Method for preparing porous carbon material from cow dung lignocellulose and application of porous carbon material

Abstract

The invention discloses a method for preparing a porous carbon material from cow dung lignocellulose and application of the porous carbon material, and belongs to the technical field of lignocellulose pretreatment and porous material adsorbents. The method comprises the following steps: pre-treating cow dung in a dilute sulfuric acid solution, and taking out to obtain pre-treated cow dung; the concentration of the dilute sulfuric acid solution is 0.25-2%; the pretreatment temperature is 80 DEG C, and the time is 2-8 hours; pre-carbonizing the pretreated cow dung in a nitrogen atmosphere to obtain a pre-carbonized material; and mixing the pre-carbonized material with an activating agent, and heating and activating in a nitrogen atmosphere to obtain the cow dung-based porous carbon material. Cellulose and hemicellulose are degraded through pretreatment, pseudo-lignin is formed, retention of cellulose and generation of pseudo-lignin are beneficial to formation of more micropores, and compared with porous carbon formed by purely taking cow dung as a carbon source, the porous carbon has better pore structure and adsorption performance.

Description

Technical Field

[0001] The invention relates to the pretreatment of biomass lignocellulose, and more specifically, to a pretreatment technology of cow dung lignocellulose and the influence of the change of lignocellulose components on the preparation of porous carbon materials. Background Art

[0002] The amount of agricultural waste generated globally is huge, and wanton disposal not only causes waste of resources, but also leads to environmental pollution. Agricultural waste mainly includes two categories: crop waste and livestock and poultry manure. Biomass carbonization technology is one of the important ways to comprehensively utilize agricultural waste. Studies have found that the transformation of crop waste pays more attention to the production of high-value resources and multifunctional materials, while the transformation of livestock and poultry manure focuses more on the utilization of energy and fertilizer. In recent years, research on the conversion of livestock and poultry manure into carbon materials has gradually increased. With the development of large-scale animal husbandry, the problem of cow dung treatment has become increasingly prominent. Cow dung not only causes serious pollution to the environment, but also restricts the sustainable development of animal husbandry. Studies have shown that converting cow dung into carbon materials is an effective solution, but this process also faces many challenges. The direct pyrolysis conversion or extraction method of cow dung to prepare carbon materials has affected its application effect due to the uncertainty of cow dung itself.

[0003] The different contents and connection modes of cellulose, hemicellulose and lignin in lignocellulose in biomass will lead to significant differences in the properties of porous carbon materials such as porosity, specific surface area and pore structure. Therefore, lignocellulose pretreatment is a key step in the preparation of porous carbon materials. Selecting a suitable pretreatment method can effectively improve the performance and application value of porous carbon materials. There are physical, chemical, biological and physical and chemical pretreatment methods for lignocellulose pretreatment. Among them, chemical methods include organic solvent treatment, alkali treatment, acid treatment, ionic liquid treatment and ozone treatment. Each treatment method has different effects on lignocellulose components. Among them, alkali treatment can degrade lignin by reacting with the ester bond between lignin and hemicellulose to disconnect the link between lignin and hemicellulose. Acid treatment can promote the breaking of ether bonds in polysaccharide polymers through hydrolysis reactions, thereby achieving the depolymerization of cellulose and hemicellulose. In addition, acid treatment can produce pseudo-lignin and degrade cellulose and hemicellulose at the same time. The change in structure provides favorable conditions for the preparation of porous carbon materials.

[0004] Most of the studies on the conversion of lignocellulose into porous carbon materials are done by separating or degrading a certain component through pretreatment. In order to improve the complexity of the preparation process and the characteristics of the lignocellulose structure, the acid pretreatment method is used to degrade or change the structure of cellulose, hemicellulose, and lignin in the biomass, and explore the effects of the common changes of cellulose, hemicellulose, and lignin on the structure and performance of carbon materials. Summary of the invention

[0005] The technical problem to be solved by the present invention is to change the component content and structure of lignocellulose in cow dung by pretreatment with dilute sulfuric acid, thereby improving the surface chemical properties of porous carbon materials, preparing a cow dung-based porous carbon material, and providing a solid adsorbent biomass porous carbon material for adsorbing CO2.

