A boron and nitrogen co-doped carbon catalyst, a preparation method thereof and a method for catalytically synthesizing unsymmetrical organic carbonates

Abstract

The application relates to a boron and nitrogen co-doped carbon catalyst, a preparation method thereof and a method for catalytically synthesizing unsymmetrical organic carbonates, and belongs to the field of catalytic organic synthesis. The application constructs a boron and nitrogen co-doped carbon material-based metal-free catalytic system, utilizes Lewis acid-base site synergism, realizes efficient synthesis of unsymmetrical organic carbonates, reduces the substrate ratio of symmetrical carbonates and alcohols, reduces separation energy consumption and cost, improves the applicability of the catalytic system to various alcohol substrates, and excellent catalytic activity is shown for alkyl alcohol, cycloalkyl alcohol, aryl alcohol, heterocyclic alcohol and other biomass alcohols, high yield and high selectivity are ensured, Lewis acid-base synergistic sites are constructed in the carbon material through boron and nitrogen co-doping, traditional metal-based catalysts are replaced, metal residues are avoided, the product can be used in fields such as medicine and food which have high purity requirements, the application scenarios are wider, the subsequent treatment pressure of catalyst separation and recovery is reduced, and the economic efficiency and greenness of the process are strengthened.

Description

Technical Field

[0001] This invention belongs to the field of catalytic organic synthesis, specifically relating to a boron and nitrogen co-doped carbon catalyst, its preparation method, and its catalytic synthesis of asymmetric organic carbonates. Background Technology

[0002] Asymmetric organic carbonates, as a class of high-value core chemical intermediates, possess irreplaceable unique properties such as high flash point, strong polarity, low viscosity, and excellent dielectric constant. They exhibit broad and indispensable application prospects in several key fields, including food additives, aprotic polar solvents, pharmaceutical synthesis, and polymer preparation, demonstrating greater market competitiveness and application potential compared to traditional symmetric organic carbonates. However, their persistently high market price has become a core bottleneck restricting their large-scale promotion and industrial application, severely hindering the upgrading and development of related downstream industries. Traditional organic carbonate preparation processes primarily rely on the phosgene method; however, as the health hazards associated with phosgene production have become increasingly recognized and emphasized, many countries worldwide have gradually banned the use of this process. Against this backdrop, green synthesis pathways for organic carbonates have become a research hotspot and core direction for researchers both domestically and internationally. In summary, current research on clean production technologies for organic carbonates mainly focuses on four methods: oxidative carbonylation, transesterification, urea alcoholysis, and direct CO2 synthesis. Currently, transesterification is the mainstream technical route for synthesizing asymmetric organic carbonates. This method uses symmetric carbonates and biomass alcohols as core raw materials, and achieves the preparation of the target product through selective mono-transesterification under the action of a catalyst. However, existing transesterification catalytic systems have significant technical drawbacks: on the one hand, the catalytic materials mostly rely on metal oxides (such as MgO and ZnO) or metal-supported carbon materials, resulting in insufficient environmental friendliness, high preparation costs, and metal ion loss; on the other hand, to ensure reaction selectivity, a large amount of excess symmetric carbonate is required (e.g., a molar ratio of dimethyl carbonate to alcohol as high as 33:2), which not only significantly increases the cost of raw materials and the difficulty of subsequent separation and purification, but also causes resource waste and environmental pollution, further exacerbating the production dilemma of asymmetric organic carbonates. Against this backdrop, developing novel catalytic systems that are environmentally friendly, highly efficient, and can significantly reduce substrate ratios has become a core research hotspot in academia and the chemical industry. Patents related to such novel catalytic systems are key to overcoming existing technical bottlenecks and solving the problem of low-cost, large-scale production of asymmetric organic carbonates. These patents not only fill gaps in existing catalytic technologies and promote the greening and efficiency upgrade of transesterification synthesis processes, but also fundamentally reduce product production costs and accelerate their large-scale application in various downstream sectors. This has immeasurable significance and enormous industrial value in driving the upgrading of the entire chemical industry chain and enhancing the market competitiveness of related industries.

[0003] Defects and shortcomings of existing technology: 1. Reliance on metal catalysts: Traditional catalytic systems often contain metal components, which not only increases production costs but may also lead to metal residues in the products, limiting their application in fields such as pharmaceuticals and food where high purity is required; 2. Excessive substrate ratio: A large amount of symmetrical carbonate is required to ensure reaction selectivity, resulting in high energy consumption and cost of subsequent distillation and separation, and poor atom economy; 3. Limited applicability: Some catalysts have poor catalytic performance on substituent-containing alcohols and heterocyclic alcohols, resulting in low product yields and selectivity; 4. Insufficient catalyst stability: Some metal-based catalysts have poor recycling performance, and their catalytic activity decreases significantly after repeated use, increasing the catalyst consumption cost in industrial production. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a boron and nitrogen co-doped carbon catalyst, its preparation method, and a method for catalytically synthesizing asymmetric organic carbonates. This invention constructs a boron-nitrogen co-doped carbon-based metal-free catalytic system, utilizing the synergistic effect of Lewis acid-base sites to achieve efficient synthesis of asymmetric organic carbonates. This reduces the substrate ratio of symmetric carbonates to alcohols, decreasing separation energy consumption and costs; improves the applicability of the catalytic system to various alcohol substrates, ensuring high yields and high selectivity; enhances the stability and recyclability of the catalyst, reducing total production costs; and avoids metal residues, broadening the application scenarios of the product.

