Facile copper ferrite/carbon quantum dot magnetic nanocomposite as an effective nanocatalyst for reduction of para-nitroaniline and ortho-nitroaniline

PNA (C₆H₆N₂O₂, 99%) and ONA (C₆H₆N₂O₂, 98%), as pollutant models, gelatin (C₁₀₂H₁₅₁N₃₁O₃₉), sodium borohydride (NaBH₄, 98%), as a reducing agent, and ethanol (C₂H₅OH, 96%) were purchased from Merck without any purification process.

A magnetic stirrer (Fan Azma Gostar, Iran) was used to dissolve the solutions to prevent agglomeration. Detailed examination of the functional groups of gelatin-derived CQDs and the CuFe₂O₄/CQD nanocomposite was recorded by Fourier transform infrared (FT-IR) spectroscopy (Nexus Model Infrared Spectrophotometer, Nicolet Co, USA). To acquire the crystal structure and size of nanoparticles and CQDs, x-ray diffraction (XRD; PW1730, Phillips, Netherlands) was carried out with Cu Kα radiation (λ = 1.540 56 Å) at room temperature. The patterns analyzed by XRD were measured between angle radii ranging from 10° to 80° (2θ) and the time of each step was reported in 1 s. To compute the crystal size of the CuFe₂O₄/CQD nanocomposite, the Debye–Scherrer formula was used. The approximate size can be calculated from [35, 36]

$$\begin{align}D = { }\frac{{K\lambda }}{{\beta \cos \theta }}\end{align} \tag{ 1 }$$

where ${D_{{\text{avg}}}}$ is the crystal size (nm), $K$ is a constant (about 0.9 for synthesized particles), $\lambda$ is the x-ray wavelength, $\beta$ is the full width at half-maximum (FWHM) of extreme and wide peaks and $\theta \,$is the Bragg angle or diffraction angle. Investigation of the morphology and diameter of nanoparticles was done by scanning electron microscopy (SEM; Carl Zeiss 1430VP L, Germany) and transmission electron microscopy (TEM; EM 208S, Philips, Netherlands). To calculate the absorbance for catalytic reduction of nitroarenes, UV–visible absorption spectra (UV–Vis; Specord 250, Analytic Jena) were used. A centrifuge (Hettich Centrifuge EBA III) was used to aid the separation process. Physical adsorption and desorption of N₂ at 77 K with a BELSORP MINI II device were employed to measure the Brunauer–Emmet–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore size distribution of the synthesized sample.

CuFe₂O₄ nanoparticles were synthesized via the facile one-step hydrothermal method. Initially, 0.574 g of tetrasodium EDTA (99%) salt as a chelator was dispersed in 50 ml of deionized water for 10–15 min in an ultrasonic bath cleaner. Then, stoichiometric amounts of 3.24 g of anhydrous FeCl₃ (98%) and 1.35 g of CuCl₂·2H₂O (99%) were added to the desired solution under a constant stirring speed. The alkaline solution, prepared by adding 17.7 g of granular sodium hydroxide (NaOH, 97%) per 100 ml of water, was added dropwise to the solution using a syringe (during the reaction, the pH of the reaction was continuously monitored using a pH meter). When the pH reached 10–11, the solution was poured into a Teflon autoclave at 185 °C for 15 h. After completion of the reaction, the magnetic sediment was separated by a strong external magnet and washed several times with ethanol and distilled water. Finally, the prepared brown sediment was dried at 60 °C for approximately 3–4 h.

Gelatin-derived CQDs were acquired by the one-pot hydrothermal method, as shown in figure 1. First, 2.0 g of gelatin was added to 50 ml of distilled water and dispersed into an ultrasonic cleaner for 20 min. Afterwards, the solution flowed into a steel Teflon autoclave for 4 h at 180 °C. After reaction and cooling, the solution was separated for half an hour by centrifugation at 6 rpm. After this process, a yellowish product was obtained via centrifugation and dried in a hot oven at 60 °C for 2–3 h.

