A method for preparing activated carbon using crop straw as raw material and its application

Abstract

The present invention relates to a method for preparing activated carbon using crop straw as raw material, comprising the following steps: (S1) pretreatment: crushing the crop straw, washing it with water, and drying it in sequence to obtain a pretreated material; (S2) desiliconization: placing the pretreated material in a hydrofluoric acid solution for heat treatment, washing it until it is neutral, and drying it to obtain a desiliconized material; (S3) preactivation: placing the desiliconized material in an impregnation solution, impregnating and activating it at 80-110°C in a sealed state, and then granulating and drying it to obtain prefabricated activated carbon; the impregnation solution is a mixed aqueous solution of phosphoric acid, zinc chloride, and aminoindole hydrochloride prepared in a mass ratio of 1:(0.15-0.3):(0.1-0.25); and (S4) carbonization: carbonizing the prefabricated activated carbon under a protective atmosphere to obtain the activated carbon. The activated carbon prepared by the present invention has excellent adsorption properties for organic solvents, a high initial adsorption value, and an adsorption value retention rate of over 90% after 30 adsorption-desorption cycles.

Description

Technical Field

[0001] The invention belongs to the technical field of activated carbon preparation, and particularly relates to a method for preparing activated carbon using crop straw as raw material and application of the method in solvent recovery. Background Art

[0002] Activated carbon is widely used in solvent recovery, and its adsorption properties are closely related to its pore structure. Solvent recovery activated carbon generally refers to activated carbon used for organic solvent recovery, primarily for high-end air purification or various gas solvent recovery applications. Currently, coal-based granular carbon and wood-based activated carbon are the most commonly used. However, coal is a non-renewable resource. Furthermore, at the same production cost, wood-based activated carbon offers superior technical specifications to coal-based carbon, leading to a growing demand for wood-based activated carbon. Currently, the production of wood-based activated carbon primarily uses trees as raw material, but trees grow very slowly, and felling trees for charcoal burning severely damages the ecological environment. To alleviate the conflict between demand for wood-based activated carbon and the protection of ecological resources, the use of crop straw to produce activated carbon is of great significance.

[0003] Traditionally, activated carbon prepared from crop straw is activated by phosphoric acid, but the reaction cycle is long, the efficiency is low, and the porosity needs to be further improved. The main reason is that crop straw has high internal binding force and dense structure, and the cellulose, hemicellulose, and lignin are entangled and restricted with each other, so its structure is stable and not easily destroyed. If CO2 and H2O atmosphere activation are combined with phosphoric acid activation, the activation efficiency can be improved, and the porosity and specific surface area can be increased. However, it is easy to cause over-activation, which leads to the collapse of the pore structure and the decline of adsorption performance. Chinese patent CN 117208901A discloses a honeycomb solvent-recovery activated carbon and its preparation method. The preparation method comprises the following steps: S1. Cleaning, drying, and sterilizing bamboo to obtain a clean raw material; S2. Adding carbon to the raw material after heating, starting a vacuum pump to control the pressure, maintaining the temperature and pressure, cooling to room temperature, and then pulverizing the raw material. The raw material is then sprayed with an acidic solution to separate the solid and liquid, thereby obtaining a mixed material; S3. Ball-milling the raw material into a powder, screening the milled raw material, and obtaining a mixed powder; S4. Adding an acid solution, mixing, and activating the mixture under a steam atmosphere, cooling, and obtaining an activated product; S5. Washing, filtering, and drying the solid product to obtain activated carbon. While the steam atmosphere in this invention facilitates the formation of mesopores in the activated carbon during phosphoric acid activation of the bamboo, the addition of microporous activated carbon in step S2 is necessary to overcome the pore structure collapse and reduced yield caused by over-activation. Summary of the Invention

[0004] In view of the above problems, the purpose of the present invention is to provide a method for preparing activated carbon using crop straw as raw material. Through the synergistic activation of phosphoric acid, zinc chloride, and aminoindole hydrochloride and desiliconization of crop straw before activation, activated carbon with rich and stable pores and a high specific surface area is prepared. The activated carbon has excellent adsorption properties for organic solvents.

