Background
The benzene ring in the system is replaced by naphthalene ring with better rigidity compared with the traditional polyethylene terephthalate (PET), so that PEN has better performance compared with PET, and the potential application of the PEN covers all fields in which PET can be applied and can provide better performance. First PEN for CO 2 And O 2 The transmittance of the PET film is only about 30% of that of the traditional PET film, and PEN has better air tightness, so that the PEN film can better store food. Meanwhile, PEN has high modulus, high strength, stretching resistance and creep resistance, so that PEN can be used for manufacturing tires, conveyor belts, high-pressure conveying pipes and the like; finally, PEN has excellent chemical resistance and ultraviolet resistance, and is an excellent material for manufacturing various cables. In addition, the crystallization rate of PEN is reduced compared to PET, and transparent blow molded articles can be obtained.
The key to the production of PEN is to obtain monomers of polymeric grade. PEN production can take two routes, one is direct polymerization of 2, 6-naphthalene dicarboxylic acid (2, 6-NDA) with ethylene glycol; the other is transesterification polymerization of 2,6-NDC with ethylene glycol. The first route is simple in process, however, the melting point of the monomer 2,6-NDA is as high as 310 ℃, and meanwhile, the vapor pressure is low, and the solubility in various organic solvents is poor, so that the 2,6-NDA is difficult to purify by adopting an efficient separation method; the second route requires esterification of 2,6-NDA with methanol, followed by purification of the esterified product and polymerization with ethylene glycol, which is relatively complex compared to the first route and produces methanol as a byproduct of the polymerization process. However, the melting point of 2,6-NDC is far lower than that of 2,6-NDA, about 190 ℃, and the solubility of 2,6-NDC in organic solvents is far higher than that of 2,6-NDA, so that the purification of 2,6-NDC can be in the forms of crystallization, rectification and the like, and the transesterification synthesis of PEN by using 2,6-NDC is still the main method of industrial production at present.
In the second route, the esterification reaction of 2,6-NDA with methanol is critical. The catalyst used in the common esterification synthesis method is an inorganic acid catalyst, such as sulfuric acid or phosphoric acid. Wherein, when the concentrated sulfuric acid is used as a catalyst, the esterification yield of more than 80 percent can be achieved at 130 ℃. Patent US5254719 teaches that when concentrated sulfuric acid is used as the catalyst in the synthesis process, the metal catalyst added to the system in the early reaction can be dissolved, thereby removing the metal catalyst in the subsequent recrystallization step, so that the recrystallized rectifying tower bottoms are free of metal compounds, thereby preventing fouling of the tower bottoms during the rectifying process. However, in the synthesis method, the catalyst cannot be recycled, and a large amount of wastewater is generated in the synthesis process; meanwhile, equipment in the synthesis process is severely corroded, the equipment material requirement is high, and side reactions of dimethyl ether can be generated in the methanol in the synthesis process. Meanwhile, when inorganic acid is used as a catalyst, the catalyst cannot be recycled, and a large amount of wastewater is generated.
The esterification of 2,6-NDA is a spontaneous reaction process at high temperature, so that the synthesis process can be carried out without a catalyst at high temperature. However, to ensure a reaction rate, the reaction temperature is required to be higher than 200 ℃. To increase the reaction rate, patent US6013831 teaches that, after adding a small amount of molybdenum trioxide to the reaction system during synthesis at a high temperature of 270 ℃, the mass fraction of 2,6-NDC in the resulting crude ester product can reach 89.3%, the unreacted 2,6-NDA is only 3.8%, and the mass fraction of the naphthalene dicarboxylic monoesters (MM-2, 6-NDC) produced incompletely is also only 1.9%. Similar catalysts are also molybdenum trioxide, ammonium molybdate, titanium sulfate, ferrous sulfate, ferric sulfate, and the like. Meanwhile, li Yinghai compares the catalysts to find that the color of the esterification product obtained by the ammonium molybdate catalyst is the deepest, which is not beneficial to the later decolorization; the catalytic activity of molybdenum trioxide, ferric sulfate and ferrous sulfate is high, and the final product yield can reach more than 85%. However, the synthesis method not only needs higher reaction temperature, but also the added metal catalyst is easy to cause the fouling of the tower bottom in the subsequent rectification process and cannot be recycled.
