Preparation method and application of porous carbon with bimodal pore size distribution

Abstract

The present invention relates to a method for preparing a bimodal pore size distribution porous carbon, comprising the following steps: (S1) polyacrylonitrile-polyvinyl alcohol diblock copolymer (PAN-PVA), carboxyl-containing polypyridine and a stabilizer are uniformly mixed, heated for reaction, and modified polyacrylonitrile is obtained; the PVA segment portion accounts for 8-15wt% in the PAN-PVA; the mass ratio of PAN-PVA to carboxyl-containing polypyridine is 100:(20-40); (S2) modified polyacrylonitrile and polyvinyl pyrrolidone are dissolved in an organic solvent at a mass ratio of 100:(10-30) to form a polymer solution; the solution is then added dropwise to water, and solid pellets are precipitated; (S3) the dried solid pellets are mixed with an inorganic base and then carbonized to obtain a bimodal pore size distribution porous carbon. The pore diameters of the bimodal pore size distribution porous carbon obtained by the present invention are concentrated in 2-3nm and 9-12nm, respectively, and have a high specific surface area.

Description

Technical Field

[0001] The present invention belongs to the technical field of porous carbon, and in particular relates to a preparation method and application of porous carbon with bimodal pore size distribution. Background Art

[0002] Precious metal resources are scarce and expensive. Precious metals in secondary resources primarily come from electronic waste, printed circuit boards, and wastewater generated during jewelry processing. Therefore, recycling precious metal secondary resources is of great significance from the perspective of resource conservation and environmental protection. Commonly used methods for precious metal recovery include ion exchange, extraction, chemical precipitation, electrolysis, and adsorption. Adsorption is more efficient and environmentally friendly than other methods. Porous carbon, due to its abundant pores and high specific surface area, is currently the primary adsorbent used in adsorption methods. However, conventional porous carbon currently has an uneven pore distribution and a single pore size, resulting in insufficient adsorption capacity for precious metals. According to the International Union of Pure and Applied Chemistry, pores with a diameter less than 2 nm are considered micropores, pores with a diameter greater than 50 nm are considered macropores, and pores in between are called mesopores. Porous carbon with two concentrated distributions of the above-mentioned different pore size ranges (double peaks appearing in the pore size distribution diagram) is a bimodal pore size distribution porous carbon. The synergistic effect of its pores of different sizes can overcome the shortcomings of a single pore structure, so its specific surface area and volume utilization are higher, and it also shows greater advantages in the fields of secondary resource recovery of precious metals.

[0003] Currently, the main methods for preparing porous carbon with bimodal pore size distribution are activation method and template method. The activation method can prepare porous carbon with a higher specific surface area, but the pore size distribution is generally wide. The template method is divided into inorganic template method and organic template method. In the inorganic template method, the carbon precursor is introduced into the molecular sieve template, and the bimodal porous carbon is prepared through carbonization and template removal. This method requires the synthesis of molecular sieve templates. The preparation process of the template itself is complicated, the raw materials used are expensive, and the adjustable range of pore size is limited. The organic template method is to mix two organic compounds with different thermal stability and prepare bimodal porous carbon only through carbonization. This method has a simple preparation process, but the pore size distribution is not concentrated and the pore distribution is uneven.

[0004] Wang Yu et al. (Acta Physico-Chimica Sinica, 2012, 28: 1525-1532) used long rod-shaped γ-alumina with a large difference between the stacked pore size and its own diameter as a template and in situ polymerized phenolic resin as a carbon source. They prepared a thin-walled carbon material with a bimodal pore distribution structure through polymerization-carbonization. The two peaks were located in the smaller mesopore region near 4 nm and the larger mesopore region near 13 nm, respectively. However, the pore size distribution near 4 nm was not concentrated. Summary of the Invention

[0005] In view of the above problems, the object of the present invention is to provide a method for preparing porous carbon with a bimodal pore size distribution, wherein the prepared porous carbon with a bimodal pore size distribution has a concentrated pore size distribution and has a high porosity and specific surface area.

