Method for preparing furfural by using heteropoly acid-based phase transfer catalyst

Abstract

The present invention discloses a method for preparing furfural, which includes: under the action of a heteropolyacid-based phase transfer catalyst, an intramolecular dehydration reaction occurs for xylose-based carbohydrates in a ternary two-phase reaction medium composed of a low-boiling organic solvent, a protic solvent, and an ionic organic ammonium chloride. The catalyst used in the present invention has the property of phase transfer, that is, its solubility in the reaction medium increases at high temperature, and it catalyzes the dehydration of xylose in the form of a liquid acid; after the reaction, the solubility decreases and it precipitates in the form of a solid, which is convenient for the recovery and separation of the catalyst, and largely solves the problems of low efficiency of solid acid catalysts and difficult separation of liquid acid catalysts. And an advanced reaction medium is coupled to increase the solubility of xylose-based carbohydrates and the solvation effect on xylose. Through the optimization of the catalyst and the reaction medium, the reaction performance of the reaction system is effectively improved.

Description

Technical Field

[0001] The present invention relates to a method for producing biomass-based furfural products from saccharide carbohydrates, and belongs to the field of fine chemicals. Background Art

[0002] At present, the annual global production of furfural is more than 300,000 tons, and China is an important producer and consumer of furfural. China currently uses biomass raw materials such as corncobs as the main raw materials for furfural production. Using the one-step sulfuric acid continuous hydrolysis method, after physically pretreating the corncobs and mixing them with dilute sulfuric acid, through tandem cooking, xylose monomer molecules are first obtained, and then dehydrated to obtain furfural-containing steam. Furfural is an important biomass-based platform molecular compound. Through further catalytic conversion, high-value-added chemicals and liquid fuels can be prepared, effectively replacing traditional fossil resources. Therefore, developing a method for efficiently using pentose carbohydrates to prepare furfural is a key issue for the effective utilization of biomass resources.

[0003] In the process of furfural formation, acid catalysts and reaction media are the two most important factors. From the current production methods, it is found that although the homogeneous acid catalysis method with dilute sulfuric acid is widely used, it has obvious disadvantages: the efficiency of the dilute sulfuric acid catalytic system is low, the furfural yield is low, less than 50%; dilute sulfuric acid will corrode the reaction vessel under hydrothermal conditions, and special material reactors are required, which are expensive; the dilute sulfuric acid system causes serious pollution problems (waste gas, waste water, waste residue) during the production process. Based on China's current strict environmental protection standards, the price of furfural has been rising continuously. To solve the problems brought by liquid acid catalysts, researchers have begun to use solid acid catalysts that are easy to separate, have little pollution and corrosion, and have received wide attention. Patent CN102391217A discloses a method for preparing furfural by catalytic vaporization of straw washing liquid with HZSM-5 type molecular sieve solid acid. After optimization, the optimal furfural yield is 37.5%. Qing et al. (Qing Qing, Qi Guo, Linlin Zhou.et al.BioresourTechnol,2016,226,247-254.) prepared a montmorillonite-supported SO 4 2- / SnO 2Solid acid catalysts, with corncob as the raw material, adding NaCl as an auxiliary agent in a toluene / water biphasic system, and reacting at 190 °C for 15 min, the furfural yield is 81.7%. After the catalyst is recycled five times, the furfural yield drops to 69.46%. However, the solid acid catalyst has a relatively large microscopic size. During the reaction process, it is affected by the proton mass transfer efficiency and the adsorption and desorption of the substrate, resulting in a decrease in the xylose dehydration rate and a decrease in the furfural yield. In addition, the preparation process of the solid acid catalyst is complex and expensive. It is easily deactivated under hydrothermal conditions and requires a complex catalyst regeneration process, which further increases the production cost of furfural. Therefore, the production and use of solid acids also greatly limit the industrial production of furfural.

[0004] A large number of literatures have reported the reaction media required in the furfural preparation process. Among them, xylose can obtain furfural in aqueous solution and supercritical aqueous solution systems (Motokucho S, Morikawa H, Nakatani H, et al. Tetrahedron Letters 2016, 57: 4742 - 4745.), organic solvent systems (Gomes G R, Rampon D S, Ramos L P. Applied Catalysis A: General, 2017, 545: 127 - 133.), two - liquid - phase reaction systems (James A. Dumesic, et al. Science, 2006, 312, 1933 - 1937) and ionic liquid systems (Bekbolat Kassanov J W, Yan F, Jie C. RSC Advances, 2017, 7: 30755 - 30762.). However, due to the stable six - membered ring structure of xylose, it is not easy to dehydrate. At present, it is still necessary to further improve the solubility and solvation of the reaction medium for xylose, and reduce the activation energy of xylose dehydration to improve the reaction performance of the reaction medium. 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 furfural by using a heteropolyacid - based phase - transfer catalyst to catalyze xylose - based carbohydrates.

