A preparation method of metal-doped multi-level porous carbon applied to co-adsorption of various VOCs

Abstract

This invention discloses a method for preparing metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs. The method employs a low-cost carbon precursor and constructs a well-developed microporous-mesoporous / macroporous hierarchical pore structure using a chemical / catalytic gradient activation method. Trace metal adsorption sites are dispersed / doped through liquid-phase impregnation / low-temperature fixation. This invention utilizes the mass transfer channel effect and hierarchical storage mechanism of the hierarchical porous carbon support to rapidly and cost-effectively dope metal adsorption sites for the efficient treatment of multiple VOCs in industrial flue gas. It solves the problems of traditional microporous carbon adsorbents, which suffer from limited pore structure, lack of selective adsorption sites for small-molecule polar VOCs, and difficulty in effectively co-adsorbing VOCs with different physicochemical properties in industrial flue gas. Furthermore, it addresses the issues of micropore blockage due to doping, excessive mass transfer resistance, and low accessibility to deep pores and adsorption sites. This provides a new approach for the development and application of VOCs co-adsorption materials.

Description

Technical Field

[0001] This invention relates to a carbon adsorbent, specifically to a method for preparing a metal adsorption site-doped hierarchical porous carbon adsorbent that enhances the synergistic adsorption of various volatile organic compounds (VOCs) with different physicochemical properties. Background Technology

[0002] With the rapid development of industry over the years, VOCs emitted from industrial sources now account for more than 50% of total anthropogenic VOC emissions. These VOCs, once released into the atmosphere, cause PM2.5 pollution, photochemical smog, and near-ground O3 pollution, seriously endangering human health. Adsorption is considered an effective method for treating VOCs, especially suitable for capturing and storing VOCs in industrial flue gas with low concentrations and large volumes. Carbon materials are widely used as adsorbents due to their low cost, wide availability of raw materials, and adjustable adsorption sites.

[0003] Numerous reports exist on carbon adsorbents, MOFs, and zeolites for the effective adsorption and removal of single-component VOCs (CN119608133A, CN114225906A, CN119819263A). However, actual industrial flue gas contains a variety of VOCs, including but not limited to aromatic hydrocarbons, alkenes, and halogenated hydrocarbons, which exhibit significant differences in physicochemical properties such as polarity, molecular dynamics, and characteristic functional groups. This complexity presents a challenge in selecting suitable carbon adsorbents. Conventional carbon adsorbents, with their relatively uniform pore size distribution and insufficient selective adsorption sites, limit their synergistic adsorption performance against VOCs.

[0004] Aromatic VOCs possess a benzene ring structure, and their π-π interactions with carbon adsorbents give them a significant advantage in adsorption capacity compared to small-molecule polar VOCs, which also have a relatively high proportion. To address this issue, studies have proposed using metal site doping to enhance the adsorption performance of polar small-molecule VOCs (Sep. Purif. Technol, 2023, 306, 122594; Chem. Eng. Data, 2013, 58(9), 2449-2454). However, for the synergistic adsorption of mixed VOCs, pore structure and surface chemical regulation need to be coupled. Metal site doping within microporous carbon adsorbents may lead to pore blockage. Furthermore, the lack of mass transfer channels for VOC adsorbates, coupled with low accessibility of deep pores and sites, may result in strong competitive adsorption between different types of VOCs with varying physicochemical properties. Therefore, carbon adsorbent supports used for metal site doping need to possess hierarchical pore characteristics. Summary of the Invention

[0005] To address the challenge of synergistic adsorption and removal of VOCs in industrial flue gas due to their complex composition and significant differences in kinetic size, polarity, and other physicochemical properties, this invention provides a method for preparing metal-doped hierarchical porous carbon for the co-adsorption of multiple VOCs. This method utilizes hierarchical pores to weaken competitive adsorption of VOCs with different physicochemical properties, providing mass transfer channels and hierarchical storage space. Simultaneously, selective adsorption sites within the pores are doped to specifically enhance the adsorption performance of weakly adsorbing VOC components, thereby achieving efficient synergistic adsorption and removal of mixed VOCs in industrial flue gas. A low-cost carbon precursor is used to prepare the hierarchical porous carbon support via a chemical / catalytic activation coupling method, and metal adsorption sites are doped using methods such as liquid-phase impregnation and molten salt impregnation.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A method for preparing metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs is disclosed. This method employs a low-cost carbon precursor, constructs a well-developed microporous-mesoporous / macroporous hierarchical pore structure using a chemical / catalytic gradient activation method, and disperses / dops trace metal adsorption sites through liquid-phase impregnation / low-temperature fixation. The method specifically includes the following steps:

