Method for preparing high-purity aromatic hydrocarbon from crude benzene liquid phase

Abstract

The invention belongs to the technical field of chemical industry, and relates to a method for preparing high-purity aromatic hydrocarbon from a crude benzene liquid phase, which comprises the following steps: filtering a crude benzene raw material, adding a certain amount of polymerization inhibitor, pre-distilling, separating light components including benzene and toluene from the tower top, extracting xylene and styrene from the side line, and separating heavy benzene and polymerization inhibitor from the tower bottom; carrying out hydrogenation reaction on light components at the top of the tower, and simultaneously carrying out hydrogenation saturation on dialkene and olefin; the method has the advantages that the process is simple, the operation is convenient, the aromatic hydrocarbon product produced at low temperature and low pressure is high in purity, stable in quality and low in energy consumption, and the economic benefit is very remarkable.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coal chemical production, and particularly relates to a process for producing benzene series aromatic hydrocarbons from crude benzene. BACKGROUND

[0002] Crude benzene is a light distillate oil recovered from the crude coal gas of coal coking, and the main components are medium benzene, toluene and dimethylbenzene, with the content of more than 90%. Since the sulfuric acid washing method is eliminated due to low product yield and serious environmental pollution of by-product acid tar, the catalytic hydrogenation method is generally used to treat the crude benzene at present.

[0003] In the 1960s, the high-temperature crude benzene hydrogenation refining process (Litol method) for producing pure benzene was successfully developed by the United States Huthinson Air Products Company. The process produces pure benzene products with a purity of 99.9% and S < 1.5 ppm through hydrogenation refining and dealkylation under high temperature (600-630 ℃) and high pressure (5.7-6.0 MPa).

[0004] In the 1980s, the United States and Germany successively developed the medium-low temperature crude benzene hydrogenation refining technology (sulfolane and KK method). The process produces benzene, toluene and dimethylbenzene products with a purity of ≥99.9% and S < 0.5 ppm through hydrogenation refining and extraction distillation under medium-low temperature (320-380 ℃) and medium pressure (3.5-3.7 MPa), and the yield is about 98%.

[0005] The above three kinds of crude benzene hydrogenation processes are relatively complex, and the hydrogenation reaction is second-order, which is divided into pre-reaction and main reaction. If the pre-reaction is not performed, the main reactor is prone to coking, plugging and other phenomena, so that the reaction cannot be normally performed.

[0006] The Chinese patent CN101028985A discloses a method for producing benzene series aromatic hydrocarbons from crude benzene. In order to solve the problem that a small amount of unstable substances such as diene, styrene, indene, indane and gum in the crude benzene are polymerized and coked when heated, thereby affecting the operation of the reactor, 1:9-9:1 constant three-line oil (straight-run diesel oil) is added to the crude benzene, so that the pre-fractionation and pre-hydrogenation are cancelled, and the hydrogenated oil is sent into the heavy component removal tower after the heavy components are removed, and then is sent into the extraction process. The characteristics of the method are that a large amount of constant three-line oil is added to the crude benzene to dilute the unstable substances in the raw material, so as to prevent the coking and plugging of the hydrogenation reaction; and the disadvantages are that the energy consumption of the production process is increased, the regeneration device of the constant three-line oil is increased, and the hydrogenated oil still needs to be removed from the heavy components, which is consistent with the traditional pre-distillation function.

[0007] Chinese patent CN1948244A discloses a process for preparing refined benzene by hydrodesulfurization and hydrodenitrogenation of coking benzene. The process is a production process combining pretreatment and three-stage hydrogenation. The pretreatment is to remove heavy components to obtain benzene, toluene and xylene fractions for hydrogenation reaction. The hydrogenation reaction is three-stage, the first stage is preliminary hydrodesulfurization by NiMo catalyst, the second stage is hydrodesulfurization and hydrodenitrogenation by CoMoXP catalyst, and the third stage is removal of inorganic sulfur by ZnO. The process has the characteristics that the obtained pure benzene content is greater than 99.9%, sulfur content is less than 1 mg / l, and nitrogen content is less than 5 mg / l; the disadvantage is that the process is complex, and the investment and operating cost is large.

