Preparation method and application of a kind of polyvinyl polyamide positively charged nanofiltration membrane modified by in-situ growth of ZIF across base membrane

Abstract

The application discloses a preparation method and application of a ZIF modified polyvinyl polyamide positively charged nanofiltration membrane which is prepared by in-situ growth across a base film. First, an organic phase solution containing a trimesoyl chloride monomer is used to coat a support film, then a mixed aqueous solution of polyethyleneimine and zinc nitrate hexahydrate is coated on the support film to perform interfacial polymerization, the support film after polymerization is turned over, an aqueous 2-methylimidazole solution is coated on the surface, ZIF is grown in-situ across the base film, and the membrane is subjected to heat treatment and alcohol activation after the growth is completed. The method can make ZIF particles grow more uniformly in the interior of the membrane, avoids the interface gap caused by MOF agglomeration, and can prepare a high positive layer by using the reverse interfacial polymerization method of polyethyleneimine as an aqueous monomer, so that the water permeation flux is greatly improved while the magnesium ion retention is kept high, the problem of mutual restriction of permeability and selectivity is solved, the process is simple, and the preparation condition is mild.

Description

Technical fields:

[0001] This invention relates to the field of nanofiltration membranes, specifically to a method for preparing and applying a positively charged polyvinyl polyamide nanofiltration membrane modified by cross-base membrane back diffusion in situ growth. Background technology:

[0002] In the field of lithium extraction from salt lake brine, nanofiltration membrane technology is considered a core solution for achieving efficient separation of magnesium and lithium due to its advantages of low energy consumption and ion selectivity. Traditional nanofiltration membranes, based on the synergistic mechanism of the Donnan effect and size sieving effect, can preferentially retain high-valence ions (such as Mg2-) while allowing monovalent ions (such as Li-) to permeate. However, in actual salt lake brine systems, the difference in hydration radii between magnesium and lithium ions is relatively small, causing a sharp decline in the separation selectivity of conventional polyamide nanofiltration membranes for magnesium and lithium.

[0003] Currently, commercial nanofiltration membranes enhance magnesium ion retention by reducing pore size. However, the trade-off between permeability and selectivity drastically reduces membrane permeation flux. Furthermore, many commercial nanofiltration membranes have negatively charged surfaces, further hindering magnesium ion retention. Additionally, traditional polysulfone substrate membranes are expensive and complex to prepare. Therefore, using a low-cost, mechanically sound polyethylene (PE) battery separator to replace polysulfone membranes has become a research hotspot.

[0004] However, the inertness of the PE surface leads to a loose and porous polyamide layer formed by conventional interfacial polymerization, resulting in a low magnesium ion rejection rate. Some researchers have attempted to construct a dense positively charged layer on polyethylene (PE) using traditional diamine monomers such as piperazine (PIP) and trimesoyl chloride (TMC) through reverse interfacial polymerization. The resulting multilayer composite polyamide (TFC-PE / PA) nanofiltration membrane still exhibits negative charge and has a poor permeation flux of only 4.8 L·m -2 ·h -1 ·bar -1 Researchers have attempted to improve water flux by directly doping nanoparticles. However, the interfacial polymerization of traditional nanomaterials in aqueous or oil phases inevitably leads to nanoparticle aggregation, forming non-selective defects and thus reducing membrane separation performance. Therefore, improving water permeation flux while maintaining magnesium-lithium separation performance is one of the current challenges in magnesium-lithium separation technology. Summary of the Invention:

[0005] To address the above shortcomings, this invention provides a method for preparing and applying a ZIF-modified positively charged polyvinyl polyamide nanofiltration membrane grown in situ via cross-base membrane back diffusion, which significantly improves water permeation flux while ensuring magnesium-lithium separation performance.

[0006] A method for preparing a ZIF-modified positively charged polyvinyl polyamide nanofiltration membrane by in-situ reverse diffusion growth across a substrate membrane, comprising the following steps:

[0007] First, an organic phase solution containing trimesoyl chloride monomer is cast onto one side of the supporting substrate membrane. Then, a mixed aqueous phase solution containing polyethyleneimine monomer and zinc nitrate hexahydrate is cast onto the substrate membrane to carry out reverse interfacial polymerization. After the reaction is completed, the imidazole solution is reverse diffused onto the side of the supporting substrate membrane that has not undergone interfacial polymerization, and a composite nanofiltration membrane with uniformly grown ZIF is formed in situ. This is the ZIF-modified positively charged polyvinyl polyamide nanofiltration membrane grown in situ through cross-substrate reverse diffusion.

