Preparation and application of a composite hydrophobically modified activated carbon for treating chlorine-containing VOCs

Abstract

The present invention belongs to the field of modified activated carbon materials, and specifically relates to the preparation and application of a composite hydrophobically modified activated carbon for treating chlorine-containing VOCs. The method first pretreats the activated carbon, and then uses ammonia solutions of different concentrations to perform alkaline solution impregnation treatment on the pretreated activated carbon to obtain alkaline solution modified activated carbon, and finally loads a porous hydrophobic polymer on the surface of the activated carbon pores by a solution impregnation method to obtain a composite hydrophobically modified activated carbon of alkaline solution and hydrophobic polymer. The modification effect of the present invention is good. When the relative humidity is 80%, the adsorption penetration time of dichloromethane by the composite hydrophobically modified activated carbon is extended by 20.69% compared with the unmodified activated carbon, and the saturated adsorption capacity is 44.43 mg / g, which is increased by 37.68% compared with the unmodified activated carbon. After the composite hydrophobic modification of the activated carbon, its adsorption performance for chlorine-containing VOCs in a high humidity environment is significantly improved.

Description

Technical Field

[0001] The present invention belongs to the field of activated carbon material modification, and in particular relates to the preparation and application of a composite hydrophobic modified activated carbon for treating chlorine-containing VOCs. Technical Background

[0002] VOCs, short for Volatile Organic Compounds, refer to organic compounds with a saturated vapor pressure exceeding 133.32 Pa at room temperature that escape into the air as gaseous molecules. These compounds pose serious risks to the environment and human health, and their control and elimination have garnered widespread attention. VOC removal technologies primarily include adsorption, catalytic combustion, biodegradation, and photocatalytic oxidation. Adsorption has become a common method for VOC treatment due to its simplicity, low cost, and high purification efficiency. Activated carbon, with its excellent adsorption properties and low price, is widely used in waste gas treatment. However, humidity significantly affects the adsorption of VOCs by activated carbon. During the production process, a high number of acidic oxygen-containing functional groups typically remain on the surface of activated carbon, making it more susceptible to binding with water molecules in the gas, thus affecting its adsorption performance for VOCs. This is particularly true for chlorine-containing VOCs such as dichloromethane. Therefore, hydrophobic modification of activated carbon is an important method for improving its VOC adsorption performance under high humidity conditions.

[0003] In previous studies, hydrophobic coatings were often used to modify activated carbon to make it hydrophobic, but this can easily cause clogging of the pore structure of the activated carbon. CN102989414A discloses a method for preparing a hydrophobic activated carbon modified material, in which trimethylchlorosilane is used as a modifier to impregnate activated carbon, but trimethylchlorosilane easily clogs the pore structure of activated carbon, which is not conducive to its adsorption of VOCs. In addition, the hydrophobicity of activated carbon can be enhanced by removing oxygen-containing functional groups on the surface of activated carbon. CN108751194A discloses a method for removing oxygen-containing functional groups on the surface of activated carbon, in which activated carbon soaked in potassium hydroxide is activated at 700°C for 3h, then mixed with a hydroquinone solution, refluxed in a water bath for 10h, and cooled to obtain modified activated carbon. However, the process is complicated and the production cost is high. Moreover, there are few studies on activated carbon modification methods suitable for treating high-humidity chlorine-containing VOCs waste gas.

[0004] In response to the above-mentioned problems, the present invention uses chlorine-containing VOCs such as dichloromethane as the treatment object, and performs hydrophobic modification on activated carbon, thereby improving the adsorption capacity of activated carbon for chlorine-containing VOCs under high humidity conditions. The present invention considers first using cheap and readily available ammonia solution to modify the activated carbon with an alkaline solution to reduce the content of oxygen-containing functional groups on its surface. Ammonia water of appropriate concentration can minimize the content of oxygen-containing functional groups on the surface of the activated carbon. Secondly, the present invention loads PDVB on the surface of the activated carbon pores by a solution impregnation method, and a thin layer of PDVB can be formed on the surface of the activated carbon pores. Compared with silane solution, PDVB not only has strong hydrophobicity, but also has the characteristics of looseness and porosity, which can improve the hydrophobicity of the activated carbon without blocking the pore structure of the activated carbon.

