Method for preparing bio-oil and biochar from waste cotton

Abstract

The invention relates to a method for preparing bio-oil and biochar from waste cotton, which comprises the following steps: by taking polar aprotic solvents including tetrahydrofuran, 4-methyltetrahydrofuran, ethyl acetate and ethyl butyrate as reaction solvents and sulfuric acid, phosphoric acid and the like as catalysts, synthesizing bio-oil and biochar from waste cotton under mild and low-energy-consumption conditions; the acid catalyst and the polar aprotic solvent effectively depolymerize cellulose in the waste cotton and catalyze pyrolysis of intermediate products to directionally dehydrate the intermediate products into high-value platform compounds such as levoglucosenone and furfural, and the solid-phase byproduct charcoal has a hierarchical porous structure and a high specific surface area and has a wide application prospect. The process provides a novel sustainable method for recycling, harmlessness and greenization of the waste cotton, and has both environmental benefits and economic benefits.

Description

Technical Field

[0001] The present invention belongs to the fields of environmental protection and comprehensive utilization of resources, and particularly relates to a method for preparing bio-oil and biochar from waste cotton. Background Art

[0002] With the rapid development of the global fast fashion industry, the consumption of clothing has been continuously increasing, and nearly 100 million tons of waste cotton textiles are generated every year. A large amount of waste cotton textiles also bring serious environmental problems. Since incineration and landfill have the lowest treatment costs, they are the most popular waste cotton textile treatment methods globally at present, but both will bring serious environmental pollution problems. Incineration will emit CO 2 , CO, nitrogen oxides, and release dioxins and heavy metals due to incomplete combustion; while landfill not only has a long degradation period and occupies land, but also may release methane with extremely strong greenhouse effect under anaerobic conditions, and pollute soil and water sources due to chemical substance leakage and microplastic generation. However, at the same time, since the cellulose content in waste cotton exceeds 90%, it has a very broad prospect of resource utilization.

[0003] The resource utilization methods of waste cotton mainly include mechanical methods, biological methods, and chemical methods. Since the main component of waste cotton, cellulose, has a dense hydrogen bond cross-linking network, while enhancing its mechanical properties, it also makes it insoluble in conventional solvent systems. Therefore, the mechanical and biological treatment methods currently used in the resource utilization of waste cotton generally have defects such as low conversion efficiency and insufficient added value of recycled products. As a waste cotton treatment method with relatively low process cost and stable degradation efficiency, thermochemical conversion has great advantages.

[0004] Thermochemical conversion usually has methods such as combustion, gasification, and pyrolysis to convert waste cotton into solids (biochar), gases (CO and H 2), and products such as liquid (bio-oil). Gasification technology is prone to cause secondary pollution problems due to the difficulty in effectively suppressing the generation of by-products, and the low energy density characteristics of syngas lead to a significant increase in the energy consumption for its storage and transportation, and its economy is limited by downstream supporting conversion facilities; although the combustion process can directly release heat energy, the emissions of polluting gases such as nitrogen oxides and sulfur oxides rely on end-treatment, and the inherent low calorific value of waste cotton limits its thermodynamic efficiency. In contrast, pyrolysis technology converts waste cotton into platform chemicals such as bio-oil through controllable thermochemical cracking reactions, realizing the efficient and low-carbon utilization of waste cotton. The bio-oil produced by cellulose pyrolysis mainly includes levoglucosenone (LGO), furfural, levulinic acid, etc. As a high-value platform compound, LGO shows excellent application potential in fields such as chemical synthesis and green solvents. Furfural can be used as a raw material for various commodities and fine chemicals such as polymers, resins, solvents, and biofuels. The solid-phase product biochar of pyrolysis not only has application value in soil improvement and food security, but also has broad industrial application prospects in pollution control and environmental safety. However, the existing pyrolysis patent technologies for waste cotton currently have problems such as high energy consumption and low bio-oil yield. For example, in Chinese Patent Application No. 202211106878.4, a pyrolysis method using a solid catalyst in the gas phase is adopted. However, due to the low mass transfer efficiency in the gas phase, it is difficult to improve the yield of high-value-added bio-oil; at the same time, the reaction temperature is as high as 700 - 900 °C, resulting in huge energy consumption. The present invention uses a polar aprotic solvent to carry out homogeneous catalytic pyrolysis treatment on waste cotton at a lower temperature, thereby significantly improving the mass transfer efficiency and reducing the energy consumption, and realizing a significant increase in the yields of bio-oil and biochar. Summary of the Invention

[0005] The object of the present invention is to provide a method for preparing bio-oil and biochar from waste cotton.

