A polyacid zeolite imidazolate framework supported single-atom catalyst and a preparation method and application thereof

Abstract

The application discloses a polyacid zeolite imidazolate framework loaded monatomic catalyst and a preparation method and application thereof, and belongs to the technical field of catalysts. The preparation method of the catalyst is as follows: (1) preparing a zeolite imidazolate framework material; (2) mixing the dried zeolite imidazolate framework material and a metal salt in a solvent and stirring; then adding a heteropoly acid, controlling the pH in the system to be 5-7, and performing a solvothermal reaction under the condition of 80-120 DEG C; after the reaction is completed, cooling and centrifugal separation are performed to obtain a precipitate; and (3) performing calcination treatment on the precipitate to obtain the polyacid zeolite imidazolate framework loaded monatomic catalyst. The polyacid zeolite imidazolate framework loaded monatomic catalyst prepared by the method has high catalytic activity, high selectivity and good cycle stability, and has a good application prospect in acetyl propionate esterification reaction.

Description

Technical Field

[0001] This invention relates to the field of catalyst technology, specifically to a single-atom catalyst supported on a polyacid zeolite imidazole ester framework, its preparation method, and its application. Background Technology

[0002] In recent years, with the rapid development of the chemical industry, the demand for efficient and environmentally friendly catalysts has been increasing. Lipylpropionate is an important organic synthesis intermediate, widely used in pharmaceuticals, pesticides, and synthetic resins. Traditional levylpropionation reactions often employ metal catalysts, such as noble metal catalysts, which offer high catalytic efficiency and good selectivity. However, these traditional catalysts typically face problems such as high cost, metal waste, and environmental pollution.

[0003] Single-atom catalysts are a class of supported metal catalysts in which the metal component on the support exists in the form of individual atoms. Due to their unique active sites and high catalytic performance, they have become a research hotspot in the field of catalysis. Single-atom catalysts have the advantage of high atom utilization, which can significantly improve catalytic efficiency and reduce costs. However, achieving stable loading of single-atom catalysts and maintaining their activity remains one of the technical problems that urgently need to be solved in the current technology. In particular, loading single-atom catalysts onto supports with suitable pore structures to achieve effective control of catalytic reactions and catalyst reuse is currently a hot research topic.

[0004] In single-atom catalysts supported by zeolite imidazole ester (ZIF) frameworks, the unique pore structure and good chemical stability of ZIFs make them excellent catalyst support materials. These materials typically possess high specific surface areas, providing a large number of active sites, and their catalytic performance can be tuned by selecting different metal precursors.

[0005] Chinese patent document CN113150291A discloses a glucose-modified bimetallic zeolite imidazole ester framework-derived catalyst and its preparation method. The invention synthesizes ZIFs framework materials in an aqueous solvent under room temperature conditions using cobalt nitrate hexahydrate, zinc nitrate hexahydrate, and 2-methylimidazole as reactants. A trace amount of glucose is added for modification. The molar ratio of glucose to 2-methylimidazole is 1:(50-60), and the molar ratio of Co(NO3)2·6H2O, Zn(NO3)2·6H2O, and 2-methylimidazole is 2:1:8. The resulting precursor is calcined at high temperature to obtain the glucose-modified bimetallic zeolite imidazole ester framework-derived catalyst.

[0006] Chinese patent document CN114029081A discloses a bimetallic copper-cobalt-nitrogen-carbon catalyst, its preparation method, and its application. This invention uses inorganic salts of copper and cobalt, sodium formate, and benzimidazole as raw materials, and N,N-dimethylformamide as a solvent. The raw materials are dissolved in the solvent, stirred, mixed, and crystallized to obtain Cu-Co-ZIF-9. The molar ratio of cobalt to copper is 10.0–50.0, and the molar ratio of benzimidazole to copper-cobalt is 2.0–8.0. Cu-Co-ZIF-9 is calcined in a hydrogen-argon mixed atmosphere to obtain the bimetallic copper-cobalt-nitrogen-carbon catalyst.

