Preparation method of zinc ion hydrophilic metal organic framework mixed polyvinyl alcohol composite film

Abstract

The present application relates to the preparation of composite membranes and the separation of methanol dehydration technology, aiming at providing a preparation method of zinc ion hydrophilic metal organic framework mixed polyvinyl alcohol composite membrane. Including: Zn(NO3)2.6H2O, PVP and 2-cyano imidazole are reacted, and the reaction product is sequentially subjected to amidation modification and hydroxyl modification reaction to obtain a zinc ion hydrophilic metal organic framework, which has a microstructure of being externally modified by hydroxyl and amide groups and internally hollow; polyether sulfone ultrafiltration membrane is used as a support membrane, polyvinyl alcohol solution containing the organic framework is dried on the upper part of the support membrane to form a polyvinyl alcohol mixed matrix membrane, and the composite membrane is formed by cross-linking at the contact interface. The present application can preferentially adsorb water molecules and improve the solubility of water in the membrane; water molecules can be selectively passed through, effectively improving the permeability and selectivity of the polyvinyl alcohol membrane to water, and high-purity methanol products can be obtained after cyclic dehydration; by adding MOF fillers and interfacial cross-linking, the present application can maintain good mechanical, chemical and thermal stability.

Description

Technical Field

[0001] This invention relates to the technology of preparing composite membranes and separating and dehydrating methanol, and particularly to the preparation method of a zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane and its application in methanol dehydration. Background Technology

[0002] Methanol production, as a clean and renewable energy source and a commonly used industrial raw material, has attracted considerable attention. While reducing carbon dioxide to methanol through hydrogenation allows for carbon dioxide capture and utilization, thus reducing carbon emissions, water is inevitably generated during the process, making dehydration a crucial step. Traditional distillation methods account for approximately 10% to 30% of the total energy consumption in methanol production, leading to increasing focus on energy conservation and emission reduction in methanol purification processes, and the development of new, efficient, and low-energy technologies to replace traditional distillation. Pervaporation (PV), a novel membrane separation technology, offers advantages such as low energy consumption, environmental friendliness, lack of gas-liquid equilibrium limitations, process simplicity, and ease of operation. The specific process is as follows: a liquid mixture flows across the upstream side of the membrane, and a vacuum is drawn on the downstream side, creating a chemical potential difference between the liquid components. Driven by this chemical potential difference, the components permeate through the membrane and escape as a gas from the downstream side. Due to the varying degrees of interaction between the membrane and different components, as well as the differences in the properties of the components themselves, the solubility and diffusion rates of each component within the polymer membrane differ, thus achieving selective separation. Pervaporation can be coupled with processes such as distillation and adsorption to modify traditional processes and achieve significant energy savings.

[0003] Currently, industrial pervaporation technology primarily utilizes hydrophilic membrane materials to remove water from organic matter. These membrane materials exhibit high preferential adsorption selectivity for water; preferentially adsorbed water molecules tend to diffuse preferentially, resulting in high separation selectivity and permeate flux. From a material perspective, inorganic membranes possess higher mechanical strength, better thermal stability, and resistance to swelling. For example, Liu et al. successfully prepared a novel template-free organic silica-rich LTA molecular sieve membrane on a tubular α-alumina support using a secondary growth method. This membrane still exhibited excellent water / methanol separation performance under high-temperature, water-rich conditions. However, inorganic membranes suffer from drawbacks such as high cost, poor film-forming properties, and difficulty in scale-up.

[0004] Therefore, compared to inorganic membranes, organic membranes are simpler to manufacture, lower in cost, and have better film-forming properties, making them easier to apply industrially. Among organic membranes, polyvinyl alcohol (PVA) is a commonly used material for high-performance pervaporation dehydration membranes due to its good selectivity for water caused by hydrogen bonding and ionic dipole interactions. However, the high swelling property of PVA makes it prone to deformation in methanol solutions, requiring the addition of supporting materials to prepare composite membranes to ensure better membrane stability. For example, Professor Zhang Liping of Tsinghua University developed a PVA / PAN hydrophilic composite membrane for methanol dehydration, which has good stability, but its separation factor is only 43, indicating poor selectivity.

[0005] The swelling problem of traditional polymer membranes leads to low permeation flux and selectivity, which can be solved by adding inorganic fillers to prepare hybrid matrix membranes. For example, Saira Bano et al. prepared sodium alginate (NaAlg) / polyvinyl alcohol (PVA) blend membranes for the pervaporation separation of methanol-water mixtures. While the membranes exhibited high hydrophilicity, they also showed interfacial defects with non-selective voids, hindering the selective separation of water and methanol. Therefore, the interfacial compatibility between inorganic fillers and organic polymer membranes must be considered when selecting fillers. Metal-organic frameworks (MOFs), as organic-inorganic hybrids, exhibit good interfacial compatibility with polymer membranes. Furthermore, MOFs possess high porosity and chemical tunability, which can better improve membrane selectivity and permeability, and have been widely used in the preparation of hybrid matrix membranes. For example, Dipeshkumar D. Kachhadiya et al. of the Sadar Walrabhai National Technical Institute prepared a mixed matrix membrane by incorporating ZIF-8 and ZIF-67 as fillers into polyvinylidene fluoride, and added PEG200 to improve the membrane's hydrophilicity. The membrane showed the best separation performance at 55°C. YCJean et al. of the University of Missouri found that adding ZIF-8 to polybenzimidazole can effectively improve the membrane's anti-swelling property in methanol-water pervaporation separation.

