A process for preparing high-purity dimethyl carbonate by ester exchange

Abstract

The present invention discloses a process for preparing high-purity dimethyl carbonate by an ester exchange method, comprising the steps of: (1) adding propylene carbonate and anhydrous methanol to a reactor having the hydrotalcite solid base catalyst, then filling the reactor with nitrogen, and heating the reactor in a sealed manner for reaction, and releasing the reaction liquid after completion. (2) performing atmospheric distillation on the reaction liquid in a distillation device to obtain a crude dimethyl carbonate liquid, and then sequentially performing cooling crystallization, heating sweating, melting, and ultrafiltration membrane separation on the crude dimethyl carbonate liquid to obtain medical-grade dimethyl carbonate. The process of the present invention not only effectively improves the reaction activity of the hydrotalcite solid base catalyst and improves the raw material conversion rate by modifying the hydrotalcite, but also prepares medical-grade high-purity dimethyl carbonate by melt crystallization technology.

Description

Technical Field

[0001] The invention relates to the technical field of dimethyl carbonate preparation, and in particular to a process for preparing high-purity dimethyl carbonate by an ester exchange method. Background Art

[0002] Dimethyl carbonate is a colorless, transparent liquid with a pungent odor at room temperature, referred to as DMC. It is insoluble in water, but soluble in organic solvents such as ethanol and ether. In terms of product purity, dimethyl carbonate mainly includes three types: industrial grade, medical grade and battery grade. The purity of medical grade dimethyl carbonate needs to reach more than 99.99%. At present, the purification processes of dimethyl carbonate mainly include: azeotropic distillation, pressure swing distillation, extractive distillation, etc. However, azeotropic distillation requires the introduction of an azeotropic agent, and the recovery of the azeotropic agent consumes a lot of energy. The pressure swing distillation method requires high-pressure operation in the tower, which not only has high requirements for equipment, but also makes it difficult to ensure safety. The extractive distillation method requires the assistance of an extractant, which not only increases the complexity of the process and brings additional separation steps, but also many extractants are toxic and difficult to recover. In addition, the ester exchange method is used to prepare dimethyl carbonate and co-produce ethylene glycol because of its mature technology. It is widely used in the preparation of dimethyl carbonate due to its advantage of being capable of large-scale industrial production. However, it has the problem of insufficient ester exchange activity of the catalyst, which affects the conversion of the raw materials into dimethyl carbonate. Summary of the invention

[0003] The present invention provides a process for preparing high-purity dimethyl carbonate by an ester exchange method, which not only effectively improves the reaction activity of a hydrotalcite solid base catalyst and improves the raw material conversion rate by modifying the hydrotalcite, but also prepares medical-grade high-purity dimethyl carbonate by melt crystallization technology. Specifically, the technical scheme of the present invention is as follows.

[0004] A process for preparing high-purity dimethyl carbonate by an ester exchange method comprises the following steps:

[0005] (1) Preparation of hydrotalcite solid base catalyst:

[0006] ① Add magnesium aluminum hydrotalcite to Ce 3+ The mixture is added to an aqueous solution and then subjected to a hydrothermal reaction. After completion, a solid product is separated and set aside.

[0007] ② heating the solid product to obtain a catalyst precursor; adding the catalyst precursor to a sodium hydroxide or potassium hydroxide solution for heating and reaction; after completion, separating the catalyst precursor, washing with water and drying to obtain a hydrotalcite solid base catalyst for standby use.

[0008] (2) Propylene carbonate and anhydrous methanol are added into a reaction kettle containing the hydrotalcite solid base catalyst, and then nitrogen is filled into the reaction kettle, and the reaction kettle is sealed and heated for reaction, and the reaction liquid is released after completion.

[0009] (3) subjecting the reaction liquid to atmospheric distillation in a distillation device to obtain a crude dimethyl carbonate liquid, and then subjecting the crude dimethyl carbonate liquid to sequential cooling crystallization, heating sweating, melting, and ultrafiltration membrane separation steps to obtain medical grade dimethyl carbonate.

