A method for preparing porous MOFs by microbial etching

Abstract

The present invention provides a method for preparing hierarchical porous MOFs by microbial etching. The microbial etching utilizes the interaction between microbial cells and metal-organic framework materials to form defects in the latter. The hierarchical pores include mesopores, macropores formed by etching, and the original micropores of the metal-organic framework. This method uses the collected microbial cells after culturing the microbial strain as a bio-etching agent, and then mixes the bio-etching agent with the metal-organic framework material under specific conditions to prepare hierarchical porous metal-organic framework materials. The method of the present invention has the advantages of being green, environmentally friendly, simple and efficient, and can be used for large-scale production. At the same time, this method is applicable to a variety of metal-organic framework materials, and the prepared hierarchical porous metal-organic framework materials still retain the original crystal structure. The mesopores and macropores in the hierarchical pore structure can effectively improve the mass transfer efficiency, and have potential application values in drug loading, efficient catalysis, gas adsorption and separation, etc.

Description

Technical Field

[0001] The present invention relates to the field of preparation of porous materials, and particularly to a method for preparing hierarchical porous MOFs by microbial etching. Background Art

[0002] Metal-organic framework materials (MOFs) are a class of organic-inorganic hybrid crystal materials formed by self-assembly of organic ligands and metal ions through coordination bonds. They have a controllable pore structure and an ultra-high specific surface area, and thus have a wide range of applications in catalytic oxidation, gas adsorption and separation, sensors, biomedicine, optical materials, etc. Usually, the pores existing in MOFs are all micropores (pore diameter less than 2 nm), which is not conducive to the diffusion of large-volume molecules and the interaction with active sites inside the MOF structure, restricting their applications in aspects such as loading of large-volume molecules and heterogeneous catalysis. Therefore, on the premise of maintaining the original crystal structure of MOFs unchanged, the preparation of hierarchical porous MOFs with mesopores (pore diameter 2 - 50 nm) or macropores (pore diameter 50 - 500 nm) has become a research hotspot in recent years.

[0003] Albolkany et al. (Angew. Chem. Int. Ed, 2021, 60(26), 14601 - 14608) developed a method for gas-phase etching of HKUST-1. By using ammonia gas to cut off the coordination bond between carboxylate and metal, pores are generated on the crystal surface. The etched HKUST-1 successfully achieved the encapsulation and controlled release of macromolecules. However, due to the limited reactivity of ammonia gas, only mesopores can be etched, macropores cannot be obtained, and it is only applicable to MOF species with relatively weak coordination bond forces; Song et al. (ACS Materials Letters, 2021, 3(2), 171 - 178) etched ZIF-8 materials using a wet chemical process. By adjusting the concentration and pH of the etching agent, ZIF-8 crystals with different shapes can be obtained, but the etching process of this process is very slow and the preparation efficiency is low; Li et al. (Advanced Energy Materials, 2020, 10(33)) used three-dimensionally ordered polystyrene microspheres as templates to in-situ synthesize HKUST-1 with a macroporous structure. When the carbon material derivative of this HKUST-1 is used to prepare supercapacitor electrodes, its diffusivity and structural stability are effectively improved, making it have excellent conductivity rate and cycle stability. However, this method requires repeated washing with anhydrous ethanol and N,N-dimethylformamide solution to remove the polystyrene template, resulting in a large consumption of chemical reagents, poor economy of the preparation process and easy pollution. Summary of the Invention

[0004] The existing preparation methods of hierarchical porous MOFs have problems such as low efficiency, complex processes, high energy consumption, and environmental unfriendliness. Moreover, it is difficult to obtain mesopores and / or macropores simultaneously by the existing methods. The present invention innovatively develops a method for preparing hierarchical porous MOFs by microbial etching. The obtained material retains the original crystal structure of MOFs, has a high specific surface area, a controllable hierarchical composite pore structure, and a porous structure at the same time.

[0005] The present invention mainly includes two parts: the preparation of a biological etchant and the biological etching regulation of MOFs. The biological etchant refers to the cell dispersion of microorganisms such as bacteria, molds, and yeasts. According to their different growth characteristics, they are respectively applicable to different microbial species or different organs of the same microorganism. The biological etching regulation of MOFs is to control the etching conditions to controllably etch new mesopores and macropores in the crystal with only a microporous structure originally by using the interaction between the microbial etchant and MOFs on the premise of retaining the original crystal structure. At the same time, for the chemical properties of different MOFs, the present invention selects different solvents and pH values to protect the crystal structure from being damaged during the etching process.