[0006] Existing research on methanol dehydration mixed matrix membranes is limited, and the membranes exhibit low selectivity, with the MOFs used as packing materials being inherently hydrophobic. The hydrophilicity or hydrophobicity of MOFs is related to their topology, geometry, and functionalized ligands. Chemical modification of MOFs by adding hydrophilic functional groups such as hydroxyl (-OH), amino (-NH2), carboxyl (-COOH), and amide (-NHCO-) ​​groups can impart hydrophilicity. For example, Zhu Tengyang et al. from Huazhong University of Science and Technology prepared a mixed matrix membrane using ZIF-90 containing aldehyde groups and PVA, improving the separation factor for ethanol dehydration; Professor Wang Yan synthesized a mixed matrix membrane for the permeation separation of ethanol and water using amino-modified NH2-ZIF-8 and PVA, improving the permeate flux. Therefore, it is speculated that adding hydrophilic MOFs to methanol dehydration membranes may further improve membrane separation performance.

[0007] Furthermore, methanol and water have similar chemical properties, so simply increasing the chemical hydrophilicity of the membrane has limited effect on improving membrane separation efficiency. Selective water passage through molecular sieving is also necessary, but this also means water molecules will be adsorbed in the narrow pores, significantly reducing the permeate flux. This has been studied in ethanol dehydration membranes. For example, C.-H. Kang et al. found that although a mixed matrix membrane prepared with ZIF-7 and chitosan facilitated the selective adsorption of water on ethanol, the narrow pore size of ZIF-7 (0.43 nm) also reduced the permeate flux from 602 g / m³. 2 / h decreased to 322g / m 2 / h. Therefore, by analogy to the more challenging methanol dehydration membrane, low-resistance mass transfer channels need to be designed to improve the permeability of water molecules. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for preparing a zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite film.

[0009] To solve the above-mentioned technical problems, the solution of the present invention is:

[0010] A method for preparing a zinc ion hydrophilic metal-organic framework-infused polyvinyl alcohol composite membrane is provided, comprising the following steps:

[0011] (1) Add Zn(NO3)2·6H2O and polyvinylpyrrolidone (PVP) to N,N-dimethylformamide and stir thoroughly to dissolve and mix; add 2-cyanoimidazole to N,N-dimethylformamide and stir thoroughly to dissolve and mix; the mass ratio of Zn(NO3)2·6H2O, polyvinylpyrrolidone (PVP) and 2-cyanoimidazole is 2.02~20.2∶2~6∶1.58~15.8;

[0012] (2) Add the two mixed solutions obtained in step (1) into the same reaction vessel, stir evenly, and react at 150°C for 24-96 h; after the reaction is completed, centrifuge the product, wash with ethanol, methanol or deionized water and centrifuge again; separate the precipitate and vacuum dry it to obtain zinc ion metal-organic framework.

[0013] (3) Disperse the zinc ion metal-organic framework in dimethyl sulfoxide, and continue to add hydrogen peroxide aqueous solution and potassium carbonate under ice-water bath conditions to carry out an exothermic reaction, controlling the solution temperature to remain constant at 20℃; the ratio of zinc ion metal-organic framework, dimethyl sulfoxide, hydrogen peroxide aqueous solution and potassium carbonate is 4~20kg : 0.1~0.5m 3 : 0.03~0.3m 3 : 1-3 kg, with a hydrogen peroxide aqueous solution concentration of 30%;

[0014] After the reaction was completed, the reaction product was centrifuged, washed with ethanol, and centrifuged again; the precipitate was separated and vacuum dried to obtain amide-modified metal-organic framework nanomaterials.

[0015] (4) Apply at a ratio of 4-40 kg: 0.2-0.5 m 3 Based on the dosage relationship, the amide-modified metal-organic framework nanomaterials were immersed in a tannic acid solution for 0.5–1 h and kept there for 0.5–1 h. The mass concentration of the tannic acid solution was 5%. After immersion, the nanomaterials were centrifuged, washed with ethanol, methanol, or deionized water, and then centrifuged again. The precipitate was separated and vacuum dried to obtain a zinc ion hydrophilic metal-organic framework with a microstructure that is externally modified by hydroxyl and amide groups and internally hollow.

[0016] (5) Add the zinc ion hydrophilic metal-organic framework to a 10% (w / w) polyvinyl alcohol solution and stir thoroughly to obtain a mixed solution; the mass ratio of the organic framework to the polyvinyl alcohol contained in the solution in the mixed solution is 0.25-2.5:1-10;

[0017] (6) The polyethersulfone ultrafiltration membrane is immersed in glutaraldehyde hydrochloric acid solution for 2-5 hours. The mass ratio of glutaraldehyde, hydrochloric acid and water in the solution is 1:1.5:97.5. The immersed polyethersulfone ultrafiltration membrane is placed in a circular groove on the surface of a polytetrafluoroethylene plate as a support membrane, and then the mixed solution in step (5) is poured in. After 12 hours of drying, the upper mixed solution is transformed into a polyvinyl alcohol mixed matrix membrane, and crosslinked with the polyethersulfone ultrafiltration membrane at the contact interface due to the action of glutaraldehyde hydrochloric acid to form a composite membrane.

[0018] The cross-linked composite membrane is removed and soaked in a 5% tannic acid solution for 10–30 minutes to obtain the final product, a zinc ion hydrophilic metal-organic framework blended polyvinyl alcohol composite membrane.

[0019] As a preferred embodiment of the present invention, the centrifugation process in steps (2)-(4) refers to using a centrifuge to run at a speed of 12000 rpm for 10 to 120 minutes, discarding the supernatant and retaining the precipitate.

[0020] As a preferred embodiment of the present invention, the number of cleaning cycles in steps (2)-(4) is 5 to 15.

[0021] As a preferred embodiment of the present invention, the vacuum drying process in steps (2)-(4) refers to drying in a vacuum oven at a constant temperature of 80-150°C for 12-36 hours.

[0022] As a preferred embodiment of the present invention, in step (6), the polyethersulfone ultrafiltration membrane has a diameter of 25 cm and a pore size of 0.45 μm.

