A method for preparing furfural and furoic acid

Abstract

The present invention discloses a method for preparing furfural and furoic acid. The method for preparing furfural comprises: under the catalytic action of a weak acid having a cyclic structure, in a two-liquid phase reaction system consisting of a low-boiling-point polar organic solvent, water, and an organic ammonium chloride, a xylose-based carbohydrate undergoes an intramolecular dehydration reaction in a reactor to obtain furfural. After the reaction is completed, the acid catalyst is retained in the mixed solution of the lower layer of water and the organic ammonium chloride, and 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 is greatly reduced, thereby improving the recovery rate of the acid catalyst. In addition, the present invention can also effectively reduce the content of humin impurities in the organic phase, achieving 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 method for preparing furfural and furoic acid. Background Art

[0002] Furfural has two key functional groups in its structure: an aldehyde group and a conjugated double bond. These form the basis of furfural's potential as a high-value-added biomass-based platform molecule. The aldehyde group allows furfural to undergo acetalization, acylation, aldol condensation, reduction to alcohols, decarbonylation, oxidation to carboxylic acids, and Grignard reactions. The conjugated double bond system allows furfural to undergo alkylation, hydrogenation, oxidation, halogenation, ring opening, and nitration. Therefore, furfural is widely used in the synthesis of biomass-based fuel additives and high-value-added chemicals. Furthermore, furfural is widely used in the petrochemical industry as an excellent extractant for aromatics from lubricating oils and diesel fuels.

[0003] Currently, furfural is primarily produced through the direct dehydration of carbohydrates, typically pentose monomers such as xylose, via protonic acid catalysis. Global annual furfural production now exceeds 300,000 tons, with the majority produced in China. my country uses corncobs as its primary raw material, producing furfural vapor through a continuous sulfuric acid digestion process. While widely used, this method has significant drawbacks: the use of sulfuric acid can severely corrode the reaction vessel; the reaction efficiency is low, with furfural yields typically below 50%; and the dilute sulfuric acid creates significant pollution. Recently, rising costs for acid wastewater treatment have led to a surge in furfural prices.

[0004] To improve the yield of furfural reactions, researchers have begun using novel reaction systems such as water-organic solvents and ionic liquids. These novel reaction systems often require chloride additives. For example, in water-organic solvent two-phase reactions, NaCl is needed to reduce the solubility of water in the organic solvent (Top Catal (2009) 52:297–303). Ionic liquids (such as imidazolium chloride, Bekbolat Kassanov JW, Yan F, Jie C. RSC Advances, 2017, 7:30755-30762) are themselves organic chlorides. When common homogeneous inorganic strong acids are used as catalysts in these reaction systems, their protons react with chloride ions in the reaction medium to form HCl. Under high temperature conditions, some HCl volatilizes, causing pipeline corrosion and catalyst spillage and loss.

[0005] In addition, common strong acidic inorganic homogeneous acid catalysts are difficult to separate from the reaction medium and the target product in the above reaction system. Although a montmorillonite-supported SO4 was prepared by immobilizing a homogeneous inorganic strong acid, for example, Qing et al. (Qing Qing, Qi Guo, Linlin Zhou. et al. Bioresource Technol, 2016, 226, 247-254.) 2- While using a solid acid catalyst containing 1% mol / snO₂ can improve acid catalyst recovery, its stability is poor. Using corncobs as feedstock and adding NaCl as an additive in a toluene / water two-phase system, the yield of furfural was 81.7% after a 15-minute reaction at 190°C. However, after five cycles of use, the yield dropped to 69.46%. The loss of acidic sites on the catalyst led to a continuous decrease in furfural yield. These sites then dissolve in the reaction medium, reverting to homogeneous acid properties. This makes separation from the target product difficult and becomes an impurity associated with the target product.

[0006] In order to avoid the loss of HCl generated by the mixing of a strong acidic catalyst with a chloride auxiliary agent in the reaction solvent system in the above process, and to avoid the problem of excessive acid impurities and low purity in the target product due to the shedding of the catalyst's acidic sites, it is still necessary to optimize the use of the acid catalyst and propose a method for the continuous preparation of furfural and furoic acid with high selectivity and high yield. Summary of the Invention

[0007] The present invention aims to overcome the above-mentioned shortcomings of the prior art and proposes a method for preparing furfural by dehydrating xylose-based carbohydrates using a weak organic acid as a catalyst. This method can reduce catalyst loss and improve the purity of furfural.