[0006] By changing the concentration of the etchant and the etching time, the number and size of the pores etched in MOFs can be controlled, so as to achieve fine regulation of the pore channels of MOFs materials at the micro-nano level. The present invention is applicable to different microbial etchants and various MOFs. The prepared hierarchical porous MOFs have good structural stability and high catalytic performance, and can be reused. This material can meet the requirements of material manufacturing in multiple aspects such as drug loading, gas adsorption and separation, and catalysis, and is of great significance for promoting the preparation and application of MOFs materials.

[0007] The key point of the present invention is that for the first time, a microbial etching method is developed for preparing hierarchical porous MOFs. There has been no relevant report on the biological etching of MOFs before. Since the etchant used in the preparation process is not a traditional chemical reagent, but various microorganisms widely existing in nature, and this microbial etchant can be recovered, separated, cultured, and recycled, a large amount of energy and harsh reaction conditions are not required during the preparation process, no toxic and dangerous chemicals are used, no large amount of chemical waste is generated, and there is no pollution to the environment, which conforms to the development concept of green environmental protection. In addition, the preparation process of this method is simple, the etching conditions are mild, no complex equipment is required, the preparation cycle is short, and the raw materials are easy to separate and recycle, so it is suitable for large-scale production.

[0008] The present invention adopts the following technical scheme:

[0009] A method for preparing porous hierarchical MOFs by microbial etching, comprising the following steps: mixing a microbial cell dispersion and MOFs together, and etching MOFs with the microbial cells to obtain porous hierarchical MOFs.

[0010] Further, the method for preparing porous hierarchical MOFs by microbial etching includes the following steps: After culturing the microbial strain, washing and drying it, formulating a cell dispersion with a mixed solution of a polar organic solvent and water, adjusting the pH of the cell dispersion, adding MOFs to the cell dispersion, and performing etching under sealed stirring conditions to obtain porous hierarchical MOFs.

[0011] The MOFs are well-known in the art, including HKUST-1, ZIF-8, UiO-66, and ZIF-67. The porous hierarchical MOFs refer to MOFs obtained by etching and having two or three of micropores (pore diameter less than 2 nm), mesopores (pore diameter between 2 - 50 nm), and macropores (pore diameter greater than 50 nm), such as micropore-mesopore, micropore-macropore, and micropore-mesopore-macropore.

[0012] Further, the microorganisms are some microbial species with certain resistance to metal ions or capable of taking up metal ions as nutrient elements, including at least one of molds, bacteria, and yeasts. The form of the microorganisms is at least one of spores and hyphae, preferably mold spores. The mold is further preferably Aspergillus niger, Penicillium chrysogenum, or Rhizopus. The bacteria are selected from Nostoc flagelliforme and Bacillus subtilis; the yeasts are selected from Saccharomyces cerevisiae A and Candida sp.

[0013] Further, the polar organic solvent can be a polar organic solvent that is non-toxic to microbial cells, such as glycerol, propylene glycol, butylene glycol, or isopentyl glycol; the mass ratio of the organic solvent in the mixed solution of the polar organic solvent and water is 20 - 50%. The concentration of the cell dispersion is 1 - 15 g / L, such as 1 g / L, 3 g / L, 5 g / L, 8 g / L, 10 g / L, 12 g / L, 13 g / L, 14 g / L, or 15 g / L. The concentration of the cell dispersion is the ratio of the mass of the dried cells to the volume of the dispersion solvent. As the concentration of the bio-etchant increases, the number of pores on the surface of the MOFs increases; as the bio-etching time extends, the pore size on the surface of the MOFs increases, thereby obtaining a hierarchical pore material with adjustable pore number and size.

[0014] The pH adjustment is to adjust the pH to 4 - 7 with a weak acid or a weak base. The weak acids include but are not limited to oxalic acid solution, maleic acid solution, and citric acid solution; the weak bases include but are not limited to solutions of at least one of sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate. Preferably, when using a weak acid to adjust the pH, a compound organic acid solution with a mass ratio of citric acid:oxalic acid:maleic acid of 2 - 3:2 - 3:1 is used. The concentrations of the weak acid and the weak base are not particularly limited. In a specific embodiment of the present invention, the concentration of the weak acid or the weak base is 0.005 - 0.025 mol / L, preferably 0.01 - 0.02 mol / L.

[0015] The inventor unexpectedly found that selecting organic acids prepared by compounding citric acid, oxalic acid and maleic acid in a certain proportion can obtain porous MOFs with more stable morphology. The possible reason is that citric acid, oxalic acid and maleic acid will be produced during microbial metabolism, and the mixed organic acid solution of these three compounds is closer to the solution environment of natural microbial metabolism.

