Preparation method and application of cobaltosic oxide / carbon nitride composite catalyst

Abstract

The invention relates to a preparation method and application of a cobaltosic oxide / carbon nitride composite catalyst. According to the method, ultrasonic dispersion is combined with an alkaline coprecipitation method, Co3O4 nanoparticles are loaded on carbon nitride in a high-dispersion manner, acid-base bifunctional sites are formed under low-temperature calcination, CO2 cycloaddition reaction can be efficiently catalyzed under mild conditions without a halogen promoter, the preparation process is remarkably simplified, and the catalytic performance is improved. The method is simple, short in preparation period and high in economic benefit.

Description

Technical Field

[0001] The present invention belongs to the field of heterogeneous catalysis, and particularly relates to a preparation method of a cobalt tetroxide / graphitic carbon nitride composite catalyst and its application in the CO2 cycloaddition reaction Background Art

[0002] With the continuous increase in carbon dioxide emissions, the global climate crisis has become increasingly severe. There is an urgent need to develop technologies for efficient conversion of CO2 to alleviate environmental pressure. The conversion of CO2 into high-value-added chemicals (such as cyclic carbonates) has attracted much attention due to its advantages of resource utilization and emission reduction. Among them, the cycloaddition reaction of CO2 with epoxides has become one of the most promising paths because of its 100% atom utilization rate and the green commercialization of the products. In the catalytic system of the CO2 cycloaddition reaction, homogeneous catalysts have high accessibility of active sites and can achieve high catalytic activity under mild conditions. Homogeneous catalysts such as ammonium salts and metal halides have been industrially applied, but homogeneous catalysts face disadvantages such as difficult separation and recovery of catalysts. Existing heterogeneous catalysts generally have disadvantages such as low activity, poor stability, the need to add a large amount of solvents and cocatalysts, etc. Therefore, the development of high-performance heterogeneous catalysts has always been the research focus of scientific researchers

[0003] In the current catalytic system, homogeneous catalysts (such as ammonium salts and metal halides) have high activity but are difficult to recover, while heterogeneous catalysts generally have problems such as low activity and the need to add halogen auxiliaries, which easily cause corrosion and separation problems. Therefore, the development of halogen-free heterogeneous catalysts with both acid and base bifunctional sites is of great significance. g-C3N4 is regarded as an ideal support because of its simple preparation, high stability and ability to adsorb and activate CO2, but it lacks acidic sites. Although researchers have introduced Lewis acidic sites (B element, P element, Zn 2+ metal ions, etc.), (-OH, -COOH groups, etc.) and nucleophilic ring-opening sites (Br - ) into g-C3N4 by regulating its morphology, element doping and loading of the second active component, ideal catalytic activity still needs to be assisted by quaternary ammonium salts

[0004] Chinese patent document with publication number CN112156799 B discloses a composite catalyst of multiple metal oxides (combination of divalent, trivalent and tetravalent metals) and graphitic carbon nitride, but this material involves multiple steps of solution mixing and high-temperature hydrothermal treatment (120 - 180 °C), and the steps are cumbersome. In addition, the calcination temperature of the metal oxide is 400 - 700 °C, an inert atmosphere (such as nitrogen) is required, and high-pressure (up to 15.0 MPa) reaction conditions are needed, with high energy consumption and strict requirements for equipment Summary of the Invention

[0005] The object of the present invention is to provide a preparation method and application of a cobalt tetroxide / carbon nitride composite catalyst in view of the deficiencies of the current technology. This method uses ultrasonic dispersion combined with an alkaline co-precipitation method to highly disperse and load Co3O4 nanoparticles on carbon nitride, and form acid-base bifunctional sites under low-temperature calcination, enabling efficient catalytic CO2 cycloaddition reaction under mild conditions without the need for a halogen cocatalyst, significantly simplifying the preparation process and improving the catalytic performance. The method of the present invention is simple, has a short preparation cycle, and high economic benefits.

[0006] In order to achieve the above object, the present invention provides the following specific technical solutions:

[0007] A preparation method of a cobalt tetroxide / carbon nitride composite catalyst, the method comprising the following steps:

[0008] (1) Preparation of carbon nitride g-C3N4: Mix and stir urea and water at 45-55 °C to obtain a precursor solution, and then heat it to 450-600 °C and calcine for 1-3 hours to obtain carbon nitride nanosheets g-C3N4;

[0009] Wherein, the mass ratio of urea to water is (0.5-1):1.