[0005] The present invention discloses a boron and nitrogen co-doped carbon catalyst, which is a metal-free boron and nitrogen co-doped carbon material, denoted as xBN@CT, where T represents the crystallization temperature under an inert atmosphere and x represents the molar ratio of boron to nitrogen. The boron source for the boron and nitrogen co-doped carbon catalyst is boric acid; The nitrogen source of the boron and nitrogen co-doped carbon catalyst is at least one of urea, melamine, chitosan, gelatin and dicyandiamide; the molar ratio of boric acid to nitrogen source is 0.2-1.1. The carbon source of the boron and nitrogen co-doped carbon catalyst includes at least one of glucose, fructose, sucrose, cellulose and citric acid. A method for preparing the above-mentioned boron and nitrogen co-doped carbon catalyst includes the following steps: S1. Dissolve carbon source and aluminum nitrate nonahydrate in water, stir until completely dissolved, add phosphoric acid, continue stirring, then add boric acid and nitrogen source and stir. Heat in a water bath to obtain a viscous prepolymer. S2. The prepolymer is calcined in air and then crystallized in argon atmosphere to obtain the crystallized product. S3. Add the crystallized product to a nitric acid solution, stir, filter, wash until neutral, and vacuum dry to obtain the xBN@CT catalyst.

[0006] The mass ratio of the carbon source and aluminum nitrate nonahydrate mentioned in step S1 is 0.9-1.1:1; The ratio of the volume of water in step S1 to the total mass of the carbon source and aluminum nitrate nonahydrate is 4-8 mL:1g. The stirring time mentioned in step S1 is 30-50 min; The mass ratio of phosphoric acid and aluminum nitrate nonahydrate in step S1 is 0.5-1:1; The mass ratio of boric acid to the total mass of the carbon source and aluminum nitrate nonahydrate in step S1 is 0.05–0.3:1. The mass ratio of the nitrogen source to the total mass of the carbon source and aluminum nitrate nonahydrate mentioned in step S1 is 0.5 to 2.0: 1; The stirring time for adding boric acid and nitrogen source in step S1 is 1-3 hours; The water bath heating temperature mentioned in step S1 is 80-90℃; The calcination temperature in step S2 is 250-350℃, and the calcination time is 6-12 min; The crystallization temperature in step S2 is 600-1000℃, and the crystallization time is 5-8 h; The concentration of the nitric acid solution in step S3 is 4 M; the ratio of the amount of the crystallized product to the amount of nitric acid solution is 1 g: 10-20 ml; The stirring temperature in step S3 is 40-60℃, and the stirring time is 4-6 h; The vacuum drying temperature in step S3 is 80-100℃, and the vacuum drying time is 6-10h.

[0007] The method for synthesizing asymmetric organic carbonates using the xBN@CT catalyst obtained by the above preparation method is as follows: dimethyl carbonate (DMC), alcohol compounds and xBN@CT catalyst are mixed and reacted in an air atmosphere. After the reaction is completed, the mixture is cooled to room temperature, and the xBN@CT catalyst is separated by filtration. The crude product is further purified by column chromatography to obtain asymmetric organic carbonates.

[0008] The alcohols include alkyl alcohols, cycloalkyl alcohols, aryl alcohols, and heterocyclic alcohols; the alkyl alcohols include n-butanol or isoamyl alcohol; the cycloalkyl alcohols include cyclohexylmethanol; the aryl alcohols include benzyl alcohol, 4-chlorobenzyl alcohol, phenethyl alcohol, or DL-1-phenethyl alcohol; and the heterocyclic alcohols include tetrahydrofurfuryl alcohol. The molar ratio of dimethyl carbonate (DMC) to alcohol is 1-8:1; The ratio of the amount of the xBN@CT catalyst to the amount of the alcohol compound is 20-50:1 mg / mmol; the reaction temperature is 60-100℃ and the reaction time is 4-7 h.