[0016] The conditions for etching are as follows: the temperature is not particularly limited, and it is only necessary to maintain the activity of the strain. Generally, it is 15 - 30°C, preferably 20 - 25°C, which can be selected according to the characteristics of different bacteria; the stirring speed is 50 - 100 rpm. If the stirring speed is too slow, it will affect the dispersion of the strain and thus affect etching. If the stirring speed is too fast, the etching will also be unstable. The etching time is not particularly limited. As the etching time prolongs, the number of pores on the MOFs increases. The etching time can be adjusted within the range of 0.5 - 10 h according to actual needs, so as to control the degree of etching.

[0017] The post-treatment steps after etching are well-known in the art. In a specific embodiment of the present invention, after etching, centrifugation, washing and drying are carried out to obtain porous MOFs.

[0018] The cultivation of the microbial strain of the present invention is to transfer the microbial strain onto a culture medium and cultivate it for a period of time under sterile, constant temperature and constant humidity conditions. Then, the microbial cells are collected, washed and dried, and then placed in a specific solvent and mixed evenly to obtain a cell dispersion liquid with controllable concentration, which is the bio-etchant.

[0019] Specifically, the culture medium for culturing the microorganism can be a solid culture medium, a liquid culture medium or a semi-solid culture medium, including but not limited to PDA medium, Martin's medium, BG110 medium, YEPD medium and various natural culture media. The preferred culture conditions are a temperature of 20 - 40°C, a humidity of 80 - 90%, and a culture time of 5 - 10 days. The specific culture scheme can be appropriately adjusted according to the growth characteristics of the selected microorganism.

[0020] For different types of fungi, the methods for collecting cells are different. For bacteria-like microorganisms, the method for collecting cells is as follows: select a solid culture medium to culture the strain, place the culture dish in a sterile incubator and culture for 5 days. When the culture dish is covered with colonies, scrape the cells with a spreading rod, wash them several times with sterile water and then dry them at room temperature under vacuum to obtain bacterial cells; for yeast-like microorganisms, the method for collecting cells is as follows: select a liquid culture medium to culture the strain, wrap the mouth of the liquid culture medium with 7 layers of gauze, place it on a shaker and shake it for 7 days. When the culture medium becomes turbid, take it out and centrifuge to remove the upper culture solution. Wash the lower precipitate several times with sterile water and then dry it at room temperature under vacuum to obtain yeast cells; for mold-like microorganisms, the cells can be further divided into hyphae (the vegetative organs of the cells) and spores (the reproductive organs of the cells).

[0021] Further, the method for collecting the mycelia of the mold is as follows: Select a liquid medium to culture the strain, wrap the mouth of the liquid medium with seven layers of gauze, place it on a shaker and shake culture for 4 - 6 days. When a large number of fluffy mycelial balls appear in the bottle, take it out, filter to remove the culture solution, wash it several times with sterile water, and then dry it at room temperature under vacuum to obtain the mold mycelia;

[0022] The method for collecting the spores of the mold is as follows: Select a solid medium to culture the strain, invert the culture dish, place it in a sterile incubator and culture for 8 - 10 days. When black spores appear on the mycelia, the spores can fall off naturally after maturation. Collect the spores on the lid of the culture dish, wash it several times with sterile water, and then dry it at room temperature under vacuum to obtain the mold spores.

[0023] The method of the present invention can conveniently and controllably prepare porous hierarchical MOFs by microbial etching, mainly including two key points. First, a suitable solution environment should be created to ensure both the stability of the MOF structure and the metabolic activity of the microorganism. In the present invention, this is achieved by adding a non-toxic polar solvent and adjusting the appropriate pH. Second, to make the etching controllable, mainly control the pore number by adjusting the concentration of the etchant; control the pore size by changing the etching time.

[0024] The effects of the present invention: The original crystal structure of the hierarchical porous MOFs prepared by the present invention is completely retained, with a high specific surface area, having a hierarchical composite pore structure of micropores, mesopores and macropores, where the macropore size can reach 300 nm. At the same time, it has high catalytic performance and reusability, and has potential application values in aspects such as macromolecular drug loading, macromolecular gas adsorption and separation, and high-efficiency catalysis. The method of the present invention has the advantages of being green, environmentally friendly, simple and efficient, and can be used for large-scale production. The biological etchants used in the present invention can be recycled and continue to be cultured, with extremely low costs. Most of the reagents used are environmentally friendly and have no pollution to the environment, meeting the requirements of sustainable development. The present invention is applicable to a variety of microbial etchants, such as molds, yeasts, bacteria, etc. Similarly, the present invention is applicable to a variety of MOF materials, such as HKUST-1, ZIF-8, UiO-66, ZIF-67, etc. The prepared hierarchical porous metal-organic framework materials still retain the original crystal structure, and the mesopores and macropores in their hierarchical pore structures can effectively improve the mass transfer efficiency, and have potential application values in aspects such as drug loading, high-efficiency catalysis and gas adsorption and separation. Description of the Drawings

