A process for the preparation of dimethyl carbonate

Abstract

The application belongs to the technical field of organic synthesis and relates to a method for preparing dimethyl carbonate. The method uses a deep eutectic solvent (DES) as a catalyst to catalyze the reaction of CO2 and methanol to prepare dimethyl carbonate in the presence of an organic solvent and a dehydrating agent, and the reaction conditions are as follows: the initial pressure of CO2 is 1-6 MPa, the temperature is 100-120 DEG C, and the time is 4-10 h. After the reaction, the layers are separated by standing, the lower layer DES is separated from the dehydrating agent by centrifugation, and then can be directly recycled, and the upper layer product phase is removed from the organic solvent by rectification to obtain the product dimethyl carbonate, and the organic solvent can be recycled. The characteristics of the application are that the DES-dehydrating agent system can realize the high-yield and high-purity synthesis of dimethyl carbonate, solves the problems of difficult recovery of the catalyst, many by-products and low yield in the traditional method, has the advantages of recyclable catalyst and solvent, low cost and simple operation, and is suitable for industrial production.

Description

Technical Field

[0001] This invention belongs to the field of organic synthesis technology, specifically relating to a method for preparing dimethyl carbonate by catalyzing the reaction of CO2 and methanol in the presence of an organic solvent using DES as a catalyst. Background Technology

[0002] Dimethyl carbonate (DMC), as an important green chemical raw material, contains functional groups such as carbonyl, methyl, and methoxy groups in its molecular structure, exhibiting good reactivity and biocompatibility. It is widely used in pharmaceuticals, pesticides, coatings, lithium battery electrolytes, and fuel additives. However, its traditional preparation methods have significant drawbacks. For example, the phosgene method, using methanol as a raw material, while a mature process, is restricted due to the highly toxic nature of phosgene. The methanol oxidative carbonylation method is divided into liquid-phase and gas-phase methods, using methanol, CO, and O2 as raw materials and employing copper-based catalysts such as CuCl. - Loss leads to problems such as rapid catalyst deactivation, difficulty in separating catalyst from product, and increased equipment wear and corrosion due to byproduct HCl. The transesterification method uses cyclic carbonates, such as ethylene carbonate (EC), to react with methanol, which has high raw material costs, requires high-pressure reaction conditions, and produces a large amount of alkylene glycols as byproducts. The urea alcoholysis method uses urea and methanol as raw materials and reacts them under the action of heterogeneous or homogeneous catalysts such as dibutyldimethyltinane and tetraphenyltin. The raw materials are inexpensive, but the reaction requires high temperature and high pressure, and the byproduct ammonia is difficult to handle, resulting in low product purity. In existing catalytic systems, ionic liquid (IL) catalysts, such as EMIM-BF4, exhibit rapid activity decline and complex separation processes. Metal oxide catalysts, such as those in Chinese patent CN106478421A, use multi-component composite metal oxides to catalyze the alcoholysis of alkylene glycols and urea to produce cyclic carbonates and ammonia. The cyclic carbonates then undergo transesterification with methanol to prepare dimethyl carbonate. However, nitrogen impurities must be removed before the transesterification reaction, making the reaction complex and energy-intensive. Supported catalysts, such as those in Chinese patent CN107649158B, use porous spherical activated alumina as a support, on which potassium carbonate and sodium carbonate are loaded. This preparation is relatively complex and requires sophisticated reaction equipment. This invention utilizes the synergistic effect of DES and organic solvents. The unique hydrogen-bonding network of DES can simultaneously activate CO2 and methanol, resulting in a stable product yield of over 91.5% and a purity of ≥99.2%, significantly superior to existing technologies.

[0003] Based on this, this invention proposes a method for reacting CO2 with methanol to produce dimethyl carbonate using DES as a catalyst in the presence of an organic solvent. The reaction conditions are mild, with an initial CO2 pressure of 1-6 MPa and a reaction temperature of 100-120°C. This eliminates the need for high-temperature and high-pressure conditions, reducing the requirements for reaction equipment and energy consumption. Furthermore, the synergistic effect of DES and the organic solvent allows for efficient catalyst recovery and recycling after the reaction through static separation and centrifugation. The product and organic solvent are efficiently separated by distillation, and the organic solvent and dehydrating agent can also be recycled, significantly reducing production costs. This method achieves high-yield and high-purity synthesis of dimethyl carbonate, with simple operation and promising prospects for industrial application. Summary of the Invention

[0004] The purpose of this invention is to provide an efficient and highly selective method for synthesizing dimethyl carbonate.

