Nanofiber membrane and carbon nanofiber containing oxidized starch, and preparation method and application thereof

Abstract

The present invention discloses a nanofiber membrane containing oxidized starch and carbon nanofibers, as well as preparation methods and applications thereof. The present invention comprises the following steps: dispersing oxidized starch in water and then gelatinizing to obtain an oxidized starch solution; dispersing polyvinyl alcohol in water and then stirring to obtain a polyvinyl alcohol solution; mixing the oxidized starch solution and the polyvinyl alcohol solution to form an oxidized starch / polyvinyl alcohol mixed solution having a solute mass fraction of 5 to 15%; electrospinning the oxidized starch / polyvinyl alcohol mixed solution to prepare a nanofiber membrane containing oxidized starch; pre-oxidizing the nanofiber membrane containing oxidized starch in air and then carbonizing it under a protective atmosphere to obtain carbon nanofibers. The nanofiber membrane containing oxidized starch of the present invention has a high starch content, low cost, and high spinnability; and the carbon nanofiber of the present invention can efficiently adsorb target gases, has low cost, and is simple to process.

Description

Technical Field

[0001] The present invention belongs to the technical field of food and chemical industry, and particularly relates to a nanofiber membrane containing oxidized starch and carbon nanofiber, as well as a preparation method and application thereof. Background Art

[0002] The plant hormone ethylene is produced during the early stages of fruit ripening and is used to produce various ripening enzymes (Kumar et al. 2019). Different types of fruits and vegetables have varying degrees of sensitivity to ethylene. Generally, an ethylene concentration of 10–100 nL / L accelerates ripening (Aghdam et al. 2019; Sadeghi et al. 2021). Furthermore, the persistent presence of ethylene can lead to overripening, senescence, reduced quality, and a shortened postharvest shelf life. Ethylene is arguably one of the most significant factors contributing to postharvest quality loss in fruits and vegetables. During transportation and storage, even at extremely low concentrations ranging from ppm (μL / L) to ppb (nL / L), ethylene can cause fresh fruits and vegetables to deteriorate more rapidly, rot, lose firmness, and shorten their shelf life (Ulloa 2007). Reducing ethylene concentrations during the storage of ethylene-sensitive, perishable commodities has significant economic benefits (Keller et al. 2013).

[0003] Porous materials have advantages such as high specific surface area, strong chemical stability, hydrothermal stability, and adjustable pore structure, making them promising for development and application in the field of gas adsorption. In recent years, research on the use of porous materials for ethylene adsorption has increased. CN115090265A discloses a method for preparing a highly efficient adsorbent for ethylene from refinery dry gas. The adsorbent is a mechanical mixture of a metal-organic framework material and activated carbon or an ordered mesoporous material. By adjusting the material ratio, the adsorption capacity of ethylene can be achieved in the range of 5 to 45 cm3. 3 / g. CN101371979 discloses an adsorbent for adsorbing trace amounts of ethylene in carbon dioxide and its preparation method. The adsorbent is composed of a monovalent copper compound, a divalent manganese compound, a divalent zinc compound, and a carrier HY, Hβ, or HZSM-5 molecular sieve. It is prepared by high-temperature roasting and has a saturated adsorption capacity for ethylene of 1.613 to 2.122 cm 3 / g. CN103285810A discloses an adsorbent for adsorbing ethylene and its preparation method. The adsorbent is composed of copper, aluminum, zinc, molecular sieve and activated carbon. When adsorbing ethylene, it can reach 22.97cm 3 / g of saturated adsorption capacity. The above-mentioned porous materials have a certain ethylene adsorption capacity, but the saturated adsorption capacity is relatively low, and the selected carbon source is relatively expensive, and the production cost is high. Carbon nanofibers (CNFs) are composed of one-dimensional nanomaterials with sp2 hybridized carbon, similar to the structure of carbon nanotubes, with rich nanoscale properties, and can present different shapes, diameters and lengths. CNF has high specific surface area, high mechanical strength, high aspect ratio, low density, environmental stability and compatibility with other matrix materials, and can interact with organic molecules through non-covalent bonds, such as electrostatic and hydrophobic interactions, hydrogen bonds, van der Waals forces and π-π conjugation, and is suitable for small molecule gas adsorption.

