CN116899570B - Metal catalyst and preparation method and application thereof - Google Patents

Abstract

The present invention provides a metal catalyst and a preparation method and application thereof. The catalyst is obtained by pyrolysis of a precursor having a general structural formula: Ni _x Co _y ‑MOF; wherein 0≤x≤2, 0≤y≤2, and x and y are not 0 at the same time; MOF represents an organic metal framework. The present invention partially decomposes the MOF framework structure through controlled pyrolysis, so that the porous nanostructure of the MOF is partially retained, thereby generating an MNPs active center in the porous MOF, and the residual MOF framework can ensure the stability of the active center, thereby producing a good catalytic effect, and can be used for catalytic hydrogenation of unsaturated bonds of organic compounds such as benzene, toluene, olefins, alkynes, aldehydes, ketones, esters, and catalytic hydrogenation of carbon dioxide.

Description

A metal catalyst and its preparation method and application

Technical Field

The present invention relates to the technical field of catalysts, and in particular to a metal catalyst and a preparation method and application thereof.

Background Art

Metal-organic frameworks (MOFs) have high surface area and structural diversity, and have important application value in catalysis, gas storage/separation, drug delivery and chemical sensing. The study of the synergistic effect between metal nanoparticles (MNPs) and MOFs is of great significance in catalysis. If MNPs and MOFs can be reasonably coordinated, the MNP/MOF composite material may not only have a large specific surface area and pore volume, but also have the advantages of adjustable composition and morphology, thereby greatly improving its catalytic performance.

At present, the methods for preparing MNP/MOF include solvothermal method, solid grinding method, solution impregnation method, chemical vapor deposition method and self-template method. Pyrolysis is a simple and effective way to expose the metal active sites of MNP/MOF materials. However, the direct high-temperature calcination (above 400°C) of MOF-derived nanomaterials reported so far causes the complete decomposition of MOF at high temperature, causing the collapse of the pores and framework structure inside MOF, so that a large number of active metal sites are embedded in the bulk phase, hindering the contact between the reactants and the metal sites. Therefore, the catalytic effect of the catalyst obtained by this method is general and difficult to promote. The solid grinding method is a method in which a volatile organic metal precursor is ground, sublimated, evaporated and diffused into the MOF cavity, and then the organic metal precursor embedded in the MOF is reduced to form an MNP/MOF composite material. The solid grinding method is simple to operate, can avoid excess solvent, and can achieve complete loading of the introduced precursor. However, it has high requirements for the metal precursor, and the loaded metal precursor needs to be volatile. Therefore, this method is expensive and not universal. The solution impregnation method uses capillary pressure to infiltrate a solution containing a metal precursor into the internal cavity of the MOF, and then reduces the metal precursor to MNP. This method also has the advantage of simple operation, but the problem is that it is difficult to evenly distribute MNPs inside and outside the MOF pores, and the MNPs adsorbed on the surface are prone to migration. These problems result in the catalytic effect of the obtained catalyst being average, and also limit the application of this method. The chemical vapor deposition method diffuses a volatile metal precursor into the desolvated MOF pores under vacuum conditions at a certain temperature. This method does not require solvents to prepare MNP/MOF and can load a large amount of MNPs, but it also requires the organic metal precursor to be highly volatile at room temperature or to be easily sublimated at elevated temperatures.

Summary of the invention

In view of the problems of poor catalytic effect, high preparation cost, and strict requirements on raw materials in the existing MNP/MOF and its preparation method, the present invention provides a metal catalyst and its preparation method and application. The technical concept of partially decomposing the MOF framework structure through controlled pyrolysis allows the porous nanostructure of MOF to be partially retained, thereby generating MNPs active centers in the porous MOF, and the residual MOF framework can ensure the stability of the active center, thereby producing a good catalytic effect.

To achieve the above object, the technical solution of the present invention is:

In a first aspect, the present invention provides a metal catalyst obtained by thermal decomposition of a precursor having a general structural formula: Ni _x Co _y -MOF; wherein 0≤x≤2, 0≤y≤2, and x and y are not simultaneously 0; MOF represents an organic metal framework.

Preferably, the precursor is pyrolyzed in a _H2 atmosphere or a _N2 atmosphere to obtain the metal catalyst; further preferably, the pyrolysis temperature in the _H2 atmosphere is 300 to 350°C, preferably 320 to 330°C; the pyrolysis time is 1 to 2 hours;

Further preferably, the pyrolysis temperature in _N2 atmosphere is 300-600°C, preferably 450-500°C; and the pyrolysis time is 1-2h.

Preferably, the method for preparing the precursor comprises: subjecting nickel salt and/or cobalt salt, an organic ligand and an alkaline regulator to a hydrothermal reaction in a solvent.

Preferably, the nickel salt is selected from: at least one of nickel nitrate, nickel acetate, nickel sulfate, nickel chloride, and nickel bromide; the cobalt salt is selected from: at least one of cobalt nitrate, cobalt acetate, cobalt chloride, cobalt bromide, and cobalt sulfate; the organic ligand is selected from: at least one of terephthalic acid, 1,3,5-benzenetricarboxylic acid, 2,5-dihydroxyterephthalic acid, 2,5-thiophenedicarboxylic acid, formic acid, and pyridine carboxylic acid; the alkaline regulator is selected from: at least one of triethylenediamine hexahydrate, sodium hydroxide, potassium hydroxide, triethylamine, triethanolamine, and polyvinylpyrrolidone; the solvent is selected from: at least one of DMF, methanol, ethanol, and deionized water.

Preferably, in the preparation method of the precursor, the molar ratio of nickel salt to cobalt salt is 0-2:0-2; the molar amount of the organic ligand is 0.9-1.1 of the sum of the molar amount of nickel in the nickel salt and the molar amount of cobalt in the cobalt salt; the amount of the alkaline regulator is 0.5-2.0 of the sum of the molar amount of nickel in the nickel salt and the molar amount of cobalt in the cobalt salt.

Preferably, in the method for preparing the precursor, the hydrothermal reaction temperature is 110 to 150° C., preferably 125 to 135° C.; the hydrothermal reaction time is 12 to 48 h, preferably 20 to 30 h.

Preferably, the preparation method of the precursor also includes: separating the solid product of the hydrothermal reaction, washing and drying; further preferably, after separation, the solid product of the hydrothermal reaction is washed multiple times with DMF, and then washed multiple times with ethanol; further preferably, the washed solid is vacuum dried at 60-80°C for more than 8h.

Preferably, the preparation method comprises: subjecting nickel salt and/or cobalt salt, organic ligand and alkaline regulator to a hydrothermal reaction in a solvent to obtain a metal precursor, and then subjecting the metal precursor to a pyrolysis reaction to obtain the metal catalyst.

