Continuous synthesis process of 3, 4-difluorobenzonitrile

Abstract

The invention belongs to the technical field of production of fine chemicals, and discloses a continuous synthesis process of 3, 4-difluorobenzonitrile, which comprises the steps of dehydration, water-carrying agent removal, product removal, solid-liquid separation, product refining and the like. According to the process, continuous reaction rectification, water removal during reaction, water-carrying agent removal, product collection and continuous water, water-carrying agent and product removal are adopted, so that the continuity of the reaction and separation process is realized, the problems of long time, dense personnel, high labor intensity and large occupied area in the intermittent operation process are avoided, unorganized emission in the filling process is eliminated, and the production cost is reduced. The raw material and energy utilization rate is improved, and the process yield is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of compound production, and particularly to a continuous synthesis process of 3,4-difluorobenzonitrile. Background Art

[0002] 3,4-Difluorobenzonitrile is an intermediate for synthesizing various pesticides, pharmaceuticals, dyes, and fluorine-containing liquid crystal materials. Cyhalofop-butyl, a herbicide synthesized from it, is a selective herbicide for rice. This herbicide is highly efficient, low-toxic, and low-residual, and is quite safe for both transplanted and direct-seeded rice. It is a recognized excellent rice herbicide. How to synthesize 3,4-difluorobenzonitrile using inexpensive raw materials and the simplest process, and then synthesize highly efficient and inexpensive cyhalofop-butyl, is the key to its large-scale popularization and application.

[0003] The synthesis methods of 3,4-difluorobenzonitrile mainly include 7 routes such as 3,4-dichlorobenzonitrile route, 3,4-difluorotoluene route, 4-chlorobenzonitrile route, 3,4-difluorobenzoic acid route, 1-bromo-3,4-difluorobenzene route, 3,4-difluoronitrobenzene route, and 3,4-dinitrotoluene route according to different raw materials used. Among them, the direct fluorination of 3,4-dichlorobenzonitrile to synthesize 3,4-difluorobenzonitrile has the best economy. This reaction is a solid-liquid heterogeneous reaction, generally using potassium fluoride as the fluorination reagent, high-boiling aprotic polar solvents such as sulfolane, dimethyl sulfoxide, dimethylformamide, and N-methylpyrrolidone as solvents, and quaternary ammonium salts, quaternary phosphonium salts, polyethers, and crown ethers as phase transfer catalysts.

[0004] Chinese Patent Invention No. CN201210176136.9, "An Industrial Preparation Process of 3,4-Difluorobenzonitrile", uses N-bis(dimethylamino)methylene-1,3-dimethylimidazolinium chloride as a catalyst and 1,3-dimethyl-2-imidazolidinone as a solvent, and synthesizes 3,4-difluorobenzonitrile by reacting and rectifying simultaneously for 20 h, with a synthesis yield of 3,4-difluorobenzonitrile reaching 80%.

[0005] Chinese Patent Invention No. CN201310533100.6, "A Synthesis Process of 3,4-Difluorobenzonitrile", uses 3,4-dichlorobenzonitrile as a raw material, potassium fluoride as the fluorination reagent, 1,3-dimethyl-2-imidazolidinone as the reaction solvent, and bis-(N-bis(dimethylamino)methylene)-chloride imide salt as the catalyst. The reactants react at 130 - 150 °C for 2 - 3 h and at 180 - 200 °C for 5 - 6 h to end. After the reaction solution is filtered, it is rectified under reduced pressure to obtain 3,4-difluorobenzonitrile with a purity of over 99%, and the yield can reach 85%.

[0006] The Chinese invention patent "A Preparation Method of 3,4-Difluorobenzonitrile" with the application number CN201810292749.6 uses 3,4-dichlorobenzonitrile as the raw material, potassium fluoride as the fluorination reagent, bis-(N,N'-1,3-dimethyl-2-imidazolinyl)-ammonium chloride salt as the phase transfer catalyst, and then adds a reducing agent and a dispersant to react to prepare 3,4-difluorobenzonitrile. In this method, the fluorination reaction temperature is relatively low, the reaction time is 5-6 h, and the total molar yield can reach about 90%.

