A method for the preparation of p-nitrotoluene

Abstract

The application discloses a preparation method of p-nitrotoluene and relates to the technical field of organic synthesis, and comprises the following steps: S1, liquid CO2 is filled into a high-pressure reaction kettle, and then toluene, mixed acid and a solid acid catalyst are sequentially added to perform a nitration reaction; wherein the mixed acid is prepared from nitric acid and concentrated sulfuric acid; S2, organic phase after the nitration reaction is separated; S3, the organic phase is sequentially washed and dried to obtain a high-selectivity p-nitrotoluene product. The preparation method of the p-nitrotoluene uses liquid CO2 as a reaction medium, and toluene is subjected to a nitration reaction with the mixed acid as a nitration agent under the action of a solid acid catalyst, and the para-position selectivity of the reaction can be improved by controlling the reaction conditions.

Description

Technical Field

[0001] This invention relates to the field of organic synthesis technology, and more specifically to a method for preparing p-nitrotoluene. Background Technology

[0002] p-Nitrotoluene is the most valuable key intermediate among nitrotoluene compounds, widely used in the synthesis of fine chemicals such as dyes, pharmaceuticals, and pesticides. The mainstream industrial process for large-scale preparation of p-nitrotoluene is the toluene-mixed-acid nitration method. This process uses a mixture of nitric acid and sulfuric acid as the nitrating agent, achieving the nitration of toluene through an electrophilic substitution reaction.

[0003] However, the existing mixed acid nitration process has three major technical defects in practical applications. 1) Low para-selectivity and high separation cost: Due to the ortho- and para-positioning effect of the methyl group in the toluene molecule, the products of the traditional mixed acid nitration reaction are mainly ortho-nitrotoluene and para-nitrotoluene, with a ratio of only 1.5~1.8:1, and a para-selectivity of only 60%~65%, accompanied by about 5% m-nitrotoluene byproduct. Because the physical properties of nitrotoluene isomers are similar, the separation and purification of p-nitrotoluene using conventional methods such as distillation and crystallization requires a large amount of energy and equipment investment, which significantly increases the production cost of the target product and restricts the economic benefits of the process; 2) Insufficient heat removal efficiency and prominent safety risks: Toluene nitration is a strongly exothermic reaction. If the large amount of heat released during the reaction cannot be removed in time, it will cause a sudden rise in the local temperature of the reaction system; Traditional processes use jacket cooling to remove heat, but due to the limitation of heat transfer efficiency, it is difficult to achieve rapid and uniform removal of the heat of reaction, which can easily lead to side reactions such as polynitration and oxidation. This not only reduces the yield and purity of the target product, but may also generate polynitrate byproducts and organic peroxides with poor stability, which may bring potential safety hazards to subsequent separation and purification processes; 3) Poor environmental friendliness and significant environmental pressure: Traditional mixed acid nitration processes require the use of a large amount of concentrated sulfuric acid as a reaction solvent and solid acid catalyst, which will generate a huge amount of waste acid during the production process. This type of waste acid has a complex composition, containing unreacted nitric acid, organic matter, and salts. Neutralization requires a large amount of alkaline solution, and the waste residue generated after treatment can easily cause secondary pollution. At the same time, the recovery and concentration of waste acid requires high investment in equipment and energy costs, which puts a heavy environmental governance pressure on enterprises and is not in line with the current development trend of green chemical industry.

[0004] Although some improvements have been proposed in the existing technology, such as adding aromatic sulfonic acid compounds to the nitration system to improve para-selectivity, or using adiabatic nitration process to recover heat from waste acid, the former still does not break away from the dependence on concentrated sulfuric acid system and the waste acid treatment problem has not been fundamentally solved, while the latter has new problems such as by-products clogging equipment and insufficient product stability at high temperatures. Summary of the Invention

[0005] The purpose of this invention is to solve the problem of poor nitration of mixed acid in the prior art. This invention provides a method for preparing p-nitrotoluene, which improves the stability of the product at high temperature while eliminating dependence on concentrated sulfuric acid system and reducing the generation of by-products.

[0006] To achieve the above objectives, the present invention specifically adopts the following technical solution: A method for preparing p-nitrotoluene includes the following steps: S1. After introducing liquid CO2 into the high-pressure reactor, toluene, mixed acid and solid acid catalyst are added in sequence to carry out the nitration reaction. The mixed acid is prepared from nitric acid and concentrated sulfuric acid. S2, Separate the organic phase after the nitration reaction; S3. The organic phase is washed and dried sequentially to obtain a highly selective p-nitrotoluene product.

[0007] Preferably, the high-pressure reactor has a reaction pressure of 5.1~5.6MPa, a reaction temperature of 14~21℃, and a reaction time of 30~40min.

[0008] Preferably, the purity of the liquid CO2 is not less than 99.9%.

