A bifunctional MOF-encapsulated metal catalyst, its preparation method and application

Abstract

This invention discloses a bifunctional MOF-encapsulated metal catalyst, its preparation method, and its application. The preparation includes: dissolving an iron precursor and a bifunctional ligand in a mixture of N,N-dimethylformamide and formic acid; obtaining a bifunctional MOF through a solvothermal reaction; activating the MOF; dispersing it in a colloidal solution of noble metal nanoparticles; stirring and mixing; and removing the solvent to obtain the catalyst. In this invention, the MOF is prepared via a solvothermal synthesis-liquid-phase reduction deposition method, and noble metal nanoparticles are encapsulated within it. This catalyst can synthesize H2O2 in situ as an oxidant. Subsequently, the high electron density metal active centers modified with the two functional groups can efficiently and rapidly decompose the H2O2 enriched within the catalyst framework, thereby efficiently activating methane. When applied to the partial oxidation of methane to methanol, a high methanol yield can be obtained.

Description

Technical Field

[0001] This invention relates to the field of catalytic materials technology, specifically to a bifunctional MOF-encapsulated metal catalyst, its preparation method, and its application. Background Technology

[0002] Methane is an important component of natural gas, and due to its abundant reserves and high energy density (55 MJ / kg), it has become a vital resource for energy and chemical production. However, the utilization of natural gas is hindered by the difficulty of long-distance gas transportation. Partial oxidation of methane into the liquid oxygen-containing compound methanol is an effective solution. H₂O₂ is a green oxidant with moderate oxidizing power, capable of converting methane to methanol under mild reaction conditions. However, the high industrial price of H₂O₂ limits the application of this process. Therefore, the in-situ generation of H₂O₂ from H₂ and O₂ to oxidize methane is a strategy worthy of further investigation.

[0003] Noble metals Pd and Au often exhibit good performance in the in-situ synthesis of H2O2. However, in the in-situ synthesis of methane oxidation from H2O2, the utilization rate of H2O2 is not high due to its self-decomposition. Recent literature reports that by coating Au-Pd nanoparticles into ZSM-5 catalyst, the concentration of H2O2 inside the catalyst is enriched, thereby increasing the methanol yield (Science 2020, 367, 193-197). In addition, the efficiency of the catalyst in decomposing H2O2 into ·OH is also an important factor limiting the methanol yield. Furthermore, studies have found that the higher the electron density of the central metal, the more it promotes the decomposition of H2O2. The electron density of the metal active site is sensitive to electron donors and can be modulated by organic ligands (Chemosphere 2022, 291, 133026). Therefore, introducing electron donor groups can increase the electron density of the central metal and improve the decomposition and utilization efficiency of H2O2.

[0004] Therefore, the focus of reaction research is to encapsulate noble metal nanoparticles on organic framework materials whose metal active centers can be modified and regulated by electron-donating groups to prepare catalysts that can efficiently generate, decompose and utilize H2O2 and activate methane. Summary of the Invention

[0005] This invention addresses the problems of high oxidant cost, low H2O2 decomposition and utilization efficiency, and low methanol yield in existing methane partial oxidation to methanol production methods. It provides a method for preparing a bifunctional MOF-encapsulated metal catalyst. This catalyst can synthesize H2O2 in situ through noble metal nanoparticles encapsulated in a metal-organic framework. At the same time, the functional group modification increases the electron density of the metal active center, rapidly decomposes hydrogen peroxide, and thus efficiently activates methane to produce methanol.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a bifunctional MOF-encapsulated metal catalyst includes the following steps:

[0008] Step 1: Dissolve the iron precursor and the bifunctional ligand in a mixture of N,N-dimethylformamide and formic acid, and obtain the bifunctional modified MOF by solvothermal reaction;

[0009] Step 2: Add a reducing agent to the noble metal precursor to obtain a colloidal solution of noble metal nanoparticles;

[0010] Step 3: After activating the bifunctional MOF modified in Step 1, disperse it in the colloidal solution of the noble metal nanoparticles and stir to mix. After removing the solvent, the MOF-encapsulated metal catalyst is obtained.

