Preparation method of nitrogen-doped coal-based carbon material for methane enrichment

Abstract

The invention discloses a preparation method of a nitrogen-doped coal-based carbon material for methane enrichment, and belongs to the technical field of carbon materials for gas separation. The method comprises the following steps: carrying out controlled change on a coal structure through pretreatment of composite organic acid, mixing biomass waste as a nitrogen source with pretreated coal, and carrying out pyrolysis to obtain a nitrogen-doped coal-based crude product; and mixing the nitrogen-doped coal-based crude product with an activating agent, and activating in a nitrogen atmosphere to obtain the nitrogen-doped coal-based carbon material. Through pretreatment, chemical bonds in organic macromolecules in the coal are broken, the pore structure of the coal is improved, and the carbon element is enriched; the composite organic acid is used as a pickling agent, the biomass waste is used as a nitrogen source for nitrogen doping, use of a traditional high-energy-consumption chemical nitrogen source is reduced, pretreated coal and the nitrogen source are co-pyrolyzed to provide more micropores and channels for the carbon material, and the adsorption performance of the carbon material is obviously improved; the preparation process is simple and effective, and green preparation of the nitrogen-doped coal-based carbon material is realized.

Description

TECHNICAL FIELD

[0001] The application relates to a preparation method of a nitrogen-doped coal-based carbon material for methane enrichment, and belongs to the technical field of carbon materials for gas separation. BACKGROUND

[0002] Unconventional natural gas contains a certain proportion of impurities such as N2, O2 and CO2, and only when the CH4 concentration in methane gas reaches a certain value, the methane gas can be used as an efficient energy source or a chemical raw material. Enriching and recycling CH4 from unconventional natural gas has positive significance in energy, environmental protection and safety.

[0003] Pressure swing adsorption (PSA) technology is considered as a promising CH4 enrichment and separation technology, and the separation effect is mainly affected by the performance of the adsorbent. Coal-based carbon materials have become an important choice in the field of methane enrichment due to their economy, efficiency and practicability, and have a significant advantage in large-scale application.

[0004] Chinese patent CN109292751A discloses a preparation method and application of silicon-doped biochar, which is prepared by pretreating bamboo shoot shells in a nitric acid solution, and then performing silicon doping, carbonization, washing and purification treatment. The prepared adsorbent material is mainly used for cement solidification treatment of medium and low-level technetium waste liquid. However, the nitric acid pretreatment in this method can easily cause corrosion of the equipment, the production cost is high, and the generated waste liquid can easily pollute the environment.

[0005] Chinese patent CN117208902A discloses a preparation method and application of a triazine-pyrazine-based nitrogen-rich nitrogen-doped porous carbon material, which is prepared by taking a high-nitrogen-content porous organic polymer as a raw material, and then performing washing and purification, drying, activation and carbonization treatment. The prepared triazine-pyrazine-based nitrogen-rich nitrogen-doped porous carbon material has a good separation effect on CO2 / N2. However, the process is complex, the operability is poor, and the use of organic polymers can easily pollute the environment.

[0006] Therefore, in view of the above environmental pollution problems, there is an urgent need for a green method to realize the enrichment and separation of CH4. SUMMARY

[0007] The application aims to provide a preparation method of a nitrogen-doped coal-based carbon material for methane enrichment, which realizes green preparation of the coal-based carbon material by using biodegradable organic acid, avoids corrosion of the equipment and pollution of the environment by strong acid, and reduces the waste liquid treatment cost by more than 80%, thereby realizing effective enrichment of methane.

