Preparation method and application of palladium catalyst

Abstract

The invention discloses a preparation method and application of a palladium catalyst. The catalyst is composed of a cross-linked polymer and palladium nanoparticles loaded on the cross-linked polymer, has high catalytic activity, selectivity and stability, and can efficiently catalyze the reduction reaction of nitro-aromatic hydrocarbons. The catalyst provided by the invention is simple in preparation method, low in cost and suitable for large-scale production. The catalyst has a wide application prospect in the fields of environmental protection and fine chemical engineering.

Description

Technical Field

[0001] The present invention relates to a method for preparing a catalyst and its application. Specifically, it is a method for preparing a cross-linked polymer supported palladium nanoparticle catalyst, and the application of this catalyst in the catalytic reduction reaction of nitroarenes. Background Art

[0002] The reduction reaction of nitroarenes is an important route for the preparation of aromatic amines, which are widely used in the synthesis of fine chemicals such as pharmaceuticals, dyes, pesticides, etc. Traditional methods for reducing nitroarenes usually use reducing agents such as iron / hydrochloric acid or hydrogen sulfide / sodium sulfide, but these methods have serious environmental pollution problems. In recent years, supported noble metal nanoparticles (such as palladium, platinum, gold, etc.) have been widely used in catalytic reduction reactions due to their high efficiency and environmental friendliness. However, noble metal nanoparticles are prone to aggregation and loss during the reaction process, resulting in catalyst deactivation. Based on this, researchers have developed a variety of support materials to stabilize noble metal nanoparticles, such as mesoporous silica, activated carbon, graphene, etc. However, the interaction between these inorganic supports and noble metal nanoparticles is weak, making it difficult to effectively prevent the aggregation and loss of nanoparticles. Therefore, developing a catalyst with higher catalytic activity for the reduction of nitroarenes has important research significance and application value. Summary of the Invention

[0003] The present invention discloses a novel palladium-based catalyst and its application in the preparation of aromatic amines by reducing nitroarenes, which has significantly improved catalytic activity compared with the existing catalytic system. At the same time, a method for preparing this catalyst and a method for preparing aromatic amines by reducing nitroarenes with this catalyst are provided.

[0004] The palladium catalyst of the present invention is composed of a cross-linked polymer and palladium nanoparticles uniformly supported thereon, and the cross-linked polymer is a product obtained by the condensation reaction of dicyandiamide and ferrocene formaldehyde.

[0005] Preferably, in the palladium catalyst of the present invention, the average particle size of the palladium nanoparticles is 1 - 3 nm.

[0006] The method for preparing the palladium catalyst of the present invention is: preparing a cross-linked polymer by the condensation reaction of ferrocene formaldehyde and dicyandiamide, and then adsorbing palladium ions thereon.

[0007] Preferably, the method for preparing the catalyst of the present invention is: 1) Preparation of cross-linked polymer Add 642 mg of ferrocene formaldehyde, 252 mg of dicyandiamide and dimethyl sulfoxide into a reaction vessel. Under nitrogen protection, heat the reaction system to 150 °C and stir the reaction for 24 hours. After the reaction is completed, cool to room temperature, filter to obtain a black gel-like product, and obtain the support after washing and drying.

[0008] 2) Catalyst Preparation The cross-linked polymer is dispersed in methanol, and the palladium acetate metal precursor is dissolved in methanol. The two solutions are mixed and stirred overnight. Since the cross-linked polymer contains abundant nitrogen atoms, the metal ions interact with the nitrogen atoms, enabling the palladium ions to be evenly adsorbed and anchored on the cross-linked polymer. Sodium borohydride is dissolved in water and added dropwise to the aforementioned mixed solution, and stirring is continued for several hours. Sodium borohydride, as a reducing agent, can reduce the palladium ions to palladium metal and attach it to the carrier. Then, the product is filtered out and collected, and after washing and drying, the palladium catalyst described above is obtained.

[0009] The catalyst of the present invention can be applied to the catalytic reduction reaction of nitroaromatics.