[0006] A method for preparing bio-oil and biochar from waste cotton proposed by the present invention specifically comprises the following steps: (1) Add a polar aprotic solvent to a reaction kettle equipped with a magnetic stirrer; (2) Dissolve an acid in the polar aprotic solvent described in step (1), stir for 1 - 10 min, and control the acid concentration to be 1 - 100 mM, to obtain a solution system; (3) Add waste cotton to the solution system obtained in step (2), seal the reaction kettle, and purge with an inert gas to discharge air; (4) Stir and heat the solution system of the reaction kettle described in step (3) to a predetermined temperature, and maintain it at this predetermined temperature for a certain period of time; (5) Quench the reaction of the reaction kettle after the reaction in step (4) in cold water, cool it to room temperature, and filter to obtain the liquid-phase product bio-oil and the solid-phase product; (6) After drying the solid-phase product obtained in step (5) in an oven, biochar is obtained.

[0007] In the present invention, the polar aprotic solvent in step (1) is any one or a mixture of several of tetrahydrofuran, ethyl acetate, ethyl butyrate, 2-methyltetrahydrofuran, or dichloromethane.

[0008] In the present invention, the acid in step (2) is any one or a mixture of several of sulfuric acid or phosphoric acid.

[0009] In the present invention, the concentration of the acid in step (2) is 0 - 100 mM.

[0010] In the present invention, the amount of waste cotton added in step (3) is any loading amount in the range of 0.1 - 50 wt% (total mass percentage of the reaction system).

[0011] In the present invention, the inert gas used for purging in step (3) is any one or a mixture of two of nitrogen or argon.

[0012] In the present invention, the initial pressure in the reactor after purging in step (3) is 0 - 7 MPa.

[0013] In the present invention, the predetermined temperature in step (4) is 150 - 270 °C.

[0014] In the present invention, the reaction time in step (4) is 5 - 120 min.

[0015] The present invention is a method for catalytic pyrolysis of waste cotton in the liquid phase to depolymerize cellulose in waste cotton into high-value bio-oil and biochar under certain temperature and time conditions. The preparation process of this technology is simple, with low cost, rapid reaction and mild conditions, providing a new and sustainable method for the resource recovery and utilization of waste cotton, and having a positive significance for the environmental pollution and energy waste problems of waste cotton textiles.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention uses a polar aprotic solvent as the reaction solvent and an acid as the catalyst to carry out liquid-phase catalytic pyrolysis of waste cotton. Based on acid catalysis to reduce the activation energy for the selective generation of bio-oil from waste cotton, and at the same time, the polar aprotic solvent effectively dissolves intermediate products to improve mass transfer efficiency and amplify the acid-catalysis effect. According to the cellulose catalytic pyrolysis method, selective catalytic pyrolysis is carried out on it, and it is efficiently converted into high-value levoglucosenone, furfural, and biochar, thereby realizing the resourceful, harmless, and high-value utilization of waste cotton.

[0017] The present invention provides a process for the liquid-phase catalytic pyrolysis of waste cotton with high production efficiency, low cost, and mild conditions. The acid can effectively depolymerize cellulose in waste cotton and catalytically pyrolyze the primary products to dehydrate and selectively convert them into high-value bio-oils such as levoglucosenone and furfural. The polar aprotic solvent dissolves intermediate products, improves mass transfer efficiency, and amplifies the acid-catalyzed effect. Compared with other pyrolysis technologies for waste cotton, the reaction temperature of this technology is lower, reducing energy consumption. At the same time, the yield of high-value bio-oil is relatively high, realizing the high-value utilization of waste cotton. The solid-phase by-product of the present invention has a hierarchical porous structure and a high specific surface area, and the surface is rich in various free radical groups, showing broad industrial application prospects and realizing waste-free production. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a basic flow chart of the process of the present invention.

[0019] Figure 2 It is the GC-MS spectrum of the liquid-phase product in Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] The following examples are used to further illustrate the present invention and are not intended to limit the present invention.

