Heteroatom modified micropore-rich silicon dioxide coated polylactic acid nano-fiber membrane and preparation method thereof

Abstract

The invention provides a heteroatom modified micropore-rich silicon dioxide coated polylactic acid nano-fiber membrane and a preparation method thereof, the method comprises the following steps: step S1, mixing ammonia water with an alcohol-water solution to prepare a catalytic solution; s2, dispersing a silicon source and a pore-forming agent in an alcohol solvent, pouring into the catalytic solution obtained in S1, and reacting to prepare micropore-rich silicon dioxide (SiO2) nano microspheres; s3, fully dissolving the micropore-rich SiO2 nano-microspheres obtained in the step S2, polylactic acid and a heteroatom carrier in an organic solvent to prepare a heteroatom modified mixed solution; and step S4, preparing the silicon dioxide coated polylactic acid nano-fiber membrane from the heteroatom modified mixed solution obtained in the step S3 through a spinning technology. The polylactic acid nano-fiber membrane prepared by the invention not only has the remarkable advantages of rich micropores, heteroatom adsorption sites, high gas selectivity and the like, but also has a unique SiO2 coated fiber structure, the preparation process is simple, the condition is mild, and the polylactic acid nano-fiber membrane is a gas adsorption and separation membrane material with excellent performance.

Description

Technical Field

[0001] The present invention relates to the technical field of toxic gas separation materials, and particularly relates to a heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane and a preparation method thereof. Background Art

[0002] As a byproduct of fossil fuel combustion, sulfur dioxide (SO 2 ) is released into the atmosphere, causing harm to the environment and inducing human respiratory diseases. At the same time, SO 2 present in industrial tail gases can poison purification materials and have an adverse effect on their performance. Generally, SO 2 is removed by ammonia scrubbing technology, but due to the high volatility and corrosiveness of ammonia, it will increase the cost of the desulfurization process. Limestone scrubbing is another commonly used technology for capturing SO 2 in industrial waste gases. However, limestone reacts irreversibly with SO 2 to form low-value refractory gypsum (CaSO 4 ), which hinders its long-term application. Reversible adsorption of solid materials is considered a promising clean technology for reducing SO 2 emissions. Given the coexistence characteristics and toxicity of SO 2 and carbon dioxide (CO 2 ) in tail gases, improving the adsorption and separation selectivity of SO 2 / CO 2 is an important issue in the development of effective desulfurization materials.

[0003] There are many methods for adsorbing and separating CO 2 and SO 2 from flue gases, and each method has its own advantages and disadvantages. Compared with other separation methods, the solid adsorption method has the advantages of low energy consumption, simple operation, strong adaptability, etc., and can handle gas flows under various working conditions. So far, significant progress has been made in gas adsorption based on solid adsorbents. Traditional adsorption materials such as zeolites, silica, and porous carbon, as well as new adsorption materials such as silica gel, metal-organic frameworks, and covalent organic frameworks, all show effective adsorption capabilities. However, due to production costs and the capture environment, for many years, the separation selectivity of materials has been a key problem in optimizing the adsorption process. Among many solid adsorption materials, porous nanofiber composites have attracted much attention due to their wide raw material sources, simple synthesis, high stability, low renewable energy consumption, and easy modification.

[0004] Currently, researchers have fabricated a series of porous materials using activation methods, template methods, and sol-gel methods. However, porous materials also have some drawbacks that urgently need to be addressed, such as irregular structures, poor pore properties, and scarce adsorption sites. Therefore, the structural and functional modification of porous materials has attracted much attention. In fact, many researchers have adopted various modification techniques, such as surface functionalization and element doping. Researchers have prepared a series of nitrogen, oxygen, and sulfur co-doped porous materials using waste distillation particles and thiourea. These materials have abundant micropores and a high specific surface area, and can adsorb up to 7.02 mmol⋅g -1 of CO 2 adsorption capacity (Sep Purif. Technol., 2023;320), providing a sustainable solution for the development of low-cost porous materials for efficient CO 2 capture. Zhang et al. proposed a clean modification method to obtain a uniformly modified biochar porous material containing nitrogen groups by co-carbonizing biochar and zeolitic imidazolate frameworks (J.Clean. Prod., 2022;355). In addition, according to DFT calculations, they concluded that C-O and graphitic nitrogen have an obvious synergistic effect on CO 2 adsorption, providing new insights for the design and modification of porous materials for adsorptive separation of mixed gases. Summary of the Invention

[0005] The object of the present invention is to prepare a heteroatom-modified micro-porous silica-coated polylactic acid nanofiber membrane to meet the requirements for selective adsorption and separation of SO 2 in industrial waste gas.

[0006] To achieve the above object, the present invention provides a silica-coated polylactic acid nanofiber membrane and a preparation method thereof. In the present invention, a silicon source and a pore-forming agent are fully dispersed in an alcohol solvent through sol-gel phase transformation, quickly poured into a catalytic solution for reaction and washed and dried to prepare silica nanospheres with a rich microporous structure, which are uniformly dispersed with polylactic acid and a heteroatom carrier in an organic solvent to prepare a heteroatom-modified mixed solution, and then a heteroatom-modified micro-porous silica-coated polylactic acid nanofiber membrane is prepared through various spinning techniques.

