A nanometer-thick square acid MOF film and a preparation method thereof

Abstract

The application discloses a kind of nanometer thickness square acid MOF film and preparation method thereof, and preparation method includes: preparation cationic surfactant, square acid, sodium hydroxide, metal salt is dissolved in water respectively and is matched into 1mg / mL solution;Square acid MOF Langmuir film is assembled using Langmuir film instrument, and square acid MOF film is transferred to solid substrate using Langmuir-Blogget method, to obtain nanometer thickness square acid MOF film;The application is induced by cationic surfactant to interface aggregation of water-soluble negative square acid molecule, interface pressure is reasonably controlled using Langmuir film preparation technology, adjust the distance between surfactant and interface charge density, then control the interface density of square acid ligand molecule and film thickness induced by it, prepare film thickness 2nm, size is centimeter level, defectless MOF film material.

Description

Technical Field

[0001] This invention belongs to the field of functional materials technology, and relates to MOF molecular membrane materials, specifically to a nano-thickness squaric acid MOF membrane and its preparation method. Background Technology

[0002] Gas separation technology is one of the fundamental separation processes in the chemical industry. Compared with traditional cryogenic distillation and adsorption methods, membrane separation technology has attracted much attention due to its greater energy-saving advantages. Currently, membrane materials that separate mixed gases based on size differences mainly include carbon molecular sieve membranes, metal-organic framework (MOF) membranes, two-dimensional nanosheet stacked membranes, and ionic liquid membranes. It is worth noting that the reported carbon molecular sieve membranes, MOF membranes, two-dimensional nanosheet stacked membranes, and ionic liquid membranes are difficult to simultaneously provide high selectivity and high permeability, which limits their commercial application. Developing high-performance gas separation membrane materials that achieve a balance between high selectivity and high permeability has significant academic importance and practical needs.

[0003] Among various separation membrane materials, MOF (Metal-Organic Fragment) membrane materials possess unique advantages. MOFs are crystalline compounds formed by the coordination of metal ions and ligands. MOFs exhibit characteristics such as large specific surface area, high porosity, ease of modification, and tunable structure, attracting significant attention in catalysis, separation, and electrochemistry in recent years. Currently, MOF materials have been used for the separation and storage of various gases such as CO2, H2, CH4, and NH3. However, separating gases with extremely small size differences, such as ethylene / ethane, remains challenging. During actual separation, the organic ligand groups in MOFs can undergo free rotation and other actions, altering the pore size and leading to decreased separation efficiency or even separation failure. Therefore, to achieve size-based separation using MOF materials to prepare high-performance separation membranes, the MOF material must possess suitable pore size, and the ligands must have a rigid structure to prevent changes in pore size. Yang et al. reported a 5 nm thick MOF gas filter membrane for precise hydrogen permeation and carbon dioxide molecule retention. The hydrogen permeability reached 2700 GPU (approximately 1.1 × 10⁻⁶ mol / m²s Pa), and the permeability selectivity was 291. Reducing the membrane thickness significantly improved the gas permeability without significantly affecting the selectivity. Low-dimensional MOF membranes can be used for ethylene / ethane enrichment and separation, achieving a balance between high selectivity and high permeability. High-selectivity gas separation can be achieved by precisely designing and controlling the nanopore size, and high-flux gas permeation can be achieved by utilizing the ultra-thin properties of the material. Currently, there are no reports on the use of low-dimensional MOF membranes for ethylene / ethane separation. Therefore, high-performance ethylene / ethane separation MOF membrane materials should possess (1) arrive (1) Appropriate pore size; (2) Rigid non-rotatable skeleton structure; (3) Nanoscale thickness to ensure efficient passage of gas molecules, thereby achieving a balance between high permeability and high selectivity in nanoporous separation membranes.

[0004] Squaric acid is an organic molecule containing a four-membered ring and four coordinating groups. With the continuous synthesis and discovery of squaric acid and its derivatives, it has attracted much attention in fields such as hydrogels, self-contained materials, and framework materials. In 2019, Chen Banglin et al. reported the synthesis of an ultramicroporous metal-organic framework [Ca(C4O4)(H2O)] MOF material with rigid one-dimensional channels using calcium nitrate and squaric acid for the separation of ethane / ethylene gases. Due to the pore size limitation, ethane was basically not adsorbed, preliminarily proving that squaric acid MOF materials can achieve ethylene / ethane gas separation based on size differences. Based on the above discussion of the characteristics of ideal MOF membrane materials for ethylene / ethane gas separation, it can be inferred that monolayer squaric acid MOF membrane materials formed by squaric acid and calcium ions may exhibit good selectivity for ethylene / ethane gas separation while maintaining high gas permeability. Therefore, the key to achieving efficient ethylene / ethane separation using squaric acid MOFs is to design and prepare nano-thickness, large-area, and highly crystalline squaric acid MOF membrane materials.

