A zinc-based metal-organic framework material and a preparation method thereof

Abstract

This invention relates to the field of gas adsorption and separation technology, and discloses a zinc-based metal-organic framework material and its preparation method. The zinc-based metal-organic framework material is synthesized by coordination self-assembly via a solvothermal method using divalent zinc ions as metal nodes and pyrazole-4-carboxylic acid and 2-methylimidazole as mixed organic ligands. The zinc-based metal-organic framework material provided by this invention possesses a specific pore chemistry environment and exhibits significant propylene / ethylene adsorption selectivity. This material can achieve efficient adsorption and separation of propylene / ethylene mixtures under ambient temperature and pressure conditions, and has advantages such as simple preparation, excellent adsorption performance, and good cycle stability, making it of significant application value in the field of industrial olefin purification and separation.

Description

Technical Field

[0001] This invention relates to the field of gas adsorption and separation technology, and more specifically, to a zinc-based metal-organic framework material and its preparation method. Background Technology

[0002] Light olefins (such as ethylene and propylene) are crucial raw materials for the manufacture of plastics and various high-value-added chemicals in modern chemical industry. Traditionally, light olefins containing 2-4 carbon atoms are mainly produced through the steam cracking of fossil resources such as naphtha. This process is highly dependent on non-renewable fossil energy and consumes a huge amount of energy. In recent years, with the increasing depletion of fossil resources and growing environmental pressure, methanol-to-olefins (MTO) technology has become an effective alternative process that can simultaneously produce ethylene and propylene. The MTO process typically produces a mixed gas containing propylene (approximately 21 wt%) and ethylene (approximately 51 wt%). To meet the high-purity industrial requirements of downstream polymerization monomers, this mixed product must be efficiently separated and purified.

[0003] Due to the similar physicochemical properties of propylene and ethylene molecules, their efficient separation is extremely challenging in industry. Currently, industrial separation of olefin mixtures mainly relies on cryogenic distillation technology; however, this method requires operation under harsh conditions, consumes a great deal of energy, and incurs high equipment investment costs, failing to meet the energy conservation and emission reduction requirements of modern green chemistry and the "dual carbon" goal. With the deepening of green chemistry and sustainable development concepts, the development of novel energy-saving technologies based on adsorption separation has become a research hotspot.

[0004] Metal-organic frameworks (MOFs) are a class of porous network crystalline materials formed by the self-assembly of metal ions or metal clusters with organic ligands based on coordination bonds. Due to their significant advantages such as high porosity, large specific surface area, precisely controllable pore size, and easy functionalization of the internal surface, MOFs have shown great application potential in the field of gas adsorption and separation. However, most existing MOF materials for propylene / ethylene separation still face several technical bottlenecks: on the one hand, most materials exhibit low selectivity for propylene / ethylene separation, making it difficult to achieve efficient purification of mixed gases; on the other hand, some materials suffer from poor chemical and thermal stability, with their frameworks easily collapsing, making them unsuitable for complex and harsh actual industrial operating conditions. Therefore, developing a novel zinc-based metal-organic framework material that combines high separation selectivity, excellent structural stability, and simple preparation method is of great practical significance. Summary of the Invention

[0005] In view of this, the present invention proposes a zinc-based metal-organic framework material and its preparation method. The zinc-based metal-organic framework material can preferentially adsorb propylene with high selectivity, thereby achieving efficient purification and separation of propylene / ethylene. It aims to solve the problems of low selectivity and poor stability of existing MOF adsorption and separation materials in the current technology.

[0006] This invention proposes a zinc-based metal-organic framework material, which is a compound synthesized by coordinating divalent zinc ions with pyrazole-4-carboxylic acid and 2-methylimidazole as organic ligands.

[0007] Furthermore, the zinc metal centers in the zinc-based metal-organic framework material are all in a four-coordinate mode; wherein each independent zinc atom is coordinated with a nitrogen atom from two different PCA ligands and a carboxylic acid oxygen atom from one PCA ligand, and the remaining coordination site is occupied by a nitrogen atom from the imidazole ring on the MIM ligand.

[0008] A method for preparing a zinc-based metal-organic framework material includes the following steps: The organic ligand and zinc source are dissolved in a mixed solvent containing N,N-dimethylformamide and water, and an acid-base adjuster is added. The zinc-based metal-organic framework material is then obtained by a solvothermal reaction under closed conditions.