[0007] According to the first aspect of the present invention, a preparation method of a silica-coated polylactic acid nanofiber membrane is provided, including the following steps: Step S1, preparing a catalytic solution: mixing ammonia water with an alcohol-aqueous solution to prepare a catalytic solution; Step S2, preparing micro-porous silica (SiO 2 2) nanospheres: dispersing a silicon source and a pore-forming agent in an alcohol solvent, quickly pouring into the catalytic solution obtained in S1, and preparing micro-porous silica (SiO 2Nanospheres; Step S3: Prepare a heteroatom-modified mixture: Dissolve the obtained microporous SiO-rich nanospheres in step S2, polylactic acid, and a heteroatom carrier in an organic solvent to prepare a heteroatom-modified mixture; Step S4: Prepare a silica-coated polylactic acid nanofiber membrane: Prepare a heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane by spinning the heteroatom-modified mixture obtained in step S3. 2 Dissolve the 2 nanospheres, polylactic acid, and a heteroatom carrier in an organic solvent to prepare a heteroatom-modified mixture; Step S4: Prepare a silica-coated polylactic acid nanofiber membrane: Prepare a heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane by spinning the heteroatom-modified mixture obtained in step S3.

[0008] Preferably, the alcohol-water solution in step S1 is composed of an alcohol solvent and water, and the volume ratio of the alcohol solvent to water in the alcohol-water solution is 1:2 to 2:1. The alcohol solvent is one or more of methanol, ethanol, isopropanol, ethylene glycol, glycerol, n-butanol, and tert-butanol.

[0009] Preferably, the mass concentration of ammonia water in step S1 is 20 to 30%, and the mass ratio (volume ratio) of ammonia water to the alcohol-water solution is 1:5 to 1:20.

[0010] Preferably, the silicon source in step S2 is one or more of methyl orthosilicate, ethyl orthosilicate, propyl orthosilicate, sodium silicate, polydimethylsiloxane, silicon tetrachloride, and methyltrichlorosilane, and the mass ratio of the silicon source to the alcohol solvent is 1:10 to 1:100.

[0011] Preferably, the pore-forming agent in step S2 is one or more of cetyltrimethylammonium bromide, polyethylene glycol, dimethyldiethoxysilane, and sodium chloride, and the mass ratio of the pore-forming agent to the silicon source is 1:2 to 1:10.

[0012] Preferably, the reaction conditions for the sol-gel phase transition in step S2 are a reaction temperature of 20 to 60°C and a reaction time of 1 to 24 h.

[0013] Preferably, the cleaning method in step S2 is one or more of solvent cleaning, acid-base cleaning, ultrasonic cleaning, and high-temperature calcination cleaning. The cleaning solvent used in the above cleaning methods is one or more of ethanol, methanol, isopropanol, water, acetone, dilute acid solution, and dilute alkali solution.

[0014] Preferably, the cleaning method in step S2 is solvent cleaning, the cleaning temperature is 20 to 60°C, the cleaning time is 0.5 to 2 h, the cleaning solvent is a mixed solvent of ethanol, acetone, and deionized water, and the mass ratio of the silica nanospheres to the cleaning solvent is 1:5 to 1:20.

[0015] Preferably, the cleaning method in step S2 is acid-base cleaning, the cleaning temperature is 20 to 60°C, the cleaning time is 5 to 20 min, the cleaning solvent is a dilute acid solution or a dilute alkali solution, and the mass ratio of the silica nanospheres to the cleaning solvent is 1:3 to 1:10.

[0016] Preferably, the cleaning method in step S2 is ultrasonic cleaning. The ultrasonic frequency is 20 - 100 kHz, the single ultrasonic time is 3 - 10 min, the number of cleaning times is 3 - 5 times. The cleaning solvent used in ultrasonic cleaning is a mixed solvent of ethanol, acetone and deionized water, and the mass ratio (volume ratio) of silica to the solvent is 1 g:10 - 30 mL.

[0017] Preferably, the cleaning method in step S2 is high-temperature calcination cleaning. The calcination temperature is 500 - 1200 °C, the calcination time is 2 - 6 h, the heating rate is 1 - 10 °C / min, the calcination atmosphere is an inert gas, and the gas flow rate is 20 - 50 mL / min.

[0018] Preferably, in step S3, the mass fraction of the micro-porous silica nanoparticles in the heteroatom modification mixture is 1 - 30 wt%, and the concentration of polylactic acid in the heteroatom modification mixture is 0.5 - 30 wt%. In step S3, the heteroatom carrier is one or more of 4,4'-diaminodiphenyl disulfide, diphenyl disulfide, dicyandiamide, urea, melamine, and citric acid. The mass ratio of the heteroatom carrier to polylactic acid is 1:1 - 1:10. In step S3, the organic solvent is one or more of N,N-dimethylformamide, tetrahydrofuran, dimethyl sulfoxide, dichloromethane, chloroform, trichloromethane, acetone, ethyl acetate, methanol, and ethanol.

