A green process for the synthesis of 4-methyl-2h-pyran-2,6(3h)-dione

Abstract

The application discloses a green process for synthesizing 4-methyl-2H-pyrane-2,6(3H)-dione, which comprises the following steps: (a) hydrolyzing 4,6-dimethyl-2-oxo-2H-pyrane-5-ethyl formate with liquid alkali to obtain a hydrolysis mixture, and performing negative pressure distillation on the hydrolysis mixture to recover an ethanol aqueous solution; (b) neutralizing the product of the negative pressure distillation to 7.0-7.5, centrifuging to obtain a filtrate and a residue, drying the residue to obtain anhydrous sodium salt, and performing three-stage extraction on the filtrate to obtain an aqueous phase and an organic phase; (c) performing cooling crystallization on the aqueous phase, centrifuging to obtain a precipitate and a first centrifugal filtrate, wherein the first centrifugal filtrate is used for preparing liquid alkali; performing concentration crystallization on the organic phase, recovering an extraction agent, cooling to crystallize when the concentration crystallization is performed to the point that crystals are precipitated, and centrifuging to obtain 3-methyl pentene diacid solid and a second centrifugal filtrate, wherein the second centrifugal filtrate is used for concentration crystallization; and (d) performing ring formation on the 3-methyl pentene diacid by dehydration reaction with a dehydrating agent, wherein the dehydrating agent is acetic anhydride or acetyl chloride. In this way, the discharge amount of pollutants is greatly reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fine chemical synthesis, and particularly relates to a green process for synthesizing 4-methyl-2H-pyrane-2,6(3H)-dione. BACKGROUND

[0002] 4-methyl-2H-pyrane-2,6(3H)-dione (also known as 3-methyl pentenedioic anhydride, beta-methyl pentenedioic anhydride, etc.) is an important intermediate for synthesizing delta-lactone products, and its structural formula is shown in the following formula (I). Figure 1 In recent years, it has been widely used in the fields of food, medicine, daily chemicals and functional chemicals.

[0003] A preparation method of 3-hydroxy-2-(hydroxymethyl)-4H-pyrane-4-ketone is disclosed in Chinese patent CN111320598A, which belongs to the field of pharmaceutical chemical industry. The method can react with ethyl hydroxyl pyran, ketone compound and one water p-toluenesulfonic acid, then the product is obtained by treatment, and then the final product is obtained by alcohol hydroxyl substitution, dehydration, rearrangement and other steps. The method provided by the application can effectively prepare the required 3-hydroxy-2-(hydroxymethyl)-4H-pyrane-4-ketone, and provides favorable conditions for further preparation of target compounds.

[0004] A method for preparing 4-hydroxy-6-methyl tetrahydro-2-pyrane is disclosed in Chinese patent CN107311970A, which comprises the following steps: triacetyl lactone raw material is mixed with alcohol compound with hydrogen donor and added into a closed high-pressure reaction kettle, and hydrogen transfer reaction is carried out under the conditions of nitrogen pressure of 1-12 MPa, reaction temperature of room temperature-150 DEG C, and presence of catalyst for 0.5-48 h to obtain 4-hydroxy-6-methyl tetrahydro-2-pyrane, and the catalyst is non-noble metal nanoparticles. The application does not need to use noble metal, has low cost, is easy to separate and can be repeatedly used.

[0005] The above-mentioned patents disclose 4-methyl-2H-pyrane-2,6(3H)-dione analogs, and the reaction conditions are more inclined to be carried out in the laboratory, and there is no green and environmentally friendly technical route. In view of the potential or important significance of the production of delta-lactone products, how to produce the compound in a green and environmentally friendly way will be a problem to be solved, and has high economic value. SUMMARY

[0006] The application aims to overcome the deficiencies of the prior art and provide a green process for synthesizing 4-methyl-2H-pyrane-2,6(3H)-dione.