[0006] The heteropolyacid - based phase - transfer catalyst used in the method of the present invention has the property of thermodynamic phase change, that is, in the reaction medium, the solubility of the catalyst gradually increases with the increase of temperature and can be completely dissolved in the reaction medium in the form of liquid acid at the reaction temperature; after the reaction ends and the temperature drops, the solubility decreases and solid particles are re - formed and precipitated from the reaction medium, which is convenient for the recovery and reuse of the catalyst. Therefore, this catalyst integrates the characteristics of high activity of liquid acid catalysts and easy recovery of solid acid catalysts.

[0007] A method for preparing furfural provided by the present invention includes:

[0008] In a ternary two-phase reaction medium composed of a low-boiling organic solvent, a protic solvent, and an ionic organic ammonium chloride, in the presence of a heteropolyacid-based phase transfer catalyst, an intramolecular dehydration reaction of xylose-based carbohydrates occurs to obtain the furfural.

[0009] The low-boiling organic solvent is selected from one or more of acetonitrile, acetone, methyl ethyl ketone, methyl isobutyl ketone, 1,4-dioxane, and tetrahydrofuran, preferably one or more of 1,4-dioxane, tetrahydrofuran, and acetonitrile.

[0010] The protic solvent is selected from one or more of water and polyols, preferably water and C1-C3 polyols, more preferably one or more of water, ethylene glycol, 1,2-propanediol, 1,3-propanediol, and glycerol.

[0011] The ionic organic ammonium chloride is selected from ionic organic quaternary ammonium chlorides with short carbon chains, preferably one or more of C1-C4 alkyltrimethylammonium chlorides, more preferably one or more of tetramethylammonium chloride, choline chloride, chlormequat chloride, allyltrimethylammonium chloride, and butyltrimethylammonium chloride.

[0012] The protic solvent and the ionic organic ammonium chloride respectively act as a hydrogen bond donor and a hydrogen bond acceptor to form a binary single-phase deep eutectic solvent by constructing a hydrogen bond network structure. The mass ratio of the ionic organic ammonium chloride to the protic solvent is 0.1:1 - 5:1, preferably 1.5:1 - 3:1.

[0013] The low-boiling organic solvent is immiscible with the deep eutectic solvent, forming a two-phase reaction medium, where the low-boiling organic solvent is located in the upper layer and the deep eutectic solvent is located in the lower layer.

[0014] The volume fraction of the deep eutectic solvent in the two-phase reaction medium is 5% - 50%, preferably 10% - 25%.

[0015] The heteropolyacid-based phase transfer catalyst can be selected from unmodified heteropolyacids, whose metal atoms are selected from one or more of W, V, Nb, Mo, Re, and Ta, and whose heteroatoms are selected from one or more of P or Si, preferably one or more of tungsten-containing heteropolyacids, more preferably one or more of phosphotungstic acid and silicotungstic acid.

[0016] The heteropolyacid-based phase transfer catalyst can also be selected from heteropolyacid-based phase transfer catalysts modified by quaternary ammonium bases and / or quaternary ammonium salts. The preparation method includes: adding a quaternary ammonium base and / or a quaternary ammonium salt to an aqueous solution of phosphotungstic acid or silicotungstic acid, heating and reacting to produce a white precipitate, and after the reaction, filtering, washing, and drying the white precipitate.

[0017] The quaternary ammonium base and / or quaternary ammonium salt is preferably a quaternary ammonium base and / or quaternary ammonium salt containing a carboxyl group, more preferably one or more of betaine, betaine hydrochloride, and C1-C18 alkyl-substituted betaine.

[0018] The molar ratio of the quaternary ammonium base and / or quaternary ammonium salt to the heteropolyacid is 0.1:1 - 10:1, preferably 0.2:1 - 5:1, and more preferably 0.5:1 - 3:1.

[0019] During the preparation of the catalyst, the heating temperature is 30°C - 100°C, preferably 50°C - 80°C; the reaction time is 0.5 - 10 hours, preferably 6 - 8 hours.

[0020] The xylose-based carbohydrate is selected from one or more of purified xylose, crude xylose, xylan, xylose syrup, hemicellulose, and industrial hemicellulose.

[0021] The mass ratio of the xylose-based carbohydrate to the two-phase reaction medium is 1:1 - 1:1000, preferably 1:5 - 1:100, and more preferably 1:5 - 1:20.

[0022] The mass ratio of the heteropolyacid-based phase transfer catalyst to the xylose-based carbohydrate is 1:1 - 1:100, preferably 1:2 - 1:10.

[0023] The temperature of the dehydration reaction of the xylose-based carbohydrate is 80°C - 200°C, preferably 110°C - 130°C; the time of the xylose dehydration reaction is 0.1 - 12 hours, preferably 0.2 - 1 hour.

[0024] According to the method of the present invention, stirring is carried out during the xylose dehydration reaction.

[0025] The reactor required for the xylose dehydration reaction can be a thick-walled pressure-resistant bottle, a stainless steel reaction kettle with a polytetrafluoroethylene lining, and reactors that can be easily conceived by those skilled in the art. The reaction does not require separate control of the reaction pressure and can be carried out under the self-generated pressure in a closed reactor at the said temperature.

[0026] 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.