[0025] Figure 1 is the scanning electron microscope image of the hierarchical porous HKUST-1 obtained in Example 1;

[0026] Figure 2 is the X-ray diffractometer image of the hierarchical porous HKUST-1 obtained in Example 1;

[0027] Figure 3It is the nitrogen adsorption instrument diagram of the hierarchical pore HKUST-1 obtained in Example 1;

[0028] Figure 4 It is the comparison of the catalytic performance between the microporous HKUST-1 and the hierarchical pore HKUST-1 obtained in Example 1; Figure 4 (a) The conversion rate of benzaldehyde at different reaction times; Figure 4 (b) The repeated catalytic performance test of HKUST-1;

[0029] Figure 5 It is the scanning electron microscope image of the hierarchical pore HKUST-1 obtained in Example 2;

[0030] Figure 6 It is the scanning electron microscope image of the hierarchical pore HKUST-1 obtained in Example 3;

[0031] Figure 7 It is the scanning electron microscope image of the hierarchical pore HKUST-1 obtained in Example 4;

[0032] Figure 8 It is the scanning electron microscope image of the hierarchical pore HKUST-1 obtained in Example 5;

[0033] Figure 9 It is the scanning electron microscope image of the hierarchical pore ZIF-8 obtained in Example 6;

[0034] Figure 10 It is the scanning electron microscope image of the hierarchical pore HKUST-1 obtained in Example 7;

[0035] Figure 11 It is the scanning electron microscope image of the hierarchical pore ZIF-8 obtained in Example 8;

[0036] Figure 12 It is the scanning electron microscope images of four kinds of hierarchical pore HKUST-1 obtained in Example 9;

[0037] Figure 13 It is the scanning electron microscope images of four kinds of hierarchical pore HKUST-1 obtained in Example 10;

[0038] Figure 14 It is the scanning electron microscope image of the hierarchical pore UiO-66 obtained in Example 11;

[0039] Figure 15 It is the scanning electron microscope image of the hierarchical pore ZIF-8 obtained in Example 12;

[0040] Figure 16 It is the scanning electron microscope image of the hierarchical pore ZIF-67 obtained in Example 13;

[0041] Figure 17 It is the scanning electron microscope images of three kinds of HKUST-1 obtained in Example 14;

[0042] Figure 18 It is the SEM image of HKUST-1 obtained in Example 15;

[0043] Figure 19 It is the SEM image of HKUST-1 obtained in Comparative Example 1. Specific Embodiments

[0044] The following combines specific embodiments to further describe in detail the specific embodiments of the present invention. The following embodiments are used to illustrate the present invention, but are not used to limit the protection scope of the present invention. The protection scope of the present invention is subject to the claims.

[0045] Unless otherwise specified, the experimental reagents and materials used in the embodiments of the present invention can be obtained commercially. Unless specifically specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0046] Example 1

[0047] The microporous MOFs used in this example are HKUST-1, purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; the microbial cells used are Aspergillus niger spores.

[0048] (1) Aspergillus niger strain culture: Select PDA solid medium. Wash 200 g of potatoes, peel them, cut them into small pieces, add an appropriate amount of water and boil for 30 min, then filter them in a gauze bag. Add 20 g of glucose and 20 - 25 g of agar to the filtrate, make up the volume to 1000 mL with water, heat again until the agar is completely melted, place it in a steam sterilizer and sterilize for 30 min (pressure 0.1 MPa, temperature 121 °C), take it out and pour it into a petri dish to cool and solidify to obtain the solid medium. Inoculate one loop of the strain into the above medium, and place the petri dish upside down in a mold incubator and culture for 10 days (temperature 30 °C, humidity 90%);

[0049] (2) Spore dispersion preparation: Collect the spores on the dish cover, wash them several times with sterile water and then vacuum dry. Take 0.45 g of spores and disperse them in 30 mL of 30% glycerol-water mixture, stir evenly, and adjust the pH to 4 with a compound organic acid solution (citric acid: oxalic acid: maleic acid = 2:1:1) with a total molar concentration of organic acids of 0.02 mol / L to obtain a spore dispersion with a concentration of 15 g / L;

[0050] (3) Biological etching of MOFs: Take 30 mg of microporous HKUST-1 and add it to the spore dispersion, seal and stir at room temperature of 25 °C (stirring speed is 50 RPM) to allow the spore etchant to etch HKUST-1, and the etching time is 3 h;

[0051] (4) After the etching is completed, the mixture is centrifuged to remove the upper liquid (centrifugation speed: 1000 RPM, centrifugation time: 6 min), and the lower precipitate is collected. After vacuum drying at 70 °C, the hierarchical pore HKUST-1 is obtained (attached Figure 1 is the SEM photograph).