[0006] One of the purposes of the present invention is to provide a method for changing the content and structure of cellulose, hemicellulose and lignin in cow dung.

[0007] Another object of the present invention is to provide a method for preparing porous carbon materials from cow dung pretreated with dilute sulfuric acid.

[0008] Another object of the present invention is to provide a method for pretreating cow dung with dilute sulfuric acid and preparing porous carbon materials for use in CO2 adsorption.

[0009] The purpose of the present invention is achieved by the following technical solutions:

[0010] The invention discloses a preparation method of a porous carbon material and a cow dung pretreated with dilute sulfuric acid. The raw material is cow dung, and the cow dung is pretreated with a dilute sulfuric acid solution to change the content and structure of cellulose, hemicellulose and lignin.

[0011] According to the present invention, the principle of the dilute sulfuric acid solution treatment is to promote the breaking of ether bonds in polysaccharide polymers through hydrolysis reaction, thereby achieving the depolymerization of cellulose and hemicellulose. In addition, acid treatment can produce pseudo-lignin.

[0012] A method for pretreating cow dung with dilute sulfuric acid and a method for preparing a porous carbon material, comprising the following steps:

[0013] 1) Pretreatment: Weigh 5 g of the dried and crushed cow dung raw material, soak it in 100 mL of dilute sulfuric acid solution under certain temperature and time conditions and then dry it. The concentration of the dilute sulfuric acid solution is 0.25-2%; the pretreatment temperature is 80° C. and the time is 2-8 hours.

[0014] 2) Pre-carbonization: The dried solid in step 1) is crushed and ground, and then pre-carbonized in a N2 atmosphere, wherein the carbonization temperature of the pre-carbonization is 600°C and the carbonization time is 1 hour.

[0015] 3) Activation: The pre-carbonized material in step 2) is ground into powder and mixed with a certain proportion of an activator, dried, heated and activated in a N2 atmosphere, cooled to room temperature, washed to neutrality, dried, and ground into powder to obtain a porous carbon material, wherein the mass ratio of the activator to the carbonized material is 1:1, the stirring time is 1h, the activation temperature is 800°C, and the activation time is 1h. The washing reagents are hydrochloric acid and deionized water.

[0016] The prepared dilute sulfuric acid pretreated cow dung and the prepared porous carbon material will be used in CO2 adsorption.

[0017] The beneficial effects of the present invention are:

[0018] The different contents and connection modes of cellulose, hemicellulose and lignin in lignocellulose in biomass will lead to significant differences in the properties of porous carbon materials such as porosity, specific surface area and pore structure. Therefore, lignocellulose pretreatment is a key step in the preparation of porous carbon materials. Selecting a suitable pretreatment method can effectively improve the performance and application value of porous carbon materials. In the process of preparing porous carbon, a large number of oxygen-containing functional groups in cellulose and hemicellulose will be eliminated in the form of H2O, CO2 and CO during pyrolysis, resulting in a microporous structure. Lignin, on the other hand, contains a relatively high aromatic structure, which is difficult to crack and produces less small molecule gas. Therefore, the presence of lignin will affect the porosity and specific surface area of ​​porous carbon materials. The present invention uses a dilute sulfuric acid solution to pretreat cow dung, and promotes the breaking of ether bonds in polysaccharide polymers through a hydrolysis reaction, thereby achieving the depolymerization of cellulose and hemicellulose. In addition, the carbohydrate monomers released during the acid pretreatment process can be degraded into compounds such as furfural, 5-hydroxymethylfurfural, and levulinic acid, which can be further degraded into carbon-rich aromatic structures, and these aromatic structures are considered to be key intermediates in the formation of pseudo-lignin. The present invention degrades cellulose and hemicellulose in cow dung through pretreatment and forms pseudo lignin. The retention of cellulose and the generation of pseudo lignin are conducive to the formation of more micropores. Compared with porous carbon formed by simply using cow dung as a carbon source, it has a better pore structure.

