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. 2023 Feb 13;8(8):7459–7469. doi: 10.1021/acsomega.2c06326

Reductive Oligomerization of Nitroaniline Catalyzed by Fe3O4 Spheres Decorated with Group 11 Metal Nanoparticles

Carlos Alberto Huerta-Aguilar †,*, Rajendra Srivastava ‡, Jesús A Arenas-Alatorre §, Pandiyan Thangarasu ∥,*
PMCID: PMC9979374  PMID: 36873030

Abstract

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The present work demonstrates a simple and sustainable method for forming azo oligomers from low-value compounds such as nitroaniline. The reductive oligomerization of 4-nitroaniline was achieved via azo bonding using nanometric Fe3O4 spheres doped with metallic nanoparticles (Cu NPs, Ag NPs, and Au NPs), which were characterized by different analytical methods. The magnetic saturation (Ms) of the samples showed that they are magnetically recoverable from aqueous environments. The effective reduction of nitroaniline followed pseudo-first-order kinetics, reaching a maximum conversion of about 97%. Fe3O4-Au is the best catalyst, its a reaction rate (kFe3O4-Au = 0.416 mM L–1 min–1) is about 20 times higher than that of bare Fe3O4 (kFe3O4 = 0.018 mM L–1 min–1). The formation of the two main products was determined by high-performance liquid chromatography-mass spectrometry (HPLC-MS), evidencing the effective oligomerization of NA through N = N azo linkage. It is consistent with the total carbon balance and the structural analysis by density functional theory (DFT)-based total energy. The first product, a six-unit azo oligomer, was formed at the beginning of the reaction through a shorter, two-unit molecule. The nitroaniline reduction is controllable and thermodynamically viable, as shown in the computational studies.

Introduction

Anilines and aromatic azo molecules are industrially produced by catalytic reduction of toxic nitrobenzene at high temperature and pressure1 and it is known that aromatic azo polymers are common precursors in the synthetic organic industry.2 However, aromatic imine production is essential for the modern industry; thus, the search for alternative less toxic methods is the topic of interest for the above production. But the reaction mechanism of the reduction of nitrobenzene is complicated as it depends on several parameters such as solvent, temperature, size, and composition of the catalyst although the Haber type of reaction mechanism is the most accepted one.3 The environmental production agency is regularly monitoring the industrial process and evaluating the environmental impact from the industrial hazardous waste.4 So, a standardized method is required for the reduction of nitroaniline, where the catalytic material plays a vital role in the large-scale production since the industrial reductive process is expensive because of poor selectivity in the purification process.5 Even though costly metals such as platinum6 and gold7 have been widely employed as catalysts for the reduction of organic compounds due to their stability and high efficiencies8 over other low-cost transition metals (TM) such as Cu,9 Ni,10 and Ru,11 the latter have started being used lately as catalysts for the organic reactions to cut the cost. The metal-based micro- or nano-sized powders are considered as potential industrial catalysts because of their exponentially greater active sites (surface area to volume ratio) caused by quantum confinement effects.12 It has been reported that nanometric catalysts have enhanced stereo- and enantio-selectivity to improve the efficiency of the reactions.13 To improve further the performance of the catalyst, it is common to mix multiple metals14 to yield hybrid organic/inorganic materials such as Pd/C,15 Pt/C, and Pd/graphene.16