[0019] Preferably, the spinning technology in step S4 is one or more of high-voltage electrospinning technology, high-speed air-flow spinning technology, high-speed centrifugal spinning technology, and melt spinning technology.

[0020] Preferably, the spinning technology in step S4 is high-voltage electrospinning, and the spinning parameters are set as follows: the working voltage is 2 - 60 kV, the spraying speed is 0.5 - 5 mL / h, the receiving distance is 15 - 25 cm, the temperature is 10 - 50 °C, the relative humidity is 10 - 70%, and the reciprocating speed is 1 - 5 cm / min.

[0021] Preferably, the spinning technology in step S4 is high-speed air-flow spinning, and the spinning parameters are set as follows: the spraying angle is 15 - 60°, the ratio of the spraying speed to the extrusion speed is 1k - 30k, the spraying pressure is 0.2 - 10 MPa, the air flow temperature is 25 - 85 °C, and the relative humidity is 40 - 80%.

[0022] Preferably, the spinning technology in step S4 is high-speed centrifugal spinning, and the spinning parameters are set as follows: the centrifugal rotation speed is 1k - 50k, the rotation radius is 5 - 50 cm, the solution supply rate is 0.1 - 10 mL / h, the receiving distance is 10 - 50 cm, the ambient temperature is 15 - 35 °C, and the relative humidity is 30 - 60%.

[0023] Preferably, in step S4, the spinning technology is melt spinning, and the spinning parameters are set as follows: the melting temperature is 100-180 °C, the heating rate is 5-20 °C / min, the pressure control is 1-10 MPa, the spinning speed is 500-5000 m / min, the draw ratio is 1-10 times, and the collection distance is 0.3-1.0 m.

[0024] To achieve the above object, according to the second aspect of the present invention, the present invention also provides a silica-coated polylactic acid nanofiber membrane obtained by the foregoing preparation method.

[0025] Preferably, the thickness of the obtained silica-coated polylactic acid nanofiber membrane is 50-200 μm, and the fiber diameter is 50-1000 nm.

[0026] Preferably, the obtained silica-coated polylactic acid nanofiber membrane has a high specific surface area (384 m 2 / g - 467 m 2 / g), small pore size (1.56 nm - 2.32 nm), rich heteroatom content (7.2-9.6 wt%), high SO 2 adsorption capacity (4.8-6.7 mmol / g) and excellent SO 2 / CO 2 separation selectivity (81-121).

[0027] The innovative act of the present invention in preparing a polylactic acid nanofiber membrane by mixing microporous silica microspheres, heteroatom carriers and polylactic acid shows unique advantages and far-reaching significance. On the one hand, polylactic acid itself has good biodegradability, which is derived from renewable biomass resources. It not only conforms to the environmental protection trend, reduces "white pollution", but also ensures the sustainability of raw materials. The addition of silica further optimizes the material properties. With its high specific surface area and rich active site characteristics, combined with the three-dimensional network structure of polylactic acid nanofibers, it greatly enhances the adsorption affinity and selectivity for SO 2 . On the other hand, the silica-coated polylactic acid nanofiber membrane shows excellent performance in the adsorption and separation of SO 2 , and is expected to provide strong technical support for fields such as industrial waste gas purification and air pollution control.

[0028] The beneficial effects of the present invention are as follows: (1) By undergoing a sol-gel phase transition process, silica nanospheres with controllable shape and size are prepared, and the appropriate size can enable their uniform and orderly coating on polylactic acid nanofibers, thereby fully exposing the pore properties; (2) The introduction of heteroatom carriers such as nitrogen and oxygen can change the surface charge distribution of the molecular skeleton, promote the change of molecular polarity, and provide selective adsorption of SO 2The process generates more intermolecular forces; (3) The introduction of microporous silica provides a larger specific surface area and a rich pore structure for the polylactic acid nanofiber membrane, exposing as many heteroatom adsorption sites as possible; (4) Through the optimization of the synthesis process, the prepared material not only has significant advantages such as rich micropores, heteroatom adsorption sites, and high gas selectivity, but also has a unique SiO 2 coated fiber structure, and the preparation process is simple and the conditions are mild, which is an excellent gas adsorption and separation membrane material.

[0029] The technical solution proposed by the present invention enables the heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane to have a high specific surface area, small pore size, rich heteroatom adsorption sites, high SO 2 adsorption capacity and excellent SO 2 / CO 2 separation selectivity. The design of this material aims to meet the requirements of selective adsorption and separation of SO 2 in industrial waste gas, and has broad application potential and market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0031] Figure 1 It is a scanning electron microscope image of the silica-coated polylactic acid nanofiber membrane in Example 1.

[0032] Figure 2 It is an element mapping image of the silica-coated polylactic acid nanofiber membrane in Example 1.

[0033] Figure 3 It is a Fourier transform infrared spectrum of the silica-coated polylactic acid nanofiber membrane in Example 1.

[0034] Figure 4 It is a powder X-ray diffraction spectrum of the silica-coated polylactic acid nanofiber membrane in Example 1.

[0035] Figure 5 It is a nitrogen adsorption-desorption isotherm curve of the silica-coated polylactic acid nanofiber membrane in Example 1.