[0008] Step 1: Preparation of microporous carbon:

[0009] Step 1: The carbon precursor is crushed and sieved to obtain particles, which are then acid-washed in HCl and HF solutions to remove inorganic ash. The carbon precursor is biomass, coal, etc.; the particle size of the crushed and sieved carbon precursor is 40-80 mesh; the concentration of HCl solution is 5 M, and the concentration of HF solution is 10 wt.%; the acid washing temperature is 60-80℃, and the time is 24-48 h.

[0010] Steps 1 and 2: Under N2 atmosphere, the deashed carbon precursor particles are mixed with a chemical activator and heated to 200-300℃ for pre-carbonization for 0.5-1h, then heated to 800-900℃ for chemical activation for 1-2h to obtain microporous carbon. The mass ratio of the deashed carbon precursor particles to the chemical activator is 1:1 to 1:4; the chemical activator is KOH; and the heating rate is 3-10℃ / min.

[0011] Step 2: Preparation of hierarchical porous carbon support:

[0012] Step 2: Under a CO2 and N2 atmosphere, mix the microporous carbon from Step 1 with alkaline earth metal salts and heat to 800-900℃ for catalytic activation for 0.5-1 h. The volume ratio of CO2 to N2 is 2:3-3:4; the amount of alkaline earth metal salt is 0.5-1.5 wt.% of the microporous carbon; the mixing time between the microporous carbon and calcium salts is 6-12 h; the alkaline earth metal salts are calcium salts (C4H6CaO4, CaCl2, CaCO3); and the heating rate is 3-10 ℃ / min.

[0013] Step 2: Rinse with deionized water and then dry to obtain a hierarchical porous carbon support. The drying temperature is 80-100℃, and the drying time is 4-6 hours. The specific surface area of ​​the hierarchical porous carbon support is greater than 1000 m². 2 ·g -1 The total pore volume is greater than 0.8 cm³. 3 ·g -1 Micropores are distributed in the range of 0.5 to 0.9 nm, mesopores are mainly distributed in the range of 1.8 to 6 nm, and the volume of mesopores / macropores accounts for more than 40% of the total pore volume and the average pore diameter is greater than 2 nm.

[0014] Step 3: Preparation of metal adsorption site-doped hierarchical porous carbon adsorbent:

[0015] Step 3: Dissolve the metal nitrate in 20-40 mL of deionized water to prepare a metal salt solution with a concentration of 0.02-0.06 mol / kg, wherein the metal salt solution is a metal nitrate solution;

[0016] Step 32: Immerse 0.1~0.2 g of hierarchical porous carbon support in a metal salt solution and ultrasonically impregnate for 1~2 h, wherein the ultrasonic power is 100~300 W;

[0017] Step 3: After filtration and recovery of excess impregnation solution, the product is dried to obtain a metal adsorption site-doped hierarchical porous carbon adsorbent. The drying temperature is 100~130℃ and the drying time is 4~6 h. The metal adsorption site loading is 0.1~2%. The metal adsorption site-doped hierarchical porous carbon adsorbent can synergistically treat a variety of VOCs with different physicochemical properties. The VOCs include at least two of the following: aromatic hydrocarbons (benzene, toluene, xylene, phenol, ethylbenzene), olefins (ethylene, propylene, butadiene, isoprene), halogenated hydrocarbons (dichloromethane, trichloromethane, dichloroethane, trichloroethane, trichloroethylene), oxygenated hydrocarbons (acetone, methanol), and lipids (ethyl acetate, acrylate).