[0010] Furthermore, in step ①, the material-liquid ratio of the magnesium-aluminum aqueous solution is 1:10-25, and the Ce 3+ Ce in aqueous solution 3+ The mass fraction of Ce is 3-7%. 3+ Any one of cerium nitrate, cerium chloride, cerium acetate and the like.

[0011] Furthermore, in step ①, the temperature of the hydrothermal reaction is 170-220°C and the time is 80-130 min. 3+ Intercalation into the interlayers of hydrotalcite can replace part of the magnesium and aluminum elements, which not only stabilizes the structure of hydrotalcite during subsequent heating treatment, but also forms alkaline active catalytic sites to promote the reaction.

[0012] Furthermore, in step ②, the heating temperature is 450-500° C. and the time is 40-55 min. The magnesium hydroxide, aluminum hydroxide, carbonate and intercalated cerium salt in the magnesium aluminum hydrotalcite are decomposed by heating to form a composite alkaline neutral of magnesium oxide, aluminum oxide and cerium oxide, while increasing the specific surface area and pore volume of the hydrotalcite.

[0013] Furthermore, in step ②, the material-liquid ratio of the catalyst precursor to the sodium hydroxide or potassium hydroxide solution is 1:15-25, and the molar concentration of hydroxide in the solution is 0.6-1.4 mol / L. The aluminum oxide in the catalyst precursor is knocked out by alkaline solution, and the basic centers of magnesium oxide and cerium oxide are retained, further expanding the specific surface area and pore volume of the hydrotalcite.

[0014] Furthermore, in step ②, the catalyst precursor is added to a sodium hydroxide or potassium hydroxide solution and then heated to 60-70° C. for reaction for 30-50 minutes.

[0015] Furthermore, in step ②, the catalyst precursor is washed with clean water until it becomes neutral, then dried at 70-85° C. for 2-3 hours, and then cooled to room temperature to obtain the hydrotalcite solid base catalyst.

[0016] Furthermore, in step (2), the molar ratio of propylene carbonate to anhydrous methanol is 1:3-6.

[0017] Furthermore, in step (2), the amount of the hydrotalcite solid base catalyst added is 0.6-1.4% of the total mass of propylene carbonate and anhydrous methanol.

[0018] Furthermore, in step (2), the temperature of the heating reaction is 130-160° C., the time is 7-9 hours, and the nitrogen pressure in the reactor is 0.35-0.5 MPa.

[0019] Furthermore, in step (2), the temperature during the atmospheric distillation is 65-85° C., so as to preliminarily extract dimethyl carbonate from the reaction solution.

[0020] Furthermore, in step (2), the temperature of the cooling crystallization is 2-4°C, and the temperature of the heating sweating is 4-6°C. Optionally, the cooling or heating rate is 0.5-1.5°C / h. The melting temperature is 8-12°C.

[0021] Furthermore, in step (2), the flow rate of the ultrafiltration membrane is in the range of 40 to 70 m / s. 3 / h, the filtration accuracy is 0.2~0.25μm and the working pressure is 0.1~0.5MPa.

[0022] Compared with the prior art, the present invention has achieved at least the following beneficial technical effects:

[0023] (1) The present invention uses melt crystallization technology coupled with ultrafiltration membrane separation technology: the crude dimethyl carbonate liquid is subjected to cooling crystallization, heating sweating, melting, and ultrafiltration membrane separation in sequence to prepare high-purity medical-grade dimethyl carbonate with a purity of more than 99.99%.

[0024] (2) The present invention effectively improves the reaction activity of hydrotalcite solid base catalysts and improves the raw material conversion rate by modifying hydrotalcite. This is because the hydrotalcite solid base catalysts currently used have low activity and are difficult to promote and apply. To this end, the present invention firstly converts Ce into 3+ Intercalation into the interlayer of magnesium aluminum hydrotalcite expands the interlayer distance, which is beneficial to the migration of components in and out during the catalytic reaction. During the subsequent heat treatment, the magnesium hydroxide, aluminum hydroxide, carbonate and intercalated cerium salt in the hydrotalcite decompose to form magnesium oxide, aluminum oxide, cerium oxide composite alkaline neutrality, while increasing the specific surface area and pore volume of the hydrotalcite, and increasing the exposure of active sites. At the same time, the cerium oxide plays a role in stabilizing the structure of the hydrotalcite, reducing the effect of the hydrotalcite in keeping its layered structure intact, and the formation of a composite alkaline center by cerium oxide and the magnesium oxide is more conducive to promoting the reaction. Further, the present invention utilizes the amphoteric characteristics of aluminum oxide to knock out aluminum oxide through alkali solution, retain magnesium oxide and cerium oxide, further expand the specific surface area and pore volume of the hydrotalcite, improve the catalyst activity, and promote the occurrence of the reaction. This hydrotalcite solid base catalyst obtained after multiple modifications effectively overcomes the problem of low reaction activity of traditional hydrotalcite solid base catalysts. DETAILED DESCRIPTION

