Ag nanoparticles immobilized on new magnetic alginate halloysite as a recoverable catalyst for reduction of nitroaromatics in aqueous media

Amines can be applied in the synthesis of various important compounds such as dyes, drugs, polymers, pharmaceutical products, and biologically active materials. The significant subject in the preparation of amines is the selection of the most effective heterogeneous catalyst to get the best catalytic efficiency, stability, recoverability, and reusability. For this target, we prepared new alginate magnetically recoverable nanocatalyst by stabilization of Ag nanoparticles on the surface of the halloysite (HS) [HS-Alginate-Ag/Fe3O4]. Several detection methods confirmed the production of HS-Alginate-Ag/Fe3O4 nanocatalyst and the results obtained were well explained in the context. HS-Alginate-Ag/Fe3O4 presented good catalytic performance for the hydrogenation of nitro compounds using NaBH4 as the reducing agent and hydrogen donor. The good activity and durability of this catalyst can be attributed to the good dispersion and nano-sized particle of silver nanoparticles.

www.nature.com/scientificreports/ appropriate supports 19 . Since some supports are costly for practical application, hence, finding affordable and accessible support is necessary. Halloysite (HS) has the potential to be used as a catalyst carrier to inhibit the aggregation of silver nanoparticles. HS with a formula Al 2 Si 2 O 5 (OH) 4 ·nH 2 O (n = 0, 2) is naturally existing in soils and weathered rocks 20,21 . It is a clay mineral having a hollow tubular structure with a multi-layer wall in the nanometer range (lengths of 300 ~ 900 nm, inside diameters of 10 ~ 25 nm, and outside diameters of 50 ~ 100 nm) and a high specific surface area 22,23 . As well as HS owns many reactive groups on inside and outside surfaces [24][25][26] . In comparison with other tubular nanostructures e.g. carbon nanotubes, HS is eco-friendly, biocompatible, and cost-effective. Due to its high specific surface area and hollow structure, HS can serve as promising support for various applications [27][28][29][30] . HS has been investigated as high-efficiency support for various nanoparticle such as Pd 31 , ZnO 32 , TiO 2 33 , Au 34 resulting in effective catalyzes with adjustable properties.
In recent years, polysaccharides including chitosan, gelatin, and alginate are investigated as a most appropriate substrate for metal nanoparticles owing to their biocompatible, biodegradable, rheological, and non-toxic features [35][36][37][38] . Among them, alginate is a non-toxic and anionic compound, naturally exiting in bacteria and brown algae. Its polysaccharide chain contains β-d-mannuronate (M)and α-l-guluronate (G), linearly linked by 1,4-glycosidic units 39 . Moreover, there are abundant of hydroxyl and carboxyl groups in this macromolecule which can be suitably connected to other compounds.
Over the last years, magnetic nanoparticles (e.g. Fe 3 O 4 ) have been broadly studied as a suitable compound for the separation of compounds in numerous biological and industrial fields 40 . Their magnetic property are them effectively and readily separated from the reaction media by using an external magnet.
In this study, we describe the synthesis of Ag nanoparticles stabilized on new alginate magnetic halloysite as a magnetically separable nanocatalyst and it was also investigated for its catalytic activity in hydrogenation of nitro aromatic compounds with NaBH 4 as a mild hydride donor. It is found that the as-synthesized HS-Alginate-Ag/ Fe 3 O 4 can be used as a new, reusable and efficient nanocatalyst for the reduction of nitro aromatic compounds into the target amine derivatives. To the best of our knowledge, HS-Alginate-Ag/Fe 3 O 4 has not been reported as the nanocatalyst for the reduction of nitro aromatic compounds so far.
For determining the crystalline and phase structure of the synthesized nanocomposite, Transmission electron microscopy (TEM) imaging was performed with a CM30, Philips, Germany operating at 300 kV. Field emission scanning electron microscopy (FESEM) images equipped with an EDS attachment were taken on VEGA3, Tescan, USA. X-ray diffraction patterns were obtained on a PW 1800 X-ray diffractometer (Philips, Netherlands) with Cu Kα radiation (l = 0.154056 nm). Fourier transform infrared (FTIR) spectra were recorded using an FTIR apparatus (SHIMADZU, Japan) and in the range of 4000-400 cm −1 . (a) Synthesis of Hs-Fe 3 O 4 : The 2.5 g of the HS was dispersed in 120 mL of deionized water for 15 min. Then, iron (III) chloride (1.37 g) and iron (II) (0.5 g) were added to the above suspension. This suspension was stirred at 70 °C under an N 2 atmosphere. Then, 11 mL of concentrated ammonia was added to the above mixture and stirred for one hour. At the completion of the reaction, the product by an external magnet was separated, washed 3 times with distillated water, and dried at room temperature. (b) Synthesis of HS-Alginate/Fe 3 O 4 : In this stage, 500 mg of halloysite/Fe 3 O 4 in 60 mL was dispersed in water for 20 min. Then 100 mg of alginate was added to the above solution and stirred for 5 h. Finally, the precipitation by using an external magnet was separated, washed 3 times with distillated water, and dried at room temperature. (c) Synthesis of HS-Alginate-Ag/Fe 3 O 4 : At first, 500 mg of HS-Alginate/Fe 3 O 4 in 50 mL of water was stirred for 90 min at 25 °C, and then 1.4 mmol AgNO 3 was added to the above mixture. In the next step, 0.25 mL of hydrazine hydrate in 1 mL of distilled water was added dropwise to the above solution and stirred at 25 °C for overnight. Subsequently, the precipitation by a magnet was isolated and washed three times with distilled water, and dried completely.

