A metal-organic framework membrane with shape-selective separation performance, its preparation method and application

Abstract

The present invention relates to the technical field of nanofiltration membrane separation, and particularly relates to a metal-organic framework membrane with shape-selective separation performance, a preparation method thereof and an application. The metal-organic framework membrane is formed on the surface of a polyacrylonitrile ultrafiltration membrane support body by using copper hydroxide nanowires as a precursor and meso-tetrakis(4-carboxyphenyl)porphine H2TCPP as a ligand. The metal-organic framework membrane can effectively retain non-planar molecules with larger cross-sectional sizes in dyes with similar molecular weights, while having a lower retention rate for planar molecules with smaller cross-sectional sizes. In addition, the metal-organic framework membrane can also effectively separate trivalent metal ions from monovalent and divalent metal ions.

Description

Technical Field

[0001] The present invention relates to the technical field of nanofiltration membrane separation, and particularly relates to a metal-organic framework membrane with shape-selective separation performance, a preparation method thereof, and an application thereof. Background Art

[0002] Using energy-saving and environment-friendly membrane separation technology for molecular separation is crucial in the water environment field, such as wastewater reuse, seawater and brackish water desalination. Currently, polymers are by far the most widely used membrane materials, mainly because they are easy to process and have low costs. However, traditional polymer membranes for water treatment usually have a dense selective layer, resulting in insufficient permeability that is difficult to overcome. At the same time, existing nanofiltration membranes have insufficient selectivity for substances with similar molecular weights but different structures, and it is difficult to achieve efficient separation of substances with different molecular structures.

[0003] Metal-organic framework (MOF) materials are one of the research hotspots in the field of membrane separation. MOF materials have a high specific surface area and porosity, diverse types and structures, and strong structural designability and pore size adjustability, and have more advantages in designing appropriate membrane materials for specific separation systems. In particular, two-dimensional MOF materials have significant advantages in preparing nanofiltration membranes with both high permeability and high selectivity. However, most of the existing two-dimensional MOF membranes are based on the filtration assembly of two-dimensional nanosheets, and the stacking of nanosheets will have an uncontrollable impact on the channel size in the membrane. In contrast, MOF membranes prepared by in-situ growth methods have a clear structure and pore size, and have more advantages in the regulation of separation selectivity. However, the in-situ preparation of two-dimensional ultrathin MOF membranes is still one of the difficulties in the field of membrane separation.

[0004] Therefore, it is urgent to conceive a new method to improve the above problems. Summary of the Invention

[0005] Aiming at the problems existing in the prior art, the present invention provides a metal-organic framework membrane with shape-selective separation performance, a preparation method thereof, and an application thereof.

[0006] The present invention is realized by the following technical solutions:

[0007] A metal-organic framework membrane with shape-selective separation performance, the metal-organic framework membrane is formed by binding on the surface of a polyacrylonitrile ultrafiltration membrane support with copper hydroxide nanowires as a precursor and meso-tetrakis(4-carboxyphenyl)porphine (H2TCPP) as a ligand.

[0008] The present invention also provides a preparation method of the metal-organic framework membrane with shape-selective separation performance, including the following steps:

[0009] S1: Dissolve copper nitrate trihydrate and aminoethanol in ultrapure water respectively, then mix them and stir for 5 min, and age for 2 days to obtain a copper hydroxide nanowire solution;

[0010] S2: Immerse the polyacrylonitrile ultrafiltration membrane in a 2 mol / L sodium hydroxide aqueous solution at 65 °C for hydrolysis for 2 h, and then wash it with ultrapure water until the pH is neutral to obtain a hydrolyzed polyacrylonitrile ultrafiltration membrane;

[0011] S3: Filter the copper hydroxide nanowire solution obtained in step S1 onto the support of the hydrolyzed polyacrylonitrile ultrafiltration membrane obtained in step S2 to form a copper hydroxide nanowire precursor layer;

[0012] S4: Immerse the ultrafiltration membrane with the copper hydroxide nanowire precursor layer obtained in step S3 into the H2TCPP solution, take it out after reacting for a period of time, and dry it in a vacuum oven to obtain a metal-organic framework membrane.