Metal oxides are also good candidates for catalysts, especially, iron oxides, which are cheap, robust, nontoxic, eco-friendly, easy to handle, and magnetically separable for reuse. They are actively considered from the environmental point of view17 since iron can be easily removed in biological reactions18 as compared to other metallic or chalcogen oxides.19 Fe3O4 (magnetite) can be used for H2/hydrogenation if it is mixed with Ni,20 and can be employed for organic reduction in the presence of CO and H2O.21 Magnetite is presented in the stoichiometric ratio of 1:1 (FeO/Fe2O3) and 1:2 (Fe2+/Fe3+) in a closed cubic lattice. Typically, Fe2+ is positioned at tetrahedral sites whereas Fe3+ occupies octahedral sites within the crystal structure. The understanding of the exact stoichiometry of Fe3O4 is limited, but the superparamagnetic and catalytic properties of the sample can be maintained without a significant change in the structure (Fe3−δO4) Fe3−δO4.22 Fe3O4, a narrow-band gap energy semiconductor, has a property of visible light-harvesting character. However, the recombination effect of e–/h+ is very high; thus, the addition of metals to Fe3O4 can reduce the recombination effect of e-/h+, decreasing the band gap energy.23 This means that the valence band (VB)–conduction band (CB) interband states can be modified by employing the metal work function;24 as a result, the Fe3O4–metal conjugation exhibits a greater photostability and higher extinction coefficients, improving the catalytical, physical, and chemical properties.25 So, the present work is focused on the deposition of Au NPs, Ag NPs, and Cu NPs on Fe3O4 and analyzes the impact of electronic and magnetic properties on the catalytic reduction of 4-nitroaniline. The formation of oligomers was optimized and the total energy determined by the density functional theory was analyzed for each product formed in the reduction, which is interesting and, to the best of our knowledge, has not been reported in the literature. One-step, recoverable, and reusable catalytic systems such as Fe3O4-M (M = Cu, Ag, and Au) are prepared and characterized completely, and employed for the selective reduction of 4-nitroaniline (NA) at room conditions.

Results and Discussion

Characterizations

X-ray Diffraction (XRD)

The characteristic XRD peaks of Fe3O4 planes are observed in all samples at 2θ = 18.0 [111], 30.1 [220], 34.8 [311], 43.7 [400], 53.2 [422], 56.8 [511], and 59.5 [400] corresponding to JCPDS 19-0629.26 The average grain size of the samples calculated by Scherrer’s equation was around 10.69 nm.27 Fe3O4-Cu corresponds to cubic Cu clusters and it is identified at 2θ = 46.5 [111] and 52 [200], coinciding with JCPDF 04-0836.28 For Fe3O4-Ag, there is a typical fcc-Ag pattern at 2θ = 27.8 [210], 32.2 [122], 38.1 [111], and 44.3 [200] (JCPDS No. 04-0783)29 despite having a secondary phase characterized as sodium carbonate, which was used to reduce Ag+ to Ag NPs (JCPDS 37-0451).30 Similarly, for Fe3O4-Au, the presence of fcc phase was seen as the signal was detected at 2θ = 37.8 [111] as reported with JCPDS No. 04-0784,31 for which, particularly, a broadening of the peaks was observed due to a lack of periodicity in the small particles (Figure 1).

Figure 1.

Figure 1

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XRD patterns of magnetite-based samples.

Scanning Electron Microscopy (SEM)

After analyzing the SEM micrographs (Figure 2), which reveal the existence of uniform spherical particles (∼75 nm),32 for example, the deposition of Ag and Au NPs on Fe3O4 exhibits the presence of small bright dots, showing dense aggregations uniformly dispersed over massive Fe3O4 particles (Figure 2c,d), while for Fe3O4-Cu, since the difference of size and mass between Fe and Cu is almost similar, it averts direct identification of Cu nanostructures.33

Figure 2.

Figure 2

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SEM analysis of magnetite nanoparticles: (a) bare Fe3O4; (b) Fe3O4-Cu; (c) Fe3O4-Ag; and (d) Fe3O4-Au.

Transmission electron microscopy (TEM) studies were performed for the samples (Figure 3), showing that Fe3O4 NPs are presented in semispherical form with the size of 70–80 nm. This observation is seen to be consistent with other samples, and also seen are small dense particles over the surface of iron oxide spheres; for Fe3O4-Cu, the particle size was around 20 nm (Figure 3a); for Fe3O4-Ag and Fe3O4-Au, it was 17–30 nm (Ag black dense particle), and for Ag NPs, 10–30 nm.

