Preparation method and application of a palladium nanoparticle catalyst supported on polyaniline

Abstract

The present invention discloses a preparation method and application of a palladium nanoparticle-loaded polyaniline catalyst. The Pd / APNI catalyst used is prepared by the "in-situ oxidation polymerization and coordination" method using manganese dioxide (MnO2) with a specific crystal form as a sacrificial template. The size of the highly dispersed metal nanoparticles is at least about 0.3 - 0.9 nm, and the stability is relatively good. It has high selectivity for the reduction coupling of ethyl 5-bromofuroate to prepare bisfuran compounds, and the reaction system uses alcohol as a green reducing agent with mild reaction conditions. This preparation method is simple, safe, green, environmentally friendly and low-cost, providing an important strategy for designing highly dispersed catalysts with controllability at the atomic level.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalytic materials, and relates to a preparation method of a palladium nanoparticle-loaded polyaniline catalyst and its application as a catalyst in the reductive coupling reaction of furfural derivatives. Background Art

[0002] The conversion of biomass into chemicals and fuels is of great strategic significance for alleviating the depletion of fossil resources and the increasingly serious environmental problems. So far, many people have been committed to realizing the directional cleavage and conversion of the main chemical bonds in the basic structural units of biomass. In particular, furan-based platform molecules represented by 5-hydroxymethylfurfural (HMF) and furfural (FF) derived from lignocellulose have great potential in the selective modification strategy of chemical bonds for the development of various high-value chemicals and biofuels.

[0003] Compounds of the bifuran structure class obtained by constructing a C-C bond between two furan rings have been proposed as key components in a variety of important industrial processes, such as the preparation of biobased polyesters and plasticizers. Therefore, the production of a series of bifuran molecules may provide a sustainable approach for biomass upgrading and the carbon neutrality strategy.

[0004] Generally speaking, there have been many studies on the synthesis of bifuran compounds from furfural derivatives. For example, the oxidative coupling of C-H bonds in the presence of a palladium catalyst, but a strong oxidant is required in the catalytic process, which limits its wide application (Chem. Sci., 2013, 4, 3508.); in another case, in the palladium-catalyzed coupling reaction of C-H (or C-B) and C-X bonds, the pre-functionalization of furan boronic acid or bromo-furan type substrates is essential, which makes the process cumbersome (Org. Chem. Front., 2017, 4, 2336 - 2342.); in addition, the nickel-catalyzed reductive coupling of C-X bonds makes the reaction process more dangerous due to the use of excessive stoichiometric metal reducing agents (such as zinc, tin or manganese) (ACS Macro Lett., 2016, 5, 332 - 336.). In recent years, a reductive coupling system using a cheap bromine source to synthesize bromo-furan substrates and an alcohol as a reducing agent has been reported (Chinese J. Chem., 2020, 39, 62 - 68.), but the main problems still exist, such as the separation and recovery of homogeneous palladium complex catalysts, the use of toxic ligands and unsatisfactory product yields. It is worth noting that many reports are devoted to regulating the support structure of Pd-based heterogeneous catalysts to improve the intrinsic activity of the reductive coupling system of aryl halides, but there are few reports on some bromo-furan compounds.

[0005] As a nitrogen-containing conjugated polymer, polyaniline (PANI) has the advantages of environmental friendliness, high stability, low toxicity, and simple preparation, and has broad application prospects in various fields. In particular, the π-conjugated ligands and abundant N atoms in PANI are potential sites for electron effects with metal d orbitals. However, in traditional preparation methods, it is difficult to precisely control the morphology and composition distribution of PANI-functionalized palladium catalysts using strong oxidants (such as ammonium persulfate). Using a controllable soft template for in-situ oxidative polymerization and Pd / PANI coordination may be a good way to solve this problem. As is well known, manganese dioxide (MnO 2 ) is a typical oxidant, and its different morphologies, crystal structures, and oxidation capabilities will affect the reaction process. Therefore, using different crystal forms of MnO 2 materials as sacrificial templates, a series of PANIs with various redox states and morphologies can be synthesized, and then Pd / APNI catalysts with different catalytic activities can be obtained. In addition, after being induced by appropriate MnO 2 , Pd / PANI forms a special atomic arrangement, which may also provide a superior environment for the high dispersion of Pd atoms, so that there are enough electrons to participate in chemisorption and activation. Correspondingly, other properties of Pd / PANI will also show certain differences, such as morphology, structure, interaction, coordination mode, etc.

