Metal-organic nanosheets, and methods of making and using the same

Abstract

The application provides a metal organic nanosheet, a preparation method and application thereof, and a chemical composition of a structural unit of the metal organic nanosheet is M 12 (trimeric indene derivative) 15 (CL)3(OH)6, wherein M is Zr or Hf, and CL is an acetate, propionate or butyrate. The metal organic nanosheet is obtained by mixing a trimeric indene derivative, a transition metal salt and an acid solvent and then performing a solvothermal reaction; the metal organic nanosheet can be dispersed in ethanol and is used to form a composite film on a polydimethylsiloxane base through vacuum-assisted suction filtration, and is used for gas separation in a methane / nitrogen system and a hydrogen / helium system. The application constructs a metal organic nanosheet with a two-dimensional sheet layer feature, and forms a composite separation film with a relatively complete structure, which is beneficial to gas permeability and separation selectivity.

Description

Technical Field

[0001] This invention relates to the field of nanomaterials and gas separation technology, and particularly to a metal-organic nanosheet, its preparation method, and its application. Background Technology

[0002] The development of high-performance gas separation materials has always been an important research direction in the fields of energy and chemical engineering, industrial gas purification, and environmental separation. Compared with traditional separation methods such as cryogenic separation and pressure swing adsorption, membrane separation technology has advantages such as short process, low energy consumption, compact equipment, and convenient operation. Therefore, it has received continuous attention in natural gas purification, hydrogen recovery, and other industrial gas separation scenarios. Especially for gas systems with similar molecular sizes, such as methane / nitrogen and hydrogen / helium, which are difficult to separate, the pore structure, interfacial properties, and mass transfer path design of the membrane material have a decisive influence on the final separation performance.

[0003] When dealing with the aforementioned difficult-to-separate systems, conventional gas separation membrane materials typically present the following problems: First, there is a clear trade-off between selectivity and flux. While membrane materials improve separation selectivity, they are often accompanied by a significant decrease in gas flux, making it difficult to achieve both high selectivity and high permeability. Secondly, some membrane materials are prone to structural relaxation, interface defects, or performance degradation in complex gas environments or during long-term operation, which affects separation stability. Third, the preparation process of existing high-performance membrane materials is often quite complex, with high requirements for film formation conditions and material structure consistency, which is not conducive to practical application.

[0004] Therefore, a metal-organic nanosheet, its preparation method, and its application are proposed. Summary of the Invention

[0005] The technical solution of this invention is implemented as follows: a metal-organic nanosheet, wherein the chemical composition of the structural units of the metal-organic nanosheet is M. 12 (Triindene derivative) 15 (CL)3(OH)6; where M is a transition metal element, and the triminin derivative is a triminin derivative with the structure shown in Formula 2;

[0006] In Formula 2, R is a straight-chain alkyl group and CL is an acid radical ion. By employing the above structural unit, triindene derivatives with rigid planar conjugated frameworks are orderly coordinated with metal cluster nodes, thereby constructing a metal-organic nanosheet structure with two-dimensional extension characteristics at the molecular level. The resulting material simultaneously possesses regular channels, high specific surface area, and a sheet morphology suitable for subsequent layered stacking into films.

[0007] Furthermore, the transition metal element is Zr or Hf. A stable coordination framework can be formed between the Zr or Hf metal center and the carboxyl ligand, which on the one hand improves the structural stability of the obtained metal-organic nanosheets in subsequent separation applications, and on the other hand helps maintain the integrity and two-dimensional extensibility of the sheet framework, thus providing a material basis for constructing continuous, dense composite membranes with directional mass transfer characteristics.

[0008] Furthermore, the anion is acetate, propionate, or butyrate. By changing the type of anion, the coordination environment, crystal growth behavior, and final nanosheet structure formation process in the reaction system can be adjusted, thereby enabling the regulation of the microstructure and film-forming adaptability of metal-organic nanosheets, and thus providing structural design space for optimizing membrane performance under different gas separation systems.

[0009] This invention also provides a method for preparing the above-mentioned organometallic nanosheets, comprising the following steps: mixing a triindene derivative with the structure shown in Formula 2, a transition metal salt, and an acid solvent, followed by a solvothermal reaction to obtain the organometallic nanosheets, wherein the transition metal salt is zirconium chloride or hafnium chloride. The above preparation method utilizes solvothermal conditions in an acidic reaction medium to induce coordination self-assembly between the organic ligand and the metal center, forming organometallic nanosheet products with a layered morphology. This method has relatively concentrated steps and a clear reaction pathway, ensuring the orderly formation of the framework structure while facilitating subsequent washing, purification, and mass production.