[0005] Based on the above, this invention relates to a method for preparing dimethyl carbonate, characterized by using DES as a catalyst in the presence of an organic solvent to catalyze the reaction of CO2 with methanol to prepare dimethyl carbonate. The initial CO2 pressure is 1-6 MPa, the molar ratio of methanol to dehydrating agent is 1:1-1:3, the molar ratio of methanol to DES is 1:2-1:4, the reaction temperature is 100-120℃, and the reaction time is 4-10 h. After the reaction, the mixture is allowed to stand and separate into layers, yielding a lower DES phase and an upper product phase containing organic solvent. The recovered lower DES can be directly recycled after centrifugation to remove the dehydrating agent. The upper product phase containing organic solvent is purified by distillation to remove the organic solvent, yielding dimethyl carbonate with a yield ≥91.5% and a purity ≥99.2%. The organic solvent obtained from distillation can be recycled. The recovered DES, after being reused 10 times under the same experimental conditions, consistently yields dimethyl carbonate with a yield ≥90.3% and a purity ≥99.1%. The hydrogen bond acceptor is selected from one of choline chloride (ChCl), polyethylene glycol (PEG), and glycerol (GLY); the hydrogen bond donor is selected from one of trifluoromethanesulfonic acid (TFMSA), trichloroacetic acid (TCA), and p-toluenesulfonic acid (PTSA); the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:1 to 1:3; the organic solvent is selected from one of toluene, methyl tert-butyl ether, and methylcyclohexane; the dehydrating agent is selected from one of 4A molecular sieve, anhydrous magnesium sulfate, and silica gel, and can be recycled after drying and dehydration.

[0006] This invention solves this technical problem through the following technical solution:

[0007] The specific technical solution is illustrated using a eutectic solvent prepared by choline chloride and trichloroacetic acid in a molar ratio of 1:2. This eutectic solvent is named [ChCl][TCA]2, and the naming method for other eutectic solvents follows the same principle.

[0008] Choline chloride and trichloroacetic acid were added to a 100 mL three-necked flask in a molar ratio of 1:2. The flask was placed on a thermostatic magnetic stirrer and heated to 70 °C under nitrogen protection. The mixture was stirred at 500 rpm for 2 hours until a homogeneous and transparent liquid was formed. The mixture was then cooled to room temperature to obtain [ChCl][TCA]2, which was stored in a dry and sealed container for later use.

[0009] 1 mol methanol, 2 mol anhydrous magnesium sulfate, 3 mol [ChCl][TCA]2 and 50 mL toluene were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced into the reactor until the initial reaction pressure was 3.5 MPa. The reactor was reacted at 110 °C for 7 hours. After the reaction, the mixture was allowed to stand and separate into two phases, yielding a lower DES phase and an upper product phase containing organic solvent. The DES and anhydrous magnesium sulfate were separated by centrifugation. The recovered DES could be directly recycled, with a single recovery rate of ≥95.3%. The toluene-containing upper product phase was purified by distillation to obtain dimethyl carbonate, with a product yield of 92.6% and a purity of 99.5%.

[0010] Compared with traditional methods, the present invention is characterized by:

[0011] 1. The basic sites and hydrogen bond network of DES activate methanol and CO2, promoting the nucleophilic addition reaction of CO2 and methanol. The organic solvent suppresses side reactions, solving the problem of many by-products in traditional methods, and achieving high yield and high purity synthesis of dimethyl carbonate.

[0012] 2. The phase separation characteristics of DES and organic solvents construct an integrated "reaction-separation" system, enabling efficient recovery and recycling of DES catalyst and organic solvent.

