Covalent organic framework film with asymmetric structure as well as preparation method and application of covalent organic framework film

Abstract

The invention discloses a covalent organic framework membrane with an asymmetric structure as well as a preparation method and application thereof, and the preparation method comprises the following steps: preparing a trialdehyde phloroglucinol solution as an aldehyde solution, preparing a 2, 5-diaminobenzene sulfonic acid solution as an amine solution, and dripping the aldehyde solution into the amine solution under stirring to obtain a COF nanosheet mixed solution; diluting with deionized water to obtain a diluent, and dialyzing to obtain a COF nanosheet dispersion liquid; adding the COF nanosheet dispersion liquid into ethanol to obtain a second dispersion liquid; carrying out vacuum filtration on the second dispersion liquid to deposit the COF nanosheets on the nylon base membrane, and drying to obtain a Primary COF membrane; carrying out vacuum filtration on a gamma-cyclodextrin aqueous solution, depositing gamma-cyclodextrin on a Primary COF membrane, and drying to obtain a covalent organic framework membrane with an asymmetric structure; through a mixed solution nanofiltration test experiment, the membrane disclosed by the invention has the capability of separating dye molecules and salt ions in salt-containing wastewater or dye-containing wastewater.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of covalent organic framework membranes, and particularly relates to a covalent organic framework membrane with an asymmetric structure and a preparation method and application thereof. BACKGROUND

[0002] Water resource shortage and water pollution have become key challenges to global industrial development, ecological protection and human life quality improvement, and efficient treatment and resource recycling of dye-containing textile industrial wastewater are one of the core problems in the current water treatment field. Dye molecules have the characteristics of structural stability, strong toxicity and difficult degradation, and if directly discharged, they can seriously destroy the ecological balance of water bodies and threaten the survival of animals and plants and human health; if the salt substances (such as NaCl, MgSO4, etc.) existing in the wastewater can be effectively separated and recycled, not only the difficulty of wastewater treatment can be reduced, but also resource recycling can be realized, which has significant environmental and economic value. Therefore, developing a separation material with high dye interception capacity, high salt passage rate and excellent water flux is of great importance to the precise separation of dye / salt separation or dye / water separation system.

[0003] Covalent organic frameworks (COFs) are a kind of crystalline porous materials formed by covalent bonds between organic monomers, which have great application potential in the field of membrane separation due to their controllable pore size, high specific surface area, good chemical stability and structural adjustability. The separation membrane prepared based on COF materials can realize efficient interception of large molecules such as dyes through molecular size sieving effect, while allowing small molecule salt ions and water molecules to permeate, which provides an ideal material basis for dye / salt separation or dye / water separation. However, the pore size of traditional COF membranes is difficult to ensure high interception rate of dyes while realizing rapid permeation of salt ions, resulting in the need to improve the selectivity of dye / salt separation or dye / water separation. SUMMARY

[0004] The purpose of the present application is to overcome the shortcomings of the prior art and provide a covalent organic framework membrane with an asymmetric structure.

[0005] The second purpose of the present application is to provide a preparation method of a covalent organic framework membrane with an asymmetric structure.

[0006] The third purpose of the present application is to provide an application of a covalent organic framework membrane with an asymmetric structure in separating dyes and or salts in industrial wastewater.

[0007] The technical scheme of the present application is summarized as follows: A preparation method of a covalent organic framework membrane with an asymmetric structure, comprising the following steps: 1) Dissolve 0.1 mmol of trialdehyde phloroglucinol and 0.15 mmol of 2,5-diaminobenzenesulfonic acid in 5 mL of dimethyl sulfoxide according to the proportions. Stir for 20 min at room temperature and sonicate for 20 min to obtain aldehyde solution and amine solution respectively. Add the aldehyde solution dropwise to the amine solution while stirring, and let it stand at 25 °C for 3 days to obtain COF nanosheet mixture. Dilute the COF nanosheet mixture with 2 times the amount of deionized water to obtain diluent. Dialyze the diluent using a dialysis bag with a molecular weight cutoff of 3500 Da for 5 days to obtain COF nanosheet dispersion. 2) Take 0.9 mL of the COF nanosheet dispersion and add it to 10 mL of ethanol. Stir at room temperature to obtain a second dispersion. Filter the second dispersion under a pressure of 0.04 MPa to deposit the COF nanosheets on the nylon-based membrane. Dry in a vacuum oven at 60°C for 30 min and let stand for 30 min to obtain the Pristine COF membrane. 3) Under a pressure of 0.04 MPa, 13 mL of an aqueous solution of γ-cyclodextrin with a concentration of 10 mg / mL was vacuum filtered to deposit γ-cyclodextrin onto a Pristine COF membrane. After drying, a covalent organic framework membrane with an asymmetric structure was obtained.

