, for example, selectively convert the aldehyde-containing functional groups of the humicin present in the dehydration product (e.g., formed during upstream dehydration of one or more carbohydrates), and thereby eliminate their tendency to otherwise undergo aldol condensation and develop color over time in the pretreated dehydration product and / or downstream products (such as FDCA-containing compositions provided after subsequent oxidation of the pretreated dehydration product). In the oxidative pretreatment step, FDCA-forming furans present in the dehydration product can be largely retained in the pretreated dehydration product (remaining unconverted), such that the oxidative pretreatment step is characterized by selective modification of the humicin, while these FDCA-forming furans are substantially unconverted, as by oxidation. For example, in a representative embodiment, the yield of FDCA-forming furan compounds in the pretreated dehydration product is at least about 80 mol-%, meaning that the number of moles of FDCA-forming furan compounds recovered from the pretreated dehydration product is at least about 80% of those present in the dehydration product prior to oxidative pretreatment. In other embodiments, the yield of FDCA-forming furan compounds obtained from oxidative pretreatment is at least about 85 mol-%, at least about 90 mol-%, or at least about 95 mol-%.

[0025] Similarly, according to some embodiments, the weight percentage of humulins present in the dehydration product can also be largely retained, although these humulins can be significantly modified by oxidative pretreatment, such that the improved (lighter) color of the pretreated dehydration product can be visually perceived and / or quantitatively measured, as described herein. For example, in representative embodiments, the yield of humulins in the pretreated dehydration product is at least about 85 wt%, meaning that the weight of humulins recovered from the pretreated dehydration product is at least about 85% of the weight of humulins present in the dehydration product prior to oxidative pretreatment. In other embodiments, the yield of humulins obtained from oxidative pretreatment is at least about 90 mol%, at least about 95 mol%, or at least about 97 mol%.

[0026] Furan compounds that form FDCA refer to monomeric and dimer molecules containing a furan ring that form FDCA through catalytic oxidation in the oxidation steps described herein. Furan compounds that form FDCA can be obtained from the dehydration of one or more carbohydrates having a hexose unit at high temperatures and in the presence of an acid catalyst that may be inherently homogeneous or heterogeneous, along with humin, in the dehydration product. Furan compounds that form FDCA include 5-(hydroxymethyl)furfural (HMF), and in the case of its formation in the presence of a lower carboxylic acid (such as acetic acid), it may include ester derivatives of HMF, such as 5-(acetoxymethyl)furfural, or in the case of its formation in the presence of a lower alcohol solvent (such as methanol), it may include ether derivatives of HMF, such as 5-(methoxymethyl)furfural. Other furan compounds that form FDCA may include derivatives of HMF, such as 2,5-diformylfuran and 5-formyl-2-furancarboxylic acid. The furan compounds that form FDCA further include HMF dimer 5,5'-[oxybis(methylene)]bis(2-furanaldehyde) and HMF oligomers. Examples of non-FDCA-forming furan compounds include, but are not limited to, furfural, 2-(hydroxyacetyl)furan, and 2-(acetoxyacetyl)furan. In preferred embodiments, the dehydration product and the pretreated dehydration product may contain one or more of HMF and / or its ester or ether derivatives, such that the FDCA-forming furan compounds may contain one or more of HMF and / or its ester or ether derivatives. When dehydration is performed using an acetic acid catalyst and solvent, the FDCA-forming furan compounds may include or may consist substantially of HMF, 5-(acetoxymethyl)furfural, and HMF dimer.

[0027] In one embodiment, dehydration can utilize hydrobromic acid other than acetic acid and can be integrated with a subsequent medieval oxidation step, as described herein, such that the dehydrated product is fed directly into the subsequent oxidation pretreatment step, and the pretreated dehydrated product obtained from this step is then fed into the oxidation step, for example, according to the oxidation step described in US 10,538,499. The integration of dehydrating and oxidation steps, such as through an intermediate oxidation pretreatment step, can provide the advantages of using the same acetic acid (or acetic acid and water) solvent in two or all three of these steps. Moreover, in addition to hydrobromic acid being suitable as an additional acid catalyst for the dehydration step, the use of a common solvent provides the added advantage that acetic acid and optionally at least a portion of the bromine source used in the medieval oxidation step (typically in the form of hydrobromic acid) can be recycled (and preferably substantially completely recycled) from the oxidation step back to the dehydration step. These components, namely hydrobromic acid and acetic acid, can also be used in the intermediate oxidation pretreatment step in a similar manner to the oxidation step. This leads to a significant reduction in the financial and operational costs associated with converting carbohydrates with hexose units into FDCA.

[0028] Generally, following oxidative pretreatment, the pretreated dehydration product can undergo an oxidation step in the entire method for preparing a monomer composition containing FDCA. This method may include contacting all or a portion of the pretreated dehydration product (e.g., as an oxidation feed) with an oxidation catalyst in the presence of oxygen (e.g., an oxygen-containing gas such as air) to provide an FDCA-containing composition. In the oxidation step, the furan compounds present in the pretreated dehydration product that form FDCA can be substantially converted (oxidized) in the FDCA-containing composition. For example, in representative embodiments, based on the furan compounds that form FDCA in the pretreated dehydration product, the yield of FDCA in the oxidation step can be at least about 60 mol-%, i.e., the number of moles of FDCA recovered in the FDCA-containing composition can be at least about 60% of the number of moles of furan compounds that form FDCA in the pretreated dehydration product. In other embodiments, the yield of FDCA obtained from oxidation is at least about 70 mol-%, at least about 80 mol-%, or at least about 90 mol-%. For example, the molar number of furan compounds that form FDCA retained in the composition containing FDCA may be less than about 40%, less than about 30%, less than about 20%, or less than about 10% of those present in the oxidation feed or pretreated dehydration product before oxidation. More specifically, the oxidation step may include feeding all or part of the pretreated dehydration product as an oxidation feed into an oxidation reactor containing an oxidation catalyst (e.g., a homogeneous or heterogeneous catalyst) and reactant oxygen.

[0029] In oxidative pretreatment, contact between the dehydration product and the oxidant can be carried out, for example, by continuously adding these oxidative pretreatment feeds to a vessel or reactor in the desired amount (e.g., a given ratio of the oxidative equivalent provided by the oxidant to the molar number of FDCA-forming furan compounds provided by the dehydration product). More generally, contact can also be carried out if the oxidant and the dehydration product are present together in the vessel or reactor in such desired amounts. Whether the oxidative pretreatment feeds are continuously added to the vessel or reactor, as in the case of continuous operation, and / or these feeds are present together in the vessel or reactor, as in the case of batch operation, charged into the vessel or reactor in the initially desired amount (a given molar ratio as described above), contact between the dehydration product and the oxidant occurs under appropriate oxidative pretreatment conditions that effectively achieve selective modification of the humicin present in the dehydration product. In this way, the resulting pretreated dehydration product can have improved properties relative to the dehydration product due to the modification of the humicin initially present in the product, while there is essentially no conversion of FDCA-forming furan compounds in the product.

