A method for preparing two-dimensional β-ketoenamine covalent organic framework nanosheets

Abstract

The invention discloses a method for preparing a two-dimensional β-ketoenamine covalent organic framework nanosheet, which belongs to the technical field of synthesis of nanomaterials. The specific preparation steps are as follows: a fatty amine aqueous solution is prepared with water as a solvent, the fatty amine aqueous solution is used as a stripping solution, wherein the fatty amine is one or more of propylamine, butylamine, di-n-propylamine, di-n-butylamine or triethylamine; a bulk material COF-JLU2, NUS-3 or TFP-DABA is placed in the stripping solution, and the fatty amine is used as an intercalation agent to enter the interlayer of the bulk material, and the stripping is performed under the assistance of ultrasound, and then the dispersion is centrifuged and washed, and dried to obtain a two-dimensional β-ketoenamine covalent organic framework nanosheet. The invention is simple to make, and can prepare a few-layer nanosheet at low cost and on a large scale; the invention can efficiently prepare a few-layer nanosheet with high selectivity.

Description

Technical Field

[0001] The invention belongs to the technical field of synthesis of nano materials, and in particular relates to a method for preparing a two-dimensional β-ketoenamine covalent organic framework nanosheet. Background Art

[0002] Two-dimensional β-ketoenamine covalent organic frameworks (2D COFs) use aldehyde and amino groups in monomers as raw materials. After Schiff base condensation, the enol form is irreversibly converted to the keto form, and finally a covalent organic framework material with a stable β-ketoenamide structure is obtained. These building units have the ability to induce electron push-pull effects and proton transfer, which makes the two-dimensional β-ketoenamine covalent organic framework materials have good photogenerated charge separation ability and visible light absorption ability, making them excellent photocatalytic materials. The solubility and processability of 2D COFs powders are poor, which greatly limits their application. In contrast, two-dimensional covalent organic framework nanosheets (2D CONs) are widely used in various fields because they expose more active sites and have good dispersion stability in solvents. So far, the preparation methods of 2D COFs nanosheets are generally divided into two categories, namely top-down and bottom-up. Generally speaking, the top-down synthesis strategy refers to the exfoliation of bulk 2D COFs crystals by physical or chemical means, mainly including mechanical exfoliation, chemical exfoliation, and ultrasonic-assisted liquid phase exfoliation. The bottom-up synthesis strategy is to directly prepare 2D COFs films on different substrates and interfaces, such as interfacial synthesis. Among them, the liquid phase exfoliation method is considered to be a method that is simple to operate, low in cost, and can be used to prepare nanosheets on a large scale. However, the current problem is that there are few studies on 2D COFs liquid phase exfoliation, the solvents used are mostly organic solvents, most 2D COFs powders have poor dispersibility in solvents, and the yield, selectivity and quality of the prepared nanosheets are low. In view of this, the development of a high-yield and high-selectivity method for preparing covalent organic framework nanosheets (CONs) is an urgent problem to be solved. Summary of the invention

[0003] The technical problem solved by the present invention is to provide a method for preparing two-dimensional β-ketoenamine covalent organic framework nanosheets with high yield, high selectivity and simple operation. The method uses ultrasonic-assisted liquid phase exfoliation of aliphatic amine aqueous solution to prepare few-layer β-ketoenamine covalent organic framework nanosheets.

[0004] The present invention adopts the following technical solution to solve the above technical problems, a method for preparing a two-dimensional β-ketoenamine covalent organic framework nanosheet, characterized by the specific steps of:

[0005] Step S1, preparing an aqueous fatty amine solution with water as a solvent, the aqueous fatty amine solution being used as a stripping solution, wherein the fatty amine is one or more of propylamine, butylamine, di-n-propylamine, di-n-butylamine or triethylamine;

[0006] Step S2, placing the bulk material COF-JLU2, NUS-3 or TFP-DABA in the stripping solution obtained in step S1, and using the fatty amine as an intercalating agent to enter the interlayer of the bulk material, and performing stripping under the assistance of ultrasound, and then centrifugally washing and drying the dispersion to obtain a two-dimensional β-ketoenamine covalent organic framework nanosheet;

[0007] The structural formula of the block material COF-JLU2, NUS-3 or TFP-DABA is:

[0008] .

