A method for preparing 5-hydroxymethylfurfural and 2,5-furandicarboxylic acid

Abstract

The invention discloses a method for preparing 5-hydroxymethylfurfural and 2,5-furandicarboxylic acid. The method for preparing 5-hydroxymethylfurfural includes: in a two-liquid phase reaction system consisting of a weak acid with a ring structure, water, an organic ammonium chloride and a low-boiling point polar organic solvent, a fructose-based carbohydrate is subjected to an intramolecular dehydration reaction in a reactor to obtain 5-hydroxymethylfurfural. After the reaction is completed, the target product can be extracted into the organic phase in situ, and the acid catalyst is retained in a mixed solution of water and the organic ammonium chloride, which greatly reduces the loss caused by the volatilization of HCl generated by the catalyst protons and chloride ions and the dissolution of the catalyst in the organic solvent. In addition, the present invention can also effectively reduce the content of humin impurities in the organic phase, and realize the in-situ separation of the target product, the catalyst and by-products.

Description

Technical Field

[0001] The invention belongs to the field of oxygen-containing compounds, and particularly relates to a preparation method of 5-hydroxymethylfurfural and 2,5-furandicarboxylic acid. Background Art

[0002] 5-Hydroxymethylfurfural (HMF) is a high-value-added biomass-based platform molecule with reactive groups such as aldehyde and hydroxyl groups. It can undergo reactions such as hydrogenation, oxidative dehydrogenation, esterification, halogenation, polymerization, and hydrolysis. It is used in the synthesis of important materials such as polymers, pharmaceuticals, resins, plastics, and fuel additives. Currently, HMF is primarily produced through the direct dehydration of carbohydrates, such as fructose, catalyzed by protonic acids. HMF can be further oxidized to produce 2,5-furandicarboxylic acid, which is also an important basic organic chemical raw material.

[0003] Aqueous solutions are ideal media for fructose dehydration reactions, but they require relatively high temperatures. Fructose and the target product are extremely unstable in high-temperature aqueous conditions, resulting in extremely low HMF yields. To improve the yield of the target product, researchers have begun using novel reaction systems such as water-organic solvents, ionic liquids, and deep eutectic solvents. These coupled with highly acidic inorganic homogeneous acid catalysts (such as hydrochloric acid, sulfuric acid, nitric acid, and hydroiodic acid) can achieve higher HMF yields. However, these novel reaction systems all require chloride salts as auxiliary agents. For example, in water-organic solvent two-phase reactions, NaCl is required to reduce the solubility of water in the organic solvent (Top Catal (2009) 52:297–303). Ionic liquids (e.g., imidazolium chloride, Journal of Molecular Catalysis A: Chemical 253 (2006) 165–169) and deep eutectic solvents (choline chloride, Industrial Crops and Products 99 (2017) 1–6) are typical chloride compounds themselves. When common inorganic strong acids are used in these reaction systems, protons react with chloride ions in the reaction medium to form HCl. Under high temperature conditions, some HCl volatilizes, causing not only pipeline corrosion but also catalyst loss. Furthermore, common strong inorganic homogeneous acid catalysts are difficult to separate from the reaction medium and target product in these reaction systems. Although methods such as spiral diffusion dialysis can remove more than 90% of the homogeneous acid catalyst (Carbohydrate Polymers 142 (2016) 177–182), removing the remaining small amount of homogeneous acid catalyst requires more complex methods, reducing the economic efficiency of the HMF production process.

[0004] In order to avoid the problems of catalyst loss and residue in the above process and further improve the feasibility and economy of the HMF production process, it is still necessary to optimize the use of acid catalysts and propose a method for the continuous preparation of HMF and 2,5-furandicarboxylic acid with high selectivity and high yield. Summary of the Invention

[0005] The purpose of the present invention is to overcome the above-mentioned shortcomings of the prior art and provide a method for preparing HMF by dehydrating fructosyl carbohydrates using a weak organic acid as a catalyst, which can reduce catalyst loss and improve the purity of HMF.

[0006] The present invention also provides a method for preparing 2,5-furandicarboxylic acid by using HMF.