[0030] Improved properties, which may result from the selective transformation of the aldehyde-containing functional groups of humulins present in the dehydration product or from other transformations occurring during oxidative pretreatment, may include an improved color of the pretreated dehydration product, which may be determined qualitatively (e.g., by observation) or quantitatively (e.g., by a reduction in absorption of ultraviolet or visible light radiation). For example, according to a specific embodiment, the pretreated dehydration product may exhibit a reduction in light absorption, as measured at a specific wavelength such as 460 nanometers (nm). The absorption relative to the untreated dehydration product may be, for example, about 80% or less, i.e., the absorption may be reduced by at least about 20%. In other embodiments, the absorption relative to the dehydration product may be about 70% or less, about 60% or less, or about 50% or less. Alternatively or in combination, other improved properties may include the ability to further process the pretreated dehydrated product, reduced color formation and / or increased color stability in downstream processing steps and in the resulting downstream products (e.g., FDCA, esterified derivatives of FDCA, prepolymers as products of the reaction of FDCA or esterified derivatives of FDCA with comonomers, and copolymers formed by the condensation of these prepolymers). Alternatively or in combination, other improved properties may include a decrease in the average molecular weight of humic acid in the pretreated dehydrated product relative to the average molecular weight of humic acid in the dehydrated product before oxidative pretreatment. For example, the decrease in molecular weight may be due to the degradation of humic acid by reaction with an oxidant. Alternatively or in combination, other improved properties may also include an increase in the amount (e.g., weight percentage) of precipitated (solid) humic acid present in the pretreated dehydrated product relative to the amount present (e.g., as suspended solids) in the dehydrated product before oxidative pretreatment. In this context, a representative method may include removing (e.g., by filtration) solid humic acid from the pretreated dehydration product prior to further processing, such as by oxidizing the furan compounds that form FDCA in the product to FDCA. Alternatively or in combination, other improved properties may include increased stability of the pretreated dehydration product as feedstock for the oxidation process, i.e., as the oxidation feedstock for the production of FDCA. That is, the pretreated dehydration product may provide improved performance with respect to any or any combination of the operating parameters described herein (e.g., those mentioned above) that are limited to homogeneous or heterogeneous catalytic oxidation due to the presence of humic acid. For example, in a given oxidation step, the pretreated dehydration product may require reduced catalyst consumption compared to the untreated dehydration product.

[0031] Representative oxidants that can be used for oxidative pretreatment include oxygen-containing compounds, such as those in which one or more oxygen atoms have an oxidation state of -1 or lower, for example, in the case of peroxides. Suitable oxidants of specific types include transition metal oxides (e.g., manganese oxide), alkali metal or alkaline earth metal halide oxides (e.g., sodium oxychloride), alkali metal or alkaline earth metal percarbonates (e.g., sodium percarbonate), alkali metal or alkaline earth metal permanganates (e.g., potassium permanganate), alkali metal or alkaline earth metal chlorates or perchlorates (e.g., potassium perchlorate), alkali metal or alkaline earth metal bromates or perbromates (e.g., potassium bromate), alkali metal or alkaline earth metal iodates or periodates (e.g., sodium periodate), and alkali metal or alkaline earth metal sulfates or persulfates (e.g., potassium monopersulfate or...). Oxidizing agents can be peroxides (e.g., hydrogen peroxide), peroxy acids (e.g., peracetic acid or m-chloroperoxybenzoic acid), or oxygen (e.g., contained in air). Combinations of these types and / or specific compounds can be used as oxidizing agents. Therefore, in one embodiment, the oxidizing agent may be selected from MnO2, NaOCl, H2O2, KHSO5, etc. One or more of Na₂CO₃·1.5H₂O₂, m-chloroperoxybenzoic acid, peracetic acid, KMnO₄, NaIO₄, and KBrO₃. In the case of alkali metal or alkaline earth metal bromates or perbromates, the oxidant can be a bromine source that generates Br₂ and facilitates the selective oxidation of humicin, such as selective “bleaching.” A bromine source can be used alone, as in the case of using alkali metal or alkaline earth metal bromates or perbromates alone; multiple bromine sources can be used; or one or more bromine sources can be used in combination with one or more bromine-free oxidants, such as those described above (e.g., MnO₂). Bromine sources include those described herein for use in the dehydration step (e.g., hydrobromic acid), and also include bromine-containing substances as described herein for use in the oxidation step. In a preferred embodiment, any oxidant or combination of oxidants as described herein can be dissolved in the dehydration product or in the reaction mixture containing the product during the oxidation pretreatment. However, some oxidants with limited water solubility can also be used in solid form, such as in the form of fine solid particles, which can be separated after oxidative pretreatment and before further processing of the pretreated dehydrated products, for example by filtration.

[0032] Oxidative pretreatment can be carried out by initially loading or continuously adding an oxidant as described herein into a container or reactor, into which the dehydration product is also loaded and / or continuously added. Advantageously, the oxidant can be used in relatively small amounts, such as substoichiometry relative to the number of moles of furan compounds that form FDCA in the dehydration product (e.g., relative to the number of moles of HMF, 5-(acetoxymethyl)furfural, and HMF dimers). In some embodiments, the oxidant can be present (in a suitable container or reactor) or added (to a suitable container or reactor) in an amount less than about 85 mol-% (e.g., about 5 mol-% to about 85 mol-%) of the oxidative equivalent relative to the furan compounds that form FDCA. The oxidative equivalent is based on the number of atoms in a given oxidant having oxidizing power (e.g., oxidizing oxygen atoms and oxidizing halogen atoms), such that, for example, 1 mole of NaOCl represents 2 moles of oxidative equivalent. In other embodiments, the oxidant can be present or added in an amount less than about 70 mol-% (e.g., about 5 mol-% to about 70 mol-%) of the oxidative equivalent. For example, the oxidant may be present or added in amounts of about 10 mol-% to about 70 mol-%, about 10 mol-% to about 50 mol-%, about 25 mol-% to about 70 mol-%, about 35 mol-% to about 70 mol-%, or about 25 mol-% to about 50 mol-%. Generally, all other parameters being equal, the increased dry solids content in the dehydration feed, corresponding to the weight percentage of hexose carbohydrates used in the feed and related to the increased content of dissolved and / or solid humin in the dehydration product, will guarantee the use of the increased amount of oxidant with respect to the oxidant equivalent present or added.

[0033] In cases where (i) a dehydration product containing a furan compound that forms FDCA and (ii) an oxidant are co-fed into a vessel or reactor for oxidative pretreatment, these feeds (i) and (ii) can be provided as separate streams (e.g., input at separate locations) to the same vessel or reactor. For example, these separate streams can enable a more desired temperature profile within the vessel or reactor, or otherwise provide improved control over this temperature profile, such as by positioning or manipulating the release of exothermic heat. Alternatively, feeds (i) and (ii) can be provided as a combined feed stream to the vessel or reactor, as in the case of combining a stream of oxidant with an effluent from a dehydration reactor containing the dehydration product. In yet another embodiment, portions of feeds (i) and (ii) can be combined upstream of the vessel or reactor and / or added as separate streams, depending on objectives related to process efficiency and process control. In the case of batch operation, the co-feed of (i) and (ii) may include initially loading these feeds (e.g., in the relative amounts described above) into a container or reactor and subjecting them to an oxidative pretreatment at a certain time and temperature, which will effectively alter the humicin content of the resulting dehydrated product as described herein.

