Low-density monolithic porous carbon and microemulsion preparation method and application thereof

Abstract

The application discloses low-density monolithic porous carbon and a microemulsion preparation method and application thereof, and relates to the field of foam carbon materials. The porous carbon material comprises a three-dimensional network structure formed by cross-linking and stacking of cavity carbon nanoparticles, and has a density of 0.16-0.35 g / cm 3 ; the phenolic resin is used as a precursor, the foaming agent, i.e., the microemulsion droplet of the imidazole compound, is used as a template, and a green solvent is used, and the monolithic porous carbon is obtained through a sol-gel reaction, drying and pyrolysis. The imidazole compound and the hydrophilic end of the foaming agent interact to form a flexible small-molecule interface film, reduce the interfacial tension and stabilize the emulsion droplet. The foaming agent is decomposed to form a cavity spherical structure during carbonization. The monolithic porous carbon has a developed porous structure and surface hydrophobicity, is beneficial to constructing a high-efficiency electrocatalytic reaction microenvironment, and is reflected in improving the surface reaction gas concentration and inhibiting excessive mass supply, so as to improve the electrocatalytic reaction selectivity. Based on the morphology, pore structure and surface properties, the monolithic porous carbon also has application potential in the fields of water purification and heat insulation and flame retardation.

Description

Technical Field

[0001] This invention belongs to the field of foamed carbon materials, and relates to a low-density monolithic porous carbon and its microemulsion preparation method and application. Background Technology

[0002] Foamed carbon is a porous carbon material with a three-dimensional network structure composed of bubbles and interconnected bubble walls. Due to its advantages such as low density, good electrical conductivity, and high chemical stability, it is widely used in advanced research fields, especially in high-tech areas such as chemical engineering, marine engineering, and aerospace. Based on the different precursor raw materials, foamed carbon materials can be divided into three categories: coal-based, pitch-based, and phenolic resin-based foamed carbon. Foamed carbon prepared from easily graphitized raw materials such as coal pitch has complex preparation process parameters and is difficult to control in terms of porous structure, limiting its application. Foamed carbon prepared from polymer resins has adjustable structure and has greater potential for application in various strategic technological development directions.

[0003] Phenolic resin polymer foams are generally composed of phenolic resin, foaming agents, and curing agents. The controllable formation mechanism of their porous structure has always been a research focus, as pore structure parameters (including pore configuration, pore size, and cell density) are the main factors determining the mechanical, thermal, and electrical properties of the derived foamed carbon. The preparation process of phenolic resin-based foamed carbon typically includes foaming, carbonization, and graphitization steps involving carbon-containing precursors. Among these, the foaming process is the most crucial step, directly affecting the porous structure and strength properties of the foamed carbon material. Foaming with foaming agents is a common and effective method, further divided into chemical foaming and supercritical phase change foaming. Chemical foaming requires the introduction of one or more foaming agents. Its foaming principle is that when the polymer precursor reaches the boiling point or decomposition temperature of the foaming agent during carbonization, gaseous volatiles are formed, achieving the foaming effect. Chemical foaming easily forms porous cells with large average diameters, but it suffers from the problems of toxic gases and high costs. Supercritical phase change foaming requires the foaming material to be injected in advance. At high temperature, a series of phase change processes will occur between the foaming agent and the polymer. However, this process is easily affected by external factors (temperature, pressure, etc.) and has problems such as complex operation, high equipment requirements, and difficulty in controlling the foam pore size.

[0004] Furthermore, in the preparation process of foamed carbon, the drying method of the phenolic resin polymer precursor is a key step and technical challenge in controlling the structural strength and density of the foamed carbon. Atmospheric pressure drying is a simple and energy-saving process. However, during the removal of water from the wet gel network structure, atmospheric pressure drying easily leads to changes in surface tension, resulting in the destruction of the integrity of the gel network and the collapse of the pore structure. Existing technologies struggle to achieve atmospheric pressure drying while maintaining polymer non-shrinkage.