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] (1) This invention utilizes the mass transfer channel effect and hierarchical storage mechanism of hierarchical porous carbon carrier to rapidly and cost-effectively dop metal adsorption sites and use them to efficiently treat various VOCs in industrial flue gas. It solves the problems of traditional microporous carbon adsorbents, which are difficult to effectively synergistically adsorb VOCs with different physicochemical properties in industrial flue gas due to the single pore structure and lack of selective adsorption sites for small molecule polar VOCs. At the same time, the sites in the micropores are doped and block the pores, resulting in excessive mass transfer resistance and low accessibility of deep pores and adsorption sites.

[0020] (2) This invention uses low-cost carbon precursors as raw materials to achieve efficient hierarchical construction of micropore-mesopore / macropore carbon adsorbent carriers: the specific surface area of ​​metal-doped hierarchical porous carbon adsorbents exceeds 1000 m². 2 ·g -1 The mesoporous / macroporous pore volume ratio exceeds 40% and the total pore volume is greater than 0.8 cm³. 3 ·g -1 The pore distribution is precisely matched to typical VOCs pollutants with different molecular dynamics dimensions. At the same time, the metal adsorption sites achieve a uniform trace loading of less than 2%, which has no significant impact on the pore structure and only changes the surface chemical properties.

[0021] (3) The metal-doped hierarchical porous carbon adsorbent prepared in this invention can efficiently and synergistically adsorb VOCs with different physicochemical properties in industrial flue gas. In typical comparative examples and embodiments, the total synergistic adsorption capacity of the metal-doped hierarchical porous carbon adsorbent for typical VOCs toluene and dichloromethane reaches 764 mg·g⁻¹. -1 The total adsorption capacity of the microporous carbon adsorbent for toluene-dichloromethane is 440 mg·g. -1 The adsorption capacity of the metal adsorption site-doped microporous carbon adsorbent for toluene-dichloromethane was 1.7 times that of the metal adsorption site-doped microporous carbon adsorbent, with a total adsorption capacity of 493 mg·g⁻¹. -1 It is 1.5 times that of the previous method, achieving efficient synergistic adsorption and removal of VOCs with different physicochemical properties under simulated industrial flue gas conditions, providing a new approach for the development and application of VOCs co-adsorption materials. Attached Figure Description

[0022] Figure 1 The N2 adsorption / desorption isotherms are for Comparative Example 1, Comparative Example 2, and Example 1.

[0023] Figure 2 For the micropore volume, mesopore and macropore volume of Comparative Example 1, Comparative Example 2 and Example 1, 1-microporous carbon adsorbent, 2-metal adsorption site doped microporous carbon adsorbent, 3-metal adsorption site doped hierarchical porous carbon adsorbent.

[0024] Figure 3 Small-angle X-ray scattering spectra of microporous carbon adsorbent (Comparative Example 1), microporous carbon support (Comparative Example 2), and hierarchical porous carbon support (Example 1);

[0025] Figure 4 Comparative Example 1 microporous carbon adsorbent, Comparative Example 2 microporous carbon support, and Example 1 hierarchical porous carbon support were compared based on low q (< 0.04 Å) small-angle X-ray scattering (SAXS) spectra. -1 The fractal characteristics of );

[0026] Figure 5 The relative loading of Al element in the Al-doped carbon adsorbents in Comparative Example 2 and Example 1 was determined by X-ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectroscopy.

[0027] Figure 6 The aperture distribution is shown in Example 1;

[0028] Figure 7 This is a scanning electron microscope image of the surface morphology of the hierarchical porous carbon support in Example 1.

[0029] Figure 8 The image shows a transmission electron microscope image of Al sites loaded on a hierarchical porous carbon support in Example 1, and the X-ray energy spectrum distribution of Al. Bright spots represent Al sites.

[0030] Figure 9 The breakthrough curve of toluene-dichloromethane dynamic synergistic adsorption is shown in Comparative Example 1.

[0031] Figure 10 The breakthrough curve of toluene-dichloromethane dynamic synergistic adsorption is shown in Comparative Example 2.