[0007] Chinese patent document CN113522317A discloses a method for preparing and applying a cobalt-based bimetallic sulfur / carbon catalyst derived from MOFs. Specifically, the invention uses zeolite imidazole ester framework-67 (ZIF-67) as a precursor to derive a layered bimetallic hydroxide with a three-dimensional structure to achieve doping with a second metal ion, and then prepares a novel carbon-based Fenton-like catalyst through a one-step sulfidation and carbonization method.

[0008] Chinese patent document CN112473736A discloses a novel supported polyacid catalyst and its application in the preparation of levulinic esters. The supported polyacid catalyst of this invention consists of molecular sieve Sapo-18 and polyacid compound H3PW. 12 O 40 The prepared product can be used to catalyze the hydrolysis of biomass to produce levulinate.

[0009] Chinese patent document CN115888827A discloses a method and application for constructing a heteropolyacid@layered bimetallic hydroxide acid-base bifunctional catalyst. The method uses ZIF-based metal-organic framework material (POM@ZIF) pre-encapsulated with POMs as a precursor and achieves complete conversion through a one-step in-situ encapsulation-recombination strategy to construct a POM@LDH hollow acid-base bifunctional catalyst. The POM@LDH hollow acid-base bifunctional catalyst has a hollow nanocage morphology. The nanocage is composed of staggered nanosheets, forming a self-supporting hollow catalyst shell. POMs are uniformly distributed on these staggered nanosheets, stably immobilized and encapsulated in the nanocage.

[0010] However, existing technologies for preparing polyacid zeolite imidazolium ester single-atom catalysts still face some challenges. For example, traditional preparation methods may result in uneven distribution of metal active sites, low impregnation and anchoring efficiency of metal ions, and poor stability of the metal active sites. These problems limit the catalyst activity and its performance in industrial applications. Therefore, there is an urgent need to develop a method for preparing polyacid zeolite imidazolium ester single-atom catalysts with uniform metal active site distribution and high loading stability to achieve high-efficiency and high-selectivity catalysis of levulinization reactions. Summary of the Invention

[0011] This invention provides a method for preparing a single-atom catalyst supported on a polyacid zeolite imidazole ester framework. The process is simple and efficient, and the prepared single-atom catalyst with high catalytic activity, high selectivity, and good cycle stability has good application prospects in acetylacetyl propionate esterification reaction.

[0012] The specific technical solution adopted is as follows:

[0013] A method for preparing a single-atom catalyst supported on a polyacid zeolite imidazole ester framework, comprising:

[0014] (1) Preparation of zeolite imidazolium ester framework material; the zeolite imidazolium ester framework material includes ZIF-8 or ZIF-67.

[0015] (2) The dried zeolite imidazole ester framework material and metal salt were mixed in a solvent and stirred; then a heteropoly acid was added, and the pH of the system was controlled at 5-7. The solvothermal reaction was carried out at 80-120℃; after the reaction was completed, the mixture was cooled and centrifuged to obtain the precipitate.

[0016] (3) The precipitate was calcined to obtain the single-atom catalyst supported on the polyacid zeolite imidazole ester framework.

[0017] The metal salt is selected from at least one of platinum salt, palladium salt, gold salt, or silver salt;

[0018] The aforementioned heteropolyacid is a Keggin-type polyacid containing [XM] 12 O 40 ] n- Ions, wherein X is selected from P, Si or Ge, M is selected from Mo, W, V or Nb, and n is an integer from 1 to 5.

[0019] This invention first uniformly loads a metal salt onto a zeolite imidazole ester framework material, and then adds a heteropoly acid. The addition of the heteropoly acid can introduce acid sites and redox sites. Under controlled pH conditions, the metal salt may be reduced to form nano-sized metal particles, which improves the dispersibility of the nanoparticles. Furthermore, since the process of this invention uses a solvothermal reaction, the heteropoly acid interacts with the surface of the metal particles to form a stable complex, which helps to disperse and fix the metal particles. This results in the catalyst prepared by the method of this invention having a uniform distribution of metal active centers and good stability.