[0021] Example 1 67 μL of 98% concentrated sulfuric acid was added to a Hastelloy reaction kettle containing 60 mL of tetrahydrofuran, and stirred for 5 min. Then, 0.5352 g of waste cotton powder was added. The reaction kettle lid was covered and the nuts on the lid top were tightened in diagonal order and all pressure relief valves were tightened. The reaction kettle was transferred to a heating protection sleeve. The gas cylinder valve was opened to introduce high-purity argon into the reaction kettle, and then the gas in the reaction kettle was pumped dry with a vacuum pump, and this was repeated six times to exhaust the air in the reaction kettle. Finally, argon was introduced into the reaction kettle to maintain an inert atmosphere in the reaction kettle and control the initial reaction pressure to 0 MPa. The rotor speed was adjusted to 600 rpm, and the reaction temperature was raised to 210 °C within 20 min and maintained for 15 min. Immediately after the reaction ended, it was placed in cold water to stop the reaction. After cooling to room temperature, the product was taken out and filtered. The solid-phase product was placed in an oven at 70 °C and dried for 12 h. n-Dodecane was added to the liquid-phase product as an internal standard and detected by gas chromatography-mass spectrometry (GC-MS). Finally, the bio-oil yield was 40.33%, and the biochar yield was 15.73%.

[0022] Example 2 67 μL of 98% concentrated sulfuric acid was added to a Hastelloy gold reactor containing 60 mL of tetrahydrofuran, and stirred for 5 min. Subsequently, 0.5352 g of waste cotton powder was added. The reactor lid was covered, and the nuts on the lid top were tightened in sequence according to the diagonal order, and all pressure relief valves were tightened. The reactor was transferred to a heating protection sleeve. The gas cylinder valve was opened to introduce high-purity argon into the reactor, and then the gas in the reactor was pumped dry with a suction pump, repeated six times to exhaust the air in the reactor. Finally, argon was introduced into the reactor to maintain an inert atmosphere in the reactor and control the initial reaction pressure at 0 MPa. The rotor speed was adjusted to 600 rpm, and the temperature was raised to the reaction temperature of 190 °C within 20 min and maintained for 15 min. Immediately after the reaction ended, it was placed in cold water to stop the reaction. After cooling to room temperature, the product was taken out and filtered. The solid-phase product was placed in an oven at 70 °C and dried for 12 h. n-Dodecane was added to the liquid-phase product as an internal standard, and gas chromatography-mass spectrometry (GC-MS) was used for detection. Finally, the bio-oil yield was 44.44%, and the biochar yield was 15.76%.

[0023] Example 3 67 μL of 98% concentrated sulfuric acid was added to a Hastelloy gold reactor containing 60 mL of ethyl butyrate, and stirred for 5 min. Subsequently, 0.5352 g of waste cotton powder was added. The reactor lid was covered, and the nuts on the lid top were tightened in sequence according to the diagonal order, and all pressure relief valves were tightened. The reactor was transferred to a heating protection sleeve. The gas cylinder valve was opened to introduce high-purity argon into the reactor, and then the gas in the reactor was pumped dry with a suction pump, repeated six times to exhaust the air in the reactor. Finally, argon was introduced into the reactor to maintain an inert atmosphere in the reactor and control the initial reaction pressure at 0 MPa. The rotor speed was adjusted to 600 rpm, and the temperature was raised to the reaction temperature of 210 °C within 20 min and maintained for 15 min. Immediately after the reaction ended, it was placed in cold water to stop the reaction. After cooling to room temperature, the product was taken out and filtered. The solid-phase product was placed in an oven at 70 °C and dried for 12 h. n-Dodecane was added to the liquid-phase product as an internal standard, and gas chromatography-mass spectrometry (GC-MS) was used for detection. Finally, the bio-oil yield was 32.38%, and the biochar yield was 16.31%.