Zhang Zhijiang it is pointed out that when 2,6-NDC is synthesized by using phosphotungstic acid as a catalyst, when the reaction temperature is 180 ℃, the molar ratio of alkyd is 80:1, the catalyst dosage is 1-3 wt% and the reaction time is 4-6 hours, the purity of the esterified product can reach 99% without recrystallization, and meanwhile, compared with the synthesis method by using sulfuric acid as the catalyst, the amount of wastewater in the process is greatly reduced. However, this synthesis method has a fatal disadvantage in that the alkyd ratio is excessively high, and the high purity of the esterified product is obtained on the basis of the consumption of raw materials.
From the above analysis, the known 2,6-NDC synthesis method has the following drawbacks: (1) The synthesis method has serious corrosion to equipment and generates a large amount of wastewater; (2) The reaction time in the synthesis process is long, the synthesis process needs to be carried out at high temperature and high pressure, and the energy consumption is huge; (3) The added catalyst in the synthesis process makes the post purification process difficult, and the catalyst is difficult to recycle.
Disclosure of Invention
The invention provides a synthesis method of 2,6-NDC based on the defects of the known synthesis method. The synthesis method has the advantages of less side reaction, high conversion rate, high selectivity, good catalyst circularity, short reaction time and the like. Meanwhile, the synthesis method disclosed by the invention is mild in reaction condition, does not introduce metal elements, and does not influence the later separation and purification process.
To achieve the above object, the solution of the present invention is as follows:
a method for synthesizing 2, 6-naphthalene dicarboxylic acid dimethyl ester comprises the following steps of reacting 2, 6-naphthalene dicarboxylic acid with methanol by using imidazole sulfonic acid functionalized ionic liquid as a catalyst.
In the technical scheme, the ionic liquid is imidazole sulfonic acid functionalized ionic liquid, and the catalyst is a compound with the following structural formula
Wherein R represents an alkyl group having 1 to 20 carbon atoms, preferably R is a methyl group, an ethyl group or a butyl group, n is an integer selected from 1 to 10, preferably 3 or 4, and X is an anion, preferably including hydrogen sulfate, hydrogen phosphate and p-toluenesulfonate.
In the above technical scheme, the addition amount of the 2, 6-dimethyl naphthalene dicarboxylate is 1-15wt%, preferably 1-10wt%, more preferably 2-7wt% of the addition amount of methanol in the reaction system.
In the technical scheme, the reaction is further added with 2, 6-dimethyl naphthalene dicarboxylate, and preferably the 2, 6-dimethyl naphthalene dicarboxylate added in the reaction system comprises a mixed solution containing 2, 6-dimethyl naphthalene dicarboxylate obtained by solid-liquid separation after the reaction of 2, 6-dimethyl naphthalene dicarboxylate and methanol is finished.
In the technical scheme, the solid-liquid separation temperature is 50-100 ℃, and the better separation temperature is 60-80 ℃.
In the technical scheme, the mixed solution containing the 2, 6-dimethyl naphthalene dicarboxylate obtained by solid-liquid separation also comprises an ionic liquid catalyst.
In the technical scheme, the adding amount of the catalyst ionic liquid is 0.1-5 wt% of the adding amount of the methanol in the reaction system, and the preferable adding amount is 0.1-2 wt%.
In the technical scheme, the mass ratio of the 2, 6-naphthalene dicarboxylic acid to the methanol is 1:3-1:15, and the preferable mass ratio is 1:5-1:10.
In the technical scheme, the reaction temperature is 60-170 ℃, the preferable reaction temperature is 80-150 ℃, the preferable reaction pressure is 0-3 Mpa, and the reaction time is 1-10 h.