[0009] It is further defined that the concentration of the fatty amine aqueous solution in step S1 is 0.01-0.05 mol / L.

[0010] It is further defined that the feeding ratio of the block material to the stripping solution in step S2 is 1 mg: 1-1.2 mL.

[0011] It is further defined that the temperature of the ultrasound-assisted effect in step S2 is 20-30°C.

[0012] It is further defined that the ultrasonic power of the ultrasonic-assisted action in step S2 is 280-360 W, and the ultrasonic-assisted action time is 7-11 h.

[0013] It is further defined that the centrifugal washing in step S2 includes two processes: low-speed centrifugation and high-speed centrifugation, wherein the low-speed centrifugation speed is 1500~2500rpm, the low-speed centrifugation time is 5~15min, centrifugation is performed twice, the upper layer of dispersion is taken and high-speed refrigerated centrifugation is performed again, the high-speed centrifugation speed is 10000~20000rpm, the high-speed centrifugation time is 3~10min, the bottom precipitate is collected and washed with secondary water at least 10 times, and washed with ethanol at least 3 times to remove residual fatty amines, and finally the washed precipitate is vacuum dried at 30~50°C to obtain pure two-dimensional β-ketoenamine covalent organic framework nanosheets.

[0014] Compared with the prior art, the excellent effects of the present invention are embodied in the following aspects: the present invention is simple to operate, and can prepare few-layer β-ketoenamine covalent organic framework nanosheets on a large scale at low cost; the present invention can efficiently prepare few-layer β-ketoenamine covalent organic framework nanosheets with high selectivity; the two-dimensional β-ketoenamine covalent organic framework nanosheets prepared by the present invention have good dispersibility in a fatty amine-aqueous solution system, and the nanosheets obtained during the ultrasonic process are not easy to precipitate; the yield of NUS-3 nanosheets obtained by peeling according to the present invention is as high as 35.6%, the selectivity of nanosheets with a thickness of less than 2 nm is 86.2%, and the lateral size is about 1 μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The FT-IR spectra and XRD spectra of the nanosheets and bulk materials obtained by peeling of the present invention are shown.

[0016] Figure 2 The AFM and thickness statistics of COF-JLU2, NUS-3 and TFP-DABA nanosheets prepared in di-n-propylamine aqueous solution according to the present invention.

[0017] Figure 3 This is a SEM comparison picture of COF-JLU2, NUS-3 and TFP-DABA nanosheets and bulk materials prepared in a di-n-propylamine aqueous solution according to the present invention.

[0018] Figure 4 TEM comparison diagram of NUS-3 nanosheets and bulk materials prepared in di-n-propylamine aqueous solution according to the present invention. DETAILED DESCRIPTION

[0019] The above contents of the present invention are further described in detail below through examples, but this should not be understood as the scope of the above subject matter of the present invention being limited to the following examples, and all technologies implemented based on the above contents of the present invention belong to the scope of the present invention.

[0020] According to the methods reported in the literature, three two-dimensional β-ketoenamine COFs with different interlayer spacings and functional groups, namely the bulk materials COF-JLU2, NUS-3 and TFP-DABA, were synthesized respectively (An azine-linked covalent organic framework: Synthesis , characterization and efficient gas storage. Chemistry:A European journal, 2015, 21(34): 12079-12084; Mixed matrix membranes (MMMs)comprising exfoliated 2D covalent organic frameworks (COFs) for efficient CO2separation. Chemistry of Materials, 2016, 28(5): 1277-1285; Synthesis of asulfonated two-dimensional covalent organic framework as an efficient solidacid catalyst for biobased chemical conversion. ChemSusChem, 2015, 8(19):3208-3212.). Example 1

[0021] Preparation of COF-JLU2 nanosheets:

[0022] Step S1, weigh 5 mg of block material COF-JLU2, disperse it in 5 mL of 0.05 mol / L di-n-propylamine aqueous solution, and perform water bath ultrasonic treatment for 7 h at an ultrasonic power of 280 W to obtain a red suspension.

