Monovalent selective cation exchange membrane coupled with covalent organic framework material and preparation method of monovalent selective cation exchange membrane

Abstract

The invention belongs to the field of cation exchange membranes, and discloses a monovalent selective cation exchange membrane coupled with a covalent organic framework material and a preparation method of the monovalent selective cation exchange membrane. The preparation method comprises the following steps: firstly, synthesizing a sulfonated chloromethylated polymer, evaporating a solvent to form a membrane, and then constructing a covalent organic framework ion sieve layer on the surface of the membrane by a solvothermal growth method. A regular porous channel of the covalent organic framework and a sulfonic acid group in the base membrane have a synergistic effect, so that good ion flux of monovalent ions is ensured. Meanwhile, the chemical reaction of chloromethyl and amido contained in the base membrane endows the covalent organic framework modified layer with better stability. Due to the charge property of the covalent organic framework, the covalent organic framework has different coulomb force effects on monovalent and multivalent ions, so that the selective separation of monovalent cations is achieved, and the prepared monovalent selective cation exchange membrane is suitable for an electrodialysis process.

Description

Technical Field

[0001] The invention belongs to the field of cation exchange membranes, and in particular relates to a monovalent selective cation exchange membrane coupled with a covalent organic framework material and a preparation method thereof. Background Art

[0002] Water is an indispensable resource, not only for water itself, but also for the precious resources it contains. Whether it is lithium extraction from salt lakes, seawater desalination or wastewater treatment, electrodialysis technology with monovalent selective cation exchange membranes as the core is particularly attractive due to its simple operation, high energy efficiency and low application requirements. Efficient MCPMs often require high flux of monovalent ions and good selectivity between monovalent / multivalent cations. Previous researchers have developed a series of effective cation exchange membranes using a variety of strategies including cross-linking, surface modification and polymer blending. However, limited by the separation structure of excessive density or high repulsion, existing methods often increase the monovalent ion transport resistance while improving the membrane selectivity, resulting in increased membrane resistance and reduced cation flux (trade-off phenomenon). In addition, the poor stability of the separation structure also restricts the commercialization of monovalent ion selective separation membranes. Therefore, it is necessary to develop new membrane materials to solve the trade-off phenomenon between ion flux and selectivity, while improving the stability of the separation structure.

[0003] Covalent organic frameworks (COFs) are a class of emerging crystalline porous polymers connected by covalent bonds, with regular pore structure, high porosity and excellent chemical stability. COFs materials have shown excellent flux performance in many studies, which can be mainly attributed to the low mass transfer resistance of their regular porous channels. This low mass transfer resistance facilitates the transport and precise screening of ions, and is expected to solve the trade-off between ion flux and selectivity. At present, many studies have been carried out by combining COFs with ion exchange membranes, but they are mainly concentrated in the fields of fuel cells, and the progress of using COFs to prepare monovalent selective cation exchange membranes is still limited. The main difficulty is that it is difficult to prepare a thin and defect-free COFs layer firmly attached to the support, and the pore size of existing COFs is usually in the range of 1-5nm, which is larger than the hydration kinetic diameter of inorganic salt ions (less than 1nm), making it difficult to achieve efficient ion screening. Therefore, designing suitable COFs and solving the structural stability problem between them and the substrate to prepare monovalent selective cation exchange membranes can achieve effective separation of specific ions while overcoming the above shortcomings. Summary of the invention

[0004] In order to solve the above technical problems existing in the prior art, a method for preparing a monovalent selective cation exchange membrane is required for the electrodialysis process. The method should enable the obtained cation exchange membrane to have good stability and monovalent and polyvalent cation selective permeability, and the preparation method should be simple and easy to operate.