[0008] The present invention also provides a method for preparing furoic acid by using furfural.

[0009] The present invention provides a method for preparing furfural using a weak organic acid as a catalyst, comprising: under the catalytic action of a weak acid having a cyclic structure, in a two-liquid phase reaction system consisting of a low-boiling-point polar organic solvent, water, and an organic ammonium chloride salt, causing xylosyl carbohydrates to undergo an intramolecular dehydration reaction in a reactor to obtain furfural.

[0010] According to the method of the present invention, after the reaction is completed, the target product furfural can be extracted in situ into the upper organic phase, and the acid catalyst is retained in the mixed solution of water and organic ammonium chloride in the lower layer. The aqueous phase and the organic phase are directly separated, so that the target product furfural and the homogeneous acid catalyst can be effectively separated, thereby improving the purity of furfural.

[0011] The weak acid having a cyclic structure is selected from carboxylic acids having a furan ring or a benzene ring structure, such as furoic acid, 2,5-furandicarboxylic acid, benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, etc., and furoic acid is most preferred.

[0012] The low-boiling point polar organic solvent is selected from acetone, butanone, methyl isobutyl ketone, tetrahydrofuran, 1,4-dioxane, acetonitrile, etc., preferably one or more of tetrahydrofuran, 1,4-dioxane, and acetonitrile.

[0013] The organic ammonium chloride is selected from organic quaternary ammonium chlorides with short carbon chains of C1-C6, preferably one of C1-C4 hydrocarbon trimethyl quaternary ammonium chlorides, more preferably one or more of tetramethylammonium chloride, choline chloride, choline chloride, allyltrimethylammonium chloride, and butyltrimethylammonium chloride, and most preferably tetramethylammonium chloride.

[0014] The mass ratio of the cyclic weak acid to water is 0.1:1-1:1, preferably 0.3:1-1:1.

[0015] The mass ratio of the organic quaternary ammonium chloride to water is 1:1-5:1, preferably 1.5:1-2.5:1.

[0016] In the xylose dehydration reaction process of the present invention, the weak acid with a cyclic structure serves as both a catalyst and a part of the reaction medium. The weak acid with a cyclic structure and water, which respectively have carboxyl and hydroxyl groups, serve as hydrogen bond donors, and 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.

[0017] 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 15%-25%.

[0018] During the xylose dehydration reaction, the xylosyl carbohydrate is selected from one of purified xylose, crude xylose, xylan, xylose syrup, hemicellulose and industrial hemicellulose.

[0019] The weight ratio of the xylosyl carbohydrate in the two-liquid phase reaction system is 1%-15%, preferably 3%-8%. The temperature of the xylosyl carbohydrate dehydration reaction is 90°C-160°C, more preferably 130°C-150°C; the xylose dehydration reaction time is 0.1-12 hours, preferably 0.2-5 hours, more preferably 0.3-1 hour.

[0020] According to the method of the present invention, the xylose 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.

[0021] The reactor required for the xylose 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 conceived by those skilled in the art.

[0022] After the xylose dehydration reaction is completed, the conversion rate of the reaction substrate, the selectivity and yield of furfural during the reaction process can be analyzed and calculated by high performance liquid chromatography.

[0023] In the xylose dehydration reaction process of the present invention, on the one hand, the acidic group of the weak acid with cyclic structure is carboxyl, and its acidity is weaker than the strong acid inorganic acid such as common sulfuric acid, hydrochloric acid, so its proton will not form HCl volatilization with the chloride ion of chloride in reaction medium, thereby reducing the overflow loss of catalyst proton. On the other hand, the weak acid with cyclic structure and water and organic ammonium chloride salt form the deep eutectic solvent of double hydrogen bond donor. Construct abundant hydrogen bond network structure, strengthen the immobilization effect of the weak acid with cyclic structure in deep eutectic solvent, reduce its solubility in organic solvent, further reduce the loss of catalyst caused by being dissolved in organic phase, can greatly improve the recovery rate of catalyst.

