A method for the synthesis of hexafluorobenzene

Abstract

The application discloses a synthesis method of hexafluorobenzene and belongs to the fine chemical field. The method is carried out according to the following process: 1) mixing pentafluorobenzene, a catalyst and an initiator to obtain a mixed solution; 2) heating a pretreated reaction device, and then feeding specific mixed gas into the device from bottom to top; then feeding the mixed solution obtained in the step 1) into the reaction device from top to bottom; 3) collecting all the crude products obtained in the step 2), and then performing rectification to obtain hexafluorobenzene. The method has the advantages of mild reaction condition, easy industrialization, low raw material cost, high product yield and purity, and energy saving and environmental protection compared with other methods.

Description

Technical Field

[0001] This invention relates to a method for synthesizing hexafluorobenzene, belonging to the field of fine chemicals. Technical Background

[0002] Hexafluorobenzene, CAS: 392-56-3, molecular formula C6F6, molecular weight 186.05, freezing point 3.7-4.1℃, boiling point 80-82℃, density 1.612 g / cm3. This substance is commonly used as an anesthetic and an excellent solvent for many organic compounds. It is also an important intermediate in the synthesis of perfluorinated aromatic compounds and has broad application prospects in chemical, pharmaceutical, and liquid crystal materials fields.

[0003] The most common method for synthesizing hexafluorobenzene is the direct fluorination of aromatic halogenated compounds. However, because the substrates for direct fluorination lack strongly electron-withdrawing groups, halogen exchange reactions are unfavorable. Therefore, this method typically requires very high temperatures and has a slow reaction rate. Prolonged high temperatures can cause denaturation of the solvent and reactants, resulting in tar formation. This severely impacts the reaction yield and increases the difficulty of post-processing, leading to high costs from both a reaction process and atom utilization perspectives.

[0004] The common fluorine sources in fluorination reactions are KF and HF. Fluorine gas is rarely used as a fluorine source because it is too violent and requires careful control under ultra-low temperature conditions to complete the reaction. This makes it difficult to industrialize. The diluent added during the reaction increases the difficulty of purification in the later stages, making the purity of the product unsatisfactory (CN 111116306A).

[0005] The preparation of hexafluorobenzene via halogen exchange, regardless of whether KF or fluorine gas is used as the fluorine source, requires relatively harsh reaction conditions, which is the main reason for low yields or low contents. Therefore, it is necessary to develop a hexafluorobenzene preparation method with milder reaction conditions and simpler operation. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for synthesizing hexafluorobenzene. Using pentafluorobenzene as a raw material, and through a pentafluoronitrobenzene catalyst and initiator, a dual reaction involving free radical substitution and electrophilic substitution occurs with fluorine gas, yielding high-purity hexafluorobenzene. This method features mild reaction conditions, is easily industrialized, has low raw material costs, and achieves high product yield and purity. Compared to other methods, it is also more energy-efficient and environmentally friendly.

[0007] Fluorine gas, due to its electronegativity, is unlikely to undergo electrophilic substitution and react directly with pentafluorobenzene. Experiments have shown that, at specific temperatures, fluorine gas is initiated by oxides, and the actual reaction involves pentafluoronitrobenzene continuously undergoing free radical substitution with fluorine to generate hexafluorobenzene and nitryl fluoride; nitryl fluoride possesses NO2... +The active particles directly react with the starting material pentafluorobenzene in an electrophilic substitution reaction to yield pentafluoronitrobenzene; the newly generated pentafluoronitrobenzene then continues the above reaction process to complete the catalytic cycle. The reaction principle is represented by the following equation:

[0008]

[0009] The reaction apparatus employs a trickle bed design, with a specific packing material filling the reaction tube. Fluorine gas is purged from bottom to top, while the reaction liquid drips from top to bottom. The reaction occurs at the surface of the packing material, and multi-stage liquid film exchange ensures a more complete reaction. Nitrogen gas, used as a dilution gas, not only mitigates the intense exothermic reaction of fluorine gas but also promptly purges lighter components from the system. This allows the high-purity reaction liquid to flow out of the reaction tube in a timely manner, avoiding heat accumulation effects such as coupling or chain termination. Finally, after purification by distillation, hexafluorobenzene is obtained.

