English

• 1. INTRODUCTION

  Aromatic amines (anilines), serve as vital precursor for producing pharmaceuticals, dyes, pigments, and agrochemicals^[1-3]. As the most common and reliable way to produce functionalized arylamines, the complete hydrogenation of nitroarenes remains a significant but challenging task in both laboratory research and industrial synthetic chemistry^[4-6]. Research works available in thermal heterogeneous catalysis often require harsh condition condition such as high temperature (mostly > 373 K) and/or high pressure 50 atm H₂. which lead to the difficulty in controlling the reaction^{[2, 7]}. Some heterogeneous catalysts can achieve the hydrogenation of aromatics under mild conditions have been well developed (r25 1 C and r3 atm H₂)^[8-11]. In most cases, the process of synthesizing such a catalyst is difficult to achieve, with the disadvantage of low activity or low stability^{[7, 12]}. Therefore, the search for easily prepared, highly active and economic reaction system that operates under mild conditions still remains a paramount challenge.

  Thin film catalysts have been widely used in recent years due to their advantages of simple fabrication and easy separation^[13-15]. At the same time, compared with the traditional catalyst, it also has the advantages of high activity, easy recycling and strong stability^[16-19]. In general, layer-by-layer (LbL) is one of the most common and easiest ways to make ultra-thin films. There are many reports on the stable presence of metal nanoparticles in thin films to form heterogeneous catalysts^{[20, 21]}. For example, Ren et al. have successfully prepared Pd-PVP multilayer films using LbL and demonstrated that they are potential heterogeneous catalysts for selective hydrogenation of conjugated aromatics^[22]. Therefore, the film has broad application prospects in catalysis. At the same time, its biggest advantage is that it can recover the catalyst at any time and reduce the waste of the catalyst. LbL has the advantages of simple synthesis, high cost efficiency and wide application, and is a powerful method for preparing thin film materials^[23].

  At present, there are many catalysts containing noble metal nanoparticles, especially platinum and palladium, which are used for the catalytic reaction of hydrogenation and exhibit good activity^[24-27]. For example, Wang et al. reported a Pd nanoparticle catalyst supported on mesoporous graphite carbonitride (Pd@mpg-C₃N₄)^[28]. The catalyst has high activity and can promote the selective formation of cyclohexanone under normal pressure in an additivefree aqueous medium. In our previous report^{[7, 29]}, Pd in situ produced within LbL films have been studied for the hydrogenation of nitroarenes by molecular hydrogen. In this paper, a multilayer film structure based on metal ion-ligand interaction was constructed. Using K₂PdCl₄ as a precursor, 3-amino-3-(4-pyridinyl)-propionitrile (P1) (ligand) was selected as the material for constructing films. The P1 was synthesized according to reference^[30]. We have comprehensively characterized the palladium multilayer film prepared by LbL method, and revealed the growth process of the film. The nitrobenzene hydrogenation reaction was carried out on the multilayer film, and its catalytic performance was verified and determined by gas chromatography. The results show that under the action of the catalyst multilayer film, it exhibits excellent catalytic performance for different nitro substrates and has a high yield of aniline. At the same time, XPS and TEM images confirmed that Pd²⁺ ions were reduced into Pd nanoparticles to the hydrogenation reaction, which would be well dispersed in the LbL multilayer film.

  2. EXPERIMENTAL

  2 1 Materials

  Benzonitrile, potassium t-butamine, toluene and acetonitrile were purchased from Aldrich Chemical Co. Diethyl ether, diethyl ether and magnesium sulfate were provided by Aladdin Reagent Company and used without further purification. Pure and dry hydrogen gas, nitrogen stream and water (Milli-Q, 18.2 MΩ cm) were used during the experiment. Poly(ethyleneimine) (PEI) with a molecular weight of approximately 60, 000 and K₂PdCl₄ were supplied by Shanghai Titan Scientific Co., Ltd. All the reagents were used as received.

  2 2 Synthesis of 3-amino-3-(4-pyridinyl)-propionitrile

  Benzonitrile (4.00 g) and potassium t-butamine (10 g) were added to a dry mixed solution of toluene (100 ml) and acetonitrile (3.20 g). The reaction was kept stirring overnight at room temperature. To separate several layers, 100 ml of water and 100 ml of diethyl ether were added. The extraction of the aqueous layer and the drying of the organic layer were performed using 100 mL of diethyl ether and magnesium sulfate, respectively. The mixture was placed under vacuum to remove the solvent, followed by a small portion of diethyl ether and a mixture of n-hexanes to afford turbid mixture. After cooling for 12 h, it was washed with n-hexane to give 3-Amino-3-(4-pyridinyl)-propionitrile as white crystals^[30].

