Metal organic framework-based catalyst, preparation method thereof and application of metal organic framework-based catalyst in photocatalytic methane oxidation

Abstract

The invention discloses a metal organic framework-based catalyst, a preparation method thereof and application of the metal organic framework-based catalyst in photocatalytic methane oxidation, and belongs to the field of metal organic framework-based catalysts. The metal organic framework-based catalyst is designed, the expression of the metal organic framework-based catalyst is Pd / In-MIL-68-bpy, and the metal organic framework-based catalyst is of an In MOF and In2O3 composite structure loaded with Pd single atoms. The preparation method of the catalyst comprises the following steps: firstly, In-MIL-68-bpy is prepared from In (NO3) 3.5 H2O and H2bpydc, then In-MIL-68-bpy powder is dissolved in a PdCl2 methanol solution to prepare Pd / In-MIL-68-bpy, and a liquid product obtained after the reaction of the catalyst in photocatalytic methane oxidation only contains methanol and formaldehyde. The brand-new MOFs material is prepared and applied to the field of photocatalytic methane oxidation for the first time, in the photocatalytic methane oxidation process of the catalyst, only methanol and formaldehyde are generated in liquid products, and the C1 product selectivity reaches 100%. After the noble metal Pd is loaded, the catalyst shows good absorption performance on ultraviolet light and visible light, which shows that Pd / In-MIL-68-bpy has good overall optical performance.

Description

Technical Field

[0001] The invention belongs to the field of metal organic framework-based catalysts, and in particular relates to a metal organic framework-based catalyst and a preparation method thereof and application thereof in photocatalytic methane oxidation. Background Art

[0002] Metal-Organic Frameworks (MOFs) catalysts are a class of catalytic materials with special structures. It is a crystalline structure formed by the interconnection of metal ions or metal clusters and organic ligands. This special structure gives MOFs materials many unique properties and a wide range of applications. MOFs have their unique advantages, such as highly controllable structure and pore properties, which means that MOFs can provide more active sites and surface area for adsorption and storage of gases, such as hydrogen storage and CH4 capture, thereby enhancing the efficiency and selectivity of catalytic reactions. In 1999, Professor Omar M. Yaghi's research team at the University of California, Berkeley, reported in Nature magazine a coordination compound with a two-dimensional structure synthesized by a rigid organic ligand trimesic acid (BTC) and a transition metal Co, and called it MOF-5. Since then, the concept of metal organic frameworks has been formally proposed and has developed at an astonishing speed in the following nearly two decades.

[0003] Although there are endless types of catalysts on the market, many different types of functional nanomaterials have been developed, including alloys, metal oxides, etc. However, studies have shown that unmodified photocatalysts still face some difficult-to-overcome defects, such as low electronic conductivity and poor apparent quantum efficiency of metal oxide materials, poor cyclic stability of carbon-based materials, etc. Therefore, it is usually necessary to modify the photocatalysts in order to further construct high-quality functional materials with excellent overall photocatalytic performance. Summary of the invention

[0004] In view of the problems described in the background technology, the present invention takes the photocatalytic methane selective oxidation reaction as the research object, designs a metal organic framework-based catalyst MOFs-Pd / In-MIL-68-bpy as the main light absorption carrier in the photocatalytic material, utilizes solar energy, and selectively oxidizes methane to generate high value-added liquid products, thereby increasing the value of methane and facilitating transportation. The following technical solutions are adopted:

[0005] A metal organic framework-based catalyst was designed, which is expressed as Pd / In-MIL-68-bpy, which is a composite structure of InMOF and In2O3 loaded with Pd single atoms.

[0006] Preferably, the structure as a whole presents a porous rod-like morphology, and the Pd single atom is anchored on the dinitrogen site, which is the active center of the catalyst.

[0007] Furthermore, a method for preparing the metal organic framework-based catalyst is designed, comprising the following steps:

[0008] S1, a certain amount of In(NO3)3·5H2O and 2,2'-bipyridine-5,5'-dicarboxylic acid are dissolved in a solvent;

[0009] S2, heating after ultrasound, collecting the precipitate after cooling, washing and drying to prepare In-MIL-68-bpy;

[0010] S3, dissolving In-MIL-68-bpy in a certain amount of PdCl2 solution, stirring the obtained mixture, washing and drying the mixture to obtain.

[0011] Preferably, in step S1, the mass ratio of In(NO3)3·5H2O to 2,2'-bipyridine-5,5'-dicarboxylic acid is 4:3 to 3:2.

[0012] Preferably, in step S1, the solvent is DMF.

