A photothermal catalyst for CO2 cycloaddition reaction and its preparation method

Abstract

The present invention relates to the technical field of catalysts, and particularly to a photothermal catalyst for CO2 cycloaddition reaction and a preparation method thereof. The photothermal catalyst of the present invention is obtained by dispersing a template in water, adding a Tb source, a B source and a carbon source, stirring, and then calcining and drying. The present invention prepares heteroatom- and rare earth metal-functionalized interconnected hollow carbon spheres by high-temperature pyrolysis. The doping of heteroatoms can adjust the coordination environment of metal centers through the difference in electronegativity, and the electron-deficient heteroatoms and single-atom rare earth metals can serve as Lewis acid sites, while the electron-rich N in the carbon material itself can serve as a Lewis base site. The simultaneous presence of Lewis acid-base sites is beneficial to the adsorption and activation of CO2 and epoxides, and promotes the effective progress of the photothermal catalytic CO2 cycloaddition reaction under normal temperature and pressure conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts, and particularly to a photothermal catalyst for CO2 cycloaddition reaction and a preparation method thereof. Background Art

[0002] With the continuous progress of society and the continuous development of industrial technologies, the concentration of carbon dioxide (CO2) in the atmosphere has risen sharply, bringing a series of environmental problems such as the greenhouse effect, sea-level rise, and ocean acidification. How to deal with the excessive emissions of CO2 has become the focus of global attention. At the same time, as a C1 resource with rich reserves, safety, low cost, and easy availability and renewability, converting CO2 into high-value-added products through chemical conversion technologies is an effective way to solve the above environmental problems.

[0003] In the past half century, people have successively developed various catalysts for catalyzing the reaction of CO2 and epoxides to synthesize cyclic carbonates. According to the phase state of the catalytic reaction, they can be divided into two categories: homogeneous catalysts and heterogeneous catalysts. Among them, homogeneous catalysts include: Schiff base metal complexes, alkali metal salts, ionic liquids, organic bases, etc. Heterogeneous catalysts mainly include: porous organic polymers, metal oxides, modified silica gels or molecular sieves, metal-organic frameworks (MOFs), and ionic liquid-based heterogeneous catalysts, etc. Under conventional thermal catalytic conditions, this reaction has few by-products and high atom utilization rate, meeting the requirements of green chemistry and atom economy development, and has become a potential way to utilize CO2 on a large scale. However, whether it is a homogeneous catalyst or a heterogeneous catalyst, this reaction requires relatively high reaction temperatures (100 - 140 °C) and pressures (1.0 - 4.0 MPa) to have relatively high reaction activity. Therefore, people are eager to develop a technology that does not require external energy consumption, has mild reaction conditions, and can utilize CO2 on a large scale, truly and effectively realizing CO2 emission reduction and carbon cycle, while solving the environmental problems caused by CO2 emissions.

[0004] Considering from the aspects of energy and cost, developing a method to directly drive the reaction of CO2 and epoxides to form cyclic carbonates with sunlight under normal temperature and pressure is a very valuable way to achieve CO2 emission reduction and resource utilization. Summary of the Invention

[0005] Based on the above, the present invention provides a photothermal catalyst for CO2 cycloaddition reaction and a preparation method thereof. The photothermal catalyst can generate heat under light irradiation, thereby realizing the efficient photothermal catalysis of the reaction of CO2 and epoxides to synthesize cyclic carbonates under normal temperature, normal pressure, and without adding an external heat source.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] One of the technical solutions of the present invention is a carbon-based material. The carbon-based material is in the shape of a hollow sphere, the diameter of the hollow sphere is 200 - 400 nm, and the shell thickness is 5 - 30 nm. The elemental composition of the carbon-based material includes B, Tb, N, and C.

[0008] Tb exists in the carbon-based material in the form of single atoms.

[0009] The present invention also attempts to replace the heteroatom element B with P and S, and replace the rare earth metal element Tb with Ce and Er. The results show that only the combination of the heteroatom element being B and the rare earth metal element being Tb has the best effect. The combination of the heteroatom elements being P and S and the rare earth metal elements being Tb, Ce, and Er respectively, or the combination of the heteroatom element being B and the rare earth metal elements being Ce and Er respectively, are all inferior to the catalytic performance of the carbon-based material (photothermal catalyst) prepared by the combination of B and Tb defined in the present invention.

