Continuous nitration preparation method of 2-amino-4-nitrotoluene in a microreactor

Abstract

The invention discloses a microreactor continuous nitration preparation method for 2-amino-4-nitrotoluene. The method comprises the following steps: salt formation: o-toluidine is dropwise added to concentrated sulfuric acid, and water is added to control the temperature to 0-35°C to obtain o-toluidine sulfate; mixed acid preparation: 98% concentrated sulfuric acid is added to 50-75% industrial nitric acid in batches, and the temperature is controlled to be 0-35°C; nitration: o-toluidine sulfate and mixed acid are respectively controlled to enter a microchannel reactor through an injection pump, and gas is introduced simultaneously, the nitration temperature is controlled to be 0-35°C, and the total residence time is 0.5-10 minutes; a microchannel end is treated with water to precipitate a solid, and a water precipitate is obtained by filtration; neutralization: the water precipitate is added to water, neutralized with alkali to neutrality, and filtered to obtain a product. The invention uses 50-75% nitric acid instead of fuming nitric acid, thereby shortening the reaction time and reducing the risk factor of the reaction while ensuring the conversion rate; and changing the size of the microchannel reactor can also increase the production capacity.

Description

Technical Field

[0001] The present invention specifically relates to a method for preparing 2-amino-4-nitrotoluene by continuous nitration in a microreactor. Background Art

[0002] 2-Amino-4-nitrotoluene (Brilliant Red G) is a golden crystalline solid and a key component of insoluble azo dyes. It is primarily used for dyeing silk, viscose, and nylon fabrics and can also be used as an organic pigment intermediate. Its current preparation method primarily uses o-toluidine as a raw material, undergoing sulfuric acid salt formation, mixed acid nitration, and soda ash neutralization followed by centrifugal separation.

[0003] The traditional batch production method uses o-toluidine as the raw material, using 98% sulfuric acid and a small amount of waste sulfuric acid with a concentration greater than 50% to form a salt. A mixed acid of 98% concentrated sulfuric acid and 98% fuming nitric acid is used as the nitrating agent. The nitration reaction is carried out at -5°C to 0°C. The product is then neutralized with liquid ammonia (or soda ash) and centrifuged and filtered to obtain the product (CN106278908A). This traditional method has problems such as long reaction time, high risk, poor selectivity, and environmental pollution.

[0004] Micro-reaction continuous flow technology is increasingly being used in the production of fine chemical products due to its high mass and heat transfer efficiency, easy temperature control, few side reactions, and low liquid holdup. In the patents currently being applied for, two technical routes are used to prepare bright red base G using micro-reaction continuous flow technology: one is to use acetic anhydride or glacial acetic acid to form a salt with o-toluidine, which is then concentrated by distillation to obtain the acetate, which is then nitrated with a mixed acid of acetic acid and nitric acid (acetic acid and nitric acid mixture) in a microchannel. The resulting nitrated solution is dissolved in ice water, crystallized, and filtered. Finally, the filter cake is acidified, dissolved, crystallized, and filtered to obtain the product (CN114478262A and CN109942434A). This technical route has complex procedures, long reaction cycles, and high costs. In addition, the acetic anhydride or glacial acetic acid used can cause occupational health problems in the workshop due to their pungent odor. The second technical route is to replace the nitration reaction in a batch reactor in the traditional production method with a continuous flow reaction in a microchannel (CN113121360A).

[0005] Traditional batch production uses impeller stirring to achieve mixing and mass transfer, resulting in a low mass transfer coefficient and poor mixing. To improve reaction efficiency, a mixed acid of 98% concentrated sulfuric acid and 98% fuming nitric acid is used as the nitrating agent to maintain a high concentration of nitroxyl ions, thereby increasing the reaction rate. This reaction is also highly exothermic, and for safety reasons, the liquid in the kettle must be maintained at a low temperature of -5°C to remove the heat of reaction. However, this low system temperature increases the viscosity of the sulfuric acid system, weakening the mass transfer and dispersion effect, further reducing the reaction rate. Consequently, the reaction time reaches 6-8 hours, making the entire process inefficient and posing a safety risk of hypothermia.