[0024] Example 4 294 μL of 90% concentrated phosphoric acid was added to a Hastelloy gold reactor containing 60 mL of tetrahydrofuran, and stirred for 5 min. Subsequently, 0.5352 g of waste cotton was added. The reactor lid was covered, and the nuts on the lid top were tightened in sequence according to the diagonal order, and all pressure relief valves were tightened. The reactor was transferred to a heating protection sleeve. The gas cylinder valve was opened to introduce high-purity argon into the reactor, and then the gas in the reactor was pumped dry with a suction pump, and this was repeated six times to exhaust the air in the reactor. Finally, argon was introduced into the reactor to maintain an inert atmosphere in the reactor and control the initial reaction pressure at 0 MPa. The rotor speed was adjusted to 600 rpm, and the temperature was raised to the reaction temperature of 210 °C within 20 min and maintained for 15 min. Immediately after the reaction ended, it was placed in cold water to stop the reaction. After cooling to room temperature, the product was taken out and filtered. The solid-phase product was placed in an oven at 70 °C and dried for 12 h. n-Dodecane was added to the liquid-phase product as an internal standard, and gas chromatography-mass spectrometry (GC-MS) was used for detection. Finally, the bio-oil yield was 26.89%, and the biochar yield was 34.25%.

[0025] Example 5 67 μL of 98% concentrated sulfuric acid was added to a Hastelloy gold reactor containing 60 mL of tetrahydrofuran, and stirred for 5 min. Subsequently, 2.81 g of waste cotton powder was added. The reactor lid was covered, and the nuts on the lid top were tightened in sequence according to the diagonal order, and all pressure relief valves were tightened. The reactor was transferred to a heating protection sleeve. The gas cylinder valve was opened to introduce high-purity argon into the reactor, and then the gas in the reactor was pumped dry with a suction pump, and this was repeated six times to exhaust the air in the reactor. Finally, argon was introduced into the reactor to maintain an inert atmosphere in the reactor and control the initial reaction pressure at 0 MPa. The rotor speed was adjusted to 600 rpm, and the temperature was raised to the reaction temperature of 210 °C within 20 min and maintained for 15 min. Immediately after the reaction ended, it was placed in cold water to stop the reaction. After cooling to room temperature, the product was taken out and filtered. The solid-phase product was placed in an oven at 70 °C and dried for 12 h. n-Dodecane was added to the liquid-phase product as an internal standard, and gas chromatography-mass spectrometry (GC-MS) was used for detection. Finally, the bio-oil yield was 24.84%, and the biochar yield was 21.43%.

[0026] Example 6 67 μL of 98% concentrated sulfuric acid was added to a Hastelloy gold reactor containing 60 mL of tetrahydrofuran, and stirred for 5 min. Subsequently, 0.5352 g of waste cotton powder was added. The reactor lid was covered, and the nuts on the lid top were tightened in sequence according to the diagonal order, and all pressure relief valves were tightened. The reactor was transferred to a heating protective sleeve. The gas cylinder valve was opened to introduce high-purity argon into the reactor, and then the gas in the reactor was pumped dry with a vacuum pump, and this was repeated six times to exhaust the air in the reactor. Finally, argon was introduced into the reactor to maintain an inert atmosphere in the reactor and control the initial reaction pressure to 0 MPa. The rotor speed was adjusted to 600 rpm, and the temperature was raised to the reaction temperature of 210 °C within 20 min and maintained for 45 min. Immediately after the reaction ended, it was placed in cold water to stop the reaction. After cooling to room temperature, the product was taken out and filtered. The solid-phase product was placed in an oven at 70 °C and dried for 12 h. n-Dodecane was added to the liquid-phase product as an internal standard, and gas chromatography-mass spectrometry (GC-MS) was used for detection. Finally, the bio-oil yield was 51.06%, and the biochar yield was 14.98%.

[0027] Example 7 67 μL of 98% concentrated sulfuric acid was added to a Hastelloy gold reactor containing 60 mL of tetrahydrofuran, and stirred for 5 min. Subsequently, 0.5352 g of waste cotton powder was added. The reactor lid was covered, and the nuts on the lid top were tightened in sequence according to the diagonal order, and all pressure relief valves were tightened. The reactor was transferred to a heating protective sleeve. The gas cylinder valve was opened to introduce high-purity argon into the reactor, and then the gas in the reactor was pumped dry with a vacuum pump, and this was repeated six times to exhaust the air in the reactor. Finally, argon was introduced into the reactor to maintain an inert atmosphere in the reactor and control the initial reaction pressure to 0 MPa. The rotor speed was adjusted to 600 rpm, and the temperature was raised to the reaction temperature of 210 °C within 20 min and maintained for 5 min. Immediately after the reaction ended, it was placed in cold water to stop the reaction. After cooling to room temperature, the product was taken out and filtered. The solid-phase product was placed in an oven at 70 °C and dried for 12 h. n-Dodecane was added to the liquid-phase product as an internal standard, and gas chromatography-mass spectrometry (GC-MS) was used for detection. Finally, the bio-oil yield was 22.93%, and the biochar yield was 19.62%.