[0019] The use of dilute sulfuric acid to pretreat cow dung and prepare porous carbon materials can not only realize the resource utilization of cow dung, but also promote the development of animal husbandry and ecological environment protection by improving resource utilization efficiency and reducing environmental pollution. The application of this technology expands the resource utilization of biomass waste and has important economic and social value. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The mass percentage of cellulose, hemicellulose and lignin in the intermediate product of step 4 of the preparation of the embodiment and the comparative example

[0021] Figure 2 FTIR graphs of the intermediate products obtained in step 4 of Examples 1-4 and Comparative Example;

[0022] Figure 3 SEM images of the intermediate products obtained in step 4 of the preparation of the examples and comparative examples;

[0023] Figure 4 The intermediate product obtained in step 4 of Examples 2, 4 and Comparative Example 13 C NMR spectra;

[0024] Figure 5The SEM images of the coke obtained in step 5 of the preparation of the examples and comparative examples are as follows;

[0025] Figure 6 The N2 adsorption-desorption isotherms and pore size distribution diagram of the coke obtained in step 5 of the preparation of the embodiment and the comparative example;

[0026] Figure 7 X-ray diffraction (XRD) patterns and Raman spectra (Raman) patterns of samples prepared for embodiments and comparative examples;

[0027] Figure 8 SEM of samples prepared for Examples 1-4;

[0028] Fig. 9 N2 adsorption-desorption isotherms and pore size distribution diagrams of samples prepared in Examples and Comparative Examples;

[0029] Fig.10 Fourier transform infrared (FTIR) and X-ray electron diffraction (XPS) patterns of samples prepared for Examples and Comparative Examples;

[0030] Fig.11 CO2 isotherm diagram of samples prepared in Example; DETAILED DESCRIPTION

[0031] The cow dung-based porous carbon material provided by the present invention, its preparation method and application are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0032] Example 1

[0033] 1) Wash the cow dung with deionized water for 4 times and then dry it in a drying oven.

[0034] 2) Dilute concentrated sulfuric acid with ultrapure water to prepare a 0.25% dilute sulfuric acid solution.

[0035] 3) Soaking the cow dung obtained in step 1) in 100 ml of the dilute sulfuric acid solution obtained in step 2) at 80° C. for 4 hours to obtain a mixture.

[0036] 4) Dry the mixture obtained in step 3) for 12 h, and grind the obtained solid.

[0037] 5) The solid obtained in step 4) was placed in a tubular furnace and pre-carbonized at 600° C. for 1 h under a nitrogen atmosphere.

[0038] 6) The pre-carbonized sample obtained in step 5) was mixed with KOH in a mass ratio of 1:1, stirred for 1 hour and dried. The dried sample was placed in a tubular furnace and heated at 800°C for 1 hour under a nitrogen atmosphere. After cooling to room temperature, it was washed with hydrochloric acid and finally washed with deionized water until neutral, and then dried at 120°C for 12 hours to obtain a porous carbon material.

[0039] Embodiment 2:

[0040] 1) Wash the cow dung with deionized water for 4 times and then dry it in a drying oven.

[0041] 2) Dilute concentrated sulfuric acid with ultrapure water to prepare a 0.5% dilute sulfuric acid solution.

[0042] 3) Soaking the cow dung obtained in step 1) in 100 ml of the dilute sulfuric acid solution obtained in step 2) at 80° C. for 4 hours to obtain a mixture.

[0043] 4) Dry the mixture obtained in step 3) for 12 h, and grind the obtained solid.

[0044] 5) The solid obtained in step 4) was placed in a tubular furnace and pre-carbonized at 600° C. in a nitrogen atmosphere for 1 h.

[0045] 6) The pre-carbonized sample obtained in step 5) was mixed with KOH in a mass ratio of 1:1, stirred for 1 hour and dried. The dried sample was placed in a tubular furnace and heated at 800°C for 1 hour under a nitrogen atmosphere. After cooling to room temperature, it was washed with hydrochloric acid and finally washed with deionized water until neutral, and then dried at 120°C for 12 hours to obtain a porous carbon material.

