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Article

Highly Efficient and Magnetically Recyclable Non-Noble Metal Fly Ash-Based Catalysts for 4-Nitrophenol Reduction

by
Iwona Kuźniarska-Biernacka
1,*,
Inês Ferreira
1,
Marta Monteiro
1,
Ana Cláudia Santos
2,
Bruno Valentim
2,
Alexandra Guedes
2,
João H. Belo
3,
João P. Araújo
3,
Cristina Freire
1 and
Andreia F. Peixoto
1,*
1
LAQV/REQUIMTE, Department of Chemistry and Biochemistry, Faculty of Science, University of Porto, Rua do Campo Alegre s/n, 4169-007 Porto, Portugal
2
Earth Science Institute—Porto Pole, Department of Geosciences, Environment and Spatial Plannings, Faculty of Sciences, University of Porto, Rua do Campo Alegre s/n, 4169-007 Porto, Portugal
3
IFIMUP, Departamento de Física e Astronomia da Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre s/n, 4169-007 Porto, Portugal
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(1), 3; https://doi.org/10.3390/catal14010003
Submission received: 9 November 2023 / Revised: 10 December 2023 / Accepted: 12 December 2023 / Published: 19 December 2023
(This article belongs to the Special Issue Advances in Green Catalysis for Sustainable Organic Synthesis, 2nd Edition)
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Versions Notes

Abstract

:
4-nitrophenol (4-NPh) is a harmful compound produced in large amounts in the chemical industry, and its reduction to aminophenol (4-APh) using noble metals as catalysts is one of the most studied processes. The development of noble metal-free catalysts represents an economic advantage in large-scale applications and contributes to the sustainability of raw materials. Coal fly ash (FA), a major waste stream from coal combustion, contains an easily recoverable magnetic fraction (FAmag sample) composed of Fe-rich particles that could substitute noble metal catalysts in 4-NPh reduction, with the concomitant advantage of being easily recovered via magnetic separation. For this purpose, a new composite material containing copper ferrite nanoparticles (FAmag@CS@CuFe) was prepared via a facile, environmentally friendly and cost-effective method based on three components: FAmag as the core, a biobased polymer chitosan (CS) as the linker and copper ferrite CuFe2O4 nanoparticles (CuFe) as the active sites. The structure, morphology, composition and magnetic properties of the FAmag@CS@CuFe material were studied to assess the efficiency of the preparation. It was found that the biopolymer prevented the aggregation of CuFe nanoparticles and enabled a synergistically outstanding activity towards the reduction of 4-NPh in comparison to the pristine FAmag and bare CuFe nanoparticles. The FAmag@CS@CuFe catalyst showed efficiency and stability in the conversion of 4-NPh of up to 95% in 3 min over four consecutive cycles. Such remarkable catalytic results demonstrate the potential of this catalyst as a substitute for expensive noble metals.

Graphical Abstract

1. Introduction

The extensive use of aromatic nitro compounds by various industries, such as in the manufacturing of papers, pharmaceuticals, leather, dyes, pesticides and herbicides, has led to environmental contamination of soil and groundwater [1]. As nitroarenes are highly toxic and cancerogenic, various techniques have been employed to reduce their concentrations in water, including physical (e.g., adsorption/desorption processes [2]), chemical (e.g., oxidative degradation [3], photodegradation [4] and catalytic reduction [5]) and biological techniques (e.g., microbial treatment via anaerobic and aerobic biodegradation [6]). An advantage of the catalytic reduction of 4-nitrophenol (4-NPh) is the selective formation of 4-aminophenol (4-APh), which is significantly less toxic and a valuable product since it is an important intermediate in the production of agrochemicals, dyes and drugs, such as paracetamol and phenacetin [7].
Noble metals, especially Au, Pt and Pd nanoparticles, have been extensively used as catalysts for 4-NPh reduction with excellent results (total 4-NPh reduction in less than 3 min [8]). However, their tendency to aggregate during the reaction leads to a significant reduction in catalytic activity [9]. This activity suppression and the high cost of noble metals make this process unattractive to be applied at an industrial scale.
Meanwhile, the search for alternatives to noble metals has been growing in recent years, and transition metal oxides have shown to be promising. Nevertheless, active site aggregation, which results in lower catalytic activity [10,11], still needs to be overcome.
In this context, nanoparticle impregnation via a proper support is being actively used as an efficient methodology to avoid aggregation. For example, the use of carbonaceous materials, such as carbon nanotubes [12], activated carbons [13], carbon black [14] or biochar [15], is excellent for this purpose due to their high surface area and active site separation. However, their production demands harsh conditions and multi-step processes [16,17,18].
Meanwhile, composite materials (natural biopolymer–metallic or metal oxide nanoparticles) have been successfully employed in catalysts for advanced oxidation processes [19], but studies regarding their application in catalytic reduction are scarce [20] despite the fact that composite preparation is simple, environmentally friendly and cost-effective and leads to highly stable, well-dispersed nanoparticles in the biopolymer matrix [21,22].
Among natural biopolymers originated from poly-saccharides, chitosan (CS) is mostly used as a dispersing or supporting agent since it readily forms hydrogels via crosslinking in an acidic medium, and also due to its polymeric chain functional groups (–OH and –NH2), which may chemically or electrostatically interact with metal ions and immobilize them in the matrix via the formation of ion pairs or ion exchange [23]. Moreover, chitosan is a very cheap and easily accessible biopolymer that can be obtained from crustacean shell wastes, thus contributing to minimizing their disposal in landfills [24].
Recently, chitosan-modified Cu2O nanoparticles were tested as a catalyst in 4-NPh reduction and exhibited improved photocatalytic abilities (K0 = 2.28 × 10−1 min−1), with the reduction rate increased by 1.7 times compared to free Cu2O nanoparticles (K0 = 1.32 × 10−1 min−1). This improvement was attributed mainly to chitosan-induced formation of (111) crystal surfaces of Cu2O. However, the catalyst efficiency dropped after three catalytic cycles to 63% and 76% for Cu2O and chitosan-modified Cu2O catalysts, respectively, in comparison to the total 4-NPh conversion in the first cycle [25]. Similarly, the catalytic activity of ruthenium supported on magnetically recyclable biopolymer-based nanocatalysts, where MnFe2O4 was used as the magnetic core and chitosan [26] or chitosan–carrageenan [27] as the biopolymer, was evaluated for 4-NPh reduction. Both catalysts showed a high catalytic performance of 100% in the reduction of 4-NPh in less than 1 min and could be reused in at least 10 cycles without losing their catalytic activity, which confirmed their high stability [26,27].
Regarding catalyst support materials, it is crucial to improve studies based on alternative low-cost materials coming from recycling streams such as coal fly ash (FA), which are mainly composed of aluminosilicate glass and minor amounts of other materials, including Fe-rich particles such as ferrospheres [28,29] that are easily recovered via magnetic separation [2,3,4]. For example, Gadore and Ahmaruzzamn [30] reviewed the use of FA-based nanocomposites as effective photocatalysts for water remediation; Saputra and co-authors [31] prepared a Co-supported fly ash catalyst to promote phenol oxidation and observed a total degradation after 90 min at 45 °C; Yusuff et al. [32] used FA-supported ZnO as a catalyst in biodiesel production from used oil and obtained an 83.2% yield after 180 min at 140 °C; Dong et al. [33] used coal fly ash as a Ni-Re catalyst support for CO2 methanation. The bimetallic catalyst achieved 99.55% of CO2 conversion and 70.27% of CH4 selectivity under the following conditions: 400 °C, 2000 h−1, 1 atm and H2:CO2:N2 = 4:1:0.5.
In a recent study, it was found that the synergy caused by the combination of two transition metals (Cu and Fe) in bimetallic NPs, and the presence of iron oxide in FA can increase the efficiency of the catalyst due to enhanced electron transfer [34]. Thus, a facile synthesis was developed and tested using a magnetically recyclable three-component composite material containing copper ferrite (CuFe), FAmag@CS@CuFe.
As far as is known, for the first time, CuFe nanoparticles were immobilized on ferrosphere surfaces using chitosan as linker, creating an innovative composite material to efficiently reduce 4-NPh and potentially other derivatives.

