Synthesis method of 3, 4-dichlorobenzonitrile

Abstract

The invention relates to a synthetic method of 3, 4-dichlorobenzonitrile, and belongs to the technical field of organic synthesis. According to the preparation method, tetramethyl silicate is creatively used as a silicon source, aluminum chloride hexahydrate is used as an aluminum source, yttrium chloride hexahydrate is used as a yttrium source / rare earth source, then based on a sol-gel method, the opportunity of adding the aluminum source and the yttrium source into a reaction solvent is controlled, and then the aluminum-yttrium doped silica sol is prepared, the preparation method comprises the following steps: adding tetramethyl silicate for the first time before dropwise adding the tetramethyl silicate into a reaction solvent, adding the tetramethyl silicate for the second time after completely dropwise adding the tetramethyl silicate into the reaction solvent, and then loading a vanadium-chromium catalyst by utilizing aluminum-yttrium doped silica sol on the basis of an impregnation method, so as to prepare the loaded catalyst. According to the present invention, the 3, 4-dichlorobenzonitrile can be well catalyzed and synthesized during the ammoxidation reaction of the 3, 4-dichlorotoluene, such that the conversion rate is high, the selectivity is good, and the excellent technical effect is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of organic synthesis, and particularly relates to a method for synthesizing 3,4-dichlorobenzonitrile. Background Art

[0002] As an important organic intermediate, 3,4-dichlorobenzonitrile is widely used in agriculture, medicine, pigments, new materials and other fields. Among them, there are many methods for the synthesis of 3,4-dichlorobenzonitrile, which can be roughly divided into four major pathways, namely, the reaction synthesis of 3,4-dichlorotrichloromethylbenzene and ammonium chloride, the dehydration synthesis of 3,4-dichlorobenzamide, the dehydration synthesis of 3,4-dichlorobenzaldehyde oxime and the ammoxidation synthesis of 3,4-dichlorotoluene. Compared with the first three pathways, the fourth pathway has more advantages in terms of both economy and environmental protection. It has gradually become a mainstream method in industrial production. The key to the ammoxidation method (ammonia oxidation reaction) often lies in the selection of catalysts. How to develop a catalyst with high catalytic activity and high selectivity is a perennial topic for technicians. Although numerous studies have shown that vanadium-chromium catalysts can effectively catalyze the one-step reaction of 3,4-dichlorotoluene to produce 3,4-dichlorobenzonitrile, existing vanadium-chromium catalysts still face the dilemma of low catalytic activity and selectivity. Even after being supported on a carrier such as silica, there is still much room for improvement. Summary of the Invention

[0003] In response to the problems existing in the prior art, the present invention aims to provide a method for synthesizing 3,4-dichlorobenzonitrile. The present invention creatively uses tetramethyl silicate as a silicon source, aluminum chloride hexahydrate as an aluminum source, and yttrium chloride hexahydrate as a yttrium source / rare earth source. Then, based on a sol-gel method, the timing of adding the aluminum source and the yttrium source to a reaction solvent is controlled to prepare an aluminum-yttrium-doped silica sol. The aluminum source and the yttrium source are added twice, the first addition being before the tetramethyl silicate is added dropwise to the reaction solvent, and the second addition being after all the tetramethyl silicate is added dropwise to the reaction solvent. Then, based on an impregnation method, the aluminum-yttrium-doped silica sol is used to load a vanadium-chromium catalyst to prepare a supported catalyst. When the supported catalyst is applied to the ammoxidation reaction of 3,4-dichlorotoluene, it can well catalyze the synthesis of 3,4-dichlorobenzonitrile, with not only a high conversion rate but also good selectivity, achieving excellent technical effects.

[0004] The purpose of the present invention can be achieved through the following technical solutions: A method for synthesizing 3,4-dichlorobenzonitrile, comprising the following steps: The reactor is loaded with a supported catalyst, and raw materials consisting of 3,4-dichlorotoluene, ammonia and air are added to react, and the reaction is performed after post-treatment to obtain the catalyst; the reaction temperature is 390-405°C; the reaction pressure is normal pressure; the reaction space velocity is 600-1000h-1 .