[0023] This invention further provides a method for methanol dehydration using a zinc ion hydrophilic metal-organic framework-infused polyvinyl alcohol composite membrane prepared by the aforementioned method, comprising the following steps:

[0024] (1) A single-layer zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane is installed inside the flat sheet membrane module. The membrane edge is sealed with a rubber ring and fixed and pressed by a flange. The inlet side of the flat sheet membrane module is connected to a storage tank containing water-containing methanol by two pipes, one is the feed pipe and the other is the return pipe. The outlet side of the flat sheet membrane module is connected to a cold trap and a vacuum pump in sequence through pipes.

[0025] (2) The water-containing methanol is pumped into the flat sheet membrane module, and the methanol is separated and dehydrated based on the pervaporation process. The water contained in the membrane is vaporized in the post-membrane chamber after passing through the flat sheet membrane module, and is collected by condensation in the -20℃ cold trap through the permeate pipeline. The methanol that fails to pass through the membrane is returned to the storage tank through the reflux pipe. During the continuous cycle dehydration process, the feed and permeate of the flat sheet membrane module are sampled at regular intervals, and the water content of the liquid in the storage tank is analyzed by gas chromatography until the target concentration of methanol purification is met.

[0026] As a preferred embodiment of the present invention, the feed temperature of the flat sheet membrane module is controlled to be 55-65°C and the feed flow rate is 0.5L / min.

[0027] As a preferred embodiment of the present invention, the aqueous methanol is processed in batches, and the methanol mass concentration of the raw material liquid is 70% to 95%; the methanol concentration is purified to 99% through continuous cyclic dehydration, which is the final methanol product.

[0028] Invention principle:

[0029] This invention presents an innovative zinc-ion hydrophilic metal-organic framework (MOF) porous material. The external microstructure is modified with hydroxyl and amide groups to enhance hydrophilicity, while the hollow internal structure increases the permeability of water molecules. Compared to fillers used in existing methanol dehydration mixed matrix membrane studies, this novel MOF significantly improves the hydrophilicity of PVA membranes and utilizes a 0.36 nm pore size to restrict methanol molecule passage, thereby improving membrane separation selectivity. The hollow structure of the MOF provides low-resistance mass transfer channels for water molecules within the membrane, enhancing permeation flux. Furthermore, a highly stable, interfacial compatibility-free polyvinyl alcohol composite membrane is formed by crosslinking a polyvinyl alcohol mixed matrix membrane with a polyethersulfone supporting membrane using glutaraldehyde. The composite membrane undergoes hydrophilic treatment to further improve selectivity. The resulting zinc-ion hydrophilic metal-organic framework-infused polyvinyl alcohol composite membrane exhibits high selectivity and high permeation while maintaining good mechanical, chemical, and thermal stability.

[0030] This invention utilizes a zinc-ion metal-organic framework modified with hydroxyl and amide groups, exhibiting strong selective adsorption capacity for water molecules. Both the polyoxyethylene mixed matrix membrane and the polyethersulfone support undergo hydrophilic modification and glutaraldehyde treatment, thereby enhancing the composite membrane's selectivity for water and its chemical stability. In the actual separation of methanol and water, due to the strong selective adsorption of water molecules by the characteristic functional groups, water molecules first dissolve in the mixed matrix membrane. Under the pressure of the downstream vacuum, they rapidly diffuse through the composite membrane, vaporize downstream, and are discharged by a pump, achieving separation. Simultaneously, methanol not selectively adsorbed flows back from upstream of the membrane to the feed tank, achieving methanol recycling and dehydration to increase the methanol product concentration.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] 1. This invention utilizes the hydroxyl and amide functional groups on a zinc ion hydrophilic metal-organic framework (MOF) with strong selective adsorption capacity for water molecules, preferentially adsorbing water molecules and improving the solubility of water in the membrane. Furthermore, the hollow structure inside the MOF provides low-resistance mass transfer channels for water molecules, allowing them to pass through rapidly and improving their diffusivity in the membrane. The MOF has a pore size of 0.36 nm, allowing selective passage of water molecules (the molecular dynamic diameters of methanol and water are 0.38 nm and 0.26 nm, respectively). Simultaneously, the composite membrane prepared from the hydrophilic MOF, polyvinyl alcohol (PVA), and polyethersulfone, after hydrophilic modification, further increases the number of hydroxyl groups on the membrane surface, enhancing the selectivity of the composite membrane for water molecules. Water contact angle tests show that the final zinc ion hydrophilic MOF-infused PVA composite membrane has a water contact angle of 37°, while the pure PVA membrane has a water contact angle of 74°. This invention effectively improves the hydrophilicity of the PVA membrane. In actual pervaporation separation experiments, under conditions of 55℃~65℃ and a feed flow rate of 0.5 / min, the permeation flux of the zinc ion hydrophilic metal-organic framework-doped polyvinyl alcohol composite membrane was 196~413 g / m³. 2 With a separation factor of 69–147 per hour, the polyvinyl alcohol membrane effectively improves water permeability and selectivity. After 3–6 hours of cyclic dehydration experiments, the methanol mass concentration increased from 70%–90% of the feed to 95%–99%, yielding a methanol product with a mass concentration of 99%.

[0033] 2. The zinc-ion hydrophilic metal-organic framework polyvinyl alcohol composite membrane of this invention improves the membrane's anti-swelling property by adding MOF filler, reducing the swelling degree of the membrane in a 70wt% methanol and water mixed solution from 53% to 24%. Furthermore, the crosslinking of polyvinyl alcohol and polyethersulfone enhances the membrane's stability. After soaking in the feed for 100 hours, the membrane's permeate flux and separation factor did not change significantly, while the uncrosslinked polyvinyl alcohol / polyethersulfone composite membrane showed a significant decrease in separation performance after 100 hours of soaking. Attached Figure Description

[0034] Figure 1 A process flow diagram for the preparation of a zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane.

[0035] Figure 2 A process flow diagram for a method to achieve methanol dehydration via pervaporation using composite membrane products.

[0036] Figure 3 Scanning electron microscope image of a zinc ion hydrophilic metal-organic framework.

[0037] Figure 4 Transmission electron microscopy image of a zinc ion hydrophilic metal-organic framework.