[0006] The present invention adopts the following technical solutions to achieve the above purpose:

[0007] A method for preparing porous carbon with a bimodal pore size distribution comprises the following steps:

[0008] (S1) uniformly mixing a polyacrylonitrile-polyvinyl alcohol diblock copolymer (PAN-PVA), a carboxyl-containing polypyridine, and a stabilizer, and heating the mixture to react to obtain a modified polyacrylonitrile; wherein the PVA segment portion in the PAN-PVA accounts for 8 to 15 wt %; and the mass ratio of the PAN-PVA to the carboxyl-containing polypyridine is 100:(20 to 40);

[0009] (S2) dissolving modified polyacrylonitrile and polyvinyl pyrrolidone in an organic solvent at a mass ratio of 100:(10-30) to form a polymer solution; dripping the polymer solution into water at room temperature to precipitate solid pellets, filtering and drying to obtain solid pellets;

[0010] (S3) mixing the solid pellets with an inorganic base, carbonizing the mixture under an inert atmosphere, cooling, washing with water, and drying the mixture to obtain a porous carbon having a bimodal pore size distribution.

[0011] The diblock copolymer of polyacrylonitrile and polyvinyl alcohol (PAN-PVA) is a hydroxyl-containing linear AB-type block polymer, with A being the polyacrylonitrile segment and B being the polyvinyl alcohol segment. When PAN-PVA is modified with carboxyl-containing polypyridine, the carboxyl groups in the carboxyl-containing polypyridine undergo esterification with the hydroxyl groups in the B segment, forming an interpenetrating crosslinked structure. This interpenetrating crosslinked structure also contains a polypyridine structure, where the pyridine groups are nitrogen-containing heterocyclic structures. This polypyridine structure exhibits relatively good thermal stability. The resulting modified polyacrylonitrile is a linear-stereomeric block polymer, with the linear molecular chains being the polyacrylonitrile segments and the stereomeric portions being crosslinked structures containing polypyridine. The combined effect of the polypyridine structure and the interpenetrating crosslinked structure imparts improved thermal stability to the modified polyacrylonitrile. Modified polyacrylonitrile, which has greater thermal stability, decomposes above 400°C, while polyvinylpyrrolidone decomposes at a lower temperature, between 250 and 300°C. In other words, during the subsequent carbonization stage, the two decompose at different temperatures, producing small molecules that escape from the matrix. This leads to hierarchical pore formation and the formation of porous carbon with two distinct pore size ranges, known as bimodal pore size distribution. Because modified polyacrylonitrile is an in-situ linear-block polymer, it achieves molecular-scale segmentation and crosslinking, resulting in a more concentrated pore size distribution and high porosity.

[0012] The proportion of PVA segments and the proportion of carboxyl-containing polypyridine in PAN-PVA must be controlled within a certain range. If the dosage is too low, the interpenetrating cross-linking structure will have fewer connection points, the thermal stability of the modified polyacrylonitrile will be limited, and the graded pore formation will not be obvious; if the dosage is too high, the overall carbonization rate of the product will be reduced.

[0013] Furthermore, in step (S1), the carboxyl-containing polypyridine is at least one of poly(pyridine-2-acetic acid), poly(pyridine-3-acetic acid), poly(pyridine-4-acetic acid), poly(pyridine-2-carboxylic acid), poly(pyridine-3-carboxylic acid), poly(pyridine-4-carboxylic acid), and poly(6-methoxy-3-pyridineacetic acid); and its number average molecular weight is 3000 to 5000.

[0014] Furthermore, in step (S1), the number average molecular weight of the PAN-PVA is 50,000 to 80,000, and the PAN-PVA can be homemade or commercially available.

[0015] Furthermore, in step (S1), the stabilizer is trimethyl phosphate, and its usage is 0.1 to 0.3 wt% of PAN-PVA.

[0016] Furthermore, in step (S1), the heating reaction is: adding PAN-PVA and carboxyl-containing polypyridine into a horizontal twin-screw mixer, heating to 50-70°C, and mixing at medium speed for 20-40 minutes; then adding a stabilizer thereto, heating to 180-190°C, and mixing at high speed for 30-50 minutes.

