Synthesis of Ag nanoparticles by Celery leaves extract supported on magnetic biochar substrate, as a catalyst for the reduction reactions

Green synthesis of a noble metal such as Ag nanoparticles is an enormously developed research area. In this study, a biochar/Fe3O4–Ag magnetic nanocatalyst was produced via a green path by using Celery stalk as a carbon-based substrate and Celery leaf extract as reducing and stabilizing agents to construct Ag nanoparticles. The synthesized nanocatalyst was determined using various techniques, such as UV–Vis spectroscopy, FT-IR spectroscopy, XRD (X-ray diffraction), SEM/EDX spectroscopy (scanning electron microscopy/energy-dispersive X-ray), TEM (transmission electron microscopy), and VSM (vibrating sample magnetometer). To survey the catalytic action of the biochar/Fe3O4–Ag nanocatalyst, it was used in the reduction reaction of disparate nitroaromatics, aldehydes, and ketones. This catalyst has demonstrated good characteristics in terms of the amount, reusability, recoverability, activity, and structural integrity of the catalyst during the reaction. In addition, biochar/Fe3O4–Ag could be detached magnetically and recycled multiple times without significantly reducing its catalytic performance.


Synthesis of biochar/Fe 3 O 4 -Ag nanocatalyst. Synthesis of biochar.
To prepare the biochar carbon substrate initially, 250 g of celery stalk was rinsed and desiccated at 60 °C for 24 h and then powdered. The green powder (4 g) with distilled water (65 mL) was poured into a 100 mL autoclave. Approximately, the autoclave was warmed up at 180 °C for 24 h and then chilled naturally to ambient temperature. The solid product was separated by centrifugation, washed, and dried. The final black powder was biochar. 3 O 4 . Generally, 0.5 g of biochar is dispersed to 120 mL of distilled water. Then, 0.5 g of FeCl 2 .4H 2 O and 1.37 g of FeCl 3 ·6H 2 O were added to the above mixture and warmed at 35 °C for 3 h. Subsequently, the temperature was up to 60 °C, and 10 mL of NH 4 OH was added drop-wise to the above mixture. Then, the mixture was stirred for an additional 60 min. After cooling to room temperature, the product was separated using an external magnet, washed multiple times with distilled water (H 2 O), and dried at ambient temperature.

Synthesis of biochar/Fe
Preparation of leaf extract. One hundred grams of green celery leaves were cut and thoroughly rinsed many times with distilled water (H 2 O) to eliminate mist particles. Afterward, the green leaves (50 g) were extracted by using 300 mL of distilled water at 100 °C for 6 h, after which they were permitted to become cold on their own. Finally, the celery leaf extract solution was filtered and dried at 55 °C. The procurement of leaf extract is shown in Fig. S1.
Green synthesis of Ag nanoparticles. In a usual reaction process, 2 mL of the leaf extract of 0.25% M was added dropwise to 5 mL of 1.5 mM aqueous AgNO 3 solution and stirred at 55 °C. The Ag nanoparticles were made from the reduction of silver ions over approximately 3 h. By UV-Vis (UV-Vis) spectroscopy, the reaction was controlled. The color change from light green to brown confirms the reduction of Ag + to Ag 0 .

Synthesis of biochar/Fe 3 O 4 -Ag.
To procure the nanocatalyst, biochar/Fe 3 O 4 (0.15 g) was added to 20 mL of distilled water (H 2 O) and stirred for 30 min. Afterward, 50 mL of AgNO 3 (2 mM) solution was appended into the blend and stirred at room temperature for 5 h. Thereafter, the temperature of the reaction was up to 65 °C, and it was stirred for another 30 min. Afterward, 10 mL of aqueous extract (0.25% M) was added to this mixture and stirred for 3 h at 65 °C. The constructed product was detached by an external magnet, washed with distilled

Reduction of nitroaromatic compounds catalyzed by biochar/Fe 3 O 4 -Ag. Catalytic reduction
reactions of nitro compounds were performed in an aqueous solution at 50 °C in the presence of NaBH 4 as the reducing agent. In a typical way, nitroaromatic compounds (0.5 mmol) and H 2 O (3.0 mL) were mixed into a 10 mL round-bottom flask and stirred at the desired temperature . Then, biochar/Fe 3 O 4 -Ag nanocatalyst (10 mg) and NaBH 4 (3 mmol) were added, and the final mixture was stirred for a suitable time. The progression of the reaction was controlled by applying TLC (normal hexane-ethyl acetate as solvent). After the finishing of the reduction reaction, the biochar/Fe 3 O 4 -Ag nanocatalyst was segregated by an outer magnet, rinsed with H 2 O and ethanol, and dried to be applied for the next cycle. Finally, to provide pure products, the obtained products were recrystallized from ethanol.
Reduction of aldehyde and ketone compounds catalyzed by biochar/Fe 3 O 4 -Ag. In a catalytic process, the reduction reactions of aldehyde and ketone compounds were performed. In a usual way, a mixture of 0.5 mmol aldehyde and ketone compounds and 2 mL of water as a solvent was added into a round bottom flask and stirred for 10 min at ambient temperature. Afterward, 5 mg of biochar/Fe 3 O 4 -Ag catalyst and NaBH 4 (3 mmol) were subjoined into the above mixture, and the whole combination was stirred for an adequate time. The reaction was monitored by TLC (normal hexane-ethyl acetate as the solvent, 2:8). After termination of the reaction, the nanocatalyst was detached by an outer magnet, rinsed with ethanol and H 2 O, and dried to be used in the next cycle. Finally, the product was extracted and purified.