[0013] Copper nitrate trihydrate dissociates in water to generate copper ions. Aminoethanol is a weak base that can partially dissociate in water to form hydroxide ions and react with copper ions at the same time. In the mixed solution, copper ions react with hydroxide ions to form copper hydroxide precipitate. Copper hydroxide may spontaneously grow along specific crystal planes through slow aging (2 days) to form a one-dimensional nanowire structure. Ultrapure water is used to ensure that there is no impurity interference in the system. Short-time stirring (5 minutes) after uniform mixing helps to form a uniform initial system, but excessive stirring may damage the formation of nanostructures.

[0014] Through the hydrolysis reaction under strong alkaline conditions, the cyano groups in the polyacrylonitrile ultrafiltration membrane are converted into negatively charged carboxyl groups, which can enhance the interaction with the positively charged copper hydroxide nanowires and strengthen the binding between the precursor layer and the support. The hydrolysis temperature is 65 °C. Too high a hydrolysis temperature may cause damage to the physical structure of the membrane and affect its mechanical strength. The increase of time and alkali concentration will deepen the degree of hydrolysis, but excessive hydrolysis may make the membrane too fragile or the pore size increase, affecting the separation performance. The hydrolyzed membrane is washed with ultrapure water until the pH value reaches neutral. This is to remove the residual sodium hydroxide on the membrane surface and avoid its impact on subsequent use.

[0015] Furthermore, in step S1, the concentration of the copper nitrate trihydrate is 4 mmol / L, and the concentration of the aminoethanol is 1.4 mmol / L. 4 mmol / L of copper nitrate provides a moderate copper ion concentration, which helps to uniformly generate copper hydroxide nanowires. If the copper ion concentration is too low, the reactants in the reaction system are insufficient, and it is difficult to form continuous and uniform nanowires; too high a concentration leads to too fast a reaction rate, forming irregular particles or aggregates instead of one-dimensional nanowires.

[0016] The concentration of aminoethanol is 1.4 mmol / L, providing a moderate alkaline environment. If the concentration of aminoethanol is too low, the alkalinity of the system is insufficient, resulting in a small amount of copper hydroxide formed and possibly irregular particles. If the ratio is too high, the alkalinity is too strong, which may lead to too fast reaction rate, forming irregular aggregates or porous structures. The concentration of 1.4 mmol / L is moderate and will not cause violent reactions or local supersaturation phenomena in the solution, which is conducive to the formation of stable one-dimensional nanostructures. Under this alkaline condition, the nanowires can grow slowly and preferentially along specific crystal planes, obtaining good crystal integrity and forming long one-dimensional structures (with a length of several micrometers). At the same time, the moderate reaction rate helps the dispersibility of the nanowires and reduces the aggregation effect.

[0017] The concentration of copper ions and the alkaline conditions provided by aminoethanol match each other, ensuring both a moderate reaction rate and avoiding disordered morphologies caused by supersaturation. The moderate reaction rate enables the crystals to grow slowly along a specific direction, ensuring the uniformity of the nanowires.

[0018] Preferably, in step S3, the volume of the copper hydroxide nanowire solution, i.e., the filtration volume, is 35.5 mL, and the effective filtration area of the support polyacrylonitrile ultrafiltration membrane is 7.065 cm 2 。

[0019] Preferably, in step S4, the solvent used to prepare the H2TCPP solution is a mixed solvent of ethanol and water, and the volume ratio of ethanol to water is 60:40. The mixed solvent with a volume ratio of 60:40 of ethanol and water is used to balance the solubility of H2TCPP, the reaction uniformity, and the film-forming quality of the membrane. This ratio provides a suitable organic and polar environment, which can not only enhance the dissolution and stability of H2TCPP but also optimize the structural properties and preparation process of the membrane.

[0020] Preferably, in step S4, the concentration of the H2TCPP solution is 0.025 mg / mL.

[0021] Preferably, in step S4, the reaction time is 4 h, and the drying temperature in the vacuum oven is 35 °C.

[0022] During the reaction process, H2TCPP molecules need to be fully coordinated with the copper ions on the surface of the support to form a uniform metal-organic framework (MOF) layer. According to the kinetics of the coordination reaction, 4 hours can provide enough time for the reaction to reach a state close to equilibrium, ensuring the integrity of the framework structure. Too short reaction time may lead to incomplete coordination reaction, insufficient surface coverage of the membrane, or unstable structure. Too long reaction time may lead to excessive deposition on the surface of the membrane material or the formation of irregular layered structures, thus affecting the flux performance and selectivity of the membrane. Long-time solution immersion will also damage the mechanical strength of the support membrane.

