Method for preparing high-purity citric anhydride

Abstract

The invention relates to the field of organic synthesis, in particular to a preparation method of a process for preparing high-purity citric anhydride. The preparation method comprises the following steps: preparation of a hierarchical pore zeolite molecular sieve, preparation of a supported molecular sieve catalyst and synthesis of the high-purity citric anhydride. According to the method, the zeolite molecular sieve is subjected to alkalization treatment, so that the connectivity of pore channels of the zeolite molecular sieve can be improved, the catalytic effect can be improved, and high-purity citric anhydride can be synthesized in an assisted manner; by preparing and using the supported molecular sieve catalyst, the generation of a citric acid dehydration reaction is facilitated, the generation of byproducts is reduced, and the catalytic efficiency and the catalytic performance are improved; in the process of synthesizing the high-purity lemon anhydride, the separated crystalline solid is washed by using the oxygen-containing organic solvent, so that the purity of the lemon anhydride can be improved.

Description

Technical Field

[0001] The invention relates to the field of organic synthesis, and more specifically to a method for preparing high-purity citric anhydride. Background Art

[0002] Citric anhydride is formed by dehydration between two carboxyl groups of citric acid. It has water absorption and acylation ability and is an important intermediate for the synthesis of pesticides, medicines, foods, beverages, detergents, cosmetics and resins. Citric anhydride is mainly synthesized by dehydration reaction of citric acid using citric acid and acetic anhydride as raw materials and acetic acid as catalyst and solvent. However, in the actual synthesis process of citric anhydride, it is easy to cause problems such as improper control of reaction conditions, low catalytic efficiency and incomplete removal of impurities, resulting in low synthesis reaction efficiency, production and doping of by-products, and thus causing a decrease in the yield and purity of citric anhydride.

[0003] In order to improve the dehydration reaction of citric acid, during the synthesis and processing of citric anhydride, producers will add other acid catalysts to improve the reaction synthesis efficiency, such as molecular sieve catalysts, heteropoly acid catalysts, metal oxide catalysts, etc. Among them, molecular sieve catalysts, as common acid catalysts, can accelerate the reaction and are very easy to separate and recycle, but their pores and pore sizes are small, and they are easy to form coke and cause carbon deposits to accumulate, resulting in pore blockage, which is not conducive to internal diffusion and mass transfer, thereby affecting the catalytic efficiency and catalytic effect, and cannot efficiently and stably catalyze the synthesis of citric anhydride. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a method for preparing high-purity citric anhydride.

[0005] A method for preparing high-purity citric anhydride comprises the following steps:

[0006] S1: preparation of hierarchical pore zeolite molecular sieve, using sodium aluminate, nano-silica sol and tetrapropylammonium hydroxide as raw materials to prepare zeolite molecular sieve, stirring and mixing sodium hydroxide, deionized water and zeolite molecular sieve, heating reaction, washing and drying to obtain hierarchical pore zeolite molecular sieve;

[0007] S2: Preparation of supported molecular sieve catalyst, using magnesium nitrate hexahydrate, deionized water and sodium hydroxide solution as raw materials to prepare magnesium oxide, using tetraethyl silicate, ionized water and dilute nitric acid solution as raw materials to prepare silicon dioxide, mixing hierarchical pore zeolite molecular sieve, deionized water, magnesium oxide and silicon dioxide, stirring and reacting, filtering, drying and calcining to obtain supported molecular sieve catalyst;

[0008] S3: Synthesis of high-purity citric anhydride. Anhydrous citric acid, organic anhydride, organic acid and supported molecular sieve catalyst are put into a distillation reactor. The relative vacuum degree and temperature of the distillation reactor are adjusted. The substances in the distillation reactor are stirred to react. After the stirring reaction is completed, heating and distillation are performed. The supported molecular sieve catalyst is fished out. A mixed organic solvent is added to the remaining liquid in the distillation reactor. After crystallization, standing, filtering, washing with an oxygen-containing organic solvent and vacuum drying, high-purity citric anhydride is obtained.

[0009] Furthermore, step S1 of preparing a multi-level pore zeolite molecular sieve specifically comprises the following steps:

[0010] S1.1: Sodium aluminate, nano silica sol and tetrapropylammonium hydroxide are used as aluminum source, silicon source and template respectively, and the molar ratio of n(Na2O):n(SiO2):n(Al2O3):n(TPAOH):n(H2O)=(8-12):(90-100):(1-5):(16-20):(2400-2500) is used for feeding. Sodium aluminate, nano silica sol and tetrapropylammonium hydroxide are stirred and mixed, and then placed in a homogeneous reactor at 75-84°C for aging for 20-24h;

[0011] S1.2: After the aging is completed, the synthetic gel obtained in the reactor is transferred to a stainless steel reactor, and the stainless steel reactor is placed in an oven for static crystallization at 90-180° C. for 72-90 hours to obtain a crystallized product, and the crystallized product is washed 2-5 times with deionized water;

[0012] S1.3: placing the washed crystallized product in an oven and drying it at 70-80°C for 8-12 hours. After drying, grinding the crystallized product into crystallized particles, and calcining the crystallized particles in a muffle furnace for 5-6 hours at a calcination temperature of 500-580°C to obtain solid particles;

[0013] S1.4: After the calcination is completed, the temperature is naturally lowered, and the solid particles are rinsed with deionized water until the pH of the deionized water after the last rinse is 6.8-7.2. The rinsed solid particles are dried at 70-80°C for 3-5 hours, and after the drying is completed, the dried solid particles are ground to obtain a zeolite molecular sieve;

[0014] S1.5: Sodium hydroxide, deionized water and zeolite molecular sieve are stirred and mixed according to the molar ratio of n(NaOH):n(H2O):n(zeolite molecular sieve)=(0.2-0.5):(9-12):(1-3), and reacted at 60-70°C for 40-90 min. After the reaction, the alkalized zeolite molecular sieve is rinsed 2-3 times with deionized water, and dried at 90-120°C for 30-60 min to obtain a multi-level pore zeolite molecular sieve.

