A method for synthesizing 2,5-norbornadiene under an acetylene atmosphere

Abstract

A method for synthesizing 2,5-norbornadiene in acetylene atmosphere, which uses cyclopentadiene (CPD) as starting material and N,N-dimethylformamide (DMF) as solvent, first prepares CPD-DMF solution, then makes the CPD-DMF solution flow through the packing ring or (and) the aluminum oxide particles in a vertical reaction tube filled with acetylene gas in the packing void space. The acetylene pressure is 0.1 MPa-0.15 MPa, the reaction temperature is 60-110 DEG C, and the material stays in the reaction tube for 7-9 minutes. The yield of 2,5-norbornadiene in the product can reach 93%.

Description

Technical Field

[0001] The present invention relates to a chemical synthesis method, in particular to a method for synthesizing 2,5-norbornadiene safely, quickly and in high yield by reacting cyclopentadiene with acetylene. Background Art

[0002] 2,5-Norbornadiene (NBD) is a widely used intermediate and chemical raw material. It can be used as a plant preservative and solar energy storage material. It can also undergo Diels-Alder polymerization with unsaturated hydrocarbons in the presence of catalysts. The resulting product, upon hydrogenation, can produce a variety of high-performance, high-density hydrocarbon fuels or additives. Furthermore, 2,5-Norbornadiene itself can be hydrogenated and used as a high-energy fuel for racing cars, missiles, and rockets.

[0003] 2,5-Norbornadiene is typically produced through the Diels-Alder reaction of cyclopentadiene and acetylene. Since cyclopentadiene readily polymerizes to form dicyclopentadiene at room temperature, the current synthesis process typically uses dicyclopentadiene as the starting material and acetone as the solvent. Dicyclopentadiene is cracked into cyclopentadiene at temperatures exceeding 170°C. This cyclopentadiene then reacts with acetylene dissolved in acetone to yield the target product, 2,5-norbornadiene.

[0004] Patent CN03142244.6 discloses a method for synthesizing 2,5-norbornadiene. Dicyclopentadiene is dissolved in acetone or dimethyl sulfoxide (DMSO) and reacted in an autoclave at 170°C to 230°C for 3 to 7 hours. The reported NBD yield can reach up to 45%. The patent does not disclose the acetylene pressure used in the reaction, but based on the physicochemical data of the acetone used, its solubility in acetylene, and the reaction temperature, it is estimated that the acetylene pressure was no less than 1.5 MPa.

[0005] Patent CN200610117484.3 discloses a method for synthesizing 2,5-norbornadiene. The method involves dissolving dicyclopentadiene in 1.5 to 3 times its weight of acetone, then introducing acetylene into a sealed autoclave. The mixture is allowed to dissolve into the solution, achieving a molar ratio of acetylene to dicyclopentadiene of 4 to 6 to 1. The mixture is then pumped into a tubular reactor, where it reacts under high temperature and pressure to produce 2,5-norbornadiene. The reaction temperature is 180°C to 200°C, and the material remains in the reactor for 10 to 30 minutes. This invention achieves a maximum NBD yield of 58.5%.

[0006] The existing 2,5-norbornadiene synthesis method has low yield, which is determined by the mechanism of the synthesis reaction of 2,5-norbornadiene. Because the dicyclopentadiene needs to be cracked into cyclopentadiene at a temperature of 170°C or above, and under high temperature and pressure, in addition to the main reaction of generating the target product 2,5-norbornadiene between cyclopentadiene and acetylene, the side reaction of further reaction between the generated 2,5-norbornadiene and cyclopentadiene to generate DMN is also obvious, especially in the case of acetylene consumption and low concentration in the material system after the reaction, the side reaction becomes the dominant reaction. In addition, the dicyclopentadiene that does not crack under high temperature and pressure will also trimerize with cyclopentadiene to generate its trimer. The higher the reaction temperature and pressure, the longer the reaction time, the side reaction will also accelerate accordingly, and the side reaction products will also increase accordingly.

