A cobalt-based MOF derivative catalyst, a preparation method and application thereof

Abstract

This invention belongs to the field of environmental catalytic materials and air pollution control technology, specifically relating to a cobalt-based MOF derivative catalyst, its preparation method, and its application. The preparation method uses CoMOF74 as a precursor, which is pyrolyzed in air at 350-450℃ for 35 hours to obtain the cobalt-based MOF derivative catalyst. This method, through precise control of the pyrolysis atmosphere and temperature, transforms the precursor into a Co3O4-based catalyst with high crystallinity, abundant surface pores, and fully exposed active sites. This catalyst exhibits excellent activity in the low-temperature catalytic oxidation of toluene. 90 The temperature can be as low as 195℃, and the preparation process is simple and reproducible, making it suitable for the efficient purification of low concentrations of toluene.

Description

Technical Field

[0001] This invention belongs to the field of environmental catalytic materials and air pollution control technology, specifically relating to a cobalt-based MOF derivative catalyst, its preparation method, and its application. Background Technology

[0002] Volatile organic compounds (VOCs) are important precursors to air pollution and photochemical smog. Toluene, a typical aromatic VOC, poses a significant threat to human health and the environment. Catalytic oxidation can completely convert VOCs into harmless CO2 and H2O at relatively low temperatures, making it a highly efficient and economical purification technology. Its core lies in developing highly active, low-cost, and stable catalysts.

[0003] Cobalt-based oxides (such as Co3O4) are characterized by their abundant oxygen species and variable valence states (Co). 2+ / Co 3+ Cobalt-based catalysts, with their excellent redox capabilities, show great potential in the catalytic oxidation of VOCs. However, conventionally prepared cobalt-based catalysts suffer from problems such as easy agglomeration of metal particles, insufficient exposure of active sites, and insufficient activity at low temperatures.

[0004] Metal-organic frameworks (MOFs) are crystalline porous materials formed by the self-assembly of metal ions / clusters and organic ligands. They have attracted much attention due to their high specific surface area, tunable structure, and highly dispersed metal sites. Using MOFs as precursors or self-sacrificing templates, metal oxide / carbon composites can be prepared through high-temperature pyrolysis. These composites not only inherit some of the porous structure of MOFs but also achieve high dispersion and interface control of metal species within the carbon matrix, thereby significantly improving catalytic performance.

[0005] Metal-organic frameworks (MOFs) can be used as precursors to prepare structure-controllable metal oxide catalysts via pyrolysis. Relevant explorations have already been undertaken in the prior art. Chinese invention patent application CN110681382A discloses a specific MOF (ZSA) 1) As a precursor, at 250 Co3O4 catalyst was prepared by calcination in air at 450℃ for 1 hour, and the optimal sample M was obtained. Co3O4 350 against 1000 ppm toluene T 90 The temperature was 239℃. This technique emphasizes short-time calcination and maintaining the octahedral morphology, but it does not systematically explore the performance changes in pyrolysis atmosphere (limited to air) and a wider temperature range, nor does it address the influence of different atmospheres.

[0006] Chinese invention patent application CN111408374A discloses a method using MOF 74 (i.e., similar materials to the precursor of this invention), ZIF 9. MOF 39 is a precursor, in 300 500℃ (preferably 350℃) Co3O4 catalyst was prepared by pyrolysis in air at 400℃ for the catalytic oxidation of o-xylene. 90 The temperature was 270°C (at high airspeed). This document confirms the MOF. The feasibility of deriving Co3O4 was discussed, but the focus was on the universality of different MOF precursors, without delving into the feasibility of Co-derived Co3O4. MOF When 74 is the precursor, the fine-tuning rules and structure-activity relationship of pyrolysis temperature (especially 400℃) and atmosphere (air vs. inert atmosphere) on the final catalyst phase, structure and catalytic performance on toluene are discussed.

[0007] Chinese invention patent application CN112657497A describes the preparation of Co by impregnation with a second metal. MOF-based composite oxides are used to improve performance, with M being the optimal catalyst. Co3O4 / CuO x T of p-toluene 90 The temperature was 208℃. This method introduces additional metal components and more complex preparation steps.

