Preparation method and application of self-assembled carboxyl type cationic membrane

Abstract

The application discloses a preparation method and application of a self-assembled carboxyl type cation membrane, and comprises the following preparation steps: firstly, a MIL-101-NH2 type MOFs material is prepared by using a solvothermal method; then, a small molecule containing a functional group with specific recognition characteristics for rare earth ions is used to post-modify and modify the MIL-101-NH2 type MOFs; and finally, the small molecule is grafted to the surface of PVDF activated by acrylic acid through an in-situ self-assembly mode, so that the self-assembled carboxyl type cation membrane is obtained. The self-assembled carboxyl type cation membrane prepared by the application has good selectivity, retention rate and stability for rare earth elements in waste FCC catalysts. In a simulated acid leaching solution with pH 6, the retention rate of the composite membrane can reach 95% within 30 min, and the retention efficiency can still be kept above 90% after 4 cycles, the membrane is simple to prepare and has controllable cost, and the application provides a new thought for efficient recycling of solid waste resources, and has good application prospect.

Description

Technical Field

[0001] The invention relates to a preparation method of a self-assembled carboxyl type cationic membrane and application thereof in efficient separation, extraction and utilization of rare earth elements in waste FCC catalysts, belonging to the field of membrane preparation and application. Background Art

[0002] Rare earth elements are considered key strategic resources necessary for the development of high-tech industries. However, their reserves are scarce and unevenly distributed worldwide. Therefore, the search, discovery, and extraction of rare earth elements are of great strategic significance to the development of a country's science and technology industries. Fluidized catalytic cracking (FCC) catalysts are important catalysts in the petrochemical industry and are used in huge quantities, with tens of thousands of tons of such catalysts discarded each year. Spent FCC catalysts use alumina as a carrier and rare earth elements as catalytically active ingredients. Their rare earth content is approximately 2%, and they are rich in valuable metal elements such as lanthanum and cerium. The rational and comprehensive utilization of this spent catalyst not only achieves the secondary recycling of valuable metal elements, thereby protecting the environment, but also complies with national policy requirements for the scientific treatment of hazardous solid waste, while also providing good economic, environmental, and social benefits.

[0003] Metal-organic frameworks (MOFs) possess advantages such as high porosity, low density, large specific surface area, regular pore structures, tunable pore size, diverse topological structures, and ease of modification, making them important applications in gas storage, chemical separation, and sensing. The introduction of specific functional groups into MOFs can improve not only their adsorption properties and hydrophilicity but also their ion selectivity. However, powdered MOFs are difficult to recycle in liquid phase separations and suffer from poor recyclability, limiting their large-scale application. Membrane separation technology has emerged as a novel separation technique that combines high efficiency, environmental friendliness, and precision. Its ease of operation and manageable cost make it particularly suitable for industrial applications. MOF composite membranes have attracted widespread attention for their excellent selectivity and adsorption performance. The ease of modification and tunable pore size of MOFs underpin their efficient and selective separations. By enhancing the interaction between guest molecules and the multifunctional metal central ions or organic ligands, the separation performance of MOF membranes is improved, broadening the application of MOFs. Existing MOFs composite membranes are mostly based on metal oxides or tubular structures. Furthermore, MOFs are large, making it difficult to form a continuous growth pattern on such supports, and thus unable to ensure good mechanical strength of the membrane attached to the support. Polyvinylidene fluoride (PVDF) is an organic polymer material with unique advantages, exhibiting high thermal stability, mechanical strength, and chemical inertness. Its advantages as a membrane material are reflected in its ease of film formation, simple preparation process, low cost, and amenability to large-scale manufacturing. It is one of the most promising porous polymer membranes and is widely used in ultrafiltration, microfiltration, and separation and concentration. In recent years, PVDF-based membranes have been applied to oil-water separation and metal ion separation (including heavy metals, rare earth metals, and platinum group metals). The application of MOFs in the field of organic / inorganic hybrid membranes can effectively improve the compatibility between hybrid particles and polymer membranes. Introducing synthesized post-modified MOFs with specific recognition properties into PVDF membranes can not only enhance the separation performance and stability of the composite membranes, but also improve the adsorption specificity of PVDF membranes for rare earth ions, thereby enhancing adsorption performance. However, the PVDF-MOFs composite membranes reported so far are usually prepared by blending. Due to the large size of MOFs particles, they are unevenly dispersed and have weak interaction with PVDF, resulting in poor stability and thus affecting their performance. Therefore, the key to overcoming this problem is to first modify the PVDF support and then prepare the MOFs membrane through chemical bonding. Summary of the Invention

