A dense transparent zif-8 block and a cold sintering preparation method thereof

Abstract

The present application relates to the technical field of material science, and particularly relates to a dense transparent ZIF-8 block and a cold sintering preparation method thereof. The method comprises the following steps: providing ZIF-8 powder; mixing the ZIF-8 powder with N,N-dimethylformamide to obtain a mixture; and performing cold sintering treatment on the mixture to obtain a dense transparent ZIF-8 block; the uniaxial pressure of the cold sintering treatment is 10-30 MPa, the temperature is 80-150 DEG C, and the holding time is 1-3 h. The block has high density, optical transmittance, complete crystal structure, high specific surface area and excellent mechanical properties, and the preparation process is simple and mild, and is easy to scale up.

Description

Technical Field

[0001] This invention relates to the field of materials science, and in particular to a dense and transparent ZIF-8 bulk material and its cold sintering preparation method. Background Technology

[0002] Metal-organic frameworks (MOFs), especially ZIF-8, have shown broad application prospects in catalysis, energy storage, gas adsorption and separation due to their excellent properties such as high specific surface area and tunable pore structure. To meet practical application requirements, ZIF-8 powder is usually processed into macroscopic bulk materials.

[0003] Currently, common methods for preparing ZIF-8 bulk materials include: 1) vapor deposition, in which a metal source and an organic ligand are mixed in gaseous form and deposited on the surface of a substrate to form a ZIF-8 thin film, which is then stacked to obtain a bulk material; 2) electrochemical synthesis, in which ZIF-8 is prepared on the surface of an electrode through an electrochemical process; 3) casting, in which a pre-synthesized ZIF-8 solution or dispersion is injected into a mold and bulk material is obtained through solvent evaporation or other appropriate treatments; 4) mechanical methods (such as ball milling), in which mechanical force is used to aggregate ZIF-8 microcrystals to form a bulk material; and 5) solvothermal or hydrothermal methods, in which organic solvents or water are used to promote the reaction of metal ions with organic ligands under high temperature and high pressure conditions to form a bulk structure.

[0004] However, the aforementioned preparation methods generally suffer from the following problems: some methods are complex to operate, requiring the use of toxic, harmful, and difficult-to-recover solvents or reagents; some methods rely on intermediate supports or require complex pretreatment steps; and some methods involve excessively high reaction temperatures. Studies have shown that the framework structure of ZIF-8 may collapse during high-temperature sintering, leading to crystal structure disintegration and even a glass transition, causing ZIF-8 to change from a crystalline state to an amorphous or glassy state. These structural changes not only result in insufficient density of the prepared bulk material but also damage internal pores, significantly reducing the specific surface area and weakening its performance in adsorption, separation, and other applications. Furthermore, due to insufficient density or the presence of internal light-scattering interfaces, ZIF-8 bulk materials prepared by existing technologies are usually opaque powders or turbid states, unable to achieve optical transparency. This severely limits the application of ZIF-8 materials in optical devices, transparent sensors, photocatalysis, and other fields.

[0005] In summary, how to prepare bulk materials that can maintain the inherent crystalline structure and high specific surface area of ​​ZIF-8 while also possessing high density, good mechanical properties, and optical transmittance, without causing structural damage during high-temperature processing, is an urgent problem to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to address the problems existing in the prior art by providing a dense and transparent ZIF-8 bulk material and its cold sintering preparation method. By mixing ZIF-8 powder with N,N-dimethylformamide and then performing cold sintering treatment, the bulk material is densified and made transparent at a lower temperature and pressure.

[0007] To achieve the above objectives, the present invention provides a method for cold sintering preparation of dense and transparent ZIF-8 bulk materials, comprising the following steps: S1. Provides ZIF-8 powder; S2. Mix ZIF-8 powder with N,N-dimethylformamide to obtain a mixture; S3. The mixture is subjected to cold sintering to obtain a dense and transparent ZIF-8 block; the uniaxial pressure of the cold sintering treatment is 10-30MPa, the temperature is 80-150℃, and the holding time is 1-3h.