[0036] Figure 6 It is a pore size distribution curve of the silica-coated polylactic acid nanofiber membrane in Example 1.

[0037] Figure 7It is the scanning electron microscope image of the silica-coated polylactic acid nanofiber membrane in Example 2.

[0038] Figure 8 It is the scanning electron microscope image of the silica-coated polylactic acid nanofiber membrane in Example 3.

[0039] Figure 9 It is the scanning electron microscope image of the silica-coated polylactic acid nanofiber membrane in Example 4.

[0040] Figure 10 It is the schematic flow chart of the method of the present invention. Detailed implementation manners

[0041] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be described in detail below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other implementation manners obtained by those of ordinary skill in the art without creative efforts shall fall within the scope protected by the present invention. The present invention will be described in detail below with reference to the embodiments.

[0042] As Figure 10 shown, Embodiment 1 of the present invention provides a method for preparing a silica-coated polylactic acid nanofiber membrane, including the following steps: Step S11, preparing a catalytic solution: mixing ammonia water (mass concentration of 25%) and an alcohol-aqueous solution (volume ratio of absolute ethanol to water is 1:1) in a mass ratio of 1:5 to prepare a catalytic solution; Step S12, preparing microporous silica (SiO 2 2) nanospheres: fully dispersing tetraethyl orthosilicate and cetyltrimethylammonium bromide (mass ratio of 1:2) in ethanol (mass ratio of 1:10), quickly pouring it into the catalytic solution obtained in S11, and reacting at 40 °C for 2 h through a sol-gel phase transition process, and preparing microporous silica (SiO 2 2) nanospheres by the solvent washing method (washing temperature is 40 °C, washing time is 2 h, washing solvent is a mixed solvent of ethanol, acetone and deionized water, and the mass ratio of silica nanospheres to the washing solvent is 1:10) and drying; Step S13, preparing a heteroatom-modified mixed solution: the microporous silica (SiO 2)The nano-microspheres, polylactic acid, and 4,4'-diaminodiphenyl disulfide are fully dissolved in N,N-dimethylformamide (the mass fraction of silica in the mixed solution is 20 wt%, the concentration of polylactic acid in the mixed solution is 10 wt%, and the mass ratio of the heteroatom carrier to polylactic acid is 1:2) to prepare a heteroatom-modified mixed solution; Step S14: Prepare a silica negatively coated polylactic acid nanofiber membrane: The heteroatom-modified mixed solution obtained in Step S13 is prepared by high-voltage electrospinning technology (the working voltage is 20 kV, the spraying speed is 1 mL / h, the receiving distance is 15 cm, the temperature is 25 °C, the relative humidity is 30%, and the reciprocating speed is 3 cm / min) to prepare a heteroatom-modified silica-coated polylactic acid nanofiber membrane with rich micropores. The specific surface area of the obtained nanofiber membrane is 467 m 2 / g, and the average pore size is 1.56 nm.

[0043] As Figure 1 shown in the figure, the scanning electron microscope image of the silica-coated polylactic acid nanofiber membrane obtained in Example 1 of the present invention shows that the silica microspheres are uniformly wrapped on the surface of the polylactic acid nanofibers to form a unique three-dimensional woven core-shell structure.

[0044] As Figure 2 shown in the figure, the elemental mapping image of the silica-coated polylactic acid nanofiber membrane obtained in Example 1 of the present invention shows that carbon, oxygen, and nitrogen elements are uniformly distributed on the surface of the polylactic acid nanofiber membrane, and the heteroatom content is 9.6 wt%.

[0045] As Figure 3 shown in the figure, the Fourier transform infrared spectrum of the silica-coated polylactic acid nanofiber membrane obtained in Example 1 of the present invention shows the stretching vibration signal peaks of Si−O−Si, H−O−H, Si−OH, and −OH bonds in the structure of the material.

[0046] As Figure 4 shown in the figure, the powder X-ray diffraction pattern of the silica-coated polylactic acid nanofiber membrane obtained in Example 1 of the present invention shows the appearance of broad diffraction peaks representing an amorphous structure, indicating that the polylactic acid nanofiber membrane has an amorphous structure controlled by kinetics.

[0047] As Figure 5 shown in the figure, the nitrogen adsorption-desorption isotherm curve of the silica-coated polylactic acid nanofiber membrane obtained in Example 1 of the present invention shows that the material has a large specific surface area and small pore size.

[0048] As Figure 6 shown in the figure, the pore size distribution curve of the silica-coated polylactic acid nanofiber membrane obtained in Example 1 of the present invention shows that the average pore size of the material is concentrated in the micropore and mesopore regions.

