UiO-66-based monovalent cation selective separation membrane as well as preparation method and application thereof

Abstract

The invention discloses a UiO-66-based monovalent cation selective separation membrane as well as a preparation method and application thereof, and belongs to the technical field of membrane separation. The preparation method comprises the following steps: mixing a zirconium n-propoxide solution with n-propyl alcohol, glacial acetic acid and DMF (Dimethyl Formamide) to prepare a zirconium cluster solution; performing oxygen plasma etching and acrylic acid grafting pretreatment on the BPPO base film to obtain a BPPO / CA film; finally, the BPPO / CA membrane, a zirconium cluster solution, terephthalic acid and triethylamine are subjected to an original growth reaction in a two-chamber reaction tank, and the UiO-66-based monovalent cation selective separation membrane is prepared. The preparation method provided by the invention is simple and mild in condition, and the prepared membrane has good cation selectivity and is suitable for the fields of industrial-grade TMAH solution purification and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of ion exchange membranes, and particularly relates to a preparation method of a monovalent cation selective separation membrane based on UiO-66 for purifying industrial-grade TMAH solution. Background Art

[0002] The research on monovalent cation selective separation membranes began in the 1960s. Early research mainly focused on polymer materials such as polyamide (PA) and polysulfone (PSF), and was applied to the removal of multivalent cations in seawater to prepare edible salt. In recent years, the domestic and foreign research on monovalent cation selective separation membranes has attracted much attention. Monovalent cation selective separation membranes have high selectivity for monovalent cations and can effectively remove pollutants such as heavy metal ions and radioactive ions in water.

[0003] Since MOF is a porous material that can provide Egyptian-sized ion transmission channels, and it is relatively easy to functionalize the surface of its pore structure, the application of MOF materials in membrane separation technology can promote the development of this technology. At present, functional membranes based on MOF materials have been widely used in gas separation, dye separation, pervaporation, and organic solvent nanofiltration. However, there are few studies on the efficient removal of metal ions such as sodium ions in industrial-grade tetramethylammonium hydroxide (TMAH) aqueous solutions using membranes prepared by MOF materials to achieve the electronic-grade TMAH required by the market, and there are still many difficulties in the selective separation of monovalent / monovalent cations using MOF membranes. This has prompted membrane scientists to continuously explore and demand solutions in the field of cation selective separation. Some researchers have used MOF materials to prepare high-performance monovalent cation selective separation membranes for the selective separation of monovalent and polyvalent cations such as lithium extraction from salt lakes and seawater, achieving rapid transmission of lithium ions. Although there are few studies on the selective separation of monovalent and polyvalent cations using MOF membranes, the current researchers can be inspired by the research on the selectivity of monovalent and polyvalent cations, providing valuable research experience for the selective separation of monovalent and polyvalent cations MOF membranes. In addition, although the above work has achieved excellent separation performance, most MOF separation membranes lack the interaction between MOF materials and substrate membranes, resulting in instability between the substrate and MOF materials. UiO-66 (Zr) series MOFs have shown great application prospects in catalysis, functional materials and adsorption due to their large specific surface area, good pore structure and flexible controllability. These materials introduce defect sites during synthesis to regulate the physical and chemical properties of the materials, such as band structure and pore structure, to obtain excellent performance. Therefore, in order to meet the electronic-grade TMAH required by domestic integrated circuits, optoelectronics and other microelectronics industries and the calls of corresponding countries, it is necessary to develop high-performance and high-stability MOF separation membranes that can efficiently remove metal ions such as sodium ions in industrial-grade TMAH aqueous solutions under strong alkaline conditions to get rid of the restrictions of being monopolized by foreign markets. Summary of the invention

[0004] The purpose of the present invention is to provide a monovalent cation selective separation membrane based on UiO-66 and a preparation method thereof. The prepared membrane shows better performance than the commercial membrane CIMS in the field of monovalent / monovalent cation selective separation, and has broad application prospects in the fields of industrial-grade TMAH aqueous solution purification, etc.

