Method for directly synthesizing dimethyl carbonate from carbon dioxide and methanol and catalytic reaction system

Abstract

The invention provides a method for directly synthesizing dimethyl carbonate from carbon dioxide and methanol and a catalytic reaction system, and the method is characterized in that in a high-pressure reaction kettle filled with a dehydrating agent, a solvent and a metal-doped rod-like cerium oxide catalyst, the carbon dioxide and the methanol are subjected to a one-pot reaction to directly synthesize the dimethyl carbonate; the dehydrating agent is 2-cyanopyridine, and the solvent is acetonitrile; the metal-doped rod-like cerium oxide catalyst has a rod-like microstructure, the microstructure of the metal-doped rod-like cerium oxide catalyst is a solid solution formed by doping a metal element M into cerium oxide crystal lattices, and the metal element M is selected from one or more of La, Zn, Ga and Co. According to the method, thermodynamic equilibrium limitation is overcome and the problem of agglomeration and deactivation of the nano-catalyst is solved in the reaction of directly synthesizing dimethyl carbonate from carbon dioxide and methanol, so that high conversion rate is achieved, and reaction products are easy to separate.

Description

Technical Field

[0001] This invention relates to the field of dimethyl carbonate synthesis technology, specifically to a method and catalytic reaction system for the direct synthesis of dimethyl carbonate from carbon dioxide and methanol. Background Technology

[0002] Dimethyl carbonate (DMC), an environmentally friendly green chemical, is widely used in methylation reagents, clean fuel additives, and lithium-ion battery electrolytes due to the presence of carbonyl, methyl, and methoxy functional groups in its molecular structure. However, the direct synthesis of DMC from carbon dioxide and methanol is severely limited by thermodynamic equilibrium; the water produced in the reaction induces a reverse reaction, resulting in extremely low feed conversion rates. To overcome this limitation, existing technologies often employ the addition of dehydrating agents to remove moisture. Although dehydrating agents can improve yield to some extent, they still face severe challenges in practical applications. First, there is the bottleneck of catalyst activity and stability. Although traditional cerium oxide catalysts have certain acid-base and redox properties, the oxygen flow of pure cerium oxide lattice is poor and the surface oxygen vacancy concentration is insufficient, making it difficult to efficiently activate cerium oxide molecules. Existing commercially available spherical or irregularly shaped cerium oxide particles have a large specific surface area, but lack the selective exposure of specific active crystal faces, resulting in low catalytic efficiency. Second, and more challenging, is the mass transfer and separation problem of the reaction system. In high-pressure reaction systems with solid dehydrating agents, high-surface-energy nano-cerium oxide catalysts are prone to agglomeration, mixing with the dehydrating agent and its hydration products to form a viscous "mud-like" substance. This agglomeration phenomenon not only severely hinders the contact between reactants and catalyst active sites, i.e., mass transfer is impeded, leading to a significant decrease in catalyst utilization, but also makes solid-liquid separation after the reaction extremely difficult, failing to meet the requirements of industrial continuous production for catalyst recycling and reuse. Summary of the Invention

[0003] This invention provides a method and catalytic reaction system for the direct synthesis of dimethyl carbonate from carbon dioxide and methanol. In the reaction of direct synthesis of dimethyl carbonate from carbon dioxide and methanol, the thermodynamic equilibrium limitation is overcome and the problem of nanocatalyst aggregation and deactivation is solved, thereby achieving high conversion rate and easy separation of reaction products.

[0004] To achieve the above objectives, the present invention provides the following technical solution: A method for directly synthesizing dimethyl carbonate from carbon dioxide and methanol, the method comprising: Dimethyl carbonate is directly synthesized from carbon dioxide and methanol in a one-pot reaction in a high-pressure reactor containing a dehydrating agent, a solvent, and a metal-doped rod-shaped cerium oxide catalyst. Wherein, the dehydrating agent is 2-cyanopyridine, and the solvent is acetonitrile; The metal-doped rod-shaped cerium oxide catalyst has a rod-shaped microstructure, and its microstructure is a solid solution formed by the doping of metal element M into the cerium oxide lattice, wherein the metal element M is selected from one or more of La, Zn, Ga, and Co.

[0005] To achieve the above objectives, the present invention also provides the following technical solutions: A catalytic reaction system for carrying out the above method, the catalytic reaction system comprising the following components: (A) Carbon dioxide and methanol as reactants; (B) Rod-shaped cerium oxide nanoparticles doped with metal element M, wherein the molar ratio of doped metal M to Ce is (0.05~0.3):1; (C) 2-Cyanopyridine as a dehydrating agent; (D) Acetonitrile as a dispersion solvent.