[0004] Electrospinning can reshape carbon precursor polymers and obtain continuous and uniform polymer nanofibers by adjusting the preparation conditions. Currently, the raw materials for preparing electrospun nanofibers include polyacrylonitrile (PAN), asphalt, polyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), phenolic resin, biomass and its derivatives. Among the many precursors, biomass and its derivatives have received widespread attention as precursors for preparing nanofibers. Electrospinning provides a solution for the simple and low-cost production of nanocarbon fibers, but the adsorption performance of the prepared nanocarbon fibers is still not high. Summary of the Invention

[0005] To address the shortcomings of the prior art, the present invention aims to provide a method for preparing electrostatically blended oxidized starch / polyvinyl alcohol nanofiber membranes and carbon nanofibers that is simple to operate, high in yield, and low in cost. This method involves oxidatively modifying starch to prepare the oxidized starch / polyvinyl alcohol nanofiber membranes using gelatinized starch and polyvinyl alcohol, thereby improving the starch's spinnability. The oxidized starch / polyvinyl alcohol nanofiber membranes are then pre-oxidized and carbonized to produce structurally uniform carbon nanofibers. The carbon nanofibers of this invention are capable of efficiently adsorbing the small molecule gas ethylene.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] A method for preparing a nanofiber membrane containing oxidized starch comprises the following steps:

[0008] (1) dispersing oxidized starch in water and then gelatinizing to obtain an oxidized starch solution;

[0009] (2) dispersing polyvinyl alcohol in water and stirring to obtain a polyvinyl alcohol solution;

[0010] (3) mixing the oxidized starch solution obtained in step (1) and the polyvinyl alcohol solution obtained in step (2) to form an oxidized starch / polyvinyl alcohol mixed solution with a solute mass fraction of 5 to 15%;

[0011] (4) electrospinning the oxidized starch / polyvinyl alcohol mixed solution prepared in step (3) to prepare a nanofiber membrane containing oxidized starch.

[0012] Preferably, in step (1), the carboxyl content of the oxidized starch is 0.08% to 0.64%;

[0013] Further preferably, the carboxyl content of the oxidized starch is 0.35% to 0.45%;

[0014] Preferably, in step (1), the oxidized starch is oxidized starch prepared by wet oxidation of starch with hydrogen peroxide and / or sodium hypochlorite.

[0015] Further preferably, the preparation method of the oxidized starch is: using hydrogen peroxide and / or sodium hypochlorite as an oxidant and copper sulfate as a catalyst to oxidize starch to obtain oxidized starch;

[0016] More preferably, the starch is at least one of corn starch, potato starch and tapioca starch; the mass ratio of copper sulfate to starch is 0.01-0.1%; the hydrogen peroxide and / or sodium hypochlorite accounts for 8.25-49.5% of the mass of starch (dry matter) (calculated based on the density of a hydrogen peroxide solution with a mass concentration of 30% in the embodiment as 1.1 g / mL); the temperature of the oxidation treatment is 30-60° C., and the time is 3-5 h.

[0017] Preferably, in step (1), the mass ratio of the oxidized starch to water is 1:9 to 3:7;

[0018] Further preferably, the mass ratio of the oxidized starch to water is 1:9 to 2:8;

[0019] Preferably, in step (1), the gelatinization is carried out in a boiling water bath, and the gelatinization time is 0.5 to 1.5 hours;

[0020] Further preferably, the gelatinization time is 1 hour;

[0021] Preferably, in step (1), the gelatinization is carried out under stirring.

[0022] Preferably, in step (2), the mass ratio of the polyvinyl alcohol to water is 1:9 to 3:7;

[0023] Further preferably, the mass ratio of the polyvinyl alcohol to water is 1:9 to 2:8;

[0024] Preferably, in step (2), the stirring is carried out in a boiling water bath for 1 to 3 hours;

[0025] Further preferably, the stirring time is 1 h;

[0026] More preferably, the polyvinyl alcohol is dispersed in water and stirred at room temperature for 2 to 4 hours and then stirred in a boiling water bath.