[0052] As Figure 2 shown, it is the X-ray diffractometer pattern. From top to bottom, they are the fitted HKUST-1, microporous HKUST-1, and the hierarchical pore HKUST-1 obtained by microbial etching according to the method of Example 1. Compared with the initial microporous HKUST-1, the characteristic peaks of the X-ray diffractometer for the hierarchical pore HKUST-1 obtained in Example 1 did not change, indicating that the crystal structure was intact during the etching process; as Figure 3 shown, the adsorption-desorption curve of the nitrogen sorption analyzer indicates that new pore structures (mesopores and macropores) were added to HKUST-1;

[0053] (5) Catalytic conversion of benzaldehyde by hierarchical pore HKUST-1: 6.6 mg of hierarchical pore HKUST-1 and 5 mL of tetrahydrofuran are placed in a 25 mL sample tube and sonicated for 1 min to mix them evenly. Then, 0.201 g of benzaldehyde (1.9 mmol) and 0.251 g of malononitrile (3.8 mmol) are added to the above mixture respectively, and the mixture is stirred at room temperature. At the end of the reaction, ethyl acetate is added to quench the reaction. The mixtures at different reaction times (0 - 8.5 h) are taken, and the hierarchical pore HKUST-1 powder is removed by centrifugation. The conversion rate of benzaldehyde is calculated using a gas chromatography-mass spectrometry instrument.

[0054] (6) Recycling of hierarchical pore HKUST-1 as a catalyst: The mixture after the catalytic reaction is centrifuged to remove the solution, and the hierarchical pore HKUST-1 is separated. After washing with tetrahydrofuran and centrifuging and drying to remove the excess solvent, it is used as a catalyst again. The reaction and calculation processes are the same as in step (5). The above steps are cycled multiple times, and the conversion rate of benzaldehyde after 3 h of catalytic reaction after the hierarchical pore HKUST-1 is recycled different times is calculated. The results are shown in Table 1 and Figure 4 (b).

[0055] Table 1

[0056] Number of cycles 1 2 3 4 5 6 7 Conversion rate (%) 99.4 98.5 96.3 93.2 90.4 86.5 81.2

[0057] Figure 4 is the performance graph of the catalytic conversion of benzaldehyde by the traditional microporous HKUST-1 and the porous HKUST-1 prepared by microbial etching in Example 1, where Figure 4(a) shows the relationship between the conversion rate of benzaldehyde and time. The porous HKUST-1 of the present invention can reach the maximum conversion rate in 3 h, while the microporous HKUST-1 takes 7 h to reach the maximum conversion rate, indicating that the porous MOFs obtained by microbial etching in the present invention have better catalytic activity; Table 1 and Figure 4 (b) shows that the hierarchically porous HKUST-1 prepared in this example has high catalytic performance and reusability.

[0058] Example 2

[0059] The MOFs used in this example are microporous HKUST-1, purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; the microbial cells used in this example are Aspergillus niger hyphae.

[0060] (1) Aspergillus niger strain culture: Select liquid modified Martin's medium. Take 1 g of KH2PO4, 0.5 g of MgSO4·7H2O, 10 g of glucose, and 5 g of peptone, dissolve them in 1 L of distilled water, then add 3.3 mL of 1% rose bengal aqueous solution and 3 mL of 1% streptomycin, stir evenly, and dispense into Erlenmeyer flasks. Sterilize in a steam sterilizer for 30 min (pressure 0.1 MPa, temperature 121 °C), take out and cool to obtain the liquid medium. Inoculate one loop of Aspergillus niger strain into 300 mL of the above medium, and shake culture at 150 RPM and 40 °C for 5 days.

[0061] (2) Preparation of hyphal dispersion: After culturing for 5 days, a large number of fluffy mycelial balls appear in the medium. Take out, filter to remove the culture solution, wash several times with sterile water, and then vacuum dry to obtain Aspergillus niger mycelial balls. Take 0.45 g of mycelial balls and disperse them in 30 mL of 30% glycerol-water mixture, stir evenly, and adjust the pH to 4 with 0.02 mol / L compound organic acid solution (citric acid: oxalic acid: maleic acid = 2:1:1) to obtain a hyphal dispersion with a concentration of 15 g / L;

[0062] (3) Etch to obtain hierarchically porous HKUST-1 according to steps (3) and (4) in Example 1. The SEM photos are shown in Figure 5 , since the spore size is smaller than that of the mycelial balls, the etching process is easier to control, and the pores etched are more uniform than the hierarchically porous HKUST-1 obtained in Example 1.