Zoom In Zoom Out Reset image size

Typically, CuFe₂O₄/CQD nanocomposite was prepared by a co-precipitation method with a weight ratio of 2:1 (figure 2). Two millmol (0.02 mg) of CuFe₂O₄ and 1 mmol (0.01 mg) of gelatin-CQDs were poured into 10 ml of ethanol and completely mixed for 15 min until dispersion. Then, the solution was heated at 70 °C in an oil bath and perfectly combined under argon gas and a magnetic stirrer for 4 h. Finally, the target brown precipitate was detached with a strong external magnet, washed alternately with distilled water and ethanol (2:1) and dried in an oven overnight at 40 °C.

Zoom In Zoom Out Reset image size

The appropriate catalyst dosage, PNA or ONA and NaBH₄ were measured in a quartz crystal cell with a path length of 1 cm and a total volume of 3.50 ml at room temperature. The following steps were performed to inspect the reduction of PNA and ONA as models of an aqueous environmental pollutant with a reductant of NaBH₄ and nanocatalyst of CuFe₂O₄/CQD. Firstly, each pollutant (separately) was dispersed into 100 ml of distilled water. Next, 5 ml of the acquired nitroaniline solution was spilled into the test tube and 100 mg of NaBH₄ as a reducing agent and 3.5 mg of CuFe₂O₄/CQD as a catalyst were added to the solution in the tube. Finally, 100 μl of the prepared solution was transferred quickly into a quartz cuvette and its absorption was measured second by second until the color of the solution completely changed. The improvement of the reduction reaction was recorded by measuring the UV–Vis absorption spectrum of the reaction mixture.

Figure 3(a) shows the FT-IR spectra of the CuFe₂O₄/CQD nanocomposite, and figure 3(b) illustrates the FT-IR spectra of gelatin-CQDs. As shown in figure 3(a), the wide peak observed at 3500 cm⁻¹ was attributed to the stretching vibration of O–H. The strong peaks around 1650 cm⁻¹ and 1500 cm⁻¹ are depicted as the metal–OH (Fe–OH and Cu–OH) bending vibration, respectively [37]. Furthermore, the high-frequency band at ∼570 cm⁻¹ refers to the deformation of Fe–O in octahedral sites [38]. The Fe–OOH bond at 999 cm⁻¹ illustrates the functional groups of copper ferrite that exist in the CuFe₂O₄/CQD nanocomposite [39]. As can be seen in figure 3(b), the bands at 900 cm⁻¹ and 1076 cm⁻¹ in the IR of gelatin-CQDs are assigned to stretching of C–O bonds, demonstrating the existence of carboxylic acid belonging to the other functional groups containing oxygen. The broad peak at 3400 cm⁻¹ corresponds to the N–H bands, proving the existence of amino groups in the CQD structure due to the gelatin [40].

Zoom In Zoom Out Reset image size

XRD was employed to determine crystal structures, nanoparticle size and material purity. Figure 4 demonstrates the XRD pattern of the CuFe₂O₄/CQD nanocomposite. All diffraction peaks can be indexed as the tetragonal phase of CuFe₂O₄ (JCPDS card no. 34-0425). The characteristic diffraction peaks seen at 2θ = 30.24°, 35.59°, 43.69°, 57.29°, 61.69° and 62.84° denote the (112), (211), (220), (321), (224) and (440) crystal planes of CuFe₂O₄, respectively [41]. The strong peak at 2θ = 42.39° is indexed to Cu [42]. Furthermore, prominent peaks seen at 2θ = 49.44°, 53.59° and 74.29° correspond to (*−202), (*020) and (*220), indicating the monoclinic CuO phase plane (JCPDS no. 80-1916) [43, 44]. Some secondary impurities, such as Fe₂O₃, are still found (peak seen at 2θ = 33.14°), which may be attributed to the insolubility of FeO, in good agreement with JCPDS card no. 33-0664 [45]. Furthermore, the nanocomposite's XRD pattern confirms its crystalline structure. A broad diffraction peak at 2θ = 19° is attributed to the (002) peak, denoting that the CQDs were added to the magnetic CuFe₂O₄ particles [46]. According to the FWHM crystal plane (211), based on Scherrer's equation, the crystallite size of nanoparticles was calculated to be about 12.2 nm [47].

Zoom In Zoom Out Reset image size

To determine the nanoparticle size, a histogram graph is presented (figure 5). The calculated particle size distribution in the ImageJ software histogram indicates that the particles were distributed in the range of 61.38 nm in the CuFe₂O₄@CQD nanocomposite.