[0010] Another technical solution of the present invention is a preparation method of the above carbon-based material, including the following steps:

[0011] Disperse the template in water, then add the Tb source, B source, and carbon source and stir, and then calcine and dry to obtain the carbon-based material.

[0012] Furthermore, the template is a mesoporous silica template or a polystyrene sphere template. Preferably, the template is a mesoporous silica template.

[0013] The polystyrene sphere template is also applicable to the present invention, but relatively speaking, the mesoporous silica template is the optimal template for the present invention.

[0014] Furthermore, the carbon source is chitosan or glucose.

[0015] Furthermore, the Tb source is terbium nitrate.

[0016] Furthermore, the B source is boric acid.

[0017] Furthermore, the mass ratio of the template to the rare earth metal source, heteroatom source, and carbon source is 2:0.4:1:0.8.

[0018] The main function of water is to dissolve the raw materials. Changing the amount of water has little effect on the catalytic performance of the photothermal catalyst. Therefore, the present invention does not limit the amount of water used.

[0019] The amounts of the template and chitosan determine the thickness of the carbon layer of the carbon-based material (photothermal catalyst), while the rare earth metal source and heteroatom source determine the degree of functionalization. Higher or lower than the above-recorded parameters will result in a decrease in the catalytic performance of the photothermal catalyst.

[0020] Further, the calcination is specifically as follows: heating to 800 °C at a heating rate of 1-5 °C / min and holding for 2 h.

[0021] Changing the heating rate and holding time described above will affect the crystallization degree of the carbon-based material (photothermal catalyst), and thus affect the catalytic performance of the prepared photothermal catalyst.

[0022] Further, after the calcination, the steps of soaking in aqua regia, washing with water, and soaking in HF are also included in sequence.

[0023] The purpose of soaking in aqua regia is to remove excess metal particles, and the purpose of soaking in HF is to remove silicon dioxide in the template.

[0024] The third technical solution of the present invention, the application of the above carbon-based material in the catalytic CO2 cycloaddition reaction, and the CO2 cycloaddition reaction is carried out under normal temperature and normal pressure conditions.

[0025] The fourth technical solution of the present invention, a method for improving the conversion rate and yield in the CO2 cycloaddition reaction, using the above carbon-based material as a photothermal catalyst and tetrabutylammonium bromide or tetrabutylammonium chloride as a co-catalyst. The CO2 cycloaddition reaction is specifically the cycloaddition reaction of CO2 with an epoxide.

[0026] The dosage ratio of the catalyst to the epoxide is 5-25 mg: 71-72 mmol.

[0027] The technical concept of the present invention:

[0028] As a photothermal material with broad-spectrum absorption, carbon-based materials can effectively absorb sunlight and convert it into heat energy. However, due to the lack of active sites and weak interaction with CO2 and epoxides in pure carbon materials, their catalytic activity in the CO2 cycloaddition reaction is poor. To solve this problem, the present invention designs to introduce Lewis acid sites (single-atom metal sites) and Lewis base sites (electron-rich N) into porous carbon materials. The simultaneous presence of Lewis acid-base sites is beneficial to the adsorption and activation of epoxides and CO2, thus accelerating the ring-opening of epoxides and promoting the progress of the CO2 cycloaddition reaction. The doping of heteroatom B with different electronegativities can regulate the electronic structure and coordination environment of the central metal atom, improve the single-atom loading rate and metal anchoring strength. In addition, the special porous structure can increase the specific surface area of the material, increase the contact between the catalyst and the reactants, which is beneficial to the catalytic reaction; change the activity of the metal center and then improve the adsorption and activation ability of CO2, so as to further enhance the catalytic activity of the catalyst. In addition, the hollow structure can make sunlight be reflected and absorbed multiple times inside the catalyst, improving the utilization efficiency of solar energy; the porous carbon material with a large specific surface area increases the exposure of active sites, enhances the adsorption ability of CO2 and epoxides, and promotes the mass transfer of substrates / products, thus promoting the progress of the CO2 cycloaddition reaction with epoxides.

