1 Introduction

Magnetic iron oxide nanoparticles being robust with greater surface area have wide spread applications in the field of drug delivery [1], magnetic resonance imaging [2], storage devices [3], catalysis [4] etc. Recently many researchers reported the iron oxide nanoparticle as catalyst support due to their easy recoverability by magnetic separation method [5]. Generally, Fe3O4 nanoparticles can be synthesized from iron precursors by various method such as chemical reduction, coprecipitation, thermal decomposition, sonochemical method etc. [6]. But while synthesizing Fe3O4 few factors to be considered like particle size, uniform shape, less aggregation, fine dispersion etc. Numerous methods have been developed to synthesis iron oxide nanoparticles by using surfactants [7, 8] organic ligands [9], polymers [10], dendrimers [11] as stabilizers. Among these methods, chemical reduction using amines is simple and convenient for the synthesis of Fe3O4 nanoparticles with enchanced features. [12]. Further, the surface of the iron oxide nanoparticles have functionalized in order to synthesis specific catalysis. The surface modifications of Fe3O4 nanoparticles can be carried out by using polymer [13], mesoporous silica [14], metals [15], metal oxides [16] etc. Besides, polymer coating strategy on the Fe3O4 nanoparticles facilitates numerous sites for further modification and fine dispersion. Many organic polymers such as polyester [17], polyethylene glycol [18], poly styrene [19] etc,. have been reported which forms an organic shell around the iron oxide nanoparticles. These polymer matrix provides an avenue to immobilize metal nanoparticles for catalysis [20]. Normally, metal nanoparticles can be immobilized on the polymer coated iron oxide nanoparticles by following two strategies i.e. to deposit pre-synthesized metal nanoparticles on the polymer coated iron oxide nanoparticle, but the metal nanoparticles can be leached after catalysis. The second method is to directly grow the metal nanoparticle on the polymeric surface with controllable size. Since polymer possess greater multifunctional atoms and favorable structural features, it provides excellent support for direct synthesis of size-controlled metal nanoparticles. Recently, noble metals such as silver, gold, platinum, palladium nanoparticles were immobilized on solid-support and used for the reduction of p-nitrophenol. As we are aware that p-nitrophenol is a significant compound in pharma sector and which remains as pollutant too. Hence, it has to be reduced to p-aminophenol by greener method in aqueous medium using recoverable and reusable catalysts [21–23,22,].

In this research work, we have demonstrated the efficient two step synthetic protocol for immobilization of platinum nanoparticle onto spherical magnetite–poly (epoxyamine) nanocomposite with uniform size, shape and monodispersion of Pt nanoparticles. Efficacy of this material has been tested for aqueous phase catalysis of reduction of p-nitrophenol to p-aminophenol.

2 Experiment and methods

2.1 Materials

Ferrous sulphate heptahydrate (FeSO4·7H2O), 1,4-diaminobutane (DAB), epichlorohydrin, sodium borohydride (NaBH4), p-nitrophenol were purchased from SRL. Chloroplatinic acid (H2PtCl6) was purchased from Sigma Aldrich. Double Distilled (DD) water was used wherever necessary. Reactions were carried out using Remi mechanical overhead stirrer provided with Teflon blade.

2.2 Characterization methods

Images were taken using High Resolution Transmission Electron Microscope (HRTEM), JEOL Japan JEM 2100 at 200 kV model with LaB6 electron gun. The grid was placed under electron beam and images were captured. X-ray diffraction patterns were recorded in Bruker USA D8 Advance, Davinci using CuKα radiations with wavelength 0.154 nm. The measurements were made between the angles 10° and 70°. Magnetic behavior of the nanocomposite materials synthesized were analyzed by using Vibrating Sample Magnetometer Lakshore, 7410 series with a maximum field of 3.1 Testla at room temperature having field setting resolution of 30 mOe. FTIR spectra was obtained from Perkin Elmer instrument in transmittance mode for measurements in the range of 4100–400 cm−1 using KBr pellets sampling method.

2.3 Synthesis of poly (epoxyamine) functionalized Fe3O4 nanocomposite (Fe3O4/PEA)

0.278 g (1 mM) of FeSO4·7H2O was dissolved in DD water and stirred vigorously. To this solution, about 10 equivalence i.e. 0.8 mL of DAB (10 mM) was added under continuous stirring, the solution turned green. This suspension turned black when heated at 80 °C for 6 h. The black particles Fe3O4–NH2 got attracted towards magnet [7]. To the above Fe3O4–NH2 black suspension, equal amount of DAB (2 mL, 20 mM) and epichlorohydrin (1.6 mL, 20 mM) were added and stirred at 80 °C for 24 h [24]. Magnetite–poly (epoxyamine) nanocomposite obtained was separated by external magnet and washed thrice with DD water, thrice with acetone and dried in vacuum for 24 h.

