Preparation method and application of atomic-scale dispersed cobalt-doped three-dimensional hollow sphere carbon nitride material

Abstract

The invention relates to a preparation method and application of an atomic-scale dispersed cobalt-doped three-dimensional hollow sphere carbon nitride material. According to the method, a unique three-dimensional hollow sphere carrier structure is constructed through supramolecular self-assembly and subsequent pyrolysis of melamine and cyanuric acid, atomic-scale dispersion of cobalt species is successfully realized by adopting a low-concentration cobalt salt solution, and a stable Co-Nx coordination center is formed. According to the catalyst, the high specific surface area and rich alkaline sites of the carrier are fully utilized, efficient adsorption of CO2 is achieved, and in-situ adsorption activation and efficient catalytic conversion of CO2 are achieved through the synergistic effect of atomic-scale dispersed Co < 2 + > sites serving as Lewis acid sites and the alkaline sites of the carrier. The catalyst obtained by the invention shows high activity and excellent stability in CO2 cycloaddition reaction, and the preparation process is simple.

Description

Technical Field

[0001] This invention belongs to the field of carbon dioxide capture and conversion, specifically relating to a method for preparing an atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material and its application in catalyzing the cycloaddition reaction of CO2 with epoxides. Background Technology

[0002] With the continuous increase in carbon dioxide emissions, the global climate crisis is becoming increasingly severe, urgently requiring the development of efficient CO2 conversion technologies to alleviate environmental pressure. Converting CO2 into high-value-added chemicals (such as cyclic carbonates) has attracted much attention due to its advantages in both resource utilization and emission reduction. Among these, the cycloaddition reaction of CO2 with epoxides is one of the most promising pathways due to its 100% atom utilization and wide range of product applications. In the catalytic system of CO2 cycloaddition reactions, homogeneous catalysts (such as ammonium salts and metal halides) have high activity but are difficult to recover. Existing heterogeneous catalysts generally suffer from low active site density and insufficient stability. Graphite-phase carbon nitride (g-C3N4) is suitable because of its simple preparation, high stability, and the presence of -NH4+ in its framework. x The functional groups can provide sites for CO2 adsorption and activation, and are considered ideal heterogeneous catalyst supports. However, g-C3N4 prepared by conventional methods has a low specific surface area (typically <10 m²). 2 The adsorption capacity and mass transfer efficiency of CO2 are severely limited by the fact that it lacks sufficient Lewis acid sites to efficiently activate epoxides.

[0003] To address the aforementioned problems, various improvement solutions have emerged in the existing technology. For example, Chinese patent CN112156799B discloses a composite catalyst of multiple metal oxides and carbon nitride. The preparation of this material involves multi-step solution mixing and high-temperature hydrothermal treatment (120-180 ℃), which is cumbersome. Furthermore, the calcination temperature of the metal oxides is high (400~700 ℃), and the CO2 pressure required for the reaction is as high as 15.0 MPa, placing stringent requirements on equipment and energy consumption.

[0004] Further improvements are reflected in the literature (DOI: 10.1016 / j.ces.2023.119206), which constructs a Y / g-C3N4 catalyst by introducing yttrium (Y) onto sheet-like carbon nitride with a larger specific surface area. This catalyst achieves the cycloaddition reaction of CO2 with epoxides through the Lewis acid-base synergistic effect between the metal and the support. However, the catalyst prepared by this technique still faces the problem of insufficient specific surface area, which limits its ability to enrich CO2 and restricts the construction of higher-density active sites and the full realization of the synergistic catalytic mechanism. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for preparing and applying atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride materials. This method constructs a unique three-dimensional hollow spherical carrier structure through supramolecular self-assembly of melamine and cyanuric acid followed by pyrolysis. Atomically dispersed cobalt species are successfully achieved using low-concentration cobalt salt solutions (0.001 mol / L to 0.010 mol / L), forming stable Co-N. x Coordination centers. This catalyst fully utilizes the high specific surface area and abundant basic sites of the support to achieve efficient CO2 adsorption, and through atomically dispersed Co... 2+ The site, acting as a Lewis acid site, synergistically interacts with the basic site of the support to achieve in-situ adsorption and activation of CO2 and efficient catalytic conversion. This catalyst exhibits high activity and excellent stability in the CO2 cycloaddition reaction, and its preparation process is simple.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing an atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material, the method comprising the following steps: (1) Preparation of three-dimensional hollow sphere carbon nitride support: Melamine solution and cyanuric acid solution were mixed at room temperature and stirred to form a white precipitate. After filtration, washing and drying, a white precursor powder was obtained. The precursor powder was calcined at 450~600℃ for 2~6 hours under an inert atmosphere to obtain a three-dimensional hollow sphere carbon nitride support, denoted as 3D g-C3N4. The molar ratio of melamine to cyanuric acid is 0.5~1.5:1; The solvent for both melamine and cyanuric acid solutions is dimethyl sulfoxide.

