A method for preparing PVDF organic-inorganic hybrid membranes with in-situ grown ZIF-8

Abstract

This invention belongs to the field of membrane separation technology and discloses a method for preparing an in-situ grown ZIF-8 PVDF organic-inorganic hybrid membrane. Using DMAc as a solvent, PVDF is dissolved to obtain a homogeneous casting solution. Zn(NO3)2 and dimethylimidazole are added to the casting solution, and the casting solution is scraped and immersed in a gel bath solution. Solvent exchange occurs at room temperature to form the membrane, simultaneously allowing Zn(NO3)2 and dimethylimidazole to react at room temperature to generate ZIF-8 particles, which are then embedded in the membrane, resulting in an antifouling ZIF-8 / PVDF membrane. This invention features a simple and low-cost preparation process, and the amount of nanoparticles added is adjustable over a wide range. The prepared hybrid membrane requires no subsequent treatment such as hydrothermal reactions, and the process is easily scaled up. The prepared hybrid membrane exhibits high water flux and good antifouling performance. The organic-inorganic hybrid membrane prepared by this invention has excellent performance and broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology and relates to a method for preparing an organic-inorganic hybrid membrane by reacting Zn(NO3)2 and dimethylimidazole (mIN) with water, ethanol or methanol in situ during the formation of a polyvinylidene fluoride (PVDF) membrane to obtain ZIF8, and uniformly dispersing it in a PVDF membrane matrix. Specifically, it is a method for preparing a uniformly dispersed ZIF-8 / PVDF hybrid membrane. Background Technology

[0002] Polyvinylidene fluoride (PVDF) membranes possess excellent heat resistance and chemical stability, maintaining stable performance even in high-temperature and corrosive environments. Their high porosity and permeability make them suitable for filtration and separation processes, such as gas and steam filtration, and high-temperature liquid filtration. PVDF membranes are easily processed into various shapes and sizes to meet diverse application requirements. However, PVDF membranes have low surface energy and are non-polar, exhibiting strong hydrophobicity due to the lack of hydrogen bonding with water. This makes the membrane surface susceptible to fouling during actual separation processes. Hydrophobic substances such as proteins and colloidal particles in the water easily adsorb onto the membrane surface, leading to pore blockage and membrane fouling. Membrane fouling can cause a decrease in flux, poorer separation efficiency, shorter service life, and a significant increase in operating costs. To overcome these drawbacks, physical or chemical modification methods are often employed to improve PVDF membrane performance and expand its application range.

[0003] Therefore, it is urgent to use hydrophilic materials to modify them for hydrophilicity, and introducing hydrophilic inorganic nanoparticles into the membrane matrix has become an effective and rapid method. Metal-organic frameworks (MOFs) are organic-inorganic hybrid materials with intramolecular pores formed by the self-assembly of organic ligands and metal ions or clusters through coordination bonds, and they have good compatibility with polymer materials. Numerous studies have introduced MOF particles into polymer casting solutions to prepare organic-inorganic hybrid membranes to improve the overall performance of the membranes, achieving positive results. However, MOF materials have poor solubility and dispersion in polymer solutions, resulting in limited addition amounts of MOF particles. Most of the particles are encapsulated by the polymer, leading to poor hydrophilicity and antifouling properties in the formed membrane.

[0004] Patent CN110102184B discloses a method for preparing a composite desulfurization membrane by in-situ layer-by-layer self-assembly of ZIF-8 on the surface of a prepared PVDF membrane. The PVDF base membrane is immersed in a suspension formed by the initial reaction of ZIF-8 raw materials zinc nitrate hexahydrate, dimethylimidazole, and water. This in-situ self-assembly of ZIF-8 partially solves the problem of severe particle agglomeration in traditional blending methods. However, to reduce defects in the membrane composite functional layer, multiple layer-by-layer assembly and a polyethylene glycol (PEG) layer are required for protection. PEG forms a uniformly distributed separation layer, partially penetrating into the gaps between ZIF-8 particles and immersing into the pores of the porous PVDF base membrane, resulting in a tighter interlayer bond. Patent CN106995531A discloses a method for in-situ synthesis of a composite material from cellulose and MOF. This patent mixes a precursor metal salt solution, a diluted cellulose mixture solution, and an organic ligand solution. After uniform mixing and high-temperature reaction, a composite solution is obtained, which is then vacuum-filtered to obtain the membrane product. Metal-organic frameworks (MOFs) are introduced in situ into cellulose membranes, ensuring the integrity of the structure and function of both cellulose and MOFs, resulting in low energy consumption and rapid, high efficiency. Due to the high-temperature reaction, ZIF-8 or other MOF particles are formed in the precursor solution beforehand. Therefore, during the filtration process, the water-insoluble cellulose and inorganic particles such as ZIF-8 are drawn into a filter cake to obtain the filter membrane. However, it is actually a physical mixture of ZIF-8 particles and the filter material cellulose. ZIF-8 has already formed particles in the early reaction, which to some extent causes the agglomeration and uneven dispersion of the particles. Patent CN116116241A discloses a method for preparing an ultrathin silicone rubber / MOF hollow fiber composite membrane. This invention uses a prepared MOFs precursor solution to wet PVDF membrane fibers, and then self-crystallizes to obtain a hollow fiber membrane module containing MOFs seeds; repeating the above steps more than once, a hollow fiber membrane module with a grown MOF membrane is obtained. The process is quite complex. MOF particles grow in situ within the existing PVDF membrane pores. Due to differences in membrane pore size, the resulting MOF particles are not uniform in size, and some particles do not have direct contact with the membrane substrate, resulting in limited stability.