[0009] Furthermore, the molar ratio of pyrazole-4-carboxylic acid and 2-methylimidazole in the organic ligand is 1:1; the molar ratio of the organic ligand to the zinc source is 1:1 to 3.

[0010] Furthermore, the zinc source is selected from one of zinc nitrate hexahydrate, zinc sulfate heptahydrate, zinc chloride, or zinc carbonate; the acid-base regulator is selected from one of nitric acid, hydrochloric acid, or tetrafluoroboric acid with a concentration of 1-3 mol / L.

[0011] Furthermore, the parameters of the solvothermal reaction are: temperature 80~150℃ and time 48~120h.

[0012] Furthermore, in the mixed solvent, the volume ratio of N,N-dimethylformamide to water is 6:1~3.

[0013] The present invention also provides the application of the zinc-based metal-organic framework material described in the above technical solution, specifically the application of the zinc-based metal-organic framework material in the selective separation of propylene / ethylene.

[0014] Furthermore, before being used for separation, the zinc-based metal-organic framework material is sequentially washed with N,N-dimethylformamide, exchanged with methanol solvent, and activated by vacuum removal of organic molecules.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Thanks to the customized pore size and specific surface coordination microenvironment, the adsorbent material in this invention achieves precise spatial and molecular recognition of propylene and ethylene molecules. Single-component gas adsorption and dynamic breakthrough experiments have confirmed that the material exhibits a high adsorption capacity for propylene at room temperature, while showing almost no adsorption for ethylene. It can preferentially adsorb propylene molecules in a mixed gas with extremely high selectivity, directly achieving one-step extraction of high-purity ethylene, overcoming the technical bottlenecks of low selectivity and insufficient separation purity of traditional adsorbents.

[0016] (2) The one-pot solvothermal synthesis process using a mixed solvent and acid-base regulator employed in this invention is simple to operate, with mild and easily controllable reaction conditions. The raw materials used, such as ligands and zinc salts, are inexpensive and readily available, and the synthesis exhibits excellent reproducibility. Furthermore, the adsorbent can be activated by simple solvent washing and vacuum drying, and its adsorption and separation performance is not lost after multiple reversible adsorption-desorption cycles. It is expected to significantly reduce the high energy consumption of traditional cryogenic distillation methods and demonstrates extremely broad economic value and application potential in petrochemical fields such as methanol-to-olefins processes and the separation and purification of low-carbon olefins from naphtha cracking products. Attached Figure Description

[0017] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings: Figure 1 This is a schematic diagram of the structure of the zinc-based metal-organic framework material in an embodiment of the present invention; Figure 2 This is the XRD powder diffraction pattern of the zinc-based metal-organic framework material in the embodiments of the present invention; Figure 3 Thermogravimetric analysis diagram of zinc-based metal-organic framework material in the embodiments of the present invention; Figure 4 The figures show the CO2 adsorption / desorption isotherms and pore distribution of the zinc-based metal-organic framework material in this embodiment of the invention. Figure 5 This is an adsorption isotherm diagram of propylene and ethylene by the zinc-based metal-organic framework material in an embodiment of the present invention; Figure 6 This is a penetration curve of the zinc-based metal-organic framework material through a propylene / ethylene mixture in an embodiment of the present invention. Figure 7 This is a bar chart of the zinc-based metal-organic framework material after 5 adsorption tests in an embodiment of the present invention. Detailed Implementation

[0018] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the present invention.

[0019] Furthermore, regarding the numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included within this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0020] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0021] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This specification and embodiments are merely exemplary.

[0022] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0023] This invention proposes a zinc-based metal-organic framework material, which is a compound synthesized by coordinating divalent zinc ions with pyrazole-4-carboxylic acid and 2-methylimidazole as organic ligands.

[0024] In this invention, the zinc metal centers in the zinc-based metal-organic framework material are all in a four-coordinate mode; wherein each independent zinc atom is coordinated with a nitrogen atom from two different PCA ligands and a carboxylic acid oxygen atom from one PCA ligand, and the remaining coordination site is occupied by a nitrogen atom from the imidazole ring on the MIM ligand.