[0005] Despite ongoing research into the synthesis of porous carbon foam materials with significant application potential, challenges remain, including complex and time-consuming preparation processes, substantial resin shrinkage during carbonization, and severe cracking. These issues in preparation and properties result in poor structural density, inability to precisely control pore structure, and a typically linear correlation between strength and density, making it difficult to meet practical application requirements. Therefore, finding a green, environmentally friendly, mild, and bulk-phase-homogeneous synthetic method for preparing porous carbon foam materials is of great significance for developing their practical applications. Summary of the Invention

[0006] To address the current technical challenges in the preparation of foamed carbon materials, this invention aims to provide a low-density monolithic porous carbon and its microemulsion preparation method and application. Using phenolic resin as a precursor and imidazole-based microemulsion droplets as templates, the invention optimizes the framework network and pore structure of the phenolic resin-based foamed carbon by utilizing the decomposition and foaming of droplets formed by the uniform dispersion of oleic acid or sodium oleate. Innovatively, imidazole compounds are used as catalysts and foam stabilizers. Imidazole compounds can form an interfacial molecular film, reducing the interfacial tension of the droplets in the solvent. First, the uniform dispersion of oleic acid or sodium oleate droplets in the solvent is ensured at room temperature. Second, the polymerization reaction of phenol and aldehyde outside the microemulsion droplets is catalyzed to form cross-linked polymer nanoparticles. Further carbonization of the polymer yields the foamed carbon, which comprises a three-dimensional network structure formed by the cross-linking and stacking of carbon nanoparticles with a cavity structure. More importantly, introducing micro-emulsions of foaming agents—imidazolium compounds—into phenolic resin-based polymers enhances the strength of the polymer while reducing the material density. Drying can be completed under normal pressure, making the operation simple and energy-saving. After drying, the foamed carbon material hardly shrinks and does not crack.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A low-density monolithic porous carbon comprises a three-dimensional network structure formed by the cross-linking and stacking of hollow carbon nanoparticles. This unique network framework structure endows the monolithic foamed carbon with good structural properties and a low density of 0.16–0.35 g / cm³. 3 The monolithic porous carbon is rod-shaped, tubular, or plate-shaped. Its strength is 0.3–1 MPa; further, its density is 0.16–0.25 g / cm³. 3 The strength is 0.8 to 1 MPa.

[0009] The low-density monolithic porous carbon is phenolic resin-based foamed carbon.

[0010] The diameter of the hollow carbon nanoparticles is 200–600 nm.

[0011] This invention provides a method for preparing a microemulsion of the low-density monolithic porous carbon, comprising the following steps:

[0012] S1 involves dissolving an imidazole compound and a blowing agent in a solvent at room temperature to form a homogeneous and transparent blowing agent-imidazole compound solution; the blowing agent is oleic acid or sodium oleate.

[0013] S2 adds phenol and aldehyde to a solvent and stirs to form a uniform, colorless, and transparent phenol-aldehyde solution; then adds the phenol-aldehyde solution to a foaming agent-imidazolium compound solution and continues stirring until a pale yellow and transparent sol solution is formed.

[0014] S3. Pour the sol solution into a sealed container, place it in an oven for polymerization and aging, and then remove the polymer and cool it to room temperature for drying.

[0015] S4 The dried polymer (dry gel polymer) is placed under an inert gas protection condition for high-temperature pyrolysis, and oleic acid is decomposed and foamed at 200-400℃ to obtain the low-density monolithic porous carbon.

[0016] The polymer does not shrink in volume during the drying process; when the dried polymer is subjected to high-temperature pyrolysis, no structural collapse occurs, and the radial shrinkage rate is only 18-22%.

[0017] The imidazole compound mentioned in step S1 is one or more selected from imidazole, 1-methylimidazolium, 2-methylimidazolium, 1-ethylimidazolium, 2-ethylimidazolium, 1-propylimidazolium, 2-propylimidazolium, and 1-butylimidazolium. The concentration of the imidazole compound in the sol solution mentioned in step S2 is 0.001–0.5 mol / L. -1 The molar ratio of the imidazole compound to the foaming agent is 1:1 to 1:5.

[0018] The molar mass ratio of phenol to aldehyde is 1:2 to 1:6.

[0019] The molar ratio of the imidazole compound to the phenol is 3:1 to 15:1.

[0020] The solvents mentioned in steps S1 and S2 are water, methanol, ethanol, isopropanol, or N,N-dimethylformamide.

[0021] The phenol mentioned in step S2 is phenol, resorcinol, hydroquinone, catechol, cresol, or bisphenol A.

[0022] The aldehyde mentioned in step S2 is formaldehyde, acetaldehyde, or furfural.

[0023] The polymerization aging temperature in step S3 is 50–120°C, and the aging time is 2–12 h.

[0024] In step S3, the drying method is either atmospheric pressure drying or freeze drying, and more specifically, atmospheric pressure drying is selected.

