A monovalent selective cation exchange membrane and its preparation method

Abstract

The present invention relates to a monovalent selective cation exchange membrane and a preparation method thereof, belonging to the technical field of cation exchange membranes. The specific steps are: weighing DA and ammonium persulfate and adding them to a buffer solution to obtain a PDA coating solution, pouring the PDA coating solution on the surface of a base membrane for soaking, depositing a PDA coating on the surface of the activated base membrane to obtain membrane I; dissolving zirconium tetrachloride and 2-aminoterephthalic acid in DMF, then solvent-thermally synthesizing an amino-functionalized MOF (UIO-66-NH2) under acidic conditions, weighing PEI and MOF (UIO-66-NH2) powders and adding them to a buffer solution, then pouring them on the surface of membrane I, depositing a PEI-MOF composite coating to obtain membrane II, and air-drying membrane II to obtain a monovalent selective cation exchange membrane. It can be seen that the present invention can prepare a low-cost, highly selective, and stable-performance monovalent selective cation exchange membrane by modifying the cation exchange membrane through a simple and easy-to-operate method.

Description

Technical Field

[0001] The present invention belongs to the technical field of cation exchange membranes, and in particular relates to a monovalent selective cation exchange membrane and a preparation method thereof. Background Art

[0002] Ion exchange membranes, as the core components of electrodialysis, are widely used in the desalination and concentration of electrolyte solutions, especially in the treatment of ionic aqueous solutions, such as seawater desalination, brackish water desalination, and the reuse of acid and alkali solutions.

[0003] At present, the performance of traditional ion exchange membranes has been greatly improved compared with the past, and can basically meet the requirements of general industrial treatment. However, general ion exchange membranes only have selective permeability to anions and cations, and do not have selective permeability to ions of different valences. During the ion migration process, water dissociates to produce OH - Easy to react with Ca in aqueous solution 2 + Mg 2+ The combination of divalent ions produces precipitation, making it difficult for conventional electrodialysis to separate monovalent and polyvalent ions of the same type. These ions adhere to the membrane surface, causing membrane fouling, which leads to low electrodialyzer current efficiency and increased energy consumption, seriously shortening the life of the membrane. With the increasing global water shortage and technological development in areas such as lithium extraction from salt lakes and industrial wastewater treatment, the importance of monovalent selective cation exchange membranes has become increasingly prominent.

[0004] Under the drive of DC electric field, the monovalent selective cation exchange membrane can preferentially make monovalent cations (such as Na + , K + ) pass through, while rejecting most divalent and higher valent cations (Mg 2+ 、Al 3+ However, the current monovalent selective cation exchange membranes are not only expensive but also need to be further improved in terms of monovalent selectivity.

[0005] Existing modification technologies such as surface coating (using a cross-linking agent to infiltrate or cover the modified material on the base membrane) and doping modification (i.e., when initially preparing the ion membrane, some additives with specific advantages are added to the casting solution to optimize the monovalent selectivity of the prepared ion exchange membrane) have the following problems: the monovalent selectivity is not significantly improved; the functional layer is easy to fall off (for example, the bonding between the nanoparticles and the base membrane is weak); and the modification process is complex (high-temperature treatment is required, etc.). Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a monovalent selective cation exchange membrane and a preparation method thereof.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is: a method for preparing a monovalent selective cation exchange membrane, comprising the following steps:

[0008] S1. Zirconium tetrachloride and 2-aminoterephthalic acid were dissolved in N,N-dimethylformamide (DMF), and then amino-functionalized MOF (UIO-66-NH2) was solvothermally synthesized under acidic conditions.

[0009] S2. At room temperature, the cation exchange membrane to be modified is used as the base membrane and is immersed in deionized water to activate the surface.

[0010] S3. Weigh dopamine hydrochloride (DA) and ammonium persulfate, add them to Tris-HCl buffer, and sonicate until completely dissolved to obtain polydopamine (PDA) coating solution.

[0011] S4, weighing polyethyleneimine (PEI) and the MOF (UIO-66-NH2) powder obtained in step S1, adding them to Tris-HCl buffer, and sonicating until completely dissolved;

[0012] S5, fixing the basement membrane obtained in step S2, pouring the PDA coating solution prepared in step S3 onto the surface of the basement membrane to soak it, depositing a PDA coating on the surface of the activated basement membrane, and reacting at 25°C to obtain membrane I;

[0013] S6. Pour the solution obtained in step S4 onto the surface of membrane I obtained after the reaction in step S5, deposit a PEI-MOF composite coating, and react at 25°C and pH=8.5 to obtain membrane II.

[0014] S7, naturally air-drying the membrane II obtained in step S6 to obtain a monovalent selective cation exchange membrane.

