A method for continuously producing 4,4'-dichloromethylbiphenyl

Abstract

The application discloses a method for continuously preparing 4,4'-dichloromethyl biphenyl, which comprises the following steps: pumping a paraformaldehyde, hydrochloric acid and a catalyst solution into a first-stage tubular reactor to perform a formaldehyde depolymerization reaction, and then pumping the depolymerization reaction solution into a second-stage tubular reactor to perform a chloromethylation reaction in the presence of biphenyl and a chlorinating reagent, and finally discharging the reaction solution from the tubular reactor to perform cooling crystallization and filtration to obtain a 4,4'-dichloromethyl biphenyl crude product, and recycling and reusing the catalyst in the system. The method has the advantages of high reaction efficiency, simple system, low cost, high yield and easy industrial production.

Description

Technical Field

[0001] This invention relates to the field of organic synthesis technology, and specifically to a method for the continuous preparation of 4,4'-dichloromethylbiphenyl. Background Technology

[0002] 4,4'-Dichloromethylbiphenyl is a key intermediate in the optical brightener 4,4'-bis(2-sulfonylstyryl)-1,1-biphenyl (CBS-351). CBS-351 is known as a super optical brightener due to its safe and non-toxic properties, strong whitening ability, rapid whitening speed, high efficiency, good stability, excellent solubility and dispersibility, and good cumulative effect. Currently, the quality and quantity of CBS-351 cannot meet market demand, mainly because the purity and yield of the key intermediate 4,4'-dichloromethylbiphenyl are low, thus limiting the purity of the finished CBS-351 product. Therefore, the preparation of high-purity 4,4'-dichloromethylbiphenyl is of great significance.

[0003] Existing synthetic processes for 4,4'-dichloromethylbiphenyl include: 1) Traditional zinc chloride catalytic batch method: Biphenyl reacts with paraformaldehyde under anhydrous ZnCl2 catalysis with HCl. The product selectivity is low (about 70%) and contains a large amount of monochloromethyl biphenyl, isomers and multi-substituted byproducts. 2) Organic acid / molecular sieve composite catalysis: using monochloroacetic acid and 4A molecular sieve as catalysts, ultrasonic assistance shortens the reaction time, but the energy consumption for molecular sieve regeneration is high; 3) Concentrated hydrochloric acid-formic acid solvent method: The reaction is carried out at high temperature, which causes severe equipment corrosion and easily generates chlorobiphenyl impurities that are directly substituted with benzene rings; The preparation methods of 4,4'-dichloromethylbiphenyl generally suffer from problems such as long reaction time, low yield, and difficult post-processing. How to find a simple, short-time, and high-yield preparation method of 4,4'-dichloromethylbiphenyl has always been a hot research topic in the industry. The inventors of this application intend to use a continuous dynamic tubular reactor to solve the problems of multiphase state and low efficiency in the reaction process. Summary of the Invention

[0004] In view of the above-mentioned technical problems existing in the prior art, the purpose of this application is to provide a method for continuous preparation of 4,4'-dichloromethylbiphenyl, (1) achieving continuous preparation, reducing the amount of chlorination reagent, and reducing parallel / series byproducts by combining kinetic simulation calculations, thereby improving selectivity. (2) enhancing mass transfer, solving problems such as low HCl utilization and liquid-liquid-solid-gas heterogeneity, reducing material consumption, improving yield and efficiency, and improving intrinsic safety. (3) realizing the recycling of the catalyst system, thereby significantly reducing process costs.

[0005] The technical solution adopted in this invention is as follows: A method for the continuous preparation of 4,4'-dichloromethylbiphenyl includes the following steps: S1: The catalyst is dispersed in concentrated hydrochloric acid to form a catalyst solution; the catalyst solution, paraformaldehyde solution, and biphenyl solution are respectively pumped into a primary tubular reactor to carry out the formaldehyde depolymerization reaction; S2: In step S1, the depolymerization reaction liquid flows out of the primary tubular reactor. The depolymerization reaction liquid, biphenyl solution, and chlorination reagent are respectively introduced into the secondary tubular reactor for chloromethylation reaction. After the reaction is completed, the heterogeneous reaction liquid that flows out is post-processed to obtain the product 4,4'-dichloromethylbiphenyl. The catalyst is reused after post-processing.