[0010] To achieve the above technical solution, a method for synthesizing hexafluorobenzene includes the following steps:

[0011] 1) Mix pentafluorobenzene, catalyst and initiator to obtain a mixture;

[0012] 2) Heat the reaction apparatus and introduce the mixed gas into the apparatus from bottom to top; then pump the mixed liquid obtained in step 1) into the reaction apparatus from top to bottom;

[0013] 3) Collect all the crude product obtained in step 2) and distill it to obtain hexafluorobenzene.

[0014] Furthermore, in the above technical solution, the catalyst mentioned in the first step is pentafluoronitrobenzene; the feeding ratio is 1-2.5% of the mass of pentafluoronitrobenzene.

[0015] Furthermore, in the above technical solution, the initiator in the first step is one or more of benzoyl peroxide, benzoyl tert-butyl peroxide, and methyl ethyl ketone peroxide, preferably benzoyl peroxide; the feeding ratio is 0.5-1% of the mass of pentafluorobenzene.

[0016] Furthermore, in the above technical solution, the second step is specifically operated as follows: filling the reaction device with packing material and purging it with high-temperature nitrogen gas, heating the treated reaction device to the temperature required for the reaction, and introducing a specific proportion of mixed gas into the device from bottom to top at a specific flow rate; then pumping the mixed liquid obtained in step 1) into the reaction device from top to bottom at a specific flow rate through a plunger pump (the crude hexafluorobenzene product is separated from the gas-liquid separator at the bottom).

[0017] Furthermore, in the above technical solution, the second step reaction device is filled with Hastelloy triangular spiral packing, the temperature is raised and maintained at 105±1℃, and high-purity nitrogen is used for purging for 30 minutes.

[0018] Furthermore, in the above technical solution, the temperature required for the second reaction step is 50–65°C. During the reaction, ice-salt water is circulated according to the degree of heat release to control the temperature and stabilize it within the reaction temperature range.

[0019] Furthermore, in the above technical solution, the mixed gas in the second step is a mixture of fluorine gas and high-purity nitrogen gas; in the mixed gas, fluorine gas accounts for 20-35% of the volume. In the above technical solution, the flow rate of the mixed gas is 0.5-1.5 L / min; the flow rate of the mixed liquid is 2-5 mL / min.

[0020] Furthermore, in the above technical solution, the third step involves using a 30cm packed distillation column to perform atmospheric distillation on the crude product, while simultaneously collecting the 80-82℃ fraction to obtain hexafluorobenzene.

[0021] Beneficial effects of the invention

[0022] 1. By using fluorine gas as the fluorine source, and through a specific reaction, the harsh reaction conditions required for the original fluorination reaction are improved, allowing the synthesis of hexafluorobenzene to be completed at a relatively mild temperature that is easy to achieve industrially.

[0023] 2. Low raw material cost and high product yield, avoiding the use of complex phase transfer catalysts or ionic liquids, greatly improving environmental protection requirements.

[0024] 3. The absence of large amounts of solvent as a diluent reduces heat loss during later product purification, avoids the generation of fluorine-depleted impurities during post-processing, and maintains a high level of product purity. Detailed Implementation

[0025] Example 1

[0026] Raw material pretreatment

[0027] Benzoyl peroxide was pre-dehydrated with hexane and stored under nitrogen protection. Pentafluorobenzene and pentafluoronitrobenzene were dehydrated (the dehydration process was a conventional technique and will not be described in detail here). 25g of pentafluoronitrobenzene and 5g of benzoyl peroxide were added to 1kg of pentafluorobenzene, mixed thoroughly, and set aside for later use.