[0025] This invention adds organic ligands to the zinc-based metal-organic framework material. The organic ligands comprise pyrazole-4-carboxylic acid (PCA) and 2-methylimidazole. PCA provides two key coordination sites: the nitrogen atom on the pyrazole ring and the oxygen atom in the carboxylic acid group. These three coordinate bonds bind to zinc ions, forming the core connecting unit of the framework. Its aromatic ring structure combines rigidity and stability, enhancing the framework's resistance to collapse. 2-methylimidazole (MIM) occupies the fourth coordination site of the zinc ion through the nitrogen atom on the imidazole ring, filling the coordination vacancy and creating a saturated four-coordinate mode at the zinc metal center, further improving the structural integrity and thermal stability of the framework. The synergistic effect of PCA, along with the interaction between pyrazole-4-carboxylic acid, 2-methylimidazole, and Zn... 2+ The coordination assembly forms a regular microporous network structure, providing physical space for gas adsorption.

[0026] Furthermore, the molecular size and coordination mechanism of PCA and MIM synergistically regulate the pore size, adapting it to the kinetic diameter of propylene molecules while creating a steric repulsion effect on ethylene molecules. The carboxylic acid oxygen atom of PCA has a certain electronegativity, allowing it to form weak hydrogen bonds with the CH bonds in propylene molecules. The aromatic rings (pyrazole and imidazole rings) of the two ligands can form π-π stacking interactions with the carbon-carbon double bonds in propylene molecules. The lone pair electrons of the nitrogen atoms in the ligands can generate dipole-dipole interactions with propylene molecules. These interactions provide specific recognition capabilities for propylene molecules, while ethylene molecules, due to their symmetrical structure and weak polarity, find it difficult to form effective interactions with the ligands, thus achieving selective separation.

[0027] In this invention, the coordination bond energy between the carboxylic acid group of PCA and zinc ions is high, and the coordination effect between the imidazole ring of MIM and zinc ions combines rigidity and flexibility. The synergy of the two ensures that the material maintains structural stability after multiple adsorption-desorption cycles, avoiding framework collapse. Both ligands have good solubility, dissolving in a mixed solvent of N,N-dimethylformamide and water, and their reactivity is matched. They can coordinate with zinc ions simultaneously under mild solvothermal conditions, ensuring the reproducibility of the synthesis process and the uniformity of the product, and reducing the difficulty of industrial scale-up.

[0028] A method for preparing a zinc-based metal-organic framework material includes the following steps: The organic ligand and zinc source are dissolved in a mixed solvent containing N,N-dimethylformamide and water, and an acid-base adjuster is added. The zinc-based metal-organic framework material is then obtained by a solvothermal reaction under closed conditions.

[0029] In this invention, the molar ratio of pyrazole-4-carboxylic acid and 2-methylimidazole in the organic ligand is 1:1; the molar ratio of the organic ligand to the zinc source is 1:1 to 3.

[0030] In this invention, the zinc source is selected from one of zinc nitrate hexahydrate, zinc sulfate heptahydrate, zinc chloride, or zinc carbonate; the acid-base regulator is selected from one of nitric acid, hydrochloric acid, or tetrafluoroboric acid with a concentration of 1-3 mol / L.

[0031] In the preparation method of the zinc-based metal-organic framework material of the present invention, an acid-base regulator is selected. This acid-base regulator indirectly affects the structure, crystallinity, and properties of the material by controlling the pH value of the reaction system and optimizing the coordination environment. Specifically, the coordination activity of the mixed organic ligands of PCA and MIM depends on a specific pH environment, and the carboxylic acid group of PCA can moderately dissociate into -COO under suitable acidic conditions. - Enhance its relationship with Zn 2+ Its coordination ability, while an acidic environment can inhibit Zn 2+ Hydrolysis produces zinc hydroxide precipitate, avoiding interference with the directional coordination reaction. The pH of the reaction system directly affects the crystal growth rate and morphology; a moderately acidic environment slows down the crystal growth rate, allowing the coordination units to arrange in an orderly manner, reducing lattice defects, and forming crystals with high crystallinity and uniform pore size. The uniformity of pore size is key to achieving selective recognition of propylene / ethylene molecules. By regulating the pores formed with acid-base modulators, the kinetic diameter is precisely matched to propylene molecules while repelling ethylene molecules, providing a structural basis for highly selective adsorption. A suitable pH environment can enhance the Zn... 2+ The bond energy of the coordination bonds between the material and the ligands enhances the thermal stability of the material (avoiding the collapse of the framework due to the breakage of coordination bonds at low temperatures) and strengthens the structural stability of the material during adsorption and desorption cycles, ensuring that the adsorption and separation performance remains unchanged after multiple uses, thus meeting the needs of continuous industrial operation. In addition, the acid-base regulator can neutralize trace impurities that may exist in the reaction system, ensuring that the reaction environment remains consistent for each synthesis, thereby improving the crystal structure, pore characteristics and adsorption performance of the product.