Synthesis of HS-Alginate
The procedure for reduction of nitro aromatic compounds. To a round bottomed flask containing nitro compound (1 mmol) and 5 mL H 2 O, 0.03 g catalyst was added and the mixture was vigorously stirred at room temperature. Then NaBH 4 was added to the suspension and the reaction temperature was raised to 50 °C. After completion of the reaction (monitored by TLC), the catalyst by an external magnetic was separated and then the precipitate was recrystallized from EtOH to give pure products (  (Fig. 3a) (Fig. 3c). However, several shifts are also observed. These shifts can be recognized as an indication that the hydroxyl and carboxyl groups are responsible for the Ag nanoparticle stabilization. The morphology and chemical composition of HS-Alginate-Ag/Fe 3 O 4 were determined through FESEM, EDS, mapping, and TEM. As shown in Figs. 4 and 5 it was clear that HS has a diameter in the range of 41-52 nm, and the HS nanotubes were open-ended. In the FESEM image, it can be revealed some roughness on the surface of HS owing to grafted alginate. Also, the HS structures have a slight agglomeration. However, it showed that its surface morphology does not change very in the presence of Fe 3 O 4 and Ag (Fig. 4a). As can be observed in Fig. 5, the HS-Alginate-Ag/Fe 3 O 4 nanocatalyst has an almost uniform size distribution. The EDS (Fig. 4b) represented that the constituents for the HS-Alginate-Ag/Fe 3 O 4 were Al, Si, O, Ag, C, and Fe. According to mapping (Fig. 4c), these elements were irregularly dispersed on the surface of HNTs.  www.nature.com/scientificreports/ The TEM image in Fig. 5 shows a defect in the HS surface that results in rough exterior walls, which is consistent with the FESEM image. The strong interaction of the polymer between the substrate (halloysite) and the nanoparticles, especially the Fe 3 O 4 nanoparticles due to the carboxylic acid functional groups, as well as the flexibility of the polymer, has caused the nanoparticle and polymer images to be seen as mixtures that are almost far apart. This could be because polymers are also present between halloysites.