[0007] The invention patent CN202110235985.6 uses the method of adding organic amine in acetone solvent, uses the physical adsorption of organic amine to acetylene molecule, increases the solubility of acetylene in acetone, so as to maintain a high molar ratio of acetylene and dicyclopentadiene, thereby improving the yield of the target product 2,5-norbornadiene, and the highest yield can be increased to 66.9%. The patent method still needs to carry out synthesis reaction under high temperature (180℃-240℃) and high pressure conditions.

[0008] The invention patent CN202110236926.0 uses the method of reacting the dicyclopentadiene-acetone solution material saturated with acetylene dissolved at low temperature through the reaction tube filled with acid microporous zeolite catalyst, and the invention patent CN202110235986.0 uses the method of reacting the dicyclopentadiene-acetone solution material saturated with acetylene dissolved at low temperature through the reaction tube filled with acid microporous zeolite catalyst with passivated outer surface. The two invention patents can respectively improve the yield of the target product 2,5-norbornadiene to the highest 75.4% and 88.6%. The two patent methods also need to carry out reaction under high temperature (160℃-240℃) and high pressure conditions.

[0009] The invention patent CN202110236911.4 first cracks dicyclopentadiene into cyclopentadiene in the upper section of the reaction tube, and then mixes with the acetone solution saturated with acetylene to carry out synthesis reaction in the lower section of the reaction tube. The synthesis reaction in the lower section of the reaction tube still needs to be carried out under high temperature (180℃-240℃) and high pressure. The yield of the target product 2,5-norbornadiene can be up to 85.8%.

[0010] The existing synthesis process of 2,5-norbornadiene represented by the above patents has the following main problems:

[0011] 1. To increase the amount of acetylene that can be dissolved, acetone, a relatively high solubility solvent, was selected. Acetone is a low-boiling-point solvent, boiling at only 56°C (760 mmHg), while the synthesis reaction is conducted at temperatures between 160°C and 240°C. Consequently, the reaction tube is subjected to a high-temperature, high-pressure environment. These high-temperature, high-pressure conditions favor the trimerization side reaction between dicyclopentadiene and cyclopentadiene, as well as the reaction of the target product, 2,5-norbornadiene, with cyclopentadiene to form the byproduct, DMN.

[0012] 2. Since the solubility of acetylene in acetone is greatly affected by liquid temperature and acetylene pressure (at 20°C, the solubility of acetylene in acetone at an acetylene partial pressure of 0.1 MPa is 27.9 g / Kg, the solubility of acetylene in acetone at an acetylene partial pressure of 0.5 MPa is 142.3 g / Kg, and the solubility of acetylene in acetone at an acetylene partial pressure of 1 MPa is 293 g / Kg; at 30°C, the solubility of acetylene in acetone at an acetylene partial pressure of 0.1 MPa is 17.9 g / Kg, the solubility of acetylene in acetone at an acetylene partial pressure of 0.5 MPa is 113 g / Kg, and the solubility of acetylene in acetone at an acetylene partial pressure of 1 MPa is 230 g / Kg), in order to increase the amount of acetylene dissolved in acetone solution, acetylene is passed into cold acetone at a higher acetylene partial pressure in a closed container before the reaction. However, the solubility of acetylene in acetone is always limited, and because acetylene cannot be further introduced during the subsequent reaction process, the amount of 2,5-norbornadiene produced increases as the reaction proceeds. At the same time, the acetylene in the reaction materials is consumed and the concentration becomes dilute. The side reaction between the generated 2,5-norbornadiene and cyclopentadiene will gradually become the dominant reaction, resulting in a decrease in the yield of the target product.

[0013] 3. The low-boiling point solvent acetone is easy to evaporate and dissipate during production and use, resulting in solvent loss and production environment pollution. Summary of the Invention

[0014] The present invention aims to provide a technical method for synthesizing 2,5-norbornadiene in high yield by using cyclopentadiene as a starting material, N,N-dimethylformamide with a relatively high boiling point as a solvent, reacting in a low-pressure acetylene atmosphere and at a relatively low reaction temperature.