[0006] The study of applying the Pd / PANI catalyst prepared by the above strategy to the reductive coupling reaction of bromo furan has never been reported. Therefore, in order to achieve the large-scale production of furan compounds, it is necessary to improve the problems of existing technologies and establish a heterogeneous catalytic system of Pd / PANI. Summary of the Invention

[0007] The purpose of the present invention is to provide a preparation method and application of a palladium nanoparticle catalyst supported on polyaniline in view of the above technical analysis and existing problems. Through the sacrificial template MnO 2 , the purpose of controllable material structure and morphology and high dispersion is achieved. The catalyst shows excellent catalytic activity, good stability, wide substrate applicability in the reductive coupling reaction of biomass furfural derivatives, and has good application prospects.

[0008] The above object of the present invention is achieved by the following technical solutions:

[0009] The palladium nanoparticle catalyst supported on polyaniline (Pd / PANI) is prepared by a low-temperature in-situ polymerization and coordination method. The size of the highly dispersed metal nanoparticles is about 0.3 - 0.9 nm at least, and the stability is good. It has high selectivity for the reductive coupling of ethyl 5-bromofuroate to prepare furan compounds, and the reaction system uses alcohol as a green reducing agent, and the reaction conditions are mild. The specific steps are as follows:

[0010] (1) Disperse MnO 2 as a sacrificial template agent into deionized water, ultrasonicate and stir to obtain solution A, and then immerse it in an ice-water bath for a period of time.

[0011] (2) Dissolve aniline monomer in an aqueous hydrochloric acid solution, then add a palladium(II) salt precursor, and stir in an ice-water bath. The resulting solution is solution B.

[0012] (3) Rapidly mix solution A and solution B, and continue to stir in an ice-water bath for 12 - 72 h. Then, filter and wash with deionized water and ethanol to remove free metals and impurities.

[0013] (4) Further immerse the obtained product in an excessive amount of NH 3 ·H 2 O aqueous solution for 12 - 72 h. Finally, further wash the residue with deionized water until the filtrate becomes neutral. Collect the solid and dry it under vacuum at 60 °C, denoted as the Pd / PANI catalyst.

[0014] In the above method, the sacrificial template agent described in step (1) is any one of α-MnO 2 , β-MnO 2 , γ-MnO 2 , δ-MnO 2 or ε-MnO 2 , denoted as Pd / α-PANI, Pd / β-PANI, Pd / γ-PANI, Pd / δ-PANI, and Pd / ε-PANI respectively.

[0015] In the above method, the palladium(II) salt precursor used in step (2) is one or a combination of palladium chloride, palladium acetate, palladium nitrate, or palladium trifluoroacetate.

[0016] In the above method, the mass fraction of palladium in the Pd / PANI catalyst is 0.5% - 5%.

[0017] In the above method, the molar ratio of MnO 2 in "solution A" to the aniline monomer in "solution B" is 0.5 - 2.

[0018] In the above method, the concentrations of the aqueous hydrochloric acid solution and the NH 3 ·H 2 O aqueous solution are 0.5 M - 2 M.