[0007] To achieve the above object, the present application adopts the technical scheme of a green process for synthesizing 4-methyl-2H-pyrane-2,6(3H)-dione, comprising the following steps:

[0008] (a) hydrolyzing 4,6-dimethyl-2-oxo-2H-pyrane-5-ethyl formate with liquid alkali to obtain a hydrolysis mixture, and subjecting the hydrolysis mixture to negative pressure distillation to recover an aqueous ethanol solution;

[0009] (b) neutralizing the product of the negative pressure distillation to 7.0-7.5, centrifuging to obtain a filtrate and a residue, drying the residue to obtain anhydrous sodium salt, and subjecting the filtrate to three-stage extraction to obtain an aqueous phase and an organic phase;

[0010] (c) subjecting the aqueous phase to temperature reduction crystallization, centrifuging to obtain a precipitate and a first centrifugal filtrate, wherein the first centrifugal filtrate is used to prepare liquid alkali; subjecting the organic phase to concentration crystallization and recovering an extraction agent, when the concentration crystallization is performed to the point that crystals are precipitated, reducing the temperature to crystallize and centrifuging to obtain 3-methyl pentene diacid solid and a second centrifugal filtrate, wherein the second centrifugal filtrate is used for concentration crystallization;

[0011] (d) subjecting the 3-methyl pentene diacid to dehydration reaction with a dehydrating agent to form a ring, wherein the dehydrating agent is acetic anhydride or acetyl chloride.

[0012] Optimally, in step (a), the mass concentration of the liquid alkali is 5-40%, preferably 30-40%;

[0013] The molar ratio of the 4,6-dimethyl-2-oxo-2H-pyrane-5-ethyl formate to sodium hydroxide in the liquid alkali is 1:2-5, preferably 1:2.5-3;

[0014] The temperature of the hydrolysis is 20-100°C, preferably 60-80°C, and the time is 0.5-8h, preferably 1.0-1.5h.

[0015] Further, in step (a), the temperature of the negative pressure distillation is 50-55°C;

[0016] The aqueous ethanol solution is introduced into a rectification tower to perform rectification to obtain industrial ethanol and first wastewater; the process conditions of the rectification are as follows: the overhead temperature is 75-80°C, the bottom temperature is 90-100°C, the pressure is normal pressure, and the mass reflux ratio is 1:1.

[0017] Optimally, in step (c), the temperature of the temperature reduction crystallization is 42-50°C;

[0018] The precipitate is subjected to recrystallization and centrifugation to obtain sodium acetate trihydrate and second wastewater;

[0019] The temperature of the recrystallization is 100-110℃, and the temperature is decreased to 25-40℃ for crystallization.

[0020] Optimally, in step (d), when the dehydrating agent is acetic anhydride, the molar ratio of 3-methyl pentene diacid to acetic anhydride is 1:1.5-5, preferably 1:2-3; the temperature of the dehydration reaction is 20℃-120℃, preferably 50℃-80℃, and the time is 0.5h-8h, preferably 1h-1.5h.

[0021] Further, in step (d), the product of the dehydration reaction is subjected to first negative pressure distillation to recover by-product acetic acid; and then subjected to second negative pressure distillation to recover acetic anhydride; the first negative pressure distillation is carried out in a first negative pressure distillation kettle, and the process parameters are temperature 60℃-80℃ and pressure 0.02MPa-0.04MPa; the first negative pressure distillation kettle uses a packed column, the packing is 3 meters high, the built-in plate corrugated ceramic packing, the diameter is 300mm, and the mass reflux ratio is 0.2.

[0022] The second negative pressure distillation is carried out in a second negative pressure distillation kettle, and the process parameters are temperature 60℃-80℃ and pressure 0.005MPa-0.015MPa.

[0023] Further, in step (d), the product of the second negative pressure distillation is dissolved in a crystallization solvent, activated carbon is added, and heat preservation decolorization is carried out at 70-80℃; the activated carbon is removed by pressure filtration, the activated carbon is treated as solid waste, the filtrate is distilled to recover the crystallization solvent, and when crystals are precipitated, the temperature is decreased for crystallization, and 4-methyl-2H-pyran-2,6(3H)-dione solid is obtained by centrifugal separation, the crystallization solvent and the filtrate are used for decolorization and impurity removal.

[0024] Optimally, in step (d), when the dehydrating agent is acetyl chloride, the molar ratio of 3-methyl pentene diacid to acetyl chloride is 1:3-8, preferably 1:4-5; the temperature of the dehydration reaction is 4℃-50℃, preferably 40℃-45℃, and the time is 0.5h-8h, preferably 6h-8h.

[0025] Further, in step (d), hydrogen chloride is absorbed with water during the dehydration reaction to recover 30% hydrochloric acid; and after the reaction is completed, the product of the dehydration reaction is subjected to atmospheric distillation to recover unreacted acetyl chloride, and then subjected to third negative pressure distillation to recover by-product acetic acid.

