Catalyst for synthesizing 2, 5-dimethylpyrazine through cyclization dehydrogenation of isopropanolamine as well as preparation method and application of catalyst

Abstract

The invention relates to the technical field of catalysts, in particular to a catalyst for synthesizing 2, 5-dimethylpyrazine through cyclization dehydrogenation of isopropanolamine, a preparation method of the catalyst and application of the catalyst in a reaction for synthesizing 2, 5-dimethylpyrazine through cyclization dehydrogenation of isopropanolamine. The catalyst consists of a nitrogen-doped carbon-based carrier, and copper salt, zinc salt, manganese salt and potassium salt which are loaded on the carrier, the particle size of the nitrogen-doped carbon-based carrier is 40-1000 meshes, the specific surface area is 500m < 2 > / g-2000m < 2 > / g, the average pore size is 1nm-30nm, a nitrogen element in the nitrogen-doped carbon-based carrier is directly doped into a carbon skeleton or is connected with a carbon material through an N-C bond, and the content of the nitrogen element is 0.5-10wt%. The catalyst is large in specific surface area, the structure and chemical properties of the catalyst can be unchanged for a long time in a catalytic reaction, and the catalytic activity and stability of the catalyst are obviously improved.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts, and more specifically, to a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine, a preparation method of the catalyst, and an application of the catalyst in the reaction of cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. Background Art

[0002] 2,5-Dimethylpyrazine is a colorless or light yellow transparent liquid with a boiling point of 155 °C and a flash point of 64 °C. It is soluble in water and organic solvents such as ethanol and ether. It has flavors such as nuts and chocolate and is a spice permitted for use in China's GB-2760-86. It is often used in the preparation of flavors for baked foods, coffee, meat, nuts, etc. In addition, 2,5-dimethylpyrazine can be oxidized to 5-methylpyrazine-2-carboxylic acid, which is an intermediate for the synthesis of new drugs such as glipizide, acipimox, and methyl 5-methylpyrazine-2-carboxylate.

[0003] The liquid-phase method is one of the main methods for synthesizing 2,5-dimethylpyrazine, and there are mainly the following several synthetic routes: (1) synthesized from ketone compounds containing active methylene groups. This route has complicated steps and low yields and is not suitable for large-scale industrial production; (2) synthesized by cyclization of acrolein. This route uses highly toxic raw materials and is prone to explosion during the reaction process, with high safety risks; (3) synthesized from α-halogenomethyl ketones. This reaction requires high-temperature and high-pressure conditions and has high requirements for equipment; (4) synthesized by condensation of 1,2-propanediamine and 1,2-propanedione. This route is prone to generating 2,6-dimethylpyrazine, which will lead to a decrease in the yield of the target product 2,5-dimethylpyrazine. In summary, the liquid-phase method faces many challenges in synthesizing 2,5-dimethylpyrazine and is not suitable for large-scale production. There are mainly two ways to synthesize 2,5-dimethylpyrazine by the gas-phase method: (1) the cyclization reaction of 1,2-propanediamine and 1,2-propanediol. This process route is relatively short, but the selectivity of 2,5-dimethylpyrazine is poor, resulting in low yields and difficult separation; (2) using isopropanolamine as the raw material and adopting gas-solid catalysis synthesis. However, the catalyst has a significant impact on the results.

[0004] The invention patent CN1513846A discloses a preparation method of a catalyst for the gas-solid phase catalytic preparation of 2,5-dimethylpyrazine. A Zn-Cu-Cr-Al composite condensation catalyst is used. The highest yield of this catalyst is 86.25%, but the reaction temperature is relatively high, ranging from 380°C to 420°C, with a large energy consumption demand, greatly increasing the production cost and being unfavorable for industrial application. The invention patent CN106582672B discloses a preparation method of a catalyst for synthesizing 2,5-dimethylpyrazine. The catalyst includes a SiO2 carrier and active components of Cr, Cu, and Zn supported on the SiO2 carrier. This catalyst is not environmentally friendly due to the use of heavy metal chromium, restricting the large-scale application of the catalyst. The invention patent CN103949266A discloses a preparation method of a catalyst for synthesizing 2,5-dimethylpyrazine using isopropanolamine as a raw material. The catalyst uses a copper, zinc, and chromium / silver mixed oxide. The highest yield of this catalyst is 86.2%, but the production cost of the silver catalyst is relatively high, and it also faces the problem of being unfriendly to the environment due to the use of heavy metal chromium. Summary of the Invention

[0005] In view of the above existing deficiencies in the industrial synthesis of 2,5-dimethylpyrazine, the present application provides a catalyst, a preparation method, and an application for the cyclization dehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine.

