Preparation of complex bipolar membrane for low carbon alcohol alkali metal salt and its electrodialysis process

Abstract

The application relates to a composite bipolar membrane for preparing low-carbon alcohol alkali metal salt and an electrodialysis process thereof, and belongs to the technical field of membrane separation and electrochemical synthesis. The composite bipolar membrane comprises a cation exchange layer, an intermediate catalytic layer and an anion exchange layer which are sequentially laminated in composite. The process comprises the following steps: feeding a salt solution containing alkali metal ions into an anode chamber of a bipolar membrane electrodialysis system, feeding a low-carbon alcohol solution into a cathode chamber; applying a direct current electric field on both sides of the composite bipolar membrane, so that the low-carbon alcohol is dissociated to generate alkoxide ions and hydrogen ions at the membrane interface; the alkoxide ions migrate to the anode chamber and combine with the alkali metal ions to generate low-carbon alcohol alkali metal salt; the membrane material has high adaptability, the membrane structure has good stability, the process parameters are mature, and the system has high integration degree.

Description

Technical Field

[0001] This invention relates to a composite bipolar membrane for preparing low-carbon alcohol alkali metal salts and its electrodialysis process, belonging to the field of membrane separation and electrochemical synthesis technology. Background Technology

[0002] Low-carbon alkali metal salts of alcohols (such as sodium methoxide, potassium methoxide, and sodium ethoxide) are important chemical raw materials, widely used as strong alkaline catalysts in biodiesel production, pharmaceutical synthesis, and pesticide production. With the deepening of green chemistry and sustainable development concepts, higher requirements are being placed on the green synthesis processes of these key chemicals. Traditional synthesis methods mainly rely on metal and alkaline methods, which suffer from poor safety (e.g., the metal method generates hydrogen), high energy consumption, and serious environmental pollution. Therefore, there is an urgent need to develop next-generation green synthesis technologies.

[0003] Table 1. Comparison of Main Synthesis Methods for Low-Carbon Alkyl Metal Salts

[0004]

[0005] In recent years, bipolar membrane technology has made significant progress in material design and process applications. In the field of organic synthesis, bipolar membrane electrodialysis has demonstrated unique advantages. Studies have shown that this technology can be used in areas such as organic acid synthesis, pH adjustment of fermentation broth, and food processing. For example, in lactic acid production, bipolar membrane electrodialysis can effectively separate electrolyte ions such as lactic acid from the fermentation broth, improving extraction efficiency. In the deacidification process of fruit juice, the OH- ions generated by the hydrolysis of the bipolar membrane... - It can adjust the pH of fruit juice without significantly affecting its composition, color, and flavor. These successful cases provide technical reference for extending bipolar membrane technology to the synthesis of low-carbon alcohol-alkali metal salts. Bipolar membrane electrodialysis, as an emerging technology combining membrane separation and electrochemical synthesis, shows great potential in the field of green synthesis of chemical products. A bipolar membrane is a specially structured ion exchange membrane composed of an anion exchange layer, a cation exchange layer, and an intermediate interface layer. Under the action of a DC electric field, the bipolar membrane can efficiently dissociate water molecules into H+. + and OH -This characteristic gives it unique advantages in fields such as acid and base preparation, organic synthesis, and resource recovery. Extending this technological concept to the field of alcohol dissociation, utilizing bipolar membranes to dissociate low-carbon alcohols into alkoxy ions and hydrogen ions provides a novel approach for the green synthesis of low-carbon alcohol alkali metal salts. For example, Chinese Patent Publication No. CN113830740A discloses a method for preparing acids and bases using bipolar membranes based on electrodialysis technology, belonging to the field of ion exchange membrane technology. Pure water is injected into hydrochloric acid concentrate tanks and sodium hydroxide concentrate tanks, respectively. An initial 5% concentration of sodium bicarbonate solution is prepared in sodium bicarbonate desalination tanks and bicarbonate concentrate tanks, respectively. High-concentration sodium chloride water is injected into a high-concentration sodium chloride desalination tank via a booster pump and a security filter. The bipolar membrane electrodialysis uses a specially structured five-channel module, assembled from a bipolar membrane, a monovalent / divalent selective anion membrane, and a standard cation membrane. However, applying bipolar membrane technology to the synthesis of low-carbon alcohol alkali metal salts still faces a series of technical bottlenecks:

[0006] Poor adaptability of membrane materials: Traditional bipolar membranes are designed for water dissociation and have limited ability to dissociate alcohols, requiring the development of a dedicated alcohol dissociation catalytic layer.

[0007] Insufficient membrane structure stability: Alcohol solvents have a strong swelling effect on polymer membrane materials, which can easily lead to membrane performance degradation, so it is necessary to improve the solvent resistance of the membrane.

[0008] The process parameters are not mature: Research on electrodialysis processes for the synthesis of alcohol-alkali metal salts is almost non-existent, and key parameters such as current density, temperature and material concentration need to be systematically optimized.

[0009] Low system integration: The processing capacity of a single membrane unit is limited, and multi-stage series systems need to be developed to improve conversion efficiency.

[0010] A series of technical problems need to be solved in order to successfully transfer this technology to the field of alkali-alcohol metal salt synthesis. Summary of the Invention

[0011] The purpose of this invention is to provide a composite bipolar membrane for preparing low-carbon alcohol alkali metal salts and its electrodialysis process, which meets the special requirements of alcohol dissociation reaction, has high membrane material adaptability, good membrane structure stability, mature process parameters, and high system integration.

[0012] The present invention describes a composite bipolar membrane for preparing low-carbon alcohol alkali metal salts, comprising a cation exchange layer, an intermediate catalytic layer, and an anion exchange layer stacked sequentially.

[0013] The cation exchange layer is a sulfonated polyphenylene fluoride nanofiber membrane that has been cross-linked with a cross-linking agent.

[0014] The intermediate catalyst layer comprises nanoscale composite metal hydroxide, graphene oxide, quaternary ammonium salt compound, and ionic liquid;

[0015] The anion exchange layer is a quaternary ammonium salt functionalized covalent organic framework material membrane.

[0016] Through the synergistic design of a three-layer functionalized structure, the problems of low dissociation efficiency, poor stability, and insufficient selectivity of traditional bipolar membranes in alcohol systems are fundamentally solved. Specifically, this is reflected in:

[0017] Highly efficient catalytic alcohol dissociation: The specially designed intermediate catalytic layer significantly reduces the activation energy barrier for alcohol dissociation, with a start-up voltage as low as below 1.5V, which is much lower than that of traditional water dissociation bipolar membranes (>2.0V).

[0018] Excellent selectivity: The anion exchange layer uses a well-structured COF material, which has precise pore size sieving and electrostatic adsorption properties, enabling it to target alkali metal ions (such as Na+). + The selectivity is over 95%;

[0019] Excellent solvent resistance and stability: The cation exchange layer uses sulfonated polyphenylene fluoride as the substrate, and its fluorinated skeleton has extremely strong resistance to alcohol swelling. The membrane exhibits minimal performance degradation during long-term operation (>1500h) in alcohol solutions.