Figure 3.

Figure 3

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TEM images of the samples: (a) Fe3O4-Cu; (b) Fe3O4-Ag; and (c) Fe3O4-Au.

Energy Dispersive X-ray Spectroscopy (EDS)

The elemental composition of the samples (Table S1) shows that in all of the samples, the contents of Fe and O are close to the theoretical values derived for Fe3O4 (O = 27.64%, 57.15% atom; Fe = 72.36% Elem., 42.85% atom).34 The metal deposition in all of the samples was less than 1.0% and the scattering of these NPs over the large magnetite surface was as follows: Cu = 0.52%, 0.30% atom; Ag = 0.83% Elem., 0.77% atom; and Au = 0.53% Elem., 0.52% atom. The EDS spectrum in Figure S1 shows peaks of Fe (Kα = 6.40 keV and Lα = 0.70 keV) and O (Kα = 0.52 keV); in Fe3O4-Cu, Cu Kα is found at E = 8.0 keV; in Fe3O4-Ag, Ag Kα at E = 2.98 keV; and for Fe3O4-Au, the peak at 10 keV was assigned to Au Kα.35 In all samples, a trace of chlorine was found, possibly, from the water.

X-ray Photoelectron Spectroscopy (XPS)

The oxidation state of the metal ion was studied by XPS (Figure S2), showing that in all of the samples (Fe3O4-Cu, Fe3O4-Ag, and Fe3O4-Au), the presence of Fe3O4 is established as a main component. There is also an Na residual (Na 1s, EB = 1085.2 eV) originated from sodium acetate and carbonate, which were used in the sample preparation; besides, there is a carbon C 1s signal at EB = 284 eV due to organic residual and chamber contamination. A typical OKLL is found between EB = 970 and 986 eV for the samples. The presence of Fe3+ is confirmed by observing the peaks Fe 2p3/2 = 710 ± 0.39 eV and Fe 2p1/2 = 724 ± 0.26 eV, which establishes the existence of magnetite due to the lack of the satellite peak (Figure S3) and is commonly found in other Fe3+ oxides.36 The additional Fe 3p signal observed is attributed to the iron present in the high-spin Fe2+ and Fe3+, 37 as seen at 53.43 ± 0.37 eV (Table S2). After observing the O1s signal, which gives a good insight into the structures,38 the presence of surface-adsorbed water and hydroxyl groups is recognized at 531.5 ± 0.46 eV as the split signals in 528–532 eV, and the peaks at 528.4 ± 0.34 eV are contributed from the lattice oxygen species (O2–).39 The surface deposition of Group 11 metal NPs (Fe3O4-Cu, Fe3O4-Ag, and Fe3O4-Au) is confirmed by XPS and the peaks corresponding to Cu (2p3/2 = 931.81 eV and 2p1/2 = 953.63 eV) are observed; nevertheless, the appearance of a satellite at EB = 942.5 eV is the footprint of Cu2+ and it is a natural tendency of Cu0 when it gets oxidized under room conditions. For Fe3O4-Ag, the signals 3d5/2 = 366.03 eV and Ag 3d3/2 = 372.27 eV were seen, showing the presence of Ag.40 Finally, for Fe3O4-Au, 4f corresponding to gold was detected at 91.44 eV41 and the signal was still detectable despite the interference of Fe 3s.

Magnetic Properties

The hysteresis curves for Fe3O4-Cu, Fe3O4-Ag, and Fe3O4-Au were obtained using the vibrating sample magnetometer (VSM) by applying the external magnetic fields at 298 and 5.0 K, and it revealed magnetic saturation (Ms) for bare Fe3O4, Fe3O4-Cu, Fe3O4-Ag, and Fe3O4-Au. The Ms values are almost similar without a significant change (56.8 and 59.9 emu/g); however, for Fe3O4-Ag, the magnetization reached to 75.5 emu/g since Ms is directly related to the nature of metals deposited on the magnetite surface (Figure 4).42 The hysteresis loops suggest the presence of single-domain grains contributing to superparamagnetic behavior.43

Figure 4.