[0007] (2) Preparation of atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material: Cobalt salt was dissolved in anhydrous ethanol to prepare a cobalt salt solution; the three-dimensional hollow spherical carbon nitride support obtained in step (1) was added to the cobalt salt solution and stirred at room temperature for 6 to 12 hours; then the solvent was removed by vacuum distillation and the resulting solid was dried to obtain the material precursor; the precursor was heated to a calcination temperature of 300 to 600 °C under an inert atmosphere and held for 2 to 6 hours, washed and dried, and finally the atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material was obtained, denoted as Co-3D g-C3N4; In this process, 0.1 to 2 g of three-dimensional hollow carbon nitride support is added to every 100 mL of cobalt salt solution; the concentration of the cobalt salt solution is 0.001 mol / L to 0.010 mol / L.

[0008] In step (1), the heating rate is 1~10℃ / min.

[0009] In step (2), the heating rate is 1~10℃ / min.

[0010] In step (2), the cobalt salt is one of cobalt nitrate, cobalt carbonate, cobalt sulfate, cobalt chloride, or cobalt acetate.

[0011] The inert atmosphere is nitrogen or argon.

[0012] The room temperature mentioned refers to 30℃ ± 5℃.

[0013] The atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material prepared by the method is used to catalyze the cycloaddition of CO2 with epoxides to prepare cyclic carbonates.

[0014] The application includes the following steps: loading the catalyst, tetrabutylammonium bromide, N,N-dimethylformamide and epoxide into a high-pressure reactor, purging CO2 to a reaction pressure of 0.1~5 MPa, and reacting at 60~150℃ for 1~6 hours to obtain the target product cyclic carbonate; wherein, the catalyst is an atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material prepared by the aforementioned method; For every 20-25 mmol of epoxide, 50-200 mg of catalyst, 0.01-0.50 mmol of tetrabutylammonium bromide, and 1-10 mL of N,N-dimethylformamide are added.

[0015] The epoxides are ethylene oxide, propylene oxide, butane oxide, epichlorohydrin, and styrene oxide.

[0016] The essential features of this invention are: In existing g-C3N4-based heterogeneous catalytic systems, metal species supported by conventional impregnation methods tend to agglomerate during heat treatment, forming metal or metal oxide nanoparticles. This results in a limited number of exposed active sites, low atomic utilization, and failure to maximize the synergistic effect between Lewis acidic sites and the support basic sites. This invention utilizes cyanuric acid-melamine supramolecular self-assembly combined with impregnation heat treatment to construct cobalt-supported three-dimensional hollow spherical carbon nitride materials. By leveraging the high specific surface area and abundant nitrogen coordination sites of the three-dimensional hollow spherical carbon nitride support as spatial anchoring points, cobalt species are successfully highly dispersed at the atomic level, forming stable Co-N... x Coordination structure. Its atomically dispersed Co 2+ The site, with its moderate Lewis acidity, efficiently activates epoxides and lowers the ring-opening energy barrier, inherently... The basic sites are responsible for adsorbing and activating CO2, forming a highly efficient synergistic mechanism to achieve the efficient conversion of CO2 and epoxides to prepare cyclic carbonates. This invention achieves atomic-level dispersion of metal sites through support structure design. The unique morphology and pore structure of the catalyst, as well as the acid-base bifunctional sites, significantly improve the adsorption and catalytic conversion performance of g-C3N4-based materials for CO2, and the material exhibits excellent cycle stability. Compared with existing technologies, the principles and beneficial effects of this invention are as follows: 1. This invention utilizes a cyanuric acid-melamine supramolecular self-assembly strategy to successfully construct g-C3N4 (3D g-C3N4) with a three-dimensional hollow sphere structure. N2 physical adsorption-desorption characterization demonstrates that this unique structure endows the material with a high molecular weight (79.7 μm). 2 The BET specific surface area per g is much higher than that of traditional bulk g-C3N4 (typically <10 m²). 2 It has a high density of 100 g / g and abundant hierarchical channels. Its unique structure not only greatly increases the exposure of active sites, but also provides ideal spatial confinement and nitrogen coordination anchoring sites for subsequent high dispersion of metal atoms, which is the structural basis for achieving efficient catalysis.