[0006] Furthermore, the catalyst comprises at least one of a supported heteropolyacid catalyst, a transition metal compound, polyphosphoric acid, and a Lewis acid, wherein the supported heteropolyacid catalyst is silica-supported phosphotungstic acid, the transition metal compound is lanthanum(III) trifluoromethanesulfonate, and the Lewis acid is at least one of zinc chloride and aluminum trichloride.

[0007] Furthermore, when the catalyst is at least one of a transition metal compound, polyphosphoric acid, and Lewis acid, the molar ratio of the catalyst to biphenyl is 0.10-0.30:1; when the catalyst is silica-supported phosphotungstic acid, the molar ratio of phosphotungstic acid to biphenyl in the catalyst is 0.10-0.30:1.

[0008] Furthermore, the paraformaldehyde solution and the biphenyl solution use the same solvent, which is selected from one or more of C5-C8 aliphatic ethers and C5-C8 aliphatic hydrocarbons, preferably cyclohexane, n-hexane or petroleum ether.

[0009] Furthermore, the molar ratio of concentrated hydrochloric acid to biphenyl is 0.2-0.4:1; the molar ratio of formaldehyde monomer to biphenyl in paraformaldehyde is 2.3-3.0:1.

[0010] Further, the chlorination reagent includes one or more of phosphorus oxychloride, phosphorus trichloride, thionyl chloride, and hydrogen chloride; the molar ratio of Cl element in the chlorination reagent in step S2 to biphenyl in step S1 is 3.0~4.5:1.

[0011] Furthermore, the temperature of the formaldehyde depolymerization reaction in step S1 is 20~50℃, preferably 25~30℃, the reaction residence time is 1-10min, preferably 4-6min, and the reaction pressure is atmospheric pressure to 1MPa, preferably 0.5~1MPa.

[0012] Furthermore, the temperature of the chloromethylation reaction in step S2 is 15~50℃, preferably 20~40℃, the reaction residence time is 0.2~0.8h, preferably 0.5-0.7h, and the reaction pressure is atmospheric pressure to 1MPa, preferably 0.5~1MPa.

[0013] Furthermore, when the catalyst is a soluble catalyst, the post-processing method in step S2 is as follows: add a small amount of water to the reaction solution to obtain a solid-liquid-liquid three-phase mixture, filter, collect the filter cake, recrystallize to obtain the target product; collect the filtrate, let it stand to separate into layers, remove water from the aqueous layer by vacuum distillation to obtain the catalyst for reuse, and recover the solvent from the organic layer by distillation.

[0014] Furthermore, when the catalyst is an insoluble catalyst, the post-treatment method in step S2 is as follows: filter, collect the filter cake, add dichloroethane and stir to dissolve the target product, then filter to separate the catalyst, then distill the product solution to remove dichloroethane, recrystallize, and obtain the target product.

[0015] Furthermore, the solvent used for recrystallization is toluene.

[0016] Compared with the prior art, the present invention has the following advantages: This invention utilizes a continuous flow reactor for continuous preparation. The reactor, through a micro-pressure method, greatly reduces the leakage of HCl gas, the chlorination reagent, significantly improving reaction efficiency and reducing material consumption.

[0017] By enhancing mass transfer, problems such as heterogeneous liquid-liquid-solid-gas processes are solved, enabling continuous preparation. Combined with kinetic simulation calculations, parallel / series byproducts are reduced, selectivity and efficiency are improved, and intrinsic safety is enhanced.