[0034] Whether the oxidation pretreatment is carried out continuously or batch-wise, the dehydration product and the oxidant can be contacted in the presence of a catalyst and / or other components, as described herein, that are also generally suitable for oxidizing the pretreated dehydration product, to provide a composition comprising FDCA derived from the oxidation of furan compounds that form FDCA in the dehydration product, or more specifically, from the oxidation of at least a portion of these FDCA-forming furan compounds retained in the pretreated dehydration product. Exemplary catalysts comprise one or more metals from Groups 5-11 of the Periodic Table (IUPAC version), such as soluble forms of any of these metals, with more specific catalysts comprising soluble forms of Co and / or Mn. Any of these metals may be present independently in an amount or in combination in a reaction mixture, for example, for carrying out the oxidation pretreatment and comprising the dehydration product and any of the oxidants described herein, representing about 0.1 mol-% to about 10 mol-%, such as about 1 mol-% to about 5 mol-%, of the furan compounds forming FDCA in the dehydration product present in such a reaction mixture. Alternatively or additionally, the catalyst may contain an acid, such as nitric acid (HNO3), which may be present in the reaction mixture in amounts within these ranges, or in larger or smaller amounts. For example, the acid may be present in amounts from about 0.05 mol-% to about 5 mol-% of the furan compounds forming FDCA in the dehydration product, such as from about 0.1 mol-% to about 1 mol-%. Exemplary catalysts or catalyst systems contain soluble forms of Co and / or Mn present in the reaction mixture, optionally together with an acid (e.g., HNO3), wherein the concentrations of the metal and optional acid components are based on the furan compounds forming FDCA within these ranges.

[0035] In some embodiments, the oxidative pretreatment of the predominantly modified humin and the subsequent oxidation, which predominantly converts the furan compounds that form FDCA, to FDCA can be carried out in the same vessel or reactor. However, regardless of whether these steps use the same vessel or reactor, according to some embodiments, the oxidative pretreatment and oxidation can be distinguished because the former is carried out under milder conditions relative to the latter. For example, the oxidative pretreatment temperature used in the oxidative pretreatment may be lower than the oxidation temperature used in the oxidation step. A representative or average temperature of the reaction mixture containing the dehydration product and the oxidant and suitable for the oxidative pretreatment can be from about room temperature (i.e., about 20°C) to about 120°C. In other embodiments, the temperature or average temperature can be below about 100°C (e.g., from about 20°C to about 100°C) or below about 80°C (e.g., from about 20°C to about 80°C). Such a temperature or average temperature can be used in conjunction with a suitable residence time during which the dehydration product and the oxidant remain in contact (e.g., in batch or continuous processes), such as a residence time of at least 1 hour (e.g., from about 1 hour to about 24 hours, from about 1 hour to about 12 hours, or from about 1 hour to about 6 hours). For example, in the case of a batch process, the dehydration product and the oxidant may (i) be charged into a container or reactor in relative amounts as described herein, (ii) undergo an oxidation pretreatment step under conditions including a given residence time at an average temperature as described herein, and (iii) after that residence time, undergo an oxidation step under more demanding conditions (including a higher average temperature and / or other conditions as described herein that are generally applicable to the oxidation of the pretreated dehydration product) to provide a composition containing FDCA.

[0036] Whether carried out continuously or in batches, the oxidative pretreatment and oxidation can be performed using the same catalyst or catalyst system, optionally using the same concentration of catalyst and / or the same ratio relative to the furan compounds forming FDCA in their respective feeds. In some embodiments, the oxidative pretreatment and oxidation can be distinguished by the use of an oxidant as described herein, which is used in the former but not in the latter. Regardless of whether these steps are distinguished in this way, oxygen (e.g., oxygen contained in the air) can be used as the oxidant in both steps. As mentioned above, the oxidative pretreatment, which may include the co-feed dehydration product and the oxidant, prior to the co-feed, can be further combined, either integrally or non-integrally, with an additional step of dehydrating one or more carbohydrates in the dehydrated feed to obtain furan compounds forming FDCA. This dehydration can be performed on one or more carbohydrates (e.g., acetic acid) in a solution containing a lower carboxylic acid (e.g., acetic acid) or a lower alcohol (e.g., methanol or ethanol) as a solvent.

[0037] Specific embodiments of the invention relate to processing choices and overall processing flexibility arising from the use of oxidative pretreatment to partially or completely restore the performance (e.g., catalyst activity) of furan compounds that form FDCA to FDCA formation, relative to comparative (or baseline) oxidation performed without oxidative pretreatment but with the removal of humic acid from the dehydration product or in the absence of humic acid. Advantageously, based on the portion of the dehydration product that can be oxidized to the desired monomer (e.g., furan compounds that form FDCA "onpath"), performance recovery can be achieved using only a small amount of oxidant (e.g., on a molar basis) relative to the dehydration product. According to the oxidative pretreatment step, representative methods for the pretreated dehydration product or for preparing monomer compositions containing FDCA may include contacting a dehydration product of one or more carbohydrates having 6-carbon sugar units with an oxidant as described herein to provide the pretreated dehydration product. In such methods, the amount of oxidant added (e.g., in the case of continuous operation) or the amount of oxidant present (e.g., in the case of bath operation) can be adjusted based on the humin content of the dehydrated product, or as a representative of that humin content, based on the dry solids content of the dehydrated feed.

[0038] Dehydrated carbohydrates used to prepare furan compounds that form FDCA

[0039] Representative methods include a dehydration step prior to the oxidative pretreatment step, for preparing the furan compounds that form FDCA as described above, wherein all or part of the furan compounds that form FDCA are used in this oxidative pretreatment step (e.g., as a component of the oxidative pretreatment feed or as a co-feed stream). The dehydration step may be carried out batchwise, thereby recovering the furan compounds that form FDCA and intermittently transferring them to the oxidative pretreatment step, but preferably the dehydration step is carried out continuously by a continuous transfer. Thus, in any case, the upstream step of the method may include dehydrating a dehydration feed comprising one or more carbohydrates having hexose units as described above. This dehydration feed (e.g., an aqueous fructose solution) may have a dry solids concentration of about 5 wt% to about 50 wt%, such as about 10 wt% to about 30 wt%, or about 5 wt% to about 35 wt% and / or may be prepared from a purified source of hexoses, such as fructose with a purity of at least 90 wt% (e.g., 97 wt% fructose).

