Synthesis and characterization of Ni nanoparticles incorporated into hyperbranched polyamidoamine–polyvinylamine/SBA-15 catalyst for simple reduction of nitro aromatic compounds

Abstract

In this study, hyperbranched polyamidoamine (PAMAM) was grown up onto the surface of polyvinyl amine-functionalized SBA-15 (PVAm/SBA-15) by a divergent method, without using organosilane precursors. Because of the surface modification of the PVAm/SBA-15 with the PAMAM hyperbranched polymer, the obtained hybrid material is able to trap water soluble metal ions such as Ni²⁺ via complex formation of PAMAM with metal ions. The reduction of trapped nickel ions in the hyperpolymeric shell of the PAMAM–PVAm/SBA-15 by sodium borohydride led to immobilized nickel nanoparticles in the structure of the composite. The obtained nanocomposite was effectively employed as a heterogeneous catalyst for the reduction of aromatic nitro compounds in the presence of NaBH₄ as a reducing agent. The physical and chemical properties of the Ni nanoparticle-PAMAM–PVAm/SBA-15 were investigated by XRD, FT-IR, BET, TEM and XPS. The catalyst showed excellent activity for the reduction of a number of aromatic nitro compounds in water at room temperature and could be reused ten times without any significant loss of activity.

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1. Introduction

The reduction of nitro compounds to amino compounds is industrially important as the amines obtained are commercially important starting materials and intermediates for a number of valuable compounds such as pharmaceuticals, agrochemicals and dyes.^1,2 In this regard, a variety of reducing systems and catalysts has been used for the catalytic reduction of nitro compounds.^3–8 However, in spite of the availability of the numerous methodologies, there is still strong need to develop new, green catalysts that should be efficient, safe, involve an easy work-up, and afford greater yields in shorter reaction times.^9,10

Metal nanoparticles have been under intensive study for applications in a wide variety of areas as they exhibit unique electronic, optical and catalytic properties.^11–14 Among them, nickel nanoparticles have received a lot of attention, due to being cheap and needing mild reaction conditions for high yields of products, in short reaction times.¹⁵ Ni nanoparticles are also greener, as compared to the traditional Raney^® nickel catalyst in respect of producing undesired toxic waste.¹⁵ However, apart from all these advantages, a key challenge in the application of these materials is the prevention of the agglomeration of the nanoparticles, which can be overcome through surface functionalization/stabilization.¹⁶ Therefore, catalytic nanoparticles are usually stabilized by stabilizers or immobilized on various supports, such as alumina, metal oxides, carbon, or polymeric materials, including polystyrene microspheres, polyelectrolyte brushes and dendrimers.^17,18

Polyamidoamine (PAMAM) dendrimers are highly branched, well-defined, synthetic macromolecules available in nanometer dimensions. They have a shape like a tree, which can behave as molecular boxes for entrapping and stabilizing metal nanoparticles.¹⁹ In addition, since PAMAM dendrimers are soft adsorbents, which permit the passage of the reactants and products of the catalytic reactions, they can be used as supports for metal nanoparticle catalysts.^20–22 Although the dendrimer-encapsulated noble metal nanoparticles show uniquely catalytic characteristics, the difficulties in separation and recycling limit the application of such catalytic materials. To overcome this problem, dendrimers were grafted onto silica-based supports and used as anchors for the metal complexes. Such dendrimer functionalized silica supports combine the advantages of the dendrimers (high density of active sites) with the ease of separation of the solid-phase catalysts.^23–26 However, in most cases, the incorporation of dendrimers into mesoporous silica and other inorganic materials are performed with the organosilica precursors.^27–31 Although they extensively improve the hydrophobicity of the mesoporous silica material, the organosilane precursors either involve complicated synthesis and purification methods, or are very expensive and toxic. Therefore, from both the environmental and economic points of view, using organic–inorganic hybrid composites, instead of organoalkoxysilane compounds as supports for the supporting of dendrimers is highly desirable.

