A two-step hydrothermal method for preparing nickel-iron bimetallic MOF derivatives and its application

Abstract

This invention discloses a two-step hydrothermal method for preparing nickel-iron bimetallic MOF derivatives and its application. By controlling the precursor structure through a two-step hydrothermal strategy, the nickel-iron bimetallic MOF derivative catalyst forms a foam-like hierarchical porous carbon framework. The metal nanoparticles are uniformly dispersed, small in size, and encapsulated by carbon layers, significantly improving the specific surface area and accessibility of active sites. It exhibits excellent hydrogen yield and anti-carbon deposition performance in plastic pyrolysis tar reforming. The synergistic effect of nickel and iron electrons effectively inhibits metal sintering and carbon deposition, and has good catalytic stability. This invention solves the problem in existing nickel-iron bimetallic MOF derivatives where phase separation easily occurs in one-step synthesis due to the difference in nucleation and growth rates between nickel and iron ions and organic ligands, leading to uneven metal distribution, increased particle size, underdeveloped pore structure, and low accessibility of active sites, thus affecting catalytic reforming activity and anti-carbon deposition ability.

Description

Technical Field

[0001] This invention relates to the field of catalyst technology and the field of hydrogen production technology from waste plastic pyrolysis tar catalytic reforming, specifically to a two-step hydrothermal method for preparing nickel-iron bimetallic MOF derivatives and its application in hydrogen production from waste plastic pyrolysis tar catalytic reforming. Background Technology

[0002] Catalytic reforming of waste plastic pyrolysis tar for hydrogen production is a key technology for realizing the resource utilization of plastic waste and the production of clean energy. Nickel-based catalysts have good cracking activity for hydrocarbons and are widely used in the catalytic reforming of waste plastic pyrolysis tar for hydrogen production. However, under high-temperature reaction conditions, the active sites of nickel-based catalysts are easily deactivated by carbon deposition, which restricts their long-term stable operation. To solve the above problems, existing technologies introduce iron into nickel-based catalysts to form a nickel-iron bimetallic system. The activation ability of iron on water vapor and carbon dioxide generates active oxygen species, which promotes the gasification and removal of carbon deposits. At the same time, by forming a nickel-iron alloy, the electronic structure of nickel is changed, inhibiting the formation of carbon deposits.

[0003] Currently, the preparation of nickel-iron bimetallic catalysts mainly employs the traditional impregnation method. This method struggles to achieve a uniform distribution of nickel and iron at the nanoscale, easily leading to metal particle agglomeration, reduced specific surface area, weakened synergistic effects between the bimetals, and consequently, impacting catalytic activity. Catalysts derived from metal-organic frameworks (MOFs) precursors can achieve composite structures where metal particles are encapsulated by a carbon framework, which helps suppress metal particle sintering at high temperatures. However, nickel-iron bimetallic MOF precursors synthesized in a one-step process are prone to phase separation due to differences in the nucleation and growth rates of nickel and iron ions with organic ligands. This results in problems such as uneven metal phase distribution, increased metal particle size, and underdeveloped pore structure in the final catalyst, reduced accessibility to active sites, and catalytic performance and resistance to carbon deposition failing to meet application requirements. Therefore, it is necessary to optimize the microstructure of the catalyst. Summary of the Invention

[0004] The purpose of this invention is to provide a two-step hydrothermal method for preparing nickel-iron bimetallic MOF derivatives and its application in the catalytic reforming of waste plastic pyrolysis tar to produce hydrogen. This method solves the problem that existing nickel-iron bimetallic MOF derivatives are prone to phase separation in one-step synthesis due to the difference in nucleation and growth rates between nickel-iron ions and organic ligands. This results in uneven metal distribution, increased particle size, underdeveloped pore structure, and low accessibility of active sites, which in turn affects the catalytic activity and anti-carbon deposition ability of catalytic reforming of waste plastic pyrolysis tar to produce hydrogen.