[0018] The process method of this invention controls parameters such as feed temperature, reaction temperature and pressure in stages to further improve reaction yield and product purity, while realizing the recycling of catalyst, which is conducive to industrial production. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the apparatus for the continuous preparation of 4,4'-dichloromethylbiphenyl according to the present invention. Detailed Implementation

[0020] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0021] A schematic diagram of the apparatus for the continuous preparation of 4,4'-dichloromethylbiphenyl according to the present invention is shown below. Figure 1 The device includes a tubular reactor, which is divided into two sections: a depolymerization reaction section at the front and a chloromethylation reaction section at the rear. Heat exchange jackets are installed on the outer sides of both the depolymerization and chloromethylation reaction sections to control the temperature of each reaction section separately.

[0022] The concentrated hydrochloric acid used in the embodiments of this invention is a 30% (w / w) aqueous solution of HCl.

[0023] Example 1: 0.20 mol of biphenyl was dissolved in cyclohexane to prepare a biphenyl solution (biphenyl concentration 2.30 mol / L, the concentration in subsequent examples is consistent with this); 0.54 mol of formaldehyde monomer from paraformaldehyde was dissolved in cyclohexane to prepare a formaldehyde solution (the concentration of formaldehyde monomer in the solution is 3.47 mol / L, the concentration in subsequent examples is consistent with this); 0.06 mol of concentrated hydrochloric acid and 0.05 mol of ZnCl2 were dissolved to prepare an aqueous catalyst solution. The formaldehyde solution and catalyst solution were separately introduced into the depolymerization reaction section through metering pumps, with a reaction back pressure of 0.5 MPa, a temperature of 30 °C, and a residence time of 5 min to complete the formaldehyde depolymerization. The depolymerized solution entered the chloromethylation reaction section, and then the prepared biphenyl solution and 0.23 mol of chlorination reagent PCl3 were separately introduced into the chloromethylation reaction section through metering pumps, with a reaction back pressure of 0.5 MPa, a reaction temperature of 40 °C, and a residence time of 0.5 h. After the reaction was complete, the material flowed out. A small amount of water was added to the reaction solution, and the mixture was cooled to 10 °C to crystallize. The crude product was then filtered to obtain the crude product. The crude product was recrystallized from toluene to obtain the target product, 4,4'-dichloromethylbiphenyl, with a yield of 93% and a purity of 98%.

[0024] Example 2: 0.20 mol of biphenyl was dissolved in petroleum ether to form a biphenyl solution; 0.54 mol of formaldehyde monomer from paraformaldehyde was dissolved in petroleum ether to prepare a formaldehyde solution; 0.06 mol of concentrated hydrochloric acid and 0.04 mol of lanthanum(III) trifluoromethanesulfonate were dissolved to prepare an aqueous catalyst solution. The formaldehyde solution and catalyst solution were separately fed into the depolymerization reaction section via metering pumps, with a back pressure of 0.5 MPa, a temperature of 25 °C, and a residence time of 2 min. The depolymerization solution entered the chloromethylation reaction section, and then the prepared biphenyl solution and 0.24 mol of chlorination reagent PCl3 were separately fed into the chloromethylation reaction section via metering pumps, with a back pressure of 0.5 MPa, a reaction temperature of 20 °C, and a residence time of 0.6 h. After post-treatment (addition of water, filtration, recrystallization), the target product 4,4'-dichloromethylbiphenyl was obtained with a yield of 95% and a purity of 98%.