[0009] Preferably, the concentration of nitric acid is 65-70%, the concentration of concentrated sulfuric acid is 95-98%, and the mass ratio of nitric acid to concentrated sulfuric acid is 1:2.5-3.

[0010] Preferably, the molar ratio of toluene to nitric acid is 1:1.2~1.5.

[0011] Preferably, the amount of the solid acid catalyst added is 5-8% of the mass of toluene, and the solid acid catalyst is Hβ zeolite or AlCl3-SiO2.

[0012] Preferably, the Hβ zeolite is activated by calcination at 550°C, the silicon-to-aluminum ratio of the Hβ zeolite is 260, and the loading of AlCl3 in AlCl3-SiO2 is 10wt%.

[0013] Preferably, the specific method for washing the organic phase includes: washing the organic phase twice with a 5% sodium carbonate solution, and then washing the organic phase with deionized water until it is neutral.

[0014] Preferably, during the nitration reaction, liquid CO2 vaporizes to absorb and remove the heat of reaction, and the vaporized CO2 is condensed and recovered.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Using liquid CO2 as the reaction medium, toluene and mixed acid as nitrating agents are nitrated under the action of a solid acid catalyst. The para-selectivity of the reaction can be improved by controlling the reaction conditions.

[0016] 2. Using liquid CO2 as a heat transfer carrier can remove the heat of reaction from the reaction site. The vaporized CO2 is compressed and condensed into liquid and then recycled back to the reaction system for reuse. This saves resources and achieves isothermal operation of the reaction system, avoiding side reactions caused by local overheating.

[0017] 3. By replacing most of the sulfuric acid as a medium with liquid CO2, the amount of waste acid emissions is reduced at the source; CO2 itself is non-toxic and non-flammable, and can be efficiently condensed and recovered, realizing the recycling of solvents. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a batch reaction device.

[0019] Legend: 1. High-pressure reactor; 2. Liquid CO2 storage tank; 3. Mixed acid feed tank; 4. Toluene feed tank; 5. Catalyst feed port; 6. Temperature sensor; 7. Pressure sensor; 8. Vaporized CO2 outlet; 9. Product outlet; 10. Agitator. Detailed Implementation

[0020] 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The materials and instruments used in the following examples are all commercially available.

[0022] A method for preparing p-nitrotoluene includes steps S1 to S3.

[0023] S1. After filling the high-pressure reactor with liquid CO2, toluene, mixed acid and solid acid catalyst are added in sequence to carry out the nitration reaction.

[0024] Specifically, the purity of the liquid CO2 is not less than 99.9%. Using liquid CO2 as the reaction medium, toluene reacts with a mixed acid as a nitrating agent under the action of a solid acid catalyst. By controlling the reaction pressure in the high-pressure reactor to 5.1–5.6 MPa, the reaction temperature to 14–21 °C, the reaction time to 30–40 min, and the molar ratio of toluene to nitric acid to 1:1.2–1.5, the para-selectivity of the reaction can be improved. Furthermore, liquid CO2 acts as a heat transfer carrier, removing the heat of reaction from the reaction site. The vaporized CO2, after compression and condensation into liquid, is recycled back to the reaction system for reuse, achieving isothermal operation of the reaction system with a temperature control accuracy of ±0.5 °C, avoiding side reactions caused by localized overheating. Liquid CO2 replaces most of the sulfuric acid as the medium, reducing waste acid emissions by more than 70% at the source. CO2 itself is non-toxic, non-flammable, and can be efficiently condensed and recovered, achieving solvent recycling.

[0025] The mixed acid is prepared by mixing nitric acid and concentrated sulfuric acid; the concentration of nitric acid is 65-70%, the concentration of concentrated sulfuric acid is 95-98%, and the mass ratio of nitric acid to concentrated sulfuric acid is 1:2.5-3.

[0026] The amount of solid acid catalyst added is 5-8% of the mass of toluene. The solid acid catalyst is Hβ zeolite or AlCl3-SiO2. Hβ zeolite is activated by calcination at 550℃. The silicon-to-aluminum ratio of Hβ zeolite is 260. The loading of AlCl3 in AlCl3-SiO2 is 10wt%.

[0027] S2. Separate the organic phase after the nitration reaction to obtain the crude product.

[0028] S3. The organic phase is washed and dried sequentially to obtain a highly selective p-nitrotoluene product. The specific method for washing the organic phase includes: washing the organic phase twice with a 5% sodium carbonate solution, and then washing the organic phase with deionized water until neutral.