[0038] Figure 5 This is a scanning electron microscope image of the surface of the composite film product of the present invention.

[0039] Figure 6 This is a scanning electron microscope image of the longitudinal cross-section of the composite membrane product of the present invention.

[0040] Figure 7 The results show the comparison of the water contact angle measurement between the composite membrane product of this invention and a pure polyvinyl alcohol membrane.

[0041] Figure 8 The results show the permeation flux and separation factor of the composite membrane product of this invention in pervaporation experiments at different temperatures.

[0042] Figure 9 The swelling test results are for different membranes. Detailed Implementation

[0043] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. These embodiments will enable those skilled in the art to gain a more comprehensive understanding of the present invention, but will not limit the invention in any way.

[0044] Part 1: Preparation of Composite Membrane Products

[0045] The preparation process of zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane is as follows: Figure 1 As shown, the preparation steps of the example include:

[0046] (1) Take 2.02–20.2 kg of Zn(NO3)2·6H2O and 2–6 kg of polyvinylpyrrolidone (PVP), and dissolve them in 1–6 ml of water. 3 In N,N-dimethylformamide, stir and mix thoroughly;

[0047] (2) Take 1.58–15.8 kg of 2-cyanoimidazole and dissolve it in 0.6–6 ml of water. 3In N,N-dimethylformamide, stir thoroughly to mix; then add to the mixed solution from step (1), and transfer the whole mixture to 1-2 ml. 3 In a hydrothermal reaction vessel, after stirring thoroughly, the mixture was reacted at 150°C for 24–96 h. After the reaction, the product precipitate was first centrifuged, then washed with ethanol, methanol, or deionized water; after centrifugation again, it was vacuum dried to obtain a zinc ion metal-organic framework.

[0048] (3) Take 4–20 kg of zinc ion metal-organic framework and disperse it in 0.1–0.5 m³. 3 In dimethyl sulfoxide, 0.03–0.3 mg / L was added to the solution in an ice-water bath environment. 3 An exothermic reaction was carried out using a 30% hydrogen peroxide aqueous solution and 1–3 kg of potassium carbonate, causing the solution temperature to rise continuously. Water was added to control the ice-water bath environment to maintain a constant solution temperature of 20°C. The resulting precipitate was first centrifuged, then washed with ethanol; after a second centrifugation, the precipitate was vacuum dried to obtain amide-modified metal-organic framework nanomaterials.

[0049] (4) Take 4–40 kg of the above-mentioned amide-modified metal-organic framework nanomaterials and place them in a 0.2–0.5 m... 3 The surface of the metal-organic framework nanomaterial was further modified with hydroxyl groups by immersing it in a 5% tannic acid solution for 0.5–1 h. The resulting precipitate was centrifuged and then washed with ethanol, methanol, or deionized water. After centrifugation again, it was vacuum dried to obtain a zinc ion hydrophilic metal-organic framework with an external modification of hydroxyl and amide groups and a hollow interior.

[0050] (5) Take 1-10 kg of polyvinyl alcohol and add 9-90 kg of deionized water to obtain a polyvinyl alcohol solution with a mass concentration of 10%; then add 0.25-2.5 kg of the zinc ion hydrophilic metal-organic framework obtained in step (4) and stir thoroughly to obtain a mixed solution;

[0051] (6) Take a polyethersulfone ultrafiltration membrane with a diameter of 25 cm and a pore size of 0.45 μm and soak it in 0.05 kg of glutaraldehyde hydrochloric acid solution for 2 to 5 hours; the mass ratio of glutaraldehyde, hydrochloric acid and water in the glutaraldehyde hydrochloric acid solution is 1:1.5:97.5.

[0052] (7) Take the polyethersulfone ultrafiltration membrane impregnated in step (6) and place it in a circular groove on the surface of a polytetrafluoroethylene plate as a support membrane. The groove depth is 80 μm. Pour the mixed solution from step (5) onto the polyethersulfone support membrane and dry it for 12 hours to allow the mixed solution to transform into a polyvinyl alcohol mixed matrix membrane. At the contact interface, glutaraldehyde and hydrochloric acid crosslink with the polyethersulfone ultrafiltration membrane to form a composite membrane. Then use 0.1–0.2 μm... 3The surface of the composite membrane was modified with hydroxyl groups by immersing it in a 5% tannic acid solution for 10–30 minutes to obtain a zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane.

[0053] As an example, the centrifugation process in steps (2)-(4) can be selected as running a centrifuge at 12000 rpm for 10 to 120 minutes, discarding the supernatant and retaining the precipitate; the number of washing cycles can be selected as 5 to 15 times; the vacuum drying process refers to drying in a vacuum oven at a constant temperature of 80 to 150°C for 12 to 36 hours.

[0054] Part Two: Separation of Aqueous Methanol

[0055] The method for separation and dehydration using the composite membrane product of the present invention is as follows: Figure 2 As shown, the example includes the following steps:

[0056] (1) A single-layer zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane is installed inside the flat sheet membrane module. The membrane edge is sealed with a rubber ring and fixed and pressed by a flange. One membrane and its membrane module constitute a separation stage. Using multiple stages simultaneously can improve separation efficiency. The accompanying drawings of this specification only show one membrane and its membrane module as an example. It is preferred to use five stages in series.

[0057] Each membrane consists of a separation layer (polyvinyl alcohol mixed matrix membrane) and a support layer (polyethersulfone support membrane). The separation layer of each membrane faces the feed direction of the water-containing methanol. The reflux liquid from the previous membrane is the feed for the next stage, and the reflux liquid from the last stage is returned to the storage tank. There are gaps between the membranes, and each membrane is fixed by a corresponding membrane module with short gaps between the modules.