Zoom In Zoom Out Reset image size

To obtain further information about the morphology and size of the prepared nanoparticles, SEM and TEM were employed. Figures 6(A) and (B) show SEM images, figure 6(C) is the TEM picture and figure 7 illustrates the energy dispersive x-ray images and map for the CuFe₂O₄/CQD magnetic nanocomposite. The results obtained from SEM pictures show that the particles have a high accumulation of spherical and cubic shapes. The agglomeration of nanoparticles is related to the interaction between the magnetic nanoparticles of copper ferrite, the time for which the solution was irradiated and the shape of the magnetic particles [48]. TEM analysis was carried out to get more information about nanocomposite morphology as a catalyst. It can be seen that some nanoparticles are agglomerated, which could be due to the large specific surface area, small particle size and high surface energy of CuFe₂O₄ nanoparticles; when the surface-to-volume ratio of nanoparticles increases, the effect of the catalyst goes up. The nanoparticle size is larger than the nanocomposite, so the surface-to-volume ratio in the nanocomposite was higher than in CuFe₂O₄ nanoparticles. As a result, the reaction rate and time of the nanocomposite were higher than for the CuFe₂O₄ nanoparticles individually. Furthermore, the EDS mapping pictures in figure 7 show that Cu, C, O and Fe elements were all distributed throughout the CuFe₂O₄/CQD nanocomposite. A map of the nanocomposite comprehensively indicated the homogeneous distribution of the mentioned elements. Different proportions of CQDs and magnetic nanoparticles were synthesized. However, when CQDs were used in a larger amount than nanoparticles it caused suffocation and reduced the performance of the catalyst. The optimum ratio of 2:1 (2 mmol magnetic nanoparticles + 1 mmol CQDs) was selected. Energy-dispersive x-ray spectroscopy proved the correct synthesis of CuFe₂O₄/CQD nanocomposite figure 8.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Textural characteristics of CuFe₂O₄/CQD nanocomposite are reported in table 1. The CuFe₂O₄/CQD nanocomposite has a high BET surface area of 120.6 m² g⁻¹, associated with a pore volume of 0.24 cm³ g⁻¹. Additionally, the measured mean pore diameter of 8.1 nm indicates that CuFe₂O₄/CQD nanocomposite has a mesoporous structure with many mesopores.

Figure 9 shows the N₂ adsorption/desorption isotherm of the CuFe₂O₄/CQD nanocomposite. Due to the substantial shape symmetry, the N₂ adsorption/desorption isotherm is classified as type II, demonstrating nanoporous texture including mesopores [49]. The CuFe₂O₄/CQD nanocomposite possesses a type H3 hysteresis loop, revealing the aggregation of plate-like particles that cause the appearance of slit-shaped pores [50, 51]. The depicted isotherm at 0 ⩽ P/P₀ ⩽ 0.2 is an apparent characterization of the mesoporous configuration, which stems from monolayer and multilayer adsorption on the inner surface [52].

Zoom In Zoom Out Reset image size

The BJH pore size distribution of the CuFe₂O₄/CQD nanocomposite at pore diameters ranging from 1 to 100 nm is depicted in figure 10. It can be observed that CuFe₂O₄/CQD nanocomposite comprises numerous mesopores with small diameters, a few high-diameter mesopores and minor macropores. It can be concluded that the preparation of CuFe₂O₄/CQD nanocomposite affected the formation of mesopores. The illustrated pore size distribution confirms the N₂ adsorption/desorption isotherm, showing the presence of mesopores.