[0010] (2) Preparation of cobalt tetroxide / carbon nitride composite catalyst Co3O4 / g-C3N4: Ultrasonically disperse g-C3N4 into a cobalt salt solution, stir and mix evenly, add an alkali solution to adjust the pH of the solution to be alkaline, carry out an aging reaction, then filter, wash, and dry to obtain a yellow-green powder. Place the yellow-green powder in a muffle furnace, heat it to 300-500 °C, and the calcination time is 1-3 h to obtain a gray-black powder Co3O4 / g-C3N4;

[0011] Wherein, the mass ratio of the cobalt salt to g-C3N4 is 0.1-1.5;

[0012] The cobalt salt is cobalt nitrate, cobalt chloride or cobalt acetate; the solvent of the cobalt salt solution is an ethanol solution. The concentration of the ethanol solution is 75% to 99.7%;

[0013] The concentration of the cobalt salt solution is 0.007 mol / L to 1.05 mol / L;

[0014] In the step (2), the alkali solution is one of KOH solution, NaOH solution, Na2CO3 solution and ammonia water, the concentration of the alkali solution is 1-3 mol / L, the pH value of the solution is 10-13, and the aging time is 2-6 h;

[0015] The heating rate in step (1) is 1-3 °C / min; the heating rate in step (2) is 1-5 °C / min;

[0016] Application of the cobalt tetroxide / graphitic carbon nitride composite catalyst prepared by the described method, for the application of preparing cyclic carbonates by CO2 cycloaddition, specifically comprising the following steps:

[0017] Load the catalyst, DMF, and epoxybutane into a stainless-steel autoclave, then flush CO2 gas into the autoclave and seal it. The reaction pressure is 0.5 - 3 MPa, the reaction temperature is 60 - 150 °C. After reacting for 1 - 5 h, stop the reaction to obtain the target product butylene carbonate;

[0018] Among them, for every 0.2 g of the obtained cobalt tetroxide / graphitic carbon nitride composite catalyst Co3O4 / g-C3N4, add 1 - 3 mL of DMF and 1 - 10 mL of epoxybutane.

[0019] The substantial features of the present invention are:

[0020] The present invention uses ultrasonic dispersion combined with alkaline co-precipitation method to highly disperse and load Co3O4 nanoparticles on graphitic carbon nitride. The catalyst obtained by calcination at low temperature has abundant Lewis acid-base sites. Co 2+ and Co 3+ ions effectively reduce the ring-opening energy barrier of epoxides by virtue of appropriate Lewis acidity. Oxygen vacancies and amino groups activate CO2 chemically and then provide an oxygen anion for nucleophilic attack on the ring-opened epoxide, enabling a high yield of cyclic carbonates under relatively mild reaction conditions without using a co-catalyst.

[0021] In the catalyst application, the catalytic system of the current technology has insufficient nucleophilicity towards epoxides and requires the participation of halide ions to promote the ring-opening reaction of epoxides.

[0022] In the present invention, the carbonate intermediate formed by the synergistic activation of carbon dioxide by amino functional groups and oxygen vacancies can attack the epoxide activated by Lewis acid instead of halide ions, thus avoiding the participation of halide ions.

[0023] Compared with the prior art, the principle and beneficial effects of the present invention:

[0024] 1. The Co3O4 / g-C3N4 material prepared by the present invention has abundant acid-base sites. Co 2+ and Co 3+ with appropriate Lewis acidity serve as epoxide adsorption sites and have the ability to activate epoxides and accelerate their ring-opening performance. Oxygen vacancies and amino groups activate CO2 chemically and then provide an oxygen anion for nucleophilic attack on the ring-opened epoxide, enabling a high yield of cyclic carbonates without using a halogen-containing co-catalyst.

[0025] 2. In the catalyst prepared by the present invention, the supported cobalt tetroxide nanoparticles are uniformly distributed on the surface of the carbon nitride nanosheets, which effectively inhibits the agglomeration of Co3O4. Small-sized Co3O4 nanoparticles (about 8.6 nm) are formed at a relatively low temperature, increasing the specific surface area and the density of active sites.

[0026] 3. The charge transfer phenomenon between Co3O4 and g-C3N4 makes the proportion of Co 2+ / Co 3+ in the composite material increase, enhancing the Lewis acidity and promoting the ring-opening of epoxides; the increase in the proportion of oxygen vacancies (O V ) enhances the Lewis basicity, and the synergistic effect of the acid-base sites improves the efficiency of the CO2 cycloaddition reaction.

[0027] 4. The catalytic performance of the cobalt tetroxide / carbon nitride composite catalyst Co3O4 / g-C3N4-1 (Example 1) prepared by the present invention for the synthesis of butylene carbonate from CO2 and epoxybutane was investigated. Under the conditions of 150 °C and 2 MPa CO2 for 3 hours, the conversion rate of epoxybutane reached 91%, the yield of butylene carbonate was 87%, and the selectivity was 96%. Compared with the single-component Co3O4 (Comparative Example 1) and g-C3N4 (Comparative Example 2) catalysts, the yields were increased by 3.5 times and 34.6 times, respectively, and the product and the catalyst could be separated by simple filtration. Under the condition of similar yields of cyclic carbonates, the metal oxide and carbon nitride composite materials disclosed in the patent literature require a higher CO2 partial pressure (68 MPa).

