An indium-gallium-cerium composite metal oxide catalyst, a preparation method thereof and application of the catalyst in catalyzing methanol carbonylation to prepare dimethyl carbonate

Abstract

This invention discloses a method for the direct carbonylation of methanol to dimethyl carbonate using an indium gallium cerium composite oxide catalyst, belonging to the field of catalyst technology. The catalyst preparation method comprises the following four steps: First, indium, gallium, and cerium metal salts are dissolved in deionized water with citric acid; second, the catalyst precursor solution is heated until the water evaporates to form a viscous sol-like mixture, which is then removed; third, the sol-like mixture is dried in an oven; fourth, the dried solid is transferred to a muffle furnace and calcined in air to obtain the indium gallium cerium composite metal oxide catalyst. This catalyst preparation method is simple, and the catalyst is composed of nanoparticles with a large specific surface area and numerous active sites. It exhibits excellent catalytic performance in the direct carbonylation of methanol and CO2 to produce dimethyl carbonate.

Description

Technical Field

[0001] This invention belongs to the technical field of direct carbonylation of methanol and CO2 to prepare dimethyl carbonate, specifically involving an indium gallium cerium composite metal oxide catalyst, its preparation method, and its application in catalyzing the direct carbonylation of methanol to prepare dimethyl carbonate. Background Technology

[0002] Dimethyl carbonate (DMC) is an important chemical raw material and organic solvent, widely used in pharmaceuticals, pesticides, solvents, lubricants, and polymer materials. Traditional methods for preparing DMC mainly include the phosgene method, the ethylene carbonate method, and the direct oxidation method, but these methods have significant drawbacks. The phosgene method uses highly toxic phosgene, resulting in extremely poor safety and generating large amounts of toxic byproducts, causing serious environmental pollution. While the ethylene carbonate method reduces some toxicity issues, the cost of the raw material ethylene carbonate is high, and the handling of byproducts generated during the reaction is cumbersome, increasing production difficulty and cost. The direct oxidation method relies on highly efficient catalysts, which are typically expensive and prone to deactivation. Furthermore, this method requires high-temperature and high-pressure conditions, making the production process complex and costly. Chinese patent CN114054043B discloses a catalyst for synthesizing dimethyl carbonate using a mixture of alumina, silica, and calcium oxide as a carrier, palladium as the active component, and copper, titanium, and nickel metals as auxiliary agents. However, the preparation process is cumbersome, and the reaction uses toxic CO gas, posing significant safety hazards.

[0003] The direct carbonylation synthesis of DMC from methanol and carbon dioxide offers significant advantages in terms of environmental protection, economy, and safety. Firstly, by utilizing carbon dioxide, a greenhouse gas, this method effectively reduces its emissions, contributing to mitigating global warming. Economically, methanol is inexpensive and widely available, and carbon dioxide can be captured from industrial waste gases, resulting in low raw material costs. Furthermore, the direct synthesis method features a simple process flow, low equipment requirements, and ease of industrial-scale production, thereby reducing overall production costs. In terms of safety, methanol and carbon dioxide are relatively safe, easy to store and transport, reducing potential safety risks. Moreover, the reaction process does not require the use of highly toxic and risky phosgene, making the operating environment safer. With continuous advancements in catalyst technology and reaction engineering, the efficiency of this method will be further improved, and production costs will be further reduced, indicating broad prospects for industrial application. Summary of the Invention

[0004] The technical problem to be solved by this invention is to overcome the problems of high toxicity of raw materials, complex processes, environmental pollution and high cost brought about by traditional DMC synthesis methods. Therefore, it provides a green and environmentally friendly DMC synthesis route with readily available raw materials and a simplified process. Thus, it provides an indium gallium cerium composite metal oxide catalyst with high conversion rate, difficult deactivation and good stability, as well as its preparation method and its application in the catalytic carbonylation of methanol to prepare dimethyl carbonate.