[0029] The present invention discloses the following technical effects:

[0030] The present invention uses mesoporous silica as a template to prepare B- and Tb-functionalized interconnected hollow carbon spheres (B-Tb-NC) by high-temperature pyrolysis. The doping of heteroatom B can regulate the coordination environment of metal center Tb through the difference in electronegativity. The electron-deficient B and single-atom Tb can serve as Lewis acid sites, and the electron-rich N in the carbon material itself can serve as a Lewis base site. The simultaneous presence of Lewis acid-base sites is beneficial to the adsorption and activation of CO2 and epoxides, promoting the progress of the CO2 cycloaddition reaction.

[0031] The photothermal catalyst prepared by the present invention has a photothermal effect and can quickly convert light energy into heat energy under light irradiation. The temperature on the surface of the catalyst can reach equilibrium in about 2 minutes of light irradiation. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0033] Figure 1Schematic flow chart for preparing mesoporous silica templates and preparing catalysts using the mesoporous silica templates in the present invention.

[0034] Figure 2 XRD diffraction patterns (a) of the catalysts prepared in Example 1 and Comparative Examples 1-3, and TEM images (b), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (d), and elemental distribution maps (e-h) of the catalyst prepared in Example 1 at different magnifications.

[0035] Figure 3 UV-Vis diffuse reflectance spectra (a) of the catalysts prepared in Example 1 and Comparative Examples 1-3, and temperature (b) of B-Tb-NC prepared in Example 1 under xenon lamp irradiation for different times at 400 mW / cm 2 xenon lamp irradiation at different times.

[0036] Figure 4 Effects of reaction time (a), light intensity (b), incident light wavelength (c), and catalyst dosage (d) on the photothermal catalytic cycloaddition performance of B-Tb-NC prepared in Example 1.

[0037] Figure 5 Cyclic stability of B-Tb-NC prepared in Example 1.

[0038] Figure 6 Reaction temperature in the catalytic system of B-Tb-NC prepared in Example 1 under different light intensities.

[0039] Figure 7 Surface temperature test of B-Tb-NC prepared in Example 1 under different incident light wavelengths (irradiation time is about 2 min); where a represents the incident light wavelength of 380-780 nm, b represents the incident light wavelength of 780-1200 nm, c represents the incident light wavelength of 380-1200 nm, and d represents the incident light wavelength of 200-380 nm.

[0040] Figure 8 Gas chromatogram of the product during the process of verifying the catalytic performance of the catalyst in the present invention. Detailed implementation manners

[0041] Now, various exemplary implementation manners of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0042] It should be understood that the terms used in this invention are only for describing particular embodiments and are not intended to limit the invention. Additionally, for the numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0043] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although this invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of this invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to those documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0044] Without departing from the scope or spirit of this invention, various improvements and changes can be made to the specific embodiments of the description of this invention, which are obvious to those skilled in the art. Other embodiments obtained from the description of this invention are obvious to those skilled in the art. The description and examples of this invention are merely exemplary.

[0045] Regarding "comprising", "including", "having", "containing", etc. used herein, they are all open-ended terms, meaning including but not limited to.

[0046] The raw materials used in the embodiments of this invention can be obtained from commercial sources without special instructions.

[0047] The "normal temperature" mentioned in this invention represents 15 - 30 °C without special instructions.

[0048] The "normal pressure" mentioned in this invention represents 101 KPa without special instructions.

[0049] In the embodiments of the present invention, the preparation of the mesoporous silica template used refers to the literature [Sun J, Zhang J, Zhang M, et al. Bioinspired hollow semiconductor nanospheres as photosynthetic nanoparticles[J]. Nature Communications, 2012, 3(1): 1139.]. The mesoporous silica template has a hollow sphere structure. The diameter of the hollow part of the hollow sphere structure is 270 nm, the shell thickness is 10 nm, and the overall diameter of the hollow sphere structure is 280 nm. Mesoporous silica templates obtained by other preparation methods or other means are equally applicable to the present invention.

[0050] The process schematic diagrams for preparing the mesoporous silica template and preparing the catalyst using the mesoporous silica template in the present invention are as Figure 1 shown. The red dots in mSiO2 / B-Tb-NC and B-TB-NC in the figure represent Tb atoms.

