A metal-free sulfur and nitrogen co-doped carbon catalyst, a preparation method thereof and application thereof in synthesis of methyl ethyl carbonate

Abstract

The application provides a metal-free sulfur-nitrogen co-doped carbon catalyst and a preparation method and application thereof in synthesis of methyl ethyl carbonate, and belongs to the technical field of carbon catalysis preparation.The metal-free sulfur-nitrogen co-doped carbon catalyst utilizes Lewis acid-base site synergistic effect, the catalyst simultaneously contains Lewis acidic sites and Lewis basic sites, double active centers are realized through sulfur and nitrogen heteroatom synergistic doping, and the efficient synthesis of methyl ethyl carbonate is realized.The application provides a metal-free, high-activity, high-selectivity and long-circulation sulfur-nitrogen co-doped carbon catalyst and a process method for efficiently synthesizing battery-grade methyl ethyl carbonate under mild conditions.The problems of metal residues, poor catalyst stability, recovery difficulty, high energy consumption, insufficient selectivity and the like in the existing ester exchange synthesis of methyl ethyl carbonate are solved, obvious side reactions are avoided, a subsequent separation and purification process is simplified, rectification energy consumption and production cost are reduced, and process atomic economy is improved.

Description

Technical Field

[0001] This invention belongs to the field of carbon catalysis preparation technology, specifically relating to a metal-free sulfur and nitrogen co-doped carbon catalyst, its preparation method, and its application in the synthesis of methyl ethyl carbonate. Background Technology

[0002] Ethyl methyl carbonate (EMC) is a non-protic polar solvent that combines high dielectric constant, low viscosity, excellent low-temperature performance, and electrochemical stability. As a core solvent in lithium-ion battery electrolytes, it is widely used in power batteries, energy storage batteries, and 3C digital battery fields. With the rapid development of the new energy industry, high-end electrolytes place stringent requirements on the purity and metal ion content of EMC. Developing green, efficient, and metal-residue-free synthesis technologies has become an urgent industry need.

[0003] Currently, the industrial production of methyl ethyl carbonate (MEC) mainly employs the transesterification route of dimethyl carbonate (DMC) and diethyl carbonate (DEC). This route utilizes widely available raw materials, operates under mild reaction conditions, and produces no highly toxic reactants, making it the mainstream technology for the clean preparation of MEC. In this method, both the raw materials and the products can be used as solvents in lithium-ion battery electrolytes, thus eliminating the need for separation. Only the moisture content needs to be controlled, significantly reducing separation and purification costs. Existing transesterification catalytic systems are mainly classified into homogeneous catalysis, heterogeneous metal catalysis, molecular sieve catalysis, and resin catalysis, but all have significant drawbacks in practical applications.

[0004] Although traditional homogeneous catalytic systems have high catalytic activity, the catalysts are not recyclable, have high consumption, and require complex post-processing. Furthermore, they introduce residual alkali metal ions, which seriously affect the stability of the electrolyte and the cycle life of the battery, making it difficult to meet the production requirements of battery-grade high-purity methyl ethyl carbonate.

[0005] Multiphase metal oxides and supported catalytic systems such as magnesium oxide, zinc oxide, and hydrotalcite-like compounds have solved the problem of separation difficulties in homogeneous catalysts, but still have significant shortcomings: catalytically active components are easily lost, resulting in poor cycle stability; high-temperature calcination preparation consumes a lot of energy; metal ions pose a risk of dissolution, leading to excessive metal residues in the product; there are many side reactions, resulting in low selectivity for the target product and increasing the difficulty of separation and purification. In the literature (J Mol Catal A: Chem, 2010, 327: 32-37.), MgO / NC-2 exhibits high catalytic performance, but this catalyst requires high-temperature calcination in an argon atmosphere after use to restore its activity, making catalyst reuse more difficult. The patent (publication number: CN1394847) uses supported catalysts such as SnO2 / Al2O3, Ga2O3 / Al2O3, MoO3 / Al2O3, ZrO2 / Al2O3, TiO2 / Al2O3 and V2O3 / Al2O3 as catalysts. Under normal pressure and 50-200℃ conditions, the reaction time is 2-48 hours, and the yield of methyl ethyl carbonate is less than 44%.

