Zeolite-like imidazolate framework material, and preparation method and application thereof

Abstract

The application discloses a zeolite-like imidazole framework material and a preparation method and application thereof, and comprises the following steps: adding benzimidazole, 4-nitroimidazole and a zinc source into an amide solvent to mix, to obtain a mixed solution; after heating and reacting the mixed solution, cooling, washing and drying, the zeolite-like imidazole framework material is obtained. In the reaction process of benzimidazole and the zinc source, 4-nitroimidazole is added, the benzimidazole in the original structure of ZIF-7 is replaced by 4-nitroimidazole, a polar group (-NO2) is introduced, the NO2-ZIF-7-0.4 zeolite-like imidazole framework material is obtained, the pore size is concentratedly distributed in the vicinity, is highly matched with the kinetic diameter of Rn atoms, the original narrow window of the ZIF-7 material is widened, the limitation of the window on the diffusion of Rn atoms in the adsorption material is avoided, the limited effect and the polarization effect are synergistically effective, and the adsorption performance of the modified ZIF-7 material on Rn is increased.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of radioactive gas adsorption, and particularly relates to a zeolite-like imidazolate framework material and a preparation method and application thereof. BACKGROUND

[0002] Radioactive gas refers to gas containing radionuclides. In the production of the atomic energy industry or the operation of nuclear facilities, different exhaust gases containing radionuclides are generated in different process engineering. Among them, radon (Rn) is a colorless, odorless, tasteless and natural radioactive noble gas, which is derived from the natural decay of long half-life 235 U (half-life of 7 x 10 8 years), 232 Th (half-life of 1.405 x 10 10 years) and 238 U (half-life of 4.458 x 10 9 years). Rn is the main source of human natural radiation, which contributes about 40% of all natural radiation doses received by humans. The most stable isotope of Rn is 222 Rn, with a half-life of 3.82 days. 222 Rn and its decay daughter, especially 218 Po and 214 Po, are all alpha and beta radionuclides, which are easily inhaled by humans and deposited in the human body environment when they are combined with aerosol particles in the air.

[0003] However, 222 Rn is difficult to form strong interaction with surrounding coordination groups as a noble gas. How to effectively capture and enrich and separate the extremely low concentration of radioactive noble gas Rn from complex air components is still an important challenge. At present, the main technologies for removing Rn are ventilation, coating plugging and physical adsorption. Forced ventilation has the characteristics of energy-intensive due to its additional equipment cost and use site limitations. Coating plugging is not the optimal solution for deep Rn removal because the coating is not moisture-resistant, easy to fall off, contains other chemical substances and is harmful to human health. The physical adsorption method based on porous materials has the advantages of simple operation and energy saving, and is the simplest and most effective deep Rn removal technology at present.

[0004] Traditional porous adsorption materials such as activated carbon and zeolite molecular sieve are widely used for the removal of Rn due to their large specific surface area, good stability, multi-level pore structure contained in the material itself, and low cost. However, the pore channels and pore volume of activated carbon are not uniformly distributed in space, and the micropores with adsorption blocking effect for Rn account for a low proportion of the effective volume mass fraction, limiting the saturated adsorption capacity for Rn. In many practical application scenarios, especially in high humidity environments, it is difficult to effectively achieve the purpose of deep purification of Rn. Silver exchanged zeolite (Ag-ETS-10 and Ag-ZSM-5) is reported to exhibit ultra-high Rn adsorption coefficient at room temperature in a nitrogen atmosphere, but the cost of silver exchanged zeolite is high, limiting its economic feasibility in large-scale applications, and humidity has a greater impact on the effect of silver exchanged zeolite adsorbing Rn. Experiments show that when humid air passes through the adsorption bed, Rn is quickly released (Scientific Reports, 2023, 13, 6811). Due to the properties of traditional porous materials caused by their own structure, the adsorption capacity of Rn is not high, and the selectivity is not ideal, making it difficult to achieve good results in the application of deep Rn removal. Therefore, how to solve the drawbacks of traditional porous materials and develop new targeted deep Rn removal materials is particularly important for the control of Rn radioactive pollution and the protection of human green living environment.

[0005] Metal-organic frameworks (MOFs) are a class of crystalline porous materials with periodic network structure formed by self-assembly of inorganic metal centers (metal ions or metal clusters) and bridging organic ligands. MOFs not only have ultra-high porosity and specific surface area, but also have adjustable pore size and easy modification characteristics. The modular construction method and highly adjustable structure of MOFs create a wide variety of ordered channels, providing conditions to meet different application needs.

