Preparation method and application of nitrogen-doped-thermally activated composite hydrophobically modified activated carbon

Abstract

This invention belongs to the field of activated carbon material modification, and relates to a method for preparing and applying nitrogen-doped-thermally activated composite hydrophobically modified activated carbon. The method involves first pretreating the activated carbon; then impregnating the pretreated activated carbon with urea impregnation solutions of different concentrations, and drying it to obtain nitrogen-doped activated carbon; then thermally activating the nitrogen-doped activated carbon in an oxygen-free environment at 500–700℃, and cooling it to obtain nitrogen-doped-thermally activated composite hydrophobically modified activated carbon. Test data from this invention show that, at a relative humidity of 80%, the saturated adsorption time of the composite hydrophobically modified activated carbon for dichloromethane in the gas is extended by 44.83% compared to unmodified activated carbon, and the saturated adsorption capacity is 53.7 mg / g, an increase of 66.65% compared to unmodified activated carbon. Composite hydrophobic modification of activated carbon is beneficial for improving its adsorption selectivity for VOCs under high humidity conditions.

Description

Technical Field

[0001] This invention belongs to the field of activated carbon material modification, specifically relating to a method for preparing and applying nitrogen-doped-thermally activated composite hydrophobic modified activated carbon. Technical Background

[0002] Volatile organic compounds (VOCs) are organic compounds with a saturated vapor pressure greater than 70 Pa at room temperature and a boiling point of 260℃ at normal pressure. They not only pose a threat to human health but may also lead to serious environmental problems such as the greenhouse effect and photochemical smog. Therefore, VOCs treatment is particularly important, and adsorption methods hold a leading position in the domestic market due to their ease of operation and low energy consumption. Among these methods, activated carbon has become one of the most popular adsorption materials due to its good adsorption performance and low price. However, in practical applications, the adsorption effect of activated carbon is greatly affected by water molecules in the waste gas. During adsorption, water molecules compete with VOCs molecules for adsorption sites on the activated carbon surface, leading to a decrease in the VOCs adsorption capacity. Therefore, hydrophobic modification treatment of activated carbon is necessary to reduce the adsorption of water vapor, thereby improving the activated carbon's adsorption capacity for target VOCs.

[0003] Currently, commonly used hydrophobic modification methods include impregnation and thermal activation. Impregnation typically involves loading hydrophobic compounds such as organosilanes onto the surface of activated carbon to achieve hydrophobicity. However, this method easily clogs the pore structure of the activated carbon, reducing its adsorption and regeneration performance, and is also costly, thus it is rarely used in actual production. Thermal activation involves treating activated carbon at high temperatures to remove oxygen-containing functional groups from its surface, reducing its surface polarity and thus decreasing the amount of water molecules adsorbed. However, this method easily causes the collapse of the internal pore structure of the activated carbon, reducing its specific surface area and hindering the adsorption of VOCs. Summary of the Invention

[0004] In view of the shortcomings of the above-described methods, the purpose of this invention is to provide a method for preparing and applying nitrogen-doped-thermally activated composite hydrophobic modified activated carbon.

[0005] The technical solution of the present invention is as follows:

[0006] A method for preparing nitrogen-doped-thermally activated composite modified hydrophobic activated carbon, comprising the following steps:

[0007] First, nitrogen-doped activated carbon is prepared by introducing urea into the pores of activated carbon through an impregnation method. Then, the activated carbon is thermally activated in an oxygen-free environment at 500–700℃. The urea crystals within the activated carbon pores decompose and volatilize at high temperature, releasing NH3 and CO2. This process increases the specific surface area and the number of micropores in the activated carbon, mitigating the problem that existing thermal activation methods may damage the pore structure and reduce the specific surface area. The activated carbon prepared by this method maintains a high specific surface area while reducing the number of oxygen-containing functional groups on the activated carbon surface and increasing the nitrogen content. It also increases the number of micropores, which can improve the hydrophobicity of the activated carbon to some extent and enhance its selective adsorption of VOCs in high-humidity environments.

[0008] To achieve the above objectives, the present invention adopts the following technical solutions, which are divided into two aspects: preparation and application:

[0009] A method for preparing nitrogen-doped-thermally activated composite hydrophobic modified activated carbon specifically includes the following steps:

[0010] The first step is the pretreatment of activated carbon: the coal-based columnar activated carbon is repeatedly rinsed with ethanol and ultrapure water and dried at 80-110℃;

[0011] The second step is the preparation of nitrogen-doped activated carbon: the pretreated activated carbon is impregnated in urea with a mass concentration of 5% to 20% and stirred for 12 to 24 hours. After being taken out and washed, it is placed in an oven at 80 to 110°C and dried for 6 hours to obtain nitrogen-doped activated carbon.