[0011] In this invention, a metal-organic framework (MOF) material modified with two amino (NH2) / methoxy (OMe) groups is formed by bridging a metal precursor and an organic ligand using a solvothermal synthesis method. Noble metal nanoparticles are then prepared and encapsulated within the MOF framework using a liquid-phase reduction deposition method. The noble metal nanoparticles within this catalyst framework can synthesize H2O2 in situ as an oxidant. Subsequently, the high electron density metal active centers modified with the two functional groups efficiently and rapidly decompose the H2O2 enriched within the catalyst framework, thereby efficiently activating methane. Applying this catalyst to the partial oxidation of methane to methanol not only reduces reaction costs but also achieves a high methanol yield.

[0012] The iron precursor is ferric nitrate and its hydrate;

[0013] The noble metal precursor includes one or more of palladium or gold halides, nitrates or acetates and their hydrates; more preferably, palladium chloride, palladium nitrate and their hydrates.

[0014] The reducing agent includes one or a combination of two or more of sodium borohydride, sodium citrate, ascorbic acid, and ethylene glycol. Sodium borohydride is preferred because it is relatively stable to water and oxygen in the air, and the particle size of the metal nanoparticles can be controlled by adjusting the concentration of the sodium borohydride solution and the reduction temperature.

[0015] The bifunctional ligand includes any one of 2,5-diaminoterephthalic acid and 2,5-dimethoxyterephthalic acid, or a mixture of either with terephthalic acid. Both the aforementioned metal precursor and organic ligand can form metal-organic framework materials modified with two amino / methoxy groups. Adding a small amount of terephthalic acid can reduce costs.

[0016] The molar ratio of the iron precursor to the bifunctional ligand is 1:3-3:1.

[0017] In step 1, the molar ratio of the iron precursor to N,N-dimethylformamide is 1:100-1:300. The volume ratio of formic acid to N,N-dimethylformamide is 1:5-50; a volume ratio that is too high or too low will decrease the crystallinity of the metal-organic framework material. Preferably, the volume ratio of formic acid to N,N-dimethylformamide is 1:10-30, more preferably 1:10-20.

[0018] In step 1, the solvothermal reaction temperature is 100-180℃, and the reaction time is 12-72 h. Too high or too low a reaction temperature and time will affect the purity and crystallinity of the metal-organic framework material. Preferably, the solvothermal reaction temperature is 110-150℃, and the reaction time is 24-60 h; more preferably, the solvothermal reaction temperature is 110-130℃, and the reaction time is 30-50 h.

[0019] In step 2, the molar ratio of reducing agent to noble metal is 1-100:1. A ratio that is too low will prevent the noble metal from being completely reduced to nanoparticles, while a ratio that is too high will increase the size of the noble metal particles. The concentration of the reducing agent is 10-100 g / L, preferably 30-90 g / L, and more preferably 60-80 g / L. An excessively high concentration of the reducing agent will increase the particle size of the noble metal nanoparticles. The reduction temperature is 0-60℃, preferably 0-40℃, and more preferably 0-10℃. An excessively high reduction temperature will result in excessively large particle sizes of the noble metal nanoparticles. The reduction time is 30 min-8 h, more preferably 30 min-2 h. Prolonged reduction times may lead to agglomeration of the noble metal nanoparticles and an increase in particle size.

[0020] In step 3, the activation of the bifunctional MOF is carried out under vacuum, with an activation temperature of 60-120℃, preferably 80-100℃, and an activation time of 8-20 h, preferably 8-16 h. If the activation temperature is too low or the activation time is too short, the residual solvent molecules in the pores of the metal-organic framework material may not be completely removed. If the activation temperature is too high or the activation time is too long, the structure of the metal-organic framework material may be damaged.