[0049] Example 2 of the present invention provides a method for preparing a silica-coated polylactic acid nanofiber membrane, which includes the following steps: Step S21, preparing a catalytic solution: mixing ammonia water (mass concentration of 20%) and an alcohol-water solution (volume ratio of anhydrous methanol to water is 1:2) in a mass ratio of 1:10 to prepare a catalytic solution; Step S22, preparing microporous silica (SiO 2 )nanospheres: fully dispersing methyl orthosilicate and polyethylene glycol (mass ratio of 1:5) in ethanol (mass ratio of 1:20), quickly pouring it into the catalytic solution obtained in S21, reacting at 20 °C for 12 h through a sol-gel phase transition process, and preparing microporous silica (SiO 2 )nanospheres by acid-base cleaning method (cleaning temperature is 30 °C, cleaning time is 10 min, cleaning solvent is 0.1 mol / L hydrochloric acid solution, mass ratio of silica nanospheres to cleaning solvent is 1:5) and drying; Step S23, preparing a heteroatom-modified mixed solution: fully dissolving the microporous silica (SiO 2 )nanospheres, polylactic acid, and diphenyl disulfide obtained in step S22 in tetrahydrofuran (mass fraction of silica in the mixed solution is 10 wt%, concentration of polylactic acid in the mixed solution is 12 wt%, mass ratio of heteroatom carrier to polylactic acid is 1:5) to prepare a heteroatom-modified mixed solution; Step S24, preparing a silica-coated polylactic acid nanofiber membrane: passing the heteroatom-modified mixed solution obtained in step S23 through high-speed air spinning (spraying angle is 30°, ratio of spraying speed to extrusion speed is 10k, spraying pressure is 3 MPa, air flow temperature is 35 °C, relative humidity is 40%) to prepare a heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane, and the specific surface area of the obtained nanofiber membrane is 441 m 2 / g, and the average pore diameter is 1.71 nm.

[0050] As Figure 7 shown, the scanning electron microscope image of the silica-coated polylactic acid nanofiber membrane obtained in Example 2 of the present invention shows that the silica microspheres are uniformly wrapped on the surface of the polylactic acid nanofibers to form a unique three-dimensional woven core-shell structure.

[0051] Example 3 of the present invention provides a method for preparing a silica-coated polylactic acid nanofiber membrane, which includes the following steps: Step S31, preparing a catalytic solution: mixing ammonia water (mass concentration of 30%) and an alcohol-water solution (volume ratio of anhydrous isopropanol to water is 2:3) in a mass ratio of 1:15 to prepare a catalytic solution; Step S32, preparing microporous silica (SiO 2)Nanospheres: Tetrapropyl orthosilicate and dimethyldiethoxysilane (mass ratio 1:7) were fully dispersed in ethanol (mass ratio 1:30), quickly poured into the catalytic solution obtained in S31, and reacted at 30 °C for 8 h through a sol-gel phase transition process. After ultrasonic cleaning (ultrasonic frequency 80 kHz, single ultrasonic time 5 min, cleaning times 5, ultrasonic cleaning solvent is a mixed solvent of ethanol, acetone and deionized water, mass / volume ratio of silica to solvent 1 g:20 mL) and drying, mesoporous silica (SiO 2 )nanospheres were prepared; Step S33, Preparation of heteroatom-modified mixed solution: The mesoporous silica (SiO 2 )nanospheres, polylactic acid and dicyandiamide obtained in step S32 were fully dissolved in acetone (mass fraction of silica in the mixed solution 15 wt%, concentration of polylactic acid in the mixed solution 15 wt%, mass ratio of heteroatom carrier to polylactic acid 1:7) to prepare a heteroatom-modified mixed solution; Step S34, Preparation of silica-coated polylactic acid nanofiber membrane: The heteroatom-modified mixed solution obtained in step S33 was prepared by high-speed centrifugal electrospinning (centrifugal speed 20k, spinning radius 10 cm, solution supply rate 3 mL / h, receiving distance 20 cm, ambient temperature 25 °C, relative humidity 30%) to prepare a mesoporous silica-coated polylactic acid nanofiber membrane with heteroatom modification. The specific surface area of the obtained nanofiber membrane was 419 m 2 / g, and the average pore size was 1.96 nm.

[0052] As Figure 8 shown, the scanning electron micrograph of the silica-coated polylactic acid nanofiber membrane obtained in Example 3 of the present invention shows that silica microspheres are uniformly wrapped on the surface of polylactic acid nanofibers to form a unique three-dimensional woven core-shell structure.

[0053] Example 4 of the present invention provides a method for preparing a silica-coated polylactic acid nanofiber membrane, including the following steps: Step S41, Preparation of catalytic solution: Ammonia water (concentration 28%) and an alcohol-water solution (volume ratio of anhydrous ethylene glycol to water 2:1) were mixed at a mass ratio of 1:20 to prepare a catalytic solution; Step S42, Preparation of mesoporous silica (SiO 2 )nanospheres: Polydimethylsiloxane and sodium chloride (mass ratio 1:10) were fully dispersed in ethylene glycol (volume ratio 1:50), quickly poured into the catalytic solution obtained in S41, and reacted at 60 °C for 1 h through a sol-gel phase transition process. After high-temperature calcination cleaning (calcination temperature 700 °C, calcination time 3 h, heating rate 5 °C / min, calcination atmosphere inert gas, gas flow rate 20 mL / min) and drying, mesoporous silica (SiO 2) nano - microspheres; Step S43, preparing a heteroatom - modified mixed solution: dispersing the obtained microporous - rich silica (SiO 2 ) nano - microspheres, polylactic acid, and urea in chloroform (the mass fraction of silica in the mixed solution is 30 wt%, the concentration of polylactic acid in the mixed solution is 20 wt%, and the mass ratio of the heteroatom carrier to polylactic acid is 1:10) to prepare a heteroatom - modified mixed solution uniformly; Step S44, preparing a silica - coated polylactic acid nanofiber membrane: passing the heteroatom - modified mixed solution obtained in Step S43 through a melt - spinning technology by heating (the melting temperature is 100 °C, the heating rate is 10 °C / min, the pressure is controlled at 6 MPa, the spinning speed is 1000 m / min, the draw ratio is 3 times, and the collection distance is 0.5 m) to prepare a microporous - rich silica - coated polylactic acid nanofiber membrane with heteroatom modification. The specific surface area of the obtained nanofiber membrane is 384 m 2 / g, and the average pore diameter is 2.32 nm.