[0006] Compared with the prior art, the present invention has achieved the following beneficial effects: 1. Significantly improved catalytic activity and conversion rate: Compared with existing commercial spherical catalysts, the metal-doped rod-shaped cerium oxide catalyst used in this invention significantly improves methanol conversion rate and dimethyl carbonate yield. 2. Overcoming the "mud-like" problem and improving mass transfer and separation: In existing technologies, high specific surface area nanocatalysts tend to form "mud-like" aggregates after the reaction, resulting in catalyst encapsulation, inactivation of active sites, and difficulty in recovery. This invention, by limiting acetonitrile to a solvent, utilizes its physical dispersion effect in combination with the morphological characteristics of rod-shaped catalysts to transform the post-reaction system from "mud-like" to "solution-like". This not only exposes more active sites but also greatly simplifies the solid-liquid separation process. 3. Microscopic synergy between crystal plane control and element doping: The rod-shaped morphology preferentially exposes the (110) and (111) crystal planes of CeO2. Combined with the lattice doping of metal ions (forming a solid solution), the surface oxygen vacancy concentration is significantly increased, thereby reducing the reaction activation energy at the microscopic level. Combined with macroscopic dehydrating agents and solvents, the whole chain optimization of "micro-macro" is achieved. Attached Figure Description

[0007] To more clearly illustrate the technical solution in one embodiment of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0008] Figure 1This is a transmission electron microscope (TEM) image of the 0.2-Ga-CeO2 rod-shaped cerium oxide nanocatalyst prepared in the embodiments of the present invention.

[0009] Figure 2 This is a high-resolution transmission electron microscope (HRTEM) image of the 0.2-Ga-CeO2 rod-shaped cerium oxide nanocatalyst prepared in the embodiments of the present invention, showing the (110) crystal plane.

[0010] Figure 3 This is a high-resolution transmission electron microscope (HRTEM) image of the 0.2-Ga-CeO2 rod-shaped cerium oxide nanocatalyst prepared in the embodiments of the present invention, showing the (111) crystal plane. Detailed Implementation

[0011] The technical solution of one embodiment of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It should be understood that the specific embodiments described herein are only for illustration and explanation of the present invention and are not intended to limit the present invention.

[0012] Any specific numerical value (including the endpoints of the numerical range) disclosed in this invention is not limited to the exact value, but should be understood to also cover values ​​close to the exact value, such as all possible values ​​within ±5% of the exact value. Furthermore, for the disclosed numerical range, one or more new numerical ranges can be obtained by arbitrarily combining the endpoint values ​​of the range, the endpoint values ​​with specific point values ​​within the range, and the specific point values. These new numerical ranges should also be considered as specifically disclosed in this invention.

[0013] The terminology used in this invention is for the purpose of describing specific exemplary embodiments only and is not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms “a,” “an,” and “the” as used herein are intended to include the plural forms as well. The terms “comprising,” “including,” “containing,” and “having” are inclusive and thus describe the presence of said features, elements, compositions, steps, integers, operations, and / or components, but do not exclude the presence or inclusion of one or more other features, integers, steps, operations, elements, components, and / or sets thereof. Although the open-ended term “comprising” should be understood as a non-limiting term used to describe and claim the various embodiments described in this invention, in some aspects it may instead be understood as a more restrictive and limiting term, such as “consisting of” or “essentially composed of.” Thus, for any given embodiment describing a composition, material, component, element, feature, integer, operation, and / or process step, the invention also particularly includes embodiments consisting of or substantially consisting of such compositions, materials, components, elements, features, integers, operations, and / or process steps. In the case of “consisting of…”, the alternative embodiments exclude any additional compositions, materials, components, elements, features, integers, operations and / or process steps. In the case of “essentially composed of…”, any additional compositions, materials, components, elements, features, integers, operations and / or process steps that substantially affect the essential and novel characteristics are excluded from such embodiments. However, any compositions, materials, components, elements, features, integers, operations and / or process steps that do not substantially affect the essential and novel characteristics may be included in the embodiments.

[0014] Any method steps, processes, and operations described in this invention should not be construed as necessarily requiring them to be performed in the specific order discussed or shown, unless explicitly specified as such. It should also be understood that, unless otherwise stated, additional or alternative steps may be used.

[0015] In this invention, except where expressly stated, any matters or issues not mentioned are directly applicable to those known in the art without any modification. Furthermore, any embodiment described in this invention can be freely combined with one or more other embodiments described in this invention, and the resulting technical solutions or concepts are considered part of the original disclosure or original record of this invention, and should not be regarded as new content not disclosed or anticipated by this invention, unless those skilled in the art consider the combination to be clearly unreasonable.

[0016] Unless otherwise stated, the terminology used in this invention has the same meaning as commonly understood by those skilled in the art. If a term is defined in this invention and its definition differs from the common understanding in the art, the definition of this invention shall prevail.

[0017] As mentioned above, existing technologies have not yet been able to overcome the limitations of thermodynamic equilibrium and solve the problem of nanocatalyst aggregation and deactivation in the direct synthesis of dimethyl carbonate from carbon dioxide and methanol. Furthermore, existing technologies have low conversion rates and cannot achieve easy separation of reaction products. In view of this, the present invention provides the following technical solutions to address the aforementioned problems.

