A method for electrochemically synthesizing metal organic framework membranes and its application in gas separation

Abstract

The present invention discloses a method for electrochemically synthesizing a metal organic skeleton membrane and its application in gas separation. Under the influence of the confinement effect of the nanosheet structure, the thin sheet precursor is converted into a metal organic skeleton membrane of smaller size through the action of an external electric field, so as to improve the gas separation performance of the metal organic skeleton nanosheet membrane. The preparation method includes pretreatment of the surface of a porous carrier, growth of a nanosheet structure precursor on the surface of a porous carrier, induction synthesis of a metal organic skeleton membrane under the action of an electric field, and gas separation application after film formation. The present invention proposes a novel method for preparing a metal organic skeleton membrane to improve the gas separation performance of the membrane. The preparation method has the advantages of being fast and efficient, and has good prospects for industrial production.

Description

Technical Field

[0001] The invention belongs to the field of membrane separation and relates to the electrochemical synthesis and application of metal organic skeleton membranes. Background Art

[0002] High-purity olefins are still the main raw materials for the production of bulk chemicals. However, the extraction of high-purity olefins from olefin and paraffin mixtures currently mainly adopts low-temperature distillation, which has high energy consumption. As a breakthrough separation technology, membrane separation can effectively reduce energy consumption in the chemical separation process, reduce greenhouse gas emissions, and lay the foundation for sustainable development. Metal-organic framework (MOF) materials are composed of metal ions or metal ion clusters and organic ligands. They have rich pore structures and adjustable pore sizes, making them ideal membrane building units. Zeolite imidazolate framework ZIF-8, with an effective pore size (~4.0 to ) between C3H6 and C3H8 The critical diameter of C3H6 / C3H8 is considered to be an ideal candidate for separating C3H6 / C3H8 by molecular sieves. The preparation process of metal organic framework membranes has always been relatively complicated, and most of the obtained membrane layers have certain defects, which affect the separation effect in the actual separation process. The electrochemical synthesis method has the advantages of being fast and efficient. The conversion under current drive is relatively uniform, and a dense membrane layer can be obtained with good separation effect. Therefore, this time an electrochemical method is used to realize the conversion of the two-dimensional precursor to obtain a metal organic framework membrane with a smaller grain size. Summary of the invention

[0003] The purpose of the present invention is to provide a method for obtaining a metal organic framework membrane by electrochemically converting a nanosheet precursor, and to test the gas separation performance of the obtained membrane material. Under the action of current driving, a uniform and dense metal organic framework membrane is obtained, the membrane material is optimized, and its gas separation performance is improved.

[0004] A method for electrochemically synthesizing a metal organic framework membrane includes the preparation of a nanosheet structure precursor, the conversion of the precursor into a membrane under the action of an external electric field, and a gas separation test after the membrane is formed. The specific process includes the following steps:

[0005] (1) The porous carrier is cleaned with deionized water and an organic solvent at 0-100° C. with stirring for 0-24 h.

[0006] (2) The cleaned carrier is subjected to surface coating treatment.

[0007] (3) A divalent zinc salt or a divalent cobalt salt is selected as the metal source, and an alkali solution, a divalent metal salt solution and an aqueous surfactant solution are sequentially mixed, stirred at 0-100° C. for 0-24 h and then allowed to stand to obtain a mixed liquid; wherein the molar ratio of each raw material is metal salt: alkali: surfactant: water = 1: 0-10: 0-5: 1000-5000, preferably 1: 0-10: 0-5: 1000-3000;

[0008] (4) placing the porous carrier on a polytetrafluoroethylene support, and placing it in the mixed solution obtained in step (3) for 0-30 days, in a constant temperature water bath of 0-60° C., to obtain a two-dimensional nanosheet precursor grown on the surface of the carrier, washing with deionized water and an organic solvent for 0-10 times respectively, and then drying, to achieve one-step synthesis of the two-dimensional nanosheet precursor;

[0009] (5) coating the precursor film obtained in step (4);

[0010] (6) mixing a metal salt, an organic ligand, an ionic liquid, and an organic solvent, and stirring at room temperature until the mixture is completely dissolved to obtain a reaction solution; wherein the molar ratio of each raw material is metal salt: organic ligand: ionic liquid: organic solvent = 0-1: 0-100: 0-10: 0-1000, and the molar ratio of the metal salt, the organic ligand, and the solvent is not zero;

[0011] (7) fixing the porous support obtained in step (5) to the cathode of the electrolytic cell, adding the reaction solution obtained in step (4) to the electrolytic cell, and electrolyzing for 0 to 24 hours at a constant current of 0 to -2A;

[0012] (8) The membrane obtained in step (7) is washed 0 to 10 times with deionized water and an organic solvent and then dried.