[0013] 3. Overcoming the problems of traditional catalysts being difficult to recover and requiring harsh conditions, the reaction conditions are mild and the operation is simple. The production cost is significantly reduced through the recycling of catalysts, dehydrating agents and organic solvents, making it suitable for industrialization. Detailed Implementation

[0014] The method of the present invention will be further described below with reference to embodiments, but this is not intended to limit the present invention.

[0015] Example 1: Choline chloride and trichloroacetic acid were added to a three-necked flask at a molar ratio of 1:2 and stirred under nitrogen protection to prepare [ChCl][TCA]2. 1 mol methanol, 2 mol anhydrous magnesium sulfate, 3 mol [ChCl][TCA]2, and 50 mL toluene were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial reaction pressure in the reactor reached 3.5 MPa. The reaction was carried out at 110 °C for 7 hours. After the reaction, the mixture was allowed to stand and separate into two phases, yielding a lower DES phase and an upper product phase containing organic solvent. The DES and anhydrous magnesium sulfate were separated by centrifugation. The recovered DES could be directly recycled, with a single-cycle recovery rate ≥95.3%. The toluene-containing upper product phase was purified by distillation to obtain dimethyl carbonate, with a product yield of 92.6% and a product purity of 99.5%.

[0016] Comparative Example 1: 0.5 g of a ZnO-Al₂O₃-ZrO₂ composite metal oxide catalyst (mass ratio 1:1:1), 1 mol of ethylene glycol, and 1.2 mol of urea were added to a 250 mL reactor and reacted at 180 °C and 1 MPa for 5 h to produce ethylene carbonate and ammonia. After the reaction, unreacted raw materials and ammonia impurities were removed by distillation. 1 mol of the generated ethylene carbonate and 3 mol of methanol were added to the reactor and reacted at 150 °C and 2 MPa for 4 h in the presence of 0.3 g of the composite metal oxide catalyst. Dimethyl carbonate was obtained by distillation with a yield of 68.3% and a purity of 97.1%. The catalyst needed to be washed with ethanol, dried, and regenerated. After three cycles, the yield decreased to 52.6%.

[0017] Comparative Example 2: A porous spherical activated alumina support was calcined at 500℃ for 3 hours, impregnated in a mixed solution of potassium carbonate and sodium carbonate in a mass ratio of 2:1, dried at 80℃, and then calcined at 550℃ for 4 hours to obtain a supported catalyst. 1 mol of methanol, 0.1 g of catalyst, and 50 mL of cyclohexane were added to a 250 mL reactor, and CO2 was introduced to a pressure of 4 MPa. The reaction was carried out at 140℃ for 8 hours. Dimethyl carbonate was obtained by distillation, with a product yield of 72.5% and a purity of 96.8%. Catalyst separation required centrifugation and washing; after 5 cycles, the yield decreased to 61.3%.

[0018] Example 2: Choline chloride and trifluoromethanesulfonic acid were added to a three-necked flask at a molar ratio of 1:1 and stirred continuously under nitrogen protection to obtain [ChCl][TFMSA]. 1 mol methanol, 1 mol 4A molecular sieve, 2 mol [ChCl][TFMSA] and 50 mL methyl tert-butyl ether were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial reaction pressure in the reactor was 1 MPa. The reaction was carried out at 100 °C for 4 hours. After the reaction, the mixture was allowed to stand and separate into two phases, yielding a lower DES phase and an upper product phase containing organic solvent. The DES and 4A molecular sieve were separated by centrifugation. The recovered DES could be directly recycled, with a single recovery rate ≥94.8%. The upper product phase containing methyl tert-butyl ether was removed by distillation to obtain dimethyl carbonate, with a product yield of 91.7% and a product purity of 99.2%.

[0019] Example 3: Polyethylene glycol and p-toluenesulfonic acid were added to a three-necked flask at a molar ratio of 1:3 and stirred continuously under nitrogen protection to obtain [PEG][PTSA]3. 1 mol methanol, 3 mol silica gel, 4 mol [PEG][PTSA]3 and 50 mL methylcyclohexane were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial reaction pressure in the reactor reached 6 MPa. The reaction was carried out at 120 °C for 10 hours. After the reaction, the mixture was allowed to stand and separate into layers, yielding a lower DES phase and an upper product phase containing organic solvent. The DES and silica gel were separated by centrifugation. The recovered DES could be directly recycled, with a single-cycle recovery rate ≥95.1%. The upper product phase containing methylcyclohexane was purified by distillation to obtain dimethyl carbonate, with a product yield of 91.8% and a product purity of 99.3%.