[0005] The technical solution provided by this invention is as follows: A method for preparing an indium gallium cerium composite metal oxide catalyst includes the following steps: 1) Dissolve the metal salts of indium, gallium, and cerium in citric acid in deionized water and stir to form a catalyst precursor solution; based on 1 mol of citric acid, the amounts of metal salts of indium, gallium, and cerium are 0.05-0.2 mol, 0-0.2 mol, and 0.6-0.9 mol, respectively. 2) Add nitric acid dropwise to the catalyst precursor solution in step 1) to adjust the pH to 1-2, heat and stir, and remove the solution after the water evaporates to form a viscous sol mixture. 3) After drying the sol-gel mixture from step 2), transfer it to a muffle furnace and calcine it in air to obtain the indium gallium cerium composite metal oxide catalyst.

[0006] Further, in step 1), the metal salts of indium, gallium, and cerium are each independently selected as chlorides, nitrates, sulfates, or acetylacetone salts of the respective metals; based on 1 mol of citric acid, the amounts of metal salts of indium, gallium, and cerium are 0.1-0.2 mol, 0.05-0.1 mol, and 0.8-0.9 mol, respectively; the ratio of the total molar amount of indium and gallium metal salts to the molar amount of citric acid is 0.2-0.25:1.

[0007] Further, in step 1), the metal salts of indium, gallium, and cerium are dissolved in deionized water by stirring. After stirring for 10-30 minutes, citric acid is added and stirring is continued until the citric acid is completely dissolved.

[0008] Furthermore, in step 2), the oil bath heating temperature of the catalyst precursor solution is 60-150 ℃, preferably 70-90 ℃, and the solution is heated while being stirred.

[0009] Further, in step 3), the furnace is calcined in stages in a muffle furnace. The temperature of the first stage is 200-500 ℃, preferably 300-350 ℃, and the calcination time is 1-3 h. In the second stage, the temperature is increased to 600-900 ℃, preferably 700-750 ℃, and the calcination time is 4-8 h.

[0010] This invention also discloses the application of the aforementioned indium gallium cerium composite metal oxide catalyst, which is used in the reaction of catalytic direct carbonylation of methanol and CO2 to prepare dimethyl carbonate. The application method is as follows: methanol and catalyst are added to a high-pressure reactor, and carbon dioxide is introduced at 0.1-10 MPa, preferably 2-6 MPa. The airtightness of the reactor is checked. Under the condition of good airtightness, the temperature is increased to 50-200 °C, preferably 90-180 °C, at a heating rate of 1-10 °C / min, and the reaction is carried out for 1-24 h, preferably 2-15 h. Then, the product is obtained by cooling.

[0011] Compared with the prior art, the technical solution of the present invention has the following advantages: 1) This invention utilizes an indium gallium cerium composite metal oxide catalyst to catalyze the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate, which offers several significant advantages. Firstly, this catalyst exhibits high selectivity and high catalytic activity during the reaction, effectively improving product yield and reducing byproduct formation. Chinese patent CN102423707B discloses an iron-zirconium oxide catalyst for the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate, with a methanol conversion rate of 0.8509% and a dimethyl carbonate selectivity of 87.12%. The catalytic activity of the catalyst in this invention represents a significant improvement over Chinese patent CN102423707B.

[0012] 2) The indium gallium cerium composite metal oxide catalyst exhibits excellent durability and stability, maintaining high catalytic performance even after multiple cycles, thus reducing catalyst usage costs. Furthermore, this method uses methanol and carbon dioxide as raw materials; the latter is an industrial byproduct, while the former is widely available and economical, ensuring a stable and economically feasible raw material supply. Most importantly, this process is green and environmentally friendly, overcoming the problems of high raw material toxicity, complex processes, and environmental pollution associated with traditional DMC synthesis methods. It helps reduce greenhouse gas emissions, meeting the requirements of green chemistry and sustainable development. In summary, the indium gallium cerium composite metal oxide catalyst demonstrates significant research value and application prospects in the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate. Attached Figure Description

[0013] Figure 1 In catalyst for the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate 0.1 Ga 0.1 Ce 0.8 O x Transmission electron microscopy (TEM) spectra.