[0006] Molecular sieve and ionic liquid catalytic systems exhibit relatively good catalytic selectivity, but suffer from problems such as poor heat resistance, low active site density, slow reaction rate, easy pore blockage, and difficult regeneration, resulting in insufficient overall catalytic efficiency and industrial economics. The literature (Appl. Catal. A:Gen., 2006, 304, 152-158) proposed acidic molecular sieve catalysts AlMCM-41 and Al-Zn-MCM-41 for the synthesis of methyl ethyl carbonate. While the initial catalysts showed high activity, their stability was poor, and they were prone to carbon deposition and deactivation during the reaction. Metal-free catalytic routes such as ionic liquids (patent publication number: CN102863339A) and organic amines (patent publication number: CN120441436A), although offering higher yields, currently face challenges such as high cost / viscosity of ionic liquids, difficulty in separating organic bases, and complex operation. In addition, existing single heteroatom-doped carbon catalysts generally suffer from problems such as insufficient activity, single site, and low catalytic efficiency. Relying solely on nitrogen doping or sulfur doping is insufficient to efficiently activate transesterification reactions and cannot simultaneously meet the industrial requirements of high activity, high selectivity, and high stability.

[0007] In summary, existing technologies for synthesizing EMC from DMC and DEC suffer from problems such as high reaction temperature, high energy consumption, long reaction time, metal ion loss, and difficulty in catalyst reuse, which limit large-scale industrial application. Therefore, developing novel metal-free, highly active, highly selective, and highly stable carbon catalysts is of great significance for overcoming technical challenges and achieving green, low-cost, large-scale production of battery-grade methyl ethyl carbonate. Summary of the Invention

[0008] To address the shortcomings of existing transesterification synthesis of ethyl methyl carbonate, such as metal residues, poor catalyst stability, difficult recovery, high energy consumption, and insufficient selectivity, this invention proposes a metal-free sulfur-nitrogen co-doped carbon catalyst, its preparation method, and its application in the synthesis of ethyl methyl carbonate. It provides a metal-free, highly active, highly selective, and long-cycle sulfur-nitrogen co-doped carbon catalyst, and a process for the efficient synthesis of battery-grade ethyl methyl carbonate under mild conditions.

[0009] The metal-free sulfur-nitrogen co-doped carbon catalyst proposed in this invention utilizes the synergistic effect of Lewis acid-base sites. The catalyst simultaneously contains both Lewis acidic and Lewis basic sites, achieving dual active centers through the synergistic doping of sulfur and nitrogen heteroatoms, thus realizing the efficient synthesis of methyl ethyl carbonate. The specific technical solution is as follows: The catalyst proposed in this invention is a metal-free sulfur-nitrogen co-doped carbon material, denoted as SN@CT, where T is the crystallization temperature under argon atmosphere; The metal-free sulfur-nitrogen co-doped carbon material includes a carbon source, a sulfur source, and a nitrogen source; the carbon source is one or more of fructose, glucose, sucrose, cellulose, and citric acid; the sulfur source is one or more of thiourea, sublimed sulfur, and thioacetamide; and the nitrogen source is one or more of urea, melamine, gelatin, dicyandiamide, and thioacetamide. This invention provides a method for preparing a metal-free sulfur-nitrogen co-doped carbon catalyst, comprising the following steps: S1. Mix carbon source and water and stir, then add sulfur source and nitrogen source and continue stirring. Heat in a water bath to obtain prepolymer. S2. The prepolymer is heated in an air atmosphere and then crystallized in an argon atmosphere to obtain the SN@CT catalyst.

[0010] The mass ratio of carbon source to water mentioned in step S1 is 1:5-20; The mass ratio of the carbon source, sulfur source, and nitrogen source mentioned in step S1 is 1:(0.1~0.5):(0.2~1.0); The mixing and stirring time in step S1 is 20-30 minutes; the continued stirring time is 3-6 hours. The water bath heating temperature in step S1 is 70-90 °C, and the water bath heating time is 36-72 h; The heating rate in step S2 is 5-10°C / min, the heating temperature is 250-350°C, and the heating time is 8-12 min. In step S2, the argon flow rate in the argon atmosphere is 40-100 mL / min; The crystallization heating rate in step S2 is 5-10°C / min, the crystallization temperature is 600-800 ℃, and the crystallization time is 5-7 h.