[0020] Robust structure: The three layers are hot-pressed together to form an integrated structure with high interfacial bonding strength, which avoids interlayer delamination under electric field and fluid shear.

[0021] Preferably, the cation exchange layer uses sulfonated polyphenylene fluoride as the substrate and constructs a nanoscale fiber membrane through electrospinning technology; polyvinyl alcohol is added during the spinning process, and then ferric chloride solution is used as a crosslinking agent for crosslinking treatment to form a three-dimensional network structure; the degree of sulfonation of the sulfonated polyphenylene fluoride is not less than 35%, and the degree of alcoholysis of the polyvinyl alcohol is not less than 99%.

[0022] Nanofiber membranes constructed using electrospinning technology possess a continuous three-dimensional network structure and extremely high specific surface area, providing numerous channels for proton transport and exhibiting low surface resistivity. The addition of polyvinyl alcohol and crosslinking with ferric chloride significantly enhances the flexibility and mechanical strength of the fiber membrane, enabling it to withstand the fluid pressure and membrane expansion stress during electrodialysis. A sulfonation degree ≥35% ensures the membrane possesses sufficient sulfonic acid groups (-SO3H), providing abundant proton conduction sites, thereby achieving highly efficient H+ ionization in alcohol dissociation reactions. + Generation and migration ensure high ion exchange capacity.

[0023] Preferably, the intermediate catalyst layer uses a nanoscale composite metal hydroxide as the main catalyst, which is blended with graphene oxide and quaternary ammonium salt to form a composite catalyst layer, and an ionic liquid is added; the nanoscale composite metal hydroxide includes magnesium hydroxide and aluminum hydroxide in a mass ratio of 1:1 to 1:3, and the ionic liquid is 1-butyl-3-methylimidazolium tetrafluoroborate.

[0024] A specific mass ratio (1:1-1:3) of Mg(OH)₂ and Al(OH)₃ forms a catalytically active center with optimal acid-base synergistic effect, capable of efficiently polarizing and breaking the OH bonds of alcohols. The addition of graphene oxide forms a three-dimensional conductive framework, significantly reducing membrane resistance; the ionic liquid [BMIM][BF₄] acts as a proton carrier and interfacial wetting agent, further enhancing the proton conduction rate and interfacial compatibility within the catalytic layer. This four-component system (metal hydroxide, GO, quaternary ammonium salt, and ionic liquid) forms a stable composite catalytic interface through strong interactions, reducing the alcohol dissociation overpotential by more than 40%.

[0025] Preferably, the anion exchange layer is obtained by reacting terephthalaldehyde and tris(4-aminophenyl)amine to form a covalent organic framework precursor membrane, followed by acetic acid-catalyzed crystallization, reduction with NaBH3CN, and then quaternization reaction with trimethylamine.

[0026] The COF membrane prepared by a four-step method of aldehyde-amine condensation, crystallization, reduction, and quaternization possesses a highly ordered pore structure (pore size of approximately 0.5 nm) and uniformly distributed quaternary ammonium groups. Based on both size and charge effects, it preferentially allows Na+ to pass through the pores. + K + Small monovalent cations can pass through, while effectively blocking Mg. 2+ It contains divalent ions with an ion selectivity of ≥96%. The covalently linked COF skeleton exhibits excellent chemical stability in alcohols, water, and weak acid and weak base environments, and is not prone to structural collapse or loss of functional groups.

[0027] Preferably, the cation exchange layer, intermediate catalyst layer and anion exchange layer are hot-pressed together at 70-100℃, 0.5-2.0MPa and 30-60min to obtain a composite bipolar membrane.

[0028] Hot pressing at specific temperatures (70-100℃) and pressures (0.5-2.0MPa) allows for the interdiffusion and physical entanglement of molecular chains at the interface of the three-layer membrane material, potentially forming covalent bonds in some areas. This results in an integrated composite membrane with low interlayer resistance and resistance to delamination during long-term use. The process parameters are clearly defined, facilitating large-scale production and ensuring batch-to-batch consistency of membrane performance.

[0029] The electrodialysis process for preparing low-carbon alcohol alkali metal salts according to the present invention includes:

[0030] (1) A salt solution containing alkali metal ions is introduced into the anode chamber of the bipolar membrane electrodialysis system, and a low-carbon alcohol solution is introduced into the cathode chamber; the bipolar membrane electrodialysis system includes at least one electrodialysis unit, the electrodialysis unit includes an anode chamber, a cathode chamber, and a composite bipolar membrane for preparing low-carbon alcohol alkali metal salts as described above, disposed between the anode chamber and the cathode chamber;

[0031] (2) Apply a DC electric field to both sides of the composite bipolar membrane to cause the low-carbon alcohol to dissociate at the membrane interface to generate alkoxy ions and hydrogen ions;

[0032] (3) The alkoxy ions migrate to the anode chamber and combine with the alkali metal ions to generate low-carbon alcohol alkali metal salts.

[0033] The entire process can be carried out at ambient temperature and pressure, without generating dangerous gases such as hydrogen, completely eliminating the safety hazards of traditional metal-based methods. Theoretically, raw materials can be completely converted into products, with no waste salt generated; the only byproduct is the corresponding organic acid (such as formic acid), which is easy to process or utilize. Utilizing an electric field to drive the directional migration of ions, the reaction rate is fast, and the single-pass conversion rate is high (≥90%). Due to the high selectivity of the membrane, the content of free alkali and heavy metal impurities in the product is extremely low, with a purity reaching over 99%.

[0034] Preferably, the bipolar membrane electrodialysis system is a multi-stage series bipolar membrane electrodialysis system, including at least two electrodialysis units, and the composite bipolar membrane on the side near the cathode chamber in the previous stage of the electrodialysis unit also serves as the composite bipolar membrane on the side near the anode chamber in the next stage of the electrodialysis unit.

[0035] The multi-stage series design breaks the thermodynamic equilibrium limitations of single-stage reactions, allowing the reaction driving force to accumulate step by step. The final product concentration can be increased by more than 80% compared to a single-stage system, reducing energy consumption in subsequent concentration. Optimized ion transport paths improve current efficiency: the series structure reduces the migration and backmixing of non-target ions, achieving a total system current efficiency of over 90%.

[0036] Preferably, the DC electric field current density is 10-200 A / m 2 The reaction temperature is 0-60℃, and the alkali metal salt solution is a formate or carbonate solution or an organic acid salt or an oxygen-containing inorganic acid salt with a concentration of 0.2-2 mol / L; the low carbon alcohol solution is a methanol or ethanol or propanol or butanol solution with a concentration of 70-99 vol.

[0037] Here, the alkali metal salt solution is a solution with water as the main solvent, in which alkali metal cations and specific anions are dissolved. This solution is passed into the anode chamber, where the anions are consumed or transformed during the electrochemical process, thereby driving the alkali metal cations to continuously pass through the bipolar membrane and combine with alkoxy ions generated in the cathode chamber to generate the target product, a low-carbon alcohol alkali metal salt. The alkali metal salt solution contains alkali metal ions (lithium, sodium, potassium, etc.) and anions that can combine with protons to generate gases or separate products; the alkali metal salt solution is preferably an aqueous solution of formate, carbonate, bicarbonate, or C1-C4 carboxylate; more preferably, it is a sodium formate or potassium carbonate solution.