[0054] As Figure 9 shown, the scanning electron microscope image of the silica - coated polylactic acid nanofiber membrane obtained in Example 4 of the present invention shows that the silica microspheres are uniformly wrapped on the surface of the polylactic acid nanofibers to form a unique three - dimensional woven core - shell structure.

[0055] Comparative Example 1 of the present invention provides a method for preparing a silica - coated polylactic acid nanofiber membrane, which basically uses the method of Example 1 to prepare a heteroatom - modified polylactic acid nanofiber membrane. The difference is that in this example, the polylactic acid nanofiber membrane is not prepared by coating with silica nano - microspheres. Specifically, polylactic acid and 4,4'-diaminodiphenyl disulfide are uniformly dispersed in N,N - dimethylformamide (the concentration of polylactic acid in the mixed solution is 10 wt%, and the mass ratio of the heteroatom carrier to polylactic acid is 1:2) to prepare a heteroatom - modified mixed solution; the obtained heteroatom - modified mixed solution is passed through a high - voltage electrospinning technology (the working voltage is 20 kV, the spraying speed is 1 mL / h, the receiving distance is 15 cm, the temperature is 25 °C, the relative humidity is 30%, and the reciprocating speed is 3 cm / min) to prepare a heteroatom - modified polylactic acid nanofiber membrane. The specific surface area of the obtained nanofiber membrane is 152 m 2 / g, and the average pore diameter is 3.69 nm.

[0056] Comparative Example 2 of the present invention provides a method for preparing a silica-coated polylactic acid nanofiber membrane, and basically uses the method of Example 2 to prepare a microporous silica-coated polylactic acid nanofiber membrane. The difference is that in this example, a heteroatom carrier is not used to prepare the polylactic acid nanofiber membrane. Specifically, ammonia water (mass concentration of 20%) and an alcohol-aqueous solution (volume ratio of anhydrous methanol to deionized water is 1:2) are mixed at a mass ratio of 1:10 to prepare a catalytic solution; methyl orthosilicate and polyethylene glycol (mass ratio of 1:5) are fully dispersed in ethanol (volume ratio of 1:20), quickly poured into the obtained catalytic solution, and reacted at 20 °C for 12 h through a sol-gel phase transition process, and prepared into microporous silica (SiO 2 )nanospheres by an acid-base cleaning method (cleaning temperature is 30 °C, cleaning time is 10 min, cleaning solvent is 0.1 mol / L hydrochloric acid solution, mass ratio of silica nanospheres to cleaning solvent is 1:5) and drying; the obtained microporous silica (SiO 2 )nanospheres and polylactic acid are fully dissolved in tetrahydrofuran (mass fraction of silica in the mixed solution is 10 wt%, concentration of polylactic acid in the mixed solution is 12 wt%) to prepare a heteroatom-modified mixed solution; the obtained spinning mixed solution is prepared into a microporous silica-coated polylactic acid nanofiber membrane by high-speed air spinning (spraying angle is 30°, ratio of spraying speed to extrusion speed is 10k, spraying pressure is 3 MPa, air flow temperature is 35 °C, relative humidity is 40%), and the specific surface area of the obtained nanofiber membrane is 214 m 2 / g, and the average pore diameter is 3.15 nm.

[0057] Comparative Example 3 of the present invention provides a method for preparing a silica-coated polylactic acid nanofiber membrane, and basically uses the method of Example 3 to prepare a heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane. The difference is that in this example, a pore-forming agent is not used to prepare the silica nanospheres. Specifically, ammonia water (mass concentration of 30%) and an alcohol-aqueous solution (volume ratio of anhydrous isopropanol to deionized water is 2:3) are mixed at a mass ratio of 1:15 to prepare a catalytic solution; propyl orthosilicate is fully dispersed in ethanol (volume ratio of 1:30), quickly poured into the obtained catalytic solution, and reacted at 30 °C for 8 h through a sol-gel phase transition process to prepare silica nanospheres; the obtained SiO 2The heteroatom-modified mixed solution was prepared by uniformly dispersing nanospheres, polylactic acid, and dicyandiamide in acetone (the mass fraction of silica in the mixed solution was 15 wt%, the concentration of polylactic acid in the mixed solution was 15 wt%, and the mass ratio of the heteroatom carrier to polylactic acid was 1:7). The obtained heteroatom-modified mixed solution was used to prepare a silica-coated polylactic acid nanofiber membrane with heteroatom modification through high-speed centrifugal electrospinning (the centrifugal speed was 20k, the rotation radius was 10 cm, the solution feeding rate was 3 mL / h, the receiving distance was 20 cm, the environmental temperature was 25°C, and the relative humidity was 30%). The specific surface area of the obtained nanofiber membrane was 189 m 2 / g, and the average pore size was 3.44 nm.

