Method for preparing toluene diisocynate by utilizing phosgene

Abstract

The invention discloses a method for preparing toluene diisocynate by utilizing phosgene, and belongs to the technical field of toluene diisocynate preparation.The preparation method comprises the following steps that a Pd-Co3O4 / C catalyst is taken and heated to 140-150 DEG C, dinitrotoluene and activated hydrogen with the molar ratio being 1: (6.2-6.4) are added into the catalyst in a countercurrent mode, after the reaction is completed, cooling and gas-liquid separation are conducted, and the toluene diisocynate is obtained. The method comprises the following steps: adding m-toluenediamine into a first-stage distillation tower, collecting a liquid component, carrying out reduced pressure distillation, collecting a component which is not evaporated, sequentially introducing the component which is not evaporated into the first-stage distillation tower and a second-stage distillation tower, and separating m-toluenediamine; taking m-toluenediamine, introducing phosgene, reacting at room temperature for 2-3 hours, heating to 145-160 DEG C after the reaction is finished, stirring at constant temperature until the reaction is complete, collecting a product, and carrying out reduced pressure distillation to obtain the toluene diisocynate. Through detection, the purity of the toluene diisocynate prepared by the method provided by the invention reaches 99.7%-99.8%.

Description

Technical Field

[0001] This invention belongs to the field of toluene diisocyanate preparation technology, and relates to a method for preparing toluene diisocyanate using phosgene. Background Technology

[0002] Toluene diisocyanate (TDI) is an important intermediate in chemical organic synthesis. Phosgeneization is currently the only industrially produced method for TDI. The phosgeneization process mainly includes five stages: nitration of toluene to produce dinitrotoluene (DNT), hydrogenation of DNT to produce toluene diamine (TDA), reaction of carbon monoxide and chlorine to produce phosgene, reaction of TDA and phosgene to produce TDI, and purification of the target product TDI.

[0003] Among these processes, the DNT hydrogenation to TDA production stage is a core focus of industry research. Firstly, TDA is a direct precursor to TDI, and its isomer ratio (meta / para) directly affects the molecular structure and properties of TDI. Meta-TDA is the main raw material for TDI synthesis. Secondly, the hydrogenation reaction in this stage consumes a large amount of hydrogen (H2 / DNT molar ratio ≥ 4:1), with hydrogen costs accounting for over 60% of the total cost. Finally, the palladium-on-carbon catalyst used in this stage exhibits only 80%–85% selectivity for meta-TDA, making catalyst improvement a key research focus for this stage.

[0004] A search revealed that invention patent application number 202111615330.8 discloses a noble metal catalyst and its preparation method, as well as its application in the catalytic hydrogenation of dinitrotoluene to toluene diamine (TDA). This invention patent focuses on improving the catalyst in the hydrogenation of DNT to TDA, achieving a final reaction conversion rate of 99.99% by optimizing the proportion of metal elements in the catalyst. However, this prior art does not further investigate the separation of the toluene diamine intermediate-TDA. Summary of the Invention

[0005] This invention provides a method for preparing toluene diisocyanate using phosgene. Specifically, this invention mainly improves the TDA preparation process by hydrogenating DNT. Compared with the prior art, this invention further improves the separation of the catalyst and meta-TDA, thereby increasing the purity of meta-toluene diamine and ultimately improving the purity of the product toluene diisocyanate.

[0006] The objective of this invention can be achieved through the following technical solutions: A method for preparing toluene diisocyanate using phosgene includes the following steps: S1. Add Pd-Co3O4 / C catalyst to reactor A and heat to 140-150℃. Then add dinitrotoluene and activated hydrogen gas in a countercurrent ratio of 1:6.2-6.4. React for 3-4 hours. After the reaction is complete, cool to 50-60℃. After gas-liquid separation, collect the liquid component and then place it under reduced pressure distillation at 170-180℃ and a vacuum of 1-5 kPa. Collect the undistilled component and then pass it through a primary distillation column and a secondary distillation column in sequence to separate m-toluenediamine. S2. Add m-toluenediamine to reactor B, then introduce phosgene, and react at room temperature for 2-3 hours. After the reaction is complete, heat reactor B to 145-160℃ and stir at a constant temperature until the reaction is complete. Collect the product and distill it under reduced pressure to obtain toluene diisocyanate.