[0014] Figure 2 The graph shows a comparison of the catalytic performance of the catalysts for the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate in Examples 1-6. Detailed Implementation

[0015] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0016] Example 1: In 0.05 Ga 0.05 Ce 0.9 O x The preparation of the catalyst and its application in the direct carbonylation of methanol and CO2 to produce dimethyl carbonate includes the following steps: In prepared using citric acid-assisted sol-gel method 0.05 Ga 0.05 Ce 0.9 O x The catalyst was prepared by weighing out a certain mass of indium nitrate, gallium nitrate, and cerium nitrate, dissolving them in 40 mL of deionized water, and heating while stirring to ensure thorough mixing and dissolution. After stirring for 30 min, 2.1 g of citric acid (AC) was added to the solution to form a catalyst precursor solution with a molar ratio of AC:In(NO3)3:Ga(NO3)3:Ce(NO3)3 = 1:0.05:0.05:0.9, where the concentration of citric acid (AC) in the solution was 0.274 mmol / mL. Nitric acid was added dropwise to adjust the pH of the precursor solution to 1-2. The precursor solution was heated and stirred in an 80 ℃ oil bath until the water evaporated to form a sol mixture. The mixture was then removed and dried overnight in a 110 ℃ oven. The dried sample was calcined in a muffle furnace at 350 ℃ in air atmosphere for 1 h, followed by calcination at 700 ℃ for 5 h to obtain In. 0.05 Ga 0.05 Ce 0.9 O x catalyst.

[0017] In prepared using Example 1 0.05 Ga 0.05 Ce 0.9 O x The catalyst was applied to the catalytic reaction: 10 ml of methanol and 0.5 g of catalyst were added to a high-pressure reactor, which was then charged with 3 MPa of carbon dioxide. The reactor was sealed, and its airtightness was checked. Ensuring good airtightness, the temperature was increased to 150 °C at a rate of 5 °C / min, and the reaction was carried out for 6 h. The reactor was then cooled to obtain the product. 0.05 Ga 0.05 Ce 0.9 O x The catalyst's performance in the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate was tested, and the results were as follows: methanol conversion rate 1.27%, and dimethyl carbonate selectivity nearly 100%. Figure 2 ).

[0018] Example 2: In 0.1 Ga0.1 Ce 0.8 O x The preparation of the catalyst and its application in the direct carbonylation of methanol and CO2 to produce dimethyl carbonate includes the following steps: In prepared using citric acid-assisted sol-gel method 0.1 Ga 0.1 Ce 0.8 O x The catalyst was prepared by weighing out a certain mass of indium nitrate, gallium nitrate, and cerium nitrate, dissolving them in 40 g of deionized water, and heating while stirring to ensure thorough mixing and dissolution. After stirring for 30 min, 2.1 g of citric acid (AC) was added to the solution to form a catalyst precursor solution with a molar ratio of AC:In(NO3)3:Ga(NO3)3:Ce(NO3)3 = 1:0.1:0.1:0.8. Nitric acid was added dropwise to adjust the pH of the precursor solution to 1-2, with the concentration of citric acid (AC) in the solution being 0.274 mmol / mL. The precursor solution was heated and stirred in an oil bath at 80 ℃ until the water evaporated to form a sol mixture. The mixture was then removed and dried overnight in an oven at 110 ℃. The dried sample was calcined in a muffle furnace at 350 ℃ in air atmosphere for 1 h, followed by calcination at 700 ℃ for 5 h to obtain In. 0.1 Ga 0.1 Ce 0.8 O x catalyst.

[0019] In prepared using Example 2 0.1 Ga 0.1 Ce 0.8 O x The catalyst was applied to the catalytic reaction: 10 ml of methanol and 0.5 g of catalyst were added to a high-pressure reactor, which was then charged with 3 MPa of carbon dioxide. The reactor was sealed, and its airtightness was checked. Ensuring good airtightness, the temperature was increased to 150 °C at a rate of 5 °C / min, and the reaction was carried out for 6 h. The reactor was then cooled to obtain the product. 0.1 Ga 0.1 Ce 0.8 O x The catalyst's performance in the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate was tested, and the results were as follows: methanol conversion rate 1.80%, and dimethyl carbonate selectivity nearly 100%. Figure 2 ).

[0020] In prepared in Example 2 0.1 Ga 0.1 Ce 0.8 O x The transmission electron microscopy (TEM) spectrum of the catalyst is shown below. Figure 1 As shown, from Figure 1 It can be seen that In0.1 Ga 0.1 Ce 0.8 O x The catalyst is made of nanoparticles with a large specific surface area, which exposes more active sites and greatly promotes the improvement of catalytic performance.