[0007] The present invention provides a method for preparing HMF using a weak organic acid as a catalyst, comprising: in a two-liquid phase reaction system consisting of a weak acid having a cyclic structure, water, an organic ammonium chloride salt, and a low-boiling-point polar organic solvent, causing fructose-based carbohydrates to undergo an intramolecular dehydration reaction in a reactor to obtain HMF.

[0008] According to the method of the present invention, after the reaction is completed, the target product can be extracted into the organic phase in situ, and the acid catalyst is retained in the mixed solution of water and organic ammonium chloride. The aqueous phase and the organic phase can be directly separated to achieve effective separation of the target product HMF and the homogeneous acid catalyst, thereby improving the purity of HMF.

[0009] The weak acid having a cyclic structure is preferably a carboxylic acid having a furan ring or a benzene ring structure, such as 2,5-furandicarboxylic acid, furanoic acid, benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, etc., and 2,5-furandicarboxylic acid is most preferred.

[0010] The low-boiling-point polar organic solvent is selected from acetone, butanone, methyl isobutyl ketone, 1,4-dioxane, tetrahydrofuran, acetonitrile, etc., preferably one or more of 1,4-dioxane, tetrahydrofuran, and acetonitrile.

[0011] The organic ammonium chloride is selected from organic quaternary ammonium chloride with a short carbon chain of C1-C6, preferably C1-C4 hydrocarbon trimethyl quaternary ammonium chloride, such as one or more of tetramethylammonium chloride, choline chloride, choline chloride, allyltrimethylammonium chloride, and butyltrimethylammonium chloride, and tetramethylammonium chloride is most preferred.

[0012] The mass ratio of the weak acid having a cyclic structure to water is 0.1:1-1:1, preferably 0.3:1-1:1.

[0013] The mass ratio of the organic quaternary ammonium chloride to water is 0.1:1-5:1, preferably 1.5:1-3:1.

[0014] In the reaction process of the present invention, the cyclic weak acid serves as both a catalyst and a part of the reaction medium. The cyclic weak acid and water, which have carboxyl and hydroxyl groups, respectively, serve as hydrogen bond donors, while the organic ammonium chloride serves as a hydrogen bond acceptor. When mixed, the three form a deep eutectic solvent that is a double hydrogen bond donor.

[0015] The volume proportion of the deep eutectic solvent of the double hydrogen bond donor in the two-liquid phase reaction system is 5%-50%, preferably 10%-25%.

[0016] The fructosyl carbohydrate is selected from one or more of purified fructose, crude fructose, polyfructose, fructose syrup and fructose-glucose syrup.

[0017] The mass proportion of the fructose-based carbohydrate in the two-liquid phase reaction system is 1%-15%, preferably 3%-8%.

[0018] The temperature of the fructose-based carbohydrate dehydration reaction is 90° C.-160° C., more preferably 120° C.-140° C.; the time of the fructose dehydration reaction is 0.1-12 hours, preferably 0.2-5 hours, more preferably 0.2-1 hour.

[0019] According to the method of the present invention, the fructose dehydration reaction is stirred during the reaction. The reaction does not require separate control of the reaction pressure and can be carried out under autogenous pressure in a closed reactor at the temperature.

[0020] The reactor required for the fructose dehydration reaction process can be a thick-walled pressure-resistant bottle, a stainless steel reactor with a polytetrafluoroethylene lining, and other reactors that can be easily imagined by those skilled in the art.

[0021] After the fructose dehydration reaction is completed, the conversion rate of the reaction substrate, the selectivity and yield of HMF during the reaction process can be analyzed and calculated by high performance liquid chromatography.

[0022] In the reaction process of the present invention, on the one hand, the acidic group of the weak acid with a cyclic structure is a carboxyl group, which is weaker than common inorganic acids such as sulfuric acid and hydrochloric acid, so its proton will not volatilize with the chloride ions in the reaction medium to form HCl, thereby reducing the loss of catalyst overflow. On the other hand, the weak acid with a cyclic structure, water, and organic ammonium chloride form a deep eutectic solvent of a double hydrogen bond donor, which has a rich hydrogen bond network structure, thereby enhancing the immobilization effect of the weak acid with a cyclic structure in the deep eutectic solvent, reducing its solubility in low-boiling point polar organic solvents, further reducing the loss of the catalyst due to dissolution in the organic phase, and greatly improving the recovery rate of the catalyst.

