Sodium alginate pervaporation hybrid membranes based on electrostatic interaction for membrane densification and preparation and application thereof

Abstract

This invention discloses a sodium alginate pervaporation hybrid membrane with electrostatic interaction-based control of membrane density. The hybrid membrane comprises a base membrane and a functional layer. The functional layer uses negatively charged sodium alginate as the polymer host and positively charged ionic covalent organic framework nanosheets as fillers, with a mass ratio of 100:5–25. Its preparation involves: obtaining ionic covalent organic framework nanosheets through solvothermal polycondensation of 1,3,5-trialdehyde phloroglucinol and amino monomers; dispersing these nanosheets in deionized water and blending them with sodium alginate to obtain a casting solution; spin-coating this solution onto the base membrane surface; and then drying, cross-linking, and drying to obtain the hybrid membrane. The process is simple, highly controllable, and universally applicable. Electrostatic interaction leads to a tighter bond between the polymer and filler, improving membrane density and selectivity. When used for pervaporation ethanol dehydration, this hybrid membrane exhibits high permeability, high selectivity, and high stability.

Description

Technical Field

[0001] This invention relates to a sodium alginate pervaporation hybrid membrane whose membrane density is controlled by electrostatic interaction, as well as its preparation and application, belonging to the field of polymer-organic hybrid membrane technology. Background Technology

[0002] The development of biomass fuel technology has received increasing attention worldwide in recent years. The technology for converting various biomass into fuel ethanol through microbial fermentation is becoming increasingly mature. However, the large amount of water in the fermentation broth forms a binary azeotrope with ethanol, significantly increasing the energy consumption of the separation step. Pervaporation membrane separation follows the dissolution-diffusion principle, achieving separation by leveraging the different adsorption and diffusion rates of different components on and inside the membrane surface. This allows for efficient separation of ethanol / water azeotropes with relatively low energy consumption. Sodium alginate is one of the most widely used pervaporation membrane materials. However, the sodium alginate molecular chain contains numerous hydrophilic hydroxyl and carboxyl functional groups, leading to severe swelling in aqueous solutions, which reduces membrane density and selectivity. Adding fillers to sodium alginate to construct hybrid membranes can effectively improve membrane separation performance. Covalent organic frameworks are emerging organic porous crystalline materials that have emerged in recent years. They are topologically linked materials with periodic covalent bonds synthesized through reversible reactions. Their framework structure is highly tunable and holds great potential in the field of molecular separation. When covalent organic frameworks are used as fillers, their size is usually in the micrometer range, and they are usually connected to the polymer host by short-range forces such as hydrogen bonding. It is still difficult to improve the compactness and stability of covalent organic framework hybrid films. Summary of the Invention

[0003] To address the aforementioned limitations of existing technologies and improve the density and stability of hybrid membranes, this invention provides a sodium alginate pervaporation hybrid membrane with electrostatically modulated membrane density. The preparation of this hybrid membrane utilizes the ease with which charged groups can be introduced into the all-organic structure of a covalent organic framework. Charged groups, such as pyridine and guanidine, are added to the covalent organic framework framework or its pores through pre-design or post-grafting methods, thus preparing an ionic covalent organic framework. Ionic covalent organic frameworks possess long-range ordered charged inherent pores and are easily exfoliated into nanosheets due to electrostatic repulsion between layers. The charged groups typically exhibit good hydration properties, resulting in excellent water dispersibility. Furthermore, the charge of the filler can be precisely controlled by designing groups with different charges, making it an ideal filler for pervaporation ethanol dehydration membranes. In the preparation of the hybrid membrane of this invention, an ionic covalent organic framework is used as a filler. This framework can form a long-range electrostatic interaction with the electronegative carboxyl groups on the sodium alginate molecular chain. By controlling the mass ratio of the filler to sodium alginate, the strength of the electrostatic interaction can be precisely controlled, thereby better controlling the membrane's density and effectively improving its selectivity. Simultaneously, the inherent nanoscale hydrophilic channels of the ionic covalent organic framework synergistically enhance the membrane's permeability and selectivity through molecular sieving within the sodium alginate. The preparation method of the hybrid membrane of this invention is simple to operate and highly controllable. The prepared hybrid membrane exhibits high density, and when applied to the pervaporation ethanol dehydration process, it demonstrates high permeation flux and separation factor, as well as good stability.