[0005] This invention is achieved through the following technical solutions: A two-step hydrothermal method for preparing nickel-iron bimetallic MOF derivatives, comprising the following steps: S1: Preparation of the first metal-organic framework precursor: The first metal salt, the organic ligand and the solvent are mixed and subjected to a first hydrothermal reaction to obtain the first metal-organic framework precursor; the first metal salt is a nickel salt or an iron salt, and the organic ligand is terephthalic acid; S2: Preparation of bimetallic organic framework precursor: The first metal-organic framework precursor obtained in S1, the second metal salt, the organic ligand, and the solvent are mixed and subjected to a second hydrothermal reaction to obtain the bimetallic organic framework precursor; the second metal salt is an iron salt or nickel salt different from the first metal salt, and the organic ligand is terephthalic acid; S3: Preparation of nickel-iron bimetallic MOF derivatives: The bimetallic organic framework precursor obtained in S2 was subjected to high-temperature carbonization reduction treatment under an inert atmosphere at a temperature of 700~900 ℃ to obtain nickel-iron bimetallic MOF derivatives.

[0006] Furthermore, the conditions for the first hydrothermal reaction and the second hydrothermal reaction are independently: reaction temperature 120~180℃, reaction time 12~36 hours.

[0007] Furthermore, the high-temperature carbonization reduction treatment is carried out under an inert atmosphere of nitrogen or argon for 2 to 6 hours.

[0008] Furthermore, the solvent is selected from one or more of N,N-dimethylformamide, anhydrous ethanol, or water.

[0009] Furthermore, the nickel salt is nickel nitrate or nickel chloride, and the iron salt is ferric chloride or ferric nitrate.

[0010] Furthermore, the molar ratio of the first metal-organic framework precursor to the second metal salt is 1:0.5 to 1:2.

[0011] This invention also protects nickel-iron bimetallic MOF derivatives prepared by the above method. These nickel-iron bimetallic MOF derivatives possess a foam-like hierarchical porous carbon framework structure with a specific surface area of ​​100–120 m². 2 / g, pore volume 0.25~0.30 cm³ 3 / g, the nickel-iron alloy nanoparticles have a particle size of 10~30 nm and are uniformly dispersed in the carbon skeleton and coated with carbon.

[0012] This invention also protects the application of the nickel-iron bimetallic MOF derivative prepared by the above method as a catalyst in the catalytic reforming of waste plastic pyrolysis tar to produce hydrogen-rich gas.

[0013] This invention allows for the acquisition of catalysts with different microstructures by controlling the types of the first and second metal salts. When the first metal salt in step S1 is an iron salt, the resulting first metal-organic framework precursor is an iron-based metal-organic framework; when the second metal salt in S2 is a nickel salt, the resulting bimetallic organic framework precursor is a nickel-iron bimetallic organic framework with a layered structure; after high-temperature carbonization and reduction in step S3, a nickel-iron bimetallic MOF derivative with a foam-like carbon framework structure is obtained, wherein the nickel-iron alloy nanoparticles are uniformly dispersed and encapsulated by carbon layers. When the first metal salt in step S1 is a nickel salt and the second metal salt in step S2 is an iron salt, the resulting bimetallic organic framework precursor is a nickel-iron bimetallic organic framework with a mixed block and layered structure; after high-temperature carbonization and reduction in step S3, a nickel-iron bimetallic MOF derivative with a coarse-grained carbon foam structure is obtained.

[0014] Compared with the prior art, the present invention has the following advantages: 1) This invention regulates the precursor structure through a two-step hydrothermal strategy, enabling the nickel-iron bimetallic MOF derivative catalyst to form a foam-like hierarchical porous carbon framework. The metal nanoparticles are uniformly dispersed, small in size, and encapsulated by carbon layers, which significantly improves the specific surface area and accessibility of active sites.

