High-performance perfluorinated ion exchange membrane for chlor-alkali and preparation method thereof

Abstract

The invention discloses a high-performance perfluorinated ion exchange membrane for chlor-alkali and a preparation method of the perfluorinated ion exchange membrane, and belongs to the technical field of ion exchange membrane materials. The membrane is synthesized from a perfluorosulfonic acid monomer, a novel fluorine-containing comonomer, a surface functionalized nano oxide, an initiator and a cross-linking agent through a copolymerization reaction. The method is characterized in that a comonomer with a structural general formula of CF2 = CF-(CF2) n-X (X =-OCF2CF2OH or-OCF2CF2OCH3) is introduced to form a hydrogen bond network with a perfluorosulfonic acid group, and an ion transmission channel is structured; meanwhile, nano TiO2 / ZrO2 is subjected to surface functionalization by adopting a fluorine-containing silane coupling agent, so that the nano TiO2 / ZrO2 is covalently bonded with a polymer matrix, and a'molecular chain-nano particle 'dual enhancement system is constructed. The preparation method comprises the steps of comonomer synthesis, nanoparticle functionalization, solution polymerization and heat treatment. According to the invention, the film resistance (less than or equal to 8.2 omega.cm < 2 >) is obviously reduced, the tensile strength (greater than or equal to 48 MPa) and the long-term operation stability (the resistance amplification after 1000 hours of electrolysis is less than or equal to 12%) are improved, and the method is suitable for an efficient and low-consumption chlor-alkali electrolysis process.

Description

Technical Field

[0001] This invention belongs to the field of chemical technology, specifically relating to a high-performance perfluorinated ion exchange membrane for chlor-alkali and its preparation method. Background Technology

[0002] Ion exchange membranes are core components of modern chlor-alkali electrolyzers, and their performance directly determines the production efficiency and energy consumption of sodium hydroxide, chlorine, and hydrogen. A high-performance chlor-alkali ion exchange membrane must achieve a difficult balance between low resistance (ensuring low energy consumption), high ion selectivity (ensuring high current efficiency and product purity), excellent mechanical strength (ensuring dimensional stability and long lifespan), and outstanding chemical stability (resisting the corrosion of active chlorine and strong alkalis).

[0003] Currently, perfluorosulfonic acid ion exchange membranes are the most widely used in commercial applications. These materials utilize a polytetrafluoroethylene (PTFE) backbone for chemical stability, while the sulfonic acid groups (-SO3H) at the end of the side chains provide ion conduction channels. However, their inherent structure leads to several major technical bottlenecks that have not yet been perfectly resolved: 1. The contradiction between membrane resistance and selectivity: Ion cluster channels formed by random copolymers often lack regularity, resulting in high ion transport resistance and thus a typically high membrane resistance (e.g., the membrane resistance of commercial membranes is mostly 9-11 Ω·cm). 2 (Range). In pursuing lower resistance, it is often necessary to increase the ion exchange capacity or change the microstructure, but this may lead to excessive swelling of the membrane, reducing its resistance to hydroxide ions (OH-). - The selective blocking ability of the current causes a decrease in current efficiency.

[0004] 2. Insufficient mechanical strength: Pure perfluorosulfonic acid resin has limited mechanical strength in a wet state, with its tensile strength typically in the range of 30-40 MPa. Under long-term electrochemical conditions, temperature fluctuations, and physical tension, the membrane is prone to creep or permanent deformation. Especially in large bipolar electrolyzers, the dimensional instability of the membrane can lead to short-circuit risks and shorten its service life.

[0005] 3. Simple blending strategies have inherent drawbacks: To enhance mechanical properties, a common improvement method is to introduce rigid nanoparticles, such as SiO2, TiO2, or Al2O3, into the polymer matrix. However, there is a serious interfacial incompatibility problem between unmodified inorganic nanoparticles and the hydrophobic-oleophilic perfluoropolymer matrix. This weak physical adsorption interface easily becomes a stress defect point under long-term electrolyte immersion and ion migration impact, leading to nanoparticle aggregation or detachment from the matrix. The result is not only ineffective enhancement but also damages the film's compactness, causing a significant decrease in ion selectivity and potentially accelerating the chemical degradation of the film due to interfacial defects.

[0006] While existing technologies include improved methods such as copolymerization using different functional monomers (e.g., perfluorocarboxylic acid monomers) or physical blending using different types of nanofillers, these methods mostly optimize only from a single dimension. For example, adjusting the proportion of comonomers may improve conductivity but sacrifice strength; adding nanofillers may increase strength but impair selectivity and stability. These methods fail to fundamentally and synergistically address the two core issues of "ordering of ion channels at the molecular chain level" and "strengthening of the two-phase interface at the nanoscale."

