Synthetic method of pyrazine

Abstract

The invention provides a synthesis method of pyrazine, which comprises the following steps: introducing monoethanolamine and water into a reactor filled with a Cu-Ni-based composite catalyst, carrying out cyclization reaction on the monoethanolamine under the conditions of 0-1MPa and 250-350 DEG C, and converting the monoethanolamine into a target product pyrazine, the Cu-Ni-based composite catalyst comprises a carrier, CuO, NiO and an auxiliary agent, wherein the CuO and the NiO are loaded on the carrier; the Cu-Ni type composite catalyst is used for catalyzing monoethanolamine to generate cyclization reaction to generate pyrazine, single raw material ethanolamine is used for cyclization to generate pyrazine, operation is easy and convenient, the technological process is remarkably shortened, the production cost is reduced, only single product pyrazine is generated by catalyzing monoethanolamine reaction through the catalyst, almost no by-product is generated, the production efficiency is high, and the method is suitable for industrial production. And the method is suitable for large-scale industrial continuous production. Besides, the Cu-Ni type composite catalyst is composed of a carrier, CuO, NiO and an auxiliary agent, the CuO, the NiO and the auxiliary agent are loaded on the carrier, and the catalyst does not contain chromium which pollutes the environment and is environmentally friendly.

Description

Technical Field

[0001] This application relates to the field of organic synthesis, and in particular to a method for synthesizing pyrazines. Background Technology

[0002] Pyrazines, as an important azahexacyclic aromatic compound, have shown indispensable application value in many fields due to their unique chemical structure.

[0003] Currently, the main methods for synthesizing pyrazines fall into two categories: biosynthesis and chemical synthesis. Biosynthesis utilizes microorganisms (such as Bacillus subtilis) to synthesize pyrazine compounds through metabolic pathways; this method is environmentally friendly. Chemical synthesis includes various specific approaches, such as the nitro compound reduction method and the Gutknecht pyrazine synthesis method.

[0004] However, existing pyrazine synthesis technologies suffer from problems such as complex raw materials, harsh reaction conditions, environmental pollution, cumbersome processes, and the inability to conduct large-scale basic synthesis of pyrazines. Summary of the Invention

[0005] To address the above problems, this application provides a method for synthesizing pyrazines, the specific scheme of which is as follows: A method for synthesizing pyrazine includes: passing monoethanolamine and water into a reactor packed with a Cu-Ni-based composite catalyst, and causing the monoethanolamine to undergo a cyclization reaction under conditions of 0-1 MPa and 250-350 °C to convert it into the target product pyrazine. The Cu-Ni based composite catalyst comprises: a support, CuO, NiO supported on the support, and an additive.

[0006] Optionally, the feed space velocity of the monoethanolamine is 0.4–1.0 h⁻¹. -1 The feed space velocity of the water is 0.1~0.3 h⁻¹. -1 .

[0007] Optionally, the monoethanolamine and water are preheated in a preheater before being introduced into the reactor for reaction; The preheating temperature is 250℃~300℃.

[0008] Optionally, the auxiliary agent is selected from at least one of ZrO2, Ce2O3, CeO2, La2O3, MgO, CaO, MnO, and BaO; The carrier is selected from at least one of Al2O3, SiO2 and carbon black.

[0009] Optionally, in the Cu-Ni based composite catalyst, the mass percentage of the support is 25-60%, the mass percentage of CuO is 12-30%, the mass percentage of NiO is 8-20%, and the mass percentage of the metal in the additive is 20-25%.

[0010] Optionally, the method further includes: introducing hydrogen gas into the reactor so that the cyclization reaction is carried out in a hydrogen-containing environment; The feed space velocity of the hydrogen is 50~500 h⁻¹. -1 .

[0011] Optionally, before passing monoethanolamine and water into the reactor packed with a Cu-Ni-based composite catalyst, the method further includes: activating the Cu-Ni-based composite catalyst, including: A mixture of hydrogen and nitrogen gas is introduced into the reactor filled with Cu-Ni-based composite catalyst, and the reactor is activated at 150-200°C for 1-10 hours.

[0012] Optionally, the volume content of hydrogen in the mixed gas is 1-20%.

[0013] Optionally, when the Cu-Ni composite catalyst is deactivated or its activity decreases, regeneration gas is introduced into the reactor to perform in-situ regeneration of the Cu-Ni composite catalyst. The feed space velocity of the regenerated gas is 50 h⁻¹. -1 ~1000h -1 ; The regenerated gas is air or nitrogen containing oxygen, wherein the oxygen in the nitrogen contains oxygen has a volume percentage of 0.1% to 10%. The regeneration process is carried out at a temperature of 200℃ to 400℃, with a heating rate of 10℃ / h to 20℃ / h and a processing time of 2h to 20h.

[0014] Optionally, the yield of the pyrazine is greater than 40% based on monoethanolamine.

[0015] Compared with the prior art, this application has the following advantages: This application provides a method for synthesizing pyrazine, utilizing a Cu-Ni type composite catalyst to catalyze the cyclization reaction of monoethanolamine to generate pyrazine under conditions of 0–1 MPa and 250–350 °C. The raw materials are readily available and inexpensive, the reaction conditions are mild, and the cyclization of ethanolamine from a single raw material to generate pyrazine is simple to operate, significantly shortens the process flow, and reduces production costs. The catalyst-catalyzed reaction of monoethanolamine produces only the single product pyrazine, with almost no byproducts. The single product and high production efficiency make it suitable for large-scale continuous industrial production. Furthermore, the Cu-Ni type composite catalyst consists of a support and CuO, NiO supported on the support, and additives. The catalyst does not contain chromium, which pollutes the environment, making it environmentally friendly. Attached Figure Description

[0016] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings: Figure 1 A schematic diagram of the pyrazine synthesis system provided in an embodiment of this application is shown; Figure 2 The gas chromatographic detection results of the pyrazine product obtained in Example 1 of this application are shown; Figure 3 The gas chromatographic detection results of the pyrazine product obtained in Example 2 of this application are shown; Figure 4 The gas chromatographic detection results of the pyrazine product obtained in Example 3 of this application are shown; Figure 5 The gas chromatographic detection results of the pyrazine product prepared in Comparative Example 1 of this application are shown.