[0006] The first object of the present invention is to provide a catalyst for the cyclization dehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The catalyst is composed of a nitrogen-doped carbon-based carrier and copper salt, zinc salt, manganese salt, and potassium salt supported on the carrier; the particle size of the nitrogen-doped carbon-based carrier is 40 mesh - 1000 mesh, the specific surface area is 500 m 2 / g - 2000 m 2 / g, the average pore diameter is 1 nm - 30 nm, and the nitrogen element in the nitrogen-doped carbon-based carrier is directly doped into the carbon skeleton or connected to the carbon material by an N-C bond, where the nitrogen element content is 0.5 wt% - 10 wt%.

[0007] The second object of the present invention is to provide a preparation method of the catalyst for the cyclization dehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine, including:

[0008] S1. Treat the carbon-based carrier with a nitric acid solution, filter and wash it to neutrality, and vacuum dry it to obtain a carbon-based carrier with oxygen-containing functional groups;

[0009] S2. Mix the carbon-based carrier with oxygen-containing functional groups with a nitrogen source and dry it to obtain a nitrogen-doped carbon-based precursor;

[0010] S3. Grind the nitrogen-doped carbon-based precursor and calcine it in a nitrogen atmosphere to obtain a nitrogen-doped carbon-based carrier;

[0011] S4. Uniformly mix manganese salt, potassium salt, copper salt and zinc salt, dissolve them, and obtain a mixed salt solution after stirring evenly. Immerse the nitrogen-doped carbon-based carrier in the mixed salt solution to obtain a precursor solution;

[0012] S5. Age, dry the precursor solution, and calcine it in a nitrogen atmosphere to obtain a 2,5-dimethylpyrazine catalyst.

[0013] In some embodiments, step S1 satisfies at least one of the following: (1) The carbon-based carrier is selected from one or more of activated carbon, carbon fiber, and mesoporous carbon; (2) In the operation of treating the carbon-based carrier with a nitric acid solution, the treatment temperature is 60°C - 100°C, and the treatment time is 6h - 10h. Preferably, the treatment temperature is 70°C - 90°C, and the preferred treatment time is 7h - 9h; (3) The treatment time for vacuum drying is 10h - 15h.

[0014] In some embodiments, step S2 satisfies at least one of the following: (1) The nitrogen source is selected from one or more of melamine, dicyandiamide, and urea, preferably dicyandiamide; (2) In the mixing operation, the mixing time is 3h - 7h, preferably the mixing time is 4h - 6h; (3) In the drying operation, the drying temperature is 100°C - 140°C, and the drying time is 10h - 15h. Preferably, the drying temperature is 110°C - 130°C, and the preferred drying time is 11h - 13h.

[0015] In some embodiments, step S3 satisfies at least one of the following: (1) In the calcination operation, the initial temperature is 30°C, and the temperature is raised to 600°C - 1000°C at a heating rate of 1°C / min - 10°C / min and held for 2h - 6h. Preferably, the temperature is raised to 700°C - 900°C at a heating rate of 3°C / min - 6°C / min and held for 3h - 5h; (2) The gas flow rate of the nitrogen atmosphere is 10 mL / min - 100 mL / min, preferably the gas flow rate is 20 mL / min - 50 mL / min.

[0016] In some embodiments, step S4 satisfies at least one of the following: (1) The manganese salt is selected from one or more of manganese acetate tetrahydrate, manganese(II) nitrate tetrahydrate, manganese(II) nitrate hexahydrate, and manganese chloride, preferably manganese(II) nitrate tetrahydrate and / or manganese acetate tetrahydrate; (2) The potassium salt is selected from one or more of potassium acetate, potassium nitrate, and potassium chloride, preferably potassium nitrate and / or potassium chloride; (3) The copper salt is selected from one or more of copper acetate monohydrate, anhydrous copper acetate, copper(II) nitrate trihydrate, and anhydrous copper(II) chloride, preferably copper(II) nitrate trihydrate and / or anhydrous copper(II) chloride; (4) The zinc salt is selected from one or more of zinc acetate, zinc nitrate hexahydrate, and zinc chloride, preferably zinc nitrate hexahydrate and / or zinc chloride; (5) The loading amount of the active component supported on the nitrogen-doped carbon-based support is calculated based on the mass percentage of the nitrogen-doped carbon-based support. In the catalyst, the mass fraction of metallic manganese element is 1 wt% - 15 wt%, the mass fraction of metallic copper element is 5 wt% - 30 wt%, the mass fraction of metallic zinc element is 5 wt% - 20 wt%, and the mass fraction of metallic potassium element is 1 wt% - 15 wt%.