[0017] 2. The unique hierarchical pore structure and high specific surface area of ​​3D g-C3N4 provide a platform for the physical adsorption and rapid mass transfer of CO2 molecules. Its inherent -NH group in its framework... x The group, acting as a Lewis basic site, exhibits excellent chemisorption performance for CO2 molecules. Physicosorption tests of CO2 show that the synergistic effect of its unique physical structure and surface chemical properties results in an adsorption capacity of 8.74 cm⁻¹ at 273.15 K. 3 / g, Example 24) is much higher than that of traditional bulk g-C3N4 (1.34 cm). 3 / g). This highly efficient CO2 adsorption and enrichment capability can establish a locally high-concentration CO2 reaction microenvironment around the active center of the catalyst, significantly increasing the probability of CO2 molecules contacting and reacting with the activated epoxide, thus laying a key foundation for comprehensively improving the cycloaddition reaction rate.

[0018] 3. Through an optimized impregnation-heat treatment process, this invention successfully achieved atomic-level dispersion of cobalt species on a three-dimensional hollow spherical carrier, forming a stable Co-N mixture. x The coordination structure successfully constructed high-density, highly stable Lewis acid sites. This atomically dispersed Co... 2+ The site and the inherent -NH of the vector x The synergistic effect of basic sites efficiently activates epoxides and CO2 molecules, thereby significantly improving the efficiency of cycloaddition reactions.

[0019] 4. The atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material Co-3D g-C3N4-0.6 prepared in this invention exhibits catalytic performance in the production of ethylene carbonate from CO2 and ethylene oxide. Under the conditions of 140 °C and 2 MPa CO2 for 3 hours, the yield of ethylene carbonate reaches 94.7%, which is significantly better than that of uncobalt-supported 3D g-C3N4 (Comparative Example 2) and cobalt-supported sheet-like carbon nitride Co / g-C3N4 (Comparative Example 3).

[0020] 5. The material of this invention can efficiently catalyze the cycloaddition reaction between CO2 and ethylene oxide under the conditions of (reaction pressure 0.5~3MPa, reaction temperature 60~50 ℃). It maintains high activity even after 10 CO2 adsorption catalytic cycles (Example 23), and its catalytic performance can be restored through simple regeneration, exhibiting excellent stability. Compared with existing catalytic systems, it has better application prospects.

[0021] 6. The preparation method of this invention is simple and operates under mild conditions, significantly reducing production energy consumption and equipment requirements. Compared with traditional metal oxide composite catalysts that require a high-temperature, high-pressure hydrothermal process (typically requiring dynamic aging at 120~180 ℃ for 6~24 hours), this invention only requires impregnation and stirring at room temperature during the metal loading stage, eliminating the need for complex co-precipitation control using multiple solutions. The entire process is simple and easy to operate, providing favorable conditions for large-scale production. Attached Figure Description

[0022] Figure 1 X-ray diffraction (XRD) spectra of 3D g-C3N4 and Co-3D g-C3N4-0.2, Co-3D g-C3N4-0.4, and Co-3D g-C3N4-0.6 catalysts.

[0023] Figure 2 The image shown is a high-angle annular dark-field-scanning transmission electron microscope (HAADF-STEM) image of the Co-3D g-C3N4-0.6 catalyst prepared in Example 1; wherein, Figure 2 (a) is a high-magnification HADDF-STEM image of the Co-3D g-C3N4-0.6 catalyst. Figure 2 (b) is a low-magnification panoramic image of the Co-3D g-C3N4-0.6 catalyst using HADDF-STEM.

[0024] Figure 3 The Fourier transform infrared (FT-IR) spectra of the Co-3D g-C3N4-0.6 catalyst prepared in Example 1 before and after use are shown. Detailed Implementation

[0025] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. It should be noted that the following embodiments are illustrative and not limiting, and should not be construed as limiting the scope of protection of the present invention. All raw materials used in the following embodiments and comparative examples are commercially available.

[0026] In this embodiment, the XRD analysis of the atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material Co-3D g-C3N4 was performed using a Rigaku D / MAX-2550 X-ray diffractometer.

[0027] In this embodiment, the HAADF-STEM morphology analysis of the atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material Co-3D g-C3N4 was performed using a JEOL JEM-F200 transmission electron microscope.