[0058] The structural characterization and performance tests are as follows.

[0059] Observation by scanning electron microscope: The microstructure of the silica-coated polylactic acid nanofiber membrane was observed through a field emission scanning electron microscope (model JSM-7900F, JEOL, Japan) ( Figure 1 , Figure 7 , Figure 8 , Figure 9 ).

[0060] Surface element distribution test: The surface element distribution of the silica-coated polylactic acid nanofiber membrane was recorded using an energy-dispersive X-ray spectrometer (model Vario EL, JEOL, Japan) ( Figure 2 ).

[0061] Functional group structure test: The functional groups of the silica-coated polylactic acid nanofiber membrane were recorded using an infrared spectrometer (model VERTEX 70, Bruker, USA) ( Figure 3 ).

[0062] Crystal structure test: The crystal structure of the silica-coated polylactic acid nanofiber membrane was recorded using an X-ray spectrometer (model D / Max-2400, Rigaku Corporation, Japan) ( Figure 4 ).

[0063] Specific surface area test: The nitrogen adsorption-desorption isotherm of the silica-coated polylactic acid nanofiber membrane was measured at 77K using a gas adsorption analyzer (ASAP 2060, Micromeritics, USA) ( Figure 5 , Figure 6 ).

[0064] Gas adsorption performance test: A gas adsorption analyzer (ASAP 2020, Micromeritics, USA) was used to adsorb CO 2 and SO 2 . High-purity gases CO 2 (99.999%) and SO 2(99.999%) is used for adsorption measurement, while the free space is measured with helium (99.999%). The isothermal adsorption is carried out at 273 K (ice-water bath).

[0065] Experimental results: As Figure 1 , Figure 7 , Figure 8 and Figure 9 shown, it can be observed that by uniformly dispersing the microporous silica nanospheres, polylactic acid, and heteroatom carriers in an organic solvent, a heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane is prepared by a spinning technique. By adjusting measures such as the ratio of reactants and the polarity of the organic solvent, the silica microspheres are uniformly wrapped on the surface of the polylactic acid nanofibers to form a unique three-dimensional woven core-shell structure.

[0066] As Figure 2 shown, it can be observed that carbon, nitrogen, and cobalt elements are uniformly distributed on the surface of the silica-coated polylactic acid nanofiber membrane, indicating that the heteroatom carriers are uniformly mixed in the mixed solution, and the heteroatom adsorption sites are fully exposed on the material surface, providing more opportunities and possibilities for the selective adsorption and separation of SO 2

[0067] As Figure 3 shown, it can be observed that there are stretching vibration signal peaks of Si−O−Si, H−O−H, Si−OH, and −OH bonds in the structure of the silica-coated polylactic acid nanofiber membrane, indicating the successful introduction of silica.

[0068] As Figure 4 shown, a broad and flat signal peak appears in the diffraction pattern of the silica-coated polylactic acid nanofiber membrane at about 20°, indicating that the polylactic acid nanofiber membrane is formed in an amorphous structure under kinetic control.

[0069] As Figure 5 shown, the adsorption-desorption isotherm curve of the silica-coated polylactic acid nanofiber membrane shows a significant increase in nitrogen adsorption in the low-pressure region, a relatively slow increase in the middle-pressure region, and a sharp increase in nitrogen adsorption in the high-pressure region, indicating that the polylactic acid nanofiber membrane is composed of continuous micropores, mesopores, and macropores.

[0070] As Figure 6 shown, the pore size distribution curve of the silica-coated polylactic acid nanofiber membrane shows a continuous distribution in the ranges of micropores, mesopores, and macropores, indicating that the polylactic acid nanofiber membrane has a hierarchical porous structure, providing more space and adsorption sites for the adsorption and separation of SO 2

[0071] Table 1 compares the specific surface area, average pore size, heteroatom content, SO 2 adsorption capacity, and SO 2 / CO 2 Separation selectivity results.

[0072] Table 1

[0073] Examples 1-4 have a relatively high specific surface area (384 m 2 / g - 467 m 2 / g) and a relatively small pore size (1.56 nm - 2.32 nm) because the introduction of micro-porous silica nanospheres endows the surface of the poly(lactic acid) nanofiber membrane with a larger specific surface area and pore structure; the specific surface areas of Comparative Examples 1-3 are only 152-214 m 2 / g, and the average pore diameter is 3.15 - 3.69 nm, because silica with an ideal microporous structure is not coated.