[0015] Furthermore, step S2, preparation of supported molecular sieve catalyst, specifically comprises the following steps:

[0016] S2.1: 5 to 30 parts by weight of magnesium nitrate hexahydrate are added to 50 to 120 parts by weight of deionized water, and stirred until completely dissolved to obtain a magnesium nitrate solution. 20 to 50 parts by weight of a sodium hydroxide solution having a mass concentration of 15% is slowly dripped into the magnesium nitrate solution, and stirred while dripping. After the dripping is completed, stirring is continued for 10 to 15 minutes to generate a precipitate A, which is allowed to stand at room temperature for 8 to 12 hours to allow the precipitate A to precipitate and age;

[0017] S2.2: The aged precipitate A is then taken out, washed with deionized water 2 to 3 times, centrifuged, dried and calcined to obtain magnesium oxide;

[0018] S2.3: Add 5 to 50 parts by weight of tetraethyl silicate to 100 to 200 parts by weight of deionized water, slowly dropwise add 20 to 80 parts by weight of a 6% dilute nitric acid solution while stirring, continue stirring for 2 to 5 hours after the addition is complete, and after the hydrolysis is completed, obtain a precipitate B, wash the precipitate B with deionized water 2 to 3 times, dry and calcine to obtain silicon dioxide;

[0019] S2.4: Add 20 to 40 parts by weight of hierarchical pore zeolite molecular sieve to 80 to 150 parts by weight of deionized water, stir evenly, then add 3 to 10 parts by weight of magnesium oxide and 3 to 10 parts by weight of silicon dioxide, stir and mix to obtain a suspension;

[0020] S2.5: The suspension is stirred uniformly for 4 to 6 hours at a stirring speed of 800 to 1000 r / min, and then filtered, dried and calcined to obtain a supported molecular sieve catalyst.

[0021] Furthermore, the calcination temperature in step S2.2 is 500-550° C., and the calcination time is 3-5 hours.

[0022] Furthermore, the calcination temperature in step S2.3 is 480-530° C., and the calcination time is 10-12 hours.

[0023] Furthermore, the calcination temperature in step S2.5 is 350-600° C., and the calcination time is 3-4 hours.

[0024] Further, step S3 of synthesizing high-purity citric anhydride specifically comprises the following steps:

[0025] S3.1: adding anhydrous citric acid, organic acid anhydride and organic acid into a distillation reactor at a material ratio of 1:(1-2.4):(1-2.8) by molar ratio, and then adding a supported molecular sieve catalyst into the distillation reactor, wherein the mass of the supported molecular sieve catalyst accounts for 10-25% of the mass of the anhydrous citric acid;

[0026] S3.2: The relative vacuum degree of the distillation reactor is adjusted to -0.1 to 0 MPa, the condenser temperature is 0 to 5°C, and the substances in the distillation reactor are stirred at 36 to 50°C for 1 to 3 hours, during which the organic acid is gradually distilled and condensed. After the stirring reaction is completed, the temperature in the distillation reactor is increased to 60 to 80°C, and the distillation is heated for 15 to 30 minutes to completely condense and precipitate the organic acid;

[0027] S3.3: Subsequently, the loaded molecular sieve catalyst is fished out from the distillation reactor, the remaining liquid in the distillation reactor is poured out, and 1 to 3 times the volume of a mixed organic solvent is added to the remaining liquid under continuous stirring, and the reaction is stirred for 10 to 20 minutes. After standing for 1 to 3 hours, filtration is performed to obtain a crystalline solid, and the crystalline solid is washed 2 to 3 times with an oxygen-containing organic solvent, and finally vacuum dried to obtain high-purity citric anhydride.

[0028] Furthermore, the organic anhydride is acetic anhydride, propionic anhydride, butyric anhydride or valeric anhydride, and the organic acid is acetic acid, propionic acid, butyric acid or valeric acid.

[0029] Furthermore, the mixed organic solvent is composed of dichloromethane or chloromethane and a weak polar organic solvent in a volume ratio of (1-4):1, and the weak polar organic compound is pentane, hexane, octane, toluene or benzene.

[0030] Furthermore, the oxygen-containing organic solvent is ethyl acetate, methyl acetate, ethanol, methanol, isopropanol, ethylene glycol, propylene glycol or diethylene glycol.