[0046] Embodiment 3:

[0047] 1) Wash the cow dung with deionized water for 4 times and then dry it in a drying oven.

[0048] 2) Dilute concentrated sulfuric acid with ultrapure water to prepare a 1% dilute sulfuric acid solution.

[0049] 3) Soaking the cow dung obtained in step 1) in 100 ml of the dilute sulfuric acid solution obtained in step 2) at 80° C. for 4 hours to obtain a mixture.

[0050] 4) Dry the mixture obtained in step 3) for 12 h and grind the obtained solid.

[0051] 5) The solid obtained in step 4) was placed in a tubular furnace and pre-carbonized at 600° C. in a nitrogen atmosphere for 1 h.

[0052] 6) The pre-carbonized sample obtained in step 5) was mixed with KOH in a mass ratio of 1:1, stirred for 1 hour and dried. The dried sample was placed in a tubular furnace and heated at 800°C for 1 hour under a nitrogen atmosphere. After cooling to room temperature, it was washed with hydrochloric acid and finally washed with deionized water until neutral, and then dried at 120°C for 12 hours to obtain a porous carbon material.

[0053] Embodiment 4:

[0054] 1) Wash the cow dung with deionized water for 4 times and then dry it in a drying oven.

[0055] 2) Dilute concentrated sulfuric acid with ultrapure water to prepare a 2% dilute sulfuric acid solution.

[0056] 3) Soaking the cow dung obtained in step 1) in 100 ml of the dilute sulfuric acid solution obtained in step 2) at 80° C. for 4 hours to obtain a mixture.

[0057] 4) Dry the mixture obtained in step 3) for 12 h and grind the obtained solid.

[0058] 5) The solid obtained in step 4) was placed in a tubular furnace and pre-carbonized at 600° C. in a nitrogen atmosphere for 1 h.

[0059] 6) The pre-carbonized sample obtained in step 5) was mixed with KOH in a mass ratio of 1:1, stirred for 1 hour and dried. The dried sample was placed in a tubular furnace and heated at 800°C for 1 hour under a nitrogen atmosphere. After cooling to room temperature, it was washed with hydrochloric acid and finally washed with deionized water until neutral, and then dried at 120°C for 12 hours to obtain a porous carbon material.

[0060] Embodiment 5:

[0061] 1) Wash the cow dung with deionized water for 4 times and then dry it in a drying oven.

[0062] 2) Dilute concentrated sulfuric acid with ultrapure water to prepare a 0.5% dilute sulfuric acid solution.

[0063] 3) Soaking the cow dung obtained in step 1) in 100 ml of the dilute sulfuric acid solution obtained in step 2) at 80° C. for 2 hours to obtain a mixture.

[0064] 4) Dry the mixture obtained in step 3) for 12 h, and grind the obtained solid.

[0065] 5) The solid obtained in step 4) was placed in a tubular furnace and pre-carbonized at 600° C. in a nitrogen atmosphere for 1 h.

[0066] 6) The pre-carbonized sample obtained in step 5) was mixed with KOH in a mass ratio of 1:1, stirred for 1 hour and dried. The dried sample was placed in a tubular furnace and heated at 800°C for 1 hour under a nitrogen atmosphere. After cooling to room temperature, it was washed with hydrochloric acid and finally washed with deionized water until neutral, and then dried at 120°C for 12 hours to obtain a porous carbon material.

[0067] Embodiment 6:

[0068] 1) Wash the cow dung with deionized water for 4 times and then dry it in a drying oven.

[0069] 2) Dilute concentrated sulfuric acid with ultrapure water to prepare a 0.5% dilute sulfuric acid solution.

[0070] 3) Soaking the cow dung obtained in step 1) in 100 ml of the dilute sulfuric acid solution obtained in step 2) at 80° C. for 8 hours to obtain a mixture.

[0071] 4) Dry the mixture obtained in step 3) for 12 h, and grind the obtained solid.

[0072] 5) The solid obtained in step 4) was placed in a tubular furnace and pre-carbonized at 600° C. in a nitrogen atmosphere for 1 h.