[0040] Dehydration can be carried out in the presence of a Brønsted acid source (which may be or may include a bromine source, such as HBr used and recycled in the context of the integrated dehydration-medieval-catalytic oxidation sequence described in US 10,538,499) and a solvent for forming FDCA-like furan compounds, and can be carried out at high temperatures and for a sufficient duration to produce an oxidative pretreatment feed comprising FDCA-like furan compounds, such as HMF, and / or derivatives thereof. Depending on the solvent, one or more derivatives may include ester derivatives, ether derivatives, and / or HMF dimers. Oxidative pretreatment feeds containing some or all of the products formed in the dehydration step (dehydration products) as components will generally also contain at least a portion of the solvent. That is, all or part of the solvent used in this step, such as a mixture of acetic acid and water, may be conveyed to the oxidative pretreatment step, except for all or part of any water produced in the dehydration step. The solvent may additionally include alcohols such as methanol, ethanol, or higher alcohols, or possibly cyclic or heterocyclic hydrocarbon compounds (e.g., dioxane). Preferably, after the oxidation step following the oxidation pretreatment step, the solvent is separated to provide a solvent recycle stream returned to the dehydration step, in addition to the monomeric composition containing FDCA as described herein. This recycle stream typically also contains a Brønsted acid source (e.g., HBr). The solvent recycle stream typically further contains furan compounds that form FDCA, such that the total amount of furan compounds forming FDCA in the dehydration product can include a portion newly generated during the dehydration step (i.e., based on "single-pass conversion") and a portion already recycled back from the downstream oxidation step.

[0041] When a bromine source is used in the dehydration step, the bromine source, and any bromine source that can be used as an oxidant in the oxidative pretreatment step, can generally be any compound that provides bromide ions or groups in the reaction mixture. Representative bromine sources used in any step include hydrogen bromide, hydrobromic acid, sodium bromide, potassium bromide, elemental bromine, benzyl bromide, and tetrabromoethane. In the case of using hydrogen bromide (e.g., in the context of the integrated method described in US 10,538,499, at least in part as recycled hydrogen bromide, now incorporated herein by reference for describing a particular dehydration method in which a bromine source is used), the compound can act as an acid catalyst in the dehydration step, decomposing to form hydrobromic acid in the presence of the dehydration feed and solvent. In some other embodiments, bromine sources such as 1-alkylpyridinium bromide and 1,3-dialkylimidazolium bromide can be used as promoters in the presence of a solvent comprising acetic acid and water. Thus, regardless of the specific bromine source, the dehydration step provides a dehydration product, some or all of which can be used as a component of subsequent oxidative pretreatment steps.

[0042] Oxidation of pretreated dehydrated products to prepare monomer compositions

[0043] Representative methods include an oxidation step in which an oxidation feedstock is contacted with an oxidation catalyst in the presence of oxygen to provide a monomer composition containing FDCA as described herein. The oxidation feedstock may contain a pretreated dehydration product as described herein, or at least a portion of that product, such as a portion obtained after removing humin as described above. The oxidation feedstock may further contain all or a portion of the solvent initially used to prepare the dehydration feedstock and / or at least one bromine-containing substance. Oxygen may be obtained using air, purified oxygen, or other oxygen-containing feedstocks as sources. The oxidation step may be carried out batchwise, but is preferably continuously, wherein at least the pretreated dehydration product (or a portion thereof), the oxygen-containing feedstock, and optionally the catalyst are continuously fed into the oxidation reactor, and the monomer composition is continuously drawn from this reactor.

[0044] Specific methods may include contacting the oxidation feed with a metal-containing catalyst and an oxygen-containing feed at a high temperature for a time sufficient to produce a monomer composition as an oxidation product, the monomer composition comprising FDCA and / or its derivatives, a solvent, and residual catalyst. For example, as shown in US 10,538,499, some or substantially all of the bromine required for medieval-style oxidation can be provided by a bromine-containing substance (such as recycled HBr) in the oxidation feed prepared from the aforementioned dehydration step. Representative bromine-containing substances include: inorganic bromides, such as HBr; metal bromides, such as lithium bromide, sodium bromide, potassium bromide, magnesium bromide, calcium bromide, cobalt bromide, and manganese bromide; and organic bromides, such as 5-(bromomethyl)furfural and its derivatives, and brominated furan oligomers. If necessary, an additional bromine source can be introduced into the oxidation step (e.g., feed to an oxidation reactor), providing the bromine content provided by the bromine-containing substance in the oxidation feed from the dehydration step and / or oxidation pretreatment step.

[0045] In the case of homogeneous (liquid-phase) oxidation catalysts, metal-containing catalysts can specifically and effectively convert HMF and / or other furan compounds (e.g., HMF esters and / or HMF ethers) in the oxidation feed into FDCA and / or their derivatives (depending on the solvent used in the dehydration step). As described herein, these metal-containing catalysts can alternatively or also be used in oxidation pretreatment to modify humic acid in the dehydration product (e.g., under milder conditions compared to those used for oxidation to form monomers). Representative metal-containing catalysts used for one or both of the oxidation pretreatment and / or oxidation steps may contain one or more transition metals, such as Co and Mn, or both, optionally in combination with Zr, Ce, Zn, Mo, Bi, V, and / or Ni. Metal-containing catalysts can react with bromine present in bromine-containing substances as described above to form metal bromides in situ. According to a specific embodiment, the metal catalyst may be present in the reaction mixture contained in an oxidation pretreatment reactor or an oxidation reactor (as described above, which may include the same or different containers), such that the concentration of one or more transition metals is independently in the range of about 5 wt-ppm to about 10,000 wt-ppm, such as about 10 wt-ppm to about 8,000 wt-ppm or about 50 wt-ppm to about 5,000 wt-ppm. For example, Co may be present in the reaction mixture at a concentration of about 10 wt-ppm to about 10,000 wt-ppm, about 10 wt-ppm to about 8,000 wt-ppm, about 59 wt-ppm to about 5,900 wt-ppm, or about 2,000 to about 4,000 wt-ppm. Mn may be present in the reaction mixture at a concentration of about 5 wt-ppm to about 10,000 wt-ppm, about 5 wt-ppm to about 8,000 wt-ppm, about 55 wt-ppm to about 5,500 wt-ppm, or about 200 to about 1,000 wt-ppm. Bromine from bromine-containing substances and / or bromine sources may be present in the reaction mixture in amounts of about 0.1 wt-ppm to about 20,000 wt-ppm, about 200 wt-ppm to about 20,000 wt-ppm, about 10 wt-ppm to about 10,000 wt-ppm, or about 1,000 wt-ppm to about 2,000 wt-ppm.

[0046] The oxidation conditions, or the conditions maintained in the oxidation reactor, are typically different in at least one respect from the conditions used for the oxidation pretreatment, even if both steps use the same reactor. These conditions may include temperatures of about 120°C to about 250°C, such as about 170°C to about 190°C, and oxygen partial pressures of about 0.02 bar to about 100 bar, about 0.02 bar to about 21 bar, about 0.2 bar to about 100 bar, or about 0.2 bar to about 21 bar. The total absolute pressure in the oxidation reactor may be about 1 bar to about 200 bar, such as about 5 bar to about 100 bar, or about 10 bar to about 20 bar. The molar yield of the monomer from the oxidation step, such as the molar yield of FDCA from the furan compounds forming FDCA in the oxidation feed, may be at least about 60%, at least about 70%, at least about 80%, at least about 90%, or at least about 95%. After the oxidation step, the resulting monomer (containing FDCA) may be separated from the oxidation reaction mixture (containing solvent) for further purification. At least a portion of the solvent separated from the monomer can then be recycled back to the dehydration reactor along with at least a portion of the unconverted furan compounds that form FDCA. Since FDCA is largely insoluble in solvents containing acetic acid or mixtures of acetic acid and water under mild conditions, in embodiments utilizing such solvents, the separation of FDCA can be readily accomplished by filtration and / or centrifugation after precipitation from the oxidation reaction mixture. Specific methods for oxidation to form the monomer are described, for example, in US 10,538,499, which is incorporated herein by reference to disclose these methods.