[0058] (2) There are two pipes upstream of the flat sheet membrane module connected to the storage tank. One is the feed pipe (equipped with a feed pump) and the other is the return pipe. Downstream, it is connected to the cold trap and vacuum pump. Under the conditions of 55-65℃ and feed flow rate of 0.5L / min, an aqueous solution with a methanol mass concentration of 70%-95% is introduced into the flat sheet membrane separation system. The methanol is separated and dehydrated by the pervaporation method of the flat sheet membrane module. After the water passes through the flat sheet membrane module, it is vaporized in the post-membrane chamber and enters the -20℃ cold trap from the permeate pipe to condense. The methanol that does not pass through the membrane returns to the storage tank from the return pipe upstream of the membrane. The dehydration is continuously circulated. The feed and permeate are sampled at regular intervals and the water content of the feed solution is analyzed by gas chromatography until the feed solution in the storage tank reaches the target methanol concentration.

[0059] (3) In actual operation, the aqueous methanol is processed in batches, with 5 kg of raw material liquid being introduced into the storage tank each time. After continuous circulation and dehydration for 3 to 6 hours, the methanol concentration in the storage tank is tested and found to be 85% to 99%. When the methanol concentration in the storage tank is purified to 99%, the purified methanol product is obtained.

[0060] Part Three: Specific Implementation Examples

[0061] Example 1

[0062] Take 2.02 kg of Zn(NO3)2·6H2O and 2 kg of polyvinylpyrrolidone (PVP), and dissolve them in 2 mL of water. 3 In N,N-dimethylformamide, mix thoroughly by stirring; take 6.96 kg of 2-cyanoimidazole and dissolve it in 4 ml of water. 3 Add N,N-dimethylformamide, stir thoroughly, and then add to the previously mixed solution. Transfer the resulting solution to a 1.5 mL container. 3 The reaction was carried out in a hydrothermal reactor at 150°C for 48 hours. The resulting product precipitate was first centrifuged, then washed with methanol, and after a second centrifugation, vacuum dried to obtain a zinc ion metal-organic framework. 20 kg of the zinc ion metal-organic framework was dispersed in a 0.3 mL... 3 In dimethyl sulfoxide, 0.3 mg / L was added to the solution in an ice-water bath environment. 3 A 30% (w / w) aqueous solution of hydrogen peroxide and 3 kg of potassium carbonate underwent an exothermic reaction, causing the solution temperature to rise continuously. Water was then added to cool the solution and maintain the temperature at 20°C. The resulting precipitate was centrifuged, then washed with ethanol, and after further centrifugation and vacuum drying, amide-modified metal-organic framework nanomaterials were obtained. 4 kg of the amide-modified metal-organic framework nanomaterials were then immersed in 0.3 m... 3 The product precipitate was first centrifuged and then washed with deionized water after being incubated in a 5% tannic acid solution for 0.6 hours. After centrifugation again, it was vacuum dried to obtain a zinc ion hydrophilic metal-organic framework with an externally modified hydroxyl and amide groups and an internally hollow structure. 10 kg of polyvinyl alcohol was taken and 90 kg of deionized water was added to obtain a 10% polyvinyl alcohol solution. Then, 2.5 kg of zinc ion hydrophilic metal-organic framework was added and stirred thoroughly to obtain a mixed solution. A polyethersulfone ultrafiltration membrane with a diameter of 25 cm and a pore size of 0.45 μm was taken and immersed in 0.05 kg of glutaraldehyde hydrochloric acid solution (the mass ratio of glutaraldehyde, hydrochloric acid and water was 1:1.5:97.5) for 2 hours. The immersed polyethersulfone ultrafiltration membrane was placed in a circular polytetrafluoroethylene plate with a thickness of 80 μm as a support membrane. The previous mixed solution was poured on the polyethersulfone support membrane and dried for 12 hours to transform the mixed solution into a polyvinyl alcohol mixed matrix membrane. Simultaneously, under the action of glutaraldehyde hydrochloric acid solution, the polyvinyl alcohol mixed matrix membrane and the polyethersulfone supporting membrane crosslink to form a polyvinyl alcohol composite membrane, which is then subjected to 0.1m... 3 A zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane was obtained by soaking in a 5% tannic acid solution for 30 minutes.

[0063] The zinc ion hydrophilic metal-organic framework-infused polyvinyl alcohol composite membrane is placed in a flat sheet membrane module, and the membrane edges are sealed with rubber rings. Two pipes connect the upstream of the flat sheet membrane module to a storage tank: one is the feed pipe, and the other is the return pipe. Downstream, a cold trap and a vacuum pump are connected. At a temperature of 55°C and a feed flow rate of 0.5 L / min, a 70% methanol aqueous solution is introduced into the flat sheet membrane separation system. Methanol is separated and dehydrated using the pervaporation method of the flat sheet membrane module. Water vaporizes in the post-membrane chamber after passing through the membrane module and condenses in the -20°C cold trap via the permeate pipe. Methanol that does not pass through the membrane returns to the storage tank via the return pipe upstream of the membrane. Continuous dehydration is performed, with periodic sampling of the feed and permeate for gas chromatography analysis of water content, until the target methanol concentration is reached in the storage tank. This method involves batch processing of the methanol aqueous solution, with 5 kg of feed solution introduced into the storage tank each time. After 4 hours of continuous dehydration, the methanol concentration in the storage tank is measured and purified from 70% to 85%.

[0064] In pervaporation separation, the hydroxyl and amide functional groups on the zinc ion hydrophilic metal-organic framework preferentially adsorb water molecules, improving water solubility in the membrane. Furthermore, its internal hollow structure provides low-resistance mass transfer channels for water molecules, allowing them to pass through rapidly and enhancing diffusivity within the membrane. Therefore, this significantly increases the membrane's water permeation flux (413 g / m³). 2 / h) and separation factor (69).