[0004] In order to improve the problem that powdered metal organic framework materials are difficult to recover and have poor recycling performance in liquid phase separation, the technical problem to be solved by the present invention is to provide a self-assembled carboxyl cationic membrane that is easy to produce and prepare, has specific selectivity for rare earth ions and is recyclable, aiming to generate chemical bonds between the MOFs material and the substrate, making the connection tighter and the resulting membrane more uniform.

[0005] The technical solution of the present invention to solve the above technical problems is as follows:

[0006] This invention first discloses a method for preparing a self-assembled carboxyl cationic membrane. The method is characterized by first preparing a MIL-101-NH2 MOF material using a solvothermal method, then post-modifying the MIL-101-NH2 MOF with small molecules containing functional groups that specifically recognize rare earth ions, and then grafting the modified materials onto an acrylic acid-activated PVDF surface via in situ self-assembly to obtain the self-assembled carboxyl cationic membrane. Research results show that the membrane exhibits excellent selectivity, retention rate, and recyclability for rare earth ions, demonstrating that the self-assembled carboxyl cationic membrane prepared by this invention has excellent stability and practical application potential for rare earth ion recovery.

[0007] Preferably, the method for preparing the self-assembled carboxyl-type cationic membrane comprises the following steps:

[0008] 1) Chromium nitrate, 2-aminoterephthalic acid, and sodium hydroxide were added to water and stirred at room temperature to thoroughly mix. The resulting solution was transferred to a stainless steel reactor and incubated at 150-200°C for 12-36 hours. After the reaction was complete, the resulting reaction solution was centrifuged and filtered, washed sequentially with DMF and ethanol, and dried in a vacuum oven at 60-80°C to obtain a green MIL-101-NH2 MOF material powder.

[0009] 2) The MIL-101-NH2 MOFs obtained in step 1) are mixed with a small molecule containing a functional group with specific recognition properties for rare earth ions. N,N-dicyclohexylcarbodiimide (DCC) is then added, and DMF is used as a solvent. The reaction is carried out at 120-150°C for 24-48 hours under nitrogen protection. After the reaction, the resulting product is centrifuged, washed 3-5 times with anhydrous ethanol, and dried at 60-80°C for 6-12 hours to obtain a green powder of post-modified MIL-101-NH2 MOFs.

[0010] 3) Immerse PVDF powder in an acetone solution of AIBN and stir until uniform. Then, add acrylic acid and react at 50-100°C under N2 protection for 4-8 hours to generate a large number of carboxyl groups on the surface. After the reaction, add the post-modified MIL-101-NH2 MOFs obtained in step 2) to the reaction solution. Stir for 30-60 minutes, then add the porogen PPA. Continue stirring in an oil bath at 50-80°C for 12-24 hours to allow the post-modified MIL-101-NH2 MOFs to be grafted onto the PVDF surface via in situ self-assembly, forming chemical bonds between the MOFs and the membrane, creating a tighter connection. The resulting casting solution is ultrasonically degassed for 1-3 hours. A film is cast onto a clean glass plate using a film caster to a desired thickness. The glass plate is then immediately placed in deionized water. After the film detaches automatically, the glass plate is removed and the detached film is soaked in water for 24-48 hours to produce a self-assembled carboxyl-type cationic membrane.

[0011] Preferably, the small molecule containing a functional group with specific recognition properties for rare earth ions is N-(phosphonomethyl)methylenediacetic acid hydrate, also known as PMIDA, and the resulting post-modified MOFs are PMIDA-M1N MOF particles (abbreviated as PMIDA-M1N). With amino groups as the center, PMIDA is used to post-modify MIL-101-NH2 MOFs, introducing abundant phosphate groups.

[0012] Preferably, in step 1), the molar ratio of chromium nitrate, 2-aminoterephthalic acid and sodium hydroxide is 1:(1-1.2):(1-2).

