Preparation method of Zn-Al-LDH ZIF-8 based on Zn-Al-LDH in-situ growth ZIF-8 metal framework

Abstract

The invention discloses a preparation method of Zn-Al-LDH (at) ZIF-8 based on a Zn-Al-LDH in-situ growth ZIF-8 metal framework, and belongs to the technical field of composite materials. The preparation method comprises the following steps: synthesizing Zn-Al-LDH particles in an alkaline solution with a pH value of 10 through a precipitation method, adding the Zn-Al-LDH particles and 2-methylimidazole particles in a methanol solution according to a mass ratio of 1: (3-4), and enabling a ZIF-8 metal framework to grow in situ on Zn-Al-LDH to synthesize Zn-Al-LDH ZIF-8. According to the preparation method disclosed by the invention, an in-situ synthesis method is adopted, ZIF-8 nanocrystals are directly and uniformly nucleated on the surface of Zn-Al-LDH, and microporous-mesoporous multilevel channels of ZIF-8 are complementarily fused with a layered structure of LDH, so that not only is the ion adsorption capacity of LDH retained, but also the active site density is greatly improved through the high specific surface area and ordered pores of ZIF-8, and high loading capacity is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of composite materials, and particularly relates to a preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH. Background Art

[0002] In the field of metal corrosion prevention, traditional single-layered double hydroxides (LDHs) are limited in their long-term applications due to low interlayer loading capacity, easy collapse of the structure in acidic / high-salt environments, and insufficient selective adsorption of competitive ions; while single metal-organic frameworks (MOFs) have a high specific surface area, but are limited by microporous diffusion kinetics, poor environmental stability, and functional singularity. Although existing LDH / MOF composites attempt to combine the advantages of both, they still face bottlenecks such as loose interfacial bonding, harsh preparation conditions (requiring a high-temperature alkaline environment leading to dissolution of the LDH layer plates), and limited performance improvement (Cl - adsorption capacity ≤ 50 mg / g, and the efficiency decreases by 30% after 3 cycles). Summary of the Invention

[0003] The present invention discloses a preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH, which solves the above problems. The prepared composite material has ion adsorption capacity and high loading rate, and relevant exploration and improvement are carried out on the chloride ion adsorption capacity of this material.

[0004] The present invention discloses a preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH. After synthesizing Zn-Al-LDH particles by precipitation method in an alkaline solution with pH = 10, in a methanol solution, Zn-Al-LDH particles and 2-methylimidazole particles are added in a mass ratio of 1:(3 - 4), and the ZIF-8 metal framework is grown in-situ on Zn-Al-LDH to synthesize Zn-Al-LDH@ZIF-8, and the particle size range of ZIF-8 is 50 - 100 nm.

[0005] Preferably, it specifically includes the following steps:

[0006] (1) Preparation of Zn-Al-LDH: Under a nitrogen atmosphere, zinc nitrate hexahydrate, aluminum nitrate nonahydrate, and sodium nitrate are added to deionized water and dissolved uniformly. Sodium hydroxide solution is dropped in to adjust the pH to 10. After centrifuging the mixed solution, it is washed with deionized water and dried at a high temperature using a vacuum drying oven to obtain a white solid. The white powder obtained by crushing and grinding the white solid is Zn-Al-LDH particles;

[0007] (2) In-situ synthesis of Zn-Al-LDH@ZIF-8: Under a nitrogen atmosphere, Zn-Al-LDH particles and 2-methylimidazole particles were separately dissolved in methanol and ultrasonically dispersed. After mixing the two methanol solutions containing different solutes, they were mixed under ultrasonic action for 30 min. The mixed suspension was allowed to stand and then centrifuged. The precipitate was taken and washed 3 times with methanol, and then dried at a high temperature and crushed and ground. The obtained white powder was Zn-Al-LDH@ZIF-8.

[0008] Preferably, in step (1), the mass ratio of zinc nitrate hexahydrate, aluminum nitrate nonahydrate, and sodium nitrate is (5 - 7.5):3:(1.4 - 2.1).

[0009] Preferably, in step (1), the density of the sodium hydroxide solution is 0.02875 g / mL.

[0010] Preferably, in step (1), the rotation speed of the centrifuge is 8000 rpm, the centrifugation time is 10 - 15 min, the temperature for high-temperature drying is 120 °C, and the drying time is 3 hours.

[0011] Preferably, in step (2), the mass-volume ratio of Zn-Al-LDH particles to methanol is 1 g:(55 - 175) mL, the mass-volume ratio of 2-methylimidazole particles to methanol is 2.1 g:(80 - 90) mL, the volume ratio of the methanol solution containing Zn-Al-LDH particles to the methanol solution containing 2-methylimidazole particles is 1:(1 - 2), and the mixing temperature is 60 °C.