[0065] Example 2

[0066] Take 10.1 kg of Zn(NO3)2·6H2O and 6 kg of polyvinylpyrrolidone (PVP), and dissolve them in 6 mL of water. 3 In N,N-dimethylformamide, mix thoroughly by stirring; take 1.58 kg of 2-cyanoimidazole and dissolve it in 0.6 ml 3 In N,N-dimethylformamide, mix thoroughly and add to the previously mixed solution. Then transfer the resulting solution to a 1m solution. 3 The reaction was carried out in a hydrothermal reactor at 150°C for 24 hours. The resulting product precipitate was first centrifuged, then washed with deionized water, and then vacuum dried after centrifugation to obtain a zinc ion metal-organic framework. 10 kg of the zinc ion metal-organic framework was dispersed in a 0.5 mL... 3 In dimethyl sulfoxide, 0.1 mg of solution was added to the solution in an ice-water bath environment. 3A 30% (w / w) aqueous solution of hydrogen peroxide and 1 kg of potassium carbonate underwent an exothermic reaction, causing the solution temperature to rise continuously. Water was then added to cool the solution and maintain the temperature at a constant 20°C. The resulting precipitate was centrifuged, then washed with ethanol, and after further centrifugation and vacuum drying, amide-modified metal-organic framework nanomaterials were obtained. 40 kg of the amide-modified metal-organic framework nanomaterials were then immersed in a 0.5 m... 3 The product precipitate was first centrifuged and then washed with ethanol after being incubated in a 5% tannic acid solution for 0.5 h. After centrifugation again, it was vacuum dried to obtain a zinc ion hydrophilic metal-organic framework with an externally modified hydroxyl and amide groups and an internally hollow structure. 1 kg of polyvinyl alcohol was taken and 9 kg of deionized water was added to obtain a 10% polyvinyl alcohol solution. Then 0.5 kg of zinc ion hydrophilic metal-organic framework was added and stirred thoroughly to obtain a mixed solution. A polyethersulfone ultrafiltration membrane with a diameter of 25 cm and a pore size of 0.45 μm was taken and immersed in 0.05 kg of glutaraldehyde hydrochloric acid solution (the mass ratio of glutaraldehyde, hydrochloric acid and water was 1:1.5:97.5) for 3 h. The immersed polyethersulfone ultrafiltration membrane was placed in a circular polytetrafluoroethylene plate with a thickness of 80 μm as a support membrane. The previous mixed solution was poured on the polyethersulfone support membrane and dried for 12 h to transform the mixed solution into a polyvinyl alcohol mixed matrix membrane. Simultaneously, under the action of glutaraldehyde hydrochloric acid solution, the polyvinyl alcohol mixed matrix membrane and the polyethersulfone supporting membrane crosslink to form a polyvinyl alcohol composite membrane, which is then coated with 0.15m... 3 A zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane was obtained by soaking in a 5% tannic acid solution for 15 minutes.

[0067] The zinc ion hydrophilic metal-organic framework-infused polyvinyl alcohol composite membrane is placed in a flat sheet membrane module, and the membrane edges are sealed with rubber rings. Two pipes connect the upstream of the flat sheet membrane module to a storage tank: one is the feed pipe, and the other is the return pipe. Downstream, a cold trap and a vacuum pump are connected. At a temperature of 60°C and a feed flow rate of 0.5 L / min, an 85% methanol aqueous solution is introduced into the flat sheet membrane separation system. Methanol is separated and dehydrated using the pervaporation method of the flat sheet membrane module. Water vaporizes in the post-membrane chamber after passing through the membrane module and condenses in the -20°C cold trap via the permeate pipe. Methanol that does not pass through the membrane returns to the storage tank via the return pipe upstream of the membrane. Continuous dehydration is performed, with periodic sampling of the feed and permeate for gas chromatography analysis of water content, until the target methanol concentration is reached in the storage tank. This method involves batch processing of the methanol aqueous solution, with 5 kg of feed solution introduced into the storage tank each time. After 3 hours of continuous dehydration, the methanol concentration in the storage tank is measured and purified from 85% to 95%.

[0068] In pervaporation separation, the hydroxyl and amide functional groups on the zinc ion hydrophilic metal-organic framework preferentially adsorb water molecules, improving water solubility in the membrane. Furthermore, its internal hollow structure provides low-resistance mass transfer channels for water molecules, allowing them to pass through rapidly and enhancing diffusivity within the membrane. Therefore, this significantly increases the membrane's water permeation flux (289 g / m³). 2 / h) and separation factor (114).

[0069] Example 3

[0070] Take 20.2 kg of Zn(NO3)2·6H2O and 5 kg of polyvinylpyrrolidone (PVP), and dissolve them in 4 mL of water. 3 In N,N-dimethylformamide, mix thoroughly by stirring; take 15.8 kg of 2-cyanoimidazole and dissolve it in 6 ml of water. 3 After thoroughly mixing N,N-dimethylformamide, add it to the previously mixed solution, and then transfer the resulting solution to a 2m³ container. 3 The reaction was carried out in a hydrothermal reactor at 150°C for 96 hours. The resulting precipitate was first centrifuged, then washed with ethanol; after a second centrifugation, it was vacuum dried to obtain a zinc ion metal-organic framework. 4 kg of the zinc ion metal-organic framework was dispersed in a 0.1 mL... 3 In dimethyl sulfoxide, 0.03 mg / L was added to the solution in an ice-water bath environment. 3 A 30% (w / w) aqueous solution of hydrogen peroxide and 2 kg of potassium carbonate underwent an exothermic reaction, causing the solution temperature to rise continuously. Water was then added to cool the solution and maintain the temperature at 20°C. The resulting precipitate was centrifuged, then washed with ethanol, and after further centrifugation and vacuum drying, amide-modified metal-organic framework nanomaterials were obtained. 20 kg of the amide-modified metal-organic framework nanomaterials were then immersed in 0.2 m... 3The product precipitate was first centrifuged and then washed with methanol after being immersed in a 5% tannic acid solution for 1 hour. After centrifugation again, it was vacuum dried to obtain a zinc ion hydrophilic metal-organic framework with an external modification of hydroxyl and amide groups and a hollow internal structure. 5 kg of polyvinyl alcohol was taken and 45 kg of deionized water was added to obtain a 10% polyvinyl alcohol solution. Then 0.25 kg of zinc ion hydrophilic metal-organic framework was added and stirred thoroughly to obtain a mixed solution. A polyethersulfone ultrafiltration membrane with a diameter of 25 cm and a pore size of 0.45 μm was taken and immersed in 0.05 kg of glutaraldehyde hydrochloric acid solution (the mass ratio of glutaraldehyde, hydrochloric acid and water was 1:1.5:97.5) for 5 hours. The immersed polyethersulfone ultrafiltration membrane was placed in a circular polytetrafluoroethylene plate with a thickness of 80 μm as a support membrane. The previous mixed solution was poured on the polyethersulfone support membrane and dried for 12 hours to transform the mixed solution into a polyvinyl alcohol mixed matrix membrane. Simultaneously, under the action of glutaraldehyde hydrochloric acid solution, the polyvinyl alcohol mixed matrix membrane and the polyethersulfone supporting membrane crosslink to form a polyvinyl alcohol composite membrane, which is then subjected to 0.2m... 3 A zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane was obtained by soaking in a 5% tannic acid solution for 10 minutes.