[0025] In step S4, the inert protective gas in the pyrolysis process is nitrogen or argon, and the pyrolysis temperature is 600-1000℃, with argon being the preferred choice.

[0026] The present invention also provides the application of the low-density monolithic porous carbon in the recovery of low-density waste oil.

[0027] The present invention also provides the application of the low-density monolithic porous carbon in flame-retardant and heat-insulating materials.

[0028] The present invention also provides the application of the low-density monolithic porous carbon in the electrocatalytic CO2 reduction reaction, wherein the electrocatalyst is based on the low-density monolithic porous carbon as a carrier. First, a nickel and ethylenediamine complex is introduced by an equal-volume impregnation method. After drying, the mixture of metal complex and carbon is successively subjected to pyrolysis under an inert atmosphere, acid washing to remove metal particles, water washing to neutrality, and drying to obtain a nickel-nitrogen-carbon electrocatalyst.

[0029] The beneficial effects of this invention are:

[0030] 1. This invention uses phenol and aldehyde as raw materials for phenolic resin-based polymers, and introduces oleic acid or sodium oleate as a foaming agent to improve the macroscopic mechanical properties and microscopic porous structure of the material. Due to interfacial tension, oleic acid or sodium oleate cannot directly form uniformly dispersed droplets in a solvent. By innovatively introducing imidazole compounds as catalysts and foaming agents, oleic acid microemulsions were successfully dispersed in the solvent. Imidazole compounds can also catalyze the polymerization of phenol and aldehyde on the outside of the microemulsions, forming an encapsulation layer. Therefore, the microstructure exhibits a spherical particle shape and generates particle cross-linking, forming a three-dimensional network structure. The synthesis reaction steps of this system are simple, without high-temperature or high-pressure processes, ensuring high safety, a short synthesis cycle, and overall system homogeneity, possessing potential for large-scale application.

[0031] 2. Dry gel polymer materials have good compressive elasticity and can adapt and transform during linear buckling to withstand local strain while maintaining their structural integrity. Combined with the material's own structural characteristics that are not easy to collapse, tubular integral porous carbon can be obtained by pyrolyzing tubular polymers.

[0032] 3. The introduction of oleic acid or sodium oleate and imidazole compounds as reinforcing phases significantly improves the mechanical properties of phenolic-derived charcoal, achieving a bulk compressive strength of 0.8–1 MPa. The organic aerogel (polymer) is dried under normal pressure, resulting in no shrinkage or cracking of the bulk material. The enhanced mechanical properties resist capillary forces generated during aqueous phase evaporation, inhibiting pore structure collapse during drying. The prepared monolithic foamed charcoal is lightweight, with a density as low as 0.16–0.25 g / cm³. 3The volume shrinkage during pyrolysis is only about 20%, and its unique structural characteristics give the material excellent application performance.

[0033] 4. Studies have found that the thermal conductivity of solids is affected by various factors such as density, pore size, and particle size. The foam (low-density monolithic porous carbon) carbon prepared in this invention benefits from: 1) its structure consists of cross-linked hollow carbon particles with a particle size of 200-600 nm and a low density, thus significantly reducing the overall heat transfer capacity of the carbon skeleton; 2) the nanopores, mainly composed of micropores and mesopores, can limit the thermal conductivity of gases, thus exhibiting good thermal insulation and flame retardant properties.

[0034] 5. The foamed carbon (low-density monolithic porous carbon) prepared by this invention has a strong hydrophobic surface and is less affected by the acidity or alkalinity of the liquid, and can be applied to the recovery of low-density waste oil in water.

[0035] 6. Thanks to the synergistic effect of porous structure and surface hydrophobicity, the transition metal and nitrogen co-doped electrocatalyst prepared by the foamed carbon of the present invention has a good three-phase interface microenvironment required for gas-consuming reactions, and exhibits high CO selectivity (greater than 90%) with a wide potential in CO2 reduction reaction. Attached Figure Description

[0036] Figure 1 The image shows the actual product of the dry gel polymer and foamed carbon A in Example 1.

[0037] Figure 2 The graph shows the strength test results of foamed carbon A in Example 1.

[0038] Figure 3 This is a scanning electron microscope image of foamed carbon A from Example 1.

[0039] Figure 4 This is a transmission electron microscope (TEM) image of foamed carbon A from Example 1.

[0040] Figure 5 This is a diagram illustrating the flame retardant properties of foamed charcoal A in Example 1.

[0041] Figure 6 This is a diagram illustrating the thermal insulation performance of foamed carbon A in Example 1.