[0007] Therefore, developing a novel perfluorinated ion exchange membrane that can overcome existing performance bottlenecks and achieve synergistic improvements in low resistance, high selectivity, high strength, and high durability is of vital importance for promoting energy conservation, emission reduction, and high-quality development in the chlor-alkali industry. Summary of the Invention

[0008] To address the problems of high membrane resistance, insufficient mechanical strength, and decreased selectivity and stability caused by weak interphase bonding in nanocomposite membranes, as described in the background art, this invention aims to provide a high-performance perfluorinated ion exchange membrane for chlor-alkali applications and its preparation method. Its core objective is to overcome the performance bottlenecks of existing technologies through the synergistic design of molecular structure and material interfaces, and to prepare an ion exchange membrane that simultaneously achieves low resistance, high selectivity, high strength, and long lifespan.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A high-performance perfluorinated ion exchange membrane for chlor-alkali applications is synthesized from the following raw materials through copolymerization and crosslinking reactions: Perfluorosulfonic acid monomers: As the main structural unit and ion conduction functional unit of the membrane, accounting for 80-85 mol%. Its typical structure is CF2=CFOCF2CF(CF3)OCF2CF2SO3H.

[0010] Novel fluorinated comonomers: comprising 15-20 mol%, with the general structural formula CF2=CF-(CF2). n -X, where n takes the value of 2-4, and X is a functional group that can form hydrogen bonds with sulfonic acid groups, preferably -OCF2CF2OH or -OCF2CF2OCH3. The introduction of this monomer aims to regulate the arrangement of ion clusters and optimize ion transport channels through hydrogen bonding forces.

[0011] The nano-reinforcing phase comprises 1-3% of the total monomer mass and consists of nano-oxides with a particle size of 20-40 nm, preferably TiO2 or ZrO2. The key feature is that the surface of the nano-oxides is grafted with a fluorinated silane coupling agent CF3(CF2)5CH2CH2Si(OCH3)3 to achieve chemical bonding with the polymer matrix.

[0012] Initiator: Potassium persulfate or azobisisobutyronitrile (AIBN) can be used, with an amount of 0.5-1% of the total mass of the monomers.

[0013] Crosslinking agent: ethylene glycol dimethacrylate is used, and the amount is 0.2-0.5% of the total mass of monomers.

[0014] This invention constructs a dual-reinforcement system of "molecular chain hydrogen bond network - nanoparticle chemical bonding" through a specific combination of the above-mentioned raw materials. Specifically, the polar functional groups of the novel fluorinated comonomer form intramolecular / intermolecular hydrogen bonds with the perfluorosulfonic acid groups, acting as "channel regulators"; while the surface-functionalized nanoparticles form a strong Si-OC covalent bond interface with the polymer chain through fluorinated silanes, acting as "nano-reinforcing rivets".

[0015] A method for preparing the high-performance perfluorinated ion exchange membrane for chlor-alkali includes the following steps: 1) Comonomer synthesis: Hexafluoropropylene and perfluoroolefins containing hydroxyl or ether bonds (such as CF2=CFOCF2CF2OH) are added to a high-pressure reactor in a 1:1 molar ratio. Perfluorooctanoic acid is used as a catalyst, and the copolymerization reaction is carried out at 80°C and 2MPa for 4 hours to obtain the novel fluorinated comonomer.

[0016] 2) Nanoparticle functionalization: The nano-oxide was dispersed in an ethanol-water mixture (volume ratio 3:1), and 3% of a fluorinated silane coupling agent by mass of the nanoparticles was added. The mixture was ultrasonically dispersed at 60°C for 2 hours, then filtered and dried to obtain a nano-reinforced phase with surface grafted fluorinated silane.

[0017] 3) Solution polymerization and film formation: Perfluorosulfonic acid monomer and novel fluorinated comonomer are dissolved in N,N-dimethylformamide (DMF) solvent at a specific molar ratio (preferably 5:1). After stirring evenly, functionalized nano-reinforcing phase, initiator and crosslinking agent are added, and polymerization reaction is carried out at 65℃ and 0.25MPa for 10h.

[0018] 4) Heat treatment: The polymerization product is poured into a mold and heat-treated at 90°C for 8 hours to promote complete cross-linking reaction and form a stable membrane microstructure, finally obtaining a perfluorinated ion exchange membrane with uniform thickness (about 50 μm).