[0011] The application of the SN@CT catalyst prepared by the above method in the synthesis of ethyl methyl carbonate (EMC) is as follows: Ethyl methyl carbonate (EMC) is obtained by mixing SN@CT catalyst, dimethyl carbonate (DMC), and diethyl carbonate (DEC) and then heating the mixture.

[0012] The ratio of the SN@CT catalyst, dimethyl carbonate (DMC), and diethyl carbonate (DEC) is 40-140 mg: 0.1-0.2 mol: 0.1 mol. The heating temperature is 80-105 °C, and the heating time is 4-10 h.

[0013] The beneficial effects of this invention are: 1. The metal-free sulfur and nitrogen co-doped carbon catalytic system is adopted. The catalyst does not contain any metal components, which avoids the dissolution and residue of metal ions from the source. The resulting EMC can meet the requirements of high-purity solvents for lithium-ion battery electrolytes, which significantly broadens the application range of the product.

[0014] 2. A Lewis acid-Lewis base dual-active-site synergistic catalytic center is formed on the catalyst surface. Sulfur atoms provide acidic sites and nitrogen atoms provide basic sites, which synergistically activate the substrate and intermediates, significantly reducing the activation energy of the transesterification reaction, and enabling efficient conversion under mild, atmospheric pressure, and solvent-free conditions.

[0015] 3. The EMC selectivity of the target product is close to 100%, with no obvious side reactions, which simplifies the subsequent separation and purification process, reduces distillation energy consumption and production costs, and improves the atom economy of the process.

[0016] 4. The catalyst preparation process is controllable and the structure is stable. After high-temperature carbonization, it has a rich pore structure, high specific surface area, and uniform heteroatom doping. The active sites are not easily lost during the recycling process. It can maintain high catalytic activity and selectivity even after more than 8 cycles, which greatly reduces the catalyst consumption cost in industrial production.

[0017] 5. The reaction process is green, safe, and easy to operate. It does not require inert gas protection or high-pressure equipment. Excess raw materials can be recycled and reused. It produces little waste, meets the requirements of green chemical industry and low-carbon production, and has good potential for industrial scale-up.

[0018] 6. Compared with traditional homogeneous alkalis, metal oxides, molecular sieves, and single heteroatom-doped carbon catalysts, this system achieves comprehensive improvements in metal-free properties, selectivity, stability, and process economy. It avoids problems such as metal ion loss, reduced catalytic efficiency, and secondary pollution during the catalytic process of metal catalysts. At the same time, it overcomes the defects of low activity and single site of single nitrogen doping and single sulfur doping, and the synergistic effect of sulfur-nitrogen dual sites significantly improves catalytic performance. Attached Figure Description

[0019] Figure 1 Here is a scanning electron microscope image of the catalyst SN@C-700 obtained in Example 1; Figure 2 The powder X-ray diffraction pattern of the SN@C-700 catalyst obtained in Example 1; Figure 3 The Raman spectrum of the SN@C-700 catalyst obtained in Example 1; Figure 4 The nitrogen adsorption-desorption isotherm of SN@C-700 obtained in Example 1; Figure 5 The reaction equation for the synthesis of EMC catalyzed by SN@C-700 obtained in Example 1; Figure 6 The gas chromatography-mass spectrum (GC-MS) of the EMC obtained in Example 1. Figure 7 Schematic diagram of EMC yield under different reaction times in Example 1; Figure 8 This is a schematic diagram of EMC yield under different temperature conditions in Example 2; Figure 9 Schematic diagram of EMC yield under different catalyst dosages in Example 3; Figure 10 The diagram shows the cycle performance of SN@C-700 obtained in Example 4 for synthesizing EMC. Detailed Implementation

[0020] In this embodiment of the invention, after the reaction is complete, the catalyst is filtered out, and the obtained reaction solution is diluted with dichloromethane. 30 μL of reaction solution corresponds to 570 μL of dichloromethane. The reaction conversion and yield are calculated using the normalization method. The correction factor for DEC is set to 1, and the correction factor for EMC relative to DEC is (F... EMC The coefficient of performance (COP) is 1.3179, indicating a very high selectivity. The correction factor for other byproducts is assumed to be 1.

