PVA catalytic composite membrane with asymmetric structure and preparation method thereof

Abstract

This invention relates to an asymmetric catalytic composite membrane and its preparation method, belonging to the field of chemical catalytic separation technology. The catalytic composite membrane adopts a three-layer asymmetric composite structure, consisting of a catalytic layer, an intermediate support layer, and a separation layer. The intermediate support layer is placed between the catalytic layer and the separation layer to achieve physical isolation between the two layers, avoiding damage to the separation layer structure during the catalytic layer preparation process. Specifically, the catalytic layer is a sulfonated polyvinyl alcohol (PVA) membrane with a porous structure prepared by freeze-drying technology. After sulfonation modification and freeze-drying to create pores, it exhibits both low mass transfer resistance and high catalytic activity. The intermediate support layer is a polyacrylonitrile (PAN) ultrafiltration membrane, providing stability and isolation. The separation layer is a dense membrane formed by spin-coating MOF-801 nanoparticles with sodium alginate (SA). The hydrophilic MOF-801 nanoparticles significantly improve the water / ethanol separation efficiency. The asymmetric structure of this invention has the characteristic of independent optimization of functional layers, which can enhance the performance of the catalytic layer and the separation layer respectively. The prepared sulfonated polyvinyl alcohol (PVA) catalytic composite membrane exhibits excellent catalytic-separation coupling performance and long-term stability in esterification reaction, and has good prospects for industrial application.

Description

Technical Field

[0001] This invention belongs to the fields of materials science and membrane separation technology, specifically relating to an asymmetric PVA catalytic composite membrane and its preparation method. The catalytic composite membrane prepared by this invention is mainly used for the esterification reaction of organic carboxylic acids and organic alcohols. Background Technology

[0002] Pervaporation catalytic membrane reactors (PVCMR) integrate catalytic and separation functions and are a key technology for enhancing esterification reactions. The catalytic composite membrane, as its core material, must possess both excellent catalytic activity and separation performance.

[0003] Currently, most common catalytic composite membranes employ a direct composite structure of a catalyst layer and a separation layer. This structure has inherent design flaws: the solvents and water used in the catalyst layer preparation process easily permeate and diffuse into the separation layer, disrupting its dense structure and leading to a significant decrease in separation selectivity, making it difficult to achieve synergistic optimization of catalytic and separation performance. For example, patent CN109772453B discloses an ionic liquid-functionalized PVA catalytic composite membrane for ester synthesis reactions. This patent introduces a strongly acidic ionic liquid into PVA to prepare the catalyst layer and directly composites it onto the cross-linked PVA separation layer. However, the strongly acidic catalyst damages the separation layer structure, resulting in a significant decrease in separation performance while improving catalytic performance. Therefore, how to achieve functional layer isolation through reasonable structural design has become a key challenge that urgently needs to be solved in the field of catalytic composite membranes.

[0004] The preparation of functional layers remains a research hotspot. For the preparation of catalytic layers, traditional porous catalytic layers are mostly constructed using the immersion phase inversion method. This process is complex, the immersion time is difficult to control precisely, and solvents and water easily penetrate into the separation layer during preparation, damaging its dense structure. For example, patent CN202510359505A discloses a three-layer heterogeneous catalytic composite membrane prepared using the immersion phase inversion method, consisting of a porous catalytic layer, a dense separation layer, and a support layer from top to bottom. Although this design increases active sites through the porous structure, synergistically enhancing catalytic reactions and separation mass transfer, the separation factor of the membrane is significantly reduced due to the damage to the integrity of the separation layer caused by water during the preparation of the catalytic layer. In contrast, freeze-drying technology eliminates the need for immersion, directly forming a continuous porous structure during drying. The process is simple and causes minimal damage to the base membrane, facilitating mass transfer and exposure of active sites. Using it as a sulfonated polyvinyl alcohol (PVA) catalytic layer can simultaneously utilize the catalytic activity and hydrophilicity of sulfonic acid groups to further enhance catalytic performance. Combined with the intermediate support layer, it constructs a functionally isolated asymmetric structure, a method rarely reported to date. The design of the separation layer is just as important as that of the catalyst layer. Sodium alginate (SA), a hydrophilic natural polysaccharide, is a commonly used pervaporation membrane material, but its separation selectivity is limited. Metal-organic framework material MOF-801 has excellent hydrophilicity and hydrothermal stability, and its doping into sodium alginate can significantly improve the water / ethanol separation efficiency of the separation layer.

[0005] To address the shortcomings of existing research, such as functional layer interference, complex catalytic layer preparation processes, and insufficient selectivity of the separation layer, this invention proposes an asymmetric PVA catalytic composite membrane. This membrane employs a three-layer asymmetric composite structure, with an intermediate support layer physically isolating the catalytic and separation layers, allowing for independent optimization of each functional layer. The catalytic layer utilizes freeze-drying technology to construct a sponge-like interconnected pore structure to reduce mass transfer resistance and is modified with sulfonic acid groups to impart a high density of catalytic active sites. The separation layer incorporates hydrophilic MOF-801 nanoparticles for doping, significantly improving water / ethanol separation efficiency. This invention solves the problems of functional layer interference and insufficient catalytic efficiency in existing catalytic composite membranes from both the membrane structure design and membrane preparation process perspectives, providing a highly efficient and stable catalytic composite membrane material for enhancing esterification reactions. Summary of the Invention