[0028] Pretreatment of experimental setup

[0029] A 600mm long, 14mm inner diameter 316L reaction tube was filled with Hastelloy triangular spiral packing. The tube was then sealed and placed in a heat exchanger. Heating was turned on, and the internal temperature of the reaction tube was maintained at 105±1℃. High-purity nitrogen gas was introduced into the lower end of the reaction tube and discharged from bottom to top for 30 minutes, allowing it to cool to room temperature.

[0030] trickle bed reaction

[0031] The center of the reaction tube was heated to 65°C. The lower gas path of the reaction tube was switched to a mixed gas path, and an F2 / N2 mixture was introduced at a mixing ratio of 35% (F2 = 0.5 L / min). The mixed raw materials were pumped into the upper part of the reaction tube using a plunger pump at a flow rate of 2 ml / min. The reaction liquid was separated by a gas-liquid separator at the lower end of the reaction tube and flowed into a receiving tank. The mixed gas at the upper end of the reaction tube was absorbed by an alkaline solution. As the reaction began to show a heating trend, ice-salt water was introduced into the coils inside the apparatus, controlling the flow rate to balance the exothermic reaction. After 6 hours, the raw material feeding was completed, the liquid in the reaction tube completely flowed out, and the reaction ended, yielding 1.13 kg of crude product.

[0032] Product post-processing

[0033] The crude product was subjected to atmospheric distillation using a 30cm packed column, while simultaneously collecting the fraction at 80-82℃, yielding 0.97kg of hexafluorobenzene with a purity of 99.2% and a yield of 86.9%.

[0034] Example 2

[0035] Raw material pretreatment

[0036] Methyl ethyl ketone peroxide was pre-dehydrated with hexane and stored under nitrogen protection. Pentafluorobenzene and pentafluoronitrobenzene were also dehydrated. 20g of pentafluoronitrobenzene and 10g of methyl ethyl ketone peroxide were added to 2kg of pentafluorobenzene, mixed thoroughly, and set aside for later use.

[0037] Pretreatment of experimental setup

[0038] A 600mm long, 14mm inner diameter 316L reaction tube was filled with Hastelloy triangular spiral packing. The tube was then sealed and placed in a heat exchanger. Heating was turned on, and the internal temperature of the reaction tube was maintained at 105±1℃. High-purity nitrogen gas was introduced into the lower end of the reaction tube and discharged from bottom to top for 30 minutes, allowing it to cool to room temperature.

[0039] trickle bed reaction

[0040] The center of the reaction tube was heated to 50°C. The lower gas path of the reaction tube was switched to a mixed gas path, and an F2 / N2 mixture was introduced at a mixing ratio of 20% (F2 = 1.5 L / min). The mixed raw materials were pumped into the upper part of the reaction tube using a plunger pump at a flow rate of 5 ml / min. The reaction liquid was separated by a gas-liquid separator at the lower end of the reaction tube and flowed into a receiving tank. The mixed gas at the upper end of the reaction tube was absorbed by an alkaline solution. As the reaction began to show a heating trend, ice-salt water was introduced into the coils inside the apparatus, controlling the flow rate to balance the exothermic reaction. After 4.5 hours, the raw material feeding was completed, the liquid in the reaction tube completely flowed out, and the reaction ended, yielding 2.11 kg of crude product.

[0041] Product post-processing

[0042] The crude product was subjected to atmospheric distillation using a 30cm packed column, while simultaneously collecting the fraction at 80-82℃, yielding 1.78 kg of hexafluorobenzene with a purity of 99.6% and a yield of 80.0%.

[0043] Example 3

[0044] Raw material pretreatment

[0045] Benzoyl peroxide was pre-dehydrated with hexane and stored under nitrogen protection. Pentafluorobenzene and pentafluoronitrobenzene were dehydrated. 15g of pentafluoronitrobenzene and 15g of benzoyl peroxide were added to 1.5kg of pentafluorobenzene, mixed thoroughly, and set aside for later use.

[0046] Pretreatment of experimental setup

[0047] A 600mm long, 14mm inner diameter 316L reaction tube was filled with Hastelloy triangular spiral packing. The tube was then sealed and placed in a heat exchanger. Heating was turned on, and the internal temperature of the reaction tube was maintained at 105±1℃. High-purity nitrogen gas was introduced into the lower end of the reaction tube and discharged from bottom to top for 30 minutes, allowing it to cool to room temperature.