[0023] The present invention also provides the application of the metal-organic framework membrane in the separation of dye molecules.

[0024] The present invention also provides the application of the metal-organic framework membrane in the separation of metal ions with different valence states.

[0025] The advantages and beneficial effects of the present invention are as follows:

[0026] 1. In the present invention, copper hydroxide nanowires are used as precursors, combined with H2TCPP ligands to form a Cu-TCPP metal-organic framework membrane. By regulating the growth of MOF through the concentration of H2TCPP ligands, an ultrathin two-dimensional MOF membrane is successfully prepared in situ. The preparation method is simple and easy to scale up and mass-produce.

[0027] 2. The Cu-TCPP metal-organic framework membrane in the present invention is prepared based on the in-situ growth method. The channel size in the membrane is clear (about 0.90 nm in the length direction and about 0.45 nm in the width direction, which is the structural pore size of the Cu-TCPP MOF material). It can efficiently retain non-planar molecules with larger cross-sectional sizes among molecules with similar molecular weights, while having a lower retention rate for planar molecules with smaller cross-sectional sizes, and has good shape-selective separation performance.

[0028] 3. The Cu-TCPP metal-organic framework membrane in the present invention can effectively retain trivalent metal ions with larger hydrated radii, and has a lower retention rate for monovalent and divalent metal ions, showing application potential in the separation of trivalent metal ions / monovalent and divalent metal ions. Description of the Drawings

[0029] Figure 1 is a schematic diagram of the film-forming process of copper hydroxide nanowires and ligand H2TCPP in the present invention;

[0030] Figure 2 is the surface scanning electron micrograph of the Cu-TCPP membrane prepared in Example 1;

[0031] Figure 3 is the cross-sectional scanning electron micrograph of the Cu-TCPP membrane prepared in Example 1;

[0032] Figure 4 is the bar chart of the water permeation flux and safranin T retention rate of the Cu-TCPP membranes prepared in Examples 1-6;

[0033] Figure 5 is the retention performance of the Cu-TCPP membrane prepared in Example 1 for different dyes;

[0034] Figure 6 is the pore structure diagram of the Cu-TCPP membrane prepared in Example 1. Detailed Embodiments

[0035] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention.

[0036] The present invention provides a metal-organic framework membrane with shape-selective separation performance. The metal-organic framework membrane is formed by binding on the surface of a polyacrylonitrile ultrafiltration membrane support using copper hydroxide nanowires as a precursor and meso-tetrakis(4-carboxyphenyl)porphine (H2TCPP) as a ligand.

[0037] The preparation method of the metal-organic framework membrane with shape-selective separation performance includes the following steps:

[0038] S1: Dissolve copper nitrate trihydrate and aminoethanol in ultrapure water respectively, then mix them, stir for 5 min, and age for 2 days to obtain a copper hydroxide nanowire solution;

[0039] S2: Immerse the polyacrylonitrile ultrafiltration membrane in a 2 mol / L sodium hydroxide aqueous solution at 65 °C for hydrolysis for 2 h, and then wash it with ultrapure water until the pH is neutral to obtain a hydrolyzed polyacrylonitrile ultrafiltration membrane;

[0040] S3: Filter the copper hydroxide nanowire solution obtained in step S1 onto the hydrolyzed polyacrylonitrile ultrafiltration membrane support obtained in step S2 to form a copper hydroxide nanowire precursor layer;

[0041] S4: Immerse the ultrafiltration membrane with the copper hydroxide nanowire precursor layer obtained in step S3 in the H2TCPP solution, take it out after reacting for a period of time, and dry it in a vacuum oven to obtain the metal-organic framework membrane.

[0042] The schematic diagram of the film-forming process of the reaction between copper hydroxide nanowires and the ligand H2TCPP is as Figure 1 shown.