[0073] 6) The pre-carbonized sample obtained in step 5) was mixed with KOH at a mass ratio of 1:1 for 1 hour and dried. The dried sample was placed in a tubular furnace and heated at 800°C for 1 hour under a nitrogen atmosphere. After cooling to room temperature, it was washed with hydrochloric acid and finally washed with deionized water until neutral, and then dried at 120°C for 12 hours to obtain a porous carbon material.

[0074] Comparative Example:

[0075] 1) Wash the cow dung with deionized water for 4 times and then dry it in a drying oven.

[0076] 2) The cow dung obtained in step 1) was placed in a tubular furnace and pre-carbonized at 600° C. for 1 h in a nitrogen atmosphere.

[0077] 3) The pre-carbonized sample obtained in step 2) was mixed with KOH in a mass ratio of 1:1, stirred for 1 hour and dried. The dried sample was placed in a tubular furnace and heated at 800°C for 1 hour under a nitrogen atmosphere. After cooling to room temperature, it was washed with hydrochloric acid and finally washed with deionized water until neutral, and then dried at 120°C for 12 hours to obtain a cow dung-based porous carbon material.

[0078] Structure and performance characterization:

[0079] 1. Analysis of intermediate products and raw material cow dung obtained in steps 1-4 of the embodiment

[0080] Dilute acid pretreatment of cow dung has a certain effect on its cellulose, hemicellulose and lignin. Figure 1 The following is a graph showing the changes in the contents of cellulose, hemicellulose, and lignin after pretreatment at different concentrations and for different times. When pretreated with dilute acid, the cellulose and hemicellulose contents of all examples decreased, and the lignin content increased (Table 1). As the concentration increased, the cellulose content decreased, and the lignin content increased, because as the concentration of dilute sulfuric acid increased, its contact surface became more comprehensive, and the reaction proceeded more thoroughly. When the treatment time was different at the same concentration, the contents of cellulose, hemicellulose, and lignin did not change much, indicating that the change in time had little effect on the pretreatment.

[0081] Table 1, cellulose, hemicellulose, lignin content of comparative examples and examples

[0082]

[0083] FTIR Figure 2 In 3300cm -1The peaks on the left and right correspond to the stretching vibration caused by the hydroxyl group of cellulose. In Example 4, the transmittance here is not obvious due to the degradation of cellulose. -1 The peak near 1731cm represents the vibration of aliphatic carbon-hydrogen bonds. Its transmittance decreases with the increase of pretreatment sulfuric acid concentration, which is related to the increase of hemicellulose and cellulose degradation degree with the increase of pretreatment sulfuric acid concentration. -1 , 1638cm -1 , 1513cm -1 ,1285cm -1 The peak at 1026 cm-1 is caused by the conjugation of the aromatic rings of pseudo-lignin and lignin (carbonyl and / or carboxyl). -1 The peak is caused by the CO stretching vibration of cellulose. After pretreatment with dilute sulfuric acid, this peak disappears and is concentrated in the range of 1300-1000 cm -1 The new peaks in the region correspond to CO stretching (in alcohols, ethers or carboxylic acids), which is caused by the degradation of cellulose and hemicellulose by dilute sulfuric acid pretreatment, thereby breaking the internal connections of lignocellulose.

[0084] SEM after pretreatment with dilute sulfuric acid Figure 3 It can be seen that after the dilute acid pretreatment, the bundle structure of the examples is destroyed to varying degrees compared with the comparative example, and irregular cracks appear on the surface or part of the structure is peeled off. When pretreated with dilute sulfuric acid of different concentrations, the bundle structures of Examples 1, 3, and 4 are sunken or broken in the middle due to the removal of cellulose and hemicellulose, while regular holes are formed on the surface of Example 2. When the pretreatment time is different, the destruction of the bundle structure is more obvious as the immersion time increases, and the formed holes decrease as the immersion time increases. The reason may be that the contact time between the dilute sulfuric acid and the sample increases with the extension of the immersion time, thereby collapsing the pores. In the lignocellulose structure, cellulose and hemicellulose are mainly connected by hydrogen bonds. This physical connection makes the cellulose molecular chains intertwined into bundles in the hemicellulose matrix, similar to the structure of reinforced concrete, providing support and protection. Therefore, with the degradation of hemicellulose, the lignocellulose structure becomes more fragile.