[0017] Figure label: 1: Monoethanolamine feed pump; 2: Water feed pump; 3: Mixed gas inlet; 4: Reactor; 5: Condenser; 6: Collector; 7: Preheater. Detailed Implementation

[0018] The present application will now be described in detail with reference to the described embodiments. Although specific embodiments of the present application are shown, it should be understood that the present application can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this application will be thorough and complete, and will fully convey the scope of the present application to those skilled in the art.

[0019] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions in the specification are preferred embodiments for carrying out this application; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of this application. The scope of protection of this application shall be determined by the appended claims.

[0020] Pyrazines are not only key structural units in many high-value-added chemicals, but their derivatives are also widely used in pharmaceuticals (such as the anti-tuberculosis drug pyrazinamide and the hypoglycemic drug glipizide), pesticides, fragrances (such as food flavoring ingredients 2-methylpyrazine and tetramethylpyrazine), food additives, and advanced functional materials. The market demand for basic pyrazines and their specific substituted derivatives continues to grow, highlighting the urgent need to develop efficient, green, and large-scale synthesis technologies.

[0021] Currently, the synthesis of pyrazines mainly relies on biosynthesis and various chemical synthesis methods: Biosynthesis: This method utilizes the metabolic pathways of microorganisms (such as Bacillus subtilis) to synthesize pyrazine. While it has the advantages of being green and environmentally friendly, it also has inherent limitations such as long strain optimization cycles, high requirements for strains, and low production efficiency, making it difficult to meet the needs of large-scale industrial production of basic pyrazine.

[0022] The chemical synthesis method has the following specific implementation methods: Nitro compound reduction method: Using nitrobenzene and other similar raw materials, pyrazine rings are constructed through reduction and deoxygenation under high temperature and pressure. This method has harsh reaction conditions, produces many byproducts (such as amine compounds) that are difficult to separate and purify, and has poor environmental performance. It is mainly limited to small-scale laboratory applications and has low industrial application value.

[0023] The Gutknecht pyrazine synthesis method uses expensive α-diketones or α-diamines as raw materials, requiring a strong base dehydrogenation process to form a 6H-pyrazine intermediate and subsequent oxidation steps. This method is lengthy, has low yields, and demands harsh reaction conditions (requiring strong acids and bases). It is mainly used for the synthesis of specific substituted pyrazine derivatives, and is costly and lacks versatility. Although improved methods have attempted to synthesize pyrazines using inexpensive and readily available ethylene glycol and ethylenediamine under oxidative catalysts, improving the availability of raw materials and yields, they are still limited by excessively high reaction temperatures, making large-scale industrial production difficult.

[0024] Specific derivative synthesis methods: For example, patent CN113058638A reported the synthesis of 2,5-dimethylpyrazine by gas-solid phase reaction using isopropanolamine as a raw material on a copper-zinc catalyst supported by ZSM-5 molecular sieve; patents CN103949266A and CN106582672A reported methods for synthesizing 2,5-dimethylpyrazine, but these methods either rely on complex catalyst preparation or use environmentally unfriendly heavy metals (such as chromium) as catalyst components, which are insufficient in terms of environmental protection and universality.

[0025] In summary, existing pyrazine synthesis technologies generally suffer from the following key problems: Raw material issues: Some methods rely on expensive (such as α-diketones) or structurally complex raw materials, or specific derivative raw materials (such as isopropanolamines), which limits the general synthesis of basic pyrazines.

[0026] Reaction conditions: It generally requires harsh conditions such as high temperature and high pressure, strong acid and strong alkali, which places high demands on equipment and consumes a lot of energy, and poses safety hazards.

[0027] Environmental and health risks: Some methods generate large amounts of byproducts that are difficult to handle, or use / generate toxic and harmful substances (such as heavy metal chromium catalysts, highly corrosive acids and alkalis), posing a high risk of environmental pollution and not in line with the trend of green chemistry development.

[0028] Complex process and yield issues: The long route, numerous steps, and low overall yield increase production costs.

[0029] The aforementioned problems (especially high temperature, harsh conditions, catalyst complexity, and environmental pressure) collectively make it difficult for existing methods to achieve efficient, economical, and green large-scale industrial production of basic pyrazines.

[0030] This application provides a method for synthesizing pyrazine, comprising: passing monoethanolamine and water into a reactor packed with a Cu-Ni-based composite catalyst, and causing the monoethanolamine to undergo a cyclization reaction under conditions of 0-1 MPa and 250-350°C to convert it into the target product pyrazine; wherein the Cu-Ni-based composite catalyst comprises: a support, CuO, NiO supported on the support, and an auxiliary agent.

[0031] It should be noted that the above-mentioned reactor is a fixed-bed reactor, and this embodiment of the application uses a fixed-bed reactor as an example for illustration.

[0032] It should be noted that the molar ratio of water to monoethanolamine mentioned above is (2~20):1. For example, the molar ratio of water to monoethanolamine can be any one or any two of the following: 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, 20:1. When water molecules are present in excess, a gaseous proton solvent environment is first formed, and the water molecules dissociate at high temperature to produce H3O. + and OH - Ion pairs are used to construct a dynamic proton transfer network. Excess water molecules surround the Ni sites of the catalyst, compressing the local concentration of the imine intermediate through a solvation effect, inhibiting the self-condensation of the imine intermediate to form trimers, and improving the selectivity of the monoethanolamine cyclization product pyrazine. When the molar ratio of water to ethanolamine is 2-5:1, water mainly acts as a reaction orienting agent, precisely neutralizing strong acid sites on the support surface to suppress the formation of side reactions. At the same time, water forms a subcritical fluid with a diffusion coefficient matching that of monoethanolamine, eliminating the microporous diffusion gradient. When the molar ratio is increased to 10-20:1, water becomes a thermal engineering medium. Superheated steam forms supercritical microcavities within the mesopores of the catalyst, compressing the local concentration of monoethanolamine, increasing the molecular collision frequency, and enhancing the conversion rate of monoethanolamine.

[0033] Specifically, monoethanolamine and water are passed into a reactor packed with a Cu-Ni-based composite catalyst, causing the monoethanolamine to undergo a cyclization reaction under conditions of 0–1 MPa and 250–350 °C, converting it into the target product, pyrazine. The catalyst, through the synergistic effect of CuO and NiO, precisely activates the CN and CO bonds in the monoethanolamine molecule, directly guiding the formation of the pyrazine ring and significantly reducing the formation of byproducts such as 2-methylpyrazine and piperazine. Monoethanolamine is the sole raw material, and the reaction byproducts are only water and ammonia, requiring no additional solvents or additives, thus conforming to the principles of green synthesis. Furthermore, the pyrazine synthesis is completed in a single step, eliminating the need for multi-step reactions or intermediate separation.

