Alumina-loaded ZIF-8 adsorbent, preparation method, and application thereof

Abstract

This invention relates to an alumina-supported ZIF-8 adsorbent, its preparation method, and its application, belonging to the field of insulating oil adsorbents. Addressing the problem of poor adsorption performance of existing adsorbents, the following technical solution is adopted: A method for preparing an alumina-supported ZIF-8 adsorbent includes the following steps: preparing ZIF-8 powder using the sol-gel method; dispersing the prepared ZIF-8 powder in an organic solvent to form an organic solution containing ZIF-8; immersing alkaline alumina in a polyethyleneimine aqueous solution, followed by ultrasonic treatment to remove the alkaline alumina; immersing the treated alkaline alumina in the ZIF-8-containing organic solution, followed by stirring and ultrasonic treatment to obtain a ZIF-8 / Al2O3 composite. The prepared composite adsorbent exhibits high thermal stability, a large specific surface area, and strong adsorption capacity; the preparation process is simple, the reaction conditions are mild, it does not pollute the environment, and it is easy to mass-produce; the adsorbent can be reused after simple treatment.

Description

Technical Field

[0001] This invention belongs to the field of insulating oil adsorbents, and specifically relates to an alumina-supported ZIF-8 adsorbent, its preparation method, and its application. Background Technology

[0002] During long-term operation, transformers are affected by a combination of factors, including temperature, electric field, oxygen, and metal ions. This causes the internal oil-paper insulation (oil and paper, including insulating oil and insulating paper) to gradually age and deteriorate, generating many polar impurities. The properties of aged vegetable insulating oil decline, and the aging products in the oil can severely damage the transformer, leading to decreased insulation performance and, in severe cases, even malfunctions. Adsorption treatment of aged transformer oil can reduce impurities and effectively slow down the aging of the oil-paper insulation. This is an economical, feasible, and environmentally friendly insulating oil regeneration technology.

[0003] Currently used adsorbents include traditional adsorbent materials such as activated clay, silica gel, and alumina, as well as newer materials such as molecular sieves and other natural adsorbents. However, activated clay production is complex, and silica gel adsorption is prone to decolorization. Considering factors such as adsorption effect, economy, and simple preparation, alkaline alumina, with its excellent adsorption effect, is chosen as the basic material. However, when the content of small-molecule acid impurities is low, the adsorption effect of alkaline alumina is not significant. Therefore, it is necessary to modify alkaline alumina by loading other substances onto its surface to improve its adsorption capacity.

[0004] Metal-organic frameworks (MOFs) are a novel class of porous materials formed by the coordination of metals and organic ligands, possessing advantages such as large specific surface area, high porosity, and tunable structure. ZIF-8 is an important subclass of MOFs, composed of transition metals and organic imidazole compounds. Its structure is a zeolite-like three-dimensional network, exhibiting extremely high stability, with a specific surface area reaching up to 1800 g / m². 2 Its properties are far superior to those of activated carbon and molecular sieves. Considering all the properties of ZIF-8, it is a better choice as a surface loading material for alkaline alumina. Patent application number 201310136268.3 provides a method for preparing a ZIF-8 membrane supported by a porous alumina carrier. This method uses ionothermal synthesis to prepare the ZIF-8 membrane, with porous alumina as the support. The alumina needs to be removed after crystallization, making the preparation process relatively cumbersome. While the resulting product has good gas separation and permeation performance, it cannot be used in liquid adsorption. There is little research on the application of alumina-supported ZIF-8 composite adsorbents in the regeneration of insulating oils, especially natural ester vegetable insulating oils. After aging, these oils contain a wide variety of substances, and the adsorption effect and performance improvement have not yet been fully confirmed. Summary of the Invention

[0005] To address the problem that the adsorption effect of existing adsorbents cannot meet production needs, this invention provides an alumina-supported ZIF-8 adsorbent, its preparation method, and its application. For the first time, this adsorbent is applied to the purification of natural ester plant insulating oil.

[0006] The technical solution adopted in this invention is as follows: A method for preparing an alumina-supported ZIF-8 adsorbent, comprising the following steps:

[0007] Step 1: ZIF-8 powder was prepared using the sol-gel method;

[0008] Step 2: Disperse the prepared ZIF-8 powder with an organic solvent to form an organic solution containing ZIF-8;

[0009] Step 3: Immerse the alkaline alumina in a polyethyleneimine aqueous solution, and remove the alkaline alumina by ultrasonic treatment;

[0010] Step 4: Immerse the treated alkaline alumina in the organic solution containing ZIF-8 prepared in Step 2. After stirring and ultrasonic treatment, obtain the ZIF-8 / Al2O3 composite, wherein the mass ratio of alkaline alumina to ZIF-8 is (0.05-0.2):1.