[0031] The present invention has the following advantages:

[0032] 1. In the present invention, by alkalizing the zeolite molecular sieve, mesoporous channels can be generated in the zeolite molecular sieve, and a multi-level pore structure can be etched in the zeolite molecular sieve to improve the connectivity of the zeolite molecular sieve channels. The obtained multi-level pore zeolite molecular sieve can effectively reduce the generation efficiency of carbon deposits, improve the catalytic effect and catalytic efficiency of the molecular sieve catalyst, and assist in improving the yield of citric anhydride.

[0033] 2. In the present invention, magnesium oxide and silicon dioxide are loaded on the hierarchical pore zeolite molecular sieve. The loading of magnesium oxide and silicon dioxide is beneficial to increase the specific surface area of ​​the hierarchical pore zeolite molecular sieve, increase the pore size and promote mass transfer, so that the loaded molecular sieve catalyst can promote the occurrence of citric acid dehydration reaction, reduce the formation of by-products, and thus improve the yield and purity of citric anhydride.

[0034] 3. In the present invention, in the process of synthesizing citric anhydride, a mixed organic solvent is used for crystallization, so that the crystal precipitate is easier to precipitate, the crystallization time is shortened, and the precipitated crystalline solid is washed by an oxygen-containing organic solvent, so that the doped organic anhydride and organic acid can be effectively dissolved and removed, thereby improving the synthesis purity of citric anhydride, which is conducive to obtaining high-purity citric anhydride with good dispersibility and fine texture. DETAILED DESCRIPTION

[0035] In order to enable persons skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0036] Example 1

[0037] A method for preparing high-purity citric anhydride comprises the following steps:

[0038] S1: Preparation of hierarchical pore zeolite molecular sieves,

[0039] S1.1: Sodium aluminate, nano silica sol and tetrapropylammonium hydroxide are used as aluminum source, silicon source and template respectively, and are added according to the molar ratio of n(Na2O):n(SiO2):n(Al2O3):n(TPAOH):n(H2O)=12:100:5:20:2500. Sodium aluminate, nano silica sol and tetrapropylammonium hydroxide are stirred and mixed, and then placed in a homogeneous reactor at 84°C for aging for 24 hours;

[0040] S1.2: After the aging is completed, the synthetic gel obtained in the reactor is transferred to a stainless steel reactor, and the stainless steel reactor is placed in an oven for static crystallization at 180°C for 90 hours to obtain a crystallized product, and the crystallized product is washed 5 times with deionized water;

[0041] S1.3: The washed crystallized product is placed in an oven and dried at 80°C for 12 hours. After drying, the crystallized product is ground into crystallized particles, and the crystallized particles are placed in a muffle furnace and calcined for 6 hours to remove the template. The calcination temperature is 580°C to obtain solid particles.

[0042] S1.4: After the calcination is completed, the temperature is naturally lowered, and the solid particles are rinsed with deionized water until the pH of the deionized water after the last rinse is 7.2. The rinsed solid particles are placed in an oven and dried at 80°C for 5 hours. After the drying is completed, the dried solid particles are ground to obtain a zeolite molecular sieve;

[0043] S1.5: Sodium hydroxide, deionized water and zeolite molecular sieve were stirred and mixed according to the molar ratio of n(NaOH):n(H2O):n(zeolite molecular sieve)=0.5:12:3, and reacted at 70°C for 90 minutes. After the reaction, the alkalized zeolite molecular sieve was rinsed 3 times with deionized water, and then placed in an oven and dried at 120°C for 60 minutes to obtain a multi-level pore zeolite molecular sieve;

[0044] S2: Preparation of supported molecular sieve catalysts,

[0045] S2.1: 30 parts by weight of magnesium nitrate hexahydrate are added to 120 parts by weight of deionized water, and stirred until completely dissolved to obtain a magnesium nitrate solution. 50 parts by weight of a sodium hydroxide solution having a mass concentration of 15% is slowly dripped into the magnesium nitrate solution while stirring. After the dripping is completed, stirring is continued for 15 minutes to generate a precipitate A, which is allowed to stand at room temperature for 12 hours to allow the precipitate A to age.

[0046] S2.2: The aged precipitate A is then taken out, washed with deionized water for three times, centrifuged, dried and calcined at a temperature of 550°C for 5 hours to obtain magnesium oxide;

[0047] S2.3: Add 50 parts by weight of tetraethyl silicate to 200 parts by weight of deionized water, slowly drop 80 parts by weight of a 6% dilute nitric acid solution while stirring, continue stirring for 5 hours after the addition is complete, and after the hydrolysis is completed, obtain a precipitate B, wash the precipitate B three times with deionized water, dry and calcine at a calcination temperature of 530° C. for 12 hours to obtain silicon dioxide;

[0048] S2.4: Add 40 parts by weight of hierarchical pore zeolite molecular sieve to 150 parts by weight of deionized water, stir evenly, then add 10 parts by weight of magnesium oxide and 10 parts by weight of silicon dioxide, stir and mix to obtain a suspension;

[0049] S2.5: The suspension was stirred at a constant speed of 1000 r / min for 6 h, and then filtered, dried and calcined at a temperature of 600° C. for 4 h to obtain a supported molecular sieve catalyst;

[0050] S3: Synthesis of high purity citric anhydride,

[0051] S3.1: adding anhydrous citric acid, acetic anhydride and acetic acid in a material ratio of 1:2.4:2.8 into a distillation reactor, and then adding a supported molecular sieve catalyst into the distillation reactor, wherein the mass of the supported molecular sieve catalyst accounts for 25% of the mass of the anhydrous citric acid;

[0052] S3.2: The relative vacuum degree of the distillation reactor is adjusted to 0 MPa, and the condenser temperature is 5°C. The substances in the distillation reactor are stirred at 50°C for 3 hours, during which the acetic acid is gradually distilled and condensed. After the stirring reaction is completed, the temperature in the distillation reactor is increased to 80°C, and the distillation is heated for 30 minutes to completely condense the acetic acid.