FT-IR spectroscopy.
To study the structure of the presented nanocatalyst in more detail and characterize the functional groups, the FT-IR spectra of the nanocatalyst fabrication steps of (a) biochar, (b) biochar-  Figure S3c shows that by adding silver nanoparticles to the surface of the biochar-Fe 3 O 4 substrate, there was no considerable change in the spectrum. Therefore, it can be concluded that the biochar-Fe 3 O 4 substrate was stable during the synthesis of silver (Ag) nanoparticles. Fig. S4, the X-ray diffraction patterns of the silver (Ag) nanoparticles synthe- UV-Vis analysis of silver (Ag) nanoparticles. UV-Vis analysis of leaf extract, silver nitrate (AgNO 3), and synthesized silver NP spectra are shown in Fig. 1. In this process, Ag nanoparticles were prepared using Celery leaf extract. Reducing Ag + into Ag 0 was confirmed by changing the color of the reaction mixture from light green to brown. As shown in Fig. 1, the leaf extract and AgNO 3 solution did not show any absorbance peak in the range of 400-800 nm, but by adding the leaf extract into the AgNO 3 solution, good absorbance was observed at 440 nm, which was relevant to the SPR (surface plasmon resonance) of silver (Ag) nanoparticles.  Fig. 4. According to the curve obtained from the VSM, biochar/Fe 3 O 4 -Ag has magnetic properties, and its saturation magnetization value was 29.4 emu/g. Additionally, due to the absence of a hysteresis loop, this nanocomposite has superparamagnetic properties. This magnetic behavior of the prepared nanocatalyst causes the particles to accumulate rapidly in the attendance of an external magnet, and the particles are easily dispersed as soon as the external magnet is removed.

X-ray diffraction (XRD). In
Catalytic performance. The nitroaromatic compounds reduction reaction. The reduction of 4-nitroaniline (0.5 mmol) was considered the model reaction to optimize the reduction reaction conditions of nitroaromatics. The quantity of biochar/Fe 3 O 4 -Ag nanocatalyst, temperature, and type of solvent were changed and evaluated to achieve the optimized value, as shown in Table S1.
To optimize the reaction conditions, first, different amounts of catalyst were examined. The results showed that in the absence of the catalyst, the reduction reaction did not occur (Table S1, Entry 1). Therefore, the presence of biochar/Fe 3 O 4 -Ag nanocatalysts is a vital factor for the reduction reaction. As seen, 10 mg of catalyst was considered a reasonable and optimal value (Table S1, Entry 4). Additionally, based on the results, it was found that by increasing the amount of nanocatalyst, the reaction time was decreased, and the yield was increased (Table S1, Entries 2-5).
After determining the optimal value of the nanocatalyst, the efficacy of temperature on the progress of the reduction reaction was surveyed. Based on the outcomes, it was observed that increasing the temperature led to higher performance and yield of the reaction as well as decreasing the reaction time (Table S1, Entries 6, 7). Therefore, due to less energy consumption, 50 °C was selected as the appropriate and optimal temperature for this reaction (Table S1, Entry 4).
Finally, to investigate the effect of the solvent, the model reaction was performed in the presence of different solvents (Table S1, Entries 8-13). Following the outcomes, H 2 O, as a green, cheap and stable solvent, indicated www.nature.com/scientificreports/ the best performance with a 98% yield (Table S1, Entry 4). These results illustrate that biochar/Fe 3 O 4 -Ag has good catalytic efficiency for the reduction of nitroaromatic compounds by utilization of NaBH 4 as the reducing agent in water. Following the results of the optimization experiments, the optimal conditions for the reduction reaction of nitroaromatic compounds were 10 mg of the biochar/Fe 3 O 4 -Ag synthesized nanocatalyst in water at 50 °C. After obtaining the optimized conditions, the reduction reactions of different nitroaromatic compounds under these conditions were investigated, and the results are illustrated in Table S2. The first compound, 4-nitroaniline, was reduced at a yield of 98% in 60 min (Table S2, Entry 1). Substituted nitroaromatic compounds such as amine, acid, and hydroxyl-nitrobenzenes were also reduced with high reaction performance and a yield of more than 95% in the reaction time range of 40-80 min (Table S2, Entries 2-8).