[0005] As a preferred technical solution of the present invention, the supported catalyst is prepared by the following steps: Step A: adding a hydrochloric acid aqueous solution to 100-110 parts by weight of an ethanol aqueous solution at room temperature until the pH is adjusted to 2.5-3, then adding 0.4-0.6 parts by weight of aluminum chloride hexahydrate and 0.2-0.3 parts by weight of yttrium chloride hexahydrate, stirring for 5-10 minutes, then adding 13-16 parts by weight of tetramethyl silicate dropwise while stirring, and continuing to stir for 15-30 minutes after all the addition is complete, finally adding 0.3-0.4 parts by weight of aluminum chloride hexahydrate and 0.08-0.1 parts by weight of yttrium chloride hexahydrate, stirring for 2-3 hours, and concentrating to obtain aluminum-yttrium-doped silica sol; Step B: Add 40-45 parts by weight of oxalic acid to 400 parts by weight of deionized water at room temperature, then heat to 80-90° C., keep warm, and stir for 10-15 minutes to mix. Then, add vanadium pentoxide and chromium trioxide, stir until no bubbles are generated, cool naturally to room temperature, add 100-110 parts by weight of aluminum yttrium-doped silica sol, concentrate, stand at room temperature, dry, calcine, and cool naturally to room temperature.

[0006] Furthermore, the mass fraction of the ethanol aqueous solution in step A is 30-35%.

[0007] Furthermore, the concentration of the hydrochloric acid aqueous solution in step A is 0.1-0.2 mol / L.

[0008] Furthermore, the rate of the dripping in step A is 3-4s / drop.

[0009] Furthermore, the concentration in step A refers to distillation under reduced pressure at 40-45° C. under a vacuum degree of 0.05 MPa until the solid content is 35-40 wt %.

[0010] Furthermore, the heating rate in step B is 3-6°C / min.

[0011] Furthermore, the mass ratio of the oxalic acid to the vanadium pentoxide in step B is 45:6-8.

[0012] Furthermore, the mass ratio of the oxalic acid to the chromium trioxide in step B is 45:7-8.

[0013] Furthermore, the concentration in step B refers to distillation under reduced pressure at 40-45° C. under a vacuum degree of 0.08 MPa until the volume becomes 30-40% of the original volume.

[0014] Furthermore, the standing time in step B is 8-10 hours.

[0015] Furthermore, the drying in step B refers to drying at 95-100° C. to constant weight.

[0016] Furthermore, the calcination in step B refers to heating the temperature to 520-570° C. at a heating rate of 5° C. / min and then keeping the temperature for 6-8 hours.

[0017] As a preferred technical solution of the present invention, the molar ratio of the 3,4-dichlorotoluene, the ammonia and the oxygen in the air is 1:3-4:4.

[0018] The beneficial effects of the present invention are: (1) The present invention creatively uses tetramethyl silicate as a silicon source, aluminum chloride hexahydrate as an aluminum source, and yttrium chloride hexahydrate as a yttrium source / rare earth source, and then controls the timing of adding the aluminum source and the yttrium source to the reaction solvent based on a sol-gel method, thereby preparing an aluminum-yttrium-doped silica sol, wherein the aluminum source and the yttrium source are added twice, the first addition is before the tetramethyl silicate is added to the reaction solvent, and the second addition is after the tetramethyl silicate is completely added to the reaction solvent. Then, based on an impregnation method, the aluminum-yttrium-doped silica sol is used to load a vanadium-chromium catalyst to prepare a loaded catalyst. When the loaded catalyst is applied to the ammoxidation reaction of 3,4-dichlorotoluene, it can well catalyze the synthesis of 3,4-dichlorobenzonitrile, with not only a high conversion rate but also good selectivity, and has achieved excellent technical effects.