[0004] To address the aforementioned technical problems, this invention proposes a sodium alginate pervaporation hybrid membrane based on electrostatic interaction to regulate membrane density. This hybrid membrane comprises a base membrane and a functional layer. The functional layer uses negatively charged sodium alginate as the polymer host and positively charged ionic covalent organic framework nanosheets as fillers. The mass ratio of the ionic covalent organic framework nanosheets to sodium alginate is 5–25:100. The ionic covalent organic framework nanosheets are formed by the condensation polymerization of 1,3,5-trialdehyde phloroglucinol and an amino monomer at a molar ratio of 1:1–1.5, and the thickness of the ionic covalent organic framework nanosheets is 3–5 nm.

[0005] Furthermore, this invention also proposes a method for preparing the aforementioned sodium alginate pervaporation hybrid membrane, comprising the following steps:

[0006] Step 1: Preparation of ionic covalent organic framework nanosheets: 1,3,5-trialdehyde phloroglucinol, an amino monomer, mesitylene, 1,4-dioxane, and an aqueous acetic acid solution were mixed and sealed. The mass-volume concentration of the 1,3,5-trialdehyde phloroglucinol was 11.7–19.1 mg / mL, the mass-volume concentration of the amino monomer was 7.8–53.7 mg / mL, the molar concentration of the aqueous acetic acid solution was 3–6 mol / L, and the volume ratio of mesitylene, 1,4-dioxane, and the aqueous acetic acid solution was 4:1:3. The mixture was then sonicated. After thorough mixing, a freeze-evacuation-thawing process was performed, including: freezing with liquid nitrogen and then evacuating the gas from the tube; thawing under vacuum to remove the gas from the liquid; repeating the freeze-evacuation-thawing process three times; heating the reaction at 120°C for 3 days; washing the precipitate sequentially with methanol, deionized water, and tetrahydrofuran; drying in a vacuum oven at 90°C for 12 hours; adding the obtained product to deionized water at a mass-volume concentration of 60 mg / mL; ball milling at 3000 rpm for 30–60 minutes; and collecting the ball-milled product, which is the ionic covalent organic framework nanosheet.

[0007] Step 2: Preparation of sodium alginate hybrid membrane with electrostatically controlled density: A certain amount of the ionic covalent organic framework nanosheet dispersion obtained in Step 1 was added to deionized water to obtain solution A, which was ultrasonically dispersed for 10 min. Then, sodium alginate was added to solution A to obtain solution B, wherein the mass percentage of sodium alginate to deionized water was 2.5%, and the mass ratio of sodium alginate to ionic covalent organic framework nanosheets was 100:5–25. Liquid B was mechanically stirred at 30℃ for 4–8 h, the resulting solution was filtered, and allowed to stand for 1 h to degas, yielding a casting solution. Using a polyacrylonitrile ultrafiltration membrane as the base membrane, the casting solution was prepared according to a volume-area ratio of 0.2 mL / cm². 2 The membrane was uniformly spin-coated onto the base membrane surface and dried at room temperature. The dried membrane was then placed in a 0.5 M CaCl2 aqueous solution and stirred for crosslinking for 5–20 min. After removal, the membrane surface was rinsed with a large amount of deionized water and dried at room temperature for 24–48 h to finally obtain a sodium alginate pervaporation hybrid membrane with membrane density controlled by electrostatic interaction.

[0008] Furthermore, in step one of the preparation method described in this invention, the amino monomer is selected from ethidium bromide, 2,5-diaminopyridine, and triaminoguanidine hydrochloride.

[0009] The hybrid membrane prepared according to this invention was used for pervaporation ethanol dehydration. Under the conditions of 76°C and a 90 / 10 wt% ethanol / water mixture, the permeation flux was 1825.4–3166.4 g / m³. -2 h -1 The separation factor ranged from 887 to 2112, and the separation performance remained stable within 120 hours.