[0008] The specific steps are as follows:

[0009] Step 1: Prepare a mixed aqueous solution of polyethyleneimine and zinc ions as an aqueous phase solution, prepare an alkane solution of aromatic acyl chloride as an organic phase solution, and prepare an aqueous or alcoholic solution containing imidazole monomer as a ligand solution for back diffusion.

[0010] Step 2: Pour the organic phase solution prepared in Step 1 onto either side of the porous polyolefin support substrate, let it soak for a period of time, and remove the excess solution.

[0011] Step 3: Use the aqueous solution prepared in Step 1 to cast the support substrate film that is wetted with organic phase monomers obtained in Step 2. After the reverse interfacial polymerization has been carried out for a period of time, remove the excess solution.

[0012] Step 4: Pour the imidazole solution prepared in Step 1 onto the side of the porous polyolefin support substrate obtained in Step 3 that has not undergone interfacial polymerization. After reverse diffusion in situ growth for a period of time, remove the excess solution.

[0013] Step 5: Heat-treat and alcohol-activate the nanofiltration membrane obtained in step 4 to obtain a ZIF-modified positively charged polyvinyl polyamide nanofiltration membrane grown in situ via cross-base membrane back diffusion.

[0014] The polyethyleneimine monomer has a molecular weight of 700 to 70,000 Daltons, the acyl chloride monomer is pyromellitic tricarboxylic acid chloride, the alkane solution is one or more of n-hexane, cyclohexane, ethyl acetate, and dodecane, the imidazole monomer is one or more of dimethylimidazolium and benzimidazole, and the ligand solution for dissolving the imidazole monomer is one or more of deionized water, methanol, and ethanol.

[0015] In step 1: the concentration of pyromellitic methyl chloride in the prepared organic phase solution is 0.01–0.1 w / v% (i.e., the ratio of aromatic methyl chloride to alkane in the organic phase solution is 0.01–0.1 g: 100 mL), the concentration of polyethyleneimine solution in the prepared aqueous phase solution is 0.1–2.0 w / v% (i.e., the ratio of polyethyleneimine to water in the aqueous phase solution is 0.1–2.0 g: 100 mL), the concentration of zinc ions in the prepared aqueous phase is 0.0002–0.002 mol / L, and the concentration of imidazole ligand is 0.003–0.03 mol / L;

[0016] In step 2: the organic phase solution is poured for 1 to 10 minutes;

[0017] In step 3: the aqueous solution is applied over a period of 1 to 10 minutes;

[0018] In step 4: the imidazole solution is applied over a period of 1 to 60 minutes;

[0019] In step 5: the drying temperature is 50-80℃, the drying time is 10-20 minutes, the alcohol type used for alcohol activation is isopropanol, and the alcohol activation time is 1-3 minutes;

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] (1) Using polyolefin base film saves costs, the operating conditions are relatively mild, the preparation process is simple, and it is suitable for industrial production.

[0022] (2) Due to the trade-off effect, the magnesium ion rejection rate will decrease significantly when the permeation flux is increased. We broke this balance by using reverse interfacial polymerization combined with cross-base membrane reverse diffusion in situ growth of ZIF. This method not only ensures the magnesium ion rejection rate but also greatly increases the permeation flux. Furthermore, the ZIF particles grown by this method are more uniformly distributed, effectively avoiding interfacial gaps caused by agglomeration.

[0023] (3) In-situ growth of ZIF-modified polyvinyl polyamide positively charged nanofiltration membranes via cross-base membrane back diffusion is beneficial for maintaining high water flux and magnesium ion rejection as well as low lithium ion rejection.

[0024] This invention discloses a method for preparing a positively charged polyvinyl polyamide nanofiltration membrane modified by ZIF in situ reverse diffusion growth across the base membrane. The method uses an aqueous solution of polyethyleneimine monomer rich in amine groups to improve the retention of magnesium ions in the composite membrane through reverse interfacial polymerization. The reverse diffusion in situ growth of ZIF improves the interfacial compatibility between the MOF material and the polyamide layer and provides additional water channels to enhance the hydrophilicity of the membrane, thereby significantly improving the membrane permeation flux. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the process for preparing the nanofiltration membrane according to the present invention.

[0026] Figure 2 Electron micrographs of the membrane surfaces of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3.

[0027] Figure 3 The diagram shows a comparison of the water contact angles of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3.

[0028] Figure 4 The roughness comparison diagrams are for Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3.

[0029] Figure 5 The images are infrared characterization images of Example 1 and Comparative Example 1.