[0007] The Chinese invention patent "A Process for Producing 3,4-Difluorobenzonitrile" with the application number CN201911178344.0 uses 3,4-dichlorobenzonitrile as the raw material, potassium fluoride as the fluorination reagent, 1,3-dimethylimidazoline as the solvent, and N-bis(dimethylamino)-1,3-dimethylimidazoline with good high-temperature stability as the phase transfer catalyst, which reduces the reaction temperature required and increases the reaction rate; at the same time, by controlling the reaction depth (the content of 3,4-difluorobenzonitrile is 89.0-89.5%), the residence time of the system at high temperature is reduced, thereby effectively reducing polymerization, dehalogenation, and coking phenomena, and increasing the product yield to more than 92%, minimizing the environmental protection pressure.

[0008] However, the above-mentioned patented technologies and industrial production processes all adopt batch reaction and separation processes, which have problems such as long reaction time, large floor area, high labor intensity, low yield, high energy consumption in the production process, and serious pollution. Therefore, there is an urgent need for a continuous synthesis process of 3,4-difluorobenzonitrile to solve the above-mentioned technical problems. Summary of the Invention

[0009] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a continuous synthesis process of 3,4-difluorobenzonitrile. Using 3,4-dichlorobenzonitrile as the raw material, potassium fluoride as the fluorination reagent, an anhydrous aprotic polar solvent as the reaction solvent, a non-protic organic solvent that forms an azeotrope with water but is immiscible with water as the water-carrying agent, under the action of a phase transfer catalyst, through continuous dehydration, removal of the water-carrying agent, removal of the product, solid-liquid separation, and product purification, 3,4-difluorobenzonitrile with a purity of more than 99% is obtained, and the total reaction yield can reach up to 95% at most. This process realizes continuous reaction and continuous rectification separation, and the unreacted raw materials, reaction solvent, and catalyst are recycled, increasing the total reaction yield, reducing the labor intensity and production cost. The continuous process is easy to achieve automatic control, the process is safe and environmentally friendly, the amount of three wastes generated is small, and it is easy to industrialize.

[0010] To achieve the above purpose, the present invention is implemented according to the following technical scheme:

[0011] A continuous synthesis process of 3,4-difluorobenzonitrile, comprising the following steps:

[0012] S1 Dehydration: Continuously add an anhydrous aprotic polar solvent, a water-carrying agent, 3,4-dichlorobenzonitrile, potassium fluoride, and a phase transfer catalyst into the bottom of the dehydration tower; after the steam at the top of the dehydration tower is condensed and phase-separated, the aqueous phase is continuously withdrawn, and the oil phase of the water-carrying agent is refluxed; the anhydrous aprotic polar solvent is 1,3-dimethylimidazolinone; the water-carrying agent is toluene;

[0013] S2 Removal of the water-carrying agent: The material in the bottom of the dehydration tower flows by gravity into the bottom of the water-carrying agent removal tower. After the steam at the top of the water-carrying agent removal tower is condensed, a part of it is refluxed, and a part of the water-carrying agent is continuously withdrawn;

[0014] S3 Removal of the product: The material in the bottom of the water-carrying agent removal tower flows by gravity into the bottom of the product removal tower. After the steam at the top of the product removal tower is condensed, a part of it is refluxed, and a part of the crude 3,4-difluorobenzonitrile product is continuously withdrawn;

[0015] S4 Solid-liquid separation: The material in the bottom of the product removal tower is cooled and subjected to solid-liquid separation to obtain a filtrate and a filter cake;

[0016] S5 Product purification: The crude 3,4-difluorobenzonitrile product enters the middle of the light component removal tower. The light components are continuously withdrawn from the top of the light component removal tower. The material in the bottom of the light component removal tower is continuously pumped into the middle of the heavy component removal tower. The heavy components are continuously withdrawn from the bottom of the heavy component removal tower and returned to the dehydration tower. The finished 3,4-difluorobenzonitrile product is obtained at the top of the heavy component removal tower.

[0017] Specifically, the dehydration tower is a dehydration tower with a reflux and water separation distillation device. The specific process of step S1 is as follows: In the bottom of the dehydration tower with a reflux and water separation distillation device, continuously add the anhydrous aprotic polar solvent 1,3-dimethylimidazolinone, the water-carrying agent toluene, 3,4-dichlorobenzonitrile, potassium fluoride, and a phase transfer catalyst. After the steam at the top of the dehydration tower is condensed and phase-separated, the aqueous phase is continuously withdrawn, and the oil phase of the water-carrying agent is refluxed.