[0018] First aspect This invention provides a method for directly synthesizing dimethyl carbonate from carbon dioxide and methanol. The method comprises: directly synthesizing dimethyl carbonate by reacting carbon dioxide and methanol in a one-pot reaction in a high-pressure reactor containing a dehydrating agent, a solvent, and a metal-doped rod-shaped cerium oxide catalyst; wherein the dehydrating agent is 2-cyanopyridine, and the solvent is acetonitrile; the metal-doped rod-shaped cerium oxide catalyst has a rod-shaped microstructure, and its microstructure is a solid solution formed by doping a metal element M into the cerium oxide lattice, wherein the metal element M is selected from one or more of La, Zn, Ga, and Co. This invention achieves multiple synergistic effects by employing a specific combination of metal-doped rod-shaped cerium oxide catalyst, acetonitrile solvent, and 2-cyanopyridine dehydrating agent: 1. Overcoming thermodynamic limitations: 2-cyanopyridine, as a chemical dehydrating agent, can remove water, a byproduct generated in the reaction, in situ, effectively overcoming the thermodynamic equilibrium limitations of the CO2-methanol reaction and significantly improving the raw material conversion rate; 2. Solving the "slurry" and mass transfer problems: Utilizing the excellent physical dispersion effect of acetonitrile on the rod-shaped catalyst, the reaction system is transformed from the traditional viscous "slurry" state to a highly fluid "solution" state, greatly improving the gas-liquid-solid three-phase mass transfer efficiency and simplifying the subsequent solid-liquid separation and catalyst recovery process; 3. Enhancing intrinsic activity: Metal M doping enters the cerium oxide lattice to form a solid solution, and combined with the specific crystal faces exposed by the rod-shaped morphology, significantly increases the oxygen vacancy concentration on the catalyst surface, reducing the reaction activation energy, thereby solving the technical problems of low activity and easy agglomeration and deactivation of existing cerium oxide-based catalysts.

[0019] In the above method for directly synthesizing dimethyl carbonate from carbon dioxide and methanol, the reaction process of carbon dioxide reacting with methanol to produce dimethyl carbonate is as follows: .

[0020] In some embodiments of the present invention, the molar ratio of doped metal M to Ce in the metal-doped rod-shaped cerium oxide catalyst is (0.05~0.3):1. By controlling the doping ratio of metal M within the range of (0.05~0.3):1, the lattice defect concentration is optimized. This ratio ensures the introduction of sufficient heteroatoms to generate abundant oxygen vacancy active sites, while avoiding the collapse of the cerium oxide fluorite crystal structure or the formation of impurity phases due to excessive doping, thereby ensuring that the catalyst possesses both high catalytic activity and structural stability.

[0021] In some embodiments of the present invention, the metal element M is Ga, and the molar ratio of Ga to Ce is 0.2:1. By limiting the element to Ga and the molar ratio of Ga to Ce to 0.2:1, the catalytic performance is maximized: under this specific doping parameter, Ga ions (Ga³) + ) and Ce ions (Ce 4+ The lattice distortion effect induced by the difference in radius and valence state of CO2 is optimal, which can maximize the activation of CO2 molecules and bring methanol conversion and dimethyl carbonate yield to their peak values.

[0022] In some embodiments of the present invention, the metal-doped rod-shaped cerium oxide catalyst exhibits lattice fringes corresponding to the (110) and (111) crystal planes of CeO2 under a transmission electron microscope, and no impurity phase lattice fringes other than the (110) and (111) crystal planes are observed. The catalytic efficiency is further improved by preparing a homogeneous solid solution structure that exposes specific (110) and (111) active crystal planes and is free of impurities. The (110) and (111) crystal planes have been confirmed as the main active planes for this carbonylation reaction; simultaneously, the single lattice fringes confirm that the doped metal has completely entered the lattice interior rather than adhered to the surface, avoiding surface oxide coverage of active sites and ensuring the persistence and selectivity of the catalytic reaction.

[0023] In some embodiments of the present invention, the mass ratio of acetonitrile to catalyst in the reaction system is (1.5~3.5):1, resulting in a uniformly dispersed solution after the reaction. By precisely defining the mass ratio of acetonitrile to catalyst, the severe agglomeration problem that easily occurs in high-pressure dehydration systems of nanocatalysts is solved. This ratio ensures that the mixture after the reaction is a low-viscosity "solution" rather than a "slurry," which not only maximizes the specific surface area utilization of the catalyst but also allows the reaction products to be rapidly separated through simple centrifugation or filtration, meeting the process operability requirements of continuous industrial production.

[0024] In some embodiments of the present invention, the molar ratio of 2-cyanopyridine to methanol is not less than 0.5:1. By limiting the stoichiometric ratio of the dehydrating agent to the raw material methanol (based on the reaction formula 2MeOH + CO2 → DMC + H2O), the amount of dehydrating agent added is ensured to be sufficient or excessive. This achieves complete removal of the water generated in the reaction, continuously pushing the chemical equilibrium towards the forward reaction to form dimethyl carbonate, thereby maximizing the conversion potential of the reaction.

[0025] In some embodiments of the present invention, the reaction conditions are: a carbon dioxide reaction pressure of 1.0–5.0 MPa, a reaction temperature of 80–140°C, and a reaction time of 2–12 h. Within this pressure and temperature range, it is possible to ensure that CO2 has high reactivity and solubility in the liquid phase, while avoiding catalyst sintering or increased side reactions due to excessively high temperatures. Simultaneously, it prevents excessive pressure from imposing harsh requirements on equipment materials, thus achieving a balance between reaction rate, equipment cost, and production safety.

[0026] In some embodiments of the present invention, the specific steps of the reaction are as follows: first, carbon dioxide is introduced to an initial pressure of 1.0 MPa; then, the temperature is raised to 100°C, and carbon dioxide is continuously added to bring the reaction pressure to 3.0 MPa, and the reaction is maintained under these conditions for 4 hours. This stepwise pressurization and temperature control strategy solves the safety hazards and unstable pressure control problems that may result from direct pressurization and heating. The initial low-pressure heating avoids the risk of overpressure, and the subsequent addition of carbon dioxide to 3.0 MPa after reaching the reaction temperature ensures a constant and sufficient partial pressure of CO2 during the reaction stage, thus optimizing the reaction kinetics and achieving the best yield.

