1. The preparation method of 2, 6-dimethylnaphthalene is characterized in that 3, 4-dichlorotoluene and isoprene are used as raw materials and react in the presence of a Pd catalyst to prepare the 2, 6-dimethylnaphthalene.

2. The method of claim 1, wherein the Pd catalyst is a supported Pd catalyst, wherein the carrier is one or more of titanium dioxide, aluminum oxide, Y molecular sieve and activated carbon, and the active component is Pd.

3. The method of claim 2, wherein the amount of the Pd catalyst added is 0.001mol% to 1mol% based on the active component Pd, based on the 3, 4-dichlorotoluene.

4. The method according to claim 1, wherein the reaction is carried out in an organic solvent, the organic solvent being one or more of N-methylpyrrolidone, N-dimethylformamide, N-dimethylacetamide, ethyl acetate, toluene, 1, 4-dioxane;

and/or the molar ratio of the organic solvent to the 3, 4-dichlorotoluene is (0.5-5): 1.

5. The method as claimed in claim 1, wherein during the reaction, an alkali compound is further added, wherein the alkali compound is one or more of potassium carbonate, cesium carbonate, sodium acetate and sodium ethoxide;

and/or the molar ratio of the alkali compound to the 3, 4-dichlorotoluene is (0.2-2): 1.

6. The method as claimed in claim 1, wherein during the reaction, an auxiliary agent is further added, wherein the auxiliary agent comprises tetrabutylammonium bromide and an iodide salt.

7. The method of claim 6, wherein the iodide salt is selected from at least one of potassium iodide, sodium iodide, and cuprous iodide.

8. The method according to claim 6, wherein the molar ratio of tetrabutylammonium bromide to 3, 4-dichlorotoluene is (0.1-2): 1;

and/or the molar content of the iodide salt is 0.01-10 mol% of the molar amount of the 3, 4-dichlorotoluene.

9. The process according to claim 1, characterized in that the reaction system is carried out under heating; the heating temperature is 50-180 ℃;

and/or the pressure of the reaction system is 0-2.0 MPa.

10. The method according to claim 1, characterized in that it comprises in particular the steps of:

3, 4-dichlorotoluene, isoprene, a Pd catalyst, an alkali compound, an organic solvent and an auxiliary agent are uniformly mixed, and are heated and reacted in an inert atmosphere to prepare the 2, 6-dimethylnaphthalene.

11. The process according to any one of claims 1 to 10, wherein the molar ratio of 3, 4-dichlorotoluene to isoprene is (0.1-2): 1.

Background

2, 6-dimethylnaphthalene (2, 6-DMN) is a precursor for synthesizing polyethylene naphthalate (PEN), and the chemical structure of the precursor is similar to that of polyethylene terephthalate (PET), except that naphthalene rings with higher rigidity are used for replacing benzene rings in the PET, so that the polymer of the precursor has higher physical and mechanical properties, gas barrier property, heat resistance, optical property, electrical property and chemical stability. In addition, the 2,6-DMN also has wide application in the industries of medicine, pesticide, dye and the like. In recent years, 2, 6-naphthalenedicarboxylic acid obtained by oxidation of 2,6-DMN as a raw material can produce a novel Liquid Crystal Polymer (LCP) which has high fluidity and can be filled into fine and thin-walled products. As electronic devices move toward thinner and more complex devices, the demand for LCP has increased dramatically, making 2,6-DMN an important organic material of new development. Therefore, the efficient synthesis of 2,6-DMN is a precondition for the large-scale production of PEN and LCP.

In the early stage, 2,6-DMN was prepared by direct extraction, i.e., by distillation and separation through a series of processes using coal tar or by-product tar refined from petroleum as raw material. However, the method has the disadvantages of low content of 2,6-DMN in the raw material, complex components, and very close boiling points of DMN isomers, and complicated separation steps, so that the method cannot meet the requirements of large-scale industrial production. The focus of current research is chemical synthesis, including routes to naphthalene or methylnaphthalene methylation, aromatics and olefin cyclization, and the like.