In the above technical scheme, the method comprises a re-reaction step, preferably 2-10 times.
The synthesis method of the invention has the following beneficial effects:
the acidic functional ionic liquid can be recycled, the ionic liquid is not required to be recovered through rectification in the recycling process, and only the reacted methanol is required to be supplemented, so that the esterification rate can still reach more than 85% after the acidic functional ionic liquid is recycled for 3-8 times; the conversion rate of the 2,6-NDA is 60-100%, and the selectivity of the target product 2,6-NDC is good and can be close to 100% (more than 99.9%); the synthetic method can effectively inhibit etherification of the reaction raw material methanol and possible side reaction of 2,6-NDA at high temperature, the color value of the obtained product is obviously improved, and the b value is-5 < b <5; according to the synthesis method, the catalyst methanol solution is recycled at the temperature of 50-100 ℃, so that the re-reaction solution contains 1-10% by weight of 2,6-NDC, the reaction time in the recycling process is greatly reduced, and the efficiency of the whole process is improved. The synthesis method of the invention can be used for high-efficiency synthesis of 2, 6-NDC.
DETAILED DESCRIPTION OF EMBODIMENT (S) OF INVENTION
[ example 1 ]
The ionic liquid synthesis according to the invention can be carried out as follows. The first step is the synthesis of the ylide. 1, 3-propane sultone was first melted into a liquid state in a water bath at 40℃and then 0.1mol of 1, 3-propane sultone was weighed into a 250ml three-necked flask, and dissolved by adding about 150ml of ethyl acetate. The three-neck flask is placed in an ice-water bath, the three-neck flask is provided with stirring, stirring is started, then 0.1mol of alkyl substituent imidazole is added dropwise, after the dropwise addition is finished, the three-neck flask is moved into a water bath kettle to be heated to 70 ℃, and stirring reaction is carried out for 6 hours. After the reaction was complete, the solid was filtered off with suction, washed 3 times with ethyl acetate and finally dried in vacuo or rotary evaporated to remove residual ethyl acetate. In the first reaction step, the main reactions are as follows:
the second reaction is to acid functionalize the ionic liquid. The ylide obtained in the reaction step 1 was put into a 250ml three-necked flask, and 150ml of water was added to dissolve the ylide. The three-necked flask was placed in an ice-water bath, the three-necked flask was equipped with stirring, stirring was started, then an equimolar amount of concentrated sulfuric acid was added dropwise, and after the dropwise addition was completed, the three-necked flask was moved into a water bath and heated to 70℃and stirred for 6 hours. After the reaction was completed, rotary evaporation and vacuum drying were employed to remove the water in the system. In the second reaction step, the main reactions are as follows:
synthesis by the above route
120g of methanol, 12g of 2,6-NDA and 1.2g of the ionic liquid catalyst described above were charged into a 500ml stainless steel reactor. The top of the stainless steel reaction kettle is provided with a packing tower, and the packing tower is filled with molecular sieves to remove water generated in a reaction system, so that the reaction moves in a positive direction. At the beginning of the reaction, the whole system was replaced with nitrogen gas 3 times to remove air in the system, and then pressurized to 1Mpa. Heating is started to raise the temperature in the kettle to 130 ℃, and stirring is started at the same time. When the temperature in the kettle is increased to 130 ℃, starting timing, and stopping the reaction after 5 hours. The reaction product was pressed into a collection tank and after cooling to 40 c, the pressure was vented. The obtained product is filtered and then is put into a vacuum oven for drying. And finally, analyzing the obtained dry product by adopting high performance liquid chromatography. />
The results obtained are shown in Table 1:
table 1: composition of the product after the reaction
As can be seen from the data in Table 1, the selectivity of the product obtained by the first reaction can be close to 100%, and the conversion rate of the raw material 2,6-NDA can be more than 94%.