[0027] The present invention has the following advantages:

[0028] The present invention improves the reaction performance of xylose from both aspects of the catalyst and the reaction medium, thereby increasing the yield of furfural. On the one hand, the preparation method of the heteropolyacid-based phase transfer catalyst provided by the present invention is simple, green, and safe, and has typical thermodynamic phase change properties, that is, its solubility in the reaction medium gradually increases with the increase of temperature and is completely miscible with the reaction medium at the reaction temperature, catalyzing the dehydration of xylose with the properties of a liquid acid, greatly increasing the dehydration rate of xylose and reducing the residence time of the target product furfural under high-temperature acidic conditions; after the reaction ends and the temperature drops, the solubility of the catalyst decreases, and it re-precipitates from the reaction medium as solid particles, facilitating the separation and recovery of the catalyst. On the other hand, the reaction medium provided by the present invention has good solubility for xylose, can improve the solvation degree of xylose, reduce the reaction activation energy of xylose dehydration, and improve the reaction performance of the reaction medium. Detailed Embodiments

[0029] In the examples, heteropolyacids phosphotungstic acid and silicotungstic acid were both purchased from InnoChem Co., Ltd.

[0030] Preparation Example 1

[0031] A certain amount of water was added to a 100 mL round-bottom flask, then 5 g of betaine was added, and then the round-bottom flask was placed in an 80 °C oil bath and heated with stirring. When the betaine was completely dissolved in water, a certain amount of aqueous phosphotungstic acid solution was slowly added, and the molar ratio of betaine to phosphotungstic acid was 1:1. Then, after stirring for 6 hours under continued oil bath heating, the white precipitate was filtered, and the precipitate was washed thoroughly with deionized water until the filtrate was neutral. The catalyst prepared in this example was denoted as Catalyst I, and the molecular structure of the catalyst was as described in Preparation Formula 1.

[0032] Preparation Formula 1

[0033]

[0034] Preparation Example 2

[0035] The catalyst was prepared according to the process of Preparation Example 1, except that the molar ratio of betaine to phosphotungstic acid was 2:1. The catalyst prepared in this example was denoted as Catalyst II, and the molecular structure of the catalyst was as described in Preparation Formula 2.

[0036] Preparation Formula 2

[0037]

[0038] Preparation Example 3

[0039] The catalyst was prepared according to the process of Preparation Example 1, except that the molar ratio of betaine to phosphotungstic acid was 3:1. The catalyst prepared in this example was denoted as Catalyst III, and the molecular structure of the catalyst was as described in Preparation Formula 3.

[0040] Preparation of Formula 3:

[0041]

[0042] Preparation Example 4,

[0043] The catalyst was prepared according to the procedure of Preparation Example 1, except that betaine was replaced with dodecyl betaine. The catalyst prepared in this example was designated as Catalyst IV, and the molecular structure of the catalyst was as described in Preparation Formula 4.

[0044] Preparation Formula 4:

[0045]

[0046] Preparation Example 5,

[0047] The catalyst was prepared according to the procedure of Preparation Example 2, except that betaine was replaced with dodecyl betaine. The catalyst prepared in this example was designated as Catalyst V, and the molecular structure of the catalyst was as described in Preparation Formula 5.

[0048] Preparation Formula 5:

[0049]

[0050] Preparation Example 6,

[0051] The catalyst was prepared according to the procedure of Preparation Example 3, except that betaine was replaced with dodecyl betaine. The catalyst prepared in this example was designated as Catalyst VI, and the molecular structure of the catalyst was as described in Preparation Formula 6.

[0052] Preparation Formula 6:

[0053]

[0054] Preparation Example 7,

[0055] The catalyst was prepared according to the procedure of Preparation Example 1, except that phosphotungstic acid was replaced with silicotungstic acid. The catalyst prepared in this example was designated as Catalyst VII, and the molecular structure of the catalyst was as described in Preparation Formula 7.

[0056] Preparation Formula 7:

[0057]

[0058] Comparative Preparation Example 1,

[0059] The catalyst was prepared according to the procedure of Preparation Example 1, except that betaine was replaced with cetylammonium chloride. The catalyst prepared in this example was designated as Catalyst D I, and the molecular structure of the catalyst was as described in Comparative Preparation Formula 1.

[0060] Comparative Preparation Formula 1:

[0061]

[0062] Comparative Preparation Example 2,

[0063] Prepare the catalyst according to the process of Preparation Example 1, except that betaine is replaced with choline chloride. The catalyst prepared in this example is denoted as Catalyst D II, and the molecular structure of the catalyst is as described in Comparative Preparation Formula 2.

[0064]

[0065] Comparative Preparation Example 3,

[0066] CN102153527A discloses a preparation method of phosphotungstic acid supported on titanium dioxide. According to the catalyst preparation method provided therein, the phosphotungstic acid catalyst is immobilized on titanium dioxide, denoted as Catalyst D III.

[0067] Example 1,

[0068] This example illustrates the method for synthesizing furfural of the present invention using phosphotungstic acid.

