WO2025113469A1 - Method for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran and catalyst used therefor - Google Patents

Abstract

A method for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran and a catalyst used therefor. In the method for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran, 2,5-dimethyl-2,5-hexanediol is subjected to a dehydration reaction in the presence of the catalyst in a reactor at a temperature of 50-200°C. The catalyst for synthesizing 2,2,5,5-tetramethyltetrahydrofuran can catalyze the reaction of 2,5-dimethyl-2,5-hexanediol at a higher conversion rate, and the reaction has very high selectivity of up to 99% or more. The catalyst used therefor has good stability and a long service life, so that the production energy consumption is further reduced, the production cost is reduced, and industrial production is easy to realize.

Description

A continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran and catalyst used therein Technical Field

The present application relates to the field of chemical synthesis, and in particular to a method for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran by dehydrating 2,5-dimethyl-2,5-hexanediol, a catalyst used in the method and a preparation method thereof.

Background Art

2,2,5,5-Tetramethyltetrahydrofuran, CAS No. 15045-43-9, referred to as TMTHF, is a colorless liquid with a freezing point of -92°C, a boiling point of 112°C, a relative density of 0.811 (25°C), a refractive index of 1.409 (19°C), and a flash point of 3.9°C. It is an important organic intermediate and an excellent solvent. Due to its low density, low boiling point, and low ETN value of 0.111, it may replace traditional hydrocarbon solvents such as toluene and hexane, and is widely used in industrial production as a new type of solvent. Although TMTHF is an ether by definition because it contains a RO-R' group (where R and R' are alkyl groups), it does not have the peroxide formation potential of other ethers (such as THF or 2-MeTHF). This is because there is no proton in the α position relative to the ether oxygen. The α-proton in traditional ethers is easily removed by low-energy light to form free radicals. Oxygen from the air can react with the free radicals to form explosive peroxides. The rate of peroxide formation potential in ethers increases with increasing radical stability: primary α-carbon << secondary α-carbon < tertiary α-carbon. Since TMTHF contains two quaternary ether carbons and no α-protons, the potential for peroxide formation is eliminated. This combination of very favorable properties makes TMTHF a rare low boiling point, low polarity molecule that has no peroxide formation potential and can be easily produced from biomass.

There are literature reports on the dehydration of 2,5-dimethyl-2,5-hexanediol (as shown above) to synthesize TMTHF, such as Denney et al. in J.Org.Chem.1984,49,p2831 disclosed a method for preparing TMTHF, comprising contacting 2,5-dimethyl-2,5-hexanediol with pentaethoxyphosphorane as a catalyst in DCM as a solvent. Vlad and Ungur disclosed a method for preparing TMTHF in Synthesis 1983,1983,p216, comprising contacting 2,5-dimethyl-2,5-hexanediol with trimethylchlorosilane as a catalyst in benzene as a solvent. Gillis & Beck disclosed a method for preparing TMTHF in J.Org.Chem.1963,28,p1388, comprising contacting 2,5-dimethyl-2,5-hexanediol with DMSO as a solvent and a catalyst. Yamaguchi et al. in Catal. Today 2012, 185, p302 disclose a method for preparing TMTHF, which comprises contacting 2,5-dimethyl-2,5-hexanediol with hot liquid water in high-pressure carbon dioxide as a catalyst and solvent. In fact, in all the above methods, the yield of TMTHF does not exceed 80% in the presence of a solvent. Higher yields are achieved in solvent-free methods. DE700036C discloses a method for preparing TMTHF, which comprises contacting 2,5-dimethylalkane-2,5-hexanediol with potassium pyrosulfate in the absence of a solvent, with a yield of 94.6%. Olah et al. in Synthesis 1981, p474 have used Nafion-H as a catalyst in the synthesis of TMTHF from 2,5-dimethyl-2,5-hexanediol as a precursor. The advantage of a solid catalyst such as Nafion-H is that it can be easily separated from the reaction mixture, and the synthesis has a yield of 94%. CN 109790134 A discloses the synthesis of TMTHF by the dehydration reaction of 2,5-dimethyl-2,5-hexanediol with H-Beta and various molecular sieve catalysts, and compares the reaction results. It is found that compared with other molecular sieves (HY, H-ZSM5) under the same conditions, H-Beta has higher reaction activity and product selectivity.

At present, the prior art often adopts intermittent reaction, which is simple to operate, but it is easy to cause carbon deposition due to the difficulty in controlling the residence time, resulting in low yield and high production cost. The other is a continuous reaction, represented by a fixed bed/fluidized bed reactor, using a solid acid catalyst, characterized by continuous reaction, and the residence time can be accurately controlled, high yield can be achieved, and safety is guaranteed. This reaction can use molecular sieves such as H-Beta, HY, and H-ZSM5 as catalysts. However, the acidity and alkalinity of the catalyst surface are generally controlled by controlling the content between the various oxide components, as well as the conditions of the synthesis process such as crystallization temperature, crystallization time, aging temperature, etc. It is difficult to accurately control the acidity and alkalinity of the catalyst surface in the above method. The synthesis process of the catalyst generally requires the use of a template with greater toxicity, and the carbon deposition generated during the reaction process easily blocks the surface of the catalyst active site, causing the catalyst to be deactivated. At the same time, continuous operation has certain requirements for the stability of the catalyst, and frequent regeneration of the catalyst increases the difficulty of operation to a certain extent.