[0005] The purpose of the present invention is to prepare a high-selectivity and high-flux monovalent selective cation exchange membrane, and to provide a method for preparing a monovalent selective cation exchange membrane of a coupled covalent organic framework material. The coupled covalent organic framework cation exchange membrane is constructed on the surface of a SPPSU-Cl membrane by a solvothermal growth method and is composed of Tp and TG. Cl In situ growth of TpTG Cl Covalent organic framework layer. When ions pass through the membrane, due to the positive charge of the TpTGCl covalent organic framework, it has a greater Coulomb repulsion for multivalent cations, thus achieving the purpose of ion separation.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] The present invention adopts a very simple solvent thermal growth method to prepare a monovalent selective cation exchange membrane with a covalent organic framework structure, and successfully constructs a covalent organic framework layer with positive charge on the membrane surface, comprising the following steps:

[0008] Step 1, weighing a certain amount of sulfonated polymer and dissolving it in N,N-dimethylacetamide, using chloromethyl ether compounds as chloromethylation reagents, and using tin tetrachloride as a catalyst to prepare sulfonated chloromethylated polymer;

[0009] Step 2: Weigh a certain amount of sulfonated chloromethylated polymer and dissolve it in dimethyl sulfoxide to prepare a casting solution, cast the casting solution on a glass plate, heat and dry it, and after the solvent is completely evaporated, take out the membrane and store it in a wet state.

[0010] Step 3, weigh a certain amount of 1,3,5-triformylphloroglucinol Tp and a positively charged amine monomer and add them into a certain amount of water to prepare a COF modified solution;

[0011] Step 4: Place the membrane in step 2 in a reactor, add the COF modified solution in step 3 to one side, heat and evaporate, and after the solution is completely evaporated, take out the membrane and store it in a wet state.

[0012] Furthermore, the sulfonated polymer in step 1 is one or more of polyphenylsulfone (PPSU), polyetheretherketone (PEEK), and polyethersulfone (PES).

[0013] Furthermore, in step one, the chloromethyl ether compound is one of chloromethyl methyl ether and chloromethyl ethyl ether.

[0014] Furthermore, the positively charged amine monomer in step 3 is triaminoguanidine chloride (TG Cl ) or ethidium bromide (EtBr).

[0015] Furthermore, in step 3, the Tp concentration is 1.5 mmol / L, and the positively charged amino monomer concentration is 1.5-3.5 mmol / L.

[0016] Furthermore, in step 4, the heating evaporation is performed at 90° C. for 12 hours.

[0017] A monovalent selective cation exchange membrane coupled with a covalent organic framework material is prepared by adopting the preparation method.

[0018] Compared with the prior art, the present invention has the following advantages: 1. The present invention adopts a very simple solvent thermal growth method to prepare a monovalent selective cation exchange membrane with a covalent organic framework structure, successfully constructing a covalent organic framework layer with positive charge on the membrane surface, without the need for special equipment, and is simple and easy to operate. 2. The coupled covalent organic framework cation exchange membrane prepared by the present invention has a positively charged covalent organic framework structure, which has a greater Coulomb repulsion for multivalent cations, thereby achieving Li + / Mg 2+ 、Na + / Mg 2+ and K + / Mg 2+ Selective separation, with separation performance higher than commercial membranes, can be applied to lithium extraction from salt lakes, crude salt refining, etc. 3. The covalent organic framework structure introduced in the present invention is expected to alleviate the balance / trade-off effect between flux and selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a physical picture of the monovalent selective cation exchange membrane prepared in Example 6;

[0020] Figure 2a A scanning electron microscope plan view of the monovalent selective cation exchange membrane prepared in Example 5;

[0021] Figure 2b A transmission electron microscopy cross-sectional view of the monovalent selective cation exchange membrane prepared in Example 5;

[0022] Figure 3 is a Fourier infrared spectrum of the sulfonated chloromethylated polyphenylsulfone, the monovalent selective cation exchange membrane prepared in Example 6;

[0023] Figure 4 TpTG Cl Schematic diagram of the chemical structure of a covalent organic framework;

[0024] Figure 5 TpTG Cl X-ray diffraction patterns of covalent organic frameworks;

[0025] Figure 6 It is a schematic diagram of the structure of the testing device of the present invention;

[0026] Reference numerals in the figure: 1-first electrode chamber, 2-dilute chamber, 3-concentrated chamber, 4-second electrode chamber, 5-first anion exchange membrane, 6-monovalent selective cation exchange membrane, 7-second anion exchange membrane, 8-anode, 9-cathode, 10-feed liquid inlet. DETAILED DESCRIPTION

[0027] The present invention will be further described below in conjunction with the accompanying drawings.