Figure 4

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Magnetic hysteresis loops of Fe3O4, Fe3O4-Cu, Fe3O4-Ag, and Fe3O4-Au.

Group 11 metals have ns1(n – 1)d10 valence electrons acting as e- scavengers and they increase the response of the materials toward external magnetic fields.44 In Fe3O4-Cu, this effect is debilitated due to the small size of Cu atoms as a higher Ms is anticipated if the size of the atom is increased; nevertheless, in the case of Au, the distortion in crystal lattices is expected by the crystal defects and it is directly related to the reduction of the magnetic moment inside particles because of the magnetocrystalline anisotropy effect.45 In particular, for Fe3O4-Ag, the Ag electron scavenger effect could affect the magnetic response while its smaller atom size will not cause strong distortions in the magnetite particle’s surface; thus, it exhibits superparamagnetic property, so, these types of samples can be efficiently recovered after the catalysis.46 The squareness ratio (SQ), defined as Mr/Ms ratio, explains the dependence of the response on the applied magnetic field. As can be observed in Table S3, Fe3O4, Fe3O4-Cu, Fe3O4-Ag, and Fe3O4-Au exhibit an extremely low magnetic squareness (SQ): 0.04 for Fe3O4-Au and 0.05 for Fe3O4-Ag. A remarkably high SQ (0.24) was obtained for bare magnetite Fe3O4 since a magnetic moment with a strong dependence on the applied field is desired (Mr/Ms → 0). These results show that the addition of metallic clusters such as Cu, Ag, and Au is beneficial from the catalytic and magnetic points of view.47

Catalytic Activity

Since it is difficult to obtain the reduction potential for the nitro group , ascorbic acid and NaBH4 were used as control (yield < 10%) to discard any spontaneous reaction (Figure 5). Fe3O4-Cu, Fe3O4-Ag, and Fe3O4-Au were used for the reduction of NA, giving the yield of 70–80%, showing that if G11 metals are presented, the yield has been increased to 82.56% for Fe3O4-Cu’s conversion of NA to oligomers, which would be further increased to 87.8 and 91.3%, respectively, if ascorbic acid or NaBH4 were added. The oligomer conversion was 93.4 and 96.6% for Fe3O4-Ag and Fe3O4-Au, respectively, without any additional reducing agents. From the conversion point of view, all catalysts show a similar behavior for a reduction reaction longer than 60 min. However, the kinetic results demonstrate that the presence of Ag and, especially, Au accelerates the reaction rate. For Fe3O4-Cu, the conversion was 82.5% after 30 min and for Fe3O4-Au, it was >90% even at less than 10 min, showing that Au influences significantly the crystalline and catalytic performances. The reduction reaction follows a first-order kinetics, showing that Fe3O4-Au is the best catalyst with a reaction rate (kFe3O4-Au = 0.416 mM L–1 min–1) of 20 times greater than that of bare Fe3O4 (kFe3O4 = 0.018 mM L–1 min–1). The reduction rate increased when we added NaBH4 (k = 1.04 mM L–1 min–1) to the medium (t1/2 < 1.6 min). In contrast, the reduction rate for Fe3O4-Cu (kFe3O4-Cu = 1.04 mM L–1 min–1) or Fe3O4-Ag is slower even in the presence of NaBH4, with half-lives significantly greater for Cu (t1/2 = 3.39 min) and Ag (t1/2 = 2.88 min) (Figure S4).

Figure 5.