[0026] The third negative pressure distillation is carried out in a third negative pressure distillation kettle, and the process parameters are temperature 60℃-80℃ and pressure 0.02MPa-0.04MPa.

[0027] Further, in step (d), the product of the third negative pressure distillation is dissolved in a crystallization solvent, activated carbon is added, and the mixture is heated and maintained at 70-80°C to remove impurities by decolorization; the activated carbon is removed by pressure filtration, and the activated carbon is treated as solid waste, and the filtrate is distilled to recover the crystallization solvent and crystallized by cooling when crystals are precipitated, and 4-methyl-2H-pyran-2,6(3H)-dione is obtained by centrifugal separation, the crystallization solvent and the filtrate are used for decolorization and impurity removal.

[0028] Due to the above technical solution, the present application has the following advantages compared with the prior art: the green process for synthesizing 4-methyl-2H-pyran-2,6(3H)-dione can produce 3-methyl pentene diacid, and then dehydrate the 3-methyl pentene diacid into a ring using acetic anhydride or acetyl chloride as a dehydrating agent, and the yield of 3-methyl pentene diacid solid can reach 98%; the by-product sodium sulfate has a content of ≥99% and appears as white crystalline solid particles, meeting the quality requirements of industrial-grade anhydrous sodium sulfate, and can be sold as a by-product; the by-product sodium acetate trihydrate has a content of ≥90% and appears as white crystalline solid particles, and can be sold as a solid carbon source for wastewater; after recrystallization, the by-product sodium acetate trihydrate has a content of ≥99% and appears as white transparent crystalline solid particles; the amount of wastewater per ton of intermediate is 0.4-0.7 tons, which is mainly a mixture of steam and a small amount of mother liquor, and the wastewater has good biochemical indicators (pH=7-7.5, COD=8000-10000 mg / L, salt content of 12000-15000 mg / L, and main components of ethanol, sodium acetate, and sodium sulfate); the yield of 4-methyl-2H-pyran-2,6(3H)-dione is 87%, and the purity is 99.6% (using acetic anhydride as a dehydrating agent); the yield of 4-methyl-2H-pyran-2,6(3H)-dione is 91%, and the purity is 98.2% (using acetyl chloride as a dehydrating agent); the by-product acetic acid has a content of ≥99.5% and appears as colorless transparent liquid, meeting the quality requirements of industrial-grade acetic acid, and can be sold as a by-product. In this way, the amount of pollutants is greatly reduced. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The chemical formula of 4-methyl-2H-pyran-2,6(3H)-dione;

[0030] Figure 2 The synthesis route of 4-methyl-2H-pyran-2,6(3H)-dione;

[0031] Figure 3 The synthesis process flow chart of 3-methyl pentene diacid;

[0032] Figure 4 The synthesis process flow chart of 4-methyl-2H-pyran-2,6(3H)-dione (using acetic anhydride as a dehydrating agent);

[0033] Figure 5 Flow chart for the synthesis process of 4-methyl-2H-pyran-2,6(3H)-dione (acetyl chloride as the dehydrating agent). DETAILED DESCRIPTION

[0034] The preferred embodiments of the present application will be described in detail below. Example 1

[0035] This example provides a green process for the synthesis of 4-methyl-2H-pyran-2,6(3H)-dione, as shown in Figure 2 , the initial raw material is ethyl acetoacetate (or methyl acetoacetate), which is subjected to a double-molecular condensation reaction to produce ethyl 4,6-dimethyl-2-oxo-2H-pyran-5-carboxylate, which is then subjected to subsequent alkaline hydrolysis and ring-forming reaction to obtain 4-methyl-2H-pyran-2,6(3H)-dione. The double-molecular condensation reaction of ethyl acetoacetate (or methyl acetoacetate) to produce ethyl 4,6-dimethyl-2-oxo-2H-pyran-5-carboxylate is patented, with an application number of 202410668263.3. To avoid insufficient disclosure in the specification, this example uses ethyl 4,6-dimethyl-2-oxo-2H-pyran-5-carboxylate as the raw material, as shown in Figure 3 and Figure 4 , which includes the following steps:

[0036] (a) 800 kg of ethyl 4,6-dimethyl-2-oxo-2H-pyran-5-carboxylate and 1630 kg of liquid alkali (the mass concentration of the liquid alkali can be 5-40%, but 5% will consume more water; therefore, 30-40% is usually selected, which has no significant effect on the quality of the product as long as the ratio with the raw material is ensured; in this example, 30% of commercially available liquid alkali is used) are added to a hydrolysis kettle (with a volume of 3000 L) for hydrolysis reaction at 60-80°C (i.e., the hydrolysis reaction temperature fluctuates within the range of 60-80°C; the hydrolysis reaction time can be 1.0-1.5 h, and in this example, it is set to 1.0 h) to obtain a hydrolysis mixture;

[0037] The hydrolysis mixture is subjected to negative pressure distillation (the equipment for negative pressure distillation is a conventional reaction kettle) at 50-55°C to recover an ethanol aqueous solution (about 310 kg); the ethanol aqueous solution is introduced into a rectification column for rectification to obtain industrial ethanol (95% industrial ethanol, about 195 kg) and first wastewater (the yield of the first wastewater is about 115 kg, and the main components are water and ethanol, with a pH of 7-7.5 and a COD of 17000-22000 mg / L); the process conditions for rectification are: the overhead temperature is 75-80°C, the bottom temperature is 90-100°C, the pressure is normal pressure, and the mass reflux ratio is 1:1.

[0038] (b) The product of the negative pressure distillation is neutralized with commercially available 98% sulfuric acid to 7.0~7.5, centrifugal separation to obtain the filtrate and residue (the residue is dried to obtain white crystalline solid sodium sulfate, about 870 kg; the by-product sodium sulfate content is ≥99%, white crystalline solid particles, meeting the quality requirements of industrial grade anhydrous sodium sulfate, sold as a by-product), and the filtrate is subjected to three-stage extraction (three-stage countercurrent extraction, the extractant can be ethyl acetate, toluene, hexane, heptane, methyl tert-butyl ether or dichloromethane, ethyl acetate is used in this embodiment, and the process parameters are: temperature control is 55~65℃, the mass flow ratio of the water phase to the organic phase (containing the extractant) is 1:0.5; the flow ratio can be reasonably selected within the range of 1:(0.3~0.6) according to actual needs);

[0039] (c) The water phase is introduced into a crystallization kettle and cooled to 42~50℃ to crystallize, and centrifugal separation is performed to obtain precipitate and first centrifugal filtrate; the first centrifugal filtrate is used to prepare liquid caustic soda; the precipitate (by-product sodium acetate trihydrate crude product, content ≥90%, white crystalline solid particles, sold as a wastewater solid carbon source) or is subjected to recrystallization (the recrystallization temperature is 100~110℃, and then cooled to 25~40℃ to crystallize), and centrifugal separation is performed to obtain sodium acetate trihydrate (by-product sodium acetate trihydrate content ≥99%, white transparent crystalline solid particles, about 560 kg) and second wastewater (the wastewater is only discharged when the quality of the sodium acetate trihydrate product exceeds the standard, and is generally recycled to the crystallization kettle, about 60 kg / batch, pH is 7~7.5, COD is 120000~150000 mg / L, salt content is 30000~50000 mg / L, and the main components are sodium acetate and a small amount of sodium sulfate);

[0040] The organic phase is concentrated and crystallized at 50~60℃, and the extractant is recovered; when the concentrated crystallization is performed to precipitate crystals, the temperature is lowered to -5~10℃ to crystallize and centrifugal separation is performed to obtain 3-methyl pentene diacid solid (yield 98%, 576 kg) and second centrifugal filtrate, and the second centrifugal filtrate is used for concentrated crystallization;

[0041] The wastewater amount (i.e. the combined first and second wastewater) per ton of intermediate (i.e. per ton of 3-methyl pentene diacid) is 0.4~0.7 tons, which is a mixture of steam-out water and a small part of mother liquor, and the wastewater quality has good biochemical indicators (pH is 7~7.5, COD is 60000~100000 mg / L, salt content is 12000~15000 mg / L, and the main components are ethanol, sodium acetate, sodium sulfate, etc.).

[0042] (d) 576 kg of 3-methylglutaconic acid is subjected to a dehydration reaction with 1200 kg of a dehydrating agent (acetic anhydride) (dehydration reaction temperature: 50-80°C, time: 1-1.5 h; in this example, the dehydration reaction temperature is 60°C, and the time is set to 1.0 h) to form a ring.

