A method for preparing cerium oxide@bimetallic organic framework core-shell structure material

Abstract

The present invention belongs to the technical field of functional materials, and specifically relates to a method for preparing a cerium oxide @ bimetallic organic framework core-shell structure material, wherein cerium oxide, aluminum salt, chromium salt and organic ligand are ultrasonically dispersed in a mixed solvent, stirred, then cooled, centrifuged, washed and dried to obtain a cerium oxide @ bimetallic organic framework core-shell structure material. The preparation method of the present invention has mild reaction conditions and simple steps, and utilizes the strong coordination ability of Al ions, Cr ions and 2-aminoterephthalic acid, and by regulating the content of N,N-dimethylformamide in the mixed solvent, effectively avoids the problem of nucleation growth of different metal ions when reacting with ligands, and realizes the successful coating of the bimetallic organic framework on the surface of cerium oxide nanoparticles.

Description

Technical Field

[0001] The invention belongs to the technical field of functional materials, and in particular relates to a method for preparing a cerium oxide@bimetallic organic framework core-shell structure material. Background Art

[0002] Metal organic frameworks (MOFs) are a new type of crystalline porous material with high specific surface area, structural diversity and customizability, which has attracted widespread attention in the fields of catalysis and gas separation. For example, in catalytic reactions, the cavity size and pore structure of MOFs play a screening role in the mass transfer behavior of reaction substrates / products, thereby changing the reaction pathway and affecting the selectivity of products. Especially in core-shell structure materials, using MOFs as shell materials can not only give play to its structural advantages, but also the MOFs shell and core may produce a synergistic effect, which is conducive to the improvement of catalytic reaction performance.

[0003] As we all know, cerium oxide is one of the most commonly used carriers for loading precious metal particles. If MOFs materials are coated on the oxide surface to form a cerium oxide @ MOFs core-shell structure, it is expected to inhibit the agglomeration of precious metal nanoparticles and exert the synergistic effect of cerium oxide, precious metal particles and MOFs in the catalytic reaction. However, due to the large lattice mismatch between metal oxides and MOFs, it is difficult to successfully grow MOFs on the surface of metal oxides by traditional wet chemical methods. In order to prevent the homogeneous nucleation of MOFs, Long et al. modified the surface of metal oxides, and then adsorbed metal ions and organic ligands multiple times, and assembled layer by layer to successfully prepare a metal oxide @ MOFs core-shell structure (Long, Yan et al, Pt / CeO2@MOFCore@Shell Nanoreactor for Selective Hydrogenation of Furfural via the Channel Screening Effect ACS Catalysis (2018)). Although this method is effective, the operation steps are cumbersome and time-consuming. The existing technology uses a sacrificial template method, with oxides such as Al2O3 and Cu2O as sacrificial templates, and forms Al2O3@Al-MOFs and Cu2O@Cu-MOFs core-shell structures by reacting the Al and Zn ions provided in situ with organic ligands. The sacrificial template method inhibits the homogeneous nucleation of the MOFs shell, which is conducive to the formation of the core-shell structure. However, not all metal oxides are suitable as sacrificial templates. For metal oxides such as CeO2 that are highly resistant to acid and alkali, it is difficult to release metal ions in the solution to react with organic ligands to form MOFs. Therefore, how to coat MOFs on the surface of metal oxides such as CeO2 has become a challenge.

[0004] Bimetallic MOFs exhibit synergistic effects in catalytic reactions due to the tunability of different metal components and the interaction between metal sites. However, due to the different coordination abilities and modes between different metal ions and organic ligands, the synthesis of bimetallic MOFs places higher demands on the selection of metal ions and organic ligands. Although the synthesis of bimetallic MOFs has been reported in many literatures, due to the difficulty in preparing metal oxide @ bimetallic MOFs core-shell structure materials, no relevant reports on metal oxide @ bimetallic MOFs core-shell structure materials have been found. Summary of the invention

[0005] In view of the shortcomings of the current cerium oxide @ bimetallic organic framework core-shell structure material preparation technology, the present invention provides a method for preparing a cerium oxide @ bimetallic organic framework core-shell structure material, by mixing cerium oxide, aluminum salt, chromium salt and organic ligand, without any sacrificial template or pre-modification of the surface of the cerium oxide particles, to prepare the cerium oxide @ bimetallic organic framework core-shell structure material.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for preparing a cerium oxide@bimetallic organic framework core-shell structure material comprises the following steps:

[0008] The cerium oxide, aluminum salt, chromium salt and organic ligand are ultrasonically dispersed in a mixed solvent, stirred for reaction, and then cooled, centrifuged, washed and dried to obtain a cerium oxide@bimetallic organic framework core-shell structure material.

[0009] The cerium oxide is in the form of nano-spherical particles with a particle size of 80-200 nm.

[0010] The aluminum salt is Al(NO3)3·9H2O, and the chromium salt is Cr(NO3)3·9H2O.