[0010] Further, the triindene derivative with the structure shown in Formula 2 is obtained through the following process: First, triindene, an alkylating agent, potassium tert-butoxide, and tetrahydrofuran solvent are mixed in an anhydrous and oxygen-free environment to carry out an alkylation reaction, yielding intermediate 1; then, intermediate 1, acetyl chloride, anhydrous aluminum chloride, and dichloromethane solvent are mixed in an anhydrous environment to carry out an acylation reaction, yielding intermediate 2; subsequently, intermediate 2, sodium hypobromite, and 1,4-dioxane solvent are mixed to carry out an oxidative cleavage reaction, followed by an acidification reaction with hydrochloric acid, yielding the triindene derivative with the structure shown in Formula 2. Through the above stepwise synthetic route, a carboxyl structure suitable for coordination framing can be introduced around the triindene skeleton, while retaining the central rigid conjugated skeleton and the peripheral alkyl substitution structure, enabling the target ligand to possess both coordination reaction activity and two-dimensional growth guidance capability.

[0011] Furthermore, the molar ratio of the triindene derivative with the structure shown in Formula 2 to the transition metal salt is 1:4. By controlling the feed ratio of the ligand to the metal salt within the above range, it is beneficial to form a stable and reproducible coordination assembly environment during the solvothermal reaction, reducing problems such as the formation of secondary phases, incomplete lamellar structures, or uncontrolled crystal morphology caused by insufficient or excessive metal source, thereby facilitating the acquisition of metal-organic nanosheets with well-defined target structures and good dispersion.

[0012] Furthermore, the solvothermal reaction is carried out at a temperature of 220°C for 3–7 days. These temperature and time conditions provide a stable environment for sufficient coordination, self-assembly, and crystal growth between the metal center and the organic ligand, enabling the system to gradually form a metal-organic framework structure with two-dimensional nanosheet characteristics. At the same time, this reaction range balances crystal structure integrity and preparation efficiency, which is beneficial for obtaining target nanosheet products that can be used for subsequent film preparation.

[0013] This invention also provides the application of the above-mentioned metal-organic nanosheets or the metal-organic nanosheets prepared by the above method in gas separation. The application method is as follows: first, the metal-organic nanosheets are dispersed in a solvent to form a casting solution; then, the casting solution is vacuum-assisted filtered on a polymer substrate to form a composite separation membrane; and gas separation is performed using the composite separation membrane. In this application method, the two-dimensional metal-organic nanosheets can be deposited and oriented layer by layer along the substrate surface under filtration drive, forming a continuous and relatively dense separation layer. This separation layer simultaneously contains pore sieving paths provided by the framework itself and interlayer mass transfer paths formed by the stacking of layers, thereby achieving selective separation of the target gas mixture.

[0014] Furthermore, the solvent is ethanol, and the polymer substrate is polydimethylsiloxane. Ethanol facilitates the formation of a uniform and stable dispersion system of the metal-organic nanosheets, thereby improving the deposition uniformity of the casting solution during the filtration process; the polydimethylsiloxane substrate has good flexibility, film-forming compatibility, and substrate support capabilities, and can form a structurally continuous composite separation membrane together with the metal-organic nanosheets.

[0015] Furthermore, the gas separation refers to the separation of methane / nitrogen systems and / or hydrogen / helium systems. Both of these mixed gas systems are typical gas systems with similar molecular sizes and high separation difficulty. Using the metal-organic nanosheets and their composite separation membranes provided by this invention, the size sieving effect of the nanosheet framework pores and the differentiated interactions of different gas molecules within the pore microenvironment can be utilized to achieve effective separation of methane / nitrogen and hydrogen / helium systems. This provides a new material system and implementation path for the development of high-efficiency and energy-saving gas membrane separation materials.

[0016] The embodiments of the present invention have the following advantages due to the adoption of the above technical solutions: I. This invention utilizes tri-indene derivative ligands to coordinate with Zr or Hf metal centers to construct metal-organic nanosheets with two-dimensional sheet-like characteristics. The tri-indene derivative ligands possess high skeletal rigidity and planar conjugation, effectively guiding the two-dimensional extension of the metal-organic framework. Furthermore, the selection of different metal centers and anions facilitates the regulation of the pore structure and pore chemical microenvironment of the nanosheets, thereby providing a stable material basis for the selective separation of difficult-to-separate gases such as methane / nitrogen and hydrogen / helium systems.