[0020] Example 4: Glycerol and trichloroacetic acid were added to a three-necked flask at a molar ratio of 1:1 and stirred continuously under nitrogen protection to obtain [GLY][TCA]. 1 mol methanol, 1 mol anhydrous magnesium sulfate, 2 mol [GLY][TCA] and 50 mL toluene were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced to the reactor until the initial reaction pressure was 1 MPa. The reaction was carried out at 110 °C for 7 hours. After the reaction, the mixture was allowed to stand and separate into layers, yielding a lower DES phase and an upper product phase containing organic solvent. The DES and anhydrous magnesium sulfate were separated by centrifugation. The recovered DES could be directly recycled, with a single recovery rate ≥95.0%. The toluene-containing upper product phase was purified by distillation to obtain dimethyl carbonate, with a product yield of 92.1% and a product purity of 99.4%.

[0021] Example 5: Choline chloride and p-toluenesulfonic acid were added to a three-necked flask at a molar ratio of 1:3 and stirred continuously under nitrogen protection to obtain [ChCl][PTSA]3. 1 mol methanol, 3 mol 4A molecular sieve, 3 mol [ChCl][PTSA]3 and 50 mL methyl tert-butyl ether were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial reaction pressure in the reactor reached 6 MPa. The reaction was carried out at 100 °C for 10 hours. After the reaction, the mixture was allowed to stand and separate into two phases, yielding a lower DES phase and an upper product phase containing organic solvent. The DES and 4A molecular sieve were separated by centrifugation. The recovered DES could be directly recycled, with a single-cycle recovery rate ≥94.9%. The upper product phase containing methyl tert-butyl ether was removed by distillation to obtain dimethyl carbonate, with a product yield of 91.7% and a product purity of 99.2%.

[0022] Example 6: Polyethylene glycol and trifluoromethanesulfonic acid were added to a three-necked flask at a molar ratio of 1:2 and stirred continuously under nitrogen protection to obtain [PEG][TFMSA]2. 1 mol methanol, 2 mol silica gel, 4 mol [PEG][TFMSA]2 and 50 mL methylcyclohexane were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial reaction pressure in the reactor was 3.5 MPa. The reaction was carried out at 120 °C for 4 hours. After the reaction, the mixture was allowed to stand and separate into layers, yielding a lower DES phase and an upper product phase containing organic solvent. The DES and silica gel were separated by centrifugation. The recovered DES could be directly recycled, with a single-cycle recovery rate ≥95.2%. The upper product phase containing methylcyclohexane was purified by distillation to obtain dimethyl carbonate, with a product yield of 92.3% and a product purity of 99.4%.

[0023] Example 7: Glycerol and trifluoromethanesulfonic acid were added to a three-necked flask at a molar ratio of 1:2 and stirred continuously under nitrogen protection to obtain [GLY][TFMSA]2. 1 mol methanol, 1 mol silica gel, 3 mol [GLY][TFMSA]2 and 50 mL toluene were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial reaction pressure in the reactor was 3.5 MPa. The reaction was carried out at 100 °C for 10 hours. After the reaction, the mixture was allowed to stand and separate into layers, yielding a lower DES phase and an upper product phase containing organic solvent. The DES and silica gel were separated by centrifugation. The recovered DES could be directly recycled, with a single recovery rate ≥95.0%. The upper product phase containing toluene was purified by distillation to obtain dimethyl carbonate, with a product yield of 92.0% and a product purity of 99.3%.