[0019] (2) The catalytic activity and selectivity of the existing vanadium-chromium catalysts still need to be improved even after being loaded on a carrier (silica sol). For this reason, the present invention creatively improves the technology of silica sol and prepares an aluminum-yttrium-doped silica sol based on the sol-gel method. On the one hand, after the silica sol is doped with aluminum-yttrium, the subsequent supported catalyst can form more Lewis acid sites, causing changes in acid-base properties, which to a certain extent helps to improve the catalytic activity and selectivity. On the other hand, aluminum-yttrium doping can also regulate the specific surface area, pore structure, interfacial activity and other properties of silica sol (colloids), enhance its interaction with the vanadium-chromium catalyst, obtain a better loading effect, and further enhance the catalytic activity and selectivity. It is worth mentioning. It should be noted that the timing of adding the aluminum source and the yttrium source needs to be controlled during the preparation of the aluminum-yttrium-doped silica sol of the present invention. Adding them twice greatly promotes the selectivity of the subsequently prepared supported catalyst. The reason is that the reaction process of tetramethyl silicate (when preparing silica sol) involves hydrolysis and polycondensation. Adding them twice can allow the aluminum yttrium to be more evenly "fixed" in the skeleton of silica, changing the electron density distribution, thereby regulating the redox reaction behavior, enhancing the stabilization of the intermediates generated by the ammoxidation reaction of 3,4-dichlorotoluene, and promoting selectivity. In addition, compared with single-element doping, dual-element doping of aluminum and rare earth yttrium can form a good complementary advantage, exert an excellent synergistic effect, and obtain better technical effects.

[0020] (3) The present invention provides a method for synthesizing 3,4-dichlorobenzonitrile, which is a technical improvement around the catalyst (carrier) and can obtain a higher yield of 3,4-dichlorobenzonitrile, and is very suitable for application in industrial large-scale production. DETAILED DESCRIPTION

[0021] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0022] The reactors in the following examples and comparative examples are all φ38 mm fluidized bed reactors with a length of 1800 mm; the amount of supported catalyst used is 250 g; Post-treatment refers to first condensing in a condenser to obtain a condensate, then sending the condensate to a separator, standing for 20 minutes, separating the organic phase, and finally sending the organic phase to a distillation tower for distillation, wherein the condensation temperature is 75°C, the temperature of the separator is maintained at 80°C, the distillation conditions are that the vacuum degree is controlled at 0.095MPa, the tower top temperature is controlled at 118°C, the tower bottom temperature is controlled at 156°C, and the reflux ratio is controlled at 4.

[0023] Example 1 A method for synthesizing 3,4-dichlorobenzonitrile, comprising the following steps: The supported catalyst is loaded into the reactor, and raw materials consisting of 3,4-dichlorotoluene, ammonia and air are added to react, and the reaction is performed after post-treatment to obtain the catalyst; the reaction temperature is 390°C; the reaction pressure is normal pressure; the reaction space velocity is 600h -1 .

[0024] The supported catalyst is prepared by the following steps: Step A: adding a hydrochloric acid aqueous solution to 100 parts by weight of an ethanol aqueous solution at room temperature until the pH is adjusted to 2.5, then adding 0.4 parts by weight of aluminum chloride hexahydrate and 0.2 parts by weight of yttrium chloride hexahydrate, stirring for 5 minutes, then adding 13 parts by weight of tetramethyl silicate dropwise while stirring, stirring for 15 minutes after all the addition is complete, finally adding 0.3 parts by weight of aluminum chloride hexahydrate and 0.08 parts by weight of yttrium chloride hexahydrate, stirring for 2 hours, and concentrating to obtain aluminum yttrium-doped silica sol; Step B: Add 40 parts by weight of oxalic acid to 400 parts by weight of deionized water at room temperature, then heat to 80°C, keep warm, and stir for 10 minutes to mix. Then, add vanadium pentoxide and chromium trioxide, stir until no bubbles are generated, cool naturally to room temperature, add 100 parts by weight of aluminum yttrium-doped silica sol, concentrate, stand at room temperature, dry, calcine, and cool naturally to room temperature.

[0025] The mass fraction of the ethanol aqueous solution in step A is 30%.

[0026] The concentration of the hydrochloric acid aqueous solution in step A is 0.1 mol / L.

[0027] The rate of dropping in step A is 3s / drop.

[0028] The concentration in step A refers to distillation under reduced pressure at 40° C. under a vacuum degree of 0.05 MPa until the solid content is 35 wt %.

[0029] The heating rate in step B is 3°C / min.

[0030] The mass ratio of the oxalic acid to the vanadium pentoxide in step B is 45:6.

[0031] The mass ratio of the oxalic acid to the chromium trioxide in step B is 45:7.

[0032] The concentration in step B refers to distillation under reduced pressure at 40° C. under a vacuum degree of 0.08 MPa until the volume becomes 30% of the original volume.

[0033] The standing time in step B is 8 hours.

[0034] The drying in step B refers to drying at 95° C. to constant weight.