[0006] This invention aims to solve the problems of mutual interference between functional layers and insufficient catalytic efficiency in existing catalytic composite membranes, and to improve the conversion rate and by-product removal efficiency of esterification reactions. The technical solution adopted in this invention is: an asymmetric PVA catalytic composite membrane and its preparation method, specifically including the following steps: (1) Preparation of MOF-801 nanoparticles: Fumaric acid and zirconium oxychloride octahydrate were dissolved in a DMF / formic acid mixed solvent with a volume ratio of 3:1, placed in a reaction vessel and heated, and then centrifuged, washed and dried to obtain MOF-801 nanoparticles. (2) Preparation of dense separation layer: Weigh a certain mass concentration of MOF-801 nanoparticles prepared in step (1) and disperse them in deionized water. Add sodium alginate (SA) powder, stir to dissolve, and let stand to remove bubbles to form MOF-801 / SA separation layer casting solution. Spin-coat the casting solution onto the smooth side of the polyacrylonitrile (PAN) ultrafiltration membrane, dry it, and then soak it in calcium chloride solution for crosslinking for 5~20 min. After washing with deionized water and drying, a dense separation layer is obtained. (3) Preparation of porous catalyst layer: Polyvinyl alcohol (PVA) with a mass concentration of 3~12% is dissolved in deionized water, and sulfonic acid group catalyst with a mass concentration of 8~12% is added for grafting reaction. The reaction is stirred at 60~80 ℃ for 4~8 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution is formed. The casting solution is spin-coated onto the rough side of the polyacrylonitrile (PAN) ultrafiltration membrane in step (2). It is pre-frozen first, and then freeze-dried to form a sulfonated PVA catalyst layer with a sponge-like through-pore structure. Finally, a PVA catalyst composite membrane with a three-layer asymmetric structure is obtained.

[0007] Preferably, the amount of each component used in step (1) is: 40-60 mmol of fumaric acid, 40-60 mmol of zirconium oxychloride octahydrate, and 250-300 mL of DMF / formic acid mixed solvent.

[0008] Preferably, the reaction temperature of the reactor in step (1) is 120~140 ℃ and the reaction time is 4~8 h.

[0009] Preferably, the mass fraction of each component in step (2) is: 2-7% MOF-801 nanoparticles, 1-4% sodium alginate in the separation layer casting solution, and 1-5% calcium chloride solution.

[0010] Preferably, the sulfonic acid group catalyst in step (3) is one of 5-sulfosalicylic acid, aminosulfonic acid and p-toluenesulfonic acid.

[0011] Preferably, in step (3), the pre-freezing temperature is -20~-10 ℃, the freezing time is 2~4h, the freeze-drying temperature is -60~-50 ℃, the drying time is 24~48h, and the vacuum degree is less than 10 Pa.

[0012] Preferably, in steps (2) and (3), a high-precision spin coater is used to spin coat the casting solution, and a segmented speed-increasing spin coat method is adopted, wherein the low speed range is 500~1900 r / 20 s and the high speed range is 800~2200 r / 40 s.

[0013] This invention also provides an asymmetric PV catalytic composite membrane prepared by the above-mentioned technical solution. The membrane adopts a three-layer asymmetric composite structure, consisting of a catalytic layer, an intermediate support layer, and a separation layer. The intermediate support layer physically isolates the catalytic layer from the separation layer, enabling independent optimization of each functional layer. The catalytic layer is disposed on the rough side of the intermediate support layer and is a sulfonated PVA membrane with a sponge-like through-pore structure. The separation layer is disposed on the smooth side of the intermediate support layer and is a MOF-801-doped SA dense membrane. The intermediate support layer is a PAN ultrafiltration membrane.

[0014] More preferably, the preparation time of the membrane is reduced to 56-72 h, the thickness of the catalyst layer is 5-50 μm, the porosity is 60-90%, and the surface of the separation layer is uniform and dense with a thickness of 0.5-2 μm.

[0015] Finally, this invention provides an application of an asymmetric PVA catalytic composite membrane in enhancing esterification reactions, with a permeation flux of 1500 g·m³. -2 ·h -1 With a separation factor of 900 or higher, the acetic acid conversion rate can reach 85% or higher, and it still maintains a conversion rate of 83% or higher after being reused 5 times.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Overcoming the interlayer interference of traditional catalytic composite membranes By constructing a three-layer asymmetric composite structure of "catalytic layer-intermediate support layer-separation layer" using PAN ultrafiltration membrane as the intermediate support layer, the physical isolation between the catalytic layer and the separation layer is achieved. This overcomes the technical barrier of solvent and water damaging the dense structure of the separation layer during the preparation of the catalytic layer in traditional direct composite membranes, and enables independent optimization of the functional layers of the catalytic composite membrane. (2) Construction of a porous catalyst layer with low mass transfer resistance using freeze-drying technology A sulfonated PVA catalyst layer with a uniform porous structure was constructed using freeze-drying technology. This method simultaneously optimized the mass transfer pathway and introduced catalytically active sites in a single preparation process, overcoming the technical bottlenecks of traditional immersion phase inversion methods, such as complex processes, difficulty in precisely controlling immersion time, and easy damage to the separation layer. This technique eliminates the need for an immersion process, avoiding secondary damage to the separation layer by the solvent. The constructed porous structure significantly reduces mass transfer resistance, and combined with sulfonic acid group grafting modification, it achieves uniform exposure of high-density catalytic sites, maintaining the integrity, uniformity, and density of the membrane separation layer surface.