[0013] Under the influence of the confinement effect of the nanosheet structure, the present invention converts the sheet precursor into a metal organic framework membrane of smaller size through the action of an external electric field, so as to improve the gas separation performance of the metal organic framework nanosheet membrane.

[0014] Based on the above technical solution, preferably, the porous carrier used in step (1) is an anodic aluminum oxide, stainless steel mesh, nickel mesh or other porous carrier, and the pore size is 5 nm to 1 μm.

[0015] Based on the above technical solution, preferably, in step (1), the porous carrier is in the shape of a sheet structure, a fiber structure, or a tubular structure.

[0016] Based on the above technical scheme, preferably, in step (1), the porous carrier needs to be pretreated before use, that is, the porous carrier is first washed with deionized water and an organic solvent at 0-100°C for 0-24h under stirring, and the washed carrier is subjected to surface coating treatment; wherein the organic solvent used for washing is anhydrous methanol, anhydrous ethanol, anhydrous acetone, anhydrous ethylene glycol, anhydrous propylene glycol, anhydrous n-heptane, anhydrous n-hexanol, deoxygenated water or anhydrous tetrahydrofuran; the type of the coated layer is a conductive film layer such as gold, platinum, carbon, etc., and the coating thickness is 0-100nm.

[0017] Based on the above technical solution, preferably, in step (1), the washing conditions are: washing with deionized water for 0-10 times, and washing with anhydrous acetone for 0-10 times, each time for 0-6 hours.

[0018] Based on the above technical scheme, preferably, in step (1) and step (6), the organic solvent is at least one of anhydrous methanol, anhydrous ethanol, anhydrous acetone, anhydrous ethylene glycol, anhydrous propylene glycol, anhydrous n-heptane, anhydrous n-hexanol, deoxygenated water, and anhydrous tetrahydrofuran.

[0019] Based on the above technical solution, preferably, in step (2) and step (5), the type of the plated layer is a conductive film layer such as gold, platinum, carbon, etc., and the plated film thickness is 0 to 100 nm.

[0020] Based on the above technical scheme, preferably, the surfactant described in step (3) is at least one of hexadecyltrimethylammonium bromide, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and triethanolamine.

[0021] Based on the above technical scheme, preferably, in step (3) and step (6), the metal salt is Zn(NO3)2, ZnCl2, Zn(CH3COO)2, CoCl2, Co(NO3)2, ZrCl4, Cr3(NO3)3·9H2O, Al(NO3)·9H2O, AlCl3·6H2O, etc., and the amount of the metal salt is not 0.

[0022] Based on the above technical solution, preferably, in step (3), the base is an inorganic base such as ammonia water.

[0023] Based on the above technical scheme, preferably, in step (3), when the alkali solution is ammonia water, the metal source is a divalent zinc salt, and the surfactant used is hexadecyltrimethylammonium bromide, the molar ratio of the raw materials is divalent zinc salt: ammonia water: hexadecyltrimethylammonium bromide: water = 1: 0-10: 0-5: 1000-5000; and the molar ratios of ammonia water and hexadecyltrimethylammonium bromide cannot be 0.

[0024] Based on the above technical solution, preferably, in step (4), the preferred constant temperature water bath temperature is 0-60°C, and the growth time of the carrier in the mixed solution is 0-30 days.

[0025] Based on the above technical solution, preferably, in step (4) and step (8), the washing conditions are rinsing with deionized water for 0-60 min and rinsing with anhydrous ethanol for 0-60 min.

[0026] Based on the above technical scheme, preferably, the organic ligand described in step (6) is at least one of 2-methylimidazole, benzimidazole, 5-chlorobenzimidazole, 3H-imidazo[4,5-c]pyridine, 5-methoxy-2-benzimidazole, terephthalic acid, 2-aminoterephthalic acid, 2-fluoroterephthalic acid, 2-chloroterephthalic acid, 2-bromoterephthalic acid, 2-hydroxyterephthalic acid, 2-methylterephthalic acid, 2,5-dichloroterephthalic acid, 2,5-dihydroxyterephthalic acid, and 2,5-dibromoterephthalic acid.

[0027] Based on the above technical scheme, preferably, in step (6), the ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium trifluoroacetate, 1-ethyl-3-methylimidazolium tetrafluoroborate, etc.