[0019] The palladium nanoparticles supported on polyaniline catalyst is applied to the reductive coupling reaction of ethyl 5-bromofuroate; this catalytic material exhibits excellent catalytic performance in the reductive coupling reaction of ethyl 5-bromofuroate. The reaction conditions are as follows: 0.1 - 2.0 mmol of substrate, 0.5 - 3.0 mL of solvent, the molar percentage of the catalytically active species Pd in the substrate is 0.1 - 0.3 mol%, 0.5 - 2.0 mmol of base, 2.0 mmol of ethanol as the reducing agent, and nitrogen as the protective gas; after reacting for 5 - 15 h under the condition of 70 - 120 °C, the conversion rate of the substrate reaches 70% - 100%, and the selectivity of the target product ethyl bifuran-2-carboxylate reaches 70% - 92%.

[0020] The catalyst used is any one of Pd / α-PANI, Pd / β-PANI, Pd / γ-PANI, Pd / δ-PANI or Pd / ε-PANI.

[0021] The reaction solvent described is any one or a combination of any of N,N-dimethylformamide, dimethyl sulfoxide, 1,4-dioxane, toluene or water.

[0022] The base described is any one of potassium acetate, triethylamine, potassium phosphate, sodium carbonate or cesium carbonate.

[0023] The advantage of the present invention is that the catalyst used in this method is simple to prepare and easy to operate, and is environmentally friendly. By selecting different crystal forms of MnO 2 as the sacrificial template, palladium nanoparticles supported on polyaniline catalysts with different morphologies and structures are prepared. It can efficiently and highly selectively catalyze the reductive coupling of ethyl 5-bromofuroate to prepare ethyl bifuran-2-carboxylate, with mild reaction conditions, high product selectivity, and easy recovery of the catalyst, having significant technical and economic effects and good application prospects. Description of the Drawings

[0024] Figure 1 are SEM photos of different MnO 2 and the corresponding Pd / PANI catalysts: (a - e) α-MnO 2 , β-MnO 2 , γ-MnO 2 , δ-MnO 2 , ε-MnO 2 , (f - j) Pd / α-PANI, Pd / β-PANI, Pd / γ-PANI, Pd / δ-PANI, Pd / ε-PANI.

[0025] Figure 2 are XRD patterns of different MnO 2 and the corresponding Pd / PANI catalysts;

[0026] Figure 3TEM images and particle size distribution results of different Pd / PANI catalysts: (a - e) Pd / α - PANI, Pd / β - PANI, Pd / γ - PANI, Pd / δ - PANI, Pd / ε - PANI. Detailed implementation manners

[0027] The present invention will be further described below by way of examples, but the implementation manners of the present invention are not limited thereto. The following descriptions are only preferred embodiments of the present invention. Any person skilled in the relevant art may use the disclosed technical content to make equivalent changes to equivalent embodiments. Any simple modification or equivalent change made to the following embodiments based on the technical essence of the present invention without departing from the content of the present invention's solution falls within the protection scope of the present invention. Unless otherwise specified, the experimental methods in the following embodiments are all conventional methods.

[0028] Example 1

[0029] Disperse 0.435 g (5.0 mmol) of α - MnO 2 as a sacrificial template agent in deionized water, ultrasonicate and stir for 0.5 h to obtain "Solution A". Dissolve 0.465 g (5.0 mmol) of aniline monomer in 0.5 M hydrochloric acid aqueous solution, then add 0.002 g (0.012 mmol) of palladium acetate, and stir in an ice - water bath for 0.5 h. The resulting solution is "Solution B". Quickly mix Solutions A and B, and continue to stir in the ice - water bath for 18 h. Then filter and wash with deionized water and ethanol to remove free metals and impurities. Subsequently, further immerse the product in an excessive amount of NH 3 ·H 2 O (0.5 M) aqueous solution for 12 h. Finally, further wash the residue with deionized water until the filtrate becomes neutral. Collect the solid and dry it in vacuo at 60 °C, denoted as Pd / α - PANI catalyst.