  2 3 The preparation of PEI-(K₂PdCl₄-P1)_n-film

  First, H₂SO₄ and H₂O₂ were mixed and heated to 70 ℃, then the quartz substrate (25 mm × 12 mm × 1 mm) was immersed in 1 h^[29]. Then, the quartz piece was taken out and immersed in a mixed solution of H₂O₂, H₂O and NH₃OH at 70 ℃ for 30 min, followed by rinsing with a large amount of water and drying the quartz piece with nitrogen. After washing with a large amount of water and drying the quartz piece with nitrogen, the quartz substrate was placed in a PEI solution (10 mg/mL) for 20 minutes to render the substrate positively charged. As demonstrated in Fig. 1, the quartz flakes were rinsed with nitrogen and a large amount of water. Then, a film was prepared by alternately growing under the interaction of an aqueous solution of K₂PdCl₄ and 3-amino-3-(4-pyridinyl)-propionitrile, respectively.

  Figure 1

  

  Figure 1.  Synthesis steps and schematic diagram of PEI-(K₂PdCl₄-P1)_n-film

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  2 4 Material characterization

  UV spectroscopy was used to monitor the growth process of film catalyst. In order to characterize surface morphology information of the prepared films, atomic force microscopy (AFM) images were recorded on single-crystal silicon slide using a Veeco Multimode NS3A-02 Nanoscope Ⅲ atomic force microscope at tapping-mode. TEM micrographs were taken on a JEM-2010 at 120 kV. To analysis Pd content in films, inductively coupled plasma OES spectrometer (ICP-OES) was used. The High-resolution XPS spectra (Physical Electronics, USA) were applied to realize the qualitative analysis and valence state of the surface elements. The catalytic process of the hydrogenation of nitroarenes was performed on a gas chromatograph (Angilent G7890A) equipped with a FID detector.

  3. RESULTS AND DISCUSSION

  3 1 Analysis of the PEI-(K₂PdCl₄-P1)_n-film

  The variation of the UV-vis absorption spectroscopy was used to study the progress of the synthesis of the PEI-(K₂PdCl₄-P1)_n-film. For the UV-vis spectrum of the PEI-(K₂PdCl₄-P1)_n-film (Fig. 2a), the absorbance between 200 and 800 nm shows a proportional increase with the increase of the number of layers due to the progress of LbL, which is attributed to the successful adsorption of the ligand on each layer of the film. which is attributed to the fact that the P1 ligand is successfully adsorbed on each film. In addition, as shown in Fig. 2b, the absorption of the PEI-(K₂PdCl₄-P1)_n-film shows a linear increase (R² = 0.9939) with the increase of the number of layers.

  Figure 2

  

  Figure 2.  Absorbance of (a) PEI-(K₂PdCl₄-P1)_n-film and (b) and the absorbance of the cyclic deposition of K₂PdCl₄ and 3-amino-3-(4-pyridinyl)-propionitrile (300 nm)

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  The content of Pd in PEI-(K₂PdCl₄-P1)_n-film was detected by ICP, and the detection environment was carried out in the aqua regia solution. Fig. 3 shows that the Pd content in PEI-(K₂PdCl₄-P1)₂-film is 1.13×10^-7 mol, which is the smallest among all PEI-(K₂PdCl₄-P1)_n-film. The Pd content of PEI-(K₂PdCl₄-P1)₁₀-film is the largest, and the concentration is 5.47×10^-7 mol. It is consistent with the results of UV, and both show a good linear increase effect. These results further confirmed that the growth of PEI-(K₂PdCl₄-P1)_n-film is only related to the deposition mechanism of LbL.

  Figure 3

  

  Figure 3.  Pd content of PEI-(K₂PdCl₄-P1)_n-film with different layer numbers

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  The AFM was used to study the change in surface morphology of the multilayer structure of PEI-(K₂PdCl₄-P1)_n-film during the manufacturing process. The observation of the entire morphology was carried out on a silicon substrate (the scan area = 1.0×1.0 μm²). As shown in Fig. 4, all AFM images show a bright nano-granular structure, demonstrating the successful growth of PEI-(K₂PdCl₄-P1)_n-film on a silicon substrate. The root-mean-square (RMS) roughness of the PEI-(K₂PdCl₄-P1)₂-film is 4.03 nm, showing relatively dense particles. The surface of the PEI-(K₂PdCl₄-P1)₂-film exhibits a rough morphology with a maximum peak height (R_max) of 42.1 nm (Fig. 4a). As the number of layers increases, the surfaces of the PEI-(K₂PdCl₄-P1)₆-film and the PEI-(K₂PdCl₄-P1)₁₀-film become rougher and more uneven. The RMS of PEI-(K₂PdCl₄-P1)₆-film (Fig. 4b) and PEI-(K₂PdCl₄-P1)₁₀-film (Fig. 4c) is 8.47 and 12.6 nm and their R_max reach up to 91.8 and 136 nm, respectively. The difference in AFM surface morphology of PEI-(K₂PdCl₄-P1)_n-film indicates that the particles can be aggregated to form a compacted film with some defects on the surface. The above results further prove that the PEI-(K₂PdCl₄-P1)_n-film is carried out in the conventional LbL mode.