[0018] The specific process of step S2 is as follows: The material in the bottom of the dehydration tower flows by gravity into the bottom of the water-carrying agent removal tower 2. The toluene is continuously withdrawn after the steam at the top of the dehydration tower is condensed.

[0019] Specifically, in step S4, the filtrate is returned to the dehydration tower, and the filter cake is washed and dried with the water-carrying agent to obtain a by-product.

[0020] The specific process of step S5 is as follows: The crude 3,4-difluorobenzonitrile product enters the middle of the light component removal tower. The toluene light components are continuously withdrawn from the top of the light component removal tower. The material in the bottom of the light component removal tower is continuously pumped into the middle of the heavy component removal tower; the heavy components such as the aprotic polar solvent are continuously withdrawn from the bottom of the heavy component removal tower and returned to the dehydration tower. The qualified 3,4-difluorobenzonitrile product is obtained at the top of the heavy component removal tower.

[0021] Preferably, in step S1, 3,4-difluorobenzonitrile is dissolved in an anhydrous aprotic polar solvent, and potassium fluoride and a phase transfer catalyst are suspended in a water-carrying agent.

[0022] Preferably, in step S1, the phase transfer catalyst is at least one of N-bis(dimethylamino)-1,3-dimethylimidazoline, N-bis(dimethylamino)methylene-1,3-dimethylimidazolin-2-chloride ammonium salt, and bis-(N-bis(dimethylamino)methylene)-chloride imide salt.

[0023] Further, the phase transfer catalyst is preferably N-bis(dimethylamino)-1,3-dimethylimidazoline.

[0024] Preferably, in step S1, the molar ratio of 3,4-dichlorobenzonitrile to potassium fluoride is 1:1.2 to 2.5; the mass of the phase transfer catalyst is 3% to 10% of the mass of 3,4-dichlorobenzonitrile; the mass ratio of 3,4-dichlorobenzonitrile to the anhydrous aprotic polar solvent is 1:1 to 3; the mass ratio of 3,4-dichlorobenzonitrile to the water-carrying agent is 1:0.5 to 1.5.

[0025] Preferably, in step S1, the water content in the bottom of the dehydration tower is always less than 0.05%. Preferably, in step S2, several stripping towers for stripping the water-carrying agent are provided; the several stripping towers for stripping the water-carrying agent are arranged in series.

[0026] Further, 2 to 3 stripping towers for stripping the water-carrying agent are provided and are arranged in series. Specifically, 2 stripping towers for stripping the water-carrying agent are provided, including a first stripping tower for stripping the water-carrying agent and a second stripping tower for stripping the water-carrying agent; the first stripping tower for stripping the water-carrying agent and the second stripping tower for stripping the water-carrying agent are arranged in series; the material in the bottom of the dehydration tower flows by gravity into the bottom of the first stripping tower for stripping the water-carrying agent, and the material in the bottom of the first stripping tower for stripping the water-carrying agent flows by gravity into the bottom of the second stripping tower for stripping the water-carrying agent.

[0027] Preferably, in step S3, several product stripping towers are provided; the several product stripping towers are arranged in series.

[0028] Further, 2 to 3 product stripping towers are provided and are arranged in series. Specifically, 3 product stripping towers can be provided, including a first product stripping tower, a second product stripping tower, and a third product stripping tower; the first product stripping tower, the second product stripping tower, and the third product stripping tower are arranged in series; the material in the bottom of the stripping tower for stripping the water-carrying agent flows by gravity into the bottom of the first product stripping tower, the material in the bottom of the first product stripping tower flows by gravity into the bottom of the second product stripping tower, and the material in the bottom of the second product stripping tower flows by gravity into the bottom of the third product stripping tower.

[0029] Specifically, two product stripping towers can be provided.