[0024] According to the method of the present invention, if the ultimate goal of the reaction is to produce furoic acid from furfural, furoic acid is preferably used as a catalyst for the xylose dehydration reaction. After the reaction is completed, the furfural is extracted in situ into the organic phase, which inevitably still contains a small amount of catalyst impurities. In this case, the use of an oxidation catalyst can further oxidize the furfural extracted from the organic phase to furoic acid. That is, the initially added catalyst and the final target product are the same substance and can be simultaneously precipitated after evaporation of the solvent. This eliminates the need for other complex and sophisticated separation operations for the small amount of catalyst impurities, thereby improving the feasibility and cost-effectiveness of the furfural preparation and reconversion process and simplifying the production process of furoic acid.

[0025] Based on this, the present invention also provides a method for preparing furoic acid from furfural, comprising: preparing furfural according to the aforementioned method of the present invention, wherein furoic acid is used as a catalyst; after the reaction is completed, separating the organic phase and the aqueous phase, and using an oxidation catalyst to oxidize the furfural in the organic phase into furoic acid.

[0026] The oxidation catalyst is selected from one or more of MnO2, Co3O4, and Fe3O4 catalysts.

[0027] In the oxidation reaction, oxygen, air, or other oxygen-containing gases are used as the oxidant, the oxygen partial pressure is 0.1-5 MPa, preferably 2-4 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, and the oxidation temperature is 110° C.-180° C., preferably 140° C.-160° C. The oxidation time is 2-12 hours, preferably 6-10 hours. Additional solvent may or may not be added during the oxidation reaction.

[0028] During the xylose dehydration reaction of the present invention, humin impurities are generated through condensation reactions due to the instability of furfural and other furan ring intermediates. The reaction solution is brown in color. Humin is usually mainly present in the organic phase. After the reaction, the organic phase needs to be further separated and purified to obtain a high-purity product, furfural. The inventors of the present application unexpectedly discovered that according to the method of the present invention, a large amount of humin can be retained 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-furoic acid), thereby reducing the concentration of humin impurities in the organic phase, improving the purity of furfural, and achieving in-situ separation of furfural and humin impurities.

[0029] The present invention has the following advantages:

[0030] (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.

[0031] (2) Weak acids with cyclic structures can also act as hydrogen bond donors, forming a rich network structure with hydrogen bond acceptor organic ammonium chloride salts, thereby enhancing the immobilization effect of the catalyst in the low eutectic solvent, reducing its solubility in the organic solvent, and reducing the loss of the catalyst caused by dissolution in the organic phase.

[0032] (3) After the reaction is completed, the furfural extracted from the organic phase can be further oxidized to furoic acid. By regulating the subsequent reactions so that the final product of furfural is identical to the catalyst, the catalyst remaining in the xylose dehydration process can be precipitated together with the product after the solvent is evaporated. This eliminates the need for other complex and sophisticated separation operations for a small amount of catalyst impurities, thereby improving the feasibility and economy of the furfural preparation and reconversion process.

[0033] (4) After the reaction is completed, the lower 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 furfural. DETAILED DESCRIPTION

[0034] The present invention will be further described below by way of examples.

[0035] The xylose in the examples is purified crystalline xylose purchased from Tokyo Chemical Industry Development Co., Ltd. (Shanghai).

[0036] Other raw materials are commercially available.

[0037] Example 1

[0038] To a 15mL pressure-resistant bottle, add 0.5g of xylose, 0.3g of furoic acid, 0.5g of water, 1.0g of tetramethylammonium chloride, and 8.5mL of 1,4-dioxane. The volume ratio of the deep eutectic solvent formed in the lower layer of the two-liquid system is 15%. After heating the multichannel heater to 140°C, the sealed pressure-resistant bottle is placed in the multichannel heater and stirred at 600 rpm for 35 minutes. After the reaction is completed, the pressure-resistant bottle is removed from the multichannel heater and naturally cooled to room temperature. The reaction solution is analyzed by high-performance liquid chromatography, which shows a xylose conversion of 96.2% and a furfural yield of 61.6%.

[0039] Example 2

[0040] 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 furoic acid content in the pressure bottle was analyzed by high performance liquid chromatography, and the catalyst recovery was calculated to be 100% based on the initial charge.