[0015] The present invention adopts the following technical solutions:

[0016] Using cyclopentadiene (CPD) as a starting material and N,N-dimethylformamide (DMF) as a solvent, 2,5-norbornadiene was produced and synthesized in an acetylene atmosphere at a pressure of ≤0.15 MPa according to the following technical scheme.

[0017] 1. Preparation of CPD–DMF Solution

[0018] Dicyclopentadiene (DCPD) is heated and cracked into cyclopentadiene (CPD), and the cracked cyclopentadiene is dissolved in N,N-dimethylformamide (DMF) solvent to prepare a CPD-DMF solution.

[0019] Conventional thermal cracking methods are used for dicyclopentadiene. For example, DCPD can be heated to above 170°C at normal or reduced pressure to crack it, followed by absorption of the distilled CPD in DMF. Alternatively, DCPD can be passed through a heating tube at above 170°C to crack it into CPD, which is then dissolved in DMF. Regardless of the method used, a DCPD cracking rate of over 98% is required.

[0020] In the CPD-DMF solution prepared here, CPD / DMF=1 / 1.5 to 1 / 3 (weight ratio). The prepared CPD-DMF solution was stored at room temperature (20° C. to 40° C.) and used within two days.

[0021] Normally, neat CPD readily polymerizes into DCPD at room temperature. The present inventors have observed through numerous experiments that CPD's stability is significantly enhanced in DMF solutions. Within a CPD / DMF ratio range of 1 / 1.5 to 1 / 3 (by weight), the polymerization rate of CPD is less than 1% after 50 hours of storage at 20°C to 40°C, and less than 3.5% after 80 hours. Therefore, CPD-DMF solutions should be prepared immediately and used within two days.

[0022] The researchers of the present invention also observed that the boiling point of neat CPD is 42.5°C, but when CPD is dissolved in the aprotic polar solvent DMF, which has a higher boiling point, CPD is not easily volatilized even when the solution temperature is raised to 60°C under normal pressure. If the pressure is increased, the CPD in the solution is even less volatile.

[0023] 2. Make a special vertical tube reactor

[0024] The synthesis reaction of the present invention is carried out in a specialized reactor consisting of several parallel stainless steel reaction tubes nested within a stainless steel cylindrical shell, similar to a shell-and-tube condenser. The reactor's outer jacket is sealed at halfway along its length, separating the outer shell into two independent compartments, one above the other, allowing for the flow of heat transfer media (thermal oil) at different temperatures. The reactor has a total length of 12 to 16 meters, with individual reaction tubes having an inner diameter of 40 to 60 mm and a length of 12 to 16 meters. The reaction tubes are packed with packing rings or / and alumina (γ-Al2O3) granules.

[0025] Each reaction tube must be straight, can not have a bend, the reactor must also be vertical type vertically placed to avoid the reaction material liquid in the reaction tube. When using, let the acetylene gas and CPD-DMF solution from the top of the reaction tube into, through the reaction tube from the bottom of the reaction tube out. The reactor shell is filled with heat transfer medium (heat conducting oil), the heat transfer medium from the shell below to the top.

[0026] The researchers of the present application have preferred two kinds of special vertical reaction tube fillers of the present application through multiple experimental comparisons: filler ring and alumina particles. The filler ring can be made of glass, ceramic or plastic, and has a circular ring appearance, with an outer diameter of 5mm-7mm and a length of 3mm-5mm. The alumina particles have a diameter of 5mm-7mm. By selecting this size range, the natural packing void ratio of the filler ring is 40%-60% of the total packing volume, and the natural packing void ratio of the alumina particles is 25%-30% of the total packing volume, which can form sufficient voids in the reaction tube of the present application to provide sufficient acetylene gas atmosphere space.