[0009] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Significant advantages of metal-free catalysis: Lewis acid-base synergistic sites are constructed in carbon materials by boron-nitrogen co-doping, which can replace traditional metal-based catalysts, avoid metal residues, and the products can be used in fields with high purity requirements such as medicine and food, with a wider range of applications; 2. Synergistic enhancement of catalytic efficiency by acid-base sites: The precisely designed bifunctional structure with both Lewis acidic and basic sites can effectively activate the active groups of the reaction substrate and reduce the activation energy of the reaction. Compared with single active site catalysts, it significantly improves the synthesis efficiency and selectivity of asymmetric organic carbonates, breaking through the bottleneck of insufficient catalytic activity of traditional single-site catalysts. 3. Substrate ratio significantly optimized: Only a DMC to alcohol molar ratio of 5:1 is required to achieve 99% yield and selectivity. Compared with the traditional process (33:2), the amount of ester substrate used is greatly reduced, reducing distillation energy consumption and cost, and resulting in higher atom economy. 4. Low catalyst dosage and significant economic benefits: Thanks to the highly efficient synergistic effect of Lewis acid-base sites, the catalytic activity of the catalyst is greatly enhanced. Only 30 mg / mmol of alcohol is needed to achieve the efficient synthesis of asymmetric organic carbonates, far lower than the dosage standards of traditional metal-based catalysts and similar carbon-based catalysts. This low dosage not only reduces the material input in the catalytic process and lowers raw material costs, but also alleviates the pressure of subsequent catalyst separation and recovery, further enhancing the economic efficiency and environmental friendliness of the process. 5. Broad substrate applicability: It exhibits excellent catalytic activity for biomass alcohols such as alkyl alcohols, cycloalkyl alcohols, aryl alcohols, and heterocyclic alcohols, with a selectivity of 99% and a product yield of 94%-99%, solving the problem of poor catalytic effect of existing catalysts on complex structured alcohols; 6. Strong catalytic stability: After treatment with nitric acid and high-temperature crystallization, the catalyst forms a stable carbon skeleton structure, with boron and nitrogen elements uniformly doped into the carbon matrix. They are not easily detached or lost, allowing the catalyst to maintain high catalytic activity even after being recycled more than 10 times. This significantly reduces the operating cost of the catalytic process and improves the economy and sustainability of the process. 7. Green and environmentally friendly and easy to industrialize: The reaction does not require additional solvents and can be carried out in air. Excess DMC can be recycled, which is in line with the concept of green chemical industry. At the same time, it can be scaled up to the gram scale, and the product yield remains above 96%, which has clear potential for industrial application. Attached Figure Description

[0010] Figure 1 Here is a SEM image of the catalyst 0.6 BN@C-800 obtained in Example 1; Figure 2 The powder X-ray diffraction pattern of the 0.6 BN@C-800 catalyst obtained in Example 1 is shown. Figure 3 The Raman spectrum of the 0.6 BN@C-800 catalyst obtained in Example 1 is shown below. Figure 4 The X-ray photoelectron spectroscopy (XPS) spectrum of the 0.6 BN@C-800 catalyst obtained in Example 1 is shown below. Figure 5 The X-ray photoelectron spectrum of boron in the 0.6 BN@C-800 catalyst obtained in Example 1 is shown. Figure 6 The nitrogen adsorption-desorption isotherm curve of 0.6 BN@C-800 obtained in Example 1; Figure 7 The synthesis equation for the 0.6 BN@C-800 asymmetric organic carbonate obtained in Example 1; Figure 8 The graph shows the cycling performance of the benzylmethyl carbonate obtained in Example 2. Figure 9 The 1H NMR spectrum of benzylmethyl carbonate obtained in Example 2; Figure 10 The structural diagrams show asymmetric carbonates synthesized from different substrates. Detailed Implementation

[0011] The catalyst recovery and recycling involved in this invention: The recovered catalyst is washed twice with ethanol and once with deionized water, and then vacuum dried at 120°C for 14 h. It can be reused in the next round of reaction, and the number of times it can be recycled can be no less than 10 times.

[0012] Example 1 Preparation of 0.6 BN@C-800 catalyst S1. Weigh 10.3 g of glucose and 10.8 g of aluminum nitrate nonahydrate, dissolve them in 100 mL of distilled water, stir until completely dissolved, add 6 mL of phosphoric acid, and continue stirring for 30 min; add 2.1 g of boric acid and 3.4 g of urea (molar ratio of boric acid to urea is 0.6), stir for 3 h, and then heat in an 80 ℃ water bath to form a viscous prepolymer; S2. The prepolymer was calcined in air at 300 °C for 10 min, and then crystallized in argon at 800 °C for 6 h to obtain a black solid product. S3. The black solid was added to a 4 M nitric acid solution and stirred at 50 °C for 5 h. After filtration, it was washed three times with deionized water (50 mL each time) until neutral. The solution was then dried under vacuum at 100 °C for 8 h to obtain the 0.6 BN@C-800 catalyst. The characterization results are shown below. Figure 1-6 .

