A coal-based carbon nanotube / activated carbon composite material for CH4 / N2 adsorption and separation and its preparation method

Abstract

This invention discloses a coal-based carbon nanotube / activated carbon composite material for CH4 / N2 adsorption and separation, and its preparation method, belonging to the field of adsorption materials. The invention uses raw coal as raw material, performs hydrothermal treatment under a carbon-rich atmosphere, then carbonizes the treated coal powder with an activator using an impregnation method, and finally cools, washes, and dries to obtain the carbon nanotube / activated carbon composite material. This invention utilizes inexpensive, readily available, and abundant coal resources as raw material, and prepares a carbon nanotube / activated carbon composite material by nitrogen doping the coal under hydrothermal conditions. This promotes the formation of microporous structures at the nanoscale, resulting in a narrower pore size distribution in the interlayer packing space of the carbon nanotubes, improving the CH4 / N2 adsorption and separation effect, exhibiting excellent selectivity for CH4 / N2 adsorption and separation, and reducing the cost of the material.

Description

Technical Field

[0001] This invention relates to a coal-based carbon nanotube / activated carbon composite material for CH4 / N2 adsorption and separation and its preparation method, belonging to the field of adsorption materials. Background Technology

[0002] Coalbed methane, as a powerful supplement to natural gas, is an indispensable resource. However, due to lagging development and utilization technologies, the methane in low-concentration coalbed methane cannot be efficiently utilized, resulting in a huge waste of resources. Low-concentration coalbed methane typically contains a high proportion of nitrogen, and the presence of nitrogen leads to the production of more nitrogen oxides (NOx) during combustion. x Methane and nitrogen, among other harmful gases, pollute the environment. Separating methane and nitrogen from coalbed methane can reduce the emission of these harmful gases, contributing to improved air quality and reduced greenhouse gas emissions. Effective separation can increase the purity of methane in coalbed methane, thereby improving its energy utilization efficiency and reducing resource waste. This is crucial for energy efficiency and environmental protection. Therefore, effectively separating low-concentration methane from nitrogen in coalbed methane can improve its utilization rate, contributing to increased energy efficiency, reduced environmental pollution, compliance with regulations, and increased economic benefits. Thus, it should be widely applied and promoted in coalbed methane extraction. Simultaneously, continued research and improvement of this technology to enhance its efficiency and reduce costs is of great significance for sustainable energy development and environmental protection.

[0003] Chinese patent CN115155533A discloses "Application of a hydrophobic long-chain vapor-deposition modified MOF adsorbent in the separation of methane and nitrogen." This invention involves depositing hydrophobic organic long-chain molecules onto the surface of MOF materials via vapor deposition, and then modifying them through hydrophobic bonds to obtain MOF material adsorbents, thereby effectively adsorbing methane to achieve the separation of CH4 / N2. Although the adsorbents obtained by this method have high specific surface area and good selectivity, most MOF materials are expensive, which increases the cost of their industrial application and limits their large-scale industrial application.

[0004] Chinese patent CN114797761A discloses "a carbon molecular sieve adsorbent for methane-nitrogen separation and its preparation method," which requires mixing, carbonization, and carbon deposition to prepare a carbon molecular sieve precursor, impregnating and doping it with transition metal salt components, followed by reduction treatment under a protective atmosphere, and finally obtaining a transition metal-modified carbon molecular sieve adsorbent for methane-nitrogen separation. Although this method modifies the transition metal and distributes it within the pore structure of the carbon molecular sieve, achieving an N2 / CH4 separation coefficient of 9.035, the calcination and reduction temperatures are 400-800℃ and 400-1000℃ respectively, requiring high energy consumption and demanding high precision in equipment and carbon deposition processes. Furthermore, it exhibits low selectivity and low methane adsorption capacity.