[0085] Solid State 13 C NMR spectra Figure 4 The main peaks are shown in the table. 13The peaks of C NMR centered at 109-100ppm, 91-81ppm, 79-68ppm, and 68-58ppm are attributed to cellulose or xylan. The C1, C2, C3, C4, and C5 of xylose, and the 60-55ppm region are related to the methoxy groups related to the aromatic ring. The 50-10ppm peak is attributed to aliphatic carbon. The signal intensity here becomes weaker after pretreatment with dilute sulfuric acid. The sulfuric acid pretreatment causes the chemical bonds connected to some aliphatic carbons to break or change their structures. The peaks at 125-102ppm, 142-125ppm, and 155-142ppm are related to CH, CC, and CO of aromatic or furan.

[0086] 182-169ppm belongs to C=O of carboxylic acid or ester. After pretreatment with dilute sulfuric acid, a new peak appeared at 120-140ppm in the aromatic region, and a double peak of equal intensity appeared at 140-160ppm, which was caused by the rearrangement of the aromatic structure of lignin and the generation of pseudo-lignin during the pretreatment process. This change indicates that pretreatment with dilute sulfuric acid may convert aliphatic structures into aromatic structures or deplete aliphatic structures, further proving that pseudo-lignin is generated during pretreatment with dilute sulfuric acid.

[0087] The main derivatives after pyrolysis of the comparative example and Example 2 are listed in Tables 2 and 3. Various compounds are produced after pyrolysis of the raw cow dung, while the compounds after pyrolysis of the sample treated with dilute sulfuric acid are mainly aliphatic, alicyclic, aromatic, alcohol, aldehyde, ether, and amine sulfide. Due to the partial degradation of hemicellulose and cellulose and the retention of lignin and the generation of pseudo-lignin during the dilute sulfuric acid pretreatment, the derivatives after pyrolysis change, among which the changes in aromatics are mainly due to the changes in lignin, and the changes in aliphatics, aldehydes, etc. are due to the changes in cellulose and hemicellulose. Furfural in the pyrolysis product of Example 2 further proves that pseudo-lignin is produced during the dilute sulfuric acid pretreatment.

[0088] Table 2, Attribution of the main pyrolysis degradation product peaks of the comparative example (Py-GC / MS)

[0089]

[0090]

[0091] Table 3, Attribution of the main pyrolysis degradation product peaks of Example 2 (Py-GC / MS)

[0092]

[0093] Example 2

[0094]

[0095]

[0096] 2. Analysis of coke obtained in step 5 of the embodiment and step 2 of the comparative example

[0097] Comparative Examples and Examples Microstructure after pre-carbonization as shown in SEM Figure 5 As shown in the figure, the biomass structure of the raw cow dung is retained after pre-carbonization, while the overall structure of the lignocellulose of other samples becomes loose, and uniform holes appear on the surface. As the concentration of dilute sulfuric acid increases, the degree of surface structure damage and pores increase. This is because the degradation of hemicellulose during the dilute sulfuric acid pretreatment breaks the connection between hemicellulose and lignin, thereby destroying the rigid structure of the lignocellulose structure, exposing more structural surfaces, and the sulfuric acid retained on the surface during the pre-carbonization process corrodes the surface.