[0013] Example 2 Catalytic synthesis of benzylmethyl carbonate In a 10 mL flask equipped with a reflux condenser, 1 mmol of benzyl alcohol, 5 mmol of DMC, and 30 mg of the 0.6 BN@C-800 catalyst obtained in Example 1 were added. The mixture was stirred in an oil bath at 80 °C for 4 h. After the reaction was completed, the mixture was cooled to room temperature, and the catalyst was separated by filtration. The filtrate was subjected to rotary evaporation to remove volatiles. The residue was purified by column chromatography using an ethyl acetate / petroleum ether mixture (volume ratio 1:5) to give benzylmethyl carbonate with a yield of 99% and a selectivity of 99%. The synthetic equation is as follows. Figure 7 As shown, the proton NMR spectrum is as follows: Figure 9 As shown.

[0014] Example 3 Catalytic synthesis of benzylmethyl carbonate The reaction conditions were the same as in Example 2, except that 5 mmol DMC was replaced with 3 mmol DMC, with a yield of 73% and a selectivity of 74%.

[0015] Example 4 Catalytic synthesis of benzylmethyl carbonate The reaction conditions were the same as in Example 2, except that 5 mmol DMC was replaced with 4 mmol DMC, with a yield of 89% and a selectivity of 90%.

[0016] Example 5 Catalytic synthesis of benzylmethyl carbonate The reaction conditions were the same as in Example 2, except that 5 mmol DMC was replaced with 8 mmol DMC, with a yield of 99% and a selectivity of 99%.

[0017] Example 6 Catalytic synthesis of 4-chlorobenzylmethyl carbonate The reaction conditions were the same as in Example 2, except that benzyl alcohol was replaced with 1 mmol of 4-chlorobenzyl alcohol, and the reaction time was extended to 6 h.

[0018] Performance data: Yield 95%, selectivity 99%.

[0019] Example 7 Catalytic synthesis of methylphenylethyl carbonate The reaction conditions were the same as in Example 2, except that benzyl alcohol was replaced with 1 mmol phenylethanol and the reaction time was extended to 6 h.

[0020] Performance data: Yield 96%, selectivity 98%.

[0021] Example 8 Catalytic synthesis of methyl (1-phenylethyl) carbonate The reaction conditions were the same as in Example 2, except that benzyl alcohol was replaced with 1 mmol DL-1-phenylethanol, and the reaction time was extended to 7 h.

[0022] Performance data: Yield 95%, selectivity 98%.

[0023] Example 9 Catalytic synthesis of cyclohexylmethylmethyl carbonate The reaction conditions were the same as in Example 2, except that benzyl alcohol was replaced with 1 mmol of cyclohexylmethanol, and the reaction time was 5 h.

[0024] Performance data: Yield 94%, selectivity 97%. The structural diagrams of the asymmetric carbonates synthesized in Examples 2-9 are shown below. Figure 10 As shown.

[0025] Example 10 Catalytic synthesis of methyl ((tetrahydrofuran-2-yl)methyl) carbonate The reaction conditions were the same as in Example 2, except that benzyl alcohol was replaced with 1 mmol of tetrahydrofurfuryl alcohol, and the reaction time was adjusted to 5 h.

[0026] Performance data: Yield 95%, selectivity 98%.

[0027] Example 11 Catalytic synthesis of butyl methyl carbonate The reaction conditions were the same as in Example 2, except that benzyl alcohol was replaced with 1 mmol of n-butanol and the reaction time was adjusted to 4 h.

[0028] Performance data: Yield 97%, selectivity 98%.

[0029] Example 12 Catalytic synthesis of methyl (2-methylbutyl) carbonate The reaction conditions were the same as in Example 2, except that benzyl alcohol was replaced with 1 mmol of isoamyl alcohol and the reaction time was adjusted to 5 h.

[0030] Performance data: Yield 97%, selectivity 99%.

[0031] Example 13 Catalyst recycling test (10th cycle) The catalyst separated by filtration in Example 2 was washed twice with ethanol and once with deionized water, then dried under vacuum at 120 °C for 14 h and used in the next round of benzyl methyl carbonate synthesis. This cycle was repeated 10 times, with each reaction under the same conditions as in Example 2. The cycle performance diagram of the catalyst obtained in Example 2 is shown in the figure. Figure 8 As shown.

[0032] The catalyst prepared by this invention can be reused after simple washing and drying, whereas existing catalysts require high-temperature calcination again, which greatly reduces the operating cost of the catalytic process and improves the economy and sustainability of the process.

[0033] Example 14 Catalytic synthesis of benzylmethyl carbonate The reaction conditions were the same as in Example 2, except that 30 mg of 0.6 BN@C-800 catalyst was replaced with 20 mg of 0.6 BN@C-800 catalyst, with a yield of 90% and a selectivity of 99%.