[0007] Further, the Pd-Co3O4 / C catalyst described in S1 is prepared by the following steps: Cobalt nitrate hexahydrate, urea, graphene oxide, and pure water were added to a high-pressure reactor and stirred at room temperature for 20–30 min to form a mixture. The concentration of cobalt nitrate hexahydrate in the mixture was 0.1–0.2 mol / L, the molar ratio of urea to cobalt nitrate hexahydrate was 5–6:1, and the concentration of graphene oxide in the mixture was 0.5–0.55 mg / mL. The high-pressure reactor was then heated to 177–180 °C and stirred for 12 h. After the reaction was complete, the product was obtained. The product was washed and dried to obtain Co(OH). Co(OH)₂ / GO precursor was calcined at 400℃ for 2 h to obtain Co₃O₄ nanosheets. The Co₃O₄ nanosheets were then dispersed in anhydrous ethanol solution to form a Co₃O₄ nano-ethanol dispersion with a concentration of 1 mg / mL. The palladium on carbon catalyst was then immersed in the Co₃O₄ nano-ethanol dispersion and ultrasonicated for 30 min. The catalyst was then rotary evaporated to dryness at 65-70℃ to obtain the supported material. The supported material was then activated in a dielectric barrier discharge plasma reactor to obtain the Pd-Co₃O₄ / C catalyst.

[0008] Furthermore, the palladium-on-carbon catalyst has a particle size of 5 nm and a Pd content of 2%.

[0009] Furthermore, the amount of the Pd-Co3O4 / C catalyst used in S1 is 4% to 5% of the mass of dinitrotoluene.

[0010] Further, the dinitrotoluene described in S1 is prepared by the following steps: First, a mixed acid is prepared by mixing 98% sulfuric acid, 68% nitric acid, and pure water in a mass ratio of 2.00–2.07:1.00:0.89. After preparation, the mixed acid is heated to 50–55°C, and toluene is added to it with stirring. The ratio of toluene to mixed acid is 1 mL:5–10 mL. The mixture is stirred at a constant temperature for 2–4 hours, then heated to 85–90°C and stirred at a constant temperature for 5–6 hours. After the reaction is complete, the product is collected. The product is washed with water, washed with alkali, and distilled to obtain dinitrotoluene.

[0011] Further, the activated hydrogen gas mentioned in S1 is prepared by the following steps: Hydrogen gas was introduced into the dielectric barrier discharge plasma reactor at a flow rate of 50 mL / min, and the back pressure was set to 1 MPa. After the hydrogen gas was introduced, the plasma power supply was started, and the power supply frequency was adjusted to 19.5–20.5 kHz to ensure that the plasma electron temperature was 2–3 eV. After the adjustment was completed, plasma treatment was carried out at a constant power supply frequency for 5 min. After completion, activated hydrogen gas was obtained.

[0012] Furthermore, the bottom temperature of the primary distillation column in S1 is set to 130–150°C and the vacuum degree is 0.5–1 kPa, while the bottom temperature of the secondary distillation column is set to 208–210°C, the top temperature is set to 200–205°C, and the vacuum degree is 0.25–0.28 kPa.

[0013] Furthermore, the phosgene mentioned in S2 is prepared by the following steps: First, activated carbon is added to the fixed-bed reactor and the temperature is raised to 110-120°C. Then, carbon monoxide and chlorine are added in a molar ratio of 1:1.05-1.1. The reaction is carried out for 20-30 seconds. After the reaction is completed, a mixed gas is obtained. The mixed gas is cooled to -10 to 0°C, and the liquid mixture is collected. The liquid mixture is then passed into a distillation column with a top temperature of 7-8°C for purification to obtain phosgene.

[0014] Furthermore, the activated carbon has a particle size of 5 mm and a specific surface area of ​​800 m². 2 / g.

[0015] Furthermore, the molar ratio of phosgene and m-toluenediamine in S2 is 2.1 to 2.3:1.

[0016] The present invention has the following beneficial effects: This invention provides a method for preparing toluene diisocyanate using phosgene. Testing shows that the purity of the toluene diisocyanate prepared by this method reaches 99.7%–99.8%, and the APHA (alpha-chroma ratio) is 22–24. The key improvements in this invention lie in the catalyst preparation, hydrogen activation, and multi-stage distillation separation of m-toluene diamine during the reduction of dinitrotoluene. The specific principles are as follows: First, this invention improves the catalyst. This invention provides a Pd-Co3O4 / C catalyst in which sheet-like Co3O4 is supported on the Pd / C surface, and then subjected to DBD plasma treatment (50 W / cm²). 3 The presence of Ar / H2 forms an atomic-level pinning effect, which enhances the bonding energy at the Co-O-Pd interface, overcoming the limitations of physical adsorption in traditional impregnation methods and thus improving the efficiency of catalytic hydrogenation. Simultaneously, this invention innovatively utilizes oxygen vacancies in Co3O4 as a hydrogen transport medium. Testing shows that the Pd-Co3O4 / C catalyst prepared in this invention significantly improves the efficiency of catalytic hydrogenation, ultimately resulting in increased purity of m-toluenediamine.

