Introduction

Pursuing sustainable and environmentally friendly methods in chemical synthesis is pivotal in contemporary organic chemistry. The imperative to minimize chemical waste and reduce organic pollutants underscores the principles of green chemistry1,2,3. Nitroaromatic compounds, known for their toxicity and environmental hazards, represent a significant challenge. In contrast, their reduction to amino derivatives is essential in various industrial sectors, including pharmaceuticals and agrochemicals4,5,6,7.

The progress of efficient and sustainable catalysts for reducing nitroarenes has recently garnered considerable attention8,9,10. Although effective, traditional methods, such as the Béchamp reduction using iron or ferrous salts, suffer from drawbacks, including the generation of substantial chemical waste and reliance on mineral acids11. In contrast, catalytic hydrogenation using transition metals has emerged as a promising alternative12,13,14,15.

Palladium catalysts have been extensively studied and utilized to reduce nitroaromatics due to their high activity. However, their widespread application is hindered by the overpriced and poisonous associated with Pd and its derivatives16,17,18,19,20. In response, there is a growing interest in exploring cost-effective and less toxic alternatives, such as nickel-based catalysts, which offer comparable reactivity and are more abundant21,22,23,24,25,26.

Recently, drastic notice has been given to using magnetite nanoparticles (Fe3O4) as excellent recyclable support for transition metals, such as palladium27, copper28, and nickel29. The high chemical stability, large surface area, simple separation by an external permanent magnet and recovery, non-toxicity, readily accessible, low cost, and environmental compatibility are some prominent advantages of magnetic nanoparticle catalysts. Also, the heterogeneous catalysts particularly supported catalysts, have been given significant attention lately30,31,32,33.

Among the various supramolecular catalysts, β-cyclodextrins (β-CDs) have gained prominence due to their environmentally benign nature and unique molecular structure. β-CDs are cyclic oligosaccharides derived from the enzymatic conversion of starch, characterized by a hydrophobic central hollow and a hydrophilic exterior cover. β-CDs can create inclusion complexes with a wide range of guest molecules without forming covalent bonds, thereby serving as versatile and biocompatible materials34,35,36,37,38. The use of β-CDs in catalysis has been widely explored across different fields, highlighting their potential in sustainable chemical transformations39,40,41,42,43,44,45,46,47.

Reducing nitroaromatic compounds to less toxic amines using β-CD-based catalysts represents a significant advancement in green chemistry9,10,48,49,50,51. Recently, Hasan et al. reported the synthesis of a heterogeneous catalyst, Fe3O4@β-CD@Pd, demonstrating promising catalytic activity for nitroarene reduction under mild conditions52. Inspired by these advances, our study focuses on developing and identifying a new catalyst system based on nickel nanoparticles supported on β-CD grafted magnetic nanoparticles of Fe3O4 (Ni@β-CD@Fe3O4).

This research addresses current limitations associated with existing catalysts, such as high cost and toxicity. As a transition metal, nickel offers a cost-effective and environmentally benign alternative to palladium, while β-CDs provide a robust and eco-friendly support matrix. Our catalyst aims to combine the high reactivity of nickel with the advantageous properties of β-CDs and magnetic nanoparticles (Fe3O4), enabling efficient catalytic conversion of nitroarenes into their corresponding amines under aqueous conditions.

The importance of current work depends on its potential contributions to sustainable chemistry, where reducing toxic nitroaromatic pollutants using a recyclable and efficient catalyst system aligns with the principles of green chemistry. By leveraging the synergistic effects of nickel nanoparticles and β-CDs grafted onto magnetic nanoparticles, we anticipate achieving high catalytic activity, selectivity, and recyclability, paving the way for practical applications in pharmaceutical and fine chemical industries.