[0030] Example 2

[0031] Disperse 0.218 g (2.5 mmol) of β - MnO 2 as a sacrificial template agent in deionized water, ultrasonicate and stir for 0.5 h to obtain "Solution A". Dissolve 0.465 g (5.0 mmol) of aniline monomer in 1.0 M hydrochloric acid aqueous solution, then add 0.005 g (0.028 mmol) of palladium chloride, and stir in an ice - water bath for 0.5 h. The resulting solution is "Solution B". Quickly mix Solutions A and B, and continue to stir in the ice - water bath for 20 h. Then filter and wash with deionized water and ethanol to remove free metals and impurities. Subsequently, further immerse the product in an excessive amount of NH 3 ·H 212 h in an aqueous solution of O(1.5 M). Finally, the residue was further washed with deionized water until the filtrate became neutral. The solid was collected and dried in vacuo at 60 °C, denoted as the Pd / β-PANI catalyst.

[0032] Example 3

[0033] Disperse 0.174 g (2.0 mmol) of γ-MnO 2 as a sacrificial template agent in deionized water, ultrasonicate and stir for 0.5 h to obtain "Solution A". Dissolve 0.465 g (5.0 mmol) of aniline monomer in an aqueous solution of 0.7 M hydrochloric acid, then add 0.003 g (0.020 mmol) of palladium nitrate, and stir in an ice-water bath for 0.5 h. The resulting solution is "Solution B". Quickly mix Solutions A and B, and continue to stir in the ice-water bath for 24 h. Then, filter and wash with deionized water and ethanol to remove free metals and impurities. Subsequently, the product was further soaked in an excess of NH 3 ·H 2 O (0.5 M) aqueous solution for 12 h. Finally, the residue was further washed with deionized water until the filtrate became neutral. The solid was collected and dried in vacuo at 60 °C, denoted as the Pd / γ-PANI catalyst.

[0034] Example 4

[0035] Disperse 0.435 g (5.0 mmol) of δ-MnO 2 as a sacrificial template agent in deionized water, ultrasonicate and stir for 0.5 h to obtain "Solution A". Dissolve 0.233 g (2.5 mmol) of aniline monomer in an aqueous solution of 0.6 M hydrochloric acid, then add 0.005 g (0.015 mmol) of palladium trifluoroacetate, and stir in an ice-water bath for 0.5 h. The resulting solution is "Solution B". Quickly mix Solutions A and B, and continue to stir in the ice-water bath for 13 h. Then, filter and wash with deionized water and ethanol to remove free metals and impurities. Subsequently, the product was further soaked in an excess of NH 3 ·H 2 O (1.5 M) aqueous solution for 10 h. Finally, the residue was further washed with deionized water until the filtrate became neutral. The solid was collected and dried in vacuo at 60 °C, denoted as the Pd / δ-PANI catalyst.

[0036] Example 5

[0037] Disperse 0.435 g (5.0 mmol) of ε-MnO 2As a sacrificial template agent, it was dispersed into deionized water, ultrasonicated and stirred for 0.5 h to obtain "Solution A". 0.372 g (4.0 mmol) of aniline monomer was dissolved in 1.0 M hydrochloric acid aqueous solution, and then 0.005 g (0.028 mmol) of palladium chloride was added. It was stirred in an ice-water bath for 0.5 h, and the resulting solution was "Solution B". The two solutions of A and B were quickly mixed and continuously stirred in an ice-water bath for 48 h, and then filtered and washed with deionized water and ethanol to remove free metals and impurities. Subsequently, the product was further immersed in an excessive amount of NH 3 ·H 2 O (1.3 M) aqueous solution for 15 h. Finally, the residue was further washed with deionized water until the filtrate became neutral. The solid was collected and vacuum dried at 60 °C, denoted as Pd / ε-PANI catalyst.

[0038] Example 6

[0039] The morphology and structure of different Pd / PANI catalysts were characterized. Figure 1 They are scanning electron microscope (SEM) pictures of five crystal forms of MnO 2 and their corresponding Pd / PANI catalysts. It can be found that the shape of MnO 2 is crucial for the formation of the final morphology of the Pd / PANI catalyst, which is basically consistent with the template, and the obtained samples have regular structures and highly distinct bending characteristics.

