A palladium-based catalyst derived from MOFs and a preparation method and application thereof

Abstract

The present application relates to the technical field of catalyst, in particular to a MOFs derived palladium-based catalyst and its preparation method and application. The present application first prepares MOF material, then prepares palladium-based MOF precursor, mixes the palladium-based MOF precursor with nitrogen-containing precursor, and calcines under inert atmosphere, and cools to room temperature. In the high-temperature pyrolysis process, the external nitrogen-containing precursor is introduced, and the palladium-based MOF precursor is calcined together, which significantly improves the nitrogen doping amount in the carbon carrier. Through enhancing the anchoring effect of nitrogen species on Pd nanoparticles and electronic regulation, high dispersion and electronic structure optimization of Pd particles are realized, so as to obtain high activity, high selectivity and high stability in 2-methyl-3-butyn-2-ol selective hydrogenation reaction.

Description

Technical Field

[0001] This invention relates to the field of catalyst technology, specifically to a MOF-derived palladium-based catalyst, its preparation method, and its application. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] In the fields of fine chemical and pharmaceutical intermediate synthesis, many reactions rely on highly efficient catalysts to achieve specific transformations. Among them, the selective hydrogenation of 2-methyl-3-butyn-2-ol to 2-methyl-3-buten-2-ol is crucial. This reaction requires the catalyst to precisely hydrogenate the carbon-carbon triple bond to a carbon-carbon double bond while avoiding over-hydrogenation to form a saturated alcohol. Palladium (Pd) is a commonly used hydrogenation catalyst, but traditional Pd catalysts (such as Pd / C) perform poorly in this reaction, exhibiting problems such as low selectivity, easy over-hydrogenation, and easy aggregation and loss of Pd nanoparticles, leading to decreased catalyst activity and stability, making it difficult to meet the reaction requirements.

[0004] Metal-organic frameworks (MOFs) are ideal precursors for preparing high-performance carbon-based catalysts due to their regular pore structure and tunable composition. High-temperature pyrolysis of MOFs can yield nitrogen-doped porous carbon supports, whose pore structure can confine metal particles, and nitrogen species can form electronic interactions with the metal, theoretically solving the dispersion and stability problems of traditional catalysts. Existing palladium-based MOF carbonization-derived catalysts, which achieve nitrogen doping through in-situ MOF carbonization, exhibit high stability in the hydrogenation reaction of terephthalic acid (PTA).

[0005] However, there are still many shortcomings when applying such MOF-derived palladium-based catalysts to the selective hydrogenation reaction of 2-methyl-3-butyn-2-ol. Firstly, existing technologies rely solely on the nitrogen-containing ligands of the MOF itself to provide the doped nitrogen source. After pyrolysis, the nitrogen content is limited, resulting in insufficient anchoring ability for Pd nanoparticles. Particle sintering still occurs during high-temperature pyrolysis, leading to the loss of active sites in the catalyst. Secondly, the low nitrogen content results in weak electronic regulation, failing to effectively adjust the electron cloud density of Pd. This leads to an uncontrolled equilibrium between the adsorption and activation of carbon-carbon triple bonds and the desorption of double bonds, increasing the proportion of excessive hydrogenation side reactions. Thirdly, the PTA hydrogenation reaction requires overcoming the limitation of macromolecular mass transfer, while the core challenge of selective hydrogenation of alkynols is the control of reaction selectivity; the catalytic requirements of the two are significantly different. Therefore, developing a palladium-based catalyst with high activity, high selectivity, and high stability in the selective hydrogenation reaction of 2-methyl-3-butyn-2-ol has become an urgent technical problem to be solved in this field. Summary of the Invention

[0006] To overcome the above problems, the present invention provides a MOF-derived palladium-based catalyst, its preparation method and application.

[0007] To achieve the above technical objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing a MOF-derived palladium-based catalyst, comprising the following steps: (1) Dissolve the metal salt in the first solvent to obtain solution A, dissolve the imidazole organic ligand in the second solvent to obtain solution B, mix solution A and solution B, stir and react, centrifuge, wash and dry after the reaction to obtain MOF material; (2) The MOF material was dispersed in a third solvent, a palladium salt solution was added, and the mixture was stirred and impregnated. Then, the mixture was centrifuged, washed, and dried to obtain a palladium-based MOF precursor. (3) The palladium-based MOF precursor is mixed with a nitrogen-containing precursor, heated and calcined under an inert atmosphere, and then cooled to room temperature to obtain the palladium-based catalyst derived from the MOFs.