[0042] Figure 7 This is a contact angle diagram of foamed carbon A at different pH values ​​in Example 1.

[0043] Figure 8 This is a diagram showing the effect of foamed carbon A in Example 1 on the adsorption of n-hexane in water.

[0044] Figure 9 The figure shows the CO Faradaic efficiency results of the nickel-nitrogen-carbon electrocatalyst derived from foamed carbon A in Example 1.

[0045] Figure 10 This is a photograph of the tubular foamed carbon L from Example 12. Detailed Implementation

[0046] The present invention will be described in detail below through some representative examples. It should be understood that the following examples are merely illustrative and should not be construed as limiting the protection of this invention. Any technical solutions implemented based on the content described in this invention should be covered within the protection scope of this invention.

[0047] Example 1

[0048] 1. Preparation of foamed carbon material: First, weigh 100 mg of 2-methylimidazole. Add 2-methylimidazole and oleic acid to 4 mL of water and stir until a transparent solution is formed, obtaining an oleic acid-2-methylimidazole aqueous solution with a molar ratio of 1:3. Prepare 4 mL of 0.1 mol / L resorcinol aqueous solution and add 0.15 g of 37 wt.% formaldehyde aqueous solution, stirring until a colorless and transparent solution is formed, obtaining a phenolic solution. Add the prepared phenolic solution to the oleic acid-2-methylimidazole aqueous solution and mix until a pale yellow transparent sol solution is formed. Pour the solution into a quartz tube and seal it. Place the tube in a 70℃ oven for polymerization reaction for 12 h. After removing the polymer, first cool it to room temperature, then place it in a 50℃ oven for normal pressure drying, with no volume shrinkage. Place the dried gel polymer in a tube furnace under an argon protective atmosphere for high-temperature pyrolysis, raising the temperature to 800℃ and maintaining it for 2 h, then allowing it to cool naturally to room temperature. Named Foamed Carbon A, it does not experience structural collapse during pyrolysis and has a radial shrinkage rate of only 20%.

[0049] 2. Preparation of electrocatalyst using foamed carbon A as support: A solution of nickel nitrate and ethylenediamine complex was prepared, with the mass percentage of nickel and carbon support being 5 wt.%. The metal was introduced into foamed carbon A by equal volume impregnation. After drying the mixture, it was pyrolyzed to 1000℃ under an argon atmosphere. The mixture was then acid-washed in 4M hydrochloric acid to remove metal particles, filtered and washed with water until neutral, and dried overnight at 50℃ to obtain a nickel-nitrogen-carbon electrocatalyst.

[0050] 3. The nickel-nitrogen-carbon catalyst is applied to the electrocatalytic CO2 reduction reaction, comprising the following steps:

[0051] (1) Electrocatalytic CO2 performance test: H-type electrolytic cell and three-electrode system were used. The cathode chamber contained the working electrode (nickel-nitrogen-carbon electrocatalyst) and the reference electrode (saturated Ag / AgCl electrode). The anode chamber was the counter electrode (platinum sheet electrode). The two chambers were separated by a Nafion 117 proton exchange membrane. The electrolyte was 0.5M KHCO3. The test potential window was -0.6 to -1.0V (relative to the reversible hydrogen electrode).

[0052] (2) Product detection: The electrocatalytic CO2 reduction products were detected and analyzed online using Agilent 7890B gas chromatography and nuclear magnetic resonance hydrogen spectroscopy. The electrocatalytic CO2 reduction product of the nickel-nitrogen-carbon catalyst prepared in this invention is CO, and there are no liquid phase products.

[0053] 4. The structure of the foamed carbon A prepared in Example 1 was characterized and its application was tested:

[0054] Depend on Figure 1 It is known that the dry gel polymer and its derived foamed carbon A both exhibit high uniformity, enabling the large-scale preparation of monolithic foamed carbon with a density of only 0.16 g / cm³. 3 , Figure 1 b and 1c also demonstrate that the foamed carbon A has a low density. Figure 2 It is known that although the foamed carbon A has a low density, it can still exhibit good mechanical strength within a certain range, with a bulk compressive strength reaching 1 MPa. Figure 3 and Figure 4 It is known that the foamed carbon A has a continuous three-dimensional network structure, composed of cross-linked particles of uniform size with a particle diameter of 300 nm. The monomers are carbon spheres with hollow cavities, stacked to form micron-level interconnected pores. The foamed carbon A has a well-developed hierarchical structure, and nitrogen adsorption results indicate that its specific surface area is 664 m². 2 / g, predominantly microporous structure, with mesopores accounting for 40%, the hierarchical pores and uniform three-dimensional framework work synergistically to reduce the overall heat transfer coefficient of the material. Figure 5 It is known that when the foamed charcoal A is placed directly above a candle flame for 60 seconds, the material shows no obvious structural damage and remains intact. Figure 6 It is known that during the first 30 seconds of burning, the upper and lower surfaces of the material can maintain a temperature difference of about 100°C. Figure 6 As shown in Figure b, placing foamed carbon A on the hand, with the upper part of foamed carbon A maintaining the ambient temperature, demonstrates its heat insulation properties. Figure 7 It is known that the foamed carbon A has good surface hydrophobicity, with a contact angle of 160° in pure water. When the solution pH is 1 or 14, the contact angle remains essentially unchanged, indicating that the surface hydrophobicity of the foamed carbon A is suitable for a wide range of acidic and alkaline conditions. A piece of the foamed carbon A was used for hexane recovery from water. Figure 8 It is known that the adsorption capacity of n-hexane is 8.5g. 正己烷 / g 炭 Furthermore, the electrocatalytic CO2 reduction reaction using a nickel-nitrogen-carbon electrocatalyst benefits from the synergistic effect of its porous structure and hydrophobic surface. Figure 9It is known that the nickel-nitrogen-carbon electrocatalyst prepared by the present invention forms a good three-phase interface microenvironment required for gas-consuming reactions, exhibiting high CO selectivity (greater than 90%) with a wide potential range in the electrocatalytic CO2 reduction reaction.

[0055] Example 2

[0056] In Example 1, the raw material 2-methylimidazole was replaced with imidazole, while the rest, including the synthesis of foamed carbon, remained the same as in Example 1. The material was named Foamed Carbon B.

[0057] Example 3

[0058] In Example 1, the 2-methylimidazole raw material was replaced with 1-methylimidazole, while the rest, including the synthesis of foamed carbon, remained the same as in Example 1. The material was named foamed carbon C.

[0059] Example 4

[0060] In Example 1, the raw material 2-methylimidazole was replaced with 2-ethylimidazole, while the rest, including the synthesis of foamed carbon, remained the same as in Example 1. The material was named foamed carbon D.

[0061] Example 5

[0062] In Example 1, the raw material 2-methylimidazole was replaced with 2-propylimidazole, while the rest, including the synthesis of foamed carbon, remained the same as in Example 1. The material was named foamed carbon E.

[0063] Example 6

[0064] The amount of 2-methylimidazole used in Example 1 was doubled, while the rest, including the synthesis of foamed carbon, remained the same as in Example 1. The material was named foamed carbon F.

[0065] Example 7

[0066] In Example 1, the polymerization time at 70°C was extended from 2 hours to 12 hours. The remaining steps, including the synthesis of carbon foam, were the same as in Example 1. The material was named Carbon Foam G.

[0067] Example 8

[0068] The drying method in Example 11 was changed to freeze drying, while the rest, including the synthesis of foamed carbon, remained the same as in Example 1. The material was named foamed carbon H.

[0069] Example 9

[0070] The carbonization temperature in Example 1 was changed to 600°C, while the rest, including the synthesis of foamed carbon, remained the same as in Example 1. The material was named Foamed Carbon I.

[0071] Example 10

[0072] The carbonization temperature in Example 1 was changed to 1000℃, while the rest, including the synthesis of foamed carbon, remained the same as in Example 1. The material was named foamed carbon J.

[0073] Example 11

[0074] In Example 1, the amount of 2-methylimidazole used in step 1 was halved, while the rest, including the synthesis of foamed carbon, remained the same as in Example 1. The material was named foamed carbon K.

[0075] Example 12

[0076] The pale yellow transparent sol solution from Example 1 was poured into a quartz tube mold with a central cylinder and sealed. The rest of the process, including the synthesis of foamed carbon, was the same as in Example 1. The material was named Foamed Carbon L, and its physical image is shown below. Figure 10 As shown.

[0077] Comparative Example 1

[0078] In Example 1, the 2-methylimidazole raw material was replaced with hexadecyltrimethylammonium bromide, while the rest of the process, including the synthesis of the foamed charcoal, remained the same as in Example 1. The prepared polymer was inelastic, brittle, and could not be demolded to form a tubular structure. It exhibited significant shrinkage before and after drying, and the derived foamed charcoal had a relatively high density of 0.88 g cm³. -3 .