In a second aspect, the present invention provides a metal precursor for synthesizing the above-mentioned metal catalyst, wherein the metal precursor has a general structural formula: NixCoy-MOF; wherein 0≤x≤2, 0≤y≤2, and x and y are not 0 at the same time; MOF represents an organic metal framework.

The third aspect is the use of the above-mentioned metal catalyst or the above-mentioned metal precursor in the catalytic hydrogenation reaction of unsaturated bonds in organic compounds.

The beneficial effects of the present invention are as follows:

The present invention prepares a metal catalyst with a MOF framework structure, uses nickel salt and cobalt salt as metal ions, selects suitable organic ligands, and regulates the content and distribution of metal species in the MOF structure to regulate the microstructure of the prepared MNP/MOF material, thereby ultimately achieving the purpose of regulating the microstructure of the metal catalyst after pyrolysis. Compared with traditional metals, the metal catalyst prepared by the present invention has a nano-scale porous MOF structure residual from pyrolysis with easily accessible active sites, and its framework structure can enrich the concentration of reactants near the active MNPs site, thereby improving the conversion rate of the reactants, and can be used for catalytic hydrogenation of unsaturated bonds of organic compounds such as benzene, toluene, alkene, alkyne, aldehyde, ketone, ester, and catalytic hydrogenation of carbon dioxide. In addition, the metal catalyst of the present invention does not contain precious metals, the preparation method is simple, the conditions are not harsh, the cost is controllable to a certain extent, and it has a good industrialization prospect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a TG analysis chart of Ni _x Co _y -MOF.

Figure 2 is the FTIR analysis graph of NixCoy-MOF precursor and metal catalyst, wherein (a) is the FTIR analysis graph of NixCoy-MOFs; (b) is the FTIR analysis graph of the catalyst obtained by decomposing NixCoy-MOFs at 325°C; (c) is the FTIR analysis graph of the catalyst obtained by decomposing Ni1Co2-MOFs at different decomposition temperatures (300°C, 325°C, 350°C).

Figure 3 shows the XRD patterns of NixCoy-MOF precursor and metal catalyst, where (a) is the XRD analysis diagram of NixCoy-MOFs; (b) is the XRD analysis diagram of the catalyst obtained by decomposing NixCoy-MOFs at 325°C; (c) is the local (43.6°～45.6°) XRD analysis diagram of the catalyst obtained by decomposing different NixCoy-MOFs at 325°C; (d) is the XRD analysis diagram of the catalyst obtained by decomposing Ni ₁ Co ₂ -MOFs at different decomposition temperatures (300°C, 325°C, 350°C).

Figure 4 shows the N2 adsorption-desorption isotherms of the metal catalysts.

Figure 5 is a scanning electron microscope (SEM) analysis of NixCoy-MOFs and metal catalysts, where the catalysts corresponding to Figures a to j are Ni-MOF, Ni2Co1-MOF, Ni1Co1-MOF, Ni1Co2-MOF, Co-MOF, Ni/MOF-325, Ni2Co1/MOF-325, Ni1Co1/MOF-325, Ni1Co2/MOF-325, and Co/MOF-325, respectively.

FIG6 is an XPS spectrum of metal catalysts, wherein (a) shows the XPS spectrum of Ni 2p in various metal catalysts; and (b) shows the XPS spectrum of Co 2p in various metal catalysts.

FIG7 is the H2-TPD curve of the metal catalyst in the range of 60 to 210°C.

Figure 8 is an analysis diagram of the catalytic activity and stability of metal catalysts, wherein (a) shows the conversion rate of toluene hydrogenation catalyzed by the catalysts obtained by pyrolysis of NixCoy-MOF precursor in nitrogen at different temperatures; (b) shows the conversion rate of toluene hydrogenation catalyzed by the catalysts obtained by pyrolysis of NixCoy-MOF precursor in hydrogen at 325°C; (c) shows the conversion rate of toluene hydrogenation catalyzed by the catalysts obtained by pyrolysis of Ni1Co2/MOF at different temperatures; (d) shows the stability of different metal catalysts at 160°C.

FIG9 is a curve showing the conversion rate of benzene hydrogenation catalyzed by the metal catalyst Ni/MOF-300 over temperature.

Note: NixCoy-MOF-325 indicates the metal catalyst obtained by pyrolysis of NixCoy-MOF at 325 °C.

DETAILED DESCRIPTION

In the description of the present invention, it should be noted that, if the specific conditions are not specified in the examples, the experiments were carried out according to conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used, if the manufacturer is not specified, are all conventional products that can be purchased commercially.

The present invention provides a metal catalyst, which is obtained by thermal decomposition of a precursor with a general structural formula: Ni _x Co _y -MOF; wherein 0≤x≤2, 0≤y≤2, x and y can both be 0, 1, 2, but x and y cannot be 0 at the same time; MOF represents an organic metal framework.

In a preferred embodiment of the present invention, the method for preparing the precursor comprises: subjecting nickel salt and/or cobalt salt, an organic ligand, and an alkaline regulator to a hydrothermal reaction in a solvent.

In the present invention, the nickel salt is preferably selected from at least one of nickel nitrate, nickel acetate, nickel sulfate, nickel chloride, and nickel bromide; the cobalt salt is selected from at least one of cobalt nitrate, cobalt acetate, cobalt chloride, cobalt bromide, and cobalt sulfate; the organic ligand is selected from at least one of terephthalic acid, 1,3,5-benzenetricarboxylic acid, 2,5-dihydroxyterephthalic acid, 2,5-thiophenedicarboxylic acid, formic acid, and pyridinecarboxylic acid. These organic ligands can provide a stable MOF skeleton structure for the metal catalyst of the present invention, and terephthalic acid is preferably used in the present invention.

In the present invention, the alkaline regulator is preferably selected from at least one of triethylenediamine hexahydrate, sodium hydroxide, potassium hydroxide, triethylamine, triethanolamine, and polyvinylpyrrolidone; preferably triethylenediamine hexahydrate. The alkaline regulator is added to improve the coordination ability of metal ions and ligands. It is currently found that the aforementioned several can be used in the present invention.

In the present invention, the solvent is preferably selected from at least one of DMF, methanol, ethanol and deionized water.

In a preferred embodiment of the present invention, in the preparation method of the precursor, the molar ratio of nickel salt to cobalt salt is 0-2:0-2; the molar amount of the organic ligand is 0.9-1.1 of the sum of the molar amount of nickel in the nickel salt and the molar amount of cobalt in the cobalt salt, and it is better to control it at around 1.0; the amount of the alkaline regulator is 0.5-2.0 of the sum of the molar amount of nickel in the nickel salt and the molar amount of cobalt in the cobalt salt, and it is preferably controlled at 0.5-1.0.

In a preferred embodiment of the present invention, in the method for preparing the precursor, the hydrothermal reaction temperature is 110-150°C, preferably 125-135°C; more preferably 130°C; the hydrothermal reaction time is 12-48h, preferably 20-30h, more preferably 24h.