In U.S. Pat. Nos. 6011190 and 6018086, monomethyl naphthalene and methanol are used as raw materials, MCM-22 is used as an alkylation catalyst, and 2,6-DMN is prepared by reaction at 400 ℃ and 0.5 MPa. In the route, the conversion rate of the monomethyl naphthalene is only 58 percent, and the selectivity of the 2,6-DMN is only 14 percent, so in the subsequent reaction step, the isomer of the byproduct DMN continuously reacts with naphthalene to obtain the monomethyl naphthalene which is recycled to the alkylation reaction. Although the raw material source of the route is wide, the yield of the 2,6-DMN is very low, and the isomers of the by-product DMN need to be recycled after further reaction treatment. The method has long process route, harsh reaction conditions and lower yield of target products, and simultaneously needs to separate DMN isomers, thus leading to higher production cost of the 2, 6-DMN.

In the patents of U.S. Pat. No. 5,5334796, U.S. Pat. No. 4,825,825, U.S. Pat. No. 4,503077, U.S. Pat. No. 5,987,5189234, U.S. Pat. No. 4,87260, U.S. Pat. No. 5,10563, U.S. Pat. No. 5,83933 and U.S. Pat. No. 5,5292934, ortho-xylene and butadiene are used as raw materials, and 1,5-DMN is obtained by side chain alkylation, cyclization and dehydroaromatization, and then 2,6-DMN with the weight percent of more than 95% is obtained by isomerization, separation and purification. The route is developed by American BP Amoco company, is the only production process for producing 2, 6-naphthalene dicarboxylic acid in large scale in the world, but the process route is long, so that the production cost of 2,6-DMN is difficult to reduce, and the development of PEN and related polyester industries is restricted.

In recent years, researchers have made improvements to the aromatic and olefin cyclization process routes. In patent CN104447179A, amyl alcohol and toluene are used as raw materials, olefin is obtained by dehydrogenation reaction, olefin is further reacted with toluene by alkylation, cyclodehydrogenation and isomerization reaction, and 2,6-DMN is finally obtained. In patent CN1192726A, p-xylene and butadiene are used as raw materials, under the action of a potassium or sodium catalyst, p- (2-methylbutenyl) toluene is firstly obtained, and then 2,6-DMN is obtained through dehydrocyclization reaction. In patent CN111217659A, isoprene and methyl p-benzoquinone are used as raw materials, and under the action of heating and Lewis acid catalyst, intermediate product dimethyl hexahydronaphthalenone is generated, and then under the action of Cu-based catalyst, dehydrogenation and deoxidation reactions occur, and finally 2,6-DMN is generated. In patent CN111233602A, isoprene and 3-cyclohexene formaldehyde are used as raw materials, and 2,6-DMN is generated through two-step reaction. Although the method improves the synthesis method of the 2,6-DMN, the method still has the defects of long reaction steps, harsh reaction conditions, low conversion rate of raw materials, poor selectivity of target products, more by-products and difficult separation.

The current commercial process route is prepared from BP Amoco through three-step reactions, in which o-xylene and butadiene are reacted under the condition of an alkaline catalyst to prepare 5- (o-tolyl) pent-2-ene, and then the 5-DMN is prepared through cyclization dehydrogenation, and finally the 2,6-DMN is prepared through isomerization. The separation of DMN isomers is a thermodynamic equilibrium process, and has the advantages of low single-pass separation yield, large circulation amount and high energy consumption. The prior art synthetic routes to 2,6-DMN all produce multiple DMN isomers, and additional separation and purification steps are required to produce pure 2, 6-DMN. DMN has 10 isomers, and is difficult to separate by conventional separation methods such as distillation or solvent extraction, and particularly, 2,6-DMN and 2,7-DMN have very similar physical properties and are difficult to separate. Therefore, it is important to develop a high-efficiency synthesis method without other isomers of 2, 6-DMN.

Disclosure of Invention

As is known, the synthesis of 2,6-DMN by taking ortho-xylene and butadiene as raw materials has complex process and long process route, and the intermediate product of each step of reaction needs to be separated to ensure the completion of the next step of reaction, so that the cost of the 2,6-DMN is difficult to reduce. Compared with the synthesis process using o-xylene and butadiene as raw materials, the process for synthesizing 2,6-DMN by using monomethyl naphthalene and methanol as raw materials is simple and has low raw material cost, but the process also has the defects of extremely low single-pass yield and more DMN isomers, thereby causing poor economic benefit.