[0023] According to the method of the present invention, if the ultimate goal of the reaction is to produce 2,5-furandicarboxylic acid from HMF, 2,5-furandicarboxylic acid is preferably used as a catalyst. After the reaction is completed, HMF is extracted in situ into the organic phase, which inevitably still contains a small amount of catalyst impurities. In this case, an oxidation catalyst can be used to further oxidize HMF to 2,5-furandicarboxylic acid. This means that the initially added catalyst and the final target product are the same substance and can be simultaneously precipitated after solvent evaporation. This eliminates the need for complex and sophisticated acid separation operations, improving the feasibility and cost-effectiveness of the HMF preparation and reconversion process and simplifying the production process of 2,5-furandicarboxylic acid.

[0024] Based on this, the present invention provides a method for preparing 2,5-furandicarboxylic acid using HMF, comprising: preparing HMF according to the aforementioned method of the present invention, wherein 2,5-furandicarboxylic acid is used as a catalyst; after the reaction is completed, separating the organic phase and the aqueous phase, and oxidizing the HMF in the organic phase to 2,5-furandicarboxylic acid using an oxidation catalyst.

[0025] The catalyst can be selected from one or more of MnO2, Co3O4, Fe3O4 catalysts and the like.

[0026] In the oxidation reaction, oxygen or an oxygen-containing gas is used as the oxidant, the oxygen partial pressure is 0.1-5 MPa, preferably 1-3 MPa, the mass ratio of the oxidation catalyst to the organic phase is 0.01:1-0.1:1, preferably 0.02:1-0.05:1, the oxidation temperature is 110° C.-180° C., preferably 130° C.-150° C., and the oxidation time is 2-12 hours, preferably 6-8 hours. Additional solvent may or may not be added during the oxidation reaction.

[0027] During the reaction process of the present invention, due to the instability of HMF and other furan ring intermediates, humin impurities are generated through condensation reactions, and the reaction solution is brown. Usually, humin is mainly present in the organic phase. After the reaction, the organic phase needs to be further separated and purified to obtain a higher purity product HMF. The inventors of the present application unexpectedly discovered that according to the method of the present invention, a large amount of humin can be separated in the low eutectic solvent of the above-mentioned double hydrogen bond donor (i.e., the aqueous phase, specifically a mixed solvent of water-organic ammonium chloride-2,5-furandicarboxylic acid), thereby reducing the concentration of humin impurities in the organic phase, improving the purity of HMF, and realizing the in situ separation of HMF and humin impurities.

[0028] Therefore, the present invention has the following advantages:

[0029] (1) The acidity of the weak acid with a ring structure is weak, which prevents its protons from reacting with chloride ions in the reaction medium to form HCl volatilization, thereby reducing the overflow loss of the catalyst.

[0030] (2) Weak acids with cyclic structures can also act as hydrogen bond donors, forming a rich network structure in the reaction system, thereby enhancing their immobilization effect in the deep eutectic solvent, improving the recovery rate of the catalyst in the deep eutectic solvent, and further reducing the loss of the catalyst caused by dissolution in the organic phase.

[0031] (3) After the reaction, the HMF extracted from the organic phase can be further oxidized to 2,5-furandicarboxylic acid. The residual 2,5-furandicarboxylic acid catalyst can be precipitated together with the product after evaporation of the solvent, avoiding the complex and delicate acid separation process and improving the feasibility and economy of the reaction process.

[0032] (4) After the reaction, the low eutectic solvent has a better adsorption capacity for humin impurities, thereby reducing the impurity content in the organic phase and improving the purity of the target product HMF. DETAILED DESCRIPTION

[0033] The present invention will be further described below by way of examples.

[0034] The fructose in the examples is purified crystalline fructose purchased from TCI (Shanghai) Chemical Industry Development Co., Ltd.

[0035] Other raw materials are commercially available.

[0036] Example 1

[0037] To a 15 mL pressure flask, 0.5 g of fructose, 0.3 g of 2,5-furandicarboxylic acid, 0.5 g of water, 1.0 g of tetramethylammonium chloride, and 8.5 mL of 1,4-dioxane were added. The volume ratio of the deep eutectic solvent formed in the lower layer of the two-liquid phase system was 15%. After heating the multichannel heater to 140°C, the sealed pressure flask was placed in the multichannel heater and stirred at 600 rpm for 20 minutes. After the reaction was completed, the pressure flask was removed from the multichannel heater and naturally cooled to room temperature. The reaction solution was analyzed by high-performance liquid chromatography, which determined that the fructose conversion was 99.6% and the HMF yield was 85.5%.