[0079] The structural information of the foamed carbon prepared in Examples 1 to 12 is shown in Table 1.

[0080] Table 1 shows the structural information of the foamed carbon materials described in Examples 1-10.

[0081]

[0082] Conclusion: Addressing the scientific challenges in the synthesis of phenolic resin-based foamed carbon materials, this novel material preparation system, based on a flexible microemulsion method (surfactant-free), aims to improve the mechanical properties of phenolic resin-based foamed carbon. Using phenol and aldehyde as polymerization raw materials, it innovatively introduces oleic acid / sodium oleate as a foaming agent and imidazole compounds as stabilizers and catalysts, achieving the preparation of a low-density monolithic porous carbon under atmospheric pressure. The results from the above examples show that the imidazole compounds in this invention can reduce the interfacial tension between the two phases, effectively stabilizing the oleic acid emulsion droplets, and exhibiting a certain catalytic effect, promoting phenolic polymerization on the outside of the droplets to form a polymer encapsulation layer structure and generating cross-links, further forming a three-dimensional network structure. During high-temperature pyrolysis, oleic acid undergoes pyrolysis and foaming, creating a cavity structure, resulting in foamed carbon with a good hierarchical porous structure. Furthermore, the introduction of the reinforcing phase improves the mechanical properties of the material, resisting the capillary forces generated by phase transformation during drying. The use of an atmospheric pressure drying process also maintains structural integrity and reduces the complexity of the preparation process. The main objective of this invention is to provide a low-density monolithic porous carbon and its microemulsion preparation method. The method not only effectively improves the structural properties of the foamed carbon, but also shows good application potential in fields such as electrocatalysis, heat insulation and flame retardancy, and oily wastewater purification.

Claims

1. A low-density monolithic porous carbon, characterized in that: It includes a three-dimensional network structure composed of cross-linked stacked hollow carbon nanoparticles, with a density of 0.16~0.35 g / cm³. 3 The low-density monolithic porous carbon is rod-shaped, tubular, or plate-shaped; the diameter of the hollow carbon nanoparticles is 200~600 nm.

2. The low-density monolithic porous carbon as described in claim 1, characterized in that: The strength of the low-density monolithic porous carbon is 0.3~1 MPa.

3. A method for preparing a microemulsion of low-density monolithic porous carbon as described in claim 1, characterized in that: Includes the following steps: S1 Under room temperature conditions, an imidazole compound and a foaming agent are dissolved in a solvent to form a homogeneous and transparent foaming agent-imidazole compound solution; the foaming agent is oleic acid or sodium oleate. S2 At room temperature, phenol and aldehyde are added to a solvent and stirred to form a uniform, colorless, and transparent phenol-aldehyde solution; then the phenol-aldehyde solution is added to a foaming agent-imidazolium compound solution and stirred until a pale yellow transparent sol solution is formed. S3 Pour the sol solution into a sealed container, place it in an oven for polymerization aging, and then remove the polymer and cool it to room temperature for drying. S4 The dried polymer is placed under inert gas protection for high-temperature pyrolysis to obtain the low-density monolithic porous carbon.

4. The method for preparing a microemulsion of low-density monolithic porous carbon as described in claim 3, characterized in that: The imidazole compound is one or more selected from imidazole, 1-methylimidazolium, 2-methylimidazolium, 1-ethylimidazolium, 2-ethylimidazolium, 1-propylimidazolium, 2-propylimidazolium, and 1-butylimidazolium, and the concentration of the imidazole compound in the sol solution is 0.001~0.5 mol / L. -1 The molar ratio of the imidazole compound to the foaming agent is 1:1 to 1:

5.

5. The method for preparing a microemulsion of low-density monolithic porous carbon as described in claim 3, characterized in that: The molar mass ratio of the phenols, aldehydes and imidazoles is 1:2~6:3~15.

6. The method for preparing a microemulsion of low-density monolithic porous carbon according to claim 3, characterized in that: The polymerization aging temperature in step S3 is 50~120 ℃, and the aging time is 2~12 h; the pyrolysis temperature in step S4 is 600~1000 ℃.

7. The application of the low-density monolithic porous carbon according to claim 1 in the recovery of low-density waste oil.

8. The application of the low-density monolithic porous carbon as described in claim 1 in flame-retardant and heat-insulating materials.

9. The application of the low-density monolithic porous carbon according to claim 1 in the CO2 electroreduction reaction.

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