Preparation method of transition metal doped hierarchical porous carbon applied to co-adsorption of multiple VOCs (Volatile Organic Compounds)

Abstract

The invention discloses a preparation method of transition metal doped hierarchical pore carbon applied to co-adsorption of various VOCs (volatile organic compounds). According to the method, a low-cost carbon precursor is adopted, developed micropore-mesopore / macropore hierarchical pores are constructed by a chemical / catalytic gradient activation method, and trace transition metal adsorption sites are dispersed / doped in a liquid-phase impregnation / low-temperature fixing process. By utilizing the mass transfer channel effect and the grading occurrence mechanism of the graded porous carbon carrier, transition metal adsorption sites can be doped quickly at low cost, and various VOCs in industrial flue gas can be efficiently treated; the problems that due to the fact that a traditional microporous carbon adsorbent is single in pore structure and lacks selective adsorption sites for micromolecular polar VOCs, VOCs with different physicochemical properties in industrial flue gas are difficult to effectively and synergistically adsorb, meanwhile, pores are blocked by site doping in micropores, mass transfer resistance is too large, and the accessibility of deep pores and adsorption sites is low are solved. A new thought is provided for development and application of the VOCs co-adsorption material.

Description

Technical Field

[0001] The present invention relates to a carbon adsorbent, and in particular to a preparation method of a transition metal adsorption site-doped hierarchical pore carbon adsorbent for enhancing the synergistic adsorption of multiple volatile organic compounds (VOCs) with different physical and chemical properties. Background Art

[0002] With the rapid growth of industry over the years, VOCs emitted from industrial sources now account for over 50% of total anthropogenic VOC emissions. These VOCs enter the atmosphere, causing PM2.5 pollution, photochemical smog, and near-surface O3 pollution, seriously endangering human health. Adsorption is considered an effective method for treating VOCs, particularly suitable for capturing and storing VOCs in industrial flue gases with low concentrations and high exhaust volumes. Carbon materials are widely used as adsorbent materials due to their low cost, wide availability of raw materials, and adjustable adsorption sites.

[0003] Numerous reports have been published on carbon adsorbents, MOFs, zeolites, and other materials for the effective adsorption and removal of single-component VOCs (CN119608133A, CN114225906A, and CN119819263A). However, industrial flue gas contains a wide variety of VOCs, including but not limited to aromatic hydrocarbons, olefins, and halogenated hydrocarbons. These VOCs exhibit significant differences in physicochemical properties, such as polarity, molecular dynamics size, and characteristic functional groups. This complexity poses a challenge in selecting suitable carbon adsorbents. Conventional carbon adsorbents have a relatively uniform pore size distribution and a lack of selective adsorption sites, limiting their synergistic adsorption performance for VOCs.

[0004] Aromatic hydrocarbon VOCs have a benzene ring structure, and the π-π interaction between them and carbon adsorbents gives them a significant advantage in adsorption capacity compared to small molecule polar VOCs with a similarly high proportion. To address this problem, 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 construction and surface chemical regulation need to be coupled. Metal site doping in microporous carbon adsorbents may lead to pore blockage. In addition, there is a lack of mass transfer channels for VOC adsorbates. Deep pores and low site accessibility may lead to strong competitive adsorption between different types of VOCs with different physical and chemical properties. Therefore, the carbon adsorbent carrier used for metal adsorption site doping needs to have hierarchical pore characteristics. Summary of the Invention

[0005] In response to the problem of difficulty in synergistic adsorption and removal of VOCs in industrial flue gas due to the complex types and significant differences in physical and chemical properties such as kinetic size and polarity, the present invention provides a method for preparing transition metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs. This method utilizes hierarchical pores to weaken the competitive adsorption of VOCs pollutants with different physical and chemical properties, provide mass transfer channels and hierarchical storage space, and at the same time, dope selective adsorption sites within the pores to specifically enhance the adsorption performance of weakly adsorbed VOCs, thereby achieving efficient synergistic adsorption and removal of mixed VOCs in industrial flue gas. A low-cost carbon precursor is used to prepare a hierarchical porous carbon carrier by a chemical / catalytic activation coupling method, and the transition metal adsorption sites are doped by methods such as liquid phase impregnation and molten salt impregnation.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] A method for preparing transition metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs uses a low-cost carbon precursor to construct a well-developed microporous-mesoporous / macroporous hierarchical pore structure through a chemical / catalytic gradient activation method, and then disperses / dopes trace transition metal adsorption sites through a liquid phase impregnation / low-temperature fixation process. The method specifically includes the following steps:

[0008] Step 1: Preparation of microporous carbon:

[0009] Step 11: crushing and sieving the carbon precursor to obtain particles, and then acid-washing them in HCl and HF solutions to remove inorganic ash, wherein: the carbon precursor is biomass, coal, etc.; the particle size of the carbon precursor after crushing and screening is 40-80 mesh; the concentration of the HCl solution is 5M, and the concentration of the HF solution is 10wt.%; the acid washing temperature is 60-80°C, and the time is 24-48 hours;

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

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

[0012] Step 21: Under a CO2 and N2 atmosphere, the microporous carbon prepared in step 1 is mixed with an alkaline earth metal salt and heated to 800-900°C for catalytic activation for 0.5-1h, wherein the gas volume ratio of CO2 and N2 is 2:3-3:4; the amount of alkaline earth metal salt is 0.5-1.5wt.% of the microporous carbon; the mixing time of the microporous carbon and the calcium salt is 6-12h; the alkaline earth metal salt is a calcium salt (C4H6CaO4, CaCl2, CaCO3); the heating rate is 3-10°C / min;

[0013] Step 22: Rinse with deionized water and then dry to obtain a hierarchical porous carbon support, wherein the drying temperature is 80-100°C and the drying time is 4-6 h; the specific surface area of ​​the hierarchical porous carbon support is greater than 1000 m 2 ·g -1 , the total pore volume is greater than 0.8 cm 3 ·g -1 , micropores are distributed in the range of 0.5~0.9nm, mesopores are mainly distributed in the range of 1.8~6nm, 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 a transition metal nitrate in 20-40 mL of deionized water to prepare a transition metal salt solution with a concentration of 0.02-0.06 mol / kg, wherein the transition metal salt solution is a transition metal nitrate solution;

[0016] Step 32: 0.1-0.2 g of the graded porous carbon support is immersed in a transition metal salt solution and ultrasonically immersed for 1-2 hours, wherein: the ultrasonic power is 100-300 W;

[0017] Step 3. Filter and recover the excess impregnation liquid and then dry it to obtain a metal adsorption site-doped hierarchical porous carbon adsorbent, wherein: the drying temperature is 100-130°C and the time is 4-6 hours; 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 physical and chemical properties, including aromatic hydrocarbons (benzene, toluene, xylene, phenol, ethylbenzene), olefins (ethylene, propylene, butadiene, isoprene), halogenated hydrocarbons (dichloromethane, chloroform, dichloroethane, trichloroethane, trichloroethylene), oxygen-containing hydrocarbons (acetone, methanol) and lipids (ethyl acetate, acrylate). At least two of them.

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

[0019] (1) The present invention utilizes the mass transfer channel effect and graded storage mechanism of the graded porous carbon carrier to quickly and inexpensively dope transition metal adsorption sites and effectively treat a variety of VOCs in industrial flue gas. This solves the problem that traditional microporous carbon adsorbents are difficult to effectively synergistically adsorb VOCs with different physical and chemical properties in industrial flue gas due to their single pore structure and lack of selective adsorption sites for small molecular polar VOCs. At the same time, the doping of sites in the micropores blocks the pores, resulting in excessive mass transfer resistance and low accessibility of deep pores and adsorption sites.

[0020] (2) The present invention uses low-cost carbon precursors as raw materials to achieve efficient hierarchical construction of micropores, mesopores, and macropores of carbon adsorbent carriers: the specific surface area of ​​transition metal-doped hierarchical porous carbon adsorbents exceeds 1000 m 2 ·g -1 , the mesopore / macroporous pore volume accounts for more than 40% and the total pore volume is greater than 0.8cm 3 ·g -1 The pore distribution precisely matches typical VOCs pollutants of different molecular kinetic sizes. At the same time, the metal adsorption sites achieve a uniform trace loading of less than 2%, with no significant impact on the pore structure, only changing the surface chemical properties.

[0021] (3) The transition metal-doped hierarchical porous carbon adsorbent prepared by the present invention can efficiently and synergistically adsorb VOCs with different physical and chemical properties in industrial flue gas. In typical comparative examples and examples, the total synergistic adsorption capacity of the transition metal-doped hierarchical porous carbon adsorbent for typical VOCs toluene and dichloromethane reached 764 mg·g -1 , the total adsorption capacity of toluene-dichloromethane for microporous carbon adsorbent is 440 mg·g -1 The total adsorption capacity of toluene-dichloromethane by metal adsorption site-doped microporous carbon adsorbent is 493 mg·g -1 It is 1.5 times that of VOCs, achieving efficient synergistic adsorption and removal of VOCs with different physical and chemical properties under simulated industrial flue gas conditions, and provides new ideas for the development and application of VOCs co-adsorption materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 N2 adsorption / desorption isotherms of Comparative Example 1, Comparative Example 2 and Example 1;