[0028] 5. The catalyst of the present invention can catalyze the cycloaddition reaction of CO2 and epoxybutane under relatively mild conditions (reaction pressure of 0.5 - 3 MPa), showing very good application prospects.

[0029] 6. The preparation method proposed by the present invention does not use organic solvents, and the catalyst can be easily recovered. It has the advantages of low cost, simple operation, and easy large-scale production. In the current technology, the composite of metal oxide and carbon nitride requires hydrothermal dynamic aging at 120 - 180 °C for 6 - 24 hours; the composite step of the present invention only needs to age at room temperature (25 °C) for 2 - 6 hours without high-temperature and high-pressure hydrothermal conditions; and the solution preparation step is omitted: the current technology requires separately preparing solution A (metal salt solution) and solution B (OH- / CO3 2 - mixed solution) and controlling the pH by co-current coprecipitation; the present invention only needs to directly adjust the pH with a single cobalt salt solution and an alkali solution (such as KOH), reducing the solution preparation step. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 XRD patterns of Co3O4, g-C3N4, and Co3O4 / g-C3N4-1 obtained in Example 1.

[0031] Figure 2 TEM images and elemental mapping images of Co3O4, g-C3N4, and Co3O4 / g-C3N4-1 obtained in Example 1.

[0032] Figure 3 FT-IR spectra of the frameworks of Co3O4, g-C3N4, and Co3O4 / g-C3N4-1 obtained in Example 1.

[0033] Figure 4 X-ray photoelectron spectra of Co3O4, g-C3N4, and Co3O4 / g-C3N4-1 obtained in Example 1; among them, Figure 4 (a) is the C1s XPS spectrum of g-C3N4 and Co3O4 / g-C3N4-1, Figure 4 (b) is the N1s XPS spectrum of g-C3N4 and Co3O4 / g-C3N4-1, Figure 4 (c) is the O1s XPS spectrum of Co3O4 and Co3O4 / g-C3N4-1, Figure 4 (d) is the Co 2p XPS spectrum of Co3O4 and Co3O4 / g-C3N4-1. Detailed implementation manners

[0034] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners. It should be noted that the following embodiments are illustrative and not restrictive, and the protection scope of the present invention cannot be limited by the following embodiments. The raw materials required in the following embodiments and comparative examples are all commercially available.

[0035] In this embodiment, the XRD of the cobalt cobalt oxide / graphitic carbon nitride composite catalyst Co3O4 / g-C3N4 was analyzed using an X-ray diffractometer of the D / MAX-2550 model produced by Rigaku.

[0036] In this embodiment, the TEM morphology analysis and EDS mapping elemental distribution of the cobalt cobalt oxide / graphitic carbon nitride composite catalyst Co3O4 / g-C3N4 were tested using a transmission electron microscope of the JEM-F200 model produced by JEOL, Japan.

[0037] In this embodiment, the framework infrared spectrum of the cobalt cobalt oxide / graphitic carbon nitride composite catalyst Co3O4 / g-C3N4 was analyzed using a Thermo Scientific Nicolet 6700 infrared spectrometer.

[0038] In this embodiment, the XPS elemental chemical state analysis of the cobalt cobalt oxide / carbon nitride composite catalyst Co3O4 / g-C3N4 was carried out using an AXIS Supra+ X-ray photoelectron spectrometer from Shimadzu Corporation.

[0039] Example 1

[0040] The preparation steps of a cobalt cobalt oxide / carbon nitride composite catalyst Co3O4 / g-C3N4-1 are as follows:

[0041] (1) Preparation of g-C3N4: Weigh 20 g of urea and place it in a beaker. Add 20 g of deionized water and continuously stir in a water bath at 50 °C for 30 min. Then, transfer the solution to a covered 100 mL ceramic crucible and heat it to 550 °C at a heating rate of 1 °C / min and hold for 3 h. Put the obtained yellow powder into a mortar and grind it for later use, denoted as g-C3N4.

[0042] (2) Preparation of the cobalt cobalt oxide / carbon nitride composite catalyst Co3O4 / g-C3N4: Weigh 1.5 g of Co(Cl)2·6H2O and add it to 60 mL of ethanol solution (concentration 99.7%) (to obtain a cobalt salt concentration of 0.105 mol / L), and stir for 30 min to ensure uniform dispersion of cobalt ions. Under ultrasonic conditions, add 1 g of g-C3N4 to it, and then ultrasonically treat the mixed solution at 20 °C for 1 h and then continue to magnetically stir at 25 °C and 600 rpm for 1 h. Dropwise add 1 M KOH solution to adjust the pH of the system to 12, age in a 25 °C water bath for 3 h, filter, wash successively with water and ethanol until neutral, and vacuum dry at 60 °C for 12 h to obtain a yellow-green powder. After grinding for 5 min, put it into a crucible and calcine it in a muffle furnace at 300 °C at a heating rate of 2 °C / min for 3 h. The obtained gray-black powder sample is named Co3O4 / g-C3N4-1. Through ICP-OES characterization, the loading amount of Co3O4 in the final product is measured to be 26.3 wt%.