[0005] To achieve the above object, the present invention adopts the following technical solution: A preparation method of a UiO-66-based monovalent cation selective separation membrane, the specific steps include: First, prepare a 20wt% BPPO / NMP solution, stir to obtain a homogeneous solution, remove bubbles by standing, and then perform film scraping on a glass plate to obtain a BPPO-based membrane; drop the zirconium propoxide solution into a mixed solution composed of n-propanol, glacial acetic acid, and N,N-dimethylformamide (DMF), stir and then heat for reaction to obtain a zirconium cluster solution; perform oxygen plasma etching and acrylic grafting pretreatment on the upper surface of the BPPO-based membrane, and name the obtained membrane BPPO / CA membrane; install the BPPO / CA membrane on a self-designed two-chamber reaction cell, inject the zirconium cluster solution, terephthalic acid, and triethylamine into the reaction side, and make the reaction side closely fit the surface of the membrane treated by plasma modification, inject pure water into the other side, and stand for reaction for 6 - 72 hours to obtain a UiO-66-based monovalent cation selective separation membrane.

[0006] The thickness of the BPPO-based membrane is 200 µm - 250 µm.

[0007] The mass fraction of the zirconium propoxide solution is 70%, and the volume ratio of zirconium propoxide, n-propanol, glacial acetic acid, and DMF is 1:0.73:56:98.

[0008] The reaction temperature of the zirconium cluster solution is 130 °C, and the reaction time is 2 hours.

[0009] The pretreatment steps of the BPPO-based membrane include: (1) Perform oxygen plasma etching on the BPPO-based membrane: The fixed voltage of the plasma etching is AC100V, the fixed current is below 15A, the fixed frequency is 50 / 60Hz, the treatment time is 2 min, the treatment atmosphere is an oxygen atmosphere, and the gas flow rate is 100 ml / min; (2) Acrylic grafting: Immerse the BPPO-based membrane treated by plasma in an acrylic acid solution, pass nitrogen for 15 min, and then react in a 70 °C water bath for 2 hours to obtain a BPPO / CA membrane.

[0010] The concentration of the acrylic acid solution is 70 vol%.

[0011] In the two-chamber reaction cell, the dosages of the zirconium cluster solution, terephthalic acid, and triethylamine on the reaction side are 39 mL, 0.27 g, and 1.5 mL respectively.

[0012] The standing reaction time is 6 - 72 hours, and the reaction temperature is room temperature.

[0013] A UiO-66-based monovalent cation selective separation membrane prepared by the above method.

[0014] Application of the above UiO-66-based monovalent cation selective separation membrane in the purification of TMAH solution.

[0015] The beneficial effects of the present invention are as follows: (1) The preparation conditions of the UiO-66-based monovalent cation selective separation membrane of the present invention are relatively mild, which reduces the preparation cost to a certain extent.

[0016] (2) For the UiO-66-based monovalent cation selective separation membrane prepared in the present invention, compared with other MOF separation membranes, carboxylic acid groups are introduced between the substrate membrane and the MOF material through oxygen plasma technology and acrylic acid grafting pretreatment, thereby providing stable interaction forces, making the prepared membrane more stable.