[0028] The monovalent selective cation exchange membrane of the coupled covalent organic framework material of the present invention comprises the following steps:

[0029] Step 1, weighing a certain amount of sulfonated polymer and dissolving it in N,N-dimethylacetamide, using chloromethyl ether compounds as chloromethylation reagents, and using tin tetrachloride as a catalyst to prepare sulfonated chloromethylated polymer;

[0030] Step 2: Weigh a certain amount of sulfonated chloromethylated polymer and dissolve it in dimethyl sulfoxide to prepare a casting solution, cast the casting solution on a glass plate, heat and dry it, and after the solvent is completely evaporated, take out the membrane and store it in a wet state.

[0031] Step 3, weighing a certain amount of 1,3,5-triformylphloroglucinol (Tp) and a positively charged amine monomer and adding them into a certain amount of water to prepare a COF modified solution;

[0032] Step 4: Place the membrane in step 4 in a reactor, add the COF modified solution in step 3 to one side, evaporate at 90° C. for 12 hours, and after all the solution is volatilized, take out the membrane and store it in a wet state.

[0033] Example 1

[0034] Monovalent selective cation exchange membranes with different polymer substrates were prepared by selecting different sulfonated polymers. 8.8 g of sulfonated polyphenylsulfone (SPPSU) was weighed and dissolved in 150 ml of DMAC, and 2 ml of chloromethyl ether and 2 ml of anhydrous tin tetrachloride were added dropwise in sequence. The reaction was carried out at 0 ° C for 48 hours. The product was precipitated and dried in water to obtain sulfonated chloromethylated polyphenylsulfone (SPPSU-Cl). 0.784 g of SPPSU-Cl was weighed and added to 6.30 g of dimethyl sulfoxide to prepare a casting solution, and dried at 60 ° C for 12 hours to obtain a SPPSU-Cl base membrane. 0.03 mmol of TP and 0.04 mmol of TG ClDissolve in 20 mL of water, add the solution into the reactor with SPPSU-Cl membrane, evaporate at 90 °C for 12 hours, take out the membrane and remove the bubbles, put it in a mixed salt solution and name it SPPSU-TG Cl 2.

[0035] Example 2

[0036] The difference from Example 1 is that the polyethersulfone is sulfonated and chloromethylated in Step 1 and Step 2, and the degree of sulfonation is the same as the degree of chloromethylation. In Step 3, 0.784 g of SPES is added to 6.30 g of dimethyl sulfoxide to prepare a casting solution. The other operations are the same as in Example 1. The membrane is named SPES-TG. Cl 2.

[0037] Example 3

[0038] The difference from Example 1 is that the polyetheretherketone is sulfonated in step 1 and step 2, and the degree of sulfonation is the same as the degree of chloromethylation. In step 2, 0.784 g of SPEEK is weighed and added to 6.30 g of dimethyl sulfoxide to prepare a casting solution. The other operations are the same as in Example 1. The membrane is named SPEEK-TG Cl 2.

[0039] Example 4

[0040] The difference from Example 1 is that in step 1, chloromethyl methyl ether is used to chloromethylate the sulfonated polyphenylsulfone, and the degree of sulfonation is the same as the degree of chloromethylation. In step 2, 0.784 g of SPPSU-Cl is weighed and added to 6.30 g of dimethyl sulfoxide to prepare a casting solution. The other operations are the same as in Example 1. The membrane is named SPPSU / 2-TG Cl 2

[0041] Example 5

[0042] This example prepares different monovalent selective cation exchange membranes by adjusting the amount of charged amine monomers. The difference from Example 1 is that 0.03 mmol of EtBr is used in step 1. The other operations are the same as Example 1. The membrane is named SPPSU-EtBr2

[0043]

[0044]

[0045] Table 1: Separation performance of the cation exchange membranes prepared in Examples 1-5 for a 0.05 mol / L lithium-magnesium mixed solution

[0046] Table 1 shows that regardless of the type of polymer, the monovalent selective cation exchange membrane obtained by the preparation method of the present invention has a good screening effect for monovalent and divalent cations. Example 1 prepared by chloromethylating sulfonated polyphenylsulfone with chloromethyl ethyl ether is the membrane with the best lithium-magnesium selectivity, Li + / Mg 2+ The selectivity reached 5.886, and the flux of lithium ions was also high, reaching 1329.906mmol m -2 h -1 Regardless of the type of charged amine monomer, it has a good screening effect for monovalent and divalent cations.