[0012] The third step is the preparation of nitrogen-doped-thermally activated composite hydrophobic modified activated carbon: nitrogen-doped activated carbon is thermally activated in an oxygen-free environment at 500-700℃ for 30-120 minutes, and then cooled to obtain nitrogen-doped-thermally activated composite hydrophobic modified activated carbon.

[0013] Preferably, the first step involves drying at 80°C.

[0014] An application of a nitrogen-doped-thermally activated composite hydrophobic modified activated carbon specifically includes the following steps:

[0015] (1) A suitable amount of nitrogen-doped-thermally activated composite hydrophobic modified activated carbon is loaded into the adsorption device;

[0016] (2) Control the VOCs concentration to 100-1000 ppm and the relative humidity to 10-80%;

[0017] (3) The above gas is passed into an adsorption device filled with composite hydrophobic modified activated carbon. The residence time of the gas through the activated carbon layer is controlled to be 0.2 to 1 second to achieve the removal of VOCs in the gas. The removal rate can reach more than 90%. The time when the VOCs removal rate is more than 90% can be controlled by adjusting the amount of activated carbon. The replacement or regeneration cycle of activated carbon can be controlled according to the initial concentration of VOCs in the gas and environmental protection requirements.

[0018] (4) The concentration of VOCs in the inlet and outlet gases of the adsorption device is measured in real time by a VOCs analyzer or a gas chromatograph. After the activated carbon is saturated, the adsorption breakthrough curve is plotted and the saturated adsorption capacity of the composite hydrophobic modified activated carbon for VOCs is calculated.

[0019] The benefits of this invention are:

[0020] (1) The effect of hydrophobic modification of activated carbon is significantly improved, and the modification process does not involve the use of toxic and harmful substances.

[0021] (2) It can effectively reduce the content of oxygen-containing functional groups on the surface of activated carbon and increase the number of micropores inside the activated carbon through nitrogen doping-thermal activation, which is conducive to improving the adsorption selectivity of activated carbon for small molecule VOCs, and improving the hydrophobicity of activated carbon and the adsorption effect under high humidity conditions.

[0022] The beneficial effects of this invention are:

[0023] This invention modifies activated carbon using a nitrogen-doped, thermally activated composite hydrophobic process. First, pretreated activated carbon is immersed in a urea impregnation solution, followed by high-temperature thermal activation. On one hand, residual urea crystals within the pores of the nitrogen-doped activated carbon pyrolyze into NH3 and CO2 gases at high temperatures. This gas volatilization process forms microporous structures within the activated carbon, increasing the number of micropores and enhancing its selective adsorption of small-molecule VOCs. On the other hand, high-temperature thermal activation effectively removes oxygen-containing functional groups from the activated carbon surface, reducing its affinity for water molecules and thus improving its hydrophobicity. Furthermore, residual nitrogen forms nitrogen-containing groups on the activated carbon surface, increasing the nitrogen / oxygen ratio (N / O ratio), which also enhances the activated carbon's VOCs adsorption performance. Attached Figure Description

[0024] Figure 1 Here are flowcharts of the adsorption devices used in each embodiment;

[0025] Figure 2 The image shows a comparison of scanning electron microscopy (SEM) images of unmodified activated carbon and the composite hydrophobic modified activated carbon in Example 4 of this invention.

[0026] Figure 3A comparison diagram of pore distribution and specific surface area between unmodified activated carbon and the composite hydrophobic modified activated carbon in Example 4 of this invention;

[0027] Figure 4 The adsorption curves of unmodified activated carbon and the composite hydrophobic modified activated carbon in Example 4 for dichloromethane at a relative humidity of 80%.

[0028] In the diagram: 1 is an N2 gas cylinder; 2 is a VOCs / N2 gas cylinder; 3 is a flow meter; 4 is an H2O bubbling bottle; 5 is a water bath; 6 is a gas mixing tank; 7 is a hygrometer; 8 is a gas mixing bottle; and 9 is an adsorption column. Detailed Implementation

[0029] The present invention is further described below through embodiments, but the present invention is not limited to the following embodiments. The embodiments are exemplary and intended to explain the present application, and should not be construed as limiting the present application.

[0030] Example 1

[0031] (1) Pretreatment of activated carbon: The activated carbon is repeatedly washed in ethanol solution and ultrapure water, and then dried thoroughly at 80°C to remove the moisture in the activated carbon, so as to obtain clean and dry activated carbon.