[0017] Then, this invention utilizes plasma activation to improve the utilization rate of hydrogen. After plasma treatment, hydrogen can generate highly active substances such as H*. Then, by countercurrent contact between activated hydrogen and DNT, the effective reaction time is extended. According to the test, under the same activated hydrogen dosage, the purity of m-toluenediamine is significantly improved. This verifies that the activation of hydrogen and the setting of countercurrent contact significantly improve the utilization rate of hydrogen.

[0018] Finally, this invention achieves efficient separation of m-TDA through a multi-stage distillation process. By utilizing the synergistic effect between catalyst design, hydrogen activation, and separation processes, this invention aims to improve the purity of m-toluene diamine and ultimately to improve the purity of toluene diisocyanate. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1 A method for preparing toluene diisocyanate using phosgene includes the following steps: Nitration of toluene: First, prepare a mixed acid solution. The mixed acid solution is prepared by mixing sulfuric acid (98% by mass; purchased from Sinopharm Chemical Reagent Co., Ltd.), nitric acid (68% by mass; purchased from Sinopharm Chemical Reagent Co., Ltd.), and pure water in a mass ratio of 2.00:1.00:0.89. After preparation, heat the mixed acid solution to 50°C, and then add toluene while stirring. The volume ratio of toluene to mixed acid is 1 mL:5 mL. The mixture is stirred at a constant temperature for 2 hours, then heated to 85°C and stirred at a constant temperature for 5 hours. After completion, collect the product. The product was washed sequentially with pure water, then with a 10% sodium hydroxide solution, and finally with alkali to obtain crude dinitrotoluene. The crude dinitrotoluene was then preheated to 60°C and pumped into a distillation column. The pressure inside the distillation column was controlled at 0.5 kPa (absolute pressure), the bottom temperature was controlled at 155°C, and the top temperature was controlled at 110°C. During the distillation process, the fore-distillate from the heating stage was removed, and the main distillate at the target temperature was collected to obtain dinitrotoluene. The purity of the dinitrotoluene was 99.2%, and the yield was 95.3%.

[0021] Preparation of Pd-Co3O4 / C catalyst: Preparation of Co3O4 nanosheets: Cobalt nitrate hexahydrate, urea, graphene oxide, and pure water were added to a high-pressure reactor and stirred at room temperature for 20 min to form a mixture. The concentration of cobalt nitrate hexahydrate in the mixture was 0.1 mol / L, the molar ratio of urea to cobalt nitrate hexahydrate was 5:1, and the concentration of graphene oxide in the mixture was 0.5 mg / mL. The high-pressure reactor was then heated to 177 °C and stirred at a constant temperature for 12 h. After the reaction was completed, the mixture was naturally cooled to room temperature. The product was washed three times alternately with deionized water and anhydrous ethanol. The product was then dried in a vacuum drying oven at 50 °C until a constant temperature was reached to obtain the Co(OH)2 / GO precursor. The Co(OH)2 / GO precursor was then placed in a muffle furnace and heated to 400 °C at a heating rate of 5 °C / min and calcined for 2 h. After cooling to room temperature, Co3O4 nanosheets were obtained.

[0022] Surface modification of Co3O4 nanosheets: Palladium-on-carbon catalyst (Pd / C, 2% Pd, 5 nm particle size) was used. The Co3O4 nanosheets prepared in the above process were dispersed in anhydrous ethanol solution to form a Co3O4 nano-ethanol dispersion with a concentration of 1 mg / mL. The palladium-on-carbon catalyst was then immersed in the Co3O4 nano-ethanol dispersion and ultrasonicated (40 kHz) for 30 min. After that, it was evaporated to dryness at 65 °C to obtain the loaded material. The loaded material was then activated by plasma treatment in a dielectric barrier discharge (DBD) plasma reactor for 3 min, with a power density of 50 W / cm². 3 The gas atmosphere was Ar / H2 (70 / 30 vol%). After activation treatment, Pd-Co3O4 / C catalyst was obtained.