[0043] The performance evaluation method of Co3O4 / g-C3N4-1 is as follows:

[0044] Weigh 0.2 g of the cobalt cobalt oxide / carbon nitride composite catalyst Co3O4 / g-C3N4-1 and place it in a 25 mL reactor. Then add 3 mL of DMF, 1 mL of epoxybutane, and seal the reactor. Open the inlet valve and add 2 MPa (and always maintain this pressure) of CO2 to the reactor. Turn on heating and magnetic stirring. After the reactor temperature rises to 150 °C, start timing the reaction for 3 h and then stop the reaction, and cool it in an ice-water bath. Using biphenyl as an external standard, quantitative analysis is carried out using gas chromatography. The conversion rate of epoxybutane is 90%, the yield of butylene carbonate is 87%, and the selectivity is 97%.

[0045] Example 2

[0046] The preparation steps of Co3O4 / g-C3N4-2 refer to Example 1, with the difference that the cobalt salt used in step (2) of Example 1 is changed from Co(Cl)2·6H2O to Co(NO3)2·6H2O, and the remaining steps are the same as those in Example 1. The prepared sample is denoted as Co3O4 / g-C3N4-2.

[0047] The performance evaluation method of Co3O4 / g-C3N4-2 refers to Example 1, with the difference that the Co3O4 / g-C3N4-1 sample in Example 1 is replaced by the Co3O4 / g-C3N4-2 sample, and the remaining steps are the same as those in Example 1. The conversion rate of epoxybutane is 83%, the yield of butylene carbonate is 77%, and the selectivity is 93%. The loading amount of Co3O4 in the final product is 26.3 wt% characterized by ICP-OES.

[0048] Example 3

[0049] The preparation steps of Co3O4 / g-C3N4-3 refer to Example 1, with the difference that the mass of Co(Cl)2·6H2O used in step (2) of Example 1 is changed from 1.5 g to 1 g, and the remaining steps are the same as those in Example 1. The prepared sample is denoted as Co3O4 / g-C3N4-3.

[0050] The performance evaluation method of Co3O4 / g-C3N4-3 refers to Example 1, with the difference that the Co3O4 / g-C3N4-1 sample in Example 1 is replaced by the Co3O4 / g-C3N4-3 sample, and the remaining steps are the same as those in Example 1. The conversion rate of epoxybutane is 89%, the yield of butylene carbonate is 86%, and the selectivity is 95%. The loading amount of Co3O4 in the final product is 22.6 wt% characterized by ICP-OES.

[0051] Example 4

[0052] The preparation steps of Co3O4 / g-C3N4-4 refer to Example 1, with the difference that the KOH solution in step (2) of Example 1 is changed to NaOH solution, and the remaining steps are the same as those in Example 1. The prepared sample is denoted as Co3O4 / g-C3N4-4.

[0053] The performance evaluation method of Co3O4 / g-C3N4-4 refers to Example 1, with the difference that the Co3O4 / g-C3N4-1 sample in Example 1 is replaced by the Co3O4 / g-C3N4-4 sample, and the remaining steps are the same as those in Example 1. The conversion rate of epoxybutane is 88%, the yield of butylene carbonate is 84%, and the selectivity is 95%. The loading amount of Co3O4 in the final product is 26.3 wt% characterized by ICP-OES.

[0054] Example 5

[0055] The preparation steps of Co3O4 / g-C3N4-5 refer to Example 1, except that the concentration of KOH solution in step (2) of Example 1 is changed from 1 mol / L to 2 mol / L, and the remaining steps are the same as those in Example 1. The prepared sample is denoted as Co3O4 / g-C3N4-5.

[0056] The performance evaluation method of Co3O4 / g-C3N4-5 refers to Example 1, except that the Co3O4 / g-C3N4-1 sample in Example 1 is replaced with the Co3O4 / g-C3N4-5 sample, and the remaining steps are the same as those in Example 1. The conversion rate of epoxy butane is 89%, the yield of butene carbonate is 80%, and the selectivity is 90%. The loading amount of Co3O4 in the final product is characterized by ICP-OES to be 26.3 wt%.