[0047] Esterified monomers

[0048] The step of esterifying monomers such as FDCA to form ester derivatives includes reacting these monomers with an esterifying agent such as an alcohol, for example, methanol if dimethyl ester is desired, or ethanol if diethyl ester is desired, or possibly phenol if diphenyl ester is desired. The reaction of FDCA or other monomers with a suitable alcohol or phenol can be carried out under suitable esterification conditions in an esterification reaction mixture comprising a high-boiling solvent (e.g., dimethyl sulfoxide, dimethyl acetamide, sulfolane, FDME, γ-butyrolactone, isosorbide or its dimethyl ether, propylene carbonate, adipic acid, isophorone, ethyl phenyl ether, diphenyl ether, dibenzyl ether, aromatic 200 fluid, butyl phenyl ether, methyl heptaketone, ethyl phenyl ketone, 2'-hydroxyacetophenone, decahydronaphthalene, tetrahydronaphthalene, etc.). These may include temperatures of about 30°C (86°F) to about 350°C (662°F) and pressures of about atmospheric pressure to about 3.5 MPa, and the esterification reaction may be carried out together with distillation (according to a reactive distillation process) to separate FDME or other ester derivatives.

[0049] In a preferred embodiment, the step of esterifying a monomeric composition containing FDCA obtained by oxidation comprises reacting the monomeric composition or a separated fraction thereof (e.g., a fraction rich in FDCA relative to the monomeric composition as a whole, which is obtained directly by oxidation) with methanol as an esterifying agent. This can provide an esterified monomeric composition containing FDME, which, due to its lower boiling point relative to the parent dicarboxylic acid FDCA, is easily separated by distillation and can therefore be a desired ester derivative monomer.

[0050] Polymers formed from monomer compositions containing FDCA or esterified derivatives

[0051] Monomer compositions containing FDCA and / or its derivatives can be used to form polymers, and particularly bio-based copolymers having a moiety related to FDCA (e.g., a furanyl dicarboxylate moiety). Generally, representative polymer-forming steps may include (i) esterification of a monomer composition containing FDCA as described herein, or a fraction thereof, or (ii) transesterification of an esterified monomer composition as described herein (e.g., containing FDME), or a fraction thereof, followed by polycondensation to produce a polyester. Representative polyester polymer-forming steps include polymerization of monomers or ester derivative monomers as described herein, such as monomer FDCA or ester derivative monomer FDME, with a suitable comonomer such as a glycol. For example, ethylene glycol can be used as a comonomer from FDCA or FDME to produce poly(ethylene furanyl dicarboxylate) (PEF). The comonomer 1,3-propanediol can be used from FDCA or FDME to produce poly(trimethyl furanyl dicarboxylate) (PTF).

[0052] Specific methods may include producing a precursor composition comprising a prepolymer, which is an esterification intermediate, such as the product of the reaction of FDCA with a comonomer, or an transesterification intermediate, such as the product of the reaction of FDME with a comonomer. The prepolymer, whether an esterification or transesterification intermediate, is functionalized with a terminal alcohol group (e.g., not the terminal carboxylic ester group of FDCA, or the terminal methyl group of FDME) and can therefore then undergo polycondensation to provide copolymers as described herein, and particularly poly(alkylene furanate) polymers. Thus, methods for producing polyester polymers (e.g., copolyesters) may include a first esterification or transesterification step to produce an intermediate (prepolymer), followed by a second polycondensation step. The first step may be catalyzed by an esterification / transesterification catalyst at a temperature of about 150°C (302°F) to about 250°C (482°F) and carried out until the concentration of the starting monomer or ester derivative monomer is reduced to less than about 3 mol-%. The catalyst may comprise an organotin (IV) compound present in the polymer-forming reaction mixture at a concentration of about 0.01 mol-% to about 0.2 mol-% relative to the monomer or ester derivative. Thus, the prepolymer as described herein may be the product of the reaction of two diol monomers and one monomer carrying a furanyl dicarboxylate moiety ultimately present in the backbone of the resulting polymer.

[0053] Other diols of interest for forming intermediates (prepolymers) include those that can be bio-derived, such as FDCA, as in the case of isohexol. These compounds are bicyclic rigid diols with different orientations of hydroxyl groups only at the C2 and C5 ring positions, and they can be obtained by ring dehydration of isohexol. For example, isomannitol can be obtained from mannitol (inner-inner), isosorbide (outer-inner) from sorbitol, and isoidide (outer-outer) from idoterol. Regardless of the specific diol, the intermediate (prepolymer) formed can optionally be separated from the reaction mixture of the first reaction step, although this is usually not necessary. The second polycondensation step can be catalyzed and carried out under reduced pressure (e.g., 100 Pa or less), at a temperature in the range from the melting point of the resulting copolymer to about 30 °C (54 °F) above this temperature, and preferably at a temperature of at least about 180 °C (356 °F). Polycondensation catalysts can contain tin(II) compounds, such as tin(II) oxide or organotin(II) compounds. Alternatively, titanium-based catalysts, such as titanium or chelated titanium compounds, can be used, which have various ligands that can include alkoxides, such as propoxides or tert-butoxides. Therefore, representative catalysts are titanium propoxide (IV) and titanium tert-butoxide (IV).

[0054] FDCA or its derivatives can also be used to form polymers other than polyester polymers. For example, polyamide polymers can be formed by reacting FDCA or its derivatives with a comonomer (e.g., a diamine) having at least two amino groups to produce a polyamide having a moiety associated with FDCA. Suitable comonomers include aliphatic diamines such as hexamethylenediamine and aromatic diamines such as p-phenylenediamine. Similarly, as described above, FDCA derivatives can be used to form polyester polymers. For example, polyester polymers can be formed by reacting a hydroxyl (alcohol) derivative of FDCA (e.g., a diol derivative of FDCA) as a comonomer with a polybasic acid (e.g., FDCA) to produce a polyester having a moiety associated with FDCA. Suitable hydroxyl derivative comonomers of FDCA include furan 2,5-diol and furan 2,5-diethanol.