[0034] Example 15 Catalytic synthesis of benzylmethyl carbonate The reaction conditions were the same as in Example 2, except that 30 mg of 0.6 BN@C-800 catalyst was replaced with 40 mg of 0.6 BN@C-800 catalyst, with a yield of 99% and a selectivity of 99%.

[0035] Example 16 Catalytic synthesis of benzylmethyl carbonate The reaction conditions were the same as in Example 2, except that 30 mg of 0.6 BN@C-800 catalyst was replaced with 50 mg of 0.6 BN@C-800 catalyst, with a yield of 99% and a selectivity of 99%.

[0036] Example 17 Preparation of 0.2 BN@C-700 catalyst S1. Weigh 10.8 g of glucose and 10.8 g of aluminum nitrate nonahydrate, dissolve them in 100 mL of distilled water, add 8 mL of phosphoric acid and stir for 30 min; add 2.1 g of boric acid and 10.2 g of urea (molar ratio of boric acid to urea is 0.2), stir for 2 h and then form a prepolymer at 85℃. S2. The prepolymer was calcined in air at 300 °C for 10 min, followed by crystallization in argon at 700 °C for 5 h to obtain a black solid. S3. The black solid was added to a 4 M nitric acid solution and stirred at 40 °C for 6 h. After filtration, it was washed three times with deionized water (50 mL each time) until neutral. It was then dried under vacuum at 80 °C for 10 h to obtain 0.2 BN@C-700 catalyst.

[0037] Catalytic performance: Used for the synthesis of benzyl methyl carbonate (reaction conditions same as in Example 2), yield 75%, selectivity 99%, verifying the effect of the molar ratio of boron and nitrogen and carbonization temperature.

[0038] Example 18 Preparation of 0.6 BN@C-700 catalyst S1. Weigh 10.3 g of glucose and 10.8 g of aluminum nitrate nonahydrate, dissolve them in 100 mL of distilled water, add 6 mL of phosphoric acid and stir for 40 min; add 2.1 g of boric acid and 3.4 g of urea (molar ratio of boric acid to urea is 0.6), stir for 3 h and then form a prepolymer at 80℃. S2. The prepolymer was calcined in air at 300 °C for 10 min, followed by crystallization in argon at 700 °C for 6 h to obtain a black solid. S3. The black solid was added to a 4 M nitric acid solution and stirred at 60 °C for 4 h. After filtration, it was washed three times with deionized water (50 mL each time) until neutral. It was then dried under vacuum at 100 °C for 6 h to obtain 0.2 BN@C-700 catalyst.

[0039] Catalytic performance: Used for the synthesis of benzyl methyl carbonate (reaction conditions same as in Example 2), yield 82%, selectivity 99%, verifying the effect of crystallization temperature.

[0040] Example 19 Preparation of 0.9 BN@C-900 catalyst S1. Weigh 11.3 g of glucose and 10.8 g of aluminum nitrate nonahydrate, dissolve them in 100 mL of distilled water, add 10 mL of phosphoric acid and stir for 30 min; add 2.1 g of boric acid and 2.3 g of urea (molar ratio of boric acid to urea is 0.9), stir for 1.5 h and then 80 °C to form a prepolymer. S2. The prepolymer was calcined in air at 250 °C for 10 min, followed by crystallization in argon at 900 °C for 5 h to obtain a black solid. S3. Subsequent washing and drying steps were the same as in Example 1, yielding a 0.9 BN@C-900 catalyst. Catalytic performance: Used for the synthesis of benzyl methyl carbonate (reaction conditions same as in Example 2), yielding 82% and selectivity 99%, further verifying the influence of parameter variables on catalytic activity.

[0041] Example 20 Scale-up synthesis experiment In a 100 mL flask, 1.2 g of the 0.6 BN@C-800 catalyst obtained in the previous example, 40 mmol of benzyl alcohol, and 0.20 mol of DMC (molar ratio 5:1) were added, and the mixture was stirred in an oil bath at 80 °C for 4 h. After the reaction, the catalyst was separated by filtration, and the excess DMC in the filtrate was recovered by distillation (recovery rate 95%). The remaining residue was purified by column chromatography to obtain benzylmethyl carbonate. The product yield was 97%, and the recovered DMC was directly used in the next round of reaction (reaction conditions unchanged), verifying the feasibility of large-scale production.

[0042] Example 21 Preparation and catalytic performance of 0.6 BN@C-800-FM (FM is an abbreviation for Fructose and Melamine) catalyst using melamine as nitrogen source and fructose as carbon source. S1. Weigh 10.3 g fructose and 10.8 g aluminum nitrate nonahydrate, dissolve them in 100 mL distilled water, stir until completely dissolved, add 6 mL phosphoric acid, and continue stirring for 50 min; add 2.1 g boric acid and 7.05 g melamine (molar ratio of boric acid to melamine 0.6), stir for 3 h, and then heat in an 80 ℃ water bath to form a viscous prepolymer; S2, same as step S2 in Example 1; the prepolymer is calcined in air at 250 °C for 12 min; S3, same as step S3 in Example 1, to obtain 0.6 BN@C-800-FM catalyst; Catalytic performance: Used for the synthesis of benzylmethyl carbonate (reaction conditions as in Example 2), yield 96%, selectivity 99%.

