A method for preparing microporous carbon fiber membranes by aqueous electrospinning and its application

Abstract

This invention relates to a method for preparing self-activated lignin-based microporous carbon fiber membranes via aqueous electrospinning and its applications. Utilizing lignin, an industrial waste pollutant, as the carbon source and eliminating organic solvents, the method employs electrospinning with an aqueous sodium hydroxide solution of industrial lignin as a precursor. Through spinning, thermal stabilization, and carbonization, the carbon fibers are in-situ self-activated using sodium hydroxide during heat treatment, resulting in a flexible carbon fiber membrane material with a surface micropore diameter of approximately 0.45 nm (the diameter of formaldehyde gas molecules). This invention avoids the environmental pollution and energy consumption problems that may arise from traditional activation methods, while also solving the problem of using toxic organic solvents in electrospinning. It features low raw material costs, high formaldehyde adsorption capacity, good adsorption selectivity and stability, one-step spinning, and a simple and environmentally friendly process. This technology provides a novel solution for the green preparation of flexible microporous carbon fiber membranes and has broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of carbon fiber preparation technology, and more specifically, relates to a method for preparing microporous carbon fiber membranes by aqueous electrospinning and its application. Background Technology

[0002] Formaldehyde possesses reproductive toxicity, genotoxicity, and carcinogenic effects. It primarily enters the human body through respiration, but can also be absorbed through the skin or ingested. Even extremely low concentrations can pose health risks, making it considered one of the most threatening indoor air pollutants. Unlike other VOCs, formaldehyde's high vapor pressure (3883 mmHg at 25°C) and relatively low boiling point (-19.3°C) mean that ordinary adsorbents cannot achieve ideal removal results. Developing highly efficient formaldehyde removal technologies, aiming for higher adsorption capacity and better adsorption stability, has become a research hotspot in environmental protection and public health.

[0003] Traditional adsorption materials such as activated carbon, zeolite, and silica gel, while possessing certain adsorption capacities, have limitations in terms of adsorption capacity, regenerability, and adsorption selectivity. In recent years, with the deepening research into green and environmentally friendly materials, carbonaceous adsorption materials developed based on biomass resources have gradually become an emerging research direction. Among them, the preparation of activated carbon fibers using industrial lignin as a carbon source has advantages such as low cost, renewability, and environmental friendliness, demonstrating enormous application potential in multiple fields such as supercapacitors, adsorption materials, filters, and flexible electronic devices.

[0004] In the preparation of activated carbon fibers, the activation step is crucial to the specific surface area and pore size distribution of the carbon fibers, which directly determine their application performance. Common activation methods include high-temperature gas activation and alkaline corrosion activation. While these methods can increase the specific surface area and pore size of carbon fibers, they also bring about problems such as energy consumption, environmental pollution, and increased production costs. Therefore, developing a low-cost, green, and environmentally friendly activation process has become a research focus in the production of lignin-based carbon fibers.

[0005] Electrospinning, as a highly efficient fiber preparation method, can produce fibrous membranes ranging from nanometers to micrometers. Due to their extremely high specific surface area and significant porosity, these membrane materials exhibit unique performance advantages in fields such as filtration, sensors, and energy storage. However, traditional electrospinning technologies typically rely on flammable, toxic, or difficult-to-handle solvents. The use of these solvents not only poses safety hazards but also potentially leads to environmental pollution and energy consumption. Specifically, halogenated solvents (such as chloroform and trifluoroethanol) and toxic solvents (such as dimethylformamide) are the most commonly used solvents in electrospinning, which to some extent limits the promotion of this technology in large-scale commercial applications. To enhance the market competitiveness of electrospinning technology and improve its sustainable development potential, developing aqueous solution electrospinning technology has become an ideal choice that is greener, more environmentally friendly, and scalable.