[0035] The calcination in step B refers to heating to 520°C at a heating rate of 5°C / min and then keeping the temperature for 6 hours.

[0036] The molar ratio of the 3,4-dichlorotoluene, the ammonia gas and the oxygen in the air is 1:3:4.

[0037] Example 2 A method for synthesizing 3,4-dichlorobenzonitrile, comprising the following steps: The supported catalyst is loaded into the reactor, and raw materials consisting of 3,4-dichlorotoluene, ammonia and air are added to react, and the reaction is performed after post-treatment to obtain the product; the reaction temperature is 405°C; the reaction pressure is normal pressure; the reaction space velocity is 1000h -1 .

[0038] The supported catalyst is prepared by the following steps: Step A: adding a hydrochloric acid aqueous solution to 110 parts by weight of an ethanol aqueous solution at room temperature until the pH is adjusted to 3, then adding 0.6 parts by weight of aluminum chloride hexahydrate and 0.3 parts by weight of yttrium chloride hexahydrate, stirring for 10 minutes, then adding 16 parts by weight of tetramethyl silicate dropwise while stirring, stirring for 30 minutes after all the addition is complete, finally adding 0.4 parts by weight of aluminum chloride hexahydrate and 0.1 parts by weight of yttrium chloride hexahydrate, stirring for 3 hours, and concentrating to obtain aluminum yttrium-doped silica sol; Step B: Add 45 parts by weight of oxalic acid to 400 parts by weight of deionized water at room temperature, then heat to 90°C, keep warm, and stir for 15 minutes to mix. Then, add vanadium pentoxide and chromium trioxide, stir until no bubbles are generated, cool naturally to room temperature, add 110 parts by weight of aluminum yttrium-doped silica sol, concentrate, stand at room temperature, dry, calcine, and cool naturally to room temperature.

[0039] The mass fraction of the ethanol aqueous solution in step A is 35%.

[0040] The concentration of the hydrochloric acid aqueous solution in step A is 0.2 mol / L.

[0041] The rate of dropping in step A is 4s / drop.

[0042] The concentration in step A refers to distillation under reduced pressure at 45° C. under a vacuum degree of 0.05 MPa until the solid content is 40 wt %.

[0043] The heating rate in step B is 6°C / min.

[0044] The mass ratio of the oxalic acid to the vanadium pentoxide in step B is 45:8.

[0045] The mass ratio of the oxalic acid to the chromium trioxide in step B is 45:8.

[0046] The concentration in step B refers to distillation under reduced pressure at 45° C. under a vacuum degree of 0.08 MPa until the volume becomes 40% of the original volume.

[0047] The standing time in step B is 10 hours.

[0048] The drying in step B refers to drying at 100° C. to constant weight.

[0049] The calcination in step B refers to heating to 570°C at a heating rate of 5°C / min and then keeping the temperature for 8 hours.

[0050] The molar ratio of the 3,4-dichlorotoluene, the ammonia gas and the oxygen in the air is 1:4:4.

[0051] Example 3 A method for synthesizing 3,4-dichlorobenzonitrile, comprising the following steps: The reactor is loaded with a supported catalyst, and raw materials consisting of 3,4-dichlorotoluene, ammonia and air are added to react, and the reaction is performed after post-treatment to obtain the product; the reaction temperature is 400°C; the reaction pressure is normal pressure; the reaction space velocity is 800h -1 .

[0052] The supported catalyst is prepared by the following steps: Step A: adding a hydrochloric acid aqueous solution to 105 parts by weight of an ethanol aqueous solution at room temperature until the pH is adjusted to 2.8, then adding 0.5 parts by weight of aluminum chloride hexahydrate and 0.25 parts by weight of yttrium chloride hexahydrate, stirring for 8 minutes, then adding 15 parts by weight of tetramethyl silicate dropwise while stirring, stirring for 20 minutes after all the addition is complete, finally adding 0.35 parts by weight of aluminum chloride hexahydrate and 0.09 parts by weight of yttrium chloride hexahydrate, stirring for 2.5 hours, and concentrating to obtain aluminum yttrium-doped silica sol; Step B: Add 44 parts by weight of oxalic acid to 400 parts by weight of deionized water at room temperature, then heat to 85°C, keep warm, and stir for 12 minutes to mix. Then, add vanadium pentoxide and chromium trioxide, stir until no bubbles are generated, cool naturally to room temperature, add 105 parts by weight of aluminum yttrium-doped silica sol, concentrate, stand at room temperature, dry, calcine, and cool naturally to room temperature.