Therefore, how to improve the stability of solid acid catalysts while maintaining high selectivity for TMTHF in the product is a difficult point. Biomass carbon materials can often show catalytic activity and product selectivity different from traditional metal oxides due to their rich surface pore structure and easily regulated surface groups. In addition, the surface properties of the material can also be regulated by doping carbon materials with heteroelements. By this means, the catalyst can be rationally designed and regulated according to the active sites required for different reactions, and efficient catalysts that can catalyze specific reactions with high selectivity can be synthesized in a targeted manner. This catalyst has the advantages of a wide range of raw material sources, renewability, easy regulation of catalyst surface properties, and no use of metal components, and is gradually gaining attention.

Summary of the invention

In view of the problems existing in the above-mentioned prior art, the object of the present invention is to provide a method for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran, which uses 2,5-dimethyl-2,5-hexanediol as a raw material for continuous dehydration reaction to prepare 2,2,5,5-tetramethyltetrahydrofuran, as well as a catalyst used in the method and a method for preparing the catalyst, wherein the catalyst can operate stably for a long time.

To achieve the above object, according to one aspect of the present invention, an object of the present invention is to provide a continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran, wherein in the presence of a catalyst, 2,5-dimethyl-2,5-hexanediol is subjected to a dehydration reaction in a reactor at 50°C to 200°C.

According to one embodiment of the present application, the reactor in the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran is selected from any one of a continuous stirred tank reactor, a plug flow reactor, a stationary phase reactor and a fluidized bed reactor, or can be a mixed reactor of two or more of these reactors connected; preferably a fixed bed reactor;

According to one embodiment of the present application, the catalyst may be in the form of strips, columns or sheets;

According to one embodiment of the present application, the dehydration reaction temperature may preferably be 80°C to 150°C;

According to one embodiment of the present application, the dehydration reaction can be carried out under one or more of a nitrogen atmosphere, a helium atmosphere or an argon atmosphere;

According to one embodiment of the present application, the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran can be carried out at a reaction pressure of 0.1MPa to 4MPa, preferably 0.1MPa to 2MPa; or at a reaction pressure of normal pressure to 4MPa, preferably normal pressure to 2MPa;

According to one embodiment of the present application, the 2,5-dimethyl-2,5-hexanediol can be reacted in the absence of a solvent or in the presence of a solvent, and the solvent is one or more selected from tetrahydrofuran, acetonitrile and 1,4-dioxane, preferably tetrahydrofuran or 1,4-dioxane;

According to one embodiment of the present application, the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran can be carried out at a reaction space velocity of 0.05 h ^-1 to 5 h ^-1 , preferably 0.1 h ^-1 to 3 h ^-1 .

According to one embodiment of the present application, the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran further comprises activating the catalyst before the reaction. Specifically, before the reaction, the catalyst is heated to an activation temperature of 300°C to 500°C and maintained for 1h to 6h. Preferably, the activation temperature may be 300°C to 400°C.

According to one embodiment of the present application, the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran further includes post-reaction treatment. Specifically, the reaction product is condensed and separated from gas and liquid, and then distilled. The condensation, gas-liquid separation and distillation are conventional methods and conditions for separating 2,2,5,5-tetramethyltetrahydrofuran and by-products in the art, and will not be repeated here.

According to another aspect of the present invention, another object of the present invention is to provide a catalyst used in the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran, wherein the catalyst is prepared by a method comprising the following steps:

(1) The biomass raw material is crushed by a pulverizer, and then added to a ball mill together with a solid acid catalyst to be ball-milled into fine powder, which is then added to a reactor, and distilled water is added. The reactor is sealed and heated to perform a hydrolysis reaction. After the reaction is completed, the temperature is lowered, the pressure is released, and the filtrate is filtered under reduced pressure, and the filtrate is distilled and concentrated into a concentrated solution;

(2) adding the acid solution to the concentrated solution in step (1) under vigorous stirring, mixing evenly, adding chitosan, transferring to a hydrothermal kettle, performing hydrothermal treatment, cooling, and relieving pressure, washing the obtained product with anhydrous ethanol and deionized water, respectively, and drying to obtain a doped carbon material;

(3) adding alkali to the doped carbon material obtained in step (2), stirring and mixing, placing in a tube furnace, heating in an inert gas atmosphere for carbonization treatment, cooling after carbonization, washing the obtained material with distilled water until the filtrate is neutral, and drying;

(4) The doped carbon material obtained in step (3) is mixed and stirred evenly with an acid or an oxidant, and subjected to heat treatment. After the treatment, the temperature is reduced and filtered, and the material is washed with distilled water until the filtrate is neutral, and then dried.

Further, in step (1), the biomass material is selected from one or more of corn cobs, corn stalks, sawdust, peanut shells, and bamboo shoots; preferably one or more of corn cobs, corn stalks, and peanut shells, and more preferably one or more of corn cobs and corn stalks;

Further, the solid acid catalyst is selected from one or more of silicon dioxide, γ-alumina, zirconium dioxide, cerium dioxide, tungsten trioxide, niobium pentoxide, zeolite molecular sieve, and ion exchange resin;

Furthermore, the solid acid catalyst is preferably one or more of silicon dioxide, γ-alumina, tungsten trioxide, niobium pentoxide, zeolite molecular sieve, and ion exchange resin; more preferably one or more of γ-alumina, zeolite molecular sieve, and ion exchange resin;

Furthermore, the zeolite molecular sieve is selected from one or more of HZSM5, HZSM11, HY, Hβ, HMOR, and SAPO-34.

Furthermore, in step (1):

The dried biomass raw material is crushed by a pulverizer, and then added to a ball mill together with a solid acid catalyst to be ball-milled into a fine powder of 200-400 meshes, and the reactor is sealed and heated to a temperature of 150-250°C;

Furthermore, the mass ratio of the distilled water to the biomass raw material is 50:1-2:1; preferably 20:1-5:1;

Further, the hydrolysis reaction temperature is 120-250°C; preferably 150-220°C; more preferably 160-210°C;

Furthermore, the hydrolysis reaction time is 4-10 hours; preferably 4-6 hours;

Furthermore, the mass concentration of the concentrated solution is 10%-30%, preferably 10%-20%.

