A method for preparing anionic membrane with asymmetric structure

Abstract

The present invention discloses a method for preparing an anion exchange membrane with an asymmetric structure. A positively charged anion exchange membrane material and a water-soluble polymer additive are dissolved in an organic solvent to form a casting solution, which is then coated on a substrate. The asymmetric anion exchange membrane is then obtained by inducing phase separation through solvent evaporation and immersing in water. This method overcomes the common drawbacks of traditional non-solvent phase transformation methods for preparing asymmetric anion exchange membranes, such as the inability to effectively control the position, number, and distribution of functional groups and the membrane microphase separation structure. The resulting asymmetric anion exchange membrane also has low surface resistance, meeting the demand for high-performance anion exchange membranes in processes centered on anion exchange membranes, such as diffusion dialysis and electrodialysis.

Description

Technical Field

[0001] The present invention belongs to the field of membrane technology, and specifically relates to a method for preparing an asymmetric ion exchange membrane. This asymmetric anion exchange membrane can be used in application fields such as diffusion dialysis, electrodialysis, and electro-nanofiltration with anion exchange membrane as the core process. Background Art

[0002] It is well known that the thickness, surface properties, and pore structure of a membrane's separation layer significantly influence its flux. Generally speaking, thinner separation layers increase flux, but excessively thin separation layers often lead to decreased mechanical properties. Asymmetric separation membranes, comprising a porous support layer and a dense separation skin structure, are an important approach to addressing these issues. This method has long been widely used to prepare high-performance separation membrane products of various types and applications, such as gas separation membranes, nanofiltration membranes, ultrafiltration membranes, and reverse osmosis membranes.

[0003] In the field of anion exchange membranes, in order to pursue high membrane flux, researchers have also made a lot of attempts in the field of preparing asymmetric anion exchange membranes. The currently reported method for preparing asymmetric anion exchange membranes is mainly the non-solvent phase inversion method (NIPS). For example: You Xinqiang et al. dissolved chloromethylated polyethersulfone in an organic solvent to form a uniform casting solution, coated the casting solution on the substrate, and then prepared a chloromethylated polyethersulfone porous base membrane by a solvent-free phase inversion method. After the porous base membrane was soaked in an ethylenediamine solution for cross-linking modification, the porous cross-linked membrane was immersed in a 1-methylimidazole solution for quaternization, thereby obtaining an asymmetric anion exchange membrane (Chinese patent application, application number: 202210277908.1). Liao Xiafeng et al. (Chinese patent application, application number: 202110373709.6) dissolved chloromethylated polyethersulfone in an organic solvent to form a casting solution, then coated the solution on a substrate. A porous base membrane was then prepared by a phase inversion method. The porous base membrane was then immersed in a pentamethyldiethylenetriamine solution for simultaneous crosslinking and quaternization modification, thereby producing a porous cross-linked anion exchange membrane suitable for diffusion dialysis. Wang Huanting et al. (Chinese patent application, application number: 201680064936.8) invented a method for preparing asymmetric porous ion exchange membranes based on ultrafiltration membranes through a post-modification strategy. However, all of the above preparation methods have the following common problems: starting from a polymer precursor, an asymmetric structure is first prepared through the NIPS method, and then functional groups are introduced through a post-modification strategy. This preparation route generally has the following disadvantages, namely, the number and position of functional groups are difficult to accurately control, and it is difficult to form a good microphase separation structure. It is well known that the control of these parameters has a decisive influence on the performance of ion exchange membranes, especially their ion conductivity. Therefore, the development of a new, simple, and precisely controllable method for preparing asymmetric ion exchange membranes with microphase separation structures is of great significance for their practical applications and the innovation of related technologies in this field. Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing an anion exchange membrane with an asymmetric structure.