Preferably, in the present invention, the method for preparing the precursor further comprises: separating the solid product of the hydrothermal reaction, washing and drying; preferably, after separation, the solid product of the hydrothermal reaction is first washed multiple times with DMF, and then washed multiple times with ethanol; preferably, the washed solid is vacuum dried at 60-80°C for more than 8h, preferably dried at 70±2°C for 24h.

The metal catalyst is applied to the catalytic hydrogenation reaction of unsaturated bonds of organic compounds. Taking the Ni ₁ Co ₂ /MOF-325 catalyst derived from the Ni/Co molar ratio of 1/2 as an example, it not only retains the excellent structure and large specific surface area similar to MOF, but also has more unoccupied d orbitals in the d energy band of Co in the Co-rich alloy catalyst, which improves the adsorption and activation ability of the catalyst to H ₂ , makes the metal active components fully contact with the H ₂ molecules, and is conducive to improving its toluene hydrogenation activity and reaction stability.

The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

Example 1

1.1 The preparation method of the metal catalyst is as follows:

Accurately weigh Ni(NO ₃ ) ₂ ·6H ₂ O, Co(NO ₃ ) ₂ ·6H ₂ O (mass see Table 1), 0.700g terephthalic acid (0.0042mol) and 0.464g (0.0021mol) triethylenediamine hexahydrate, dissolve in 50mL N,N-dimethylformamide (DMF), ultrasonically vibrate for 10min, magnetically stir for 20min, transfer the stirred solution to a 100mL polyethylene hydrothermal autoclave and seal it, keep it at 130℃ for 24.0h. After cooling naturally, the product is washed twice with DMF and twice with ethanol, centrifuged at 4000rpm for 7min each time, and vacuum dried at 70℃ overnight. The obtained crystalline powder is the Ni _x Co _y -MOF precursor, which is recorded as Ni-MOF, Ni ₂ Co ₁ -MOF, Ni ₁ Co ₁ -MOF, Ni ₁ Co ₂ -MOF and Co-MOF according to the Ni/Co molar ratio. After pyrolysis in a H ₂ atmosphere at 325°C for 1 h, the metal catalysts Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₁ /MOF-325, Ni ₁ Co ₂ /MOF-325 and Co/MOF-325 of the present invention are obtained.

Table 1 Ni(NO ₃ ) ₂ ·6H ₂ O and Co(NO ₃ ) ₂ ·6H ₂ O with different Ni/Co molar ratios

1.2 Characterization of catalysts

1.2.1 Thermogravimetric analysis (TG)

The catalyst precursor was subjected to thermogravimetric analysis (TG) using a STA4493F3 synchronous thermal analyzer (Netzsch Instrument Manufacturing Co., Ltd., Germany). The test atmosphere was nitrogen, the temperature range was 25-900°C, and the heating rate was 10°C/min.

1.2.2 X-ray diffraction (XRD)

The X-ray diffraction (XRD) of the sample was tested on XRD-6100 (Shimadzu Corporation, Japan). The ray tube was a 2.2kW Cu target, a Kα ray source, a tube voltage of 40kV, a current of 30mA, a scanning step of 0.02°, a scanning rate of 4°/min, and a scanning range of 5 to 80°.

1.2.3 Specific surface area and pore size distribution test (BET)

The specific surface area and pore structure of the samples were measured using an ASAP2020HD88 fully automatic specific surface and micropore adsorption instrument (BET, Micromeritics, Inc., USA). Before the BET measurement, the accurately weighed samples were vacuum degassed at 150 °C for 3.0 h in a nitrogen flow and then tested at liquid nitrogen temperature (-196 °C).

1.2.4 Fourier Transform Infrared Spectroscopy (FTIR)

The functional group composition of the samples was tested and analyzed by IRPrestige-21 Fourier transform infrared spectrometer (FTIR, Hitachi, Japan). KBr was used as the test background, and the test wave number range was 500-4000 cm ^–1 .

1.2.5H _{2 -TPR}

A fully automatic chemisorption instrument (Belcat-II, Japan Microchem Belcat Co., Ltd.) was used for _{H 2} -TPR. 50 mg of catalyst precursor was weighed and placed in a quartz tube. In a 6% H ₂ /Ar flowing mixed gas, the temperature was raised from room temperature to the set temperature at a rate of 10 ^°C /min. The hydrogen consumption was detected by a thermal conductivity detector (TCD).

1.2.6H ₂ temperature programmed desorption (H ₂ -TPD)

A fully automatic chemisorption instrument (Belcat-II, Japan Micro-Bel Co., Ltd.) was used for H ₂ temperature-programmed desorption (H ₂ -TPD) test. 50 mg of the precursor was weighed and transferred to a quartz tube. It was first pyrolyzed at 325 °C for 1.0 h in a H ₂ atmosphere, then cooled to 50 ^°C in a He atmosphere, and then a 6% H ₂ /Ar mixed gas was introduced into the catalyst bed for 30 min. Subsequently, He gas was purged at this temperature to remove physically adsorbed H ₂ molecules. Finally, the sample was heated from 50 ^°C to the set temperature at 10 ^°C /min, and the hydrogen desorption amount of the sample was obtained by TCD detection.

1.2.7 Scanning Electron Microscope (SEM)

The morphological characteristics of the samples were characterized using a SU1510 scanning electron microscope (SEM) (Hitachi, Japan) with an operating voltage of 15 kV.

1.2.8 X-ray Photoelectron Spectroscopy (XPS)

The X-ray photoelectron spectroscopy (XPS) of Ni 2p and Co 2p was measured on an X-ray photoelectron spectrometer (Thermo Scientific K-Alpha, Thermo Fisher Scientific, USA), and all binding energy calibrations were based on the C1s peak at 284.8 eV. The operating voltage was 12 kV, the filament current was 6 mA, and Al Kα was used as the excitation source (hν = 1486.6 eV).

1.3 Evaluation of catalytic activity of metal catalysts

The toluene hydrogenation reaction was carried out in a fixed bed reactor. Weigh 50 mg of the Ni _x Co _y -MOF precursor sample prepared in step 1.1 and place it in the middle of the quartz tube reactor. Then fix the quartz tube in a tube furnace. In 20 mL/min of flowing H ₂ , heat the temperature to the pyrolysis temperature of the catalyst at a rate of 10 °C/min and keep it at this temperature for 1.0 h. After the pyrolysis is completed, when the temperature drops to 80 °C, the obtained metal catalyst is then subjected to in-situ hydrogenation reaction: the raw gas composed of toluene and H ₂ is introduced into the reactor at 20 mL/min, and the toluene hydrogenation activity on the catalyst is evaluated in the range of 80-200 °C. The reaction products are analyzed online using a gas chromatograph equipped with FID.