[0008] This invention introduces an external nitrogen-containing precursor during high-temperature pyrolysis and co-calcines it with a palladium-based MOF precursor, significantly increasing the nitrogen doping content in the carbon support. This solves the problem of insufficient nitrogen content in existing MOF-derived catalysts that rely solely on their own ligand nitrogen sources. By enhancing the anchoring effect and electronic regulation of nitrogen species on Pd nanoparticles, it achieves high dispersion and optimized electronic structure of Pd particles, thereby simultaneously obtaining high activity, high selectivity, and high stability in the selective hydrogenation reaction of 2-methyl-3-butyn-2-ol.

[0009] In one or more embodiments, in step (1), the metal salt is zinc nitrate or cobalt nitrate, preferably cobalt nitrate. Zinc nitrate or cobalt nitrate, as metal sources, can form ZIF-8 or ZIF-67 structures, respectively. After pyrolysis, these structures generate a porous carbon matrix, providing confined space for Pd nanoparticles. When cobalt nitrate is preferred, the cobalt-nitrogen co-doped carbon formed by pyrolysis can further exhibit a synergistic effect with Pd, regulating the electronic structure and improving hydrogenation selectivity.

[0010] In one or more embodiments, in step (1), the imidazole organic ligand is 2-methylimidazole. As an organic ligand, 2-methylimidazole can self-assemble with metal ions to form a stable ZIF structure, retaining a high specific surface area and porous structure after pyrolysis, thus laying the foundation for nitrogen doping and metal loading.

[0011] In one or more embodiments, in step (1), the mass ratio of the metal salt to the organic ligand is 1:2.0~2.5.

[0012] In one or more embodiments, in step (2), the palladium salt includes any one of palladium chloride, palladium nitrate, and palladium acetate.

[0013] In one or more embodiments, in step (2), the concentration of the palladium salt solution is 0.05~0.2mol / L.

[0014] In one or more embodiments, in step (3), the nitrogen-containing precursor is urea or melamine. Urea or melamine, as an external nitrogen-rich precursor, decomposes during pyrolysis to generate a large number of nitrogen-containing groups, significantly increasing the nitrogen doping level of the carbon support, enhancing the anchoring strength of Pd nanoparticles, preventing sintering and loss, and simultaneously regulating the electron density of Pd through nitrogen-Pd electron interactions, optimizing the adsorption-desorption balance of alkynol molecules, and improving selectivity.

[0015] In one or more embodiments, in step (3), the mass ratio of the palladium-based MOF precursor to the nitrogen-containing precursor is 1:0.8-1.5, preferably 1:1. Under this condition, a sufficient supply of external nitrogen source is provided to form a highly nitrogen-doped carbon support, while avoiding excessive nitrogen precursor from clogging the pores or affecting the integrity of the carbon structure, thereby achieving the best nitrogen doping effect and catalytic performance.

[0016] In one or more embodiments, in step (3), the inert atmosphere is nitrogen or argon. The inert atmosphere prevents the carbon materials and metals from being oxidized during high-temperature pyrolysis, ensuring the formation of a stable nitrogen-doped carbon structure and highly dispersed metal active sites.

[0017] In one or more embodiments, in step (3), the heating and calcination process is as follows: the temperature is increased to 600-900℃ at a rate of 1-5℃ / min, and held for 1-4 hours. Under these conditions, the MOF material can be fully pyrolyzed, the nitrogen-containing precursor can be completely decomposed, and a stable nitrogen-doped carbon support and uniformly dispersed metal nanoparticles can be formed.

[0018] A second aspect of the present invention provides a MOF-derived palladium-based catalyst prepared by the method described in the first aspect. This catalyst comprises highly dispersed Pd nanoparticles confined within a nitrogen-doped porous carbon matrix, wherein the introduction of an external nitrogen source significantly enhances the nitrogen doping level, giving the catalyst a combination of high specific surface area, uniform pore structure, and optimized electronic structure, exhibiting excellent activity and selectivity in the selective hydrogenation of alkynols.

[0019] A third aspect of the present invention provides the application of a MOF-derived palladium-based catalyst prepared by the method described in the first aspect in the selective hydrogenation of unsaturated alkynols to unsaturated enols.

[0020] In one or more embodiments, the unsaturated alkynol is 2-methyl-3-butyn-2-ol, and the unsaturated enol is 2-methyl-3-buten-2-ol.

