Full heat exchange membrane with rough surface structure containing heteromorphic zif-8 microcrystal leaves and preparation method thereof

Abstract

The application discloses a full heat exchange film with an ultra-rough surface structure based on a special ZIF-8 microcrystal leaf and a preparation method thereof. The existing full heat exchange film cannot simultaneously have the problems of high energy recovery and high carbon dioxide blocking, so that the film has higher water vapor permeability while still maintaining higher enthalpy exchange efficiency. In the application, a hydrophilic polymer is used as a crosslinking agent to grow special ZIF-8 crystal microleaves on a support layer in situ to construct a thin film composite full heat exchange film, so that the energy recovery efficiency and the carbon dioxide blocking performance are greatly improved. Based on the close interaction between the polymer matrix and the formed ZIF-L, an ultra-rough composite full heat exchange film is formed, which has a high water permeability of 1800-1900 g·m ‑2 ·24h ‑1 and a low carbon dioxide permeability of 1-8 GPU.

Description

Technical Field

[0001] This invention relates to an ultra-rough total heat exchange membrane based on irregularly shaped ZIF-8 crystalline microleafs and its preparation method. Background Technology

[0002] Metal-organic frameworks (MOFs) are organic-inorganic hybrid materials with intramolecular pores, formed by the self-assembly of organic ligands and metal ions or clusters through coordination bonds. Their porosity, large specific surface area, structural and functional diversity, and unsaturated metal sites make MOFs important for applications in hydrogen storage, gas adsorption and separation, sensors, drug delivery, and catalysis. Therefore, more and more researchers are combining MOFs with membrane separation materials, hoping to overcome the trade-off effect. This idea is supported not only by gas-phase separation but also by numerous experiments in liquid-phase separation. Whether in gas-phase separation, liquid-phase separation, or even pervaporation, the introduction of MOFs provides significant performance improvements to membrane separation.

[0003] Membrane technology is increasingly being studied in energy recovery and energy conservation processes, evolving from initial applications in industrial exhaust gas recovery and distillation to the current use of total heat exchangers to recover energy from the air. In recent decades, ventilation systems have become increasingly popular in buildings. In summer and winter, direct ventilation often increases the load on air conditioning systems due to heat loss during ventilation. Energy Recovery Ventilation (ERV) refers to the recovery of heat from exhausted stale air through heat or enthalpy exchange with fresh air during ventilation, representing an energy-saving method. Total heat exchange materials are the core components of ERV devices and have been a hot topic in experimental and theoretical research for many years. Air energy recovery devices can be classified into simple sensible heat exchange and total heat exchange (i.e., including both sensible and latent heat) based on the form of energy exchange. Sensible heat is energy absorbed and released without chemical change or phase change, mainly manifested in temperature changes; latent heat is the energy change resulting from a phase change. Therefore, high-performance total heat exchange membranes need to possess high total heat (i.e., enthalpy) exchange efficiency, encompassing both sensible and latent heat exchange. The main approaches to solving the energy recovery problem should be considered from two aspects: first, controlling the amount of gas permeable membrane material to prevent pollution of fresh air; and second, maintaining high water vapor permeability to obtain higher latent heat of vaporization in the air.

[0004] Leveraging the unique pore structure, large surface area, and controllable morphology and size of MOF materials, combining them with a support layer possessing excellent mechanical properties and high porosity can effectively control the moisture and gas permeability of the membrane. The particle size, shape, internal pore structure, and surface functional groups of ZIFs (Zeolitic Imidazolate Frameworks) nanomaterials can be used to construct various gas and liquid separation membranes. In this study, we utilized the complexation of PEI (Polyethyleneimine) molecules with metal ions to uniformly disperse metal ions on the surface of a polysulfone ultrafiltration membrane. We then coated a polysulfone support layer with ZIF-L (Zeolitic Imidazolate Framework-leaves), the transient crystalline forms of ZIF-8 at various growth stages, as a separation layer, thus preparing a series of hybrid membranes. Experimental verification showed that the PEI / ZIF-L hybrid membrane exhibited excellent moisture permeability and gas barrier properties, and is expected to represent a new breakthrough in the field of total heat exchange membranes, leading international products. Summary of the Invention