[0008] During pretreatment, the carboxyl and hydroxyl groups in the composite organic acid attack oxygen-containing bridges in the coal through hydrogen bonding, while simultaneously reacting with metal ions in the coal to form a chelate reaction, effectively disrupting the coal's dense cross-linked network. Gentle heating at 50-100°C induces thermal expansion of the coal's microcrystalline structure, further expanding the permeation channels for the organic acid and accelerating the oxidative cleavage of C-C bonds in the aromatic side chains, forming a mesoporous framework structure rich in carboxylic acid groups. This pretreatment (i.e., mixing coal powder and the composite organic acid in a suitable proportion, followed by heating and stirring) not only removes ash from the coal but also achieves pyrolysis and pore formation. This chemical modification provides abundant active sites for subsequent nitrogen doping, resulting in an ordered, localized enrichment of carbon. Using the composite organic acid as a pickling agent reduces the use of traditional inorganic acids, contributing to environmental protection. During the nitrogen doping process, the pyrolysis of the biomass nitrogen source is dynamically coupled with the thermochemical reconstruction of the coal, reducing the use of traditional, energy-intensive chemical nitrogen sources. When the temperature rises to 400-600°C, the proteins in the biomass decompose to produce NH3, HCN, and nitrogen-containing free radicals. These active substances undergo in situ condensation reactions with aromatic free radicals produced by coal pyrolysis. Through the cyclodehydration process of aminoaromatic intermediates, stable pyridinic and pyrrolic nitrogen-doped structures are ultimately formed. Simultaneously, the carboxylic acid groups pre-introduced into the pretreated coal skeleton act as electron acceptors, promoting the chemical adsorption of nitrogen-containing precursors on the carbon matrix surface, achieving uniform embedding of nitrogen atoms through an "anchoring-rearrangement" mechanism. The co-pyrolysis of pretreated coal and nitrogen source provides more micropores and channels for carbon materials, increases the specific surface area and pore volume of carbon materials, and the significantly increased microporosity and the generation of carbon nanotubes significantly improve the adsorption performance of carbon materials. The entire process realizes the systematic optimization of coal-based carbon materials from molecular structure to macroscopic performance through the progressive regulation of "directional breakage of chemical bonds-in-situ anchoring of nitrogen species-multi-level pore coordinated growth", and avoids the use of highly corrosive reagents throughout the preparation process. By replacing traditional inorganic acids with composite organic acids and chemical nitrogen sources with biomass nitrogen sources, the concept of green manufacturing of nitrogen-doped coal-based carbon materials is demonstrated while improving the adsorption performance.

[0009] The present invention provides a method for preparing a nitrogen-doped coal-based carbon material for methane enrichment, comprising the following steps: (1) Coal pretreatment: Grind the coal to 100-120 mesh, then mix the coal powder and composite organic acid in proportion, stir at a temperature of 50-100 ° C for 3-5 hours, wash with deionized water until neutral, and dry to obtain pretreated coal; (2) Pretreated coal-based nitrogen doping treatment: After the pretreated coal and nitrogen source are evenly mixed in proportion, they are placed in a tube furnace, the reaction temperature is controlled at 400-600 ° C, and the reaction is carried out at a constant temperature for 1-2 hours under a N2 atmosphere to obtain a nitrogen-doped coal-based crude product; (3) Activation treatment: The nitrogen-doped coal-based crude product obtained in step (2) and the activator are mixed uniformly by grinding and placed in a reactor; the reactor containing the sample is placed in a muffle furnace, and the activation temperature is controlled to be 600-1000°C and the activation time is 1-2 hours; a crude product of the nitrogen-doped coal-based carbon material is obtained; (4) The crude product obtained in step (3) is washed with deionized water until it becomes neutral, and dried to obtain a nitrogen-doped coal-based carbon material.

[0010] The coal in step (1) is one or more of weakly caking coal, lignite, long flame coal, and non-caking coal.

[0011] The composite organic acid described in step (1) is a mixture of light coal pyrolysis oil and an organic acid; the organic acid is one or more of formic acid, citric acid, oxalic acid, glycolic acid, maleic acid, p-toluenesulfonic acid, and salicylic acid, and the acid composite system composed of several selected acids can be any proportion combination; the light coal pyrolysis oil is rich in substances such as aliphatic and phenolic compounds.

[0012] The composite organic acid in step (1) is composed of light coal pyrolysis oil and an organic acid solution (referring to an aqueous solution of organic acid). The mass concentration of the organic acid solution is 5%-30%. In the composite organic acid, the mass ratio of the light coal pyrolysis oil to the organic acid solution is 4-6:4-6.

[0013] In step (1), the mass ratio of coal powder to the volume ratio of the composite organic acid is 1-10 g: 12-120 ml.

[0014] The nitrogen source in step (2) is one or more biomass materials with a high natural nitrogen content selected from corn gluten meal, soybean meal, or soybean dregs. When using a mixture of several nitrogen sources, they can be mixed in any proportion.