[0010] Cross-linked polymers are a class of organic polymer materials with rich pore structures and high specific surface areas, and have received extensive attention due to their unique physical and chemical properties. Cross-linked polymers can be prepared by simple synthesis methods, and the interaction with noble metal nanoparticles can be enhanced by introducing functional groups (such as heteroatoms like nitrogen, phosphorus, sulfur, etc.), thereby achieving effective fixation and dispersion of the nanoparticles. However, the existing cross-linked polymer materials are mainly used for gas storage and separation, and the preparation process usually requires the use of expensive cross-linking agents or catalysts, increasing the production cost and complexity. The present invention provides a new use of cross-linked polymer materials as carriers for noble metal nanoparticles. The preparation method of the cross-linked polymer materials used is relatively simple, with the advantages of low cost and high efficiency.

[0011] The catalyst of the present invention can be used in the reduction reaction with 4-nitrophenol and 2-nitroaniline as model substrates. The experimental results show that: for the reduction reaction of 4-nitrophenol, the rate constant of the Pd@SP catalyst of the present invention is 156 s -1 g -1 , and the reported rate constants of Pd NTs (Green Chem. 2016, 18, 558 - 564), Pd@MIL-100 (MicroporousMesoporous Mater. 2018, 255, 1 - 6) and Pd / Fe 3 O 4 @C (New J. Chem. 2017, 41, 4014 - 4021) catalysts in the literature are 71 s -1 g -1 , 12.3 s -1 g -1 and 2.8 s -1 g -1Under the same reaction conditions, the catalytic activity of the catalyst of the present invention is 2 - 55 times that of the above-mentioned catalyst. Moreover, under mild reaction conditions, this catalyst can efficiently reduce a variety of nitroarenes to the corresponding aromatic amines, further expanding its scope of application. Among them, in the reduction reaction of halogenated nitroarenes (X = F, Cl, and Br), no dehalogenation phenomenon occurs, showing excellent functional group tolerance. In addition, this catalyst also exhibits high stability in the catalytic reduction reaction of 4-nitrophenol and can be reused at least 10 times without deactivation. Description of the Drawings

[0012] Figure 1 : (a) TEM and (b) particle size statistical distribution diagrams of the Pd@SP catalyst.

[0013] Figure 2 : XRD pattern of the Pd@SP catalyst.

[0014] Figure 3 : XPS spectrum of the Pd@SP catalyst.

[0015] Figure 4 : UV-visible spectra of the Pd@SP catalyst for the reduction reactions of (a) 4-nitrophenol and (b) 2-nitroaniline.

[0016] Figure 5 : Cyclic reaction diagram of the Pd@SP catalyst for the reduction of 4-nitrophenol. Detailed Description of the Invention

[0017] The present invention will be illustrated with examples (I) Preparation of the catalyst (hereinafter simply referred to as the Pd@SP catalyst) 1: Preparation of the crosslinked polymer 642 mg of ferrocene formaldehyde, 252 mg of dicyandiamide, and 20 mL of dimethyl sulfoxide were added to a 100 mL single-necked flask, and a condenser was installed. Under nitrogen protection, the reaction system was heated to 150 °C and stirred for 24 hours. After the reaction was completed, it was cooled to room temperature, and the black gel-like product was obtained by filtration. It was washed successively with N,N-dimethylformamide, tetrahydrofuran, and methanol, and finally dried overnight in a vacuum drying oven to obtain the crosslinked polymer.

[0018] 2: Preparation of the catalyst Pd@SP 100 mg of the crosslinked polymer was dispersed in 20 mL of methanol, and 10 mg of the palladium acetate metal precursor was dissolved in 20 mL of methanol. The two solutions were mixed and stirred overnight to allow the palladium ions to be evenly adsorbed on the crosslinked polymer. 28 mg of sodium borohydride (NaBH 4( ) It was dissolved in 2 mL of water and added dropwise to the above mixed solution, and stirring was continued for several hours. The product was collected by filtration, and the solid was washed several times with water and dried to obtain the Pd@SP catalyst.