[0071] The zinc ion hydrophilic metal-organic framework-infused polyvinyl alcohol composite membrane is placed in a flat-sheet membrane module, and the membrane edges are sealed with rubber rings. Two pipes connect the upstream of the flat-sheet membrane module to a storage tank: one is the feed pipe, and the other is the return pipe. Downstream, a cold trap and a vacuum pump are connected. Under conditions of 65°C and a feed flow rate of 0.5 L / min, a 95% methanol aqueous solution is introduced into the flat-sheet membrane separation system, and methanol is separated and dehydrated using the pervaporation method of the flat-sheet membrane module. Water passes through the flat-sheet membrane module and then... The methanol is vaporized in the rear chamber and condensed in a -20°C cold trap through the permeate pipe. Methanol that does not pass through the membrane returns to the storage tank through the reflux pipe upstream of the membrane. Continuous dehydration is performed, and the feed and permeate are sampled periodically to analyze the water content of the feed solution using gas chromatography until the feed solution in the storage tank reaches the target methanol concentration. This method involves batch processing of methanol-water solution. Each time, 5 kg of feed solution is introduced into the storage tank. After 6 hours of continuous dehydration, the methanol concentration in the storage tank is tested and purified from 95% to 99%, ultimately yielding a methanol product with a mass concentration of 99%.

[0072] In pervaporation separation, the hydroxyl and amide functional groups on the zinc ion hydrophilic metal-organic framework preferentially adsorb water molecules, enhancing water solubility in the membrane. Furthermore, its internal hollow structure provides low-resistance mass transfer channels for water molecules, allowing for rapid passage and improving water molecule diffusivity within the membrane. Therefore, this significantly increases the membrane's water permeation flux (196 g / m³). 2 / h) and separation factor (147).

[0073] Product performance testing:

[0074] Taking the zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane product in Example 1 as an example, electron microscopy observation, water contact angle measurement, pervaporation flux and separation factor of the membrane in pervaporation experiments at different temperatures, and swelling degree of the membrane in different liquids were tested.

[0075] Figure 3 This is a scanning electron microscope image of a zinc ion hydrophilic metal-organic framework. Figure 4 This is a transmission electron microscope image of a zinc ion hydrophilic metal-organic framework, which reveals the hollow microstructure of the organic framework. Figure 5 The image shown is a scanning electron microscope image of the surface of the composite membrane product of the present invention. It can be seen that the filler and polyvinyl alcohol have good interfacial compatibility and no voids. Figure 6 The image shows a longitudinal cross-sectional scanning electron microscope image of the composite membrane product of the present invention. It can be seen that the polyvinyl alcohol mixed matrix membrane and the polyethersulfone support membrane are tightly cross-linked without gaps (the upper part of the image is the separation layer: a dense polyvinyl alcohol mixed matrix membrane, and the lower part is the support layer: a loose and porous polyethersulfone support membrane).

[0076] Figure 7 The results of water contact angle measurements of the composite membrane products of the present invention are shown, wherein (a) is the water contact angle of one surface of the polyvinyl alcohol mixed matrix membrane, showing a result of 37°; and (b) is the water contact angle of the pure polyvinyl alcohol membrane as a comparison, showing a result of 74°.

[0077] Figure 8 The figure shows the permeation flux and separation factor of the composite membrane product of the present invention in pervaporation experiments at different temperatures. As can be seen from the figure, as the temperature increases, the membrane permeation flux and separation factor show an inverse trend, that is, the higher the temperature, the higher the separation factor, but the permeation flux decreases.

[0078] Swelling degree tests were performed on different membranes according to the methods specified in the following two published documents:

[0079] [1]Haaz E,Toth AJ.Methanol dehydration with pervaporation:Experimentsand modeling.Separation and Purification Technology.2018;205:121-129.https: / / doi.org / 10.1016 / j.seppur.2018.04.088;

[0080] [2]Selim A,Toth AJ,Fozer D,Szanyi A,Mizsey P.PervaporativeDehydration of Methanol Using PVA / Nanoclay Mixed Matrix Membranes:Experimentsand Modeling.Membranes.2020;10:435.https: / / doi.org / 10.3390 / membranes10120435.

[0081] Test results are as follows Figure 9 As shown in the figure, the composite membrane product of this invention improves the membrane's anti-swelling property by adding MOF filler, reducing the swelling degree of the membrane in a 70wt% methanol and water mixed solution from 53% to 24%. Simultaneously, the crosslinking of polyvinyl alcohol and polyethersulfone enhances the membrane's stability. When the same experiment was conducted after soaking in aqueous methanol for 100 hours, the permeate flux and separation factor of the composite membrane product of this invention did not change significantly, while the separation performance of the uncrosslinked polyvinyl alcohol / polyethersulfone composite membrane decreased significantly after 100 hours of soaking.