[0029] Example 1 99.9% liquid CO2 was introduced into a 10L high-pressure reactor, and the system pressure was adjusted to 5.17 MPa. The temperature was stabilized at 15℃ using jacket cooling. 5.3g of toluene, 18.6g of mixed acid, and 0.42g of Hβ zeolite were added sequentially. The concentrations of nitric acid and concentrated sulfuric acid were 68% and 98% respectively, with a mass ratio of 1:2.5. The Hβ zeolite was activated by calcination at 550℃, and its silica-to-alumina ratio was 260. The mixture was stirred at 800 rpm for 35 min. During the reaction, the vaporized CO2 was discharged through the outlet and condensed for recovery, with system temperature fluctuations less than 0.3℃. After the reaction, the pressure was released, the organic phase was separated, washed twice with 5% sodium carbonate solution, then washed with deionized water until neutral, and dried to obtain 7.2g of product. Gas chromatography analysis showed a toluene conversion rate of 98.5%, a selectivity of 99.8% for p-nitrotoluene (significantly inhibiting the formation of ortho-products), and a purity of 99.8% for p-nitrotoluene.

[0030] Example 2 99.9% liquid CO2 was introduced into a 10L high-pressure reactor, and the system pressure was adjusted to 5.5MPa. The temperature was stabilized at 20℃ using jacket cooling. 5.3g of toluene, 19.2g of mixed acid, and 0.27g of AlCl3-SiO2 were added sequentially; the concentrations were 65% nitric acid, 96% concentrated sulfuric acid (mass ratio of nitric acid to concentrated sulfuric acid 1:3), and the AlCl3 loading in the AlCl3-SiO2 was 10%. The mixture was stirred at 800rpm for 40min. During the reaction, the vaporized CO2 was discharged through the outlet and condensed for recovery, with system temperature fluctuations less than 0.3℃. After the reaction, the pressure was released, the organic phase was separated, washed twice with 5% sodium carbonate solution, then washed with deionized water until neutral, and dried to obtain 7.1g of product. Gas chromatography analysis showed a toluene conversion rate of 97.2%, a selectivity of 99.6% for p-nitrotoluene (significantly inhibiting the formation of ortho-products), and a purity of 99.6% for p-nitrotoluene.

[0031] Example 3 99.9% liquid CO2 was introduced into a 10L high-pressure reactor, and the system pressure was adjusted to 5.1 MPa. The temperature was stabilized at 14℃ using jacket cooling. 5.3 g of toluene, 18.6 g of mixed acid, and 0.42 g of Hβ zeolite were added sequentially. The concentrations of nitric acid and concentrated sulfuric acid were 68% and 98% respectively, with a mass ratio of 1:2.5. The Hβ zeolite was activated by calcination at 550℃, and its silica-to-alumina ratio was 260. The mixture was stirred at 800 rpm for 40 min. During the reaction, the vaporized CO2 was discharged through the outlet and condensed for recovery, with system temperature fluctuations less than 0.3℃. After the reaction, the pressure was released, the organic phase was separated, washed twice with 5% sodium carbonate solution, then washed with deionized water until neutral, and dried to obtain 7.2 g of product. Gas chromatography analysis showed a toluene conversion rate of 96.8% and a selectivity of 99.5% for p-nitrotoluene, indicating that the formation of ortho-products was essentially suppressed.

[0032] Example 4 99.9% liquid CO2 was introduced into a 10L high-pressure reactor, and the system pressure was adjusted to 5.6 MPa. The temperature was stabilized at 21℃ using jacket cooling. 5.3 g of toluene, 18.6 g of mixed acid, and 0.42 g of Hβ zeolite were added sequentially. The concentrations were: nitric acid 68%, concentrated sulfuric acid 98%, and a mass ratio of nitric acid to concentrated sulfuric acid of 1:2.5. The Hβ zeolite was activated by calcination at 550℃, and its silica-to-alumina ratio was 260. The mixture was stirred at 800 rpm for 30 min. During the reaction, the vaporized CO2 was discharged through the outlet and condensed for recovery, with system temperature fluctuations less than 0.3℃. After the reaction, the pressure was released, the organic phase was separated, washed twice with 5% sodium carbonate solution, then washed with deionized water until neutral, and dried to obtain 7.2 g of product. Gas chromatography analysis showed a toluene conversion rate of 98.9% and a selectivity of 99.0% for p-nitrotoluene, indicating that the formation of ortho-products was essentially suppressed.

[0033] Comparative Example 1 99.9% liquid CO2 was introduced into a 10L high-pressure reactor, and the system pressure was adjusted to atmospheric pressure. The temperature was stabilized at 30℃ using jacket cooling. 5.3g of toluene and 40g of mixed acid were added sequentially, with nitric acid at a concentration of 68% and concentrated sulfuric acid at a concentration of 98%, in a mass ratio of 1:4. The mixture was stirred at 800rpm for 60min. During the reaction, the vaporized CO2 was discharged through the outlet and condensed for recovery. The system temperature fluctuation was less than 0.3℃. After the reaction, the pressure was released, the organic phase was separated, washed twice with 5% sodium carbonate solution, and then washed with deionized water until neutral. After drying, 7.2g of product was obtained. Gas chromatography analysis showed a toluene conversion rate of 99.0%, a selectivity of 64.2% for p-nitrotoluene, and a purity of 35.1%. A small amount of dinitrotoluene was also detected.