[0006] The pore size, pore volume, and pore density of the adsorbent material are key factors affecting the gas adsorption process and capacity. Therefore, it is important to design the pore properties of the framework material according to the kinetic diameter of the adsorbed gas. If the pore size is too narrow, the adsorbed gas cannot enter the pore, and the overall adsorption efficiency of the adsorbent material will be affected. If the pore size is too large, the pore cannot produce confinement effect and interaction with the guest molecules, and cannot achieve the effect of adsorbing guest molecules. Therefore, designing a material with suitable pore properties can greatly improve the deep capture capacity of the adsorbent material for Rn. On the other hand, the polar structure in the structure of the adsorbent material can produce polarized adsorption of noble gases. For example, the Ag nanoparticles in the Ag@Ni-MOF-74 channel can induce the polarization of Xe (which has a similar kinetic diameter to Rn) and enhance the interaction with Xe. Density functional theory calculations show that the charge transfer from Xe to Ni or CoNi nanoparticles can result in higher binding energy, so the adsorbent material has a high affinity for Xe (Chem Commun, 2014, 50, 466). Since Xe has a similar kinetic diameter to Rn, and the polarizability of noble gases increases with the atomic number in the periodic table, the polarized ability of the polar structure in the structure of the adsorbent material should be higher for Rn.

[0007] Zeolitic imidazolate frameworks (ZIFs) materials are an important subclass of metal-organic frameworks (MOFs) materials, which are formed by bridging imidazoles and their derivatives with four-coordinated metal ions to form a material with zeolite topology. This kind of framework material not only has ultra-high porosity and specific surface area, and adjustable and easy-to-modify structural pore size, but also has excellent chemical stability and thermal stability, and inherent hydrophobicity. Among them, zeolitic imidazolate framework-ZIF-7 is a spherical cage-like cavity with a crystal six-membered ring (6MR) pore size of 6.8 A, which is formed by self-assembly of benzimidazole (BIm) as an organic ligand combined with metal nodes-zinc metal ions with sodalite (SOD) topology. The narrow window connecting two adjacent cage-like cavities is 4.8 A. The size of the ZIF-7 cavity is very matched with the kinetic diameter of the Rn atom between 6.8 A and 4.8 A, and has sufficient van der Waals interaction force for Rn; however, the narrow window has a high gate energy barrier to limit the diffusion of Rn atoms in the ZIF-7 structure. Therefore, it is of great significance to develop a zeolitic imidazolate framework material with suitable pore size and polar adsorption sites for the adsorption of radioactive gases. SUMMARY

[0008] The technical problem solved by the present application is to provide a zeolite-like imidazole framework material and a preparation method and application thereof, wherein 4-nitroimidazole (4-NO2-Im) with a smaller size is used to replace benzimidazole in the original structure of ZIF-7 without changing the topological type of the ZIF material, so as to expand the window and introduce a polar group (-NO2), so that the confinement effect and polarization effect work together to increase the adsorption performance of the modified ZIF-7 material on Rn, and solve the problems of poor adsorption effect, low selectivity, high cost and inability to mass production of the radioactive gas adsorption material in the prior art.

[0009] To solve the above technical problem, the present application provides a preparation method of a zeolite-like imidazole framework material, comprising the following steps:

[0010] S1, benzimidazole (BIm), 4-nitroimidazole (4-NO2-Im) and a zinc source are added to an amide solvent to obtain a mixed solution;

[0011] S2, after heating and reacting the mixed solution, cooling, washing and drying are performed to obtain the zeolite-like imidazole framework material.

[0012] In the present application, 4-nitroimidazole is added in the reaction process of benzimidazole and the zinc source, 4-nitroimidazole with a smaller size is used to replace benzimidazole in the original structure of ZIF-7, a polar group (-NO2) is introduced, and a NO2-ZIF-7-0.4 zeolite-like imidazole framework material is obtained, the pore size of which is concentrated in The kinetic diameter of the Rn atom is highly matched, the original narrow window of the ZIF-7 material is widened, the diffusion of the Rn atom in the adsorption material is avoided, the confinement effect and the polarization effect work together, and the adsorption performance of the modified ZIF-7 material on Rn is increased.

[0013] Further, the zinc source is one or more of zinc nitrate, zinc chloride, zinc sulfate and zinc nitrate hexahydrate, and is not limited thereto, and can also be other zinc-containing compounds that can completely ionize zinc ions in the solvent.

[0014] Further, the molar ratio of the benzimidazole, 4-nitroimidazole and zinc source is 1:(0.4-2.1):(1-1.5), and the preferred molar ratio is 1:0.4:1.