[0034] Specifically, the Cu-Ni based composite catalyst, after activation, converts CuO and NiO into Cu and Ni. Monoethanolamine HOCH2CH2NH2 is adsorbed at the Cu site, and the hydroxyl group dehydrogenates to form a C=O bond, generating aminoacetaldehyde H2N-CH2-CHO. The aldehyde group in aminoacetaldehyde is an electrophilic center, and the amino group nucleophilically attacks the carbonyl carbon, further dehydrating to generate the imine intermediate H2C=CH-NH. The imine intermediate is extremely unstable and further dehydrates at the Ni site to form stable aminoethylene, providing the diene H2C=CH-NH for subsequent cyclization reactions. A six-membered ring dihydropyrazine is formed, releasing two molecules of H2. Under the synergistic effect of Cu and Ni, the dihydropyrazine loses two molecules of H2 and aromatizes to generate a stable pyrazine.

[0035] In this application, the addition of water plays a crucial role. When monoethanolamine and water are passed into a reactor packed with a Cu-Ni-based composite catalyst, water molecules first associate with the hydroxyl and amino groups of monoethanolamine through a hydrogen bond network, reducing intermolecular forces and making it easier for the catalyst to adsorb and activate on the catalyst surface. On the one hand, pyrazine is readily soluble in water, and water vapor accelerates the removal of the product from the catalyst surface through stripping, reducing secondary reactions. Water, as a proton transfer medium, directly participates in the dehydrogenation step. At the Cu active sites of the catalyst, the α-CH bond of monoethanolamine breaks to generate hydrogen atoms, and water molecules capture the active hydrogen to form the intermediate H3O. + The dehydrogenation kinetics are significantly accelerated by subsequent dehydration to generate gaseous H2. Water adsorbs on the strong Lewis acid sites of the support, inhibiting the occurrence of catalytic side reactions at these sites and directing the reaction toward the imine intermediate pathway. On the other hand, water vapor reacts with carbon on the catalyst surface via a water-gas shift reaction (C + H2O → H2), reducing the low-carbon coverage; the heat of vaporization of water can also absorb the heat of reaction, preventing local overheating that could lead to catalyst sintering; and the in-situ steam reforming continuously removes soft coke precursors from the catalyst surface, increasing the catalyst's lifespan.

[0036] In this application, the Cu-Ni-based composite catalyst uses a porous support as a matrix. A heterogeneous catalytic system is constructed by loading the main active component CuO and the synergistic active component NiO, and adding composite promoters. This achieves the efficient and directional conversion of ethanolamine to pyrazine, significantly reducing byproducts such as 2-methylpyrazine and piperazine. The promoters optimize the surface acidity and alkalinity of the support, suppressing side reactions such as the cracking of monoethanolamine to generate small molecule amines or the condensation to generate piperazine compounds, while further improving the dispersion and stability of the active components within the catalyst.

[0037] It should be noted that the cyclization reaction of monoethanolamine is carried out under conditions of 0–1 MPa and 250–350 °C. For example, the reaction pressure can be any combination of 0 MPa, 0.1 MPa, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa, and 1 MPa. At a reaction pressure greater than or equal to 0 MPa, the reaction can be achieved through continuous gas-phase flow under normal pressure, resulting in low equipment cost. At a reaction pressure less than or equal to 1 MPa, the partial pressure of monoethanolamine can be increased, accelerating the molecular collision frequency and improving the conversion rate of monoethanolamine, while avoiding increased equipment complexity and safety hazards caused by excessively high pressure. For example, the reaction temperature can be any one or a combination of 250℃, 255℃, 260℃, 265℃, 270℃, 275℃, 280℃, 285℃, 290℃, 295℃, 300℃, 305℃, 310℃, 315℃, 320℃, 325℃, 330℃, 335℃, 340℃, 345℃, and 350℃. A reaction temperature greater than or equal to 250℃ avoids incomplete reactions due to insufficient temperature, and at this temperature, the catalyst can effectively catalyze the dehydrogenation step to form the imine intermediate. A reaction temperature less than or equal to 350℃ avoids a significant increase in side reactions due to excessively high temperatures.

[0038] In one specific embodiment, the feed space velocity of the monoethanolamine is 0.4–1.0 h⁻¹. -1 The feed space velocity of the water is 0.1~0.3 h⁻¹. -1 .

[0039] It should be noted that liquid hourly space velocity (LHSV) refers to the volume of liquid feed passing through a unit volume of catalyst per unit time (unit: h). −1 The space velocity directly determines the residence time of monoethanolamine and water in the catalyst bed; the space velocity is inversely proportional to the residence time of the reactants in the catalyst bed.

[0040] The feed space velocity for monoethanolamine is 0.4–1.0 h⁻¹. -1 For example, the feed space velocity for monoethanolamine can be 0.4 h⁻¹. -1 0.5h -1 0.6h -1 0.7h -1 0.8h -1 0.9h -1 1.0h -1 The range of one or both of these values. The feed space velocity of water is 0.1~0.3 h⁻¹. -1 For example, the feed space velocity of water can be 0.10 h⁻¹. -1 0.12h -1 0.15h -1 0.18h-1 0.20h -1 0.22h -1 0.25h -1 0.28h -1 0.30h -1 The range of one or both of these values. During mixing, monoethanolamine and water enter the mixer at different space velocities. When monoethanolamine is injected into the mixer at a higher space velocity, its molecular kinetic energy is sufficient to overcome the gas-liquid interfacial tension and preferentially disperse into droplets, while water, when input at a lower space velocity, forms a continuous phase coating layer, resulting in stepwise activation during the preheating stage.

[0041] In this embodiment, the feed space velocity of monoethanolamine is controlled at 0.4–1.0 h⁻¹. -1 The feed air velocity of water is controlled at 0.1~0.3 h⁻¹. -1 The synergistic regulation of monoethanolamine space velocity and water space velocity is beneficial to improving the raw material utilization and conversion rate of monoethanolamine, avoiding over-reaction, and suppressing side reactions, which facilitates the improvement of pyrazine selectivity.