[0023] Reduction of Dinitrotoluene: The reduction reaction of dinitrotoluene was carried out using a tubular reactor (hereinafter referred to as the reactor). The reactor was filled with Pd-Co3O4 / C catalyst, with the amount of Pd-Co3O4 / C catalyst being 4% of the mass of dinitrotoluene. The packing density of the Pd-Co3O4 / C catalyst was 0.55 kg / L. After the catalyst was completely filled, the reactor was heated to 140℃, and then dinitrotoluene and activated hydrogen were added to the reactor at a molar ratio of 1:6.2. The activation process of hydrogen was as follows: Hydrogen was introduced into the dielectric barrier discharge (DBD) plasma reactor at a flow rate of 50 mL / min, and the back pressure was set to 1 MPa. After the hydrogen was introduced, the plasma power supply was started, and the power supply frequency was adjusted to 19.5 kHz to ensure that the plasma electron temperature was 2 eV. After adjustment, plasma treatment was carried out at a constant power supply frequency for 5 min. After this, the activation of hydrogen was completed. After the dinitrotoluene and activated hydrogen were completely introduced into the reactor... To ensure countercurrent contact between dinitrotoluene and hydrogen, the partial pressure of the activated hydrogen was set to 4 MPa, and the residence time of dinitrotoluene in the reactor was 3 hours. After the reaction was complete, the reactor was cooled to 50°C, and the liquid component was collected after gas-liquid separation. The liquid was then subjected to vacuum distillation at 170°C under a vacuum of 1 kPa. The undistilled component was collected, and analysis revealed that the content of meta-toluenediamine (m-TDA) in the undistilled component was 85.4%, and the content of para-toluenediamine (p-TDA) was 11.0%. The undistilled components are then subjected to multi-stage distillation separation. Specifically, the undistilled components are sequentially fed into a primary distillation column and a secondary distillation column. The bottom temperature of the primary distillation column is set at 130°C and the vacuum degree is 0.5 kPa. The bottom temperature of the secondary distillation column is set at 208°C, the top temperature is set at 200°C, and the vacuum degree is 0.25 kPa. The light components are removed by the primary distillation column, and then m-toluenediamine is separated by the secondary distillation column. The purity of m-toluenediamine is found to be 99.8%.

[0024] Phosgene production: Phosgene synthesis is carried out using a fixed-bed reactor (hereinafter referred to as the reactor). First, activated carbon (particle size 5 mm; specific surface area 800 m²) is added to the reactor as a catalyst. 2 / g), the catalyst filling thickness is 1m. After the catalyst is filled, the reactor is heated to 110℃, and carbon monoxide and chlorine are added to the reactor at a molar ratio of 1:1.05. A slight excess of chlorine ensures that the carbon monoxide reacts completely. By controlling the flow rate of carbon monoxide and chlorine, both carbon monoxide and chlorine are kept in the reactor for 20s. After the reaction is complete, a mixed gas is obtained. The mixed gas is cooled to -10℃, and the liquid mixture (boiling point 8.2℃) is collected. The liquid mixture is then passed into a distillation column with a top temperature of 7℃ to purify to a purity of ≥99.9% to obtain high-purity phosgene.

[0025] Phosgene reaction: A glass-lined reactor equipped with a jacketed temperature control and a high-shear stirrer (hereinafter referred to as the reactor) was selected. m-Toluenediamine was added to the reactor, followed by phosgene. The molar ratio of phosgene to m-Toluenediamine was 2.1:1. After the phosgene was introduced, the reactor was stirred at room temperature for 2 hours. After the reaction was completed, the reactor was heated to 145°C at a rate of 30°C / h. After the heating was completed, the release rate of HCl in the reactor was detected and recorded as the initial value. The reactor was stirred at a constant temperature until the release rate of HCl in the reactor dropped to less than 2% of the initial value, which was considered as the complete reaction. The gas generated during the reaction was treated to recover phosgene. The remaining product was collected and distilled under reduced pressure (100°C, 10 kPa) to obtain toluene diisocyanate. The purity of toluene diisocyanate was 99.7%, and the color (APHA) was 24.

[0026] Example 2 A method for preparing toluene diisocyanate using phosgene includes the following steps: Nitration of toluene: First, prepare a mixed acid solution. The mixed acid solution is prepared by mixing sulfuric acid (98% by mass; purchased from Sinopharm Chemical Reagent Co., Ltd.), nitric acid (68% by mass; purchased from Sinopharm Chemical Reagent Co., Ltd.), and pure water in a mass ratio of 2.05:1.00:0.89. After preparation, heat the mixed acid solution to 50°C, and then add toluene while stirring. The volume ratio of toluene to mixed acid is 1 mL:8 mL. The mixture is stirred at a constant temperature for 4 hours, then heated to 88°C and stirred at a constant temperature for 6 hours. After completion, collect the product. The sample was washed sequentially with pure water, then with a 10% sodium hydroxide solution, and finally with alkali to obtain crude dinitrotoluene. The crude dinitrotoluene was then preheated to 70°C and pumped into a distillation column. The pressure inside the distillation column was controlled at 1 kPa (absolute pressure), the bottom temperature was controlled at 158°C, and the top temperature was controlled at 120°C. During the distillation process, the fore-distillate from the heating stage was removed, and the main distillate at the target temperature was collected to obtain dinitrotoluene. The purity of the dinitrotoluene was 99.7%, and the yield was 97.7%.