[0082] Finally, it should be noted that the above examples are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments and many variations are possible. All variations that can be directly derived or conceived by those skilled in the art from the disclosure of this invention should be considered within the scope of protection of this invention.

Claims

1. A method for preparing a zinc ion hydrophilic metal-organic framework-infused polyvinyl alcohol composite membrane, characterized in that, Includes the following steps: (1) Add Zn(NO3)2·6H2O and polyvinylpyrrolidone (PVP) to N,N-dimethylformamide and stir thoroughly to dissolve and mix; add 2-cyanoimidazole to N,N-dimethylformamide and stir thoroughly to dissolve and mix; the mass ratio of Zn(NO3)2·6H2O, polyvinylpyrrolidone (PVP) and 2-cyanoimidazole is 2.02~20.2∶2~6∶1.58~15.8; (2) Add the two mixed solutions obtained in step (1) into the same reaction vessel, stir evenly, and react at 150°C for 24-96 h; after the reaction is completed, centrifuge the product, wash with ethanol, methanol or deionized water and centrifuge again; separate the precipitate and vacuum dry it to obtain zinc ion metal-organic framework. (3) zinc ion metal organic framework is dispersed in dimethyl sulfoxide, and under ice water bath condition, hydrogen peroxide aqueous solution and potassium carbonate are continuously added to carry out exothermic reaction, the temperature of solution is controlled to be constant at 20℃ all the time; the dosage relationship of zinc ion metal organic framework, dimethyl sulfoxide, hydrogen peroxide aqueous solution and potassium carbonate is 4-20 kg: 0.1-0.5 m 3 L 0.03-0.3 m 3 L 1-3 kg, and the mass concentration of hydrogen peroxide aqueous solution is 30%; After the reaction was completed, the reaction product was centrifuged, washed with ethanol, and centrifuged again; the precipitate was separated and vacuum dried to obtain amide-modified metal-organic framework nanomaterials. (4) Apply at a ratio of 4-40 kg: 0.2-0.5 m 3 Based on the dosage relationship, the amide-modified metal-organic framework nanomaterials were immersed in a tannic acid solution for 0.5–1 h and kept there for 0.5–1 h. The mass concentration of the tannic acid solution was 5%. After immersion, the nanomaterials were centrifuged, washed with ethanol, methanol, or deionized water, and then centrifuged again. The precipitate was separated and vacuum dried to obtain a zinc ion hydrophilic metal-organic framework with a microstructure that is externally modified by hydroxyl and amide groups and internally hollow. (5) Add the zinc ion hydrophilic metal-organic framework to a 10% (w / w) polyvinyl alcohol solution and stir thoroughly to obtain a mixed solution; the mass ratio of the organic framework to the polyvinyl alcohol contained in the solution in the mixed solution is 0.25-2.5:1-10; (6) The polyethersulfone ultrafiltration membrane is immersed in glutaraldehyde hydrochloric acid solution for 2-5 hours. The mass ratio of glutaraldehyde, hydrochloric acid and water in the solution is 1:1.5:97.

5. The immersed polyethersulfone ultrafiltration membrane is placed in a circular groove on the surface of a polytetrafluoroethylene plate as a support membrane, and then the mixed solution in step (5) is poured in. After 12 hours of drying, the upper mixed solution is transformed into a polyvinyl alcohol mixed matrix membrane, and crosslinked with the polyethersulfone ultrafiltration membrane at the contact interface due to the action of glutaraldehyde hydrochloric acid to form a composite membrane. The cross-linked composite membrane is removed and soaked in a 5% tannic acid solution for 10–30 minutes to obtain the final product, a zinc ion hydrophilic metal-organic framework blended polyvinyl alcohol composite membrane.

2. The method according to claim 1, characterized in that, The centrifugation process in steps (2)-(4) refers to running a centrifuge at 12000 rpm for 10 to 120 minutes, discarding the supernatant and retaining the precipitate.

3. The method according to claim 1, characterized in that, The number of cleaning cycles in steps (2)-(4) is 5 to 15.

4. The method according to claim 1, characterized in that, The vacuum drying process in steps (2)-(4) refers to drying in a vacuum oven at a constant temperature of 80-150℃ for 12-36 hours.

5. The method according to claim 1, characterized in that, In step (6), the polyethersulfone ultrafiltration membrane has a diameter of 25 cm and a pore size of 0.45 μm.

6. A method for methanol dehydration using a zinc ion hydrophilic metal-organic framework-infused polyvinyl alcohol composite membrane prepared by the method of claim 1, characterized in that, Includes the following steps: (1) A single-layer zinc ion hydrophilic metal-organic framework mixed with polyvinyl alcohol composite membrane is installed inside the flat sheet membrane module. The membrane edge is sealed with a rubber ring and fixed and pressed by a flange. The inlet side of the flat sheet membrane module is connected to a storage tank containing water-containing methanol by two pipes, one is the feed pipe and the other is the return pipe. The outlet side of the flat sheet membrane module is connected to a cold trap and a vacuum pump in sequence through pipes. (2) The water-containing methanol is pumped into the flat sheet membrane module, and the methanol is separated and dehydrated based on the pervaporation process. The water contained in the membrane is vaporized in the post-membrane chamber after passing through the flat sheet membrane module, and is collected by condensation in the -20℃ cold trap through the permeate pipeline. The methanol that fails to pass through the membrane is returned to the storage tank through the reflux pipe. During the continuous cycle dehydration process, the feed and permeate of the flat sheet membrane module are sampled at regular intervals, and the water content of the liquid in the storage tank is analyzed by gas chromatography until the target concentration of methanol purification is met.

7. The method according to claim 6, characterized in that, The feed temperature of the flat sheet membrane module is controlled at 55-65℃ and the feed flow rate is 0.5L / min.

8. The method according to claim 6, characterized in that, The aqueous methanol is processed in batches, with the methanol mass concentration in the raw liquid being 70% to 95%. After continuous cyclic dehydration, the methanol concentration is purified to 99%, which is the final methanol product.

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