This introduction outlines the critical importance of developing green and sustainable catalytic systems for organic transformations. It provides a rationale for exploring novel catalysts based on nickel nanoparticles supported on β-CD grafted magnetic nanoparticles of Fe3O4 to reduce nitroarenes, highlighting the potential benefits over existing methodologies. The subsequent sections will detail the experimental methods, characterization techniques, and results obtained, culminating in a comprehensive evaluation of the catalytic performance and prospects for future research directions.

Results and discussion

β-CD, as a green renewable cyclic oligosaccharide and inexpensive chemically stable available material, has excellent potential to be used as a support and ligand to stabilize metal nanoparticles. In this regard, our recent review explained the advances in β-CD-based catalysts for reducing toxic nitroaromatic10. This paper shows further significant applications of the β-CD-supported nickel nanoparticles for reducing toxic nitroaromatic using low nickel loading at 70 °C in water.

The stepwise synthesis pathway of the catalyst is shown in Fig. 1. Fe3O4 nanoparticles were made ready based on the strategy of Massart employing FeCl3·6H2O and FeCl2·4H2O53. The β-CD was sonicated with magnetic nanoparticles at 70 °C overnight to produce β-CD@Fe3O4 NPs. The Ni@β-CD@Fe3O4 was gained quickly by dissolving Ni(OAc)2 in distilled water and adding β-CD@Fe3O4, also dissolved in H2O by sonication, at 70 oC for one day.

Fig. 1
figure 1

Preparation steps of Ni@β-CD@Fe3O4 NPs.

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Characterization

FT-IR analysis

The obtained Ni@β-CD@Fe3O4 nanoparticles were identified utilizing the FT-IR technique, which is displayed in Fig. 2. The spectrum of β-CD indicated the absorption bands at ν = 3000–3900 cm− 1 that belong to the stretching vibrations of the hydroxyl group and peak at ν = 1157 cm− 1 relevant to asymmetric glycosidic C–O–C vibration.

The band at 570 cm− 1 corresponds to O–Fe stretch and strong band absorption at 3400 cm− 1 in accordance with –OH stretching vibration for magnetic nanoparticle bonding. For Ni@β-CD@Fe3O4, spectra were found to be narrowed in the FT-IR spectrum after being bonded with Ni(OAC)2, which is suitable proof of the formation of the inclusion complex and did not display a carbonyl group absorption band, which verifies that the acetate anion has been deleted after the process of reduction of Ni(II) to Ni(0). Here, we didn’t use a reducer agent. Based on our previous works with β-CD and nickel, β-CD acts as a reducer agent and reduces Ni(II) to Ni(0). Ni(0) species is generated in the presence of β-CD. We assume that the role of β-CD is to stabilize the active catalyst44,45. The FT-IR results prove that magnetite nanoparticle functionalization has been done successfully.

Fig. 2
figure 2

FT-IR spectra of β-CD (a), Fe3O4 (b), and Ni@β-CD@Fe3O4 (c).

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FE-SEM and TEM analysis

The FE-SEM image of the Ni@β-CD@Fe3O4 indicated the construction of mono-dispersed and uniform spherical particles (Fig. 3a–c). The TEM image in Fig. 3d indicates that the Ni@β-CD@Fe3O4 NPs are almost spherical with an average size of approximately 10–20 nanometers. The boundary between the particles (low contrast) was not detectable in the TEM of the Ni@β-CD@Fe3O4, which may be due to interactions between the neighboring particles. No significant layer of Ni@β-CD could be observed on these nanoparticles54.

Fig. 3
figure 3

FE-SEM images of Ni@β-CD@Fe3O4 in different magnifications (a–c), TEM of Ni@β-CD@Fe3O4 (d), and Fe3O4 (e).

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EDX analysis

The energy-dispersive spectrum (EDX) obtained from FE-SEM analysis proved the presence of diverse elements in the material’s structure, like Ni, Fe, O, and C species (Fig. 4). Quantitative results of EDX data are shown in Table 1.