[0008] A covalent organic framework membrane with an asymmetric structure was prepared by the above preparation method.

[0009] The above-mentioned covalent organic framework membrane with an asymmetric structure is used in the separation of saline wastewater or dye-containing wastewater.

[0010] Advantages of this invention: This invention discloses a covalent organic framework membrane with an asymmetric structure. Its in-plane oriented mass transfer channels enhance size sieving capacity and shorten the mass transfer path, thereby enabling rapid transport of water molecules. The mass transfer channels consist of a wide hydrophilic conical inlet (CD) of γ-cyclodextrin (CD), a narrow hydrophobic outlet, and intrinsic COF nanopores. The wide CD-shaped hydrophilic conical inlet facilitates water molecule entry, while the combination of the narrow CD-shaped hydrophobic outlet and the intrinsic COF nanopores improves membrane selectivity, thus achieving a synergistic improvement in separation performance. Nanofiltration tests using mixed solutions were conducted to evaluate the separation capability of this asymmetric covalent organic framework membrane for dye molecules and salt ions in saline wastewater or dye-containing wastewater.

[0011] γ-Cyclodextrin (CD) is a natural oligosaccharide with a unique cavity structure, and the cavity size can be adapted to small molecules. By combining CD with COF materials, a novel separation membrane with high hydrophilicity and multi-level mass transfer channels can be constructed, thereby simultaneously improving the membrane's water flux and separation selectivity. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 Separation performance of COF-CD-13 membrane to four dyes and NaCl mixed system.

[0013] Figure 2 Surface micro-morphology images of COF membranes, wherein (a) is a pristine COF membrane; (b) is COF-CD-4; (c) is COF-CD-7; (d) is COF-CD-10; (e) is COF-CD-13; (f) is COF-CD-16; Figure 3 is a schematic diagram of the relationship between the CD assembly amount and the thickness of the COF-CD membrane; Figure 4 is a schematic diagram of the relationship between the CD assembly amount and the surface roughness of the COF-CD membrane; Figure 5 is a schematic diagram of the relationship between the CD assembly amount and the water contact angle of the COF-CD membrane; Figure 6 is a separation performance diagram of the COF-CD membrane; Figure 7 is the rejection performance of the COF-CD-13 membrane to dyes and salts; Figure 8 is the separation performance of COF-CD membranes with different CD contents to CR / NaCl mixed systems; DETAILED DESCRIPTION

[0014] The application will be further described below in conjunction with the examples and drawings.

[0015] Example 1 A preparation method of a covalent organic framework membrane with an asymmetric structure, comprising the following steps: 1) 0.1 mmol of tri-aldehyde-based phloroglucinol and 0.15 mmol of 2,5-diaminobenzenesulfonic acid were respectively dissolved in 5 mL of dimethyl sulfoxide, stirred at room temperature for 20 min, and ultrasonicated for 20 min to obtain an aldehyde solution and an amine solution; the aldehyde solution was added dropwise to the amine solution under stirring, and the mixture was left to stand at 25°C for 3 days to obtain a COF nanosheet mixture; 2 times of deionized water was added to the COF nanosheet mixture to obtain a dilution liquid, and the dilution liquid was dialyzed for 5 days using a dialysis bag with a molecular weight cut-off of 3500 Da to obtain a COF nanosheet dispersion; 2) 0.9 mL of the COF nanosheet dispersion was added to 10 mL of ethanol, and the mixture was stirred uniformly at room temperature to obtain a second dispersion; the second dispersion was suction-filtered at a pressure of 0.04 MPa, and the COF nanosheets were deposited on a filter paper with an area of 12.5 cm 2Nylon-based membrane (organic Nylon membrane with pore size of 0.22 pm from DeFiltration Technology) was dried in a vacuum oven at 60 °C for 30 min and rested for 30 min to obtain a Pristine COF membrane (also referred to as an initial COF membrane); 3) Under a pressure of 0.04 MPa, 4 mL, 7 mL, 10 mL, 13 mL and 16 mL of a 10 mg / mL aqueous solution of γ-cyclodextrin were vacuum-filtered onto the Pristine COF membrane, respectively, to obtain five COF-CD membranes with asymmetric structures, namely COF-CD-4, COF-CD-7, COF-CD-10, COF-CD-13 and COF-CD-16, respectively.

[0016] By adjusting the amount of CD assembly, the mass transfer channel in the membrane was regulated. As shown in Figure 2 SEM characterization showed that the introduction of CD did not destroy the structural integrity of the membrane, and the surface of the COF-CD membrane was still dense and had no defects, which indicated that the introduction of CD maintained the overall structural uniformity of the COF-CD membrane.

