A method for preparing a hydrophobic hierarchical-pore carbon dioxide adsorption material for catalytic coal pyrolysis

Abstract

The application aims to provide a method for preparing a hydrophobic hierarchical pore carbon dioxide adsorption material through catalytic coal pyrolysis, and belongs to the technical field of carbon materials. The surface polarity and surface basicity of the porous carbon are enhanced through nitrogen doping, and the carbon dioxide adsorption capacity is effectively improved. Meanwhile, the method adopts a secondary activation modification mode of KOH, urea and thiourea, adjusts the structure of the carbon nanotube in the composite material, controls the porosity of the composite material, and prepares the carbon nanotube composite material with high specific surface area, rich in high micropores and mesopores, good hydrophobicity, and stable and good adsorption of carbon dioxide.

Description

Technical Field

[0001] This invention belongs to the field of carbon materials technology, specifically relating to a method for preparing hydrophobic cascade porous carbon dioxide adsorbent materials by catalytic coal pyrolysis. Background Technology

[0002] Currently, carbon materials are used for CO2 capture in flue gas due to their excellent microporous, hydrophobic, corrosion-resistant, and surface chemical properties. However, the large amount of moisture in flue gas affects the CO2 adsorption performance.

[0003] As research progresses, the requirements for the adsorption performance of carbon materials are becoming increasingly stringent. The surface chemical properties of carbon materials depend on the presence of heteroatoms, such as O, N, and S. These heteroatoms exist in the form of acidic, basic, or neutral organic functional groups. By introducing basic functional groups into their structure, the pore structure of carbon materials can be altered. Microporous activated carbon and coal-based carbon nanotubes in a mixture can be used to construct micro-mesoporous composite materials. Increasing the tiered pore structure of carbon materials and introducing basic groups onto the surface facilitates the capture of acidic CO2 gas, thereby improving the adsorption performance of carbon dioxide.

[0004] Chinese patent CN202011202081.5 discloses a bifunctional oxygen electrode catalyst and its preparation method using N and S co-doped porous carbon-coated carbon nanotubes, belonging to the field of energy materials and electrochemical technology. First, a polysulfide precursor is dispersed in a solvent and then transferred to a reaction vessel, where it is reacted at 50–300°C to obtain the polysulfide precursor material. Second, the precursor for the N and S co-doped porous carbon-coated CNT catalyst is prepared.

[0005] Chinese patent 201710258088.0 discloses a Co / N / S doped porous carbon nanotube and its preparation method. The method uses a polyacrylonitrile fiber film containing precursors as a template, and obtains a polyacrylonitrile@BMZIF fiber film through a solution reaction method; through a series of surface treatments, BMZIF@PDA / S nanotubes are obtained; and the BMZIF@PDA / S nanotubes are calcined to obtain Co / N / S doped porous carbon nanotubes. Although this patent produces porous carbon nanotubes with a certain high specific surface area, it does not control the content of ultrapores.

[0006] Chinese patent CN202210208344.6 discloses a method for preparing nitrogen-doped carbon nanotube materials for carbon dioxide adsorption. The method involves modifying the carbon material with an ionomer in an acidic system to increase the microporosity and improve the adsorption performance of carbon dioxide.

[0007] Chinese patent CN202010448273.8 discloses a method for preparing carbon nanotubes using high-sulfur and high-sodium materials. This method uses iron and sodium from high-sulfur iron ore in coal as catalysts for carbon nanotube formation. Sodium catalyzes the formation of coal-based carbon nanotubes while simultaneously opening pores in the coal, resulting in carbon nanotubes with sulfur adsorption capacity. Although this patent generates a certain specific surface area and pore size under high-temperature alkaline catalysis, it cannot adjust the content of ultramicropores.