[0030] Figure 6 The graph shows a comparison of water flux and salt interception performance between Examples 1-4 and Comparative Examples 1-4.

[0031] Figure 7 Mg from Examples 1, 2, and 3 2+ / Li + Separation performance diagram.

[0032] Figure 8 The pressure resistance and operational stability of Example 1 are shown. Detailed Implementation

[0033] To illustrate the present invention in more detail, a detailed description will be provided below, but it is not limited thereto. All matters not described in detail in the present invention are based on conventional techniques in the art.

[0034] like Figure 1 As shown, the method of the present invention includes the following steps:

[0035] Step 1: Prepare a mixed aqueous solution of polyethyleneimine and zinc ions as an aqueous phase solution, prepare an alkane solution of aromatic acyl chloride as an organic phase solution, and prepare an aqueous or alcoholic solution containing imidazole monomer as a reverse diffusion imidazole solution.

[0036] Step 2: Pour the organic phase solution prepared in Step 1 onto one side of the porous polyolefin support substrate, and remove the excess solution after wetting.

[0037] Step 3: Use the aqueous solution prepared in Step 1 to cast the support substrate film that is wetted with organic phase monomers obtained in Step 2. After reverse interfacial polymerization, remove the excess solution to obtain a porous polyolefin support material.

[0038] Step 4: The surface of the porous polyolefin support material obtained in Step 3 that has not undergone interfacial polymerization is coated with the reverse diffusion imidazole solution prepared in Step 1. After reverse diffusion in situ growth, the excess solution is removed to obtain the nanofiltration membrane.

[0039] Step 5: Heat-treat and alcohol-activate the nanofiltration membrane obtained in step 4 to obtain a ZIF-modified positively charged polyvinyl polyamide nanofiltration membrane grown in situ via cross-base membrane back diffusion.

[0040] Materials used in this invention: There are no special restrictions on the source of all raw materials in this invention and the following embodiments; they can be commercially available.

[0041] Membrane flux testing method for high-selectivity lithium-magnesium separation membranes: A membrane permeation selectivity performance testing system was used to test the membrane's water permeation flux and salt rejection rate. The testing system included a pump, membrane tank, piping, regulating valves, and pressure and flow detectors. The effective membrane area tested was 19.64 cm². 2 The test pressure was 6 bar, and the test temperature was 25 ± 0.5 °C. The salt concentrations for testing the single salt rejection rate were 1000 ppm for both MgCl2 and LiCl.

[0042] Example 1:

[0043] Prepare 50 mL of a hexane solution containing 0.05% trimesoyl chloride (i.e., the ratio of trimesoyl chloride to hexane is 0.05 g: 100 mL) as the organic phase solution; prepare 50 mL of a mixed aqueous solution containing 1.0% polyethyleneimine (70,000 molecular weight) and 0.0006 mol / L zinc nitrate hexahydrate as the aqueous phase solution (i.e., the ratio of polyethyleneimine to water is 1.0 g: 100 mL); and prepare 50 mL of an aqueous solution containing 0.009 mol / L 2-methylimidazole as the anti-diffusion ligand solution.

[0044] First, the organic phase solution was poured onto the surface of the polyolefin support membrane and kept for 4 minutes before removing the excess solution. Then, the aqueous phase solution was poured onto the membrane surface wetted by the organic phase and the interfacial polymerization reaction was carried out for 4 minutes before removing the excess solution. The liquid on the membrane surface was then dried. The membrane was then flipped to the side that had not undergone interfacial polymerization and a 2-methylimidazolium ligand aqueous solution was poured onto it and reacted for 30 minutes before removing the excess solution. The membrane was then placed in a 60°C oven to dry for 15 minutes. After drying, the membrane was immersed in isopropanol for 1 minute and the membrane surface was washed with water before testing.

[0045] The test result is: water flux 15.11 L·m -2 ·h -1 ·bar -1 The retention rate for magnesium chloride was 96.6%, and the retention rate for lithium chloride was 40.1%.

[0046] like Figure 2 As shown, the membrane surface of Example 1 exhibits a significant wrinkled morphology. This unique surface microstructure effectively increases the surface roughness of the membrane material, which is achieved through… Figure 4 The results of atomic force microscopy roughness analysis were further validated. Increased surface roughness is beneficial for enhancing the wettability of material surfaces, which is related to... Figure 3 The results of the contact angle test are consistent: Example 1 showed the smallest water contact angle of 69°, and due to its excellent surface hydrophilicity, the membrane material exhibited the highest water flux in the filtration experiment.