[0008] In an optional embodiment, in S1, the preparation process of the ZIF-8 powder includes: mixing a zinc source, an organic ligand, and a solvent, reacting them, and drying them to obtain ZIF-8 powder.

[0009] In one optional embodiment, the zinc source is at least one of zinc chloride and zinc nitrate; the organic ligand is 2-methylimidazole; and the solvent is methanol.

[0010] In an optional embodiment, the molar ratio of the zinc source to the organic ligand is 1:8; and the molar ratio of the zinc source to the solvent is 1:(9-11).

[0011] In one optional embodiment, the reaction is carried out at a temperature of 20-40°C for a time of 22-26 hours.

[0012] In an optional embodiment, after the reaction is complete, the resulting mixed solution is centrifuged to obtain ZIF-8 particles; the ZIF-8 particles are washed and finally dried to obtain ZIF-8 powder. The centrifugation temperature is 20-30℃, the rotation speed is 1000-10000 r / min, and the time is 10-20 min; the washing reagent is methanol, and the washing is performed ≥2 times; the drying temperature is 55-65℃, and the time is 12-24 h.

[0013] In an optional embodiment, in S2, the mass ratio of the ZIF-8 powder to N,N-dimethylformamide is 100:(3-7).

[0014] In an optional embodiment, step S3, prior to the cold sintering process, includes a pre-compression step. Specifically, the mixture is placed in a stainless steel mold, the mold is placed in a tablet press for pre-compression, and then transferred to a hot press for cold sintering.

[0015] In one optional embodiment, the pre-compression treatment is performed at a temperature of 20-30°C, a uniaxial pressure of 3-5 MPa, and a holding time of 3-7 min.

[0016] In an optional implementation, in S3, the heating rate of the cold sintering treatment is 3-7°C / min.

[0017] In an optional embodiment, in S3, after the cold sintering treatment is completed, the temperature is cooled to 20-30°C at a cooling rate of 3-7°C / min, the mold is removed, and dried at 115-125°C for 22-26 hours to obtain a dense and transparent ZIF-8 block.

[0018] The present invention also provides a dense and transparent ZIF-8 bulk material, which is prepared by cold sintering according to the aforementioned method for preparing dense and transparent ZIF-8 bulk material.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses a cold sintering method to prepare ZIF-8 bulk material at a lower temperature (80-150℃) and a lower pressure (10-30MPa), achieving a relative density of 99% and good optical transmittance.

[0020] (2) This invention avoids the collapse of the ZIF-8 framework, amorphization, or glass transition caused by traditional high-temperature sintering. The resulting bulk material maintains a complete crystalline structure with a specific surface area of ​​618.7561 m². 2 / g, total pore volume is 0.20745cm³ 3 / g, exhibiting good adsorption potential.

[0021] (3) According to the nano-indentation test, the ZIF-8 block prepared by the present invention has a hardness of 1.21±0.15GPa and an elastic modulus of 9.83±0.8GPa. Compared with loose ZIF-8 powder, the mechanical properties are significantly improved, which can meet the requirements of practical application for the mechanical strength of block materials.

[0022] (4) The present invention does not require complex equipment or harsh conditions, the solvent used is easy to handle and non-toxic, the preparation process is simple and conducive to large-scale production.

[0023] (5) The nitrogen adsorption-desorption test results show that the ZIF-8 block after cold sintering is still composed of a microporous framework and retains some of the inherent pore structure of ZIF-8, which provides a structural basis for its application in gas adsorption, separation and other fields. Attached Figure Description