[0027] Preferably, in step (3), the mass ratio of oxidized starch to polyvinyl alcohol in the oxidized starch / polyvinyl alcohol mixed solution is 1:4 to 4:1.

[0028] Further preferably, the mass ratio of oxidized starch to polyvinyl alcohol in the oxidized starch / polyvinyl alcohol mixed solution is 3:7 to 7:3.

[0029] Preferably, in step (3), the mass fraction of the solute (dry matter, i.e., oxidized starch and polyvinyl alcohol) in the oxidized starch / polyvinyl alcohol mixed solution is 7.5-12.5%.

[0030] Preferably, in step (3), the solvent in the oxidized starch / polyvinyl alcohol mixed solution is water.

[0031] Preferably, in step (3), the mixing configuration is stirring in a boiling water bath for 5-50 minutes.

[0032] Preferably, in step (4), the ambient temperature of the electrospinning is 15-35° C., and the ambient humidity is 35%-55%;

[0033] Preferably, in step (4), the electrospinning conditions are: voltage 12-24 kV, receiving distance 5-25 cm, and pushing speed 0.5-2.5 mL / h.

[0034] Preferably, in step (4), the diameter of the electrospinning needle is 0.3 to 0.8 mm.

[0035] The nanofiber membrane containing oxidized starch is prepared by the above preparation method.

[0036] A method for preparing carbon nanofibers comprises the following steps:

[0037] The nanofiber membrane containing oxidized starch is pre-oxidized in air and then carbonized under a protective atmosphere to obtain carbon nanofibers; the pre-oxidation temperature is 180-240°C, the pre-oxidation time is 1-3 hours, the carbonization temperature is 600-1000°C, and the carbonization time is 1-4 hours.

[0038] Preferably, the heating rate of the pre-oxidation is 5 to 15° C. / min; and the heating rate of the carbonization is 5 to 15° C. / min.

[0039] The carbon nanofibers prepared by the above preparation method have the ability to adsorb small molecule gases (ethylene).

[0040] The above-mentioned carbon nanofibers are used in ethylene adsorption, such as for preserving fruits and vegetables.

[0041] The present invention utilizes electrospinning to produce a nanofiber membrane with uniform diameter. The membrane is then pre-oxidized and carbonized to produce microporous carbon nanofibers. The resulting carbon nanofiber system has a carbon content of 89.3% to 90.8%, and the micropores are uniform in diameter, effectively maintaining the stability of the carbon framework. This creates a microporous carbon material ethylene carrier, enabling efficient gas adsorption. The present invention features a simple preparation process, readily available raw materials, low cost, and ease of operation.

[0042] Currently, there is limited research on the electrospinning of carbon nanofibers from biomass. Furthermore, the complex composition of biomass precludes electrospinning alone, requiring polymers (such as PAN and PEO) as spinning aids to produce nanofibers. Compared to existing precursors, starch offers advantages such as widespread availability, low cost, diverse variety, and renewable properties. Using modified starch as a raw material and dissolving it in water under heating can shorten processing time, improve production efficiency, and effectively reduce the use of other organic polymers.

[0043] Currently, the common methods for preparing porous carbon nanofibers are: (1) post-activation treatment after carbonization, physical activation (steam or carbon dioxide) and chemical activation (potassium hydroxide or zinc chloride); (2) template method, in which a second component is introduced into the precursor as a sacrificial template (sacrificial pore-forming agent), and then the template is removed during carbonization or post-treatment to form pores inside the carbon nanofiber. The latter has the advantage of a one-step method, requiring no post-treatment, while maintaining a high carbon yield. By controlling the type and concentration of the sacrificial polymer, the pore content and specific surface area of ​​the carbon nanofiber can be adjusted (Wang et al. 2018).