[0015] Furthermore, hydrochloric acid is added in step S1.

[0016] Furthermore, in step S5, the mixture was placed on a shaker at 25° C. and a rate of 100 rpm / min for reaction for 4 h.

[0017] Furthermore, in step S6, the mixture was placed on a shaker at 25° C. and 100 rpm / min for reaction for 1 h.

[0018] Among them, PDA is evenly coated on the surface of the base membrane as an intermediate layer, enhancing the bonding force between MOF (UIO-66-NH2) and the base membrane and preventing MOF particles from falling off; and the catechol / quinone groups (-OH, C=O) of PDA improve the wettability of the base membrane surface and promote the rapid transmission of hydrated ions.

[0019] Among them, the flexible chain of PEI can fill the gaps in the MOF (UIO-66-NH2) pores to form a dense but hydrophilic transport network, enhancing the transport of high-valent ions (Mg 2+ The pore size of MOF (UIO-66-NH2) (0.8~1.2nm) preferentially allows hydrated Na + (hydration radius is about 0.36nm) through, while Mg 2+ (The hydration radius is about 0.43nm) and is partially blocked because its size is close to the pore limit. The -NH2 of MOF (UIO-66-NH2) is partially protonated (-NH3 + ), synergistically enhances the Mg 2+ electrostatic repulsion (same charge repulsion), while selectively adsorbing low-valent Na through the Donnan effect + In addition, due to the difference in hydration energy of ions, Na + Due to its low hydration energy (about -424 kJ / mol), it is easy to pass through the MOF pores after removing part of the hydration shell; Mg 2+ The hydration energy is high (about -1920 kJ / mol), requiring higher energy for dehydration, and its migration is hindered in the dense pores of PDA / PEI-UIO-66-NH2.

[0020] Another object of the present invention is to provide a monovalent selective cation exchange membrane prepared by a method for preparing a monovalent selective cation exchange membrane.

[0021] Furthermore, the Na / Mg monovalent selectivity of the monovalent selective cation exchange membrane can reach a maximum of 47.87.

[0022] The beneficial effects of the present invention are:

[0023] This invention uses MOF (UIO-66-NH2) with angstrom-level pores, PDA, and PEI to modify a cation exchange membrane to improve its monovalent selectivity. Physical barrier is achieved through MOF pore size screening, structural stabilization is achieved through PDA interfacial adhesion, and chemical strengthening is achieved through PEI charge repulsion, forming a three-level synergistic functional layer, significantly improving the monovalent selectivity of cations.

[0024] The present invention adopts a low-temperature co-deposition process: the PEI-MOF composite coating is deposited at 25°C and pH=8.5, abandoning the traditional method of synthesizing MOF on the membrane at high temperature, avoiding the energy consumption problem of traditional high-temperature sintering, and also avoiding the damage of high temperature to the membrane structure.

[0025] The process of the present invention is simple and easy to operate, and the three-level synergistic structural functional layer enables the unit price selectivity of the cation exchange membrane prepared by the present invention to reach up to 47.87, which has a very significant selectivity improvement effect.

[0026] In summary, the present invention can prepare a low-cost, highly selective, and stable-performance monovalent selective cation exchange membrane by modifying the cation exchange membrane through a simple and easy-to-operate method. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be described in detail below with reference to the accompanying drawings and in combination with examples, and the advantages and implementation modes of the present invention will become more apparent. The contents shown in the accompanying drawings are only used to illustrate the present invention and do not constitute any limitation to the present invention. In the accompanying drawings:

[0028] Figure 1 Schematic diagram of the structure of the electrodialysis device of the present invention.

[0029] In the picture:

[0030] 1. First electrode chamber; 2. Desalination chamber; 3. Concentration chamber; 4. Second electrode chamber; 5. Anode; 6. Cathode; 7. Monovalent selective cation exchange membrane; 8. Anion exchange membrane. DETAILED DESCRIPTION

[0031] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with the best embodiment.

[0032] A method for preparing a monovalent selective cation exchange membrane comprises the following steps:

[0033] S1. Dissolve 0.5 g zirconium tetrachloride and 0.53 g 2-aminoterephthalic acid in 60 ml N,N-dimethylformamide (DMF). Then add 4 ml 36% hydrochloric acid and solvothermally synthesize amino-functionalized UiO-66-type MOF (UIO-66-NH2) under acidic conditions.

[0034] S2. At room temperature, the existing cation exchange membrane to be modified is used as the base membrane and is immersed in deionized water for 24 hours to activate the surface.

[0035] S3. Weigh 0.04 g dopamine hydrochloride (DA) and 0.04 g ammonium persulfate, add them to 20 ml Tris-HCl buffer, and sonicate until completely dissolved to obtain polydopamine (PDA) coating solution.