[0028] Example 8 67 μL of 98% concentrated sulfuric acid was added to a Hastelloy gold reactor containing 60 mL of tetrahydrofuran, and the mixture was stirred for 5 min. Subsequently, 0.5352 g of waste cotton powder was added. The reactor lid was covered, and the nuts on the lid top were tightened in sequence according to the diagonal order, and all pressure relief valves were tightened. The reactor was transferred to a heating protection sleeve. The gas cylinder valve was opened to introduce high-purity argon into the reactor, and then the gas in the reactor was pumped dry with a vacuum pump and repeated six times to expel the air in the reactor. Finally, argon was introduced into the reactor to maintain an inert atmosphere in the reactor and control the initial reaction pressure to 7 MPa. The rotor speed was adjusted to 600 rpm, and the temperature was raised to the reaction temperature of 270 °C within 20 min and maintained for 60 min. Immediately after the reaction ended, it was placed in cold water to stop the reaction. After cooling to room temperature, the product was taken out and filtered. The solid-phase product was placed in an oven at 70 °C and dried for 12 h. n-Dodecane was added to the liquid-phase product as an internal standard, and gas chromatography-mass spectrometry (GC-MS) was used for detection. Finally, the bio-oil yield was 13.78%, and the biochar yield was 25.42%.

[0029] Example 9 67 μL of 98% concentrated sulfuric acid was added to a Hastelloy gold reactor containing 60 mL of tetrahydrofuran, and the mixture was stirred for 5 min. Subsequently, 0.5352 g of waste cotton powder was added. The reactor lid was covered, and the nuts on the lid top were tightened in sequence according to the diagonal order, and all pressure relief valves were tightened. The reactor was transferred to a heating protection sleeve. The gas cylinder valve was opened to introduce high-purity nitrogen into the reactor, and then the gas in the reactor was pumped dry with a vacuum pump and repeated six times to expel the air in the reactor. Finally, nitrogen was introduced into the reactor to maintain an inert atmosphere in the reactor and control the initial reaction pressure to 0 MPa. The rotor speed was adjusted to 600 rpm, and the temperature was raised to the reaction temperature of 150 °C within 20 min and maintained for 30 min. Immediately after the reaction ended, it was placed in cold water to stop the reaction. After cooling to room temperature, the product was taken out and filtered. The solid-phase product was placed in an oven at 70 °C and dried for 12 h. n-Dodecane was added to the liquid-phase product as an internal standard, and gas chromatography-mass spectrometry (GC-MS) was used for detection. Finally, the bio-oil yield was 11.06%, and the biochar yield was 28.55%.

[0030] Example 10 Add 0.5352 g of waste cotton powder into a Hastelloy reaction kettle containing 60 mL of tetrahydrofuran. Cover the reaction kettle lid and tighten the nuts on the lid top in diagonal order and tighten all pressure relief valves. Transfer the reaction kettle to a heating protective sleeve. Open the gas cylinder valve to introduce high-purity gas into the reaction kettle, and then use a vacuum pump to pump the gas in the reaction kettle dry. Repeat this six times to exhaust the air in the reaction kettle. Finally, introduce argon gas into the reaction kettle to maintain an inert atmosphere in the reaction kettle and control the initial reaction pressure to 0 MPa. Adjust the rotor speed to 600 rpm, heat up to a reaction temperature of 210 °C within 20 min and maintain for 15 min. Immediately put it into cold water to stop the reaction after the reaction ends. After cooling to room temperature, take out the product and filter it. The solid-phase product is placed in an oven at 70 °C and dried for 12 h. Add n-dodecane as an internal standard to the liquid-phase product and use gas chromatography-mass spectrometry (GC-MS) for detection. Finally, the bio-oil yield is 8.30% and the biochar yield is 14.31%.