[0017] (3) Since constructing Å-level ion confinement transport channels based on MOF has the advantages of being simple and easy to implement and having a stable structure. The inherently sub-nanometer-sized pores of the UiO-66-based monovalent cation selective separation membrane prepared in the present invention enable the membrane to perform selective separation based on the hydrated diameter of cations, and its window size (6 Å) falls between the hydrated diameter (7.16 Å) and the bare ion diameter (2.04 Å) of Na + ions. When it passes through the window, it only needs to be slightly dehydrated, rehydrates after entering the cavity, and dehydrates again when transported to the window for passage. For Me 4 N + ions (hydrated diameter 6.94 Å), which are larger than the window size and require more dehydration energy, thus facing higher transport resistance. Therefore, the UiO-66-based monovalent cation selective separation membrane prepared in the present invention has excellent monovalent / monovalent cation selective separation performance. Among them, for the separation of Na + / Me 4 N + , the selectivity can reach 46.354, and the ion permeability of Na + can reach 0.665 mol m -2 h -1 . For the separation of K + / Me 4 N + , the selectivity can reach 57.140, and the ion permeability of K + can reach 0.725 mol m -2 h -1 . Under the same performance test conditions, the monovalent / monovalent cation selective separation performance of the prepared membrane is much higher than that of the commercial membrane CIMS. The Na + / Me 4 N + selectivity of the CIMS membrane is 21.6, and the K + / Me 4 N+ The selectivity is 20.5. Description of the Drawings

[0018] Figure 1 It is a self-designed reaction cell device diagram in Example 3.

[0019] Figure 2 It is the infrared spectrum diagram of the membranes prepared in Examples 1-8.

[0020] Figure 3 It is the full-spectrum scanning XPS spectrum diagram of the membranes prepared in Examples 1-8.

[0021] Figure 4 It is the SEM diagram of the membranes prepared in Examples 1-8.

[0022] Figure 5 It is the XRD diagram of the membranes prepared in Examples 1-3 and Example 6. Detailed Description of the Invention

[0023] To make the content of the present invention easier to understand, the technical solutions of the present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited thereto.

[0024] The chemical reagents used in the examples of the present invention are all commercially available.

[0025] Example 1 First, the BPPO polymer material was dissolved in the organic solvent N-methylpyrrolidone (NMP) at a mass ratio of 1:4, and a homogeneous solution was obtained by magnetic stirring. After standing to remove bubbles, a film was cast on a glass plate to obtain a BPPO-based film with a thickness of 200 µm - 250 µm.

[0026] Using the BPPO-based film prepared in Example 1, the selective separation performance of monovalent / divalent cations was tested at 25 °C using electrodialysis technology. 100 ml of deionized water and 100 ml of 0.1 M Na + / Me 4 N + or K + / Me 4 N + mixed solutions were circulated, and the replenishing solution in the electrode chamber was 100 ml of 0.3 M Na 2 SO 4 , the circulation flow rate was 20 ml·min -1 , and the operating current was kept constant at 0.004 A. Among them, for the separation of Na + / Me 4 N + , its selectivity was only 0.979, and its Na +The ionic permeability can reach 1.242 mol m -2 h -1 For the separation of K + / Me 4 N + its selectivity is only 1.165, and its K + The ionic permeability can reach 1.355 mol m -2 h -1 .

[0027] Example 2 The BPPO-based membrane prepared in Example 1 was used for pretreatment: (1) Plasma treatment: The BPPO membrane was cut into samples of appropriate size, and the base membrane was placed in the sample chamber of the plasma instrument for treatment. The fixed voltage of the plasma was AC100V; the fixed current was below 15A; the fixed frequency was 50 / 60Hz; the treatment time was 2min; the treatment atmosphere was an oxygen atmosphere; the gas flow rate was 100ml / min; the treated sample was put into a sealed bag for standby. (2) Graft polymerization of AAc (acrylic acid) on the BPPO membrane after plasma treatment: Preparation of the monomer solution: 100 mL of AAc solution with a concentration of 70 vol% was prepared in a 150 mL conical flask; Graft polymerization reaction: The BPPO membrane sample treated with plasma was immersed in a 150 mL conical flask containing 100 mL of AAc solution with different concentrations, purged with nitrogen (15 min), and then placed in a constant temperature water bath for reaction. The reaction temperature was 70 °C and the reaction time was 2 h. The grafted BPPO membrane was named BPPO / CA; Washing: The BPPO / CA membrane was ultrasonically cleaned with deionized water five times to remove acrylic acid homopolymers and residual monomers. Subsequently, the BPPO / CA membrane was placed in ultrapure water for standby. For the separation of Na + / Me 4 N + its selectivity is only 1.004, and its Na + The ionic permeability can reach 1.324 mol m -2 h -1 For the separation of K + / Me 4 N + its selectivity is only 1.042, and its K + The ionic permeability can reach 1.388 mol m -2 h -1 .