2. Results and Discussion

2.1. Materials Characterization

The XRF results are listed in Table 1. The FAmag sample had the highest Si and Al concentrations, compared to the modified samples. However, after covering of FAmag with CS using acetic acid (sample FAmag@CS), Si decreased from 36.77 to 16.73 wt.%, and the Al concentration decreased from 19.01 to 7.82 wt.%, respectively, which was due to some leaching promoted by acetic acid [35,36,37]. Consequently, the relative concentration of Fe increased from 41.63 wt.% in FAmag to 68.49 wt.% in FAmag@CS.
Further modifications of FAmag@CS, by the incorporation of Cu or CuFe, resulted in the increase in Cu content (from absence to 0.91 and 14.37 wt.%, respectively, for samples FAmag@CS, FAmag@CS@Cu and FAmag@CS@CuFe) and Fe content (from 41.63 wt.% to 61.68 wt.%, respectively, for samples FAmag and FAmag@CS@CuFe). These results confirmed the successful formation of the tri-component composites FAmag@CS@Cu and FAmag@CS @CuFe.
The FTIR spectrum of chitosan (Figure 1) shows characteristic bands for carbohydrate, amide and amine groups [38]. A broad, intensive band at 3430 cm−1 is attributed to the O–H stretching vibration of adsorbed water and carbohydrate ring (CS), and a weak shoulder at 3240 cm−1 is due to N–H stretching vibration. The weak bands at 2925 and 2875 cm−1 are usually associated with C–H stretching vibrations. The amine deformation vibrations usually produce strong to very strong bands in the 1660–1575 cm−1 region [39], and the band observed at 1654 cm−1 can be associated with the N–H bending amide vibration. In addition, C–N stretching vibrations occur in the 1190–920 cm−1 region and overlap the vibrations of the carbohydrate ring (observed as a band at 1075 cm−1) [40,41]. These most intensive bands due to O–H, C–H, N–H and C–N vibrations were observed in the modified FAmag@CS, FAmag@CS@Cu and FAmag@CS@CuFe materials, confirming the successful coating of FAmag by CS (Figure 1).
The band at 580 cm−1 in FTIR spectra, usually assigned to metal–oxygen stretching vibrations [42,43,44], was only presented in FAmag@CS@Cu and FAmag@CS@CuFe samples. This agrees with XRF results (Table 1) and corroborates the success of the preparation of the multicomponent FAmag@CS@Cu and FAmag@CS@CuFe samples.
To further support the formation of composites containing Cu and CuFe, Raman spectroscopic analysis was performed for the as-prepared respective samples, which allowed us to identify magnetite and hematite in the FAmag sample (Figure 2A), as previously observed [45].
The Raman spectra of the CuFe nanoparticles (Figure 2B) show four Raman active modes at 670 (A1g), 530 (F2g), 470 (T2g) and 300 (E1g) cm−1 and the position of the bands in agreement with the literature data [46,47,48]. The band at 670 cm−1 is usually attributed to the local lattice effect in the tetrahedral sub-lattice, and the band at 470 cm−1 is typically related to local lattice in the octahedral sublattice [47,48]. However, in the Raman spectrum of the tri-component sample (FAmag@CS@CuFe), these bands are overlapped with characteristic bands of iron oxides (from FAmag), which confirm the presence of CuFe nanoparticles in the composite material. The Cu–O modes at ~600 cm−1 and ~300 cm−1 in the Raman spectrum of FAmag@CS@Cu are expected, but they are overlapped by higher intensity bands of iron oxides present in FAmag.
Scanning electron microscopy (SEM-EDS) experiments were performed to characterize the samples of FAmag and FAmag-composite materials (samples FAmag@CS, FAmag@CS@Cu and FAmag@CS@CuFe). The detailed imaging of the FAmag sample (Figure 3) confirms that it is mainly composed by Fe-rich morphotypes. The morphotypes are spheres with variable amounts of aluminosilicate glass embedding Fe-minerals (e.g., wüstite-magnetite dendritic crystals, maghemite, hematite, and magnesioferrite) with variable amounts of Fe (ferrospheres), Fe and Mg (magnesiaspheres and magnesiaferrospheres), and Fe, Ca and Mg (calcimagnesiaferrospheres) (Figure 3A–D) [31].
Chitosan coating of FAmag particles is clearly observed in Figure 3E, as well as the successful incorporation of Cu (Figure S1, Supporting Materials) and CuFe nanoparticles on the chitosan surface, Figure 3F–H. It is clear that CuFe nanoparticles are uniformly dispersed (Figure 3G,H) in the chitosan film (FAmag@CS@CuFe material), as observed by the EDS mapping images of FAmag@CS@CuFe, presented in Figure S2 (Supporting Material). These observations are in accordance with ones observed in the literature for composites using different metals (Ag–Au, Ag–Pd, and Au–Pd) and gum kondagogu as natural biopolymer [20].
The XRD diffractograms of FAmag, FAmag@CS, FAmag@CS@CuFe and CuFe NP samples are presented in Figure 4. The CuFe nanoparticles show diffraction lines at 18.4°, 30.1°, 35.6°, 38.8°, 43.2°, 53.8°, 57.1°, and 62.6°, corresponding to the tetragonal structure of the copper ferrite [49,50]. The XRD pattern of FAmag confirms the presence of SiO2 (quartz) with a diffraction line localized at 26.6° and iron oxides with diffraction peaks at 2θ = 18.2°, 30.2°, 35.6°, 43.2°, 53.8°, 57.1° and 62.8°, corresponding to the spinel structure of Fe2O3 or Fe3O4. In the FAmag@CS@CuFe composite, the diffraction peaks of CuFe were detected along with a characteristic diffraction peak of FAmag at around 26.6° (2θ), corresponding to quartz, indicating the successful preparation of the material.