[0043] The product of the dehydration reaction is also subjected to first negative pressure distillation to recover by-product acetic acid (by-product acetic acid content: ≥99.5%, yield: about 240 kg, appearance: colorless transparent liquid, meeting the quality requirements of industrial-grade acetic acid, and sold as a by-product); the first negative pressure distillation is performed in a first negative pressure distillation kettle, and the process parameters are a temperature of 60-80°C and a pressure of 0.02-0.04 MPa; the first negative pressure distillation kettle uses a packed column, the packing is 3 meters high, the built-in plate corrugated ceramic packing has a diameter of 300 mm, and the mass reflux ratio is 0.2.

[0044] Second negative pressure distillation is then performed to recover acetic anhydride; the second negative pressure distillation is performed in a second negative pressure distillation kettle, and the process parameters are a temperature of 60-80°C and a pressure of 0.005-0.015 MPa.

[0045] The product of the second negative pressure distillation is dissolved in a crystallization solvent (the crystallization solvent can be toluene, benzene, cyclohexane, or dioxane, etc., and toluene is used in this example), activated carbon is added, and heat preservation decolorization is performed at 70-80°C; the activated carbon is removed by pressure filtration, and the activated carbon is treated as solid waste and outsourced, and the filtrate is distilled (temperature: 60-80°C) to recover the crystallization solvent and crystallize when crystals are precipitated (temperature: 30-40°C), and about 441 kg of 4-methyl-2H-pyrane-2,6(3H)-dione solid is obtained by centrifugal separation, and the crystallization solvent and the filtrate are used for decolorization and impurity removal.

[0046] The yield of 4-methyl-2H-pyrane-2,6(3H)-dione is 87%, and the purity is 99.6%. Example 2

[0047] This example provides a green process for synthesizing 4-methyl-2H-pyrane-2,6(3H)-dione, which is basically the same as that in Example 1, except that in step (d), 576 kg of 3-methylglutaconic acid is subjected to a reaction with 2040 kg of a dehydrating agent (acetic anhydride) to obtain about 445 kg of 4-methyl-2H-pyrane-2,6(3H)-dione solid.

[0048] The yield of 4-methyl-2H-pyrane-2,6(3H)-dione is 88%, and the purity is 99.5%. Example 3

[0049] The present embodiment provides a green process for synthesizing 4-methyl-2H-pyrane-2,6(3H)-dione, which is basically the same as that in Embodiment 1, except that in step (d), 576 kg of 3-methyl pentene diacid is reacted with 612 kg of dehydrating agent (acetic anhydride) to obtain about 414 kg of 4-methyl-2H-pyrane-2,6(3H)-dione solid.

[0050] The yield of 4-methyl-2H-pyrane-2,6(3H)-dione is 82%, and the purity is 94.8%. Embodiment 4

[0051] The present embodiment provides a green process for synthesizing 4-methyl-2H-pyrane-2,6(3H)-dione, which is basically the same as that in Embodiment 1, except that in step (d), the dehydrating agent is acetyl chloride.

[0052] That is, 576 kg of 3-methyl pentene diacid is dehydrated with 1550 kg of dehydrating agent (acetyl chloride) (dehydration reaction temperature is 40-45°C, time is 6-8h; in the present embodiment, it is set to 6h) to form a ring.

[0053] During the dehydration reaction, water is used to absorb hydrogen chloride (falling film absorption) to recover 30% hydrochloric acid (about 490 kg); after the reaction is completed, the product of the dehydration reaction is subjected to atmospheric distillation (50-80°C) to recover unreacted acetyl chloride;

[0054] A third vacuum distillation is then performed to recover byproduct acetic acid; the third vacuum distillation is performed in a third vacuum distillation kettle, and the process parameters are temperature 60-80°C and pressure 0.02-0.04 MPa.

[0055] The product of the third vacuum distillation is dissolved in a crystallization solvent (the crystallization solvent is toluene, benzene, cyclohexane, dioxane, etc.), activated carbon is added, and heat preservation decolorization is performed at 70-80°C; the activated carbon is removed by pressure filtration, and the activated carbon is treated as solid waste and outsourced, and the filtrate is distilled (60-80°C) to recover the crystallization solvent and crystallize when crystals are precipitated (30-40°C), and 4-methyl-2H-pyrane-2,6(3H)-dione solid is obtained by centrifugal separation, about 460 kg, and the crystallization solvent and the filtrate are used for decolorization and impurity removal.