[0028] Based on the above technical scheme, preferably, in step (6), when the metal salt is a divalent zinc salt, the organic ligand is methylimidazole, the ionic liquid is 1-butyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide salt, and the organic solvent is anhydrous methanol, the molar ratio of the raw materials is divalent zinc salt: methylimidazole: 1-butyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide salt: organic solvent = 0-1: 0-100: 0-10: 0-1000, wherein the metal salt, the organic ligand and the solvent are not 0;

[0029] Based on the above technical solution, preferably, in step (7), the size of the electrolytic cell is 1-1000 cm 3 , the volume of electrolyte added is 1-1000cm 3 The method used is the constant current method, the current passed is DC 0~-2A, and the power-on time is 0-24h.

[0030] Based on the above technical solution, preferably, the cathode and anode of the electrolysis process are both graphite, and the porous carrier is fixed to the cathode of the electrolytic cell.

[0031] Based on the above technical solution, preferably, in step (4) and step (8), the drying conditions are: temperature 0-200° C., time 1-24 h.

[0032] The present invention also relates to a metal organic skeleton membrane prepared by the method described above.

[0033] The metal organic framework membrane described in the present invention is a metal organic framework material such as ZIF series, UIO series, MIL series, CAU series and their modified derivatives. The metal organic framework material can be prepared in a controllable manner by regulating the carrier treatment method, ligand type and dosage, and current density.

[0034] The present invention also relates to the use of the metal organic framework membrane described above in gas separation, liquid separation and dye separation, especially propylene / propane separation.

[0035] Beneficial effects: The present invention first grows a two-dimensional nanosheet precursor in situ on the surface of a porous carrier, and then converts the nanosheet precursor into a metal organic framework membrane under the action of current drive. By controllably adjusting the thickness of the nanosheet precursor, the ligand concentration, and the current density, the size of the metal organic framework can be effectively controlled, and an excellent metal organic framework membrane material can be obtained to improve its separation performance of propylene and propane. The current-driven method has the advantages of being fast, uniform, and efficient, and has good industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention is attached Figure 5 The widths are:

[0037] Figure 1 This is the X-ray diffraction pattern of the two-dimensional ZnO nanosheet film synthesized in Example 1;

[0038] Figure 2 This is a scanning electron microscope photograph of the surface of the two-dimensional ZnO nanosheet membrane synthesized in Example 1;

[0039] Figure 3 Scanning electron microscope photo of the cross section of the two-dimensional ZnO nanosheet film synthesized in Example 1

[0040] Figure 4 This is the X-ray diffraction pattern of the metal organic framework membrane electrochemically synthesized in Example 2;

[0041] Figure 5 This is a scanning electron microscope photograph of the surface of the metal organic framework membrane electrochemically synthesized in Example 2; DETAILED DESCRIPTION

[0042] The present invention will be further described in the following examples, but the present invention is not limited thereto.

[0043] Example 1 Preparation of two-dimensional ZnO nanosheet precursor

[0044] Dissolve 11.67g of hexadecyltrimethylammonium bromide in 300ml of distilled water to obtain a hexadecyltrimethylammonium bromide mixed solution; dissolve 5.95g of zinc nitrate hexahydrate in 100ml of distilled water to obtain a zinc nitrate solution; mix 14.87g of ammonia water with 72ml of distilled water to obtain an ammonia water mixed solution. Stir the above three liquids for 20 minutes, add the ammonia water mixed solution to the hexadecyltrimethylammonium bromide mixed solution, stir for 20 minutes, then add the zinc nitrate solution to the above mixed solution, stir at room temperature for 10 minutes to obtain a mixed solution. Fix an anodized aluminum oxide carrier (circular with a radius of 0.9cm) with a pore size of 100nm on a polytetrafluoroethylene bracket and soak it in the above mixed solution. After 2 hours, take out the carrier, rinse it with deionized water and anhydrous ethanol 10 times respectively, and dry it at room temperature. The obtained membrane is plated, and the plated layer is 2nm Pt.

[0045] X-ray diffraction confirmed that the product was zinc oxide (such as Figure 1 ), scanning electron microscope surface and cross-sectional images show that the support surface has the morphology of nanosheet precursors (e.g. Figure 2 , 3 ).