[0017] II. The present invention disperses the obtained metal-organic nanosheets on a polymer substrate and forms a composite membrane by vacuum-assisted filtration. This enables the continuous deposition and directional stacking of two-dimensional nanosheets on the substrate surface, thereby forming a composite separation membrane with a relatively complete structure and a relatively continuous separation layer. It can achieve effective selective separation of difficult-to-separate gas systems while maintaining high gas permeability, and has good feasibility and application value. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a flowchart illustrating the preparation process of the tri-indene derivative of the present invention; Figure 2 The Fourier transform infrared spectra of Zr-hmtt-AA and Zr-hett-AA of the present invention are shown below. Figure 3 The thermogravimetric (TG) curve and derivative thermogravimetric (DTG) curve of Zr-hmtt-AA in this invention; Figure 4 The thermogravimetric (TG) curve and derivative thermogravimetric (DTG) curve of Zr-hett-AA in this invention; Figure 5 The powder X-ray diffraction (PXRD) curves and simulated curves of Zr-hmtt-AA of the present invention are shown. Figure 6 The powder X-ray diffraction (PXRD) curves and simulated curves of Zr-hett-AA of the present invention are shown. Figure 7 The X-ray diffraction (XRD) curve of the Zr-hmtt-AA composite film of the present invention is shown below. Figure 8This is a scanning electron microscope (SEM) image of the surface of the Zr-hmtt-AA composite film of the present invention; Figure 9 This is a scanning electron microscope (SEM) image of the cross-section of the Zr-hmtt-AA composite membrane of the present invention; Figure 10 This is a diagram showing the mixed gas separation performance of the Zr-hmtt-AA composite membrane of the present invention; Figure 11 The powder X-ray diffraction (PXRD) curves of the materials of this invention series are shown below. Figure 12 The X-ray diffraction (XRD) curves are shown for the composite films of the series of materials of this invention. Detailed Implementation

[0020] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0021] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0022] like Figure 1-12 As shown, this invention provides a metal-organic nanosheet, its preparation method, and its application. The chemical composition of the nanosheet structural unit is: M 12 (Triindene derivative) 15 (CL)3(OH)6; M is a transition metal element; The chemical structural formula of the tri-indene derivative is shown in Formula 2: Formula 2; In Formula 2, the R group is a straight-chain alkyl group; CL is an acid radical ion.

[0023] In this invention, the chemical composition of the metal-organic nanosheets is: [M 12 (Triindene derivative) 15 (CL)3(OH)6]n.

[0024] In this invention, the transition metal element preferably includes Zr or Hf, more preferably Zr. The R group in Formula 2 is preferably methyl or ethyl, more preferably methyl.

[0025] In a specific embodiment of the present invention, the tri-indene derivative is preferably:

[0026] In this invention, the acid radical ions preferably include acetate, propionate or butyrate, and more preferably acetate.

[0027] In this invention, the triindene derivative is a triindene-derived ligand containing carboxyl coordination sites, which is formed by introducing straight-chain alkyl substituents around the triindene backbone and further constructing carboxyl functional groups. This type of ligand has strong skeletal rigidity and planar conjugation characteristics, and can undergo ordered coordination assembly with transition metal centers, thereby facilitating the formation of metal-organic nanosheet structures with two-dimensional extension characteristics.

[0028] The present invention provides a method for preparing the above-mentioned metal-organic nanosheets, comprising the following steps: mixing the triindene derivative with the structure shown in Formula 2, a transition metal salt and an acid solvent and then carrying out a solvothermal reaction to obtain metal-organic nanosheets, wherein the transition metal salt is zirconium chloride or hafnium chloride.

[0029] Unless otherwise specified, all raw materials, reagents and solvents used in this invention are commercially available products that are accessible to those skilled in the art.

[0030] In this invention, the preferred method for preparing the triminin derivative with the structure shown in Formula 2 includes the following steps: mixing triminin, an alkylating agent, potassium tert-butoxide, and tetrahydrofuran solvent in an anhydrous and oxygen-free environment to carry out an alkylation reaction, thereby obtaining intermediate product 1; mixing intermediate product 1, acetyl chloride, anhydrous aluminum chloride, and dichloromethane solvent in an anhydrous environment to carry out an acylation reaction, thereby obtaining intermediate product 2; mixing intermediate product 2, sodium hypobromite, and 1,4-dioxane solvent to carry out an oxidative pyrolysis reaction, followed by adding hydrochloric acid to carry out an acidification reaction, thereby obtaining the triminin derivative with the structure shown in Formula 2.