[0063] Example 3

[0064] The MOFs used in this example and other operation steps are the same as those in Example 1, except that the microbial cells selected in this example are Penicillium Chrysogenum spores. The hierarchically porous HKUST-1 prepared in this example is as shown in Figure 6 shown.

[0065] Example 4

[0066] The MOFs used in this example and other operation steps are the same as those in Example 1, except that the microbial cells selected in this example are Rhizopus spores. The hierarchically porous HKUST-1 prepared in this example is as shown in Figure 7 shown.

[0067] Aspergillus niger, Penicillium chrysogenum, and Rhizopus are representatives of molds, indicating that the microbial etching method is applicable to mold microorganisms.

[0068] Example 5

[0069] The MOFs used in this example are HKUST-1, purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; the microbial cells used are Nostoc flagelliforme.

[0070] (1) Cultivation of Nostoc flagelliforme: Select solid BG110 medium. Weigh 15 g of agar powder, 6 g of citric acid, 1 g of EDTA·2Na, 36 g of CaCl2·2H2O, 23.8 g of K2HPO4·2H2O, 75 g of MgSO4·7H2O, 2.86 g of H3BO3, 1.8 g of MnCl2·4H2O, 0.2 g of ZnSO4·7H2O, 0.4 g of NaMoO4·2H2O, 0.08 g of CuSO4·5H2O, 0.01 g of CoCl2, 4.8 g of FeSO4·7H2O, and 58 g of NaSiO3, dissolve them in 1000 mL of distilled water, stir evenly, dispense into Erlenmeyer flasks, sterilize in a steam sterilizer for 30 min (pressure 0.1 MPa, temperature 121 °C), take out and pour into petri dishes to cool and solidify to obtain the solid medium. Spread the Nostoc flagelliforme bacterial solution evenly on the above medium and place it in a bacterial incubator for 5 days (temperature 25 °C, humidity 80%);

[0071] (2) Preparation of cell dispersion: After 5 days of cultivation, the petri dish is covered with colonies. Scrape the cells with a spreading rod, wash them several times with sterile water, and then vacuum dry to obtain Nostoc flagelliforme cells. Take 0.45 g of cells and disperse them in 30 mL of 30% glycerol-water mixture, stir evenly, and adjust the pH to 4 with 0.02 mol / L compound organic acid solution (citric acid: oxalic acid: maleic acid = 2:1:1) to obtain a cell dispersion with a concentration of 15 g / L;

[0072] (3) Etch to obtain hierarchically porous HKUST-1 according to steps (3) and (4) in Example 1. The SEM photos are shown in Figure 8 , which is the same as the hierarchically porous HKUST-1 obtained in Example 1 ( Figure 1)Compared with [reference], the etching is uneven, and the diameter and depth of the etched holes are smaller.

[0073] Example 6

[0074] The MOFs used in this example were self-synthesized ZIF-8; the microbial cells used were Bacillus subtilis.

[0075] (1) MOFs synthesis: Dissolve 1.8511 g of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 1.6212 g of 2-methylimidazole (2-MIM) in 70 mL of N,N-dimethylformamide respectively. Then, while stirring, slowly pour the 2-methylimidazole solution into the zinc nitrate solution. After the two are mixed evenly, stop stirring. Transfer the mixed solution to a 350 mL pressure-resistant bottle and seal it. Let it stand in an oil bath at 140 °C for 24 h. After cooling to room temperature, centrifuge to collect the lower-layer precipitate, add methanol, stir and wash for 2 h, then centrifuge. Repeat the washing 3 times. Place it in a vacuum dryer at 70 °C to obtain the ZIF-8.

[0076] (2) Cultivate Bacillus subtilis according to step (1) in Example 5.

[0077] (3) Collect the spores, wash them several times with sterile water and then dry them in vacuum. Take 0.3 g of spores and disperse them in 30 mL of sterilized water. Stir evenly and adjust the pH to 7 with 0.01 mol / L sodium hydroxide to obtain a spore dispersion with a concentration of 10 g / L.

[0078] (4) Biological etching of MOFs: Add 30 mg of ZIF-8 to the spore dispersion, seal and stir at room temperature (stirring speed is 50 RPM) to allow the spore etchant to etch ZIF-8 for 8 h.

[0079] (5) Centrifuge and wash according to step (4) in Example 1 to obtain hierarchical porous ZIF-8. The SEM photos are shown in Figure 9 .

[0080] Cyanobacterium flagelliforme and Bacillus subtilis are representatives of bacteria, indicating that the microbial etching method is also applicable to bacterial microorganisms.

[0081] Example 7

[0082] The MOFs used in this example were microporous HKUST-1, purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; the microbial cells used were Saccharomyces sp. A.