[0006] This invention utilizes lignin, an industrial waste pollutant, as a carbon source, eliminating organic solvents and employing electrospinning with an aqueous sodium hydroxide solution of industrial lignin as a precursor. Through spinning, thermal stabilization, and carbonization, sodium hydroxide is used to in-situ self-activate the carbon fibers during heat treatment, resulting in a flexible carbon fiber membrane material with a surface micropore diameter of approximately 0.45 nm (the diameter of formaldehyde gas molecules). The introduction of the self-activation process further reduces production costs while avoiding the environmental pollution and energy consumption problems that may arise from traditional activation methods. It features low raw material costs, high formaldehyde adsorption capacity, good adsorption selectivity and stability, one-step spinning, and a green, environmentally friendly, and sustainable process. This technology provides a novel solution for the green preparation of flexible microporous carbon fiber membranes and has broad application prospects. Summary of the Invention

[0007] To address the problems of expensive raw materials, the use of organic solvents in spinning preparation, the necessity of activation and pore formation, and weak adsorption capacity of existing carbon fiber, this invention provides an aqueous electrospinning method for preparing a self-activated lignin-based flexible microporous carbon fiber membrane. The micropore size is concentrated at around 0.5 nm, which is close to the diameter of formaldehyde gas molecules (0.45 nm), enabling more effective capture of formaldehyde while improving adsorption stability and reducing secondary pollution.

[0008] Another object of the present invention is to protect the lignin-based flexible microporous carbon fiber membrane prepared by the method described in the present invention.

[0009] The final objective of this invention is to protect the application of the flexible microporous carbon fiber membrane described herein in the fields of supercapacitors, adsorption materials, filters, and flexible electronic devices.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

[0011] (1) Lignin is spun into precursor (lignin) filaments by electrospinning technology.

[0012] In the presence of a spinning aid, lignin is dissolved in a solvent to prepare a spinning solution, which is then electrospun using a CO2 airflow-assisted process to obtain precursor (lignin) filaments.

[0013] The lignin is an industrial-grade sulfate-process lignin, a byproduct of sulfate pulping and papermaking, and its biomass source includes at least one of coniferous wood, broadleaf wood, and herbaceous plants (such as pine, poplar, and corn stalks).

[0014] Preparation of spinning solution: Excluding sodium hydroxide, the mass fraction of the solute (industrial lignin and spinning aid) in the spinning solution is 5-20%, wherein the mass ratio of the spinning aid polyethylene oxide (PEO) to lignin is 5-20:95-80. The solvent is deionized water or tap water. Sodium hydroxide is used as an activator, and its concentration in the spinning solution is 0.25-2 mol / L. The spinning solution can be prepared using a one-pot method: At 40-70℃, industrial lignin, PEO, and sodium hydroxide are dissolved in the solvent under stirring. After complete dissolution, the spinning solution is cooled to room temperature and allowed to stand. Electrospinning is then carried out within 0-24 hours.

[0015] Electrospinning: CO2 gas flow is used to assist spinning at a flow rate of 0.25–1.5 L / min; the spinning voltage is 12–20 kV; the feed pump speed is 0.5–5 ml / h; spinning is carried out at room temperature, and the humidity is controlled below 60%. After spinning, the resulting fiber membrane is dried overnight in an oven at 60–80°C.

[0016] (2) The lignin precursor filament spun in step (1) is subjected to thermal stabilization and carbonization treatment to obtain lignin-based flexible microporous carbon fibers. This invention does not require any activation step; self-activation and pore formation occur during the carbonization process, resulting in carbon fiber micropores with a diameter concentrated in the range of 0.4–0.6 nm, exhibiting a high specific surface area and a stable micro / nano structure.

[0017] Heat stabilization treatment: Place the dried fiber membrane in a tube furnace or atmosphere furnace and keep it at 250℃ for 0.5 to 2 hours in an air atmosphere, with a heating rate of 2 to 5℃ / min.

[0018] Carbonization: After thermal stabilization pretreatment, high-purity nitrogen is introduced into a tube furnace, and the temperature is raised to 700-1000℃ in a nitrogen atmosphere and held for 0.5-1h at a heating rate of 5-10℃ / min. After the holding period ends and the temperature is cooled to room temperature, microporous carbon fibers can be obtained.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] (1) Industrial-grade sulfate lignin is a byproduct of sulfate pulping and papermaking, which is a high-value utilization of industrial byproducts and solves the problem of high cost of carbon fiber raw materials from the source.

[0021] (2) The solvent for preparing the spinning solution is water, which can be directly used by tap water, thus completely solving the problem of organic solvent pollution in electrostatic spinning.

[0022] (3) The spinning solution is prepared in one pot and activated in situ during carbonization, which greatly simplifies the production process and saves energy consumption.