[0020] Optionally, in step (2), the solvent is at least one of water, ethanol, methanol or xylene.

[0021] Preferably, the metal salt is selected from chloroplatinic acid hexahydrate (H2PtCl6·6H2O), palladium chloride (PdCl2), palladium acetate (Pd(CH3COO)2), or iridium nitrate (H2IrCl6).

[0022] Preferably, in step (2), the mass ratio of zeolite imidazole ester skeleton material, metal salt and heteropoly acid is 1:0.001-0.05:0.1-0.5; the mass-volume ratio of zeolite imidazole ester skeleton material to solvent is 1g:10-100mL.

[0023] Preferably, in step (2), the solvothermal reaction time is 2-6 hours.

[0024] Preferably, in step (3), the calcination treatment is carried out in a hydrogen atmosphere at a temperature of 200-600℃ for 2-4 hours.

[0025] The present invention also provides a method for preparing a single-atom catalyst supported on a polyacid zeolite imidazole ester framework, which yields a single-atom catalyst supported on a polyacid zeolite imidazole ester framework.

[0026] The catalyst consists of a metal single atom and a polyacid zeolite imidazole ester framework support. The pore size of the polyacid zeolite imidazole ester framework support is 3-10 angstroms, which meets the material diffusion requirements of the catalytic reaction.

[0027] The present invention also provides the application of the single-atom catalyst supported on the polyacid zeolite imidazole ester framework in the catalytic acetylation reaction.

[0028] The present invention also provides a method for preparing levulinic acid ester, comprising: using levulinic acid and low alcohol as raw materials, and using the single-atom catalyst supported on the polyacid zeolite imidazole ester framework as a catalyst, to prepare levulinic acid ester by reaction.

[0029] Specifically, lower alcohols include methanol, ethanol, isopropanol, or n-propanol.

[0030] Preferably, the reaction conditions are a temperature of 60-110℃, a pressure of 0.1-0.5MPa, and a time of 0.5-6h.

[0031] Specifically, after the reaction was completed, the system was cooled to room temperature, the catalyst was separated by filtration, and the remaining part was purified by rotary evaporation and gel chromatography to prepare levulinic acid ester.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] (1) The present invention achieves uniform dispersion and stable loading of metal single atoms on the polyacid zeolite imidazole ester framework through a specific preparation method, and prepares a catalyst with high specific surface area and high catalytic activity.

[0034] (2) The catalyst prepared by the method of the present invention is composed of a metal single atom and a polyacid zeolite imidazole ester framework support. The pore size of the polyacid zeolite imidazole ester framework support is 3-10 angstroms, which meets the material diffusion requirements of the catalytic reaction. The metal sites are open and the polyacid components are embedded in the framework. The catalyst has excellent catalytic selectivity and can efficiently catalyze and promote the acetylation reaction. At the same time, the catalyst can be recovered from the reaction system by a simple separation method after the reaction, and has good regenerability and stability. The acetylation ester product is also easy to separate, which is conducive to the realization of industrial production.

[0035] (3) The single-atom catalyst supported on the polyacid zeolite imidazole ester framework has good cycling stability. After at least 5 reaction cycles, its catalytic activity remains at more than 80% of the initial activity, reducing costs and environmental impact. Attached Figure Description

[0036] Figure 1 TEM image and HRTEM lattice stripe pattern of the Pd / HSiW@ZIF-67 catalyst prepared in Example 2. Detailed Implementation

[0037] The present invention will be further illustrated below with reference to the embodiments and accompanying drawings. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0038] Unless otherwise specified in the following examples, all techniques or conditions described in the literature in this field, or the product instructions, shall be followed. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased through legitimate channels or prepared using existing techniques.