[0043] Example 22 Preparation and catalytic performance of 0.6 BN@C-800-SC (SC is an abbreviation for Sucrose and Chitosan) catalyst using chitosan as nitrogen source and sucrose as carbon source. S1. Weigh 10.3 g sucrose and 10.8 g aluminum nitrate nonahydrate, dissolve them in 100 mL distilled water, stir until completely dissolved, add 6 mL phosphoric acid, and continue stirring for 30 min; add 2.1 g boric acid and 9.1 g chitosan (boric acid to chitosan molar ratio 0.6), stir for 3 h, and then heat in an 80 ℃ water bath to form a viscous prepolymer; S2, same as step S2 in Example 1, followed by crystallization at 800 °C for 8 h in an argon atmosphere to obtain a black solid; S3, the same as step S3 in Example 1, yields a 0.6 BN@C-800-SC catalyst.

[0044] Catalytic performance: Used for the synthesis of benzylmethyl carbonate (reaction conditions as in Example 2), yield 95%, selectivity 99%.

[0045] Example 23 Preparation and catalytic performance of 0.6 BN@C-800-CG (CG is an abbreviation for Cellulose and Gelatin) catalyst using gelatin as nitrogen source and cellulose as carbon source. S1. Weigh 10.3 g of ball-milled refined cellulose and 10.8 g of aluminum nitrate nonahydrate, dissolve them in 100 mL of distilled water, stir until completely dissolved, add 6 mL of phosphoric acid, and continue stirring for 30 min; add 2.1 g of boric acid and 5.1 g of gelatin (molar ratio of boric acid to gelatin 0.6), stir for 3 h, and then heat in an 80 ℃ water bath to form a viscous prepolymer; S2, Same as step S2 in Example 1; the prepolymer is calcined in air at 350°C for 6 min. S3, the same as step S3 in Example 1, yields a 0.6 BN@C-800-CG catalyst.

[0046] Catalytic performance: Used for the synthesis of benzylmethyl carbonate (reaction conditions as in Example 2), yield 94%, selectivity 98%.

[0047] Example 24 Preparation and catalytic performance of 0.6 BN@C-800-CC (CC is an abbreviation for Citric acid and Dicyandiamide) catalyst using dicyandiamide as nitrogen source and citric acid as carbon source. S1. Weigh 10.3 g of citric acid and 10.8 g of aluminum nitrate nonahydrate, dissolve them in 100 mL of distilled water, stir until completely dissolved, add 6 mL of phosphoric acid, and continue stirring for 30 min; add 2.1 g of boric acid and 4.8 g of dicyandiamide (molar ratio of boric acid to dicyandiamide 0.6), stir for 3 h, and then heat in an 80 ℃ water bath to form a viscous prepolymer; S2, same as step S2 in Example 1; S3, the same as step S3 in Example 1, yields a 0.6 BN@C-800-CC catalyst.

[0048] Catalytic performance: For the synthesis of benzylmethyl carbonate (reaction conditions same as in Example 2), yield 97%, selectivity 98%. For 4-chlorobenzylmethyl carbonate (reaction conditions same as in Example 3), yield 94%, selectivity 99%. For methylphenylethyl carbonate (reaction conditions same as in Example 4), yield 95%, selectivity 96%. For methyl(1-phenylethyl) carbonate (reaction conditions same as in Example 5), yield 94%, selectivity 97%.

[0049] Example 25 Preparation and catalytic performance of 0.6 BN@C-800-MC (MC is an abbreviation for Mixed Carbon & Nitrogen) catalyst from mixed nitrogen and carbon sources. S1. Weigh 5.15 g glucose + 5.15 g fructose mixed carbon source and 10.8 g aluminum nitrate nonahydrate, dissolve in 100 mL distilled water, stir until completely dissolved, add 6 mL phosphoric acid, and continue stirring for 30 min; add 2.1 g boric acid, 1.7 g urea + 2.4 g dicyandiamide mixed nitrogen source (molar ratio of boric acid to mixed nitrogen source 0.6), stir for 3 h, and then heat in an 80 ℃ water bath to form a viscous prepolymer; S2, same as step S2 in Example 1; S3, the same as step S3 in Example 1, yields a 0.6 BN@C-800-MC catalyst.

[0050] Catalytic performance: Used for the synthesis of benzylmethyl carbonate (reaction conditions as in Example 2), yield 98%, selectivity 99%.

[0051] Comparative Example 1 S1. Same as step S1 in Example 1, except that the molar ratio of boric acid to urea is adjusted to 1.5 (3.5g boric acid, 2.3g urea). S2, same as step S2 in Example 1; S3, the same as step S3 in Example 1, yields a 1.5 BN@C-800 catalyst.