[0021] In step 3, the mass of the noble metal in the noble metal nanoparticle colloidal solution is 0.1-3 wt% of the mass of the bifunctional MOF; preferably 0.5-1 wt%. Reducing the noble metal loading is beneficial to reducing the catalyst preparation cost. The stirring and mixing are carried out at room temperature for 4-12 h, preferably 6-8 h.

[0022] The present invention also provides a bifunctional MOF-encapsulated metal catalyst prepared by the above preparation method.

[0023] This invention also provides the application of the MOF-encapsulated metal catalyst in the catalytic production of methanol from methane. Preferably, in the methanol production reaction, the reaction temperature is 30-120℃; the reaction time is 0.5-2 h; the oxygen pressure is 0.3-0.8 MPa; the hydrogen pressure is 0.6-1.5 MPa; the methane pressure is 0.5-3 MPa; and the catalyst mass ratio to methane mass is 1:5-1:50.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The present invention uses a simple solvothermal synthesis method to bridge metal and organic ligands modified with two functional groups to form a metal-organic framework material, which improves the electron density of the central active center. The catalyst exhibits excellent catalytic performance in efficiently decomposing and utilizing H2O2, thereby improving the yield of methanol.

[0026] (2) This invention uses a simple liquid-phase reduction deposition method to disperse and load noble metal nanoparticles into a metal-organic framework material with functional group modification, thereby realizing the coupling process of in-situ synthesis, in-situ rapid decomposition and methane oxidation of H2O2, reducing reaction costs. Moreover, the catalyst preparation process is simple and easy to implement, and the raw materials are simple and readily available, showing good prospects for industrial application. Attached Figure Description

[0027] Figure 1 The images show the XRD patterns of the Fe MOF-2NH2 material, Pd@Fe MOF-2NH2 catalyst, and catalysts synthesized under alkaline conditions and using ferric chloride as a precursor, prepared in Example 1.

[0028] Figure 2 The image shows the SEM images of the Fe MOF-2NH2 material and Pd@Fe MOF-2NH2 catalyst prepared in Example 1.

[0029] Figure 3 This is a TEM image of the Pd@Fe MOF-2NH2 catalyst prepared in Example 1.

[0030] Figure 4 The image shows the XPS plot of Pd in ​​the Pd@Fe MOF-2NH2 catalyst prepared in Example 1.

[0031] Figure 5 The image shows a TEM image of the Pd / Fe MOF-2NH2 catalyst prepared in Comparative Example 1.

[0032] Figure 6 The graph shows the performance evaluation of the partial oxidation of methane to methanol in Examples 1-5.

[0033] Figure 7 The graph shows the performance evaluation of the partial oxidation of methane to methanol in Comparative Examples 1-4. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Modifications or equivalent substitutions made by those skilled in the art based on their understanding of the technical solutions of this invention, without departing from the spirit and scope of the invention, should be covered within the protection scope of this invention.

[0035] All raw materials used in the following specific implementation methods were purchased from the market.

[0036] Example 1: 0.5 wt% Pd catalyst encapsulated in Fe MOF-2NH2

[0037] (1) 1 mmol of ferric nitrate and 1 mmol of 2,5-diaminoterephthalic acid were dispersed in 10 mL of N,N-dimethylformamide, and then 300 μL of formic acid was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 120 °C for 48 h to obtain Fe MOF-2NH2 material. Ferric nitrate and 2,5-diaminoterephthalic acid were added to the high-pressure reactor and reacted therein. Under the control of formic acid, crystal nucleation and growth will form a metal-organic framework material with Fe-O clusters linked by 2,5-diaminoterephthalic acid.

[0038] (2) Prepare a 20 mL solution of 1.25 mg of the noble metal precursor Pd(NO3)2·2H2O and place it in an ice-water bath. Prepare a fresh NaBH4 solution with a mass ratio of 30:1 to the noble metal precursor and slowly add it dropwise. Continue stirring at room temperature for 2 h to obtain a colloidal solution containing Pd nanoparticles.