In continuing our efforts towards the development of efficient and environmentally benign heterogeneous catalysts,^32–34 herein, hyperbranched polyamidoamine–polyvinylamine/SBA-15 containing Ni nanoparticles was prepared as a heterogeneous catalyst, without using organosilane precursors. The obtained catalyst was used as an efficient heterogeneous catalyst for the rapid reduction of aromatic nitro compounds to the corresponding aromatic amino compounds in the presence of sodium borohydride (NaBH₄) as a mild reducing agent. In addition, the catalytic activity of this hybrid hyperbranched polymeric catalyst in the reduction reaction was compared with Ni nanoparticle-polyvinylamine/SBA-15, which was reported in our previous work,³² in order to investigate the effect of hyperbranched polymer material on the activity and stability of the catalyst.

2. Experimental

2.1. Chemicals supply

All the chemicals were obtained from Sigma-Aldrich and Merck, and were used without further purification.

2.2. Instruments and characterization

The samples were analyzed using FT-IR spectroscopy (using a Perkin Elmer 65 in a KBr matrix in the range 4000–400 cm⁻¹). The BET specific surface areas and BJH pore size distribution of the samples were determined by the adsorption–desorption of nitrogen at the liquid nitrogen temperature, using a Series BEL SORP 18. The X-ray powder diffraction (XRD) of the catalyst was carried out on a Bruker D8Advance X-ray diffractometer, using nickel filtered Cu K_α radiation at 40 kV and 20 mA. The scanning electron microscope (SEM) studies were performed on a Philips, XL30, SE detector. The transmission electron microscope (TEM) observations were performed on a Phillips CM10 microscope at an accelerating voltage of 150 kV, in order to obtain information on the size of the Ni nanoparticles and the X-ray photoelectron spectra (XPS) were recorded on an ESCA SSX-100 (Shimadzu) using a non-monochromatized Mg K_α X-ray as the excitation source.

The products were characterized by ¹H NMR and ¹³C NMR spectroscopy (Bruker DRX-500 Avance spectrometer at 400 and 100 MHz, respectively). The melting points were measured on an Electrothermal 9100 apparatus and they were uncorrected. All the products were known compounds and they were characterized by FT-IR, ¹H NMR and ¹³C NMR. All the melting points were compared satisfactorily with those reported in the literature.

2.3. Catalyst preparation

2.3.1. Preparation of polyvinylamine/SBA-15 composite. Mesoporous silica SBA-15 was prepared through the method described in the literature recently.³⁵

The polyvinylamine/SBA-15 (PVAm/SBA-15) composite was synthesized in two steps without using any organosilica precursors via an in situ polymerization method.³⁶ At first, SBA-15 (0.5 g) and acrylamide (0.25 g) in THF (7 mL) were placed in a round bottom flask. Benzoyl peroxide (0.025 g) was added and the mixture was heated to 70–75 °C for 5 h, while being stirred. The solvent was removed and the precipitate was dried at 60 °C overnight under a vacuum to gain 0.7 g of a white powder of a polyacrylamide/SBA-15 (PAA/SBA-15) composite. In the second step, Hoffmann degradation on the PAA/SBA-15 was carried out by Ca(OCl)₂ in order to prepare PVAm/SBA-15 composite (conversion of the amide groups into amines).³⁶ PAA/SBA-15 (0.1 g), H₂O (15 mL) and Ca(OCl)₂ (0.4 g) were placed into a round bottom flask and refluxed for 6 h under vigorous stirring. Then, the mixture was filtered, washed with water, then n-hexane and finally dried at 60 °C overnight under a vacuum, to yield the yellow PVAm/SBA-15 composite (the reaction of calcium dihypochlorite (Ca(OCl)₂) with polyacrylamide, transforms the primary amide into an isocyanate intermediate. Then, the isocyanate intermediate is hydrolyzed to a primary amine, giving off carbon dioxide).

2.3.2. Preparation of hyperbranched polyamidoamine–polyvinylamine/SBA-15 (PAMAM–PVAm/SBA-15). The grafting reaction and propagation of hyperbranched polyamidoamine from the PVAm/SBA-15 surface was achieved in two steps:^37,38 (1) Michael addition of methyl acrylate (MA) to the amino groups on the surface; and (2) amidation of the terminal ester groups by ethylenediamine (EDA).

The Michael addition of MA to the amino groups on the surface was carried out as follows. At first, 0.42 g of PVAm/SBA-15, 7 mL of methanol and 0.84 g (9.7 mmol) of MA were placed in a round bottom flask. Then, the mixture was stirred with a magnetic stirrer at 50 °C. After 24 h, the resulting powder (MA–PVAm/SBA-15) was separated, washed several times with methanol and dried at room temperature (Scheme 1a).

