Nitroaromatic compounds are dangerous and poisonous to the environment (humans, fishes, and invertebrates) due to the presence of the nitro group. They are widespread environmental contaminants in the soil and groundwater as a result of their wide use for manufacturing dyes, drugs (several analgesic and antipyretic drugs), pesticides, herbicides, fungicides, paints, and explosives [1, 2, 3, 4, 5, 6]. In the dye industry, 4-nitroaniline is also an important nitroaromatic compound and a major component of hazardous wastewater. These refractory organics are hard to decompose with conventional ways such as the active sludge method. They are used as the precursor in several chemical synthesis of various azo dyes, poultry medicine, antioxidants, antiseptic agents, pesticides, fuel additives as well as an important corrosion inhibitors [7]. Their disposal in wastewater results in high toxicity, carcinogenicity, and mutagenicity to several organisms [8, 9, 10, 11, 12]. However, aminophenol compounds are important for the preparation of several analgesic and antipyretic grugs such as parcetamol, acetanilide, and phenacetin.
A nicotinic acid complex with a metal, Mn, Co, Ni, Zn, or Cu, has been synthesized and reported in several research works [13]. Due to the electron donor behavior of the N atom of nicotinic acid, it has the ability to attract the metal through the N atom of the pyridine [14, 15]. Fe3O4 nanoparticles (NPs) have been extensively researched for many years for their supermagnetic behavior and catalytic properties, including their use in many applications such as microwave absorption, medical diagnostics, and catalytic degradation [16, 17, 18, 19]. Ag NPs have been studied and found to be an effective catalytic material [20, 21, 22, 23, 24, 25]. The chemical grafting of a homogeneous metal complex onto a solid magnetic surface is a versatile method to synthesize a metal complex with a regulated metal- coordination structure having special catalytic properties [26, 27, 28, 29, 30].
In recent years, due to their high surface-to-volume ratio and unique electronic and surface properties, nanoparticles have gained much attention for catalysis [31, 32]. There are many reports in the literature on the application of metal nanoparticles (Au, Ag, Pd, and Pt NPs are the most common) as catalyst for the reduction of nitrophenols to aminophenols in the presence of NaBH4 [33, 34]. Due to the easy separation of magnetic nanoparticles from the reaction medium (due to the superparamagnetic nature of the material), nanocomposites which combine a noble metal with a magnetic material have been extensively studied. Ag and Fe3O4 both have catalytic activity and several nanocatalysts have been synthesized and reported, e.g., magnetic photocatalyst Fe3O4@C@Ag [35], Fe3O4@C@Ag, and Ag-coated Fe3O4@TiO2 [36, 37], and Au-Fe3O4 heterostructures for nitrophenol reduction [38]. The use of a Fe3O4@Nico@Ag magnetic nanocatalyst for the reduction of 4-nitrophenol and 4-nitroaniline and their mixture has not been reported so far.
FeCl3·6H2O, FeCl2·4H2O, nicotinic acid, AgNO3, NaBH4, 4-nitroaniline, 4-nitrophenol, and NH3 were obtained from Merck and used without further purification. Fourier transform infrared (FT-IR) spectra were recorded in the transmission mode with a PerkinElmer BX FT-IR spectrometer. The powder samples were ground with KBr and pressed into a pellet. FT-IR spectra in the range 4000-400 cm−1 were recorded to investigate the nature of the chemical bonds formed.
The crystalline structure of the nanoparticles was determined with X-ray diffraction (XRD) measurements using a Rigaku D/Max-IIIC instrument with Cu-Kα radiation in the 2θ range of 20°-70°. The surface morphology of the composite was analyzed with a JEOL JSM 7001F scanning electron microscope (SEM).
The ultraviolet-visible (UV-Vis) spectrometer used was a Model Shimadzu UV-Vis 2600 in the range of 300-800 nm. The thermal stability was determined by thermogravimetric analysis (TGA, PerkinElmer Instruments model, STA 6000). The TGA thermograms were recorded with 6 mg of powder sample at a heating rate of 10 °C/min in the temperature range of 30-800 °C under a N2 atmosphere. Vibrating sample magnetometry (VSM) measurements were performed using a vibrating sample magnetometer (LDJ Electronics Inc., Model 9600). The magnetization measurements were carried out in an external field up to 15 kOe at room temperature.