[0005] There are many methods for preparing MOF membranes, among which in-situ growth and secondary growth are the two most commonly used methods. In-situ growth involves directly immersing the support in the synthesis solution during the synthesis process, allowing MOF crystals to nucleate and grow under specific reaction conditions, ultimately forming a dense MOF film on the support surface. While this method is simple to operate, the simultaneous nucleation and growth processes often make it difficult to precisely control the membrane growth process. Furthermore, in-situ growth requires a high-quality substrate, making film formation more challenging.

[0006] The secondary growth method involves pre-introducing a seed layer onto the surface of a support, then immersing the support with the seed layer in a synthesis solution, and allowing it to grow into a dense film under specific reaction conditions. Compared to the in-situ growth method, the secondary growth method is advantageous for controlling film thickness and dominant orientation. However, MOF films prepared using the secondary growth method often suffer from insufficient adhesion to the support surface and inadequate mechanical stability.

[0007] Langmuir-Blodgett (LB) membrane technology is a monolayer deposition technique jointly invented by Langmuir and Blodgett. An LB membrane is a single-layer or multi-layer molecular membrane obtained by depositing one or more layers of molecular membrane spread on the surface of a liquid onto a substrate using a specific dip-coating method. LB membranes have many advantages over other systems: (1) they are ultrathin and their thickness is precisely controlled, with a thickness in the nanometer range; (2) the molecules in the membrane are highly ordered and anisotropic, controllable at the molecular level, allowing for the design and realization of molecular-level organization as needed; (3) the membrane preparation conditions are mild and the operation is simple. Generally, molecules that can form LB membranes are amphiphilic molecules, while hydrophilic molecules such as squaric acid cannot be prepared into LB membranes using conventional methods.

[0008] As previously stated, the controllable preparation of large-area, highly crystalline MOF thin films with nanometer-thickness remains extremely difficult. MOF films are generally prepared using traditional methods such as hydrothermal and solvothermal processes, but these methods result in materials with difficult-to-control thickness, coarse grains, and numerous defects. Interfacial methods offer unique advantages in MOF material preparation. It is important to note that the ligands used in interfacial MOF preparation are all amphiphilic or hydrophobic structures; therefore, ligands with good water solubility, such as squaric acid molecules, cannot be directly prepared into MOF films using interfacial methods. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide a nano-thickness squaric acid MOF membrane and its preparation method, which uses a cationic surfactant to induce the interfacial aggregation of negatively charged squaric acid molecules to prepare a highly crystalline, nano-thickness squaric acid MOF membrane.

[0010] To achieve the above objectives, the present invention employs the following technical solution:

[0011] A method for preparing a nano-thickness squaric acid MOF film includes the following steps:

[0012] Step 1: Measure or prepare the cationic surfactant by dissolving it in chloroform to make a 1 mg / ml solution;

[0013] Step 2: Dissolve squaric acid, sodium hydroxide, and metal salt separately in water to prepare 1 mol / L solutions;

[0014] Step 3: Add 20 μL of the cationic surfactant solution prepared in Step 1 dropwise onto a surface with an area of ​​50 cm². 2At the water-air interface, after the chloroform has completely evaporated, the squaric acid solution, sodium hydroxide solution and metal salt solution prepared in step two are added to the aqueous phase according to the molar ratio of squaric acid, sodium hydroxide and metal salt of 1:(0.25~4):(1~2). The interface pressure is monitored to be 5~150mN / m by the Wilhelmy plate of the Langmuir thin film preparation instrument. After two hours, the squaric acid MOF molecules complete the ordered assembly on the water surface to form a tightly packed ultrathin ordered squaric acid MOF Langmuir film.

[0015] The volume ratio of the cationic surfactant solution to the squaric acid solution is 1:(50-100);

[0016] Step 4: Insert the cleaned solid substrate into the solution tank, maintain a constant interfacial pressure, and perform vertical interfacial deposition and transfer to obtain a nano-thickness squaric acid MOF film transferred to the solid substrate.