[0006] Prior art CN113121360A discloses a method for preparing a bright red base G, comprising reacting o-toluidine with sulfuric acid to obtain a first product, which is then separately transferred to a microchannel reactor to react with fuming nitric acid to obtain a second product, which is then reacted with soda ash until neutral to obtain the bright red base G. The method uses 98% fuming nitric acid as the nitrating reagent, resulting in a high concentration of nitroxyl cations. Combined with the high specific surface area provided by the microchannel reactor, the reaction is completed in a very short time. However, this method is prone to hotspot concentration, requiring a strong heat exchange system to remove heat promptly, resulting in significant energy consumption. Hotspot concentration also increases side reactions and reduces selectivity. Furthermore, transient reactions are completed at the front end of the microchannel, rendering the back end of the microchannel reactor inoperative, resulting in equipment waste. Furthermore, the microchannels currently used have relatively small diameters, typically ranging from 300 to 1000 μm, which limits production capacity.

[0007] The diameter of the pipe is inversely proportional to the heat transfer coefficient and specific surface area of ​​the system. If the pipe diameter is increased in order to increase production capacity, the specific surface area and heat transfer coefficient will decrease. This will lead to two results: First, the mixing efficiency of the liquid-liquid heterogeneous reaction will drop significantly, the selectivity of the reaction will also drop, the by-products will increase, and the yield will decrease. Second, in the system with 98% fuming nitric acid as the nitrating reagent, the concentration of nitroxyl cations is very high and the reaction rate is very fast. If the pipe diameter is increased in order to increase production capacity and the input of raw materials is increased, the heat accumulation problem in the pipe will inevitably occur due to the concentration of hot spots, and higher requirements will be placed on the heat exchange system. Otherwise, hypothermia will occur and bring safety hazards. If the concentration of nitric acid is reduced for safety, it will inevitably cause NO 2+ The decrease in concentration leads to low conversion rate and incomplete reaction, which affects the yield. If the conversion rate is to be increased, the reaction residence time must be significantly extended, which in turn leads to a significant increase in equipment costs. At the same time, the extended residence time also reduces production efficiency.

[0008] When preparing 2-amino-4-nitrotoluene (2-amino-4-nitrotoluene G) using a microchannel reactor, how to reduce the nitric acid concentration and increase safety while taking into account the conversion rate and production efficiency has become a technical problem that needs to be solved urgently. Summary of the Invention

[0009] The present invention aims to provide a microreactor continuous nitration preparation method for 2-amino-4-nitrotoluene, which adopts 50-75% industrial nitric acid instead of 98% fuming nitric acid, reduces the risk factor of the reaction while ensuring the conversion rate without prolonging the residence time.

[0010] In order to achieve the above purpose, the technical solutions adopted are as follows:

[0011] The method for preparing 2-amino-4-nitrotoluene by continuous nitration in a microreactor comprises the following steps:

[0012] (1) Salt formation: o-toluidine is added dropwise to concentrated sulfuric acid, and water is added to control the temperature to 0-35°C to obtain o-toluidine sulfate;

[0013] (2) Preparation of mixed acid: Add 98% concentrated sulfuric acid to 50-75% industrial nitric acid in batches, and control the temperature at 0-35°C;

[0014] (3) Nitration: o-toluidine sulfate and mixed acid are respectively controlled by an injection pump to enter the microchannel reactor, and gas is introduced at the same time. The nitration temperature is controlled at 0-35°C, and the total residence time is 0.5-10 min. The end of the microchannel is treated with water to precipitate solids, which are filtered to obtain a water precipitate.

[0015] (4) Neutralization: Add the precipitated material into water, neutralize it with alkali until it is neutral, and filter it to obtain the product.

[0016] According to the above scheme, the molar ratio of concentrated sulfuric acid to o-toluidine in step (1) is (4-10):1.