2 Results and discussion

2.1TG analysis

The thermal decomposition behaviors of Ni-MOF, Ni ₂ Co ₁ -MOF, Ni ₁ Co ₁ -MOF, Ni ₁ Co ₂ -MOF and Co-MOF in nitrogen atmosphere were investigated using a simultaneous thermal analyzer.

As shown in Figure 1, the thermal degradation of _{NixCoy -} MOF in a nitrogen atmosphere mainly includes three temperature stages: (1) When the temperature is 25-370℃, each precursor has a small mass loss, which is attributed to the release of solvent DMF molecules and absorbed water molecules encapsulated in the skeleton, as well as the decomposition of part of the MOFs skeleton structure; (2) When the temperature is 370-500℃, there is an obvious mass loss in this stage, which is caused by the large-scale decomposition of ligands in MOFs; (3) When the temperature is 500-800℃, Ni-MOF, _Ni1Co1 -MOF _{, Ni1Co2 -} MOF and _{Ni2Co1 -} MOF all have a small mass loss, while the mass of Co-MOF increases slightly, which may be because in this temperature range, Co-MOF further decomposes and releases a large amount of gas phase products, resulting in a sudden increase in local pressure. The above results show that the pyrolysis temperature range of Ni-MOF, Ni ₂ Co ₁ -MOF, Ni ₁ Co ₁ -MOF, Ni ₁ Co ₂ -MOF and Co-MOF in an inert atmosphere is 370-500°C, and the MOFs framework structure will be completely decomposed at a temperature above 500°C. Therefore, controlled pyrolysis in nitrogen at 370-500°C may partially decompose the structure of MOFs, thereby retaining part of the MOFs framework structure.

2.2 FTIR study

We used FTIR analysis to study the functional groups of Ni _x Co _y -MOF precursors and catalysts.

Figure 2(a) shows the FTIR spectra of Ni _x Co _y -MOF precursors with different Ni/Co molar ratios. The bands at 3603 cm ^-1 and 3430 cm ^-1 are attributed to OH in the Ni _x Co _y -MOF precursor, which is caused by the coordinated water in MOFs, indicating that water molecules exist in the Ni _x Co _y -MOF structure. The peaks at 2927, 1502, 815 and 753 cm ^-1 are characteristic peaks of para-aromatic CH stretching bands. The strong bands at 1572 ^{cm -1} and 1383 cm ^-1 are respectively attributed to the asymmetric and symmetric stretching modes of the coordination group (-COO ^– ), indicating that the -COO ^– of terephthalic acid is coordinated with Ni ²⁺ and Co ²⁺ through a bidentate ligand mode (coordination between metal ions and linker molecules is successful). In addition, the peaks at 608 cm ^-1 and 698 cm ^-1 are attributed to the characteristic peaks of Ni-O or Co-O, indicating that Ni or Co atoms successfully formed metal oxygen bonds with ^-COO- of terephthalic acid. Therefore, Ni and Co species have been successfully introduced into the framework of MOFs, thereby forming a _{NixCoy -} MOF structure.

Figure 2(b) shows the FTIR spectrum of Ni _x Co _y /MOF-325 catalyst obtained by pyrolysis of Ni _x Co _y -MOF at 325℃ and H ₂ flow for 1.0h. Compared with Ni/MOF, Ni ₂ Co ₁ /MOF, and Ni ₁ Co ₁ /MOF (2(a)), the characteristic peaks of MOFs structure on Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, and Ni ₁ Co ₁ /MOF-325 catalysts in Figure 2(b) almost disappear, significantly weaken or even disappear, indicating that the MOFs framework structure in Ni/MOF-325 and Ni ₂ Co ₁ /MOF-325 catalysts has been decomposed under 325℃ and H ₂ flow. Similarly, compared with the Ni ₁ Co ₁ /MOF precursor, the characteristic peaks of MOFs structure on the Ni ₁ Co ₁ /MOF-325 catalyst are significantly weakened, indicating that its MOFs framework structure is destroyed. In addition, the FTIR spectra of Co/MOF-325 and Ni ₁ Co ₂ /MOF-325 are similar to those of Co/MOF and Ni ₁ Co ₂ /MOF precursors, indicating that the Co/MOF-325 and Ni ₁ Co ₂ /MOF-325 catalysts retain the framework structure of MOFs well.

Figure 2(c) shows the FTIR spectra of the catalysts obtained after Ni ₁ Co ₂ -MOF was treated at different pyrolysis temperatures. Compared with Ni ₁ Co ₂ /MOF-300 and Ni ₁ Co ₂ /MOF-325, the characteristic peaks of the MOFs structure on Ni ₁ Co ₂ /MOF-350 almost disappeared, indicating that its MOFs framework structure was seriously disintegrated, while the MOFs framework structure of Ni ₁ Co ₂ /MOF-300 and Ni ₁ Co ₂ /MOF-325 was relatively complete. Therefore, the pyrolysis temperature is also one of the important factors affecting the MOFs framework structure of the catalyst.

2.3 XRD study

Figure 3(a) shows the XRD spectra of Ni-MOF, Ni ₂ Co ₁ -MOF, Ni ₁ Co ₁ -MOF, Ni ₁ Co ₂ -MOF and Co-MOF. They have similar XRD crystal phase diffraction peaks because their lattice constants and crystal structures are similar, indicating that Ni and Co metal ions have almost no effect on the crystal structure of the synthesized MOFs. Among them, the diffraction peak of Co-MOF is weak, indicating that the crystallinity of the corresponding phase is poor, which may be because Co atoms promote the growth of MOF materials in amorphous form.