[0005] In summary, the present invention first uses ammonia water to reduce the content of oxygen-containing functional groups on the surface of activated carbon, and then loads a porous hydrophobic polymer on the surface of the activated carbon to further improve the hydrophobicity of the activated carbon. The purpose is to provide a method for preparing hydrophobic activated carbon with a simple modification method that can be used for the adsorption of chlorine-containing VOCs. Summary of the Invention

[0006] To achieve the above objectives, the present invention adopts the following technical solutions, which are divided into two aspects: preparation and application:

[0007] A method for preparing a composite hydrophobically modified activated carbon for treating chlorine-containing VOCs comprises the following steps:

[0008] (1) Pretreatment of activated carbon: The coal-based columnar activated carbon was repeatedly washed with ethanol and ultrapure water, and then heated and dried at 80-100°C;

[0009] (2) Preparation of alkaline solution modified activated carbon: The pre-treated and dried activated carbon was immersed in an ammonia solution with a mass concentration of 5% to 25%, stirred in a water bath at 30 to 60°C for 12 to 24 hours, then taken out and washed with ultrapure water until neutral and dried to obtain alkaline solution modified activated carbon;

[0010] (3) Synthesis of PDVB powder: First, prepare an appropriate amount of tetrahydrofuran / water solution (volume ratio of 10-15:1). Add divinylbenzene to the solution at a volume ratio of 3-6:25. Then, add an appropriate amount of azobisisobutyronitrile as an initiator. After stirring the solution at room temperature for 3 hours, transfer it to a hydrothermal reactor and heat it at 110°C for 12-24 hours. After the reaction is complete, grind the white PDVB solid into a powder in a mortar and pestle and place it in a fume hood for 24 hours to evaporate the remaining solvent, thereby obtaining PDVB powder.

[0011] (4) Preparation of alkaline solution-polymer PDVB composite hydrophobically modified activated carbon: Take an appropriate amount of the above-mentioned PDVB powder and add it to a certain volume of anhydrous ethanol, magnetically stir for 12 to 24 hours until the PDVB is evenly dispersed, and prepare a PDVB solution with a mass concentration of 1.5 to 3.5%; immerse the alkaline solution modified activated carbon in the PDVB solution for 6 to 12 hours, then separate the activated carbon from the PDVB solution with a filter and place it in an oven at 60 to 100°C for drying, so that the PDVB is loaded on the surface of the activated carbon pores, to obtain an alkaline solution-polymer composite hydrophobically modified activated carbon.

[0012] An application of composite hydrophobically modified activated carbon for treating chlorine-containing VOCs specifically comprises the following steps:

[0013] (1) A certain amount of alkaline solution-polymer composite hydrophobically modified activated carbon is loaded into an adsorption device;

[0014] (2) Control the concentration of chlorine-containing VOCs to 100-1000 ppm and the relative humidity to 10-80%;

[0015] (3) Start the fan in the gas pipeline and pass the chlorine-containing VOCs gas into the adsorption device filled with composite hydrophobic modified activated carbon, control the residence time of the gas through the activated carbon layer to 0.2 to 2 seconds, and achieve the removal of chlorine-containing VOCs. The removal rate can reach more than 90%. The time when the chlorine-containing VOCs removal rate exceeds 90% can be controlled by adjusting the activated carbon loading amount. The replacement or regeneration cycle of the activated carbon can be controlled according to the initial concentration of chlorine-containing VOCs in the gas and environmental protection requirements;

[0016] (4) The concentration of chlorinated VOCs in the inlet and outlet gases of the adsorption device can be measured online in real time by a VOCs analyzer or a gas chromatograph to determine the saturation end point of activated carbon adsorption and calculate the saturated adsorption capacity of the composite hydrophobically modified activated carbon for chlorinated VOCs.

[0017] The benefits of the present invention are:

[0018] (1) The preparation process of composite hydrophobically modified activated carbon is simple and the reaction conditions are relatively mild;

[0019] (2) Compared with hydrophobic substances such as silane, PDVB not only has strong hydrophobicity, but also has the characteristics of looseness and porosity. It can improve the hydrophobicity of activated carbon and the adsorption effect under high humidity conditions without blocking the pore structure of activated carbon. The basic principle of the present invention is: impregnating activated carbon with ammonia water can effectively remove the oxygen-containing functional groups on the surface of activated carbon, reduce the affinity of the activated carbon surface to water molecules, and thus improve the hydrophobicity of activated carbon and improve the adsorption performance of activated carbon to high humidity chlorine-containing VOCs; secondly, the hydrophobic polymer polydivinylbenzene has strong hydrophobicity and loose porosity. The PDVB thin layer loaded on the surface of activated carbon can not only improve the hydrophobicity of activated carbon, but also help to increase the specific surface area of activated carbon. After loading PDVB, the specific surface area of activated carbon increases from 889.23m 2 / g increased to 915.06m 2 / g, thereby improving its adsorption performance under high humidity conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a flow chart of the adsorption device used in Example 1;