[0017] According to the above scheme, the molar ratio of concentrated sulfuric acid to water in step (1) is (4-10):1.

[0018] According to the above scheme, the molar ratio of 98% concentrated sulfuric acid and 50-75% nitric acid in step (2) is (0.3-1.0):1.

[0019] According to the above scheme, in step (3), o-toluidine sulfate and mixed acid enter the microchannel reactor at a volume ratio of (5-7):1.

[0020] According to the above scheme, the gas in step (3) includes any one of air, carbon dioxide, nitrogen, and argon.

[0021] According to the above scheme, the gas introduced in step (3) controls the gas-liquid volume ratio in the reaction system to be (10-1000):1.

[0022] According to the above scheme, the inner diameter of the microchannel of the microchannel reactor in step (3) ranges from 0.3 to 4 mm.

[0023] The chemical reaction synthesis route of the present invention is as follows:

[0024]

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) The system uses 50-75% nitric acid to replace 98% of fuming nitric acid in the traditional process, thereby reducing the cost of raw materials.

[0027] (2) The reaction is mild and controllable, and side reactions are reduced.

[0028] (3) The system temperature is increased from about -5°C in the traditional process to above 10°C, reducing energy consumption.

[0029] (4) The safety of the reaction is greatly improved while ensuring the conversion rate without extending the residence time.

[0030] (5) The system uses microchannels with an inner diameter ranging from 0.3 to 4 mm, which increases the flux and production capacity by an order of magnitude compared to traditional microreactors. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 : The typical preparation process flow chart of 2-amino-4-nitrotoluene of the present invention.

[0032] Figure 2 : The liquid chromatogram of embodiment 1 product. DETAILED DESCRIPTION

[0033] The following examples further illustrate the technical solutions of the present invention but are not intended to limit the scope of protection of the present invention.

[0034] The specific embodiment provides a 2-amino-4-nitrotoluene microreactor continuous nitration preparation method, which is as follows:

[0035] (1) Salt formation: o-toluidine is added dropwise to concentrated sulfuric acid, and water is added to control the temperature to 0-35°C to obtain o-toluidine sulfate;

[0036] (2) Preparation of mixed acid: Add 98% concentrated sulfuric acid to 50-75% industrial nitric acid in batches, and control the temperature at 0-35°C;

[0037] (3) Nitration: o-toluidine sulfate and mixed acid are respectively controlled by an injection pump to enter the microchannel reactor, and gas is introduced at the same time. The nitration temperature is controlled at 0-35°C, and the total residence time is 0.5-10 min. The end of the microchannel is treated with water to precipitate solids, which are filtered to obtain a water precipitate.

[0038] (4) Neutralization: Add the precipitated material into water, neutralize it with alkali until it is neutral, and filter it to obtain the product.

[0039] Specifically, in step (1), the molar ratio of concentrated sulfuric acid to o-toluidine is (4-10):1. The molar ratio of concentrated sulfuric acid to water is (4-10):1.

[0040] Specifically, in step (2), the molar ratio of 98% concentrated sulfuric acid to 50-75% nitric acid is (0.3-1.0):1.

[0041] Specifically, in step (3), o-toluidine sulfate and mixed acid are introduced into the microchannel reactor at a volume ratio of (5-7):1. The gas comprises any one of air, carbon dioxide, nitrogen, and argon. When the gas is introduced, the gas-liquid volume ratio in the reaction system is controlled to be (10-1000):1.

[0042] Specifically, the inner diameter of the microchannel of the microchannel reactor in step (3) ranges from 0.3 to 4 mm.

[0043] The typical preparation process flow chart of 2-amino-4-nitrotoluene of the present invention is shown in the attached Figure 1 As shown, tanks A and B contain the salt-forming material and mixed acid, respectively. Two pumps deliver these two streams to the reactor. A gas flowmeter adjusts the gas flow rate before it enters the salt-forming material pipeline. The entire system's temperature and pressure are monitored by built-in temperature and pressure sensors. The two streams enter the microchannel reactor at a T-junction, where nitration occurs. The tail end of the reactor is then introduced into water, filtered, and sampled for liquid phase analysis.