[0057] Example 6

[0058] The preparation steps of Co3O4 / g-C3N4-6 refer to Example 1, except that the pH value of the solution in step (2) of Example 1 is changed from 12 to 13, and the remaining steps are the same as those in Example 1. The prepared sample is denoted as Co3O4 / g-C3N4-6.

[0059] The performance evaluation method of Co3O4 / g-C3N4-6 refers to Example 1, except that the Co3O4 / g-C3N4-1 sample in Example 1 is replaced with the Co3O4 / g-C3N4-6 sample, and the remaining steps are the same as those in Example 1. The conversion rate of epoxy butane is 75%, the yield of butene carbonate is 70%, and the selectivity is 93%. The loading amount of Co3O4 in the final product is characterized by ICP-OES to be 26.3 wt%.

[0060] Example 7

[0061] The preparation steps of Co3O4 / g-C3N4-7 refer to Example 1, except that the aging time in step (2) of Example 1 is changed from 3 h to 6 h, and the remaining steps are the same as those in Example 1. The prepared sample is denoted as Co3O4 / g-C3N4-7.

[0062] The performance evaluation method of Co3O4 / g-C3N4-7 refers to Example 1, except that the Co3O4 / g-C3N4-1 sample in Example 1 is replaced with the Co3O4 / g-C3N4-7 sample, and the remaining steps are the same as those in Example 1. The conversion rate of epoxy butane is 69%, the yield of butene carbonate is 67%, and the selectivity is 97%. The loading amount of Co3O4 in the final product is characterized by ICP-OES to be 26.3 wt%.

[0063] Example 8

[0064] The preparation steps of Co3O4 / g-C3N4-8 refer to Example 1, with the difference that the calcination temperature in the muffle furnace in step (2) of Example 1 is changed from 300 °C to 500 °C, and the remaining steps are the same as those in Example 1. The prepared sample is denoted as Co3O4 / g-C3N4-8.

[0065] The performance evaluation method of Co3O4 / g-C3N4-8 refers to Example 1, with the difference that the Co3O4 / g-C3N4-1 sample in Example 1 is replaced with the Co3O4 / g-C3N4-8 sample, and the remaining steps are the same as those in Example 1. The conversion rate of epoxy butane is 65%, the yield of butylene carbonate is 60%, and the selectivity is 93%. The loading amount of Co3O4 in the final product is characterized by ICP-OES to be 26.3 wt%.

[0066] Example 9

[0067] The preparation steps of Co3O4 / g-C3N4-9 refer to Example 1, with the difference that the calcination time in the muffle furnace in step (2) of Example 1 is changed from 3 h to 5 h, and the remaining steps are the same as those in Example 1. The prepared sample is denoted as Co3O4 / g-C3N4-9.

[0068] The performance evaluation method of Co3O4 / g-C3N4-9 refers to Example 1, with the difference that the Co3O4 / g-C3N4-1 sample in Example 1 is replaced with the Co3O4 / g-C3N4-9 sample, and the remaining steps are the same as those in Example 1. The conversion rate of epoxy butane is 67%, the yield of butylene carbonate is 65%, and the selectivity is 97%. The loading amount of Co3O4 in the final product is characterized by ICP-OES to be 26.3 wt%.

[0069] Example 10

[0070] The preparation steps of Co3O4 / g-C3N4-10 refer to Example 1, with the difference that the heating rate of the muffle furnace in step (2) of Example 1 is changed from 2 °C / min to 5 °C / min, and the remaining steps are the same as those in Example 1. The prepared sample is denoted as Co3O4 / g-C3N4-10.

[0071] The performance evaluation method of Co3O4 / g-C3N4-10 refers to Example 1, with the difference that the Co3O4 / g-C3N4-1 sample in Example 1 is replaced with the Co3O4 / g-C3N4-10 sample, and the remaining steps are the same as those in Example 1. The conversion rate of epoxy butane is 75%, the yield of butylene carbonate is 73%, and the selectivity is 97%. The loading amount of Co3O4 in the final product is characterized by ICP-OES to be 26.3 wt%.

[0072] Example 11

[0073] The performance evaluation method of Co3O4 / g-C3N4-1 refers to Example 1, except that the reaction time in the performance evaluation method in Example 1 is changed from 3 h to 0.5 h, and the remaining steps are the same as those in Example 1. The conversion rate of epoxy butane is 56%, the yield of butene carbonate is 55%, and the selectivity is 98%.

[0074] Example 12

[0075] The performance evaluation method of Co3O4 / g-C3N4-1 refers to Example 1, except that the reaction pressure in Example 1 is changed from 2 MPa to 1 MPa, and the remaining steps are the same as those in Example 1. The conversion rate of epoxy butane is 78%, the yield of butene carbonate is 75%, and the selectivity is 96%.