[0031] In the above alkylation reaction, the alkylating agent is preferably iodomethane or bromoethane, more preferably iodomethane. An anhydrous and oxygen-free environment is preferably achieved by using ultra-dry reagents and evacuating the reaction system before purging with high-purity nitrogen. The molar ratio of trimerin, alkylating agent, and potassium tert-butoxide is preferably 1:15:20. The alkylation reaction temperature is preferably 80°C, the reaction time is preferably 12 h, and the reaction is carried out under stirring. After the reaction, it is preferable to first remove solid impurities to obtain a liquid product, then remove the solvent to obtain a solid product, and finally wash with n-hexane to obtain intermediate product 1. The preferred method for removing solid impurities is centrifugation, and the preferred method for removing the solvent is rotary evaporation.

[0032] In the above acylation reaction, an anhydrous environment is preferably achieved by using an ultra-dry reagent. The molar ratio of intermediate product 1, acetyl chloride, and anhydrous aluminum chloride is preferably 1:70:70. The acylation reaction temperature is preferably 25°C, the reaction time is preferably 3 h, and the reaction is carried out under stirring. After the reaction, the reaction solution is preferably quenched and purified to obtain a miscible liquid product, then aqueous impurities are removed to obtain an oil-phase liquid product, followed by solvent removal to obtain a solid product, which is then washed with n-hexane to obtain intermediate product 2. The quenching and purification method is preferably ice-water bath quenching, the removal of aqueous impurities is preferably extraction, and the removal of solvent is preferably rotary evaporation.

[0033] In the above-mentioned oxidative cracking and acidification reactions, the molar ratio of intermediate product 2 to sodium hypobromite is preferably 1:15. The preferred temperature for the oxidative cracking reaction is 60℃, and the preferred reaction time is 3 h; the preferred temperature for the acidification reaction is 25℃, and the preferred reaction time is 12 h, and both reactions are preferably carried out under stirring conditions. The preferred hydrochloric acid is 1 mol / L hydrochloric acid, and its addition amount is preferably used to adjust the pH value of the system to 1-3. After the oxidative cracking reaction is completed, a quencher is preferably added for impurity removal, and the preferred quencher is hydroxylamine hydrochloride; after the acidification reaction is completed, liquid impurities are preferably removed by vacuum filtration, the solid product is collected, and washed with water and n-hexane to obtain the tri-indene derivative with the structure shown in Formula 2. The preferred water is deionized water.

[0034] In this invention, the transition metal salt is preferably a metal chloride salt, more preferably zirconium chloride or hafnium chloride. The acid solvent is preferably acetic acid, propionic acid, or butyric acid, more preferably acetic acid. The molar ratio of the tri-indene derivative with the structure shown in Formula 2 to the transition metal salt is preferably 1:4. The solvothermal reaction is preferably carried out by mixing the tri-indene derivative and the transition metal salt in an acid solvent. The solvothermal reaction temperature is preferably 220°C, and the reaction time is preferably 3 to 7 days, more preferably 3 days.

[0035] After the solvothermal reaction is completed, the resulting reaction solution is preferably subjected to solid-liquid separation. The solid product is collected and washed with N,N-dimethylformamide and ethanol, respectively, to obtain metal-organic nanosheets. The preferred solid-liquid separation method is centrifugation, and the washing with N,N-dimethylformamide and ethanol is preferably performed three times each.

[0036] In one embodiment of the present invention, metal-organic nanosheets can be dispersed in ethanol to form a dispersion, and then vacuum-assisted filtration is performed on a polymer substrate to form a composite membrane.

[0037] In this invention, the amount of ethanol used is preferably sufficient to fully disperse the organometallic nanosheets. The amount of organometallic nanosheets in the dispersion is preferably 0.9 mg. The dispersion is preferably subjected to ultrasonic treatment for 30 min. The polymer substrate is preferably a polydimethylsiloxane substrate.

[0038] After the composite membrane filtration is completed, the resulting composite membrane is preferably dried. The drying process preferably includes: first drying the composite membrane in an atmospheric pressure forced-air oven, and then transferring it to a vacuum oven for further drying. The preferred temperature of the atmospheric pressure forced-air oven is 60℃, and the preferred drying time is 30 min; the preferred temperature of the vacuum oven is 60℃, and the preferred drying time is 6 h.

[0039] This invention also provides the application of the metal-organic nanosheets and membrane materials obtained by the above preparation method in gas separation. The application involves using the metal-organic nanosheets or membrane materials prepared therefrom for gas separation; the gas separation system preferably includes a methane / nitrogen system and a hydrogen / helium system.