[0008] Currently available literature indicates that there is no method to improve carbon dioxide adsorption performance by adjusting the micropore and mesopore content of coal-based carbon nanotubes during the preparation of activated carbon-based carbon nanotubes via coal pyrolysis. Therefore, there is an urgent need for a method to prepare carbon nanotube materials with higher specific surface area, good hydrophobicity, a high mesopore ratio, and excellent carbon dioxide adsorption performance. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing hydrophobic, tiered-porous carbon dioxide adsorbent materials through catalytic coal pyrolysis. This invention enhances the surface polarity and alkalinity of porous carbon by doping with nitrogen, effectively improving the adsorption capacity of carbon dioxide. Simultaneously, this invention employs a secondary activation modification method using KOH, urea, and thiourea to adjust the structure of carbon nanotubes and the porosity of the composite material, preparing a carbon nanotube composite material with a high specific surface area and a structure rich in micropores and mesopores. This material exhibits good hydrophobicity and stable and good adsorption properties for carbon dioxide.

[0010] The present invention adopts the following technical solution:

[0011] A method for preparing a hydrophobic cascade-porous carbon dioxide adsorbent material by catalytic coal pyrolysis includes the following steps:

[0012] The first step is to select bituminous coal, crush and screen it to obtain coal powder A;

[0013] The second step involves selecting coal powder A obtained in the first step, adding activator and catalyst B to coal powder A, and performing the first activation through impregnation to obtain mixture C;

[0014] The third step involves placing the mixture C obtained in the second step into a sealed stainless steel reaction vessel, heating it to 440-460℃, and holding it at 440-460℃ for activation and pore expansion. Then, it is heated to 880℃-920℃ at the same heating rate and held at that temperature for 1-3 hours to complete the activated carbon activation and carbon nanotube catalysis process of the mixture C, thus obtaining coal-based nanotubes D.

[0015] The fourth step involves cooling the coal-based carbon nanotubes D obtained in the third step to room temperature, acid washing the coal-based carbon nanotubes D until the pH is 6.5-7.5, repeatedly washing with deionized water, and then filtering. Finally, the coal-based carbon nanotubes E with hydrophobic function are obtained.

[0016] The fifth step involves adding a secondary activator F to the coal-based carbon nanotubes E obtained in the fourth step for a second activation. This activation process is repeated according to the micropore size, until a hydrophobic cascade porous carbon dioxide adsorption material with ultra-micropore content is finally obtained.

[0017] Furthermore, the bituminous coal mentioned in the first step includes long-flame coal, and the coal powder A is screened through an 80-mesh sieve to obtain a particle size of less than 0.2 mm.

[0018] Furthermore, in the second step, the activation catalyst B is composed of potassium hydroxide and urea, with a mass ratio of potassium hydroxide to urea of ​​1-3:1. The first activation process involves the activation catalyst B activating coal powder A, with a mass ratio of activation catalyst B to coal powder A of 1-3:1.

[0019] Furthermore, in the third step, the heating rate of the mixture C is 3-10℃ / min, the residence time at 440-460℃ is 1-5h, and the mass fraction of the obtained coal-based carbon nanotubes D is 5-10 wt%.

[0020] Furthermore, the pickling solvent mentioned in step four is 1 mol / L hydrochloric acid with a pH range of 6.5-7.5.

[0021] Furthermore, the secondary activator F mentioned in step five is composed of potassium hydroxide and thiourea in a mass ratio of 1-3:1, and the mass ratio of the secondary activator F to the coal-based carbon nanotubes E is 1-2:1.

[0022] Furthermore, the hydrophobic cascade porous carbon dioxide adsorbent material with ultramicropore content obtained in step 5 has an ultramicropore content ranging from 0.79% to 0.83% of the total pore volume.

[0023] Furthermore, the hydrophobic, tiered porous carbon dioxide adsorbent material with ultramicropores obtained in the fifth step has a contact angle of 46-93° with the liquid.

[0024] Furthermore, the second activation described in step five is performed at a temperature of 550-650℃ for a time of 1.5-2.5 h.

[0025] This invention utilizes N and S co-doping to activate the substrate, improve the dispersion of the catalyst, increase the yield of carbon nanotubes, and increase the pore content of ultrafine pores, ultimately for CO2 adsorption performance.

[0026] This invention uses coal as raw material and obtains a porous material with activated carbon as the substrate and carbon nanotubes grown on the surface through secondary activation. At the same time, the content of ultra-micropores and mesopores in the porous structure is further adjusted to have a more profound impact on its carbon dioxide adsorption performance.