[0028] From Examples 1-2, it can be seen that the BPPO membrane without loaded UiO-66 and the BPPO / CA membrane do not have excellent selective separation performance for monovalent cations.

[0029] Example 3 The BPPO-based membrane prepared in Example 1 was used to load UiO-66. The BPPO-based membrane prepared in Example 1 was installed on a self-designed reaction pool device. The reaction pool device used was an H-type diffusion device (such as Figure 1 ), the H-type diffusion device consists of a central channel and diffusion chambers located on the left and right sides of the central channel; the porous base membrane is fixed in the middle of the central channel of the diffusion device, and one side of the left and right chambers is arbitrarily selected as the reaction side. 250 microliters of 70% wt zirconium n-propoxide was added to 0.1817 mL n-propanol, 14 mL glacial acetic acid, and 24.5 mL DMF. After mixing evenly, the mixture was reacted at 130°C for 2 hours, and then allowed to stand at room temperature to prepare a zirconium cluster solution. 39 mL of zirconium cluster solution, 0.27 g of terephthalic acid, and 1.5 mL of triethylamine were injected into the reaction side of the device, and the mixture was stirred thoroughly to ensure that the solution was evenly mixed and that the reaction side was tightly attached to the upper surface of the membrane. An equal amount of pure water was injected on the other side to maintain the solvent balance of the system. The reaction was allowed to stand for 24 hours at room temperature, and the prepared UiO-66 cation selective separation membrane was named BPPO-UiO-66. Its BPPO-UiO-66 membrane is resistant to Na + / Me 4 N + The selectivity of separation is only 0.9249, and its Na + The ion permeability can reach 0.9946 mol m -2 h -1 , for K + / Me 4 N + The selectivity of separation is only 1.012, and its K + The ion permeability can reach 0.9836 mol m -2 h -1 .

[0030] Example 4 A UiO-66-based monovalent cation selective separation membrane was prepared by a method similar to that of Example 3. The base membrane was changed to the BPPO / CA membrane prepared in Example 2, and the static reaction time was changed to 6 h. The prepared UiO-66 cation selective separation membrane was named BPPO / CA-UiO-66-6h. The BPPO / CA-UiO-66-6h membrane was + / Me 4 N + The selectivity of Na + The ion permeability can reach 0.73 mol m -2 h -1 , for K + / Me 4 N+ Separation, with a selectivity up to 37.417 and its K + The ionic permeability of can reach 0.785 mol m -2 h -1 Example 5 A monovalent cation-selective separation membrane based on UiO-66 was prepared using a method similar to that of Example 3. The substrate membrane was changed to the BPPO / CA membrane prepared in Example 2, and the static reaction time was changed to 12 h. The prepared UiO-66 cation-selective separation membrane was named BPPO / CA-UiO-66-12h. For the separation of Na + / Me 4 N + Separation, with a selectivity up to 35.717 and its Na + The ionic permeability of can reach 0.705 mol m -2 h -1 For the separation of K + / Me 4 N + Separation, with a selectivity up to 44.975 and its K + The ionic permeability of can reach 0.76 mol m -2 h -1 .

[0031] Example 6 A monovalent cation-selective separation membrane based on UiO-66 was prepared using a method similar to that of Example 3. The substrate membrane was changed to the BPPO / CA membrane prepared in Example 2, and the static reaction time was changed to 24 h. The prepared UiO-66 cation-selective separation membrane was named BPPO / CA-UiO-66-24h. For the separation of Na + / Me 4 N + Separation, with a selectivity up to 46.354 and its Na + The ionic permeability of can reach 0.665 mol m -2 h -1 For the separation of K + / Me 4 N + Separation, with a selectivity up to 57.140 and its K + The ionic permeability of can reach 0.725 mol m -2 h -1 .