[0015] The mass ratio of the pretreated coal to the nitrogen source in step (2) is 1-1.5:1-3.

[0016] The activator in step (3) is at least one of KOH, K2CO3, KHCO3, C6H5K3O7, and triethanolamine, and the mass ratio of the nitrogen-doped coal-based crude product to the activator is 1-1.5:2-3.

[0017] The present invention provides the use of the nitrogen-doped coal-based carbon material in methane enrichment.

[0018] Beneficial effects of the present invention: (1) The use of composite organic acid to pretreat coal in this experiment has many significant effects. First, the aliphatic compounds rich in light coal pyrolysis oil act as non-polar solvents that can penetrate the coal matrix, while phenolic substances combine with oxygen-containing functional groups in the coal through hydrogen bonding, jointly promoting coal swelling and fluffing of the pore structure, providing channels for subsequent diffusion of activators. Second, the acidic environment of organic acids weakens the π-π interactions and hydrogen bonding in the coal macromolecular network through protonation, significantly reducing the energy barrier for chemical bond breakage, and lowering the starting temperature of reactions such as aromatic ring side chain breakage and oxygen-containing bridge bond dissociation during subsequent pyrolysis. Even at the same pyrolysis temperature, the number of broken bonds can be increased by a certain ratio, significantly increasing the release of small molecular volatiles such as carbon monoxide. Finally, the selective etching of minerals in the coal by the acidic components during pretreatment and the synergistic thermal condensation of the active components in the pyrolysis oil form a hierarchical porous structure within the coal char. Its mesopore ratio increases to several times that of the original coal, and the specific surface area is also significantly increased, providing an ideal mass transfer channel and active site carrier for subsequent catalytic conversion. This pretreatment technology achieves dual optimization of reducing energy consumption in the coal pyrolysis process and improving product quality through the coupling effects of physical swelling, chemical bond weakening and microstructure regulation.

[0019] (2) The present invention mixes coal and a nitrogen source, and co-pyrolyzes them during the carbonization and activation process, providing the carbon material with more micropores and channels, increasing the specific surface area and pore volume of the carbon material, and changing the pore structure of the carbon material prepared from pure coal. The significantly increased microporosity and the formation of carbon nanotubes significantly improve the adsorption performance of the carbon material. At the same time, biomass waste is used as a nitrogen source instead of a chemical reagent nitrogen source, reducing the use of traditional high-energy-consuming chemical nitrogen sources and being more environmentally friendly.

[0020] (3) The preparation process of the present invention has good controllability and does not rely on sophisticated production equipment. The reaction conditions are mild, the energy consumption is low, and it is safe and environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is an SEM image of the nitrogen-doped coal-based carbon material prepared in Example 1 of the present invention; Figure 2 This is a N2 adsorption-desorption isotherm diagram of the nitrogen-doped coal-based carbon material prepared in Example 1 of the present invention; Figure 3 This is a micropore distribution diagram of the nitrogen-doped coal-based carbon material prepared in Example 1 of the present invention; Figure 4 This is the mesopore distribution diagram of the nitrogen-doped coal-based carbon material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0022] The present invention is further illustrated by the following examples, but is not limited to the following examples. Example 1

[0023] (1) Long flame coal was crushed and ground to 120 mesh, 10 g of long flame coal powder was taken, 100 ml of composite organic acid, the long flame coal was immersed in the composite organic acid, stirred at 60℃ for 3-5h, the obtained mixture was washed repeatedly with deionized water until neutral, placed in an oven at 105℃ for 12h, and the pretreated coal was obtained; The composite organic acid was composed of light coal pyrolysis oil and formic acid solution with a mass ratio of 5:5, and the mass concentration of the formic acid solution was 5%; (2) 5g of pretreated coal and 10g of corn protein powder were mixed uniformly and placed in a tube furnace, heated to 500℃ at a heating rate of 10℃ / min under N2 atmosphere, and then kept at constant temperature for 2h to obtain a nitrogen-doped coal-based crude product.