[0027] This invention uses coal as raw material to prepare activated carbon-based carbon nanotubes with a volumetric porous structure. The well-developed microporous structure of activated carbon can provide space for CO2 to be contained. The overlap of electric field potentials of adjacent pore walls can make the interaction between pore structures stronger, which is conducive to CO2 capture. The smaller mesopores, mainly composed of carbon nanotubes, can promote the rapid diffusion of CO2, which is conducive to reducing the diffusion resistance of molecules and shortening the diffusion path.

[0028] This invention not only expands pores through alkali catalysis, but also introduces nitrogen and sulfur elements to enhance the surface polarity and alkalinity of porous carbon, and adjusts the content of ultra-micropores in coal-based carbon nanotubes, effectively improving the carbon dioxide adsorption capacity.

[0029] This invention utilizes a prepared carbon nanotube tiered porous material for adsorption, and increasing the surface carbon nanotube content enhances the material's hydrophobic properties. Increasing the surface carbon nanotube content significantly improves hydrophobicity; when the carbon nanotube content increases from 0 to 8-10 wt%, the wetting angle increases from 35° to 46°, and if it increases to over 20 wt%, the wetting angle can reach over 60°.

[0030] The beneficial effects of this invention are as follows:

[0031] This invention prepares a N / O co-doped carbon nanotube cascade porous composite material with a layered porous structure by co-pyrolysis of KOH and urea. The doping of nitrogen and oxygen increases the surface basicity and surface polarity of the carbon material, enhancing its affinity for carbon dioxide, resulting in a rich microporous structure in the prepared carbon nanotubes with a micro-to-mesopore ratio as high as 0.79–0.83. It exhibits excellent selective adsorption of carbon dioxide. Furthermore, after multiple cyclic carbon dioxide adsorption experiments, the original carbon dioxide adsorption capacity is retained, demonstrating good adsorption stability, while simultaneously improving the hydrophobicity of the carbon material. Attached Figure Description

[0032] Figure 1 Comparison of CO2 adsorption isotherms at 0 °C for carbon nanotubes prepared in this invention;

[0033] Figure 2 Comparison of CO2 and N2 adsorption isotherms at 25 °C for carbon nanotubes prepared in this invention;

[0034] Figure 3 The contact angle diagram is shown between the 8wt% carbon nanotubes prepared in this invention and the liquid. Detailed Implementation

[0035] The present invention will be further described in conjunction with specific embodiments.

[0036] Example 1

[0037] The first step is to select long-flame bituminous coal, crush it, and sieve it through an 80-mesh sieve to obtain coal powder A with a particle size of 0.18 mm;

[0038] The second step involves selecting coal powder A obtained in the first step and adding activation and catalyst B according to a mass ratio of potassium hydroxide to urea of ​​1:1. The mass ratio of activation and catalyst B to coal is 3:1. The mixture is then activated for the first time by impregnation to obtain mixture C.

[0039] The third step involves placing the mixture C obtained in the second step into a sealed stainless steel reaction vessel and heating it to 450°C at a heating rate of 10°C / min. The mixture is then held at 450°C for 1 hour to activate and expand the pores. Finally, it is heated to 900°C at a heating rate of 10°C / min and held at that temperature for 2 hours. This completes the activated carbon activation and carbon nanotube catalysis process of the mixture C, yielding coal-based nanotubes D with a mass fraction of 8 wt%.

[0040] The fourth step involves cooling the coal-based carbon nanotubes D obtained in the third step to room temperature, then washing them with 1 mol / L acid until the pH reaches 7. The mixture is then repeatedly washed with deionized water and filtered. Finally, it is dried at 100 °C for 12 h to obtain coal-based carbon nanotubes E with hydrophobic properties.

[0041] In the fifth step, the coal-based carbon nanotubes E obtained in the fourth step are added with secondary activator F at a mass ratio of potassium hydroxide to thiourea of ​​1:1. The material is then activated for a second time at 650 °C for 1.5 h. The activation process is adjusted according to the micropore content. This step is repeated until a tiered porous carbon dioxide adsorbent material with a micropore content of 0.82% of the total pore volume and a liquid contact angle of 46.33° is obtained. Finally, a hydrophobic tiered porous carbon dioxide adsorbent material with a certain micropore content is obtained.