[0030] Preferably, in step S1, the reaction space velocity of the material during the dehydration process is 1 to 3 h-1 。

[0031] Preferably, in the step S2, the reaction space velocity of the material in the process of removing the water-carrying agent is 0.3-1.0 h -1 。

[0032] Preferably, in the step S3, the reaction space velocity of the material in the process of removing the product is 0.15-0.3 h -1 。

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] 1. The process of the present invention adopts continuous reactive distillation, removing water, water-carrying agent and collecting products while reacting, continuously removing water, water-carrying agent and products, realizing the continuity of the reaction process, avoiding the long time, dense personnel, large labor intensity and large floor area in the intermittent operation process, eliminating the unorganized emission in the process of packing fillers, improving the utilization rate of raw materials and energy, and improving the process yield;

[0035] 2. The reaction crude product adopts continuous distillation, and through a continuous light removal tower and a continuous heavy removal tower, a qualified 3,4-difluorobenzonitrile product is obtained, avoiding the generation of excessive fractions in the intermittent distillation process, eliminating the problem of frequent tower blockage caused by the intermittent heating and cooling process, reducing the separation energy consumption, and improving the separation yield;

[0036] 3. Continuous dehydration can ensure that the water content in the bottom of the dehydration tower is always at a low level (less than 0.05%), ensuring the reaction activity of KF at the initial stage of the reaction, and avoiding the adverse effect of high water content on the reaction during the dehydration process at the initial stage of the intermittent reaction.

[0037] 4. The filtrate of the solid-liquid separation, the washing liquid of the filter cake and the bottom liquid of the heavy removal tower are returned to the dehydration tower, improving the utilization rate of raw materials and increasing the total process yield to 90%-95%.

[0038] 5. Except for the intermittent operation in part of the solid-liquid separation process, the whole process realizes continuous operation, which is easy for automatic control and industrial production. Specific Embodiments

[0039] The following further describes the present invention with specific embodiments. The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but not to limit the present invention.

[0040] For all raw materials of the present invention, there is no particular limitation on their sources, and they are purchased on the market or prepared by conventional methods well-known to those skilled in the art.

[0041] For the devices involved in the present invention, those without particular limitations are common devices in the art, and those skilled in the art know their operations and usage methods.

[0042] Example 1:

[0043] A continuous synthesis process of 3,4-difluorobenzonitrile, comprising the following steps:

[0044] 1) Continuously add anhydrous aprotic polar solvent 1,3-dimethylimidazolinone, water-carrying agent toluene, raw material 3,4-dichlorobenzonitrile, potassium fluoride and phase transfer catalyst N-bis(dimethylamino)-1,3-dimethylimidazole into the bottom of the dehydration tower equipped with a reflux water-separating distillation device. The molar ratio of 3,4-dichlorobenzonitrile to potassium fluoride is 1:1.2, and the mass of the phase transfer catalyst is 3% of the mass of 3,4-dichlorobenzonitrile; the mass ratio of 3,4-dichlorobenzonitrile to 1,3-dimethylimidazolinone is 1:1; the mass ratio of 3,4-dichlorobenzonitrile to toluene is 1:0.5;

[0045] The top temperature of the dehydration tower is 108 °C. The overhead steam of the dehydration tower is condensed to 60 °C and then phase-separated. The aqueous phase is continuously withdrawn, and toluene is refluxed; the bottom temperature of the dehydration tower is 143 °C, the water content in the bottom of the dehydration tower is 0.048%, and the reaction space velocity of the material in the bottom of the dehydration tower is 1 h -1 ;

[0046] 2) The material in the bottom of the dehydration tower flows by gravity into the bottom of the toluene removal tower (also called the water-carrying agent removal tower); the top temperature of the toluene removal tower is 112 °C. A part of the toluene steam at the top of the toluene removal tower is withdrawn after condensation, and a part is refluxed; the bottom temperature of the toluene removal tower is 195 °C, and the reaction space velocity of the material in the bottom of the toluene removal tower is 0.3 h -1 ;

[0047] 3) The material in the bottom of the toluene removal tower flows by gravity into the bottom of the product removal tower. The top temperature of the product removal tower is 170 °C, and the bottom temperature of the product removal tower is 216 °C. Approximately 86% of the crude product 3,4-difluorobenzonitrile is continuously withdrawn from the top of the product removal tower, and the reaction space velocity of the material in the bottom of the product removal tower is 0.15 h -1 ;