[0023] (4) Using CO2 airflow-assisted electrospinning can effectively suppress the generation of electric sparks during aqueous electrospinning, and at the same time solve the problem of spontaneous combustion of carbonized samples when exposed to air, thus fully ensuring experimental safety and sample yield.

[0024] (5) The microporous carbon fiber prepared has excellent flexibility and is easy to device. The micropore diameter is concentrated at about 0.5 nm, which is close to the diameter of formaldehyde gas molecules (0.45 nm). It can capture formaldehyde more effectively, while improving adsorption stability and reducing secondary pollution. It is an ideal material for preparing air purification filter elements. Attached Figure Description

[0025] Figure 1 Scanning electron microscope (SEM) images of the carbon fiber membranes prepared in Examples 1-6;

[0026] Figure 2 Nitrogen adsorption-desorption curves and micropore size distribution of the carbon fiber membrane prepared in Example 6;

[0027] Figure 3 Dynamic formaldehyde performance tests in Examples 1-6;

[0028] The background formaldehyde concentration was 12±1ppm, the air flow rate was 0.6L / min, and the carbon fiber adsorbent mass was 30mg.

[0029] Figure 4 Photographs of the spontaneous combustion of the carbon fiber membrane prepared in Comparative Example 1;

[0030] Figure 5 Microscopic photograph of the electrospinning product of the spinning solution prepared in Comparative Example 2.

[0031] Figure 6 Flocculation photograph of the spinning solution prepared in Comparative Example 3. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments.

[0033] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0034] In the embodiments, unless otherwise specified, all methods used are conventional methods in the art.

[0035] The terms “comprising,” “including,” or any other variations thereof, as used herein, are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0036] Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0037] Example 1:

[0038] Raw material: Poplar lignin produced by sulfate process.

[0039] Preparation of spinning solution: Excluding sodium hydroxide, the mass fraction of the solute (industrial lignin and PEO) in the spinning solution is 10%, with a PEO to lignin mass ratio of 20:80. The solvent is tap water. The concentration of sodium hydroxide in the spinning solution is 0.25 mol / L. The spinning solution can be prepared using a one-pot method: at 70℃, industrial lignin, PEO, and sodium hydroxide are dissolved in the solvent under stirring. After complete dissolution, the spinning solution is cooled to room temperature, and electrospinning is immediately performed.

[0040] Electrospinning: CO2 gas flow was used to assist spinning at a flow rate of 0.25 L / min; the spinning voltage was 20 kV, the feed pump speed was 0.5 ml / h, and spinning was carried out at room temperature with humidity controlled below 60%. After spinning, the resulting fiber membrane was dried overnight in an oven at 60°C.

[0041] Heat stabilization treatment: Place the dried fiber membrane in a tube furnace or atmosphere furnace and keep it at 250℃ for 0.5h in an air atmosphere, with a heating rate of 2℃ / min.

[0042] Carbonization: After heat stabilization pretreatment, high-purity nitrogen is introduced into a tube furnace, and the temperature is raised to 900℃ in a nitrogen atmosphere and held for 0.5 h at a heating rate of 5℃ / min. After cooling to room temperature, the microporous carbon fiber membrane of Example 1 is obtained, and its scanning electron microscope results are as follows: Figure 1 As shown in Table 1, nitrogen adsorption-desorption tests were performed on the sample. The pore size characteristics and specific surface area were measured.

[0043] Example 2:

[0044] Raw material: Pine wood sulfate lignin.

[0045] Preparation of spinning solution: Excluding sodium hydroxide, the mass fraction of the solute (industrial lignin and PEO) in the spinning solution is 5%, with a PEO to lignin mass ratio of 10:90. The solvent is tap water. The concentration of sodium hydroxide in the spinning solution is 1.0 mol / L. The spinning solution can be prepared using a one-pot method: at 70℃, industrial lignin, PEO, and sodium hydroxide are dissolved in the solvent under stirring. After complete dissolution, the spinning solution is cooled to room temperature and allowed to stand for 24 hours before electrospinning.

[0046] Electrospinning: CO2 gas flow was used to assist spinning at a flow rate of 1 L / min; the spinning voltage was 18 kV, the feed pump speed was 2 ml / h, and spinning was carried out at room temperature with humidity controlled below 60%. After spinning, the resulting fiber membrane was dried overnight in an oven at 60°C.