[0008] In summary, while existing technologies point to the direction of MOF-derived cobalt-based oxides, they fail to fully reveal the process through a single variable—especially for Co. MOF 74 precursors, systematically regulating pyrolysis atmosphere and temperature—to maximize the potential of single cobalt-based oxide catalytic toluene oxidation performance. How to obtain materials with significantly better low-temperature activity than existing single cobalt-based catalysts, and even comparable to some composite catalysts, through simple and precise process control without introducing other metals, remains a technical problem to be solved. Summary of the Invention

[0009] The technical problem to be solved by this invention is to provide a cobalt-based MOF derivative catalyst with controllable process and good repeatability, as well as its preparation method and application.

[0010] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a method for preparing a cobalt-based MOF derivative catalyst, comprising the following steps: S1. Precursor preparation: Cobalt source and organic ligand are dissolved in a mixed organic solvent, and after ultrasonic mixing, a hydrothermal reaction is carried out. After the reaction is completed, solid-liquid separation, washing and drying are performed to obtain the cobalt-based metal-organic framework material precursor. S2. Pyrolysis Conversion: The cobalt-based metal-organic framework material precursor obtained in step S1 is placed in an oxygen-containing atmosphere and subjected to pyrolysis treatment at a heating rate of 5-12℃ / min within a temperature range of 350℃ to 450℃ for 3-5 hours to obtain the cobalt-based MOF derivative catalyst with cobalt tetroxide as the main active component.

[0011] Furthermore, in step S2 of the above-mentioned method for preparing cobalt-based MOF derivative catalysts, the oxygen-containing atmosphere is an air atmosphere, the pyrolysis treatment is carried out in a porcelain crucible, and natural calcination is performed in a muffle furnace using ambient air; the temperature of the pyrolysis treatment is 390°C to 410°C.

[0012] As described above, air, as a cheap and readily available gas source, offers excellent operability and economy. More importantly, pyrolysis in an air atmosphere ensures the full oxidation of cobalt ions in the precursor, guaranteeing the formation of highly reactive Co3O4 (Co... 2+ / Co 3 + The mixed valence state of CoO, rather than the low valence state CoO that might be generated under an inert atmosphere (such as nitrogen), is present. Comparative experiments show that, at the same 400℃, the toluene oxidation activity (T0) of the pyrolysis products in air atmosphere is significantly higher. 90 =190℃) is significantly better than nitrogen atmosphere products (T 90 =200℃). This clarifies that an air atmosphere is a necessary and preferred condition for achieving the highly active catalyst of this invention.

[0013] Firing with ambient air eliminates the need for additional atmosphere control, making the method of this invention simple, economical, easy to repeat, and industrializable.

[0014] Pyrolysis within the temperature range of 390℃ to 410℃ most effectively transforms the precursor into a material with moderate crystallinity, suitable grain size, and surface Co. 3+ / Co 2+ Optimized spinel-type Co3O4. Too low a temperature (e.g., 350℃) may result in residual organic components or incomplete Co3O4 crystallization; too high a temperature (e.g., above 450℃) can easily cause significant Co3O4 grain growth and sintering, leading to a reduction in active sites. Experimental data (T... 90 =195℃) proved that the catalyst obtained at this preferred temperature has significantly better performance than the pyrolysis products obtained at 600℃ or 800℃ (T 90 (215℃ and 255℃ respectively).

[0015] Furthermore, in step S1 of the above-mentioned method for preparing cobalt-based MOF derivative catalysts, the cobalt source is at least one of cobalt nitrate hexahydrate, cobalt nitrate, cobalt chloride, and cobalt sulfate; The organic ligand is at least one of 2,5-dihydroxyterephthalic acid, 1,2,4-triscarboxylic acid, and pyromellitic acid; The organic solvent includes at least one of N,N-dimethylformamide, methanol, ethanol, and water.

[0016] Furthermore, the reaction vessel in the above-mentioned method for preparing the cobalt-based MOF derivative catalyst is a threaded glass reactor lined with polytetrafluoroethylene. The cobalt source is cobalt nitrate hexahydrate; The organic solvent is a mixture of N,N-dimethylformamide, methanol, and ethanol.