[0039] (3) 100 mg of Fe MOF-2NH2 material was activated in vacuum at 100℃ for 16 h, and then added to the above solution of Pd nanoparticles. The mass of Pd metal was 0.5 wt% of MOF material. Stirring was continued for 8 h, and then the solution was dried in a rotary evaporator to obtain the catalyst encapsulated with Pd in ​​Fe MOF-2NH2 material, denoted as Pd@Fe MOF-2NH2.

[0040] Example 2: 0.5 wt% Pd catalyst encapsulated in Fe MOF-2NH2

[0041] (1) 1 mmol of ferric nitrate and 1 mmol of 2,5-diaminoterephthalic acid were dispersed in 10 mL of N,N-dimethylformamide. Then, 1 mL of formic acid was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 150 °C for 24 h to obtain Fe MOF-2NH2 material.

[0042] (2) Prepare a 20 mL solution of 1.25 mg of the noble metal precursor Pd(NO3)2·2H2O and place it in an ice-water bath. Prepare a fresh NaBH4 solution with a mass ratio of 30:1 to the noble metal precursor and slowly add it dropwise. Continue stirring at room temperature for 2 h to obtain a colloidal solution containing Pd nanoparticles.

[0043] (3) 100 mg of Fe MOF-2NH2 material was activated in vacuum at 100℃ for 16 h, and then added to the above solution of Pd nanoparticles. The mass of Pd metal was 0.5 wt% of MOF material. Stirring was continued for 8 h, and then the solution was dried in a rotary evaporator to obtain the catalyst encapsulated with Pd in ​​Fe MOF-2NH2 material, denoted as Pd@Fe MOF-2NH2.

[0044] The main differences from Example 1 are the amount of formic acid added and the synthesis temperature and time. Increasing the amount of formic acid will affect the crystallinity of the product, while the synthesis temperature and time will affect the morphology of the product. Through optimization of the synthesis conditions, Example 1 was selected as the optimal synthesis condition.

[0045] Example 3: Fe MOF-2NH2 encapsulation of 0.5 wt% Pd catalyst

[0046] (1) 1 mmol ferric nitrate, 0.5 mmol 2,5-diaminoterephthalic acid and 0.5 mmol terephthalic acid were dispersed in 10 mL N,N-dimethylformamide, and then 300 μL formic acid was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 120 °C for 48 h to obtain Fe MOF-2NH2 material.

[0047] (2) Prepare a 20 mL solution of 1.25 mg of the noble metal precursor Pd(NO3)2·2H2O and place it in an ice-water bath. Prepare a fresh NaBH4 solution with a mass ratio of 30:1 to the noble metal precursor and slowly add it dropwise. Continue stirring at room temperature for 2 h to obtain a colloidal solution containing Pd nanoparticles.

[0048] (3) 100 mg of Fe MOF-2NH2 material was activated in vacuum at 100℃ for 16 h, and then added to the above solution of Pd nanoparticles. The mass of Pd metal was 0.5 wt% of MOF material. Stirring was continued for 8 h, and then the solution was dried in a rotary evaporator to obtain the catalyst encapsulated with Pd in ​​Fe MOF-2NH2 material, denoted as Pd@Fe MOF-2NH2.

[0049] The main difference from Example 1 is that some of the 2,5-diaminoterephthalic acid is replaced with terephthalic acid. Although it can still form a metal-organic framework material, the total number of amino groups in the material is reduced.

[0050] Example 4: Fe MOF-2NH2 encapsulation of 0.5 wt% Au catalyst

[0051] (1) 1 mmol of ferric nitrate and 1 mmol of 2,5-diaminoterephthalic acid were dispersed in 10 mL of N,N-dimethylformamide. Then, 300 μL of formic acid was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 120 °C for 48 h to obtain Fe MOF-2NH2 material.

[0052] (2) Prepare a 20 mL solution of 1.05 mg of noble metal precursor HAuCl4·4H2O and place it in an ice-water bath. Prepare a fresh NaBH4 solution with a mass ratio of 20:1 to the noble metal precursor and slowly add it dropwise. Continue stirring at room temperature for 2 h to obtain a colloidal solution containing Au nanoparticles.

