Bio-inspired synthesis of palladium nanoparticles fabricated magnetic Fe3O4 nanocomposite over Fritillaria imperialis flower extract as an efficient recyclable catalyst for the reduction of nitroarenes

This current research is based on a bio-inspired procedure for the synthesis of biomolecule functionalized hybrid magnetic nanocomposite with the Fe3O4 NPs at core and Pd NPs at outer shell. The central idea was the initial modification of magnetic NP by the phytochemicals from Fritillaria imperialis flower extract, which was further exploited in the green reduction of Pd2+ ions into Pd NPs, in situ. The flower extract also acted as a capping agent for the obtained Pd/Fe3O4 composite without the need of additional toxic reagents. The as-synthesized Fe3O4@Fritillaria/Pd nanocomposite was methodically characterized over different physicochemical measures like FT-IR, ICP-AES, FESEM, EDX, TEM, XPS and VSM analysis. Thereafter, its catalytic potential was evaluated in the reduction of various nitrobenzenes to arylamines applying hydrazine hydrate as reductant in ethanol/water (1:2) medium under mild conditions. Furthermore, the nanocatalyst was retrieved using a bar magnet and recycled several times without considerable leaching or loss of activity. This green, bio-inspired ligand-free protocol has remarkable advantages like environmental friendliness, high yields, easy workup and reusability of the catalyst.

www.nature.com/scientificreports/ MNPs. The polyphenolic compounds of the flower extract contain hydroxyl and ketonic groups that chelate Pd 2+ ions and subsequently reduce them green metrically (Scheme 1). The structural and physicochemical characteristics of the nanomaterial was characterized using diverse analytical techniques like FT-IR, ICP-AES, FE-SEM, TEM, EDX, XPS and VSM studies. Figure 2 depicts comparative FT-IR spectra of bare Fe 3 O 4 NPs, Fritillaria extract, Fe 3 O 4 @Fritillaria and Fe 3 O 4 @Fritillaria/Pd nanocomposite in order to illustrate the stepwise synthesis. In the spectrum of Fe 3 O 4 NP (Fig. 2a), two broad peaks at 1622 and 3419 cm −1 correspond to the physisorbed H 2 O and the surface OH groups. The characteristic peaks appeared at 584 and 439 cm −1 are due to the stretching and bending vibrations of Fe-O bond. Pure Fe 3 O 4 structure is characterized by a peak at 632 cm −1 . Figure 2b represents the spectrum of Fritillaria extract which displays the significant peaks at 3385 cm −1 due to O-H groups of polyols 49 and C-H stretching vibration from hydrocarbons and flavonoids at 2926 cm −150 . Additionally, due to the presence of quinones, ketones, and carboxylic acids functions in the biomolecules contained in it, the distinctive peaks of C=O and O-C-O appears at 1709 cm −1 and 1072 cm −1 respectively 51 . An FT-IR band is observed in the range of 1400-1600 cm −1 owing to aromatic C=C stretching vibrations. The corresponding spectrum of Fe 3 O 4 @Fritillaria NPs is depicted in Fig. 2c. It is literally a combination of Fig. 2a,b indicating the successful functionalization of Fritillaria molecules over the ferrite NPs. These biomolecules actually perform as excellent capping agent, preventing the NPs from agglomeration and oxidation 52 . It also acts as reducing and stabilizing agent for immobilizing Pd NPs on the ferrite surface. The FT-IR spectrum of Fe 3 O 4 @Fritillaria/Pd NPs (Fig. 2d) is almost alike The structural morphology, size and shape of the Fe 3 O 4 , Fe 3 O 4 @Fritillaria, and Fe 3 O 4 @Fritillaria/Pd nanocomposite were investigated with the FE-SEM analysis as shown in Fig. 3. The materials are of nanometric size and of quasi-spherical shape (Fig. 3a). In addition, a continuous biopolymer layer is seen on the nanocomposite surface indicating the surface modification (Fig. 3b,c). The bright spots in Fig. 3c signifies the in situ synthesized Pd NPs being spread over the Fe 3 O 4 @Fritillaria composite.
EDX analysis of the material was conducted in order to know the chemical composition. The spectrum obtained on recording of signals at random points of the catalyst surface showed the presence of Fe, Pd as www.nature.com/scientificreports/ metallic and C, O as non-metallic components. The non metals justify the attachment of phyto-compounds in the composite (Fig. 4).
In addition to the EDX analysis, elemental mapping of Fe 3 O 4 @Fritillaria/Pd nanocomposite also carried out to have the knowledge of component distributions over the catalyst surface. X-ray scanning of a segment of the FE-SEM image reveals the homomorphic dispersion of all the components on the nanocomposite (Fig. 5). The uniform distribution of the active site definitely has a significant role behind its catalytic superiority.
The TEM image of the Fe 3 O 4 @Fritillaria/Pd NPs exhibits that the Pd NPs are formed with almost globular morphology (Fig. 6). As can be seen in the image (Fig. 6a), the ferrite NPs are of 10-20 nm in dimension that are coated by thin layers of Fritillaria extract. The biomolecular layers from the extract acts as the green reducing  Magnetic characteristics of the Fe 3 O 4 @Fritillaria/Pd NPs was assessed through VSM analysis and the magnetization curve has been shown in Fig. 8. From the corresponding hysteris curve, the maximal saturation  After resolving the required optimizations, the next endeavor was to generalize them over a range of differently functionalized (electron-donating and electron-withdrawing groups) nitroarenes. The results in terms of reaction yield and TOF are shown in Table 2. All the reactions were executed superbly with all kind of substrates without noticeable influence of functional groups on the reaction. All the reactions were completed within 0.5-2 h.