[0083] (1) Cultivation of Saccharomyces cerevisiae: Select YEPD liquid medium. Dissolve 100 g of glucose, 10 g of peptone, and 2 g of diammonium hydrogen phosphate in 1000 mL of distilled water. After stirring evenly, dispense it into Erlenmeyer flasks and sterilize it in a steam sterilizer for 30 min (pressure 0.1 MPa, temperature 121 °C). Inoculate one loop of Saccharomyces cerevisiae strain into 300 mL of the above medium and shake-culture it at 150 RPM and 20 °C for 7 days.

[0084] (2) Preparation of cell dispersion: After 7 days of cultivation, take it out and centrifuge to remove the upper culture solution (centrifugation speed is 8000 RPM, centrifugation time is 10 min). Wash the lower precipitate several times with sterile water and then vacuum-dry it to obtain yeast cells. Take 0.45 g of cells and disperse them in 30 mL of 30% glycerol-water mixture. After stirring evenly, adjust the pH to 4 with 0.02 mol / L compound organic acid solution (citric acid:oxalic acid:maleic acid = 2:1:1) to obtain a cell dispersion with a concentration of 15 g / L;

[0085] (3) Etch to obtain hierarchical pore HKUST-1 according to steps (3) and (4) in Example 1. The SEM photos are shown in Figure 10 , compared with the hierarchical pore HKUST-1 obtained in Example 1 ( Figure 1 ), the etching degree is insufficient and the etched pores are fewer.

[0086] Example 8

[0087] The MOFs used in this example are self-synthesized ZIF-8, and the selected microbial cells are Candida. Cultivate and collect the cells according to step (1) in Example 7. All other etching operation steps are the same as those in Example 6. The obtained hierarchical pore ZIF-8 is as shown in Figure 11 .

[0088] Saccharomyces cerevisiae A and Candida are representatives of yeast, indicating that the microbial etching method is also applicable to yeast microorganisms.

[0089] Example 9

[0090] The MOFs used in this example are microporous HKUST-1; the microbial cells used are Aspergillus niger spores. By adjusting the concentration of the microbial etchant, control the number of etched pores in HKUST-1. The specific steps are as follows:

[0091] (1) Cultivate Aspergillus niger according to step (1) in Example 1;

[0092] (2) Prepare four kinds of spore dispersions with concentrations of 1 g / L, 5 g / L, 10 g / L, and 15 g / L respectively according to step (2) in Example 1;

[0093] (3) Four kinds of hierarchical pore HKUST-1 samples were etched according to steps (3) and (4) in Example 1.

[0094] In this example, by adjusting the concentration of the microbial etchant while keeping the microbial species and etching time unchanged, the regulation of the number of etched pores in the hierarchical pore HKUST-1 sample can be achieved. Figure 12 It can be seen that as the concentration of the etchant increases, the number of pores on the surface of HKUST-1 increases. Hierarchical pore HKUST-1 with a specific number of pores can be obtained by the preparation method of the present invention.

[0095] Example 10

[0096] The MOFs used in this example were HKUST-1; the microbial cells used were Aspergillus niger spores. By adjusting the etching time, the size of the etched pores in HKUST-1 was controlled, and the specific steps were as follows:

[0097] (1) Aspergillus niger was cultured according to step (1) in Example 1.

[0098] (2) A spore dispersion with a concentration of 15 g / L was prepared according to step (2) in Example 1.

[0099] (3) Four kinds of hierarchical pore HKUST-1 samples were etched according to steps (3) and (4) in Example 1, and the etching times were 0.5 h, 1 h, 3 h, and 5 h respectively.

[0100] In this example, by adjusting the etching time while keeping the microbial species and etchant concentration unchanged, the regulation of the size of the etched pores in the hierarchical pore HKUST-1 sample can be achieved. Figure 13 It can be seen that as the etching time increases, the size of the pores on the surface of HKUST-1 increases. Hierarchical pore HKUST-1 with a specific pore size can be obtained by the preparation method of the present invention.

[0101] Example 11

[0102] The MOFs used in this example were UiO-66, purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; the microbial cells used were Aspergillus niger spores.

[0103] All the operation steps in this example were the same as those in Example 1, except that the selected MOFs were UiO-66.

[0104] UiO-66 is a representative of Zr-based MOFs. The hierarchical pore UiO-66 prepared in this example is as Figure 14As shown, it shows that the microbial etching method is also applicable to Zr-MOFs.

[0105] Example 12

[0106] The MOFs used in this example are self-synthesized ZIF-8; the microbial cells used are Aspergillus niger spores.

[0107] The cultivation of Aspergillus niger and the collection of spores in this example are the same as those in Example 1, and the synthesis and etching operations of other ZIF-8 are the same as those in Example 6.