[0027] The researchers of the present application have found in experiments that the advantage of filling the reaction tube with a filler ring is that the reaction material liquid is distributed more widely on the surface of the filler ring during the flow through the reaction tube, the liquid film is thinner, the absorption and dissolution speed of acetylene is faster, and it is more conducive to keeping the material system in acetylene saturation state, thereby achieving better inhibition of side reactions.

[0028] The researchers of the present application have found in experiments that the alumina (γ-Al2O3) particles as reaction tube fillers can promote the main reaction of cyclopentadiene and acetylene, and in the case of using alumina particles as reaction tube fillers, the reaction conversion speed of CPD is faster, but the side reaction impurities in the reaction product are slightly more. Analysis and judgment of the more side reaction impurities is because the surface of the alumina packing void is relatively small, and the distribution area of the reaction material liquid on the surface of the packing is relatively small, and the effect of maintaining the acetylene dissolution speed and dissolution amount during the reaction process is not as good as that of the filler ring.

[0029] The researchers of the present application have found that if the filler ring and alumina particles are mixed and used, and the two kinds of filling materials are alternately filled according to the filling of 0.3m length of the filler ring and then the filling of 0.2m length of the alumina particles, a more comprehensive effect can be achieved.

[0030] 3. Pre-pass acetylene

[0031] The CPD-DMF solution prepared in the foregoing is loaded into a sealed reaction kettle, the solution is kept at 20℃-30℃, acetylene is introduced into the solution, the acetylene partial pressure is controlled at 0.1MPa-0.15MPa, and the solution is kept for 0.5-1 hour to make the acetylene reach saturation in the solution.

[0032] 4. Synthesis reaction operation under acetylene atmosphere

[0033] The synthesis reaction operation of the present application is combined with a special reactor and adopts a unique synthesis reaction operation mode under acetylene atmosphere:

[0034] The temperature of the flow heat transfer medium in the upper section of the reactor shell is controlled at 60-80°C, the temperature of the flow heat transfer medium in the lower section is controlled at 90-110°C, acetylene gas is continuously introduced from the top end of the reactor, the acetylene pressure is 0.1-0.15 MPa, and the acetylene gas is always maintained to fill the internal gap space of the reaction tube. Then the CPD-DMF solution saturated with acetylene is pumped from the top end of the reactor, and the reaction of CPD and acetylene occurs in the process of the CPD-DMF solution passing through the reaction tube to generate the target product 2,5-norbornadiene. The addition mode of the CPD-DMF solution must meet the following two conditions:

[0035] (1) The addition rate (addition amount) of the CPD-DMF solution is controlled to maintain that the CPD-DMF solution does not produce liquid accumulation in the reaction tube, i.e. the CPD-DMF solution only flows in the form of a thin liquid layer along the surface of the packing particles from top to bottom at random, and only flows through 70-90% of the surface of the packing particles, without forming a pocket in the reaction tube. The liquid surface of the CPD-DMF solution is fully exposed to the acetylene atmosphere during the entire flow process in the reaction tube, and the thin liquid layer always dissolves acetylene to maintain acetylene in a saturated state in the solution.

[0036] (2) The addition rate (addition amount) of the CPD-DMF solution is controlled to ensure that more than 99% of the CPD in the CPD-DMF solution has been converted when it flows from the top end of the reactor to the bottom end of the reactor.

[0037] N,N-dimethylformamide (DMF) is an aprotic polar solvent, which is inert to acetylene and cyclopentadiene. The boiling point of DMF is 152.8°C, and the solubility of its solvent for acetylene is better than that of acetone. At 20°C, the solubility of acetylene in DMF is 42.7 g / Kg under an acetylene partial pressure of 0.1 MPa, and the solubility of acetylene in DMF is 154 g / Kg under an acetylene partial pressure of 0.5 MPa; at 30°C, the solubility of acetylene in DMF is 32.3 g / Kg under an acetylene partial pressure of 0.1 MPa, and the solubility of acetylene in DMF is 125 g / Kg under an acetylene partial pressure of 0.5 MPa.