[0021] Figure 2 : This is a scanning electron microscopy comparison of unmodified activated carbon and the composite hydrophobically modified activated carbon in Example 1 of the present invention;

[0022] Figure 3 : is a comparison diagram of the optical water contact angle of unmodified activated carbon and the composite hydrophobically modified activated carbon in Example 1 of the present invention;

[0023] Figure 4 This is the adsorption curve of dichloromethane by unmodified activated carbon and the composite hydrophobically modified activated carbon in Example 1 at a relative humidity of 80%.

[0024] In the figure: 1 is an N2 gas cylinder; 2 is a chlorine-containing 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 device. DETAILED DESCRIPTION

[0025] The following will be combined with the embodiments and drawings to clearly and completely describe the concept, specific structure and technical effects of the present invention so as to fully understand the purpose, scheme and effect of the present invention. It should be noted that the embodiments and features in the embodiments of this application can be combined with each other unless there is a conflict.

[0026] The preparation and application of a composite hydrophobically modified activated carbon for treating chlorine-containing VOCs specifically comprises the following steps:

[0027] Example 1:

[0028] (1) Pretreatment of activated carbon: The activated carbon was washed repeatedly in ethanol solution and ultrapure water, and dried thoroughly at 100°C to remove moisture from the activated carbon and obtain clean and dry activated carbon;

[0029] (2) Preparation and adsorption performance evaluation of alkaline solution modified activated carbon: Four batches of pretreated activated carbon were immersed in ammonia solutions with mass fractions of 5%, 10%, 15%, and 25%, respectively. After stirring in a 50°C water bath for 12 hours, the activated carbon was taken out and washed with ultrapure water until neutral. The activated carbon was then dried in an oven at 110°C for 6 hours to obtain alkaline solution impregnated activated carbon. According to the different ammonia concentrations, the alkaline solution modified activated carbons were named a%-NAC, where a represents the ammonia concentration and NAC represents alkaline solution modified activated carbon.

[0030] The total amount of oxygen-containing functional groups on the surface of unmodified and alkaline solution modified activated carbon was quantified using the Boehm titration method. Figure 1 The adsorption performance of the above-mentioned activated carbon for dichloromethane was measured using an adsorption device. The specific operation method was as follows: a certain amount of unmodified or alkali solution modified activated carbon was loaded into the adsorption device, the fan in the gas pipeline was started, and dichloromethane gas was passed into the adsorption device filled with activated carbon. The dichloromethane concentration was controlled to be 1000 ppm and the relative humidity was 80%. The residence time of the gas passing through the activated carbon layer was 0.71 s. The inlet and outlet dichloromethane concentrations were recorded by gas chromatograph and the adsorption breakthrough curve was drawn to calculate the saturated adsorption capacity of the unmodified or alkali solution modified activated carbon for dichloromethane. The results are shown in Table 1.

[0031] Table 1 Total amount of oxygen-containing functional groups and adsorption properties of unmodified and alkaline solution modified activated carbon

[0032]

[0033] As shown in Table 1, the saturated adsorption capacity of dichloromethane by activated carbon modified by soaking in different concentrations of ammonia initially increases with increasing ammonia concentration. When the ammonia concentration is less than 15%, the adsorption effect is good, with a saturated adsorption capacity of dichloromethane around 41 mg / g. However, as the ammonia concentration increases, the saturated adsorption capacity decreases. This is because the activated carbon, after being soaked in high-concentration ammonia, suffers structural damage such as fracture and erosion in its internal pore structure, resulting in a decrease in its adsorption performance. Therefore, 15%-NAC was selected as the alkaline solution-modified activated carbon (NAC) for subsequent treatment.

[0034] (3) Synthesis of PDVB powder: First, prepare an appropriate amount of tetrahydrofuran / water (10:1 by volume) solution and dissolve divinylbenzene in the solution at a volume ratio of 3:25. Then, add an appropriate amount of azobisisobutyronitrile as an initiator. The mixture is magnetically stirred at room temperature for 3 h. The solution is then transferred to a hydrothermal reactor and heated at 110°C for 24 h. After the reaction is complete, the white PDVB solid is ground into a powder in a mortar and placed in a fume hood for 24 h to evaporate the remaining solvent, thereby obtaining PDVB powder.