Zoom In Zoom Out Reset image size

Reduction of PNA and ONA with NaBH₄ as a reducer was studied in the presence and absence of a nanocatalyst (figure 11). The UV–Vis spectrum with the typical absorption peak at 380 nm for PNA showed a slight decrease: only 3% of nitroanilines were reduced even after 24 h [53]. This suggests that electron transfer from the electron donor, NaBH₄, to the nitroaniline receptor may be inhibited by a kinetic obstacle. The ability of the obtained CuFe₂O₄/CQD nanocomposite, as a heterogeneous catalyst, to catalyze the reduction of PNA to PPDA and ONA to ortho-phenylenediamine (OPDA) in the presence of NaBH₄ was evaluated. The top of the absorption peak for PNA is at λ_max = 385 nm (figures 11(a) and (c)), and for ONA it is at λ_max = 285 nm and 420 nm (figures 11(b) and (d)) [54, 55]. In an assessment of the reduction (the diagram shown in figures 11(a) and (b)) there was no reaction for hours when only the reducing agent of sodium borohydride was included before the addition of nanocatalyst to the solution. However, after addition of the magnetic catalyst, as exhibited in figure 11(c), the absorption of PNA at λ_max = 385 nm declined from 0.741 to 0.010. During this process a new absorption peak at approximately 310 nm began to increase after 13 s. The reason for this descrease at 385 nm was related to the decrease in PNA concentration and the cause for the rise, and the creation of a new peak, was related to the creation of a new amine product, PPDA [56]. In order to investigate the reduction of ONA, two main peaks are observed at λ_max = 285 nm and 420 nm. With passing time, the absorption diminished from 0.609 to 0.023 λ_max = 420 nm and shifted from 284 nm to 294 nm. The main reason for the decrease was related to the decline in ONA concentration, and the shift was due to the formation of OPDA [57]. This study showed that the nanomaterials created excellent reduction profiles for nitroanilines. As is clear from figure 12, the results indicate that efficient electron transfer from the BH₄ ^– anion to nitroanilines (PNA and ONA) as electron acceptors occurs by the Fermi level shift of the nanoparticles, indicating its noticeable catalytic activity [58]. Typically, the catalyst's performance depends on the available active surface and the number of active sites on the surface. Additionally, while the nanomaterial reacts with the BH₄ ^– anion, hydride can be formed and may subsequently interact with nitroaniline molecules that may be adsorbed on the metal surface. Consequently, after a rapid reaction at the active sites, the reaction was rapidly completed while the product was desorbed from the nanomaterial surface almost immediately [53]. Figure 13 illustrates the color change during reduction. It is evident that the PNA and ONA are bright yellow and orange-yellow solids, respectively; after adding reductant, there were no changes in color, but after adding the synthesized nanocatalyst, the color changed to gray (colorless) [59].

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Considering the catalytic effect of the nanocatalyst on nitro group reduction with respect to the specific structural roles of its parts, −NO₂ is a nitroaniline group. The –NO₂ group is an electron-withdrawing group. Since an electron-withdrawing group is present, there will be a −I effect. In the ONA and PNA compounds, only pair displacement is observed. Therefore, both the −I and the −m effect are observed. However, compared with PNA, ONA will have a close induction effect. Therefore, ONA is less basic than PNA compounds. The synthesized nanocatalyst reduced PNA in less time and faster than ONA.

The efficiency of the CuFe₂O₄/CQD nanocomposite as a catalyst of PNA and ONA was measured using equation (2). In this formula, ${A_0}$ and A_t are the absorbance of the solution in time = 0 (absorbance of PNA or ONA without any additive) and time = t, respectively [60]

$$\begin{align}\left( \% \right){\text{conversion }}= \frac{{{A_0} - {A_t}}}{{{A_0}}} \times 100.\end{align} \tag{ 2 }$$

A linear correlation between ${\text{ln }}\left({{C_t}/{C_0}} \right)$ and reduction time ($t$) for PNA and ONA reduction using the CuFe₂O₄/CQD magnetic nanocatalyst is seen in figures 14(a) and (c). Because the concentration of NaBH₄ is higher than that of the nitroanilines, the catalytic system follows pseudo-first-order kinetics. To calculate the rate constant (${K_{{\text{app}}}}$) the following equation was utilized:

$$\begin{align}\ln \left( {\frac{{{C_t}}}{{{C_0}}}} \right) = \ln \left( {\frac{{{A_t}}}{{{A_0}}}} \right) = - {K_{{\text{app}}}}.t.\end{align} \tag{ 3 }$$

Zoom In Zoom Out Reset image size

In this equation, the definition of all parameters is the same as in equation (2) [61]. To calculate the rate constant in more detail ($k'$) for the catalyst used to catalyze the reaction (the nanocatalyst), equation (4) can be used

$$\begin{align}\begin{array}{*{20}{c}} {k' = {K_{{\text{app}}}}/M.} \end{array}\end{align} \tag{ 4 }$$

In equation (4), $k'$ is equal to the ratio of ${K_{{\text{app}}}}$ (apparent rate constant; s⁻¹ or min⁻¹) to $M$ (the exact catalyst dosage in g or mg used to catalyze the PNA or ONA solution reduction) [32]. Using equations (2)–(4), the conversion yield, apparent rate constant and rate constant per unit of catalyst were measured (table 2).