Figure 5

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Reduction of 4-nitroaniline by magnetite catalysts (Fe3O4, Fe3O4-Cu, Fe3O4-Ag, and Fe3O4-Au): (a) reduction yield and (b) first-order kinetics; (c) half-life of the conversion of NA to oligomers under the catalytic process.

Magnetite is a stable material and it has 2/3 of Fe3+ in Fe3O4, making it an electron-rich center for the interaction with aromatic amines to form azo bonds, liberating H+, while 1/3 of the Fe2+ in magnetite is less stable. After the Fe3+ active sites are engaged with NA and iron(II) is involved in the reaction in the presence of water.48

Among the samples, for Fe3O4-Ag, Ag0 slowly transforms into Ag+, which improves its efficiency. For the Fe3O4-Au sample, on the other hand, gold is resistant, behaving as a strong oxidizing agent. Moreover, the SPR of noble metals (Ag and Au) can increase the electron mobility, acting as electron sinks at the material interface centers that reduce the electron density (δ–) in alcohol (in the solvent system), enhancing further the reduction potential.49 It has also been theorized that at the surface of the catalysts, the Fe3O4 matrix can react with H+ and H3O+ to form Fe3O3-Au-H type intermediates.50 The recyclability studies of catalysts were not attempted; however, these materials have been steadily prepared in the laboratory and in our previous works, our group demonstrated the feasibility of the sample recuperation due to their magnetic properties.51

Product Analysis

The NA reduction was studied by 1H NMR (Figure S5a). In the aromatic region (Figure S6b), three signals are presented corresponding to aromatic amines (δ ≈ 6.5 ppm) at the beginning of the reduction (T0) and deshielded hydrogens within the aromatic center (δ ≈ 8.2 ppm). The formation of imine/azo groups typically detected at δ ≈ 10.1 ppm shows that NA exists in equilibrium state with an azo adduct. The H2 production from the catalyst reduces the nitro group as the signal at δ ≈ 6.5 ppm quickly fades, and after 60 min (T1), the imine signal (N=N−) is steadily presented. After T5 = 300 min, only the presence of aromatic hydrogens and imines (δ ≈ 8.2 ppm) is observed, suggesting the reduction of NA, which is consistent with the reported studies.52

The product formation was also analyzed by liquid chromatography (Figure 6). At time = 0 min, in the reduction reaction, NA shows small amounts of a mixture of two semicompounds (molecule A, TR = 13.6 min, C6H4N3O2+, m/z = 150, and molecule B, TR = 15.89 min, C6H6N4, m/z = 135). These two molecules are part of a residual emerging from the natural decay of NA.53 After 3.0 h of the reaction, a new peak corresponding to product P1 at TR = 11.2 min (C36H37N12O4+, m/z = 701) is detected, and after 5.0 h, along with the signal of P1, another peak corresponding to product P2 at TR = 2.6 min (C12H12N4O, m/z = 228) is observed. Both of these products are formed from the reduction of R-NO2 groups through imine linkage. For P1, the oligomer structure consists of six aromatic groups, while for P2, it contains two rings. The additional signals evidently correspond to aniline, m/z = 94.

Figure 6.

Figure 6

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High-performance liquid chromatography-mass spectrometry (HPLC-MS) studies for product analysis in reduction of NA by Fe3O4-Au.

After analyzing the above results, a mechanism is proposed whereby NA undergoes polymeric catalytic reactions in which molecules A and B are formed as cationic fragments, as observed by LC-MS (Scheme 1). In the catalytic reaction, the catalytic material gives away two electrons, which interact with water to produce H2 and OH– ions. In the first step, with two electrons, the −NO2 group converts to –N=O (I), which undergoes a reduction to the –NH-OH group (II). In the second step, −N=O and −NH-OH undergo a reaction to form the diazo link –N=N– (III), which can be reduced to –NH–NH– (IV). Furthermore, another molecule of –N=O (I) reacts with molecule III to form molecule IV, which undergoes a reduction to give molecule V through catalytic reduction. This reaction is repeated three times after taking three molecules of I with molecule V to form VI. The reduction of nitroaniline is normally related to the Haber reaction: the condensation of nitrosoaniline with another nitroaniline to form azoxyaniline structures. If this process further proceeds, a secondary process takes place, involving the formation of hydrazoaniline, yielding imines and H2O, O2, and H+.54 For Fe3O4-Au, NPs enhance amine reduction and facilitate azo polymerization by dendrimeric growth.55 Under the studied conditions (20 °C, 101.3 kPa), a maximum number of 6 bound units is detected and there could be a greater degree of oligomerization if the temperature and pressure are increased.