[0042] In one specific embodiment, the monoethanolamine and water are preheated in a preheater before being introduced into the reactor for reaction; The preheating temperature is 250℃~300℃.

[0043] It should be noted that the preheating temperature is 250℃~300℃. For example, the preheating temperature can be any one or both of the following: 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, 310℃, 320℃, 330℃, 340℃, and 350℃. Within the preheating range of 250℃~300℃, the mixture of monoethanolamine and water undergoes three stages: hydrogen bond reconstruction and cleavage, β-dehydrogenation pre-activation, and supercritical phase transition. In the hydrogen bond reconstruction and cleavage stage, the OH···N hydrogen bonds between monoethanolamine molecules break at temperatures above 250℃, forming a low-energy associative complex HO-CH2-CH2-NH2···HOH with water. This recombination enhances the polarity of the C2-H bond and reduces its bond energy. During the β-dehydrogenation pre-activation stage, thermal energy is focused on the β-carbon site of monoethanolamine, triggering partial dehydrogenation to generate the enamine intermediate HO-CH=CH-NH2. In the supercritical phase transition stage, the water-ethanolamine mixture reaches a supercritical state, its viscosity decreases, and its diffusion coefficient increases, achieving molecular-level diffusion which is beneficial for subsequent contact with the catalyst and subsequent reaction.

[0044] In this embodiment, the preheating temperature is greater than or equal to 250°C to ensure that the CO bond of monoethanolamine is not nucleophilically attacked by water molecules to generate acetaldehyde and ammonia. The preheating temperature is less than or equal to 300°C to avoid the activation energy of the C-C bond cleavage of monoethanolamine being thermally excited at temperatures above 300°C, further avoiding the generation of byproducts such as methane and ethylene; at the same time, it avoids the runaway release of lattice oxygen from the catalyst promoters, which could cause irreversible collapse of oxygen vacancies and thus affect the catalyst's lifespan.

[0045] In one specific embodiment, the additive is selected from at least one of ZrO2, Ce2O3, CeO2, La2O3, MgO, CaO, MnO, and BaO; the support is selected from at least one of Al2O3, SiO2, and carbon black.

[0046] It should be noted that when the auxiliary agent is selected from at least one of ZrO2, Ce2O3, CeO2, La2O3, MgO, CaO, MnO, and BaO, the structure of the active component can be optimized, selectivity can be regulated, and resistance to deactivation can be enhanced. Specifically, Ce2O3 and CeO2 can be activated through Ce... 4+ / Ce 3+ The oxidation state cycle generates oxygen vacancies, accelerating the dehydrogenation of monoethanolamine to aminoacetaldehyde, while simultaneously adsorbing H2 generated in the reaction to inhibit carbon deposition. ZrO2 anchors Cu and Ni through lattice defects generated by the tetragonal-monoclinic phase transition, preventing the sintering of active components in the catalyst. La2O3 forms a high-temperature barrier, increasing the sintering temperature of the catalyst and extending its service life. MgO accelerates the cyclization reaction by optimizing the adsorption configuration of imine intermediates, while inhibiting the formation of byproducts. The strong basicity of CaO and BaO neutralizes strong acid sites on the catalyst surface, reducing excessive dehydration of monoethanolamine to olefins and improving pyrazine selectivity. MnO, through Mn... 3+ / Mn 4+ The valence cycle accelerates the dehydrogenation aromatization of dihydropyrazine while inhibiting the polymerization of aminoethylene to form piperazine.

[0047] It should be noted that the support in the Cu-Ni based composite catalyst can support the active component, regulate interfacial interactions, and optimize mass / heat transfer. In the embodiments of this application, one or more of Al2O3, SiO2, and carbon black are selected as the support, which can suppress deep side reactions and improve pyrazine selectivity in the monoethanolamine and water cyclization reactions.

[0048] In this embodiment, the support, CuO, NiO, and auxiliary metals in the Cu-Ni based composite catalyst work synergistically to determine the catalyst's activity, selectivity, and stability. When the support is selected from one or more of Al2O3, SiO2, and carbon black, and the auxiliary is selected from one or more of ZrO2, Ce2O3, CeO2, La2O3, MgO, CaO, MnO, and BaO, high activity, high selectivity, and long lifetime of the Cu-Ni based composite catalyst in monoethanolamine and water cyclization reactions can be achieved.

[0049] In one specific embodiment, in the Cu-Ni based composite catalyst, the mass percentage of the support is 25-60%, the mass percentage of CuO is 12-30%, the mass percentage of NiO is 8-20%, and the mass percentage of the metal in the additive is 20-25%.

[0050] It should be noted that in Cu-Ni based composite catalysts, the mass percentage of the support is 25% to 60%. For example, the mass percentage of the support can be any one or any two of the following: 25%, 30%, 35%, 40%, 45%, 50%, and 60%. Controlling the mass percentage of the support at 25% to 60% optimizes the specific surface area, pore volume, and chemical interactions of the support, while balancing dispersibility and mass transfer efficiency.

[0051] It should be noted that the mass percentage of CuO is 12% to 30%, for example, it can be any one or two of 12%, 15%, 20%, 25%, and 30%. A CuO mass percentage greater than or equal to 12% ensures the formation of a continuous Cu active phase on the support surface, providing sufficient dehydrogenation sites to meet the minimum activation requirement for the breaking of the CO bond in monoethanolamine molecules. A CuO mass percentage less than or equal to 30% is limited by the maximum monolayer dispersion capacity of the support; when it exceeds 30%, CuO particles agglomerate, reducing the specific surface area of ​​the catalyst and thus reducing the catalytic efficiency.

[0052] It should be noted that the mass percentage of NiO is 8-20%, for example, it can be any one or two of the following values: 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, and 20%. A NiO mass percentage greater than or equal to 8% ensures sufficient Ni sites to catalyze the intramolecular condensation of the imine intermediate with another molecule of hydroxyacetaldehyde, forming a dihydropyrazine ring structure. A NiO mass percentage less than or equal to 20% avoids excessive NiO covering the active sites of CuO, disrupting the dehydrogenation-cyclization synergy in the monoethanolamine cyclization reaction, and when the NiO content is greater than 20%, it promotes CC cracking, leading to catalyst coking and affecting the catalyst's catalytic efficiency.