[0017] In some embodiments, step S5 satisfies at least one of the following: (1) In the aging operation, the aging time is 10 h - 24 h, preferably the aging time is 12 h - 20 h; (2) In the drying operation, the drying temperature is 60 °C - 120 °C, and the drying time is 10 h - 24 h, preferably the drying temperature is 80 °C - 110 °C, and preferably the drying time is 15 h - 18 h; (3) In the calcination operation, the initial temperature is 30 °C, and the temperature is raised to 350 °C - 650 °C at a heating rate of 1 °C / min - 10 °C / min and held for 3 h - 8 h, preferably the temperature is raised to 400 °C - 600 °C at a heating rate of 3 °C / min - 6 °C / min and held for 4 h - 6 h; (4) The gas flow rate of the nitrogen atmosphere is 10 mL / min - 100 mL / min, preferably the gas flow rate is 20 mL / min - 50 mL / min.

[0018] The third object of the present invention is to provide a method for synthesizing 2,5-dimethylpyrazine by cyclodehydrogenation of isopropanolamine. 2,5-Dimethylpyrazine is synthesized by cyclodehydrogenation of isopropanolamine, and the catalyst used is prepared by the above preparation method.

[0019] In some embodiments, the catalyst is loaded into a fixed-bed reaction tube, N2 is continuously introduced and heated to the reaction temperature, and isopropanolamine is fed into the reaction tube. Isopropanolamine undergoes cyclodehydrogenation reaction to obtain 2,5-dimethylpyrazine.

[0020] In some embodiments, at least one of the following is satisfied: (1) The gas flow rate of the N2 atmosphere is 5 mL / g cat / min - 20 mL / g cat / min, preferably the gas flow rate is 8 mL / gcat / min; (2) The reaction temperature is 200°C - 320°C, preferably 250°C; (3) The feeding rate of isopropanolamine is 1 g / g cat / h - 3 g / g cat / h, preferably the feeding rate is 1.5 g / g cat / h; (4) The dosage of the catalyst is 0.33 g / g - 1 g / g based on the mass of isopropanolamine.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] (1) The present invention relates to a catalyst for the cyclization dehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The catalyst is prepared by the method of nitrogen-doped carbon-based support. The obtained catalyst has a large specific surface area. The incorporation of electron-rich nitrogen elements can increase the π electron density of the carbon-based support, promote the exposure of active sites to the reactants, and can maintain its structure and chemical properties unchanged for a long time during the catalytic reaction, significantly improving the catalytic activity and stability of the catalyst.

[0023] (2) The present invention relates to a preparation method of a catalyst for the cyclization dehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The preparation method uses a nitrogen-doped carbon-based support and manganese salt, potassium salt, copper salt, and zinc salt supported on the nitrogen-doped carbon-based support as active components. The preparation method is inexpensive. The prepared catalyst has high activity, high stability, few side reactions, and low energy consumption. Based on the above characteristics, the catalyst obtained by applying this preparation method is suitable for large-scale industrial production of 2,5-dimethylpyrazine.

[0024] (3) The present invention relates to a method for the cyclization dehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. For the first time, the method of synthesizing 2,5-dimethylpyrazine without hydrogen reduction is adopted. Compared with the existing methods for synthesizing 2,5-dimethylpyrazine, the method provided in this application has less energy consumption and cost savings. The yield of 2,5-dimethylpyrazine > 70%, and the purity of the synthesized 2,5-dimethylpyrazine is generally high, and it can operate stably for 120 h under the reaction conditions. Description of the Drawings

[0025] Figure 1 It is the catalyst stability test curve graph of Example 2 of this application;

[0026] Figure 2 It is the catalyst stability test curve graph of Example 4 of this application;

[0027] Figure 3 It is the catalyst stability test curve graph of Example 7 of this application. Detailed Embodiments

[0028] The present invention will be further described below in conjunction with specific embodiments. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention. Those not described in detail in this patent application for the present invention can be understood as common general knowledge in the art.

[0029] Example 1

[0030] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The catalyst includes a nitrogen-doped mesoporous carbon support and metal elements supported on the nitrogen-doped mesoporous carbon support. The loading amount of the active component supported on the nitrogen-doped mesoporous carbon support is calculated based on the mass percentage of the nitrogen-doped mesoporous carbon support. In the catalyst, the mass fraction of metal manganese element is 1 wt%, the mass fraction of metal potassium element is 1 wt%, the mass fraction of metal copper element is 5 wt%, and the mass fraction of metal zinc element is 5 wt%.