[0005] Therefore, developing a PVDF organic-inorganic hybrid membrane with excellent high permeation flux and hydrophilicity is of great significance for improving the long-term performance of PVDF separation membranes. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies by preparing a uniformly dispersed ZIF-8-doped ZIF-8 / PVDF organic-inorganic hybrid membrane using an immersion precipitation phase inversion method. This membrane is prepared using a specific process, in which Zn(NO3)2, dimethylimidazole (mIN), and PVDF are dissolved in N,N-dimethylacetamide (DMAC) to form a uniform casting solution. The solution is then coated onto a glass plate or nonwoven fabric and immersed in a gel bath (water, ethanol-water solution, or methanol-water solution) to allow solvent exchange and the formation of a nascent membrane. Water or alcohol in the gel bath enters the membrane matrix, causing Zn(NO3)2 and dimethylimidazole (mIN) to react in situ, generating ZIF-8 nanoparticles. Because the gelling agent (water or alcohol) slowly permeates into the nascent PVDF membrane, the nucleation and growth rates of ZIF-8 are limited, ensuring slow growth of ZIF-8 particles and obtaining relatively uniform, small-diameter particles that are uniformly dispersed in the PVDF matrix.

[0007] The technical solution of this invention:

[0008] A method for preparing an in-situ grown ZIF-8 PVDF organic-inorganic hybrid membrane, comprising the following steps:

[0009] Using DMAc as a solvent, PVDF is dissolved to obtain a uniform casting solution. Zn(NO3)2 and dimethylimidazole (mIN) are added to the casting solution. The casting solution is then scraped and immersed in a gel bath solution. Solvent exchange occurs at room temperature to form a membrane. Simultaneously, Zn(NO3)2 and dimethylimidazole (mIN) react at room temperature to generate ZIF-8 particles, which are then embedded in the membrane, resulting in an antifouling ZIF-8 / PVDF membrane.

[0010] The PVDF content is 5%-15 wt.%.

[0011] The molar ratio of Zn(NO3)2 to dimethylimidazole is 1:2-1:4, and the molar concentration of Zn(NO3)2 in the casting solution is 1% to 8%.

[0012] The gel bath is water, an aqueous solution of ethanol, or an aqueous solution of methanol, wherein the mass percentage of ethanol in the aqueous solution of ethanol is no more than 80%, or the mass percentage of methanol in the aqueous solution of methanol is no more than 80%.

[0013] The beneficial effects of this invention are:

[0014] (1) This invention utilizes the solubility of Zn(NO3)2 and dimethylimidazole in DMAc to obtain uniformly dispersed ZIF-8 nanoparticles through an in-situ reaction under aqueous conditions, which are then embedded in the membrane. Since the nanoparticles are obtained through an in-situ reaction, they exhibit high uniformity, good dispersibility, and high antifouling performance. Furthermore, the nanoparticles are embedded in the membrane matrix, making them less prone to detachment and exhibiting high stability over long-term use, thereby effectively extending the membrane's lifespan and reducing the frequency of cleaning.

[0015] (2) The preparation process of this invention is simple and inexpensive, and the amount of nanoparticles added can be adjusted within a wide range. The prepared hybrid membrane does not require subsequent treatment such as hydrothermal reaction, and the process is easy to scale up. The prepared hybrid membrane has high water flux and good antifouling performance.