[ example 2 ]
Synthesis by the synthetic route of example 1
120g of methanol, 12g of 2,6-NDA,3.4g of 2,6-NDC and 1.2g of the ionic liquid catalyst described above were charged into a 500ml stainless steel reaction vessel. The top of the stainless steel reaction kettle is provided with a packing tower, and the packing tower is filled with molecular sieves to remove water generated in a reaction system, so that the reaction moves in a positive direction. At the beginning of the reaction, the whole system was replaced with nitrogen gas 3 times to remove air in the system, and then pressurized to 1Mpa. Heating is started to raise the temperature in the kettle to 130 ℃, and stirring is started at the same time. And after the temperature in the kettle is increased to 130 ℃, starting timing, and respectively carrying out experiments of reaction time periods of 2 hours, 3 hours and 4 hours. The reaction product was pressed into a collection tank and after cooling to 40 c, the pressure was vented. The obtained product is filtered and then is put into a vacuum oven for drying. And finally, analyzing the obtained dry product by adopting high performance liquid chromatography. The results obtained are shown in Table 2:
table 2: composition of the product after the reaction
As can be seen from the data in Table 2, the system reaches equilibrium after 3 hours for the first reaction by using the synthesis method of the present invention, which shows that the reaction rate is greatly improved after 2,6-NDC is added.
[ example 3 ]
Synthesis by the synthetic route of example 1
120g was charged into a 500ml stainless steel reaction vessel12g of 2,6-NDA and 1.2g of the ionic liquid catalyst described above. The system was purged with nitrogen 3 times to remove air in the system, and then pressurized to 1Mpa. Heating is started to raise the temperature in the kettle to 120 ℃, and stirring is started at the same time. When the temperature in the kettle is raised to 120 ℃, starting timing, and stopping the reaction after 5 hours. The reaction product was pressed into a collection tank and after cooling to 40 c, the pressure was vented. The obtained product is filtered and then is put into a vacuum oven for drying.
Comparative example 1
120g of methanol, 12g of 2,6-NDA and 0.6g of 98% concentrated sulfuric acid were charged into a 500ml stainless steel reaction vessel. The system was purged with nitrogen 3 times to remove air in the system, and then pressurized to 1Mpa. Heating is started to raise the temperature in the kettle to 120 ℃, and stirring is started at the same time. When the temperature in the kettle is raised to 120 ℃, starting timing, and stopping the reaction after 5 hours. The reaction product was pressed into a collection tank and after cooling to 40 c, the pressure was vented. The obtained product is filtered and then is put into a vacuum oven for drying.
The products obtained in example 3 and comparative example 1 were analyzed by liquid chromatography, and the composition of the reacted materials is shown in Table 3.
Table 3: composition of the product after the reaction
[ example 4 ]
Synthesis by the synthetic route of example 1
120g of methanol, 12g of 2,6-NDA and 1.2g of the ionic liquid catalyst described above were charged into a 500ml stainless steel reactor. The system was purged with nitrogen 3 times to remove air in the system, and then pressurized to 0.5Mpa. Heating is started to raise the temperature in the kettle to 100 ℃, and stirring is started at the same time. When the temperature in the kettle is raised to 100 ℃, starting timing, focusing on the reaction phenomenon in the kettle at any time, and recording the reaction time when the reaction system presents a clear liquid phaseAnd (3) the room(s). The reaction was continued until 5h later, the reaction was terminated. The reaction product was pressed into a collection tank and after cooling to 40 c, the pressure was vented. The obtained product is filtered by suction and then is put into a vacuum oven for drying, and the color of the dried product is observed.
Comparative example 2
120g of methanol, 12g of 2,6-NDA and 0.6g of 98% concentrated sulfuric acid were charged into a 500ml stainless steel reaction vessel. The system was purged with nitrogen 3 times to remove air in the system, and then pressurized to 0.5Mpa. Heating is started to raise the temperature in the kettle to 100 ℃, and stirring is started at the same time. When the temperature in the kettle is raised to 100 ℃, starting timing, focusing on the reaction phenomenon in the kettle at any time, and recording the reaction time when the reaction system presents a clear liquid phase. The reaction was continued until 5h later, the reaction was terminated. The reaction product was pressed into a collection tank and after cooling to 40 c, the pressure was vented. The obtained product is filtered by suction and then is put into a vacuum oven for drying, and the color of the dried product is observed.