[0047] Example 6

[0048] In this example, different monovalent selective cation exchange membranes were prepared by adjusting the amount of charged amine monomers. 0.784 g of SPPSU-Cl was added to 6.30 g of dimethyl sulfoxide to prepare a casting solution, which was dried at 60°C for 12 hours to obtain a SPPSU-Cl based membrane. 0.03 mmol of TP and 0.03 mmol of TG were added to the solution. Cl Dissolve in 20 mL of water, add the solution into the reactor with SPPSU-Cl membrane, evaporate at 90 °C for 12 h, take out the membrane and remove the bubbles in a mixed salt solution, named SPPSU-COF1.

[0049] Example 7

[0050] The difference from Example 4 is that in step 5, 0.03 mmol of TP and 0.04 mmol of TG are added. Cl Dissolved in 20 mL of water, other operations were the same as in Example 4, and the membrane was named SPPSU-COF2.

[0051] Example 8

[0052] The difference from Example 4 is that in step 5, 0.03 mmol of TP and 0.05 mmol of TG are added. Cl Dissolved in 20 mL of water, other operations were the same as in Example 4, and the membrane was named SPPSU-COF3.

[0053] Example 9

[0054] The difference from Example 4 is that in step 5, 0.03 mmol of TP and 0.06 mmol of TG are added. Cl Dissolved in 20 mL of water, other operations were the same as in Example 4, and the membrane was named SPPSU-COF4.

[0055] Example 10

[0056] The difference from Example 4 is that in step 5, 0.03 mmol of TP and 0.07 mmol of TG are added.Cl Dissolved in 20 mL of water, other operations were the same as in Example 4, and the membrane was named SPPSU-COF5.

[0057]

[0058] Table 2: Separation performance of the cation exchange membranes prepared in Examples 6-10 for 0.05 mol / L lithium-magnesium mixed solution

[0059] Table 2 shows that Example 5 is the membrane with the best lithium-magnesium selectivity, Li + / Mg 2+ The selectivity reached 5.886, and the flux of lithium ions was also high, reaching 1329.906mmol m -2 h -1 The covalent organic framework modified layer achieves the separation of monovalent / divalent cations through the charge repulsion effect. At the same time, the regular porous channels of the covalent organic framework and the sulfonic acid groups in the SPPSU-Cl membrane ensure good ion flux of monovalent ions. Cl The increase in dosage makes the modified layer too positively charged, increases the ion transmission resistance, maximizes the membrane resistance, and reduces the ion flux.

[0060]

[0061] Table 3: Separation performance of the cation exchange membranes prepared in Examples 6-10 for 0.05 mol / L sodium-magnesium mixed solution.

[0062] As shown in Table 3, the experimental results of Examples 4-8 show that Na + Flux changes with Li + The trend is similar, when Example 5 Na + / Mg 2+ The selectivity is the best, reaching 10.567.

[0063]

[0064]

[0065] Table 4: Separation performance of the cation exchange membranes prepared in Examples 6-10 for 0.05 mol / L potassium-magnesium mixed solution.

[0066] As shown in Table 4, the cation exchange membranes prepared in Examples 6-10 have higher selectivity for potassium and magnesium than for sodium and magnesium and lithium and magnesium. + / Mg 2+ The separation performance is the best, with a selectivity of 20.273.

[0067] In summary, the SPPSU-Cl membrane modified with coupled covalent organic framework prepared in this experiment has good + / Mg 2+ 、Na + / Mg 2+ and K + / Mg 2+ The system shows that Li + 、Na + and K + The excellent selectivity can be inferred to be due to the positive charge of the TpTGCl covalent organic framework, which has a greater Coulomb repulsion for multivalent cations, thereby achieving the purpose of ion separation. + >Na + >K + , the corresponding dehydration energy is also Li + >Na + >K + , so in binary mixed solutions, selective Li + / Mg 2+ <Na + / Mg 2+ <K + / Mg 2+ The best selectivities were 5.886, 10.567, and 20.273, respectively.