[0005] A method for preparing activated carbon using crop straw as raw material comprises the following steps:

[0006] (S1) Pretreatment: The crop straw is crushed, washed, and dried in sequence to obtain pretreated material;

[0007] (S2) Desiliconization treatment: placing the pretreated material in a hydrofluoric acid solution for heat treatment, cooling, washing until neutral, and drying to obtain a desiliconized material;

[0008] (S3) Preactivation: placing the desiliconized material in an impregnation solution, impregnating and activating it in a sealed state at 80-110° C., and then granulating and drying it in sequence to obtain prefabricated activated carbon; the impregnation solution is a mixed aqueous solution of phosphoric acid, zinc chloride, and aminoindole hydrochloride prepared in a mass ratio of 1: (0.15-0.3): (0.1-0.25);

[0009] (S4) Carbonization: The prefabricated activated carbon is carbonized, cooled, washed, and dried under a protective atmosphere to obtain activated carbon.

[0010] This invention uses a highly active impregnation solution composed of phosphoric acid, zinc chloride, and aminoindole hydrochloride to pre-activate desiliconized crop straw before carbonizing it to produce activated carbon. The resulting activated carbon has a high specific surface area, is stable, and has abundant pores. Phosphoric acid, a Lewis acid, accelerates the decomposition of cellulose and hemicellulose in crop straw and promotes aromatic condensation during cellulose carbonization. Zinc chloride strongly catalyzes dehydroxylation and dehydration, allowing hydrogen and oxygen in the raw material to escape as water vapor, thereby suppressing tar production and preventing pore blockage. Aminoindole hydrochloride is a chloride salt containing a benzene ring and a nitrogen heterocycle. The inventors have discovered that aminoindole hydrochloride can synergistically activate crop straw with phosphoric acid and zinc chloride, resulting in the prepared activated carbon having richer pores and a higher specific surface area. This may be due to, on the one hand, the interaction between the benzene ring and the nitrogen heterocycle in aminoindole hydrochloride and the benzene ring in the biomass, thereby promoting the activation of the crop straw by phosphoric acid and zinc chloride; and, on the other hand, the presence of a large number of hydroxyl groups on the cellulose molecular chains in the crop straw, resulting in strong hydrogen bonding between or within the cellulose molecules. Aminoindole hydrochloride can form hydrogen bonds with the hydroxyl groups on the cellulose molecular chains, thereby weakening the hydrogen bonding between or within the cellulose molecules and increasing the spatial volume of the cellulose, thereby making the cellulose looser. The present invention prepares high-performance activated carbon through the synergistic effect of phosphoric acid, zinc chloride and aminoindole hydrochloride. However, the three cannot be combined in any proportion. If the zinc chloride content is too low, its effect is limited, but if its content is too high, it will block the formed pores. If the aminoindole hydrochloride content is too low, its effect is also limited, but if its content is too high, the phosphoric acid content will be too low, thereby reducing the activation and pore-forming effect. Therefore, the ratio of the three needs to be controlled within an appropriate range to achieve the synergistic effect.

[0011] In addition, the silicon dioxide contained on the surface of the crop straw will hinder the contact between phosphoric acid, zinc chloride, aminoindole hydrochloride and the fiber and the activation reaction, thereby hindering the formation and development of pores in the activated carbon; the present invention desiliconizes the crop straw before carbonization and activation, and the desiliconization treatment will leave pores at the original position of the silicon dioxide, which will extend and expand the pores in the subsequent carbonization and activation process.

[0012] Furthermore, the crop straw in step (S1) is one or more of corn straw, wheat straw, rice straw, and soybean straw; the crushing is crushing to 100-400 mesh, and the crushing method is air flow crushing, high-speed crushing or ball milling; and the drying is drying at 80-100° C. for 24-36 hours.

[0013] Furthermore, in step (S2), the concentration of the hydrofluoric acid solution is 5-10 wt%, the solid-to-liquid ratio of the pretreated material to the hydrofluoric acid solution is 1 kg:1-3 L, and the heat treatment is performed at 50-70°C for 30-90 minutes. Using hydrofluoric acid heat treatment not only removes harmful silicon-containing components from the crop straw raw material but also further dissolves non-cellulose components in the crop straw, reducing resistance to subsequent high-temperature carbonization.