[0069] Add 0.5 g of xylose, 0.1 g of phosphotungstic acid, 0.5 mL of water, 1.0 g of tetramethylammonium chloride, and 8.5 mL of 1,4-dioxane into a 15 mL pressure-resistant bottle. After the temperature of the multi-channel heater is heated to 110 °C, place the pressure-resistant bottle in the multi-channel heater and stir the reaction at a speed of 600 r / min for 30 minutes. After the reaction is completed, take out the pressure-resistant bottle from the multi-channel heater and cool it naturally to room temperature. After filtering the catalyst, analyze the reaction solution by high-performance liquid chromatography. The conversion rate of xylose is measured to be 97.6%, and the yield of furfural is 62.9%. After separating the catalyst, wash and dry it, and then calculate the recovery rate of phosphotungstic acid to be 91.2% according to the weight difference method.

[0070] Example 2,

[0071] This example illustrates the method for synthesizing furfural of the present invention using silicotungstic acid.

[0072] Carry out the reaction according to Example 1, except that phosphotungstic acid is replaced with silicotungstic acid, the mass of the catalyst is increased to 0.12 g, and the reaction temperature is raised to 120 °C. The conversion rate of xylose is measured to be 97.7%, the yield of furfural is 62.5%, and the recovery rate of silicotungstic acid is 91.0%.

[0073] Example 3,

[0074] This example illustrates the method for synthesizing furfural of the present invention using Catalyst I.

[0075] The reaction was carried out according to Example 1, except that phosphotungstic acid was replaced with Catalyst I, the mass of the catalyst was increased to 0.15 g, the reaction temperature was raised to 120 °C, and the reaction time was extended to 40 minutes. The conversion rate of xylose was measured to be 97.5%, the yield of furfural was 62.1%, and the recovery rate of the catalyst was 95.8%.

[0076] Example 4

[0077] In this example, the method for synthesizing furfural of the present invention is illustrated with Catalyst II.

[0078] The reaction was carried out according to Example 3, except that Catalyst I was replaced with Catalyst II, the mass of the catalyst was increased to 0.15 g, the reaction temperature was raised to 125 °C, and the reaction time was extended to 45 minutes. The conversion rate of xylose was measured to be 98.0%, the yield of furfural was 62.1%, and the recovery rate of the catalyst was 97.6%.

[0079] Example 5

[0080] In this example, the method for synthesizing furfural of the present invention is illustrated with Catalyst III.

[0081] The reaction was carried out according to Example 3, except that Catalyst I was replaced with Catalyst III, the mass of the catalyst was increased to 0.15 g, the reaction temperature was raised to 130 °C, and the reaction time was extended to 50 minutes. The conversion rate of xylose was measured to be 97.4%, the yield of furfural was 62.3%, and the recovery rate of the catalyst was 99.0%.

[0082] Example 6

[0083] In this example, the method for synthesizing furfural of the present invention is illustrated with Catalyst IV.

[0084] The reaction was carried out according to Example 3, except that Catalyst I was replaced with Catalyst IV, the mass of the catalyst was increased to 0.2 g, the reaction temperature was raised to 125 °C, and the reaction time was extended to 45 minutes. The conversion rate of xylose was measured to be 97.5%, the yield of furfural was 62.0%, and the recovery rate of the catalyst was 96.5%.

[0085] Example 7

[0086] In this example, the method for synthesizing furfural of the present invention is illustrated with Catalyst V.

[0087] The reaction was carried out according to Example 3, except that Catalyst I was replaced with Catalyst V, the mass of the catalyst was increased to 0.2 g, the reaction temperature was raised to 130 °C, and the reaction time was extended to 50 minutes. The conversion rate of xylose was measured to be 96.8%, the yield of furfural was 61.4%, and the recovery rate of the catalyst was 98.5%.

[0088] Example 8

[0089] In this example, the method for synthesizing furfural according to the present invention is described using catalyst VI.

[0090] The reaction was carried out according to Example 3, except that catalyst I was replaced with catalyst VI, the mass of the catalyst was increased to 0.2 g, the reaction temperature was raised to 130 °C, and the reaction time was extended to 60 minutes. The conversion rate of xylose was determined to be 96.7%, the yield of furfural was 62.0%, and the recovery rate of the catalyst was 99.3%.

[0091] Example 9

[0092] In this example, the method for synthesizing furfural according to the present invention is described using catalyst III.

[0093] The reaction was carried out according to Example 3, except that tetramethylammonium chloride was replaced with choline chloride. The conversion rate of xylose was determined to be 97.6%, the yield of furfural was 61.4%, and the recovery rate of the catalyst was 95.8%.

[0094] Example 10

[0095] In this example, the method for synthesizing furfural according to the present invention is described using catalyst III.

[0096] The reaction was carried out according to Example 3, except that tetramethylammonium chloride was replaced with chlormequat chloride (cycocel). The conversion rate of xylose was determined to be 97.0%, the yield of furfural was 61.1%, and the recovery rate of the catalyst was 95.6%.

[0097] Example 11

[0098] In this example, the method for synthesizing furfural according to the present invention is described using catalyst III.

[0099] The reaction was carried out according to Example 3, except that tetramethylammonium chloride was replaced with allyltrimethylammonium chloride. The conversion rate of xylose was determined to be 97.0%, the yield of furfural was 60.8%, and the recovery rate of the catalyst was 95.4%.