[0040] Figure 2 They are the transmission electron microscope (TEM) and particle size distribution results of the Pd / PANI catalyst. The distribution of Pd nanoparticles on different carriers is different. Among them, the metal particles on Pd / ε-PANI are the smallest and relatively uniform, mainly in the range of 0.3 - 0.6 nm.

[0041] Figure 3 They are the characterization results of the infrared spectrum (FT-IR) of the Pd / PANI catalyst. Five absorption peaks appear at 1560 cm -1 , 1480 cm -1 , 1300 cm -1 , 1135 cm -1 and 801 cm -1 , corresponding to the quinone ring structure, benzene ring structure, C-N stretching, in-plane C-H vibration and out-of-plane C-H bending vibration respectively. Among them, the area ratios of the characteristic peaks of the quinone ring structure and benzene ring structure are different, which is caused by the different oxidation degrees of the carriers in the Pd / PANI catalyst.

[0042] In summary, during the preparation process of the catalytic material, due to the sacrificial template MnO 2Different crystal forms and morphologies will lead to differences in the structure, morphology, metal particle dispersion, and interaction forces of the Pd / APNI catalyst, and thus exhibit different catalytic activities.

[0043] Performance test of the Pd / PANI catalyst, Example 7: 0.219 g of ethyl 5-bromofuroate was added to 2.5 mL of toluene to prepare a reactant substrate solution. 0.010 g of 1 wt% Pd / α-PANI catalyst, 0.092 g of ethanol, and 0.147 g of potassium acetate were added. After mixing, it was placed in a sealed Schlenk tube, and the air was displaced 3 - 5 times with nitrogen. Under the conditions of a temperature of 110 °C and a stirring speed of 300 revolutions per second, the reaction was carried out for 4 h to obtain ethyl bifuran-2-carboxylate. After the sample was separated, the conversion rate of ethyl 5-bromofuroate was 80.5%, and the selectivity of ethyl bifuran-2-carboxylate was 60.5%.

[0044] Example 8: 0.219 g of ethyl 5-bromofuroate was added to 2.5 mL of dimethyl sulfoxide to prepare a reactant substrate solution. 0.012 g of 1 wt% Pd / β-PANI catalyst, 0.092 g of ethanol, and 0.318 g of potassium phosphate were added. After mixing, it was placed in a sealed Schlenk tube, and the air was displaced 3 - 5 times with nitrogen. Under the conditions of a temperature of 110 °C and a stirring speed of 300 revolutions per second, the reaction was carried out for 10 h to obtain ethyl bifuran-2-carboxylate. After the sample was separated, the conversion rate of ethyl 5-bromofuroate was 90.6%, and the selectivity of ethyl bifuran-2-carboxylate was 92.9%.

[0045] Example 9: 0.219 g of ethyl 5-bromofuroate was added to 2.5 mL of 1,4-dioxane to prepare a reactant substrate solution. 0.009 g of 1 wt% Pd / γ-PANI catalyst, 0.092 g of ethanol, and 0.207 g of potassium carbonate were added. After mixing, it was placed in a sealed Schlenk tube, and the air was displaced 3 - 5 times with nitrogen. Under the conditions of a temperature of 110 °C and a stirring speed of 300 revolutions per second, the reaction was carried out for 7 h to obtain ethyl bifuran-2-carboxylate. After the sample was separated, the conversion rate of ethyl 5-bromofuroate was 68.7%, and the selectivity of ethyl bifuran-2-carboxylate was 53.0%.

