Preparation method of branched polybenzimidazole side chain grafted monovalent / divalent high-selectivity cationic membrane

Abstract

The invention discloses a preparation method of a branched polybenzimidazole side chain grafted monovalent / divalent high-selectivity cationic membrane, which is characterized in that a branched unit is introduced into a main chain to construct a microcavity structure based on the unique tight stacking and rich hydrogen bond network characteristics of polybenzimidazole (PBI), and the pore diameter in the membrane and the free volume of a polymer are regulated and controlled to prepare the branched polybenzimidazole side chain grafted monovalent / divalent high-selectivity cationic membrane. The stable and rapid transmission of monovalent ions is promoted, and a high interception effect on divalent ions is kept. Then molecules containing sulfonic acid groups and hydrophobic side chain molecules are grafted to the branched PBI, the ionic conduction performance is improved, meanwhile, a microphase separation structure can be constructed, the ionic transmembrane energy barrier difference is increased through the ionic hydration energy difference of monovalent and divalent ions, selective transmission of ions is achieved, and the ionic conductivity is improved. Finally, the ion separation membrane with outstanding stability and excellent comprehensive performance of selectivity and flux is obtained.

Description

Technical Field

[0001] The invention belongs to the field of membrane technology, and in particular relates to a method for preparing a branched polybenzimidazole side-chain branched monovalent / divalent high-selectivity cationic membrane. Background Art

[0002] With the rapid development of global industrial production, the demand for efficient and precise ion separation is becoming increasingly prominent in fields such as environmental protection and water treatment, energy and resource development, and industrial production and manufacturing. Traditional technologies, such as adsorption and extraction, are often limited by the dual bottlenecks of high energy consumption and high costs when addressing large-scale, complex separation tasks, making them unable to meet the demands of modern industry for green, low-carbon, and efficient production. Against this backdrop, electrodialysis has emerged and continues to mature as an electrically driven membrane separation solution. This technology, driven by a direct current electric field, selectively transmits ions through ion exchange membranes, demonstrating excellent separation efficiency and playing a key role in the fine separation of monovalent and divalent ions. However, the current global market for monovalent and divalent selective cation membranes is primarily dominated by Japanese manufacturers. Their high price, limited selectivity, membrane stability issues, and susceptibility to detachment severely hinder their widespread adoption in applications such as large-scale water treatment. Consequently, the development of highly selective monovalent and divalent cation membranes with superior performance, scalability, and lower cost has become a critical challenge in the field of ion separation technology.

[0003] The efficient separation of monovalent and divalent ions is primarily based on the inherent differences in charge and hydration radius. Divalent ions carry a higher charge, increasing their likelihood of adsorption or rejection by the membrane material, which in turn affects membrane permeation rate and selectivity. Furthermore, ions typically exist in solution as hydrated ions, and the size of the hydration radius directly correlates to the ease with which they cross the membrane. Therefore, the design and development of membrane materials must precisely exploit these differences, optimize membrane structure and performance, and achieve fine-tuned control of ion transport behavior, thereby achieving the dual goals of improving separation efficiency and selectivity.

[0004] Existing technologies attempt to address these issues through various approaches. For example, in patent CN109758917A, a specific polymer mixture is coated on the surface of a cation exchange membrane, utilizing surface charge regulation to achieve selective separation of monovalent and divalent ions. However, this method carries the risk of coating loss or peeling, impacting long-term stable use. Patent CN116196767A, on the other hand, utilizes a chloromethyl polymer / polyethyleneimine cross-linked layer to modify the membrane surface. While this enhances the stability of the modified layer, it also results in increased membrane resistance and decreased flux.

