Preparation Method and Application of a MOF@Polymer Composite Membrane

Abstract

The present invention discloses a preparation method and application of a MOF@polymer composite membrane. The preparation method includes: a) pre-synthesizing MOF seeds; b) blending the MOF seeds with a polymer and obtaining a polymer membrane embedding the seeds by means of immersion precipitation phase inversion; c) adopting a solvothermal method to contact the polymer membrane embedding the seeds with a MOF precursor and a crystallization regulator, and generating MOF crystals with a specific morphology on the surface of the polymer membrane. The present invention conducts various morphology regulations on ZIF crystals based on the surface-confined growth method, realizing the controllable growth and preparation of zero-dimensional to three-dimensional crystals. The synthesized porous MOF crystals have the advantages of high porosity, large specific surface area, and a large number of active adsorption sites on the surface, and are expected to show good application prospects in complex water treatment fields such as oil-water separation, dye adsorption, and photocatalytic degradation.

Description

Technical Field

[0001] The invention belongs to the field of multifunctional polymer membranes, and in particular relates to a method for regulating ZIF crystal morphology based on a crystallization regulator and an application thereof. Background Art

[0002] At present, with the rapid development of industry, the discharge of oily wastewater is increasing, which poses a huge challenge to the field of environmental protection. Oily wastewater can generally be divided into suspended oil, emulsified oil, dispersed oil, dissolved oil and other forms. Among them, suspended oil generally floats on the water surface, and the particle size is usually greater than 100 microns; dispersed oil generally suspends in water, but will float to the surface after standing for a long time, and the particle size is between 10 and 100 microns; the surface of emulsified oil contains a layer of surfactant as a protective layer, which is stably dispersed in water, with a particle size between 0.1 and 10 microns, and is the most difficult oil substance to remove; dissolved oil has a small diameter and exists in the water in a dissolved state. Most of them are molecules and have little effect on water quality. In the face of oil pollution, traditional treatment methods such as gravity method, centrifugation method, adsorption method, etc., have significant disadvantages, such as low separation efficiency, easy to produce secondary pollution, high energy consumption, low emulsion separation efficiency, etc.

[0003] In addition to various oil substances, wastewater also contains a variety of organic dyes, which not only pollute the environment, but also harm the human body and have strong teratogenic and carcinogenic properties. Traditional methods for treating organic dyes include biological treatment, flocculation, chemical oxidation, electrochemical methods, etc. These methods often have disadvantages such as high cost, difficult recovery, and easy secondary pollution when treating organic dyes.

[0004] Based on the above discussion, in the face of the treatment difficulties of complex sewage and the limitations of traditional treatment methods, we need to develop new wastewater treatment materials that are multifunctional, efficient, and green. Membrane separation technology has been proven to be a more promising method for separating various oil-water emulsions because of its high separation efficiency, low cost, and easy operation. Among them, membrane wettability is an important factor affecting the separation function of polymer membranes, and membrane surface roughness and chemical functional components are key factors affecting membrane wettability.

[0005] Metal-organic frameworks (MOFs) are network structure crystals formed by the coordination of metal central ions and organic ligands, also known as multi-void coordination polymers. Compared with other porous materials, MOFs have high porosity, high specific surface area, and excellent thermochemical stability. They can provide a certain degree of roughness on the membrane surface and improve wettability. However, when MOFs materials are used as separate adsorption materials, they are easy to agglomerate and fall off during use, causing environmental pollution; at the same time, MOFs themselves are granular crystals, which limits the further improvement of their specific surface area, which greatly limits the application of MOFs materials in the field of water treatment.

[0006] Patent CN116116241A discloses a MOF composite membrane. The MOF composite membrane is obtained by secondary growth through a seed-assisted method. The morphology of MOF crystals on the membrane surface is granular, which limits the further increase of the specific surface area of MOF crystals. Patent CN117691035A deposits MOF crystals on a zinc foil through an in-situ growth method. The morphology of MOF crystals is granular, and there is no anchor point to fix the MOF crystals, which are easy to fall off, resulting in performance loss and environmental pollution. Summary of the Invention

[0007] In view of this, based on the regulation characteristics of the crystallization regulator on the deprotonation rate of raw materials, relying on the seed-assisted method, the present invention realizes the controllable preparation of ZIF crystals on the surface of a polymer membrane through surface-confined growth, and synthesizes a MOF composite membrane with high porosity, a further increased specific surface area, a controllable and stable structural morphology (such as granular, rod-shaped, sheet-shaped), so as to be effectively applied to water treatment fields such as oil-water separation and dye adsorption.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] A MOF@polymer composite membrane is composed of a porous polymer membrane embedding MOF nanoparticles and MOF crystals grown in a confined manner on the surface of the porous polymer membrane. The preparation steps are as follows:

[0010] a) Pre-synthesize MOF nanoparticles;

[0011] b) Blend the MOF nanoparticles with a polymer casting solution, and use the immersion precipitation phase inversion method to obtain a porous polymer membrane embedding MOF nanoparticles;

[0012] c) Using the solvothermal method, immerse the porous polymer membrane embedding MOF nanoparticles in a solution containing a MOF precursor and a crystallization regulator. The MOF nanoparticles on the surface of the porous polymer membrane act as seeds to promote the formation of MOF crystals with a specific morphology on the surface of the porous polymer membrane.