[0053] S3.3: Subsequently, the loaded molecular sieve catalyst is fished out from the distillation reactor, the remaining liquid in the distillation reactor is poured out, and 3 times the volume of a mixed organic solvent is added to the remaining liquid under continuous stirring, wherein the mixed organic solvent is composed of a mixture of dichloromethane and pentane in a volume ratio of 4:1, and the reaction is stirred for 20 minutes. After standing for 3 hours, a crystalline solid is obtained by filtering, and the crystalline solid is washed 3 times with ethylene glycol, and finally vacuum dried to obtain high-purity citric anhydride.

[0054] Then, the yield and purity of the obtained high-purity citric anhydride were detected, and the results are shown in the following table:

[0055] Citric acid conversion rate (%) Citric anhydride yield (%) Citric anhydride purity (%) 96.2 95.6 99.40

[0056] Example 2

[0057] A method for preparing high-purity citric anhydride comprises the following steps:

[0058] S1: Preparation of hierarchical pore zeolite molecular sieves,

[0059] S1.1: Sodium aluminate, nano silica sol and tetrapropylammonium hydroxide are used as aluminum source, silicon source and template respectively, and are added according to the molar ratio of n(Na2O):n(SiO2):n(Al2O3):n(TPAOH):n(H2O)=12:100:5:20:2500. Sodium aluminate, nano silica sol and tetrapropylammonium hydroxide are stirred and mixed, and then placed in a homogeneous reactor at 75°C for aging for 20 hours;

[0060] S1.2: After the aging is completed, the synthetic gel obtained in the reactor is transferred to a stainless steel reactor, and the stainless steel reactor is placed in an oven for static crystallization at 90°C for 72 hours to obtain a crystallized product, and the crystallized product is washed twice with deionized water;

[0061] S1.3: The washed crystallized product is placed in an oven and dried at 70°C for 8 hours. After drying, the crystallized product is ground into crystallized particles, and the crystallized particles are placed in a muffle furnace and calcined for 5 hours to remove the template. The calcination temperature is 500°C to obtain solid particles.

[0062] S1.4: After the calcination is completed, the temperature is naturally lowered, and the solid particles are rinsed with deionized water until the pH of the deionized water after the last rinse is 6.8. The rinsed solid particles are placed in an oven and dried at 70°C for 3 hours. After the drying is completed, the dried solid particles are ground to obtain a zeolite molecular sieve;

[0063] S1.5: Sodium hydroxide, deionized water and zeolite molecular sieve were stirred and mixed according to the molar ratio of n(NaOH):n(H2O):n(zeolite molecular sieve)=0.5:12:3, and reacted at 60°C for 40 minutes. After the reaction, the alkalized zeolite molecular sieve was rinsed twice with deionized water, and then placed in an oven and dried at 90°C for 30 minutes to obtain a multi-level pore zeolite molecular sieve;

[0064] S2: Preparation of supported molecular sieve catalysts,

[0065] S2.1: 30 parts by weight of magnesium nitrate hexahydrate are added to 120 parts by weight of deionized water, and stirred until completely dissolved to obtain a magnesium nitrate solution. 50 parts by weight of a sodium hydroxide solution having a mass concentration of 15% is slowly dripped into the magnesium nitrate solution while stirring. After the dripping is completed, stirring is continued for 10 minutes to generate a precipitate A, which is allowed to stand at room temperature for 8 hours to allow the precipitate A to precipitate and age;

[0066] S2.2: The aged precipitate A is then taken out, washed twice with deionized water, centrifuged, dried and calcined at a temperature of 500°C for 3 hours to obtain magnesium oxide;

[0067] S2.3: Add 50 parts by weight of tetraethyl silicate to 200 parts by weight of deionized water, and slowly drop 80 parts by weight of a 6% dilute nitric acid solution while stirring. After the dropwise addition is completed, continue stirring for 2 hours. After the hydrolysis is completed, a precipitate B is obtained. The precipitate B is washed twice with deionized water, dried and calcined at a calcination temperature of 480°C for 10 hours to obtain silicon dioxide;

[0068] S2.4: Add 40 parts by weight of hierarchical pore zeolite molecular sieve to 150 parts by weight of deionized water, stir evenly, then add 10 parts by weight of magnesium oxide and 10 parts by weight of silicon dioxide, stir and mix to obtain a suspension;

[0069] S2.5: The suspension was stirred at a constant speed of 800 r / min for 4 h, and then filtered, dried and calcined at a temperature of 350° C. for 3 h to obtain a supported molecular sieve catalyst;

[0070] S3: Synthesis of high purity citric anhydride,

[0071] S3.1: adding anhydrous citric acid, acetic anhydride and acetic acid in a material ratio of 1:2.4:2.8 into a distillation reactor, and then adding a supported molecular sieve catalyst into the distillation reactor, wherein the mass of the supported molecular sieve catalyst accounts for 25% of the mass of the anhydrous citric acid;