[0005] Polybenzimidazole (PBI) polymer has a stable polymer skeleton, and the π-π stacking between the main chains gives it a regular structure, showing great potential as a monovalent / divalent ion selective membrane material. However, the main research on PBI is currently concentrated in the field of proton exchange membranes for fuel cells. For example, in patents CN115763918A and CN117069985A, PBI, as a membrane material with outstanding stability, can achieve ion conduction through modification. However, although PBI performs well in terms of stability, its tightly stacked molecular structure limits the further improvement of ion transport performance to a certain extent. In patent CN117700741A, a branched polybenzimidazole containing sulfonic acid groups was prepared, but it still relied on phosphoric acid doping to achieve proton transport, and no side chain groups were constructed that could achieve differentiated transport of monovalent and divalent ions. Summary of the Invention

[0006] Based on the above-mentioned problems existing in the existing technology, the present invention provides a method for preparing a branched polybenzimidazole side-chain grafted mono / divalent highly selective cationic membrane. Based on the unique tight stacking and rich hydrogen bond network characteristics of polybenzimidazole (PBI), the present invention starts from the perspective of optimizing the spatial structure of the polymer, introduces branching units into the main chain, adjusts the spatial distribution of the polymer chains, constructs a unique framework structure, increases the free volume in the membrane, promotes ion transport, and improves the polymer solubility, broadening the processing and applicability. Subsequently, molecules containing sulfonic acid groups and hydrophobic side chain molecules are grafted onto the branched polybenzimidazole polymer BPBI, while improving the ion conductivity performance and achieving differentiated transmembrane transport of different ions. Through the constructed microphase separation structure, an ion membrane with excellent comprehensive selectivity and flux performance is obtained.

[0007] To achieve the above objectives, the present invention adopts the following technical solutions:

[0008] A method for preparing a branched polybenzimidazole side-chain branched monovalent / divalent highly selective cationic membrane comprises the following steps:

[0009] Step 1: preparing branched polybenzimidazole polymer BPBI;

[0010] Step 2: Reaction grafting of branched polybenzimidazole polymer BPBI with molecules containing sulfonic acid groups and hydrophobic molecules to obtain a polymer SH-BPBI containing cation exchange groups and hydrophobic side chains;

[0011] Step 3: dissolving the polymer SH-BPBI in an organic solvent to obtain a membrane solution, and then drying the solution to obtain a branched polybenzimidazole side-chain branched monovalent / divalent highly selective cationic membrane.

[0012] Furthermore, in step 1, the BPBI is prepared by polycondensation reaction of the following three types of monomers, and the degree of polymerization of the BPBI is within the range of 25 to 2500 by controlling the molar fraction of the monomers and the reaction conditions.

[0013] The first type of monomer is a tetraamine monomer, and the general structural formula is or its derivatives R1 is one of -O-, -SO2- and aliphatic or aromatic derivatives thereof. The specific structure of the tetraamine monomer is shown in any one of formula (1):

[0014]

[0015] The second type of monomer is a dicarboxyl monomer, which has the general structural formula HOOC-R2-COOH, where R2 is an aliphatic hydrocarbon group, an aromatic group, or a derivative thereof. The specific structural formula of the dicarboxyl monomer is shown in any one of formula (2):

[0016]

[0017] The third type of monomer is a branched monomer containing multiple carboxyl groups, which contains at least three carboxyl groups. The specific structural formula is shown in any one of formula (3).

[0018]

[0019] Furthermore, in step 1, the polycondensation reaction is carried out in a nitrogen atmosphere at a reaction temperature of 120-250°C for a reaction time of 0.5-48 hours, and the reaction solvent is polyphosphoric acid or Eaton's reagent. The molar amounts of the tetraamine monomer, the dicarboxyl monomer, and the branched monomer containing x carboxyl groups are set to n1, n2, and n3, respectively, and the ratio of amino groups to carboxyl groups in the monomers is controlled to be 0.5-1.5:1, that is, 4n1 / (2n2+xn3) is in the range of 0.5-1.5:1.

[0020] Furthermore, in step 2: the small molecule containing a sulfonic acid group is y is any integer not less than 1; the hydrophobic molecule may be a long-chain hydrocarbon group, such as etc., z is any integer not less than 1.