[0019] Compared with the prior art, the present invention has the following significant advantages: 1. Significantly optimized ion transport performance: The introduction of a novel fluorinated comonomer capable of forming hydrogen bonds with sulfonic acid groups effectively improves the regularity and order of ion cluster channels, significantly reducing ion migration resistance. The membrane resistivity prepared by this invention can be reduced to 8.0-8.2 Ω·cm. 2 Compared with traditional commercial membranes, it reduces voltage by more than 20%, thereby effectively reducing the voltage and energy consumption of the electrolyzer.

[0020] 2. Significantly Enhanced Mechanical Strength: Thanks to the strong covalent bonds between the surface-functionalized nanoparticles and the polymer matrix, the interfacial bonding force is greatly enhanced, eliminating interfacial defects found in traditional composite membranes. The tensile strength of the membrane reaches 48-50 MPa, which is more than 26% higher than that of unreinforced pure polymer membranes, enabling it to better withstand mechanical stress under long-term electrolysis conditions.

[0021] 3. Excellent long-term operational stability: Due to the dual stabilizing effect of the "hydrogen bond network" and "chemical bonding interface," the membrane structure is more stable in harsh electrochemical environments. Functionalized nanoparticles are not easily detached, and the membrane microstructure is not easily degraded. After 1000 hours of continuous electrolysis testing, the resistance increase of the membrane of this invention is only 10-12%, far lower than that of the comparative example (20-25%), which greatly extends the service life of the membrane and reduces the replacement frequency and production costs. Detailed Implementation

[0022] The present invention will be further described below with reference to the embodiments, but the scope of the present invention is not limited to these embodiments.

[0023] To ensure that this invention can be clearly and completely reproduced, the general raw materials, equipment, and testing methods used in the experiment are first clarified: Main raw material specifications: Perfluorosulfonic acid monomer (CF2=CFOCF2CF(CF3)OCF2CF2SO3H): purity ≥ 99.5%, moisture content <200ppm, sourced from Zhejiang Juhua Co., Ltd.

[0024] Hexafluoropropylene (C3F6): purity ≥99.9%, sourced from Zhonghao Fluorochemicals.

[0025] Hydroxyl-containing perfluoroolefin (CF2=CFOCF2CF2OH): Prepared in the laboratory, with a purity of ≥98.5% as determined by gas chromatography (GC).

[0026] Nano-oxides: TiO2 (anatase type, average particle size 20±5nm, specific surface area 50±10m²) 2 / g), ZrO2 (monoclinic phase, average particle size 30±5nm, specific surface area 35±5 m² / g), 2 All reagents (g) were purchased from Aladdin Reagents.

[0027] Fluorinated silane coupling agent (CF3(CF2)5CH2CH2Si(OCH3)3): purity ≥97%, purchased from Geist reagent.

[0028] Solvents: N,N-dimethylformamide (DMF), γ-butyrolactone (GBL), dried for one week by 4A molecular sieve before use, with a moisture content of <50ppm.

[0029] Initiators and crosslinking agents: potassium persulfate (KPS), azobisisobutyronitrile (AIBN), and ethylene glycol dimethacrylate (EGDMA) were all of analytical grade and were not further purified before use.

[0030] Main equipment and instruments: High-pressure reactor: Parr 4500 series, with mechanical stirring and temperature control system.

[0031] Ultrasonic Dispersor: Scientz-IIID Ultrasonic Cell Disruptor, 900W, probe type.

[0032] Polymerization apparatus: A 5000mL three-necked flask equipped with magnetic stirring, reflux condenser, and nitrogen inlet tube, placed in a constant temperature oil bath.

[0033] Hot press: Carver 4122 hot press.

[0034] Performance testing equipment: Membrane resistance: Measured using an AC impedance method in 0.5M NaCl solution with a Shanghai Chenhua CHI660e electrochemical workstation (frequency range 100kHz-1Hz, amplitude 10mV).

[0035] Ion selectivity: OH groups were measured using a laboratory-constructed electrolytic cell at 90°C and 32% NaOH. - The transmittance was calculated.

[0036] Tensile strength: Tested using an Instron 3365 universal testing machine, in accordance with standard ASTM D882.

[0037] Lifetime test: at 90℃, current density 4kA / m 2 Under these conditions, a continuous electrolysis experiment was conducted in a 32% NaOH solution.

[0038] Example 1 This embodiment details the preparation process of membrane M1.