[0021] The beneficial effects of this invention are as follows: (1) This invention provides regular channels using MOF as a precursor and, in conjunction with the introduction of an external nitrogen-containing precursor, forms a highly nitrogen-doped porous carbon matrix during pyrolysis. Its abundant nitrogen species (such as pyridine nitrogen) serve as anchoring sites, strongly immobilizing Pd nanoparticles and effectively preventing particle migration and aggregation during high-temperature pyrolysis and use, thus ensuring the high dispersion and stability of the active centers. Compared with existing technologies that rely solely on the nitrogen source of the MOF itself, the addition of an external nitrogen source significantly increases the nitrogen content, solving the problem of insufficient anchoring ability in traditional MOF-derived catalysts.

[0022] (2) The pyrolysis of the external nitrogen-containing precursor introduces a large number of nitrogen species, which form strong electronic interactions with Pd nanoparticles, regulating the electron cloud density of Pd and reducing its over-adsorption capacity for carbon-carbon double bonds. This optimizes the adsorption-desorption balance of 2-methyl-3-butyn-2-ol and 2-methyl-3-buten-2-ol at the active sites. This electronic effect enables the catalyst to catalyze the hydrogenation of triple bonds to double bonds with high selectivity, suppressing the formation of saturated alcohols. The selectivity of the target product can be stably maintained above 90%, overcoming the defect of poor selectivity of existing Pd-based catalysts in the hydrogenation of alkynols.

[0023] (3) In the catalytic hydrogenation of 2-methyl-3-butyn-2-ol to methyl-3-buten-2-ol, the high-nitrogen-doped carbon support not only stabilizes the Pd particles through confinement but also enhances the hydrogenation activity of Pd through electronic effects, giving the catalyst high catalytic activity. At the same time, the enhanced anchoring effect reduces the loss and sintering of Pd particles during the reaction, and the catalyst maintains high activity and selectivity after multiple cycles, solving the problem of insufficient stability of traditional catalysts.

[0024] (4) The preparation process of this invention involves simple mixing of an external nitrogen-containing precursor and a palladium-based MOF precursor followed by pyrolysis. No complex post-processing is required, the raw materials are readily available, the process is simple, and it is suitable for industrial-scale production. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0026] Figure 1 The images show the XRD patterns of the Pd / C catalyst prepared in Example 1 and the Pd-Co / C catalyst prepared in Example 2, where (a) is Example 1 and (b) is Example 2. Figure 2 The images are scanning electron microscope images of the Pd / C catalyst prepared in Example 1 and the Pd-Co / C catalyst prepared in Example 2, where (a) is Example 1 and (b) is Example 2. Detailed Implementation

[0027] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0028] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0029] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0030] Example 1 Synthesis of Pd / C catalysts: (1) Dissolve 1.23g of zinc nitrate in 36mL of methanol (solution A), dissolve 2.70g of 2-methylimidazole in 36mL of methanol (solution B), quickly pour solution B into solution A, and stir magnetically in a water bath at 35℃ for 4 hours. After the reaction is complete, collect the white precipitate by centrifugation, wash it three times with methanol, and dry it in a vacuum drying oven at 60℃ for 12 hours to obtain ZIF-8 powder; (2) Take 0.5g of the above ZIF-8 powder and disperse it in 5mL of methanol. Disperse it evenly by ultrasonication. Add 0.5mL of 0.1mol / L Pd(NO3)2 methanol solution and stir at room temperature for 12 hours. Then dry it under vacuum at 60℃ to obtain Pd / ZIF-8 precursor. (3) The obtained 0.5g Pd / ZIF-8 precursor was mixed with 0.5g urea and placed in a ceramic boat. The boat was then placed in a tube furnace and heated to 800℃ at a rate of 2℃ / min under a nitrogen atmosphere. The temperature was maintained for 2 hours and then naturally cooled to room temperature to obtain a black powdery Pd / C catalyst.

[0031] Figure 1 The image shows the XRD pattern of the Pd / C catalyst prepared in Example 1. Figure 1The results show that the peaks at 40.0°, 46.5°, 67.9°, and 82.1° can be attributed to metallic Pd, which fully confirms the successful synthesis of the Pd / C catalyst.

[0032] Figure 2 These are scanning electron microscope (SEM) images of the Pd / C catalyst prepared in Example 1 of this study. Figure 2 As can be seen, the carbon material exists in block form with uneven size.