[0005] To achieve the above tasks, the present invention adopts the following technical solutions:

[0006] A method for preparing an asymmetric anion exchange membrane is characterized by selecting a positively charged anion exchange membrane material as a substrate, using a water-soluble polymer as an additive, and selecting an organic solvent that is compatible with the positively charged anion exchange membrane material and the additive, and preparing the asymmetric anion exchange membrane according to the following steps:

[0007] 1) First, a positively charged anion exchange membrane material is fully stirred and dissolved in an organic solvent to obtain a first casting solution L-1;

[0008] 2) Adding an appropriate proportion of additives to the first casting solution L-1, stirring and dissolving the mixture to obtain a second casting solution L-2;

[0009] 3) filtering, vacuum or ultrasonically degassing the second casting solution L-2 to obtain a third casting solution L-3;

[0010] 4) coating the third L-3 on the substrate, evaporating the solvent at a suitable temperature, and finally fully soaking it in water to obtain an asymmetric anion exchange membrane.

[0011] According to the present invention, the positively charged anion exchange membrane material is a polymer material containing positively charged functional groups, the main chain of the polymer structure is one or more of polyarylethersulfone, polyaryletherketone, polyphenylene oxide, polystyrene, polyvinyl chloride, polyimide, polyamide, polyurethane, polyolefin, and polybenzimidazole, and the positively charged functional group is one or more of quaternary ammonium and heterocyclic cations. The specific chemical structure is as follows:

[0012]

[0013] Among them, the positively charged functional group is connected to the polymer main chain through R1, and R1, R2, R3, and R4 respectively represent groups with a fatty chain structure composed of 0-6 C and O skeleton atoms.

[0014] Specifically, the organic solvent is any one or more of polar aprotic solvents such as N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) or N-methylpyrrolidone (NMP); and has the following characteristics:

[0015] (1) Good solubility for positively charged anion exchange membrane materials and additives;

[0016] (2) It has a high boiling point, which is beneficial to controlling the phase conversion rate of anion exchange membrane materials during solvent evaporation.

[0017] Furthermore, the water-soluble polymer is one or more of natural water-soluble polymers such as starches, proteins, alginates, gelatin, etc., semi-synthetic water-soluble polymers such as carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, etc., and synthetic water-soluble polymers such as polyacrylamide, polyacrylic acid and salts, polyvinyl pyrrolidone, polyvinyl alcohol, polymaleic anhydride, polyethers (polyether polyols), polyquaternary ammonium salts, polyethylene glycol, polyether amine, polyamide, polyethyleneimine, etc., and its mass average molecular weight is between 200-100000.

[0018] The concentration of the first casting solution L-1 is 5wt% to 40wt%; the mass ratio of the additive to the positively charged anion exchange membrane material is 0.05:1 to 2:1.

[0019] The substrate is a metal plate, a glass plate, a ceramic, a polymer plate or a fabric.

[0020] The temperature range for evaporating the solvent is 25° C. to 180° C., the evaporation time is 1 hour to 48 hours, and the soaking time in water is 1 hour to 120 hours.

[0021] The method for preparing an asymmetric structure anion exchange membrane of the present invention, by taking advantage of the differences in physical and chemical properties between the anion exchange membrane material and the water-soluble polymer additive and the good compatibility between the two, realizes the preparation of an asymmetric structure anion exchange membrane with an ultra-thin separation layer and a uniform large-pore support layer for the first time through solvent evaporation induction and water etching steps of the additive.

[0022] Compared with the prior art, the technical innovation of the present invention lies in:

[0023] 1. This method is universal, simple, easy to operate, low-cost and easy to apply on a large scale in industry.

[0024] 2. It overcomes the defects commonly faced by the traditional non-solvent phase conversion method for preparing asymmetric anion exchange membranes, such as the inability to effectively control the position, number, distribution of functional groups and the membrane microphase separation structure.

[0025] 3. The obtained asymmetric structure anion exchange membrane has low membrane surface resistance, which can meet the demand for high-performance anion exchange membranes in process processes such as diffusion dialysis and electrodialysis with anion exchange membranes as the core. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1These are morphology diagrams of the asymmetric anion exchange membrane prepared in Example 1, where (a) shows the surface morphology of the dense layer and (b) shows the overall cross-sectional morphology of the membrane.

[0027] Figure 2 These are morphology diagrams of the asymmetric anion exchange membrane prepared in Example 2, where (a) shows the surface morphology of the dense layer and (b) shows the overall cross-sectional morphology of the membrane.