[0043] Example 1

[0044] S1: Dissolve 0.8 mmol of copper nitrate trihydrate and 0.28 mmol of aminoethanol in 200 mL of ultrapure water respectively, then mix them, stir for 5 min, and age for 2 days to obtain a copper hydroxide nanowire solution;

[0045] S2: Put the polyacrylonitrile ultrafiltration membrane into a 2 mol / L sodium hydroxide aqueous solution, place it in an oven at 65 °C for treatment for two hours, and then wash it with ultrapure water until the pH is neutral to obtain a hydrolyzed polyacrylonitrile ultrafiltration membrane;

[0046] S3: Filter 35.5 mL of the copper hydroxide nanowire solution obtained in step 1 onto the hydrolyzed polyacrylonitrile ultrafiltration membrane to form a copper hydroxide nanowire precursor layer. The filtration area of the polyacrylonitrile ultrafiltration membrane is 7.065 cm 2 ;

[0047] S4: Dissolve 1 mg of H2TCPP in 40 mL of an ethanol / water mixed solvent with a volume ratio of ethanol to water of 60:40 to form a ligand solution with a concentration of 0.025 mg / mL. Then, place the ultrafiltration membrane with the copper hydroxide nanowire precursor layer face down into the H2TCPP ligand solution. After reacting for 4 h, take it out and place it in a vacuum oven for drying at a drying temperature of 35 °C to obtain the metal-organic framework membrane described above.

[0048] Example 2

[0049] The difference between Example 2 and Example 1 lies in that in step 4, the concentration of the H2TCPP ligand is 0.100 mg / ml.

[0050] Example 3

[0051] The difference between Example 3 and Example 1 lies in that in step 4, the concentration of the H2TCPP ligand is 0.050 mg / ml.

[0052] Example 4

[0053] The difference between Example 4 and Example 1 lies in that in step 4, the concentration of the H2TCPP ligand is 0.010 mg / ml.

[0054] Example 5

[0055] The difference between Example 5 and Example 1 lies in that in step 4, the concentration of the H2TCPP ligand is 0.005 mg / ml.

[0056] Example 6

[0057] The difference between Example 6 and Example 1 lies in that in step 4, the concentration of the H2TCPP ligand is 0.001 mg / ml.

[0058] See Figure 2 , the metal-organic framework membrane described in the present invention is formed by converting on the surface of a polyacrylonitrile ultrafiltration membrane support with copper hydroxide nanowires as the precursor and H2TCPP as the ligand. H2TCPP is dissolved in a mixed solvent of ethanol and water to form a ligand solution. After contacting with the copper hydroxide nanowire precursor, the copper hydroxide nanowires are gradually converted into Cu-TCPP to form a dense Cu-TCPP membrane layer. The surface and cross-sectional morphologies of the Cu-TCPP membrane obtained in Example 1 are as shown in Figure 2 and Figure 3 shown.

[0059] Retention performance test of the Cu-TCPP membranes obtained in Examples 1-6

[0060] The retention performance of the membrane was tested using a 0.1 g / L safranine T aqueous solution under a pressure of 0.5 MPa. The test results are shown in Table 1 and Figure 4As shown, it can be seen that the Cu-TCPP membrane obtained in Example 1 has the highest rejection rate for safranine T (95.75%), and the rejection rates of the membranes at other concentrations are all lower in comparison. Thus, it can be seen that the preferred concentration of H2TCPP is 0.025 mg / mL.

[0061] Table 1. Test results of the rejection performance of the Cu-TCPP membranes obtained in Examples 1-6

[0062]

[0063] Example 7

[0064] Rejection test of the Cu-TCPP membrane obtained in Example 1 for different dyes

[0065] Using 0.1 g / L methylene blue, congo red, eriochrome black T, methyl orange, methylene blue, safranine T and hematoxylin aqueous solutions to continue testing the Cu-TCPP membrane prepared in Example 1 under a pressure of 0.5 MPa, the results are as Figure 5 shown, and the results show that:

[0066] The obtained Cu-TCPP membranes have rejection rates of over 99% for dyes with relatively large molecular weights (greater than 400 g / mol) (methylene blue, congo red, eriochrome black T). For the four dyes with relatively close molecular weights (safranine T, 350.85 g / mol; methyl orange, 327.33 g / mol; methylene blue, 319.85 g / mol; hematoxylin, 302.28 g / mol), the rejection rates of the non-planar molecules safranine T and hematoxylin are 95.75% and 96.69% respectively, while the rejection rates of the planar molecules methyl orange and methylene blue are 59.50% and 70.03%, and hematoxylin with the smallest molecular weight has a higher rejection rate than methyl orange and methylene blue.