[0046] Example 2 Ultrathin ZnO nanosheets converted into ZIF-8 film under electric field drive

[0047] Dissolve 0.957g zinc nitrate hexahydrate, 0.82g methylimidazole, and 21.4g 1-ethyl-3-methylimidazole bromide in 100ml anhydrous methanol to obtain a reaction solution. Fix the carrier of the grown zinc oxide nanosheets prepared in Example 1 on the surface of a graphite plate (size 30*30*2mm) electrode and place it in a quartz electrolytic cell (30*40*60mm) as a cathode, and a graphite plate of the same size as an anode. 3 The reaction solution was added into the electrolytic cell with a constant current of -0.5 mA. After 2 hours, the membrane was taken out, rinsed 10 times with deionized water and anhydrous ethanol respectively, and dried at room temperature.

[0048] X-ray diffraction confirmed that the product was ZIF-8 (such as Figure 4 ), the scanning electron microscope surface images show that the metal organic framework material on the support surface is small in size (e.g. Figure 5 ).

[0049] Example 3 Membrane separation performance of propylene / propane gas separation test supported on ZIF-8 membrane

[0050] The supported membrane prepared in Example 2 was encapsulated in a Wicke-Kallenbach membrane module and tested for separation of propylene / propane mixed gas at room temperature and ΔP=0 bar, with argon as the purge gas. (Under standard conditions, 1 GPU=1×10-6 cm 3 / cm 2 ·s·cmHg). The ZIF-8 supported membranes No. 1, 2, and 3 in the table are three supported membranes prepared by the method in Example 2. The data in the table show that the membrane material has excellent gas separation performance.

[0051]

[0052] Example 4 Dye separation test of ZIF-8 supported membrane Membrane separation performance

[0053] The supported membrane obtained in Example 2 was sealed in a filtration device, with the edges covered with silicone rubber pads and stainless steel washers, and a hole with a diameter of 10 mm in the center to prevent the O-ring from scratching the membrane. The effective membrane area was 0.785 cm 2 . The membrane was first subjected to a 1-hour water compression process at 5 bar, and pure water permeation reached equilibrium after about 30 minutes. Then a water purification test was performed at 1 bar for 1 hour. The separation performance test was performed using an Evans blue aqueous solution (10 mol / L) as the raw material solution. The concentrations of the feed solution and the permeation solution were measured by UV / Vis spectroscopy. In order to eliminate the concentration polarization on the feed side, the feed concentration gradually increases as water molecules are transmitted to the permeation side through the membrane, and the volume of the feed solution is constant at 500 mL. Among them, the ZIF-8 supported membranes No. 1, 2, and 3 in the table are three ZIF-8 supported membranes prepared by the method in Example 2, respectively.

[0054] ZIF-8 supported membrane number Evans blue dye removal rate / % <![CDATA[Water flux (kg / m -2 h -1 )]]> 1 99.6 67 2 99.3 74 3 99.8 62

[0055] Example 5: Liquid separation performance of ZIF-8 supported membranes for ethanol / water separation tests

[0056] The synthesized ZIF-8 supported membrane was evaluated for ethanol (90wt% ethanol / water) dehydration at different feed temperatures and concentrations by pervaporation technology, and the steam permeating through the ZIF-8 membrane was collected after condensation in liquid nitrogen. The real-time component concentrations of the feed solution and the condensed permeate solution were studied using a gas chromatograph (Agilent 7890A equipped with a thermal conductivity detector and an Agilent 6Ft 1 / 8 2mm Porapak Q 80 / 100SS stainless steel column). The ZIF-8 supported membrane numbers 1, 2, and 3 in the following table are three ZIF-8 supported membranes prepared by the method in Example 2.

[0057] ZIF-8 supported membrane number Ethanol / water separation coefficient <![CDATA[Total flux (kg / m -2 h -1 )]]> 1 96 2.2 2 105 3.4 3 121 1.9

[0058] Example 6 Ultrathin ZnO nanosheets converted into ZIF-95 films under electric field drive

[0059] Dissolve 0.957g zinc nitrate, 1.53g 5-chlorobenzimidazole, and 21.4g 1-ethyl-3-methylimidazolium bromide in 100ml anhydrous methanol to obtain a reaction solution. Fix the carrier of zinc oxide nanosheets grown in Example 1 on the surface of a graphite plate (size 30*30*2mm) electrode and place it in a quartz electrolytic cell (30*40*60mm) cathode. Use a graphite plate of the same size as an anode. 3 The reaction solution was added into the electrolytic cell with a constant current of -0.5 mA. After 2 hours, the membrane was taken out, rinsed 10 times with deionized water and anhydrous ethanol respectively, and dried at room temperature.