[0108] ZIF-8 is a representative of Zn-based MOFs. The hierarchically porous ZIF-8 prepared in this example is as Figure 15 shown, indicating that the microbial etching method is also applicable to Zn-MOFs.

[0109] Example 13

[0110] The MOFs used in this example are ZIF-67, purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; the microbial cells used are Aspergillus niger spores.

[0111] The operating steps in this example are the same as those in Example 1, except that the selected MOFs are ZIF-67, and the etching time in step (4) is changed to 6 h.

[0112] ZIF-8 is a representative of Co-based MOFs. The hierarchically porous ZIF-67 prepared in this example is as Figure 16 shown, indicating that the microbial etching method is also applicable to Co-MOFs.

[0113] Example 14

[0114] The MOFs used in this example are HKUST-1, purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; the microbial cells used are Aspergillus niger spores.

[0115] Other operations are the same as those in Example 1, except that the organic acid solutions are citric acid, oxalic acid, and maleic acid respectively, and the concentration of each single acid is 0.02 mol / L. The SEM images are as Figure 17 shown. It can be seen that the etching degree of the single acid is insufficient, and strip cracks are prone to occur in HKUST-1, indicating that selecting a compound organic acid solution to adjust the pH in the present invention is more beneficial to the etching effect of microorganisms.

[0116] Example 15

[0117] The MOF used in this example is HKUST-1, purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; the microbial cells used are Aspergillus niger spores.

[0118] Other operations are the same as in Example 1, except that the 30% glycerol-water mixture is replaced with a 30% ethylene glycol-water mixture in step (2). Finally, porous HKUST-1 is obtained. The SEM photograph is shown in Figure 18 .

[0119] Comparative Example 1: Pure water was used as the dispersion solvent for the microbial etchant. The specific operation steps are as follows:

[0120] The MOF used in this comparative example is HKUST-1; the microbial cells used are Aspergillus niger spores.

[0121] All the operation steps of this example are the same as those of Example 1, except that pure water is selected as the solvent for dispersing spores in step 2. The obtained HKUST-1 sample is as shown in Figure 19 . It can be seen that serious structural degradation has occurred in HKUST-1, indicating that the selection of the glycerol-water mixture as the dispersion solvent in the present invention is the key to ensuring the structural stability of HKUST-1 during etching.

[0122] Application Example

[0123] The catalytic activity and catalytic stability data of the porous HKUST-1 in the above examples are listed in Table 2.

[0124] Table 2

[0125] Catalyst Benzaldehyde conversion rate (%) Benzaldehyde conversion rate (%) after 5 cycles Example 1 99.4％ 90.4％ Example 2 95.1％ 86.6％ Example 3 97.5％ 88.9％ Example 4 95.0％ 86.3％ Example 5 88.3％ 76.2％ Example 7 92.4％ 84.6％ Example 14 98.7％ 90.1％

Claims

1. A method for preparing porous MOFs by microbial etching, comprising the following steps: After culturing the microbial strain, it is washed and dried, and then formulated into a cell dispersion with a mixed solution of a polar organic solvent and water. The pH of the cell dispersion is adjusted, and MOFs are added to the cell dispersion. Etching is carried out under sealed stirring conditions to obtain porous MOFs; the porous MOFs refer to MOFs obtained by etching having two or three of micropores, mesopores, and macropores; the microorganisms are selected from at least one of molds, bacteria, and yeasts; the form of the microorganisms is at least one of spores and hyphae; the polar organic solvent is a polar organic solvent non-toxic to microbial cells; adjusting the pH is to adjust the pH to 4-7 with a weak acid or a weak base; the weak base includes at least one solution of sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate; the concentration of the weak acid or weak base is 0.005-0.025 mol / L; the mold is at least one of Aspergillus niger, Penicillium chrysogenum, and Rhizopus; the bacteria are selected from at least one of Nostoc flagelliforme and Bacillus subtilis; the yeast is Candida; the polar organic solvent is glycerol, and the mass ratio of the organic solvent in the mixed solution of the polar organic solvent and water is 20%-50%; when adjusting the pH with a weak acid, a compound organic acid solution with a mass ratio of citric acid:oxalic acid:maleic acid of 2-3:2-3:1 is used; the MOFs are selected from HKUST-1, ZIF-8, UiO-66, and ZIF-67.

2. The method according to claim 1, wherein The concentration of the cell dispersion is 1-15 g / L.

3. The method according to claim 1, wherein The concentration of the weak acid or weak base is 0.01-0.02 mol / L.

4. The method according to claim 1, characterized in that The conditions for carrying out the etching are that the temperature is 15-30 °C; the stirring speed is 50-100 rpm; the etching time is 0.5-5 h.

5. The method according to claim 4, wherein The conditions for carrying out the etching are that the temperature is 20-25 °C.

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