Table 6 shows the time required for the reaction systems in example 4 and comparative example 2 to reach a clear point and the color value b of the dried product. As can be seen from Table 4, the quality of the product is higher by adopting the synthesis method disclosed by the invention, and the side reaction in the system is reduced.
Table 4: clear point time of different reaction processes and color of product after reaction
[ example 5 ]
Synthesis by the synthetic route of example 1
120g of methanol, 12g of 2,6-NDA and 1.2g of the ionic liquid catalyst described above were charged into a 500ml stainless steel reactor. The top of the stainless steel reaction kettle is provided with a packing tower, and the packing tower is filled with molecular sieves to remove water generated in a reaction system, so that the reaction moves in a positive direction. At the beginning of the reaction, the whole system is replaced by nitrogen for 3To remove air from the system, and then pressurized to 1Mpa. Heating is started to raise the temperature in the kettle to 130 ℃, and stirring is started at the same time. When the temperature in the kettle is increased to 130 ℃, starting timing, and stopping the reaction after 5 hours. When the temperature in the reaction kettle is changed to 60 ℃, solid-liquid separation is carried out, the solid phase is taken as a first reaction product, the liquid phase enters a collecting tank, and the liquid temperature in the collecting pipe is maintained at 60 ℃. 20g of fresh methanol was added to the liquid phase collection tank using a pump so that the 2,6-NDC content of the methanol solution was maintained at 2.32%. The obtained solution was subjected to the second and third reactions under the same conditions as the first reaction, and the reaction time period of both reactions was 3 hours. The three obtained products were dried and analyzed for product composition, and the results are shown in Table 5.
Table 5: influence of the number of cycles on the catalytic effect of the catalyst
As can be seen from Table 5, after the catalyst is recycled for 3 times by adopting the synthesis method of the invention, the esterification rate can still reach more than 90%, and the esterification rate is only reduced by 2.5%.
[ example 6 ]
Synthesis by the synthetic route of example 1
120g of methanol, 12g of 2,6-NDA and 1.2g of the ionic liquid catalyst described above were charged into a 500ml stainless steel reactor. The top of the stainless steel reaction kettle is provided with a packing tower, and the packing tower is filled with molecular sieves to remove water generated in a reaction system, so that the reaction moves in a positive direction. At the beginning of the reaction, the whole system was replaced with nitrogen gas 3 times to remove air in the system, and then pressurized to 1Mpa. Heating is started to raise the temperature in the kettle to 130 ℃, and stirring is started at the same time. When the temperature in the kettle is increased to 130 DEG CAfter that, the time was started, and after 5 hours of reaction, the reaction was terminated. When the temperature in the reaction kettle is changed to 80 ℃, solid-liquid separation is carried out, the solid phase is taken as a first reaction product, the liquid phase enters a collecting tank, and the temperature of the liquid in the collecting pipe is maintained at 80 ℃. 20g of fresh methanol was added to the liquid phase collection tank using a pump so that the 2,6-NDC content of the methanol solution was maintained at 4.32%. The obtained solution was subjected to the second and third reactions under the same conditions as the first reaction, and the reaction time period of both reactions was 2 hours. The three obtained products were dried and analyzed for product composition, and the results are shown in Table 6.