[0068] Application test experiment:

[0069] This experiment uses a laboratory-made membrane stack to test the unit price selectivity of the membrane. Figure 6 First, prepare 0.05 mol / L Na + / Mg 2+ , Li + / Mg 2+ , K + / Mg 2+ The mixed solution was used as the test solution and 0.05 mol / L Na2SO4 solution was used as the circulating polar solution. The modified membrane was immersed in 0.05 mol / L Na + / Mg 2+ , Li + / Mg 2+ , K + / Mg 2+ The mixed solution was equilibrated for 24 hours. Then the modified cation exchange membrane was fixed between two commercial anion membranes (blue, alloy membranes), with 100 mL of Na + / Mg 2+ , Li + / Mg 2+ , K + / Mg 2+Mixed solution, 500mL Na2SO4 solution circulates in the polar chambers on both sides of the anion membrane through a water pump. The modified membrane has an effective area of ​​7.065cm 2 The constant current density is 5.0 mA cm -2 , and the stable operation was carried out for 1 h. The ion concentration in the fresh water chamber before and after the operation was determined by a cation ion chromatograph (SH-CC-3L, Shenghan, China).

[0070] The monovalent selective cation exchange membrane was operated stably for 1 h, and the ion flux was calculated using the following equation:

[0071]

[0072] where c0 and c t It is a monovalent ion M + (K + 、Na + and Li + ) or Mg 2+ The concentration is 0 and th in the desalination chamber, V is the volume of the solution in the desalination chamber, A m Represents the effective test area of ​​the membrane (7.065 cm 2 ), t represents the test duration.

[0073] The selectivity calculation formula is as follows:

[0074]

[0075] Where: J represents the membrane ion flux, in mmol m -2 h -1 ; c0 represents the ion concentration before separation, in mol / L. Na+ and c Mg2+ Represents the monovalent ion M after separation + (K + 、Na + and Li + )Na + Mg 2+ Plasma concentration, in mol / L.

[0076] The above contents are only preferred embodiments of the present invention and are not intended to limit the implementation scheme of the present invention. A person skilled in the art can easily make corresponding changes or modifications based on the main concept and spirit of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope required by the claims.

Claims

1. A method for preparing a monovalent selective cation exchange membrane coupled with a covalent organic framework material, characterized in that The following steps are involved: Step 1, weighing a certain amount of sulfonated polymer and dissolving it in N,N-dimethylacetamide, using chloromethyl ether compounds as chloromethylation reagents, and using tin tetrachloride as a catalyst to prepare a sulfonated chloromethylated polymer; Step 2: Weigh a certain amount of sulfonated chloromethylated polymer and dissolve it in dimethyl sulfoxide to prepare a casting solution, cast the casting solution on a glass plate, heat and dry it, and after the solvent is completely evaporated, take out the membrane and store it in a wet state.

2. Step 3, weigh a certain amount of 1,3,5-triformyl phloroglucinol Tp and a positively charged amine monomer and add them into a certain amount of water to prepare a COF modified solution; Step 4: Place the membrane in step 2 in a reactor, add the COF modified solution in step 3 to one side, heat and evaporate, and after the solution is completely evaporated, take out the membrane and store it in a wet state.

3. The preparation method according to claim 1, characterized in that: In step 1, the sulfonated polymer is one or more of polyphenylsulfone (PPSU), polyetheretherketone (PEEK), and polyethersulfone (PES).

4. The preparation method according to claim 1, characterized in that: Step 1: The chloromethyl ether compound is one of chloromethyl methyl ether and chloromethyl ethyl ether.

5. The preparation method according to claim 1, characterized in that: The positively charged amine monomer in step 3 is triaminoguanidine chloride (TG Cl ) or ethidium bromide (EtBr).

6. The preparation method according to claim 1, characterized in that: The Tp concentration in step 3 is 1.5 mmol / L, and the concentration of the positively charged amino monomer is 1.5-3.5 mmol / L.

7. The preparation method according to claim 1, characterized in that: In step 4, the heating evaporation is carried out at 90° C. for 12 hours.

8. A monovalent selective cation exchange membrane coupled with a covalent organic framework material, characterized in that: The method is described in any one of claims 1 to 6.

Images