[0032] Example 7 A monovalent cation-selective separation membrane based on UiO-66 was prepared using a method similar to that in Example 3. The substrate membrane was changed to the BPPO / CA membrane prepared in Example 2, and the static reaction time was changed to 48 h. The prepared UiO-66 cation-selective separation membrane was named BPPO / CA-UiO-66-48h. For the separation of Na + / Me 4 N + , its selectivity can reach 45.423, and the ionic permeability of Na + can reach 0.645 mol m -2 h -1 . For the separation of K + / Me 4 N + , its selectivity can reach 57.882, and the ionic permeability of K + can reach 0.72 mol m -2 h -1 .

[0033] Example 8 A monovalent cation-selective separation membrane based on UiO-66 was prepared using a method similar to that in Example 3. The substrate membrane was changed to the BPPO / CA membrane prepared in Example 2, and the static reaction time was changed to 72 h. The prepared UiO-66 cation-selective separation membrane was named BPPO / CA-UiO-66-72h. For the separation of Na + / Me 4 N + , its selectivity can reach 43.864, and the ionic permeability of Na + can reach 0.64 mol m -2 h -1 . For the separation of K + / Me 4 N + , its selectivity can reach 56.758, and the ionic permeability of K + can reach 0.725 mol m -2 h -1 .

[0034] It is not difficult to find from Examples 1-8 that under other identical conditions, the membranes prepared by loading UiO-66 on the BPPO membrane without plasma technology and acrylic grafting pretreatment have selectivities for Na + / Me 4 N + and K + / Me 4 N +excellent Na was not demonstrated in the separation system + / Me 4 N + 、K + / Me 4 N + selective separation performance. However, the BPPO membrane pretreated by plasma technology and acrylic grafting has excellent Na + / Me 4 N + 、K + / Me 4 N + selective separation performance. This is because carboxylic acid groups that can form stable interaction forces with UiO-66 have been introduced on the surface of the BPPO / CA membrane, so it can promote the loading of UiO-66 material on the membrane, thereby improving its cation selective separation performance. In addition, by comparing a series of prepared UiO-66 cation selective separation membranes (BPPO / CA-UiO-66-Xh (X = 6, 12, 24, 48, 72)), it can be seen that as the loading time of UiO-66 on the membrane increases from 6 h to 24 h, its Na + / Me 4 N + 、K + / Me 4 N + selective separation performance also increases accordingly, and its Na + 、K + ion permeability decreases slightly. When the time continues to increase to 72 h, its selective separation performance and ion permeability tend to balance.

[0035] The tested infrared spectra are as Figure 2 shown. Compared with the infrared spectra of the BPPO-based membrane, BPPO / CA membrane, and BPPO-UiO-66 membrane, a series of BPPO / CA-UiO-66-Xh (X = 6, 12, 24, 48, 72) membranes prepared by in-situ growth of UiO-66 on the membrane after plasma treatment and acrylic liquid-phase grafting pretreatment have strong infrared absorption peaks at 659 cm -1 and 743 cm -1 respectively, which are the characteristic peaks of Zr-O in aromatic compounds. At 1505 cm -1 、1395 cm -1New infrared characteristic peaks also appear at this position, which are respectively attributed to two strongly coupled C−O bonds in the carboxylic acid groups of the organic ligand terephthalic acid. The results prove that a continuous and dense UiO-66 crystal layer has been successfully loaded on the upper surface of a series of prepared BPPO / CA-UiO-66-Xh (X = 6, 12, 24, 48, 72) membranes, demonstrating that carboxylic acid functionalization of the membrane upper surface promotes the in-situ growth of UiO-66 crystals on the membrane upper surface. In addition, the C-O symmetric stretching vibration peak at 1395 cm -1 is significantly enhanced. This is because the upper surface of the membrane after carboxylic acid functionalization pretreatment has carboxylic acid groups, and the carboxylic acid groups coordinate with the Zr 6 clusters of UiO-66 (usually binding in a bidentate or monodentate form), thus significantly enhancing the C-O symmetric stretching vibration peak, making the UiO-66 crystals more firmly attached to the membrane upper surface and improving the stability of the prepared UiO-66 membrane.