Scheme 1 Systematic procedures for the preparation of hyperbranched PAMAM polymer supported on SBA-15.

The amidation of the resulting terminal ester groups of the composite was carried out as follows. At first, the obtained MA–PVAm/SBA-15 powder, 7 mL of methanol and 0.84 g (13.9 mmol) of EDA were placed in a round bottom flask. Then, the mixture was stirred with a magnetic stirrer at 50 °C. After 24 h, the resulting powder (EDA–MA–PVAm/SBA-15) was separated, washed several times with methanol and dried at room temperature (Scheme 1b).

The two reactions of Michael addition and amidation were repeated for the propagation of the hyperbranched polyamidoamine from the PVAm/SBA-15 surface, in order to yield PAMAM–PVAm/SBA-15 (Scheme 1c).

2.3.3. Preparation of Ni nanoparticle-hyperbranched polyamidoamine–polyvinylamine/SBA-15. 1 mL aqueous solution of NiCl₂·6H₂O (1 M) was added to the obtained PAMAM–PVAm/SBA-15 (0.1 g) together with 3 mL of H₂O and the solution pH was adjusted to 3, by the addition of 0.1 M HCl aqueous solution. The mixture was stirred at 80 °C for 5 h. Then, a solution of NaBH₄ (0.23 g, 6 mmol) dissolved in 5 mL methanol was added to the mixture drop by drop over 20–30 min. After that, the solution was stirred for 3 h. Then, the addition of the same amount of NaBH₄ was repeated and again the mixture was stirred for 3 h. Afterwards, the suspension was filtered, washed sequentially with deionized water and methanol to remove excess NaBH₄ and NiCl₂ and dried at room temperature to obtain the Ni nanoparticle–hyperbranched polyamidoamine–polyvinylamine (Ni–PAMAM–PVAm/SBA-15).

The Ni content of the catalyst was estimated by decomposing a known amount of the catalyst by perchloric acid, nitric acid, fluoric acid and hydrochloric acid and the Ni content was measured by an atomic absorption spectrometer. The Ni content of the catalyst estimated by the atomic absorption spectrometer was 2.29 mmol g⁻¹.

2.4. General procedure for the reduction of aromatic nitro compounds

In a typical procedure, a mixture of nitrobenzene (2 mmol), NaBH₄ (6 mmol), Ni–PAMAM–PVAm/SBA-15 (0.1 g) and water (3 mL) was placed in a 25 mL round bottom flask. The suspension was stirred at room temperature. Completion of the reaction was monitored by thin layer chromatography (TLC), using n-hexane–ethyl acetate (15 : 5) as an eluent. After the completion of the reaction, the catalyst was removed from the reaction mixture by filtration, and aniline was extracted from the aqueous medium by ether (3 × 5 mL) as an organic phase. Then, the solvent was removed under reduced pressure. The crude product was purified by flash column chromatography (hexane or hexane–ethyl acetate) to afford the desired product. The products were identified with ¹H-NMR, ¹³C-NMR and FT-IR spectroscopy techniques.

3. Results and discussion

3.1. Catalyst characterization

The powder X-ray diffraction patterns of the mesoporous silica SBA-15, PAMAM–PVAm/SBA-15 and Ni–PAMAM–PVAm/SBA-15 samples are shown in Fig. 1. Typically, the low angle diffraction pattern of purely siliceous SBA-15 exhibits three peaks at 2θ values of 0.5–3°, including one strong peak (100) and two weak peaks (110) and (200) (Fig. 1a)‚ which corresponds to a highly ordered hexagonal mesoporous silica framework.³⁹ The PAMAM–PVAm/SBA-15 and Ni–PAMAM–PVAm/SBA-15 samples show the same patterns at 2θ values of 0.5–3° which indicate that the long-range order of the SBA-15 framework is well retained after the immobilization of the nickel and polymer species (Fig. 1b and c).

Fig. 1 XRD patterns of (a) mesoporous silica SBA-15, (b) PAMAM–PVAm/SBA-15 (c), Ni–PAMAM–PVAm/SBA-15 (2θ = 0.5–5°), (d) Ni–PAMAM–PVAm/SBA-15 burned in 200 °C for 2 h (2θ = 15–70°) and (e) Ni–PAMAM–PVAm/SBA-15 burned in 400 °C for 2 h (2θ = 15–70°).