[0038] Example 2

[0039] The reaction was carried out according to Example 1, except that the pressure bottle was not sealed. When the volume of the solution in the pressure bottle was reduced to half during the heating process, the pressure bottle was removed and naturally cooled to room temperature. The residual 2,5-furandicarboxylic acid content in the pressure bottle was analyzed by high performance liquid chromatography, and the catalyst recovery rate was calculated to be 100% based on the initial charge amount.

[0040] Example 3

[0041] The reaction was carried out according to Example 1. After the reaction, the two phases were separated. The 2,5-furandicarboxylic acid content in the lower deep eutectic solvent was analyzed by high performance liquid chromatography. The catalyst recovery rate in the deep eutectic solvent was calculated to be 99.0% based on the initial charge amount.

[0042] Example 4

[0043] The reaction was carried out according to Example 1. After completion of the reaction, the two phases were separated, and 2 g of activated carbon was added to each phase and stirred at room temperature for 5 hours to adsorb the humin impurities in the two phases. The mixture was then filtered through filter membranes. After drying, the masses of humin in the upper and lower phases were calculated by differential gravimetry to be 0.019 g and 0.035 g, respectively. The mass of humin in the lower deep eutectic solvent layer accounted for 64.8% of the total mass of humin.

[0044] Example 5

[0045] The reaction and separation were carried out according to Examples 1-4, except that 2,5-furandicarboxylic acid was replaced with an equal molar amount of terephthalic acid. At the completion of the reaction, the fructose conversion was 99.5%, the HMF yield was 85.3%, the catalyst recovery in the lower deep eutectic solvent was 99.0%, and the humin retention rate was 65.0%. After solvent evaporation, the catalyst recovery rate was 100.0%.

[0046] Example 6

[0047] The reaction was carried out according to Example 1, except that the catalyst dosage was increased to 0.4 g, the reaction temperature was lowered to 130° C., and the reaction time was extended to 35 minutes. The fructose conversion rate was determined to be 99.5%, and the HMF yield was 85.2%.

[0048] Example 7

[0049] The reaction was carried out according to Example 1, except that the catalyst dosage was increased to 0.5 g, the reaction temperature was lowered to 120° C., and the reaction time was extended to 50 minutes. The fructose conversion rate was determined to be 99.7%, and the HMF yield was 85.0%.

[0050] Example 8

[0051] The reaction was carried out in the same manner as in Example 1, except that the organic solvent was replaced with tetrahydrofuran. The fructose conversion rate was determined to be 99.5%, and the HMF yield was 84.9%.

[0052] Example 9

[0053] The reaction was carried out in the same manner as in Example 1, except that the organic solvent was replaced with acetonitrile. The fructose conversion rate was determined to be 99.8%, and the HMF yield was 85.7%.

[0054] Example 10

[0055] The reaction was carried out in the same manner as in Example 1, except that tetramethylammonium chloride was replaced with choline chloride. The fructose conversion rate was determined to be 99.5%, and the HMF yield was 85.3%.

[0056] Example 11

[0057] The reaction was carried out in the same manner as in Example 1, except that tetramethylammonium chloride was replaced with choline chloride. The fructose conversion rate was determined to be 99.6%, and the HMF yield was 85.3%.

[0058] Example 12

[0059] The reaction was carried out in the same manner as in Example 1, except that tetramethylammonium chloride was replaced with allyltrimethylammonium chloride. The fructose conversion rate was determined to be 99.5%, and the HMF yield was 85.1%.

[0060] Example 13

[0061] The reaction was carried out in the same manner as in Example 1, except that tetramethylammonium chloride was replaced with butyltrimethylammonium chloride. The fructose conversion rate was determined to be 99.4%, and the HMF yield was 85.0%.

[0062] Example 14

[0063] The reaction was carried out according to Example 1, except that the amount of tetramethylammonium chloride was reduced to 0.9 g, the water content was increased to 0.6 g, and the reaction time was extended to 40 minutes. The fructose conversion rate was determined to be 99.6%, and the HMF yield was 85.6%.