[0044] Starch, composed of polymerized glucose molecules, can serve as a sacrificial template for preparing high-quality porous carbon nanofibers. At high temperatures, starch undergoes thermal degradation through a complex, multi-step chemical reaction, beginning with free radical reactions caused by intramolecular or intermolecular dehydration. Oxidized starch begins to decompose into gaseous substances such as CO2 and CO at around 300°C, and the decomposition process continues until approximately 600°C. Using modified starch as a sacrificial component in the preparation of porous carbon nanofibers can achieve green and sustainable energy development, high-value utilization of starch resources, and promote the development of small-molecule gas adsorption technology.

[0045] Compared with the prior art, the advantages of the present invention are:

[0046] (1) The method of preparing nanofiber membranes containing oxidized starch by electrostatically blending oxidized starch / polyvinyl alcohol of the present invention uses starch as one of the raw materials for preparing nanofiber membranes. Starch is abundant in source and safe and non-toxic, which reduces the cost of use.

[0047] (2) In the method of preparing a nanofiber membrane containing oxidized starch by electrostatically blending oxidized starch / polyvinyl alcohol of the present invention, the starch is preliminarily subjected to a wet oxidation modification treatment to improve the compatibility and spinnability of the starch with the polymer material.

[0048] (3) The present invention discloses a method for preparing carbon nanofibers by electrostatically blending oxidized starch / polyvinyl alcohol. The nanofiber membrane containing oxidized starch is prepared by electrostatically blending oxidized starch and polyvinyl alcohol. The carbon nanofibers prepared after carbonization have a uniform diameter of 120 to 200 nm, and micropores with a pore size of about 0.8 nm are distributed on the surface and inside of the carbon nanofibers. The present invention successfully prepares carbon nanofibers with good morphological characteristics and a good degree of carbonization.

[0049] (4) The solid carrier carbon nanofiber prepared by the present invention can significantly improve the gas adsorption performance under normal pressure conditions (can adsorb 30.66-157.47 cm 3 / g ethylene gas), significantly reducing the technical requirements for product preparation, shortening the time consumption, being easy to operate, low in cost, and having a simple process, which is more conducive to industrial production and large-scale promotion and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 These are the surface and microscopic morphologies of the nanofiber membranes obtained in Example 1 and Comparative Example 1.

[0051] Figure 2 These are the surface and microscopic morphologies of the carbon nanofibers obtained in Example 2 and Comparative Example 2.

[0052] Figure 3 The ethylene adsorption capacity at room temperature and pressure of the carbon nanofibers of Example 1, Example 2, Example 3, and Comparative Example 2 and the starch / polyvinyl alcohol nanofiber film of Comparative Example 1.

[0053] Figure 4 The carboxyl content of the oxidized starch in Example 1, Comparative Example 3, Comparative Example 4, Comparative Example 5, and Comparative Example 6.

[0054] Figure 5 These are the microscopic morphology and diameter distribution diagrams of the oxidized starch / polyvinyl alcohol nanofiber membranes of Example 1, Comparative Example 3, Comparative Example 4, Comparative Example 5, and Comparative Example 6. DETAILED DESCRIPTION

[0055] For a better understanding of the present invention, the present invention is further described below with reference to the embodiments. However, the scope of protection claimed by the present invention is not limited to the scope described in the embodiments.

[0056] The morphology of the oxidized starch / polyvinyl alcohol nanofiber membrane and the carbon nanofiber system was observed by the following method: the obtained sample was adhered to a stage with a conductive double-sided tape, and then the morphology was observed using a scanning electron microscope.

[0057] In the examples, the ethylene content adsorbed by carbon nanofibers at room temperature and pressure was determined as follows: a certain amount (30 mg) of carbon nanofibers was placed in a sealed container, filled with pure ethylene gas, and maintained at room temperature and pressure for 12 hours. The carbon nanofibers were transferred to a headspace bottle, 1 mL of 0.1 M sodium hydroxide solution was added, and the bottle cap was quickly tightened. The mixture was stirred overnight in a water bath at 60°C at 400 rpm to fully release ethylene, and the ethylene content was determined by headspace gas chromatography. The gas chromatograph was equipped with a flame ionization detector (FID) and a DB-5 column (30 m × 0.32 mm × 0.25 μm), with high-purity nitrogen as the carrier gas at a flow rate of 40 mL / min. The test conditions of the headspace sampler were: oven temperature 60°C, injection temperature 250°C, and headspace bottle pressurization time 0.2 min. The ethylene peak area was converted to ethylene concentration based on an ethylene standard.