[0021] Furthermore, in step 2, the sites for grafting the sulfonic acid group and the hydrophobic side chain onto BPBI are both -NH- sites on the imidazole. The reaction temperature is 50-120°C, and the reaction time is 1-48 hours. A catalyst with an acid-binding effect, such as sodium hydride, potassium carbonate, potassium tert-butoxide, or sodium tert-butoxide, is added to the reaction. The molar ratio of BPBI to catalyst molecules is 1:0.01-10.

[0022] Furthermore, in step 2, the molar ratio of BPBI to the molecule containing a sulfonic acid group is 1:0.01-2, and the molar ratio of BPBI to the hydrophobic molecule is 1:0.01-2.

[0023] Furthermore, in step 3, the organic solvent used is a mixture of one or more of dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, etc., and the concentration of the obtained membrane solution is 1-20 wt%.

[0024] Furthermore, in step 3, the drying temperature is room temperature-150° C., and the thickness of the prepared film is 20-200 μm.

[0025] The present invention provides a branched polybenzimidazole side-chain grafted monovalent / divalent highly selective cationic membrane. By grafting onto the branched polybenzimidazole backbone, this membrane avoids the complex processes, increased membrane resistance, lower separation performance, and reduced stability associated with surface modification methods. Its beneficial effects are specifically reflected in the following aspects:

[0026] (1) The -NH- and other groups in the BPBI polymer can form a rich hydrogen bond network as ion transition sites. The π-π interaction between the aromatic rings of benzimidazole gives the polymer a regular and compact chain stacking structure, which can have a high retention effect on larger divalent ions through the size screening effect. The branched structure can not only effectively construct a framework structure and achieve molecular-level pore size regulation to form local microcavities, but its unique pore structure can also reduce the transport resistance of monovalent ions and improve the flux. In addition, the specific groups in the branched units can play a role in ion-specific recognition, constructing the transport differences of single and multivalent ions.

[0027] (2) The sulfonated groups are evenly distributed on the BPBI polymer chain, serving as cation binding and transport centers to form ion exchange sites and effectively construct selective ion transport channels. Compared with traditional polymers, the ion transport channels of BPBI are more ordered and controllable, significantly improving the ion transport path and further expanding the difference in transmembrane efficiency of different ions, thereby improving ion selectivity. At the same time, the sulfonated groups interact with the imidazole groups in the polymer, promoting the hopping transport of ions between the interaction sites, while reducing the swelling and deformation of the membrane during long-term use, thereby improving its service life and performance stability.

[0028] (3) The introduction of hydrophobic side chains onto BPBI polymers can induce microphase separation between the main chain and the side chains, forming specific ion transport channels and promoting the rapid transport of monovalent ions. At the same time, due to the higher Gibbs hydration free energy, divalent ions form a tighter hydration shell when combined with water molecules. When entering the polymer membrane containing hydrophobic side chains, they are hindered by the larger volume and stronger hydration, further expanding the difference in transmembrane energy barriers between monovalent and divalent ions and improving ion selectivity.

[0029] (4) The synthetic route of the present invention is based on liquid phase reaction, which is simple to operate and easy to control. By introducing branching units and side chain grafting methods, solubility can be improved, and processing and applicability can be broadened. Compared with traditional surface charge layer modification, the film casting by solvent casting not only reduces the membrane resistance, but also greatly simplifies the preparation process, ensuring the reliability of the ion transport channel of the membrane material during long-term use, maintaining high flux and high selectivity.

[0030] In summary, compared with the existing technology, the present invention has prepared a new type of ion separation membrane polymer - side-chain branched polybenzimidazole. The homogeneous cation exchange membrane prepared based on it has good solubility, stability, excellent flux and monovalent and divalent ion selectivity, and is expected to achieve large-scale industrial production and application in the fields of monovalent and multivalent ion separation membranes. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the membrane material prepared in Example 1 of the present invention.

[0032] Figure 2 This is the result of hydrogen nuclear magnetic resonance spectrum characterization of the branched polymer BPBI prepared in Example 3 of the present invention.