[0013] Preferably, in step 2), the molar ratio of the MIL-101-NH2 type MOFs material, small molecule, and N,N-dicyclohexylcarbodiimide is 1:(1-1.5):(1-2).

[0014] Preferably, in step 3), the usage ratio of PVDF, AIBN, acrylic acid, PPA and post-modified MIL-101-NH2 type MOFs is 4g:(0.5-2)g:(2-4)mL:(0.2-0.8)g:(2-4)g.

[0015] Preferably, in step 3), the thickness of the push film is 20-100 μm.

[0016] The self-assembled carboxyl type cationic membrane prepared by the preparation method can be used for rare earth element recovery, especially for efficient separation, extraction and utilization of rare earth elements in waste FCC catalysts.

[0017] The beneficial effects of the present invention are:

[0018] (1) In the self-assembled carboxyl cationic membrane prepared by the present invention, a chemical bond is generated between the MOFs material and the membrane, resulting in a tighter connection and a more uniform membrane;

[0019] (2) The preparation method of the self-assembled carboxyl cationic membrane of the present invention is simple and easy to operate, uses cheap raw materials, can be produced in large quantities, and is suitable for industrial application;

[0020] (3) The composite membrane prepared by the present invention has specific selectivity for rare earth ions and has good stability and recyclability. In addition, the membrane material obtained by the present invention can not only exert the chelation effect of the functional groups on rare earth ions and the pore size screening of ions by MOFs, but also fully utilize the high efficiency and green environmental protection advantages of membrane separation. This material can be used in the separation process of rare earth ions and has unique advantages in practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the preparation process of a self-assembled carboxyl-type cationic membrane of the present invention;

[0022] Figure 2 1 is an XRD analysis diagram of the self-assembled carboxyl-type cationic membrane prepared in Example 1 of the present invention;

[0023] Figure 3 This is an infrared spectrum of the self-assembled carboxyl-type cationic membrane prepared in Example 1 of the present invention;

[0024] Figure 4 Figure 1 is a SEM analysis of the self-assembled carboxyl cationic membrane prepared in Example 1 of the present invention, wherein a is a surface SEM analysis of PMIDA-M1N@PVDF, and b is a cross-sectional analysis of the membrane;

[0025] Figure 5 Graph showing chemical stability analysis of the self-assembled carboxyl-type cationic membrane prepared in Example 1 of the present invention;

[0026] Figure 6 Graph showing ion selectivity analysis of the self-assembled carboxyl-type cationic membrane prepared in Example 1 of the present invention;

[0027] Figure 7 The effect of the thickness of the self-assembled carboxyl cationic membrane prepared in Example 1 of the present invention on the rejection rate and water flux;

[0028] Figure 8 This is a recycling analysis diagram of the self-assembled carboxyl-type cationic membrane prepared in Example 1 of the present invention;

[0029] Figure 9This is an XRD stability analysis diagram of the self-assembled carboxyl cationic membrane prepared in Example 1 of the present invention after recycling, where a is the structure of PMIDA-M1N, b is the structure of PMIDA-M1N@PVDF before recycling, and c is the structure of PMIDA-M1N@PVDF after recycling. DETAILED DESCRIPTION

[0030] In order to more fully understand the present invention, this embodiment describes the present invention in more detail through the following non-limiting examples or comparative examples, and provides detailed implementation methods and specific operating processes, but the protection scope of the present invention is not limited to the following examples.

[0031] Example 1: Preparation of self-assembled carboxyl cationic membrane

[0032] 1) Preparation of MIL-101-NH2 MOFs: Chromium nitrate, 2-aminoterephthalic acid, and sodium hydroxide were added to water at a molar ratio of 1:1:2. The mixture was stirred at room temperature for 30 minutes to thoroughly mix. The resulting solution was transferred to a stainless steel reactor and incubated at 150°C for 12 hours. After the reaction, the resulting solution was centrifuged and filtered, washed sequentially with DMF and ethanol, and dried in a vacuum oven at 60°C for 12 hours to obtain a green MIL-101-NH2 MOF powder, designated M1N.