[0028] In this embodiment, the FT-IR analysis of the atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material Co-3D g-C3N4 was performed using a Nicolet iS10 infrared spectrometer from Thermo Fisher Scientific, USA.

[0029] Example 1: The preparation steps of an atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material, Co-3D g-C3N4-0.6, are as follows: (1) Preparation of three-dimensional hollow spherical carbon nitride support (3D g-C3N4): 19.82 mmol of melamine was dissolved in 100 mL of dimethyl sulfoxide (DMSO), and an equimolar amount of cyanuric acid was dissolved in 50 mL of DMSO. The solutions were ultrasonically treated for 30 min to ensure complete dissolution. The melamine solution was added dropwise to the cyanuric acid solution at 25 °C to form a white precipitate, and the mixture was stirred for 1 h. The solution was then filtered and washed three times each with deionized water and anhydrous ethanol. The washed sample was placed in a vacuum drying oven and dried overnight at 60 °C to obtain a white powder precursor. The ground precursor was heated to 550 °C at a rate of 3 °C / min in a high-purity nitrogen atmosphere (flow rate 30 mL / min) and held for 4 h to obtain a brownish-yellow three-dimensional hollow spherical carbon nitride support, denoted as 3D g-C3N4.

[0030] (2) Preparation of atomically dispersed cobalt-doped catalyst Co-3D g-C3N4-0.6: 0.6 mmol of Co(NO3)2·6H2O was dissolved in 100 mL of anhydrous ethanol to prepare a cobalt salt solution with a concentration of 0.006 mol / L. 1.0 g of the 3D g-C3N4 support prepared in step (1) was slowly added to the solution and stirred at 25 °C for 10 h to achieve full impregnation. The solvent was then removed by rotary evaporation, and the sample was placed in a vacuum drying oven and dried at 60 °C overnight to obtain the catalyst precursor. The precursor was transferred to a tube furnace and heated to 400 °C at a heating rate of 2 °C / min under the protection of high-purity nitrogen at 30 mL / min, and held for 3 h. The final product was washed three times by centrifugation with anhydrous ethanol and dried in a vacuum environment at 60 °C for 12 h to obtain the target catalyst Co-3D g-C3N4-0.6.

[0031] The performance evaluation method for Co-3D g-C3N4-0.6 is as follows: 200 mg of Co-3D g-C3N4-0.6 catalyst was weighed and placed in a 50 mL high-pressure reactor. Then, 0.075 mmol of tetrabutyl bromide, 6 mL of N,N-dimethylformamide (DMF), and 2 mL (22.84 mmol) of ethylene oxide were added. The reactor was sealed, and the air inside was replaced three times with CO2, followed by CO2 purging to 2 MPa. Heating and magnetic stirring were started. Timing began when the reactor temperature reached 140 °C, and the reaction was stopped after 3 h. The reactor was then cooled in an ice-water bath. After slowly releasing unreacted CO2, the catalyst was separated by filtration, and the liquid product was collected. Quantitative analysis was performed using gas chromatography with biphenyl as an internal standard, and the yield of ethylene carbonate reached 94.7%.

[0032] Example 2: The preparation steps of Co-3D g-C3N4-0.2 are the same as in Example 1, except that the mass of Co(NO3)2·6H2O used in step (2) of Example 1 is changed from 0.6 mmol to 0.2 mmol. The rest of the steps are the same as in Example 1. The prepared sample is recorded as Co-3D g-C3N4-0.2.

[0033] The performance evaluation method for Co-3D g-C3N4-0.2 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.2 sample. The remaining steps are the same as in Example 1. The yield of ethylene carbonate is 67.2%.

[0034] Example 3: The preparation steps of Co-3D g-C3N4-0.4 are the same as in Example 1, except that the mass of Co(NO3)2·6H2O used in step (2) of Example 1 is changed from 0.6 mmol to 0.4 mmol. The rest of the steps are the same as in Example 1. The prepared sample is recorded as Co-3D g-C3N4-0.4.

[0035] The performance evaluation method for Co-3D g-C3N4-0.4 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.4 sample. All other steps are the same as in Example 1. The yield of ethylene carbonate is 74.6%.

[0036] Example 4: The preparation steps of Co-3D g-C3N4-0.6-1 are the same as in Example 1, except that the molar ratio of melamine to cyanuric acid in step (1) of Example 1 is changed from 1:1 to 1.5:1. The rest of the steps are the same as in Example 1. The prepared sample is recorded as Co-3D g-C3N4-0.6-1.