[0025] Example 3: 0.20 mol of biphenyl was dissolved in cyclohexane to prepare a biphenyl solution; 0.54 mol of formaldehyde monomer from paraformaldehyde was dissolved in cyclohexane to prepare a formaldehyde solution; 0.06 mol of concentrated hydrochloric acid and 0.05 mol of ZnCl2 were dissolved to prepare an aqueous catalyst solution. The formaldehyde solution and catalyst solution were separately introduced into the depolymerization reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the temperature was 30 °C, and the residence time was 5 min to complete the formaldehyde depolymerization. The depolymerized solution entered the chloromethylation reaction section, and then the prepared biphenyl solution and 0.23 mol of chlorination reagent PCl3 were separately introduced into the chloromethylation reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the reaction temperature was maintained at 40 °C, and the residence time was 0.5 h. After the reaction was completed, the material flowed out, a small amount of water was added to the reaction solution, the temperature was lowered to 10 °C for crystallization, and the crude product was obtained by filtration. The crude product was recrystallized from toluene to obtain the target product 4,4'-dichloromethylbiphenyl, with a yield of 93% and a purity of 97%.

[0026] Example 4: 0.20 mol of biphenyl was dissolved in cyclohexane to prepare a biphenyl solution; 0.54 mol of formaldehyde monomer from paraformaldehyde was dissolved in cyclohexane to prepare a formaldehyde solution; 0.06 mol of concentrated hydrochloric acid and 0.05 mol of ZnCl2 were dissolved to prepare an aqueous catalyst solution. The formaldehyde solution and catalyst solution were separately introduced into the depolymerization reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the temperature was 30 °C, and the residence time was 5 min to complete the formaldehyde depolymerization. The depolymerized solution entered the chloromethylation reaction section, and then the prepared biphenyl solution and 0.30 mol of chlorination reagent SOCl2 were separately introduced into the chloromethylation reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the reaction temperature was maintained at 35 °C, and the residence time was 0.5 h. After the reaction was completed, the material flowed out, a small amount of water was added to the reaction solution, the temperature was lowered to 10 °C for crystallization, and the crude product was obtained by filtration. The crude product was recrystallized from toluene to obtain the target product 4,4'-dichloromethylbiphenyl, with a yield of 90% and a purity of 99%.

[0027] The steps for recovering the catalyst ZnCl2 in the post-processing are as follows: After the reaction is completed, the feed liquid flowing out is filtered to obtain a filter cake of 4,4'-dichloromethylbiphenyl product. The filtrate is a two-phase mixture of cyclohexane and water. After separation, an aqueous phase is obtained. Finally, the aqueous phase is distilled to remove water and obtain the catalyst ZnCl2 for reuse in the feed reaction of Example 9.

[0028] Example 5: 0.20 mol of biphenyl was dissolved in cyclohexane to prepare a biphenyl solution; 0.54 mol of formaldehyde monomer from paraformaldehyde was dissolved in cyclohexane to prepare a formaldehyde solution; 0.06 mol of concentrated hydrochloric acid and 0.02 mol of lanthanum(III) trifluoromethanesulfonate were dissolved to prepare an aqueous catalyst solution. The formaldehyde solution and catalyst solution were separately fed into the depolymerization reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the temperature was 30 °C, and the residence time was 5 min to complete the formaldehyde depolymerization. The depolymerized solution entered the chloromethylation reaction section, and then the prepared biphenyl solution and 0.29 mol of chlorination reagent PCl3 were separately fed into the chloromethylation reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the reaction temperature was maintained at 40 °C, and the residence time was 0.5 h. After the reaction was completed, the material flowed out, a small amount of water was added to the reaction solution, the temperature was lowered to 10 °C for crystallization, and the crude product was obtained by filtration. The crude product was recrystallized from toluene to obtain the target product 4,4'-dichloromethylbiphenyl, with a yield of 91% and a purity of 96%.