[0010] Compared with the prior art, the beneficial effects of the present invention are:

[0011] In the preparation method of this invention, the ionic covalent organic framework filler binds tightly to sodium alginate through long-range electrostatic interactions, effectively improving the membrane's density and thus enhancing selectivity. The inherent hydrophilic nanochannels of the ionic covalent organic framework filler provide additional water mass transfer and sieving channels within the hybrid membrane, synergistically improving the selectivity and permeability of the hybrid membrane. The hybrid membrane preparation method of this invention is simple, highly controllable, and universally applicable. The prepared hybrid membrane, used for pervaporation ethanol dehydration, exhibits high permeability, high selectivity, and high stability. Attached Figure Description

[0012] Figure 1 This is a surface electron microscope image of membrane 1 obtained in Example 1.

[0013] Figure 2 This is a cross-sectional electron microscope image of membrane 1 obtained in Example 1.

[0014] Figure 3 This is a surface electron microscope image of membrane 2 obtained in Example 2.

[0015] Figure 4 This is a cross-sectional electron microscope image of membrane 2 obtained in Example 2.

[0016] Figure 5 This is a surface electron microscope image of film 3 obtained in Example 3.

[0017] Figure 6 This is a cross-sectional electron microscope image of membrane 3 obtained in Example 3.

[0018] Figure 7 This is a surface electron microscope image of film 4 obtained in Example 4.

[0019] Figure 8 This is a cross-sectional electron microscope image of membrane 4 obtained in Example 4.

[0020] Figure 9 This is a surface electron microscope image of film 5 obtained in Example 5.

[0021] Figure 10 This is a cross-sectional electron microscope image of membrane 5 obtained in Example 5.

[0022] Figure 11 This is a comparison graph of the permeation flux and separation factor of the membranes obtained in Examples 1-5 and the comparative examples;

[0023] Figure 12 This is a graph showing the trend of membrane permeation flux and separation factor over time in Example 1. Detailed Implementation

[0024] The design concept of this invention, a sodium alginate hybrid membrane with electrostatic interaction-controlled density, is as follows: Ionic covalent organic framework nanosheets provide additional mass transfer channels with sieving function within the dense polymer, improving membrane permeability and selectivity. Simultaneously, the ionic covalent organic framework nanosheets, possessing opposite charges to the polymer host, can generate electrostatic interactions with the polymer host, enhancing membrane density. This hybrid membrane comprises a base membrane and a functional layer. In the functional layer, negatively charged sodium alginate serves as the polymer host, while positively charged ionic covalent organic framework nanosheets serve as the filler. The ionic covalent organic framework nanosheets and sodium alginate are mixed at a mass ratio of 5–25:100. The electrostatic interaction strength can be controlled by adjusting the mass ratio of the filler to sodium alginate, achieving precise control of membrane density. The ionic covalent organic framework material is synthesized by solvothermal polycondensation of 1,3,5-trialdehyde phloroglucinol and an amino monomer at a molar ratio of 1:1–1.5. The preparation steps are as follows: First, ionic covalent organic framework nanosheets are obtained through solvothermal reaction and ball milling; then, the ionic covalent organic framework nanosheets are dispersed in deionized water and blended with sodium alginate to prepare a casting solution. The casting solution is spin-coated onto the surface of a polyacrylonitrile ultrafiltration membrane, dried, cross-linked with CaCl2 aqueous solution, and dried again to obtain the hybrid membrane. This invention provides a simple, highly controllable, and universally applicable preparation method. The prepared hybrid membrane is used for pervaporation ethanol dehydration and exhibits high permeability, high selectivity, and high stability.

[0025] The technical solution of the present invention will be further described in detail below with reference to specific examples and appendices. The specific implementation examples described are only for explanation and illustration of the present invention and are not intended to limit the present invention.