Fig. 4
figure 4

EDX spectrum of Ni@β-CD@Fe3O4.

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Table 1 Quantitative results of EDX data.
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XRD analysis

Powder X-ray diffraction (XRD) study of Ni@β-CD@Fe3O4 displayed peaks at 2θ values 2θ = 30.3°, 35.7°, 37.3°, 43.3°, 53.8°, 57.4°, and 63.0° are relevant to (220), (311), (222), (400), (511), and (400) planes of NiFe2O4 structure (JCPDS card no. 54–0964). (Fig. 5)

Fig. 5
figure 5

XRD pattern Fe3O4 and Ni@β-CD@Fe3O4 NPs.

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VSM analysis

The magnetic features of this nanocatalyst were analyzed with a VSM. The superparamagnetic character of Ni@β-CD@Fe3O4, Fe3O4 NPs, and reused catalysts was also investigated. The observed results demonstrated that a decline in the magnetization value of the Ni@β-CD@Fe3O4 (50 emug− 1) in comparison with that of Fe3O4 NPs (64 emug− 1), which shows the magnetic property of Fe3O4 is reduced by coating and approves the modification of Fe3O4 NPs (Fig. 6). However, the Ms. Value of Ni@β-CD@Fe3O4 was sufficiently high, which can still be separated quickly from the solution using a magnet bar.

Fig. 6
figure 6

VSM image of Ni@β-CD@Fe3O4 NPs and recycle catalyst after five runs.

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TGA analysis

The TGA curve of Ni@β-CD@Fe3O4 nanoparticles (NPs) provides valuable insights into the composite material’s thermal stability and decomposition patterns (Fig. 7). The TGA profile reveals several distinct weight-loss stages corresponding to different thermal events within the sample.

Stage 1: Weight Loss Below 150 °C.

The initial weight loss of 1.37% observed below 150 °C is attributed to the evaporation of the sample’s physically adsorbed solvents and water molecules. This low-temperature weight loss is typical for many nanoparticle systems where residual solvents and moisture are present on the surface or within the material’s pores.

Stage 2: Weight Loss from 200 to 400 °C.

In the temperature range of 200 to 400 °C, a more significant weight loss of 4.17% is observed. This stage indicates the decomposition of organic components within the Ni@β-CD@Fe3O4 NPs. The β-CD component, an organic molecule, decomposes in this temperature range. The observed weight loss can be correlated with the thermal degradation of β-CD as it breaks down into more minor volatile compounds.

Stage 3: Weight Loss from 400 to 550 °C.

As the temperature increases from 400 to 550 °C, a second notable weight loss of 3.61% is recorded. This weight loss is predominantly associated with the further decomposition of β-CD and any remaining organic residues. The β-CD undergoes complete thermal degradation at these elevated temperatures, resulting in additional weight loss.

The TGA curve of Ni@β-CD@Fe3O4 NPs indicates a total weight loss of approximately 9.15% up to 550 °C. This weight loss is primarily owing to the decomposition of the β-CD component and the desorption of physically adsorbed solvents and water. The TGA data confirm the presence of β-CD in the composite material and estimate its thermal stability. The observed thermal degradation pattern is consistent with the expected behavior of β-CD-based nanocomposites, where initial weight loss corresponds to desorption of adsorbed species followed by decomposition of the organic matrix.

These thermal properties are essential for understanding the stability and potential applications of the Ni@β-CD@Fe3O4 NPs, especially in processes that involve elevated temperatures. The material’s thermal stability ensures its suitability for catalytic applications, where temperature resilience is often required.

Fig. 7
figure 7

Thermogravimetric analysis of β-CD (a), Fe3O4 (b), and Ni@β-CD@Fe3O4 (c).