[0032] (2) Preparation of nitrogen-doped activated carbon: The pretreated activated carbon was soaked in a urea impregnation solution with a mass concentration of 5%, and magnetically stirred for 12 hours. After being taken out, washed, and dried in an oven at 110°C for 6 hours, nitrogen-doped activated carbon was obtained.

[0033] (3) Preparation of nitrogen-doped-thermally activated composite hydrophobic modified activated carbon: The dried nitrogen-doped activated carbon was thermally activated in an oxygen-free environment at 500℃ for 120 min, and then cooled to obtain nitrogen-doped-thermally activated composite hydrophobic modified activated carbon.

[0034] (4) The total amount of oxygen-containing functional groups on the surface of unmodified activated carbon and the composite hydrophobic modified activated carbon in Example 1 above was quantified using the Boehm titration method; the relative content of nitrogen and oxygen on the surface of activated carbon was determined using X-ray photoelectron spectroscopy, and the N / O ratio was calculated; [further details are needed for accurate translation.] Figure 1 The adsorption device shown was tested for its dichloromethane adsorption performance and the saturated adsorption capacity of activated carbon for dichloromethane was calculated: A certain amount of unmodified activated carbon or nitrogen-doped-thermally activated composite hydrophobic modified activated carbon was packed into the adsorption device. The concentration of gas dichloromethane was controlled to be 1000 ppm and the relative humidity to be 80% by a flow meter. The residence time through the activated carbon layer was 0.71 s. The concentration of dichloromethane at the inlet and outlet of the adsorption device was recorded by a gas chromatograph and the adsorption breakthrough curve was plotted to calculate the saturated adsorption capacity of unmodified activated carbon or composite hydrophobic modified activated carbon for dichloromethane. The results are shown in Table 1.

[0035] Table 1 Comparison of main parameters between composite hydrophobic modified activated carbon and unmodified activated carbon in Example 1

[0036]

[0037] Example 2

[0038] (1) Pretreatment of activated carbon: The activated carbon is repeatedly washed in ethanol solution and ultrapure water in sequence, and then dried thoroughly at 80°C to remove the moisture in the activated carbon and obtain clean and dry activated carbon.

[0039] (2) Preparation of nitrogen-doped activated carbon: The pretreated activated carbon was soaked in a urea impregnation solution with a mass concentration of 10% and magnetically stirred for 12 hours. After being taken out, washed, and dried in an oven at 110°C for 6 hours, nitrogen-doped activated carbon was obtained.

[0040] (3) Preparation of nitrogen-doped-thermally activated composite hydrophobic modified activated carbon: The dried nitrogen-doped activated carbon was thermally activated in an oxygen-free environment at 500℃ for 120 min, and then cooled to obtain nitrogen-doped-thermally activated composite hydrophobic modified activated carbon.

[0041] (4) The total amount of oxygen-containing functional groups on the surface of unmodified activated carbon and the composite hydrophobic modified activated carbon in Example 1 above was quantified using the Boehm titration method; the relative content of nitrogen and oxygen on the surface of activated carbon was determined using X-ray photoelectron spectroscopy, and the N / O ratio was calculated; [further details are needed for accurate translation.] Figure 1 The adsorption device shown was tested for its dichloromethane adsorption performance and the saturated adsorption capacity of activated carbon for dichloromethane was calculated: A certain amount of unmodified activated carbon or nitrogen-doped-thermally activated composite hydrophobic modified activated carbon was packed into the adsorption device. The concentration of gas dichloromethane was controlled to be 1000 ppm and the relative humidity to be 80% by a flow meter. The residence time through the activated carbon layer was 0.71 s. The concentration of dichloromethane at the inlet and outlet of the adsorption device was recorded by a gas chromatograph and the adsorption breakthrough curve was plotted to calculate the saturated adsorption capacity of unmodified activated carbon or composite hydrophobic modified activated carbon for dichloromethane. The results are shown in Table 2.

[0042] Table 2 Comparison of main parameters between composite hydrophobic modified activated carbon and unmodified activated carbon in Example 2

[0043]

[0044] Example 3

[0045] (1) Pretreatment of activated carbon: The activated carbon is repeatedly washed in ethanol solution and ultrapure water in sequence, and then dried thoroughly at 90°C to remove the moisture in the activated carbon and obtain clean and dry activated carbon.

[0046] (2) Preparation of nitrogen-doped activated carbon: The pretreated activated carbon was soaked in a urea impregnation solution with a mass concentration of 20% and magnetically stirred for 12 hours. After being taken out, washed, and dried in an oven at 110°C for 6 hours, nitrogen-doped activated carbon was obtained.