[0021] Catalyst recovery and recycling: The recovered catalyst, after being thoroughly washed with ethanol and deionized water and dried under vacuum at 120 °C for 6 h, can be reused in the next round of reaction, with a recycling rate of no less than 8 times.

[0022] Example 1 (1) Catalyst preparation Weigh 10.0 g fructose, dissolve it in 150 mL distilled water, and stir for 30 min; then add 5 g thiourea and 5 g urea, stir for 5 h, and heat in an 85 °C water bath until a blocky black solid prepolymer is formed; raise the temperature of the prepolymer to 300 °C at a heating rate of 5 °C / min, calcine it in air for 10 min, and then transfer it to an argon atmosphere (argon flow rate 60 mL / min), raise the temperature to 700 °C at a heating rate of 5 °C / min, and crystallize for 6 h to obtain the SN@C-700 catalyst.

[0023] The SN@C-700 catalyst exhibits a three-dimensional cross-linked honeycomb porous structure, assembled from numerous interconnected micron-sized carbon sheets, with no obvious particle agglomeration. Compared to bulk carbon materials prepared by traditional direct carbonization, the catalyst prepared by this invention through a process controlled by prepolymerization-air pre-oxidation-argon crystallization forms a rich hierarchical pore network, providing ample mass transfer channels for the reaction substrate. This open framework structure effectively avoids the embedding of active sites and provides more binding sites for subsequent heteroatom doping, significantly differentiating it from existing S / N co-doped carbon catalysts with dense surfaces and single pores, such as... Figure 1 As shown.

[0024] The SN@C-700 catalyst exhibits a typical amorphous / low-graphitized carbon structure. Compared to high-graphitized carbon materials prepared by traditional high-temperature carbonization, the in-situ doping of S and N atoms in this invention effectively suppresses the ordered stacking of the carbon framework, resulting in significantly broadened diffraction peaks. This indicates an increase in the interplanar spacing and defect density of the carbon material. This structural feature retains the abundant defect sites of amorphous carbon materials as catalytic active centers while avoiding the problem of active site burial caused by high graphitization. It provides more binding sites for heteroatom doping and is one of the key structural foundations for its excellent catalytic activity. Figure 2 As shown.

[0025] like Figure 3 As shown, two characteristic peaks of carbon materials appeared in the Raman spectrum, corresponding to the defect / disorder structure of carbon materials (D peak) and graphitized sp, respectively. 2 In-plane stretching vibrations of hybrid carbon (G peak). Calculated peak intensity ratio (I... D / I GThe S / N ratio of 1.06 indicates that the catalyst has a high defect density and low graphitization degree, which is consistent with the PXRD results. Compared with carbon catalysts prepared by direct carbonization in the prior art, the S and N co-doping of this catalyst successfully introduces a large number of edge defects and topological defects. These defect sites can significantly improve the catalytic activity by regulating the charge distribution and substrate adsorption energy.

[0026] The nitrogen adsorption-desorption isotherm of the catalyst exhibits typical type IV isotherm characteristics. Rapid adsorption in the low-pressure region indicates the presence of a microporous structure, providing abundant active sites for the catalytic reaction. The obvious hysteresis loop appearing in the medium- and high-pressure regions confirms its mesoporous structure. The hierarchical pore structure of the catalyst obtained in this invention synergistically combines the high active site density of micropores with the efficient mass transfer advantages of mesopores, effectively overcoming the shortcomings of traditional microporous carbon catalysts such as high mass transfer resistance and low active site utilization. Figure 4 As shown.