The wide-angle XRD pattern of the Ni–PAMAM–PVAm/SBA-15 nanocomposite (2θ = 15–60°) exhibited an amorphous pattern, which did not show any peak for nickel nanoparticles (data not shown). This may occur because of the homogeneity of the Ni–PAMAM–PVAm/SBA-15 nanocomposite, so the nickel nanoparticles cannot be seen as a distinct peak.

Therefore, in order to prove the existence of the Ni nanoparticles in the composite, the composite catalyst was exposed to high temperatures (200 °C and 400 °C). It was observed that the Ni nanoparticles display two characteristic peaks, which are mainly due to a change of the amorphous nickel to crystalline, because of heating (Fig. 1d and e).³² The XRD patterns of the Ni–PAMAM–PVAm/SBA-15 sample, which was burned at two temperatures (200 °C and 400 °C) can be seen in Fig. 1d and e, respectively. After burning of the samples, the XRD patterns of the residues show the characteristic peaks at 2θ = 44.4° and 51.9° which are attributed to the planes (111) and (200) of a fcc nickel nanoparticle.⁴⁰ The crystallite size of the Ni nanoparticles was also evaluated using the Scherrer equation for the (111) peak and was found to be approximately 8 nm in size. The size of the Ni nanoparticles determined by TEM analysis (average diameter of ∼2–4 nm) is more reliable than using the Scherrer formula in XRD analysis.

The FT-IR spectra of PVAm/SBA-15, EDA–MA–PVAm/SBA-15, PAMAM–PVAm/SBA-15 and Ni–PAMAM–PVAm/SBA-15 are shown in Fig. 2. The characteristic bands at around 1090, 810 and 450 cm⁻¹ may be assigned to the Si–O–Si asymmetrical stretching vibration, symmetrical stretching vibration and bending vibration, respectively, which are related to the SBA-15 structure (Fig. 2a–c). The existence of PVAm in the PVAm/SBA-15 composite is evidenced by the appearance of a typical PVAm vibration in the FT-IR spectrum (Fig. 2a). In the FT-IR spectrum of PVAm/SBA-15 (Fig. 2a), the new band at 1428 cm⁻¹ corresponds to the bending vibration absorption of the N–H bond. The absorption peaks at 1620 cm⁻¹ (shoulder) and 1426 cm⁻¹ correspond to the C=O stretching and N–H bending vibrations of amide groups, respectively, indicating the complete amidation of MA with EDA on the surface of the PVAm/SBA-15 (Fig. 2b). Furthermore, the characteristic band of an ester group, which is generally present at about 1730 cm⁻¹, is absent in the spectrum of the sample, suggesting that amidation has successfully occurred on the surface of PVAm/SBA-15 and the EDA–MA–PVAm/SBA-15 is obtained (Fig. 2b). The increase in the relative intensity of the amide and methylene peaks in the spectrum of PAMAM–PVAm/SBA-15 (Fig. 2c) in comparison to those of EDA–MA–PVAm/SBA-15 (Fig. 2b) indicates the propagation of hyperbranched polyamidoamine from the PVAm/SBA-15 surface and means that the hyperbranched polymer was successfully constructed on the surface of PVAm/SBA-15. It is also clear that the –NH₂ stretching bands in the composites, which are generally located at about 3100–3300 cm⁻¹, are overlapped by the OH stretching broad band (∼3430 cm⁻¹) of adsorbed water molecules (Fig. 2a–c). Moreover, the presence of peaks at around 2800–3000 cm⁻¹ corresponds to the aliphatic C–H stretching of the methylene groups in the samples (Fig. 2a–c). The characteristic band of a tertiary amino group, which generally appears at approximately 1030 cm⁻¹, was also overlapped by the Si–O–Si stretching band of SBA-15 (Fig 2b and c). Furthermore, the Ni–PAMAM–PVAm/SBA-15 spectrum (Fig. 2d) shows that the bending vibration absorption band of N–H at 1430 cm⁻¹ is shifted to lower wavenumbers (1430 → 1378 cm⁻¹), which is possibly due to the strong interaction of N groups with the metal nanoparticles. In addition, the band at 1615 cm⁻¹, which corresponds to carbonyl bonds of the hyperbranched polymer, is also shifted to lower wavenumbers (1615 → 1593 cm⁻¹) (Fig. 2d). These observations may be attributed to the strong interaction between the Ni nanoparticles and the functional groups in the structure of the supported polymer.³² According to the FT-IR analyses, it can be concluded that the hyperbranched polymer was constructed on the surface of the PVAm/SBA-15 and Ni–PAMAM–PVAm/SBA-15 was successfully prepared.