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To study the performance of the prepared Ni@β-CD@Fe3O4 NPs as a nanocatalyst for the nitro reduction reactions, the hydrogenation of 4-nitrotoluene was selected as a pattern reaction, and the influence of different items like solvent, reducing agent, and quantity of catalyst were studied (Table 2). Various solvents were listed, and the results indicated that the amine product was provided with down-to-average yields (Table 2, entries 1–11). The influence of some reducing agents was also checked, and the results indicated that the role of sodium borohydride is crucial for this reaction (Table 2, entries 13–16). Reducing the quantity of nickel NPs to 1 mol% induced a notable drop in yields (Table 2, entry 17).

To investigate the catalytic performance of the synthesized Ni@β-CD@Fe3O4 NPs in nitro reduction reactions, we selected the reduction of 4-nitrotoluene as a pattern reaction. We evaluated the effects of diverse parameters on the reaction efficiency, including solvent, the reducing agent, and catalyst quantity (Table 2).

Optimization of reaction conditions

Nickel loading on the Ni(II)-β-CD complexes was 1.37 ppm, measured by atomic absorption spectroscopy (AAS). We began by screening different solvents for reducing 4-nitrotoluene using Ni@β-CD@Fe3O4 NPs (15 mg, 1.5 mol% of Ni) and NaBH4 (0.8 mmol) at 25 ℃. The solvents tested included DMF, THF, toluene, DMSO, dichloromethane, 1,4-dioxane, ethyl acetate, PEG200, ethanol, acetone, methanol, and water. The yields of the reduction product varied significantly, with water proving to be the most effective solvent, providing a 100% yield (Table 2, entry 12). Other solvents yielded lower efficiencies, with ethanol (71%), acetone (65%), and methanol (35%) following (Table 2, entries 9, 10, and 11).

Next, we explored the influence of different reducing agents on the reaction in water. The reducing agents tested were isopropyl alcohol, ammonium formate, glycerol, and hydrazine. NaBH4 was found to be essential for the reaction, with other agents resulting in no reaction or significantly lower yields (Table 2, entries 13–16).

We also examined the effect of reducing the catalyst quantity. Lowering the catalyst loading to 1 mol% led to a notable drop in yield to 42%, demonstrating the importance of using an adequate amount of catalyst for optimal performance (Table 2, entry 17).

Table 2 The reduction of 4-nitrotoluene under optimization reaction status.
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By having the optimized conditions, the reduction of a variety of nitroaromatic compounds was evaluated (Table 3). Results demonstrated that the hydrogenation of nitroarenes carrying electron-withdrawing groups like –F, –Br, –Cl, and –I performed successfully, and expected products were provided in high yields. The hydrogenation of nitroarenes containing electron-donating groups like –OH, –Me, –OMe, –CH2OH, and –NH2 was conducted, and the desired amines were afforded in good to superb yields (Table 3, entries 1–20).

Evaluation of catalytic performance

With the optimized reaction conditions established (water as the solvent, NaBH4 as the reducing agent, and 1.5 mol% catalyst loading), we proceeded to evaluate the catalytic performance of Ni@β-CD@Fe3O4 NPs for the reduction of diverse nitroaromatic compounds (Table 3).

Nitroarenes having electron-withdrawing groups (e.g., –F, –Br, –Cl, and –I) were efficiently reduced to their corresponding amines in high yields, showcasing the catalyst’s effectiveness (Table 3, entries 1–8). Similarly, nitroarenes with electron-donating groups (e.g., –NH2, –CH2OH, –OMe, and –Me) were also reduced effectively, yielding the desired amines in good to high yields (Table 3, entries 9–20).

Table 3 The reduction of diverse nitro compounds using Ni@β-CD@Fe3O4 NPs as a catalyst.
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Comparison of catalytic activity

A comparison of the results of the present system with the previous reports is shown in Table 4.

Table 4 A comparison of the results of the present system with the previous reports.
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Catalyst recycling

The hydrogenation of 4-nitrotoluene under the optimal reaction conditions was examined to evaluate the recoverability power of this nanocatalyst. For this purpose, in each cycle, the aqueous medium containing the nanocatalyst was transferred to a new flask and employed in the next run. The catalyst was recyclable five successive times with slight decreases in its catalytic activity (Fig. 8).