[0035] (4) Preparation of alkaline solution-polymer PDVB composite hydrophobically modified activated carbon: Take an appropriate amount of the above-mentioned PDVB powder and add it to a certain volume of anhydrous ethanol, stir it magnetically for 24 hours until the PDVB is evenly dispersed, and prepare a PDVB solution with a mass concentration of 1.5-3.5%; immerse the alkaline solution modified activated carbon in the PDVB solution for 6 hours, then use a filter to separate the alkaline solution modified activated carbon from the PDVB solution and put it into an oven at 80°C for 12 hours to load PDVB on the surface of the activated carbon pores, thereby preparing alkaline solution-polymer composite hydrophobically modified activated carbon. According to the different PDVB concentrations, they are named PDVB-1.5%-NAC, PDVB-2.5%-NAC, and PDVB-3.5%-NAC.

[0036] (5) Evaluation of the adsorption performance of composite hydrophobically modified activated carbon: A certain amount of unmodified or alkaline solution-polymer composite hydrophobically modified activated carbon was loaded into the adsorption device, the blower in the gas pipeline was started, and dichloromethane gas was passed into the adsorption device filled with activated carbon. The dichloromethane concentration was controlled to be 1000 ppm and the relative humidity was 80%. The residence time of the gas passing through the activated carbon layer was 0.71 s. The inlet and outlet dichloromethane concentrations were recorded by gas chromatograph and the adsorption penetration curve was drawn to calculate the saturated adsorption capacity of unmodified or alkaline solution-polymer composite hydrophobically modified activated carbon for dichloromethane. The results are shown in Table 2.

[0037] Table 2 Adsorption properties of unmodified and alkaline solution-polymer composite hydrophobically modified activated carbon

[0038]

[0039] This example compares the saturated adsorption capacity of chlorine-containing VOCs by activated carbon after impregnation with ammonia water of different concentrations. It is found that when the ammonia concentration is 15%, 15%-NAC has the best adsorption effect. Therefore, 15%-NAC is used as the alkaline solution to modify the activated carbon for subsequent PDVB loading experiments. Tetrahydrofuran is used as a solvent to synthesize PDVB, and the difference in adsorption performance of NAC after impregnation with PDVB solutions of different concentrations is explored. It is found that when the concentration of the PDVB impregnation solution is 2.5%, PDVB-2.5%-NAC has the best adsorption effect. This is because when the impregnation solution concentration is 1.5%, the loading amount of PDVB in the activated carbon pores is low, and the improvement effect is not obvious; when the impregnation solution concentration is 3.5%, the impregnation solution concentration is relatively high, and PDVB is easy to clump and fall off during drying, which is not conducive to improving the adsorption effect of the modified activated carbon.

[0040] Figure 2 This is a scanning electron microscope comparison of unmodified activated carbon and the composite hydrophobic modified activated carbon in Example 1 of the present invention. It can be found that after the composite hydrophobic modification, a layer of white porous substance is loaded on the surface of the activated carbon, indicating that PDVB is successfully loaded. Figure 3 This is a comparison diagram of the optical water contact angles of unmodified activated carbon and the composite hydrophobically modified activated carbon in Example 1 of the present invention. The water contact angle of the activated carbon after composite hydrophobic modification increases from 0° to 124.3°, and the macroscopic hydrophobicity of the activated carbon is significantly enhanced. Figure 4 This is a graph showing the adsorption penetration curve of dichloromethane by the composite hydrophobically modified activated carbon prepared in Example 1 under the above conditions. The penetration curve refers to a curve showing the concentration of the outflowing component changing with time when a multi-component mixed gas / steam flows through a fixed bed adsorption column. The abscissa in the figure is time, and the ordinate is the ratio of the outlet concentration C to the inlet concentration C0 at time t. The corresponding point of the penetration curve corresponding to when the outlet concentration C reaches 5% of the inlet concentration C0 is called the penetration point, and the longer the penetration time corresponding to the penetration point, the greater the adsorption capacity. By calculation, when the relative humidity is 80%, the adsorption penetration time of dichloromethane by the composite hydrophobically modified activated carbon can be extended by 20.69%, and the saturated adsorption capacity is 44.43 mg / g, which is 37.68% higher than that of unmodified activated carbon.