[0024] (3) The nitrogen-doped coal-based crude product was loaded with 2 times the amount of K2CO3 on the product obtained in step (2) by grinding method, and placed in a reaction kettle. The reaction kettle containing the sample was placed in a muffle furnace and activated at a heating rate of 10℃ / min to 800℃, and the activation time was 1.5h. The crude product of nitrogen-doped coal-based carbon material was obtained.

[0025] (4) The crude product obtained in step (3) was washed with deionized water until the filtrate was neutral, and then dried to obtain the nitrogen-doped coal-based carbon material.

[0026] The nitrogen-doped carbon material obtained in Example 1 was subjected to nitrogen adsorption analysis, and the sample showed type I isotherm characteristics. At low pressure (P / P0<0.1, P / P0 is the relative pressure), the N2 adsorption amount increased rapidly, indicating that the sample had high micropore characteristics. With further increase of P / P0, the N2 adsorption amount tended to be stable. In the range of P / P0>0.45, the sample contained hysteresis loop, indicating that all samples contained different amounts of mesopores. Combined with the pore size distribution shown in Figure 3 and Figure 4 It can be seen that the sample mainly has micropores of 0.6-0.75nm and mesopores of 3.5-4nm.

[0027] According to Figure 3 the nitrogen adsorption-desorption isotherm, the specific surface area of the adsorbed carbon material was calculated as 1578m 2 / g, the total pore volume was 0.65cm 3 / g, the micropore volume was 0.59cm 3 / g, and the average pore size was 1.79nm, the micropore accounted for 91%, and the number of micropores increased greatly, which was more conducive to the adsorption and separation of gas.

[0028] In this embodiment, the nitrogen content of the nitrogen-doped carbon material is 4.08 at%. The prepared nitrogen-doped coal base is used for CH4 / N2 / CO2 separation, with an adsorption capacity of 1.88 mmol / g for CH4, an adsorption capacity of 0.49 mmol / g for N2, an adsorption selectivity of CH4 / N2 of 5.70, an adsorption capacity of 4.10 mmol / g for CO2, and an adsorption selectivity of CO2 / CH4 of 8.09. Example 2

[0029] (1) Grind the non-sticky coal to 120 mesh, take 10g of non-sticky coal powder and 110ml of composite organic acid, soak the non-sticky coal in the composite organic acid, stir at 60℃ for 3-5h, wash the resulting mixture repeatedly with deionized water until neutral, and dry it in an oven at 105℃ for 12h to obtain pretreated coal; The composite organic acid is composed of light coal pyrolysis oil and glycolic acid solution in a mass ratio of 4:6, and the mass concentration of the glycolic acid solution is 10%.

[0030] (2) Take 6 g of pretreated coal and 14 g of soybean meal, mix them evenly, place them in a tube furnace, heat them to 500 °C at a heating rate of 10 °C / min under N2 atmosphere, and keep the temperature constant for 2 h to obtain nitrogen-doped coal-based crude product.

[0031] (3) Using a grinding method, KOH (twice as much as the crude nitrogen-doped coal-based product) was loaded onto the product obtained in step (2) and placed in a reactor. The reactor containing the sample was placed in a muffle furnace and heated to 700°C at a heating rate of 10°C / min for 1.5 hours before activation. A crude nitrogen-doped coal-based carbon material was obtained.

[0032] (4) The crude product obtained in step (3) was washed with deionized water until the filtrate was neutral, and then dried to obtain a nitrogen-doped coal-based carbon material.

[0033] Nitrogen adsorption analysis of the nitrogen-doped carbon material obtained in Example 2 demonstrated that it was a micro-mesoporous mixed material, with the sample primarily comprising micropores of 0.5-0.7 nm and mesopores of 3.4-3.9 nm.

[0034] The specific surface area of ​​the adsorbed carbon material was calculated to be 1368 m 2 / g, total pore volume of 0.43cm 3 / g, micropore volume is 0.37cm 3 / g and the average pore size is 1.83nm, with micropores accounting for 86%.