[0012] Preferably, in step (2), the rotation speed of the centrifuge is 5000 rmp, and the centrifugation time is 10 min.

[0013] Preferably, in step (2), the drying temperature is 70 °C.

[0014] Therefore, the present invention provides a preparation method of Zn-Al-LDH@ZIF-8 based on the in-situ growth of ZIF-8 metal framework on Zn-Al-LDH, having the following beneficial effects:

[0015] (1) For the Zn-Al-LDH@ZIF-8 prepared by the present invention, ZIF-8 nanocrystals directly nucleate uniformly on the surface of Zn-Al-LDH, forming a strong interfacial bond, ensuring the structural stability of the composite material. The microporous-mesoporous hierarchical pore channels of ZIF-8 and the layered structure of LDH are complementary and fused, not only retaining the ion adsorption ability of LDH, but also greatly increasing the density of active sites through the high specific surface area and ordered pores of ZIF-8, and having a high loading capacity.

[0016] (2) The in-situ growth strategy of the present invention, compared with ZIF-8 synthesized by the solution synthesis method with a particle size range of 100-200 nm, has a smaller particle size range between 50-100 nm due to the flaky structure of LDH restricting the growth of ZIF-8. It can inhibit the stacking of LDH and the aggregation of ZIF-8, enhance mechanical stability and cycling performance, and at the same time, the pore size distribution and surface functional groups can be precisely designed by regulating the ZIF-8 growth parameters.

[0017] (3) The preparation method provided by the present invention, compared with the traditional physical mixing method, the in-situ hydrothermal synthesis process does not require toxic binders and has mild conditions, which conforms to the concept of green chemistry. The one-step synthesis feature, without complex templates or post-treatment steps, significantly simplifies the preparation process, strengthens the combination of ZIF-8 and the LDH substrate, with uniform nucleation, strong interfacial bonding and process simplification, providing a better solution for the preparation of high-performance functional composite materials. Description of the Drawings

[0018] Figure 1 It is a synthesis route diagram of a preparation method of Zn-Al-LDH disclosed in Example 1 of the present invention;

[0019] Figure 2 It is a synthesis route diagram of a preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH disclosed in Example 2 of the present invention;

[0020] Figure 3 It is an XRD curve diagram of Zn-Al-LDH, Zn-Al-LDH@ZIF-8 and ZIF-8 nanoparticles prepared in Example 1, Example 2 and the comparative example of the present invention;

[0021] Figure 4 It is a scanning electron microscope picture of ZIF-8 prepared in the comparative example of the present invention. Among them, (a) is a scanning electron microscope picture magnified 10,000 times, and (b) is a scanning electron microscope picture magnified 50,000 times;

[0022] Figure 5 It is a scanning electron microscope picture of Zn-Al-LDH nanoparticles prepared in Example 1 of the present invention. Among them, (a) is a scanning electron microscope picture magnified 10,000 times, and (b) is a scanning electron microscope picture magnified 50,000 times;

[0023] Figure 6 It is a scanning electron microscope picture of Zn-Al-LDH@ZIF-8 prepared in Example 2 of the present invention. Among them, (a) is a scanning electron microscope picture magnified 10,000 times, and (b) is a scanning electron microscope picture magnified 50,000 times;

[0024] Figure 7Nitrogen adsorption - desorption isotherm and pore size distribution diagram of the Zn - Al - LDH nanoparticles prepared in Example 1 of the present invention;

[0025] Figure 8 Nitrogen adsorption - desorption isotherm and pore size distribution diagram of the ZIF - 8 nanoparticles prepared in the comparative example of the present invention;

[0026] Figure 9 Nitrogen adsorption - desorption isotherm and pore size distribution diagram of the Zn - Al - LDH@ZIF - 8 prepared in Example 2 of the present invention;

[0027] Figure 10 Chloride ion isothermal adsorption curve diagrams of the Zn - Al - LDH, Zn - Al - LDH@ZIF - 8, and ZIF - 8 nanoparticles prepared in Example 1, Example 2, and the comparative example of the present invention. Detailed implementation manners

[0028] The present invention will be further described below through specific examples, but it should be understood that these examples are only used for more detailed description and should not be construed as limiting the present invention in any form, that is, it is not intended to limit the protection scope of the present invention.