[0053] (3) 100 mg of Fe MOF-2NH2 material was activated in vacuum at 100℃ for 16 h, and then added to the above solution of Au nanoparticles. The mass of Au metal was 0.5 wt% of the MOF material. The mixture was stirred for 8 h, and then the solution was dried in a rotary evaporator to obtain the catalyst encapsulated with Au in Fe MOF-2NH2 material, denoted as Au@Fe MOF-2NH2.

[0054] Example 5: Fe MOF-2OMe encapsulation of 0.5 wt% Pd catalyst

[0055] (1) 1 mmol of ferric nitrate and 1 mmol of 2,5-dimethoxyterephthalic acid were dispersed in 10 mL of N,N-dimethylformamide. Then, 300 μL of formic acid was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 120 °C for 48 h to obtain Fe MOF-2OMe material.

[0056] (2) Prepare a 20 mL solution of 1.25 mg of the noble metal precursor Pd(NO3)2·2H2O and place it in an ice-water bath. Prepare a fresh NaBH4 solution with a mass ratio of 30:1 to the noble metal precursor and slowly add it dropwise. Continue stirring at room temperature for 2 h to obtain a colloidal solution containing Pd nanoparticles.

[0057] (3) 100 mg of Fe MOF-2OMe material was activated in vacuum at 100℃ for 16 h, and then added to the above solution of Pd nanoparticles. The mass of Pd metal was 0.5 wt% of the MOF material. The mixture was stirred for 8 h, and then the solution was dried in a rotary evaporator to obtain the catalyst encapsulated with Pd in ​​Fe MOF-2OMe material, denoted as Pd@Fe MOF-2OMe.

[0058] Comparative Example 1: 0.5 wt% Pd catalyst supported on the surface of Fe MOF-2NH2

[0059] (1) 1 mmol of ferric nitrate and 1 mmol of 2,5-diaminoterephthalic acid were dispersed in 10 mL of N,N-dimethylformamide. Then, 300 μL of formic acid was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 120 °C for 48 h to obtain Fe MOF-2NH2 material.

[0060] (2) Prepare a 20 mL solution of 1.25 mg of the noble metal precursor Pd(NO3)2·2H2O and place it in an ice-water bath. Prepare a fresh NaBH4 solution with a mass ratio of 60:1 to the noble metal precursor and slowly add it dropwise. Continue stirring at room temperature for 20 min to obtain a colloidal solution containing Pd nanoparticles.

[0061] (3) 100 mg of Fe MOF-2NH2 material was added to the above solution of Pd nanoparticles. The mass of Pd metal was 0.5 wt% of MOF material. The solution was dried in a rotary evaporator without stirring to obtain a catalyst with Pd loaded on the surface of Fe MOF-2NH2 material, denoted as Pd / Fe MOF-2NH2.

[0062] Figure 5 This is a TEM image of the Pd / Fe MOF-2NH2 catalyst prepared in Comparative Example 1. Figure 5 It can be seen that Pd nanoparticles are loaded on the Fe MOF-2NH2 surface.

[0063] Comparative Example 2: Fe MOF encapsulation of 0.5 wt% Pd catalyst

[0064] (1) 1 mmol of ferric nitrate and 1 mmol of terephthalic acid were dispersed in 10 mL of N,N-dimethylformamide, and then 300 μL of formic acid was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 120 °C for 48 h to obtain Fe MOF material.

[0065] (2) Prepare a 20 mL solution of 1.25 mg of the noble metal precursor Pd(NO3)2·2H2O and place it in an ice-water bath. Prepare a fresh NaBH4 solution with a mass ratio of 30:1 to the noble metal precursor and slowly add it dropwise. Continue stirring at room temperature for 2 h to obtain a colloidal solution containing Pd nanoparticles.

