Method for preparing dimethyl carbonate

Abstract

The invention belongs to the technical field of organic synthesis, and relates to a method for preparing dimethyl carbonate, the method is characterized in that an ionic liquid (ILs) is used as a catalyst, in the presence of an organic solvent and a dehydrating agent, CO2 and methanol are catalyzed to react to prepare dimethyl carbonate, and the reaction conditions are that the initial pressure of CO2 is 2-6 MPa, the temperature is 100-120 DEG C, and the time is 4-10 hours. After the reaction, standing for layering, directly recycling the lower ILs phase after the dehydrating agent is centrifugally separated, rectifying the upper product phase to remove the organic solvent to obtain the product dimethyl carbonate, and recycling the organic solvent. The method has the characteristics that the ILs-dehydrating agent system can realize high-yield and high-purity synthesis of dimethyl carbonate, solves the problems of difficult catalyst recovery, more byproducts, low yield and the like in the traditional method, has the advantages of recyclable catalyst and solvent, low cost and simplicity and convenience in 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 ILs as catalysts. Background Technology

[0002] Dimethyl carbonate (DMC), an important green chemical raw material, contains carbonyl, methyl, and methoxy functional groups in its molecular structure, exhibiting good reactivity and biocompatibility. It is widely used in pharmaceuticals, pesticides, coatings, lithium battery electrolytes, and fuel additives. Its preparation methods mainly include direct synthesis, urea alcoholysis, direct / indirect methanol oxidative carbonylation, and transesterification. Among these, the direct reaction of CO2 with methanol results in a low yield due to the chemical inertness of CO2 and the inability to promptly remove the byproduct water from the reaction system. The urea alcoholysis method uses urea and methanol as raw materials, reacting under heterogeneous or homogeneous catalysts such as dibutyldimethyltinane and tetraphenyltin. While the raw materials are inexpensive, the reaction requires high temperature and pressure, and the byproduct ammonia is difficult to handle, resulting in low product purity. 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. However, the yield is low due to the chemical inertness of CO2 and the inability to promptly remove the byproduct water from the reaction system. - The loss of catalyst leads to rapid deactivation, making catalyst-product separation difficult, and the byproduct HCl exacerbates equipment wear and corrosion. The transesterification method, using cyclic carbonates such as ethylene carbonate (EC) with methanol, has high raw material costs, requires high-pressure reaction conditions, and produces large amounts of alkylene glycols as byproducts. Chinese patent CN113563189B uses the strongly basic ionic liquid 1-ethyl-3-methylimidazolium salt [Emim]IM as a catalyst for a one-step catalytic reaction of CO2, methanol, and propylene oxide to prepare dimethyl carbonate. This method is simple and energy-efficient, but produces propylene glycol as a byproduct, resulting in a low yield of only 83.6%. Chinese patent CN115279726B uses ethylene oxide as a water scavenger in the direct reaction of CO2 and methanol, showing better performance than dehydrating agents, but it is relatively expensive, produces ethylene glycol as a byproduct, and the catalyst cannot be recycled. This invention activates methanol and CO2 with the alkaline anions of ILs to regulate the reaction pathway, optimizes the reaction microenvironment with the assistance of cations, and optimizes the mass transfer of the system in synergy with organic solvents. The product yield reaches more than 93.4% and the purity is ≥99.2%, which is significantly better than the prior art.

[0003] Based on this, this invention proposes a method for reacting CO2 with methanol to produce dimethyl carbonate in the presence of an organic solvent using solvents (ILs) as catalysts. The reaction conditions are mild, with an initial CO2 pressure of 2-6 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 the ILs and organic solvents 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 solvents (ILs) as catalysts to catalyze the reaction of CO2 with methanol in the presence of an organic solvent to prepare dimethyl carbonate. The initial CO2 pressure is 2-6 MPa, the molar ratio of methanol to dehydrating agent is 1:1-3:1, the molar ratio of methanol to ILs 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 ILs phase and an upper product phase containing the organic solvent. The recovered lower ILs can be directly recycled after centrifugation to remove the dehydrating agent. The upper product phase containing the organic solvent is purified by distillation to remove the organic solvent, yielding dimethyl carbonate with a yield ≥93.4% and a purity ≥99.2%. The organic solvent obtained from distillation can be recycled. When the recovered ILs are reused 10 times under the same experimental conditions, the yield of the obtained dimethyl carbonate is consistently ≥91.2%, and the purity is consistently ≥98.4%. The cation of the ILs is 1-butyl-3-methylimidazolium ([Bmim)). + ), 1-ethylpyridinium ([Epy) + ), Tetrabutylammonium ([N4444)) + ) or tributylmethylphosphine ([P4441]) + One of the ILs, wherein the anion of the ILs is hydroxide (OH-). - Acetate (CH3COO) - ), bicarbonate (HCO3) 3- ) or methoxy (CH3O - One of the following: the organic solvent is selected from cyclohexane, ethyl acetate, and toluene; the dehydrating agent is selected from anhydrous calcium chloride, anhydrous sodium sulfate, and 3A molecular sieve, and can be recycled after drying and dehydration.