[0053] It should be noted that the mass percentage of metal in the additive is 20-25%, for example, it can be one or any two of the following: 20%, 21%, 22%, 23%, 24%, 25%. A metal mass percentage of 20% or more ensures complete coverage of the support by the additive, avoiding the inability to effectively control the catalyst surface properties. A metal mass percentage of 25% or less avoids increased mass transfer resistance of reactants on the catalyst, further preventing a decrease in the catalyst's space velocity tolerance.

[0054] In one specific embodiment, the method for synthesizing pyrazine further includes: introducing hydrogen gas into a reactor to allow the cyclization reaction to proceed in a hydrogen-containing environment; the hydrogen feed space velocity is 50-500 h⁻¹. -1 .

[0055] It should be noted that the hydrogen feed space velocity is 50~500 h⁻¹. -1 For example, the feed space velocity for hydrogen can be 50 h⁻¹. -1 80h -1 100h -1 150h -1 170h -1 200h -1 220h -1 260h -1 280h -1 300h -1 350h -1 400h -1 450h -1 500h -1 The range of one or any two of them.

[0056] In this embodiment, monoethanolamine can undergo side reactions involving the elimination of hydroxyl and β-H groups to produce imine and water. The hydrogen feed space velocity is controlled at 50-500 h⁻¹. -1 This allows an active hydrogen atom layer to form on the catalyst surface, occupying both acidic and basic sites and further blocking the β-hydrogen elimination pathway. Furthermore, the N and O in monoethanolamine readily coordinate with metals, leading to cracking and dehydrogenation to form tar and carbon deposits. Introducing hydrogen gas blocks the cracking reaction, thereby eliminating carbon deposits and extending the catalyst's lifespan.

[0057] In one specific embodiment, before introducing monoethanolamine and water into the reactor packed with Cu-Ni-based composite catalyst, the method further includes: activating the Cu-Ni-based composite catalyst, including: introducing a mixture of hydrogen and nitrogen into the reactor packed with Cu-Ni-based composite catalyst, and activating it at 150~200°C for 1h~10h.

[0058] It should be noted that the activation temperature is 150~200℃. For example, the activation temperature can be any one or any two of 150℃, 160℃, 170℃, 180℃, 190℃, and 200℃. The activation time is 1~10h. For example, it can be any one or any two of 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, and 10h.

[0059] In this embodiment, the catalyst is activated at 150-200°C for 1-10 hours. CuO and NiO can be reduced to catalytically active metallic copper and nickel under the action of hydrogen. During the reduction process, the adsorption-dissociation of H2 can induce moderate migration and recombination of metal particles, increasing the number of active sites and dispersion uniformity, and avoiding sintering and agglomeration. At the same time, nitrogen can reduce the partial pressure of hydrogen, slow down the violent reduction reaction, and avoid local overheating that leads to catalyst sintering and deactivation. Nitrogen can also increase the heat transfer efficiency of gas flow, making the catalyst temperature uniform. In addition, nitrogen can form an inert atmosphere to prevent hydrogen from exploding when heated and to prevent secondary oxidation or carbon deposition of metals.

[0060] In one specific embodiment, the volume content of hydrogen in the mixture of hydrogen and nitrogen is 1-20%.

[0061] For example, it can be a range of one or both of the following: 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%.

[0062] In this embodiment, the volume fraction of hydrogen is greater than or equal to 1% to ensure sufficient H2 is provided to initiate the surface reduction reaction of CuO and NiO, while also ensuring effective diffusion of hydrogen molecules within the catalyst channels. The volume fraction of hydrogen is less than or equal to 20% to limit the increase in Cu atom migration rate induced by high concentrations of H2, thereby further suppressing Cu sintering.

[0063] In one specific embodiment, when the Cu-Ni composite catalyst is deactivated or its activity decreases, regeneration gas is introduced into the reactor to perform in-situ regeneration treatment of the Cu-Ni composite catalyst. The feed space velocity of the regenerated gas is 50 h⁻¹. -1 ~1000h -1 ; The regenerated gas is air or nitrogen containing oxygen, wherein the oxygen in the nitrogen contains oxygen has a volume percentage of 0.1% to 10%. The regeneration process is carried out at a temperature of 200℃ to 400℃, with a heating rate of 10℃ / h to 20℃ / h and a processing time of 2h to 20h.

[0064] It should be noted that the deactivation or reduced activity of Cu-Ni composite catalysts is achieved by monitoring the conversion rate of monoethanolamine. For example, at the beginning of the reaction, the conversion rate of monoethanolamine is at a high level. Using this conversion rate as a benchmark, when the conversion rate decreases or decreases significantly, it can be determined that the activity of Cu-Ni composite catalysts has decreased or deactivated.

[0065] It should be noted that the feed space velocity of the regenerated gas is 50 h⁻¹. -1 ~1000h -1 For example, the feed air velocity can be 50 h. -1 100h -1 150h -1 200h -1 300h -1 500h -1 800h -1 1000h -1 The range of one or any two of them.

[0066] It should be noted that the volume percentage of oxygen is 0.1% to 10%; for example, the volume percentage of oxygen can be any value within the range of 0.1%, 0.2%, 0.5%, 0.8%, 1.0%, 2.0%, 3.0%, 4.0%, 5.0%, 6.0%, 7.0%, 8.0%, 9.0%, and 10%, or any combination thereof. This can slow down the oxidation rate and prevent localized overheating. An oxygen volume percentage greater than or equal to 0.1% ensures that the oxidation reaction on the carbon deposits can be initiated and that oxygen penetrates deep into the micropores of the Cu-Ni-based composite catalyst, preventing the outer surface of the Cu-Ni-based composite catalyst from being oxidized while carbon deposits remain in the core. An oxygen volume percentage less than or equal to 10% avoids excessive heat release per unit time, which could lead to a rapid increase in the regeneration reaction temperature, causing high-temperature migration of Cu and Ni ions and reducing the catalytic activity of the Cu-Ni-based composite catalyst.

[0067] It should be noted that the regeneration temperature is 200℃~400℃. For example, the regeneration temperature can be any value within the range of 200℃, 220℃, 250℃, 280℃, 300℃, 330℃, 350℃, 370℃, and 400℃. During the reaction process, coke or other non-reactive substances are easily deposited on the catalyst surface, covering the active sites. A regeneration temperature of 200℃~400℃ can achieve the removal of impurities by oxidation (air / oxygen) and hydrogen reduction, restoring the active surface.