[0056] The yield of 4-methyl-2H-pyrane-2,6(3H)-dione is 91%, and the purity is 98.2%. Embodiment 5

[0057] The present example provides a green process for synthesizing 4-methyl-2H-pyran-2,6(3H)-dione, which is basically the same as that in Example 4, except that in step (d), 576 kg of 3-methyl pentene diacid is reacted with 2510 kg of dehydrating agent (acetyl chloride) to obtain about 468 kg of 4-methyl-2H-pyran-2,6(3H)-dione solid.

[0058] The yield of 4-methyl-2H-pyran-2,6(3H)-dione is 93%, and the purity is 97.7%. Example 6

[0059] The present example provides a green process for synthesizing 4-methyl-2H-pyran-2,6(3H)-dione, which is basically the same as that in Example 4, except that in step (d), 576 kg of 3-methyl pentene diacid is reacted with 942 kg of dehydrating agent (acetyl chloride) to obtain about 424 kg of 4-methyl-2H-pyran-2,6(3H)-dione solid.

[0060] The yield of 4-methyl-2H-pyran-2,6(3H)-dione is 84%, and the purity is 92.5%.

[0061] Comparative Example 1

[0062] The present example provides a process for synthesizing 4-methyl-2H-pyran-2,6(3H)-dione, which is basically the same as that in Example 1, except that the basic hydrolysis temperature is 120°C, 0.02 MPa, and about 323 kg of 4-methyl-2H-pyran-2,6(3H)-dione solid is obtained.

[0063] The yield of 4-methyl-2H-pyran-2,6(3H)-dione is 64%, and the purity is 99.2%.

[0064] Comparative Example 2

[0065] The present example provides a process for synthesizing 4-methyl-2H-pyran-2,6(3H)-dione, which is basically the same as that in Example 1, except that after the dehydration reaction is completed, no decolorization and impurity removal treatment is performed, and after the concentrated crystallization mother liquor is reused three times, the product quality has been reduced to 86.4%, and it is difficult to remove by recrystallization.

[0066] Comparative Example 3

[0067] The present example provides a process for the synthesis of 4-methyl-2H-pyran-2,6(3H)-dione, which is substantially identical to that of Example 1, except that 576 kg of 3-methyl pentene dioic acid is subjected to a dehydration reaction with 408 kg of a dehydrating agent (acetic anhydride). Too little dehydrating agent results in incomplete dissolution and a lower reaction conversion rate, and about 310 kg of 4-methyl-2H-pyran-2,6(3H)-dione solid is obtained.

[0068] The yield of 4-methyl-2H-pyran-2,6(3H)-dione is 61%, and the purity is 94.8%.

[0069] Comparative Example 4

[0070] The present example provides a process for the synthesis of 4-methyl-2H-pyran-2,6(3H)-dione, which is substantially identical to that of Example 4, except that 576 kg of 3-methyl pentene dioic acid is subjected to a dehydration reaction with 315 kg of a dehydrating agent (acetyl chloride). Too little dehydrating agent results in incomplete dissolution, difficulty in stirring, a lower reaction conversion rate, and about 197 kg of 4-methyl-2H-pyran-2,6(3H)-dione solid is obtained.

[0071] The yield of 4-methyl-2H-pyran-2,6(3H)-dione is 39%, and the purity is 92.5%.

[0072] The above examples are only for illustrating the technical concept and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and to implement it, and it cannot limit the protection scope of the present application. Any equivalent changes or modifications made in accordance with the spirit and essence of the present application should be covered within the protection scope of the present application.