2.2. Catalytic Performance

Prior to catalytic tests, the 4-NPh adsorption studies were performed using the as-prepared samples (FAmag, CuFe and composites: FAmag@CS, FAmag@CS@Cu and FAmag@CS@CuFe). It was found that the materials had a negligible adsorption of 4-NPh after 180 min of contact. The maximum of 4-NPh adsorption capacity (3.6%) was observed for the pristine FAmag sample. Thus, no kinetic studies of the 4-NPh adsorption process were further performed.
The reduction of 4-NPh, performed in the presence of NaBH4 at room temperature and using the catalysts listed above, is expected to take place in two steps [51]. Firstly, the 4-NPh is spontaneously converted to p-nitrophenolate ion (4-NPh-) by the addition of NaBH4 (pHreaction medium~11, pKa(4-NPh) = 7.1) [52], which changes the color of the solution from pale to deep yellow. In the second step, in the presence of catalysts, the hydrogenation of 4-NPh− takes place to form a colorless solution of 4-APh.
The catalytic reduction of 4-NPh in the presence of CuFe nanoparticles, FAmag and multicomponent composites is shown in Figure 5, and the evolution in the UV-Vis spectrum of the 4-NPh solution during the catalytic reduction is presented in Figure S3, Supporting Material and the catalytic reduction profiles presented in Figure 5. The catalyst FAmag@CS@CuFe showed the best catalytic performance, with total 4-NPh conversion in 3 min (Table 2, Figure 5).
The bare CuFe, under the same experimental conditions, showed 88% conversion of 4-NPh in 3 min. The FAmag@CS@Cu also presented catalytic activity in 4-NPh reduction (substrate conversion 88% in 30 min), but the efficiency was far less than that of pristine CuFe or the FAmag@CS@CuFe composite, due to the need for an induction period. The FAmag and FAmag@CS showed almost no activity after 30 min (less than 10% of substrate conversion) and led to conversion of only 53% and 8% of 4-NPh, respectively, after 180 min.
The reproducibility of FAmag utilization as catalyst in the 4-NPh reduction was tested in three independent experiments under the same experimental conditions. The kinetic profiles and the substrate conversion (Figure 6; Figure S4 Supporting Material) suggest that the active sites in FAmag are uniformly distributed, resulting in Δ %C4-NPh < 1, as previously observed by the authors using magnetic size-fractions of FA [45].
Kinetic studies of the catalytic reduction of 4-NPh were performed. The reactions were performed using a significant excess of NaBH4 compared to the substrate concentration, under the assumption that the BH4− concentration remains constant during the reaction cycle. These studies were assessed using pseudo-first-order kinetics. The rate constants obtained from linear correlation of ln(C/C0) vs. time for 4-NPh reduction were 1.98 min−1, 1.02 min−1, 0.0818 min−1, 0.004 min−1 and 0.0005 min−1 for FAmag@CS@CuFe, CuFe, FAmag@CS@Cu, FA and FA@CS catalysts, respectively (Figure 5B). The calculated activity parameter (reaction rate constant per unit mass) was 19.800 min−1g−1, 10.200 min−1g−1, 0.818 min−1g−1, 0.040 min−1g−1 and 0.005 min−1g−1, respectively (Table 2). Using FAmag@CS@CuFe as catalyst, the reaction rate and catalyst activity increased 495 times in comparison with FAmag, 24 times in comparison with FAmag@CS@Cu, and 2 times in comparison with bare CuFe. These high activities may derive from abundant amine and primary and secondary hydroxyl groups on chitosan that prevented CuFe nanoparticle aggregation on the material surface (Figure 3G,H). A similar effect was observed using polydopamine-graphene/Ag–Pd nanocomposite as the catalyst for reduction of nitrophenol in the presence of NaBH4 as reducing agent. The high activity of the catalyst originated from numerous amine and catechol groups on polydopamine, which ensured the dispersion of bimetallic Ag/Pd nanoparticles on the surface of the nanocomposite [53].
The details of 4-NPh reduction reaction conditions along with the catalytic rate constants are summarized in Table S1 (Supplementary Material). The FAmag@CS@CuFe catalyst exhibited superior catalytic activity compared with the other FA-based catalysts, even those containing noble metals. For example, FA-supported Pd–Ag bimetallic nanoparticles led to faster 4-NP reduction in the presence of NaBH4 in water (K1 = 0.7176 min−1) than their monometallic analogues FA–Pd (K1 = 0.5449 min−1) and FA–Ag (K1 = 0.5572 min−1), as well as their physical mixture (K1 = 0.4075 min−1) [54], which was 2.7 times slower than the rate obtained with FAmag@CS@CuFe (K1 = 1.9761 min−1) in this work. Furthermore, the rate constant of 4-NPh reduction over core–shell fly ash@polypyrrole/Au composite microspheres (K1 = 0.474 min−1) [55] was 4.2 times lower than the one obtained for FAmag@CS@CuFe (K1 = 1.9761 min−1).
Similarly, the FAmag@CS@CuFe catalyst exhibited enhanced catalytic activity compared to the core–shell structured CuFe2O4/Ag@COF nanocomposites (K1 = 0.77 min−1) as well as its two-component composite CuFe2O4/Ag (K1 = 0.25 min−1) or CuFe2O4@COF (K1 = 0.15 min−1) [56], eggshell membrane-CuFe2O4 nanocomposite (K1 = 0.748 min−1) [57], cellulose nanocrystal (CNC)-supported magnetic CuFe2O4@Ag@ZIF-8 nanospheres (K1 = 0.64 min−1) [58] or reduced graphene oxide (RGO) nanocomposite with the best catalytic performance among other RGO/CuFe2O4 composites, i.e., material consisting of 96 wt.% CuFe2O4 and 4 wt.% RGO (K1 = 1.032 min−1) [59] vs. K1 = 1.9761 min−1 obtained in this work. The superior activities of FAmag@CS@CuFe can be ascribed to the synergistic effect of CuFe2O4, CS and FAmag and the homogeneous distribution of CuFe nanoparticles.
Other examples with good performance in 4-NPh reduction were found for the nanocomposite catalysts consist of carbon dots and CuFe nanoparticles [60], possibly due to the smaller particle size of CuFe2O4 in composite than pure CuFe2O4, and consequently the larger surface area of the composite and better adsorption of the substrate and reductant and good electron-transport properties of the C-dot shell [61,62]. However, carbon dot preparation is complex and demands high energy-consuming hydrothermal conditions for coating [62] and core–shell [61] catalysts.