[ comparative example 3 ]
Synthesis by the synthetic route of example 1
120g of methanol, 12g of 2,6-NDA and 1.2g of the ionic liquid catalyst described above were charged into a 500ml stainless steel reactor. The top of the stainless steel reaction kettle is provided with a packing tower, and the packing tower is filled with molecular sieves to remove water generated in a reaction system, so that the reaction moves in a positive direction. At the beginning of the reaction, the whole system was replaced with nitrogen gas 3 times to remove air in the system, and then pressurized to 1Mpa. Heating is started to raise the temperature in the kettle to 130 ℃, and stirring is started at the same time. When the temperature in the kettle is increased to 130 ℃, starting timing, and stopping the reaction after 5 hours. When the temperature in the reaction kettle is changed to 30 ℃, solid-liquid separation is carried out, the solid phase is taken as a first reaction product, and the liquid phase enters a collecting tank. To the liquid phase collection tank, 20g of fresh methanol was added, and the obtained solution was subjected to the second reaction and the third reaction under the same conditions as those of the first reaction, and the reaction time period of both the two reactions was 2 hours. The three obtained products were dried and analyzed for product composition, and the results are also shown in Table 6.
Table 6: influence of the number of cycles on the catalytic effect of the catalyst
As can be seen from Table 6, the cyclic reaction of the synthesis method of the present invention can reach equilibrium in a short period of time, and the reaction time is greatly reduced.
[ example 7 ]
Synthesis by the synthetic route of example 1
120g of methanol, 12g of 2,6-NDA, 5.3g of 2,6-NDC and 1.2g of the ionic liquid catalyst described above were charged into a 500ml stainless steel reaction vessel. The top of the stainless steel reaction kettle is provided with a packing tower, and the packing tower is filled with molecular sieves to remove water generated in a reaction system, so that the reaction moves in a positive direction. At the beginning of the reaction, the whole system was replaced with nitrogen gas 3 times to remove air in the system, and then pressurized to 1Mpa. Heating is started to raise the temperature in the kettle to 130 ℃, and stirring is started at the same time. When the temperature in the kettle is increased to 130 ℃, starting timing, and stopping the reaction after 2 hours. When the temperature in the reaction kettle is changed to 80 ℃, solid-liquid separation is carried out, the solid phase is taken as a first reaction product, the liquid phase enters a collecting tank, and the temperature of the liquid in the collecting pipe is maintained at 80 ℃. 20g of fresh methanol was added to the liquid phase collection tank using a pump so that the 2,6-NDC content of the methanol solution was maintained at 4.32%. The obtained solution was subjected to the second and third reactions under the same conditions as the first reaction, and the reaction time period of both reactions was 2 hours. The three obtained products were dried and analyzed for product composition, and the results are shown in Table 7.
Table 7: influence of the number of cycles on the catalytic effect of the catalyst
As can be seen from Table 7, the reaction time is greatly reduced by adopting the synthesis method of the invention, and the catalyst is simple and convenient to recycle, and the methanol raw material can be recycled for a plurality of times.
[ example 8 ]
Synthesis by the synthetic route of example 1
120g of methanol, 12g of 2,6-NDA, 5.3g of 2,6-NDC and 1.2g of the ionic liquid catalyst described above were charged into a 500ml stainless steel reaction vessel. The top of the stainless steel reaction kettle is provided with a packing tower, and the packing tower is filled with molecular sieves to remove water generated in a reaction system, so that the reaction moves in a positive direction. At the beginning of the reaction, the whole system was replaced with nitrogen gas 3 times to remove air in the system, and then pressurized to 1Mpa. Heating is started to raise the temperature in the kettle to 130 ℃, and stirring is started at the same time. When the temperature in the kettle is increased to 130 ℃, starting timing, and stopping the reaction after 2 hours. When the temperature in the reaction kettle is changed to 80 ℃, solid-liquid separation is carried out, the solid phase is taken as a first reaction product, the liquid phase enters a collecting tank, and the temperature of the liquid in the collecting pipe is maintained at 80 ℃. 20g of fresh methanol was added to the liquid phase collection tank using a pump so that the 2,6-NDC content of the methanol solution was maintained at 4.32%. The obtained solution was subjected to the second and third reactions under the same conditions as the first reaction, and the reaction time period of both reactions was 2 hours. The three obtained products were dried and analyzed for product composition, and the results are shown in Table 8.
Table 8: influence of the number of cycles on the catalytic effect of the catalyst