[0011] The ZIF-8 / Al2O3 composite prepared in this application combines the advantages of basic alumina and ZIF-8, resulting in superior performance. The composite adsorbent exhibits higher thermal stability and a larger specific surface area. Polyethyleneimine has the highest cation density, which, after treating basic alumina, enhances the adhesion and adsorption of polar small molecules. The alkalinity of basic alumina neutralizes acidic substances in the oil, while the active sites of ZIF-8 coordinate with impurities. The porous nature of both allows for impurity adsorption; the combination of chemical and physical adsorption enhances the adsorption capacity, overcoming the drawback of basic alumina's difficulty in adsorbing trace amounts of acidic small molecules. The preparation process is simple, the reaction conditions are mild, it does not pollute the environment, and it is easy to mass-produce. The adsorbent can be separated from the insulating oil through filtration, and the adsorbent can be reused after simple treatment. Therefore, the composite adsorbent prepared in this application is also easy to recycle and reuse, aligning with the concept of green development.

[0012] Furthermore, the process of preparing ZIF-8 powder in step 1 is as follows:

[0013] Step 1.1: Prepare organic solutions of zinc source and HMIM respectively, and then mix the two organic solutions to form a mixture; wherein the mass ratio of zinc source to HMIM is 1:(0.8~1.3); used to form zinc imidazole zinc (Zn-Im-Zn) structure;

[0014] Step 1.2: After allowing the mixture to stand, centrifuge it to separate the mixture and facilitate the collection of precipitate.

[0015] Step 1.3: After removing the supernatant from centrifugation, dry the product to obtain ZIF-8 powder.

[0016] Furthermore, in step 1.1, methanol or ethylene glycol is used as the zinc source and the organic solvent for HMIM, and the zinc source is Zn(CH3COO)2·2H2O.

[0017] Furthermore, in step 1.2, the settling time is 18-30 hours and the centrifugation time is 5-10 minutes, which can achieve sufficient and effective separation.

[0018] Furthermore, in step 1.3, the drying method used is low-temperature drying, with a time of 20-35 hours and a temperature of 40-60℃. This can prevent the prepared material from decomposing at high temperatures.

[0019] Furthermore, in step 2, the organic solvent used is methanol or ethylene glycol. Using the same organic solvent in both steps 1 and 2 avoids the problem of layering due to differences in the density of different organic solvents. Alcohol solvents can slow down the reaction rate in this application, resulting in a more complete and uniform reaction and a more consistent morphology.

[0020] Furthermore, in step 3, a 2-5% aqueous solution of polyethyleneimine is used, and the ultrasonic treatment time is 5-10 minutes.

[0021] The second inventive aspect of this application is the ZIF-8 / Al2O3 composite prepared using the above-described method. Polyethyleneimine has the highest cation density, which can improve the adsorption and adhesion of alumina to polar small molecules.

[0022] The third inventive point of this application is the application of the ZIF-8 / Al2O3 composite as an adsorbent in improving the performance of aged plant insulating oil.

[0023] The beneficial effects of this invention are as follows: The ZIF-8 / Al2O3 composite prepared in this application combines the advantages of alkaline alumina and ZIF-8, exhibiting higher thermal stability, a larger specific surface area, and superior performance; polyethyleneimine has the highest cation density, and after treating alkaline alumina, it can improve the adhesion and adsorption of polar small molecules to alumina; moreover, the preparation process is simple, the reaction conditions are mild, it does not pollute the environment, and it is easy to mass-produce; when applied to the treatment of aging vegetable insulating oil, the ZIF-8 / Al2O3 composite can be separated from the insulating oil by filtration, and the adsorbent can be reused after simple treatment. Attached Figure Description

[0024] Figure 1 The XRD pattern of the ZIF-8 sample prepared in Example 1;

[0025] Figure 2 The image shows a scanning electron microscope (SEM) image of the ZIF-8 sample prepared in Example 1.

[0026] Figure 3 XRD pattern of the basic alumina-supported ZIF-8 sample prepared in Example 1;

[0027] Figure 4 The image is a scanning electron microscope (SEM) image of the alkaline alumina-supported ZIF-8 sample prepared in Example 1.

[0028] Figure 5 This is a comparison chart of adsorption test results. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be explained and described below with reference to the accompanying drawings. However, the following embodiments are only preferred embodiments of the present invention and not all of them. Other embodiments obtained by those skilled in the art based on the embodiments in the implementation methods without creative effort are all within the protection scope of the present invention.