[0031] The preparation method of this catalyst is as follows:

[0032] S1. Take 2 g of mesoporous carbon, treat the mesoporous carbon with 100 mL of 5 mol / L nitric acid solution at 60 °C for 6 h, then filter and wash it with deionized water until neutral, and vacuum dry it for 10 h to obtain mesoporous carbon with oxygen-containing functional groups.

[0033] S2. Stir the treated mesoporous carbon with oxygen-containing functional groups and 2 g of melamine in 100 mL of deionized water at room temperature for 3 h, and then place it in a blast drying oven at 100 °C and dry it for 10 h to obtain a nitrogen-doped mesoporous carbon precursor.

[0034] S3. Grind the nitrogen-doped mesoporous carbon precursor and put it into a tube furnace. Under a nitrogen atmosphere of 10 mL / min, raise the temperature to 600 °C at a heating rate of 1 °C / min, and keep it warm for 2 h. Finally, cool it naturally to room temperature to obtain a nitrogen-doped mesoporous carbon support.

[0035] S4. Weigh 0.0457 g of Mn(NO3)2·4H2O, 0.0259 g of KNO3, 0.1901 g of Cu(NO3)2·3H2O, and 0.2275 g of Zn(NO3)2·6H2O respectively, mix them evenly, dissolve them in 100 mL of deionized water, stir evenly to obtain a mixed salt solution, and immerse 1 g of the nitrogen-doped mesoporous carbon support in the mixed salt solution to obtain a precursor solution.

[0036] S5. Age the precursor solution obtained in step S4 for 12 h, then place it in a blast drying oven and dry it at 60 °C for 12 h. Then place it in a tube furnace. Under a nitrogen atmosphere of 10 mL / min, raise the temperature to 350 °C at a heating rate of 1 °C / min and keep it warm for 3 h, and cool it naturally to room temperature to obtain the catalyst.

[0037] Example 2

[0038] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The prepared catalyst and its preparation method are basically the same as those in Example 1, except that the carbon-based carrier is an equal amount of activated carbon.

[0039] Example 3

[0040] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The prepared catalyst and its preparation method are basically the same as those in Example 1, except that the carbon-based carrier is an equal amount of carbon fiber.

[0041] Example 4

[0042] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The catalyst includes a nitrogen-doped activated carbon carrier and metal elements supported on the nitrogen-doped activated carbon carrier. The loading amount of the active component supported on the nitrogen-doped activated carbon carrier is calculated based on the mass percentage of the nitrogen-doped activated carbon carrier. In the catalyst, the mass fraction of metal manganese element is 5wt%, the mass fraction of metal potassium element is 5wt%, the mass fraction of metal copper element is 15wt%, and the mass fraction of metal zinc element is 10wt%.

[0043] The preparation method of this catalyst is as follows:

[0044] S1. Take 2 g of activated carbon, treat the activated carbon with 100 mL of 5 mol / L nitric acid solution at 80 °C for 8 h, then filter and wash it with deionized water until neutral, and dry it in vacuum for 12 h to obtain activated carbon with oxygen-containing functional groups;

[0045] S2. Stir the treated activated carbon with oxygen-containing functional groups and 2 g of dicyandiamide in 100 mL of deionized water at room temperature for 5 h, and then dry it in a forced-air drying oven at 120 °C for 12 h to obtain a nitrogen-doped activated carbon precursor;

[0046] S3. Grind the nitrogen-doped activated carbon precursor and put it into a tubular furnace. Under a nitrogen atmosphere of 20 mL / min, raise the temperature to 800 °C at a heating rate of 5 °C / min, and keep it at this temperature for 4 h. Finally, cool it naturally to room temperature to obtain a nitrogen-doped activated carbon carrier;

[0047] S4. Weigh 0.2284 g of Mn(NO3)2·4H2O, 0.129 g of KNO3, 0.5703 g of Cu(NO3)2·3H2O, and 0.455 g of Zn(NO3)2·6H2O respectively, mix them evenly, dissolve them in 100 mL of deionized water, stir well to obtain a mixed salt solution, and immerse 1 g of nitrogen-doped activated carbon support in the mixed salt solution to obtain a precursor solution;

[0048] S5. Age the precursor solution obtained in step S4 for 15 h, then place it in a blast drying oven and dry it at 80 °C for 15 h. After that, place it in a tubular furnace, under a nitrogen atmosphere of 50 mL / min, raise the temperature to 450 °C at a heating rate of 5 °C / min and hold for 5 h, and then cool it naturally to room temperature to obtain the catalyst.