[0017] The present invention also has the following technical features:

[0018] Preferably, the cationic surfactant mentioned in step one includes any one of benzalkonium bromide, dodecyl pyridine chloride, or hexadecyl amethyst.

[0019] Furthermore, the preparation method of the hexadecyl amethyst includes:

[0020] The intermediate 1-methyl-4,4'-bipyridine was prepared by refluxing bipyridine and iodomethane in dichloromethane at a molar ratio of 2:1:20. Then, the cationic surfactant hexadecyl amethyst was prepared by refluxing 1-methyl-4,4'-bipyridine and hexadecyl bromide in acetonitrile at 82°C in air overnight.

[0021] Preferably, the metal salt mentioned in step two includes any one of calcium nitrate, cobalt chloride, zinc chloride, and chromium nitrate.

[0022] Preferably, the waiting time for the complete evaporation of chloroform in step three is 15 to 30 minutes.

[0023] Furthermore, the solid substrate described in step four is treated in a plasma cleaner at 60W power for 30 seconds.

[0024] Preferably, during the vertical deposition and transfer of the interface in step four, the interface pressure is maintained at any certain value between 5 and 15 mN / m.

[0025] The present invention also protects a centimeter-sized, defect-free, nanometer-thickness squaric acid MOF membrane prepared by the method described above.

[0026] Compared with the prior art, the present invention has the following technical effects:

[0027] This invention uses cationic surfactants to induce the interfacial aggregation of water-soluble negatively charged squaric acid molecules. By employing Langmuir thin film preparation technology to rationally control the interfacial pressure, adjust the distance between surfactants and the interfacial charge density, and thus control the interfacial density and film thickness of the induced aggregated squaric acid ligand molecules, a defect-free MOF film material with a film thickness of 2 nm and a size on the centimeter scale is prepared. Attached Figure Description

[0028] Figure 1 The structure of benzalkonium bromide, a cationic surfactant;

[0029] Figure 2 The structure of the cationic surfactant dodecylpyridine chloride;

[0030] Figure 3 The preparation route for the cationic surfactant hexadecyl amethyst;

[0031] Figure 4 The isotherm of the acid-MOF interface in Example 2;

[0032] Figure 5 This is an optical microscope image of the squaric acid MOF deposited on a silicon wafer in Example 2.

[0033] Figure 6 This is an atomic force microscopy image of the squaric acid MOF in Example 2. Detailed Implementation

[0034] The specific content of the present invention will be further explained in detail below with reference to the embodiments.

[0035] In the following examples, the surfactants benzalkonium bromide and dodecyl pyridine chloride can be purchased directly. Benzalkonium bromide was purchased from Merck (95% purity, 5g), and dodecyl pyridine chloride was purchased from Ron (98% purity, 2g).

[0036] Square acid, also known as square acid, square acid, or 3,4-dihydroxy-3-cyclobutene-1,2-dione, was purchased from Aladdin (purity 98%, 1g).

[0037] Example 1

[0038] 1. Preparation of squaric acid solution:

[0039] Weigh 1.14g of squaric acid and add it to 10mL of deionized water to form a 1mol / L squaric acid solution;

[0040] 2. Preparation of NaOH solution:

[0041] Weigh 0.41 g NaOH and add it to 10 mL of deionized water to obtain a 1 mol / L sodium hydroxide solution;

[0042] 3. Preparation of Ca(NO3)2 solution:

[0043] Weigh 1.64g Ca(NO3)2 and add it to 10mL of deionized water to obtain a 1mol / L calcium nitrate solution;

[0044] 4. Preparation of benzalkonium bromide solution for cationic surfactant:

[0045] Weigh 10 mg of the cationic surfactant benzalkonium bromide, dissolve it in 10 mL of chloroform solution, and stir to obtain a 1 mg / mL cationic surfactant benzalkonium bromide solution;

[0046] 5. Assembling the Langmuir MOF membrane of squaric acid:

[0047] Add an appropriate amount of deionized water to the water tank of the Langmuir membrane apparatus. Insert the silicon wafer, which has been cleaned by 60W plasma cleaning for 30s, into the water tank. Use a microsyringe to add 20μL of the cationic surfactant benzalkonium bromide solution evenly to the water-air interface in the Langmuir membrane apparatus. Wait 20 minutes for the solvent to evaporate, then slowly compress the molecules by controlling the sliding barrier. Once the surface pressure reaches 30mN / m, fix the sliding barrier. Use a syringe to draw 2mL of 1mol / L calcium nitrate solution and add it to the water tank. Use a syringe to draw 2mL of 1mol / L squaric acid solution and slowly inject it into the water tank from the outside of the sliding barrier. At the same time, slowly inject 0.5mL of 1mol / L NaOH solution. Wait two hours for the squaric acid MOF molecules to complete the ordered assembly on the water surface, forming a tightly packed ultrathin ordered squaric acid MOF Langmuir membrane.

[0048] 6. Transfer of Squaric acid MOF membrane:

[0049] The Langmuir-Blogget method was used to transfer the squaric acid MOF film. The interfacial pressure was maintained at 15 mN / m. The silicon wafer immersed in the water bath was vertically lifted at a speed of 1 mm / min until it was completely removed from the water surface. After deposition, it was dried for 5 minutes to remove moisture, resulting in a squaric acid MOF film with a monolayer thickness transferred to the solid substrate.

[0050] Example 2

[0051] 1. Preparation of squaric acid solution:

[0052] Weigh 1.14g of squaric acid and add it to 10mL of deionized water to form a 1mol / L squaric acid solution;

[0053] 2. Preparation of NaOH solution:

[0054] Weigh 0.41 g NaOH and add it to 10 mL of deionized water to obtain a 1 mol / L sodium hydroxide solution;

[0055] 3. Preparation of Ca(NO3)2 solution:

[0056] Weigh 1.64g Ca(NO3)2 and add it to 10mL of deionized water to obtain a 1mol / L calcium nitrate solution;

[0057] 4. Preparation of a solution of the cationic surfactant dodecyl pyridine chloride:

[0058] Weigh 10 mg of the cationic surfactant dodecyl pyridine chloride, dissolve it in 10 mL of chloroform solution, and stir to obtain a 1 mg / mL cationic surfactant dodecyl pyridine chloride solution;

[0059] 5. Assembling the Langmuir MOF membrane of squaric acid:

[0060] Add an appropriate amount of deionized water to the water tank of the Langmuir membrane apparatus. Use a syringe to draw 2 mL of 1 mol / L calcium nitrate solution and add it to the water tank. Insert the silicon wafer, which has been cleaned by 60W power for 30 seconds in a plasma cleaner, into the water tank. Use a microsyringe to take 20 μL of cationic surfactant dodecyl pyridine chloride solution and add it evenly to the water-air interface in the Langmuir membrane apparatus. Wait for 30 minutes. After the solvent evaporates, control the barrier to slowly compress the molecules. After the surface pressure reaches 10 mN / m, fix the barrier. Use a syringe to draw 2 mL of 1 mol / L squaric acid solution and slowly inject it into the water tank on the outside of the barrier. At the same time, slowly inject 0.5 mL of 1 mol / L NaOH solution. Wait for two hours. The squaric acid MOF molecules complete the ordered assembly on the water surface to form a tightly packed ultrathin ordered squaric acid MOF Langmuir membrane.

[0061] 6. Transfer of Squaric acid MOF membrane:

[0062] The Langmuir-Blogget method was used to transfer squaric acid MOF films. The interfacial pressure was maintained at 15 mN / m. The silicon wafer immersed in the water bath was vertically lifted at a speed of 1 mm / min until it was completely removed from the water surface. After deposition, it was dried for 5 minutes to remove moisture, resulting in a squaric acid MOF film with a monolayer thickness transferred to the solid substrate. The film was then washed with chloroform for 2 minutes to remove surfactants before testing.

[0063] Figure 4 The isotherms at the squaric acid MOF interface are shown in Example 2; the black line represents the surfactant; the red line represents the isotherm after the addition of squaric acid; and the blue line represents the isotherm before the addition of squaric acid.

[0064] Figure 5This is an optical microscope image of the squaric acid MOF deposited on a silicon wafer in Example 2; left side of the line: blank silicon wafer; right side of the line: MOF film deposited on the silicon wafer.

[0065] Figure 6 The image shows an atomic force microscope image of the squaric acid MOF in Example 2; the height distribution of the area within the black box in the image proves that the material thickness is 2 nm.