[0037] The performance evaluation method for Co-3D g-C3N4-0.6-1 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.6-1 sample. The remaining steps are the same as in Example 1. The yield of ethylene carbonate is 88.9%.

[0038] Example 5: The preparation steps of Co-3D g-C3N4-0.6-2 are the same as in Example 1, except that the heating rate in step (1) of Example 1 is changed from 3 ℃ / min to 1.0 ℃ / min. The rest of the steps are the same as in Example 1. The prepared sample is denoted as Co-3D g-C3N4-0.6-2.

[0039] The performance evaluation method for Co-3D g-C3N4-0.6-2 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.6-2 sample. All other steps are the same as in Example 1. The yield of ethylene carbonate is 92.1%.

[0040] Example 6: The preparation steps of Co-3D g-C3N4-0.6-3 are the same as in Example 1, except that the heat treatment temperature in step (1) of Example 1 is changed from 550 ℃ to 450 ℃. The rest of the steps are the same as in Example 1. The prepared sample is denoted as Co-3D g-C3N4-0.6-3.

[0041] The performance evaluation method for Co-3D g-C3N4-0.6-3 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.6-3 sample. The remaining steps are the same as in Example 1. The yield of ethylene carbonate is 88.5%.

[0042] Example 7: The preparation steps of Co-3D g-C3N4-0.6-4 are the same as in Example 1, except that the thermal polymerization time in step (1) of Example 1 is changed from 4 h to 2 h. The rest of the steps are the same as in Example 1. The prepared sample is denoted as Co-3D g-C3N4-0.6-4.

[0043] The performance evaluation method for Co-3D g-C3N4-0.6-4 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.6-4 sample. The remaining steps are the same as in Example 1. The yield of ethylene carbonate is 84.8%.

[0044] Example 8: The preparation steps of Co-3D g-C3N4-0.6-5 are the same as in Example 1, except that the cobalt salt used in step (1) of Example 1 is changed from Co(NO3)2·6H2O to Co(CH3COO)2·4H2O. The rest of the steps are the same as in Example 1. The prepared sample is recorded as Co-3D g-C3N4-0.6-5.

[0045] The performance evaluation method for Co-3D g-C3N4-0.6-5 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.6-5 sample. The remaining steps are the same as in Example 1. The yield of ethylene carbonate is 85.3%.

[0046] Example 9: The preparation steps of Co-3D g-C3N4-0.6-6 are the same as in Example 1, except that the soaking and stirring time in step (2) of Example 1 is changed from 10 h to 6 h. The rest of the steps are the same as in Example 1. The prepared sample is recorded as Co-3D g-C3N4-0.6-6.

[0047] The performance evaluation method for Co-3D g-C3N4-0.6-6 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.6-6 sample. The remaining steps are the same as in Example 1. The yield of ethylene carbonate is 90.8%.

[0048] Example 10: The preparation steps of Co-3D g-C3N4-0.6-7 are the same as in Example 1, except that the heating rate in step (2) of Example 1 is changed from 2 ℃ / min to 5 ℃ / min. The rest of the steps are the same as in Example 1. The prepared sample is denoted as Co-3D g-C3N4-0.6-7.

[0049] The performance evaluation method for Co-3D g-C3N4-0.6-7 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.6-7 sample. The remaining steps are the same as in Example 1. The yield of ethylene carbonate is 89.6%.

[0050] Example 11: The preparation steps of Co-3D g-C3N4-0.6-8 are the same as in Example 1, except that the heat treatment temperature in step (2) of Example 1 is changed from 400 ℃ to 300 ℃. The rest of the steps are the same as in Example 1. The prepared sample is denoted as Co-3D g-C3N4-0.6-8.

[0051] The performance evaluation method for Co-3D g-C3N4-0.6-8 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.6-8 sample. All other steps are the same as in Example 1. The yield of ethylene carbonate is 89.2%.

[0052] Example 12: The preparation steps of Co-3D g-C3N4-0.6-9 are the same as in Example 1, except that the heat treatment time in step (2) of Example 1 is changed from 3 h to 2 h. The rest of the steps are the same as in Example 1. The prepared sample is denoted as Co-3D g-C3N4-0.6-9.

[0053] The performance evaluation method for Co-3D g-C3N4-0.6-9 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 is replaced with the Co-3D g-C3N4-0.6-9 sample. The remaining steps are the same as in Example 1. The yield of ethylene carbonate is 90.3%.