[0027] Preparation of Pd-Co3O4 / C catalyst: Preparation of Co3O4 nanosheets: Cobalt nitrate hexahydrate, urea, graphene oxide, and pure water were added to a high-pressure reactor and stirred at room temperature for 30 min to form a mixture. The concentration of cobalt nitrate hexahydrate in the mixture was 0.2 mol / L, the molar ratio of urea to cobalt nitrate hexahydrate was 6:1, and the concentration of graphene oxide in the mixture was 0.55 mg / mL. The high-pressure reactor was then heated to 180 °C and stirred at a constant temperature for 12 h. After the reaction was completed, the mixture was naturally cooled to room temperature. The product was washed three times alternately with deionized water and anhydrous ethanol. The product was then dried in a vacuum drying oven at 60 °C until a constant temperature was reached to obtain the Co(OH)2 / GO precursor. The Co(OH)2 / GO precursor was then placed in a muffle furnace and heated to 400 °C at a heating rate of 10 °C / min and calcined for 2 h. After cooling to room temperature, Co3O4 nanosheets were obtained.

[0028] Surface modification of Co3O4 nanosheets: Palladium-on-carbon catalyst (Pd / C, 2% Pd, 5 nm particle size) was used. The Co3O4 nanosheets prepared in the above process were dispersed in anhydrous ethanol solution to form a Co3O4 nano-ethanol dispersion with a concentration of 1 mg / mL. The palladium-on-carbon catalyst was then immersed in the Co3O4 nano-ethanol dispersion and ultrasonicated (40 kHz) for 30 min. The mixture was then evaporated to dryness at 70 °C to obtain the loaded material. The loaded material was then subjected to plasma activation by placing it in a dielectric barrier discharge (DBD) plasma reactor for 4 min, with a power density of 50 W / cm². 3 The gas atmosphere was Ar / H2 (70 / 30 vol%). After activation treatment, Pd-Co3O4 / C catalyst was obtained.

[0029] Reduction of Dinitrotoluene: The reduction reaction of dinitrotoluene was carried out using a tubular reactor (hereinafter referred to as the reactor). The reactor was filled with Pd-Co3O4 / C catalyst, with the amount of Pd-Co3O4 / C catalyst being 5% of the mass of dinitrotoluene. The packing density of the Pd-Co3O4 / C catalyst was 0.6 kg / L. After the catalyst was completely filled, the reactor was heated to 150°C, and then dinitrotoluene and activated hydrogen were added to the reactor at a molar ratio of 1:6.4. The activation process of hydrogen was as follows: Hydrogen was introduced into the dielectric barrier discharge (DBD) plasma reactor at a flow rate of 50 mL / min, and the back pressure was set to 1 MPa. After the hydrogen was introduced, the plasma power supply was started, and the power supply frequency was adjusted to 20 kHz to ensure that the plasma electron temperature was 3 eV. After adjustment, plasma treatment was carried out at a constant power supply frequency for 5 min. After this, the activation of hydrogen was completed. After the dinitrotoluene and activated hydrogen were completely introduced into the reactor... To ensure countercurrent contact between dinitrotoluene and hydrogen, the partial pressure of the activated hydrogen was set to 5 MPa, and the residence time of dinitrotoluene in the reactor was 4 hours. After the reaction was complete, the reactor was cooled to 55°C, and after gas-liquid separation, the liquid component was collected. Then, it was subjected to vacuum distillation at 175°C and a vacuum degree of 2 kPa. The undistilled component was collected, and the content of meta-toluenediamine (m-TDA) in the undistilled component was 87.1%, and the content of para-toluenediamine (p-TDA) was 10%. 4% was then subjected to multi-stage distillation to separate the undistilled components. Specifically, the undistilled components were sequentially fed into a primary distillation column and a secondary distillation column. The bottom temperature of the primary distillation column was set at 140°C and the vacuum degree at 1 kPa. The bottom temperature of the secondary distillation column was set at 208°C, the top temperature at 205°C, and the vacuum degree at 0.2 kPa. The light components were removed by the primary distillation column, and then m-toluenediamine was separated by the secondary distillation column. The purity of m-toluenediamine was found to be 99.9%.

[0030] Phosgene production: Phosgene synthesis is carried out using a fixed-bed reactor (hereinafter referred to as the reactor). First, activated carbon (particle size 5 mm; specific surface area 800 m²) is added to the reactor as a catalyst. 2 / g), the catalyst filling thickness is 1m. After the catalyst is filled, the reactor is heated to 120℃, and carbon monoxide and chlorine are added to the reactor at a molar ratio of 1:1.1. A slight excess of chlorine ensures that the carbon monoxide reacts completely. By controlling the flow rate of carbon monoxide and chlorine, it is ensured that both carbon monoxide and chlorine remain in the reactor for 30s. After the reaction is complete, a mixed gas is obtained. The mixed gas is cooled to -10℃, and the liquid mixture (boiling point 8.2℃) is collected. The liquid mixture is then passed into a distillation column with a top temperature of 8℃ to purify to a purity ≥99.9% to obtain high-purity phosgene.