[0038] The aforementioned CPD-DMF solution dosing method eliminates liquid accumulation in the reactor. While the acetylene partial pressure is low, the acetylene gas consistently fills the entire void space of the reaction tube. The CPD-DMF solution flowing over the packing surface is constantly exposed to the acetylene atmosphere, continuously dissolving acetylene and maintaining acetylene saturation in the reactant liquid. This maintains a significant excess of acetylene molecules relative to CPD molecules from the start to the end of the reaction. This fundamentally differs from all existing 2,5-norbornadiene synthesis processes in terms of concept, equipment, and operational approach.

[0039] It typically takes about 7-9 minutes for the CPD-DMF solution to enter the reactor from the top and exit from the bottom. After exiting the reactor, the reactant liquid is immediately cooled to room temperature through a cooling tube before being collected in a collector. Acetylene gas escaping from the reactor bottom along with the reactant is also collected and reused.

[0040] 4. Subsequent Operations

[0041] The reaction liquid flowing out from the bottom of the reactor is collected and distilled to separate the target product 2,5-norbornadiene according to a conventional method.

[0042] The technical solution of the present invention can achieve the following effects:

[0043] 1. The acetylene pressure is maintained within the safe pressure (≤0.15MPa) during the entire acetylene introduction and synthesis reaction process.

[0044] 2. Maintaining the number of acetylene molecules in the reaction material system far exceeds the number of cyclopentadiene molecules from beginning to end can ensure that the reaction of acetylene with cyclopentadiene to produce the target product 2,5-norbornadiene is always the dominant reaction, while effectively suppressing the side reaction between the generated target product 2,5-norbornadiene and cyclopentadiene.

[0045] 3. The relatively low reaction temperature in the reaction tube allows the primary reaction to proceed rapidly. Furthermore, the absence of dicyclopentadiene in the reaction materials and the low reaction temperature prevent the trimerization of cyclopentadiene and dicyclopentadiene. The reaction liquid remains heated in the reaction tube for only 7-9 minutes before being discharged and cooled to terminate the reaction, effectively suppressing side reactions.

[0046] 4. The conversion rate of cyclopentadiene in the reaction system reaches more than 99%, and the yield of the target product 2,5-norbornadiene can reach up to 93%.

[0047] 5. The solvent N,N-dimethylformamide has a high boiling point and is a commonly used safe solvent that is easy to use and handle during the production process. DETAILED DESCRIPTION

[0048] Example 1:

[0049] In this embodiment, a reaction tube with a length of 16 m and an inner diameter of 50 mm was used for the synthesis operation. Ceramic rings with a diameter of 5 mm to 7 mm and a length of 3 mm to 5 mm were filled in the reaction tube.

[0050] The CPD-DMF solution was prepared at a weight ratio of CPD / DMF = 1 / 1.5. The void space within the reaction tube was constantly filled with acetylene gas, with an acetylene partial pressure of 0.12 MPa to 0.14 MPa. The thermal oil temperature in the upper shell of the reaction tube was controlled at 78°C ± 2°C, and in the lower shell at 108°C ± 2°C.

[0051] Acetylene was introduced into the CPD-DMF solution at a temperature of 25°C to 30°C and an acetylene pressure of 0.12 MPa to 0.14 MPa for 0.5 hours to achieve saturation. The CPD-DMF solution was then pumped into the reaction tube from the top at a rate of approximately 330 g / min. The material flowed out of the bottom of the reaction tube after approximately 8-9 minutes. The outflowing material was cooled to 30°C to 35°C in a cooling tube and collected. The target product, 2,5-norbornadiene, was then separated and recovered by distillation in an experimental distillation apparatus.

[0052] In this example, a total of 10 kg of CPD-DMF solution was added, resulting in a calculated net CPD content of 4 kg. The residual CPD content in the discharged reaction material was 0.3%, and the calculated CPD conversion rate of the starting material was 99.3%. The reaction material was fractionated and separated by distillation to yield 5.15 kg of 98.8% pure 2,5-norbornadiene, equivalent to 5.09 kg of pure 2,5-norbornadiene. The calculated yield of the target product, 2,5-norbornadiene, was 91.4%.