[0033] 2) Preparation of PMIDA-modified MIL-101-NH2 MOFs: MIL-101-NH2 MOFs, PMIDA (N-(phosphonomethyl)methylenediacetic acid hydrate), and DCC (N,N-dicyclohexylcarbodiimide) were added to a three-necked flask at a molar ratio of 1:1.5:2. DMF was added as the solvent and the mixture was reacted at 150°C for 48 hours under nitrogen. Upon completion of the reaction, the product was centrifuged, washed four times with anhydrous ethanol, and dried in a vacuum oven at 60°C for 6 hours to obtain a green powder, designated PMIDA-M1N.

[0034] 3) Preparation of a self-assembled carboxyl-type cationic membrane using an in situ self-assembly method: In a three-necked flask, 4g of dry PVDF powder was immersed in 10mL of acetone solution containing 1.8g of AIBN and stirred for 30min. 3mL of acrylic acid (AA) was then added to the flask, and the reaction was carried out in a 70°C oil bath under N2 protection for 5h. After the reaction, 3g of PMIDA-M1N was added to the flask and stirred for 30min. Then, 0.6g of the porogen PPA was added and stirred in a 60°C oil bath for 24h to form a uniform casting solution. The casting solution was ultrasonically degassed for 1h, and a film was cast onto a clean glass plate using a film caster to a desired thickness (20-100μm). The glass plate was then immediately placed in deionized water. After the film detached automatically, the glass plate was removed and the detached film was soaked in water for 48h to obtain the PMIDA-M1N@PVDF cationic membrane.

[0035] Example 2: Characterization of self-assembled carboxyl-type cationic membranes

[0036] The XRD analysis of PMIDA-M1N@PVDF ion membrane prepared in Example 1 is as follows: Figure 2 Figure 1 shows a PMIDA-M1N membrane (a), a PMIDA-M1N membrane (b), and a PMIDA-M1N@PVDF membrane (c). The PMIDA-M1N-PVDF membrane exhibits consistent diffraction peaks characteristic of MOFs, demonstrating that this composite membrane fabrication method does not disrupt the MOFs crystal structure and preliminarily confirming the successful preparation of PMIDA-M1N@PVDF.

[0037] The FT-IR analysis of PMIDA-M1N@PVDF cationic membrane prepared in Example 1 is as follows: Figure 3 As shown in the figure, the comparative analysis of a, b, and c shows that the characteristic peak of PVDF CF bond appears at 1400 cm -1 and 1180cm -1 , respectively -CF2 deformation vibration peak and stretching vibration, while 875cm -1 The characteristic absorption peak of the amorphous region of polyvinylidene fluoride. The comparison between PMIDA-M1N MOFs material and PMIDA-M1N@PVDF cationic membrane shows that the MOFs material has been successfully grafted onto the PVDF membrane.

[0038] The SEM of PMIDA-M1N@PVDF cationic membrane prepared in Example 1 is as follows: Figure 4 As shown by Figure 4 It can be seen from a that PMIDA-M1N@PVDF cationic membrane has a good porous structure. Figure 4The cross-sectional view in b shows that there are through-holes inside the membrane. The cross-linked structure generated between the substrate and MOF can greatly enhance the hardness and compressive resistance of the fiber and can be regarded as an ion transmission channel, indicating that PMIDA-M1N@PVDF cationic membrane has good ion transmission potential.

[0039] The chemical stability analysis of PMIDA-M1N@PVDF cationic membrane prepared in Example 1 is as follows Figure 5 As shown, the composite membrane was cut into the same size and immersed in acid or alkaline solutions with pH 2, 4, 6, 8, 10, and 12 for 12 hours respectively. After being taken out and dried, XRD scanning was performed. It can be clearly seen that there is a characteristic diffraction peak of MOFs at 2θ=8-10° and a characteristic diffraction peak of PVDF near 2θ=20°. The PMIDA-M1N@PVDF cationic membrane material can maintain a complete crystal structure under different acid and alkaline conditions and has good chemical stability and structural stability.