[0066] (3) 100 mg of Fe MOF material was added to the above solution of Pd nanoparticles, the mass of Pd metal was 0.5 wt% of MOF material, and the mixture was stirred for 8 h. The solution was then dried in a rotary evaporator to obtain the catalyst encapsulated with Pd in ​​Fe MOF material, denoted as Pd@Fe MOF.

[0067] Comparative Example 3: Al MOF encapsulated 0.5 wt% Pd catalyst

[0068] (1) 2 mmol aluminum nitrate and 1 mmol terephthalic acid were dispersed in 3 mL deionized water, and then 300 μL formic acid was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 220 °C for 72 h to obtain Al-MOF material.

[0069] (2) Prepare a 20 mL solution of 1.25 mg of the noble metal precursor Pd(NO3)2·2H2O and place it in an ice-water bath. Prepare a fresh NaBH4 solution with a mass ratio of 30:1 to the noble metal precursor and slowly add it dropwise. Continue stirring at room temperature for 2 h to obtain a colloidal solution containing Pd nanoparticles.

[0070] (3) 100 mg of Al MOF material was added to the above solution of Pd nanoparticles, the mass of Pd metal was 0.5 wt% of MOF material, and the mixture was stirred for 8 h. The solution was then dried in a rotary evaporator to obtain the catalyst encapsulated with Pd in ​​Al MOF material, denoted as Pd@Al MOF.

[0071] Comparative Example 4: 0.5 wt% Pd catalyst encapsulated in Fe MOF-NH2

[0072] (1) 1 mmol of ferric nitrate and 1 mmol of 2-aminoterephthalic acid were dispersed in 10 mL of N,N-dimethylformamide, and then 300 μL of formic acid was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 120 °C for 48 h to obtain Fe MOF-NH2 material.

[0073] (2) Prepare a 20 mL solution of 1.25 mg of the noble metal precursor Pd(NO3)2·2H2O and place it in an ice-water bath. Prepare a fresh NaBH4 solution with a mass ratio of 30:1 to the noble metal precursor and slowly add it dropwise. Continue stirring at room temperature for 2 h to obtain a colloidal solution containing Pd nanoparticles.

[0074] (3) 100 mg of Fe MOF-NH2 material was activated in vacuum at 100℃ for 16 h, and then added to the above solution of Pd nanoparticles. The mass of Pd metal was 0.5 wt% of the MOF material. The mixture was stirred for 8 h, and then the solution was dried in a rotary evaporator to obtain the catalyst encapsulated with Pd in ​​Fe MOF-NH2 material, denoted as Pd@Fe MOF-NH2.

[0075] Comparative Example 5: Synthesis of Fe MOF-2NH2 in an alkaline environment

[0076] (1) 1 mmol of ferric nitrate and 1 mmol of 2,5-diaminoterephthalic acid were dispersed in 10 mL of N,N-dimethylformamide. Then, 200 μL of NaOH solution (4 mol / L) was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 120 °C for 48 h. Fe MOF-2NH2 material could not be obtained. Ferric nitrate and 2,5-diaminoterephthalic acid cannot crystallize to form MOF material under conventional alkaline conditions. Only acidic environment is conducive to the coordination of Fe with 2,5-diaminoterephthalic acid.

[0077] Comparative Example 6: Synthesis of Fe MOF-2NH2 from FeCl3·6H2O

[0078] (1) 1 mmol of ferric chloride and 1 mmol of 2,5-diaminoterephthalic acid were dispersed in 10 mL of N,N-dimethylformamide, and then 300 μL of formic acid was added to the above solution to form a mixed solution. The mixed solution was transferred to a high-pressure reactor and reacted at 120 °C for 48 h. Fe MOF-2NH2 material could not be obtained. The chloride ions in conventional ferric chloride may compete with 2,5-diaminoterephthalic acid for the coordination of Fe, which will interfere with the orderly assembly of the MOF skeleton and generate amorphous products.