[0041] Example 3

[0042] The reaction was carried out according to Example 1. After the reaction was completed, the two phases were separated. The furoic acid content in the lower eutectic solvent was analyzed by high performance liquid chromatography. The catalyst recovery rate was calculated to be 99.0% based on the initial charge amount.

[0043] Example 4

[0044] 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 6 hours to adsorb the humin impurities in the two phases. The mixture was then filtered through a filter membrane. After drying, the masses of humin in the upper and lower phases were calculated by differential gravimetry to be 0.064 g and 0.12 g, respectively. The mass of humin in the deep eutectic solvent was 65.2% of the total mass of humin.

[0045] Example 5

[0046] The reaction and separation were carried out according to Examples 1-4, except that furoic acid was replaced with an equal molar amount of benzoic acid. After completion of the reaction, the xylose conversion was 96.5%, the furfural yield was 62.0%, the catalyst recovery in the lower deep eutectic solvent layer was 99.0%, and the retained humin was 66.0%. After evaporation of the solvent, the catalyst recovery was 100.0%.

[0047] Example 6

[0048] 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 40 minutes. The xylose conversion rate was determined to be 96.5%, and the furfural yield was 61.2%.

[0049] Example 7

[0050] 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 130° C., and the reaction time was shortened to 30 minutes. The xylose conversion rate was determined to be 96.7%, and the furfural yield was 61.5%.

[0051] Example 8

[0052] The reaction was carried out in the same manner as in Example 1, except that the organic solvent was replaced with tetrahydrofuran. The xylose conversion rate was determined to be 96.1%, and the furfural yield was 61.9%.

[0053] Example 9

[0054] The reaction was carried out in the same manner as in Example 1, except that the organic solvent was replaced with acetonitrile. The xylose conversion rate was determined to be 96.8%, and the furfural yield was 62.2%.

[0055] Example 10

[0056] The reaction was carried out in the same manner as in Example 1, except that tetramethylammonium chloride was replaced with choline chloride. The xylose conversion rate was determined to be 96.5%, and the furfural yield was 61.5%.

[0057] Example 11

[0058] The reaction was carried out in the same manner as in Example 1, except that tetramethylammonium chloride was replaced with choline chloride. The xylose conversion rate was determined to be 96.6%, and the furfural yield was 61.3%.

[0059] Example 12

[0060] The reaction was carried out in the same manner as in Example 1, except that tetramethylammonium chloride was replaced with allyltrimethylammonium chloride. The xylose conversion rate was determined to be 96.6%, and the furfural yield was 61.1%.

[0061] Example 13

[0062] The reaction was carried out in the same manner as in Example 1, except that tetramethylammonium chloride was replaced with butyltrimethylammonium chloride. The xylose conversion rate was determined to be 99.4%, and the furfural yield was 61.0%.

[0063] Example 14

[0064] The reaction was carried out according to Example 1, except that the amount of tetramethylammonium chloride was reduced to 0.9 g, the amount of water was increased to 0.6 g, and the reaction was extended to 50 minutes. The xylose conversion rate was determined to be 96.9%, and the furfural yield was 62.0%.

[0065] Example 15

[0066] The reaction was carried out according to Example 1, with the amount of tetramethylammonium chloride remaining unchanged, except that the amount of water was reduced to 0.4 g and the reaction time was shortened to 20 minutes. The xylose conversion rate was determined to be 96.7%, and the furfural yield was 61.8%.

[0067] Example 16

[0068] 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 20% and the reaction time was extended to 45 minutes. The xylose conversion rate was determined to be 96.7% and the furfural yield was 61.7%.

[0069] Example 17

[0070] 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%, the temperature was raised to 150°C, and the reaction time was increased to 40 minutes. The xylose conversion rate was determined to be 96.6%, and the furfural yield was 61.5%.