[0015] 2) The nickel-iron bimetallic MOF derivative catalyst prepared in this invention exhibits excellent hydrogen yield and anti-carbon deposition performance in plastic pyrolysis tar reforming. The synergistic effect of nickel and iron electrons effectively inhibits metal sintering and carbon deposition, and has good catalytic stability. Attached Figure Description

[0016] Figure 1 These are scanning electron microscope images of Fe-Ni@C (bottom) prepared in Example 1 and Ni-Fe@C (top) prepared in Example 2; Figure 2 This is a schematic diagram of a two-stage fixed-bed reactor for a mixed plastic pyrolysis tar catalytic reforming experiment. Detailed Implementation

[0017] The following is a further description of the invention, but not a limitation thereof.

[0018] Example 1: A method for preparing a nickel-iron bimetallic MOF derivative (hereinafter referred to as Fe-Ni@C) for the production of hydrogen-rich gas from tar reforming, and its catalytic performance was evaluated.

[0019] The specific steps are as follows: S1. Preparation of the first metal-organic framework precursor: Weigh 2.162 g (8 mmol) of FeCl3·9H2O (the first metal salt, iron salt) and 1.33 g of terephthalic acid (H2BDC, organic ligand), dissolve them in 60 mL of N,N-dimethylformamide (solvent), and stir magnetically for 30 minutes to ensure homogeneity. Transfer the mixed solution to a 100 mL polytetrafluoroethylene-lined high-pressure reactor and carry out the first hydrothermal reaction at 150 °C for 24 hours. After the reaction is complete, allow it to cool naturally to room temperature, centrifuge the obtained product, discard the supernatant, and collect the solid product. Wash twice with anhydrous ethanol, and then dry overnight in a vacuum drying oven at 60 °C to obtain the iron-based metal-organic framework precursor Fe-BDC.

[0020] S2. Preparation of the second metal-organic framework precursor: All the Fe-BDC precursor obtained in S1, 2.326 g (8 mmol) of Ni(NO3)2·6H2O (the second metal salt, nickel salt), and 1.33 g of terephthalic acid were dissolved together in 60 mL of DMF and magnetically stirred for 30 minutes. The mixed solution was transferred to a new high-pressure reactor and subjected to a second hydrothermal reaction at 150 °C for 24 hours. After the reaction was completed, the product was collected by centrifugation, washed twice with anhydrous ethanol, and dried overnight in a vacuum drying oven at 60 °C to obtain the nickel-iron bimetallic organic framework precursor Fe-Ni-BDC with a layered structure.

[0021] S3. Preparation of MOF Derivatives: The Fe-Ni-BDC precursor obtained in S2 was placed in a tube furnace and heated to 800 °C at a heating rate of 5 °C / min under a nitrogen atmosphere. It was then subjected to high-temperature carbonization reduction treatment at this temperature for 4 hours. After natural cooling to room temperature, the nickel-iron bimetallic MOF derivative Fe-Ni@C was obtained. This catalyst has a foam-like carbon framework structure, in which nickel-iron alloy nanoparticles are uniformly dispersed and encapsulated by a carbon layer. (See [link to relevant documentation]). Figure 1 .

[0022] The catalytic performance of the Fe-Ni@C catalyst prepared in this embodiment was evaluated. A catalytic reforming experiment was conducted using a two-stage fixed-bed reactor to produce 1 g of pyrolysis tar from polyolefin plastics. Figure 2 As shown. The catalyst loading was 0.5 g, the reaction temperature was 800 ℃, the steam feed rate was 10 mL / h, and the nitrogen carrier gas flow rate was 20 mL / min. After the reaction, the collected gas was analyzed by gas chromatography-thermal conductivity detector. The gas yield was calculated by equation (1): Equation (1); The results showed that the Fe-Ni@C catalyst achieved an H2 yield of 144.37 mmol / g, a CO yield of 72.83 mmol / g, a CH4 yield of 13.3 mmol / g, and a CO2 yield of 21.47 mmol / g (see Table 1). In the subsequent catalytic cycles 2-5, the H2 yield decreased to 64% of the original yield and then stabilized, at 90.96 mmol / g, 91.50 mmol / g, 92.01 mmol / g, and 92.99 mmol / g, respectively.