[0033] Figure 3 This is the nuclear magnetic resonance hydrogen spectrum characterization result of the side-chain grafted branched polymer SH-BPBI prepared in Example 3 of the present invention.

[0034] Figure 4 2 is a diagram of an ion selectivity testing device used in an embodiment of the present invention.

[0035] Figure 5 These are the monovalent and divalent ion selectivity and flux of the membranes prepared in Examples 1-6 of the present invention and the comparative example.

[0036] Figure 6 This is a test of the ion selectivity stability of the membrane prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0037] The technical solutions of the present invention are further illustrated by examples below. These technical solutions are only intended to make the present invention easier to understand by researchers in the field, and are not intended to limit the scope of protection of the present invention. All variations and extensions made using the present invention are within the scope of protection of the present invention.

[0038] 1. Preparation of monovalent / divalent selective cation membrane

[0039] Example 1

[0040] The preparation steps of a branched polybenzimidazole side-chain branched mono / divalent highly selective cationic membrane provided in this embodiment are as follows:

[0041] Preparation of BPBI branched polymer: 500 g of polyphosphoric acid was weighed into a round-bottom three-necked flask, and 0.1 mol of 3,3'-diaminobenzidine, 0.09 mol of 4,4'-dicarboxydiphenyl ether, and 0.0033 mol of trimesic acid were added. After stirring to dissolve, the mixture was reacted at 200°C under a nitrogen atmosphere for 2 h. The reactant was repeatedly washed with water until neutral and dried to obtain a branched polybenzimidazole polymer BPBI (polymer repeating unit molecular weight approximately 400).

[0042] Preparation of polymer SH-BPBI containing cation exchange groups and hydrophobic side chains: 4 g BPBI (0.01 mol) product was dissolved in 76 g DMSO solution, 0.02 g NaH was added, and after stirring to dissolve, 0.6107 g 1,3-propane sultone (0.005 mol) and 0.4990 g (0.001 mol) perfluorooctane bromide were added. The reaction was carried out at 60°C for 12 h. After the reaction, the crude product was repeatedly washed with water until the washing liquid was neutral, and the product was dried to obtain polymer SH-BPBI.

[0043] Membrane preparation: 1g of SH-BPBI was placed in a round-bottom flask, and 19g of dimethyl sulfoxide solution was added. The solution was stirred and dissolved to obtain a membrane solution. The membrane solution was cast on a glass plate and dried at 80°C for 24 hours to remove the solvent and form a membrane. A branched polybenzimidazole side-chain branched mono / divalent highly selective cationic membrane was obtained, which was named SH-BPBI-1. The prepared membrane material is as follows Figure 1 shown.

[0044] Use Figure 4 The device shown tests the Li + / Mg 2+ Ion selectivity and flux, tested at 5 mA cm -2 , the test results are as follows Figure 5 The ion separation stability of SH-BPBI-1 was tested repeatedly, and the test results were as follows. Figure 6 shown.

[0045] Example 2

[0046] The preparation steps of a branched polybenzimidazole side-chain branched mono / divalent highly selective cationic membrane provided in this embodiment are as follows:

[0047] Preparation of BPBI branched polymer: The preparation method is the same as that in Example 1, except that the branching monomer trimesic acid is replaced by 1,3,5-tris(4-carboxyphenyl)benzene.

[0048] Preparation of polymer SH-BPBI containing cation exchange groups and hydrophobic side chains: The same preparation method as in Example 1 was used.

[0049] Preparation of membrane: The same preparation method as in Example 1 was adopted, and the prepared membrane material was named SH-BPBI-2.

[0050] Use Figure 4 The device shown tests the Li + / Mg 2+ Ion selectivity and flux, tested at 5 mA cm -2 , the test results are as follows Figure 5 shown.

[0051] Example 3

[0052] The preparation steps of a branched polybenzimidazole side-chain branched mono / divalent highly selective cationic membrane provided in this embodiment are as follows:

[0053] Preparation of BPBI branched polymer: The preparation method is the same as that in Example 1, except that the branching monomer trimesic acid is replaced by 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine.