[0076] Example 13

[0077] The performance evaluation method of Co3O4 / g-C3N4-1 refers to Example 1, except that the reaction temperature in Example 1 is changed from 150 °C to 120 °C, and the remaining steps are the same as those in Example 1. The conversion rate of epoxy butane is 57%, the yield of butene carbonate is 56%, and the selectivity is 99%.

[0078] Comparative Example 1

[0079] Comparative sample 1, the preparation steps of pure-phase Co3O4 are as follows:

[0080] The preparation steps of Co3O4 refer to Example 1, except that g-C3N4 in step (2) of Example 1 is not added, and the remaining steps are the same as those in Example 1.

[0081] The performance evaluation method of Co3O4 refers to Example 1, except that the Co3O4 / g-C3N4-1 sample in Example 1 is replaced with a Co3O4 sample, and the catalyst dosage is 0.02 g, which is equivalent to the Co3O4 loading in the Co3O4 / g-C3N4-1 sample in Example 1, and the remaining steps are the same as those in Example 1. The conversion rate of epoxy butane is 29%, the yield of butene carbonate is 25%, and the selectivity is 85%.

[0082] Comparative sample 2, set g-C3N4 as Comparative sample 2

[0083] The performance evaluation method of g-C3N4 refers to Example 1. The conversion rate of epoxy butane is 3%, the yield of butene carbonate is 2%, and the selectivity is 82%.

[0084] Example 14

[0085] The crystal structures of Co3O4, g-C3N4 and Co3O4 / g-C3N4-1 catalysts were characterized by X-ray diffraction (XRD), as Figure 1As shown. The XRD pattern of g-C3N4 shows a strong diffraction peak at 27.0°, corresponding to the (002) crystal plane, which results from the periodic stacking of its layered structure; a weak diffraction peak appears at 13.3°, corresponding to the (100) crystal plane, which results from the in-plane arrangement of heptazine ring units. Characteristic peaks with 2θ angles at 18.9°, 31.0°, 36.7°, 44.7° and 61.3° are detected in the XRD patterns of both Co3O4 and Co3O4 / g-C3N4-1 samples, which are respectively attributed to the (111), (220), (311), (400), (511) and (440) crystal planes of the spinel Co3O4 structure (PDF#43-1003), and no diffraction peaks of impurity Co, CoO, Co(OH)2 phases are observed. XRD characterization indicates the effective loading of Co3O4 nanoparticles on the surface of g-C3N4. Subsequently, the crystallite sizes of the catalysts were calculated by the Scherrer formula. The crystallite size of pure-phase Co3O4 is 13.1 nm, and that of Co3O4 / g-C3N4-1 is 10.9 nm. This indicates that the abundant tertiary nitrogen functional groups on the surface of the g-C3N4 support can provide uniform anchoring sites for Co3O4 precursors, promoting the dispersion of Co3O4 nanoparticles and resulting in a significantly smaller crystallite size of supported Co3O4 than that of unsupported Co3O4 nanoparticles.

[0086] Example 15

[0087] The porous structure and composition of the catalysts were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) and the corresponding energy-dispersive spectrometer (EDS-Mapping). As Figure 2 shown in (a). It can be seen that Co3O4 presents a porous nanosheet structure, and irregularly shaped nanoparticles are distributed on the catalyst surface, with an average crystallite size of the particles being 13.6 nm. Figure 2 Shown in (b) is the HR-TEM image of Co3O4. Two different lattice fringe spacings are observed, which are 0.20 nm and 0.24 nm respectively, corresponding to the (400) and (311) crystal planes of cubic Co3O4. As Figure 2 shown in (c), g-C3N4 presents a typical layered structure. Figure 2 Shown in (d) is the TEM image of the Co3O4 / g-C3N4-1 sample. Due to the strong coordination effect between Co 2+ / Co 3+ metal ions and tertiary nitrogen, black Co3O4 nanoparticles are uniformly distributed on the surface of flaky carbon nitride, and its average particle size is 8.6 nm. From Figure 2In (e), lattice fringes of 0.20 nm and 0.24 nm can be observed, corresponding to the (400) and (311) crystal planes of cubic Co3O4, respectively, which confirms that the black nanoparticles are Co3O4. In addition, EDS-Mapping was used to determine the distribution of C, N, Co, and O elements in Co3O4 / g-C3N4-1, as shown in Figure 2 (f) of. Among them, C and N are from g-C3N4, and Co and O are from Co3O4. All elements are evenly dispersed in the prepared catalyst, indicating that Co3O4 nanoparticles are successfully introduced onto the g-C3N4 nanosheets.