[0005] This invention provides a total heat exchange membrane with an ultra-rough surface structure composed of ZIF-8 microcrystalline leaves and its preparation method. Through this method, ZIF-L particles are formed in situ on the membrane surface. PEI molecules, through complexation with zinc ions, uniformly disperse the metal units of ZIF-L on the polysulfone ultrafiltration membrane surface, enhancing the adhesion of ZIF-L to the substrate. Compared with PEI / ZIF-8 hybrid membranes, the unique structure of ZIF-L creates a membrane surface with a rich three-dimensional pore structure, ensuring high porosity and a highly rough surface. The prepared total heat exchange membrane exhibits high gas barrier and moisture permeability, as well as excellent total heat exchange performance, along with superior stability and a simple and easy-to-operate process.

[0006] The technical solution adopted in this invention is:

[0007] A full heat exchange membrane with an ultra-rough surface structure based on irregular ZIF-8 microcrystalline leaves. The full heat exchange membrane is divided into a polymer support layer and a functional layer. The functional layer is a mixed matrix membrane containing hydrophilic polymer and irregular ZIF-8 microcrystalline leaf material.

[0008] The polymer support layer is composed of a nonwoven or textile material made of one or more polymers such as polyester, polyolefin, and nylon, with a thickness of 30-300 micrometers.

[0009] The side chains of the hydrophilic polymer contain one or more of the following hydrophilic groups and their salts: -NH, -NH2, -OH, -COOH, -SO3H, -PO(OH2), such as polyethyleneimine, polyvinyl alcohol, polydopamine, etc.

[0010] The irregular ZIF-8 microcrystalline leaf material includes various shapes such as leaf-shaped, cross-shaped, sword-shaped, disc-shaped, pyramidal, cross-pyramidal, petal-shaped, spherical, and rhombic dodecahedron; the particle size of the irregular ZIF-8 microcrystalline leaf is between 50nm and 20μm.

[0011] A method for preparing a total heat exchange membrane with an ultra-rough surface structure based on irregularly shaped ZIF-8 microcrystalline leaves includes the following steps:

[0012] (1) Dissolve the hydrophilic polymer and zinc compound in deionized water;

[0013] (2) Fix the support material to the base plate with a square frame, and add the above solution into the frame and let it stand for a period of time;

[0014] (3) Dissolve the imidazole ligand in deionized water and add it to the above plate and frame to react for a period of time;

[0015] (4) Discard the supernatant in the frame and dry the finished film at a certain temperature.

[0016] The zinc ion-containing substrate includes one or more of zinc nitrate, zinc chloride, zinc sulfate, and zinc acetate.

[0017] The imidazole ligands mentioned above include 2-methylimidazole;

[0018] In step (1), the mass fraction of polyethyleneimine is 1-5%; the molar concentration of zinc ions is 0.001-0.01 mol / L.

[0019] The settling time in step (2) is 0.01 to 6 hours;

[0020] In step (3), the amount of imidazole ligand added is 0.01–0.1 mmol; the reaction temperature is 5–90 °C; and the reaction time is 0.1–48 h.

[0021] In step (3), the molar ratio of zinc ions to imidazole in the reaction solution is 1:1-1:60, and the optimized ratio is 1:5-1:40.

[0022] In step (4), the film drying temperature is 20-90℃;

[0023] The polymer separation membrane includes flat sheet membranes, hollow fiber homogeneous membranes, or hollow composite membranes and tubular membranes.