[0027] (2) Test of transesterification reactivity 0.1 mol DMC and 0.1 mol DEC were added to a 50 mL reaction flask, and 120 mg of SN@C-700 catalyst was used. The mixture was stirred and reacted for 8 h at 85 °C. After filtering off the catalyst, the EMC content was determined by gas chromatography-mass spectrometry, yielding an EMC yield of 51.4%. The reaction equation for the catalytic synthesis of EMC is as follows: Figure 5 As shown; Gas chromatography-mass spectrometry (GC-MS) chromatogram as shown Figure 6 As shown, EMC is the only product.

[0028] Example 2 (1) Catalyst preparation Weigh 10.0 g of glucose, dissolve it in 200 mL of distilled water, and stir for 20 min. Then add 1 g of thiourea and 2 g of urea, stir for 3 h, and heat in a 70 °C water bath for 72 h to obtain a blocky black solid prepolymer. The blocky black solid prepolymer is heated to 250 °C at a heating rate of 10 °C / min and calcined in air for 8 min. Then it is transferred to an argon atmosphere with an argon flow rate of 40 mL / min and heated to 600 °C at a heating rate of 10 °C / min for 5 h to crystallize, thus obtaining the SN@C-600 catalyst.

[0029] (2) Test of transesterification reactivity The process is the same as step (2) in Example 1, and the yield of EMC is 51.1%.

[0030] Example 3 (1) Catalyst preparation Weigh 10.0 g of cellulose, dissolve it in 50 mL of distilled water, and stir for 20 min. Then add 5 g of thiourea and 10 g of urea, stir for 6 h, and heat in a 90 °C water bath for 36 h to obtain a blocky black solid prepolymer. The blocky black solid prepolymer is heated to 350 °C at a heating rate of 8 °C / min and calcined in air for 12 min. Then it is transferred to an argon atmosphere with an argon flow rate of 100 mL / min and heated to 800 °C at a heating rate of 10 °C / min to crystallize for 7 h to obtain the SN@C-800 catalyst.

[0031] The process is the same as step (2) in Example 1, and the yield of EMC is 51.2%.

[0032] Example 4 (1) Catalyst preparation The process is the same as step (1) of Example 1, except that melamine is used as the nitrogen source, that is, urea is replaced with melamine to obtain the S-NMel@C-700 catalyst.

[0033] (2) Test of transesterification reactivity The process is the same as step (2) in Example 1, and the yield of EMC is 49.7%.

[0034] Example 5 (1) Catalyst preparation The process is the same as step (1) of Example 1, except that gelatin is used as the nitrogen source, that is, urea is replaced with gelatin to obtain the S-Gel@C-700 catalyst.

[0035] (2) Test of transesterification reactivity The process is the same as step (2) in Example 1, and the EMC yield is 45.3%.

[0036] Example 6 (1) Catalyst preparation The process is the same as step (1) of Example 1, except that dicyandiamide is used as the nitrogen source, that is, urea is replaced with dicyandiamide to obtain the S-Dicy@C-700 catalyst.

[0037] (2) Test of transesterification reactivity The process is the same as step (2) in Example 1, and the EMC yield is 47.8%.

[0038] Example 7 (1) Catalyst preparation The process is the same as step (1) of Example 1, except that sublimed sulfur is used as the sulfur source, that is, thiourea is replaced with sublimed sulfur to obtain the S-Sub@C-700 catalyst.

[0039] (2) Test of transesterification reactivity The process is the same as step (2) in Example 1, and the EMC yield is 48.6%.

[0040] Example 8 (1) Catalyst preparation The process is the same as step (1) of Example 1, except that thioacetamide is used as the sulfur source, that is, thiourea is replaced with thioacetamide to obtain the S-TAA@C-700 catalyst.

[0041] (2) Test of transesterification reactivity The process is the same as step (2) in Example 1, and the EMC yield is 42.5%.

[0042] Example 9 (1) Catalyst preparation The process is the same as step (1) of Example 1, except that urea and melamine with a mass ratio of 1:1 are used as nitrogen sources, that is, 5 g of urea is replaced with 2.5 g of urea and 2.5 g of melamine to obtain the mixed nitrogen source SN-Mix@C-700 catalyst.

[0043] (2) Test of transesterification reactivity The process is the same as step (2) in Example 1, and the EMC yield is 50.7%.