[0040] Example 3: Application of PMIDA-M1N@PVDF cationic membrane

[0041] Example 1 Analysis of ion selectivity of PMIDA-M1N@PVDF cationic membrane prepared: In order to understand the selectivity of the synthesized composite membrane to rare earth ions La and Ce, a waste FCC catalyst acid leaching simulation solution (K + , Ca 2+ 、Zn 2+ 、La 3+ 、Ce 3+ 、Ni 3+ 、Fe 3+ The concentrations of La and Ce are 15.85 mg / L and 1.995 mg / L, respectively. + , Ca 2+ 、Zn 2+ 、Ni 3+ 、Fe 3 + The concentrations were 38.06 mg / L, 84.08 mg / L, 13.09 mg / L, 15.79 mg / L, 15.79 mg / L), and the test K + , Ca 2+ 、Zn 2 + 、La 3+ 、Ce 3+ 、Ni 3+ 、Fe 3+ The retention of ions through the low-pressure membrane flux tester is as follows: Figure 6 As shown, the vertical axis is the separation coefficient β, and the calculation formula is shown in formulas (1) and (2):

[0042]

[0043]

[0044] Where R is the ion retention rate of the membrane, C0 is the initial concentration of the ion, C is the equilibrium concentration after retention, β A / B is the separation coefficient of ion A to B, obtained by comparing the retention rates of the two. Figure 6 It can be seen that among these metal ions, PMIDA-M1N@PVDF cationic membrane has the highest 3+ The retention rate of 3+ , and the third is Fe 3+ , the separation coefficients β of La, Ce and coexisting ions all follow β La / Zn >β La / Ca >β La / K >β La / Ni >β La / Fe The order of the maximum separation coefficient for Zn ions can reach 31.9. The results show that PMIDA-M1N@PVDF cationic membrane has a good separation effect on the rare earth element La in the acid leaching simulation solution of the spent catalyst. 3+ 、Ce 3+ Has good separation selectivity.

[0045] Effect of PMIDA-M1N@PVDF cationic membrane thickness on rejection rate R and water flux J prepared in Example 1: Membranes with thicknesses of 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm and 90 μm were placed in a low-pressure membrane flux tester, and a waste FCC catalyst acid leaching simulation solution (i.e., La 3+ and Ce 3+ A mixed solution of La and Ce (with concentrations of 15.85 mg / L and 1.995 mg / L, respectively) was passed through a low-pressure membrane flux tester. A certain amount of sample solution was taken out at 30 minutes, filtered, and diluted to measure ICP. The effect of membrane thickness on the rejection rate and water flux of PMIDA-M1N@PVDF cationic membrane for rare earth ions La and Ce was explored. The results are shown in Figure 2. Figure 7 The retention rate is calculated according to formula (1), and the water flux is calculated according to formula (3). The water flux and the retention rate of rare earth ions La and Ce of PMIDA-M1N@PVDF cationic membrane increase with the increase of membrane thickness. When the thickness increases to above 40μm, the retention rate of rare earth ions La and Ce of PMIDA-M1N@PVDF cationic membrane decreases with the increase of membrane thickness, indicating that the thicker the membrane, the more adsorption sites there are. However, if the membrane is too thick, the steric hindrance is too great, which is not conducive to the adsorption and retention of the membrane, reflecting the trade-off effect of the membrane.

[0046]

[0047] Where: J is the water flux, unit is mL·min -1 cm -2 ; t is the filtration time, unit is min; Q is the volume of solution that passes through in t time, unit is mL; A is the filtration area of ​​the membrane, unit is cm 2 .

[0048] The recyclability of the PMIDA-M1N@PVDF cationic membrane prepared in Example 1: After the PMIDA-M1N@PVDF cationic membrane was first adsorbed with La and Ce ions for 30 minutes by a membrane separation device, it was immersed in 1 mol / L dilute hydrochloric acid for 24 hours to fully elute the adsorbed metal ions. It was then immersed in pure water for 24 hours and naturally dried before the next adsorption. The dried membrane was repeated with the above experimental process 4 times. The results are as follows Figure 8 As shown, the vertical axis is the relative retention rate A%, and the calculation formula is shown in (4).

[0049]

[0050] Where R1 is the retention rate of the membrane during the first adsorption, R n is the retention rate of the membrane when it is used for the nth time.