[0055] For example, FDCA or its derivatives can be used to form polyurethane polymers by reacting a hydroxyl (alcohol) derivative (e.g., a diol derivative) of FDCA as a comonomer with a polyisocyanate to produce a polyurethane having an FDCA-related moiety. Suitable comonomers for hydroxyl derivatives of FDCA include those described above with respect to the formation of polyester polymers. Suitable polyisocyanates include diisocyanates, and in particular aromatic diisocyanates such as toluene diisocyanate, methylene diphenyl diisocyanate, and polymeric methylene diisocyanate. In other embodiments, polyamide polymers can be formed by reacting an amino derivative (e.g., a diamino derivative) of FDCA as a comonomer with a polybasic acid (e.g., FDCA) to produce a polyamide having an FDCA-related moiety. Suitable comonomers for amino derivatives of FDCA include furan 2,5-diamines and furan 2,5-dialkylamines, such as furan 2,5-dimethylamine. In other embodiments, the polyamide polymer can be formed by reacting an acyl derivative of FDCA (e.g., a diacyl chloride derivative of FDCA) with a comonomer (e.g., a diamine) having at least two amino groups to produce a polyamide having an FDCA-related moiety. Suitable acyl derivatives of FDCA include furan 2,5-dicarboxyl chloride and furan 2,5-dialkyl chlorides, such as furan 2,5-diacetyl chloride. Suitable comonomers include aliphatic diamines such as hexamethylenediamine and aromatic diamines such as p-phenylenediamine. In other embodiments, the polyurethane polymer can be formed by reacting an isocyanate derivative of FDCA (e.g., a diisocyanate derivative of FDCA) alone or in combination with a comonomer (e.g., a diol) having at least two hydroxyl groups to produce a polyurethane having an FDCA-related moiety. Suitable isocyanate derivatives of FDCA include furan 2,5-diisocyanates and furan 2,5-dialkyl isocyanates, such as furan 2,5-dimethyl isocyanate. Suitable comonomers include diols, such as those described above for forming intermediates (prepolymers) used in the final production of polyester polymers. In other embodiments, the polycarbonate polymer can be formed by reacting a hydroxyl (alcohol) derivative of FDCA (e.g., a diol derivative of FDCA) with phosgene to produce a polycarbonate having a moiety associated with FDCA. Suitable hydroxyl derivatives of FDCA include those described above for the formation of comonomer polyester polymers.

[0056] As described above, the benefits obtained from modifying humic acid in oxidative pretreatment, and especially those resulting from improved properties such as color reduction and increased color stability, can be advantageously extended to the downstream products described herein, including monomeric compositions containing FDCA, compositions containing esterified derivatives of FDCA (e.g., FDME), compositions containing prepolymers (e.g., esterified and transesterified derivatives of FDCA), and bio-based copolymers.

[0057] The following examples are presented as representative of the invention. These examples are illustrative and are not to be construed as limiting the scope of the invention as defined by the appended claims.

[0058] Example

[0059] The effect of oxidative pretreatment on the color of dehydrated products

[0060] Experiments were conducted to evaluate the effects of treating the product with various oxidants, which was obtained by dehydrating fructose using a dehydration feed having 10 wt% dry solids. These oxidants and their amounts, expressed as molar percentages and molar oxidative equivalents relative to the furan compounds forming FDCA in the dehydrated product (or relative to the fructose sugar in the dehydrated feed), are summarized in Table 1 below.

[0061] Table 1 - Oxidizing Agents and Mole Percentages

[0062]

[0063] In each case, the oxidant was added in water to 1–2 ml of the dehydrated product in a 20 ml scintillation vial, and the contents were stirred at room temperature for 2–3 hours with a 5 mol-% loading sample, and overnight with a 10 mol-% loading sample at room temperature. Following oxidation at these time intervals, humin and “pathway” furans, or FDCA-forming furans HMF, 5-(acetoxymethyl)furfural, and HMF dimers, were analyzed in the samples. Based on these analyses, the yields of humin and FDCA-forming furans were minimally affected. Even for the 10 mol-% loading sample, approximately 80–95 wt% of the initial amount of humin was retained after oxidation. These yields were even higher for the 5 mol-% loading sample. Analysis of the time intervals before and after oxidation confirmed a humin yield of 100 wt% in the control sample without the oxidant. Furthermore, in all cases, the yield of furan compounds forming FDCA was close to 100 mol-%, and analysis of control samples confirmed that the yield of furan compounds forming FDCA was also 100 mol-%.

[0064] The absorption spectra of each of these samples in the UV and visible wavelength range of 360 nm to 710 nm were evaluated compared to the control samples, and it was shown that absorption in these wavelengths was generally reduced, corresponding to typically lighter colors. The absorbance of the 10 mol-% loaded samples was specifically measured at 460 nm, and the results showed absorbance reductions as low as approximately 42% of the control samples, representing untreated dehydration products or starting compositions. The largest reduction in absorbance was achieved using KBrO3 as the oxidant. However, absorbance values ​​of 80% or lower were generally obtained relative to the control samples, and in some cases, absorbance values ​​of 60% or lower were obtained. The change in absorbance at 460 nm per mol-% oxidation equivalent was also evaluated. Based on this, hydrogen peroxide (H₂O₂) and mCPBA (m-chloroperoxybenzoic acid) were the most effective oxidants.

[0065] Bromine source as oxidant

[0066] Molecular bromine (Br2) is a bromine that produces Br in aqueous solution. - The bromine source of the ion was also investigated, and its effect on the color of the dehydrated product as described above, i.e., the product obtained from the dehydration of fructose using a dehydration feed having 10 wt.% dry solids. In each case, different amounts of this bromine source / oxidant were added in water to 1–2 ml of the dehydrated product in a 20 ml scintillation flask, and the contents were stirred overnight at room temperature. These different amounts corresponded to different molar oxidation equivalent percentages relative to the furan compounds that form FDCA in the dehydrated product (or relative to the fructose in the dehydrated feed). Using an oxidation equivalent of up to 50 mol.% resulted in minimal impact on the humin yield, as the initial amount of humin remained above 95 wt.% after oxidation. Furthermore, at this loading, the yield of furan compounds that form FDCA exceeded 80 mol.%, and in the case of the pretreated dehydrated product, the absorption of light at a wavelength of 460 nm was approximately 60% of that measured in the initial dehydrated product. At a loading of approximately 90 mol-% oxidative equivalent, the yields of humin and the furan compounds forming FDCA decreased to approximately 95 wt-% and less than 60 mol-%, respectively. However, the absorbed light at a wavelength of 460 nm was approximately 30% of that measured for the initial dehydration product. Therefore, it can be understood that Br - Sources can be effective oxidants / bleaching agents, as long as they can significantly reduce the color of the pretreated dehydrated product, with only a moderate or negligible effect on the amount of furan compounds present in the product that form FDCA. Large amounts of humin are also largely retained. Unbound by theory, it is believed that Br in aqueous solution... -The source can be oxidized to Br2 at some stage of the oxidation pretreatment / bleaching, and this oxidized form can then contribute to the selective bleaching of humulin.

[0067] Hydrogen peroxide, as an oxidant, is optionally used in conjunction with a catalyst and / or acid.