[0048] 4) The material in the bottom of the product removal tower is cooled to 90 °C by a cooler, and then solid-liquid separated by a centrifuge. The filtrate is returned to the bottom of the dehydration tower, and the filter cake is washed and dried with toluene as a by-product. The toluene washing liquid is returned to the bottom of the dehydration tower;

[0049] 5) Approximately 86% of the crude product 3,4-difluorobenzonitrile withdrawn from the top of the column in step 3) enters the middle of the light-end column. The top temperature of the light-end column is 111°C. Toluene light components are continuously withdrawn from the top of the light-end column. The bottom temperature of the light-end column is 174°C. The bottom material of the light-end column is continuously fed into the middle of the heavy-end column by a pump. The bottom temperature of the heavy-end column is 220°C. Heavy components such as 1,3-dimethylimidazolinone are continuously withdrawn from the bottom of the heavy-end column and returned to the bottom of the dehydration column. The top temperature of the heavy-end column is 178°C. A 99.6% 3,4-difluorobenzonitrile product is obtained from the top of the heavy-end column, and the yield is 90%.

[0050] Example 2:

[0051] A continuous synthesis process of 3,4-difluorobenzonitrile, comprising the following steps:

[0052] 1) Continuously add anhydrous aprotic polar solvent 1,3-dimethylimidazolinone, water-carrying agent toluene, raw material 3,4-dichlorobenzonitrile, potassium fluoride and phase transfer catalyst N-bis(dimethylamino)-1,3-dimethylimidazolinium into the bottom of the dehydration column equipped with a reflux and water separation distillation device. The molar ratio of 3,4-dichlorobenzonitrile to potassium fluoride is 1:2.0. The mass of the phase transfer catalyst is 6% of the mass of 3,4-dichlorobenzonitrile. The mass ratio of 3,4-dichlorobenzonitrile to 1,3-dimethylimidazolinone is 1:2. The mass ratio of 3,4-dichlorobenzonitrile to toluene is 1:1.0;

[0053] The top temperature of the dehydration column is 109°C. The top steam of the dehydration column is condensed to 60°C and then phase-separated. The aqueous phase is continuously withdrawn, and toluene is refluxed. The bottom temperature of the dehydration column is 140°C. The water content in the bottom of the dehydration column is 0.031%. The reaction space velocity of the material in the bottom of the dehydration column is 2.2 h -1 ;

[0054] 2) The material in the bottom of the dehydration column flows by gravity into the bottom of the toluene removal column (which can also be called the water-carrying agent removal column). The toluene removal column is set to operate in series with two toluene removal columns, including the first toluene removal column and the second toluene removal column. The top temperature of the first toluene removal column is 111°C, and the bottom temperature of the first toluene removal column is 187°C. The material in the bottom of the first toluene removal column flows by gravity into the bottom of the second toluene removal column. The top temperature of the second toluene removal column is 112°C, and the bottom temperature of the second toluene removal column is 202°C. Part of the toluene steam at the top of the first toluene removal column and the second toluene removal column is withdrawn after condensation, and part is refluxed. The total reaction space velocity of the material in the bottoms of the first toluene removal column and the second toluene removal column is 0.6 h -1 ;

[0055] 3) The material in the kettle of the toluene removal tower flows by gravity into the kettle of the product removal tower, and the product removal tower is configured as two product removal towers operated in series; the two product removal towers include a first product removal tower and a second product removal tower, the top temperature of the first product removal tower is 167° C., and the kettle temperature of the first product removal tower is 214° C.; the material in the kettle of the first product removal tower flows by gravity into the kettle of the second product removal tower, the top temperature of the second product removal tower is 169° C., and the kettle temperature of the second product removal tower is 222° C. About 85% of the crude product 3,4-difluorobenzonitrile is continuously extracted from the top of the first product removal tower and the second product removal tower, and the reaction space velocity of the material in the kettles of the first product removal tower and the second product removal tower is 0.21h -1 ;

[0056] 4) The material in the kettle of the dehydration tower is cooled to 90°C by a cooler, and the solid-liquid separation is performed by a centrifuge. The filtrate is returned to the kettle of the dehydration tower. The filter cake is washed and dried with toluene as a by-product, and the toluene washing liquid is returned to the kettle of the dehydration tower;