In order to improve the technical problems, the invention provides a method for preparing 2, 6-dimethylnaphthalene by one-step reaction of 3, 4-dichlorotoluene and isoprene, which takes the 3, 4-dichlorotoluene and isoprene as raw materials, and efficiently synthesizes the 2, 6-dimethylnaphthalene (2, 6-DMN) by one step under mild conditions through C-C coupling reaction so as to overcome the defects of long process route, harsh reaction conditions and low yield of the 2,6-DMN in the prior art.

The technical scheme of the invention is as follows:

the preparation method of 2, 6-dimethylnaphthalene takes 3, 4-dichlorotoluene and isoprene as raw materials and prepares the 2, 6-dimethylnaphthalene by reaction in the presence of a Pd catalyst.

According to the invention, the Pd catalyst is a supported Pd catalyst, wherein the carrier is one or more of titanium dioxide, aluminum oxide, a Y molecular sieve and active carbon, and the active component is Pd.

According to the invention, the precursor of the active component Pd is one or more of palladium acetate, palladium nitrate, palladium chloride and palladium oxalate.

According to the invention, in the supported Pd catalyst, the mass percentage content of the active component Pd is 0.1-10%, preferably 0.2-1%, based on the mass of the carrier. For example, the concentration may be 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination of the foregoing.

According to the invention, the amount of the Pd catalyst is 0.001mol% to 1mol%, preferably 0.01mol% to 0.5mol%, most preferably 0.05mol% to 0.3mol% of the 3, 4-dichlorotoluene based on the content of the active component Pd.

Compared with the traditional catalyst for C-C coupling reaction, the Pd catalyst, such as the supported Pd catalyst, adopted by the invention reduces the use of organic phosphine ligands, reduces the cost of the catalyst, has no pollution, and is convenient for the recovery and recycling of the catalyst.

According to the invention, the reaction is carried out in an organic solvent, which facilitates the dissolution, dispersion and mass transfer of the reaction starting materials. Illustratively, the organic solvent is one or more of N-methylpyrrolidone, N-dimethylformamide, N-dimethylacetamide, ethyl acetate, toluene, 1, 4-dioxane. Specifically, the molar ratio of the organic solvent to the 3, 4-dichlorotoluene is (0.5 to 5):1, preferably (1 to 5):1, and may be, for example, 0.5:1, 1:1, 1.5:1, 2:1, 2.4:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, or any combination of two or more of the foregoing.

According to the invention, during the reaction, an alkali compound is also added. Illustratively, the alkali compound is one or more of potassium carbonate, cesium carbonate, sodium acetate and sodium ethoxide. Specifically, the molar ratio of the alkali compound to 3, 4-dichlorotoluene is (0.2 to 2):1, preferably (0.5 to 1.5):1, and may be, for example, 0.2:1, 0.5:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, or any combination of the foregoing. The purpose of adding the alkali compound is as follows: 3, 4-dichlorotoluene and isoprene react with the generated palladium (II) compound to generate HCl, and the HCl is reduced and eliminated after alkali is added, so that the palladium (II) compound is converted into zero-valent palladium to continuously participate in the reaction.

According to the invention, an auxiliary agent is also added during the reaction. Illustratively, the adjuvant comprises tetrabutylammonium bromide and an iodide salt. The iodine salt is at least one selected from potassium iodide, sodium iodide and cuprous iodide. Preferably, the auxiliary agent at least comprises tetrabutylammonium bromide and at least one of potassium iodide, sodium iodide and cuprous iodide.

According to the invention, the molar ratio of tetrabutylammonium bromide to 3, 4-dichlorotoluene is (0.1-2): 1, preferably (0.2-0.6): 1. For example, the ratio may be 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.8:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, or any combination of the above two points.

According to the invention, the molar content of the iodine salt is 0.01mol% to 10mol%, preferably 0.05mol% to 2mol%, and most preferably 0.1mol% to 1mol% of the molar amount of 3, 4-dichlorotoluene. In the invention, the addition of the iodide salt helps to activate 3, 4-dichlorotoluene with low activity in C-C coupling reaction and generate 2,6-DMN with high chemoselectivity.

According to the invention, the reaction is carried out under an inert atmosphere.