[0039] Step 1: Synthesize hydroxyl-containing perfluorinated comonomer (CF2=CFOCF2CF2OH): In a 1L clean and dry high-pressure reactor, 500 mL of pre-dried perfluorohexane was added as a solvent and 5.0 g of perfluorooctanoic acid was added as a catalyst.

[0040] After purging the reactor with nitrogen three times to replace the air, 0.5 mol of CF2=CFOCF2CF2OH was added sequentially through a metering pump, and 0.5 mol of hexafluoropropylene gas was also introduced.

[0041] The reaction system was heated to 80°C and maintained at a pressure of 2.0 MPa (gauge pressure). The system was mechanically stirred at 300 rpm for 4 hours.

[0042] After the reaction was complete, the mixture was cooled to room temperature and the pressure was released. The reaction solution was poured into a large amount of ice-cold n-hexane to precipitate the solid, which was then filtered to obtain a white solid. The solid was washed three times with ethanol and dried in a vacuum drying oven at 60°C for 12 hours to obtain the target comonomer.

[0043] Structural verification: 50 mg of the product was dissolved in deuterated acetone and analyzed by ¹H-NMR. A broad singlet at a chemical shift of δ=4.5 ppm, attributed to the -OH group, was observed in the spectrum, confirming the successful synthesis of the target structure.

[0044] Step 2: Surface functionalization of nano-TiO2: 20g of nano-TiO2 powder was dispersed in a mixture of 800mL of ethanol and deionized water (ethanol:water volume ratio = 3:1).

[0045] Slowly add 0.6 g (3% of the mass of TiO2) of fluorinated silane coupling agent CF3(CF2)5CH2CH2Si(OCH3)3 to the above suspension.

[0046] Transfer the mixture to a 1000mL beaker, place it in a 60℃ water bath, and treat it with an ultrasonic probe (power set to 400W, operating mode: 2 seconds of ultrasound followed by 2 seconds of intermittent ultrasound) for 2 hours.

[0047] After processing, the solid was filtered through a 0.22 μm polytetrafluoroethylene microporous membrane. The resulting solid was washed three times with ethanol and dried in a vacuum oven at 80 °C for 6 hours to obtain functionalized nano-TiO2 powder with fluorosilane grafted onto its surface.

[0048] Step 3: Membrane preparation: Solution preparation: In a nitrogen-filled glove box, add 1000 mL of dry DMF solvent to a 2000 mL three-necked flask. Then, accurately weigh 10.0 mol of perfluorosulfonic acid monomer and 2.0 mol of the hydroxyl-containing perfluoro copolymer monomer synthesized in step 1 (molar ratio 5:1) and add them to the flask. In a 50 °C water bath, magnetically stir at 300 rpm for 30 minutes until the monomer is completely dissolved, yielding a clear and transparent solution.

[0049] Nanodispersion: Add 20g of functionalized nano-TiO2 (2% of the estimated total polymer mass) to the above monomer solution. Transfer the system to an ultrasonic water bath and ultrasonically disperse at 40°C for 50 minutes to form a uniform milky white suspension.

[0050] Polymerization and crosslinking: 0.096 mol of potassium persulfate (0.8% of the total monomer mass) and 0.036 mol of crosslinking agent EGDMA (0.3% of the total monomer mass) were added sequentially to the suspension. Nitrogen gas was bubbled through the suspension for 15 minutes to remove dissolved oxygen. The system was then placed in a constant-temperature oil bath at 65°C and reacted for 10 hours at 0.25 MPa (nitrogen pressure, gauge pressure) with slow stirring (100 rpm). During the reaction, the viscosity of the system gradually increased, eventually forming a viscous gel.

[0051] Molding and heat treatment: The obtained gel-like polymer was poured into a 30cm×30cm polytetrafluoroethylene mold, and the thickness was controlled with a scraper. The mold was then transferred to a circulating air oven and heat-treated at 90°C for 8 hours.

[0052] After heat treatment, the membrane is naturally cooled to room temperature and peeled off from the mold to obtain a transparent and flexible perfluorinated ion exchange membrane with a thickness of 50±5μm, denoted as M1.

[0053] Example 2 This example illustrates the preparation of membrane M2 using different comonomers and nano ZrO2.

[0054] Step 1: Synthesize the ether-containing comonomer (CF2=CFOCF2CF2OCH3): The synthesis steps are similar to those in Example 1, but the hydroxyl-containing perfluoroolefin is replaced with an equimolar amount of CF2=CFOCF2CF2OCH3.