[0098] Figure 6 (ab) are N2 adsorption-desorption and pore size distribution diagrams of pre-carbonized samples. All samples present type IV isotherms. From the pore size distribution diagram and pore structure parameters (Table 4), it can be concluded that all samples have a microporous structure. Compared with the comparative example, the specific surface area, micropore volume and area of ​​the samples pretreated with dilute sulfuric acid increased significantly. This is because after pretreatment with dilute sulfuric acid, the internal structure of cellulose is peeled off and the connection between cellulose components is broken due to the removal of hemicellulose and cellulose, as well as the corrosion effect of residual sulfuric acid. The micropore area of ​​all embodiments is between 254-302m 2 / g range, the specific surface area, micropore area and volume of the examples after pre-carbonization are not much different, among which Example 2 has the largest specific surface area and total pore volume, and its cellulose content is 21.89%, and its lignin content is 36.10%, indicating that the retention of cellulose helps to form pores. Example 2 has the largest micropore area and volume, and its cellulose and lignin contents are 2.06% and 41.28% respectively, indicating that the generation of pseudo-lignin helps to form pores. It shows that Example 6 has the smallest micropore area and volume, but its total pore volume is not much different from other samples. This may be because part of the surface of the material is corroded due to the extended immersion time during the pretreatment with dilute sulfuric acid, and the corroded holes collapse after pre-carbonization to form macropores.

[0099] Table 4, pore structure parameters of the pre-carbonization step of the comparative example and the embodiment

[0100]

[0101] 3. Analysis of the intermediate product obtained in step 6 of the embodiment

[0102] XRD of Examples and Comparative Examples Figure 7(a), the broad peaks at 2θ angles of 22°-24° and 44° are associated with the (002) interface of the amorphous carbon material and the (100) crystal plane of graphite, respectively, which indicates that the material has a high degree of disorder, a low degree of graphitization and a small amount of graphitized carbon structure. Raman spectra of the embodiments and comparative examples Figure 7 (b) at about 1361 cm -1 The peak at about 1602cm -1 There are two obvious peaks. They belong to the defect peak and graphitization peak of D band and G band respectively. D band mainly comes from the disordered structure or defects in carbon materials. G band is composed of sp 2 The ID / IG ratio is close to 1, indicating that the material has a high degree of disorder and a low degree of graphitization.

[0103] SEM Figure 8 It can be seen that the surface structure of the examples and comparative examples after KOH activation is peeled off into longitudinal ridges, and uniform pores appear on the longitudinal ridges. Compared with the examples and comparative examples after pre-carbonization, the degree of surface peeling increases, and more pores appear on the surface. This is attributed to the degradation of hemicellulose and cellulose during the pretreatment process, the connection between the wood cellulose components is broken and the structure changes, so that the internal structure is revealed or the contact surface increases, which provides a favorable place for KOH corrosion to form pores.

[0104] The N2 adsorption-desorption isotherms (9a) of the examples and comparative examples belong to type I isotherms. The adsorption amount increases rapidly at low relative pressures (P / P0<0.1), which is due to micropore filling. As the relative pressure increases, the adsorption amount tends to stabilize and forms a platform, which indicates that multilayer adsorption has occurred. In addition, when the relative pressure exceeds 0.5, a very small hysteresis loop appears in the isotherm, indicating the presence of a mesoporous structure. It can be seen from the pore size distribution diagram (9b) that most of the pore sizes of the material are in the range of <2nm. With the reduction of cellulose in the pretreatment examples and the increase in the retention of lignin and the generation of pseudo-lignin, the specific surface area and total pore volume of the activated samples increase (Table 5). The micropore volume and area of ​​Examples 2 and 4 are the smallest. This may be because the pores generated during precarbonization collapse during activation carbonization, so that their micropore volume and area are significantly lower than those of other examples.

[0105] Table 5. Pore structure parameters of the embodiments and comparative examples

[0106]