Recyclability of Fe 3 O 4 @Fritillaria/Pd catalyst.
For every heterogeneous catalytic system, the isolation and recycling of catalyst is a crucial feature in view of sustainable and industrial concern. The reusability of Fe 3 O 4 @Fritillaria/Pd was examined over the reduction of nitrobenzene under optimized conditions. After finishing a fresh batch of reaction the catalyst was recovered using a bar magnet and washed several times with ethanol and water. It was regenerated after drying at moderate temperature. To our delight, we could have reused it for eight consecutive cycles of reaction without noticeable loss in its activity (Fig. 9). We further analyzed the structural morphology of Fe 3 O 4 @Fritillaria/Pd nanocomposite after recycling 7 times by using TEM and EDX. As clearly can be seen from the TEM image (Fig. 10), the catalyst retains its initial morphology and particles size without any sign of agglomeration. Alongside, there occurs no change in elemental composition as evident from EDX data (Fig. 10), which in turn validates the robustness of our material.
Heterogeneity test for Fe 3 O 4 @Fritillaria/Pd catalyst. The Sheldon's test was carried out to assure heterogeneous nature of the synthesized material, whether any Pd species leached out in the filtrate solution. The reduction of nitrobenzene was continued over the catalyst under optimized state for 15 min and then the reaction mixture was divided into two-halves. From one portion of the reaction mixture the catalyst was removed by a magnetic bar and both the part reactions were further continued for another 15 min. On gas chromatographic analysis, it was revealed that no significant progress in reaction was achieved under non-catalytic conditions www.nature.com/scientificreports/ (60% conversion) while the other portion leaded to completion. The result is shown in Fig. 11. This further suggests that there was hardly any leaching of Pd NPs took place in the reaction mixture justifying its true heterogeneity. www.nature.com/scientificreports/ Study of reaction mechanism. Based on earlier published works, a probable reaction pathway has been documented in Scheme 2 [54][55][56][57] . The reaction goes through several intermediates. At the outset, hydrazine gets adsorbed on the surface of Pd NPs (I) which subsequently generates N 2 and nascent hydrogen by bond cleavage. This hydrogen is captured by the active Pd NPs to form metal hydride (II). In the meantime the substrate nitroarenes also get adsorbed over the catalyst surface and gets reduced by hydride transfer from II to form active nitroso derivative (III). This moiety is then further reduced to amine via hydroxylamine (IV) intermediate through hydrogen transfer. The hydrogenation of hydroxylamine is considered to be slow and rate determining step. Finally, the desired product leaves behind the catalyst surface to be used for the next cycle.  Table 3 which evidently displays that the Fe 3 O 4 @Fritillaria/Pd nanocomposite is much superior to others in terms of reaction time and yield.

Conclusion
We introduce a facile procedure for the synthesis of a heterogeneous and reusable Pd NPs decorated on Fritillaria imperialis flower extract modified magnetic ferrite nanoparticles by post functionalization approach. Catalytic performance of the Fe 3 O 4 @Fritillaria/Pd nanocomposite material was studied in the competent reduction of nitroarenes without the use of any added base. The protocol worked proficiently using hydrazine hydrate as the reducing agent under eco-friendly conditions affording various aromatic amines with excellent yields. In addition, due to strong magnetic nature, the Fe 3 O 4 @Fritillaria/Pd nanocatalyst could be reused as much as eight cycles in the reduction process emphasizing its true heterogeneity. In view of the outstanding catalytic behavior, the engineered material is anticipated to be a versatile support to feed many other noble metals like Ag, Au, Cu etc. towards many catalytic transformations and might find an excellent exposure in chemical industry.