[0072] S3.2: The relative vacuum degree of the distillation reactor is adjusted to -0.1MPa, the condenser temperature is 0°C, and the substances in the distillation reactor are stirred at 36°C for 1 hour, during which acetic acid is gradually distilled and condensed. After the stirring reaction is completed, the temperature in the distillation reactor is increased to 60°C, and the distillation is heated for 15 minutes to completely condense and precipitate the acetic acid;

[0073] S3.3: Subsequently, the loaded molecular sieve catalyst is fished out from the distillation reactor, the remaining liquid in the distillation reactor is poured out, and 3 times the volume of a mixed organic solvent is added to the remaining liquid under continuous stirring, wherein the mixed organic solvent is composed of a mixture of dichloromethane and pentane in a volume ratio of 4:1, the reaction is stirred for 10 minutes, and after standing for 1 hour, a crystalline solid is obtained by filtering, the crystalline solid is washed twice with ethylene glycol, and finally vacuum dried to obtain high-purity citric anhydride.

[0074] Then, the yield and purity of the obtained high-purity citric anhydride were detected, and the results are shown in the following table:

[0075] Citric acid conversion rate (%) Citric anhydride yield (%) Citric anhydride purity (%) 93.7 92.7 98.9

[0076] Example 3

[0077] A method for preparing high-purity citric anhydride comprises the following steps:

[0078] S1: Preparation of hierarchical pore zeolite molecular sieves,

[0079] S1.1: Sodium aluminate, nano silica sol and tetrapropylammonium hydroxide are used as aluminum source, silicon source and template respectively, and are added according to the molar ratio of n(Na2O):n(SiO2):n(Al2O3):n(TPAOH):n(H2O)=8:90:1:16:2400. Sodium aluminate, nano silica sol and tetrapropylammonium hydroxide are stirred and mixed, and then placed in a homogeneous reactor at 84°C for aging for 24 hours;

[0080] S1.2: After the aging is completed, the synthetic gel obtained in the reactor is transferred to a stainless steel reactor, and the stainless steel reactor is placed in an oven for static crystallization at 180°C for 90 hours to obtain a crystallized product, and the crystallized product is washed 5 times with deionized water;

[0081] S1.3: The washed crystallized product is placed in an oven and dried at 80°C for 12 hours. After drying, the crystallized product is ground into crystallized particles, and the crystallized particles are placed in a muffle furnace and calcined for 6 hours to remove the template. The calcination temperature is 580°C to obtain solid particles.

[0082] S1.4: After the calcination is completed, the temperature is naturally lowered, and the solid particles are rinsed with deionized water until the pH of the deionized water after the last rinse is 7.2. The rinsed solid particles are placed in an oven and dried at 80°C for 5 hours. After the drying is completed, the dried solid particles are ground to obtain a zeolite molecular sieve;

[0083] S1.5: Sodium hydroxide, deionized water and zeolite molecular sieve were stirred and mixed according to the molar ratio of n(NaOH):n(H2O):n(zeolite molecular sieve)=0.2:9:1, and reacted at 70°C for 90 minutes. After the reaction, the alkalized zeolite molecular sieve was rinsed 3 times with deionized water, and then placed in an oven and dried at 120°C for 60 minutes to obtain a multi-level pore zeolite molecular sieve;

[0084] S2: Preparation of supported molecular sieve catalysts,

[0085] S2.1: 5 parts by weight of magnesium nitrate hexahydrate are added to 50 parts by weight of deionized water, and stirred until completely dissolved to obtain a magnesium nitrate solution. 20 parts by weight of a sodium hydroxide solution having a mass concentration of 15% is slowly dripped into the magnesium nitrate solution while stirring. After the dripping is completed, stirring is continued for 15 minutes to generate a precipitate A, which is allowed to stand at room temperature for 12 hours to allow the precipitate A to age;

[0086] S2.2: The aged precipitate A is then taken out, washed with deionized water for three times, centrifuged, dried and calcined at a temperature of 550°C for 5 hours to obtain magnesium oxide;

[0087] S2.3: Add 5 parts by weight of tetraethyl silicate to 100 parts by weight of deionized water, slowly drop 20 parts by weight of a 6% dilute nitric acid solution while stirring, continue stirring for 5 hours after the addition is complete, and after the hydrolysis is completed, obtain a precipitate B, wash the precipitate B three times with deionized water, dry and calcine at a calcination temperature of 530°C for 12 hours to obtain silicon dioxide;

[0088] S2.4: Add 20 parts by weight of hierarchical pore zeolite molecular sieve to 80 parts by weight of deionized water, stir evenly, then add 3 parts by weight of magnesium oxide and 3 parts by weight of silicon dioxide, stir and mix to obtain a suspension;

[0089] S2.5: The suspension was stirred at a constant speed of 1000 r / min for 6 h, and then filtered, dried and calcined at a temperature of 600° C. for 4 h to obtain a supported molecular sieve catalyst;

[0090] S3: Synthesis of high purity citric anhydride,

[0091] S3.1: adding anhydrous citric acid, acetic anhydride and acetic acid in a material ratio of 1:1:1 into a distillation reactor, and then adding a supported molecular sieve catalyst into the distillation reactor, wherein the mass of the supported molecular sieve catalyst accounts for 10% of the mass of the anhydrous citric acid;

[0092] S3.2: The relative vacuum degree of the distillation reactor is adjusted to 0 MPa, and the condenser temperature is 5°C. The substances in the distillation reactor are stirred at 50°C for 3 hours, during which the acetic acid is gradually distilled and condensed. After the stirring reaction is completed, the temperature in the distillation reactor is increased to 80°C, and the distillation is heated for 30 minutes to completely condense the acetic acid.