[0057] 5) About 85% of the crude product 3,4-difluorobenzonitrile extracted from the top of the tower in step 3) enters the middle part of the light-removing tower, the top temperature of the light-removing tower is 112° C., the light component of toluene is continuously extracted from the top of the light-removing tower, the bottom temperature of the light-removing tower is 175° C., the bottom material of the light-removing tower is continuously pumped into the middle part of the heavy-removing tower, the bottom temperature of the heavy-removing tower is 220° C., the bottom of the heavy-removing tower continuously extracts heavy components such as 1,3-dimethylimidazolidinone and returns to the bottom of the dehydration tower, the top temperature of the heavy-removing tower is 178° C., and 99.5% of the 3,4-difluorobenzonitrile product is obtained from the top of the heavy-removing tower, with a yield of 93%.

[0058] Embodiment 3:

[0059] A continuous synthesis process of 3,4-difluorobenzonitrile comprises the following steps:

[0060] 1) adding anhydrous aprotic polar solvent 1,3-dimethylimidazolidinone, water-carrying agent toluene, raw materials 3,4-dichlorobenzonitrile and potassium fluoride and phase transfer catalyst N-bis(dimethylamino)-1,3-dimethylimidazoline into the kettle of a dehydration tower with a reflux water separation distillation device at one time, wherein the molar ratio of 3,4-dichlorobenzonitrile to potassium fluoride is 1:2.5, the mass of the phase transfer catalyst is 10% of the mass of 3,4-dichlorobenzonitrile; the mass ratio of 3,4-dichlorobenzonitrile to 1,3-dimethylimidazolidinone is 1:3; and the mass ratio of 3,4-dichlorobenzonitrile to toluene is 1:1.5;

[0061] The top temperature of the dehydration tower is 110°C. The top steam of the dehydration tower is condensed to 60°C and then separated into phases. The water phase is continuously extracted and toluene is refluxed. The bottom temperature of the dehydration tower is 138°C. The water content in the bottom is 0.016%. The reaction space velocity of the material in the bottom of the dehydration tower is 3h -1 ;

[0062] 2) The material in the bottom of the dehydration tower flows by gravity into the bottom of the toluene removal tower (which can also be called the water-carrying agent removal tower); the top temperature of the toluene removal tower is 112 °C, and a part of the toluene vapor at the top of the toluene removal tower is taken out after condensation, and a part is refluxed; the bottom temperature of the toluene removal tower is 204 °C, and the reaction space velocity of the material in the bottom of the toluene removal tower is 1.0 h -1 ;

[0063] 3) The material in the bottom of the toluene removal tower flows by gravity into the bottom of the product removal tower. The top temperature of the product removal tower is 167 °C, and the bottom temperature of the product removal tower is 222 °C. About 83% of the crude product 3,4-difluorobenzonitrile is continuously taken out from the top of the product removal tower. The reaction space velocity of the material in the bottom of the product removal tower is 0.3 h -1 ;

[0064] 4) The material in the bottom of the product removal tower is cooled to 90 °C by a cooler, and then undergoes solid-liquid separation by a centrifuge. The filtrate is returned to the bottom of the dehydration tower, and the filter cake is washed and dried with toluene and used as a by-product. The toluene washing liquid is returned to the bottom of the dehydration tower;

[0065] 5) About 83% of the crude product 3,4-difluorobenzonitrile taken out from the top in step 3) enters the middle of the light component removal tower. The top temperature of the light component removal tower is 112 °C, and the toluene light components are continuously taken out from the top of the light component removal tower. The bottom temperature of the light component removal tower is 176 °C. The material in the bottom of the light component removal tower is continuously sent to the middle of the heavy component removal tower by a pump. The bottom temperature of the heavy component removal tower is 218 °C, and the heavy components such as 1,3-dimethylimidazolinone are continuously taken out from the bottom of the heavy component removal tower and returned to the bottom of the dehydration tower. The top temperature of the heavy component removal tower is 178 °C, and 99.2% of the 3,4-difluorobenzonitrile product is obtained at the top of the heavy component removal tower, and the yield is 95%.