[0005] Although the above classification methods have achieved certain results in terms of selectivity and performance, the complexity of the process, the high requirements for equipment, the extremely high cost and high energy consumption limit their application in many aspects. Summary of the Invention

[0006] To address the issues of complexity, high precision requirements for equipment operation, and high production costs associated with existing CH4 / N2 adsorption and separation technologies, this invention provides a coal-based carbon nanotube / activated carbon composite material for CH4 / N2 adsorption and separation, along with its preparation method. The carbon nanotube / activated carbon composite material, exhibiting excellent methane adsorption capacity and CH4 / N2 separation effect, can be obtained by nitrogen-doping and hydrothermal treatment of coal powder, followed by pyrolysis.

[0007] This invention provides a coal-based carbon nanotube / activated carbon composite material for CH4 / N2 adsorption and separation. The composite material is prepared by nitrogen doping of raw coal followed by hydrothermal treatment. Coal, as a low-cost and abundant carbon resource, undergoes hydrothermal treatment, which breaks down the macromolecules in the coal, providing more pores and numerous active sites. Nitrogen doping alters the hydrophilicity / hydrophobicity and adsorption capacity of the composite material surface. During pyrolysis, the release of methylene groups and the formation of carbon nanotubes create a favorable pore structure, enhancing the composite material's adsorption capacity. The narrow pore size effectively adsorbs methane and repels nitrogen molecules; it exhibits excellent selectivity for CH4 / N2 adsorption and separation, ensuring good performance while controlling costs, and shows promising prospects for industrial application.

[0008] This invention provides a method for preparing a coal-based carbon nanotube / activated carbon composite material for CH4 / N2 adsorption and separation, comprising the following steps: The first step involves mixing 80-150 mesh raw coal, nitrogen source, and water in a mass ratio of 30-40 wt%: 20-30 wt%: 30-50 wt%, and adding 0.5-2 wt% fly ash of the total mass of the three raw materials into a high-pressure reactor. The reactor is then filled with a carbon-rich atmosphere, the pressure is adjusted to 2-4 MPa, and the mixture is stirred until homogeneous. The temperature is raised to 280-340℃ and maintained for 20-60 minutes. After the reaction is complete, heating is stopped, and the mixture is cooled to room temperature to obtain a solid-liquid mixture. The second step is to filter the liquid from the solid-liquid mixture obtained in the first step and dry it to obtain hydrothermal carbon. The third step involves using an impregnation method, with deionized water as the solvent to load the activator onto the hydrothermal carbon obtained in the second step. The mass ratio of the added activator to the hydrothermal carbon is 1:1-3. The resulting solid-liquid mixture is stirred with a magnetic stirrer for 8-12 hours, and then placed in a sealed stainless steel reactor. The mixture is heated to 600-900℃ at a heating rate of 3-10℃ / min, held at that temperature for 20-60 minutes, and then cooled to room temperature to obtain the pyrolysis product. The fourth step involves cooling the pyrolysis product to room temperature, washing it with distilled water until neutral, and drying it to obtain a carbon nanotube / activated carbon composite material.

[0009] The raw coal mentioned in the first step has a volatile matter content (Vdaf) > 30% and a caking index < 60.

[0010] The nitrogen source mentioned in the first step includes any one of urea, melamine, humic acid, and propylamine. The mass concentration of the urea is 10-20%.

[0011] The fly ash mentioned in the first step has the following mass content: 20-40% calcium, 5-8% iron, and less than 0.8% sodium or potassium.

[0012] The volume content of CH4+CO in the carbon-rich atmosphere described in the first step is greater than 70%, and the volume ratio of CO / CH4 is greater than 3. During the preparation process, the above volume ratio is controlled by a gas cylinder and a flow meter to maintain a nitrogen-rich atmosphere, thereby fully carrying out the water-gas shift reaction.

[0013] The activator mentioned in the third step is any one of potassium oxalate, potassium citrate, potassium hydroxide, or potassium carbonate.

[0014] This invention provides a coal-based carbon nanotube / activated carbon composite material obtained by the above preparation method.