[0100] Example 12

[0101] In this example, the method for synthesizing furfural according to the present invention is described using catalyst III.

[0102] The reaction was carried out according to Example 3, except that tetramethylammonium chloride was replaced with butyltrimethylammonium chloride.

[0103] The conversion rate of xylose was determined to be 96.8%, the yield of furfural was 60.5%, and the recovery rate of the catalyst was 95.4%.

[0104] Example 13

[0105] In this example, the method for synthesizing furfural according to the present invention is described using catalyst III.

[0106] The reaction was carried out according to Example 3, except that 1,4-dioxane was replaced with tetrahydrofuran. The conversion rate of xylose was determined to be 97.2%, the yield of furfural was 61.5%, and the recovery rate of the catalyst was 95.8%.

[0107] Example 14

[0108] This example illustrates the method for synthesizing furfural of the present invention using catalyst III.

[0109] The reaction was carried out according to Example 3, except that 1,4-dioxane was replaced with acetonitrile. The conversion rate of xylose was determined to be 97.6%, the yield of furfural was 62.0%, and the recovery rate of the catalyst was 95.6%.

[0110] Example 15

[0111] This example illustrates the method for synthesizing furfural of the present invention using catalyst III.

[0112] The reaction was carried out according to Example 3, except that water was replaced with ethylene glycol. The conversion rate of xylose was determined to be 96.7%, the yield of furfural was 62.6%, and the recovery rate of the catalyst was 95.5%.

[0113] Example 16

[0114] This example illustrates the method for synthesizing furfural of the present invention using catalyst III.

[0115] The reaction was carried out according to Example 3, except that water was replaced with 1,2-propanediol. The conversion rate of xylose was determined to be 96.4%, the yield of furfural was 62.0%, and the recovery rate of the catalyst was 95.8%.

[0116] Example 17

[0117] This example illustrates the method for synthesizing furfural of the present invention using catalyst III.

[0118] The reaction was carried out according to Example 3, except that water was replaced with 1,3-propanediol. The conversion rate of xylose was determined to be 96.5%, the yield of furfural was 62.1%, and the recovery rate of the catalyst was 95.5%.

[0119] Example 18

[0120] This example illustrates the method for synthesizing furfural of the present invention using catalyst III.

[0121] The reaction was carried out according to Example 3, except that water was replaced with glycerol. The conversion rate of xylose was determined to be 96.4%, the yield of furfural was 61.0%, and the recovery rate of the catalyst was 95.6%.

[0122] Example 19

[0123] This example uses catalyst VII to illustrate the method for synthesizing furfural of the present invention.

[0124] The reaction was carried out according to Example 2, except that silicotungstic acid was replaced with catalyst VII, the reaction temperature was raised to 125 °C, and the reaction time was extended to 45 minutes. The conversion rate of xylose was measured to be 98.0%, the yield of furfural was 62.4%, and the recovery rate of the catalyst was 95.5%.

[0125] Example 20

[0126] This example uses catalyst I to illustrate the method for synthesizing furfural of the present invention.

[0127] The reaction was carried out according to Example 3, except that the mass of the catalyst was reduced to 0.05 g, the reaction temperature was raised to 130 °C, and the reaction time was extended to 55 minutes. The conversion rate of xylose was measured to be 98.2%, the yield of furfural was 62.6%, and the recovery rate of the catalyst was 95.8%.

[0128] Comparative Example 1

[0129] The reaction was carried out according to Example 3, except that catalyst I was replaced with catalyst D I. The conversion rate of xylose was measured to be 70.6%, the yield of furfural was 46.3%, and the recovery rate of the catalyst was 99.8%.

[0130] Comparative Example 2

[0131] The reaction was carried out according to Example 3, except that catalyst I was replaced with catalyst D II. The conversion rate of xylose was measured to be 65.7%, the yield of furfural was 43.9%, and the recovery rate of the catalyst was 99.9%.

[0132] Comparative Example 3

[0133] The reaction was carried out according to Example 3, except that catalyst I was replaced with catalyst D III. The conversion rate of xylose was measured to be 69.5%, the yield of furfural was 45.0%, and the recovery rate of the catalyst was 99.8%.

[0134] Comparative Example 4

[0135] The reaction was carried out according to Example 3, except that tetramethylammonium chloride was replaced with octyltrimethylammonium chloride. The conversion rate of xylose was measured to be 79.0%, the yield of furfural was 51.2%, and the recovery rate of the catalyst was 95.6%.

[0136] Comparative Example 5

[0137] The reaction was carried out according to Example 3, except that water was replaced with 1,4-butanediol. The conversion rate of xylose was determined to be 97.2%, the yield of furfural was 51.9%, and the recovery rate of the catalyst was 95.7%.

[0138] Comparative Example 6

[0139] The reaction was carried out according to Example 3, except that tetramethylammonium chloride was replaced with an equimolar amount of NaCl (0.53 g), the water content was increased to 1.5 mL, and the reaction time was extended to 2.5 h. The conversion rate of xylose was determined to be 96.7%, the yield of furfural was 45.7%, and the recovery rate of the catalyst was 93.7%.