[0025] It should be noted that the following detailed descriptions are all illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0026] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof. The technical solution of the present invention is further described by the following specific embodiments.

[0027] Example 1

[0028] A process for preparing high-purity dimethyl carbonate by an ester exchange method comprises the following steps:

[0029] (1) Preparation of hydrotalcite solid base catalyst:

[0030] ① Mix magnesium aluminum hydrotalcite and 5wt.% cerium nitrate solution in a ratio of 1:15 and stir for 10 minutes, then add the obtained mixture into a high-pressure reactor and heat to 200°C for hydrothermal reaction for 90 minutes. After completion, cool to room temperature, filter to obtain a solid product, and set aside.

[0031] ② Heat the solid product to 480°C for 45 minutes, and cool it to obtain a catalyst precursor. Mix the precursor with 1.0 mol / L sodium hydroxide solution at a solid-liquid ratio of 1:20, stir, and then heat to 70°C for 40 minutes. After completion, filter the reaction liquid to obtain a catalyst precursor. Wash the catalyst precursor with clean water until it is neutral, then dry it in an oven at 80°C for 2 hours, and cool it to room temperature to obtain the hydrotalcite solid base catalyst for standby use.

[0032] (2) Propylene carbonate and anhydrous methanol were added into a stainless steel autoclave at a molar ratio of 1:5, wherein the hydrotalcite solid base catalyst prepared in this embodiment accounted for 0.9% of the total mass of propylene carbonate and anhydrous methanol. Then, 0.5 MPa of nitrogen was charged into the autoclave, heated to 150° C. for reaction for 8 hours, and the reaction liquid was discharged after completion.

[0033] (3) The reaction solution is subjected to atmospheric distillation at 80° C. in a distillation device to obtain a crude dimethyl carbonate liquid. The crude dimethyl carbonate liquid is then added to a crystallizer and cooled to 2° C. (cooling rate 0.5° C. / h) to allow dimethyl carbonate to crystallize, and then the dimethyl carbonate crystals are separated.

[0034] (4) The dimethyl carbonate crystals are heated to 4° C. (heating rate 1.0° C. / h) to sweat and remove moisture therein. Finally, the dimethyl carbonate crystals are melted by heating to 10° C. at a rate of 5.0° C. / h to obtain liquid purified dimethyl carbonate.

[0035] (5) The purified dimethyl carbonate is subjected to ultrafiltration treatment using ultrafiltration membrane separation technology, wherein the ultrafiltration membrane flow rate is 50 m 3 / h, the filtration accuracy is 0.22μm, and the working pressure is 0.5MPa. Medical grade dimethyl carbonate is obtained.

[0036] Example 2

[0037] A process for preparing high-purity dimethyl carbonate by an ester exchange method comprises the following steps:

[0038] (1) Preparation of hydrotalcite solid base catalyst:

[0039] ① Mix magnesium aluminum hydrotalcite and 3wt.% cerium acetate solution in a ratio of 1:10 and stir for 10 minutes, then add the obtained mixture into a high-pressure reactor and heat to 220°C for hydrothermal reaction for 80 minutes. After completion, cool to room temperature, filter to obtain a solid product, and set aside.

[0040] ② Heat the solid product to 450°C for 55 minutes, and then cool it to obtain a catalyst precursor. Mix the precursor with 0.5 mol / L sodium hydroxide solution at a solid-liquid ratio of 1:25, stir, and then heat to 65°C for 50 minutes. After completion, filter the reaction liquid to obtain a catalyst precursor. Wash the catalyst precursor with clean water until it is neutral, then dry it in an oven at 85°C for 2 hours, and cool it to room temperature to obtain the hydrotalcite solid base catalyst for standby use.