[0017] As shown in Figure 3 SEM images of the cross-section of the membrane showed that the thickness of the Pristine COF membrane without CD was only 0.19 ± 0.01 pm. After the introduction of CD, the separation layer thickness of the COF-CD membrane increased significantly, and the thickness of the membrane gradually increased from 0.82 ± 0.01 pm to 1.81 ± 0.04 pm as the amount of CD assembly increased from 4 mL to 16 mL.

[0018] Further analysis of the surface roughness of the COF-CD membrane was carried out by AFM characterization. As shown in Figure 4 Compared with the Pristine COF membrane without CD, the roughness of the COF-CD membrane increased. The increase in the surface roughness of the COF-CD membrane resulted from the introduction of surface CD, which produced additional surface characteristics, leading to pore distribution and material aggregation, making the COF-CD surface more irregular compared with the smooth surface of the Pristine COF membrane.

[0019] The wettability of the membrane surface was characterized by the water contact angle of the COF-CD membrane in air. As shown in Figure 5As shown, the contact angle of COF-CD membranes decreased with the increase of CD assembly amount, indicating that the hydrophilicity of the membrane surface was enhanced. The pristine COF membrane had the highest contact angle of 75.1 ± 0.8°, with lower hydrophilicity. With the increase of CD assembly amount, the contact angle gradually decreased, with the contact angle of COF-CD-4 membrane being 66.2 ± 1.5°, and the contact angle of COF-CD-7 membrane being slightly lower at 66.1 ± 3.2°. The contact angle of COF-CD-10 membrane decreased more obviously to 63.3 ± 1.7°. After further introduction of CDs, the contact angle of COF-CD-13 membrane was 55.2 ± 2.5°, and the contact angle of COF-CD-16 membrane was the lowest at 47.2 ± 1.7°. This is because CDs have hydrophilic groups, which increase the hydrophilicity of COF-CD membranes.

[0020] COF membrane separation performance test experiment Test Example 1 Step 2) in Example 1 to obtain a Pristine COF membrane (also known as an initial COF membrane).

[0021] The Pristine COF membrane was tested for water flux permeability in dead-end filtration, and the selectivity test for the interception of organic dyes such as Alcian Blue (AB), Congo Red (CR), Eriochrome Black T (EBT), and Direct Red (DR). The results showed that the Pristine COF membrane had a pure water flux of 45 L m -2 h -1 at an operating pressure of 0.02 MPa, and the interception rate of the four dyes remained above 98%.

[0022] The COF-CD-4 membrane was tested for dead-end filtration performance, to evaluate its water flux permeability, and the selectivity test for the interception of organic dyes such as AB, CR, EBT, and DR. The results showed that the COF-CD-4 membrane had a pure water flux of 51 L m -2 h -1 at an operating pressure of 0.02 MPa, and the interception rate of the four target dyes remained above 98%.

[0023] The COF-CD-7 membrane was tested for dead-end filtration performance, to evaluate its water flux permeability, and the selectivity test for the interception of organic dyes such as AB, CR, EBT, and DR. The results showed that the COF-CD-7 membrane had a pure water flux of 51 L m -2 h -1 at an operating pressure of 0.02 MPa, and the interception rate of the four target dyes remained above 98%.

[0024] Dead-end filtration performance tests were conducted on the COF-CD-10 membrane to evaluate its water flux permeability, and retention selectivity tests were performed on organic dyes such as AB, CR, EBT, and DR. The results showed that the COF-CD-10 membrane achieved a pure water flux of 88 L / m³ at an operating pressure of 0.02 MPa. -2 h -1 Furthermore, the retention rate for all four target dyes remained above 99%.

[0025] Dead-end filtration performance tests were conducted on the COF-CD-13 membrane to evaluate its water flux permeability, and retention selectivity tests were performed on organic dyes such as AB, CR, EBT, and DR. The results showed that the COF-CD-13 membrane achieved a pure water flux of 99 L / m³ at an operating pressure of 0.02 MPa. -2 h -1 Furthermore, the retention rate for all four target dyes remained above 99%.

[0026] like Figure 6 As shown, the observed water flux increases with increasing CD content, peaking at COF-CD-13. This change in water flux in the COF-CD membrane can be attributed to the unique structure of the mass transfer channels. In the Pristine COF membrane, the water flux is limited due to the homogeneous pore structure of the COF nanosheets, failing to effectively reduce water mass transfer resistance. With increasing CD content, the formation of mass transfer channels facilitates the passage of water molecules, reducing inlet resistance and increasing water flux.