[0041] In summary, the composite hydrophobic modification method used in the present invention has a simple process, mild reaction conditions, does not involve corrosive or toxic drugs, and is environmentally friendly; the hydrophobic properties of the modified activated carbon are enhanced, and the water contact angle increases from 0° to 124.3°; after using the composite hydrophobic modification method of the present invention, the adsorption penetration time and saturated adsorption amount of dichloromethane of the composite hydrophobic modified activated carbon are significantly improved compared with the unmodified activated carbon, which helps to improve the adsorption performance of the activated carbon for chlorine-containing VOCs in a high humidity environment.

Claims

1. A method for preparing a composite hydrophobically modified activated carbon for treating chlorine-containing VOCs, characterized in that: Here are the steps: The first step is the pretreatment of activated carbon: the coal-based columnar activated carbon is washed with ethanol and ultrapure water, and then heated and dried at 80-100°C; The second step is to prepare the alkaline solution modified activated carbon: the pre-treated and dried activated carbon is immersed in an ammonia solution with a mass concentration of 5% to 25%, stirred in a water bath at 30 to 60°C for 12 to 24 hours, taken out and washed with ultrapure water until neutral and dried to obtain the alkaline solution modified activated carbon; The third step is the synthesis of PDVB powder: first, a tetrahydrofuran / water solution is prepared, with the volume ratio of tetrahydrofuran / water being 10 to 15:1; divinylbenzene is added to the above solution at a volume ratio of 3 to 6:25, and then azobisisobutyronitrile is added as an initiator. The solution is stirred at room temperature and then transferred to a hydrothermal autoclave and heated at 110°C for 12 to 24 hours; after the reaction is completed, the white PDVB solid is ground into powder in a mortar and placed in a fume hood to evaporate the remaining solvent, thereby obtaining PDVB powder; The fourth step is the preparation of alkaline solution-polymer PDVB composite hydrophobically modified activated carbon: take an appropriate amount of the above-mentioned PDVB powder and add it to anhydrous ethanol, magnetically stir for 12 to 24 hours until the PDVB is evenly dispersed, and prepare a PDVB solution with a mass concentration of 1.5 to 3.5%; immerse the alkaline solution modified activated carbon in the PDVB solution for 6 to 12 hours, then separate the activated carbon from the PDVB solution with a filter and place it in an oven at 60 to 100°C for drying, so that the PDVB is loaded on the surface of the activated carbon pores, to obtain an alkaline solution-polymer PDVB composite hydrophobically modified activated carbon.

2. The method for preparing a composite hydrophobically modified activated carbon for treating chlorine-containing VOCs according to claim 1, wherein: In the third step, the solution was stirred at room temperature for 3 hours and then transferred to a hydrothermal reactor. After the reaction was completed, the white PDVB solid was ground into powder using a mortar and placed in a fume hood for 24 hours to evaporate the remaining solvent to obtain PDVB powder.

3. The use of the composite hydrophobically modified activated carbon obtained by the preparation method of the composite hydrophobically modified activated carbon for treating chlorine-containing VOCs according to claim 1 or 2, characterized in that: The specific steps include: (1) loading the alkaline solution-polymer PDVB composite hydrophobically modified activated carbon into an adsorption device; (2) Control the concentration of chlorine-containing VOCs to 100-1000 ppm and the relative humidity to 10-80%; (3) Start the fan in the gas pipeline and pass the chlorine-containing VOCs gas into the adsorption device filled with composite hydrophobic modified activated carbon, control the residence time of the gas passing through the activated carbon layer to 0.2 to 2 seconds, and achieve the removal of chlorine-containing VOCs with a removal rate of more than 90%. By adjusting the activated carbon loading amount, the time when the chlorine-containing VOCs removal rate is more than 90% is controlled. According to the initial concentration of chlorine-containing VOCs in the gas and environmental protection requirements, the replacement or regeneration cycle of the activated carbon is controlled; (4) The concentration of chlorinated VOCs in the inlet and outlet gases of the adsorption device can be measured online in real time by a VOCs analyzer or a gas chromatograph to determine the saturation end point of activated carbon adsorption and calculate the saturated adsorption capacity of the composite hydrophobically modified activated carbon for chlorinated VOCs.

4. The use of the composite hydrophobically modified activated carbon obtained by the preparation method of the composite hydrophobically modified activated carbon for treating chlorine-containing VOCs according to claim 3, characterized in that: The chlorine-containing VOCs are one or a combination of dichloromethane, chloroform and chlorobenzene.

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