2.4 Immobilization of Pt nanoparticles on Fe3O4/PEA nanocomposite (Fe3O4/PEA/Pt)

100 mg of the Fe3O4/PEA nanocomposite was suspended in 50 mL DD water, to this suspension 0.2 mL of chloroplatinic acid (4.8 μM) were added under continuous stirring for 15 min. Then Pt4+ was reduced with aqueous NaBH4 (2.4 mM, 91 mg) in cold condition, stirring was continued for 30 min at room temperature. Fe3O4/PEA/Pt nanoparticles obtained was washed thrice with DD water, thrice with acetone and dried in vacuum for 24 h.

2.5 Determination of amine concentration by conductometric titration method

Exactly 10 mg of the nanomaterial was dispersed in 20 mL of standardized hydrochloric acid (1 mmol) and stirred mechanically for 30 min. Afterwards, the nanomaterial was isolated by magnetic separation, 5 mL of supernatant solution was titrated against standardized NaOH solution, the variations of conductance was measured for each 0.2 mL addition of NaOH solution. From the measured conductance, the value of concentration of HCl remaining in the solution which were not absorbed by the Fe3O4/PEA/Pt nanocomposite was determined. Henceforth, HCl consumed by the nanomaterial can be determined and that is equivalent to the concentration of amine groups available on the nanocomposite material.

2.6 Catalysis of reduction of p-nitrophenol

1 mM of p-nitrophenol solution was stirred with 7 mg of Fe3O4/PEA/Pt catalyst at room temperature. 10 mM of NaBH4 was added to the above suspension in cold condition under continuous stirring. During the course of the reaction the light-yellow colored p-nitrophenol solution turned  intense yellow colored solution, it has faded gradually and turned brown in 21 min.  This colour change conformed the conversion of p-nitrophenol into p-aminophenol.

3 Results and discussion

Synthesis of  Pt nanoparticle immobilised Fe3O4 poly (epoxyamine) nanocomposite was obtained from convenient one-step synthetic protocol. Initially, amine stabilized Fe3O4 nanoparticle was synthesized from the reduction of FeSO4 using DAB in the ratio 1:10. DAB acts as the reducer as well as stabilizer to afford spherical shaped monodispersed diamino butane protected Fe3O4 nanoparticles. Further, the presence of amino groups on the Fe3O4 nanoparticles provides added advantage for polymer functionalization on it. Additionally, DAB is one of the monomers for the polymer functionalization on iron oxide nanoparticles. Hence, removal of excess DAB has not been implemented and then equimole of DAB and epichlorohydrin were added to the diamine stabilized Fe3O4 nanoparticles, which was allowed to undergo polycondensation for a period of 24 h to yield poly (epoxyamine) functionalized magnetite nanocomposite.

Polymer functionalization on the Fe3O4 nanoparticle protects the iron oxide core from chemical degradation, prevents agglomeration, to enhance the spherical morphology and to achieve well dispersibility in solvents. Besides, the magnetic property of Fe3O4, also preserved to exhibit the material for high throughput application especially as recyclable catalyst support. Subsequently, Pt nanoparticles were immobilized on the Fe3O4/PEA nanocomposite by a simple wet impregnation method. The actual formation of Pt nanoparticle was taken place while mixing the H2PtCl6 with magnetite–poly (epoxyamine) composites, which enables chemical interaction between Pt4+ and amine group. The Pt4+ coordinated to the amine was reduced to Pt0 by means of aq. NaBH4 solution to produce Pt nanoparticle decorated magnetite–poly (epoxyamine) composite. The precise size and uniform distribution of the Pt nanoparticle on the surface of the nanocomposite was attributed by effective stabilization of Pt nanoparticles on polymer functionalized magnetite particles (Scheme 1).