[0017] Furthermore, in step (S1), the carboxyl-containing polypyridine is prepared by a method comprising the following steps:

[0018] (L1) dissolving a pyridinate ester in an organic solvent, adding an oxidant under nitrogen protection, and stirring the reaction at 0-5°C for 5-8 hours to obtain a reactant; then pouring the reactant into an alcohol solvent, allowing it to stand, filtering, washing, and drying to obtain a polypyridinate ester;

[0019] (L2) adding polypyridinium ester to an aqueous solution of an inorganic base and performing a hydrolysis reaction at 90-100° C.; then adding hydrochloric acid to obtain a precipitate, filtering, washing, and drying to obtain a carboxyl-containing polypyridine.

[0020] Preferably, in step (L1), the molar ratio of the pyridinic acid ester to the oxidant is 1:(1.2-2.5).

[0021] Preferably, in step (L1), the pyridinium ester is at least one of methyl pyridine-2-acetate, methyl pyridine-3-acetate, methyl polypyridine-4-acetate, ethyl pyridine-2-carboxylate, ethyl pyridine-3-carboxylate, ethyl pyridine-4-carboxylate, and methyl 6-methoxy-3-pyridineacetate; the organic solvent is at least one of chloroform, carbon tetrachloride, and dichloromethane; the oxidant is at least one of FeCl3, MoCl5, and RuCl3; the alcohol solvent is at least one of methanol, ethanol, and isopropanol; and the washing and drying is washing with water 2 to 3 times and then vacuum drying at 50 to 60°C for 10 to 24 hours.

[0022] Preferably, in step (L2), the molar ratio of the pyrrolidone group to the inorganic base in the polypyrrolidone is 1:(1.1-1.3).

[0023] Preferably, in step (L2), the inorganic base is NaOH or KOH, and the concentration of its aqueous solution is 1-2 mol / L; the hydrolysis reaction is a hydrolysis reaction for 12-24 hours; the concentration of the hydrochloric acid solution is 1-2 mol / L, and the amount of hydrochloric acid used is adjusted to a pH of 5-6; the washing and drying is washing with water 2-3 times and then vacuum drying at 50-60°C for 10-24 hours.

[0024] Furthermore, in step (S2), the number average molecular weight of the polyvinyl pyrrolidone is 1000 to 5000; the organic solvent is at least one of N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA); and the concentration of the polymer solution is 10 wt% to 15 wt%. A high polymer concentration makes it difficult to drip, while a low polymer concentration makes it difficult to form solid pellets, resulting in a low yield. Dissolving and mixing the modified polyacrylonitrile and polyvinyl pyrrolidone before forming solid pellets ensures thorough mixing of the two at the molecular level, which helps improve pore uniformity and specific surface area during subsequent carbonization.

[0025] Furthermore, in step (S2), the polymer solution is added at a rate of 10 to 20 mL / h. The addition rate affects the particle size of the solid pellets. The slower the addition rate, the smaller the particle size of the solid pellets. The smaller the particle size of the solid pellets, the smaller the pore size formed after carbonization, the larger the pore volume, and the larger the specific surface area. Conversely, the larger the particle size, the smaller the specific surface area of ​​the porous carbon obtained. Considering efficiency and performance, the addition rate is controlled within a certain range.

[0026] Furthermore, in step (S3), the mass ratio of the solid pellets to the inorganic base is 100:(30-50), and the inorganic base is KOH and / or NaOH. The inorganic base, as an activating agent, has strong dehydration and intercalation effects, which facilitates the formation of a rich pore structure in the porous carbon, thereby increasing its specific surface area.

[0027] Furthermore, in step (S3), the inert atmosphere is at least one of nitrogen, argon, and helium; and the carbonization conditions are: heating to 700-900°C at a heating rate of 3-8°C / min and keeping warm for 3-8h.

[0028] Furthermore, in step (S3), the water washing is performed by using deionized water to neutrality; and the drying is performed in an oven at 80 to 120° C. for 24 to 36 hours.

[0029] In a second aspect, the present invention provides the use of the bimodal pore size distribution porous carbon prepared by the above preparation method in the recovery of precious metals.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The present invention uses carboxyl-containing polypyridine to modify PAN-PVA to produce a modified polyacrylonitrile with a linear-block structure. This modified polyacrylonitrile has better thermal stability and decomposes at temperatures above 400°C. Polyvinylpyrrolidone, on the other hand, decomposes at a lower temperature, between 250°C and 300°C. During the subsequent carbonization stage, the two decompose at different temperatures to produce small molecules that escape from the matrix, thereby achieving hierarchical pore formation, i.e., forming a porous carbon with a bimodal pore size distribution. Because the modified polyacrylonitrile is an in-situ formed linear-block polymer, molecular-scale segmentation and crosslinking are achieved, resulting in a more concentrated pore size distribution and a high porosity.