[0041] (2) Propylene carbonate and anhydrous methanol were added into a stainless steel autoclave at a molar ratio of 1:6, wherein the hydrotalcite solid base catalyst prepared in this embodiment accounted for 1.4% of the total mass of propylene carbonate and anhydrous methanol. Then, 0.35 MPa of nitrogen was charged into the autoclave, heated to 130° C. for reaction for 9 hours, and the reaction liquid was discharged after completion.

[0042] (3) The reaction solution is subjected to atmospheric distillation at 85° C. in a distillation device to obtain a crude dimethyl carbonate liquid. The crude dimethyl carbonate liquid is then added to a crystallizer and cooled to 3° C. (cooling rate 1.0° C. / h) to allow dimethyl carbonate to crystallize, and then the dimethyl carbonate crystals are separated.

[0043] (4) The dimethyl carbonate crystals are heated to 5° C. (heating rate 1.5° C. / h) to sweat and remove moisture therein. Finally, the dimethyl carbonate crystals are melted by heating to 8° C. at a rate of 4.0° C. / h to obtain liquid purified dimethyl carbonate.

[0044] (5) The purified dimethyl carbonate is subjected to ultrafiltration treatment using ultrafiltration membrane separation technology, wherein the ultrafiltration membrane flow rate is 40 m 3 / h, the filtration accuracy is 0.20μm, and the working pressure is 0.3MPa. Medical grade dimethyl carbonate is obtained.

[0045] Example 3

[0046] A process for preparing high-purity dimethyl carbonate by an ester exchange method comprises the following steps:

[0047] (1) Preparation of hydrotalcite solid base catalyst:

[0048] ① Mix magnesium aluminum hydrotalcite and 7wt.% cerium chloride solution in a ratio of 1:25 and stir for 15 minutes, then add the obtained mixture into a high-pressure reactor and heat to 170°C for hydrothermal reaction for 130 minutes. After completion, cool to room temperature, filter to obtain a solid product, and set aside.

[0049] ② Heat the solid product to 500°C for 40 minutes, and obtain a catalyst precursor after cooling. Mix the precursor with 1.2 mol / L potassium hydroxide solution at a solid-liquid ratio of 1:15, stir, and then heat to 60°C for 30 minutes. After completion, filter the reaction liquid to obtain a catalyst precursor. Wash the catalyst precursor with clean water until it is neutral, then dry it in an oven at 70°C for 3 hours, and cool it to room temperature to obtain the hydrotalcite solid base catalyst for standby use.

[0050] (2) Propylene carbonate and anhydrous methanol were added into a stainless steel autoclave at a molar ratio of 1:3, wherein the hydrotalcite solid base catalyst prepared in this embodiment accounted for 0.6% of the total mass of propylene carbonate and anhydrous methanol. Then, 0.4 MPa of nitrogen was charged into the autoclave, heated to 160° C. for reaction for 7 hours, and the reaction liquid was discharged after completion.

[0051] (3) The reaction solution is subjected to atmospheric distillation at 65° C. in a distillation device to obtain a crude dimethyl carbonate liquid. The crude dimethyl carbonate liquid is then added to a crystallizer and cooled to 4° C. (cooling rate 1.5° C. / h) to allow dimethyl carbonate to crystallize, and then the dimethyl carbonate crystals are separated.

[0052] (4) The dimethyl carbonate crystals are heated to 6° C. (heating rate 1.5° C. / h) to sweat and remove moisture therein. Finally, the dimethyl carbonate crystals are melted by heating to 12° C. at a rate of 6.0° C. / h to obtain liquid purified dimethyl carbonate.

[0053] (5) The purified dimethyl carbonate is subjected to ultrafiltration treatment using ultrafiltration membrane separation technology, wherein the ultrafiltration membrane flow rate is 70 m 3 / h, the filtration accuracy is 0.25μm, and the working pressure is 0.5MPa. Medical grade dimethyl carbonate is obtained.