[0017] Furthermore, in the above-mentioned method for preparing cobalt-based MOF derivative catalysts, the molar ratio of the cobalt salt to the 2,5-dihydroxyterephthalic acid is 1:(0.5-2); and in the mixed solvent, the volume ratio of N,N-dimethylformamide, ethanol, and water is 1:(0.8-1.2):(0.8-1.2).

[0018] Furthermore, in the above-mentioned method for preparing cobalt-based MOF derivative catalysts, the hydrothermal reaction temperature is 95°C to 110°C, and the reaction time is 10-14 hours.

[0019] Another technical solution provided by the present invention is: to provide a cobalt-based MOF derivative catalyst, which is prepared by the above-mentioned cobalt-based MOF derivative catalyst preparation method.

[0020] Another technical solution provided by the present invention is: the application of the above-mentioned cobalt-based MOF derivative catalyst in the catalytic oxidation of volatile organic compounds.

[0021] In the above applications, the volatile organic compound includes at least one of toluene, benzene, or xylene.

[0022] In the above applications, the volatile organic compound is toluene.

[0023] The beneficial effects of this invention are as follows: (1) This invention successfully prepared a Co3O4 catalyst with high intrinsic activity by precisely controlling the pyrolysis of the Co-MOF-74 precursor in air at approximately 400°C for 3-5 hours. This catalyst exhibits excellent low-temperature catalytic oxidation capability for low concentrations of toluene (200 ppm), with a T0.05... 90 It can reach temperatures as low as 195°C. This performance is significantly superior to the prior art mentioned in the three background technologies, where the prior art's T... 90 Temperatures are generally above 200℃.

[0024] (2) This invention systematically reveals and applies for the first time the optimized pyrolysis law for Co-MOF-74 precursor: air atmosphere is a necessary condition for generating active Co3O4; 400℃ is the "golden section" of pyrolysis temperature, which balances the contradiction between complete conversion and prevention of sintering; 3-5 hours of pyrolysis time ensures full reaction.

[0025] (3) The method of the present invention only involves conventional hydrothermal synthesis and one-step programmed temperature-controlled pyrolysis, without the need to introduce a second metal or complex templates or post-processing. The raw materials used are inexpensive and readily available (cobalt nitrate, common organic ligands, air), the process conditions are mild and controllable, the repeatability is good, and the energy consumption is relatively low, which is very beneficial for large-scale production and practical application. Attached Figure Description

[0026] Figure 1a Figure 1b The X-ray diffraction (XRD) patterns of the Co-MOF-74 precursor prepared in Example 1 and the catalysts obtained by pyrolysis under different conditions are compared. Figure 2a , Figure 2b The images shown are of the finished Co-MOF-74 products prepared in Example 1. Figure 3 The curves showing the relationship between toluene conversion and temperature at different pyrolysis temperatures for the Co-MOF samples prepared in Example 1 are shown. Figure 4 The curves showing the relationship between toluene conversion rate and temperature under different pyrolysis atmospheres for the Co-MOF series samples prepared in Example 1 are shown. Figure 5 The left figure shows the effect of flow rate on toluene conversion; the right figure shows the relationship between flow rate and temperature (T). 90% and T 100% Relationship bar chart; Figure 6 The left figure shows the effect of indoor exhaust gas concentration on toluene conversion rate; the right figure shows the relationship between indoor exhaust gas concentration and T. 90% and T 100% Relationship bar chart. Detailed Implementation

[0027] To explain in detail the technical content, objectives, and effects of the present invention, the following description is provided in conjunction with the embodiments and accompanying drawings.

[0028] Example 1 The preparation of a cobalt-based MOF derivative catalyst includes the following steps: S1. Synthesis of precursor Co-MOF: Accurately weigh 1.485 g of cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 5.0 mmol) and 0.301 g of 2,5-dihydroxyterephthalic acid (DHTA, 1.5 mmol) and place them in a beaker. Add a mixed solvent (volume ratio 1:1:1) consisting of 41 mL of N,N-dimethylformamide (DMF), 41 mL of anhydrous ethanol, and 41 mL of deionized water to the beaker. Stir the mixture magnetically at room temperature for 30 minutes, then sonicate it in an ultrasonic cleaner for 30 minutes until a homogeneous, clear or slightly turbid pink solution is formed.