[0008] Catalytic hydrogenation can be divided into high-temperature hydrogenation and low-temperature hydrogenation according to the reaction temperature. The most typical way of high-temperature hydrogenation is to separate the crude benzene into light benzene and heavy benzene in a pre-fractionating column, and then the light benzene is mixed with hydrogen gas in an evaporator by a high-pressure pump, and the mixture of aromatic hydrocarbon vapor and hydrogen gas is introduced into a pre-reactor from the top of the column. The hydrogenation conditions are that the pre-reactor temperature is 200-250°C, the pressure is 5.7 MPa, the catalyst is CoMo catalyst, the pre-hydrogenation reaction can hydrogenate the styrene homologues which are easy to polymerize at high temperature, so as to prevent polymerization in the main reactor, and the main reactor temperature is generally 610-630°C, the pressure is 5-10 MPa, and the catalyst is Cr-based catalyst. The hydrogenation cracking, dealkylation, desulfurization and other reactions are completed in the main reactor.

[0009] The reaction conditions of high-temperature hydrogenation reaction are relatively harsh, the equipment requirements are high, and the implementation difficulty is large. The low-temperature hydrogenation process mainly includes low-temperature gas phase hydrogenation process and low-temperature gas-liquid two-phase hydrogenation process. The low-temperature gas phase hydrogenation process is to remove heavy components from the crude benzene to obtain light benzene with a boiling point less than 185°C in a two-benzene column, vaporize the light benzene through an evaporation system, and then carry out desulfurization, denitrogenation and olefin saturation reactions under the action of Ni-Mo catalyst at a temperature of 190-220°C and a pressure of 2.4-2.8 MPa, so that the unsaturated compounds are converted into saturated compounds. The obtained saturated compounds are extracted and rectified to obtain pure benzene, toluene and xylene with good purity.

[0010] The existing technology shows that the existing crude benzene refining process has many disadvantages, such as waste of raw materials, easy coking and plugging of the reaction system, and high requirement for reaction temperature. SUMMARY

[0011] The purpose of this invention is to overcome the defects of the existing technology. This purpose is achieved as follows: A method for preparing high-purity aromatics from crude benzene in liquid phase: After filtering the crude benzene feedstock, a certain amount of polymerization inhibitor is added. After pre-distillation, a light component including benzene and toluene is separated from the top of the column, xylene and styrene are collected from the side stream, and heavy benzene and polymerization inhibitor are separated from the bottom of the column. The light component from the top of the column undergoes a hydrogenation reaction in a single-stage reactor equipped with a hydrogenation catalyst, while simultaneously hydrogenating saturated dienes and olefins. The hydrogenated oil obtained from the hydrogenation reaction is desulfurized and denitrogenated, and then extracted and distilled to obtain high-purity aromatics. The specific process includes the following steps: (1) Pre-distillation: After filtering the crude benzene raw material, a certain amount of polymerization inhibitor is added and then sent to the first distillation column for pre-distillation. The light benzene fraction is collected from the top of the column and sent to the reflux tank. Part of it is used as the top reflux liquid and the rest is sent to the hydrogenation process. Xylene and a small amount of toluene and styrene fractions are collected from the side stream in the middle of the column and sent to the xylene column for rectification. Heavy benzene containing polymerization inhibitor is collected from the bottom of the column and sent to the heavy component product storage tank.

[0012] The pre-distillation column operates at a pressure of 50 kPa and a temperature of 80–140 °C; the reflux ratio at the top of the column is 0.5–2.0. The polymerization inhibitor is selected from one or more of the following: 2,2,4-trimethyl-1,2-dihydroquinoline polymer or monomer, 2,2,6,6-tetramethylpiperidine oxide (TEMPO), N-alkyl-N'-phenyl-p-phenylenediamine, N-(1,4-dimethylpentyl)-4-nitrosoaniline, N-(1,4-dimethylpentyl)-N'-phenyl-p-phenylenediamine, 4,6-dinitro-o-sec-butylphenol, 4,6-dinitro-p-cresol, and 4,6-dinitro-o-cresol.

[0013] The polymerization inhibitor is added at a rate of 0.001% to 0.01% of the crude benzene feedstock. (2) Hydrogenation: The light benzene fraction sent from the pre-distillation process is hydrogenated in a hydrogenation reactor containing a hydrogenation catalyst, and then sent to the stripping process.