[0051] Example 1

[0052] Weigh 2 g of the mesoporous silica template, disperse it in 100 mL of deionized water, stir for 1 h, and then sequentially add 0.4 g of terbium nitrate hexahydrate, 0.8 g of chitosan, and 1 g of boric acid to the above mixture. Stir for 12 h, then calcine at 800 °C for 2 h under an argon atmosphere with a heating rate of 2 °C / min. After calcination, soak it with aqua regia to remove the excess metal particles, then centrifuge and wash it 4 times with deionized water, and then soak it with 6M HF to remove the excess silica. Finally, dry it overnight at 80 °C to obtain the catalyst (labeled as B-Tb-NC). The synthesis method is as Figure 1 shown.

[0053] Comparative Example 1

[0054] The difference from Example 1 is only that the addition of 0.4 g of terbium nitrate hexahydrate is omitted, and the obtained catalyst is labeled as B-C-N.

[0055] Comparative Example 2

[0056] The difference from Example 1 is only that the addition of 1 g of boric acid is omitted, and the obtained catalyst is labeled as Tb-N-C.

[0057] Comparative Example 3

[0058] The difference from Example 1 is only that the addition of 0.4 g of terbium nitrate hexahydrate and 1 g of boric acid is omitted, and the obtained catalyst is labeled as C-N.

[0059] Figure 2XRD diffraction patterns (a) of the catalysts prepared in Example 1 and Comparative Examples 1-3, and TEM images (b), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (d), and elemental distribution maps (e-h) of the catalyst prepared in Example 1 at different magnifications. In the XRD diffraction patterns ( Figure 2 in a), only two broad diffraction peaks were observed for C-N, B-Tb-NC, B-C-N, and Tb-C-N, which were attributed to the diffraction peaks of the (002) crystal plane and (101) crystal plane of graphite carbon, respectively, indicating that all four materials had an amorphous graphite carbon structure. In the XRD diffraction patterns of Tb-C-N and B-Tb-NC, no characteristic peaks related to Tb species appeared. Transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images both showed ( Figure 2 in b-d) that B-Tb-NC was interconnected hollow spheres, with the diameter of the hollow spheres being approximately 280 nm and the thickness of the carbon shell layer being approximately 10 nm. High-angle annular dark-field scanning transmission electron microscopy (HADDF-STEM) images and their corresponding EDS mapping diagrams ( Figure 2 in e-h) showed that C, N, and B in the B-Tb-NC nanomaterial were mainly distributed around the shell layer, while Tb was evenly distributed in the material. No presence of Tb species was found in the high-resolution (HRTEM) pattern of B-Tb-NC either, which was consistent with the XRD diffraction pattern results. This indicated that Tb in the B-Tb-NC material might exist in atomic form.

[0060] Figure 3 UV-Vis diffuse reflectance spectra (a) of the catalysts prepared in Example 1 and Comparative Examples 1-3, and the temperature (b) of B-Tb-NC prepared in Example 1 under xenon lamp irradiation at 400 mW / cm 2 for different times.

[0061] Figure 3 In a, the light absorption ability of the catalysts prepared in Example 1 and Comparative Examples 1-3 in the solar spectrum range was tested by UV-Vis DRS. The results showed that C-N, B-C-N, Tb-C-N, and B-Tb-NC all exhibited excellent light absorption ability in the solar spectrum region of 350 - 1400 nm (i.e., visible light and near-infrared light), indicating that all four catalysts had excellent solar light utilization efficiency in the spectral range of 350 - 1400 nm. Subsequently, an infrared thermometer was also used to monitor the temperature of solid B-Tb-NC under xenon lamp irradiation at 400 mW / cm 2 As shown in Figure 3 b, under xenon lamp (400 mW / cm 2)After 2.0 min of downward irradiation, the local temperature of the powder B-Tb-NC sample rapidly increased to 191 °C, indicating that B-Tb-NC can effectively absorb sunlight and convert it into heat energy.