According to the invention, the reaction is carried out under heating. For example, the heating temperature is 50 to 180 ℃, preferably 90 to 160 ℃, and more preferably 110 to 160 ℃; for example, the temperature may be 50 ℃, 60 ℃, 80 ℃, 90 ℃, 100 ℃, 110 ℃, 130 ℃, 160 ℃, 180 ℃ or any one of the above ranges in which two of them are combined. The reaction of the invention can be carried out at a lower temperature, thereby saving energy and ensuring safety.

According to the invention, the pressure of the reaction is 0-2.0 MPa, preferably 0.2-1 MPa, and more preferably 0.3-0.8 MPa; for example, it may be 0, 0.1Mpa, 0.2Mpa, 0.3Mpa, 0.5Mpa, 0.8Mpa, 1Mpa, 1.5Mpa, 1.8Mpa, 2Mpa or any combination of the above two. The method can be carried out under lower pressure, saves energy and can ensure safety.

According to the invention, the preparation method of the 2, 6-dimethylnaphthalene specifically comprises the following steps:

3, 4-dichlorotoluene, isoprene, a Pd catalyst, an alkali compound, an organic solvent and an auxiliary agent are uniformly mixed, and are heated and reacted in an inert atmosphere to prepare the 2, 6-dimethylnaphthalene.

According to the invention, the reaction time is 0.5-24 h, preferably 1-12 h, and more preferably 3-8 h. For example, it may be 0.5h, 1h, 2h, 3h, 5h, 8h, 10h, 12h, 15h, 18h, 20h, 24h, or any one of the above ranges in combination of two.

As a preferred embodiment of the present invention, the method for preparing 2, 6-dimethylnaphthalene specifically comprises the following steps:

adding raw materials of 3, 4-dichlorotoluene, isoprene, a Pd catalyst, an alkali compound, an organic solvent and an auxiliary agent into a reaction kettle with condensing reflux and stirring, introducing nitrogen to replace air in the reaction kettle before the reaction starts, stirring and reacting for 0.5-24 hours under the conditions of 0-2.0 Mpa and 50-180 ℃, and synthesizing 2, 6-dimethylnaphthalene by one-step C-C coupling reaction.

According to the present invention, the Pd catalyst, the alkali compound, the organic solvent and the auxiliary have the above definitions.

In the invention, the specific reaction process of the reaction system is as follows:

according to the invention, the molar ratio of the 3, 4-dichlorotoluene to the isoprene is (0.1-2): 1, preferably (0.5-1.5): 1. For example, the ratio may be 0.1:1, 0.2:1, 0.5:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, or any combination of the above two.

According to the invention, after the reaction is finished, the reaction mixture is a solid phase and a liquid phase, and the solid-liquid phase separation is realized after centrifugal separation; the obtained liquid phase is the crude product of 2, 6-DMN.

According to the invention, after deionized water is added into the solid matter obtained by solid-liquid phase separation to dissolve soluble salt, centrifugal separation is continued, and the obtained solid matter is washed and dried by ethanol and deionized water to obtain the recovered Pd catalyst which can be recycled in a reaction system.

In the invention, the liquid after the first centrifugation is qualitatively and quantitatively analyzed by a gas-mass spectrometer and a gas chromatograph.

Advantageous effects

1. Isoprene in the present invention can be fermented from lignocellulose or by CO₂The 3, 4-dichlorotoluene is obtained by biological fermentation, has relatively low price and obvious raw material advantages; 3, 4-dichlorotoluene and isoprene are used as raw materials, and the 2,6-DMN can be obtained through one-step reaction based on C-C coupling reaction.

2. The reaction can be carried out at lower temperature and pressure, the conversion per pass of the 3, 4-dichlorotoluene can reach more than 99 percent, the yield of the 2,6-DMN can reach more than 90 percent, and compared with the existing synthesis method, the yield of the 2,6-DMN is high, and the reaction condition is mild.

3. The invention creatively provides that the addition of an auxiliary agent of iodine salt (such as potassium iodide) in a reaction system is helpful for activating 3, 4-dichlorotoluene, initiating the reaction and generating 2,6-DMN with high chemoselectivity, and the invention has few reaction byproducts and is easy to refine and separate.