Figure 3(b) shows the _XRD spectra of Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, Ni 1 Co 1 /MOF-325, Ni ₁ Co ₂ /MOF-325 and Co/MOF-325 catalysts obtained by pyrolysis of the _{above MOFs} materials at 325°C under H 2 flow for 1.0 h. No characteristic diffraction peaks of Ni/MOF and Ni ₂ Co ₁ /MOF were observed in the spectra of Ni/MOF-325 and Ni ₂ Co ₁ /MOF-325, indicating that Ni/MOF and Ni ₂ Co ₁ /MOF were completely decomposed under 325°C and H ₂ flow. Compared with Ni ₁ Co ₁ -MOF, the intensity of the MOFs characteristic diffraction peak in Ni ₁ Co ₁ /MOF-325 was significantly reduced, while Ni ₁ Co ₂ /MOF-325 basically maintained the MOF characteristic diffraction peak intensity consistent with that of Ni ₁ Co ₂ -MOF. This indicates that Ni ₁ Co ₂ /MOF-325, after pyrolysis of Ni ₁ Co ₂ -MOF at 325°C under H ₂ flow, maintains the framework structure of MOFs materials well, further indicating that the introduction of Co into Ni-based MOFs to form NiCo-based MOFs materials can enhance their high temperature resistance in H ₂ atmosphere. In addition, it can be clearly observed in Figure 3(b) that the Ni/MOF-325 catalyst has diffraction peaks at 2θ=44.5°, 51.8°, and 76.4°, which are respectively attributed to the {111}, {200}, and {220} crystal planes of face-centered cubic metal Ni, which are Ni NPs generated by the complete decomposition of Ni/MOF. Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₁ /MOF-325 and Ni ₁ Co ₂ /MOF-325 catalysts also showed diffraction peaks at 2θ=44.4°, 51.6° and 76.2°, as shown in Figure 3(c) (partial enlargement of Figure 3(b)), which are the {111}, {200}, and {220} crystal planes of face-centered cubic Ni-Co alloy, respectively. Among the five samples, the XRD spectrum of Ni/MOF-325 showed sharp metallic Ni diffraction peaks, indicating high crystallinity of metallic Ni, which explained that the pyrolysis of Ni/MOF in H ₂ atmosphere at 325℃ caused the MOFs structure to collapse sharply and formed huge Ni agglomerates. The metal particle sizes of Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₁ /MOF-325 and Ni ₁ Co ₂ /MOF-325 were calculated from the half-peak width of the {111} diffraction peak by the Scherrer equation and were 25.5, 15.2, 12.3 and 10.2 nm, respectively. Obviously, the introduction of Co into Ni-based MOFs significantly reduced the metal particle size of the catalyst after pyrolysis, which was attributed to the stronger high temperature resistance of Co-MOF in H ₂ atmosphere (see the activity test section), preventing the decomposition of the MOF framework structure and the aggregation of metal species during high temperature pyrolysis, and improving the dispersion of metal particles produced by pyrolysis in the MOFs carrier.

Figure 3(d) shows the XRD spectra of Ni ₁ Co ₂ /MOF-300, Ni ₁ Co ₂ /MOF-325 and Ni ₁ Co ₂ /MOF-350 catalysts obtained at different pyrolysis temperatures (300, 325 and 350°C). In the figure, the MOFs characteristic peak intensity of Ni ₁ Co ₂ /MOF-300 _is significantly stronger than that of Ni ₁ Co ₂ / _MOF -325, while no characteristic diffraction peak of Ni ₁ Co ₂ /MOF is observed in Ni ₁ Co ₂ /MOF-350, indicating that only a small amount of MOF structure may be decomposed under 300°C and H ₂ flow, so the generated Ni-Co alloy nanoparticles are small; while under 350°C and H ₂ flow, due to the high pyrolysis temperature, the MOFs framework structure is completely decomposed, resulting in the aggregation of metal species and the disappearance of the characteristic diffraction peak of MOFs. In particular, the XRD pattern of the Ni ₁ Co ₂ /MOF-325 catalyst has diffraction peaks of both MOFs and Ni-Co alloys, indicating that the controllable pyrolysis path allows the Ni ₁ Co ₂ /MOF-325 catalyst to retain the MOF framework structure well, and at the same time, a large number of well-dispersed Ni-Co alloy nanoparticles (NiCo NPs) are formed in the Ni ₁ Co ₂ /MOF, which helps to improve its catalytic hydrogenation performance. Therefore, the appropriate pyrolysis temperature has an important influence on the phase structure of Ni-Co alloy catalysts derived from NiCo-based MOFs.

2.4BET Research

The specific surface area and pore structure of MOFs derived catalysts Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₁ /MOF-325, Ni ₁ Co ₂ /MOF-325 and Co/MOF-325 were studied by BET technique, and the results are shown in Figure 4. Among them, Figure 4 shows the N ₂ adsorption-desorption isotherms of these catalysts, with a relative pressure range of 0.0 to 1.0 (P/P ₀ ) and an adsorption amount range of 0 to 400 (cm ³ /g STP).

As shown in Figure 4, in the relative pressure range of 0.7 to 1.0P/P ₀ , all isotherms show typical IV type curves with H3 type hysteresis loops, indicating the presence of mesoporous structures in these samples. The specific surface areas of Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₁ /MOF-325, Ni ₁ Co ₂ /MOF-325 and Co/MOF-325 are 4.67, 5.12, 9.67, 46.55 and 23.64 m ² /g, respectively, that is, the order of specific surface area is: Ni ₁ Co ₂ /MOF-325>Co/MOF-325>Ni ₁ Co ₁ /MOF-325>Ni ₂ Co ₁ /MOF-325>Ni/MOF-325. This shows that at a pyrolysis temperature of 325°C, the molar ratio of Ni to Co in the catalyst has a greater influence on its specific surface area. When Ni/Co is 1/2, the specific surface area of the corresponding Ni ₁ Co ₂ /MOF-325 catalyst reaches the maximum value. The specific surface areas of Ni ₂ Co ₁ /MOF-325 and Ni/MOF-325 are smaller, which may be due to the serious decomposition of their MOF framework structure during the pyrolysis process.

The average pore size and total pore volume data of Ni _x Co _y /MOF-325 catalyst calculated by Barrett-Joyner-Halenda (BJH) method are shown in Table 2. The total pore volumes of Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₁ /MOF-325, Ni ₁ Co ₂ /MOF-325 and Co/MOF-325 are 0.02, 0.01, 0.02, 0.26 and 0.17 cm ³ /g, respectively, and the average pore sizes are 6.84, 6.83, 7.33, 22.66 and 28.61 nm, respectively. It can be seen that Ni ₁ Co ₂ /MOF-325 not only has a high specific surface area, but also has a large total pore volume, which may be beneficial to improving its catalytic activity.

The BET analysis results in this section show that different Ni/Co molar ratios can adjust the specific surface area and nanopore structure of the metal catalyst.

Table 2 Average pore size and total pore volume data of _{Ni x} Co _y -MOF-325

2.5 Scanning electron microscopy (SEM) analysis

The morphology of the prepared MOFs and Ni _x Co _y /MOF-325 catalyst was characterized by scanning electron microscopy, and the results are shown in Figure 5. As can be seen from the figure, among the prepared MOFs, Ni-MOF, Ni ₂ Co ₁ -MOF, Ni ₁ Co ₁ -MOF and Ni ₁ Co ₂ -MOF (a, b, c, d in Figure 5) have columnar structures with a wide size distribution, while block structures can be observed in Co-MOF (e in Figure 5). In the SEM of the Ni _x Co _y /MOF-325 catalyst after H ₂ pyrolysis, it can be observed that the columnar structure of Ni/MOF-325 (f in Figure 5) and Ni ₂ Co ₁ /MOF-325 (g in Figure 5) catalysts collapsed significantly, indicating that the MOFs skeleton structure was completely decomposed by H ₂ pyrolysis at 325℃. The surfaces of Ni ₁ Co ₁ /MOF-325 (h in Figure 5) and Ni ₁ Co ₂ /MOF-325 (i in Figure 5) catalysts are much rougher than their corresponding MOFs. In particular, the Ni ₁ Co ₂ /MOF-325 catalyst not only retains the framework structure of MOF before pyrolysis, but also forms obvious honeycomb pores on the surface, indicating that pyrolysis in hydrogen at 325°C can partially decompose the structure of Ni ₁ Co ₂ -MOF, thereby retaining part of the framework structure of MOFs while forming metal particles. In addition, compared with its corresponding MOF, the surface of the Co/MOF-325 (j in Figure 5) catalyst has neither structural collapse nor rough surface formation, which may be due to the decomposition of only a small amount of Co-MOF in the hydrogen environment at 325°C.