[0036] Figure 3 Figure shows the full-spectrum scanning XPS spectra of BPPO porous substrate membrane, BPPO / CA membrane, BPPO-UiO-66 membrane, and a series of BPPO / CA-UiO-66-Xh (X = 6, 12, 24, 48, 72) membranes. As shown, characteristic peaks of O1s and C1s orbitals appear at the same positions for all membranes, because the upper surfaces of all membranes contain the corresponding chemical elements. At the same time, characteristic peaks of Br3d orbitals appear at the same positions for BPPO substrate membrane, BPPO / CA membrane, and BPPO-UiO-66 membrane, while they are not observed for BPPO / CA-UiO-66-Xh (X = 6, 12, 24, 48, 72) membranes. This is because the upper surface of BPPO / CA-UiO-66-Xh (X = 6, 12, 24, 48, 72) membranes has a dense and continuous UiO-66 crystal layer, resulting in the inability to detect the characteristic peaks of Br3d orbitals contained in the upper surface of the BPPO substrate membrane. In addition, characteristic peaks of Zr3d orbitals are not observed on the upper surfaces of BPPO substrate membrane, BPPO / CA membrane, and BPPO-UiO-66 membrane, while characteristic peaks of Zr3d orbitals appear at the same positions for BPPO / CA-UiO-66-Xh (X = 6, 12, 24, 48, 72) membranes. This further verifies that a continuous and dense UiO-66 crystal layer has been successfully loaded on the membrane after plasma treatment and acrylic acid grafting pretreatment, and a defect-free UiO-66 separation membrane has been successfully prepared.