[0028] Figure 3 These are morphology diagrams of the asymmetric anion exchange membrane prepared in Example 3, where (a) shows the surface morphology of the dense layer and (b) shows the overall cross-sectional morphology of the membrane.

[0029] Figure 4 These are morphology diagrams of the asymmetric anion exchange membrane prepared in Example 4, where (a) shows the surface morphology of the dense layer and (b) shows the overall cross-sectional morphology of the membrane.

[0030] Figure 5 These are morphology diagrams of the asymmetric anion exchange membrane prepared in Example 5, where (a) shows the surface morphology of the dense layer and (b) shows the overall cross-sectional morphology of the membrane.

[0031] Figure 6 These are morphology diagrams of the asymmetric anion exchange membrane prepared in Example 6, where (a) shows the surface morphology of the dense layer and (b) shows the overall cross-sectional morphology of the membrane.

[0032] Figure 7 These are morphology diagrams of the asymmetric anion exchange membrane prepared in Example 7, where (a) shows the surface morphology of the dense layer and (b) shows the overall cross-sectional morphology of the membrane.

[0033] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. DETAILED DESCRIPTION

[0034] In the following embodiments, the positively charged anion exchange membrane material (P) is a polymer material containing positively charged functional groups. The main chain structure of these polymer materials is mainly one or more of polyarylethersulfone, polyaryletherketone, polyphenylene oxide, polystyrene, polyvinyl chloride, polyimide, polyamide, polyurethane, polyolefin, and polybenzimidazole. The positively charged functional groups can be one or more of quaternary ammonium, cyclic ammonium, or imidazole (heterocyclic) cations. The structural formula is as follows:

[0035]

[0036] Among them, the positively charged functional group is connected to the polymer main chain through R1, and R1, R2, R3, and R4 respectively represent groups with a fatty chain structure composed of 0-6 C and O skeleton atoms.

[0037] In the following embodiments, one or more of a quaternary ammonium group, a pyrrolidine ammonium group, a morpholine ammonium group, a piperidine ammonium group, a piperazine ammonium group, a thiomorpholine ammonium group, and an imidazolium cationic group are preferred;

[0038] The additives include one or more of natural water-soluble polymers such as starch, protein, alginic acid, gelatin, etc.; semi-synthetic water-soluble polymers such as carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, etc.; and synthetic water-soluble polymers such as polyacrylamide, polyacrylic acid and salts, polyvinyl pyrrolidone, polyvinyl alcohol, polymaleic anhydride, polyethers (polyether polyols), polyquaternary ammonium salts, polyethylene glycol, polyetheramine, polyamide, polyethyleneimine, etc., and the mass average molecular weight is between 200 and 100,000.

[0039] The solvent is an organic matter that has good solubility in the positively charged anion exchange membrane material and the additive.

[0040] The organic solvent is mainly any one or more polar aprotic solvents such as N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) or N-methylpyrrolidone (NMP). And has the following characteristics:

[0041] (1) Good solubility for positively charged anion exchange membrane materials and additives;

[0042] (2) It has a high boiling point, which is beneficial to controlling the phase conversion rate of anion exchange membrane materials during solvent evaporation.

[0043] This example provides a method for preparing an anion exchange membrane with an asymmetric structure, and the steps are as follows:

[0044] Step 1: Dissolve the positively charged anion exchange membrane material in the polar aprotic solvent to prepare a first casting solution L-1 with a mass fraction of 5 wt% to 40 wt%. The concentration of the first casting solution L-1 is preferably 5 wt% to 20 wt%.

[0045] Step 2: Add additives to the first casting solution L-1.

[0046] Additives have the following requirements:

[0047] First, the solubility in polar aprotic solvents is much greater than that of positively charged anion exchange membrane materials; this allows for rapid phase transformation of the positively charged anion exchange membrane material on the surface during dissolution and evaporation, rather than additives.