[0029] The above mixed solution was transferred to a 50 mL threaded glass reactor with a PTFE liner. The reactor was placed in a forced-air drying oven and reacted at 100°C for 12 hours. After the reaction was complete, it was allowed to cool naturally to room temperature. The reactor was opened, the supernatant was poured off, and the reddish-brown solid product at the bottom was collected by centrifugation.

[0030] The solid product was washed three times by centrifugation with DMF and methanol, respectively, to remove unreacted raw material and solvent molecules. The washed solid was placed in a vacuum drying oven and dried at 150°C for 5 hours to obtain a dry reddish-brown powder, which is the cobalt-based metal-organic framework precursor, denoted as Co-MOF-74. Its XRD pattern (Figure 1) shows characteristic diffraction peaks at 7.8° and 13.6°, consistent with the reported structure of Co-MOF-74.

[0031] S2. Catalyst preparation by pyrolysis: Take 0.5~1 gram of the Co-MOF-74 precursor powder prepared above and place it in a clean porcelain crucible.

[0032] Place the porcelain crucible into a box-type muffle furnace. Using the ambient air (static air) inside the muffle furnace as the reaction atmosphere, heat the furnace from room temperature to 400°C at a rate of 10°C / min. After reaching 400°C, maintain this temperature for 4 hours for pyrolysis.

[0033] After pyrolysis, the heating was turned off, and the muffle furnace was allowed to cool naturally to room temperature. The porcelain crucible was removed, and a black powdery solid was obtained, which is the cobalt-based MOF derivative catalyst described in this invention, denoted as Co-MOF-400.

[0034] Comparative Example 1-1: Preparation of catalysts at different pyrolysis temperatures The Co-MOF-74 precursor was prepared by repeating step S1 of Example 1. In step S2, the pyrolysis temperature was changed to 600℃ and 800℃ respectively, while other conditions (air atmosphere, 10℃ / min heating, 4 hours isothermal) remained unchanged. The resulting catalysts were designated as Co-MOF-600 and Co-MOF-800 respectively.

[0035] Comparative Examples 1-2: Preparation of catalysts under different pyrolysis atmospheres The Co-MOF-74 precursor was prepared by repeating step S1 of Example 1. In step S2, pyrolysis was performed using a tube furnace. The precursor was placed in the isothermal zone of the tube furnace, and high-purity nitrogen (flow rate 100 mL / min) was introduced as a protective gas. The temperature was increased to 400°C at a rate of 10°C / min and maintained under a nitrogen atmosphere for 4 hours, followed by natural cooling. The resulting catalyst was designated Co-MOF-400-N2.

[0036] Referring to Figure 1(a), the two characteristic peaks of the prepared Co-MOF-74 at 7.8° and 13.6° correspond to the (110) and (300) planes of Co-MOF-74, respectively, indicating that Co-MOF-74 was successfully synthesized. The samples after high-temperature pyrolysis all showed the disappearance of the two characteristic diffraction peaks at 7.8° and 13.6°, indicating the formation of new compounds. Among them, the sample after nitrogen inert gas pyrolysis showed obvious broad diffraction peaks at 42.2°, 49.3°, and 72.1°, which correspond to the characteristic diffraction peaks of the (111), (200), and (220) planes of amorphous CoO (PDF#78-0431), respectively.

[0037] Referring to Figure 1(b), the sample after pyrolysis in air exhibits characteristic diffraction peaks at 22.2°, 36.6°, 43.1°, 45.2°, 52.6°, and 65.7°, which correspond to the characteristic diffraction peaks of the (111), (220), (311), (222), (400), and (422) planes of the single-phase cubic Co3O4 crystal (PDF#43-1003), respectively. The peak intensity of Co-MOF-T increases with increasing pyrolysis temperature, which may be attributed to the removal and decomposition of organic ligands in the form of CO2 at higher pyrolysis temperatures, resulting in the exposure of more Co3O4 crystal phases and a continuous reduction in carbon groups. Furthermore, under inert atmosphere conditions, the metal ions Co in the framework... 2+ It is reduced to form amorphous CoO crystals, and oxidized in air to form Co3O4 crystals.