[0052] Experimental results: After calcination, the product showed boron agglomeration and could not form a uniform boron-nitrogen co-doped carbon framework. Catalytic performance: Used for the synthesis of benzylmethyl carbonate (reaction conditions as in Example 2), yield 42%, selectivity 65%.

[0053] In Comparative Example 1, the boron-nitrogen molar ratio was 1.5. Excessive boron agglomeration disrupted the acid-base site ratio balance, reducing the number of effective active sites and triggering side reactions, resulting in a double decrease in yield and selectivity. In contrast, the boron-nitrogen molar ratio specified in this invention ensures that boron (Lewis acid sites) and nitrogen (Lewis base sites) are uniformly doped in the carbon framework, forming an acid-base synergistic site pair of approximately 1:1, achieving efficient activation of the substrate (borate activates the OH bond of alcohol hydroxyl groups, and amino groups activate the C=O bond of DMC).

[0054] Comparative Example 2 S1, same as step S1 in Example 1; S2, Same as step S2 in Example 1, except that crystallization is carried out at 600 °C for 6 h in an argon atmosphere; S3, the same as step S3 in Example 1, yields a 0.6 BN@C-600 catalyst.

[0055] Experimental results: The carbon materials have a low degree of graphitization, and Lewis acid-base synergistic sites are not effectively formed; Catalytic performance: Used for the synthesis of benzyl methyl carbonate (reaction conditions as in Example 2), yield 58%, selectivity 81%, activity decreased significantly after two cycles.

[0056] Comparative Example 3 S1, same as step S1 in Example 1; S2, same as step S2 in Example 1; the difference is that crystallization is carried out at 1100 °C for 6 h in an argon atmosphere. S3, the same as step S3 in Example 1, yields a 0.6 BN@C-1100 catalyst.

[0057] Experimental results: The carbon skeleton underwent excessive sintering, resulting in a significant reduction in specific surface area (only 1 / 5 of that of 0.6 BN@C-800), and the active sites were encapsulated. Catalytic performance: Used for the synthesis of benzylmethyl carbonate (reaction conditions as in Example 2), yield 35%, selectivity 78%.

[0058] Comparative Example 2 had an excessively low crystallization temperature, resulting in insufficient graphitization and ineffective formation of acid-base sites. Comparative Example 3 had an excessively high crystallization temperature, leading to carbon framework sintering and encapsulation of active sites, both significantly reducing catalytic efficiency. The crystallization temperature specified in this invention ensures the formation of a mesoporous graphitized framework in the carbon material and stable boron and nitrogen doping, achieving effective graphitization of the carbon material, forming a stable conductive framework, providing channels for electron transfer, and preventing excessive sintering of the carbon framework. This preserves sufficient specific surface area and mesoporous structure, ensuring adequate contact between the substrate and active sites.

[0059] Comparative Example 4 Same as Example 2, except that the oil bath temperature is adjusted to 50°C, while all other parameters remain unchanged; Catalytic performance: The activation energy of the reaction is insufficient, the transesterification reaction rate is extremely slow, the conversion rate is extremely low within 4 hours, the yield is 31%, and the selectivity is 95% (no obvious side reactions, but the reaction is not fully carried out).

[0060] The reaction temperature specified in this invention can meet the activation energy requirements of transesterification reaction, while avoiding side reactions such as DMC decomposition and alcohol oxidation caused by high temperature.

[0061] Comparative Example 5 Phosphoric acid-free modification (blank control) S1, Same as step S1 in Example 1, except that phosphoric acid was not added during the preparation process, and the other parameters remained unchanged; S2, same as step S2 in Example 1; S3. Same as step S3 in Example 1, to obtain 0.6 BN@C-800-NP (NP stands for No Phosphoric acid) catalyst.

[0062] Experimental results: The prepolymer has poor formability, and the carbon material structure is loose after calcination, with uneven boron and nitrogen doping. Catalytic performance: Used for the synthesis of benzylmethyl carbonate (reaction conditions as in Example 2), yield 51%, selectivity 85%.

[0063] The optimization mechanism of catalyst structure by phosphoric acid modification Phosphoric acid plays a dual role as a pore-forming agent and a dispersant in catalyst preparation: on the one hand, it forms an aluminum phosphate complex with aluminum nitrate, which decomposes after calcination to form a mesoporous structure, thereby increasing the specific surface area of ​​the catalyst; on the other hand, it disperses carbon, boron and nitrogen sources, preventing raw material agglomeration and ensuring uniform boron and nitrogen doping.

[0064] Comparative Example 5, without the addition of phosphoric acid, had a loose prepolymer structure, uneven boron and nitrogen doping, a significantly reduced number of effective active sites, and a marked decrease in catalytic performance.