[0068] The study specifically investigated the effect of the oxidant hydrogen peroxide (H2O2) on the color of the dehydration product as described above, with and without the addition of catalytic metals Co and / or Mn and / or the addition of nitric acid (HNO3). The dehydration product was obtained from fructose dehydration using a feed containing 10 wt% dry solids. In the absence of any catalyst or acid, in each case, 1-2 ml of the dehydration product in 20 ml scintillation bottles was added in water at amounts of 10 mol%, 15 mol%, 20 mol%, 30 mol%, and 40 mol% relative to the furan compounds forming FDCA in the dehydration product (or relative to the fructose in the dehydration feed), and the contents were kept at room temperature for 3 days. The preparation of these samples was repeated, with a separate oxidation pretreatment step performed at 50 °C for 4 hours. Based on visual observation, in both room temperature and high temperature experiments, the pretreated dehydration product exhibited a gradually lighter color with increasing H2O2 loading. Furthermore, under a loading of 30 mol-% H₂O₂, the yields of humicin and the furan compounds forming FDCA were favorable in both room temperature and high temperature experiments, at 80 wt-% and 90 mol-% or higher, respectively, relative to the initial dehydration product. Additionally, under this loading, the absorption of light at a wavelength of 460 nm for the pretreated dehydration product obtained after oxidation pretreatment at room temperature and 50 °C was approximately 36% of the absorption measured for the initial dehydration product in each case. At a loading of 40 mol-% H₂O₂, the yield loss of the furan compounds forming FDCA became more pronounced.

[0069] Based on these results, a loading of 30 mol-% H₂O₂ was chosen for further investigation of the effects of adding catalytic metals Co and / or Mn and / or adding nitric acid. The metals were added in the form of their soluble acetates. Specifically, according to further experiments, in each case, 30 mol-% H₂O₂ in water was added to 1-2 ml of the dehydration product in a 20 ml scintillation flask as a percentage of the furan compounds forming FDCA in the dehydration product (or as a percentage of the fructose in the dehydration feed). In addition to this loading of H₂O₂, the contents of the individual vials were added as follows: (i) 2 mol% Co relative to the furans forming FDCA, (ii) 2 mol% Mn relative to the furans forming FDCA, (iii) 2 mol% each of Co and Mn relative to the furans forming FDCA, (iv) 0.5 mol% HNO₃ relative to the furans forming FDCA, (v) 2 mol% and 0.5 mol% Co and HNO₃ relative to the furans forming FDCA, (vi) 2 mol% and 0.5 mol% Mn and HNO₃ relative to the furans forming FDCA, and (vii) 2 mol% each of Co and Mn relative to the furans forming FDCA, and 0.5 mol% HNO₃ relative to the furans forming FDCA. In each case, the contents of these vials were stirred overnight at room temperature. Based on visual observation, the colors of vials (i)-(vii) were all lighter than those of control vials containing dehydration products that were kept under the same conditions but without the addition of H2O2, catalysts or metals.

[0070] Further experimental investigations investigated the effect of different loadings of H₂O₂ combined with 2 mol% Co relative to the FDCA-forming furan compounds on the color of the dehydration product obtained from fructose dehydration using a feed containing 10 wt% dry solids, as described above. In each case, the oxidant and catalyst were added in water to 1–2 ml of the dehydration product in a 20 ml scintillation flask, and the contents were maintained at room temperature for 3 days. Co was added in the form of its soluble acetate. At a loading of 20 mol% H₂O₂, the yields of humin and the FDCA-forming furan compounds were favorable, at 80 wt% and 95 mol% or higher, respectively, relative to the initial dehydration product. Furthermore, at this loading, the absorption of light at a wavelength of 460 nm was reduced to approximately 40% of the absorption measured for the initial dehydration product for the pretreated dehydration product. Under a loading of 40 mol-% H2O2, the yield of furan compounds forming FDCA still exceeded 95 mol-%, and for the pretreated dehydrated product, the absorption of light at a wavelength of 460 nm was reduced to about 20% of the absorption measured for the initial dehydrated product.

[0071] Dehydration products obtained from dehydration feeds with high dry solids content

[0072] Further experiments were conducted to investigate the effect of hydrogen peroxide on the color of the dehydration product, in this case obtained from fructose dehydration using a feedstock with 20 wt% dry solids. This higher dry solids content, compared to studies in other experiments, resulted in the use of a larger amount of H₂O₂, up to 200 mol% relative to the furans forming FDCA in the dehydration product (or relative to fructose in the dehydration feed). In each case, the oxidant was added in water to 1–2 ml of the dehydration product in a 20 ml scintillation flask, and the contents were maintained at room temperature for 3 days. At loadings close to 50 mol% H₂O₂, the yields of humin and the furans forming FDCA were favorable, at 95 wt% and 85 mol% or higher, respectively, relative to the initial dehydration product. Furthermore, at these loadings, the absorption of light at a wavelength of 460 nm was reduced to approximately 60% of the absorption measured for the initial dehydration product. Under a loading of 80-100 mol-% H2O2, the yield loss of furan compounds forming FDCA becomes more pronounced.

[0073] Further experimental studies investigated the effect of different loadings of H₂O₂ combined with 20 mol% Co relative to the furan compounds forming FDCA on the color of the dehydration product as described above, obtained from a feed with a high dry solids content, i.e., the product obtained from the dehydration of fructose using a dehydration feed with 20 wt% dry solids. Again, this higher dry solids content, compared to studies in other experiments, resulted in the use of a larger amount of H₂O₂, up to 200 mol% relative to the furan compounds forming FDCA in the dehydration product (or relative to the fructose in the dehydration feed). In each case, the oxidant and catalyst were added in water to 1–2 ml of the dehydration product in a 20 ml scintillation flask, and the contents were kept at room temperature for a period of 3 days. Co was added in the form of its soluble acetate. At a loading of approximately 50 mol-% H₂O₂, the yields of humin and the furan compounds forming FDCA were favorable, at 80 wt-% and 95 mol-% or higher, respectively, relative to the initial dehydrated product. Furthermore, at this loading, the absorption of light at a wavelength of 460 nm for the pretreated dehydrated product decreased to approximately 32% of the absorption measured for the initial dehydrated product. At a loading of 100 mol-% H₂O₂, the yield of the furan compounds forming FDCA was close to 90 mol-%, and the absorption of light at a wavelength of 460 nm for the pretreated dehydrated product decreased to approximately 18% of the absorption measured for the initial dehydrated product. In the case of obtaining a dehydrated product from fructose using a dehydration feed with 20 wt-% dry solids, oxidative pretreatment was found to reduce the molecular weight of humin, while this effect was not observed in experiments using a dehydrated product obtained from a dehydration feed with 10 wt-% dry solids.