[0040] Example 1 1. Synthesis of the organic ligand hmtt 0.60 g (1.75 mmol) of indene trimer and 3.93 g (35.00 mmol) of potassium tert-butoxide were weighed and dispersed in tetrahydrofuran. The system was evacuated and then purged with high-purity nitrogen, with this process repeated three times to establish an anhydrous and oxygen-free reaction environment. Subsequently, 3.73 g (26.25 mmol) of iodomethane was slowly added dropwise under ice-water bath and vigorous stirring. After the addition was complete, stirring was continued to ensure thorough mixing, and the mixture was then heated to 80 °C and refluxed for 12 h. After the reaction was complete, the mixture was cooled to room temperature, and solid impurities were removed by centrifugation. The reaction solution was then rotary evaporated to obtain a crude solid product. This crude product was washed three times with n-hexane to obtain a white powdery intermediate product 1.

[0041] 10.92 g (81.9 mmol) of anhydrous aluminum chloride was weighed and dispersed in 5.82 mL (81.9 mmol) of acetyl chloride. Under ice-water bath conditions and vigorous stirring, 20 mL of dichloromethane solution containing 10.50 g (1.17 mmol) of intermediate product 1 was slowly added dropwise to the above system. After the addition was complete, the reaction was continued at 25 °C for 3 h. After the reaction was complete, the reaction solution was slowly added dropwise to 100 mL of an ice-water mixture and stirred thoroughly. Dichloromethane was then added to fully dissolve the crude product in the system. After extraction to separate the lower oil phase, the solvent was removed by rotary evaporation to obtain a solid crude product. This was then washed three times with n-hexane to obtain a white powdery intermediate product 2.

[0042] 86.12 g (10.86 mmol) of 15% sodium hypobromide aqueous solution was added to 30 mL of a 1,4-dioxane solution containing 0.40 g (0.72 mmol) of intermediate product 2, and the reaction was carried out at 60 °C for 3 h under vigorous stirring. After the system was cooled to room temperature, 20 mL of an aqueous solution of 1.00 g (14.39 mmol) of hydroxylamine hydrochloride was added for quenching; then excess 1 mol / L hydrochloric acid was added to adjust the pH of the system to 1, and the reaction was continued at room temperature for 12 h. After the reaction was completed, the solid crude product was collected by vacuum filtration and washed three times each with deionized water and n-hexane to obtain the white powdered target ligand hmtt. This route completed the stepwise construction from the trimerinene framework to the carboxylated trimerinene ligand, providing a ligand basis for subsequent coordination with metal centers to form two-dimensional metal-organic nanosheets.

[0043] 2. Synthesis of Zr-hmtt-AA metal-organic nanosheets 55.8 mg (0.1 mmol) of hmtt and 93.2 mg (0.4 mmol) of zirconium chloride were weighed and dispersed in 3.0 mL of acetic acid. The resulting suspension was sonicated for 3 min to ensure uniform dispersion of the ligand and metal salt in the acidic medium. The suspension was then transferred to a hydrothermal synthesis reactor and reacted at 220 °C for 3 days. After the reaction was completed, the mixture was allowed to cool to room temperature, and the crude solid product was collected by centrifugation. The crude product was washed three times with N,N-dimethylformamide and ethanol, respectively, to obtain the target product Zr-hmtt-AA.

[0044] The obtained Zr-hmtt-AA was characterized by infrared spectroscopy, and its characteristic absorption peak was found to be at 2964 cm⁻¹. -1 2931 cm -1 2872 cm -1 1597 cm -1 and 1408 cm -1 Thermogravimetric analysis results showed that the material began to lose coordinating hydroxyl groups and solvent molecules at 110.2℃ and 357.7℃, respectively. When the temperature increased to 490.3℃, the framework began to decompose, indicating that it has good structural stability in the medium and low temperature range. Powder X-ray diffraction results showed that the first five major diffraction peaks were located at 3.16°, 4.67°, 5.64°, 6.33° and 7.87°, corresponding to the (001), (100), (101), (002) and (102) crystal planes, respectively, indicating that the obtained product has a clear crystal structure and good crystallinity.

[0045] 3. Synthesis of the organic ligand hett The overall synthetic route for the organic ligand hett is the same as that for hmtt, except that the alkylating agent used in the first alkylation reaction is replaced with bromoethane instead of iodomethane. Specifically, 0.60 g (1.75 mmol) of trimerin and 3.93 g (35.00 mmol) of potassium tert-butoxide were weighed and dispersed in tetrahydrofuran. After vacuuming and nitrogen purging three times, 2.86 g (26.25 mmol) of bromoethane was slowly added dropwise under ice-water bath and vigorous stirring. The reaction was then refluxed at 80 °C for 12 h. After cooling, the reaction mixture was centrifuged and rotary evaporated, and then washed three times with n-hexane to obtain a white powdery intermediate 3.