[0024] Example 8: Polyethylene glycol and trichloroacetic acid were added to a three-necked flask at a molar ratio of 1:1. Under nitrogen protection, the mixture was continuously stirred to obtain [PEG][TCA]. 1 mol of methanol, 3 mol of anhydrous magnesium sulfate, 2 mol of [PEG][TCA], and 50 mL of methyl tert-butyl ether were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial reaction pressure in the reactor reached 6 MPa. The reaction was carried out at 110 °C for 4 hours. After the reaction, the mixture was allowed to stand and separate into two phases, yielding a lower DES phase and an upper product phase containing organic solvent. The DES and anhydrous magnesium sulfate were separated by centrifugation. The recovered DES could be directly recycled, with a single-cycle recovery rate ≥94.7%. The upper product phase containing methyl tert-butyl ether was removed by distillation to obtain dimethyl carbonate, with a product yield of 91.6% and a product purity of 99.2%.

[0025] Example 9: The DES recovered in Example 1 was added to a 250 mL reactor along with 1 mol of methanol, 2 mol of anhydrous magnesium sulfate, and 50 mL of toluene. The mixture was stirred until homogeneous, and CO2 was introduced until the initial reaction pressure in the reactor reached 3.5 MPa. The reaction was carried out at 110 °C for 7 hours. After the reaction, the mixture was allowed to stand and separate into layers. The upper product phase containing toluene was removed by distillation to obtain dimethyl carbonate, with a yield of 92.3% and a purity of 99.5%. The lower DES phase was centrifuged to separate DES from anhydrous magnesium sulfate. The recovered DES could be directly recycled. Under the same experimental conditions, the product was reused 10 times. The yield of dimethyl carbonate obtained from each of the 10 reuses was ≥91.2%, and the purity was ≥99.1%.

[0026] Example 10: The DES recovered in Example 2 was further mixed with 1 mol of methanol, 1 mol of 4A molecular sieve, and 50 mL of methyl tert-butyl ether in a 250 mL reactor. The mixture was stirred until homogeneous, and CO2 was introduced until the initial reaction pressure in the reactor reached 1 MPa. The reaction was carried out at 100°C for 4 hours. After the reaction, the mixture was allowed to stand and separate into layers. The upper product phase containing methyl tert-butyl ether was removed by distillation to obtain dimethyl carbonate, with a yield of 91.5% and a purity of 99.2%. The lower DES phase was centrifuged to separate DES and 4A molecular sieve. The recovered DES could be directly recycled. Under the same experimental conditions, the product was reused 10 times. The yield of dimethyl carbonate obtained from each of the 10 reuses was ≥90.5%, and the purity was ≥99.1%.

[0027] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. However, these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions claimed by the present invention.

Claims

1. A method for preparing dimethyl carbonate, characterized in that, Includes the following steps: Dimethyl carbonate was prepared by reacting CO2 with methanol in the presence of an organic solvent and a dehydrating agent using eutectic solvent DES as a catalyst. After the reaction, the mixture was allowed to stand and separate into layers to obtain a lower DES phase and an upper product phase containing organic solvent. The lower DES phase could be recycled directly after centrifugation to remove the dehydrating agent. The upper product phase containing organic solvent was distilled to remove the organic solvent, and the resulting product, dimethyl carbonate, was obtained. The organic solvent obtained from the distillation could be recycled. The DES consists of a hydrogen bond acceptor and a hydrogen bond donor; the hydrogen bond acceptor is selected from choline chloride, polyethylene glycol, and glycerol; the hydrogen bond donor is selected from trifluoromethanesulfonic acid, trichloroacetic acid, and p-toluenesulfonic acid; the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:1 to 1:

3.

2. The method for preparing dimethyl carbonate according to claim 1, characterized in that, The conditions for the catalytic reaction are: initial CO2 pressure of 1-6 MPa, molar ratio of methanol to dehydrating agent of 1:1-1:3, molar ratio of methanol to DES of 1:2-1:4, reaction temperature of 100-120℃, and reaction time of 4-10 h.

3. The method for preparing dimethyl carbonate according to claim 1, characterized in that, The organic solvent is selected from toluene, methyl tert-butyl ether, and methylcyclohexane.

4. The method for preparing dimethyl carbonate according to claim 1, characterized in that, The dehydrating agent is selected from one of 4A molecular sieve, anhydrous magnesium sulfate, and silica gel, and can be recycled after drying and dehydration.