Fig. 2 FT-IR spectra of (a) PVAm/SBA-15, (b) EDA–MA–PVAm/SBA-15, (c) PAMAM–PVAm/SBA-15 and (d) Ni–PAMAM–PVAm/SBA-15.

The morphology and the distribution of the Ni nanoparticles in the Ni–PAMAM–PVAm/SBA-15 nanocomposite were studied by TEM observations (Fig. 3). Looking at the TEM images, it can be seen that although they are smeared and dark after the encapsulation of the hyperbranched polymer and Ni nanoparticles, the pore structure of SBA-15 is retained and no damage of the framework is observed (Fig 3a and b). Moreover, Fig. 3c clearly shows the uniform dispersion of Ni nanoparticles mainly inside the SBA-15 channels. The places with a darker contrast can be assigned to the presence of Ni nanoparticles with diameters in the range 2–4 nm (Fig. 3c). The TEM images (Fig. 3a–c) depict that the hyperbranched polymer can effectively entrap and stabilize Ni nanoparticles in a relatively uniform dispersion.

Fig. 3 TEM images of Ni–PAMAM–PVAm/SBA-15.

The specific surface area, pore volume and the pore size of the mesoporous silica SBA-15, PVAm/SBA-15, PAMAM–PVAm/SBA-15 and Ni–PAMAM–PVAm/SBA-15 samples are summarized in Table 1. It is clear that the PAMAM–PVAm/SBA-15 exhibits a smaller specific surface area, and pore volume in comparison to those of pure SBA-15 (Table 1), suggesting the successful incorporation of the hyperbranched PAMAM–PVAm polymer into the internal pores of SBA-15. However, there is an increase in the pore diameter for the PAMAM–PVAm/SBA-15 in comparison to PVAm/SBA-15 and SBA-15, respectively. It is due to the incorporation and growth of hyperbranched polymers inside the pores of the SBA-15. This incorporation can produce some pressure on the mesoporous structures (physical pressure on the wall of the channels) and it can increase the pore diameters. After adsorption of NiCl₂ and the subsequent reduction, the specific surface area, pore volume and pore size further decrease, suggesting that Ni nanoparticles are located inside the pores of the SBA-15. Although there are significant decreases in the surface area, the Ni–PAMAM–PVAm/SBA-15 pores were not blocked by deposition of the hyperbranched polymer and Ni nanoparticles. Furthermore, the BJH pore size distribution curve of the Ni–PAMAM–PVAm/SBA-15 exhibited a narrow pore size distribution (data not shown) which means that the Ni nanoparticles are well distributed on the channels of the PAMAM–PVAm/SBA-15. This result is in good agreement with that obtained from the TEM analysis and shows the effective role of the hyperbranched polymer to entrap and uniformly disperse Ni nanoparticles.

Table 1 Surface area analyses of samples

Sample	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
Mesoporous silica SBA-15	1430	1.9	9.9
PVAm/SBA-15	856	1.1	8.8
PAMAM–PVAm/SBA-15	81.6	0.23	11.6
Ni–PAMAM–PVAm/SBA-15	53.9	0.1	6.5

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In order to investigate the state of the Ni in the nanocomposite, an XPS study was carried out on the Ni–PAMAM–PVAm/SBA-15 catalyst (Fig. 4). The Ni 2p spectrum exhibits two main peaks at 851.8 and 868.4 eV, which are attributed to Ni(0) 2p_3/2 and Ni(0) 2p_1/2, respectively (Fig. 4). It means that most of the Ni species are stable as the metallic state in the PAMAM–PVAm/SBA-15 media. Furthermore, the Ni(0) 2p_3/2 and 2p_1/2 binding energy of 851.8 and 868.4 eV shown in Fig. 4 are slightly lower than those of typical Ni(0) nanoparticles (852.3 eV for Ni 2p_3/2 and 869.7 eV for Ni 2p_1/2 (ref. 41)). This may be due to the interaction between the N atoms of the hyperbranched polymer and Ni nanoparticles, which causes an increase in the charge density around Ni⁰ and a decrease in the binding energy. In general, the binding energy of Ni 2p is sensitive to the surrounding chemical environment, which means that when Ni interacts with polymers, the binding energy will decrease slightly.