[0054] Example 4

[0055] A process for preparing high-purity dimethyl carbonate by an ester exchange method comprises the following steps:

[0056] (1) Preparation of hydrotalcite solid base catalyst:

[0057] ① Mix magnesium aluminum hydrotalcite and water in a ratio of 1:15 and stir for 10 minutes, then add the obtained mixture into a high-pressure reactor and heat to 200°C for hydrothermal reaction for 90 minutes. After completion, cool to room temperature, filter to obtain a solid product, and set aside.

[0058] ② Heat the solid product to 480°C for 45 minutes, and cool it to obtain a catalyst precursor. Mix the precursor with 1.0 mol / L sodium hydroxide solution at a solid-liquid ratio of 1:20, stir, and then heat to 70°C for 40 minutes. After completion, filter the reaction liquid to obtain a catalyst precursor. Wash the catalyst precursor with clean water until it is neutral, then dry it in an oven at 80°C for 2 hours, and cool it to room temperature to obtain the hydrotalcite solid base catalyst for standby use.

[0059] (2) Propylene carbonate and anhydrous methanol were added into a stainless steel autoclave at a molar ratio of 1:5, wherein the hydrotalcite solid base catalyst prepared in this embodiment accounted for 0.9% of the total mass of propylene carbonate and anhydrous methanol. Then, 0.5 MPa of nitrogen was charged into the autoclave, heated to 150° C. for reaction for 8 hours, and the reaction liquid was discharged after completion.

[0060] (3) The reaction solution is subjected to atmospheric distillation at 80° C. in a distillation device to obtain a crude dimethyl carbonate liquid. The crude dimethyl carbonate liquid is then added to a crystallizer and cooled to 2° C. (cooling rate 0.5° C. / h) to allow dimethyl carbonate to crystallize, and then the dimethyl carbonate crystals are separated.

[0061] (4) The dimethyl carbonate crystals are heated to 4° C. (heating rate 1.0° C. / h) to sweat and remove moisture therein. Finally, the dimethyl carbonate crystals are melted by heating to 10° C. at a rate of 5.0° C. / h to obtain liquid purified dimethyl carbonate.

[0062] (5) The purified dimethyl carbonate is subjected to ultrafiltration treatment using ultrafiltration membrane separation technology, wherein the ultrafiltration membrane flow rate is 50 m 3 / h, the filtration accuracy is 0.22μm, and the working pressure is 0.5MPa. Medical grade dimethyl carbonate is obtained.

[0063] Example 5

[0064] A process for preparing high-purity dimethyl carbonate by an ester exchange method comprises the following steps:

[0065] (1) Preparation of hydrotalcite solid base catalyst:

[0066] ① Mix magnesium aluminum hydrotalcite and 3wt.% cerium acetate solution in a ratio of 1:10 and stir for 10 minutes, then add the obtained mixture into a high-pressure reactor and heat to 220°C for hydrothermal reaction for 80 minutes. After completion, cool to room temperature, filter to obtain a solid product, and set aside.

[0067] ② The solid product was mixed with 0.5 mol / L sodium hydroxide solution at a solid-liquid ratio of 1:25, stirred, and then heated to 65°C for reaction for 50 minutes. After completion, the reaction liquid was filtered to obtain a catalyst precursor. The catalyst precursor was washed with clean water until neutral, and then dried in an oven at 85°C for 2 hours, cooled to room temperature, and the hydrotalcite solid base catalyst was obtained for standby use.

[0068] (2) Propylene carbonate and anhydrous methanol were added into a stainless steel autoclave at a molar ratio of 1:6, wherein the hydrotalcite solid base catalyst prepared in this embodiment accounted for 1.4% of the total mass of propylene carbonate and anhydrous methanol. Then, 0.35 MPa of nitrogen was charged into the autoclave, heated to 130° C. for reaction for 9 hours, and the reaction liquid was discharged after completion.

[0069] (3) The reaction solution is subjected to atmospheric distillation at 85° C. in a distillation device to obtain a crude dimethyl carbonate liquid. The crude dimethyl carbonate liquid is then added to a crystallizer and cooled to 3° C. (cooling rate 1.0° C. / h) to allow dimethyl carbonate to crystallize, and then the dimethyl carbonate crystals are separated.