[0024] More preferably, a controllable preparation method for an ultra-rough total heat exchange membrane composed of irregularly shaped ZIF-8 crystalline microleaves (ZIF-L) is provided, the method comprising the following steps:

[0025] (1) Prepare an aqueous solution of PEI with a mass fraction of 0.1-5 wt% (preferably 0.4-0.8 wt%) using deionized water as solvent and polyethyleneimine (PEI) as crosslinking agent;

[0026] (2) Prepare a zinc compound with a molar concentration of 0.001-0.01 mol / L (preferably 0.002-0.006 mol / L) and dissolve it in the above PEI aqueous solution and name it solution A;

[0027] (3) Using a polysulfone ultrafiltration membrane as a support layer, fix it on the base plate with a square plate frame, add solution A into it and deposit for 0.01-6h (preferably 2-4h);

[0028] (4) Using deionized water as solvent, prepare a solution of 2-methylimidazole with a molar concentration of 0.05-4.8 mol / L (preferably 0.3-2.4 mol / L), name it solution B, and add it to solution A in the above plate frame to allow for complete reaction;

[0029] (5) Pour out the supernatant in the plate frame, put the membrane into an oven at 20-90℃ (preferably 30-60℃) to dry, and seal and dry for storage.

[0030] Because the polymer (PEI) contains a large number of amino groups, it complexes with metal ions in solution, uniformly coating zinc ions onto the membrane surface through complexation, allowing them to firmly adhere to the pore structure of the support layer. During the formation of ZIF-8 particles, weak connections are formed between particles complexed by different PEI branches, resulting in good interfacial bonding between ZIF-8 particles on the membrane surface, forming a complete and defect-free mixed matrix membrane.

[0031] The growth of ZIF-8 is significantly influenced by synthesis conditions. Particularly with a fixed solvent content, the molar ratio of the central metal ion to the ligand controls the crystallinity of ZIF-8 crystals, thus affecting the macroscopic morphology and size of the particles, resulting in ZIF-8 particles of varying sizes. Specifically, due to insufficient protonated (2-methylimidazole: Hmim) Hmim ligands in water, two-dimensional or similar two-dimensional products are transient crystalline phases of ZIF-8. In other words, the morphology of the two-dimensional layered structure is linked by hydrogen bonds, rather than by free mim-stacks. Due to the presence of Hmim ligands in water, intermediate products also rapidly increase and then randomly attach between the two-dimensional layer units, causing initial growth to follow an isomeric growth mechanism and grow uniformly along the anisotropic growth direction of the crystal. However, due to the insufficient protonated Hmim ligands in water, the two-dimensional layered structure units are linked by hydrogen bonds in each direction instead of free mim-stacks, thus forming a layered structure in each direction. This also explains why, as the proportion increases, cruciform flower-like structures and some bamboo leaf-like and continuously thickening petal-like flower-like structures appear. When the Hmim content increases to an excess, the excess Hmim undergoes Zn-Hmim recombination. The material undergoes a deprotonation reaction to form ZIF-8 crystals, and the stack is decomposed into smaller crystals. In this stage, zinc-metal bonds break and new zinc-metal-zinc coordination bonds are formed, resulting in a developed ZIF-8 structure. Finally, the metastable ZIF-8 nanocrystals undergo the Ostwald maturation process to generate ZIF-8 with a pure-phase homogeneous rhombohedral structure.

[0032] Furthermore, changes to the temperature and reaction time of the reaction solution also have a significant impact on the ZIF-8 particles on the membrane surface. Reaction temperature accelerates particle crystallization and increases reactivity, while reaction time determines the completeness of particle crystallinity, thus affecting the macroscopic morphology of the particles.

[0033] The zinc compound is one of zinc nitrate, zinc chloride, zinc acetate, zinc oxide, and zinc sulfide (preferably zinc nitrate hexahydrate);

[0034] In step (4), the molar ratio of zinc ions to 2-methylimidazole in the mixed solution of A and B is controlled at 1:1-1:60 (preferably 1:5-1:40).