[0032] (2) The pore sizes of the bimodal pore size distribution porous carbon prepared by the present invention are concentrated in 2-3 nm and 9-12 nm, respectively, and have high porosity and specific surface area; the two pores of different sizes work synergistically to show strong adsorption performance for precious metals. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the pore size distribution diagram of the bimodal pore size distribution porous carbon prepared in Example 1. DETAILED DESCRIPTION

[0034] In order to better explain the present invention, the following description will be made in detail with reference to the embodiments of the present invention, and the main contents of the present invention will be further clarified in conjunction with specific examples. However, the contents of the present invention are not limited to the following examples.

[0035] Unless otherwise specified, the "parts" in the examples of the present invention are all parts by weight. All reagents used are commercially available reagents in the art.

[0036] Polyacrylonitrile-polyvinyl alcohol diblock copolymer (PAN-PVA) was purchased from Xi'an Ruixi Biotechnology Co., Ltd., with the PVA segment accounting for about 12 wt % and the number average molecular weight of PAN-PVA being about 65,000.

[0037] Polyvinyl pyrrolidone was purchased from Aladdin Reagent Co., Ltd., and its number average molecular weight was about 3,000.

[0038] Polyacrylonitrile (PAN) was purchased from Beijing Bailingwei Technology Co., Ltd., with a number average molecular weight of approximately 58,000.

[0039] Preparation of carboxyl-containing polypyridine

[0040] Preparation Example 1

[0041] (L1) 4.53 g (30 mmol) of methyl pyridine-2-acetate was dissolved in 200 mL of chloroform. Under nitrogen protection, 9.73 g (60 mmol, i.e., the molar ratio of pyridine ester to oxidant was 1:2) of anhydrous FeCl3 was added. The mixture was stirred at 2±1°C for 6 h to obtain a reactant. The reactant was then poured into ethanol, allowed to stand, filtered, washed twice with pure water, and dried at 60°C for 12 h to obtain poly(methyl pyridine-2-acetate).

[0042] (L2) All the poly(methyl pyridine-2-acetate) prepared in step (L1) was added to 36 mL of a 1 mol / L aqueous solution of NaOH (36 mmol of NaOH) and hydrolyzed at 100°C for 12 h; then, a 1 mol / L hydrochloric acid solution was added to adjust the pH of the system to 5.5 to obtain a precipitate, which was filtered, washed twice with pure water, and dried at 60°C for 12 h to obtain poly(pyridine-2-acetic acid).

[0043] The number average molecular weight of poly(pyridine-2-acetic acid) was determined to be about 4,000 by gel permeation chromatography.

[0044] Preparation Example 2

[0045] (L1) 4.53 g (30 mmol) of ethyl pyridine-3-carboxylate was dissolved in 200 mL of chloroform. Under nitrogen protection, 9.84 g (36 mmol, i.e., the molar ratio of pyridine ester to oxidant was 1:1.2) of anhydrous MoCl5 was added. The mixture was stirred at 0°C for 5 h to obtain a reactant. The reactant was then poured into an ethanol solvent, allowed to stand, filtered, washed twice with pure water, and dried at 60°C for 12 h to obtain poly(ethyl pyridine-3-carboxylate);

[0046] (L2) All the poly(ethyl pyridine-3-carboxylate) prepared in step (L1) was added to 36 mL of a 1 mol / L aqueous NaOH solution (36 mmol of NaOH) and hydrolyzed at 100°C for 12 h; then, a 1 mol / L hydrochloric acid solution was added to adjust the pH of the system to 5.5 to obtain a precipitate, which was filtered, washed twice with pure water, and dried at 60°C for 12 h to obtain poly(pyridine-3-carboxylic acid).

[0047] The number average molecular weight of poly(pyridine-3-carboxylic acid) was determined to be about 2500 by gel permeation chromatography.