[0093] S3.3: Subsequently, the loaded molecular sieve catalyst is fished out from the distillation reactor, the remaining liquid in the distillation reactor is poured out, and 1 volume of a mixed organic solvent is added to the remaining liquid under continuous stirring, wherein the mixed organic solvent is composed of a mixture of dichloromethane and pentane in a volume ratio of 1:1, the reaction is stirred for 20 minutes, and after standing for 3 hours, a crystalline solid is obtained by filtering, the crystalline solid is washed 3 times with ethylene glycol, and finally vacuum dried to obtain high-purity citric anhydride.

[0094] Then, the yield and purity of the obtained high-purity citric anhydride were detected, and the results are shown in the following table:

[0095] Citric acid conversion rate (%) Citric anhydride yield (%) Citric anhydride purity (%) 94.1 93.2 99.0

[0096] Comparative Example 1

[0097] Compared with Example 1, the difference of Comparative Example 1 is that step S1.5 is removed, the multi-level pore zeolite molecular sieve in step S2.4 is replaced with an equal weight portion of zeolite molecular sieve, and the other steps remain unchanged to prepare a loaded molecular sieve catalyst and synthesize high-purity citric anhydride, which is recorded as Comparative Example 1.

[0098] Then, the yield and purity of the obtained high-purity citric anhydride were detected, and the results are shown in the following table:

[0099] Citric acid conversion rate (%) Citric anhydride yield (%) Citric anhydride purity (%) 82.8 80.1 97.8

[0100] By comparing the results of Example 1 and Comparative Example 1, it can be seen that in the synthesis of citric anhydride, the multi-level pore zeolite molecular sieve is used as a raw material, and the catalytic effect of the prepared supported molecular sieve catalyst is more significant, which can effectively accelerate the citric acid dehydration reaction, improve the citric acid conversion rate and the yield of citric anhydride. Under the same reaction conditions as Example 1, the supported molecular sieve catalyst prepared in Comparative Example 1 directly using the unalkalized zeolite molecular sieve as a raw material has a poor catalytic effect, and the conversion rate of citric acid is only 82.8%. It shows that the zeolite molecular sieve is alkalized and then prepared into a supported molecular sieve catalyst, which can effectively improve the catalytic effect, is beneficial to improve the conversion of citric acid, and then assist in improving the yield of citric anhydride.

[0101] Comparative Example 2

[0102] Compared with Example 1, the difference of Comparative Example 2 is that the magnesium oxide in step S2.4 is replaced by an equal weight portion of silicon dioxide, and only silicon dioxide is loaded on the multi-pore zeolite molecular sieve. The other steps remain unchanged to prepare a loaded molecular sieve catalyst and synthesize high-purity citric anhydride, which is recorded as Comparative Example 2.

[0103] Then, the yield and purity of the obtained high-purity citric anhydride were detected, and the results are shown in the following table:

[0104] Citric acid conversion rate (%) Citric anhydride yield (%) Citric anhydride purity (%) 86.5 78.8 97.3

[0105] Comparative Example 3

[0106] Compared with Example 1, the difference of Comparative Example 3 is that the silicon dioxide in step S2.4 is replaced by an equal weight portion of magnesium oxide, and only magnesium oxide is loaded on the multi-pore zeolite molecular sieve. The other steps remain unchanged to prepare a loaded molecular sieve catalyst and synthesize high-purity citric anhydride, which is recorded as Comparative Example 3.

[0107] Then, the yield and purity of the obtained high-purity citric anhydride were detected, and the results are shown in the following table:

[0108] Citric acid conversion rate (%) Citric anhydride yield (%) Citric anhydride purity (%) 89.3 80.2 96.2

[0109] Comparative Example 4

[0110] Compared with Example 1, the difference of Comparative Example 4 is that step S2 is removed, and the supported molecular sieve catalyst in step S3.2 is replaced with an equal mass portion of a multi-level pore zeolite molecular sieve, and the other steps remain unchanged to synthesize high-purity citric anhydride, which is recorded as Comparative Example 4.