[0038] Operating within this range ensures high current efficiency (≥80%) and product purity, while avoiding side reactions (such as excessive oxidation of alcohols) and membrane damage. This range is applicable to a variety of low-carbon alcohols (methanol, ethanol) and alkali metal salts (sodium formate, potassium carbonate), demonstrating the process's versatility.

[0039] Preferably, the DC electric field current density is 35-45 A / m², and the reaction temperature is 35-45℃.

[0040] At a current density of 40 A / m 2 Under the condition of 40℃, the yield (>98%), purity (>99%), current efficiency (>85%) and energy consumption per unit product (approximately 2.1 kWh / kg) of sodium methoxide synthesis are optimal, providing a clear and efficient parameter benchmark for industrial operation.

[0041] Preferably, the multi-stage series bipolar membrane electrodialysis system employs gradient current density.

[0042] Applying different current densities at different stages (e.g., low in the initial stage and high in the later stage) matches the kinetic requirements of reactant concentration changes at each stage. This gradient strategy is effective:

[0043] ① Suppress final-stage concentration polarization to prevent voltage surge;

[0044] ② Reduce overall energy consumption; the energy consumption per unit product can be further reduced compared to a symmetrical series system.

[0045] ③ Reduce membrane fouling, slow down the deposition of byproducts on the membrane surface from the source, and extend the membrane's service life.

[0046] The composite bipolar membrane for preparing low-carbon alcohol alkali metal salts and its electrodialysis process described in this invention have the following beneficial effects:

[0047] 1. Membrane material innovation: For the first time, sulfonated polyphenylene fluoride was combined with covalent organic framework materials to prepare nanoscale ion channel bipolar membranes through electrospinning technology, which significantly improved the ion selectivity and chemical stability of the membrane.

[0048] 2. Catalytic layer innovation: Targeting the characteristics of alcohol dissociation reaction, a nano-composite metal hydroxide / graphene oxide composite catalytic layer was designed. A stable alcohol dissociation interface was constructed through in-situ polymerization, which significantly reduced the alcohol dissociation potential.

[0049] 3. Process innovation: A multi-stage series bipolar membrane electrodialysis system was developed to achieve continuous and efficient synthesis of low-carbon alcohol-alkali metal salts, solving the problem of low single-stage conversion rate.

[0050] 4. System integration innovation: Adding composite catalysts and ionic liquids to the BP layer, acid chamber and base chamber creates a synergistic promoting effect, significantly improving reaction efficiency and energy utilization. Attached Figure Description

[0051] Figure 1 This is a schematic diagram of the structure of a composite bipolar membrane for preparing low-carbon alcohol-alkali metal salts according to the present invention;

[0052] Figure 2 This is a flowchart of an electrodialysis process for preparing low-carbon alcohol alkali metal salts according to the present invention;

[0053] Figure 3 A graph of FTIR data;

[0054] Figure 4 This is a long-term voltage diagram.

[0055] In the diagram: 1. Cation exchange layer; 2. Intermediate catalyst layer; 3. Anion exchange layer. Detailed Implementation

[0056] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0057] like Figure 1 As shown, this invention addresses the specific requirements of alcohol dissociation reactions by designing a composite bipolar membrane for preparing low-carbon alcohol alkali metal salts, comprising:

[0058] Cation exchange layer 1: A nanofiber membrane is constructed using sulfonated poly(phenylene fluoride) as the substrate via electrospinning. This material exhibits excellent chemical stability and proton conductivity, and the fluorinated backbone effectively resists the erosion of alcohol solvents. Polyvinyl alcohol (PVA) is added during the spinning process to improve the fiber's flexibility and mechanical strength, followed by cross-linking treatment with ferric chloride solution to form a stable three-dimensional network structure.

[0059] Intermediate Catalytic Layer 2: A dedicated catalytic system designed for alcohol dissociation reactions. Nanoscale composite metal hydroxides (such as magnesium hydroxide and aluminum hydroxide) are used as the main catalyst, blended with graphene oxide and quaternary ammonium salts to form a composite catalytic layer. The nanocatalyst provides abundant active sites for alcohol dissociation, graphene oxide enhances ion transport channels, and quaternary ammonium salts improve interfacial compatibility. A small amount of ionic liquid (such as [BMIM][BF4]) is added to further improve the membrane conductivity.

[0060] Anion exchange layer 3: A highly selective anion exchange membrane was prepared using a covalent organic framework (COF) material. A TD-amorphous membrane was prepared via aldehyde-amine condensation reaction, followed by acetic acid-catalyzed C=N recrystallization and NaBH3CN reduction to obtain a structurally regular RTD-COF membrane. Finally, surface functionalization was achieved through in-situ polymerization of small-molecule quaternary ammonium salts, resulting in anion exchange layer 3 with high ionic conductivity and high selectivity.

[0061] The membrane composite is produced using a hot-pressing process (70-100℃, 0.5-2.0MPa, 30-60min), which enables the three-layer structure to form a strong covalent bond, ensuring the stability of the membrane during long-term use.

[0062] 2. This invention also designs a dedicated electrodialysis process for the synthesis of low-carbon alcohol alkali metal salts:

[0063] Reactor design: A BP-C type two-compartment structure is adopted. The anode compartment is filled with an alkali metal salt solution (such as sodium formate or potassium carbonate), and the cathode compartment is filled with a low-carbon alcohol solution (such as methanol or ethanol). A bipolar membrane is placed between the two compartments to achieve alcohol dissociation and directional ion migration under the action of an electric field.

[0064] Operating parameter optimization: Based on previous experimental results, key parameters such as temperature (0-60℃, optimal 30-45℃), current density (10-200A / m², optimal 25-50A / m²), and feed concentration (alkali metal salt 0.2-2mol / L, alcohol concentration 70-99%) were optimized. These parameters ensured high current efficiency while reducing energy consumption.

[0065] Multi-stage series system: To address the limited conversion rate of single-stage systems, an innovative multi-stage series system was designed, where the second bipolar membrane of the previous stage serves as the first bipolar membrane of the next stage, and so on. This design significantly improves the feed conversion rate and product concentration.

[0066] Specifically, when a multi-stage series electrodialysis system is working, the raw material (R) ， M) is added from the first stage, and alcohol (ROH) is added to each of the cathode chambers. Under the influence of the electric field, the bipolar membrane dissociates the ROH to produce RO. - and H + RO - With R ，M in + Combined with the generation of the target product ROM, H + With R ， Combined to generate by-product acid R ， H. The ROM produced in each stage enters the next stage for further concentration and purification, ultimately yielding a high-purity product.