[0037] Figure 4SEM images of the morphologies of BPPO porous base membranes, BPPO / CA membranes, and a series of UiO-66 separation membranes. Clear nano-scale hydrophobic pores can be observed on the upper surface of the BPPO base membrane. Comparing with the upper surface morphology of the BPPO / CA membrane, it can be seen that the pores on the upper surface of the BPPO / CA membrane are more obvious. This is because the plasma treatment etched the upper surface and the acrylic acid solution slightly corroded the membrane surface, resulting in an increase in pore size. To further explore the effects of plasma treatment and acrylic acid grafting pretreatment on the loading of UiO-66 on the membrane, in this experiment, with other reaction conditions unchanged, the in-situ growth of UiO-66 was carried out on the BPPO base membrane and the BPPO / CA membrane respectively. By observing the morphological characteristics of their upper surfaces, it can be clearly seen that a dense and continuous UiO-66 seed layer cannot grow on the BPPO base membrane without plasma treatment and acrylic acid grafting pretreatment, and it was named BPPO-UiO-66 membrane. However, a continuous and dense UiO-66 seed layer grew on the pretreated BPPO / CA membrane. This is because after plasma treatment and acrylic acid grafting pretreatment, carboxylic acid groups that can form stable interaction forces with UiO-66 will be introduced on the membrane surface, thus further promoting the growth of UiO-66 seed layer on the membrane surface. By studying the principle of crystal growth, it is known that there are common factors affecting crystal growth for different growth methods, such as reaction time, reaction concentration, reaction temperature, etc. In this experiment, a series of BPPO / CA-UiO-66-Xh (X = 6, 12, 24, 48, 72) membranes were prepared by controlling different growth reaction times. By observing the SEM images of the upper surfaces of this series of UiO-66 membranes, it can be seen that as the in-situ growth time on the upper surface of the membrane increases, the UiO-66 crystals grown become denser. Until the growth time reaches 24 h, continuing to increase the growth time, the denseness of the membrane does not increase significantly. This is because when the growth time reaches 24 h, a continuous and dense UiO-66 selective layer has been formed on the upper surface of the membrane. Continuing to increase the growth time, the crystals will accumulate on the already formed continuous and dense selective layer, thus forming another continuous and dense UiO-66 selective layer. Observing the SEM images of the lower surface morphologies of the BPPO porous base membrane, the BPPO / CA membrane, and a series of UiO-66 separation membranes, clear micron-scale pore structures can be seen on the lower surface. In addition, by observing the lower surfaces of a series of UiO-66 membranes, it can be observed that the UiO-66 seed layers are few and discontinuous. This is because a small amount of UiO-66 crystals enter the lower surface through the pore channels of the membrane for growth, which also further demonstrates that the experiment successfully carried out the precise in-situ growth of UiO-66 crystals on the upper surface.From the cross-sections of the BPPO porous substrate membrane and the BPPO / CA membrane, it can be observed that the cross-sections of both are relatively flat. By further observing the cross-section of the BPPO-UiO-66 membrane and comparing it with those of the BPPO substrate membrane and the BPPO / CA membrane, it can be seen that no dense UiO-66 selective layer is formed on the cross-section of the BPPO-UiO-66 membrane. When comparing its cross-section with that of the BPPO / CA-UiO-66-24h membrane, it can be seen that there is an obvious UiO-66 selective layer with a thickness of 871.4 nm on the cross-section of the BPPO / CA-UiO-66-24h membrane. This further indicates that after plasma treatment and acrylic acid grafting pretreatment, carboxylic acid groups can be introduced onto the membrane surface, and stable interaction forces can be formed between the carboxylic acid groups on the upper surface of the membrane and UiO-66 crystals, promoting the in-situ growth of UiO-66 crystals on the upper surface of the membrane. By observing a series of UiO-66 membranes prepared with different UiO-66 crystal growth times and comparing their cross-sectional morphologies with those of the BPPO substrate membrane, the BPPO / CA membrane, and the BPPO-UiO-66 membrane, a dense and continuous UiO-66 selective layer can be clearly observed on the cross-sections of this series of UiO-66 membranes, indicating that UiO-66 crystals have been successfully loaded on the upper surface of the membrane. In addition, it can also be observed that as the growth time increases, the thickness of the UiO-66 crystal selective layer increases, with thicknesses of 585.7 nm, 642.9 nm, 871.4 nm, 928.6 nm, and 950 nm respectively. This is because as the growth time increases, UiO-66 crystals continuously accumulate on the upper surface of the membrane.

[0038] In this study, XRD tests were carried out on the representative membrane BPPO / CA-UiO-66-24h in a series of prepared UiO-66 membranes, and the results are as Figure 5 shown. It can be observed that although the diffraction peaks in the XRD pattern of the BPPO / CA-UiO-66-24h membrane are not as obvious as those of the UiO-66 powder, its main characteristic peaks and their positions can be clearly observed from the figure and are consistent with the positions of the main characteristic peaks of the UiO-66 powder. This indicates that the crystals loaded on the membrane are pure-phase UiO-66 crystal materials, and it also shows that a pure-phase UiO-66 membrane has been successfully prepared. In addition, XRD tests were also carried out on the BPPO substrate membrane, the BPPO / CA membrane, and the BPPO-UiO-66 membrane, and the results are as Figure 5As shown, it can be observed from the test results that the XRD patterns of the BPPO-based membrane, the BPPO / CA membrane, and the BPPO-UiO-66 membrane with in-situ growth of UiO-66 crystals on the BPPO-based membrane without carboxylic acid functionalization modification are consistent, and no obvious UiO-66 characteristic peaks appear. Compared with the XRD pattern of the BPPO / CA-UiO-66-24h membrane, the main diffraction peaks consistent with UiO-66 powder do not appear in its XRD pattern either. This indicates that it is difficult to grow continuous and dense UiO-66 crystals on the BPPO-based membrane without carboxylic acid functionalization pretreatment. This also shows that the carboxylic acid groups introduced on the membrane surface after pretreatment by plasma treatment and acrylic liquid-phase grafting can form stable interactions with UiO-66. This interaction can not only attract UiO-66 crystals to grow in-situ on the upper surface of the membrane, but also enable UiO-66 crystals to form a firm and dense crystal layer on the upper surface of the membrane.