[0074] The SO 2 adsorption and separation selectivity of the silica-coated poly(lactic acid) nanofiber membrane are closely related to the specific surface area, pore size, and heteroatom adsorption sites. Examples 1-4 with a larger specific surface area, a smaller pore size, and abundant heteroatom adsorption sites have an SO 2 adsorption capacity of more than 4.8 mmol / g, and an SO 2 / CO 2 separation selectivity of more than 81, showing good SO 2 adsorption and separation selectivity. Among them, Example 1 with the highest specific surface area, the smallest pore size, and the most heteroatom adsorption sites shows the best performance in gas adsorption separation tests. The SO 2 adsorption capacity is 6.7 mmol / g, and the SO 2 / CO 2 separation selectivity is 121, far higher than that of Comparative Examples 1-2 with a low specific surface area, a large pore size, and no silica and heteroatom adsorption sites (SO 2 adsorption capacity ≤ 1.1 mmol / g, SO 2 / CO 2 separation selectivity ≤ 12).

[0075] The present invention provides a silica-coated poly(lactic acid) nanofiber membrane and a preparation method thereof. There are many specific methods and ways to implement this technical solution. The above description is only a preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention. Each component not clearly defined in this embodiment can be realized by existing technologies.

Claims

1. A method for preparing a heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane, characterized in that: The following steps are involved: Step S1, mixing aqueous ammonia and an alcohol-water solution to prepare a catalytic solution; Step S2, dispersing the silicon source and the pore-forming agent in an alcohol solvent, mixing the dispersed silicon source and the catalytic solution obtained in S1, and then purifying the dispersed silicon source and the pore-forming agent by a washing method to prepare microporous silica nanospheres; Step S3, dissolving polylactic acid, the heteroatom carrier and the microporous silica nanospheres obtained in step S2 in an organic solvent to prepare a heteroatom-modified mixed solution; Step S4, using the heteroatom modified mixed solution obtained in step S3 through spinning technology to prepare a heteroatom modified microporous silica-coated polylactic acid nanofiber membrane.

2. The method for preparing the heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane according to claim 1, characterized in that: The alcohol-water solution in step S1 is composed of an alcohol solvent and water, the volume ratio of the alcohol solvent to water in the alcohol-water solution is 1:2-2:1, and the alcohol solvent is one or more of methanol, ethanol, isopropanol, ethylene glycol, glycerol, n-butanol, and tert-butanol.

3. The method for preparing the heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane according to claim 1, characterized in that: The mass concentration of the ammonia water in step S1 is 20-30%, and the mass ratio of the ammonia water to the alcohol-water solution is 1:5-1:

20.

4. The method for preparing the heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane according to claim 1, characterized in that: In step S2, the silicon source is one or more of methyl orthosilicate, ethyl orthosilicate, propyl orthosilicate, sodium silicate, polydimethylsiloxane, silicon tetrachloride, and methyltrichlorosilane, and the mass ratio of the silicon source to the alcohol solvent is 1:10-1:

100.

5. The method for preparing the heteroatom modified microporous silica-coated polylactic acid nanofiber membrane according to claim 1, characterized in that: In step S2, the pore-forming agent is one or more of hexadecyltrimethylammonium bromide, polyethylene glycol, dimethyldiethoxysilane, and sodium chloride, and the mass ratio of the pore-forming agent to the silicon source is 1:2-1:

10.

6. The method for preparing the heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane according to claim 1, characterized in that: The mixing conditions in step S2 are a temperature of 20-60° C. and a reaction time of 1-24 h.

7. The method for preparing the heteroatom-modified microporous silica-coated polylactic acid nanofiber membrane according to claim 1, characterized in that: The cleaning method in step S2 is one or more of solvent cleaning, acid-base cleaning, ultrasonic cleaning, and high-temperature calcination cleaning. The cleaning solvent used in the cleaning method is one or more of ethanol, methanol, isopropanol, water, acetone, dilute acid solution, and dilute alkali solution.

8. The method for preparing the heteroatom modified microporous silica-coated polylactic acid nanofiber membrane according to claim 1, characterized in that: In the step S3, the mass fraction of the microporous silica nanospheres in the heteroatom-modified mixed solution is 1-30 wt%, and the concentration of the polylactic acid in the heteroatom-modified mixed solution is 0.5-30 wt%; in the step S3, the heteroatom carrier is one or more of 4,4'-diaminodiphenyl disulfide, diphenyl disulfide, dicyandiamide, urea, melamine, and citric acid, and the mass ratio of the heteroatom carrier to the polylactic acid is 1:1-1:10; in the step S3, the organic solvent is one or more of N,N-dimethylformamide, tetrahydrofuran, dimethyl sulfoxide, dichloromethane, trichloromethane, chloroform, acetone, ethyl acetate, methanol, and ethanol.

9. The method for preparing the heteroatom modified microporous silica-coated polylactic acid nanofiber membrane according to claim 1, characterized in that: The spinning technology in step S4 is one or more of high-voltage electrostatic spinning technology, high-speed airflow spinning technology, high-speed centrifugal spinning technology, and heated melt spinning technology.

10. A polylactic acid nanofiber membrane prepared by the method for preparing a heteroatom-modified microporous silica-rich polylactic acid nanofiber membrane according to any one of claims 1 to 9.

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