[0067] This multi-stage series design has the following advantages:

[0068] Improving raw material conversion rate: Increasing the number of stages makes the reaction more complete;

[0069] Achieve product concentration: Gradually increase product concentration;

[0070] Reduce energy consumption: Optimize ion transport paths to reduce energy loss;

[0071] Enhanced system flexibility: The number of levels can be adjusted according to processing needs.

[0072] Membrane performance indicators: alcohol dissociation start-up voltage ≤1.5V (at 50mA / cm) 2 (Under current density); film surface resistivity ≤ 3Ω·cm 2 Ion selectivity ≥95%; Stability in alcohol solution ≥1500h.

[0073] Process parameters: Single-pass conversion rate ≥90%; Product purity ≥99%; Current efficiency ≥85%; Energy consumption is reduced by more than 30% compared with traditional methods.

[0074] Product specifications: Alkali metal alkoxide content ≥99%; Free alkali ≤1.0%; Heavy metal content meets industry standards.

[0075] Economic indicators: Production costs are reduced by more than 15% compared to traditional methods; emissions of waste gas, wastewater, and solid waste are reduced by more than 80%.

[0076] Example 1: Preparation and Characterization of Composite Bipolar Film

[0077] Experimental objective: To prepare sulfonated poly(phenylene fluoride) / COF composite bipolar membranes and characterize their basic properties.

[0078] Experimental steps:

[0079] I. Preparation of Cation Exchange Layer 1

[0080] Preparation of spinning solution: Accurately weigh 1.5g of sulfonated polystyrene fluoride (SPFE) (sulfonation degree ≥35%) and 0.5g of polyvinyl alcohol (PVA) (hydrolysis degree ≥99%), dissolve them in 100mL of 50% acetic acid solution, heat and stir in a water bath at 55℃ for 12 hours until completely dissolved, prepare a homogeneous spinning solution with a mass fraction of 2%, and let it stand for 2 hours to remove bubbles before use.

[0081] Electrospinning: Using an electrospinning device, under conditions of 50%±5% humidity, an iron-nickel alloy mesh was used as the receiving device. The spinning solution flow rate was set to 1.0 mL / h, the voltage to 20 kV, the nozzle distance to 15 cm, the inner diameter of the spinneret to 0.5 mm, and the spinning time to 4 hours to prepare nanofiber membranes.

[0082] Crosslinking treatment: The nanofiber membrane was soaked in a 5% ferric chloride-anhydrous ethanol solution for 30 minutes for crosslinking, washed with distilled water, and then vacuum dried at 40°C for 12 hours.

[0083] II. Construction of Intermediate Catalytic Layer 2

[0084] Catalytic slurry preparation: 0.33 g of magnesium hydroxide and 0.67 g of aluminum hydroxide (mass ratio 1:2) were thoroughly ground and mixed. 0.05 g of graphene oxide (0.5%) and 1.0 g of tetrabutylammonium hydroxide (1%) were added and dispersed in a mixed solvent containing 5 mL of [BMIM][BF4] ionic liquid and 95 mL of methanol. The mixture was ultrasonically dispersed for 30 minutes to form a uniform suspension.

[0085] Film formation by spraying: The catalytic slurry is uniformly coated on the surface of the cation exchange layer 1 by spraying. The spray gun diameter is 0.3 mm, the pressure is 0.2 MPa, the spraying distance is 20 cm, the wet film thickness is 50 μm, and the catalytic layer is formed by hot air drying at 60℃ for 1 hour.

[0086] III. Preparation of Anion Exchange Layer 3

[0087] TD-COF membrane was prepared by aldehyde-amine condensation reaction: 0.5 mmol of terephthalaldehyde and 0.33 mmol of tris(4-aminophenyl)amine were dissolved in 50 mL of LDM and reacted at 120 °C for 72 hours.

[0088] Acetic acid-catalyzed crystallization: Treat with 100 mL of acetic anhydride solution containing 1% acetic acid at 60 °C for 6 hours to promote the recrystallization of C=N bonds.

[0089] NaBH3CN reduction: Use 0.1 mol / L NaBH3CN aqueous solution (pH=7) to reduce the C=N bond to C-NH bond at room temperature for 12 hours.

[0090] Quaternization functionalization: The above membrane was immersed in 100 mL of methanol solution containing 10% trimethylamine and reacted at 60 °C for 6 hours to perform surface quaternization functionalization, thus obtaining RTD-COF-TMA anion exchange layer 3.

[0091] IV. Hot-pressed composite

[0092] The prepared cation exchange layer 1, intermediate catalyst layer 2 and anion exchange layer 3 are stacked in sequence and hot-pressed at 90℃±2℃ and 1.0MPa pressure for 40 minutes to form a strong bond between the layers, thus obtaining a composite bipolar membrane.

[0093] Experimental Results and Discussion:

[0094] The membrane thickness is 125±5μm (cation exchange layer 150μm, intermediate catalyst layer 215μm, anion exchange layer 360μm).

[0095] like Figure 3 As shown, FTIR reveals sulfonic acid groups (1020 cm⁻¹). -1 ) and quaternary ammonium groups (950cm) -1 The presence of distinct characteristic peaks indicates successful recombination.

[0096] The product has a compact three-layer structure, clear interfaces, and a uniformly distributed catalytic layer.

[0097] As shown in Table 2, the swelling degree test results indicate that the composite bipolar membrane has a swelling degree of 15.2% in methanol, which is lower than that of the traditional membrane (>25%), demonstrating excellent alcohol resistance. Here, a traditional bipolar membrane was used as Comparative Example 1, and a simple SPFE / PVA composite membrane without an intermediate catalyst layer and without special crosslinking treatment was used as Comparative Example 2.

[0098] Table 2. Swelling test data of composite bipolar membrane in methanol

[0099]

[0100] Conclusion: A composite bipolar membrane with stable structure and good alcohol resistance was successfully prepared, meeting the requirements of subsequent experiments.

[0101] Bipolar membrane electrochemical performance testing

[0102] Experimental objective: To test the alcohol dissociation voltage, membrane surface resistance, and ion selectivity of the composite bipolar membrane.

[0103] 1. Test equipment and conditions

[0104] Electrochemical workstation: LK2010 model, supports impedance measurement frequency from 0.00001Hz to 1MHz, current measurement range from 1nA to 1000mA, and voltage control accuracy of ±0.1%.

[0105] Electrolytic cell design: two-chamber structure (anode chamber and cathode chamber each with a volume of 500mL), made of polytetrafluoroethylene (PTFE), with a temperature range of 0 to 100℃.

[0106] Electrode configuration: The anode is a titanium-coated ruthenium mesh (effective area 10cm²). 2 The cathode is a 316L stainless steel plate (2mm thick) with a coating thickness of 5μm, and the reference electrode is a saturated calomel electrode (SCE).

[0107] Temperature control: A constant temperature water bath circulation system (accuracy ±0.5℃) was used to maintain the experimental temperature at 25±1℃.

[0108] 2. Solution preparation and feed amount

[0109] Cathode chamber solution: Add 900 mL of methanol (analytical grade) and 100 mL of deionized water to prepare a 90% methanol solution. The total feed volume is 1000 mL.