[0013] Specifically, the porous polymer membrane includes, but is not limited to, ultrafiltration and microfiltration. The polymer membranes used include, but are not limited to, PVDF membranes, polysulfone membranes, and polyethersulfone membranes. Correspondingly, the polymer casting solution includes a polymer and an organic solvent, and the polymers include, but are not limited to, PVDF, polysulfone, and polyethersulfone.

[0014] Specifically, the MOF nanoparticles include, but are not limited to, ZIF-8, MILs, HKUST-1, ZIF-67, etc. The MOF precursor is the precursor for preparing MOF nanoparticles, specifically a uniformly mixed solution of an organic ligand and a metal precursor. The metal precursors include, but are not limited to, zinc precursors, cobalt precursors, iron precursors, chromium precursors, etc.

[0015] Preferably, a method for preparing a MOF@polymer composite membrane comprises the following steps:

[0016] 1) Add PVDF powder and ZIF8 nanoparticles into an organic solvent, and stir until dissolved uniformly to obtain a PVDF casting solution containing ZIF-8 nanoparticles;

[0017] 2) Perform vacuum degassing on the PVDF casting solution containing ZIF-8 nanoparticles;

[0018] 3) Cast the PVDF casting solution containing ZIF-8 nanoparticles onto a substrate, and then transfer the substrate to a coagulation bath for nonsolvent-induced phase inversion to obtain a PVDF membrane containing ZIF-8 nanoparticles;

[0019] 4) Dry the PVDF membrane containing ZIF-8 nanoparticles for later use;

[0020] 5) Uniformly mix a zinc precursor, a crystallization regulator, and a solvent to obtain solution A; uniformly mix an organic ligand and a solvent to obtain solution B;

[0021] 6) Add solution B into solution A, stir and mix uniformly to obtain solution C, and form a MOF precursor in solution C;

[0022] 7) Fix the membrane in step 4) in a reaction vessel, and use the solvothermal method to contact the PVDF membrane containing ZIF-8 nanoparticles with the MOF precursor and the crystallization regulator to generate MOF crystals with a specific morphology on the surface of the PVDF membrane;

[0023] 8) Wash and dry the product obtained in step 7) to obtain a ZIF crystal@polymer composite membrane with different morphologies.

[0024] The preparation method of the ZIF-8 nanoparticles in step 2) is as follows: Add the uniformly dissolved organic ligand solution into the uniformly dissolved zinc precursor solution, stir, centrifuge, and dry to obtain ZIF-8 nanoparticles. The zinc precursor is selected from one of zinc acetylacetonate Zn(acac)2, zinc nitrate Zn(NO3)2, zinc sulfate ZnS04, and zinc perchlorate Zn(ClO4)2; the organic ligand is selected from one of 2-methylimidazole, ethylenediamine, and polyvinylpyrrolidone; the solvent in the organic ligand solution and the zinc precursor solution is selected from one of water, N,N-dimethylformamide, N,N-dimethylacetamide, and methanol; the mass ratio of the zinc precursor is preferably 0.1-10%, the mass ratio of the organic ligand is preferably 0.05-20%, the mass ratio of the solvent is 70-99.85%, the stirring rate is 100-1600 rpm, the centrifugation rate is 1000-10000 r / min, and the drying temperature is 40-100 °C.

[0025] Preferably, in step 1), the PVDF powder is selected from one of the following.

[0026] Preferably, in step 1), the organic solvent is selected from one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone.

[0027] Preferably, in step 1), the mass ratio of the PVDF powder is 10-20%, the mass ratio of the ZIF-8 powder is 0.1-2%, the mass ratio of the organic solvent is 80-90%, the stirring rate is 100-600 rpm, the stirring time is 4-24 h, and the stirring temperature is 40-120 °C.

[0028] Preferably, in step 2), the vacuum degree for degassing is 0.01-0.1 MPa, and the degassing temperature is 25-120 °C.

[0029] Preferably, in step 3), the thickness of the doctor blade used is 100-600 μm, the coagulation bath used is one of a pure water coagulation bath or a mixed coagulation bath, and the temperature used is 25-50 °C.

[0030] Preferably, in step 4), the vacuum degree during drying is 0.01-0.1 MPa, and the temperature is 25-120 °C.

[0031] Preferably, in step 5), the zinc precursor is selected from one of zinc acetylacetonate Zn(acac)2, zinc nitrate Zn(NO3)2, zinc sulfate ZnSO4, and zinc perchlorate Zn(ClO4)2.

[0032] Preferably, in step 5), the organic ligand is selected from one of 2-methylimidazole, ethylenediamine, and polyvinylpyrrolidone.

[0033] Preferably, in step 5), the crystallization regulator is selected from one of cetyltrimethylammonium bromide CTAB, 1-methylimidazole, sodium formate, and n-butylamine.

[0034] Preferably, in step 5), the solvent is selected from one of water, N,N-dimethylformamide, N,N-dimethylacetamide, and methanol.

[0035] Preferably, in step 6), in solution C, the mass ratio of the zinc precursor is 0.05-4.5%, the mass ratio of the crystallization regulator is 0.01-4.5%, the mass ratio of the organic ligand is 0.01-4.5%, and the mass ratio of the organic solvent is 86.5-99.93%.

[0036] Preferably, in step 6), solution B is added to solution A and stirred uniformly for 10 - 120 min at a stirring rate of 100 - 1200 rpm.

[0037] Preferably, in step 7), the temperature of the solvothermal reaction is 40 - 160 °C, and the reaction time is 2 - 24 h.