[0068] It should be noted that the heating rate is 10℃ / h to 20℃ / h. For example, it can be one or any two of the following values: 10℃ / h, 12℃ / h, 14℃ / h, 15℃ / h, 17℃ / h, 19℃ / h, and 20℃ / h. This is to avoid local overheating that could cause "hot spots" and prevent copper and nickel particles from agglomerating due to thermal stress.

[0069] It should be noted that the processing time is 2h to 20h. For example, it can be one or any two of the following values: 2h, 3h, 5h, 8h, 10h, 12h, 15h, 18h, and 20h, to ensure complete combustion of carbon deposits.

[0070] In this embodiment, when the Cu-Ni-based composite catalyst catalyzes the cyclization reaction of monoethanolamine, carbon deposits, mainly composed of adsorbed hydrocarbons, graphitic carbon, and metal carbides, are formed on the surface of the Cu-Ni-based composite catalyst. The carbon deposits react completely with oxygen, eliminating the carbon deposits on the surface of the Cu-Ni-based composite catalyst, extending its service life, and preventing the formation of oxygen-containing intermediates that clog the pores of the catalyst during low-temperature oxidation. Simultaneously, O2 induces the oxidation of Cu and Ni metals above 200°C, eliminating dislocations and lattice distortions. Regeneration and reduction treatment can effectively recover and reuse the catalyst, reducing the generation of waste liquid and waste gas, and mitigating environmental pollution. Strengthening equipment maintenance and management ensures the long-term stable operation of the fixed-bed reactor, enabling continuous production.

[0071] In one specific embodiment, the yield of the pyrazine is greater than 40% based on monoethanolamine.

[0072] In this embodiment, the pyrazine yield is greater than 40%, the product has high purity, and the economic benefits are good.

[0073] Figure 1 A schematic diagram of the pyrazine synthesis system provided in an embodiment of this application is shown. (Refer to...) Figure 1 As shown, the synthesis system includes a monoethanolamine feed pump 1, a water feed pump 2, a mixed gas inlet 3, a reactor 4, a condenser 5, a collector 6, and a preheater 7.

[0074] In practice, a Cu-Ni-based composite catalyst is loaded into reactor 4. After activating the catalyst by introducing a mixed gas of hydrogen and nitrogen through the mixed gas inlet 3, monoethanolamine feedstock and water are introduced into preheater 7 via monoethanolamine feed pump 1 and water feed pump 2, respectively. The preheated and vaporized monoethanolamine and water vapor mixture in preheater 7 is then piped into the reactor to react with the activated Cu-Ni-based composite catalyst. During the reaction, hydrogen is continuously introduced into reactor 4 through the mixed gas inlet 3. The reaction products are condensed in condenser 5 and collected in collector 6. This process, through the synergistic design of Cu-Ni bimetallic activation and regeneration with reaction-separation, achieves efficient and continuous production of pyrazine from monoethanolamine.

[0075] Example 1 (1) Activation of Cu-Ni based composite catalyst A Cu-Ni based composite catalyst was packed into reactor 4, with a packing volume of 15 cm³. 3 The catalyst uses CuO (30% by mass) and NiO (10% by mass) as the active components, with CaO and MgO added as promoters (the total mass percentage of metal ions in the promoters is 20%), and SiO2 as the support (40% by mass). A mixture of hydrogen and nitrogen is introduced through the mixed gas inlet 3 to activate the Cu-Ni based composite catalyst. The activation conditions are: nitrogen flow rate of 250 mL / min, hydrogen flow rate of 10 mL / min, and the volume hourly space velocity (VHSV) of the hydrogen and nitrogen mixture of 100 h⁻¹. -1 The reduction temperature is 180℃ and the reduction time is 10h.

[0076] (2) Synthesis of pyrazine After reduction, fresh monoethanolamine feedstock and water enter preheater 7 via monoethanolamine feed pump 1 and water feed pump 2, respectively. The molar ratio of water to monoethanolamine is 5:1, and the feed space velocity for monoethanolamine is 0.6 h⁻¹. -1 The feed space velocity of water is 0.15 h⁻¹. -1 The monoethanolamine and water vapor mixture, preheated to 260℃ in preheater 7, enters reactor 4 via pipeline and reacts with the activated Cu-Ni-based composite catalyst. During the reaction, H2 is continuously introduced into reactor 4 through mixed gas inlet 3 at a space velocity of 50 h⁻¹. -1 The reaction temperature was 290℃, the pressure was set at 0.5MPa, and the reaction time was 5 hours. The reaction products were condensed by condenser 5 and collected in collector 6.

[0077] The liquid product from Example 1 was subjected to gas chromatography analysis, such as... Figure 2As shown, the product peaks eluted first according to their boiling points, with the lowest boiling points eluting first. Specifically, the dilution solvent peak eluted first at 2.0 min, followed by monoethanolamine at 2.5 min, and the target product pyrazine at 2.9 min. The reaction results indicate a monoethanolamine conversion rate of 58%, and a pyrazine yield of 50% based on monoethanolamine. Furthermore, as shown in the figure, almost no byproducts were detected.

[0078] Example 2 The difference between Example 2 and Example 1 is as follows: In the activation step of (1) Cu-Ni based composite catalyst, the composition of Cu-Ni based composite catalyst was adjusted to 40% Al2O3 support, 20% CuO, 20% NiO, and a total of 20% CaO and MgO additives; the reduction temperature was adjusted to 150℃.

[0079] In the synthesis step of (2) pyrazine, the reaction temperature, the feed space velocity of monoethanolamine, the feed space velocity of water, and the preheating temperature were adjusted to 270℃ and 0.4h, respectively. -1 0.1h -1 280℃.

[0080] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0081] The liquid product from Example 2 was subjected to gas chromatography analysis, such as... Figure 3 As shown, the product peaks eluted first according to their boiling points, with the lowest boiling points eluting first. Specifically, the dilution solvent peak eluted first at 2.0 min, followed by monoethanolamine at 2.5 min, and the target product pyrazine at 2.8 min. The reaction results indicate a monoethanolamine conversion rate of 45%, and a pyrazine yield of 41% based on monoethanolamine. Furthermore, the graph shows that almost no byproducts were detected.