[0035] The surface of the total heat exchange membrane is uniformly covered on the support layer by ZIF-L microcrystals with different morphologies cross-linked with PEI, wherein the ZIF-L microcrystals have different morphological sizes. The total heat exchange membrane has a smooth but rough membrane surface and is divided into a functional separation layer and a support layer. It is stored in a dry state.

[0036] Compared with the prior art, the technical advantages of the present invention are as follows:

[0037] 1) No organic solvents are involved in the reaction, and there is no pollution or interference from other organic solvents;

[0038] 2) The process is simple, the operation is straightforward, and the synthesis method and preparation conditions are easy;

[0039] 3) By complexing polyethyleneimine (PEI) with zinc ions, nanoparticles are uniformly dispersed in the casting solution, which to some extent solves the problems of nanoparticles being difficult to form films and uneven dispersion.

[0040] 4) The total heat exchange membrane prepared by introducing nanoparticles has a rough surface structure due to the different morphological sizes of the irregular ZIF-8 particles, which gives it good moisture permeability and gas barrier properties as well as high enthalpy exchange efficiency.

[0041] 5) This invention primarily addresses the problem that existing total heat exchange membranes cannot simultaneously achieve high energy recovery and high carbon dioxide barrier properties, enabling the membrane to maintain high enthalpy exchange efficiency while allowing for high water vapor permeability. In this invention, a thin-film composite total heat exchange membrane is constructed by in-situ growth of irregularly shaped ZIF-8 crystalline microleaves on a support layer using a hydrophilic polymer as a crosslinking agent, significantly improving energy recovery efficiency and carbon dioxide barrier performance. Based on the close interaction between the polymer matrix and the formed ZIF-L, an ultra-rough (Ra: 600-900 nm) composite total heat exchange membrane is formed, exhibiting a high water permeability of 1800-1900 g·m⁻²·24 h⁻¹ and a low carbon dioxide permeability of 1-8 GPU. This membrane also possesses ultra-high sensible heat recovery efficiency (>95%), significantly improving humidity exchange efficiency (50%) and enthalpy exchange efficiency by 65-75%. Attached Figure Description

[0042] Figure 1 These are scanning electron microscope images of the surface morphology of the total heat exchange membranes prepared in Comparative Example 1 (top left), Example 14 (top right), Comparative Example 16 (bottom left), and Example 18 (bottom right).

[0043] Figure 2 The XRD powder diffraction pattern (a) and infrared spectrum (b) of the total heat exchange membranes prepared in Comparative Example 1, Example 14, and Example 18 are shown.

[0044] Figure 3 These are atomic force microscopy images of the total heat exchange membranes prepared in Examples 1, 2, 3, 4, and 5, and their water contact angles.

[0045] Figure 4 The data (a) shows the water vapor transmission rate and the total heat exchange performance test data (b) of the polysulfone support layer and the total heat exchange membranes prepared in Comparative Examples 1, 2, 14, and 18. Detailed Implementation

[0046] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0047] Comparative Example 1:

[0048] Dissolve 0.8g of PEI in 50ml of deionized water;

[0049] Using a polysulfone ultrafiltration membrane as a support layer, it was fixed on a base plate with a square frame. The above-mentioned PEI aqueous solution was poured into the frame and deposited at 25°C for 4 hours. After that, it was poured out and placed in an oven at 60°C for 10 minutes to dry and store, thus obtaining a blank control pure PEI membrane without ZIF-L particles.

[0050] Comparative Example 2:

[0051] Dissolve 0.95g of zinc nitrate hexahydrate in 50ml of deionized water;

[0052] Using a polysulfone ultrafiltration membrane as a support layer, it was fixed on a base plate with a square plate frame. The above solution was added to the support layer pressed on the base plate and deposited for 4 hours.

[0053] Dissolve 1.31g of 2-methylimidazole in 50ml of deionized water, pour it into the above zinc ion aqueous solution, and react at 25℃ for 12h.