[0023] Figure 2 The micropore volume, mesopore volume 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 the microporous carbon adsorbent of Comparative Example 1, the microporous carbon support of Comparative Example 2, and the hierarchical pore carbon support of Example 1;

[0025] Figure 4 The microporous carbon adsorbent of Comparative Example 1, the microporous carbon support of Comparative Example 2 and the hierarchical pore carbon support of Example 1 were characterized by low q (< 0.04 Å) based on the small angle X-ray scattering spectra. -1 )’s typing characteristics;

[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 optical emission spectroscopy;

[0027] Figure 6 is the pore size distribution of Example 1;

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

[0029] Figure 8 This is the transmission electron microscope image of the hierarchical porous carbon support loaded with Al sites and the X-ray energy spectrum distribution of the Al element in Example 1. The bright spots are Al element sites;

[0030] Figure 9 This is the toluene-dichloromethane dynamic synergistic adsorption breakthrough curve of Comparative Example 1;

[0031] Figure 10 This is the toluene-dichloromethane dynamic synergistic adsorption breakthrough curve of Comparative Example 2;

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

[0033] Figure 12 The adsorption capacity of toluene, dichloromethane, and toluene+dichloromethane in Example 1, Comparative Example 1, and Comparative Example 2 under the conditions of toluene-dichloromethane co-adsorption is shown in FIG1 , 1 - microporous carbon adsorbent, 2 - metal adsorption site-doped microporous carbon adsorbent, and 3 - metal adsorption site-doped hierarchical porous carbon adsorbent. DETAILED DESCRIPTION

[0034] The technical solution of the present invention is further illustrated below in conjunction with comparative examples and embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0035] Comparative Example 1:

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

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

[0038] Comparative Example 2:

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

[0040] Small angle X-ray scattering data and classification characteristics of microporous carbon supports Figure 3 、 Figure 4 The N2 adsorption / desorption isotherms and micropore, mesopore / macroporous volumes of Al metal adsorption sites doped microporous carbon adsorbent are shown in Figure 2. Figure 1 、 Figure 2 The results of X-ray photoelectron spectroscopy and inductively coupled plasma emission spectroscopy for Al element loading are shown in Figure 5 shown.

[0041] Example 1:

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

[0043] The N2 adsorption / desorption isotherms and micropore, mesopore / macroporous volumes of Al metal adsorption sites doped hierarchical porous carbon adsorbents are shown in Figure 2. Figure 1 、 Figure 2 As shown, small angle X-ray scattering data and typing characteristics are as follows Figure 3 、 Figure 4 As shown, the pore size distribution is Figure 6 As shown, the surface morphology is Figure 7 Scanning electron microscope images show that the Al element dispersion is as follows Figure 8 Transmission electron microscopy and X-ray energy spectrum show that the loading amount is measured by X-ray photoelectron spectroscopy and inductively coupled plasma emission spectroscopy. Figure 5 shown.

[0044] according to Figure 1 The carbon adsorbent constructed by this embodiment has N2 adsorption / desorption isotherms with both type I and IV characteristics, indicating that it has 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. Figure 2 , intuitively showing that Example 1 has a hierarchical pore structure compared to Comparative Examples 1 and 2 and the pore volume is higher than the latter. Figure 3 Since the low-q region reflects scattering from the mesopores, the hierarchical pore structure of the carbon support of this embodiment is further verified. Figure 4 The linear slopes of the carbon carrier of this embodiment, the microporous carbon adsorbent of comparative example 1 and the microporous carbon carrier of comparative example 2 are between 1 and 3, indicating that the pore fractal characteristics are obvious. At the same time, the hierarchical pore carrier and the microporous carrier D pThe difference is not significant. The formation of mesopores / macropores does not reduce the branched structure of the hierarchical pore carrier. The gradient catalytic activation based on micropores solves the problem of Ca only inducing surface etching. Figure 6 The pore distribution of Al adsorption site doped hierarchical porous carbon adsorbent is mainly micropores 0.6~0.8nm and mesopores 2~6nm. Figure 7 , dense narrow pores can be observed on the surface of the carbon adsorbent constructed by this embodiment. Figure 5 and Figure 8 The Al adsorption sites are loaded in trace amounts on the hierarchical porous carbon adsorbent and have good dispersion. This demonstrates that the preparation method adopted in this example can directionally construct a hierarchical porous structure in the carbon precursor and uniformly dope trace amounts of metal adsorption sites.