Scheme 1. Proposed Mechanism for the Reduction of 4-Nitroaniline by Fe3O4-M (Au, Ag, Cu).

Scheme 1

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Carbon Balance Analysis

The total carbon balance of oligomerization of nitroaniline was analyzed (Figure 7), observing that the total amount of carbon in the system remains stable without any further carbon fixation from the environment.56 For example, the TOC is 84.67 mg/L at the initial time, coinciding approximately to the theoretical value of 72 mg/L for NA (1.0 mM). However, this value is increased modestly to 95.20, 97.27, and 96.34 mg/L for Fe3O4-Au, Fe3O4-Ag, and Fe3O4-Cu, respectively (calculated error, σ = 8.65). The slight increase of organic carbon is probably attributed to the reduction of trace amounts of carbonate salts in water.57 The concentration of COD in the oligomerization shows a considerable change; for instance, initially, the COD concentration (control) is 157.1 ± 8.38 mg/L and it has been increased if oligomerization occurs from the reduction of NA in the presence of Fe3O4-Cu, Fe3O4-Ag, or Fe3O4-Au. This means that azo linkages involve in the reduction process, requiring an oxidizing agent such as O2, which is inversely proportional to the number of oligomers formed upon the catalyzed reduction. Thus, for bare Fe3O4, the COD was 197 mg/L, while for Fe3O4-Au, Fe3O4-Ag, and Fe3O4-Cu, the value was increased to 230, 223.26, and 200.93 mg/L, respectively (standard deviation or error σ = 8.84).

Figure 7.

Figure 7

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Carbon–oxygen balance for oligomerization of 4-nitroaniline by Fe3O4-based catalysts.

Mechanism Modeling

The reduction of 4-NA was computationally studied using density functional theory (DFT) calculations, and the geometries at the stationary point were optimized at each reaction step with B3LYP58 at 6-311G(d,p).58b The reduction of NA by Fe3O4 yields the formation of two products; the total energy for the feasibility formation was analyzed thermodynamically (Figure S6).

In the proposed pathway (Scheme 1), several steps were involved in the reduction of NA and they were modeled by DFT (Figure S7). The total energy of each system allowed us to find out the minimum energy of the system that supports the oligomerization of nitroaniline under the catalytic reduction by Fe3O4-Au. The geometrical optimization associated with a minimum energy of the nitroaniline reduction products was calculated in an aqueous environment (Figures 8 and S8). In step 1, −NO2 undergoes a reduction by the catalytic material as this is favored at −0.8 kJ/mmol. Step 2 involves further reduction of the nitro group to form an oxime although we were unable to detect this in the chromatographic analysis as its formation required a greater energy (ΔE = 0.29 kJ/mmol), which would be associated with the rate-determining step. In step 3 and step 4, the low energy couplings are favored because of the reaction of a nitroso group with amine to release of 0.69 kJ/mmol for the dimer formation, while the energy release of 1.01 kJ/mmol is for the trimerization. The metallic clusters promote e- mobility, which generates additional adducts to trigger finally the oligomeric production in step 5. The final product consists of 6 aromatic rings formed by an exothermic reaction (ΔEt = −5.09 kJ/mmol) and the formation of the immediate precursor with ΔE = −4.0 kJ/mmol (step 7). This additional release of energy could further contribute to a self-maintaining reaction, and it enhances further the reduction potential of the system. This observation suggests that a metallic surface allows the reduction reaction. This explains the identification of simple dimers P1 after several hours of reaction (t > 3 h) and the product P2 can be easily formed, but it was not seen in the reduction reaction. So, in the presence of Fe3O4-M (M = Cu, Ag, Au), NA undergoes oligomerization.