[0030] Example 1

[0031] Measure 100 ml of methanol into a beaker using a graduated cylinder. Accurately weigh 2.1682 g of Zn(CH3COO)2·2H2O (sample is a white powder) using an electronic analytical balance and dissolve it in methanol to form solution A. Stir with a magnetic stirrer for 1 hour. Weigh 2.2430 g of HMIM and dissolve it in 100 ml of methanol to form solution B, and stir magnetically for 1 hour. After stirring solutions A and B separately, slowly pour solution B into solution A and continue stirring magnetically for 1 hour. After stirring, remove the rotor and seal the resulting white suspension with plastic wrap, allowing it to stand at room temperature for 24 hours. After standing, centrifuge at 8000 rpm for 5 minutes. After centrifugation, pour off the supernatant and place the centrifuge tube in a constant temperature oven at 45°C for 24 hours. After drying, remove the sample to obtain ZIF-8 powder. Test the surface morphology and composition of the ZIF-8 powder, as follows... Figure 1 and Figure 2 As shown, from Figure 1 As can be seen, the X-ray diffraction pattern of the prepared sample is consistent with the standard values ​​of the standard spectrum, proving that the prepared ZIF-8 crystal structure is standard and free of impurities. Figure 2The prepared ZIF-8 crystals exhibit a polygonal spherical structure with uniform size, approximately 1 μm. The prepared ZIF-8 powder was dispersed in methanol to form a methanol solution containing ZIF-8. Subsequently, basic alumina was immersed in a 2-5% polyethyleneimine aqueous solution, followed by ultrasonic treatment. The treated basic alumina was then removed and immersed in the ZIF-8-containing methanol solution. After stirring and ultrasonic treatment, a ZIF-8 / Al₂O₃ composite was obtained, with the morphology and composition as shown below. Figure 3 and Figure 4 As shown.

[0032] Example 2

[0033] Measure 100 ml of methanol into a beaker using a graduated cylinder. Accurately weigh 2.1682 g of Zn(CH3COO)2·2H2O (sample is a white powder) using an electronic analytical balance and dissolve it in methanol to form solution A. Stir with a magnetic stirrer for 1 hour. Weigh 1.7346 g of HMIM and dissolve it in 100 ml of methanol to form solution B, and stir magnetically for 1 hour. After stirring solutions A and B separately, slowly pour solution B into solution A and continue stirring magnetically for 1 hour. After stirring, remove the rotor and seal the resulting white suspension with plastic wrap, allowing it to stand at room temperature for 24 hours. After standing, centrifuge at 8000 rpm for 5 minutes. After centrifugation, pour off the supernatant and place the centrifuge tube in a constant temperature oven at 45°C for 24 hours. After drying, remove the sample to obtain ZIF-8 powder. The prepared ZIF-8 crystals have a polygonal spherical structure with uniform size, approximately 1 μm. The prepared ZIF-8 powder was dispersed in methanol to form a methanol solution containing ZIF-8. Subsequently, basic alumina was immersed in a 2-5% polyethyleneimine aqueous solution and ultrasonically treated. The treated basic alumina was then removed and immersed in the ZIF-8-containing methanol solution. After stirring and ultrasonic treatment, the ZIF-8 / Al₂O₃ composite was obtained.

[0034] Example 3

[0035] Measure 100 ml of methanol into a beaker using a graduated cylinder. Accurately weigh 2.1682 g of Zn(CH3COO)2·2H2O (sample is a white powder) using an electronic analytical balance and dissolve it in methanol to form solution A. Stir with a magnetic stirrer for 1 hour. Weigh 2.8187 g of HMIM and dissolve it in 100 ml of methanol to form solution B, and stir magnetically for 1 hour. After stirring solutions A and B separately, slowly pour solution B into solution A and continue stirring magnetically for 1 hour. After stirring, remove the rotor and seal the resulting white suspension with plastic wrap, allowing it to stand at room temperature for 24 hours. After standing, centrifuge at 8000 rpm for 5 minutes. After centrifugation, pour off the supernatant and place the centrifuge tube in a constant temperature oven at 45°C for 24 hours. After drying, remove the sample to obtain ZIF-8 powder. The prepared ZIF-8 crystals have a polygonal spherical structure with uniform size, approximately 1 μm. The prepared ZIF-8 powder was dispersed in methanol to form a methanol solution containing ZIF-8. Subsequently, basic alumina was immersed in a 2-5% polyethyleneimine aqueous solution and ultrasonically treated. The treated basic alumina was then removed and immersed in the ZIF-8-containing methanol solution. After stirring and ultrasonic treatment, the ZIF-8 / Al₂O₃ composite was obtained.