Furthermore, in step (2):

The acid is selected from one or more of formic acid, acetic acid, propionic acid, and hydrochloric acid;

The mass concentration of the acid solution is 1%-30%, preferably 3%-10%;

The mass ratio of the acid solution to the concentrated solution is 1:1-10:1; preferably 1:1-5:1;

The mass ratio of chitosan to concentrated solution is 1:10-1:100;

The hydrothermal treatment temperature is 160-220°C; preferably 180-210°C;

The hydrothermal treatment time is 4-20 hours; preferably 5-10 hours;

The obtained product was washed with anhydrous ethanol and deionized water three times respectively and dried at 110 °C for 12 h.

Furthermore, in step (3):

The base is selected from one or more of sodium hydroxide, potassium hydroxide, sodium methoxide, potassium methoxide, sodium ethoxide, and potassium ethoxide;

The mass ratio of the base to the doped carbon material is 1:1-10:1; preferably 1:1-5:1; more preferably 1:1-3:1;

The inert gas used in the carbonization process is selected from one or more of nitrogen, helium and argon; preferably one or more of nitrogen and argon.

The carbonization treatment temperature is 300-700°C, and the treatment time is 4-20h; after the carbon is washed, it is dried at 110°C for 12h.

Furthermore, in step (4):

The acid is one or more of sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid, and phosphoric acid;

The oxidant is one or more of hydrogen peroxide with a mass concentration of 30wt% and sodium hypochlorite with an effective chlorine content of 6%;

The mass ratio of the acid to the doped carbon material is 1:1-10:1;

The mass ratio of the oxidant to the doped carbon material is 1:1-10:1.

The heating treatment temperature is 60-90°C;

The heating treatment time is 4-10h;

After washing, the mixture was dried at 110°C for 12 h.

Furthermore, after the reaction in step (1) is completed, the filter cake obtained by suction filtration contains the solid acid catalyst. The filter cake is calcined at 350-550° C. in an air atmosphere for 3-6 hours to remove organic matter to obtain the solid acid catalyst. The obtained solid acid catalyst can be repeatedly recycled.

According to another aspect of the present invention, another object of the present invention is to provide use of the catalyst in the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran according to the present invention.

Beneficial Effects

The catalyst for synthesizing 2,2,5,5-tetramethyltetrahydrofuran provided in the present application can catalyze the reaction of 2,5-dimethyl-2,5-hexanediol with a high conversion rate, and the reaction has a very high selectivity (up to 99% or more). The catalyst used has good stability and long life, thereby further reducing production energy consumption, reducing production costs, and facilitating industrial production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of a synthesis reaction device for 2,2,5,5-tetramethyltetrahydrofuran according to one embodiment of the present application.

FIG. 2 is ammonia temperature-programmed desorption test results of the adsorption performance of the catalyst products of Preparation Example 6, Comparative Example 1 and Comparative Example 2.

FIG. 3 is a graph showing the reaction stability test results in Reaction Example 2.

FIG. 4 is a graph showing the reaction stability test results in Reaction Example 3.

FIG. 5 is a graph showing the reaction stability test results in Reaction Example 6.

FIG6 is a graph showing the reaction stability test results in Comparative Example 1.

FIG. 7 is a graph showing the reaction stability test results in Comparative Example 2.

DETAILED DESCRIPTION

Hereinafter, the preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terms used in the specification and the appended claims should not be interpreted as limited to the general and dictionary meanings, but should be interpreted based on the principle of allowing the inventor to appropriately define the terms for the best interpretation, based on the meaning and concept corresponding to the technical level of the present invention. Therefore, the description herein is only a preferred example for illustrative purposes, and is not intended to limit the scope of the present invention, so it should be understood that other equivalent implementations and modifications can be made without departing from the spirit and scope of the present invention.

The following examples are merely listed as examples of implementation schemes of the present application and do not constitute any limitation to the present application. Those skilled in the art will understand that modifications within the scope of the essence and concept of the present application fall within the protection scope of the present application.

Preparation of 2,2,5,5-Tetramethyltetrahydrofuran

In the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran according to the present application, 2,5-dimethyl-2,5-hexanediol is used as a raw material, and a cyclic ether is obtained by dehydration reaction. The product obtained after post-treatment is passed through a 0.22 μm filter membrane and analyzed and detected by gas chromatography (GC). The low-boiling point product was qualitatively analyzed by gas chromatography-mass spectrometry (GC-MS) and the GC retention time of the standard substance, and it was determined that the reaction product was mainly 2,2,5,5-tetramethyltetrahydrofuran. The low-boiling point substance was quantitatively determined by Shimadzu-GC 2020 gas chromatograph, and the quantitative analysis was performed by comparing the retention time and peak area size with the standard substance. The relevant calculation formula is as follows:

The unit of the flow rate of 2,5-dimethyl-2,5-hexanediol is g/min, and the unit of the amount of the catalyst is g.

As shown in Figure 1, it is a schematic diagram of a synthesis reaction device of 2,2,5,5-tetramethyltetrahydrofuran according to an embodiment of the present application. Among them, the reaction tube is filled with a catalyst for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran according to the present application. First, the carrier gas is passed into the reaction tube by controlling the flow rate through a mass flow meter to create a carrier gas atmosphere, and then the heating furnace can be heated to activate the catalyst. Then, the temperature of the reaction tube is maintained, and 2,5-dimethyl-2,5-hexanediol is fed into the reaction tube through a feed pump, and in the case of a carrier gas atmosphere and a catalyst catalyzed, the reaction generates a product containing 2,2,5,5-tetramethyltetrahydrofuran. After condensation and gas-liquid separation, 2,2,5,5-tetramethyltetrahydrofuran can be collected.