[0016] (3) The innovative organic-inorganic hybrid membrane of this invention has excellent comprehensive performance and broad application prospects. Attached Figure Description

[0017] Figure 1 This is a SEM image of the ZIF-8 / PVDF membrane of the present invention. The nanoparticles in the image are ZIF-8. Detailed Implementation

[0018] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0019] Example 1:

[0020] (1) Preparation of hybrid membrane

[0021] Take 8.5g of DMAc, add 1.2g of PVDF to the DMAc, stir and dissolve to obtain a uniform casting solution; add 0.2975g of Zn(NO3)2·6H2O and 0.192g of dimethylimidazole, mix well; after the casting solution is scraped onto a polyester nonwoven fabric, immerse it in an aqueous gel bath solution to allow solvent exchange, and let it stand for more than 12 hours to obtain a uniformly dispersed ZIF-8 / PVDF membrane.

[0022] (2) Performance Testing

[0023] The water flux of the membrane is 65 L / m at 0.1 MPa. 2 h. When the membrane was used to treat 20 mg / L methylene blue dye wastewater, the dye removal rate was 94%.

[0024] See attached Figure 1 ZIF-8 nanoparticles are uniformly dispersed on the surface of the hybrid film.

[0025] Example 2:

[0026] (1) Preparation of hybrid membrane

[0027] Take 8.5g of DMAc, add 1.2g of PVDF to the DMAc, stir and dissolve to obtain a uniform casting solution; add 0.2975g of Zn(NO3)2·6H2O and 0.192g of dimethylimidazole, mix well; after the casting solution is scraped onto a polyester nonwoven fabric, immerse it in an aqueous gel bath solution to allow solvent exchange, and let it stand for more than 12 hours to obtain a uniformly dispersed ZIF-8 / PVDF membrane.

[0028] (2) Performance Testing

[0029] The water flux of the membrane is 65 L / m at 0.1 MPa. 2 h. When the membrane was used to treat 20 mg / L methylene blue dye wastewater, the dye removal rate was 97%.

[0030] Comparative Example 1:

[0031] (1) Membrane preparation

[0032] Take 8.5g of DMAc, add PVDF to DMAc, where the PVDF content is 1.2g, stir to dissolve and obtain a uniform casting solution; add 0.4g of ZIF-8 particles and stir to mix; due to the low solubility of the particles, the solution is viscous and it is difficult to obtain a uniform and dense film.

[0033] This invention utilizes the characteristic of ZIF-8 raw material dissolved in PVDF membrane-forming solution, reacting under aqueous conditions to generate ZIF-8. This ingeniously achieves the synchronous growth of PVDF membrane and ZIF-8 particles in the membrane substrate, effectively realizing the controllable growth of ZIF-8 nanoparticles. The nanoparticles exhibit high uniformity, good dispersibility, and are embedded in the membrane substrate. Therefore, the resulting hybrid membrane possesses high hydrophilicity and antifouling properties. Furthermore, the nanoparticles, embedded in the membrane matrix, are not prone to detachment, exhibiting high stability over long-term use, thus effectively extending the membrane's lifespan and reducing cleaning frequency. The preparation process of this invention is simple and low-cost, and the amount of nanoparticles added is adjustable within a wide range. It possesses excellent comprehensive performance and broad application prospects.

Claims

1. A method for preparing an in-situ grown ZIF-8 PVDF organic-inorganic hybrid membrane, characterized in that, The steps are as follows: Using DMAc as a solvent, PVDF was dissolved to obtain a uniform casting solution. Zn(NO3)2 and dimethylimidazole were added to the casting solution. The casting solution was scraped and immersed in a gel bath solution. Solvent exchange and membrane formation occurred at room temperature. Simultaneously, Zn(NO3)2 and dimethylimidazole reacted at room temperature to generate ZIF-8 particles and embedded in the membrane, thus obtaining a PVDF organic-inorganic hybrid membrane with antifouling properties. The molar ratio of Zn(NO3)2 to dimethylimidazole is 1:2-1:4, and the molar concentration of Zn(NO3)2 in the casting solution is 1%~8%. The gel bath is water, an aqueous solution of ethanol, or an aqueous solution of methanol, wherein the mass percentage of ethanol in the aqueous solution of ethanol is not greater than 80%, or the mass percentage of methanol in the aqueous solution of methanol is not greater than 80%.

2. The preparation method according to claim 1, characterized in that, The casting solution has a PVDF content of 5%-15 wt.%.

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