[0032] In this invention, the parameters of the solvothermal reaction are: temperature 80~150℃ and time 48~120h.

[0033] This invention employs a solvothermal method in the preparation method of the zinc-based metal-organic framework material. This solvothermal method controls reaction kinetics, crystal growth, and pore structure. Through the synergistic effect of a closed system and moderate to high temperatures, the solvothermal method can provide Zn... 2+The coordination reaction with mixed ligands of PCA and MIM creates an optimal kinetic environment. Furthermore, by adjusting process parameters, the crystal quality and pore structure of the material can be precisely controlled, directly affecting adsorption and separation performance. Optimization of reaction temperature and time allows the coordination assembly of PCA and MIM to form micropores that are compatible with propylene molecules and repel ethylene molecules. Simultaneously, the aromatic ring structure of the ligands and the coordination environment jointly shape specific action sites within the pores, providing a structural basis for highly selective propylene adsorption. The flow and diffusion of the mixed solvent in the solvothermal method reduces unreacted ligand residue, ensures unobstructed pores, and improves gas adsorption mass transfer efficiency. This also adapts to the industrial application requirements of this invention, reducing preparation difficulty and ensuring product consistency through process characteristics, thereby enhancing the material's practicality and industrial potential.

[0034] In this invention, the volume ratio of N,N-dimethylformamide to water in the mixed solvent is 6:1~3.

[0035] The present invention also provides the application of the zinc-based metal-organic framework material described in the above technical solution, specifically the application of the zinc-based metal-organic framework material in the selective separation of propylene / ethylene.

[0036] In this invention, the zinc-based metal-organic framework material is subjected to N,N-dimethylformamide washing, methanol solvent exchange, and activation treatment to remove organic molecules under vacuum before being used for separation.

[0037] The zinc-based metal-organic framework material preferentially adsorbs propylene. Firstly, although propylene and ethylene have similar molecular structures, they exhibit precisely identifiable physicochemical differences, including different kinetic diameters. The pore size of the zinc-based metal-organic framework material, precisely regulated by mixed ligands, falls within the critical size range suitable for propylene and slightly repelling ethylene. Furthermore, their molecular polarities and interaction sites differ; propylene contains a methyl side chain (-CH3), making it asymmetrical and more polar, with its double bond electron cloud more easily polarized; ethylene has a symmetrical structure and extremely weak polarity, resulting in far fewer intermolecular forces than propylene. Secondly, the zinc-based metal-organic framework material is composed of Zn... 2+ The material is constructed using a four-coordinate framework of pyrazole-4-carboxylic acid (PCA) and 2-methylimidazole (MIM), forming a uniform and rigid microporous channel. The inner diameter of the channel is highly matched with the kinetic diameter of propylene, allowing propylene to smoothly enter the channel and be adsorbed. The pore size creates steric hindrance for ethylene, making it difficult for ethylene to enter the channel, resulting in only trace amounts of surface physical adsorption, macroscopically manifested as almost no adsorption. Furthermore, the inner wall of the pores is modified by the functional groups of PCA and MIM, forming a chemical microenvironment that strongly binds to propylene but has no strong effect on ethylene. The two form three types of specific interactions: ① hydrogen bonding, where the carboxylic acid oxygen (-COO) of PCA... -① As a hydrogen bond acceptor, PCA can form a weak hydrogen bond with the CH bond of the methyl group in the propylene molecule, significantly increasing the adsorption binding energy; ethylene has no methyl side chain and cannot form this interaction; ② π–π stacking interaction: the pyrazole ring of PCA and the imidazole ring of MIM are aromatic ring systems, which can form π–π stacking with the C=C double bond of propylene; ③ Dipole-dipole interaction: the double bond of ethylene is symmetrical and the electron cloud is evenly distributed, so the interaction with the aromatic ring is very weak; propylene is more polar and forms a dipole interaction with the electronegative N / O atoms in the pores; ethylene is nonpolar and has no such interaction.

