A two-dimensional porous MOF nanosheet and its preparation method

Abstract

The present invention belongs to the field of preparation of advanced functional materials and gas adsorption applications, and discloses a two-dimensional porous MOF nanosheet and a preparation method thereof. The two-dimensional porous MOF nanosheet has a hexagonal morphology and is composed of metal ions and ligands as basic units. The metal ions are iron ions, the ligands are BDC and TTP, and the structural formula of the basic unit is Fe3(BDC)3(TTP), where BDC is terephthalic acid anion with the molecular formula C8H4O2 2‑ , TTP is a terpyridine-derived ligand with the molecular formula C 20 H 14 N4; The present invention provides a novel two-dimensional porous MOF nanosheet that can be prepared by a simple method and has a regular geometric morphology. The nanosheet has a hexagonal morphology and a high aspect ratio; its synthesis method has a simple route, easily available raw materials, and low cost.

Description

Technical Field

[0001] The present invention relates to the field of preparation of advanced functional materials and gas adsorption applications, and particularly relates to a two-dimensional porous MOF nanosheet and a preparation method thereof. Background Art

[0002] Metal-Organic Framework (MOF) materials are a class of porous polymer crystal materials assembled from transition metal nodes and organic ligands. There are various types of MOF materials. The transition metals serving as nodes are inexpensive and readily available, and the bridging organic small molecule ligands have high designability. There are a large number of regular nano-scale pores in the highly crystalline framework constructed, and it can be used as a good porous material in the fields of gas storage, adsorption, and separation, etc. Compared with traditional inorganic zeolite pore materials, the specific surface area of microporous MOF materials can reach more than 3000 m 2 / g, and the synthesis is simple and the structure is stable. It is considered to be one of the most promising materials in the future nano field. However, most of the reported MOF materials at present are three-dimensional bulk structures at the micron scale, and the internal metal ions are deeply wrapped by organic ligands, which greatly hinders the exposure of their surface active sites.

[0003] Two-dimensional MOF nanosheets are a branch of metal-organic frameworks, generally referring to metal-organic framework materials with a size of about several nanometers in at least one dimension. Compared with traditional bulk metal-organic frameworks, the most prominent feature of two-dimensional MOF nanosheets is their ultra-thin thickness. Two-dimensional MOF nanosheets combine the advantages of two-dimensional layered nanomaterials and MOFs, have a large specific surface area and a highly open structure, and expose a large number of surface active sites. Therefore, controllable synthesis of two-dimensional MOF nanomaterials is an effective means to increase the specific surface area of the material and enhance the surface activity of the material. However, it is difficult to achieve controllable synthesis of high-quality MOF nanosheets. At present, there are mainly two types of methods for preparing ultra-thin metal-organic framework nanosheets. One is to directly exfoliate large-volume metal-organic frameworks into nanosheets, with a low yield and uncontrollable morphology of the nanosheets. The other is to introduce small molecule inhibitors with directional coordination ability to carry out a "substitution-inhibition" process on the connecting ligands on specific crystal planes. Highly directional surface modification can effectively inhibit the growth of crystals in a specific dimensional direction, making them gradually evolve from three-dimensional bulk to two-dimensional sheet morphology. However, at present, most small molecule inhibitors cannot well match the framework structure of MOFs, cannot achieve precise directional inhibition of crystal planes, the orientation of surface modification is not strong, and the aspect ratio improvement effect of crystals is not good. Summary of the Invention

[0004] In view of the above problems, the present invention provides a novel two-dimensional porous MOF nanosheet and a preparation method thereof. Through a new synthesis method, the nanosheet has a hexagonal morphology, a high aspect ratio, and the synthesis method has a simple route, easily available raw materials, and low cost.

[0005] The object of the present invention is achieved by the following technical solutions:

[0006] A two-dimensional porous MOF nanosheet, which has a hexagonal morphology and consists of metal ions and ligands as basic units. The metal ions are iron ions, the ligands are BDC and TTP, and the structural formula of the basic unit is Fe3(BDC)3(TTP), where BDC is terephthalic acid anion with the molecular formula C8H4O2 2- , TTP is a terpyridine-derived ligand with the molecular formula C 20 H 14 N4, and the molecular structure is as follows:

[0007]

[0008] In some preferred embodiments, the lateral size of the nanosheet is 1 μm, and the thickness is 1 - 50 nm.