[0047] (3) Preparation of nitrogen-doped-thermally activated composite hydrophobic modified activated carbon: The dried nitrogen-doped activated carbon was thermally activated in an oxygen-free environment at 500℃ for 120 min, and then cooled to obtain nitrogen-doped-thermally activated composite hydrophobic modified activated carbon.

[0048] (4) The total amount of oxygen-containing functional groups on the surface of unmodified activated carbon and the composite hydrophobic modified activated carbon in Example 1 above was quantified using the Boehm titration method; the relative content of nitrogen and oxygen on the surface of activated carbon was determined using X-ray photoelectron spectroscopy, and the N / O ratio was calculated; [further details are needed for accurate translation.] Figure 1 The adsorption device shown was tested for its dichloromethane adsorption performance and the saturated adsorption capacity of activated carbon for dichloromethane was calculated: A certain amount of unmodified activated carbon or nitrogen-doped-thermally activated composite hydrophobic modified activated carbon was packed into the adsorption device. The concentration of gas dichloromethane was controlled to be 1000 ppm and the relative humidity to be 80% by a flow meter. The residence time through the activated carbon layer was 0.71 s. The concentration of dichloromethane at the inlet and outlet of the adsorption device was recorded by a gas chromatograph and the adsorption breakthrough curve was plotted to calculate the saturated adsorption capacity of unmodified activated carbon or composite hydrophobic modified activated carbon for dichloromethane. The results are shown in Table 3.

[0049] Table 3 Comparison of main parameters between composite hydrophobic modified activated carbon and unmodified activated carbon in Example 3

[0050]

[0051] Example 4

[0052] (1) Pretreatment of activated carbon: The activated carbon is repeatedly washed in ethanol solution and ultrapure water in sequence, and then dried thoroughly at 100°C to remove the moisture in the activated carbon and obtain clean and dry activated carbon.

[0053] (2) Preparation of nitrogen-doped activated carbon: The pretreated activated carbon was soaked in a urea impregnation solution with a mass concentration of 10% and magnetically stirred for 12 hours. After being taken out, washed, and dried in an oven at 110°C for 6 hours, nitrogen-doped activated carbon was obtained.

[0054] (3) Preparation of nitrogen-doped-thermally activated composite hydrophobic modified activated carbon: The dried nitrogen-doped activated carbon was thermally activated in an oxygen-free environment at 700℃ for 120 min, and then cooled to obtain nitrogen-doped-thermally activated composite hydrophobic modified activated carbon.

[0055] (4) The total amount of oxygen-containing functional groups on the surface of unmodified activated carbon and the composite hydrophobic modified activated carbon in Example 1 above was quantified using the Boehm titration method; the relative content of nitrogen and oxygen on the surface of activated carbon was determined using X-ray photoelectron spectroscopy, and the N / O ratio was calculated; [further details are needed for accurate translation.] Figure 1 The adsorption device shown was tested for its dichloromethane adsorption performance and the saturated adsorption capacity of activated carbon for dichloromethane was calculated: A certain amount of unmodified activated carbon or nitrogen-doped-thermally activated composite hydrophobic modified activated carbon was packed into the adsorption device. The concentration of gas dichloromethane was controlled to be 1000 ppm and the relative humidity to be 80% by a flow meter. The residence time through the activated carbon layer was 0.71 s. The concentration of dichloromethane at the inlet and outlet of the adsorption device was recorded by a gas chromatograph and the adsorption breakthrough curve was plotted to calculate the saturated adsorption capacity of unmodified activated carbon or composite hydrophobic modified activated carbon for dichloromethane. The results are shown in Table 4.

[0056] Table 4 Comparison of main parameters between composite hydrophobic modified activated carbon and unmodified activated carbon in Example 4

[0057]

[0058] Comparing Examples 1-3, it is evident that, under the same thermal activation temperature, impregnating activated carbon with a 10% mass concentration of urea solution is beneficial for nitrogen retention, resulting in a higher N / O ratio, which improves the adsorption performance of activated carbon for VOCs. Comparing Examples 3 and 4, it is evident that, under the same urea impregnation concentration, increasing the activation temperature from 500℃ to 700℃ can further reduce the oxygen-containing tubular groups on the activated carbon surface, while maintaining a higher N / O ratio. Furthermore, 700℃ is more conducive to promoting the pyrolysis of urea within the activated carbon pores, optimizing the specific surface area of ​​the activated carbon, and further increasing the saturated adsorption capacity of activated carbon for VOCs.