[0071] Example 18

[0072] React according to embodiment 1, after reaction finishes, two phases are separated, and get upper organic phase, quality is about 8.9g, places the stainless steel reactor with polytetrafluoroethylene lining, adds 0.3g MnO then Catalyzer, oxygen partial pressure is 2MPa.Utilize heating jacket to begin timing reaction after temperature is raised to 150 ℃ after the sealing, react after 8 hours reactor is taken out and is cooled to room temperature, reaction solution analyzes with high performance liquid chromatography, and the transformation efficiency of measuring furfural is 100.0%, and furoic acid yield is 91.2%.Utilize Rotary Evaporators that the organic solvent in the reaction solution is removed, white furoic acid is separated out, and utilizing nuclear magnetic analysis to measure the furoic acid purity of preparation is 97.0%.

[0073] Example 19

[0074] The xylose dehydration reaction was carried out according to Example 5, and then the furfural oxidation reaction was carried out according to Example 18, except that 0.4 g of Co3O4 was used as a catalyst. The reaction was carried out at 150°C for 9 hours. After the reaction, the furfural conversion rate was 100.0%, the furoic acid yield was 90.5%, and the furoic acid purity was 96.5% after evaporation of the organic solvent.

[0075] Comparative Example 1

[0076] The reaction was carried out according to Example 1, except that the furoic acid catalyst was replaced with sulfuric acid in an equal molar amount of protons and the temperature was lowered to 130° C. The xylose conversion rate was determined to be 97.0%, and the furfural yield was 62.0%.

[0077] Comparative Example 2

[0078] The reaction was carried out according to Example 2, except that the furoic acid catalyst was replaced with an equimolar amount of sulfuric acid. The residual sulfuric acid content in the pressure bottle was quantitatively analyzed using elemental analysis techniques, and the catalyst recovery rate was calculated to be 97.5% based on the initial charge amount.

[0079] Comparative Example 3

[0080] The reaction was carried out according to Example 3, except that the furoic 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 in the deep eutectic solvent was calculated to be 95.0%.

[0081] Comparative Example 4

[0082] The reaction was carried out according to Example 4, except that the furoic 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 6 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 31.2% of the total mass of humin.

[0083] Comparative Example 5

[0084] The reaction was carried out according to Example 4, except that the furoic 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, followed by stirring at room temperature for 6 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 33.4% of the total mass of humin.

[0085] Comparative Example 6

[0086] The reaction was carried out in the same manner as in Example 1, except that tetramethylammonium chloride was replaced with octyltrimethylammonium chloride. The xylose conversion rate was determined to be 95.5%, and the furfural yield was 72.2%.

[0087] Comparative Example 7

[0088] Xylose dehydration reaction was carried out according to Comparative Example 1, and then furfural oxidation reaction was carried out according to Example 18. The conversion rate of furfural was determined to be 94.8%, the yield of furoic acid was 74.2%, and the purity of furoic acid after evaporation of the organic solvent was 90.5%.

[0089] 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 furoic acid is weaker than that of sulfuric acid, and therefore a higher reaction temperature is required to achieve a higher xylose conversion rate and furoic acid yield.

[0090] Comparing the results of Example 2 and Comparative Example 2 shows that, because furoic acid is less acidic than sulfuric acid, its protons do not react with chloride ions in the reaction medium to form HCl, thereby preventing catalyst spillage and loss due to HCl volatilization under high temperature conditions. In contrast, sulfuric acid is more acidic, and its protons can react with chloride ions in the reaction medium to form HCl, which volatilizes at high temperatures, resulting in approximately 2.5% catalyst spillage and loss after each reaction.

[0091] The result of embodiment 3 and comparative example 3 is relatively known. After each reaction finishes, the rate of recovery of furoic acid in the deep eutectic solvent is 99.0%, which is higher than the sulfuric acid rate of recovery 95.0%. This is because the carboxyl of furoic acid and the hydroxyl of water can simultaneously serve as hydrogen bond donors to form the deep eutectic solvent of double hydrogen bond donors with hydrogen bond acceptor tetramethylammonium chloride. And after changing to sulfuric acid, only water can be used as hydrogen bond donor to form the deep eutectic solvent of single hydrogen bond donor. Because the hydrogen bond network in the double hydrogen bond deep eutectic solvent is more abundant, the hydrogen bonding effect is stronger, so the immobilization of furoic acid in the deep eutectic solvent is stronger, and the loss caused by being dissolved in the organic phase is less, and the rate of recovery in the deep eutectic solvent is higher.