[0048] trickle bed reaction

[0049] The center of the reaction tube was heated to 60°C. The lower gas path of the reaction tube was switched to a mixed gas path, and an F2 / N2 mixture was introduced at a mixing ratio of 30% (F2 = 1 L / min). The mixed raw materials were pumped into the upper part of the reaction tube using a plunger pump at a flow rate of 3.5 ml / min. The reaction liquid was separated by a gas-liquid separator at the lower end of the reaction tube and flowed into a receiving tank. The mixed gas at the upper end of the reaction tube was absorbed by an alkaline solution. As the reaction began to show a heating trend, ice-salt water was introduced into the coils inside the apparatus, controlling the flow rate to balance the exothermic reaction. After 5 hours, the raw material feeding was completed, the liquid in the reaction tube completely flowed out, and the reaction ended, yielding 1.65 kg of crude product.

[0050] Product post-processing

[0051] The crude product was subjected to atmospheric distillation using a 30cm packed column, while simultaneously collecting the fraction at 80-82℃, yielding 1.41 kg of hexafluorobenzene with a purity of 99.3% and a yield of 84.3%.

[0052] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for synthesizing hexafluorobenzene, characterized in that, Includes the following steps: 1) A mixture is prepared by mixing pentafluorobenzene, a catalyst and an initiator to obtain a mixture; the catalyst is pentafluoronitrobenzene; the initiator is one or more of benzoyl peroxide, benzoyl tert-butyl peroxide and methyl ethyl ketone peroxide. 2) Heat the reaction apparatus and introduce the mixed gas into the apparatus from bottom to top; The mixture obtained in step 1) is then pumped into the reaction apparatus from top to bottom; 3) Collect all the crude product obtained in step 2) and distill it to obtain hexafluorobenzene.

2. The method for synthesizing hexafluorobenzene according to claim 1, characterized in that: In step 1), the catalyst feed ratio is 1-2.5% of the mass of pentafluorobenzene.

3. The method for synthesizing hexafluorobenzene according to claim 1, characterized in that: In step 1), the initiator is added at a ratio of 0.5% to 1% of the mass of pentafluorobenzene.

4. The method for synthesizing hexafluorobenzene according to claim 1, characterized in that: Step 2) Specifically, the reaction device is filled with packing material and purged with high-temperature nitrogen gas. The treated reaction device is then heated to the required reaction temperature. A specific proportion of mixed gas is introduced into the device from bottom to top at a specific flow rate. The mixture obtained in Step 1) is then pumped into the reaction device from top to bottom at a specific flow rate through a plunger pump.

5. The method for synthesizing hexafluorobenzene according to claim 4, characterized in that: In step 2), the reaction apparatus is filled with Hastelloy triangular spiral packing, the temperature is raised and maintained at 105±1℃, and high-purity nitrogen is used to purge for 30 minutes.

6. The method for synthesizing hexafluorobenzene according to claim 1 or 4, characterized in that: In step 2), the reaction temperature is raised to 50-65°C; during the reaction, ice-salt water is passed through according to the degree of heat release to control the temperature and keep it stable within the reaction temperature range.

7. The method for synthesizing hexafluorobenzene according to claim 1 or 4, characterized in that: In step 2), the mixed gas is a mixture of fluorine gas and high-purity nitrogen gas; in the mixed gas, fluorine gas accounts for 20-35% of the volume.

8. The method for synthesizing hexafluorobenzene according to claim 1 or 4, characterized in that: In step 2), the flow rate of the mixed gas is 0.5 to 1.5 L / min; the flow rate of the mixed liquid is 2 to 5 mL / min.

9. The method for synthesizing hexafluorobenzene according to claim 1, characterized in that: In step 3), the crude product is distilled at atmospheric pressure using a 30cm packed distillation column, while the fraction at 80-82℃ is collected to obtain hexafluorobenzene.

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