[0009] Another object of the present invention is to provide a preparation method of the two-dimensional porous MOF nanosheet, including the following steps:

[0010] Mix the terpyridine-derived ligand, iron salt, terephthalic acid, benzoic acid analog and pyridine in N,N-dimethylformamide and reflux. After the reaction is completed, separate the precipitate, wash it, and obtain the product.

[0011] The benzoic acid analog is benzoic acid, 4-fluorobenzoic acid, 4-iodobenzoic acid or 3-thiophenecarboxylic acid.

[0012] In some preferred embodiments, the preparation method of the terpyridine-derived ligand includes the following steps:

[0013] Dissolve 4-pyridinecarboxaldehyde and 2-acetylpyridine in a reaction solvent, add granular sodium hydroxide and stir at room temperature, then add ammonia water, heat to reflux, separate the precipitate after the reaction is completed, wash it, and obtain the product.

[0014] In some preferred embodiments, the molar ratio of the terpyridine-derived ligand, iron salt and terephthalic acid is 1:3:3.

[0015] In some preferred embodiments, the molar ratio of the benzoic acid analog to the terpyridine-derived ligand is 5:1.

[0016] In some preferred embodiments, the temperature of the reflux reaction is 100 °C, and the reaction time is 12 - 24 h.

[0017] In some preferred embodiments, the mixing ratio of the pyridine to the terpyridine-derived ligand is 0.2 mL / 0.3 mmol.

[0018] The beneficial effects of the present invention are as follows:

[0019] (1) The present invention provides a novel two-dimensional porous MOF nanosheet that can be prepared by a simple method and has a regular geometric morphology. The nanosheet has a hexagonal morphology and a high aspect ratio; its synthesis method has a simple route, easily available raw materials, and low cost.

[0020] (2) Using FeCl3·6H2O, terephthalic acid, 4-pyridinecarboxaldehyde, 2-acetylpyridine, etc. as the main raw materials, compared with the prior art, the present invention uses inexpensive metal ions as the connection sites, with low raw material costs and rich sources; the ligand synthesis method is simple, and the reaction yield is high; the MOF crystal constructed by the two ligands has strong crystallinity and stable physical and chemical properties; the thickness control method is convenient and can widely adapt to various small molecule inhibitors with a benzoic acid-like structure; the morphology maintenance ability is strong, and the prepared nanosheets have a uniform morphology, and the overall preparation strategy can demonstrate extremely high controllability.

[0021] (3) Using a benzoic acid analog as an additive, the MOF crystal with the structure of Fe3(BDC)3(TTP) is modified and maintained as a two-dimensional MOF nanohexagonal sheet morphology. While there are a large number of micropores in the crystal, the two-dimensional sheet-like MOF crystal can stack into a honeycomb-like mesopore, further enhancing the gas permeation and adsorption capacity of the material. It can be applied to N2 adsorption, and due to the porosity of the material, it also has great application potential in the adsorption of toxic gases and pollutants. Description of the Drawings

[0022] The present invention is further illustrated by the accompanying drawings, but the embodiments in the drawings do not constitute any limitation to the present invention. For those of ordinary skill in the art, other drawings can be obtained based on the following drawings without creative efforts.