Stability of Catalysts

The recycling of the catalysts is essential to make the process more attractive for application at industrial scale. In this sense, to assess the reusability of the best performing catalyst (FAmag@CS@CuFe), trials were conducted under the same reaction conditions in a sealed quartz cell (optical path l = 1 cm, V = 3 mL). After the first cycle, the catalyst was fixed at the cuvette bottom using an external magnet, the reaction mixture was removed by decantation, and a new portion of 4-NPh and NaBH4 was added to the remaining catalyst. This process was repeated four times. The results are summarized in Figure 7, and the corresponding UV-Vis spectra are shown in Figure S5 (Supporting Materials).
The FAmag@CS@CuFe catalyst was highly active during five consecutive cycles and led to 85% conversion of 4-NPh in the first 10 min. However, the apparent K1 gradually decreased in each cycle (from K1 1st cycle = 1.98 min−1 to K1 5th cycle = 1.82 × 10−1 min−1), which may have been due to some leaching of CuFe. The XRD pattern of the recycled catalyst (Figure S6, Supporting Material) was similar to that of the as-prepared sample, suggesting that the spinel structure of the oxides did not change during five consecutive cycles. However, after the last catalytic cycle, the copper content of the FAmag@CS@CuFe composite was significantly reduced (14.34 wt.% for the as-prepared sample vs. 0.84 wt.% after the fifth cycle), suggesting Cu leaching from the sample (Table 1), whereas the Fe concentration did not change (61.68 wt.% vs. 61.64 wt.%). However, the Raman spectrum of the FAmag@CS@CuFe catalyst after the fifth reaction cycle showed no features pertaining to Cu- or Fe-oxides, indicating their partial conversion into metallic Cu0 or Fe0, respectively [60]. Another possibility is the adsorption of borohydride on the particle surface (Cu0), decreasing the reduction potential of the metallic nanoparticle [63]. This can promote the oxidative dissolution of reduced Cu0 due to high susceptibility toward oxidation by oxygen, and may cause the copper leaching.
Figure 8 displays the results of room-temperature magnetization vs. applied magnetic field curves for all samples. All samples followed a soft ferromagnetic-like behavior where magnetization reached saturation at relatively low fields (<5 kOe), and small remanence magnetization and coercive field values (<3 emu g−1 and 58 Oe) were observed. Such behavior is consistent with the presence of both CuFe2O4 and Fe3O4 magnetic phases. The two most striking features were (i) the larger saturation magnetization displayed by the FAmag@CS@CuFe material after the last catalytic cycle, particularly in comparison with the as-prepared FAmag@CS@CuFe (blue curve), and (ii) the larger paramagnetic-like linear behavior exhibited by CuFe nanoparticles. The latter can typically be attributed to the linear susceptibility associated with uncompensated spins at the nanoparticle surface [64]. More importantly, the former feature can be explained by the enhancement of the higher-saturation magnetization Fe3O4 phase at the expense of the lower-saturation magnetization CuFe2O4 phase—leading to an overall increase in the saturation magnetization of the sample [65,66]. These results corroborate the XRF results (Table 1) and support the hypothesis of Cu leaching from the sample during catalytic cycles.
The formation of Cu0 was observed by Kang et al. [67] during reductive decomposition of CuFe nanoparticles in a methane atmosphere, where Cu segregation and simultaneous formation of magnetic Fe3O4 occurred in the first reduction step of CuFe. The Cu2+ reduction to metallic Cu0 was also observed by Park et al. [68] for water-washed coal fly ash-supported copper catalysts in the presence of NaBH4. The authors suggested that the Cu0/Fe3+ and Cu0/Fe2+ were the active species that led to the enhanced reduction of 4-NPh. This conclusion suggests that the catalysts should be more active in consecutive cycles, and it agrees with the results from the second cycle of FAmag@CS@Cu (Table 2), where the apparent K1 increased in the second cycle from 8.18 × 10−2 to 1.22 × 10−1 min−1.
Summarizing, the Raman spectrum of the FAmag@CS@CuFe material after the catalytic cycle suggests partial reduction of Cu2+ to Cu0 (from CuFe) and Fe2+ or Fe3+ to Fe0 (from CuFe or from ferrospheres, i.e., Fe2O3 or Fe3O4). The study of magnetic properties of FAmag@CS@CuFe suggests the formation of Fe3O4 from CuFe2O4. All of these data support the statement that Cu0 is formed. In aqueous solution, the BH4− ions (from NaBH4) are adsorbed on the surface of the catalysts, and the active species acts as intermediate for electron transfer from BH4− (donor) to substrate 4-NPh (acceptor). During this process, Cu2+ can be partially converted into Cu0 and induce the changes in the valence state between Cu+ –Cu2+ and Fe2+–Fe3+ ion pairs in CuFe2O4, which can enhance catalytic activity, which was suggested by H Zheng et al. [69]. Thus, the presence of Cu0 plays a key role in enhancing the catalytic reduction of 4-NPh, as it can boost the electron transfers of CuFe2O4 and directly transfer electrons from BH4− ions to 4-NPh. Thus, the oxidative leaching of Cu0 to solution causes that activity of FAmag@CS@CuFe to decrease in the following cycles.