[0066] Example 3

[0067] 1. Preparation of squaric acid solution:

[0068] Weigh 1.14g of squaric acid and add it to 10mL of deionized water to form a 1mol / L squaric acid solution;

[0069] 2. Preparation of NaOH solution:

[0070] Weigh 0.41 g NaOH and add it to 10 mL of deionized water to obtain a 1 mol / L sodium hydroxide solution;

[0071] 3. Preparation of Ca(NO3)2 solution:

[0072] Weigh 1.64g Ca(NO3)2 and add it to 10mL of deionized water to obtain a 1mol / L calcium nitrate solution;

[0073] 4. Preparation of a solution of the cationic surfactant cetyl amethyst:

[0074] A nitrogen-methyl bipyridine salt intermediate was prepared by refluxing bipyridine (5 g, 32 mmol) with iodomethane (1 mL, 16 mmol) in dichloromethane. Then, the intermediate was refluxed with hexadecane (100 g, 320 mmol) in acetonitrile to prepare the cationic surfactant hexadecyl amethyst.

[0075] Weigh 10 mg of the cationic surfactant hexadecyl amethyst, dissolve it in 10 mL of chloroform solution, and stir to obtain a 1 mg / mL cationic surfactant hexadecyl amethyst solution;

[0076] 5. Assembling the Langmuir MOF membrane of squaric acid:

[0077] Add an appropriate amount of deionized water to the water tank of the Langmuir membrane apparatus. Use a syringe to draw 2 mL of 1 mol / L calcium nitrate solution and add it to the water tank. Insert the silicon wafer, which has been cleaned by 60W power for 30s in a plasma cleaner, into the water tank. Use a microsyringe to take 20 μL of cationic surfactant cetyl amethyst solution and add it evenly to the water-air interface in the Langmuir membrane apparatus. Wait 15 min for the solvent to evaporate, and then control the barrier to slowly compress the molecules. After the surface pressure reaches 20 mN / m, fix the barrier. Use a syringe to draw 2 mL of 1 mol / L squaric acid solution and slowly inject it into the water tank on the outside of the barrier. At the same time, slowly inject 0.5 mL of 1 mol / L NaOH solution. Wait two hours for the squaric acid MOF molecules to complete the ordered assembly on the water surface and form a tightly packed ultrathin ordered squaric acid MOF Langmuir membrane.

[0078] 6. Transfer of Squaric acid MOF membrane:

[0079] The Langmuir-Blogget method was used to transfer the squaric acid MOF film. The interfacial pressure was maintained at 5 mN / m. The silicon wafer immersed in the water bath was vertically lifted at a speed of 1 mm / min until it was completely removed from the water surface. After deposition, it was dried for 5 minutes to remove moisture, resulting in a squaric acid MOF film with a monolayer thickness transferred to the solid substrate.

[0080] Example 4

[0081] 1. Preparation of squaric acid solution:

[0082] Weigh 1.14g of squaric acid and add it to 10mL of deionized water to form a 1mol / L squaric acid solution;

[0083] 2. Preparation of NaOH solution:

[0084] Weigh 0.41 g NaOH and add it to 10 mL of deionized water to obtain a 1 mol / L sodium hydroxide solution;

[0085] 3. Preparation of CoCl2 solution:

[0086] Weigh 1.30 g of CoCl2 and add it to 10 mL of deionized water to obtain a 1 mol / L cobalt chloride solution;

[0087] 4. Preparation of benzalkonium bromide solution for cationic surfactant:

[0088] Weigh 10 mg of the cationic surfactant benzalkonium bromide, dissolve it in 10 mL of chloroform solution, and stir to obtain a 1 mg / mL cationic surfactant benzalkonium bromide solution;

[0089] 5. Assembling the Langmuir MOF membrane of squaric acid:

[0090] Add an appropriate amount of deionized water to the water tank of the Langmuir membrane apparatus. Insert the silicon wafer, which has been cleaned by 60W plasma cleaning for 30s, into the water tank. Use a microsyringe to add 20μL of the cationic surfactant benzalkonium bromide solution evenly to the water-air interface in the Langmuir membrane apparatus. Wait 25 minutes for the solvent to evaporate, then slowly compress the molecules by controlling the sliding barrier. Once the surface pressure reaches 5mN / m, fix the sliding barrier. Use a syringe to draw 3mL of 1mol / L cobalt chloride solution and add it to the water tank. Use a syringe to draw 2mL of 1mol / L squaric acid solution and slowly inject it into the water tank outside the sliding barrier. At the same time, slowly inject 1mL of 1mol / L NaOH solution. Wait two hours for the squaric acid MOF molecules to complete the ordered assembly on the water surface, forming a tightly packed ultrathin ordered squaric acid MOF Langmuir membrane.