Claims

1. A green process for synthesis of 4-methyl-2H-pyran-2,6(3H)-dione characterized by, The method comprises the following steps: (a) hydrolyzing ethyl 4,6-dimethyl-2-oxo-2H-pyran-5-carboxylate with liquid alkali to obtain a hydrolysis mixture, and subjecting the hydrolysis mixture to negative pressure distillation to recover an ethanol aqueous solution; the mass concentration of the liquid alkali is 30-40%, and the molar ratio of the ethyl 4,6-dimethyl-2-oxo-2H-pyran-5-carboxylate to sodium hydroxide in the liquid alkali is 1:2-5; the hydrolysis temperature is 20-100 ℃, and the hydrolysis time is 0.5-8 h; (b) neutralizing the product of the negative pressure distillation to 7.0-7.5, and centrifuging to obtain a filtrate and a residue; drying the residue to obtain anhydrous sodium salt, and subjecting the filtrate to three-stage extraction to obtain an aqueous phase and an organic phase; (c) subjecting the aqueous phase to cooling crystallization and centrifuging to obtain a precipitate and a first centrifugal filtrate, and using the first centrifugal filtrate to prepare liquid alkali; subjecting the organic phase to concentration crystallization and recovering an extractant; when the concentration crystallization is performed to the point that crystals are precipitated, cooling is performed to crystallize, and centrifuging is performed to obtain 3-methyl pentene diacid solid and a second centrifugal filtrate, and the second centrifugal filtrate is used for concentration crystallization; (d) subjecting the 3-methyl pentene diacid to dehydration reaction with a dehydrating agent to form a ring, and the dehydrating agent is acetyl chloride; when the dehydrating agent is acetyl chloride, the molar ratio of the 3-methyl pentene diacid to acetyl chloride is 1:3-8, and the dehydration reaction temperature is 4 ℃-50 ℃, and the dehydration reaction time is 0.5 h-8 h.

2. A green process for synthesis of 4-methyl-2H-pyran-2,6(3H)-dione as claimed in claim 1, wherein: In step (a), the molar ratio of the ethyl 4,6-dimethyl-2-oxo-2H-pyran-5-carboxylate to sodium hydroxide in the liquid alkali is 1:2.5-3; The hydrolysis temperature is 60-80 ℃, and the hydrolysis time is 1.0-1.5 h.

3. A green process for synthesis of 4-methyl-2H-pyran-2,6(3H)-dione according to claim 1 or 2, characterized by: In step (a), the negative pressure distillation temperature is 50-55 ℃. The ethanol aqueous solution is introduced into a rectification tower to perform rectification to obtain industrial ethanol and first wastewater; the rectification process conditions are as follows: the tower top temperature is 75-80 ℃, the tower bottom temperature is 90-100 ℃, the pressure is normal pressure, and the mass reflux ratio is 1:

1.

4. A green process for synthesis of 4-methyl-2H-pyran-2,6(3H)-dione as claimed in claim 1, wherein: In step (c), the cooling crystallization temperature is 42-50 ℃. The precipitate is subjected to recrystallization and centrifuging to obtain sodium acetate trihydrate and second wastewater. The concentration crystallization temperature is 100-110 ℃, and cooling is performed to 25-40 ℃ to crystallize.

5. A green process for synthesis of 4-methyl-2H-pyran-2,6(3H)-dione as claimed in claim 1, wherein: In step (d), when the dehydrating agent is acetyl chloride, the molar ratio of the 3-methyl pentene diacid to acetyl chloride is 1:4-5; the dehydration reaction temperature is 40 ℃-45 ℃, and the dehydration reaction time is 6 h-8 h.

6. A green process for synthesis of 4-methyl-2H-pyran-2,6(3H)-dione as claimed in claim 5, wherein: In step (d), hydrogen chloride is absorbed with water during the dehydration reaction to recover 30% hydrochloric acid; and after the reaction is completed, the dehydration reaction product is subjected to normal pressure distillation to recover unreacted acetyl chloride, and then subjected to third negative pressure distillation to recover by-product acetic acid; The third negative pressure distillation is performed in a third negative pressure distillation kettle, and the process parameters are as follows: the temperature is 60 ℃-80 ℃, and the pressure is 0.02 MPa-0.04 MPa.

7. A green process for synthesis of 4-methyl-2H-pyran-2,6(3H)-dione as claimed in claim 6, wherein: In step (d), the product of the third negative pressure distillation is dissolved in a crystallization solvent, activated carbon is added, and the mixture is incubated at 70-80°C for decolorization and impurity removal. The activated carbon is removed by pressure filtration, and the activated carbon is disposed of as solid waste. The filtrate is distilled to recover the crystallization solvent, and the mixture is cooled to crystallize when crystals precipitate. The 4-methyl-2H-pyran-2,6(3H)-dione solid is obtained by centrifugal separation. The crystallization solvent and the filtrate are used for decolorization and impurity removal.

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