Fig. 4 XPS spectrum of Ni 2p of Ni–PAMAM–PVAm/SBA-15.

The full XPS spectrum of Ni–PAMAM–PVAm/SBA-15 shows the peaks of carbon, oxygen, silicium and nickel. The silicium peaks correspond to the mesoporous silica SBA-15 structure and the carbon is related to the hyperbranched polymer structure (Fig. 5).

Fig. 5 Full XPS spectrum of Ni–PAMAM–PVAm/SBA-15.

3.2. Catalytic activity

This heterogeneous hyperbranched polymeric nanocomposite (Ni–PAMAM–PVAm/SBA-15) was prepared by an inexpensive method, without using any expensive organosilane or bridged organosilane compounds (the organosilica precursors either involve complicated synthesis and purification method or are very expensive). The main goal of this catalytic synthesis was to compare this hybrid catalyst in the reduction reaction with Ni–polyvinylamine/SBA-15, which was prepared in our previous work,³² in order to investigate the effect of the hyperbranched polymer material on the activity and stability of the catalyst and to expand the use of these types of hyperbranched polymeric nanocomposite for organic reactions. In this regard, the reduction of nitrobenzene was chosen as a model reaction to test the catalytic activity of Ni–PAMAM–PVAm/SBA-15 for the reduction of aromatic nitro compounds. This reduction reaction was carried out in water, as a green solvent, at room temperature.

To identify the effect of the nickel nanoparticle concentration on the reduction reaction, different amounts of NiCl₂·6H₂O (0 mmol to 2.0 mmol) were employed for the preparation of the nanocomposite and the nickel content of the prepared catalyst was estimated using atomic absorption spectroscopy. As shown in Table 2, the catalytic activity was improved by increasing the nickel content of the catalyst and the amount of 1.0 mmol of NiCl₂·6H₂O shows the best catalytic activity. Moreover, by further increasing the amount of NiCl₂·6H₂O (1.5 mmol), the nickel content of catalyst was not changed and the reaction yield remained constant, which means that all sites of the support are occupied by nickel species and no more site exists for the adsorption of more nickel species. As the catalytic reaction mechanism involves a Ni nanoparticle mediated electron transfer from BH₄⁻ ion to the nitro compounds, the amount of H⁻ sites on the catalyst surface increased by increasing the nickel nanoparticles, and a larger amount of hydride can be transferred to the nitro compounds, through the catalyst. Therefore, higher concentrations of Ni nanoparticles on the catalyst provide more sites for the adsorption of hydride ions and cause an increase in the catalytic performance. According to the results, 1.0 mmol of NiCl₂·6H₂O was chosen as an optimized amount for the preparation of the catalyst.

Table 2 Effect of NiCl₂·6H₂O molar ratio on the catalytic activity^a

NiCl2·6H2O (mmol)	Ni content of catalyst^b (mmol in 0.1 g catalyst)	Yield^c (%)
a Reaction conditions: nitrobenzene (2 mmol), Ni–PAMAM–PVAm/SBA-15 (0.1 g), H2O (3 mL), NaBH4 to NiCl2 molar ratio = 6, room temperature, reaction time = 10 min.b Estimated by AAS.c Isolated yield.
0	—	—
0.6	0.135	85
1	0.229	98
1.5	0.229	98
2	0.197	80

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The effect of the NaBH₄ amount on the reduction reaction was investigated using 0.1 g of the catalyst and 2 mmol of nitrobenzene. The results show that by increasing the amount of NaBH₄ from 4 mmol to 6 mmol, the yield of the reaction also rises (Table 3), while with a further increase in the NaBH₄ amount, the yield remains constant. Therefore, 6 mmol of NaBH₄ was chosen as the optimum amount of the reducing agent for the further steps.