[0024] Figure 1 This is a physical representation of the dense, transparent ZIF-8 block in Embodiment 1 of the present invention; Figure 2 This is the XRD characterization image of the dense transparent ZIF-8 block in Embodiment 1 of the present invention; Figure 3 This is a SEM characterization image of the ZIF-8 powder in Example 1 of this invention; Figure 4 This is a SEM characterization image of the dense transparent ZIF-8 block in Embodiment 1 of the present invention; Figure 4 In the image, 'a' represents the SEM image at a scale of 500 nm. Figure 4 In the image, b represents the SEM image at a scale of 200 nm. Figure 5 This is a SEM characterization image of the dense and transparent ZIF-8 block in Embodiment 2 of the present invention; Figure 6 This is a SEM characterization image of the dense transparent ZIF-8 block in Embodiment 3 of the present invention; Figure 7 This is a SEM characterization image of the dense transparent ZIF-8 block in Embodiment 4 of the present invention; Figure 8 This is a SEM characterization image of the dense transparent ZIF-8 block in Embodiment 5 of the present invention; Figure 9 This is a SEM characterization image of the dense and transparent ZIF-8 block in Embodiment 6 of the present invention; Figure 10 This is a SEM characterization image of the dense and transparent ZIF-8 block in Embodiment 7 of the present invention; Figure 11 This is a SEM characterization image of the dense transparent ZIF-8 block in Embodiment 8 of the present invention; Figure 12 This is a SEM characterization image of the dense transparent ZIF-8 block in Embodiment 9 of the present invention; Figure 13 These are the adsorption-desorption curves of ZIF-8 powder and dense transparent ZIF-8 bulk material in Example 1 of this invention; Figure 14 This is a microscopic indentation image of the dense, transparent ZIF-8 block in Embodiment 1 of the present invention; Figure 14 In the image, 'a' represents the micro-indentation pattern at a scale bar of 20 μm. Figure 14 In the diagram, b represents the micro-indentation pattern at a scale of 2 μm; Figure 15 This is the displacement-load curve of the dense transparent ZIF-8 block in Embodiment 1 of the present invention. Detailed Implementation

[0025] The following embodiments are provided to better understand the present invention and are not limited to the described embodiments. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0026] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0027] Example 1 This embodiment provides a method for cold sintering preparation of dense and transparent ZIF-8 bulk materials, including the following steps: Zinc chloride, 2-methylimidazole, and methanol were mixed in a molar ratio of 1:8:10. The mixture was stirred and ground until homogeneous, and then reacted at 30°C for 24 hours. After the reaction, the resulting solution was centrifuged at 5000 rpm for 15 minutes at 25°C to obtain ZIF-8 particles. The ZIF-8 particles were washed three times with methanol to remove residual solution from the surface, and then dried at 60°C for 24 hours to obtain ZIF-8 powder.

[0028] ZIF-8 powder and N,N-dimethylformamide were mixed at a mass ratio of 100:5 to obtain a paste-like mixture.

[0029] The paste-like mixture was placed in a stainless steel mold and pre-compressed: the mold containing the paste-like mixture was placed in a tablet press, and a uniaxial pressure of 4 MPa was applied at 25°C for 5 minutes. After pre-compression, the mold was transferred to a hot press for cold sintering. The hot press was heated to 100°C at a heating rate of 5°C / min, while a uniaxial pressure of 20 MPa was applied, and the temperature and pressure were held for 3 hours. After cold sintering, the mold was cooled to 25°C at a cooling rate of 5°C / min, removed, and dried in a drying oven at 120°C for 24 hours to obtain a dense and transparent ZIF-8 block.

[0030] Example 2 This embodiment is basically the same as Embodiment 1, except that the target temperature for cold sintering is adjusted to 80°C.

[0031] Example 3 This embodiment is basically the same as Embodiment 1, except that the target temperature for cold sintering is adjusted to 120°C.

[0032] Example 4 This embodiment is basically the same as Embodiment 1, except that the target temperature for cold sintering is adjusted to 140°C.

[0033] Example 5 This embodiment is basically the same as Embodiment 1, except that the holding and pressure holding time of the cold sintering treatment is adjusted to 1 hour.

[0034] Example 6 This embodiment is basically the same as Embodiment 1, except that the holding and pressure holding time of the cold sintering treatment is adjusted to 2 hours.

[0035] Example 7 This embodiment is basically the same as Embodiment 1, except that the uniaxial pressure of the cold sintering process is adjusted to 10 MPa.

[0036] Example 8 This embodiment is basically the same as Embodiment 1, except that the uniaxial pressure of the cold sintering process is adjusted to 15 MPa.

[0037] Example 9 This embodiment is basically the same as Embodiment 1, except that the uniaxial pressure of the cold sintering process is adjusted to 25 MPa.