Scheme 1
scheme 1

Synthesis of Pt nanoparticle decorated on magnetite–poly (epoxyamine) nanocomposite

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The formation of iron oxide nanoparticles and the presence of amino groups on their periphery has been confirmed primarily through FTIR spectrum (Fig. 1A). Absorbance band around 580 cm−1 attributes for Fe–O vibrations of Fe3O4 and absorbance around 3350 cm−1 attributes to the –NH stretching, 1625 cm−1 attributes to the –NH bending of the amine present on the periphery of the iron oxide nanoparticle. The X-ray diffraction pattern of amine stabilized iron oxide nanoparticle has been proved to be magnetite (inverse spinel cubic structure) showing sharp peaks for 2θ values at 30.25°, 35.66°, 45.34°, 57.37°, 62.7° corresponds to the respective lattice planes (220), (311), (400), (511), (440) (Fig. 2A). These XRD patterns matched well with JCPDS patterns of magnetite (JCPDS card No. 19-0629). The magnetic behavior of the synthesized amine stabilized Fe3O4 nanoparticles were evidenced to be superparamagnetic through the VSM studies (Fig. 3A). These nanoparticles acquired magnetic saturation value of 68.8 emu/g even when high magnetic field in the order of 1500 KOe was applied. Additionally, these nanoparticles have less magnetic retentivity such that negligible hysteresis loop was observed. Thus, the magnetometry studies authorizes the amine stabilized Fe3O4 nanoparticles behaves like temporary magnet which acquires magnetic moment only in the presence of an external magnet. The superparamagnetic property of Fe3O4–NH2 nanoparticle provides credential to the material to serve as excellent catalyst support. The concentration of amine groups present on the periphery of amine stabilized Fe3O4 nanoparticle were determined by conductometric titrations and was found to be 2.14 mmol/g of amine groups.

Fig. 1
figure 1

FTIR spectra of (A) Fe3O4 nanoparticle, (B) Fe3O4/PEA nanocomposite, (C) Fe3O4/PEA/Pt nanocomposite

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Fig. 2
figure 2

X-ray diffraction patterns of (A) Fe3O4 nanoparticle, (B) Fe3O4/PEA nanocomposite, (C) Fe3O4/PEA/Pt nanocomposite

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Fig. 3
figure 3

VSM studies of (A) Fe3O4 nanoparticle, (B) Fe3O4/PEA nanocomposite

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The morphology of amine stabilized Fe3O4 nanoparticle were evidently clearly seen from SEM images. SEM images observed for amine stabilized Fe3O4 nanoparticle shows spherical morphology with minimal agglomeration (Fig. 4a), since the Fe3O4 nanoparticle were stabilized with DAB. These observations of amine stabilized Fe3O4 nanoparticle were further confirmed from HRTEM images (Fig. 5a). The size of the nanoparticles was found to be around 35 ± 5 nm from HRTEM images, which holds in good agreement with the size calculated from XRD from Scherrer formula. The FWHM was determined for 311 lattice plane and the crystallite size of the magnetite nanoparticle was calculated from the Scherrer equation D = (K * λ)/(β * cosθ) where K = 0.69, λ = wavelength of X-ray, β = FWHM.

Fig. 4
figure 4

SEM images of a Fe3O4 nanoparticle, b Fe3O4/PEA nanocomposite

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Fig. 5
figure 5

HRTEM images of a Fe3O4/PEA nanocomposite, b Fe3O4/PEA/Pt nanocomposite, c EDS spectrum of Fe3O4/PEA/Pt nanocomposite

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Fe3O4/PEA nanocomposite material was synthesized by the condensation polymerization of DAB and epichlorohydrin on amine stabilized Fe3O4 nanoparticle. It was confirmed through FTIR (Fig. 1B) from increased intensity of absorption band around 3350 cm−1 for –NH as well as –OH stretching, appearance of new band at 3250 cm−1 for –CH stretching and decreased intensity of absorption band at 580 cm−1 for Fe–O vibration due to functionalization of PEA on iron oxide nanoparticles compared with the FTIR spectrum obtained for amine stabilized Fe3O4 nanoparticle. Hence, these absorption band depicts that the formation of Fe3O4/PEA nanocomposite. The XRD pattern for Fe3O4/PEA nanocomposite does not show any new characteristic peak other than the patterns observed for magnetite nanoparticle, confirming that the magnetite crystallite structure has not been disturbed even after polymer functionalization (Fig. 2B). In addition to these analytical observations, VSM study validates the persistence of superparamagnetic character of Fe3O4/PEA nanocomposite. The observed magnetic saturation values for Fe3O4 and Fe3O4/PEA is 68.8 emu/g and 60.2 emu/g respectively. The decline in magnetic saturation values from 68.8 to 60.2 emu/g provides the insight information about the polymer functionalization of magnetite nanoparticle (Fig. 3B). Moreover, the superparamagnetic behavior still exists favoring the easy recoverability of the nanoparticle by using external magnet. The polymer functionalization on Fe3O4 nanoparticle was also observed from HRTEM images (Fig. 5b). Independent Fe3O4/PEA nanocomposite were found, having boundary for each entity of Fe3O4/PEA nanocomposite. The diamine present on iron oxide nanoparticles serves as monomer and it facilitates the polymerization with epichlorohydrin on Fe3O4 nanoparticle consistently, such that they appear to be monodispersed with less agglomeration.