[0017] (3) The three-layer asymmetric structure achieves synergistic enhancement of catalytic-separation performance. The asymmetric composite structure constructed in this invention achieves a synergistic effect between catalytic and separation functions: the catalytic layer, with its interconnected porous structure and high-density sulfonic acid groups, enables rapid mass transfer and efficient catalysis, while the separation layer enhances the preferential permeability of water molecules by introducing hydrophilic MOF-801 nanoparticles. This design enables simultaneous catalytic reaction and product separation, with a separation permeation flux ≥1500 g·m³. -2 ·h -1 Separation factor ≥900, acetic acid conversion rate ≥85%, performance remains above 83% after 5 reuses. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 (a) The structure of the PVA catalytic composite membrane in the comparative example; (b) The PVA catalytic composite membrane with an asymmetric structure in the example; Figure 2 (a) SEM surface image of the PVA catalytic composite membrane in the comparative example; (b) cross-sectional image of the PVA catalytic composite membrane in the comparative example; (c) SEM surface image of the asymmetric PVA catalytic composite membrane in the example; (d) SEM cross-sectional image of the asymmetric PVA catalytic composite membrane in the example. Detailed Implementation

[0020] The present invention will be further described below with reference to specific embodiments.

[0021] Comparative Example 1 (1) Preparation of MOF-801 nanoparticles: 40 mmol fumaric acid and 40 mmol zirconium oxychloride octahydrate were dissolved in 250 mL of DMF / formic acid mixed solvent with a volume ratio of 3:1. The mixture was placed in a reaction vessel and heated at 120 °C for 4 h. After centrifugation, washing and drying, MOF-801 nanoparticles were obtained. (2) Preparation of dense separation layer: A certain amount of MOF-801 nanoparticles with a mass concentration of 2% prepared in step (1) were weighed and dispersed in deionized water. Sodium alginate (SA) powder with a mass concentration of 2.5% was added. After stirring and dissolving, the mixture was allowed to stand to remove bubbles and form MOF-801 / SA separation layer casting solution. The casting solution was spin-coated onto the surface of polyacrylonitrile (PAN) ultrafiltration membrane. After drying, the membrane was soaked in 1% calcium chloride solution for crosslinking for 5 min. After washing with deionized water and drying, a dense separation layer was obtained. (3) Preparation of porous catalyst layer: 10% polyvinyl alcohol (PVA) was dissolved in deionized water, and 8% 5-sulfosalicylic acid was added for grafting reaction. The reaction was stirred at 60 °C for 4 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution was formed. The casting solution was spin-coated onto the separation layer that had been dried in step (2). The undried membrane was immersed in a 10% ethanol solution for 2.5 h using the traditional immersion phase inversion method. Finally, it was dried at room temperature for 30 h to obtain a PVA catalyst composite membrane with a three-layer structure.

[0022] The prepared catalytic composite membrane has a three-layer structure and a preparation time of 96 h, in which the catalytic layer and the separation layer are tightly bonded. Due to the permeation of solvent and water during the preparation of the catalytic layer, the structure of the separation layer is significantly damaged, resulting in uneven thickness and local thinning. Below the separation layer is a support layer. The thickness of the catalytic layer is 55 μm, the thickness of the separation layer is 1 μm, and the porosity of the catalytic layer is 48%. A 90% ethanol-water solution was used as the separation system. During the reaction, the vacuum degree was maintained at 300 Pa, the water bath temperature was controlled at 75℃, and the permeate in the cold trap was collected every 0.5 h. The permeate composition was determined by gas chromatography, and the separation factor and permeation flux were calculated. The permeation flux was 1300 g·m³. -2 ·h -1 The separation factor was 600. Using the synthesis reaction of ethyl acetate as a model reaction, the molar ratio of ethanol to acetic acid was 3:1, the water bath temperature was controlled at 75℃, and the reaction was stopped once every 1 h. Samples were taken, and the mass of ethyl acetate in the reaction solution and permeate was determined by gas chromatography to calculate the conversion rate. The conversion rate was found to be 80% after 12 h of reaction. After repeating the reaction 5 times, the conversion rate decreased to 69%.

[0023] Example 1 (1) Preparation of MOF-801 nanoparticles: 40 mmol fumaric acid and 40 mmol zirconium oxychloride octahydrate were dissolved in 250 mL of DMF / formic acid mixed solvent with a volume ratio of 3:1. The mixture was placed in a reaction vessel and heated at 120 °C for 4 h. After centrifugation, washing and drying, MOF-801 nanoparticles were obtained. (2) Preparation of dense separation layer: A certain amount of 2% MOF-801 nanoparticles prepared in step (1) were weighed and dispersed in deionized water. Sodium alginate (SA) powder with a mass concentration of 1% was added. After stirring and dissolving, the mixture was allowed to stand to remove bubbles and form a casting solution for the MOF-801 / SA separation layer. The casting solution was spin-coated onto one side of a polyacrylonitrile (PAN) ultrafiltration membrane. After drying, the membrane was soaked in 1% calcium chloride solution for crosslinking for 5 min. After washing and drying with deionized water, a dense separation layer was obtained. (3) Preparation of porous catalyst layer: 3% polyvinyl alcohol (PVA) was dissolved in deionized water, and 8% 5-sulfosalicylic acid was added for grafting reaction. The reaction was stirred at 60 °C for 4 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution was formed. The casting solution was spin-coated onto the other side of the polyacrylonitrile (PAN) ultrafiltration membrane in step (2). The freeze-drying technology was used. The membrane was first pre-frozen at -20 °C for 2 h. During the drying process, the vacuum degree was maintained at 8 Pa. The membrane was dried at -60 °C for 24 h to form a sulfonated PVA catalyst layer with a sponge-like through-pore structure. Finally, a PVA catalyst composite membrane with a three-layer asymmetric structure was obtained.