[0039] Example 1: Preparation of Pt / HPW@ZIF-8

[0040] (1) Disperse 2.90g of 2-methylimidazole and 1.79g of zinc nitrate hexahydrate into 50mL of methanol solution. After stirring at room temperature for 12 hours, centrifuge and wash three times to obtain ZIF-8 crystals.

[0041] (2) Dry the ZIF-8 crystals at 80°C for 24 hours. Mix 1.0g of dried zeolite imidazole ester framework material ZIF-8 with 0.02g of chloroplatinic acid hexahydrate in 100mL of ethanol solvent. Stir the mixture at room temperature for 24 hours to uniformly load the platinum precursor onto the ZIF-8 material. After stirring, add 0.1g of phosphotungstic acid to the system and control the pH of the system to 6.5. Perform a solvothermal reaction at 100°C for 6 hours. After the reaction, cool, centrifuge, and wash to remove unreacted platinum precursor and obtain the precipitate.

[0042] (3) In a hydrogen atmosphere, the precipitate obtained in step (2) is heated to 300°C at a heating rate of 5°C / min and kept at the temperature for 2 hours to obtain the single-atom catalyst Pt / HPW@ZIF-8 supported on the polyacid zeolite imidazole ester framework.

[0043] Example 2: Preparation of Pd / HSiW@ZIF-67

[0044] (1) Disperse 3.94g of 2-methylimidazole and 3.49g of cobalt nitrate hexahydrate into 300mL of methanol solution. After stirring at room temperature for 6 hours, centrifuge and wash three times to obtain ZIF-67 crystals.

[0045] (2) Dry ZIF-67 crystals at 80℃ for 24 hours. Mix 2.0g of dried zeolite imidazole ester framework material ZIF-67 with 0.1g of palladium chloride (PdCl2) in 100mL of anhydrous ethanol solvent. Stir the mixture at room temperature for 6 hours to uniformly load the palladium precursor onto the ZIF-67 material. After stirring, add 0.2g of silicotungstic acid to the system and control the pH of the system to 6.5. Perform a solvothermal reaction at 100℃ for 6 hours. After the reaction, cool, centrifuge, and wash to remove unreacted palladium precursor and obtain precipitate.

[0046] (3) In a hydrogen atmosphere, the precipitate obtained in step (2) is heated to 300°C at a heating rate of 5°C / min and kept at the temperature for 2 hours to obtain the single-atom catalyst Pd / HSiW@ZIF-67 supported on the polyacid zeolite imidazole ester framework.

[0047] TEM images and HRTEM lattice fringes of the Pd / HSiW@ZIF-67 catalyst are shown below. Figure 1As shown in the image, the obvious Pd lattice fringe spacing of 0.356 nm and the clearly identifiable single-crystal diffraction pattern indicate that the single-atom catalyst Pd / HSiW@ZIF-67 supported on the polyacid zeolite imidazole ester framework has been successfully synthesized.

[0048] Example 3: Aleucopropionic acid esterification reaction

[0049] In a 50 mL reaction flask, 1 mmol of levulinic acid, 10 mmol of a lower alcohol, and 0.1 g of the single-atom catalyst supported on the polyacid zeolite imidazole ester framework prepared in Example 1 or 2 were added. A control without catalyst was also included. After nitrogen purging, the reaction flask was placed in an oil bath and reacted at 60-110 °C for 0.5-6 h. After the reaction was complete, the system was cooled to room temperature, and the catalyst was separated by filtration. The remaining liquid fraction was evaporated using a rotary evaporator to obtain a crude product. The crude product was further purified by gel permeation chromatography, and the purified ethyl levulinate product was collected. Specific parameters such as the lower alcohol used, reaction temperature, and reaction time are shown in Table 1. Gas chromatography-mass spectrometry (GC-MS) was used to analyze the purified product to determine its structure and purity. The recovered catalyst was washed and reused in the next round of catalytic reactions after simple washing and drying.