  Figure 4

  

  Figure 4.  AFM images of (a) PEI-(K₂PdCl₄-P1)₂-film, (b) PEI-(K₂PdCl₄-P1)₆-film, and (c) PEI-(K₂PdCl₄-P1)₁₀-film

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  3 2 Catalysis of PEI-(K₂PdCl₄-P1)_n-film

  Palladium nanoparticles have been reported to be widely used as catalysts in hydrogenation reactions because of their high specific surface area^[31]. The catalyst PEI-(K₂PdCl₄-P1)_n-film was prepared and its effect on the hydrogenation of nitrobenzene with different space and electron groups was studied. As shown in Table 1, even after the nitrobenzene substrate was replaced with 2-nitrobenzoic acid, the catalyst brought the conversion of the substrate to nearly 100% under the conditions of the reaction within 3 hours (Table 1, entry 1). It is interesting to note that in the case of a reaction condition of 35 ℃ and 1.0 atm, most of the reaction substrate can achieve complete conversion with the extension of the reaction time (Table 1, entries 3, 5, 6 and 7). However, in this reaction system, the conversion rate of 3-nitrophenol and 4-nitrophenol cannot reach 100%.(Table 1, entries 2 and 4). The 3-Hydroxy-1-nitrobenzene and 4-nitrofenol with an electron donating group have a lower reactivity in a reaction time of 3 hours (Table 1, entries 2 and 4). The yield of the nitroarenes containing an electron withdrawing group such as o-nitrobenzoic acid, ethyl p-nitrobenzoate or p-nitroacetophenone to the corresponding product is greater than 90%. This proves that these substrates have excellent catalytic properties (Table 1, entries 1, 6, and 8). At the same time, the reduction yield of 2-nitrophenol reached 48% (Table 1, entry 3), while the conversion of 4-nitrophenol and 3-nitrophenol reached only 19% and 12% after 3 hours of reaction (Table 1, entries 2 and 4). This phenomenon is attributed to the spatial effect. Thus, Phenol, 2-nitrylphemol and 3-nitrylphemol are converted to the corresponding products in different times.

  Table 1

  

  Table 1.  Hydrogenation of Different Types of Aromatic Nitro Compounds by PEI-(K₂PdCl₄-P1)₁₀-Film Catalysts

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  Entry	Aromatic nitro compounds	Conversion product	Time (h)	Yield (%)
  1			3 h	> 99
  2			3 h 12 h	19 71
  3			3 h 8.5 h	48 > 99
  4			3 h 9 h	12 37
  5			3 h 10.5 h	50 97
  6			3 h 5.5 h	72 > 99
  7			3 h 7h	36 > 99
  8			3 h 4.5 h	78 96
  9			3 h 4.5 h	84 > 99

  We removed the catalyst PEI-(K₂PdCl₄-P1)_n-film as a control experiment in the nitrobenzene hydrogenation reaction. After 2 h of reaction, no conversion of nitrobenzene was caused under the same conditions. This is enough to prove that the catalyst is effective in the conversion of nitrobenzene.

  We further verified the effect of palladium content on the catalytic activity of the film, and selected nitrobenzene as the test substrate. Fig. 5 shows that the conversion per mol of Pd increases as the number of the first 6 layers increases. However, when the number of film layers is more than 4 layers, one mole conversion rate of Pd rises relatively slowly. Therefore, the content of Pd and the number of layers simultaneously affect its catalytic activity.

  Figure 5

  

  Figure 5.  Relationship between catalyst activity and PEI-(K₂PdCl₄-P1)₁₀-film catalysts with different Pd amounts under the reaction conditions of 2.5 h

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  The cyclability of the catalyst is important for the service life and is also significant for the investigation of heterogeneous catalysis. After one cycle of circulation, the stabilized PEI-(K₂PdCl₄-P1)₁₀-film was taken directly from the reaction unit and washed dry to do the next catalytic cycle. The results show that even after the catalyst was recycled for 5 times, a high conversion rate was maintained (Fig. 6).