[0079] Figure 1 The images show the XRD patterns of the Fe MOF-2NH2 material and Pd@Fe MOF-2NH2 catalyst prepared in Example 1. Figure 1 It is evident that the Fe MOF-2NH2 material was successfully prepared, and the encapsulation of Pd within the Fe MOF-2NH2 material did not alter the material's crystal structure. Furthermore, the absence of diffraction peaks from Pd nanoparticles indicates that the Pd nanoparticles have a small particle size and high dispersion.

[0080] XRD patterns of the catalysts synthesized under alkaline conditions in Comparative Example 5 and using ferric chloride as a precursor in Comparative Example 6 are also shown. Figure 1 In the text, they are labeled as "NaOH" and "FeCl3·6H2O" respectively. This invention discovered that if the alkaline environment commonly used for MOF synthesis is employed, MOF materials cannot be successfully synthesized. It was also unexpectedly found that conventional iron precursors such as ferric chloride cannot effectively synthesize MOFs.

[0081] Figure 2 SEM images of the Fe MOF-2NH2 material and Pd@Fe MOF-2NH2 catalyst prepared in Example 1. Figure 2 It can be seen that the Fe MOF-2NH2 material is a spindle-shaped octahedron, and the encapsulation of Pd in ​​the Fe-MOF-2NH2 material does not change the morphology of the material itself.

[0082] Figure 3 This is a TEM image of the Pd@Fe MOF-2NH2 catalyst prepared in Example 1. Figure 3 It is evident that the Pd nanoparticles are encapsulated within Fe MOF-2NH2.

[0083] Figure 4 The image shows the XPS plot of Pd in ​​the Pd@Fe MOF-2NH2 catalyst in Example 1. It can be seen that the Pd nanoparticles have a zero valence, and Pd is in a metallic state.

[0084] Catalyst performance evaluation

[0085] The performance of the catalysts prepared in the examples and comparative examples for the partial oxidation of methane to methanol was tested and evaluated. The performance evaluation was carried out in a high-pressure autoclave reactor, and the reaction temperature was controlled by thermocouples.

[0086] Application Example 1

[0087] Step 1: Weigh 10 mg of the catalyst from the examples and comparative examples respectively, transfer it to the quartz liner of the batch reactor, then add 30 mL of water to the liner and seal the batch reactor.

[0088] Step 2: Purge the air in the reactor with O2 for 5 min to replace it, then purge with 0.3 MPa O2, 0.8 MPa H2 and 1.5 MPa CH4. Set the reaction temperature to 70 ℃ and the reaction time to 0.5 h.

[0089] Step 3: After the reaction is complete, place the reactor in an ice-water bath to rapidly cool it. After the gaseous product is detected by chromatography, centrifuge the reaction liquid, take the supernatant for liquid chromatography-nuclear magnetic resonance analysis, and calculate the product yield and methanol selectivity using the external standard method. Table 1 and Figure 6 This is a comparison of the reaction performance of the partial oxidation of methane to methanol in Examples 1-4 and Comparative Examples 1-3.

[0090] Table 1. Performance evaluation results of catalytic partial oxidation of methane to methanol in Examples 1-5 and Comparative Examples 1-4

[0091]

[0092] from Figure 6 , Figure 7 As shown in Table 1, the catalyst synthesized when the molar ratio of metal precursor to 2,5-diaminoterephthalic acid is 1:1 and the volume ratio of formic acid to DMF in the solvent is 3:50 performs best in the oxidation of methane to methanol. The performance of Examples 3 and 4 is worse than that of Example 1 because the catalysts in Examples 3 and 4 have relatively fewer amino groups, resulting in a relatively lower electron density at the metal active centers. The catalyst in Comparative Example 2, without any amino modification, exhibits even lower activity, indicating that amino modification can promote the oxidation of methane to methanol. The performance of Example 4 is generally poor because Au, as the active metal for the in-situ synthesis of H2O2 from H2 and O2, yields less H2O2 than Pd, but overall it is worse than Comparative Examples 1-3. The performance of Example 5 is worse than that of Example 1 because the electron-donating ability of the methoxy group is weaker than that of the amino group, but its performance is still better than the comparative examples with a single amino group (Comparative Example 4) and no amino group (Comparative Example 2).