[0015] This invention provides the application of the above-mentioned coal-based carbon nanotube / activated carbon composite material in the adsorption and separation of CH4 / N2. Specifically, an adsorption and separation experiment of a binary gas mixture of CH4 and N2 was conducted on a fixed-bed experimental setup; the mixture was passed through an adsorption column containing 1 g of carbon material at a flow rate of 10 mL / min under conditions of 298 K and 101 kPa; the adsorption curve of porous carbon was obtained by monitoring the change in the ratio of the concentration of (CH4 or N2) in the outlet and inlet gases over time.

[0016] The beneficial effects of this invention are: (1) The carbon nanotube / activated carbon composite material after hydrothermal treatment increases the functional groups on the surface, introduces more active sites, increases the specific surface area of ​​the composite material, increases the proportion of micropores, endows the composite material with excellent adsorption capacity, improves the adsorption capacity of the composite material, and can efficiently adsorb methane in methane-nitrogen mixture, providing higher separation efficiency and processing capacity.

[0017] (2) The carbon nanotube interlayer stacking space formed in the composite material results in narrow pore sizes and a special structure for carbon nanotubes / activated carbon, providing a large number of adsorption sites for CH4 adsorption. Simultaneously, it improves the pore size distribution of the material's micropores and channels, enabling effective separation of CH4 and N2. It can be used to adsorb gaseous or solute molecules. These micropores and channels greatly promote the separation of CH4.

[0018] (3) Nitrogen doping provides a large number of nitrogen-containing functional groups, which provide more active sites, resulting in physical and chemical adsorption and increasing the adsorption capacity for methane. At the same time, nitrogen doping changes the charge distribution on the surface of the composite material, affecting the electrostatic interaction between methane and the surface of the composite material, and altering the surface hydrophilicity / hydrophobicity and adsorption capacity of the carbon nanotube / activated carbon composite material, thus giving the material a wide range of application prospects. Attached Figure Description

[0019] Figure 1 The dynamic breakthrough curve for the adsorption of CH4 / N2 by the carbon nanotube / activated carbon composite material prepared in Example 1; Figure 2 The static adsorption curves of CH4 / N2 adsorbed by the carbon nanotube / activated carbon composite material prepared in Example 1 are shown. Figure 3 Nitrogen adsorption isotherm for the preparation of carbon nanotube / activated carbon composite material in Example 2; Figure 4 Scanning electron microscope (SEM) image of the carbon nanotube / activated carbon composite material prepared in Example 3; Figure 5 Scanning electron microscope (SEM) image of the carbon nanotube / activated carbon composite material prepared in Example 4. Detailed Implementation

[0020] The present invention will be further illustrated by the following embodiments, but is not limited to the following embodiments. Example 1

[0021] (1) Grind long-flame coal into 80-150 mesh and take 12g. In addition, add 8g of urea, 20g of deionized water and 0.8g of fly ash to a high-pressure reactor, charge it with 80% CO and 10% CH4 by volume, adjust the pressure to 2MPa, stir evenly, raise the temperature to 280℃, maintain the reaction temperature for 30min, stop heating after the reaction is completed and cool to room temperature.

[0022] (2) The product obtained in (1) was filtered to remove the liquid from the solid product, and then the modified coal sample was dried in an oven at 80°C for 12 h.

[0023] (3) Using deionized water as solvent, weigh 4g of the product obtained in (2), load 4g of KOH onto the product, stir with a magnetic stirrer for 12 h, place the mixture in a sealed stainless steel reactor, and heat to 900℃ at a heating rate of 10 ℃ / min. After reacting for 60 min, cool to room temperature to obtain the pyrolysis product.

[0024] (4) Cool the pyrolysis product to room temperature, wash the sample with distilled water to neutral pH, and dry to obtain carbon nanotube / activated carbon composite material. Example 2

[0025] (1) Grind non-sticky coal powder into 80-150 mesh and take 12g. In addition, add 8g of urea, 20g of deionized water and 0.8g of fly ash to a high-pressure reactor, fill it with 80% CO and 10% CH4 by volume, adjust the pressure to 2MPa, stir evenly, heat to 300℃, maintain the reaction temperature for 40min, stop heating after the reaction is completed and cool to room temperature.

[0026] (2) The product obtained in (1) was filtered to remove the liquid from the solid product, and then the modified coal sample was dried in an oven at 80°C for 12 h.