[0038] Preferably, in step 8), it is washed with one of water, N,N-dimethylformamide, N,N-dimethylacetamide, and methanol, and then dried. The washing method is to alternately wash with water and ethanol 1 - 5 times; the drying method is one or a combination of methods such as air drying at room temperature, vacuum drying, and atmospheric pressure heating drying.

[0039] The MOF@polymer composite membrane is used for oil-water separation and the removal of dyes in solution.

[0040] The present invention provides the ZIF crystals obtained by the method for regulating the morphology of ZIF crystals based on a crystallization regulator described in the above technical solution.

[0041] The MOF nanoparticles in the porous polymer membrane, as crystal seeds, promote the continuous secondary growth of MOF nanoparticles in the precursor solution, not only forming special morphologies different from traditional nanoparticle crystals (such as sheet-like, rod-like, etc.), with the crystal size range changing from the nanoscale to the microscale, but also the nanoparticles growing on the MOF crystal seeds being more tightly combined with the ultrafiltration membrane. The MOF crystal seeds play a role in anchoring the MOFs on the membrane surface, ensuring that the MOFs grown on the surface have significant stability. The MOF material generated on the membrane surface endows the membrane surface with a large roughness, significantly improving the superwetting property of the membrane and helping to increase the permeation flux.

[0042] Compared with the prior art, the present invention has the following advantages and beneficial effects: The present invention uses the surface-confined growth method to regulate the morphology of ZIF nanocrystals, controls the deprotonation rate of ZIF nanocrystals and reaction raw materials by regulating the content of the crystallization regulator or the content of the zinc precursor and the organic ligand, and can realize the growth control process from zero-dimensional to three-dimensional nanocrystals at a relatively low temperature. A porous ZIF crystal material with a high porosity and a large specific surface area is prepared on the surface of the porous polymer membrane. The preparation method using the crystallization regulator to control the morphology has low cost, simple process, and strong practicability. The obtained ZIF crystals have good stability on the membrane surface, and after surface-confined growth, the specific surface area of the ZIF crystals is further increased, which makes the MOF composite membrane have great application potential in oil-water separation and dye adsorption. Description of the Drawings

[0043] Figure 1 It is a scanning electron microscope image of the ZIF nanocrystals prepared in step (1) of Example 1.

[0044] Figure 2 XRD images of the ZIF-8 prepared in step (1) of Example 1 compared with the simulated ZIF-8 standard card.

[0045] Figure 3 SEM images of the surface crystal morphology of the MOF composite membranes prepared in Example 1 (left) and Comparative Example 1 (right).

[0046] Figure 4 SEM images of the surface crystal morphology of the MOF composite membranes prepared in Example 2 (left) and Comparative Example 2 (right).

[0047] Figure 5 SEM images of the surface crystal morphology of the MOF composite membranes prepared in Example 3 (left) and Comparative Example 3 (right).

[0048] Figure 6 SEM images of the surface crystal morphology of the MOF hybrid composite membrane prepared in Example 4.

[0049] Figure 7 SEM images of the surface crystal morphology of the MOF composite membrane prepared in Example 5.

[0050] Figure 8 SEM images of the surface crystal morphology of the MOF composite membrane prepared in Comparative Example 1.

[0051] Figure 9 XRD images of the samples of Examples 1 to 3.

[0052] Figure 10 Images of the oil-water emulsion separation effect of the MOF composite membrane in Example 1.

[0053] Figure 11 Images of the dye adsorption effect of the MOF composite membrane in Example 1. Detailed implementation manners

[0054] The present invention provides a method for regulating the crystal morphology of ZIF based on a crystallization regulator, comprising the following steps:

[0055] 1) Dissolve 1 to 12 g of a zinc precursor and 0.5 to 26.4 g of an organic ligand uniformly in 80 to 1264 g of a solvent, stir and then centrifuge and dry to obtain ZIF-8 nanoparticles.

[0056] 2) Add 10 to 20 g of PVDF powder and 0.1 to 2 g of ZIF-8 powder to 80 to 90 g of an organic solvent, stir until dissolved uniformly to obtain a PVDF casting solution containing ZIF-8 nanoparticles;

[0057] 3) Place the PVDF casting solution containing ZIF-8 nanoparticles in a vacuum oven and degas it under vacuum at a certain temperature;

[0058] 4) Cast the PVDF casting solution containing ZIF-8 nanoparticles onto a substrate, and then transfer the substrate to a coagulation bath for non-solvent induced phase inversion to obtain a PVDF membrane containing ZIF-8 nanoparticles;

[0059] 5) Place the PVDF membrane containing ZIF-8 nanoparticles in a vacuum oven and dry it for later use;

[0060] 6) Uniformly mix 0.5 - 5 g of a zinc precursor, 0.1 - 5 g of a crystallization regulator, and 60 - 300 ml of a solvent to obtain solution A; uniformly mix 0.1 - 4.8 g of an organic ligand and 60 - 300 ml of a solvent to obtain solution B;

[0061] 7) Add solution B to solution A and stir it uniformly at a rate of 100 - 1200 rpm for 10 - 120 min to obtain solution C;

[0062] 8) Fix the membrane in step 5) in a reaction vessel, add solution C in step 7), and use the solvothermal method to bring the polymer membrane embedded with seeds into contact with the solution, and grow MOF crystals with a specific morphology on the surface of the polymer membrane.