4. Compared with the traditional C-C coupling reaction, the supported Pd catalyst is used in the reaction process, so that the use of organic phosphine ligands is reduced, the cost of the catalyst is reduced, no pollution is caused, and the catalyst is convenient to recover and recycle.

Drawings

FIG. 1 is a reaction scheme of the present invention.

Detailed Description

The technical solution of the present invention will be further described in detail with reference to specific embodiments. It is to be understood that the following examples are only illustrative and explanatory of the present invention and should not be construed as limiting the scope of the present invention. All the technologies realized based on the above-mentioned contents of the present invention are covered in the protection scope of the present invention.

Unless otherwise indicated, the raw materials and reagents used in the following examples are all commercially available products or can be prepared by known methods.

Example 1

100mmol of 3, 4-dichlorotoluene, 200mmol of isoprene, 40mmol of tetrabutylammonium bromide, 240 mmol of N, N-dimethylformamide and 100mmol of K₂CO₃0.1mmol of potassium iodide and Pd-based catalyst supported on different carriers at a loading of 0.5wt% (wherein, different kinds of carriers and the amounts of the added Pd-based catalysts are shown in Table 1) were charged into a 250ml reaction vessel. After sealing, the air in the kettle was slowly replaced with nitrogen three times, and stirring was turned on (600 rmp/min). Then nitrogen is supplemented into the reaction kettle, the pressure in the reaction kettle is increased to 0.7Mpa, heating is started, the temperature in the reaction kettle is increased to 140 ℃, and the reaction is stopped after a constant temperature reaction is kept for a certain time (the reaction time is shown in Table 1). And centrifuging the reacted mixture to realize solid-liquid phase separation and recovering the liquid phase, namely the crude product of the 2, 6-dimethylnaphthalene.

And (3) qualitatively analyzing the product of the liquid phase by a gas-mass spectrometer, and quantitatively analyzing the reaction by a gas chromatograph to obtain the conversion rate of the raw material 3, 4-dichlorotoluene and the yield of the product 2, 6-dimethylnaphthalene.

And adding deionized water into the solid mixture to dissolve soluble salts, continuing centrifugal separation, washing the solid matters obtained again with ethanol and water, and drying to obtain the recovered Pd-based catalyst.

The kind of catalyst and the reaction results in example 1 are shown in Table 1.

TABLE 1 influence of catalyst type and reaction time on the reaction in example 1

Serial number	Catalyst and process for preparing same	Amount of catalyst added (mmol in Pd)	Reaction time (h)	Conversion ratio of 3, 4-dichlorotoluene (%)	Yield of 2, 6-dimethylnaphthalene (%)
						1	Pd/Al2O3	0.1	2	62	59
2	Pd/Al2O3	0.1	5	81	77
						3	Pd/Al2O3	0.1	8	94	79
4	Pd/C	0.1	2	56	31
						5	Pd/C	0.1	5	85	46
6	Pd/C	0.1	8	98	51
						7	Pd/TiO2	0.1	2	19	3
8	Pd/TiO2	0.1	5	25	5
						9	Pd/TiO2	0.1	8	39	10
10	Pd/Y molecular sieve	0.1	2	21	15
						11	Pd/Y molecular sieve	0.1	5	40	27
12	Pd/Y molecular sieve	0.1	8	55	36

The above results show that Al is used₂O₃Activated carbon, TiO₂And the Y molecular sieve is used as a carrier, an active Pd component is loaded, and the C-C coupling reaction of the 3, 4-dichlorotoluene and isoprene can be catalyzed under the conditions of 0.7Mpa and 140 ℃ to generate the 2, 6-dimethylnaphthalene, wherein the yield is between 3 and 79 percent. The catalyst is Pd/Al₂O₃Preferably, the reaction time is 5-8 h.

Example 2

100mmol of 3, 4-dichlorotoluene, 200mmol of isoprene, 40mmol of tetrabutylammonium bromide, 240 mmol of N, N-dimethylformamide and 100mmol of K₂CO₃0.1mmol of potassium iodide and 0.5wt% of Pd/Al₂O₃The catalyst was added to a 250ml reaction kettle. After sealing, the air in the kettle was slowly replaced with nitrogen three times, and stirring was turned on (600 rmp/min). Then, nitrogen gas was supplied to the reaction vessel and the reaction pressure was increased (the pressure of the reaction system is shown in Table 2) (when the pressure was increased, the step of supplying nitrogen gas was not included, and only the pressure generated during the reaction of the reaction materials was used), heating was started (the temperature of the reaction system is shown in Table 2), the temperature in the reaction vessel was increased to the reaction temperature,keeping the constant temperature reaction for 5h, and stopping the reaction to prepare the 2, 6-DMN.