[0048] Secondly, the additives contain charged hydrophilic groups such as carboxylic acid, sulfonic acid, phosphoric acid, and quaternary ammonium groups, or contain hydroxyl, amino, amide, and other groups or elements that are easy to form hydrogen bond donors or acceptors. They can form certain interactions with the positively charged anion exchange membrane material molecules through electrostatic effects or hydrogen bonds, which is beneficial to the highly uniform dispersion of the positively charged anion exchange membrane material and the additives in the solvent, and further beneficial to the formation of a relatively uniform microphase separation structure during the solvent evaporation phase transformation process.

[0049] Third, the differences in the physical and chemical properties of the additive and the positively charged anion exchange membrane material facilitate microphase separation between the positively charged anion exchange membrane material and the additive molecules during the later stages of solvent evaporation. Fourth, the additive has good solubility in water and can be removed by subsequent immersion in aqueous solution, further forming a uniform asymmetric macroporous structure.

[0050] In this embodiment, the additive can be selected from water-soluble polymers, preferably any one or more of carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, polyacrylamide, polyvinyl pyrrolidone, polyvinyl alcohol, polyethers (polyether polyols), polyquaternary ammonium salts, polyethylene glycol, polyetheramine, polyamide, etc., and the mass average molecular weight of the additive is preferably between 200 and 60,000.

[0051] The mass ratio of the additive to the positively charged anion exchange membrane material is 0.05:1 to 2:1, preferably 0.1:1 to 1.8:1. The specific addition amount can be adjusted according to the chemical structure of the positively charged anion exchange membrane material and the physicochemical structure of various types of additives.

[0052] After the additives are added, they are stirred at 20° C. to 80° C. for 1 to 24 hours until they are completely dissolved to obtain the second casting solution L-2.

[0053] Step 3: Filter the second casting liquid L-2 and perform vacuum or ultrasonic degassing for 1 minute to 24 hours to obtain the third casting liquid L-3.

[0054] Step 4: Uniformly apply the third casting solution L-3 to a substrate selected from the group consisting of a metal plate, a glass plate, a ceramic plate, a polymer plate, or a fabric. Evaporate the solution at 25°C to 150°C for 1 to 48 hours, preferably at 50°C to 100°C for 3 to 24 hours. The key objectives of solvent evaporation are to achieve phase inversion of the positively charged anion exchange membrane material (forming a dense separation layer and a preliminary asymmetric structure) and microphase separation between the positively charged anion exchange membrane material and the additive molecules.

[0055] Step 5: After the solvent evaporation step, the substrate membrane is immersed in water at room temperature for 1 to 120 hours. Preferably, the immersion time is 1 to 48 hours. The primary purpose of this thorough immersion in water is to remove additive molecules from the membrane and form a uniform, asymmetric, macroporous support layer structure.

[0056] The following are specific embodiments given by the inventor.

[0057] Example 1:

[0058] This embodiment provides a method for preparing an asymmetric anion exchange membrane using a polymer: The embodiment starts with phenolphthalein polyarylethersulfone containing pyrrolidine cyclic ammonium groups, and the polymer structure is as follows:

[0059]

[0060] 0.5 g of the above polymer was dissolved in 5 mL of the organic solvent DMAc, and stirred with a magnetic stirrer at room temperature for 1 h to obtain the first casting solution L-1.

[0061] 0.3 g of polypropylene glycol 400 was weighed and added to the first casting solution L-1, and the mixture was stirred and dissolved at room temperature for 6 h to obtain the second casting solution L-2.

[0062] The second casting liquid L-2 was filtered using a G2 sand core funnel and vacuum degassed for 30 minutes to obtain a third casting liquid L-3.

[0063] The third casting membrane L-3 liquid was uniformly poured onto a glass plate and dried in an oven at 100° C. for 6 h. After the solvent evaporated, the glass plate was immersed in deionized water for 12 h to obtain an asymmetric anion exchange membrane.

[0064] Morphology test of the asymmetric structure anion exchange membrane: the morphology of the membrane was observed using a scanning electron microscope.

[0065] The surface resistance test of the asymmetric structure anion exchange membrane was conducted using a self-made four-cell device.