[0046] Example 10: 0.219 g of ethyl 5-bromofuroate was added to 2.5 mL of N,N-dimethylformamide to prepare a reactant substrate solution. 0.008 g of 1 wt% Pd / δ-PANI catalyst, 0.092 g of ethanol, and 0.151 g of triethylamine were added. After mixing, it was placed in a sealed Schlenk tube, and the air was displaced 3 - 5 times with nitrogen. Under the conditions of a temperature of 110 °C and a stirring speed of 300 revolutions per second, the reaction was carried out for 6 h to obtain ethyl bifuran-2-carboxylate. After the sample was separated, the conversion rate of ethyl 5-bromofuroate was 96.4%, and the selectivity of ethyl bifuran-2-carboxylate was 79.3%.

[0047] Example 11: 0.219 g of ethyl 5-bromofuroate was added to 2.5 mL of toluene to prepare a reactant substrate solution. 0.001 g of 1 wt% Pd / ε-PANI catalyst, 0.092 g of ethanol and 0.318 g of potassium phosphate were added. After mixing, the mixture was placed in a sealed Schlenk tube and purged with nitrogen 3 - 5 times to remove air. The reaction was carried out at 110 °C with a stirring speed of 300 revolutions per second for 12 h to obtain ethyl furan-2,5-dicarboxylate. After separation of the sample, the conversion rate of ethyl 5-bromofuroate was 97.0% and the selectivity of ethyl furan-2,5-dicarboxylate was 95.4%.

Claims

1. Preparation method of a palladium nanoparticle catalyst supported on morphology - controllable and highly - dispersed polyaniline, characterized in that, The preparation process used ε-MnO 2 as a sacrificial template agent to obtain the Pd / ε-PANI catalyst; It includes the following steps: dispersing ε-MnO 2 in deionized water, ultrasonically treating and stirring to obtain solution A; dissolving aniline monomer in an aqueous hydrochloric acid solution, adding a palladium (II) salt precursor, and stirring in an ice-water bath, and the resulting solution is solution B; quickly mixing solution A and solution B, and continuously stirring in an ice-water bath for 12 to 72 h, then filtering and washing with deionized water and ethanol to remove free metals and impurities; subsequently, further soaking the product in excessive ammonia water for 12 to 72 h; finally, further washing the residue with deionized water until the filtrate becomes neutral, collecting the solid, and drying it under vacuum at 40 to 100 °C to obtain the Pd / ε-PANI catalyst; the palladium (II) salt precursor is one or more of palladium chloride, palladium acetate, palladium nitrate or palladium trifluoroacetate; the mass fraction of palladium in the Pd / ε - PANI catalyst is 0.5% - 5%; The ε-MnO of the solution A 2 The molar ratio with aniline monomer of the solution B is 0.5 to 2.

2. The preparation method of the polyaniline - supported palladium nanoparticle catalyst according to claim 1, characterized in that, the concentration of the hydrochloric acid aqueous solution is 0.5 M - 2 M.

3. The preparation method of the polyaniline - supported palladium nanoparticle catalyst according to claim 1, characterized in that, the concentration of the ammonia water is 0.5 M - 2 M.

4. Application of the polyaniline - supported palladium nanoparticle catalyst prepared by the method of claim 1, characterized in that, the polyaniline - supported palladium nanoparticle catalyst is applied to the reductive coupling reaction of ethyl 5 - bromofuroate; the reaction conditions are: 0.1 - 2.0 mmol of substrate, 0.5 - 3.0 mL of solvent, the molar percentage of the catalytically active species Pd in the substrate is 0.1 - 0.3 mol%, 0.5 - 2.0 mmol of base, 2.0 mmol of ethanol as the reducing agent, and nitrogen as the protective gas; after reacting for 5 - 15 h under the condition of 70 - 120 °C, the conversion rate of the substrate reaches 70% - 100%, and the selectivity of the target product ethyl difurylfuroate reaches 70% - 92%.

5. The application according to claim 4, characterized in that, the solvent is any one or a combination of N,N - dimethylformamide, dimethyl sulfoxide, 1,4 - dioxane, toluene.

6. The application according to claim 4, characterized in that, the base is any one of potassium acetate, triethylamine, potassium phosphate, sodium carbonate or cesium carbonate.

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