[0062] To study the photothermal catalytic activities of the B-Tb-NC prepared in Example 1 and the catalysts prepared in Comparative Examples 1-3, the present invention used the photothermal cycloaddition reaction of CO2 and epichlorohydrin as a model reaction, and tested the catalytic performances of the B-Tb-NC and the catalysts prepared in Comparative Examples 1-3 under the conditions of normal pressure without using an external heat source (the specific experimental conditions are as follows: normal temperature and pressure, CO2 atmosphere, catalyst dosage 15 mg, epichlorohydrin dosage: 71.4 mmol, cocatalyst dosage 0.25 g, biphenyl 0.15 g, light intensity 400 mW / cm 2 , light source wavelength 380 - 1200 nm, irradiation time 6 h, the reaction product is 4-chloromethyl-1,3-dioxolan-2-one, the determination of the reaction product is determined by gas detection, and the gas chromatogram is shown in Figure 8 , the structure is as follows The results are shown in Table 1. In the case of no catalyst added, the conversion rate of epichlorohydrin was only 7.0% (Table 1, serial number 1). When using C-N as the catalyst, the conversion rate of epichlorohydrin increased to 61.4% (Table 1, serial number 2). When using B-C-N and Tb-C-N as the catalysts, the conversion rates of CO2 cycloaddition increased to 78.3% and 65.0% (Table 1, serial numbers 3-4). Compared with the C-N material, when using B-C-N and Tb-C-N as the catalysts, the conversion rates of epichlorohydrin were both improved. This is because after doping B or Tb into the C-N material, B and Tb can act as Lewis acid sites, which is beneficial to the activation of epoxides and thus improves the catalytic activity. When using B-Tb-NC as the catalyst, the conversion yield of epichlorohydrin rapidly increased to 92.1%, and the yield increased to 87.9% (Table 1, serial number 5). The key to the strong catalytic activity of B-Tb-NC is that the electron-deficient B and the metal single atom Tb can act as Lewis acid sites, while the N in the carbon material itself can act as a Lewis base site. The simultaneous presence of Lewis acid-base sites is beneficial to the adsorption of epoxides and CO2 molecules, accelerating the ring-opening of epoxides (the rate-determining step), thereby improving the reaction activity of B-Tb-NC for photothermal catalytic CO2 cycloaddition. When only using B-Tb-NC as the catalyst without adding TBAB (tetrabutylammonium bromide) (Table 1, serial number 6), the cycloaddition reaction hardly occurred, indicating that the nucleophile is crucial for the CO2 cycloaddition reaction. Under the same conditions, when using tetrabutylammonium chloride (TBAC) as the cocatalyst, the cycloaddition reaction activity decreased (Table 1, serial number 7). This is mainly related to the nucleophilicity and leaving ability of the halide ions in the nucleophile. The ionic radius of Br - is larger than that of Cl- and its electronegativity is weak, so Br - is more likely to leave from TBA + to generate a stronger nucleophilic ability. Therefore, Br - is more conducive to the ring-opening of epoxides to form cyclic carbonate intermediates. Therefore, the ability of TBAB to activate epoxides is superior to that of TBAC. From the above results, it can be seen that the presence of both nucleophiles and Lewis acid-base sites plays a key role in the successful progress of the CO2 cycloaddition reaction. The electron-deficient B and single-atom Tb in B-Tb-NC can act as Lewis acid sites to activate epoxides, and Br - on the nucleophile TBAB attacks the oxygen atom of the epoxide to promote its ring-opening. The simultaneous presence of these two active species makes B-Tb-NC exhibit excellent catalytic activity in the photothermal catalytic CO2 cycloaddition reaction.

[0063] Table 1 Catalytic performance of different catalysts

[0064]

[0065] In Table 1, Yield (%) = (molar amount of cyclic carbonate / molar amount of epoxide) × 100%

[0066] Selectivity (%) = (molar amount of cyclic carbonate / molar amount of all products) × 100%