[0045] Example 2:

[0046] Using dichloromethane and toluene as cooperative adsorption probes, the carbon adsorbents of Example 1, Comparative Example 1 and Comparative Example 2 were tested for cooperative adsorption. The dynamic cooperative adsorption penetration curves are shown in Figure 2. Figure 9 、 Figure 10 and Figure 11 The test conditions are 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°C and the mass of adsorbent added was 0.08 g. The adsorption capacities of toluene, dichloromethane and dichloromethane + toluene under co-adsorption conditions were as follows: Figure 12 The dichloromethane adsorption capacity of the metal adsorption site-doped hierarchical porous carbon adsorbent in Example 1 is 134 mg·g -1 , toluene adsorption capacity 630 mg g -1 , total adsorption capacity 764 mg·g -1 , which are 3.1 times that of the microporous carbon adsorbent in Comparative Example 1 (44 mg·g -1 ), 1.6 times (396 mg·g -1 ) and 1.7 times (440 mg·g -1 ), 2.7 times that of the metal-doped microporous carbon adsorbent in Comparative Example 2 (44 mg·g -1 ), 1.4 times (443 mg g -1 ) and 1.5 times (493 mg·g -1 ).

Claims

1. A method for preparing transition metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs, characterized in that The method comprises the following steps: Step 1: Preparation of microporous carbon: Step 11: crushing the carbon precursor and then screening it to obtain particles, and then acid-washing them in HCl and HF solutions to remove inorganic ash; Step 1 and 2: Under a nitrogen atmosphere, the deashed carbon precursor particles are mixed with a chemical activator and heated to 200-300° C. for pre-carbonization for 0.5-1 hour, and then heated to 800-900° C. for chemical activation for 1-2 hours to obtain microporous carbon, wherein the mass ratio of the deashed carbon precursor particles to the chemical activator is 1:1-1:4; Step 2: Preparation of hierarchical pore carbon support: Step 21: Under a CO2 and N2 atmosphere, mix the microporous carbon prepared in step 1 with an alkaline earth metal salt and heat to 800-900°C for catalytic activation for 0.5-1h, wherein the amount of alkaline earth metal salt is 0.5-1.5wt.% of the microporous carbon; Step 22: Rinse with deionized water and then dry to obtain a hierarchical porosity carbon support; Step 3: Preparation of metal adsorption site-doped hierarchical porous carbon adsorbent: Step 3: Dissolve the transition metal nitrate in 20-40 mL of deionized water to prepare a transition metal salt solution with a concentration of 0.02-0.06 mol / kg. Step 32: immerse 0.1-0.2 g of the graded porosity carbon support in a transition metal salt solution and ultrasonically immerse for 1-2 h; Step 3: Filter and recover the excess impregnation liquid, and then dry it to obtain a metal adsorption site-doped hierarchical porous carbon adsorbent.

2. The method for preparing transition metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs according to claim 1, characterized in that In step 1, the carbon precursor is one of biomass and coal; the particle size of the carbon precursor after crushing and screening is 40-80 mesh; the concentration of the HCl solution is 5M, and the concentration of the HF solution is 10 wt.%; the acid washing temperature is 60-80°C, and the time is 24-48 hours.

3. The method for preparing transition metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs according to claim 1, characterized in that In steps 1 and 2, the chemical activator is KOH; and the heating rate is 3-10°C / min.

4. The method for preparing transition metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs according to claim 1, characterized in that In step 21, the gas volume ratio of CO2 and N2 is 2:3~3:4; the mixing time of the microporous carbon and the alkaline earth metal salt is 6~12 hours; the alkaline earth metal salt is calcium salt; and the heating rate is 3~10°C / min.

5. The method for preparing transition metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs according to claim 1, characterized in that In the step 22, the drying temperature is 80-100°C and the drying time is 4-6 hours; the specific surface area of ​​the hierarchical porous carbon carrier is greater than 1000 m 2 ·g -1 , the total pore volume is greater than 0.8 cm 3 ·g -1 The micropores are distributed in the range of 0.5~0.9nm, the mesopores are distributed in the range of 1.8~6nm, the volume of mesopores / macropores accounts for more than 40% of the total pore volume and the average pore diameter is greater than 2nm.

6. The method for preparing transition metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs according to claim 1, characterized in that In step 31, the transition metal salt solution is a transition metal nitrate solution.

7. The method for preparing transition metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs according to claim 1, characterized in that In step 32, the ultrasonic power is 100-300W.

8. The method for preparing transition metal-doped hierarchical porous carbon for co-adsorption of multiple VOCs according to claim 1, characterized in that In the step 33, the drying temperature is 100-130° C., the drying time is 4-6 hours, and the metal adsorption site loading is 0.1-2%.

9. Use of the transition metal-doped hierarchical porous carbon prepared by the method according to any one of claims 1 to 8 in the co-adsorption of VOCs.

10. The use of the transition metal-doped hierarchical porous carbon in VOCs co-adsorption according to claim 9, characterized in that The VOCs include at least two of aromatic hydrocarbons, olefins, halogenated hydrocarbons, oxygenated hydrocarbons and lipids.

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