[0110] Anode chamber solution: Weigh 13.64 g of sodium formate (HCOONa·2H2O, AR grade), dissolve it in deionized water and bring the volume to 500 mL, the concentration is 1.0 mol / L.

[0111] Ion selectivity test solution: Accurately weigh 5.844 g of sodium chloride (NaCl) and 20.33 g of magnesium chloride (MgCl2·6H2O), dissolve them in deionized water and bring the volume to 1 L to obtain a mixed solution of 0.1 mol / L NaCl and 0.1 mol / L MgCl2.

[0112] 3. Test Procedure and Parameters

[0113] Alcohol dissociation voltage test: current density scanned in constant current mode (10 to 100 mA / cm²) 2 ), scan rate 5mA / cm 2 • min, record the stable voltage value, focusing on 50mA / cm 2 The voltage below is used as the alcohol dissociation initiation voltage.

[0114] Film surface resistance test: frequency range 100kHz to 0.1Hz, AC amplitude 10mV, formula for calculating film surface resistance: R 面 =R m *A (A represents the effective area; in this embodiment, the effective area is 10cm²) 2 R m R is the membrane resistance. 面 (This refers to the film surface resistance).

[0115] Ion selectivity test: Calculation of Na using the Nernst equation + / Mg 2+ Number of migrations, formula: P Na+ / Mg2+ =[C Na+ ] / [C Mg2 + ]*Z Mg2 + / Z Na+ Where Z is the ion charge, C is the ion concentration, and P is the ion concentration. Na+ / Mg2+ for Na + / Mg 2+ Number of migrations.

[0116] Long-term stability test: constant current density 50mA / cm 2 The system was run continuously for 24 hours, with voltage changes recorded every 30 minutes. The electrolyte was replaced every 8 hours to avoid the accumulation of byproducts that could affect accuracy.

[0117] Results and Discussion:

[0118] Alcohol dissociation performance: As shown in Tables 3 and 4, the alcohol dissociation starting voltage is 1.42V (50mA / cm). 2 The voltage is significantly lower than that of traditional bipolar films (typically greater than 2.0V).

[0119] Table 3. Performance Comparison of Composite Bipolar Membranes and Traditional Bipolar Membranes

[0120]

[0121] Analysis of the cause: The magnesium hydroxide-aluminum hydroxide nanoparticles in the composite catalyst layer provide abundant active sites for alcohol dissociation, while the ionic liquid [BMIM][BF4] enhances the proton conduction pathway and synergistically lowers the reaction energy barrier.

[0122] The relationship between current density and voltage shows that in the range of 10 to 60 mA / cm² 2 The voltage increases linearly within this range (slope 0.02V / mA), indicating that the polarization of the membrane is controllable within this range.

[0123] Membrane surface resistance and ion conduction mechanism: As shown in Table 4, the membrane surface resistance is 2.8 Ω·cm. 2 It is lower than that of traditional bipolar films (8.1 Ω·cm). 2 ).

[0124] Ion selectivity: Na + / Mg 2+ The migration number was calculated to be 96.5 percent, which is better than most conventional bipolar films (typically less than 90 percent).

[0125] Mechanism: The regular channels (pore size approximately 0.5 nm) of the anion exchange layer 3 (RTD-COF-TMA) exhibit a size sieving effect, and the quaternary ammonium groups have a positive effect on Na+. + Electrostatic adsorption preferentially occurs over Mg 2+ .

[0126] Long-term operational stability: such as Figure 4 Within 24 hours, the voltage fluctuation ranged from 1.45 to 1.48V (deviation less than 2.1%), the current efficiency decreased from 85.3% to 82.1%, and the attenuation rate was only 3.7%.

[0127] Membrane structure stability verification: After testing, the membrane was disassembled and observed. No delamination was observed at the three-layer interface. The sulfonic acid groups in the FTIR (1020cm) were stable.-1 ) and quaternary ammonium groups (950cm) -1 The characteristic peak intensity did not decrease, confirming the good chemical stability of the material.

[0128] Table 4 Key Test Parameter Comparison Table

[0129]

[0130] Experimental conclusion: Under the conditions of clearly defined feed amount, temperature and solution concentration, this test verified the low energy consumption (low voltage), high efficiency (high ion selectivity) and high stability of the composite bipolar membrane.

[0131] Its performance advantages mainly stem from material design: the synergistic effect of nano-metal hydroxides and ionic liquids in the catalyst layer reduces the alcohol dissociation overpotential; structural optimization: electrospun fibers and COF channels together provide directional ion transport pathways;

[0132] Process parameters: Hot pressing (90℃ / 10MPa / 40min) ensures the interlayer bonding strength, enabling the membrane to remain stable during long-term operation;

[0133] This membrane is suitable for the continuous electrosynthesis of low-carbon alcohol alkali metal salts, and the coupling relationship between current density and temperature can be further optimized to reduce energy consumption.

[0134] Sodium methoxide synthesis process optimization

[0135] Experimental objective: To optimize the effects of current density and temperature on the yield and purity of sodium methoxide.

[0136] 1. Raw material preparation and feeding amount

[0137] Anode chamber solution: Dissolve 20.4g of sodium formate (HCOONa·2H2O, AR grade) in deionized water and bring the volume to 100mL to prepare a 1.5mol / L solution. Total feed volume: 500mL.

[0138] Cathode chamber solution: Mix 450 mL of methanol (purity ≥ 998%) with 50 mL of deionized water to prepare a 90% methanol solution, with a total feed volume of 500 mL;

[0139] Electrolyte additive: 0.1 mol / L NaCl (2.92 g / L) is added to the anode chamber as a supporting electrolyte to enhance conductivity.

[0140] 2. Electrodialysis system parameters

[0141] Equipment configuration: BP-C type electrodialysis unit, effective membrane area 10cm² 2 The intermembrane spacing is 2mm;

[0142] Electrode materials: The anode uses titanium-coated ruthenium mesh (coating thickness 5μm), and the cathode uses 316L stainless steel plate;

[0143] Circulation system: The feed pump has a flow rate of 50L / h, and the temperature is controlled by a constant temperature circulating water bath (accuracy ±0.5℃).

[0144] Power supply requirements: DC regulated power supply (0–100V / 0–50A), voltage range 10–60V, automatically adjusted according to current density.

[0145] 3. Test Matrix and Operation Procedure

[0146] Variable design: Current density: 30, 40, 50, 60 mA / cm² 2 (Corresponding voltage range 12–35V);

[0147] Temperature: 30, 40, 50, 60℃ (water bath control);

[0148] Electrolysis time: fixed at 120 min, with sampling every 30 min;

[0149] Reaction procedure: The solutions in the anode and cathode chambers are circulated separately for 10 minutes for premixing and equilibration;

[0150] Start the DC power supply, apply current at the set current density, and record the real-time voltage.

[0151] After the reaction was completed, the solution in the cathode chamber was taken, concentrated by vacuum evaporation at 80°C, and methanol was recovered to obtain solid sodium methoxide.