[0048] Preparation Example 3

[0049] The rest of the reaction was the same as in Example 1, except that in step (L1), methyl pyridine-4-acetate was used instead of methyl pyridine-2-acetate, and the reaction was stirred at 4±1°C for 8 h; poly(pyridine-4-acetic acid) was finally obtained. The number average molecular weight of poly(pyridine-4-acetic acid) was determined by gel permeation chromatography to be approximately 5500.

[0050] Example 1

[0051] (S1) adding 100 parts of polyacrylonitrile-polyvinyl alcohol diblock copolymer (PAN-PVA) and 20 parts of poly(pyridine-2-acetic acid) obtained in Preparation Example 1 to a horizontal twin-screw mixer, heating to 50-70° C., and mixing at a medium speed for 30 minutes, then adding 0.1 parts of trimethyl phosphate as a stabilizer, heating to 180° C., and mixing at a high speed for 40 minutes, and extruding and cooling to obtain modified polyacrylonitrile;

[0052] (S2) dissolving 100 parts of modified polyacrylonitrile and 10 parts of polyvinylpyrrolidone in 800 parts of N,N-dimethylformamide (DMF) to form a polymer solution with a concentration of 12 wt %; transferring the polymer solution to a propeller and dripping it into water at room temperature at a rate of 15 mL / h to precipitate solid pellets, which were filtered and dried to obtain solid pellets;

[0053] (S3) After mixing 100 parts of solid pellets with 30 parts of NaOH, the mixture was heated to 700°C at a rate of 5°C / min under an argon atmosphere and kept warm for 6 hours; carbonized, cooled, washed with deionized water until neutral, and dried in an oven at 100°C for 24 hours to obtain a porous carbon with a bimodal pore size distribution.

[0054] Example 2

[0055] The rest is the same as Example 1, except that in step (S1), the poly(pyridine-3-carboxylic acid) prepared in Preparation Example 2 is used instead of the poly(pyridine-2-acetic acid) prepared in Preparation Example 1.

[0056] Example 3

[0057] The rest is the same as Example 1, except that in step (S1), the poly(pyridine-4-acetic acid) prepared in Preparation Example 3 is used instead of the poly(pyridine-2-acetic acid) prepared in Preparation Example 1.

[0058] Example 4

[0059] The rest is the same as implementation 1, except that:

[0060] The amounts of the materials used in step (S1) are different, specifically: 100 parts of PAN-PVA, 30 parts of poly(pyridine-2-acetic acid) prepared in Preparation Example 1;

[0061] The amounts of the materials used in step (S1) are different, specifically: 100 parts of modified polyacrylonitrile and 20 parts of polyvinyl pyrrolidone.

[0062] Example 5

[0063] The rest is the same as implementation 1, except that:

[0064] The amounts of the materials used in step (S1) are different, specifically: 100 parts of PAN-PVA, 30 parts of poly(pyridine-2-acetic acid) prepared in Preparation Example 1;

[0065] The amounts of the materials used in step (S1) are different, specifically: 100 parts of modified polyacrylonitrile and 30 parts of polyvinyl pyrrolidone.

[0066] Example 6

[0067] The rest is the same as implementation 1, except that:

[0068] The amounts of the materials used in step (S1) are different, specifically: 100 parts of PAN-PVA, 40 parts of poly(pyridine-2-acetic acid) prepared in Preparation Example 1;

[0069] The amounts of the materials used in step (S1) are different, specifically: 100 parts of modified polyacrylonitrile and 30 parts of polyvinyl pyrrolidone.

[0070] Comparative Example 1

[0071] The rest is the same as in Example 1, except that step (S1) is omitted, that is, the carboxyl-containing polypyridine is not used to modify PAN-PVA, that is:

[0072] (1) 100 parts of PAN-PVA and 10 parts of polyvinylpyrrolidone were dissolved in 800 parts of N,N-dimethylformamide (DMF) to form a polymer solution with a concentration of 12 wt%; the polymer solution was transferred to a propeller and dripped into water at room temperature at a rate of 15 mL / h to precipitate solid pellets, which were filtered and dried to obtain solid pellets;

[0073] (2) After mixing 100 parts of solid pellets with 30 parts of NaOH, the mixture was heated to 700°C at a rate of 5°C / min under an argon atmosphere and kept warm for 6 hours; carbonized, cooled, washed with deionized water until neutral, and dried in an oven at 100°C for 24 hours to obtain porous carbon.