[0063] 9) Wash the product obtained in step 8) to obtain a ZIF crystal @ polymer composite membrane with different morphologies;

[0064] In the present invention, a zinc precursor, an organic ligand, and a crystallization regulator are added to an organic solvent and mixed uniformly to obtain a precursor solution for regulating the morphology of ZIF crystals. The polymer membrane coated with ZIF seeds is placed in the precursor solution for a solvothermal reaction, and ZIF crystals with different morphologies can be obtained. The present invention successfully realizes the morphology regulation of ZIF crystals from zero-dimensional to three-dimensional by the surface-confined growth method, and prepares multi-morphology, high-porosity, large specific surface area, and structurally stable multi-dimensional ZIF crystals.

[0065] In the present invention, when synthesizing ZIF-8 nanoparticles, the zinc precursor is preferably selected from one of zinc acetylacetonate Zn(acac)2, zinc nitrate Zn(NO3)2, zinc sulfate ZnSO4, and zinc perchlorate Zn(ClO4)2, and more preferably one of zinc nitrate Zn(NO3)2 and zinc sulfate ZnSO4. In the present invention, the zinc precursor is used to provide metal center ions, which coordinate with the organic ligand to form ZIF crystals.

[0066] In the present invention, when synthesizing ZIF-8 nanoparticles, the mass ratio of the zinc precursor is preferably 0.1-10%, more preferably 0.5-3.3%. The present invention preferably controls the mass of the zinc precursor within the above range, which is beneficial to the coordination effect.

[0067] In the present invention, when synthesizing ZIF-8 nanoparticles, the organic ligand is preferably selected from one of 2-methylimidazole, ethylenediamine, and polyvinylpyrrolidone, more preferably one of 2-methylimidazole and ethylenediamine. In the present invention, after the organic ligand is deprotonated, it coordinates with the metal center ion to form a ZIF crystal.

[0068] In the present invention, when synthesizing ZIF-8 nanoparticles, the mass ratio of the organic ligand is preferably 0.05-20%, more preferably 0.2-7%. The present invention preferably controls the mass of the organic ligand within the above range, which is beneficial to the coordination effect.

[0069] In the present invention, when synthesizing ZIF-8 nanoparticles, the solvent is selected from one of water, N,N-dimethylformamide, N,N-dimethylacetamide, and methanol, more preferably one of water and methanol. In the present invention, the solvent deprotonates the organic ligand, which is beneficial to the coordination effect.

[0070] In the present invention, when synthesizing ZIF-8 nanoparticles, the mass ratio of the solvent is 70-99.85%, more preferably 89.7-99.3%. The present invention preferably controls the mass of the solvent within the above range, which is beneficial to the coordination effect.

[0071] In the present invention, when synthesizing ZIF-8 nanoparticles, the stirring rate is 100-1600 rpm, more preferably 500-1000 rpm, the centrifugation rate is 1000-10000 r / min, more preferably 5000-10000 r / min, and the drying temperature is 40-100 °C, more preferably 60-80 °C;

[0072] In the present invention, the PVDF powder is selected from one of them, more preferably one of them. In the present invention, PVDF is used as a polymer membrane to coat the ZIF-8 seeds, and after surface-confined growth, a ZIFF-8@PVDF composite membrane is formed.

[0073] In the present invention, the mass ratio of the PVDF powder is 10-20%, more preferably 11-15%. The present invention preferably controls the mass of the PVDF powder within the above range, which is beneficial to phase inversion film formation.

[0074] In the present invention, the mass ratio of the ZIF-8 powder is 0.1 to 2%, more preferably 0.5 to 1%. The present invention preferably controls the mass of the ZIF-8 powder within the above range, which is beneficial to the growth of crystal seeds and reduces the impact on the casting solution.

[0075] In the present invention, the organic solvent is selected from one of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone, more preferably one of N,N-dimethylformamide and N,N-dimethylacetamide. In the present invention, the solvent dissolves the PVDF powder to form a casting solution.

[0076] In the present invention, the mass ratio of the organic solvent in step 2 is 80 to 90%, more preferably 85 to 89%. The present invention preferably controls the mass of the solvent within the above range, which is beneficial to the preparation of the casting solution.

[0077] In the present invention, the stirring rate is 100 to 600 rpm, more preferably 200 to 400 rpm; the stirring time is 4 to 24 h, more preferably 6 to 12 h; the stirring temperature is 40 to 120 °C, more preferably 60 to 90 °C. The present invention controls the above conditions within the above range, which is beneficial to the successful preparation of the casting solution.

[0078] In the present invention, the defoaming vacuum degree of the casting solution is 0.01 to 0.1 MPa, more preferably 0.05 to 0.1 MPa; the defoaming temperature is 25 to 120 °C, more preferably 40 to 90 °C. The present invention preferably controls the above conditions within the above range, which is beneficial to the defoaming of the casting solution and forms a homogeneous casting solution without bubbles.

[0079] In the present invention, the casting solution containing ZIF nanocrystals is scraped onto a substrate, and then the substrate is placed in a coagulation bath for phase inversion to obtain a film containing ZIF nanocrystals.

[0080] In the present invention, the scraping is specifically preferably: pouring the casting solution onto the smooth side of the substrate, and then evenly and quickly scraping from one side to the other side with a scraper.

[0081] In the present invention, the substrate is preferably a glass plate.