[0111] Then, the yield and purity of the obtained high-purity citric anhydride were detected, and the results are shown in the following table:

[0112] Citric acid conversion rate (%) Citric anhydride yield (%) Citric anhydride purity (%) 80.9 71.3 92.9

[0113] Comparative Example 1 and Comparative Example 2-4 result can be known, under the same reaction conditions as Example 1, comparative example 2-3 single load has the catalytic action played by the supported molecular sieve catalyst of silicon dioxide or magnesium oxide relatively weak, the dehydration reaction speed of citric acid is slow, resulting in a lower citric acid conversion rate, comparative example 4 directly uses multi-stage hole zeolite molecular sieve as catalyst use, and can not play an efficient catalytic effect, and the dehydration reaction efficiency of citric acid is low.In addition, compared to embodiment 1-3, the citric acid conversion rate and citric anhydride yield in comparative example 2-4 differ greatly in pairs, illustrating that citric acid is in the process of dehydration reaction, and other side reaction efficiencies are also affected by the supported molecular sieve catalyst or multi-stage hole zeolite molecular sieve in comparative example 2-4, resulting in the occurrence of other side reactions affecting the synthesis yield of citric anhydride.It can be concluded that by simultaneously loading magnesium oxide and silicon dioxide on multi-stage hole zeolite molecular sieve, magnesium oxide and silicon dioxide mutually cooperate to promote the selectivity of citric acid dehydration reaction, while reducing the production of by-products, improving the yield and purity of citric anhydride.

[0114] Comparative Example 5

[0115] Compared with Example 1, the difference of Comparative Example 5 is that the mixed organic solvent in step S3.3 is replaced by an equal volume of chloroform, and the other steps remain unchanged to synthesize high-purity citric anhydride, which is recorded as Comparative Example 5.

[0116] Then, the yield and purity of the obtained high-purity citric anhydride were detected, and the results are shown in the following table:

[0117] Citric acid conversion rate (%) Citric anhydride yield (%) Citric anhydride purity (%) 92.8 76.4 94.5

[0118] Comparative Example 6

[0119] Compared with Example 1, the difference of Comparative Example 6 is that the oxygen-containing organic solvent in step S3.3 is replaced by chloroform, and the other steps remain unchanged to synthesize high-purity citric anhydride, which is recorded as Comparative Example 6.

[0120] Then, the yield and purity of the obtained high-purity citric anhydride were detected, and the results are shown in the following table:

[0121] Citric acid conversion rate (%) Citric anhydride yield (%) Citric anhydride purity (%) 91.9 85.7 91.2

[0122] By comparing the results of Example 1 and Comparative Examples 5-6, it can be seen that in the process of synthesizing citric anhydride, using a mixed organic solvent for crystallization and an oxygen-containing organic solvent for washing and purification can effectively improve the yield and purity of citric anhydride and obtain high-purity citric anhydride.

[0123] It should be understood that those skilled in the art can make improvements or changes based on the above description, and all such improvements and changes should fall within the scope of protection of the appended claims of the present invention. Parts not described in detail in this specification belong to the prior art known to those skilled in the art.

Claims

1. A method for preparing high-purity citric anhydride, characterized in that: The following steps are involved: S1: preparation of hierarchical pore zeolite molecular sieve, using sodium aluminate, nano-silica sol and tetrapropylammonium hydroxide as raw materials to prepare zeolite molecular sieve, stirring and mixing sodium hydroxide, deionized water and zeolite molecular sieve, heating reaction, washing and drying to obtain hierarchical pore zeolite molecular sieve; S2: Preparation of supported molecular sieve catalyst, using magnesium nitrate hexahydrate, deionized water and sodium hydroxide solution as raw materials to prepare magnesium oxide, using tetraethyl silicate, ionized water and dilute nitric acid solution as raw materials to prepare silicon dioxide, mixing hierarchical pore zeolite molecular sieve, deionized water, magnesium oxide and silicon dioxide, stirring and reacting, filtering, drying and calcining to obtain supported molecular sieve catalyst; S3: Synthesis of high-purity citric anhydride. Anhydrous citric acid, organic anhydride, organic acid and supported molecular sieve catalyst are put into a distillation reactor. The relative vacuum degree and temperature of the distillation reactor are adjusted. The substances in the distillation reactor are stirred to react. After the stirring reaction is completed, heating and distillation are performed. The supported molecular sieve catalyst is fished out. A mixed organic solvent is added to the remaining liquid in the distillation reactor. After crystallization, standing, filtering, washing with an oxygen-containing organic solvent and vacuum drying, high-purity citric anhydride is obtained.

2. The method for preparing high-purity citric anhydride according to claim 1, characterized in that: Step S1: preparation of multi-level pore zeolite molecular sieve, specifically comprising the following steps: S1.1: Sodium aluminate, nano silica sol and tetrapropylammonium hydroxide are used as aluminum source, silicon source and template respectively, according to n(Na2O):n(SiO2):n(Al2O3):n(TPAOH):n(H2O)=(8~12):(90~100): Sodium aluminate, nano silica sol and tetrapropylammonium hydroxide are mixed in a molar ratio of (1-5): (16-20): (2400-2500), and then placed in a homogeneous reactor at 75-84° C. for aging for 20-24 hours; S1.2: After the aging is completed, the synthetic gel obtained in the reactor is transferred to a stainless steel reactor, and the stainless steel reactor is placed in an oven for static crystallization at 90-180° C. for 72-90 hours to obtain a crystallized product, and the crystallized product is washed 2-5 times with deionized water; S1.3: placing the washed crystallized product in an oven and drying it at 70-80°C for 8-12 hours. After drying, grinding the crystallized product into crystallized particles, and calcining the crystallized particles in a muffle furnace for 5-6 hours at a calcination temperature of 500-580°C to obtain solid particles; S1.4: After the calcination is completed, the temperature is naturally lowered, and the solid particles are rinsed with deionized water until the pH of the deionized water after the last rinse is 6.8-7.