[0047] Heat stabilization treatment: Place the dried fiber membrane in a tube furnace or atmosphere furnace and keep it at 250℃ for 1 hour in an air atmosphere, with a heating rate of 5℃ / min.

[0048] Carbonization: After heat stabilization pretreatment, high-purity nitrogen is introduced into a tube furnace, and the temperature is raised to 900°C in a nitrogen atmosphere and held for 1 hour at a heating rate of 10°C / min. After cooling to room temperature, the microporous carbon fiber membrane of Example 2 is obtained, and its scanning electron microscope results are as follows: Figure 1 As shown in Table 1, nitrogen adsorption-desorption tests were performed on the sample. The pore size characteristics and specific surface area were measured.

[0049] Example 3:

[0050] The difference from Example 2 is that the carbonization temperature is 700°C.

[0051] Example 4:

[0052] The difference from Example 2 is that the carbonization temperature is 1000°C.

[0053] Example 5:

[0054] Raw material: Straw sulfate lignin.

[0055] Preparation of spinning solution: Excluding sodium hydroxide, the mass fraction of the solute (industrial lignin and PEO) in the spinning solution is 10%, with a PEO to lignin mass ratio of 10:90. The solvent is tap water. The concentration of sodium hydroxide in the spinning solution is 1.5 mol / L. The spinning solution can be prepared using a one-pot method: at 40℃, industrial lignin, PEO, and sodium hydroxide are dissolved in the solvent under stirring. After complete dissolution, the spinning solution is cooled to room temperature and allowed to stand for 12 hours before electrospinning.

[0056] Electrospinning: CO2 gas flow was used to assist spinning at a flow rate of 0.6 L / min; the spinning voltage was 16 kV, the feed pump speed was 3 ml / h, and spinning was carried out at room temperature with humidity controlled below 60%. After spinning, the resulting fiber membrane was dried overnight in an oven at 80°C.

[0057] Heat stabilization treatment: Place the dried fiber membrane in a tube furnace or atmosphere furnace and keep it at 250℃ for 1 hour in an air atmosphere, with a heating rate of 5℃ / min.

[0058] Carbonization: After heat stabilization pretreatment, high-purity nitrogen is introduced into a tube furnace, and the temperature is raised to 900℃ in a nitrogen atmosphere and held for 1 hour at a heating rate of 10℃ / min. After cooling to room temperature, the microporous carbon fiber membrane of Example 5 is obtained, and its scanning electron microscope results are as follows: Figure 1 As shown in Table 1, nitrogen adsorption-desorption tests were performed on the sample. The pore size characteristics and specific surface area were measured.

[0059] Example 6:

[0060] Raw material: Pine wood sulfate lignin.

[0061] Preparation of spinning solution: Excluding sodium hydroxide, the mass fraction of the solute (industrial lignin and PEO) in the spinning solution is 20%, with a PEO to lignin mass ratio of 5:95. The solvent is deionized water. The concentration of sodium hydroxide in the spinning solution is 2.0 mol / L. The spinning solution can be prepared using a one-pot method: at 40℃, industrial lignin, PEO, and sodium hydroxide are dissolved in the solvent under stirring. After complete dissolution, the spinning solution is cooled to room temperature and allowed to stand for 24 hours before electrospinning.

[0062] Electrospinning: CO2 gas flow was used to assist spinning at a flow rate of 1.5 L / min; the spinning voltage was 12 kV, the feed pump speed was 5 ml / h, and spinning was carried out at room temperature with humidity controlled below 60%. After spinning, the resulting fiber membrane was dried overnight in an oven at 80°C.

[0063] Heat stabilization treatment: Place the dried fiber membrane in a tube furnace or atmosphere furnace and keep it at 250℃ for 1 hour in an air atmosphere, with a heating rate of 5℃ / min.

[0064] Carbonization: After thermal stabilization pretreatment, high-purity nitrogen is introduced into a tube furnace, and the temperature is raised to 900°C in a nitrogen atmosphere and held for 1 hour at a heating rate of 10°C / min. After cooling to room temperature, the microporous carbon fiber membrane of Example 6 is obtained, and its scanning electron microscope results are as follows: Figure 1 As shown in Table 1, nitrogen adsorption-desorption tests were performed on the sample. The pore size characteristics and specific surface area were measured. The nitrogen adsorption-desorption curves and micropore size distribution are shown in Table 1. Figure 2 .