Figure 8.

Figure 8

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DFT total energy determined for the products (oligomerization) formed in the 4-NA reduction in the presence of Fe3O4-Au (pathway 1 in blue; pathway 2 in red).

Experimental Section

Materials and Methods

All reagents and solvents used in the studies were as received from the chemical company without any further purification: HAuCl4, FeCl3·6H2O, CH3COONa, Ni(NO3)2, Cu(NO3)2, NaBH4, AgNO3, and Na2CO3 (Merck-Aldrich).

Characterization

XRD analyses were conducted using a D8 Advance Davini diffractometer with Ni-filtered Cu Kα radiation (λ = 1.541Å) with a Theta-Theta Bruker AXS configuration (diffraction angle at 2θ: 20–80° operating at a voltage of 40 kV, 30 mA). Scanning electron microscopy was performed using a JEOL JSM-5900 at 20 kV. Energy dispersive X-ray spectroscopy analyses were also performed using an Oxford ISIS EDS analyzer coupled to the SEM, for which 15 sample spots were taken, the elemental composition was measured, and the average results were used to determine the elemental composition of the sample.59 Transmission electron microscopy was carried out using a JEOL-JEM 2010 microscope equipped with a LaB6 thermo-ionic cannon and the crystalline nature was studied.60 XPS studies were performed using a K-Alpha Thermo-Fisher; the images were collected with a pass energy of 80 eV, and the oxidation state of metal ions was analyzed. Magnetic studies were done using a Quantum Design MPMS3 Squid AC magnetometer at 300 K and in the range of −50 kOe < H < 50 kOe (H ± 5.0 T). Absorbance measurements were performed using a Perkin Elmer Lambda 25 UV-Vis spectrophotometer. In the reduction of 4-NP, the product analysis was studied by 1H NMR using a 400 MHz Varian VNRMS; the separation of products was done by an Agilent 1200 HPLC equipped with a 300 mm C-18 column and the effluent was injected into an Agilent 6410 triple quadrupole for ESI+ analysis.

Synthesis of Nanocatalysts

Magnetite nanoparticles were prepared as reported previously.61 FeCl3·6H2O was mixed with CH3COONa in ethylene glycol, and the resulting mixture was stirred and transferred to a stainless-steel autoclave for heating for 8 h at 473 K. The product isolated was washed three times with ethanol to yield a fine black powder Fe3O4, to which Cu NPs or Ag NPs or Au NPs were doped by the reduction of Cu(NO3)2 or AgNO3 or HAuCl4 to obtain Fe3O4-Cu NPs or Fe3O4-Ag NPs or Fe3O4-Au NPs, respectively (Scheme 2).

Scheme 2. Preparation of Fe3O4 Doping of Group 11 Metals.

Scheme 2

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Catalytic Reduction of Nitrobenzene

To analyze the photocatalytic activity of magnetite nanoparticles, the reduction kinetics of 4-nitroaniline (NA) was determined as follows: A certain amount of NA (0.01 mmol) was added to a flask containing 10.0 mL of a 5:95 of 2-propanol/H2O solution as a solvent system. In this mixture, the prepared catalysts were dispersed in the solution with a concentration of 50.0 ppm. This mixture is allowed to react at room conditions (STR) under stirring in the absence of light. The solution was taken at different time intervals to measure the NA concentration by recording UV-Vis spectra from 200 to 900 nm and the intensity of the peak corresponding to 4-nitroaniline (λmax, 380 nm) was determined. In product analysis using 1H NMR, a sample of Fe3O4-Au (50 ppm) was left to react with NA (0.1 mM) for 5 h, and the substrate and 2-propanol (0.01 mM) were used in the ratio 5:95 in D2O. In the case of HPLC-MS studies, the same procedure was followed with a subsequent injection using a mobile phase consisting of water and CN (90:10) with 0.1% of formic acid.