[0024] The prepared catalytic composite membrane has a three-layer asymmetric structure and was prepared over 72 h. The support layer is in the middle, the catalytic layer is on the rough side of the support layer, and the separation layer has a uniform and dense surface on the smooth side of the membrane. Below the separation layer is the support layer. The catalytic layer has a thickness of 20 μm, the separation layer has a thickness of 1 μm, and the porosity of the catalytic layer is 60%. A 90% ethanol-water solution was used as the separation system. During the reaction, the vacuum was maintained at 300 Pa, and the water bath temperature was controlled at 75℃. Permeate was collected from the cold trap every 0.5 h, weighed, and its composition was determined by gas chromatography. The separation factor and permeate flux were calculated, and the permeate flux was found to be 1500 g·m³. -2 ·h -1 The separation factor was 900. Using the synthesis reaction of ethyl acetate as a model reaction, the molar ratio of ethanol to acetic acid was 3:1, the water bath temperature was controlled at 75℃, and the reaction was stopped once every 1 h. Samples were taken, and the mass of ethyl acetate in the reaction solution and permeate was determined by gas chromatography to calculate the conversion rate. The conversion rate was found to be 85% after 12 h of reaction. The reaction was repeated 5 times, and the conversion rate remained at 83%.

[0025] Example 2 (1) Preparation of MOF-801 nanoparticles: 45 mmol fumaric acid and 50 mmol zirconium oxychloride octahydrate were dissolved in 260 mL of DMF / formic acid mixed solvent with a volume ratio of 3:1. The mixture was placed in a reaction vessel and heated at 120 °C for 5 h. After centrifugation, washing and drying, MOF-801 nanoparticles were obtained. (2) Preparation of dense separation layer: A certain mass concentration of 3% of the MOF-801 nanoparticles prepared in step (1) was weighed and dispersed in deionized water. Sodium alginate (SA) powder with a mass concentration of 3% was added. After stirring and dissolving, the mixture was allowed to stand to remove bubbles and form a casting solution for the MOF-801 / SA separation layer. The casting solution was spin-coated onto one side of a polyacrylonitrile (PAN) ultrafiltration membrane. After drying, the membrane was soaked in a 3% calcium chloride solution for crosslinking for 10 min. After washing and drying with deionized water, a dense separation layer was obtained. (3) Preparation of porous catalyst layer: 8% polyvinyl alcohol (PVA) was dissolved in deionized water, and 9% 5-sulfosalicylic acid was added for grafting reaction. The reaction was stirred at 70 °C for 5 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution was formed. The casting solution was spin-coated onto the other side of the polyacrylonitrile (PAN) ultrafiltration membrane in step (2). The freeze-drying technology was used. The membrane was first pre-frozen at -10 °C for 3 h. During the drying process, the vacuum degree was maintained at 8 Pa. The membrane was dried at -50 °C for 24 h to form a sulfonated PVA catalyst layer with a sponge-like through-pore structure. Finally, a PVA catalyst composite membrane with a three-layer asymmetric structure was obtained.

[0026] The prepared catalytic composite membrane has a three-layer asymmetric structure and was prepared over 72 h. The support layer is in the middle, the catalytic layer is on the rough side of the support layer, and the separation layer has a uniform and dense surface on the smooth side of the membrane. Below the separation layer is the support layer. The catalytic layer has a thickness of 15 μm, the separation layer has a thickness of 0.5 μm, and the porosity of the catalytic layer is 74%. A 90% ethanol-water solution was used as the separation system. During the reaction, the vacuum was maintained at 300 Pa, and the water bath temperature was controlled at 75℃. Permeate was collected from the cold trap every 0.5 h, weighed, and its composition was determined by gas chromatography. The separation factor and permeate flux were calculated, and the permeate flux was found to be 1900 g·m³. -2 ·h -1 The separation factor was 1300. Using the synthesis of ethyl acetate as a model reaction, an ethanol-acetic acid molar ratio of 3:1 was added, and the water bath temperature was controlled at 75℃. The reaction was stopped once every 1 hour, and samples were taken. The mass of ethyl acetate in the reaction solution and permeate was determined by gas chromatography to calculate the conversion rate. The conversion rate was found to be 94% after 12 hours of reaction. The reaction was repeated 5 times, and the conversion rate remained at 90%.