[0015] Further, the concentration of benzimidazole in the mixed solution is 0.014-0.028 mol·L -1 , and the preferred concentration is 0.019 mol·L -1 .

[0016] Further, the amide solvent is one or more of dimethylformamide (DMF) and dimethylacetamide, and the organic ligand is deprotonated by the amide solvent.

[0017] Further, in S2, the heating reaction is performed at a temperature of 85-150 DEG C for 3-5 days.

[0018] Further, in S2, the washing is performed by washing with DMF for multiple times and then washing with one or more of methanol, ethanol and acetone for multiple times.

[0019] Further, in S2, the drying condition is a temperature of 100-120 DEG C and a vacuum degree of <100 mTorr.

[0020] The second aspect of the present application provides the zeolite-like imidazolate framework material prepared by the preparation method of the first aspect.

[0021] The third aspect of the present application provides the application of the zeolite-like imidazolate framework material of the second aspect in adsorbing radon (Rn).

[0022] The present application has the following beneficial effects:

[0023] The present application substitutes 4-nitroimidazole for benzimidazole in the original structure of ZIF-7 to prepare NO2-ZIF-7-0.4 material, realizes the expansion of the narrow window of the original material, introduces a polar group, and makes the limited effect and polarization effect work together to increase the adsorption performance of the modified ZIF-7 material to Rn.

[0024] The preparation method of the present application is simple, the raw material cost is low, and the present application can be produced on a large scale, and can be used to solve the key problem of deep adsorption of radioactive gas in the environment, and has very important application prospect and commercial value. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is the X-ray powder diffraction pattern of NO2-ZIF-7-0.4 and ZIF-7 of the present application;

[0026] Figure 2 is the scanning electron microscope pattern of NO2-ZIF-7-0.4 and ZIF-7 of the present application;

[0027] Figure 3 is the nuclear magnetic hydrogen spectrum of BIm, 4-NO2-Im, ZIF-7 and NO2-ZIF-7-0.4 of the present application;

[0028] Figure 4 is the infrared spectrum of NO2-ZIF-7-0.4 and ZIF-7 of the present application;

[0029] Figure 5 This is the CO2 adsorption isotherm and pore size distribution diagram of NO2-ZIF-7-0.4 of the present invention at 273K.

[0030] Figure 6 This is a thermal stability analysis diagram of NO2-ZIF-7-0.4 of the present invention;

[0031] Figure 7 This is the powder diffraction pattern of NO2-ZIF-7-0.4 of the present invention after γ-irradiation;

[0032] Figure 8 In the figure, (a) is the Xe adsorption isotherm of NO2-ZIF-7-0.4 and ZIF-7 at 298 K, and (b) is the Henry's coefficient fitting line of NO2-ZIF-7-0.4 and ZIF-7 at 298 K.

[0033] Figure 9 In the diagram, (a) is a schematic diagram of the radon penetration device, (b) is the Rn penetration curve of ZIF-7, and (c) is the Rn penetration curve of NO2-ZIF-7-0.4. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0035] Example 1

[0036] This embodiment relates to a method for preparing a zeolite-like imidazole framework material, including the following steps:

[0037] 5.6 mmol (1.324 g) of benzimidazole (BIm), 2.24 mmol (0.506 g) of 4-nitroimidazole (4-NO2-Im), 5.6 mmol (3.332 g) of Zn(NO3)2·6H2O and 300 mL of N,N-dimethylformamide (DMF) were placed in a 500 mL blue-capped bottle and heated at 100 °C for 3 days. After cooling naturally to room temperature, the sample was washed three times with DMF and CH3OH respectively, and then dried under vacuum at 115 °C to obtain a white granular powder sample (NO2-ZIF-7-0.4).

[0038] Comparative Example 1

[0039] The difference between this comparative example and Example 1 is that no 4-nitroimidazole was added for substitution, and the ZIF-7 powder sample was prepared.

[0040] Test case

[0041] Structural characterization

[0042] Powder diffraction is an important criterion for detecting the purity of crystal product. The X-ray diffraction analysis was performed on the NO2-ZIF-7-0.4 prepared in Example 1 and the ZIF-7 prepared in Comparative Example 1, as shown in the powder diffraction patterns of Example 1 and Comparative Example 1 and the powder diffraction pattern of the simulated ZIF-7 structure are basically consistent, indicating that the synthesized material is a pure phase. Figure 1 Figure 2 Figures (a) and (b) are scanning electron micrographs of ZIF-7 and NO2-ZIF-7-0.4, respectively, and it can be seen that the particle shape of NO2-ZIF-7-0.4 is very similar to that of ZIF-7, and the particle size distribution is uniform in general.