The Fe3O4@Nicotinic acid nanocomposite was synthesized by the reflux method. Stoichiometric amounts of metal salts, Fe (III) and Fe (II) chlorides with the mole ratio of Fe3+/Fe2+ equal to 2, were dissolved in 40 mL of distilled water in a three neck round-bottom flask and their homogeneous solution was prepared using magnetic stirring. A stoichiometric amount of nicotinic acid was added to this mixture under vigorous stirring and then concentrated NH3 solution was added dropwise under constant stirring to adjust the pH to 10 at which the precipitation of the ferrites takes place. After transferring the flask to the heating mantle, it was refluxed under a N2 atmosphere at 80 °C for 5 h. Finally, the synthesized Fe3O4@Nicotinic acid nanocomposite was separated by a permanent magnet and washed with distilled water and ethanol solution several times to remove impurities. A black powder product was obtained and dried at 80 °C for 4 h.
The synthesized Fe3O4@Nico nanocomposite (150 mg) was dispersed in 50 mL of deionized water and sonicated for 30 min. This was followed by the addition of 30 mL of AgNO3 solution (0.2 mmol/L). The solution was vigorously stirred for 30 min and then 0.6 g of NaBH4 was quickly added and the mixture was allowed to react for 3 h under rapid stirring. The product was separated magnetically and washed several times with deionized water and ethanol to eliminate impurities. The synthesis of the Fe3O4@Nico@Ag nanocomposite is outlined in the Scheme 1.
The XRD powder pattern of the Fe3O4@Nico@Ag nanocatalyst is given in Fig. 1(a). It indicated the presence of both Fe3O4 ((220), (311), (400), (422), (511), (440)) [39] and Ag ((111) and (200)) [40]. All the diffraction peaks matched well with Ag (JCPDS 87-0720) NPs and Fe3O4 NPs (JCPDS 75-0033). Using Scherer’s formula and the FWHM of the strongest peak of the Fe3O4 NPs, the average crystallite size was estimated as 10.1 nm.
The FT-IR spectra of the Fe3O4@Nicotinic acid@Ag magnetic nanocatalyst, Fe3O4@Nicotinic acid, and nicotinic acid are presented in Fig. 1(b). The surface molecules in the adsorbed state were subjected to the field of the solid surface. It is worth noting that the C=O stretching band of the carboxyl group, which was present at 1696 cm-1 in the pure nicotinic acid, was absent in the nanocomposite. Tao et al. [41] and Ahn et al. [42] suggested two binding modes for the surface carboxylate bonding. When the carboxylate is bonded symmetrically to the surface, only the symmetric C=O stretching band appears at 1404 cm-1 [41, 43]. In our case, only one peak at 1403 cm-1 corresponding to the symmetric stretching was observed. Therefore, our conclusion was that both the carboxylic acid oxygen atoms were symmetrically bonded to the Fe3O4 nanoparticle surface [44]. The bonding of nitroaniline (NA) to the Fe3O4 surface was explained in detail in our previous paper [45].
The following FT-IR stretches can be assigned to the pristine NA: υ(C=O) = 1700 cm-1, υ(C-O) = 1300 cm-1, υ(C-N) = 1326 cm-1 [46]. The characteristic free NA C-N stretching absorption of the pyridine ring (υ = 1321 cm-1) was shifted to a higher frequency at 1340 cm-1 due to the binding between the N group of NA and the Ag NPs. This suggested that the Ag nanoparticles were formed by the coordination of the pyridine ring N atom [45, 47].
The TG curves of pristine nicotinic acid and Fe3O4@Nico- @Ag magnetically recycable nanocatalyst (MRC) are presented in Fig. 1(c). The pristine nicotinic acid has mass losses in two steps up to 180 °C (due to the evaporation of adsorbed water) and at 271 °C (due to the decomposition of the organic layer) [48]. Above this temperature, nicotinic acid was converted into gaseous products. On the other hand, the product (Fe3O4@Nico@Ag MRC) also has two step mass losses. The first mass loss was at 200 °C (due to dehydration). Then the anhydrous compound was stable up to 300 °C, and above this temperature the thermal decomposition of organic layer (oxidation of the organic matter) took place. Up to the end temperature, decomposition of the inorganic layer (due to the inorganic content, Fe3O4 and Ag) did not occur (Fig. 1(b)). The TG analysis showed that the Fe3O4@Nico@Ag MRC consisted of 90% inorganic and 10% organic layers.
Figure 1(d) shows the room temperature magnetization curve of Fe3O4@Nico@Ag. The material has no remanence or coercivity at 27 °C, which indicated that Fe3O4@Nico@Ag was superparamagnetic at room temperature. The saturation magnetization of the product was 52.4 emu/g.
SEM analysis was performed to evaluate the morphology of the composite nanoparticles, and the results are presented in Fig. 2. The particles of iron oxide were observed to form agglomerates of 20-50 nm in both Fig. 2(a) and (b). EDX analysis was also performed to detect the presence of all the components and the co-presence of Fe, N, C, O, and Ag for the Fe3O4@Nico@Ag MRC and Fe and O for the Fe3O4 NPs, which were confirmed. Using the EDX data and assuming a particle size of 20 nm for Fe3O4 and 3-4 nm for the Ag NPs, the surface coverage density of Ag NPs on Fe3O4 was estimated as 50%.