[0067] Dyes with relatively large molecular weights are not easily passed through the pores of the membrane due to their large molecular sizes, so they are efficiently rejected by the Cu-TCPP membrane. The relatively large molecules of these dyes cause them to be blocked by the pore screening effect in the membrane structure and cannot pass through freely, thus showing a high rejection rate (>99%). For the four dyes with relatively small and close molecular weights (safranine T, methyl orange, methylene blue, hematoxylin), the difference in their rejection rates is mainly related to the molecular structure of the dyes (especially planarity): safranine T (350.85 g / mol) and hematoxylin (302.28 g / mol) are non-planar molecules, restricted by the pore size of the Cu-TCPP material structure ( Figure 6Constrained in the width direction, it is difficult to diffuse into the pores, resulting in a relatively high rejection rate (95.75% and 96.69% respectively). Methyl orange (327.33 g / mol) and methylene blue (319.85 g / mol) are planar molecules. Less restricted by the pore size of the material, the molecules can more easily pass through the pores of the membrane, so the rejection rates are relatively low (59.50% and 70.03% respectively). For the above reasons, although hematoxylin has the smallest molecular weight (302.28 g / mol) among the above four dyes, the Cu-TCPP membrane described in Example 1 has the highest rejection rate for it. In contrast, although the molecular weights of methyl orange and methylene blue are greater than that of hematoxylin, their planar structures make it easier for them to pass through the membrane pores, resulting in lower rejection rates.

[0068] Example 8 Rejection test of the Cu-TCPP membrane obtained in Example 1 for different metal ions

[0069] Solutions containing Al 3+ , Na + and Ca 2+ were prepared respectively. The Cu-TCPP membrane was pretreated and washed. Under a pressure of 0.5 MPa, the above metal ion solutions were filtered through the membrane. The rejection rate of each ion was calculated. The results were as follows: the rejection rate of the Cu-TCPP membrane material for trivalent Al 3+ ions was 81.26%, the rejection rate for monovalent Na + ions was only 21.08%, and the rejection rate for divalent Ca 2+ ions was 25.46%, showing good separation ability for trivalent metal ions / monovalent and divalent metal ions.

Claims

1. Application of a metal-organic framework membrane with shape-selective separation performance in the separation of dye molecules, characterized in that: The metal-organic framework membrane has a high rejection rate for non-planar molecules with a larger cross-sectional size among molecules with similar molecular weights and a low rejection rate for planar molecules with a smaller cross-sectional size; The metal-organic framework membrane is formed by binding on the surface of a polyacrylonitrile ultrafiltration membrane support with copper hydroxide nanowires as the precursor and meso-tetrakis(4-carboxyphenyl)porphine H2TCPP as the ligand; The preparation method of the metal-organic framework membrane with shape-selective separation performance comprises the following steps: S1: Dissolve copper nitrate trihydrate and aminoethanol in ultrapure water respectively and then mix them, stir for 5 min, and age for 2 days to obtain a copper hydroxide nanowire solution; S2: Place the polyacrylonitrile ultrafiltration membrane in a 2 mol / L sodium hydroxide aqueous solution at 65 °C for hydrolysis for 2 h, and then wash it with ultrapure water until the pH is neutral to obtain a hydrolyzed polyacrylonitrile ultrafiltration membrane; S3: Filter the copper hydroxide nanowire solution obtained in step S1 on the hydrolyzed polyacrylonitrile ultrafiltration membrane support obtained in step S2 to form a copper hydroxide nanowire precursor layer; S4: Immerse the ultrafiltration membrane with the copper hydroxide nanowire precursor layer obtained in step S3 into the H2TCPP solution, take it out after reacting for 4 h, and place it in a vacuum oven for drying to obtain the metal-organic framework membrane; the solvent used for preparing the H2TCPP solution is a mixed solvent of ethanol and water, the volume ratio of ethanol to water is 60:40, and the concentration of the H2TCPP solution is 0.025 mg / mL.

2. Use of the metal-organic framework membrane with shape-selective separation performance according to claim 1 in the separation of dye molecules, characterized in that: In step S1, the concentration of copper nitrate trihydrate is 4 mmol / L, and the concentration of aminoethanol is 1.4 mmol / L.

3. Use of the metal-organic framework membrane with shape-selective separation performance according to claim 1 in the separation of dye molecules, characterized in that: In step S3, the volume of the copper hydroxide nanowire solution, which is the suction filtration volume, is 35.5 mL, and the effective suction filtration area of the support polyacrylonitrile ultrafiltration membrane is 7.065 cm 2 .

4. Use of the metal-organic framework membrane with shape-selective separation performance according to claim 1 in the separation of dye molecules, characterized in that: In step S4, the drying temperature of the vacuum oven is 35 °C.

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