[0044] Example 1

[0045] (1) Salt Formation: Add 98% concentrated sulfuric acid to a three-necked glass bottle with stirring at a molar ratio of 5:1. The temperature in the bottle is controlled at 25°C. Add o-toluidine dropwise to the three-necked glass bottle. The o-toluidine is added dropwise in about 1 hour. Then, add water at a molar ratio of 4:1. Stir for 5 minutes and then discharge the product.

[0046] (2) Preparation of mixed acid: Add concentrated sulfuric acid to nitric acid in batches according to a molar ratio of 98% concentrated sulfuric acid to 68% nitric acid of 0.6:1, and control the temperature at 20°C.

[0047] (3) Nitration: Nitrogen gas was introduced into the microchannel system via an inlet pump at a volume ratio of 5:1 between o-methylaniline sulfate and mixed acid, with the volume of nitrogen being 200 times the total volume of the liquid. The gas and liquid were combined via a tee and then flowed into a reaction tube with an inner diameter of 1 mm, a wall thickness of 0.5 mm, and a length of 10 m. The temperature was set at 10°C. The reaction residence time was 1 min.

[0048] (4) Neutralization: Add the prepared pure alkali water (mass concentration 30%) dropwise to the precipitate, neutralize to pH = 7, and filter to obtain the product. The product yield is 85.7%, the product purity is 98.26%, and the space-time yield is: 8 kg / cm 3 .h. The liquid chromatogram of the product obtained in this example is attached Figure 2 shown.

[0049] Example 2

[0050] (1) Salt Formation: Add 98% concentrated sulfuric acid to a three-necked glass bottle with stirring at a molar ratio of 4:1. The temperature in the bottle is controlled at 25°C. Add o-toluidine dropwise to the three-necked glass bottle. The o-toluidine is added dropwise in about 1 hour. Then, add water at a molar ratio of 6:1. Stir for 5 minutes and then discharge the product.

[0051] (2) Preparation of mixed acid: Add concentrated sulfuric acid to nitric acid in batches according to the molar ratio of 98% concentrated sulfuric acid and 60% nitric acid of 0.5:1, and control the temperature at 20°C.

[0052] (3) Nitration: Nitrogen gas was introduced into the microchannel system via an injection pump at a volume ratio of 6:1 between o-methylaniline sulfate and mixed acid, with the volume of nitrogen being 100 times the total liquid volume. The gas and liquid materials were combined via a tee and then flowed into a reaction tube with an inner diameter of 3 mm, a wall thickness of 0.5 mm, and a length of 15 m. The set temperature was 20°C, and the reaction residence time was 2 min.

[0053] (4) Neutralization: Add the prepared pure alkali water (mass concentration 30%) dropwise to the precipitate, neutralize to pH = 7, and filter to obtain the product. The product yield is 82.6%, the product purity is 98.13%, and the space-time yield is: 11 kg / cm 3 .h

[0054] Example 3

[0055] (1) Salt Formation: Add 98% concentrated sulfuric acid to a three-necked glass bottle with stirring at a molar ratio of 6:1. The temperature in the bottle is controlled at 25°C. Add o-toluidine dropwise to the three-necked glass bottle. The o-toluidine is added dropwise in about 1 hour. Then, add water at a molar ratio of 5:1. Stir for 5 minutes and then discharge the product.

[0056] (2) Preparation of mixed acid: Concentrated sulfuric acid was added to nitric acid in batches according to a molar ratio of 98% concentrated sulfuric acid to 55% nitric acid of 0.5:1, and the temperature was controlled at 20°C.

[0057] (3) Nitration: Nitrogen gas was introduced into the microchannel system via an injection pump at a volume ratio of 7:1 between o-methylaniline sulfate and mixed acid, with the volume of nitrogen being 100 times the total volume of the liquid. The gas and liquid materials were combined via a tee and then flowed into a reaction tube with an inner diameter of 3 mm, a wall thickness of 0.5 mm, and a length of 15 m. The temperature was set at 20°C, and the reaction residence time was 2 min.