2.6XPS study

The surface composition and chemical state of Ni and Co in the prepared Ni _x Co _y /MOF-325 catalysts (Ni/MOF-325, Ni ₁ Co ₁ /MOF-325, Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₂ /MOF-325 and Co/MOF-325) were studied by XPS, and the results are shown in Figure 6, where (a) and (b) in Figure 6 are the XPS spectra of Ni 2p and Co 2p, respectively.

In the Ni 2p and Co 2p spectra, two main characteristic peaks corresponding to Ni 2p _3/2 (856.2eV) and Ni 2p _1/2 (873.6eV) and two main characteristic peaks of Co 2p _3/2 (781.5eV) and Co 2p _1/2 (797.2eV) are shown, respectively. Two shock peaks belonging to Ni ²⁺ and Co ²⁺ species can be observed at 862.2, 880.1eV and 787.8, 803.7eV, respectively, indicating that there is a strong interaction between Ni and Co species on the surface of each catalyst. The two main peaks of Ni 2p and Co 2p can be divided into three and two sub-peaks, respectively. Among them, the three sub-peaks of Ni 2p _3/2 at electron binding energies of 852.9±0.9eV, 854.8±1.5eV, and 858.0±2.5eV and the three sub-peaks of Ni 2p _1/2 at electron binding energies of 871.6±1.9eV, 873.5±1.4eV, and 876.3±2.2eV are respectively attributed to Ni ⁰ , Ni ²⁺ , and Ni ³⁺ species on the catalyst surface in their respective regions. The two sub-peaks of Co 2p _3/2 at 780.5±0.9eV and 782.6±1.1eV and the two sub-peaks of Co 2p _1/2 at 796.3±1.0eV and 798.2±1.0eV are respectively attributed to Co ⁰ and Co ²⁺ species on the catalyst surface.

As shown in Figure 6, there are a large number of oxidized species on the surfaces of these five catalysts, which may be oxidized species produced by oxidation of the catalyst in the air after reduction. The XPS peak intensity and peak area of Ni ⁰ species in Ni-MOF-325 are the highest, and the XPS peak intensity and peak area of Co ⁰ species in Co-MOF-325 are lower. This is because under the pyrolysis conditions of 325℃, the framework structure of Ni-MOF completely collapses, and more Ni ⁰ species are exposed on its surface; while the structure of Co-MOF is still in a steady state after pyrolysis at 325℃, resulting in less exposure of Co ⁰ species, which is consistent with the analysis results of XRD.

For Ni ₁ Co ₁ /MOF-325, Ni ₂ Co ₁ /MOF-325 and Ni ₁ Co ₂ /MOF-325, since Ni ₁ Co ₂ /MOF-325 retains more MOF framework structure (see XRD research results), the detection amount of Ni ⁰ and Co ⁰ species on its surface is reduced, and the corresponding XPS peak intensity and peak area are reduced; the XPS peak areas of Ni 0 and Co ⁰ species in Ni ₁ Co ₁ /MOF-325 and Ni ₂ Co ₁ /MOF-325 are larger, which is attributed to the serious damage of their MOFs framework structure. The Ni-Co alloy is embedded in the catalyst, but its crystallite size is larger (see XRD research results). Ni ⁰ and Co ⁰ species can still be detected on its surface, indicating that the alloy catalyst prepared when the Ni/Co molar ratio is 1/1 and 2/1 ^is unstable, and it is more stable when the Ni/Co molar ratio is 1/2, which is beneficial to improve the Ni ₁ Co ₂ /Catalytic hydrogenation performance of MOF-325.

The above XPS results show that the Ni/Co molar ratio affects the composition of the precursor, which in turn affects the distribution of the Ni-Co alloy in the catalyst after pyrolysis, and further affects the catalytic activity.

2.7H ₂ -TPD study

The H ₂ adsorption and desorption performance of Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₁ /MOF-325, Ni ₁ Co ₂ /MOF-325 and Co-MOF-325 catalysts was studied by H 2 -TPD, and the results are shown in Figure 7. All catalysts formed a H ₂ desorption peak in the range of 60-210 °C, which was attributed to the desorption of weakly chemically adsorbed H species on the catalyst surface. The reaction temperature of toluene hydrogenation in this work (80-200 °C) is just within this desorption temperature range. Therefore, the desorbed H ₂ in this _range provides the required active H species for the toluene hydrogenation reaction.

The H ₂ -TPD desorption peak temperature on the catalyst can be used to determine the strength of H ₂ adsorption. The H 2 desorption peak temperatures of Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₁ /MOF-325, Ni ₁ Co ₂ /MOF-325 and Co/MOF-325 catalysts are 101°C, 89°C, 103°C, 121°C and 107°C, respectively. Among them, the H ₂ desorption peak temperature of _{Ni 1} Co ₂ -MOF-325 is significantly higher, indicating that it has a stronger H ₂ adsorption strength in this area. The area of the H ₂ -TPD desorption peak is positively correlated with the amount of H ₂ desorption. The order of the H ₂ desorption peak area of the studied catalysts is: Ni ₁ Co ₂ /MOF-325>Co/MOF-325>Ni ₁ Co ₁ /MOF-325>Ni ₂ Co ₁ /MOF-325≈Ni/MOF-325. Among them, the H ₂ desorption peak area of Ni/MOF-325 and Ni ₂ Co ₁ /MOF-325 is smaller, which may be due to the complete decomposition of their MOFs framework structure (see FTIR and XRD analysis results). On the one hand, the complete decomposition of the MOFs framework structure leads to low specific surface area and pore volume of Ni/MOF-325 and Ni ₂ Co ₁ /MOF-325 catalysts, which weakens the catalyst's ability to adsorb and activate H _2. On the other hand, the decomposition of the MOFs structure forms larger metal agglomerates, which embeds the active metal sites in the bulk phase, which is not conducive to the contact between H ₂ and the active metal sites, thereby reducing the amount of H ₂ adsorption. Compared with Ni/MOF-325, Co/MOF-325 has a larger H ₂ desorption peak area, which is attributed to the surviving MOFs structure in the Co/MOF-325 catalyst. Among the catalysts studied, Ni ₁ Co ₂ /MOF-325 has the largest H ₂ desorption peak area, mainly because the catalyst retains the framework structure of MOFs, which gives it a larger surface area and pore volume, which is conducive to the adsorption and activation of H _2. Secondly, the surviving MOFs structure improves the dispersion of metal particles produced by pyrolysis in the MOFs carrier, forming smaller nano-metal particles, which are conducive to the weak chemical adsorption of H on the metal surface.