According to the present application, the catalyst is applied to the process of preparing 2,2,5,5-tetramethyltetrahydrofuran from 2,5-dimethyl-2,5-hexanediol as raw material. By reducing the generation of by-products, the selectivity of 2,2,5,5-tetramethyltetrahydrofuran is improved and the difficulty of separation is reduced. The method for preparing 2,2,5,5-tetramethyltetrahydrofuran provided by the present application has easy-to-obtain raw materials, a greener route, a simple process, high efficiency, and can be produced continuously.

Unless otherwise specified, the raw materials used in this application are all commercially available, and the methods and equipment used are all conventional methods and equipment in the art.

In the following examples, 2,5-dimethyl-2,5-hexanediol, sodium hydroxide, potassium hydroxide, formic acid, acetic acid, hydrochloric acid, sulfuric acid, and nitric acid were purchased from Sinopharm Chemical Reagent Co., Ltd., high-purity nitrogen, high-purity helium, and air were purchased from Qingdao Dehai Weiye Technology Co., Ltd., and corn cobs, corn stalks, and peanut shells were purchased locally.

Catalyst preparation

Preparation Example 1

A catalyst for preparing 2,2,5,5-tetramethyltetrahydrofuran is prepared by a method comprising the following steps:

1. Crush 150g of dried corn cobs with a pulverizer, add them together with 15g of HZSM5 catalyst into a ball mill and grind them into fine powder of 200-400 mesh, add them into a reactor, add 800ml of distilled water, seal the reactor and heat to 200℃ for hydrolysis reaction for 6h. After the reaction is completed, cool down, release the pressure, filter under reduced pressure, and distill and concentrate the filtrate to obtain 143ml of concentrated solution.

2. Add 200 ml of 10% by mass formic acid solution to the concentrated solution in step 1 under vigorous stirring, add 5 g of chitosan, mix well, add to a hydrothermal kettle, hydrothermally treat at 180°C for 10 hours, cool and release the pressure, wash the resulting product with anhydrous ethanol and deionized water for 3 times respectively, and then dry at 110°C for 12 hours to obtain the doped carbon material.

3. Take 20 g of the doped carbon material obtained in step 2, add 60 g of potassium hydroxide thereto, stir and mix evenly, place in a tube furnace, heat to 500°C in an inert gas atmosphere for carbonization treatment for 5 hours. After the carbonization is completed, cool down, wash the obtained material with distilled water until the filtrate is neutral, and dry at 110°C for 12 hours.

4. Add 100 ml of 20 wt% nitric acid aqueous solution to 10 g of the doped carbon material obtained in step 3, heat to 60°C for 6 h, cool and filter after treatment, wash the material with distilled water until the filtrate is neutral, and dry at 110°C for 12 h. After cooling, take out to obtain catalyst 1.

Preparation Example 2

1. Crush 150g of dried bamboo shoots with a crusher, add them together with 15g of HY catalyst into a ball mill and grind them into fine powder of 200-400 mesh, add them into a reactor, add 800ml of distilled water, seal the reactor and heat to 200℃ for hydrolysis reaction for 6h. After the reaction is completed, cool down, release the pressure, filter under reduced pressure, and distill and concentrate the filtrate to obtain 140ml of concentrated solution.

2. Add 200 ml of 10% acetic acid solution by mass to the concentrated solution in step 1 under vigorous stirring, then add 5 g of chitosan and mix well, add to a hydrothermal kettle, hydrothermally treat at 180°C for 10 hours, cool and release the pressure, wash the resulting product with anhydrous ethanol and deionized water for 3 times respectively, and then dry at 110°C for 12 hours to obtain the doped carbon material.

3. Take 20 g of the doped carbon material obtained in step 2, add 60 g of potassium hydroxide thereto, stir and mix evenly, place in a tube furnace, heat to 600°C in an inert gas atmosphere for carbonization treatment for 5 hours. After the carbonization is completed, cool down, wash the obtained material with distilled water until the filtrate is neutral, and dry at 110°C for 12 hours.

4. Add 100 ml of 30 wt% hydrochloric acid to 10 g of the doped carbon material obtained in step 3, heat to 70°C for 6 h, cool and filter after treatment, wash the material with distilled water until the filtrate is neutral, and dry at 110°C for 12 h. After cooling, take out to obtain catalyst 2.

Preparation Example 3

1. Crush 300g of dried peanut shells with a crusher, add them together with 30g of Hβ catalyst into a ball mill and grind them into fine powder of 200-400 mesh, add them into a reactor, add 1500ml of distilled water, seal the reactor and heat to 200℃ for hydrolysis reaction for 6h. After the reaction is completed, cool down, release the pressure, filter under reduced pressure, and distill and concentrate the filtrate to obtain 380ml of concentrated solution.

2. Add 400 ml of 10% hydrochloric acid solution by mass to the concentrated solution in step 1 under vigorous stirring, add 10 g of chitosan, mix well, add to a hydrothermal kettle, hydrothermally treat at 180°C for 10 h, cool and release the pressure, wash the resulting product with anhydrous ethanol and deionized water for 3 times respectively, and dry it at 110°C for 12 h to obtain the doped carbon material.