[0023] Step S2, transfer the red suspension obtained in step S1 into a 5 mL centrifuge tube and centrifuge it twice at 2000 rpm, each time for 10 min, and the collected supernatant is the COF-JLU2 nanosheet dispersion obtained by peeling.

[0024] Step S3, centrifuge the COF-JLU2 nanosheet dispersion obtained in step S2 at a speed of 15000 rpm for 5 minutes, and the precipitate after centrifugation is the COF-JLU2 nanosheet, and then wash the COF-JLU2 nanosheet with secondary water 10 times and ethanol 3 times to remove residual fatty amines, and finally put the washed precipitate into a vacuum drying oven at 40°C and dry it overnight to obtain relatively pure COF-JLU2 nanosheets. Example 2

[0025] Preparation of NUS-3 nanosheets:

[0026] Step S1, weigh 5 mg of block material NUS-3, disperse it in 5 mL of 0.01 mol / L di-n-propylamine aqueous solution, and perform water bath ultrasonic treatment for 11 h at an ultrasonic power of 360 W to obtain a yellow suspension.

[0027] Step S2, transfer the yellow suspension obtained in step S1 into a 5 mL centrifuge tube and centrifuge it twice at a speed of 2000 rpm, each time for 10 min, and the collected supernatant is the NUS-3 nanosheet dispersion obtained by peeling.

[0028] Step S3, the NUS-3 nanosheet dispersion obtained in step S2 is centrifuged at a speed of 15000 rpm for 5 minutes, and the precipitate after centrifugation is the NUS-3 nanosheet, and then the NUS-3 nanosheet is washed 10 times with secondary water and 3 times with ethanol to remove residual fatty amines, and finally the washed precipitate is placed in a vacuum drying oven at 40°C and dried overnight to obtain relatively pure NUS-3 nanosheets. Example 3

[0029] Preparation of TFP-DABA nanosheets:

[0030] Step S1, weigh 5 mg of bulk material TFP-DABA, disperse it in 5 mL of 0.01 mol / L di-n-propylamine aqueous solution, and perform water bath ultrasonic treatment for 11 h at an ultrasonic power of 360 W to obtain a dark red suspension.

[0031] Step S2, transfer the dark red suspension obtained in step S1 into a 5 mL centrifuge tube, and centrifuge it twice at a speed of 2000 rpm, each time for 10 minutes, and the collected supernatant is the TFP-DABA nanosheet dispersion obtained by peeling.

[0032] Step S3, centrifuging the TFP-DABA nanosheet dispersion obtained in step S2 at a speed of 15000 rpm for 5 minutes, and the precipitate after centrifugation is the TFP-DABA nanosheet, and then washing the nanosheet with secondary water 10 times and ethanol 3 times to remove residual fatty amines, and finally placing the washed precipitate in a vacuum drying oven at 40°C and drying overnight to obtain relatively pure TFP-DABA nanosheets.

[0033] Table 1 shows the yield of nanosheets obtained by optimizing the concentration of aliphatic amine, ultrasonic power and ultrasonic time by the control variable method of the present invention. When NUS-3 is peeled in a di-n-propylamine aqueous solution, the yield of nanosheets obtained is the highest, reaching 35.6%.

[0034] Table 1

[0035]

[0036] Figure 1 The FT-IR spectra and XRD spectra of the nanosheets obtained by stripping of the present invention and their bulk materials are shown in the figure. In the infrared spectrum, the characteristic peaks of the nanosheets obtained after stripping are all present compared with the three bulk materials, and the positions have not changed, indicating that the chemical structure and bond connection mode of the material after stripping remain unchanged, but the XRD characteristic peaks of the nanosheets after stripping are greatly weakened or disappear, which is because the layered stacking of the material after stripping is destroyed, thereby forming a few-layer nanosheet.

[0037] Figure 2 The AFM and thickness statistics of COF-JLU2, NUS-3 and TFP-DABA nanosheets prepared in di-n-propylamine aqueous solution of the present invention are shown in Figure 1. The nanosheets obtained by exfoliation of the three two-dimensional β-ketoenamine COFs in the di-n-propylamine aqueous solution system have a lateral size of 1-2 μm, and the proportion of nanosheets with a thickness of less than 2 nm is as high as 86.2%, indicating that the successful exfoliation has obtained a few-layer nanosheet with high selectivity and quality.