[0043] H nuclear magnetic resonance (H NMR) is a method for analyzing and characterizing the molecular structure of organic compounds by using nuclear magnetic resonance technology. By analyzing the chemical shift, coupling constant, peak area and peak shape and other data on the H NMR spectrum, the structural characteristics of the molecule can be inferred. 1 1 H NMR spectra of ZIF-7 and NO2-ZIF-7-0.4 were characterized, as shown in 1 Figure 3 Figures (a), (b), (c) and (d) are H NMR spectra of BIm, 4-NO2-Im, ZIF-7 and NO2-ZIF-7-0.4, respectively. The characteristic shift peak (8.26) of 4-nitroimidazole appears in the H NMR spectrum of NO2-ZIF-7-0.4, proving that the -NO2 group has been successfully introduced into the ZIF-7 structure in NO2-ZIF-7-0.4.

[0044] Fourier transform infrared spectroscopy (FT-IR) characterizes chemical bonds and functional groups in molecules by measuring the absorption of specific frequency infrared light by the molecules. Fourier transform infrared spectroscopy analysis was performed on NO2-ZIF-7-0.4 and ZIF-7, as shown in Figure 4 Figure, a characteristic infrared absorption peak of -NO2 appears at an absorption wavelength of 1530 cm -1 , proving that the -NO2 group has been successfully introduced into the ZIF-7 structure in Example 1.

[0045] The specific surface area and pore size of porous adsorbent materials are important factors affecting the gas adsorption performance. Generally, the larger the specific surface area of the material and the more suitable the pore size, the more suitable the material is for adsorbing guest molecules. As shown in Figure 5 Based on the CO2 adsorption isotherm measured at 273 K and 1 bar for NO2-ZIF-7-0.4, the specific surface area of the material was calculated to be 105.59 m2 / g using the BET (Brunauer-Emmett-Teller) formula.​​​2 ·g -1 Further, the pore size distribution of the NO2-ZIF-7-0.4 material can be obtained by using the NLDFT method model, which is concentrated in the range of 3.5-4.5 A, matching the kinetic diameter of Rn.

[0046] Stability analysis

[0047] The thermal stability and irradiation stability of the NO2-ZIF-7-0.4 prepared in Example 1 were studied. As shown in FIG. 6, the thermal gravimetric experiment proved that the material can be stable to 400°C under nitrogen atmosphere, indicating that the material has good thermal stability. Figure 6

[0048] Since Rn is radioactive, the adsorption material will be irradiated by a certain amount of radiation in the process of adsorbing radioactive Rn gas. In view of this, the irradiation stability of the material is of great significance to the practical application of the adsorption material. As shown in FIG. 7, the powder diffraction of the NO2-ZIF-7-0.4 material was analyzed after being irradiated by 50 kGy, 100 kGy and 150 kGy of γ rays, respectively, and compared with the diffraction pattern of the (As synthesized) NO2-ZIF-7-0.4 synthesized in Example 1 and the (Simulated) diffraction pattern. The results show that the synthesized NO2-ZIF-7-0.4 material has good irradiation stability, indicating that the structure of NO2-ZIF-7-0.4 will not be destroyed in the process of adsorbing radioactive Rn. Figure 7

[0049] Xe adsorption analysis

[0050] Since the kinetic diameter of Xe atom is very close to the kinetic diameter of Rn atom , and the conventional Xe gas is not radioactive. Therefore, we will use the adsorption performance of Xe by the adsorption material to predict the adsorption effect of Rn before carrying out the adsorption experiment of radioactive Rn gas. Therefore, the Xe adsorption performance of NO2-ZIF-7-0.4 at room temperature was studied by static adsorption experiment. As shown in FIG. 8(a), the Xe adsorption isotherm of ZIF-7 material has a typical hysteresis loop, and the adsorption isotherm and the desorption isotherm do not coincide, which also proves the existence of the narrow window of ZIF-7 material. The Xe adsorption curve of NO2-ZIF-7-0.4 conforms to the characteristics of type I isotherm, which proves that the addition of 4-nitroimidazole expands the narrow window in the structure of ZIF-7 material, further improving the adsorption capacity of the material for Xe. Figure 8

[0051] ​​​​To more clearly understand the enhancement of the Xe adsorption capacity of the NO2-ZIF-7-0.4 material, the slope of the straight line obtained by linear fitting of the adsorption isotherm under low pressure conditions is the Henry constant, which can be used to characterize the strength of the interaction between the adsorbent material and the adsorbed gas under low pressure conditions. By comparing the Henry constants of ZIF-7 and NO2-ZIF-7-0.4 materials in the low pressure region, the effect of introducing -NO2 functional groups into the original structure on the Xe adsorption capacity can be clearly understood, as shown in Figure 8 (b). It is calculated that the Xe adsorption Henry constants of ZIF-7 and NO2-ZIF-7-0.4 materials at 298K are 7.1 mmol·g -1 ·bar -1 and 73.96 mmol·g -1 ·bar -1 respectively, indicating that the introduction of -NO2 functional groups has increased the interaction of the material with Xe by nearly 10 times. This data also indicates that the modified ZIF-7 material will have good adsorption effect on the radioactive gas Rn.