[0027] In some embodiments of the present invention, the metal-doped rod-shaped cerium oxide catalyst is prepared by the following steps: (1) dissolving a cerium source and a metal M source in water, adding a sodium hydroxide solution, and stirring and mixing at room temperature to obtain a precursor; (2) subjecting the precursor to a hydrothermal reaction at 100°C for 24 hours in a closed high-pressure reactor; (3) washing and drying the product, and then calcining it at 500°C for 4 hours to obtain the catalyst. Through a specific low-temperature hydrothermal (100°C) combined with medium-temperature calcination (500°C) process, precise control of the catalyst morphology and crystallinity is achieved. This process condition induces the precursor to grow along a specific direction to form a rod-shaped structure, and stabilizes the position of the doped metal in the crystal lattice during calcination, while avoiding the decrease in specific surface area caused by high-temperature sintering, thereby preparing a high-performance catalyst with uniform morphology and stable structure.

[0028] In some embodiments of the present invention, in step (1), the cerium source is cerium nitrate hexahydrate, and the metal M source is a nitrate of metal M. The excellent solubility of cerium nitrate hexahydrate and the nitrate of metal M in the aqueous phase enables uniform mixing of cerium ions and doped metal ions. This facilitates the formation of a homogeneous solid solution precursor during subsequent co-precipitation and hydrothermal processes, avoiding component segregation.

[0029] In some embodiments of the present invention, in step (1), the cerium source is cerium nitrate hexahydrate, and the metal M source is gallium nitrate nonahydrate. The excellent solubility of cerium nitrate hexahydrate and gallium nitrate nonahydrate in the aqueous phase enables uniform mixing of cerium and gallium ions. This facilitates the formation of a homogeneous solid solution precursor during subsequent co-precipitation and hydrothermal processes, avoiding component segregation.

[0030] Second aspect This invention provides a catalytic reaction system for carrying out the method described in the first aspect, comprising the following components: (A) carbon dioxide and methanol as reactants; (B) a rod-shaped cerium oxide nanoparticle catalyst doped with metal element M, wherein the molar ratio of the doped metal M to Ce is (0.05~0.3):1; (C) 2-cyanopyridine as a dehydrating agent; and (D) acetonitrile as a dispersing solvent. This invention provides a highly matched catalytic reaction system by systematically integrating a specific Ga-doped rod-shaped catalyst, a specific chemical dehydrating agent, and a specific dispersing solvent. This system can be directly applied as a single product (such as a reaction reagent package) to the industrial production of DMC. The synergistic effect (dispersion, dehydration, activation) among the components ensures efficient reaction and easy product separation, thereby solving the problems of difficulty in matching individual components and poor overall process performance in the prior art.

[0031] Example Unless otherwise stated, all raw materials, materials, equipment, and instruments mentioned in this specification are available through general commercial channels.

[0032] Table 1. Reagents used in the examples and their sources.

[0033] Table 2. Equipment used in the embodiments and their sources

[0034] Preparation Example 1 Preparation of 0.2-Ga-CeO2 rod-shaped cerium oxide nanocatalysts 4 mmol of cerium nitrate hexahydrate (Ce(NO3)3·6H2O, 1.737 g) and 0.8 mmol of gallium nitrate nonahydrate (Ga(NO3)3·9H2O) were dissolved in 60 mL of deionized water and stirred for 10 min to ensure homogeneity. Then, 60 mL of a 6 mol / L sodium hydroxide solution (NaOH) was added, and the mixture was stirred at room temperature for 30 min to form a homogeneous precursor slurry. This slurry was transferred to a 100 mL stainless steel autoclave lined with polytetrafluoroethylene (PTFE), sealed, and subjected to a hydrothermal reaction at 100 °C for 24 h. After the reaction, the mixture was allowed to cool naturally to room temperature, and the solid product was collected by centrifugation. The product was washed three times alternately with deionized water and ethanol, and dried at 80 °C for 12 h. Finally, the dried powder was placed in a muffle furnace and calcined at 500 °C for 4 h to obtain a pale yellow rod-shaped 0.2-Ga-CeO2 nanocatalyst.

[0035] Preparation Example 2 Preparation of rod-shaped cerium oxide nano-catalysts (undoped control sample) The preparation steps are the same as in Preparation Example 1, except that gallium nitrate nonahydrate is not added.

[0036] Preparation Example 3 Preparation of cubic cerium oxide nanoparticles catalyst (morphology comparison sample) Cerium oxide with a cubic morphology was prepared using a high-concentration alkaline solution (NaOH, 19.198 g) and high-temperature hydrothermal conditions (180 °C, 24 h). The remaining post-treatment steps were the same as in Preparation Example 1.

[0037] Preparation Example 4 Preparation of octahedral cerium oxide nano-catalysts (morphology comparison sample) Cerium oxide with an octahedral morphology was prepared by hydrothermal treatment at 170℃ for 12 hours and calcination at 600℃ using sodium phosphate (Na3PO4) as a mineralizing agent.