3. Take 20 g of the doped carbon material obtained in step 2, add 60 g of potassium hydroxide thereto, stir and mix evenly, place in a tube furnace, heat to 600°C in an inert gas atmosphere for carbonization treatment for 5 hours. After the carbonization is completed, cool down, wash the obtained material with distilled water until the filtrate is neutral, and dry at 110°C for 12 hours.

4. Add 100 ml of sodium hypochlorite aqueous solution (effective chlorine 6%) to 15 g of the doped carbon material obtained in step 3, heat to 60°C for 6 h, cool and filter after treatment, wash the material with distilled water until the filtrate is neutral, and dry at 110°C for 12 h. After cooling, take out to obtain catalyst 3.

Preparation Example 4

1. Crush 300g of dried peanut shells with a crusher, add them together with 30g of Hβ catalyst into a ball mill and grind them into fine powder of 200-400 mesh, add them into a reactor, add 1500ml of distilled water, seal the reactor and heat to 200℃ for hydrolysis reaction for 6h. After the reaction is completed, cool down, release the pressure, filter under reduced pressure, and distill and concentrate the filtrate to obtain 369ml of concentrated solution.

2. Add 400 ml of 10% by mass formic acid solution to the concentrated solution in step 1 under vigorous stirring, add 6 g of chitosan and mix well, add to a hydrothermal kettle, hydrothermally treat at 180°C for 10 hours, cool and release the pressure, wash the resulting product with anhydrous ethanol and deionized water for 3 times respectively, and then dry at 110°C for 12 hours to obtain the doped carbon material.

3. Take 20 g of the doped carbon material obtained in step 2, add 60 g of potassium hydroxide thereto, stir and mix evenly, place in a tube furnace, heat to 600°C in an inert gas atmosphere for carbonization treatment for 5 hours. After the carbonization is completed, cool down, wash the obtained material with distilled water until the filtrate is neutral, and dry at 110°C for 12 hours.

4. Add 100 ml of 20 wt% sulfuric acid aqueous solution to 18 g of the doped carbon material obtained in step 3, heat to 80°C for 6 h, cool and filter after treatment, wash the material with distilled water until the filtrate is neutral, and dry at 110°C for 12 h. After cooling, take out to obtain catalyst 4.

Preparation Example 5

1. 300g of dried corn stalks were crushed by a pulverizer, and then added to a ball mill together with 30g _of γ- _Al2O3 catalyst to form a fine powder of 200-400 mesh. The powder was added to a reactor, and 1500ml of distilled water was added. The reactor was sealed and heated to 200℃ for hydrolysis reaction for 6h. After the reaction, the temperature was lowered, the pressure was released, and the filtrate was filtered under reduced pressure. The filtrate was distilled and concentrated to obtain 369ml of concentrated solution.

2. Add 400 ml of 10% acetic acid solution by mass to the concentrated solution in step 1 under vigorous stirring, add 4 g of chitosan and mix well, add to a hydrothermal kettle, hydrothermally treat at 180°C for 10 hours, cool and release the pressure, wash the resulting product with anhydrous ethanol and deionized water for 3 times respectively, and then dry at 110°C for 12 hours to obtain the doped carbon material.

3. Take 20 g of the doped carbon material obtained in step 2, add 60 g of potassium hydroxide thereto, stir and mix evenly, place in a tubular furnace, heat to 600°C in a nitrogen atmosphere for carbonization treatment for 5 hours. After the carbonization is completed, cool down, wash the obtained material with distilled water until the filtrate is neutral, and dry at 110°C for 12 hours.

4. Add 100 ml of 20 wt% nitric acid aqueous solution to 15 g of the doped carbon material obtained in step 3, heat to 60°C for 6 h, cool and filter after treatment, wash the material with distilled water until the filtrate is neutral, and dry at 110°C for 12 h. After cooling, take out to obtain catalyst 5.

Preparation Example 6

1. 300g of dried corn cobs were crushed by a pulverizer, and then added to a ball mill together with 30g of _Nb2O5 to _form a fine powder of 200-400 mesh. The powder was added to a reactor, and 1500ml of distilled water was added. The reactor was sealed and heated to 200℃ for hydrolysis reaction for 6h. After the reaction, the temperature was lowered, the pressure was released, and the filtrate was filtered under reduced pressure. The filtrate was distilled and concentrated to obtain 341ml of concentrated solution.

2. Add 400 ml of 10% acetic acid solution by mass to the concentrated solution in step 1 under vigorous stirring, add 5 g of chitosan, mix well, add to a hydrothermal kettle, hydrothermally treat at 180°C for 10 hours, cool and release the pressure, wash the resulting product with anhydrous ethanol and deionized water for 3 times respectively, and then dry at 110°C for 12 hours to obtain the doped carbon material.

3. Take 20g of the doped carbon material obtained in step 2, add 60g of sodium hydroxide thereto, stir and mix evenly, place in a tubular furnace, heat to 600°C in a nitrogen atmosphere for carbonization treatment for 5h. After the carbonization is completed, cool down, wash the obtained material with distilled water until the filtrate is neutral, and dry at 110°C for 12h.

4. Add 100 ml of 40 wt% sulfuric acid aqueous solution to 15 g of the doped carbon material obtained in step 3, heat to 60°C for 6 h, cool and filter after treatment, wash the material with distilled water until the filtrate is neutral, and dry at 110°C for 12 h. After cooling, take out to obtain catalyst 6.

Comparative Example 1

1. 300g of dried corn cobs were crushed by a pulverizer, and then added to a ball mill together with 30g of _Nb2O5 to _form a fine powder of 200-400 mesh. The powder was added to a reactor, and 1500ml of distilled water was added. The reactor was sealed and heated to 200℃ for hydrolysis reaction for 6h. After the reaction, the temperature was lowered, the pressure was released, and the filtrate was filtered under reduced pressure. The filtrate was distilled and concentrated to obtain 341ml of concentrated solution.