[0050] Table 1 Preparation of levulinate under different reaction conditions

[0051] experiment alcohol catalyst Reaction temperature / ℃ Reaction time / h Product yield / % 1 Isopropanol Pt / HPW@ZIF-8 85 6 82.6 2 Isopropanol none 85 6 49.2 3 Isopropanol Pd / HSiW@ZIF-67 85 6 81.8 4 n-Propanol Pt / HPW@ZIF-8 85 6 86.9 5 ethanol Pt / HPW@ZIF-8 85 6 84.6 6 methanol Pt / HPW@ZIF-8 85 6 86.6 7 Isopropanol Pt / HPW@ZIF-8 85 3 75.3 8 Isopropanol Pt / HPW@ZIF-8 85 0.5 52.4 9 Isopropanol Pt / HPW@ZIF-8 60 6 79.3 10 Isopropanol Pt / HPW@ZIF-8 100 6 85.3 11 Isopropanol Pt / HPW@ZIF-8 110 6 80.2

[0052] The results above show that without the addition of a single-atom catalyst supported on the polyacid zeolite imidazole ester framework, the yield of levulinic ester is only 49.2% (Experiment 2); while with the single-atom catalyst supported on the polyacid zeolite imidazole ester framework, the yield of levulinic ester is above 79%, and can reach up to 86.6% (Experiment 6).

[0053] The embodiments described above provide a detailed explanation of the technical solutions of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, or similar substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. The application of a single-atom catalyst supported on a polyacid zeolite imidazole ester framework in the catalytic acetylation reaction, characterized in that, The method for preparing the single-atom catalyst supported on the polyacid zeolite imidazole ester framework includes: (1) Preparation of zeolite imidazole ester framework material; the zeolite imidazole ester framework material includes ZIF-8 or ZIF-67; (2) The dried zeolite imidazole ester framework material and metal salt are mixed in a solvent and stirred; then a heteropoly acid is added, and the pH of the system is controlled at 5-7. The solvothermal reaction is carried out at 80-120℃; after the reaction is completed, the mixture is cooled and centrifuged to obtain the precipitate. (3) The precipitate was calcined to obtain the single-atom catalyst supported on the polyacid zeolite imidazole ester framework. The metal salt is selected from at least one of platinum salt, palladium salt, gold salt, or silver salt; The aforementioned heteropolyacid is a Keggin-type polyacid containing [XM] 12 O 40 ] n- Ions, wherein X is selected from P, Si or Ge, M is selected from Mo, W, V or Nb, and n is an integer from 1 to 5; In step (3), the calcination treatment is carried out in a hydrogen atmosphere at a temperature of 200-600 ℃ for 2-4 h.

2. The application according to claim 1, characterized in that, The metal salt is selected from chloroplatinic acid hexahydrate, palladium chloride, or palladium acetate.

3. The application according to claim 1, characterized in that, In step (2), the mass ratio of zeolite imidazole ester skeleton material, metal salt and heteropoly acid is 1:0.001-0.05:0.1-0.5; the mass-volume ratio of zeolite imidazole ester skeleton material to solvent is 1g:10-100mL.

4. The application according to claim 1, characterized in that, In step (2), the solvothermal reaction takes 2-6 hours.

5. A method for preparing levulinic ester, characterized in that, include: Aleucopropionic acid and lower alcohols were used as raw materials, and a single-atom catalyst supported on the polyacid zeolite imidazole ester framework as described in claim 1 was used as a catalyst to prepare levulinic acid ester.

6. The method according to claim 5, characterized in that, Lower alcohols include methanol, ethanol, isopropanol, or n-propanol; the reaction conditions are a temperature of 60-110 °C and a time of 0.5-6 h.

7. The method according to claim 5, characterized in that, After the reaction was completed, the system was cooled to room temperature, the catalyst was separated by filtration, and the remaining part was purified by rotary evaporation and gel chromatography to prepare levulinic acid ester.

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