[0031] Phosgene reaction: A glass-lined reactor equipped with a jacketed temperature control and a high-shear stirrer (hereinafter referred to as the reactor) was selected. m-Toluenediamine was added to the reactor, followed by phosgene. The molar ratio of phosgene to m-Toluenediamine was 2.2:1. After the phosgene was introduced, the reactor was stirred at room temperature for 3 hours. After the reaction was completed, the reactor was heated to 150°C at a rate of 30°C / h. After the heating was completed, the release rate of HCl in the reactor was detected and recorded as the initial value. The reactor was stirred at a constant temperature until the release rate of HCl in the reactor dropped to less than 2% of the initial value, which was considered as the complete reaction. The gas generated during the reaction was treated to recover phosgene. The remaining product was collected and distilled under reduced pressure (110°C, 15 kPa) to obtain toluene diisocyanate. The purity of toluene diisocyanate was 99.8%, and the color (APHA) was 22.

[0032] Example 3 A method for preparing toluene diisocyanate using phosgene includes the following steps: Nitration of toluene: First, prepare a mixed acid solution. The mixed acid solution is composed of sulfuric acid (98% by mass; purchased from Sinopharm Chemical Reagent Co., Ltd.), nitric acid (68% by mass; purchased from Sinopharm Chemical Reagent Co., Ltd.), and pure water in a mass ratio of 2.07:1.00:0.89. After preparation, heat the mixed acid solution to 55°C, and then add toluene while stirring. The volume ratio of toluene to mixed acid is 1 mL:10 mL. The mixture is stirred at a constant temperature for 4 hours, then heated to 90°C and stirred at a constant temperature for 6 hours. After completion, collect the solution. The product was washed sequentially with pure water and a 10% sodium hydroxide solution, followed by an alkali wash, to obtain crude dinitrotoluene. The crude dinitrotoluene was then preheated to 80°C and pumped into a distillation column. The pressure inside the distillation column was controlled at 5 kPa (absolute pressure), the bottom temperature was controlled at 160°C, and the top temperature was controlled at 130°C. During the distillation process, the fore-distillate from the heating stage was removed, and the main distillate at the target temperature was collected to obtain dinitrotoluene. The purity of the dinitrotoluene was 99.6%, and the yield was 97.2%.

[0033] Preparation of Pd-Co3O4 / C catalyst: Preparation of Co3O4 nanosheets: Cobalt nitrate hexahydrate, urea, graphene oxide, and pure water were added to a high-pressure reactor and stirred at room temperature for 30 min to form a mixture. The concentration of cobalt nitrate hexahydrate in the mixture was 0.2 mol / L, the molar ratio of urea to cobalt nitrate hexahydrate was 6:1, and the concentration of graphene oxide in the mixture was 0.55 mg / mL. The high-pressure reactor was then heated to 180 °C and stirred at a constant temperature for 12 h. After the reaction was completed, the mixture was naturally cooled to room temperature. The product was washed three times alternately with deionized water and anhydrous ethanol. The product was then dried in a vacuum drying oven at 60 °C until a constant temperature was reached to obtain the Co(OH)2 / GO precursor. The Co(OH)2 / GO precursor was then placed in a muffle furnace and heated to 400 °C at a heating rate of 10 °C / min and calcined for 2 h. After cooling to room temperature, Co3O4 nanosheets were obtained.

[0034] Surface modification of Co3O4 nanosheets: Palladium-on-carbon catalyst (Pd / C, 2% Pd, 5 nm particle size) was used. The Co3O4 nanosheets prepared in the above process were dispersed in anhydrous ethanol solution to form a Co3O4 nano-ethanol dispersion with a concentration of 1 mg / mL. The palladium-on-carbon catalyst was then immersed in the Co3O4 nano-ethanol dispersion and ultrasonicated (40 kHz) for 30 min. The mixture was then evaporated to dryness at 70 °C to obtain the loaded material. The loaded material was then subjected to plasma activation by placing it in a dielectric barrier discharge (DBD) plasma reactor for 4 min, with a power density of 50 W / cm². 3 The gas atmosphere was Ar / H2 (70 / 30 vol%). After activation treatment, Pd-Co3O4 / C catalyst was obtained.