[0053] The mass fraction of the ethanol aqueous solution in step A is 33%.

[0054] The concentration of the hydrochloric acid aqueous solution in step A is 0.15 mol / L.

[0055] The rate of dropping in step A is 3.5s / drop.

[0056] The concentration in step A refers to distillation under reduced pressure at 42° C. under a vacuum degree of 0.05 MPa until the solid content is 38 wt %.

[0057] The heating rate in step B is 5°C / min.

[0058] The mass ratio of the oxalic acid to the vanadium pentoxide in step B is 45:7.

[0059] The mass ratio of the oxalic acid to the chromium trioxide in step B is 45:7.3.

[0060] The concentration in step B refers to distillation under reduced pressure at 44° C. under a vacuum degree of 0.08 MPa until the volume becomes 36% of the original volume.

[0061] The standing time in step B is 9 hours.

[0062] The drying in step B refers to drying at 98° C. to constant weight.

[0063] The calcination in step B refers to heating to 550°C at a heating rate of 5°C / min and then keeping the temperature for 7 hours.

[0064] The molar ratio of the 3,4-dichlorotoluene, the ammonia gas and the oxygen in the air is 1:3.5:4.

[0065] Comparative Example 1 The difference between Comparative Example 1 and Example 3 is that in step A, aluminum chloride hexahydrate is replaced with an equal molar amount of yttrium chloride hexahydrate, and the rest remains unchanged.

[0066] Comparative Example 2 The difference between Comparative Example 2 and Example 3 is that the yttrium chloride hexahydrate in step A is replaced by an equimolar amount of aluminum chloride hexahydrate, and the rest remains unchanged.

[0067] Comparative Example 3 Comparative Example 3 differs from Example 3 in that: Step A is modified to add aqueous hydrochloric acid solution to 105 parts by weight of aqueous ethanol at room temperature until the pH is adjusted to 2.8, then add 0.85 parts by weight of aluminum chloride hexahydrate and 0.34 parts by weight of yttrium chloride hexahydrate, stir and mix for 8 minutes, then add 15 parts by weight of tetramethyl silicate dropwise while stirring, and continue stirring for 170 minutes after all the addition is complete, and concentrate to obtain aluminum yttrium-doped silica sol; the rest remains unchanged.

[0068] Comparative Example 4 Comparative Example 4 differs from Example 3 in that: Step A is modified to add aqueous hydrochloric acid solution to 105 parts by weight of aqueous ethanol at room temperature until the pH is adjusted to 2.8, then add 15 parts by weight of tetramethyl silicate dropwise with stirring, and continue stirring for 20 minutes after all the addition is complete. Finally, 0.85 parts by weight of aluminum chloride hexahydrate and 0.34 parts by weight of yttrium chloride hexahydrate are added, stirred for 2.5 hours, and concentrated to obtain aluminum yttrium-doped silica sol; the rest remains unchanged.

[0069] Comparative Example 5 Comparative Example 5 differs from Example 3 in that: Step A is modified to include adding aqueous hydrochloric acid solution to 105 parts by weight of aqueous ethanol solution at room temperature until the pH is adjusted to 2.8, then adding 15 parts by weight of tetramethyl silicate dropwise with stirring, and continuing to stir and mix for 170 minutes after all the addition is complete, and concentrating to obtain silica sol; Step B is modified to include adding 44 parts by weight of oxalic acid to 400 parts by weight of deionized water at room temperature, then heating to 85° C., keeping the temperature, stirring and mixing for 12 minutes, then adding vanadium pentoxide and chromium trioxide, stirring until no bubbles are generated, cooling naturally to room temperature, adding 105 parts by weight of silica sol, concentrating, standing at room temperature, drying, calcining, and cooling naturally to room temperature; all other procedures remain unchanged.

[0070] Evaluation of conversion rate and yield: The conversion rate of 3,4-dichlorotoluene and the yield of 3,4-dichlorobenzonitrile were calculated for Examples 1-3 and Comparative Examples 1-5 (GC-MASS qualitative analysis and gas chromatography quantitative analysis using an external standard method).