[0064] Example 15

[0065] The reaction was carried out according to Example 1, except that the water content was reduced to 0.4 g, the amount of tetramethylammonium chloride was increased to 1.1 g, and the reaction time was reduced to 12 minutes. The fructose conversion rate was determined to be 99.7%, and the HMF yield was 85.7%.

[0066] Example 16

[0067] The reaction was carried out according to Example 1, except that the volume ratio of the deep eutectic solvent in the two-liquid phase system was reduced to 10% and the reaction time was shortened to 15 minutes. The fructose conversion rate was determined to be 99.7% and the HMF yield was 85.7%.

[0068] Example 17

[0069] The reaction was carried out according to Example 1, except that the volume ratio of the deep eutectic solvent in the two-liquid phase system was increased to 25% and the reaction time was increased to 40 minutes. The fructose conversion rate was determined to be 99.6%, and the HMF yield was 85.6%.

[0070] Example 18

[0071] The reaction was carried out according to Example 1. After the reaction was completed, the two phases were separated and the upper organic phase was taken. The mass was about 9g and placed in a stainless steel reactor lined with polytetrafluoroethylene. 0.3g MnO2 catalyst was then added and the oxygen partial pressure was 2MPa. After sealing, the temperature was raised to 150°C using a heating jacket and the timing reaction was started. After reacting for 7 hours, the reactor was taken out and cooled to room temperature. The reaction solution was analyzed by high performance liquid chromatography. The conversion of HMF was 100.0% and the yield of 2,5-furandicarboxylic acid was 89.1%. The organic solvent in the reaction solution was removed by rotary evaporator, and white 2,5-furandicarboxylic acid was precipitated. The purity of the 2,5-furandicarboxylic acid prepared by nuclear magnetic analysis was 97.3%.

[0072] Example 19

[0073] Fructose dehydration reaction was carried out according to Example 5, and then HMF oxidation reaction was carried out according to Example 18, except that 0.4 g of Fe3O4 was used as a catalyst. The reaction was carried out at 140°C for 8 hours. After the reaction, the HMF conversion rate was 100.0%, the 2,5-furandicarboxylic acid yield was 89.3%, and the purity of 2,5-furandicarboxylic acid after evaporation of the organic solvent was 96.7%.

[0074] Comparative Example 1

[0075] The reaction was carried out according to Example 1, except that the 2,5-furandicarboxylic acid catalyst was replaced with sulfuric acid in an equal molar amount of protons. The reaction time was shortened to 15 minutes. The fructose conversion rate was determined to be 99.7%, and the HMF yield was 85.3%.

[0076] Comparative Example 2

[0077] The reaction was carried out according to Example 2, except that the 2,5-furandicarboxylic acid catalyst was replaced with an equiprotic molar amount of sulfuric acid. The residual sulfuric acid content in the pressure bottle was quantitatively analyzed using elemental analysis technology, and the catalyst recovery rate was calculated to be 97.5% based on the initial charge amount.

[0078] Comparative Example 3

[0079] The reaction was carried out according to Example 3, except that the 2,5-furandicarboxylic acid catalyst was replaced with an equiprotic molar amount of sulfuric acid. After the reaction, the two phases were separated, and the sulfuric acid content in the lower deep eutectic solvent was analyzed using elemental analysis. Based on the initial charge, the catalyst recovery rate in the deep eutectic solvent was calculated to be 95.0%.

[0080] Comparative Example 4

[0081] The reaction was carried out according to Example 1, except that the 2,5-furandicarboxylic acid catalyst was replaced with an equiprotic molar amount of sulfuric acid. After the reaction, the two phases were separated, and 2 g of activated carbon was added to each phase and stirred at room temperature for 5 hours to adsorb the humin impurity. The mixture was then filtered through a filter membrane, dried, and the mass of humin in each phase was calculated using differential gravimetry. The mass of humin adsorbed in the lower deep eutectic solvent was calculated to be 30.7% of the total mass of humin.