[0082] Example 3 The difference between Example 3 and Example 1 is as follows: In the activation step of (1) Cu-Ni based composite catalyst, the composition of Cu-Ni based composite catalyst was adjusted to 50% SiO2 support, 20% CuO, 8% NiO, and a total of 22% CaO and MgO promoters; the flow rates of nitrogen and hydrogen were adjusted to 300 mL / min and 20 mL / min, respectively; the reduction temperature was adjusted to 150℃ and the reduction time was adjusted to 8h.

[0083] In the synthesis step of (2) pyrazine, the reaction temperature and preheating temperature were adjusted to 320℃ and 300℃, respectively.

[0084] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0085] The liquid product from Example 3 was subjected to gas chromatography analysis, such as... Figure 4 As shown, the product peaks eluted first according to their boiling points, with the lowest boiling points eluting first. Specifically, the dilution solvent peak eluted first at 2.0 min, followed by monoethanolamine at 2.5 min, and the target product pyrazine at 2.8 min. The reaction results indicate a monoethanolamine conversion rate of 72%, and a pyrazine yield of 69% based on monoethanolamine. Furthermore, the graph shows that almost no byproducts were detected.

[0086] Example 4 The difference between Example 4 and Example 1 is as follows: In the synthesis step of (2) pyrazine, the molar ratio of water to monoethanolamine is adjusted to 20:1.

[0087] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0088] Example 5 The difference between Example 5 and Example 1 is: In the synthesis step of (2) pyrazine, the molar ratio of water to monoethanolamine is adjusted to 2:1.

[0089] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0090] Example 6 The difference between Example 6 and Example 1 is: In the activation step of (1) Cu-Ni based composite catalyst, the composition of Cu-Ni based composite catalyst is adjusted to a total of 20% Ce2O3 and CeO2 promoters.

[0091] In the synthesis step of (2) pyrazine, the reaction pressure is adjusted to 0.

[0092] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0093] Example 7 The difference between Example 7 and Example 1 is as follows: In the activation step of (1) Cu-Ni based composite catalyst, the composition of Cu-Ni based composite catalyst is adjusted to a total of 20% ZrO2 and MgO.

[0094] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0095] Example 8 The difference between Example 8 and Example 1 is: In the synthesis step of (2) pyrazine, the feed space velocity of monoethanolamine and the feed space velocity of water were adjusted to 1.0 h⁻¹. -1 0.3h -1 .

[0096] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0097] Examples 9-11 The difference between Examples 9-11 and Example 1 is that: In the synthesis step of (2) pyrazine, the reaction temperature was adjusted to 250℃, 300℃ and 350℃ respectively.

[0098] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0099] Example 12 In the activation step of (1) Cu-Ni based composite catalyst, the reduction temperature was adjusted to 200℃, and the flow rates of nitrogen and hydrogen were adjusted to 100mL / min and 20mL / min, respectively.

[0100] In the synthesis step of pyrazine (2), the hydrogen feed space velocity was adjusted to 200 h⁻¹. -1 .

[0101] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0102] The gas chromatograms of Examples 4-12 are similar to those of Examples 1-3, and will not be shown again here.

[0103] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is as follows: In the activation step of (1) Cu-Ni based composite catalyst, the composition of the catalyst is adjusted to 55% CuO, 44% ZnO, and 1% ZMS-5 molecular sieve.

[0104] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0105] The liquid product from Comparative Example 1 was subjected to gas chromatography analysis, such as... Figure 5 As shown, the dilution solvent peak first appears at 2.0 min, followed by monoethanolamine at 2.5 min. The target product, pyrazine, peaks at 2.8 min, while the byproducts 2-methylpyrazine and piperazine peak at 3.6 min and 4.6 min, respectively. The reaction results indicate a monoethanolamine conversion rate of 66%, and a pyrazine yield of 30% based on monoethanolamine. Furthermore, the graph shows significant impurity peaks, indicating a large amount of byproducts and poor product quality. Additionally, a significant amount of residual monoethanolamine further reduces the overall conversion rate.

[0106] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is as follows: In the synthesis step of pyrazine (2), the feed space velocity of water was adjusted to 0.05 h⁻¹. -1 .

[0107] The other steps and dosages are the same as in Example 1, and the target product pyrazine is obtained.

[0108] The chromatogram of Comparative Example 2 was similar to that of Comparative Example 1. The reaction results showed that the conversion rate of monoethanolamine was 60%, and the yield of pyrazine was 24% based on monoethanolamine. Other byproducts such as 2-methylpyrazine, 2-ethylpyrazine, and piperazine were also present, resulting in poor quality of the pyrazine product.

[0109] The monoethanolamine conversion rate and pyrazine yield of each embodiment are shown in Table 1.

[0110] Table 1

[0111] Table 1 shows that comparing Examples 1 and 2, the conversion rate of monoethanolamine and the yield of pyrazine in Example 1 (SiO2 support) are about 10% higher than those in Example 2 (Al2O3 support). Therefore, the Cu-Ni composite catalyst with SiO2 support exhibits superior catalytic performance in pyrazine synthesis compared to the Al2O3 support. Comparing Examples 1-3, when the NiO content in the Cu-Ni composite catalyst is around 10%, the conversion rate of monoethanolamine and the yield of pyrazine are relatively high. When the NiO content is 20%, the conversion rate of monoethanolamine is less than 50%, and the yield of pyrazine is less than 45%. This demonstrates that an appropriate NiO content in the Cu-Ni composite catalyst has a significant impact on directly guiding the formation of the pyrazine ring and significantly reducing the formation of byproducts such as 2-methylpyrazine and piperazine.

[0112] Comparing Examples 1, 4, and 5, the molar ratio of water to monoethanolamine was different. When the molar ratio of water to monoethanolamine was too large (e.g., 20:1 in Example 4) or too small (e.g., 2:1 in Example 5), the yield of pyrazine decreased. Therefore, by controlling the molar ratio of water to monoethanolamine to be between 2:1 and 20:1, the yield of pyrazine was higher.

[0113] Comparing Examples 1 and 9-10, it can be seen that the yield of pyrazine gradually increases with increasing reaction temperature. This shows that reaction temperature increases the efficiency of pyrazine synthesis catalyzed by the catalyst. When the reaction temperature is between 250 and 350°C, a suitable temperature can avoid a significant increase in side reactions caused by excessively high temperatures, while also reducing equipment energy consumption.