[0029] Example 6: 0.20 mol of biphenyl was dissolved in cyclohexane to prepare a biphenyl solution; 0.54 mol of formaldehyde monomer from paraformaldehyde was dissolved in cyclohexane to prepare a formaldehyde solution; 0.06 mol of concentrated hydrochloric acid and silica-supported phosphotungstic acid (equivalent to 0.05 mol H3PW) were added. 12 O 40 A catalyst aqueous solution was prepared by dissolving phosphotungstic acid (with a loading of 25 wt%). Formaldehyde solution and catalyst solution were separately introduced into the depolymerization reaction section via metering pumps. The reaction back pressure was 0.5 MPa, the temperature was 30 °C, and the residence time was 5 min to complete the formaldehyde depolymerization. The depolymerized solution entered the chloromethylation reaction section. HCl gas (0.73 mol continuously) was introduced into the chloromethylation reaction section via metering pumps. The reaction back pressure was 0.5 MPa, and the reaction temperature was 35 °C. Then, the prepared biphenyl solution and chlorination reagent were added, and the residence time was 0.7 h. After the reaction, the material flowed out, cooled to 10 °C for crystallization, and the filter cake was collected. Dichloroethane was added and stirred to dissolve the target product, and then the catalyst was separated by filtration. The crude product solution was then distilled to remove dichloroethane. The crude product was recrystallized from toluene to obtain the target product 4,4'-dichloromethylbiphenyl, with a yield of 90% and a purity of 98%.

[0030] Example 7: 0.20 mol of biphenyl was dissolved in cyclohexane to prepare a biphenyl solution; 0.54 mol of formaldehyde monomer from paraformaldehyde was dissolved in cyclohexane to prepare a formaldehyde solution; 0.06 mol of concentrated hydrochloric acid and 0.02 mol of p-toluenesulfonic acid were dissolved to prepare an aqueous catalyst solution. The formaldehyde solution and catalyst solution were separately introduced into the depolymerization reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the temperature was 30 °C, and the residence time was 5 min to complete the formaldehyde depolymerization. The depolymerized solution entered the chloromethylation reaction section, and then the prepared biphenyl solution and 0.29 mol of chlorination reagent SOCl2 were separately introduced into the chloromethylation reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the reaction temperature was maintained at 40 °C, and the residence time was 0.5 h. After the reaction was completed, the material flowed out, a small amount of water was added to the reaction solution, the temperature was lowered to 10 °C for crystallization, and the crude product was obtained by filtration. The crude product was recrystallized from toluene to obtain the target product 4,4'-dichloromethylbiphenyl, with a yield of 84% and a purity of 97%.

[0031] Example 8: 0.20 mol of biphenyl was dissolved in cyclohexane to prepare a biphenyl solution; 0.54 mol of formaldehyde monomer from paraformaldehyde was dissolved in cyclohexane to prepare a formaldehyde solution; 0.06 mol of concentrated hydrochloric acid and 0.03 mol of polyphosphoric acid were dissolved to prepare an aqueous catalyst solution. The formaldehyde solution and catalyst solution were separately fed into the depolymerization reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the temperature was 30 °C, and the residence time was 5 min to complete the formaldehyde depolymerization. The depolymerized solution entered the chloromethylation reaction section, and then the prepared biphenyl solution and 0.29 mol of chlorination reagent PCl3 were separately fed into the chloromethylation reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the reaction temperature was maintained at 35 °C, and the residence time was 0.6 h. After the reaction was completed, the material flowed out, a small amount of water was added to the reaction solution, the temperature was lowered to 10 °C for crystallization, and the crude product was obtained by filtration. The crude product was recrystallized from toluene to obtain the target product 4,4'-dichloromethylbiphenyl, with a yield of 78% and a purity of 96%.