[0021] In addition, for the catalytic reaction in Example 2, a catalyst reuse experiment was conducted. After the catalytic reaction was carried out at 150°C for 6 hours, it was cooled to room temperature, the catalyst was centrifuged and washed with methanol, and then used in the next catalytic experiment (considering that there is some loss during the catalyst recovery process, 0.01g of fresh catalyst was added each time the catalyst was reused). According to the above experimental procedure, the results of the direct carbonylation reaction of methanol and CO2 after 8 cycles of catalyst use are shown in Table 1.

[0022] Table 1 .

[0023] As can be seen from Table 1, the indium gallium cerium composite metal oxide catalyst of the present invention has excellent durability and stability, and can maintain high catalytic performance even after multiple cycles.

[0024] Example 3: In 0.2 Ga 0.2 Ce 0.6 O x The preparation of the catalyst and its application in the direct carbonylation of methanol and CO2 to produce dimethyl carbonate includes the following steps: In prepared using citric acid-assisted sol-gel method 0.2 Ga 0.2 Ce 0.6 O x The catalyst was prepared by weighing out a certain mass of indium nitrate, gallium nitrate, and cerium nitrate, dissolving them in 40 mL of deionized water, and heating while stirring to ensure thorough mixing and dissolution. After stirring for 30 min, 2.1 g of citric acid (AC) was added to the solution to form a catalyst precursor solution with a molar ratio of AC:In(NO3)3:Ga(NO3)3:Ce(NO3)3 = 1:0.2:0.2:0.6. Nitric acid was added dropwise to adjust the pH of the precursor solution to 1-2, with the concentration of citric acid (AC) in the solution being 0.274 mmol / mL. The precursor solution was heated and stirred in an 80 ℃ oil bath until the water evaporated to form a sol mixture. The mixture was then removed and dried overnight in a 110 ℃ oven. The dried sample was calcined in a muffle furnace at 350 ℃ in air atmosphere for 1 h, followed by calcination at 700 ℃ for 5 h to obtain In. 0.2 Ga 0.2 Ce 0.6 O x catalyst.

[0025] In prepared using Example 3 0.2 Ga 0.2 Ce 0.6 O x The catalyst was applied to the catalytic reaction: 10 ml of methanol and 0.5 g of catalyst were added to a high-pressure reactor, which was then charged with 3 MPa of carbon dioxide. The reactor was sealed, and its airtightness was checked. Ensuring good airtightness, the temperature was increased to 150 °C at a rate of 5 °C / min, and the reaction was carried out for 6 h. The reactor was then cooled to obtain the product. 0.2 Ga 0.2 Ce 0.6 O x The catalyst's performance in the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate was tested, and the results were as follows: methanol conversion rate 0.96%, and dimethyl carbonate selectivity nearly 100%. Figure 2 ).

[0026] Comparing the experimental results of Example 3 and Example 2, it can be seen that the amount of Ce salt used during catalyst preparation has a significant impact on catalytic performance.

[0027] Example 4: Ga 0.2 Ce 0.8 O x The preparation of the catalyst and its application in the direct carbonylation of methanol and CO2 to produce dimethyl carbonate includes the following steps: Ga was prepared using the citric acid-assisted sol-gel method. 0.2 Ce 0.8 O x For the catalyst, a certain mass of gallium nitrate and cerium nitrate were weighed and dissolved in 40 mL of deionized water. The solution was heated while stirring to ensure complete mixing and dissolution. After stirring for 30 min, 2.1 g of citric acid (AC) was added to the solution to form a catalyst precursor solution with a molar ratio of AC:Ga(NO3)3:Ce(NO3)3 = 1:0.2:0.8. Nitric acid was added dropwise to adjust the pH of the precursor solution to 1-2, with the concentration of citric acid (AC) in the solution being 0.274 mmol / mL. The precursor solution was heated and stirred in an 80 ℃ oil bath until the water evaporated to form a sol mixture. This mixture was then removed and dried overnight in a 110 ℃ oven. The dried sample was calcined in a muffle furnace at 350 ℃ in air atmosphere for 1 h, followed by calcination at 700 ℃ for 5 h to obtain Ga. 0.2 Ce 0.8 O x catalyst.