[0038] Furthermore, the preferential adsorption mechanism of the zinc-based metal-organic framework material for propylene does not require low temperature and high pressure, but can be completed at room temperature and pressure; and the material framework is stable, and the selectivity and adsorption capacity do not decrease after multiple adsorption-desorption cycles, which is fully suitable for the continuous operation requirements of industrial olefin purification.

[0039] In this invention, unless otherwise specified, all raw materials required for preparation are commercially available products well known to those skilled in the art.

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the content of this invention will be further explained below in conjunction with the accompanying drawings and embodiments. However, these examples do not limit the scope of protection of this invention. Based on the technical solutions of this invention, various modifications or variations that can be made by those skilled in the art without creative effort are still within the scope of protection of this invention.

[0041] Example 1: Preparation of Zinc-Based Metal-Organic Framework Materials Ingredients: Zinc nitrate hexahydrate (0.2 mmol), PCA (0.1 mmol), MIM (0.1 mmol) Zinc nitrate hexahydrate, PCA, and MIM were mixed in a 20 mL polytetrafluoroethylene liner. 6.0 mL of N,N-dimethylformamide was added to the mixture, and it was stirred thoroughly at room temperature. Then, 2 mL of water and 160 μL of tetrafluoroboric acid were added to ensure the reactants were thoroughly mixed. The liner was then sealed in a stainless steel reactor and reacted at 80 °C for 72 h. After naturally cooling to room temperature, the mixture was washed and dried to obtain the product, a zinc-based metal-organic framework material.

[0042] Test Example 1: Stability Characterization of Zinc-Based Metal-Organic Framework Materials To determine the thermal stability of the zinc-based metal-organic framework material synthesized in Example 1, thermogravimetric analysis (TG) was used to characterize its thermal stability. The sample was subjected to a temperature increase of 5°C / min from room temperature. -1 The heating rate was increased to 1000℃, and the results were as follows: Figure 3As shown, the sample lost all the solvent molecules in the channels before 180℃, followed by a plateau until the framework began to collapse at 340℃, indicating that the synthesized zinc-based metal-organic framework material has good thermal stability.

[0043] To determine the framework structure of the zinc-based metal-organic framework material synthesized in Example 1, the results are as follows: Figure 1 As shown, the zinc-based metal-organic framework material is a four-coordinated, well-ordered crystal framework composed of Zn. 2+ The precise coordination of pyrazole-4-carboxylic acid (PCA) and 2-methylimidazole (MIM) forms a framework with uniform microporous channels. The inner walls of the channels have specific interaction sites such as the carboxylic acid oxygen of PCA and the imidazole nitrogen of MIM, which provide a structural basis for spatial matching and preferential adsorption of propylene by chemical recognition.

[0044] Depend on Figure 2 It is known that the above-mentioned zinc-based metal-organic framework material has high crystallinity, pure phase, and no impurities or amorphous impurities; its crystal structure is regular and orderly, and the pore size is uniform, ensuring stable and reliable size selectivity for propylene / ethylene.

[0045] Depend on Figure 4 It can be seen that the zinc-based metal-organic framework material is a typical microporous material, with a specific surface area and pore volume that conform to the characteristics of a high-efficiency adsorbent. The pore size distribution is narrow and uniform, precisely matching the kinetic diameter of propylene, and forming steric hindrance to ethylene, which directly confirms the spatial matching effect.

[0046] Depend on Figure 7 It can be seen that after five adsorption test cycles, the zinc-based metal-organic framework material does not show a significant decrease in the amount of propylene it can adsorb, meaning that the zinc-based metal-organic framework material does not lose its adsorption and separation performance after undergoing multiple reversible adsorption-desorption cycles.