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

[0007] 1-Butyl-3-methylimidazolium ([Bmim) + ) and acetate (CH3COO) - Taking an ILs (abbreviated as Ac) as an example, the specific technical solution is explained. This IL is named [Bmim]Ac, and the naming method for other ILs follows the same pattern.

[0008] 1-Methylimidazole was refluxed with excess n-butane bromo in ethanol at 70-80°C for 24 hours. After washing and drying, the intermediate [Bmim]Br was obtained. Then, it was stirred with an equimolar amount of silver acetate in anhydrous ethanol in the dark for 24 hours. The silver bromide precipitate was removed by filtration, and the filtrate was distilled and dried to obtain [Bmim]Ac.

[0009] 0.2 mol methanol, 0.1 mol anhydrous calcium chloride, 0.6 mol [Bmim]Ac and 50 mL ethyl acetate were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced into the reactor until the initial reaction pressure was 4 MPa. The reactor was reacted at 110 °C for 7 hours. After the reaction was completed, the mixture was allowed to stand and separate into layers, yielding a lower ILs phase and an upper product phase containing organic solvents. The ILs phase and anhydrous calcium chloride were separated by centrifugation. The recovered ILs can be directly recycled, with a single recovery rate of ≥94.8%. The upper product phase containing ethyl acetate was purified by distillation to obtain dimethyl carbonate, with a product yield of 93.8% and a purity of 99.5%.

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

[0011] 1. The basic anions of ILs activate methanol and CO2, regulate the reaction pathway, and the cations assist in optimizing the reaction microenvironment. They also work synergistically with organic solvents to optimize system mass transfer and effectively suppress side reactions. This solves the problems of numerous byproducts and rapid decline in catalyst activity in traditional methods, enabling the high-yield and high-purity synthesis of dimethyl carbonate.

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

[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: 1-Methylimidazole, n-butane bromide, and silver acetate were added to a three-necked flask and stirred under nitrogen protection to prepare [Bmim]Ac. 0.2 mol methanol, 0.1 mol anhydrous calcium chloride, 0.6 mol [Bmim]Ac, and 50 mL ethyl acetate were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the reactor pressure reached 4 MPa, and 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 ILs phase and anhydrous calcium chloride were separated by centrifugation. The recovered ILs phase could be directly recycled, with a single recovery rate ≥94.8%. The upper product phase was purified by distillation to remove ethyl acetate, yielding dimethyl carbonate with a yield of 93.8% and a purity of 99.5%.

[0016] Comparative Example 1: 0.5 mol imidazole was dissolved in 500 mL of anhydrous ethanol at room temperature. 0.5 mol KOH powder was slowly added under ice bath conditions, and stirring was continued for 24 h until completely dissolved. The mixture was filtered, and the filtrate was rotary distilled under reduced pressure at 80 °C for 1 h to separate the generated water and ethanol solvents. After cooling to room temperature, a brown solid imidazole potassium was obtained. This solid reacted with a methanol solution containing 75% 1-ethyl-3-methylimidazole bromide and anhydrous ethanol. The mixture was stirred at room temperature for 20 min, and then stirred for 24 h. The precipitate was filtered off, and rotary distilled for 2 h to obtain a viscous yellow liquid, which was [Emim]IM. Using [Emim]IM as a catalyst, the reaction was carried out at 120 °C and 2.8 MPa for 5.5 h to catalyze the synthesis of propylene carbonate from propylene oxide and CO2. This propylene carbonate then reacted with methanol to produce a dimethyl carbonate and methanol azeotrope, with 1,2-propanediol as a byproduct. The product yield was 83.6%.