[0036] Comparative Example

[0037] The ZIF-8 / Al2O3 complex prepared in Example 1 was used for adsorption treatment of aged vegetable oil, and compared with adsorption by alkaline alumina alone. The specific steps are as follows: 2L of vegetable oil (BHT antioxidant, BHT content of 0.3% of oil mass) was placed on a copper sheet, aged for 72 hours, and then the copper sheet was removed to obtain an aged vegetable oil sample. The aged oil sample was divided into three groups of 600ml each, and experiments were conducted on each group. The first group served as the control group: the acid value, dielectric loss factor, initial oxidation temperature, and resistivity of the aged oil samples were directly tested. The second group was an alkaline alumina adsorption test: 5% alkaline alumina (by weight of the aged oil) was added, magnetically stirred and dispersed for 5 minutes, placed in a 50°C oven for 3 hours, then filtered and collected. The acid value, dielectric loss factor, initial oxidation temperature, and resistivity of the oil samples were tested using the same standard. The third group was an adsorption test of the alkaline alumina-supported ZIF-8 prepared in Example 1: 5% alkaline alumina (by weight of the aged oil) was added, magnetically stirred and dispersed for 5 minutes, placed in a 50°C oven for 3 hours, then filtered and collected. The acid value, dielectric loss factor, initial oxidation temperature, and resistivity of the oil samples were tested using the same standard. The tests were repeated three times, and the average value of the test results was taken.

[0038] Test results are as follows Figure 5As shown, the properties of aged natural ester oil samples were significantly improved after adsorption treatment with alkaline alumina-supported ZIF-8. Compared with adsorption by alkaline alumina alone, the performance improvement was more significant, thus proving that the alkaline alumina-supported ZIF-8 adsorbent prepared in this application has a stronger adsorption capacity.

[0039] The solution in the third group of experiments underwent solid-liquid separation, specifically using methods such as centrifugation, filtration, or precipitation, to separate the ZIF-8 / Al2O3 composite from the insulating oil. The separated solid was then washed with water, alcohol, or other solutions. Excess moisture was removed by drying methods such as natural air drying, vacuum drying, or infrared drying. Finally, pyrolysis, acid washing, and alkali washing were used to restore the adsorption performance of the adsorbent. Five more adsorption tests were conducted under the same conditions as in the third group, and the adsorption of aged vegetable oil remained good. While the effects of reducing acidity, dielectric loss, and resistivity were slightly lower than in the third group, the results were still better than those treated with pure alkaline alumina.

[0040] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that the present invention includes, but is not limited to, the contents described in the accompanying drawings and the specific embodiments above. Any modifications that do not depart from the functional and structural principles of the present invention will be included within the scope of the claims.

Claims

1. A method for preparing an alumina-supported ZIF-8 adsorbent, characterized in that, Includes the following steps: Step 1: ZIF-8 powder is prepared using the sol-gel method; the process is as follows: Step 1.1: Prepare organic solutions of zinc source and HMIM respectively, and then mix the two organic solutions to form a mixture; wherein the mass ratio of zinc source to HMIM is 1:(0.8~1.3); used to form zinc imidazole zinc structure; methanol or ethylene glycol is used as organic solvent for zinc source and HMIM, and zinc source is Zn(CH3COO)2·2H2O; Step 1.2: After the mixture has stood, centrifuge it to separate the mixture. Step 1.3: After removing the supernatant from centrifugation, dry the product to obtain ZIF-8 powder; the drying method used is low-temperature drying at a temperature of 40-60℃. Step 2: Disperse the prepared ZIF-8 powder with an organic solvent to form an organic solution containing ZIF-8; Step 3: Immerse the alkaline alumina in a polyethyleneimine aqueous solution, and remove the alkaline alumina by ultrasonic treatment; Step 4: Immerse the treated alkaline alumina in the organic solution containing ZIF-8 prepared in Step 2. After stirring and ultrasonic treatment, obtain the ZIF-8 / Al2O3 composite, wherein the mass ratio of alkaline alumina to ZIF-8 is (0.05-0.2):

1.

2. The method for preparing the alumina-supported ZIF-8 adsorbent according to claim 1, characterized in that, In step 1.2, the settling time is 18-30 hours and the centrifugation time is 5-10 minutes.

3. The method for preparing the alumina-supported ZIF-8 adsorbent according to claim 1, characterized in that, In step 1.3, the drying time is 20-35 hours.

4. The method for preparing alumina-supported ZIF-8 adsorbent according to claim 1, characterized in that, In step 2, the organic solvent used is methanol or ethylene glycol.

5. The method for preparing alumina-supported ZIF-8 adsorbent according to claim 1, characterized in that, In step 3, a 2-5% aqueous solution of polyethyleneimine is used, and the ultrasonic treatment time is 5-10 minutes.

6. The alumina-supported ZIF-8 adsorbent prepared by the preparation method according to any one of claims 1 to 5.

7. The application of the alumina-supported ZIF-8 adsorbent according to claim 6 in the treatment of aged vegetable insulating oil.

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