[0046] Subsequently, using the same acylation and oxidative cleavage / acidification steps as in the HMTT synthesis, intermediate 3 (0.60 g, 1.17 mmol) was converted to intermediate 4, and then intermediate 4 (0.46 g, 0.72 mmol) was converted to the target product HETT. Finally, after washing with deionized water and n-hexane, a white powdery HETT ligand was obtained. Due to the change of the alkyl substituent from methyl to ethyl, HETT maintains the tri-indene planar conjugated framework while altering its peripheral microenvironment, allowing it to be used to construct another type of metal-organic nanosheets and compare its impact on final separation performance.

[0047] 4. Synthesis of Zr-hett-AA metal-organic nanosheets 64.3 mg (0.1 mmol) of hett and 93.2 mg (0.4 mmol) of zirconium chloride were weighed and dispersed in 3.0 mL of acetic acid. After sonication for 3 min, the mixture was transferred to a hydrothermal synthesis reactor and reacted at 220 °C for 3 days. After cooling, the crude solid product was collected by centrifugation and washed three times with N,N-dimethylformamide and ethanol, respectively, to obtain the target product Zr-hett-AA. Infrared spectroscopy showed that its characteristic absorption peak was located at 2962 cm⁻¹. -1 2933 cm -1 2875 cm -1 1597 cm -1 and 1404 cm -1 Thermogravimetric analysis showed that it began to lose solvent molecules and coordinating hydroxyl groups at 115.2℃ and 351.3℃, and the framework began to decompose at 501.0℃. Powder X-ray diffraction results showed that the first five diffraction peaks were located at 3.14°, 4.63°, 5.59°, 6.28°, and 7.80°, corresponding to the (001), (100), (101), (002), and (102) crystal planes, respectively. Compared with Zr-hmtt-AA, the overall framework structure of this material remained consistent, with the diffraction peak positions shifting only due to changes in the substituents surrounding the ligands.

[0048] Example 2 This example illustrates the applicability of changes in acid radical ions to the preparation of organometallic nanosheets. In Example 2, the preparation methods for the two ligands, hmtt and hett, are consistent with the corresponding steps in Example 1, and will not be repeated. Only the acid solvent used in the solvothermal reaction is adjusted.

[0049] 55.8 mg (0.1 mmol) of hmtt and 93.2 mg (0.4 mmol) of zirconium chloride were dispersed in 3.0 mL of propionic acid. After sonication for 3 min, the mixture was transferred to a hydrothermal synthesis reactor and reacted at 220 °C for 3 days. The mixture was then cooled, centrifuged, and washed three times sequentially with N,N-dimethylformamide and ethanol to obtain the target product Zr-hmtt-PA. Using the same process, hmtt was dispersed in 3.0 mL of butyric acid and subjected to a hydrothermal reaction to obtain Zr-hmtt-BA.

[0050] Similarly, Zr-hett-PA was obtained by dispersing 64.3 mg (0.1 mmol) of hett and 93.2 mg (0.4 mmol) of zirconium chloride in 3.0 mL of propionic acid, followed by sonication and a solvothermal reaction at 220 °C for 3 days. Zr-hett-BA was obtained by dispersing hett in 3.0 mL of butyric acid and using the same post-treatment process. This example demonstrates that when the ratio of ligand to metal source remains constant, the corresponding organometallic nanosheet products can still be stably obtained by changing the type of acid solvent.

[0051] Example 3 This example illustrates the applicability of changes in the metal center to the preparation of metal-organic nanosheets. In Example 3, the preparation methods for the two ligands, hmtt and hett, are the same as the corresponding steps in Example 1, except that the metal source is replaced by hafnium chloride instead of zirconium chloride.

[0052] 55.8 mg (0.1 mmol) of hmtt and 128.1 mg (0.4 mmol) of hafnium chloride were dispersed in 3.0 mL of acetic acid, propionic acid and butyric acid, respectively. After sonication for 3 min, the mixture was transferred to a hydrothermal synthesis reactor and reacted at 220 °C for 3 days. After cooling, the mixture was centrifuged and washed three times with N,N-dimethylformamide and ethanol, respectively, to obtain Hf-hmtt-AA, Hf-hmtt-PA and Hf-hmtt-BA.

[0053] Similarly, 64.3 mg (0.1 mmol) of hett and 128.1 mg (0.4 mmol) of hafnium chloride were dispersed in 3.0 mL of acetic acid, propionic acid, and butyric acid, respectively. Using the same ultrasonic, hydrothermal reaction, and washing steps, Hf-hett-AA, Hf-hett-PA, and Hf-hett-BA were obtained, respectively. This example illustrates that the tri-indene derivative ligand of the present invention can not only form target two-dimensional metal-organic nanosheets with Zr metal centers, but also form corresponding products with Hf metal centers, demonstrating the applicability of this technical solution in metal node selection.