[0093] In Comparative Example 1, due to the different mass ratio of reducing agent NaBH4 to noble metal precursor in step 3, the resulting Pd nanoparticles were slightly larger. Furthermore, since the noble metal Pd did not undergo a long stirring and encapsulation process, it remained on the surface of the MOF. Therefore, Pd could not fully contact the large number of Fe sites in the MOF channels, thus limiting the synergistic effect of the two in the catalytic cycle.

[0094] In Comparative Example 3, Pd@Al MOF only has Pd sites for in-situ synthesis of H2O2, but no Fe sites for decomposing H2O2, and therefore has no catalytic activity.

Claims

1. A method for preparing a bifunctional MOF-encapsulated metal catalyst, characterized in that, Including the following steps: Step 1: Dissolve the iron precursor and the bifunctional ligand in a mixture of N,N-dimethylformamide and formic acid, and obtain the bifunctional MOF by solvothermal reaction; the iron precursor is ferric nitrate and its hydrate. Step 2: Add a reducing agent to the noble metal precursor to obtain a colloidal solution of noble metal nanoparticles; Step 3: After activating the bifunctional MOF modified in Step 1, disperse it in the noble metal nanoparticle colloidal solution and stir to mix. After removing the solvent, the MOF-encapsulated metal catalyst is obtained. The noble metal precursor includes one or more of the following: palladium or gold halides, nitrates or acetates and their hydrates. The bifunctional ligand includes any one of 2,5-diaminoterephthalic acid and 2,5-dimethoxyterephthalic acid, or a mixture of any one with terephthalic acid. In step 3, the activation of the bifunctional MOF was carried out under vacuum, at an activation temperature of 60-120℃, and for an activation time of 8-20 h.

2. The method for preparing the bifunctional MOF-encapsulated metal catalyst according to claim 1, characterized in that, The reducing agent includes one or a combination of two or more of sodium borohydride, sodium citrate, ascorbic acid, and ethylene glycol.

3. The method for preparing the bifunctional MOF-encapsulated metal catalyst according to claim 1, characterized in that, The molar ratio of the iron precursor to the bifunctional ligand is 1:3-3:

1.

4. The method for preparing the bifunctional MOF-encapsulated metal catalyst according to claim 1, characterized in that, In step 1, the volume ratio of formic acid to N,N-dimethylformamide is 1:5-50; the molar ratio of the iron precursor to N,N-dimethylformamide is 1:100-1:

300. And / or, in step 1, the solvothermal reaction temperature is 100-180℃ and the reaction time is 12-72 h.

5. The method for preparing the bifunctional MOF-encapsulated metal catalyst according to claim 1, characterized in that, In step 2, the molar ratio of reducing agent to precious metal is 1-100:1; the concentration of reducing agent is 10-100 g / L; the reduction temperature is 0-60℃; and the reduction time is 30 min-8h.

6. The method for preparing the bifunctional MOF-encapsulated metal catalyst according to claim 1, characterized in that, In step 3, the mass of the noble metal in the colloidal solution of the noble metal nanoparticles is 0.1-3 wt% of the mass of the bifunctional MOF; the stirring and mixing is carried out at room temperature for 4-12 h.

7. A bifunctional MOF-encapsulated metal catalyst prepared by the preparation method according to any one of claims 1-6.

8. The application of the MOF-encapsulated metal catalyst according to claim 7 in the catalytic production of methanol from methane, characterized in that, In the methanol preparation process, the reaction temperature is 30-120℃; the reaction time is 0.5-2 h; the oxygen pressure is 0.3-0.8 MPa; the hydrogen pressure is 0.6-1.5 MPa; the methane pressure is 0.5-3 MPa; and the mass ratio of the catalyst to methane is 1:5-1:50.

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