[0066] The surface morphology of the dense layer of the asymmetric anion exchange membrane prepared in this example is as follows: Figure 1 As shown in (a), Figure 1 (b) shows the overall cross-sectional morphology of the asymmetric anion exchange membrane. It can be seen that the asymmetric anion exchange membrane prepared in this example exhibits a distinct asymmetric structure. The cross-sectional view reveals that the porous layer is relatively uniform and sponge-like. The measured surface resistance of the asymmetric anion exchange membrane is 2.28Ω·cm. 2 .

[0067] Example 2:

[0068] This example uses the same polymer as Example 1. The difference is that the additive is fatty alcohol polyoxyethylene ether (Mw ~ 315, CAS: 68131-39-5), and the added mass is 0.5g. The conditions are the same as those in Example 1. The morphology characterization method and the asymmetric anion exchange membrane surface resistance test method of the asymmetric anion exchange membrane prepared in this example are the same as those in Example 1. The measured surface resistance of the asymmetric anion exchange membrane is 2.08Ω·cm 2 .

[0069] The surface morphology of the dense layer of the asymmetric anion exchange membrane prepared in this example is as follows: Figure 2 As shown in (a), Figure 2 (b) is the overall cross-sectional morphology of the asymmetric anion exchange membrane.

[0070] Example 3:

[0071] This example uses the same polymer as Example 1. The difference is that the additive is polyethylene glycol 1000, and the added mass is 0.7g. The conditions are the same as those in Example 1. The morphology characterization method and the asymmetric anion exchange membrane surface resistance test method of the asymmetric anion exchange membrane prepared in this example are the same as those in Example 1. The measured surface resistance of the asymmetric anion exchange membrane is 1.44Ω·cm 2 .

[0072] The surface morphology of the dense layer of the asymmetric anion exchange membrane prepared in this example is as follows: Figure 3 As shown in (a), Figure 3 (b) is the overall cross-sectional morphology of the asymmetric anion exchange membrane.

[0073] Example 4:

[0074] The difference between this embodiment and embodiment 3 is that the quaternized polymer used in this embodiment contains morpholine cyclic ammonium groups, and its specific structural formula is:

[0075]

[0076] The additive used was polyethylene glycol 600, with an added mass of 0.7 g. The conditions were the same as those in Example 3. The morphology characterization method and the surface resistance test method of the asymmetric anion exchange membrane prepared in this example were the same as those in Example 1. The measured surface resistance of the asymmetric anion exchange membrane was 0.58 Ω·cm 2 .

[0077] The surface morphology of the dense layer of the asymmetric anion exchange membrane prepared in this example is as follows: Figure 4 As shown in (a), Figure 4(b) is the overall cross-sectional morphology of the asymmetric anion exchange membrane.

[0078] Example 5:

[0079] The difference between this embodiment and embodiment 1 is that this embodiment starts from polyphenylene ether containing quaternary ammonium groups, whose ion exchange capacity is 1.5 mmol / g, and whose polymer structure is as follows:

[0080]

[0081] The additive used was polyoxyethylene polyoxypropylene ether (L43), and the added mass was 0.5 g. The conditions were the same as those in Example 1.

[0082] The morphology characterization method and the surface resistance test method of the asymmetric anion exchange membrane prepared in this example are the same as those in Example 1. The measured surface resistance of the asymmetric anion exchange membrane is 4.01Ω·cm 2 .

[0083] The surface morphology of the dense layer of the asymmetric anion exchange membrane prepared in this example is as follows: Figure 5 As shown in (a), Figure 5 (b) is the overall cross-sectional morphology of the asymmetric anion exchange membrane.

[0084] Example 6:

[0085] The difference between this embodiment and embodiment 1 is that this embodiment starts from polyarylethersulfone containing quaternary ammonium groups, whose ion exchange capacity is 1.3 mmol / g, and whose polymer structure is as follows:

[0086]

[0087] The additive used was polyetheramine ED-600, with an added mass of 0.5 g. The conditions were the same as those in Example 1. The morphology characterization method and the surface resistance test method of the asymmetric anion exchange membrane prepared in this example were the same as those in Example 1. The measured surface resistance of the asymmetric anion exchange membrane was 5.18 Ω·cm 2 .