[0038] Preparation Example 5 Preparation of spindle-shaped cerium oxide nanocatalysts (morphology comparison sample) 4.8 mmol of cerium nitrate hexahydrate (Ce(NO3)3·6H2O, 2.084 g) and 12.8 mmol of urea (0.769 g) were dissolved in 160 mL of deionized water, and the mixture was stirred at 600 rpm for 1 hour at room temperature. The solution was transferred to a 200 mL stainless steel autoclave lined with polytetrafluoroethylene, sealed, and placed in a drying oven for hydrothermal reaction at 120 °C for 12 hours. After the reaction, the mixture was allowed to cool naturally to room temperature, and the precipitate was collected by filtration. The precipitate was washed alternately with deionized water and ethanol, and then dried at 80 °C for 12 hours. Finally, the dried product was calcined at 600 °C for 5 hours to obtain a light yellow spindle-shaped cerium oxide nano-catalyst.

[0039] Preparation Example 6 Preparation of La-doped rod-shaped cerium oxide The preparation steps are the same as in Preparation Example 1, except that gallium nitrate is replaced with an equimolar amount of lanthanum nitrate.

[0040] Preparation Example 7 Preparation of Co-doped rod-shaped cerium oxide The preparation steps are the same as in Preparation Example 1, except that gallium nitrate is replaced with an equimolar amount of cobalt nitrate.

[0041] Preparation Example 8 Preparation of Zn-doped rod-shaped cerium oxide The preparation steps are the same as in Preparation Example 1, except that gallium nitrate is replaced with an equimolar amount of zinc nitrate.

[0042] Example 1 In this embodiment, a rod-shaped cerium oxide catalyst (0.2-Ga-CeO2) with a Ga-doped molar ratio of 0.2:1 was prepared and used for the synthesis of dimethyl carbonate. First, 4 mmol of cerium nitrate hexahydrate and 0.8 mmol of gallium nitrate nonahydrate were dissolved in deionized water. Sodium hydroxide solution was added and the mixture was stirred at room temperature. The resulting precursor slurry was then placed in an autoclave and subjected to a hydrothermal reaction at 100°C for 24 hours. The product was washed, dried, and calcined at 500°C for 4 hours to obtain the rod-shaped catalyst. Next, 3.2 g of methanol, 1.02 g of the prepared catalyst, 5.2 g of 2-cyanopyridine as a dehydrating agent, and 2.05 g of acetonitrile as a solvent were added to a 25 mL autoclave. Carbon dioxide was introduced into the autoclave to an initial pressure of 1.0 MPa, and the temperature was raised to 100°C. Carbon dioxide was then continuously added until the reaction pressure reached 3.0 MPa, and the reaction was maintained for 4 hours. After the reaction was completed, the system was a uniformly dispersed solution. Gas chromatography analysis showed that the methanol conversion rate was 90.95%, the dimethyl carbonate selectivity was 94.19%, and the yield was 41.94 mmol / g.

[0043] Example 2 This example investigated the performance of a Ga-doped rod-shaped cerium oxide catalyst (0.1-Ga-CeO2) with a low doping concentration (0.1:1). The catalyst preparation process was basically the same as in Example 1, except that the amount of gallium nitrate nonahydrate was adjusted to 0.4 mmol to reduce the doping concentration while maintaining the rod-shaped morphology. In the catalytic performance test, the exact same reaction system ratio (methanol, 2-cyanopyridine, acetonitrile) and process conditions (100℃, 3.0 MPa CO2, 4 hours) as in Example 1 were used. The experimental results showed that the low-doped catalyst could still maintain a good dispersion in the reaction system. The final measured methanol conversion rate was 87.38%, the dimethyl carbonate selectivity was 95.64%, and the yield was 40.91 mmol / g, demonstrating that the catalyst still has good activity at this doping ratio.

[0044] Example 3 This example investigated the effect of a high-doping (0.3:1) Ga-doped rod-shaped cerium oxide catalyst (0.3-Ga-CeO2) on the reaction. In the catalyst preparation stage, the amount of gallium nitrate nonahydrate was increased to 1.2 mmol, while the remaining hydrothermal and calcination steps remained consistent with Example 1 to prepare a rod-shaped solid solution with high Ga content. The catalyst was then placed in a high-pressure reactor containing methanol, 2-cyanopyridine, and acetonitrile, and reacted for 4 hours at 100°C and 3.0 MPa CO2 pressure. The results showed that although the methanol conversion remained at a high level of 87.43%, the selectivity of dimethyl carbonate decreased to 70.39%, with a final yield of 30.13 mmol / g, indicating that excessively high doping levels may trigger side reactions, thus affecting product selectivity.

[0045] Example 4 This example verifies the applicability of the Co (cobalt)-doped rod-shaped cerium oxide catalyst (0.2-Co-CeO2) in this reaction system. 0.8 mmol of cobalt nitrate nonahydrate was used instead of gallium nitrate in catalyst preparation, while the remaining precipitation, hydrothermal, and calcination process parameters were identical to those in Example 1, successfully preparing the Co-doped rod-shaped catalyst. In the subsequent DMC synthesis reaction, this cobalt-doped catalyst was used in conjunction with acetonitrile solvent and 2-cyanopyridine dehydrating agent, reacting at 100°C and 3.0 MPa CO2 for 4 hours. The reaction mixture was a homogeneous solution. Analysis showed a methanol conversion rate of 86.14%, a dimethyl carbonate selectivity of 94.46%, and a yield of 39.84 mmol / g, confirming that the rod-shaped morphology combined with acetonitrile solvent process platform of this invention has good universality for different transition metal doping.