[0035] Reduction of Dinitrotoluene: The reduction reaction of dinitrotoluene was carried out using a tubular reactor (hereinafter referred to as the reactor). The reactor was filled with a Pd-Co3O4 / C catalyst, the amount of which was 5% of the mass of dinitrotoluene, and the packing density of the Pd-Co3O4 / C catalyst was 0.6 kg / L. After the catalyst was completely filled, the reactor was heated to 150°C, and then dinitrotoluene and activated hydrogen were added to the reactor at a molar ratio of 1:6.4. The activation process of hydrogen was as follows: Hydrogen was introduced into the dielectric barrier discharge (DBD) plasma reactor at a flow rate of 50 mL / min, and the back pressure was set to 1 MPa. After the hydrogen was introduced, the plasma power supply was started, and the power supply frequency was adjusted to 20.5 kHz to ensure that the plasma electron temperature was 3 eV. After adjustment, plasma treatment was carried out at a constant power supply frequency for 5 min. After the hydrogen was introduced, the activation of hydrogen was completed. After the dinitrotoluene and activated hydrogen were completely introduced into the reactor... To ensure countercurrent contact between dinitrotoluene and hydrogen, the partial pressure of the activated hydrogen was set to 5 MPa, and the residence time of dinitrotoluene in the reactor was 4 hours. After the reaction was complete, the reactor was cooled to 60°C, and the liquid component was collected after gas-liquid separation. The liquid was then subjected to vacuum distillation at 180°C under a vacuum of 5 kPa. The undistilled component was collected, and analysis revealed that the content of meta-toluenediamine (m-TDA) in the undistilled component was 86.8%, and the content of para-toluenediamine (p-TDA) was 10.5%. The undistilled components are then subjected to multi-stage distillation separation. Specifically, the undistilled components are sequentially fed into a primary distillation column and a secondary distillation column. The bottom temperature of the primary distillation column is set at 150°C and the vacuum degree is 1 kPa. The bottom temperature of the secondary distillation column is set at 210°C, the top temperature is set at 205°C, and the vacuum degree is 0.28 kPa. The light components are removed by the primary distillation column, and then m-toluenediamine is separated by the secondary distillation column. The purity of m-toluenediamine is found to be 99.9%.

[0036] Phosgene production: Phosgene synthesis is carried out using a fixed-bed reactor (hereinafter referred to as the reactor). First, activated carbon (particle size 5 mm; specific surface area 800 m²) is added to the reactor as a catalyst. 2 / g), the catalyst filling thickness is 1m. After the catalyst is filled, the reactor is heated to 120℃, and carbon monoxide and chlorine are added to the reactor at a molar ratio of 1:1.1. A slight excess of chlorine ensures that the carbon monoxide reacts completely. By controlling the flow rate of carbon monoxide and chlorine, both carbon monoxide and chlorine are ensured to remain in the reactor for 30s. After the reaction is complete, a mixed gas is obtained. The mixed gas is cooled to 0℃, and the liquid mixture (boiling point 8.2℃) is collected. The liquid mixture is then passed into a distillation column with a top temperature of 8℃ to purify to a purity ≥99.9% to obtain high-purity phosgene.

[0037] Phosgene reaction: A glass-lined reactor equipped with a jacketed temperature control and a high-shear stirrer (hereinafter referred to as the reactor) was selected. m-Toluenediamine was added to the reactor, followed by phosgene. The molar ratio of phosgene to m-Toluenediamine was 2.3:1. After the phosgene was introduced, the reactor was stirred at room temperature for 3 hours. After the reaction was completed, the reactor was heated to 160°C at a rate of 30°C / h. After the heating was completed, the release rate of HCl in the reactor was detected and recorded as the initial value. The reactor was stirred at a constant temperature until the release rate of HCl in the reactor dropped to less than 2% of the initial value, which was considered as the complete reaction. The gas generated during the reaction was treated to recover phosgene. The remaining product was collected and distilled under reduced pressure (120°C, 15 kPa) to obtain toluene diisocyanate. The purity of toluene diisocyanate was 99.8%, and the color (APHA) was 23.

[0038] Comparative Example 1 Comparative Example 1 served as the control group for Example 2. The Pd-Co3O4 / C catalyst in Example 2 was replaced with the raw material palladium on carbon catalyst (Pd / C, 2% Pd, particle size 5 nm). The remaining preparation steps and processes were consistent with those in Example 2. The purity of m-toluene diamine was 98.1%, and the purity of toluene diisocyanate was 97.4%, with an APHA of 35.

[0039] Comparative Example 2 Comparative Example 2 served as the control group for Example 2. The activated hydrogen in Example 2 was replaced with raw material hydrogen, i.e., the hydrogen activation process was removed. The remaining preparation steps and processes were consistent with those in Example 2. The purity of m-toluenediamine was 99.5%, and the purity of toluene diisocyanate was 99.1%, with an APHA of 27.

[0040] It should be noted that, in this document, terms such as “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0041] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing toluene diisocyanate using phosgene, characterized in that, Includes the following steps: S1. Add Pd-Co3O4 / C catalyst to reactor A and heat to 140-150℃. Then add dinitrotoluene and activated hydrogen gas in a countercurrent ratio of 1:6.2-6.

4. React for 3-4 hours. After the reaction is complete, cool to 50-60℃. After gas-liquid separation, collect the liquid component and then place it under reduced pressure distillation at 170-180℃ and a vacuum of 1-5 kPa. Collect the undistilled component and then pass it through a primary distillation column and a secondary distillation column in sequence to separate m-toluenediamine. S2. Add m-toluenediamine to reactor B, then introduce phosgene, and react at room temperature for 2-3 hours. After the reaction is complete, heat reactor B to 145-160℃ and stir at a constant temperature until the reaction is complete. Collect the product and distill it under reduced pressure to obtain toluene diisocyanate.