[0054] Preparation of polymer SH-BPBI containing cation exchange groups and hydrophobic side chains: The same preparation method as in Example 1 was used.

[0055] Preparation of membrane: The same preparation method as in Example 1 was adopted, and the prepared membrane material was named SH-BPBI-3.

[0056] The structures of the BPBI polymer and the side-chain grafted SH-BPBI polymer prepared in this example were characterized by hydrogen nuclear magnetic resonance spectroscopy. Figure 2 and 3 shown.

[0057] Use Figure 4 The device shown tests the Li + / Mg 2+ Ion selectivity and flux, tested at 5 mA cm -2 , the test results are as follows Figure 5 shown.

[0058] Example 4

[0059] The preparation steps of a branched polybenzimidazole side-chain branched mono / divalent highly selective cationic membrane provided in this embodiment are as follows:

[0060] Preparation of BPBI branched polymer: The preparation method is the same as that in Example 1, except that the branching monomer trimesic acid is replaced by 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine.

[0061] Preparation of polymer SH-BPBI containing cation exchange groups and hydrophobic side chains: The preparation method was the same as that in Example 1, except that the mass of perfluorooctyl bromide added was changed from 0.4990 g (0.001 mol) to 0.

[0062] Preparation of membrane: The same preparation method as in Example 1 was adopted, and the prepared membrane material was named SH-BPBI-4.

[0063] Use Figure 4 The device shown tests the Li + / Mg 2+ Ion selectivity and flux, tested at 5 mA cm -2 , the test results are as follows Figure 5 shown.

[0064] Example 5

[0065] The preparation steps of a branched polybenzimidazole side-chain branched mono / divalent highly selective cationic membrane provided in this embodiment are as follows:

[0066] Preparation of BPBI branched polymer: The preparation method is the same as that in Example 1, except that the branching monomer trimesic acid is replaced by 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine.

[0067] Preparation of polymer SH-BPBI containing cation exchange groups and hydrophobic side chains: The preparation method was the same as that in Example 1, except that the mass of perfluorooctane bromide added was changed from 0.4990 g (0.001 mol) to 0.2495 g (0.0005 mol).

[0068] Preparation of membrane: The same preparation method as in Example 1 was adopted, and the prepared membrane material was named SH-BPBI-5.

[0069] Use Figure 4 The device shown tests the Li + / Mg 2+ Ion selectivity and flux, tested at 5 mA cm -2 , the test results are as follows Figure 5 shown.

[0070] Example 6

[0071] The preparation steps of a branched polybenzimidazole side-chain branched mono / divalent highly selective cationic membrane provided in this embodiment are as follows:

[0072] Preparation of BPBI branched polymer: The preparation method is the same as that in Example 1, except that the branching monomer trimesic acid is replaced by 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine.

[0073] Preparation of polymer SH-BPBI containing cation exchange groups and hydrophobic side chains: The preparation method was the same as that in Example 1, except that the mass of perfluorooctane bromide added was changed from 0.0499 g (0.001 mol) to 0.9980 g (0.002 mol).

[0074] Preparation of membrane: The same preparation method as in Example 1 was adopted, and the prepared membrane material was named SH-BPBI-6.

[0075] Use Figure 4 The device shown tests the Li + / Mg 2+ Ion selectivity and flux, tested at 5 mA cm -2 , the test results are as follows Figure 5 shown.

[0076] Comparative Example

[0077] This comparative example prepared a cationic membrane as a comparison according to the following steps:

[0078] Preparation of PBI polymer: Weigh 500 g of polyphosphoric acid into a round-bottom three-necked flask, add 0.1 mol of 3,3'-diaminobenzidine and 0.1 mol of 4,4'-dicarboxydiphenyl ether, stir to dissolve, react at 200°C under nitrogen atmosphere for 2 h, then repeatedly wash the reactant with water until neutral, and dry to obtain PBI polymer.