[0027] Dead-end filtration performance tests were conducted on the COF-CD-16 membrane to evaluate its water flux permeability, and retention selectivity tests were performed on organic dyes such as AB, CR, EBT, and DR. The results showed that the COF-CD-16 membrane achieved a pure water flux of 83 L / m³ at an operating pressure of 0.02 MPa. -2 h -1 Furthermore, the retention rate for all four target dyes remained above 99%.

[0028] like Figure 6 As shown, compared to the COF-CD-13 membrane, the pure water flux of the COF-CD-16 membrane is reduced. This is because when the CD content increases further, the excessive aggregation of CD may lead to structural damage or blockage of the mass transfer channel, resulting in a narrower effective size of the mass transfer channel, thereby reducing the membrane permeability.

[0029] The rejection rates of all four dyes were consistently high on both Pristine COF and COF-CD membranes. Despite the difference in water flux, the dye rejection rates of all membranes remained above 98%. This phenomenon is due to the size sieving effect dominating the rejection process. The nanoscale pores of Pristine COF and COF-CD membranes are small enough to effectively block the passage of larger dye molecules while allowing smaller salt ions or water molecules to pass through. Even as the CD content increases and the membrane structure becomes more complex, the pore size remains within the optimal range for dye rejection, ensuring that the rejection rate does not decrease significantly.

[0030] COF-CD-13 membrane, used to test dye / salt separation in mixed solutions of dyes and salts. As shown in Figure 7 , the COF-CD-13 membrane has a high rejection rate for different dyes (rejection rate > 99%) while having a low rejection rate for different salt solutions (rejection rate < 25%). The COF-CD membrane achieves efficient rejection of macromolecular dyes and selective permeation of small-sized salt ions through a precise pore size sieving mechanism, exhibiting significant dye / salt separation performance. The effective diameter of the membrane is larger than the hydrated diameter of the ions, and the ions are more easily transported across the membrane due to the size sieving effect.

[0031] COF-CD membrane, used to test CR / NaCl mixed solution separation performance. As shown in Figure 8 , as the CD content increases, the molecular sieving effect of the mass transfer channel increases. This structure allows larger dye molecules to be more effectively rejected, while smaller ions are more easily transported through the mass transfer channel, increasing the separation factor. The COF-CD-13 membrane has a high selectivity for the CR / NaCl system, with a separation factor of 676. However, when the CD content is high, such as in the COF-CD-16 membrane, excessive aggregation of CDs can cause pore blockage or irregular channel structure, reducing the separation capacity of the COF-CD membrane, which explains the decrease in the CR / NaCl separation factor to 491.

[0032] COF-CD-13 membrane was tested in a mixed system of four dyes and NaCl. As shown in Figure 1 , the DR / NaCl and AB / NaCl systems exhibit the highest separation factors (1092 and 928) due to the largest size of the dye molecules, while the CR / NaCl (676) and EBT / NaCl (432) systems have smaller dye sizes and exhibit smaller separation factors.

Claims

1. A method for preparing a covalent organic framework membrane with an asymmetric structure, characterized in that... Includes the following steps: 1) Dissolve 0.1 mmol of trialdehyde phloroglucinol and 0.15 mmol of 2,5-diaminobenzenesulfonic acid in 5 mL of dimethyl sulfoxide according to the proportions. Stir for 20 min at room temperature and sonicate for 20 min to obtain aldehyde solution and amine solution respectively. Add the aldehyde solution dropwise to the amine solution while stirring, and let it stand at 25 °C for 3 days to obtain COF nanosheet mixture. Dilute the COF nanosheet mixture with 2 times the amount of deionized water to obtain diluent. Dialyze the diluent using a dialysis bag with a molecular weight cutoff of 3500 Da for 5 days to obtain COF nanosheet dispersion. 2) Take 0.9 mL of the COF nanosheet dispersion and add it to 10 mL of ethanol. Stir at room temperature to obtain a second dispersion. Filter the second dispersion under a pressure of 0.04 MPa to deposit the COF nanosheets on the nylon-based membrane. Dry in a vacuum oven at 60°C for 30 min and let stand for 30 min to obtain the Pristine COF membrane. 3) Under a pressure of 0.04 MPa, 13 mL of an aqueous solution of γ-cyclodextrin with a concentration of 10 mg / mL was vacuum filtered to deposit γ-cyclodextrin onto a Pristine COF membrane. After drying, a covalent organic framework membrane with an asymmetric structure was obtained.

2. A covalent organic framework membrane with an asymmetric structure prepared by the preparation method of claim 1.

3. The application of a covalent organic framework membrane with an asymmetric structure according to claim 2 in the separation of saline wastewater or dye-containing wastewater.

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