[0019] (II) Pd@SP Catalytic Reduction Reaction 1. Catalytic Reduction of 4-Nitrophenol (4-NP) 250 μL of NaBH 4 solution (0.19 M), 30 μL of 4-NP solution (0.01 M) and 2.7 mL of water were mixed to obtain a bright yellow solution. 10 μL of the Pd@SP catalyst suspension (10 mg / mL) was quickly added and stirred rapidly. After the reaction was completed, the solution became colorless, indicating that 4-NP was completely reduced to 4-aminophenol (4-AP). The reaction process was monitored by UV-visible spectroscopy.

[0020] 2. Catalytic Reduction of 2-Nitroaniline (2-NA) 250 μL of NaBH 4 solution (0.19 M), 30 μL of 2-NA solution (0.01 M) and 2.7 mL of water were mixed. 20 μL of the Pd@SP catalyst suspension (10 mg / mL) was added and stirred rapidly. After the reaction was completed, the solution became colorless, indicating that 2-NA was completely reduced to 2-aminobenzeneamine (o-PDA). The reaction process was monitored by UV-visible spectroscopy.

[0021] 3. Catalytic Reduction of Nitroarenes to the Corresponding Aromatic Amines 1 mmol of nitroarene, 10 mmol of NaBH 4 , 5 mg of the catalyst and 20 mL of the solvent (V 水 :V 乙醇 = 3:2) were added to a 25 mL single-necked flask and reacted at room temperature for 15 minutes. The product in the solution was extracted with ethyl acetate, and the yield of the product was determined by gas chromatography-mass spectrometry.

[0022] The relevant data for the reduction reaction of nitroarenes to prepare different aromatic amines using the catalyst of the present invention are shown in Table 1.

[0023]

[0024]

[0025] As can be seen from the results in Table 1, the Pd@SP catalyst has a wide range of applicability to various nitroarene substrates, showing excellent activity and selectivity. Taking the reduction of 4-nitrophenol as a model reaction, under the same reaction conditions, the activity of the catalyst is 2 - 55 times that of the reported Pd catalysts. In addition, the Pd@SP catalyst does not show dehalogenation in the reduction reaction of halogenated nitroarenes (X = F, Cl, and Br), showing excellent functional group tolerance.

Claims

1. A palladium catalyst, characterized in that The palladium catalyst is composed of a cross-linked polymer and palladium nanoparticles uniformly loaded thereon, and the cross-linked polymer is a product obtained by the condensation reaction of dicyandiamide and ferroceneformaldehyde.

2. The palladium catalyst according to claim 1, characterized in that The particle size of the palladium nanoparticles in the catalyst is 1-3 nm.

3. The method for preparing a catalyst according to claim 1 or 2, characterized in that A cross-linked polymer is prepared by condensation reaction of ferrocene formaldehyde and dicyandiamide, and is used as a carrier to adsorb palladium ions.

4. The method for preparing a catalyst according to claim 3, characterized in that 1) Preparation of cross-linked polymers 642 mg of ferrocene carboxaldehyde, 252 mg of dicyandiamide and 20 mL of dimethyl sulfoxide were added to a reaction vessel. Under nitrogen protection, the reaction system was heated to 150 °C and stirred for 24 hours. After the reaction was completed, it was cooled to room temperature and filtered to obtain a black gel product, which was washed and dried to obtain the product. 2) Catalyst preparation The cross-linked polymer is dispersed in methanol, the palladium acetate metal precursor is dissolved in methanol, the two solutions are mixed, and stirred overnight to allow the palladium ions to be uniformly adsorbed on the cross-linked polymer. Sodium borohydride is dissolved in water and added dropwise to the mixed solution, and the product is filtered and collected after continued stirring for several hours, and the palladium catalyst is obtained after washing and drying.

5. Use of the catalyst according to claim 1 or 2 in the catalytic reduction of nitroaromatics.

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