3. Materials and Methods

3.1. Reagents and Materials

In this work, the following reagents were used: chitosan (CS, medium molecular weight, 75–85% deacetylated, Merck KGaA, Darmstadt, Germany), sodium tripolyphosphate (TPP, technical grade, 85%, Merck KGaA, Darmstadt, Germany), iron(III) chloride hexahydrate (FeCl3 × 6H2O, for analysis, Merck KGaA, Darmstadt, Germany), copper(II) chloride dihydrate (CuCl2 × 4H2O, ACS reagent, ≥99.0%, Merck KGaA, Darmstadt, Germany), potassium bromide (≥99%, FTIR grade, Merck KGaA, Darmstadt, Germany), 4-nitrophenol (4-NPh, >99%, Merck KGaA, Darmstadt, Germany), sodium borohydride (NaBH4, >99%, Merck KGaA, Darmstadt, Germany), 4-aminophenol (≥99%, Merck KGaA, Darmstadt, Germany), sodium hydroxide (NaOH, ≥99.5%, Merck KGaA, Darmstadt, Germany), potassium carbonate, (K2CO3, 98%, VWR Chemicals, Carnaxide, Portugal). All experiments were conducted using ultrapure water (Millipore, specific resistivity 18 MΩ·cm−1, Merck Millipore, Darmstadt, Germany).
The Fe-rich fraction (sample FAmag) was magnetically recovered from a fly ash sample from a pulverized coal-fired thermal power plant (Abrantes, Portugal) [45].

3.2. Preparation of Materials

3.2.1. Synthesis of CuFe2O4 Nanoparticles

The CuFe2O4 nanoparticles were prepared by an aqueous coprecipitation method described elsewhere [70]. Briefly, 4.0 mmol CuCl2 × 4H2O and 8 mmol FeCl3 × 6H2O were dissolved in 100 mL distilled water, then 15 mL of NaOH solution (5.0 M) was added dropwise (1.5 mL min−1). The mixture was heated at 100 °C and mechanically stirred for 5 h. The obtained dark brown precipitate was separated by external magnet, washed with water until pH 7, then washed with ethanol and acetone, and then dried at 80 °C for 12 h. Henceforward, this sample will be denominated as “CuFe”.

3.2.2. Preparation of Chitosan-Coated Fe-Rich Morphotypes

The sample of “chitosan-coated Fe-rich morphotypes” was prepared according to a modification of a method described by Calvo et al. based on the ionic gelation of CS with TPP anions [71]. An acetic acid solution (0.5%, v/v, 25 mL), stirred at RT for 1 h, was used to disperse 0.5 g of FAmag sample while dissolving CS (0.7 g). The TPP solution (2.5 g/L, 50 mL) was injected dropwise into the CS solution by using a syringe pump with 1 mL/min injection rate. After the complete addition of TPP, the mixture was mechanically stirred at 1000 rpm for 1 h. The formed powder product was then collected with an external magnet, washed with water until pH 7, and dried under vacuum at RT for 24 h. Henceforward, this sample will be denominated as “FAmag@CS”.

3.2.3. Modification of FAmag@CS with CuCl2

Briefly, 0.5 g FAmag was dispersed in 0.5% acetic acid together with CuCl2 × 4H2O (0.3 mmol) and CS (0.7 g), and mechanically stirred for 1 h. Then, 50 mL of TPP solution (2.5 g/L) was injected dropwise, and the mixture was stirred for 1 h. The powder product was separated with an external magnet, washed with water and dried under vacuum at RT for 24 h. Henceforward, this sample will be denominated as “FAmag@CS@Cu”.