[0049] Example 5

[0050] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The prepared catalyst and its preparation method are basically the same as those in Example 4, except that the carbon-based support is an equal amount of mesoporous carbon.

[0051] Example 6

[0052] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The prepared catalyst and its preparation method are basically the same as those in Example 4, except that the carbon-based support is an equal amount of carbon fiber.

[0053] Example 7

[0054] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The catalyst includes a nitrogen-doped carbon fiber support and metal elements supported on the nitrogen-doped carbon fiber support. The loading amount of the active component supported on the nitrogen-doped carbon fiber support is calculated based on the mass percentage of the nitrogen-doped carbon fiber support. In the catalyst, the mass fraction of metal manganese element is 5 wt%, the mass fraction of metal potassium element is 15 wt%, the mass fraction of metal copper element is 30 wt%, and the mass fraction of metal zinc element is 5 wt%.

[0055] The preparation method of this catalyst is as follows:

[0056] S1. Take 2 g of carbon fiber, treat the activated carbon with 100 mL of 5 mol / L nitric acid solution at 100 °C for 10 h, then filter and wash it with deionized water until neutral, and dry it in vacuum for 15 h to obtain carbon fiber with oxygen-containing functional groups;

[0057] S2. Stir the carbon fiber with oxygen-containing functional groups after treatment and 2 g of urea in 100 mL of deionized water at room temperature for 7 h, and then dry it in a forced-air drying oven at 140 °C for 14 h to obtain a nitrogen-doped carbon fiber precursor;

[0058] S3. Grind the nitrogen-doped carbon fiber precursor and put it into a tubular furnace. Under a nitrogen atmosphere of 30 mL / min, raise the temperature to 1000 °C at a heating rate of 10 °C / min and hold for 6 h, and finally cool it naturally to room temperature to obtain a nitrogen-doped carbon fiber support;

[0059] S4. Weigh 0.2284 g of Mn(NO3)2·4H2O, 0.3879 g of KNO3, 1.1406 g of Cu(NO3)2·3H2O, and 0.2275 g of Zn(NO3)2·6H2O respectively, mix them evenly, dissolve them in 100 mL of deionized water, stir evenly to obtain a mixed salt solution, and immerse 1 g of the nitrogen-doped carbon fiber support in the mixed salt solution to obtain a precursor solution;

[0060] S5. Age the precursor solution obtained in step S4 for 24 h, then place it in a forced-air drying oven and dry it at 120 °C for 24 h. Then place it in a tubular furnace. Under a nitrogen atmosphere of 100 mL / min, raise the temperature to 650 °C at a heating rate of 10 °C / min and hold for 8 h, and cool it naturally to room temperature to obtain the catalyst.

[0061] Example 8

[0062] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The prepared catalyst and its preparation method are basically the same as those in Example 7, except that the carbon-based support is an equal amount of activated carbon.

[0063] Example 9

[0064] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The catalyst includes a nitrogen-doped mesoporous carbon support and metal elements supported on the nitrogen-doped mesoporous carbon support. The loading amount of the active component supported on the nitrogen-doped mesoporous carbon support is calculated based on the mass percentage of the nitrogen-doped mesoporous carbon support. The mass fraction of metal manganese element in the catalyst is 15 wt%, the mass fraction of metal potassium element is 5 wt%, the mass fraction of metal copper element is 10 wt%, and the mass fraction of metal zinc element is 20 wt%.

[0065] The preparation method of the catalyst is as follows:

[0066] S1. Take 2 g of mesoporous carbon, treat the activated carbon with 100 mL of 5 mol / L nitric acid solution at 100 °C for 10 h, then filter and wash it with deionized water until neutral, and dry it in vacuum for 15 h to obtain mesoporous carbon with oxygen-containing functional groups;

[0067] S2. Stir the mesoporous carbon with oxygen-containing functional groups obtained after treatment and 2 g of urea in 100 mL of deionized water at room temperature for 7 h, then place it in a blast drying oven at 140 °C and dry it for 14 h to obtain a nitrogen-doped mesoporous carbon precursor;

[0068] S3. Grind the nitrogen-doped mesoporous carbon precursor and put it into a tubular furnace. Under a nitrogen atmosphere of 100 mL / min, raise the temperature to 1000 °C at a heating rate of 10 °C / min and hold for 6 h, and finally cool it naturally to room temperature to obtain a nitrogen-doped mesoporous carbon support;