[0014] The hydrogenation reaction is carried out at a temperature of 50–150°C, a pressure of 0.5–1.5 MPa, a hydrogen-to-oil ratio (volume ratio of hydrogen to light benzene fraction) of 1000–2000:1, and a volume hourly space velocity (VHSV) of 0.1–3 h⁻¹. -1 ; The hydrogenation reactor can be one or more of a fixed bed, a fluidized bed, and a reaction vessel; The hydrogenation catalyst described above has a platinum-cobalt active component, which can essentially remove unsaturated olefins from crude benzene in a liquid phase system and at a relatively low reaction temperature of 50~150℃.

[0015]

[0016]

[0017]

[0018]

[0019]

[0020] The catalysts in the above reaction formula are all supported platinum-cobalt catalysts; Further, the hydrogenation catalyst is a columnar supported catalyst, the component of which is a supported catalyst containing Pt-Co, the content of Pt is 0.2% to 1.0%, the content of Co is 0.2% to 1.0%, the carrier in the catalyst is coconut shell charcoal, the specific surface area of which is ≥1000m 2 / g, the pore volume is 1.5 to 2.5g / ml, the pore size is 1.5 to 2.5nm, the ash content is ≤2%, and the water absorption rate is 50-60%; (3) Stripping: the material from the hydrogenation process is sent into the stripping tower, the light component H2S is stripped from the top of the tower, and the benzene and toluene fractions are cooled after being taken from the bottom of the tower and then go to the extraction process, the operating pressure of the stripping tower is 0.7MPa, and the temperature is 118-125℃; (4) Extraction: the benzene and toluene fractions from the stripping process are added from the middle of the extraction tower, and the solvent is added from the upper part of the tower, after being condensed at the top of the tower, part of the reflux extraction tower, and part goes to the non-aromatic hydrocarbon product tank, the reflux ratio is 1.0-2.5; the bottom distillate goes to the solvent recovery process.

[0021] The operating pressure of the extraction tower is 110-150KPa, and the temperature is 150-165℃; the extraction solvent is sulfolane.

[0022] (5) Solvent recovery: the bottom distillate from the extraction process is sent into the solvent recovery tower for distillation, the operating pressure of the tower is 10-50KPa, and the temperature is 60-120℃, the aromatic hydrocarbon gas at the top of the tower is cooled and then separated into gas and liquid, part of which returns to the top of the tower as reflux liquid, and part goes to the rectification process, the reflux ratio is 1.0-2.5; 10-20% of the bottom distillate goes to the regeneration tower for purification, and the remaining part returns to the extraction tower as solvent, the operating pressure of the regeneration tower is 10-50KPa, and the temperature is 110-120℃.

[0023] (6) Rectification of pure benzene and toluene: the aromatic hydrocarbon from the solvent recovery process is added from the middle of the benzene tower, the operating pressure of the tower is 100-110KPa, and the operating temperature is 135-141℃, the gas phase at the top of the tower is cooled and then part of it enters the benzene tower as reflux liquid, and part returns to the second distillation tower for extraction, the reflux ratio is 2.0, pure benzene product is taken from the upper part of the tower, and toluene product is taken from the bottom of the tower; (7) The distillate taken from the middle side line of the first distillation column is introduced into a xylene column, the column is operated at a pressure of 70 KPa and a temperature of 90-104 DEG C, the gas phase at the top of the column is cooled, 3-5% of the cooled gas phase is sent to the crude benzene raw material tank, the rest is returned to the xylene column as reflux liquid, xylene product is taken from the upper side line of the column, and heavy components taken from the bottom of the column are sent to a product storage tank.

[0024] The application provides a preparation method of a platinum-cobalt catalyst, the catalyst is composed of a carrier and an active component, the carrier is sulfided activated carbon, and the active component is platinum-cobalt, and the specific preparation method comprises the following steps: step S01: 200g of activated carbon and 900-1100ml of 2.0-2.5% (wt) inorganic acid are weighed, constant temperature heating is carried out for 1.5-2.5h, and then the mixture is filtered and washed until neutral, hydrogen peroxide and sodium hypochlorite are sequentially added to the mixture at room temperature, the mixture is treated for 3-5h, the mixture is filtered and washed, and the mixture is dried in an oven at 100-120 DEG C for 4-6h; step S02: 100g of the treated activated carbon is added to a beaker, a prepared chloroplatinic acid / cobalt chloride solution is added to the beaker, the mixture is left to stand for 3-5h, a sodium carbonate solution is added to the mixture and reacts for 3-5h, a sodium hypophosphite solution is added to the mixture for reduction, the mixture is washed until neutral, and the mixture is dried in an oven at 100-120 DEG C for 6-10h; and step S03: a prepared sulfide (20% sodium hydrosulfide, 3ml) solution is added to the Pt / C mixed solution, constant temperature stirring is carried out at 50-70 DEG C for 5-7h, the mixture is filtered and washed until no sulfide is detected in the filtrate by GC (gas chromatography) analysis, and the catalyst is obtained by drying the solid phase.