[0082] Comparative Example 5

[0083] The reaction was carried out according to Example 1, except that the 2,5-furandicarboxylic acid catalyst was replaced with an equiprotic molar amount of acetic acid. After the reaction, the two phases were separated, and 2 g of activated carbon was added to each phase and stirred at room temperature for 5 hours to adsorb the humin impurity. The mixture was then filtered through a filter membrane, dried, and the mass of humin in each phase was calculated using differential gravimetry. The mass of humin adsorbed in the lower deep eutectic solvent was calculated to be 33.2% of the total mass of humin.

[0084] Comparative Example 6

[0085] The reaction was carried out in the same manner as in Example 1, except that tetramethylammonium chloride was replaced with octyltrimethylammonium chloride. The fructose conversion rate was determined to be 95.5%, and the HMF yield was 72.2%.

[0086] Comparative Example 7

[0087] Fructose dehydration reaction was carried out according to Comparative Example 1, and then HMF oxidation reaction was carried out according to Example 18. The conversion rate of HMF was determined to be 95.2%, the yield of 2,5-furandicarboxylic acid was 76.2%, and the purity of 2,5-furandicarboxylic acid after evaporation of the organic solvent was 90.7%.

[0088] Comparing the results of Example 1 and Comparative Example 1, it can be seen that under the condition of equimolar proton amounts, the acidity of 2,5-furandicarboxylic acid is weaker than that of sulfuric acid, and therefore a longer time is required to achieve similar conversion rates and yields.

[0089] Comparing the results of Example 2 and Comparative Example 2 shows that because 2,5-furandicarboxylic acid is weaker in acidity than sulfuric acid, its protons do not react with chloride ions to form HCl, preventing catalyst loss due to HCl volatilization at high temperatures. However, sulfuric acid is more acidic, and its protons react with chloride ions to form HCl, which volatilizes at high temperatures, resulting in approximately 2.5% catalyst loss after each reaction.

[0090] Comparing the results of Example 3 and Comparative Example 3, it can be seen that after each reaction, the recovery rate of 2,5-furandicarboxylic acid in the deep eutectic solvent is 99.0%, while the recovery rate of sulfuric acid is lower, only 95.0%. This is because the carboxyl group of 2,5-furandicarboxylic acid and water can simultaneously act as hydrogen bond donors, forming a deep eutectic solvent with a double hydrogen bond donor with the hydrogen bond acceptor tetramethylammonium chloride. After switching to sulfuric acid, only water can now act as a hydrogen bond donor, thus forming a deep eutectic solvent with a single hydrogen bond donor. Because the hydrogen bond network in the double hydrogen bond deep eutectic solvent is richer, the immobilization effect of 2,5-furandicarboxylic acid in the deep eutectic solvent is stronger, resulting in a higher recovery rate in the deep eutectic solvent and less loss due to dissolution in the organic phase.

[0091] Comparing the results of Example 4 with those of Comparative Examples 4 and 5, it can be seen that after the catalyst was replaced by 2,5-furandicarboxylic acid with sulfuric acid and acetic acid, the adsorption capacity of the deep eutectic solvent formed by the catalyst-water-tetramethylammonium chloride for humin decreased significantly. Among them, 2,5-furandicarboxylic acid and acetic acid are both organic carboxylic acids, and the corresponding humin adsorption capacity is higher than that of the inorganic acid sulfuric acid, indicating that when a weak acid with a carboxylic acid structure is used as a component of the deep eutectic solvent, it is helpful for its adsorption and separation of humin. In addition, 2,5-furandicarboxylic acid with a furan ring structure has a better adsorption capacity for humin than acetic acid, and its corresponding deep eutectic solvent has a nearly doubled retention of humin, indicating that the ring-structured catalyst is more conducive to the adsorption of humin by the deep eutectic solvent, thereby reducing the transfer of humin to the organic phase and improving the purity of HMF in the organic phase.

[0092] Comparing the results of Examples 1-4 and 5 shows that the catalytic performance is essentially the same when the catalyst is replaced with terephthalic acid instead of 2,5-furandicarboxylic acid, demonstrating that the catalyst's cyclic structure is applicable to both furan and benzene rings. Although both catalysts achieve a 99.0% recovery rate in the lower eutectic solvent after the reaction, approximately 1.0% of the catalyst remains in the organic phase, forming an impurity in HMF. Furthermore, due to the low impurity content of the catalyst, a more complex method is required to completely separate it from HMF, reducing the economic efficiency of HMF production and separation and purification. If an oxidation catalyst such as MnO2 is used to further oxidize HMF to 2,5-furandicarboxylic acid, even if the initially added catalyst and the final target product are identical, they can be simultaneously precipitated upon solvent evaporation. This eliminates the need for complex and delicate acid separation procedures, improving the feasibility and economic efficiency of the HMF production and reconversion process. Therefore, 2,5-furandicarboxylic acid is the preferred cyclic carboxylic acid catalyst.