[0046] Comparative Example 1 This comparative example was used to verify the crucial dispersing role of acetonitrile in the reaction system. The experiment used undoped pure rod-shaped cerium oxide catalyst, methanol, and 2-cyanopyridine, but without adding acetonitrile solvent. After reacting for 4 hours at the same conditions of 100°C and 3.0 MPa CO2, a viscous, mud-like solid was observed deposited at the bottom of the vessel. The catalyst was encapsulated by the hydration products of the dehydrating agent, making dispersion difficult. The results showed that the methanol conversion rate was only 68.75%, and the dimethyl carbonate yield was 33.41 mmol / g, far lower than in Example 1, demonstrating that acetonitrile plays a decisive role in de-agglomerating the rod-shaped catalyst and improving mass transfer efficiency.

[0047] Comparative Example 2 This comparative example aims to verify the synergistic effect of the rod-shaped morphology and acetonitrile solvent. Commercially available spherical cerium oxide nanoparticles were used instead of the rod-shaped catalyst in Example 1, while the types and amounts of the remaining dehydrating agent (2-cyanopyridine) and solvent (acetonitrile) remained unchanged. After reacting for 4 hours at 100°C and 3.0 MPa, although the reaction system was dispersed, the methanol conversion rate was only 44.13%, and the dimethyl carbonate yield was 21.46 mmol / g. Compared with Example 1, it is clear that without a matching rod-shaped morphology, a high conversion rate cannot be achieved with only the solvent, confirming a significant synergistic mechanism between the specific morphology and the solvent in this invention.

[0048] Comparative Example 3 This comparative study investigated the performance of pure rod-shaped cerium oxide catalysts without metal doping. No gallium nitrate was added during the preparation process; only pure rod-shaped cerium oxide (CeO2) was prepared. It was placed in a system containing methanol, 2-cyanopyridine, and acetonitrile, and reacted under the same process conditions. The experimental results showed a methanol conversion of 75.03% and a dimethyl carbonate yield of 36.58 mmol / g. While these results are superior to those of commercially available spherical catalysts, they are significantly lower than those of Example 1 (90.95%), demonstrating that lattice doping with metallic Ga can further activate the catalyst and is a necessary condition for achieving high conversion rates.

[0049] Comparative Example 4 This comparative study tested the activity of an octahedral cerium oxide catalyst. Octahedral cerium oxide was hydrothermally synthesized using sodium phosphate as a mineralizing agent, and then used for DMC synthesis in the presence of acetonitrile and 2-cyanopyridine. Under the same reaction conditions, this catalyst exhibited extremely low activity, with a methanol conversion of only 15.65% and a dimethyl carbonate yield of 7.66 mmol / g, further demonstrating the importance of the specific crystal facets exposed by the rod-shaped morphology for this catalytic reaction.

[0050] Test case The microstructure and crystal structure of the prepared metal-doped cerium oxide catalyst were characterized using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Specifically, TEM was used to observe the overall morphology of the catalyst sample to confirm whether it had successfully grown into the expected rod-shaped, cubic, or octahedral structure. HRTEM was then used to observe the lattice fringes of the sample, and the main exposed crystal plane types on the catalyst surface were determined by measuring the interplanar spacing, such as the (110) and (111) crystal planes corresponding to CeO2. At the same time, by observing the clarity and uniformity of the lattice fringes, no independent lattice fringes corresponding to the doped metal oxide were found, thus confirming that the doped metal atoms had successfully entered the interior of the cerium oxide lattice to form a uniform solid solution structure.

[0051] The catalytic performance of the direct synthesis of dimethyl carbonate from carbon dioxide and methanol was evaluated in a closed, high-pressure reactor system equipped with a stirrer. During the experiment, measured amounts of methanol, dehydrating agent, solvent, and catalyst were added to the reactor. After sealing, carbon dioxide was introduced at room temperature to the initial pressure, followed by stirring and heating to the reaction temperature. Carbon dioxide was then introduced again to reach the target reaction pressure, and the reaction was maintained at a constant temperature. After the predetermined reaction time, heating was stopped, and the reaction system was cooled to room temperature. Unreacted carbon dioxide gas was slowly released by opening the exhaust valve. Because acetonitrile was added as a dispersing solvent, the mixture after the reaction was a well-dispersed solution, facilitating subsequent solid-liquid separation by centrifugation.

[0052] Qualitative and quantitative analyses of the reaction products were performed using gas chromatography (GC). A suitable amount of the supernatant sample after the reaction was injected into the GC for detection, and various catalytic performance indicators were calculated based on the chromatographic analysis data. Specifically, methanol conversion rate was defined as the percentage of the molar amount of methanol consumed in the reaction relative to the total molar amount of methanol in the feedstock; dimethyl carbonate selectivity was defined as the percentage of the molar amount of dimethyl carbonate produced relative to the molar amount of methanol consumed in the reaction (converted to stoichiometric proportions); and the yield of dimethyl carbonate was calculated as the number of millimoles of dimethyl carbonate (mmol / g) produced per gram of catalyst, comprehensively reflecting the catalyst's production efficiency in the reaction system.