[0027] Example 3 (1) Preparation of MOF-801 nanoparticles: 45 mmol fumaric acid and 50 mmol zirconium oxychloride octahydrate were dissolved in 260 mL of DMF / formic acid mixed solvent with a volume ratio of 3:1. The mixture was placed in a reaction vessel and heated at 120 °C for 5 h. After centrifugation, washing and drying, MOF-801 nanoparticles were obtained. (2) Preparation of dense separation layer: A certain mass concentration of 3% of the MOF-801 nanoparticles prepared in step (1) was weighed and dispersed in deionized water. Sodium alginate (SA) powder with a mass concentration of 3% was added. After stirring and dissolving, the mixture was allowed to stand to remove bubbles and form a casting solution for the MOF-801 / SA separation layer. The casting solution was spin-coated onto one side of a polyacrylonitrile (PAN) ultrafiltration membrane. After drying, the membrane was soaked in a 3% calcium chloride solution for crosslinking for 10 min. After washing and drying with deionized water, a dense separation layer was obtained. (3) Preparation of porous catalyst layer: 8% polyvinyl alcohol (PVA) was dissolved in deionized water, and 9% aminosulfonic acid was added for grafting reaction. The reaction was stirred at 70 °C for 5 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution was formed. The casting solution was spin-coated onto the other side of the polyacrylonitrile (PAN) ultrafiltration membrane in step (2). The freeze-drying technology was used. The membrane was first pre-frozen at -10 °C for 3 h. During the drying process, the vacuum degree was maintained at 8 Pa. The membrane was dried at -50 °C for 24 h to form a sulfonated PVA catalyst layer with a sponge-like through-pore structure. Finally, a PVA catalyst composite membrane with a three-layer asymmetric structure was obtained.

[0028] The prepared catalytic composite membrane has a three-layer asymmetric structure and was prepared over 68 h. The support layer is in the middle, the catalytic layer is on the rough side of the support layer, and the separation layer has a uniform and dense surface on the smooth side of the membrane. The support layer lies below the separation layer. The catalytic layer has a thickness of 17 μm, the separation layer has a thickness of 1.2 μm, and the porosity of the catalytic layer is 67%. A 90% ethanol-water solution was used as the separation system. During the reaction, the vacuum was maintained at 300 Pa, and the water bath temperature was controlled at 75℃. Permeate was collected from the cold trap every 0.5 h, weighed, and its composition was determined by gas chromatography. The separation factor and permeate flux were calculated, and the permeate flux was found to be 1700 g·m³. -2 ·h -1 The separation factor was 1200. Using the synthesis of ethyl acetate as a model reaction, an ethanol-acetic acid molar ratio of 3:1 was added, and the water bath temperature was controlled at 75℃. The reaction was stopped once every 1 hour, and samples were taken. The mass of ethyl acetate in the reaction solution and permeate was determined by gas chromatography to calculate the conversion rate. The conversion rate was found to be 88% after 12 hours of reaction. This process was repeated 5 times, and the conversion rate remained at 85%.

[0029] Example 4 (1) Preparation of MOF-801 nanoparticles: 45 mmol fumaric acid and 50 mmol zirconium oxychloride octahydrate were dissolved in 260 mL of DMF / formic acid mixed solvent with a volume ratio of 3:1. The mixture was placed in a reaction vessel and heated at 120 °C for 5 h. After centrifugation, washing and drying, MOF-801 nanoparticles were obtained. (2) Preparation of dense separation layer: A certain mass concentration of 3% of the MOF-801 nanoparticles prepared in step (1) was weighed and dispersed in deionized water. Sodium alginate (SA) powder with a mass concentration of 3% was added. After stirring and dissolving, the mixture was allowed to stand to remove bubbles and form a casting solution for the MOF-801 / SA separation layer. The casting solution was spin-coated onto one side of a polyacrylonitrile (PAN) ultrafiltration membrane. After drying, the membrane was soaked in a 3% calcium chloride solution for crosslinking for 10 min. After washing and drying with deionized water, a dense separation layer was obtained. (3) Preparation of porous catalyst layer: 8% polyvinyl alcohol (PVA) was dissolved in deionized water, and 9% p-toluenesulfonic acid was added for grafting reaction. The reaction was stirred at 70 °C for 5 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution was formed. The casting solution was spin-coated onto the other side of the polyacrylonitrile (PAN) ultrafiltration membrane in step (2). The freeze-drying technology was used. The membrane was first pre-frozen at -10 °C for 3 h. During the drying process, the vacuum degree was maintained at 8 Pa. The membrane was dried at -50 °C for 24 h to form a sulfonated PVA catalyst layer with a sponge-like through-pore structure. Finally, a PVA catalyst composite membrane with a three-layer asymmetric structure was obtained.

[0030] The prepared catalytic composite membrane has a three-layer asymmetric structure and was prepared over 68 h. The support layer is in the middle, the catalytic layer is on the rough side of the support layer, and the separation layer has a uniform and dense surface on the smooth side of the membrane. The support layer lies below the separation layer. The catalytic layer has a thickness of 18 μm, the separation layer has a thickness of 1.2 μm, and the porosity of the catalytic layer is 63%. A 90% ethanol-water solution was used as the separation system. During the reaction, the vacuum was maintained at 300 Pa, and the water bath temperature was controlled at 75℃. Permeate was collected from the cold trap every 0.5 h, weighed, and its composition was determined by gas chromatography. The separation factor and permeate flux were calculated, and the permeate flux was found to be 1600 g·m³. -2 ·h -1 The separation factor was 976. Using the synthesis reaction of ethyl acetate as a model reaction, the molar ratio of ethanol to acetic acid was 3:1, the water bath temperature was controlled at 75℃, and the reaction was stopped once every 1 h. Samples were taken, and the mass of ethyl acetate in the reaction solution and permeate was determined by gas chromatography to calculate the conversion rate. The conversion rate was found to be 87% after 12 h of reaction. After repeating the reaction 5 times, the conversion rate remained at 84%.