[0038] Experimental Example 1 The dense, transparent ZIF-8 block prepared in Example 1 was placed on top of a piece of paper with printed text, and an optical transmittance test was performed. The resulting physical characterization image of the dense, transparent ZIF-8 block in Example 1 is shown below. Figure 1 As shown. From Figure 1 The text below the block is clearly visible, indicating that the ZIF-8 block prepared by this invention has excellent optical transmittance and achieves transparency.

[0039] Experiment Example 2 The dense, transparent ZIF-8 bulk material prepared in Example 1 was subjected to X-ray diffraction (XRD) analysis, and the XRD characterization pattern of the dense, transparent ZIF-8 bulk material in Example 1 was obtained, as shown below. Figure 2 As shown. By Figure 2 It can be seen that the diffraction peaks of the obtained ZIF-8 bulk body are in good agreement with the ZIF-8 standard card (PDF#62-1030), indicating that the ZIF-8 bulk body prepared by the method of the present invention maintains a complete crystalline structure and does not undergo the amorphization or glass transition common in high-temperature sintering.

[0040] Experimental Example 3 The ZIF-8 powder and dense, transparent ZIF-8 bulk material from Example 1 were observed using scanning electron microscopy (SEM). The SEM characterization images of the ZIF-8 powder from Example 1 are shown below. Figure 3 As shown; SEM characterization image of the dense transparent ZIF-8 block in Example 1, as shown. Figure 4 As shown; Figure 4 In the image, 'a' represents the SEM image at a scale of 500 nm. Figure 4 In the image, b represents the SEM image at a scale of 200 nm. Figure 3 It can be seen that ZIF-8 powder is in a loose granular state. From Figure 4 It is known that after cold sintering, the ZIF-8 particles are tightly connected, forming a dense bulk structure. The densification mechanism is as follows: during the cold sintering process, ZIF-8 particles form liquid bridges under the action of N,N-dimethylformamide, and some crushed ZIF-8 particles fill the contact area between particles. Under uniaxial pressure, the edges and corners of the particles preferentially dissolve, and the dissolved part flows in the gaps and precipitates in the low-pressure area, thereby achieving densification.

[0041] To further investigate the effects of cold sintering conditions on the density and microstructure of ZIF-8 bulk materials, the dense and transparent ZIF-8 bulk materials prepared in Examples 2-9 were subjected to SEM morphology observation.

[0042] SEM characterization image of the dense transparent ZIF-8 block in Example 2, as shown below. Figure 5 As shown; SEM characterization image of the dense transparent ZIF-8 block in Example 3, as shown. Figure 6 As shown; SEM characterization image of the dense transparent ZIF-8 block in Example 4, as shown. Figure 7 As shown. Analysis Figure 4 as well as Figure 5-7 It can be seen that at a temperature of 80℃, only a few neck connections are formed between particles, and the overall structure is relatively loose; when the temperature is increased to 100℃, the liquid-phase mass transfer between particles is sufficient, forming a highly dense bulk; when the temperature is further increased to 120℃ and 140℃, the density is similar to that at 100℃, and no significant difference is observed. Considering that excessively high temperatures may increase energy consumption and equipment requirements, a cold sintering temperature of 100℃ is preferred.

[0043] SEM characterization image of the dense transparent ZIF-8 block in Example 5, as shown below. Figure 8 As shown; SEM characterization image of the dense transparent ZIF-8 block in Example 6, as shown. Figure 9 As shown. Analysis Figure 4 as well as Figure 8-9It can be seen that when the heat preservation and pressure holding time is 1 hour, there are still many gaps between the particles in the block, and the degree of densification is low; when the time is extended to 2 hours, the connection between particles is enhanced and the porosity is significantly reduced; when the time reaches 3 hours, the block is completely densified, the particle boundaries are blurred, and a continuous and dense whole is formed. Therefore, the preferred heat preservation and pressure holding time is 3 hours.