[0065] The proposed solution constructs a metal-free boron-nitrogen co-doped carbon catalytic system with stable structure and uniform active sites, achieving efficient synergistic effects of Lewis acid-base sites. The system fundamentally solves the problems of metal residue, excessively high substrate ratio, poor stability, and narrow applicability of traditional catalytic systems. The comparative experimental results further verify the necessity and optimization of the parameter limits of the present invention, demonstrating the innovation of the technical solution and its industrial application value.

Claims

1. A boron and nitrogen co-doped carbon catalyst, characterized in that, The boron source of the boron and nitrogen co-doped carbon catalyst is boric acid; the nitrogen source of the boron and nitrogen co-doped carbon catalyst is at least one of urea, melamine, chitosan, gelatin and dicyandiamide; the molar ratio of boric acid to nitrogen source is 0.2-1.

1. The carbon source of the boron and nitrogen co-doped carbon catalyst includes at least one of glucose, fructose, sucrose, cellulose, and citric acid.

2. A method for preparing the boron and nitrogen co-doped carbon catalyst according to claim 1, characterized in that, Includes the following steps: S1. Dissolve carbon source and aluminum nitrate nonahydrate in water, stir until completely dissolved, add phosphoric acid, continue stirring, then add boric acid and nitrogen source and stir. Heat in a water bath to obtain a viscous prepolymer. S2. The prepolymer is calcined in air and then crystallized in argon atmosphere to obtain the crystallized product. S3. Add the crystallized product to a nitric acid solution, stir, filter, wash until neutral, and vacuum dry to obtain a boron and nitrogen co-doped carbon catalyst.

3. The method for preparing the boron and nitrogen co-doped carbon catalyst according to claim 2, characterized in that, In step S1, the mass ratio of carbon source to aluminum nitrate nonahydrate is 0.9-1.1:1; the volume ratio of water to the total mass of carbon source and aluminum nitrate nonahydrate is 4-8 mL:1 g; the mass ratio of phosphoric acid to aluminum nitrate nonahydrate is 0.5-1:1; and the stirring time is 30-50 min.

4. The method for preparing the boron and nitrogen co-doped carbon catalyst according to claim 2, characterized in that, In step S1, the mass ratio of boric acid to the total mass of carbon source and aluminum nitrate nonahydrate is 0.05–0.3:1; the mass ratio of nitrogen source to the total mass of carbon source and aluminum nitrate nonahydrate is 0.5–2.0:1; the stirring time for adding boric acid and nitrogen source is 1–3 h; and the water bath heating temperature is 80–90 °C.

5. The method for preparing the boron and nitrogen co-doped carbon catalyst according to claim 2, characterized in that, The calcination temperature in step S2 is 250-350℃, and the calcination time is 6-12 min; the crystallization temperature is 600-1000℃, and the crystallization time is 5-8 h.

6. The method for preparing the boron and nitrogen co-doped carbon catalyst according to claim 2, characterized in that, The concentration of the nitric acid solution in step S3 is 4 M; the ratio of the amount of the crystallized product to the amount of nitric acid solution is 1 g: 10-20 ml.

7. The method for preparing the boron and nitrogen co-doped carbon catalyst according to claim 2, characterized in that, The stirring temperature in step S3 is 40-60℃, and the stirring time is 4-6 h; the vacuum drying temperature is 80-100℃, and the vacuum drying time is 6-10 h.

8. A method for synthesizing asymmetric organic carbonates catalyzed by a boron and nitrogen co-doped carbon catalyst as described in claim 1 or 2, characterized in that, The process includes: mixing dimethyl carbonate (DMC), alcohol compounds and boron and nitrogen co-doped carbon catalyst, reacting them in an air atmosphere, cooling to room temperature after the reaction is completed, filtering to separate the boron and nitrogen co-doped carbon catalyst, and further purifying the crude product by column chromatography to obtain an asymmetric organic carbonate.

9. The method for catalytic synthesis of asymmetric organic carbonates according to claim 8, characterized in that, The alcohols include alkyl alcohols, cycloalkyl alcohols, aryl alcohols, and heterocyclic alcohols; the alkyl alcohols include n-butanol or isoamyl alcohol; the cycloalkyl alcohols include cyclohexylmethanol; the aryl alcohols include benzyl alcohol, 4-chlorobenzyl alcohol, phenethyl alcohol, or DL-1-phenethyl alcohol; and the heterocyclic alcohols include tetrahydrofurfuryl alcohol.

10. The method for catalytic synthesis of asymmetric organic carbonates according to claim 8, characterized in that, The molar ratio of dimethyl carbonate (DMC) to the alcohol compound is 1-8:1; the ratio of boron and nitrogen co-doped carbon catalyst to the alcohol compound is 20-50:1 mg / mmol; the reaction temperature is 60-100℃, and the reaction time is 4-7 h.

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