[0067] Conversion rate (%) = Yield / Selectivity × 100%

[0068] The influence of reaction parameters on the catalytic performance of the catalyst and the practical application of the catalyst are of great significance. In this invention, the influence of reaction time, light intensity, incident light wavelength, and the amount of catalyst on the photothermal catalytic cycloaddition performance of B-Tb-NC prepared in Example 1 was investigated to determine the optimal reaction conditions for the photothermal catalytic CO2 cycloaddition reaction of B-Tb-NC (specific reaction conditions: under normal temperature and pressure, in a CO2 atmosphere, 71.4 mmol of epichlorohydrin, 0.25 g of TBAB; except for the above variables, Figure 4 in a-d respectively also correspond to the following reaction conditions: a light intensity of 400 mW / cm 2 , incident light wavelength of 380 - 1200 nm, catalyst weight of 15 mg, reaction time of 4 - 8 h; b incident light wavelength of 380 - 1200 nm, catalyst weight of 15 mg, reaction time of 6 h, light intensity of 300 - 500 mW / cm 2 ; c light intensity of 400 mW / cm 2 , catalyst weight of 15 mg, reaction time of 6 h, incident light wavelength of 400 - 800 cm -1 , 200 - 800 cm -1 , 200 - 1200 cm-1 、 380 - 1200 cm -1 、; d light intensity 400 mW / cm 2 , incident light wavelength 380 - 1200 nm, reaction time 6 h, catalyst dosage 5 mg - 25 mg), and the results are as Figure 4 shown ( Figure 4 where Conversity represents the conversion rate of epichlorohydrin and Selectivity represents selectivity). Figure 4 In [a], it shows the influence of reaction time on its catalytic performance. Within the first 4 h of the reaction, the reaction rate is relatively slow, and at this time, the conversion rate of epichlorohydrin is only 69.9%. As the reaction time extends, the reaction rate increases sharply. When the reaction proceeds to 6 h, the conversion rate of epichlorohydrin can reach 92.1%. Further extending the reaction time to 8 h, the conversion rate of the reaction only increases to 92.8%. This is because the concentration of epoxide decreases in the later stage of the reaction, which leads to a reduction in the interaction between the reactants and the catalyst, thereby resulting in a decrease in the reaction rate. Therefore, 6 h is selected here as the optimal reaction time for the catalytic reaction.

[0069] From Figure 4 [b], it can be seen that the light intensity has a great influence on the catalytic activity of B - Tb - NC. When the light intensity increases from 300 mW / cm 2 to 400 mW / cm 2 , the conversion rate of epichlorohydrin also increases rapidly from 52.8% to 92.1%. This is because B - Tb - NC can convert light energy into heat energy under light irradiation, thus accelerating the progress of the catalytic reaction. And when the light intensity is further increased to 500 mW / cm 2 , the conversion rate of epichlorohydrin only increases to 92.4%. This is because under the irradiation of a xenon lamp at 500 mW / cm 2 , the temperature in the catalytic system only increases from 90 °C to 95 °C (see Figure 6 ), and the temperature change in the reaction system is not significant, resulting in the conversion rate of epichlorohydrin remaining basically unchanged. Therefore, 400 mW / cm 2 is selected here as the optimal light intensity.

[0070] Figure 4 In [c], the influence of the incident light wavelength on the catalytic performance of B - Tb - NC is investigated. When only visible light is used, the conversion rate of epichlorohydrin is only 48.9%; if only near - infrared light is used, the conversion rate of epichlorohydrin can reach 73.1%. This is because infrared light can generate heat on the catalyst surface through photothermal conversion, thereby increasing the temperature of the catalytic system (see Figure 7)。When a light source with a wavelength range of 380 - 1200 nm is used, its conversion rate can reach 92.1%. The results show that B-Tb-NC can effectively absorb visible light and near-infrared light and convert it into heat energy to catalyze the cycloaddition reaction of CO2 and epoxides. Therefore, this system selects 380 - 1200 nm (visible light and infrared light) as the optimal incident light wavelength.

[0071] Figure 4 The effect of the amount of B-Tb-NC on the catalytic activity of the photothermal catalytic CO2 cycloaddition reaction was evaluated. As shown in the figure, when the amount of the catalyst increased from 5 mg to 15 mg, the conversion rate of the reaction also rapidly increased from 52.8% to 92.1%. This is mainly due to the increase in the number of active sites participating in the reaction caused by the increase in the amount of the catalyst. Continuing to increase the amount of the catalyst to 25 mg, the conversion rate of the epoxide did not increase significantly. The reason is that the excessive catalyst hindered the mass transfer process between the catalyst and the reactants. Therefore, 15 mg was used as the optimal amount of the catalyst for the cycloaddition reaction in this system.