[0079] Preparation of polymer SH-PBI containing cation exchange groups and hydrophobic side chains: 4 g PBI (0.01 mol) product was dissolved in 76 g DMSO solution, 0.02 g NaH was added, and after stirring to dissolve, 0.6107 g 1,3-propane sultone (0.005 mol) and 0.0499 g (0.0001 mol) perfluorooctane bromide were added. The reaction was carried out at 60°C for 12 h. After the reaction, the crude product was repeatedly washed with water until the washing liquid was neutral, and the product was dried to obtain polymer SH-PBI.

[0080] Preparation of membrane: The same preparation method as in Example 1 was used, and the prepared membrane material was named SH-PBI.

[0081] Use Figure 4 The device shown tests the Li + / Mg 2+ Ion selectivity and flux, tested at 5 mA cm -2 , the test results are as follows Figure 5 shown.

[0082] 2. Ion Selectivity and Flux Testing

[0083] Use Figure 4 The device shown is at 5.0 mA cm -2 The electric drive permeation test was carried out at a current density of 100 nm to test the Li + / Mg 2+ Ion selectivity and flux testing. The specific method is:

[0084] The device is divided into four chambers using three membranes: cathode chamber | concentration chamber | desalination chamber | cathode chamber (anode). The membranes between the cathode and concentration chambers, and between the anode and desalination chambers, are commercial anion exchange membranes (AGUs). Their primary purpose is to separate the cathode chamber solution from the internal chamber solution. The membrane to be tested is located between the desalination chamber and the concentration chamber. 100 mL of 0.3 M Na2SO4 solution was added to the cathode chamber, 100 mL of LiCl / MgCl2 solution (both LiCl and MgCl2 concentrations were 0.1 M) was added to the desalination chamber, and 100 mL of 0.01 M KCl solution was added to the concentration chamber. All solutions were circulated by a peristaltic pump at a constant flow rate of 80 mL min. -1 The Li in the concentration chamber was determined by inductively coupled plasma technology after 1 h of electrodialysis. + and Mg 2+ The concentrations of C t (Li + ), C t (Mg 2+ ). Ion selectivity P=J(Li + )·C0(Li + ) / J(Mg 2+ )·C0(Mg 2+ ), Li + Flux J(Li + )=C t (Li + )V / At,Mg 2+ Flux J(Mg 2+ )=C t (Mg 2+ )V / At, where V is the volume of the solution in the concentration chamber, A is the effective membrane area, t is the test time, and the initial Li + and Mg 2+The concentrations of C0(Li + ) and C0(Mg 2+ ).

[0085] like Figure 5 As shown in the figure: the ion flux of SH-PBI prepared in the comparative example is low. This is because PBI itself has a tightly stacked structure and ions need to overcome greater resistance to achieve transmembrane transport. Different branching units were added in Examples 1-3 to prepare branched polymers SH-BPBI-1, SH-BPBI-2, and SH-BPBI-3. The addition of branching units increased the ion flux and improved the selectivity, which is due to the better segment mobility, expanded free volume and unique framework structure of the branched polymer for smaller Li + It provides a fast transport channel. BPBI also retains a compact structure, which is very suitable for Mg 2+ Still maintain a high retention effect. Therefore, the branched polymer SH-BPBI can achieve a simultaneous improvement in selectivity and flux compared to SH-PBI. In Examples 3 to 6, on the basis of the same branched polymer, molecules containing sulfonic acid monomers were grafted, and hydrophobic side chains of different proportions were introduced. The grafting of hydrophobic side chains will induce microphase separation between the main chain and the side chain, forming a specific ion transport channel, optimizing the ion transport path, and making the ion transport in the membrane more orderly and efficient. In addition, the Gibbs hydration free energy of different ions in the solution is also different, Mg 2+ The ion hydration energy is much greater than Li + , more water molecules need to be removed and a higher energy barrier needs to be overcome before they can enter the membrane, which increases the difficulty of entering the membrane and further differentiates Li + and Mg 2+ SH-BPBI has excellent mechanical properties and structural stability, and can maintain extremely high stability in long-term testing. Figure 6 As shown, through multiple tests of ion selectivity, it was found that the flux of ions and Li + / Mg 2+ Selectivity remains at a high level.