[0011] The organic ligand is 2-aminoterephthalic acid.

[0012] The mass ratio of the chromium salt to the aluminum salt is 1:(5-10), otherwise it is difficult to form bimetallic MOFs due to the coordination kinetics of the two metal ions; preferably, the mass ratio of the chromium salt to the aluminum salt is 1:(7-9).

[0013] The mass ratio of the organic ligand to the aluminum salt is (2-5):1.

[0014] The mass ratio of the cerium oxide to the aluminum salt is (2-10):1.

[0015] The mass ratio of the organic ligand to the mixed solvent is (2-5):1.

[0016] The mixed solvent is a mixed solvent of water and DMF, wherein the volume ratio of water to DMF is (1-7 / 3): 1, preferably 1: 1. When the volume ratio of water to DMF is too small or too large, it is difficult to form a core-shell structure due to the change in the deprotonation ability of the organic solvent and the coordination environment.

[0017] The stirring rate is 100-500 rpm, preferably 200-400 rpm;

[0018] The stirring time is 4 to 96 h, preferably 8 to 24 h;

[0019] The reaction temperature is 60-100°C.

[0020] The cerium oxide@bimetallic organic framework core-shell structure material is used in the fields of separation and catalysis.

[0021] In the prior art, metal oxide surface coated bimetallic MOFs, there are huge challenges in the preparation process, one of the reasons is that the metal oxide and MOFs have a large lattice mismatch, and it is difficult to successfully grow on the metal oxide surface through traditional wet chemical methods. Second, the coordination ability and mode between different metal ions and organic ligands are different, which makes it more difficult to coat bimetallic MOFs on the metal oxide surface. However, the present invention adopts the strong coordination ability of Al, chromium ions and 2-aminoterephthalic acid, and regulates the content of DMF in the mixed solvent, which effectively avoids the problem of nucleation of different metal ions when reacting with ligands, thereby preparing cerium oxide @ bimetallic organic framework core-shell structure material.

[0022] Beneficial effects:

[0023] The preparation method of the present invention has mild reaction conditions and simple steps. It utilizes the strong coordination ability of Al ions, Cr ions and 2-aminoterephthalic acid, and by regulating the content of DMF in the mixed solvent, effectively avoids the problem of nucleation of different metal ions when reacting with ligands, and realizes the successful coating of bimetallic MOFs on the surface of cerium oxide nanoparticles. The present invention breaks the limitation of the current metal organic framework as shell material, expands the single metal organic framework into a bimetallic organic framework, and has mild preparation conditions and simple operation, which provides reference significance for the synthesis of bimetallic MOFs-based shell structure materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the synthesis of cerium oxide @Al / Cr-BDC-NH2 core-shell structure materials;

[0025] Figure 2 This is a low-magnification transmission electron microscopy image of the cerium oxide @Al / Cr-BDC-NH2 core-shell structure material prepared in Example 1;

[0026] Figure 3 Element distribution diagram of the cerium oxide @ Al / Cr-BDC-NH2 core-shell structure material prepared in Example 1;

[0027] Figure 4 This is a low-magnification transmission electron microscopy image of the sample prepared in Comparative Example 1;

[0028] Figure 5 This is a low-magnification transmission electron microscope image of the sample prepared in Comparative Example 3. DETAILED DESCRIPTION

[0029] The present invention is further described below in conjunction with the accompanying drawings and examples, which are only used to illustrate the present invention, but do not constitute any limitation to the scope of the present invention. Unless otherwise specified, the raw materials used in the following examples and comparative examples are all commercially available.

[0030] In the following examples and comparative examples, the CeO2 nanoparticles are obtained by reacting ethylene glycol with cerium nitrate under hydrothermal conditions. The size of the CeO2 nano-spherical particles is changed by adjusting the ratio of ethylene glycol and water in the solvent. The CeO2 particle size used in the present invention is 150 nm.

[0031] Example 1

[0032] Schematic diagram of the synthesis of cerium oxide @ Al / Cr-BDC-NH2 core-shell structure materials Figure 1 As shown, 25 mg CeO2, 9 mg Al(NO3)3·9H2O, 1 mg Cr(NO3)3·9H2O and 25 mg H2BDC were ultrasonically dispersed in 10 mL of a mixed solvent of deionized water and DMF (volume ratio of 1:1), stirred at 60 °C with a stirring rate of 200 rpm, reacted for 24 h, cooled to room temperature, centrifuged, washed with DMF and ethanol respectively, and dried in vacuum at 80 °C overnight to obtain cerium oxide@Al / Cr-BDC-NH2 core-shell structure material.