[0069] S4. Weigh 0.6853 g of Mn(NO3)2·4H2O, 0.1293 g of KNO3, 0.3802 g of Cu(NO3)2·3H2O, and 0.91 g of Zn(NO3)2·6H2O respectively, mix them evenly, dissolve them in 100 mL of deionized water, stir evenly to obtain a mixed salt solution, and immerse 1 g of the nitrogen-doped mesoporous carbon support in the mixed salt solution to obtain a precursor solution;

[0070] S5. Age the precursor solution obtained in step S4 for 12 h, then place it in a blast drying oven and dry it at 100 °C for 12 h. Then place it in a tubular furnace. Under a nitrogen atmosphere of 30 mL / min, raise the temperature to 500 °C at a heating rate of 3 °C / min and hold for 6 h, and cool it naturally to room temperature to obtain the catalyst.

[0071] Example 10

[0072] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The prepared catalyst and its preparation method are basically the same as those in Example 9, except that the carbon-based support is an equal amount of carbon fiber.

[0073] Example 11

[0074] This example provides a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine. The prepared catalyst and its preparation method are basically the same as those in Example 9, except that the carbon-based support is an equal amount of activated carbon.

[0075] Test Example 1

[0076] To verify the stability of the catalysts in each example, the catalysts prepared in Examples 2, 4, and 7 were respectively selected for stability testing, and the results are as Figures 1 to 3 shown.

[0077] Figure 1 It shows that after the catalyst prepared in Example 2 was continuously reacted for 120 h under the conditions of a reaction temperature of 250 °C, a rate of raw material isopropanolamine of 1.2 g / h, and an N2 flow rate of 10 mL / min, the catalyst activity decreased by about 30.5% and dropped to 69.5%; the yield of 2,5-dimethylpyrazine decreased by about 25.3% and dropped to 35.2%.

[0078] Figure 2 It shows that after the catalyst prepared in Example 4 was continuously reacted for 120 h under the conditions of a reaction temperature of 280 °C, a rate of raw material isopropanolamine of 1.5 g / h, and an N2 flow rate of 8 mL / min, the catalyst activity decreased by about 5.65% and dropped to 94.35%; the yield of 2,5-dimethylpyrazine decreased by about 17.2% and dropped to 57.3%.

[0079] Figure 3 It shows that after the catalyst prepared in Example 7 was continuously reacted for 120 h under the conditions of a reaction temperature of 320 °C, a rate of raw material isopropanolamine of 3 g / h, and an N2 flow rate of 20 mL / min, the catalyst activity decreased by about 19.6% and dropped to 80.4%; the yield of 2,5-dimethylpyrazine decreased by about 21.2% and dropped to 44.7%.

[0080] Test Example 2

[0081] To verify the catalyst activity in each example, 1 g of the catalyst prepared in the above examples was used in the reaction of synthesizing 2,5-dimethylpyrazine by cyclodehydrogenation of isopropanolamine. The yield and purity of the product 2,5-dimethylpyrazine were detected by a Fuli GC9790Plus gas chromatograph and an STI501 isocratic high-performance liquid chromatograph to judge the catalyst activity.

[0082] This application uses a continuous gas-solid phase catalytic reaction to synthesize 2,5-dimethylpyrazine, and the reaction formula is:

[0083]

[0084] The specific method is as follows: The catalyst is loaded into a fixed-bed reaction tube, N2 is continuously introduced and heated to the reaction temperature, and isopropanolamine is fed into the reaction tube. The isopropanolamine undergoes a cyclodehydrogenation reaction to obtain 2,5-dimethylpyrazine. The specific reaction conditions include the following 5 types:

[0085] (1) Load 1 g of the catalyst into a fixed-bed reaction tube, introduce N₂ at a flow rate of 20 mL / min. When the reactor temperature rises to 200 °C, feed the raw material isopropanolamine into the reactor at a rate of 1 g / h, and continuously introduce N₂ at a flow rate of 5 mL / min to cause the cyclization dehydrogenation reaction of isopropanolamine to obtain 2,5-dimethylpyrazine.

[0086] (2) Load 1 g of the catalyst into a fixed-bed reaction tube, introduce N₂ at a flow rate of 20 mL / min. When the reactor temperature rises to 280 °C, feed the raw material isopropanolamine into the reactor at a rate of 1.5 g / h, and continuously introduce N₂ at a flow rate of 8 mL / min to cause the cyclization dehydrogenation reaction of isopropanolamine to obtain 2,5-dimethylpyrazine.