[0072] As a heterogeneous catalyst, the stability and recyclability of the B-Tb-NC prepared in Example 1 are extremely important. Therefore, under the optimal reaction conditions (i.e., normal temperature and pressure, CO2 atmosphere, 71.4 mmol of epoxide, 15 mg of the amount of the catalyst, 0.25 g of the amount of the cocatalyst, light intensity 400 mW / cm 2 , reaction time 6 h, incident light wavelength 380 - 1200 nm), the recycling experiment of B-Tb-NC in the photothermal catalytic cycloaddition reaction of CO2 and epichlorohydrin was tested. After each test was completed, B-Tb-NC was washed repeatedly with ethanol and acetone and directly used for the next recycling test after vacuum drying. The results are as Figure 5 shown. After B-Tb-NC was continuously used 5 times, the conversion rate of epichlorohydrin only decreased slightly. Nevertheless, the conversion rate of epichlorohydrin still remained above 89%.

[0073] In addition, in order to investigate the applicability of the catalyst B-Tb-NC to other epoxides, under the same test conditions (test conditions: normal temperature and pressure, CO2 atmosphere, 71.4 mmol of epoxide, 0.25 g of the amount of TBAB, 15 mg of the amount of the catalyst, light intensity 400 mW / cm 2 , reaction time 6 h, incident light wavelength 380 - 1200 nm), the catalytic activity of this catalyst for different epoxides was explored, and the results are shown in Table 2. When using B-Tb-NC for the photothermal catalytic reaction of CO2 and epichlorohydrin, at 400 mW / cm 2React for 6 h under the conditions, and the conversion rate of epichlorohydrin is 92.1% (Table 2, Serial No. 1). If epibromohydrin is used as the reactant, it is found that it has a reaction activity comparable to that of epichlorohydrin (Table 2, Serial No. 2). This is because both epibromohydrin and epichlorohydrin have halogen functional groups with electron-withdrawing groups on the ring, which can effectively reduce the charge density of the oxygen atom in the epoxide and promote the activation of the epoxide at the Lewis acid site. For cyclohexene oxide (Table 2, Serial No. 3), its conversion rate is only 67.5%. This is because the steric hindrance on the β-carbon atom of the epoxide is relatively large, which further limits the attack of the nucleophile Br - on the β-carbon atom of the epoxide, resulting in a decrease in the ring-opening rate and thus a decrease in the conversion rate of epichlorohydrin. Similarly, for other epoxides without halogen functional groups and with relatively large steric hindrance, the catalytic effect of B-Tb-C-N is also not good. The conversion rate of styrene oxide is only 54.3% (Table 2, Serial No. 4), while for cyclohexene oxide with a larger volume at the β-carbon atom (Table 2, Serial No. 5), its conversion rate is only 34.3%. This result is mainly due to the larger the substituent of the epoxide, the greater its steric hindrance, and the more unfavorable it is for the nucleophile to attack the epoxide, so the reaction rate is lower. And epoxides with smaller volumes are more likely to contact the active sites, resulting in better catalytic effects.

[0074] Table 2 Catalytic performance of B-Tb-NC for different epoxides

[0075]

[0076] The embodiments described above are only descriptions of the preferred modes of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A carbon-based material for CO2 cycloaddition reaction, characterized in that, The carbon-based material is hollow spherical, with the diameter of the hollow sphere being 200 - 400 nm and the shell thickness being 5 - 30 nm; the elemental composition of the carbon-based material includes B, Tb, N, and C; The Tb exists in the form of single atoms in the carbon-based material; In the CO2 cycloaddition reaction, the carbon-based material is used as a photothermal catalyst, and tetrabutylammonium bromide or tetrabutylammonium chloride is used as a co-catalyst; The CO2 cycloaddition reaction is carried out under the conditions of 15 - 30 °C and 101 kPa.

2. The preparation method of the carbon-based material according to claim 1, characterized in that, It includes the following steps: Disperse the template in water, then add the Tb source, B source, and carbon source and stir, and then calcine and dry to obtain the carbon-based material; The template is a mesoporous silica template or a polystyrene sphere template; the carbon source is chitosan or glucose; The Tb source is terbium nitrate; The B source is boric acid; The mass ratio of the template to the Tb source, B source, and carbon source is 2:0.4:1:0.8; The specific calcination is: heating to 800 °C at a heating rate of 1 - 5 °C / min and holding for 2 h; After the calcination, it also includes the steps of soaking in aqua regia, washing with water, and soaking in HF in sequence.

3. Use of the carbon-based material according to claim 1 in the catalytic CO2 cycloaddition reaction.

Images