[0031] Example 11 Add 67 μL of 98% concentrated sulfuric acid into a Hastelloy reaction kettle containing 60 mL of tetrahydrofuran, stir for 5 min, and then add 0.5352 g of waste cotton powder. Cover the reaction kettle lid and tighten the nuts on the lid top in diagonal order and tighten all pressure relief valves. Transfer the reaction kettle to a heating protective sleeve. Open the gas cylinder valve to introduce high-purity argon gas into the reaction kettle, and then use a vacuum pump to pump the gas in the reaction kettle dry. Repeat this six times to exhaust the air in the reaction kettle. Finally, introduce argon gas into the reaction kettle to maintain an inert atmosphere in the reaction kettle and control the initial reaction pressure to 0 MPa. Adjust the rotor speed to 600 rpm, heat up to a reaction temperature of 210 °C within 20 min and maintain for 120 min. Immediately put it into cold water to stop the reaction after the reaction ends. After cooling to room temperature, take out the product and filter it. The solid-phase product is placed in an oven at 70 °C and dried for 12 h. Add n-dodecane as an internal standard to the liquid-phase product and use gas chromatography-mass spectrometry (GC-MS) for detection. Finally, the bio-oil yield is 40.33% and the biochar yield is 15.73%.

[0032] Example 12 320 μL of 98% concentrated sulfuric acid was added to a Hastelloy gold reactor containing 60 mL of ethyl butyrate, and the mixture was stirred for 5 min. Subsequently, 0.5338 g of waste cotton powder was added. The reactor lid was covered, and the nuts on the lid top were tightened in sequence according to the diagonal order, and all pressure relief valves were tightened. The reactor was transferred to a heating protective sleeve. The gas cylinder valve was opened to introduce high-purity argon gas into the reactor, and then the gas in the reactor was pumped dry with a suction pump, and this was repeated six times to exhaust the air in the reactor. Finally, argon gas was introduced into the reactor to maintain an inert atmosphere in the reactor and control the initial reaction pressure at 0 MPa. The rotor speed was adjusted to 600 rpm, and the temperature was raised to the reaction temperature of 210 °C within 20 min and maintained for 15 min. Immediately after the reaction ended, it was placed in cold water to stop the reaction. After cooling to room temperature, the product was taken out and filtered. The solid-phase product was placed in an oven at 70 °C and dried for 12 h. n-Dodecane was added to the liquid-phase product as an internal standard, and gas chromatography-mass spectrometry (GC-MS) was used for detection. Finally, the bio-oil yield was 11.23%, and the biochar yield was 51.25%.

Claims

1. A method for preparing bio-oil and bio-char from waste cotton, characterized in that The specific steps are as follows: (1) Adding a polar aprotic solvent into a reaction kettle equipped with a magnetic stirrer; (2) dissolving the acid in the polar aprotic solvent described in step (1), stirring for 1-10 min, and controlling the acid concentration to be 1-100 mM. A solution system is obtained; (3) adding the waste cotton to the solution system obtained in step (2), sealing the reaction kettle, and purging with inert gas to expel air; (4) stirring and heating the solution system in the reaction kettle of step (3) to a predetermined temperature, and maintaining the solution system at the predetermined temperature for a certain period of time; (5) placing the reactor after the reaction in step (4) in cold water to quench the reaction, cooling to room temperature, and filtering to obtain a liquid product bio-oil and a solid product; (6) The solid phase product obtained in step (5) is dried in an oven to obtain biochar.

2. The method according to claim 1, characterized in that: The polar aprotic solvent in step (1) is any one of tetrahydrofuran, ethyl acetate, ethyl butyrate, 2-methyltetrahydrofuran or dichloromethane, or a mixture of several of them.

3. The method according to claim 1, characterized in that: The acid in step (2) is any one of sulfuric acid and phosphoric acid or a mixture of several thereof, and the concentration of the acid in step (2) is 0-100 mM.

4. The method according to claim 1, characterized in that: The amount of waste cotton added in step (3) is any loading amount in the range of 0.1-50wt% of the total mass of the reaction system.

5. The method according to claim 1, characterized in that: The inert gas used for purging in step (3) is either nitrogen or argon or a mixture of the two.

6. The method according to claim 1, characterized in that: The initial pressure in the reactor after purging in step (3) is 0-7 MPa.

7. The method according to claim 1, characterized in that: The predetermined temperature in step (4) is 150-270°C.

8. The method according to claim 1, characterized in that: The reaction time in step (4) is 5-120 min.

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