[0025] Further, the chloroplatinic acid solution can also be prepared by using platinum nitrate, platinum chloride and the like; and the cobalt chloride can also be prepared by using cobalt nitrate and the like.

[0026] Beneficial effects: Compared with the prior art, although the application uses a Pt noble metal catalyst, the application has the advantages of simple process, convenient operation, high purity of aromatic hydrocarbon products produced at low temperature and low pressure, stable quality, low energy consumption, reduced equipment of the whole production device, low investment, greatly reduced corrosion of the equipment during production, improved operation cycle of the equipment, and very significant economic benefits. The application does not produce new heavy benzene byproducts, does not waste raw materials, improves the added value of products, and is not prone to coking and plugging, It should be noted that the embodiments in the application and the features in the embodiments can be combined with each other without conflict. The application will be described in detail below with reference to the embodiments.

[0027] Embodiment 1 (1) Pre-distillation: the crude benzene raw material (coking crude benzene, its components and mass fraction are as follows: benzene: 76.52%, toluene: 13.31%, xylene: 2.03%, light components: 0.85%, heavy components: 7.29%, total sulfur content: 6432.1 ppm, total nitrogen content: 695.4 ppm) is filtered through a filter, a certain amount of polymerization inhibitor is added, and after being heated to 70°C, it is sent to the first distillation column for pre-distillation. The column operating pressure is 50 KPa, and the temperature is 80-140°C. The light benzene fraction is collected at the top of the column and enters the reflux tank, part of which is used as the column top reflux liquid, and part of which is sent to the hydrogenation process. The reflux ratio is 0.5-2.0. The xylene and a small amount of toluene, styrene fraction are collected at the middle side of the column and sent to the xylene column for rectification. The heavy benzene containing polymerization inhibitor is collected at the bottom of the column and sent to the heavy component product storage tank. The polymerization inhibitor is 2,2,4-trimethyl-1,2-dihydroquinoline, and the amount added is 0.005% of the mass of the crude benzene raw material; (2) Hydrogenation: the light benzene fraction from the pre-distillation process is mixed with hydrogen at a volume ratio of 1:1000, and hydrogenation is carried out in a fixed bed hydrogenation reactor containing a supported catalyst with a Pt content of 0.6% and a Co content of 0.8%, at a temperature of 70°C, a pressure of 0.8 MPa, a hydrogen / oil volume ratio of 1000:1, and a volume space velocity of 0.5h -1 -1. The hydrogenated product has a total sulfur content of 0.2 ppm and a total nitrogen content of less than 0.3 ppm. 2 The support of the catalyst has a specific surface area of 1125 m 2 / g, a pore volume of 2.0 g / ml, a pore size of 2.5 nm, an ash content of 1.2%, and a water absorption rate of 56%. (3) Stripping: the material from the hydrogenation process is fed into the column for stripping. The column operating pressure is 0.7 MPa, and the temperature is 118-125°C. The light components H2S are stripped from the top of the column, and the benzene and toluene fractions are cooled and then sent to the extraction process.

[0028] (4) Extraction: the benzene and toluene fractions from the stripping process are fed into the middle of the extraction column, and the solvent is fed from the top of the column. The column operating pressure is 130 KPa, and the temperature is 150-160°C. The non-aromatic gas extracted at the top of the column is separated into gas and liquid, part of which is used as the reflux of the extraction column, and part of which is sent to the non-aromatic product storage tank. The reflux ratio is 1.5. The bottom distillate is sent to the solvent recovery process. The extraction solvent is sulfolane.