[0034] Comparative Example 2 20 mL of dichloromethane was introduced into a 10 L high-pressure reactor, and the system pressure was adjusted to atmospheric pressure. The temperature was stabilized at 15 °C using jacket cooling. 5.3 g of toluene, 18.6 g of mixed acid, and 0.42 g of Hβ zeolite were added sequentially. The concentrations of nitric acid and concentrated sulfuric acid were 68% and 98% respectively, with a mass ratio of 1:2.5. The Hβ zeolite was activated by calcination at 550 °C, and its silica-to-alumina ratio was 260. The mixture was stirred at 800 rpm for 35 min. During the reaction, the vaporized CO2 was discharged through the outlet and condensed for recovery. The system temperature fluctuation was less than 0.3 °C. After the reaction, the pressure was released, the organic phase was separated, washed twice with 5% sodium carbonate solution, then washed with deionized water until neutral, and dried to obtain 7.2 g of product. Gas chromatography analysis showed a toluene conversion rate of 95.5% and a selectivity of 86.3% for p-nitrotoluene. Strong external cooling and dichloromethane recovery are required during the reaction.

[0035] This method exhibits high toluene conversion, high selectivity for p-nitrotoluene, and high purity of p-nitrotoluene. The overall reaction conditions are mild, resulting in less equipment corrosion, making it suitable for intermittent or continuous industrial-scale production.

[0036] The batch reaction apparatus used in this method is also disclosed, such as... Figure 1 As shown.

[0037] The batch reaction unit includes a liquid CO2 storage tank 2, a mixed acid feed tank 3, a toluene feed tank 4, and a high-pressure reactor 1. The outlets of the liquid CO2 storage tank 2, the mixed acid feed tank 3, and the toluene feed tank 4 are all connected to the inlet of the high-pressure reactor 1. The high-pressure reactor 1 is made of 316L stainless steel. The top of the high-pressure reactor 1 is equipped with a catalyst feed port 5, a vaporized CO2 outlet 8, a temperature sensor 6, and a pressure sensor 7. The bottom of the high-pressure reactor 1 is equipped with a product outlet 9. An agitator 10 is installed inside the high-pressure reactor 1. The vaporized CO2 outlet 8 is connected to a condensation recovery unit.

Claims

1. A method for preparing p-nitrotoluene, characterized in that, Includes the following steps: S1. After introducing liquid CO2 into the high-pressure reactor, toluene, mixed acid and solid acid catalyst are added in sequence to carry out the nitration reaction. The mixed acid is prepared from nitric acid and concentrated sulfuric acid. S2, Separate the organic phase after the nitration reaction; S3. The organic phase is washed and dried sequentially to obtain a highly selective p-nitrotoluene product.

2. The method for preparing p-nitrotoluene according to claim 1, characterized in that, The high-pressure reactor has a reaction pressure of 5.1~5.6MPa, a reaction temperature of 14~21℃, and a reaction time of 30~40min.

3. The method for preparing p-nitrotoluene according to claim 1, characterized in that, The purity of the liquid CO2 is not less than 99.9%.

4. The method for preparing p-nitrotoluene according to claim 1, characterized in that, The concentration of nitric acid is 65-70%, the concentration of concentrated sulfuric acid is 95-98%, and the mass ratio of nitric acid to concentrated sulfuric acid is 1:2.5-3.

5. The method for preparing p-nitrotoluene according to claim 4, characterized in that, The molar ratio of toluene to nitric acid is 1:1.2~1.

5.

6. The method for preparing p-nitrotoluene according to claim 1, characterized in that, The amount of solid acid catalyst added is 5-8% of the mass of toluene, and the solid acid catalyst is Hβ zeolite or AlCl3-SiO2.

7. The method for preparing p-nitrotoluene according to claim 6, characterized in that, The Hβ zeolite was activated by calcination at 550℃, and the silicon-to-aluminum ratio of the Hβ zeolite was 260, with the AlCl3 loading in AlCl3-SiO2 being 10wt%.

8. The method for preparing p-nitrotoluene according to claim 1, characterized in that, The specific method for washing the organic phase includes: washing the organic phase twice with a 5% sodium carbonate solution, and then washing the organic phase with deionized water until it is neutral.

9. The method for preparing p-nitrotoluene according to claim 1, characterized in that, During the nitration reaction, liquid CO2 vaporizes to absorb and remove the heat of reaction, and the vaporized CO2 is condensed and recovered.

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