[0044] Example 10 (1) Catalyst preparation The process is the same as step (1) in Example 1; (2) Test of transesterification reactivity 0.1 mol DMC and 0.1 mol DEC were added to a 50 mL reaction flask. 120 mg of SN@C-700 catalyst was used. The reaction was stirred for 2 h, 4 h, 6 h, 8 h, and 10 h at a temperature of 85 °C. After filtration, the EMC content was determined by gas chromatography-mass spectrometry, yielding EMC yields of 35.4%, 48.5%, 51.1%, 51.4%, and 51.4%, respectively. A schematic diagram of EMC yields at different reaction times is shown below. Figure 7 As shown.

[0045] Example 11 (1) Catalyst preparation The process is the same as step (1) in Example 1; (2) Test of transesterification reactivity 0.1 mol DMC and 0.1 mol DEC were added to a 50 mL reaction flask. 120 mg of SN@C-700 catalyst was used. The reaction was stirred and carried out at temperatures of 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, and 105 °C for 8 h. After the reaction was complete, the catalyst was filtered off, and the EMC content was determined by gas chromatography-mass spectrometry. The yields of EMC were approximately 32.1%, 37.3%, 44.6%, 51.4%, 51.4%, 51.4%, 51.3%, and 51.2%. A schematic diagram of the EMC yield at different temperatures is shown below. Figure 8 As shown.

[0046] Example 12 (1) Catalyst preparation The process is the same as step (1) in Example 1; (2) Test of transesterification reactivity 0.1 mol DMC and 0.1 mol DEC, along with SN@C-700 catalyst, were added to a 50 mL reaction flask. The mixture was stirred and reacted at 85 °C for 8 h. The catalyst dosages were 20 mg, 40 mg, 60 mg, 80 mg, 100 mg, 120 mg, and 140 mg. After the reaction was complete, the catalyst was filtered off, and the EMC content was determined by gas chromatography-mass spectrometry. The yields of EMC were 37.6%, 40.2%, 42.4%, 45.8%, 48.6%, 51.4%, and 51.4%, respectively. A schematic diagram illustrating the EMC yields under different catalyst dosages is shown below. Figure 9 As shown.

[0047] Example 13 (1) Catalyst preparation The process is the same as step (1) in Example 1; (2) Test of transesterification reactivity 0.2 mol DMC and 0.1 mol DEC, along with 120 mg of SN@C-700 catalyst, were added to a 50 mL reaction flask; the reaction temperature was 85 °C, and the reaction time was 8 h. EMC yield: 51.2%.

[0048] The transesterification reaction is a reversible transesterification reaction. It is limited by thermodynamic equilibrium. Increasing the proportion of DMC can promote the equilibrium to the right. However, due to the upper limit of the system equilibrium constant, the increase in the EMC equilibrium yield is very small. Therefore, it is close to the yield of the equimolar ratio system.

[0049] Cyclic activity test of transesterification reaction: 0.1 mol DMC and 0.1 mol DEC, along with 120 mg of SN@C-700 catalyst, were added to a 50 mL reaction flask. The mixture was stirred and reacted at 85 °C for 8 h. After the reaction was complete, the catalyst was filtered off, and the EMC content was determined by gas chromatography-mass spectrometry to obtain the EMC yield. The catalyst was then washed with simple ethanol, and the transesterification reaction activity test was repeated eight times. The yields of EMC were approximately 51.4%, 51.3%, 51.4%, 51.4%, 51.3%, 51.4%, 51.3%, and 51.2%. The cycling performance is shown in the figure below. Figure 10 As shown in the figure, the catalyst retains high catalytic activity and selectivity after being washed with ethanol and can be cycled 8 times. The catalyst has excellent structural stability and economic applicability.

[0050] Comparative Example 1 (1) Catalyst preparation The catalyst was prepared without adding a sulfur source, only a nitrogen source. The specific process was as follows: 10.0 g of fructose was weighed and dissolved in 150 mL of distilled water and stirred for 30 min; 5 g of urea was added and stirred for 5 h, then heated in an 85 °C water bath until a blocky black solid prepolymer was formed; the prepolymer was heated to 300 °C at a heating rate of 5 °C / min and calcined in air for 10 min, then transferred to an argon atmosphere (argon flow rate 60 mL / min) and heated to 700 °C at a heating rate of 5 °C / min for 6 h to crystallize, thus obtaining the nitrogen-doped carbon catalyst N@C-700.