[0082] In the present invention, the notch thickness of the scraper used for scraping is preferably 100 to 600 μm, more preferably 150 to 300 μm. The present invention preferably uses a scraper with the above notch thickness for scraping to ensure that the mechanical properties of the film can meet the application requirements.

[0083] In the present invention, the phase inversion is preferably carried out at room temperature; the phase inversion time is preferably 2 min to 48 h. In the present invention, the more sufficient the phase inversion time, the higher the degree of phase inversion of the film.

[0084] In the present invention, the coagulation bath preferably comprises water or a mixed solution of water and an organic solvent, and the temperature is 25 to 50 °C; the mixed coagulation bath is composed of water and an organic solvent, wherein the organic solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone, and the volume ratio of water to the organic solvent is 1:(0.1 to 2.0); the drying method is one or a combination of methods such as air drying at room temperature, vacuum drying, and atmospheric pressure heating drying.

[0085] In the present invention, the zinc precursor is preferably selected from one of zinc acetylacetonate Zn(acac)2, zinc nitrate Zn(NO3)2, zinc sulfate ZnSO4, and zinc perchlorate Zn(ClO4)2, and more preferably one of zinc nitrate Zn(NO3)2 and zinc sulfate ZnSO4. In the present invention, the zinc precursor is used to provide metal central ions, so as to coordinate with organic ligands to form ZIF crystals.

[0086] In the present invention, the mass ratio of the zinc precursor in solution C is preferably 0.05 to 4.5%, more preferably 0.25 to 2.4%. The present invention preferably controls the mass ratio of the zinc precursor within the above range, which is beneficial to the coordination effect.

[0087] In the present invention, the organic ligand is preferably selected from one of 2-methylimidazole, ethylenediamine, and polyvinylpyrrolidone, and more preferably one of 2-methylimidazole and ethylenediamine. In the present invention, after the organic ligand is deprotonated, it coordinates with metal central ions to form ZIF crystals.

[0088] In the present invention, the mass ratio of the organic ligand in solution C is preferably 0.01 to 4.5%, more preferably 0.15 to 2.2%. The present invention preferably controls the mass ratio of the organic ligand within the above range, which is beneficial to the coordination effect.

[0089] In the present invention, the crystallization regulator is preferably selected from one of cetyltrimethylammonium bromide CTAB, 1-methylimidazole, sodium formate, and n-butylamine, and more preferably one of 1-methylimidazole and sodium formate. In the present invention, the crystallization regulator can regulate the pH of the precursor solution, thereby controlling the deprotonation rate, and thus regulating the morphology of ZIF crystallization.

[0090] In the present invention, the mass ratio of the crystallization regulator in solution C is preferably 0.01 to 4.5%, more preferably 0.25 to 2.4%. The present invention preferably controls the mass of the crystallization regulator within the above range, which is beneficial to precisely regulating the morphology.

[0091] In the present invention, the solvent is preferably selected from one of water, N,N-dimethylformamide, N,N-dimethylacetamide, methanol, and ethanol, and more preferably one of N,N-dimethylformamide, methanol, and ethanol. In the present invention, the organic solvent can also play a certain role in the process of deprotonating the raw materials, accelerating the deprotonation process, and being beneficial to the synthesis of ZIF crystals.

[0092] In the present invention, the mass ratio of the organic solvent is preferably 86.5-99.93%, more preferably 93-99.35%. The present invention preferably controls the mass ratio of the organic solvent within the above range, which is beneficial to the synthesis of ZIF crystals.

[0093] In the present invention, when stirring solution C, the stirring rate is 100-1200 rpm, more preferably 500-1000 rpm; the stirring time is 10-120 min, more preferably 10-60 min.

[0094] In the present invention, the temperature of the solvothermal reaction is preferably 40-160 °C, more preferably 60-100 °C; the reaction time is preferably 2-24 h, more preferably 4-12 h. The present invention preferably conducts the reaction under the above conditions, which can enable the rapid and uniform coordination of the zinc precursor and the organic ligand, and is beneficial to the regulation of the morphology of ZIF crystals.

[0095] The present invention uses the surface confinement growth method to regulate the morphology of ZIF nanocrystals. By adjusting the content of the crystallization regulator or the content of the zinc precursor and the organic ligand, the deprotonation rate of ZIF nanocrystals and reaction raw materials can be controlled. The growth control process from zero-dimensional to three-dimensional nanocrystals can be realized at a relatively low temperature, and a porous ZIF crystal material with high porosity, further increased specific surface area, and stable structural morphology can be prepared. The prepared MOF composite membrane has great potential in oil-water separation and dye adsorption.

[0096] The present invention provides an MOF composite membrane prepared by the preparation method described in the above technical solution.

[0097] The present invention also provides the application of the MOF composite membrane described in the above technical solution in oil-water emulsion separation and organic dye adsorption.

[0098] In the present invention, the application of the MOF composite membrane in separating oil-water emulsion is preferably as follows: adding oil and surfactant into deionized water, and obtaining a surfactant-stabilized oil-water emulsion through mechanical stirring; then separating it using the MOF composite membrane.

[0099] In the present invention, the oil substance is preferably toluene. In the present invention, the surfactant is preferably the non-ionic surfactant Tween 80. In the present invention, the volume ratio of the oil substance to deionized water is preferably 1:99; the concentration of the surfactant is 0.1 mg / ml.