[0060] Example 7 Membrane separation performance of propylene / propane gas separation test supported on ZIF-95 membrane

[0061] The ZIF-95 supported membrane prepared in Example 6 was encapsulated in a Wicke-Kallenbach membrane module and subjected to a hydrogen / carbon dioxide mixed gas separation test at room temperature and ΔP = 0 bar, with argon as the purge gas. (Under standard conditions, 1 GPU = 1 × 10 -6 cm 3 / cm 2 ·s·cmHg). In the table below, the ZIF-95 supported membranes No. 1, 2, and 3 are three ZIF-95 supported membranes prepared by the method in Example 6. The data in the table show that the membrane material has excellent gas separation performance.

[0062] ZIF-95 support membrane number <![CDATA[H2 / CO2 separation coefficient]]> <![CDATA[H2 Permeation (GPU)]]> <![CDATA[CO2 Permeation Rate (GPU)]]> 1 521 2117 4.1 2 506 2213 4.4 3 485 2328 4.8

Claims

1. A method for preparing an electrochemically synthesized metal-organic skeleton membrane, characterized in that: The following steps are involved: (1) The porous carrier is washed with deionized water and an organic solvent at 0-100°C for 0-24 hours by stirring; (2) The cleaned carrier is subjected to surface coating treatment; (3) Mixing the metal salt solution, the alkali solution and the surfactant solution, stirring at 0-100° C. for 0-60 min to obtain a mixed liquid; wherein the molar ratio of each raw material is metal salt: alkali: surfactant: water = 1: 0-10: 0-5: 1000-5000; (4) placing the porous carrier obtained in step (2) on a polytetrafluoroethylene support, and placing it in the mixed solution obtained in step (3) and soaking it at 0-60° C. for 0-30 days to obtain a two-dimensional nanosheet precursor grown on the surface of the carrier, washing it with deionized water and an organic solvent for 0-10 times respectively, and then drying it; (5) subjecting the precursor obtained in step (4) to a coating process; (6) Mixing a metal salt, an organic ligand, an ionic liquid, and an organic solvent, and stirring at room temperature until they are completely dissolved to obtain a reaction solution; wherein the molar ratio of each raw material is metal salt: organic ligand: ionic liquid: organic solvent = 0~1: 0~100: 0~10: 0~1000, and the metal salt, organic ligand, and solvent are not 0; (7) Fixing the porous support to the cathode of the electrolytic cell, adding the reaction solution obtained in step (6) to the electrolytic cell, and electrolyzing for 0 to 24 hours at a constant current of 0 to -2A; (8) The membrane obtained in step (7) is washed with deionized water and an organic solvent for 0 to 10 times respectively and then dried; In step (3), the metal salt solution is at least one of Zn(NO3)2, ZnCl2, and Zn(CH3COO)2 solutions; the surfactant used is hexadecyltrimethylammonium bromide; and the alkaline solution is aqueous ammonia; In step (6), the metal salt is at least one of Zn(NO3)2, ZnCl2, and Zn(CH3COO)2; the ligand is at least one of 2-methylimidazole, benzimidazole, 5-chlorobenzimidazole, 3H-imidazo[4,5-c]pyridine, and 5-methoxy-2-benzimidazole; and the ionic liquid is at least one of 1-butyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazole bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazole chloride, 1-ethyl-3-methylimidazole bromide, 1-ethyl-3-methylimidazole trifluoroacetate, and 1-ethyl-3-methylimidazole tetrafluoroborate.

2. The preparation method according to claim 1, characterized in that: In step (1), the porous carrier used is anodized aluminum or stainless steel mesh, and the pore size of the porous carrier is 5 nm to 1 μm; the shape of the porous carrier is a sheet structure, a fiber structure or a tubular structure.

3. The preparation method according to claim 1, characterized in that: In steps (1), (4) and (8), the organic solvent is at least one of anhydrous ethanol, anhydrous methanol, anhydrous acetone, anhydrous ethylene glycol, anhydrous propylene glycol, anhydrous n-heptane, anhydrous n-hexanol and anhydrous tetrahydrofuran.

4. The preparation method according to claim 1, characterized in that: In step (2) and step (5), the type of the coating is a conductive film layer of platinum, gold, chromium, or carbon, and the coating thickness is 0 to 100 nm.

5. A metal-organic skeleton membrane prepared by the preparation method according to any one of claims 1 to 4.

6. Use of the metal-organic framework membrane described in claim 5 in gas separation, liquid separation and dye separation.

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