[0087] (3) Load 1 g of the catalyst into a fixed-bed reaction tube, introduce N₂ at a flow rate of 20 mL / min. When the reactor temperature rises to 320 °C, feed the raw material isopropanolamine into the reactor at a rate of 3 g / h, and continuously introduce N₂ at a flow rate of 20 mL / min to cause the cyclization dehydrogenation reaction of isopropanolamine to obtain 2,5-dimethylpyrazine.

[0088] (4) Load 1 g of the catalyst into a fixed-bed reaction tube, introduce N₂ at a flow rate of 20 mL / min. When the reactor temperature rises to 300 °C, feed the raw material isopropanolamine into the reactor at a rate of 2 g / h, and continuously introduce N₂ at a flow rate of 15 mL / min to cause the cyclization dehydrogenation reaction of isopropanolamine to obtain 2,5-dimethylpyrazine.

[0089] (5) Load 1 g of the catalyst into a fixed-bed reaction tube, introduce N₂ at a flow rate of 20 mL / min. When the reactor temperature rises to 250 °C, feed the raw material isopropanolamine into the reactor at a rate of 1.2 g / h, and continuously introduce N₂ at a flow rate of 10 mL / min to cause the cyclization dehydrogenation reaction of isopropanolamine to obtain 2,5-dimethylpyrazine.

[0090] The catalysts prepared in Examples 1 - 11 were respectively applied to the synthesis reactions of 2,5-dimethylpyrazine under 5 different conditions to obtain the product 2,5-dimethylpyrazine, and the yield and purity of 2,5-dimethylpyrazine were measured. The results are shown in Table 1.

[0091] Table 1 Yield and purity of the product 2,5-dimethylpyrazine

[0092]

[0093] From the catalytic test results of the above catalyst for the cyclization dehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine, it can be seen that the catalyst prepared with an appropriate loading amount and calcination temperature has excellent catalytic activity and stability. Compared with other existing methods for synthesizing 2,5-dimethylpyrazine, the present invention first synthesizes 2,5-dimethylpyrazine by a method without hydrogen reduction, which consumes less energy, saves costs, and generally has a higher purity of the synthesized 2,5-dimethylpyrazine.

[0094] Based on the ideal embodiments of the present invention as inspiration, through the above description, relevant personnel can completely make various changes and modifications without departing from the technical idea of the present invention. The technical scope of the present invention is not limited to the content in the specification, and the technical scope must be determined according to the scope of the claims.

Claims

1. A catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine, characterized in that, The catalyst is composed of a nitrogen-doped carbon-based support and copper salt, zinc salt, manganese salt, and potassium salt supported on the support; the particle size of the nitrogen-doped carbon-based support is 40 mesh - 1000 mesh, the specific surface area is 500 m 2 / g - 2000 m 2 / g, and the average pore diameter is 1 nm - 30 nm. In the nitrogen-doped carbon-based support, nitrogen elements are directly doped into the carbon skeleton or connected to the carbon material by N-C bonds, and the nitrogen element content is 0.5 wt% - 10 wt%.

2. A preparation method of a catalyst for the cyclodehydrogenation of isopropanolamine to synthesize 2,5-dimethylpyrazine, characterized in that, Including: S1. Treat the carbon-based carrier with a nitric acid solution, filter and wash it until neutral, and then dry it under vacuum to obtain a carbon-based carrier with oxygen-containing functional groups; S2. Mix the carbon-based carrier with the oxygen-containing functional groups with a nitrogen source and dry it to obtain a nitrogen-doped carbon-based precursor; S3. Grind the nitrogen-doped carbon-based precursor and calcine it in a nitrogen atmosphere to obtain a nitrogen-doped carbon-based carrier; S4. Uniformly mix manganese salt, potassium salt, copper salt and zinc salt, dissolve them, and stir evenly to obtain a mixed salt solution. Immerse the nitrogen-doped carbon-based carrier in the mixed salt solution to obtain a precursor solution; S5. Age, dry and calcine the precursor solution in a nitrogen atmosphere to obtain a 2,5-dimethylpyrazine catalyst.

3. The preparation method according to claim 2, characterized in that, The step S1 satisfies at least one of the following: (1) The carbon-based carrier is selected from one or more of activated carbon, carbon fiber and mesoporous carbon; (2) In the operation of treating the carbon-based carrier with the nitric acid solution, the treatment temperature is 60°C - 100°C, the treatment time is 6h - 10h, preferably the treatment temperature is 70°C - 90°C, and preferably the treatment time is 7h - 9h; (3) The treatment time of the vacuum drying is 10h - 15h.