For the catalytic reduction experiment, 4-nitroaniline solution (100 µL, 10 mmol/L) and aqueous freshly prepared NaBH4 (1 mL, 100 mmol/L) were added to a UV cuvette. The volume of the mixture was adjusted to 3 mL with distilled water. Finally, 1 mg of Fe3O4@Nico@Ag nanocatalyst was used to catalyze the 4-nitroaniline solution, during which the color of the solution vanished, indicating the degradation of the 4-nitroaniline solution. The reduction was monitored with a UV-vis spectrophotometer. After the degradation was complete, the Fe3O4@Nico@Ag nanocatalyst was separated using a magnet and the process was repeated to investigate the recyclability of the catalyst. A similar procedure was used for the catalytic reduction of 4-nitrophenol. At the end of the reaction, the color of the solution vanished. The color of each solution changed to colorless, which indicated the degradation of 4-nitrophenol and 4-nitroaniline. The results of the reduction of 4-nitrophenol to 4-aminophenol and 4-nitroaniline to p-phenylenediamine by the Fe3O4@Nico@Ag magnetic nanocatalyst in an excess of NaBH4 are presented in Fig. 3.
To investigate the catalytic performance of Fe3O4@Nico- @Ag, 4-nitrophenol (4-NP) and 4-nitroaniline (4-NA) reduction reactions were carried out as model reactions in the presence of NaBH4. In the absence of the Fe3O4@Nico@Ag magnetic nanocatalyst, as seen in Fig. 4(a) and (b), there was no significant change in the absorption intensity with time. On the other hand, as shown in Fig. 4(c) and 4(d), when the Fe3O4@Nico@Ag magnetic nanocatalyst was introduced into the solution, the absorption at 400 nm for 4-NP and 380 nm for 4-NA decreased within 6 and 4 min, respectively, which indicated the formation of 4-aminophenol and p- phenyl- enediamine [49, 50, 51, 52, 53, 54, 55, 56, 57].
Since the concentration of NaBH4 was higher than those of 4-NP and 4-NA, the rate of reduction was independent of the concentration of NaBH4, thus the rate constants for the reduction of 4-NP and 4-NA was evaluated by pseudo-first order kinetics. As the absorbance of 4-NP or 4-NA is proportional to its concentration, the ratio of the absorbance at time t (At) to that at t = 0 (A0) is proportional to the concentration at time t (Ct) to that of t = 0 (C0), i.e., At/A0 α Ct/C0. The reaction rate constant K was calculated from the linear correlation between Ln(At/A0) and time as shown in Fig 5. The reaction rate constant k was found to be 0.34 and 0.51 min−1 for 4-NP and 4-NA, respesctively. We have also investigated the catalytic activity of the Fe3O4@Nico@Ag magnetic naocatalyst for a mixture of 4-NP and 4-NA in the presence of NaBH4, which clearly demonstrated that the reduction of both compounds occurred within 7 min as shown in Fig. 6. The rate contanst of the reaction, k, obtained from the slope the straight line was found to be 0.35 min-1 for the mixture, which is shown in Table 1. Our results also showed that the reaction rate constant was higher than in previous studies [32, 58, 59]. The reusability of the Fe3O4@Nico@Ag catalyst was examined. The results are shown in Fig. 5(b). With a high saturation magnetization, Fe3O4@Nico@Ag can be easily separated from the reaction mixture using a magnet. The recycled Fe3O4@Nico@Ag was washed with water and ethanol and then dried for the next cycle. As shown in Fig. 5(b), the catalyst activity did not show any significant change for the 4-NP and 4-NA reduction reactions up to three cycles.
A novel magnetic recoverable Fe3O4@Nico@Ag nanocatalyst was synthesized using a combination of reflux and a chemical reduction method. The Fe3O4@Nico@Ag magnetic nanocatalyst was a good catalyst for the reduction of 4-NP to 4-AP and 4-NA to dinitrobenzene at room temperature in the presence of NaBH4 as the reducing agent. The reduction reaction followed pseudo-first order kinetics. The Fe3O4@Nico@Ag magnetic nanocatalyst exhibited good stability. Its activity showed no significant decrease after reuse for 4 cycles. Therefore this catalyst can be used in wastewater treatment and the conversion of 4-NP to 4-AP in aqueous solution under mild condition.
This work was supported by Fatih University under BAP Grant No. P50021301-Y (3146). Md. Amir also thanks the Turkish Research Council for foreign student master program scholarship program of 2015.
2015, Vol. 36 Issue (5): 705-711
PDF (704 KB)
2015, Vol. 36 Issue (5): 705-711
PDF (704 KB)