[0017] Comparative Example 2: CO2, methanol, and ethylene oxide were added to the reactor at a feed ratio of 1:1:1 and reacted at 150°C and 5 MPa. The first reaction was the reaction of CO2 and methanol to produce dimethyl carbonate and water. The second reaction was the reaction of ethylene oxide as a water scavenger with water to produce ethylene glycol as a byproduct. Ethylene oxide is not recyclable. The conversion rates of methanol and CO2 were both greater than 90%, and the selectivity of dimethyl carbonate was greater than 90%.

[0018] Example 2: 1-Ethylpyridine, bromoethane, and silver acetate were added to a three-necked flask and stirred under nitrogen protection to prepare [Epy]Ac. 0.2 mol methanol, 0.1 mol anhydrous calcium chloride, 0.4 mol [Epy]Ac, and 50 mL ethyl acetate were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial pressure reached 2 MPa, and the reaction was carried out at 100 °C for 4 h. After the reaction, the mixture was allowed to stand and separate into layers. The ILs phase and anhydrous calcium chloride were separated by centrifugation. The recovered ILs phase could be directly recycled, with a single recovery rate ≥94.8%. The upper product phase was purified by distillation to remove ethyl acetate, yielding dimethyl carbonate with a yield of 93.5% and a purity of 99.4%.

[0019] Example 3: Tetrabutylammonium bromide and potassium hydroxide were added to a three-necked flask and stirred under nitrogen protection to prepare [N4444]OH. 0.2 mol methanol, 0.067 mol 3A molecular sieve, 0.8 mol [N4444]OH, and 50 mL cyclohexane were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial pressure reached 6 MPa, and the reaction was carried out at 120 °C for 10 h. After the reaction, the mixture was allowed to stand and separate into layers. The ILs phase and 3A molecular sieve were separated by centrifugation. The recovered ILs phase could be directly recycled, with a single recovery rate ≥94.8%. The upper product phase was purified by distillation to remove cyclohexane, yielding dimethyl carbonate with a yield of 93.4% and a purity of 99.3%.

[0020] Example 4: Tributylmethylphosphine and sodium bicarbonate were added to a three-necked flask and stirred under nitrogen protection to prepare [P4441]HCO3. 0.2 mol methanol, 0.2 mol anhydrous sodium sulfate, 0.6 mol [P4441]HCO3, and 50 mL toluene were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial pressure reached 4 MPa, and the reaction was carried out at 110 °C for 7 h. After the reaction, the mixture was allowed to stand and separate into layers. The ILs phase and anhydrous sodium sulfate were separated by centrifugation. The recovered ILs phase could be directly recycled, with a single recovery rate ≥94.8%. The upper product phase was purified by distillation to remove toluene, yielding dimethyl carbonate with a yield of 93.7% and a purity of 99.5%.

[0021] Example 5: 1-Methylimidazole, n-butane bromide, and sodium methoxide were added to a three-necked flask and stirred under nitrogen protection to prepare [Bmim]OCH3. 0.2 mol methanol, 0.1 mol 3A molecular sieve, 0.4 mol [Bmim]OCH3, and 50 mL cyclohexane were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced at an initial pressure of 6 MPa, and the reaction was carried out at 100 °C for 10 h. After the reaction, the mixture was allowed to stand and separate into layers. The ILs phase and 3A molecular sieve were separated by centrifugation. The recovered ILs phase could be directly recycled, with a single recovery rate ≥94.8%. The upper product phase was purified by distillation to remove cyclohexane, yielding dimethyl carbonate with a yield of 93.4% and a purity of 99.4%.

[0022] Example 6: 1-Ethylpyridine, bromoethane, and potassium hydroxide were added to a three-necked flask and stirred under nitrogen protection to prepare [Epy]OH. 0.2 mol methanol, 0.067 mol anhydrous calcium chloride, 0.8 mol [Epy]OH, and 50 mL toluene were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial pressure reached 2 MPa, and the reaction was carried out at 120 °C for 4 h. After the reaction, the mixture was allowed to stand and separate into layers. The ILs phase and anhydrous calcium chloride were separated by centrifugation. The recovered ILs phase could be directly recycled, with a single recovery rate ≥94.8%. The upper product phase was purified by distillation to remove toluene, yielding dimethyl carbonate with a yield of 93.9% and a purity of 99.3%.