[0152] Analysis and testing: Yield calculation: weighing method, formula: yield percentage equals actual product quality divided by theoretical output multiplied by 100%.

[0153] Purity determination: Hydrochloric acid titration method (refer to GB / T601-2016), using phenolphthalein as an indicator, to titrate the free base and total base content;

[0154] Energy consumption calculation: Energy consumption (kWh / kg) equals average voltage multiplied by current multiplied by time divided by product mass.

[0155] 4. Experimental Results and Discussion:

[0156] Table 5 Comparison of Key Optimization Parameters

[0157]

[0158] Optimal parameter combination: current density 40mA / cm 2 At a temperature of 40℃, the yield is 98.7%, the purity is 99.3%, and the energy consumption is 2.1kWh / kg, showing the best overall performance.

[0159] Effect of current density: below 40 mA / cm 2 At this stage, the reaction kinetics are slow, and the yield is low (e.g., 30 mA / cm). 2 (Hourly yield less than 97%)

[0160] Above 50 mA / cm 2 At this time, side reactions intensify (such as the oxidation of methanol to formic acid), causing the yield to decrease to 93.8% (60 mA / cm). 2 );

[0161] Temperature effect: The ion migration rate and reactivity are in optimal balance at 40℃. When the temperature rises above 50℃, methanol volatilization loss increases (volatilization rate is greater than 15%), and the yield decreases significantly.

[0162] Energy consumption comparison: The energy consumption of this process (2.1kWh / kg) is significantly lower than that of the traditional alkaline process (greater than 3.5kWh / kg) because the low dissociation voltage of the bipolar membrane reduces ohmic losses.

[0163] Verification of multi-stage serial systems

[0164] Experimental objective: To verify the improvement of sodium methoxide synthesis efficiency by a three-stage series electrodialysis system.

[0165] 1. System Configuration

[0166] Single-stage system: BP-C type electrodialysis unit, effective membrane area 10cm² 2 .

[0167] Two-stage series system: two BP-C units are connected in series, with the second bipolar membrane of the first unit serving as the first bipolar membrane of the second unit.

[0168] Three-stage series system: Three BP-C units are connected in series, with the connection method as above.

[0169] Common operating parameters: Current density 40mA / cm 2 The temperature was 40℃, the anolyte was 1.5mol / L sodium formate solution, the catholyte was 90% methanol solution, the total feed flow rate was 50L / h, the feed was countercurrent, and the total residence time was 360 minutes.

[0170] 2. Testing and Analysis Methods

[0171] Product concentration: After the catholyte from each stage is concentrated by evaporation, the mass fraction of sodium methoxide is calculated by weighing.

[0172] Current efficiency: Calculated based on Faraday's law by the total amount of charge and the actual amount of product.

[0173] Energy consumption: Energy consumption (kWh / kg) equals (average voltage multiplied by current multiplied by time) divided by sodium methoxide production.

[0174] Voltage drop: Records the total voltage difference between the system's inlet and outlet.

[0175] 3. Results and Discussion: Performance Comparison Analysis of Multi-Level Systems

[0176] Table 6 Performance Comparison of Multilevel Systems

[0177]

[0178] 4. Mechanism Discussion and Optimization Analysis

[0179] Product concentration enhancement mechanism: The core advantage of the multi-stage series system lies in the realization of progressive product concentration. The sodium methoxide solution generated in the previous stage is used as the feed for the next stage. Through countercurrent design, the product is continuously enriched in the system. Therefore, the final concentration of the three-stage system (18.5%) is much higher than that of the single-stage system (10.2%).

[0180] Current efficiency and energy consumption optimization: The series structure optimizes the ion transport path, reducing the migration of non-target ions and side reactions, thereby increasing the current efficiency with increasing stage number (from 85.3% to 91.5%). Although the total system voltage drop increases significantly with increasing stage number (from 12V to 32V), the higher current efficiency means that more electrical energy is used for the synthesis of the target product, resulting in a reduction in energy consumption per unit product (from 2.1 kWh / kg to 1.8 kWh / kg).

[0181] Challenges and Solutions for System Voltage Drop: Increasing the number of stages leads to a significant increase in the total system resistance and voltage drop. This is a major challenge for the large-scale application of series systems. Solutions can refer to advanced power management technologies, such as voltage clamping circuits or dynamic voltage balancing units. These technologies can monitor voltage at key points in the series circuit and dynamically adjust the voltage at those points through rapid charging and discharging, thereby ensuring that each stage operates within its optimal voltage window without excessively increasing the total input voltage, ultimately improving overall energy efficiency.

[0182] Performance advantages: The three-stage series system significantly outperforms single-stage and two-stage systems in terms of product concentration, current efficiency, and unit energy consumption, demonstrating the great potential of multi-stage series design in improving reaction efficiency and reducing energy consumption.

[0183] Key technologies: Countercurrent feeding and staged concentration are crucial for increasing product concentration. Addressing the challenge of high pressure drop is central to further scaling up the system (e.g., developing a four- or five-stage system).

[0184] Long-term stability test

[0185] Experimental objective: To evaluate the stability of the composite bipolar membrane and electrodialysis system during long-term operation.

[0186] Experimental steps:

[0187] The product was continuously operated for 72 hours under optimal process conditions, and samples were taken every 12 hours to analyze the product purity, membrane voltage, and current efficiency.

[0188] After operation, the membrane module was disassembled to observe the surface fouling of the membrane and test changes in membrane performance.

[0189] Analyze the changes in elements on the membrane surface and use FTIR to analyze the stability of the chemical structure.

[0190] Experimental Results and Discussion:

[0191] The yield remained at 98.5–97.2% within 72 hours, with a purity >99.0%.

[0192] The transmembrane voltage slowly increased from 1.42V to 1.52V, and the current efficiency decreased from 85.3% to 82.1%, with a low attenuation rate.

[0193] The membrane surface showed no obvious damage, and FTIR indicated that the chemical structure was stable with no loss of functional groups.

[0194] Conclusion: The composite bipolar membrane exhibits excellent long-term operational stability, meeting the requirements for industrial applications.

[0195] Catalyst layer composition optimization and structure-activity relationship study

[0196] Experimental objective: To systematically study the effects of three key variables—the Mg / Al molar ratio of nano-metal hydroxide, the content of graphene oxide (GO), and the content of ionic liquid ([BMIM][BF4])—on membrane performance (alcohol dissociation start-up voltage, membrane surface resistance, and operational stability) in the intermediate catalyst layer 2 of the composite bipolar membrane, determine the optimal composition range, and explore their synergistic mechanism.

[0197] Experimental methods:

[0198] Membrane preparation: The preparation process of fixed cation exchange layer 1 (SPFE / PVA nanofiber membrane) and anion exchange layer 3 (RTD-COF-TMA membrane) was used. Only the slurry formulation of the intermediate catalyst layer 2 was changed.

[0199] Variable design:

[0200] Series A (Mg / Al ratio): With a fixed GO content of 0.5 wt% and an ionic liquid content of 5 wt%, the molar ratio of Mg(OH)2 to Al(OH)3 is changed to 3:1, 2:1, 1:1, 1:2, and 1:3.