[0086] The method of the present invention is simple in process and easy to implement on a large scale. In addition, the prepared membrane material has outstanding performance and shows great application potential and broad market prospects in multiple fields such as seawater desalination, precision separation, and energy development.

Claims

1. A method for preparing a branched polybenzimidazole side chain branched mono / divalent highly selective cationic membrane, characterized in that: The steps include: Step 1: preparing branched polybenzimidazole polymer BPBI; Step 2: Reaction grafting of branched polybenzimidazole polymer BPBI with molecules containing sulfonic acid groups and hydrophobic molecules to obtain a polymer SH-BPBI containing cation exchange groups and hydrophobic side chains; Step 3: dissolving the polymer SH-BPBI in an organic solvent to obtain a membrane solution, and then drying the solution to obtain a branched polybenzimidazole side-chain branched monovalent / divalent highly selective cationic membrane.

2. The preparation method according to claim 1, wherein: In step 1, the BPBI is prepared by polycondensation of a tetraamine monomer, a dicarboxyl monomer and a branched monomer containing multiple carboxyl groups, and the degree of polymerization of the BPBI is in the range of 25 to 2500.

3. The preparation method according to claim 1, wherein: The general structural formula of the tetraamine monomer is or its derivatives R1 is one of -O-, -SO2- and aliphatic or aromatic derivatives thereof; the general structural formula of the dicarboxyl monomer is HOOC-R2-COOH, R2 is one of aliphatic hydrocarbon group, aromatic group and derivatives thereof; the branched monomer containing multiple carboxyl groups contains at least three carboxyl groups.

4. The preparation method according to claim 3, characterized in that The specific structural formula of the tetraamine monomer is shown in any one of formula (1): The specific structural formula of the dicarboxyl monomer is shown in any one of formula (2): The specific structural formula of the branched monomer containing multiple carboxyl groups is shown in any one of formula (3):

5. The preparation method according to claim 2, wherein the polycondensation reaction is carried out in a nitrogen atmosphere, the reaction temperature is 120-250° C., the reaction time is 0.5-48 hours, polyphosphoric acid or Eaton's reagent is selected as the reaction solvent, the molar amounts of the tetraamine monomer, the dicarboxyl monomer, and the branched monomer containing x carboxyl groups are n1, n2, and n3, respectively, and the ratio of the amino group to the carboxyl group of the monomer is controlled to be 0.5-1.5:1, that is, the range of 4n1 / (2n2+xn3) is 0.5-1.5:

1.

6. The preparation method according to claim 1, wherein: In step 2, the molecule containing the sulfonic acid group is y is any integer not less than 1; the hydrophobic molecule is z is any integer not less than 1.

7. The preparation method according to claim 1, wherein: In step 2, the sites for the reaction grafting of the sulfonic acid group and the hydrophobic side chain on BPBI are both -NH- sites on the imidazole, the reaction temperature is 50-120°C, the reaction time is 1-48h, and a catalyst with an acid-binding effect is added during the reaction, with a molar ratio of BPBI to catalyst molecules of 1:0.01-10.

8. The preparation method according to claim 1 or 7, characterized in that: In step 2, the molar ratio of BPBI to the molecule containing a sulfonic acid group is 1:0.01-2, and the molar ratio of BPBI to the hydrophobic molecule is 1:0.01-2.

9. The preparation method according to claim 1, wherein: In step 3, the organic solvent used is a mixture of one or more of dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone. The concentration of the obtained membrane solution is 1-20wt%, the drying temperature is room temperature-150°C, and the thickness of the prepared membrane is 20-200μm.

10. A branched polybenzimidazole side-chain branched mono- / divalent highly selective cationic membrane prepared by the preparation method according to any one of claims 1 to 9.

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