[0054] Example 13: The performance evaluation method for Co-3D g-C3N4-0.6 is the same as in Example 1, except that the reaction temperature in Example 1 was changed from 140 °C to 110 °C. The yield of ethylene carbonate was 66.0%.

[0055] Example 14: The performance evaluation method for Co-3D g-C3N4-0.6 is the same as in Example 1, except that the reaction temperature in Example 1 was changed from 140 °C to 120 °C. The yield of ethylene carbonate was 80.1%.

[0056] Example 15: The performance evaluation method for Co-3D g-C3N4-0.6 is the same as in Example 1, except that the reaction temperature in Example 1 was changed from 140 °C to 130 °C. The yield of ethylene carbonate was 88.2%.

[0057] Example 16: The performance evaluation method for Co-3D g-C3N4-0.6 is the same as in Example 1, except that the reaction temperature in Example 1 was changed from 140 °C to 150 °C. The yield of ethylene carbonate was 93.7%.

[0058] Example 17: The performance evaluation method for Co-3D g-C3N4-0.6 is the same as in Example 1, except that the reaction pressure in Example 1 was changed from 2 MPa to 1 MPa, while the other steps were the same. The yield of ethylene carbonate was 75.2%.

[0059] Example 18: The performance evaluation method for Co-3D g-C3N4-0.6 is the same as in Example 1, except that the reaction pressure in Example 1 was changed from 2 MPa to 3 MPa, and the rest of the steps were the same as in Example 1. The yield of ethylene carbonate was 93.0%.

[0060] Example 19: The performance evaluation method for Co-3D g-C3N4-0.6 is the same as in Example 1, except that the reaction pressure in Example 1 was changed from 2 MPa to 5 MPa, while the other steps were the same. The yield of ethylene carbonate was 85.9%.

[0061] Example 20: The performance evaluation method for Co-3D g-C3N4-0.6 is the same as in Example 1, except that the reaction time in Example 1 was changed from 3 h to 0.5 h, while the other steps were the same. The yield of ethylene carbonate was 59.9%.

[0062] Example 21: The performance evaluation method for Co-3D g-C3N4-0.6 is the same as in Example 1, except that the reaction time in Example 1 was changed from 3 h to 1 h, while the other steps were the same. The yield of ethylene carbonate was 73.4%.

[0063] Example 22: The performance evaluation method for Co-3D g-C3N4-0.6 is the same as in Example 1, except that the reaction time in Example 1 was changed from 3 h to 4 h, while the other steps were the same. The yield of ethylene carbonate was 95.0%.

[0064] Comparative Example 1: Comparative Example 1 is a blank experiment The performance evaluation method for the blank experiment is the same as in Example 1, except that no catalyst is added, and all other steps are the same as in Example 1. The yield of ethylene carbonate is 24.1%.

[0065] Comparative Example 2: The control sample was 3D g-C3N4 The performance evaluation method for 3D g-C3N4 was the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 was replaced with 100 mg of 3D g-C3N4 carrier, and all other steps were the same as in Example 1. The yield of ethylene carbonate was 27.8%.

[0066] Comparative Example 3: The control sample was Co / g-C3N4 The preparation method of cobalt-supported sheet-like carbon nitride Co / g-C3N4 is as follows: (1) Synthesis of sheet-like carbon nitride g-C3N4: According to the method in the literature (DOI: 10.1016 / j.ces.2023.119206). 3 g of melamine and 15 g of ammonium chloride were thoroughly mixed in a mortar. The mixture was heated to 550 °C at a heating rate of 3 °C / min and held for 4 hours. The resulting yellow powder was ground in a mortar to obtain g-C3N4 nanosheets, denoted as g-C3N4.

[0067] (2) Synthesis of cobalt-supported sheet-like carbon nitride Co / g-C3N4: 1 g g-C3N4 was suspended in 100 mL of anhydrous ethanol, and 0.6 mmol Co(NO3)2·6H2O was added. The mixture was stirred vigorously for 1 hour. The solvent was then removed by rotary evaporation at 60 °C to obtain a solid. The solid was heated to 400 °C under a nitrogen stream at a heating rate of 2 °C / min and held for 3 hours to obtain the Co / g-C3N4 catalyst.