[0088] Example 16

[0089] Fourier transform infrared spectroscopy was used to perform skeletal infrared characterization of Co3O4, g-C3N4, and Co3O4 / g-C3N4-1, and their infrared spectra are as shown in Figure 3 . Two strong absorption peaks were detected in pure-phase Co3O4 at 572 cm -1 and 666 cm -1 , which were assigned to the stretching vibrations of Co 3+ -O octahedral sites and Co 2+ -O tetrahedral sites, further proving the formation of the cubic spinel Co3O4 structure. In addition, two characteristic vibration bands were detected at 1628 cm -1 and 3400 cm –1 . The former is attributed to the bending vibration of the -OH group of chemisorbed water molecules, and the latter is attributed to the stretching vibration of the O–H bond in physically adsorbed H2O. Characteristic peaks of g-C3N4 at 812 cm -1 , 1200 - 1700 cm -1 , 2181 cm -1 and 3000 - 3400 cm -1 are attributed to the out-of-plane bending vibration of the heptazine ring, C-N / C=N bonds, C≡N groups, and the stretching vibration of N-H bonds, respectively. For the Co3O4 / g-C3N4-1 catalyst, the main characteristic peaks belonging to Co3O4 and g-C3N4 can be clearly observed, indicating the effective loading of Co3O4 nanoparticles on the surface of g-C3N4.

[0090] Example 17

[0091] The surface element composition, chemical valence states, and interfacial interactions of Co3O4, g-C3N4, and Co3O4 / g-C3N4-1 were studied by X-ray photoelectron spectroscopy. Compared with g-C3N4, the binding energies of C1s and N1s in the Co3O4 / g-C3N4-1 composite material shift towards higher binding energies. Specifically, as shown in Figure 4(a) As shown, the three peaks at the binding energies of 284.5 eV, 286.1 eV, and 288.0 eV in the g-C3N4 spectrum are respectively attributed to sp 2 hybridized C-C bonds, sp 2 hybridized N-C=N bonds, and C-NH x bonds. In Co3O4 / g-C3N4-1, the above peaks are respectively shifted to 284.8 eV, 286.3 eV, and 288.4 eV. Figure 4 (b) The N 1s XPS spectra of g-C3N4 and Co3O4 / g-C3N4-1 are shown. The three characteristic peaks of g-C3N4 at the binding energies of 398.5 eV, 400.0 eV, and 401.0 eV are respectively attributed to N-C=N bonds, N-(C)3 bonds, and -NH x functional groups. In Co3O4 / g-C3N4-1, the above characteristic peaks are shifted to 399.1 eV, 400.5 eV, and 401.6 eV in the direction of higher binding energy. The positive shift of the C 1s and N 1s binding energies indicates that the electron cloud density of g-C3N4 in Co3O4 / g-C3N4-1 decreases, which is attributed to the charge transfer between g-C3N4 and Co3O4, that is, the electron-rich g-C3N4 transfers part of its electrons to Co3O4, resulting in a decrease in the electron density of the former and an increase in the electron density of the latter.

[0092] The O 1s spectrum and Co 2p fine spectrum also prove the charge transfer effect. Compared with Co3O4, the O 1s and Co 2p binding energies of the Co3O4 / g-C3N4-1 composite material both shift in the direction of lower binding energy. As Figure 4 (c) shown, the three peaks at the binding energies of 529.7 eV, 531.4 eV, and 533.2 eV in the Co3O4 spectrum are respectively attributed to lattice oxygen (O L ), oxygen vacancies (O V ), and surface adsorbed oxygen (O A ), corresponding to the three peaks of Co3O4 / g-C3N4-1 at the binding energies of 529.4 eV, 531.1 eV, and 532.9 eV. It should be noted that in the O 1s spectrum of Co3O4 / g-C3N4-1, the proportion of the peak area of lattice oxygen (O L ) decreases, while the proportion of the peak area of oxygen vacancies (O V ) increases significantly. We use the O V / O L value to represent the oxygen vacancy content. This indicates that electron transfer increases the oxygen vacancy concentration of Co3O4 / g-C3N4-1. Oxygen vacancies, as Lewis basic sites, are beneficial to enhancing the chemical adsorption ability of CO2 of the composite material. The Co 2p XPS spectra of Co3O4 and Co3O4 / g-C3N4-1 are shown in Figure 4As shown in (d). All samples have two main peaks and two satellite peaks. The peaks of Co3O4 at 779.9 eV and 794.6 eV are attributed to Co 2p 3 / 2 and Co 2p 1 / 2 main peaks. The peaks at 788.0 eV and 803.0 eV are attributed to satellite peaks (Sat.). Among them, Co 3+ and Co 2+ are the main valence states of Co3O4, and the double-peak interval of the same valence state is about 15 eV. The peaks of Co3O4 / g-C3N4-1 at 779.6 eV and 794.3 eV are attributed to Co 2p 3 / 2 and Co 2p 1 / 2 main peaks. The peaks at 787.7 eV and 802.7 eV are attributed to satellite peaks (Sat.). The peaks of Co3O4 at 779.6 eV and 794.6 eV are attributed to Co 3+ 2p 3 / 2 and Co 3+ 2p 1 / 2 . The peaks at 781.2 eV and 796.5 eV are attributed to Co 2+ 2p 3 / 2 and Co 2+ 2p 1 / 2 . The peaks of Co3O4 / g-C3N4-1 at 779.3 eV and 794.2 eV are attributed to Co 3+ 2p 3 / 2 and Co 3+ 2p 1 / 2 . The peaks at 780.8 eV and 796.0 eV are attributed to Co 2+ 2p 3 / 2 and Co 2+ 2p 1 / 2 . In addition, in the Co 2p spectrum of Co3O4 / g-C3N4-1, the proportion of the peak area of Co 2+ significantly increases, and the proportion of the peak area of Co 3+ decreases, indicating that electron transfer promotes the partial reduction of Co 3+ to Co 2+ , increasing the Co 2+ / Co 3+ ratio. The Co 2+ sites, as unsaturated Lewis acid sites, are more favorable for the adsorption and activation of epoxides.