[0053] Table 3. Chromatographic analysis results of existing commercially available nanosphere cerium oxide catalysts with different acetonitrile contents.

[0054] As shown in Table 3, the amount of acetonitrile used in the range of 1.025 g to 3.075 g did not improve the methanol conversion rate of existing commercial spherical catalysts. This proves that the main role of acetonitrile is to physically disperse catalyst particles, improve mass transfer, and significantly optimize the solid-liquid separation performance after the reaction.

[0055] Table 4. Chromatographic analysis results of acetonitrile on rod-shaped nano-cerium oxide catalysts

[0056] Comparing the data from Comparative Example 2 (without acetonitrile) and Experimental Example 4 (with acetonitrile), it can be seen that for the rod-shaped nano-cerium oxide catalyst, the addition of acetonitrile not only increased the selectivity of dimethyl carbonate from 99.27% ​​to 99.60% and the methanol conversion rate from 68.75% to 75.03%, but also changed the post-reaction system from a "slurry-like" state to a "solution-like" state. This proves that the physical dispersion effect of acetonitrile can effectively release the high specific surface area active sites of the rod-shaped catalyst, greatly simplify the separation process, and improve the operability of the process.

[0057] Table 5. Chromatographic analysis results of cerium oxide nanocatalysts with different morphologies

[0058] Table 6. Chromatographic analysis results of nano-cerium oxide catalysts with different metal doping

[0059] Table 7. Chromatographic analysis results of nano-cerium oxide catalysts with different Ga doping contents

[0060] Table 8. Chromatographic analysis results of 0.2Ga-CeO2 nano-cerium oxide catalyst with different contents of acetonitrile

[0061] Table 9 Comparison of data between 0.2-Ga-CeO2 prepared in this invention and Comparative Example 1

[0062] Table 3 shows that as the acetonitrile content increased (from 0 g to 3.075 g), the methanol conversion rate not only did not increase, but actually slightly decreased from 46.13% to 39.81%, and the yield of dimethyl carbonate remained at a low level of 19-22 mmol / g. This objectively proves from the opposite perspective that for conventional commercial spherical catalysts without specific morphology, simply adding acetonitrile solvent cannot stimulate additional catalytic activity. At this time, acetonitrile only plays a physical dispersing role and cannot break through the intrinsic activity limit of the catalytic system itself.

[0063] Table 4 compares the catalytic performance of undoped pure rod-shaped cerium oxide nanoparticles under conditions without acetonitrile and with acetonitrile (2.05 g). Table 2 shows that after introducing acetonitrile, the methanol conversion rate increased from 68.75% to 75.03%, and the yield also increased from 33.41 mmol / g to 36.58 mmol / g. Combined with the data in Table 1, it can be seen that the acetonitrile solvent not only prevents the reaction system from becoming "muddy," but also produces a unique synergistic effect with the "rod-shaped morphology," effectively releasing the hindered active sites on the surface of the rod-shaped catalyst.

[0064] Table 5 compares five different morphologies of cerium oxide catalysts—rod-shaped, cubic, octahedral, spindle-shaped, and commercially available spherical—under the same reaction conditions (with the addition of 2.05 g acetonitrile and 5.2 g 2-cyanopyridine). The results show that the methanol conversion (75.03%) and yield (36.58 mmol / g) of the rod-shaped catalyst significantly outperformed those of other morphologies (e.g., the octahedral conversion was only 15.65%). This, from a crystal plane engineering perspective, confirms that the specific crystal planes preferentially exposed by the rod-shaped morphology (such as the (110) and (111) crystal planes) are the optimal structure for activating the reaction molecules.

[0065] Table 6 verifies the further enhancement of the intrinsic activity of the rod-shaped catalyst by specific transition metal (especially Ga) doping. Based on the determination of the rod morphology, Table 4 examines the effects of four different metals (La, Ga, Co, and Zn) doping into the cerium oxide lattice when the doping ratio is fixed at 0.2:1. The data show that all four metal dopings can increase the conversion rate to over 77%, with Ga doping (0.2-Ga-CeO2) showing the best performance, achieving a methanol conversion rate of 90.95% and a high yield of 41.94 mmol / g. This proves that the metal doping successfully constructs a solid solution and increases the concentration of lattice defects such as oxygen vacancies.

[0066] Table 7 shows that when the doping ratio is 0.2:1, the catalyst achieves the optimal balance between conversion (90.95%) and selectivity (94.19%). It is noteworthy that when the doping ratio is further increased to 0.3:1, although the conversion remains at 87.43%, the selectivity of dimethyl carbonate sharply decreases to 70.39%, resulting in an overall yield drop to 30.13 mmol / g.

[0067] Table 8 shows the catalytic performance of the catalyst under different acetonitrile contents when the doping ratio is 0.2:1. Table 6 data shows that as the acetonitrile content increases, the methanol conversion rate increases from 85.47% to 90.95%, then decreases to 84.71%, while the DMC selectivity first slightly increases from 92.63% to 94.19% and then stabilizes at 94.22%. This indicates that an appropriate amount of acetonitrile effectively breaks the "slurry" structure, allowing the catalyst particles to be well dispersed in the reaction system. When acetonitrile is excessive, the relative concentration of reactants (methanol and CO2) is diluted, reducing the probability of reactant molecules colliding with the active centers of the catalyst, leading to a decrease in conversion rate.