[0068] The material was characterized by N2 physical adsorption-desorption, and its molecular weight was found to be 43 μm. 2The BET specific surface area of ​​Co / g is between that of conventional bulk g-C3N4 and three-dimensional hollow spherical carbon nitride carrier 3D g-C3N4. The performance evaluation method for Co / g-C3N4 is the same as in Example 1, except that the Co-3D g-C3N4-0.6 sample in Example 1 was replaced with Co / g-C3N4; all other steps were the same as in Example 1. The yield of ethylene carbonate was 68.2%.

[0069] Example 23: Catalytic cycling tests were conducted on the Co-3D g-C3N4-0.6 catalyst with the best performance. The catalyst obtained by filtration separation in Example 1 was successively washed thoroughly with methanol, dried under vacuum at 60 °C, and regenerated by calcination at 400 °C under a high-purity nitrogen atmosphere for 3 h, and then used for the next evaluation. The evaluation method was the same as in Example 1, with one evaluation considered as one cycle. After 10 cycles, the yield of ethylene carbonate was 92.4%. The catalyst after 10 cycles was designated as Co-3D g-C3N4-0.6-10. After 10 repeated uses, the Co-3D g-C3N4-0.6-10 catalyst still maintained its high activity, exhibiting excellent catalytic cycling stability.

[0070] Example 24: To directly investigate the adsorption and enrichment capacity of the catalyst of this invention for the reaction substrate CO2, CO2 physical adsorption tests were conducted on the catalysts 3D g-C3N4, Co-3D g-C3N4-0.6, Co-3D g-C3N4-0.6-1, and Co-3D g-C3N4-0.6-10. After pretreatment by vacuum degassing at 200 °C for 4 h, the tests were performed at 273.15 K.

[0071] Test results show that the CO2 adsorption capacity of 3D g-C3N4 is 8.74 cm⁻¹ at 273.15 K and 1 bar. 3 / g; After cobalt loading, the CO2 adsorption capacity of the Co-3D g-C3N4-0.6 catalyst is 8.16 cm⁻¹. 3 / g. Both are significantly higher than the measured value of bulk C3N4 prepared by traditional thermal polymerization under the same conditions (1.34 cm). 3 / g). When the molar ratio of melamine to cyanuric acid was increased from 1:1 to 1.5:1, the CO2 adsorption capacity of the prepared Co-3D g-C3N4-0.6-1 catalyst increased to 9.01 cm⁻¹. 3 / g. This result indicates that increasing the proportion of melamine in the precursor helps to enrich more -NH on the 3D g-C3N4 backbone. xThe catalyst possesses alkaline sites, thereby enhancing its chemisorption capacity for CO2. After 10 catalytic cycles, the CO2 adsorption capacity of the catalyst Co-3D g-C3N4-0.6-10 is 8.10 cm⁻¹. 3 The CO2 adsorption capacity is basically consistent with that of fresh catalyst. The CO2 adsorption data directly proves that the three-dimensional hollow sphere structure constructed in this invention significantly enhances the physical adsorption and enrichment capacity of CO2 gas due to its high specific surface area and well-developed porosity. This provides favorable conditions for the high concentration distribution and rapid diffusion of CO2 molecules around the active sites in the reaction system, and is a key physical structural factor for enhancing catalytic performance.

[0072] Example 25: The crystal structures of 3D g-C3N4 and Co-3D g-C3N4C-m catalysts were characterized by X-ray diffraction, such as... Figure 1 As shown. The XRD pattern of 3D g-C3N4 exhibits two characteristic diffraction peaks near 13.1 ° and 27.1 °, which are attributed to the (100) and (002) crystal planes of g-C3N4, respectively. The (100) peak corresponds to the in-plane periodic arrangement of the tri-s-triazine rings, while the (002) peak corresponds to the aromatic interlayer π-π stacking structure. Compared with 3D g-C3N4, the intensity of the two characteristic diffraction peaks of Co-3D g-C3N4-m decreases with increasing Co loading, but no diffraction peaks of any cobalt species (such as Co, CoO, Co3O4, etc.) are observed, indicating that the cobalt species do not form a crystalline phase but exist in a highly dispersed form.