[0092] 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 sulfuric acid and acetic acid from furoic acid, the adsorption capacity of the deep eutectic solvent formed by the catalyst-water-tetramethylammonium chloride on humin showed a significant decrease. Furfural and acetic acid are both organic carboxylic acids, and the adsorption capacity of the corresponding humin is higher than that of the inorganic acid sulfuric acid, indicating that when a weak acid with a carboxylic acid structure is used as the composition of the deep eutectic solvent, it contributes to its adsorption and separation of humin. In addition, the humin separation capacity of furoic acid with a furan ring structure is better than acetic acid, and its corresponding deep eutectic solvent has increased the retention of humin by nearly double, indicating that the catalyst with a ring structure 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 furfural in the organic phase.

[0093] Comparing the results of Examples 1-4 and 5, it is known that when the catalyst is changed to p-benzoic acid from furic acid, the catalyst effect is basically the same, indicating that the cyclic structure feature of the catalyst is applicable to both furan rings and benzene rings. Although the recovery rate of these two catalysts in the lower floor's deep eutectic solvent can reach 99.0% after the xylose dehydration reaction ends, about 1.0% of the catalyst still enters the organic phase and becomes an impurity of the target product, furfural. And because the catalyst impurity content is small, a more complicated method is needed to thoroughly separate from furfural, which reduces the economy and feasibility of furfural production and separation and purification process. If furfural is further oxidized to furic acid using an oxidation catalyst at this time, that is, by regulating and controlling the reaction process, the catalyst initially added and the final target product are the same substance, which can be separated out simultaneously after evaporating the solvent, then there is no need to use additional complicated and delicate acid separation operation, which improves the feasibility and economy of furfural preparation and reconversion process. Therefore, the carboxylic acid catalyst with cyclic structure is preferably furic acid.

[0094] Comparing the results of Example 1 and Examples 6-7, it can be seen that when the amount of furoic acid added is increased, the reaction rate is accelerated and the reaction temperature can be gradually lowered to obtain similar xylose conversion rates and furfural yields.

[0095] Comparing the results of Example 1 with those of Examples 8-9 shows that the xylose conversion rate and furfural yield remain essentially unchanged when using 1,4-dioxane, tetrahydrofuran, and acetonitrile. Substituting other low-boiling-point organic solvents significantly reduces the xylose dehydration rate and furfural yield. Therefore, the preferred organic solvents in the two-phase reaction medium are 1,4-dioxane, tetrahydrofuran, and acetonitrile.

[0096] Comparing the results of Example 1 and Examples 10-13 with Comparative Example 6, it can be seen that when tetramethylammonium chloride is replaced with choline chloride, choline chloride (chlormequat), allyltrimethylammonium chloride and butyltrimethylammonium chloride, that is, after a methyl side chain (C1) of tetramethylammonium chloride is replaced by a side chain (C2-C4) of a long carbon chain, the furfural yield slowly decreases, but the final furfural yield is still higher than 60%. However, when the substituent side chain continues to extend (C8), the furfural yield further decreases, and due to the increase in the carbon number of the side chain, the deep eutectic solvent on the lower floor and the organic solvent on the upper floor are no longer able to form a two-liquid phase reaction system, but are instead a uniformly mixed single-liquid phase reaction system. Therefore, it is impossible to achieve in-situ separation of the target product furfural from the acid catalyst and humin impurities.

[0097] 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 water content increases, which is not conducive to the dehydration of xylose, the xylose dehydration rate slows down, and the time required to achieve the same reaction effect is prolonged. When the mass ratio of tetramethylammonium chloride to water is increased to 2.5:1, the water content decreases, the xylose dehydration rate accelerates, and the time required to achieve the same reaction effect is shortened.

[0098] Comparing the results of Example 1 and Examples 16-17, it can be seen that when the volume ratio of the deep eutectic solvent gradually increases, the dehydration rate of xylose decreases due to the increase in water content, and the temperature required to achieve the same reaction effect increases and the time required is prolonged.