[0038] Figure 3 The present invention is a SEM comparison of COF-JLU2, NUS-3 and TFP-DABA nanosheets prepared in a di-n-propylamine aqueous solution and bulk materials. The unpeeled bulk materials COF-JLU2, NUS-3 and TFP-DABA are spherical in size, but the COF-JLU2 and TFP-DABA nanosheets after peeling become smaller blocks, while the NUS-3 nanosheets obviously become smaller fragments. The lateral dimensions of the nanosheets of the three bulk materials COFs after peeling are smaller than those of the bulk materials. Similar phenomena are also observed in other fatty amine aqueous solutions.

[0039] Figure 4The TEM comparison of the NUS-3 nanosheets prepared in the di-n-propylamine aqueous solution of the present invention and the bulk material. The NUS-3 nanosheets are stacked in thin sheets, and compared with the unpeeled bulk material, the nanosheets are more transparent and thinner. This shows that we have successfully obtained thinner nanosheets in the fatty amine aqueous solution system. This phenomenon has also been observed in other fatty amine aqueous solutions.

[0040] The present invention uses a fatty amine aqueous solution to disperse and stabilize bulk material two-dimensional β-ketoenamine COFs, and prepares high-yield and high-quality COFs nanosheets with the assistance of ultrasound. The NUS-3 nanosheets prepared by the present invention have better photocatalytic CO2 reduction performance than the bulk material NUS-3. The technical advantages of the present invention are: (1) The preparation process of the present invention is simple and efficient, and easy to scale up; (2) During the stripping process of the present invention, the material can be stably present in the fatty amine aqueous solution and is not easy to precipitate; (3) The yield of the NUS-3 nanosheets obtained by the stripping of the present invention is as high as 35.6%, the selectivity of the nanosheets with a thickness of less than 2nm is 86.2%, and the lateral size is about 1μm.

[0041] The above embodiments describe the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for illustrating the principles of the present invention. Without departing from the scope of the principles of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a two-dimensional β-ketoenamine covalent organic framework nanosheet, characterized in that The specific steps are: Step S1, preparing an aqueous fatty amine solution with water as a solvent, the aqueous fatty amine solution being used as a stripping solution, wherein the concentration of the aqueous fatty amine solution is 0.01 to 0.05 mol / L; Step S2, placing the bulk material COF-JLU2, NUS-3 or TFP-DABA in the stripping solution obtained in step S1, the feeding ratio of the bulk material to the stripping solution is 1 mg: 1-1.2 mL, the fatty amine is used as an intercalation agent to enter the interlayer of the bulk material, and the stripping is performed at 20-30° C. under the assistance of ultrasound, wherein the ultrasonic power of the ultrasonic assistance is 280-360 W, and the ultrasonic assistance time is 7-11 hours, and then the dispersion is centrifuged and washed, and dried to obtain a two-dimensional β-ketoenamine covalent organic framework nanosheet, wherein the centrifugal washing includes two processes of low-speed centrifugation and high-speed centrifugation, wherein the low-speed centrifugation speed is 1500-2500 rpm, the low-speed centrifugation time is 5-15 minutes, centrifugation is performed twice, the upper layer of the dispersion is taken and high-speed centrifugation is performed again, the high-speed centrifugation speed is 10000-20000 rpm, and the high-speed centrifugation time is 3-10 minutes, the bottom precipitate is collected, and it is washed with secondary water at least 10 times and ethanol at least 3 times to remove residual fatty amine; The structural formula of the block material COF-JLU2, NUS-3 or TFP-DABA is: When the block material COF-JLU2 is selected for stripping in step S2, the aliphatic amine in step S1 is one or more of propylamine, di-n-propylamine, di-n-butylamine or triethylamine; When the block material NUS-3 is selected for stripping in step S2, the aliphatic amine in step S1 is one or more of propylamine, butylamine, di-n-propylamine, di-n-butylamine or triethylamine; When the bulk material TFP-DABA is selected for stripping in step S2, the aliphatic amine in step S1 is one or more of propylamine, butylamine, di-n-propylamine or triethylamine.

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