[0044] SEM characterization image of the dense transparent ZIF-8 block in Example 7, as shown below. Figure 10 As shown; SEM characterization image of the dense transparent ZIF-8 block in Example 8, as shown. Figure 11 As shown; SEM characterization image of the dense transparent ZIF-8 block in Example 9, as shown. Figure 12 As shown. Analysis Figure 4 as well as Figure 10-12 It can be observed that at a pressure of 10 MPa, some unfilled pores still exist in the bulk material; when the pressure is increased to 15 MPa, the density improves somewhat, but small pores are still visible in some areas; when the pressure reaches 20 MPa, the bulk material is completely dense, and the particles are tightly bound together. Further increasing the pressure to 25 MPa does not significantly improve the density; therefore, a uniaxial pressure of 20 MPa is preferred.

[0045] The above results indicate that, under the preferred conditions of a cold sintering temperature of 100℃, a uniaxial pressure of 20MPa, and a holding time of 3h, a dense and transparent ZIF-8 block with a complete structure and no visible pores can be obtained.

[0046] Experiment Example 4 The ZIF-8 powder (labeled as ZIF-8 powder in the figure) and the dense, transparent ZIF-8 bulk (labeled as ZIF-8 bulk in the figure) from Example 1 were subjected to nitrogen adsorption-desorption tests. The test conditions were a liquid nitrogen environment, and the adsorption-desorption time was approximately 4 hours. The adsorption-desorption curves of the ZIF-8 powder and the dense, transparent ZIF-8 bulk in Example 1 were obtained, as shown below. Figure 13 As shown. By Figure 13 The adsorption-desorption curves of the two samples largely overlap, indicating that the samples have fewer mesopores and the adsorption and desorption processes are reversible. At extremely low relative pressures, nitrogen adsorption is mainly contributed by micropores; when the relative pressure exceeds 0.1P / P0, the adsorption amount tends to stabilize. This isotherm characteristic suggests that the samples are composed of a microporous framework and still retain some of the inherent pore structure of ZIF-8.

[0047] Calculations show that the specific surface area of ​​ZIF-8 powder is 1291 m². 2 / g, total pore volume is 0.53cm³ 3 / g; The specific surface area of ​​the dense, transparent ZIF-8 bulk material is 618.7561 m². 2 / g, total pore volume is 0.20745cm³ 3 / g. It can be seen that although the specific surface area decreases after cold sintering, it still maintains a large surface area and uniform pore size, and has good adsorption potential.

[0048] Furthermore, the density of the dense and transparent ZIF-8 block prepared in Example 1 was tested using the Archimedes displacement method, and its relative density was found to be 99%, indicating that the method of the present invention can achieve high densification of ZIF-8 material.

[0049] Experimental Example 5 The dense, transparent ZIF-8 bulk material prepared in Example 1 was subjected to nanoindentation testing to evaluate its hardness and elastic modulus. A Berkovich-type triangular pyramidal diamond indenter with a tip curvature radius of approximately 5 μm and a maximum load of 20 mN was used. Load and displacement data were recorded in real time during loading to obtain load-displacement curves. The test was conducted at room temperature, and indentations were performed at five different locations; the results were averaged.

[0050] After testing, the indentation morphology was obtained using a scanning probe microscope, resulting in a microscopic indentation image of the dense, transparent ZIF-8 block in Example 1, as shown below. Figure 14 As shown; Figure 14 In the image, 'a' represents the micro-indentation pattern at a scale bar of 20 μm. Figure 14 In the diagram, 'b' represents the micro-indentation pattern at a scale of 2 μm. Figure 14 It can be seen that under a load of 20mN, the edge of the indentation area is clear and no cracks are generated. The material does not exhibit plastic deformation or spalling, indicating that the prepared ZIF-8 block has sufficient compressive strength and can withstand local indentation loads without structural failure.

[0051] Furthermore, the displacement-load curve of the dense transparent ZIF-8 block in Example 1, as shown... Figure 15 As shown in the figure, 1, 2, 3, 4, and 5 correspond to five independent indentation tests. All curves are smooth and continuous, without abrupt jumps or step-like changes, indicating that the material did not experience sudden fracture or instability during loading. The five curves are basically overlapping, indicating that the micromechanical properties of the sample are uniform.