[0023] Example 2: Referring to Example 1, the difference lies in the following: In step S1, the first metal salt is Ni(NO3)2·6H2O, and 2.326 g (8 mmol) is weighed to obtain the nickel-based metal-organic framework precursor Ni-BDC; In step S2, the second metal salt is FeCl3·9H2O, and 2.162 g (8 mmol) is weighed. After washing and centrifugation, the Ni-Fe-BDC precursor is obtained, and the obtained bimetallic organic framework precursor is a nickel-iron bimetallic organic framework with a mixed structure of block and lamellar structures; In step S3, after carbonization and reduction treatment under the same conditions, the nickel-iron bimetallic MOF derivative Ni-Fe@C is obtained, which has a coarse-grained carbon foam structure, see [link to example]. Figure 1 .

[0024] The catalyst obtained was evaluated for its catalytic performance using the same method as in Example 1. The results are as follows: H2 yield was 107.13 mmol / g, CH4 yield was 14.67 mmol / g, CO yield was 59.43 mmol / g, and CO2 yield was 15.80 mmol / g. See Table 1 for details.

[0025] Comparative Example 1: Referring to Example 1, the difference is that steps S1 and S2 are combined into one step, i.e., a one-step hydrothermal method is used to prepare the bimetallic MOF precursor; the molar ratio of Ni(NO3)2·6H2O and FeCl3·9H2O is 0.5:0.5, and 1.163 g (4 mmol) and 1.081 g (4 mmol) of Ni(NO3)2·6H2O and FeCl3·9H2O are weighed out respectively, and dissolved together with 1.33 g of terephthalic acid in 60 mL of DMF. The hydrothermal reaction is carried out at 150 °C for 24 h, and Ni is obtained after washing and centrifugation. 0.5 Fe 0.5 -BDC precursor; in step S3, Ni is obtained by carbonization and reduction at 800 °C for 4 h under a nitrogen atmosphere. 0.5 Fe 0.5 @C catalyst.

[0026] The catalyst obtained was evaluated for its catalytic performance using the same method as in Example 1. The results are as follows: H2 yield was 31.95 mmol / g, CH4 yield was 6.46 mmol / g, CO yield was 12.27 mmol / g, and CO2 yield was 2.61 mmol / g. For details, please refer to Table 1.

[0027] A comparison of Examples 1 and 2 with Comparative Example 1 shows that Comparative Example 1, which uses a one-step hydrothermal method to directly synthesize a nickel-iron bimetallic MOF precursor, is prone to phase separation due to significant differences in the coordination nucleation and growth rates of nickel and iron ions with terephthalic acid. This results in uneven metal particle distribution and underdeveloped pore structure, ultimately leading to gas yields far lower than in Examples 1 and 2. This clearly demonstrates that the present invention, through a two-step hydrothermal strategy—first constructing a single-metal MOF framework and then introducing a second metal—can effectively avoid phase separation problems and achieve uniform metal dispersion at the nanoscale.

[0028] Comparative Example 2: The process is the same as in Example 1, except that step S1 is omitted. In step S2, only Ni(NO3)2·6H2O (2.326 g, 8 mmol) and 1.33 g terephthalic acid are reacted in 60 mL DMF at 150 °C for 24 h. After washing and centrifugation, the Ni-BDC precursor is obtained. In step S3, the catalyst is carbonized and reduced at 800 °C for 4 h under a nitrogen atmosphere to obtain the Ni@C catalyst.

[0029] The catalyst obtained was evaluated for its catalytic performance using the same method as in Example 1. The results are as follows: H2 yield was 67.08 mmol / g, CH4 yield was 11.85 mmol / g, CO yield was 23.92 mmol / g, and CO2 yield was 7.40 mmol / g. See Table 1 for details.

[0030] A comparison of Examples 1 and 2 with Comparative Example 2 shows that Comparative Example 2, a single-metal nickel catalyst, while possessing some cracking activity, lacks iron components and cannot effectively activate water vapor to generate reactive oxygen species, resulting in insufficient carbon deposition inhibition and significantly lower catalytic performance than Examples 1 and 2. In Examples 1 and 2, the synergistic effect of the nickel-iron bimetallic catalyst significantly improved hydrogen yield and carbon deposition resistance.