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0030] Example 1

[0031] This example provides a preparation method of Zn - Al - LDH, and its synthesis route is as shown in the attached specification Figure 1 Specifically, it includes the following steps: Weigh 5.75 g of NaOH with an electronic balance and dissolve it in 200 ml of deionized water. Then weigh 11.25 g of Zn(NO3)2·6H2O, 4.50 g of Al(NO3)3·9H2O, and 3.15 g of NaNO3. Under the protection of N2, pour all the weighed reagents into 150 ml of deionized water, and titrate with the NaOH solution during the process of introducing nitrogen, adjust the pH of the solution to about 10, stir the obtained solution at 25 °C for 5 h, then centrifuge at 8000 rpm for 15 min, wash it 3 times with deionized water, and finally put it into an oven and dry it at 90 °C for 12 h. Crush and grind the white solid, and the obtained white powder is the Zn - Al - LDH particles.

[0032] Example 2

[0033] This embodiment provides a preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH, and its synthesis route is as follows Figure 2 shown. Specifically, it includes the following steps. In this embodiment, the preparation of Zn-Al-LDH in step (1) is exactly the same as that in Example 1; step (2) is to weigh 2.1 g of Zn-Al-LDH particles, disperse them into 120 ml of methanol under ultrasonic action, then weigh 6.3 g of 2-methylimidazole and dissolve it in 240 ml of methanol, and then mix the two under ultrasonic action for 30 min. Let the mixed suspension stand for 12 h, centrifuge the suspension at 5000 rmp for 10 min, wash the precipitate with methanol three times, and then dry it at 80 °C for 4 h and crush and grind it. The obtained white powder is Zn-Al-LDH@ZIF-8.

[0034] Example 3

[0035] This embodiment provides a preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH, including the following steps:

[0036] Step 1: Prepare Zn-Al-LDH. Pass 200 ml of deionized water through nitrogen for 15 minutes, then add 15 g of zinc nitrate hexahydrate, 9 g of aluminum nitrate nonahydrate, and 4.2 g of sodium nitrate into the deionized water respectively, and stir at room temperature. Subsequently, add 10% sodium hydroxide solution to adjust the pH to 10, and stir at room temperature for 3 hours. Then centrifuge at 8000 rpm for 10 minutes, then wash with deionized water several times, and finally dry in a vacuum drying oven at 120 °C for 3 hours to obtain Zn-Al-LDH.

[0037] Step 2: Weigh 2.1 g of the prepared Zn-Al-LDH, disperse it in 360 ml of methanol solution by ultrasonic, pass nitrogen through the solution as a protective gas, then add 8.4 g of 2-MIM to the solution, and stir at 60 °C for 4 hours. Centrifuge the obtained solution at 5000 rpm for 10 minutes, wash the obtained precipitate with methanol three times, and finally put the precipitate into a vacuum drying oven and dry it at 70 °C for 3 hours to obtain Zn-Al-LDH@ZIF-8.

[0038] Comparative Example

[0039] This comparative example provides a method for synthesizing ZIF-8 nanoparticles, which includes the following steps: Weigh 0.9 g of Zn(NO3)2·6H2O and 2.0 g of 2-methylimidazole with an electronic balance. Dissolve the two in 50 ml of methanol respectively. After complete dissolution, mix them at 25 °C for five minutes, then seal and let it stand for 6 h. Centrifuge the obtained white suspension at a speed of 5000 rpm for 10 min. Wash the centrifuged white precipitate with methanol three times. Dry the product at 80 °C for 4 h, and then crush and grind the white solid. The obtained white powder is ZIF-8 nanoparticles.

[0040] Perform XRD detection on the Zn-Al-LDH particles, Zn-Al-LDH@ZIF-8, and ZIF-8 nanoparticles prepared in the above Example 1, Example 2, and comparative example. The curve graph is as Figure 3 shown. It can be seen from the XRD curve graph that the ZIF-8 synthesized by the solution synthesis method shows characteristic peaks of (011), (002), (112), and (222) belonging to the ZIF-8 mirror plane at 7.53°, 10.58°, 12.93°, and 18.24° respectively. This can prove that the white powdery material synthesized by the solution synthesis method is ZIF-8 nanoparticles, and using this method for synthesis will not affect the structure of the synthesized ZIF-8. The XRD curve of Zn-Al-LDH nanoparticles shows characteristic peaks of double-layer hydroxide of (003), (100), (009), and (012). After synthesizing ZIF-8 on Zn-Al-LDH by the in-situ synthesis method, characteristic peaks of ZIF-8 of (011), (002), (112), and (222) appear in the curve, and no other characteristic peaks appear, indicating that the effect of preparing Zn-Al-LDH@ZIF-8 by the in-situ synthesis method is significant and will not destroy the original structure of Zn-Al-LDH.