[0047] Example 2: Refer to Example 1, except that the back diffusion time of the 2-methylimidazolium ligand aqueous solution is 45 minutes.

[0048] The test result was: water flux 14.77 L·m -2 ·h -1 ·bar -1 The retention rate for magnesium chloride was 95.7%, and the retention rate for lithium chloride was 42.2%.

[0049] Example 3: Refer to Example 1, except that the back diffusion time of the 2-methylimidazolium ligand aqueous solution is 60 minutes.

[0050] The test result was: water flux 13.8 L·m -2 ·h -1 ·bar -1 The retention rate for magnesium chloride was 95.85%, and the retention rate for lithium chloride was 40%.

[0051] Example 4: Similar to Example 1, except that the 2-methylimidazole ligand solution is a mixture of 50% methanol and water.

[0052] The test result was: water flux 14.78 L·m -2 ·h -1 ·bar -1 The retention rate for magnesium chloride was 95.65%, and the retention rate for lithium chloride was 41%.

[0053] Comparative Example 1:

[0054] Prepare 50 mL of a hexane solution containing 0.05% trimesoyl chloride as the organic phase solution, and prepare 50 mL of an aqueous solution containing 1.0% polyethyleneimine (70,000 molecular weight) as the aqueous phase solution.

[0055] First, the organic phase solution is poured onto the surface of the polyolefin support membrane and kept for 4 minutes before removing the excess solution. Then, the aqueous phase solution is poured onto the membrane surface that is wetted by the organic phase. After the interfacial polymerization reaction is carried out for 4 minutes, the excess solution is removed. The liquid on the membrane surface is dried by blowing, and the membrane is placed in a 60°C oven to dry for 15 minutes. After drying, it is immersed in isopropanol for 1 minute and the membrane surface is washed with water before testing.

[0056] The test result was: water flux 5.8 L·m -2 ·h -1 ·bar -1 The retention rate for magnesium chloride was 97.9%, and the retention rate for lithium chloride was 50.2%.

[0057] Comparative Example 2: Similar to Comparative Example 1, except that the aqueous phase solution was a mixed aqueous solution of 1.0% polyethyleneimine (70,000 molecular weight) and 0.0006 mol / L zinc nitrate hexahydrate.

[0058] The test result was: water flux 7.65 L·m -2 ·h -1 ·bar -1 The retention rate for magnesium chloride was 96.1%, and the retention rate for lithium chloride was 38.5%.

[0059] Comparative Example 3:

[0060] Prepare 50 mL of a hexane solution containing 0.05% trimesoyl chloride as the organic phase solution; prepare 50 mL of a mixed aqueous solution containing 1.0% polyethyleneimine (70,000 molecular weight) and 0.0006 mol / L zinc nitrate hexahydrate as the aqueous phase solution; and prepare 50 mL of an aqueous solution containing 0.009 mol / L 2-methylimidazole as the anti-diffusion ligand solution.

[0061] First, the organic phase solution was poured onto the surface of the polyolefin support membrane and kept for 4 minutes before removing the excess solution. Then, the aqueous phase solution was poured onto the surface of the polyolefin support membrane that was wetted by the organic phase and the interfacial polymerization reaction was carried out for 4 minutes before removing the excess solution. The liquid on the membrane surface was then dried. The 2-methylimidazolium ligand aqueous solution was poured onto the side that had undergone interfacial polymerization and reacted for 30 minutes before removing the excess solution. The membrane was then placed in a 60°C oven to dry for 15 minutes. After drying, the membrane was immersed in isopropanol for 1 minute and the membrane surface was washed with water before testing.

[0062] The test result was: water flux 11.2 L·m -2 ·h -1 ·bar -1 The retention rate for magnesium chloride was 96.05%, and the retention rate for lithium chloride was 37.65%.

[0063] Comparative Example 4: Prepare 50 mL of a hexane solution containing 0.05% trimesoyl chloride as the organic phase solution, and prepare 50 mL of a mixed solution containing 1.0% polyethyleneimine (70,000 molecular weight) and 0.02% ZIF-8 nanoparticles as the aqueous phase solution. First, pour the organic phase solution onto the surface of the polyolefin support membrane, keep it for 4 minutes, and then remove the excess solution. Then, pour the aqueous phase solution onto the membrane surface wetted by the organic phase, react for 4 minutes, and then remove the excess solution. Blow the liquid off the membrane surface, place the membrane in a 60°C oven to dry for 15 minutes, immerse it in isopropanol for 1 minute, and then wash the membrane surface with water for testing.