[0054] After pouring out the solution from the plate frame, the membrane was placed in an oven at 60°C for 20 minutes and then sealed for storage to obtain a PEI-free blank control pure ZIF-L membrane.

[0055] Example 1:

[0056] The specific steps for preparing a total heat exchange membrane material covered with irregularly shaped ZIF-8 microcrystalline leaves are as follows:

[0057] Dissolve 0.8g of PEI (Shanghai Aladdin Biochemical Technology Co., Ltd., polyethyleneimine, 10000 Daltons) in 50ml of deionized water; dissolve 0.95g of zinc nitrate hexahydrate in the above PEI aqueous solution and name it solution A.

[0058] Dissolve 1.31 g of 2-methylimidazole in 50 ml of deionized water and name it solution B;

[0059] Using a polysulfone ultrafiltration membrane as a support layer, it was fixed on a base plate with a square plate frame. Solution A was poured into the plate frame and allowed to stand for 4 hours. Then, solution B was added and reacted at 25°C for 12 hours.

[0060] The supernatant in the above-mentioned plate frame was poured out, dried at 60℃ for 20 min, and then sealed for storage. A total heat exchange membrane with a cross-shaped ZIF-L separation layer on the membrane surface was obtained. At this point, the leaf-shaped ZIF-L layers uniformly covered the support layer surface in a cross-shaped pattern. The ultra-rough surface structure increased water vapor permeation, thus improving the latent heat exchange efficiency, while the uniformly covered ZIF layer blocked gas permeation, enhancing the membrane's gas barrier properties.

[0061] Examples 1a-d:

[0062] Examples 1a-d are the same as Example 1, except that the drying temperatures are 25°C, 30°C, 40°C, and 50°C, respectively. Despite the change in drying conditions, measurements showed that the performance of the total heat exchange membrane did not change significantly.

[0063] Examples 2-5:

[0064] Examples 2-5 follow the same method as Example 1, except that the PEI mass is 0.4g, 0.6g, 1.0g, and 1.2g, respectively. The optimal crosslinking concentration of PEI and ZIF-L is controlled by adjusting the concentration of the PEI solution, allowing ZIF-L to stably form an ultra-rough membrane surface structure. Experiments showed that as the PEI concentration increases, the amount of ZIF adhering to the membrane surface gradually decreases at the same zinc ion concentration, meaning the ZIF content on the membrane surface gradually decreases.

[0065] Examples 6-10:

[0066] Examples 6-10 follow the same method as Example 1, except that the mass of nitric acid hexahydrate is 0.3g, 0.6g, 0.9g, 1.19g, and 1.48g, respectively. The optimal ZIF content that can cover the membrane surface is controlled by adjusting the concentration of the zinc ion solution. Experiments showed that as the zinc ion concentration increases, the amount of ZIF-L covering the membrane surface also gradually increases, which to some extent increases the difficulty of gas permeation. However, because a thicker separation layer delays and hinders water vapor permeation, the water vapor permeability of the membrane decreases with increasing zinc ion concentration. At the same PEI concentration, an increase in zinc ion concentration increases the thickness of the ZIF-L separation layer.

[0067] Examples 11-18:

[0068] Examples 11-18 follow the same method as Example 1, except that the mass of 2-methylimidazole in solution B is 0.79 g, 2.10 g, 3.15 g, 3.93 g, 5.24 g, 6.55 g, 7.86 g, and 10.49 g, respectively. By fixing the zinc ion content and adjusting the molar ratio of zinc ions to ligands in the solution, the morphology and size of ZIF-L particles on the membrane surface were controlled, thus exploring the effect of different morphological sizes of ZIF particles on the performance of the total heat exchange membrane. Experiments showed that as the ratio of zinc ions to ligands changed, the morphology of the ZIF-L microcrystalline leaf particles also changed, from aggregated particles to blade-shaped, cross-shaped, cross-shaped pyramidal, flower-shaped, spherical, and the classic rhombic dodecahedral shape of ZIF-8. Because of the different morphologies of ZIF-L particles on the membrane surface, the total heat exchange performance of the total heat exchange membrane also underwent subtle changes.