[0027] (3) Using deionized water as solvent, weigh 4g of the product obtained in (2), load 4g of K2C2O onto the product, and then stir with a magnetic stirrer for 12 h. Place the mixture in a sealed stainless steel reactor and heat it to 900℃ at a heating rate of 10 ℃ / min. After reacting for 60 min, cool it to room temperature to obtain the pyrolysis product.

[0028] (4) Cool the pyrolysis product to room temperature, wash the sample with distilled water to neutral pH, and dry to obtain carbon nanotube / activated carbon composite material. Example 3

[0029] (1) Grind non-sticky coal powder into 80-150 mesh and take 12g. In addition, add 8g of urea, 20g of deionized water and 0.8g of fly ash into a high-pressure reactor, fill it with CO with a volume content of 80% and CH4 with 10% of the volume, adjust the pressure to 2MPa, stir evenly, raise the temperature to 320℃, maintain the reaction temperature for 50min, stop heating after the reaction is completed and cool to room temperature.

[0030] (2) The product obtained in (1) was filtered to remove the liquid from the solid product, and then the modified coal sample was dried in an oven at 80°C for 12 h.

[0031] (3) Using deionized water as solvent, weigh 4g of the product obtained in (2), load 4g of C6H5K3O7 onto the product, stir with a magnetic stirrer for 12 h, place the mixture in a sealed stainless steel reactor, and heat to 900℃ at a heating rate of 10 ℃ / min. After reacting for 60 min, cool to room temperature to obtain the pyrolysis product.

[0032] (4) Cool the pyrolysis product to room temperature, wash the sample with distilled water to neutral pH, and dry to obtain carbon nanotube / activated carbon composite material. Example 4

[0033] (1) Grind lignite into powder below 80-150 mesh and take 12g. In addition, add 8g urea, 20g deionized water and 0.8g fly ash to a high-pressure reactor, fill it with CO with a volume content of 80% and CH4 with 10% by volume, adjust the pressure to 2MPa, stir evenly, raise the temperature to 340℃, maintain the reaction temperature for 60min, stop heating after the reaction is completed and cool to room temperature.

[0034] (2) The product obtained in (1) was filtered to remove the liquid from the solid product, and then the modified coal sample was dried in an oven at 80°C for 12 h. Finally, it was crushed and ground to 100 mesh.

[0035] (3) Using deionized water as solvent, weigh 4g of the product obtained in (2), load 4g of K2CO3 onto the product, stir with a magnetic stirrer for 12 h, place the mixture in a sealed stainless steel reactor, and heat to 900℃ at a heating rate of 10 ℃ / min. After reacting for 60 min, cool to room temperature to obtain the pyrolysis product.

[0036] (4) Cool the pyrolysis product to room temperature, wash the sample with distilled water to neutral pH, and dry to obtain carbon nanotube / activated carbon composite material.

[0037] Figure 1 The dynamic breakthrough curves of CH4 / N2 adsorption of the carbon nanotube / activated carbon composite material prepared in Example 1 are shown. The experimental results show that nitrogen begins to break through in the first 60 seconds, indicating that the composite material reaches saturation for nitrogen in about 60 seconds, while the saturation time for methane is after 350 seconds. This shows that the composite material adsorbs methane much better than nitrogen and can effectively separate methane and nitrogen. Figure 1 The results show that the composite material adsorbs methane for 350 s, indicating that the composite material of the present invention changes the surface hydrophilicity / hydrophobicity and adsorption capacity of carbon nanotube / activated carbon composite material while increasing the number of active sites, thus giving the material a wide range of application prospects.

[0038] Figure 2The static adsorption curves of CH4 / N2 on the carbon nanotube / activated carbon composite material prepared in Example 1 are shown. The adsorption capacities of the material for CH4 and N2 at 298 K were experimentally determined. It can be found that the material exhibits better adsorption of methane than nitrogen across the entire pressure range of 0-101 kPa. Specifically, at 101 kPa, the methane adsorption capacity reaches 53.69 cm⁻¹. 3 / g, while the nitrogen adsorption capacity is only 9.05 cm³. 3 / g indicates that the composite material has a high selectivity for methane.