[0033] Example 2 Synthesis of Pd-Co / C catalysts: (1) Dissolve 0.87g cobalt nitrate in 36mL methanol (solution A), dissolve 1.97g 2-methylimidazole in 36mL methanol (solution B), add solution A dropwise to solution B, stir magnetically for 3 hours, let stand for 24 hours, after the reaction is complete, collect the purple precipitate by centrifugation, wash it three times with methanol, and dry it in a vacuum drying oven at 60℃ for 12 hours to obtain ZIF-67 powder; (2) Take 0.5g of the above ZIF-67 powder and disperse it in 5mL of methanol. Disperse it evenly by ultrasonication. Add 0.5mL of 0.1mol / L Pd(NO3)2 methanol solution and stir at room temperature for 12 hours. Then dry it under vacuum to obtain Pd / ZIF-67 precursor. (3) The obtained 0.5g Pd / ZIF-67 precursor was mixed with 0.5g urea and placed in a ceramic boat. The boat was then placed in a tube furnace and heated to 800℃ at a rate of 2℃ / min under a nitrogen atmosphere. The temperature was maintained for 2 hours and then naturally cooled to room temperature to obtain a black powdery Pd-Co / C catalyst.

[0034] Figure 1 The image shows the XRD pattern of the Pd-Co / C catalyst prepared in Example 2. Figure 1 The results show that the peaks at 40.0°, 46.5°, 67.9°, and 82.1° are attributed to Pd metal, and the peak at 43.8° is attributed to Co. This result fully confirms the successful synthesis of the Pd-Co / C catalyst.

[0035] Figure 2 The images shown are scanning electron microscope (SEM) images of the Pd-Co / C catalyst prepared in Example 2 of this study. Figure 2 As can be seen, the carbon material exists in a honeycomb structure.

[0036] Example 3 Synthesis of Pd / C catalysts: (1) Dissolve 1.23g of zinc nitrate in 36mL of methanol (solution A), dissolve 2.70g of 2-methylimidazole in 36mL of methanol (solution B), quickly pour solution B into solution A, and stir magnetically in a water bath at 35℃ for 4 hours. After the reaction is complete, collect the white precipitate by centrifugation, wash it three times with methanol, and dry it in a vacuum drying oven at 60℃ for 12 hours to obtain ZIF-8 powder; (2) Take 0.5g of the above ZIF-8 powder and disperse it in 5mL of methanol. Disperse it evenly by ultrasonication. Add 0.5mL of 0.1mol / L Pd(NO3)2 methanol solution and stir at room temperature for 12 hours. Then dry it under vacuum at 60℃ to obtain Pd / ZIF-8 precursor. (3) The obtained 0.5g Pd / ZIF-8 precursor was mixed with 0.5g melamine and placed in a ceramic boat. The boat was then placed in a tube furnace and heated to 800℃ at a rate of 2℃ / min under a nitrogen atmosphere. The temperature was maintained for 2 hours and then naturally cooled to room temperature to obtain a black powdery Pd / C catalyst.

[0037] Example 4 Synthesis of Pd-Co / C catalysts: (1) Dissolve 0.87g cobalt nitrate in 36mL methanol (solution A), dissolve 1.97g 2-methylimidazole in 36mL methanol (solution B), add solution C dropwise to solution D, stir magnetically for 3 hours, let stand for 24 hours, after the reaction is complete, collect the purple precipitate by centrifugation, wash it three times with methanol, and dry it in a vacuum drying oven at 60℃ for 12 hours to obtain ZIF-67 powder; (2) Take 0.5g of the above ZIF-67 powder and disperse it in 5mL of methanol. Disperse it evenly by ultrasonication. Add 0.5mL of 0.1mol / L Pd(NO3)2 methanol solution and stir at room temperature for 12 hours. Then dry it under vacuum to obtain Pd / ZIF-67 precursor. (3) The obtained 0.5g Pd / ZIF-67 precursor was mixed with 0.5g melamine and placed in a ceramic boat. The boat was then placed in a tube furnace and heated to 800℃ at a rate of 2℃ / min under a nitrogen atmosphere. The temperature was maintained for 2 hours and then naturally cooled to room temperature to obtain a black powdery Pd-Co / C catalyst.

[0038] Comparative Example 1 Traditional activated carbon supported Pd catalyst (Pd / AC) Take 0.5g of commercial activated carbon (AC), mix it with 5mL of methanol, add 0.5mL of 0.1mol / L Pd(NO3)2 methanol solution, stir at room temperature for 12 hours, vacuum dry, then place it in a porcelain boat, put it in a tube furnace, heat it to 800℃ at a rate of 2℃ / min under nitrogen atmosphere, and hold it at that temperature for 2 hours. After naturally cooling to room temperature, the Pd / AC catalyst is obtained.

[0039] Comparative Example 2 The difference between this comparative example and Example 1 is that no urea is added in this comparative example, but the other preparation methods are the same as in Example 1.

[0040] Comparative Example 3 The difference between this comparative example and Example 2 is that no urea is added in this comparative example, but the other preparation methods are the same as in Example 2.