2. Add 400 ml of 10% acetic acid solution by mass to the concentrated solution in step 1 under vigorous stirring, mix well, add to a hydrothermal kettle, hydrothermally treat at 180°C for 10 hours, cool and release the pressure, wash the resulting product with anhydrous ethanol and deionized water for three times respectively, and then dry at 110°C for 12 hours to obtain the doped carbon material.

3. Take 20g of the doped carbon material obtained in step 2, add 60g of sodium hydroxide thereto, stir and mix evenly, place in a tubular furnace, heat to 600°C in a nitrogen atmosphere for carbonization treatment for 5h. After the carbonization is completed, cool down, wash the obtained material with distilled water until the filtrate is neutral, and dry at 110°C for 12h.

4. Add 100 ml of 40 wt% sulfuric acid aqueous solution to 15 g of the doped carbon material obtained in step 3, heat to 60°C for 6 h, cool and filter after treatment, wash the material with distilled water until the filtrate is neutral, and dry at 110°C for 12 h. After cooling, take out to obtain comparative catalyst 1.

Comparative Example 2

1. 300g of dried corn cobs were crushed by a pulverizer, and then added to a ball mill together with 30g of _Nb2O5 to _form a fine powder of 200-400 mesh. The powder was added to a reactor, and 1500ml of distilled water was added. The reactor was sealed and heated to 200℃ for hydrolysis reaction for 6h. After the reaction, the temperature was lowered, the pressure was released, and the filtrate was filtered under reduced pressure. The filtrate was distilled and concentrated to obtain 341ml of concentrated solution.

2. Add 400 ml of 10% acetic acid solution by mass to the concentrated solution in step 1 under vigorous stirring, mix well, add to a hydrothermal kettle, hydrothermally treat at 180°C for 10 hours, cool and release the pressure, wash the resulting product with anhydrous ethanol and deionized water for three times respectively, and then dry at 110°C for 12 hours to obtain the doped carbon material.

3. Take 20 g of the doped carbon material obtained in step 2, add 60 g of sodium hydroxide thereto, stir and mix evenly, place in a tubular furnace, heat to 600°C in a nitrogen atmosphere for carbonization treatment for 5 hours. After the carbonization is completed, cool down, wash the obtained material with distilled water until the filtrate is neutral, and dry at 110°C for 12 hours to obtain comparative catalyst 2.

Characterization of Catalyst Products

1. Catalyst element analysis

Table 1 below shows the elemental analysis results of the catalyst products prepared in Preparation Examples 1-6 and Comparative Examples 1-2.

Table 1: Elemental analysis of catalysts

From the elemental analysis results, it can be seen that in the synthesis process of the carbon-based catalyst material, the nitrogen element in chitosan can be well doped into the carbon material, and the nitrogen content in the synthesized doped carbon material is about 10wt%. In Comparative Example 1 without adding chitosan, the nitrogen content in the obtained carbon material is low, indicating that the above synthesis method can dope the nitrogen element in chitosan into the carbon material.

2. Quantitative detection of catalyst surface groups

The Boehm titration method was used to quantitatively detect the surface groups of the prepared catalyst as follows:

Preparation:

1) Boil deionized water in an oil bath at 160°C for several minutes and store in a sealed container.

2) Prepare standard titration solutions of NaOH, HCl, Na ₂ CO ₃ , and NaHCO ₃ and determine the concentration of the standard titration solutions.

Boehm titration:

1) Weigh 3 samples of about 1.0 g each and place them in 3 conical flasks with stoppers (made of plastic and thoroughly dried), and add 50 mL of 0.05 mol/L NaOH, Na ₂ CO ₃ , and NaHCO ₃ solution respectively.

2) Place the conical flask on an oscillator for 4 hours and then leave it at room temperature for 24 hours (generally the longer the better).

3) Filter the activated carbon slurry once and take 20 mL of the filtrate.

4) Add 20 mL of 0.05 mol/L hydrochloric acid to 20 mL of the filtrate (add 40 mL of hydrochloric acid to the filtrate containing Na ₂ CO ₃ , and boil the filtrate containing Na ₂ CO ₃ or NaHCO ₃ again after adding hydrochloric acid to remove the CO ₂ ).

5) Use phenolphthalein as an indicator and back-titrate the excess acid with 0.05 mol/L standard NaOH titration solution until the solution turns slightly red.

Alkali consumption:

a＝(V*C _NaOH +20*C ₀ -20*C _HCl )*2.5/M(NaOH,NaHCO ₃ calculation formula)

a＝(V*C _NaOH +20*C ₀ -40*C _HCl )*2.5/M(Na ₂ CO ₃ calculation formula)

Calculation formula description:

The concentration _C0 in the formula is based on the equivalent _{concentration} . Since _Na2CO3 is a diprotic base, _when calculating _Na2CO3 , _C0 should be the molar concentration _{of Na2CO3} multiplied by 2.

V is the volume of NaOH consumed, _C0 is the concentration of the added alkali solution, _CHCl is the concentration of the hydrochloric acid solution used, and M is the mass of the activated carbon.

calculate:

The number of carboxyl groups is represented by the consumption of NaHCO ₃ , aNaHCO ₃ ; the number of lactone groups is represented by the difference between the consumption of Na ₂ CO ₃ and NaHCO ₃ , a Na 2 CO ₃ -a NaHCO ₃ ; the number of phenolic hydroxyl groups is represented by the difference between the consumption of NaOH and Na ₂ CO _{3 ,} aNaOH-a Na ₂ CO ₃

Table 2 below shows the results of surface group quantity analysis of the catalyst products prepared in Preparation Examples 1-6 and Comparative Examples 1-2.