[0065] Formaldehyde performance testing: Dynamic formaldehyde performance tests in Examples 1-6 are as follows Figure 3 As shown in the figure. Air was used as the carrier gas in the test, with a background formaldehyde concentration controlled at 12±1 ppm, an airflow rate of 0.6 L / min, and a carbon fiber adsorbent mass of 30 mg. The tests showed that Examples 2, 3, and 6 exhibited better resistance to formaldehyde penetration. Therefore, it can be concluded that micropores larger than 0.45 nm and a higher micropore area play a crucial role in resisting formaldehyde penetration.

[0066] Comparative Example 1:

[0067] The difference from Example 2 is that no CO2 airflow was used during the electrospinning process, and the airflow rate was 0 L / min. After carbonization, the sample spontaneously combusted upon contact with air, as shown in the comparison photographs. Figure 4 As shown.

[0068] Comparative Example 2:

[0069] The difference from Example 2 is that the concentration of sodium hydroxide in the spinning solution was 0.2 mol / L. During electrospinning, droplets were ejected instead of fibers, thus failing to successfully prepare lignin precursor fibers. Comparative optical microscope images are shown below. Figure 5 As shown.

[0070] Comparative Example 3:

[0071] The difference from Example 2 is that the concentration of sodium hydroxide in the spinning solution was 2.5 mol / L. During the preparation of the spinning solution, flocculation occurred, making spinning impossible. Comparison photos of the spinning solutions are shown below. Figure 6 As shown.

[0072] As shown in the comparative examples, CO2-assisted spinning is an essential process for successful fiber carbonization, which is related to the thermal stability of sodium hydroxide and sodium carbonate. Utilizing CO2-assisted spinning to convert sodium hydroxide in the solution into sodium carbonate effectively reduces the residual elemental sodium after carbonization, thereby reducing the risk of spontaneous combustion. Simultaneously, sodium hydroxide acts as an activator, and its concentration in the spinning solution should be between 0.25 and 2 mol / L; concentrations that are too low or too high will hinder normal spinning. Finally, flexible microporous carbon fiber membranes can be successfully prepared at carbonization temperatures between 700 and 1000℃, but a carbonization temperature of 900℃ is preferred for obtaining a higher specific surface area.

[0073] Table 1 shows the pore size characteristics and specific surface area of ​​Examples 1-6.

[0074]

[0075]

[0076] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A method for preparing lignin-based microporous carbon fiber membranes by aqueous electrospinning, characterized in that, The lignin is an industrial-grade sulfate-process lignin, a byproduct of sulfate pulping and papermaking, and is used as a raw material for carbon fiber preparation, including the following steps: (1) Lignin is spun into precursor lignin filaments by aqueous electrospinning technology; wherein, the spinning solution is prepared by one-pot method, and lignin, spinning aid and activator are dissolved together in solvent. The solvent of the spinning solution is deionized water or tap water; the activator is sodium hydroxide, and its concentration in the spinning solution is 0.25~2 mol / L; the spinning aid is polyethylene oxide (PEO), and the mass ratio of PEO to lignin is 5~20:95~80; excluding the activator, the mass fraction of lignin and spinning aid in the spinning solution is 5~20%; CO2 airflow is used to assist spinning, with an airflow rate of 0.25~1.5 L / min; (2) The lignin precursor spun in step (1) is subjected to thermal stabilization and carbonization treatment to obtain a flexible microporous carbon fiber membrane; wherein the micropore size of the carbon fiber is concentrated in 0.4~0.6 nm.

2. A lignin-based flexible microporous carbon fiber membrane, characterized in that, It is prepared by the method described in claim 1.

3. The lignin-based flexible microporous carbon fiber membrane according to claim 2, characterized in that, The micropore area accounts for 56% to 81% of the total specific surface area, and the micropore size is concentrated in the range of 0.4 to 0.6 nm.

4. The application of the lignin-based flexible microporous carbon fiber membrane prepared by the preparation method of claim 1 or the lignin-based flexible microporous carbon fiber membrane of claims 2-3 in the fields of supercapacitors, adsorption materials, filters and flexible electronic devices.

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