2.8 Catalyst activity and stability test

Under normal pressure and reaction temperature of 80-200°C, the gas phase toluene hydrogenation performance of Ni ₁ Co ₂ /MOF-325, Ni ₁ Co ₂ /MOF-400, Ni ₁ Co ₂ /MOF-450, Ni ₁ Co ₂ /MOF-500, Ni ₁ Co ₂ /MOF-600 catalysts prepared by pyrolysis in a nitrogen atmosphere and Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₂ /MOF-325, Ni ₁ Co ₁ /MOF-325, Co/MOF-325 catalysts prepared by pyrolysis in a hydrogen atmosphere were tested. In the present invention, the only product detected by toluene hydrogenation is methylcyclohexane. The change of toluene conversion rate of each catalyst with temperature is shown in Figures 8(a) and 8(b). At the same time, the toluene hydrogenation activities of Ni ₁ Co ₂ /MOF-N, Ni ₁ Co ₂ /MOF-300, Ni ₁ Co ₂ /MOF-325 and Ni ₁ Co ₂ /MOF-350 catalysts prepared by pyrolysis in a hydrogen atmosphere were compared, as shown in Figure 8(c). In addition, the stability of Ni/MOF-325, Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₂ /MOF-325, Ni ₁ Co ₁ /MOF-325 and Co/MOF-325 catalysts was tested at 160°C for 10.0 h using toluene hydrogenation as a probe reaction, and the results are shown in Figure 8(d).

As shown in Figure 8(a), the highest conversion rates of the catalysts obtained by pyrolysis in nitrogen are all at a reaction temperature of 180°C, and the order of hydrogenation activity of each catalyst is: Ni ₁ Co ₂ /MOF-500-N>Ni ₁ Co ₂ /MOF-450-N>Ni ₁ Co ₂ /MOF-600-N>Ni ₁ Co ₂ /MOF-400-N>Ni ₁ Co ₂ /MOF-325-N. Their highest conversion rates for toluene are all below 10.0%, which is far less than the hydrogenation activity of the catalysts obtained by pyrolysis in a hydrogen atmosphere. Therefore, hydrogen is selected as the pyrolysis and reduction gas flow for MNP-MOF in this paper.

As shown in Figure 8(b), the toluene hydrogenation conversion rate on each catalyst shows a trend of increasing first and then decreasing with the increase of reaction temperature. In the high temperature region, their toluene conversion rates all decrease significantly, which may be related to the dehydrogenation of the generated methylcyclohexane at high temperature. The conversion rate of toluene hydrogenation on Co/MOF-325 catalyst is 0, and the toluene hydrogenation conversion rate of Ni/MOF-325 catalyst is the highest at 180℃, which is 27.0%. The toluene hydrogenation conversion rates of Ni ₂ Co ₁ /MOF-325, Ni ₁ Co ₂ /MOF-325 and Ni ₁ Co ₁ /MOF-325 catalysts are the highest at 160℃, which are 81.3%, 95.7% and 87.3%, respectively. That is, the toluene conversion rates on each catalyst follow the rule: Ni ₁ Co ₂ /MOF-325>Ni ₁ Co ₁ /MOF-325>Ni ₂ Co ₁ /MOF-325>Ni/MOF-325>Co/MOF-325. It shows that Ni ₁ Co ₂ /MOF-325 catalyst has good toluene hydrogenation performance.

In order to verify that 325℃ is the optimal pyrolysis temperature, the toluene hydrogenation activities of Ni ₁ Co ₂ /MOF-U, Ni ₁ Co ₂ /MOF-300, Ni ₁ Co ₂ /MOF-325 and Ni ₁ Co ₂ /MOF-350 were compared. As shown in Figure 8(c), the order of toluene hydrogenation activities of each catalyst within the reaction temperature range is: Ni ₁ Co ₂ /MOF-325>Ni ₁ Co ₂ /MOF-350>Ni ₁ Co ₂ /MOF-300>Ni ₁ Co ₂ /MOF-N. Among them, the conversion rate of toluene of Ni ₁ Co ₂ /MOF-325 catalyst pyrolyzed at 325℃ is significantly greater than that of catalysts pyrolyzed at other temperatures within the reaction temperature range, indicating that the catalyst has excellent catalytic hydrogenation performance when the pyrolysis temperature is 325℃.

As shown in Figure 8(d), after the catalysts were reacted at 160°C for 10.0 h, the conversion rate of toluene on Co/MOF-325 was 0, and the extreme differences in the conversion rates of toluene for Ni ₁ Co ₂ /MOF-325, Ni ₁ Co ₁ /MOF-325, Ni ₂ Co ₁ /MOF-325 and Ni/MOF-325 were 2.1%, 7.3%, 6.9% and 4.3%, respectively, that is, the stability order was Ni ₁ Co ₂ /MOF-325>Ni/MOF-325>Ni ₂ Co ₁ /MOF-325>Ni ₁ Co ₁ /MOF-325. Among them, Ni ₁ Co ₂ /MOF-325 had better stability, which may be related to the structure of Ni-Co alloy on Ni ₁ Co ₂ /MOF-325.

The structure and properties of the active components determine the hydrogenation performance of the catalyst. XRD, BET, FTIR, XPS and H ₂ -TPD results show that both pyrolysis temperature and Ni/Co molar ratio can regulate the microstructure of Ni-Co alloy, thereby improving the hydrogenation performance of the catalyst. On the one hand, taking Ni ₁ Co ₂ -MOF as the research object and keeping other conditions unchanged, when the pyrolysis temperature is 300℃, it is not enough to pyrolyze the MOF framework structure, lacks hydrogenation active sites, and its hydrogenation activity is relatively low; when the pyrolysis temperature is 350℃, it provides enough energy to completely decompose the MOF framework structure, and the Ni-Co alloy is exposed on the catalyst surface. The metal species on the catalyst surface are severely agglomerated, and the ability to adsorb and desorb H ₂ is weakened, thereby weakening its hydrogenation ability; 325℃ is its critical pyrolysis temperature. The Ni ₁ Co ₂ /MOF-325 catalyst derived from pyrolysis at this temperature can not only retain most of the porous structure of MOF in Ni ₁ Co ₂ /MOF for the dispersion of Ni-Co alloy and the adsorption of H ₂ , but also expose a large number of Ni-Co alloy active sites. Therefore, the Ni ₁ Co ₂ /MOF-325 catalyst exhibits excellent toluene hydrogenation activity. On the other hand, keeping the pyrolysis temperature at 325℃ and other related conditions unchanged, when the Ni/Co molar ratio is 1/0 and 0/1, Ni/MOF-325 and Co/MOF-325 catalysts can be derived. Due to the collapse of the porous structure of Ni/MOF-325, the Co nanoparticles of Co/MOF-325 are wrapped in the amorphous structure and not activated, resulting in insufficient hydrogenation activity, which is difficult to compare with the bimetallic Ni-Co alloy catalyst. When the Ni/Co molar ratio is 1/1, the derived Ni ₁ Co ₁ /MOF-325 catalyst has a serious decomposition of the MOF framework structure compared with its precursor, providing fewer active sites for the Ni-Co alloy and reducing its stability. When the Ni/Co molar ratio is 2/1, the MOF framework structure of the prepared Ni ₂ Co ₁ /MOF-325 catalyst is completely decomposed and collapsed, so that the active components of the Ni-Co alloy are embedded in the catalyst body phase, and the adsorption and desorption ability of H ₂ decreases accordingly, but its stability is slightly higher than that of Ni ₁ Co ₁ /MOF-325 catalyst; the Ni ₁ Co ₂ /MOF-325 catalyst derived with a Ni/Co molar ratio of 1/2 not only retains the excellent structure and large specific surface area similar to MOF, but also the Co-rich alloy catalyst has more unoccupied d orbitals in the d energy band of Co, which improves the catalyst's adsorption and activation ability for H ₂ , allowing the active components of the Ni-Co alloy to fully contact with H ₂ molecules, which is beneficial to improving its toluene hydrogenation activity and reaction stability.