[0031] Comparative Example 3: Referring to Example 1, the difference is that step S2 is omitted. In step S1, only FeCl3·9H2O (2.162 g, 8 mmol) and 1.33 g of terephthalic acid were reacted in 60 mL of DMF at 150 °C for 24 h. After washing and centrifugation, the Fe-BDC precursor was obtained. In step S3, the Fe@C catalyst was obtained by carbonization and reduction at 800 °C for 4 h under a nitrogen atmosphere.

[0032] The catalysts obtained were evaluated for their catalytic performance using the same method as in Example 1. The results are as follows: H2 yield was 17.84 mmol / g, CH4 yield was 6.82 mmol / g, CO yield was 7.98 mmol / g, and CO2 yield was 3.17 mmol / g. For details, please refer to Table 1.

[0033] A comparison of Example 1 and Comparative Example 3 shows that Comparative Example 3, using a single metallic iron catalyst (Fe@C), exhibits a sharp decrease in the yield of each gas. This is mainly attributed to the dense structure of the iron-based MOF precursor (Fe-BDC), resulting in a low specific surface area and poor accessibility of active sites after carbonization. Examples 1 and 2, by introducing nickel to form a foam-like carbon framework structure, significantly increased the specific surface area and pore volume, thereby greatly enhancing catalytic activity.

[0034] Table 1. Gas yields from tar catalytic reforming in Examples 1, 2 and Comparative Examples 1-3

[0035] The above description of the embodiments is only for the purpose of helping to understand the technical solution of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principle of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A two-step hydrothermal method for preparing nickel-iron bimetallic MOF derivatives, characterized in that, The method includes the following steps: S1: A first metal salt, an organic ligand, and a solvent are mixed and subjected to a first hydrothermal reaction to obtain a first metal-organic framework precursor; the first metal salt is a nickel salt or an iron salt, and the organic ligand is terephthalic acid; S2: The first metal-organic framework precursor, the second metal salt, the organic ligand obtained in S1 are mixed with a solvent and subjected to a second hydrothermal reaction to obtain a bimetallic organic framework precursor; the second metal salt is an iron salt or a nickel salt different from the first metal salt, and the organic ligand is terephthalic acid; S3: The bimetallic organic framework precursor obtained in S2 is subjected to high-temperature carbonization and reduction treatment under an inert atmosphere at a temperature of 700~900 ℃ to obtain a nickel-iron bimetallic MOF derivative.

2. The method according to claim 1, characterized in that, The conditions for the first and second hydrothermal reactions are independent: reaction temperature 120~180 ℃, reaction time 12~36 hours.

3. The method according to claim 1, characterized in that, The high-temperature carbonization reduction treatment is performed under an inert atmosphere of nitrogen or argon for 2 to 6 hours.

4. The method according to claim 1, characterized in that, The solvent is selected from one or more of N,N-dimethylformamide, anhydrous ethanol, or water.

5. The method according to claim 1, characterized in that, The nickel salt is nickel nitrate or nickel chloride, and the iron salt is ferric chloride or ferric nitrate.

6. The method according to claim 1, characterized in that, The molar ratio of the first metal-organic framework precursor to the second metal salt is 1:0.5 to 1:

2.

7. The nickel-iron bimetallic MOF derivative prepared by the method of claim 1, characterized in that, It has a foam-like hierarchical porous carbon framework structure with a specific surface area of ​​100~120 m². 2 / g, pore volume 0.25~0.30 cm³ 3 / g, the nickel-iron alloy nanoparticles have a particle size of 10~30 nm and are uniformly dispersed in the carbon skeleton and coated with carbon.

8. The application of the nickel-iron bimetallic MOF derivative prepared by the method of claim 1 as a catalyst in the catalytic reforming of waste plastic pyrolysis tar to produce hydrogen-rich gas.

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