[0073] Example 26: The microstructure and elemental distribution of the catalyst were characterized using transmission electron microscopy. For example... Figure 2 (a) A high-magnification HADDF-STEM image of the Co-3Dg-C3N4-0.6 catalyst shows numerous bright, isolated dots with significantly higher contrast than the surrounding C, N, and O elements, corresponding to cobalt atoms with higher atomic numbers, confirming that cobalt exists in an atomically dispersed form. Figure 2 (b) The HADDF-STEM low-magnification panoramic image of the Co-3D g-C3N4-0.6 catalyst shows that Co-3D g-C3N4-0.6 is a three-dimensional spherical structure assembled from ordered stacked nanosheets with a diameter of 1-3 μm. Its interior is a hollow structure, and the spherical wall is assembled from nanosheets with abundant surface pores, with a wall thickness of about 500 nm.

[0074] Example 27: To investigate its structural stability, FT-IR was used to compare and analyze the chemical structures of the fresh catalyst and the catalyst after five cycles. Figure 3As shown, the spectra of the catalyst before and after the reaction did not show significant changes in the position and intensity of the key characteristic peaks. All samples exhibited typical graphitic carbon nitride structural features, with the 814 cm⁻¹ peak being the most prominent. -1 The sharp peak at 1200-1650 cm is due to the respiratory vibration of the heptamethrin ring. -1 The range exhibits typical stretching vibrations of the aromatic ring, 3000-3400 cm. -1 The absorption peaks within the range are attributed to the stretching vibrations of NH and OH bonds. These results demonstrate that the basic chemical framework structure of the Co-3D g-C3N4-0.6 catalyst remains intact after multiple reactions and regeneration processes, fully proving its excellent structural stability and reusability in the CO2-BO cycloaddition reaction.

[0075] The present invention has been described above by way of example. It should be noted that any simple modifications, alterations or other equivalent substitutions that can be made by those skilled in the art without creative effort without departing from the core of the present invention fall within the protection scope of the present invention.

[0076] Matters not covered in this invention are common knowledge.

Claims

1. A method for preparing an atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material, characterized in that, The method includes the following steps: (1) Preparation of three-dimensional hollow sphere carbon nitride support: Melamine solution and cyanuric acid solution are mixed at room temperature and stirred to form a white precipitate. After filtration, washing and drying, a white precursor powder is obtained. The precursor powder is calcined at 450~600℃ for 2~6 hours under an inert atmosphere to obtain a three-dimensional hollow sphere carbon nitride support. The molar ratio of melamine to cyanuric acid is 0.5~1.5:1; (2) Preparation of atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material: Cobalt salt was dissolved in anhydrous ethanol to prepare a cobalt salt solution; the three-dimensional hollow spherical carbon nitride support obtained in step (1) was added to the cobalt salt solution and stirred at room temperature for 6 to 12 hours; the solvent was then removed by vacuum distillation and the resulting solid was dried to obtain the material precursor; the precursor was heated to a calcination temperature of 300 to 600 °C under an inert atmosphere and held for 2 to 6 hours, washed and dried to finally obtain the atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material. In this process, 0.1 to 2 g of three-dimensional hollow carbon nitride support is added to every 100 mL of cobalt salt solution; the concentration of the cobalt salt solution is 0.001 mol / L to 0.010 mol / L; and the cobalt salt is one of cobalt nitrate, cobalt carbonate, cobalt sulfate, cobalt chloride, or cobalt acetate.

2. The method for preparing atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material as described in claim 1, characterized in that, In step (1), the solvent for the melamine solution and the cyanuric acid solution is dimethyl sulfoxide.

3. The method for preparing atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material as described in claim 1, characterized in that, In steps (1) and (2), the heating rate is 1~10℃ / min.

4. The method for preparing atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material as described in claim 1, characterized in that, The inert atmosphere is nitrogen or argon.

5. The application of the atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material prepared by the method described in claim 1, characterized in that, It is used to catalyze the cycloaddition of CO2 with epoxides to prepare cyclic carbonates.

6. The application as described in claim 5, characterized in that, The process includes the following steps: a catalyst, tetrabutylammonium bromide, N,N-dimethylformamide, and an epoxide are loaded into a high-pressure reactor, CO2 is introduced to a reaction pressure of 0.1–5 MPa, and the reaction is carried out at 60–150 °C for 1–6 hours to obtain the target product, cyclic carbonate; wherein the catalyst is an atomically dispersed cobalt-doped three-dimensional hollow spherical carbon nitride material prepared by the aforementioned method. For every 20-25 mmol of epoxide, 50-200 mg of catalyst, 0.01-0.50 mmol of tetrabutylammonium bromide, and 1-10 mL of N,N-dimethylformamide are added. The epoxides are ethylene oxide, propylene oxide, butane oxide, epichlorohydrin, and styrene oxide.

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