[0059] Figure 2 The images show a comparison of scanning electron microscopy (SEM) images of unmodified activated carbon and the composite hydrophobic modified activated carbon in Example 4 of this invention. After nitrogen doping-thermal activation composite hydrophobic modification, the surface texture of the activated carbon becomes deeper, and its roughness and etching degree increase, which is beneficial to the expansion of the internal pore structure and the increase of specific surface area of ​​the activated carbon. Figure 3 The image shows a comparison of pore distribution and specific surface area between unmodified activated carbon and composite hydrophobically modified activated carbon. It can be seen that the number of micropores with a diameter less than 1 nm significantly increased after composite hydrophobic modification. Simultaneously, the specific surface area of ​​the composite hydrophobically modified activated carbon increased from 889.23 m² before modification. 2 / g increased to 918.14m 2 / g, this is because the urea in the pores of activated carbon decomposes into NH3 and CO2 when heated, and the process of the gas volatilizing from the pores of activated carbon is conducive to the increase of the specific surface area and the number of micropores of activated carbon. Figure 4The graph shows the adsorption curves of unmodified activated carbon and the composite hydrophobic modified activated carbon in Example 4 for dichloromethane at a relative humidity of 80%. The composite hydrophobic modified activated carbon exhibits excellent adsorption performance for VOCs under high humidity conditions, with a saturated adsorption capacity greater than that of unmodified activated carbon. Its saturated adsorption time for dichloromethane in the gas is extended by 44.83% compared to unmodified activated carbon, and the saturated adsorption capacity is 53.7 mg / g, which is 66.65% higher than that of unmodified activated carbon.

[0060] In summary, this invention employs nitrogen doping and thermal activation to perform composite hydrophobic modification on activated carbon. First, nitrogen sources such as urea are doped into the activated carbon via impregnation. Subsequently, a heat treatment method is used to optimize the pore structure of the activated carbon. Urea in the pores of the activated carbon decomposes into NH3 and CO2 upon heating. This process is beneficial to increasing the specific surface area and the number of micropores of the activated carbon, which can improve the problem of pore blockage that may occur during thermal activation alone. At the same time, the heat treatment can decompose the polar oxygen-containing functional groups on the surface of the activated carbon, reducing its surface polarity. The resulting nitrogen-doped-thermally activated composite hydrophobic modified activated carbon has good hydrophobicity and stable performance, which helps to improve the adsorption performance of activated carbon for VOCs in high humidity environments.

[0061] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An application of a nitrogen-doped-thermally activated composite hydrophobic modified activated carbon, characterized in that, The preparation method of the nitrogen-doped-thermally activated composite hydrophobic modified activated carbon is as follows: The first step is the pretreatment of activated carbon: the coal-based columnar activated carbon is repeatedly rinsed with ethanol and ultrapure water and dried at 80~110℃; The second step is the preparation of nitrogen-doped activated carbon: the pretreated activated carbon is immersed in a urea impregnation solution with a mass concentration of 5% to 20% and stirred for 12 to 24 hours. After being taken out and washed, it is placed in an oven at 80 to 110 ℃ and dried for 6 hours to obtain nitrogen-doped activated carbon. The third step is the preparation of nitrogen-doped-thermally activated composite hydrophobic modified activated carbon: nitrogen-doped activated carbon is thermally activated in an oxygen-free environment at 500~700 ℃ for 30~120 min, and then cooled to obtain nitrogen-doped-thermally activated composite hydrophobic modified activated carbon. The application includes the following steps: (1) Nitrogen-doped-thermally activated composite hydrophobic modified activated carbon is packed into the adsorption device; (2) Control the VOCs concentration to 100~1000 ppm and the relative humidity to 80%; (3) The above gas is passed into an adsorption device filled with composite hydrophobic modified activated carbon. The residence time of the gas through the activated carbon layer is controlled to be 0.2~1 s to achieve the removal of VOCs in the gas with a removal rate of more than 90%. The time when the VOCs removal rate is more than 90% is controlled by adjusting the amount of activated carbon. The replacement or regeneration cycle of activated carbon is controlled according to the initial concentration of VOCs in the gas and environmental protection requirements. (4) The concentration of VOCs in the inlet and outlet gases of the adsorption device is measured in real time by a VOCs analyzer or a gas chromatograph. After the activated carbon is saturated, the adsorption breakthrough curve is plotted and the saturated adsorption capacity of the composite hydrophobic modified activated carbon for VOCs is calculated. The VOCs mentioned are dichloromethane.

2. The application as described in claim 1, characterized in that, The first step, as described above, involves drying at 80°C.

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