[0038] Please refer to Figure 2a and Figure 2bCo-MOF-74 exhibits a regular morphology and smooth surface, displaying a polyhedral prism-like needle-like structure and a flower-like morphology formed by aggregation. After pyrolysis, the prism-like structure of Co-MOF-74 fractures, and long cracks and grooves appear on the surface, leading to the accumulation of metal oxide agglomerates and forming a rough surface. The more metal oxide agglomerates appear with increasing pyrolysis temperature, which may be attributed to the pyrolysis disappearance of the carbon skeleton and the formation of metal oxides in the metal center. This aligns with the increase in the intensity of the Co3O4 crystal phase peak as the pyrolysis temperature increases, as shown in Figure 1(b) of the XRD. Among these, Co-MOF-400 pyrolyzed in an air atmosphere exhibits a significantly rougher surface than Co-MOF-400-N2 pyrolyzed in a nitrogen atmosphere, displaying numerous pores and pits of irregular size and shape.

[0039] Experiments and Analysis 1. To further investigate the effect of different pyrolysis temperatures on the catalytic performance of toluene, Co-MOF-74 was pyrolyzed at 400℃, 600℃, and 800℃, and its effect on toluene and catalytic performance was tested. The results are as follows: Figure 3 As shown.

[0040] Among them, Co-MOF-400 (T 90% =195℃)>Co-MOF-600 (T 90% =215℃)>Co-MOF-800 (T 90% =255℃). With increasing pyrolysis temperature, the catalytic performance of Co-MOF-400 for toluene decreases. However, Co-MOF-400 exhibits excellent toluene catalytic performance. 90% =195℃.

[0041] 2. Effect of different pyrolysis atmospheres on toluene catalytic performance: Co-MOF-74 was pyrolyzed at 400℃ under nitrogen and oxygen atmospheres to prepare Co-MOF-400-Air (i.e., Co-MOF-400) and Co-MOF-400-N2, respectively. Their toluene catalytic performance was evaluated, and the results are as follows: Figure 4 As shown.

[0042] Although Co-MOF-400-Air has a small surface area (7.608m²), 2 / g), but some carbon atoms in the framework react with oxygen atoms to form CO2, causing the framework to collapse and thus exposing more metal oxides, which is beneficial for Co-MOF-400-Air to exhibit better catalytic oxidation performance, such as Figure 4 As shown, Co-MOF-400-Air (T 90% =190℃)>Co-MOF-400-N2 (T 90% =200℃).

[0043] 3. Gas hourly space velocity (GHSV) is a crucial parameter for designing the external dimensions of waste gas treatment devices. Therefore, under an inlet gas concentration of 200 ppm, the effects of four gas flow rates (40 mL / min, 50 mL / min, 60 mL / min, and 70 mL / min, corresponding to GHSVs of 24000, 30000, 36000, and 42000 mL / g / h, respectively) on the toluene conversion rate of Co-MOF-400 were investigated. The test results are as follows: Figure 5 Show.

[0044] Depend on Figure 5 It can be seen that within the concentration range of 40-70 mL / min, as the flow rate increases, T... 90% As the flow rate increased, the T90% of toluene increased by 30°C from 40 mL / min to 70 mL / min. Similarly, the T100% increased by 40°C from 40 mL / min to 70 mL / min. This is attributed to the reduced residence time of toluene molecules on the Co-MOF-400 surface with increasing flow rate. Toluene is often blown away by the gas flow before it has fully reacted, hindering the catalytic reaction and reducing catalytic performance. Furthermore, since the toluene catalysis is endothermic, excessively high flow rates remove a significant amount of heat, further reducing the degradation efficiency. As the GHSV increased from 20,000 to 120,000 mL / g / h, the T50% and T90% of toluene conversion increased by 17°C and 15°C, respectively.