[0032] Example 9: 0.20 mol of biphenyl was dissolved in cyclohexane to prepare a biphenyl solution; 0.54 mol of formaldehyde monomer from paraformaldehyde was dissolved in cyclohexane to prepare a formaldehyde solution; 0.06 mol of concentrated hydrochloric acid and 0.05 mol of ZnCl2 recovered in Example 4 were dissolved to prepare a catalyst aqueous solution. The formaldehyde solution and catalyst solution were separately fed into the depolymerization reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the temperature was 30 °C, and the residence time was 5 min to complete the formaldehyde depolymerization. The depolymerization solution entered the chloromethylation reaction section, and then the prepared biphenyl solution and 0.30 mol of chlorination reagent SOCl2 were separately fed into the chloromethylation reaction section through metering pumps. The reaction back pressure was 0.5 MPa, the reaction temperature was maintained at 35 °C, and the residence time was 0.5 h. After the reaction was completed, the material flowed out, a small amount of water was added to the reaction solution, the temperature was lowered to 10 °C for crystallization, and the crude product was obtained by filtration. The crude product was recrystallized from toluene to obtain the target product, 4,4'-dichloromethylbiphenyl, with a yield of 90% and a purity of 99%. The results were basically consistent with those in Example 4.

[0033] Comparative Example 1: The experimental procedure was repeated in Example 2, except that 0.06 mol of concentrated hydrochloric acid was replaced with 0.06 mol of sulfuric acid, which was a 30% (w / w) aqueous solution of sulfuric acid. All other conditions remained unchanged. The target product, 4,4'-dichloromethylbiphenyl, was obtained with a yield of 52% and a purity of 82%. This is significantly lower than the 95% yield and 98% purity achieved in Example 2 with the hydrochloric acid system. Furthermore, the subsequent recovery of the catalyst from the filtrate was more difficult in the sulfuric acid system.

[0034] Comparative Example 2: The experimental procedure was repeated in Example 2, except that 0.06 mol of concentrated hydrochloric acid was replaced with 0.06 mol of glacial acetic acid, which was a 30% aqueous solution of acetic acid, and the residence time of the chloromethylation reaction section was 0.8 h. All other conditions remained unchanged. The target product, 4,4'-dichloromethylbiphenyl, was finally obtained with a yield of 61% and a purity of 89%. This is significantly lower than the 95% yield and 98% purity achieved in the hydrochloric acid system of Example 2. The catalytic effect was poor in the acetic acid system because the high boiling point of acetic acid resulted in high energy consumption during the subsequent recovery of the catalyst from the filtrate.

[0035] Comparative Example 3: The experimental process, material preparation, and feed amount of Comparative Example 3 were the same as in Example 4, except that in the experiment of Comparative Example 3, biphenyl solution, paraformaldehyde solution, concentrated hydrochloric acid, and ZnCl2 were dissolved to prepare a catalyst aqueous solution, and SOCl2 was simultaneously introduced into a single-stage tubular reactor (depolymerization reaction section). The formaldehyde depolymerization reaction section and the chloromethylation reaction section were no longer distinguished. The remaining reaction conditions (reaction temperature 35 ℃, pressure 0.5 MPa, total residence time equivalent to Example 4) remained the same. Due to incomplete formaldehyde depolymerization, the chlorinating agent participated in the reaction too early, and the side reactions in the system increased significantly. After the reaction, the same post-treatment was performed to obtain the target product 4,4′-dichloromethylbiphenyl, with a yield of 59% and a purity of 84%. This is significantly lower than the yield of over 90% and the purity of about 99% under the segmented feeding conditions of this invention. At the same time, more monochloromethylated and multi-substituted byproducts were detected in the reaction solution, the crystallization performance deteriorated, and the difficulty of filtration and recrystallization increased significantly.

[0036] The contents described in this specification are merely an enumeration of the implementation forms of the inventive concept, and the scope of protection of this invention should not be regarded as limited to the specific forms described in the embodiments.

Claims

1. A method for the continuous preparation of 4,4'-dichloromethylbiphenyl, characterized in that, Includes the following steps: S1: Disperse the catalyst in concentrated hydrochloric acid to form a catalyst solution; The catalyst solution, paraformaldehyde solution, and biphenyl solution are respectively pumped into a single-stage tubular reactor to carry out the formaldehyde depolymerization reaction. S2: In step S1, the depolymerization reaction liquid flows out of the primary tubular reactor. The depolymerization reaction liquid, biphenyl solution, and chlorination reagent are respectively introduced into the secondary tubular reactor for chloromethylation reaction. After the reaction is completed, the heterogeneous reaction liquid that flows out is post-processed to obtain the product 4,4'-dichloromethylbiphenyl. The catalyst is reused after post-processing.