[0013] Preferably, in step S2, ultrasonication is performed in a polytetrafluoroethylene liner for 10-15 minutes, and then the mixture is placed in a high-pressure reactor and heated at 100-120°C for 24-26 hours; after cooling to room temperature, the precipitate is collected by centrifugation, washed three times with ethanol by centrifugation, and dried overnight at 60-80°C; finally, the mixture is washed three times with acetone and freeze-dried for 12-14 hours.

[0014] Preferably, in step S3, the solvent of the PdCl2 solution is methanol, and the resulting mixture is stirred in a screw-mouth vial wrapped with PTFE tape at room temperature for 24-26 hours; then centrifuged and washed four times with methanol and dried naturally; finally, washed three times with acetone and freeze-dried for 12-14 hours.

[0015] Furthermore, the liquid products after the catalytic reaction are only methanol and formaldehyde.

[0016] Preferably, under light irradiation, both H2O and CH4 participate in the reaction, and CH3OH and HCHO are generated by forming ·CH2 and ·OCH3 free radical intermediates.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] In the photocatalytic methane oxidation, the catalyst of the present invention produces only methanol and formaldehyde as liquid products, and the selectivity of C1 product reaches 100%. After loading the precious metal Pd, the catalyst shows good absorption performance for both ultraviolet and visible light, indicating that Pd / In-MIL-68-bpy has good overall optical properties.

[0019] This study prepared a new MOFs material and applied it in the field of photocatalytic methane oxidation for the first time.

[0020] Compared with traditional thermal catalysis, photocatalysis has higher efficiency and can utilize higher frequency energy, such as ultraviolet and visible light. The utilization efficiency of light energy is higher, which can make the reaction rate faster. At the same time, there is less energy consumption. No additional energy input is required in the photocatalytic reaction. The catalyst can be activated by light energy, so the energy loss is very small. Moreover, photocatalysis has a wider range of applications. It can be used in some areas that thermal catalytic reactions cannot cover. For example, the use of light can promote the cleaning of metal surfaces under mild conditions and other reactions.

[0021] Compared with traditional electrocatalysis, photocatalysis also has certain advantages. In terms of reaction efficiency, photocatalysis can increase the reaction rate and achieve high selectivity, because the light irradiated on the catalyst surface can directly stimulate the chemical reaction. Electrocatalysis needs to provide electrons to complete the chemical reaction, which often requires a long electrode reaction time and a high voltage. Therefore, the efficiency of photocatalysis is much higher than that of electrocatalysis. In terms of the use environment, photocatalysis can be used in different environments, while electrocatalysis can only be used in liquid phase systems. At the same time, photocatalysis has a lower cost. Electrocatalysis requires the use of expensive electrode materials and a large amount of energy, resulting in high costs. Photocatalysis does not require electrodes, and only direct light can activate the reaction. Moreover, photocatalysis is more environmentally friendly than electrocatalysis. The electrodes used in electrocatalysis often produce a lot of waste, while photocatalysis does not have this problem. Photocatalysis can also use solar energy to promote chemical reactions, achieving true green environmental protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is the Pd / In-MIL-68-bpy product in Example 2 of the present invention 1 H NMR spectra.

[0023] Figure 2 (aj) is a schematic diagram of the morphology and structural analysis results of Pd / In-MIL-68-bpy in Example 3 of the present invention.

[0024] Figure 3 It is a schematic diagram of the characterization of the surface elements and molecular composition of Pd / In-MIL-68-bpy in Example 4 of the present invention, wherein a, b, d, and e are XPS graphs, and c and f are simulation structure diagrams corresponding to a, b and d, e.

[0025] Figure 4 It is the O1s XPS spectrum of Pd / In-MIL-68-bpy in Example 4 of the present invention.

[0026] Figure 5 It is the UV-visDRS diagram in Example 5 of the present invention.

[0027] Figure 6 This is the In suit-FTIR chart of Pd / In-MIL-68-bpy in Example 6 of the present invention. DETAILED DESCRIPTION

[0028] The specific implementation of the present invention is described below in conjunction with the accompanying drawings and examples, but the following examples are only used to illustrate the present invention in detail and do not limit the scope of the present invention in any way. The instruments and equipment used in the present invention are all conventional experimental equipment in the field.

[0029] Example 1: A method for preparing a metal organic framework-based catalyst Pd / In-MIL-68-bpy, the steps are as follows:

[0030] (1) Preparation of In-MIL-68-bpy

[0031] 0.4082 g In(NO3)3·5H2O and 0.2940 g 2,2'-bipyridine-5,5'-dicarboxylic acid (H2bpydc) were dissolved in 5 mL DMF and ultrasonicated in a polytetrafluoroethylene liner for 10 min. Then, the mixture was placed in an autoclave and heated at 100 °C for 24 h. After cooling to room temperature, the precipitate was collected by centrifugation, washed three times with ethanol by centrifugation, and dried at 60 °C overnight. Finally, it was washed three times with acetone and freeze-dried for 12 h.