[0014] Furthermore, the aminoindole hydrochloride in step (S3) is at least one of 5-aminoindole hydrochloride, 4-aminoindole hydrochloride, 3-aminoindole hydrochloride, 2-aminoindole hydrochloride, 3-(2-aminoethyl)indole hydrochloride, and 3-(2-aminopropyl)indole hydrochloride.

[0015] Furthermore, in step (S3), the mass ratio of phosphoric acid, zinc chloride, and aminoindole hydrochloride in the impregnation solution is 1: (0.2-0.25): (0.15-0.2).

[0016] Furthermore, in step (S3), the concentration of the impregnation liquid is 50-75 wt%; the solid-liquid ratio of the desiliconized material to the impregnation liquid is 1 kg:1-3 L; and the impregnation activation is performed for 1.5-3 h.

[0017] In step (S3), the 80-110°C is the temperature for immersion activation, such as 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, and 110°C.

[0018] Furthermore, the granulation in step (S3) is to obtain columnar or spherical granules by a granulator, for example, by a plunger granulator, cutting into rods or strips with a diameter of 2 to 5 mm, such as 2 mm, 3 mm, 4 mm, and a length of 5 to 8 mm; by a needle granulator, spherical particles are formed, and the particle size can be 1.5 mm, 2 mm, 2.5 mm, or 3 mm; and the drying is drying in an oven at 100 to 150° C. for 24 to 36 hours.

[0019] Furthermore, in step (S4), the protective atmosphere is nitrogen, argon or helium; the carbonization conditions are: heating to 500-700°C at a heating rate of 3-8°C / min in a rotary kiln, keeping warm for 2-5 hours, and the rotary kiln speed is 15-20 rpm; the washing is boiling in water for 30-60 minutes to remove residual phosphoric acid in the carbonized material, so that the phosphoric acid in the carbonized material is less than 200 ppm, and the residual phosphoric acid can also be recovered for use in step (S1); the drying is drying to a moisture content of <5wt%, such as baking in an oven at 100-150°C for 24-36 hours.

[0020] In a second aspect, the present invention provides activated carbon prepared by the above method for preparing activated carbon.

[0021] In a third aspect, the present invention provides the use of activated carbon prepared by the above method for preparing activated carbon in solvent recovery.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention prepares porous activated carbon by impregnating and activating desiliconized crop straw with a high-activity impregnation solution formed by phosphoric acid, zinc chloride, and aminoindole hydrochloride in a specific mass ratio and then carbonizing the impregnated crop straw. The prepared activated carbon has abundant and stable pores and a high specific surface area, and has excellent adsorption properties for organic solvents. Not only is the initial adsorption value high, but the adsorption value retention rate after 30 adsorption-desorption cycles is above 90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the SEM image of the activated carbon prepared. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with specific examples. Unless otherwise specified, the "parts" in the examples of the present invention are all parts by weight, and the reagents used are all commercially available reagents in this area.

[0026] Example 1

[0027] (S1) Pretreatment: The wheat straw was crushed to 200 mesh by air flow, washed twice with water, and then dried in an oven at 90°C for 24 hours to obtain a pretreated material;

[0028] (S2) Desiliconization treatment: the pretreated material was placed in a 5 wt% hydrofluoric acid solution at a solid-liquid ratio of 1 kg:2 L, treated at 60° C. for 60 min, then cooled, washed with pure water until neutral, and dried to obtain a desiliconized material;

[0029] (S3) Preactivation: First, phosphoric acid, zinc chloride, and 4-aminoindole hydrochloride are prepared into a mixed aqueous solution with a concentration of 60 wt% at a mass ratio of 1:0.15:0.1, i.e., an impregnation solution; then, the desiliconized material is placed in the impregnation solution at a solid-liquid ratio of 1 kg:2 L, and impregnated and activated in a kneader at 100° C. for 2 hours to obtain a kneaded material; then, the kneaded material is formed into strip-shaped particles with a particle size of 3 mm and a length of 7 mm by a plunger granulator, and dried in an oven at 100° C. for 36 hours to obtain a prefabricated material;

[0030] (S4) Carbonization: The preform was placed in a rotary kiln and heated to 600°C at a heating rate of 6°C / min under a nitrogen atmosphere, and kept at this temperature for 2.5 h. The rotary kiln speed was 20 rpm. After carbonization, the preform was naturally cooled to room temperature, boiled in water for 40 min, and dried in an oven at 120°C for 24 h to obtain activated carbon.