In figures 14(a) and (c) the line slope of $- \ln ( {\frac{{{A_t}}}{{{A_0}}}} )$ with respect to time can be defined as ${K_{{\text{app}}}}$. It is obvious that with passing time the slope of the line increases slightly, demonstrating noticeable agreement with pseudo-first-order kinetics. It is noteworthy in figures 14(b) and (d) that the concentration of substrates, as a pollutant model, was decreased in the aqueous medium due to substrate consumption during reduction. The concentration finally reached approximately zero.

The CuFe₂O₄/CQD nanocomposite as a nanocatalyst to catalyze the reduction reaction of PNA and ONA was compared with other reported investigations (tables 3 and 4, respectively). For many reasons, the prepared catalyst is the best and most effective catalyst among the synthesized catalysts that are candidates in other studies. Firstly, the other catalysts were either expensive or required scarce precursors or combinations of several materials. These reasons are not negligible in terms of cost-effectiveness and availability. Secondly, those with cost-effective raw materials had a long reaction time. Hence, the as-obtained nanocatalyst is the best catalyst for time-saving, availability of raw materials, cost-effectiveness and facile synthesis method.

To study the industrial capability of the synthesized nanocatalyst, turnover number (TON) and turnover frequency (TOF) were calculated. A TOF in the range 10⁻²–10² s⁻¹ (enzymes 10³–10⁷ s⁻¹) is desirable for industrial applications [78]. Considering that each CuFe₂O₄/CQD nanocomposite has at least one active catalytic site, the TON and TOF are calculated related to moles of CuFe₂O₄/CQD nanocomposite as follows [79]:

$$\begin{align*}{\text{TON }} = \frac{{{\text{moles of reagents} \times {\text{yield}}}}}{{{\text{moles of nanoparticles}}}}\end{align*}$$

$$\begin{align*} = >{\text{ TON }}\left( {{\text{PNA}}} \right) = \left( {{\text{0}}{\text{.0521}}} \right){{ * }}\left( {{\text{98}}{\text{.54}}} \right) / \left( {{\text{1}}{\text{.463}}} \right) = {\text{3}}{\text{.509}}\end{align*}$$

$$\begin{align*} = >{\text{ TON }}\left( {{\text{ONA}}} \right) = \left( {{\text{0}}{\text{.0521}}} \right){{ * }}\left( {{\text{96}}{\text{.16}}} \right) / \left( {{\text{1}}{\text{.463}}} \right) = {\text{3}}{\text{.424}}\end{align*}$$

$$\begin{align*}{\text{TOF }} = \frac{{{\text{TON}}}}{{{\text{time of reaction }}\left( {\text{s}} \right)}}\end{align*}$$

$$\begin{align*} = >{\text{ TOF }}\left( {{\text{PNA}}} \right) = {\text{3}} .509/13 = 0 {\text{.269 }}{{\text{s}}^{ - {\text{1}}}}\end{align*}$$

$$\begin{align*} = >{\text{ TOF }}\left( {{\text{ONA}}} \right) = {\text{3}} .424/35 = 0 {\text{.0978 }}{{\text{s}}^{ - {\text{1}}}}.\end{align*}$$

The reusability of nanocatalyst in reduction of nitroanilines should be considered when evaluating the efficiency of catalytic applications. In this examination, the recyclability of magnetic catalyst in six repeated cycles for the reduction of PNA and ONA was evaluated, as exhibited in figure 15. After completion of each catalytic reduction reaction, the CuFe₂O₄/CQD nanocomposite was collected from the reaction solution by a strong external magnet, washed with ethanol and water to reactivate it, then filtered with a pump and dried after each reaction. Recycled and dried catalyst was reused in subsequent catalytic reactions. The outcomes of the recyclability study revealed that the magnetic nanocatalyst showed higher recyclability performance. In contrast, the catalytic performance of reusable catalyst changed negligibly from 98.8% to 96% for PNA reduction and from 96.16% to 90% for ONA after six cycles. These findings illustrated that CuFe₂O₄/CQD nanocomposite is an efficacious and reusable catalyst for reduction of PNA and ONA.

Zoom In Zoom Out Reset image size