[0053] Example 2:

[0054] The synthesis was also performed using only one reaction tube, which was 12 meters long and 50 mm in inner diameter. The tube was filled with alumina particles with a diameter between 5 mm and 7 mm.

[0055] The CPD-DMF solution used was prepared at a CPD / DMF ratio of 1 / 2 (by weight). The void space within the reaction tube was constantly filled with acetylene gas, with an acetylene partial pressure of 0.12 MPa to 0.14 MPa. The thermal oil temperature in the upper shell of the reaction tube was controlled at 62°C ± 2°C, and in the lower shell at 95°C ± 2°C.

[0056] Acetylene was introduced into the CPD-DMF solution at a temperature of 25°C to 30°C and an acetylene pressure of 0.12 MPa to 0.14 MPa for 0.5 hours to saturate it. The CPD-DMF solution was then pumped into the reaction tube from the top at a rate of approximately 420 g / min. The material flowed out of the bottom of the reaction tube after approximately 7 to 8 minutes. The outflowing material was cooled to 30°C to 35°C in a cooling tube and collected. The target product, 2,5-norbornadiene, was then separated and recovered by distillation in an experimental distillation apparatus.

[0057] In this example, a total of 15 kg of CPD-DMF solution was added, resulting in a calculated net CPD content of 4.95 kg. The residual CPD content in the discharged reaction material was 0.1%, and the calculated CPD conversion rate of the starting material was 99.7%. The reaction material was fractionated and separated by distillation to yield 6.37 kg of 98.2% pure 2,5-norbornadiene, equivalent to 6.26 kg of pure 2,5-norbornadiene. The calculated yield of the target product, 2,5-norbornadiene, was 90.7%.

[0058] Example 3:

[0059] The synthesis was performed using a single reaction tube, 14 m long and 50 mm in inner diameter. The tube was packed alternately, with ceramic packing rings (0.3 m) and alumina particles (0.2 m) each. The sizes of the packing rings and alumina particles were the same as in Examples 1 and 2, respectively.

[0060] The CPD-DMF solution used was prepared at a ratio of CPD / DMF = 1 / 3 (by weight). The void space within the reaction tube was always filled with acetylene gas, with an acetylene partial pressure of 0.12 MPa to 0.14 MPa. The thermal oil temperature in the upper shell of the reaction tube was controlled at 70°C ± 2°C, and in the lower shell at 105°C ± 2°C.

[0061] Acetylene was introduced into the CPD-DMF solution at a temperature of 25°C to 30°C and an acetylene pressure of 0.12 MPa to 0.14 MPa for 0.5 hours to saturate it. The CPD-DMF solution was then pumped into the reaction tube from the top at a rate of approximately 380 g / min. After approximately 7 to 8 minutes, the material flowed out of the bottom of the reaction tube. The outflowing material was cooled to 30°C to 35°C in a cooling tube and collected. The target product, 2,5-norbornadiene, was then separated and recovered by distillation in an experimental distillation apparatus.

[0062] In this example, a total of 15 kg of CPD-DMF solution was added, resulting in a calculated net CPD content of 3.75 kg. The residual CPD content in the discharged reaction material was 0.15%, and the calculated CPD conversion rate of the starting material was 99.4%. The reaction material was fractionated and separated by distillation to yield 4.93 kg of 98.7% pure 2,5-norbornadiene, equivalent to 4.86 kg of pure 2,5-norbornadiene. The calculated yield of the target product, 2,5-norbornadiene, was 93.2%.