[0026] Example 1: Preparation of a sodium alginate pervaporation hybrid membrane with membrane density controlled by electrostatic interaction, the steps are as follows:

[0027] Step 1: Preparation of ionic covalent organic framework nanosheets:

[0028] 42 mg of 1,3,5-trialdehyde phloroglucinol, 118.3 mg of ethidium bromide, 0.8 mL of mesitylene, 0.2 mL of 1,4-dioxane, and 0.6 mL of 6 mol / L aqueous acetic acid solution were placed in a Piezx tube and sealed. The mixture was sonicated for 30 min until homogeneous. After freezing with liquid nitrogen, the gas inside the tube was evacuated. The tube was then thawed under vacuum to remove the gas from the liquid. This freezing-evacuation-thawing cycle was repeated three times. The mixture was then heated at 120 °C for 3 days. The precipitate was washed with methanol, deionized water, and tetrahydrofuran. The precipitate was dried in a vacuum oven at 90 °C for 12 h to obtain an ionic covalent organic framework.

[0029] Add deionized water and 300 mg of the above-mentioned ionic covalent organic framework to a 5 ml grinding tube, then add 15 grinding balls with a diameter of 3 mm and 3 grinding balls with a diameter of 5 mm. Grind at 3000 rpm for 30 min and collect the grinding product, which is the ionic covalent organic framework nanosheet.

[0030] Step 2: Preparation of sodium alginate hybrid membrane with electrostatically controlled density:

[0031] Take 113.1 mg of the ionic covalent organic framework nanosheet dispersion prepared in step one into 25 mL of deionized water, and ultrasonically disperse for 10 min. Then add 641 mg of sodium alginate (i.e., the mass ratio of ionic covalent organic framework nanosheet to sodium alginate is 15:100). The mass fraction of sodium alginate in this solution is 2.5 wt%. The mixed solution is mechanically stirred at 30 °C for 8 h. The resulting solution is filtered and allowed to stand for 1 h to remove bubbles, thus obtaining a uniform casting solution.

[0032] According to a volume-area ratio of 0.2 mL / cm² 2 The casting solution was uniformly spin-coated onto the surface of a polyacrylonitrile ultrafiltration membrane and dried at room temperature to obtain a hybrid membrane. The hybrid membrane was then placed in a 0.5 M CaCl2 aqueous solution and stirred for 15 min for crosslinking. After removal, the membrane surface was rinsed with a large amount of deionized water and dried at room temperature for 48 h. Finally, a sodium alginate pervaporation hybrid membrane with electrostatically controlled membrane density was obtained, designated as membrane 1. Surface and cross-sectional electron micrographs of membrane 1 are shown below. Figure 1 , Figure 2 As shown.

[0033] Membrane 1 was used for pervaporation of ethanol for dehydration. Under conditions of 76°C and a 90 / 10 wt% ethanol / water mixture, the permeation flux was 2460 g / m³. -2 h -1 The separation factor is 2112, and its separation performance remains stable within 120 hours. Figure 11 and Figure 12 As shown.

[0034] Example 2: Preparation of a sodium alginate pervaporation hybrid membrane with membrane density controlled by electrostatic interaction, the steps are as follows:

[0035] The preparation process is basically the same as in Example 1, except that in step one, the amount of 1,3,5-trialdehyde phloroglucinol is changed from 42 mg to 63 mg, and the amount of ethidium bromide is changed from 118.3 mg to 49.1 mg of 2,5-diaminopyridine; in step two, the amount of ionic covalent organic framework nanosheets is changed from 113.1 mg to 71.2 mg, i.e., the mass ratio of ionic covalent organic framework nanosheets to sodium alginate is 10:100, and the crosslinking time is changed from 10 min to 15 min; the final membrane is designated as membrane 2, and the surface and cross-sectional electron micrographs of membrane 2 are shown in the figure. Figure 3 , Figure 4 As shown.

[0036] Membrane 2 was used for pervaporation of ethanol for dehydration. Under conditions of 76°C and a 90 / 10 wt% ethanol / water mixture, the permeation flux was 1997 g m³. -2 h -1 The separation factor is 887, such as Figure 11 As shown.

[0037] Example 3: Preparation of a sodium alginate pervaporation hybrid membrane with membrane density controlled by electrostatic interaction, the steps are as follows:

[0038] The preparation process is basically the same as in Example 1, except that in step one, the 118.3 mg of ethidium bromide is replaced with 28 mg of triaminoguanidine hydrochloride; in step two, the amount of ionic covalent organic framework nanosheets is changed from 113.1 mg to 160.3 mg, i.e., the mass ratio of ionic covalent organic framework nanosheets to sodium alginate is 20:100, and the crosslinking time is changed from 10 min to 18 min; the final membrane is designated as membrane 3, and the surface and cross-sectional electron micrographs of membrane 3 are shown in the figure. Figure 5 , Figure 6 As shown.