Fig. 8
figure 8

Recycling Ni@β-CD@Fe3O4 NPs catalyst in nitro aromatic reduction.

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FT-IR spectrum of the recycled nanocatalyst after five times indicated the conservation of the nanocatalyst (Fig. 9).

Fig. 9
figure 9

FT-IR photograph of recycled nanocatalyst five times.

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Also, the DRS-UV-Vis spectra of the recycle nanocatalyst after five runs indicated a similar pattern to that of the fresh catalyst (Fig. 10).

Fig. 10
figure 10

DRS-UV-Vis spectra of Ni@β-CD@Fe3O4 NPs after recyclability test five times.

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The VSM of Ni@β-CD@Fe3O4 reused the catalyst five times (Fig. 11).

Fig. 11
figure 11

VSM curves of Ni@β-CD@Fe3O4 and reused catalyst five runs.

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Experimental

Chemicals and instruments

Chemicals were provided by Merck, Across, and Sigma-Aldrich. Reactions were followed by GC on a Varian CP-3800 device. The progress of the nitro reduction reactions was monitored by Analytical TLC (silica gel). Infrared spectra were conducted using utilizing a Bruker Vector 22 spectrometer. The crystallographic structures of the catalyst were characterized by X-ray diffraction (XRD) using a Philips X’Pert Pro apparatus. The FE-SEM mapping and TEM images were captured with Philips CM30 and EOL JEM-2010, respectively. The magnetic feature was measured with a VSM at 25 ℃ from − 15,000 to + 15,000 Oe. TGA was conducted from 30 to 800 °C under a nitrogen flow rate of 20 mL min− 1 with a NETZSCH STA apparatus. The Varian SpectrAA 110 atomic absorption spectrometer was employed to measure the amount of nickel in the nanocatalyst. DRS-UV-Vis was examined on a Perkin-Elmer Lambda 25 apparatus.

Synthesis of Fe3O4 NPs

Fe3O4 nanoparticles were made ready in a co-precipitation according to Massart’s method by employing FeCl3·6H2O and FeCl2·4H2O. Fe3O4 nanoparticles were synthesized by the reaction of ferrous salts (FeCl2·4H2O and FeCl3·6H2O with a molar ratio of 1:2). Usually, FeCl3·6H2O (0.0216 mmol, 5.838 g) and FeCl2·4H2O (0.0108 mmol, 2.147 g) was solvated in distilled water (100 mL) under an Ar atmosphere and strong stirring. Next, ammonia (25%, 10 mL) was added up leisurely under an Ar atmosphere, and the resulting combination was stirred at 70 ℃ for 24 h. Adding the base to the Fe2+/Fe3+ salt solution led to the construction of nanoparticles that were removed with a magnet, washed several runs by EtOH and distilled H2O, and dried in an oven overnight to provide magnetic nanoparticles.

The preparation of β-CD@Fe3O4

A solution of β-CD (1 g) in deionized H2O (10 mL) was stirred at 25 ℃ for 15 min, and the mixture was moved to a flask including Fe3O4 NPs (500 mg in 10 mL of water) and the mixture sonicated for 15 min. Afterwards, this mixture was dispersed utilizing a mechanical stirrer at 70 °C for 24 h. The mixture was cooled to room temperature, and the attained material was centrifuged. The resulting β-CD@Fe3O4 was exposed to magnetic separation, and the gained material was rinsed with H2O/EtOH and dried under vacuum.