[0041] Observe the morphologies of the Zn-Al-LDH particles, Zn-Al-LDH@ZIF-8, and ZIF-8 nanoparticles prepared in the above Example 1, Example 2, and comparative example. The results are as Figures 4 - 6 shown. By analyzing the scanning electron microscope images of ZIF-8 ( Figure 4 (a) and Figure 4 (b)), it can be observed that the morphology of ZIF-8 synthesized by the solution synthesis method has obvious prismatic microstructures, and basically all are hexagonal prismatic microstructures. When analyzing the scanning electron microscope results of Zn-Al-LDH ( Figure 5 (a) and Figure 5(b)), it was found that the Zn-Al-LDH synthesized by the precipitation method was irregular flakes in structure and had a relatively smooth surface. ZIF-8 was synthesized on Zn-Al-LDH by in-situ synthesis. The scanning electron microscope results are as shown in Figure 6 (a) and Figure 6 (b). It can be seen from the microscopic morphology that granular structures with a particle size of 50-100 nm have successfully grown on the smooth surface of the previous Zn-Al-LDH. These granular structures can be determined to be the crystal microstructure of ZIF-8 in combination with the XRD pattern. Thus, it can be shown that the Zn-Al-LDH@ZIF-8 synthesized by the in-situ generation method has a good synthesis effect.

[0042] The synthesized samples of Zn-Al-LDH particles, Zn-Al-LDH@ZIF-8, and ZIF-8 nanoparticles prepared in Example 1, Example 2, and the comparative example above were subjected to nitrogen adsorption-desorption and pore size distribution observations. Before measurement, the samples were degassed at 120 °C for 2 hours to remove all physically adsorbed substances on the particle surface. Subsequently, a Micromeritics ASAP2460 analyzer was used to measure the N2 adsorption-desorption isotherm and pore size data. As shown in Figure 7 the adsorption-desorption curve of Zn-Al-LDH, it can be seen that the curve fitting basically conforms to the type III isotherm, and the pore size distribution is mainly concentrated in the mesopore and macropore range. The specific surface area is approximately 12.394 m 2 / g, the pore volume is 0.066401 cm 3 / g, and the average pore size is 21.4553 nm. From Figure 8 the adsorption-desorption isotherm of ZIF-8 as shown, it can be seen that the isotherm conforms to the Langmuir curve, that is, the type I isotherm, which is a typical isotherm characteristic of zeolites and has an obvious inflection point B. And from the pore size distribution diagram, it can be seen that the pore size of ZIF-8 is mainly distributed in the micropore and mesopore range. The specific surface area is 309.7443 m 2 / g, the pore volume is 0.166480 cm 3 / g, and the average pore size is 2.1499 nm. The nitrogen adsorption-desorption curve of Zn-Al-LDH@ZIF-8 synthesized by in-situ synthesis is as shown in Figure 9 . The still existing inflection point B is mainly attributed to ZIF-8 on the surface of Zn-Al-LDH. And the synthesized material combines the specific surface areas of Zn-Al-LDH and ZIF-8 and has a significant increase. The increased specific surface area can load more rust inhibitors and adsorb chloride ions better, while the pore volume also increases to 0.527966 cm 3 / g. The increase in pore volume enables the material to have more volume for adsorbing chloride ions. The specific data of the material are: the specific surface area is 1058.1244 m 2 / g, with a pore volume of 0.527966 cm 3 / g and an average pore diameter of 1.9959 nm.

[0043] Isothermal chloride adsorption experiments were carried out on the Zn-Al-LDH, Zn-Al-LDH@ZIF-8, and ZIF-8 nanoparticles prepared in Example 1, Example 2, and the comparative example. The results are as Figure 10 shown. It can be found from the chloride isothermal adsorption curve that the chloride adsorption performance is Zn-Al-LDH@ZIF-8 > Zn-Al-LDH > ZIF-8. The adsorption of Zn-Al-LDH mainly depends on interlayer anion exchange and specific surface area. The [Zn / Al] lamellar of LDH is positively charged, and the exchangeable anions in the interlayer (mainly and ) react with Cl - through an exchange reaction, with chemical adsorption being the main and surface adsorption being the secondary. The adsorption of ZIF-8 has pore size limitations. Since the material is mainly dominated by micropores and the micropore size of ZIF-8 is close to the diameter of Cl - hydrated ions (~0.66 nm), there is a large diffusion resistance and a low adsorption capacity. Therefore, the imidazole ester skeleton of ZIF-8 lacks strong binding sites and only relies on van der Waals forces or defect adsorption. As a composite material, Zn-Al-LDH@ZIF-8 has a synergistic effect (interlayer ion exchange of Zn-Al-LDH + micropore confinement of ZIF-8), which significantly improves the adsorption capacity and rate. The Zn-Al-LDH component provides ion exchange sites to quickly capture Cl - , and the micropores of ZIF-8 enrich Cl - through the confinement effect, enhancing the local concentration. After compounding, hierarchical pores (micropores + mesopores) are formed, accelerating the diffusion of Cl - to the active sites of Zn-Al-LDH. From a kinetic perspective, the initial adsorption rate of Zn-Al-LDH@ZIF-8 is the fastest, depending on the synergistic effect of the composite material, and it tends to balance in the later stage. The initial rate of Zn-Al-LDH is slightly slower than that of Zn-Al-LD H@ZIF-8, mainly relying on interlayer diffusion and having a longer equilibrium time. The adsorption capacity of ZIF-8 is the lowest and the rate is slow, limited by micropore diffusion. From the final adsorption amount of the material, Zn-Al-LDH@ZIF-8 has a synergistic effect, and the maximum adsorption amount reaches 66.26 mg / g, far higher than 46.87 mg / g of Zn-Al-LDH and 21.3 mg / g of ZIF-8.