[0039] Membrane 3 was used for pervaporation of ethanol for dehydration. Under conditions of 76°C and a 90 / 10 wt% ethanol / water mixture, the permeation flux was 2044 g / m³. -2 h -1 The separation factor is 1371, such as Figure 11 As shown.

[0040] Example 4: Preparation of a sodium alginate pervaporation hybrid membrane with membrane density controlled by electrostatic interaction, the steps are as follows:

[0041] The preparation process is basically the same as in Example 1, except that in step two, the amount of ionic covalent organic framework nanosheets is changed from 113.1 mg to 33.7 mg, that is, the mass ratio of ionic covalent organic framework nanosheets to sodium alginate is 5:100. The final membrane is designated as membrane 4. The surface and cross-sectional electron micrographs of membrane 4 are shown in the figure. Figure 7 , Figure 8As shown.

[0042] Membrane 4 was used for pervaporation of ethanol for dehydration. At 76°C and with a 90 / 10 wt% ethanol / water mixture, the permeation flux was 1825.4 g / m³. -2 h -1 The separation factor is 970, such as Figure 11 As shown.

[0043] Example 5: Preparation of a sodium alginate pervaporation hybrid membrane with membrane density controlled by electrostatic interaction, the steps are as follows:

[0044] The preparation process is basically the same as in Example 1, except that in step two, the amount of ionic covalent organic framework nanosheets is changed from 113.1 mg to 213.7 mg, that is, the mass ratio of ionic covalent organic framework nanosheets to sodium alginate is 25:100. The final membrane is designated as membrane 5. The surface and cross-sectional electron micrographs of membrane 5 are shown in the figure. Figure 9 , Figure 10 As shown.

[0045] Membrane 4 was used for pervaporation of ethanol for dehydration. At 76°C and with a 90 / 10 wt% ethanol / water mixture, the permeation flux was 3166.4 g / m³. -2 h -1 The separation factor is 945, such as Figure 11 As shown.

[0046] Comparative example: Preparation of pure sodium alginate membrane, the steps are as follows:

[0047] Add 641 mg of sodium alginate to 25 mL of deionized water. The mass fraction of sodium alginate in the solution is 2.5 wt%. The solution is mechanically stirred at 30 °C for 8 h. The resulting solution is filtered and allowed to stand for 1 h to remove bubbles, thus obtaining a uniform casting solution.

[0048] The casting solution was uniformly spin-coated onto the surface of a polyacrylonitrile ultrafiltration membrane and dried at room temperature to obtain a hybrid membrane. The hybrid membrane was placed in a 0.5 M CaCl2 aqueous solution and stirred for crosslinking for 15 min. After removal, the membrane surface was rinsed with a large amount of deionized water and dried at room temperature for 48 h to obtain a pure sodium alginate membrane. This membrane was designated as the control membrane.

[0049] The comparative example was used for pervaporation dehydration of ethanol. Under the conditions of 76°C and a 90 / 10 wt% ethanol / water mixture, the permeation flux was 1542 g m. -2 h -1 The separation factor is 437, such as Figure 11 As shown.

[0050] Figure 11Table 1 shows the permeation flux and separation factor of the comparative examples and Examples 1-5 under the conditions of 76°C and 90 / 10wt% ethanol / water mixture. Table 1 also lists the membrane thickness of the membranes obtained in Examples 1-5 and the comparative examples.

[0051] Table 1

[0052]

[0053] The comparison of the above comparative examples and Examples 1-5 shows that the addition of ionic covalent organic framework nanosheets can effectively improve the pervaporation ethanol dehydration performance of sodium alginate membranes. Comparing Examples 1, 4, and 5 (using the same filler in preparation), controlling the mass ratio of ionic covalent organic framework nanosheets to sodium alginate effectively regulates the electrostatic interaction strength. With increasing addition, the electrostatic interaction strengthens, the membrane thickness decreases significantly, surface defects are reduced, and the membrane becomes more compact. Therefore, the sodium alginate pervaporation hybrid membrane proposed in this invention, which regulates membrane compactness through electrostatic interaction, has broad application potential.