[0054] Example 4 1. Preparation of Zr-hmtt-AA composite separation membrane In one specific embodiment, 0.9 mg of Zr-hmtt-AA obtained in step 2 of Example 1 was dispersed in 100 mL of ethanol to obtain a metal-organic nanosheet dispersion. The dispersion was ultrasonically treated for 30 min to ensure uniform dispersion of the nanosheets in ethanol and reduce agglomeration. Subsequently, the dispersion was vacuum-assisted filtration on a polydimethylsiloxane substrate, allowing the two-dimensional nanosheets to gradually deposit onto the substrate surface under pressure differential, forming a composite membrane. After filtration, the composite membrane was first dried in a 60°C atmospheric pressure forced-air oven for 30 min to remove most of the volatile solvents in the separation layer; then it was transferred to a 60°C vacuum oven for drying for 6 h to further stabilize the membrane and reduce the influence of residual solvents on the test results.

[0055] 2. Mixed Gas Separation Performance Test The prepared composite membrane was cut to the appropriate size according to the membrane module specifications. After cutting, it was placed under a sealing rubber gasket and installed into the membrane module, and fixed diagonally with hexagonal bolts to ensure good airtightness of the testing system. The inlet gas flow rate and the proportion of each component were adjusted through the gas mixing system, and the inlet gas pressure of the mixed gas was adjusted using the main valve. The test temperature was controlled by the temperature control unit. After the test conditions stabilized, automatic sample injection and analysis were performed at 5-minute intervals. Each group was measured 10 times consecutively, and the average of the last three stable results was taken as the final test data. The test gas system was set as methane / nitrogen and hydrogen / helium, and the test conditions were set as 0.20 MPa and 298 K.

[0056] Test results show that after Zr-hmtt-AA nanosheets are deposited on a PDMS substrate using a vacuum filtration method, the X-ray diffraction pattern of the nanosheets simultaneously exhibits characteristic peaks of both Zr-hmtt-AA and polydimethylsiloxane, indicating that the nanosheet selective layer and the substrate film are effectively composited. Furthermore, the intensity of the (001) crystal plane and its order diffraction peaks of the two-dimensional nanosheets is significantly higher than that of other crystal planes, indicating that the nanosheets exhibit obvious preferred orientation and directional arrangement during film formation. Scanning electron microscopy results of the film surface and cross-section further demonstrate that the resulting film structure is continuous and the separation layer is intact.

[0057] Under the above test conditions, the Zr-hmtt-AA composite membrane exhibits a methane permeability of 1101.5 GPU and a methane / nitrogen selectivity of 3.07 in the methane / nitrogen system; and a hydrogen permeability of 30173.8 GPU and a hydrogen / helium selectivity of 1.70 in the hydrogen / helium system. This demonstrates that the two-dimensional metal-organic nanosheets obtained in this invention, after being deposited as a membrane through filtration, can achieve effective selective separation of difficult-to-separate gas systems while maintaining a high gas flux.

[0058] Example 5: Preparation of representative metal-organic nanosheet composite membranes and testing of mixed gas separation performance. Based on the membrane preparation and testing methods in Example 4, composite membranes were prepared and mixed gas separation performance was tested on the representative metal-organic nanosheets obtained in Examples 1 to 3. The selected representative materials included Zr-hmtt-AA, Zr-hett-AA, Zr-hmtt-PA, Zr-hett-PA, and Hf-hett-PA. The preparation method of each composite membrane was the same as that of the Zr-hmtt-AA composite membrane in Example 4, i.e., the corresponding metal-organic nanosheets were dispersed in ethanol, ultrasonically treated, and then vacuum-assisted filtered on a polydimethylsiloxane substrate. After filtration, the membranes were successively dried under normal pressure and then under vacuum to obtain the corresponding composite separation membranes.

[0059] The obtained composite separation membranes were all tested for mixed gas separation performance using the same test method as in Example 4. Specifically, the prepared composite membranes were cut to a suitable size and then installed into the membrane module. The inlet gas flow rate and component ratio were adjusted through the gas mixing system, and the test pressure was controlled at 0.20 MPa and the test temperature at 298 K. After the test conditions stabilized, automatic sample injection and analysis were performed at 5-minute intervals. Each group was measured 10 times consecutively, and the average of the last 3 stable results was taken as the final test result.

[0060] The separation performance of representative composite separation membranes in the methane / nitrogen system is shown in Table 1.