Claims

1. A method for synthesizing 2,5-norbornadiene, characterized in that: The following steps are involved: (1) Preparation of CPD–DMF solution Dicyclopentadiene (DCPD) is heated and cracked into cyclopentadiene (CPD), and the cracked cyclopentadiene is dissolved in N,N-dimethylformamide (DMF) solvent to prepare a CPD-DMF solution; (2) Make a special vertical tube reactor The reactor is composed of several stainless steel reaction tubes arranged in parallel, with a stainless steel cylindrical shell nested outside, similar to a shell-and-tube condenser. The outer jacket shell of the reactor is sealed at half the total length of the reactor, dividing the outer shell into two independent spaces, upper and lower, so that heat transfer medium thermal oil of different temperatures can flow separately. Each single reaction tube in the reactor is 12 to 16 meters long and has an inner diameter of 40 to 60 mm. The reaction tubes are filled with fillers. Each reaction tube must be straight without any bends, and the reactor must be placed vertically to avoid accumulation of reaction material liquid in the reaction tube. When in use, acetylene gas and CPD-DMF solution are allowed to enter from the top of the reaction tube, pass through the reaction tube, and flow out from the bottom of the reaction tube. The reactor shell is filled with heat transfer medium thermal oil, and the heat transfer medium flows from the bottom of the shell to the top. (3) Pre-introduction of acetylene The CPD-DMF solution prepared above was placed in a sealed reactor, and the solution was kept at 20°C to 30°C. Acetylene was introduced into the solution, and the acetylene partial pressure was controlled at 0.1MPa to 0.15MPa. The temperature was maintained for 0.5 to 1 hour to allow the acetylene to reach saturation in the solution. (4) Synthesis reaction operation under acetylene atmosphere At the specified reaction temperature, acetylene gas is continuously introduced from the top of the reactor to ensure that the acetylene gas always fills the void space inside the reaction tube. Then, a CPD-DMF solution saturated with acetylene is pumped from the top of the reactor. As the CPD-DMF solution passes through the reaction tube, CPD reacts with acetylene to produce the target product, 2,5-norbornadiene. The method of adding the CPD-DMF solution must simultaneously meet the following two conditions: ① Control the addition rate of the CPD-DMF solution to ensure that the CPD-DMF solution does not accumulate in the reaction tube. That is, the CPD-DMF solution flows randomly from top to bottom along the surface of the filler particles in the form of a thin liquid surface, and only flows through 70% to 90% of the surface of the filler particles, without forming pockets in the reaction tube. During the entire flow process of the CPD-DMF solution in the reaction tube, the liquid surface is fully exposed to the acetylene atmosphere. The flowing thin liquid surface layer can always dissolve acetylene, thereby keeping the acetylene in the solution in a saturated state. ②Control the addition rate of CPD-DMF solution to ensure that when the CPD-DMF solution flows from the top of the reactor to the bottom of the reactor, the CPD in it has been converted by more than 99%; The CPD-DMF solution enters the top of the reactor, flows for 7 to 9 minutes, flows out from the bottom of the reactor, immediately flows through a cooling pipe to cool to room temperature, and then flows into a collector; the reaction materials are separated by distillation to collect the target product 2,5-norbornadiene.

2. The method for synthesizing 2,5-norbornadiene according to claim 1, wherein In the step (1), the weight ratio of CPD / DMF in the prepared CPD-DMF solution is 1 / 1.5 to 1 / 3.

3. The method for synthesizing 2,5-norbornadiene according to claim 1, wherein In step (2), the fillers used are filler rings and alumina particles. The filler rings are circular rings with an outer diameter of 5 mm to 7 mm and a length of 3 mm to 5 mm. The diameter of the alumina particles is between 5 mm and 7 mm. The filler rings and alumina particles are filled separately, or alternately mixed in a manner of filling 0.3 m of filler rings and then filling 0.2 m of alumina particles.

4. The method for synthesizing 2,5-norbornadiene according to claim 1, wherein In the step (4), the acetylene partial pressure is 0.1 MPa to 0.15 MPa, and the acetylene gas is always kept filling the void space inside the reaction tube.

5. The method for synthesizing 2,5-norbornadiene according to claim 1, wherein In the step (4), the temperature of the flowing heat transfer medium in the upper section of the reactor shell is 60°C to 80°C, and the temperature of the flowing heat transfer medium in the lower section is 90°C to 110°C.