Table 3 Effect of reducing agent on the nitrobenzene reduction^a

NaBH4 (mmol)	Reaction time (min)	Yield^b (%)
a Reaction conditions: nitrobenzene (2 mmol), Ni–PAMAM–PVAm/SBA-15 (0.1 g), H2O (3 mL), room temperature.b Isolated yield.
4	10	80
6	10	98
8	10	98

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The reduction reaction of nitrobenzene with NaBH₄ over different amounts of the Ni–PAMAM–PVAm/SBA-15 was investigated (Table 4). It was observed that while the amount of catalyst increased from 0 to 0.1 g, the product yield rose significantly from 0% to 98%, which is probably due to the availability of more active sites (Ni nanoparticles). After that, the yield remained stable between 0.1 g and 0.12 g. Therefore, 0.1 g was chosen as the optimized amount of catalyst for the further steps.

Table 4 Effect of catalyst amount on the nitrobenzene reduction^a

Amount of catalyst (g)	Reaction time (min)	Yield^b (%)
a Reaction conditions: nitrobenzene (2 mmol), catalyst (Ni–PAMAM–PVAm/SBA-15), NaBH4 (6 mmol), H2O (3 mL), room temperature.b Isolated yield.
0	120	—
0.08	60	90
0.1	10	98
0.12	10	98

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In order to have a better understanding of the intermediates involved in the reduction of nitro aromatic compounds over Ni–PAMAM–PVAm/SBA-15, LC/MS analysis was carried out for the optimized reaction of nitrobenzene. In fact, almost no intermediates, nitrosobenzene and phenylhydroxylamine were detected during the reaction process, indicating that this catalytic hydrogenation can be considered as a one-step reaction. The main reason for this should be ascribed to the fact that the Ni nanoparticles are uniformly dispersed onto the surface of the PAMAM–PVAm/SBA-15 support, which significantly enhances the accessibility of the reactants to the active sites. It causes the reaction to run faster and makes the catalytic hydrogenation a one-step reaction.

In further studies, the reductions of different nitro aromatic compounds were carried out over the Ni–PAMAM–PVAm/SBA-15, after ascertaining the optimum experimental conditions. The results are summarized in Table 5. In all cases, the reduction reactions proceeded rapidly to give the corresponding aniline derivatives in excellent yield/selectivity and short reaction times.

Table 5 Reduction of various nitro aromatic compounds over Ni–PAMAM–PVAm/SBA-15^a

Entry	Substrate	Product	Time (min)	Yield^b (%)
a Reaction conditions: nitro aromatic compound (2 mmol), Ni–PAMAM–PVAm/SBA-15 (0.1 g), NaBH4 (6 mmol), H2O (3 mL), room temperature.b Isolated yield.
1			10	98
2			Immediately	98
3			Immediately	98
4			Immediately	98
5			Immediately	98
6			30	98
7			5	95

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Another important issue concerning the application of a heterogeneous catalyst is its reusability and stability under the reaction conditions. To gain an insight into this issue, catalyst recycling experiments were carried out, using the reduction of nitrobenzene over Ni–PAMAM–PVAm/SBA-15. The results are represented in Table 6. After each cycle, the catalyst was filtered off, and washed with water (10 mL) and diethyl ether (5 mL). Then, it was dried in an oven at 60 °C and reused in the subsequent run. The results show that the Ni–PAMAM–PVAm/SBA-15 could be reused 10 times without any modification, and no significant loss of activity/selectivity performance was observed. Moreover, there is very low Ni leaching (about 3%) during the reaction and the catalyst exhibits a high stability, even after 10 recycles (Table 6). In addition, the low angle XRD pattern of the used catalyst after ten runs showed that the ordered structure of the catalyst was maintained (Fig. 6).

Table 6 The catalyst reusability for the reduction reaction^a

Cycle	Ni content of catalyst^b (mmol in 0.1 g catalyst)	Yield^c (%)
a Reaction conditions: nitrobenzene (2 mmol), Ni–PVAm/SBA-15 (0.1 g), H2O (3 mL), NaBH4 (6 mmol), room temperature, reaction time = 10 min.b Estimated by AAS.c Isolated yield.
Fresh	0.229	98
1	0.229	98
2	0.229	98
3	0.228	98
4	0.228	98
5	0.226	98
6	0.226	97
7	0.225	97
8	0.224	96
9	0.223	94
10	0.222	93

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Fig. 6 Low angle XRD pattern of Ni–PAMAM–PVAm/SBA-15 after 10 cycles of the reaction.