[0061] Table 1: Separation performance of different composite membranes in methane / nitrogen system

[0062] Table 1 shows that the composite separation membranes obtained under different metal centers, different ligand periphery substituents, and different anion conditions all exhibit high methane permeability and stable separation selectivity in the difficult-to-separate methane / nitrogen system. Among them, the Zr-hett-PA composite membrane achieved a methane / nitrogen selectivity of 3.31, demonstrating superior separation performance; the Hf-hett-PA composite membrane achieved a methane permeability of 1882.7 GPU, exhibiting high gas mass transfer capability. These results indicate that the metal-organic nanosheet material constructed in this invention has good applicability in the methane / nitrogen system.

[0063] The separation performance of representative composite separation membranes in the hydrogen / helium system is shown in Table 2.

[0064] Table 2: Separation performance of different composite membranes in hydrogen / helium systems

[0065] Table 2 shows that the aforementioned composite separation membranes also exhibit high hydrogen permeability and good separation selectivity in the hydrogen / helium system. Among them, the Zr-hett-PA composite membrane showed the best hydrogen / helium selectivity at 2.04, indicating superior performance among the tested samples; the Zr-hmtt-PA composite membrane achieved a hydrogen permeability of 34393.8 GPU, demonstrating its excellent rapid mass transfer capability. These results demonstrate that the metal-organic nanosheets and their composite membrane system constructed in this invention also possess good separation capabilities in the difficult-to-separate hydrogen / helium system, where molecular sizes are highly similar.

[0066] As can be seen from Tables 1 and 2, this invention achieves stable production of organometallic nanosheets with two-dimensional sheet-like characteristics through the coordination of tri-indene derivative ligands with Zr or Hf metal centers. After dispersion with ethanol and vacuum-assisted filtration, the obtained nanosheets can form a structurally continuous and well-oriented composite separation membrane on a polydimethylsiloxane substrate. The composite separation membrane exhibits good permeation and separation performance in both methane / nitrogen and hydrogen / helium systems, demonstrating the good feasibility and versatility of the material system, preparation method, and application scheme of this invention.

[0067] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various variations or substitutions within the technical scope disclosed in the present invention, and these should all be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A metal-organic nanosheet, characterized in that, The chemical composition of the structural units of the metal-organic nanosheets is M 12 (Triindene derivative) 15 (CL)3(OH)6; Wherein, M is a transition metal element, and the triminium derivative is a triminium derivative with the structure shown in Formula 2; ， In Formula 2, R is a straight-chain alkyl group and CL is an acid radical ion.

2. The metal-organic nanosheets according to claim 1, characterized in that, M is either Zr or Hf.

3. The metal-organic nanosheets according to claim 1, characterized in that, The CL is acetate, propionate, or butyrate.

4. A method for preparing metal-organic nanosheets according to any one of claims 1 to 3, characterized in that, The process includes the following steps: mixing the triindene derivative with the structure shown in Formula 2, a transition metal salt, and an acid solvent, and then performing a solvothermal reaction to obtain the metal-organic nanosheets, wherein the transition metal salt is zirconium chloride or hafnium chloride.

5. The method for preparing metal-organic nanosheets according to claim 4, characterized in that, The preparation method of the triindene derivative with the structure shown in Formula 2 includes the following steps: S1. Tri-indene, alkylating agent, potassium tert-butoxide and tetrahydrofuran solvent were mixed in an anhydrous and oxygen-free environment to carry out alkylation reaction to obtain intermediate product 1; S2. The intermediate product 1, acetyl chloride, anhydrous aluminum chloride and dichloromethane solvent are mixed in an anhydrous environment to carry out an acylation reaction to obtain intermediate product 2. S3. The intermediate product 2, sodium hypobromite and 1,4-dioxane solvent are mixed and subjected to oxidative pyrolysis reaction, followed by the addition of hydrochloric acid for acidification reaction to obtain the tri-indene derivative with the structure shown in Formula 2.

6. The method for preparing metal-organic nanosheets according to claim 4, characterized in that, The molar ratio of the triindene derivative with the structure shown in Formula 2 to the transition metal salt is 1:

4.

7. The method for preparing metal-organic nanosheets according to claim 4, characterized in that, The solvothermal reaction is carried out at a temperature of 220°C for 3 to 7 days.

8. The application of the metal-organic nanosheets prepared by the method of any one of claims 1 to 3 or any one of claims 4 to 7 in gas separation, characterized in that, The metal-organic nanosheets are dispersed in a solvent to form a casting solution, and then vacuum-assisted filtration is performed on a polymer substrate to form a composite separation membrane. The composite separation membrane is then used for gas separation.

9. The application according to claim 8, characterized in that, The solvent is ethanol, and the polymer substrate is polydimethylsiloxane.

10. The application according to claim 9, characterized in that, The gas separation is the separation of a methane / nitrogen system and / or a hydrogen / helium system.

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