[0074] Effect of oxidative pretreatment on subsequent oxidation of furan compounds that form FDCA

[0075] Experiments were conducted to evaluate the effect of oxidative pretreatment on the subsequent oxidation of FDCA-forming furans, namely "pathway" furans of HMF, 5-(acetoxymethyl)furfural, and HMF dimers, representing the products obtained from the dehydration of fructose in the presence of an acetic acid and aqueous solvent system. The untreated (baseline) and pretreated dehydrated products used in the comparative experiments each contained 8.5 wt% of these compounds in a solution containing this solvent system. In each case, homogeneous catalyst compositions containing cobalt, manganese, and bromine were charged into the oxidative reactors used in the experiments at concentrations of 1000 wt-ppm, 932 wt-ppm, and 1926 wt-ppm, respectively. The oxidation reaction conditions included a temperature of 180 °C and a total pressure of 15.2 bar (220 psi), with the feed rates of the dehydrated products and air to the oxidation reactor maintained at 0.9 mL / min and 550 sccm, respectively, for a target reaction time of 150 min. This target reaction time represents the maximum time the reaction can be carried out in each experiment, where shorter reaction times may result in “ignition” losses, as indicated by a decrease in oxygen consumption. During each reaction, the contents of the oxidation reactor were analyzed to determine the amount of FDCA present, as well as the amounts of intermediates that form FDCA, such as 5-hydroxymethyl-2-furanoic acid and 5-formyl-2-furanoic acid, which remain valuable reaction products in terms of their ability to undergo further oxidation to form FDCA.

[0076] Importantly, the baseline experiment lasted only about 120 minutes, at which point the reaction ignited and no further conversion occurred. In contrast, using the pretreated dehydrated product, the reaction could be sustained for almost the entire target reaction time of 150 minutes. Therefore, the use of this product advantageously resulted in a stabilizing effect on the oxidation of furan compounds that form FDCA to FDCA.

[0077] Results of the oxidant / condition investigation

[0078] Given these results, based on the retention of humulins before and after oxidation, the color characteristics of these contaminants appear to have been altered (by humulin “bleaching”) due to the presence of the studied chemical oxidant, even at substoichiometric levels. Oxidation conditions can be adjusted to achieve selective oxidation of humulin functional groups relative to furan compounds that form FDCA, with minimal impact on humulin content in many cases. Furthermore, for a given system, the oxidation (bleaching) rate and the oxidation selectivity for humulin functional groups can be increased by using catalysts such as soluble Co and / or Mn. Increasing the temperature can also be used to accelerate oxidation. Moreover, significantly higher oxidant loadings are required for the efficient oxidation of higher molecular weight humulins present in higher amounts in dehydrated products obtained from feeds with higher dry solids content. Furthermore, an oxidation pretreatment step using a 1 mol-% Co-Mn-HNO3 catalyst system and 30 mol-% H2O2, where the molar percentages are relative to the furan compounds forming FDCA, was selected for scale-up and treatment of 500 g of dehydrated product obtained from fructose dehydration using a dehydration feed having 10 wt-% dry solids. The efficacy of this catalyst system was thus demonstrated on a pilot / demonstration scale.

[0079] Based on the knowledge gained from this disclosure, those skilled in the art can effectively adjust conditions and catalysts when using a wide range of oxidants to achieve selective oxidation / bleaching of humulants, thereby obtaining the advantages taught herein, such as improved stability and other beneficial effects when oxidizing furans that form FDCA to FDCA. It will also be appreciated that various modifications can be made to the oxidation pretreatment process steps, oxidants, catalysts, and conditions to obtain these and other advantages without departing from the scope of this disclosure.

Claims

1. A method for pretreating dehydrated products of one or more carbohydrates having 6-carbon sugar units, wherein the carbohydrates having 6-carbon sugar units are selected from hexoses and polymers of hexoses, the method comprising: In the oxidative pretreatment step, the dehydrated product is contacted with an oxidant to provide a pretreated dehydrated product having improved properties due to modification of the humicin present in the dehydrated product. The oxidant mentioned is H2O2. In the aforementioned oxidative pretreatment step, selective modification of humin is performed, while the furan compounds forming FDCA are substantially not converted. Furthermore, the molar number of FDCA-forming furan compounds recovered from the pretreated dehydration product is at least 80% of the FDCA-forming furan compounds present in the dehydration product before oxidative pretreatment. These FDCA-forming furan compounds are selected from 5-(hydroxymethyl)furfural (HMF), 5-(acetoxymethyl)furfural, 5-(methoxymethyl)furfural, 2,5-diformylfuran, 5-formyl-2-furancarboxylic acid, and / or HMF dimers. The improved characteristics include reduced absorption of ultraviolet or visible light radiation, reduced color of downstream products, reduced average molecular weight of humic acid, increased amount of precipitated humic acid, or increased stability as an oxidative feedstock for the production of FDCA.

2. The method as described in claim 1, wherein, The improved property is a reduction in the absorption of light at a wavelength of 460 nm.

3. The method as described in claim 2, wherein, The absorption of light at a wavelength of 460 nm is 60% or less relative to the dehydrated product.

4. The method of claim 1, wherein, In the oxidation pretreatment step, the oxidant is present or added in an amount of 5 mol% to 70 mol% of the oxidation equivalent relative to the furan compounds that form FDCA in the dehydration product.

5. The method of claim 1, wherein, In the oxidation pretreatment step, the average residence time is at least 1 hour.

6. The method of claim 1, wherein, In the oxidation pretreatment step, the temperature is between 20°C and 120°C.

7. The method of claim 1, wherein, In the oxidation pretreatment step, the dehydration product and the oxidant are brought into contact in the presence of a catalyst.

8. The method of claim 7, wherein, The catalyst is selected from Co and / or Mn.

9. The method of claim 8, wherein, The Co and / or Mn are present independently in a certain amount, ranging from 0.1 mol% to 10 mol% of the furan compounds that form FDCA in the dehydration product, or in a combined amount.

10. The method of claim 7, wherein, The catalyst is an acid.

11. The method of claim 10, wherein, The acid is present in an amount of 0.05 mol% to 5 mol% of the furan compounds that form FDCA in the dehydration product.

12. The method according to any one of claims 1-11, wherein, The carbohydrate having a 6-carbon sugar unit is an oligomer of a 6-carbon sugar.

13. A method comprising the method of any one of claims 1-12, and further comprising: In the oxidation step, the pretreated dehydrated product is contacted with an oxidation catalyst in the presence of oxygen to provide a composition containing FDCA. In the oxidation step, based on the formation of furan compounds of FDCA in the pretreated dehydration product, the yield of FDCA is at least 60 mol.

14. The method of claim 13, wherein, The oxidation pretreatment temperature used in the oxidation pretreatment step is lower than the oxidation temperature used in the oxidation step.

15. The method of claim 14, wherein, The oxidation pretreatment temperature is between 20°C and 120°C.

16. A method comprising the method of any one of claims 1-12, and further comprising: In the oxidation step, the pretreated dehydrated product is contacted with an oxidation catalyst in the presence of oxygen to provide a composition containing FDCA. The amount of the oxidant added or present, relative to the furan compounds that form FDCA in the dehydration product, is adjusted based on the dry solids content of the dehydration feed that has undergone a dehydration step to obtain the dehydration product or based on the humin content of the dehydration product, wherein the dry solids content of the dehydration feed is the weight percentage of carbohydrates with 6-carbon sugar units used in the dehydration feed.

17. The method of claim 16, wherein, The dry solids content of the dehydrated feed is 5 wt% to 35 wt%.

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