[0032] Figure 11 The toluene-dichloromethane dynamic synergistic adsorption breakthrough curve of Example 1;

[0033] Figure 12 The adsorption capacities of toluene, dichloromethane, and toluene + dichloromethane in Example 1 and Comparative Examples 1 and 2 under toluene-dichloromethane co-adsorption conditions are: 1-microporous carbon adsorbent, 2-metal adsorption site-doped microporous carbon adsorbent, and 3-metal adsorption site-doped hierarchical porous carbon adsorbent. Detailed Implementation

[0034] The technical solution of the present invention will be further described below with reference to comparative examples and embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0035] Comparative Example 1:

[0036] Raw coal from Zhundong, Xinjiang, was crushed and sieved to collect coal particles with a diameter of 40-80 mesh. These particles were then mixed sequentially with 5 M HCl solution and 10 wt.% HF solution at a mass ratio of 1:5. The mixture was magnetically stirred in an 80 ℃ water bath for 24 h, followed by washing with deionized water until the pH of the washing solution reached 6-7. Finally, the mixture was dried at 80 ℃ for 24 h to obtain de-ashed coal particles. These de-ashed coal particles were mixed with KOH at a mass ratio of 1:2 and heated to 250 ℃ at a heating rate of 5 ℃ / min, held for 0.5 h for pre-carbonization, and then chemically activated at 900 ℃ at a heating rate of 5 ℃ / min for 2 h to obtain a microporous carbon adsorbent.

[0037] N2 adsorption / desorption isotherms and micropore, mesopore / macropore volumes of microporous carbon adsorbents, as shown in... Figure 1 , Figure 2 As shown, small-angle X-ray scattering data and classification characteristics are as follows: Figure 3 , Figure 4 As shown.

[0038] Comparative Example 2:

[0039] Raw coal from Zhundong, Xinjiang, was crushed and sieved to collect coal particles with a diameter of 40-80 mesh. These particles were then mixed sequentially with 5 M HCl solution and 10 wt.% HF solution at a mass ratio of 1:5. The mixture was magnetically stirred in an 80 ℃ water bath for 24 h, followed by washing with deionized water until the pH of the washing solution reached 6-7. Finally, the mixture was dried at 80 ℃ for 24 h to obtain de-ashed coal particles. These de-ashed coal particles were mixed with KOH at a mass ratio of 1:2 and heated to 250 ℃ at a heating rate of 5 ℃ / min, held for 0.5 h for pre-carbonization, and then chemically activated at 900 ℃ at a heating rate of 5 ℃ / min for 2 h to obtain a microporous carbon support. The microporous carbon support was impregnated in a 0.04 mol / kg Al(NO3)3 salt solution, sonicated at 200 W for 1 h, and after filtration to recover excess impregnation solution, dried at 120 ℃ for 4 h to obtain an Al metal adsorption site-doped microporous carbon adsorbent.

[0040] Small-angle X-ray scattering data and typing characteristics of microporous carbon supports, such as Figure 3 , Figure 4 As shown. The N2 adsorption / desorption isotherms and micropore, mesopore, and macropore volumes of the Al metal adsorption site-doped microporous carbon adsorbent are shown in the figure. Figure 1 , Figure 2 As shown. The X-ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectrometry results of Al element loading are as follows. Figure 5 As shown.

[0041] Example 1:

[0042] Raw coal from Zhundong, Xinjiang, was crushed and sieved to collect coal particles with a diameter of 40-80 mesh. These particles were then mixed sequentially with 5 M HCl solution and 10 wt.% HF solution at a mass ratio of 1:5. The mixture was magnetically stirred in an 80 ℃ water bath for 24 h, followed by washing with deionized water until the pH of the washing solution reached 6-7. Finally, the mixture was dried at 80 ℃ for 24 h to obtain de-ashed coal particles. These de-ashed coal particles were mixed with KOH at a mass ratio of 1:2 and heated to 250 ℃ at a heating rate of 5 ℃ / min, held for 0.5 h for pre-carbonization, and then chemically activated at 900 ℃ at a heating rate of 5 ℃ / min for 2 h to obtain microporous carbon. Microporous carbon was mixed with 1 wt.% calcium acetate (C4H6CaO4) and catalytically activated for 1 h at 900 °C under a CO2 and N2 atmosphere by heating at a rate of 5 °C / min. The mixture was then rinsed with 500 mL of deionized water and dried at 80 °C for 4 h to obtain a hierarchical porous carbon support. The hierarchical porous carbon support was impregnated in a 0.04 mol / kg Al(NO3)3 salt solution, sonicated at 200 W for 1 h, and after filtration to recover excess impregnation solution, dried at 120 °C for 4 h to obtain an Al metal adsorption site-doped hierarchical porous carbon adsorbent.