[0052] Rn breakthrough experiment

[0053] The dynamic Rn adsorption performance of ZIF-7 and NO2-ZIF-7-0.4 materials at 297K was studied using the device shown in Figure 9 (a). The closed Rn chamber with an initial concentration of 2000 Bq·m -3 maintained a constant Rn concentration, which came from the decay of 226 Ra. The Rn breakthrough data are shown in Figure 9 (b) and 9(c). Unlike the Rn concentration curve of ZIF-7, which remained unchanged, the Rn concentration began to drop rapidly a few minutes after the NO2-ZIF-7-0.4 sample column was introduced into the gas path. After the Rn concentration curve of NO2-ZIF-7-0.4 dropped to the lowest value (about 63 Bq·m -3 ), the concentration curve began to rise, and after about 360 min, it returned to the initial concentration. The change in the Rn concentration curve can directly indicate that the introduction of -NO2 into the ZIF-7 structure not only widens the originally narrow window to allow Rn atoms to diffuse well in the material, but also greatly enhances the interaction of the ZIF-7 material with Rn atoms.

[0054] By fitting the descending and rising sections of the Rn breakthrough curve, and then calculating by the formula, the dynamic adsorption coefficient (K d) and the saturated adsorption capacity (Q), the calculation method is known to those skilled in the art, and here is not described. NO2-ZIF-7-0.4 shows the highest Rn adsorption performance (Q = 20.57 Bq·g -1 ,K d = 10.78 g·L -1 ), which is more than twice the adsorption performance of the best commercial activated carbon (Q = 14.1 Bq·g -1 ,K d = 5.2 g·L -1 ).

[0055] In summary, by replacing the benzimidazole in the original structure of ZIF-7 with 4-nitroimidazole, the narrow window of the original material is expanded, and a polar group is introduced, so that the confinement effect and polarization effect work together to increase the adsorption performance of the modified ZIF-7 material for Rn

[0056] The above examples are only preferred examples for fully illustrating the present application, and the protection scope of the present application is not limited thereto. Any equivalent replacement or transformation made by those skilled in the art on the basis of the present application is within the protection scope of the present application. The protection scope of the present application is subject to the claims.

Claims

1. Use of a zeolite-like imidazolate framework material for adsorbing radon gas, characterized in that, The preparation method of the zeolite-like imidazole framework material comprises the following steps: S1, benzimidazole, 4-nitroimidazole and zinc source are added into an amide solvent according to a molar ratio of 1:0.4:1 to mix, to obtain a mixed solution; S2, the mixed solution is heated to react, 4-nitroimidazole substitutes benzimidazole in ZIF-7 generated by the reaction of benzimidazole and zinc source to generate NO2-ZIF-7-0.4, and then the product is cooled, washed and dried to obtain the zeolite-like imidazole framework material.

2. Use of a zeolite imidazolate framework material according to claim 1 for adsorbing radon, characterized in that, The zinc source is one or more of zinc nitrate, zinc chloride, zinc sulfate and zinc nitrate hexahydrate.

3. Use of a zeolite imidazolate framework material as claimed in claim 1 for adsorbing radon gas, characterized in that, The concentration of benzimidazole in the mixed solution is 0.014-0.028 mol·L -1 .

4. Use of a zeolite imidazolate framework material according to claim 1 for adsorbing radon, characterized in that, The amide solvent is one or more of dimethylformamide and dimethylacetamide.

5. Use of a zeolite imidazolate framework material according to claim 1 for adsorbing radon, characterized in that, In S2, the temperature of the heating reaction is 85-150 DEG C, and the time is 3-5 days.

6. Use of a zeolite imidazolate framework material according to claim 1 for adsorbing radon gas, characterized in that, In S2, the drying condition is that the temperature is 100-120 DEG C, and the vacuum degree is less than 100 mTorr.

7. Use of a zeolite imidazolate framework material according to claim 1 for adsorbing radon gas, characterized in that, In S2, the washing is specifically that the product is first washed by dimethylformamide, and then washed by one or more of methanol, ethanol or acetone.

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