Carbon Balance Analysis

The carbon balance and oxygen demands were determined after the formation of oligomerization from 4-nitroaniline. Total Organic Carbon (TOC) and Chemical Oxygen demand (COD) were estimated on a GE Sievers Innovox instrument using KMnO4 as an oxidizing agent.

Theoretical Studies

DFT Modeling

Energy determination was carried out using an unrestricted HSEh1PBE/LANL2DZ basis set for a full-set population analysis. The calculated model consists of 4 cubic spinel Fe3O4 cells attached to an Au8 gold bipyramidal (D5h) cluster.

Structural Optimization of Oligomers

The reduction of 4-NA was computationally studied using the DFT calculations,55−57 and the geometries at stationary point at each reaction step were studied with B3LYP58 at 6-311G(d,p).58b The unrestricted B3LYP calculations were used wherever radical systems were involved, and for relative reactivity trend, the UB3LYP/6-311G(d,p) was employed because of its high reliability.62 In a PCM model, the solvent medium (water as a solvent) was considered. The total energy and vibrational frequencies were determined in each step of the mechanism using the optimized geometries as inputs, and the corresponding transition states (TS) were determined by a QST3 calculation using the BB1K set developed by Becke.63 The DFT modeling consisted of a system containing 126 atoms (C = 36, H = 52, O = 22, N = 16) and 556 electrons. The starting geometry (C36H52N16O22) consisted of four 1,4-nitroaniline and two 1,4-bis(dianzenyl)benzene molecules complemented with 14 –OH groups in an aqueous solvent system. For every step in the mechanism, the total system energy was determined, and individual molecular local optimizations were conducted. For the computational work, a B3LYP functional coupled to a 6-311G basis set was used in a polarizable continuum model of water.

Conclusions

In the present work, the oligomerization of p-nitroaniline was achieved via azo bonding using Fe3O4-Cu NPs, Fe3O4-Ag NPs, or Fe3O4-Au NPs. The reduction reaction follows a pseudo-first-order kinetics, and for Fe3O4-Au, the reaction rate is k = 1.04 mM L–1 min–1 (97%). HPLC-MS showed the formation of a six-unit oligomer (C36H37N12O4) and a dimer (C12H12N4O) as the main products, for which a mechanism of NA reduction is proposed. To prove further the feasibility of the reaction, a DFT model was applied to a system consisting of 36 C, 52 H, 16 N, and 22 O in a PCM model. The present study describes the development of nanocatalysts for the reduction of NA to avoid its environmental implications.

Acknowledgments

This research was co-founded by CAPEX Project: Renewable Energies Lab at Tecnológico de Monterrey, Puebla. The authors thank DGAPA for supporting this research through project IN-202622 and CONACYT-DST (266150) and also thank Yiying Wu, Ohio State University, for the support of XPS.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c06326.

  • Preparation details of nanoparticles, powder X-ray diffraction (PXRD), EDS elemental analysis, X-ray photoelectron spectroscopy (XPS), magnetic parameters, minetics of nitroaniline reduction, 1H NMR kinetics for reduction of nitroaniline by Fe3O4-Au; proposed mechanism for formation of P1 by oligomeric reduction of p-nitroaniline by Fe3O4-Au; electrostatic potential mapping of key states during NA reduction (PDF)

The authors declare no competing financial interest.

Notes

The authors declare no competing financial interest. The data that support the findings of this study are available in the supplementary material of this article.

Supplementary Material

ao2c06326_si_001.pdf (1.3MB, pdf)

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