[0035] In this embodiment, the nitrogen content of the nitrogen-doped carbon material is 3.50 at%. The prepared nitrogen-doped coal base is used for CH4 / N2 / CO2 separation, and the adsorption capacity for CH4 is 1.58 mmol / g, the adsorption capacity for N2 is 0.41 mmol / g, the adsorption selectivity for CH4 / N2 is 4.78, the adsorption capacity for CO2 is 3.79 mmol / g, and the adsorption selectivity for CO2 / CH4 is 7.35. Example 3

[0036] (1) Grind the lignite to 100 mesh, take 10g of lignite powder and 120ml of composite organic acid, soak the lignite in the composite organic acid, stir at 60℃ for 3-5h, wash the resulting mixture repeatedly with deionized water until neutral, and dry it in an oven at 105℃ for 12h to obtain pretreated coal; The composite organic acid is composed of light coal pyrolysis oil and maleic acid solution in a mass ratio of 6:4, wherein the mass concentration of the maleic acid solution is 15%.

[0037] (2) Take 4 g of pretreated coal and 9 g of bean dregs, mix them evenly, place them in a tube furnace, heat them to 500 °C at a heating rate of 10 °C / min under N2 atmosphere, and keep the temperature constant for 2 h to obtain nitrogen-doped coal-based crude product.

[0038] (3) Using a grinding method, KHCO₃ in an amount twice that of the crude nitrogen-doped coal-based product was loaded onto the product obtained in step (2) and placed in a reactor. The reactor containing the sample was placed in a muffle furnace and heated to 900°C at a heating rate of 10°C / min for 1.5 hours before activation. A crude nitrogen-doped carbon-based carbon material was obtained.

[0039] (4) The crude product obtained in step (3) was washed with deionized water until the filtrate was neutral, and then dried to obtain a nitrogen-doped coal-based carbon material.

[0040] Nitrogen adsorption analysis of the nitrogen-doped carbon material obtained in Example 3 showed that it was a micro-mesoporous mixed material. The sample mainly contained micropores of 0.5-0.7 nm and mesopores of 3.3-3.9 nm.

[0041] The specific surface area of ​​the adsorbed carbon material was calculated to be 1678 m 2 / g, total pore volume of 0.68cm 3 / g, micropore volume is 0.61cm 3 / g and the average pore size is 1.85nm, with micropores accounting for 90%.

[0042] The nitrogen content of the nitrogen-doped carbon material in this embodiment is 3.13 at%, and the prepared nitrogen-doped coal base is used for CH4 / N2 / CO2 separation. The CH4 adsorption capacity is 1.67 mmol / g, the N2 adsorption capacity is 0.43 mmol / g, the CH4 / N2 adsorption selectivity is 4.89, the CO2 adsorption capacity is 3.86 mmol / g, and the CO2 / CH4 adsorption selectivity is 7.56. Example 4

[0043] (1) The weakly cohesive coal was crushed and ground to 110 mesh. 10 g of weakly cohesive coal powder was taken, 105 ml of composite organic acid was taken, and the weakly cohesive coal was immersed in the composite organic acid solvent. The mixture was stirred at 60°C for 3-5h, and then washed with deionized water until neutral. The mixture was placed in an oven at 105°C for 12h to obtain the pretreated coal. The composite organic acid was composed of light coal pyrolysis oil and glycolic acid solution with a mass ratio of 5.5:4.5, and the mass concentration of the glycolic acid solution was 12%.

[0044] (2) 6g of the pretreated coal and 13g of soybean dregs were mixed uniformly and placed in a tube furnace. The temperature was raised to 500°C at a rate of 10°C / min under N2 atmosphere, and then kept constant for 2h to obtain the nitrogen-doped coal base crude product.

[0045] (3) The nitrogen-doped coal base crude product was loaded with 2 times of triethanolamine by grinding method, and then placed in a reaction kettle. The reaction kettle containing the sample was placed in a muffle furnace and activated at a rate of 10°C / min to 1000°C, and the activation time was 1.5h. The nitrogen-doped coal base carbon material was obtained.

[0046] (4) The crude product obtained in step (3) was washed with deionized water until the filtrate was neutral, and then dried to obtain the nitrogen-doped coal base carbon material.

[0047] The nitrogen-doped carbon material obtained in Example 4 was subjected to nitrogen adsorption analysis, and the results proved that it was a micro-mesoporous mixed material. The sample mainly had micropores of 0.5-0.7nm and mesopores of 3.2-3.8nm.