Table 2: Number of groups on the catalyst surface

As can be seen from Table 2 above, the doped carbon material catalysts synthesized in this application all have abundant surface groups, and the amounts of carboxyl, lactone, and phenolic hydroxyl groups on the surface of the catalyst material are respectively about 0.5-0.6mmol/g, 0.6-0.7mmol/g, and 0.1mmol/g. In Comparative Example 2, which is not treated with an acid or/oxidant, the amounts of carboxyl, lactone, and phenolic hydroxyl groups on the surface of the obtained carbon material are respectively 0.33mmol/g, 0.21mmol/g, and 0.06mmol/g, which are significantly lower than the contents of other carbon materials described in this patent application, indicating that acid/oxidant treatment can greatly enrich the number of groups on the surface of the carbon material.

3. Detection of temperature-programmed desorption of ammonia on catalyst

The NH ₃ characterization of the catalyst product was carried out on a Micromeritics AutoChem 2920 chemical adsorption instrument. The specific experimental steps are as follows: 0.1g of sample was placed in a U-shaped quartz tube, purged at 150°C for 2h in an Ar gas atmosphere, then cooled to 100°C, and adsorbed 5wt% NH ₃ /Ar mixed gas at 100°C for 2h. Then, the physically adsorbed ammonia was purged under Ar atmosphere for 1h, and after the baseline was leveled, the temperature was raised to 800°C at a heating rate of 10°C/min. The NH ₃ signal was recorded using a TCD detector. The results are shown in Figure 2.

As can be seen from Figure 2, the NH ₃ -TPD comparison results of catalyst 6 and comparative catalysts 1-2 are shown in Figure 2. It can be seen that comparative catalyst 1 has an obvious NH ₃ desorption peak at 200°C, indicating that there are certain weak acid sites. Comparative catalyst 2 has a desorption peak at a higher temperature (about 440°C) in addition to an NH ₃ -desorption peak near 200°C, indicating that it has stronger acidity. Catalyst 6 has a stronger NH ₃ desorption peak at 230°C, which shows that acid or/oxidant treatment can significantly enhance the acidity of the catalyst surface, and the desorption temperature of NH ₃ is significantly increased, indicating that the acid strength of the catalyst is stronger and the acid amount is higher. This result is consistent with the catalyst surface group content result obtained by the above-mentioned Boehm titration test, indicating that acid/oxidant treatment can significantly increase the number of groups on the catalyst surface, thereby enhancing the catalyst surface acidity.

Reaction Example

2,2,5,5-Tetramethyltetrahydrofuran was prepared by the following steps:

2g of the above-prepared catalyst was added to a fixed bed reactor, heated to 400°C under _N2 atmosphere and maintained for 3h for activation, and then cooled to 95-110°C; at a reaction temperature of 110°C and normal pressure, 2,5-dimethyl-2,5-hexanediol was introduced into the reactor at a space velocity of 0.2-0.3h ^-1 for reaction. The reaction product was condensed and separated from the gas and liquid, and then GC was performed. The results of each catalyst are shown in Table 3 below:

Table 3. Reaction results using different catalysts

According to the reaction results of the above reaction examples, the conversion rate of comparative catalyst 2 is significantly enhanced compared with comparative catalyst 1, but its stabilization time is relatively short. This result is consistent with the above catalyst characterization results that comparative catalyst 2 has strong acidity, indicating that the treatment of acid/oxidant can significantly increase the number of groups on the catalyst surface, thereby enhancing the surface acidity of the catalyst. When molecular sieves are used as catalysts for the reaction, higher conversion rates and selectivity can be achieved, but the stability of the catalyst is poor. When the catalysts prepared according to Preparation Examples 1 to 6 of the present application are used, the selectivity of TMTHF is increased while the service life of the catalyst is significantly enhanced, so that the target product TMTHF can be produced and the efficiency is improved, thereby further reducing production energy consumption, reducing production costs, and facilitating industrial production.

The above-mentioned specific embodiments of the present application are merely preferred embodiments for explaining the present application, and are not limitations of the present application. After reading this specification, those skilled in the art may make modifications without creative contribution as needed. However, any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims (10)

A continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran is characterized in that, in the presence of a catalyst, 2,5-dimethyl-2,5-hexanediol is subjected to a dehydration reaction in a reactor at 50° C. to 200° C. The continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran according to claim 1, characterized in that the reactor in the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran is selected from any one of a continuous stirred tank reactor, a plug flow reactor, a fixed phase reactor and a fluidized bed reactor, or a mixed reactor of two or more of these reactors connected; preferably a fixed bed reactor. The method for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran according to claim 2, characterized in that it comprises at least one of the following features (a) to (g): (a) The catalyst may be in the form of strips, columns or sheets; (b) The dehydration reaction temperature may preferably be 80°C to 150°C; (c) the dehydration reaction may be carried out under one or more of a nitrogen atmosphere, a helium atmosphere and an argon atmosphere; (d) The continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran can be carried out at a reaction pressure of 0.1 MPa to 4 MPa, preferably 0.1 MPa to 2 MPa; or at a reaction pressure of normal pressure to 4 MPa, preferably normal pressure to 2 MPa; (e) the 2,5-dimethyl-2,5-hexanediol may be reacted in the absence of a solvent or in the presence of a solvent, wherein the solvent is one or more selected from tetrahydrofuran, acetonitrile and 1,4-dioxane, preferably tetrahydrofuran or 1,4-dioxane; (f) The continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran can be carried out at a reaction space velocity of 0.05 h ^-1 to 5 h ^-1 , preferably 0.1 h ^-1 to 3 h ^-1 . A method for the continuous preparation of 2,2,5,5-tetramethyltetrahydrofuran according to claim 3, characterized in that the method comprises at least one of the following features (a) and (b): (a) The preparation method further comprises activating the catalyst before the reaction: before the reaction, the catalyst is heated to an activation temperature of 300°C to 500°C and maintained for 1h to 6h; preferably, the activation temperature may be 300°C to 400°C; (b) The preparation method further comprises post-reaction treatment: the reaction product is condensed and separated into gas and liquid, and then distilled. The continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran according to claim 1, characterized in that the catalyst used therein is prepared by a method comprising the following steps: (1) The biomass raw material is crushed by a pulverizer, and then added to a ball mill together with a solid acid catalyst to be ball-milled into fine powder, which is then added to a reactor, and distilled water is added. The reactor is sealed and heated to perform a hydrolysis reaction. After the reaction is completed, the temperature is lowered, the pressure is released, and the filtrate is filtered under reduced pressure, and the filtrate is distilled and concentrated into a concentrated solution; (2) adding the acid solution to the concentrated solution in step (1) under vigorous stirring, mixing evenly, adding chitosan, transferring to a hydrothermal kettle, performing hydrothermal treatment, cooling, and relieving pressure, washing the obtained product with anhydrous ethanol and deionized water, respectively, and drying to obtain a doped carbon material; (3) adding alkali to the doped carbon material obtained in step (2), stirring and mixing, placing in a tube furnace, heating in an inert gas atmosphere for carbonization treatment, cooling after carbonization, washing the obtained material with distilled water until the filtrate is neutral, and drying; (4) The doped carbon material obtained in step (3) is mixed and stirred evenly with an acid or an oxidant, and subjected to heat treatment. After the treatment, the temperature is reduced and filtered, and the material is washed with distilled water until the filtrate is neutral, and then dried. The method for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran according to claim 5, characterized in that in the step (1), the biomass material is selected from one or more of corn cobs, corn stalks, sawdust, peanut shells, and bamboo shoots; preferably one or more of corn cobs, corn stalks, and peanut shells, and more preferably one or more of corn cobs and corn stalks; The solid acid catalyst is selected from one or more of silicon dioxide, γ-alumina, zirconium dioxide, cerium dioxide, tungsten trioxide, niobium pentoxide, zeolite molecular sieve, and ion exchange resin; The solid acid catalyst is preferably one or more of silicon dioxide, γ-alumina, tungsten trioxide, niobium pentoxide, zeolite molecular sieve, and ion exchange resin; more preferably one or more of γ-alumina, zeolite molecular sieve, and ion exchange resin; The zeolite molecules are selected from one or more of HZSM5, HZSM11, HY, Hβ, HMOR, and SAPO-34; In the step (1): The dried biomass raw material is crushed by a pulverizer, and then added to a ball mill together with a solid acid catalyst to be ball-milled into a fine powder of 200-400 meshes, and the reactor is sealed and heated to a temperature of 150-250°C; The mass ratio of distilled water to biomass raw material is 50:1-2:1; preferably 20:1-5:1; The hydrolysis reaction temperature is 120-250°C; preferably 150-220°C; more preferably 160-210°C; The hydrolysis reaction time is 4-10h; preferably 4-6h; The mass concentration of the concentrated solution is 10%-30%, preferably 10%-20%. The method for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran according to claim 5, characterized in that in the step (2): The acid is selected from one or more of formic acid, acetic acid, propionic acid, and hydrochloric acid; The mass concentration of the acid solution is 1%-30%, preferably 3%-10%; The mass ratio of the acid solution to the concentrated solution is 1:1-10:1; preferably 1:1-5:1; The mass ratio of chitosan to concentrated solution is 1:10-1:100; The hydrothermal treatment temperature is 160-220°C; preferably 180-210°C; The hydrothermal treatment time is 4-20 hours; preferably 5-10 hours; The obtained product was washed with anhydrous ethanol and deionized water three times respectively, and dried at 110 °C for 12 h. The method for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran according to claim 5, characterized in that in the step (3): The base is selected from one or more of sodium hydroxide, potassium hydroxide, sodium methoxide, potassium methoxide, sodium ethoxide, and potassium ethoxide; The mass ratio of the base to the doped carbon material is 1:1-10:1; preferably 1:1-5:1; more preferably 1:1-3:1; The inert gas used in the carbonization process is selected from one or more of nitrogen, helium and argon; preferably one or more of nitrogen and argon; The carbonization treatment temperature is 300-700°C, and the treatment time is 4-20h; after the carbon is washed, it is dried at 110°C for 12h. The method for continuously preparing 2,2,5,5-tetramethyltetrahydrofuran according to claim 5, characterized in that in the step (4): The acid is one or more of sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid, and phosphoric acid; The oxidant is one or more of hydrogen peroxide with a mass concentration of 30wt% and sodium hypochlorite with an effective chlorine content of 6%; The mass ratio of the acid to the doped carbon material is 1:1-10:1; The mass ratio of the oxidant to the doped carbon material is 1:1-10:1; The heating treatment temperature is 60-90°C; The heating treatment time is 4-10h; After washing, dry at 110°C for 12h; After the reaction in step (1) is completed, the filter cake obtained by suction filtration contains the solid acid catalyst. The filter cake is calcined at 350-550° C. in an air atmosphere for 3-6 hours to remove organic matter to obtain the solid acid catalyst. The obtained solid acid catalyst can be repeatedly recycled. Use of the catalyst in the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran according to any one of claims 1 to 9 in the continuous preparation method of 2,2,5,5-tetramethyltetrahydrofuran.