[0036] S4, weighing 0.02 g of polyethyleneimine (PEI) and 0.02 g of MOF (UIO-66-NH2) powder obtained in step S1, adding them to 20 ml of Tris-HCl buffer, and sonicating until completely dissolved;

[0037] S5. Fix the basement membrane obtained in step S2, pour the PDA coating solution prepared in step S3 onto the surface of the basement membrane to soak it, deposit a PDA coating on the surface of the activated basement membrane, and shake it on a shaker at 25° C. and 100 rpm / min for 4 hours to obtain membrane I;

[0038] S6. Pour the solution obtained in step S4 onto the surface of membrane I obtained after the reaction in step S5 to deposit a PEI-MOF composite coating. The mixture was reacted on a shaker at 25°C and pH 8.5 at a speed of 100 rpm / min for 1 hour to obtain membrane II.

[0039] S7, naturally air-drying the membrane II obtained in step S6 to obtain a monovalent selective cation exchange membrane.

[0040] Among them, PDA is evenly coated on the surface of the base membrane as an intermediate layer, enhancing the bonding force between MOF (UIO-66-NH2) and the base membrane and preventing MOF particles from falling off; and the catechol / quinone groups (-OH, C=O) of PDA improve the wettability of the base membrane surface and promote the rapid transmission of hydrated ions.

[0041] Among them, the flexible chain of PEI can fill the gaps in the MOF (UIO-66-NH2) pores to form a dense but hydrophilic transport network, enhancing the transport of high-valent ions (Mg 2+ The pore size of MOF (UIO-66-NH2) (0.8~1.2nm) preferentially allows hydrated Na + (hydration radius is about 0.36nm) through, while Mg 2+ (The hydration radius is about 0.43nm) and is partially blocked because its size is close to the pore limit. The -NH2 of MOF (UIO-66-NH2) is partially protonated (-NH3 + ), synergistically enhances the Mg 2+ electrostatic repulsion (same charge repulsion), while selectively adsorbing low-valent Na through the Donnan effect + In addition, due to the difference in hydration energy of ions, Na + Due to its low hydration energy (about -424 kJ / mol), it is easy to pass through the MOF pores after removing part of the hydration shell; Mg 2+ The hydration energy is high (about -1920 kJ / mol), requiring higher energy for dehydration, and its migration is hindered in the dense pores of PDA / PEI-UIO-66-NH2.

[0042] Another object of the present invention is to provide a monovalent selective cation exchange membrane prepared by a method for preparing a monovalent selective cation exchange membrane.

[0043] Furthermore, the Na / Mg monovalent selectivity of the monovalent selective cation exchange membrane can reach up to 47.87.

[0044] like Figure 1 As shown, another object of the present invention is to provide an electrodialysis device for evaluating the monovalent selectivity of a monovalent selective cation exchange membrane.

[0045] Furthermore, the electrodialysis device includes a first electrode chamber 1, a desalination chamber 2, a concentration chamber 3 and a second electrode chamber 4 arranged in sequence. An anode 5 is provided on one side of the first electrode chamber 1, and a cathode 6 is provided on one side of the second electrode chamber 4.

[0046] Furthermore, a monovalent selective cation exchange membrane 7 is provided between the desalination chamber 2 and the concentration chamber 3 , and an anion exchange membrane 8 is provided between the first electrode chamber 1 and the desalination chamber 2 and between the second electrode chamber 4 and the concentration chamber 3 .

[0047] Furthermore, the anode 5 and the cathode 6 are both titanium electrodes coated with ruthenium.

[0048] Furthermore, the desalination chamber 2 contains 0.1 mol / L NaCl and 0.1 mol / L MgCl2, the concentration chamber 3 contains 0.01 mol / L KCl, and the first electrode chamber 1 and the second electrode chamber 4 both contain 0.2 mol / L Na2SO4.

[0049] Under the action of the DC electric field, cations migrate to the cathode through the cation exchange membrane, but are blocked by the anion exchange membrane; anions migrate to the anode through the anion exchange membrane, but are blocked by the cation exchange membrane; at this time, cations in the desalination chamber 2 enter the concentration chamber 3 through the cation exchange membrane. When the cation exchange membrane is the monovalent selective cation exchange membrane 7 of the present invention, Na + Through Mg 2+ is retained, so it can be tested by the concentration chamber 3 Na + and Mg 2+ The concentration is calculated to calculate the Na / Mg monovalent selectivity of the monovalent selective cation exchange membrane 7 of the present invention.