Reusability of the biochar/Fe 3 O 4 -Ag nanocatalyst for the nitroaromatic reduction reaction.
Recyclability is an important factor to evaluate a catalyst. Therefore, recycling experiments were accomplished to appraise the stability and activity of the catalyst. Under the optimized conditions, the biochar/Fe 3 O 4 -Ag nanocatalyst was separated by an external magnet after the nitroaromatic compound reduction reaction was completed, and then it was washed, dried, and used for subsequent cycles. The recovered catalyst was reused up to 5 times without substantial reduction in catalytic activity, and the results are demonstrated in Fig. 5. These results demonstrated that the biochar/Fe 3 O 4 -Ag catalyst has privileged properties, such as good catalytic performance, cost-effectiveness, facile and green synthesis, good stability, and recyclability, which made it an adequate catalyst for the reduction of nitroaromatic compounds.
Aldehyde and ketone compounds reduction reaction. To optimize the reduction reaction conditions of aldehydes and ketones, such as the amount of biochar/Fe 3 O 4 -Ag nanocatalyst, temperature, type of solvent, and the amount of NaBH 4 as a reducing agent, the reduction of benzaldehyde (0.5 mmol) as a model reaction was examined, and it is demonstrated in Table 1.   Entries 2-4). Therefore, 5 mg of biochar/Fe 3 O 4 -Ag was selected as the optimal value because of the short reaction time and high yield ( Table 1, Entry 4).
After opting for the appropriate amount of catalyst, the efficacy of temperature on the reaction progress was determined. Depending on the results (Table 1, Entry 4), 25 °C was determined to be the optimal temperature for this reaction because this temperature complies with the laws of green chemistry. Additionally, it was observed that increasing the temperature led to a decrease in the reaction time (Table 1, Entry 5).
After determining the appropriate temperature, the next step is to peruse the effect of the solvent on the attendance of the biochar/Fe 3 O 4 -Ag catalyst in the reaction progress. As a function of the outcomes, H 2 O with a yield of 98% (Table 1, Entry 4) and THF with a yield of 96% (Table 1, Entry 9) were suitable solvents for this reaction, but H 2 O was selected as the optimal solvent due to its green, inexpensive and high yield. Additionally, other solvents, such as ethanol, acetonitrile, and DMF, had a yield between 60 and 87% (Table 1, Entries 6, 8, 11). www.nature.com/scientificreports/ Finally, NaBH 4 was used as a reducing agent, and its different concentrations were investigated to assess the optimum value while maintaining that the other parameters were constant. With the enhancement of the concentration of NaBH 4 , the yield was enhanced (Table 1, Entries 13, 14). Therefore, the concentration of 1 mmol NaBH 4 was determined as the optimized value (Table 1, Entry 4). These results elucidate that biochar/Fe 3 O 4 -Ag is an outstanding catalyst for the reduction of aldehyde and ketone compounds in the presence of NaBH 4 as the reducing agent in water.   www.nature.com/scientificreports/ As a first example, benzaldehyde was reduced at a yield of 98% in 3 min ( Table 2, Entry 1). Substituted aldehyde and ketone compounds were also reduced with great reaction performance; the yield was more than 85% in the reaction time range from 3 to 10 min for aldehydes, and the yield was more than 30% in the reaction time range from 6 to 60 min for ketones.
For sample, IR spectra were taken from six derivatives, the peaks of which are as follows:   The IR spectra of these derivatives are shown in Fig. S6.

Reusability of the biochar/Fe 3 O 4 -Ag nanocatalyst for aldehyde and ketone reduction reactions.
Catalyst recovery and reusability are fundamental factors in the nomination of the efficiency and performance of the catalyst. In this regard, the recoverability of the biochar/Fe 3 O 4 -Ag nanocatalyst was investigated in the model reaction of the reduction of aldehydes and ketones under optimal conditions. The results of the performance testing demonstrated that the synthesized nanocatalyst could be applied in at least 10 successive runs without a substantial reduction in catalytic performance, which affirmed the heterogeneous nature of the catalyst. The results are shown in Fig. 6. Based on the outcomes, good catalytic performance, excellent stability, and recoverability, biochar/Fe 3 O 4 -Ag is an appropriate catalyst for the reduction of aldehydes and ketones.

Conclusion
This study was centralized on the green synthesis of biochar as a carbon-based substrate and Ag nanoparticles using leaf extract of Celery. In general, a fast and ecofriendly synthesis process for silver nanoparticles in the presence of a green precursor and solvent has been illustrated. The structural, morphological, and optical properties of the Ag nanoparticles were determined by diverse techniques. The prepared biochar/Fe 3 O 4 -Ag nanocatalyst exhibited excellent catalytic efficiency for the reduction of nitroaromatic compounds, aldehydes, and ketones in the presence of NaBH 4 as a reducing agent and H 2 O as a green solvent and possessed appropriate reusability. The advantages of this heterogeneous catalyst include green conditions such as low reaction temperature, green solvent, short reaction time, easy separation, low cost, and eco-friendliness. The biochar/Fe 3 O 4 -Ag nanocatalyst could be segregated by the utilization of an external magnet and reused five to ten times without appreciable loss of its catalytic performance.  1  2  3  4  5  6  7  8  9  10   3  3  3  3  3  3  3  3  3  3   98  98  98  98  98  97  97  97  95  95 Time & Yield Run Catalyst reusibility Figure 6. Reusability of the biochar/Fe 3 O 4 -Ag nanocatalyst for aldehydes and ketones reduction reaction.