[0140] Comparing the results of Examples 1 and 2, according to the common knowledge of those skilled in the art, ordinary commercially available heteropolyacids are highly soluble in water and thus also soluble in a mixed solution of water and an organic solvent, and do not have the property of thermodynamic phase change. However, the inventors of the present application unexpectedly found that in the presence of an organic quaternary ammonium chloride, commercially available heteropolyacids have the property of thermodynamic phase change when a homogeneous solvent is formed through hydrogen bonding between water and the organic quaternary ammonium chloride. Specifically manifested in the present invention as: after the two-phase reaction system is heated above 100 °C, ordinary commercially available heteropolyacid catalysts dissolve in the reaction system; after the reaction is completed, the catalyst gradually re-precipitates as the temperature decreases and settles at the bottom of the reactor. This may be because after adding an organic quaternary ammonium chloride to the aqueous phase, the newly formed eutectic solvent changes the polarity and the strength of the hydrogen bond network structure of the original aqueous solvent, affecting the solubility of conventional heteropolyacids in the aqueous phase and endowing them with the property of thermodynamic phase change. In addition, phosphotungstic acid is slightly stronger in acidity than silicotungstic acid. Therefore, to achieve the same reaction result, the catalyst feeding amount and reaction temperature required for silicotungstic acid are slightly higher than those for phosphotungstic acid. However, regardless of the acidity of the two heteropolyacids, commercial heteropolyacids exhibit thermosensitive properties in the lower aqueous phase under the action of an organic quaternary ammonium chloride, demonstrating the universality of the thermosensitive properties of ordinary commercially available heteropolyacids in this system.

[0141] Comparing the results of Example 1 and Examples 3-5, it can be seen that the hydrophobic property of commercial phosphotungstic acid is enhanced after being modified with betaine, resulting in enhanced thermosensitive properties, and the thermosensitive properties gradually increase with the increase in the addition amount of betaine. Therefore, the recovery rates of Catalysts I to III gradually increase after the reaction ends and the temperature drops. According to Preparation Formula 1, the more the addition amount of betaine, the more the strong acid protons on phosphotungstic acid are replaced, resulting in the proton sites of the catalyst changing from the strong acid proton sites provided by phosphotungstic acid to the weak acid proton sites provided by the carboxyl groups on the modifier, and the acidity of the catalyst weakens. Therefore, higher catalyst feeding amounts, reaction temperatures, and longer reaction times are required to achieve the same reaction result. To ensure the recovery rate and acidity of the catalyst, the molar ratio of betaine to phosphotungstic acid is not higher than 3:1, preferably 0.5:1 - 3:1.

[0142] Comparing the results of Examples 3-5 and Examples 6-8, it can be seen that dodecyl betaine has a similar effect to betaine. Phosphotungstic acid still has thermosensitive properties after being modified by dodecyl betaine. When the molar ratio of the modifier to phosphotungstic acid is the same, the catalyst modified by dodecyl betaine has a higher recovery rate after the reaction. This is because dodecyl betaine has a more hydrophobic alkyl side chain, and the catalyst obtained by modifying phosphotungstic acid is more hydrophobic, so the catalyst recovery rate is higher. However, due to the relatively large molecular weight of dodecyl betaine, the amount of protons per unit mass of the catalyst obtained by modification at the same molar ratio decreases. Therefore, to obtain similar reaction results, it is necessary to increase the catalyst dosage, reaction temperature, and appropriately extend the reaction time.

[0143] Comparing the results of Example 3 and Comparative Example 1, it can be seen that when the modifier only contains a hydrophobic aliphatic side chain and no hydrophilic oxygen-containing side chain, the modified phosphotungstic acid catalyst does not have thermosensitive properties and always exists in a solid form during the reaction. The dissociation degree and acidity of protons are affected, and the adsorption of the product by the solid acid catalyst is relatively strong, which affects the desorption and diffusion of the product. The target product will be further decomposed to obtain by-products, resulting in a decrease in furfural selectivity and yield. Therefore, after phosphotungstic acid loses its thermosensitive properties, both the furfural selectivity and yield are lower than those of the thermosensitive solid acid catalyst in the present invention. Therefore, when using quaternary ammonium base / salt as the modifier, the modifier needs to contain a certain amount of hydrophilic oxygen-containing functional groups so that the prepared catalyst has thermosensitive properties.

[0144] Comparing the results of Example 3 and Comparative Example 2, it can be seen that when choline chloride is used as the modifier, although the modifier contains a hydrophilic oxygen-containing side chain: hydroxyl group, the modified phosphotungstic acid catalyst still does not have thermosensitive properties. The obtained catalyst is similar to the result of Comparative Example 2, and the catalyst always exists in a solid form during the reaction, resulting in a decrease in furfural selectivity and yield. Therefore, when using quaternary ammonium base / salt as the modifier, the hydrophilic oxygen-containing functional group on the modifier needs to have a certain degree of proton dissociation. Since the proton dissociation degree of the hydroxyl group in choline chloride is less than that of the carboxyl group in betaine, the prepared catalyst still does not have thermosensitive properties under the reaction conditions of the present invention. Therefore, quaternary ammonium base / salt containing carboxyl hydrophilic functional groups such as betaine, dodecyl betaine, and dodecyl betaine hydrochloride are preferred options for the modifier in the present invention.