[0032] (2) Preparation of Pd / In-MIL-68-bpy

[0033] 30 mg of In-MIL-68-bpy powder was dissolved in 3 mL of PdCl2 methanol solution (5 mg), and the resulting mixture was stirred at room temperature for 24 hours in a screw-neck vial wrapped with PTFE tape. It was then washed four times by centrifugation with methanol and dried naturally. Finally, it was washed three times with acetone and freeze-dried for 12 hours.

[0034] Example 2: Application of metal organic framework-based catalyst Pd / In-MIL-68-bpy in photocatalytic methane oxidation. The Pd / In-MIL-68-bpy photocatalyst prepared in Example 1 was subjected to photocatalytic methane oxidation. The liquid product was subjected to 1 H NMR detection ( Figure 1 ), except for the water peak at 4.7 ppm and the DMSO peak at 2.6 ppm, only the methanol peak appeared at 3.2 ppm, and the yield was 2746 μmol·g cat -1At the same time, the formaldehyde content was detected by colorimetry and was 5826 μmol·g cat -1 , the liquid products only produced methanol and formaldehyde, and the C1 product selectivity reached 100%.

[0035] Example 3: Morphology and structural analysis of Pd / In-MIL-68-bpy. The Pd / In-MIL-68-bpy prepared in Example 1 was first characterized by XRD ( Figure 2 ab), in the process of synthesis, in order to precipitate MOF, an excess of In source is added. When MOF is precipitated, the remaining In in the solution 2+ In2O3 will be generated under hydrothermal reaction, and finally a composite material of MOF and oxide will be formed. It can be found that the classic MOF peak is present at the 2θ diffraction angle of 3° to 15°, confirming that the MOF structure has been successfully synthesized. The peak at 43° belongs to the In2O3 (332) crystal plane, which preliminarily indicates that the synthesized structure is not a simple MOF structure, but a material with a composite structure of In MOF and In2O3. In order to analyze its morphology, AC-HAADF-STEM characterization was performed ( Figure 2 c) It was found that the whole structure showed a porous rod-like morphology. Usually, the morphology of MOF should be more crystalline, and the porous morphology here is considered to be the effect of In2O3.

[0036] In order to further determine its crystal structure, a lattice gap of 0.3 nm was found on high-power AC-STEM. Figure 2 d), which belongs to the (222) crystal plane of In2O3, further verifying the crystal structure. At the same time, many white bright spots were found at a size of 2nm ( Figure 2 e), it is preliminarily believed that the Pd single atom loading was successful. Combined with the mapping, the Pd single atom has a high dispersion and exhibits good dispersion ( Figure 2 fj).

[0037] Example 4: The surface elements and molecular composition of Pd / In-MIL-68-bpy prepared in Example 1 were determined and characterized by XPS. From the full spectrum, it can be seen that the surface chemical composition is five elements: Pd, In, C, N, and O ( Figure 3 a). The high-resolution spectrum of C 1s revealed characteristic peaks at 284.81 eV and 288.36 eV for the CC bond and CN bond, respectively, which correspond to the bonding of the bpydc ligand ( Figure 3 b), the corresponding structure is as follows Figure 3c; The high-resolution spectrum of N 1s shows that after the introduction of Pd, the metal-N-pyridine peak intensity of the Pd / In-MIL-68-bpy catalyst at a binding energy of 399.79 eV increases, and the shoulder peak attributed to NH disappears, reflecting that the Pd single atom is anchored on the dinitrogen site of the bpydc ligand to replace the original H( Figure 3 d). At the same time, in the high-resolution spectrum of Pd 3d, characteristic peaks of Pd appeared at binding energies of 342.9 eV and 337.7 eV, further confirming that the Pd single atom was anchored on the dinitrogen site ( Figure 3 e), the corresponding structure is as follows Figure 3 f.

[0038] The O 1s spectrum reflects the information about the active sites ( Figure 4 ), the combined peaks centered at 534.419 eV and 531.294 eV on the O1s spectrum of Pd / In-MIL-68-bpy are attributed to adsorbed oxygen and lattice oxygen, respectively. At the same time, In-MIL-68-bpy does not have adsorbed oxygen. After loading Pd, a peak of adsorbed oxygen appears, that is, active oxygen is generated around Pd, indicating that Pd is very likely to be the active center of the entire material.