[0023] Figure 1 It is the structure of the MOF nanosheet crystal prepared in the embodiment of the present invention within the ab plane, where the gray small balls are carbon atoms, the blue small balls are nitrogen atoms, the red small balls are oxygen atoms, and the light blue small balls are iron atoms. All hydrogen atoms have been omitted;

[0024] Figure 2It is the interlayer structure of the MOF nanosheet crystal prepared in the embodiment of the present invention, where the gray spheres are carbon atoms, the blue spheres are nitrogen atoms, the red spheres are oxygen atoms, and the light blue spheres are iron atoms. All hydrogen atoms have been omitted;

[0025] Figure 3 It is the 1H NMR spectrum of the ligand L1 synthesized in Example 1;

[0026] Figure 4 It is the XRD comparison spectrum of the MOF nanosheet synthesized in Example 2 and the standard isophase MOF crystal;

[0027] Figure 5 It is the SEM image of the MOF nanosheet synthesized in Example 2;

[0028] Figure 6 It is the thermogravimetric spectrum of the MOF nanosheet synthesized in Example 2;

[0029] Figure 7 It is the XRD spectrum of the MOF nanosheet synthesized in Example 3;

[0030] Figure 8 It is the SEM spectrum of the MOF nanosheet synthesized in Example 3;

[0031] Figure 9 It is the XRD spectrum of the MOF nanosheet synthesized in Example 4;

[0032] Figure 10 It is the SEM spectrum of the MOF nanosheet synthesized in Example 4;

[0033] Figure 11 It is the XRD spectrum of the MOF nanosheet synthesized in Example 5;

[0034] Figure 12 It is the SEM spectrum of the MOF nanosheet synthesized in Example 5;

[0035] Figure 13 It is the XRD spectrum of the MOF nano dodecahedron synthesized in Example 6;

[0036] Figure 14 It is the SEM spectrum of the MOF nano dodecahedron synthesized in Example 6;

[0037] Figure 15 It is the XRD spectrum of the MOF nano truncated polyhedron synthesized in Example 7;

[0038] Figure 16 It is the SEM spectrum of the MOF nano truncated polyhedron synthesized in Example 7;

[0039] Figure 17is the N2 adsorption - desorption curve of the MOF nanosheets synthesized in Example 2;

[0040] Figure 18 is the AFM thickness test result diagram of the MOF nanosheets synthesized in Example 2. Detailed implementation manners

[0041] The present invention will be further described in conjunction with the following examples.

[0042] See Appendix Figure 1-18 In the MOF nanosheets and their preparation method provided by the embodiments of the present invention, on the premise of introducing benzoic acid and its analogs as inhibitors, by regulating the content of the inhibitors, the growth of the MOF framework structure is inhibited in a specific dimension, the morphology of the MOF crystal is gradually adjusted from a dodecahedron to a truncated polyhedron, and finally a nanosheet morphology with a thickness below 300 nm and a significantly increased aspect ratio is formed, and good gas adsorption performance is exhibited. The embodiments of the present invention also verify that a series of analogs with the benzoic acid structure as a template have good regulation effects on this MOF framework, proving that the preparation method of the present invention has advantages such as strong applicability and simple synthesis route, and demonstrates excellent application potential.

[0043] Example

[0044] The two - dimensional porous MOF nanosheets provided by the present invention have a hexagonal morphology, and the basic unit is composed of metal ions and ligands. The metal ions are iron ions, the ligands are BDC and TTP, and the structural formula of the basic unit is Fe3(BDC)3(TTP), where BDC is terephthalic acid anion with the molecular formula C8H4O2 2- , TTP is a terpyridine - derived ligand with the molecular formula C 20 H 14 N4, and the molecular structure is as follows:

[0045]

[0046] In some preferred implementation manners, the lateral size of the nanosheets is 1 μm, and the thickness is 1 - 50 nm.

[0047] Example 1

[0048] The preparation method of the terpyridine - derived ligand L1 of the two - dimensional porous MOF nanosheets provided in this example includes the following steps:

[0049] Dissolve 4-pyridinecarboxaldehyde and 2-acetylpyridine in ethanol at a ratio of 1:2.1, add 20 molar equivalents of granular NaOH to 4-pyridinecarboxaldehyde, stir at room temperature for 8 h, then add an excess of ammonia water, reflux at 85 °C for 12 h, cool and filter by suction. Wash the solid twice with water and ethanol respectively, and then recrystallize in DMSO. The obtained white solid is ligand L1 with a yield of 74%.

[0050] The 1H NMR spectrum of ligand L1 is as Figure 3 shown.