[0039] The results of the above examples show that the loading time of UiO-66 in the monovalent cation selective separation membrane of UiO-66 prepared by the present invention is 24 h. In the case of Na + / Me 4 N + 、K + / Me 4 N + aspect, it shows better performance and is suitable for the application of industrial-grade TMAH aqueous solution purification. At the same time, there are few studies on the separation of monovalent / divalent cations by MOF separation membranes, and there is almost no research in the fields such as industrial-grade TMAH aqueous solution purification. Therefore, it has broad development prospects.

[0040] The above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims

1. A method for preparing a monovalent cation selective separation membrane based on UiO-66, characterized in that: First, a 20wt% BPPO / NMP solution is prepared, stirred to obtain a uniform solution, and then the solution is allowed to stand to remove bubbles and then scraped on a glass plate to obtain a BPPO-based membrane; the n-propanol zirconium solution is added dropwise to a mixed solution consisting of n-propanol, glacial acetic acid and N,N-dimethylformamide, stirred and heated to react to obtain a zirconium cluster solution; the upper surface of the BPPO-based membrane is subjected to oxygen plasma etching and acrylic acid grafting pretreatment, and the resulting membrane is named BPPO / CA membrane; the BPPO / CA membrane is installed on a two-chamber reaction cell, the zirconium cluster solution, terephthalic acid and triethylamine are injected into the reaction side, and the reaction side is closely attached to the membrane surface treated with plasma modification, and pure water is injected into the other side, and the reaction is allowed to stand for 6-72 hours to obtain a UiO-66-based monovalent cation selective separation membrane.

2. The method according to claim 1, characterized in that: The thickness of BPPO base film is 200µm-250µm.

3. The method according to claim 1, characterized in that: The mass fraction of the zirconium n-propoxide solution is 70%, and the volume ratio of zirconium n-propoxide, n-propanol, glacial acetic acid and DMF is 1:0.73:56:

98.

4. The method according to claim 1, characterized in that: The reaction temperature of the zirconium cluster solution is 130° C., and the reaction time is 2 hours.

5. The method according to claim 1, characterized in that: The pretreatment step of the BPPO base film comprises: (1) The BPPO base film is subjected to oxygen plasma etching: the rated voltage of plasma etching is AC100V, the rated current is less than 15A, the rated frequency is 50 / 60Hz, the processing time is 2min, the processing atmosphere is oxygen atmosphere, and the gas flow rate is 100ml / min; (2) Acrylic acid grafting: The BPPO-based film after plasma etching was immersed in an acrylic acid solution, and after nitrogen flow for 15 minutes, it was reacted in a 70°C water bath for 2 hours to obtain a BPPO / CA film.

6. The method according to claim 1, characterized in that: The concentration of the acrylic acid solution was 70 vol %.

7. The method according to claim 1, characterized in that: In the two-chamber reaction cell, the amounts of zirconium cluster solution, terephthalic acid and triethylamine used on the reaction side were 39 mL, 0.27 g and 1.5 mL respectively.

8. The method according to claim 1, characterized in that: The static reaction time is 6-72 hours, and the reaction temperature is room temperature.

9. A monovalent cation selective separation membrane based on UiO-66 prepared according to the method according to any one of claims 1 to 8.

10. Use of a UiO-66-based monovalent cation selective separation membrane prepared according to the method of any one of claims 1 to 8 in purifying an industrial-grade TMAH solution.

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