[0041] Experimental Example 1 The MOF-derived palladium-based catalysts prepared in Examples 1-3 and the materials in Comparative Examples 1-3 were used as catalysts to catalyze the hydrogenation of 2-methyl-3-butyn-2-ol to synthesize 2-methyl-3-buten-2-ol.

[0042] The reaction synthesis conditions are as follows: 25 g of 2-methyl-3-butyn-2-ol was dispersed in 120 mL of ethanol, and 0.8 g of catalyst was added. The mixture was then passed through hydrogen gas (1.2 MPa) at a temperature of 80 °C. o The reaction was carried out at C for 4 hours. After the reaction was completed, the reaction products were detected by gas chromatography.

[0043] Table 1. Catalytic results of catalysts in Examples 1-4 and Comparative Examples 1-3

[0044] As can be seen from the catalytic results in Table 1, the MOF-derived palladium-based composite materials prepared in this invention (Examples 1-4) exhibit significantly superior comprehensive performance compared to traditional catalysts (Comparative Example 1) and MOF-derived catalysts without an external nitrogen source (Comparative Examples 2-3) in the selective hydrogenation of 2-methyl-3-butyn-2-ol to 2-methyl-3-buten-2-ol. Specifically, this is manifested in high catalytic activity, high selectivity, and high stability. This is mainly attributed to the fact that MOFs provide regular channels for the precursor, and combined with the introduction of external nitrogen-containing precursors, a highly nitrogen-doped porous carbon matrix is ​​formed during pyrolysis. This support structure not only effectively confines Pd nanoparticles, preventing their aggregation, but also provides strong electronic interactions between the abundant nitrogen species and Pd, optimizing the electron cloud density of Pd. This optimization precisely controls the adsorption strength of reactants (alkynes) and products (olefins) at the active sites, enabling the catalyst to efficiently activate the hydrogenation of carbon-carbon triple bonds while promoting the desorption of carbon-carbon double bond products. Thus, while achieving high conversion, it effectively suppresses excessive hydrogenation side reactions, resulting in excellent selectivity.

[0045] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a MOF-derived palladium-based catalyst, characterized in that, Includes the following steps: (1) Dissolve the metal salt in the first solvent to obtain solution A, dissolve the imidazole organic ligand in the second solvent to obtain solution B, mix solution A and solution B, stir and react, centrifuge, wash and dry after the reaction to obtain MOF material; (2) The MOF material was dispersed in a third solvent, a palladium salt solution was added, and the mixture was stirred and impregnated. Then, the mixture was centrifuged, washed, and dried to obtain a palladium-based MOF precursor. (3) The palladium-based MOF precursor is mixed with a nitrogen-containing precursor, heated and calcined under an inert atmosphere, and then cooled to room temperature to obtain the palladium-based catalyst derived from the MOFs. In step (1), the imidazole organic ligand is 2-methylimidazole; In step (1), the mass ratio of the metal salt to the organic ligand is 1:2.0~2.5; In step (2), the concentration of the palladium salt solution is 0.05~0.2 mol / L; In step (3), the nitrogen-containing precursor is urea or melamine; In step (3), the mass ratio of the palladium-based MOF precursor to the nitrogen-containing precursor is 1:0.8-1.5; In step (3), the inert atmosphere is nitrogen or argon; In step (3), the heating and roasting process is as follows: the temperature is increased to 600-900℃ at a rate of 1-5℃ / min and held for 1-4 hours.

2. The preparation method according to claim 1, characterized in that, In step (1), the metal salt is zinc nitrate or cobalt nitrate.

3. The preparation method according to claim 2, characterized in that, In step (1), the metal salt is cobalt nitrate.

4. The preparation method according to claim 1, characterized in that, In step (1), the first solvent is one or more of methanol, ethanol, and water, and the second solvent is one or more of methanol, ethanol, and water; In step (2), the third solvent is one or more of methanol, ethanol, and water.

5. The preparation method according to claim 1, characterized in that, In step (2), the palladium salt includes any one of palladium chloride, palladium nitrate, and palladium acetate.

6. The preparation method according to claim 1, characterized in that, In step (3), the mass ratio of the palladium-based MOF precursor to the nitrogen-containing precursor is 1:

1.

7. The MOF-derived palladium-based catalyst prepared by the preparation method according to any one of claims 1 to 6.

8. The application of the MOFs-derived palladium-based catalyst prepared by the preparation method according to any one of claims 1 to 6 in the selective hydrogenation of unsaturated alkynols to unsaturated enols.

9. The application as described in claim 8, characterized in that, The unsaturated alkynol is 2-methyl-3-butyn-2-ol, and the unsaturated enol is 2-methyl-3-buten-2-ol.

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