[0054] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many improvements and changes under the guidance of the present invention without departing from the spirit of the present invention, and these improvements and changes are all within the protection scope of the present invention.

Claims

1. A method for preparing a sodium alginate pervaporation hybrid membrane based on electrostatic interaction to regulate membrane density, the hybrid membrane comprising a base membrane and a functional layer, wherein the functional layer uses negatively charged sodium alginate as the polymer host and positively charged ionic covalent organic framework nanosheets as fillers, the ionic covalent organic framework nanosheets being mixed with sodium alginate at a mass ratio of 5-25:100; wherein, The ionic covalent organic framework nanosheets are formed by the condensation polymerization of 1,3,5-trialdehyde phloroglucinol and amino monomers in a molar ratio of 1:1 to 1.5, and the thickness of the ionic covalent organic framework nanosheets is 3 to 5 nm; the method for preparing the hybrid film includes the following steps: Step 1: Preparation of ionic covalent organic framework nanosheets: 1,3,5-Trialdehyde phloroglucinol, an amino monomer, mesitylene, 1,4-dioxane, and an aqueous acetic acid solution were mixed and sealed. The 1,3,5-trialdehyde phloroglucinol had a mass-to-volume concentration of 11.7–19.1 mg / mL, the amino monomer had a mass-to-volume concentration of 7.8–53.7 mg / mL, the aqueous acetic acid solution had a molar concentration of 3–6 mol / L, and the volume ratio of mesitylene, 1,4-dioxane, and the aqueous acetic acid solution was 4:1:

3. The mixture was sonicated until homogeneous and then subjected to a freeze-evacuation-thawing process, including: freezing with liquid nitrogen and then evacuating the gas from the tube; thawing under vacuum to remove the gas from the liquid; repeating the freeze-evacuation-thawing process three times; heating the reaction at 120 °C for 3 days; washing the precipitate sequentially with methanol, deionized water, and tetrahydrofuran; and drying in a vacuum oven at 90 °C for 12 h. The obtained product was added to deionized water at a mass-volume concentration of 60 mg / mL and ball-milled at 3000 rpm for 30-60 min. The ball-milled product was collected as ionic covalent organic framework nanosheets. Step 2: Preparation of sodium alginate hybrid membrane with electrostatically controlled density: A certain amount of the ionic covalent organic framework nanosheet dispersion prepared in step one was added to deionized water to obtain solution A, which was ultrasonically dispersed for 10 min. Then, sodium alginate was added to solution A to obtain solution B, wherein the mass percentage of sodium alginate to deionized water was 2.5%, and the mass ratio of sodium alginate to ionic covalent organic framework nanosheets was 100:5~25. Liquid B was mechanically stirred at 30°C for 4~8 h, the obtained solution was filtered, and allowed to stand for 1 h to degas, thus obtaining the casting solution. Using a polyacrylonitrile ultrafiltration membrane as the base membrane, the casting solution was prepared at a volume area ratio of 0.2 mL / cm². 2 The membrane was uniformly spin-coated onto the base film surface and dried at room temperature. The dried membrane was then placed in a 0.5 M CaCl2 aqueous solution and stirred for crosslinking for 5-20 min. After removal, the membrane surface was rinsed with a large amount of deionized water and dried at room temperature for 24-48 h to finally obtain a sodium alginate pervaporation hybrid membrane with membrane density controlled by electrostatic interaction.

2. The preparation method according to claim 1, characterized in that, In step one, the amino monomer is selected from ethidium bromide, 2,5-diaminopyridine, and triaminoguanidine hydrochloride.

3. An application of a sodium alginate pervaporation hybrid membrane based on electrostatic interaction to regulate membrane density, characterized in that, The hybrid membrane prepared according to the method of claim 1 or 2 was used for pervaporation ethanol dehydration. Under the conditions of 76 °C and a 90 / 10 wt% ethanol / water mixture, the permeation flux was 1825.4~3166.4 gm. -2 h -1 The separation factor was 887~2112, and the separation performance remained stable within 120h.

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