[0031] The SEM images of the activated carbon prepared are as follows Figure 1 shown.

[0032] Example 2

[0033] The rest is the same as Example 1, except that the mass ratio of phosphoric acid, zinc chloride, and 4-aminoindole hydrochloride in the impregnation solution of step (S3) is 1:0.15:0.15.

[0034] Example 3

[0035] The rest is the same as Example 1, except that the mass ratio of phosphoric acid, zinc chloride, and 4-aminoindole hydrochloride in the impregnation solution of step (S3) is 1:0.2:0.15.

[0036] Example 4

[0037] The rest is the same as Example 1, except that the mass ratio of phosphoric acid, zinc chloride, and 4-aminoindole hydrochloride in the impregnation solution of step (S3) is 1:0.25:0.2.

[0038] Example 5

[0039] The rest is the same as Example 1, except that the mass ratio of phosphoric acid, zinc chloride, and 4-aminoindole hydrochloride in the impregnation solution in step (S3) is 1:0.3:0.25.

[0040] Example 6

[0041] The rest is the same as Example 1, except that 3-(2-aminoethyl)indole hydrochloride is used instead of 4-aminoindole hydrochloride in step (S3).

[0042] Example 7

[0043] The rest is the same as Example 1, except that rice straw is used instead of wheat straw in step (S1).

[0044] Comparative Example 1

[0045] The rest is the same as Example 1, except that the impregnation solution in step (S3) is an aqueous solution of phosphoric acid and zinc chloride in a mass ratio of 1:0.15, that is, it does not contain 4-aminoindole hydrochloride.

[0046] Comparative Example 2

[0047] The rest is the same as Example 1, except that the impregnation solution in step (S3) is an aqueous solution of phosphoric acid and 4-aminoindole hydrochloride in a mass ratio of 1:0.1, that is, it does not contain zinc chloride.

[0048] Comparative Example 3

[0049] The rest is the same as Example 1, except that the mass ratio of phosphoric acid, zinc chloride, and 4-aminoindole hydrochloride in the impregnation solution of step (S3) is 1:0.4:0.3.

[0050] Comparative Example 4

[0051] The rest is the same as Example 1, except that the mass ratio of phosphoric acid, zinc chloride, and 4-aminoindole hydrochloride in the impregnation solution of step (S3) is 1:0.1:0.05.

[0052] Comparative Example 5

[0053] The rest is the same as Example 1, except that step (S2) is omitted, that is, the crop straw is not subjected to desiliconization treatment.

[0054] Testing and Analysis

[0055] 1) Specific Surface Area Determination: A low-temperature nitrogen adsorption experiment was performed on the activated carbons prepared in the Examples and Comparative Examples using a Tristar II 3020 fully automatic specific surface analyzer produced by Micromeritics Instrument Corporation of the United States to determine the specific surface area of ​​the activated carbons.

[0056] 2) Adsorption performance test:

[0057] According to GB / T 12496.8-1999, the iodine adsorption values ​​of the activated carbons prepared in the examples and comparative examples were measured;

[0058] The toluene adsorption values ​​of the activated carbons prepared in the examples and comparative examples were determined according to GB / T 35815-2018.

[0059] 3) Adsorption Performance Retention Test: After the activated carbons prepared in the Examples and Comparative Examples were subjected to 30 cycles of toluene adsorption-desorption (water vapor desorption at 100°C for 60 minutes), their iodine adsorption and toluene adsorption values ​​were tested again. The adsorption retention rates were calculated by comparing the values ​​with those of the initial test.

[0060] ;

[0061] Table 1 Specific surface area and adsorption performance test

[0062] .

[0063] As can be seen from Table 1, the activated carbons prepared in Examples 1 to 7 of the present invention have a high specific surface area, an initial iodine adsorption value of more than 1200 mg / g, and an initial toluene adsorption value of 300 mg / g, both of which are significantly higher than those of the comparative example; and the iodine adsorption value retention rate and the toluene adsorption value retention rate after 30 adsorption-desorption cycles are both above 90%.

[0064] By comparing Example 1 with Comparative Examples 1 and 2, it can be seen that the adsorption effect of the prepared activated carbon is significantly reduced due to the lack of aminoindole hydrochloride or zinc chloride in the impregnation solution.