[0047] Test Example 2: Gas Adsorption and Separation Performance Tests of Zinc-Based Metal-Organic Framework Materials To characterize the single-component adsorption performance of the zinc-based metal-organic framework material prepared in Example 1 for propylene and ethylene, single-component adsorption isotherms of the zinc-based metal-organic framework material for propylene and ethylene were tested. The results are as follows: Figure 5 As shown. According to Figure 5 It can be seen that the propylene adsorption capacity of this material is 44.8 cm⁻¹. 3 ·g -1 The framework exhibits minimal adsorption of ethylene, while simultaneously demonstrating stronger interactions with propylene gas molecules. This phenomenon demonstrates the preferential capture of propylene gas by zinc-based metal-organic framework materials in propylene / ethylene mixtures.

[0048] To further determine the separation performance of the prepared zinc-based metal-organic framework material for the propylene / ethylene mixed system, the microporous zinc-based metal-organic framework material was washed with N,N-dimethylformamide at 40 °C and then immersed in methanol solvent before being used for separation. Solvent exchange was performed 3-5 times over a period of 3 days. The exchanged samples were then degassed at 90 °C under vacuum for 10 h. The results are as follows. Figure 6 As shown, the breakthrough curve of the zinc-based metal-organic framework material at 298 K for a propylene / ethylene mixture (20:50, volume ratio) indicates that the zinc-based metal-organic framework material has good separation capability for the propylene / ethylene mixture. Within a certain time, propylene is adsorbed in the separation column, thus directly yielding pure ethylene product. This phenomenon demonstrates that the zinc-based metal-organic framework material has excellent propylene / ethylene separation capability.

[0049] In summary, the zinc-based metal-organic framework material provided by this invention has a simple preparation process, good stability, and the ability to selectively adsorb propylene in propylene / ethylene mixtures, thus effectively separating propylene / ethylene mixtures. This invention provides a useful reference for the structural design and gas separation performance of metal-organic framework materials and has great application potential in industrial olefin separation.

[0050] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention. The scope of protection of the present invention is determined by the appended claims.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A zinc-based metal-organic framework material, characterized in that, The zinc-based metal-organic framework material is a compound synthesized by coordinating divalent zinc ions with pyrazole-4-carboxylic acid and 2-methylimidazolium as organic ligands.

2. The zinc-based metal-organic framework material of claim 1, wherein, The zinc metal centers in the zinc-based metal-organic framework materials are all in a four-coordinate mode; wherein each independent zinc atom is coordinated with a nitrogen atom from two different PCA ligands and a carboxylic acid oxygen atom from one PCA ligand, and the remaining coordination site is occupied by a nitrogen atom from the imidazole ring on the MIM ligand.

3. A method for preparing a zinc-based metal-organic framework material according to any one of claims 1 to 2, characterized in that, Includes the following steps: The organic ligand and zinc source are dissolved in a mixed solvent containing N,N-dimethylformamide and water, and an acid-base adjuster is added. The zinc-based metal-organic framework material is then obtained by a solvothermal reaction under closed conditions.

4. The method for preparing a zinc-based metal-organic framework material according to claim 3, characterized in that, The molar ratio of pyrazole-4-carboxylic acid and 2-methylimidazole in the organic ligand is 1:1; the molar ratio of the organic ligand to the zinc source is 1:1 to 3.

5. The method for preparing a zinc-based metal-organic framework material according to claim 3, characterized in that, The zinc source is selected from one of zinc nitrate hexahydrate, zinc sulfate heptahydrate, zinc chloride, or zinc carbonate; the acid-base regulator is selected from one of nitric acid, hydrochloric acid, or tetrafluoroboric acid with a concentration of 1-3 mol / L.

6. The method for preparing a zinc-based metal-organic framework material according to claim 3, characterized in that, The parameters for the solvothermal reaction are: temperature 80~150℃ and time 48~120h.

7. The method for preparing a zinc-based metal-organic framework material according to claim 3, characterized in that, In the mixed solvent, the volume ratio of N,N-dimethylformamide to water is 6:1~3.

8. An application of the zinc-based metal-organic framework material according to claim 1, characterized in that, The specific application is the use of the zinc-based metal-organic framework material in the selective separation of propylene / ethylene.

9. The application of the zinc-based metal-organic framework material according to claim 8, characterized in that, Before being used for separation, the zinc-based metal-organic framework material is sequentially washed with N,N-dimethylformamide, exchanged with methanol solvent, and activated by vacuum removal of organic molecules.

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