In order to prove the heterogeneous nature of the catalyst, a heterogeneity test was performed for the optimized reduction reaction of nitrobenzene, in which the catalyst was separated from the reaction mixture at approximately a 50% conversion of the starting material through centrifugation. The reaction progress in the filtrate was then monitored. No further oxidation reaction occurred even at extended times, indicating that the nature of reaction process is heterogeneous and there is not any reaction progress in the homogeneous phase.

The catalytic activity of the Ni–PAMAM–PVAm/SBA-15 in the reduction of nitro aromatic compounds was compared with Ni–PVAm/SBA-15, which was reported in our previous work.³² The results are summarized in Table 7. As can be seen, compared to Ni–PVAm/SBA-15, the Ni–PAMAM–PVAm/SBA-15 depicts shorter reaction time, higher loaded nickel nanoparticles and lower amounts of catalyst and reducing agent in the reduction reaction. The turn-over frequency (TOF) value indicates that the Ni–PAMAM–PVAm/SBA-15 is a more active catalyst for this kind of reaction. The TOF is defined as the mol product per mol catalyst per hour and this was calculated from the isolated yield, the amount of nickel used and the reaction time. Moreover, by comparing the reusability of the catalysts, it is obvious that the Ni–PAMAM–PVAm/SBA-15 shows a higher stability and reusability for the reduction reaction. It may be related to the higher number of reachable amine sites in the Ni–PAMAM–PVAm/SBA-15 in comparison to the Ni–PVAm/SBA-15. Since there are more basic sites for the coordination of nickel nanoparticles in the structure of the hyperbranched polymeric support (Ni–PAMAM–PVAm/SBA-15) than the previous reported catalyst (Ni–PVAm/SBA-15), higher loaded nickel nanoparticles, lower leaching of nickel nanoparticles and consequently higher activity and stability for the Ni–PAMAM–PVAm/SBA-15 can be expected.

Table 7 Comparison to catalytic activity of Ni–PAMAM–PVAm/SBA-15 and Ni–PVAm/SBA-15 in the reduction reaction of nitrobenzene^a

Catalyst	Time (min)	Yield^c (%)	TOF^d (h^−1)	Reusability	Ref.
a Reaction conditions: nitrobenzene (2 mmol), catalyst (0.1 g), H2O (3 mL), NaBH4 (6 mmol), room temperature.b Reaction conditions: nitrobenzene (2 mmol), catalyst (0.12 g), H2O (3 mL), NaBH4 (8 mmol), room temperature.c Isolated yield.d Turn-over frequency.
Ni–PAMAM–PVAm/SBA-15^a	10	98	51.35	10 cycles	This work
Ni–PVAm/SBA-15^b	20	98	29.52	5 cycles	32

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4. Conclusion

In conclusion, a hyperbranched PAMAM polymer was grown onto the surface of polyvinylamine/SBA-15 (prepared without using any expensive organosilane or bridged organosilane compounds) by a divergent method. Detailed characterization techniques showed that the hyperbranched polymer forms both on the surface of the support and inside the mesoporous channels. The PAMAM–PVAm/SBA-15 hybrid material was effectively employed as a scaffold for the in situ generation of Ni nanoparticles. Instrumental analysis confirmed that there is a strong interaction between the nickel nanoparticles and the functional groups in the structure of the polymeric support. The Ni–PAMAM–PVAm/SBA-15 nanocomposite was found to be a highly efficient and reusable catalyst in the reduction of various nitro aromatic compounds with short reaction times and high yields. Moreover, the catalyst could be easily recovered and reused ten times without any significant loss of the activity/selectivity performance. In addition, it was shown that the reported heterogeneous catalyst (Ni–PAMAM–PVAm/SBA-15) based on a hyperbranched polymeric support, exhibited a higher activity and stability compared with Ni–PVAm/SBA-15, which is possibly due to the existence of more basic sites in the PAMAM–PVAm/SBA-15 structure, compared to the PVAm/SBA-15 composite. These unique results open new perspectives for the application of these types of hyperbranched polymeric heterogeneous nanocomposites in other organic reactions.

Acknowledgements

The support by the Islamic Azad University, Shahreza Branch (IAUSH) Research Council and the Center of Excellence in Chemistry is gratefully acknowledged.

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