2. The method for continuous preparation of 4,4'-dichloromethylbiphenyl as described in claim 1, characterized in that, The catalyst includes at least one of a supported heteropolyacid catalyst, a transition metal compound, polyphosphoric acid, and a Lewis acid. The supported heteropolyacid catalyst is silica-supported phosphotungstic acid, the transition metal compound is lanthanum(III) trifluoromethanesulfonate, and the Lewis acid is at least one of zinc chloride and aluminum trichloride.

3. The method for continuous preparation of 4,4'-dichloromethylbiphenyl as described in claim 2, characterized in that, When the catalyst is at least one of a transition metal compound, polyphosphoric acid, and Lewis acid, the molar ratio of the catalyst to biphenyl is 0.10-0.30:1; when the catalyst is silica-supported phosphotungstic acid, the molar ratio of phosphotungstic acid to biphenyl in the catalyst is 0.10-0.30:

1.

4. The method for continuous preparation of 4,4'-dichloromethylbiphenyl as described in claim 1, characterized in that, The paraformaldehyde solution and the biphenyl solution use the same solvent, which is selected from one or more of C5-C8 aliphatic ethers and C5-C8 aliphatic hydrocarbons, preferably cyclohexane, n-hexane or petroleum ether.

5. The method for continuous preparation of 4,4'-dichloromethylbiphenyl as described in claim 1, characterized in that, The molar ratio of concentrated hydrochloric acid to biphenyl is 0.2-0.4:1; the molar ratio of formaldehyde monomer to biphenyl in paraformaldehyde is 2.3-3.0:

1.

6. The method for continuous preparation of 4,4'-dichloromethylbiphenyl as described in claim 1, characterized in that, The chlorination reagent includes one or more of phosphorus oxychloride, phosphorus trichloride, thionyl chloride, and hydrogen chloride; the molar ratio of Cl element in the chlorination reagent in step S2 to biphenyl in step S1 is 3.0~4.5:

1.

7. The method for continuous preparation of 4,4'-dichloromethylbiphenyl as described in claim 1, characterized in that, The temperature of the formaldehyde depolymerization reaction in step S1 is 20~50℃, preferably 25~30℃, the reaction residence time is 1-10min, preferably 4-6min, and the reaction pressure is atmospheric pressure to 1MPa.

8. The method for continuous preparation of 4,4'-dichloromethylbiphenyl as described in claim 1, characterized in that, The temperature of the chloromethylation reaction in step S2 is 15~50℃, preferably 20~40℃, the reaction residence time is 0.2~0.8h, preferably 0.5-0.7h, and the reaction pressure is atmospheric pressure to 1MPa.

9. The method for continuous preparation of 4,4'-dichloromethylbiphenyl as described in claim 2, characterized in that, When the catalyst is a soluble catalyst, the post-processing method in step S2 is as follows: add a small amount of water to the reaction solution to obtain a solid-liquid-liquid three-phase mixture, filter, collect the filter cake, recrystallize to obtain the target product; collect the filtrate, let it stand to separate into layers, remove water from the aqueous layer by vacuum distillation to obtain the catalyst for reuse, and recover the solvent from the organic layer by distillation. When the catalyst is an insoluble catalyst, the post-treatment method in step S2 is as follows: filter, collect the filter cake, add dichloroethane and stir to dissolve the target product, then filter to separate the catalyst, then distill the crude product solution to remove dichloroethane, recrystallize to obtain the target product.

10. The method for continuous preparation of 4,4'-dichloromethylbiphenyl as described in claim 9, characterized in that, The solvent used for recrystallization is toluene.

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