[0058] Example 1

[0059] (1) 20 g of starch (tapioca starch) was dissolved in distilled water to prepare a starch milk with a mass concentration of 40%, copper sulfate (0.04 wt%, relative to the amount of starch) was added, 1 M sodium hydroxide solution was added to adjust the pH to 8.35, and after stirring evenly, 15 mL of hydrogen peroxide solution (mass concentration 30%) was added. The mixture was stirred at a constant speed at 45° C. for 4 h, and 5 mL of sodium bisulfite solution (mass concentration 10%) was added to terminate the reaction. 1 M sodium hydroxide solution was added to adjust the pH to 7, and then the mixture was filtered, washed 5 times, and dried at 45° C. to obtain oxidized starch;

[0060] (2) dispersing the oxidized starch obtained in step (1) in distilled water (mass concentration of 10%) and stirring in a boiling water bath for 1 hour to obtain an oxidized starch solution; and dispersing polyvinyl alcohol in distilled water (mass concentration of 10%) and stirring at room temperature for 2 hours, and then stirring in a boiling water bath under uniform stirring for 1 hour to obtain a polyvinyl alcohol solution;

[0061] (3) The oxidized starch solution obtained in step (2) was mixed with the polyvinyl alcohol solution in a solution mass ratio of 3:7, and stirred in a boiling water bath for 15 minutes to prepare an oxidized starch / polyvinyl alcohol mixed solution with a dry matter mass fraction of 10%, which was used as an electrospinning solution; and the oxidized starch / polyvinyl alcohol nanofiber membrane was prepared by electrospinning under the conditions of an electrospinning voltage of 18 kV, a spinning distance of 15 cm, and a spinning solution pushing speed of 1 mL / h.

[0062] (4) The obtained oxidized starch / polyvinyl alcohol nanofiber membrane was placed in a muffle furnace, and the temperature was raised to 220°C at 5°C / min in an air atmosphere and kept warm for 3 hours for pre-oxidation. Then, the pre-oxidized oxidized starch / polyvinyl alcohol nanofiber membrane was placed in a vacuum tube furnace, and the temperature was raised to 800°C at 5°C / min under nitrogen protection for carbonization, and kept warm for 2 hours to obtain oxidized starch / polyvinyl alcohol carbon nanofibers.

[0063] (5) The carbon nanofibers obtained in step (4) are placed in a sealed container filled with ethylene gas and adsorbed at room temperature and pressure for 12 hours.

[0064] Example 2

[0065] (1) 20 g of starch (tapioca starch) was dissolved in distilled water to prepare a starch milk with a mass concentration of 40%, copper sulfate (0.04 wt%, relative to the amount of starch) was added, 1 M sodium hydroxide solution was added to adjust the pH to 8.35, and after stirring evenly, 15 mL of hydrogen peroxide solution (mass concentration 30%) was added. The mixture was stirred at a constant speed at 45° C. for 4 h, and 5 mL of sodium bisulfite solution (mass concentration 10%) was added to terminate the reaction. 1 M sodium hydroxide solution was added to adjust the pH to 7, and then the mixture was filtered, washed 5 times, and dried at 45° C. to obtain oxidized starch;

[0066] (2) dispersing the oxidized starch obtained in step (1) in distilled water (mass concentration of 10%) and gelatinizing in a boiling water bath with stirring for 1 hour to obtain an oxidized starch solution; and dispersing polyvinyl alcohol in water (mass concentration of 10%), stirring at room temperature for 2 hours, and then gelatinizing under uniform stirring for 1 hour to obtain a polyvinyl alcohol solution;

[0067] (3) The modified oxidized starch and polyvinyl alcohol were mixed in a solution mass ratio of 1:1, and stirred in a boiling water bath for 15 min to prepare an oxidized starch / polyvinyl alcohol mixed solution with a dry matter mass fraction of 10%, which was used as the electrospinning solution; the oxidized starch / polyvinyl alcohol nanofiber membrane was prepared by electrospinning under the conditions of an electrospinning voltage of 18 kV, a spinning distance of 15 cm, and a spinning solution pushing speed of 1 mL / h.