[0114] Example 1 differs from Comparative Example 1 only in the type of catalyst. Comparative Example 1 used CuO, ZnO, and molecular sieves without any additives; Example 1 used SiO2 support, CuO, and NiO. In addition to the additives CaO and MgO, the experimental results showed that the pyrazine yield in Comparative Example 1 was only 30% and had a lot of byproducts, while the pyrazine yield in Example 1 was 50% and had almost no byproducts. Therefore, the catalytic effect of NiO and the additives in the catalyst for the synthesis of pyrazine was significantly better than that in Comparative Example 1.

[0115] The only difference between Example 1 and Comparative Example 2 is the water feed space velocity. In Example 1 and Comparative Example 2, the water feed space velocities are 0.15 h⁻¹ and 0.15 h⁻¹, respectively. -1 0.05h -1 The yields of pyrazine were 50% and 24%, respectively. Example 1 showed almost no byproducts, while Comparative Example 2 showed more byproducts. This demonstrates that the appropriate water content in the pyrazine synthesis process is crucial; the feed space velocity of water should be below 0.1 h⁻¹. -1 The yield of synthesized pyrazine decreased significantly, which had a substantial impact on the catalyst.

[0116] Cu-Ni based composite catalyst regeneration treatment The conversion rate of monoethanolamine in each embodiment was monitored. When the conversion rate decreased or decreased significantly, it indicated that the activity of the Cu-Ni-based composite catalyst in each embodiment was reduced or deactivated. In this case, the Cu-Ni-based composite catalyst was regenerated in situ as follows: Nitrogen regeneration gas containing 8% oxygen was introduced into a reactor packed with Cu-Ni-based composite catalyst. The feed space velocity of the regeneration gas was 200 h⁻¹. -1 The Cu-Ni-based composite catalyst was heated to 280℃ at a heating rate of 12℃ / h to regenerate it in situ for 10h. After cooling to room temperature, the regenerated Cu-Ni-based composite catalyst was used to catalyze the cyclization reaction of monoethanolamine and water.

[0117] In this embodiment, monoethanolamine and water are passed into a reactor packed with a Cu-Ni-based composite catalyst, causing the monoethanolamine to undergo a cyclization reaction under conditions of 0–1 MPa and 250–350 °C, converting it into the target product pyrazine. This method abandons the traditional multi-step dehydrogenation and condensation route, using a single raw material, ethanolamine, to generate pyrazine, significantly shortening the process and reducing production costs. The use of a fixed-bed reactor for continuous production results in high efficiency and stable product quality, making it more suitable for large-scale continuous production compared to the batch production methods in existing technologies. Furthermore, the reaction can be carried out at atmospheric pressure and 250–350 °C, avoiding the need for high-pressure equipment and ensuring high operational safety. Compared to existing technologies, the synthesis temperature is controlled below 350 °C, resulting in high synthesis efficiency. The reaction raw materials only require the addition of ethanolamine, a nitrogen / hydrogen mixture, and water, without the addition of external solvents or additives, resulting in a single raw material and good atom economy. Finally, the experimental results obtained in this application show only the single product of pyrazine and unreacted raw materials, with no other by-products detected, demonstrating a single product and high production efficiency.

[0118] The above-described embodiments are merely preferred embodiments of this application and are not intended to limit this application in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this application without departing from the content of the technical solution of this application shall still fall within the protection scope of the technical solution of this application.

Claims

1. A method for synthesizing pyrazine, characterized in that, include: Monoethanolamine and water are passed into a reactor packed with a Cu-Ni-based composite catalyst, and the monoethanolamine undergoes a cyclization reaction under conditions of 0-1 MPa and 250-350 °C to be converted into the target product pyrazine. The Cu-Ni based composite catalyst comprises: a support, CuO, NiO supported on the support, and an additive.

2. The method for synthesizing pyrazine according to claim 1, characterized in that, The feed space velocity of the monoethanolamine is 0.4–1.0 h⁻¹. -1 The feed space velocity of the water is 0.1~0.3 h⁻¹. -1 .

3. The method for synthesizing pyrazine according to claim 1, characterized in that, The monoethanolamine and water are preheated in a preheater before being introduced into the reactor for reaction. The preheating temperature is 250℃~300℃.

4. The method for synthesizing pyrazine according to claim 1, characterized in that, The auxiliary agent is selected from at least one of ZrO2, Ce2O3, CeO2, La2O3, MgO, CaO, MnO, and BaO; The carrier is selected from at least one of Al2O3, SiO2 and carbon black.

5. The method for synthesizing pyrazine according to claim 4, characterized in that, In the Cu-Ni based composite catalyst, the mass percentage of the support is 25-60%, the mass percentage of CuO is 12-30%, the mass percentage of NiO is 8-20%, and the mass percentage of the metal in the additive is 20-25%.

6. The method for synthesizing pyrazine according to claim 1, characterized in that, The method further includes: Hydrogen gas is introduced into the reactor so that the cyclization reaction takes place in a hydrogen-containing environment; The feed space velocity of the hydrogen is 50~500 h⁻¹. -1 .

7. The method for synthesizing pyrazine according to claim 1, characterized in that, Before passing monoethanolamine and water into the reactor packed with a Cu-Ni-based composite catalyst, the method further includes: activating the Cu-Ni-based composite catalyst, including: A mixture of hydrogen and nitrogen gas is introduced into the reactor filled with Cu-Ni-based composite catalyst, and the reactor is activated at 150-200°C for 1-10 hours.

8. The method for synthesizing pyrazine according to claim 7, characterized in that, In the mixed gas, the volume content of hydrogen is 1-20%.

9. The method for synthesizing pyrazine according to claim 1, characterized in that, When the Cu-Ni composite catalyst is deactivated or its activity decreases, regeneration gas is introduced into the reactor to perform in-situ regeneration of the Cu-Ni composite catalyst. The feed space velocity of the regenerated gas is 50 h⁻¹. -1 ~1000h -1 ; The regenerated gas is air or nitrogen containing oxygen, wherein the oxygen in the nitrogen contains oxygen has a volume percentage of 0.1% to 10%. The regeneration process is carried out at a temperature of 200℃ to 400℃, with a heating rate of 10℃ / h to 20℃ / h and a processing time of 2h to 20h.

10. The method for synthesizing pyrazine according to claim 1, characterized in that, The yield of the pyrazine is greater than 40% based on monoethanolamine.

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