[0069] Please note to all technical personnel: Although the present invention has been described according to the specific embodiments above, the inventive concept of the present invention is not limited to this invention. Any modifications that utilize the inventive concept will be included within the scope of protection of the patent rights of the present invention.

Claims

1. A method for preparing a total heat exchange membrane with a rough surface structure based on irregularly shaped ZIF-8 microcrystalline leaves, characterized in that: Includes the following steps: (1) Polyethyleneimine and zinc compounds are dissolved in deionized water to obtain a polymer solution; (2) Fix the support material to the base plate with a square frame, and add the polymer solution into the frame and let it stand; (3) Dissolve the imidazole ligand in deionized water and add it to the plate and frame for reaction; (4) Pour off the supernatant in the frame and dry the finished membrane to obtain a full heat exchange membrane with a rough surface structure based on the irregular ZIF-8 microcrystalline leaf, which is a full heat exchange membrane with a rough surface structure based on the irregular ZIF-8 microcrystalline leaf, including a support layer and a functional layer disposed on the support layer. The functional layer contains polyethyleneimine and irregular ZIF-8 microcrystalline leaf material.

2. The method for preparing a total heat exchange membrane with a rough surface structure based on irregularly shaped ZIF-8 microcrystalline leaves according to claim 1, characterized in that, In step (1), the zinc compound is one or more of zinc nitrate, zinc chloride, zinc sulfate, and zinc acetate.

3. The method for preparing a total heat exchange membrane with a rough surface structure based on irregularly shaped ZIF-8 microcrystalline leaves according to claim 1, characterized in that, In step (1), the mass fraction of polyethyleneimine in the polymer solution is 1-5%; The molar concentration of zinc ions in the polymer solution is 0.001–0.1 mol / L.

4. The method for preparing a total heat exchange membrane with a rough surface structure based on irregularly shaped ZIF-8 microcrystalline leaves according to claim 1, characterized in that, In step (2), the settling time is 0.01 to 6 hours.

5. The method for preparing a total heat exchange membrane with a rough surface structure based on irregularly shaped ZIF-8 microcrystalline leaves according to claim 1, characterized in that, In step (3), the imidazole ligand is 2-methylimidazole; In step (3), the reaction temperature is 5–90℃ and the reaction time is 0.1–48h; In step (3), the molar ratio of zinc ions to imidazole in the reaction solution is 1:1 to 1:

60.

6. The method for preparing a total heat exchange membrane with a rough surface structure based on irregularly shaped ZIF-8 microcrystalline leaves according to claim 1, characterized in that, In step (4), the drying temperature for film formation is 20-90℃.

7. The total heat exchange membrane with a rough surface structure based on irregularly shaped ZIF-8 microcrystalline leaves prepared by the preparation method according to any one of claims 1 to 6, characterized in that: The support layer is a polymer support layer, which is composed of one or more textile materials selected from polyester, polyolefin, and nylon, and has a thickness of 30-300 micrometers.

8. A total heat exchange membrane with a rough surface structure based on irregularly shaped ZIF-8 microcrystalline leaves, prepared by the preparation method according to any one of claims 1 to 6, characterized in that: The irregular ZIF-8 microcrystalline leaf material has one or more of the following shapes: leaf-shaped, cross-shaped, sword-shaped, disc-shaped, pyramidal, cross-pyramidal, petal-shaped, spherical, and rhombic dodecahedron.

9. A total heat exchange membrane with a rough surface structure based on irregularly shaped ZIF-8 microcrystalline leaves, prepared by the preparation method according to any one of claims 1 to 6, characterized in that: The particle size of the irregular ZIF-8 microcrystalline leaf material is 50nm-20μm.

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