2. The rinsed solid particles are dried at 70-80°C for 3-5 hours, and after the drying is completed, the dried solid particles are ground to obtain a zeolite molecular sieve; S1.5: Sodium hydroxide, deionized water and zeolite molecular sieve are stirred and mixed according to the molar ratio of n(NaOH):n(H2O):n(zeolite molecular sieve)=(0.2-0.5):(9-12):(1-3), and reacted at 60-70°C for 40-90 min. After the reaction, the alkalized zeolite molecular sieve is rinsed 2-3 times with deionized water, and dried at 90-120°C for 30-60 min to obtain a multi-level pore zeolite molecular sieve.

3. The method for preparing high-purity citric anhydride according to claim 1, characterized in that: Step S2: preparation of supported molecular sieve catalyst, specifically comprising the following steps: S2.1: 5 to 30 parts by weight of magnesium nitrate hexahydrate are added to 50 to 120 parts by weight of deionized water, and stirred until completely dissolved to obtain a magnesium nitrate solution. 20 to 50 parts by weight of a sodium hydroxide solution having a mass concentration of 15% is slowly dripped into the magnesium nitrate solution, and stirred while dripping. After the dripping is completed, stirring is continued for 10 to 15 minutes to generate a precipitate A, which is allowed to stand at room temperature for 8 to 12 hours to allow the precipitate A to precipitate and age; S2.2: The aged precipitate A is then taken out, washed with deionized water 2 to 3 times, centrifuged, dried and calcined to obtain magnesium oxide; S2.3: Add 5 to 50 parts by weight of tetraethyl silicate to 100 to 200 parts by weight of deionized water, slowly dropwise add 20 to 80 parts by weight of a 6% dilute nitric acid solution while stirring, continue stirring for 2 to 5 hours after the addition is complete, and after the hydrolysis is completed, obtain a precipitate B, wash the precipitate B with deionized water 2 to 3 times, dry and calcine to obtain silicon dioxide; S2.4: Add 20 to 40 parts by weight of hierarchical pore zeolite molecular sieve to 80 to 150 parts by weight of deionized water, stir evenly, then add 3 to 10 parts by weight of magnesium oxide and 3 to 10 parts by weight of silicon dioxide, stir and mix to obtain a suspension; S2.5: Stir the suspension uniformly for 4 to 6 hours at a stirring speed of 800 to 1000 r / min, and then filter, dry and calcine to obtain a loaded molecular sieve catalyst.

4. The method for preparing high-purity citric anhydride according to claim 3, characterized in that: The calcination temperature in step S2.2 is 500-550° C., and the calcination time is 3-5 hours.

5. The method for preparing high-purity citric anhydride according to claim 4, characterized in that: The calcination temperature in step S2.3 is 480-530° C., and the calcination time is 10-12 hours.

6. The method for preparing high-purity citric anhydride according to claim 5, characterized in that: The calcination temperature in step S2.5 is 350-600° C., and the calcination time is 3-4 hours.

7. The method for preparing high-purity citric anhydride according to claim 1, characterized in that: Step S3: Synthesis of high-purity citric anhydride, specifically comprising the following steps: S3.1: adding anhydrous citric acid, organic acid anhydride and organic acid into a distillation reactor at a material ratio of 1:(1-2.4):(1-2.8) by molar ratio, and then adding a supported molecular sieve catalyst into the distillation reactor, wherein the mass of the supported molecular sieve catalyst accounts for 10-25% of the mass of the anhydrous citric acid; S3.2: The relative vacuum degree of the distillation reactor is adjusted to -0.1 to 0 MPa, the condenser temperature is 0 to 5°C, and the substances in the distillation reactor are stirred at 36 to 50°C for 1 to 3 hours, during which the organic acid is gradually distilled and condensed. After the stirring reaction is completed, the temperature in the distillation reactor is increased to 60 to 80°C, and the distillation is heated for 15 to 30 minutes to completely condense and precipitate the organic acid; S3.3: Subsequently, the loaded molecular sieve catalyst is fished out from the distillation reactor, the remaining liquid in the distillation reactor is poured out, and 1 to 3 times the volume of a mixed organic solvent is added to the remaining liquid under continuous stirring, and the reaction is stirred for 10 to 20 minutes. After standing for 1 to 3 hours, filtration is performed to obtain a crystalline solid, and the crystalline solid is washed 2 to 3 times with an oxygen-containing organic solvent, and finally vacuum dried to obtain high-purity citric anhydride.

8. The method for preparing high-purity citric anhydride according to claim 7, characterized in that: The organic anhydride is acetic anhydride, propionic anhydride, butyric anhydride or valeric anhydride, and the organic acid is acetic acid, propionic acid, butyric acid or valeric acid.

9. The method for preparing high-purity citric anhydride according to claim 8, characterized in that: The mixed organic solvent is composed of dichloromethane or chloromethane and a weak polar organic solvent in a volume ratio of (1-4):1, and the weak polar organic compound is pentane, hexane, octane, toluene or benzene.

10. The method for preparing high-purity citric anhydride according to claim 9, characterized in that: The oxygen-containing organic solvent is ethyl acetate, methyl acetate, ethanol, methanol, isopropanol, ethylene glycol, propylene glycol or diethylene glycol.