The preparation of Ni@β-CD@Fe3O4

β-CD@Fe3O4 (1 g) was sonicated in H2O (15 mL) for 15 min. In another flask, nickel acetate (300 mg) was sonicated and dissolved in deionized H2O (5 mL). The resulting mixture was added to the flask, including β-CD@Fe3O4, under an Ar atmosphere, and the mixture was stirred at 70 °C for one night. Next, the resultants were gathered employing a magnet and rinsed with water and EtOH to eliminate the unattached substrates and dry them under a vacuum at 60 °C.

Typical procedure for nitroarenes reduction

Nitroarene (0.2 mmol), NaBH4 (0.8 mmol), catalyst (15 mg, 1.5 mol% nickel), and solvent were added to a 5 mL flask, and the resulting solution was stirred for a proper time at 25 ℃. After completion of the reaction, the reaction mixture was extracted by ethyl acetate and purified using column or plate chromatography.

A plausible mechanism

Based on the previous report55, a suggested pathway was indicated in Fig. 12. The hydrogenation of the nitroaromatic material has happened through an H− transfer from sodium borohydride to the –NO2 group to yield a –NO group, which reacted with more H− to produce the amine derivatives.

Based on previous reports and our experimental results, a possible mechanism for the nitroaromatic reduction using the Ni@β-CD@Fe3O4 catalyst is proposed (Fig. 12). The mechanism involves the following steps:

  • Hydride transfer: The reduction process begins with the transfer of hydride ions from NaBH4 to the nitro group of the aromatic compound, converting it to a nitroso intermediate.

  • Formation of hydroxylamine: Subsequent hydride transfers further reduce the nitroso group to form a hydroxylamine intermediate.

  • Final reduction to amine: Additional hydride ions reduce the hydroxylamine to the final amine product.

Mechanistic insights:

Role of Ni@β-CD@Fe3O4: The Ni nanoparticles provide active sites for the adsorption and activation of the nitroarene and NaBH4, facilitating efficient hydride transfer.

Stabilization by β-CD: The β-CD grafted onto Fe3O4 enhances the dispersion and stability of Ni nanoparticles, ensuring consistent catalytic activity.

This proposed mechanism aligns with the observed high conversion rates and selectivity for amine products, underscoring the effectiveness of Ni@β-CD@Fe3O4 as a green and recyclable catalyst for nitroaromatic reductions.

These interpretations comprehensively understand the catalyst’s performance and underlying reduction mechanism, contributing to green chemistry and sustainable catalytic processes.

Fig. 12
figure 12

A suggested mechanism for the hydrogenation of nitrobenzene in the existence of Ni@β-CD@Fe3O4.

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Conclusion

This study successfully synthesized a novel, highly active, and separable heterogeneous catalyst by immobilizing nickel nanoparticles onto β-CD grafted magnetic Fe3O4 nanoparticles (Ni@β-CD@Fe3O4). This catalyst demonstrated exceptional catalytic activity and recyclability in reducing nitroaromatic compounds in aqueous media. The incorporation of β-CD provided a biocompatible, green, and non-toxic environment, enhancing the overall sustainability of the catalyst.

Key findings from our research include:

  1. 1.

    High catalytic efficiency: The Ni@β-CD@Fe3O4 catalyst effectively reduced various nitroarene derivatives, achieving high conversion rates and selectivities under mild conditions.

  2. 2.

    Recyclability and stability: The catalyst exhibited excellent stability and could be easily separated from the reaction mixture utilizing an external magnetic, allowing for multiple reuse cycles without significant activity loss.

  3. 3.

    Green chemistry principles: The synthesis and application of this catalyst align with green chemistry principles, emphasizing the use of non-toxic materials, energy efficiency, and waste reduction.

These results highlight the potential of Ni@β-CD@Fe3O4 as a viable alternative to traditional palladium-based catalysts, offering a more sustainable and cost-effective solution for industrial applications involving the reduction of nitroaromatic compounds. Future research will optimize the synthesis process and explore the catalyst’s performance in a broader range of chemical reactions. Additionally, scaling up the production and evaluating the economic feasibility of industrial applications will be crucial next steps.