[0068] (4) The obtained oxidized starch / polyvinyl alcohol nanofiber membrane was placed in a muffle furnace, and the temperature was raised to 220°C at 5°C / min in an air atmosphere and kept warm for 3 hours for pre-oxidation. Then, the pre-oxidized oxidized starch / polyvinyl alcohol nanofiber membrane was placed in a vacuum tube furnace, and the temperature was raised to 1000°C at 5°C / min under nitrogen protection for carbonization, and kept warm for 2 hours to obtain oxidized starch / polyvinyl alcohol carbon nanofibers.

[0069] (5) The carbon nanofibers obtained in step (4) are placed in a sealed container filled with ethylene gas and adsorbed at room temperature and pressure for 12 hours.

[0070] Example 3

[0071] (1) 20 g of starch (tapioca starch) was dissolved in distilled water to prepare a starch milk with a mass concentration of 40%, copper sulfate (0.04 wt%, relative to the amount of starch) was added, 1 M sodium hydroxide solution was added to adjust the pH to 8.35, and after stirring evenly, 15 mL of hydrogen peroxide solution (mass concentration 30%) was added. The mixture was stirred at a constant speed at 45° C. for 4 h, and 5 mL of sodium bisulfite solution (mass concentration 10%) was added to terminate the reaction. 1 M sodium hydroxide solution was added to adjust the pH to 7, and then the mixture was filtered, washed 5 times, and dried at 45° C. to obtain oxidized starch;

[0072] (2) dispersing the oxidized starch obtained in step (1) in distilled water (mass concentration of 10%) and gelatinizing in a boiling water bath for 1 hour to obtain an oxidized starch solution; and dispersing polyvinyl alcohol in water (mass concentration of 10%), stirring at room temperature for 2 hours, and then gelatinizing under uniform stirring for 1 hour to obtain a polyvinyl alcohol solution;

[0073] (3) The modified oxidized starch and polyvinyl alcohol were mixed in a solution mass ratio of 7:3, and stirred in a boiling water bath for 15 min to prepare an oxidized starch / polyvinyl alcohol mixed solution with a dry matter mass fraction of 10%, which was used as the electrospinning solution; the oxidized starch / polyvinyl alcohol nanofiber membrane was prepared by electrospinning under the conditions of an electrospinning voltage of 18 kV, a spinning distance of 15 cm, and a spinning solution pushing speed of 1 mL / h.

[0074] (4) The obtained oxidized starch / polyvinyl alcohol nanofiber membrane was placed in a muffle furnace, and the temperature was raised to 220°C at 5°C / min in an air atmosphere and kept warm for 3 hours for pre-oxidation. Then, the pre-oxidized oxidized starch / polyvinyl alcohol nanofiber membrane was placed in a vacuum tube furnace, and the temperature was raised to 1000°C at 5°C / min under nitrogen protection for carbonization, and kept warm for 2 hours to obtain oxidized starch / polyvinyl alcohol carbon nanofibers.

[0075] (5) The carbon nanofibers obtained in step (4) are placed in a sealed container filled with ethylene gas and adsorbed at room temperature and pressure for 12 hours.

[0076] Comparative Example 1

[0077] According to steps (2) and (3) of Example 1, starch / polyvinyl alcohol nanofiber membranes were prepared by electrospinning using unoxidized starch (cassava starch) and polyvinyl alcohol as raw materials. The electrospinning differences between starch / polyvinyl alcohol and oxidized starch / polyvinyl alcohol and the possibility of preparing carbonized membranes were observed.

[0078] Comparative Example 2

[0079] According to steps (2) (3) (4) of Example 1, polyvinyl alcohol was used as the raw material to prepare polyvinyl alcohol nanofiber membrane by electrospinning and then pre-oxidized and carbonized without adding oxidized starch to prepare pure polyvinyl alcohol carbon nanofiber and measure the ethylene adsorption capacity.