[0042] Example 2

[0043] First step, select the second step, select the coal powder A obtained in the first step, add activation and catalyst B according to the mass ratio of potassium hydroxide to urea 2:1, the mass ratio of activation and catalyst B to coal is 2:1, and perform the first activation by impregnation method to obtain mixture C;

[0044] The third step involves placing the mixture C obtained in the second step into a sealed stainless steel reaction vessel and heating it to 440°C at a heating rate of 8°C / min. The mixture is then held at 440°C for 1 hour to activate and expand the pores. Finally, it is heated to 880°C at a heating rate of 8°C / min (the heating rates should be the same for both steps and need to be adjusted accordingly). After reaching the temperature, the mixture is held at that temperature for 3 hours to complete the activated carbon activation and carbon nanotube catalysis process of the mixture C, resulting in coal-based nanotubes D with a mass fraction of 5 wt%.

[0045] The fourth step involves cooling the coal-based carbon nanotubes D obtained in the third step to room temperature, then acid-washing the coal-based carbon nanotubes D with 1 mol / L until the pH reaches 7.5, repeatedly washing with deionized water, filtering, and then drying at 110℃ for 12 h to finally obtain coal-based carbon nanotubes E with hydrophobic function.

[0046] In the fifth step, the coal-based carbon nanotubes E obtained in the fourth step are added with secondary activator F at a mass ratio of potassium hydroxide to thiourea of ​​2:1. The material is then activated for a second time at 600 °C for 2.5 h. The activation process is adjusted according to the micropore content. This step is repeated until a tiered porous carbon dioxide adsorbent material with an ultramicropore content of 0.81% of the total pores and a liquid contact angle of 93° is obtained. Finally, a hydrophobic tiered porous carbon dioxide adsorbent material with a certain ultramicropore content is obtained.

[0047] Example 3

[0048] The first step is to select long-flame bituminous coal, crush it, and screen it through an 80-mesh sieve to obtain coal powder A with a particle size of less than 0.2 mm;

[0049] The second step involves selecting coal powder A obtained in the first step and adding activation and catalyst B according to a mass ratio of potassium hydroxide to urea of ​​3:1. The mass ratio of activation and catalyst B to coal is 1:1. The mixture is then activated for the first time by impregnation to obtain mixture C.

[0050] The third step involves placing the mixture C obtained in the second step into a sealed stainless steel reaction vessel, heating it to 460°C at a heating rate of 3°C / min, and holding it at 460°C for 1 hour to activate and expand the pores. Then, it is heated to 920°C at a heating rate of 3°C / min and held at that temperature for 1 hour to complete the activated carbon activation and carbon nanotube catalysis process of the mixture C, yielding coal-based nanotubes D with a mass fraction of 10 wt%.

[0051] The fourth step involves cooling the coal-based carbon nanotubes D obtained in the third step to room temperature, then acid-washing the coal-based carbon nanotubes D with 1 mol / L until the pH reaches between 6.5, repeatedly washing with deionized water, and then filtering. Finally, drying at 105℃ for 12 h yields coal-based carbon nanotubes E with hydrophobic function.

[0052] In the fifth step, the coal-based carbon nanotubes E obtained in the fourth step are added with secondary activator F at a mass ratio of potassium hydroxide to thiourea of ​​3:1. The material is then activated for a second time at 550℃ for 1.5 hours. The activation process is adjusted according to the micropore content. This step is repeated until a tiered porous carbon dioxide adsorbent material with an ultramicropore content of 0.79% of the total pores and a liquid contact angle of 60° is obtained. Finally, a hydrophobic tiered porous carbon dioxide adsorbent material with a certain ultramicropore content is obtained.

[0053] Depend on Figure 1 The CO2 adsorption isotherm of the carbon nanotubes prepared in this invention at 0 ℃ shows that the CO2 adsorption capacity of the prepared double-activated carbon nanotubes (KN-NTC) at 0 ℃ is stronger than that of ordinary single-activated carbon nanotubes.