The solid-liquid mixture after the reaction was subjected to the treatment method of example 1 to perform qualitative and quantitative analyses of the liquid sample, and the Pd-based catalyst was recovered. The reaction results are shown in table 2.

TABLE 2 Effect of temperature and pressure on the reaction

In Table 2, self-boosting means that the system does not apply pressure during the reaction but only depends on the pressure generated by the reaction raw materials during the reaction.

The above results show that the temperature and pressure have a large influence on the yield of 2, 6-dimethylnaphthalene. The 3, 4-dichlorotoluene and isoprene react for 5 hours at 90-160 ℃ and 0.5-1.0MPa to generate C-C coupling reaction to obtain the 2, 6-dimethylnaphthalene, the yield is between 7% and 77%, the temperature is between 140 ℃ and 160 ℃, and the pressure is 0.7 MPa.

Example 3

100mmol of 3, 4-dichlorotoluene, 200mmol of isoprene, 40mmol of tetrabutylammonium bromide, 240 mmol of organic solvent (the type of the organic solvent is shown in Table 3), 100mmol of different bases (the type of the base is shown in Table 3), 0.1mmol of potassium iodide and 0.5wt% of Pd/Al supported₂O₃The catalyst was added to a 250ml reaction kettle. After sealing, the air in the kettle was slowly replaced with nitrogen three times, and stirring was turned on (600 rmp/min). And then supplementing nitrogen into the reaction kettle, raising the pressure in the reaction kettle to 0.7Mpa, starting heating, raising the temperature in the reaction kettle to 140 ℃, keeping the constant temperature for reaction for 5 hours, and stopping reaction to prepare the crude product of the 2, 6-DMN.

The solid-liquid mixture after the reaction was subjected to the treatment method of example 1 to perform qualitative and quantitative analyses of the liquid sample, and the Pd-based catalyst was recovered. The reaction results are shown in Table 3.

TABLE 3 influence of the kind of basic auxiliary and solvent on the reaction

Serial number	Amount of catalyst added (mmol in Pd)	Alkali	Solvent(s)	Conversion ratio of 3, 4-dichlorotoluene (%)	Yield of 2, 6-dimethylnaphthalene (%)
						1	0.1	K2CO3	N, N-dimethylformamide	81	77
2	0.1	Cs2CO3	N, N-dimethylformamide	＞99	48
						3	0.1	Sodium acetate	N, N-dimethylformamide	94	91
4	0.1	Sodium ethoxide	N, N-dimethylformamide	98	62
						5	0.1	K2CO3	N, N-dimethyl acetamide	78	71
6	0.1	Sodium acetate	N, N-dimethyl acetamide	92	86
						7	0.1	K2CO3	N-methyl pyrrolidone	65	58
8	0.1	Sodium acetate	N-methyl pyrrolidone	88	70
						9	0.1	Sodium acetate	Ethyl acetate	68	41
10	0.1	Sodium acetate	Toluene	53	27
						11	0.1	Sodium acetate	1, 4-dioxane	32	14

The above results show that with K₂CO₃，Cs₂CO₃Sodium acetate and sodium ethoxide are used as alkaline auxiliary agents, N, N-dimethylacetamide, N, N-dimethylformamide, N-methylpyrrolidone, ethyl acetate, toluene and 1, 4-dioxane are used as solvents, 3, 4-dichlorotoluene and isoprene can perform C-C coupling reaction under the reaction conditions of 140 ℃ and 0.7MPa to generate 2, 6-dimethylnaphthalene, and the yield is 14% -91%. The alkaline assistant is preferably sodium acetate, and the solvent is preferably N, N-dimethylformamide.