[0029] (5) Solvent recovery: the tower bottom distillate from the extraction process is sent to the solvent recovery tower for distillation, the tower operating pressure is 30 KPa, the temperature is 80-120℃, the aromatics gas at the top is cooled and then separated into liquid and gas, part of which is returned to the top as reflux liquid, part of which is sent to the rectification process, the reflux ratio is 1.5; 15% of the tower bottom distillate is sent to the regeneration tower for purification, the remaining part is returned to the extraction tower as solvent.

[0030] (6) Rectification of pure benzene and toluene: the aromatics sent from the solvent recovery process is added from the middle of the benzene tower, the tower operating pressure is 100-110 KPa, the operating temperature is 138-140℃, the gas phase at the top is cooled and then part of it is returned to the benzene tower as reflux liquid, part of it is returned to the second distillation tower for extraction, the reflux ratio is 2.0, pure benzene product is collected from the upper part of the tower, and toluene product is collected from the bottom of the tower; (7) Rectification of xylene: the fraction collected from the middle side line of the first distillation tower is sent to the xylene tower, the tower operating pressure is 70 KPa, the temperature is 95-100℃, 5% of the gas phase at the top is sent to the crude benzene raw material tank, the rest is returned to the xylene tower as top reflux liquid, xylene product is collected from the upper side line of the tower, and heavy components are collected from the bottom and sent to the product storage tank.

[0031] Product analysis:

[0032] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes, such as changing the conditions and reactant ratio of the system, temperature, addition method and time, etc. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing high-purity aromatics from crude benzene using liquid phase, characterized in that: After filtration of crude benzene feedstock, a certain amount of polymerization inhibitor is added. Following pre-distillation, light components including benzene and toluene are separated from the top of the column, while xylene and styrene are collected from the side stream. Heavy benzene and polymerization inhibitor are separated from the bottom of the column. The light components from the top of the column undergo hydrogenation, simultaneously hydrogenating saturated dienes and olefins. The hydrogenated oil obtained from the hydrogenation reaction is desulfurized and denitrogenated, and then extracted and distilled to obtain high-purity aromatics. The specific process includes the following steps: (1) Pre-distillation: After filtering the crude benzene raw material, a certain amount of polymerization inhibitor is added and then sent to the first distillation column for pre-distillation. The light benzene fraction is collected from the top of the column and sent to the reflux tank. Part of it is used as the top reflux liquid and the rest is sent to the hydrogenation process. Xylene and a small amount of toluene and styrene fractions are collected from the side stream in the middle of the column and sent to the xylene column for rectification. The heavy benzene containing polymerization inhibitor is collected from the bottom of the column and sent to the heavy component product storage tank. The pre-distillation column operates at a pressure of 50 kPa and a temperature of 80–140 °C; the reflux ratio at the top of the column is 0.5–2.

0. The polymerization inhibitor is selected from one or more of the following: 2,2,4-trimethyl-1,2-dihydroquinoline polymer or monomer, 2,2,6,6-tetramethylpiperidine oxide (TEMPO), N-alkyl-N'-phenyl-p-phenylenediamine, N-(1,4-dimethylpentyl)-4-nitrosoaniline, N-(1,4-dimethylpentyl)-N'-phenyl-p-phenylenediamine, 4,6-dinitro-o-sec-butylphenol, 4,6-dinitro-p-cresol, and 4,6-dinitro-o-cresol; (2) Hydrogenation: The light benzene fraction from the pre-distillation process is subjected to hydrogenation reaction in a hydrogenation reactor equipped with a hydrogenation catalyst, and then sent to the stripping process; the hydrogenation reaction is carried out at a temperature of 50-150℃, a pressure of 0.5-1.5MPa, a hydrogen-to-oil ratio (volume ratio of hydrogen to light benzene fraction) of 1000-2000:1, and a volume hourly space velocity of 0.1-3h. -1 ; The hydrogenation reactor described is one of a fixed bed, a fluidized bed, or a reaction vessel; The hydrogenation catalyst described above has a platinum-cobalt active component, which can essentially remove unsaturated olefins from crude benzene in a liquid phase system and at a relatively low reaction temperature of 50~150℃. (3) Stripping: The material from the hydrogenation process is stripped in the tower. The light component H2S is stripped from the top of the tower, while the benzene and toluene fractions are cooled from the bottom of the tower and sent to the extraction process. The operating pressure of the stripping tower is 0.7 MPa and the temperature is 118-125℃. (4) Extraction: The benzene and toluene fractions from the stripping process are added from the middle of the extraction tower, while the solvent is added from the top of the tower. After condensation at the top of the tower, part of the solvent is refluxed back to the extraction tower, and part is sent to the non-aromatic product storage tank. The reflux ratio is 1.0 to 2.