[0066] Comparative Example 1:

[0067] The batch synthesis process of 3,4-difluorobenzonitrile includes the following steps:

[0068] 1) Add anhydrous aprotic polar solvent 1,3-dimethylimidazolinone, water-carrying agent toluene, and 3,4-dichlorobenzonitrile into the reaction kettle with a reflux and water separation distillation device at one time. The mass ratio of 3,4-dichlorobenzonitrile to 1,3-dimethylimidazolinone is 1:3; the mass ratio of 3,4-dichlorobenzonitrile to toluene is 1:1.5. After adding, heat up to 90-120 °C for reflux and separate water for 1-2 h until the water content in the kettle is <0.05%;

[0069] 2) Continue to add potassium fluoride and the phase transfer catalyst N-bis(dimethylamino)-1,3-dimethylimidazoline into the reaction kettle. The molar ratio of 3,4-dichlorobenzonitrile to potassium fluoride is 1:2.5, and the mass of the phase transfer catalyst is 10% of the mass of 3,4-dichlorobenzonitrile. Heat up to 130 - 150 °C and carry out a reflux water separation reaction for 2 - 3 h;

[0070] 3) Continue to heat up. When the temperature of the reaction kettle rises to 160 - 190 °C and the temperature at the top of the reaction kettle is between 110 - 113 °C, toluene is removed. The toluene removal reaction is carried out for 3 - 5 h;

[0071] 4) Continue to heat up. When the temperature inside the reaction kettle rises to 190 - 222 °C and the temperature at the top of the tower reaches 170 - 180 °C, crude 3,4-difluorobenzonitrile is collected. The average content is 80 - 90%, and the reaction time is 6 - 10 h;

[0072] 5) Turn off the hot oil pump and turn on the cold oil system. When the temperature inside the reaction kettle drops below 90 °C, the materials inside the reaction kettle are put into a centrifuge. After solid-liquid separation by the centrifuge, the filtrate is pumped back into the reaction kettle for the next feeding. The filter cake is washed with toluene and dried to be used as a by-product, and the toluene washing liquid is pumped back into the reaction kettle for the next feeding;

[0073] 6) The crude 3,4-difluorobenzonitrile is pumped into a distillation kettle. Start stirring and collect the fraction at 179 - 181 °C under normal pressure. After distillation, the residual liquid at the bottom of the kettle is sent back to the reaction kettle for the next feeding. 99.3% of 3,4-difluorobenzonitrile product is obtained at the top of the tower, and the yield is 85%.

[0074] It can be seen from the above examples and comparative examples that there are the following differences between the process of Comparative Example 1 and the process of the present invention:

[0075] In step 1) of Comparative Example 1, "heat up to 90 - 120 °C for reflux", the temperature during this reflux process gradually rises from 90 °C to 120 °C, and this process is an intermittent non-steady state process.

[0076] In step 1) of Comparative Example 1, "water separation is carried out for 1 - 2 h until the water content in the kettle < 0.05%". In the intermittent operation of Comparative Example 1, the water content gradually decreases from a high water content to less than 0.05%. The dehydration process of the present invention is a continuous operation, and the water content at the bottom of the dehydration tower always remains below 0.05%.

[0077] In step 2) of Comparative Example 1, "reflux water separation reaction is carried out for 2 - 3 h". In the process of continuous dehydration of the present invention, the first two steps of water separation are combined into one step, and the total residence time is less than 33.3 min.

[0078] In step 3) of Comparative Example 1, "toluene removal reaction is carried out for 3 - 5 h", and the continuous toluene removal space velocity in Example 3 is 1.0 h -1 , corresponding to a residence time of 1.0 h.

[0079] In step 4) of Comparative Example 1, "reaction time is 6 - 10 h", and the continuous product discharge space velocity of Example 3 is 0.3 h -1 , corresponding to a residence time of 3.3 h.

[0080] In step 5) of Comparative Example 1, "close the hot oil pump and start the cold oil system", while the process of the present invention is a continuous operation and does not require frequent closing and starting of the hot oil pump, thus enabling stable operation.

[0081] The technical solution of the present invention is not limited to the limitations of the above specific embodiments. Any technical deformation made according to the technical solution of the present invention falls within the protection scope of the present invention.