[0070] (4) The dimethyl carbonate crystals are heated to 5° C. (heating rate 1.5° C. / h) to sweat and remove moisture therein. Finally, the dimethyl carbonate crystals are melted by heating to 8° C. at a rate of 4.0° C. / h to obtain liquid purified dimethyl carbonate.

[0071] (5) The purified dimethyl carbonate is subjected to ultrafiltration treatment using ultrafiltration membrane separation technology, wherein the ultrafiltration membrane flow rate is 40 m 3 / h, the filtration accuracy is 0.20μm, and the working pressure is 0.3MPa. Medical grade dimethyl carbonate is obtained.

[0072] Example 6

[0073] A process for preparing high-purity dimethyl carbonate by an ester exchange method comprises the following steps:

[0074] (1) Preparation of hydrotalcite solid base catalyst:

[0075] ① Mix magnesium aluminum hydrotalcite and 7wt.% cerium chloride solution in a ratio of 1:25 and stir for 15 minutes, then add the obtained mixture into a high-pressure reactor and heat to 170°C for hydrothermal reaction for 130 minutes. After completion, cool to room temperature, filter to obtain a solid product, and set aside.

[0076] ② The solid product is heated to 500°C for 40 minutes, and then cooled to obtain a catalyst precursor. The precursor is mixed with water at a solid-liquid ratio of 1:15 and stirred, and then heated to 60°C for 30 minutes. After completion, the reaction liquid is filtered to obtain a catalyst precursor, which is dried in an oven at 70°C for 3 hours and cooled to room temperature to obtain the hydrotalcite solid base catalyst for standby use.

[0077] (2) Propylene carbonate and anhydrous methanol were added into a stainless steel autoclave at a molar ratio of 1:3, wherein the hydrotalcite solid base catalyst prepared in this embodiment accounted for 0.6% of the total mass of propylene carbonate and anhydrous methanol. Then, 0.4 MPa of nitrogen was charged into the autoclave, heated to 160° C. for reaction for 7 hours, and the reaction liquid was discharged after completion.

[0078] (3) The reaction solution is subjected to atmospheric distillation at 65° C. in a distillation device to obtain a crude dimethyl carbonate liquid. The crude dimethyl carbonate liquid is then added to a crystallizer and cooled to 4° C. (cooling rate 1.5° C. / h) to allow dimethyl carbonate to crystallize, and then the dimethyl carbonate crystals are separated.

[0079] (4) The dimethyl carbonate crystals are heated to 6° C. (heating rate 1.5° C. / h) to sweat and remove moisture therein. Finally, the dimethyl carbonate crystals are melted by heating to 12° C. at a rate of 6.0° C. / h to obtain liquid purified dimethyl carbonate.

[0080] (5) The purified dimethyl carbonate is subjected to ultrafiltration treatment using ultrafiltration membrane separation technology, wherein the ultrafiltration membrane flow rate is 70 m 3 / h, the filtration accuracy is 0.25μm, and the working pressure is 0.5MPa. Medical grade dimethyl carbonate is obtained.

[0081] Performance Testing

[0082] The conversion rate of propylene carbonate and the purity of the prepared medical-grade dimethyl carbonate in Examples 1 to 6 were calculated, and the results are shown in Table 1. From the purity test results of Examples 1 to 6, it can be seen that the present invention uses melt crystallization technology coupled with ultrafiltration membrane separation technology to prepare high-purity medical-grade dimethyl carbonate with a purity of more than 99.99%. From the purity test results of Examples 1 to 6, it can be seen that, compared with Examples 4 to 6, Examples 1 to 3 effectively improve the reaction activity of the hydrotalcite solid base catalyst by modifying the hydrotalcite, and significantly improve the conversion rate of the raw material propylene carbonate.