[0039] Figure 3 The nitrogen adsorption isotherm of the carbon nanotube / activated carbon composite material prepared in Example 2 is shown at 298 K. Nitrogen adsorption increases rapidly at low pressure (below 0.1 bar) and then tends to stabilize, exhibiting a typical H4-type adsorption isotherm. This indicates that the carbon nanotube / activated carbon composite material is mainly composed of micropores, which means that it has a large number of micropores and channels that can be used to adsorb gas or solute molecules. These micropores provide a large number of active sites, which greatly promotes the adsorption and separation of CH4 / N2.

[0040] Figure 4 The image shown is a scanning electron microscope image of the carbon nanotube / activated carbon composite material prepared in Example 3. It shows that the carbon nanotubes have interlayer stacking spaces, and the diameter of each space is about 0.344 nm. This indicates that the carbon nanotubes / activated carbon have narrow pores and a special structure. The narrow pores and the interlayer stacking spaces can effectively adsorb methane molecules to achieve separation.

[0041] Figure 5 The image shown is a scanning electron microscope image of the carbon nanotube / activated carbon composite material prepared in Example 4. The observed structure is tubular and has different diameters and lengths; it exhibits a typical cylindrical morphology and the outer wall surface is relatively smooth; indicating that a carbon nanotube structure has been formed.

Claims

1. A method for preparing a coal-based carbon nanotube / activated carbon composite material for CH4 / N2 adsorption and separation, characterized in that... Includes the following steps: The first step involves mixing 80-150 mesh raw coal, nitrogen source, and water in a mass ratio of 30-40 wt%: 20-30 wt%: 30-50 wt%, and adding 0.5-2 wt% fly ash of the total mass of the three raw materials into a high-pressure reactor. Under a carbon-rich atmosphere, the pressure is adjusted to 2-4 MPa and stirred evenly. The temperature is raised to 280-340℃ and maintained for 20-60 minutes. After the reaction is completed, heating is stopped, and the mixture is cooled to room temperature to obtain a solid-liquid mixture. The nitrogen source includes any one of urea, melamine, humic acid, and propylamine; The fly ash is rich in calcium, iron, sodium or potassium; its mass content is: calcium: 20-40%, iron: 5-8%, sodium or potassium: less than 0.8%; The second step is to obtain hydrothermal carbon by separating the solid-liquid mixture obtained in the first step and drying it. The third step involves using an impregnation method, with deionized water as the solvent to load the activator onto the hydrothermal carbon obtained in the second step. The mass ratio of the activator to the hydrothermal carbon is 1:1-3. After stirring with a magnetic stirrer for 8-12 hours, the mixture is placed in a sealed stainless steel reactor and heated to 600-900℃ at a heating rate of 3-10℃ / min, held at that temperature for 20-60 minutes, and then cooled to room temperature to obtain the pyrolysis product. The activator is any one of potassium oxalate, potassium citrate, potassium hydroxide, or potassium carbonate. The fourth step involves cooling the pyrolysis product to room temperature, washing it with distilled water until neutral, and drying it to obtain a carbon nanotube / activated carbon composite material.

2. The method for preparing coal-based carbon nanotube / activated carbon composite material for CH4 / N2 adsorption and separation according to claim 1, characterized in that: The raw coal mentioned in the first step has a volatile matter content (Vdaf) > 30% and a caking index < 60.

3. The method for preparing coal-based carbon nanotube / activated carbon composite material for CH4 / N2 adsorption and separation according to claim 1, characterized in that: In the first step, the volume content of CH4+CO in the carbon-rich atmosphere is greater than 70%, and the volume ratio of CO to CH4 is greater than 3.

4. A coal-based carbon nanotube / activated carbon composite material prepared by the preparation method according to any one of claims 1 to 3.

5. The application of the coal-based carbon nanotube / activated carbon composite material according to claim 4 in the adsorption and separation of CH4 / N2.

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