Example 4

100mmol of 3, 4-dichlorotoluene, 200mmol of isoprene, 40mmol of tetrabutylammonium bromide, 240 mmol of N, N-dimethylformamide and 100mmol of CH₃COONa, potassium iodide of different masses (the content of potassium iodide is shown in Table 3) and Pd/Al of different contents₂O₃The catalyst (the amount of the catalyst added is shown in Table 4, wherein the amount of the active component Pd supported on the alumina is 0.5% by weight) was charged into a 250ml reaction vessel. After sealing, the air in the kettle was slowly replaced with nitrogen three times, and stirring was turned on (600 rmp/min). And then supplementing nitrogen into the reaction kettle, raising the pressure in the reaction kettle to 0.7Mpa, starting heating, raising the temperature in the reaction kettle to 140 ℃, keeping the constant temperature for reaction for 5 hours, and stopping the reaction to prepare the crude product of the 2, 6-DMN.

The solid-liquid mixture after the reaction was subjected to qualitative and quantitative analysis of the liquid sample by the treatment method of example 1, thereby recovering the Pd-based catalyst. The reaction results are shown in Table 4.

TABLE 4 Pd/Al₂O₃Effect of the amount of catalyst added on the reaction

Serial number	Amount of catalyst added (mmol in Pd)	Potassium iodide (mmol)	Conversion ratio of 3, 4-dichlorotoluene (%)	Yield of 2, 6-dimethylnaphthalene (%)
					1	0.01	0.1	59	34
2	0.05	0.1	78	59
					3	0.1	0.1	94	91
4	0.5	0.1	71	47
					5	1	0.1	58	35
6	0.1	0	11	6
					7	0.1	1	98	80
8	0.1	5	＞99	56

The above results show that 3, 4-dichlorotoluene and isoprene are reacted in the presence of potassium iodide at 140 ℃ under 0.7MPa in the presence of Pd/Al₂O₃The yield of the 2, 6-dimethylnaphthalene generated by the C-C coupling reaction under the action of the catalyst is between 34 and 91 percent, and a better effect is achieved when the addition amount of the catalyst is 0.1mol percent. Under the condition that no potassium iodide participates in the reaction system, the conversion rate of the 3, 4-dichlorotoluene is extremely low, the addition of the potassium iodide is beneficial to the activation of the raw materials, and the effect is good when the addition amount of the potassium iodide is in the range of 0.1mol percent to 1mol percent.

Example 5

The recycled catalyst is subjected to 5 times of circulation experiments, and the method comprises the following specific steps:

100mmol of 3, 4-dichlorotoluene, 200mmol of isoprene, 40mmol of tetrabutylammonium bromide, 240 mmol of N, N-dimethylformamide and 100mmol of CH₃COONa, 0.1mmol of potassium iodide and 0.1mmol of Pd supported at 0.5wt% of Pd/Al₂O₃The catalyst was added to a 250ml reaction kettle. After sealing, the air in the kettle was slowly replaced with nitrogen three times, and stirring was turned on (600 rmp/min). And then supplementing nitrogen into the reaction kettle, raising the pressure in the reaction kettle to 0.7Mpa, starting heating, raising the temperature in the reaction kettle to 140 ℃, keeping the constant temperature for reaction for 5 hours, and stopping the reaction to prepare the 2, 6-DMN. The procedure was a cycle of 0 experiments.

The solid-liquid mixture after the reaction was subjected to qualitative and quantitative analysis of a liquid sample by the method of example 1 to recover the Pd-based catalyst.

Carrying out experiment on the recovered Pd catalyst by adopting the method again to prepare 2, 6-DMN; the process is the first cycle experiment and the Pd catalyst is recovered.

This was repeated 5 times. The test results are shown in table 5.

TABLE 5 catalyst recycle experiment

Serial number	Number of cycles	Conversion ratio of 3, 4-dichlorotoluene (%)	Yield of 2, 6-dimethylnaphthalene (%)
				1	0	94	91
2	1	89	86
				3	2	86	82
4	3	85	80
				5	4	86	80
6	5	83	78

The results show that the catalyst is recycled for 5 times, and the yield of the 2, 6-dimethylnaphthalene generated by the C-C coupling reaction of the 3, 4-dichlorotoluene and the isoprene is between 78 and 91 percent under the reaction conditions of 140 ℃ and 0.7MPa in the presence of potassium iodide.

The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiment. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.