[0058] (4) Neutralization: Add the prepared pure alkali water (mass concentration 30%) dropwise to the precipitate, neutralize to pH = 7, and filter to obtain the product. The product yield is 78.6%, the product purity is 97.6%, and the space-time yield is: 9 kg / cm 3 .h

[0059] Comparative Example 1

[0060] Example 1 was repeated, but without introducing gas, and the specific operations were as follows.

[0061] (1) Salt Formation: Add 98% concentrated sulfuric acid to a three-necked glass bottle with stirring at a molar ratio of 5:1. The temperature in the bottle is controlled at 25°C. Add o-toluidine dropwise to the three-necked glass bottle. The o-toluidine is added dropwise in about 1 hour. Then, add water at a molar ratio of 4:1. Stir for 5 minutes and then discharge the product.

[0062] (2) Preparation of mixed acid: Add concentrated sulfuric acid to nitric acid in batches according to a molar ratio of 98% concentrated sulfuric acid to 68% nitric acid of 0.6:1, and control the temperature at 20°C.

[0063] (3) Nitration: o-Toluidine sulfate and mixed acid were introduced into the microchannel system via an injection pump at a volume ratio of 5:1. No nitrogen was introduced into the system. The materials were combined through a tee and then entered into a reaction tube with an inner diameter of 1 mm, a wall thickness of 0.5 mm, and a length of 10 m. The set temperature was 10°C. The reaction residence time was 1 min.

[0064] (4) Neutralization: Add the prepared pure alkali water (mass concentration 30%) dropwise to the precipitate, neutralize to pH = 7, and filter to obtain the product. The product yield is 58.7%, the product purity is 86.36%, and the space-time yield is: 6 kg / cm 3 .h.

Claims

1. 2-amino-4-nitrotoluene microreactor continuous nitration preparation method, characterized in that The steps include: (1) Salt formation: o-toluidine is added dropwise to concentrated sulfuric acid, and water is added to control the temperature to 0-35°C to obtain o-toluidine sulfate; (2) Prepare mixed acid: Add 98% concentrated sulfuric acid to 50-75% industrial nitric acid in batches, and control the temperature at 0-35°C; the molar ratio of 98% concentrated sulfuric acid to 50-75% nitric acid is (0.3-1.0):1; (3) Nitration: o-toluidine sulfate and mixed acid are respectively controlled to enter the microchannel reactor through the injection pump, and gas is introduced into the reaction system to control the gas-liquid volume ratio (100-200):

1. The nitration temperature is controlled at 0-35 °C, and the total residence time is 0.5-10 min. The end of the microchannel is treated with water to precipitate solids, which are filtered to obtain a water precipitate. The inner diameter of the microchannel of the microchannel reactor ranges from 1 to 3 mm. (4) Neutralization: Add the precipitated material into water, neutralize it with alkali until it is neutral, and filter it to obtain the product.

2. the continuous nitration preparation method of 2-amino-4-nitrotoluene microreactor as claimed in claim 1, is characterized in that In step (1), the molar ratio of concentrated sulfuric acid to o-toluidine is (4-10):

1.

3. 2-amino-4-nitrotoluene microreactor continuous nitration preparation method as claimed in claim 1, is characterized in that In step (1), the molar ratio of concentrated sulfuric acid to water is (4-10):

1.

4. 2-amino-4-nitrotoluene microreactor continuous nitration preparation method as claimed in claim 1, is characterized in that In step (3), o-toluidine sulfate and mixed acid enter the microchannel reactor at a volume ratio of (5-7):

1.

5. 2-amino-4-nitrotoluene microreactor continuous nitration preparation method as claimed in claim 1, is characterized in that The gas in step (3) includes any one of air, carbon dioxide, nitrogen and argon.

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