[0100] In the present invention, the rate of the mechanical stirring is preferably 200 - 2000 rpm; the time of the mechanical stirring is 2 - 24 h. In the present invention, the driving force for separation is preferably gravity or transmembrane pressure; the pressure for separation is preferably 0.01 - 0.2 MPa.

[0101] There are no special limitations on the operation of the application of the MOF composite membrane in removing organic dyes in the present invention, and the application of crystal particles well-known to those skilled in the art in removing organic dyes can be adopted.

[0102] In the present invention, the application of the MOF composite membrane in the adsorption of organic dyes is preferably as follows: adding the organic dye into deionized water, and obtaining an organic dye solution through mechanical stirring; then using multi-morphology ZIF crystals for adsorption treatment.

[0103] In the present invention, the organic dye is preferably methylene blue, and the concentration of the organic dye is 400 mg / L.

[0104] In the present invention, the removal method of the MOF composite membrane for organic dyes is preferably the oscillating adsorption method; the oscillation rate is preferably 20 - 100 rpm; the oscillation time is preferably 0.5 - 6 h; the oscillation temperature is preferably 20 - 50 °C. In the present invention, the organic dyes preferably include cationic dyes or anionic dyes; the cationic dyes preferably include rhodamine B, crystal violet, malachite green or methylene blue; the anionic dyes preferably include orange G or methylene blue.

[0105] In order to make the technical means, creative features, achieved purposes and functions of the present invention easy to understand, the technical solutions in the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the protection scope of the present invention.

[0106] Example 1

[0107] (1) Dissolve 3 g of Zn(NO3)2 in 100 ml of methanol solvent uniformly. Dissolve 6.6 g of 2-methylimidazole in 100 ml of methanol uniformly. Pour the 2-methylimidazole solution into the Zn(NO3)2 solution, stir at a rate of 800 rpm for 12 h, add methanol repeatedly and centrifuge three times at a rate of 8000 r / min, and dry at 80 °C to obtain ZIF-8 nanoparticles.

[0108] (2) Add 11 g of PVDF powder and 1 g of ZIF-8 powder to 89 g of N,N-dimethylacetamide, and stir at a rate of 300 rpm for 8 h to obtain a homogeneous casting solution containing ZIF-8 nanoparticles.

[0109] (3) Obtain a polymer membrane coated with ZIF-8 seeds by the non-solvent induced phase separation method, and dry for later use.

[0110] (4) Add 3 g of Zn(NO3)2 and 0.69 g of sodium formate to 90 ml of methanol solvent, and stir and mix evenly to obtain solution A.

[0111] (5) Add 1.2 g of 2-methylimidazole to 90 ml of methanol solvent, and stir and mix evenly to obtain solution B.

[0112] (6) Pour solution B into solution A, and stir and mix evenly to obtain solution C.

[0113] (7) Fix the polymer membrane coated with ZIF-8 seeds in solution C, and use the solvothermal method to react at 90 °C for 6 h for surface-confined growth to obtain a MOF composite membrane.

[0114] (8) Wash the obtained MOF composite membrane with methanol repeatedly three times, and place it in an oven at 60 °C to dry.

[0115] Figure 1 is the scanning electron microscope image of the ZIF nanocrystals prepared in step (1) of Example 1. It can be seen from the image that crystalline particles with an initial shape of rhombic dodecahedron are prepared, belonging to zero-dimensional nanocrystalline particles.

[0116] Figure 2 is the XRD picture of the comparison between the ZIF-8 prepared in step (1) of Example 1 and the ZIF-8 standard card (simulated ZIF-8). It can be seen from the figure that the prepared ZIF crystals have a high degree of coincidence with the XRD of the standard card, and ZIF crystals are successfully prepared.

[0117] From Figure 3 (left), it can be seen that in this example, a continuous sheet crystal structure different from the nanocrystals is grown, and the crystal size range changes from the nanoscale to the microscale.

[0118] Example 2

[0119] This example is the same as Example 1 except for steps (4) and (5). Steps (4) and (5) are specifically as follows: (4) Add 1.5 g of Zn(NO3)2 and 0.69 g of sodium formate to 90 ml of methanol solvent, and stir and mix evenly to obtain solution A.

[0120] (5) Add 0.6 g of 2-methylimidazole to 90 ml of methanol solvent, and stir and mix evenly to obtain solution B.

[0121] From Figure 4 (left), it can be seen that a continuous rod-like structure different from the nanocrystals has grown in this example, and the crystal size range has changed from the nanoscale to the micron scale.

[0122] Example 3

[0123] This example is the same as Example 1 except for steps (4) and (5). Steps (4) and (5) are specifically as follows:

[0124] (4) Add 0.75 g of Zn(NO3)2 and 0.69 g of sodium formate to 90 ml of methanol solvent, and stir and mix evenly to obtain solution A. (5) Add 0.3 g of 2-methylimidazole to 90 ml of methanol solvent, and stir and mix evenly to obtain solution B.

[0125] From Figure 5 (left), it can be seen that a continuous blocky stacked structure different from the nanocrystals has grown in this example, and the crystal size range has changed from the nanoscale to the micron scale.

[0126] It can be seen from the SEM images that the contents of zinc nitrate and 2-methylimidazole in Examples 1 - 3 decrease in turn, while the content of the crystallization regulator remains unchanged. The crystallization regulator promotes the deprotonation of the organic ligand during the growth process. The lower the content of the organic ligand, the faster the deprotonation rate, resulting in fast nucleation, slow growth, and reduced size.