[0201] Series B (GO content): With a fixed Mg / Al molar ratio of 1:2 and an ionic liquid content of 5wt%, the GO content in the dry weight of the catalyst layer was changed to 0%, 0.25%, 0.5%, 1.0%, and 2.0%.

[0202] Series C (ionic liquid content): With a fixed Mg / Al molar ratio of 1:2 and GO content of 0.5wt%, the mass percentage of ionic liquid in the total weight of the slurry is changed to 0%, 2%, 5%, 10%, and 15%.

[0203] Performance testing: Following the bipolar membrane electrochemical performance testing method, the membrane formulations were tested at 50 mA / cm². 2 The alcohol dissociation start-up voltage and film resistance at current density were measured, and the constant current operation was performed for 24 hours, recording the voltage change rate (ΔV%).

[0204] Key experimental results and discussion:

[0205] 1. Effect of Mg / Al molar ratio (Series A)

[0206] Data trend: The alcohol dissociation initiation voltage first decreases and then increases with the increase of Al ratio, reaching the lowest value (1.42V) when Mg / Al=1:2. The voltages are 1.50V and 1.55V when Mg / Al=1:1 and 1:3 respectively, while the voltage is as high as 1.65V when Mg / Al=3:1.

[0207] Magnesium-rich end (e.g., 3:1): Mg 2+ It possesses moderate Lewis acidity, which is beneficial for the adsorption and polarization of methanol OH bonds. However, excessive Mg leads to an excessively low charge density on the plates, insufficient basic sites, and the deprotonation step becomes the rate-controlling step, hence the higher voltage.

[0208] Aluminum-rich end (e.g., 1:3): Al 3+ The introduction of this technology increases the charge density of the laminates and the interlayer anions (OH-). - Concentration and increased alkalinity are beneficial for RO. - Generation. However, excessively high Al content makes the laminate too rigid, and Al... 3+ The strong Lewis acidity may lead to excessive adsorption of alcohol molecules or even side reactions, disrupting the balance of the catalytic cycle.

[0209] Optimal ratio (1:2): At this ratio, a typical magnesium-aluminum layered double hydroxide (Mg-AlLDH) structure is formed. This structure realizes acidic sites (Mg... 2+ ) and basic sites (interlayer OH) - and Al 3+ The induced coordinated arrangement of adjacent O sites at the atomic scale provides the most suitable microenvironment for the adsorption-dissociation-desorption of alcohols, thereby significantly reducing the reaction energy barrier. Simultaneously, the nanoparticles prepared at this ratio exhibit the most uniform size, which is beneficial for forming a dense and highly active catalytic layer.

[0210] 2. The Influence of GO Content (Series B)

[0211] Data trend: The film surface resistivity decreases significantly with increasing GO content, reaching a plateau at 0.5% (approximately 2.8 Ω·cm). 2 The resistance of the sample without GO was as high as 8.5 Ω·cm. 2 However, when the GO content exceeds 1.0%, the mechanical brittleness of the membrane increases, and voltage fluctuations increase during long-term operation.

[0212] The two-dimensional layered structure of GO constructs a three-dimensional ionic / electronic conductive network in the catalytic layer. The oxygen-containing functional groups (-COOH, -OH) on its surface form strong hydrogen bonds or coordination bonds with the metal hydroxyl groups at the edges of Mg-AlLDH nanoparticles, achieving firm anchoring and uniform dispersion of nanoparticles and preventing aggregation and deactivation of active sites.

[0213] An appropriate amount of GO (0.5%) is sufficient to form a continuous network. Excessive GO, due to its insulating properties and stacking tendency, will hinder the transport of ionic liquids and reactants, and reduce the interfacial compatibility between the catalyst layer and the upper and lower film layers, leading to a decline in performance.

[0214] 3. The effect of ionic liquid content (Series C)

[0215] Data trends: Both the start-up voltage and the 24-hour voltage change rate initially decreased and then increased with increasing ionic liquid content. At a content of 5%, the start-up voltage was the lowest (1.42V), and the 24-hour voltage increase was the smallest (<2%). Without ionic liquid, the start-up voltage was as high as 1.85V, and the voltage was unstable in the initial stage of operation. When the content exceeded 10%, the membrane underwent over-swelling, and after long-term operation, there was slight loss of the catalyst layer.

[0216] Ionic liquids play a triple role in this system:

[0217] ① Plasticizers improve the flexibility of the catalyst layer;

[0218] ②Proton carrier, [BF4] in [BMIM][BF4] - Anions readily bind and dissociate protons, forming ultrafast proton conduction under the Grothuss mechanism;

[0219] ③ Interface stabilizers, whose ion pairs can effectively shield the electrostatic repulsion between nanoparticles and GO sheets, and interact with polymer proton exchange groups to form a stable "ion bridge" at the organic / inorganic interface.

[0220] At the optimal concentration (5%), the ionic liquid just barely wets the entire catalytic network, forming a continuous proton transport channel without causing significant swelling. If the concentration is too low, the conduction network will be discontinuous; if it is too high, the integrity of the catalytic layer structure will be compromised.

[0221] Optimal combination verification: The composite bipolar membrane prepared with the optimized formula of Mg / Al=1:2, GO=0.5%, [BMIM][BF4]=5% showed that the alcohol dissociation voltage was stable at 1.45±0.03V and the current efficiency was maintained above 82% during 120 hours of continuous operation test. The performance degradation rate was much lower than that of other non-optimized combinations.

[0222] Conclusion of this embodiment: The performance of the catalyst layer is not a simple sum of the functions of each component, but depends on the precise balance and strong synergistic effect among the intrinsic catalytic activity of Mg-AlLDH nanocrystals, the conductive and stable framework constructed by GO, and the interfacial proton transport microenvironment modulated by ionic liquid.

[0223] Study on the long-term operational stability of gradient-optimized multi-stage series systems

[0224] Experimental objective: To compare and verify the performance differences between the "symmetric current density series system" and the "gradient current density series system" under long-term continuous operation, especially the suppression effect of voltage rise and the ability to maintain energy efficiency.

[0225] System design and operating conditions:

[0226] System: Three-stage series electrodialysis system (construction method is the same as that used in the verification of multi-stage series systems).

[0227] Raw materials: The anolyte is 1.5 mol / L sodium formate, and the catholyte is 90% methanol solution.

[0228] Total stay: 360 minutes.

[0229] Comparison of options:

[0230] Option 1 (Symmetrical System): All three stages use the same current density of 40mA / cm². 2 .

[0231] Option 2 (Gradient Optimization System): Gradient current density is used.

[0232] Level 1: 30mA / cm 2 (Gentle transformation, avoiding side effects);

[0233] Level 2: 40mA / cm 2 (High-efficiency conversion and concentration);

[0234] Level 3: 45mA / cm 2 (Deep concentration to overcome mass transfer resistance at high concentrations).

[0235] The overall average current density is similar to that of Scheme 1.