[0093] Comparing the results of Example 1 and Examples 6-7, it can be seen that when the amount of 2,5-furandicarboxylic acid added is increased, the reaction rate is accelerated and the reaction temperature can be gradually lowered to obtain similar fructose conversion rates and HMF yields.

[0094] Comparing the results of Example 1 with Examples 8-9 shows that the corresponding fructose conversion rate and HMF yield remain essentially unchanged when using different organic solvents, including 1,4-dioxane, tetrahydrofuran, and acetonitrile. Substituting other low-boiling-point organic solvents significantly reduces the fructose dehydration rate and HMF yield. Therefore, the preferred organic solvents in the two-phase reaction medium are 1,4-dioxane, tetrahydrofuran, and acetonitrile.

[0095] Comparing the results of Examples 1 and 10-13 with those of Comparative Example 6, it can be seen that when tetramethylammonium chloride is replaced with choline chloride, choline chloride (chlormequat chloride), allyltrimethylammonium chloride, and butyltrimethylammonium chloride, that is, when one of the methyl side chains (C1) of tetramethylammonium chloride is replaced with a long carbon chain side chain (C2-C4), the HMF yield decreases slightly, but the final HMF yield is still not less than 85%. When the substituent side chain is further extended (C8), the HMF yield decreases significantly. Furthermore, due to the increased carbon number of the side chain, the surface activity of the hydrogen bond acceptor of the deep eutectic solvent is enhanced. At this time, a two-liquid phase reaction system can no longer be formed, but a single-liquid phase reaction system is formed. Therefore, in situ separation of the target product HMF from the acid catalyst and humin impurities cannot be achieved.

[0096] Comparing the results of Example 1 and Examples 14-15, it can be seen that when the mass ratio of tetramethylammonium chloride to water is reduced to 1.5:1, the fructose dehydration rate slows down due to the increase in water content, and the time required to achieve the same reaction effect is prolonged. When the mass ratio of tetramethylammonium chloride to water is increased to 2.75:1, the fructose dehydration rate accelerates due to the decrease in water content, and the time required to achieve the same reaction effect is shortened.

[0097] Comparing the results of Example 1 and Examples 16-17, it can be seen that when the volume ratio of the deep eutectic solvent is reduced to 10%, the fructose dehydration rate is accelerated due to the reduced water content, and the time required to achieve the same reaction effect is shortened. When the volume ratio of the deep eutectic solvent is increased to 25%, the fructose dehydration rate decreases due to the increased water content, and the time required to achieve the same reaction effect is prolonged.

[0098] Comparing the results of Example 18 and Comparative Example 7, it can be seen that when 2,5-furandicarboxylic acid is used as the fructose dehydration catalyst, the HMF in the upper organic solvent can be directly oxidized to 2,5-furandicarboxylic acid using a common MnO2 oxidation catalyst after the reaction is completed. After evaporation of the solvent, the residual catalyst and product in the organic phase are precipitated simultaneously, without the need for an additional acid separation process. However, when the catalyst is replaced with sulfuric acid, more humin remains in the organic phase. Therefore, during the HMF oxidation reaction, humin easily deposits on the surface of the oxidation catalyst, resulting in reduced catalyst activity, HMF conversion rate, and 2,5-furandicarboxylic acid yield. In addition, due to the low sulfuric acid recovery rate, the residual sulfuric acid in the organic phase is more likely to cause a secondary condensation reaction of HMF to produce more humin, thereby further reducing the yield and purity of 2,5-furandicarboxylic acid.