[0127] Example 4

[0128] This example is the same as Example 1 except for steps (4) and (5). Steps (4) and (5) are specifically as follows:

[0129] (4) Add 6 g of Zn(NO3)2 and 0.69 g of sodium formate to 90 ml of methanol solvent, and stir and mix evenly to obtain solution A.

[0130] (5) Add 2.4 g of 2-methylimidazole to 90 ml of methanol solvent, and stir and mix evenly to obtain solution B.

[0131] From Figure 6It can be seen that a large number of homogeneous nucleated large ZIF-8 particles are formed on the membrane surface in this example. The large ZIF-8 particles cover the polymer membrane, block the membrane pores, and are difficult to clean.

[0132] Example 5

[0133] In this example, all steps except step (2) are the same as those in Example 1. Step (2) is specifically as follows:

[0134] (2) Add 11 g of PVDF powder and 0.1 g of ZIF-8 powder to 89 g of N,N-dimethylacetamide, and stir at a rate of 300 rpm for 8 h to obtain a homogeneous casting solution containing ZIF-8 nanoparticles.

[0135] Figure 7 This is the scanning electron microscope image of the MOF composite membrane prepared by surface-confined growth in this example. It can be seen from the image that a flower-like structure different from nanocrystals has grown, and the crystal size has changed from the nanoscale to the microscale. However, due to too few seeds on the surface of the polymer membrane, the continuously growing nanoparticles are difficult to form a continuous microscale morphology, and the improvement of the membrane performance is small.

[0136] Comparative Example 1

[0137] The difference between this comparative example and Example 1 is only that no crystallization regulator is added in step (4), and the other steps are the same. Step (4) is specifically as follows:

[0138] (4) Add 3 g of Zn(NO3)2 to 90 ml of methanol solvent, and stir and mix evenly to obtain solution A.

[0139] From Figure 3 (right) it can be seen that there is no special MOF morphology.

[0140] Comparative Example 2

[0141] The difference between this comparative example and Example 2 is only that no crystallization regulator is added in step (4), and the other steps are the same. Step (4) is specifically as follows:

[0142] (4) Add 1.5 g of Zn(NO3)2 to 90 ml of methanol solvent, and stir and mix evenly to obtain solution A.

[0143] From Figure 4 (right) it can be seen that there is no special MOF morphology.

[0144] Comparative Example 3

[0145] The difference between this comparative example and Example 3 is only that no crystallization regulator is added in step (4), and the other steps are the same. Step (4) is specifically as follows:

[0146] (4) Add 0.75 g of Zn(NO3)2 to 90 ml of methanol solvent, and stir and mix evenly to obtain solution A.

[0147] As can be seen from Figure 5 (right), there is no special MOF morphology.

[0148] Comparative Example 4

[0149] (1) Add 11 g of PVDF powder to 89 g of N,N-dimethylacetamide, and stir at a rate of 300 rpm for 8 h to obtain a homogeneous casting solution.

[0150] (2) Obtain a polymer membrane by non-solvent induced phase separation method and dry it for later use.

[0151] (3) Add 3 g of Zn(NO3)2 to 90 ml of methanol solvent, and stir and mix evenly to obtain solution A.

[0152] (4) Add 1.2 g of 2-methylimidazole to 90 ml of methanol solvent, and stir and mix evenly to obtain solution B.

[0153] (5) Pour solution B into solution A, and stir and mix evenly to obtain solution C

[0154] (6) Fix the polymer membrane in solution C, and soak it at room temperature for 12 h to obtain a MOF composite membrane;

[0155] (7) Wash the obtained MOF composite membrane with methanol three times repeatedly, and dry it in an oven at 60 °C.

[0156] The difference from Example 1 is only that: it does not contain ZIF-8 seeds and crystallization regulators

[0157] Figure 8 This is the scanning electron microscope image of the MOF composite membrane prepared in this comparative example. It can be seen that a layer of ZIF-8 nanoparticles is deposited on the membrane surface, and these nanoparticles are particularly easy to wash off.

[0158] Figure 9 These are the XRD pictures of Examples 1 to 3. As can be seen from the figures, the various ZIF morphologies grown still conform to the peak positions of ZIF crystallization, which are indeed the results of surface-confined growth of ZIF nanocrystals.

[0159] Application Example 1

[0160] 1 mL of toluene and 0.1 g of Tween 80 were added to 99 mL of deionized water, and mechanically stirred at 1000 rpm for 24 h to obtain a surfactant-stabilized emulsion. Then, the emulsion was poured into the separation device of the MOF composite membranes prepared in Examples 1-4 and Comparative Examples 1-3, and the oil-water separation test was carried out under the drive of pressure.

[0161] Figure 10 Figure 4 shows the physical and optical microscope images of the MOF composite membrane prepared by surface-confined growth in Example 1 before and after the oil-water separation test. It can be seen from the figure that there are many fine emulsion droplets in the emulsion before separation, and the corresponding physical image shows milky turbidity; while the physical image of the emulsion after separation is clear and transparent, and there are no emulsion droplets in the corresponding optical microscope image, indicating that the MOF composite membrane prepared by the invention has high separation ability for the emulsion and successfully retains the emulsion droplets.

[0162] Application Example 2

[0163] 0.02 g of methylene blue was added to 1000 mL of deionized water to prepare an organic dye solution with a concentration of 20 mg / L. Then, the MOF composite membrane in Example 1 was added to a certain volume of the organic dye solution and oscillated for the adsorption test.