[0236] Operation and Monitoring: The two systems were operated in parallel continuously for 120 hours under the same conditions. Every 8 hours, the inlet and outlet voltages at each stage, the total system voltage drop, the final product concentration and current efficiency were recorded, and the cumulative energy consumption per unit product was calculated.

[0237] Key experimental results and discussion:

[0238] 1. Comparison of system voltage stability

[0239] Symmetrical system: The total voltage drop of the system was 32V at the beginning of operation. As the operating time increased, the total voltage drop continued to rise linearly, reaching 48V at 120 hours, an increase of 50%. Among them, the voltage drop of the third stage contributed the main increase. Analysis showed that the excessively high product concentration in the cathode chamber led to intensified concentration polarization, and slight contamination or precipitation may have occurred on the membrane surface.

[0240] Gradient optimization system: In the initial stage of operation, the total voltage drop of the system was 30V (the voltage of the first stage was relatively low). During 120 hours of operation, the total voltage drop stabilized between 31-33V, with a fluctuation range of <7%. The voltage of each stage remained stable, and there was no sharp increase in voltage of any single stage.

[0241] The gradient design optimizes the "reaction load" of each stage. In the first stage, a lower current density is used to treat high-concentration feedstocks, reducing localized overheating caused by excessively rapid reactions and the accumulation of byproducts (such as formate) on the membrane surface, thus mitigating membrane fouling at its source. In subsequent stages, as feedstock concentration decreases and product concentration increases, the current density is appropriately increased to maintain sufficient reaction driving force, while avoiding severe polarization in the final stage due to excessively high current density at the limiting concentration. This dynamic balance strategy of "pre-stage anti-fouling and post-stage anti-polarization" is key to the long-term stable operation of the system.

[0242] 2. Comparison of long-term operating performance

[0243] Table 7 Data Comparison Table (Average Value over 120 Hours)

[0244]

[0245] 3. Optimization Analysis

[0246] The advantage of gradient optimization systems lies in achieving "process matching." By adjusting the current density at each stage, the electrochemical driving force of each stage is matched with the reactant / product concentration environment, thereby maintaining the operating point of the entire series system at a consistently high efficiency, low consumption, and low pollution level. This is a system-level active control strategy based on a deep understanding of the kinetics of multi-stage electrodialysis processes.

[0247] Conclusion of this embodiment: Compared with simple symmetrical series connection, the gradient current density optimization strategy proposed in this invention can significantly suppress voltage rise and efficiency decay during long-term system operation, reduce energy consumption, and effectively mitigate membrane fouling. This proves that the innovation of this invention lies not only in the system hardware, but also in the innovation of the accompanying intelligent and adaptive operation strategy, which together constitute an efficient, stable, and reliable solution for the synthesis of low-carbon alcohol-alkali metal salts.

[0248] The specific embodiments are merely illustrative of the present invention and are not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to these embodiments without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. A composite bipolar membrane for preparing low-carbon alcohol alkali metal salts, characterized in that, It includes a cation exchange layer (1), an intermediate catalyst layer (2), and an anion exchange layer (3) stacked sequentially. The cation exchange layer (1) is a sulfonated polyphenylene fluoride nanofiber membrane that has been cross-linked with a cross-linking agent; The intermediate catalyst layer (2) comprises nanoscale composite metal hydroxide, graphene oxide, tetrabutylammonium hydroxide and ionic liquid; The anion exchange layer (3) is a quaternary ammonium salt functionalized covalent organic framework material membrane; The intermediate catalyst layer (2) uses nanoscale composite metal hydroxide as the main catalyst, which is blended with graphene oxide and tetrabutylammonium hydroxide to form a composite catalyst layer, and an ionic liquid is added; the nanoscale composite metal hydroxide includes magnesium hydroxide and aluminum hydroxide, with a mass ratio of 1:1 to 1:3, and the ionic liquid is 1-butyl-3-methylimidazolium tetrafluoroborate. The anion exchange layer (3) is obtained by forming a covalent organic framework precursor membrane by aldehyde-amine condensation reaction of terephthalaldehyde and tris(4-aminophenyl)amine, followed by acetic acid-catalyzed crystallization, reduction with NaBH3CN, and then quaternization reaction with trimethylamine.

2. The composite bipolar membrane for preparing low-carbon alcohol alkali metal salts according to claim 1, characterized in that, The cation exchange layer (1) is made of sulfonated polyphenylene fluoride as substrate and is constructed as a nanofiber membrane by electrospinning technology; Polyvinyl alcohol is added during the spinning process, and then ferric chloride solution is used as a crosslinking agent to crosslink the material, forming a three-dimensional network structure; the degree of sulfonation of the sulfonated polyphenylene fluoride is not less than 35%, and the degree of alcoholysis of the polyvinyl alcohol is not less than 99%.

3. The composite bipolar membrane for preparing low-carbon alcohol alkali metal salts according to claim 1, characterized in that, The cation exchange layer (1), intermediate catalyst layer (2) and anion exchange layer (3) are hot-pressed together at 70-100℃, 0.5-2.0MPa and 30-60min to obtain a composite bipolar membrane.

4. An electrodialysis process for preparing low-carbon alcohol alkali metal salts, characterized in that, include: (1) A salt solution containing alkali metal ions is introduced into the anode chamber of the bipolar membrane electrodialysis system, and a low-carbon alcohol solution is introduced into the cathode chamber; the bipolar membrane electrodialysis system includes at least one electrodialysis unit, the electrodialysis unit includes an anode chamber, a cathode chamber, and a composite bipolar membrane for preparing low-carbon alcohol alkali metal salts as described in any one of claims 1 to 3 disposed between the anode chamber and the cathode chamber; (2) Apply a DC electric field to both sides of the composite bipolar membrane to cause the low-carbon alcohol to dissociate at the membrane interface to generate alkoxy ions and hydrogen ions; (3) The alkoxy ions migrate to the anode chamber and combine with the alkali metal ions to generate low-carbon alcohol alkali metal salts.

5. The electrodialysis process for preparing low-carbon alcohol alkali metal salts according to claim 4, characterized in that, The bipolar membrane electrodialysis system adopts a multi-stage series bipolar membrane electrodialysis system, including at least two electrodialysis units, and the composite bipolar membrane on the side near the cathode chamber in the previous stage of the electrodialysis unit also serves as the composite bipolar membrane on the side near the anode chamber in the next stage of the electrodialysis unit.

6. The electrodialysis process for preparing low-carbon alcohol alkali metal salts according to claim 4, characterized in that, The DC electric field current density is 10-200 A / m², the reaction temperature is 0-60℃, the alkali metal salt solution is sodium formate with a concentration of 0.2-2 mol / L, and the low-carbon alcohol solution is methanol with a concentration of 70-99 vol.

7. The electrodialysis process for preparing low-carbon alcohol alkali metal salts according to claim 6, characterized in that, The DC electric field current density is 35-45 A / m², and the reaction temperature is 35-45℃.

8. The electrodialysis process for preparing low-carbon alcohol alkali metal salts according to claim 5, characterized in that, The multi-stage series bipolar membrane electrodialysis system employs gradient current density.

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