[0023] Example 7: Tetrabutylammonium bromide and sodium acetate were added to a three-necked flask and stirred under nitrogen protection to prepare [N4444]Ac. 0.2 mol methanol, 0.2 mol anhydrous sodium sulfate, 0.6 mol [N4444]Ac, and 50 mL ethyl acetate were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial pressure reached 2 MPa, and the reaction was carried out at 120 °C for 7 h. After the reaction, the mixture was allowed to stand and separate into layers. The ILs phase and anhydrous sodium sulfate were separated by centrifugation. The recovered ILs phase could be directly recycled, with a single recovery rate ≥94.8%. The upper product phase was purified by distillation to remove ethyl acetate, yielding dimethyl carbonate with a yield of 93.5% and a purity of 99.5%.

[0024] Example 8: Tributylmethylphosphine and sodium methoxide were added to a three-necked flask and stirred under nitrogen protection to prepare [P4441]OCH3. 0.2 mol methanol, 0.1 mol 3A molecular sieve, 0.8 mol [P4441]OCH3, and 50 mL cyclohexane were added to a 250 mL reactor and stirred until homogeneous. CO2 was introduced until the initial pressure reached 6 MPa, and the reaction was carried out at 100 °C for 4 h. After the reaction, the mixture was allowed to stand and separate into layers. The ILs phase and 3A molecular sieve were separated by centrifugation. The recovered ILs phase could be directly recycled, with a single recovery rate ≥94.8%. The upper product phase was purified by distillation to remove cyclohexane, yielding dimethyl carbonate with a yield of 94.0% and a purity of 99.4%.

[0025] Example 9: The ILs recovered in Example 1 were added to a 250 mL reactor along with 0.2 mol methanol, 0.1 mol anhydrous calcium chloride, 0.6 mol [Bmim]Ac, and 50 mL ethyl acetate. The mixture was stirred until homogeneous, and CO2 was introduced until the initial reaction pressure in the reactor reached 4 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 was distilled to remove ethyl acetate, yielding dimethyl carbonate with a yield of 93.6% and a purity of 99.5%. The lower ILs phase was centrifuged to separate the ILs from anhydrous magnesium sulfate. The recovered ILs could be directly recycled. Under the same experimental conditions, the product was reused 10 times. The yield of dimethyl carbonate obtained from the 10 reuses was ≥91.2%, and the purity was ≥98.4%.

[0026] Example 10: The ILs recovered in Example 2 were added to a 250 mL reactor along with 0.2 mol methanol, 0.1 mol anhydrous calcium chloride, 0.4 mol [Epy]Ac, and 50 mL ethyl acetate. The mixture was stirred until homogeneous, and CO2 was introduced until the initial pressure reached 2 MPa. The reaction was carried out at 100 °C for 4 h. After the reaction, the mixture was allowed to stand and separate into layers. The upper product phase was distilled to remove ethyl acetate, yielding dimethyl carbonate with a yield of 93.3% and a purity of 99.3%. The lower ILs phase was centrifuged to separate the ILs from the anhydrous calcium chloride. The recovered ILs could be directly recycled. Under the same experimental conditions, the product was reused 10 times. The yield of dimethyl carbonate obtained from the 10 reuses was ≥90.8%, 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: Using ionic liquids (ILs) as catalysts, dimethyl carbonate is prepared by catalyzing the reaction of CO2 and methanol in the presence of organic solvents and dehydrating agents. After the reaction, the mixture is allowed to stand and separate into layers, yielding a lower ILs phase and an upper product phase containing organic solvents. The lower ILs phase can be recovered and directly recycled after centrifugation to remove the dehydrating agent. The upper product phase containing organic solvents is distilled to remove the organic solvent, yielding the product dimethyl carbonate. The organic solvent obtained from the distillation can be recycled.

2. The reaction conditions according to claim 1 are: initial CO2 pressure of 2-6 MPa, molar ratio of methanol to dehydrating agent of 1:1-3:1, molar ratio of methanol to ILs of 1:2-1:4, reaction temperature of 100-120℃, and reaction time of 4-10 h.

3. The cation of the ILs in claim 1 is 1-butyl-3-methylimidazolium ([Bmim]). + ), 1-ethylpyridinium ([Epy) + ), Tetrabutylammonium ([N4444)) + ) or tributylmethylphosphine ([P4441]) + One of the ILs, wherein the anion of the ILs is hydroxide (OH-). - Acetate (CH3COO) - ), bicarbonate (HCO3) 3- ) or methoxy (CH3O - One of them.

4. The organic solvent of claim 1 is selected from cyclohexane, ethyl acetate, and toluene.

5. The dehydrating agent according to claim 1 is selected from anhydrous calcium chloride, anhydrous sodium sulfate, and 3A molecular sieve, and can be recycled after drying and dehydration.