[0099] Comparing the results of Examples 18 and 19 shows that when the catalyst was replaced with terephthalic acid instead of 2,5-furandicarboxylic acid, there was no significant change in the activity of either the fructose dehydration reaction or the HMF oxidation reaction. However, after the oxidation reaction was completed and the solvent was separated, the terephthalic acid remaining in the organic phase precipitated, resulting in a slight decrease in the purity of the resulting 2,5-furandicarboxylic acid. Therefore, 2,5-furandicarboxylic acid is the preferred catalyst for fructose dehydration.

[0100] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0101] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0102] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for preparing 5-hydroxymethylfurfural, comprising: In a two-liquid phase reaction system consisting of a weak acid with a cyclic structure, water, an organic ammonium chloride salt and a low-boiling point polar organic solvent, a fructose-based carbohydrate undergoes an intramolecular dehydration reaction in a reactor to obtain 5-hydroxymethylfurfural, wherein the weak acid with a cyclic structure is selected from 2,5-furandicarboxylic acid, furancarboxylic acid, benzoic acid, o- / m- / terephthalic acid, and the organic ammonium chloride salt is selected from tetramethylammonium chloride, choline chloride, choline chloride, allyltrimethylammonium chloride, butyltrimethyl One or more of ammonium chloride, the low boiling point polar organic solvent is selected from acetone, butanone, methyl isobutyl ketone, 1,4-dioxane, tetrahydrofuran, and acetonitrile, the weak acid having a cyclic structure and water form a deep eutectic solvent that is a double hydrogen bond donor with the organic ammonium chloride salt, the volume proportion of the deep eutectic solvent in the two-liquid phase reaction system is 5%-50%, the temperature of the fructose-based carbohydrate dehydration reaction is 120°C-140°C; and the fructose dehydration reaction time is 0.2-5 hours.

2. The method according to claim 1, wherein The mass ratio of the cyclic weak acid to water is 0.1:1-1:

1.

3. The method according to claim 1, wherein The mass ratio of the cyclic weak acid to water is 0.3:1-1:

1.

4. The method according to claim 1, wherein The mass ratio of the organic ammonium chloride to water is 0.1:1-5:

1.

5. The method according to claim 1, wherein The mass ratio of the organic ammonium chloride to water is 1.5:1-3:

1.

6. The method according to claim 1, wherein The volume proportion of the deep eutectic solvent in the two-liquid phase reaction system is 10%-25%.

7. The method according to claim 1, wherein The low boiling point polar organic solvent is selected from one or more of 1,4-dioxane, tetrahydrofuran, and acetonitrile.

8. The method according to claim 1, wherein The fructosyl carbohydrate is selected from one or more of purified fructose, crude fructose, polyfructose, fructose syrup and fructose-glucose syrup.

9. The method according to claim 1, wherein The mass proportion of the fructose-based carbohydrate in the two-liquid phase reaction system is 1%-15%.

10. The method according to claim 1, wherein The mass proportion of the fructose-based carbohydrate in the two-liquid phase reaction system is 3%-8%.

11. A method for preparing 2,5-furandicarboxylic acid by using 5-hydroxymethylfurfural, comprising: 5-Hydroxymethylfurfural is prepared according to the method according to any one of claims 1 to 10, wherein 2,5-furandicarboxylic acid is used as a catalyst; After the reaction is completed, the organic phase and the aqueous phase are separated, and the 5-hydroxymethylfurfural in the organic phase is oxidized to 2,5-furandicarboxylic acid using an oxidation catalyst.

12. The method according to claim 11, wherein The oxidation catalyst is selected from one of MnO2, Co3O4, and Fe3O4 catalysts.

13. The method according to claim 11, wherein In the oxidation reaction, oxygen or oxygen-containing gas is used as the oxidant, the oxygen partial pressure is 0.1-5 MPa, and the oxidation temperature is 110° C.-180° C.

14. The method according to claim 11, wherein In the oxidation reaction, oxygen or oxygen-containing gas is used as the oxidant, the oxygen partial pressure is 1-3 MPa, and the oxidation temperature is 130° C.-150° C.

15. The method according to claim 11, wherein In the oxidation reaction, the mass ratio of the oxidation catalyst to the organic phase is 0.01:1-0.1:1, and the oxidation time is 2-12 hours.

16. The method according to claim 11, wherein In the oxidation reaction, the mass ratio of the oxidation catalyst to the organic phase is 0.02:1-0.05:1, and the oxidation time is 6-8 hours.