[0052] Hardness and elastic modulus were calculated using the Oliver-Pharr method based on the principle of nanoindentation testing. Specific data were obtained from the testing instrument display, and the values ​​were derived from the following formulas: Hardness H is derived from the maximum load P. max Dividing by the indentation projection contact area A, we can calculate: H=P max / A. Where A is represented by the pressure head area function A=f(h c ) Determined, h c This represents the actual contact depth. The calculation of the elastic modulus first obtains the reduced elastic modulus E. rIts relationship with the initial slope of the unloading curve (contact stiffness S) and the projected contact area A is as follows: In the formula, β is the geometric correction factor for the indenter, taken as 1.034. Then, the elastic modulus E of the specimen is calculated according to the following formula: Where v is the Poisson's ratio of the sample (taken as 0.2-0.3), E i and v i These are the elastic modulus (1141 GPa) and Poisson's ratio (0.07) of the diamond indenter, respectively. The specific calculation formula and calibration process refer to GB / T 21838.1-2019 standard.

[0053] Calculations showed that the hardness of the dense, transparent ZIF-8 block prepared in Example 1 was 1.21 ± 0.15 GPa, and the elastic modulus was 9.83 ± 0.8 GPa. Specific data from the five tests are shown in Table 1.

[0054] Table 1. Specific data from the 5 tests

[0055] The above results show that, compared with loose ZIF-8 powder, the dense and transparent ZIF-8 bulk material prepared by the cold sintering method of this invention has significantly improved mechanical properties and can meet the requirements of practical applications for the mechanical strength of bulk materials.

[0056] In summary, this invention provides a dense and transparent ZIF-8 bulk material and its cold sintering preparation method. The bulk material has high density, optical transmittance, complete crystalline structure, high specific surface area and excellent mechanical properties. Moreover, the preparation process is simple and mild, and easy to scale up for mass production.

[0057] Finally, it should be noted that the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for the cold sintering preparation of a dense transparent ZIF-8 bulk, characterized in that, Includes the following steps: S1. Provides ZIF-8 powder; S2. Mix ZIF-8 powder with N,N-dimethylformamide to obtain a mixture; the mass ratio of ZIF-8 powder to N,N-dimethylformamide is 100:(3-7). S3. The mixture is subjected to cold sintering to obtain a dense and transparent ZIF-8 block; the uniaxial pressure of the cold sintering treatment is 10-30MPa, the temperature is 80-150℃, and the holding time is 1-3h.

2. The method of cold sintering densified transparent ZIF-8 bulk according to claim 1, wherein, In S1, the preparation process of the ZIF-8 powder includes: mixing a zinc source, an organic ligand, and a solvent, reacting them, and drying them to obtain ZIF-8 powder.

3. The method of claim 2, wherein the cold sintering of the dense, transparent ZIF-8 bulk is performed at a temperature of from 300 °C to 500 °C. The zinc source is at least one of zinc chloride and zinc nitrate; the organic ligand is 2-methylimidazole; and the solvent is methanol.

4. The method of claim 2, wherein the cold-sintered densified transparent ZIF-8 bulk is characterized by, The molar ratio of the zinc source to the organic ligand is 1:8; the molar ratio of the zinc source to the solvent is 1:(9-11).

5. The method of claim 2, wherein the cold-sintered densified transparent ZIF-8 bulk body is characterized by, The reaction is carried out at a temperature of 20-40℃ for 22-26 hours.

6. The method of claim 1, wherein the cold-sintered densified transparent ZIF-8 bulk body is characterized by, In S3, before the cold sintering process, a pre-pressing process is also included.

7. The method for cold sintering preparation of dense and transparent ZIF-8 bulk material according to claim 6, characterized in that, The pre-compression treatment is performed at a temperature of 20-30℃, a uniaxial pressure of 3-5MPa, and a holding time of 3-7min.

8. The method for cold sintering preparation of dense and transparent ZIF-8 bulk material according to claim 1, characterized in that, In S3, the heating rate of the cold sintering treatment is 3-7℃ / min.

9. A dense, transparent ZIF-8 bulk material, characterized in that, The dense and transparent ZIF-8 bulk material was prepared by cold sintering according to any one of claims 1 to 8.

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