[0164] Figure 11 Figure 14 shows the physical images of the MOF composite membrane prepared by surface-confined growth in this example before and after the organic dye adsorption test. It can be seen from the figure that the solution before adsorption is a dark blue solution with a high degree of dyeing, and it becomes a clear solution with a very low degree of dyeing after adsorption, indicating that the prepared MOF composite membrane has high adsorption ability for organic dyes.

[0165] Table 1 shows the statistical results of the test on the surface wettability, oil-water separation and dye adsorption performance of the MOF composite membranes prepared in Examples 1-4 and Comparative Examples 1-3.

[0166] Table 1

[0167]

[0168]

[0169] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made in form and details without departing from the scope defined by the claims of the present invention.

Claims

1. A method for preparing a MOF@polymer composite membrane for oil-water separation, characterized in that: The following steps are involved: 1) adding PVDF powder and ZIF-8 nanoparticles to an organic solvent and stirring until they are uniformly dissolved to obtain a PVDF casting solution containing ZIF-8 nanoparticles, wherein the mass proportion of the PVDF powder is 10-20%, the mass proportion of the ZIF-8 powder is 0.1-2%, and the mass proportion of the organic solvent is 80-90%; 2) vacuum degassing the PVDF casting solution containing ZIF-8 nanoparticles; 3) applying a PVDF film casting solution containing ZIF-8 nanoparticles onto a substrate, and then transferring the substrate to a coagulation bath for non-solvent-induced phase transformation to obtain a PVDF film containing ZIF-8 nanoparticles; 4) drying the PVDF membrane containing ZIF-8 nanoparticles for later use; 5) uniformly mixing the zinc precursor, the crystallization modifier and the solvent to obtain solution A; uniformly mixing the organic ligand and the solvent to obtain solution B; 6) adding solution B to solution A and stirring and mixing uniformly to obtain solution C, wherein a MOF precursor is formed in solution C, wherein the mass proportion of zinc precursor is 0.05-4.5%, the mass proportion of crystallization modifier is 0.01-4.5%, the mass proportion of organic ligand is 0.01-4.5%, and the mass proportion of solvent is 86.5-99.93%; 7) fixing the PVDF membrane containing ZIF-8 nanoparticles after drying in step 4) in a reaction container, adding solution C in step 6), and contacting the PVDF membrane containing ZIF-8 nanoparticles with a MOF precursor and a crystallization regulator by a solvothermal method to generate MOF crystals with a specific morphology on the surface of the PVDF membrane containing ZIF-8 nanoparticles; the MOF crystals with a specific morphology are continuous lamellar crystal structures, rod-like structures or block-like stacked structures; 8) The product obtained in step 7) is washed to obtain ZIF crystal@polymer composite membranes with different morphologies.

2. The method for preparing a MOF@polymer composite membrane for oil-water separation according to claim 1, characterized in that: In step 1), the mass proportion of ZIF-8 powder is 1%. In step 6), in solution C, the mass proportion of zinc precursor is 0.25-2.4%, the mass proportion of crystallization modifier is 0.25-2.4%, the mass proportion of organic ligand is 0.15-2.2%, and the mass proportion of solvent is 93-99.35%.

3. The method for preparing a MOF@polymer composite membrane for oil-water separation according to claim 1, characterized in that: In step 1), the mass proportion of ZIF-8 powder is 1%. Step 5) is: add 3g Zn(NO3)2 and 0.69g sodium formate into 90ml methanol solvent, stir and mix to obtain solution A; add 1.2g 2-methylimidazole into 90ml methanol solvent, stir and mix to obtain solution B.

4. The method for preparing a MOF@polymer composite membrane for oil-water separation according to claim 1, characterized in that: Step 5) is: add 1.5g Zn(NO3)2 and 0.69g sodium formate to 90ml methanol solvent, stir and mix to obtain solution A; add 0.6g 2-methylimidazole to 90ml methanol solvent, stir and mix to obtain solution B; or, add 0.75g Zn(NO3)2 and 0.69g sodium formate to 90ml methanol solvent, stir and mix to obtain solution A; add 0.3g 2-methylimidazole to 90ml methanol solvent, stir and mix to obtain solution B.

5. The method for preparing a MOF@polymer composite membrane for oil-water separation according to claim 1, characterized in that: In step 5), the crystallization modifier is selected from one of hexadecyltrimethylammonium bromide (CTAB), 1-methylimidazole, sodium formate, and n-butylamine.

6. The method for preparing a MOF@polymer composite membrane for oil-water separation according to claim 1, characterized in that: In step 5), the zinc precursor is selected from one of zinc acetylacetonate, zinc nitrate, zinc sulfate, and zinc perchlorate; the organic ligand is selected from one of 2-methylimidazole, ethylenediamine, and polyvinylpyrrolidone; and the solvent is selected from one of water, N,N-dimethylformamide, N,N-dimethylacetamide, and methanol.

7. The method for preparing a MOF@polymer composite membrane for oil-water separation according to claim 1, characterized in that: In step 7), the solvent thermal reaction conditions are: 40 ~ 160 ° C / 2 ~ 24 h.

8. A MOF@polymer composite membrane for oil-water separation prepared by any one of the preparation methods of claims 1-7.

9. Use of the MOF@polymer composite membrane according to claim 8 in oil-water separation.

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