[0039] Example 5: The optical properties of Pd / In-MIL-68-bpy prepared in Example 1 were analyzed. In order to investigate the light absorption performance, UV-vis DRS characterization was performed ( Figure 5 ), it can be seen from the UV-visible diffuse reflectance spectrum that In-MIL-68-bpy only shows high absorption performance in the UV region less than 400nm. When loaded with precious metal Pd, the material shows good absorption performance for both UV and visible light, further indicating that Pd / In-MIL-68-bpy has good overall optical properties.

[0040] Example 6: A preliminary mechanism analysis of the photocatalytic methane oxidation in Example 2 was conducted, and the free radical transformation during the reaction was analyzed by testing In suit-FTIR ( Figure 6 ), thus making a preliminary prediction of the entire reaction. -1 The peak corresponds to ·OH of H2O at 3000cm -1 The peaks at 1149 and 1363 cm are attributed to CH4. -1The peaks of ·OH and CH4 were attributed to ·CH2 and ·OCH3, respectively. These two groups are considered to be the key intermediates for the direct oxidation of CH4 to CH3OH and HCHO. CH4, O2 and water vapor were introduced during the test. Under dark conditions, the peaks of ·OH and CH4 continued to increase with time, indicating that the material adsorbed CH4 and H2O. When exposed to light, the peaks of ·OH and CH4 dropped rapidly, and the peaks of ·CH2 and ·OCH3 began to increase, further indicating that under light, H2O and CH4 both participated in the reaction, and CH3OH and HCHO were generated by forming ·CH2 and ·OCH3 free radical intermediates.

[0041] The present invention has been described in detail above with reference to the accompanying drawings and embodiments; however, those skilled in the art will appreciate that, without departing from the concept of the present invention, the specific parameters in the above embodiments may be changed, or related components and structures may be replaced by equivalents, thereby forming multiple specific embodiments, which are all within the common variation range of the present invention and will not be described in detail one by one.

Claims

1. A metal organic framework-based catalyst, expressed as Pd / In-MIL-68-bpy, is a composite structure of In MOF and In2O3 loaded with Pd single atoms.

2. The metal organic framework-based catalyst according to claim 1, characterized in that The structure as a whole presents a porous rod-like morphology, and the Pd single atom is anchored on the dinitrogen site, which is the active center of the catalyst.

3. A method for preparing the metal organic framework-based catalyst according to claim 1, characterized in that: The following steps are involved: S1, a certain amount of In(NO3)3·5H2O and 2,2'-bipyridine-5,5'-dicarboxylic acid are dissolved in a solvent; S2, heating after ultrasound, collecting the precipitate after cooling, washing and drying to prepare In-MIL-68-bpy; S3, dissolving In-MIL-68-bpy in a certain amount of PdCl2 solution, stirring the obtained mixture, washing and drying the mixture to obtain.

4. The method for preparing a metal organic framework-based catalyst according to claim 3, characterized in that: In step S1, the mass ratio of In(NO3)3·5H2O to 2,2'-bipyridine-5,5'-dicarboxylic acid is 4:3 to 3:

2.

5. The method for preparing a metal organic framework-based catalyst according to claim 3, characterized in that: In step S1, the solvent is DMF.

6. The method for preparing a metal organic framework-based catalyst according to claim 3, characterized in that: In step S2, ultrasonication is performed in a polytetrafluoroethylene liner for 10-15 minutes, and then the mixture is placed in a high-pressure reactor and heated at 100-120°C for 24-26 hours; after cooling to room temperature, the precipitate is collected by centrifugation, washed three times with ethanol by centrifugation, and dried overnight at 60-80°C; finally, the mixture is washed three times with acetone and freeze-dried for 12-14 hours.

7. The method for preparing a metal organic framework-based catalyst according to claim 3, characterized in that: In step S3, the solvent of the PdCl2 solution is methanol, and the resulting mixture is stirred at room temperature for 24-26 hours in a screw-mouth vial wrapped with PTFE tape; then, it is centrifuged and washed four times with methanol and dried naturally; finally, it is washed three times with acetone and freeze-dried for 12-14 hours.

8. Use of the metal organic framework-based catalyst according to claim 1 in photocatalytic methane oxidation, characterized in that: The liquid products after the catalytic reaction are only methanol and formaldehyde.

9. The use of the metal organic framework-based catalyst in photocatalytic methane oxidation according to claim 8, characterized in that: Under light, both H2O and CH4 participated in the reaction, and CH3OH and HCHO were generated by forming ·CH2 and ·OCH3 free radical intermediates.

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