[0051] As can be seen from the figure, after four repeated cycles, the relative interception rate of PMIDA-M1N@PVDF cationic membrane for rare earth ions La and Ce is still above 90%, which shows that the prepared PMIDA-M1N@PVDF cationic membrane has good recyclability. The membrane used in four cycles was dried and then subjected to XRD analysis. The results are as follows Figure 9 As shown in the figure, a is the structure of PMIDA-M1N, b is the structure of PMIDA-M1N@PVDF before recycling, and c is the structure of PMIDA-M1N@PVDF after recycling. After recycling, the membrane still maintains its original crystal structure, indicating that the membrane has good cycle stability.

[0052] The above description is merely an exemplary embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for preparing a self-assembled carboxyl-type cationic membrane, characterized in that: First, the MIL-101-NH2 type MOFs material was prepared by a solvothermal method. Then, the MIL-101-NH2 type MOFs was post-modified with a small molecule N-(phosphonomethyl)methylenediacetic acid hydrate containing a functional group with specific recognition characteristics for rare earth ions. Finally, it was grafted onto the acrylic acid-activated PVDF surface by in situ self-assembly to obtain a self-assembled carboxyl cationic membrane. The specific steps include: 1) Chromium nitrate, 2-aminoterephthalic acid, and sodium hydroxide were added to water and stirred at room temperature to thoroughly mix. The resulting solution was transferred to a stainless steel reactor and kept at 150-200°C for 12-36 hours. After the reaction, the resulting reaction solution was centrifuged, filtered, washed, and vacuum-dried to obtain the MIL-101-NH2 type MOFs material. 2) mixing the MIL-101-NH2 MOFs material obtained in step 1) with a small molecule N-(phosphonomethyl)methylenediacetic acid hydrate containing a functional group having specific recognition characteristics for rare earth ions, adding N,N-dicyclohexylcarbodiimide, and adding DMF as a solvent, and reacting at 120-150° C. for 24-48 hours under N2 protection; after the reaction, the resulting reactant is centrifuged, washed, and dried to obtain post-modified MIL-101-NH2 MOFs; 3) Immerse the PVDF powder in an acetone solution of AIBN, stir evenly, add acrylic acid, and react at 50-100°C under N2 protection for 4-8 hours to generate a large number of carboxyl groups on its surface; after the reaction, add the post-modified MIL-101-NH2 type MOFs obtained in step 2) to the reaction solution, stir for 30-60 minutes, and then add the porogen PPA, and continue stirring in an oil bath at 50-80°C for 12-24 hours to allow the post-modified MIL-101-NH2 type MOFs to be grafted onto the PVDF surface through in situ self-assembly to obtain a casting solution; ultrasonically degas the casting solution for 1-3 hours, and cast a film on a clean glass plate using a film caster to the desired thickness. Then, immediately and steadily place the glass plate in deionized water. After the film automatically falls off, remove the glass plate, and soak the fallen film in water for 24-48 hours to obtain a self-assembled carboxyl cationic membrane.

2. The preparation method according to claim 1, characterized in that In step 1), the molar ratio of chromium nitrate, 2-aminoterephthalic acid and sodium hydroxide is 1:1-1.2:1-2.

3. The preparation method according to claim 1, characterized in that In step 2), the molar ratio of the MIL-101-NH2 type MOFs material, the small molecule, and N,N-dicyclohexylcarbodiimide is 1:1-1.5:1-2.

4. The preparation method according to claim 1, characterized in that In step 3), the usage ratio of PVDF, AIBN, acrylic acid, PPA and post-modified MIL-101-NH2 type MOFs is 4 g: 0.5-2 g: 2-4 mL: 0.2-0.8 g: 2-4 g.

5. The preparation method according to claim 1, characterized in that In step 1), the washing is carried out sequentially with DMF and ethanol solutions, and the vacuum drying temperature is 60°C-80°C; in step 2), the washing is carried out with anhydrous ethanol, and the drying temperature is 60°C-80°C.

6. The preparation method according to claim 1, characterized in that In step 3), the thickness of the push film is 20-100 μm.

7. A self-assembled carboxyl cationic membrane prepared by the preparation method according to any one of claims 1 to 6.

8. Use of the self-assembled carboxyl cationic membrane according to claim 7 in the recovery of rare earth elements.

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