[0145] Comparing the results of Example 3 and Comparative Example 3, it can be seen that when the acidic sites are all phosphotungstic acid, phosphotungstic acid does not have thermosensitive properties after being immobilized on titanium dioxide. Similar to the results of Comparative Examples 1 and 2, the catalyst always exists in solid form during the reaction, resulting in a decrease in furfural selectivity and yield. Therefore, the supported phosphotungstic acid catalyst does not have thermosensitive properties, and both furfural selectivity and yield are lower than those of the thermosensitive solid acid catalyst in the present invention. In addition, due to the low loading amount of phosphotungstic acid on titanium dioxide, when the mass of the solid acid catalyst is the same, the number of protons provided is less, so the xylose conversion rate and furfural yield are much lower than those of the thermosensitive solid acid catalyst in the present invention.

[0146] Comparing the results of Example 3, Examples 9 - 12 and Comparative Example 4, it can be seen that when tetramethylammonium chloride is replaced with choline chloride, chlormequat chloride (CCC), allyltrimethylammonium chloride and butyltrimethylammonium chloride, that is, when one methyl side chain (C1) of tetramethylammonium chloride is replaced by a side chain with a long carbon chain (C2 - C4), the xylose dehydration rate and furfural yield decrease slowly, but the final furfural yield is still higher than 60%. When the substituent side chain continues to extend, the furfural yield further decreases, and due to the increase in the number of carbon atoms in the side chain, the surface activity of the hydrogen bond acceptor of the deep eutectic solvent is enhanced, and at this time, a two - liquid - phase reaction system cannot be formed, but a single - liquid - phase reaction system. Therefore, considering both the stability of the two - liquid - phase system and the furfural yield, the hydrogen bond acceptor of the deep eutectic solvent is selected as C 1 -C 4 chloride of substituted or unsubstituted alkyltrimethyl quaternary ammonium salt.

[0147] Comparing the results of Example 3 and Examples 13 - 14, it can be seen that when using different organic solvents such as 1,4 - dioxane, tetrahydrofuran and acetonitrile, the corresponding xylose conversion rate and furfural yield are basically unchanged. If other low - boiling organic solvents are used, the xylose dehydration rate and furfural yield will decrease significantly. Therefore, the organic solvents in the two - liquid - phase reaction medium are preferably 1,4 - dioxane, tetrahydrofuran and acetonitrile.

[0148] Comparing the results of Example 3, Examples 15 - 18 and Comparative Example 5, it can be seen that when water is replaced by ethylene glycol, 1,2 - propanediol, 1,3 - propanediol, glycerol (glycerin), the corresponding xylose conversion rate, furfural selectivity and yield are basically unchanged. When water is replaced by 1,4 - butanediol with a longer carbon chain length, at this time, the solvation effect of the deep eutectic solvent in the lower layer of the two - liquid - phase reaction medium on xylose weakens, and the xylose dehydration rate and the yield of the target product furfural decrease. Therefore, the hydrogen bond donor of the deep eutectic solvent is preferably one of water and C1 - C3 polyols, including one of water, ethylene glycol, 1,2 - propanediol, 1,3 - propanediol, glycerol (glycerin).

[0149] Comparing the results of Example 2 and Example 19, it can be seen that when silicotungstic acid is replaced with Catalyst VII, common commercially available phosphotungstic acid and silicotungstic acid both exhibit better thermosensitive properties under the modification of betaine with the same molar ratio, which further demonstrates the universality of the thermosensitive properties of the heteropolyacid catalyst modified by betaine in this system.

[0150] Comparing the results of Example 3 and Example 20, it can be seen that when the catalyst dosage is reduced to 1 / 10 of xylose, a higher reaction temperature and an extended reaction time are required to achieve similar reaction results.

[0151] Comparing the results of Example 3 and Comparative Example 6, it can be seen that when tetramethylammonium chloride is replaced with the inorganic salt NaCl, NaCl simply dissolves in water and cannot act as a hydrogen bond acceptor to form a hydrogen bond structure with water to form a deep eutectic solvent. Therefore, to ensure that the volume of the lower reaction phase remains unchanged, the water content needs to be increased. In addition, due to the inability to form a deep eutectic solvent, the solvation effect of the reaction phase on xylose weakens, the conversion rate of xylose decreases, and the time required to reach the optimal reaction result needs to be extended to 2.5 h, and the yield significantly decreases. In addition, due to the slowdown of the xylose dehydration rate and the increase in the reaction time, the exposure time of the catalyst under hydrothermal conditions is extended, and the catalyst instability increases, resulting in a decrease in the catalyst recovery rate. Therefore, the present invention preferably uses organic chlorides rather than inorganic chlorides.