[0028] Ga prepared in Example 4 0.2 Ce 0.8 O xThe catalyst was applied to the catalytic reaction: 10 ml of methanol and 0.5 g of catalyst were added to a high-pressure reactor, which was then purged with 3 MPa of carbon dioxide. The reactor was sealed, and its airtightness was checked. Ensuring good airtightness, the temperature was increased to 150 °C at a rate of 5 °C / min, and the reaction was carried out for 6 h. The reactor was then cooled to obtain the product, Ga. 0.2 Ce 0.8 O x The catalyst's performance in the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate was tested, and the results were as follows: methanol conversion rate 1.1%, and dimethyl carbonate selectivity nearly 100%. Figure 2 ).

[0029] Example 5: In 0.2 Ce 0.8 O x The preparation of the catalyst and its application in the direct carbonylation of methanol and CO2 to produce dimethyl carbonate includes the following steps: In prepared using citric acid-assisted sol-gel method 0.2 Ce 0.8 O x For the catalyst, a certain mass of gallium nitrate and cerium nitrate were weighed and dissolved in 40 mL of deionized water. The solution was heated while stirring to ensure complete mixing and dissolution. After stirring for 30 min, 2.1 g of citric acid (AC) was added to the solution to form a catalyst precursor solution with a molar ratio of AC:In(NO3)3:Ce(NO3)3 = 1:0.2:0.8. Nitric acid was added dropwise to adjust the pH of the precursor solution to 1-2, with the concentration of citric acid (AC) in the solution being 0.274 mmol / mL. The precursor solution was heated and stirred in an 80 ℃ oil bath until the water evaporated to form a sol mixture. This mixture was then removed and dried overnight in a 110 ℃ oven. The dried sample was calcined in a muffle furnace at 350 ℃ in air atmosphere for 1 h, followed by calcination at 700 ℃ for 5 h to obtain In. 0.2 Ce 0.8 O x catalyst.

[0030] In prepared using Example 5 0.2 Ce 0.8 O x The catalyst was applied to the catalytic reaction: 10 ml of methanol and 0.5 g of catalyst were added to a high-pressure reactor, which was then charged with 3 MPa of carbon dioxide. The reactor was sealed, and its airtightness was checked. Ensuring good airtightness, the temperature was increased to 150 °C at a rate of 5 °C / min, and the reaction was carried out for 6 h. The reactor was then cooled to obtain the product. 0.2 Ce 0.8 O xThe catalyst's performance in the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate was tested, and the experimental results were as follows: methanol conversion rate 1.7%, and dimethyl carbonate selectivity nearly 100%. Figure 2 ).

[0031] Example 6: Ni 0.2 Ce 0.8 O x The preparation of the catalyst and its application in the direct carbonylation of methanol and CO2 to produce dimethyl carbonate includes the following steps: Ni was prepared using a citric acid-assisted sol-gel method. 0.2 Ce 0.8 O x The catalyst was prepared by weighing out a certain mass of nickel nitrate and cerium nitrate, dissolving them in 40 mL of deionized water, and heating while stirring to ensure thorough mixing and dissolution. After stirring for 30 min, 2.1 g of citric acid (AC) was added to the solution to form a catalyst precursor solution with a molar ratio of AC:Ni(NO3)3:Ce(NO3)3 = 1:0.2:0.8. Nitric acid was added dropwise to adjust the pH of the precursor solution to 1-2, with the concentration of citric acid (AC) in the solution being 0.274 mmol / mL. The precursor solution was heated and stirred in an oil bath at 80 ℃ until the water evaporated to form a sol mixture. The mixture was then removed and dried overnight in an oven at 110 ℃. The dried sample was calcined in a muffle furnace at 350 ℃ in air atmosphere for 1 h, followed by calcination at 700 ℃ for 5 h to obtain Ni. 0.2 Ce 0.8 O x catalyst.

[0032] Ni prepared using Example 6 0.2 Ce 0.8 O x The catalyst was applied to the catalytic reaction: 10 ml of methanol and 0.5 g of catalyst were added to a high-pressure reactor, which was then charged with 3 MPa of carbon dioxide. The reactor was sealed, and its airtightness was checked. Ensuring good airtightness, the temperature was increased to 150 °C at a rate of 5 °C / min, and the reaction was carried out for 6 h. The reactor was then cooled to obtain the product, Ni. 0.2 Ce 0.8 O x The catalyst's catalytic performance in the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate was tested, and the results were as follows: methanol conversion rate of 0.18% and dimethyl carbonate selectivity of nearly 100%. Figure 2 ).