[0074] Comparative Example 2

[0075] The rest is the same as in Example 1, except that step (S1) is omitted, and polyacrylonitrile (PAN) is used instead of modified polyacrylonitrile in step (S2), that is:

[0076] (1) 100 parts of PAN and 10 parts of polyvinylpyrrolidone were dissolved in 800 parts of N,N-dimethylformamide (DMF) to form a polymer solution with a concentration of 12 wt%; the polymer solution was transferred to a propeller and dripped into water at room temperature at a rate of 15 mL / h to precipitate solid pellets, which were filtered and dried to obtain solid pellets;

[0077] (2) After mixing 100 parts of solid pellets with 30 parts of NaOH, the mixture was heated to 700°C at a rate of 5°C / min under an argon atmosphere and kept warm for 6 hours; carbonized, cooled, washed with deionized water until neutral, and dried in an oven at 100°C for 24 hours to obtain porous carbon.

[0078] Testing and Analysis

[0079] The porous carbons prepared in the examples and comparative examples were subjected to the following tests:

[0080] (1) Specific surface area and pore size: The porous carbons prepared in the Examples and Comparative Examples were subjected to a low-temperature nitrogen adsorption experiment using a Tristar II 3020 fully automatic specific surface and pore size analyzer manufactured by Micromeritics Instrument Corporation of the United States to determine the specific surface area and pore size distribution of the porous carbons. The specific data are shown in Table 1.

[0081] Figure 1 This is the pore size distribution diagram of the bimodal pore size distribution porous carbon prepared in Example 1.

[0082] (2) Noble Metal Ion Adsorption Test: A platinum ion solution and a palladium ion solution with a pH of 3 and a concentration of 100 mg / L were prepared; 0.08 g of the porous carbon prepared in each embodiment and comparative example was added to 20 mL of the above platinum ion solution and palladium ion solution, respectively, and adsorbed at 25°C and 250 rpm for 3 h; after the adsorption was completed, the solid and liquid were separated and the supernatant was collected. The platinum ion concentration and palladium ion concentration in the supernatant were detected by atomic absorption spectroscopy (AAS). The adsorption rate was obtained by calculating the concentration difference before and after adsorption. The specific data are shown in Table 2.

[0083]

[0084] Table 1 Specific surface area agent pore size distribution

[0085]

[0086]

[0087] It can be seen from Table 1 that the porous carbon prepared in the embodiment of the present invention has a bimodal pore size distribution, with pore sizes concentrated in 2-3 nm and 9-12 nm respectively; Figure 1 It can be seen that both peaks are narrow, indicating that the pore size distribution is concentrated. However, the porous carbons prepared in Comparative Examples 1 and 2 cannot achieve a bimodal pore size distribution and have low specific surface areas.

[0088] Table 2 Adsorption performance of precious metal ions

[0089] Group Platinum ion absorption rate (%) Palladium ion absorption rate (%) Example 1 99.5 98.8 Example 2 99.3 98.4 Example 3 99.4 98.5 Example 4 99.7 99.2 Example 5 99.6 99.1 Example 6 99.5 99.1 Comparative Example 1 87.2 84.9 Comparative Example 2 81.7 79.8

[0090] As can be seen from Table 2, the porous carbon with a bimodal pore size distribution prepared in the embodiment of the present invention has strong adsorption properties for platinum ions and palladium ions, and the adsorption rates for platinum ions and palladium ions are above 99% and 98%, respectively.

[0091] The above detailed description is a specific description of one feasible embodiment of the present invention. This embodiment is not intended to limit the patent scope of the present invention. Any equivalent implementation or modification that does not depart from the present invention should be included in the scope of the technical solution of the present invention.