[0031] Example 5 (1) Preparation of MOF-801 nanoparticles: 60 mmol fumaric acid and 50 mmol zirconium oxychloride octahydrate were dissolved in 280 mL of DMF / formic acid mixed solvent with a volume ratio of 3:1. The mixture was placed in a reaction vessel and heated at 120 °C for 8 h. After centrifugation, washing and drying, MOF-801 nanoparticles were obtained. (2) Preparation of dense separation layer: A certain amount of MOF-801 nanoparticles with a mass concentration of 7% prepared in step (1) were weighed and dispersed in deionized water. Sodium alginate (SA) powder with a mass concentration of 5% was added. After stirring and dissolving, the mixture was allowed to stand to remove bubbles and form a casting solution for the MOF-801 / SA separation layer. The casting solution was spin-coated onto one side of a polyacrylonitrile (PAN) ultrafiltration membrane. After drying, the membrane was soaked in a 5% calcium chloride solution for crosslinking for 5 min. After washing with deionized water and drying, a dense separation layer was obtained. (3) Preparation of porous catalyst layer: 12% polyvinyl alcohol (PVA) was dissolved in deionized water, and 11% 5-sulfosalicylic acid was added for grafting reaction. The reaction was stirred at 80 °C for 8 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution was formed. The casting solution was spin-coated onto the other side of the polyacrylonitrile (PAN) ultrafiltration membrane in step (2). The freeze-drying technology was used. The membrane was first pre-frozen at -20 °C for 3 h. During the drying process, the vacuum degree was maintained at 9 Pa. The membrane was dried at -50 °C for 24 h to form a sulfonated PVA catalyst layer with a sponge-like through-pore structure. Finally, a PVA catalyst composite membrane with a three-layer asymmetric structure was obtained.

[0032] The prepared catalytic composite membrane has a three-layer asymmetric structure and a preparation time of 56 h. The support layer is in the middle, the catalytic layer is on the rough side of the support layer, and the separation layer has a uniform and dense surface on the smooth side of the membrane. Below the separation layer is the support layer. The catalytic layer has a thickness of 14 μm, the separation layer has a thickness of 2 μm, and the porosity of the catalytic layer is 80%. A 90% ethanol-water solution was used as the separation system. During the reaction, the vacuum was maintained at 300 Pa, and the water bath temperature was controlled at 75℃. Permeate was collected from the cold trap every 0.5 h, weighed, and its composition was determined by gas chromatography. The separation factor and permeate flux were calculated, and the permeate flux was found to be 1950 g·m³. -2 ·h -1 The separation factor was 1600. Using the synthesis of ethyl acetate as a model reaction, an ethanol-acetic acid molar ratio of 3:1 was added, and the water bath temperature was controlled at 75℃. The reaction was stopped once every 1 hour, and samples were taken. The mass of ethyl acetate in the reaction solution and permeate was determined by gas chromatography to calculate the conversion rate. The conversion rate was 97.5% after 12 hours of reaction. The reaction was repeated 5 times, and the conversion rate remained at 93%.

[0033] Example 6 (1) Preparation of MOF-801 nanoparticles: 50 mmol fumaric acid and 60 mmol zirconium oxychloride octahydrate were dissolved in 300 mL of DMF / formic acid mixed solvent with a volume ratio of 3:1. The mixture was placed in a reaction vessel and heated at 140 °C for 6 h. After centrifugation, washing and drying, MOF-801 nanoparticles were obtained. (2) Preparation of dense separation layer: A certain mass concentration of 5% of the MOF-801 nanoparticles prepared in step (1) was weighed and dispersed in deionized water. Sodium alginate (SA) powder with a mass concentration of 2% was added. After stirring and dissolving, the mixture was allowed to stand to remove bubbles and form a casting solution for the MOF-801 / SA separation layer. The casting solution was spin-coated onto one side of a polyacrylonitrile (PAN) ultrafiltration membrane. After drying, the membrane was soaked in a 3% calcium chloride solution for crosslinking for 10 min. After washing and drying with deionized water, a dense separation layer was obtained. (3) Preparation of porous catalyst layer: 12% polyvinyl alcohol (PVA) was dissolved in deionized water, and 9% 5-sulfosalicylic acid was added for grafting reaction. The reaction was stirred at 80 °C for 8 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution was formed. The casting solution was spin-coated onto the other side of the polyacrylonitrile (PAN) ultrafiltration membrane in step (2). The freeze-drying technology was used. The membrane was first pre-frozen at -20 °C for 3 h. During the drying process, the vacuum degree was maintained at 9 Pa. The membrane was dried at -50 °C for 24 h to form a sulfonated PVA catalyst layer with a sponge-like through-pore structure. Finally, a PVA catalyst composite membrane with a three-layer asymmetric structure was obtained.

[0034] The prepared catalytic composite membrane has a three-layer asymmetric structure and was prepared over 72 h. The support layer is in the middle, the catalytic layer is on the rough side of the support layer, and the separation layer has a uniform and dense surface on the smooth side of the membrane. Below the separation layer is the support layer. The catalytic layer has a thickness of 5 μm, the separation layer has a thickness of 0.5 μm, and the porosity of the catalytic layer is 80%. A 90% ethanol-water solution was used as the separation system. During the reaction, the vacuum was maintained at 300 Pa, and the water bath temperature was controlled at 75℃. Permeate was collected from the cold trap every 0.5 h, weighed, and its composition was determined by gas chromatography. The separation factor and permeate flux were calculated, and the permeate flux was found to be 2360 g·m³. -2 ·h -1 The separation factor was 1800. Using the synthesis of ethyl acetate as a model reaction, an ethanol-acetic acid molar ratio of 3:1 was added, and the water bath temperature was controlled at 75℃. The reaction was stopped once every 1 hour, and samples were taken. The mass of ethyl acetate in the reaction solution and permeate was determined by gas chromatography to calculate the conversion rate. The conversion rate was 98.6% after 12 hours of reaction. After repeating the reaction 5 times, the conversion rate remained at 94.5%.