[0055] Structural verification: 1 The H-NMR spectrum showed a sharp singlet at δ=3.5 ppm, attributed to the proton in -OCH3, confirming the correct structure.

[0056] Step 2: Surface functionalization of nano-ZrO2: The steps are the same as step 2 in Example 1, but the nano TiO2 is replaced with 30g of nano ZrO2.

[0057] Step 3: Membrane preparation: The steps are similar to step 3 of Example 1, except that: The solvent was replaced with 1200 mL of γ-butyrolactone (GBL).

[0058] The amount of perfluorosulfonic acid monomer used is 12.0 mol, and the amount of ether-containing comonomer is 3.0 mol (molar ratio 4:1).

[0059] The nano-reinforcing phase uses 30g of functionalized nano ZrO2.

[0060] Finally, membrane M2 was obtained.

[0061] Comparative Example 1 This comparative example is used to illustrate the negative impact of unfunctionalized nanoparticles.

[0062] The preparation process was exactly the same as in Example 1, with the only difference being that in the "nanodispersion" step, an equal amount of untreated raw nano-TiO2 was used. The final film D1 was obtained.

[0063] Comparative Example 2 This comparative example is used to illustrate the membrane performance without the addition of a nano-reinforcing phase.

[0064] The preparation process was basically the same as in Example 1, except that no nanoparticles were added in the "nanodispersion" step. The final membrane D2 was obtained.

[0065] Performance test results comparison: The membranes prepared in the above embodiments and comparative examples were subjected to performance tests, and the results are summarized in the table below: Table 1. Comparison of membrane performance data between the examples and comparative examples. As shown in the table above, the ion exchange membranes prepared in Examples 1 and 2 of this invention exhibit significant and synergistic improvements in membrane resistance, ion selectivity, mechanical strength, and long-term operational stability compared to the comparative examples. This fully demonstrates the effectiveness and superiority of the "molecular chain hydrogen bond network-nanoparticle chemical bonding" dual-enhancement system proposed in this invention.

Claims

1. A high-performance perfluorinated ion exchange membrane for chlor-alkali applications, characterized in that, It is synthesized from the following raw materials: 80-85 mol% perfluorosulfonic acid monomer, 15-20 mol% novel fluorinated comonomer, 1-3 wt% nano-oxide of surface-grafted fluorinated silane coupling agent CF3(CF2)5CH2CH2Si(OCH3)3, 0.5-1 wt% initiator, and 0.2-0.5 wt% crosslinking agent; The novel fluorinated comonomer has the general structural formula CF2=CF-(CF2). n -X, where n=2-4, and X is -OCF2CF2OH or -OCF2CF2OCH3; The nano-oxide particles have a diameter of 20-40 nm.

2. The high-performance perfluorinated ion exchange membrane for chlor-alkali applications according to claim 1, characterized in that, The molar ratio of the perfluorosulfonic acid monomer to the novel fluorinated comonomer is 5:

1.

3. The high-performance perfluorinated ion exchange membrane for chlor-alkali applications according to claim 1, characterized in that, The nano-oxide is TiO2 or ZrO2.

4. A method for preparing a high-performance perfluorinated ion exchange membrane for chlor-alkali as described in any one of claims 1-3, characterized in that, Includes the following steps: (1) Comonomer synthesis: Hexafluoropropylene and perfluoroolefins containing hydroxyl or ether bonds were copolymerized at a molar ratio of 1:1 with perfluorooctanoic acid as a catalyst at 80°C and 2MPa for 4h to obtain a novel fluorinated comonomer. (2) Functionalization of nanoparticles: The nano-oxides were dispersed in an ethanol-water mixture, and 3% of the mass of the nanoparticles of fluorinated silane coupling agent CF3(CF2)5CH2CH2Si(OCH3)3 was added. The mixture was ultrasonically dispersed at 60℃ for 2 h, filtered and dried to obtain surface-functionalized nano-oxides. (3) Solution polymerization: Perfluorosulfonic acid monomer and novel fluorinated comonomer were dissolved in N,N-dimethylformamide solvent, functionalized nano-oxide, initiator and crosslinking agent were added, and the polymerization reaction was carried out at 65℃ and 0.25 MPa for 10 h. (4) Heat treatment: The polymerization product is poured into a mold and heat-treated at 90°C for 8 h to obtain a perfluorinated ion exchange membrane.

5. The preparation method according to claim 4, characterized in that, The volume ratio of the ethanol-water mixture is 3:1.

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