[0043] N2 adsorption / desorption isotherms and micropore, mesopore, and macropore volumes of Al metal adsorption site-doped hierarchical porous carbon adsorbents are shown in the figure. Figure 1 , Figure 2 As shown, small-angle X-ray scattering data and classification characteristics are as follows: Figure 3 , Figure 4 As shown, the aperture distribution is as follows Figure 6 As shown, the surface morphology is as follows Figure 7 As shown in the scanning electron microscope image, the dispersion of Al is as follows: Figure 8 The transmission electron microscope and X-ray energy dispersive spectroscopy (TEM) results, along with the X-ray photoelectron spectroscopy and inductively coupled plasma atomic emission spectrometry (ICP-AES) results for the loading, are shown below. Figure 5 As shown.

[0044] according to Figure 1 The carbon adsorbent constructed in this embodiment exhibits both Type I and Type IV N2 adsorption / desorption isotherms, indicating that it possesses both microporous and mesoporous / macroporous structures. Its N2 adsorption capacity is higher than that of Comparative Examples 1 and 2, and its specific surface area is larger. According to... Figure 2 The example visually demonstrates that Example 1, compared to Comparative Examples 1 and 2, possesses a hierarchical pore structure and has a higher pore volume. According to... Figure 3 The reflection of scattering from the mesopores in the low-q region further verifies the hierarchical pore structure of the carbon support in this embodiment. According to... Figure 4 In this embodiment, the linear slopes of the carbon support, the microporous carbon adsorbent of Comparative Example 1, and the microporous carbon support of Comparative Example 2 are between 1 and 3, indicating obvious fractal characteristics of the pores. Simultaneously, the hierarchical porous support and the microporous support D...p The differences were not significant; the formation of mesopores / macropores did not reduce the branching structure of the hierarchical porous support. Gradient catalytic activation based on micropores solved the problem of Ca only inducing surface etching. Figure 6 The pore distribution of the Al adsorption site-doped hierarchical porous carbon adsorbent is mainly micropores (0.6–0.8 nm) and mesopores (2–6 nm). According to... Figure 7 Dense slit pores can be observed on the surface of the carbon adsorbent constructed in this embodiment. According to... Figure 5 and Figure 8 The Al adsorption sites are loaded in trace amounts onto the hierarchical porous carbon adsorbent and exhibit good dispersibility. This demonstrates that the preparation method used in this embodiment can directionally construct a hierarchical porous structure on the carbon precursor and uniformly dope trace metal adsorption sites.

[0045] Example 2:

[0046] Using dichloromethane and toluene as synergistic adsorption probes, synergistic adsorption tests were performed on the carbon adsorbents of Example 1, Comparative Example 1, and Comparative Example 2, respectively. The dynamic synergistic adsorption breakthrough curves are shown below. Figure 9 , Figure 10 and Figure 11 As shown. The test conditions were 400 ppm toluene, 500 ppm dichloromethane, N2 as carrier gas, and a total gas flow rate of 600 mL / min. -1 The bed temperature was controlled at 25 ℃, and the added adsorbent mass was 0.08 g. The adsorption capacities of toluene, dichloromethane, and dichloromethane + toluene under the co-adsorption conditions are as follows: Figure 12 As shown. In Example 1, the dichloromethane adsorption capacity of the metal adsorption site-doped hierarchical porous carbon adsorbent was 134 mg·g. -1 Toluene adsorption capacity: 630 mg·g -1 Total adsorption capacity: 764 mg·g -1 This is 3.1 times (44 mg·g) more effective than the microporous carbon adsorbent in Comparative Example 1. -1 ), 1.6 times (396 mg·g) -1 ) and 1.7 times (440 mg·g -1 The metal-doped microporous carbon adsorbent in Comparative Example 2 was 2.7 times (44 mg·g) more effective. -1 ), 1.4 times (443 mg·g) -1 ) and 1.5 times (493 mg·g -1 ).