3.2.4. Modification of FAmag@CS with CuFe2O4

One three-component composite was prepared by wet impregnation based on ionic gelation using sodium tripolyphosphate as cross-linking agent. Briefly, 1.0 g FAmag was dispersed in 0.5% acetic acid together with freshly prepared CuFe nanoparticles (0.5 g) and CS (1.3 g) and mechanically stirred for 1 h. Then, 100 mL of TPP solution (2.5 g/L) was injected dropwise, and after complete addition, the mixture was stirred for 1 h. The powder product was separated with an external magnet and washed with water and dried under vacuum at RT for 24 h. Henceforward, this sample will be denominated as “FAmag@CS@CuFe”.

3.3. Sample Characterization Methods

The chemical characterization of FAmag, FAmag@CS, FAmag@CS@Cu and FAmag@CS@CuFe included elemental analysis using a portable X-ray fluorescence analyzer (X-MET7500, Oxford Instruments, Bristol, UK) equipped with a 45 kV Rh tube and a high-resolution silicon-drift detector (SDD). Element contents were presented as an average value calculated by the analyzer based on three separate measurements for each sample (20 s per measure).
For the detailed imaging and semi-quantitative chemical analysis, experiments were carried out in a Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) FEI Quanta 400 FEGESEM/EDAX Genesis X4M at the Materials Centre of the University of Porto—CEMUP, Porto, Portugal.
The FTIR spectra were obtained in KBr pellets (Merck, KGaA, Darmstadt, Germany, spectroscopic grade) containing 0.4 wt.% material, 1:250 sample KBr ratio using a Jasco, (Tokyo, Japan) FT-IR 460 Plus spectrometer. All spectra were collected at room temperature, in the 400−4000 cm−1 range using a resolution of 4 cm−1 and 32 scans.
Raman analyses were performed on powder samples at room temperature using a Jobin–Yvon LabRaman spectrometer (Horiba, France) equipped with a CCD camera and a He–Ne laser at an excitation wavelength of 632.8 nm. An optical microscope from Olympus (Tokyo, Japan) with a 100× objective lens was used to focus the laser beam on the sample surface and to collect the scattered radiation. A neutral density filter was used to reduce the power of the laser by 75% (D.06) to avoid laser-induced transformation of the Fe-bearing phases. Scans from 100 cm−1 to 1000 cm−1 were performed on the particles’ surface. The acquisition time and respective accumulations were individually adjusted to acquire an optimized spectrum at spectral resolutions near 1 cm−1.
X-ray diffraction (XRD) analyses were performed at IFIMUP (Departamento de Física e Astronomia da Universidade do Porto, Portugal) on a Rigaku Smartlab diffractometer (Tokyo, Japan). The XRD measurements were performed at room temperature over the range 2θ = 15−80° using Cu Kα radiation (λ = 1.5406 Å) and the Bragg−Brentano θ/2θ configuration.
Measurements of the magnetic properties of materials were investigated by using a commercial Quantum Design MPMS 3 SQUID magnetometer. The magnetization (M), as a function of the applied magnetic field (H) measurement, was performed at 300 K for a maximum H of 50 kOe.

3.4. Catalytic Reduction

The catalytic reduction of 4-NPh to 4-APh was carried out at room temperature using NaBH4 as the reducing agent. During the experiments, 30.0 mL of a 5.0 × 10−5 M stock solution of 4-NPh and 56.7 mg of NaBH4 were constantly stirred. With the addition of NaBH4, the pale-yellow color of the solution turned to bright yellow due to the formation of nitrophenolate ions. After addition of FAmag-based catalysts, the yellow color of the solution progressively vanished, following the reduction of 4-NPh. The samples were collected at fixed intervals of time and centrifuged. The conversion of the substrate to the 4-APh was monitored by UV-Vis spectroscopy measurements of the absorption spectra in the range 200–500 nm, using a quartz cell (l = 1 cm).
After each catalytic cycle, the catalysts were separated using an external magnet and washed with water to remove potential traces of substrate product and reductant adsorbed on the catalyst surface. The recovered catalysts were dried at 60 °C for 12 h under vacuum and reused in a new reaction cycle under identical experimental conditions. Adsorption experiments were carried out in the presence of the catalysts in the dark, and a blank experiment in the absence of catalyst was also performed. Reusability of the material with the best catalytic performance was evaluated for five continuous cycles.

4. Conclusions

Catalysts based on coal fly ash, a coal combustion residue, were prepared by an efficient method using a bio-degradable polymer (chitosan) as linker and CuFe as catalytic active sites, avoiding the use of noble metal-based catalysts. Multi-technique characterizations (XRF, FTIR, SEM-EDS and RAMAN) confirmed the successful preparation of the target tri-component composite, FAmag@CS@CuFe, which exhibited catalytic activity superior to that of the other catalysts studied and, to some extent, those described in the literature, which can be ascribed to the specific characteristics of its structure and the synergistic effect of CuFe and chitosan. The recyclability of this composite is remarkable, although with some negligible losses after the fifth cycle, showing that the reported catalyst is very promising for the industrial-scale use of coal fly ash as support for active transition-metal oxides to be used in the reduction of nitro compounds or other typical catalytic reduction reactions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14010003/s1, Figure S1: Detailed imaging and X-ray microanalysis of the FAmag@CS@Cu sample; Figure S2: Elemental area-mappings for FAmag@CS@Cu sample; Figure S3: Change of UV–Vis spectrum of 4-NPh during the reduction of 4-NPh by NaBH4 in the presence of: A—FAmag@CS@CuFe, B—CuFe, C—FAmag@CS@Cu, D—FAmag and E—FAmag@CS catalysts (initial 4-NPh concentration c = 5.5 × 10−5 M, catalyst dose = 3.3 mg L−1, NaBH4 concentration c = 0.05 M); Figure S4: Reproducibility tests of FAmag catalyst (initial 4-NPh concentration c = 5.5 ×10 −5 M, catalyst dose = 3.3 mg L−1, NaBH4 concentration c = 0.05 M); Figure S5: Stability tests of FAmag@CS@CuFe catalyst (initial 4-NPh concentration c = 5.5 × 10−5 M, catalyst dose = 3.3 mg L−1, NaBH4 concentration c = 0.05 M); Figure S6: XRD patterns of CuFe nanoparticles, samples FAmag, and composite FAmag@CS@CuFe, where FAmag@CS@CuFe* is the catalyst after the last catalytic cycle, Table S1: Comparison of catalytic conditions and apparent first-order rate constants for reduction of 4-NPh by FAmag@CS@CuFe and similar catalytic systems recently reported.