[0083] Table 1

[0084] Example No. 1 2 3 4 5 6 Propylene carbonate conversion rate / % 96.74 94.39 95.27 86.81 64.52 82.06 Dimethyl carbonate purity / % 99.995 99.992 99.996 99.993 99.991 99.995

[0085] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A process for preparing high-purity dimethyl carbonate by an ester exchange method, characterized in that: The steps include: (1) Preparation of hydrotalcite solid base catalyst: ① Add magnesium aluminum hydrotalcite to Ce 3+ The solid product is separated and set aside after the reaction is completed; ② heating the solid product to obtain a catalyst precursor; adding the catalyst precursor to a sodium hydroxide or potassium hydroxide solution and heating the catalyst precursor to react; after completion, separating the catalyst precursor, washing with water and drying the resulting solid base catalyst of hydrotalcite for standby use; (2) adding propylene carbonate and anhydrous methanol into a reaction kettle containing the hydrotalcite solid base catalyst, then filling the reaction kettle with nitrogen, sealing and heating the reaction kettle for reaction, and releasing the reaction liquid after completion; (3) subjecting the reaction solution to atmospheric distillation in a distillation device to obtain a crude dimethyl carbonate liquid, and then subjecting the crude dimethyl carbonate liquid to the steps of cooling crystallization, heating sweating, melting, and ultrafiltration membrane separation in sequence to obtain medical grade dimethyl carbonate; In step ②, the heating temperature is 450-500° C. and the time is 40-55 min; In step ②, the material-liquid ratio of the catalyst precursor to the sodium hydroxide or potassium hydroxide solution is 1:15-25, and the molar concentration of hydroxide in the solution is 0.6-1.4 mol / L; In step ②, the catalyst precursor is added to a sodium hydroxide or potassium hydroxide solution and heated to 60-70° C. for reaction for 30-50 minutes; In step (2), the flow rate of the ultrafiltration membrane is in the range of 40 to 70 m / s. 3 / h, the filtration accuracy is 0.2~0.25μm and the working pressure is 0.1~0.5MPa.

2. The process for preparing high-purity dimethyl carbonate by the ester exchange method according to claim 1, characterized in that: In step ①, the material-liquid ratio of the magnesium aluminum hydrolyzate and the aqueous solution is 1:10-25, and the Ce 3+ Ce in aqueous solution 3+ The mass fraction is 3 to 7%.

3. The process for preparing high-purity dimethyl carbonate by the transesterification method according to claim 1, characterized in that: In step ①, the Ce 3+ Provided by any one of cerium nitrate, cerium chloride, and cerium acetate.

4. The process for preparing high-purity dimethyl carbonate by the ester exchange method according to claim 1, characterized in that: In step ①, the temperature of the hydrothermal reaction is 170-220° C., and the time is 80-130 min.

5. The process for preparing high-purity dimethyl carbonate by the ester exchange method according to claim 1, characterized in that: In step ②, the catalyst precursor is washed with clean water until it becomes neutral, then dried at 70-85° C. for 2-3 hours, and then cooled to room temperature to obtain the hydrotalcite solid base catalyst.

6. The process for preparing high-purity dimethyl carbonate by the ester exchange method according to claim 1, characterized in that: In step (2), the molar ratio of propylene carbonate to anhydrous methanol is 1:3-6.

7. The process for preparing high-purity dimethyl carbonate by the transesterification method according to claim 1, characterized in that: In step (2), the amount of the hydrotalcite solid base catalyst added is 0.6-1.4% of the total mass of propylene carbonate and anhydrous methanol.

8. The process for preparing high-purity dimethyl carbonate by the ester exchange method according to claim 1, characterized in that: In step (2), the temperature of the heating reaction is 130-160° C., the time is 7-9 hours, and the nitrogen pressure in the reactor is 0.35-0.5 MPa.

9. The process for preparing high-purity dimethyl carbonate by the ester exchange method according to any one of claims 1 to 8, characterized in that: In step (2), the temperature during the atmospheric distillation is 65-85°C.

10. The process for preparing high-purity dimethyl carbonate by the ester exchange method according to any one of claims 1 to 8, characterized in that: In step (2), the temperature of the cooling crystallization is 2-4°C, the temperature of the heating sweating is 4-6°C; and the melting temperature is 8-12°C.

11. The process for preparing high-purity dimethyl carbonate by the ester exchange method according to any one of claims 1 to 8, characterized in that: The rate of cooling or heating is 0.5-1.5°C / h.