Example 2

The Ni/MOF prepared in Example 1 was pyrolyzed in a hydrogen atmosphere at 300°C for 1h (Ni/MOF-300) and further used for benzene hydrogenation investigation. The benzene hydrogenation reaction was carried out in a fixed bed reactor. 50 mg of the Ni-MOF precursor sample prepared in step 1.1 of Example 1 was weighed and placed in the middle of a quartz tube reactor. The quartz reaction tube was then fixed in a tube furnace. In a flowing _H2 of 13.4 mL/min, the temperature was raised to the pyrolysis temperature of the catalyst at a rate of 10°C/min and maintained at this temperature for 1.0 h. After the pyrolysis was completed, when the temperature dropped to 80°C, the obtained metal catalyst was then subjected to in-situ hydrogenation reaction: the raw gas composed of benzene and _H2 was introduced into the reactor at 13.4 mL/min, and the benzene hydrogenation activity on the catalyst was evaluated in the range of 80-200°C. The reaction product was analyzed online using a gas chromatograph equipped with FID. The specific results are as follows: As shown in Figure 9, the benzene hydrogenation conversion rate on the Ni/MOF-300 catalyst shows a trend of first increasing and then decreasing with the increase of reaction temperature. The decrease in the benzene hydrogenation conversion rate in the high temperature zone may be due to the further dehydrogenation of the generated cyclohexane into benzene. The results show that the metal catalyst of the present invention also has high hydrogenation activity for benzene, and the conversion rate of benzene at 180°C reaches more than 80%.

In summary, the present invention prepares a metal catalyst with a MOF framework structure, the metal catalyst has a nano-scale porous MOF structure residual from pyrolysis, the MOF structure has accessible active sites, and its framework structure can enrich the concentration of reactants near the active MNPs sites, thereby improving the conversion rate of the reactants. The metal catalyst of the present invention can also be used for the catalytic hydrogenation of unsaturated bonds of organic compounds such as benzene, toluene, olefins, alkynes, aldehydes, ketones, esters, and the catalytic hydrogenation of carbon dioxide.

The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims (11)

1. A metal catalyst, characterized in that the catalyst is obtained by pyrolysis of a precursor having the general structural formula: Ni _x Co _y -MOF; wherein 1≤x≤2, 1≤y≤2, and MOF represents an organic metal framework; the precursor is pyrolyzed in a H ₂ atmosphere to obtain the metal catalyst; the pyrolysis temperature is 320-330°C; and the pyrolysis time is 1-2h. 2. A metal catalyst according to claim 1, characterized in that the preparation method of the precursor comprises: subjecting nickel salt and cobalt salt, organic ligand and alkaline regulator to hydrothermal reaction in a solvent. 3. A metal catalyst according to claim 2, characterized in that the nickel salt is selected from: at least one of nickel nitrate, nickel acetate, nickel sulfate, nickel chloride, and nickel bromide; the cobalt salt is selected from: at least one of cobalt nitrate, cobalt acetate, cobalt chloride, cobalt bromide, and cobalt sulfate; the organic ligand is selected from: at least one of terephthalic acid, 1,3,5-benzenetricarboxylic acid, 2,5-dihydroxyterephthalic acid, 2,5-thiophenedicarboxylic acid, formic acid, and pyridinecarboxylic acid; the alkaline regulator is selected from: at least one of triethylenediamine hexahydrate, sodium hydroxide, potassium hydroxide, triethylamine, triethanolamine, and polyvinylpyrrolidone; the solvent is selected from: at least one of DMF, methanol, ethanol, and deionized water. 4. A metal catalyst according to claim 2 or 3, characterized in that in the preparation method of the precursor, the molar ratio of nickel salt to cobalt salt is 1~2:1~2; the molar amount of organic ligand is 0.9~1.1 of the sum of the molar amount of nickel in the nickel salt and the molar amount of cobalt in the cobalt salt; the amount of alkaline regulator is 0.5~2.0 of the sum of the molar amount of nickel in the nickel salt and the molar amount of cobalt in the cobalt salt. 5. A metal catalyst according to claim 2, characterized in that in the preparation method of the precursor, the hydrothermal reaction temperature is 110-150°C; the hydrothermal reaction time is 12-48h. 6. A metal catalyst according to claim 5, characterized in that the hydrothermal reaction temperature is 125-135°C and the hydrothermal reaction time is 20-30h. 7. A metal catalyst according to claim 2, characterized in that the method for preparing the precursor further comprises: separating the solid product of the hydrothermal reaction, washing and drying it. 8. A metal catalyst according to claim 7, characterized in that the solid product of the hydrothermal reaction is washed multiple times with DMF and then washed multiple times with ethanol after separation. 9. A metal catalyst according to claim 8, characterized in that the washed solid is vacuum dried at 60-80°C for more than 8 hours. 10. The method for preparing a metal catalyst according to claim 1, characterized in that the preparation method comprises: subjecting a nickel salt and a cobalt salt, an organic ligand, and an alkaline regulator to a hydrothermal reaction in a solvent to obtain a metal precursor, and then subjecting the metal precursor to a pyrolysis reaction in a _H2 atmosphere, wherein the pyrolysis temperature is 320-330°C and the pyrolysis time is 1-2 hours; and obtaining the metal catalyst. 11. Use of the metal catalyst according to any one of claims 1 to 9 in catalytic hydrogenation of unsaturated bonds in organic compounds.