Author Contributions

I.K.-B.: Conceptualization, Investigation, Validation, Data curation, Writing—Original draft, review and editing, Project Administration and Funding Acquisition; I.F.: Investigation; M.M.: Investigation; A.C.S.: Investigation, Data curation, Writing—Original draft, review and editing, B.V.: Conceptualization, Validation, Data curation, Writing—Original draft, review and editing, Project Administration and Funding Acquisition; A.G.: Investigation, Validation, Review and editing; J.H.B.: Investigation, Data curation, Writing—Original draft; J.P.A.: Investigation, Data curation, Writing—Original draft; C.F.: Project Administration and Funding Acquisition, Review and editing; A.F.P.: Conceptualization, Review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by project PhotoBioTrans (FCT ref. EXPL/CTM-CTM/0790/2021, http://doi.org/10.54499/EXPL/CTM-CTM/0790/2021).

Data Availability Statement

Data are contained within the article and supplementary materials.

Acknowledgments

The authors thank Fundação para a Ciência e Tecnologia (FCT) and Ministério da Ciência, Tecnologia e Ensino Superior (MCTES) for support of this work through the projects UIDB/50006/2020, UIDB/04683/2020, UIDP/04683/2020, UIDB/04968/2020, UIDP/04968/2020, PTDC/EME-TED/3099/2020, INSUB (FCT ref. DRI-India/0315/2020), PINFRA/22096/NECL/2016, and authors I.K.-B. for contract funding through program DL 57/2016—Norma transitória REQUIMTE/EEC2018/14, A.C.S. for PhD scholarship Ref: SFRH/BD/131713/2017 and COVID/BD/151941/2021) J. H. Belo for his contract DL57/2016 reference SFRH-BPD-87430/2012 and A.F. P. for contract funding under the Scientific Employment Stimulus (FCT ref: 2020.01614.CEECIND/CP1596/CT0007). The authors also thank Pego power plant (Portugal) for the fly ash samples.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