[0050] The specific working process is:

[0051] The effective area of ​​the monovalent selective cation exchange membrane 7 in the electrodialysis device is 8.6 cm 2 The electrodialysis time was controlled at 20 min and the current density was 5 mA / cm 2 , Na in concentration chamber 3 was determined by cation chromatography + and Mg 2+ concentration and calculate the ion flux and monovalent selectivity.

[0052] The calculation formula is as follows:

[0053] The cation flux through the monovalent selective cation exchange membrane 7 is:

[0054]

[0055] Where, J i is the target cation flux through the monovalent selective cation exchange membrane, C i is the concentration of the target cation in the desalination chamber, A is the effective area of ​​the monovalent selective cation exchange membrane, V is the volume of the concentration chamber, and t is the electrolysis time.

[0056] Monovalent selective cation exchange membrane 7 in Na + and Mg 2+ The unit price options are:

[0057]

[0058] Where, For monovalent selective cation exchange membrane in Na + and Mg 2+ The unit price selectivity between A + for Na + , B 2 + Mg 2+ , J A + is the Na + Flux, J B 2+ is the Mg2+ passing through the monovalent selective cation exchange membrane 2+ Flux, C A + To desalinate indoor Na + The concentration, C B 2+ Mg for desalination chamber 2+ concentration.

[0059] The selectivity of the base membrane is known to be 2.11.

[0060] The above process yields Na + The flux is 2.61×10 -8 mol·cm -2 ·s -1 , Mg 2+ The flux is 0.54×10 -9 mol·cm -2 ·s -1 , the selectivity can reach up to 47.87.

[0061] It can be seen that the present invention significantly improves the monovalent selectivity of the cation exchange membrane through multi-layer structure design, charge synergistic regulation and size screening, breaking through the limitation of traditional cation exchange membranes relying on a single mechanism and achieving efficient ion separation through the following multi-dimensional synergistic effects.

[0062] Charge gradient synergy: -NH2 of MOF(UIO-66-NH2) is partially protonated at pH=8.5 (-NH 3+ ), synergistically enhances the Mg 2+ electrostatic repulsion (same charge repulsion), while selectively adsorbing low-valent Na through the Donnan effect + .

[0063] Dynamic coordination screening: amino group (-NH2) and Mg 2+ The reversible weak coordination effect of Na + No such effect.

[0064] Size-hydration energy synergistic screening: MOF pores (0.8~1.2nm) and hydrated ion radius differences (Na + :0.36nm,Mg 2+ : 0.43nm) to achieve physical screening.

[0065] The embodiments of the present invention are described in detail above, but the contents are only preferred embodiments of the present invention and should not be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the present invention.

Claims

1. A method for preparing a monovalent selective cation exchange membrane, characterized in that: The following steps are involved: S1, zirconium tetrachloride and 2-aminoterephthalic acid were dissolved in DMF, and then the amino-functionalized MOF (UIO-66-NH2) was solvothermally synthesized under acidic conditions; S2, using the cation exchange membrane to be modified as the base membrane and soaking it in deionized water to activate the surface; S3. Weigh dopamine hydrochloride and ammonium persulfate, add them to Tris-HCl buffer, and sonicate until completely dissolved to obtain a polydopamine coating solution; S4. Weigh polyethyleneimine and the MOF (UIO-66-NH2) powder obtained in step S1, add them to Tris-HCl buffer, and sonicate until completely dissolved; S5, fixing the basement membrane obtained in step S2, pouring the polydopamine coating solution prepared in step S3 onto the surface of the basement membrane to soak it, depositing a polydopamine coating on the surface of the activated basement membrane, and reacting at 25°C to obtain membrane I; S6. Pour the solution obtained in step S4 onto the surface of membrane I obtained after the reaction in step S5 to deposit a PEI-MOF composite coating, and react at 25° C. and pH 8.5 to obtain membrane II. S7, naturally air-drying the membrane II obtained in step S6 to obtain a monovalent selective cation exchange membrane.

2. The method for preparing a monovalent selective cation exchange membrane according to claim 1, wherein: In step S1, hydrochloric acid is added.

3. The method for preparing a monovalent selective cation exchange membrane according to claim 1, wherein: In step S5, the mixture was placed on a shaker at 25° C. and 100 rpm / min for 4 h.

4. The method for preparing a monovalent selective cation exchange membrane according to claim 1, wherein: In step S6, the mixture was placed on a shaker at 25° C. and 100 rpm / min for reaction for 1 h.

5. A monovalent selective cation exchange membrane, characterized in that: The membrane is prepared by the method for preparing a monovalent selective cation exchange membrane according to any one of claims 1 to 4.

6. The monovalent selective cation exchange membrane according to claim 5, characterized in that: The Na / Mg monovalent selectivity of the monovalent selective cation exchange membrane reaches a maximum of 47.87.

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