[0065] Comparison of Example 1 with Comparative Examples 2 and 3 shows that the synergistic effect can only be effectively exerted when the mass ratio of phosphoric acid, zinc chloride and aminoindole hydrochloride is controlled within an appropriate range.

[0066] By comparing Example 1 with Comparative Example 4, it can be seen that desiliconization of crop straw is beneficial to the increase of specific surface area, which is beneficial to the formation of rich pores and thus can enhance the adsorption effect.

[0067] The above detailed description is a specific description of one feasible embodiment of the present invention. This embodiment is not intended to limit the patent scope of the present invention. Any equivalent implementation or modification that does not depart from the present invention should be included in the scope of the technical solution of the present invention.

Claims

1. A method for preparing activated carbon using crop straw as raw material, characterized in that: The following steps are involved: (S1) Pretreatment: The crop straw is crushed, washed, and dried in sequence to obtain pretreated material; (S2) Desiliconization treatment: placing the pretreated material in a hydrofluoric acid solution for heat treatment, cooling, washing until neutral, and drying to obtain a desiliconized material; (S3) Preactivation: placing the desiliconized material in an impregnation solution, impregnating and activating it in a sealed state at 80-110° C., and then granulating and drying it in sequence to obtain prefabricated activated carbon; the impregnation solution is a mixed aqueous solution of phosphoric acid, zinc chloride, and aminoindole hydrochloride prepared in a mass ratio of 1: (0.15-0.3): (0.1-0.25); (S4) Carbonization: The prefabricated activated carbon is carbonized, cooled, washed, and dried under a protective atmosphere to obtain activated carbon.

2. The method for preparing activated carbon using crop straw as raw material according to claim 1, wherein: In step (S1), the crop straw is one or more of corn straw, wheat straw, rice straw, and soybean straw; the crushing is to crush to 100-400 mesh, and the crushing method is air flow crushing, high-speed crushing or ball milling; and the drying is drying at 80-100° C. for 24-36 hours.

3. The method for preparing activated carbon using crop straw as raw material according to claim 1, characterized in that: In step (S2), the concentration of the hydrofluoric acid solution is 5-10 wt %, the solid-liquid ratio of the pretreated material to the hydrofluoric acid solution is 1 kg:1-3 L; and the heat treatment conditions are: 50-70° C. for 30-90 min.

4. The method for preparing activated carbon using crop straw as raw material according to claim 1, characterized in that: The aminoindole hydrochloride in step (S3) is at least one of 5-aminoindole hydrochloride, 4-aminoindole hydrochloride, 3-aminoindole hydrochloride, 2-aminoindole hydrochloride, 3-(2-aminoethyl)indole hydrochloride, and 3-(2-aminopropyl)indole hydrochloride.

5. The method for preparing activated carbon using crop straw as raw material according to claim 1, characterized in that: In step (S3), the mass ratio of phosphoric acid, zinc chloride, and aminoindole hydrochloride in the impregnation solution is 1: (0.2-0.25): (0.15-0.2).

6. The method for preparing activated carbon using crop straw as raw material according to claim 1, characterized in that: In step (S3), the concentration of the impregnation liquid is 50-75 wt %; the solid-liquid ratio of the desiliconized material to the impregnation liquid is 1 kg:1-3 L; and the impregnation activation is performed for 1.5-3 h.

7. The method for preparing activated carbon using crop straw as raw material according to claim 1, characterized in that: In step (S3), the granulation is performed by a granulator to obtain columnar or spherical granules; and the drying is performed in an oven at 100-150° C. for 24-36 hours.

8. The method for preparing activated carbon using crop straw as raw material according to claim 1, characterized in that: In step (S4), the protective atmosphere is nitrogen, argon or helium; the carbonization conditions are: heating to 500-700°C at a heating rate of 3-8°C / min in a rotary kiln, keeping warm for 2-5 hours, and the rotary kiln speed is 15-20 rpm; the washing is boiling in water for 30-60 minutes; and the drying is drying to a moisture content of <5wt%.

9. Activated carbon prepared by the method for preparing activated carbon according to any one of claims 1 to 8.

10. Use of the activated carbon prepared by the method for preparing activated carbon according to any one of claims 1 to 8 in solvent recovery.

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