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Catalysts 14 00003 g001
Figure 1. FTIR spectra of pristine CS, FAmag, CuFe nanoparticles and FAmag based composite materials in 4000–500 cm−1 region.
Figure 1. FTIR spectra of pristine CS, FAmag, CuFe nanoparticles and FAmag based composite materials in 4000–500 cm−1 region.
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Figure 2. Raman spectra of the Fe-bearing morphotypes (A) and FAmag based composite materials (B) in 150–1000 cm−1.
Figure 2. Raman spectra of the Fe-bearing morphotypes (A) and FAmag based composite materials (B) in 150–1000 cm−1.
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Figure 3. Detailed imaging and X-ray microanalysis of samples FAmag, FAmag@CS and FAmag@CS@CuFe (SEM-EDS, BSE mode): (A) overview of Fe-rich spherical morphotypes in sample FAmag (×2000); (B) FAmag, example of ferrosphere (Z10) and magnesiaferrosphere (Z11) (×5000); (C,D) EDS spectra Z10 and Z11 of black square areas in “B”; (E) FAmag@CS sample, chitosan (dark grey) binding magnetic spheres, and Z5 EDS spectrum of chitosan layer (×2500); (F–H) micrographs of FAmag@CS@CuFe sample showing homogeneous distribution of CuFe nanoparticles (EDS spectrum Z4 in (F); ×10,000) on chitosan ((G,H); ×1750 and ×7500, respectively).
Figure 3. Detailed imaging and X-ray microanalysis of samples FAmag, FAmag@CS and FAmag@CS@CuFe (SEM-EDS, BSE mode): (A) overview of Fe-rich spherical morphotypes in sample FAmag (×2000); (B) FAmag, example of ferrosphere (Z10) and magnesiaferrosphere (Z11) (×5000); (C,D) EDS spectra Z10 and Z11 of black square areas in “B”; (E) FAmag@CS sample, chitosan (dark grey) binding magnetic spheres, and Z5 EDS spectrum of chitosan layer (×2500); (F–H) micrographs of FAmag@CS@CuFe sample showing homogeneous distribution of CuFe nanoparticles (EDS spectrum Z4 in (F); ×10,000) on chitosan ((G,H); ×1750 and ×7500, respectively).
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Figure 4. XRD patterns of CuFe nanoparticles, parent FAmag and composite FAmag@CS@CuFe.
Figure 4. XRD patterns of CuFe nanoparticles, parent FAmag and composite FAmag@CS@CuFe.
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Figure 5. Catalytic reduction process for 4-NPh with CuFe nanoparticles, FAmag, and two- and three-component composites: (A) kinetic plot; (B) linear fit between ln (C/C0) of 4-NPh and time.
Figure 5. Catalytic reduction process for 4-NPh with CuFe nanoparticles, FAmag, and two- and three-component composites: (A) kinetic plot; (B) linear fit between ln (C/C0) of 4-NPh and time.
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Figure 6. The results of catalytic reduction of 4-NPh using FAmag as catalyst, triplicate test (A) kinetic curves and (B) conversion of 4-NPh during the catalytic reduction process, 1-3Dup—the number of experiment duplications. Conditions: mcatalyst = 100 mg, c4-NPh = 5 × 10−5 M, 30 mL, 56.7 mg of NaBH4.
Figure 6. The results of catalytic reduction of 4-NPh using FAmag as catalyst, triplicate test (A) kinetic curves and (B) conversion of 4-NPh during the catalytic reduction process, 1-3Dup—the number of experiment duplications. Conditions: mcatalyst = 100 mg, c4-NPh = 5 × 10−5 M, 30 mL, 56.7 mg of NaBH4.
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Figure 7. Plot of five consecutive cycles for reduction of 4-NPh over FAmag@CS@CuFe.
Figure 7. Plot of five consecutive cycles for reduction of 4-NPh over FAmag@CS@CuFe.
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Figure 8. Room-temperature magnetization vs. magnetic field curves of CuFe nanoparticles, and bare FAmag and composite FAmag@CS@CuFe, where FAmag@CS@CuFe* is the catalyst after the last catalytic cycle.
Figure 8. Room-temperature magnetization vs. magnetic field curves of CuFe nanoparticles, and bare FAmag and composite FAmag@CS@CuFe, where FAmag@CS@CuFe* is the catalyst after the last catalytic cycle.
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Table 1. XRF results (wt.%) regarding elements of the initial and modified samples.
Table 1. XRF results (wt.%) regarding elements of the initial and modified samples.
SampleAlSiPKCaTiMnFeCuZnSr
FAmag19.0136.770.01.300.250.550.3041.630.000.050.08
FAmag_AA a13.7527.400.002.041.420.780.4754.060.000.040.04
FAmag@CS7.8216.734.450.571.010.650.2168.400.000.030.05
FAmag@CS@Cu4.8318.100.081.011.300.480.5072.720.910.010.04
FAmag@CS@CuFe5.7213.173.640.380.300.330.2661.6814.370.000.04
FAmag@CS@CuFe b5.2313.160.641.070.870.450.3161.640.840.010.02
a The FAmag_AA sample—FAmag washed with diluted acetic acid (AA). Conditions: 500 mg of FAmag sample with 150 mL of 0.5% AA was stirred for 60 min at room temperature. b after 5th catalytic cycle.
Table 2. Results of catalytic reduction of 4-NPh with pristine FAmag and FAmag composite-based catalysts a.
Table 2. Results of catalytic reduction of 4-NPh with pristine FAmag and FAmag composite-based catalysts a.
EntryRunCatalystTime b
(min)		%C cK1 d
(min−1)		R2 eK′1 f
(min−1g−1)
11CuFe3(1)881.02 × 1000.9810.2
21FAmag180534.00 × 10−30.990.04
32FAmag20(5)981.86 × 10−11.00
41FAmag@CS18085.00 × 10−40.970.005
52FAmag@CS180(20)524.70 × 10−31.00
61FAmag@CS@Cu30888.18 × 10−20.950.818
72FAmag@CS@Cu30(5)941.22 × 10−10.98
81FAmag@CS@CuFe g3(1)991.98 × 1000.9919.8
92FAmag@CS@CuFe g6(2.5)814.96 × 10−11.00
103FAmag@CS@CuFe g10(3)974.79 × 10−10.99
114FAmag@CS@CuFe g10(3)964.02 × 10−10.99
125FAmag@CS@CuFe g10(3)851.82 × 10−10.99
131FAmag_AA h180483.8 × 10−30.980.04
a Experimental conditions: initial 4-NPh concentration (5 × 10−5 mol·dm−3), NaBH4 (5 × 10−2 mol·dm−3) and catalyst loading (100 mg), volume total V = 30 mL under stirring. b Reaction time, in parenthesis induction time. c Conversion of 4-NPh calculated from %C = 100 − [(Af/A0) × 100%], where A0 and Af are absorbance values of 4-NPh (λ = 400 nm) at t = 0 and at the end of the reaction (min).d K1 is apparent first-order kinetic rate constant (min−1), calculated from ln(Af/A0) = −K1 × t. e Correlation coefficient. f K’1 is activity parameter calculated from K´1 = K1/m, where m is the total catalyst mass (g) used in the reaction. g Reaction was performed in sealed spectroscopic cell (V = 3 mL) experimental conditions: initial 4-NPh concentration (5 × 10−5 mol·dm−3), NaBH4 (5 × 10−2 mol·dm−3) and catalyst loading (10 mg). h The FAmag_AA sample—the parent FAmag washed with diluted acetic acid (AA); conditions: a mixture containing 500 mg of FAmag sample with 150 mL of 0.5% AA was stirred for 60 min at room temperature.
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Kuźniarska-Biernacka, I.; Ferreira, I.; Monteiro, M.; Santos, A.C.; Valentim, B.; Guedes, A.; Belo, J.H.; Araújo, J.P.; Freire, C.; Peixoto, A.F. Highly Efficient and Magnetically Recyclable Non-Noble Metal Fly Ash-Based Catalysts for 4-Nitrophenol Reduction. Catalysts 2024, 14, 3. https://doi.org/10.3390/catal14010003

AMA Style

Kuźniarska-Biernacka I, Ferreira I, Monteiro M, Santos AC, Valentim B, Guedes A, Belo JH, Araújo JP, Freire C, Peixoto AF. Highly Efficient and Magnetically Recyclable Non-Noble Metal Fly Ash-Based Catalysts for 4-Nitrophenol Reduction. Catalysts. 2024; 14(1):3. https://doi.org/10.3390/catal14010003

Chicago/Turabian Style

Kuźniarska-Biernacka, Iwona, Inês Ferreira, Marta Monteiro, Ana Cláudia Santos, Bruno Valentim, Alexandra Guedes, João H. Belo, João P. Araújo, Cristina Freire, and Andreia F. Peixoto. 2024. "Highly Efficient and Magnetically Recyclable Non-Noble Metal Fly Ash-Based Catalysts for 4-Nitrophenol Reduction" Catalysts 14, no. 1: 3. https://doi.org/10.3390/catal14010003

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