[0080] After testing, such as Figure 2 As shown in the figure, the carbon nanofiber membrane system has a low carbon yield and is fragile. The ethylene adsorption capacity at room temperature and pressure is 15.83 cm 3 / g.

[0081] Comparative Example 3

[0082] Oxidized starch / polyvinyl alcohol nanofiber membranes were prepared according to steps (1), (2), and (3) of Example 1; the only difference was that the starch was corn starch and the amount of hydrogen peroxide solution was 30 mL, and everything else was the same.

[0083] Comparative Example 4

[0084] The oxidized starch / polyvinyl alcohol nanofiber membrane was prepared according to steps (1) (2) (3) of Example 1; the only difference was that the starch was corn starch, and everything else was the same.

[0085] Comparative Example 5

[0086] The oxidized starch / polyvinyl alcohol nanofiber membrane was prepared according to steps (1), (2), and (3) of Example 1; the only difference was that the starch was potato starch, and everything else was the same.

[0087] Comparative Example 6

[0088] Oxidized starch / polyvinyl alcohol nanofiber membranes were prepared according to steps (1), (2), and (3) of Example 1; the only difference was that the starch was corn starch and the amount of hydrogen peroxide solution was 5 mL, and everything else was the same.

[0089] Figure 1 These are the surface and microscopic morphologies of the nanofiber membranes obtained in Example 1 and Comparative Example 1.

[0090] Figure 2 These are the surface and microscopic morphologies of the carbon nanofibers obtained in Example 2 and Comparative Example 2.

[0091] Figure 3 The ethylene adsorption capacity at room temperature and pressure of the carbon nanofibers of Example 1, Example 2, Example 3, and Comparative Example 2 and the starch / polyvinyl alcohol nanofiber film of Comparative Example 1.

[0092] Figure 4 The carboxyl content of the oxidized starch in Example 1, Comparative Example 3, Comparative Example 4, Comparative Example 5, and Comparative Example 6.

[0093] Figure 5 These are the microscopic morphology and diameter distribution diagrams of the oxidized starch / polyvinyl alcohol nanofiber membranes of Example 1, Comparative Example 3, Comparative Example 4, Comparative Example 5, and Comparative Example 6.

[0094] Electrospinnability can be evaluated by visual morphology (presence of droplets, diameter ≥ 1 mm) and microscopic morphology (beading and fiber diameter). During the electrospinning process, if there is too strong entanglement in the spinning solution structure, droplets will form in the fiber membrane. If the electric field strength cannot overcome the viscoelasticity of the electrospinning solution and the entanglement network is weak, beads may form in the fiber (Zhang et al, 2005). It is believed that fiber membranes containing a small amount of droplets and beads have good electrospinnability. For electrospun fiber membranes without droplets and beads, fiber diameter is also an important evaluation index of spinnability (Kada 2016). Figure 1 As shown, the oxidized starch-polyvinyl alcohol blended solution in Example 1 has good electrospinning properties, and the fiber film is smooth and thick. The starch-polyvinyl alcohol in Comparative Example 1 hardly forms a Taylor cone during the electrospinning process, and many droplets appear on the receiving plate (marked with red circles). The average number of droplets is 188, and no fiber deposition is formed. This shows that oxidized starch-polyvinyl alcohol has excellent electrospinning properties, which may be related to the strong molecular entanglement of the oxidized starch and polyvinyl alcohol mixed solution. Starch is mainly composed of glucose units containing multiple hydroxyl groups. During the oxidation reaction, carboxyl and carbonyl groups are introduced into the starch molecular chain. Therefore, the degree of oxidation of oxidized starch can be determined using the carboxyl content as an indicator. As Figure 4 and 5 As shown, the carboxyl content in starch was determined using alkaline titration. Within the research range, the degree of oxidation increased with the increase of the oxidant content. Oxidized starch with a carboxyl content of 0.35-0.45% had good electrospinning performance. If the carboxyl content was too high or too low, the electrospinning performance would deteriorate, which would result in the presence of droplets in the obtained nanofiber membrane and beads could be observed in the microstructure.