2. The method for preparing toluene diisocyanate using phosgene according to claim 1, characterized in that, The Pd-Co3O4 / C catalyst described in S1 is prepared by the following steps: Cobalt nitrate hexahydrate, urea, graphene oxide, and pure water were added to a high-pressure reactor and stirred at room temperature for 20–30 min to form a mixture. The concentration of cobalt nitrate hexahydrate in the mixture was 0.1–0.2 mol / L, the molar ratio of urea to cobalt nitrate hexahydrate was 5–6:1, and the concentration of graphene oxide in the mixture was 0.5–0.55 mg / mL. The high-pressure reactor was then heated to 177–180 °C and stirred for 12 h. After the reaction was complete, the product was obtained. The product was washed and dried to obtain Co(OH). Co(OH)₂ / GO precursor was calcined at 400℃ for 2 h to obtain Co₃O₄ nanosheets. The Co₃O₄ nanosheets were then dispersed in anhydrous ethanol solution to form a Co₃O₄ nano-ethanol dispersion with a concentration of 1 mg / mL. The palladium on carbon catalyst was then immersed in the Co₃O₄ nano-ethanol dispersion and ultrasonicated for 30 min. The catalyst was then rotary evaporated to dryness at 65-70℃ to obtain the supported material. The supported material was then activated in a dielectric barrier discharge plasma reactor to obtain the Pd-Co₃O₄ / C catalyst.

3. The method for preparing toluene diisocyanate using phosgene according to claim 2, characterized in that, The palladium-on-carbon catalyst has a particle size of 5 nm and a Pd content of 2%.

4. The method for preparing toluene diisocyanate using phosgene according to claim 1, characterized in that, The amount of Pd-Co3O4 / C catalyst used in S1 is 4% to 5% of the mass of dinitrotoluene.

5. The method for preparing toluene diisocyanate using phosgene according to claim 1, characterized in that, The dinitrotoluene described in S1 is prepared by the following steps: First, a mixed acid is prepared by mixing 98% sulfuric acid, 68% nitric acid, and pure water in a mass ratio of 2.00–2.07:1.00:0.

89. After preparation, the mixed acid is heated to 50–55°C, and toluene is added to it with stirring. The ratio of toluene to mixed acid is 1 mL:5–10 mL. The mixture is stirred at a constant temperature for 2–4 hours, then heated to 85–90°C and stirred at a constant temperature for 5–6 hours. After the reaction is complete, the product is collected. The product is washed with water, washed with alkali, and distilled to obtain dinitrotoluene.

6. The method for preparing toluene diisocyanate using phosgene according to claim 1, characterized in that, The activated hydrogen gas mentioned in S1 is prepared by the following steps: Hydrogen gas was introduced into the dielectric barrier discharge plasma reactor at a flow rate of 50 mL / min, and the back pressure was set to 1 MPa. After the hydrogen gas was introduced, the plasma power supply was started, and the power supply frequency was adjusted to 19.5–20.5 kHz to ensure that the plasma electron temperature was 2–3 eV. After the adjustment was completed, plasma treatment was carried out at a constant power supply frequency for 5 min. After completion, activated hydrogen gas was obtained.

7. The method for preparing toluene diisocyanate using phosgene according to claim 1, characterized in that, The reboiler temperature of the first-stage distillation column described in S1 is set to 130–150°C, and the vacuum degree is 0.5–1 kPa. The reboiler temperature of the second-stage distillation column is set to 208–210°C, the top temperature is set to 200–205°C, and the vacuum degree is 0.25–0.28 kPa.

8. The method for preparing toluene diisocyanate using phosgene according to claim 1, characterized in that, The phosgene described in S2 is prepared by the following steps: First, activated carbon is added to the fixed-bed reactor and the temperature is raised to 110-120°C. Then, carbon monoxide and chlorine are added in a molar ratio of 1:1.05-1.

1. The reaction is carried out for 20-30 seconds. After the reaction is completed, a mixed gas is obtained. The mixed gas is cooled to -10 to 0°C, and the liquid mixture is collected. The liquid mixture is then passed into a distillation column with a top temperature of 7-8°C for purification to obtain phosgene.

9. The method for preparing toluene diisocyanate using phosgene according to claim 8, characterized in that, The activated carbon has a particle size of 5 mm and a specific surface area of ​​800 m² / g.

10. The method for preparing toluene diisocyanate using phosgene according to claim 1, characterized in that, The molar ratio of phosgene and m-toluenediamine in S2 is 2.1 to 2.3:1.