[0152] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0153] In addition, it should be noted that, in the case of no contradiction, the various specific technical features described in the above specific embodiments can be combined in any appropriate manner. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

[0154] Furthermore, any combination can be made between various different embodiments of the present invention as long as it does not violate the idea of the present invention, and it should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing furfural, comprising: In a ternary two-phase reaction medium composed of a low-boiling organic solvent, a protic solvent, and an ionic organic ammonium chloride, in the presence of a heteropoly acid-based phase transfer catalyst, an intramolecular dehydration reaction of a xylose-based carbohydrate is carried out to obtain the furfural, wherein the low-boiling organic solvent is selected from one or more of acetonitrile, acetone, methyl ethyl ketone, methyl isobutyl ketone, 1,4-dioxane, and tetrahydrofuran, the protic solvent is selected from one or more of water and polyols, the ionic organic ammonium chloride is selected from one or more of C1-C4 alkyltrimethylammonium chlorides, and the heteropoly acid-based phase transfer catalyst is selected from a heteropoly acid-based phase transfer catalyst modified by a quaternary ammonium base and / or a quaternary ammonium salt containing a carboxyl group, and the quaternary ammonium base and / or the quaternary ammonium salt containing a carboxyl group are selected from one or more of betaine, betaine hydrochloride, and C 1 -C 18 -alkyl-substituted betaines.

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

3. According to the method of claim 1, wherein, the protic solvent is selected from one or more of water and C1-C3 polyols.

4. According to the method of claim 1, wherein, the ionic organic ammonium chloride is selected from one or more of tetramethylammonium chloride, choline chloride, chloro-choline chloride, allyltrimethylammonium chloride, and butyltrimethylammonium chloride.

5. According to the method of claim 1, wherein, the mass ratio of the ionic organic ammonium chloride to the protic solvent is 0.1:1 - 5:

1.

6. According to the method of claim 1, wherein, the mass ratio of the ionic organic ammonium chloride to the protic solvent is 1.5:1 - 3:

1.

7. According to the method of claim 1, the protic solvent and the ionic organic ammonium chloride form a binary single-phase deep eutectic solvent, and the volume ratio of the deep eutectic solvent in the two-phase reaction medium is 5% - 50%.

8. According to the method of claim 7, the volume ratio of the deep eutectic solvent in the two-phase reaction medium is 10% - 25%.

9. According to the method of claim 1, wherein, The preparation method of the heteropolyacid-based phase transfer catalyst modified by a quaternary ammonium base and / or quaternary ammonium salt containing carboxyl groups includes: adding a quaternary ammonium base and / or quaternary ammonium salt containing carboxyl groups to an aqueous solution of heteropolyacid, heating for reaction to produce a white precipitate, filtering, washing, and drying the white precipitate after the reaction ends. The heteropolyacid is selected from phosphotungstic acid or silicotungstic acid, and the heteropolyacid-based phase transfer catalyst modified by a quaternary ammonium base and / or quaternary ammonium salt containing carboxyl groups is selected from betaine, betaine hydrochloride, and one or more of C 1 -C 18 alkyl-substituted betaines.

10. According to the method of claim 9, wherein, the molar ratio of the quaternary ammonium base and / or quaternary ammonium salt containing a carboxyl group to the heteropolyacid is 0.1:1 - 10:

1.

11. According to the method of claim 9, wherein, the molar ratio of the quaternary ammonium base and / or quaternary ammonium salt containing a carboxyl group to the heteropolyacid is 0.2:1 - 5:

1.

12. According to the method of claim 9, wherein, the molar ratio of the quaternary ammonium base and / or quaternary ammonium salt containing a carboxyl group to the heteropolyacid is 0.5:1 - 3:

1.

13. According to the method of claim 9, wherein, the heating temperature during the preparation of the catalyst is 30°C - 100°C, and the reaction time is 0.5 - 10 hours.

14. According to the method of claim 9, wherein, the heating temperature during the preparation of the catalyst is 50°C - 80°C, and the reaction time is 6 - 8 hours.

15. According to the method of claim 1, wherein, the xylose-based carbohydrate is selected from one of purified xylose, crude xylose, xylan, xylose syrup, and hemicellulose.

16. According to the method of claim 1, wherein, the mass ratio of the xylose-based carbohydrate to the two-phase reaction medium is 1:1 - 1:1000.

17. According to the method of claim 1, wherein, the mass ratio of the xylose-based carbohydrate to the two-phase reaction medium is 1:5 - 1:

100.

18. According to the method of claim 1, wherein, the mass ratio of the xylose-based carbohydrate to the two-phase reaction medium is 1:5 - 1:

20.

19. According to the method of claim 1, wherein, the mass ratio of the heteropolyacid-based phase transfer catalyst to the xylose-based carbohydrate is 1:1 - 1:

100.

20. According to the method of claim 1, wherein, the mass ratio of the heteropolyacid-based phase transfer catalyst to the xylose-based carbohydrate is 1:2 - 1:

10.

21. According to the method described in claim 1, Among them, the temperature of the dehydration reaction of the xylose-based carbohydrate is 80°C - 200°C, and the time of the xylose dehydration reaction is 0.1 - 12 hours.

22. According to the method described in claim 1, wherein, the temperature of the dehydration reaction of the xylose-based carbohydrate is 110°C - 130°C, and the time of the xylose dehydration reaction is 0.2 - 1 hour.

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