  Figure 6

  

  Figure 6.  PEI-(K₂PdCl₄-P1)₁₀-film catalyzed nitrobenzene hydrogenation cycle reaction for 2.5 h

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  XPS was used to investigate the cause of the activity of the catalyst. The Pd valence state in the film before and after the reaction was measured by XPS, and the change was compared. Fig. 7a shows two typical Pd(Ⅱ) characteristic peaks, Pd 3d_5/2 (336.68 eV) and 3d_3/2 (341.83 eV), respectively^{[22, 32]}. Fig. 7b shows the Pd valence state of the film after the reaction. It can be seen that the signal of Pd (3d) can be divided into two pairs of double signals. The Pd(Ⅱ) and Pd(0) is attributable to the 341.78/336.48 eV and 340.58/335.18 eV, respectively^[33]. The results demonstrated that the Pd(Ⅱ) is reduced to Pd nanoparticles during the reaction under H₂ atmosphere.

  Figure 7

  

  Figure 7.  (a) XPS survey spectra for PEI-(K₂PdCl₄-P1)₁₀-film before reaction and (b) XPS survey spectra for PEI-(K₂PdCl₄-P1)₁₀-film after reaction

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  Spherical nanoparticles were examined by TEM, which further verified that Pd²⁺ was partially reduced to Pd nanoparticles during the whole reaction (Fig. 8a). The size of the Pd nanoparticles was calculated to be 3.51 nm by the particle size distribution map (Fig. 8b). It was further verified that the nanoparticles have good catalytic properties^[34].

  Figure 8

  

  Figure 8.  TEM images about different substrates (a) nitrobenzene, (b) Pd particle size distribution obtained from (a), (c) p-nitroacetophenone, and (d) o-nitrobenzoic acid after hydrogenation reaction

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  4. CONCLUSION

  In summary, we have used the LbL method to prepare a multilayer film in which Pd and the ligand 3-amino-3-(4-pyridinyl)-propionitrile coexist. The LbL method was also confirmed by UV-visible spectroscopy and ICP. Through performance testing, the thin film catalyst can be applied to the hydrogenation reaction of various substituted nitroaromatics, showing good catalytic activity, and the stability of the catalyst is good. TEM and XPS further confirmed that during the catalytic reaction, Pd²⁺ ions in the multilayer film were reduced to Pd nanoparticles, thereby improving the catalytic activity.

  
  
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• 

  Figure 1  Synthesis steps and schematic diagram of PEI-(K₂PdCl₄-P1)_n-film

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  Figure 2  Absorbance of (a) PEI-(K₂PdCl₄-P1)_n-film and (b) and the absorbance of the cyclic deposition of K₂PdCl₄ and 3-amino-3-(4-pyridinyl)-propionitrile (300 nm)

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  Figure 3  Pd content of PEI-(K₂PdCl₄-P1)_n-film with different layer numbers

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  Figure 4  AFM images of (a) PEI-(K₂PdCl₄-P1)₂-film, (b) PEI-(K₂PdCl₄-P1)₆-film, and (c) PEI-(K₂PdCl₄-P1)₁₀-film

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  Figure 5  Relationship between catalyst activity and PEI-(K₂PdCl₄-P1)₁₀-film catalysts with different Pd amounts under the reaction conditions of 2.5 h

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  Figure 6  PEI-(K₂PdCl₄-P1)₁₀-film catalyzed nitrobenzene hydrogenation cycle reaction for 2.5 h

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  Figure 7  (a) XPS survey spectra for PEI-(K₂PdCl₄-P1)₁₀-film before reaction and (b) XPS survey spectra for PEI-(K₂PdCl₄-P1)₁₀-film after reaction

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  Figure 8  TEM images about different substrates (a) nitrobenzene, (b) Pd particle size distribution obtained from (a), (c) p-nitroacetophenone, and (d) o-nitrobenzoic acid after hydrogenation reaction

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  Table 1.  Hydrogenation of Different Types of Aromatic Nitro Compounds by PEI-(K₂PdCl₄-P1)₁₀-Film Catalysts

  Entry	Aromatic nitro compounds	Conversion product	Time (h)	Yield (%)
  1			3 h	> 99
  2			3 h 12 h	19 71
  3			3 h 8.5 h	48 > 99
  4			3 h 9 h	12 37
  5			3 h 10.5 h	50 97
  6			3 h 5.5 h	72 > 99
  7			3 h 7h	36 > 99
  8			3 h 4.5 h	78 96
  9			3 h 4.5 h	84 > 99

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