[0035] Example 7 (1) Preparation of MOF-801 nanoparticles: 50 mmol fumaric acid and 60 mmol zirconium oxychloride octahydrate were dissolved in 300 mL of DMF / formic acid mixed solvent with a volume ratio of 3:1. The mixture was placed in a reaction vessel and heated at 140 °C for 6 h. After centrifugation, washing and drying, MOF-801 nanoparticles were obtained. (2) Preparation of dense separation layer: A certain mass concentration of 5% of the MOF-801 nanoparticles prepared in step (1) was weighed and dispersed in deionized water. Sodium alginate (SA) powder with a mass concentration of 2% was added. After stirring and dissolving, the mixture was allowed to stand to remove bubbles and form a casting solution for the MOF-801 / SA separation layer. The casting solution was spin-coated onto one side of a polyacrylonitrile (PAN) ultrafiltration membrane. After drying, the membrane was soaked in a 3% calcium chloride solution for crosslinking for 10 min. After washing and drying with deionized water, a dense separation layer was obtained. (3) Preparation of porous catalyst layer: 12% polyvinyl alcohol (PVA) was dissolved in deionized water, and 9% 5-sulfosalicylic acid was added for grafting reaction. The reaction was stirred at 80 °C for 8 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution was formed. The casting solution was spin-coated onto the other side of the polyacrylonitrile (PAN) ultrafiltration membrane in step (2). The freeze-drying technology was used. The membrane was first pre-frozen at -20 °C for 3 h. During the drying process, the vacuum degree was maintained at 9 Pa. The membrane was dried at -50 °C for 24 h to form a sulfonated PVA catalyst layer with a sponge-like through-pore structure. Finally, a PVA catalyst composite membrane with a three-layer asymmetric structure was obtained.

[0036] The prepared catalytic composite membrane has a three-layer asymmetric structure and was prepared over 72 h. The support layer is in the middle, the catalytic layer is on the rough side of the support layer, and the separation layer has a uniform and dense surface on the smooth side of the membrane. Below the separation layer is the support layer. The catalytic layer has a thickness of 5 μm, the separation layer has a thickness of 0.5 μm, and the porosity of the catalytic layer is 80%. A 90% ethanol-water solution was used as the separation system. During the reaction, the vacuum was maintained at 300 Pa, and the water bath temperature was controlled at 75℃. Permeate was collected from the cold trap every 0.5 h, weighed, and its composition was determined by gas chromatography. The separation factor and permeate flux were calculated, and the permeate flux was found to be 2360 g·m³. -2 ·h -1 The separation factor was 1800. Using the synthesis of ethyl formate as a model reaction, the molar ratio of ethanol to formic acid was 4:1. The water bath temperature was controlled at 65℃. The reaction was stopped once every 1 hour, and samples were taken. The mass of ethyl acetate in the reaction solution and permeate was determined by gas chromatography to calculate the conversion rate. The conversion rate was 86.2% after 12 hours of reaction. After repeating the reaction 5 times, the conversion rate remained at 83.5%.

[0037] Example 8 (1) Preparation of MOF-801 nanoparticles: 50 mmol fumaric acid and 60 mmol zirconium oxychloride octahydrate were dissolved in 300 mL of DMF / formic acid mixed solvent with a volume ratio of 3:1. The mixture was placed in a reaction vessel and heated at 140 °C for 6 h. After centrifugation, washing and drying, MOF-801 nanoparticles were obtained. (2) Preparation of dense separation layer: A certain mass concentration of 5% of the MOF-801 nanoparticles prepared in step (1) was weighed and dispersed in deionized water. Sodium alginate (SA) powder with a mass concentration of 2% was added. After stirring and dissolving, the mixture was allowed to stand to remove bubbles and form a casting solution for the MOF-801 / SA separation layer. The casting solution was spin-coated onto one side of a polyacrylonitrile (PAN) ultrafiltration membrane. After drying, the membrane was soaked in a 3% calcium chloride solution for crosslinking for 10 min. After washing and drying with deionized water, a dense separation layer was obtained. (3) Preparation of porous catalyst layer: 12% polyvinyl alcohol (PVA) was dissolved in deionized water, and 9% 5-sulfosalicylic acid was added for grafting reaction. The reaction was stirred at 80 °C for 8 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution was formed. The casting solution was spin-coated onto the other side of the polyacrylonitrile (PAN) ultrafiltration membrane in step (2). The freeze-drying technology was used. The membrane was first pre-frozen at -20 °C for 3 h. During the drying process, the vacuum degree was maintained at 9 Pa. The membrane was dried at -50 °C for 24 h to form a sulfonated PVA catalyst layer with a sponge-like through-pore structure. Finally, a PVA catalyst composite membrane with a three-layer asymmetric structure was obtained.