[0107] Using FTIR ( Fig.10 a) The surface functional groups of the materials were analyzed as follows: -1 , 1620cm -1 , 1091cm-1 There are obvious diffraction peaks, which correspond to the stretching vibration of hydroxyl OH or NH, the symmetric stretching vibration of C=C, and the asymmetric stretching vibration of CO. The presence of C, O and N can be clearly seen from the full XPS spectrum of the sample (10b) and Table 6. Since cow dung itself contains nitrogen, the final sample also contains a small amount of nitrogen. Among them, the carbon, nitrogen and oxygen content of Example 1 and the comparative example are not much different. The oxygen content of Example 2 is the highest, and the carbon content of Examples 5 and 6 is the highest and the oxygen content is the lowest. The C1s peak of the sample ( Fig.10 c) can be deconvoluted into three peaks at 284.8, 285.7, and 289.3 eV, corresponding to CC / C=C, CO, and C=O, respectively. Fig.10 d) The spectrum is deconvoluted into three peaks with binding energies of approximately 531.3-531.6, 533.2-533.7, 534-534.8, and 535.3-536.8 eV, corresponding to O=C, CO, C-OH / O=CO, and oxygen in water molecules, respectively. Fig.10 e) The peaks at 399.7, 400.7, 401.2, and 404.9 eV after spectral deconvolution correspond to pyridinic nitrogen, pyrrolic nitrogen, graphitic nitrogen, and oxidized nitrogen, respectively.

[0108] Table 6, element contents of examples and comparative examples

[0109]

[0110] Fig.11 The CO2 adsorption capacity of the sample under 298K and 1bar conditions is shown in Figure 2. The adsorption capacity of the materials is respectively Example 1: 2.93mmol / g, Example 2: 3.89mmol / g, Example 3: 3.44mmol / g, Example 4: 3.29mmol / g, Example 5: 2.87mmol / g, and Example 6: 2.86mmol / g. For Examples 2, 3, and 4, the specific surface area and total pore volume increase, while the CO2 adsorption capacity decreases. The reason why the adsorption capacity of Example 2 is higher than that of other samples is attributed to its high oxygen content compared to other modified samples, and the nitrogen functional groups are mainly pyrrole nitrogen. The oxygen-containing groups of the material can change the polarity of the carbon material, introduce more surface defects and active sites, enhance the dispersion force, induction force and electrostatic attraction between the material and CO2, and play a positive role in improving the CO2 adsorption capacity. As an electron donor, the pyrrole nitrogen in the material can increase the alkalinity of the material surface, thereby enhancing the interaction force with the acidic CO2 molecules, promoting the combination of CO2 molecules with the surface of the carbon material, and increasing the adsorption capacity.

[0111] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing porous carbon material from cow dung lignocellulose, characterized in that: The following steps are involved: Step 1) pretreatment: soaking the dried and crushed cow dung raw material in a dilute sulfuric acid solution under certain temperature and time conditions and then drying; Step 2) Pre-carbonization: The dried solid in step 1) is crushed and ground and then pre-carbonized under a protective atmosphere. Step 3) Activation: Grind the pre-carbonized material of step 2) into powder and mix it with a certain proportion of an activator, dry it, heat it under a protective atmosphere for activation, cool it to room temperature, wash it to neutrality, dry it, and grind it into powder to obtain a porous carbon material.

2. The method for preparing the porous carbon material by pretreatment of cow dung with dilute sulfuric acid according to claim 1, characterized in that: In step 1), the concentration of the dilute sulfuric acid solution is 0.25% to 2%; the pretreatment temperature is 80° C. and the time is 2 to 8 hours.

3. The method for preparing a porous carbon material by pretreatment of cow dung with dilute sulfuric acid according to claim 1, characterized in that: In step 2), the carbonization temperature is 400-600° C. and the carbonization time is 1-2 hours.

4. The method for preparing a porous carbon material by pretreatment of cow dung with dilute sulfuric acid according to claim 1, characterized in that: In step 3), the activator includes one or more of potassium hydroxide, potassium acetate and potassium carbonate.

5. The method for preparing a porous carbon material by pretreatment of cow dung with dilute sulfuric acid according to claim 1, characterized in that: In step 3), the mass ratio of the pre-carbonized material to the activator is 1:(1-2).

6. The method for preparing a porous carbon material by pretreatment of cow dung with dilute sulfuric acid according to claim 1, characterized in that: In step 3), the activation temperature is 700-900° C. and the time is 1-2 hours.

7. The method for preparing a porous carbon material by pre-treating cow dung with dilute sulfuric acid according to claim 1, characterized in that: The prepared dilute sulfuric acid pretreated cow dung and the prepared porous carbon material are applied to CO2 adsorption.

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