[0044] Therefore, the present invention discloses a preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH. By using the in-situ synthesis method, ZIF-8 nanocrystals are directly and uniformly nucleated on the surface of Zn-Al-LDH. The microporous-mesoporous hierarchical pores of ZIF-8 are complementarily integrated with the layered structure of LDH, which not only retains the ion adsorption ability of LDH, but also greatly improves the active site density through the high specific surface area and ordered pores of ZIF-8, and has a high loading capacity.

[0045] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and do not limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH, characterized in that, After synthesizing Zn-Al-LDH particles by precipitation method in an alkaline solution with pH = 10, in a methanol solution, Zn-Al-LDH particles and 2-methylimidazole particles are added in a mass ratio of 1:(3 - 4), enabling the in-situ growth of ZIF-8 metal framework on Zn-Al-LDH to synthesize Zn-Al-LDH@ZIF-8, and the particle size range of ZIF-8 is 50 - 100 nm.

2. The preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH according to claim 1, characterized in that, Specifically, it includes the following steps: (1) Preparation of Zn-Al-LDH: Under a nitrogen atmosphere, zinc nitrate hexahydrate, aluminum nitrate nonahydrate, and sodium nitrate are added to deionized water and dissolved evenly. Sodium hydroxide solution is dropped in to adjust the pH to 10. After centrifuging the mixed solution, it is washed with deionized water and dried at a high temperature using a vacuum drying oven to obtain a white solid. The white powder obtained by crushing and grinding the white solid is Zn-Al-LDH particles; (2) In-situ synthesis of Zn-Al-LDH@ZIF-8: Under a nitrogen atmosphere, Zn-Al-LDH particles and 2-methylimidazole particles are respectively dissolved in methanol and ultrasonically dispersed. The two methanol solutions dissolved with different solutes are mixed and mixed under ultrasonic action for 30 min. After the mixed suspension is allowed to stand and then centrifuged, the precipitate is taken and washed 3 times with methanol, and then dried at a high temperature and crushed and ground. The obtained white powder is Zn-Al-LDH@ZIF-8.

3. The preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH according to claim 2, characterized in that, In step (1), the mass ratio of zinc nitrate hexahydrate, aluminum nitrate nonahydrate, and sodium nitrate is (5 - 7.5):3:(1.4 - 2.1).

4. The preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH according to claim 2, characterized in that, In step (1), the density of the sodium hydroxide solution is 0.02875 g / mL.

5. The preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH according to claim 2, characterized in that, In step (1), the rotation speed of the centrifuge is 8000 rpm, the centrifugation time is 10 - 15 min, the temperature of high-temperature drying is 120 °C, and the drying time is 3 hours.

6. The preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH according to claim 2, characterized in that, In step (2), the mass-volume ratio of Zn-Al-LDH particles to methanol is 1 g:(55 - 175) mL, the mass-volume ratio of 2-methylimidazole particles to methanol is 2.1 g:(80 - 90) mL, the volume ratio of the methanol solution dissolved with Zn-Al-LDH particles to the methanol solution dissolved with 2-methylimidazole particles is 1:(1 - 2), and the mixing temperature is 60 °C.

7. The preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH according to claim 2, characterized in that, In step (2), the rotation speed of the centrifuge is 5000 rmp, and the centrifugation time is 10 min.

8. The preparation method of Zn-Al-LDH@ZIF-8 based on in-situ growth of ZIF-8 metal framework on Zn-Al-LDH according to claim 2, characterized in that, In step (2), the drying temperature is 70 °C.

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