[0088] The surface morphology of the dense layer of the asymmetric anion exchange membrane prepared in this example is as follows: Figure 6 As shown in (a), Figure 6 (b) is the overall cross-sectional morphology of the asymmetric anion exchange membrane.

[0089] Example 7:

[0090] The difference between this embodiment and embodiment 1 is that this embodiment starts from phenolphthalein polyarylethersulfone containing piperidine cyclic ammonium groups, and its polymer structure is as follows:

[0091]

[0092] The additive used was polyvinyl pyrrolidone K12, with an added mass of 0.3 g. The conditions were the same as in Example 1. The morphology characterization method and the surface resistance test method for the asymmetric anion exchange membrane prepared in this example were the same as in Example 1. The measured surface resistance of the asymmetric anion exchange membrane was 2.27 Ω·cm. 2 .

[0093] The surface morphology of the dense layer of the asymmetric anion exchange membrane prepared in this example is as follows: Figure 7 As shown in (a), Figure 7 (b) is the overall cross-sectional morphology of the asymmetric anion exchange membrane.

Claims

1. A method for preparing an anion exchange membrane with an asymmetric structure, characterized in that: A positively charged anion exchange membrane material is selected as a base material, a water-soluble polymer is used as an additive, and an organic solvent that is compatible with the positively charged anion exchange membrane material and the additive is selected. The asymmetric anion exchange membrane is prepared according to the following steps: 1) First, the positively charged anion exchange membrane material is fully stirred and dissolved in an organic solvent to obtain a first casting solution L-1; 2) Adding an appropriate proportion of additives to the first casting solution L-1, stirring and dissolving the additives thoroughly to obtain a second casting solution L-2; 3) filtering, vacuuming or ultrasonically degassing the second casting solution L-2 to obtain a third casting solution L-3; 4) coating the third L-3 on the substrate, evaporating the solvent at a suitable temperature, and finally fully soaking it in water to obtain an asymmetric anion exchange membrane; The positively charged anion exchange membrane material is a polymer material containing positively charged functional groups, the polymer main chain of which is one or more of polyarylethersulfone, polyaryletherketone, polyphenylene oxide, polystyrene, polyvinyl chloride, polyimide, polyamide, polyurethane, polyolefin, and polybenzimidazole, and the positively charged functional groups are one or more of quaternary ammonium and heterocyclic cations. The specific chemical structure is shown below: The positively charged functional group is connected to the polymer main chain through R1, and the groups represented by R1, R2, R3, and R4 are respectively fatty chain structures composed of 0-6 C and O skeleton atoms, and the lengths of the groups represented by R1, R2, R3, and R4 are not 0; The organic solvent is any one or more of N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide or N-methylpyrrolidone polar aprotic solvents; and has the following characteristics: (1) Good solubility for positively charged anion exchange membrane materials and additives; (2) It has a high boiling point, which is beneficial to the control of the phase conversion rate of the positively charged anion exchange membrane material during the solvent evaporation process; The water-soluble polymer is a natural water-soluble polymer, a semi-synthetic water-soluble polymer, or a synthetic water-soluble polymer, and has a mass average molecular weight between 200 and 100,000. The natural water-soluble polymer is one or more of starch, protein, alginic acid and gelatin; The semi-synthetic water-soluble polymer is one or more of carboxymethyl cellulose, methyl cellulose, ethyl cellulose and hydroxyethyl cellulose; The synthetic water-soluble polymer is one or more of polyacrylamide, polyacrylic acid and its salts, polyvinyl pyrrolidone, polyvinyl alcohol, polymaleic anhydride, polyether polyol, polyquaternary ammonium salt, polyethylene glycol, polyetheramine, polyamide and polyethyleneimine; The concentration of the first casting solution L-1 is 5wt% to 40wt%; the mass ratio of the additive to the positively charged anion exchange membrane material is 0.6:1 to 2:1; The substrate is a metal plate, a glass plate, a ceramic, a polymer plate or a fabric; The temperature range for evaporating the solvent is 25° C. to 180° C., the evaporation time is 1 hour to 48 hours, and the soaking time in water is 1 hour to 120 hours.

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