[0095] like Figure 2 As shown, in Comparative Example 2 (polyvinyl alcohol carbon nanofiber system), the carbon nanofibers prepared without adding oxidized starch have a low yield and are easily broken, while the carbon nanofibers obtained in Example 2 have uniform diameters. Figure 3 As shown in FIG. 2 , the ethylene adsorption capacity of the polyvinyl alcohol carbon nanofiber in Comparative Example 2 is 15.83 cm 3 / g, the oxidized starch-polyvinyl alcohol carbon nanofiber system prepared in Example 1 can significantly improve the gas adsorption properties. After 12 hours of adsorption, the adsorption capacity of ethylene is 30.66 cm 3 / g; Example 2 and Example 3 increase the content of oxidized starch in the system to prepare carbon nanofibers, which have more microporous structures and further improve the gas adsorption properties. The adsorption capacity of ethylene is 99.97 and 157.47 cm 3 The above results indicate that the oxidized starch / polyvinyl alcohol carbon nanofiber system is a potential ethylene adsorbent and has the potential to preserve fruits and vegetables.

[0096] It should be noted that for ordinary technicians in the technical field to which the present invention belongs, the implementation methods of the present invention are not limited to the above-mentioned embodiments, and any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A carbon nanofiber membrane, characterized in that Prepared by the following steps: (1) dispersing oxidized starch in water and then gelatinizing to obtain an oxidized starch solution; the mass ratio of the oxidized starch to water is 1:9 to 3:7; the gelatinization is carried out in a boiling water bath for 0.5 to 1.5 hours; The preparation method of the oxidized starch comprises: using hydrogen peroxide and / or sodium hypochlorite as an oxidant and copper sulfate as a catalyst to oxidize starch to obtain the oxidized starch; the carboxyl content of the oxidized starch is 0.35% to 0.45%; The starch is at least one of corn starch, potato starch, and tapioca starch; the mass ratio of copper sulfate to starch is 0.01-0.1%; the hydrogen peroxide and / or sodium hypochlorite accounts for 8.25-49.5% of the starch mass; the oxidation treatment temperature is 30-60° C., and the time is 3-5 hours; (2) dispersing polyvinyl alcohol in water and stirring to obtain a polyvinyl alcohol solution; the mass ratio of the polyvinyl alcohol to water is 1:9 to 3:7; the stirring is performed in a boiling water bath for 1 to 3 hours; (3) mixing the oxidized starch solution obtained in step (1) and the polyvinyl alcohol solution obtained in step (2) to form an oxidized starch / polyvinyl alcohol mixed solution having a solute mass fraction of 5 to 15%; the mass ratio of oxidized starch to polyvinyl alcohol in the oxidized starch / polyvinyl alcohol mixed solution is 1:4 to 4:1; (4) electrospinning the oxidized starch / polyvinyl alcohol mixed solution prepared in step (3) to prepare a nanofiber membrane containing oxidized starch; the nanofiber membrane containing oxidized starch is pre-oxidized in air and then carbonized under a protective atmosphere to obtain carbon nanofibers; the pre-oxidation temperature is 180 to 240° C., the pre-oxidation time is 1 to 3 hours, the carbonization temperature is 600 to 1000° C., and the carbonization time is 1 to 4 hours; The carbon content of the prepared carbon nanofiber membrane is 89.3% to 90.8%; the diameter of the carbon nanofiber is uniform, ranging from 120 to 200 nm.

2. The carbon nanofiber membrane according to claim 1, characterized in that: In step (3), the mixing configuration is to stir in a boiling water bath for 5-50 minutes.

3. The carbon nanofiber membrane according to claim 1, characterized in that: In step (4), the ambient temperature of the electrospinning is 15 to 35° C., and the ambient humidity is 35% to 55%; In step (4), the electrospinning conditions are: voltage 12-24 kV, receiving distance 5-25 cm, and pushing speed 0.5-2.5 mL / h.

4. The carbon nanofiber membrane according to claim 1, characterized in that: The heating rate of the pre-oxidation is 5 to 15° C. / min; the heating rate of the carbonization is 5 to 15° C. / min.

5. Use of the carbon nanofiber membrane according to claim 1 in ethylene adsorption.

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