As discussed earlier, Pt nanoparticle were immobilized on Fe3O4/PEA via coordination of Pt4+ with amino group of the polymer followed by reduction of Pt4+ to Pt0 using aq. NaBH4. Pt nanoparticles were evenly distributed on the surface of the Fe3O4/PEA nanocomposite that can be attributed due to the presence of more availability of functional atoms such as nitrogen and oxygen available on the polymer stabilizes the metal nanoparticle. The Pt nanoparticles immobilized on the matrix was confirmed from the XRD pattern obtained for Fe3O4/PEA/Pt nanocomposite which showed new peak for platinum crystal structure for the lattice plane 111, 200, 311 and this in addition to the magnetite crystallite lattice planes [25]. Further, HRTEM images envisages Fe3O4/PEA nanoparticle entity contains ultrafine Pt nanoparticle that additionally supports the increase in loading capacity of the polymer (Fig. 5c). Hence, the synthesized Fe3O4/PEA/Pt nanocomposite will serve as better catalyst for reduction reactions.

4 Catalysis

4.1 Reduction of p-nitrophenol to p-aminophenol using Fe3O4/PEA/Pt nanocomposite

The catalytic activity examination of Fe3O4/PEO/Pt nanocomposite was evaluated for the reduction reaction of p-nitrophenol to p-aminophenol (Scheme 2). Usually, Palladium and gold nanoparticles were employed for various reduction reactions. Platinum metal being stable and inert, it was chosen for immobilization on magnetic nanoparticle support in order to carry out the reduction reaction of p-nitrophenol to p-aminophenol which has been the major source for preparation of paracetamol. In normal practice p-Nitrophenol does not undergo reduction reaction with NaBH4 in the absence of catalyst. Therefore the Fe3O4/PEA/Pt was used as semiheterogenous catalyst of activate NaBH4 and p-nitrophenol which inturn facilitates the reduction reaction with much faster rate. Since the particle size of the catalyst is in nanodimension, it is easily dispersed in reaction medium and leads to semiheterogenous phase catalyst. The yellow colored solution started to fade and turned brown at 20 min, which indicated the reduction of p-nitrophenol to p-aminophenol. Further, the conversion of products was confirmed through UV–Vis absorption measurement, p-nitrophenol showed absorbance spectrum at 400 nm for the protonation of p-nitrophenolate ion with aq. NaBH4 .  After the addition of the catalyst, absorption peak at 400 nm was diminished and disappeared after 20 min, that infers the complete reduction of p-nitrophenol to p-aminophenol (Fig. 6). Finally, the Fe3O4/PEO/Pt nanocatalyst was separated by magnetic attraction and regenerated by washing thrice with water, acetone and dried (Fig. 7). The recycling efficiency of the regenerated catalyst was tested up to five cycles. The catalyst showed remarkable efficiency even after fifth cycle for reduction of p-nitrophenol, without any significant loss in catalytic activity and loss in weight of the catalyst also.

Scheme 2
scheme 2

Reduction of p-nitrophenol to p-amino phenol

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Fig. 6
figure 6

UV–Vis absorbance for the reduction of p-nitrophenol to p-aminophenol

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Fig. 7
figure 7

Photograph for the magnetic separation of the Fe3O4/PEA/Pt nanocomposite

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4.2 Mechanism for the catalytic reduction of p-nitrophenol

The actual mechanism for the reduction of p-nitrophenol using platinum nanoparticle immobilized on poly (epoxyamine) coated iron oxide nanoparticle catalyst has described in Scheme 3. The typical mechanism involves the steps such as (I) adsorption of the borohydride anion and p-nitrophenol on the surface of the platinum nanoparticle, (II) formation of p-nitrosophenol, (III) formation of p-hydroxyaminophenol, (IV) hydride transfer for the formation of p-aminophenol compound, (V) desorption of the product p-aminophenol and borohydride ions [26, 27].

Scheme 3
scheme 3

Proposed mechanism for the catalytic reduction of p-nitrophenol to p-aminophenol

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5 Conclusion

We have successfully demonstrated the one-pot synthesis of magnetite–poly (epoxyamine) nanocomposite with controlled particle size, uniform shape and fine dispersion. Platinum nanoparticles were decorated onto magnetic polymer matrix. The synthesized nanocomposite was proved to be an efficient catalyst for the reduction of p-nitrophenol, also the recycle efficiency of catalyst was examined up to five cycles without any significant loss of their catalytic activity. Hence, the nanocomposite material will replace the existing catalyst for reduction reactions in industries and further many organic transformations can be achieved.