[0099] The result of embodiment 18 and comparative example 7 is relatively known, when using furoic acid as wood sugar dehydration catalyst, reaction utilizes common MnO after finishing the back Oxidizing catalyst can directly be oxidized to furoic acid by the furfural in the upper organic solvent, and after the evaporation solvent, residual catalyst and product in organic phase are separated out simultaneously, without the need to the separation process of extra acid.But after catalyzer changes to sulfuric acid, residual humin in organic phase is more, therefore in furfural oxidation reaction, humin is easily deposited on the oxidation catalyst surface, causes catalyst oxidation activity to reduce, and furfural conversion efficiency and furoic acid yield reduce.And because the sulfuric acid recovery rate is low, remaining in the organic phase content is higher and acidity is stronger, therefore in furfural oxidation reaction, residual sulfuric acid is easier to cause the secondary condensation reaction of furfural to generate more humin, thereby further reduces yield and the purity of furoic acid.

[0100] Comparing the results of Examples 18 and 19, it can be seen that when the catalyst is replaced by p-benzoic acid instead of furoic acid, there is no significant change in the xylose dehydration reaction and the furfural oxidation reaction activity. However, after the oxidation reaction is completed and the solvent is separated, the p-benzoic acid remaining in the organic phase is simultaneously precipitated, and the purity of the resulting furoic acid is slightly reduced. Therefore, furoic acid is preferably used as the wood dehydration catalyst.

[0101] 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.

[0102] 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.

[0103] 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 furfural, comprising: Under the catalytic action of a weak acid with a cyclic structure, in a two-liquid phase reaction system consisting of a low-boiling point polar organic solvent, water, and an organic ammonium chloride salt, a xylosyl carbohydrate undergoes an intramolecular dehydration reaction in a reactor to obtain furfural. The weak acid with a cyclic structure is selected from furoic acid, 2,5-furandicarboxylic acid, benzoic acid, o- / m- / terephthalic acid, the low-boiling point polar organic solvent is selected from acetone, butanone, methyl isobutyl ketone, tetrahydrofuran, 1,4-dioxane, and acetonitrile, the organic ammonium chloride salt is selected from one or more of tetramethylammonium chloride, choline chloride, choline chloride, allyltrimethylammonium chloride, and butyltrimethylammonium chloride, the weak acid with a cyclic structure, water, and the organic ammonium chloride salt form a deep eutectic solvent that is a double hydrogen bond donor, the volume proportion of the deep eutectic solvent in the two-liquid phase reaction system is 5%-50%, and the temperature of the xylosyl carbohydrate dehydration reaction is 90°C-160°C.

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

3. The method according to claim 1, wherein The mass ratio of the weak acid having a ring structure to water is 0.1:1-1:

1.

4. The method according to claim 1, wherein The mass ratio of the weak acid having a ring structure to water is 0.3:1-1:

1.

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

1.

6. The method according to claim 1, wherein The mass ratio of the organic ammonium chloride to water is 1.5:1-2.5:

1.

7. The method according to claim 1, wherein A weak acid with a cyclic structure, water and an organic ammonium chloride salt form a low eutectic solvent as a double hydrogen bond donor, and the volume proportion of the low eutectic solvent as the double hydrogen bond donor in the two-liquid phase reaction system is 15%-25%.

8. The method according to claim 1, wherein The xylosyl carbohydrate is selected from one of purified xylose, crude xylose, xylan, xylose syrup and hemicellulose.

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

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

11. The method according to claim 1, wherein The temperature of the xylosyl carbohydrate dehydration reaction is 130° C.-150° C.; the time of the xylose dehydration reaction is 0.1-12 hours.

12. A method for preparing furoic acid by furfural, comprising: Furfural is prepared according to the method of any one of claims 1 to 11, wherein furoic acid is used as a catalyst; After the reaction is completed, the organic phase and the aqueous phase are separated, and the furfural in the organic phase is oxidized to furoic acid using an oxidation catalyst.

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

14. The method according to claim 12, wherein: In the oxidation reaction, oxygen, air or other oxygen-containing gases are used as oxidants, the oxygen partial pressure is 0.1-5 MPa, and the oxidation temperature is 110° C.-180° C.

15. The method according to claim 12, wherein: The oxygen partial pressure is 2-4 MPa, and the oxidation temperature is 140°C-160°C.

16. The method according to claim 12, 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.

17. The method according to claim 12, 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 6-10 hours.