[0038] The prepared catalytic composite membrane has a three-layer asymmetric structure and a preparation time of 72 h. The support layer is in the middle, the catalytic layer is on the rough side of the support layer, and the separation layer has a uniform and dense surface on the smooth side of the membrane. Below the separation layer is the support layer. The catalytic layer has a thickness of 5 μm, the separation layer has a thickness of 0.5 μm, and the porosity of the catalytic layer is 80%. A 90% methanol-water solution was used as the separation system. During the reaction, the vacuum was maintained at 300 Pa, and the water bath temperature was controlled at 55℃. Permeate was collected from the cold trap every 0.5 h, weighed, and its composition was determined by gas chromatography. The separation factor and permeate flux were calculated, and the permeate flux was found to be 1660 g·m³. -2 ·h -1 The separation factor was 924. Using the synthesis of ethyl formate as a model reaction, the molar ratio of ethanol to formic acid was 4:1. The water bath temperature was controlled at 55℃. The reaction was stopped once every 1 hour, and samples were taken. The mass of ethyl acetate in the reaction solution and permeate was determined by gas chromatography to calculate the conversion rate. The conversion rate was 88.9% after 12 hours of reaction. After five repetitions, the conversion rate remained at 84.7%.

Claims

1. A method for preparing an asymmetric PVA catalytic composite membrane, characterized in that, Includes the following steps: (1) Preparation of MOF-801 nanoparticles: Fumaric acid and zirconium oxychloride octahydrate were dissolved in a DMF / formic acid mixed solvent with a volume ratio of 3:1, placed in a reaction vessel and heated, and then centrifuged, washed and dried to obtain MOF-801 nanoparticles. (2) Preparation of dense separation layer: Weigh a certain mass concentration of MOF-801 nanoparticles prepared in step (1) and disperse them in deionized water. Add sodium alginate (SA) powder, stir to dissolve, and let stand to remove bubbles to form MOF-801 / SA separation layer casting solution. Spin-coat the casting solution onto the smooth side of the polyacrylonitrile (PAN) ultrafiltration membrane, dry it, and then soak it in calcium chloride solution for crosslinking for 5~20 min. After washing with deionized water and drying, a dense separation layer is obtained. (3) Preparation of porous catalyst layer: Polyvinyl alcohol (PVA) with a mass concentration of 3~12% is dissolved in deionized water, and sulfonic acid group catalyst with a mass concentration of 8~12% is added for grafting reaction. The reaction is stirred at 60~80 ℃ for 4~8 h. After standing and degassing, a sulfonated modified PVA catalyst layer casting solution is formed. The casting solution is spin-coated onto the rough side of the polyacrylonitrile (PAN) ultrafiltration membrane in step (2). It is pre-frozen first, and then freeze-dried to form a sulfonated PVA catalyst layer with a sponge-like through-pore structure. Finally, a PVA catalyst composite membrane with a three-layer asymmetric structure is obtained.

2. The preparation method according to claim 1, characterized in that, The amounts of each component in step (1) are: 40-60 mmol of fumaric acid, 40-60 mmol of zirconium oxychloride octahydrate, and 250-300 mL of DMF / formic acid mixed solvent.

3. The preparation method according to claim 1, characterized in that, The reaction temperature of the reactor in step (1) is 120~140 ℃ and the reaction time is 4~8 h.

4. The preparation method according to claim 1, characterized in that, The mass fractions of each component in step (2) are: 2-7% MOF-801 nanoparticles, 1-4% sodium alginate in the separation layer casting solution, and 1-5% calcium chloride solution.

5. The preparation method according to claim 1, characterized in that, The sulfonic acid group catalyst mentioned in step (3) is one of 5-sulfosalicylic acid, aminosulfonic acid and p-toluenesulfonic acid.

6. The preparation method according to claim 1, characterized in that, In step (3), the pre-freezing temperature is -20~-10 ℃, the freezing time is 2~4 h, the freeze-drying temperature is -60~-50 ℃, the drying time is 24~48 h, and the vacuum degree is less than 10 Pa.

7. The preparation method according to claim 1, characterized in that, In steps (2) and (3), a high-precision spin coater is used to spin coat the casting solution. A segmented speed-increasing spin coat method is adopted, with a low speed range of 500~1900 r / 20 s and a high speed range of 800~2200 r / 40 s.

8. The asymmetric PVA catalytic composite membrane prepared by the preparation method according to any one of claims 1 to 7, characterized in that, The membrane adopts a three-layer asymmetric composite structure, consisting of a catalytic layer, an intermediate support layer, and a separation layer. The intermediate support layer physically isolates the catalytic layer from the separation layer, enabling independent optimization of each functional layer. The catalytic layer is located on the rough side of the intermediate support layer and is a sulfonated PVA membrane with a sponge-like through-pore structure. The separation layer is located on the smooth side of the intermediate support layer and is a MOF-801-doped SA dense membrane. The intermediate support layer is a PAN ultrafiltration membrane.

9. The asymmetric PVA catalytic composite membrane according to claim 8, characterized in that, The preparation time of the membrane is reduced to 56-72 h, the thickness of the catalyst layer is 5-50 μm, the porosity is 60-90%, and the surface of the separation layer is uniform and dense with a thickness of 0.5-2 μm.

10. The application of the asymmetric PVA catalytic composite membrane according to claim 8 in the enhancement of esterification reaction, characterized in that, The permeation flux is 1500 g·m -2 ·h -1 With a separation factor of 900 or higher, the acetic acid conversion rate can reach 85% or higher, and it still maintains a conversion rate of 83% or higher after being reused 5 times.

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