[0091] 6. Transfer of Squaric acid MOF membrane:

[0092] The Langmuir-Blogget method was used to transfer the squaric acid MOF film. The interfacial pressure was maintained at 12 mN / m. The silicon wafer immersed in the water bath was vertically lifted at a speed of 1 mm / min until it was completely removed from the water surface. After deposition, it was dried for 5 minutes to remove moisture, resulting in a squaric acid MOF film with a monolayer thickness transferred to the solid substrate.

[0093] Example 5

[0094] 1. Preparation of squaric acid solution:

[0095] Weigh 1.14g of squaric acid and add it to 10mL of deionized water to form a 1mol / L squaric acid solution;

[0096] 2. Preparation of NaOH solution:

[0097] Weigh 0.41 g NaOH and add it to 10 mL of deionized water to obtain a 1 mol / L sodium hydroxide solution;

[0098] 3. Preparation of CoCl2 solution:

[0099] Weigh 1.36g of CoCl2 and add it to 10mL of deionized water to obtain a 1mol / L zinc chloride solution;

[0100] 4. Preparation of benzalkonium bromide solution for cationic surfactant:

[0101] Weigh 10 mg of the cationic surfactant benzalkonium bromide, dissolve it in 10 mL of chloroform solution, and stir to obtain a 1 mg / mL cationic surfactant benzalkonium bromide solution;

[0102] 5. Assembling the Langmuir MOF membrane of squaric acid:

[0103] Add an appropriate amount of deionized water to the water tank of the Langmuir membrane apparatus. Insert the silicon wafer, which has been cleaned by 60W plasma cleaning for 30 seconds, into the water tank. Use a microsyringe to add 30 μL of the cationic surfactant benzalkonium bromide solution evenly to the water-air interface in the Langmuir membrane apparatus. Wait 20 minutes for the solvent to evaporate, then slowly compress the molecules by controlling the sliding barrier. Once the surface pressure reaches 50 mN / m, fix the sliding barrier. Use a syringe to draw 4 mL of 1 mol / L zinc chloride solution and add it to the water tank. Use a syringe to draw 2 mL of 1 mol / L squaric acid solution and slowly inject it into the water tank outside the sliding barrier. At the same time, slowly inject 2 mL of 1 mol / L NaOH solution. Wait two hours for the squaric acid MOF molecules to complete the ordered assembly on the water surface, forming a tightly packed ultrathin ordered squaric acid MOF Langmuir membrane.

[0104] 6. Transfer of Squaric acid MOF membrane:

[0105] The Langmuir-Blogget method was used to transfer the squaric acid MOF film. The interfacial pressure was maintained at 8 mN / m. The silicon wafer immersed in the water bath was vertically lifted at a speed of 1 mm / min until it was completely removed from the water surface. After deposition, it was dried for 5 minutes to remove moisture, resulting in a squaric acid MOF film with a monolayer thickness transferred to the solid substrate.

[0106] Example 6

[0107] 1. Preparation of squaric acid solution:

[0108] Weigh 1.14g of squaric acid and add it to 10mL of deionized water to form a 1mol / L squaric acid solution;

[0109] 2. Preparation of NaOH solution:

[0110] Weigh 0.41 g NaOH and add it to 10 mL of deionized water to obtain a 1 mol / L sodium hydroxide solution;

[0111] 3. Preparation of Cr(NO3)3 solution:

[0112] Weigh 2.38g of Cr(NO3)3 and add it to 10mL of deionized water to obtain a 1mol / L chromium nitrate solution;

[0113] 4. Preparation of benzalkonium bromide solution for cationic surfactant:

[0114] Weigh 10 mg of the cationic surfactant benzalkonium bromide, dissolve it in 10 mL of chloroform solution, and stir to obtain a 1 mg / mL cationic surfactant benzalkonium bromide solution;

[0115] 5. Assembling the Langmuir MOF membrane of squaric acid:

[0116] Add an appropriate amount of deionized water to the water tank of the Langmuir membrane apparatus. Insert the silicon wafer, which has been cleaned by 60W plasma cleaning for 30s, into the water tank. Use a microsyringe to add 40μL of the cationic surfactant benzalkonium bromide solution evenly to the water-air interface in the Langmuir membrane apparatus. Wait 15 minutes for the solvent to evaporate, then slowly compress the molecules by controlling the sliding barrier. Once the surface pressure reaches 150mN / m, fix the sliding barrier. Use a syringe to draw 4mL of 1mol / L chromium nitrate solution and add it to the water tank. Use a syringe to draw 2mL of 1mol / L squaric acid solution and slowly inject it into the water tank outside the sliding barrier. At the same time, slowly inject 8mL of 1mol / L NaOH solution. Wait two hours for the squaric acid MOF molecules to complete the ordered assembly on the water surface, forming a tightly packed ultrathin ordered squaric acid MOF Langmuir membrane.

[0117] 6. Transfer of Squaric acid MOF membrane:

[0118] The Langmuir-Blogget method was used to transfer the squaric acid MOF film. The interfacial pressure was maintained at 10 mN / m. The silicon wafer immersed in the water bath was vertically lifted at a speed of 1 mm / min until it was completely removed from the water surface. After deposition, it was dried for 5 minutes to remove moisture, resulting in a squaric acid MOF film with a monolayer thickness transferred to the solid substrate.

[0119] It should be noted that, although embodiments of the present invention have been shown and described, various changes, modifications, substitutions and variations made by those skilled in the art without departing from the principles and spirit of the present invention are all within the scope of protection of the present invention.

Claims

1. A method for preparing a nano-thickness squaric acid MOF membrane, characterized in that, Includes the following steps: Step 1: Measure or prepare the cationic surfactant by dissolving it in chloroform to make a 1 mg / ml solution. Step 2: Dissolve squaric acid, sodium hydroxide, and metal salt separately in water to prepare 1 mol / L solutions; Step 3: Add 20 μL of the cationic surfactant solution prepared in Step 1 dropwise onto a surface with an area of ​​50 cm². 2 At the water-air interface, after the chloroform has completely evaporated, the squaric acid solution, sodium hydroxide solution and metal salt solution prepared in step two are added to the aqueous phase according to the molar ratio of squaric acid, sodium hydroxide and metal salt of 1:(0.25~4):(1~2). The interface pressure is monitored to be 5~150 mN / m by the Wilhelmy plate of the Langmuir thin film preparation instrument. After two hours, the squaric acid MOF molecules are ordered to assemble on the water surface to form a tightly packed ultrathin ordered squaric acid MOF Langmuir film. The volume ratio of the cationic surfactant solution to the squaric acid solution is 1:(50-100). Step 4: Insert the cleaned silicon wafer into the solution bath, maintain a constant interfacial pressure, and perform vertical deposition and transfer at the interface to obtain a nano-thickness squaric acid MOF film transferred to the silicon wafer. The cationic surfactant mentioned in step one includes any one of benzalkonium bromide, dodecyl pyridine chloride, or hexadecyl amethyst. The metal salt mentioned in step two includes any one of calcium nitrate, cobalt chloride, zinc chloride, and chromium nitrate.

2. The method for preparing a nano-thickness squaric acid MOF membrane as described in claim 1, characterized in that, The method for preparing the hexadecyl amethyst includes: The intermediate 1-methyl-4,4'-bipyridine was prepared by refluxing bipyridine and iodomethane in dichloromethane at a molar ratio of 2:1:

20. Then, the cationic surfactant hexadecyl amethyst was prepared by refluxing 1-methyl-4,4'-bipyridine and hexadecyl bromide in acetonitrile at 82 °C in air overnight.

3. The method for preparing a nano-thickness squaric acid MOF film as described in claim 1, characterized in that, The waiting time for chloroform to completely evaporate in step three is 15-30 minutes.

4. The method for preparing a nano-thickness squaric acid MOF membrane as described in claim 1, characterized in that, The silicon wafer described in step four is processed in a plasma cleaner at 60 W power for 30 seconds.

5. The method for preparing a nano-thickness squaric acid MOF film as described in claim 1, characterized in that, During the vertical deposition and transfer of the interface in step four, the interface pressure is maintained at any constant value between 5 and 15 mN / m.

6. A cubic acid MOF membrane with centimeter size, no defects, and nanometer thickness prepared by the method of any one of claims 1-5.

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