[0048] According to the nitrogen adsorption-desorption isotherm, the specific surface area of the adsorbed carbon material was calculated to be 1702m 2 / g, the total pore volume was 0.86cm 3 / g, the micropore volume was 0.69cm 3 / g, and the average pore size was 2.10nm, and the micropore accounted for 80%.

[0049] The nitrogen content of the nitrogen-doped carbon material in this embodiment is 3.01 at%, and the prepared nitrogen-doped coal base is used for CH4 / N2 / CO2 separation. The adsorption capacity for CH4 is 1.49 mmol / g, the adsorption capacity for N2 is 0.39 mmol / g, the adsorption selectivity for CH4 / N2 is 4.53, the adsorption capacity for CO2 is 3.63 mmol / g, and the adsorption selectivity for CO2 / CH4 is 7.26.

Claims

1. A method for preparing nitrogen-doped coal-based carbon materials for methane enrichment, characterized in that The steps include: (1) Coal pretreatment: Grind the coal to 100-120 mesh, then mix the coal powder and composite organic acid in proportion, stir at a temperature of 50-100 ° C for 3-5 hours, wash with deionized water until neutral, and dry to obtain pretreated coal; (2) Pretreated coal-based nitrogen doping treatment: After the pretreated coal and nitrogen source are evenly mixed in proportion, they are placed in a tube furnace, the reaction temperature is controlled at 400-600 ° C, and the reaction is carried out at a constant temperature for 1-2 hours under a N2 atmosphere to obtain a nitrogen-doped coal-based crude product; (3) Activation treatment: The nitrogen-doped coal-based crude product obtained in step (2) and the activator are mixed uniformly by grinding and placed in a reactor; the reactor containing the sample is placed in a muffle furnace, and the activation temperature is controlled to be 600-1000°C and the activation time is 1-2 hours; a crude product of the nitrogen-doped coal-based carbon material is obtained; (4) The crude product obtained in step (3) is washed with deionized water until it becomes neutral, and dried to obtain a nitrogen-doped coal-based carbon material.

2. The method for preparing nitrogen-doped coal-based carbon material for methane enrichment according to claim 1, characterized in that: The coal in step (1) is one or more of weakly caking coal, lignite, long flame coal, and non-caking coal.

3. The method for preparing nitrogen-doped coal-based carbon materials for methane enrichment according to claim 1, characterized in that: The composite organic acid in step (1) is a mixture of light coal pyrolysis oil and an organic acid solution; the organic acid is one or more of formic acid, citric acid, oxalic acid, glycolic acid, maleic acid, p-toluenesulfonic acid, and salicylic acid; and the light coal pyrolysis oil is rich in aliphatic and phenolic substances.

4. The method for preparing nitrogen-doped coal-based carbon material for methane enrichment according to claim 3, characterized in that: The mass concentration of the organic acid solution is 5%-30%. In the composite organic acid, the mass ratio of the light coal pyrolysis oil to the organic acid solution is 4-6:4-6.

5. The method for preparing nitrogen-doped coal-based carbon material for methane enrichment according to claim 1, characterized in that: In step (1), the mass ratio of coal powder to the volume ratio of the composite organic acid is 1-10 g: 12-120 ml.

6. The method for preparing nitrogen-doped coal-based carbon material for methane enrichment according to claim 1, characterized in that: The nitrogen source in step (2) is one or more biomass materials with high natural nitrogen content selected from corn gluten meal, soybean meal or soybean dregs.

7. The method for preparing nitrogen-doped coal-based carbon material for methane enrichment according to claim 1, characterized in that: The mass ratio of the pretreated coal to the nitrogen source in step (2) is 1-1.5:1-3.

8. The method for preparing nitrogen-doped coal-based carbon material for methane enrichment according to claim 1, characterized in that: The activator in step (3) is at least one of KOH, K2CO3, KHCO3, C6H5K3O7, and triethanolamine, and the mass ratio of the nitrogen-doped coal-based crude product to the activator is 1-1.5:2-3.

9. A nitrogen-doped coal-based carbon material for methane enrichment obtained by the preparation method according to any one of claims 1 to 8.

10. Use of the nitrogen-doped coal-based carbon material according to claim 9 in methane enrichment.

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