[0051] Example 2

[0052] A two-dimensional porous MOF nanosheet, and its preparation method includes the following steps:

[0053] Disperse 0.3 mmol of ligand L1, 0.9 mmol of terephthalic acid, 0.9 mmol of FeCl3·6H2O and 1.5 mmol of benzoic acid in 40 mL of DMF, ultrasonically form a yellow turbid solution, then heat it to 100 °C, and after several minutes, a yellow clear solution is formed. Then add 0.2 mL of pyridine, stop the reaction after 16 h, cool and centrifuge. Wash the solid twice with DMF and methanol respectively to obtain a dark green solid with a yield of 17%.

[0054] The comparative XRD pattern of the product with the standard MOF crystal Fe-MIL-88B-tpy is as Figure 4 shown, and the morphology is as Figure 5 shown, and the thermogravimetric pattern is as Figure 6 shown.

[0055] Example 3

[0056] A two-dimensional porous MOF nanosheet, and its preparation method includes the following steps:

[0057] Disperse 0.3 mmol of ligand L1, 0.9 mmol of terephthalic acid, 0.9 mmol of FeCl3·6H2O and 1.5 mmol of 4-fluorobenzoic acid in 40 mL of DMF, ultrasonically form a yellow turbid solution, then heat it to 100 °C, and after several minutes, a yellow clear solution is formed. Then add 0.2 mL of pyridine, stop the reaction after 16 h, cool and centrifuge. Wash the solid twice with DMF and methanol respectively to obtain a dark green solid with a yield of 19%.

[0058] The XRD pattern of the product is as Figure 7 shown, and the morphology is as Figure 8 shown.

[0059] Example 4

[0060] A two-dimensional porous MOF nanosheet, and its preparation method includes the following steps:

[0061] Disperse 0.3 mmol of ligand L1, 0.9 mmol of terephthalic acid, 0.9 mmol of FeCl3·6H2O and 1.5 mmol of 4-iodobenzoic acid in 40 mL of DMF, and form a yellow turbid solution by ultrasonic treatment. Then heat it to 100 °C, and after several minutes, a yellow clear solution is formed. Then add 0.2 mL of pyridine, stop the reaction after 16 h, cool it and centrifuge. Wash the solid twice with DMF and methanol respectively to obtain a dark green solid with a yield of 10%.

[0062] The XRD pattern of the product is as Figure 9 shown, and the morphology is as Figure 10 shown.

[0063] Example 5

[0064] A two-dimensional porous MOF nanosheet, and its preparation method includes the following steps:

[0065] Disperse 0.3 mmol of ligand L1, 0.9 mmol of terephthalic acid, 0.9 mmol of FeCl3·6H2O and 1.5 mmol of 3-thiophenecarboxylic acid in 40 mL of DMF, and form a yellow turbid solution by ultrasonic treatment. Then heat it to 100 °C, and after several minutes, a yellow clear solution is formed. Then add 0.2 mL of pyridine, stop the reaction after 16 h, cool it and centrifuge. Wash the solid twice with DMF and methanol respectively to obtain a dark green solid with a yield of 12%.

[0066] The XRD pattern of the product is as Figure 11 shown, and the morphology is as Figure 12 shown.

[0067] Example 6

[0068] A porous MOF nano dodecahedron, and its preparation method includes the following steps:

[0069] Disperse 0.3 mmol of ligand L1, 0.9 mmol of terephthalic acid and 0.9 mmol of FeCl3·6H2O in 40 mL of DMF, and form a yellow turbid solution by ultrasonic treatment. Then heat it to 100 °C, and after several minutes, a yellow clear solution is formed. Then add 0.2 mL of pyridine, stop the reaction after 16 h, cool it and centrifuge. Wash the solid twice with DMF and methanol respectively to obtain a dark green solid with a yield of 32%.

[0070] The XRD pattern of the product is as Figure 13 shown, and the morphology is as Figure 14 shown.