[0093] The above binding energy shift and electron transfer direction prove that a strong interaction is formed between Co3O4 and g-C3N4, rather than a simple physical mixture. This interaction increases Co 2+ / Co 3+The ratio and oxygen vacancy concentration are optimized, and the number of acid-base active sites is beneficial to enhancing the adsorption and activation capabilities of epoxides and CO2, further improving the activity of the CO2 cycloaddition reaction.

[0094] The above provides an exemplary description of the present invention. It should be noted that without departing from the core of the present invention, any simple deformation, modification, or equivalent substitution that can be made by those skilled in the art without creative efforts falls within the protection scope of the present invention.

[0095] Matters not covered by the present invention are well-known technologies.

Claims

1. A preparation method of a cobalt tetroxide / carbon nitride composite catalyst, characterized in that, The method comprises the following steps: (1) Preparation of graphitic carbon nitride g-C3N4: Urea and water are mixed and stirred at 45 - 55 °C to obtain a precursor solution, which is then calcined at 450 - 600 °C for 1 - 3 hours to obtain graphitic carbon nitride nanosheets g-C3N4; Among them, the mass ratio of urea to water is (0.5 - 1):1; (2) Preparation of cobalt cobalt oxide / graphitic carbon nitride composite catalyst Co3O4 / g-C3N4: g-C3N4 is ultrasonically dispersed in a cobalt salt solution, stirred and mixed evenly, an alkali solution is added to adjust the solution to be alkaline, and aged for 2 - 6 h; Subsequently, after filtration, washing and drying, a yellow-green powder is obtained. The yellow-green powder is placed in a muffle furnace, heated to 300 - 500 °C, and the calcination time is 1 - 3 h to obtain a gray-black powder Co3O4 / g-C3N4, that is, cobalt cobalt oxide / graphitic carbon nitride composite catalyst; Among them, the mass ratio of cobalt salt to g-C3N4 is 0.1 - 1.5; The cobalt salt is cobalt nitrate, cobalt chloride or cobalt acetate.

2. The preparation method of the cobalt tetroxide / carbon nitride composite catalyst according to claim 1, characterized in that, The concentration of the cobalt salt solution is 0.007 mol / L - 1.05 mol / L; the solvent of the cobalt salt solution is an ethanol solution; the concentration of the ethanol solution is 75% - 99.7%.

3. The preparation method of the cobalt tetroxide / carbon nitride composite catalyst according to claim 1, characterized in that, In the step (2), the alkali solution is one of KOH solution, NaOH solution, Na2CO3 solution and ammonia water, the concentration of the alkali solution is 1 - 3 mol / L, and the pH value of the solution is 10 - 13.

4. The preparation method of the cobalt tetroxide / carbon nitride composite catalyst according to claim 1, characterized in that, The heating rate in step (1) is 1 - 3 °C / min; the heating rate in step (2) is 1 - 5 °C / min.

5. Use of the cobalt tetroxide / carbon nitride composite catalyst prepared by the method according to claim 1, characterized in that, For the cycloaddition of CO2 to prepare cyclic carbonates.

6. The application according to claim 5, characterized in that, Comprises the following steps: The catalyst, DMF and epoxy butane are loaded into a stainless steel reaction kettle, and then CO2 gas is introduced into the reaction kettle and sealed. The reaction pressure is 0.5 - 3 MPa, the reaction temperature is 60 - 150 °C, and the reaction is stopped after 1 - 5 h to obtain the target product butylene carbonate; Among them, for every 0.2 g of the obtained cobalt cobalt oxide / graphitic carbon nitride composite catalyst Co3O4 / g-C3N4, 1 - 3 mL of DMF and 1 - 10 mL of epoxy butane are added.

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