Claims

1. A method for preparing a metal-doped hierarchical porous carbon for application in the co-adsorption of various VOCs, characterized in that The method includes the following steps: Step 1: Preparation of microporous carbon: Step 11: After crushing the carbon precursor, it is sieved to obtain particles, which are then acid-washed in HCl and HF solutions to remove inorganic ash. Step 1 and 2: Under N2 atmosphere, mix the deashed carbon precursor particles with the chemical activator and heat to 200~300℃ for pre-carbonization for 0.5~1h, then heat to 800~900℃ for chemical activation for 1~2h to obtain microporous carbon, wherein the mass ratio of deashed carbon precursor particles to chemical activator is 1:1~1:

4. Step 2: Preparation of hierarchical porous carbon support: Step 2: Under a CO2 and N2 atmosphere, the microporous carbon from Step 1 is mixed with an alkaline earth metal salt and heated to 800-900℃ for catalytic activation for 0.5-1 h. The amount of alkaline earth metal salt used is 0.5-1.5 wt.% of the microporous carbon, and the alkaline earth metal salt is a calcium salt. Step 22: Rinse with deionized water and then dry to obtain a hierarchical porous carbon support; Step 3: Preparation of metal adsorption site-doped hierarchical porous carbon adsorbent: Step 3: Dissolve the metal nitrate in 20-40 mL of deionized water to prepare a metal salt solution with a concentration of 0.02-0.06 mol / kg. The metal salt solution is a metal nitrate solution, and the metal nitrate is aluminum nitrate. Step 32: Immerse 0.1~0.2 g of hierarchical porous carbon support in a metal salt solution and ultrasonically impregnate for 1~2 h; Step 3: After filtration to recover excess impregnation solution, dry the product to obtain a metal adsorption site-doped hierarchical porous carbon adsorbent with a metal adsorption site loading of 0.1~2%.

2. The method for preparing metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs according to claim 1, characterized in that... In step one, the carbon precursor is either biomass or coal; the particle size of the crushed and screened carbon precursor is 40-80 mesh; the concentration of HCl solution is 5 M, and the concentration of HF solution is 10 wt.%; the pickling temperature is 60-80℃, and the time is 24-48 h.

3. The method for preparing metal-doped hierarchical porous carbon for the co-adsorption of multiple VOCs according to claim 1, characterized in that In steps one and two, the chemical activator is KOH; the heating rate is 3~10 ℃ / min.

4. The method for preparing metal-doped hierarchical porous carbon for the co-adsorption of multiple VOCs according to claim 1, characterized in that In step two, the gas volume ratio of CO2 to N2 is 2:3 to 3:4; the mixing time of microporous carbon and alkaline earth metal salt is 6 to 12 h; and the heating rate is 3 to 10 ℃ / min.

5. The method for preparing metal-doped hierarchical porous carbon for the co-adsorption of multiple VOCs according to claim 1, characterized in that In step two, the drying temperature is 80-100℃, and the time is 4-6 hours; the specific surface area of ​​the hierarchical porous carbon support is greater than 1000 m². 2 ·g -1 The total pore volume is greater than 0.8 cm³. 3 ·g -1 Micropores are distributed in the range of 0.5–0.9 nm, mesopores in the range of 1.8–6 nm, and the volume of mesopores / macropores accounts for more than 40% of the total pore volume with an average pore diameter greater than 2 nm.

6. The method for preparing metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs according to claim 1, characterized in that... In step 3.2, the ultrasonic power is 100~300 W.

7. The method for preparing metal-doped hierarchical porous carbon for the co-adsorption of multiple VOCs according to claim 1, characterized in that In step three, the drying temperature is 100~130℃ and the time is 4~6 hours.

8. The application of metal-doped hierarchical porous carbon prepared by the method of any one of claims 1-7 in VOCs co-adsorption.

9. Use of the metal-doped hierarchical porous carbon according to claim 8 in the co-adsorption of VOCs, characterized in that The VOCs include aromatic hydrocarbons and halogenated hydrocarbons.

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