[0071] Example 7

[0072] A porous MOF nano polyhedron, and its preparation method includes the following steps:

[0073] 0.3 mmol of ligand L1, 0.9 mmol of terephthalic acid, 0.9 mmol of FeCl3·6H2O and 1.0 mmol of benzoic acid were dispersed in 40 mL of DMF, and ultrasonic treatment was carried out to form a yellow turbid solution. Then the temperature was raised to 100 °C, and a yellow clear solution was formed after several minutes. Then 0.2 mL of pyridine was added, and the reaction was stopped after 16 h. After cooling, centrifugation was carried out, and the solid was washed twice with DMF and methanol respectively to obtain a dark green solid with a yield of 25%.

[0074] The XRD pattern of the product is as Figure 15 shown, and the morphology is as Figure 16 shown.

[0075] Experimental example

[0076] The product obtained in Example 2 was soaked in DMF for 12 h, centrifuged and then soaked in methanol for 12 h. After repeating this three times, the solid was filtered by suction and vacuum activated at 180 °C for 2 h, and then N2 adsorption was carried out at 77 K.

[0077] Its adsorption - desorption curve is as Figure 17 shown.

[0078] The AFM thickness test results of the MOF nanosheets synthesized in Example 2 are as Figure 18 shown.

[0079] The two - dimensional porous MOF nanosheets and their preparation methods provided in the above embodiments of the present invention can make the two - dimensional porous MOF nanosheets have a hexagonal morphology. It is composed of metal ions and ligands as basic units. The metal ion is iron ion, and the ligands are BDC and TTP. The structural formula of the basic unit is Fe3(BDC)3(TTP), where BDC is terephthalic acid anion with the molecular formula C8H4O2 2- , TTP is a terpyridine - derived ligand with the molecular formula C 20 H 14 N4; The present invention provides a novel two - dimensional porous MOF nanosheet that can be prepared by a simple method and has a regular geometric morphology. The nanosheet has a hexagonal morphology and a high aspect ratio. Its synthesis method has a simple route, easily available raw materials, and low cost.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A two-dimensional porous MOF nanosheet, characterized in that, The two-dimensional porous MOF nanosheets are hexagonal in morphology and consist of metal ions and ligands as basic units. The metal ions are iron ions, the ligands are BDC and TTP, and the structural formula of the basic unit is Fe3(BDC)3(TTP), where BDC is terephthalic acid anion and TTP is a terpyridine-derived ligand, and the molecular structure is as follows:

2. The two-dimensional porous MOF nanosheet according to claim 1, wherein The lateral size of the nanosheets is 0.5 - 2 μm, and the thickness is 50 - 300 nm.

3. The preparation method of the two-dimensional porous MOF nanosheets according to claim 1 or 2, characterized in that, It includes the following steps: Mix the terpyridine-derived ligand, iron salt, terephthalic acid, benzoic acid analogue and pyridine in N,N-dimethylformamide and reflux for reaction. After the reaction is completed, separate the precipitate, wash it, and obtain the product. The benzoic acid analogue is benzoic acid, 4-fluorobenzoic acid, 4-iodobenzoic acid or 3-thiophenecarboxylic acid.

4. The preparation method according to claim 3, characterized in that, The preparation method of the terpyridine-derived ligand includes the following steps: Dissolve 4-pyridinecarboxaldehyde and 2-acetylpyridine in a reaction solvent, add granular sodium hydroxide and stir at room temperature for reaction, then add ammonia water, heat up to reflux for reaction. After the reaction is completed, separate the precipitate, wash it, and obtain the product.

5. The preparation method according to claim 3, characterized in that, The molar ratio of the terpyridine-derived ligand, iron salt and terephthalic acid is 1:3:

3.

6. The preparation method according to claim 3, characterized in that, The molar ratio of the benzoic acid analogue to the terpyridine-derived ligand is 3:1 - 8:

1.

7. The preparation method according to claim 3, characterized in that, The temperature of the reflux reaction is 80 °C - 120 °C, and the reaction time is 12 - 24 h.

8. The preparation method according to claim 3, wherein The mixing ratio of the pyridine and the terpyridine-derived ligand is 0.5 mL / mmol - 1 mL / mmol.

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