Claims

1. A continuous synthesis process of 3,4-difluorobenzonitrile, characterized in that: It includes the following steps: S1 Dehydration: Continuously add an anhydrous aprotic polar solvent, a water-carrying agent, 3,4-dichlorobenzonitrile, potassium fluoride, and a phase transfer catalyst into the bottom of a dehydration tower; after the steam at the top of the dehydration tower is condensed and phase-separated, the aqueous phase is continuously withdrawn, and the water-carrying agent oil phase is refluxed; the anhydrous aprotic polar solvent is 1,3-dimethylimidazolidinone, and the water-carrying agent is toluene; S2 Removal of the water-carrying agent: The material in the bottom of the dehydration tower flows by gravity into the bottom of a water-carrying agent removal tower. After the steam at the top of the water-carrying agent removal tower is condensed, a part of it is refluxed, and a part of the water-carrying agent is continuously withdrawn; S3 Removal of the product: The material in the bottom of the water-carrying agent removal tower flows by gravity into the bottom of a product removal tower. After the steam at the top of the product removal tower is condensed, a part of it is refluxed, and a part of the crude 3,4-difluorobenzonitrile product is continuously withdrawn; S4 Solid-liquid separation: The material in the bottom of the product removal tower is cooled and subjected to solid-liquid separation to obtain a filtrate and a filter cake; S5 Product refinement: The crude 3,4-difluorobenzonitrile product enters the middle of a light component removal tower. The light components are continuously withdrawn from the top of the light component removal tower. The material in the bottom of the light component removal tower is continuously pumped into the middle of a heavy component removal tower. The heavy components are continuously withdrawn from the bottom of the heavy component removal tower and returned to the dehydration tower. The 3,4-difluorobenzonitrile finished product is obtained at the top of the heavy component removal tower.

2. The continuous synthesis process of 3,4-difluorobenzonitrile according to claim 1, characterized in that: In the step S1, 3,4-difluorobenzonitrile is dissolved in the anhydrous aprotic polar solvent, and potassium fluoride and the phase transfer catalyst are suspended in the water-carrying agent.

3. The continuous synthesis process of 3,4-difluorobenzonitrile according to claim 1, characterized in that: In the step S1, the phase transfer catalyst is at least one of N-bis(dimethylamino)-1,3-dimethylimidazoline, N-bis(dimethylamino)methylene-1,3-dimethylimidazoline-2-chloride ammonium salt, and bis-(N-bis(dimethylamino)methylene)-chloride imide salt.

4. The continuous synthesis process of 3,4-difluorobenzonitrile according to claim 1, characterized in that: In the step S1, the molar ratio of 3,4-dichlorobenzonitrile to potassium fluoride is 1:1.2 - 2.5; the mass of the phase transfer catalyst is 3% - 10% of the mass of 3,4-dichlorobenzonitrile; the mass ratio of 3,4-dichlorobenzonitrile to the anhydrous aprotic polar solvent is 1:1 - 3; the mass ratio of 3,4-dichlorobenzonitrile to the water-carrying agent is 1:0.5 - 1.

5.

5. The continuous synthesis process of 3,4-difluorobenzonitrile according to claim 1, characterized in that: In the step S1, the water content in the bottom of the dehydration tower is always less than 0.05%.

6. The continuous synthesis process of 3,4-difluorobenzonitrile according to claim 1, characterized in that: In the step S2, several water-carrying agent removal towers are provided; several water-carrying agent removal towers are arranged in series.

7. The continuous synthesis process of 3,4-difluorobenzonitrile according to claim 6, characterized in that: In the step S2, two water-carrying agent removal towers are provided.

8. The continuous synthesis process of 3,4-difluorobenzonitrile according to claim 1, characterized in that: In the step S3, several product removal towers are provided; several product removal towers are arranged in series.

9. The continuous synthesis process of 3,4-difluorobenzonitrile according to claim 8, characterized in that: In the step S3, two product removal towers are provided.

10. A continuous synthesis process of 3,4-difluorobenzonitrile according to claim 1, characterized in that: In the step S1, the reaction space velocity of the material during the dehydration process is 1 to 3 h -1 ; in the step S2, the reaction space velocity of the material during the removal of the water-carrying agent process is 0.3 to 1.0 h -1 ; in the step S3, the reaction space velocity of the material during the removal of the product process is 0.15 to 0.3 h -1 .