[0033] A comparison of the catalytic performance of catalysts for the direct carbonylation of methanol and CO2 to prepare dimethyl carbonate in Examples 1-6 is shown in the figure. Figure 2 ,from Figure 2 All catalyst test performances show that In0.1 Ga 0.1 Ce 0.8 O x The catalyst exhibited excellent catalytic activity. This is mainly attributed to the abundant oxygen vacancies provided by CeO2, which is crucial for promoting CO2 activation and reaction. CeO2 also possesses good oxygen reduction properties, enhancing the catalyst's stability and catalytic activity. The combination of In2O3 and Ga2O3 provides multiple active sites in the catalyst. In2O3 effectively adsorbs and activates carbon dioxide, while Ga2O3 exhibits good adsorption and activation capabilities for methanol. The combination of In2O3 and Ga2O3 plays a synergistic role in the reaction mechanism, making it easier for methanol and carbon dioxide to react to form dimethyl carbonate. In terms of reaction rate, the addition of Ga2O3 accelerates the reaction, resulting in a faster formation of dimethyl carbonate. Furthermore, the presence of CeO2 also gives the catalyst good resistance to poisoning, especially as the water byproduct generated during the reaction does not easily deactivate the catalyst. Therefore, In... 0.1 Ga 0.1 Ce 0.8 O x The catalyst exhibits good stability and high catalytic performance. Clearly, the above embodiments are merely illustrative examples and not intended to limit the implementation. Those skilled in the art will recognize that various variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom remain within the scope of this invention.

[0034] The contents described in this specification are merely an enumeration of the implementation forms of the inventive concept, and the scope of protection of this invention should not be regarded as limited to the specific forms described in the embodiments.

Claims

1. Use of an indium-gallium-cerium complex metal oxide catalyst, characterized in that The catalyst is used in the reaction of catalytic direct carbonylation of methanol and CO2 to prepare dimethyl carbonate; The catalyst preparation method includes the following steps: 1) Dissolve the metal salts of indium, gallium, and cerium in citric acid in deionized water and stir to form a catalyst precursor solution; 2) Add nitric acid dropwise to the catalyst precursor solution in step 1) to adjust the pH to 1-2, heat and stir, and remove the solution after the water evaporates to form a viscous sol mixture. 3) After drying the sol-gel mixture from step 2), transfer it to a muffle furnace and calcine it in air to obtain the indium gallium cerium composite metal oxide catalyst; In step 1), the metal salts of indium, gallium, and cerium are each independently selected as chlorides, nitrates, sulfates, or acetylacetone salts of their respective metals; based on 1 mol of citric acid, the amounts of indium, gallium, and cerium metal salts are 0.1-0.2 mol, 0.05-0.1 mol, and 0.8-0.9 mol, respectively; the ratio of the total molar amount of indium and gallium metal salts to the molar amount of citric acid is 0.2-0.25:

1. In step 2), the catalyst precursor solution is heated in an oil bath at a temperature of 70-90 ℃ while stirring. Step 3) Calcination in a muffle furnace in stages. The first stage temperature is 300-350 ℃ and the calcination time is 1-3 h. In the second stage, the temperature is increased to 700-750 ℃ ​​and the calcination time is 4-8 h.

2. Use according to claim 1, wherein In step 1), in the deionized water, the metal salts of indium, gallium and cerium are dissolved by stirring. After stirring for 10-30 minutes, citric acid is added and stirring is continued until the citric acid is completely dissolved.

3. The application as described in claim 1, characterized in that... Add methanol and catalyst to the high-pressure reactor, and purge with 0.1-10 MPa of carbon dioxide. Check the airtightness of the reactor. If the airtightness is good, raise the temperature to 50-200 ℃ at a rate of 1-10 ℃ / min and react for 1-24 h. Then cool to obtain the product.

4. The application as described in claim 3, characterized in that... The carbon dioxide pressure is 2-6 MPa, the rate is increased to 90-180 °C, and the reaction time is 2-15 h.

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