5. The bottom distillate is sent to the solvent recovery process. The operating pressure of the extraction tower is 110 to 150 kPa, and the temperature is 150 to 165 °C. The extraction solvent is sulfolane. (5) Solvent recovery: The bottom distillate from the extraction process is fed into the solvent recovery tower for distillation. The tower operates at a pressure of 10-50 kPa and a temperature of 60-120°C. The aromatic gas at the top of the tower is cooled and then separated into gas and liquid. Part of it is returned to the top of the tower as reflux liquid, and the other part is sent to the rectification process of pure benzene and toluene. The reflux ratio is 1.0-2.

5. 10-20% of the bottom distillate is sent to the solvent regeneration tower for purification, and the remaining part is returned to the extraction tower as solvent. The regeneration tower operates at a pressure of 10-50 kPa and a temperature of 110-120°C. (6) Distillation of pure benzene and toluene: Aromatics from the solvent recovery process are added from the middle of the benzene tower. The tower operating pressure is 100-110 kPa and the operating temperature is 135-141 °C. After cooling, part of the vapor phase at the top of the tower is used as reflux liquid and enters the benzene tower, while part is returned to the second distillation tower for extraction. The reflux ratio is 2.

0. Pure benzene product is collected from the top of the tower and toluene product is collected from the bottom of the tower. (7) Distillation of xylene: The fraction collected from the side stream in the middle of the first distillation column is fed into the xylene column. The column operating pressure is 70 kPa and the temperature is 90-104 °C. 3-5% of the vapor phase at the top of the column is cooled and sent to the crude benzene feed tank. The rest is refluxed back to the xylene column as the top reflux liquid. Xylene product is collected from the side stream at the top of the column, and the heavy fraction collected from the bottom is sent to the product storage tank.

2. The preparation method according to claim 1, characterized in that: The catalyst consists of two parts: a support and an active component. The support is sulfide activated carbon, and the active component is platinum-cobalt. The specific preparation method includes the following steps: Step S01: Weigh 200g of activated carbon, add 900-1100ml of 2.0-2.5% (wt) inorganic acid, heat at a constant temperature for 1.5-2.5h, filter and wash until neutral, add hydrogen peroxide and sodium hypochlorite sequentially, treat at room temperature for 3-5h, filter and wash, and dry in an oven at 100-120℃ for 4-6h; Step S02: Add the treated activated carbon to a beaker... 00g, add the prepared chloroplatinic acid / cobalt chloride solution to a beaker, let stand for 3-5h, then add Na2CO3 solution and react for 3-5h, then add sodium hypophosphite solution for reduction, wash until neutral, and dry in an oven at 100-120℃ for 6-10h; Step S03: add the prepared sulfide (20% sodium hydrosulfide, 3ml) solution to the Pt / C mixed solution, stir at a constant temperature of 50-70℃ for 5-7h, filter and wash until GC (gas chromatography) analysis shows no sulfide in the filtrate, and dry the solid phase to obtain the catalyst.

3. The hydrogenation catalyst is a columnar supported catalyst, which is a supported catalyst containing Pt-Co, with Pt content of 0.2%~1.0% and Co content of 0.2%~1.0%; the support in the catalyst is coconut shell carbon, with a specific surface area ≥1000m2 / g, pore volume 1.5~2.5g / ml, pore size 1.5~2.5nm, ash content ≤2%, and water absorption rate 50-60%.

4. The preparation method according to claim 2, characterized in that: Chloroplatinic acid solution can also be made with platinum nitrate or platinum chloride.

5. The preparation method according to claim 2, characterized in that: Cobalt chloride can also be replaced by cobalt nitrate.

6. The preparation method according to claim 1, characterized in that: The polymerization inhibitor is selected from 2,2,4-trimethyl-1,2-dihydroquinoline monomer.

7. The preparation method according to claim 1, characterized in that: The amount of polymerization inhibitor added is 0.001% to 0.01% of the crude benzene raw material.

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