Investigation of the catalytic activity of HS-Alginate-Ag/Fe 3 O 4 .
Subsequently, hydrogenation of nitrobenzene was selected as the model reaction to investigate the catalytic activity of the HS-Alginate-Ag/ Fe 3 O 4 . Therefore, the model reaction was firstly tested in the presence of (0.04 g) at 70 °C in H 2 O as solvent that was mild and environmentally benign conditions. Such conditions gave aniline in 50% yields after 1 h. To improve the yield of model reaction and decrease its time, the used solvent, the amount of HS-Alginate-Ag/ Fe 3 O 4 , and the temperature were changed. It was found that running the reaction at ambient temperature gives the desired products in higher yields. By studying more temperatures, 50 °C was chosen as the optimized reaction temperature. Accordingly, by investigating the amounts of HS-Alginate-Ag/Fe 3 O 4 , it was found that the optimized amount is 0.04 g HS-Alginate-Ag/Fe 3 O 4 and further escalation didn't increase the product yield. Furthermore, among the tested solvents, water was the best one. Under the optimized conditions, hydrogenation of nitrobenzene gave anilines in 100% yields, respectively, after 30 min (Table 1,    www.nature.com/scientificreports/ The generality of this protocol was then examined using different starting materials to produce various aromatic amines. The results confirmed that HS-Alginate-Ag/Fe 3 O 4 can catalyze the hydrogenation reaction of all applied substrates to give the corresponding amine compounds at short reaction times and in high efficiencies ( Table 2).
Kinetic study. The catalytic reduction of 4-nitrophenol was selected as a model reaction to appraise the catalytic activity of the HS-Alginate-Ag/Fe 3 O 4 nanocatalysts. UV-Vis absorption spectra were used to monitor the concentration changes. The kinetics of this reaction was investigated with HS-Alginate-Ag/Fe 3 O 4 nanocatalyst. It followed the pseudo-first-order kinetics concerning the concentration of 4-nitrophenol as follows: where C t and C 0 are the 4-nitrophenol concentrations at time t and at the beginning time, respectively, and k is the apparent rate constant. The plot of ln (C t /C 0 ) vs time was obtained and a good linear correlation was observed (0.99). This phenomenon also showed that this reaction followed pseudo-first-order kinetics. From the slope of this equation, the apparent rate constant (K) for the reduction of 4-nitrophenol was obtained of 0.047 s −141,42 .

Study of reusability.
The results obtained from the reusability evaluation of HS-Alginate-Ag/Fe 3 O 4 in the model reaction are as follows. By completion of each cycle, the catalyst was separated, rinsed with EtOH, dried and reused in the next cycle. As shown by the results in Fig. 6, recycling the catalyst up to 6 runs showed no important loss of its catalytic activity. Since the catalyst contains magnetic Fe 3 O 4 nanoparticles, it is easily sepa- www.nature.com/scientificreports/ rated from the reaction mixture by an external magnetic field. Now, after six times of recovery, observing good efficiency as well as ICP (Ag content: 1.2% to 1.1% after 6 times) results showed that the catalyst has good magnetic properties and high stability.

Conclusions
The construction of cost-effective and reusable catalytic systems by using low-priced compounds and simple methods is an attractive research field for organic chemists. This study focused on the modification of halloysite material through three strategies including doping of alginate, deposition of silver nanoparticles, and Fe 3 O 4 dispersion on the support surface which was synthesized for the first time. The as-prepared HS-Alginate-Ag/ Fe 3 O 4 was utilized as a suitable and recoverable nanocatalyst for the reduction of nitro aromatic compounds to the related amine compounds. The obtained HS-Alginate-Ag/Fe 3 O 4 nanocatalyst presented a good performance in the reduction of nitro aromatic compounds to the amine aromatic compounds for a broad range of materials under moderate conditions. Recycling experiment of HS-Alginate-Ag/Fe 3 O 4 was performed using water as a green solvent and NaBH 4 as a hydrogen donor. This heterogeneous HS-Alginate-Ag/Fe 3 O 4 nanocatalyst exhibited acceptable stability and recovered by a magnet and reutilized several times with a low decrease in its efficiency. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.