[0054] Depend on Figure 2 The adsorption isotherms of CO2 and N2 at 25 °C of the carbon nanotubes prepared in this invention show that the carbon nanotubes prepared in this invention have good CO2 and N2 adsorption capacity at 25 °C.

[0055] Depend on Figure 3 It can be seen that the contact angle between the 8wt% carbon nanotubes prepared by this invention and the liquid is 46.33°. When the carbon nanotube content increases, the surface hydrophobicity of the carbon nanotubes is enhanced.

Claims

1. A method for producing a hydrophobic hierarchical-pore carbon dioxide adsorbent material for catalytic coal pyrolysis, characterized by: It comprises the following steps: The first step is to select bituminous coal, crush and sieve to obtain coal powder A; The second step is to add potassium hydroxide and urea to the coal powder A obtained in the first step, and perform the first activation by impregnation method to obtain a mixture C; The mass ratio of potassium hydroxide and urea is 1-3:1, the first activation process is that potassium hydroxide and urea activate coal powder A, and the mass ratio of the total mass of potassium hydroxide and urea to the mass of coal powder A is 1-3:1; The third step is to place the mixture C obtained in the second step in a sealed stainless steel reaction container, heat to 440-460℃, and stay at 440-460℃ for activation and hole expansion, then heat to 880-920℃ at the same heating rate, and keep the temperature constant for 1-3h after reaching the temperature, complete the activation of the activated carbon and the catalysis of carbon nanotubes of the mixture C, and obtain coal-based nanotubes D; The fourth step is to cool the coal-based carbon nanotubes D obtained in the third step to room temperature, acid wash the coal-based carbon nanotubes D until the pH is 6.5-7.5, repeatedly wash with deionized water, and perform suction filtration, then dry at 100-110℃ for 12h, and finally obtain coal-based carbon nanotubes E with hydrophobic function; The fifth step is to add secondary activator F to the coal-based carbon nanotubes E obtained in the fourth step to perform the second activation, and repeat this step until the hydrophobic hierarchical pore carbon dioxide adsorption material with super-micropore content is finally obtained. The secondary activator F consists of potassium hydroxide and thiourea, and the mass ratio of the two is 1-3:1, and the mass ratio of the secondary activator F to the coal-based carbon nanotubes E is 1-2:

1.

2. The method of claim 1, wherein the method of producing a hydrophobic hierarchical- pore carbon dioxide adsorbent material by catalytic coal pyrolysis is characterized by: The bituminous coal in the first step includes long flame coal, and the coal powder A is sieved through an 80-mesh sieve to obtain a particle size of less than 0.2mm.

3. The method of claim 1, wherein the method of producing a hydrophobic hierarchical- pore carbon dioxide adsorbent material by catalytic coal pyrolysis is characterized by: The heating rate of the mixture C in the third step is 3-10℃ / min, the residence time at 440-460℃ is 1-5h, and the mass fraction of the obtained coal-based carbon nanotubes D is 5-10 wt%.

4. The method of claim 1, wherein the method of producing a hydrophobic hierarchical- pore carbon dioxide adsorbent material by catalytic coal pyrolysis is characterized by: The acid washing solvent in the fourth step is 1 mol / L hydrochloric acid, and the pH range is 6.5-7.

5.

5. The method of claim 1, wherein the method of producing a hydrophobic hierarchical- pore carbon dioxide adsorbent material by catalytic coal pyrolysis is characterized by: The hydrophobic hierarchical pore carbon dioxide adsorption material with super-micropore content prepared in the fifth step has a super-micropore content of 0.79-0.83 of the total pore volume.

6. The method of claim 1, wherein the method of producing a hydrophobic hierarchical- pore carbon dioxide adsorbent material by catalytic coal pyrolysis is characterized by: The hydrophobic hierarchical pore carbon dioxide adsorption material with super-micropore prepared in the fifth step has a contact angle with liquid of 46-93°.

7. The method of claim 1, wherein the method of producing a hydrophobic hierarchical- pore carbon dioxide adsorbent material by catalytic coal pyrolysis is characterized by: The temperature of the second activation in the fifth step is 550-650℃, and the time is 1.5-2.5h.

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