[0051] (2) Test of transesterification reactivity The process is the same as step (2) in Example 1, and the EMC yield is 30.2%.

[0052] Comparative Example 2 (1) Catalyst preparation The catalyst was prepared without adding a sulfur source. The specific process was as follows: 10.0 g of fructose was weighed and dissolved in 150 mL of distilled water and stirred for 30 min; 5 g of thiourea was added and stirred for 5 h, and then heated in an 85 °C water bath until a black prepolymer was formed; the prepolymer was heated to 300 °C at a heating rate of 5 °C / min and calcined in air for 10 min, and then transferred to an argon atmosphere (argon flow rate 60 mL / min) and heated to 700 °C at a heating rate of 5 °C / min for 6 h to obtain the sulfur-doped carbon catalyst S@C-700.

[0053] (2) Test of transesterification reactivity The process is the same as step (2) in Example 1, and the EMC yield is 28.5%.

[0054] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it. They should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A metal-free sulfur-nitrogen co-doped carbon catalyst, characterized in that, The metal-free sulfur-nitrogen co-doped carbon catalyst includes a carbon source, a sulfur source, and a nitrogen source; The carbon source is one or more of fructose, glucose, sucrose, cellulose, and citric acid; The sulfur source is one or more of thiourea, sublimed sulfur, and thioacetamide; The nitrogen source is one or more of urea, melamine, gelatin, dicyandiamide, and thioacetamide; The preparation method of the metal-free sulfur-nitrogen co-doped carbon catalyst includes the following steps: S1. Mix carbon source and water and stir, then add sulfur source and nitrogen source and continue stirring. Heat in a water bath to obtain prepolymer. S2. The prepolymer is heated in an air atmosphere and then crystallized in an argon atmosphere to obtain the SN@CT catalyst.

2. The metal-free sulfur-nitrogen co-doped carbon catalyst according to claim 1, characterized in that, In step S1, the mass ratio of carbon source to water is 1:5-20; the mass ratio of carbon source to sulfur source to nitrogen source is 1:(0.1~0.5):(0.2~1.0).

3. The metal-free sulfur-nitrogen co-doped carbon catalyst according to claim 1, characterized in that, The mixing and stirring time in step S1 is 20-30 min; the continued stirring time is 3-6 h.

4. The metal-free sulfur-nitrogen co-doped carbon catalyst according to claim 1, characterized in that, The water bath heating temperature in step S1 is 70-90 °C, and the water bath heating time is 36-72 h.

5. The metal-free sulfur-nitrogen co-doped carbon catalyst according to claim 1, characterized in that, The heating rate in step S2 is 5-10°C / min, the heating temperature is 250-350°C, and the heating time is 8-12 min.

6. The metal-free sulfur-nitrogen co-doped carbon catalyst according to claim 1, characterized in that, In step S2, the argon flow rate in the argon atmosphere is 40-100 mL / min.

7. The metal-free sulfur-nitrogen co-doped carbon catalyst according to claim 1, characterized in that, The crystallization heating rate in step S2 is 5-10°C / min, the crystallization temperature is 600-800 ℃, and the crystallization time is 5-7 h.

8. The application of the metal-free sulfur-nitrogen co-doped carbon catalyst according to any one of claims 1-7 in the synthesis of ethyl methyl carbonate.

9. The application according to claim 8, characterized in that, The specific method involves mixing a metal-free sulfur and nitrogen co-doped carbon catalyst, dimethyl carbonate (DMC), and diethyl carbonate (DEC), and then heating the mixture to obtain ethyl methyl carbonate (EMC).

10. The application according to claim 9, characterized in that, The ratio of the metal-free sulfur and nitrogen co-doped carbon catalyst, dimethyl carbonate (DMC), and diethyl carbonate (DEC) is 40-140 mg: 0.1-0.2 mol: 0.1 mol; the heating temperature is 80-105 °C, and the heating time is 4-10 h.

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