[0045] 4. Effect of gas concentration Indoor exhaust gas concentration fluctuates due to the influence of climate and operating conditions. Therefore, the toluene conversion rate of Co-MOF-400 at four different toluene concentrations of 160ppm, 200ppm, 240ppm, and 280ppm was studied under an inlet flow rate of 50mL / min. The test results are as follows: Figure 6 As shown.

[0046] Depend on Figure 6 It is observed that within the concentration range of 160-280 ppm, the T90% increases with increasing toluene concentration; when the toluene concentration increases from 160 ppm to 280 ppm, the T90% increases by 30°C. Similarly, the T100% also increases with increasing toluene concentration; when the toluene concentration increases from 160 ppm to 280 ppm, the T100% increases by 40°C. This is attributed to the fact that when the inlet gas concentration increases, toluene molecules entering the reaction bed compete with each other to adsorb active sites on the catalyst surface, and some pollutants are stripped from the reactor because they are not oxidized in time. Increasing the reaction temperature, however, helps to increase the energy of reactant molecules, promoting their activation and thus enhancing the oxidation rate for treating high-concentration waste gas.

[0047] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing a cobalt-based MOF derivative catalyst, characterized in that, Includes the following steps: S1. Precursor preparation: Cobalt source and organic ligand are dissolved in a mixed organic solvent, and after ultrasonic mixing, a hydrothermal reaction is carried out. After the reaction is completed, solid-liquid separation, washing and drying are performed to obtain the cobalt-based metal-organic framework material precursor. S2. Pyrolysis Conversion: The cobalt-based metal-organic framework material precursor obtained in step S1 is placed in an oxygen-containing atmosphere and subjected to pyrolysis treatment at a heating rate of 5-12℃ / min within a temperature range of 350℃ to 450℃ for 3-5 hours to obtain the cobalt-based MOF derivative catalyst with cobalt tetroxide as the main active component.

2. The method for preparing the cobalt-based MOF derivative catalyst according to claim 1, characterized in that, In step S2, the oxygen-containing atmosphere is an air atmosphere, the pyrolysis treatment is carried out in a porcelain crucible, and natural firing is performed in a muffle furnace using ambient air; the temperature of the pyrolysis treatment is 390°C to 410°C.

3. The method for preparing the cobalt-based MOF derivative catalyst according to claim 1, characterized in that, In step S1, the cobalt source is at least one of cobalt nitrate hexahydrate, cobalt nitrate, cobalt chloride, and cobalt sulfate; The organic ligand is at least one of 2,5-dihydroxyterephthalic acid, 1,2,4-triscarboxylic acid, and pyromellitic acid; The organic solvent includes at least one of N,N-dimethylformamide, methanol, ethanol, and water.

4. The method for preparing the cobalt-based MOF derivative catalyst according to claim 1, characterized in that, The reaction vessel is a threaded glass reactor lined with polytetrafluoroethylene. The cobalt source is cobalt nitrate hexahydrate; The organic solvent is a mixture of N,N-dimethylformamide, methanol, and ethanol.

5. The method for preparing the cobalt-based MOF derivative catalyst according to claim 1, characterized in that, The molar ratio of the cobalt salt to the 2,5-dihydroxyterephthalic acid is 1:(0.5-2); the volume ratio of N,N-dimethylformamide, ethanol and water in the mixed solvent is 1:(0.8-1.2):(0.8-1.2).

6. The method for preparing the cobalt-based MOF derivative catalyst according to claim 1, characterized in that, In step S1, the temperature of the hydrothermal reaction is 95°C to 110°C, and the reaction time is 10-14 hours.

7. A cobalt-based MOF derivative catalyst, characterized in that, It is prepared by the method described in any one of claims 1 to 6 for the preparation of cobalt-based MOF derivative catalysts.

8. The application of a cobalt-based MOF derivative catalyst as described in claim 7 in the catalytic oxidation of volatile organic compounds.

9. The application according to claim 8, characterized in that, The volatile organic compound includes at least one of toluene, benzene, or xylene.

10. The application according to claim 9, characterized in that, The volatile organic compound is toluene.

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