[0071] Conversion rate of 3,4-dichlorotoluene = (molar amount of 3,4-dichlorotoluene reacted / molar amount of 3,4-dichlorotoluene fed) × 100%, and the result is rounded to the nearest integer.

[0072] Yield of 3,4-dichlorobenzonitrile = (molar amount of generated 3,4-dichlorobenzonitrile / molar amount of obtained 3,4-dichlorobenzonitrile) × 100%, and the result is rounded off to the nearest integer.

[0073] Table 1. Conversion rate and yield evaluation results ; As can be seen from Table 1, the synthesis method of the present invention can obtain a higher conversion rate (3,4-dichlorotoluene) and yield (3,4-dichlorobenzonitrile), and has achieved excellent technical effects; the catalyst supports of Comparative Examples 1-2 were only doped with a single element, resulting in varying degrees of reduction in conversion rate and yield; Comparative Examples 3-4 changed the timing of adding the aluminum source and the yttrium source (all added at one time, not twice), resulting in a significant decrease in yield, however, the decrease in conversion rate was not large, which indirectly indicates that the selectivity has deteriorated; the catalyst support of Comparative Example 5 was not doped with any element, and the effect was not ideal.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for synthesizing 3,4-dichlorobenzonitrile, characterized in that: The synthesis method comprises the following steps: The reactor is loaded with a supported catalyst, and raw materials consisting of 3,4-dichlorotoluene, ammonia and air are added to react, and the reaction is performed after post-treatment to obtain the catalyst; the reaction temperature is 390-405°C; the reaction pressure is normal pressure; the reaction space velocity is 600-1000h -1 .

2. A method for synthesizing 3,4-dichlorobenzonitrile according to claim 1, characterized in that: The supported catalyst is prepared by the following steps: Step A: adding a hydrochloric acid aqueous solution to 100-110 parts by weight of an ethanol aqueous solution at room temperature until the pH is adjusted to 2.5-3, then adding 0.4-0.6 parts by weight of aluminum chloride hexahydrate and 0.2-0.3 parts by weight of yttrium chloride hexahydrate, stirring for 5-10 minutes, then adding 13-16 parts by weight of tetramethyl silicate dropwise while stirring, and continuing to stir for 15-30 minutes after all the addition is complete, finally adding 0.3-0.4 parts by weight of aluminum chloride hexahydrate and 0.08-0.1 parts by weight of yttrium chloride hexahydrate, stirring for 2-3 hours, and concentrating to obtain aluminum-yttrium-doped silica sol; Step B: Add 40-45 parts by weight of oxalic acid to 400 parts by weight of deionized water at room temperature, then heat to 80-90° C., keep warm, and stir for 10-15 minutes to mix. Then, add vanadium pentoxide and chromium trioxide, stir until no bubbles are generated, cool naturally to room temperature, add 100-110 parts by weight of aluminum yttrium-doped silica sol, concentrate, stand at room temperature, dry, calcine, and cool naturally to room temperature.

3. A method for synthesizing 3,4-dichlorobenzonitrile according to claim 2, characterized in that: The mass fraction of the ethanol aqueous solution in step A is 30-35%.

4. A method for synthesizing 3,4-dichlorobenzonitrile according to claim 2, characterized in that: The rate of dropping in step A is 3-4s / drop.

5. A method for synthesizing 3,4-dichlorobenzonitrile according to claim 2, characterized in that: The concentration in step A refers to distillation under reduced pressure at 40-45° C. under a vacuum degree of 0.05 MPa until the solid content is 35-40 wt %.

6. A method for synthesizing 3,4-dichlorobenzonitrile according to claim 2, characterized in that: The mass ratio of the oxalic acid to the vanadium pentoxide in step B is 45:6-8.

7. A method for synthesizing 3,4-dichlorobenzonitrile according to claim 2, characterized in that: The mass ratio of the oxalic acid to the chromium trioxide in step B is 45:7-8.

8. A method for synthesizing 3,4-dichlorobenzonitrile according to claim 2, characterized in that: The concentration in step B refers to distillation under reduced pressure at 40-45° C. under a vacuum degree of 0.08 MPa until the volume becomes 30-40% of the original volume.

9. A method for synthesizing 3,4-dichlorobenzonitrile according to claim 1, characterized in that: The molar ratio of the 3,4-dichlorotoluene, the ammonia and the oxygen in the air is 1:3-4:4.