[0033] like Figure 2 The figure shows a low-magnification transmission electron microscope image of the cerium oxide @ Al / Cr-BDC-NH2 core-shell structure material. It can be seen from the figure that the Al / Cr-BDC-NH2 shell is evenly coated on the surface of the cerium oxide. Compared with the shell in patent CN117285722A, which is flaky and coated on the surface of the cerium oxide in a "bush" manner, the cerium oxide @ Al / Cr-BDC-NH2 core-shell structure material prepared in Example 1 has a more regular and uniformly thick shell, which is more conducive to the uniform diffusion of the reaction substrate and product molecules in the catalytic reaction (such as liquid phase hydrogenation reaction). Figure 3The figure shows the element distribution of cerium oxide @Al / Cr-BDC-NH2 core-shell structure material. It can be seen from the figure that there are obvious Al and Cr element signals in the shell material, and the core has a strong Ce element signal, which further illustrates that the bimetallic MOFs are coated on the surface of CeO2 particles.

[0034] Example 2

[0035] This example is the same as Example 1, except that the amount of Al(NO3)3·9H2O is 8.75 mg, the amount of Cr(NO3)3·9H2O is 1.25 mg, and the cerium oxide @Al / Cr-BDC-NH2 core-shell structure material is prepared, and the Al / Cr-BDC-NH2 shell layer is uniformly coated on the surface of cerium oxide.

[0036] Example 3

[0037] This example is the same as Example 1, except that the reaction temperature is 80 o C, the cerium oxide @Al / Cr-BDC-NH2 core-shell structure material was prepared, and the Al / Cr-BDC-NH2 shell layer was uniformly coated on the surface of cerium oxide.

[0038] Example 4

[0039] This example is the same as Example 1, except that the volume ratio of water to DMF in the mixed solvent is 7:3, and cerium oxide @ Al / Cr-BDC-NH2 core-shell structure material is prepared, and the Al / Cr-BDC-NH2 shell layer is uniformly coated on the surface of cerium oxide.

[0040] Comparative Example 1

[0041] This comparative example is the same as Example 1, except that the Al(NO3)3·9H2O is replaced by Ni(NO3)2. The low-magnification transmission electron microscopy image of the sample prepared after the reaction is as follows: Figure 4 As shown in the figure, MOFs are not evenly coated on the surface of CeO2 nanoparticles, but nucleate and grow separately. The reason may be that Ni ions cannot nucleate and grow with organic ligands under this condition.

[0042] Comparative Example 2

[0043] This comparative example is the same as Example 1, except that the CeO2 nanoparticles are replaced with SiO2. In the sample prepared after the reaction, MOFs are not evenly coated on the surface of the CeO2 nanoparticles, but nucleate and grow separately. The reason may be that amorphous SiO2 is not conducive to the nucleation and growth of MOFs on its surface.

[0044] Comparative Example 3

[0045] This comparative example is the same as Example 1, except that the volume ratio of water to DMF in the mixed solvent is 0:10. The low-magnification transmission electron microscopy image of the sample prepared after the reaction is as follows: Figure 5 As shown in the figure, MOFs are not evenly coated on the surface of CeO2 nanoparticles, but nucleate and grow separately. The reason is that excessive DMF affects the coordination ability of organic ligands and the growth kinetics of MOFs.

Claims

1. A method for preparing a cerium oxide@bimetallic organic framework core-shell structure material, characterized in that: The steps include: The cerium oxide, aluminum salt, chromium salt and organic ligand are ultrasonically dispersed in a mixed solvent, stirred to react, and then cooled, centrifuged, washed and dried to obtain a cerium oxide@bimetallic organic framework core-shell structure material; The mass ratio of the organic ligand to the mixed solvent is (2-5):1; The mixed solvent is a mixed solvent of water and DMF; Among them, the volume ratio of water to DMF is (1~7 / 3): 1; The organic ligand is 2-aminoterephthalic acid.

2. The method for preparing the cerium oxide@bimetallic organic framework core-shell structure material according to claim 1, characterized in that: The aluminum salt is Al(NO3)3·9H2O.

3. The method for preparing the cerium oxide@bimetallic organic framework core-shell structure material according to claim 1, characterized in that: The chromium salt is Cr(NO3)3·9H2O.

4. The method for preparing the cerium oxide@bimetallic organic framework core-shell structure material according to claim 1, characterized in that: The mass ratio of the chromium salt to the aluminum salt is 1:(5~10).

5. The method for preparing the cerium oxide@bimetallic organic framework core-shell structure material according to claim 1, characterized in that: The mass ratio of the cerium oxide to the aluminum salt is (2-10):

1.

6. The method for preparing the cerium oxide@bimetallic organic framework core-shell structure material according to claim 1, characterized in that: The mass ratio of the organic ligand to the aluminum salt is (2-5):

1.

7. The method for preparing the cerium oxide@bimetallic organic framework core-shell structure material according to claim 1, characterized in that: The stirring rate is 100-500 rpm; The stirring time is 4 to 96 h; The reaction temperature is 60-100°C.

8. The method for preparing the cerium oxide@bimetallic organic framework core-shell structure material according to claim 1, characterized in that: The cerium oxide is in the form of nano-spherical particles with a particle size of 80-200 nm.

Images