4. The preparation method according to claim 2, wherein The step S2 satisfies at least one of the following: (1) The nitrogen source is selected from one or more of melamine, dicyandiamide and urea, preferably dicyandiamide; (2) In the mixing operation, the mixing time is 3h - 7h, preferably the mixing time is 4h - 6h; (3) In the drying operation, the drying temperature is 100°C - 140°C, the drying time is 10h - 15h, preferably the drying temperature is 110°C - 130°C, and preferably the drying time is 11h - 13h.

5. The preparation method according to claim 2, characterized in that, The step S3 satisfies at least one of the following: (1) In the calcination operation, the initial temperature is 30°C, and it is heated to 600°C - 1000°C at a heating rate of 1°C / min - 10°C / min and held for 2h - 6h, preferably heated to 700°C - 900°C at a heating rate of 3°C / min - 6°C / min and held for 3h - 5h; (2) The gas flow rate of the nitrogen atmosphere is 10mL / min - 100mL / min, preferably the gas flow rate is 20mL / min - 50mL / min.

6. The preparation method according to claim 2, characterized in that, The step S4 satisfies at least one of the following: (1) The manganese salt is selected from one or more of manganese acetate tetrahydrate, manganese(II) nitrate tetrahydrate, manganese(II) nitrate hexahydrate and manganese chloride, preferably manganese(II) nitrate tetrahydrate and / or manganese acetate tetrahydrate; (2) The potassium salt is selected from one or more of potassium acetate, potassium nitrate and potassium chloride, preferably potassium nitrate and / or potassium chloride; (3) The copper salt is selected from one or more of copper acetate monohydrate, anhydrous copper acetate, copper nitrate trihydrate and anhydrous copper(II) chloride, preferably copper nitrate trihydrate and / or anhydrous copper(II) chloride; (4) The zinc salt is selected from one or more of zinc acetate, zinc nitrate hexahydrate and zinc chloride, preferably zinc nitrate hexahydrate and / or zinc chloride; (5) The loading amount of the active component supported on the nitrogen-doped carbon-based support is calculated based on the mass percentage of the nitrogen-doped carbon-based support. In the catalyst, the mass fraction of metallic manganese element is 1 wt% - 15 wt%, the mass fraction of metallic copper element is 5 wt% - 30 wt%, the mass fraction of metallic zinc element is 5 wt% - 20 wt%, and the mass fraction of metallic potassium element is 1 wt% - 15 wt%.

7. The preparation method according to claim 2, characterized in that, (5) The step S5 satisfies at least one of the following: (1) In the aging operation, the aging time is 10 h - 24 h, preferably the aging time is 12 h - 20 h; (2) In the drying operation, the drying temperature is 60 °C - 120 °C, the drying time is 10 h - 24 h, preferably the drying temperature is 80 °C - 110 °C, and preferably the drying time is 15 h - 18 h; (3) In the calcination operation, the initial temperature is 30 °C, and the temperature is raised to 350 °C - 650 °C at a heating rate of 1 °C / min - 10 °C / min and held for 3 h - 8 h. Preferably, the temperature is raised to 400 °C - 600 °C at a heating rate of 3 °C / min - 6 °C / min and held for 4 h - 6 h; (4) The gas flow rate of the nitrogen atmosphere is 10 mL / min - 100 mL / min, preferably the gas flow rate is 20 mL / min - 50 mL / min.

8. A method for synthesizing 2,5-dimethylpyrazine by cyclodehydrogenation of isopropanolamine, characterized in that, 2,5-Dimethylpyrazine is synthesized by the cyclodehydrogenation of isopropanolamine, and the catalyst used is prepared by the preparation method described in any one of claims 2 - 7.

9. The method according to claim 8, characterized in that The catalyst is loaded into a fixed-bed reaction tube, N2 is continuously introduced and heated to the reaction temperature, and isopropanolamine is fed into the reaction tube. The isopropanolamine undergoes cyclodehydrogenation reaction to obtain 2,5-dimethylpyrazine.

10. The method according to claim 9, wherein (2) At least one of the following is satisfied: (1) The flow rate of the N2 atmosphere is 5 mL / g cat / min - 20 mL / g cat / min, and the preferred flow rate of the atmosphere is 8 mL / g cat / min; (2) The reaction temperature is 200 °C - 320 °C, preferably the reaction temperature is 250 °C; (3) The feeding rate of the isopropanolamine is 1 g / g cat / h - 3 g / g cat / h, and the preferred feeding rate is 1.5 g / g cat / h; (4) The dosage of the catalyst is 0.33 g / g - 1 g / g based on the mass of isopropanolamine.

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