Claims

1. A method for preparing porous carbon with bimodal pore size distribution, characterized in that: The following steps are involved: (S1) uniformly mixing a polyacrylonitrile-polyvinyl alcohol diblock copolymer (PAN-PVA), a carboxyl-containing polypyridine, and a stabilizer, and heating the mixture to react to obtain a modified polyacrylonitrile; wherein the PVA segment in the PAN-PVA accounts for 8 to 15 wt %; and the mass ratio of the PAN-PVA to the carboxyl-containing polypyridine is 100:(20 to 40); (S2) dissolving modified polyacrylonitrile and polyvinyl pyrrolidone in an organic solvent at a mass ratio of 100:(10-30) to form a polymer solution; dropping the polymer solution into water at room temperature to precipitate solid pellets, filtering and drying to obtain solid pellets; (S3) The solid pellets are mixed with an inorganic base, carbonized under an inert atmosphere, cooled, washed with water, and dried to obtain a porous carbon with a bimodal pore size distribution.

2. The preparation method according to claim 1, characterized in that In step (S1), the carboxyl-containing polypyridine is at least one of poly(pyridine-2-acetic acid), poly(pyridine-3-acetic acid), poly(pyridine-4-acetic acid), poly(pyridine-2-carboxylic acid), poly(pyridine-3-carboxylic acid), poly(pyridine-4-carboxylic acid), and poly(6-methoxy-3-pyridineacetic acid); and its number average molecular weight is 3000-5000.

3. The preparation method according to claim 1, characterized in that In step (S1), the number average molecular weight of the PAN-PVA is 50,000-80,000.

4. The preparation method according to claim 1, characterized in that In step (S1), the stabilizer is trimethyl phosphate, and its usage is 0.1-0.3 wt% of PAN-PVA; the heating reaction is as follows: PAN-PVA and carboxyl-containing polypyridine are added to a horizontal twin-screw mixer, heated to 50-70°C, and mixed at medium speed for 20-40 minutes; then the stabilizer is added thereto, heated to 180-190°C, and mixed at high speed for 30-50 minutes.

5. The preparation method according to claim 1, characterized in that In step (S1), the carboxyl-containing polypyridine is prepared by a method comprising the following steps: (L1) dissolving a pyridinate ester in an organic solvent, adding an oxidant under nitrogen protection, and stirring the reaction at 0-5°C for 5-8 hours to obtain a reactant; then pouring the reactant into an alcohol solvent, allowing it to stand, filtering, washing, and drying to obtain a polypyridinate ester; (L2) Adding polypyridinium ester to an aqueous solution of an inorganic base and performing a hydrolysis reaction at 90-100°C; then adding hydrochloric acid to obtain a precipitate, filtering, washing, and drying to obtain a carboxyl-containing polypyridine.

6. The preparation method according to claim 5, characterized in that In step (L1), the molar ratio of the pyridinic acid ester to the oxidant is 1:(1.2-2.5); and / or The pyridinium ester is at least one of methyl pyridine-2-acetate, methyl pyridine-3-acetate, methyl polypyridine-4-acetate, ethyl pyridine-2-carboxylate, ethyl pyridine-3-carboxylate, ethyl pyridine-4-carboxylate, and methyl 6-methoxy-3-pyridineacetate; the organic solvent is at least one of chloroform, carbon tetrachloride, and dichloromethane; the oxidant is at least one of FeCl3, MoCl5, and RuCl3; the alcohol solvent is at least one of methanol, ethanol, and isopropanol; and the washing and drying is washing with water 2 to 3 times and then vacuum drying at 50 to 60°C for 10 to 24 hours.

7. The preparation method according to claim 5, characterized in that In step (L2), the molar ratio of the pyridinate group in the polypyridinate to the inorganic base is 1:(1.1-1.3).

8. The preparation method according to claim 1, characterized in that In step (S2), the number average molecular weight of the polyvinyl pyrrolidone is 1000-5000; the organic solvent is at least one of N,N-dimethylformamide and N,N-dimethylacetamide; the concentration of the polymer solution is 10wt%-15wt%; and the dripping rate of the polymer solution is 10-20mL / h.

9. The preparation method according to claim 1, characterized in that In step (S3), the mass ratio of the solid pellets to the inorganic base is 100:(30-50), and the inorganic base is KOH and / or NaOH; the inert atmosphere is at least one of nitrogen, argon, and helium; and the carbonization conditions are: heating to 700-900°C at a heating rate of 3-8°C / min and keeping warm for 3-8 hours.

10. Use of the porous carbon with bimodal pore size distribution prepared by the preparation method according to any one of claims 1 to 9 in the recovery of precious metals.

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