[0068] Table 9 shows that the methanol conversion rate increased from 46.13% to 90.95%, and the dimethyl carbonate yield increased from 22.32 mmol / g to 41.94 mmol / g, which proves that the solution provided by the present invention achieves further effects.

[0069] Figure 1 This is a transmission electron microscope (TEM) image of the 0.2-Ga-CeO2 metal-doped rod-shaped cerium oxide nanocatalyst prepared in the embodiments of the present invention. As shown in the figure, the catalyst exhibits a rod-shaped microstructure with uniform morphology and good dispersion. Figure 2 yes Figure 1 The catalyst is shown in a high-resolution transmission electron microscope (HRTEM) image. The image clearly shows lattice fringes with a spacing of 0.19 nm, which correspond to the (110) crystal plane of cubic fluorite cerium oxide (CeO2). Figure 3 yes Figure 1 Another high-resolution transmission electron microscope (HRTEM) image of the catalyst is shown. The image clearly shows lattice fringes with a spacing of 0.31 nm, corresponding to the (111) crystal plane of cubic fluorite cerium oxide (CeO2). Combined with... Figure 2 and Figure 3 It can be seen that only lattice fringes corresponding to the CeO2 structure were observed in the sample, and no independent lattice fringes of gallium oxide or other impurity phases were found. This indicates that the doped Ga ions have successfully entered the CeO2 lattice and formed a uniform solid solution.

[0070] It is worth noting that 0.2La-CeO2 and 0.3Ga-CeO2 exhibit high methanol conversion rates but low dimethyl carbonate selectivity, resulting in lower yields compared to pure rod-shaped CeO2. The surface of pure rod-shaped CeO2 primarily provides moderate acid-base sites and oxygen vacancies; doping with La (0.2) enhances the basicity of the catalyst surface, while increasing the Ga proportion (0.3) leads to stronger acidity. The surface structure of the catalyst changes, so not all metal doping can improve its performance.

[0071] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims. Furthermore, specific examples have been used in the specification to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of the present invention, and the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for directly synthesizing dimethyl carbonate from carbon dioxide and methanol, characterized in that, The method is as follows: Dimethyl carbonate is directly synthesized from carbon dioxide and methanol in a one-pot reaction in a high-pressure reactor containing a dehydrating agent, a solvent, and a metal-doped rod-shaped cerium oxide catalyst. Wherein, the dehydrating agent is 2-cyanopyridine, and the solvent is acetonitrile; The metal-doped rod-shaped cerium oxide catalyst has a rod-shaped microstructure, and its microstructure is a solid solution formed by the doping of metal element M into the cerium oxide lattice, wherein the metal element M is selected from one or more of La, Zn, Ga, and Co.

2. The method as described in claim 1, characterized in that, In the metal-doped rod-shaped cerium oxide catalyst, the molar ratio of doped metal M to Ce is (0.05~0.3):1; Preferably, the metallic element M is Ga, and the molar ratio of Ga to Ce is 0.2:

1.

3. The method as described in claim 1, characterized in that, The metal-doped rod-shaped cerium oxide catalyst showed lattice fringes corresponding to the (110) and (111) crystal planes of CeO2 under a transmission electron microscope, and no impurity phase lattice fringes other than the (110) and (111) crystal planes were observed.

4. The method as described in claim 1, characterized in that, The mass ratio of the solvent acetonitrile to the catalyst in the reaction system is (1.5~3.5):1, so that the mixture after the reaction is completed is a uniformly dispersed solution.

5. The method as described in claim 1, characterized in that, The molar ratio of 2-cyanopyridine to methanol is not less than 0.5:

1.

6. The method as described in claim 1, characterized in that, The reaction conditions are as follows: the reaction pressure of carbon dioxide is 1.0 to 5.0 MPa, the reaction temperature is 80 to 140°C, and the reaction time is 2 to 12 hours.

7. The method as described in claim 6, characterized in that, The specific steps of the reaction are as follows: first, carbon dioxide is introduced to an initial pressure of 1.0 MPa, the temperature is raised to 100°C, and then carbon dioxide is added to increase the reaction pressure to 3.0 MPa. The reaction is maintained under these conditions for 4 hours.

8. The method as described in claim 1, characterized in that, The metal-doped rod-shaped cerium oxide catalyst is prepared by the following steps: (1) Dissolve the cerium source and the metal M source in water, add sodium hydroxide solution, and stir and mix at room temperature to obtain the precursor; (2) The precursor was subjected to a hydrothermal reaction at 100°C for 24 hours in a closed autoclave; (3) The product was washed and dried, and then calcined at 500°C for 4 hours to obtain the catalyst.

9. The method as described in claim 8, characterized in that, In step (1), the cerium source is cerium nitrate hexahydrate, and the metal M source is a nitrate of metal M; Preferably, the metal M source is gallium nitrate nonahydrate.

10. A catalytic reaction system for carrying out the method of claim 1, characterized in that, The catalytic reaction system consists of the following components: (A) Carbon dioxide and methanol as reactants; (B) Rod-shaped cerium oxide nanoparticles doped with metal element M, wherein the molar ratio of doped metal M to Ce is (0.05~0.3):1; (C) 2-Cyanopyridine as a dehydrating agent; (D) Acetonitrile as a dispersion solvent.

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