[0064] The test result was: water flux 11.2 L·m -2 ·h -1 ·bar -1 The retention rate for magnesium chloride was 96.5%, and the retention rate for lithium chloride was 42.8%.

[0065] From the following Figure 6 As can be seen, Examples 1, 2, 3, and 4 have higher water flux and maintain a higher magnesium ion rejection rate compared to Comparative Examples 1, 2, 3, and 4. Figure 7 It can also be seen that Example 1 has a relatively high magnesium-lithium separation ratio of 17.65, slightly lower than that of Comparative Example 1 (21), but its water permeation flux is 2 to 3 times higher than that of Comparative Example 1, showing a significant advantage. In the infrared characterization image, the Zn-N tensile vibration mode of Example 1 can be observed at 421 cm⁻¹. -1 The observations indicate the successful growth of ZIF-8 nanoparticles. Electron micrographs show that Example 1 has a large number of wrinkled structures on its surface, significantly increasing its roughness. The roughness values ​​of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 are 95 nm, 12.3 nm, 19.7 nm, and 61.9 nm, respectively, indicating that Example 1 has better hydrophilicity. This is also evident from the water contact angle diagrams. The contact angle values ​​of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 are 69°, 94°, 84°, and 87°, respectively. Example 1 has the smallest contact angle, indicating the best hydrophilicity and permeability. Furthermore, Example 1 also exhibits good pressure resistance and operational stability.

Claims

1. A method for preparing a ZIF-modified positively charged polyvinyl polyamide nanofiltration membrane by in-situ reverse diffusion growth across a substrate membrane, characterized in that, Includes the following steps: Step 1: Prepare a mixed aqueous solution of polyethyleneimine and zinc ions as an aqueous phase solution, prepare an alkane solution of aromatic acyl chloride as an organic phase solution, and prepare an aqueous or alcoholic solution containing imidazole monomer as a reverse diffusion imidazole solution. The molecular weight of the polyethyleneimine monomer is between 700 and 70,000. The aromatic acyl chloride is pyromellitic acid chloride, and the solvent in the alkane solution of the aromatic acyl chloride is one or more of n-hexane, cyclohexane, ethyl acetate, and dodecane; The imidazole monomer is one or more of dimethylimidazolium and benzimidazole, the water is deionized water, and the alcohol is one or more of methanol and ethanol; Step 2: The organic phase solution prepared in Step 1 is poured onto one side surface of the porous polyolefin support substrate, and the excess solution is removed after wetting. Step 3: Use the aqueous solution prepared in Step 1 to cast the support substrate film that is wetted with organic phase monomers obtained in Step 2. After reverse interfacial polymerization, remove the excess solution to obtain a porous polyolefin support material. Step 4: The surface of the porous polyolefin support material obtained in Step 3 that has not undergone interfacial polymerization is coated with the reverse diffusion imidazole solution prepared in Step 1. After reverse diffusion in situ growth, the excess solution is removed to obtain the nanofiltration membrane. Step 5: Heat-treat and alcohol-activate the nanofiltration membrane obtained in step 4 to obtain a ZIF-modified positively charged polyvinyl polyamide nanofiltration membrane grown in situ via cross-base membrane back diffusion.

2. The preparation method according to claim 1, characterized in that, In step 1, the ratio of polyethyleneimine to water in the aqueous solution is 0.1~2.0g:100mL; the zinc ion concentration in the aqueous solution is 0.0002~0.002mol / L. The ratio of aromatic acyl chloride to alkane in the organic phase solution is 0.01~0.1g:100mL; The concentration of imidazole ligand in the reverse diffusion imidazole solution is 0.003~0.03 mol / L.

3. The preparation method according to claim 1, characterized in that, In step 2: the application time of the organic phase solution is 1 to 10 minutes.

4. The preparation method according to claim 1, characterized in that, In step 3: the application time for the aqueous solution is 1 to 10 minutes.

5. The preparation method according to claim 1, characterized in that, In step 4: the pouring time for the reverse diffusion imidazole solution is 1 to 60 minutes.

6. The preparation method according to claim 1, characterized in that, In step 5, the heat treatment conditions are: drying temperature of 50~80℃ and drying time of 10~20 minutes.

7. The preparation method according to claim 1, characterized in that, In step 5: the alcohol used for alcohol activation is isopropanol, and the activation time is 1-3 minutes.

8. The application of the ZIF-modified polyvinyl polyamide positively charged nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 7 in the separation of magnesium and lithium in water treatment.

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